Flexible Carbon Nanotube Based Temperature Sensor for Ultra-small

7/21/2019 Flexible Carbon Nanotube Based Temperature Sensor for Ultra-small

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Northeastern University

Mechanical Engineering Undergraduate CapstoneProjects

Department of Mechanical and IndustrialEngineering

December 04, 2007

Flexible carbon nanotube based temperaturesensor for ultra-small-site applications

Brendan CrawfordNortheastern University

Dan EspositoNortheastern University

Vishal JainNortheastern University

David PelletierNortheastern University

Tis work is available open access, hosted by Northeastern University.

Recommended CitationCrawford, Brendan; Esposito, Dan; Jain, Vishal; and Pelletier, David, "Flexible carbon nanotube based temperature sensor for ultra-small-site applications" (2007).Mechanical Engineering Undergraduate Capstone Projects. Paper 55. hp://hdl.handle.net/2047/d10012904
http://iris.lib.neu.edu/mech_eng_capstonehttp://iris.lib.neu.edu/mech_eng_capstonehttp://iris.lib.neu.edu/mech_ind_enghttp://iris.lib.neu.edu/mech_ind_enghttp://iris.lib.neu.edu/mech_ind_enghttp://iris.lib.neu.edu/mech_ind_enghttp://iris.lib.neu.edu/mech_eng_capstonehttp://iris.lib.neu.edu/mech_eng_capstone


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Flexible Carbon Nanotube Based Temperature Sensor for

Ultra-Small-Site Applications

MIMU 701-702

Technical Design Report

December 4, 2007

Department of Mechanical and Industrial Engineering

College of Engineering, Northeastern University

Boston, MA 02115

Flexible Carbon Nanotube Based Temperature

Sensor for Ultra-Small-Site Applications

Final Report

Design Advisor: Prof. Constantinos Mavroidis, Prof.

Yung Joon Jung, Dr. Azadeh Khanicheh

Design Team

Brendan Crawford, Dan Esposito,


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Flexible Carbon Nanotube Based Temperature Sensor for Ultra-

Small-Site Applications

Design TeamBrendan Crawford, Dan Esposito,

Vishal Jain, David Pelletier

Design Advisor

Prof. Dinos Mavroidis, Prof. Yung Joon Jung,

Dr. Azadeh Khanicheh

Abstract

There are many current and future temperature measurement applications that would benefitsignificantly from a commercially available, low power consuming, ultra-small-scaletemperature sensor. The superior material properties of carbon nanotubes (CNTs) havesuggested that it is possible to create a device that will meet these requirements and also havebetter performance when compared to devices currently available in the market However, thereare currently no commercially available temperature sensors that make use of the advantages inusing CNTs as a small-scale sensing element due to relatively high cost of manufacturing. Thisreport outlines the design, fabrication, and testing of a novel CNT-based Temperature Sensorencapsulated in a thin film of flexible Parylene C. The design employs a single-walled carbonnanotube (SWNT) network between two micro-scale electrodes as the sensing element. Theresulting device exploits the extremely small size of SWNTs as well as their superior thermal

and electrical properties by deriving the temperature based on a change in electrical resistancethat is induced by thermal strain. The sensitivity of the sensor is maximized by making use of anexpansion element (with high thermal coefficient of expansion), which is a novel method forregulating strain in the SWNT network, and was a key factor in relating thermal strain toelectrical resistance. The sensor requires power in the microwatt rangeorders of magnitudeless than that of typical micro-scale sensors. The basic concept prototype sensors beingfabricated can be characterized to measure temperature and predictions can be made successfullyabout the improvement in overall performance of the expanding element concept, whencompared against a device with it.


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TABLE OF CONTENTS

CHAPTER # 1: INTRODUCTION.............................................................................................................. 71.1 Problem Statement ........................................................................................................................71.2 General Information and Considerations for Nanotechnology ....................................................7

1.3 Nano-scale Challenges.....................................................................................................................91.4 Advantages of Carbon Nanotubes ..................................................................................................91.5 Summary and Significance of Nanotechnology Research .........................................................11

CHAPTER # 2: BACKGROUND............................................................................................................. 122.1 Introduction...................................................................................................................................... 122.2 Patents..............................................................................................................................................122.3 Literature Review.............................................................................................................................152.4 Summary of Current Technology ..................................................................................................212.5 Other Temperature Measurement Devices ...................................................................................222.6 Why Use CNTs for Temperature Measurement?..........................................................................22

CHAPTER # 3: Design Development.................................................................................................... 23

3.1 Introduction...................................................................................................................................... 233.2 Interviews.........................................................................................................................................233.3 Needs Assessment..........................................................................................................................283.4 Specifications ..................................................................................................................................303.5 Initial Concepts................................................................................................................................ 323.6 Concept Selection...........................................................................................................................403.7 Summary of Initial Design Concepts.............................................................................................41

CHAPTER # 4: DETAILED DESIGN ...................................................................................................... 434.1 Introduction...................................................................................................................................... 434.2 Detailed Design Concept................................................................................................................434.3 Modeling...........................................................................................................................................444.4 CAD Drawings.................................................................................................................................. 53

CHAPTER # 5: NANOMANUFACTURING PROCESS..........................................................................57

CHAPTER # 6: PROTOTYPING AND TESTING ...................................................................................636.1 Prototyping ......................................................................................................................................636.2 Testing..............................................................................................................................................70

CHAPTER # 7: CONCLUSIONS AND FUTURE WORK........................................................................ 747.1 Conclusion ....................................................................................................................................... 747.2 Future Work .....................................................................................................................................74

APPENDIX A: Material Properties ........................................................................................................ 76APPENDIX B: Detailed Manufacturing Steps...................................................................................... 76

REFERENCES ........................................................................................................................................82


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LIST OF FIGURES

Figure 1:Wilson Tennis Balls Containing Nanoengineered Materials...........................................................8Figure 2: Chirality of Carbon Nanotubes.....................................................................................................10Figure 3: Multi-walled Carbon Nanotube ....................................................................................................11Figure 4: Cross-section of an overlapped nano sensor configuration of the invention...............................13

Figure 5: Response of nano-scale temperature sensor to a 24C temperature change ............................13Figure 6: Plan view of a sensor assembly depicting aligned CNT-based conductors between twoelectrodes.................................................................................................................................................... 14Figure 7: Fabrication Process of the electrode PC board...........................................................................15Figure 8: Completed Nano Sensor Chip bonded on PC board...................................................................15Figure 9: Linear Resistance vs Temperature for CNT grown at gap width of 0.9 m and 1.8m..............16Figure 10: Temperature Co-efficient of Resistance for a typical bulk MWNT device in differentmeasurements ............................................................................................................................................17Figure 11: I-V characteristics of a typical bulk MWNT device.....................................................................17Figure 12: The micro-machined platform for DEP assembly ......................................................................19Figure 13: Schematic of the fabrication process .........................................................................................19Figure 14: SEM micrograph of the 3D assembled SWNT bundles............................................................. 19Figure 15: Measured resistance vs. temperature from a SWNT bridge .....................................................20Figure 16: The measured I-V curves from the 3D SWNT bundles before and after encapsulation...........20Figure 17: Temperature sensor using a single CNT................................................................................... 33Figure 18: SEM image showing a single CNT............................................................................................34Figure 19: A single CNT shown between two gold leads ...........................................................................34Figure 20: Sensor Concept Employing a Complex Pattern for Increased Sensitivity to Temperature....... 35Figure 21: Gold thermistor sensor with protective parylene layer...............................................................35Figure 22: Design Concept using a CNT network ......................................................................................36Figure 23: Bimetallic Position 1...................................................................................................................37Figure 24: Bimetallic Position 2...................................................................................................................38Figure 25: Bimetallic Sensor using CNTs ...................................................................................................38Figure 26 : Thermocouple Concept ............................................................................................................39Figure 27: Improved Expanding Element Concept .....................................................................................44Figure 28: Stress-Strain Curve for 20um CNT-parylene film......................................................................45Figure 29: Change in resistance.................................................................................................................45

Figure 30: Graph showing the maximum stress induced from a 100C temperature change, with changingPMMA width and insulating Parylene C layer thickness.............................................................................49Figure 31: Calibration curve for the dual PMMA expanding element, CNT-based temperature sensor ....51Figure 32: Predicted calibration curve comparison between the same device with and without PMMAexpanding elements....................................................................................................................................52Figure 33: Dual Expanding Element Layout ...............................................................................................54Figure 34: Dimension of the sensing element of the dual expansion device..............................................55Figure 35: SCS Parylene Deposition System............................................................................................. 57Figure 36 : ICP Plasma Therm 7900 ..........................................................................................................58Figure 37: Contact angle measurement of Parylene C...............................................................................59Figure 38: Contact angle measurement of Parylene C after roughening ...................................................59Figure 39: Hydrophobic recovery of Parylene C after 72 hours..................................................................60Figure 40: Brewer 100CB Resist Spinner/Bake..........................................................................................60

Figure 41: Zeiss Supra 25 FESEM .............................................................................................................61Figure 42: MRC/MAT-VAC Sputtering System...........................................................................................62Figure 43: The minimum intact features, 3um Trenches 9um Spacing in 500nm Shipley 1805 on 10um ofParylene C. .................................................................................................................................................64Figure 44: Optical Mask, 6um lines x 9um Spacing....................................................................................65Figure 45: Sonication of the SWNT solution suspended by tape ...............................................................65Figure 46: (a) Patterned wafer at 10

owith droplets of SWNT solution and (b) dried SWNT Solution........66

Figure 47: Trench in photo resist coated after dry casting SWNT solution ................................................66Figure 48: CNT Network after acetone bath, with slight overhangs ...........................................................67Figure 49: Parylene Lifting off Si Substrate after Sonication ......................................................................68


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Figure 50: CNT lift off after pressurized acteone stream............................................................................ 69Figure 51: Ti Electrodes (Light Grey) over 6um wide CNT Network (vertical lines) over Parylene (Blue) .70

LIST OF TABLES

Table 1: Contacts List for Email Interviews.................................................................................................23Table 2: Email Interviews with Local Professors and Scholars ..................................................................24Table 3: Interview with Professor Mehmet Dokmeci...................................................................................25Table 4: Interview with Nurse Rosa Morelli.................................................................................................26Table 5: Interview with Professor Sinan Mft............................................................................................ 27Table 6: Temperature Sensor Design Specifications .................................................................................30Table 7: Concept Design Selection Matrix..................................................................................................41Table 8: Change in maximum normal stress, and thus strain, with varying temperature...........................50Table 9: Interpolation of the previous strain-resistance relationship...........................................................50Table 10: Material Properties Used in Modeling Analysis ..........................................................................76


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Acknowledgments

The authors would like to thank the advisors Prof. Dinos Mavroidis, Prof. Yung Joon Jung, and Dr. AzadehKhanicheh for their continuous guidance and valuable support during the entire course of this project.

We would also like to thank Prof. Sinan Muftu, Northeastern University for putting several concepts into

perspective through his valuable insight and suggestions.

We are greatly indebted to the members of Kostas Center , especially Dr. Siva Subramanian, Selvapraba Selvarasah,Liala Jaberansari, Prashanth Makaram and Scott McNamara for their contribution towards the better understandingof the manufacturing processes.

We are also thankful to Prof. Mehmet Dokmech, Northeastern Universityi for sharing his knowledge on MEMSdevices.

Last but not least, we would like to thank Prof. Gregory Kowalski for continuously encouraging and being there forany help needed to take this project forward.

Copyright

We the team members,

Brendan Crawford Dan Esposito Vishal Jain David Pelletier

Dinos Mavroidis Yung Joon Jung Azadeh Khanicheh

Hereby assign our copyright of this report and of the corresponding Executive Summary to the Mechanical, andIndustrial Engineering (MIE) Department of Northeastern University. We also hereby agree that the video of our

Oral Presentations ifs the full property of the MIE Department.

Publication of this report does not constitute approval by Northeastern University, the MIE Department or its facultymembers of the findings or conclusions contained herein. It is published for the exchange and stimulation of ideas.


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CHAPTER # 1: INTRODUCTION

1.1 Problem Statement

As scientific research moves towards technological advancements on a smaller scale than ever before, it is becoming

increasingly necessary to develop sensing capabilities on an equally small scale in order to successfully study these

technologies. The superior thermal, electrical, and mechanical properties of carbon nanotubes (CNTs) have

suggested that using them as small-scale sensors would lead to sensor performance that is better than other devices

of similar size, or likewise, would lead to a smaller size than other devices of comparable performance. However, no

commercially available sensors exist which exploit the superior properties of CNTs that have been demonstrated in

research labs, primarily because the required nanomanufacturing processes are still too costly to offer a sufficient

potential for revenue.

This report focuses on ultra-small-scale temperature measurement, which is currently limited to devices no smaller

than two or three millimeters across in terms of whats commercially available. The challenge is to design,

fabricate, and characterize an ultra-small, low power consumption CNT-based temperature sensor that is

encapsulated in a flexible substrate. It is necessary to justify why using CNTs for obtaining temperature

measurements is advantageous, compared with traditional temperature measurement methods. Because of the goal

for an ultra-small sensor size, the device falls under strict restraints in terms of current nanomanufacturing

capabilities and the overall time constraint on the project. In addition, the device needs to be tested and validated

based on certain criteria such as repeatability of measurements, ease of calibration, and implementationconsiderations. The first step is to present information justifying the advantage of using CNTs in various aspects of

technology.

1.2 General Information and Considerations for Nanotechnology

A significant advancement in dimensional circuitry for electrical engineers was stepping into the micro scale, where

one micrometer is equal to 1,000thof a millimetera realm that can barely be recognized by the human eye. It is

clear to anyone in the technology field the importance of saving space; smaller products mean better portability, as

well as saving money on the physical supplies. As the technology advances, the ability to build smaller has grown

and science is currently entering the nano-scale age. One nanometer is equal to 1,000thof a micrometer. This realm

is no longer visible by the human eye. In fact, it is impossible to see by any optical equipment, since optical

wavelengths themselves are at the nanometer scale.

Besides the dimensional differences between micro-scale and nano-scale, there are some important considerations

for manufacturing differences. Physical manipulation is still possible at the micro-scale even though it requires


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absolute precision and quality equipment. However, it is particularly challenging to physically move, assemble or

modify at the nano-scale using standard micromanufacturing equipment. More detail on current nanomanufacturing

practices are discussed in Chapter 4 of this report.

Currently there are many products on the market that employ some sort of nanotechnology. Some examples are:

longer lasting tennis balls made by Wilson, digital cameras containing nano-optic technology, and advanced

pharmaceuticals using buckyballs for selective drug delivery. Buckyballs are individual spherically shaped

molecules composed only of carbon atoms. Carbon nanotubes are the cylindrical equivalents of buckyballs, also

called buckytubes. The structurally ideal molecule geometries are what lead to their superior mechanical, electrical,

and thermal properties [1].

Figure 1:Wilson Tennis Balls Containing Nanoengineered Materials

Impact on Temperature Nanosensor Design

It is clear the there are often significant advantages to using nanoengineering in many types of products. As

engineers are learning to control nanotechnology, implementing it and exploiting the advantages are becoming

cheaper, to the point where there is a clear financial advantage to using nanotechnology in consumer products, aswell as in other industries. Employing this technology not only offers improvements in the performance of many

products, but can give a company a competitive and attractive edge in the eyes of customers as well. Remaining on

the forefront of technology is a proven strategy for running a successful company and the advantages to doing so on

the business level .

The same concepts apply for small-scale sensing. Characterizing the performance of nanotechnology elements for

use in sensors is expected, and has been shown, to be effective for making existing technology better. Doing this

also enables appropriate measurements to be made for future high-tech and small-scale applications that dont yet

have adequate sensing capabilities. Although nano-scale sensing is being researched by institutions worldwide, thecommercial availability has not yet been established because no companies see the potential for immediate revenue.

Characterizing a nanosensor to measure temperature and including a plan for implementation in several applications

might bridge the gap between promising research and actual revenue.


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1.3 Nano-scale Challenges

It is important to discuss the major challenges in designing and manufacturing at the nano-scale in order to

understand what steps are needed to successfully fabricate and characterize a temperature sensor on this scale. The

following discussion addresses these challenges.

One of the most significant differences between the behavior of objects on the meso-scale (measured in meters) and

on the nano-scale is the dominant forces controlling the matter. The dominating forces at meso-scale are gravity and

friction. The micro-scale deals with different dominant forces, specifically surface forces. Surface forces include

stiction, friction, electrostatic and Van der Waals forces. At the nano-scale, the prominent forces are intermolecular

and atomic forces, the effects of which are often neglected in meso-scale analysis. It is therefore important that

effort be put towards understanding what assumptions can and cant be made in order to accurately predict the

behavior of ultra-small-scale devices.

Assembly at the meso-scale can be as basic as using ones hands or some automated macro-scale manufacturing

equipment. On the contrary, micro-scale manufacturing requires the use of high precision mechanical equipment for

assembly, but this is not a feasible solution on the nano-scale. Manufacturing nano-scale products requires the use

of directed self-assembly, which involves indirect methods for manipulating materials.

Currently, cost is a big hurdle for the development of nanotechnology. Nanomanufacturing processes still have a

very low output of products considering the lack of metrology technology and commercial infrastructure [2]. CNTs,

for example, are actually grown but do not develop in a consistent manner. Detailed CNT properties and challenges

are discussed in the sections to follow. The CNT synthesis process is slow and costly, which places a difficult

challenge on developing various facets of nanotechnology. Even though it is expected that nanomanufacturing costs

will go down, until then it will be very hard to commercialize nanomanufactured technology

1.4 Advantages of Carbon Nanotubes

CNTs are one of the basic building blocks in nanotechnology. Understanding the properties of CNTs is essential in

understanding the feasibility of using them to improve the performance of temperature sensors. This section covers

the advantages in basic properties of CNTs which suggest that using them in small-scale sensors could yield

unprecedented sensor performance. Chapter 2 of this report will cover some specific examples of CNT-based

temperature measurement that has been demonstrated in research labs and will provide further evidence that a CNT-

based design is advantageous.

CNTs contain some amazing properties that lead to superior performance in mechanical, electrical, and thermal

comparisons. CNTs have a tensile strength of 65 300 GPa and can stretch to 20 % of their original size [3]. They

can have high thermal conductivity, high current density (higher than copper), and are 100 times stronger than steel


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[3]. Unfortunately the processes used to create CNTs are very time consuming. These processes include arc

discharge, laser ablation, high pressure carbon monoxide (HiPCO), and chemical vapor deposition (CVD). Each

method carries with it a set of challenges but can produce a large amount of CNTs at one time. Because of the batch

fabrication capabilities, CNTs are readily available and can be purchased in solution form from many different

companies for competitive prices. Because of their commercial availability, CNTs dont need to be grown in order

to use them in new products. However, implementing them in an effective and repeatable way is very important for

bringing the overall cost of a CNT device down.

Next, it is necessary to discuss the variations of CNTs, and their respective properties. CNTs form in three variations

of chirality. Chirality is the geometric structure in which these hexagonal shapes form together. The three types are

armchair, zigzag and chiral displayed below (Figure 2) [3].

Figure 2: Chirality of Carbon Nanotubes

These variations are responsible for forming metallic, semi-metallic and semi-conductive nanotubes respectively.

The lack of chirality control is a current obstacle in nanomanufacturing. This can be overcome using procedures

such as electrophoresis, which uses electric fields to migrate only the metallic tubes [4]. Synthesis processes for

CNTs can form either single-walled nanotubes (SWNT) or multi-walled nanotubes (MWNT). MWNTs are

concentric CNTs of various diameters held together by Van der Waals forces (Figure 3). [5]


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Figure 3: Multi-walled Carbon Nanotube

MWNTs can have adversely affected conductive properties since physical contact may not be made by the inner

tubes. Each variation can be beneficial depending on the specific scenario.

When creating CNT-based circuitry, as would be the case for a CNT temperature sensor, either individual CNTs or anetwork of CNTs can be utilized. CNT networks involve a vast amount of contacts in the mesh, which create a very

conductive carpet. This is compatible with creating complex designs on a substrate; A layer of CNT networks

can be applied and many variations of designs and can be created on the surface using electrical burn out to

remove unwanted areas. Employing a network helps to average out electrical and mechanical properties that might

otherwise be hard to predict [5].

1.5 Summary and Significance of Nanotechnology Research

Several important conclusions were made from nanotechnology research related to the design of an ultra-small-scale

temperature sensor:

Firstly, it can be concluded that manufacturing cost is the primary factor preventing CNT devices from being

commercialized. It follows that reducing the costs associate with nanomanufactured devices would lead to sensors

becoming commercially available. Due to the advanced properties of CNTs, it is possible that a single CNT sensor

design could achieve the performance required for use in many small-scale applications, to sense many different

properties, whereas other sensors are usually designed for a specific high-technology micro or nano-scale

application. Interestingly, if the superior properties of CNTs could be used to create a highly applicable temperature

sensing device, the possibility for batch fabrication of a single generic sensor design becomes realistic. With

increased potential for batch fabrication comes lower manufacturing costs.

The following chapter includes specific examples where CNTs were used in temperature measurement. These

examples will be used to confirm that it is advantageous to use a SWNT network, rather than MWNTs or single

CNTs, especially when the pattern of the sensing element needs to be relatively complex.


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CHAPTER # 2: BACKGROUND

2.1 Introduction

For this project, relevant patents and scholarly articles were examined in order to obtain a thorough understanding of

current ultra-small-scale temperature sensing options in terms of performance, size, and commercial ability. In

addition to this, the goal was to identify any gaps in the intellectual property and previous work that can be

improved upon, which has not yet been addressed. Special attention was focused on the use of CNTs in the

applications of temperature sensors because the superior properties of CNTs discussed in the previous chapter

suggest that all other methods of temperature measurement will be inferior in some aspect of the measurement

capabilities.

After a considerable amount of research, it was observed that there are relatively few patents related to a CNT based

temperature sensor. However, quite a few journals and articles have been published on this topic, indicating that a

great amount of work has been done in the field of nanotechnology in the last few years, especially related to CNTs.

Additionally, the number of available papers and the current discussion among scholars suggests that CNTs show

promise in terms of their applicability and performance in both current and developing technology fields.

This chapter presents the patents and papers that were examined for the project. Because the specific use of CNTs

in temperature sensors has not been commercialized, the following sections incorporate allof the information the

group was able to find that is directly related to the topic. The large amount of attention that is put towards each

article in this report is important because it provides a thorough understanding of what has been accomplished, and

where there is room for improvement.

2.2 Patents

2.2.1 Fabrication of Nano-Scale Temperature Sensors and Heaters [6]

This patent claims to design a nano-scale temperature sensor and heaters by depositing two strips of nano sized

metals on an electrical insulator substrate using a Focused Ion Beam (FIB) deposition process. The second metal

strip is deposited in such a way that it is slightly overlapping the first strip forming a bimetal sensing junction

(Error! Reference source not found.). The two metal strips are of different materials each of 50 nm thickness. The

two metals used are tungsten (W) and platinum (Pt). The sensor designed in this patent works on the principle of a

thermocouple and is claimed to have a sensitivity of approximately 5.4 mV/C ( Error! Reference source not

found.).


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Figure 4: Cross-section of an overlapped nano sensor configuration of the invention

Figure 5: Response of nano-scale temperature sensor to a 24C temperature change

The patent claims that the sensor can also be used to measure other properties besides temperature. The resistive

segments of the heater can be made up of alternative segments of two different FIB deposited metals. The deposited

metals are not pure, and will contain gallium (Ga) originating from the FIB process and other contamination,

causing the deposited metal to have a resistivity that will allow heating when electric current is passed through the

metal. The ability of FIB to mill or deposit at high lateral resolution permits the preparation of site specific sensors.

The sensors can then be adapted in the fabrication of micro-electromechanical systems (MEMS).

The impact of this patent on the project is that it demonstrates that ultra-small-scale temperature measurement has

been patented. However, this device does not use CNTs and it is expected that the performance could increase if

CNTs could be properly characterized as a temperature sensor. Additionally, this patent makes use of tungsten and

platinum as the two sensor metals, which provides a starting point for the electrode and wire materials in the new

design.


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2.2.2 Integrated Carbon Nanotube Sensor [7]

This patent claims to have designed a method and structure for an integrated circuit (IC) comprising a first transistor

and an embedded carbon nanotube field effect transistor (CNT FET) proximate to the first transistor. The CNT FET

is dimensionally smaller than the first transistor.

The CNT FET is adapted to sense signals from the first transistor, where the signals consists of temperature, voltage,

current, electric field, and the magnetic field signals. Moreover the CNT FET is adapted to measure the stress and

strain in the IC, wherein the stress and strain comprise any of the mechanical and thermal stress and strain.

Although this patent makes use of CNTs in its application, it does not discuss or provide any explicit details as to

how the CNTs are involved with the primary transistor to detect/determine the various parameters of sensing. Also,

this design is geared towards a specific application. Designing a new sensor to sense only temperature, but for a

variety of applications, is a novel concept.

2.2.3 Carbon Nanotube Based Sensor and Method for Continually Sensing Changes in a Structure [8]

This patent claims a sensor for detecting changes in a structure. The CNT conductors are operatively positioned on

the substrate with at least one pair of spaced apart electrodes coupled to the opposing ends of the conductor ( Error!

Reference source not found.). The electrodes and the CNT conductors are coupled to the substrate and this

assembly is in turn coupled to the point of interest. Using the principle of resistance of CNTs and CNT strain,

changes in the portion of the structure to which the sensor is coupled induces a change in the electrical properties of

the conductor. It claims to use either SWNTs or MWNTs and the fabrication is done by controlled deposition using

an electric field to align the CNTs with respect to the electrodes.

Figure 6: Plan view of a sensor assembly depicting aligned CNT-based conductors between two electrodes

Electrodes

Single CNTs


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The final sensor assembly is flexible and hence can be used for the purpose of strain measurements of the surface

its coupled with. Baseline parameters, level parameters of strain, temperature and pressure are measured initially.

The electrical properties of the sensors are measured over time to determine any changes from the baseline response

which is indicative of change in one of the parameters experienced by that portion of the structure [Error!

Bookmark not defined.]. This patent is not only geared towards a specific application, but it makes use of single

CNTs between electrodes. The processes used to selectively grow CNTs (specifically chemical vapor deposition)

are costly, hard to control, and lead to limitations in performance.

2.3 Literature Review

2.3.1 Nano Temperature Sensor Using Selective Lateral Growth of Carbon Nanotube between Electrodes [9]

This paper presents lateral growth of CNTs between two electrodes and its use as nano temperature sensor (Error!

Reference source not found.and Error! Reference source not found.). The fabrication of electrodes is by MEMS

techniques. The CNTs are grown selectively by MPCVD (Microwave Plasma Chemical Vapor Deposition) between

the two electrodes. The growing conditions of the CNT, such as the flow rate of CH4(methane) or N2(nitrogen) are

varied to obtain a high quality CNT sensor. Experiments were performed that used the following variables: the

distance between the electrodes, the amount of source gas, CH4(between 10 -60 %), and different amounts of N2to

make the CNTs grow horizontally. Results showed that the greater the gap between the electrodes, the more difficult

it becomes for the CNTs to connect to the electrodes.

Figure 7: Fabrication Process of the electrode PC board

Figure 8: Completed Nano Sensor Chip bonded on PC board


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The current-voltage (I-V) curve of the CNT was measured at different temperatures and a linear relationship

between the electrical resistance of the CNT and temperature was obtained (Error! Reference source not found.).

It also shows that using titanium as an electrode provides an ohmic contact. This accounts for the fact that CNTs can

be metallic as well as semiconductive in nature.

Figure 9: Linear Resistance vs Temperature for CNT grown at gap width of 0.9 m and 1.8m

This article has a few important implications for the current project. First, the CNTs in this study are grown

selectively by using the CVD (Chemical Vapor Deposition) process, which is costly and difficult to control. This

could be part of the reason this device has not been commercialized. The use of CVD also requires bake

temperatures that are well over the transition temperature of Parylene C. This means that using current

nanomanufacturing methods, a biocompatible, flexible substrate cannot be used if the sensing element is to be a

single CNT. Even beyond the limitations in using CVD, good results have already been achieved and designing a

sensor using this concept would not present anything novel. It is therefore advisable to find other

nanomanufacturing methods that are not as difficult to optimize given the short time frame for the project.

2.3.2 Bulk Carbon Nanotube as Thermal Sensing and Electronic Circuit Elements [10]

In this paper, bulk MWNT were successfully and repeatedly manipulated by AC electrophoresis to form resistive

elements between the Au (gold) microelectrodes and were demonstrated to potentially serve as novel temperature

sensors and simple electronic circuits.

The temperature coefficient of resistance (TCR) of the MWNT bundles was determined by measuring the resistance

of the bulk MWNT with the corresponding temperature (Error! Reference source not found.). From the

experimental measurements on a typical bulk MWNT device, the resistance dropped with increasing temperature

(Error! Reference source not found.). The TCR varied by experiment but converged within -0.1 to -0.2 %/C.


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Figure 10: Temperature Co-efficient of Resistance for a typical bulk MWNT device in different

measurements

Figure 11: I-V characteristics of a typical bulk MWNT device


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This could be attributed to variations in room temperature, mismatch in thermal coefficients of expansion between

the gold electrode and the bulk MWNTs, and contamination in the sample such as moisture. The I-V measurements

have revealed that their power consumption was in W range . This is important because it shows that using a CNT

network as a resistive element for a thermal sensor will produce power dissipation in the W range and will allow

for an ultra small device.

2.3.3 Ultra-low-power Polymer Thin Film Encapsulated Carbon Nanotube Thermal Sensors [11]

A novel polymer thin film embedded carbon nanotube (PECNT) sensor was developed for ultra low power micro

thermal sensing. Basic fabrication process for this sensor includes AC electrophoresis manipulation of the MWNT

bundles on a silicon substrate and embedding them inside Parylene C layers provide a robust protection for the

bundled MWNTs. This encapsulation process ensures that the MWNT elements can be protected from moisture and

contaminates in an operating environment. In addition, Parylene C is a flexible material, which presents the

possibility of a flexible sensor. The gap distance between the electrodes of the PECNT sensors is between 3m and

10m. The additional advantage of using Parylene C is that it is deposited as a conformal coating at room

temperature.

Experimental results showed that the TCR of the PECNT sensor was around -0.15%/C to -0.18%/C which closely

matched the previous results. This shows that the thin Parylene C layer does not affect the intrinsic thermal sensing

properties of the CNTs due to the extremely high thermal conductivity of the CNTs. The TCR is of particular

importance because it determines how sensitive the device will be and what overall temperature range it will work

over. This calculation will be need for the new sensor to be designed in later chapters of the report. In the

experimental results of the TCR on the CNT sensors without the Parylene C protection, considerable room

temperatures resistance drifting was observed in repeated measurements during the thermal annealing cycles. Hence

the Parylene C layer provides a better fixation to the CNTs with the microelectrodes when compared with those

without the Parylene C protected CNT sensors.

2.3.4 Three Dimensional Dielectrophoretic Assembly of Nanostructures on a Micro-machined Platform [12]

This paper discusses a novel platform for selectively assembling conductive nanostructures in a three dimensional

structure (Error! Reference source not found.). To realize this three dimensional assembly, a versatilemicroplatform utilizing a two mask process has been designed and fabricated (Error! Reference source not

found.). The assembly process is achieved at room temperature and is compatible with conventional semiconductor

fabrication and large scale nano assembly. It utilizes dielectrophoretic (DEP) assembly and is demonstrated with

high yield. Specifically created 3D micro-bridge structures from single walled carbon nanotubes (SWNTs) for

vertical interconnect applications and 3D micro-bridges using gold nanoparticles are potential sensors (Error!

Reference source not found.).


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Figure 12: The micro-machined platform for DEP assembly

Figure 13: Schematic of the fabrication process

Figure 14: SEM micrograph of the 3D assembled SWNT bundles

After assembly, the I-V curves were first measured from the SWNT bridge, verifying that the assembly was

successful (Error! Reference source not found.). The measured current-voltage behavior is linear (~545slope).

To enhance the adhesion of the SWNTs to the metal electrodes and to protect the SWNTs from the environment,

assembled SWNTs were encapsulated using a thin (1m) Parylene-C layer. Contacts were next opened on the top

Parylene layer using a reactive ion etcher. Current-voltage measurements from the encapsulated 3D SWNT bridge

demonstrate a lower metal-SWNT-metal resistance (approx. 380 ) possibly attributed to the top Parylene layer

compressing the SWNTs and leading to an improved CNT-metal contact. To investigate the effect of temperature on

SWNTs, the 3D thermal sensor was placed on a SUSS PM5 analytical probe system and the change in resistance

recorded as the temperature was varied from 25C to 65C with 10C increments ( Error! Reference source not

found.).


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As the temperature is increased from 25 C to 65 C the resistance value dropped by more than 10%. The measured

temperature coefficient of resistance (TCR) value from the single electrode device varied from -0.154 to -0.24%.

Figure 15: Measured resistance vs. temperature from a SWNT bridge

Figure 16: The measured I-V curves from the 3D SWNT bundles before and after encapsulation

The importance of this paper for the project is that is shows the advantage of using SWNTs in a network, versus

MWNTs. This paper reports very good results based on using an SWNT network as a resistive circuit element that

is encapsulated in Parylene. This basic concept for nano-scale temperature measurement is promising and shows the

potential for many applications that the group is looking for, keeping in mind that effort will still be made to create a

novel product.


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2.3.5 Measuring the Thermal Conductivity of a Single Carbon Nanotube [13]

A novel method of reliably measuring the thermal conductivity of a single carbon nanotube using a suspended

sample-attached T-type nanosensor was reported in this article. The experimental results show that the thermal

conductivity of a carbon nanotube at room temperature increases as its diameter decreases, and exceeds 2000 W/mK

for a diameter of 9.8 nm. The temperature dependence of the thermal conductivity for a carbon nanotube with a

diameter of 16.1 nm appears to have an asymptote near 320 K. The present method is, in principle, applicable to any

kind of a single nanofiber, nanowire, and even single-walled carbon nanotube. The diameter-dependent thermal

conductivity indicates that the interactions of photons and electrons between multiwalled layers affect the thermal

conductivity. The thermal conductivity increases as the number of multiwalled layers decreases. A single-walled

carbon nanotube is expected to have much higher thermal conductivity, which is of particular importance to later

chapters of this report .

2.4 Summary of Current Technology

Gathering information on the patent information and other literature provided a better understanding of the behavior

of CNTs and their application in thermal sensing. The first conclusion that can be made from this research is that it

is possible to characterize CNT-based devices as temperature sensors but the research verifies that the concept has

not yet been commercialized. The patent and literature review also revealed that CNT-based temperature sensors

can achieve comparable performance to industry standard sensors currently in use. The size of the devices that have

been fabricated is small enough to enable application in current ultra-small-scale applications as well as future

applications. However, the performance has yet to be optimized for high applicability.

Research also showed that the nature of the nanomanufacturing processes leads to limitations in what can actually be

fabricated on the nano-scale. For example, the process CVD that is involved with growing a single carbon nanotube

is not compatible with the flexible substrate Parylene C, nor is it a cost-effective or timely option for completing the

design of a novel sensor. Using Parylene C would allow a device to be flexible as well as protecting it from

chemical and moisture damage. Parylene C is also a biocompatible material, suggesting a possible application in

biomechanics. This means that other deposition processes than CVD must be used since Parylene C has clear

advantages when used on nanosensor designs, in terms of the abovementioned details and in regards to the project

goals.

A network of SWNTs has been shown in research to provide the best overall performance for a CNT-based sensor in

terms of sensitivity and repeatability in measurements and fabrication. Devices that made use of single CNTs were

able to be characterized to measure temperature, but they are extremely fragile and were able to operate over a

relatively small temperature range. Similarly, MWNTs werent able to achieve the sensitivity performance that


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SWNTs can. If the decision was made to use a CNT-based design, a network of SWNTs was to be used. However,

different methods of measuring temperature had to be examined and compared against basic CNT concepts.

2.5 Other Temperature Measurement Devices

There are other types of sensors used to measure temperature that must be considered. Some of them include

thermocouple, semiconductor diodes, metallic resistors, thermistors, infrared thermometry, near field thermometry,

and other less-common methods such as Plasmonics and other optical methods. Each method has its own

disadvantages and advantages. For example, a thermocouple is simple in design and is inexpensive, but has

limitations in terms of its temperature range. A semiconductor diode is not suitable to measure very high

temperatures but is suitable to measure cryogenic temperatures. The spatial resolution of the above mentioned

devices is about 10 microns except for near field optical thermometry which can have a spatial resolution in the

order of 50nm. However, this method for measuring temperature involves complicated optical instrumentation and

is not very user friendly. All of the alternate methods for measuring temperature were considered during concept

brainstorming, which is discussed further in Chapter 3.

2.6 Why Use CNTs for Temperature Measurement?

The basic concept decision making process is discussed in the next chapter, but through background research and

the performance advantages discussed above, the following preliminary argument was formed regarding the design

of an ultra-small-site temperature sensor:

It is important to note that the only devices the group was able to find that were capable of taking temperature

measurements on the nano scale were CNT-based devices. Since the processes associated with manufacturing CNT-

based devices are still relatively costly and complex, it is clear that the choice would have been made by researchers

to avoid using them if it was possible to use other types of devices that might be easier and cheaper to fabricate. It is

the significant increase in performance due to the heightened properties of CNTs that make these devices attractive

in the eyes of researchers and to the community. For the case of micro-scale devices (including temperature

sensors), the manufacturing costs began very high but have since been improved until the costs were decreased

significantly. Now, MEMS devices have led to entire industries, such as microprocessors and all of the resulting

industries that came with increased computer speed. Researchers building CNT-based sensors know that it will be a

while before the comparable nanomanufacturing costs go down but when this happens, CNT-based sensors could

quickly become extremely profitable, not to mention that they would contribute directly to the development of many

future high technology and ultra-small-scale industries that have yet to even be conceptualized.


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CHAPTER # 3: Design Development

3.1 Introduction

This section covers the beginning of the design development process, including needs assessment, specification

development, concept evolution, concept drawings, and selecting a satisfying design. The needs were determined

from interviews with local professionals and scholars, which the group conducted strategically based on the results

of the background and literature search. From the interviews, a list was made of design specifications that will

satisfy the discussed needs. Once the needs were established, brainstorming was conducted, which took into

account several different ways to measure temperature and the feasibility of each on the nanoscale in terms of

manufacturing. The concepts were then compared based on weighted criteria and a selection was made.

3.2 Interviews

3.2.1 Interviews with Professors and Researchers

The following people were contacted in regards to the application and industry needs for our sensor, as well as the

manufacturability of concept designs on the nano scale (Error! Reference source not found.).

Table 1: Contacts List for Email Interviews

Name Organization Area(s) of Expertise

Xin Zhang BUCenter for Nanoscience and

NanobiotechnologyThomas Burg MIT Nanoscale Sensing @ MIT

Michel Godin MIT Nanoscale Sensing @ MIT

Will Grover MIT Nanoscale Sensing @ MIT

Scott Knudsen MIT Nanoscale Sensing @ MIT

Chris Dames UCAProperties and Applications ofNanotubes and Nanowires.

Ken crozier Harvard

Electrical Engineering andAssociated with Nano EngineeringLab. Experience withNanophotonics.

Raj Mohanty BUNanoscale Biosensors,Nanomechanics, Physics, Heads

Nano Research at BURobert Badzey BU Postdoctoral Research at BU

Tyler Dunn BUPhD candidate at BU. Nano ResearchExperience.

Dr Zhuomin ZhangGA Institute of

TechnologyHeat Transfer, EngineeringThermophysics

Dr. William P.King

GA Institute ofTechnology

Head of Nano research at GAInstitute of Technology


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The following information was compiled based on responses from and discussion with these individuals (Error!

Reference source not found.):

Table 2: Email Interviews with Local Professors and Scholars

Interview Date and Type: 9/2007 - 10/2007, EmailInterviews

Group Members whoConducted theInterview:

David Pelletier

Statement Interpretation for Project

Most sensors which make use of CNTs are very sensitive totemperature

The properties of CNTs show a sufficientchange with temperature in order to achieve

suitable performance in terms of accuracy andsensitivity

Currently, there are many small-scale technologies beingdeveloped, such as microfluidic chips, which will eventuallyrequire good temperature measurement. Technologies that

are relatively new do not yet have suitable devices for

measuring temperature.

There is a potential for application in areas thatare currently in development, where

commercially available sensors fall short of thetemperature sensing requirements

One specific microfluidic application is to monitor the chiptemperature in a process called PCR, which is used to

amplify DNA for studying and manipulation purposes. Thereis no temperature sensor built into these particular chips

This is a specific recommended application for asmall-scale temperature sensor, where no

commercially available equipment can be used

Measuring the temperature of very small devices, locally, isvery difficult. Typically, measurements are perform at acertain distance from the area of interest. It is oftentimes

assumed that the temperature is uniform everywhere,although this might not be always true. However, in many

applications, such small temperature offset might not be aproblem

A small-scale temperature sensor could be usedto take local measurements, which would

improve upon temperature uniformityassumptions and methods. However, effects ofthese assumptions are usually minimal and so

the project cannot rely solely on these types ofapplications

Encapsulating the sensor in a material that makes the deviceresistant to chemicals and to moisture is a specification that is

important for the types of applications being investigated(especially the microfluids applications). However, this isnot going to make the sensor novel since most temperature

sensors have these qualities

Moisture and chemical resistance are crucialspecifications to have but will not make the

sensor novel

Designing for novelty in both size and sensitivity/accuracywould require more time for manufacturing and testing than

the group has available. The calibration of ultra sensitivedevices with a large temperature range is complicated and it

would probably require different sensing elements fordifferent parts of the range

It is not advisable to attempt to make a sensor

that is novel in both size andaccuracy/sensitivity while maintaining a

relatively broad temperature range


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In using CNTs as the sensing element, there may be issueswith the inability to separate metallic behavior from

semiconducting. If these issues arise, using many CNTs in anetwork will average out the important properties of the

element

The unpredictability of behavior that is inherentin CNTs can be compensated for by using a

CNT network. This has been demonstrated inseveral papers

3.2.2 Interview with Professor Mehmet Dokmeci of Northeastern University ECE Department

The following table contains the information that was collected from an in-person interview with Professor

Mehment Dokmeci of the Northeastern University ECE department. Professor Dokmeci has experience in

nanosensor design, characterization, and testing (Error! Reference source not found.).

Table 3: Interview with Professor Mehmet Dokmeci

Interviewee Name: Mehmet Docmeci Group Members whoConducted the Interview:

Dan Esposito,Vishal Jain

Interviewee Addressor Institution: Northeastern UniveristyECE DepartmentInterview Date: 10/15/07


Many applications for flexible substrates for thermalsensor

Parylene or another flexible substrate can be used

CMOS sensors cant achieve the spatial resolution thatthe CNT based sensors can achieve (less than 50um)

Using CNTs is advantageous in terms of sensorresolution and sensitivity

If the device is made of Grade 6 biocompatible material,it can be used as an implant in human body

Parylene is a grade 6 biocompatible material. Needto find out if the presense of other materials (i.e.

metal electrodes) prevents implantability

Using the resitance of the CNTs, the device can becharacterized to provide temperature change, strainchanges and functionalized gas sensor

Measuring the resistance of a network of CNTs is a

feasible and appropriate method for creating a CNTbased sensor. Possibility for future modifications to

use as a strain or gas sensor

Parylene C is expensive but can be used as abiocompatable material

Possible biomedical application (human implants)

The advantage of using Parylene C is that it presses theCNTs to the electrode, thus increasing the contact

resistance of the device and averaging out the resistanceof the CNTs

It can be expected that the change in resistance withtemperature will be measurable and consistent.

Good for repeatability in measurements

At around 400C, metal particles start to bubble, whereasCNTs stay stable at even higher temperatures

Possible high-temperature application by usingCNTs. Justifying the advantage of using CNTs

instead of other metal resistive element

There should not be any strain-based changes inresistance of the CNT network due to percolation theory

and the overall small size of the sensor

Using a flexible substrate (parylene) will not preventaccurate calibration or temperature measurement

characterization

Can use the SUSS PM5 device to obtain the I-V curveCommon nanomanufacturing lab equipment can be

used to characterize and test the sensor. CHN atNortheastern has this equipment

Use a heatable chuck to obtain the current at varioustemperatures. This will characterize the device to be a

temperature sensor.

Common nanomanufacturing lab equipment can beused to characterize and test the sensor. CHN at

Northeastern has this equipment


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Contact Prabhu (PhD student) for assistance oncalibration

---

Use op-amp to determine the frequency response of thedevice. This is a very important part of the

functionalization process

It is feasible to functionalize the device as atemperature sensor

This group would be the first group to functionalize a

CNT temperature sensor

Justifying novelty of design and possibility for a

patent

3.2.3 Interview with Nurse Rosa Morelli

The following table contains information that was collected in a phone interview with Nurse Rosa Morelli. The

interview was conducted with the intention of exploring applications for a small-scale temperature sensor in the

biomedical field (Error! Reference source not found.).

Table 4: Interview with Nurse Rosa Morelli

Interviewee Name: Nurse Rosa Morelli Group Members who

Conducted theInterview:

Dan EspositoInterviewee Address

or Institution:Brigham and Womens

HospitalInterview Date: 10/10/2007


Catheters are prone to infection. With infection, bodytemperature rises. Local heating helps to fight

infection. Accurate temperature sensing needed tooptimize and study infection fighting with heat

Possible catheter application: could attach a smallsensor to catheter.

Catheter type #1 to focus on: Bladder: No need for fastresponse. Needs to be in vivo and resistent to a wet

environment

A biocompatible sensor would need to be chemicaland moisture resistant. Sensor response not as

important for this application

Catheter type #2: Endotracheal: No need for fastresponse. Needs to be In Vivo and resistance to a wet

environment

A biocompatible sensor would need to be chemicaland moisture resistant. Sensor response not as

important for this application

Catheter type #3: Heart (Angioplasty): Smaller sensorneeded to move through arteries. Faster response time

would be better.

Small-size sensor needed. CNT temperature sensor isa good solution for this application due to small size,low power consumption, overall temperature range.

Core body temperature sensing: Current method isrectal thermometry. Sometimes, false conclusions aremade using this method (especially for people who are

pronounced dead due to cold)

There is a need for better method of measuring corebody temperature

Core body temperature sensing: No current way tomeasure core temperature directly

Possibility for signal-transmitting temperature sensingdevice. Needs to be in vivo, chemical/moisture

resistant, ultra small, very low power consumption,biocompatable materials, ~1 second response time,

wide temp range


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3.2.4 Interview with Professor Sinan Mft

The following information was collected from an in-person interview with Professor Muftu of the Northeastern

University Mechanical Engineering Department. Professor Muftu was asked questions having to do with the

mechanics and modeling of the device. This interview was conducted after the concept brainstorming and

evaluations were completed. The rest of chapter 3 reflects the absence of this knowledge at that time. However, this

new information is discussed in chapter 4 and plays a key role in the detailed design development for the sensor

(Error! Reference source not found.):

Table 5: Interview with Professor Sinan Mft

IntervieweeName:

Professor SinanMft Group Members who

Conducted the Interview:Vishal Jain

IntervieweeAddress orInstitution:

NortheasternUniversity Mechanical

EngineeringDepartment

Interview Date: 10/15/2007


Expansion element below (or above the CNTnetwork) might or might not work

If an expanding element is added above (or below) the CNTnetwork, the CNT behavior is unpredictable. The CNT might

bend to form curve or might just stick to the expandingelement or might do something else

Instead of adding an expanding element aboveor below the CNT network, subject the CNTnetwork to shear forces causing it to strain

The previous design consisted of the CNTs above (or below)the CNT network. When there is a change in temperature, the

expansion element will expand greater in longitudinaldirection and less in the lateral direction. With the CNTs beingsubjected to change in the lateral direction, the effect might benegligible for the range of temperature the device is supposed

to work.

---

To overcome this, add expansion elements above and belowthe CNT network such that expansion elements and the CNT

network have the same longitudinal axes. When the expansionelements expand, they will induce a shear force in the CNT

network causing it to strain

Add parylene above and below the CNTnetwork (in between the CNTs and the

expanding element)

Parylene will be the insulator between the expanding elementand the CNTs. Use this model to thermal analysis

Perform thermal analysis to determine materialfor expanding elements, its dimensions etc.

Thermal analysis needs to be done to determine the materialfor expanding element, its dimensions such the when the

device is exposed to the change in temperature (-50C to 150C),the CNT network does not strain beyond 2% (elastic)

Use correction factor for properties

Using a correction of 0.1, 0.3 and 0.7 for the thermalproperties (linear coefficient of expansion and thermal

quantities) of CNTs and expanding element. Using correctionfactor will provide an output within a certain range


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3.3 Needs Assessment

Based on the background research and the interviews that were conducted, the need for a novel temperature sensor

was discussed. The following paragraphs summarize the discussion:

Temperature measurement is currently a very well-covered area. There are commonly accepted sensing methods

and products for most industries and applications where temperature measurement is needed. Competing with well-

designed, industry standard sensors would be very challenging, especially when trying to sell a sensor of comparable

performance to what is currently available. Attempting to improve temperature sensing for a specific application is

not advisable for this reason.

Companies in many application areas use slightly older methods to provide temperature measurement which remain

both lawful and satisfactory for their industry. These companies rely on the fact that the temperature measurement,

in general, is not very difficult and have been well-studied for many years. Such organizations may see it as a

financial disadvantage to make use of expensive state-of-the-art methods that are designed specifically for their

application when their current method still works fine. However, the trade-off is that using state-of-the-art

temperature sensing techniques can give a company a competitive edge in the eyes of customers, especially when

the company is trying to expand their capabilities to newer technology fields while maintaining high measurement

credibility and stature. These companies have a specific need for cost-effective, yet state-of-the-art temperature

measurement.

Additionally, there are also applications where previous measurement methods fall short of what is possible usingstate-of-the-art technology of today and of the future. One example of this, as shown in Table 4, is the measurement

of the human body core temperature (the temperature of the heart). Currently, rectal thermometry is used to take

core temperature measurements. In situations such as a person being affected by extreme cold, the body can go into

an unconscious state that is absolutely mistakable for death using observation and normal medical field tests. The

core body temperature measurement is critical in determining if there is a chance that the person is still alive.

However, rectal thermometry doesnt measure the bodys core directly, but takes an indirect measurement of an area

thats close to the core. There are numerous cases where a person is assumed to be deceased based on field tests and

rectal temperature measurements, only to wake up later. In this case, there is a definite need for a method of

obtaining core temperatures without surgery. This could be done with an implantable that transmits temperaturemeasurements to equipment outside the patients body. Technology for transmitting signals from a microscopic

device to external equipment is currently being developed, which will necessitate a temperature sensor on the size

scale of individual blood cells (micro scale). This is one example of an application for temperature measurement

where the current methods unacceptably fall short of what is possible using technology that is currently in

development.


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Designing a generic, inexpensive temperature sensor could provide companies, in many application areas, with a

perfect opportunity to upgrade their techniques; even if the existing technology is sufficient, or in cases where it is

not. Designing a generic temperature sensor would be the basic task, and then starting a company to sell the design

and modify it for each customers individual application would be a necessary next step to ensure smooth

implementation.

It is important to note that if a generic temperature sensor (i.e. one with carefully selected specifications that

encompass a wide variety of applications) is designed, there is huge potential for application in technologies that

have not yet moved beyond the research phase. Currently, there are many technologies being developed which

require temperature measurement on a smaller scale than ever before. For example, some microfluid applications

are being developed which will require very intensive temperature monitoring to make it out of the research phase.

More specifically, microfluid technologies such as novel methods for performing PCR (again, a process for DNA

amplification) require precise temperature measurement. In this application, and in many others, there is no current

method for measuring temperature that can satisfy the requirements of the process. Most third party sensorcompanies will not justify designing such a method or device because they dont see the immediate potential for

revenue. However, designing a sensor that is novel in areas such as small size could lead to a patent in the near

future, and the device would be easily integrated into current or future nanotechnology applications. Since size is

such an important factor in distinguishing future technology from past technology, designing an ultra small-scale

temperature sensor shows promising applicability.

List of needs

The following list presents a summary of the needs that came from interviews and the above discussion:

- There is a need for an ultra small-scale sensor (overall size in micrometers)

- The sensor should be biocompatible if it is to be used for human implant applications

- The temperature range needs to be relatively large in order to encompass many applications. The

temperature range is limited by the response of the materials to different temperature ranges

- The sensor should be comparable in sensitivity and accuracy to what is currently available as industry

standard sensors

- Since the sensor will be used for local small-scale temperature measurement, the power consumption

should be low so the heat produced does not affect the surroundings and the measurement significantly.

- The sensor needs to be easily integrated in many applications

- The sensor should be resistant to chemicals and moisture effects.

- The manufacturing time cannot exceed approximately one month, given the time constraints on the project.

- The cost should be considered and minimized in order to sell the device and be competitive. There should

be batch fabrication capabilities.


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- The device should be easy to calibrate and test given the time constraints on the project and the goal for

overall low cost.

3.4 Specifications

The following specifications were determined based on the developed needs that govern the design of the sensor(Error! Reference source not found.):

Table 6: Temperature Sensor Design Specifications

Spec Item/DescriptionWeight

(1 = Highest)Specification

Electrical Properties

Range of Use 5 -50 C to 150C

Sensitivity 4 Medium/High

Accuracy 4 +/- 1C

Power Consumption 3 Very Low, in WResponse Time 6 1s

Physical/Chemical

Overall Dimensions(smallest possible) 1

150mx50mx12m

In-Situ/In-Vivo 8 Yes

CMOS Compatible 8 Yes

Moisture Resistance 7 Yes

Chemical Resistance 7 Yes

Mechanical

Effects of strain 3 Proportional

Manufacturability

Manufacturing Time 2 Short, ~1 month

Batch Fabrication 7 Yes

Calibration

Ease of calibration 7 Easy

Others

Repeatability 8 Yes

Supply Voltage 3 Low, 0-3V

Ease of Integration 6 Easy

Environmental effects 6 Negligible

Cost 9 Medium/Low

Based on the needs assessment discussed in the previous section, it was determined that the temperature sensor

should be smaller than other industry standard sensors of comparable performance. The specifications listed in the

table above provide a method for characterizing the overall performance of a temperature sensor. Below, the

importance of each specification is justified and discussed briefly.


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- Temperature Range: The selection of a temperature sensor for a certain application is based largely on the

temperature range that the sensor is rated for. The selected range of -50C to 150C for this design

encompasses a broad range of applications. Most industry standard temperature sensors are characterized

to function within a smaller temperature range, since most of them are application-specific. Designing a

new sensor with a slightly larger range will enable its use for a broader set of applications than most

commercially available sensors. This, coupled with small size, should give the new design an advantage

when compared against other sensor options in a given application.

- Sensitivity: This specification is important because it determines the accuracy to which measurements can

be taken. This comes into play when measurement credibility and quality are particularly important in a

given application. It is desirable for the sensor being designed to take temperature measurements with

gradations that are less than one degree Celsius to compete with the existing products.

- Accuracy: Any commercial sensors reputation is directly related to its accuracy. It is imperative to design

a sensor with strict accuracy values. Having said this, the decided accuracy value of +/- 1C was chosenwith possible applications in mind. While there is potential for an even lower value, there is no valid

requirement to do so.

- Power Consumption: Since the possible applications could be battery operated or simply require a small

power supply, it is important to design the sensor using the lowest amount of power requirements possible.

In addition, the possibility of creating a temperature variant if too much current is applied is a very real

concern.

- Response Time: After intense research of possible applications it was determined that an average response

time was completely acceptable for all scenarios. A fast response time is not necessary.

- Overall Size: The most important specification for the sensor design has been determined to be the overall

size, since the project objective is to create a sensor that is novel in size, while the performance remains

comparable with that of other industry standard temperature sensors.

- In-Situ/In-Vivo: This characteristic would allow for a bio-compatible sensor. Since, there are a few

possible applications that would require a bio-compatible sensor, and this specification can be achieved

simply by changing the encapsulating material, it was decided to go forward with this specification.

- CMOS Compatible: Possible applications included several silicon technology based products. It is of the

utmost importance that the design will not disrupt surrounding circuitry whatever the application.

- Moisture Resistance: The prospective design is a basic electronic circuit, and, like any other electrical

equipment, will corrode easily by exposure to moisture or liquid. It is incredibly important to design the

product so it is protected from all outside contaminants to ensure repeatability.

- Chemical Resistance: To ensure a large variation of possible applications and since the specification can

easily be met by choosing a suitable encapsulating material, chemical resistance is desired.


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- Effects of Strain: A valid concern for this design is to ensure there are no external strain loads applied to

the sensor. Since the sensor is primarily a strain sensor it is very important to guarantee the readings are

invariantly due to the expansion element.

- Manufacturing Time: This specification is generally kept as low as possible in order to ensure the best turn

around of the product. There is a specific manufacturing process that would shorten the total time, but cost

for the use of the equipment is not acceptable for preliminary modeling. The process being used, E-Beam

vs. Optical lithography, would be changed once mass production is required.

- Batch Fabrication: A parallel process of manufacturing will allow for batch fabrication, which means the

same time required to create one sensor can produce 21 sensors with a basic fabrication modification.

- Ease of Calibration: This has more to do with customer satisfaction. A sensor that is easy to calibrate is

more attractive to scientists then one that is not. A user friendly product is the ultimate goal.

- Repeatability: This is another fundamental specification that any quality sensor should have. The ability to

obtain the same values for consecutive measurements is a requirement for this design.

- Supply Voltage: The desire for little power consumption was discussed earlier in this section, but there isstill a numerical value needed to create a basic design. This value was found from existing products.

- Ease of Integration: It is preferable that the customer do as little as possible to integrate our product into

their system. This again has to do with keeping the product user-friendly.

- Environmental Effects: It is very important that our product not interfere with the outside environment.

Additionally, the outside environment should not interfere with the sensor. This is where the packaging

material is very important and will need to be considered intensely.

- Cost: This is an obvious specification that should be kept to a minimum by all means. It is however last on

the list of specification requirements since a quality sensor is of utmost importance and should not be

compensated due to cost.

3.5 Initial Concepts

This section presents the basic concept designs that were drafted, which explore several different options for ultra-

small-scale temperature measurement. The concepts are then compared using weighted criteria in the sections to

follow and finally, a selection is made. It is important to note that not all of the options employ CNTs. Also, the

following concepts include discussion on some nanomanufacturing processes. It is important to discuss each

concept based on the issues that are related to manufacturing. Descriptions and further information on these

processes is discussed in Chapter 4 of this report.

3.5.1 Concept # 1: Temperature Sensor Using a Single Carbon Nanotube, Oriented Using Dielectrophoresis

This design would involve using the thermal properties of a single carbon nanotube (CNT) to measure temperature.

It would consist of placing a pair of metal electrodes on a substrate. These electrodes would be separated by a single

CNT (Figure 17: Temperature sensor using a single CNT).


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Figure 17: Temperature sensor using a single CNT

Unlike bulk CNTs, measurements from a single CNT will not average out due to the absence of tube to tube

junctions [14]. Therefore, by making use of a single CNT to measure resistance as a function of temperature, the

repeatability of the sensors manufactured from various batches will increase significantly, provided the CNTs

obtained from the various batches remain consistent.

The manufacturing process usually used for single CNT fabrication is Atomic Force Microscope (AFM)manipulation and dielectrophoresis.

Some of the challenges of this design are

Precision manipulation of CNTs

High initial equipment cost

Cannot perform batch fabrication using AFM; hence lower throughput and high production cost

Dielectrophoresis uses AC field for CNT alignment, which results in inconsistency in CNTlocation/placement

50 - 100micrometers


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Figure 18: SEM image showing a single CNT

Figure 19: A single CNT shown between two gold leads

3.5.2 Design Concept # 2: Temperature Sensor Using Nano Scale Metallic Resistive Element

In order to obtain a higher spatial resolution and sensitivity, the sensor devices must have smaller resolutions.

One of the potential concepts would be to design a micro/nano scale temperature sensor device that includes a

metallic wire fabricated on a suitable substrate (Error! Reference source not found.). Using the

resistive/conductive properties of the metal, a correlation with change in temperature can be obtained. Gold is

one of the proven metals that has been previously used to design a thermistor based sensor [15]. The electrical

properties of gold as well as other potential metallic nano particles materials needs investigation to determine

the best option for nanoscale temperature sensor applications.


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Figure 20: Sensor Concept Employing a Complex Pattern for Increased Sensitivity to Temperature

The fabrication of metallic element on the substrate is usually done using the process

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Flexible Carbon Nanotube Based Temperature Sensor for Ultra-small