iii

UNIVERSITY OF WATERLOO

Faculty of engineering

Nanotechnology Engineering

Design and Fabrication of Electrospray Sources for Electric

Propulsion

Prof. Mario Lanza

Dr. Enric Grsutan

Institute of Functional Nano and Soft Materials, Soochow University,

199 Renai Road, Suzhou Industrial Park,

Suzhou, Jiangsu Province, China

Prepared by

Chuqi (Steven) Wei

ID number: 20518399

Userid: c27wei

Previous academic term: 2B

Completion date: Sept. 15, 2016

iv

Suite 218, 50 Clegg Road,

Markham, Ontario, L1C 0C6

Sept.15, 2016

Dr. Shirley Tang, director

Nanotechnology Engineering

University of Waterloo

Waterloo, Ontario

N2L 3G1

Dear Madam,

The following report, titled “The design and fabrication of electrospray source for electric

propulsion”, was prepared as my 2B Work Term Report for Institute of Functional Nano and Soft

Materials (FUNSOM), at Soochow University. This report is submitted specifically for WKPRT

300 course, in fulfillment of the WatPD-Engineering program as required by my BASc

Nanotechnology Engineering degree. The purpose of this report is to present in details the design

and fabrication of a new type of electrospray source as well as the apparatus for performance testing

in the newly mounted laboratory. Some preliminary characterizations are also included in the report.

Soochow University is a public institution located in the city of Suzhou, Jiangsu Province, China.

The Institute of Functional Nano and Soft Materials (FUNSOM) is a leading institute for

nanotechnology research in China. I was employed by Professor Mario Lanza to work as a Research

Assistant on a new project with the focus of electrospray. This is a new area of research for

Professor Lanza whose main focus has been Resistive Random Access Memory devices and

Atomic Force Microscopy. The job not only requires me to do research work such as design and

fabrication, but I was also in charge of mounting a new laboratory.

I would like to thank Prof. Mario Lanza and Dr. Enric Grustan for providing me valuable advice

and resources, including critical trainings for numerous equipment, academic literatures,

fabrication experience and guidance for the design. I would also like to thank FUNSOM and

Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) of Chinese Academy of Science for

providing the facilities and equipment for me to do my job. They also proofread this report.

I also wish to thank all members in Prof. Lanza’s group, Bingru Wang, Yuanyuan Shi, Tingting

Han, Fei Hui, Lanlan Jiang, Na Xiao, Yanfeng Ji, Xiaoxue Song, Xu Jing and Chengbing Pan for

helping me with issues such as navigating through bureaucracy and paperwork, and most

importantly, the constant support to make me feel welcomed. I hereby confirm that I have received

no further help in writing this report, other than what is mentioned above. I also confirm that this

report has not been previously submitted for academic credit at this, or any other academic

institution.

Sincerely,

Chuqi (Steven) Wei 20518399

iii

Contributions

The team I worked with consisted only two people. During my cooperative work term, I was mainly

working exclusively with my supervisor, Dr. Enric Grustan who has a PhD degree in Mechanical

and Aerospace Engineering. Our work is under the oversight of Prof. Mario Lanza from FUNSOM

who grants me access to the cleanroom along with its fabrication and characterization equipment.

The overall goal of this project is to mount a new laboratory based on the model of Dr. Grustan’s

previous lab in Irvine, California, and use this new facility to explore more novel applications for

electrospray. Consequently, my work contains two major aspects, mounting the lab and working

on the electrospray source including design and fabrication. It’s noteworthy that while the lab was

being mounted, our research didn’t remain stand still, instead, processes were made on both fronts

at the same time.

For mounting the lab, my tasks include allocating funds, searching for suppliers for equipment,

devices and materials and acting as liaison between Dr. Grustan and Soochow University. Some of

the critical procured equipment, devices and materials consisted of a custom designed vacuum

chamber for prototype performance testing; mechanical and turbo-molecular pumps for

establishing vacuum environment; the electronic apparatus for performance testing which includes

a pulse generator, a pico-ammeter, an oscilloscope, two high voltage sources; an industrial camera

set to observe and document the working status of the prototype; ionic liquid 1-ethyl-3-

methylimidazolium bis(triflouromethylsulfonyl)amide (EMI-Im) to fuel the prototype; two types

of micro silica thread to act as feeding thread between the ionic liquid reservoir and the vacuum

chamber; hard and soft masks for photolithography; silicon wafers with different doping and

polishing requirements; and other laboratory supplies. These tasks are crucial for mounting the lab

successfully and achieving the overarching goal set by Prof. Lanza.

On the other hand, I was also in charge of design and fabrication of the electrospray source

prototype under the supervision of Dr. Grustan. There are two types of model, the single emitter

set for physical sputtering and the multi-emitter array for electric propulsion. The single emitter set

consists of two metal extractor layers, two plastic layers and a plastic case encompassing the whole

structure. There are holes extending from the outside to different layers for the purposes such as

fixation and acting as contact electrodes. I was in charge of designing this prototype using

Solidworks software and the final version was approved for prototype manufacture. The multi-

emitter array pattern was designed by Dr. Grustan and the fabrication and characterization part was

iv

carried out by me. Each mask for photolithography consists of seven arrays of emitters, while each

array consisting of sixty-four emitters. Using micro-electro-mechanical system (MEMS)

fabrication techniques, including photolithography, and deep reactive ion etching, emitter arrays

were fabricated and characterized. It’s worth mentioning that, since the multi-emitter array is

fabricated using MEMS techniques, many of the previous obstacles such as oversize and low power

to thrust efficiency have been overcome. Theoretically, the prototype should be able to generate a

thrust that is sufficient for micro-spacecraft.

The purpose of this report is to summarize the design process, document details and parameters for

singe emitter set, elaborate on the fabrication steps and characterization results for multi-emitter

array and present the apparatus for performance testing. The content of this report takes into account

of both aspects of my work and it’s a fair reflection of my experience in the institute.

Looking at the broader scheme of things, first and foremost, the successfully mounted lab is one of

its kind in China, so it puts our research group in a very advantageous position to further our

research in the field of electrospray and its potential applications. Secondly, electric propulsion has

immediate applications. Electric propulsion provides an alternative solution for thrust used for a

new generation of micro-spacecraft, given the competitiveness of 21st century space race, micro-

spacecraft with its high cost-performance ratio will be a major player in the field of civil and

military satellite competition.

v

Summary

The purpose of this report is to present the outcomes of the 8-month cooperative work term in

Institute of Functional Nano and Soft Materials of Soochow University, China. The goals of the

project are to mount a new laboratory for electrospray experimentation and to design and fabricate

electrospray source for physical sputtering and electric propulsion.

The main points covered in this report are the introduction to the field of electrospray thus

explaining the motivation for this project, then the background knowledge about electrospray and

electric propulsion, the design process and fabrication steps of single emitter set and multi-emitter

array with a detailed analysis for fabrication characterization, and finally presentation of the

performance testing apparatus setup including different working modes with general conclusions

and recommendations.

A major conclusion in this report is that during the deep reactive ion etching (DRIE) during the

multi-emitter array fabrication, adding oxygen to the etching gas can significantly reduce the

formation of silica grass at the base of the emitter well and the emitter rod. Different amount of

oxygen addition have been tested including constant 45 sccm, constant 35 sccm, constant 25 sccm

and ramping addition from 0 to 30 sccm. Detailed results will be discussed in the report body.

Another major conclusion is that after intensive design process, the single emitter set can now meet

the electronic and mechanical mounting requirements for fabrication and performance testing.

The major recommendations in this report are to further complete and calibrate the performance

testing apparatus since by the time of the submission of this report, the mounting process is not yet

100% complete. After the completion of the mounting process, it is recommended to perform a test

run for single emitter set prototype since it has a lower degree of complexity and easier to make

modification based on the test results. For the fabrication of the multi-emitter array, it is

recommended that to try a different DRIE etching recipe with a slower rate but less chance of over-

etching the emitter rods. It would also be beneficial to investigate further into the relationship

between the mercury lamp power of photolithography machine and its effect on the etching process

since the prototype fabrication involves multiple photolithography and DRIE steps

vi

Table of Content Summary .......................................................................................................................................... v

List of Figures ................................................................................................................................ vii

List of Tables ................................................................................................................................ viii

1.0 Introduction .......................................................................................................................... 1

2.0 Background ................................................................................................................................ 1

2.1 Electrospray ionization for electric propulsion .................................................................... 2

3.0 Prototype design and fabrication.......................................................................................... 4

3.1 The Design and Fabrication of the Single Emitter Set ........................................................... 4

3.2 Multi-Emitter Array Fabrication and Characterization .......................................................... 7

3.2.1 Fabrication Process ......................................................................................................... 8

3.2.2 Photolithography ............................................................................................................. 9

3.2.3 Deep Reactive Ion Etching (DRIE) .............................................................................. 10

4.0 Performance Testing Apparatuses ..................................................................................... 14

4.1 Vacuum apparatus ................................................................................................................ 14

4.2 Electronic apparatus ............................................................................................................. 18

5.0 Conclusions .............................................................................................................................. 18

6.0 Recommendations .................................................................................................................... 20

Glossary ......................................................................................................................................... 21

Reference ....................................................................................................................................... 22

Appendix A: RCA-1 clean process ................................................................................................ 23

Appendix B: Parameters of all DRIE trials .................................................................................... 25

vii

List of Figures page

Figure 1: Demonstration of electrospray ionization. ....................................................................... 1

Figure 2: A structural breakdown of multi-emitter array prototype. ............................................... 3

Figure 3: The experimental setup for testing single emitter set. ...................................................... 5

Figure 4: The isometric view and front plane section view of the prototype................................... 6

Figure 5: Illustration of metal contacts in the inner plastic layer and inner metal layer. ................. 6

Figure 6: The base structure of the prototype and the feeding thread connector . ........................... 7

Figure 7: Breakdown for emitter and channel fabrication process. ................................................. 8

Figure 8: Images of developed emitter array wafer and channel wafer. ........................................ 10

Figure 9: SEM characterization of emitter well for trial No. 1. ..................................................... 11

Figure 10: SEM characterization of emitter array and single emitter well for trial No. 2. ............ 12

Figure 11: Ideal shape and geometry of emitter well and emitter rod. .......................................... 13

Figure 12: Comparison between different mode of O2 injection. ................................................. 13

Figure 13: Air circuit of the vacuum system .................................................................................. 15

Figure 14: Schematics of pumping mode. ..................................................................................... 15

Figure 15: The mounting of the prototype for performance testing at Dr. Grustan’s lab at UCI. . 16

Figure 16: Schematics of experimentation mode. .......................................................................... 17

Figure 17: Schematics of standby mode. ....................................................................................... 17

Figure 18: Electronic Setup of performance testing. ..................................................................... 18

viii

List of Tables page

Table 1: Comparison of photolithography parameters at FUNSOM and SINANO facilities ......... 9

Table 2: DRIE parameters for trial No. 1....................................................................................... 11

Table 3: DRIE parameters for trial No. 2....................................................................................... 12

Table B-1: Trial No.1: only etching gas and passivation gas ........................................................ 25

Table B-2: Trial No.2: adding O2 gas (10% of SF6 flow) in the etching phase ............................ 25

Table B-3: Trial No.3: reduce O2 flow to 25 sccm ........................................................................ 25

Table B-4: Trial No. 4: increase passivation time and O2 flow ..................................................... 25

Table B-5: Trial No. 5: reset passivation time and reduce O2 flow .............................................. 25

Table B-6: Trial No.6: reduce O2 flow .......................................................................................... 25

Table B-7: Trial No.7: Increase O2 flow in a ramping fashion ..................................................... 26

1

1.0 Introduction

Electrospray, also known as electrospray ionization (ESI), is a technique that ionizes or atomizes

particles or molecules from its original form to charged ions for various purposes. The most

commonly used method to achieve such effect is to apply high voltage to transform the source

liquid to a gaseous phase. Nano-sized neutral-charged droplets will form at the tip of the

electrospray emitter and due to the strong electric field generated by the high voltage sources as

shown in Fig. 1, the droplets will reach the ejection threshold and be ejected with accordance of

electric field [1][2][3]. The concept of ESI was first proposed by Lord Rayleigh in 1882 and after

a century of development, in the late 1980s, mass spectrometry with ESI was achieved by J. B.

Fenn, who was awarded the Nobel Prize for Chemistry in 2002 [4].

2.0 Background Electrospray ionization’s prime application is mass spectrometry, which can be used to analyze the

mass-to-charge ratio of ions within the sample. Signals from the analyzer generate a spectrum

which provides useful information regarding the composition of the sample, making mass

spectrometry an extremely useful characterization tool for biochemistry, gas analysis, space

technology and many other fields of research [5]. Although mass spectrometry remains the most

prominent application of ESI, recent explorations to expand the use of electrospray ionization have

yielded promising results in numerous fields.

Figure 1: Demonstration of electrospray ionization [1].

2

Physical sputtering is a widely used deposition, etching technique in semiconductor industry,

micro-electronics industry, 3D printing industry and so on. Some traditional methods of performing

physical sputtering such as reactive ion etching (RIE) and deep reactive ion etching (DRIE) are

effective against substrate that is susceptible of chemical attack, such as silicon. However, if the

substrate is chemically inert, those aforementioned techniques will not be as effective as ion

bombardment techniques such as ion beam milling or cluster ion beam. ESI will produce highly

energized nano-droplets in the process, and bombarding inert material’s surface with those droplets

will greatly increase the sputtering rate and yield [6]. Moreover, the size and energy the projectile

can be tuned by applying different magnitude of electric field and source fluid, so that the surface

properties of the target can be tuned mechanically. In addition to simply etching the surface,

bombarding the surface with nano-droplets can also alter the roughness of the target substrate or

achieve amorphization of a thin layer. ESI bombardment can also be a potential substitute of

focused ion beam (FIB) to create pillars, nanorods, and nanowires [7].

One of ESI’s more novel applications is electric propulsion for micro spacecraft which will be the

focus of this report. The idea of this application can be traced back to 1960s, Thanks to the

development of microfabrication techniques, using electrospray ionization becomes a feasible

solution to space propulsion problem.

2.1 Electrospray ionization for electric propulsion

As mentioned above, ESI has numerous applications, one of the more novel and promising paths

is electric propulsion, as known as colloid thruster, for micro spacecraft. Entering 21st century, the

need for smarter and more cost-effective spacecraft has become an urgent concern for those

countries that wish to share the enormous benefit from the exploration of space. So to develop a

cheaper, more controllable and more reliable source of propulsion is becoming a popular research

area. Not only for micro spacecraft, there are some missions that require the position of the

spacecraft reaches nanometer precision, such as the joint Laser Interferometer Space Antenna

(LISA) project by European Space Agency (ESA) and NASA. To achieve such effect, the required

thrust ranges from micro to milinewton with a resolution no greater than 0.1 micro newton [5].

Contestants for propulsion source include laser propulsion, electromagnetic drive, ESI and other

propulsion system.

The idea of using electrospray ionization to power spacecraft was proposed in the 1960s and its

feasibility was also discussed by NASA. The problem back then was the magnitude of the thrust

generated was insufficient to power spacecraft since only a handful of emitters can be fabricated

3

on the limited surface area. In addition, the US government cut the budget for aerospace research

during that time period, so the research for ESI propulsion entered a halt. The development of

micro-electro-mechanical system (MEMS) techniques helps to bring ESI propulsion back to the

table for it is now possible to fabricate much more emitters on a limited surface than it could in the

1960s. In theory, if a single emitter can generate a thrust in the order of nano-newton, an array of

100 emitters could reach the order of micro-newton, and the 100-emitter array can now be

fabricated using MEMS techniques [8][9].

Some advantages of using ESI propulsion over other contestants are its extremely controllable flow

rate and its high power-thrust efficiency. For a satellite mission, in most of the cases, the power is

provided via solar panels, and how to utilize the limited power to generate the maximum thrust is

a mission-critical problem. In a colloid thruster, the amount of thrust is controlled by the magnitude

of the electric field across the extractors, and the magnitude of the electric field is controlled by the

voltage input which relies on the solar panel. A high power-thrust efficiency will not only enhance

the thrust generated, but can also extend the lifetime of the solar panels, making the mission more

cost-effective. Dr. Grustan’s lab at the University of California at Irvine (UCI) produced the

prototype with the maximum thrusting efficiency of 70%, which is the highest among all the other

colloid thruster prototypes previously fabricated [1].

To facilitate the reading of this report, a brief breakdown of the multi-emitter prototype structure

will be provided. There are three main parts of the prototype, an extractor, an emitter and a feeding

system, as shown in Fig. 2. The extractor(s) will be connected to a voltage source to generate an

electric field, thus “extracting” the ions from the emitter. The emitter array and channels underneath

it (integrate into the emitter die) will act as exits and conduits respectively. A silica feeding thread

will connect the reservoir filled with ionic fluid to the aforementioned structures to provide fuel to

the system.

Figure 2: A structural breakdown of multi-emitter array prototype [1].

4

The work that is undertaken by Institute of Functional Nano and Soft Materials (FUNSOM) at

Soochow University is to reproduce some the results from the UCI experiments and modify the

design to improve the performance.

3.0 Prototype design and fabrication

There are two types of ESI source prototypes in which the team was involved of designing and

fabricating. The first one is the multi-emitter array and the other one is a single emitter set. The

design for the multi-emitter array was completed by Dr. Grustan when he was still at UCI, though

some minor modifications were required due to the difference in the fabrication equipment. Due to

time constraint, a fully functional multi-emitter array prototype was not yet finished, however, the

team was able to fabricate the emitter array and the channel to conduit the ionic fluid using

microfabrication techniques such as photolithography and DRIE, and was able to characterize the

quality of the fabrication via scanning electron microscopy. For the single emitter set, Dr. Grustan

made some major modifications based on his experience working with the previous prototype at

UCI. Building upon the rough sketch he provided, a new ESI source with single emitter was

designed using the software Solidworks, some new features such as an extra extractor and adjusted

dimensions, appear in the new design. This new single emitter prototype is currently being

fabricated by a 3D-printing company, which is also a development from the previous one, which

was fabricated via conventional means.

3.1 The Design and Fabrication of the Single Emitter Set

In order to understand how multi-emitter array works, it’s logical to understand the mechanism of

the single emitter set first. Similar to the aforementioned multi-emitter prototype breakdown, the

single emitter set also consists of three major parts, the extractor, the emitter and the feeding system.

However, there are also some features that are unique to the single emitter set, such as the

alternating metal/plastic layers encompassing the emitter rod, screws acting as metal contacts to

create a potential difference between different layers and a customized flange to fit the testing

chamber.

The mechanism for the single emitter set is quite straight forward. As shown in Fig. 3, the reservoir

contains ionic liquid and during experimentation, carefully adjust the pressurized system denoted

as P in the schematic, the pressure difference will draw the liquid to the emitter and the strong

electric field between the extractor and the emitter tip will atomize the ionic liquid thus forming

5

nano-scale droplets. First, apply voltage to the extractor creating a potential difference between the

first extractor and the emitter rod, this difference will generate an electric field. Then ionic fluid

which is fueled with silica thread via pressure difference will migrate toward the tip of the rod,

when a certain threshold is reached, those liquid particles will be ionized and ejected toward the

extractor,

Figure 3: The experimental setup for testing single emitter set [1].

However, to facilitate the formation of droplets instead of pure ions, a second extractor is added on

top of the first extractor acting as an accelerator. Because the single emitter set is mainly used as

an ESI source for physical sputtering, so it is necessary to use a lower voltage to form electrospray

consisting mostly of energized nano-droplets instead of ion beam, then to further accelerate it to

the target.

There are five layers of alternating metal/plastic layers encompassing the emitter rod. The metal is

aluminum and the plastic is a special material called polyether ether ketone (PEEK). The PEEK

material has a low leakage coefficient making it suitable for vacuum experiments. Three screws act

as contact electrodes which extend to various layers in the prototype body. The metal layers are

made of aluminum, which has great conductivity. The screws will connect the metal layer and the

voltage sources so when turned on, strong electric field can be generated in between layers,

specifically between the extractor layer and the innermost PEEK layer, and between the extractor

layer and acceleration layer (the outer metal layer). Fig. 4 (a) is the isometric view of the single

emitter set prototype in Solidworks. Metal part is painted orange and the PEEK part is painted

white. In the body of the outer PEEK shell, three holes can be observed, those are for the screws

extending to different layers inside the emitter body. On top of the emitter set is a metal “coin”

which has an opening in the middle, this is to block any excess spray and control the diameter of

6

the output beam Fig. 4 (b) is the front plane section view of the emitter body, the layers have been

better illustrated and the tube in the center represents the tip of where the nano-droplets are created.

The inner metal layer is the extractor layer which generates electrospray droplets and the outer

metal layer which is the acceleration layer will energize (accelerate) those droplets so they carry

enough energy to bombard the surface of the target.

(a) Isometric view (b) Front plane section view

Figure 4: (a) The isometric view and (b) front plane section view of the prototype.

Fig 5 shows two of the three screw holes, the screw in 5 (a) will extend to the innermost layer of

the prototype which is a PEEK layer, and then connect to the ground; the screw in 5 (b) will extend

to the inner metal layer acting as a metal contact, and then connect to a high voltage source and

create a strong electric field between the extractor and tip, as shown in Fig. 4 (b). The height of this

prototype is 8.3 cm. This will ensure the emitter set can fit into the testing apparatus nicely.

The screw bodies have a dimension of metric 2mm. Since the screws are contacts, so it’s important

that they don't touch other layers, especially the other metal layers. So the holes are drilled with a

slightly larger diameter, 3mm for the PEEK layers and 4mm for the metal layers.

(a) Screw hole No. 1 (b) Screw hole No. 2

Figure 5: Illustration of metal contacts in the inner plastic layer and inner metal layer.

7

The base of the emitter set will be fixed on top of a PEEK flange by two metric 3mm screws as

shown in Fig. 6 (a). The flange will seal the testing chamber opening. A connector which connects

the feeding thread to the flange and upper portion of the emitter is shown in Fig. 6 (b). An ultra-

thin silica thread will act as a feeding thread and when extending into the connector the feeding

thread will go through the center of the flange and into the main body to fuel the emitter tip. The

internal structure of the connector is designed such that it will enclose any space between the thread

and side walls and essentially become a seal for the flange with the thread as the only exit. This

structure will later be elaborated in section 4.1.

(a) The PEEK flange base (b) schematics of the feeding connector

Figure 6: (a) The base structure of the prototype and (b) the feeding thread connector.

As of the time of the completion of this report, the design has met all requirements set by Dr.

Grustan including dimensions and positions of metal contacts and the prototype has been 3D-

printed. The advantage of 3D-printing is that the entire structure can be manufactured at the same

time and it offers greater precision than the traditional methods such as a machine tool.

3.2 Multi-Emitter Array Fabrication and Characterization

As mentioned in the structure breakdown in section 2.1, there are three parts in the multi-emitter

array prototype, extractor(s), an emitter array and a feeding system. Due to time constraint and

current resource limitation, only sample emitter arrays and sample channels were fabricated,

complete fabrication process and system assembly are not yet finished. The fabrication was

conducted in the cleanrooms at FUNSOM and Suzhou Institute of Nano-Tech and Nano-Bionics

(SINANO) of Chinese Academy of Science. Techniques such as photolithography, and deep

reactive ion etching (DRIE) were used in the process, the detailed procedures using those

techniques will be further elaborated. To evaluate the quality of fabricated arrays and channels,

scanning electron microscopy (SEM) was used for characterization.

8

3.2.1 Fabrication Process To complete a functional prototype, multiple steps of photolithography, DRIE, plasma-enhanced

chemical vapor deposition (PECVD), reactive ion etching (RIE) are needed. Fig. 7 is a step-by-step

breakdown of this process.

Figure 7: Breakdown for emitter and channel fabrication process [1].

The wafer used has a thickness of 450 µm. Step (1): the double-polished wafer is cleaned through

standard RCA-1 procedure (refer to Appendix A); step (2): pattern the channel via photolithography;

step (3): etch 20 µm anisotropically via DRIE, strip the photoresist afterward; step (4): oxidize the

wafer by heating the entire wafer in an oxidation oven for 2 hours, grow a 1 µm silicon dioxide

(SiO2) layer (denoted as thermal oxide in Fig. 7) at 1100 °C, this is to protect the channel from

external damage and smooth any etching defects; step (5): flip the wafer and use PECVD to grow

a 4 µm SiO2 layer (denoted as PECVD oxide in Fig. 7); step (6): pattern the emitter array (well

only) via photolithography; step (7): use RIE to etch the oxide layer and dip the wafer in

hydrofluoric acid to remove the unprotected oxide; step (8): pattern the emitter rod via

photolithography; step (9): use DRIE to etch 250 µm into the wafer and strip the photoresist; step

(10): the last DRIE carves the emitter rod to 300 µm deep, connecting the channels at the backside

with the emitter rod, also note the formation of the side well; step (11): clean the wafer and strip

the oxide via hydrofluoric acid.

9

3.2.2 Photolithography

Photolithography is crucial in the fabrication process for it’s the technique for pattern transfer. The

quality of photolithography directly impacts the quality of latter steps and the performance of the

prototype. All the photomasks used in this process were designed by Dr. Grustan back at UCI.

Photolithography contains several steps which can be briefly summarized as the following: 1. Clean

the wafers and photomasks, normally with the RCA-1 procedure, but sometimes for convenience,

acetone and ethanol rinse is sufficient; 2. Apply bias (trimethylsilyl) amine, which is also known

as HMDS to the surface of the wafer. This is to enhance the adhesiveness between the wafer and

photoresist. In the cleanroom at FUNSOM, HMDS was applied via a spinner while at SINANO, a

programed treatment system is used to spray HMDS to the wafer; 3. Apply photoresist and evenly

spread it on the wafer; 4. Bake the coated wafer; 5. Load the photomask and wafer onto

photolithography machine, expose the wafer to UV light; 6. Unload the wafer and immerse it in

developer solution for development; 7. Rinse the developed wafer with deionized water and dry it

with nitrogen gun; 8. Hard-bake the wafer at 110°C for 8mins. Table 1 details the parameters of

some of the aforementioned steps and compares the difference between the FUNSOM and

SINANO facilities.

Table 1: Comparison of photolithography parameters at FUNSOM and SINANO facilities

Name of procedure

Parameters (FUNSOM)

Parameters (SINANO)

HMDS coating 1000 RPM (10s) /3500 RPM

(20s)

10mins standard procedure in

HMDS pre-treating machine

Photoresist (AZ4620) coating

500 RPM (10s) /1500 RPM

(30s)

500 RPM (10s) /1500 RPM

(30s)

Baking

90°C for 30mins 100°C for 6mins

Exposure

Soft contact for 25s (mercury

lamp power 20mW/cm2)

Soft contact for 27s (mercury

lamp power 9mW/cm2)

Developing

AZ400K Developer: water=1: 4

Time: 3mins 33s; the time varies

due to the quality of previous

steps

RZX-3038 Developer:

water=1:8

Time: 3mins; the time varies due

to the quality of previous steps

10

The parameters used at FUNSOM are customized based on Dr. Grustan’s experience working on

microfabrication at UCI while the parameters at SINANO are standardized. The effect of those

differences is an issue worth investigating given the importance of photolithography in this

fabrication process. Fig. 8 shows pictures of developed wafer pieces, 8(a) is the emitter array

whose photolithography process was conducted at FUNSOM cleanroom; 8(b) is the channel

whose photolithography process was conducted at SINANO cleanroom.

(a)developed emitter array wafer piece (b) developed channel wafer piece

Figure 8: (a) Images of developed emitter array wafer and (b)channel wafer.

Generally speaking, the cleanroom at FUNSOM is at a more inferior quality compared to the

facility at SINANO which has a much stricter system of regulations. For photolithography,

especially for the fabrication of the channel part, the class of cleanroom is a critical factor because

even the slightest blockage in the channel could cause the failure of the entire prototype.

3.2.3 Deep Reactive Ion Etching (DRIE)

After the photolithography process is completed, the wafer is ready to be etched. And since in the

prototype, the emitter well is 300 µm deep in average with high aspect ratio, deep reactive ion

etching is the most suitable method. This process was carried out in SINANO facility.

DRIE is a cyclic process, alternating between an etching and passivation phase. It’s well-

established that SF6 works as the etching gas and CF4 as the passivation gas. The etching

environment is under Argon protection and etching gas will be ionized and form plasma from

where radical fluorine particles bombard the surface of the target guided by a strong electric field.

The areas covered by photoresist are protected from fluorine attacks and after a set amount of time,

normally in seconds, a passivation phase will follow. The purpose of passivation is to protect side

edges from etchant’s chemical attacks so that the etching can maintain largely isotropic along the

vertical axis. However, too much of passivation gas will form a deposition layer at the bottom of

11

the emitter well thus preventing further etching. Multiple trials were run to find the optimal recipe

for the process, table 2 is the conditions for trial No. 1 and Fig. 9 corresponds to its scanning

electron microscope (SEM) characterization. The sample holder was tilted 45° to get a better look

at the depth profile, and it can be clearly observed that at the bottom of the emitter well and the

bottom of the emitter rod, large amount of under-etched sharp silicon known as the silicon “grass”

are present. Their presence is not desirable thus the recipe needs to be modified.

Table 2: DRIE parameters for trial No. 1

Etching time 8s

SF6 flow 450 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 30mins

* Sccm corresponds to gas flow unit which is standard cubic centimeters per minute.

Figure 9: SEM characterization of emitter well for trial No. 1.

After the first trial, Dr. Grustan suggested adding oxygen gas during the etching phase to increase

the etching efficiency. The theoretical basis for this is that when SF6 molecule is ionized, fluorine

radicals have the tendency to recombine with sulfur ions, so by steadily adding O2 gas, sulfur ions

can bind with O atoms thus preventing this recombination. With more fluorine radicals available,

the etching efficiency is improved, and a second effect of adding O2 gas is that it can consume any

passivation deposition at the bottom of the well so the etchant can bombard the target’s surface

without any hindrance thus also improving the etching efficiency. Trial No. 2 is modified based on

Dr. Grustan’s suggestion of adding O2 gas during the etching phase, parameters are shown in Table

3. Fig. 10 is the corresponding SEM characterization image, Fig. 10 (a) is the overview of the

emitter array, and smooth surface at the bottom of the wells is observed, which is a significant

improvement compared to the first trial, Fig. 10 (b) gives a closer look at an individual well, and

12

further confirmed that no “grass” is formed at either the bottom of the side wall or the bottom of

emitter rod.

Table 3: DRIE parameters for trial No. 2

Etching time 8s

SF6 flow 450 sccm

O2 flow 45 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 50mins

(a) SEM image of the emitter array (b) SEM image of a single emitter well

Figure 10: (a) SEM characterization of emitter array and (b) single emitter well for trial No. 2.

Trial No. 2 revealed promising results which proved that adding oxygen gas during the etching

phase can significantly increase the etching efficiency, thus eliminating the formation of silicon

“grass”. However, it was also clear that the diameter at the bottom of the rod is narrower than that

of the top of the rod. The diameter is designed to be 100 µm and by estimation, the diameter at the

bottom is almost halved, which has exceeded the tolerable margin of error. Ideally, there should

not have been any reduction in diameter of the rod, a margin of 10-20 um reduction is considered

within the margin of error. This “over-etching” effect could cause the rod to break and increase the

impedance of fluid flow, which potentially can compromise the integrity and performance of the

prototype. Fig. 11 shows the ideal shape of the emitter rod, which was fabricated at UCI, 11 (a) is

the overview of the array while 11 (b) shows an individual emitter rod which largely maintains a

uniform diameter along the vertical axis. To achieve such effect, further modification of the recipe

is required.

13

(a) the overview of the emitter array (b) an individual emitter rod

Figure 11: Ideal shape and geometry of (a) emitter well and (b) emitter rod [1].

To prevent this “over-etching” effect, reduction of the oxygen flow was applied to limit the number

of fluorine radicals so it can etch “less”, but the problem persisted. Increasing the passivation time

was also experimented aiming to give the edges more protection, but the deposit layer ended up too

thick to etch, Fig 12 shows the SEM characterization from different attempts to solve the “over-

etching” issue.

(a) O2 flow decreased (b) passivation time increased (c) ramping O2 increase

Figure 12: Comparison between different mode of O2 injection:

Fig. 12 (a) shows the result after decreasing the steady O2 flow to 27 sccm, though there’s

improvement compared to 45 sccm, but the shrink in diameter is still too significant to tolerate; Fig.

12 (b) shows the result after increasing the passivation from 3s to 5s and as it clearly shows, the

amount of silicon “grass” present increased and the etching depth is not enough. This is due to the

excess passivation gas forming a deposition layer at the bottom of the well during passivation cycles,

so the etchant couldn’t penetrate and attack the surface of the wafer; Fig. 12 (c) shows the result

after adding the O2 in a ramping fashion instead of a steady flow at a constant rate. However, due

to a functional problem of the machine, a consistent ramping wasn’t applicable, so a step-increase

was adopted instead. The result was not good, not only broken rods were present, but it also had

14

deformed rod and presence of silicon “grass”. Parameters of all DRIE trials are listed in Appendix

B.

It’s worth mentioning that the cost of using SINANO’s DRIE machine is 1000 RMB/hr which

approximately $200/hr, and every trial required at least two hours due to the preparation process.

So with a limited budget, after several unsuccessful trials, it was decided that it was more

economical and convenient to adjust the geometry of the photomask instead of modifying the recipe.

The newly designed pattern has a decreased diameter of the emitter rod and emitter well, making

it more durable during the DRIE process.

Photolithography and DRIE were the main focus of this co-op term, other steps outlined in Fig. 7

were not yet completed due to time constraint and resource limitation. It is expected that a working

prototype can be fabricated before August. The total cost of fabrication including photolithography,

DRIE and the use of SINANO cleanroom is estimated to be 15000$ for the completion of the

prototype.

4.0 Performance Testing Apparatuses

After a prototype is fabricated, performance testing is required to analysis its properties and

determine what modifications are required. The procurement of performance testing apparatuses

was a major task, the total cost of equipment exceeded 40,000$. Equipment can be grouped into

three categories, electronic apparatus, data acquisition (DAQ) system and the vacuum apparatus

which is the most expensive part costing about 25000$. The vacuum setup consists of a main

vacuum chamber, two mechanical pumps, two turbo-molecular pumps and valves. The electronic

setup consists of two high voltage sources, a pulse generator, an oscilloscope, a pico-ammeter and

a 3D positioning system which includes three sets of tracks and step-motors. The data acquisition

system consists of a central DAQ card, an optical camera and a computer to process collected data.

This report focuses on the vacuum setup and electronic apparatus.

4.1 Vacuum apparatus

The performance testing will be conducted in a vacuum environment whose pressure is maintained

below 9*10-5 Pa. The general air circuit of the vacuum system is shown in Fig. 13. In the figure, T

represents turbo-molecular pumps, M1 and M2 represent mechanical pumps, P represents a

pressurized system which in our case, the atmosphere. The reservoir and chamber are connected

15

with a silica thread (painted orange in the figure), and the orange box at the bottom right corner is

a pressure gauge.

Figure 13: Air circuit of the vacuum system

The vacuum system has three different modes, pumping, experimentation and standby.

For the pumping mode, first, the mechanical pump No. 1 (denoted as M1 in Fig. 13) will start pre-

pumping, the purpose of this step is to accelerate the pumping rate by quickly pumping large

amounts of air out of the chamber as illustrated in Fig 14 (a). At the step, all valves are open so that

both the reservoir and the chamber can be pumped at the same time; when the chamber pressure

drops to 10-1 Pa, the turbo-molecular pumps are turned on to further pump air molecules such as

N2 and O2 out of the system as illustrated in Fig 14 (b). The reason for turning on turbo-molecular

pump only when a certain pressure threshold is reached is that if a large amount of air molecules

enter the pumps, it will damage the turbans in the pumps and therefore damage the pumps.

Normally, the pumping process will last for two hours for the chamber pressure to drop to 9*10-5

Pa. In Fig. 14, 16 and 17, orange arrows indicate the air flow directions.

(a) Mechanical pre-pumping (b) turbo-molecular pumping

Figure 14: Schematics of pumping mode.

16

The experimentation will start once the pressure of the chamber drops to 9*10-5 Pa. The pressurized

system (denoted P in Fig. 13) will be connected to the reservoir to pressure the ionic fluid through

silica thread into the prototype which will be fixed in the chamber. The thread is connected to the

PEEK flange which acts as a seal of the chamber, Fig. 15 shows the mounting the prototype inside

the chamber for performance testing. To maintain the vacuum environment, the turbo-molecular

pumps will stay on, so it’s crucial to close the valve connecting the chamber and the reservoir so

that ionic fluid won’t eject into the chamber due to pressure difference and protect the pumps from

a sudden pressure change due to the exposure of pressurized system as illustrated in Fig. 16 (a).

The ionic liquid used in the testing is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)

imide (EMI-Im). This particular ionic fluid has been tested multiple times by different research

groups and is believed to be the ideal source for electric propulsion testing [10]. After the

experiment is concluded, it’s necessary to level the pressure between the reservoir and the chamber

so the system can proceed to standby mode. To achieve that, mechanical pump No. 2 (denoted as

M2 in Fig. 13 will be turned on to lower the pressure in the reservoir so that it reaches approximately

the same level as the main chamber as illustrated in Fig. 16 (b), and the system is now ready to

proceed to standby.

Figure 15: The mounting of the prototype for performance testing at Dr. Grustan’s lab at UCI

[1].

17

(a) experimentation initiation (b) experimentation termination

Figure 16: Schematics of experimentation mode.

The standby mode preserves the vacuum after the experiments are concluded. When the pressure

between the reservoir and the main chamber reaches equilibrium, turn off the M2 pump and close

the valve between the M2 pump and the reservoir. To further lower the pressure by opening the

valves as the pre-pumping step as illustrated in Fig 17 (a). When the pressure stabilizes at 5*10-4

or lower, close the valve between the chamber and the turbo-molecular pumps and the valve

between the M1 pump and turbo-molecular pumps to seal the system. When the system is sealed

as illustrated in Fig. 17 (b), there’s a minimum air exchange with the atmosphere since a complete

isolation is virtually impossible. The preservation of vacuum reduces the time required for pumping

when next time the experiment is conducted. The pressurized system has been tested and the time

needed to reach the desired pressure met the requirement which is two hours if the chamber had

been previously exposed to the atmosphere or half an hour if the chamber had been sealed under

standby mode.

(a) standby mode---lower system pressure (b) standby mode---seal the system

Figure 17: Schematics of standby mode.

18

4.2 Electronic apparatus

The electronic setup which is illustrated in Fig. 18, is responsible for data measurements. The

high voltage supplies will be connected to the prototype to generate the electric field needed for

electrospray. The operation voltage ranges from negative 1500V to positive 1500V. When the

electrospray beam is formed, it will shoot toward the main chamber and be collected in the

Faraday cup which is connected with a pico-ammeter. The grid placed in front of the cup is to

suppress secondary electrons [5]. Current and current intensity are measured as a function of

experimentation time, those data will provide valuable information about the stability, emitting

efficiency, emitting intensity and so on. The thrust generated is closely related to the intensity and

magnitude of the current from ionic beam, and a vacuum chamber can simulate the working

environment of the thruster which offers researchers a more accurate assessment of the prototype.

The interior setup will be placed on an XYZ positioner which can move the Faraday cup in all

three directions. However, the assembly and calibration of the electronic setup were not

completed by the time the co-op term ended.

Figure 18: Electronic Setup of performance testing [5].

5.0 Conclusions

From the analysis of the report body, it was concluded that during the deep reactive ion etching

(DRIE), adding oxygen gas as part of etching gas can help eliminate the formation of silica grass

at the base of the emitter well and emitter rods. The ideal amount of oxygen added was thoroughly

investigated. The main etching gas was SF6, adding oxygen can prevent the recombination between

sulfur ions and fluorine ions which is mainly responsible for the etching. The addition of O2 gas

enhanced the efficiency of etching by allowing more fluorine ions to attack the surface of the silicon

wafer, thus increasing the etching rate and eliminating silicon grass.

19

The increased etching rate and efficiency is preferable, but it also comes with over-etching which

is an undesirable effect that weakens the emitter structure. Over-etching thins the base of the emitter

rods and in some cases even cause the rod to break. The integrity of the emitter rods is crucial to

the performance of the prototype and after several adjustments to the DRIE etching recipe, the

problem remains. Eventually, a modified design for multi-emitter array has been adapted. The new

array consists of the same number of emitters with higher durability to withstand the bombardment

from etchants.

The design of the single emitter set must meet the mounting requirements for performance testing.

Those include mechanical requirements such as the diameter, height of the emitter set and the

position of the fixation screws, which are important for placing the prototype onto the apparatus

and electrical requirements such as the positions of contact electrodes for the extractors to work

properly which are important for data collection. The final version of the design met all

requirements and is approved by Dr. Grustan for prototype fabrication.

Considering the economical aspect of the project, the mounting of the new laboratory, the testing

trails leading to the fabrication of prototype and the prototype fabrication itself would cost more

than 55000$. And if the first prototypes (single emitter and multi-emitter array) don’t deliver good

results, modifications are required either to the design or to the fabrication parameters. Both types

of modification cost a considerable amount of resources which include the cost of using SINANO

equipment, 3D-print new emitter set and so on. However, if the project can move on to the next

stage after promising results are delivered, then its potential application in the aerospace industry

can draw more attention and investment. Electric propulsion is a candidate for next generation

satellite propulsion system, so the potential economic gain is enormous.

Finally, the main components of the performance testing apparatus which consists of a vacuum

chamber, two turbo-molecular pumps, two mechanical pumps, an industrial camera set and

electronic measuring devices, is mounted and ready to perform some preliminary tests. However,

the mounting process is not yet completed, several key pieces are still in the process of procurement,

including an XYZ positioning system, a data acquisition system and a computer to process

experimental data. The chance of acquiring the aforementioned pieces within a short period of time

is promising and in the meantime, some preliminary results can be obtained so that Dr. Grustan’s

team can learn more about the prototypes and the performance testing apparatus.

20

6.0 Recommendations

Based on the analysis and conclusions drawn in this report, it is recommended that more research

should be done on the relationship between the flow of etching gas injected, etching/passivation

cycle time and their effects at a different aspect ratio. The study would be beneficial to improve the

quality of the DRIE process. While the over-etching problem was solved by redesigning the

geometry of the emitter rod, it comes at the cost of compromising some performance indices. It is

evident that injecting O2 during the etching cycle can help eliminate silica grass, but cause over-

etching at the base of the emitter rods, so to avoid that, it’s recommended that a new recipe should

be developed with a gentler rate of etching but producing a more evenly etched surface.

Another factor that significantly affects the quality of multi-emitter array fabrication is the quality

of photolithography. The cleanroom facilities in Institute of Functional Nano and Soft Materials

(FUNSOM) and Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) use different

parameters for photolithography, specifically the power of the mercury lamp. Due to time constraint,

the relationship between the power of the lamp and its effect on the quality of the photolithography

was not thoroughly studied, so it is recommended that more research should be done on this topic

to ensure the quality of photolithography which can significantly affect the quality of the DRIE

process thus affecting the final quality of the prototype.

Lastly, it is recommended that the missing pieces aforementioned in the conclusion section should

be purchased and assembled as soon as possible so that prototypes can be tested and modified

accordingly. Based on the degree of complexity of the two electrospray sources, it’s recommended

that the single emitter set should be tested first since it’s more easily manufactured and can help

the team to understand the capacity of the testing apparatus in the meantime.

21

Glossary

This list contains some of the acronyms appeared throughout the text with a brief description for

each term.

ESI: Electrospray ionization. It’s a technique which ionizes molecules via strong electric field. It’s

a technique mainly used for mass spectrometry. In the context of this report, the main focus is to

use ESI as a source for electric propulsion.

MEMS: Micro-Electronic-Mechanical-System. Electronic and mechanical devices which have a

dimension equal or below micro-scale. The techniques to fabricate these devices are different from

conventional methods and more sophisticated.

FUNSOM: Institute for Nano and Soft Materials. It’s an advanced research institute at Soochow

University, China. It is located in the Suzhou Industrial Park and it’s the place where I did most of

my work during the coop terms.

SINANO: Suzhou Institute of Nano-Tech and Nano-Bionics. It’s an advanced research and

fabrication facility belongs to Chinese Academy of Science. I conducted most of the fabrication

work for the prototype using their facility and equipment. Its facility contains a complete set of

micro-fabrication equipment and state-of-art cleanroom.

PEEK: Polyether ether ketone. It’s a special plastic which has a very low leakage coefficient,

making it an ideal choice for vacuum experiments. The plastic layers and the base of the single

emitter prototype and the flange to seal the chamber are made of this material.

DRIE: Deep reactive ion etching. It’s a micro-fabrication technique to perform the high aspect ratio

etching of silicon.

SEM: Scanning electron microscopy. It’s a widely used method to characterize micro-scale samples.

The imaging principle is to collect backscattering electrons and secondary electrons from the

sample after bombarding it with an electron gun.

Sccm: Standard cubic centimeter. It’s a gas flow unit which measures the different gas flow into

the DRIE chamber.

PECVD: Plasma enhanced chemical vapor deposition. It’s a technique to deposit thin films which

involves exciting the reaction gas to its plasma state. This technique was used to deposit a thin layer

of SiO2 on top of the silicon wafer.

RCA: Radio Corporation of America.

UCI: University of California at Irvine, it is the university which Dr. Grustan worked as the director

of the cleanroom and received his PhD. Many of the experimental parameters were based on his

work there.

22

Reference [1] E. Grustan Gutiérrez, "Multiplexing of electrospray sources for space propulsion and

physical sputtering", Ph.D, University of California at Irvine, 2015.

[2] C. S. Ho, C. W. K. Lam, M. H. M. Chan, R. C. K. Cheung, L. K. Law, L. C. W. Lit, K. F.

Ng, M. W. M. Suen, and H. L. Tai, “Electrospray ionisation mass spectrometry: principles and

clinical applications.,” Clin. Biochem., vol. 24, no. 1, pp. 3–12, 2003.

[3] J. J. Pitt, “Principles and applications of liquid chromatography-mass spectrometry in

clinical biochemistry.,” Clin. Biochem. Rev., vol. 30, no. 1, pp. 19–34, 2009.

[4] R. Wang, A. J. Gröhn, L. Zhu, R. Dietiker, K. Wegner, D. Günther, and R. Zenobi, “On

the mechanism of extractive electrospray ionization (EESI) in the dual-spray configuration,” Anal.

Bioanal. Chem., vol. 402, no. 8, pp. 2633–2643, 2012.

[5] R. Krpoun and H. R. Shea, “Integrated out-of-plane nanoelectrospray thruster arrays for

spacecraft propulsion,” J. Micromechanics Microengineering, vol. 19, 2009.

[6] R. Borrajo-Pelaez, E. Grustan-Gutierrez, and M. Gamero-Castaño, “Sputtering of Si, SiC,

InAs, InP, Ge, GaAs, GaSb, and GaN by electrosprayed nanodroplets,” J. Appl. Phys., vol. 114, no.

18, 2013.

[7] P. Sigmund, “Mechanisms and theory of physical sputtering by particle impact,” Nucl. Inst.

Methods Phys. Res. B, vol. 27, no. 1, pp. 1–20, 1987.

[8] M. S. Alexander, K. L. Smith, M. D. Paine, and J. P. W. Stark, “Voltage-Modulated Flow

Rate for Precise Thrust Control in Colloid Electrospray Propulsion,” J. Propuls. Power, vol. 23,

no. 5, pp. 1042–1048, 2007.

[9] L. Konermann, E. Ahadi, A. D. Rodriguez, and S. Vahidi, “Unraveling the mechanism of

electrospray ionization,” Anal. Chem., vol. 85, no. 1, pp. 2–9, 2013.

[10] M. Gamero-Castaño, “Characterization of the electrosprays of 1-ethyl-3-

methylimidazolium bis(trifluoromethylsulfonyl) imide in vacuum,” Phys. Fluids, vol. 20, no. 3, pp.

1–11, 2008.

[11] W. Kern, Ed, Handbook of Semiconductor Cleaning Technology, Noyes publishing; Park

Ridge, NJ, 1993, Ch 1.

23

Appendix A: RCA-1 clean process11

Overview:

RCA-1 process was developed by Werner Kern in late 1960s. It’s an effective cleaning process to

remove organic residue and thin films from silicon wafer. Time required for the process is

approximately 30 mins.

Material required:

Ammonium Hydroxide (27%)

Hydrogen Peroxide (30%)

Bath container

Hot plate

Preparation:

The recipe requires the ratio of DI water, Hydrogen Peroxide (H2O2) and Ammonium Hydroxide

(NH4OH) to be 5:1:1. Turn on the hot plate for pre-heating. Pour sufficient amount of

aforementioned reactants to beakers based on the size of the bath container, for example 325ml of

H2O, 65ml of H2O2 and 65ml of NH4OH.

Procedure:

Add 65ml NH4OH to DI water, and heat the solution to about 70°C, remove the solution from the

hot plate and add 65ml H2O2 to the solution. The solution will start to bubble and violent

exothermic reaction will start which indicates the solution is ready for use. Fully immerse the

wafer into the solution and soak it for 15 mins, then retrieve the wafer and rinse it with DI water.

Repeat the process for all wafers needed for the fabrication.

11 W. Kern, Ed, Handbook of Semiconductor Cleaning Technology, Noyes publishing; Park Ridge, NJ,

1993, Ch 1.

25

Appendix B: Parameters of all DRIE trials

Table B-1: Trial No.1: only etching gas and passivation gas

Etching time 8s

SF6 flow 450 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 30mins

Table B-2: Trial No.2: adding O2 gas (10% of SF6 flow) in the etching phase

Etching time 8s

SF6 flow 450 sccm

O2 flow 45 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 50mins

Table B-3: Trial No.3: reduce O2 flow to 25 sccm

Etching time 8s

SF6 flow 450 sccm

O2 flow 25 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 50mins

Table B-4: Trial No. 4: increase passivation time and O2 flow

Etching time 8s

SF6 flow 450 sccm

O2 flow 45 sccm

Passivation time 5s

CF4 flow 190 sccm

Total time 50mins

Table B-5: Trial No. 5: reset passivation time and reduce O2 flow

Etching time 8s

SF6 flow 450 sccm

O2 flow 35 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 50mins

Table B-6: Trial No.6: reduce O2 flow

Etching time 8s

SF6 flow 450 sccm

O2 flow 27 sccm

Passivation time 3s

CF4 flow 190 sccm

Total time 50mins

26

Table B-7: Trial No.7: Increase O2 flow in a ramping fashion

Etching time 8s

SF6 flow 450 sccm

O2 flow 0 sccm for 10mins

9 sccm for 15mins

18 sccm for 15mins

27 sccm for 10mins

30 sccm for 20mins

Passivation time 3s

CF4 flow 190 sccm

Total time 70mins