NAME: SOHAM DEY ROLL:

001010801063 UG 4TH YEAR

DEPARTMENT OF ELECTRICAL ENGINEERING

JADAVPUR UNIVERSITY

MICROELECTROMECHANICAL SYSTEM

INTRODUCTION

Micro-electro-mechanical systems or in short MEMS is a technology which is based on a number of tools and methodologies used to form small structures with dimensions in the micrometer scale.

It uses similar micro fabrication steps used in Integrated Circuits industry to build these microstructures on a silicon substrate. Miniaturized sensors and actuators can be developed .Moreover the sensors and actuators can be merged with electronic circuit on a single silicon chip.

Thus in this single unit the sensors can gather information from the environment by measuring mechanical, thermal, biological, chemical or magnetic phenomena. The electronic circuit’s then process the information derived from the sensors and through some decision making capability direct the actuators to respond by either moving, positioning, regulating or pumping thereby controlling the environment.

The characteristic size of the structures ranges from a nanometer to a few micrometer. The figure below shows a MEMS device to demonstrate scale.

Figure : The Drive gear chain and linkages having a size of about 50um is shown along with a grain of pollen (top right) and coagulated red blood cells (lower right, top left) each having a size of about 10 um.

Purpose of building MEMS Devices

Do we gain anything by scaling down the sensors and actuators to micro level?

In the table below the parameters of a Conventional accelerometer has been compared with a MEMS based accelerometer.

Parameters Conventional MEMS Based

Weight 1567 gm. 10gm

Power Consumed 35 W 1 mW

Size 15cm*8cm*5cm 2cm*2cm*0.5cm

Cost $ 10,000 $ 100

Comparing the two we can see that it is possible to drastically reduce the weight, size as well as power consumed and cost of a unit.

Conventional Accelerometer

MEMS accelerometer

ADVANTAGES

All MEMS fabrication approaches share the following three key advantages:

1. Miniaturization a) Small and light structures lead to devices that have relatively high resonant

frequencies. These high resonant frequencies in turn mean higher operating frequencies and bandwidths for sensors and actuators.

b) Thermal time constants are smaller which allows the structures to absorb and release heat quickly.

c) Miniature systems are less invasive. They can be easily embedded in objects.

2. Multiplicity a) It possible to fabricate ten thousand or a million components as easily and quickly

as one .This helps in reducing unit cost.b) The designer can design the form of interconnection among these components to

realize diverse functionalities. Thus it provides flexibility in building massively parallel and interconnected electromechanical system.

3. Microelectronics a) Microelectronics integrated into MEMS devices provides the latter with

intelligence, allows closed-loop feedback systems and helps in controlling massively parallel actuator arrays.

b) Since Localized signal conditioning is possible, addition of noise to signals from the sensors is reduced.

LIMITATIONS

1. Scaling Limit : All MEMS device have a lower limit to the size we can miniaturize them.

At micro scale surface forces like electrostatic force, capillary force & molecular Van der Waals force dominate .They cause stiction between microscopic structures when their surfaces come into contact .Thermal noises can alter the signals generated.

Different difficulties are encountered while scaling various systems.

For a fluidic system as the dimensions is reduced by x, the measure of flow turbulence is also reduced by x, and the fluid flow becomes increasingly laminar .Because of this behavior, it is very challenging to achieve thorough mixing in microfluidic systems. The scaling of chemical and biological systems is limited by a fundamental tradeoff between sample size and detection limit. As the sample size is reduced, the total molecules which need to be detected are also decreased, and therefore an increasingly sensitive detector will be needed.

2. Mechanical power : Mechanical power transfer capability of the mechanical components is very less.

3. Packaging issues : Firstly silicon based MEMS devices are complex 3D structure while IC’s are primarily 2D structure .Secondly MEMS devices have moving solid components and have to interface with the environment .So it has to protect the electrical circuits from mechanical and environmentally induced damages. These make packaging much more difficult and complex.

4. No industrial standards : For MEMS there are no industrial standards regarding design, materials selections, fabrication processes and assembly-packaging-testing. It has limited the commercial success of MEMS devices.

REFERENCES

1. D.Senturia, S. “ Microsystem Design”. Kluwer Academic Publishers.

2. Stanmirovic, I., & Stanimirovic, Z. (2009). Reliability Of MEMS,Micro Electronic And Mechanical Systems.

3. Andrew A.Berlin, K. J. (1997). Distributed Mems: New Challenges For Computation. Computational Science &Engineering,IEEE (vol:4 Issue:1), 12-16.

4. S.Eddy, D., & R.Sparks, D. (1998). Application Of MEMS Technology in automotive sensors and actuators. Proceedings of the IEEE (volume:86 issue:8), (pp. 1747-1755).

5. http://mems.sandia.gov/gallery/images.html

MICROFABRICATION PROCESSES

Micro fabrication is based on Planar Technology where the substrates are initially flat wafers. Single crystal silicon is mostly used as substrate. Huge advancement made by Integrated Circuit industry in Planar Technology allows MEMS designers to exploit similar processing steps to develop electrical and mechanical components .

It is a repetitive process involving three steps

1. Deposition 2.Patterning 3. Etching .

The illustration below shows the steps along with their techniques.

A brief description of the processing steps is given below

DEPOSITION

It is the process of growing a thin film over the substrate .These films are generally metals, crystalline silicon, silicon dioxide or silicon nitride. They have thickness ranging from 1um to 20um

The purpose of the thin film depends on the type of device . It can be reflective, transparent, conducting or insulating.

Doping if required is generally done after first depositing a thin film. So it has been classified under deposition.

Doping

It is the process of adding minute quantity of impurity atoms to the substrate to modify its electrical characteristics. It is used to create an n-type or p-type region.

The process of doping a wafer consists of two steps:

• Deposition : A particle accelerator shoots a beam of dopant atoms directly into the wafer and introduces a very thin sheet of a known number of dopant atoms at a particular depth.

Depth and Location can be controlled by masking regions with photoresist layers as illustrated below

Drive-in : The system is annealed at high temperature so that the dopant atoms diffuse to achieve required distribution

PATTERNING

Pattern transfer is done using a process known as lithography .In this process the desired geometric pattern is photographically transferred from an optical plate called mask to a photosensitive film coating the wafer.

Photolithography consists of two key steps:

(1) Pattern generation : It begins with mask design. A mask is Quartz plate with a patterned emulsion of metal film on one side.

(2) Pattern transfer : A layer of resist is coated on the substrate and then the pattern on the mask is transferred on to the resist.

Following steps are followed sequentially:

a) Cleaning : The substrate is first cleaned by a procedure known as RCA Cleaning procedure to remove organic and inorganic impurities.

b) Resist Coating : A viscous, liquid solution of photo-resist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer between 0.5um and 2.5um thick.

c) Soft Baking : The system is baked to evaporate the solvent and to make the resist coating dense. Baking also makes the resist photosensitive.

d) Mask Alignment & Exposure: The mask is now properly aligned and the resist is exposed to UV light through the mask. If the resist is positive, the chemical structure of the exposed portion changes in such a way that these portions become more soluble in a developer solution .

e) Development: After exposure the system is immersed in a developer solution which washes away only the exposed portions, leaving a pattern. Converse happens if it is a negative resist.

The illustration explains the difference in the structure obtained with positive and

negative resist.

f) Hard baking : Finally the system is hard baked to increase the stability of the developed structure for subsequent processes.

This completes the process of pattern transfer.

ETCHING

In this process we remove material along the patterns to develop shapes and structures .

The chemical used for etching is called etchant. The effectiveness of an etchant depends on its ability to effectively etch along the exposed layer without significantly etching the masking layer.

Etching can be either Isotropic or Anisotropic in nature . In isotropic etching the etch rate is same in all directions. On the other hand Anisotropic etching is dependent on the orientation of crystal plane. Etch rate is quick along some planes and slow in others.

The illustration shows the difference in the shapes. In isotropic etching since etch rate is same in lateral and vertical directions a typical undercut is obtained as shown.

Depending on where the materials are removed by etching the overall process is classified as Bulk Micromachining or Surface Micromachining.

Bulk Micromachining

When structures are formed by etching inside the substrate, the overall process is classified as Bulk Micromachining. The etching is generally anisotropic in nature.

Why etching is anisotropic?

The mechanism responsible for anisotropic etching is the different bond structure that are revealed in different surface planes of single crystal silicon. The possible planes are{100} plane, {110} plane and {111} plane ( the numbers are Miller Indices). Crystals that are oriented with one of the {100} planes as its surface are called (100) wafers. It has been found that the atoms in {111} planes are more tightly bound than the other planes . As a result {111} plane Etch significantly less quickly compared to other planes when placed in a suitable etchant.

The illustration below shows shows a (100) wafer with mask aligned to <110> direction.Exposed {100} planes etch rapidly while {111} planes etch slowly .

Structures like V-grooves, pyramidal pits can be developed as shown below.

SURFACE MICROMACHINING

In Surface Micromachining layers are deposited over the substrate and these layers are etched to develop structures on the substrate. The etching process is generally isotropic in nature.

It in involves the following steps as illustrated in the figure above

a) First a sacrificial layer is deposited and patterned.b) Then, a structural material is deposited and patterned so that part of it extends

over the sacrificial layer and part of it provides an anchor to the substrate.c) Finally a Release Etchant removes the sacrificial layer leaving free standing

mechanical structure.

Using the two micromachining processes different electrical and mechanical components can be developed.

Surface Micromachining is more popular than Bulk Micromachining as it allows the creation of more complex structures but it requires more fabrication steps and is costly.