2010 International Conference on Mechanical and Electrical Technology (ICMET 2010)

DESIGN OF MEMS BASED CAPACITIVE ACCELEROMETER

T.K. SETHURAMALINGAM Research Scholar, Dept. ofinstrumentation & Control

Engg. SRM University, Kattankulathur Tamilnadu ,India

email: tksethuramalingam@yahoo.co.in

Abstract- MEMS are the manufacturing of a wide variety of items that are electronic and mechanical in nature. In addition to sensors, small motors, pumps, hydraulic systems, warhead fuses, high resolution displays, mass data storage devices are but a few of the devices that can be manufactured using MEMS technology. The characteristics of MEMS fabrication are miniaturization, multiplicity, and microelectronics. Miniaturization not only allows for small, lightweight devices, but these same devices have high resonant frequencies which mean higher operating frequencies and bandwidths for microsensors and microactuators. An accelerometer measures proper acceleration, which is the acceleration it experiences relative to freefall, and is the acceleration that is felt by people and objects. Such accelerations are popularly measured in terms of g-force. At any point in space time the equivalence principle guarantees the existence of a local inertial frame, and an accelerometer measures the acceleration relative to that frame. As a consequence an accelerometer at rest relative to the Earth's surface will indicate approximately 1 g upwards, because any point on the earth's surface is accelerating upwards relative to the local inertial frame, which would be the frame of a freely falling object at the surface. To obtain the pure acceleration due to motion with respect to the Earth, this "gravity offset" must be subtracted. This is generally true of any gravitational field, since gravity does not produce proper acceleration, and an accelerometer is not sensitive to it, and cannot measure it directly. An accelerometer behaves as a damped mass on a spring. When the accelerometer experiences acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration. There are many different ways to make an accelerometer. Some accelerometers use the piezoelectric effect - they contain microscopic crystal structures that get stressed by accelerative forces, which cause a voltage to be generated. Another way to do it is by sensing changes in capacitance. Capacitive interfaces have several attractive features. In most micromachining technologies no or minimal additional processing is needed. Capacitors can operate both as sensors and actuators. They have excellent sensitivity and the transduction mechanism is intrinsically insensitive to temperature.

Keywords- accelerometer, microactutor, micromachining

MEMS,

I. INTRODUCTION

microsensors,

MEMS accelerometers are one of the simplest but also most applicable micro-electromechanical systems.

978-1-4244-8102-6/10/$26.00 CO 2010 IEEE 565

Dr. A. VIMALAJULIET Head, Dept. ofinstrumentation & Control Engg.,

SRM University, Kattankulathur Tamil Nadu, India - 603 203 e-mail: vimlala@yahoo.co.in

They became indispensable in automobile industry, computer and audio-video technology. Micro machined accelerometers are a highly enabling technology with a huge commercial potential. They provide lower power, compact and robust sensing. Multiple sensors are often combined to provide multi-axis sensing and more accurate data.

This model for the development and implementation of MEMS based capacitive accelerometer. When selecting an accelerometer, it is important to determine whether one is trying to measure motion or vibration. whereas in vibration measurement, one is after the vibratory responses of the object under test, in motion measurement, one is interested in the speed or the displacement of the rigid body. While using an accelerometer to measure motion accurately, it is to be ensured that the measured acceleration data do not contain any zero offset error. A very small amount of zero offset in the acceleration output can lead to gross amount of velocity or displacement errors after numerical integrations. Since all piezoelectric based accelerometers and other AC-coupled designs will produce zero offset errors while trying to follow a slow motion, they should not be considered for motion measurements. The design process and simulation are done using Intellisuite software. An accelerometer is an electromechanical device that will measure acceleration forces. These forces may be static, like the constant force of gravity pulling at your feet, or they could be dynamic -caused by moving or vibrating the accelerometer. If an accelerative force moves one of the structures, then the capacitance will change. Add some circuitry to convert from capacitance to voltage, and you will get an accelerometer.

II. SCALING ADVANTAGES AND ISSUES When miniaturizing any device or system, it is critical to

have a good understanding of the scaling properties of the transduction mechanism, the overall design, the materials and the fabrication processes involved. The scaling properties of any one of these components could present a formidable barrier to adequate performance or economic feasibility. Due to powerful scaling functions and the sheer magnitude of the scaling involved (Le., MEMS can be more than 1000 times smaller than their macroscopic counterpart), our experience and intuition of macroscale phenomena and designs will not transfer directly to the micro scale.

2010 International Conference on Mechanical and Electrical Technology (ICMET 2010)

III. STRENGTH OF POLY SILICON FOR MEMS DEVICES

The safe, secure and reliable application of Microelectromechanical Systems (MEMS) devices requires knowledge about the distribution in material and mechanical properties of the small-scale structures. A new testing program at Sandia is quantifying the strength distribution using polysilicon samples that reflect the dimensions of critical MEMS components. The strength of poly silicon fabricated at Sandia's Microelectronic Development Laboratory was successfully measured using samples 2.5 microns thick, 1.7 microns wide with lengths between 15 and 25 microns. These tensile specimens have a freely moving hub on one end that anchors the sample to the silicon die and allows free rotation. Each sample is loaded in uniaxial tension by pulling laterally with a flat tipped diamond in a computer-controlled Nanoindenter. The stress-strain curve is calculated using the specimen cross section and gage length dimensions verified by measuring against a standard in the SEM. Fracture strength measurements grouped into three strength levels, which matched three observed failure modes. The samples in the highest strength group failed in the gage section, those in the moderate strength group failed at the gage section fillet and those in the lowest strength group failed at a dimple in the hub. With this technique, mUltiple tests can be programmed at one time and performed without operator assistance at a rate of 20-30 per day allowing the collection of significant populations of data. Since the new test geometry has been proven, the project is moving to test the distributions seen from real geometric features characteristic of MEMS, such as the effect of gage length, fracture toughness, bonding between layers, etch holes, dimples and shear of gear teeth.Maintaining the Integrity of the Specifications

Poly-Si MEMS Technology Finger cross sectIOn vth slight squeeze film damping

Moyabie

Fixed Rxed

Figure 1. Poly - Si MEMS Design Technology

IV. DESIGN AND ANALYSIS REPORT The mask layout of a comb structure capacitive

accelerometer is given here.

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Figure 2. Mask layout of a comb structure capacitive accelerometer

Figure 3. View of the capacitive accelerometer

Poly silicon springs suspend the MEMS structure above the substrate such that the body of the sensor (also known as the proof mass) can move in the X and Y axes. Acceleration causes deflection of the proof mass from its centre position. Around the four sides of the square proof mass are 32 sets of radial fingers. These fingers are positioned between plates that are fixed to the substrate. Each finger and pair of fixed plates make up a differential capacitor, and the deflection of the proof mass is determined by measuring the differential capacitance.

The obtain mask layout is undergone with different MEMS material analysis and finally the mask layout is fabricated.

TABLEr MEMS MATERIAL PROPERTY Si3N4 PECVD Ar

Property Value Units Comments STRESS 467.793 MPa avg

DENSITY 2.55 -

g/cm3 meas CTExp 16 10(7)1C meas YOUNG 300 GPa meas POISSON 0.27 const meas

REFRJN 2.05 const meas

The above table indicates the material property and the analysis is obtained

2010 International Conference on Mechanical and Electrical Technology (ICMET 2010)

Figure 4. Material Property Analysis

The designed mask is fabricated and the output is given here.

Figure 5. Fabricated Design of a Comb Structure Capacitive Accelerometer

The fabricated comb structure capacitive accelerometer is undergone for thermoelectromechanical analysis at a displacement on y axis and the output is given here

Figure 6. Thermoelectromechanical analysis on displacement at y axis

The final design output of the comb structure capacitive accelerometer is given here

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Figure 7. Design output of the comb structure capacitive accelerometer

MEMS-based accelerometer contains a small heater at the bottom of a very small dome, which heats the air inside the dome to cause it to rise. A thermocouple on the dome determines where the heated air reaches the dome and the deflection off the center is a measure of the acceleration applied to the sensor. Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-ofplane device three axes can be measured. Such a combination always has a much lower misalignment error than three discrete models combined after packaging.

Micromechanical accelerometers are available in a wide variety of measuring ranges, reaching up to thousands of g's. The designer must make a compromise between sensitivity and the maximum acceleration that can be measured.

V. MEASURING VIBRATION ON OBJECTS

There are many applications where the test articles are no bigger than a tennis ball. Making shock and vibration measurements under such conditions require sensors with unique physical characteristics. Accelerometer selection considerations in this application are:

Mass-Loading Effect - Mass-loading effect can change the dynamic responses of the measurement. Size and weight of the accelerometer must not be out of proportion with the test article. The rule-of-thumb is not to exceed 10: 1. There are PE, ISOTRON and PR accelerometer models that are very small and lightweight (as low as 0. 14 gm) which help minimize mass-loading problems.

Mounting Method - Drilling threaded holes for studmount type sensors in a small test article is impractical. Adhesive mounting is the only logical method. Adhesive mounting/removal instructions should be followed religiously to prevent damage to the accelerometer body.

Surface curvature - Care should be taken to provide a flat surface for the accelerometer. This might require manufacturing special mounting blocks with matched curvature of the lower surface.

2010 International Conference on Mechanical and Electrical Technology (ICMET 2010)

Resonance - Small structures usually have high frequency modes. Accelerometers with higher resonance (>50 kHz) may be required.

Cable - When the test structure is very small and lightweight, even the stiffuess of the cable can affect the dynamic responses. Small-gauge, flexible cable should be used in these situations.

VI. BUILDING & STRUCTURAL MONITORING

Accelerometers are used to measure the motion and vibration of a structure that is exposed to dynamic loads. Dynamic loads originate from a variety of sources including:

Human activities - walking, running, dancing or skipping

Working machines - inside a building or in the surrounding area

Construction work - driving piles, demolition, drilling and excavating

Moving loads on bridges Vehicle collisions Impact loads - falling debris Concussion loads - internal and external explosions Collapse of structural elements Wind loads and wind gusts Air blast pressure Loss of support because of ground failure Earthquakes and aftershocks

Measuring and recording how a structure responds to these inputs is critical for assessing the safety and viability of a structure. This type of monitoring is called Dynamic Monitoring.

VII. CONCLUSION

MEMS accelerometers are inertial sensing devices that address the high performance, low power, integrated functionality, and small size requirements in countless applications. Intelligent sensor accelerometers offer further integration and improved performance including applicationtargeted functionality, comprehensive factory calibration that saves costs and production test time, and a simple programmable interface that ensures highly precise integration that is simple to implement. Standardization of production, testing and packaging MEMS would certainly do a big part at it. The relatively long and expensive development cycle for a MEMS component is a hurdle that needs to be lowered and also less expensive microfabrication method than photolithography has to be pursued.

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T.K.Sethuramalingam. This author was born in Tamil Nadu, India, in 1981 and received the B.Sc. degree from the Manonmaniam Sundaranar

University, India, in 2001. Further he received his M.Sc . . and M.Phil degrees from Bharathidasan University, India, in 2003 and 2005. He has been teaching at Mohamed Sathak college of Arts & Science, Chennai, India since 2003. He is currently doing his Ph.D under the guidance of Dr. A. Vimala Juliet. He visited foreign countries and presented his research publications. He is a Member in IEEE, ISSS, IACSIT. His research interests and

publications have been in the areas of VLSI design, Signal & Image processing, communication electronics and MEMS.

A. Vimala Juliet. This author was born in Chennai, India, in 1969. She received the B.E degree from the Bharathiar University, India, in 1992 and acquired her M.E. and Ph.D. degrees from Anna University, India, in 1994 and 2005 respectively. Dr. A. Vimala Juliet has been teaching at SRM University, India since 1995. She has been a senior member of IS A since 1999. In 2006, she was promoted Professor and Head of the

Instrumentation and Control Engineering Department of SRM University. She visited UC Davis, USA under SAP programme during October 2008. Her research interests and publications have been in the areas of Sensors, Virtual Instrumentation, MEMS and Control systems.