Design,fabrication and Application of MEMS accelerometer.

1. MEMS CAPACITIVE
ACCELEROMETER
(Design and Fabrication)
Presented by:-
PRASHANT SINGH

2. OUTLINE

Introduction

MEMS




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


Accelerometer
Basic operation principle
Capacitive Accelerometer


MEMS vs. IC’s
Capacitance basics
Sensing mechanism
Structure analysis (spring stiffness)
Advantages
Accelerometer fabrication
Applications

3. INTRODUCTION

MEMS



MEMS:



MEMS move and Sense Mass.
MEMS act as transducer (Sensor & Actuator).
IC



Micro-Electro-mechanical-system
Integration of mechanical unit, electrical unit, sensor and
actuator on a single substrate.
IC’s move & sense electrons.
IC’s acts as sensor not actuator.
MEMS Advantage



Smaller
Lighter
Economical

4. ACCELEROMETER
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Inertial sensor:
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Device used to measure:
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Newton’s 1st law (Mass of inertia).
Acceleration
Displacement
Force
Inclination
Design Approach:
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
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Piezoelectric
Piezoresistive
Tunneling
Capacitive, etc.
Basic capacitive Accelerometer.

5. BASIC OPERATION PRINCIPLE


6. CAPACITIVE ACCELEROMETER




Based on Change in capacitance between Comb
fingers.
{Capacitance change} α {Force applied on Proof
Mass}
Comb structure
Large capacitance value
Advantages





High resolution
Good DC response
Linear output
low power dissipation
Easy incorporation with CMOS

7. CAPACITANCE BASICS


8. SENSING MECHANISM

(a)
Two types:

Out-of-Plane Sensing
 Change

in overlapping width (w).
In-Plane sensing
 Lateral

sensing (a)
Change in overlapping length (L)
 Transverse

sensing (b)
Change in finger gap (d)
(b)

9. STRUCTURE ANALYSIS

Unguided and guided beam structure

10. CAPACITIVE ACCELEROMETER ADVANTAGES
High sensitivity.
 Easy readout circuitry.
 Independent of temperature variation.
 Easy fabrication (two level mask).
 Large noise margin.
 Fabrication on silicon.
 Compatible with CMOS technology.


11. FABRICATION PROCESS

Silicon on insulator(SOI) wafer(i)
15µm
2µm
 Cleaning
-RCA 1
-RCA 2

Oxidation(ii)


475
µm
thickness of oxide layer ::7700 Å at 1050ºC
Photolithography(iii)
-1st level mask
-prebaking
-UV light exposer-15 sec
-PR develop
silicon
oxide
silicon
oxide
silicon
oxide
(i)
(ii)
silicon
oxide
photoresist
Silicon
oxide
silicon
oxide
(iii)

12. FABRICATION PROCESS CONTD..

SiO2 etch (iv)
–buffer HF
-etch rate ≈ 1000Å/min.

Silicon etch(v)
Silicon
oxide
silicon
oxide
(iv
oxide
-PR removal
-Anisotropic Si etch
-THAH solution used
-V groove formation
oxide
silicon
oxide
(v

13. FABRICATION PROCESS CONTD..

SiO2 removal (vi)
-Buffer HF
-hanging structure

oxide
(vi
silicon
Metallization (vii)
-2nd level mask
-Al deposited
-temp. 650ºC
oxide
silicon
(vi

14. ACCELEROMETER FABRICATION CONTD..
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Accelerometer (1st level mask, PR developed)
photoresist

Accelerometer (SiO2 etched)
SiO2
Si
SiO2
Si

15. APPLICATIONS
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Automotive
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Consumer Electronics
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hard disk protection(laptops)
screen rotation (mobile)
Image stabilization (camera)
Industrial
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Crash detection & Air bag deployment.
Vibration detection (machine)
crack detection (pulley)
Aerospace & Defence
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Navigation
Missile guidance
Thrust detection
Accelerometer application

16. THANK YOU