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Design,fabrication and Application of MEMS accelerometer.
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Presented by:-
PRASHANT SINGH
MEMS CAPACITIVE
ACCELEROMETER (Design and Fabrication)
OUTLINE Introduction
MEMS MEMS vs. IC’s
Accelerometer Basic operation principle Capacitive Accelerometer
Capacitance basics Sensing mechanism Structure analysis (spring stiffness) Advantages
Accelerometer fabrication Applications
INTRODUCTION MEMS
Micro-Electro-mechanical-system Integration of mechanical unit, electrical unit, sensor and actuator on a
single substrate. MEMS:
MEMS move and Sense Mass. MEMS act as transducer (Sensor & Actuator).
IC IC’s move & sense electrons. IC’s acts as sensor not actuator.
MEMS Advantage Smaller Lighter Economical
ACCELEROMETER Inertial sensor:
Newton’s 1st law (Mass of inertia). Device used to measure:
Acceleration Displacement Force Inclination
Design Approach: Piezoelectric Piezoresistive Tunneling Capacitive, etc.
Basic capacitive Accelerometer.
BASIC OPERATION PRINCIPLE Newton’s 1st law (Mass of inertia). Newton’s 2nd law: F=ma. Accelerometer model: 2nd order spring-damper
model. Force ‘Fe’ applied on the frame. Reference frame displacement- Xf Proof mass displacement- Xm
{}
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
CAPACITANCE BASICS
A= overlapping area
d= separation between fingers
C change depends on ‘l’, ‘w’ & ‘d’.
Capacitance change
, ,
SENSING MECHANISM
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)
(a)
(b)
STRUCTURE ANALYSIS Three elements:
Proof mass Comb structure Proof mass support (beam or spring)
Spring stiffness (k) Also defined for beam.
K
Unguided and guided beam structure
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.
FABRICATION PROCESS
silicon
oxidesilicon
oxidesiliconoxide
Silicon on insulator(SOI) wafer(i) Cleaning -RCA 1
-RCA 2
Oxidation(ii) thickness of oxide layer ::7700 Å at 1050ºC
Photolithography(iii)-1st level mask
-prebaking
-UV light exposer-15 sec
-PR develop
silicon
475µm
2µm15µm
silicon
oxideSilicon
photoresist
(i)
(ii)
(iii)
oxide
oxide
FABRICATION PROCESS CONTD..
SiO2 etch (iv)
–buffer HF
-etch rate ≈ 1000Å/min.
Silicon etch(v)-PR removal
-Anisotropic Si etch
-THAH solution used
-V groove formation
silicon
oxideSilicon
oxide
oxide
silicon
oxide
oxide
(iv)
(v)
FABRICATION PROCESS CONTD..
SiO2 removal (vi)-Buffer HF
-hanging structure
Metallization (vii)-2nd level mask
-Al deposited
-temp. 650ºC
silicon
oxide
(vi)
silicon
oxide
(vii)
ACCELEROMETER FABRICATION CONTD..
Accelerometer (1st level mask, PR developed)
Accelerometer (SiO2 etched)
SiO2
Si
SiO2
Si
photoresist
APPLICATIONS
Automotive Crash detection & Air bag deployment.
Consumer Electronics hard disk protection(laptops) screen rotation (mobile) Image stabilization (camera)
Industrial Vibration detection (machine) crack detection (pulley)
Aerospace & Defence Navigation Missile guidance Thrust detection Accelerometer application
THANK YOU