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