B/ Silicon as a Structural Material for Mechanical Sensors C/ Pressure Sensors and Accelerometers (mechanical aspects)

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Brussels, September 10-19, 2001

Laboratoire de Mécanique Appliquée – UMR CNRS 6604 – Université de Franche Comté Institut des Microtechniques de Franche Comté

4 Silicon-Based

Mechanical Microsensors

A/ Economic Aspects

B/ Silicon as a Structural Material for Mechanical Sensors C/ Pressure Sensors and Accelerometers (mechanical aspects)

D/ Some Examples of Industrial Devices (including electronic interface)

Mechanical Microsensors: Market & Industrial Perspectives

Market Volume:

- US $ 15 Billion Annualy (year 2000) Production Price:

- US $1000 in the 1960’s

- Bellow 1 Dollar per Piece Today (Pressure Sensors)

Similar Development Expected from Acceleration Sensors Spectacular Market Development Mainly Due to the Way Microsensors are Fabricated:

- Batch Processing Production Method

e.g. A Large Number of Components are Made at the Same Time

The First Silicon Micromachining Entry in the Commercial World

NATO Advanced Study Institute RESPONSIVE SYSTEMS … / MEMS Course Brussels, September 10-19, 2001

Automotive

Industrial

Biomedical Computer Consumer

World Sensor Market Breakdown

World Market for Micromachined Devices by Function

Pressure Sensors

Optical Switches

Inertial Sensors Fluid

Regulation and Control Other

Mass Storage

Si-Based Microsensors & Car Industry

From web site : mems.colorado.edu

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Single Crystal Silicon as a Structural Material for Mechanical Sensors

SCS is an Optimal Material for Mechanical Sensors:

- Free from Mechanical Hysteresis ( no Plastic Deformation < 800°C)

Creep

Reproducible Signal

Low Mechanical Losses (Q Factor = 10

⁸

in Vacuum) - SCS Fails Before it is Deformed Plastically (Brittle Material)

Overloaded Sensor Brakes Instead of Giving False Signal - Critical Stress of SCS Higher than the Yield Stress of Steels

SCS has a High Piezoresistivity

SCS Allows Integration of Sensing Elements and Electronics on the Same Substrate

MECHANICAL PROPERTIES OF STEEL Young modulus : 210GPa

Elastic Limit : from hundreds MPa up to 1GPa Ultimate Yield Strength : below 4GPa (steel)

below 2 GPa (Stainless Steel)

STRAIN

STRESS

ELASTIC LIMIT

MECHANICAL PROPERTIES OF SILICON

Young modulus : 190GPa Elastic Limit

= up to 7GPa Ultimate Tensile Strength

Comparing SCS and Steel Mechanical Properties

(Theoretically)

Laboratoire de Mécanique Appliquée – UMR CNRS 6604 – Université de Franche Comté

Basic Transduction Mechanisms / Architectures & Materials

The Two Most Important Transduction Mechanisms (e.g. The Way Mechanical Deformations due to External Mechanical Forces are Measured) :

- Piezoresistive Sensors - Capacitive Sensors

The Two Main Classes of Mechanical Architectures:

- Membrane-Type Structures (Pressure & Flow Sensors) - Cantilever Beam Structures ( Acceleration Sensors) The Two Manufacturing Options & Corresponding Materials

- Monocrystalline Silicon (Bulk-Micromachined Sensors) - Polycristalline Silicon * (Surface-Micromachined Sensors)

* Note that Mechanical Properties of Polycristalline Silicon are Still Process Dependent !

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Membrane-Type Structures

Cantilever Beam Structures

Mechanical Architectures

Pressure & Flow Sensors

Accelerometers

Force & Pressure Sensors

d: Plate separation of the unloaded diaphragm h: Thickness of the membrane

Spring

Element Sensor

Element

Force Pressure

Mechanical Deformation

Electrical Output Signal

Ebh

= F ε

3 0 4

32 1

dh r E

p C

C = δ

Sensor Beam Strain Gauge

Piezoresistive

Capacitive

d

p

Pressure Sensors

Architecture of the Spring Element:

- Membrane (Always) Structural Material:

- Metal Membranes Monocrystalline Silicon Membranes Read-Out Mechanism:

- 1. Piezoresistive (Still the Most Widely Used)

- 2. Capacitive (Higher Sensitivity)

- 3. Resonant Pressure Sensors (Highest Accuracy) Early 1980’s

Suffer Much Less From:

- Creep - Fatigue - Hysteresis - Small Size

- High Elastic Modulus / Low Density Very High Resonance Frequency 1. Change in Resistance: 2 to 5%

2. Capacitance Change: 30 to 50%

Piezoresistive Pressure Sensors

Most Currently Available Silicon Pressure Sensors Use a Piezoresistive Read-Out Mechanism

Typical Bulk Micromachined

Piezoresistive Pressure Sensor Surface Micromachined Piezoresistive Pressure Sensor With Polysilicon Membrane Polysilicon Membrane

Thickness: 2µm Monocrystalline Silicon Membrane

Thickness: Several Tens µm

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Capacitive Pressure Sensors

Two Configurations of Diaphragm Capacitive Pressure Sensors

The deflection of a circular membrane loaded by a pressure p is:

( ) 3 ( 0 ² ² ) ²

1 2

16 3 p r r

r Eh

w − −

= ν

d r

0

h

( ) _rdr d w C 2

^ro

d ( 1 ^r )

0 −

= ^π ∫ ^ε

Non-Linear Capacity Response

Capacitance Changes as a function of the Contact Area

Much Closer to Linearity than a

Comparing Piezoresistive & Capacitive Pressure Sensors

Membrane deformation has quite different consequences for the relative changes of capacity and resistance…

0 4 3

0 4

32 1 32

1 



 



≈ 

= h

r E

p dh

r E

p C

δ C

0 2

6 1 



 



= 

∆ =

h r E G p R G

R ε

h d ≈

Since r

0

/h >>1, sensitivity of capacitive pressure sensors is much higher…

Approximate solutions must be used for rectangular & square membranes.

Polysilicon Strain Gauge r

0

0 3 r

Gauge Factor Average Strain

Relative change of resistance Relative change of capacitance

Intra-Ocular Pressure Sensor [BAC 90]

SEM Picture of the Membrane Showing the Bonded Region

Comparing Bulk & SM Capacitive Pressure Sensors

Silicon-Based Accelerometers

The Next Big Silicon Micromachining Market Entry in the Industrial World After Pressure Sensors

Pressure Sensors Various Ranges of Pressure Measurements & Many Different Packages

Accelerometers Little Difference in Packaging & Operating Ranges Two Main Accelerometer Types cover all the Needs

“High g” “Low g”

- Airbag Accelerometers Operating Range: 50g

- Car Suspensions

- Anti-Lock Braking Systems (ABS)

Operating Range: 0 – 2g

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Comparing Piezoresistive & Capacitive Accelerometers

Early Accelerometer Structure using Piezoresistors to Sense the Motion of a Proof Mass in Response to Acceleration.

- Poor matching & Linearity Characteristics - Large Variation with Temperature - Poor Sensitivity…

Bulk-Micromachined Accelerometers Moved to Capacitive Sensing Methods using Bonded Wafer Technique…

Capacitors Drift Little over Temperature Temperature Compensation Circuit is Minimal

Bulk-Micromachined Accelerometers in Single Crystal Silicon

Basic 1D Bulk- Micromachined Capacitive Accelerometer

From [PUE 98]

3D Bulk- Micromachined Capacitive Accelerometer (a)Rest position, (b)Vertical Displacement due to z-axis acceleration, (c) Tilt due to x-axis acceleration

Surface-Micromachined Accelerometers: Toward Integration of Electronics

Monolithic processing and reduced parts count yield high reliability at low cost.

Today’s Force-Balance

Integrated Accelerometer

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Surface-Micromachined Accelerometers in Poly Si

Restricted Proof Mass !

3-Axis Accelerometer Example (From BSAC) - 3 sensors x/y/z

- 500 kHz sampling rate - 2µm CMOS - 4 x 4 mm

²

die - 5V/9Ma per axis

Surface-Micromachined Accelerometers in Poly Si

Commercialized 50-g Surface Micromachined Accelerometer

Analog Device’s ADXL50 Accelerometer

Capacitive sensor

Some Commercialized Accelerometers

5 to 200g Single Axis Accelerometers Machined Through Electroplated Nickel (From Silicon Design,Inc.).