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ES1050 – Introductory Engineering Design and Innovation Studio

ECE Case Study

Accelerometers in Interface Design – Part IIProf. Jayshri Sabarinathan

TEB 259 jsabarinathan@eng.uwo.ca

2009 11 27

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Outline

Recap of Design process for WII Introduction to electronics The microelectronics process MEMS MEMS Accelerometer

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Recap

Problem definition Functionality -> existing technology to solve

some features Objectives Constraints Concepts – decision making -> accelerometers Analysis/ Calculations Simplify Next step : Iteration- need something smaller

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Accelerometer Theory

Accelerometer for into plane acceleration or pitch

Strain gauge

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Wii First Proposal

We can build something like this:

Acc 1

Acc 2

Acc 3

Computer monitors and integrates

acceleration data

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Introduction to electronics It all started with the vacuum tube

Amplification mode (eg: radio)

Switching mode (eg: computing)

They are inefficient, bulky and slow

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From Electronics to Microelectronics

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Introduction to microelectronics In the late 1940s, we moved to

semiconductor technology The primary semiconductor is

silicon

Pure silicon crystal BouleSingle silicon wafer

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Progression of Silicon Wafer sizes

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Introduction to electronics Silicon forms a crystal lattice structure

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

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Introduction to electronics n-type doping with (eg) Phosphorus

PSi Si P SiPSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiP P Si SiSiSi Si

SiSi P Si SiSiSi Si

Free electrons

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Introduction to electronics n-type doping with (eg) Phosphorus

PSi Si P SiPSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiP P Si SiSiSi Si

SiSi P Si SiSiSi Si

- +Electric field

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Introduction to electronics n-type doping with (eg) Phosphorus

PSi Si P SiPSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiP P Si SiSiSi Si

SiSi P Si SiSiSi Si

- +Electric field

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Introduction to electronics n-type doping with (eg) Phosphorus

PSi Si P SiPSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiP P Si SiSiSi Si

SiSi P Si SiSiSi Si

- +Electric field

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Introduction to electronics p-type doping with (eg) Boron

BSi Si B SiBSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiB B Si SiSiSi Si

SiSi B Si SiSiSi Si

Free holes

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Introduction to electronics p-type doping with (eg) Boron

BSi Si B SiBSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiB B Si SiSiSi Si

SiSi B Si SiSiSi Si

+ -Electric field

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Introduction to electronics p-type doping with (eg) Boron

BSi Si B SiBSi Si

SiSi Si Si SiSiSi Si

SiSi Si Si SiSiSi Si

SiB B Si SiSiSi Si

SiSi B Si SiSiSi Si

+ -Electric field

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Introduction to electronics Once we have n-type and p-type silicon,

we have electronics These devices do everything tubes do,

only faster, cheaper and smaller

p n

Diode

n np

p pn

Transistor

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Microelectronics Process The real breakthrough was the monolithic

(integrated) circuit

p n p n

Integrated circuits are built onto a single silicon substrate, not from discrete parts

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Microelectronics Process Integrated circuits are created using a

process called lithography

Lithography uses masks and resists to dope and create circuit elements

Resist

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Microelectronics Process Masks define the locations of circuit

elements and light (or other beam) cures the resist

Mask

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Microelectronics Process Uncured resist can be washed away,

leaving only cured resist behind

Cured resist

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Microelectronics Process Dopants can then be flooded over the

wafer. They will only penetrate where they should

Boron solution

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Microelectronics Process Finally, the resist can be removed, to yield

doped silicon

P-type silicon

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Microelectronics Process A second sequence (with different mask)

continues to build the circuit

Resist

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Microelectronics Process A second sequence (with different mask)

continues to build the circuit

Mask

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Microelectronics Process A second sequence (with different mask)

continues to build the circuit

Cured resist

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Microelectronics Process A second sequence (with different mask)

continues to build the circuit

Phosphorus solution

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Microelectronics Process A second sequence (with different mask)

continues to build the circuit

Integrated diode

p n p n

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Microelectronics Process Devices can be made as small as we can

focus the exposing beam (~ 20 nm)

We can make as many simultaneously as will fit on a wafer

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Several Microchips from a single wafer

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Clicker Question #1 Which of the following combination of p-type and

n-type material is NOT a transistor?

A. B. C.

n np p pnp n

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Introduction to MEMS That’s nice, but what does this have to do

with accelerometers?

It turns out that silicon can also be etched, vertically or at a 55 degree angle

This allows us to build microelectromechanical systems (MEMS) using the lithography process

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Accelerometer Theory

Suppose we want to build this accelerometer

Strain gauge

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Accelerometer Theory

First, we create the negative of the shape as a mask

Strain gauge

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Accelerometer Theory

If we use this mask in the lithography process then etch, we can cut away the center portion with vertical etching

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Accelerometer Theory

Edge regions

Central regions

Next, an anisotropic wet (KOH) etch from the bottom creates the thin beam

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Accelerometer Theory

Edge regions

Central regions

Next, an anisotropic wet (KOH) etch from the bottom creates the thin beam

Thin, flexible support beam

Large inertial mass

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Accelerometer Theory

The strain gauges are not required, because the resistance of silicon depends on stress

Piezoresistors

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Suspended MEMS Bridge

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Wii First Proposal

Recall that we needed a computer to process the data

Acc 1

Acc 2

Acc 3

Computer monitors and integrates

acceleration data

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Accelerometer TheoryBut this is still silicon. So, we can just build the electronics on the same wafer, using more steps of the same process.

Control circuit

Accelerometer

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MEMS Process Devices can be made as small as

practical, given the needed function

We can still make as many simultaneously as will fit on a wafer!

We can build the needed electronics (to communicate with Wii, for example) right on the same chip.

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MEMS Process Can you think of other applications for the

MEMS accelerometer?

Can you think of other applications for MEMS?

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Other Applications

Cellphones Laptop Screen rotation Laptop safety

MEMS Switches Tunable electronics with Biotechnology, RF-MEMS for

communication