1. ECE Case Study Accelerometers in Interface Design – Part II
Prof. Jayshri Sabarinathan
TEB 259

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

3. 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

4. Accelerometer Theory
Accelerometer for into plane acceleration or pitch
Strain gauge

5. Computer monitors and integrates acceleration data
Wii First Proposal
We can build something like this:
Computer monitors and integrates acceleration data
Acc 1
Acc 3
Acc 2

6. Introduction to electronics
It all started with the vacuum tube
Amplification mode (eg: radio)
Switching mode (eg: computing)
They are inefficient, bulky and slow

7. From Electronics to Microelectronics

8. Introduction to microelectronics
In the late 1940s, we moved to semiconductor technology
The primary semiconductor is silicon
Single silicon wafer
Pure silicon crystal Boule

9. Progression of Silicon Wafer sizes

10. Introduction to electronics
Silicon forms a crystal lattice structure Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si

11. Introduction to electronics
n-type doping with (eg) Phosphorus Si Si P Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si P Si Si Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si Si Si
Free electrons

12. Introduction to electronics
n-type doping with (eg) Phosphorus Si Si P Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si P Si Si Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si Si Si - +
Electric field

13. Introduction to electronics
n-type doping with (eg) Phosphorus Si Si P Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si P Si Si Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si Si Si - +
Electric field

14. Introduction to electronics
n-type doping with (eg) Phosphorus Si Si P Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si P Si Si Si P Si Si Si Si Si Si Si Si Si Si Si Si P Si Si Si - +
Electric field

15. Introduction to electronics
p-type doping with (eg) Boron Si Si B Si B Si Si Si Si Si Si Si Si Si Si Si Si B Si B Si Si Si B Si Si Si Si Si Si Si Si Si Si Si Si B Si Si Si
Free holes

16. Introduction to electronics
p-type doping with (eg) Boron Si Si B Si B Si Si Si Si Si Si Si Si Si Si Si Si B Si B Si Si Si B Si Si Si Si Si Si Si Si Si Si Si Si B Si Si Si +
Electric field -

17. Introduction to electronics
p-type doping with (eg) Boron Si Si B Si B Si Si Si Si Si Si Si Si Si Si Si Si B Si B Si Si Si B Si Si Si Si Si Si Si Si Si Si Si Si B Si Si Si +
Electric field -

18. 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 n p p n
Diode
Transistor

19. 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

20. Microelectronics Process
Integrated circuits are created using a process called lithography
Resist
Lithography uses masks and resists to dope and create circuit elements

21. Microelectronics Process
Masks define the locations of circuit elements and light (or other beam) cures the resist
Mask

22. Microelectronics Process
Uncured resist can be washed away, leaving only cured resist behind
Cured resist

23. Microelectronics Process
Dopants can then be flooded over the wafer. They will only penetrate where they should
Boron solution

24. Microelectronics Process
Finally, the resist can be removed, to yield doped silicon
P-type silicon

25. Microelectronics Process
A second sequence (with different mask) continues to build the circuit
Resist

26. Microelectronics Process
A second sequence (with different mask) continues to build the circuit
Mask

27. Microelectronics Process
A second sequence (with different mask) continues to build the circuit
Cured resist

28. Microelectronics Process
A second sequence (with different mask) continues to build the circuit
Phosphorus solution

29. Microelectronics Process
A second sequence (with different mask) continues to build the circuit
Integrated diode p n p n

30. 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

31. Several Microchips from a single wafer

32. Clicker Question #1
Which of the following combination of p-type and n-type material is NOT a transistor?
A B. C. p n n p p n

33. 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

34. Accelerometer Theory Suppose we want to build this accelerometer
Strain gauge

35. Accelerometer Theory
First, we create the negative of the shape as a mask
Strain gauge

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

37. Accelerometer Theory Edge regions Central regions
Next, an anisotropic wet (KOH) etch from the bottom creates the thin beam

38. Thin, flexible support beam
Accelerometer Theory
Thin, flexible support beam
Edge regions
Large inertial mass
Central regions
Next, an anisotropic wet (KOH) etch from the bottom creates the thin beam

39. Accelerometer Theory
The strain gauges are not required, because the resistance of silicon depends on stress
Piezoresistors

40. Suspended MEMS Bridge

41. Computer monitors and integrates acceleration data
Wii First Proposal
Recall that we needed a computer to process the data
Computer monitors and integrates acceleration data
Acc 1
Acc 3
Acc 2

42. Accelerometer Theory
But 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

43. 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.

44. MEMS Process
Can you think of other applications for the MEMS accelerometer?
Can you think of other applications for MEMS?

45. Other Applications Cellphones Laptop Screen rotation Laptop safety
MEMS Switches
Tunable electronics with
Biotechnology, RF-MEMS for communication