1. ES050 – Introductory Engineering Design and Innovation Studio 1 ECE Case Study Accelerometers in Interface Design – Part II Prof. Ken McIsaac 2008 11 21

2. 2 Outline Review of generated concepts Introduction to electronics The microelectronics process MEMS MEMS Accelerometer

3. 3 Accelerometer Theory Accelerometer for into plane acceleration or pitch Strain gauge

4. 4 Wii First Proposal We can build something like this: Acc 1 Acc 2 Acc 3 Computer monitors and integrates acceleration data

5. 5 Introduction to electronics It all started with the vacuum tube Amplification mode (eg: radio) Switching mode (eg: computing) They are inefficient, bulky and slow

6. 6 Introduction to electronics In the late 1940s, we moved to semiconductor technology The primary semiconductor is silicon Pure silicon crystalSingle silicon wafer

7. 7 Introduction to electronics Silicon forms a crystal lattice structure Si

8. 8 Introduction to electronics n-type doping with (eg) Phosphorus PSi P P PP P Free electrons

9. 9 Introduction to electronics n-type doping with (eg) Phosphorus PSi P P PP P - + Electric field

10. 10 Introduction to electronics n-type doping with (eg) Phosphorus PSi P P PP P - + Electric field

11. 11 Introduction to electronics n-type doping with (eg) Phosphorus PSi P P PP P - + Electric field

12. 12 Introduction to electronics p-type doping with (eg) Boron BSi B B BB B Free holes

13. 13 Introduction to electronics p-type doping with (eg) Boron BSi B B BB B + - Electric field

14. 14 Introduction to electronics p-type doping with (eg) Boron BSi B B BB B + - Electric field

15. 15 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 pn Diode nnp ppn Transistor

16. 16 Microelectronics Process The real breakthrough was the monolithic (integrated) circuit pn pn Integrated circuits are built onto a single silicon substrate, not from discrete parts

17. 17 Microelectronics Process Integrated circuits are created using a process called lithography Lithography uses masks and resists to dope and create circuit elements Resist

18. 18 Microelectronics Process Masks define the locations of circuit elements and light (or other beam) cures the resist Mask

19. 19 Microelectronics Process Uncured resist can be washed away, leaving only cured resist behind Cured resist

20. 20 Microelectronics Process Dopants can then be flooded over the wafer. They will only penetrate where they should Boron solution

21. 21 Microelectronics Process Finally, the resist can be removed, to yield doped silicon P-type silicon

22. 22 Microelectronics Process A second sequence (with different mask) continues to build the circuit Resist

23. 23 Microelectronics Process A second sequence (with different mask) continues to build the circuit Mask

24. 24 Microelectronics Process A second sequence (with different mask) continues to build the circuit Cured resist

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

26. 26 Microelectronics Process A second sequence (with different mask) continues to build the circuit Integrated diode p n pn

27. 27 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

28. 28 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

29. 29 Accelerometer Theory Suppose we want to build this accelerometer Strain gauge

30. 30 Accelerometer Theory First, we create the negative of the shape as a mask Strain gauge

31. 31 Accelerometer Theory If we use this mask in the lithography process then etch, we can cut away the center portion with vertical etching

32. 32 Accelerometer Theory Edge regions Central regions Next, an anisotropic wet (KOH) etch from the bottom creates the thin beam

33. 33 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

34. 34 Accelerometer Theory The strain gauges are not required, because the resistance of silicon depends on stress Piezoresistors

35. 35 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

36. 36 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

37. 37 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.

38. 38 MEMS Process Can you think of other applications for the MEMS accelerometer? Can you think of other applications for MEMS?