## 7.3 Summary

### 7.3.1 Summary to 7: MEMS

| MEMS are "Micro Electro Mechanical Systems" including also micro optics, micro fluidics and generally meaning micro systems. |  |
|  | MEMS uses Si substrates and technologies because "it is there and cheap" for the non-electronic part and because electronic components can be integrated on the same chip. |
|  | Examples of high-volume MEMS products are (Pressure) sensors. Accelerometers. Gyros "Beamer" chips (DLP) |
|  | More products are to come; MEMS is an emerging and often an enabling technology |
| Gyros are particular complex MEMS sensor products with a huge range of applications. |
|  | There must be a physical principle behind the sensor design; different approaches can be used. |
|  | One approach uses the Coriolis force causing detectable additional vibrations in an oscillator with two degrees of freedom if some rotation is experienced. |
| Many MEMS devices are either sensors or actuators. |  |
|  | Looking only at mechanical MEMS, there is a need to couple mechanical movements to electrical signals and vice verse. |
|  | Ways to do this include. Capacitive coupling Piezoelectric and piezoresisitive coupling. Thermal coupling (expansion, resistivity changes). Magnetic coupling. Optical coupling. |
|  | There is no "ideal" coupling; all methods suffer from certain problems. |

| MEMS uses all of "known" Si technology and has some specifics of its own. |  |
Works | Sticks
--- | ---
|  | Making cantilevers and membranes necessitates making "large" cavities. |
|  | Staying absolutely planar and stress-free is essential |
|  | Packaging can be far more demanding than for chips (e.g. transparent tops for OMEMS, keeping defined pressures for > 10 a in gyros). |
| The bane of MEMS is stiction. |
|  | If you can't lubricate, it will stick sooner or later. Never bring moving parts in contact! |
|  | MEMS design therefore cannot just miniaturize exiting mechanical designs; it must look for new approaches. |
| MEMS employs some special processes and materials; they are the drivers of progress |  |
|  | Anisotropic chemical etching |  |
Making "large" cavities and extremely deep "holes" Planarization Free-standing structures
|  | High-rate plasma etching ("Bosch process") |
|  | Chemical-mechanical polishing |
|  | Sacrificial layers and removal (including chemical etching with "vapors") |
|  | Wafer bonding; in particular for packaging. |
| Process integration looks simple if compared to an advanced CMOS process, but is actually rather involved due to the special processes needed and quality requirements |  |
Exercise 7.3-1
All Questions to 7.

© H. Föll (Semiconductor Technology - Script)