1. Making a Capacitive Accelerometer
EE147/247A
Fall 2016
Week 2, Lecture 1
K. Pister

2. Outline Simple SOI process Simple capacitive accelerometer Performance
Etching
Design rules
Simple capacitive accelerometer
Anchor, spring, mass, capacitor
Performance
Spring constant, mass, frequency response
Capacitance vs. acceleration

3. Simple SOI process Start with 20um thick SOI on 2um SiO2
spin, expose, develop, hardbake photoresist with mask STRUCT
DRIE (deep reactive ion etch) silicon etch (plasma)
strip photoresist
5 minute etch in 49% HF (liquid)
rinse, dry

4. Minimum size for an anchor?

5. Maximum width of a free beam?

6. Maximum distance between etch holes in a free plate?

7. Design Rules: example minimum line/space: 2um
etch hole size: 2um square
etch hole spacing: 6um center-to-center
minimum width, fixed beam: 20um
max width, free beam: 8um

8. Design Rules
Design rules are a contract between the manufacturer and the designer
“obey these rules and we guarantee this level of performance”
free beams will be free
fixed beams will be fixed
2um lines (e.g. springs) will still be there
2um gaps (e.g. capacitors) will be cleared

9. Design rules, cont.
Note that this does not guarantee that a 2um line will be 2um wide when it is fabbed
Design rules are a good idea even when you are both the designer and the manufacturer (maybe “especially when”)

10. Simplest accelerometer

11. ST 3 axis accelerometer LIS44AHL

12. ST 3 axis accelerometer LIS44AHL

13. Possibly the ST LIS331EB? (probably not)
Moving mass
(with etch holes)
Spring
Capacitor
fingers?
Anchor

14. MEMS racetrack and racecar
Pham, Dao, Sugiyama, “A micro transportation system […] J. Micromech. Microeng. 2007

15. Spring, mass, damper, capacitor

16. Spring

17. Mass

18. Capacitor