1. Isotropic Anisotropic
Bulk micromachining
Wet Chemical etching:
Masking layer
Bulk Si
Bulk Si
Isotropic Anisotropic

2. Surface micromachining
Carving of layers put down sequentially on the substrate by using selective etching of sacrificial thin films to form free-standing/completely released thin-film microstructures
HF can etch Silicon oxide but does not affect Silicon
Release step
crucial

3. MEMS: Foundry services
SAMPLES: Sandia Agile MEMS Prototyping, Layout tools,
Education and Services
(Current process: SUMMIT V)
Sandia’s Ultra-planar Multi-level MEMS Technology)
- 5 levels of poly-silicon
- $10,000 / design
MUMPS: Multi-user MEMS processes
- Derived from the BSAC processes at U.C. Berkeley
- 3-levels of poly-Si

4. Process steps for fabricating a MEMS
device
MUMPS: Multi-user MEMS processes
> Commercially operated, a repository of processing, design
libraries
> Standard processing steps, can be custom-designed
Poly-MUMPS: Three-layer polysilicon process
Metal-MUMPS: Ni electroplating process
SOI-MUMPS: Silicon-on-Insulator micromachining process

5. The CRONOS process for a micro-motor
e.g., Synchronous motor
Stator
Rotor

6. The CRONOS process for a micro-motor
Poly-silicon (POLY): Structural Material
Silicon Oxide/PSG (OXIDE): Sacrificial material
Silicon Nitride (NITRIDE): for isolation
- 8 photo-masks: 8 levels of processing

7. The cross-sections are depicted in MEMS processing …

8. Photoresist washed away
Photoresist (PR)
Photoresist
RIE removes POLY0
Photoresist washed away
Oxide sacrificial layer deposited
by LPCVD
PR applied, dimples patterned,
and PR washed away
(PSG : OXIDE)
Oxide patterned and etched,
Poly1 deposited

9. -contd. OXIDE 2 Pattern POLY1 (4th level), OXIDE & POLY etched: RIE
Deposit & pattern OXIDE 2,
(Level 5)
Deposit PR (Level 6) and
pattern an ANCHOR
contacting POLY 0

10. - contd. Deposit POLY 2 and OXIDE (PSG) Pattern POLY 2 (7th level) and

11. STATOR ROTOR STATOR -contd. Deposit and pattern METAL (Level 8)
POLY 2
RELEASE structure,
OXIDES are sacrificial
STATOR ROTOR STATOR

12. Case Studies in MEMS Case study Technology Transduction Packaging
Pressure sensor Bulk micromach. Piezoresistive sensing Plastic
+ bipolar circuitry of diaphragm deflection
Accelerometer Surface micromach. Capacitive detection of Metal can
proof of mass motion
Electrostatic Surface micromach Electrostatic torsion of Glass bonded
projection displays XeF2 release suspended tensile beams
Catalytic combustible Surface micromach Resistance change due Custom mount
gas sensor to heat of reaction
RF switches Surface micromach Cantilever actuation Glass bonded
DNA amplification Bonded etched glass Pressure driven flow Microcapillaries
with PCR across T-controlled zones
Lab on a chip Bulk & Surface Electrophoresis & Microfluidics
micromachining electrowetting & Polymers

13. A project on the frontier application areas of MEMS/NEMS
Required: A written report + Presentation
The project should address the following issues:
(1) What is new or novel about this application?
(2) Is there any new physical principle being used
(3) Where is this headed?
(commercial potential, offshoot into new areas of engineering …)
(4) Most importantly, YOUR ideas for improvement.
Presentations (15 minutes/team of two)

14. A Piezoresistive Pressure Sensor
Piezoresistance: the variation of electrical resistance with strain
Origin in the deformation of semiconductor energy bands
NOT the same as piezo-electricity
Transduction of stress into voltage
Application: Manifold-Absolute-Pressure (MAP) sensor: Motorola
One of the largest market segments of mechanical MEMS devices

15. Piezoresistivity
Piezoresistive effect is described by a fourth-rank tensor
E = re [1 + Π · s] · J at small strains
Electric field
Resistivity tensor (2nd rank) Stress Current density

16. Tensor notation Stress Strain sij eij sij = Cijkl ekl
sxx txy txz
tyx syy tyz
tzx tzy szz
sij ≡
exx gxy gxz
gyx eyy gyz
gzx gzy ezz
eij ≡
4th rank tensor (81 elements)
sij = Cijkl ekl
From symmetry (no net force in equilibrium) sij = sji
 6 independent variables
sxx
syy
szz
tyz
tzx
txy
exx
eyy
ezz
gyz
gzx
gxy

17. Contracted tensor notation
C C C C C C16
C12 C22 C23 C24 C25 C26
C13 C23 C33 C C C36
C C24 C C C45 C46
C C25 C35 C45 C55 C56
C C26 C36 C46 C56 C66
sxx
syy
szz
tyz
tzx
txy
exx
eyy
ezz
gyz
gzx
gxy ≡
(6 X 6) matrix,
21 independent elements (as, Cij = Cji)

18. For cubic materials, e.g. single crystal Silicon, there are only
3 independent constants
C C C
C12 C11 C
C12 C12 C C C
C44

19. Piezoresistivity for Silicon

20. Piezoresistivity
Piezoresistive effect is described by a fourth-rank tensor
E = re [1 + Π · s] · J
Electric field
Resistivity tensor (2nd rank) Stress Current density
x 1, y2, z 3, [11, 22, 33, 23, 31, 12]  [1, 2, 3, 4, 5, 6]
Piezoresistive coefficients
E1 = [1+ p11s1 + p12(s2 + s3)] J1 + p44(t12J2+ t13J3) re
re p11 = Π1111
re p12 = Π1122
E2 = [1+ p11s2 + p12(s1 + s3)] J2 + p44(t12J1+ t23J3) re
re p44 = 2Π2323
E3 = [1+ p11s3 + p12(s1 + s2)] J3 + p44(t13J1+ t23J2) re

21. Measurement of Piezoresistance coefficients

22. Practical Piezoresistance measurements

23. Slide courtesy: M. Wu

24. Longitudinal & transverse piezoresistance
DR = plsl + ptst l: longitudinal, t: transverse R
Longitudinal & Transverse piezoresistance coefficients
Longitudinal pl Transverse pt
direction direction
(100) p (010) p12
(001) p (110) p12
(111) /3 (p11+p12+ 2 p44) (110) /3 (p11+2 p12- 2 p44)
(110) 1/2 (p11+p12+ p44) (111) /3 (p11+2 p12- p44)
(110) 1/2 (p11+p12+ p44) (001) p44
(110) 1/2 (p11+p12+ p44) (110) /2 (p11 + p12 - p44)

25. Piezoresistive coefficients of Si
- decrease as the doping level/temperature increases
Type
Resistivity
p11
p12
p44
Units
W-cm
10-11 Pa-1
n-type
11.7
-102.2
53.4
-13.6
p-type
7.8
6.6
-1.1
138.1
C.S. Smith, Phys. Rev. B, vol. 94, pp.42-59, (1954).

26. Concept of a piezoresistive sensing scheme
Max. surface stress
Proof Mass
Substrate
Flexure
If piezo-resistor is along [110]:
n-type: pl: · Pa-1, pt: · Pa-1
p-type: pl: 71.8 · Pa-1, pt: · Pa-1
Transverse
Longitudinal
- more sensitive
- easier to align

27. Principle of measurement
Diaphragm
Poisson ratio, n = 0.06
DR1 R1
= (pl + npt)sl = (67.6 · 10-11) sl
CROSS-SECTION
TOP VIEW
DR2
= - (61.7· 10-11) sl R2 R2
WHEATSTONE BRIDGE R1 R3 V + - R2 R3 R1 R4 R4 Vo
R1 = R3 = (1+ a1) Ro
R2 = R4 = (1 - a2) Ro
ai = Σ pisi

28. Resistance change due to stress
Lc: cantilever length
x: distance from support
t: thickness
Support Cantilever
Piezoresistors x
Cantilever tip displacement (w) for a point load =
wmax x Lc 2
3Lc 1- 3
d2w
dx2
Radius of curvature =
1/r =
= 3 wmax (Lc - x)
Lc3 sl
= E
[(t/2)/r] DR
= pl sl
Stress = E · Strain R

29. The Motorola MAP sensor
MAP: Manifold Absolute Pressure
Sensor measures mass airflow into the engine, to control
air-fuel ratio
Uses piezoresistance to measure diaphragm bending with
integrated signal-conditioning and calibration circuitry
S. Senturia, page 461, Microsystem design

30. Process flow for MAP sensor
Bipolar (NPN) instead of MOS processing on (100) wafers
uses only one piezo-resistor: Xducer
<100> p-Si substrate
n+ - buried layer
p-type piezoresistor
n-epi
Al metallization
OXIDE
n+ - Emitter
p-base
n+ - collector

31. Pressure sensor fabrication and packaging
Piezoresistor
element
DIAPHRAGM
Glass frit/Anodic
bond