Transcript Spintronics Integrating magnetic materials with semiconductors

Bulk micromachining
• Wet Chemical etching:
Masking layer
Bulk Si
Isotropic
Bulk Si
Anisotropic
Surface micromachining
http://www.darpa.mil/mto/mems
Carving of layers put down sequentially on the substrate by
using selective etching of sacrificial thin films to form freestanding/completely released thin-film microstructures
HF can etch Silicon oxide but does not affect Silicon
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
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
The CRONOS process for a micromotor
e.g., Synchronous motor
Stator
Rotor
The CRONOS process for a micromotor
Poly-silicon (POLY): Structural Material
Silicon Oxide/PSG (OXIDE): Sacrificial material
Silicon Nitride (NITRIDE): for isolation
- 8 photo-masks: 8 levels of processing
http://mems.sandi
a.gov/
The cross-sections are depicted in MEMS
processing …
Photoresist
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
-contd.
Pattern POLY1 (4th level),
OXIDE & POLY etched: RIE
OXIDE 2
Deposit & pattern OXIDE 2,
(Level 5)
Deposit PR (Level 6) and
pattern an ANCHOR
contacting POLY 0
- contd.
Deposit POLY 2 and
OXIDE (PSG)
Pattern POLY 2 (7th level) and
OXIDE
-contd.
Deposit and pattern
METAL (Level 8)
POLY 2
STATOR
ROTOR
RELEASE structure,
OXIDES are sacrificial
STATOR
Case Studies in MEMS
Case study
Pressure sensor
Technology
Bulk micromach.
+ bipolar circuitry
Transduction
Packaging
Piezoresistive sensing
of diaphragm deflection
Plastic
Accelerometer
Surface micromach.
Capacitive detection of
proof of mass motion
Metal can
Electrostatic
projection displays
Surface micromach.
+ XeF2 release
Electrostatic torsion of
suspended tensile beams
Glass bonded
Catalytic combustible Surface micromach.
gas sensor
Resistance change due
to heat of reaction
RF switches
Cantilever actuation
Surface micromach.
DNA amplification Bonded etched glass
with PCR
Pressure driven flow
across T-controlled zones
Lab on a chip
Electrophoresis &
electrowetting
Bulk & Surface
micromachining
Custom mount
Glass bonded
Microcapillaries
Microfluidics
& Polymers
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)
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
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
Tensor notation
Stress
Strain
sxx txy txz
sij ≡ tyx syy tyz
tzx tzy szz
eij
exx gxy gxz
≡ gyx eyy gyz
gzx gzy ezz
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
Contracted tensor notation
C11 C12 C13 C14 C15 C16
sxx
syy
szz
tyz
tzx
txy
≡
C12
C22 C23
C24
C25
C26
C13
C23 C33
C34 C35 C36
C14
C24
C34 C44 C45
C46
C15
C25
C35 C45
C55
C56
C16
C26
C36 C46
C56
C66
exx
eyy
ezz
gyz
gzx
gxy
(6 X 6) matrix,
21 independent elements (as, Cij = Cji)
For cubic materials, e.g. single crystal Silicon, there are only
3 independent constants
C11 C12 C12
0
0
0
C12
C11
C12
0
0
C12
C12 C11
0
0
0
0
0
0
C44
0
0
0
0
C44
0
0
0
0
0
0
0
0
0
C44
Piezoresistivity for Silicon
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]
E1 = [1+ p11s1 + p12(s2 + s3)] J1 + p44(t12J2+ t13J3)
re
E2 = [1+ p11s2 + p12(s1 + s3)] J2 + p44(t12J1+ t23J3)
re
E3 = [1+ p11s3 + p12(s1 + s2)] J3 + p44(t13J1+ t23J2)
re
Piezoresistive coefficients
re p11 = Π1111
re p12 = Π1122
re p44 = 2Π2323
Measurement of Piezoresistance coefficients
Practical Piezoresistance measurements
Slide courtesy: M. Wu
Longitudinal & transverse piezoresistance
DR = plsl + ptst l: longitudinal, t: transverse
R
Longitudinal & Transverse piezoresistance coefficients
Longitudinal
direction
pl
(100)
(001)
(111)
(110)
(110)
(110)
p11
p11
1/3 (p11+p12+ 2 p44)
1/2 (p11+p12+ p44)
1/2 (p11+p12+ p44)
1/2 (p11+p12+ p44)
Transverse
direction
(010)
(110)
(110)
(111)
(001)
(110)
pt
p12
p12
1/3 (p11+2 p12- 2 p44)
1/3 (p11+2 p12- p44)
p44
1/2 (p11 + p12 - p44)
Piezoresistive coefficients of Si
- decrease as the doping level/temperature increases
Type
Resistivity
p11
p12
p44
Units
W-cm
10-11 Pa-1 10-11 Pa-1 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).
Concept of a piezoresistive sensing scheme
Max. surface stress
Proof Mass
Substrate
Flexure
If piezo-resistor is along [110]:
n-type: pl: -31.2 · 10-11 Pa-1, pt: -17.6 · 10-11 Pa-1
p-type: pl: 71.8 · 10-11 Pa-1, pt: -66.3 · 10-11 Pa-1
Longitudinal
- easier to align
Transverse
- more sensitive
Principle of measurement
Poisson ratio, n = 0.06
Diaphragm
DR1 = (p + np )s = (67.6 · 10-11) s
l
t l
l
R1
CROSS-SECTION
DR2 = - (61.7· 10-11) s
l
R2
R2
TOP VIEW
WHEATSTONE BRIDGE
R3
R1
R4
V
-
R1 = R3 = (1+ a1) Ro
R2 = R4 = (1 - a2) Ro
ai = Σ pisi
R3
R2
+
Vo
R1
R4
Resistance change due to stress
Support
Lc: cantilever length
x: distance from support
t: thickness
Cantilever
x
Piezoresistors
3 w
Cantilever tip displacement (w) for a point load =
max
2
Radius of curvature = 1/r =
sl = E [(t/2)/r]
Stress = E · Strain
d2w
dx2
= 3 wmax (Lc - x)
Lc3
DR = p s
l l
R
x
Lc
2
1- x
3Lc
The Motorola MAP sensor
S. Senturia, page 461, Microsystem design
http://www.motorola.com/automotive/prod_sensors.html
- 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
Process flow for MAP sensor
-Bipolar (NPN) instead of MOS processing on (100) wafers
- uses only one piezo-resistor: Xducer
Al metallization
n+ - collector
n+ - Emitter
p-base
n+ - buried layer
OXIDE
n-epi
<100> p-Si substrate
p-type piezoresistor
Pressure sensor fabrication and packaging
Piezoresistor
element
DIAPHRAGM
Glass frit/Anodic
bond