Spintronics Integrating magnetic materials with semiconductors

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Bulk micromachining

• Wet Chemical etching

Masking layer
Bulk Si
Bulk Si
Isotropic Anisotropic
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Surface micromachining
Carving of layers put down sequentially on the
substrate by using selective etching of
sacrificial thin films to form free-standing/compl
etely released thin-film microstructures
http//www.darpa.mil/mto/mems
HF can etch Silicon oxide but does not affect
Silicon
Release step
crucial
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MEMS Foundry services
SAMPLES Sandia Agile MEMS Prototyping, Layout
tools, Education and Services (Current
process SUMMIT V) Sandias 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
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Process steps for fabricating a MEMS device
MUMPS Multi-user MEMS processes gt Commercially
operated, a repository of processing, design
libraries gt 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
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The CRONOS process for a micro-motor
e.g., Synchronous motor
Stator
Rotor
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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
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The cross-sections are depicted in MEMS
processing
http//mems.sandia.gov/
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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
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-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
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- contd.
Deposit POLY 2 and OXIDE (PSG)
Pattern POLY 2 (7th level) and OXIDE
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-contd.
Deposit and pattern METAL (Level 8)
POLY 2
RELEASE structure, OXIDES are sacrificial
STATOR ROTOR STATOR
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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
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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)
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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

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Piezoresistivity
Piezoresistive effect is described by a
fourth-rank tensor
E re 1 ? s J at small strains
Electric field Resistivity tensor (2nd
rank) Stress Current density
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Tensor notation
Stress Strain
4th rank tensor (81 elements)
sij Cijkl ekl
From symmetry (no net force in equilibrium) sij
sji ? 6 independent variables
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Contracted tensor notation
C11 C12 C13 C14 C15
C16 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

(6 X 6) matrix, 21
independent elements (as, Cij Cji)
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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
0 C12 C12 C11 0 0
0 0 0 0 C44 0
0 0 0 0 0 C44
0 0 0 0 0 0
C44
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Piezoresistivity for Silicon
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Piezoresistivity
Piezoresistive effect is described by a
fourth-rank tensor
E re 1 ? 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
re p11 ?1111
re p12 ?1122
re p44 2?2323
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Measurement of Piezoresistance coefficients
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Practical Piezoresistance measurements
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Slide courtesy M. Wu
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Longitudinal transverse piezoresistance
DR plsl ptst l longitudinal, t transverse
R
Longitudinal Transverse piezoresistance
coefficients

• Longitudinal pl Transverse pt
• direction direction
• (100) p11 (010) p12
• (001) p11 (110) p12
• (111) 1/3 (p11p12 2 p44) (110)
  1/3 (p112 p12- 2 p44)
• (110) 1/2 (p11p12 p44) (111) 1/3
  (p112 p12- p44)
• (110) 1/2 (p11p12 p44) (001)
  p44
• (110) 1/2 (p11p12 p44) (110) 1/2
  (p11 p12 - p44)

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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).
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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
Transverse
Longitudinal
- more sensitive
- easier to align
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Principle of measurement
Diaphragm
Poisson ratio, n 0.06
CROSS-SECTION TOP VIEW
DR2
- (61.7 10-11) sl
R2
R2
WHEATSTONE BRIDGE
R3
R1
R4
Vo
R1 R3 (1 a1) Ro R2 R4 (1 - a2) Ro
ai S pisi
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Resistance change due to stress
Lc cantilever length x distance from support t
thickness
x
3 wmax (Lc - x)
Radius of curvature
1/r
Lc3
(t/2)/r
sl
E
DR
pl sl
R
Stress E Strain
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The Motorola MAP sensor
http//www.motorola.com/automotive/prod_sensors.ht
ml

• 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
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Process flow for MAP sensor

• Bipolar (NPN) instead of MOS processing on (100)
  wafers
• uses only one piezo-resistor Xducer

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