22 - Engineering - University of Victoria

Transcription

MECH 466
Microelectromechanical Systems
University of Victoria
Dept. of Mechanical Engineering
Lecture 22:
MEMS Course Review
© N. Dechev, University of Victoria
1
MEMS are Components within a System
The name MEMS generally refers to the micro-scale ‘components’ or
micro-scale ‘devices’ within a system. Not the entire system itself.
In order to create a complete functional ‘System’ that makes use of
MEMS, the system will require various other sub-systems, such as:
power, microelectronics, communication and software.
Power
MEMS Sensor
Electronics
Communication
Software
MEMS Actuator
Complete MEMS Based ‘System’
2
With MEMS, the Key Question is:
‘Why Go Small?’
Scaling devices down to the micro-scale may allow for the
following advantages:
For micro-scale sensors:
- Higher Sensitivity
- Better Linearity
- Better Response
- Dynamic Range
- Cost Reduction from Batch Fabrication
For micro-scale actuators:
- Dynamic Response Speed
- Lower Power Consumption
- Footprint
- Cost Reduction from Batch Fabrication
Physical size is only one of many considerations.
3
Scaling Laws
The scaling laws can help answer the question of “Why go small ?”
They allow us to determine whether physical phenomena will scale
more favorably or will scale poorly.
Generally, smaller things are less effected by volume dependent
phenomena such as mass and inertia, and are more effected by
surface area dependent phenomena such as contact forces or heat
transfer.
Friction and Stiction > Inertia
Heat Dissipation > Heat Storage
4
Scaling Laws
a
Recall the example of a cube held by a steel rod:
r
Mass
Therefore, σ ∝ L.
l
h
w
- as L increases, σ increases proportionally.
- as L decreases, σ decreases proportionally.
Question: If the dimensions of this system were all increased by a
factor of 10x, (l, w, h, and r), is there a greater risk of rod failure?
Answer: Yes. Internal rod stress is 10x greater than before.
5
Microfabrication From Sand to Chips
Silicon Ingot [Kayex]
Wafer is processed [Siemens]
Ingots are Sliced with Multi-Wire
Sawing (MWS) [WaferNet Inc.]
Wafers are Lapped, Wet Etched and
Polished [WaferNet Inc.]
Wafer is sliced into individual
chip die [Majelac Technologies]
Die pads are wire bonded to
chip carrier leads [image from S. He]
6
[001]
Miller Indices, Directions
[100]
z
[010]
y
x
Crystal Direction [100], [010] and [001].
Direction family <100>.
z
[110]
Step 1:
Find the parallel vector that begins at the origin.
[111]
y
x
Step 2:
Reduce the three coordinates to the smallest set of integers
[h,k and l]. For the bottom left example, the vector
originates at (0,0,0) and ends at (1,1,1). Therefore, the
Miller Index direction is [1,1,1].
7
Semiconductor Properties and Doping
An n-type material has a surplus of electrons in the bulk, usually
done with phosphorus doping
A p-type material has a surplus of holes in the bulk, usually done
with Boron doping
Doping Atoms
N-Type Silicon Semiconductor
P-Type Silicon Semiconductor
[Images from HyperPhysics, C.R. Nave, Georgia State]
8
Sheet Resistivity
The concept of Sheet Resistivity is a simple way to compute the
resistance of doped areas or routing.
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9
Electrostatic MEMS Devices
General Model of Electrostatic Devices:
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10
Concept of ‘Pull-in’ Voltage
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Occurs at 1/3 of displacement
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11
The ‘Comb Drive” Actuator/Sensor:
FEM Simulation of Electric Potential of Comb Drive [Chang Liu]
12
Electrostatic Device Demo
13
Thermal MEMS Devices
Thermal Actuators:
Gap
Anchor
Pads
Hot Arm
Flexture
Cold Arm
q’’convection
Cross-Section
q’’convection
Beam
q’’conduction
‘Still’ Air
Si Substrate, Troom
Free Air
Troom
q’’convection
Conduction:
Convection:
Radiation:
Thermal Resistance:
Thermal Capacitance:
14
Lateral Thermal Actuators:
‘Arc’ is length of hot arm
h‘Arc’ is length of hot arm
δ-deflection
c-‘chord’ is
length of
cold arm +
flexture
do
r-radius of curvature
r
θ
15
Ink-Jet Nozzle:
)(%'*
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)(%'*
+&')(
,-.
+*",/
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40),2
0('%,-5
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Operation of Ink-Jet [Chang Liu]
Beam Bi-Morphs:
",
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4,
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rsinθ
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16
Piezoresistive Sensors
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Piezoresistive Sensors Made by Doped Pathways [Chang Liu]
17
Piezoresistive Sensors
Direct measurement of material strain
!.#$1-3
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Piezoresistive-based Accelerometer [Chang Liu]
18
Piezo-Electric MEMS
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19
Piezo-Electric MEMS
Piezoelectric plate
Silicon cantilever
Copper sheet
High-energy
electrons
Radioactive source
1 Beta particles (high-energy electrons) fly spontaneously
from the radioactive source and hit the copper sheet,
where they accumulate.
2 Electrostatic attraction between the copper sheet and
the radioactive source bends the silicon cantilever
and the piezoelectric plate on top of it.
3 When the cantilever bends to the point where the copper
sheet touches the radioactive source, the electrons
flow back to it, and the attractive force ceases.
4 The cantilever then oscillates, and the mechanical stress in
the piezoelectric plate creates an imbalance in its
charge distribution, resulting in an electric current.
HD Scan head without and with ‘error correction’ from a microactuator
[IBM-Almaden-Research-Center]
Operation of Piezoelectric Beam Generator
[IEEE Spectrum, Sept 2004]
SEM image of hard drive head (upside down)
[IBM-Almaden-Research-Center]
20
Magnetic-Based MEMS
emf
B Field
Inductive Coils within Magnetic Fields
[Fig. 8.5 from ‘Foundations of MEMS’, Chang Liu]
21
Magnetic-Based MEMS
High-Q, Microcoils [PARC Research]
Microcoils [N. Dechev]
Bio-Micro-robots [B. Nelson, et. al.],
Institute of Robotics and Intelligent Systems,
Swiss Federal Institute
Micromachined, 3D micro-inductors [Microfabrica.com]
22
Microfabrication Technologies
Time T = 4
Time T = 50
Simulation of Wet Etching of Bulk Silicon
using ACES Simulation Software [C. Liu]
Gas Phase Isotropic Etching with XeF2
30 um Deep Etch (Gas Phase Iso Etch)
[Image from Chang Liu]
Deep Reactive Ion Etching (DRIE)
[www.bosch-sensortec.com]
23
Bulk Micromachining
SEM images of samples:
Deep Reactive Ion Etching (RIE) using Bosch Process
[Tyndall National Institute]
Anisotropic Wet Etching using KOH with
wafer edge aligned along <110> vectors
[Asia Pacific Microsystems, Inc.]
24
Surface Micromachining
Multi-User MEMS Processes (MUMPs)
MUMPs fabrication process to create a microgripper tip.
Oxide 2
Poly 1 Lift Structure
Poly 2
Poly 1
Substrate
Oxide 1
Anchor 1
Poly 1
Poly 2
Poly 2
Gripper Tip
Section A
Poly 2
Gripper Tip
Poly 1 Lift
Structure
Gripper Tip
Upper Level
2 μm
2.75 μm
Substrate
Microgripper tip fabricated with MUMPs
[N. Dechev]
25
Stiction in MEMS
There are four major phenomena that individually contribute to the
‘overall effect’ of stiction. These are:
Capillary Forces
Hydrogen Bridging
Electrostatic Forces
Van der Waals forces
N Tas et al
© N.
Stiction in surface micromachining
water layers. Stengl et al [17] have calculated an adhesion
energy of about 100 mJ m 2 based on this bonding
model. From wafer bonding experiments [18, 19] and
stiction experiments [4, 20], adhesion energies between 60
and 270 mJ m 2 have been reported for hydrophilic surfaces
(T < 200 ⌅ C).
Figure
5. Capillary
condensation
between
contactingSurfaces
Capillary
Condensation
Between
Two two
Contacting
surfaces. The meniscus curvatures are equal to the Kelvin
[Imagethe
from
N. Tas,
et al.]
2.3. Electrostatic forces between mobile charges
radius;
contact
angles
satisfy Young’s equation.
Electrostatic attractive forces across the interface can arise
dependence
on t, g,and
E andfrom
✏s . aOnly
the in
numerical
constant
difference
work functions
or from electrostatic
a general form, which is valid
for both the spreading
charging
of opposed
surfaces [21,
18, 22].
the non-spreading condition [2]:
in (11) should be changed,
yielding
a critical
length
of Difference in
the work function leads to the formation of an electrical
clamped beams
is layer
about
times oflarger
and
(4c) that
Es = C 2Adoubly
double
by a2.9
net transfer
electrons
from one surface
b ◆la cos C
to the
other. Contactthat
potentials
are generally
a critical radius of circular
membranes
is about
2.4 below 0.5 V,
where ◆la cos C is the adhesion tension, and C takes into
and the
resulting
charge densities
times
larger
the critical
length
of surface
cantilevers
[4]. Toare smaller than
account the constant terms in
(4a, b).
The than
importance
1013 elementary charges per square centimetre [21]. At
obtain
an by
idea
strength
ofseparations
adhesion,
can evaluate
liquid mediated
supported
bothofstiction
Critical
Length
ofthe
Cantilever
Beam
Figure
6. Stuck
A cantilever
of length l andofthickness
t , adhesion is
small
the we
electrostatic
pressure between flat
Cantilever
Down on beam
Substrate
and friction
experiments.
Stiction
of by
released
structures
Stiction
surfacesand
is generally
than thebeams,
van der Waals pressure
the held
critical
length
of cantilever
doublylower
clamped
anchored
atN.a Tas,
initial
gap spacing g . The beam
attaches
the
[Image from
et al.]
can show a large dependence on the relative humidity
[21]. Temporary charging can 2occur during processing
substrate at distance x from the anchor.
assuming
an and
adhesion
✏s = 100 mJ m and a
of air [11]. Friction measurements
of silicon
silicon energy
[22] or operation.
Examples of this are tribocharging of
compounds [12] show a strong
dependence
of the static
Young’s
modulus
of 150 GPa.
Figure[15]
7 shows
the
length in insulators
rubbing surfaces
and charge
accumulation
friction coefficient on relativeof
humidity.
In
macrotribology
electrostatic
operated
micromotorsas[23].
the beams that are just ofkept
down to
the substrate,
a Permanent
surfaces,the adhesion of hydrophobic surfaces
might
be
it is well known that adhesion of solids can strongly
stiction is not expected due to these effects because the
of beam
thickness,
for
three
different
gap
spacings.
more sensitive to surface roughness because
smoothing
byhumidityfunction
depend
on relative
[13]. This
is caused
by
non-equilibrium charging will relax in time.
capillary condensation. Liquids
thatdotted
wetLength
or line
haveofain
small
The
figure 7(b)
condensed water is absent.
Critical
Cantilever
Beamshows the critical length of
contact angle on surfaces willdoubly
spontaneously
condense
into
beams, if2.4.stiffening
dueforces
to stretching of
held byclamped
Capillary Force
Van der Waals
cracks, pores, and into small gaps surrounding the points
the
beam
is
taken
into
account
[4].
Even
spacings
of
contact
between
the
contacting
surfaces.
At
equilibrium
The
van
der
Waals
dispersion
forces
between two bodies are
26 at gap
Dechev,
University
of Victoria
3. Critical
dimensions
of beams and membranes
the meniscus curvature is equal
to the
[10]:
that
areKelvin
fourradius
times
the thickness
of theelectric
beaminteraction
(t = 1ofµm,
caused by mutual
the induced dipoles
in the is
two
bodies.
Dispersion
forces generally
dominate
g
=
4
µm),
the
critical
length
only
slightly
increased
by
◆la V
As soon as a structure touches the substrate, the total surface
over orientation and induction forces except for strongly
rK =
(5)
this effect.
s)
polar molecules [24]. The interaction energy per unit area
energy is lowered. The structure will permanently stick to RT log(p/p
due to van der Waals interaction between two flat surfaces
The
figure
shows
where
V
is
the
molar
volume,
p
is
the
vapor
pressure
and that even 10 µm thick cantilevers
the substrate if during peel-off the total energy of the system
Bio-MEMS
Desired Cell
Desired Cell
Desired Cell
Desired Cell
Desired Cell
Desired Cell
Desired Cell
Desired Cell
Cell capture results with (a) actual test, (b) control test
(no external field applied). [William Liu]
Illustration of Arraying of
Immunomagnetically labelled Cells
[N. Dechev]
27
Microfluidics
r
[Image from Chang Liu]
Lab on a CD [NASA Ames Research Center ]
- - - - - - - - - - + + + + + + + + +
+
+ -
+
_
_
+ + + + + + + + +
- - - - - - - - - - -
Ion layer termed ‘electric double layer’ allowing for
Electro-Osmotic Flow
+ -
+
Where: Q - Charge
P - Polarization
E - Electric Field
Electrophoresis used to separate electrically polar particles
28
Microfluidics
v
Concept of Hydraulic Diameter:
s
h
s
Square cross-section
where:
b
Triangular cross-section
A = Cross-sectional Area
Pwet = ‘Wetted/Submersed’ Perimeter
Concept of Pressure Drop in Microchannel Flow
Q or u
Dh
Concept of Laminar or Turbulent Flow in Microchannels based upon Reynolds Number
laminar flow:
Re < 1000
Transitional flow: 1000 < Re < 2000
Turbulent flow:
Re > 2000
29
Micro-Optics
DLP Micromirror array (Single Axis, On/Off)
[Texas Instruments]
Two-Axis (pitch and yaw) rotatable micro-mirror
for optical switching [Image from Chang Liu]
Constituent Parts of the Complete DLP Projection System
[Texas Instruments]
Colorized SEM image of 1x2 fiber optic switch,
[David Bishop, Bell Labs, Lucent Technologies]
30
Micro-Optics
Illustration of 1xN Mirror for Optical Switching
[M. Basha, N. Dechev]
SEM image of 1xN Mirror for Optical Switching
Incomming Fibre Optic
Light Guides
Outgoing Fibre Optic
Light Guides
Illustration of NxN Mirror
for Optical Switching
SEM of two Mirrors
for NxN Optical Switching
31
Microassembly of MEMS
Illustration of Grasping Task [N. Dechev]
Microgripper Bonded to Probe Tip
[N. Dechev]
32
MECH 466
The End
33

22 - Engineering - University of Victoria

Transcription

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