Transcript Spintronics Integrating magnetic materials with semiconductors

Are mechanical laws different at small
scales? YES!
If we scale quantities by a factor ‘S’
Area a S2
Volume a S3
Surface tension a S
Electrostatic forces a S2
Magnetic forces a S3
Gravitational forces a S4
• Surface Area/Volume effects
• Stiction: “Sticky friction”, due to molecular forces
- surface tension pulls things together
SCALING OF: Mechanical systems
Fluidic systems
Thermal systems
Electrical and Magnetic systems
Chemical and Biological systems
Which dynamical variables are scaled?
- depends on our choice
e.g.
Mechanical systems
Constant stress  Scale independent elastic
deformation, scale independent shape
Electromagnetic systems
Constant electrostatic stresses/field strengths
Thermal systems
Constant heat capacity & thermal conductivity
Scaling Issues in Fluids
Viscosity & Surface Tension
• Definition: A fluid cannot resist shear stresses
vρl
Reynold's number (Re) 
η
Re is the ratio of inertial and viscous forces,
v: velocity, r: density. l: linear dimension
Viscosity dominates at: Re < 1
Re for whale swimming at 10 m/second ~ 300,000,000
Re for a mosquito larva , moving at 1mm/sec ~ 0.3
Re marks the transition between
Laminar/Smooth flow & Turbulent Flow (mixing)
In MEMS: always laminar flow!
Thermal Issues
Easier to remove heat from a smaller sample
• Thermal Mass (specific heat X Volume) scales
as l3, but heat removal scales as l2 (proportional
to area)
• Evaporation or Heat loss increases as Surface
Area/Volume increases
Electrophoresis
- Stirring vs. Diffusion, Diffusion is the dominant mixing process in MEMS
- Separation of bio-molecules, cells by the application of electric fields
E=0
E>0
Separation of different types of blood cells
Miniature Clinical Diagnostic Systems
Fast, on-site, real time testing
Principle: High Isolation, Low Mass, Localized heating possible
• Polymerase
Chain Reaction (PCR)
for DNA amplification
Micro-fabricated DNA capture chip
(Cepheid, CA)
Scaling of Minimal Analytic Sample Size
Scaling in Electricity and
Magnetism
• Potentiometric devices (measure voltage) are scale
invariant
• Amperometric devices (measure current) are more
sensitive when miniaturized
e.g., m-array electrochemical detectors (Kel-F) for trace
amounts of ions
Electroplating is faster in MEMS
Courtesy: M. Schoning
Scaling in electromagnetic systems
Constant electrostatic stresses/field strengths
Voltage  Electrostatic field · length  L
Resistance  Length  L-1
Area
Ohmic current  Voltage  L2
Resistance
Current density (I/A) is scale invariant
Scaling in Electricity and Magnetism
Rotor
Stator
Electric:
e: dielectric permittivity (8.85 . 10-12 F/m)
E: electric field
(Breakdown for air: 30 kV/cm)
Magnetic:
m: permeability (4p . 10-7 N/A2)
B: Magnetic field
1
U electric  ε E 2
2
1  B2 
U magnetic   
2 μ 
Sandia MEMS
Human Hair !
Electrostatics is more commonly used in MEMS
Macroscopic machines: Magnetic based
Microscopic machines: Electrostatics based
Judy, Smart Mater. Struc, 10, 1115, (2001)
Electrostatics vs. magnetostatics
Electrostatic force  Area · (Electrostatic field)2  L2
Electrostatic energy  Volume · (Electrostatic field)2  L3
Magnetic field  Current  L
distance
Magnetic force  Area · (magnetic field)2  L4
Magnetic forces are much weaker compared to
electrostatic forces
Magnetic energy  Volume · (Magnetic field)2  L5
Power and Power density scaling
Power  Force · speed  L2
Power density  Power  L-1
Volume
Small devices made through strong materials
can have very large power densities
e.g. 10 nN force in a 1mm3 volume ~ 103 J/mm2
c.f. a thin-film battery  ~ 1J/mm2
Power in MEMS
Compact power sources needed, but Power scales by mass
Currently: Fuel cells, micro-combustors, Radio frequency/optical sources
Energy stored in 1 mm3
Power capacitor
4 mJ/mm3
1 mW for 4 s
Thick Film Battery
1 J/mm3
270 mW for 1
hour
Thin Film Battery
2.5 J/mm3
0.7 mW for 1
hour
0.1 mW
Gasoline
300 J/mm3
3 mW for 1 day
178 Hf
> 10
MJ/mm3
160 mW
Solar Cell (1 X 1 X 0.1 mm3)
MEMS devices: How do we make them?
A mechanism
Gear chain
Sandia MEMS
Hinge
Gear within a gear
Making MEMS
• How to make a MEMS device
- deposit and etch out materials
• Introduction to Micro-machining
- Wet and Dry etching
- Bulk and surface micro-machining
• What kinds of materials are used in MEMS?
-Semiconductors
- Metals
- Polymers
Basic MEMS materials
Silicon and its derivatives, mostly
• Micro-electronics heritage
Si is a good semiconductor, properties can be tuned
Si oxide is very robust
Si nitride is a good electrical insulator
Substrate Cost
Metallization
Machinability
Silicon
High
Good
Very good
Plastic
Low
Poor
Fair
Ceramic Medium Fair
Poor
Glass
Low
Good
Poor
Materials in MEMS
Dominant: SEMICONDUCTORS (Silicon centric)
MEMS technology borrows heavily from the Si micro-electronics industry
The fabrication of MEMS devices relies on the processing of
Silicon and silicon compounds (silicon oxide, nitride …)
METALS: used in electrical contacts (Al,Cu),
magnetic elements (Ni, NiFe)
POLYMERS: used as sacrificial layers, for
patterning (photoresist/polyimide)
Making MEMS
• Planar technology,
constructing components (MEMS & electronics) on
initially flat wafers
> Wafer level processes
> Pattern transfer
• Introduction to Micro-machining
- Wet and Dry etching
- Bulk and surface micro-machining
• What kinds of materials are used in MEMS?
-Semiconductors
- Metals
- Polymers
Photolithography
Light
MASK
Deposit
Metal
Photoresist
Silicon substrate
Positive photoresist
Light
MASK
Silicon substrate
Negative photoresist
-Deposit and remove materials precisely to
create desired patterns
The photo-lithography process
Negative
J. Judy, Smart Materials & structures, 10, 1115, 2001
Positive
Remove deposit and etch
Surface micromachining
http://www.darpa.mil/mto/mems
How a cantilever is made:
http://mems.sandia.gov/
One can make devices as complex as one wishes
using deposition and micromachining processes
Any MEMS device is made from the processes
of deposition and removal of material
e.g. a state-of-the art MEMS electric motor
www.cronos.com
The History of MEMS
Y.C.Tai, Caltech