Nanotribology Lab NC StateNanotribology Lab NC State
Nanoscale Friction and RF MEMS
Chris Brown, NCSU Physics
Nanotribology Lab NC State
Introduction
• Generally believed by academics, military and industry that MEMS devices will be in forefront of next generation technological developments.
• In particular, RF MEMS devices have the potential to enhance many telecom and military applications due wide bandwidth ranges and operation with lows signal loss.
• However, MEMS devices, especially those which must make perpendicular or sliding contact are plagued by tribological issues.
• Goal: define a set of tribological design rules limiting stiction, friction and adhesion failures to increase low contact resistance (< 1Ω) switch lifetime from 10-25 billion cycles to 100+ billion cycles.
Nanotribology Lab NC State
Emerging Crisis or Already Here?• Presently unsure if nano-scale structures can be made
mechanically and chemically resistant enough to withstand extreme operating conditions.
• Getting devices from the laboratory to the marketplace is far from guaranteed.
• Not enough trained professionals to deal with the problems, now or in the future.– Scientists and Engineers make up 5%
of the total US workforce and over half are 40 years or older. Graduate and undergrad student populations continue to decrease.
– Other countries are making the investment to catch up with the United States.
Nanotribology Lab NC State
Focus on Fundamentals• Chemical and mechanical stability of moving nano-structures
underlie the field of nanotribology.
• Role of surface science and friction has received less thought than it relative importance.
• The fundamental problems stem from a lack of work in atomic scale tribology and surface science.– Real contact area of RF MEMS devices tend to be on the order of 75
atoms across.
– Shearing of even a single layer of atoms can spell death for a nanomachine.
• Eliminate fundamental problems at the laboratory phase. Industry is too busy firefighting existing problems to conduct the basic research needed to really answer these problems.
Nanotribology Lab NC State
System Needs
Nanotribology Lab NC State
Applications
LCCMD - 35GHz ESA Antenna
Phase Shifter
Antenna Slat
Antenna
2-D MEM Lens
Optical or RF Projection System
Multi-RFChannel
Transmitters&
Receivers
Beam controlIllumination
RFIllumination
MEM-Tenna
Control Chip
Micro-switchMetallic Pad
Control Chip
Micro-switchMetallic Pad
1
RECAP
LNATo Receive
Beamformer
LNA
TX
BandpassFilters
To Receive
Beamformer
Tunable Notch Filter
Nanotribology Lab NC State
• Large bandwidth operational range
• High linearity
• Low insertion loss
• Reduced size
• High shock resistance
• Wide temperature operational range
• Low power consumption
• Good Isolation
• Low cost
• MEMS switches pair the performance of electromechanical switches with low cost and size of solid state switches.
Why RF MEMS?
Nanotribology Lab NC State
wiSpry RF MEMS Switch
Lower Actuation Electrode Contact Dimple
1.5mm
Nanotribology Lab NC State
MEMS Switch SummaryLow Current High Current
Good
Asperity Creep
Better Durability
No Nanowire formation
Good
Lower Resistance
Near Zero Adhesion
No Bounce
Bad
Higher Resistance
Switch Induced Adhesion
Switch Bouncing
Bad
Poor Durability
Switch Shorting by Nanowire
Welding
Our group’s MURI Grant research will be looking at this in depth to understand switch failures in RF MEMS. It appears that reliability / durability will not be improved by balancing the current known variables. It will require the use of coatings and lubricants as well as non-standard environmental conditions to maintain optimum switching conditions.
Nanotribology Lab NC State
• Exploration of nanotribological failure modes at contact points.– Adhesion– Melting / Nanowire formation– Welding– Surface films
• Next Generation contact materials• Failure acceleration mechanisms
Opportunities for Improvement
0 50 100 150 200 250
0
50
100
150
200
250
Nanotribology Lab NC State
Contact Resistance
Resistance
UnstableResistance
TransitionZone
Stable Contact Resistance
Fc,min
Rc,min
Force
Property Au Au(95) Ni(5)
Fc (N) 100 300
R (m) 15 60
FB (N) 0-270 0-300
Nanotribology Lab NC State
Resistance Failures• Progressively increasing resistance during cycling is the most
prevalent failure mode for MEMS switches. – Current – decreasing current elevates resistance
– Thermochemical gradient – absorption of hydrocarbons and carbon dioxide when exposed to air.
– Electromigration – electrons conducted through metal collide with atoms displaced in the lattice due to higher temperatures. The scattering creates resistivity.
– Contact area• For radii smaller than the mean
free path, electrons are projected ballistically through the contact spot (Sharvin Mechanism).
• For radii larger than the mean free path, resistance in dominated by diffuse scattering.
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5 6 7 8 9 10
Log10 Cumulative Cycles
Res
ista
nce
(Ω
)
Nanotribology Lab NC State
• Limited work has been done on failure mechanisms and switch durability.
• Lack of correlation between test environments and data
• Time to failure measurements have limited meaning if not correlated to operating conditions.
Previous Work
Nanotribology Lab NC State
Current Work• Vacuum
– May help eliminate formation of oxide layers on gold surfaces.– Working to understand problems with actuation at low pressure. Die are
designed for dampening due to air in normal atmosphere. Q values in vacuum increase ten fold.
• Cryogenic– Initial tests show the die can survive 77Kelvin. Next step is to go down to
3Kelvin and cycle switches.– Lower temps will lessen softening / melting effects. This will in turn diminish
adhesion problems by maintaining surface roughness.
4
222 U
TTL o
Nanotribology Lab NC State
Current Work
• Variable Atmospheres– Operation of switches in inert gasses such as dry nitrogen and argon at normal
atmospheric pressures may overcome operational issues in the vacuum environment while stopping oxide formation.
• Problem: working devices.
• Future work: accelerated test methods.
Nanotribology Lab NC State
• Gain understanding of failure statistics under at range of operating parameters in various environmental conditions.
• Identify the physical phenomena associated with failures.• Develop accelerated lifecycle testing methods to
statistically determine the most detrimental failure modes and test new materials.
• Apply knowledge to a range of MEMS devices to ensure findings are not device specific.
• Use this knowledge to build a set of tribological design rules that will control frictional problems to a degree where micromachines and switches will be an economically viable option for general application.
Goals
Top Related