Nanotribology Lab

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Nanoscale Friction and RF MEMS
Chris Brown, NCSU Physics
Nanotribology Lab
NC State
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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 (lifetime from 10-25 billion cycles to 100
  billion cycles.

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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.

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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.

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System Needs
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Applications
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Why RF MEMS?

• 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.

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wiSpry RF MEMS Switch
1.5mm
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MEMS Switch Summary
Our groups 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.
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Opportunities for Improvement

• Exploration of nanotribological failure modes at
  contact points.
  
• Adhesion
• Melting / Nanowire formation
• Welding
• Surface films
• Next Generation contact materials
• Failure acceleration mechanisms

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Contact Resistance
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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.

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Previous Work

• 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.

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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.

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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.

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Goals

• 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.