Rf mems

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.


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.


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.


System Needs


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


• 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?


wiSpry RF MEMS Switch

Lower Actuation Electrode Contact Dimple

1.5mm


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.


• 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


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


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

)


• 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


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


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.


• 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

Technology

Rf mems