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RF MEMS Contact Tribology and Contact Materials Development : 

RF MEMS Contact Tribology and Contact Materials Development Angus Kingon, Materials Science with Jackie Krim, Physics Art Morris, wiSpry Don Brenner, Materials Science NCSU, October 13, 2005

Acknowledgments: 

Acknowledgments Graduate students, including Zhenyin Yang Febin Skaria Chris Brown (Jackie Krim) Stephan Menzel (Intern from RWTH-Aachen Collaborations Krim - Kingon groups Zikry group (earlier presentation) wiSpry, Inc (Art Morris, Ph.D) Brenner group - still to come

Outline: 

Outline Background Progress in materials development/characterization facility Alloy selection and development

Objectives: 

Objectives Understand the wear performance, and improve the contact materials utilized in RF MEMS contact switches Utilize wiSpry switches as a primary vehicle for the studies Develop a materials characterization/development test facility Develop improved materials

RF Switch: 

RF Switch Can achieve 10E10 cycles, but under specific conditions

Switch Lifetime: Good News: 

Switch Lifetime: Good News Operate to >2 billion cycles without failure unpackaged Contact stable with average of <1 m increase per million cycles Statistical numbers of switches tested using automated equipment (packaged and unpackaged)

The switches have become pretty darn good ………under the optimum operating conditions: 

The switches have become pretty darn good ………under the optimum operating conditions

Switch Current Handling: 

Switch Current Handling Hot Switching High-current Cold Switching Upper contact AFM after 500mA cold switching Lower contact AFM after 500 mA cold switching Contact welding metric

Specialized Characterization Facilities: 

Specialized Characterization Facilities Three separate and complementary facilities: Controlled environment cryogenic test chamber (whole switches and arrays) QCM-STM for tribological characterization Contact materials characterization and development facility

Progress on Contact Materials Test and Development Facility: 

Progress on Contact Materials Test and Development Facility

Characterization Facilities: 

Characterization Facilities Contact materials development and test facility - requirements: Close to field operational conditions Allow local examination contact surfaces Allow simultaneous electrical test over a range of operating conditions Straightforward inclusion of new contact materials

Contact Materials Characterization and Development Facility: 

Contact Materials Characterization and Development Facility Demonstration of concept AFM-based system Method for demounting wiSpry cantilever from switch and mounting onto AFM tip carrier Align chip carrier with cantilever in AFM Contact cantilever against replaceable lower electrode (contact) structure Resonate upper cantilever (or lower electrode structure against the top cantilever) Analyze contact areas in AFM

Key Steps: 

Key Steps Reliable demounting of top cantilever from real switches, and remounting on AFM chip carrier Alignment of top cantilever/chip carrier in AFM for contact measurements to bottom electrode In-situ measurements, varying the bottom electrode material

Top cantilever: wiSpry switch mounting: 

Top cantilever: wiSpry switch mounting Specially developed transfer technology is used to remove a top cantilever from the wiSpry switch chip, and then attach the cantilever to a AFM tip carrier. Thus, the cantilever acts as a AFM tip and AFM non-contact mode may do the switching during the test. SEM image of a top electrode optical image of a wiSpry switch

Top cantilever mounting: 

Top cantilever mounting Front and rear views of a cantilever demounted from the switch and mounted on an AFM chip carrier prior to characterization in the scanning probe

Fabrication of the bottom electrode: 

Fabrication of the bottom electrode Optical Images of bottom electrode Fabrication Procedures: Pattern dots on silicon wafer 2. etching MBE growth of SiO2 insulating layer 4. Cr/Mo/Au alloy layer growth Pattern alignment lines and Pads 6. Sub-divide the big pattern 7. Ion beam etching of Au and Mo 8. Solution etch of Cr 9. Acetone rinsing

Alignment in the AFM: 

Alignment in the AFM AFM chip carrier with cantilever must now be aligned in the AFM such that the top contact is aligned to the bottom electrode contacts Almost a fatal flaw!! The alignment requirement is an order of magnitude more precise in three axes of rotation than normal AFM measurements

Assessment of the geometric tolerances: 

Assessment of the geometric tolerances Using optical microscope and AFM the detail geometric information of the dimple area and top cantilever were assessed The dimple - plane height difference is only ~ 100 nm which requires special approaches for reliable contact and alignment purposes

High Frequency Dynamic Switching: 

High Frequency Dynamic Switching Fixed in-contact and out-of-contact positions make high-frequency (hundreds of Hz to KHz) dynamic switching possible Dynamic Switching allows us to investigate the evolution of the contact area Piezoelectric Actuator Amplifier Wave Generator

Effort for the force calibration: 

Effort for the force calibration Contact Force: Measurement of Young’s Modulus of the cantilever by frequency sweeping technique in AFM E = 93.3 GPa Measurement of geometry parameters of the cantilever: the dimple height is only ~0.1µm, and the distance between front edge and dimple distance is ~11 µm when it fully contacts with bottom electrode. Beam deflection calculation is valid based on a circular shape model 11µm

Force calculation Model: 

Force calculation Model For the two-dimple contact measurement, the contact forces on the two dimples are nearly equal even with a little bit cantilever tilting. ( 5° tilting only gives less than 20% difference between the two contact Force, usually the tilting is smaller than 5°) Thus:

Effort for the force calibration: 

Effort for the force calibration Adhesion Force: Measurement of Hysteresis Displacement of a uniform cantilever (∆d = d1 – d2) d1: Initial contact z position; d2: Out of contact position Estimate of Adhesion Force 11µm Δd

Adhesion forces : 

Adhesion forces The hysteresis in the displacement curves allows one to estimate the adhesion forces 100 micron (our test cantilever) of free end gives ~1 mN contact force for each dimple according to the model ∆dmeasured = 2 micron (on ~1 at.% Ni bottom electrode), indicating small adhesion force (~0.3 mN) Four point probe measures the total resistance of two contacts plus the bridge. Under such estimated force, the measured total resistance values is ranged from ~0.5 to ~0.75 ohms corresponded to partial and full contact state

Manually Switching : 

Manually Switching Slow Manually Switching and Static Measurement Technique shows repeatable and reasonable results. The technique can be used to investigate the MEMS contact resistance for different alloy candidates

New Long Cantilever: 

New Long Cantilever

New Longer Cantilever : 

New Longer Cantilever

Force Control and Optimization: 

Force Control and Optimization Using longer and narrower cantilevers can reduce the force to the region MEMS devices usually operate (60 microN to 500 microN) Longer cantilever may allow better analysis of adhesion force. Longer cantilever may allow us get force vs. contact resistance information

Status of technique development: 

Status of technique development √ Reliable method of demounting and remounting RF switch cantilevers √ Improved methods of aligning in the AFM - on third generation of lower electrode contact jig √ Developed bottom electrode process √ Developed method for contacting and undertaking contact meaurements in the AFM - quasistatic X Currently working on increasing contact cycling frequency to 100s kHz

Contact Materials Development: 

Contact Materials Development

Contact materials development: 

Contact materials development Resistivity represents tight constraint - big impact on insertion loss, especially as one moves to high frequency devices Phase I: work on small modifications to Au, primarily solid solutions, possibly smart materials(?) Phase II: look at second phase modifications with hard materials

Switch Electrical Properties: 

Switch Electrical Properties 2.4GHz Non-deembedded results! Loss on HRS High-Isolation Version 1.5mm

Metal Resistivity: 

Metal Resistivity Consider resistivity of noble metals and base metals of interest for solid solutions

Thermal Expansion Matching: 

Thermal Expansion Matching Consider CTE of noble metals and base metals of interest for alloys

Alloy selection: 

Alloy selection Au-Ni Alloy: Low Ni composition (1~2 wt at%) to retain the inertness and electrical resistivity of gold As temperature arising, low Ni composition alloy falls into a stable solid solution region up to 1000o C according to the phase diagram

Au-rich Au-Ni Alloy phase behavior: 

Au-rich Au-Ni Alloy phase behavior

Ion beam deposition : 

Ion beam deposition Cr adhesion layer, Mo diffusion barrier and gold alloy layer are deposited consequently in Xenon atmosphere (4*10-4 torr) Composition of the alloyed metal is determined by the area covered by the second metal ‘strip’ on the gold target Rotation of the target ensures uniformity of the composition. In situ moderate heating can modify the properties of the thin films

X-Ray Diffraction of Au-Ni Alloys: 

X-Ray Diffraction of Au-Ni Alloys Strongly (111) fiber textured, increasingly (100) or polycrystalline at elevated deposition temperatures

Lattic parameter : 

Lattic parameter Note: Implication is that films remain single phase solid solution (metastable)

AFM Images of Au and Au-Ni: 

AFM Images of Au and Au-Ni Gold film Au-Ni RT Au-Ni 250oC Au-Ni 200oC Alloyed with Ni, the grain size of the thin film will be reduced, which should produce more grain boundary in addition to solid solution hardening if solid solution is verified Moderate In situ heating during deposition produces much bigger grains, making the surface rougher, also confirm the XRD data result of high orientation loss.

Slide40: 

AFM series

Slide41: 

Roughness trends for the alloy series

Hardness of Au-Ni alloys: 

Hardness of Au-Ni alloys

Slide43: 

But resistivity is also increasing!

Anneal studies: 

Anneal studies The contact materials will see local elevation of temperature - therefore let’s understand anneal effects in these alloys

Slide45: 

Annealing in N2

Slide46: 

Annealing in N2

Slide47: 

Annealing in N2

Slide48: 

Annealing in N2 Conclusions: Annealing in N2 yields Ni segregation and NiO formation The NiO ends up on the Au surface, non-uniformly distributed

Forming gas anneals: 

Forming gas anneals

Slide50: 

Annealing in Forming Gas

Conclusions: 

Conclusions In-situ test approach in final stage of development Beginning to understand the behavior of Au-Ni alloys Some considerations for alternative alloys

Slide54: 

End

The Need: Applications of RF MEMS Switches: 

The Need: Applications of RF MEMS Switches Wide range of applications for the replacement of semiconductor switches Examples: Frequency agile communications front ends (tunable filters, for military and commercial) Antenna switching Radar beam steering (phase shifters)

Military/Aerospace Systems: 

Source: DARPA Military/Aerospace Systems

Military/Aerospace Systems: 

Military/Aerospace Systems

RF MEMS Switch: Opportunities : 

RF MEMS Switch: Opportunities The RF MEMS switch has the potential for integration and miniaturization, and offers lower power consumption, lower losses, higher linearity and higher Q factor than conventional communications components. Compound Semi & Micro-technology chip 26 Hyman, D

The switches have become pretty darn good ………: 

The switches have become pretty darn good ………

Thrust Objectives: 

Thrust Objectives General: Develop a generalized framework for understanding failure in RF MEMS contact switches

Thrust Objectives: 

Thrust Objectives Specific (using wiSpry switches as a vehicle): Characterize failure under standard operating conditions Characterize and understand failure under an extended range of operating conditions (current, atmosphere, temperature, force, and hot/cold) Develop advanced contact materials yielding improved switch performance Develop accelerated test methods

Thrust Approach: 

Thrust Approach How? Develop/utilize tribological characterization systems Develop a framework for understanding failure (using the wiSpry switch as the vehicle) Design and develop new contact materials Correlate materials and operating conditions with failure mechanisms

Characterization Facilities I: 

Characterization Facilities I Controlled environment cryogenic test chamber Includes operating at various currents, switch power, contact force, hot switching and cold switching. Complements wiSpry statistical testbeds Environments will include temperature variability from high temperature to 4 Kelvin. Atmospheric conditions can include air, vacuum, and inert gases such as Argon or Nitrogen

Characterization Facilities III: 

Characterization Facilities III QCM-STM Discussed by Jackie Krim (next) Operate in impact mode (not sliding contact) Au AFM tip now becomes the upper contact Contact onto real lower electrodes (from wiSpry switch), or new contact materials

Characterization Facilities: Summary: 

Characterization Facilities: Summary Three facilities cover testing over range of conditions Speed/convenience - realism spectrum wiSpry switches provide a state-of-the-art switch to ensure practical relevance and comparative metrics Maintain focus on fundamental mechanisms

Slide66: 

Material type and Synthesis Device Configuration Design Failure Mechanisms Device Performance Observation and Characterization identify determines Material Properties determines 2. Framework for Failure Mechanisms determines

Failure Mechanisms: 

Failure Mechanisms Performance Yield failures Closed - stiction Closed - beam stress (curvature) Open - beam stress Service failure Shorting Resistance increase Observations (couple to electrical) Topographic Plastic deformation Melting Grain pullout Pitting Plasma etching and deposition Chemical Surface (eg C migration) Near-surface Local material change Local plastic deformation Local recrystallization

Failure Mechanisms: 

Failure Mechanisms Characterization tools SEM, AFM Surface analytical Backscattered electron Kikuchi patterns Observations (couple to electrical) Topographic Plastic deformation Melting Grain pullout Pitting Chemical Surface (eg C migration) Near-surface Local material change Local plastic deformation Local recrystallization

Failure Mechanisms: 

Failure Mechanisms Mechanisms (examples) Plastic deformation => surface smoothing => adhesion (short) Plastic deformation (shear component) => surface roughening => contact area changes => resistance increase Plastic deformation (shear component) => surface roughening => wear debris => resistance increase Local heating at asperities => local welding => wear debris Local heating at asperities => local welding => material transfer => surface roughening => resistance increase Local plastic deformation gives local increase in material resistivity

3. Design New Contact Materials: 

3. Design New Contact Materials In parallel with mechanism studies Begin with Au lower electrode Modify not replace (resistance issue) Reviewed approaches Hardening without compromising resistance Solid solution Second phase with controlled scale of microstructure (some plastic deformation but control the magnitude of topography changes)

Slide71: 

Gold Alloying: Candidates Au-Cu, Au-Ag, Au-Ni, Au-Rh (> 1.6 %, FCC ): Continuous solid solubility

4. Correlations: 

4. Correlations Correlate materials and conditions with failure mechanisms Postulate accelerated test methods

End: 

End