contact evolution in micromechanical switches · contact evolution in micromechanical switches an...
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Contact Evolution in Micromechanical Switches
an experimental investigation using a contact test station
Lei Chen
Ph.D. DissertationPh.D. Committee: Dean Zavracky, Prof. Adams, Prof. McGruer
Boston, August 2007
Advisor: Nicol McGruerCo-Advisor: George Adams
MICROFABRICATION LABORATORY Lei Chen, August 07,07
MEMS SwitchesCompared to PIN or FET’s switches:
• Higher Linearity • Wider Frequency
Response Range
• Higher Isolation
• Lower Insertion Loss
• Lower Power Consumption
http://www.memagazine.org/backissues/oct03/features/littknow/littknow.html
Problems: Slow Response, Low Power Handling and Low Reliability
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Switch Failure Modes• Adhesion prevents contact separation.• High resistance due to contamination.• Adhesion on large contact peel material off
substrate• Excessive material transfer, reshaping
– Contact damage– Material transfer prevents separation
Contamination
104 cycles
106 cycles
No cycles
Material Transfer
Object: understand dominant physical failure mechanismsin switch lifetime test
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Outline• Experiment Setup and Cantilever Fabrication• Pull Off Force and Contact Evolution
– Separation Modes: Brittle and Ductile.– Rate Dependent Pull Off force– Force Evolution and Separation Modes– Size and Material Effects
• Resistance and Contact Evolution– Contamination buildup rates and their relation to
contact materials.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
SPM Setup for Contact Study
Test Cantilever
Substrate
PL022
Gas Inlet
wedge
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Pull Off Force Measurement
Signal Detected
Pull Off Force
Piezo Driving Signal
Piezo
Time
Loading Force
PD Response
Stiffness of the cantilever is 1~1.5x104N/m, with force measurement resolution of 10~15µN.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Fabrication Flow
1. SOI Wafer 6. Spin PR
7. Pattern PR
8. Reflow PR
9. Anisotropic Etching
13. Remove Al/Ti
10. Coat Al/Ti
12. ICP Silicon Etching
11. Pattern Al/Ti
2. Oxidation
3. Pattern Back
4. TMAH Etching
5. HF Release
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Round Bump FabricationShipley 1818 Shipley 1818 The shape of the photo
resist is transferred to the silicon by using SF6/O2/Ar ICP silicon etching process.
Photo Resist Before Reflow Photo Resist After Reflow
O2:SF6:Ar=20:10:25
Silicon Bump
O2:SF6:Ar=15:10:25
Silicon Bump• Critical issues for
profile transfer:– Process Pressure– Biased Power– Gas Ratio
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Contact Angles
32
hX h ZL
θ ⋅⎛ ⎞∆ = ⋅∆ = ∆⎜ ⎟⋅⎝ ⎠
:θ∆ Cantilever deflection angle
Horizontal movement comes along with vertical indentation
Single Wedges
Assume h=15µm and L=170µm, ∆Z=20nm will result in 2.6nm sliding
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Sliding Cancellation
• Wedge 1 provides a contact angle
• Wedge 2 provides an actuation angle
⎥⎦
⎤⎢⎣
⎡∆
−∆⋅
⋅⋅
= )(23arctan
sub
sub
ZZ
Lh δϕ
Compensation angel is related to the indentation depth!
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Outline• Experiment Setup and Fabrications• Pull Off Force and Contact Evolution
– Separation Modes: Brittle and Ductile.– Rate Dependent Pull Off force– Force Evolution and Separation Modes– Size and Material Effects
• Resistance and Contact Evolution– Contamination buildup rates and their relations to
contact materials.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Two Separation Modes
Brittle Separation: on separation, there is no plastic deformation before the rupture.
Ductile Separation: on separation, there is varying degree of plastic deformation during the rupture.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Physical Mechanisms of Separation Modes [1]
Dislocation Nucleation (Ductile)
Cleaving (Brittle)
:cleaveG Energy dissipation associated with cleaving surfaces
(=w, work of adhesion)Energy dissipation associated with dislocation nucleation
(crystalline, grain boundary, defects and material hardness):dislG
[1]: J.W.Kysar, Journal of the Mechanics and Physics of Solids, 51, 795-824, 2003MICROFABRICATION LABORATORY Lei Chen, August 07,07
Ductility of Gold • Au is FCC metal, with 111 slip planes and
<110> slip directions.• Assume a (100) crack intersected by (111) slip
plan, based on theoretical calculation [2]:– Critical surface energy needed for dislocation
nucleation is 1.27J/m2 (Pure Mode I)– Critical surface energy needed for dislocation
nucleation is 0.58J/m2(with 10% Mode II)
Surface energy of gold is 1.56J/m2, gold is an intrinsically ductile metal.
[2]: J.R. Rice, Journal of the Mechanics and Physics of Solids, 40, 239-271, 1992
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Force-Displacement Separation Curve
The “plateau” region in the force-displacement curve is characteristic of ductile separation.
Displacement
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Pull Off Force and Separation Modes
• In Brittle Mode( JKR Model) :
RwFpulloff ⋅⋅⋅= π23
HrFpulloff ⋅⋅≈ 2π
:R
:r:w
radius of curvaturework of adhesion
radius of ductile area:H hardness
:pulloffF pull off force• In Ductile Mode:
In brittle mode, magnitude of the pull off force is affected by surface conditions. Whereas, in ductile mode, magnitude of the pull off force is related to bulk properties of materials.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Rate Dependent in Brittle ModeMaximum Loading Force =200µN
• Gold contacts were tested in ambient condition (R.H=30~40%).
• In brittle mode, surface events affect magnitude of the pull off force.
• Longer time in contact, larger pull off force.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Rate Dependent in Ductile Mode
On ductile separation, higher loading and faster unloading can lead to larger pull off force.
• There is viscous effectduring ductile separation.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Atoms Flow in Nanostructure• For nanostructure, internal
cohesive bonding strength can be affected by the surface tension.
⎟⎠⎞
⎜⎝⎛ +−=
ldTT
mb
m 246
1 β
ldβ
mT
mbT
Melting temperature for nanostructure
Melting temperature for bulk material
Diameter (nm) Length (nm)
Material constant (1.12 for Au)
l
d
Liquid Drop Model:
Pulling out ductile tips
For Au, Tmb = 1337K,
Tm=1212K for d=10nm, l=20nm
Smaller melting temperature Tm
Lower activation energy for diffusion Ea
Larger diffusion coefficient D
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Viscous Force Modeling nanotips as viscous liquid bridges during ductile separation:
vmpulloff FFF +=pulloffF
mF
vF: Capillary Force, surface tension, independent of rate
: Viscous Force, viscosity, depend on unloading velocity
:Pull Off Force during ductile separation.
a0=5nmD0=20nm
a0=15nmD0=20nm
a0=30nmD0=20nm
a0=50nmD0=20nm
Tm(K) 1212 1279 1295 1302
D(×10-15m2.s-1) 1.49 0.4 0.29 0.258
η(Pa.s)
Fm(nN)
Fv1(nN)@ 35µm/s
Fv2(nN) @0.05µm/s
a0=5nmD0=20nm
a0=15nmD0=20nm
a0=30nmD0=20nm
a0=50nmD0=20nm
Tm(K) 1212 1279 1295 1302
D(×10-15m2.s-1) 1.49 0.4 0.29 0.258
η
MICROFABRICATION LABORATORY Lei Chen, August 07,07
(Pa.s) 555 2043 2829 3223
Fm(nN)
Fv1(nN)@ 35µm/s
Fv2(nN) @0.05µm/s
a0=5nmD0=20nm
a0=15nmD0=20nm
a0=30nmD0=20nm
a0=50nmD0=20nm
Tm(K) 1212 1279 1295 1302
D(×10-15m2.s-1) 1.49 0.4 0.29 0.258
η(Pa.s) 555 2043 2829 3223
Fm(nN) 7.8 70 282 785
Fv1(nN)@ 35µm/s 26 461 2174 6444
Fv2(nN) @0.05µm/s 0.04 17 82 243
pulloffF
0D
02 a⋅
vUnloading Velocity
Evolution of Brittle Separation
After 10 cycles After 102 cycles After 103 cycles After 104 cycles
• For contact evolution in the brittle mode, the nominal contact area increase during the cycling test.
• The measured pull off force were the same for all four cases. (~100µN with the maximum loading force of 200µN).
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Force and Ductile Separation
Cycle Number
1e+3 1e+4 1e+5 1e+6
Adh
esio
n (u
N)
40
60
80
100
120
140
160
Cycle Number
1e+3 1e+4 1e+5 1e+6
Adh
esio
n (u
N)
40
60
80
100
120
140
160
Cycling test leads to large area material transfer
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Evolution of Ductile Separation (a)• Evolution of the ductile separation will cause
material transfer and lead to unstable pull off force.
After 10 Cycles After 20 Cycles After 30 CyclesDetect Ductile
The ductile separation is indicated by the plateau region in the force-displacement separation curve. It shows the pull off force increases after the ductile separation.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Evolution of Ductile Separation (b)
Pull off force decreases after the ductile separation.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Force, Modes and Evolution
Displacement (nm)
16 18 20 22 24
Forc
e ( µΝ)
-120-100-80-60-40-20
02040
3x104
8x104
The contact evolution start from a ductile separation, and evolve to a brittle separation.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Size Effects
• The tests are all for 200nm sputtered gold contacts with maximum loading force of 200µN.
• The large contact bump need large pull off force.
• Ductile separation have been observed on all four sizes.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Au-5%Ru Contact
• Hardness:– Au-5%Ru: 2.42GPa– Au: 1.04GPa
• Compared to Au contacts, 5%Ru alloy element can:– Reduce ductile
separation– Lower pull off force.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Materials EffectsAu Ru Rh Pt Au5%Ru Au10%Ru Au10%Pt
3.99 2.79
Maximum Pull Off Force (µN)
200 <50 50 50 50~75 50 75
Surface Damage Severe None Minimal Minimal Medium Medium Medium
Hardness (GPa) 1.04 15.3 9.75 5.39 2.42
Materials are prepared in Air Force Research Laboratory. The pull off force value are measured with the maximum loading of 200µN. Tests are performed between the same material pair contact tests.
• For hard material, pull off force is low and surface damage is minimal
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Outline• Experiment Setup and Fabrications• Pull Off Force and Contact Evolution
– Separation Modes: brittle and ductile.– Rate Dependent Pull Off force– Force Evolution and Separation Modes– Size and Material Effects
• Resistance and Contact Evolution– Contamination buildup rates and their relations to
contact materials.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Resistance Measurement• Contact resistance measured with 1mA current source and a 2.1V
compliance voltage. • Measured resistance includes the sheet resistance component for
the cantilever and contamination film between the contacts.
Test layout for contact resistance measurement
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Resistivity and Hardness of Au Alloys
* Data were measured in Air Force Research Laboratory by Dr. Kevin Leedy.
•Ru, Pt, and Rh show higher hardness than Au.
•There is an increase in contact resistance and decrease in hardness of Pt, Rh and Ru by alloying with Au.
•Ideal contact materials: moderate hardness, low resistivity.MICROFABRICATION LABORATORY Lei Chen, August 07,07
Resistance Evolution Test
Cycle Number1e+3 1e+4 1e+5 1e+6 1e+7
Res
ista
nce
(ohm
)
0
5
10
15
20
Au Rh
RuPt
Test conditions: Contact Force, 200µN; Identical contacting materials, ~300nm thick sputtered thin film; Cold switching in room air
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Au and Au Alloys
Cycle Number1e+3 1e+4 1e+5 1e+6 1e+7
Res
ista
nce
(ohm
)
0
5
10
15
20
90%Au,10%Pt
30%Au,70%Ru70%Au,30%Ru
30%Au,70%Rh70%Au,30%Rh
50%Au,50%Pt
Au
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Ru and Au-Ru Alloys
Cycle Number1e+3 1e+4 1e+5 1e+6 1e+7
Res
ista
nce(
ohm
)
0
5
10
15
20
Ru
95%Au,5%Ru
80%Au,20%Ru
30%Au,70%Ru
Au
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Surface Reactivity [3]
Building-up contamination around Ru bump.
• Pt, Ru, Rh are transition metals with unfilled d-band.
• Au is a noble metal with filled d-band.
• Transition metals show strong surface reactivity.
[3] B.Hammer and J.K.Norskov, Nature, 376, 238-240, 1995.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
EDX SpectrumAu-30%Rh
A
B
C
A:
B:
• Contaminants are carbon based materials.
C:
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Au-5%Ru and Au-10%Ru• Cold Switching for 106 cycles
Au-5%Ru Au-10%Ru
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Conclusion• Developed practical approaches for contact
evolution study: such as monitoring rate-dependent pull off force, force-displacement curve.
• Gained some understanding about ductile separation and contact evolution.
• Material intrinsic properties are important for contact reliability: such as hardness, electron structure, resistivity, melting point.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Future Research• Plasma Cleaning• Current, rate effects on
contact evolution.
• New test structures
Structures were designed by Jim Guo
• Contact materials
MICROFABRICATION LABORATORY Lei Chen, August 07,07
AcknowledgementThis research has been supported by Northeastern University, and
DARPA under its HERMIT program through research contract F33615-03-1-7002
• I would like to thank Prof. McGruer and Prof.Adams for their guidance and support;
• I am grateful to Dean Zavracky for serving as my Ph.D. committee member;
• I wish to thank Dr. Kevin Leedy in Air Force Research Lab for collaboration.
• Also express my gratitude to the rest of faculty, staff, and colleagues in MFL group.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Questions?
MICROFABRICATION LABORATORY Lei Chen, August 07,07
MICROFABRICATION LABORATORY Lei Chen, August 07,07
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Inert Gas Control
⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢
⎣
⎡
⋅⋅=
2/10 )4(flow
pathab
A
VL
D
TerfcCC
:T Thickness of gas flow :pathL Length of gas flow
:flowV Gas flow rate :abD Diffusion coefficient
:0C Concentration in the air
:AC Concentration in the contact area
The set-up can keep the organic vapor concentration at the contact area five orders of magnitude lower than the concentration in the air
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Force Measurement
cPD ZLd⋅
=∆3
ccc ZkF ⋅=
Fc: contact force.
kc: stiffness of cantilever
Zc: deflection of the cantilever
∆PD: shift of the laser spot on PD.
L: length of the cantilever
d: the distance between the cantilever and the PD
Stiffness of the cantilever is 1~1.5x104N/m, with force measurement resolution of 10~15µN.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Test Method• Testing method: cantilever integrated with
contact bump + laser beam– Convenient to vary the contact shapes and contact
materials; – Easy to control the contact force and the actuation
methods; – In-situ measure the pull off force and contact
resistance;– Easy to inspect contact area.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
Rate Dependent at Brittle ModeMaximum Loading Force =200µN
• Gold contacts were tested in the ambient condition (R.H=40%).
• Magnitude of the pull off force affected by the time in contact and the time between contacts.
MICROFABRICATION LABORATORY Lei Chen, August 07,07
RuO2 Contact Tests
The film with high resistivity shows slow contamination rate. ρRuO2=80µΩ.cm with different film thickness
ρRuO2=167µΩ.cm 50Ǻ 200Ǻ
400Ǻ 2500Ǻ
With film thickness of 2500Ǻ
MICROFABRICATION LABORATORY Lei Chen, August 07,07