Download - Micro-thermo-mechanical Actuator for
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Micro-thermo-mechanical Actuator for in situ Wafer Temperature Mapping
SFR WorkshopApril 17, 2002
Ranju Arya, Mark Rashid, Dwight Howard, Scott Collins and Rosemary Smith
MicroInstruments and Systems LaboratoryElectrical & Computer Engineering
Mechanical & Aeronautical EngineeringUC Davis
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Problem Statement
• Small feature reproducibility requires film thickness uniformity
• Uniformity of PECVD films depends on spatial control of process parameters, including:– plasma composition (gas flow rates and pressure) – plasma energy (power)– substrate surface temperature
• Substrate surface temperature relies on – thermal conductivity of substrate and coatings– contact of substrate to platen– surface chemical reactions
Monitoring and control of surface temperature --> improved film uniformity
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State of the Art• Wafer temperature is generally not measured during deposition
(assumed to be same as platen temperature)• During process development, wafer surface temperature may
be spot tested, using either a spring bimorph temperature indicator or thermo-chromic polymer, to calibrate platen temperature to wafer surface temperature.
Our Approach• Design and test a simple, tool independent, low cost, in situ
wafer temperature mapping method for PECVD process development using an array of micro-thermo-mechanical actuators.
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Justification
• Structures are relatively simple to fabricate, are non-intrusive, and non-contaminating to process.
• Readout can be real time, in situ, or post-process, by optical imaging of wafer surface.
• Structures can be configured for electrical readout• Structural design, fabrication, density, sensitivity and range
of operation can all be tailored for a particular process, independent of wafer diameter.
• Reusable wafer platform
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Concept
Bi-stable SiMembranes
Aluminum, 1 µmSilicon, 10 µmSilicon
Thermal expansion coefficient (α ) mismatch strain displacement
TC depends on relative thicknesses of oxide and aluminum.
An array of membranes with varying oxide thickness
→ membranes of same size will “snap” at different temperatures.
αAl=23(10-6) C-1 αSi=2.3(10-6) C-1 αSiO2=0.55(10-6) C-1
SiO2, 0.3 µm
T= T0 T = TC> T0 T > TC
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Theory of Operation
Simplified Mechanical Model of Buckling Membrane
l
L
x
k1
k2
φ
L (zero stress spring length)
x
Oxide Compressive Stress
Al-Si BimorphThermal Stress
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Conditions for Membrane “Snapping”• Zero-Force Spring Length :
Spring 1 : L + ∆Spring 2: L + αT
• According to Hooke’s Law :F1 = k1 [ ( L2 + x2 )1/2 – ( L + ∆ ) ] F2 = k2 [ x + ( LαT) ]
• At equilibrium:F1sinφ = F2
Solving this equation for x gives membrane position(s) where the net vertical force is zero (equilibrium).
But, not all solutions are stable! Only zero crossings with positive slope are stable (achievable) positions.
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Membrane Position vs Net ForcePlot shows effect of increasing and decreasing temperature on membrane position.
At T= TC, membrane snaps from +x to -x position (up to down).
T3
T2
T1
Net
For
ce
Stable positions, T=T1
Stable position, T=T3Stable position, T=T2
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Preliminary Results
Glancing angle image of two membranes on wafer surface, deflected in opposite states .down up
Top view image of a single membrane, in up and down deflected states.
TC ≅ 380 C.
down up
1mm
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Discussion• For given silicon membrane dimensions and Al film
thickness, TC can be varied by oxide thickness. • Oxide color changes with thickness, therefore…
membranes of different COLOR will snap at different temperatures!
• Employing color filtering of wafer image enables determination of temperature variance across wafer.
Tempgradient
TC >TC
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Current and Future Developments
• Fabricate of membranes with varying side lengths, membrane and oxide thicknesses and measure Tc
• Develop an imaging scheme that readily determines membrane deflection (up or down)
• Test in PECVD tool• Develop a model with which one can predict TC
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• Establish design parameters (membrane size, oxide thickness, etc.) by 7/1/2002.
• Design, fabricate and test thermal actuator array by 9/30/2002.
2002 Goals