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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