GE Energy Silicon Wafer Measurement System

Team 10Olin Biddy

Scott Johnson

Chetwyn Jones

Rob McCoy

Tim Weber

Project Overview

Design a device that can measure the thickness of a silicon wafer

Silicon wafer production has to be precise Wafers come off a conveyer belt from oven Immediate feedback for production adjustments Important Requirements

– Accurate within 10 microns– 9 or more measurement points– Completes process in 30 seconds

– Repeatable and Reproducible

Goals of Prototype

Detects faulty wafers Outputs thickness

measurements in user friendly format

Store data for further research / analysis

Easy to operate for workstation user

Graphical representation of surface geometry

Design Requirements: Wants

Final % Rate ofRanking Importance

1 412 203 174 145 46 3

Easy to Use

Wants

PreciseAccurate

EconomicalStable

Quick

Precision- Degree to which future measurements will show similar results.Accuracy- Degree of conformity of measured value to its true value.

Design Requirements: Metrics

Prioritized List of Metrics

Gage Reproducibility and Repeatability – Measures Precision using Statistical Process Control equation.

Metrics % Rate Target ValueImportance

Gage R&R Error 32 Gage R&R Error less than 10%Measurement Accuracy 22 Accurate to less than 10 microns# Measurement Points 13 Greater than 9 points of measurementCompletion Time 12 Less than 30 seconds# Operater Steps 11 Less than 10 operater stepsCost 5 Less than 25,000 dollarsRange of wafer size measureable 5 Ablility to measure thickness less than 2 mm

and width and length range of 15.2 - 22.9 cm

Concept Selection

Infrared Laser- Two laser heads fixed in place. Wafer slides between lasers on XY TablePrecision Linear Gauges- Two grids of 9 linear gauges fixed, one grid on each side of wafer

Key Metrics Infrared Laser Linear GaugesGage R&R Small MediumMeasurement Accuracy .05 microns 3 microns# Measurement Points 20,000/sec 9Completion Time 25 seconds 10 secondsCost 15,400 13,000

Concepts

Complete Concept

Two lasers for subtraction measurement method

XY Table to transverse wafer

Stage to hold wafer between laser heads

Stationary frame for laser heads

Labview to control XY Table, lasers and analyze data

System Components

LK-G Laser

Accurate up to .05 microns Fast 20,000 measurements a second Auto-adjusts intensity for changing reflectivity Wide Spot of 850 microns to average over pits on surface Connectivity with Labview for data collection

XY- Table (Velmex, Inc.)

Stable movement – Stepper motor drives– 9”x9” travel of 12”x12” work surface– .001” resolution – Vmax = 4.0 in/s

LabView interface Easy attachment for wafer stage

Wafer Stage

Two stage platforms to accommodate both 6”x6” and 9”x7” wafers Press fit pins allow for quick change between top wafer platforms Rubber surface to eliminate wafer movement (not shown)

Laser Mounting Frame

Prefab Aluminum Framing System (Bosch Rexroth)– 30mm x 30mm cut-to-length extrusion– Prefabricated fasteners/accessories– Highly Modular– Rapid build time– Stable

“H” Frame– Accommodates both laser heads– Data cable conduit

Completed Prototype

RoadblocksUnforeseen Obstacles

Electronics Box– Safely house laser controller and power supply module (~120V)– Provide a point of electrical entry from wall outlet

Connectivity with lasers and Labview

– Changed from USB to RS232 input

Labview Programming– Particular version of Labview

limited graphical data output

Calibration & Testing Accuracy

Calibrate- Use known thickness object (high tolerance steel gauge blocks)

Subtraction method to calculate thickness

Test- Measure multiple repeated points on steel gauge blocks (5) to validate thickness accuracy to 10 microns

Testing Gauge R&R (Abbreviated Overview)

Gauge R&R is a Statistical Process Control measure of an instrument in Six Sigma practices

Measure 10 wafers using standard measurement process under normal operating conditions

Find statistical range & average Calculate combined repeatability (instrument error) &

reproducibility (operator error) from a standardize expression:

Gauge Error (Gauge R&R)

Gauge Error < 10% to be successful

19.1

15.5 R

Testing Steps and Time

One combined test to validate:– Operator steps– Average measurement time (per wafer)

Run of 10 wafers by a familiarized operator– Define and count steps– Divided total test time by number of wafers to find average time per

wafer Under 10 operator steps meets target Under 30 seconds average measurement time meets target

Testing Results

Completion Time: 35 seconds Operator Steps: 5 simple steps Accuracy: 6.3 microns Gauge R&R: 24.6% (point repeatability error included) Gauge R&R: 5.4% (without changing placement)

Final Bill of Materials

Laser System $11,233

XY Table $3,014

Wafer Platform $177

Laser Support System $138

Electronics Box $60

Platform $14

Total $14,693

Conclusion

Successful system designed to measure true thickness of a silicon wafer

Design can be immediately implemented upon completion Further programming needed to add Statistical Process Control

graphs as well as graphical representation of warp Instructions and materials provided for duplicate systems in

other GE applications

Working System

Questions?

Subtraction Method

Computer Display