Stepper Fault Detection in TI’s Freising Fab

Gernot Biese

Overview• Fault Detection Software: TIMS

•Method of Data collection for steppers

•Data analysis to detect errors

• Some Examples

• Side‐Effects: Usage of data to analyze and improve stepper‐throughput

Introduction to TIMS‐ Tool Interdiction Monitoring System ‐

Data CollectionAnalogDigitalSECS

Sensor

Time- seriesData and events

Historical/ Statistical Data

Real-Time Data

Equipment

Real-Time Analysis

Email

Lot Hold

Alarm log

Warning

Interdiction

SPC Chart

Notificationand Interdictionexecution

Helmut Lackermeier, Make IT

Main Benefits of TIMS

• Scrap reduction

•Optimized equipment utilization

• Improved factory efficiency

• Enhanced process and machine knowledge

•Applicable for almost all type of machines

•Rapid and easy development of process and equipment monitor models

Helmut Lackermeier, Make IT

TIMS: Most interesting for Engineers…•Model Editor: 

– Types of primitives: Boolean‐, String‐, Matrix‐, arithmetic operations, Timers/Inputs/Outputs

– Trigger for actions: Interdiction / Email / Lot‐Hold, etc.

TIMS: also interesting for Engineers…•Historical Viewer:Analysis of data‐sets in order to build new models

• 5 Misaligned wafers:– Detection possible

– Avoid such errors in the future

Steppers/scanners report important information to automation•Give data values more value by setting into a wider context

•Detect faults of stepper, which are not detected by the stepper itself.

• Compare the performance different of steppers of same type, e.g. alignment quality, throughput, assists.

Stepper Alignment procedure•Wafer Alignment (Pre‐Alignment): Define basic grid

– Search Area and detection of alignment marks

– Correction of waferRotation

– Define initial Coordinate system

•So far just ideas:– Use properties to judge Wafer‐Loaderrepeatability, etc.

x

y

Stepper Alignment procedure

• Fine‐Alignment: Define best grid to match previous layer

• Finetuning of coordinate system: minimize errors

• Derived properties:Xoffset, Yoffset

Xscale, Yscale

Ortho, RotationXresidual, Yresidual(higher order errorsor just higher variation)

• Use these properties to judgethe quality of pattern‐alignment !

Some real examples

Detection of half pitch error

• Some wafers withinlot fail…

•Misalignment is about 10µ

•Root cause: wrong detection of alignment marksCorrect:                                      Wrong: 

Solution: Outlier detection of residual error•Original TIMS Modelhad a constant limit,no wafers detected

•Outlier detection:– Exclude all pointsoutside mean+1sig

– New limit: mean+6sig’

– Check with overlaymeasure or visual insp.

• 4 bad wafers detectedre

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Detection by x/y‐offset•Again misalignment: 

•Residual 3Sigma does not indicate an error

• But yoffset exactly represents this error

Solution: Definition of Limits for X/Y‐offset• limit for |X(Y)OFF|+3Sig(X(Y)OFF)

Implementation in TIMS•Calculation of |MEAN|+3Sigma by composite model

• Limit of 2µm:

• Lot is put on hold and message is sent:

Enhance Detection for x/y‐offset•There were still cases, where residual error analysisand X/Y‐offset did not detect a bad wafer:

• Solution: Improved outlier detection– But much more complex, since TIMS has no feature to calculate outlier statistics. 

Comparison Outlier Detection•Only positive valuese.g. residual errors                                            

•positive & negative valuese.g. x/y‐offsets                         

•Calculation uses Matrix primitives

Detection of wafer scaling problems• Engineering analysis triggered by some lots with alignment and overlay problems:

TIME

Y-s

cale

3σ[a

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its] • Root Cause:Bad Lots had wafer scaling variations because of a bad cool plate in cluster TI01.

Y-s

cale

3σ

TI01 TI02 TI03 TI04

Detection of wafer scaling problems, cont’ed• Action1 : Repair Coolplate• Action2 : Introduce new TIMS model

Wafer scaling seen on stepper

3σ

3σ

Reporting

Limit check

If 3σ of x-scale and y-scale exceeds a limit an email is sent and lot is put on hold

Data Logging and Analysis with TIMS

Purpose• Motivation: Detailed TIMS data are only stored short‐term and difficult to access.

• Use Lot specific data calculated in a Stepper TIMS‐model, transfer these into a database and link these data to external parameters.

Current Content of TIMS‐DB• Equipment, Lot‐No, Device, Technology…• Average time per wafer with and without prepare.• Alignment quality: Residual 3σ, Max, Avg,

#Excursion Wafersx/y‐shift, ortho, etc.

• The information can be linked to other databases. 

Feeding of TIMS Database

1. String with data entries is built.

2. String is written to the database at the end of lot‐process.

3. Each single property(e.g. avg_3σ) defines one DB‐entry.

4. One dataset consists of 13 single DB‐entries and can be easily extended.

Effect of simultaneous stepper Alignment marks on throughput and performance• Steppers need only one position 

(instead of two)to        get x,y information.

X

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X Y X

Effect on Throughput and alignment quality

• Throughput:  ~6% better               Residual error: 40% betterTi

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Using TIMS‐data for throughput Improvement

• Link external DB to TIMS‐DB

• Interesting Data: – Time per Wafer / Prepare‐time / Waiting time, etc.

– Resist‐/Stepper‐Program (Bake condition, Alignment‐type) 

Influence of Different Alignment types•Difference of Mean‐Median is an indication for Assist‐times (all times given as deviation from baseline)

• Jobs with TYP II marks runsignificantly worse

•Potential to increasecapacity by about 4%

[%] TYP I TYP IIMean 0 33.7Median -3.8 2.6Difference 3.8 31.1

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Comparison between steppers

• Stepper 3 is significantlyslower compared to Stepper 1.

•Root cause was found to be stage travel speed.

• Potential to increasecapacity by about 5%

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# Alignment Shot Reduction•Reduce number of alignment shots from 8 to 6

•No negative impact to alignment quality.

• Potential to increase capacity by about 3%

Results of Throughput Actions

•Overall capacity increase of about 23%

Action Items

Reduction of  Assists

Speed up Stepper 3

Reduction of Alignment Shots

Reduction of  Assists

Optimization of  Maintenance Work

Optimization of Job parmameter

Reduction of waiting times fortemperature adjustment

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Summary•TIMS software is used for fault detection:

– Outlier wafers due to “residual alignment error”

– Outlier wafers due to “x/y offsets”

– “Temperature measurement” by observation of stepper’s wafer scaling.

•And for data‐tracking and analysis:– Stepper recipe optimization in terms of quality

– And Thoughput

– Comparison of stepper’s performance

• Overall Improvement of quality and wafer fab efficiency