A Comparison of Wafer Retest Methods: In vs. Post sort ...

A Comparison of Wafer Retest Methods: In‐Situ vs. Post‐sort SelectiveMark Banke, Altera Corp, [email protected]

1Silicon Valley Test Conference 2010

Agenda

• What is Wafer Retest, and Why do We do It?• What are the Types of Wafer Retest?• Wafer Retest Attributes• Software Issues Involving Retest• Results from Altera Wafer Retest Data• Cost Savings Potential• Retest Yield Distribution• Rule Based Retest• Summary


What is Wafer Retest, and Why do We do it?• The semiconductor business is driven by yield:

• Testing doesn’t always yield the expected amount on the first pass.

• To reclaim possible false failures, wafers may be retested.– False Failure: Dice which fail on the first test, but pass on subsequent retest(s). Can be caused by test hardware, test program, wafer / probe contamination, etc.

• The result is money is reclaimed from the scrap bin.

3

Yield $

Silicon Valley Test Conference 2010

What are the Types of Wafer Retest?

4

•Whole Wafer Retest

•In-situ and Selective Wafer Retest

Whole Wafer: Retest every die after complete lot has finished test

In-situ: Retest only specific dice immediately after whole wafer test

Selective: Retest only specific dice after lot has finished test


Wafer Retest Attributes

• Whole Wafer:– Verifies all dice. May eliminate false passes and false fails due to test program

issues, setup error, hardware problems

– No additional test software – just retest the wafer and nullify past results

– Increases chance for bond pad damage

– Increases sort cycle time


Wafer Retest Attributes (continued)

• In‐situ:– Only retests failing dice (or specified dice based on whole wafer binning)

immediately upon whole wafer completion

– Saves on setup time, reduces setup error frequency, catches most of the false failures

– May not catch gross failures due to initial setup errors.

– May subject passing dice in multi‐site probing setup to more probe damage

– Requires more complicated software

6

These passing dice get extra probe

contact


Wafer Retest Attributes (continued)• Selective:

– More likely to catch gross failures due to setup.– Single site probe card less likely to cause damage to adjacent dice

– Effective for testing dice at non‐yielding steps– Increases number of setups, may increase setup errors– May increase test cost– Increase in test time – tester controlled prober indexing

– Possible increase in cycle time– Requires more complicated test software


Software Issues Involving Retest

• In‐Situ: Matching summaries. Total die qty passing on retest needs to be added to original passing die total, but total tested shouldn’t be incremented.

8

W1F W1I W2F W2I W3F W3I Grand TotalTotal

Tested XXX 65 XXX 70 XXX 48 XXXX

TotalPassed

YYY 3 YYY 2 YYY 0 YYYY

TotalFailed

65 62 70 68 48 48 361

Grand Total Tested = only sum of “F” Totals

Grand Total Passed= Sum of all Passing

Why? Need to keep accurate yield data!

Yield based on Total Passed / Total Tested (Full Wafer Dice)


Software Issues Involving Retest (continued)

• Selective: Summary data must be downloaded from server to tester so only specified failure dice are re‐tested

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W1 FS

W2 FS

W3 FS

Grand Total

TotalTested

XXX XXX XXX XXX

Total Passed YYY YYY YYY YYYTotalFailed

46 63 44 153

W1 SR

W2 SR

W3SR

Grand Total

TotalTested

46 63 44 153

TotalPassed

5 25 10 40

TotalFailed

41 38 34 113

Selective Retest Summary

Full Sort Summary

Final Yield = (Total Passed Full + Total Passed Selective) / Total Tested Full


Software Issues Involving Retest (continued)

Wafer OCR / Barcode reader must be used ‐ prevent wrong wafer associated with Retest data

Tester must gather summary data from network so it can direct prober to re‐test die locations.

10

TesterProber

Network Server


Which retest method provides the best compromise between efficiency and yield?

11

In-Situ Vs SelectiveIn‐Situ Selective


Results from Altera Wafer Sort Data

Total Wafers

Avg Retest Recovery Yield

Total Unique Wafers: 9428 ‐# In‐situ Recovery

"Winners" 5439 9.0%# Selective Recovery

"Winners"

1091 4.6%

No Die Difference 2898 ‐

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Over 6 Million Dice Tested


In‐Situ vs. Selective Retest :Difference of Die Yield Reclaimed

0.0%5.0%

10.0%15.0%20.0%25.0%30.0%35.0%40.0%

1.0%2.7% 4.1% 5.3%

7.9% 8.4%

27.4%

20.7%

35.6%

21.6%

13.3%

6.8%

0.4%

% o

f Waf

ers

Test

ed

% Reclaim Yield Difference by Retest Method



14

0%

5%

10%

15%

20%

25%

30%

35%

1% 1% 0% 1% 2%

7%

31% 31%

13%

6%

2% 2% 3%% o

f Waf

ersT

este

d

Difference in Dice Reclaimed by Retest MethodSilicon Valley Test Conference 2010

In‐Situ vs. Selective Retest :• So, looks like the winner is In‐situ…

• Upon “further review…”– It’s not necessarily the Retest method recovery difference, it’s the amount of dice actually recovered because of the Retest…

15

?


In‐Situ vs. Selective Retest :

Total Wafers

Avg Retest Recovery Yield

Additional Recovery Dice

Total Unique Wafers: 9428 ‐ ‐# In‐situ Recovery

"Winners" 5439 9.0% 50,014# Selective Recovery

"Winners"

1091 4.6% 21,199

No Die Difference 2898 ‐ ‐



0%

5%

10%

15%

20%

25%

30%

35%

1% 1% 0% 1% 2%

7%

31% 31%

13%

6%

2% 2% 3%% o

f Waf

ersT

este

d

Difference in Dice Reclaimed by Retest Method

What about these?


In‐Situ vs. Selective Retest : Difference of Die Yield Reclaimed – by Product Family

0%

10%

20%

30%

40%

50%

60%

1% 0% 0% 0% 1%4%

55%

30%

7%

2% 0% 0% 0%

1% 1% 1% 2% 3%

13%

19%

42%

12%

4% 2% 0% 0%% o

f Waf

ers

Test

ed

Difference in Dice Reclaimed by Retest Method

Low Cost Families Medium Cost Families High Cost Families


Cost Savings Potential

• Possible savings may accrue if the test cost model charges extra for selective retest setup time.


Cost of Setup Time vs # of Setups with 10 minute Average Setup Time

$0

$5,000

$10,000

$15,000

$20,000

$25,000

10 20 50 75 100 200 500 1000

Cos

t of S

elec

tive

Ret

est S

etup

Ti

me

# of Setups

$50.00 $75.00 $100.00 $125.00

20

Test Cost $ / Hr


Cost of Setup Time vs # of Setups with 10 minute Average Setup Time

$-

$2,000

$4,000

$6,000

$8,000

$10,000

$12,000

$14,000

$16,000

$18,000

$20,000

1 Unit 50 Units

100 Units

200 Units

300 Units

400 Units

500 Units

750 Units

1000 Units

Pote

ntia

l $ V

alue

of U

nits

not

R

ecov

ered

Number of Units Not Recovered

Number of Units vs $ Cost of Non-Recovered Part

$1 $5 $10 $35 $50 $100 $500

Selective Retest Setup Costs @ 500 Setups


Cost Savings Potential with In‐Situ Retest

# of Retest Units

22

Setup Cost

Unit Cost

IF….


Cost Savings Potential with In‐Situ Retest

• The results favored in‐situ Retest on average when average yield from first initial test was greater than certain yield thresholds, but selective sort proved necessary when problematic test setup conditions occurred.


In‐Situ Recovery Rate vs Increase over Yield Threshold

24

2% 1% 1% 3% 3%

2017

23

25

6

0

5

10

15

20

25

30

0%10%20%30%40%50%60%70%80%90%

100%

A B C D E

# of Dice R

ecoverd by Selective Retest

% o

f Waf

ers

Test

ed

PRODUCT

% Wafers with No Selective Resort Dice Recovered% Wafers with Selective Resort Dice Recovered% > Yield Threshold# of Lost Dice if Selective Resort Not Used


Retest Yield Distribution

• Insitu Retest does in fact, on average, increase yield vs selective Retest. But the data suggest that one pass Retest may not always be enough to capture all available yield.


Distribution of Additional Dice Recovered

0%

10%

20%

30%

40%

50%

60%

70%

0 < 5 5 to 9 >= 10

26% 25%

8% 9%

64%

24%

5% 8%

% o

f Tot

al W

afer

s Te

sted

Additional Dice Recovered

1st In-situ Resort AttemptAll Selective Resort Attempts


Distribution of Additional Dice Recovered Per Retest Attempt

0%5%

10%15%20%25%30%35%40%45%50%

0 < 5 5 to 9 >= 10

26% 25%

8% 9%

48%

16%

2% 2%

16%

7%3%

5%

% of Total W

afers Tested

Additional Dice Recovered

1st In‐situ Resort Attempt

1st Selective Resort Attempt

2nd Selective Resort Attempt

3rd Selective Resort Attempt


Rule Based Retest• In‐Situ Retest will recover the majority of false failures. But there are cases where Selective Retest should be used.– Rule Based Retest

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Type of Part Condition Action

Any 1st Pass Yield << Std(Bad Setup)

Selective Retest

Low Cost Die 1st Pass Low Yield In‐Situ Retest

Long Test Time Particular Failure Type Selective Retest on ParticularFailure Dice

High Cost Die 1st Pass In‐Situ Retest, thenSelective Retest on Sample Wafers. If yield improvement, Selective Retest on all wafers


Summary

• Both In‐Situ and Selective Retest Methods are Viable

• In‐Situ will likely Recover most invalid failures, usually on the first attempt

• Rule‐based Retest will likely provide the most cost effective solution


Thank you!

• Questions?


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