# "Zero" Defect IC Quality - Auburn University

Semiconductor

Nov 1, 2013 (4 years and 8 months ago)

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Outlier Screening For “Zero” Defect
IC Quality

Electrical & Computer Engineering

Auburn University

05/18/01 V4.3

Outline

Many integrated circuits contain
fabrication defects upon manufacture

Die yields may only be 20
-
50% for high
end circuits

ICs must be carefully tested to screen out
faulty parts before integration in systems

Small “latent” defects that cause
early life
failure

must also be eliminated

problem

IC Testing is a Difficult Problem

Need 2
3
= 8 input patterns to
exhaustively test a 3
-
input NAND

2
N

tests needed for N
-
input circuit

Many ICs have > 100 inputs

Only a very few input combinations
can be applied in practice

2
100

= 1.27 x 10
30

Applying 10
30

tests at 10
9

per second (1 GHZ)
will require

10
21

secs = 400 billion centuries!

3
-
input NAND

IC Testing in Practice

For high
-
end ICs

20
-
100 seconds of test time on very expensive
production testers

Several thousand test patterns applied

Test patterns chosen to detect likely faults

High economic impact

-
total test costs are approaching total

manufacturing costs

Despite the costs, testing is imperfect

How well must we test?

Approximate order
-
of
-
magnitude estimates

Number of parts per typical system: 100

Acceptable system defect rate: 1% (1 per 100)

Therefore, required part reliability

1 defect in 10,000

100 Defects Per Million (100 DPM)

Requirement ~100 DPM for commercial ICs

~500 DPM for ASICs

How well must we test?

Assume 2 million ICs manufactured with 50% yield

1 million GOOD >> shipped

1 million BAD >> test escapes cause defective

parts to be shipped

For 100 BAD parts in 1M shipped (DPM=100)

Test must detect 999,900

For 100 DPM: Needed Test Coverage = 99.99%

DPM Depends on incoming Yield

Test Coverage: 99.99% (Escapes 100 per million defective)

1 Million Parts @ 10% Yield

0.1 million GOOD >> shipped

0.9 million BAD >> 90 test escapes

DPM = 90 /0.1 = 900

1 Million Parts @ 90% Yield

0.9 million GOOD >> shipped

0.1 million BAD >> 10 test escapes

DPM = 10/0.9 = 11

Defect Clustering on Wafers

Defects on semiconductor
wafers are not uniformly
distributed but are
clustered

Local regions of low yield
can give high DPM

x

x

x

x

x

x

Good die from “Bad Neighborhoods” must be
more carefully tested to ensure no test escapes

x

x

IC Reliability: Early Life Failures

Manufacturing defects cause ICs to fail the
production test
-

“Killer” defects

-

ICs also experience significant early life or
“infant mortality” failures

Reliability problem

Infant mortality results from
“Latent” defects

-

manufacturing flaws undetectable at initial test

Defect Types

Killer Defect

Latent Defect

Resistive open due to unfilled via causing a
TDF [
Madge et al, IEEE D&T 2003
]

A Resistive Via with an unfilled Void

Stress Testing

Infant mortality results from latent
manufacturing flaws that are undetectable
at initial wafer probe testing

Tested using accelerated life cycle or
stress tests

Burn
-
in tests exercise circuits at elevated
voltages and temperatures for a few hours
up to a few days in temperature controlled
burn
-
in “ovens”

Burn
-
in is Expensive

High end circuits have nanometer feature
sizes and operate on low voltages

Stress voltages and temperatures must be
carefully (individually) controlled to avoid
damaging the circuits >> expensive ovens

Needed burn
-
in times are growing because
voltage/temperature stress levels can only
be marginally increased from the nominal

Most ICs today cannot afford Burn
-
in!

Long warrantees ( ~ 4 years)

High warrantee repair costs (\$1000)

Large number of parts per auto

High volumes ( ~30 million US sales)

Requirement:

< 10 DPM

“Zero Defects” Quality

New Automotive Reliability Specs

Motivated by

Focus: DPM Due to Latents

Assume Latent Defect Density

-

1% Killer Defect Density

Average No. (per die)

Killer Defects
(
l
)

1.0 0.5 0.1 0.01

Latent Defects
0.01 0.005 0.001 0.0001

Die Yield (e
-
l
)
37% 60% 90% 99%

Probability of Latent
0.01 0.005 0.001 0.0001

PPM Latents

10,000 5,000 1,000 100

Poisson Yield Model

Assume Latent Defect Density

-

1% Killer Defect Density

Die Yield (e
-
l
)
37% 60% 90% 99%

Probability of Latent
0.01 0.005 0.001 0.0001

PPM Latents

10,000 5,000 1,000 100

DPM from Latents:

Digital

10,000 5,000 1,000 100

Analog
(Latents 0.1%?) 1,000 500 100 10

Analog Parts

Reliability Screening

suspicion

without proof of functional error

Excessive
yield loss

*
100
-
1000X overkill

because there is no

test information from the die itself

“Outlier” detection methods can screen out
potential reliability failures with less overkill

-

but at higher test cost

Exploiting parameter correlation
“outlier" screening

Key Idea:

Analog circuit performance measures within die
or between nearby die on the wafer should be
correlated because of parameter matching

Any anomalies, even if within functional
specifications, indicate a defect which could be
a test escape and fail in the field, or result in a
reliability problem

Exploiting parameter correlation
for Reliability Screening

Application to

-

Digital Delay Testing

-

Analog Testing

Looks for an “outlier” electrical signature of a
latent defect

Timing Tests

Two
-
pattern test vectors <V
1
V
2
> cause a change at
the outputs

Switching delay

is the time from the application
(launch) of V
2

until change at the output

Worst case

switching delay < clock period

V
1

V
2

Limitation of Testing for Timing
Fails at Functional Clock Rate

Timing margins to allow for parameter
variations, clock skew, variations in test
conditions can make “small” defects
undetectable.

<V
1
V
2
>

critical path

Timing Margin

minVDD Testing

minVDD Testing finds the lowest VDD for
which the circuit passes a transition delay
fault (TDF) test for a given clock speed

An abnormal minVDD value with respect to
the expected value for the lot/neighborhood
indicates a defect that may be a test escape
or reliability failure

minVDD Testing

minVDD is found by repeatedly running the
test vectors at different VDD voltages and
performing a binary search until the failing
voltage is identified within desired accuracy

Since binary searches on full vector sets can
be expensive, methods have been developed
to work with reduced test sets
.

MinVDD Test

Reducing VDD slows the gates and
increases circuit delay until the circuit fails
at the rated clock

A delay defect can make it fail earlier,
exposing hidden “latent” defects

<V
1
V
2
>

critical path

Timing Margin

MinVDD vs Device Speed

Two different lots showing min VDD outliers and lot
-
to
-
lot intrinsic variation.

Minimum VDD results for different functional tests
clearly showing min VDD outliers (circled)

minVDD Testing

Applying Outlier Screening to
Analog Parts

“Outliers Screening with Multiple
-
Parameter Correlation Testing for
Analogue ICs”

Liguan Fang, Mohammed Lemnawar and Yizi Xing

Philips Semiconductors

Nijmegen, The Netherlands

European Test Symposium, Southampton May 2006

Outliers Screening with Multiple
-
Parameter Correlation Testing for
Analogue ICs

Goal:

Achieve zero
-
defect product quality in
automotive application through outlier
screening

In Vehicle Network Communication IC

(interface between the protocol controller and
the physical bus)

How
Multiple
-
Parameter Correlation
Testing

Works for Analog ICs

Specifications can have wide limits to allow for
normal process based parameter variations

However parameters within the same device
track

One test result can often be more tightly
predicted based on others for the same part

-

Two identical channels have matched gain

-

Blocks using the same core amplifier layout

should have measured gain reflecting the

feedback ratios

How
Multiple
-
Parameter Correlation
Testing

Works

Block Diagram of the test Vehicle

In Vehicle Network Communication IC

Correlation in transmitter performance
in two modes (A+B and A+C)

FA indicates 10% of field returns caused by

particle defects at C4 and C5

Correlated Tests 540 and 550

Test 540: Blocks A and C active

Test 550: Blocks A and B active

New Test 555

Test 555 = Test 550
-

0.35 * Test 540 + 57.94

Mean value (from data) 0; standard deviation 2.2

6 Sigma Test limits for Test 555 = [
-
13.2, 13.2]

Verification based on historic data logs

One 6
-
sigma New Test 555 outlier out of 160K
devices tested and passed over 4 months

Test introduced in Production

Six screened parts in Batch A, mostly from
expected wafer locations: near wafer edge or near
other failed die

Defects observable in 4 out of 6 cases using only
visual inspection with microscope

Defect Visualization

Defect Visualization

Defect Visualization

Reduction Trend in Customer Returns

Conclusions

(Philips)

Virtually all the “Outliers” could be traced to
physical defects suggesting the potential for
reliability failure and customer returns

Reduction is customer returns since
introducing the test is strong evidence of its
effectiveness

The correlation test needs only minimum post
data calculation and no extra measurement

The extra test time was less than 1ms

Overall very low cost and effective approach

European Test Symposium, Southampton U.K.

May 2006

Auburn University

Outlier Screening

What is screening?

Discarding some “suspect” die without conclusive
evidence that they will fail in operation

Based on:

“company you keep”

Outliers: Behave differently in some way

“something doesn’t look right”

Basis for Screening

Profiling: “company you keep”

Outliers: “something doesn’t look right”

Airport Security “Screens”

because exhaustive testing

-

“strip searching and x
-
raying”
-

every airline passenger is cost prohibitive

-
off!

What happens next?

-

(bumped off the flight)

Suspects are tested further

Test Optimization

-

saves high cost testing of exhaustive

testing of every passenger

-

Appropriate tests are applied

depending on “what looks different”

Extreme cases!

More generally

Questions?