Effectiveness Measures for VLSI Testing: Defective Parts per Million, Defect Coverage and Fault Coverage

1
Effectiveness Measures for VLSI Testing
Defective Parts per Million, Defect Coverage and
Fault Coverage

• Vishwani D. Agrawal
• James J. Danaher Professor
• Department of Electrical and Computer Engineering
• Auburn University, Auburn, AL 36849
• http//www.eng.auburn.edu/vagrawal
• vagrawal_at_eng.auburn.edu

2
VLSI Chip Yield

• A manufacturing defect is a finite chip area with
  electrically malfunctioning circuitry caused by
  errors in the fabrication process.
• A chip with no manufacturing defect is called a
  good chip.
• Fraction (or percentage) of good chips produced
  in a manufacturing process is called the yield.
  Yield is denoted by symbol Y.
• Cost of a chip

Cost of fabricating and testing a
wafer ??????????????????? Yield Number of chip
sites on the wafer
3
Clustered VLSI Defects
Good chips
Faulty chips
Defects
Wafer
Clustered defects (VLSI) Wafer yield 17/22
0.77
Unclustered defects Wafer yield 12/22 0.55
4
Yield Parameters

• Defect density (d ) Average number of defects
  per unit of chip area
• Chip area (A )
• Clustering parameter (a)
• Negative binomial distribution of defects,
  p (x ) Prob(number of defects on a chip x )

G (a x ) (Ad / a) x ????? .
????????? x ! G (a) (1Ad / a) ax
where G is the gamma function a 0, p (x ) is a
delta function (max. clustering) a ? , p (x )
is Poisson distr. (no clustering, William/Brown)
5
Yield Equation
Y Prob( zero defect on a chip ) p (0)
Y ( 1 Ad / a ) a

Example Ad 1.0, a 0.5, Y 0.58
Unclustered defects a ?
, Y e Ad
Example Ad 1.0, a ?, Y 0.37
too pessimistic !
6
Defect Level or Reject Ratio

• Defect level (DL) is the ratio of faulty chips
  among the chips that pass tests.
• DL is measured as defective parts per million
  (dpm, or simply ppm).
• DL is a measure of the effectiveness of tests.
• DL is a quantitative measure of the manufactured
  product quality
• For commercial VLSI chips a DL higher than 500
  dpm is considered unacceptable.
• Chip manufacturers strive for much lower defect
  levels. Below 100 dpm means high quality.
• Zero-defect refers to 3.4 dpm or below.

7
Determination of DL

• From field return data Chips failing in the
  field are returned to the manufacturer. The
  number of returned chips normalized to one
  million chips shipped is the DL.
• From test data Fault coverage of tests and chip
  fallout rate are analyzed. A modified yield
  model is fitted to the fallout data to estimate
  the DL.

8
Modified Yield Equation

• Three parameters
• Fault density, f average number of stuck-at
  faults per unit chip area
• Fault clustering parameter, b
• Stuck-at fault coverage, T
• The modified yield equation

Y (T ) (1 TAf / ß) ß
Assuming that tests with 100 fault coverage (T
1.0) remove all faulty chips,
Y Y (1) (1 Af / ß) ß
9
Defect Level
Y (T ) - Y (1) DL (T ) ???????
Y (T ) ( ß TAf ) ß
1 ????????
( ß Af ) ß
Where T is the fault coverage of tests, Af is
the average number of faults on the chip of area
A, ß is the fault clustering parameter. Af and
ß are determined by test data analysis.
b ?
, Y (T ) e TAf and DL(T ) 1 Y (1)1 T
10
Example SEMATECH Chip

• Bus interface controller ASIC fabricated and
  tested at IBM, Burlington, Vermont
• 116,000 equivalent (2-input NAND) gates
• 304-pin package, 249 I/O
• Clock 40MHz, some parts 50MHz
• 0.8m CMOS, 3.3V, 9.4mm x 8.8mm area
• Full scan, 99.79 fault coverage
• Advantest 3381 ATE, 18,466 chips tested at 2.5MHz
  test clock
• Data obtained courtesy of Phil Nigh (IBM)

11
Test Coverage from Fault Simulator
Stuck-at fault coverage
Vector number, V
12
Measured Chip Fallout
Measured chip fallout
Vector number, V
13
Model Fitting
Unclustered faults 1 e TAf Af 0.31, ß ?
Y (1) 0.7348
Clustered faults 1 (1TAf/ß) ß Af 2.1, ß
0.083
Chip fallout and computed 1-Y (T )
Y (1) 0.7623
Measured chip fallout
Stuck-at fault coverage, T
14
Computed Defect Level
(1 0.7348)106
(1 0.7623)106
Unclustered faults, ß ?
Clustered faults, ß 0.083
Defect level (dpm)
Stuck-at fault coverage ()
15
Reexamine Assumption

• Assumption 100 fault coverage leads to zero
  defect level.
• Reality 100 defect coverage leads to zero
  defect level.
• Must examine the two coverages.

16
Fault vs. Defect Coverage
Fault coverage, T(V ) Defect
coverage, D(V )

• Coverage of stuck-at faults detected by
  vectors.
• Faults are countable.
• Alternative definition T (V ) Prob
  (detection by V vectors a fault is present)
• All faults assumed equally probable on a faulty
  chip.
• Determined theoretically.

• Coverage of real defects detected by vectors.
• Many types, large numbers.
• Alternative definition D (V ) Prob
  (detection by V vectors a defect is
  present)
• Each defect may have a different probability of
  occurrence.
• Determined experimentally.

17
Defect Coverage
D (V ) Prob (detection by V vectors chip is
defective) Prob (failure by V
vectors) ?????????????
1 Y (1) 1 Y (d )
????? 1 Y
(1) Measured yield, Y (d ), and estimated true
yield, Y (1), can provide a statistical estimate
for defect coverage. Source of inaccuracy true
yield, Y(1), is not known.
18
Defect and Fault Coverages
Defect coverage D(V ) from test data
Y(1) 0.7623
Fault coverage T(V ) from fault simulator
Coverage
Vector number (V )
19
Defect vs. Fault Coverage
D gt T
Defect coverage, D
D lt T
Fault coverage, T
20
Conclusion

• Defect coverage can be determined from the
  measured test data.
• Assumption
• Either, tests are capable of activating the
  defect (Q Can a delay defect be detected by
  slow-speed stuck-at fault tests?)
• Or, the real defect is clustered with faults
  detectable by the tests.
• The above assumption, DL 0 at f 100, may
  be justified since fault coverage appears to be
  more pessimistic than defect coverage.
• Defect coverage D (V ) is a transformation of
  test data
• Vector 0 ? coverage 0
• Vector ? ? coverage 100
• Unclustered fault assumption adds pessimism.

21
Future Directions

• Defect density, d, should not be confused with
  defect coverage, D (V )
• d number of defects per unit area
• D (V ) percentage of all possible defects
  detected by V vectors
• Analyze test data for yield, defect coverage and
  defect level without involving modeled faults.

Experiment
Y
Chip fallout fraction
Fraction of chips
Vectors, V
0
1.0
Prob(defect occurrence)
22
Directions . . .

• Diagnosis Defects do not conform to any single
  fault model.
• Question Which is better?
• 100 coverage for one fault model, or
• some coverage for multiple fault models

23
Directions . . .

• Generate tests for defect coverage and diagnosis.
• Question which is better?
• 100 stuck-at fault coverage, or
• 100 diagnostic coverage of stuck-at faults, or
• N-detect tests (longer tests), or
• Any of the above random vectors.

24
References

• The clustered fault model used for Sematech data
  is described in the book M. L. Bushnell and V.
  D. Agrawal, Essentials of Electronic Testing for
  Digital, Memory and Mixed-Signal VLSI Circuits,
  Springer, 2000, Chapter 3.
• The unclustered defect model is from the paper
  T. W. Williams and N. C. Brown, Defect Level as
  a Function of Fault Coverage, IEEE Trans.
  Computers, vol. C-30, no. 12, pp. 987-988, Dec.
  1981.
• The discussion on defect coverage is from a
  presentation J. T. de Sousa and V. D. Agrawal,
  An Experimental Study of Tester Yield and Defect
  Coverage, IEEE International Test Synthesis
  Workshop, Santa Barbara, California, March 2001.
• A direct analysis of defect level without
  involving the stuck-at fault coverage is given in
  the paper S. C. Seth and V. D. Agrawal, On the
  Probability of Fault Occurrence, Defect and
  Fault Tolerance in VLSI Systems, I. Koren,
  editor, Plenum Publishing Corp., 1989, pp. 47-52.