New Graphical IDDQ Signatures
Reduce Defect Level
and Yield Loss
(U. S. Patent Pending)
Lan Rao
Michael L. Bushnell
Vishwani Agrawal
ECE Dept., Rutgers U., Piscataway, NJ
IDDQ Signatures
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IDDQ Testing
VDD = 5V
IDD
Ground
IDDQ
fault
Ref. Chakravarty and Thadikaran, 1997
IDDQ Signatures
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IDDQ (microA) vs. Vector #
A good chip
A bad chip
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Two Proposed Approaches:
New way #1



Use 135 IDDQ test vectors, instead of 10-20 vectors in
real production line, collect all current measurements
and plot current as function of test vector index.
Use classifier software running on Automatic Test
Equipment to classify chip as good or bad.
Without using ‘stuck-at fault’ voltage test, but keeping
the other voltage tests.
New way #2

Only use graphical IDDQ testing method defined above.
Potentially large cost saving for VLSI chip manufacturing test.
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Economics Analysis*
Method
# Measurements
Existing Current &
With New Graphical
Voltage Testing
Current Testing
8023(stuck-at)+53k
135(IDDQ)+15232(delay)
(functional)+15232(delay)
+53k(functional)
+ 20(IDDQ)
Use Only
Graphical Test
135(IDDQ)
Test time
14.55s
10.23 s
2.90s
Production test
cost
5 cents/s X 14.55s =
72.8cents
5 cents/s X 10.23s =
51.2cents
5 cents/sX2.9s=
14.5cents
Cost savings
0%
30.0%
80.1%
% Defective chips
found
98.30%
99.85 %
96.1%
Pattern gen.
Difficult
Simpler
Much simpler
Diagnosis
Complex
Complex
Complex
*With help from Dr. Phil Nigh, IBM.
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New Classification Features
The shape of the entire curve of current
measurements –
# of bands that measurements cluster
into.
Width and separation of bands.
Current glitch or smearing detection
among all IDDQ measurements.
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IIDDQ Distribution Over Vectors
is not Gaussian
# of IDDQ
values
1
Good
Devices
Bad
Devices
IDDQ Only
5684
1592
351
2
5674
2429
546
3
487
1144
104
4
12
367
31
>4
1
44
0
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Classification of DUT
Good devices
Devices passing all tests and all test steps.
 Devices that fail wafer probe test, but pass packaged
test and burn-in, (poor wafer probe registration).

Bad devices
Devices that failed the tests other than the IDDQ test.
 Devices with extremely high IDDQ current.

IDDQ only
• Devices that failed on the IDDQ test with less than the
absolute IDDQ threshold.
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Good vs. Faulty Chip
(Single Band)
Single band of a good chip
Smeared (noisy) single band of a
bad chip
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Good vs. Faulty Chip
(Single Band with Spike)
Good Chip Plot
Faulty Chip with Noise Spikes
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Good vs. Faulty Chip
(Multiple Bands)
Good 2 Band Chip
Faulty 2 Band Chip with Smearing
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Comparing IDDQ Test Methods
Statistics
(A)
Method
Singlethreshold
Test
Escape
(8)
Overkill
(8)
Test
escape
(25)
Overkill
(25)
Test
escape
(450)
Overkill
(450)
6.4
%
1.6%
6.8%
2.2%
7.5%
2.3%
Current
difference
35.3
%
3%
34%
3%
35%
3.1%
IDDQ
(4 A)
IDDQ
(ó =0.35)
Graphical
IDDQ
8.9
%
7.3
%
5.0
%
1.1%
8.6
0.8%
8.6%
1.0%
6.6%
7%
6.7%
7.6%
6.8%
-2.8%
5.4%
-2.7%
5.97%
-2.5%
Observation: The absolute threshold value is not critical for this
technique.
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Test Method Efficiencies
Statistics
Test Method
% Bad Chips Detected
8 A
450 A
IBM Functional Test
IBM Stuck-at Test
IBM Delay Test
IBM IDDQ Test
Graphical IDDQ Test
52.6 %
71.3 %
70.3 %
93.6 %
96.1 %
61.5%
83.4%
82.2%
93.5%
96.1%
IBM (functional+
stuck-at+delay)
75.8%
88.6%
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Test Vector Set Truncation Results
# of IDDQ vectors
Good devices out
of 11,858
Bad devices out
of 5,576
IDDQ only devices
pass out of 1,032
# 1 (125)
11,711
5,229
167
# 1 + # 2 (135)
11,715
5,240
165
# 1 + # 3 (185)
11,715
5,235
154
# 1 + # 2 + # 3 (195)
11,719
5,243
153
50 random
11,657
5,152
222
100 random
11,726
5,168
176
140 random
11,735
5,209
172
Proper selection of test vector set can further improve
the test quality.
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Conclusion
Gaussian distribution of IDDQ measurements may
not be true.
We present a new way of IDDQ testing with lower
test escape and lower yield loss (overkill rate).
The new method



does not rely on a single threshold or a single delta
threshold, but on the shape of the measurement set,
hence it is promising for deep submicron technology;
can replace at least some voltage tests;
the accuracy can be further improved by careful choice
of the test vector set.
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Thank you!
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