HIGH-SPEED VLSI TESTING WITH SLOW TEST EQUIPMENT

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HIGH-SPEED VLSI TESTING WITH SLOW TEST EQUIPMENT

• Vishwani D. Agrawal
• Agere Systems
• Processor Architectures and Compilers Research
• Murray Hill, NJ 07974
• va_at_agere.com
• http//cm.bell-labs.com/cm/cs/who/va
• January 16, 2002

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MEANING OF DELAY TEST
V1 V2
Transient region
Inputs
Combinational logic
Outputs
Flip- flop
Time
Clock period
Inputs and outputs synchronized with clock
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PROBLEM STATEMENT

• Available automatic test equipment (ATE) speed is
  100-200MHz VLSI chip speed is 0.5-1GHz
• No coverage of delay faults is obtained when ATE
  applies vectors and samples outputs at slow clock
  rate
• A slow ATE can test delay faults in combinational
  circuits by skewing the output sampling times
• Skewed output sampling method tests very few
  (mostly PI to PO) paths in sequential circuits
• Problem Develop a delay test method for slow
  ATEs that will give similar path coverage as
  obtained with an at-speed ATE

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PREVIOUS WORK

• BIST (built-in self-test) with externally
  supplied high-speed clock (hardware overhead,
  non-functional paths tested)
• ATE pin multiplexing (limited vector capability)
• Reduced supply voltage, Wagner and McCluskey,
  ICCAD85 (may change critical paths, reduce noise
  margins)
• Latch designed to slow the circuit down in test
  mode, Agrawal and Chakraborty, US Patent
  5,606,567 (1997), ITC95 (needs special hardware,
  performance penalty)
• Fast clocking of flip-flops with slow vector
  application and slow output sampling, Krstic,
  Cheng and Chakradhar, VTS99 (low path coverage)

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A NEW METHOD

• Given a vector-set with specific at-speed PDF
  coverage
• Tester generates two clock signals
• Test-clock, N times slower than rated chip clock
  where N test-speed reduction factor
• Rated-clock, obtained by multiplexing N skewed
  test-clocks
• (a) Apply vectors at test-clock speed
• (b) Apply rated clock to flip-flops
• (c) Synchronize output sampling with test-clock,
  using a skew, s rated-clock period
• Repeat steps (a)-(c) with skew 2s, 3s, Ns
• Test application time (TAT) N 2 x (at-speed TAT)

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TEST APPLICATION
Speed reduction Factor, N 4
Test inputs
Primary inputs
FF clock
Output monitor strobes
Application 1
Application 2
Application 3
Application 4
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TESTING FOR FOUR TYPES OF PATHS
PO
PI
I
III
IV
II
Path Types I PI PO II FF FF III PI
FF IV FF PO
FF
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SOME PROPERTIES OF THE METHOD

• All types of paths can be tested
• Test application time (TAT) N 2 x (at-speed
  TAT)
• Coverage determined by simulation
• Path-specific test generation possible

Future detection
State c
i1/o3
i1/o5
Non- detection
i1/o1
i1/o2
i1/o3
State a
State b
State c
i1/o5
V1(i1,a)
State d
V2(i1,b)
i1/o4
State c
Fault detected
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SIMULATED PDF COVERAGE
s510 - 5,000 random vectors s5378 - 5,000 random
vectors
50
At-speed ATE
40
Slow ATE
30
PDF Coverage
20
10
1
2
3
4
Slowdown factor (N)
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A LAB EXPERIMENT

• Device CD4029B (Texas Instruments)
• Function 4 bit binary/decimal presettable
  up/down counter
• Package 16 pin DIP
• Gate count 103
• Flip-flop count 12
• I/O count 9/5
• Clock frequency 4MHz _at_5V
• Tests Fault coverage vectors from Gentest (90
  vectors)
• Path delay fault simulation for rated-speed
  operation and for high-speed test (Parodi et al.,
  ITC99)
• Tests performed by C. Parodi and J. David at
  Holmdel using HP 82000/400MHz ATE

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RESULTS OF CD4029B TEST
Three chips tested (A, B, and C)
Maximum all-test-pass clock-rate (MHz)
Vector application speed reduction factor, N
Chips
N4 (1/4 speed)
N1 (At-speed)
N2 (Half-speed)
A
3.922
4.367
3.937
B
4.367
4.167
4.167
4.132
4.115
4.367
C
Simulation showed that slow testing perhaps
activated paths that are longer than those
activated by at-speed testing.
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A VLSI CHIP EXPERIMENT

• BSM2 Chip Boundary Scan Master Version 2
  (Higgins and Srinivasan, VTS00)
• Agere 0.16 micron CMOS process
• 65MHz clock _at_1.5V
• Gate count 18,823 Flip-flop count 1,368 I/O
  count 34/34
• Production Tests
• 453,195 vectors, 96 coverage of stuck-at faults
• 164,578 tested path faults (total 400 million
  paths)
• Longest tested paths - 58 gates (longest physical
  path - 74 gates)
• Path delay fault simulation for rated-speed
  operation (Parodi et al., ITC99)
• Functional Vectors 68,608
• Rated-speed test fails above 85MHz
• ½-speed test fails above 53MHz
• ¼-speed test fails above 53MHz
• 1/8-speed test fails above 53MHz

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CONCLUSION

• It is possible to obtain same or higher PDF
  coverage with a slow ATE as with an at-speed ATE
• A slow test-clock is used for input application
  and output monitoring
• A rated-clock signal is applied to flip-flops a
  slow ATE can generate fast rated-clock by pin
  multiplexing
• Test application time (TAT) increases as square
  of speed reduction factor (N)
• TAT N 2 x V
• where V number of vectors
• (for variable clock testing, TAT N 2x V 2)
• Test application time can be reduced by test
  optimization
• Use PDF simulation
• Generate path-specific tests
• Proposed method only tests functional paths