HIGH-SPEED VLSI TESTING WITH SLOW TEST EQUIPMENT Vishwani D. Agrawal Agere Systems Processor Architectures and Compilers Research Murray Hill, NJ January 16, 2002

Jan. 16, '02Agrawal: High-speed test2 MEANING OF DELAY TEST Combinational logic Transient region Clock period Time Inputs Outputs Inputs and outputs synchronized with clock Flip- flop V1 V2

Jan. 16, '02Agrawal: High-speed test3 PROBLEM STATEMENT  Available automatic test equipment (ATE) speed is MHz; 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

Jan. 16, '02Agrawal: High-speed test4 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, ICCAD’85 (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), ITC’95 (needs special hardware, performance penalty)  Fast clocking of flip-flops with slow vector application and slow output sampling, Krstic, Cheng and Chakradhar, VTS’99 (low path coverage)

Jan. 16, '02Agrawal: High-speed test5 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)

Jan. 16, '02Agrawal: High-speed test6 TEST APPLICATION Primary inputs FF clock Application 1 Application 2 Application 3 Application 4 Test inputs Speed reduction Factor, N = 4 Output monitor strobes

Jan. 16, '02Agrawal: High-speed test7 TESTING FOR FOUR TYPES OF PATHS FF POPI I II III IV Path Types: IPI PO IIFF FF IIIPI FF IVFF PO

Jan. 16, '02Agrawal: High-speed test8 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 State a State b State c State c’ i1/o1i1/o2 V1=(i1,a) V2=(i1,b) i1/o3 i1/o4 State c’’ i1/o3 Fault detected State d i1/o5 Future detection Non- detection

Jan. 16, '02Agrawal: High-speed test9 SIMULATED PDF COVERAGE 50% PDF Coverage 1 Slowdown factor (N) 40% 30% 20% 10% 234 s ,000 random vectors s ,000 random vectors At-speed ATE Slow ATE

Jan. 16, '02Agrawal: High-speed test10 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:  Tests: Fault coverage vectors from Gentest (90 vectors)  Path delay fault simulation for rated-speed operation and for high-speed test (Parodi et al., ITC’99)  Tests performed by C. Parodi and J. David at Holmdel using HP 82000/400MHz ATE

Jan. 16, '02Agrawal: High-speed test11 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 A B C N=1 (At-speed ) N=2 (Half-speed) N=4 (1/4 speed) Simulation showed that slow testing perhaps activated paths that are longer than those activated by at-speed testing.

Jan. 16, '02Agrawal: High-speed test12 A VLSI CHIP EXPERIMENT  BSM2 Chip: Boundary Scan Master Version 2 (Higgins and Srinivasan, VTS’00) Agere 0.16 micron CMOS process 65MHz 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., ITC’99)  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

Jan. 16, '02Agrawal: High-speed test13 CONCLUSIONCONCLUSION  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 2 x V 2 )  Test application time can be reduced by test optimization Use PDF simulation Generate path-specific tests  Proposed method only tests functional paths