1ECE 269Krish Chakrabarty1ECE 269VLSI System TestingKrish ChakrabartyMemory TestingECE 269Krish Chakrabarty2Density TrendsDensity Trends• 1970 -- DRAM Invention (Intel) 1024 bits• 1993 -- 1st 256 MBit DRAM papers• 1997 -- 1st 256 MBit DRAM samples– 1 /bit --> 120 X 10-6 /bit¢¢2ECE 269Krish Chakrabarty3Memory Cells Per ChipMemory Cells Per ChipECE 269Krish Chakrabarty4Test Time in Seconds(Memory Size n Bits)Test Time in Seconds(Memory Size n Bits)n1 Mb4 Mb16 Mb64 Mb256 Mb1 Gb2 Gbn0.060.251.014.0316.1164.43128.9n log2n1.265.5424.16104.7451.01932.83994.4n3/264.5515.41.2 hr9.2 hr73.3 hr586.4 hr1658.6 hrn218.3 hr293.2 hr4691.3 hr75060.0 hr1200959.9 hr19215358.4 hr76861433.7 hrSize Number of Test Algorithm Operations3ECE 269Krish Chakrabarty5Outline• RAM structure• Fault models– Opens, shorts and stuck-at faults– Address decoder faults (inaccessible cells or multiple cell accesses)– Coupling and pattern-sensitive faults (interactions between neighboring cells)• Standard RAM tests (March tests)– Finite sequence of March elementts, i.e. finite sequence of operations applied to every cell before proceeding to next cell– Checkerboard, Walking 1s and 0s, Galloping pattern of 1s and 0s (GALPAT)Reference: A. J. Van de Goor, “Testing Semiconductor Memories:Theory and Practice”, John Wiley, 1991.ECE 269Krish Chakrabarty6RAM StructureRow decodersStorage arraySense amplifierData bufferColumn decoderData In/OutAddressbufferRefreshControl(DRAM)AddressControl4ECE 269Krish Chakrabarty7Fault Models• Fault models are technology-dependent• Stuck-at fault (SAF)– A cell is always in state 0 (s-a-0) or state 1 (s-a-1)• Transition fault (TF)– Special case of SAF– Cell unable to make 0→1 transition when 1 is written to it (up transition fault), symbol: <↑/ 0>– Cell unable to make 1→0 transition when 0 is written to it (down transition fault), symbol: <↓/ 1>ECE 269Krish Chakrabarty8Fault ModelsS1S0State diagram of fault-free cellw0 w1w0w1S0w0w1State diagram of s-a-0 cellS1w0w1State diagram of s-a-1 cellS1S0State diagram of cell with <↑/ 0> faultw0 w1w0w15ECE 269Krish Chakrabarty9Fault Models• Coupling faults– Used mostly for dense DRAMs– Cells are close to each other hence interactions are more likely– Assumptions: – Read operation will not cause an error (in absence of faults in address decoder)– Non-transition write will not cause an error– 2-coupling faults: A write operation which generates ↑or ↓ transition in one cell changes contents of a second cellECE 269Krish Chakrabarty10Coupling Faults (CF)• Inversion coupling fault–An ↑or ↓ transition in one cell (coupling cells) inverts the contents of a second cell (coupled cell)• Idempotent coupling fault–An ↑or ↓ transition in one cell (coupling cells) forces the contents of a second cell (coupled cell) to a certain value (0 or 1).6ECE 269Krish Chakrabarty11Coupling FaultsS01S00State diagram of two fault-free cellsw0/i, w0/jw0/i, w1/jw0/jw1/jS11S10w1/i, w0/jw1/i, w1/jw0/jw1/jw0/iw1/iw0/iw1/iState diagram of inversioncoupling fault??ECE 269Krish Chakrabarty12Coupling FaultsState diagram foridempotentcoupling fault(force to 0)??7ECE 269Krish Chakrabarty13Pattern Sensitive Faults (PSF)• The contents of a cell, or ability to change the contents of a cell, is influenced by contents of other cells• PSF is the general case of k-coupling faultBase cellNeighborhood cellECE 269Krish Chakrabarty14Zero-One Tests• Write (and read) 1s and 0s to every cell• Also called Memory Scan (MSCAN)• Length of test: 4. n = O(n)• Detects only SAFs, not all TFs, CFs or PSFsMemoryaddressStep12 43w0 r0w1 r18ECE 269Krish Chakrabarty15Checkerboard Tests• Write 1 in all odd cells and 0 in all even cells• Read and verify• Write 0 in all odd cells and 1 in all even cells• Read and verify• Test length = 4. n = O(n)oddoddoddodd oddevenevenevenevenevenevenevenevenoddodd odd00000001111111100011001100110011ECE 269Krish Chakrabarty16Walking 1s and 0sWrite 0 to every cell;for i = 0 to n-1 doWrite 1 in test cell Ci;Read and verify all Cj≠ Ci;Read Ci. Write 0 in test cell Ci;i := i+1; Repeat with 0 and 1 switched• Total number of steps = 2(n + n(1 + n-1 + 2)) = 2n2+ 6n = O(n2)• Detects all TFs and CFs• GALPAT: same as walking 1s and 0s, except that each Read Cjis followed by Read Ci. • Total number of steps = 2(n + n(1 + 2(n-1) + 2)) = 4n2+ 4n =
View Full Document
Unlocking...