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Copyright 2012, AgrawalLecture 12: Alternate Test1 VLSI Testing Lecture 12: Alternate Test Dr. Vishwani D. Agrawal James J. Danaher Professor of Electrical and Computer Engineering Auburn University, Alabama 36849, USA vagrawal@eng.auburn.edu http://www.eng.auburn.edu/~vagrawal IIT Delhi, July 30, 2012, 4:00-5:00PM
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Copyright 2012, AgrawalLecture 12: Alternate Test2 Contents Setting thresholds in model-based test Alternate test Summary References
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Setting Thresholds in Model-Based Test In model-based test, component values are determined. Preset “thresholds” for component variation classify the device under test as good or faulty. How do we determine the “thresholds”? For example, Circuit is good if R1’ ≤ R1 ≤ R1’’ Copyright 2012, AgrawalLecture 12: Alternate Test3
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An Operational Amplifier Copyright 2012, AgrawalLecture 12: Alternate Test4 Gain = V2/V1 = R2/R1 V1 V2 R1 R2 +_+_
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Pessimism in Model- Based Test Copyright 2012, AgrawalLecture 12: Alternate Test5 0 R1 R2 Only good devices accepted Yield loss Slope = G 0
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Reducing Yield Loss Copyright 2012, AgrawalLecture 12: Alternate Test6 0R1 R2 Faulty devices accepted Reduced yield loss 0 Slope = G
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Yield Loss and Defect Level Yield loss: Amount of yield reduction because some good devices fail non-functional tests. Defect level (DL): Fraction of faulty devices among those that pass non-functional tests. Example: 1,0000 devices are fabricated. 7,000 are good. True yield, y = 0.7. Test passes 6,900 good and 150 bad devices. Then, Yield loss = (7,000 – 6,900)/10,000 = 0.01 or 1% DL = 150/(6,900+150) = 0.02128 or 2.128% or 21,280 DPM (defective parts per million) Copyright 2012, AgrawalLecture 12: Alternate Test7
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Yield Loss and Defect Level Copyright 2012, AgrawalLecture 12: Alternate Test8 All fabricated devices Good devices Yield loss Defect level Devices passing test
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Component Variation (Statistical) Copyright 2012, AgrawalLecture 12: Alternate Test9 Component (R or C) value Mean Component (R or C) value Mean UniformGaussian
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Monte Carlo Simulation Consider operational amplifier example. R1 and R2 are random variables with given (uniform or Gaussian) probability density functions with Mean = nominal value Standard deviation based on manufacturing data Generate large number of samples for R1 and R2 Simulate each sample using spice Determine gain for each sample For each set of tolerance limits, determine yield loss and defect level. Copyright 2012, AgrawalLecture 12: Alternate Test10
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Monte Carlo Simulation Data Copyright 2012, AgrawalLecture 12: Alternate Test11 R1 0 R2 0 Slope = G
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Setting Test Limits Copyright 2012, AgrawalLecture 12: Alternate Test12 0R1 R2 Minimize yield loss Minimize defect level 0 Slope = G
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Alternate Test Besides components (e.g., R1 and R2 for operational amplifier) easily measurable parameters used for testing. An example is the supply current IDD of the operational amplifier. A simple test is to measure IDD(0) for 0V input. Monte Carlo simulation is then used to set the limits on IDD(0). Large number of sample circuits with component variations are simulated to determine thresholds for IDD(0). Additional measurements can improve test. Copyright 2012, AgrawalLecture 12: Alternate Test13
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Alternate Test: Setting Thresholds Copyright 2012, AgrawalLecture 12: Alternate Test14 0 IDD(0) Gain Minimize yield loss Minimize defect level 0 Within spec. gain FailPassFail
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References P. N. Variyam, S. Cherubal and A. Chatterjee, “Prediction of Analog Performance Parameters Using Fast Transient Testing,” IEEE Trans. Computer-Aided Design, vol. 21, no. 3, pp. 349- 361, March 2002. H.-G. Stratigopoulos and Y. Makris, “Error Moderation in Low-Cost Machine-Learning- Based Analog/RF Testing,” IEEE Trans. Computer-Aided Design, vol. 27, no. 2, pp. 339- 351, February 2008. Copyright 2012, AgrawalLecture 12: Alternate Test15
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