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VLSI Testing Lecture 12: Alternate Test

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1 VLSI Testing Lecture 12: Alternate Test
Dr. Vishwani D. Agrawal James J. Danaher Professor of Electrical and Computer Engineering Auburn University, Alabama 36849, USA IIT Delhi, Aug 24, 2013, 11:00AM-12:00PM Copyright 2012, Agrawal Lecture 12: Alternate Test

2 Lecture 12: Alternate Test
Contents Setting thresholds in model-based test Alternate test Summary References Copyright 2012, Agrawal Lecture 12: Alternate Test

3 Setting Thresholds in Model-Based Test
In model-based test, component values are tested. 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, Agrawal Lecture 12: Alternate Test

4 An Operational Amplifier
+ _ Gain = V2/V1 = R2/R1 V1 V2 Copyright 2012, Agrawal Lecture 12: Alternate Test

5 Pessimism in Model-Based Test
Slope = G Yield loss R2 Only good devices accepted R1 Copyright 2012, Agrawal Lecture 12: Alternate Test

6 Lecture 12: Alternate Test
Reducing Yield Loss Slope = G Reduced yield loss R2 Faulty devices accepted R1 Copyright 2012, Agrawal Lecture 12: Alternate Test

7 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, ) = or 2.128% or 21,280 DPM (defective parts per million) Copyright 2012, Agrawal Lecture 12: Alternate Test

8 Yield Loss and Defect Level
All fabricated devices Devices passing test Good devices Defect level Yield loss Copyright 2012, Agrawal Lecture 12: Alternate Test

9 Component Variation (Statistical)
Uniform Gaussian Mean Mean Component (R or C) value Component (R or C) value Copyright 2012, Agrawal Lecture 12: Alternate Test

10 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 on R1 and R2, determine yield loss and defect level. Copyright 2012, Agrawal Lecture 12: Alternate Test

11 Monte Carlo Simulation Data
Slope = G R2 R1 Copyright 2012, Agrawal Lecture 12: Alternate Test

12 Lecture 12: Alternate Test
Setting Test Limits Minimize yield loss Slope = G R2 Minimize defect level R1 Copyright 2012, Agrawal Lecture 12: Alternate Test

13 Lecture 12: 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, Agrawal Lecture 12: Alternate Test

14 Alternate Test: Setting Thresholds
Minimize yield loss Within spec. gain Gain Minimize defect level Fail Pass Fail IDD(0) Copyright 2012, Agrawal Lecture 12: Alternate Test

15 Lecture 12: Alternate Test
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 , 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 , February 2008. Copyright 2012, Agrawal Lecture 12: Alternate Test


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