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Elusive Analog Fault Simulation Finally Grasped

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By Stephen Sunter, Mentor Graphics

The test time per logic gate in ICs has greatly decreased in the last 20 years, thanks to scan-based design-for-test (DFT), automatic test pattern generation (ATPG) tools, and scan compression. But for analog circuits, test time per transistor has not decreased at all. And to make matters worse, the test time for the analog portion of an IC can dominate total test time. A new approach is needed for analog tests to achieve higher coverage in less time, or to improve defect tolerance.

Source: ON Semiconductor

Analog designers and test engineers do not have DFT tools comparable to those used by their digital counterparts. It has been difficult to improve the number of defective parts per million (DPPM) because it has been too challenging to measure defect coverage. These are typically measured by the rate of customer returns, which can occur months after the ICs are tested.

Analog fault simulation has only been discussed in academic papers and recently, in a few industrial papers that describe proprietary software. Why haven’t the analog fault simulation techniques described in all those papers led to commercially-available fault simulators that are used in industry? Mostly because there is no industry-accepted analog fault model and simulating all potential faults requires an impractically long time.

Potential Solutions for Reducing Simulation Time

Many methods for reducing simulation have been proposed over the years in published papers, including:

  • Simulate only shorts and opens in the schematic netlist without variations;
  • Analyze a circuit’s layout to find the shorts and opens that can actually occur (and the likelihood of those defects occurring);
  • Simulate only in the AC domain;
  • Simulate the sensitivities of each tested performance to variations in each circuit element;
  • Use a simplified, time domain simulation to measure the impact of injected shorts and opens on output signals, only within a few clock cycles;
  • Measure analog toggle coverage.

Even if these techniques were very efficient and reduced simulation time dramatically, the large number of defects simulated would mean that the number of undetected defects to diagnose would be large. For example, if there were 100,000 potential faults in a circuit and 90% were detected, there would be 10,000 undetected faults to investigate. Analyzing each defect is a very time-consuming task that requires detailed knowledge of the circuit and tests. Therefore, reducing the number of defects simulated can save a lot of time, in multiple ways. The methods to reduce the number of defects include:

  • Randomly select defects from a list of all potential defects;
  • Randomly select defects, after grouping them according to defect likelihoods;
  • Select only principal parameters of the circuit elements, such as voltage, gate length, width, and oxide thickness;
  • Select representative defects based on circuit analysis.

Potential Standard Analog Fault Models

Currently, there is no accepted analog fault model standard in the industry. Proposals such as simulating only short and open defects and simulating defective variations in circuit elements or in high-level models have been rejected. Because of the lack of a standard, a group of about a dozen companies (including Mentor Graphics) has been meeting regularly since mid-2014 to develop such a fault model. The group has reported their progress publicly several times, and hopes to develop an IEEE standard by 2018.

The Tessent DefectSim Solution

Tessent® DefectSim™ incorporates lessons learned from all previous approaches, combining the best aspects of each while avoiding their pitfalls. Simulation time is reduced using a variety of techniques that all together reduce total simulation time by many orders of magnitude compared to some of the previous approaches, without introducing a new simulator, reducing existing simulator accuracy, or restricting the types of tests. The analog defect models can be shorts and opens, just variations, or both. Or, users can substitute their own proprietary defect models. The defects can be injected at the schematic level, at the layout level, or a combination of both.

To be realistic, defects should be injected in a layout-extracted netlist. But higher-level netlist descriptions or hardware description language (HDL) models, such as Verilog-A or Verilog RTL, can reduce simulation time by one or two orders of magnitude. In practice, the highest level netlist of a subcircuit is often just its schematic; nevertheless, it typically simulates an order of magnitude faster than the layout-extracted netlist. DefectSim runs Eldo® when the circuit contains only SPICE and Verilog-A models, and Questa® ADMS™ when Verilog-AMS or RTL models are also used.

DefectSim introduces a new statistical technique called likelihood-weighted random sampling (LWRS) to minimize the number of defects to simulate. This new technique uses stratified random sampling in which each stratum contains only one defect. The likelihood of randomly selecting each defect is proportional to the likelihood of the defect occurring. Each likelihood of occurrence is computed based on designer-provided global parameters, and parameters of each circuit element.

For example, shorts are the most common. In state-of-the-art production processes, shorts are 3~10X more likely than opens. When the range of defect likelihoods is large, as it is for mixed-signal circuits, LWRS requires up to 75% fewer samples than simple random sampling (SRS) for a given confidence interval (the variation in an estimate that would occur if the random sampling was done many times). In practice, when coverage is 90% or higher, this means that it is usually sufficient to simulate a maximum 250 defects, regardless of the circuit size or the number of potential defects, to estimate coverage within 2.5%, for a 99% confidence level. Simulating as few as one hundred defects is sufficient to get ±4% estimate precision. For small circuits, or when time permits, all defects can be simulated.

DefectSim allows you to combine almost all of the previously-published techniques for reducing simulation time, including random sampling, high-level modeling, stop-on-detection, AC mode, and parallel simulation. All together, these techniques can reduce simulation time by up to six orders of magnitude compared to simulating the production test of all potential defects in a flat, layout-extracted netlist. The same techniques can be applied to the measurement of defect tolerance.

For more information about Tessent DefectSim, read the whitepaper at:
https://www.mentor.com/products/silicon-yield/resources/overview/part-1-analog-fault-simulation-challenges-and-solutions-f9fd7248-3244-4bda-a7e5-5a19f81d7490?cmpid=10167

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