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Posts Tagged ‘simulation’

Elusive Analog Fault Simulation Finally Grasped

Tuesday, September 27th, 2016

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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

3DIC Technology Drivers and Roadmaps

Monday, June 22nd, 2015

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By Ed Korczynski, Sr. Technical Editor

After 15 years of targeted R&D, through-silicon via (TSV) formation technology has been established for various applications. Figure 1 shows that there are now detailed roadmaps for different types of 3-dimensional (3D) ICs well established in industry—first-order segmentation based on the wiring-level/partitioning—with all of the unit-processes and integration needed for reliable functionality shown. Using block-to-block integration with 5 micron lines at leading international IC foundries such as GlobalFoundries, systems stacking logic and memory such as the Hybrid Memory Cube (HMC) are now in production.

Fig. 1: Today’s 3D technology landscape segmented by wiring-level, showing cross-sections of typical 2-tier circuit stacks, and indicating planned reductions in contact pitches. (Source: imec)

“There are interposers for high-end complex SOC design with good yield,” informed Eric Beyne, Scientific Director Advanced Packaging & Interconnect for imec in an exclusive interview with Solid State Technology. ““For a systems company, once you’ve made the decision to go 3D there’s no way back,” said Beyne. “If you need high-bandwidth memory, for example, then you’re committed to some sort of 3D. The process is happening today.” Beyne is scheduled to talk about 3D technology driven by 3D application requirements in the imec Technology Forum to be held July 13 in San Francisco.

Adaptation of TSV for stacking of components into a complete functional system is key to high-volume demand. Phil Garrou, packaging technologist and SemiMD blogger, reported from the recent ConFab that Hynix is readying a second generation of high-bandwidth memory (HBM 2) for use in high performance computing (HPC) such as graphics, with products already announced like Pascal from Nvidia and Greenland from AMD.

For a normalized 1 cm2 of silicon area, wide-IO memory needs 1600 signal pins (not counting additional power and ground pins) so several thousand TSV are needed for high-performance stacked DRAM today, while in more advanced memory architectures it could go up by another factor of 10. For wide-IO HVM-2 (or Wide-IO2) the silicon consumed by IO circuitry is maybe 6 cm2 today, such that a 3D stack with shorter vertical connections would eliminate many of the drivers on the chip and would allow scaling of the micro-bumps to perhaps save a total of 4 cm2 in silicon area. 3D stacks provide such trade-offs between design and performance, so the best results are predicted for 3DICs where the partitioning can be re-done at the gate or transistor level. For example, a modern 8-core microprocessor could have over 50% of the silicon area consumed by L3-cache-memory and IO circuitry, and moving from 2D to 3D would reduce total wire-lengths and interconnect power consumptions by >50%.

There are inherent thresholds based on the High:Width ratio (H:W) that determine costs and challenges in process integration of TSV:

-    10:1 ratio is the limit for the use of relatively inexpensive physical vapor deposition (PVD) for the Cu barrier/seed (B/S),

-    20:1 ratio is the limit for the use of atomic-layer deposition (ALD) for B/S and electroless deposition (ELD) for Cu fill with 1.5 x 30 micron vias on the roadmap for the far future,

-    30:1 ratio and greater is unproven as manufacturable, though novel deposition technologies continue to be explored.

TSV Processing Results

The researchers at imec have evaluated different ways of connecting TSV to underlying silicon, and have determined that direct connections to micro-bumps are inherently superior to use of any re-distribution layer (RDL) metal. Consequently, there is renewed effort on scaling of micro-bump pitches to be able to match up with TSV. The standard minimum micro-bump pitch today of 40 micron has been shrunk to 20, and imec is now working on 10 micron with plans to go to 5 micron. While it may not help with TSV connections, an RDL layer may still be needed in the final stack and the Cu metal over-burden from TSV filling has been shown by imec to be sufficiently reproducible to be used as the RDL metal. The silicon surface area covered by TSV today is a few percents not 10s of percents, since the wiring level is global or semi-global.

Regarding the trade-offs between die-to-wafer (D2W) and wafer-to-wafer (W2W) stacking, D2W seems advantageous for most near-term solutions because of easier design and superior yield. D2W design is easier because the top die can be arbitrarily smaller silicon, instead of the identically sized chips needed in W2W stacks. Assuming the same defectivity levels in stacking, D2W yield will almost always be superior to W2W because of the ability to use strictly known-good-die. Still, there are high-density integration concepts out on the horizon that call for W2W stacking. Monolithic 3D (M3D) integration using re-grown active silicon instead of TSV may still be used in the future, but design and yield issues will be at least comparable to those of W2W stacking.

Beyne mentioned that during the recent ECTC 2015, EV Group showed impressive 250nm overlay accuracy on 450mm wafers, proving that W2W alignment at the next wafer size will be sufficient for 3D stacking. Beyne is also excited by the fact the at this year’s ECTC there was, “strong interest in thermo-compression bonding, with 18 papers from leading companies. It’s something that we’ve been working on for many years for die-to-wafer stacking, while people had mistakenly thought that it might be too slow or too expensive.”

Thermal issues for high-performance circuitry remain a potential issue for 3D stacking, particularly when working with finFETs. In 2D transistors the excellent thermal conductivity of the underlying silicon crystal acts like a built-in heat-sink to diffuse heat away from active regions. However, when 3D finFETs protrude from the silicon surface the main path for thermal dissipation is through the metal lines of the local interconnect stack, and so finFETs in general and stacks of finFETs in particular tend to induce more electro-migration (EM) failures in copper interconnects compared to 2D devices built on bulk silicon.

3D Designs and Cost Modeling

At a recent North California Chapter of the American Vacuum Society (NCCAVS) PAG-CMPUG-TFUG Joint Users Group Meeting discussing 3D chip technology held at Semi Global Headquarters in San Jose, Jun-Ho Choy of Mentor Graphics Corp. presented on “Electromigration Simulation Flow For Chip-Scale Parametric Failure Analysis.” Figure 2 shows the results from use of a physics-based model for temperature- and residual-stress-aware void nucleation and growth. Mentor has identified new failure mechanisms in TSV that are based on coefficient of thermal expansion (CTE) mismatch stresses. Large stresses can develop in lines near TSV during subsequent thermal processing, and the stress levels are layout dependent. In the worst cases the combined total stress can exceed the critical level required for void nucleation before any electrical stressing is applied. During electrical stress, EM voids were observed to initially nucleate under the TSV centers at the landing-pad interfaces even though these are the locations of minimal current-crowding, which requires proper modeling of CTE-mismatch induced stresses to explain.

Fig. 2: Calibration of an Electronic Design Automation (EDA) tool allows for accurate prediction of transistor performance depending on distance from a TSV. (Source: Mentor Graphics)

Planned for July 16, 2015 at SEMICON West in San Francisco, a presentation on “3DIC Technology Past, Present and Future” will be part of one of the side Semiconductor Technology Sessions (STS). Ramakanth Alapati, Director of Packaging Strategy and Marketing, GLOBALFOUNDRIES, will discuss the underlying economic, supply chain and technology factors that will drive productization of 3DIC technology as we know it today. Key to understanding the dynamic of technology adaptation is using performance/$ as a metric.