Production Mode For The Virtual Wafer Fab

Introduction

The ability to derive a Response Surface Model (RSM) for SPICE model parameters based on process variables has been a dream of many technologists for several years. This capability has now arrived and is being taken up enthusiastically in industry. The Virtual Wafer Fab (VWF) controls process and device simulators, and parameter extraction software. It automatically creates large, virtual split lots, capable of imitating the equivalent results from thousands of experimental runs. From these runs response surface models of a process can be created. Analysis of these models introduces new statistical methods for device design and optimization.


VWF Production Mode

The VWF production mode software targets a wider range of engineers than the current T-CAD specialist. A problem in the past has been that T-CAD has been limited in use to only those few engineers with the time to learn the complex series of commands required to drive a process or device simulator. The Production Mode of the VWF completely sidesteps this requirement, allowing full use of the VWF results, without having to learn a single T-CAD simulator command. Many of the VWF Production Mode functions are directly applicable to the day-to-day production and IC design problems:

 

  • Failure Analysis
  • Yield Analysis
  • Process Synthesis
  • Disposition Synthesis
  • Automated Multivariant Calibration

 

Figure 1. VWF Production Mode gives different engineering departments access to TCAD results.

Failure Analysis Mode

The Failure Analysis (FA) mode requires the measured values that fail the parametric probe specification to be fed into the system. They are compared with the internal multi-dimensional response surface model of the process to determine the most likely cause of failure.

The resulting `probability of failure mechanism' histogram plots are generated in a matter of seconds (Figure 2). This new capability is invaluable to FA engineers trying to diagnose the reason for a strange set of values seen on a wafer at wafer probe. The FA mode runs completely independently from the Virtual Wafer Fab and requires no knowledge of simulation techniques.

 

Figure 2. The results of the Failure Analysis function
indicate that the "bl implant dose" is the probable
cause of this set of failed parametric results.

Yield Analysis

The Yield Analysis function introduces the use of TCAD to design processes for improved parametric yield distributions. Processing variations described in terms of standard deviations, or Cp and Cpk values, taken directly from production SPC charts, may be used to predict Spice Model Parameter Yield distributions. Further, in-fab processing distributions may be varied to study the effect upon the yield. This new capability will be of direct use to those engineers interested in improving both the mean value and the yield distributions of SPICE model parameters.

 

 

Figure 3. An analysis of the yield distribution of SPICE
parameter VT given certain natural process variations.

Process Synthesis

One problem encountered in industry is the lack of a tight feedback loop between IC design and technology development. The ability to synthesize a process has long been a target of the Virtual Wafer Fab environment. Now, advanced optimization strategies have been integrated into the VWF Production Mode to offer interactive process synthesis for the first time. Within a few seconds, a set of target SPICE model parameters can be entered into the system and a process derived to best fit the targeted values. IC designers can use this tool remotely from the central VWF hub, now allowing them to feedback real technological processing suggestions to technology development engineers. The ability to synthesize a calibrated and optimized BSIM3 SPICE model is a major breakthrough in the use of simulation.

 

 

Figure 4. The VWF Production Mode synthesis function
shows the highly interactive capabilities for the study of
multi-dimensional RSMs of SPICE parameters.

Disposition

Scrapping a lot off line in a production environment is an expensive risk. The Disposition Module allows an interactive check to see if it is possible to save the lot by dispositioning the lot somewhere downstream in the remaining process flow. The type of problems that might be tackled with this module include for example: mis-processing during implantation or mis-processing during diffusion (a tube abort perhaps). The disposition module will suggest a process disposition to be attached to the lot in question.

Automated Multivariant Calibration

The difficulty in calibrating TCAD simulators has been seen as a major block to the widespread use of these tools within industry. Tuning of default simulator parameters is required but the analysis of the many possible variables has been difficult. With the production mode software, split lots using simulator tuning parameters can now be analyzed to determine the best fit to measured data.

Summary

The VWF Production Mode brings the results of T-CAD simulations within the reach of many new engineers. New practical tasks may now be performed with the added insight provided by TCAD.