Книги2 / 1993 P._Lloyd,__C._C._McAndrew,__M._J._McLennan,__S._N

16 P. Lloyd et al.: Technology CAD at AT&T

Table 2. Correlation Coefficients Between BJT Electrical and Process Parameters

required for aggressive linear design. The resulting profiles, along with geometry specifications, are used as input to device simulators that generate DC, C - V and transient characteristics. These simulations are repeated for the various geometries, temperatures, and extreme processing cases. Parameter extraction tools are then used to create model parameter files used in circuit simulations. This approach gives predictive process characterizations, and enables silicon designers to evaluate a technology before it is completed. Early test wafers are then analyzed, and used as part of the process of ongoing verification and improvement of our TCAD system. This allows out tools to be used with some confidence to predict the behavior of next generation technologies.

Our simulation system is also coupled to manufacturing line measurement databases, to verify and monitor the system's predictive capability. Much of this verification is done using the statistical and graphical tools provided by the S language [37]. The manufacturing databases track production variations, which are plotted against control limits and the predictions from compact models. Statistical analyses and graphical reports are generated to ensure conformance to specifications, and to reduce design risk. Figure 8 shows a typical report, that summarizes process variations in Ion for a nominal length n-channel transistor. The left segment shows the trend over a six month period, the middle section shows individual measurements made during the last month, and the right segment is a Q - Q plot of data for the last month. Shaded regions represent the manufacturing limits, and the solid horizontal lines are the predictions from compact models generated for nominal and extreme cases.

For certain classes of circuits, the matching between n- and p-channel transistors is critical. Figure 9 shows a scatter plot of the threshold voltage for both n- and p-channel devices, for data from one month from the manufacturing database. The unshaded region, inside the box, represents 20" limits for the technology. The six sided polygon, drawn with a solid line, is the convex hull formed from the predicted compact model parameter files for the extreme processing cases, which are the vertices of the polygon. To maximize yield, the process is controlled so that the number of sample points falling within the convex hull is maximized.

functions, such as circuit simulation, parameter extraction, and stand alone evaluation, are guaranteed to be consistent with each other.

Figure 11 shows playbacks of ASIM3, against PADRE data that was used for extraction in CAMELOT. The data is for an n-channel MOSFET of Lm =0.25JllD. The top left plot of figure 11 shows threshold characteristics, I d as a function of V gs for V ds = O. IV and Vbs=0.0,-0.6,-1.2 and -2.4V. The top right plot shows subthreshold characteristics, I d as a function of V gs for V ds=O.l, 1.0,2.0 and 3.0V and V bs=O.OV.

We have used this system for the ASIM3 and AUSSIM MOSFET models, for many flavors of BIT models, for diode models, and for 3-terminal resistor models. We have found that the system greatly simplifies compact model development and implementation in ADVICE and CELERITY. All our new compact models are defined using the CAMELOT system.

5.Conclusion

TCAD is an essential part of technology development and technology characterization at AT&T. Critical to the success of our TCAD system is on-going tool improvement to ensure accuracy, robustness, and a practical approach to software and data compatibility. We also recognize that people are an important part of the overall system. Our TCAD system is evolving from separate point tools bound in shell scripts, that communicate via files, to integrated tools that are modules in a common extension language environment, that operate on common objects that represent the structures being simulated. This paper has presented the important aspects of this evolution.

Interoperability and compatibility of point tools and simulation data significantly simplifies the job of process characterization, and allows high level tasks such as optimization and sensitivity analysis to be used effectively during technology development. We have given examples of the use of our TCAD system for high level tasks.

Acknowledgements

We would like to thank R. V. Booth, W. T. Cochran, W. M. Coughran Jr., E. Grosse, H. K. Gummel, G. A. Howlett, P. A. Layman, S. Liu, L. W. Nagel, R. K. Smith, M. J. Thoma, G. Zaneski, and many other colleagues at AT&T Bell Laboratories for contributions to this work. The use of the AT&T Bell Laboratories DDL facility in Allentown is gratefully acknowledged.

0.1

0.05

0.5

1E-

Figure 11. Data Fits of the ASIM3 Compact Model, L m =O. 25 11m

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