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

Technology CAD at IBM

R.W. Knepper, J.B. Johnsont, S. Furkayt, J. Slinkmant , X. Tiant , E.M. Buturlat , R Young+, G. Fiorenza+, R Logan+, Y.S. Huang+, RR. O'Brien+, C.S. Murthy+, P.C. Murley+, J. Peng+, H.H.K. Tang+, G.R. Srinivasan+, M.M. Pelella+,

D.A. Sunderland+, J. Mandelman+, D. Lieber*, E. Farrell*, and M. Kurasic*

IBM Semiconductor Research and Development Center, East Fishkill Facility, Hopewell Junction, NY 12533, USA

hBM Technology Products Division, Burlington, VT, USA

+IBM SRDC, Hopewell Junction, NY, USA

*IBM T.J. Watson Res. Lab., Yorktown Heights, NY, USA

+AMC, Santa Barbara, CA, USA

Abstract

The IBM suite of TCAD tools for semiconductor process and device modeling is described. The series includes FEDSS for process modeling, FIELDAY for device modeling, FOXi/FIERCE for resistance and capacitance calculation, EXCALIBR and MGP for compact device model generation, and SEMM for soft-error failure probability prediction. Comprising the VATS series of tools, the programs interact through a common database representation, are accessible via a graphics-user-interface WIZARD, and provide for inputs, outputs, and meshing through a number of preand postprocessor programs.

1.Introduction

As the cost of developing new semiconductor technology at ever higher bit/gate densities continues to grow, the value of using accurate TCAD simulation tools for design and development becomes more and more of a necessity to compete in today's business. The ability to tradeoff wafer starts in an advanced piloting facility for simulation analysis and optimization utilizing a "virtual fab" S/W tool set is a clear economical asset for any semiconductor development company. Consequently, development of more sophisticated, accurate, physics-based, and easy-to-use device and process modeling tools will receive continuing attention over the coming years.

The cost of maintaining and paying for one's own internal modeling tool development effort, however, has caused many semiconductor development companies to consider replacing some or all of their internal tool development effort with the purchase of vendor modeling tools. While some (noteably larger) companies have insisted on maintaining their own internal modeling tool development organization, others have elected to depend totally on the tools offered by the TCAD vendors and have consequently reduced their modeling staffs to a bare minimal support function. Others are seeking to combine the best of their internally developed tool suite with "robust", "proven" tools provided by the vendors, hoping to achieve a certain synergy as well as savings through this approach.

In the following sections we describe IBM's internally developed suite of TCAD modeling tools and show several applications of the use of these tools. Beginning with the methodology of modeling within IBM, we will show the similarities and differences in the approaches taken for both FET and bipolar modeling. After describing features and applications of the tools, we discuss the programs being developed for interfacing the simulators, as well as for obtaining output graphics. The intermediate data representation, as well as the installation platform(s), will be discussed briefly.

2.SjW Tool Set in Use at IBM

At IBM an internally-developed process and device modeling set of TCAD tools [1-16J has been in place and seen extensive use for the past three or four generations of semiconductor technology development. Numerical simulation tools were originally developed independently at several locations throughout the company with the specific needs of either bipolar or FET device development in mind. For example, in the original 2-dimensional finite element process simulator developed for bipolar technology (SAFEPRO), particular attention was placed in the algorithm and methodology for modeling the two species interaction between arsenic and boron in order to correctly model the effect of the steep arsenic emitter profile on the more mobile but highly sensitive boron base profile [1,2]. SAFEPRO was later "converted"

into a 2D finite difference simulator FINDPRO with enhanced diffusion physics for point defects and their effect on the impurity diffusion [6,7]. This S/W tool development for the bipolar device technology was done at the East Fishkill site and was invaluable in the design and optimization of the recent high performance (20 GHz) ATX4 double polysilicon bipolar 'technology [17].

On the other hand, the 2D finite element process simulator FEDSS originally developed for the FET technology business, took a somewhat different approach. For impurity diffusion the simulator used an algorithm based on the assumption of the adequacy of single species diffusion, and concentrated instead on developing an early tool for modeling oxidation, etch, and deposition. Both FEDSS and SAFEPRO/FINDPRO received various enhancements and were used extensively in process simulation for technology design, development, and optimization in their respective intended arenas throughout the 1980's and early 90's in mM research and development at the Yorktown, Burlington, and East Fishkill locations.

The development of numerical device simulation S/W tools also occurred separately at two locations devoted somewhat independently to either bipolar or FET development. At the IBM Burlington, VT (FET) location the 2D/3D finite element FIELDAY program was written and used extensively for FET device modeling and analysis. Likewise, a 2D finite difference program 2DP was developed at the East Fishkill (bipolar) location and also heavily used for bipolar device studies, design, and model generation. Both programs solve the same semiconductor equations, i.e. the carrier continuity equations and Poisson's equation, and assuming that the same mobility, ionization, and band-gap narrowing formulations are used, both programs give the same results. 2DP used Boltzman statistics whereas FIELDAY was further enhanced to include Fermi-Dirac statistics, as well as a number of other physics-based additions including the Hydrodynamic energy-balance equations and, more recently, the lattice energy equation.

In 1991 the Semiconductor Research and Development Center (SRDC) was formed, combining the TCAD S/W development efforts at both the East Fishkill and Burlington locations into one project. At that time the decision was made to continue the development of the FIELDAY program and discontinue further development of 2DP. Likewise, when both FEDSS and FINDPRO were examined, it was decided to combine the best of both programs and apply these to the further development of the FEDSS process simulator. Internal development is therefore continuing on the FEDSS and FIELDAY programs for process and device modeling within the mM SRDC/TP organization(s).

Figure 1 is a block diagram illustration of the mM Technology ComputerAided Design (TCAD) suite of tools (coined VATS for VLSI Analysis Tools Series), including simulators, preand post-processors, meshing programs, and a graphics-user-interface, along with the intermediate database representation.

28 R.W. Knepper et al.: Technology CAD at IBM

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Figure 1. Block diagram illustrating relationship of VATS (VLSI Analysis Tools Series) simulation programs and pre/post-processors to database representation.

These tools are supported and in use today within the IBM SRDC, Technology Products, and Research organizations at the Fishkill, Burlington, and Yorktown locations, as well as at other sites involved in semiconductor design/development. Internally developed simulation tools in use today for semiconductor process and device modeling and described in this paper are the following:

•FIELDAY (2D/3D Device Simulation)

•FEDSS (2D Process Simulation)

•FOXi/FIERCE (2D/3D Capacitance, Resistance Simulation)

•DMG Program (Bipolar Model Generator)

•EXCALIBR (Extraction Program for CMOS Model Generation)

•SEMM (Soft Error Modeling of Alpha and Cosmic Particles)

These are used in connection with auxiliary preand post-processor programs for gridding, profile inputs, outputs, and a graphics user interface:

• WIZARD (Graphics User Interface)

•TRIM (mesh generation using ASCII control file)

•DELAUX (interactive mesh generation)

•REGRID (mesh refinement and optimization)

•MESH3d (extrudes a 2D mesh into 3D)

•DOPING (generates 2D doping profiles)

•BRIDGE (interpolates doping unto the mesh)

•FEMPLOT (graPHIGs-based plotting program)

•VIDS (3D Graphics Plotting for Output and Visualization)

Communication and interaction between the above mM TCAD tools is made possible by use of a common intermediate data representation (VATS/DB) for data storage. VATS/DB consists of six files having the same basename but different suffixes -

.dict - index file to all the data

.dope - contains doping distribution

.geom - contains mesh and contacts

.rest - contains nodal results

.vars - snapshot of the FIELDAY input file

.restart - restart information for FEDSS

The above programs are all currently accessable through the WIZARD Graphics User Interface with the exception of DMG (device model generator) and SEMM (soft error rate modeling).

The tools are installed and maintained on the IBM RS/6000 workstation platform running the AIX operating system, as well as MVS/ESA for mainframe application. In addition to the Fortran compiler, use of the tool set also requires the "C" compiler for dynamic memory allocation (feature of FIELDAY), graPHIGs for plotting and meshing, and DATA EXPLORER for output visualization capability utilizing VIDS. Partial support is also provided at this time for the new AIX/ESA mainframe operating system.

In addition to the above suite of internally developed tools, a number of university and vendor (process, device, and extraction) modeling programs are also available, such as SUPREM3, SUPREM4, etc., most of which are also accessable via the WIZARD GUI. The above list has included only a subset of the S/W tools normally defined as TCAD tools. For the purposes of this paper we exclude other important technology simulation tools such as lithography modeling S/W, stress simulation programs, circuit analysis tools, placement and wiring systems, and tool and equipment modeling programs.

After discussing the overall modeling methodology, we will describe the features and demonstrate the application of several of the key simulation programs listed above.

3.Modeling Methodology

As described above, bipolar and FET modeling tool development has evolved somewhat independently within IBM. In the bipolar arena a methodology, illustrated by Figure 2a, was conceived and implemented [1 - 7J beginning with a process description and ending up with a lumped equivalent circuit model for circuit analysis through the steps of 2D process modeling, 2D device simulation, and device model generation. The programs have been linked through a common intermediate data representation so that flexibility was obtained in allowing the use of either FEDSS or FINDPRO for the 2D process simulation, FIELDAY or 2DP for the 2D device simulation, DMG for compact circuit model generation, and the IBM ASTAP (now ASX) program for the circuit analysis. Recently, the DMG program concept has been extended to allow the generation of SPICE bipolar models which are desired for external OEM customers utilizing IBM advanced bipolar technology.

In the simulation of FET devices and processes a slightly modified approach has been in use, as described in Figure 2b.

1 DC STATISTICAL and

TRANSIENT RESPONSE

Figure 2. TCAD Modeling Methodology: (a) Bipolar and (b) MOSFET Tech· nology Simulations.

As in the bipolar case FEDSS and FIELDAYare used for process and device simulation, but at this point due to the more 2D nature of the FET device, parameter extraction techniques can be used to construct an accurate source·

drain current function, which then is used to build a complete FET compact device model with the addition of appropriate capacitances, resistances, and parasitics for use in either ASTAP (ASX) or SPICE circuit analysis. This alleviates any necessity for constructing a fully 3B distributed model as an interim step, as is done in the case of the bipolar modeling methodology in 2a.

As shown in Figure 2b, the use of FEDSS and FIELDAY for technology development often ends with the device sunulation results themselves, which are then studied and iterated upon for the purpose of tuning and optimizing the technology. FEDSS is available only as a 2D simulator, while FIELDAY is used for both 2D and 3D simulation in the design and optimization of FET devices. Device and process studies and design optimization clearly represent the majority of the usage for these two tools in the area of FET technology development. Although the FIELDAY results are often used to develop equivalent circuit ASTAP (or SPICE) models, as is the case for bipolar modeling, measured data is usually preferred for extracting model parameters when available.

4.FEDSS Process Simulator

The 2D semiconductor process simulator FEDSS (Finite Element Diffusion Simulation System) contains the following features and capability:

•Diffusion

•Oxidation

•Ion Implantation

•Pre-deposition and evaporation

•Epitaxy

•Material deposition

•Etching

FEDSS was initially written in the early and mid 1980's at the IBM Burlington location by Salsburg, Hansen, Borucki, et.al. [n, 12]. The simulator was first created as a diffusion program for FET diffusion analysis and was shortly thereafter modified to include oxidation, implant, etch, and deposition. Diffusion models were originally written as single species models for arsenic, boron, and phosphorous which were thereafter enhanced to include two species interactions via a four (diffusion) coefficient coupling matrix for each pair of impurity species [12]. Additional species including Sb, and In have more recently been added to the program capability [18]. FEDSS includes four arsenic clustering models, in order to correctly compute the active dopant in an NPN transistor emitter, for example.

The oxidation model implemented in FEDSS is based on a two stage approach involving first oxidation utilizing a physical viscous oxidation model published originally by Chin, et.al. [19] in the Stanford program SOAP, which is then followed by redistribution of dopants [12]. This oxidation algorithm has