Chapter 4. Computer-Aided Fabrication
Chapter 4. Computer-Aided Fabrication System
Structure
Academic and Research Staff
Professor Donald E. Troxel
Visiting Scientists and Research Affiliates
Michael B. Mcllrath
Graduate Students
Michael Heytens, Abbas Kashani
Undergraduate Students
Creighton Eldridge, Kenneth Ishii, Joseph Kalisewzki
Technical and Support Staff
Francis M. Doughty
4.1
Introduction
Sponsor
Defense Advanced Research Projects Agency
Contract MDA 972 88-K-0008
The Computer-Aided Fabrication (CAF)
system structure carried out within RLE is
part of a larger project within the Microsystems Technology Laboratories (MTL). The
overall goal of the CAF project is to integrate
computers into the control, data collection,
modeling, and scheduling of the integrated
The goal of
circuit fabrication process.
Computer-Aided Fabrication (CAF) of integrated circuits is to provide effective management of information associated with the
fabrication of integrated circuits to improve
flexibility, portability, and quality and to minimize turn around time, development cost,
confusion, error, and manufacturing cost.
4.2 Computer-Aided
Fabrication Environment
CAFE (Computer Aided Fabrication Environment) is a software system being developed
at MIT for use in the manufacture of integrated circuits. CAFE is intended to be used
in all phases of process design, development,
planning, and manufacturing of integrated
circuit wafers. While still under active development, CAFE presently provides day-to-day
support to several research and production
facilities at MIT with both standard and flexible product lines.
The MIT CAFE architecture is a framework
provided for a wide variety of software
modules including both development tools
and on-line applications. The key components of the CAFE architecture are the data
model and database schema, the process
flow representation (PFR), the user interface,
and the application programming and database interfaces.
Our database schema is based on GESTALT,
an object-oriented, extensible data model
which provides an extended set of intrinsic
data types including various temporal types
as well as inexact, interval, and null values.
This powerful base allows us to design a
schema which captures important aspects of
plant and process management such as fabrication facilities and equipment, users, equipment reservations, lots, lot tracking, wafers,
process flow descriptions, work in progress
(wip) tracking, and lab activity information,
in an especially natural and effective way.
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Chapter 4. Computer-Aided Fabrication
We developed mechanisms to facilitate
changes in the CAFE schema. Application
programmers can experiment with new
schema definitions and test them by using
the new schema in their programs. This
ability to create and modify new object types
facilitated more rapid development of applications.
We developed an application interface to the
database through the Common Lisp Object
The interface provides
System (CLOS).
transparent access to persistent objects,
which are described and manipulated via
CLOS constructs. This transparency simplifies application programming because it frees
the programmer from the task of translating
between programming language structures
and database structures, which is required in
many systems. Application programmers may
utilize the rich object modeling and generic
functions of CLOS in the integrated environment of Common Lisp to aid in program
development.
The use of a common, unified representation
of the manufacturing process throughout the
cycle of design, simulation, fabrication, and
maintenance is central to CAFE. The Process
provides an
Flow Representation (PFR)
extensible framework for knowledge about
process steps, including instructions to operators and equipment, scheduling requirements, changes effected to to the wafer
product, and physical process model parameters. In particular, the PFR accommodates a
two-stage process step model which relates
the goal of a change in wafer state first to
the physical treatment parameters and finally
to the actual machine settings used to
process the wafers. Its arbitrarily complex
hierarchical structure makes it easy for the
process designer to abstract away cumbersome details and to focus on the issues of
Applications are built
particular interest.
from specialized "process flow interpreters"
which execute a PFR as if it were a program.
For example, one such application drives
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RLE Progress Report Number 132
wafer lots through the fabrication line. Other
applications include various process simulators and production planning support.
Process flows are developed and maintained
using a process flow editor which enables
users to create process flows from basic, predefined steps. Using this editor, preparing
new flows which are minor variations on
existing flows is easy. During the creation of
a new flow, the editor provides a number of
syntactical and semantic checks, prompting
the user concerning the availability of a
library of subprocess flows. Starting with
these process flows, a simulation interpreter
prepares input data suitable for SUPREM III,
invokes the SUPREM III simulation, transforms the processed output, and generates
various graphical presentations of the simuUsers can then run
lated process flow.
SUPREM IIIsimulations without having to
know anything about the details of SUPREM
Ill. A key feature of the simulation manager
is the "validator," which checks the validity
of previous simulation computations after the
process flow representation has been modified and retains computations which are still
valid. Thus, computation time is minimized
while maintaining correctness of the final
simulations. This software also allows the
user to conveniently generate reports such as
plots of impurity concentrations and calculations of sheet resistance, etc.
Ease of use requires a coherent user interface. At the same time, this user interface
must be flexible enough to be suitable for a
wide variety of applications, many of which
remain to be defined. It also must support a
wide variety of devices for user interaction,
including ASCII terminals and bit-mapped
graphic stations. It is quite important to
require the provision of a programmatic interface to all application modules including user
interfaces. Without this, it becomes virtually
impossible to build on previously developed
software modules and to integrate them into
other systems.