Technology computer-aided design (technology CAD or TCAD) is a branch of electronic design automation that models semiconductor fabrication
and semiconductor device operation. The modeling of the fabrication
is termed Process TCAD, while the modeling of the device operation is
termed Device TCAD. Included are the modelling of process steps (such as diffusion and ion implantation), and modelling of the behavior of the electrical devices based on fundamental physics, such as the doping profiles of the devices. TCAD may also include the creation of compact models (such as the well known SPICE transistor
models), which try to capture the electrical behavior of such devices
but do not generally derive them from the underlying physics. (However,
the SPICE simulator itself is usually considered as part of ECAD rather than TCAD.)
Hierarchy
of technology CAD tools building from the process level to circuits.
Left side icons show typical manufacturing issues; right side icons
reflect MOS scaling results based on TCAD (CRC Electronic Design
Automation for IC Handbook, Chapter 25)
From the diagram on the right:
- See SPICE for an example of a circuit simulator
- See semiconductor device modeling for a description of modeling devices from dopant profiles.
- See semiconductor process simulation for the generation of these profiles
- See BACPAC for an analysis tool that tries to take all of these into account to estimate system performance
Introduction
Technology files and design rules are essential building blocks of the integrated circuit design
process. Their accuracy and robustness over process technology, its
variability and the operating conditions of the IC — environmental,
parasitic interactions and testing, including adverse conditions such as
electro-static discharge — are critical in determining performance,
yield and reliability. Development of these technology and design rule
files involves an iterative process that crosses boundaries of
technology and device development, product design and quality assurance.
Modeling and simulation play a critical role in support of many
aspects of this evolution process.
The goals of TCAD start from the physical description of
integrated circuit devices, considering both the physical configuration
and related device properties, and build the links between the broad
range of physics and electrical behavior models that support circuit
design. Physics-based modeling of devices, in distributed and lumped
forms, is an essential part of the IC process development. It seeks to
quantify the underlying understanding of the technology and abstract
that knowledge to the device design level, including extraction of the
key parameters that support circuit design and statistical metrology.
Although the emphasis here is on Metal Oxide Semiconductor
(MOS) transistors — the workhorse of the IC industry — it is useful to
briefly overview the development history of the modeling tools and
methodology that has set the stage for the present state-of-the-art.
History
The
evolution of technology computer-aided design (TCAD) — the synergistic
combination of process, device and circuit simulation and modeling tools
— finds its roots in bipolar
technology, starting in the late 1960s, and the challenges of junction
isolated, double-and triple-diffused transistors. These devices and
technology were the basis of the first integrated circuits; nonetheless,
many of the scaling issues and underlying physical effects are integral
to IC design, even after four decades of IC development. With these
early generations of IC, process variability and parametric yield were
an issue — a theme that will reemerge as a controlling factor in future
IC technology as well.
Process control issues — both for the intrinsic devices and all
the associated parasitics — presented formidable challenges and mandated
the development of a range of advanced physical models for process and
device simulation. Starting in the late 1960s and into the 1970s, the
modeling approaches exploited were dominantly one- and two-dimensional
simulators. While TCAD in these early generations showed exciting
promise in addressing the physics-oriented challenges of bipolar
technology, the superior scalability and power consumption of MOS
technology revolutionized the IC industry. By the mid-1980s, CMOS
became the dominant driver for integrated electronics. Nonetheless,
these early TCAD developments set the stage for their growth and broad deployment as an essential
toolset that has leveraged technology development through the VLSI and
ULSI eras which are now the mainstream.
IC development for more than a quarter-century has been dominated by the MOS technology. In the 1970s and 1980s NMOS
was favored owing to speed and area advantages, coupled with technology
limitations and concerns related to isolation, parasitic effects and
process complexity. During that era of NMOS-dominated LSI and the
emergence of VLSI, the fundamental scaling laws of MOS technology were
codified and broadly applied.
It was also during this period that TCAD reached maturity in terms of
realizing robust process modeling (primarily one-dimensional) which then
became an integral technology design tool, used universally across the
industry.
At the same time device simulation, dominantly two-dimensional owing
to the nature of MOS devices, became the work-horse of technologists in
the design and scaling of devices. The transition from NMOS to CMOS
technology resulted in the necessity of tightly coupled and fully 2D
simulators for process and device simulations. This third generation of
TCAD tools became critical to address the full complexity of twin-well
CMOS technology (see Figure 3a), including issues of design rules and
parasitic effects such as latchup. An abbreviated but prospective view of this period, through the mid-1980s, is given in; and from the point of view of how TCAD tools were used in the design process.
Modern TCAD
Today
the requirements for and use of TCAD cross-cut a very broad landscape
of design automation issues, including many fundamental physical limits.
At the core are still a host of process and device modeling challenges
that support intrinsic device scaling and parasitic extraction. These
applications include technology and design rule development, extraction
of compact models and more generally design for manufacturability (DFM).
The dominance of interconnects for giga-scale integration (transistor
counts in O(billion)) and clocking frequencies in O (10 gigahertz)) have
mandated the development of tools and methodologies that embrace
patterning by electro-magnetic simulations—both for optical patterns and
electronic and optical interconnect performance modeling—as well as
circuit-level modeling. This broad range of issues at the device and
interconnect levels, including links to underlying patterning and
processing technologies, is summarized in Figure 1 and provides a
conceptual framework for the discussion that now follows.
Figure
1: Hierarchy of technology CAD tools building from the process level to
circuits. Left side icons show typical manufacturing issues; right side
icons reflect MOS scaling results based on TCAD (CRC Electronic Design
Automation for IC Handbook, Chapter 25)
Figure 1 depicts a hierarchy of process, device and circuit levels of
simulation tools. On each side of the boxes indicating modeling level
are icons that schematically depict representative applications for
TCAD. The left side gives emphasis to Design For Manufacturing (DFM) issues such as: shallow-trench isolation (STI), extra features required for phase-shift masking (PSM) and challenges for multi-level interconnects that include processing issues of chemical-mechanical planarization (CMP), and the need to consider electro-magnetic effects using electromagnetic field solvers.
The right side icons show the more traditional hierarchy of expected
TCAD results and applications: complete process simulations of the
intrinsic devices, predictions of drive current scaling and extraction
of technology files for the complete set of devices and parasitics.
Figure 2 again looks at TCAD capabilities but this time more in
the context of design flow information and how this relates to the
physical layers and modeling of the electronic design automation (EDA)
world. Here the simulation levels of process and device modeling are
considered as integral capabilities (within TCAD) that together provide
the "mapping" from mask-level information to the functional capabilities
needed at the EDA level such as compact models ("technology files") and
even higher-level behavioral models. Also shown is the extraction and
electrical rule checking (ERC); this indicates that many of the details
that to date have been embedded in analytical formulations, may in fact
also be linked to the deeper TCAD level in order to support the growing
complexity of technology scaling.
Providers
Current major suppliers of TCAD tools include Synopsys, Silvaco, Crosslight, Cogenda Software, Global TCAD Solutions and Tiberlab. The open source GSS, Archimedes, Aeneas, NanoTCAD ViDES, DEVSIM, and GENIUS have some of the capabilities of the commercial products.
