Subsections

• 3.1.1 Process Simulation
  • 3.1.1.1 Generic Process Simulators
  • 3.1.1.2 Equipment Simulators
  
  
• 3.1.2 Device Simulators
  • 3.1.2.1 Generic Device Simulators
  • 3.1.2.2 Specialized Device Simulators
  
  
• 3.1.3 TCAD Work Flow Environment Software

3.1 Introduction and State-Of-The-Art

Multiple commercially and openly available TCAD simulation tools are available. In the following a short overview of the history of solutions for integrating them into a consistent work flow will be given. The following overview is far from being complete. However the main tools are reflected.
The history of commercial TCAD began with the formation of the company Technology Modeling Associates (TMA) in 1979. The software was a result of research performed at Stanford University under the guidance of Professors Dutton and Plummer. The most famous of the Stanford TCAD software programs are SUPREM and PISCES. SUPREM3 [70], [71] is a one-dimensional process simulator, while SUPREM4 [72], [73] can handle two dimensions. PISCES [74], [75] is the corresponding two-dimensional device simulator. These are general purpose simulators designed to work with fairly arbitrary semiconductor structures. TMA's versions of these programs were TSUPREM4 [76],[73] and MEDICI [77],[78]. Silvaco later licensed these programs from Stanford University too and offered a commercial alternative (ATHENA [79],[80] and ATLAS [81]). The third major TCAD vendor was Integrated Systems Engineering (ISE). Their equivalent product offerings were DIOS [82] and DESSIS [83].
TMA was later acquired by Avant! which was then acquired by Synopsys. Recently Synopsys acquired their main competitor ISE, which leaves only Synopsys and Silvaco as the main competitors in the market. Synopsys owns now more than 80% of the market share, which results in a market situation close to a monopoly.

3.1.1 Process Simulation

Process flow simulators are tools which simulate the full semiconductor manufacturing flow in a certain level of detail. For special applications and special tools (like lithography) the level of detail of the models implemented in generic process simulators is not sufficient to cover effects like, e.g., simulation of proximity effects in lithography or detailed etch sidewall shapes in plasma etching. However for the routine task of, e.g. generating a accurate representation of a semiconductor device suitable for device simulation the available process simulators are sufficient.

3.1.1.1 Generic Process Simulators

1. SUPREM3 the ``mother of all process simulators'' is now completely outdated. It is a one-dimensional process simulator incorporating already sophisticated models like diffusion in polycrystalline layers and point-defect diffusion.
2. SUPREM4 is the basis for the two commercial tools TSUPREM4 and ATHENA. Developed at Standford University in the group of Prof. Dutton, it was the first consistent approach to simulate the physical behavior of dopants in a layered two-dimensional cut through a semiconductor wafer during semiconductor processing.
3. FEDSS is based on the finite element method too. It was developed inside IBM [84] for generating suitable device structures for the device simulator FIELDAY [85]. It was a fully featured process simulator. However the level of detail of especially the diffusion models was much less sophisticated than that inside of DIOS or SUPREM4.
4. PROPHET is comparable to FEDSS and was developed in AT&T [86]. Where the main focus was the simulation of BiCMOS technology.
5. DIOS is a two-dimensional process simulator developed initially by the Swiss company ISE. Among the strengths is the adaptive meshing for structures with difficult aspect ratios (e.g. smart power devices) and the big variety of implemented models. Drawbacks are the inconsistencies in the different granularities of the models. For instance in the diffusion models the equilibrium diffusion parameters cannot be used as a basis for the point-defect diffusion models.
6. TSUPREM4 is a two-dimensional process simulator descendant from the Stanford process simulator SUPREM4. Strengths are the stable and clear programming interface and the consistent set of simulation model parameters. A weakness is the clumsy meshing and the need for setting up an appropriate initial mesh. Especially for automated process split simulations it often breaks down because of an inappropriate initial mesh.
7. TAURUS-PROCESS was a very ambitious approach to implement a process simulator based mainly on the level-set algorithm [87], [88]. The main problem of three-dimensional process simulation, to cope with moving internal and external three-dimensional boundaries and especially topological changes, when certain parts or entire layers are ``consumed'' during a semiconductor manufacturing process step, was shifted from surface meshing to bulk meshing. However the development failed to provide a stable three-dimensional process simulator yet.
8. ATHENA is the third commercial process simulation code and an additional descendant from SUPREM4. It is nearly identical to the TSUPREM4 simulator.
9. FLOOPS is an object oriented level-set based process simulator from the University of Florida [89]. It uses the algorithmic language ALAGATOR to enable the implementation of new models into the internal discretization scheme. Recently Synopsys is developing a commercial version of FLOOPS for three-dimensional process simulation and as a successor of DIOS

3.1.1.2 Equipment Simulators

Equipment simulation is still an area which is strongly under development. The first stable equipment simulators were lithography simulators which try to analyse the complex sequence of resist spin-on, illumination, development and strip with finite-element methods. Equipment simulators for etching and deposition are still strongly limited to certain manufacturing tool-sets (like those of Applied Materials) [90],[91],[92] and are normally not able to cover the full range of machine parameters which can be tuned at a certain equipment. There are also no commercially available general equipment simulators on the market.

1. ILLUM2D/3D is a tool developed by the Institute for Microelectronics at the Technical University of Vienna, which models the full lithography process flow. It is well suited to capture the physics of the process module like illumination. However the chemical effects during post-exposure bake and development are not well covered by the tool.
2. SPLAT/SAMPLE2D/3D are tools for aerial image simulation and two-,three-dimensional lithography and topology simulation developed by the University of Berkeley in the group of Prof. Neureuther [93].
3. PROLITH is a fully featured lithography simulator now distributed by KLA Tencor [94]. It offers two-dimensional and three-dimensional functionalities and is a standard tool used by the lithography groups worldwide.
4. SOLID E is the competitor to PROLITH and offers a comparable set of features for three-dimensional lithography simulation [95].
5. ACES (Anisotropic Crystalline Etch Simulation) is a tree-dimensional etch simulator using a continuous cellular automata (CA) model and a dynamic structure update method [96]. The program can simulate silicon etching with different surface orientations in selected etchants with variable etch rate ratios. It can receive two-dimensional mask designs in common mask formats (including CIF, GDSII, BMP) and generate three-dimensional profiles in standard solid-modeling languages.

3.1.2 Device Simulators

Device simulators are the counterparts for the process simulators shown above. However historically device simulation was done much earlier than process simulation. Based on assumptions on the input structure of semiconductor devices pioneering work on device simulation was carried out at ATT [97] and IBM [98] leading to major university efforts such as TU Vienna [99] and Stanford [100], finally culminating in a rapid growth of TCAD vendors and development of commercial platforms that support a broad and heterogeneous set of users.

3.1.2.1 Generic Device Simulators

1. PISCES 2ET is a dual energy transport (for carrier temperatures and lattice thermal diffusion) semiconductor device simulator. Some advanced features are the simulation of the carrier and lattice temperatures, and heterostructures in compound semiconductors. Hence, various non-stationary phenomena such as hot carrier effects and velocity overshoot can be analyzed using this program. The electrical behavior of optoelectronic devices can also be simulated with reasonable accuracy. Most of the material parameters have been calibrated and thoroughly surveyed with the help of industry.
2. FIELDAY [85] is a simulator for devices of arbitrary shape in one- up to three-dimensions. The models are especially tailored for the analysis of bipolar devices but field effect transistors can be modeled too. The complementing program FEDSS is used for the generation of input structures.
3. PADRE [101] is comparable to FIELDAY and an internal development of AT&T. It is a moment based device simulator.
4. MINIMOS-NT is a general-purpose semiconductor device simulator providing steady-state, transient, and small-signal analysis of arbitrary two- and three-dimensional device structures [102]. It was recently compared to devices simulated with DESSIS, and it yields the same quality of results as the commercial tool.
5. MEDICI is the counterpart of TSUPREM4 on the device simulation side. It is a pretty stable hence fairly old device simulator which can deal with a variety of physical effects in two-dimensional semiconductor structures. The code was licensed from Stanford University and bases entirely on the PISCES code.
6. DESSIS is a very sophisticated device simulator which deals with two- and three-dimensional device structures. It has a fairly similar feature list compared to MEDICI. However, it is much more stable and based on newer source code (C++ instead of FORTRAN) than MEDICI [103].
7. ATLAS is nearly identical to MEDICI in terms of features [81].
8. FLOODS is the counterpart to FLOOPS on the device simulation side [89].

3.1.2.2 Specialized Device Simulators

Specialized device simulators work on a ``device template'' input structure. These templates are predefined or even hardcoded in the device simulator. The simulator assumes a certain type of device (e.g. a MOSFET for MINIMOS [104] and PISCES [105] or a bipolar transistor for BIPOLE) and takes values for predefined dimensions (e.g. gate oxide thickness, gate width etc.) and doping profiles (e.g. gate channel doping profile assumed as GAUSSIAN distribution) of the semiconductor device.

1. MINIMOS is the predecessor of MINIMOS-NT. It is the famous MOSFET device simulator which worked on an orthogonal grid generated internally [106]. The simulator was a major breakthrough in the theoretical investigations of semiconductor devices, because a lot of physical effects were in reach of detailed analysis for the first time.
2. PISCES is a simulator comparable to MINIMOS, developed by Stanford University [100].
3. SEQUOIA [107] device designer is a simulator comparable to MINIMOS, developed by Sequoia Design Systems.
4. BIPOLE [108],[109] is a simulator for device simulation of bipolar transistors. The core code deals with the analysis of a one-dimensional cut through the emitter/base/collector region of an integrated bipolar transistor device. It adds additional parasitic elements into the analysis like the collector resistance by analysing the geometric dimensions and sheet resistances of the device.

The tools that define the TCAD field - process, device and circuit modeling - have evolved steadily over the past three decades, moving from research prototypes (both in industry and academia) towards robust workhorses that support both research and manufacturing applications. Figure 3.1 shows a schematic time line of evolution for process and device simulation. It is obvious that the development efforts of the commercial vendors have been steadily increasing since the 80's.

Figure 3.1: Schematic time-line of TCAD R&D for device analysis

3.1.3 TCAD Work Flow Environment Software (``Workbenches'')

Every commercial TCAD vendor is including an environment software for automated or at least semi-automated setup of the TCAD work flow shown in Figure 3.2 in Section 3.2. The three workbench software tools are GENESISE from the former ISE AG, WORKBENCH from Synopsys Inc. and VIRTUAL WAFER FAB from Silvaco Int.. Another free TCAD environment software is VISTA [110],[111] from the Institute for Microelectronics, TU Vienna. However this software is not under active development at present.
All four environments are fairly similar in their architecture. They offer macros to schedule and generate Design of Experiments (DOE) [112] simulation runs. For this purpose the simulator command files can be parametrized by a special syntax which identifies the position of a dedicated parameter in the command file. The types of parameters supported are shown in Figure 3.6 in Section 3.4. Furthermore, the extraction of dedicated results from process or device simulation (values given in certain simulation logs or result files) is supported by defining regular expressions or output templates. However all of these environments do NOT support interfaces from semiconductor manufacturing equipment or metrology tools. They offer a more or less well integrated work flow for performing simulations, but the support for interfaces from and to the simulation environment is very little or even not implemented. In the following sections a concept is layed out, how to set up such interfaces in a most effective and stable manner.