SIMULATION of Fabrication Processes and Device Operation
Using SILVACO Framework
SILVACO Virtual Wafer Fab http://www.silvaco.com/products/virtual_wafer/vwf.html allows for simulation of the whole fabrication process of Si devices (ATHENA) http://www.silvaco.com/products/process_simulation/athena.html and analyses of their operation (ATLAS) http://www.silvaco.com/products/device_simulation/atlas.html. Several, but not all, modules in the software packages are available for the UH users with Athena_Elite http://www.silvaco.com/products/vwf/athena/elite/elite_br.html and Athena_ Suprem4 http://www.silvaco.com/products/vwf/athena/ss4/ss4_br.html being the most important ones for fabrication. Models used in both Athena and Atlas simulators can be selected from the simple ones that assume only basic features to the state of the art models that include the most recent advances in physics of processing as well as physics of devices. The programs take parameters set by the users, to solve (differential) equations describing various processing and device operation. The mesh (or grid), defined also by the user, helps to adjust the 2D simulation accuracy especially for scaled down devices, which correspond to larger curvatures and therefore require more densely spaced grid points. We will focus on examples that are included for the instructional purpose and will be followed by assignment to observe the influence of process conditions on fabricated structures.
The program works in the Interactive Runtime Environment accessible via DeckBuild. It allows, through the graphical user interface, to specify (or easily modify) input information (process types, parameters and models as well as device operation conditions and models), run simulation (fabrication of devices and/or their operation), and monitor results (on DeckBuild) such as those of currently performed simulation steps, and to invoke Visualization Tool (TonyPlot) to observe the obtained intermediate or final results.
ATHENA and ATLAS are complex and powerful simulation tools. You are encouraged to browse through the examples (others than those suggested in this handout) both for the fabrication and device operation training. Unfortunately, from the whole set of the software packages, there are a few that are not licensed to us, so not all examples can be accessible. Please do not be discouraged since numerous examples which are available are very informative and educational. For your reference, the SILVACO manual is at the help desk, in case if you need more information, and there is Help also available on-line.
Set your path it in your .cshrc file in your Eng. account to usr/local/SILVACO/VWF/bin to allow for the Silvaco access; please follow instructions in to establish the access to Silvaco. To start DeckBuild enter: deckbuild & (and the return key). That will activate the DeckBuild window, with ATHENA as a default. You can set also Atlas as a current simulator since analytically defined structures can be also simulated by Atlas http://www.silvaco.com/products/vwf/atlas/spisces/spisces_br.html. The upper window in the DeckBuild is the input (your device structure that will be fabricated or a device that will operate) and the lower one is the "traced" output. You will see commands buttons that enable program control such as run, quit, etc., and its monitoring by tracing line numbers.
Manuals are available from the Main Menu of the Deckbuild and more complete (VWF with tutorials) in TonyPlots Help Menu.
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We will start with "Examples" that can be loaded from the Main Control Menu and we will modify various fabrication processes according to the assignment.
Choose any example by releasing the right hand button of the mouse at the selected item (or select and LOAD the example). Then run the simulation. You can do it as one step at a time by using the "next" button, which will enable to step through the program while displaying the corresponding output or you can run it continuously. At the end of the simulation extraction step (EXTRACT action in the place of ATHENA) will provide final parameters such as oxide thickness etc. Analyze device dimensions and the mesh. Observe processing steps including models used,
Any step either an intermediate or a final result (written in the file "results.final" in your working directory) can be displayed using TonyPlot form the DeckBuild. Both material structures and electrical properties can be plotted; they have the .str extensions. You can select the structure files, then click the Tools button with the left mouse button pressed. From the dropdown menu select Plot, then Plot Structure. TonyPlot will display your graph. Modification of the plot is possible by selecting Plot button in the TonyPlot menu. Visit these options. You can display any plot obtained by the simulation if you Load Structure (your file name) from the File menu in the TonyPlot. In particular, look for the files .history##.str (ex: .history07.str) that are listed in the output window of the DeckBuild after specific processes. After loading these files in the TonyPlot you can choose 2D display, which adds to the clarity of the plots and in many cases is indispensable. For this purpose, go to Display from Plot, and the 2-D Display Control window will pop up. Several options are available here, such as grid structure, xj etc. These options are self explanatory. Experiment with them. The Contours can be modified (material-color identification, etc.) by selecting Contours from the Define menu.
2-D contour plot can be displayed as a 1-D plot at specific cross-sections defined by the Cutline (x,y) from the Tool menu or when a process is run as one-dimensional http://www.silvaco.com/products/vwf/athena/ss3/ss3_br.html. x and y can be selected on the graph directly if you click on a horizontal- or vertical-arrows windows and draw a line on your contour. Use Shift position button for x to y transitions.
A note related to plotting. Each plot has a specific display format that includes enough information about the obtained structures. If you change the simulation such as ex. dopant type etc. used in your first run, you have to make changes in the formatting files (*.set). To do that for our first example open the set file (in your home folder) from your first run i.e. aniiex03.set (used for B implantation) and change the dopant to Phosphorus. Rename the file, for instance aniiex03_P.set and use this new file in your program that runs P implant. If you run the program w/o changing your set file, Tony will not plot anything. You can see how it works by first removing -set aniiex03.set from line 70 in the program when you run P implant instead of B implantation. When the plot is drawn in Tony, you can use various features from the Plot Menu to modify/adjust your plot appearance such as to set scales for axis, put labels, make annotations etc. This new "creation" can be saved as a set file (by going to set files in File manu in Tony) and used when applicable.
Dynamic changes in the fabricated structures can be displayed by using a Movie option form the TonyPlot (from the main Tools menu). Several plots are displayed sequentially resulting in the animation effect with typical video control options (you can rewind it, slow down etc.).
Processes used for IC fabrication include oxidation (labeled as "diffusion"), ion implantation, annealing (again labeled as "diffusion"), etching, and deposition. Observe what models are used in each step and compare them with the available options for each specific step. Try to explain why a specific model has been chosen. Observe also conditions of the processing, such as source/drain implantation and annealing/oxidation processes necessary for specific device structure. Notice the way low doped drain/source regions are made. Observe side wall oxidation of the gate oxide (a bird beak as seen in LOCOS).
BOTTOM LINE: observe the relations between processing conditions and resulting devices.
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Silicon device simulator is included in the ATLAS package. Several other modeling tools http://www.silvaco.com/products/device_simulation/atlas.html. such as those for optoelectronics, compound semiconductor devices, noncrystalline material elements etc. (see the manual) are not licensed to the University of Houstons. It is desirable that you refer to the manual for detailed understanding of the operation and capabilities of ATLAS or consult the on-line help.
ATLAS simulates two and three (not available for UH) dimensional devices,s which can either be fabricated by Athena or can be specified analytically. The first approach allows for virtual IC chip/device fabrication that comprises testing of both silicon processes and monitoring their influence on the device operation. The input file for ATLAS contains information about the device structure obtained then by a process simulator (here ATHENA) or created analytically in DevEdit. The Solution Files, which store 2D and 3D data solution for various variables in the devices and the Log Files that store summaries of electrical output information, can be displayed using TonyPlots.
ATLAS can be used through the DeckBuilder either by starting it as the default simulator (deckbuild -as) or from an existing input file (deckbuild -as <<input filename>). We will access it via examples listed in the MainControl menu. These examples are listed by technology and device type/application. You can select any of these example but only those with the licensed simulator extension will run. As in the case of process simulator examples, here the example has to be loaded (just press the button). First, the ATHENA program will simulate fabrication of your device, then automatic interface will be created under DeckBuild for ATLAS to start its operation and to model device electrical characterization. Structure can be defined also (but not in this assignment) directly in ATLAS using The Command Language by specifying a mesh and device materials/regions.
Material parameters, such as dopant type/concentration and its ionization, work function, and carrier transport properties (lifetime and mobility defined by various physical models and parameters) etc. can be selected as shown in the summary tables (p. 2-20 - 2-22).
Selected parameter (or several parameters) of devices can be simulated for any given physical structure. Numerical methods used to obtain simulation solutions can be selected for ATLAS calculations and the results can be in the form of DC, AC small signal or transients characteristics. Initial guess that improves convergence is incorporated in the initial solution (SOLVE INIT) that assumes zero bias case and is followed by two small voltage steps.
An important feature of ATLAS (and also in ATHENA) is the EXTRACT command that allows for extraction of specified parameters (you can specify a parameter name and its calculation formula) based on the performed simulation. Results of the extraction will be displayed in the run-time output and stored in a "results.final" file. These results (or others, that you specify for plotting purpose only) can be plotted in the TonyPlot using GRAPH FUNCTION text fields. Solution files can be plotted for evaluation of the device operation at particular bias point (a "snap shot" of device operation).
Projects selected as the course assignments are listed as Silvaco Assignment.
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