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the wafer tree each icon represents a wafer, the result of a simulaton module. The simulation status of the wafer (done, queued, aborted, running, etc.) is identified by appropriate symbols.

As modules are executed during the simulation of the split, the status of the wafers changes accordingly. Thus the wafer tree is a natural representation of the simulated split during its execution, allowing visual monitoring of the simulation status. Error recovery, which is neccessary in cases such as simulator divergence due to physical or numerical reasons, system failure, etc. is much easier in a visual environment. Changing a parameter value in one of the simulation modules to resolve the problem will result in an automatic re-simulation of all dependent parts of the tree (incremental simulation, see below). This can be especially important when independent parts of the tree are executed on a network. Automatic saving of intermediate wafer states guarantees high robustness and stability of the simulated experiment. In the event of a simulator failure or even a computer system or network failure, the experiment can be re-started from the failed module, thus avoiding re-running the whole simulation and minimizing the error-recovery effort.

In addition, the wafer tree serves as a graphical data viewer, representing all intermediate and [mal results of a simulated split. The user can access the data at any time during the experiment by selecting a wafer icon in the wafer tree and applying one from a selection of available tools. These tools can perform post-processing, visualization, extraction or a user-defined set of operations on the wafer representation, such as for instance extracting a peak electric field value and its location from a device simulation result. '

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either the input wafer state or module/step parameter values result in the re-simulation of the module and all modules whose input depends on the output of this module.

This functionality facilitates so-called incremental simulation, meaning that if a part of a complex simulation flow is modified only what is neccessary is re-simulated. This feature is obviously especially important for performing large-scale simulations with complex dependencies. However, even for a relatively simple linear simulation flow it relieves the user from the tedious and error-prone task of creating intermediate simulation fIles and making sure that changes made in the middle of the process get propagated.

Data management features of CAESAR include automatic naming of fIles associated with wafers in the wafer tree, removing wafer data whenever the part of the simulation flow is removed which created that data and supplying wafer data to post-processing tools. If a user wishes to examine a wafer state using for instance a graphical postprocessor (structural data) or applying a threshold voltage extractor (IV data), he simply selects the desired wafer in the wafer tree window and chooses an appropriate postprocessing tool from the "Tools" menu (see Figure I and section 7 on Visualization).

CAESAR's wafer tree window can thus be seen as a graphical data viewer, providing visual information about status of the data and interactive access to it. Completed experiments can be "frozen" for archiving purposes by automatically removing unnecessary intermediate wafer states and compressing the relevant ones to reduce memory requirements.

6. The Run-Sheet Interface

A natural interface for visualizing and controlling a simulation, especially one with splits, is a run-sheet. The run-sheet interface shows for each experiment, i.e. each leaf of the tree, a list of relevant control parameter and their values. Control parameters may or may not be used for splits. They represent important simulation parameters, determining the outcome of the simulation in some sense. In the case of splits, control parameters are the most significant parameters that are different in the branching modules. The choice of control parameters is not restricted by CAESAR. As discussed above (section 5), the user can declare any parameter in any module/step to be a control one.

Figure 7. Run-Sheet Interface showing control parameters and their values for an NMOS process and device simulation.

A number of important details can be seen in Figure 7:

• columns in the table (spread-sheet) represent final results of the split, i.e. wafers at leaves of the tree. The wafer status icons used are identical to those used in the wafer tree window. Rows in the table represent control parameter data: Tag, Name, values for each final result wafer.

•Rows are sorted according to their occurence in the simulation flow, so that the table has a similar layout to the wafer tree. The main difference is in the way how common parts of the simulation are shown: branching in the tree and selecting multiple columns in the table (See Figure 7).

•Tags are user-specified names for control parameters (see Figure 6). This name is usually used in the company-specific documentation of the process module. If the user does not wish to specify a name for the control parameter a default name is provided by the system (name of the module).

•The Name field in the table represent the parameter name as used in the simulator command (see Figure 6).

•Values of control parameters can be edited either in-place or through the "Module/Step Editors" shown in Figure 6.

The run-sheet interface is a transparent and convenient way of keeping track of simulations. It is similar to what is being used in real-world semiconductor R&D fabs and as such is expected to be intuitive to engineers. The run-sheet intelface is being extended to show extracted scalar results in additional to simulation control parameters. Extracted results would be quantities shown in Figure 2, for instance threshold voltage, oxide thickness, junction depth, etc. (see also section 8).

This combination of the run-sheet with extracted quantities results in a "split-sheet", an interface to show both simulation controls and simulation results. The split-sheet is a natural interface to both statistical Design of Experiment (DOE) tools as well as software packages for statistical analysis of simulation results. It is also a natural interface for optimization software to perform optimization of for instance electrical properties of devices as a function of process parameters and tool calibration, i.e. adjusting simulator model parameters to achieve agreement between experimental and simulated data.

7.Visualization with Michelangelo and STUDIO Viz

Data management features of CAESAR allow a tight integration with visualization tools. The main visualization tools currently used from CAESAR are Michelangelo (Structure Editor & Visualizer) and STUDIO Viz - a tool for visualization of IV data.

To visually examine a wafer structure the user simply selects a wafer icon in the wafer tree and chooses "Michelangelo" from the "Tools" menu. The result of this operation may look like Figure 8, showing the electric potential distribution in an NMOS device. In addition to visualizing simulation results such as carrier concentrations, temperatures, doping concentration, etc., Michelangelo [4] possesses structure editing capabilities. The user can modify the geometry of the device or even move individual nodes, create new material regions, change material types of regions, etc. Michelangelo can also create a completely new mesh for the device structure and interpolate all solution values onto this

new mesh. Two meshing algorithms are accessible: a general unstructured triangular Delaunay-type gridder, and a quad-tree based gridder Meshbuild from ETH, Zurich [6] including improvements done by Texas Instruments [7].

Figure 8. Example for a device structure visualized in Michelangelo (0.35 /lm NMOS). The color fill shows the electric potential in the device.

The procedure to examine IV Data is very similar. The user selects the wafer, which in this case must be the result of a device simulation, otherwise IV data will not be present, and chooses "IV Plot" from the "Tools" menu.