Exact2: Interconnect Parasitic Capacitance Simulator from Silvaco

Introduction

Exact2 from Silvaco is a sophisticated, physically-based simulation tool for calculating semiconductor interconnect capacitance values. Its purpose is to build a capacitance coefficient database that is usable by any layout parasitic capacitance (LPE) tool.

In order to accurately calculate these coefficients, it is important that the actual interconnect structures are accurately defined. Exact2 achieves this by means of an internal, physically-based 3D process simulator. Included with the process simulator is an internal 3D field solver that calculates the capacitance for each device layer and structure combination. Exact2 also creates capacitance rule files readable by any LPE tool through the use of analysis script files, written in LISA code which is, Silvaco’s dynamic scripting language.

This article presents an overview of Exact2’s features, capabilities, and use.

 

Overview of Selected Features and Improvements

Exact2’s 3D process simulation engine simulates many varieties of arbitrarily complex interconnects, including:

  • multiple dielectrics including low-k materials
  • multiple metal materials
  • non-planar dielectrics
  • damascene processing
  • conformal deposition
  • lithographic effects

Exact2’s processing options are geometric etches and depositions, or realistic etching and deposition. An integral optical solver takes photolithography effects into account. All the relevant properties of realistic etching, deposition, and optolithographic models are defined by the user, including the isotropic degree of an etch or deposition, the critical intensity at which the photoresist will develop, and finally the wavelength, aperture, and shape of the exposure source. Exact2 simplifies worst case modeling and data analysis by applying powerful statistical analysis to geometry dependent parasitics.

A new paramaterized layout editor and support for the Language for Interfacing Silvaco Applications (LISA) enhances custom layouts and analysis capabilities. LISA also enhances Exact2’s open interface by outputting data to formats recognized by any chip-level LPE tool. In addition, custom equations are easy fit to raw parasitic data.

Exact2 dramatically improves upon its predecessor in several ways, including:

  • Extensive use of tool tips throughout the application
  • Detailed output logs simplify easier development of models
  • Results are now saved in plain text for easier retrieval
  • Greater control over the simulation domain
  • Simpler, more robust GUI
  • Intuitive approach to results analysis
  • Plain text configuration files make it easy to run Exact2 without the GUI

 

The Seven Stages of An Exact2 Experiment

Figure 1 is the main start-up screen of Exact2’s Graphical User Interface (GUI). There are generally seven distinct stages that comprise a complete Exact2 experiment, and these stages are identified in Figure 1 by the corresponding icons running down the left hand side of the main screen.


Figure 1. Main Exact2 Window.

The stages are briefly described as:

  1. Process definition: define layer thickness and film properties
  2. Layout definition: choose test structures, layouts, and layer combinations for each layout
  3. Field Solver: control internal field solver attributes
  4. Output: specify the result parameters and save location
  5. Design of Experiments (DOE): describe the upcoming experiments
  6. Run: perform the calculations and generate the database
  7. Analysis: analyze, manipulate, and visualize the generated database

Exact2 features two modes of operation (Figure 1): standard and advanced. The following descriptions are based on the standard mode of operation.

Stage 1: Process definition
User-specific process are easily created with the process GUI (Figure 2a). This screen is used to input layer definition and thickness, material properties, and parameter variables. Subjects are brought to the foreground by clicking on the relevant folder heading (Figure 2a). The user may preview and modify the created process stack at any time by simply clicking on the preview button in the process GUI. Figure 2b shows a preview of the process defined in Figure 2a.


Figure 2a. The Process window allows each
layer of a process to be defined.

 


Figure 2b. Preview of the process definition
created by the process window.

 

Stage 2: Test structure (layout) definition
Once device layers are identified, the test structures’ mask layout designs are quickly chosen and added to the experiment with the layout GUI (Figure 3a). Test structures are easily parameterized and defined in any combination of selected layers. For example, combination 1 is chosen (left side of Figure 3a) that corresponds to process layers (right side of Figure 3a). Users can preview the test structure in both plan and side views (Figure 3b), and cut lines through any part of the structure are easily implemented.


Figure 3a. The Layout window allows layout files to
be added and defines the combination of
layers to be included in the simulation.

 


Figure 3b. When a layout file is loaded this preview window shows the shape
of the layer and the variables that allow its dimensions to be changed.

 

Stage 3: Field solver
Figure 4 shows the field solver GUI. This screen is used to adjust of some of the field solver attributes, such as tolerance and accuracy levels. The probes function specifically chooses of which pairs of wires will serve as targets of capacitance calculation. The probes function is brought to the foreground by clicking on the respective folder heading.


Figure 4. The field solver used by Exact2 can have different
accuracy levels that control the final capacitance extracted.

 

Stage 4: Output
Figure 5 shows the output GUI that is used to specify the calculation targets and output directories.


Figure 5. The output window allows the user to specify the
database path and output data to be saved.

 

Stage 5: Design of experiments (DOE)
This GUI (Figure 6) define the experiment using predefined variables from the process and layout stages. The only required values are the initial value, final value, number of data points, and the variation form.


Figure 6. The design of experiment GUI is used to control the
variation of any layout and/or process variable.

 

Stage 6: Run
The run stage performs all user-specified calculations and reports the status and progress back to the user. Figure 7a shows the screen before calculations are performed, while Figure 7b shows the status further on into the simulation. Clicking the View log… button (Figure 8) launches the simulation run time output dialog box, which is useful for reference and error checking. Exact2 outputs a file structure and files that contain each layer’s simulation results, as well as their respective combinations and sessions (Figure 9). Users must check out result files before viewing or modifying them..


Figure 7a.When the experiments are ready to be executed
this Run window shows the status of the submitted jobs.

 


Figure 7b. Simulation monitor window as
it appears during the simulations.

 


Figure 8. The Run time output window shows the output
from the 3D process simulator and the field solver
capacitance extraction for the current job.

 


Figure 9. Exact2 output file structure.

 

Stage 7: Analysis.
After a successful simulation run, the script files need to analyze, manipulate, and visualize the generated data-base are loaded with the analysis GUI (Figure 10). A selected script file appear in a text box to the left of the Browse… function and is into the Analysis stage by clicking Add. The Run button executes any highlighted file. The Edit... button launches the built-in text editor for quick modification to the script file(s).


Figure 10. Analysis window used to load and run scripts
to export data files that export the capacitance rule files.

 

A simple script file that outputs capacitance data in both comma separated values (CSV) format and in TonyPlot format, is shown below. CSV files are easily loaded into many data management and spreadsheet programs, such as Microsoft™ Excel™. The resulting TonyPlot file is shown in Figure 11.


Figure 11. Simulated total capacitance versus conductor spacing.

 

db = DatabaseLoad(".");

extract_name("m0Ctotal_sub", "substrate", "m0_c");
extract_name("m4Ctotal_sub", "substrate", "m4_tmc");
m0_combinations = {1};
m4_combinations = {1};

table_m0 = select(db, "model_0", m0_combinations,{"m0_subwidth"}, {"m0Ctotal_sub"});
column_scalar_op(table_m0, "m0Ctotal_sub", table_m0, "m0Ctotal_sub", "*", 1e15)
save_table(table_m0, CSV, "m0_a.csv");

table_m4= select(db, "model_4", m4_combinations,{"m4_cspace"}, {"m4Ctotal_sub"});
column_scalar_op(table_m4, "m4Ctotal_sub", table_m4, "m4Ctotal_sub", "*", 1e15);

save_table(table_m4, CSV, "m4_sim_stan.csv");
save_table(table_m4, TONYPLOT, "m4_sim_stan.str");

 

Conclusion.

Exact2 brings flexibility and simplicity to the creation of a comprehensive and accurate interconnect capacitance database. Exact2 creates the database by means of a 3D process simulator and 3D internal field solver in one self-contained package. Exact2 files are easily imported into LPE tool formats through the use of the LISA scripting language

 

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