Cross-Sectional Viewer in Expert

 

1. Introduction

Expert is extremely powerful circuit layout editing tool with the integrated ability of geometric design rule checking and LVS design verification.

IC technologies continue to develop and become more and more complex. With increase of complexity in the contemporary design processes the designer must have knowledge not only of layout design, but in simulation and manufacturing as well. The more the design engineer knows about the process with respect to layout and technology, the higher the performance he can design.

Cross-Sectional Viewer is a tool within Expert to simulate the cross sectional view of ICs along an arbitrary drawn cut-line on the layout. Cross-sectional Viewer is a link between the layout and the resulting device. It allows the designer to examine cross-sections of the device being designed. Cross-sectional drawings are useful for understanding design rules, parasitic coupling and other design and fabrication problems.

The current Expert's implementation of Cross-Sectional Viewer is based on the simplified simulation of the process steps using rough processing parameters from Expert technology file.

Future enhancements of the Cross-Sectional Viewer will include interfacing with SILVACO process and device simulation tools that will allow the users to obtain physically accurate representations of the physical structure of ICs in the problematic areas of the layout.

 

2. Cross-Sectional Viewer Setup

Approximate parameters for the simulation of fabrication step must be specified in the technology file. For simplicity we assume that each fabrication step is associated with a separate layer in the layout. This layer may be either a drawn layer or a derived one, i.e., produced by boolean, resizing or any other DRC operations on the drawn layers.

There are two ways to create and modify process simulation parameters:

  • manually editing Expert technology file or

  • interactively, using dialog panel named "Cross-sectional View"(Setup>>Cross-sectional Views), see Figure 1.


Figure 1. Cross-sectional viewer setup.

 

The Processing and Material sections of layer definition must be presented in the technology file for each processing step as follows:

Layer

{

Name = "NWELL"

...

Material

{

...

Thickness = 4

}

...

Processing

{

ProcessingStep = 1

Operation = DIFFUSE

Undercut = 0

Angle = 100

}

}

Each processing step is described by the following parameters: Layer Name, ProcessingStep, Operation, Thickness, Undercut and Angle. The 'Layer Name' is the name of layer in the technology. In some cases we must add special layers for the cross-sectional representation of layout. These layers may be without geometry or derived from other layers using derivation rules. 'ProcessingStep' is the number of the step in the fabrication sequence. 'Operation' must be one of the following fabrication steps

  • DEPOSIT
  • ETCH
  • IMPLANT, DIFFUSE
  • OXIDIZE.

'Thickness', 'Undercut' and 'Angle' fields are additional parameters for simulation operations described in the following sections.

DEPOSIT
This operation is used to simulate metal deposition or oxide growth above the whole surface of the structure, see Figure 2. Parameter 'Thickness' is used for the description of this fabrication step. Thickness must be specified in the units of the technology file for manually editing and in the measurement units of the current cell view if the dialog panel "Cross-sectional View" is used.


Figure 2. Thickness parameter for deposit step.

 

ETCH
Etch operation simulates the etching or patterning of a material layer. Parameter 'Thickness' defines the depth of the etching. Note, in the current implementation all layers are etched uniformly regardless. If you want to simulate "undercut" and slant effects that take place during etching, you can use 'Undercut' and 'Angle' parameters, see Figure 3.


Figure 3. Etching step parameters.

 

IMPLANT, DIFFUSION
Ion implantation and impurity diffusion processes are described using this step. These operations are used to create n+ and p+ doped regions as well as n- and p-wells. Parameter 'Thickness' indicates the depth of the p-n junction. 'Undercut' and 'Angle' parameters can be used for simulating lateral effects that take place during high-temperature diffusion. For describing implantation and diffusion, the same model as for etch is used, see Figure 3.

OXIDIZE
This step is to simulate local oxidation of silicon. Parameter 'Thickness' defines thickness of the field oxide. It is assumed that field oxide both recesses into the surface (depth T1 = 0.45 * Thickness) and grows above the surface (T2 = Thickness - T1). Using additional parameters, "Undercut" and "Angle", you can simulate "bird's beak" effect, see Figure 4.


Figure 4. Local oxidation parameters.

 

3. Using the Cross-sectional Viewer

Cross-sectional Viewer requests specific information to be presented in the technology of the opened project. The Setup>>Cross Sectional Views or Tools>>Cross Sectional Views>>Setup commands pop up the "Cross-sectional View" dialog panel for adding and editing parameters of simulation of fabrication steps, see Figure 1. The dialog parameters are as follows. 'Add' button is to add a new step into fabrication sequence. You must select the type of the process step from the drop-down list of the 'Type' field and specify the depth or thickness of the fabrication layer using 'Thickness' field. Thickness is measured in the technology units or units of the current cell view if the latter is opened. Additional fields 'Undercut' and 'Angle' can be specified for more realistic simulation of the described processes, see "Cross-sectional viewer setup" section. A fabrication sequence may be changed by dragging the selected operation in the Operation/Layer window. You may delete the selected fabrication step by pressing the 'Delete' button. The 'Settings' tab in the "Cross-sectional View" dialog panel is to select the representation of the opened cell view and the calculated cross-sectional view for a specified cut-line.

To generate a cross-sectional view for the opened cell, one must select "Tools>>Cross-sectional Views>> Generate" command. If no cross views for the current cell exist, the cross-sectional view tool will appear at your cursor. To specify a cut-line, one must move the cursor to a point on the layout view, press and release the left button, drag a line across layout in an arbitrary direction, and finally press the left button to finish the cut-line. After drawing the cut-line, the Cross-sectional Viewer Tool generates the corresponding cross view. The Cross View will appear according to the settings in the "Cross-sectional View" dialog panel, see Figure 5. ('Tile Cross View and Cell View'), 'Show cross lines on Cell View' options are checked). The Cross View window is like an ordinary Cell View. It can be zoomed to a more close investigation of separate parts of the Cross View. You can also print Cross View as any Cell View to make hard copy using "Project>>Plot" command for documenting the layout.


Figure 5. Cross-sectional line and the resulting cross-view.

 

The "Tools>>Cross-sectional Views>> Generate" pops up the "Cross Views" dialog if there are cross views for the current cell, see Figure 6. You can select the precalculated cross view by pressing the mouse left button or using the 'Down' or 'Up' keys. The cut-line corresponding to the Cross View will be selected on the Cell View and the fields 'First point' and 'Last point' will provide the cut-line coordinates on the layout. The 'New' button is to define a new cut-line and generate the correspond cross-sectional view. You can specify a newcross-sectional view by filling the 'First point' and 'Last point' fields at the 'Cross Views' dialog panel and pressing the 'Apply' button. All cross views are stored in a temporary file while the library is loaded. This allows the user to quickly review the cross views of interest. This feature is handy to get a three-dimensional feeling of critical pieces of the layout: one may precalculate several closely spaced cross-sections and then view them in rapid succession by clicking the 'Down' or 'Up' keys obtaining an animated view of the cross-sected "terrain".


Figure 6. Selection from precalculated cross-views.

 

To remove an unnecessary cross view, one can use the 'Delete' button. The "Setup" button pops up the "Cross-sectional View" dialog panel to edit the fabrication sequence and process step parameters. Expert Cross-sectional Viewer has a feature that allows the user to to view the actual individual fabrication processes one step at a time and in the exact sequence that they occur. "Cross-sectional Views" menu command, see Figure 7, allows the user to choose the first, last, next or previous fabrication step for viewing. The cross view will be updated up to the specified process step. You can assign shortcuts for these menu commands to simplify the usage of this feature. "Fabrication Steps" dialog panel is a more powerful tool of Cross-sectional Viewer to show "step by step" simulation of fabrication sequence, see Figure 8. To invoke this dialog use "Tools>>Cross-sectional Views>>Step by Step" menu command. Press left mouse button or keys 'Down' or 'Up' to select the processing step you want to view. The Cross View will be updated to reflect the device structure after this process step.


Figure 7. Step-by step viewing of fabrication sequence.

 


Figure 8. Random access for fabrication steps.

 

For automatic generation of Cross Views for the current Cell View you can use XI command 'cross view (start_x) (start_y) (end_x) (end_y)', see Figure 9. This command is useful for the creation of "animated views" described above.


Figure 9. Cross-view animation example using xi-script.