Hints & Tips

DEVEDIT: A Flexible Tool for Structure Editing and Mesh Generation

Mesh generation has traditionally been one of the most difficult topics in the use of TCAD tools. DevEdit was released in 1992 as a structure pre-processor including interactive mesh generation. Over the previous five years significant enhancements have allowed DevEdit to become a general purpose structure editing and mesh generation tool.

An important feature of all DevEdit generated meshes is an option to not include any obtuse triangles. Obtuse triangles lead to roughness and errors in the device simulation solution. In extreme cases obtuse triangles lead to non-convergence.

The primary application of DevEdit has always been mesh generation for device simulation. Applications have fallen into three basic groups:

  • remeshing of ATHENA structures before S-Pisces simulation
  • pre-processing of III-V structures for Blaze
  • editing of ATHENA geometries for use in ATLAS

For 3D applications, DevEdit3D is used to perform the same functions. Hints and Tips on the use of DevEdit3D for 2D to 3D interfacing are given in [1]

 

Re-meshing ATHENA Structures

The grid dependence of device simulation results has been documented in several places including [2]. Results do become grid independent at certain grid spacing but this may be finer than users expect. A simple example is shown in figure 1. Unless grid density is sufficient to resolve the electric field and the carrier concentration in the channel, erroneous results may be seen. Since both electric field and carrier concentrations have steep gradients in MOS channels the surface grid spacing must generally be on the order of 0.5nm. Even the least grid sensitive modified Watt model (MOD.WATT) requires 5nm grid spacing which can be difficult to achieve without any remeshing.

 

Figure 1. MOS Id/Vgs curve as function of mesh density. A finer channel mesh gives better resolution of fields and carriers which lower mobility in this case. At increasingly finer mesh spacing the results do tend to a fixed value.

 

If the structure is generated within ATLAS it is simple to create a fine surface mesh. However with structures generated by ATHENA it is generally impossible to ensure such a fine surface mesh spacing. Even if such a mesh is defined by the initial mesh generation in ATHENA or by initial adaptive mesh rules the surface processing steps generally degrade the mesh. DevEdit provides a solution by allowing users to discard the mesh used in ATHENA and recreate a new mesh for ATLAS based on simple rules.

Figure 2 shows a MOSFET from ATHENA with a mesh generated by DevEdit . The menus within DevEdit allow users to specify rules for the new mesh generation. For MOS structures these rules focus on a fine mesh in the channel. For MOS it is important that the meshing rules can be made completely independent of solution quantities. Remeshing on doping or even potential gradients in the channel is usually insufficient. For bipolar devices the rules focus the mesh on the base-emitter junction. More details of this approach including syntax examples can be found in [3].

 

Figure 2. DEVEDIT screen illustrating a MOSFET with fine channel mesh defined using simple menu.

 

Pre-processing Structures for BLAZE

For heterojunction device simulation it is necessary to construct a structure consisting of several layers of different semiconductor materials. In many cases there is no process simulation required so ATHENA/ Flash is not appropriate. DevEdit is the easiest and most flexible program for creating III-V device structures. DevEdit allows the user to draw regions of the device interactively using the mouse. It can handle the recess profiles of HEMT or MESFET mesa structures and the stepped profiles used in HBTs. DevEdit also allows the definition of graded and abrupt heterojunctions. Following structure creation the user can mesh the structure based on the same style of meshing rules indicated above.

 

Figure 3. DevEdit allows pre-processing of all types of III-V structures.
The mesh generation rules allow focused mesh refinement in
the channel of this measured gate HEMT.

 

ATHENA Structure Editing for ATLAS

Since the CPU time required to run process simulation is super-linear with the number of grid points there can be significant CPU time savings available by splitting large simulations into sections. These sections can be joined together at the end of the process simulation using DevEdit .

Figure 4 illustrates the JOIN function in DevEdit allowing users to select several ATHENA structure files and join them together.

 

Figure 4. A single complete device structure can be constructed in DevEdit by joining
two or more ATHENA results. The silicon surface is auto-aligned to leave a smooth surface

 

Automated Mesh Generation

Although interactive mesh generation is easy-to-use, there is a need to automate the structure editing and mesh generation process. This is done though the batch mode of DevEdit . DevEdit is implemented into DeckBuild, see Figure 5, as a fully batch mode simulator including automatic interfaces to ATHENA and ATLAS.

 

Figure 5. DeckBuild with batchmode syntax of DevEdit. Running in
batchmode allows automation of all DevEdit features.

 

It is possible to construct DevEdit syntax using popup menus in DeckBuild. However the most convenient way is to use the interactive DevEdit to define the structure or mesh rules. A DevEdit command file can then be saved that includes all the commands performed in interactive mode.

These command files can be edited to allow grid definitions based on previously results of EXTRACT statements. This allows grids of similar structures to be formed with the same set of rules to create consistent meshes. Such an approach can be used inside the VWF automation tools to remove grid dependency when experimenting with process parameters.

Conclusion

Mesh generation using DevEdit allows users to create grids for arbitrary structures based on simple rules. The interactive mode makes the program easy to use while a batch mode allows automated mesh generation for similar structures. In addition to remeshing structures, DEVEDIT is used to edit structures prior to ATLAS device simulation. Features such as the JOIN function allow users to create large structures for device simulation from a a set of smaller process simulation results.

References

[1] Hint, Tips and Solutions, "Simulation Standard", April 1997.

[2] TCAD Calibration: Challenges and Opportunities, Michael Duane,
Chipps '97 conference, (www.prismnet.com/~naomi/tcad)

[3] Hints, Tips and Solutions, "Simulation Standard", June 1996.