Hints & Tips

Q: I use SSUPREM4 for process simulation, but I need more realistic models for
deposition and etch. How can I use the ELITE module of ATHENA to do this?
How does the interface from ELITE to SSUPREM4 work?

A: ATHENA is a general purpose two-dimensional process simulator that includes modules for implant, diffusion and oxidation for silicon (SSUPREM4), implant and diffusion models for compound semiconductors (FLASH), topography (ELITE) and lithography (OPTOLITH). This means that it is simple to include physical etch or deposition steps using ELITE models in an existing SSUPREM4 input file.

As device dimensions shrink the need for more physical simulation of the deposition and etch steps in a process increases. ELITE provides these physical deposition and etch models. SSUPREM4 users can only use conformal deposition and geometrical etch features built into ATHENA. These simple models may not be sufficient to describe certain steps in the process satisfactorily.

For example, in a typical sub-micron CMOS process, ELITE models might be required for:

1. Trench isolation

2. Spacer formation

3. Reflow of oxides over non-planar surfaces

4. Metal to active area contact cuts

5. Metal deposition over step

6. Inter-metal dielectric formation.

In general ELITE should be used for any etch process with a degree of isotropy, since perfectly anisotropic etches can be handled geometrically in SSUPREM4. For deposition processes ELITE is appropriate when the deposition is significantly non-conformal.

Many topography simulators exist, but interfacing them to process simulation programs such as SSUPREM4 has traditionally been a problem. Without the tight integration of ATHENA, the interface has traditionally been one way (for example, creating a non-planar topography such as a trench and then using the surface to create the initial structure for a SSUPREM4 simulation).

In ATHENA the bi-directional interface between topography and process simulation is completely automatic and transparent to the user. Figure 1 shows this interface used to form a self-aligned trench isolation for a sub-micron CMOS process. The initial part of the simulation uses SSUPREM4 to set up a LOCOS oxidation next to a nitride spacer. ELITE is then used to remove the nitride and etch a trench into the silicon. SSUPREM4 is used to oxidize the trench sidewalls. Then the ELITE deposition models are used to fill the trench with oxide. Finally a planarizing etch is performed.

Figure 1. Simulation of self aligned trench isolation process using the ELITE and SSUPREM4 modules of ATHENA. SSUPREM4 is used for the LOCOS and trench oxidation. ELITE is used for the trench etch and refill. The interface between SSUPREM4 and ELITE is completely automatic and transparent to the user.

The syntax needed to access the ELITE models can be found using the DeckBuild Command Menus. The main parameters are RATE.ETCH MACHINE=<name> to set up parameters for the etch machine and ETCH MACHINE=<name> TIME=<value> to run that machine for a given time. Analagous commands exist for depositions.

One key parameter for users of ELITE is DX.MULT=<value> on the ETCH statement. This parameter sets the ratio between the grid spacing used by SSUPREM4 and the surface accuracy used by ELITE. The default is 1.0. Lower DX.MULT values will improve the accuracy and smoothness of etch shapes at the expense of some additional CPU time.