GaAs Process Simulation Using ATHENA and Flash

In response to requests from many customers, Silvaco has developed GaAs process simulation capabilities. These capabilities are contained in a new product, Flash, which works as part of ATHENA.

Introduction to ATHENA

ATHENA is a modular 2-D process simulator. The other ATHENA modules are: Optolith, for simulating lithography steps; Elite, for simulating deposition and etch steps; and SSuprem4 for simulating oxidation, diffusion, and ion implantation in silicon. SSuprem4 in turn has two secondary modules: Silicide to simulate silicide processing; and MC Implant for very detailed Monte Carlo based simulation of ion implantation.

The structures predicted by ATHENA can be used as input by the ATLAS II device simulator. By running ATHENA and ATLAS II in combination, users can predict the influence of processing variations, such as implant dose and energy, or oxidation times and temperatures, on any DC, small-signal AC, or transient device characteristics.

Comprehensive Capabilities

ATHENA is a significant milestone in the development of process simulation tools. It is pioneering in several respects:

 

  • The individual modules of ATHENA provide many unique capabilities, and are continuously being extended in response to inputs from customers.
  • ATHENA supports full bi-directional transfer, between all modules, of information pertaining to general 2-D non-planar structures. This gives ATHENA the ability to simulate complete process flows, rather than just isolated steps.
  • ATHENA makes full use of the MASTER tools which support powerful yet convenient interactive use.
  • ATHENA is tightly integrated within the Virtual Wafer Fab, which automates simulation studies at the design task level.

 

As a result of these factors ATHENA provides the most advanced capabilities available anywhere for process simulation.

Meeting GaAs Process Simulation Needs

Flash simulates ion implantation, impurity diffusion, and epitaxy, all within the ATHENA environment. It incorporates many sophisticated models that were previously available only to silicon technologists. For example, predictive Monte Carlo based ion implant modeling that incorporates full crystal structure is available.

The other ATHENA modules provide comprehensive models for optical lithography, deposition, etch, and lift-off. Critical patterning steps such as gate formation can be performed with the full sophistication of Optolith; and the evolution of topography during steps such as spacer formation for self-aligned processes is simulated using Elite. The realistic non-planar GaAs device structures predicted using ATHENA and Flash can be used directly as input by ATLAS II/BLAZE.

An Example: GaAs MESFET Fabrication

We will now show how ATHENA equipped with Flash , Optolith and Elite is used to simulate the complete fabrication of a GaAs MESFET The channel of the MESFET is formed by implanting and annealing silicon and beryllium. The one-dimensional profiles produced by the implantation are shown in Figure 2 (a). These are calculated using the 1D mode of Flash . All subsequent simulations are performed in 2D mode.

After channel formation, the structure is capped with nitride in preparation for gate formation via dielectric-assisted lift-off. Optolith is used to simulate the critical imaging, exposure, and development steps. Optolith handles phase shifting masks, and simulates resist bleaching, post-exposure bake, development, and post-development bake in a detailed resist modeling sequence. Figure 2 (b) shows the photoactive compound following exposure. Figure 2 (c) shows the simulated structure following resist development, and Figure 2 (d) shows the structure following an etch to form the lift-off structure.

The Monte Carlo based deposit model in Elite is used to simulate the deposition of titanium. The deposited material is shown in Figure 2 (e). Elite includes an explicit lift-off model that predicts the structure following photoresist expansion during lift-off. The structure at this point is shown in figure 2 (f). State-of-the-art deposition and etch models are used to simulate spacer formation. The final processing involves source and drain formation by silicon implantation, and contact formation and alloying. The completed structure, with a plot of electrically active impurities, is shown in Figure 1. This structure is ready for electrical analysis with ATLAS II!

 

Figure 1. A GaAs MESFET structure simulated
using ATHENA and Flash.

 

 

(a) Channel profiles after implanting silicon and beryllium.

 

 

(b) PAC following exposure for gate formation.

 

 

(c) Photoresist development shows standing waves produced
during complete exposure calculation.

 

(d) Etch inclduing undercut for dielectric assisted liftoff.

 

 

(e) Gate metalization including shadowing effect.

 

 

(f) The gate structure following liftoff.

Figure 2. Simulation of GaAs MESFET fabrication

 

Conclusions

ATHENA has been extended to meet the needs of developers of GaAs technologies. A new ATHENA module called Flash provides the ability to simulate ion implantation and diffusion in GaAs. The combination of Flash , Optolith , and a newly extended version of Elite, supports the simulation of complete GaAs process flows.

Flash is in beta test at sites in the U.S., Japan, and Europe. It will be available as part of the next release of ATHENA, which is scheduled for January 1994. For more information about ATHENA, ATLAS II, or the Virtual Wafer Fab, please contact your local Silvaco sales office.