Introduction

SUPREM-IV.GS is the first version of SUPREM-IV which models GaAs and its dopants in addition to modeling silicon fabrication technology. SUPREM-IV is an advanced 2D process simulator originally developed for submicron silicon structures. It provides cross-sections of arbitrary de- vice structures based on physical models for ion implantation, diffusion, oxidation and annealing. It is designed to interface with other programs that accurately simulate etching and deposition of thin films on the semi- conductor surface, although it includes basic models for these processes as well. The program structure is designed to handle moving boundaries that commonly occur during oxidation or during deposition or etching of thin films. SUPREM-IV can model stress gradients produced by overlaying films. The implantation, diffusion annealing and oxidation models are point defect based and are believed to be the most advanced models used in any process simulation program. While the oxidation capability will not be needed for simulating GaAs structures, all the other modules can be di- rectly applied to compound materials.

In version 9305 of SUPREM-IV.GS, we started with version 9130 of SUPREM-IV and incorporated most of SUPREM 3.5's 1D GaAs models and parameters. This involved modifying SUPREM-IV so that it recogniz- es a new semiconductor material and new dopants, and allows for new ex- pressions and parameters for the GaAs dopants in terms of the various fabrication processes.

SUPREM 3.5 was developed to provide 1D concentration-depth profiles through arbitrary cross-sections of GaAs devices for the specific process- ing conditions being simulated. SUPREM 3.5 is primarily designed for modeling processes used to make simple ion implanted MESFET and JFET structures in semi-insulating GaAs, with or without buried p-layers. The main processes modeled are ion implantation (into bare GaAs or through a silicon nitride or oxide cap) and the activation anneal (which in- cludes dopant diffusion and activation). The dopants modeled are: Si, Se, Ge, and Sn (the 4 common n-type dopants); and Be, Mg, Zn, and C (the 4 common p-type dopants). Also modeled is the diffusion of non-implanted dopants, such as those incorporated during MBE or MOCVD GaAs growth, which sometimes have different effective diffusion coefficients than implanted dopants. Incorporating GaAs and SUPREM 3.5's models and parameters into SUPREM-IV is only the first step. The simulator must be able to take into account effects such as implant damage, substrate sto- ichiometry, and defects, all of which are important in GaAs processing. More advanced models and parameters must be developed to model such things as time-dependent diffusion, uphill diffusion, and time-dependent activation and other "anomalous" phenomena, most of which are believed to be related to point defects and their interactions with dopants. In addi- tion, 2D effects during implantation, diffusion, etc. must be modeled. It is planned that a version of SUPREM-IV.GS that models many of these ef- fects and phenomena be released in 1994.

Even without those advanced models, a version of SUPREM-IV with GaAs and SUPREM 3.5's 1D parameters will be very useful. For one, it will allow one to use SUPREM-IV's point defect and 2D capabilities to help develop the more advanced models for GaAs. Secondly, there are many cases where simple SUPREM 3.5 type models are quite adequate to model the fabrication process and having 2D capability will be very use- ful.

The following briefly summarizes what has been added to SUPREM-IV to make this version 9305 of SUPREM-IV.GS. GaAs and eight GaAs dopants have been added to SUPREM-IV. The Pearson-IV implantation parameters have been included. The electron or hole dependent diffusion coefficients have been put in for these dopants. Other parameters for GaAs, such as segregation coefficients, intrinsic carrier concentrations, and defect energy levels, have also been added. In addition, compensation mechanisms for dopant activation and different diffusivities for implanted versus grown-in dopants have been added. (These two capabilities were not in the silicon-only versions of SUPREM-IV and were added especially for GaAs. However, they may be useful for silicon modeling as well.) SUPREM-IV.GS can now simulate implantation, film deposition and etch- ing, and dopant diffusion for GaAs, as well as all the silicon capabilities.

In the following sections, the addition of GaAs and its SUPREM 3.5 pro- cess models to SUPREM-IV is discussed in more detail.