Книги2 / 1993 Dutton , Yu -Technology CAD_Computer Simulation

Chapter 2

Introduction to SUPREM

2.1Introduction

While the exact form that VLSI technology would take in the future was uncertain in late 1970s, it seemed evident that costly and timeconsuming empirical approaches to developing and optimizing such technology are a luxury few will be able to afford or wish to justify. A far more attractive alternative is the formulation of accurate models of the basic physical processes involved, and their implementation in a comprehensive computer program. Such a program would allow predictions of device structures resulting from any proposed fabrication sequence and would minimize the need for empirical iterative attempts. Since its inception, the process simulator SUPREM has been one such attempt to realize this goal. Beginning with SUPREM I and proceeding to SUPREM II and III, each version has drawn from the models and physical understanding of fabrication processes then available.

The overall objective of SUPREM is to permit a technology designer to accurately simulate complete technology sequences. The program input is, in essence, a processing schedule specifying a sequence of times, temperatures, ambients, depositions, implants, and other necessary process specification inputs. The program output is available after each step in the sequence and includes one-dimensional impurity distributions in all layers including dielectrics and polysilicon. Also available are layer thicknesses, junction depths, and electrical data such as sheet resistance and threshold voltage.

To see how the SUPREM III program works, we will consider the

38 CHAPTER 2. INTRODUCTION TO SUPREM

SELECT FIELD

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Figure 2.1: (a) Process flow for the n-well diffusion and local oxidation to create the isolation regions for an n-well CMOS process. Both schematic process description and representative technology crosssections are shown.

process sequence that creates the n-well in the Stanford CMOS technology. Figure 2.1 (a) illustrates the process flow. The wafer is oxidized and dopant atoms are ion implanted. A short well drive-in is carried out in an oxidizing ambient, and is then followed by a longer drive-in in an inert ambient. Next, a nitride layer is deposited and patterned for the local oxidation step used to isolate the devices. The p+ channel-stop diffusions are also patterned during this local oxidation sequence. (A third mask, not shown in Figure 2.1 (a), is used to exclude boron from the n-well regions.)

A sequence of steps used for the SUPREM III input is listed in Figure 2.1 (b). The first input commands give essential data concerning the starting material and grid structure needed to model the ensuing steps. The total window of interest is less than 51lm with a minimum grid spacing of 100 A; the starting substrate material is < 100 > silicon doped to 9 x 1014 cm-3 with boron. Subsequent input commands model

Figure 2.2: (a) Process flow for a portion of an epitaxial buried layer bipolar process sequence. Both schematic process description and technology cross-sections are shown.

the sequence of steps as discussed above and shown in Figure 2.1 (a), and the remaining commands relate to the display of results. Note that some layers are grown/deposited, then etched away. Figure 2.1

(c) shows the cross-section of the n-well diffusion after the three steps listed in Figure 2.1 (b). Some of the details shown in Figure 2.1 (b) are representative of the real process but non-essential for simulation. In particular, the temperature ramping (t.rate) steps reflect actual furnace conditions but, for long drive-in conditions such as the n-well, are not essential.

Figures 2.2 detail another useful example - the process flow for a standard double-diffused bipolar technology. Figure 2.2 (a) shows the process flow in schematic form. Note that the surface becomes terraced following the various masking and oxidation steps. Figure 2.2 (b) shows the process flow in terms of the input file for SUPREM simulation. In

this example, a buried layer of dopant is introduced before the growth of an epitaxial layer. After the epitaxial growth, the structure contains a highly doped n+ layer buried below the n-epitaxiallayer. Figure 2.2 (c) shows the output results for the SUPREM input given in Figure 2.2 (b). The n- region results in a low collector capacitance, while the n+ region provides a low-resistance path to the external collector contact. Following the epitaxial layer growth, sequential masking steps are used to create the base and emitter regions.

Figures 2.3 shows the structure and profiles obtained from the sequence of diffusions following epitaxial growth as detailed above. Figure 2.3 (b) profiles the extrinsic base region, without emitter diffusion, while Figure 2.3 (c) shows the cross-section including the emitter. The high-concentration phosphorus present in the emitter enhances the diffusion of boron in the underlying base region. The net result is that the intrinsic base is "pushed out" beyond its depth in the extrinsic base region.

Base push-out is just one example of the situation in which boundary conditions can dramatically alter dopant diffusion. The enhanced diffusion of dopants under oxidation is another such effect. These effects, and others, are built-in features of the process models. The remainder of this chapter will consider these models in greater detail and will discuss the users' means to the controlling of model parameters.

2.2 Ion Implantation

Ion implantation is a critical tool in achieving process control for both bipolar and MOS devices. For MOS devices, ion implantation's precise control of dose and range facilitates the exact tailoring of parameters such as threshold and punch-through voltages. In this section, we will look in greater detail at the ion implantation models used in SUPREM III.

Modeling can be useful for many aspects of the ion implantation. If the implanted profile experiences sufficient thermal cycling, the initial profile shape will be masked and the primary quantities of interest in the simulation will be simply the ion dose and range. Figure 2.4 (a) is an example of a SUPREM III program in which only the "dose" and

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Figure 2.2: (C) Semi-logarithmic plot of doping versus depth for the SUPREM III input sequence shown in Figure 2.2 (b). The profiles shown correspond to the n+ Sb implant, diffusion, and n- epitaxial growth.