Introduction to VLSI Design Flow

In the previous chapters, we discussed the tasks required for designing an IC. Once we have obtained the final layout, the design process is complete. Subsequently, the GDS file corresponding to the final layout is employed for making a chip using tasks such as mask preparation, wafer fabrication, testing, and packaging. In this book, we have grouped all chip-making tasks carried out after obtaining the GDS file as post-GDS processes.

Though post-GDS processes are not directly related to the design flow, we need to understand them to appreciate the challenges of fabrication, and possibly address some of these challenges during the design phase. Therefore, we briefly explain post-GDS processes in this chapter. For a detailed understanding of post-GDS processes, readers can refer to dedicated books on these topics such as [1–5].

MASK FABRICATION

We have discussed in Chapter 2 (“Introduction to integrated circuits”) that an essential step in IC fabrication technology is photolithography. It involves transferring the patterns in a layout for a given layer to the silicon wafer. We carry out photolithography separately for each layer.

To start with, we create a replica of the pattern of a given layer on a substrate such as glass. This replica of the pattern is known as mask or reticle. After creating a mask, we use it many times for carrying out photolithography during high-volume manufacturing [4].

We can fabricate a mask using several techniques. Nevertheless, a typical mask fabrication flow consists of the following steps:

• 1. Data preparation

• 2. Mask writing and chemical processing

• 3. Quality checks and adding protections

We explain these steps briefly in the following paragraphs.

Data Preparation

First, we prepare the given layout data for mask writing. We translate the GDS-specified mask information to a format comprehended by a mask writing tool. It involves converting complicated polygons to simpler rectangles and trapeziums. This process is popularly known as fracturing. It simplifies the task for the mask writing hardware. Additionally, data preparation involves augmenting the mask data to enhance the resolution. We will describe some of these techniques later in this chapter.