Wafer steppers, introduced briefly in Chapter 1, are discussed further in this chapter, paying particular attention to the key subsystems of the modern reduction stepper, such as light sources, illuminators, reduction lenses, and the wafer stage. Alignment systems will be discussed in more detail in Chapter 6. In all of these discussions, the viewpoint will be that of the user.
Prior to the advent of wafer steppers, circuit patterns were transferred from masks to wafers by contact or proximity printing, or by using full wafer scanners. In contact printing, a mask that had the pattern for all chips on the wafer was brought into contact with a resist-coated wafer. The mask was illuminated, thereby exposing the resist under the clear features on the mask. This method of exposure was used in the earliest days of the semiconductor electronics industry, but the mechanical contact caused defects on both the mask and wafer, reducing productivity. Proximity printing, where masks and wafers were brought close to each other, but not into contact, was one approach to reducing the problem of defects that arises with contact printing. Unfortunately, resolution was poor when the gap between the mask and the wafer was a practical size, as a consequence of diffraction. The first workable solution to this problem was the full wafer scanner, which also used a mask that contained the patterns of all chips that were transferred 1:1 to the wafer.
The most common method for making the masks used for contact, proximity or full-wafer scanning lithography involved a photorepeater. The photorepeater had a stage on which an unpatterned mask could be placed and moved precisely (Fig. 5.1). A reduction lens was used to image the pattern of a single chip onto the resist-coated mask blank, exposing the mask one chip at a time and using the precise stage to move the mask between exposures. In order to distinguish between the mask being made with multiple chips and the master object containing the pattern of only one chip, the single-chip mask was called a reticle. Eventually it occurred to someone to eliminate the intermediate mask and exposure tool, and essentially to use a photorepeater to expose the wafer directly. Thus, the wafer stepper was born.
A cutaway view of a modern wafer stepper is shown in Fig. 5.2 (see Color Plates Section). This view shows all the major subsystems: reduction lens and illuminator, excimer laser light source, wafer stage, reticle stage, wafer cassettes, and operator workstation. In this particular figure, wafers are being taken from cassettes.
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