Step and Repeat
What good is it to make just one copy of a little chip onto a big wafer? Don't we need to fill the whole wafer with hundreds of copies of the chip? Indeed we do, and that's why these machines are called steppers. After projecting one image onto the wafer long enough for the photoresist to harden, the stepper moves the wafer slightly to the left and projects the image through the reticle again. After that chip is done, the wafer moves over one more position, and again the process is repeated. Like plowing a field from one corner to the other, a stepper passes from left to right, and top to bottom until the entire wafer has been exposed (see Figure 4.7). The entire process takes just a few minutes.
Figure 4.7. This wafer has been through several stages of the stepping and etching processes. The chips inscribed on it are clearly visible, making a grid pattern on the wafer's surface. Later, the wafer will be cut apart with a diamond-tipped saw, separating each of the square chips. Courtesy of Texas Instruments. Used with permission.
Why not use a bigger piece of film and expose the entire wafer at once? Wouldn't it take less time and cost less money to do all the chips in a single exposure? The reason is one of focus. As owners of big-screen TVs know, the sharpness of a picture deteriorates as it gets bigger. Photographers also know that big poster-size prints are blurrier than small 5 × 7 portraits. That's just the nature of photography and light. Steppers can't afford to sacrifice sharpness for speed. Each chip has to be absolutely sharp and perfect. It doesn't make sense to mass-expose an entire wafer if all the chips around the edges will be thrown away.
Amazingly, even the corners of normal chips are a bit blurry. Only the center of the chip is truly in focus. Intel produced an extraordinarily large chip in the early 1990s that was octagonal instead of square. The corners had to be cut off because they would have been too badly out of focus to work.