Briefly, let me attack it a different way. A stepper has a field size that it exposes in discret units of roughly 22mm X 22mm therefore the system has to be able to control focus, intensity, uniformity, and other exposure parameters over that unit area (~500mm2). Other than that, it must be able to resolve the feature size desired (0.25u and up) and be able to reposition (step) the wafer under the lens precisely to re-align the new unit area under the reticle such that it provides good registration between the reticle filed and the wafer field. All this represents discrete packets of processing. These packets of processing must be reproducible from one field to the next across the wafer processed and then conmsistent wafer to wafer. Even if we look at some of the larger field systems we are not really talking about anything larger than 30mm X 30mm or just under 1000mm2.
Practically every other operation of the process requires some sort of uniformity across the entire wafer surface, which at 200mm means 31,500mm2 and at 300mm means 70,700mm2. This is the big issue going to 300mm from 200mm, since this increase of 100mm in physical diameter and doubling of the area requires just as uniform a process as the smaller 200mm diameter wafer received. Hence the heating element uniformity analogy. You need to design a whole new set of tooling.
The comparison of processing control is the stepper's 500-1000mm2 to the other equipment's 31.5K-71K mm2 area or 30-100 times as much area to control by other disciplines.
Step and Scan systems beyond the traditional steppers has some challenges but I do not believe they are as monumental as with other types of equipment. I will try to describe. You have a 22mm X 22mm field size. In a traditional stepper, the wafer stage moves under the reticle which is fixed into place under the lens/optics system. Suppose we create a reticle field that is 22mm X 44mm long or twice the area the lens/optics can handle. We could utilize our vast experience in the old Perkin Elmer scanning technology where the wafer and the mask(we did not call them reticles back then), after being aligned (registered) to each other, scanned across the lamp/exposure optics package, to create a high tech version of this. In the step and scan, we will align the 22mm X 22mm field and then have a similar projection printer like slit that will exposre the field as both the reticle and wafer stage now move. Hence, step and scan across. You could effectively increase overall throughput.
This is not exactly what happens but it is close enough to give you a visual impression of things.
Andrew |
