Overview

Most of us design systems based upon experience. This tends to mean that each new system is very similar to the previous one with some minor modifications aimed at reducing the problems that the last machine had. This approach has some merit in that if every time we started with a blank sheet of paper and ignored all the good things about previous designs we would be repeating an assortment of mistakes.

There are some aspects of design, that I see repeatedly, which are detrimental to the process which in many cases, once the system is built, cannot be rectified without a complete system rebuild. The most common pair of problems, by far, is that for reactive processes most systems are (a) underpumped and (b) the pumping is not uniform (Fig. 22.1).

Figure 22.1: A schematic representation of the pressure gradient produced from nonuniform pumping. Many large vessels are pumped via the end plate, which would maximize this gradient.

Part of the advantage of roll-to-roll coating is that the web moving across the sources gives the opportunity for good coating uniformity in the down-the-web orientation. There is then a struggle to get similar uniformity of coating in the across-the-web orientation. Often there are elaborate shields and source designs made to aid across-the-web uniformity but little may have been done with respect to the pumping performance and uniformity. Let us now look at these two aspects of the process.

22.1 Pumping

Consider the deposition of an oxide layer such as...