8.4.1 Open-loop control
Open-loop is the crudest way of controlling etch depth. It relies on ensuring that every aspect of the process that can affect the rate of progress of the etch is kept under tight control. This can add up to a sizeable list. Table 5 shows just some parameters that affect both wet and dry etching.
Whether it involves wet or dry etching, once all these variables have been fixed, the etch system must then be calibrated for etch rate. A sample wafer is etched for a known amount of time after which the etch depth is accurately measured, either with a surface profiler or by looking at a cross section under an SEM.
Pure open-loop control of etch depth is the last resort. With so many parameters needing control, there are many ways in which the process can go wrong. It is rarely possible to guarantee the etch depth to better than a few per cent and in many cases this is simply not good enough. What is really needed is some way of keeping track of the etching while it is going on, and this leads us to the next approach to the problem.