8.4 Stopping the etch
Just as important as being able to remove material is being able to stop doing so once the intended etch depth has been reached. Success or failure in this aspect of etching determines whether or not any of the benefits of parallel processing of thousands of devices will be obtained. Uniformity of result from device to device, and repeatability from wafer to wafer, are crucial to the economic viability of the whole exercise.
There are three broad categories of approach to this problem:
Open-loop control – by close attention to maintaining consistent process conditions, calibrating the system for etch rate and then using timing.
Closed-loop control – by monitoring in real time some indicator of the progress of the etch.
Self-limiting – by designing the device in such a way that upon completion of the etch, there's no more etchable material in contact with the etchant.
The first two approaches entail the active intervention of the person or machine doing the etching, while the last is a ‘fail-safe’ approach whereby leaving the wafer longer than necessary in the etchant has no significant consequences.