Wet etch is typically used to remove a top layer from a semiconductor wafer, without harming the structure underneath that film, or etch cavities into a wafer - parts of which are protected by a mask. Wafers are either immersed in a bath, or sprayed on, with chemistries depending on the film that requires removal.
The active chemicals are consumed during etch, and need to be spiked in order to maintain concentration and maintain etch rate without damaging underlying structures.
An important Si Nitric etching (H3PO4) is used at very elevated temperatures (~160C) . At these elevated temperatures, CI's NTM W was especially designed to allow accurate wafer temparature monitoring and control.