Chemical milling is also called "Chemical contour machining", and is a metal milling technique by utilizing chemical etching process. With this technique the etching depth is limited to the thickness of the material to create three dimensional features, unlike through etching techniques.
Chemical milling typically follows a process shown in [Fig.1]. There are four major process blocks: Pre-process; Masking; Etching; and Post process.

When milling the entire surface by etching, proceeds directly to the etching process since the masking process is not needed. When creating steps on the surface, multiple "etch and mask removal" processes are repeated.

(1) Masking

Masking techniques for chemical milling are: (1) with photo resist; (2) screen or offset printing; (3) by scribe and peel; (4) mechanical masking.
The photo resist method, as described previously, creates very fine and accurate but thin resist patterns that provide small chemical resistances to strong alkali and acidic etchants. It is mostly used only for small depth applications.

The screen printing method uses screens (mesh) made of synthetic fiber such as nylon and polyester, or fine filament of stainless steel. Negative images of the needed patterns are created on the screen, then resistive ink is press-roller applied onto the substrate through the screen to create the final patterns.

With the scribe and peel method, masking material is first applied on the entire surface of the substrate, then the masking is cut along a template with a sharp cutter and peeled off to expose the area to be milled.

Mechanical masking method uses a flexible and corrosion resistant rubber template mechanically affixed to the substrate to cover the non-milled area of the substrate. Due to its nature, the mechanical masking methods can only be used for simple patterns.

An appropriate method is chosen from these methods based on required accuracy, repeatability, process speed, and economy.