How high is the plasma etch resistance of photoresists?

Photoresists of the AR-series 3000 to 4000 display a very high etch resistance due to the polymers which are used for their production. This is especially the case for dry etch processes such as e.g. argon sputter, CF₄ and CF₄/O₂-mixtures. A postbake at 110 °C prior to the etch process for re-stabilization increases the etch stability slightly. The etch stability can be further improved by adding aromatic conjugated compounds.

Resist etch rates are highly dependent on the etch conditions. In addition to the equipment used (plasma etcher), the rate is also influenced by etch gas composition, pressure, temperature, and voltage.

The following parameters were determined for our photoresists at 5 Pa and 240 – 250 V bias:
Argon sputter:           3 – 8 nm/min
CF₄:                       31 – 42 nm/min
80 CF₄ + 16 O₂:       81 – 93 nm/min
O₂-plasma:           122 – 174 nm/min
Respective parameters for each type of photoresist are specified in our Parameter Collection (see “Plasma Etching Rates”)

Careful etch procedures (low pressure and voltage) lead to a higher resistance of the resist, but the etch process requires more time. Cooling during the etching improves the resistance accordingly. If resists are etched too rigidly (e.g. too high temperatures), the subsequent removal step will cause major problems.