17. How high is the plasma etch resistance of photoresists?
Photoresists of the AR-product lines 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 restabilization 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 – 173 nm/min
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 resistance accordingly. If resists are etched too much (e.g. too high temperatures), the subsequent removal step will cause major problems.