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The following is a greatly simpe example of how a photoresist masking layer would be photodefined and used to serve as a proctective mask for wet etching of a pattern into a silicon oxide layer.
1. Select silicon wafer which is p-type substrate
2. A layer of silicon oxide is grown on the wafer
3. A solution of the polymer resist is spun on the oxide layer and then the wafer is baked to remove the solvent; thus to form a thin film
4. The semiconductor design can be implemented by utilizing a pattern mask and exposing it to a homogenous radiation source, or design pattern can be directly written on the resist. In this case we will use a glass mask to pattern.
5. Once the mask is properly aligned on the resist, the substrate is ready for exposure. The mask is designed in a such a manner that it contains transparent and non-transparent areas; thus upon exposure, light passes through transparent areas of the mask and is absorbed by the photoresist, while the non-transparent areas of the mask absorbs the light, and keeping that area of photoresist unexposed.
6. Chemical properties of the photoresist material changes when it is exposed to any type of radiation source (e.g. UV light). Photoresist can be classified as positive, meaning degradation of the polymer resist upon exposure, and negative meaning the crosslink of the polymer upon exposure.
7. The part of the photoresist that was exposed to the radiation source is hardened (yellow), while the chemical properties of the unexposed photoresist stays the same (green).
8. After the exposure the pattern mask is lifted for etching. The feature to be etched is 'imaged' by unhardened photoresist.
9. To etch the unhardened resist, the substrate is exposed to " 'special' paint-thinner" which is a photoresist solvent; therefore where exposed to radiation, it resists solvent
10. In this step, solvent for the unhardened photoresist is applied in order to etch the unhardened photoresist
11. After etching of the photoresist the substrate is expose to Hydrofluoric Acid to etch the oxide. In this step the resist layer acts as a mask.
12. Since the polymer photoresist 'resists' the hydrofluoric acid, exposed oxide layer is etched by HF acid.
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