Allresist offers the protective coating SX AR-PC 5000/40 for HF and KOH etchings already for a few years. Layers of 5 μm are able to withstand a 48 % hydrofluoric acidic solution for several hours. Also 40 % KOH solutions for silicon etchings do not attack the layer. The protective resist is however not particularly temperature-stable. At 65 °C, melting processes are initiated which make this resist unsuitable for KOH etching at 85 °C.
For this reason, we developed an improved protective coating. SX AR-PC 5000/41 (Black-Protect) is stable up to 130 °C and can thus also be used for KOH etching at 85 °C. At a sufficiently high thickness of > 5 µm, the protective layer resists both concentrated hydrofluoric acid and 40 % potassium hydroxide solutions. Prerequisite however is that especially edges are flawlessly coated. This can be achieved with an optimized coating procedure during spin coating. Using an edge covering resist of the EV-group, a very good layer quality can be obtained which is sufficient for back layer protection.
The old protective coating SX AR-PC 5000/40 could be structured together with a photoresist as two-layer system, and this is also possible with the new resist. The processing of Black-Protect is however easier due to simplified process parameters.
| 1. Coating SX AR-PC 5000/41 | 4.000 rpm, 5 µm film thickness |
| 2. Soft bake | 95 °C, 25 min oven |
| 3. Coating AR-P 3250 | 4.000 rpm, 5 µm film thickness |
| 4. Soft bake | 95 °C, 25 min oven |
| 5. Exposure | UV broad band |
| 6. Development photoresist | AR 300-26 (3:2 diluted) |
| 7. Development protective coating (isotropic) | X AR 300-74/5 (solvent) |
| 8. Removal of photoresists | AR 300-26 undiluted |
| 9. Etching | User-specific process |
Fig. 1 Developed Black-Protect layer, film thickness 5 µm
Fig. 1 shows the funnel-shaped edges of developed holes after removal of the photoresist layer. These are due to the isotropic development of Black-Protect; the light-insensitive polymer is quickly evenly removed in all directions which makes e.g. holes slightly larger as compared to the original hole in the photoresist. This effect must be taken into account during the conception of the desired structures.