Resist Wiki: Photoresists – Protective Coating

Black-Protect – stable protective coating for HF and KOH etchings

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 1.000 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.

Protective coating to prevent mechanical damage

ensitive substrates can be protected against damage during handling with resist SX AR-PC 5000/3.1, which is especially important in the case of double-sided procedures.
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PMMA protective coating: reduction of cotton candy effect

Since problems may occur when PMMA protective coatings are applied, new ideas emerged how to optimise the coating properties of these resists.
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Safer solvent PMMA protective coating

The protective coatings AR-PC 503 and AR-PC 504 have been successfully used for many years in aggressive KOH etching procedures.
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Improved protective coating SX AR-PC 5000/31

Protective coatings are in most cases composed of novolacs or PMMA polymers. Both polymers ensure a sufficiently high protective effect against mechanical damages as well as sufficiently high resistance against acidic solutions.
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Protective coating for KOH-etching

Protective coatings AR-PC 503 and 504 are able to withstand the presence of 40%, 85°C warm KOH for several hours. The polymer PMMA is not attacked under these conditions. These resists can consequently be used to protect front and back surface of wafers during deep silicon etching.
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