Which photoresists are suitable for hydrofluoric acid (HF) etching?

The positive tone photoresist AR-P 5910 is highly adhesion-enhanced and thus suitable for HF etching of up to 5 % HF. In particular on glass or silicon oxide, a pre-treatment of substrates with the adhesion promoter AR 300-80 is strongly recommended.  With AR-P 5910, a film thickness of 5 µm can be achieved at 4 000 rpm. This high thickness is advantageous to obtain a high etch resistance since HF is able to diffuse through the layer and to remove it consequently.

After the coating step, the hotbake should be performed on a hot plate at 85 – 90 °C for 2 min. In comparison to standard resists, this resist is less sensitive and consequently requires longer exposure times.

For the development of exposed resist films, undiluted developer AR 300-26 is recommended. The development time should amount to approximately 60 s.

A bake of developed structures at 105 – 115 °C increases stability and adhesion of the resist mask to the substrate substantially. Higher temperature should however be avoided. On well-adhesive surfaces such as e.g. silica, the resist mask is stable for hours in 5 % HF or HF/isopropanol mixtures. In the case of deeper etchings however, the resist begins to peel off at the edges of structures.

The protective coating SX AR-PC 5000/40 is HF and KOH resistant (KOH: caustic potash solution). This coating is used to generate stable protective films which reliably protect the backside of wafers while the etch process is performed on the front (e.g. 40 % KOH or 50 % HF).

In addition, the protective coating can be patterned in a two layer system using photoresist AR-P 3250 which allows a transfer of structures into glass or silicon oxide.