Hydrofluoric acid etchings, even those in highly concentrated acids (48%), are technologically used despite the high risk involved with HF applications. Technological problems are discussed in the section „Wet-chemical etching“.
A new polymer offers a solution for at least some of these problems. Mixtures of hydrocarbon polymers (HCPs) drastically reduce the diffusion rate of HF through the protective film, while all other polymers are penetrated significantly faster. HC-polymers can in addition be patterned using a two-layer system. First results are already available, both for the use of this polymer as unstructured protective coating as well as for the selective structuring thereof in positive- and negative two-layer patterning processes:
The unstructured protective film was investigated with respect to long-term stability in the presence of different HF-concentrations. In the presence of concentrated HF etching solutions (48%), a layer of 20 µm is stable for at least 4 hours. This time span is correspondingly extended to up to 10 hours in the presence of diluted hydrofluoric acid solutions (24%), thus reaching the technically reasonable upper limit. Thinner resist films of 5-10 µm withstand concentrated acids for up to 2 hours, while the protective effect is extended again to several hours in the presence of diluted hydrofluoric acid solutions (10% or 24%).
We were able to successfully use a negative-tone ( AR-N 4400-10 ) as well as a positive-tone resist ( AR-P 3250 ) as top-layer for two-component processes. In addition to a higher flexibility for the user to choose freely between positive or negative imaging, the optimization resulted in a higher resolution (10 µm film thickness = > 30 µm) and improved manageability of processes. Unexposed areas of the negative resist which were completely developed during the first aqueous-alkaline development can be easily removed. During the following second organic development, the lower HF-resistant polymer film can be dissolved and now exposes the desired structure. By a careful mixing of organic solvents, the recommended developer X AR 300-74/5 was adjusted to provide an optimal development rate.
This procedure is similar for positive-tone resists, only that exposed areas of the photoresist are here dissolved in the first development step.
To obtain a high film quality it is recommended to pre-treat glass substrates with the adhesion promoter AR 300-80 . Resist films should furthermore not be dried above a temperature of 60°C. Removal and cleaning of the equipment can easily be carried out with remover X AR 300-74/1.
Undercut profiles are inevitable during isotropic etchings. If the adhesion between glass surface and protective coating is high, undercutting occurs to a lesser extent. If adhesion is insufficient, the resist is basically removed during etching. Measurements of width and depth of etched trenches provides information about the adhesion. A higher trench width is correlated with poorer adhesion. (See “SX AR-N 5000/40″)