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. The resists cannot be structured with UV-lithography alone (see “UV structuring of PMMA resists”).
Coating of wafers
The main problem to achieve an optimal protection is the uniform coating of wafers. At high spin speeds, edges of the vertical surface are not sufficiently covered during spin coating. In this case, KOH migrates under the PMMA-layer and starts to detach the protective coating from the outside. If lower spin speeds (800 – 1000 rpm) are used, the resist wraps around the edges which results in an improved protection of the vertical surfaces. The best edge protection is achieved with an additional coating of wafers at the edges. Using pipettes or a dispenser, only the edge is covered with the protective coating at extremely low spin speeds (< 100 rpm), followed by a slow spin/dry step. The flats of wafers (short, straight edges) pose additional problems. Since the bottom side is also coated to some extent during this procedure, drying on a hot plate has to be performed in a small distance (proximity bake) since the wafer will otherwise thick to the plate.
To avoid the above described detachment of PMMA films from the wafer, the use of adhesion promoters is strongly recommended. A bake of wafers at high temperatures (if possible > 200°C) before coating is helpful. In this case, cooling should be fast and ideally take place in an exsiccator. The most commonly used adhesion promoter HMDS (hexamethyldisilazane) is not recommended for PMMA; adhesion features are greatly improved if AR 300-80 is used for PMMA films. This adhesion promoter containing an organic silicone compound (diphenylsilanediol) is applied via spin coating and baked at 170°C. The resist can subsequently be coated. Cotton candy effect With all PMMA-Resists, in particular however with highly viscous protective coatings AR-PC 503 and 504 , an effect can be observed which resembles effects in the preparation of cotton candy. Fine resist strings appear during a coating at high spin speeds which are deposited on the wafer. This effect is absent at low spin speeds. If a high spin speed is unavoidable, a glass rod may be attached laterally above the chuck to catch these strings during the coating.