15. How can thick films of > 10 μm be processed in an optimal way?
Coating: As a general rule for the coating with thick positive and negative tone resists, resists should be left undisturbed for several hours up to one day before processing ( → Question 6 Optimum coating
For layers < 10 μm, coating and bake steps are conducted according to the protocol for standard resists ( → AR-P 3500). For layers between 10 - 30 μm, spin times of 30 - 60 s are recommended. For layers above > 30 μm thickness, the coating should be performed in several steps. After a low initial rotational speed of 200 – 400 rpm for 10 s, rotation is continued at final speed for further 2 – 5 min. A subsequent short spin at a 50 % higher speed reduced the formation of edge beads.
→The general rule is that the film thickness increases with shorter exposure times. For highly viscous resists such as AR-N 4400-50, spin numbers of more 1 000 rpm are disadvantageous due to a negative impact on the film quality. With increasing film thickness, the process window generally decreases.
Softbake: Positive resist system AR-P 3200: Films with high thicknesses between 10 – 30 μm should be dried on a hot plate (95–100 °C, 8–15 min). For higher thicknesses > 30 μm, a bake in two steps
Negative resist system AR-N 4400 (CAR 44): The bake step at 85 – 95 °C is highly dependent on the film thickness. For films of 10–30 μm, bake times on the hot plate are in the range of 8–20 min, while 40–50 μm films require already a bake of 30–60 min. Films with thicknesses above 50 μm need even more bake time, which has to be determined individually by the respective user. It is absolutely recommended to use temperature ramps, since a fast cooling step (cool plate) may result in stress
Below, detailed information is given concerning process steps such as exposure and, for CAR 44, theadditional crosslinking bake at 95-105 °C which is required for this resist. The time for this additional bake step is again dependent on the film thickness and in the range of 8-30 min.
Development: Thick resist films are developed in an aqueous-alkaline environment (→Question 10 Influence of developer concentration and time).
For the development of AR-P 3220, developer AR 300-26 is optimally suited. Depending on the respective film thickness and type of coating, a development in dilutions of 1:0.5 to 1:3 in deionised water for 2 – 10 min is recommended.
For AR-N 4400, the MIF developer product line AR 300-40 is best suited. Thinner films of up to 10 μm demand developers in low concentrations such as AR 300-47 (in concentrations ranging from 3:2 dilution with deionised water up to undiluted use). 50-μm films require more concentrated developers such as AR 300-44 or 300-46, depending on the intensity of the bake step. The development time is strongly determined by factors such as film thickness and developer concentration. For example, a 30μm film is developed after approximately 8 min in developer AR 300-47, while thicker films require even longer. By combining a prolonged development with a just about sufficient exposure time, undercut profiles (lift-offs) can be obtained with AR-N 4450-10.
After development, resist films have to be rinsed with deionised water. The now following removal step can easily performed with remover AR 300-70 (AR-P 3220) or, in the case of AR-N 4400, with remover AR 600-70 or AR 300-72 (→Product information).