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 parameters, and 7: Air bubbles). Above a solids content of more than 50 % (AR-P 3220 and AR-N 4400-25 and -50), degassing by ultrasound is recommended. To avoid bubbles, thick coatings should be applied slowly and from a low distance to the substrate.  It is recommended to always apply equal quantities of resist, e.g. approximately 8 – 12 ml for a wafer of 4 inches, depending on the viscosity of the resist.

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 30 s, rotation is continued at the final speed for further 2 – 5 min. A subsequent short spin (250 – 500 rpm) at a higher speed (600 – 800 rpm) reduces 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 1000 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 is advisable: 1.) 75 °C, 5 min and 2.) 90 °C, 15 – 30 min. A longer bake step reduces the sensitivity as well as the tendency to form undercut profiles. These profiles are formed when layers still contains a relatively high amount of solvent, in particular in the bottom sections of the layers. A slow cooling step is recommended to avoid possible stress cracks.

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 cause stress cracks. Drying times are extended by a factor of three in the convection oven. Long, rigid bake steps result in a low sensitivity during development.

Drying times are here also film thickness-dependent and in a range between 8 – 30 min on a hot plate and 30 – 60 min in the convection oven.
Below, detailed information is given concerning process steps such as exposure and, for CAR 44, the additional cross-linking 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 a range of 8 – 30 min (hot plate).

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 2 : 1 to in deionised water for 2 – 5 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 30-µm 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.

The general rule is that longer development times in lower developer concentrations result in a higher image quality.

After development, resist films have to be rinsed with deionised water. The 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)