Spray Coating Negative Tone Resists
Spray coating is an essential coating technology for depositing resists across deeply etched structures, such as a MEMS device etched deeper than 100 μm. Conventional spin coating has limitations when processing structured wafers with severe topography. Spin coated resist often fills the entire structure. This leads to difficulties in the following lithography steps. Thus, industry requires a method to apply resist conformally. Spray coating technology is an effective method for performing conformal coating over deep etched structures. In view of conformal coating, positive tone resist is desirable, as already adopted for many applications such as etch mask or electroplating. An important property of positive tone resists – such a DNQ/Novolak resist - is the comparably high glass transition temperature that is above the soft bake temperature. During soft bake the resist thickness changes depending on corner angle and location. Typically the resists thickness reduces at the top corner and increases at the bottom corner. However, when using positive acting resists, reasonable conformal coating can be achieved. The negative tone resist is a UV plastic material that is hardened during UV exposure. Unlike the previous case, during the soft bake step, the resist becomes soft and sur - face tension deteriorates the conformality of the resist layer. Often only a very thin residual resist coverage or no resist at all is observed on the top corner after soft bake. For these reasons, industry preferably adopts positive tone spray resists. Nevertheless, using negative tone resists is still important for some specific applications. Examples are the SU8 resist from MicroChem/ Nippon Kayaku and the THB 151N resist from JSR. SU8 is often used as a permanent resist structure and THB151N is an important plating resist.