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Overview
The adhesion of spun photoresist on a silicon wafer, or other substrate, may not adequate without taking specific steps. Adhesion depends mainly of the surface state of the substrate when photoresist is spun, and ideal methods for promoting adhesion depend both on the substrate and on the spun film.
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References
Main Wiki PageSurface States
Common substrates including Si, SiO2, and SiN naturally have a hydrophilic surface due to adsorbed water and exposed OH bonds in the native oxide. This can pose a problem for photoresist surface adhesion, which are typically hydrophobic compounds. HMDS (hexamethyldisilazane, [(CH3)3Si]2NH) can react with the surface oxide to form a hydrophobic surface, and is currently used at BNC as a spin-on adhesion promoter (as was common in the 1970s, per YES). However, for fine features, this poses problems for feature definition (HMDS - MicroChemicals). Spin coating results in a thick film that does not properly displace water or bond to the surface. Although spun on HMDS does indeed form a hydrophobic top layer, ammonia is released during the resist pre-exposure bake as a result of unreacted NH groups. The ammonia diffuses into the resist and crosslinks the resist near the substrate. Per Microchem [3]:
“In case of spin coating of HMDS, a too thick HMDS film forms on the surface. After resist coating during the softbake, this excess of HMDS releases ammonia which diffuses into the resist and crosslinks the resin near the substrate. As a consequence, through-development sometimes becomes impossible. For the same reason, NEVER apply HMDS in a spin coater. HMDS vapour will diffuse into all resist films subsequently coated, and partially crosslink the resist film during softbake, which lowers the development rate and can deteriorate the resist profile and attainable resolution.”
Therefore, proper application of undiluted HMDS requires the use of a bubbler (e.g. [4]) to apply an ammonia-free monolayer of HMDS on the substrate surface [2]. Such vapor deposition equipment is not currently available at BNC, motivating both the use of alternative adhesion promoters, and the phase out of undiluted HMDS use in photoresist spinners.