I know HMDS is a popular primer in the MEMS field. It replace the hydroxyl groups on silicon surface with methyl groups which believes to bond with photoresist better and thus improve the adhesion. However, HMDS also makes the surface more hydrophobic. Hydrophobic surfaces usually have bad adhesion properties (for example, Teflon and PDMS surfaces). Why does HMDS improve adhesion while increasing the hydrophobicity of the wafer surface?
Zhaohui Li You are exactly right. It does make the surface hydrophobic. The thing to remember is the photoresist isn't a water based solution. Water and Photo resist have different chemistries and when the wafer is hydrophobic it has a higher affinity for bonding with the photoresist.
See the attached link! :)
https://www.microchemicals.com/technical_information/substrate_cleaning_adhesion_photoresist.pdf
The action of HMDS as an adhesion promoter is due to the fact that by creating a hydrophobic surface at the interphase between the substrate and the photoresist it precludes the intrusion of water which act to debunk the resist. A general discussion is in the attached article.
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