In the silylation process, HMDS reacts with the oxide surface and forms a strong bond to the surface. At the same time, free bonds are left which readily react with the photoresist, enhancing adhesion.
This process requires diffusion of a silicon containing silane, typically HMDS through an exposed resist layer. Where the resist is exposed the acid is neutralized, and the silicon molecules in the silane are left behind only in the exposed resist areas.
Time, temperature, and pressure are the factors that control diffusion. So doubling the pressure reduces the time. (Normal pressure for HMDS is 14 Torr, so the normal pressure for silylation should be 250 Torr or higher for a reduction in time of 1/20).
In operation, the process is similar to vacuum vapor priming, but the silane delivery is a heated vapor and a diaphragm pressure gauge is set to the required process pressure. This closes off the valve to the process chamber as soon as the desired process pressure is reached.