Silylation is a surface imaging technique which allows silicon to absorb into photoresist in specified areas. Photoresist containing silicon creates a hard-mask which is resistant to oxygen plasma. This allows for the development process to occur strictly in dry etch systems. Surface imaging is a desirable technique because it removes the need to expose through the photoresist stack. Traditional lithography techniques require a high dose of ultra-violet (UV) light in order to penetrate to the bottom of a photoresist stack. It is critical, for quality imaging, that the photoresist not be over or under exposed. This creates a process window dependent on two main variables; the photoresist thickness and the dose needed to penetrate through the photoresist stack. Surface imaging allows for a larger process window because the dose is not depended on the thickness of the resist. In surface imaging, the UV light only exposes the top-most layer of the photoresist material. Surface imaging removes the resist thickness variable, this will allow for a wider process window. Variation in photoresist thickness is also important; traditional lithography requires a uniform photoresist thickness across the surface of the wafer in order to optimize depth of focus (DOF). Surface imaging tends to be less effected by photoresist variation due to short depth requirements of the exposure. This allows for a relaxed depth of focus (DOF) requirement, as well as a larger process window.
Plourde, Chelsea R.
"Surface Imaging Through Silylation,"
Journal of the Microelectronic Engineering Conference: Vol. 18
, Article 15.
Available at: http://scholarworks.rit.edu/ritamec/vol18/iss1/15