YES introduced a
process now commonly known in the semiconductor industry as vacuum
bake/vapor prime, a key step in the front end processing of silicon
chips. The process dramatically improves the application of
hexamethyldisilizane (HMDS) as a surface priming treatment, which is
used to enhance adhesion of photoresist on a wafer surface.
Early application of HMDS used immersion or spraying of liquid chemical
onto the surface of the wafer in a room atmosphere, but wet HMDS
application processes have limited surface longevity. Process engineers
must apply photoresist within a few hours of HMDS treatment or the
surface modification affects of the HMDS are degraded or lost.
Investigation into the limited surface longevity revealed the culprit
was water vapor, which is always entrained on surfaces exposed to
atmosphere. Effectively, the HMDS was bonding to a microscopic water
layer rather than to the wafer surface.
In order to extend the lifespan, first, wafers must be completely
dehydratedónot only surface moisture, but chemically bound water
molecules as well. To achieve this, Yield Engineering Systems developed
a process combining high temperature with low pressure. The YES-1224
removes the moisture that is chemically bound to the surface so the
silane/substrate bond isnít impaired.
Then, when chemical is applied, a superior bond is formed that is stable
even after exposure to atmospheric moisture. Wafers properly treated
will last for weeks with no change to surface adhesion.