A method for fabricating a semiconductor trench structure includes forming a
trench
in a semiconductor substrate and filling it with a filler. A first thermal process
having a first maximum temperature cures the filler. Removing the filler from an
upper region of the trench as far as a boundary surface defines a collar region.
In a second thermal process having a second maximum temperature that is not significantly
higher than the first maximum temperature, a liner is deposited on the collar region
and the boundary surface. The liner is removed from the boundary surface, thereby
exposing the filler. The filler is then removed from a lower region of the trench.
