You're describing a re-entrant resist profile common in lift-off
techniques.  This can be accomplished two ways.  Any positive Novolak
resist can be image reversed by subjecting it to an amine vapor oven bake
after exposure, then flood exposing the wafer to convert remaining PAC to
soluble acid.  After develop the non imaged area will clear forming a
re-entrant profile whose angle depends on the acid gradient in the resist.
A second approach is to use one of the available negative resists which
forms a decent re-entrant profile on its own.  Clariant makes such a
product called NLOF.

Russell Cox
Process Engineer-Clare, Inc.
978-524-6846 Work Phone
978-479-2767 Cell Phone