High sensitivity positive resist layers and mask formation process

DETAILED DESCRIPTION The resists useful in the practice of the invention are those which are degraded under high energy radiation at dosage levels above about 1 .
times.
10.
sup.
.
sup.
-6 coulombs per square centimeter and which are conventionally employed in positive high energy exposure resist systems.
Such resists include, for example, vinyl type polymers such as the lower alkyl esters of methacrylate, n-butylmethacrylate and t-butylmethacrylate, diazo sensitized novolac resins such as described in U.
S.
Pat.
No.
3,201,239.
The plural layers of resist can be made up of the same polymer of different polymers so long as the underlayers are faster dissolving in developer solution than the overlying layer.
Depending upon the final resist relief profile which is desired, two or more layers can be employed with slow dissolving layers over faster dissolving layers.
The solubility rate differences of the layers can be established by using one or a combination of known chemical and structural differences in the resist layer compositions.
A convenient way of achieving the required solubility rate differences is by varying the molecular weight of the polymer in each layer since lower average molecular weight materials of a family of polymers are faster dissolving.
Another way of establishing the required solubility rate differences is by isomerism such as the tacticity of the polymers, with polymers of the same molecular weight but different spacial arrangement of monomer units having different solubility rates.
Co-polymers with varying ratios of monomers can also be employed as well as layers of polymers having functional groups of varying polarity.
Where molecular weight differences are relied upon, the ranges of molecular weights generally found to be useful are for the underlayer an Mn of 5,000-100,000 (where Mn is the number average molecular weight) and for the overlayer an Mn of 20,000-10,000,000.
Because any polymer material is a mixture of polymer chains of varying length, the solubility rate differences can be enhanced if polymer layers each having a relatively narrow molecular weight range, or dispersivity Mw/Mn, are selected (where Mw is the weight average molecular weight)