Contrast enhanced photolithography

DETAILED DESCRIPTION OF THE INVENTION A number of positive photolithographic resists utilise an alkali based developer which is conveniently used to remove the CEL material in the developing step.
The CEL materials which are used in the process of the present invention may be generally described as reversible photochromic compounds which are derivatives of a succinic anhydride, a succinimide or a succinic ester.
Typical photochromic materials are fulgides and fulgimides which are converted to their coloured form by heating to a temperature of about 100.
degree.
C.
or irradiation with UV light or by a combination of both treatments.
Such compounds are rapidly bleached with white light, their maximum absorbance being generally in the range of 400 to 550 nm.
Compounds having absorbance maxima at or close to 400 nm are preferred, since many conventional photoresist materials are designed for exposure to illuminating radiation in this part of the visible spectrum.
Suitable photochromic materials may be represented by the general formula (I) below: ##STR1## wherein Y represents ##STR2## R' and R" being hydrogen or alkyl, X represents oxygen or ##STR3## in which A is hydrogen or an alkyl group or aryl group, R.
sub.
1, R.
sub.
2, R.
sub.
3 and R.
sub.
4 represent aryl, alkyl, aralkyl, hydrogen or heterocyclic groups.
Typical examples of suitable fulgides or fulgimides (and their preparation) are described in British Patents Nos.
1,442,628, 1,464,603, 2,002,752 and U.
S.
Pat.
Nos.
4,186,002, 4,145,536 and 4,220,708 the disclosure of which is specifically incorporated herein.
As described in these British and U.
S.
patents, these fulgides and fulgimides are converted into their cyclised coloured form by heating to about 100.
degree.
C.
or irradiation with U.
V.
light or by a combination of both.
Generally the compounds of formula (I) above in which X represents an imide group have absorbance maxima in the shorter wavelength and may be preferred for use with some photoresist materials.
It is also possible to use the succinic acid esters and half esters corresponding to the above described succinic anhydride compounds