Spin castable photobleachable layer forming compositions

Spin castable mixtures having aryl nitrones are provided which are useful in making photobleachable layers for use in contrast enhanced photolithography.

The present invention is directed to spin castable mixtures having aryl 
nitrones which are useful in making photobleachable layers for enhancing 
the contrast of images of objects such as masks for photolithography in 
the manufacture of integrated circuits. 
Lithography in the production of integrated circuits is predominantly 
carried out by optical means. In the drive to reduce circuit dimensions, 
improve performance and increase yield, optical systems have provided the 
required resolution with each successive generation of circuit technology. 
The image resolution of projection lithographic systems has recently begun 
to approach the physical limits imposed by practical constraints on 
numerical aperture and wavelength. While further improvements in 
lithographic technology are anticipated, dramatic improvements in inherent 
lens resolution are not. In order to continue to reduction of minimum 
feature size achievable by optical techniques, it is necessary to alter 
some other aspect of the lithographic process for further improvements. 
One area in which further improvements are possible is in the photoresist 
process. Each photoresist is characterized by some degree of incident 
contrast necessary to produce patterns usable for subsequent processing. 
This minimum required contrast of illumination is referred to as the 
contrast threshold of the resist. Depending on substrate properties, the 
required pattern thickness and resist edge profiles, conventionally used 
positive photoresist has a contrast threshold between 85% and 90% 
contrast. Currently, most production is done at 90% incident contrast or 
more. If the contrast threshold of the resist is reduced, the resolution 
obtainable with a given optical system is improved due to the fact that 
image contrast is a decreasing function of the spatial frequencies present 
in the image. 
The present invention is directed to provide a photoresist process in which 
the contrast of an aerial image utilized in the process is enhanced prior 
to incidence on the photoresist. 
An object of the present invention is to lower the minimum contrast 
required to produce usable images in a photoresist. 
Another object of the present invention is to provide new photobleachable 
compounds and materials. 
In carrying out the invention in an illustrative embodiment thereof, a 
layer of photoresist of a first thickness and having a predetermined 
contrast threshold is provided. An object or mask is provided having 
opaque and transparent areas. An image having a contrast less than the 
predetermined contrast threshold of the photoresist is formed of the 
object on the layer of photoresist by projecting light of a predetermined 
wavelength through the object. A layer of photobleachable material 
including a photobleachable compound is provided between the object and 
the layer of photoresist and adjacent a surface of the layer of 
photoresist. The photobleachable compound is sensitive to the 
aforementioned wavelength of light and has an extinction coefficient to 
molecular weight ratio in liters per gram-centimeter in the unbleached 
state greater than about 10. The ratio of the extinction coefficient for 
the unbleached state to the extinction coefficient for the bleached state 
of the photobleachable material is also greater than about 10. Light of 
the aforementioned wavelength and of a predetermined intensity is 
projected through the object onto the layer of photobleachable material 
for a time to obtain a reduction in optical density of the layer of 
photobleachable material in direct proportion to the dosage of light of 
the aforementioned wavelength incident thereof, whereby the integrated 
contrast of the image transmitted by the layer of photobleachable material 
increases with dosage transmitted thereby, reaches a maximum value and 
thereafter decreases. Parameters of the layer of photobleachable material 
are selected such that the maximum value of integrated contrast is greater 
than the predetermined threshold contrast of the layer of photoresist. The 
sensitivity of the layer of photoresist and the thickness thereof are 
selected such that the layer of photoresist is fully exposed by a dosage 
in a predetermined range transmitted by the layer of photobleachable 
material and provides an integrated contrast in the transmitted image 
above the predetermined contrast threshold of the photoresist. Light of 
the aforementioned predetermined wavelength is projected through the 
object for a time to provide the dosage in aforementioned predetermined 
range transmitted through the photobleachable layer. The layer of 
photobleachable material is removed and the layer of photoresist is 
developed whereby a pattern representing an enhancement in contrast of the 
image of reduced contrast of the object is formed in the layer of 
photoresist. 
Another aspect of the present invention is directed to a spin castable 
mixture capable of forming a photobleachable layer having an absorption 
maximum in the range of 300 to 450 nanometers comprising by weight 
(A) 100 parts of organic solvent, 
(B) 1 to 30 parts, preferably 5 to 15 parts, of an inert organic polymer 
binder, 
(C) 1 to 30 parts, preferably 5 to 15 parts, of an aryl nitrone.

A large fraction of optical lithography is done currently by projection 
techniques in which an aerial image of a mask is used to expose the 
photoresist. For an aerial image of low contrast, even those parts of the 
image that correspond to the dark regions of the mask have significant 
intensities. As the contrast is reduced, discrimination of the darker area 
from the lighter area becomes increasingly difficult. In accordance with 
the present invention a method is provided which enhances the contrast of 
the image incident on the photoresist and thereby improves this 
discrimination. The contrast enhancement is based on the use of 
photobleachable materials which are initially relatively opaque, but 
following some dose of radiation, become relatively transparent. The 
optical transmission of an idealized bleachable layer is shown in FIG. 1. 
When the aerial image of a mask is incident on such a layer, the regions 
of the bleachable layer that are exposed to the highest intensities bleach 
through first, while those parts of the layer that receive the lowest 
intensities bleach through at a later time. 
The dynamics of this bleaching process are depicted in FIGS. 2A-2F. FIG. 2A 
shows the relative transmission of an object such as a mask consisting of 
opaque regions producing zero transmission 11 separated by open or 
transparent regions producing 100% transmission 12. I.sub.o is incident 
intensity of radiation. I is transmitted intensity of radiation. E.sub.b 
is the dosage of radiation required to cause bleaching of the layer. FIG. 
2B shows a graph 13 of the relative intensity as a function of position of 
an aerial image of the mask when placed in optical projection apparatus 
for producing an aerial image used, for example, for exposure of a layer 
of photoresist. It is assumed that the wavelength of light used and the 
dimensions of the regions of the mask are such as to produce the contrast 
shown. I.sub.max is the maximum intensity of the image and I.sub.min is 
the minimum intensity of the image. FIG. 2C shows the cross-section of a 
layer 15 of an idealized photobleachable material. FIG. 2D shows the 
bleaching of the layer 15 as delineated by the dotted line 16 at the end 
of a first period of time. FIG. 2E shows the bleaching of the layer 15 as 
delineated by dotted line 17 at a latter period of time. FIG. 2F shows the 
bleaching of the layer 15 as delineated by dotted lines 18 and 19 at the 
time the exposure is terminated and bleaching stopped. If the exposure is 
stopped at a time corresponding to FIG. 2F, the transmission of the 
bleachable layer corresponds to that of the original mask. When such a 
material is coated on top of a conventional photoresist layer, the 
resulting composite can have lower contrast threshold than the contrast 
threshold of resist layer alone. This will be true if the photoresist is 
sensitive enough to be exposed in a time short compared to the bleaching 
time. The bleachable layer essentially forms an in situ contact mask for 
the photoresist layer. The net effect of this in situ mask is to increase 
the contrast which is incident on the photoresist over the contrast of the 
aerial image. 
The application of contrast enhancing techniques to submicron optical 
lithography raises several physical and chemical constraints on the 
contrast enhancing layer itself. The contrast enhancing layer must be 
simultaneously thin and optically dense. The thickness requirement arises 
because of the narrow depth of focus of high resolution optical systems. 
This limits the thickness to a range of less than about 1 micron. Because 
the contrast enhancing layer must be optically dense, it is necessary that 
the photochemical constituent of the layer be strongly absorbing. Since 
the optical transmission following bleaching is determined by the 
absorption of photoproducts, the photoproducts must have a much smaller 
extinction coefficient than the parent molecule. Extinction coefficient is 
defined by the equation 
##EQU1## 
where A is absorbance, 
b is path length (cm.), and 
c is concentration (mole per liter). 
Absorbance is defined by equation 
##EQU2## 
where I.sub.o is intensity of incident radiation, and 
I is intensity transmitted radiation. The extinction coefficient .epsilon. 
of a material is obtained by determining the parameters A, b and c in 
equation (1). The parameter A is obtained from equation (2). Initially, a 
known amount of a material by volume is dissolved in a known amount of a 
solvent by volume to obtain the concentration c of the material in the 
solution. This solution is put into a cell of known dimensions and placed 
in the light path of a spectrophotometer and radiation of known intensity 
I.sub.o is directed onto the cell. The intensity of the radiation 
transmitted from the cell is measured. The solvent alone is also put into 
another cell of the same known dimensions and placed in the light path of 
the spectrophotometer and radiation of known intensity I.sub.o is directed 
onto the cell. The intensity of the radiation transmitted from the cell is 
measured. The intensity measurements of solvent alone is used to correct 
the transmitted intensity I for cell and solvent absorption. Thus using 
these values of incident intensity I.sub.o and transmitted intensity I in 
equation (2), absorbance A is obtained. Dimension b is obtained from the 
cell of known dimensions. Thus, the extinction coefficient for the 
material is obtained by substitution of values b, c and A in equation (1). 
The measurement of the extinction coefficients of materials is also 
described on pages 644-652 in "Fundamentals of Analytic Chemistry", 2nd 
Edition (1969) by Douglas A. Skoog and Donald M. West, published by Hold, 
Rinehart and Winston, Inc. of New York, N.Y. Continuing, in order to 
minimize the necessary increase in exposure time, the quantum yield of the 
bleaching reaction must be as high as possible. Also, since photoresists 
are conventionally applied by spin coating techniques, it would be 
convenient if the contrast enhancing layer could also be applied by 
similar methods. The solvent in which the bleachable material is dissolved 
must be compatible with photoresist layers. Further, it is required that 
it be possible to spin coat contrast enhancing layers of good optical 
quality. Finally, the wavelength range over which these bleachable 
materials operate must be the same as the wavelength range over which the 
optical projection system operates. Most direct-step-on-the-wafer systems 
operate at 405 nm (nanometers) or 436 nm. In this case, the 405 nm 
wavelength was chosen for use on an Optimetrix 10:1 DSW system available 
from Optimetrix Co. of Mountain View, Calif. The search for appropriate 
bleaching materials was made with regard to these constraints. 
Particularly, the search for appropriate bleaching materials was made with 
regard to absorption maxima in the wavelength range of 300 to 450 
nanometers. 
A model of the bleaching process was developed and utilized in the 
evaluation of bleachable materials suitable for use in the contrast 
enhancing layer. The parameters of the model are set forth in the 
following table: 
TABLE 1 
______________________________________ 
Quantity Explanation 
______________________________________ 
.epsilon..sub.A 
Extinction coefficient 
of unbleached molecules 
.epsilon..sub.B 
Extinction coefficient 
of bleached molecules 
.PHI. Quantum yield of the 
bleaching reaction 
N.sub.o Initial density of 
unbleached molecules 
F.sub.o Flux density of photons 
incident on contrast 
enhancing layer 
t.sub.o Thickness of contrast 
enhancing layer 
n.sub.f Index of refraction 
of bleached layer 
n.sub.s Index of refraction 
of glass substrate on 
which the contrast enhancing 
layer is situated 
______________________________________ 
Based on the analysis above and the model of the bleaching process three 
criteria for material parameters were developed and are set forth in Table 
2. 
TABLE 2 
______________________________________ 
Quantity Value 
______________________________________ 
(1) .epsilon./Molecular Weight 
.gtoreq.100 liters/gram-cm 
(2) .PHI. .gtoreq.0.2 
(3) .epsilon..sub.unbleached 
.gtoreq.30 
.epsilon..sub.bleached 
______________________________________ 
The first criterion is based on the need for an optically dense film, and 
is essentially related to the packing density of absorbing centers in the 
contrast enhancing layer. The second criterion is based on the need for as 
abrupt a transition from the unbleached to the bleached state as is 
possible. Acceptability of a given quantum yield is to some extent related 
to the first criterion, because improvements in the first can compensate 
for deficiencies in the second criterion. The third criterion is based on 
the need for the contrast enhancing layer to be transparent following the 
bleaching process. These criteria were used in the initial search for 
appropriate bleachable materials. 
Selection of suitable bleachable compound was determined by evaluation in 
the model of the bleaching process and by test of the compound in a layer 
thereof to determine the relative transmission as a function of time or 
dosage of radiation with radiation intensity being held constant. A number 
of different bleachable compounds, the bleaching properties of which were 
based on different bleaching mechanisms were evaluated. The bleachable 
compounds dependent on photoisomerization were found to be particularly 
suitable. Of these compounds, aryl nitrones represented by formula 1, were 
found particularly suitable. 
##STR1## 
In formula 1, Z is a monovalent group selected from (R.sup.3).sub.a 
--Q--R.sup.4 -- or R.sup.5 -- and Z' is a monovalent group selected from 
--R.sup.6 (X).sub.b, R, R.sup.1, R.sup.2 and R.sup.3 are monovalent 
radicals selected from the class of hydrogen, C.sub.(1-8) alkyl, 
C.sub.(1-8) substituted alkyl, C.sub.(6-13) aryl hydrocarbon and 
C.sub.(6-13) substituted aryl hydrocarbons. Q is a monovalent, divalent or 
trivalent atom selected from the group F, C, Br, I, O, S, N, where a can 
have values of 0, 1 or 2. R.sup.4 is a C.sub.(6-13) aryl hydrocarbon or a 
C.sub.(6-13) substituted aryl hydrocarbon. R.sup.5 is selectable from the 
group of substituted or unsubstituted C.sub.(6-20) aromatic heterocyclic 
compounds incorporating one or more atoms from the group O, N or S. 
R.sup.6 is selected from the group of C.sub.(6-20) aromatic hydrocarbons 
and X is selected from the group of halo, cyano, alkyl carbonyl, 
C.sub.(1-8) alkyl, C.sub.(1-8) substituted alkyl, C.sub.(6-13) aryl 
hydrocarbon, C.sub.(6-13) substituted aryl hydrocarbons, or alkoxy 
carbonyl in any combination for values of b of 0, 1, 2 or 3. n can have 
values of 0, 1, 2, 3 or 4. The above compounds can be prepared using 
procedures such as those described in "Methoden der Organischen Chemie 
(Houben-Weyl), Vol. 10, part 4 (1968), pgs. 315-416, or those described in 
Chemical Reviews (1(64), Nitrones, by Jan Hamer and Anthony Macaluso, pgs. 
476-483. 
Various aryl ring systems with a variety of substituents may be constructed 
to suit the particular needs of the optical system employed in the 
photoimaging process. The aryl nitrones exhibit extinction coefficients of 
2 to 5.times.10.sup.4 liter mole.sup.-1 cm.sup.-1 and bleach with quantum 
yields in the range of 0.1 to 0.5. 
For direct-step-on-the-wafer systems capable of imaging at 405 nm, the 
nitrones of the general structure 2 were found to be particularly useful. 
##STR2## 
Included among this subclass of p-dialkylaminoaryl nitrones are 
heterocyclic compounds such as 3. 
##STR3## 
Suitable binders for use in providing a spin castable mixture for formation 
of a photobleachable layer incoporating the aryl nitrones of formula (1) 
are: vinyl acetate polymers (homopolymers and copolymers) and their 
partially saponified products (e.g., polyvinylacetate), copolymers of 
styrene or its derivatives, polymers and copolymers of acrylate or 
methacrylate esters, acetal resins, acrylonitrile/butadiene copolymers, 
ethyl cellulose and other hydrocarbon-soluble cellulose ethers, cellulose 
propionate and other hydrocarbon-soluble cellulose esters, 
poly(chloroprene), poly(ethylene oxide, poly(vinylpyrrolidone). 
Suitable solvents for use in providing a spin castable mixture for 
formation of a photobleachable layer incorporating the aryl nitrone of 
formula (1) are: aromatic hydrocarbons (e.g. toluene xylenes, ethyl 
benzene, chlorobenzene) with or without aliphatic hydrocarbons (e.g. 
cyclohexane), halogenated aliphatic compounds e.g. trichloroethylene, 
methyl chloroform, alcohols (e.g. propanol, butanol). 
The diaryl nitrone 2 where R --CH.sub.3 CH.sub.2 and n=0 was found to be 
particularly suitable. This nitrone, referred to as 
.alpha.-(4-diethylaminophenyl)-N-phenylnitrone, was found to absorb 
strongly at 405 nm and bleaches to near transparentcy with high efficiency 
at the same wavelength by undergoing unimolecular cyclization to an 
oxaziridine. The nitrone is very soluble in solvents of moderately low 
polarity (e.g., toluene, ethylbenzene) and forms good films at high 
loading densities with a variety of polymers such as polystyrene, 
poly(hydroxyethylmethacrylate), poly-.alpha.-methylstyrene, poly(methyl 
methacrylate), polyvinylpyrrolidone, vinylpyridine/styrene copolymers and 
allyl alcohol/styrene copolymers. The material 
.alpha.-(4-diethylaminophenyl)-N-phenylnitrone has an extinction 
coefficient to weight ratio of 130 liters/gram-cm at 405 nm. The material 
was formed into a contrast enhancing layer as follows: A solution of 
.alpha.-(4-diethylaminophenyl)-N-phenylnitrone (5% by weight of solution) 
and a binder, styrene/allyl alcohol copolymer (5% by weight of solution) 
are dissolved in toluene. A glass substrate is spin coated to a thickness 
of 250 nm. The relative transmission of the sample was tested at 405 nm 
and was found to have the relative transmission versus time characteristic 
at 405 nm, shown in FIG. 3. 
The model of the bleaching process was used to calculate the improvements 
in contrast as a function of exposure time for the contrast enhancing 
layer of FIG. 3. This is accomplished by calculating the bleaching for two 
representative points in a given pattern. In this example, two incident 
intensities that correspond to the maxima and minima of line and space 
grating pattern are chosen. The contrast C that these two levels of 
intensity correspond to can be calculated from the definition of contrast: 
##EQU3## 
Using the model of the bleaching process, the transmitted intensity as a 
function of incident dose for both the maxima and minima are determined. 
From these quantities, both the instantaneous and the integrated contrast 
can be calculated as a function of incident dosage. FIG. 4 shows graphs of 
such determinations and calculations for a 30% contrast pattern incident 
on the layer of the sample described above. Graph 21 shows the relative 
transmission at a relative maximum as a function of incident dosage in 
joules. Graph 22 shows the relative transmission at a mininum as a 
function of incident dosage. Graph 23 shows the instantaneous contrast 
obtained from graphs 21 and 22 using equation 3, as a function of incident 
dosage. Graph 24 shows the integrated contrast obtained from equation 3 
using integrated values of I.sub.max and I.sub.min instead of 
instantaneous values. Since bleaching is a dynamic process, the contrast 
which results from the transmitted radiation is also a function of dosage. 
A more useful graphical representation is shown in FIG. 5 wherein the 
corresponding graphs are plotted as a function of transmitted dosage. This 
representation allows the degree of contrast enhancement to be estimated 
as a function of the sensitivity of photoresist to be used in connection 
with the contrast enhancing layer. Graph 26 shows relative transmission at 
a relative maximum as a function of transmitted dosage. Graph 27 shows 
relative transmission at a relative minimum as a function of transmitted 
dosage. Graph 28 shows the instantaneous contrast obtained from graphs 26 
and 27 using equation 3, as a function of transmitted dosage. Graph 29 
shows the integrated contrast obtained from equation 3 using integrated 
values of I.sub.max and I.sub.min, as a function of transmitted dosage. 
The process used for utilizing the contrast enhancing layer will now be 
described and thereafter the results obtained thereby will be compared 
with the results obtained under the same conditions but without the use of 
a contrast enhancing layer. Reference is now made to FIGS. 6A-6E which 
illustrate the various steps of the process for providing a pattern of 
photoresist on a suitable substrate. FIG. 6A shows a substrate 31 on which 
is provided a layer 32 of a suitable photoresist such as Shipley 1400 
series of positive photoresists available from the Shipley Company of 
Newton, Mass. 
Such positive resists consist of a novolac resin or poly(vinylphenol), 
diazonaphthoquinone esters, and solvents (e.g. cellosolve acetate, 
xylenes). 
The liquid photoresist is deposited on the surface of the substrate which 
is then spun to provide a layer of desired thickness. After baking of the 
photoresist layer to remove solvent, a constrast enhancing layer 33 is 
provided on the surface of the photoresist. The contrasting enhancing 
layer 33 is constituted of a solution of a styrene/allyl alcohol copolymer 
binder (5% by weight of solution) and 
.alpha.-(4-diethylaminophenyl)-N-phenylnitrone (5% by weight of solution) 
in the solvent toluene. The solution is deposited on the surface of the 
photoresist 32, spun and thereafter baked to remove solvent to provide the 
layer 33 of desired thickness, as shown in FIG. 6B. The resultant 
structure is then exposed to a pattern of radiation consisting of 
illuminated regions 35 underlying the arrows 37 indicating radiation and 
nonilluminated regions 36 for a time to produce a contrast enhanced image 
in the range of dosages to which the photoresist layer is sensitive and 
which fully expose the layer of photoresist, as shown in FIG. 6C. 
Thereafter, the contrast enhancing layer 33 is removed by using a suitable 
stripping solvent, such as trichloroethylene, which removes the contrast 
enhancing layer without affecting the photoresist, as shown in FIG. 6D. 
Thereafter, the exposed portions of the photoresist are removed leaving 
retaining portions 38, 39 and 40 which are unexposed or insufficiently 
exposed as shown in FIG. 6E. 
In order to determine the improvements in contrast threshold obtained for 
the composite layer of the contrast enhancing layer and photoresist in 
accordance with the present invention, the aforementioned photobleachable 
material constituted of .alpha.-(4-diethylaminophenyl)-N-phenylnitrone and 
a styrene/allyl alcohol binder was utilized in conjunction with Shipley 
1400 series of positive photoresist to fabricate various patterns. A wafer 
or substrate of silicon was coated with only photoresist having a 
thickness of 1.6 microns and another wafer of silicon was coated with a 
layer of photoresist 1.6 microns thick over which was coated a layer of 
the photobleachable material specified above to a thickness of 0.25 
microns. An object consisting of an opaque line 2 microns wide, a 
transparent space 2 microns wide, an opaque line 0.8 microns wide and a 
transparent space 0.8 microns wide was imaged using the Optimetrix 10:1 
projection system at 405 nanometers onto the wafer with just the layer of 
photoresist with a range of dosages to form a number of patterns in the 
photoresist and was also imaged on the wafer which included photoresist 
and contrast enhancing layer with a range of dosages to form a number of 
patterns in the photoresist. The photoresist on each of the wafers was 
developed to obtain the patterns of lines and spaces formed in the 
photoresist. Patterns produced in the wafer with just photoresist using 
the minimum exposure or dosage that opened the 0.8 micron space at the 
photoresist substrate interface were compared with the patterns produced 
in the wafer with photoresist and contrast enhancing layer using the 
minimum exposure or dosage that opened 0.8 microns space at the 
photoresist substrate interface. The 2.0 micron wide line and the 2.0 
micron wide space, were nearly correct for both the wafer using just the 
photoresist and the wafer using the photoresist and the contrast enhancing 
layer thereon. The 0.8 micron wide line was grossly overexposed on the 
wafer having just the photoresist using the exposure which produced a 0.8 
micron wide space in the photoresist. The 0.8 micron wide line was 
properly exposed producing a 0.8 wide line in the photoresist on the wafer 
having both photoresist and contrast enhancing layer using the exposure 
which produced a 0.8 wide space in the photoresist. Furthermore, the 
profiles of the walls of the contrast enhanced patterns were nearly 
vertical. The poorer resultant patterns produced in the photoresist 
without the contrast enhancing layer is due to the much lower aerial image 
contrast obtained at the output of the Optometrix projection system due to 
the higher spatial frequencies present in the image or pattern of 
radiation obtained therefrom. 
While the invention has been described in connection with a particular 
positive photoresist, other positive and negative photoresists may be 
utilized. Also, while a particular thickness of photoresist and a 
particular thickness contrast enhancing layer were utilized in one example 
describing the invention, it will be understood that other thicknesses of 
photoresist and contrast enhancing layers may be utilized. 
Preferably the layer of photoresist should have a thickness less than about 
3 microns and the contrast enhancing layer should have a thickness less 
than about 1 micron. 
While in an exemplary composition of the contrast enhancing layer equal 
weight proportions of the photobleachable compound and the binder therefor 
were utilized, other proportions may be utilized, if desired. 
While in connection with photoresists it was mentioned that they have a 
characteristic called contrast threshold, it should be noted that this 
characteristic is a function of conditions of usage of the photoresist as 
well as of the resist itself, for example, the nature of the substrate on 
which used and reflections therefrom. 
While values of the extinction coefficient to molecular weight ratio in the 
unbleached state for the photobleachable compound greater than about 100 
are preferred and a ratio of the extinction coefficient for the unbleached 
state to the extinction coefficient for the bleached state of the 
photobleachable compound greater than about 30 is preferred, values of the 
above ratios as low as about 10 would be satisfactory. 
While a particular class of photobleachable compounds, namely, the aryl 
nitrones, dependent on unimolecular cyclization were utilized, it will be 
understood that other photobleachable compounds dependent on unimolecular 
cyclization and on other bleaching mechanisms such as photofragmentation, 
for example, may be utilized in accordance with the present invention. 
The aforementioned .alpha.-(4-diethylaminophenyl)-N-phenylnitrone, also set 
forth in Table 1, was prepared by condensing p-diethylaminobenzaldehyde 
(18.5 g, 0.1 mole) with freshly prepared phenylhydroxylamine (11.4 g, 0.1 
mole) in 40 ml of absolute ethanol at room temperature for 18 hours. 
Evaporation of the solvent yielded a red oil which was twice crystallized 
from toluene/petroleum ether to afford 13.0 g (0.05 mole) of the nitrone, 
mp 103.degree.-105.degree. C. Further crystallization of an analytical 
sample raised the melting point to 110.degree.-112.degree. C. 
Other nitrones having absorption maxima in the wavelength range of 300 to 
450 nanometers are also set forth in Table 1. In Table 1, .lambda..sub.max 
(nm) designates absorption maximum in nanometers, .epsilon..sub.max 
designates extinction coefficients at wavelength of maximum absorption, mp 
designates melting point in degrees Centigrade. 
These other nitrones were prepared in a manner similar to the manner of 
preparation to the aforementioned nitrones by the condensation of the 
appropriate aldehydes and phenylhydroxylamines in polar solvents. 
TABLE 1 
__________________________________________________________________________ 
##STR4## 
Ar Ar' .lambda..sub.max (nm) 
.epsilon..sub.max 
mp (.degree.C.) 
__________________________________________________________________________ 
.alpha.-(4-diethylaminophenyl)-Nphenylnitrone 
##STR5## 
##STR6## 388 41,000 
110-112 
.alpha.-(4-diethylaminophenyl)-N(4-chlorophenyl)nitrone 
##STR7## 
##STR8## 398 42,500 
176-179 
.alpha.-(4-diethylaminophenyl)-N(3,4-dichlorophenyl)nitrone 
##STR9## 
##STR10## 409 43,700 
154-157 
.alpha.-(4-diethylaminophenyl)-N(4-ethoxycarbonylphenyl)nitrone 
##STR11## 
##STR12## 418 31,300 
107-109 
.alpha.-(4-diethylaminophenyl)-N(4-acetylphenyl)nitrone 
##STR13## 
##STR14## 424 27,000 
115 
.alpha.-(4-dimethylaminophenyl)-N(4-cyanophenyl)nitrone 
##STR15## 
##STR16## 420 33,000 
202-203 
.alpha.-(4-methoxyphenyl)-N(4-cyanophenyl)nitrone 
##STR17## 
##STR18## 368 13,000 
181-182 
.alpha.-(9-julolidinyl)-Nphenylnitrone 
##STR19## 
##STR20## 405 38,000 
132-133 
.alpha.-(9-julolidinyl)-N(4-chlorophenyl)nitrone 
##STR21## 
##STR22## 420 36,600 
99-103 
.alpha.-[2-(1,1-diphenylethenyl)]-Nphenylnitrone 
##STR23## 
##STR24## 366 27,800 
140-142 
.alpha.-[2-(1-phenylpropenyl)]-Nphenylnitrone 
##STR25## 
##STR26## 340 19,000 
468 
__________________________________________________________________________ 
In the method of the invention the photobleachable layer is in the form of 
an in situ mask on the layer of photoresist. The formation of such a 
composite structure has a number of advantages. The photobleachable layer 
conforms to the surface of the layer of photoresist and avoids the 
formation of gaps between the high points of the surfaces of the layer of 
photobleachable material and the layer of photoresist. Such gaps would be 
formed when the two layers are formed on separate supports and then 
brought together. Such gaps would be highly detrimental to the resolution 
of any image formed in the photoresist particularly when the features to 
be imaged are of the order of a few microns and also when the depth of 
field of the projection system is a few microns. Also, bringing a layer of 
photobleachable material into contact with a layer of photoresist and 
thereafter separating them runs the risk of having pieces of one layer 
adhering to the other layer thereby damaging the layers of photoresist. 
While in describing the invention in an exemplary embodiment, the contrast 
enhancing layer was situated in contact with the layer of the photoresist, 
the contrast enhancing layer could have been spaced apart from the layer 
of the photoresist, if desired, for example, by a thin conformal layer of 
neutral material formed in situ. 
While the invention has been described in specific embodiments, it will be 
understood that modifications such as those described above may be made by 
those skilled in the art, and it is intended by the appended claims to 
cover all such modifications and changes as fall within the true spirit 
and scope of the invention.