Process for the development of relief structures based on radiation-crosslinked polymeric precursors of polymers which are resistant to high temperature

An improved development process for relief structures based on radiation-crosslinked polymeric precursors of polymers which are resistant to high temperature in which the development fluid used is an aliphatic or cycloaliphatic ketone having 3 to 7 carbon atoms, is distinguished by rapid and complete removal of soluble polymeric material from the substrate with, at the same time, a minimum amount of attack on the radiation-crosslinked polymeric structures.

BACKGROUND OF THE INVENTION 
The invention relates to an improved development process for forming relief 
structures based on radiation-crosslinked polymeric precursors of polymers 
which are resistant to high temperature and which are based on 
radiation-sensitive photoresists. 
Photoresists for forming relief structures from polymers which are 
resistant to high temperature are used widely in the production of 
electrical and electronic components of etch resists and plating resists 
or of printing molds. One of the most accurate processes for structuring 
insulating materials and semiconducting and conducting materials in 
electrotechnology and electronics is the photo technique. This entails 
resist relief structures produced by the photo technique being copied on 
substrates by suitable processes, such as etching, vapour deposition and 
metallising non-electrically or electrically. 
Moreover, resist relief structures can assume a permanent protective 
function, for example as an insulating material. 
Processes for the production of relief structures of these types from 
polymers which are resistant to high temperature, and photoresists 
suitable for these purposes, and the photosensitive soluble polymeric 
precursors used for this are known, for example, from German Pat. No. 
2,308,830, German Pat. No. 2,437,348, German Pat. No. 2,437,368 and German 
Pat. No. 2,437,422, and German Patent Application 3,227,584 and German 
Patent Application 3,233,912, corresponding to U.S. Pat. Nos. 3,957,512, 
Re. 30,186, 4,045,223, 4,008,489, and U.S. Patent Applications Ser. Nos. 
516,399 and 531,781, respectively. The entire disclosures of these 
references are incorporated herein by reference. 
As a rule, photoresists of the type described contain radiation-sensitive 
soluble polymeric precursors. They are applied to a substrate in the form 
of a layer or film, the layer or film is dried and then irradiated through 
a negative mask. This leads to crosslinking taking place on the irradiated 
areas and this drastically reduces the solubility of the material applied 
there. The nonirradiated parts of the layer or film are then removed by 
dissolving or detaching using a developer; the remaining relief structures 
can then be converted by heat treatment into polymers which are resistant 
to high temperature and which survive temperatures of 
250.degree.-400.degree. C. without adverse effects on the edge sharpness 
and resolution. 
The soluble polymeric precursors used are polyaddition or polycondensation 
products of polyfunctional carbocyclic or heterocyclic compounds having 
radiation-sensitive radicals with diamines, diisocyanates, bis(acid 
chlorides) or dicarboxylic acids as are described in the patents referred 
to above. 
Particularly preferred soluble polymeric precursors are polycondensates of 
pyromellitic acid, which has two radicals which react to radiation and are 
bonded in the manner of esters to carboxyl groups, and a diamine which 
contains at least one cyclic structural element, such as, for example, 
4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 
4,4'-diaminodiphenyl sulfone or 2,4-diaminopyridine. 
As a rule, soluble polymeric precursors of this type which are used in 
photoresists have molecular weights between 2,000 and 100,000, preferably 
between 4,000 and 60,000. 
These soluble polymeric precursors are processed in a manner known per se 
to give the corresponding photoresists which, apart from a suitable 
solvent or mixture of solvents, can, where appropriate, contain additives 
which are known and customary in this technology, such as 
photosensitisers, photoinitiators, copolymerisable monomers or resins 
which react to radiation, adhesion promoters, plasticisers and pigments, 
dyestuffs and fillers. 
The photoresists can be applied to the clean surface of the substrates at 
the layer thickness advantageous in the individual case by spraying, flow 
coating, rolling, spin coating and dip coating, the solvent then being 
removed by evaporation so that a radiation-sensitive layer remains on the 
surface of the substrate. The removal of the solvent can, where 
appropriate, be accelerated by heating the layer to temperatures up to 
100.degree. C. Subsequently, the layer of photoresist is exposed to 
radiation which causes the groups which react to radiation to react so as 
to crosslink in the layer. The irradiation or illumination can be carried 
out through a mask, but it is also possible to guide a collimated beam 
over the surface of the radiation-sensitive layer. UV lamps are normally 
used for the irradiation, these emitting radiation of a wavelength of 200 
to 500 nm and an intensity of 0.5 to 60 mw/cm.sup.2. An image pattern is 
now developed in the layer with exposure of parts of the substrate by 
treating the layer with a developer solution which removes the 
nonirradiated areas of the photoresist material. The resist images are 
obtained after customary washing and drying. 
Now, the process step of development has particular importance in the 
production of radiatin-crosslinked photoresist structures since it exerts 
a considerable influence on the resolution and edge sharpness of the 
remaining resist structures as well as on their adhesion to the substrate 
and their material properties. 
A good developer should remove the non-irradiated, soluble polymeric layers 
from the substrate in a short time which is suitable for practice. At the 
same time, the partial dissolution and erosion of the crosslinked resist 
layers, and the loss of thickness of the layer caused by this, should be 
kept to the minimum possible. In addition, constituents of the developer 
should penetrate into the crosslinked polymeric layers as little as 
possible since this is associated with swelling of the crosslinked resist 
layer and a decrease in its adhesion to the substrate. Furthermore, there 
is a requirement for a development process which is suitable for practice, 
that is to say the safety margin in the development time should be as wide 
as possible in that, while the erosion of crosslinked polymeric material 
is still within tolerable limits, the complete removal of non-crosslinked 
resist is ensured. 
According to the state of the art, the developer solutions used are 
mixtures which consist of one or more solvents customary for the 
production of the photoresist and of a precipitant normally used in the 
production of the polymeric precursor of the photoresist. Examples of 
known and typical developer solutions are 4-butyrolactone/toluene, 
dimethylformamide/ethanol, dimethylformamide/methanol, methyl ethyl 
ketone/ethanol and methyl i-butyl ketone/i-propanol, each in the ratio 2:1 
to 4:1, each of the substances mentioned first being solvents and each of 
those mentioned second being precipitants. After development it is 
customary to rinse with the precipitant, and this merely serves to remove 
the developer completely. 
The disadvantages of developer solutions of these types according to the 
state of the art are that they always attack, to a greater or lesser 
extent, the crosslinked polymeric layers, and this is manifested by 
erosion of the layer, swelling of the layer and a diminution in its 
adhesion to the substrate, and by the safety margin in the development 
time being too narrow. Hitherto, in order to decrease the extent of 
attack, a precipitant has always been added to the developer fluid in 
addition to the solvent. 
OBJECTS OF THE INVENTION 
One object of the invention is to provide an improved process for the 
development of relief structures based on radiation-crosslinked polymeric 
precursors of polymers which are resistant to high temperature, which 
process fulfills the requirements mentioned and does not have the 
above-mentioned disadvantages of the developers according to the state of 
the art. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art. 
SUMMARY OF THE INVENTION 
These objects can be achieved in a process for forming relief structures of 
highly heat-resistant polymers, comprising the steps of applying a 
photoresist comprising a soluble prepolymer being a poly-addition or 
poly-condensation prepolymer containing radiation-reactive radicals, said 
photoresist being applied in the form of a layer or sheet to a substrate, 
drying the layer or sheet, irradiating the dried, radiation-sensitive 
layer or sheet through a negative original or using a guided collimated 
beam, dissolving out the non-irradiated portions of the layer or sheet 
using a developer fluid, optionally rinsing the resultant developed relief 
structure with a rinsing fluid, and optionally heat-treating the relief 
structure, by the improvement wherein said developer fluid consists 
essentially of a C.sub.3-7 -aliphatic or C.sub.3-7 -cycloaliphatic ketone; 
whereby said non-irradiated portions are rapidly removed with minimal 
erosion and swelling of the crosslinked layer, and the safety margin in 
the development time can be significantly increased. 
Further improvement is achieved by rinsing the developed relief structure 
with a rinsing fluid consisting essentially of a C.sub.1-4 -aliphatic 
alcohol. 
DETAILED DISCUSSION 
Suitable developer fluids according to the invention are straight-chain or 
branched aliphatic and cycloaliphatic or alkyl-substituted cycloaliphatic 
ketones having 3 to 7 carbon atoms, such as, for example, acetone, methyl 
ethyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, methyl 
tertiary-butyl ketone, cyclopentanone, cyclohexanone, cycloheptanone, 2- 
or 3-methylcyclopentanone or ethylcyclopentanone, or 2-, 3- or 
4-methylcyclohexanone. 
Acetone, methyl ethyl ketone, cyclopentanone and cyclohexanone are 
preferred, and of these acetone and cyclopentanone are in turn 
particularly preferred. 
Some of these ketones have in fact already been mentioned as solvents for 
photoresists. However, it could not have been predicted that exactly these 
compounds fulfil, without the addition of a precipitant, all the 
requirements made of a developer so well and are suitable in such an 
excellent manner as developers for relief structures based on 
radiation-crosslinked precursors, especially of polyimides which are 
resistant to high temperature. 
The embodiment of the development process according to the invention is 
particularly advantageous when, after development, the residual developer 
fliud is removed with a rinsing fluid. 
In principle, all precipitants miscible with the developer are suitable as 
rinsing fluids, and it will be understood that the development step 
according to the invention can be followed by rinsing with any 
conventional rinsing fluid. Straight-chain or branched aliphatic alcohols 
having 1 to 4 carbon atoms are preferred; ethanol, propanol and 
isopropanol are particularly suitable. 
The development process according to the invention for 
radiation-crosslinked polyimide photoresist structures and the use 
according to the invention of the ketones mentioned makes it possible to 
remove from the substrate, rapidly and without residues, the 
non-irradiated soluble polymeric layers. The new process is particularly 
distinguished by a minimal attack on the crosslinked polymeric layers, 
contrary to the result achieved by prior art developers to which a 
precipitant is purposely added for the purpose of decreasing the amount of 
attack. In the process according to the invention, the erosion of the 
crosslinked polymeric layer is considerably reduced compared with prior 
art developers, swelling of the crosslinked layer and the associated 
diminution in its adhesion to the substrate is avoided. Surprisingly, the 
process according to the invention also makes it possible for the safety 
margin in the development time to be markedly widened with no adverse 
effect on the crosslinked polymeric layers. 
Erosion of the radiation-crosslinked polymer layer is measured by the 
percentage loss in thickness of the layer, which is also related to the 
intensity and duration of irradiation. Use of a C.sub.3-7 -ketone 
developer according to the invention can reduce the loss in layer 
thickness especially at lower irradiation energies, thereby increasing the 
efficiency of the process. Lengthening of development times by factors of 
3-5, for example, can be achieved using developer fluids according to the 
invention rather than prior art developers. 
Furthermore, the much lower toxicity of the fluids used in the process 
according to the invention compared with the constituents of prior art 
developers diminishes any adverse effect on the health of the operators 
who come into contact with these substances. 
The radiation-crosslinked insolube intermediates produced in the resist 
layer undergo cyclisation on heat treatment at 200.degree. to 400.degree. 
C., and at the same time polymers are formed which are resistant to high 
temperature and, depending upon the polymeric precursors used in the 
photoresist, belong to the classes of polyimides, polyamideimides or 
polyesterimides. The class of polyimides is preferred for the production 
of relief structures resistant to high temperature. 
These polymers which are resistant to high temperature have excellent 
chemical, electrical and mechanical properties. Thus, for example, 
photoresists of the type described are suitable, to a particular extent, 
for the production of protective layers, for example in the production of 
semiconductor components, dielectric layers in multilayer integrated 
circuits, as the final passivating layer on electrical devices and as the 
orienting layer of liquid crystal display cells.

Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore, to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever. In the following 
examples, all temperatures are set forth uncorrected in degrees Celsius; 
unless otherwise indicated, all parts and percentages are by weight. 
EXAMPLE 1 
(a) A photoresist comprises 5 g of a polymeric polyimide precursor 
(obtained by reacting pyromellitic dianhydride with 2-hydroxyethyl 
methacrylate and then with thionyl chloride and 4,4'-diaminodiphenyl ether 
in accordance with German Pat. No. 2,437,348, (U.S. Pat. No. Re. 30,186) 
0.25 g of N-phenylamaleimide 
0.1 g of Michler's ketone 
0.05 g of vinyltrimethoxyethoxysilane dissolved in 10.5 ml of 
N-methylpyrrolidone. 
(b) The photoresist from (a) is spin coated onto substrate samples having 
an SiO.sub.2 surface and these are dried by heating. The 1.0 .mu.m thick 
layers which are obtained are then, under nitrogen in a contact process, 
illuminated through a pattern of lines with a 200 W Hg lamp having an 
intensity of 9-10 mw/cm.sup.2. The energy of illumination is increased 
stepwise from 960 mj/cm.sup.2 to 2,400 mj/cm.sup.2. 
(c) Then the non-illuminated parts of the photoresist are washed out by 
development with cyclopentanone and then rinsed with isopropanol. Images 
having sharp edges and a resolution of less than 3 .mu.m are obtained. The 
loss in thickness of the layer of the illuminated parts of the photoresist 
due to the process described above depends on the energy of illumination 
and decreases with increasing energy of illumination as shown: 
______________________________________ 
Energy of illumination 
Loss in thickness of the layer 
______________________________________ 
960 mJ/cm.sup.2 
70% 
1,250 " 65% 
1,540 " 60% 
1,830 " 50% 
2,110 " 45% 
2,400 " 40% 
______________________________________ 
COMISON EXAMPLE A 
Under the same conditions as in Example 1, but using 
4-butyrolactone/toluene, 1:1 (developer according to the state of the 
art), the loss in thickness of the layer is 70% irrespective of the 
illumination energy. 
EXAMPLE 2 
(a) A photoresist comprises 5 g of a polymeric polyimide precursor (as in 
Example 1) 
0.25 g of N-phenylmaleimide 
0.1 g of Michler's ketone 
0.05 g of vinyltrimethoxyethoxysilane 
1 g of 2,4,6-trisallyloxy-1,3,5-triazine dissolved in 6.5 ml of 
dimethylformamide. 
(b) The photoresist from (a) is spin coated as indicated in Example 1 and 
illuminated, the thickness of the layer being 7.3 .mu.m. 
(c) Then the non-illuminated parts of the photoresist are washed out by 
spray development with cyclopentanone and then rinsed with isopropanol. 
The loss in thickness of the layer and the resolution are not affected by 
increasing the development time from 10 to 50 seconds. 
COMISON EXAMPLE B 
Under the same conditions as in Example 2, but using 
4-butyrolactone/toluene, 1:1, well resolved structures are only obtained 
with spray development times of 10 to 15 seconds. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.