Positive-working photosensitive polyimide operated by photo induced molecular weight changes

An insoluble photosensitive polyimide having the formula ##STR1## can be exposed by a pattern of light to render the exposed areas soluble. The exposed areas can then be dissolved using a solvent to leave the pattern which can be used directly as an insulator layer in a semiconductor device. A process for preparing the photosensitive soluble polyimide utilizes maleic anhydride which is irradiated by ultraviolet light to form a cyclobutane unit which is reacted with oxydianiline to form polymic acid. The polymic acid is cured using heat into the photosensitive soluble polyimide.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates in general to photolithography techniques for 
producing semiconductor devices and the like, and in particular to a new 
and useful method of forming a pattern in an insulator layer for a 
semiconductor structure utilizing a photosensitive polyimide which becomes 
soluble when exposed to light. 
In the fabrication of multi-level metal-insulator integrated circuit 
structures, a polyimide known as Kapton (a trademark of DuPont) has proven 
to be a good insulator when applied between metallization layers in 
integrated circuit structures. This is because of the high thermal 
stability, chemical resistance and dielectric properties of Kapton 
polyimide. Kapton polyimide has the following formula: 
##STR2## 
An intermediate polyamic acid which is subjected to heat to form the Kapton 
polyimide, is itself soluble and can be spun into films which can be cured 
into the insoluble polyimide structures. Once the material is cured, it is 
generally insoluble and infusible and is extremely thermally stable. The 
insolubility and the infusibility of the Kapton polyimide requires that 
patterning of the polyimide layers be accomplished indirectly by 
photoresist technology. This process entails the spinning and curing of 
the polyimide layer, formation of a polysulfone lift-off layer and 
deposition of an SiO.sub.2 masking layer followed by a top layer of resist 
coating. 
The pattern is defined by either electron beam or optical lithography and 
the underlying layers are etched with reactive ion etching. Metal is 
deposited onto the pattern and the polysulfone and excess are lifted off 
with a solvent. 
If the polyimide layer itself could be made intrinsically photosensitive, 
the formation of a pattern in the polyimide layer would be greatly 
simplified. Several photonegative polyimide systems have been developed 
which utilize photosensitive polyimides. These are generally made by the 
reaction of the corresponding polyamic acid with a photosensitive group. 
In the most common case, the intermediate polyamic acid is partially 
esterified with photo-crosslinkable alcohols. Irradiation of these esters 
causes them to become insoluble and enables them to be used to form 
negative images upon treatment with solvent. After imaging, the films are 
thermally converted to the polyimide which itself is not sensitive to 
light. 
A photopositive system would be more desirable because of the swelling 
attendant upon solvent development of negative images. A photopositive 
polyimide containing photosensitive sulfonium salt units has been 
described in a article by Crivello et al. entitled "Synthesis and 
Characterization of Photosensitive Polyimides", Journal of Polymer 
Science: Part A: Polymer Chemistry, Vol. 25, 3293-3309 (1987). 
A polyimide resin which has good transparency and is useful to produce 
molded products with substantially no coloring and good thermal 
resistance, is disclosed in European Patent Application 0 130 481 to 
Noriaki et al. This reference does not consider or discuss the possibility 
of photosensitivity for the polyimide product. 
SUMMARY OF THE INVENTION 
The present invention involves a process for making and using an 
intrinsically photosensitive polyimide so that direct, positive 
photoimaging can be accomplished even after a complete thermal curing 
which forms the polyimide structure has taken place. 
The process for preparing the intrinsically photosensitive polyimide 
comprises irradiating maleic anhydride (MA) solution with ultraviolet 
light. See "Photodimerization of Maleic Anhydride in Carbon 
Tetrachloride", Boulet et al., Tetrahedron Letters, No. 11, pp. 865-868 
(1976). This excites the anhydride molecule to dimerize and form a 
cyclobutane unit. Because the olefin is consumed in the dimerization 
process, ultraviolet absorption of the cyclobutane unit shifts to a 
shorter wavelength. Thus, once the anhydride is formed, it can be used to 
form a polyimide which can be imaged by irradiation at a wavelength which 
corresponds to energy absorption by the cyclobutane structure. 
The cyclobutane unit is reacted with an aromatic diamine, such as 
oxydianiline (ODA) to form a polyamic acid which can thereafter be heat 
cured to produce the corresponding and intrinsically photosensitive, 
polyimide which, depending on the diamine used, may be insoluble. 
THe invention also includes a method of forming a patterned insulator layer 
useful for example in a semiconductor structure, which comprises forming a 
layer of the intrinsically photosensitive soluble or insoluble polyimide, 
irradiating the layer with a pattern of light to produce exposed and 
unexposed areas in the layer, the exposed areas becoming soluble and 
applying a solvent to the layer to remove the soluble exposed areas to 
form the patterned insulator layer. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
EXAMPLE 1 
The inherently photosensitive positive acting polyimide in accordance with 
the present invention is prepared by irradiating a solution of maleic 
anhydride (I) in carbon tetrachloride or other suitable solvent with light 
at a maximum wavelength of 280 nm from a high power mercury arc lamp for 
one hour. This forms 1, 2, 3, 4-cyclobutane tetracarboxylic 
1,2:3,4-dianhydride (CBDA) shown at II in the Equation 1. 
##STR3## 
CBDA precipitates from solution as it forms and is collected by filtration. 
CBDA (maximum wavelength of absorption is 232 nm) is then purified by 
successive recrystallization from acetic anhydride (until the filtrate is 
colorless) to yield a white solid. 
To form the polyamic acid shown at IV in Equation 2, 0.62714 g (3.1977 
mmols) of CBDA (II) is added to 60 ml of dry DMAC (dimethylacetamide) and 
0.64031 g (3.1977 mmols) of oxydianilinetemperatureold Label, III) are 
mixed together and reacted in a 3-necked, 100 ml round-bottomed flask 
fitted with a mechanical stirrer, a nitrogen inlet and a condenser. The 
reaction was allowed to proceed at room temperature under dry nitrogen for 
18.5 hours. The resulting polyamic acid (IV) was precipitated twice into 
methanol and dried in vacuo at room temperature for 24 hours. 
The intrinsic viscosity of the resulting polyamic acid was 1.47 dl/g 
measured with an Ubbelohde viscometer at 25.00.degree. C. in DMAC. 
##STR4## 
The polyamic acid is cured to the polyimide (V), as shown in Equation 2, by 
heating films or layers cast by solvent evaporation in a watch glass, in 
an oven for two hours at 100.degree. C., two hours at 175.degree. C. and 
two hours at 250.degree. C. 
The resulting uniform polyimide film (V) is colorless. 
Irradiating a film made of polyimide (V), with ultraviolet light in the 
range of 230 to 254 nm at incident doses ranging from 10 to 1000 
mJ/cm.sup.2, results in the scission which breaks the polyimide chain as 
shown at VI in Equation 2. While the photosensitive polyimide (V) is 
insoluble in solvents like DMAC and DMF (dimethyl formamide) the exposed 
compound VI is readily soluble therein. 
Samples of the intrinsically photosensitive insoluble polyimide as prepared 
above, were prepared for dissolution rate studies. The samples were 
irradiated by flood exposures. In those instances where pattern were 
generated, appropriate masking procedures were used. Development was 
carried out by immersing the samples in DMAC, a non-solvent for the 
unirradiated polyimide, for 45 seconds and then immersing the samples in 
deionized water for 15 seconds. After developing the samples, they were 
baked at 140.degree. C. for 30 minutes. 
FIG. 1 shows a differential scanning calorimogram, under nitrogen, of the 
polyamic acid. From this scan, it is apparent that conversion of polyimic 
acid to polyimide takes place in the range of 150.degree. C. to 
250.degree. C. Subsequent re-scanning of the sample showed no transitions 
in this range. 
FIG. 2 shows the thermogravimetric behavior, under nitrogen, of the 
polyamic acid. The weight loss corresponding to the curing of the polyamic 
acid into the insoluble polyimide structure, can be seen. Once the 
polyimide is formed, 50% weight loss occurs at about 460.degree. C. and 
after heating the sample to 1200.degree. C. there is a residual weight of 
approximately 35%. Thus, despite the incorporation of an aliphatic repeat 
unit in the chain, the thermal stability of the polyimide is quite high. 
FIG. 3 shows the sensitivity curve for the polyimide. In FIG. 3, doses up 
to 500 mJ/cm.sup.2 remove approximately 20% of the initial film thickness. 
Re-irradiation of the film after solvent development and drying, 
solubilizes a further portion of the film. Presumably the photoproduct is 
more absorptive than the cyclobutane ring as is to be expected. 
Measurements at lower doses yield a sensitivity in the range of 45 
mJ/cm.sup.2. 
According to the present invention thus a readily synthesized polysensitive 
polyimide is provided which has utility for image generation. 
The polyimide V can also be produced by reacting ODA and CBDA in other 
polar, aprotic solvents, such as DMF, to yield polyamic acid that is cured 
to the polyimide. 
EXAMPLE 2 
Dimethyl cyclobutane dianhydride [DMCBDA--see G. O. Schenk et al., Ber. 95, 
1642 (1962)] 
A solution of methyl maleic anhydride, also known as citraconic anhydride, 
and 7 wt.-% of benzophenone in 1,4-dioxane was irradiated with a high 
pressure mercury arc for 18 hr. To form 1,3-dimethyl-1,2,3,4-cyclobutane 
dianhydride (DMCBDA). The product was collected by filtration and purified 
by recrystallization from ethyl acetate-hexanes. The product does not melt 
below 300.degree. C. 
DMCBDA--ODA polyimide 
To a dry, 100 ml, 3-necked, round-bottomed flask fitted with a mechanical 
stirrer, nitrogen inlet and condenser was added 0;93016 g (4.6452 mmols) 
of oxydianiline (ODA), 60 ml of dry DMAC and 1.04132 g (4.6452 mmols) of 
DMCBDA. The reaction was allowed to proceed to room temperature under dry 
nitrogen for 24 hours and 80.degree. C. for 24 hours. The resulting 
polyamic acid was precipitated twice into methanol and then dried in vacuo 
at room temperature. Films of the polyimide derived from this material by 
cyclization could be prepaed as described above. When irradiated under the 
conditions previously described, this polymer also proved to be 
photosensitive and micron-sized images could be generated by washing the 
irradiated films with ethyl acetate. This result indicates that 
substituents on the cyclobutane ring do not interfere with the 
photosensitivity of the polymers of the this class. 
COMATIVE EXAMPLE 
CBDA-alicyclic diamine polyimide 
To a dry, 100 mL, 3-necked, round-bottomed flask fitted with a mechanical 
stirrer, nitrogen inlet and condenser was added 0.74067 g (3.5208 mmols) 
of 4,4'-methylene bis (cyclohexylamine) which had been previously 
distilled (163.degree.-4.degree. C., 6 torr), 60 mL of dry DMAC and 
0.69049 g. (3.5208 mmols) of CBDA. The reaction was allowed to proceed 
room temperature under dry nitrogen for 20 hr. The resulting polyamic acid 
was not isolated. Silicon wafers were spincoated with this solution and 
then terminally converted to polyimide according to the previously given 
heating cycle. When irradiated under the conditions previously described, 
this polymer was not found to be photosensitive, indicating that the 
aromatic portion of the diamine may perform a role (sensitizer) in the 
imaging process. 
Turning now to FIG. 4, a method of forming a patterned layer for 
semiconductor structures is disclosed, which utilizes the inherently 
photosensitive insoluble polyimide of the present invention. 
FIG. 4 illustrates steps A through D of a trench method used in accordance 
with the present invention. At step A, a substrate 10 advantageously made 
of semiconductor material has a covering layer 12 of polyimide made in 
accordance with the present invention. A transparent mask layer 14 covers 
the upper surface of polyimide layer 12 and includes opaque mask regions 
16. The structure of step A is irradiated, its mask is removed, and the 
pattern is solvent developed to form the structure of step B which 
includes the substrate 10 and the un-irradiated polyimide portions 12a and 
12b that had been under the opaque regions 16 in step A. 
The structure of step B is then subjected to etching of the substrate to 
form the trenched substrate shown in step C where substrate 10 is provided 
with raised regions 10a and 10b positioned under polyimide regions 12a and 
12b. 
The structure of step C is then subjected to removal of the polyimide, for 
example, by plasma etching to form the structure of step D, having 
substrate 10 with raised regions 10a and 10b. 
Another technique which can be used in accordance with the present 
invention is to subject the structure of step B to metal deposition which 
deposits metal between the polyimide regions 12a and 12b. This structure 
can then be subject to plasma etching to remove the polyimide regions 12a 
and 12b and leave the metal regions which are now spaced from each other. 
While a specific embodiment of the invention has been showed and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.