Process for forming heat-resistant resin films of polyimide and organosilicic reactants

A process for forming heat resistant resin films comprising the steps of admixing a polyimide resin precursor solution, for example, a solution containing a reactant obtained from pyrromellitic dianhydride, 3,3',4,4'-benzophenyltetracarboxylic acid dianhydride, 4,4'-diaminodiphenyl ether, and 4,4'-diaminodiphenyl ether-3-carbonic amide with an organosilicic compound solution, coating a silicon substrate with the resulting admixture, and subjecting said coated silicon substrate to heat treatment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for forming a heat resistant resin film 
used in semiconductor integrated circuit devices, and more particularly to 
a process for forming a heat resistant resin film which can be used as an 
intermediate insulating layer. 
2. Description of the Prior Art 
These days, a multi-layer interconnection technique is essential to 
increase the packing density of an integrated circuit device. In the 
multi-layer technique, multi-leveled conductive layers are electrically 
insulated from one another by insulating layers. The insulating layer is 
necessarily subjected to a relatively high temperature of about 
500.degree. C. for at least 10-20 minutes during a heating process. 
Therefore, it is required that the insulating layer material has its 
superior heat-resistant property and is chemically stable during a heating 
process. Well-known polyimide resins and chemically-produced silicon 
dioxide have been used as such insulating materials. For example, a 
polyimide resin is described in detail in Japanese Patent Publication No. 
44871/1976. 
In order to form a polyimide resin film, a polyamic acid solution is coated 
on the surface of a silicon substrate. The polyamic acid is a polyimide 
resin precursor and has the following molecular structure. 
##STR1## 
When the polyamic acid is heated at a temperature of approximately 
200.degree. C. to vaporize the solvent therein, it turns through 
dehydration into a polyimide resin film having the following ring closure 
structure. 
##STR2## 
However, such polyimide resin film gradually decreases in weight and 
thickness when heated in an oxygen atmosphere at a temperature of 
420.degree.-470.degree. C. At 530.degree. C., this resin film decomposes 
by being baked within about 10 minutes as shown as the line 1A in FIG. 1. 
Therefore, the polyimide resin film shows poor heat resistant properties. 
In addition, the polyimide resin has a relatively large ion mobility, so 
that it is not proper as a passivation material for suppressing the leak 
current in a silicon substrate. 
On the other hand, a chemically produced silicon dioxide is described in 
detail in Japanese Patent Publication Nos. 20825/1977 and 16488/1977. An 
organosilicic compound, for example, is obtained by dissolving a silicon 
acetate as a principal constituent into ethyl alcohol. When the silicon 
acetate solution obtained is coated on the surface of a silicon substrate, 
and heated at a temperature of approximately 400.degree. C. in an oxygen 
atmosphere, the organosilicic compound turns into a chemical SiO.sub.2 
film. Such SiO.sub.2 film features a high heat resistant temperature of 
about 1000.degree. C. and can be used as a mask against impurities. 
However, it is difficult to form a thick film of 1-2 .mu.m from an 
organosilicic compound, because it shrinks by heat treatment and cracks. 
As described above, both a polyimide resin and a chemically produced 
SiO.sub.2 are not enough as intermediate insulating materials for 
electrically insulating laminated conductive layers in semiconductive ICs.

SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a process 
for forming a resin film having a high heat resistant property. 
Another object of the present invention is to provide a process for forming 
a thick resin film which electrically insulates conductive layers from one 
another. 
Still another object of the present invention is to provide a process for 
forming a heat resistant resin film which provides a good passivation 
effect. 
Other objects and advantages of the invention will be apparent from the 
following description and the appended claims. 
DETAILED DESCRIPTION OF THE INVENTION 
According to the invention, a polyamic acid solution and an organosilicic 
compound solution are firstly prepared. The polyamic acid solution is 
obtained by dissolving both a primary diamine and a dicarboxylic anhydride 
with a solvent. Specific examples of diamines include diaminocarbonic 
amide, m-phenylenediamine, p-phenylene diamine, 
2,2'-bis(4-aminophenyl)propane, 4,4'-methylenedianiline, benzidine, 
4,4'-diaminodiphenylsulfide, 4,4'-diaminodiphenylsulfone, 
4,4'-diaminodiphenyl ether, hexamethylenediamine, heptamethylenediamine, 
octamethylenediamine, nonamethylenediamine, decamethylenediamine, 
3-methylheptamethylenediamine, 4,4'-dimethylheptamethylenediamine and the 
like. 
Also, examples of the dicarboxylic anhydride include pyromellitic 
anhydride, 2,3,6,7-naphthalenetetracarboxylic anhydride, 
3,3',4,4'-diphenyltetracarboxylic anhydride, 1,2,5,6 
-naphthalenetetracarboxylic anhydride, 2,2',3,3'-diphenyltetracarboxylic 
anhydride, thiophene-2,3,4,5-tetracarboxylic anhydride, 
2,2'-bis(3,4-biscarboxyphenyl)propane anhydride, 
3,4-dicarboxyphenylsulfonic anhydride, perylene-3,4,9,10-tetracarboxylic 
anhydride, bis(3,4-dicarboxyphenyl)ether anhydride, 
ethylenetetracarboxylic anhydride, 3,3',4,4'-benzophenonetetracarboxylic 
anhydride and the like. 
Furthermore, specific examples of solvent include N,N-dimethylformamide, 
N,N-dimethylacetamide, dimethyl sulfoxide, N-methyl-2-pyrrolidone, 
tetramethylurea, N-methylcaprolactam, pyridine, aromatic hydrocarbons and 
the like, and any of these solvents may be used either alone or in 
combination with one another. 
Solvents according to the invention are reqired to strongly attach two 
solutes described above to each other and to maintain them, not so as to 
set them, at low temperatures. 
On the other hand, the organosilicic compound solution is prepared by 
dissolving a silicon compound having the following formula: 
EQU [R.sub.n Si(OH).sub.4-n ] 
wherein R is a monovalent hydrocarbon group, and n is 0 or an integer of 
1-4, and an additive such as a glass forming agent and organic binder into 
an organic solvent such as an alcohol, an ester and a ketone. The 
resulting solution may contain 20% by weight or less of silicon compound, 
based on the total weight of the solution. 
Next, both the polyamic acid solution and organosilicic compound solution 
are mixed together. The resulting mixture is coated on the surface of a 
silicon substrate, and heated at a temperature of 80.degree.-500.degree. 
C. to form an insulating resin film of about 1-2 .mu.m in thickness having 
an Si--O--Si and polyimide molecular structures thereon. The resulting 
resin film showed remarkably elevated resistant properties and a smooth 
surface without cracks. 
According to this invention, phosphorus may be added to the silicon 
compound to improve the passivation effect at the surface of a silicon 
substrate. In this case, phosphorus is added by dissolving the 
organosilicic compound RnSi(OH).sub.4-n and phosphorus compound 
##STR3## 
into such organic solvent as described above. The content of phosphorus is 
preferably within a range of 1-25% by weight based on the total weight of 
the silicon compound. 
EXAMPLE 1 
First, a polyamic acid solution was prepared by dissolving a 
diaminocarbonic amide and a dicarboxylic dianhydride into an 
N-methyl-2-pyrrolidone solution so as to prepare a solution of about 14.2% 
by weight of solid content, based on the total weight of the solution. 
Also, an organosilicic compound solution was prepared by dissolving a 
silicon acetate and an additive into an organic solvent so the solvent 
contains about 3% of organosilicic compound component, based on the total 
weight of the solution. 
Next, 22 cc of the polyamic acid solution and 10 cc of the organosilicic 
compound solution were mixed. The mixed solution was applied on the 
surface of a silicon substrate by means of a spinner to form a relatively 
thick layer, then heated at about 80.degree.-500.degree. C. for 1 hour to 
form a silicon polyimide film of 1-2 .mu.m in thickness thereon. 
To investigate the changes in film thickness, the silicon polyimide film 
was placed in a furnace under a forced-air circulation at a temperature of 
530.degree. C. for 10-20 minutes. As a result, the silicon-polyimide resin 
film according to the invention proved to sufficiently withstand a high 
temperature of about 530.degree. C. for at least 20 minutes at an about an 
80% film survival ratio. (See the solid line 2A in FIG. 1) Also, in a 
microscopic examination, cracks were not particularly observed on the 
surface of the silicon-polyimide resin film. 
EXAMPLE 2 
The silicon-polyimide resin film produced by mixing both 22 cc of the 
polyamic acid solution and 22 cc of the organosilicic compound solution 
was subjected to the same resistant tests as that of Example 1. As a 
result, the resin film according to the invention proved to possess a 
higher heat resistant property than that of Example 1. 
EXAMPLE 3 
A heat resistant insulating resin film having good passivation properties 
was obtained by the following ways. First, polyamic acid solution was 
prepared by dissolving both diaminocarbonic amide and an acid dianhydride 
into an N-methyl-2-pyrrolidone solution so that the solution contains 
about 14.2% by weight of a solid content, based on the total weight of the 
solution. Also, an organosilicic compound solution was prepared by adding 
1 gram of phosphorus into 100 cc of a silicon acetate solution. 
Next, 22 cc of the polyamic acid solution and 10 cc of the organosilicic 
compound solution containing phosphorus were mixed. The mixed solution was 
applied on the surface of a silicon substrate by means of a spinner to 
form a relatively thick layer, and then heated at about 
80.degree.-500.degree. C. for 1 hour to form a silicon polyimide film of 
1-2 .mu.m in thickness thereon. As a result, the silicon polyimide resin 
obtained proved to have both a Si--O--Si structure containing phosphorus 
and a polyimide structure. 
In the silicon polyimide resin film obtained, a thermogrovinetric analysis 
(TGA) was performed by measuring the decreasing weight of the resin under 
an air of 500.degree. C., and a differential thermal analysis (DTA) was 
performed by measuring the exothermic temperatures generated by the 
decomposition of the resin under the air at 500.degree. C. 
In FIG. 2, curves 1 and 2 respectively show TGA and DTA characteristics for 
the resin according to the invention. For comparison, the TGA and DTA 
characteristics for a polyimide resin alone are shown respectively by 
curves 3 and 4 in FIG. 2. From this fact, it is clear that the resin 
according to the invention has a higher decomposition temperature than 
that of the polyimde resin alone. 
In addition, the passivation effect of the resin thus obtained was 
examined. In FIG. 3, the curve 5 shows the leak current characteristics 
for the resin of the invention, while curve 6 shows such characteristics 
for the polyimide resin alone. From FIG. 3, it is clear that the resin 
according to the invention has better passivation effects. 
EXAMPLE 4 
The silicon polyimide resin film produced by mixing both 22 cc of the 
polyamic acid solution and 22 cc of the organosilicic compound solution 
containing phosphorus was subjected to the same heat resistant tests as 
those of Example 3. As a result, the resin film according to the invention 
proved to have higher heat resistant properties than those of Example 3. 
EXAMPLE 5 
The same procedure as that of Example 4 was effected except that phosphorus 
was added to an organo-silicic compound solution within a range of 0.1 g-5 
g. As a result, a resin film having characteristics equivalent to those of 
Example 4 could be formed. 
EXAMPLE 6 
The same procedure with that of Example 4 was carried out except that 
silicon hydroxide was used in place of silicon acetate. As a result, it 
was confirmed that a resin film having such characteristics equivalent to 
those of the resin films prepared in the above described Examples could be 
obtained. 
EXAMPLE 7 
First, a reactant solution (14.2% resin content) obtained by dissolving 
pyrromellitic dianhydride, 3,3',4,4'-benzophenyltetracarboxylic acid 
dianhydride, 4,4'-diaminodiphenyl ether, and 4,4'-diaminodiphenyl 
ether-3-carbonic amide into an N-methyl-2-pyrrolidone solution was 
previously prepared as a polyimide precursor. Also, an organosilicic 
compound solution was prepared, which, for example, is a silicon acetate 
containing about 3% of a organosilicic compound component. 
Next, 20 cc of the reactant solution and 10 cc of the aforementioned 
organosilicic compound solution were admixed each other. The resulting 
admixture was applied on the surface of a silicon substrate by means of a 
spinner. Then, the silicon substrate was heated at 100.degree. C. for 1 
hour. As a result, a silicon polyimide resin film having a thickness of 
1-2 .mu.m was formed on the surface of the silicon substrate. The silicon 
polyimide resin film obtained was subjected to the TGA and DTA under air 
at 500.degree. C. 
In FIG. 5, the curves 7 and 8 show the TGA and DTA characteristics 
respectively, for the resin according to the invention. 
For comparison, the TGA and DTA characteristics on a resin film obtained by 
using polyimide only are respectively shown by the curves 9 and 10. 
Judging from the peaks of the curves 8 and 10 showing exothermic 
reactions, it is clear that the resin according to the invention has a 
better heat resistance than the resin film produced by using polyimide 
only. In a microscopic examination, cracks were not particularly observed 
on the surface of the silicon polyimide resin film. Moreover, a FTIR 
(Fourier Transform Infrared) absorption spectrum analysis was carried out 
to the silicon-polyimide resin film cured at 350.degree. C. for 1 hour. In 
FIG. 4, the absorption due to a Si--O--Si structure appears at a wave 
number of 1070 and the absorption characteristics due to an polyimide bond 
structure appears at wave numbers of 1725 and 1775. These results exhibit 
that the resin of the invention can be obtained by uniformly compounding a 
Si--O--Si component with a polyimide component without damaging their 
individual good properties. 
EXAMPLE 8 
The same procedure as that of Example 8 was carried out except that an 
admixture consisting of 20 cc of the polyimide precursor solution of 
Example 7 and 20 cc of an organosilicic compound solution was used. Then, 
the resulting resin film was examined in respect of the aforementioned 
heat resistance, and as a result the present resin film exhibited a higher 
heat resistance than that of the resin film of Example 7. 
EXAMPLE 9 
The same procedure with that of Example 7 was effected except that silicon 
hydroxide was used in place of silicon acetate. As a result, it was 
confirmed that a resin film having such characteristics equivalent to 
those of the resin films prepared in the above described Examples could be 
obtained. 
EXAMPLE 10 
First, a reactant solution (14.2% resin content) obtained by dissolving 
pyrromellitic dianhydride, 3,3',4,4'-benzophenyltetracarboxylic acid 
dianhydride, 4,4'-diaminodiphenyl ether, and 4,4'-diaminodiphenyl 
ether-3-carbonic amide into N-methyl-2-pyrrolidone solution was previously 
prepared as a polyimide precursor. Also, an organosilicic compound 
solution was prepared by adding about 1 gram of phosphorus into 100 cc of 
a silicon acetate solution having 3% organosilicic compound component. 
Next, 20 cc of the reactant solution and 10 cc of the aforementioned 
organosilicic compound solution were admixed with each other. The 
admixture obtained was applied on the surface of a silicon structure by 
means of a spinner. Then, the silicon substrate was heated at 
80.degree.-500.degree. C. for 1 hour. As a result, a silicon polyimide 
resin having a thickness of 1-2 .mu.m was formed on the surface of the 
silicon substrate. To examine heat resistant characteristics, the silicon 
polyimide resin film was subjected to the TGA and DTA under the air at 
500.degree. C. 
In a microscopic examination, cracks were not particularly observed on the 
surface of a silicon polyimide resin film. In addition, the passivation 
effect was examined in the resin obtained. In FIG. 6, the curve 11 is a 
leak current characteristic for the resin of the invention, while the 
curve 12 is for that of the polyimide resin only. 
Judging from the two curves, it is clear that the resin of the invention 
has an improved passivation effect. 
EXAMPLE 11 
The same procedure as that of Example 11 was carried out except that an 
admixture of 22 cc of the polyimide precursor solution of Example 11 and 
22 cc of an organic silicone compound solution (a phosphorus content being 
lg/100 cc organic silicone solution) was utilized. Then, the resulting 
resin film was examined in respect of the aforementioned heat resistance 
and leakage current, and as a result more favorable values than those of 
Example 11 could be obtained. 
EXAMPLE 12 
The same procedure with that of Example 11 was effected except that 
phosphorus was added to the aforesaid organic silicone compound solution 
within a range of 0.1 g-5 g. As a result, a resin film having 
characteristics equivalent to those of the resin film of Example 11 could 
be formed. 
EXAMPLE 13 
The same procedure as that of Example 11 was carried out except that 
silicon hydroxide was used in place of silicon acetate. As a result, it 
was confirmed that a resin film having such characteristics equivalent to 
those of the resin films prepared in the above described Examples could be 
obtained.