Method for the formation of silicon oxide films

Disclosed is a method for the formation of a thick silicon oxide film on the surface of a substrate. The method comprises forming a hydrogen silsesquioxane resin film on the surface of a substrate and converting the hydrogen silsesquioxane resin into silicon oxide ceramic by heating the resin film-bearing substrate in an inert gas atmosphere at 250.degree. C. to 500.degree. C. until the content of silicon-bonded hydrogen in the silicon oxide product has reached .ltoreq.80% of the content of silicon-bonded hydrogen in the hydrogen silsesquioxane.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for the formation of a silicon 
oxide film on the surface of a substrate. More specifically, the present 
invention relates to a method for the formation of a thick silicon oxide 
film that is free of cracks and pinholes and that is insoluble in organic 
solvents. 
The formation of a protective film on the surface of a substrate is a 
technique in general use for the protection of the surface of a substrate. 
In the particular case of the electric/electronic industries, there has 
been a very substantial increase in the complexity of semiconductor 
devices and in topographical variations on the surface of semiconductor 
devices in association with recent increases in the degree of integration 
and layer count of such devices. An interlevel dielectric layer may be 
formed on the surface of a semiconductor device in order to planarize the 
topographical variations on the surface of the device, while a passivation 
coating can be laid down on the surface of a semiconductor device in order 
to protect it from mechanical damage, chemical damage, damage due to 
static, ionic contaminants, nonionic contaminants, radiation damage, and 
so forth. 
Silicon oxide films are typically used for the interlevel dielectric layers 
and passivation coatings formed on semiconductor device surfaces. 
Chemical-vapor deposition (CVD) and spin-coating are examples of the 
methods used to form these silicon oxide films. In one such spin-coating 
method, a film of hydrogen silsesquioxane resin is formed on the surface 
of the substrate (e.g., the semiconductor device, etc. ), and the resin 
film-bearing substrate is then heated to 500.degree. C. to 1,000.degree. 
C. in an inert gas atmosphere in order to form a silicon oxide film 
(Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 
3-183675 [183,675/1991]). 
However, the method proposed in Japanese Patent Application Laid Open 
Number Hei 3-183675 is not able to produce a silicon oxide film thicker 
than 0.6 micrometers (6,000 angstroms). As a result, this method cannot 
completely planarize the topographical variations encountered on the 
surfaces of semiconductor devices, i.e. , topographical variations or 
height differences in excess of 0.6 micrometers (6,000 angstroms). In 
addition, when the production of a thick silicon oxide film is attempted 
by this method, cracks and pinholes are produced in the silicon oxide film 
and the reliability of the semiconductor device is drastically reduced. 
The inventors conducted extensive research into the cause of the inability 
of the method proposed in Japanese Patent Application Laid Open Number Hei 
3-183675 to produce thick silicon oxide films. It was discovered that this 
inability is due to the excessively high-temperature heating (500.degree. 
C. to 1,000.degree. C.) that is used in order to give a 0% content of 
silicon-bonded hydrogen in the silicon oxide product. It was also 
discovered that the silicon oxide film could perform well as an interlevel 
dielectric layer or passivation coating on the surface of a semiconductor 
device when the Si-bonded hydrogen content in the silicon oxide film 
product did not exceed 80% of the Si-bonded hydrogen content in the 
starting hydrogen silsesquioxane resin. Accordingly, the present invention 
was achieved as a result of extensive research into a silicon oxide film 
formation method that would be capable of producing a crack-free and 
pinhole-free thick silicon oxide film that could function as an interlevel 
dielectric layer or passivation coating on tile surface of a semiconductor 
device and that would also be able to thoroughly planarize the 
topographical variations on the surfaces of semiconductor devices. 
The present invention takes as its object the introduction of a method for 
the formation of an organic solvent-insoluble, crack-free and pinhole-free 
silicon oxide thick film by the formation of a hydrogen silsesquioxane 
resin film on the surface of a substrate and then heating this resin 
film-bearing substrate. 
SUMMARY OF THE INVENTION 
The present invention relates to a method for the formation of a silicon 
oxide film. The method is characterized by the formation of a hydrogen 
silsesquioxane resin film on the surface of a substrate and subsequent 
conversion of said hydrogen silsesquioxane resin into silicon oxide 
ceramic by heating the resin film-bearing substrate in an inert gas 
atmosphere at 250.degree. C. to 500.degree. C. (not including 500.degree. 
C.) until the content of silicon-bonded hydrogen in the silicon oxide 
product has reached .ltoreq.80% of the content of silicon-bonded hydrogen 
in the aforesaid hydrogen silsesquioxane resin. 
The present invention also relates to a method for the formation of silicon 
oxide film on the surface of a semiconductor. The method is characterized 
by the formation of a hydrogen silsesquioxane resin film on the surface of 
a semiconductor device and subsequent conversion of said hydrogen 
silsesquioxane resin into silicon oxide ceramic by heating the resin 
film-bearing semiconductor device in an inert gas atmosphere at 
250.degree. C. to 500.degree. C. (not including 500.degree. C.) until the 
content of silicon-bonded hydrogen in the silicon oxide product has 
reached .ltoreq.80% of the content of silicon-bonded hydrogen in the 
aforesaid hydrogen silsesquioxane resin. 
The present invention further relates to a method for the formation of 
silicon oxide film which is characterized by the planarization of the 
topographical variations on the surface of a semiconductor device by the 
formation thereon of a hydrogen silsesquioxane resin film, and the 
subsequent conversion of said hydrogen silsesquioxane resin into silicon 
oxide ceramic by heating the resin film-bearing semiconductor device in an 
inert gas atmosphere at 250.degree. C. to 500.degree. C. (not including 
500.degree. C.) until the content of silicon-bonded hydrogen in the 
silicon oxide product has reached .ltoreq.80% of the content of 
silicon-bonded hydrogen in the aforesaid hydrogen silsesquioxane resin. 
DETAILED DESCRIPTION OF THE INVENTION 
The method of the present invention relates to the formation of silicon 
oxide films which are thick, free of cracks and pinholes and insoluble in 
organic solvents. Generally, the method of the present invention involves 
forming a hydrogen silsesquioxane resin film on the surface of a substrate 
and heating the hydrogen silsesquioxane resin to convert it into a silicon 
oxide ceramic. The hydrogen silsesquioxane resin used by the present 
invention to coat the substrate surface is a compound with the following 
formula: 
EQU (HSiO.sub.3/2).sub.n 
wherein n is an integer. Its terminal groups, molecular weight, and 
structure are not specifically restricted, although molecular weights of 
approximately 400 to 100,000 are preferred. Its physical properties, such 
as viscosity, softening point, etc., are also not specifically restricted. 
In addition, the con tent of silicon-bonded hydrogen in the hydrogen 
silsesquioxane resin used by the invention is not specifically restricted. 
This value will vary with the molecular weight and type of terminal 
groups, and in general the silicon-bonded hydrogen content is 1.5 to 2.5 
weight % calculated on the hydrogen silsesquioxane resin. 
The method for synthesis of the subject hydrogen silsesquioxane is also not 
specifically restricted. Methods for hydrogen silsesquioxane synthesis are 
specifically exemplified by the hydrolysis of trichlorosilane using the 
crystal water of benzenesulfonic acid or toluenesulfonic acid (U.S. Pat. 
No. 3,615,272) and by the hydrolysis of trichlorosilane in dilute solution 
using a small quantity of water (Japanese Patent Application Laid Open 
Number Sho 60-86017 [86,017/1985]). 
The procedure used in the present invention to form the hydrogen 
silsesquioxane resin film on the surface of the substrate is not 
specifically restricted. This procedure is specifically exemplified by the 
following two methods: (1) preparation of an organic solvent solution of 
the hydrogen silsesquioxane resin, application of this solution by 
spin-coating, spraying, or immersion, then removal of the solvent to yield 
a film of the hydrogen silsesquioxane resin on the surface of the 
substrate; (2) heating a low-molecular-weight hydrogen silsesquioxane 
resin at reduced pressure in order to bring about vapor deposition of the 
resin on the surface of the substrate. The former method is preferred. 
In the former method, there is no specific restriction on the organic 
solvent used to dissolve the hydrogen silsesquioxane resin. The structure 
of this organic solvent preferably does not contain active hydrogen. The 
organic solvent t under consideration is specifically exemplified by 
aromatic solvents such as toluene, xylene, and so forth; aliphatic 
solvents such as hexane, heptane, octane, and so forth; ketone solvents 
such as methyl ethyl ketone, methyl isobutyl ketone, and so forth; and 
ester solvents such as butyl acetate, isoamyl acetate, and so forth. 
Additional examples of this solvent are silicone solvents, for example, 
linear siloxanes such as 1,1,1,3,3,3-hexamethyldisiloxane, 
1,1,3,3-tetramethyldisiloxane, and so forth; cyclic siloxanes such as 
1,1,3,3,5,5,7,7-octamethyltetracyclosiloxane, 
1,3,5,7-tetramethyltetracyclosiloxane, and so forth; and silanes such as 
tetramethylsilane, dimethyldiethylsilane, and so forth. Mixtures of two or 
more of these organic solvents can also be used. 
No specific restrictions apply to tile substrates operable in the present 
invention for formation of the hydrogen silsesquioxane resin film. The 
substrate is specifically exemplified by glass substrates, ceramic 
substrates, metal substrates, and semiconductor devices, with 
semiconductor devices being particularly preferred. The surface of the 
semiconductor device may present topographical variations, in which event 
these topographical variations can be planarized by the silicon oxide film 
formation method of tile present invention. 
The substrate bearing the hydrogen silsesquioxane resin film is 
subsequently heated in an inert gas atmosphere at 250.degree. C. to 
500.degree. C. (excluding 500.degree. C.) until the content of 
silicon-bonded hydrogen in tile produced silicon oxide becomes .ltoreq.80% 
of the content of silicon-bonded hydrogen in said hydrogen silsesquioxane 
resin. 
The inert gas operable for the present invention is not specifically 
restricted, and it is specifically exemplified by nitrogen, argon, helium, 
and neon. Nitrogen is preferred for its low cost and ease of acquisition 
on an industrial basis. 
The substrate carrying the hydrogen silsesquioxane resin film is heated at 
temperatures in the range of 250.degree. C. to 500.degree. C. (excluding 
500.degree. C.). The bases for this range are as follows: When the heating 
temperature is below 250.degree. C., the hydrogen silsesquioxane resin is 
not thoroughly converted into ceramic silicon oxide and as a result 
remains soluble in organic solvent. The product in this case is therefore 
unfit for use as a passivation coating or interlevel dielectric layer. On 
the other hand, the formation of a crack- and pinhole-free silicon oxide 
thick film is no longer possible when the heating temperature is 
500.degree. C. or above. No specific restrictions apply to the heating 
time except that heating must be carried out for a period of time 
sufficient for the content of silicon-bonded hydrogen in the silicon oxide 
product to become .ltoreq.80% of the content of silicon-bonded hydrogen in 
the starting hydrogen silsesquioxane resin. When the content of 
silicon-bonded hydrogen in the produced silicon oxide exceeds 80% of the 
content of silicon-bonded hydrogen in the starting hydrogen silsesquioxane 
resin, the silicon oxide product remains soluble in organic solvent and is 
therefore not capable of functioning as a passivation coating or 
interlevel dielectric layer. Although the heating time cannot be rigidly 
stipulated because the required heating time varies as a function of the 
heating temperature, the following heating times are given as preferred 
examples: approximately 1 hour at 450.degree. C., approximately 2 hours at 
400.degree. C., approximately 3 hours at 350.degree. C., approximately 3 
hours at 300.degree. C., and approximately 4 hours at 250.degree. C. 
An infrared spectrophotometer can be used to measure the silicon-bonded 
hydrogen content in both tile hydrogen silsesquioxane resin film and 
silicon oxide film formed on the surface of the substrate. The point at 
which the silicon-bonded hydrogen content in the silicon oxide .film 
reaches .ltoreq.80% of the silicon-bonded hydrogen content in tile 
starting hydrogen silsesquioxane resin film is readily determined in the 
present invention using an infrared spectrophotometer from tile intensity 
ratio K'/K wherein K is tile intensity of the SiH peak (vicinity of 2250 
cm.sup.-1) relative to the SiOSi peak (vicinity of 1100 cm.sup.-1) in the 
hydrogen silsesquioxane resin film and K' is the intensity of the SiH peak 
(vicinity of 2250 cm.sup.-1) relative to the SiOSi peak (vicinity of 1100 
cm.sup.-1) in the silicon oxide that is produced. 
The silicon oxide film formation method of the present invention can 
produce a thick (greater than 0.6 micrometers), crack-free, and 
pinhole-free silicon oxide film that is capable of functioning as a 
passivation coating or interlevel dielectric layer. For example, this 
method can produce crack- and pinhole-free silicon oxide films with 
thicknesses greater than 1.0 micrometers. Furthermore, the crosslink 
density in the silicon oxide film can be freely controlled or adjusted in 
the method of the present invention. This provides the additional effect 
of making possible relaxation of the internal stresses in the silicon 
oxide film that is produced. Moreover, because the method of the present 
invention produces a silicon oxide film by heating at temperatures of 
250.degree. C. to 500.degree. C. (excluding 500.degree. C.), it is useful 
for the formation of an interlevel dielectric layer or passivation coating 
on a semiconductor device surface because it avoids the melting-based 
deterioration of the aluminum that is used for semiconductor device 
interconnections. 
The method of the present invention is useful for the formation of the 
interlevel dielectric layer in multilayer semiconductor devices because an 
organic resin layer, silicon oxide layer, and so forth, can additionally 
be formed on the surface of a substrate carrying the silicon oxide film 
formed by the method of the present invention. 
The present invention is explained in greater detail below through working 
and comparison examples. The method described below was used to measure 
the value of the silicon-bonded hydrogen content in the silicon oxide film 
relative to the silicon-bonded hydrogen content in the hydrogen 
silsesquioxane resin film formed on the surface of the semiconductor 
device: 
Using an infrared spectrophotometer, the intensity I.sub.SiOSi of the SiOSi 
peak (vicinity of 1100 cm.sup.-1) and the intensity I.sub.SiH of the SiH 
peak (vicinity of 2250 cm.sup.-1) were determined for the hydrogen 
silsesquioxane resin film formed on the surface of the semiconductor 
device, and their ratio K was calculated from K= I.sub.SiH /I.sub.SiOSi. 
The intensity I'.sub.SiOSi of the SiOSi peak (vicinity of 1100 cm.sup.-1) 
and the intensity I'.sub.SiH of the SiH peak (vicinity of 2250 cm.sup.-1) 
were also determined for the silicon oxide film subsequently formed on the 
surface of the semiconductor device, and their ratio K' was calculated 
from K'=I'.sub.SiH /I'.sub.SiOSi. The ratio K'/K was then calculated.

REFERENCE EXAMPLE 1 
Hydrogen silsesquioxane resin was prepared by the method taught in Japanese 
Patent Publication Number Sho 47-31838 [31,838/1972 ] as follows: 
Toluenesulfonic acid monohydrate was prepared by dripping 6 moles toluene 
over a period of 1 hour into a mixture of 3.75 moles sulfuric acid and 
2.25 moles fuming sulfuric acid at a mixture temperature of 45.degree. C. 
to 60.degree. C. and then aging for an additional 30 minutes at 45.degree. 
C. Into this product was then dripped the mixture of 1 mole 
trichlorosilane and 6.6 moles toluene over a period of 5 hours at 
30.degree. C. followed by ageing for 30 minutes at 45.degree. C. After 
cooling and layer separation, the toluenesulfonic acid layer (lower layer) 
was removed. In order to remove the acid present in the upper layer, it 
was washed with suitable quantities of sulfuric acid/water (50/50 weight 
ratio), then sulfuric acid/water (25/75 weight ratio), and finally water. 
The water was then completely eliminated by azeotropic drying for 1 hour 
to afford a toluene solution. Removal of the toluene front this toluene 
solution by reduced pressure (vacuum pump) at 60.degree. C. gave hydrogen 
silsesquioxane resin A. This hydrogen silsesquioxane resin A had a 
number-average molecular weight. (M.sub.n) of 1,650, and the value of its 
weight-average molecular weight/number-average molecular weight ratio 
(M.sub.w /M.sub.n) was 19.4. 
20 g hydrogen silsesquioxane resin A was then placed in a thoroughly dried 
1 L roundbottom flask made of high-quality glass. 80 g thoroughly dried 
toluene was added and a thorough dissolution was effected. The entire 
system was maintained at 25.degree. C., and the interior of the system was 
purged with nitrogen at a rate that did not remove solvent from the 
system. This purging was continued until the completion of fractionation. 
While vigorously stirring the solution, 50 g thoroughly dried acetonitrile 
was dripped in over a period of 1 hour. The precipitate was eliminated 
after quiescence for approximately 12 hours. After elimination of the 
precipitate, another 200 g thoroughly dried acetonitrile was dripped into 
the solution over a period of 4 hours. Collection of tile resulting 
precipitate and removal of the residual solvent therefrom by vacuum drying 
at ambient temperature yielded a hydrogen silsesquioxane resin B. The 
M.sub.n of this hydrogen silsesquioxane resin B was 11,400 and its M.sub.w 
/M.sub.n was 2.88. The ionic and metal impurities were each .ltoreq.1 ppm. 
EXAMPLE 1 
Hydrogen silsesquioxane resin B was dissolved in methyl isobutyl ketone 
(MIBK) to prepare a 30 weight % solution. This solution was spin-coated on 
a substrate for semiconductor device fabrication (height variation=1.0 
micrometers) to give a hydrogen silsesquioxane resin film having a maximum 
thickness of 1.39 micrometers. After this film formation step, the 
semiconductor device substrate was held for 20 hours at 25.degree. C. and 
then heated for 2 hours at 400.degree. C., in each case in a pure nitrogen 
atmosphere. This was followed by gradual cooling in a pure nitrogen 
atmosphere to room temperature. Evaluation of the properties of the 
silicon oxide film formed on the semiconductor device substrate confirmed 
that the maximum thickness was 1.23 micrometers and the topographical 
variations of a semiconductor device surface were able to be planarized to 
uniformity and that there were no pinholes or cracks in the silicon oxide 
film. Based on tile results of infrared spectrophotometric analysis, the 
silicon-bonded hydrogen content in the silicon oxide film was 51% of the 
silicon-bonded hydrogen content in the hydrogen silsesquioxane resin film 
prior to heating. It was also confirmed that the silicon oxide film was 
insoluble in organic solvents such as MIBK and so forth. 
EXAMPLE 2 
Hydrogen silsesquioxane resin B was dissolved in MIBK to prepare a 30 
weight % solution. This solution was spin-coated on a substrate for 
semiconductor device fabrication (height variation=0.75 micrometers) to 
give a hydrogen silsesquioxane resin film having a maximum thickness of 
1.20 micrometers. After this film formation step, the semiconductor device 
substrate was held for 20 hours at 25.degree. C. and then heated for 1 
hour at 450.degree. C., in each case in a pure nitrogen atmosphere. This 
was followed by gradual cooling in a pure nitrogen atmosphere to room 
temperature. Evaluation of the properties of the silicon oxide film formed 
on the semiconductor device substrate confirmed that the maximum thickness 
was 1.00 micrometers and the topographical variations of a semiconductor 
device surface were able to be planarized to uniformity and that there 
were no pinholes or cracks in the silicon oxide film. Based on the results 
of infrared spectrophotometric analysis, the silicon-bonded hydrogen 
content in the silicon oxide film was 28% of the silicon-bonded hydrogen 
content in the hydrogen silsesquioxane resin film prior to heating. It was 
also confirmed that this silicon oxide film was insoluble in organic 
solvents such as MIBK and so forth. 
COMISON EXAMPLE 1 
Hydrogen silsesquioxane resin B was dissolved in MIBK to prepare a 30 
weight % solution. This solution was spin-coated on a substrate for 
semiconductor device fabrication (height variation=0.75 micrometers) to 
give a hydrogen silsesquioxane resin film having a maximum thickness of 
1.20 micrometers. After this film formation step, the semiconductor device 
substrate was held for 20 hours at 25.degree. C. and then heated for 1 
hour at 450.degree. C., in each case in an oxygen atmosphere. This was 
followed by gradual cooling in a nitrogen atmosphere to room temperature. 
While the maximum thickness of the silicon oxide film formed on the 
semiconductor device substrate was 0.82 micrometers, large numbers of 
cracks were produced in the surface of the silicon oxide film and the 
topographical variations of a semiconductor device surface were not able 
to be planarized to uniformity. Based on the results of infrared 
spectrophotometric analysis, the silicon-bonded hydrogen content in the 
silicon oxide film was 8% of the silicon-bonded hydrogen content in the 
hydrogen silsesquioxane resin film prior to heating. 
EXAMPLE 3 
Hydrogen silsesquioxane resin B was dissolved in MIBK to prepare a 30 
weight % solution. This solution was spin-coated on a substrate for 
semiconductor device fabrication (height variation=0.75 micrometers) to 
give a hydrogen silsesquioxane resin film having a maximum thickness of 
1.15 micrometers. After this film formation step, the semiconductor device 
substrate was held for 20 hours at 25.degree. C. and then heated for 4 
hours at 250.degree. C., in each case in a pure nitrogen atmosphere. This 
was followed by gradual cooling in a pure nitrogen atmosphere to room 
temperature. Evaluation of the properties of the silicon oxide film formed 
on the semiconductor device substrate confirmed that the maximum thickness 
was 1.03 micrometers and the topographical variations of a semiconductor 
device surface were able to be planarized to uniformity and that there 
were no pinholes or cracks in the silicon oxide film. Based on the results 
of infrared spectrophotometric analysis, the silicon-bonded hydrogen 
content in the silicon oxide film was 80% of the silicon-bonded hydrogen 
content in the hydrogen silsesquioxane resin film prior to heating. It was 
also confirmed that this silicon oxide film was insoluble in organic 
solvents such as MIBK and so forth. 
COMISON EXAMPLE 2 
Hydrogen silsesquioxane resin B was dissolved in MIBK to prepare a 30 
weight % solution. This solution was spin-coated on a substrate for 
semiconductor device fabrication (height variation=0.75 micrometers) to 
give a hydrogen silsesquioxane resin film having a maximum thickness of 
1.15 micrometers. After this film formation step, the silicon substrate 
was held for 20 hours at 25.degree. C. and then heated for 4 hours at 
200.degree. C., in each case in a pure nitrogen atmosphere. This was 
followed by gradual cooling in a pure nitrogen atmosphere to room 
temperature. When the properties of the silicon oxide film formed on the 
semiconductor device substrate were examined, it was found that this 
silicon oxide film was free of crack and pinholes but could be redissolved 
in toluene. Based on the results of infrared spectrophotometric analysis, 
the silicon-bonded hydrogen content in the silicon oxide film was 100% of 
the silicon-bonded hydrogen content in the hydrogen silsesquioxane resin 
film prior to heating, which confirmed the complete absence of 
ceramification. 
COMISON EXAMPLE 3 
Hydrogen silsesquioxane resin B was dissolved in MIBK to prepare a 30 
weight % solution. This solution was spin-coated on a substrate for 
semiconductor device fabrication (height variation=1.0 micrometer) to give 
a hydrogen silsesquioxane resin film having a maximum thickness of 1.24 
micrometers. After this film formation step, the semiconductor device 
substrate was held for 20 hours at 25.degree. C. and then heated for 1 
hour at 600.degree. C., in each case in a pure nitrogen atmosphere. This 
was followed by gradual cooling in a pure nitrogen atmosphere to room 
temperature. It was found that large numbers of cracks had been produced 
in the silicon oxide film formed on the semiconductor device substrate. 
Based on the results of infrared spectrophotometric analysis, the 
silicon-bonded hydrogen content in the silicon oxide film was 0% of the 
silicon-bonded hydrogen content in the hydrogen silsesquioxane resin film 
prior to heating, which confirmed a complete ceramification.