Pattern forming method

A pattern forming method for semiconductor devices in which a film layer of a compound containing silicon and nitrogen is formed between a substrate and a resist layer with a desired pattern readily formed utilizing a lift-off technique. A first film layer of a compound such as Si.sub.3 N.sub.4 is formed on a semiconductor substrate with a resist film layer formed in a desired pattern upon the first film layer. The first film layer is etched using the resist film layer as a mask. A second film layer is then formed on the substrate after which the first film layer is removed with an etchant which does not attack the second film layer.

BACKGROUND OF THE INVENTION 
The present invention relates to a pattern forming method in which a 
desired pattern is formed utilizing a lift-off method. 
In general, a lift-off method uses resist made of organic material which is 
soluble in organic solvent. 
A conventional lift-off patterning method will be described with reference 
to FIG. 1. After resist 2 has been applied to one surface of a substrate 
as shown in FIG. 1a, a resist pattern is formed by photo-etching. It is 
desirable that the resist pattern thus formed be such that the resist in 
cross-section has a greatly inverted taper as shown in FIG. 1b. Then, film 
layers 3 are formed on the surfaces of the resist layer and on the exposed 
substrate 1 by vacuum evaporation or sputtering the material in a desired 
pattern as shown in FIG. 1c. The substrate 1 with the resist layer and the 
film layers 3 is immersed in a solvent. As a result, the resist 2 is 
dissolved in the solvent and therefore the film layer 3 on the resist 
layer 2 is removed. Thus, only the film layer 3 on the surface of the 
exposed substrate 1 remains to provide the desired pattern. 
With this method, it is necessary to form the resist layer 2 relatively 
thick so that the film layer 3 on the surface of the exposed substrate 1 
does not touch the film layer 3 on the resist layer 2. Accordingly, it is 
difficult to etch the resist layer 2 with a high accuracy and therefore 
the pattern formed by the film layer 3 is low in accuracy. This is one of 
the drawbacks accompanying the above-described conventional method. 
A conventional method of providing a finer pattern than that formed by the 
method of FIG. 1 is illustrated in FIG. 2. 
According to the second conventional method, a first resist layer 4 and a 
second resist layer 2 are successively formed on a substrate. Then, a 
first pattern is formed by photoetching the second resist layer 2. The 
pattern thus formed must have a considerably high dimensional accuracy and 
it must be opposite to or complementary to the pattern of a film layer 3 
which is formed later. 
Thereafter, a second pattern is formed by photoetching the first resist 
layer 4 or the like with the pattern of the second resist layer 2 as a 
mask. In this case, the pattern of the first resist layer 4 is developed 
more than the pattern of the second resist layer 2 so as to provide a 
resist section having overhanging portions as shown in FIG. 2b. 
In the following step, a desired pattern is obtained by treating the 
aforementioned film layers 3 formed on the surfaces of the second resist 
layer 2 and of the exposed substrate 1. In this step, the above-described 
structure contributes greatly to the lifting off of the film layer 3 
formed on the second resist layer 2 by vacuum-evaporation or the like. 
That is, in the lifting-off operation, a solvent dissolves the resist 
layers beginning with the lower resist layer 4 and therefore the 
lifting-off operation is readily accomplished to thereby obtain the 
desired pattern. 
In accordance with this method, the resist layer 4 is interposed between 
the resist layer 2 and the substrate so that it is thus possible to make 
the resist layer 2 quite thin. Accordingly, the method of FIG. 2 can 
provide a finer pattern than the method of FIG. 1. However, the second 
method is still disadvantageous in that it is impossible to completely 
avoid the etching of the resist layer 2 while the resist layer 4 is being 
etched and hence the finally formed film layer 3 has a relatively low 
dimensional accuracy. 
In addition to the above-described conventional methods, methods of using a 
polyamide material and an oxide film, respectively, instead of the resist 
layer 4 are known in the art. However, the first of these methods is 
disadvantageous in that it does not provide a sufficient resolution to 
form a fine pattern. The latter method is disadvantageous in that during 
etching of the oxide film, the undercut control is rather difficult if 
chemical etching is employed and, if plasma etching is employed for 
etching the oxide film, then the resist layer 2 is also etched and 
therefore it is difficult to obtain a pattern with the desired dimensional 
accuracy because the oxide film etching speed is not very high. 
The above-described method in which the oxide film is interposed between 
the resist layer 2 and the substrate 1 is further disadvantageous in the 
following point. In the case where an oxide film has been previously 
formed on the substrate for a different purpose, it is impossible to etch 
only the oxide film in the middle stop. Accordingly, the method cannot be 
applied to a material in which at least an oxide film has previously been 
formed on the substrate. 
For the methods in which the resist layer 4, the polyamide or the oxide 
film is interposed between the resist layer 2 and the substrate 1, it is 
difficult to satisfactorily control the dimensions of the overhanging 
portions with a high accuracy. Especially, in the method employing the 
resist layer 4, the resist is considerably thick and therefore it is 
extremely difficult to produce a very fine pattern in which the dimensions 
must be controlled in the sub-micron range. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to form a fine pattern by 
interposing a film layer of a compound containing at least silicon and 
nitrogen, which may further contain hydrogen or oxygen and which is formed 
by plasma reaction and is essentially different in chemical etching 
characteristics or plasma etching characteristics from the conventional 
material described above. 
According to the invention, a film layer of a compound containing silicon 
and nitrogen is formed between a substrate and a resist layer so that a 
desired pattern may be readily formed by the lift-off method. The desired 
pattern depends on a pattern formed with the resist layer. 
Thus, the invention can be used to form a wiring layer made of metal or 
polycrystalline film on a substrate and furthermore the invention can be 
employed to form a wiring pattern with a film layer which has been formed 
on the wiring layer by a chemical vapor deposition method (hereinafter 
referred to as "the CVD method") or vacuum evaporation method with the 
wiring layer used as a mask. 
The invention makes it possible to readily form a fine pattern less than 
one or two microns in line width the formation of which has heretofore 
been exceedingly difficult.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred methods in accordance with the present invention will now be 
described. Referring now to FIGS. 3a-3d, a silicon nitride film layer 5 is 
formed on the surface of a substrate using a plasma CVD method. Then, the 
upper surface of the film layer 5 is coated with resist to form a resist 
layer 2. Thereafter, a pattern complementary to a desired pattern, that is 
a negative pattern, is formed on the resist layer 2 using an exposure 
method such as electron beam exposure, X-ray exposure or light exposure. 
Next, the film layer 5 formed using a plasma CVD method is subjected to 
plasma etching with the negative pattern as a mask. 
With respect to the formation of the film layer 5 using a plasma CVD 
method, a gas mixture of silane and ammonia, silane and nitrogen, or 
silane, ammonia and nitrogen at a temperature of from 100.degree. C. to 
400.degree. C. is supplied to a plasma generating reaction tube where the 
silane is decomposed under a pressure of about 0.1-1.5 Torr. to obtain 
silicon nitride. To form ordinary silicon nitride, the silane is 
decomposed at a high temperature of the order of about 600.degree. C. to 
800.degree. C. and therefore the chemical formula of the silicon nitride 
is Si.sub.3 N.sub.4. However, under the above-described conditions, the 
film which is formed contains excessive amounts of hydrogen or nitrogen 
and its reflection refractive index is 1.6 to 1.9 smaller than the 
ordinary value 2.0. The silicon nitride film layer 5 having such 
characteristics is etched at a rate of about 1000-6000 .ANG./min in a 
cylindrical plasma etching equipment under a CF.sub.4 gas pressure of 1.0 
Torr. and at high frequency power of 200 watts. Thus, the etching speed of 
the silicon nitride film layer 5 is more than several to ten times that a 
layer made with ordinary silicon nitride. 
Because of this, a film layer several thousands of .ANG.ngstroms in 
thickness can be etched in two or three minutes while during this etching 
operation the resist layer 2 is hardly etched at all. As a result, the 
pattern thereby produced has a high dimensional accuracy. If, in this 
connection, the conditions of plasma etching are changed to control the 
lateral etching speed, then it is possible to provide uppercut and 
overhanging portions having dimensions as required thereby facilitating 
the removal of the resist layers which is carried out by a lifting-off 
method in the following manufacturing step. 
Thereafter, film layers 3 are formed on the resist layer 2 and the surface 
of the exposed substrate 1 by vacuum evaporating a metal such as aluminum 
similar to the ordinary lift-off technique as shown in of FIG. 3c. Then, 
the substrate 1 with the resist layers 2 and 5 and the metal film layers 3 
is immersed in a solvent capable of dissolving the resist as a result of 
which the resist layer 2 is dissolved in the solvent and accordingly the 
metal film layer 3 on the resist layer 2 is removed. The film layer 5 is 
then removed by etching in a plasma atmosphere which does not etch 
aluminum as a result of which a pattern of metal such as one of aluminum 
is left on the substrate 1 as shown in FIG. 3d. 
In the above-described method of the invention, aluminum is employed as the 
wiring pattern forming material. However, it is obvious that other metals 
and other materials such as for instance polycrystalline silicon may be 
employed. 
The pattern of aluminum film 3 formed as described above can be utilized as 
follows. That is, if an under-film layer which has been formed in advance 
is subjected to plasma etching or sputter etching with the aluminum 
pattern used 3 as a mask, then a pattern can be formed with the under-film 
layer as shown in FIG. 4a. More specifically, a film such as a silicon 
nitride film, a polycrystalline silicon film or a molybdenum film which is 
different in plasma etching or chemical etching characteristics from the 
film 3 is employed as an under-film layer 6 formed on an oxide film layer 
7. Then, these layers are subjected to plasma etching with a gas mixture 
of CF.sub.4 and several percent of O.sub.2. In this etching operation, the 
film layer 3 is not etched at all thus serving as the mask while the 
under-film layer 6, namely, the silicon nitride film layer or the 
polycrystalline silicon film layer, is sufficiently etched under suitable 
conditions as a result of which the pattern of the under-film layer 6 is 
formed as shown in FIG. 4b. In subjecting the under-film layer 6 to plasma 
etching with the aluminum film layer 3 as the mask, not only CF.sub.4 
gases but also CCl.sub.4 gases at low temperatures (lower than 185.degree. 
C.) can be employed. 
If plasma etching is carried out with a gas mixture of CF.sub.4 and 
H.sub.2, for instance, before the under-film layer 6 is etched, then only 
the oxide film layer 8 will be etched. After the film layers 8 and 6 have 
been etched, the remaining layers are immersed in phosphoric acid as a 
result of which the aluminum film layer 3 is removed and accordingly a 
pattern is formed as shown in FIG. 4c. 
As is clear from the above description, according to the invention, the 
final pattern to be formed depends on the pattern of the resist layer on 
the silicon nitride film layer which is formed by the plasma CVD method. 
As it is possible to make the resist layer considerably thin, it may be 
formed having very fine dimensions and therefore the final pattern 
provided therefrom is considerably high in dimensional accuracy. This is 
one of the merits of the invention. 
Furthermore, as the silicon nitride film layer formed by the plasma CVD 
method is considerably uniform in thickness and quality, the undercut, 
that is, the overhanging portion, can be readily formed with a high 
accuracy by plasma etching the silicon nitride film layer. Therefore, the 
distance between wiring parts of aluminum, for instance, can be reduced 
with high accuracy. For instance, in the case where the resist layer 2 is 
formed to a 4000 .ANG. thickness using an electronic resist PMMA and the 
silicon nitride film layer 5 is formed to a thickness of 3000 .ANG. by a 
plasma CVD method, a considerably fine pattern can be obtained in which 
the width of a line of aluminum film 3 is 0.54 .mu.m and the distance 
between such lines is 0.46 .mu.m. The width of the line and the distance 
between the lines can be further decreased by improving the processing 
accuracy. 
Thus, the invention can be applied extensively to various processes in 
manufacturing semiconductor devices and memory elements such as bubble 
memories, in forming diffraction gratings, and in processing substrates 
having fine patterns such as printed circuit boards.