Pattern fabricating method

When a pattern is to be fabricated by a lithography using radiation on a heavy metal layer formed on a substrate, secondary electrons are generated in a diverging form from the heavy metal layer by irradiation with a radioactive ray to expose the resist. As a result, the accuracy of the pattern to be formed on the resist is reduced. In order to prevent this, a layer to be transferred, a substrate to be worked, a mask and so on are formed with a film capable of absorbing the secondary electrons so that secondary electrons generated from the heavy metal layer may not reach the resist film. Although a pattern having a thickness of 2 microns or less could not be fabricated according to the prior art, a pattern as thin as 1.5 microns can be fabricated by the method of the present invention.

BACKGROUND OF THE INVENTION 
The present invention relates to a fine patterning method using an X-ray 
and a mask and, more particularly, to a technique for making a pattern 
with great accuracy by reducing the secondary electrons generated in 
accordance with the exposure to the X-ray. 
In the lithography using radiation, especially in the X-ray lithography, it 
is known in the art that the secondary electrons generated from an 
absorber of an X-ray mask are generated in a diverging form to act upon a 
resist thereby reducing the line width accuracy of a resist pattern. One 
example of counter measures against this reduced accuracy is disclosed in 
the Japan Journal of Applied Physics, Vol. 20 (1981), pp. 20 to 24. 
However, no report has been found regarding the influence of secondary 
electrons which come from a layer to be transferred or a substrate to be 
worked in accordance with the exposure to the X-ray. 
The worst defect of X-ray lithography for transferring a mask pattern to a 
layer by using the X-ray is that an absorber constructing the X-ray mask, 
a resist constructing the layer, a layer to be transferred, a substrate 
and so on absorb the X-ray, while they are being exposed to the X-ray, to 
emit secondary electrons such as photoelectrons or Auger electrons, which 
act upon the resist to degrade the accuracy of a transfer pattern. As a 
method of solving that problem, there is disclosed in "Submicron pattern 
replication using a high contrast mask and two-layer resist in X-ray 
lithography" by Y. Saitoh et al (J. Vac. Sci. Technol. Vol. B2 (1), 
Jan.-Mar. 1984, pp. 63 to 67) a method of absorbing secondary electrons by 
applying a high-molecular, film to the mask or the layer to be worked. In 
order to form a highly accurate pattern, however, the formation of that 
high-molecular film may have subsidiary defects due to either an increase 
in the film thickness or irregularities that reduce productivity. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of fabricating a 
highly accurate resist pattern in lithography using radiation. 
We have found that, when a film (i.e., a film to be worked) made of a 
substance or such a heavy metal, e.g., gold (Au), tungsten (W) which is 
placed on a semiconductor substrate and which is liable to generate 
secondary electrons is to be worked in the radiation lithography, the 
secondary electrons generated as a result of radio active irradiation of 
that substance will diverge to irradiate a resist film, thereby reducing 
the accuracy of the resultant resist pattern. It has also been found that 
the maximum of the penetration depth (i.e., the escape depth) of those 
secondary electrons into the resist film is 1,000 .ANG.. 
From these findings, we have investigated the countermeasures to develop a 
method of fabricating a resist film by forming a thin film, hereafter 
referred to as a "passivation film", which is made of a substance emitting 
fewer secondary electrons in accordance with the exposure to the 
radioactive ray than the film to be worked, i.e., which is made of an 
element having a lower atomic number than that of the element making the 
film to be worked, on a substrate to be worked, which is constructed of a 
substrate and an overlying film to be worked. It is also found that the 
maximum of the thickness of that passivation film is 1,000 .ANG., although 
it varies with the quality of the material and that even the minimum 
thickness of about 50 .ANG. or the limit value of the film forming 
technique has an effect. However, it can be easily inferred that the 
present invention can be applied to a thinner film in accordance with the 
improvement in the film forming technique. 
A radiation for inducing secondary electron emission from an element is 
exemplified mainly by X-rays or ultraviolet light, which are also 
frequently used for lithography. 
On the other hand, it is desired that the radiation (or the X-ray) to be 
used for a pattern transfer has a wavelength as monochromatic as possible. 
Despite this fact, an X-ray generated from its general source is 
frequently composed, as shown in FIG. 1, of not only an X-ray component 
100 having a single wavelength but also a continuous X-ray component 200 
having a low intensity but a wide wavelength distribution. The latter 
continuous component exposes the resist and generates the secondary 
electrons to degrade the transfer pattern. Since the continuous X-ray 
component has a low intensity but a wide wavelength distribution, 
moreover, its energy is so high that the degradation in the transfer 
pattern is usually increased. On the basis of the considerations made 
above, we have investigated the counter-measures to invent a variety of 
methods of forming an X-ray mask 12 with a layer or film, which will 
hereafter be referred to as a "filter film", made of such a substance, 
which will hereafter be referred to as a "filter substance", as is 
characterized to absorb a continuous X-ray component well but not the 
single wavelength, as shown in FIGS. 1(c ) to 1(f). One example of the 
absorbing constant of the above-specified filter substance is plotted by a 
curve 300 in FIG. 1(b) against the wavelength. We have also found a method 
of using, as that filter substance, either: all or a portion of the 
substance, which makes a layer to be irradiated with the X-ray so that it 
may be worked, such as a substrate, a patterned layer formed on the 
substrate, or a resist formed on the patterned layer; or a substance 
containing the former substance. 
When X-ray lithography is to be applied to a process for fabricating a 
semiconductor device, for example, the aforementioned substrate is a 
semiconductor wafer made of silicon, Ga or As, and the patterned layer 
formed on the substrate is a semiconductor layer doped with an impurity, 
an insulator such as SiO.sub.2 or Si.sub.3 N.sub.4, a wiring material such 
as Al or Au, or an electrode material such as Mo or W. 
Since the emission of secondary electrons to be generated by irradiation 
with X-rays can be drastically reduced, according to the present 
invention, its influence upon the resist film can be accordingly reduced. 
This makes it possible to establish the X-ray lithography technique which 
can transfer patterns of high fineness of 1 microns or less and high 
quality. As a result, there can be attained an effect that the high 
resolution, miniaturization and at reduced cost can be achieved in the 
technical field using the lithography such as the fabrication of a 
semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EXAMPLE 1 
The following description will be made with reference to FIGS. 2a to 2d. 
First of all, a substrate 10 to be worked was prepared by forming a W film 
(i.e., a film to be worked) 2 having a thickness of 0.2 microns on a 
silicon wafer 1. The W film 2 was covered with a passivation film, i.e., a 
polycrystalline silicon film 3 which was formed by the CVD method. This 
CVD reaction was conducted at a substrate temperature of 600.degree. C. by 
using monosilane (SiH.sub.4) as the reaction gas. The film thickness was 
200 .ANG.. Next, the polycrystalline silicon film 3 was covered with an 
X-ray resist film 4 (as shown in FIG. 2a) having a thickness of 1 micron 
and made of NPR (which is the product of Hitachi Kasei, Ltd.) and was then 
baked at 80.degree. C. for 10 minutes. Subsequently, the X-ray resist film 
4 was exposed to an X-ray 6 through an X-ray mask 5. The X-ray 6 used was 
the MoL Lray, and the exposed dosage was 100 mJ/cm.sup.2 (as shown in FIG. 
2b). 
Next, a development for 3 minutes was conducted by the use of a developer 
NMD-3, a product of Tokyo Ohka, Ltd., to form a resist pattern 4' (as 
shown in FIG. 2c, and a post baking was then conducted at 80.degree. C. 
for 20 minutes. Subsequently, the passivation film 3 and the W film 2 were 
etched by reactive ion etching using sulfur hexafluoride (SF.sub.6) and 
that resist pattern 4' as a mask. The gas pressure of the SF.sub.6 was 20 
Pa, and the etching period was 2 minutes. Next, the pattern fabrication 
was completed by removing the remaining resist film. As a result, a 
pattern having a thickness of 0.3 microns could be formed on the W film. 
Incidentally, the thickness of the pattern was limited to 0.5 microns when 
no passivation film was used. 
Since, in that etching, the etching rate of the polycrystalline silicon 
film 3 was about six times as high as that of the W film, the increase in 
the etching period as a result of the introduction of the passivation film 
raised no substantial, practical problem. 
On the other hand, the passivation film left might be removed or not, if 
necessary. 
In the present invention, the substrate to be worked had not be formed with 
the film to be worked all over the surface of the semiconductor substrate, 
and the present invention could naturally be applied even to the case in 
which two or more films to be worked were present. On the other hand, the 
number of the passivation film had not always be one but might be two or 
more, and the present invention could be put into practice a film even if 
a film failing to act as the passivation film was sandwiched between the 
two layers. 
Although the present Example used polycrystalline silicon as the material 
of the passivation film, it is evident that the passivation film can be 
made of not only SiO.sub.2 or Si.sub.3 N.sub.4 but also an element having 
a lower atomic number, or its compound, or other various organic 
substances. 
EXAMPLE 2 
As in Example 1, a substrate to be worked was formed with a passivation 
film, to which a photo resist (AZ1350J: a product of Hoechist, Ltd.) was 
applied to have a film thickness of 0.7 microns. 
Next, a high-pressure mercury lamp was used to effect exposure of 45 
mJ/cm.sup.2 thereby to fabricate a pattern. Although a pattern having a 
thickness of 2 microns or less could not be fabricated in the absence of 
the passivation film, a pattern having a thickness of 1.5 microns could be 
fabricated in the presence of the passivation film. 
EXAMPLE 3 
This Example will be described with reference to FIGS. 3 to 5. The single 
wavelength X-ray generated from a rotary cathode type X-ray source using a 
Mo target was the MoL characteristic X-ray and had a wavelength of 5.4 
.ANG.. Therefore, an X-ray mask 12 was formed with a Mo film 15 having an 
absorption end wavelength of 4.8 .ANG.. The thickness of the Mo film was 
set at 3,500 .ANG.. Reference numerals 13 and 14 indicate a mask 
supporting frame of silicon and a support of the PIQ, respectively. 
Numeral 16 indicates an Au absorber for forming a mask pattern. FIG. 3b 
shows the structure of the X-ray mask of the prior art. 
Next, there is presented the result of the pattern transfer using that 
mask. An NPR resist, a product of Hitachi Kasei, Ltd., was applied to the 
silicon substrate to form a resist film having a thickness of 1 micron, to 
which a pattern was transferred by using the above-specified ray source 
and mask. The ray source was operated at a voltage of 20 KV to emit an 
X-ray having an intensity of 150 mJ/cm.sup.2. The Mo film 15 having the 
thickness of 3,500 .ANG. absorbed about 30% of the X-ray having a 
wavelength of 5.4 .ANG. and about 90% of the X-ray having a wavelength 
around 4.8 .ANG.. The transfer pattern obtained is plotted in the resist 
pattern width in FIG. 4. A curve 30 corresponds to the case in which the 
mask of the present Example was used, whereas a curve 31 corresponds to 
the case in which the mask of the prior art was used. The resist pattern 
width in high fidelity to the mask pattern width was attained in the case 
of the curve 30, but the in the case of the curve 31 was smaller by about 
0.2 microns than the mask pattern width. Incidentally, since the NPR 
resist was of the positive type, as should be noted, the fact that the 
pattern width was smaller than the mask pattern width means that the 
pattern width of the exposed portion of the transfer pattern was larger 
than the mask pattern width. 
FIG. 5 plots the thickness of the resist film after the pattern transfer. 
From a curve 32 when the mask of the present Example was used, the film 
thickness of the curve 33 when the mask of the prior art was used was 
smaller by about 0.25 microns. This means that more secondary electrons 
were emitted with the use of the conventional mask to irradiate the entire 
surface of the resist so that the surface of the resist film reacted 
accordingly more. That portion of the positive type resist, which had been 
irradiated with the X-ray or the electrons, i.e., which had reacted, 
dissolved into the developer so that it was removed. 
For the thickness of the Mo film 15 of 1,000 .ANG. or more, similar results 
were obtained. Despite this fact, the cost for fabricating filter film 
rose so high for the thickness 10,000 .ANG. that the practicality was 
lost. 
EXAMPLE 4 
This Example will be described with reference to FIGS. 6 and 7. 
Au was used to fabricate an absorber pattern (or a mask pattern) 16, on 
which a Mo film 15 was formed. This Mo film 15 was used as the filter film 
and the passivation film for the mask pattern 16. The X-ray mask could be 
elongated by this method. The relationship between the mask size and the 
resist size when pattern transfer was conducted by using that mask is 
plotted in FIG. 7. From this Figure, it is found that the mask size and 
the resist size are equal so that the pattern transfer conducted was of 
high quality. The practical conditions such as the ray source, the resist, 
the exposed dosage or the thickness of the filter film were identical to 
those of the Example 3. 
EXAMPLE 5 
A synchrotron orbital radiation (which will be abbreviated to "SOR") was 
used as the ray source. The SOR X-ray when the electron energy of an 
accurative ring was set at 1 GeV has its wavelength distribution shown in 
FIG. 8 and had its maximum intensity of about 20 .ANG.. In this case, an 
Al film having an absorption end wavelength of 8 .ANG. was used. The 
structure of the mask is shown in FIG. 9. On the mask support 14, there 
was formed as the filter film an Al film 18 having a thickness of 1.5 
microns, on which the mask pattern 6 was formed, thus fabricating the 
mask. The experimental results obtained from the cases in which both the 
above-specified mask and the mask of the prior art were used are shown in 
FIGS. 10 and 11, as in the Example 3. With close reference to FIGS. 10 and 
11, the effect of the present invention can be confirmed like the Example 
3. It has been confirmed that the present invention could be practiced 
when the filter film (or the Al film) had a thickness of 0.1 microns or 
more, but the cost for forming the filter film rose for the film thickness 
of 1.0 micron or more. 
EXAMPLE 6 
A rotary cathode type ray source which used a target made of Al was used. 
The X-ray in this case was Al K characteristic X-ray having a wavelength 
of 8.3 .ANG.. A layer to be worked was prepared by covering a silicon 
substrate by the vacuum deposition with a tungsten (W) film having a 
thickness of 3,000 .ANG. and by applying an NPR resist film to the 
tungsten film. The tungsten forming part of the worked layer was used as 
the filter substance of the mask. The structure of the mask is shown in 
FIG. 12. A filter film 19 had a thickness of 3,000 .ANG.. The exposed 
dosage of the X-ray was set at 200 mJ/cm.sup.2 on the wafer. FIG. 13a is a 
sectional view showing the worked layer after the pattern transfer was 
conducted by the use of that mask, i.e., after an irradiation with the 
X-ray and then a development were conducted. Reference numeral 20 
indicates a resist film to which the pattern was transferred in a 
remarkably high quality. FIG. 13b is a sectional view showing the worked 
layer when the mask of the prior art was used. Numeral 23 indicates the 
resist film after the transfer, the portions of which contacting a W film 
21 were deformed by the irradiation with the secondary electrons coming 
from the W film 21. FIG. 14 plots the relationship between the reduction 
in the thickness of the resist film due to that deformation and the 
exposed dosage. A curve 40 corresponds to the case of the present 
invention whereas a curve 41 corresponds to the case in which the mask of 
the prior art was used. From FIG. 14, it is found that the thickness 
reduction of the curve 40 was substantially zero whereas the number of the 
secondary electrons increased in the case of the curve 41 with the 
increase of the exposed dosage of the X-ray so that the thickness 
reduction also increased. The present invention could be practised for a 
thickness of the film of 1,000 .ANG. or more, but the cost for forming the 
filter film rose for a film thickness of 10,000 .ANG. or more.