Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-05-19
2001-03-06
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C378S035000
Reexamination Certificate
active
06197457
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an X-ray mask employed in fabrication of a semiconductor device, and more particularly to such an X-ray mask including an X-ray absorber composed of an alloy having low stress. The invention relates further to a method of fabricating such an X-ray mask, and still further to a method of fabricating a semiconductor device through the use of such an X-ray mask.
2. Description of the Related Art
As a semiconductor device has been integrated highly and highly, there X-ray lithography has been employed for formation of a minute pattern. The X-ray lithography is characterized by features that X-ray has a short wavelength, and that it is relatively easy to have a great depth of focus. The X-ray lithography is suitable particularly for making a small pattern having a length of 10 nm or smaller, or a pattern having a great aspect ratio. The X-ray lithography is employed also in fabrication of a liquid crystal display panel, a charge coupled device (CCD), a thin film magnetic head, and a micro-machine as well as LSI such as memory and logic.
In the X-ray lithography, a resist film is deposited on a wafer, and then, X-ray is radiated onto the resist film through an X-ray mask to thereby make a pattern. Specifically, an X-ray mask composed of X-ray absorber and having a pattern corresponding to a pattern of a semiconductor device to be fabricated is used. The X-ray mask is positioned in the close vicinity of a surface of a wafer on which an X-ray resist film has already been deposited. Then, X-ray is radiated onto the resist film through the X-ray mask to thereby transfer a pattern of the X-ray mask to the resist film.
The above-mentioned X-ray mask is usually comprised of a membrane through which X-ray can pass, X-ray absorbing material arranged on the membrane, a silicon substrate, and a support such as a glass plate for cooperating with the silicon substrate to support the membrane at its periphery.
FIG. 1
illustrates an example of an X-ray mask. The illustrated X-ray mask is comprised of a membrane
2
composed of silicon nitride (SiN), silicon carbide (SiC) or diamond (C) in the form of a thin film, an X-ray absorber
1
formed on the membrane
2
and having a desired pattern, a silicon substrate
3
supporting the membrane
2
at its periphery, and a support ring
4
composed of silicon carbide or quartz glass and cooperating with the silicon substrate
3
to thereby support the membrane
2
.
In conventional X-ray masks, the X-ray absorber
1
has usually been composed of simple metal such as tungsten (W) and tantalum (Ta).
A method of fabricating the X-ray mask illustrated in
FIG. 1
is explained hereinbelow with reference to
FIGS. 2A
to
2
D.
First, thin films
2
a
are deposited on opposite surfaces of a silicon substrate
3
by chemical vapor deposition. The silicon substrate
3
has a thickness of 1 to 2 mm. The thin films
2
a
are composed of silicon carbide (SiC) and have a thickness of about 1 to 2 &mgr;m. One of the thin films
2
a
will make a membrane
2
.
Then, as illustrated in
FIG. 2A
, a support ring
4
is adhered to a lower surface of the silicon substrate
3
at its periphery by means of an adhesive such as epoxy resin. The support ring
4
is composed of glass or silicon carbide (SiC), and has a thickness of about 5 mm.
Then, as illustrated in
FIG. 2B
, the silicon substrate
3
is back etched from a lower surface thereof by anisotropic etching through the use of KOH aqueous solution. As a result of the etching, a portion of the silicon substrate
3
is removed, and there is formed a membrane
2
on the silicon substrate
3
.
Then, as illustrated in
FIG. 2C
, an X-ray absorbing material
1
is deposited on the membrane
2
by sputtering.
Then, as illustrated in
FIG. 2D
, the X-ray absorbing material
1
is patterned into a desired pattern
1
a
by dry etching. Thus, there is completed an ray mask having a desired pattern
1
a.
In the method illustrated in
FIGS. 2A
to
2
D, although the silicon substrate
3
is back-etched prior to depositing and patterning the X-ray absorbing material
1
, the silicon substrate
3
may be back-etched after deposition of the X-ray absorbing material
1
.
In order to make a minute pattern of a semiconductor device by means of X-ray lithography, an X-ray absorbing material of which an X-ray mask is composed is required to have the following characteristics.
First, an X-ray absorbing material has to have high ability of disallowing X-ray to pass therethrough, in order to provide sufficient contrast in X-ray exposure. Herein, such ability of disallowing X-ray to pass therethrough is defined as a product of a mass absorption coefficient and a density of an X-ray absorbing material. An X-ray absorbing material is particularly required to have high ability of disallowing passage therethrough of X-ray having a wavelength of about 1 nm, which X-ray wavelength is usually used in X-ray lithography.
If an X-ray absorbing material has smaller ability of disallowing X-ray to pass therethrough, a film composed of an X-ray absorbing material, to be formed on a membrane, has to have a greater thickness. In such case, it would be quite difficult to make a minute pattern of the X-ray absorbing material.
In addition, if a film composed of an X-ray absorbing material has a great thickness, problems such as inaccurate transfer of a pattern to an X-ray absorbing material, and difficulty of controlling stress remaining in a film composed of an X-ray absorbing material, would be cause.
Second, an X-ray absorbing material has to have a stress as small as possible, and further have high controllability of stress.
If a film composed of an X-ray absorbing material, formed on a membrane has a great internal stress, positional accuracy with which a pattern is transferred to an X-ray absorbing material from an X-ray mask would be deteriorated, and as a result, misalignment would be caused in a semiconductor device pattern. Accordingly, an X-ray absorbing material is required to have almost zero internal stress all over a surface of an X-ray mask.
In addition, taking productivity of an X-ray mask into consideration, an ray mask is required to not only have an almost zero internal stress, but also be able to be repeatedly fabricated in the same configuration. Furthermore, since an X-ray mask is repeatedly employed, an X-ray mask is required to have stability in stress.
Third, an X-ray absorbing material is required to have a densified crystal structure.
Most metals having been used as an X-ray absorbing material is changed into a polycrystalline film having a columnar structure when deposited into a film by sputtering. If such a polycrystalline film is patterned, grain boundary would appear at a sidewall thereof, resulting in a side surface of a pattern having much roughness, in which case, it is no longer possible to form a desired pattern of a semiconductor device.
Apart from the above-mentioned characteristics, an X-ray absorbing material is required to have conformity to dry-etching to be carried out in patterning, and to have chemical stability.
However, conventional X-ray absorbing materials used so far cannot meet all of the above-mentioned requirements.
Tungsten (W) and tantalum (Ta) have been conventionally and widely used as an X-ray absorbing material, because these metals meet the above-mentioned first requirement. That is, these metals have sufficient ability of disallowing passage of X-ray therethrough. However, tungsten (W) and tantalum (Ta) cannot meet the above-mentioned second and third requirements, and hence, these metals cannot be used for an X-ray mask when a minute pattern of a semiconductor device is to be formed.
If a film is formed of tungsten (W) or tantalum (Ta) by sputtering, a resultant film would be a polycrystalline film having a columnar structure. Hence, when a minute pattern is to be formed of tungsten (W) or tantalum (Ta) by sputtering, grain boundary would be generated at a sidewall of a pattern to thereby rough the sidewall, which is a bi
NEC Corporation
Rosasco S.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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