Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-12-22
2003-05-13
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06562524
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a photomask based on which a pattern is defined, used in photolithography.
2. Description of the Related Art
A photomask is grouped into an emulsion mask including a photographic dry plate having a high resolution and a hard mask including a glass substrate and a patterned light-impermeable or thin metal film formed on the glass substrate. Though the hard mask is expensive relative to the emulsion mask, the hard mask is superior to the emulsion mask with respect to an ability of accomplishing a small line and a mechanical strength. Hence, the hard mask is predominantly used as a photomask in fabrication of a semiconductor device.
Presently, photolithography in which a so-called reticle, a hard mask having a pattern about five times greater in dimension than an original pattern, is used for step and repeat exposure is predominantly used in fabrication of a semiconductor device with respect to an ability of mass-production and readiness in fabricating a mask.
A pattern in a reticle used in photolithography is fabricated almost by electron beam painting. Hereinbelow is explained a conventional reticle and a conventional method of fabricating a reticle, with reference to
FIGS. 1A
,
1
B and
2
A to
2
E.
FIG. 1A
is a plan view of a conventional reticle
4
, and
FIG. 1B
is a cross-sectional view taken along the line
1
B—
1
B in FIG.
1
A.
As illustrated in
FIG. 1B
, the conventional reticle
4
is comprised of a glass substrate
51
and a light-impermeable film
52
formed on the glass substrate
51
. The light-impermeable film
52
has a pattern comprised of a chip pattern
41
and a frame pattern
42
formed around the chip pattern
41
.
The chip pattern
41
is a mask pattern used for fabricating a gate electrode of a semiconductor chip and other parts, and is fabricated by forming openings in the light-impermeable film
52
in selected areas.
The glass substrate
51
is almost entirely covered with the light-impermeable film
52
in the frame pattern
42
.
Around a periphery of the chip pattern
41
is formed an external frame line
43
as an opening of the light-impermeable film
52
.
When a resist is to be exposed to a light through the reticle
4
, at first, an object (not illustrated) to be etched, on which a resist has been coated, is mounted on an X-Y table equipped in an exposure unit (not illustrated), and the reticle
4
is mounted on a reticle stage (not illustrated) positioned above the X-Y table. After requisite alignment has been made, a light emitted from a light source is radiated to both the external frame line
43
and the chip pattern
41
through a condensing lens. The light having been radiated to the chip pattern
41
and the external frame line
43
passes through the reticle
4
. The light having passed through the reticle
4
is condensed by another condensing lens, and then, projected onto the resist coated on the object to be etched.
Hereinbelow, a method of fabricating the reticle
4
is explained with reference to
FIGS. 2A
to
2
E.
FIGS. 2A
to
2
E are cross-sectional views of the reticle
4
, taken along the line
1
B—
1
B in
FIG. 1A
, illustrating respective steps of a method of fabricating the reticle
4
.
In
FIGS. 2A
to
2
E, a broken line
55
indicates a boundary between the chip pattern
41
and the frame pattern
42
. An area located at the left of the boundary line
55
is the chip pattern
41
, and an area located at the right of the boundary line
55
is the frame pattern
42
. As illustrated in
FIG. 2A
, it is assumed that the chip pattern
41
is comprised of a peripheral portion
41
a
and a central portion
41
b
surrounded by the peripheral portion
41
a
. An area indicated with “C” indicates an area extending in the vicinity of the boundary line
55
, including the peripheral portion
41
a.
As illustrated in
FIG. 2A
, there is first prepared a blank comprised of a glass substrate
51
and a light-impermeable film
52
formed on the glass substrate
51
by sputtering. The glass substrate
51
is composed of soda-lime glass, heat resistance glass or synthesized quartz, for instance. The light-impermeable film
52
is comprised of a thin metal film such as a chromium film. The light-impermeable film
52
may be comprised of a silicide film such as a molybdenum silicon compound (MoSi
2
) film.
Then, a positive resist
53
is applied onto the light-impermeable film
52
, as illustrated in FIG.
2
A.
Then, electron beams are radiated to the positive resist
53
in predetermined areas thereof. As illustrated in
FIG. 2B
, the electron beams are radiated to the resist
53
in areas a
1
, a
2
and a
3
, and the electron beams are not radiated to the resist
53
in areas b
1
, b
2
, b
3
and b
4
.
In place of the electron beams, ion beams may be radiated to the resist
53
.
Then, the resist
53
having been exposed to the electron beams is soaked into a developing agent for developing the resist
53
. By soaking the resist
53
into a developing agent, the areas a
1
, a
2
and a
3
are dissolved, and hence, removed. As a result, there are formed openings c
1
, c
2
and c
3
, as illustrated in FIG.
2
C.
The areas b
1
, b
2
, b
3
and b
4
are not dissolved, that is, remain as they are, and define resist patterns d
1
, d
2
, d
3
and d
4
.
Then, the light-impermeable film
52
is dry-etched with the resist patterns d
1
, d
2
, d
3
and d
4
being used as a mask. As an etching gas, there is used a mixture gas containing Cl
2
at 75 volume % and O
2
at 25 volume %.
By carrying out dry etching, portions of the light-impermeable film
52
exposed through the openings c
1
, c
2
and c
3
are removed. A portion of the light-impermeable film
52
exposed through the opening c
1
is removed to thereby define the external frame line
43
, as illustrated in FIG.
2
D. Portions of the light-impermeable film
52
exposed through the openings c
2
and c
3
are removed to thereby define the chip pattern
41
, as illustrated in FIG.
2
D.
Then, the resist
53
is removed. By removing the resist
53
, portions of the light-impermeable film
52
covered with the resist patterns d
1
, d
2
, d
3
and d
4
appear to define light-impermeable film patterns e
1
, e
2
, e
3
and e
4
, as illustrated in FIG.
2
E.
Thus, the reticle
4
is completed.
The above-mentioned conventional reticle
4
is accompanied with a problem that a numerical aperture in the central portion
41
b
of the chip pattern
41
is higher than a numerical aperture in the area C. In other words, a darkness rate in the central portion
41
b
is lower than a darkness rate in the area C. This is because the area C includes the peripheral portion
41
a
and the frame pattern
42
having almost no openings, whereas a circuit pattern is formed entirely in the central portion
41
b
, and hence, the central portion
41
b
has many openings.
For instance, a gate pattern of a logic circuit has a darkness rate of 30% or smaller, that is, a numerical aperture of 70% or greater, resulting in a remarkable difference in a numerical aperture between the area C and the central portion
41
b
of the chip pattern
41
.
Such a difference in a numerical aperture between the area C and the central portion
41
b
causes unbalanced etching such as partially excessive etching or partial shortage in etching in accordance with a numerical aperture due to local loading effect.
If such unbalanced etching is caused not in a depth-wise direction of an object to be etched, but in a plane-wise direction of the same, etching rates would become different in portions of an object to be etched, in accordance with numerical apertures in a plane-wise direction of the object, even if a resist is exposed to a light in accordance with a properly designed pattern.
This results in a problem that an accurate mask pattern designed in accordance with a circuit pattern cannot be obtained. In other words, the above-mentioned difference in a numerical aperture causes reduction in an accuracy with which a resist is etched for forming a desired pattern
NEC Corporation
Rosasco S.
Scully Scott Murphy & Presser
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