Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-06-29
2003-06-10
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S323000
Reexamination Certificate
active
06576374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase shift mask used to fabricate semiconductor devices, and to a method of fabricating a phase shift mask from a mask blank. More particularly, the present invention relates to a mask blank and to a method of fabricating an alternating phase shift mask from the mask blank.
2. Description of the Related Art
The degree of integration of a semiconductor device is proportional to the resolution power of exposure equipment used to fabricate the semiconductor device. Accordingly, research has been conducted with an aim towards increasing the resolution power of semiconductor exposure equipment. The optical source of the exposure equipment is an important factor dictating the resolution power of the equipment. Therefore, much of the above-mentioned research centers around exposure equipment having an optical source which emits light of a short wavelength.
The resolution power of the exposure equipment must be in accordance with the degree of integration of the semiconductor devices being fabricated. To avoid the enormous expense that would be associated with providing new equipment for fabricating more highly integrated devices, a method of increasing the resolution power of conventional exposure equipment is used.
The method can be accomplished by using a phase shift mask, for example, an alternating phase shift mask.
A conventional method of fabricating an alternating phase shift mask, and problems thereof will now be described with reference to
FIGS. 1 through 7
. Referring first to
FIG. 1
, a chromium layer
12
is formed on a transparent substrate
10
, and photosensitive film patterns
14
which expose predetermined areas on the chromium layer
12
are formed on the chromium layer
12
. The entire surface of the chromium layer
12
is etched using the photosensitive patterns
14
as an etch mask. Then, the photosensitive film patterns
14
are removed.
Referring to
FIG. 2
, chromium layer patterns
12
a
are formed on the transparent substrate
10
by the etching. Also, the exposed areas of the chromium layer
12
are etched away, thereby exposing the surface
13
of the transparent substrate
10
at a first area A
1
and a second area A
2
. The first area A
1
is a phase non-shift area, and the second area A
2
will eventually constitute a phase shift area.
Referring to
FIG. 3
, photosensitive film patterns
16
are formed on the resultant structure on which the chromium layer patterns
12
a
are formed, so that the first area A
1
of the transparent substrate
10
and the chromium layer patterns
12
a
are covered while the second area A
2
is exposed.
Referring to
FIG. 4
, a groove
18
is formed in the transparent substrate
10
at the second area A
2
, using the photosensitive film patterns
16
as an etch mask. In this way, the phase shift area is formed on the transparent substrate
10
. The formation of the groove
18
makes the second area A
2
of the transparent substrate
10
thinner than the first area A
1
.
The photosensitive film patterns
16
are removed, resulting in a phase shift mask including the first area A
1
and the second area A
2
as shown in FIG.
5
.
However, according to the conventional method of fabricating a phase shift mask as described above, material is left on the surface of the transparent substrate
10
and at the bottom of the groove
18
in the processes of forming the chromium layer patterns
12
a
and forming the groove
18
in the transparent substrate
10
.
For example, residue from a previous step, e.g., the step of forming the photosensitive film patterns
14
, can remain on the exposed surfaces of the chromium layer
12
. The residue obstructs the etching of the exposed portions of the chromium layer
12
. The residue in fact acts as an etching mask. Therefore, although the surfaces
13
and
13
a
of the transparent substrate
10
are exposed by removing the exposed portions of the chromium layer
12
, a defect (not shown), that is, unwanted material, including chromium, is formed on the exposed surfaces
13
and
13
a
of the transparent substrate
10
. The defect is repaired by a laser. In this process, the chromium absorbs the laser, is melted, and is sublimated, whereby it is removed. Simultaneously, heat absorbed from the laser is transmitted to the portion of the transparent substrate
10
lying beneath the chromium. Consequently, the exposed surfaces
13
and
13
a
of the transparent substrate
10
are fused and then solidified. The change in physical state makes the material of the transparent substrate
10
in the neighborhood of the exposed surfaces
13
and
13
a
different from that of other portions of the transparent substrate
10
. Thus, what often happens is that material of the exposed portions
13
and
13
a
of the transparent substrate
10
does not react to an etch gas or is slow to react thereto in the process of forming the groove
18
in the transparent substrate
10
. As a result, a portion
20
(
FIG. 6
) of the substrate
10
remains unetched or only partially etched after the groove
18
is completely formed. The portion
20
is a defect because it changes the phase of light passing through the phase shift area A
2
. Also, light is diffracted at the sides of the portion
20
of the substrate, thus adversely affecting the formation of patterns using the mask.
Referring to
FIG. 7
, in the conventional method of fabricating a phase shift mask, a residual layer
22
is also formed at the phase shift area A
2
as a result of the process of etching the groove
18
in the substrate
10
. The material of the residual layer
22
is a compound of chromium (Cr) and fluorine (F), formed by the reaction of the etch gas, which is used to form the groove
18
, with the chromium layer patterns
12
a.
The residual layer
22
inhibits the transmittance of light incident upon the second region A
2
and abnormally changes the phase of light passing therethrough. Also, the residual layer
22
is formed prior to the completion of the groove
18
and impedes the etching of the substrate
10
during the process of forming the groove
18
. Accordingly, the residual layer
22
prevents a complete forming of the groove
18
.
SUMMARY OF THE INVENTION
Accordingly, a first object of the present invention is to solve the above-described problems by providing a mask blank by which defects can be prevented from being formed on a substrate during a process of fabricating a phase shift mask, and in particular, an alternating phase shift mask, from the mask blank.
To achieve the first object, the present invention provides a mask blank comprising a substrate which is transparent to incident light (exposure light of a given wavelength), a light shield layer formed on the entire surface of the transparent substrate, and a protective layer formed on the entire surface of the light shield layer. The protective layer, among other things, protects the light shield layer from etch gas during a process of etching the substrate to form a phase shift region. The light shield layer is prevented by the protective layer from reacting with the etch gas, whereby the groove in the substrate formed as a result of the etching is free of residue. The protective layer itself produces no residue by-product when exposed to the etch gas.
In this respect, the protective layer can be formed of a material which adheres well to the light shield layer and evaporates in the presence of the etch gas. For instance, when the light shield layer comprises chromium, and the etch gas contains fluorine, the protective layer can be molybdenum silicon oxynitride (MoSiON).
A second object of the present invention is to provide a method of fabricating a phase shift mask, wherein the method prevents undesirable residue from accumulating on the substrate of the mask at the bottom of a groove constituting the phase shift region.
To achieve the second object, the present invention provides a method of fabricating a phase shift mask comprising the steps of forming a light shield layer o
Huff Mark F.
Mohamedulla Saleha R.
Samsung Electronics Co,. Ltd.
Volentine & Francos, PLLC
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