Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-04-26
2004-04-20
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
06723475
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the following subject matter: reflection-type masks for use in pattern exposure for projecting mask patterns thereof by reflecting exposure light, manufacturing methods therefor, exposure apparatuses using the reflection-type masks for use in pattern exposure, methods of manufacturing devices by using the exposure apparatuses, and semiconductor devices manufactured by the methods of manufacturing devices.
2. Description of the Related Art
In recent years, semiconductor integrated circuits having smaller features have been designed, and concomitant with this trend, exposure apparatuses for transferring circuit patterns on wafers are required to have the ability to transfer even finer circuit patterns. The abilities of exposure apparatuses largely depend on the wavelength of the exposure light, and hence, exposure apparatuses for exposing finer circuit patterns tend to use exposure light having shorter wavelengths. As exposure apparatuses having the ability to transfer a finer circuit pattern as described above, soft x-ray reduction projection exposure apparatuses have been investigated. The soft x-ray reduction projection exposure apparatus is an apparatus in which x-rays are generated from a light source, a reflection-type mask having a mask pattern formed therein is irradiated by the x-rays, and reductive projection is then performed onto a resist coated on a wafer by the x-rays reflected at the reflection-type mask.
As a reflection-type mask for use in soft x-ray reduction projection exposure apparatuses, a multilayer reflection film reflecting x-rays is typically provided on a substrate having a predetermined shape, which is formed by alternately depositing two types of materials having different refractive indexes. The combination of materials constituting a multilayer reflection film is changed in accordance with the wavelength of x-rays to be reflected. For example, for x-rays having a wavelength of approximately 13 nm, molybdenum (Mo) and silicon (Si) are used, and for x-rays having a wavelength of approximately 5 nm, chromium (Cr) and carbon (C) are typically used. In general, as the topmost surface of the multilayer reflection film, a material is selected having a refractive index largely differing from that in a vacuum or a processing atmosphere, and among the materials mentioned above, a Mo layer or a Cr layer may be used. Since a multilayer reflection film composed of Mo and Si most stably gives a high reflectance at present, the practical use thereof is hopefully expected.
The thicknesses of individual layers constituting the multilayer reflection film described above are determined by the wavelength of the incident x-rays, the incident angle thereof, constituent materials for the multilayer reflection film, and the like. When incident x-rays are perpendicular to the surface of the multilayer reflection film, the thickness of a pair of layers adjacent to each other is approximately one-half of the wavelength of the incident x-rays. In addition, in general, comparing the two layers mentioned above, a layer having a smaller absorption coefficient of x-rays is slightly thicker than the other layer having a larger absorption coefficient of x-rays. Accordingly, when x-rays having a wavelength of thirteen nm are used, the thickness of the Mo layer of the multilayer reflection film is slightly smaller than three nm, that is, the layer is extremely thin.
There are two major methods for manufacturing mask patterns. One method is that non-reflection areas, which do not reflect x-rays, are formed by covering a multilayer reflection film with a patterned absorption layer so as to form a mask pattern. The other method is that non-reflection areas are formed by removing or destroying parts of a multilayer reflection film so as to directly form a mask pattern in the multilayer reflection film.
FIG. 1
is a cross-sectional view of a conventional reflection-type mask for use in a soft x-ray reduction projection exposure apparatus. As shown in
FIG. 1
, a multilayer reflection film
9
is formed on a substrate
3
having a predetermined shape by alternately depositing Mo layers
1
and Si layers
5
, which reflect x-rays. On the upper surface of the multilayer reflection film
9
, an absorption layer
4
is formed for absorbing x-rays and is patterned so as to cover parts of the multilayer reflection film
9
.
The absorption layer
4
is formed of a material having a large absorption coefficient of x-rays to be used. In general, a heavy element, such as gold (Au), tungsten (W), or tantalum (Ta), is used. The absorption coefficients of these materials mentioned above differ in accordance with the wavelength of the incident x-rays, and hence, a material constituting the absorption layer
4
is preferably selected based on the wavelength of x-rays to be used.
As a method for forming a mask pattern in the absorption layer
4
, an electroplating method may be used. In this method, a resist layer (not shown) is first formed on the multilayer reflection film
9
, and the resist layer is then patterned by using an electron beam lithographic method. Subsequently, the absorption layer
4
is formed by using an electroplating method, and a mask pattern is then formed in the absorption layer
4
by removing the resist layer by dry etching. In addition to the method using an electroplating method, a sputtering deposition method, an evaporation method, or the like method may be used, so as to form the absorption layer
4
on the multilayer reflection film
9
.
An electroplating method has advantages in that the absorption layer
4
can be easily formed with less damage imposed on the multilayer reflection film
9
. In “Surface Technology”, Vol. 49, No. 8, (1998), pp. 47 to 51, it is reported that a superior absorption layer
4
composed of nickel (Ni) is obtained by an electroplating method.
When the absorption layer
54
is formed by an electroplating method, the topmost surface layer of the multilayer reflection film
9
must be composed of a conductive material. According to “Surface Technology”, Vol. 49, No. 8, (1998), pp. 47 to 51, described above, when the topmost surface layer of the multilayer reflection film
9
is formed of a Mo layer
1
, an absorption layer
4
composed of Ni is preferably formed, and when the topmost surface layer of the multilayer reflection film
9
is formed of a Si layer
5
, Ni in a spherical shape is only formed, whereby no absorption layer
4
is practically obtained.
The reflectance of the absorption layer
4
depends on the thickness thereof. For example, when the thickness of the absorption layer
4
, which consists of Au on the multilayer reflection film having a reflectance of 70%, is 30 nm, the x-ray reflectance thereof is 6%, and when the thickness of the absorption layer
4
is decreased by 10%, that is, when the thickness is decreased to 27 nm, the reflectance thereof is 7.7%. Accordingly, when the thickness of the absorption layer
4
is not uniform, the reflectance thereof varies within the absorption layer
4
. The variation in reflectance in the absorption layer
4
results in errors in line widths of a circuit pattern on a wafer. Hence, the thickness of the absorption layer
4
must be uniform. The variation in reflectance of the absorption layer
4
can be controlled by sufficiently increasing the thickness of the absorption layer
4
. However, incident x-rays to a reflection-type mask are not ideally perpendicular to the surface of the reflection-type mask and are inclined at an angle of some degrees thereto. In the case described above, when the thickness of the absorption layer is large, errors are generated in the circuit pattern on the wafer due to shadows cast by the thickness of the absorption layer
4
. Accordingly, the thickness of the absorption layer
4
is preferably reduced to a minimum level as long as the absorption layer
4
serves as expected.
In addition, when the multilayer reflection film
9
is directly patterned, patterning may be perfo
Chiba Keiko
Tsukamoto Masami
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Rosasco S.
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