Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-02-14
2004-06-15
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06749973
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention broadly relates to an exposure technique used in a semiconductor process and, in particular, to a reflection type mask blank for EUV (Extreme Ultra Violet) exposure, a reflection type mask for EUV exposure, and a method of producing the same as well as a method of producing a semiconductor device using the same.
It is noted here that EUV light, which is described in the present specification, is a radiation having a wavelength within a soft X-ray region or a vacuum ultraviolet region, specifically, a wavelength within a range between about 0.2 and 100 nm.
In the semiconductor industry, an integrated circuit comprising fine patterns is formed on an Si substrate by the use of a pattern transfer technique. As the pattern transfer technique, use has typically been made of photolithography utilizing visible light or ultraviolet light. Following accelerated development of a semiconductor device having finer patterns, a shorter wavelength is required as an exposure wavelength to achieve higher resolution. On the other hand, limitation is imposed upon achievement of such shorter wavelength in existing optical exposure using the above-mentioned photolithography so that the limit of resolution is approaching.
In case of the photolithography, it is known that a pattern resolution limit is generally a half of the exposure wavelength. Even if an F
2
laser beam having a wavelength of 157 nm is employed, it is predicted that the resolution limit is on the order of 70 nm. As an exposure technique capable of achieving the resolution beyond 70 nm, an EUV lithography (will be abbreviated to EUVL hereinafter) using EUV light is promising because the EUV light has a wavelength of 13 nm, much shorter than that of the F
2
laser beam. The EUVL is similar to the photolithography in respect of the principle of image formation. For the EUV light, however, all substances exhibit high absorption and a refractive index is substantially equal to 1. Consequently, in the EUVL, a refraction optical system used in the photolithography can not be employed and, instead, a reflection optical system is exclusively used.
As a mask used in the EUVL, a transmission type mask with a membrane has been suggested recently. However, the transmission type mask is disadvantageous in that sufficient throughput can not be assured because the membrane exhibits high absorption for the EUV light and consequently an exposure time becomes long. Under the circumstances, a reflection type mask for exposure is generally used at present.
Referring to
FIG. 1
, description will briefly be made of first through third existing techniques for obtaining the above-mentioned reflection type mask for EUV exposure. Then, description will proceed to the necessity of an etching stopper used in a production process of the reflection type mask.
(First Existing Technique)
The production process for obtaining the reflection type mask for EUV exposure includes (1) a substrate preparation step of preparing a substrate, (2) a deposition step of depositing a multilayer film onto the substrate, (3) a deposition step of depositing an intermediate layer, (4) a deposition step of depositing an absorber layer, (5) an EB (Electron Beam) resist application step of applying an EB resist, (6) an EB resist writing step, (7) a dry-etching step, and (8) a removing step of removing the intermediate layer. Each of the above-mentioned steps will be explained hereinafter.
(1) Substrate Preparation Step:
Preferably, the substrate
11
has a low coefficient of thermal expansion and is superior in smoothness, flatness, and resistance to a cleaning method used for cleaning the EUV mask. As the substrate
11
, a glass having a low coefficient of thermal expansion is generally used.
(2) Multilayer Film Deposition Step:
The multilayer film
12
contains Mo and Si in many cases.
For example, a single-period thickness is assumed to be 28 Å and 42 Å for Mo and Si, respectively. Then, by forming a laminate structure of at least 30 periods, it is possible to realize the multilayer film which reflects the EUV light having a peak wavelength of 13.4 nm. In case of the multilayer film containing Mo and Si, a Si film is deposited as a topmost layer.
(3) Intermediate Layer Deposition Step:
On the multilayer film
12
for reflecting the EUV light, an SiO
2
film is deposited as the etching stopper constituting the intermediate layer. For example, the deposition may be carried out by RF magnetron sputtering using an SiO
2
target.
(4) Absorber Layer Deposition Step:
The absorber layer
14
for absorbing the EUV light is deposited by sputtering. As a deposition material, Ta or Cr may be used. For example, the deposition may be carried out by DC magnetron sputtering. By this step, an EUV mask blank is obtained.
(5) EB Resist Application Step:
By forming a resist pattern on the absorber layer
14
of the EUV mask blank thus obtained, the EUV mask can be produced. The EB resist is applied on the EUV mask blank obtained in the step (4), and is baked at 200° C.
(6) EB Resist Writing Step:
On the EUV mask blank with the EB resist applied thereon, the resist pattern is formed by the use of an EB writing machine.
(7) Dry-Etching Step:
With the above-mentioned resist pattern used as a mask, the EUV absorber layer
14
is dry-etched with chlorine to thereby form a pattern on the absorber layer.
(8) Intermediate Layer Removing Step:
The intermediate layer remaining on an EUV reflection surface, namely, the etching stopper
23
comprising the SiO
2
film is removed with a dilute HF solution. Thus, the reflection type mask for EUV exposure is completed.
(Necessity of Etching Stopper and Problems Thereof)
The multilayer film
12
reflecting the EUV light must have high reflectivity after completion of the production of the mask. Therefore, it is required to prevent the multilayer film
12
reflecting the EUV light from being damaged during the production process. In particular, during the patterning step, the patterning must be performed without a damage, such as reduction in film thickness and roughening of the surface, given to the multilayer film
12
.
In the patterning of the absorber layer
14
absorbing the EUV light, high dimensional accuracy can be obtained by dry-etching. However, it is impossible to perform the etching without damaging an underlayer of the absorber layer
14
absorbing the EUV light. In view of the above, it is necessary to deposit the etching stopper
23
as the intermediate layer between the multilayer film
12
and the EUV absorber layer
14
.
As the etching stopper
23
, use is generally made of an SiO
2
film having a film thickness not smaller than several hundreds of angstroms. This film sufficiently serves as the stopper in the dry-etching with a Cl
2
gas. However, If the SiO
2
film remaining in a patternless area is not perfectly removed upon completion of the patterning step, the reflectivity of the multilayer film
12
reflecting the EUV light will be considerably lowered.
It is therefore necessary to perfectly remove the SiO
2
film. However, if the dry-etching is carried out to remove the SiO
2
film, the Si film as the uppermost layer of the multilayer film
12
reflecting the EUV light is inevitably etched. This also results in low reflectivity. For this reason, the SiO
2
film must be removed by wet-etching with HF solution or the like. The wet-etching with the HF solution or the like is effective because no damage is given to the Si film as the underlayer of the SiO
2
film. On the other hand, the wet etching with the HF solution has isotropic etchability so that the pattern is laterally eroded and may possibly be peeled off.
In addition, the SiO
2
film having a film thickness not smaller than several hundreds angstroms has high surface roughness as well as high compression stress. Furthermore, abnormal discharge readily occurs during deposition of the SiO
2
film by sputtering. It is therefore difficult to achieve a low fault rate required for the EUV mask.
(Second Existing Technique)
Japanese Unexam
Hosoya Morio
Shoki Tsutomu
Hoya Corporation
Rosasco S.
Sughrue & Mion, PLLC
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