U.S. patent application number 11/836304 was filed with the patent office on 2008-02-07 for mask blanks.
This patent application is currently assigned to ASAHI GLASS CO., LTD.. Invention is credited to Yasuhiko Akao, Shinya KIKUGAWA, Yosuke Sato, Akira Takada, Satoru Takaki.
Application Number | 20080032213 11/836304 |
Document ID | / |
Family ID | 36177936 |
Filed Date | 2008-02-07 |
United States Patent
Application |
20080032213 |
Kind Code |
A1 |
KIKUGAWA; Shinya ; et
al. |
February 7, 2008 |
MASK BLANKS
Abstract
The present invention provides a mask blank which comprises a
substrate made of a synthetic quartz glass and a light-shielding
film laminated on a surface of the substrate and is for use in a
semiconductor device production technique employing an exposure
light wavelength of 200 nm or shorter, wherein the mask blank has a
birefringence, as measured at a wavelength of 193 nm, of 1 nm or
less per substrate thickness. According to the present invention,
mask blanks suitable for use in the immersion exposure technique
and the polarized illumination technique are provided.
Inventors: |
KIKUGAWA; Shinya; (Tokyo,
JP) ; Takada; Akira; (Tokyo, JP) ; Takaki;
Satoru; (Tokyo, JP) ; Sato; Yosuke; (Tokyo,
JP) ; Akao; Yasuhiko; (Yamagata, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
ASAHI GLASS CO., LTD.
Tokyo
JP
|
Family ID: |
36177936 |
Appl. No.: |
11/836304 |
Filed: |
August 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2006/302575 |
Feb 8, 2006 |
|
|
|
11836304 |
Aug 9, 2007 |
|
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Current U.S.
Class: |
430/5 |
Current CPC
Class: |
C03C 2201/12 20130101;
C03C 2217/26 20130101; G03F 1/60 20130101; C03C 2201/23 20130101;
C03C 3/06 20130101; C03C 2218/154 20130101; C03C 17/09
20130101 |
Class at
Publication: |
430/005 |
International
Class: |
G03F 1/00 20060101
G03F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 2005 |
JP |
2005-033098 |
Claims
1. A mask blank which comprises a substrate made of a synthetic
quartz glass and a light-shielding film laminated on a surface of
the substrate and is for use in a semiconductor device production
technique employing an exposure light wavelength of 200 nm or
shorter, wherein the mask blank has a birefringence, as measured at
a wavelength of 193 nm, of 1 nm or less per substrate
thickness.
2. A mask blank which comprises a substrate made of a synthetic
quartz glass and a light-shielding film laminated on a surface of
the substrate and is for use in a semiconductor device production
technique employing an exposure light wavelength of 200 nm or
shorter, wherein when a light-transmitting area of 260
nm.times.1,040 nm is formed in the light-shielding film, then the
birefringence at this light-transmitting area, as measured at a
wavelength of 193 nm, is 1 nm or less per substrate thickness.
3. A mask blank which comprises a substrate made of a synthetic
quartz glass and a light-shielding film laminated on a surface of
the substrate and is for use in a semiconductor device production
technique employing an exposure light wavelength of 200 nm or
shorter, wherein the substrate has a birefringence, as measured at
a wavelength of 193 nm, of 0.5 nm or less per substrate thickness,
and wherein the light-shielding film has a film stress of 800 MPa
or lower.
4. A mask blank which comprises a substrate made of a synthetic
quartz glass and a light-shielding film laminated on a surface of
the substrate and is for use in a semiconductor device production
technique employing an exposure light wavelength of 200 nm or
shorter, wherein the substrate has a birefringence, as measured at
a wavelength of 193 nm, of 0.5 nm or less per substrate thickness,
and wherein the mask blank has a warpage amount of 2 .mu.m or
smaller.
5. The mask blank of claim 1, wherein the synthetic quartz glass
has an OH group concentration of 100 ppm or lower.
6. The mask blank of claim 1, wherein the synthetic quartz glass
has a fluorine concentration of 100 to 10,000 ppm.
7. The mask blank of claim 1, wherein the synthetic quartz glass
has a concentration of oxygen-deficient defects of
5.times.10.sup.14 defects per cm.sup.3 or lower.
8. The mask blank of claim 1, wherein the synthetic quartz glass
has an OH group concentration of 100 ppm or lower and a fluorine
concentration of 100 to 10,000 ppm.
Description
TECHNICAL FIELD
[0001] The present invention relates to mask blanks for use in a
semiconductor device production technique employing an exposure
light wavelength of 200 nm or shorter. More particularly, the
invention relates to mask blanks for use with a lithographic
exposure tool employing an ArF excimer laser (wavelength, 193 nm),
F.sub.2 laser (wavelength, 157 nm), or the like as a light
source.
BACKGROUND ART
[0002] In the production of semiconductor integrated circuits,
lithographic exposure tools for reductively projecting and
transferring a fine circuit pattern drawn in a photomask onto a
wafer are extensively used. With the trend toward higher degrees of
integration and higher functions in circuits, the circuits are
becoming finer and the lithographic exposure tools have come to be
required to form a high-resolution circuit pattern image on a wafer
surface while attaining a large focal depth. The wavelengths of
exposure light sources are becoming shorter. KrF excimer lasers
(wavelength, 248 nm) and ArF excimer lasers (wavelength, 193 nm)
are being used as exposure light sources in place of the g-line
(wavelength, 436 nm) and i-line (wavelength, 365 nm) heretofore in
use.
[0003] Photomask substrates mainly used for lithographic exposure
tools employing such exposure light sources are ones made of a
synthetic quartz glass, because synthetic quartz glasses have
advantages, for example, that they have excellent transparency to
light in a wide range of from the near infrared region to the
ultraviolet region, have an extremely low coefficient of thermal
expansion, and can be processed relatively easily. Photomask
substrates for, e.g., ArF excimer lasers are required to have a
surface flatness of about 0.5 .mu.m, a parallelism of about 5
.mu.m, and a birefringence of about 4 to 10 nm/cm besides
resistance to ArF excimer laser light.
[0004] Recently, the technique of immersion exposure is known in
which exposure with a lithographic exposure tool is conducted while
filling the space between the projection lens of the lithographic
exposure tool and the wafer with a liquid in order to attain a
higher resolution with an ArF excimer laser. The shorter the
exposure light wavelength and the larger the NA (numerical
aperture) of the projection lens, the higher the resolution for the
lithographic exposure tool becomes. The resolution can be
represented by the following expressions. Resolution=[k(process
coefficient).times..lamda.(exposure light wavelength)]/NA
NA=n.times.sin .theta. In the expressions, n indicates the
refractive index of the medium through which the exposure light
passes. In exposure techniques heretofore in use, n is 1.0 because
the medium is the air. In this immersion exposure, however, pure
water, which has an n of 1.44, is used as the medium and the
lithographic exposure tool can hence attain an even higher
resolution.
[0005] Furthermore, the polarized illumination technique is known
in which polarized lights which exert an adverse influence on
resolution are diminished to thereby heighten image-forming
contrast and improve resolution, in contrast to the exposure
techniques heretofore in use which employ an exposure light
composed of random polarized lights having various polarization
directions.
[0006] The photomasks for use in such immersion exposure technique
and/or polarized illumination technique are required to have low
birefringence so as not to disorder the polarization of the
exposure light which passes therethrough. A photomask substrate
having a birefringence reduced to 2 nm/cm or less has hence been
proposed (see, for example, patent document 1).
[0007] Patent Document 1: JP-T-2003-515192
[0008] In patent document 1, the birefringence of the photomask
substrate is specified. This birefringence of the photomask
substrate is mainly attributable to a residual strain in the
synthetic quartz glass used as the photomask substrate. However, in
the case of a mask blank comprising a photomask substrate and a
light-shielding film laminated thereon, the birefringence thereof
is attributable also to the stress imposed by the light-shielding
film laminated on a surface of the photomask substrate. It is
therefor necessary that this film stress should be taken into
account in regulating the birefringence of a mask blank comprising
a photomask substrate and a light-shielding film laminated on a
surface of the substrate.
DISCLOSURE OF THE INVENTION
[0009] The invention has been achieved in view of the problems
described above.
[0010] An object of the invention is to provide mask blanks
suitable for use in the immersion exposure technique and/or the
polarized illumination technique.
[0011] The invention provides a mask blank which comprises a
substrate made of a synthetic quartz glass and a light-shielding
film laminated on a surface of the substrate and is for use in a
semiconductor device production technique employing an exposure
light wavelength of 200 nm or shorter, wherein the mask blank has a
birefringence, as measured at a wavelength of 193 nm, of 1 nm or
less per substrate thickness.
[0012] The invention further provides a mask blank which comprises
a substrate made of a synthetic quartz glass and a light-shielding
film laminated on a surface of the substrate and is for use in a
semiconductor device production technique employing an exposure
light wavelength of 200 nm or shorter, wherein when a
light-transmitting area of 260 nm.times.1,040 nm is formed in the
light-shielding film, then the birefringence at this
light-transmitting area, as measured at a wavelength of 193 nm, is
1 nm or less per substrate thickness.
[0013] The invention still further provides a mask blank which
comprises a substrate made of a synthetic quartz glass and a
light-shielding film laminated on a surface of the substrate and is
for use in a semiconductor device production technique employing an
exposure light wavelength of 200 nm or shorter, wherein the
substrate has a birefringence, as measured at a wavelength of 193
nm, of 0.5 nm or less per substrate thickness, and wherein the
light-shielding film has a film stress of 800 MPa or lower.
[0014] The invention furthermore provides a mask blank which
comprises a substrate made of a synthetic quartz glass and a
light-shielding film laminated on a surface of the substrate and is
for use in a semiconductor device production technique employing an
exposure light wavelength of 200 nm or shorter, wherein the
substrate has a birefringence, as measured at a wavelength of 193
nm, of 0.5 nm or less per substrate thickness, and wherein the mask
blank has a warpage amount of 2 .mu.m or smaller.
[0015] The mask blanks of the invention have a low birefringence
and are suitable for use in the immersion exposure technique and/or
the polarized illumination technique.
BEST MODE FOR CARRYING OUT THE INVENTION
[0016] The mask blanks of the invention are constituted of a
photomask substrate made of a synthetic quartz glass and a
light-shielding film laminated on a surface thereof. The synthetic
quartz glass constituting the photomask substrate can be produced,
for example, by the following methods.
[0017] First, a silicon-containing compound as a raw material and
oxygen gas, hydrogen gas, nitrogen gas, etc. are supplied to a
burner made of quartz glass. The raw material is caused to undergo
a hydrolysis reaction or oxidation reaction in an oxyhydrogen flame
to thereby synthesize a quartz glass. Examples of this synthesis
method include the direct process and the soot process (e.g., the
VAD process, OVD process, or indirect process).
[0018] The direct process is a process in which a
silicon-containing compound is subjected to flame hydrolysis at a
temperature of 1,500 to 2,000.degree. C. to synthesize SiO.sub.2
particles and the particles are deposited on and fused to a target
to thereby directly synthesize a transparent synthetic quartz glass
body.
[0019] On the other hand, the soot process is a process which
comprises subjecting a silicon-containing compound to flame
hydrolysis at a temperature of 1,000 to 1,500.degree. C. to
synthesize SiO.sub.2 particles, depositing the particles on a
target to thereby first obtain a porous synthetic quartz glass
body, and then heating this porous synthetic quartz glass body to a
temperature of 1,400 to 1,500.degree. C. to thereby densify it and
obtain a transparent synthetic quartz glass body.
[0020] The VAD process is preferred because the reaction
temperature during synthesis is relatively low and it is possible
to relatively freely regulate the composition and the concentration
of defects. In particular, the low reaction temperature during
synthesis has an advantage that the synthetic quartz glass
synthesized from a chlorine-containing raw material such as
SiCl.sub.4 by the VAD process has a lower chlorine concentration
than that by the direct process. In this point also, the VAD
process is preferred.
[0021] The raw material for the synthetic quartz glass is not
particularly limited as long as it can be gasified. Examples
thereof include silicon halide compounds such as chlorides, e.g.,
SiCl.sub.4, SiHCl.sub.3, SiH.sub.2Cl.sub.2, and SiCH.sub.3Cl.sub.3,
and fluorides, e.g., SiF.sub.4, SiHF.sub.3, and SiH.sub.2F.sub.2,
and halogen-free silicon compounds such as alkoxysilanes
represented by R.sub.nSi (OR).sub.4-n (wherein R is an alkyl group
having 1 to 4 carbon atoms and n is an integer of 0 to 3) and
(CH.sub.3).sub.3Si--O--Si (CH.sub.3).sub.3.
[0022] Use of a chloride as a raw material gives a synthetic quartz
glass which contains residual chlorine derived from the raw
material. It is therefore preferred to use as a raw material an
organosilicon compound or fluoride which contains no chlorine. It
should, however, be noted that use of a fluoride as a raw material
frequently poses a problem concerning safety and handling because
hydrofluoric acid (HF) is generated as a by-product during the
synthesis. Consequently, the raw material preferably is an
organosilicon compound containing no halogen.
[0023] In the case where the VAD process is used for synthesizing a
synthetic quartz glass, the concentrations of oxygen-excess
defects, dissolved oxygen molecules, and oxygen-deficient defects
in the synthetic quartz glass can be regulated by several methods.
Examples of the methods include (1) a method in which the
proportions of oxygen gas and hydrogen gas in the feed gases are
regulated, (2) a method in which a porous synthetic quartz glass
body is treated with a reducing substance such as a compound
containing fluorine or chlorine, and (3) a method in which the
conditions under which a porous synthetic quartz glass body is
densified to obtain a transparent synthetic quartz glass body are
regulated.
[0024] Method (1) is a technique in which the proportion of
hydrogen gas to oxygen gas in the feed gases is regulated to a
value higher than 2 which is the stoichiometric ratio, i.e., set at
a value in the range of 2.0 to 2.5, to synthesize a porous
synthetic quartz glass body.
[0025] Method (2) is a technique in which a porous synthetic quartz
glass body is heat-treated at a temperature of from room
temperature to 1,200.degree. C. in an atmosphere containing a
reducing substance such as a fluorine-containing compound, hydrogen
gas, or CO gas. Examples of the fluorine-containing compound
include CF.sub.4, SiF.sub.4, and SF.sub.6. In the case where a
compound containing fluorine or chlorine or CO gas is used, it is
desired to use a mixed gas prepared by diluting such gas with an
inert gas (e.g., nitrogen, helium, or argon) to a concentration in
the range of 0.01 to 10 vol %, preferably 0.05 to 5 vol %, because
those gases have extremely high reducing properties. The treatment
in this case is preferably conducted at a temperature of from room
temperature to about 1,000.degree. C. at a pressure of the
atmosphere of 1 to 101 kPa. In the case where hydrogen gas is used,
it is preferred to conduct the heat treatment in an inert gas
containing 50 to 100 vol % hydrogen gas under the conditions of 1
to 10 atm and 800 to 1,200.degree. C. In the treatment with any of
those reducing gases, the porous synthetic quartz glass body is
first held in a reduced-pressure atmosphere and the gas is
introduced until the reduced pressure is elevated to a given
pressure, whereby the porous quartz glass body can be evenly
treated efficiently.
[0026] Method (3) is a technique in which the porous synthetic
quartz glass body synthesized is held at a temperature of 1,100 to
1,300.degree. C., preferably 1,200 to 1,300.degree. C., for 20 to
200 hours in a reduced-pressure atmosphere consisting of a 100 vol
% inert gas, such as helium or nitrogen, and having a pressure of
from 10 Pa to 10 kPa using a graphite furnace employing high-purity
carbon in the heater and as the heat insulator and being capable of
atmosphere control or using a metal furnace employing tungsten or
molybdenum as a reflector and in the heater, and then densified by
heating to 1,400 to 1,500.degree. C. in that atmosphere to thereby
obtain a transparent synthetic quartz glass body.
[0027] The concentrations of oxygen-excess defects, dissolved
oxygen molecules, and oxygen-deficient defects in the synthetic
quartz glass can be regulated by conducting any one of methods (1)
to (3) described above or by conducting two or more thereof in
combination.
[0028] Distorted bonding structures in the synthetic quartz glass
are precursors for defects such as the E' centers and NBOHC which
are generated upon ultraviolet irradiation. The concentration
thereof preferably is lower. Specifically, in a laser Raman
spectrum, the ratios of the intensity of the scattering peak at 495
cm.sup.-1 (I.sub.495) and that of the scattering peak at 606
cm.sup.-1 (I.sub.606) to that of the scattering peak at 440
cm.sup.-1 (I.sub.440), i.e., I.sub.495/I.sub.440 and
I.sub.606/I.sub.440, are preferably 0.585 or lower and 0.136 or
lower, respectively.
[0029] It is also preferred to reduce the sodium concentration in
the synthetic quartz glass. To reduce the sodium concentration in
the synthetic quartz glass to 5 ppb or lower is effective. The
sodium concentration is especially preferably 3 ppb or lower. To
regulate the sodium concentration so that the difference between
the maximum and minimum values thereof is 3 ppb or smaller is
effective in reducing fluctuations in birefringence in the range of
exposure light wavelengths. The term "maximum value" or "minimum
value" of sodium concentration herein means the largest value or
smallest value among the found sodium concentration values
determined for respective points.
[0030] By regulating the synthetic quartz glass so as to have a
chlorine concentration of 10 ppm or lower, preferably to contain
substantially no chlorine, the change in refractive index and
decrease in transmittance which occur upon ultraviolet irradiation
can be reduced to a sufficiently low level. The chlorine
concentration in the synthetic quartz glass can be determined by
fluorescent X-ray spectroscopy. The detection limit for this
analysis is 10 ppm. When the chlorine concentration in the
synthetic quartz glass exceeds the upper limit in that range, there
is a possibility that a larger decrease in transmittance and a
larger change in refractive index might be caused by ultraviolet
irradiation.
[0031] Furthermore, by regulating the synthetic quartz glass so as
to have an OH group concentration of 100 ppm or lower, preferably
50 ppm or lower, the change in refractive index and decrease in
transmittance which occur upon ultraviolet irradiation can be
reduced to a sufficiently low level. The OH group concentration can
be determined with an infrared spectrophotometer by the method
according to a document (Cer. Bull., 55(5), 524 (1976)). The
detection limit for this analysis is 1 ppm. When the OH group
concentration in the synthetic quartz glass exceeds the upper limit
in that range, there is a possibility that a larger decrease in
transmittance and a larger change in refractive index might be
caused by ultraviolet irradiation.
[0032] Metal impurities present in the synthetic quartz glass, such
as alkali metals (e.g., Na, K, and Li), alkaline earth metals
(e.g., Mg and Ca), and transition metals (e.g., Fe, Ni, Cr, Cu, Mo,
W, Al, Ti, and Ce), not only reduce light transmittance in the
range of from the ultraviolet region to the vacuum ultraviolet
region, but also are causative of light resistance deterioration.
Because of this, the content of such metal impurities preferably is
as low as possible. Specifically, the total content of metal
impurities is preferably 100 ppb or lower, especially preferably 50
ppb or lower.
[0033] Hydrogen molecules can be incorporated in the synthetic
quartz glass in an amount in the range of from 5.times.10.sup.15 to
1.times.10.sup.19 molecules per cm.sup.3. Hydrogen molecules in the
synthetic quartz glass serve to repair paramagnetic defects such as
the E' centers and nonbridging oxygen radicals which are generated
upon ultraviolet irradiation and thereby have the effect of
inhibiting the transmittance from decreasing upon ultraviolet
irradiation.
[0034] Fluorine can be incorporated in the synthetic quartz glass
in an amount of 100 to 10,000 ppm. Fluorine is effective in
diminishing unstable structures in the synthetic quartz glass and
improving ultraviolet resistance. However, when the fluorine
content in the glass is lower than 100 ppm, there are cases where
unstable structures in the synthetic quartz glass are not
diminishing to a sufficient level. When the glass contains fluorine
in an amount exceeding 10,000 ppm, there is a possibility that
reduced defects might be generated, resulting in reduced
ultraviolet resistance.
[0035] In order that the synthetic quartz glass having the
composition described above might have a reduced birefringence so
as to be used as an optical member, it is preferred to suitably
conduct heat treatments for imparting optical properties required
of the optical member, such as homogenization, molding, and
annealing (hereinafter referred to as optical heat treatments).
Such optical heat treatments are conducted after a dense and
transparent synthetic quartz glass has been obtained.
[0036] Among those optical heat treatments, annealing is closely
related to the birefringence of the synthetic quartz glass to be
obtained. In order to impart a low birefringence to the synthetic
quartz glass, the glass is held at a temperature of 1,250.degree.
C. or higher for 5 hours or longer and then gradually cooled to
1,050.degree. C. at a cooling rate of preferably 5.degree. C./hr or
lower, more preferably 3.degree. C./hr or lower. Although this
annealing may be conducted in air, it is effective to conduct the
treatment under vacuum. The degree of vacuum is preferably 10 Pa or
a lower pressure, more preferably 1 Pa or a lower pressure. That
is, the annealing treatment is preferably carried out in the
atmosphere having a pressure of 10 Pa or lower, more preferably 1
Pa or lower.
[0037] A photomask substrate is produced from the synthetic quartz
glass thus obtained. The photomask substrate preferably has such
durability that when it is irradiated using an Xe excimer lamp at
an irradiance of 13.2 mW/cm.sup.2 for 20 minutes, then the decrease
in light transmittance as measured at a wavelength of 217 nm, in
terms of the difference between the light transmittance before the
irradiation and that after the irradiation, is 1.0% at the
most.
[0038] The photomask substrate having such high durability is
preferably obtained in the following manner. Namely, as long as the
synthetic quartz glass contains substantially no oxygen-excess
defects and substantially no dissolved oxygen molecules, the
decrease in transmittance and change in refractive index which
occur upon ultraviolet irradiation can be sufficiently reduced.
That the synthetic quartz glass contains substantially no
oxygen-excess defects and substantially no dissolved oxygen
molecules means that the concentrations as determined respectively
by the detection methods described below are not higher than the
detection limits.
[0039] The concentration of dissolved oxygen molecules can be
determined by Raman spectrometry according to a literature (L.
Skuja et al., J. Appl. Phys., Vol. 83, No. 11, pp. 6106-6110
(1998)). The detection limit for this method is 1.times.10.sup.17
molecules per cm.sup.3. The concentration of oxygen-excess defects
can be evaluated based on the increase in OH group concentration
through a heat treatment at 700 to 1,000.degree. C. in an
atmosphere comprising hydrogen gas. For example, a test piece of
the synthetic quartz glass having dimensions of
10.times.10.times.100 mm is heat-treated at 800.degree. C. for 100
hours in a 1-atm atmosphere consisting of 100% hydrogen gas and the
increase in OH group concentration through this heat treatment is
determined with an infrared spectrophotometer according to the
method described in a document (Cer. Bull., 55(5), 524 (1976)). The
detection limit for this method is 1.times.10.sup.16 molecules per
cm.sup.3.
[0040] As long as the synthetic quartz glass contains substantially
no reduced defects, the decrease in transmittance and change in
refractive index which occur upon ultraviolet irradiation can be
reduced to a sufficiently low level. That the synthetic quartz
glass contains substantially no reduced defects means that no peak
attributable to SiH is observed at around 2,250 cm.sup.-1 in Raman
spectrometry.
[0041] With respect to the concentration of oxygen-deficient
defects in the synthetic quartz glass, the concentration thereof is
reduced to 5.times.10.sup.14 defects per cm.sup.3 or lower. Thus,
the decrease in transmittance which occurs upon ultraviolet
irradiation can be sufficiently inhibited.
[0042] The concentration of oxygen-deficient defects in the
synthetic quartz glass can be determined from the intensity of the
blue fluorescence which is emitted by ultraviolet irradiation and
has a peak at around the wavelength range of 280 to 300 nm. Namely,
a fiber lightguide type spectrophotometer equipped with a
multi-channel photodiode (e.g., spectrophotometer MCPD 2000,
manufactured by Otsuka Electronics Co., Ltd.) or the like is used
to measure the intensity of the scattered light derived from ArF
excimer laser light and the intensity of the blue fluorescence peak
centering at around a wavelength of 280 to 300 nm. When the
proportion of the intensity of the blue fluorescence peak to the
intensity of the scattered light having a wavelength of 193 nm is
5.times.10.sup.-3 or lower, the concentration of oxygen-deficient
defects in the synthetic quartz glass can be judged to be within
the range shown above. When that intensity ratio exceeds
5.times.10.sup.-3, this means that the concentration of
oxygen-deficient defects in the synthetic quartz glass exceeds
5.times.10.sup.14 defects per cm.sup.3 and there is hence a
possibility that a decrease in transmittance might occur upon
ultraviolet irradiation.
[0043] The relationship between that intensity ratio and the
concentration of oxygen-deficient defects was determined from the
absorption band attributable to oxygen-deficient defects and
centering at 163 nm. Namely, the concentration of oxygen-deficient
defects was determined from the intensity of the absorption at a
wavelength of 163 nm, and a synthetic-quartz-glass sample of which
the concentration of the oxygen-deficient defects had been known
was examined for the intensity of blue fluorescence. Thus, the
relationship between the intensity ratio I of the blue fluorescence
to the scattered light having a wavelength of 193 nm and the
concentration of oxygen-deficient defects C.sub.ODC
(defects/cm.sup.3) was obtained, which is represented by the
following equation. C.sub.ODC=1.16.times.10.sup.17.times.I
[0044] That surface of the photomask substrate on which a
light-shielding film is to be laminated (hereinafter, the surface
is often referred to as "pattern formation side") preferably has a
flatness of 0.25 .mu.m or a smaller value, and the opposite side
thereof preferably has a flatness of 1 .mu.m or a smaller value. In
addition, the parallelism of the two sides preferably is 5 .mu.m or
a smaller value. The photomask substrate satisfying these
requirements enables sufficient exposure precision to be secured
even when polarized illumination is used or immersion exposure is
conducted.
[0045] The photomask substrate having such properties can be
produced, for example, by the following method.
[0046] A synthetic-quartz-glass plate having outside dimensions
larger by at least 10 mm than the dimensions of the photomask
substrate is first polished and then cut into the given dimensions
to thereby produce the photomask substrate. Alternatively, a
synthetic-quartz-glass plate having a dummy processing part
attached to the periphery thereof is polished to thereby produce
the photomask substrate.
[0047] Namely, a synthetic-quartz-glass plate larger by at least 10
mm than the outside dimensions of the photomask substrate to be
used is polished and finished so as to have a given thickness, and
a peripheral part is then cut off. Thus, a photomask substrate
having satisfactory thickness fluctuations is obtained. In the
other method, a dummy processing part having the same thickness as
the photomask substrate to be produced is disposed in place of the
cutting allowance on the periphery of a synthetic-quartz-glass
plate, and this glass plate is set on a carrier like the photomask
substrate, whereby sagging at the periphery is diminished.
[0048] The dummy processing part preferably has a width of 10 mm or
larger. The dummy processing part preferably is one made of a
synthetic quartz glass because this dummy processing part can be
polished at the same rate as the synthetic-quartz-glass plate being
processed and the fine particles generated by the polishing can be
prevented from marring the photomask substrate during the
processing. However, the dummy processing part may be made of a
resin having the same properties.
[0049] The polishing apparatus to be used for polishing the raw
plate preferably has such a size that at least one such raw plate
can be placed within the radius of the carrier or has such a size
that the dummy processing part can be wholly held within the radius
of the carrier. This is intended to minimize the influence of a
difference in polishing rate between the central part and
peripheral part of the carrier.
[0050] An example of those processes is explained below. A quartz
glass ingot synthesized by a known method is cut into a given
thickness with an inner diameter saw slicer. Thereafter, the glass
plate obtained is beveled with a commercial NC beveling machine so
as to result in given outside dimensions and radiused edges.
[0051] Subsequently, this synthetic-quartz-glass plate is immersed
in a 5% by weight HF solution in order to prevent the cracks
generated by the cutting and the cracks generated by the beveling
from propagating. This synthetic-quartz-glass plate is then lapped
to a given thickness with a both-side-lapping machine using an
abrasive slurry.
[0052] The synthetic-quartz-glass plate thus lapped is subjected to
the same etching treatment as described above. Subsequently, this
synthetic-quartz-glass plate is polished with a slurry containing
cerium oxide as a main component and a polyurethane pad using a
both-side-polishing machine and then subjected to finish polishing
with a slurry containing silica sol as a main component and a
foamed polyurethane pad using the same type of machine. Thus, a
photomask substrate having a given thickness is obtained.
[0053] The pattern formation side preferably has a surface
roughness of 0.3 nm or less in terms of Rrms value. The term "rms"
stands for "Root-Mean-Square" and it represents a square root of
the mean of squared values of deviations with respect to the
average value. Referring to one-dimensional case, for example, the
surface roughness Rrms value is given by the following formula:
Rrms= (.SIGMA..sub.i(f(x.sub.i)-m).sup.2/n) wherein f(x) represents
a cross-sectional profile of the surface shape image in x-axis
direction, m represents an arithmetic mean of f(x), and n
represents the number of points of surface roughness measurement.
Thus, the rectilinear propagation of the light which has passed
through the pattern on the photomask substrate can be easily
secured even when polarized illumination is used or immersion
exposure is conducted. In addition, defects of the kinds described
above which have a size of 150 nm or larger can be easily detected
by irradiation with scattered light.
[0054] The photomask substrate produced in the manner described
above can have a birefringence, as measured at a wavelength of 193
nm, of 0.5 nm or less per thickness of the substrate. Although
birefringence generally is measured with an He--Ne laser having a
wavelength of 633 nm, this found value can be converted to the
birefringence at wavelength of 193 nm by multiplying that value by
1.32. Incidentally, the thickness of the photomask substrate
typically is about 6.35 mm.
[0055] The mask blanks of the invention are produced by superposing
a light-shielding film on a surface of the photomask substrate
obtained in the manner described above. As the light-shielding film
is generally used a thin metal film made of chromium. The thickness
thereof is typically 100 to 160 nm.
[0056] This thin metal film made of chromium can be formed by
sputtering in the following manner. The photomask substrate and a
target comprising chromium as the main component are set in a film
deposition chamber. The chamber is evacuated to a high vacuum to
sufficiently discharge residual gases from the apparatus.
Thereafter, while evacuating the chamber with a vacuum pump, a rare
gas such as argon is introduced to form a reduced-pressure
atmosphere whose pressure is kept at a given value by regulating
the gas flow rate or evacuation rate. In this reduced-pressure
atmosphere, a negative high voltage is applied to the cathode to
generate a glow discharge. The glow discharge yields rare-gas ions,
which are accelerated by the cathode voltage and impinge and
collide against the target. Chromium atoms are thus dislodged from
the target and are deposited on the substrate to thereby form a
thin film. Examples of the glow discharge include a direct-current
discharge generated by application of a direct-current voltage and
a high-frequency discharge generated by application of a
high-frequency voltage. Although either of these can be used, it is
preferred to use a direct-current discharge formed by application
of a direct-current voltage because the application of a high
voltage can be easily conducted stably and the glow discharge can
be concentrated around the target. It is also preferred to use a
direct-current pulse discharge for the purpose of inhibiting the
generation of an abnormal discharge or improving suitability for
the control of film deposition conditions.
[0057] There are various techniques of sputtering. However, the
magnetron sputtering method, in which a magnetic field is used to
heighten the plasma density around the target, is preferred because
a film having excellent uniformity in film thickness and
homogeneity can be formed with satisfactory productivity. The
explanation given below is on the magnetron sputtering method
unless otherwise indicated. Besides the magnetron sputtering
method, the ion beam sputtering method may, for example, be used in
which an ion beam generated by an ion gun is caused to impinge on
the target and target atoms thus dislodged from the target are
deposited on a substrate. In this case, the sputtering atmosphere
gas is less apt to come into the film being deposited. This method
further has an advantage that film thickness and film homogeneity
can be controlled highly satisfactorily.
[0058] Argon, which is inexpensive, is frequently employed as the
rare gas for use as the sputtering atmosphere. Although argon is
preferred, use may be made of helium, neon, krypton, xenon, or the
like.
[0059] It is known that a chromium film formed by sputtering is
generally in a tensile-stress state because of a film stress due to
the structural defects and holes generated during film deposition.
In case where the light-shielding film has a high film stress, this
warps the substrate, resulting in a birefringence. It is therefore
preferred to reduce the film stress in the light-shielding
film.
[0060] Film stress is known to vary depending on the conditions
under which the film is deposited by sputtering. Namely, by
suitably selecting film deposition conditions, a light-shielding
film can be formed while regulating the film stress so as to be in
a desired range. Methods for reducing the film stress of a
light-shielding film are explained below.
[0061] It is known that properties of the film to be deposited by
sputtering with a glow discharge vary depending on the pressure of
the atmosphere in which the sputtering is conducted (hereinafter
referred to as sputtering pressure). In the deposition of a
chromium film by sputtering, a sputtering pressure on the order of
10.sup.-1 Pa results in a tensile stress of 1 to 2 GPa. The stress
decreases with decreasing sputtering pressure, and an even lower
pressure results in a compressive stress. Consequently, the
sputtering pressure is preferably regulated to 1.0.times.10.sup.-2
to 1.0.times.10.sup.-1 Pa. On the other hand, excessively reduced
sputtering pressures may pose a process problem such as an unstable
glow discharge or a reduced film deposition rate. Consequently, the
sputtering pressure is preferably regulated to 2.0.times.10.sup.-2
to 8.0.times.10.sup.-2 Pa from the standpoints of attaining a more
reduced tensile stress and avoiding such process problems.
Furthermore, the residual gas in the apparatus is causative of
property fluctuations of the film deposited. It is therefore
preferred for property stabilization that the film deposition
chamber be evacuated to a vacuum of at least 1.times.10.sup.-3 Pa
prior to film deposition. More preferably, the chamber is evaluated
to a vacuum higher than 1.times.10.sup.-4 Pa.
[0062] The structural defects and holes which are generated during
film deposition can be controlled by regulating the composition of
the film, i.e., by adding one or more other ingredients. By adding
oxygen or nitrogen to the argon as the sputtering atmosphere gas,
the tensile stress can be greatly reduced. On the other hand,
addition of oxygen or nitrogen in too large an amount may result in
a film which has reduced light-shielding properties and is hence
unsuitable for use as a light-shielding film. Consequently, the
proportion of the flow rate of oxygen to the overall gas flow rate,
i.e., the total flow rate of argon and oxygen, is preferably
regulated to 30% or lower, and is 25% or lower from the standpoint
of obtaining sufficient light-shielding properties. In the case of
adding nitrogen, the proportion of the flow rate of nitrogen to the
overall gas flow rate, i.e., the total flow rate of argon and
nitrogen, is regulated to preferably 30% or lower, more preferably
20% or lower. In the case of simultaneously adding oxygen and
nitrogen, the total addition amount thereof is preferably 30% or
less, more preferably 20% or less, in terms of the proportion of
the total flow rate of oxygen and nitrogen to the overall gas flow
rate, i.e., the total flow rate of argon, oxygen, and nitrogen. The
lower limit of the amount of each of oxygen and nitrogen to be
added is preferably 5% from the standpoint of sufficiently
obtaining the effect of the addition.
[0063] A reduction in tensile stress can be attained also by using
an alloy target comprising chromium and one or more other elements
in place of the chromium target to deposit a chromium alloy film as
a light-shielding film. The alloying ingredients to be used
preferably are metals which, through sputtering, give a film having
a compressive stress. Namely, it is preferred to use a chromium
alloy target containing one or more metals selected from the group
consisting of Mo, Ta, Nb, W, Ti, and Zr as alloying ingredients.
The total amount of such alloying ingredients to be added is
preferably regulated so as to be in the range of 10 to 40 atomic %
in terms of the proportion of the ingredients to the number of all
atoms constituting the target excluding oxygen and nitrogen. In
case where the amount of the alloying ingredients is smaller than
10%, a sufficient stress-reducing effect is not obtained. On the
other hand, amounts thereof exceeding 40% result in a possibility
that chemical resistance required of photomask blanks might be
reduced. It is also preferred that carbon or boron be added as an
alloying ingredient to the chromium in place of or simultaneously
with the alloying ingredients described above. Carbon and boron can
be added as alloying ingredients to the target like the alloying
ingredients described above. However, in the case of carbon, in
particular, a gas containing carbon can be added to a sputtering
atmosphere gas to deposit a film containing carbon incorporated
therein. Preferred examples of the gas containing carbon include
CO.sub.2 and CH.sub.4. The amount of the carbon-containing gas to
be added is preferably 30% or smaller, more preferably 20% or
smaller, in terms of the proportion of the flow rate of the
carbon-containing gas to the overall gas flow rate, i.e., the total
flow rate of argon and the carbon-containing gas.
[0064] Also preferred is a technique in which in place of or in
combination with the formation of a chromium alloy film, the
chromium-based film having a tensile stress and a metal film having
a compressive stress are laminated on each other to cause the two
stresses to countervail each other. Examples of the metal
constituting the metal film having a compressive stress include Mo,
Ta, Nb, W, Ti, and Zr. One chromium film and one metal film made of
any of such metals may be laminated, or these two kinds of films
may be alternately laminated to form a film composed of many
layers.
[0065] It is preferred that the chromium film or chromium alloy
film formed or the multilayer film comprising a chromium film and a
film of another metal be subjected to a heat treatment to thereby
mitigate structural defects and holes in the film and reduce the
stress. This heat treatment may be accomplished by holding the film
in an atmosphere such as dry air, nitrogen, or argon at 200 to
350.degree. C. for 5 to 60 minutes.
[0066] The light-shielding film thus formed can have a film stress
reduced to 800 MPa or lower. As a result, the photomask substrate
having this light-shielding film laminated on a surface thereof can
have a warpage amount reduced to 2 .mu.m or smaller.
[0067] In the mask blank having the constitution described above,
when a light-transmitting area of 260 nm.times.1,040 nm, which
corresponds to the pattern of a gate electrode, is formed in the
light-shielding film, then the birefringence at this
light-transmitting area can be as low as 1 nm or less per substrate
thickness as measured at a wavelength of 193 nm. In addition, the
whole mask blank including the light-transmitting area can have a
birefringence as low as 1 nm or less per substrate thickness as
measured at a wavelength of 193 nm. Consequently, the photomask
produced from this mask blank is suitable for use in the immersion
exposure technique and the polarized illumination technique.
EXAMPLES
[0068] The present invention will be illustrated in greater detail
with reference to the following Examples, but the invention should
not be construed as being limited thereto.
Example 1
[0069] By the known soot method, SiCl.sub.4 is hydrolyzed in an
oxyhydrogen flame and the resultant fine SiO.sub.2 particles are
deposited on a target to produce a porous quartz glass body of a
cylindrical shape having a diameter of 35 cm and a height of 100
cm. This porous quartz glass body is placed in an electric furnace
capable of atmosphere control. While the atmosphere in the furnace
is kept at a reduced pressure of 10 Pa or lower, the glass body is
heated to 1,450.degree. C. and held at this temperature for 10
hours to produce a transparent synthetic quartz glass body.
[0070] This transparent synthetic quartz glass body is placed in a
high-temperature heating furnace equipped with a graphite heater.
The glass body is heated to 1,750.degree. C. to deform it by its
own weight and thereby mold it into a block product having
dimensions of 17.times.17.times.25 cm. After completion of the
molding, the block product is annealed by gradual cooling under
vacuum. In this annealing, the block product is held at
1,300.degree. C. for 16 hours and then gradually cooled from
1,300.degree. C. to 1,050.degree. C. at a cooling rate of 2.degree.
C./hr.
[0071] A platy product having a length of 153 mm, width of 153 mm,
and thickness of 6.4 mm is cut out of the block product obtained,
and is placed in an electric furnace capable of atmosphere control.
This platy product is subjected to a hydrogenation treatment by
holding it at 500.degree. C. in a 10 to 100% hydrogen atmosphere of
1 to 10 atm. Thereafter, the platy product is polished to produce a
photomask substrate.
[0072] This photomask substrate is examined for the concentration
of OH groups and the concentration of oxygen-deficient defects by
the methods described above. The concentration of OH groups is 78
ppm and the concentration of oxygen-deficient defects is
5.times.10.sup.14 defects per cm.sup.3 or lower. Furthermore, the
photomask substrate is examined for birefringence, fluorine
concentration, H.sub.2 concentration, and Xe excimer lamp
resistance by the methods which will be described later.
[0073] Subsequently, a light-shielding chromium film is formed by
the magnetron sputtering method on a surface of the photomask
substrate obtained. A chromium target having a diameter of 30 cm
and a thickness of 5 mm is attached to the magnetron cathode in the
vacuum chamber of a film deposition apparatus. The photomask
substrate 6 inches square and 6.35 mm thick is set on the substrate
stage in the chamber. The distance between the target and the
substrate is adjusted to 20 cm. The vacuum chamber is evacuated
roughly and then to a high vacuum of 10.sup.-4 Pa or a lower
pressure. Thereafter, 30 sccm argon gas is introduced with a gas
introduction system while evacuating the chamber with a turbo-pump,
and the evacuation conductance is regulated to adjust the pressure
in the chamber to 7.0.times.10.sup.-2 Pa. Subsequently, a
direct-current voltage of 2.5-kW constant power is applied to the
cathode from an external sputtering power source to generate a glow
discharge. Thus, a light-shielding chromium film having a thickness
of 100 nm is deposited. The film stress of the light-shielding film
and the warpage amount of the substrate are measured by the methods
which will be described later.
Example 2
[0074] On a surface of a photomask substrate produced in the same
manner as in Example 1 is formed a light-shielding film of
nitrogen-doped chromium with the same apparatus as in Example 1 in
the same manner. In this Example, 24.0 sccm argon gas and 6.0 sccm
nitrogen gas are introduced in place of the argon gas alone in
Example 1. After the chamber is evacuated to 2.times.10.sup.-4 Pa,
these gases are introduced into the chamber with the gas
introduction system and the evacuation conductance is regulated to
thereby adjust the pressure in the chamber to 7.0.times.10.sup.-2
Pa. Sputtering is conducted while applying a direct-current voltage
of 2.5-kW constant power to deposit a nitrogen-doped chromium film
having a thickness of 150 nm on the substrate. The film stress of
the light-shielding film and the warpage amount of the substrate
are measured by the methods which will be described later.
Evaluation
Evaluation Method 1: Birefringence of Photomask Substrate
[0075] Each of 121 points selected in the photomask substrate which
are distributed in a lattice pattern at an interval of 14.2 mm in a
central area of 142.times.142 mm in the substrate was examined for
birefringence with EXICOR, manufactured by HINDS Instruments, Inc.,
which employs an He--Ne laser as a light source. The maximum value
of birefringence is determined. The birefringence of the photomask
substrate of Example 1 is 0.38 nm per 6.35 mm, which is the
thickness of the substrate.
Evaluation Method 2: Fluorine Concentration
[0076] A sample having dimensions of 15 mm.times.15 mm.times.6.3 mm
is cut out of a central part of the photomask substrate and
examined for fluorine concentration by the following method.
[0077] According to the method described in Nippon Kagakukai-shi,
1972(2), 350, the sample is thermally melted with anhydrous sodium
carbonate, and distilled water and hydrochloric acid (1:1) are
added to the resultant melt to prepare a sample liquid. The
electromotive force of this sample liquid is measured with a
radiometer using a fluorine-ion-selective electrode and using each
of No. 945-220 and No. 945-468, both manufactured by Radiometer
Trading K.K., as a reference electrode. The concentration of
fluorine is determined based on a calibration curve obtained
beforehand using standard fluorine ion solutions. The detection
limit for this method is 10 ppm. The fluorine concentration in the
substrate of Example 1 is 389 ppm.
Evaluation Method 3: Hydrogen Molecule Concentration
[0078] The photomask substrate is analyzed by Raman spectrometry.
The concentration of hydrogen molecules (molecules/cm.sup.3) is
determined from the intensity ratio (=I.sub.4135/I.sub.800) between
the intensity of the scattering peak at 4,135 cm.sup.-1
(I.sub.4135) and the intensity of the scattering peak at 800
cm.sup.-1 attributable to the fundamental vibration of silicon and
oxygen (I.sub.800) in the laser Raman spectrum (V. S. Khotimchenko
et al., Zhurnal Prikladnoi Spektroskopii, Vol. 46, pp. 987-997,
1986). The detection limit for this method is 1.times.10.sup.16
molecules per cm.sup.3. The hydrogen molecule concentration in the
substrate of Example 1 is lower than 2.9.times.10.sup.18 molecules
per cm.sup.3.
Evaluation Method 4: Suitability for Lithography Light Source
[0079] The photomask substrate is irradiated using an Xe excimer
lamp of 13.2 mW/cm.sup.2 for 20 minutes. The decrease in light
transmittance as measured at a wavelength of 217 nm which has
resulted from the Xe excimer lamp irradiation is determined to
evaluate the suitability. The decrease in light transmittance of
the substrate of Example 1 is 0.092%. The chlorine concentration
therein is 10 ppm or lower.
Evaluation Method 5: Film Stress
[0080] The film stress of the thin chromium film can be determined
by X-ray diffractometry. Namely, the lattice constant d of the
crystals in the thin chromium film on the substrate is determined
by X-ray diffractometry. From the difference .DELTA.d between the
thus-determined lattice constant d of the thin chromium film and
the lattice constant d.sub.0 of the bulk material, the lattice
distortion in the thin-film thickness direction
(.epsilon.=.DELTA.d/d.sub.0) is determined. The film stress .sigma.
which is the in-plane stress of the thin film can be determined
from that lattice distortion .epsilon. and the Young's modulus E
and Poisson's ratio .nu. of the thin film, i.e., determined using
the relationship .sigma.=E.epsilon./2.nu.. The chromium films in
Examples 1 and 2 each have a tensile stress of 800 MPa or lower.
Besides being determined by the method based on X-ray
diffractometry, the stress of the light-shielding chromium film may
be determined by a method in which the warpage of the substrate is
examined with an optical interferometer before and after the film
formation and the stress is determined from the change thereof.
Evaluation Method 6: Warpage Amount
[0081] The photomask substrate on which a light-shielding film has
been laminated is evaluated for warpage amount. The substrates of
Examples 1 and 2 each have a warpage amount of 2 .mu.m or less.
Evaluation Method 7: Birefringence of Mask Blank
[0082] The birefringence of the photomask substrate on which a
light-shielding film has been laminated is determined by examining
a phase difference between a reference light and a reflected light
using an He--Ne laser. The photomask substrates of Examples 1 and 2
each have a birefringence of 0.8 nm or less per 6.35 mm, which is
the thickness of the substrate.
INDUSTRIAL APPLICABILITY
[0083] The mask blanks of the invention have a low birefringence
and are suitable for use in the immersion exposure technique and
the polarized illumination technique.
[0084] While the present invention has been described in detail and
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope
thereof.
[0085] This application is based on Japanese Patent Application No.
2005-33098 filed Feb. 9, 2005, the contents thereof being herein
incorporated by reference.
* * * * *