U.S. patent application number 10/809523 was filed with the patent office on 2004-09-30 for method of producing a glass substrate for a mask blank and method of producing a mask blank.
This patent application is currently assigned to HOYA CORPORATION. Invention is credited to Koike, Kesahiro.
Application Number | 20040192171 10/809523 |
Document ID | / |
Family ID | 32985291 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040192171 |
Kind Code |
A1 |
Koike, Kesahiro |
September 30, 2004 |
Method of producing a glass substrate for a mask blank and method
of producing a mask blank
Abstract
A method of producing a glass substrate for a mask blank has the
steps of measuring a convex/concave profile of a surface of the
glass substrate, controlling a flatness of the surface of the glass
substrate to a value not greater than a predetermined reference
value by specifying the degree of convexity of a convex portion
present on the surface of the glass substrate with reference to a
result of measurement obtained in the profile measuring step and
executing local machining upon the convex portion under a machining
condition depending upon the degree of convexity, and polishing,
after the flatness control step, the surface of the glass substrate
subjected to the local machining by the action of a machining
liquid interposed between the surface of the glass substrate and a
surface of a polishing tool without direct contact
therebetween.
Inventors: |
Koike, Kesahiro; (Tokyo,
JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
HOYA CORPORATION
|
Family ID: |
32985291 |
Appl. No.: |
10/809523 |
Filed: |
March 26, 2004 |
Current U.S.
Class: |
451/5 ; 430/311;
430/5; 451/41; 451/6 |
Current CPC
Class: |
B24B 49/00 20130101;
B24B 37/042 20130101; G03F 1/60 20130101 |
Class at
Publication: |
451/005 ;
451/006; 451/041 |
International
Class: |
B24B 049/00; B24B
051/00; B24B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2003 |
JP |
90682/2003 |
Claims
What is claimed is:
1. A method of producing a glass substrate for a mask blank, the
method comprising: a profile measuring step of measuring a
convex/concave profile of a surface of the glass substrate for a
mask blank; a flatness control step of controlling a flatness of
the surface of the glass substrate to a value not greater than a
predetermined reference value by specifying the degree of convexity
of a convex portion present on the surface of the glass substrate
with reference to a result of measurement obtained in the profile
measuring step and executing local machining upon the convex
portion under a machining condition depending upon the degree of
convexity; and a non-contact polishing step of polishing, after the
flatness control step, the surface of the glass substrate subjected
to the local machining by the action of a machining liquid
interposed between the surface of the glass substrate and a surface
of a polishing tool without direct contact therebetween.
2. A method according to claim 1, wherein the non-contact polishing
step is carried out by float polishing.
3. A method according to claim 1 or 2, wherein: the machining
liquid comprises: an aqueous solution selected from water, an
acidic aqueous solution, and an alkaline aqueous solution; or a
mixture of the aqueous solution and at least one kind of fine
powder particles selected from colloidal silica, cerium oxide,
zirconium oxide, and aluminum oxide.
4. A method according to any one of claims 1 through 3, wherein the
local machining is carried out by plasma etching or a gas cluster
ion beam.
5. A method according to any one of claims 1 through 4, wherein the
reference value is not greater than 0.25 .mu.m.
6. A method of producing a mask blank, the method comprising the
steps of preparing the glass substrate obtained by the method
according to any one of claims 1 to 5, and forming a thin film as a
transferred pattern on the glass substrate.
7. A method of producing a transfer mask, the method comprising the
steps of preparing the mask blank obtained by the method according
to claim 6 and patterning the thin film of the mask blank to form a
thin film pattern on the glass substrate.
8. A method of producing a semiconductor device, the method
comprising the steps of preparing the transfer mask obtained by the
method according to claim 7 and transferring the thin film pattern
of the transfer mask onto a semiconductor substrate by lithography.
Description
[0001] This invention claims priority to prior Japanese application
JP 2003-90682, the disclosure of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a method of producing a glass
substrate for a mask blank and a method of producing a mask blank
and, in particular, to a method of producing a glass substrate for
a mask blank for use with light in an ultrashort wavelength range,
such as F2 excimer laser (fluorine: having a wavelength of 157 nm)
and EUV light (extreme ultra violet: having a wavelength of 13 nm),
as an exposure light source and a method of producing a mask blank
of the type.
[0003] Following the improvement of a ULSI device having a higher
density and a higher accuracy in recent years, a glass substrate
for a mask blank is required to have a substrate surface of a finer
structure. Such tendency towards the finer structure of the
substrate surface becomes more and more accelerated year by year.
In particular, as an exposure light source of a shorter wavelength
is used, the demand for a profile accuracy (flatness) and a quality
(defect size) of the substrate surface becomes strict. Thus, the
glass substrate for a mask blank is required to have an extremely
high flatness and to be free from a microscopic defect.
[0004] For example, in case where F2 excimer laser is used as the
exposure light source, the glass substrate is required to have a
flatness of 0.25 .mu.m or less and a defect size of 0.07 .mu.m or
less. In case where EUV light is used as the exposure light source,
the glass substrate is required to have a flatness of 0.05 .mu.m or
less and a defect size of 0.05 .mu.m or less.
[0005] Upon production of a glass substrate for a mask blank,
proposal has already been made of a precision polishing technique
intended to reduce a surface roughness (for example, see Japanese
Patent Application Publication (JP-A) No. 64-40267 (Reference
1)).
[0006] The precision polishing technique described in Reference 1
comprises the steps of polishing the substrate surface by the use
of an abrasive primarily comprising cerium oxide and then
finish-polishing the substrate surface by the use of colloidal
silica. In case where the glass substrate is polished by the
above-mentioned technique, use is typically made of a double-sided
polishing apparatus of a batch type capable of receiving a
plurality of glass substrates and simultaneously polishing opposite
surfaces of the glass substrates.
[0007] In the precision polishing technique mentioned above, it is
theoretically possible to achieve a desired flatness by reducing an
average grain size of abrasive grains. Actually, however, under the
influence of a mechanical accuracy of various components of the
polishing apparatus, including a carrier for holding the glass
substrate, a surface table for clamping the glass substrate, and a
planetary gear mechanism for moving the carrier, and so on, the
flatness of the glass substrate stably obtained is limited to about
0.5 .mu.m.
[0008] In view of the above, proposal has recently been made of a
leveling method for leveling or flattening the glass substrate by
local machining using plasma etching or a gas cluster ion beam (for
example, see Japanese Patent Application Publication (JP-A) No.
2002-316835 (Reference 2) and Japanese Patent Application
Publication (JP-A) No. H08-293483 (Reference 3)).
[0009] The leveling method disclosed in References 2 and 3
comprises the steps of measuring a surface profile (convexity and
concavity, peak and valley) of the glass substrate and executing
local machining upon a convex portion under a machining condition
(such as the amount of plasma etching or the amount of the gas
cluster ion beam) depending upon the degree of convexity of the
convex portion so as to flatten the surface of the glass
substrate.
[0010] In case where the flatness of the surface of the glass
substrate is adjusted by the local machining using the plasma
etching or the gas cluster ion beam, a roughened surface or a
surface defect, such as a flaw and a machining-affected layer (a
damaged layer), is formed on the glass substrate after the local
machining. Therefore, it is necessary to polish the surface of the
glass substrate after the local machining in order to repair the
roughened surface or to remove the surface defect.
[0011] However, if a surface of a polishing tool, such as a
polishing pad, is directly contacted with the surface of the glass
substrate during polishing after the local machining, the flatness
of the surface of the glass substrate may be deteriorated.
Therefore, the polishing time is limited to an extremely short time
period. This makes it impossible to sufficiently repair the
roughened surface and to sufficiently remove the surface
defect.
SUMMARY OF THE INVENTION
[0012] It is therefore an object of this invention to provide a
method of producing a glass substrate for a mask blank, which
includes a polishing step of polishing a surface of the glass
substrate subjected to local machining in order to repair a
roughened surface resulting from the local machining and to remove
a surface defect resulting from the local machining, and which is
capable of providing a glass substrate high in flatness and
smoothness and free from the surface defect by repairing the
roughened surface of the glass substrate and removing the surface
defect of the glass substrate during the polishing step while
maintaining the flatness of the surface of the glass substrate.
[0013] It is another object of this invention to provide a method
of producing a mask blank by the use of the above-mentioned glass
substrate.
[0014] According to this invention, there is provided a method of
producing a glass substrate for a mask blank, the method
comprising:
[0015] a profile measuring step of measuring a convex/concave
profile of a surface of the glass substrate for a mask blank;
[0016] a flatness control step of controlling a flatness of the
surface of the glass substrate to a value not greater than a
predetermined reference value by specifying the degree of convexity
of a convex portion present on the surface of the glass substrate
with reference to a result of measurement obtained in the profile
measuring step and executing local machining upon the convex
portion under a machining condition depending upon the degree of
convexity; and
[0017] a non-contact polishing step of polishing, after the
flatness control step, the surface of the glass substrate subjected
to the local machining by the action of a machining liquid
interposed between the surface of the glass substrate and a surface
of a polishing tool without direct contact therebetween.
[0018] In the above-mentioned method, during the polishing step of
polishing the surface of the glass substrate subjected to the local
machining for the purpose of repairing a roughened surface
resulting from the local machining and removing a surface defect
resulting from the local machining, the surface of the glass
substrate is polished by non-contact polishing by the action of the
machining liquid interposed between the surface of the glass
substrate and the surface of the polishing tool without direct
contact therebetween. Thus, it is possible to repair the roughened
surface of the glass substrate and to remove the surface defect on
the surface of the glass substrate while maintaining the flatness
of the surface of the glass substrate.
[0019] Specifically, the non-contact polishing step may be carried
out by float polishing, EEM (Elastic Emission Machining), or
hydroplane polishing.
[0020] In the method of producing a glass substrate for a mask
blank according to this invention, the non-contact polishing step
is carried out by float polishing.
[0021] In the above-mentioned method, the surface of the glass
substrate is polished with an extremely small force by contacting
the machining liquid with the surface of the glass substrate while
the glass substrate is floated or by making fine powder particles
collide with the surface of the glass substrate while the glass
substrate is floated. Therefore, it is possible not only to repair
the roughened surface resulting from the local machining into an
ultrafine surface roughness while maintaining the flatness of the
surface of the glass substrate but also to remove a microscopic
surface defect (a fine surface defect).
[0022] In the method of producing a glass substrate for a mask
blank according to this invention, the machining liquid comprises
an aqueous solution selected from water, an acidic aqueous
solution, and an alkaline aqueous solution, and a mixture of the
aqueous solution and at least one kind of fine powder particles
selected from colloidal silica, cerium oxide, zirconium oxide, and
aluminum oxide.
[0023] In the above-mentioned method, a polishing force acting upon
the surface of the glass substrate is minimized so as to reliably
avoid deterioration in flatness resulting from polishing. If the
machining liquid containing the alkaline aqueous solution is used,
it is possible not only to improve a polishing rate but also to
expose a potential defect, such as a flaw, present on the surface
of the glass substrate.
[0024] In the method of producing a glass substrate for a mask
blank according to this invention, the local machining is carried
out by plasma etching or a gas cluster ion beam.
[0025] In the above-mentioned method, by controlling the moving
rate of the ion beam or the moving rate of a plasma source chamber
or housing depending upon the degree of convexity of a convex
portion on the surface of the glass substrate, it is possible to
properly perform the local machining upon the convex portion on the
surface of the glass substrate and to control the flatness to a
value not greater than a predetermined reference value.
[0026] Alternatively, an ion beam intensity or a plasma intensity
may be controlled depending upon the degree of convexity of a
convex portion on the surface of the glass substrate.
[0027] In the method of producing a glass substrate for a mask
blank according to this invention, the reference value is not
greater than 0.25 .mu.m.
[0028] In the above-mentioned method, by performing the local
machining with the reference value of the flatness being 0.25
.mu.m, the glass substrate for a F2 excimer laser exposure mask
blank required to have a flatness of 0.25 .mu.m or less can be
obtained.
[0029] By performing the local machining with the reference value
of the flatness being 0.05 .mu.m, the glass substrate for an EUV
mask blank required to have a flatness of 0.05 .mu.m or less can be
obtained.
[0030] A method of producing a mask blank according to this
invention comprises the steps of preparing a glass substrate
obtained by the method of producing a glass substrate for a mask
blank and forming a thin film as a transferred pattern on the glass
substrate.
[0031] In the above-mentioned method, the F2 excimer laser exposure
mask blank or the EUV mask blank having a desired flatness, free
from a surface defect, and having a high quality is obtained.
[0032] A method of producing a transfer mask according to this
invention comprises the steps of preparing the mask blank obtained
by the above-mentioned method and patterning the thin film of the
mask blank to form a thin film pattern on the glass substrate.
[0033] A method of producing a semiconductor device according to
this invention comprises the steps of preparing the transfer mask
obtained by the above-mentioned method and transferring the thin
film pattern of the transfer mask onto a semiconductor substrate by
lithography.
BRIEF DESCRIPTION OF THE DRAWING
[0034] FIG. 1 is a flow chart for describing a production process
of a glass substrate for a mask blank according to this
invention;
[0035] FIG. 2 is a schematic sectional view of a polishing
apparatus used in the production process according to this
invention;
[0036] FIG. 3 is a schematic sectional view of a float polishing
apparatus used in the production process according to this
invention;
[0037] FIG. 4 is a sectional view of a characteristic part of the
float polishing apparatus illustrated in FIG. 3;
[0038] FIG. 5 is a schematic sectional view of an EEM
apparatus;
[0039] FIG. 6A is a sectional view of an EUV reflective mask blank
which uses the glass substrate according to this invention;
[0040] FIG. 6B is a sectional view of an EUV reflective mask which
uses the glass substrate according to this invention; and
[0041] FIG. 7 is a view for describing pattern transfer using the
reflective mask.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0042] Now, an embodiment of this invention will be described with
reference to the drawing.
[0043] [Method of Producing a Glass Substrate for a Mask Blank]
[0044] At first referring to FIG. 1, description will be made of a
method of producing a glass substrate for a mask blank according to
this invention.
[0045] Referring to FIG. 1, a production process of the glass
substrate for a mask blank in this invention includes a preparing
step (P-1) of preparing a glass substrate having a surface
subjected to precision polishing, a profile measuring step (P-2) of
measuring a convex/concave profile of the surface of the glass
substrate, a flatness control step (P-3) of controlling a flatness
of the surface of the glass substrate by local machining, and a
non-contact polishing step (P-4) of polishing the surface of the
glass substrate in a non-contact manner.
[0046] <Preparing Step (P-1)>
[0047] In the preparing step (P-1), preparation is made of a glass
substrate with its one surface or opposite surfaces
precision-polished by the use of a polishing apparatus which will
later be described.
[0048] The glass substrate is not particularly restricted but may
be any substrate which is suitably used as a mask blank. For
example, use may be made of a quartz glass, a soda lime glass, an
aluminosilicate glass, a borosilicate glass, and an alkali-free
glass.
[0049] In case of a glass substrate for a F2 excimer laser exposure
mask blank, use may be made of a quartz glass doped with fluorine
so as to suppress absorption of exposure light as small as
possible.
[0050] In case of a glass substrate for an EUV mask blank, use may
be made of a glass material having a low thermal expansion
coefficient within a range of 0.+-.1.0.times.10.sup.-7/.degree. C.,
preferably within a range of 0.+-.0.3.times.10.sup.-7/.degree. C.
in order distortion of a transferred pattern due to heat during
exposure.
[0051] In the EUV mask blank, a number of films are formed on the
glass substrate. Therefore, use is made of a glass material having
a high rigidity and capable of suppressing deformation due to film
stress. In particular, a glass material having a high Young's
modulus of 65 GPa or more is preferable. For example, use may be
made of an amorphous glass, such as a SiO.sub.2--TiO.sub.2 glass
and a quartz glass, and a crystallized glass with .beta.-quartz
solid solution deposited therein.
[0052] Referring to FIG. 2, a polishing apparatus 10 has a
polishing portion of a planetary gear system comprising a lower
surface table 11, an upper surface table 12, a sun gear 13, an
internal gear 14, a carrier 15, and an abrasive supply member 16.
The polishing portion polishes the opposite surfaces of the glass
substrate by holding the glass substrate in the carrier 15,
clamping the glass substrate between the upper and the lower
surface tables 11 and 12 with polishing pads 11a and 12a attached
thereto, respectively, supplying an abrasive in an area between the
upper and the lower surface tables 11 and 12, and rotating and
revolving the carrier 15. Hereinafter, the structure of the
polishing portion will be described in detail.
[0053] Each of the lower and the upper surface tables 11 and 12 is
a disk member having a ring-shaped horizontal plane. The lower and
the upper surface tables 11 and 12 have opposite surfaces to which
the polishing pads 11a and 12a are attached. The lower and the
upper surface tables 11 and 12 are supported to be rotatable around
a vertical shaft A (vertical shaft passing through the center of
the polishing portion) and are associated with surface table
rotation driving portions (not shown), respectively. Driven by the
surface table rotation driving portions, the lower and the upper
surface tables 11 and 12 are rotated.
[0054] The upper surface table 12 is supported to be movable upward
and downward along the vertical shaft A. Driven by an upper surface
table up/down driving portion (not shown), the upper surface table
12 is moved upward and downward.
[0055] The sun gear 13 is disposed at the center of the polishing
portion to be rotatable. Driven by a sun gear rotation driving
portion (not shown), the sun gear 30 is rotated around the vertical
shaft A.
[0056] The internal gear 14 is a ring-shaped gear having a series
of teeth on an inner peripheral side and is disposed outside the
sun gear 13 to be concentric therewith. The internal gear 14
illustrated in FIG. 2 is fixed to be unrotatable. Alternatively,
the internal gear 14 may be rotatable around the vertical shaft
A.
[0057] The carrier (planetary gear) 15 is a thin-plate disk member
having a series of teeth on an outer peripheral side and is
provided with one or a plurality of work holding apertures for
holding the glass substrate.
[0058] The polishing portion generally has a plurality of carriers
15. These carriers 15 are engaged with the sun gear 13 and the
internal gear 14 and are rotated and revolved around the sun gear
13 in accordance with the rotation of the sun gear 13 (and/or the
internal gear 14).
[0059] Each of the upper and the lower surface tables 12 and 11 has
an outer diameter smaller than an inner diameter of the internal
gear 14. An actual polishing region is a doughnut-like region
between the sun gear 13 and the internal gear 14 and between the
upper and the lower surface tables 12 and 11.
[0060] The abrasive supply member 16 comprises an abrasive storage
16a for storing the abrasive and a plurality of tubes 16b for
supplying the abrasive stored in the abrasive storage 16a to the
polishing region between the upper and the lower surface tables 12
and 11.
[0061] The abrasive comprises fine abrasive grains dispersed in a
liquid. For example, the abrasive grains may be silicon carbide,
aluminum oxide, cerium oxide, zirconium oxide, manganese dioxide,
and colloidal silica. Depending upon the material and the surface
roughness of the glass substrate, the abrasive grains are
appropriately selected. The abrasive grains are dispersed in a
liquid, such as water, an acidic solution, or an alkaline solution,
to be used as the abrasive.
[0062] The preparing step (P-1) at least comprises a lapping step
of lapping the opposite surfaces of the glass substrate and a
precision-polishing step of precision-polishing the opposite
surfaces of the glass substrate after subjected to lapping. Thus,
stepwise polishing is carried out.
[0063] For example, the lapping step is carried out by the use of
an abrasive obtained by dispersing cerium oxide as relatively large
abrasive grains while the precision-polishing step is carried out
by the use of an abrasive obtained by dispersing colloidal silica
as relatively small abrasive grains.
[0064] <Profile Measuring Step (P2)>
[0065] The profile measuring step (P-2) is a step of measuring the
convex/concave profile (flatness) of the surface of the glass
substrate prepared in the previous step (P-1).
[0066] Preferably, the convex/concave profile is measured by a
flatness measurement apparatus or profilometer utilizing optical
interference in view of a measuring accuracy. The flatness
measurement apparatus carries out measurement by irradiating the
surface of the glass substrate with coherent light, which is then
reflected as reflected light, and detecting a phase difference of
the reflected light corresponding to a height difference on the
surface of the glass substrate.
[0067] For example, the flatness is defined as a difference between
the maximum value and the minimum value of a measured plane of the
surface of the glass substrate with respect to a virtual absolute
plane (focal plane) calculated from the measured plane by a least
square method.
[0068] The result of measurement of the convex/concave profile is
stored in a recording medium such as a computer. Thereafter, the
result of measurement is compared with a predetermined reference
value (desired flatness) preliminarily be selected. The difference
between the result of measurement and the reference value is
calculated, for example, by an arithmetic unit of the computer. The
difference is calculated for each predetermined region on the
surface of the glass substrate. The predetermined region is
determined to be coincident with a machining region in the local
machining. Thus, the difference in each predetermined region
corresponds to a required removed amount to be removed in the local
machining for each machining region.
[0069] The above-mentioned calculation may be carried out in either
the profile measuring step (P-2) or the flatness control step
(P-3).
[0070] <Flatness Control Step (P-3)>
[0071] The flatness control step (P-3) is a step of specifying the
degree of convexity of a convex portion present on the surface of
the glass substrate with reference to the result of measurement
obtained in the profile measuring step (P-2) and carrying out local
machining upon the convex portion under the machining condition
corresponding to the degree of convexity to control the flatness of
the surface of the glass substrate to a value not greater than the
predetermined reference value.
[0072] The local machining is carried out under the machining
condition selected for each predetermined region on the surface of
the glass substrate. The machining condition is determined with
reference to the convex/concave profile of the surface of the glass
substrate measured by the flatness measurement apparatus and the
difference from the predetermined reference value of the flatness
(required removed amount in the local machining).
[0073] Depending upon a machining apparatus, parameters of the
machining condition are different. At any rate, the parameters are
determined so that a greater amount is removed as the degree of
convexity of the convex portion is greater. For example, in case
where the local machining is carried out by the use of an ion beam
or plasma etching, the moving rate of the ion beam or the moving
rate of a plasma source chamber is controlled to be slower as the
degree of convexity is greater. Alternatively, the ion beam
intensity or the plasma intensity may be controlled.
[0074] As a local machining method used in the flatness control
step (P-3), not only the ion beam machining and the plasma etching
mentioned above but also various other methods, such as MRF
(MagnetoRheological Finishing) and CMP (Chemical-Mechanical
Polishing) may be used.
[0075] In the MRF, an object to be machined (glass substrate) is
locally polished by bringing abrasive grains contained in a
magnetic fluid into contact with the object at a high speed and
controlling a holding time of a contacted portion between the
abrasive grains and the object.
[0076] The CMP comprises the steps of polishing a convex portion of
the surface of the object by the use of a small-diameter polishing
pad and an abrasive (containing abrasive grains such as colloidal
silica) and by controlling the holding time of a contacted portion
between the small-diameter polishing pad and the object (glass
substrate).
[0077] Among the local machining methods mentioned above, local
machining by the ion beam, plasma etching, or the CMP leaves a
roughened surface or a machining-affected layer on the surface of
the glass substrate. Therefore, non-contact polishing (which will
later be described) is particularly effective.
[0078] Hereinafter, description will be made of the local machining
by plasma etching and the ion beam particularly suitable in the
flatness control step (P-3).
[0079] The local machining method by the plasma etching comprises
the steps of positioning the plasma source chamber above a surface
portion to be removed and flowing an etching gas to thereby etch
the portion to be removed. By flowing the etching gas, neutral
radical species generated in plasma isotropically attack the
surface of the glass substrate so that the above-mentioned portion
is removed. On the other hand, a remaining portion where the plasma
source chamber is not located is not etched by collision of the
etching gas because no plasma is produced.
[0080] When the plasma source chamber is moved on the glass
substrate, the removed amount is adjusted by controlling the moving
rate of the plasma source chamber or the plasma intensity in
accordance with the required removed amount of the surface of the
glass substrate.
[0081] The plasma source chamber may have a structure in which the
glass substrate is clamped by a pair of electrodes. Plasma is
generated between the substrate and the electrodes by a
high-frequency wave and the etching gas is supplied to thereby
generate radical species. Alternatively, the plasma source chamber
may comprise a waveguide tube through which the etching gas flows.
Plasma is generated by oscillation of microwave to produce a stream
of radical species, which impinges on the surface of the glass
substrate.
[0082] The etching gas is appropriately selected depending upon the
material of the glass substrate. For example, use is made of a gas
of halogen compound or a mixed gas containing halogen compound.
More specifically, use may be made of tetrafluoromethane,
trifluoromethane, hexafluoroethane, octafluoropropane,
decafluorobutane, hydrogen fluoride, sulfur hexafluoride, nitrogen
trifluoride, carbon tetrachloride, silicon tetrafluoride,
trifluorochloromethane, and boron trichloride.
[0083] The local machining method by the ion beam (irradiation by
the gas cluster ion beam) comprises the steps of preparing a
substance, such as oxide, nitride, carbide, a rare gas, having a
gaseous phase at normal temperature and normal pressure (room
temperature and atmospheric pressure) or a mixed gas thereof (a
substance as a mixed gas obtained by mixing the above-mentioned
substances at an appropriate ratio), forming a gas cluster of the
substance, ionizing the gas cluster by electron irradiation to form
the gas cluster ion beam, and irradiating a solid surface (surface
of the glass substrate) with the gas cluster ion beam in an
irradiated region which may be controlled if necessary.
[0084] Generally, the cluster comprises a group of several hundreds
of atoms or molecules. Even if an accelerated voltage is 10 kV,
irradiation occurs as an ultraslow ion beam having energy not
greater than several tens eV per atom or molecule. Therefore, the
surface of the glass substrate is machined with extremely low
damage.
[0085] When the surface of the glass substrate is irradiated by the
gas cluster ion beam, the molecules or the atoms forming cluster
ions collide with atoms of the surface of the glass substrate in
multiple stages to produce reflected molecules or atoms having a
lateral or horizontal kinetic component. As a result, selective
sputtering occurs at the convex portion on the surface of the glass
substrate so as to flatten the surface of the glass substrate. Such
flattening phenomenon is also obtained by the effect of
preferentially sputtering those atoms present on the surface or
grains and having a weak bond, by the energy concentrated to the
surface of the glass substrate.
[0086] The generation of the gas cluster itself is already known.
That is, the gas cluster can be produced by blowing a gaseous
substance in a compressed state into a vacuum apparatus through an
expansion nozzle. The gas cluster thus produced can be ionized by
irradiation with electrons.
[0087] Herein, the gaseous substance may be oxide, such as
CO.sub.2, CO, N.sub.2O, NOx, and CxHyOz, O.sub.2, N.sub.2, and a
rare gas such as Ar and He.
[0088] The flatness required to the glass substrate for a mask
blank is determined in correspondence to the wavelength of an
exposure light source used for the mask blank. Depending upon the
required flatness, the reference value for controlling the flatness
in the flatness control step (P-3) is determined.
[0089] For example, in case of the glass substrate for a F2 excimer
laser exposure mask blank, the reference value for controlling the
flatness is not greater than 0.25 .mu.m. In case where the glass
substrate for an EUV mask blank, the reference value for
controlling the flatness is not greater than 0.5 .mu.m. By the use
of the reference value, the local machining is carried out.
[0090] <Non-Contact Polishing Step (P-4)>
[0091] The non-contact polishing step (P-4) is a step of polishing
the surface of the glass substrate subjected to the local machining
in the flatness control step (P-3) by the action of a machining
liquid interposed between the surface of the glass substrate and a
surface of a polishing tool without direct contact
therebetween.
[0092] A non-contact polishing method used in this step is not
particularly limited. For example, use may be made of float
polishing, EEM, and hydroplane polishing.
[0093] As fine powder particles to be contained in the machining
liquid used in non-contact polishing, abrasive grains having a
small average grain size are selected in order to reduce the
surface roughness of the glass substrate. Preferably, the average
grain size is not greater than several tens nanometers, more
preferably not greater than several nanometers. As the abrasive
grains having a small average grain size, use may be made of cerium
oxide, silica (SiO.sub.2), colloidal silica, zirconium oxide,
manganese dioxide, and aluminum oxide. Among others, colloidal
silica is preferable in view of the surface smoothness in case
where the glass substrate is used.
[0094] In the non-contact polishing, the machining liquid may be an
aqueous solution selected from water, an acidic aqueous solution,
and an alkaline aqueous solution. Alternatively, the machining
liquid may be a mixture of the aqueous solution and the
above-mentioned fine powder particles.
[0095] If the water is used, pure water and ultra pure water are
preferable.
[0096] As the acidic aqueous solution, use may be made of sulfuric
acid, hydrochloric acid, hydrofluoric acid, and fluorosilicic acid.
If the acidic aqueous solution is contained in the machining liquid
used in the non-contact polishing, the polishing rate is improved.
However, depending upon the type of the acid or if the
concentration of the acidic aqueous solution is high, the glass
substrate may be roughened. Therefore, the type of the acid and the
concentration are appropriately selected so as not to roughen the
glass substrate.
[0097] As the alkaline aqueous solution, use may be made of an
aqueous solution of potassium hydroxide or sodium hydroxide. If the
alkaline aqueous solution is contained in the machining liquid used
in the non-contact polishing, the polishing rate is improved.
Further, if a potential microscopic defect (crack, flaw, or the
like) is present on the surface of the glass substrate, such
potential microscopic defect is exposed. It is therefore possible
to reliably detect the microscopic defect in an inspection step
subsequently carried out. The alkaline aqueous solution is adjusted
within a range such that the abrasive grains contained in the
machining liquid are not dissolved. It is preferable to adjust the
alkaline aqueous solution so that the machining liquid has a pH of
9-12.
[0098] Hereinafter, description will be made of the principle of
machining by each of the float polishing, the EEM, and the
hydroplane polishing.
[0099] A polishing plate used in the float polishing has a surface
provided with a plurality of grooves for leading the machining
liquid and formed into a shape such that a dynamic or kinetic
pressure is generated. As the machining liquid, use is made of fine
powder particles having an average grain size of several nanometers
to several tens nanometers and suspended in a solvent (such as pure
water or an alkaline aqueous solution). In the machining liquid,
the polishing plate and an object to be machined (glass substrate)
are simultaneously rotated in the same direction in the state where
a polishing plate axis (main shaft) and a rotation shaft of the
object are eccentric from each other at a predetermined
distance.
[0100] At this time, the object is allowed to freely float up and
down and to receive only a rotation torque transmitted thereto.
According to a dynamic pressure effect, a small gap is formed
between the object and the polishing plate and the object floats
up. The fine powder particles passing through the gap collide with
a machined surface of the object so that microscopic destruction is
repeated. Thus, machining of the object proceeds. Because of the
above-mentioned principle, the object can be machined to an
ultrafine surface roughness. In addition, machining itself is
carried out with a small force so that the machined surface is
finished without a machining-affected layer.
[0101] In case where the object is a glass substrate, CeO.sub.2
(having a ultra high purity) or colloidal silica may be used as the
fine powder particles.
[0102] The EEM is a non-contact polishing method in which fine
powder particles of 0.1 .mu.m or less are contacted with the object
in a substantially no load condition. By an interaction (a sort of
chemical bond) produced at an interface between the fine powder
particles and the object, atoms on the surface of the object are
removed per atom. According to the principle of machining mentioned
above, machining characteristics greatly depend upon the affinity
between the fine powder particles and the object. In order to
efficiently machine the object, the fine powder particles are
appropriately selected depending upon the material of the object.
For example, in case where the object is a glass substrate,
zirconium oxide, aluminum oxide, and colloidal silica may be used
as the fine powder particles. In order to improve the machining
rate, the fine powder particles are suspended in a solvent causing
erosion of the object to obtain the machining liquid, which is
contacted with the object.
[0103] In the hydroplane polishing, the object is fixed to a
disk-shaped plate having a conical outer periphery to face a
polishing pad. The outer periphery of the disk-shaped plate is
supported by three rollers so that the object is separated from the
surface of the polishing pad by about 100 .mu.m. When an abrasive
layer is formed between the polishing pad and the object and a
space between the polishing pad and the object is filled with the
abrasive, the object and the disk-shaped plate follow the rotation
of the polishing pad and machining proceeds.
[0104] Next, description will be made of a float polishing
apparatus and an EEM apparatus.
[0105] Referring to FIG. 3, the float polishing apparatus 20
comprises a rotary table 21, a cylindrical machining tank 22 placed
on the rotary table 21 and storing a machining liquid, a main shaft
23 which is a rotation shaft of the rotary table 21, a polishing
plate 24 disposed on the rotary table 21 to be eccentric at a
predetermined distance with respect to the main shaft 23, a work
holder shaft 25 concentric with the polishing plate 24, a work
holder 26 faced to the polishing plate 24 and rotatable around the
work holder shaft 25, and a machining liquid supply member 27 for
supplying the machining tank 22 with the machining liquid
containing fine powder particles.
[0106] The rotary table 21 is required to have a high rigidity and
a resistance against the machining liquid. Therefore, the rotary
table 21 is made of a material having the above-mentioned
characteristics. Preferably, a stainless steel is used. Further,
the rotary table 21 requires a high rotation accuracy and a high
vibration absorbability. Therefore, the rotary table 21 is
preferably supported by a high-performance bearing such as a
hydrostatic oil bearing.
[0107] The rotary table 21 is provided with a discharge port (not
shown) for discharging the machining liquid supplied from the
machining liquid supply member 27. Ahead of the discharge port, a
collecting mechanism (not shown) for collecting machining scraps
produced by the float polishing is disposed. During machining, the
discharge port is kept opened. By controlling the amount of the
machining liquid supplied from the machining liquid supply member
27, a liquid level of the machining liquid in the machining tank 22
is maintained.
[0108] Driven by a rotation driving member (not shown), the rotary
table 21 is rotated around the main shaft 23 at a rotation speed of
several tens rpm to several hundreds rpm.
[0109] Driven by a rotation driving member (not shown), the work
holder 26 is rotated around the work holder shaft 200 at a rotation
speed of several tens rpm to several hundreds rpm. The work holder
26 is supported so as to float up and down and receives only a
rotation driving torque transmitted thereto. Thus, the work holder
26 is allowed to float up and down during machining. The work
holder 26 is rotated in a rotating direction same as that of the
rotary table 21.
[0110] The object to be machined is held in a manner such that the
object is not given a damage such as a flaw. For example, the
object is fixed to the work holder 26 by vacuum suction or an
adhesive.
[0111] The polishing plate 24 has a doughnut-like shape around the
main shaft 23 of the rotary table 21 and has a width at least
greater than the size of the object. Since the object is rotated
around the work holder shaft 200 on the polishing plate 24, the
width of the polishing plate 24 is greater than the diagonal length
of the object if the object has a square shape and is greater than
the long diagonal length of the object if the object has a
rectangular shape.
[0112] Referring to FIG. 4, the polishing plate 24 has an upper
surface of a non-flat shape or a convex/concave shape. Between a
plurality of convex portions 24a, a plurality of grooves 24b for
leading the machining liquid are formed. Each of the convex
portions 24a has an upper part formed into a tapered shape so as to
produce a dynamic pressure upon the object. By an inclination angle
of the tapered shape, a floating force (floating distance) of the
object is controlled. The inclination angle of the tapered shape is
appropriately adjusted within a range of 1.degree. and 20.degree.
depending upon the size of the object or the like so that the
floating distance of the object is several microns. Herein, the
floating distance is a distance between the convex portion 24a of
the polishing plate 24 and the object, i.e., a gap in which the
machining liquid is present. The width, the depth, and the pitch of
the groove 24b controls leading of the machining liquid. The groove
24b has a width appropriately selected between 1 and 5 mm, a depth
appropriately selected between 1 and 10 mm, and a pitch
appropriately selected between 0.5 and 30 mm.
[0113] The polishing plate 24 is made of a material resistant
against the machining liquid. For example, a stainless steel, tin,
ceramics may be used.
[0114] Depending upon a liquid temperature of the machining liquid,
the polishing plate 24, the rotary table 21, the work holder 26,
and the object may be thermally deformed so that the machining
accuracy is affected. Therefore, the machining liquid is accurately
controlled in temperature.
[0115] For example, the machining liquid comprises a solvent, such
as pure water, ultra pure water, an alkali, or an acid, or a
mixture of the solvent and fine powder particles contained therein.
The concentration of the fine powder particles is within a range of
0.1-40 wt %.
[0116] The machining liquid supply member 27 may circulate the
machining liquid in the manner such that the machining liquid
discharged from the discharge port is supplied again into the
machining tank 22 after the machining scraps contained in the
machining liquid are removed by a dust collector. Alternatively,
the machining liquid supply member 27 may supply a new machining
liquid into the machining tank 22 in an amount corresponding to the
machining liquid discharged from the discharge port. In the float
polishing, the thickness of a machining liquid layer interposed
between the polishing plate 24 and the object is an important
factor. Therefore, the amount of the machining liquid supplied from
the machining liquid supply member 27 is controlled with high
accuracy in order to strictly control the amount of the machining
liquid in the machining tank 22.
[0117] Referring to FIG. 5, the EEM apparatus 30 comprises a
machining tank 31 storing a machining liquid, an object holding
member 32 for holding an object in the machining tank 31, a
rotation shaft 33 extending towards a surface of the object, a
rotary member 34 rotatable around the rotation shaft 33 so that the
machining liquid (fine powder particles) is preferentially
contacted with a specific region on the surface of the object, a
moving member 35 for moving the rotary member 34 upward, downward,
leftward, rightward with respect to the object, and a machining
liquid supply member 36 for supplying the machining liquid
containing the fine powder particles into the machining tank
31.
[0118] The machining tank 31 is made of a material resistant
against the machining liquid. The machining tank 31 is provided
with a discharge port 31a for discharging the machining liquid
supplied from the machining liquid supply member 36. Ahead of the
discharge port 31a, a collecting mechanism (not shown) for
collecting machining scraps produced by the EEM is disposed. During
machining, the discharge port 31a is kept opened. By controlling
the amount of the machining liquid supplied from the machining
liquid supply member 36, a liquid level of the machining liquid in
the machining tank 31 is maintained.
[0119] The object to be machined is held in a manner such that the
object is not given a damage such as a flaw.
[0120] The shape of the rotary member 34 is appropriately selected
in correspondence to the specific region on the surface of the
object as a region which is to be preferentially contacted
(reacted) with the machining liquid. In case where the machining
liquid is to be preferentially contacted with a relatively narrow
region, the rotary member 34 has a spherical shape or a linear
shape. In case where the machining liquid is to be preferentially
contacted with a relatively large region, the rotary member 34 has
a cylindrical shape.
[0121] The rotary member 34 is made of a material resistant against
the machining liquid and having a low elasticity. If the rotary
member 34 has a high elasticity (relatively soft), deformation may
occur during rotation and the shape may become unstable so that the
machining accuracy is degraded. For example, the rotary member 34
may be made of polyurethane, glass, ceramics.
[0122] [Method of Producing a Mask Blank]
[0123] Next, description will be made of a method of producing a
mask blank according to one embodiment of this invention.
[0124] The method of producing a mask blank according to this
invention comprises the steps of preparing a glass substrate
obtained by the above-mentioned method of producing a glass
substrate for a mask blank and forming a thin film as a transferred
pattern on the glass substrate.
[0125] The mask blank is classified into a transmissive mask blank
and a reflective mask blank. In either mask blank, the thin film as
the transferred pattern is formed on the glass substrate. A resist
film may be formed on the thin film.
[0126] The thin film formed on the transmissive mask blank causes
optical change in exposure light (light emitted from the exposure
light source) used in pattern transfer to a transfer object. For
example, the thin film may be a light shielding film (an opaque
film) for shielding the exposure light or a phase shift film for
changing the phase of the exposure light.
[0127] Generally, the light shielding film may be a Cr film, a Cr
alloy film selectively containing oxygen, nitrogen, carbon, or
fluorine in addition to Cr, a laminated film thereof, a MoSi film,
a MoSi alloy film selectively containing oxygen, nitrogen, or
carbon in addition to MoSi, and a laminated film thereof.
[0128] The phase shift mask may be a SiO.sub.2 film having a phase
shift function alone, a metal silicide oxide film, a metal silicide
nitride film, a metal silicide oxynitride film, a metal silicide
oxycarbide film, a metal silicide oxycarbonitride film (metal:
transition metal such as Mo, Ti, W, Ta) each of which has a phase
shift function and a light shielding function, and a halftone film
such as a CrO film, a CrF film, and a SiON film.
[0129] The reflective mask blank comprises a glass substrate and a
laminated film formed thereon and including a reflective multilayer
film (reflective multilayer film) and a light absorber film
(absorber layer) as a transferred pattern.
[0130] The reflective multilayer film may comprise a Ru/Si periodic
multilayer film, a Mo/Be periodic multilayer film, a
Mo-compound/Si-compound periodic multilayer film, a Si/Nb periodic
multilayer film, a Si/Mo/Ru periodic multilayer film, a Si/Mo/Ru/Mo
periodic multilayer film, and a Si/Ru/Mo/Ru periodic multilayer
film.
[0131] The light absorber film may be made of a material such as
Ta, Ta alloy (for example, a material containing Ta and B, a
material containing Ta, B, and N), Cr, Cr alloy (for example, a
material containing Cr and at least one element selected from
nitrogen, oxygen, carbon, and fluorine).
[0132] For the transmissive mask blank, g ray (having a wavelength
of 436 nm), i ray (having a wavelength of 365 nm), KrF (having a
wavelength of 246 nm), ArF (having a wavelength of 193 nm), or F2
(having a wavelength of 157 nm) may be used as the wavelength of
the exposure light source. For the reflective mask blank, EUV
(having a wavelength of 13 nm) may be used as the wavelength of the
exposure light source.
[0133] For example, the above-mentioned thin film may be formed by
sputtering such as DC sputtering, RF sputtering, ion beam
sputtering.
EXAMPLES AND COMPARATIVE EXAMPLES
[0134] Hereinafter, description will be made of examples of this
invention in conjunction with a method of producing a glass
substrate for an EUV reflective mask blank (hereinafter simply be
referred to as a glass substrate) and a method of producing an EUV
reflective mask blank. It will readily be understood that this
invention is not limited to the following examples.
Example 1
Float Polishing
[0135] Preparation was made of a glass substrate (having a size of
152.4 mm.times.152.4mm and a thickness of 6.35 mm) which has been
polished stepwise by cerium oxide abrasive grains and colloidal
silica abrasive grains by the use of the above-mentioned polishing
apparatus 10.
[0136] The surface profile (flatness) of the glass substrate was
measured by a flatness measurement apparatus utilizing optical
interference. As a result, the glass substrate had a flatness of
0.2 .mu.m (convex) and a surface roughness of 0.15 nm as a
root-mean-square roughness Rq (=RMS). The root-mean-square
roughness Rq is also disclosed in U.S. Pat. No. 6,544,893 B2.
[0137] The result of profile measurement of the surface of the
glass substrate was stored in a computer and compared with a
reference value of 0.05 .mu.m (convex) as a required flatness for
the glass substrate for an EUV mask blank. The difference (required
removed amount) between the measured flatness and the reference
value was calculated by the computer.
[0138] Next, for every predetermined region (5 mm square) within
the plane of the glass substrate, the machining condition for local
plasma etching was determined in correspondence to the required
removed amount. According to the machining condition thus
determined, the profile was adjusted by the local plasma etching so
that the flatness of the glass substrate is not greater than the
reference value (flatness of 0.05 .mu.m).
[0139] The local plasma etching was carried out by the use of
tetrafluoromethane as an etching gas and a plasma source chamber of
a high-frequency type having a cylindrical electrode.
[0140] After the profile was adjusted by the local plasma etching,
the flatness of the surface of the glass substrate was measured. As
a result, the flatness was as excellent as 0.05 .mu.m. The surface
roughness Rq of the surface of the glass substrate was equal to
about 1 nm. Thus, the surface was roughened as a result of the
plasma etching.
[0141] The glass substrate was mounted to the float polishing
apparatus 20 mentioned above and subjected to non-contact
polishing.
[0142] The polishing condition in the float polishing was as
follows:
1 Machining liquid pure water + fine powder particles (Polishing
slurry): (concentration of 2 wt %) Fine powder particles: silica
(SiO.sub.2) having average grain size of about 70 nm Rotation speed
of rotary table: 5-200 rpm Rotation speed of work holder: 10-300
rpm Polishing time: 5-30 min
[0143] Thereafter, the glass substrate was cleaned by an alkali
aqueous solution to obtain the glass substrate for an EUV mask
blank.
[0144] The flatness and the surface roughness of the glass
substrate thus obtained were measured. As a result, the flatness
was as excellent as 0.05 .mu.m, i.e., the level before the float
polishing was maintained. The surface roughness Rq was 0.09 nm.
Thus, the roughened surface of the glass substrate before the float
polishing was repaired.
[0145] The surface defect of the surface of the glass substrate was
inspected by a defect inspection apparatus described in Japanese
Patent Application Publication (JP-A) No. H11-242001. The
inspection apparatus carries out defect inspection by introducing a
laser beam from a chamfered surface of the substrate, confining the
laser beam by total reflection, and detecting light scattered by
the defect and leaking out from the substrate. As a result of the
defect inspection, no flaw having a size exceeding 0.05 .mu.m was
found.
[0146] Thus, the glass substrate thus obtained satisfied the
specification required for a glass substrate for an EUV mask
blank.
Example 2
Machining Liquid for Float Polishing
[0147] A glass substrate was produced in the manner similar to
Example 1 except that the float polishing was carried out under the
following condition.
2 Machining liquid alkali aqueous solution (NaOH) + fine (Polishing
slurry): powder particles (concentration of 2 wt %), pH: 11 Fine
powder particles: colloidal silica having an average grain size of
70 nm Rotation speed of rotary table: 5-200 rpm Rotation speed of
work holder: 10-300 rpm Polishing time: 3-25 min
[0148] Thereafter, the glass substrate was cleaned by an alkali
aqueous solution (NaOH) to obtain the glass substrate for an EUV
mask blank.
[0149] The flatness and the surface roughness of the glass
substrate thus obtained were measured. As a result, the flatness
and the surface roughness were substantially same as those of the
glass substrate obtained in Example 1. The surface defect of the
surface of the glass substrate was inspected by the defect
inspection apparatus described in Japanese Patent Application
Publication (JP-A) No. H11-242001. As a result, no flaw having a
size exceeding 0.05 .mu.m was found. By the use of the alkali
aqueous solution as a solvent of the machining liquid, the
polishing rate was improved and the polishing time was
shortened.
Example 3
Machining Liquid 2 for Float Polishing
[0150] A glass substrate was produced in the manner similar to
Example 1 except that the float polishing was carried out under the
following condition.
3 Machining liquid alkali aqueous solution (NaOH) (Polishing
slurry): 5 vol % Fine powder particles: none Rotation speed of
rotary table: 5-200 rpm Rotation speed of work holder: 10-300 rpm
Polishing time: 7-45 min
[0151] Thereafter, the glass substrate was cleaned by pure water to
obtain the glass substrate for an EUV mask blank.
[0152] The flatness and the surface roughness of the glass
substrate thus obtained were measured. As a result, the flatness
and the surface roughness were substantially same as those of the
glass substrate obtained in Example 1. The surface defect of the
surface of the glass substrate was inspected by the defect
inspection apparatus described in Japanese Patent Application
Publication (JP-A) No. H11-242001. As a result, no flaw having a
size exceeding 0.05 .mu.m was found. By the use of the alkali
aqueous solution as a solvent of the machining liquid, the
polishing rate was improved and the polishing time was
shortened.
Example 4
EEM
[0153] A glass substrate was produced in the manner similar to
Example 1 except that the EEM was carried out as the non-contact
polishing after the flatness was adjusted by the local plasma
etching. The EEM was carried out in the following condition.
4 Machining liquid pure water + fine powder particles (Polishing
slurry): (concentration: 3 wt %) Fine powder particles: zirconium
oxide (ZrO.sub.2) having an average particle size of about 60 nm
Rotary member: polyurethane roll Rotation speed of rotary member:
10-300 rpm Rotation speed of work holder: 10-100 rpm Polishing
time: 5-30 min
[0154] Thereafter, the glass substrate was cleaned by an alkali
aqueous solution to obtain the glass substrate for an EUV mask
blank.
[0155] The flatness and the surface roughness of the glass
substrate thus obtained were measured. As a result, the flatness
was as excellent as 0.05 .mu.m, i.e., the level before the float
polishing was maintained. The surface roughness Rq was 0.11 nm.
Thus, the roughened surface of the glass substrate before execution
of the EEM was repaired. The surface roughness is slightly greater
than those in Examples 1 to 3 presumably under the influence of
hardness of the fine powder particles.
[0156] The surface defect of the surface of the glass substrate was
inspected by a defect inspection apparatus described in Japanese
Patent Application Publication (JP-A) No. H11-242001. As a result,
no flaw having a size exceeding 0.05 .mu.m was found.
[0157] Thus, the glass substrate obtained as mentioned above
satisfied the specification required for a glass substrate for an
EUV mask blank.
Comparative Example
[0158] A glass substrate was prepared in the manner similar to
Example 2 except that, as polishing after the flatness was adjusted
by the local plasma etching, one-side polishing was carried out in
the following manner. The glass substrate was mounted to a
polishing plate faced to a polishing surface table. The glass
substrate was rotated and pressed downward against a polishing pad
region on the polishing surface table which is rotated. The
one-side polishing was carried out in the following condition.
5 Machining liquid alkali aqueous solution (NaOH) + fine (Polishing
slurry): powder particles (concentration of 2 wt %), pH: 11 Fine
powder particles: colloidal silica having an average grain size of
about 70 nm Rotation speed of 1-50 rpm polishing surface table:
Rotation speed of 1-50 rpm polishing plate: Machining pressure:
0.1-10 kPa Polishing time: 1-10 min
[0159] Thereafter, the glass substrate was cleaned by an alkali
aqueous solution (NaOH) to obtain the glass substrate for an EUV
mask blank.
[0160] The flatness and the surface roughness of the glass
substrate thus obtained were measured. As a result, the surface
roughness Rq was as excellent as 0.15 nm. The flatness was 0.25
.mu.m which was degraded as compared with that before the one-side
polishing and that before the flatness was adjusted by the local
plasma etching.
[0161] The surface defect of the surface of the glass substrate was
inspected by the defect inspection apparatus described in Japanese
Patent Application Publication (JP-A) No. H-242001. As a result, a
number of flaws exceeding 0.05 .mu.m were found. This is presumably
because foreign matters present in the polishing pad damaged the
glass substrate during the polishing since the polishing is
performed in the state where the glass substrate is contacted with
the polishing pad.
[0162] As a result, the glass substrate obtained in Comparative
Example did not satisfy the specification required for a glass
substrate for an EUV mask blank.
[0163] <Production of EUV Reflective Mask Blank and EUV
Reflective Mask>
[0164] Referring to FIGS. 6A and 6B, production of the EUV
reflective mask blank and the EUV reflective mask will be
described.
[0165] On a glass substrate 101 obtained either in each of Examples
1 to 4 or in Comparative Example, 40 periods of Si films and Mo
films were laminated by DC magnetron sputtering. It is noted here
that a single period of deposition includes a Si film having the
thickness of 4.2 nm and a Mo film having the thickness of 2.8 nm.
Then, another Si film having the thickness of 11 nm was formed.
Thus, a reflective multilayer film 102 was produced. Next, by DC
magnetron sputtering, a chromium nitride (CrN) film having the
thickness of 30 nm as a buffer layer 103 and a TaBN film having a
thickness of 60 nm as an absorber layer 104 were formed on the
reflective multilayer film 102. Thus, the EUV reflective mask blank
100 was obtained.
[0166] Next, by the use of the EUV reflective mask blank 100, an
EUV reflective mask 100a with a 16 Gbit-DRAM pattern having a
design rule of 0.07 .mu.m was produced.
[0167] At first, an EB resist was applied to the EUV reflective
mask blank 100. By EB writing and development, a resist pattern was
formed.
[0168] Next, using the resist pattern as a mask, the absorber layer
104 was dry-etched using chlorine to form an absorber pattern 104a
on the EUV reflective mask blank 100.
[0169] The resist pattern left on the absorber pattern 104a was
removed by hot sulfuric acid. Thereafter, the buffer layer 103 was
dry-etched following the absorber pattern 104a by the use of a
mixed gas of chlorine and oxygen to form a patterned buffer layer
103a. Thus, the EUV reflective mask 100a was obtained.
[0170] Next referring to FIG. 7, description will be made of a
method of transferring a pattern by EUV light onto a semiconductor
substrate with a resist by the use of the EUV reflective mask
100a.
[0171] A pattern transfer apparatus 120 illustrated in the figure
comprises a laser plasma X-ray source 121, the EUV reflective mask
100a, and a reducing optical system 122. The reducing optical
system 122 comprises an X-ray reflection mirror. The pattern
reflected by the EUV reflective mask 100a is reduced to about 1/4.
Since the wavelength band of 13-14 nm is used as the exposure
wavelength, an optical path is preliminarily positioned in
vacuum.
[0172] In the above-mentioned state, EUV light emitted from the
laser plasma X-ray source 121 is incident to the EUV reflective
mask 100a. The light reflected by the EUV reflective mask 100a is
transferred to the semiconductor substrate 110 with a resist
through the reducing optical system 122.
[0173] Specifically, the light incident to the EUV reflective mask
100a is absorbed by the absorber layer 104 and is not reflected in
an area where the absorber pattern 104a is present. On the other
hand, the light incident to a remaining area where the absorber
pattern 104a is not present is reflected by the reflective
multilayer film 102. Thus, a pattern formed by the reflected light
from the EUV reflective mask 100a is transferred through the
reducing optical system 122 to a resist layer on the semiconductor
substrate 110.
[0174] By the use of the EUV reflective mask 100a comprising the
glass substrate 101 obtained in each of Examples 1-4 and
Comparative Example, pattern transfer onto the semiconductor
substrate was carried out by the pattern transfer method mentioned
above. As a result, it was confirmed that the EUV reflective mask
100a in each of Examples 14 had an accuracy of 16 nm or less, as
required in the 0.07 .mu.m design rule. On the other hand, the EUV
reflective mask 100a in Comparative Example did not satisfy the
accuracy of 16 nm or less as required in the 0.07 .mu.m design
rule.
[0175] As described above, according to this invention, it is
possible to provide a method of producing a glass substrate for a
mask blank, which includes a polishing step of polishing a surface
of the glass substrate subjected to local machining in order to
repair a roughened surface resulting from the local machining and
to remove a surface defect resulting from the local machining, and
which is capable of providing a glass substrate high in flatness
and smoothness and free from the surface defect by repairing the
roughened surface of the glass substrate and removing the surface
defect of the glass substrate during the polishing step while
maintaining the flatness of the surface of the glass substrate. It
is also possible to provide a method of producing a mask blank by
the use of the above-mentioned glass substrate.
[0176] Although the present invention has been shown and described
in conjunction with a few preferred embodiments or examples
thereof, it will readily be understood by those skilled in the art
that the present invention is not limited to the foregoing
description but may be changed and modified in various other
manners without departing from the spirit and scope of the present
invention as set forth in the appended claims.
* * * * *