U.S. patent application number 10/284478 was filed with the patent office on 2003-07-03 for glass ceramic product with variably adjustable zero crossing of the cte-t curve.
This patent application is currently assigned to SCHOTT GLAS. Invention is credited to Mitra, Ina.
Application Number | 20030125184 10/284478 |
Document ID | / |
Family ID | 7710602 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030125184 |
Kind Code |
A1 |
Mitra, Ina |
July 3, 2003 |
Glass ceramic product with variably adjustable zero crossing of the
CTE-T curve
Abstract
The present invention relates to a process for producing a
glass-ceramic with a defined zero crossing of the CTE-T curve (CTE:
coefficient of thermal expansion), and glass-ceramic products
produced using the process according to the invention.
Inventors: |
Mitra, Ina;
(Stadecken-Elsheim, DE) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Assignee: |
SCHOTT GLAS
Mainz
DE
|
Family ID: |
7710602 |
Appl. No.: |
10/284478 |
Filed: |
October 31, 2002 |
Current U.S.
Class: |
501/7 ;
65/33.8 |
Current CPC
Class: |
C03C 10/0027 20130101;
G03F 7/70958 20130101; C03B 32/02 20130101 |
Class at
Publication: |
501/7 ;
65/33.8 |
International
Class: |
C03C 010/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2001 |
DE |
10163597.4-45 |
Claims
1. A process for producing a glass-ceramic which has a zero
crossing of the CTE-T curve at a selected temperature
T.sub.A.+-.10.degree. C., comprising the steps of providing a
glass-ceramic or a green body of a glass-ceramic and ceramicizing
in accordance with a ceramicizing program, the determination of
which comprises the step of determining the CTE-T curve.
2. The process as claimed in claim 1, in which a .DELTA.l/l.sub.0-T
set of curves is compiled in order to determine the ceramicizing
program.
3. The process as claimed in claim 1, in which the determination of
the CTE-T curve comprises the following steps: providing a green
body or a pre-ceramicized glass-ceramic, determining
.DELTA.l/l.sub.0-T measurement points for the glass-ceramic in a
temperature range T.sub.A.+-.50.degree. C. matching a polynomial to
the .DELTA.l/l.sub.0-T measurement points, and deriving the
polynomial to obtain a CTE-T curve.
4. The process as claimed in claim 1, in which the zero crossing of
the CTE-T curve is set at T.sub.A.+-.7.degree. C.
5. The process as claimed claim 1, in which the zero crossing of
the CTE-curve is set at T.sub.A.+-.5.degree. C.
6. The process as claimed in one of the preceding claims, in which
a glass-ceramic belonging to the
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 system is provided.
7. The process as claimed in claim 1, in which ceramicizing is
carried out at a temperature at which the mean CTE
[T.sub.A-50;T.sub.A+50] substantially does not change.
8. The process as claimed in claim 1, in which a glass-ceramic with
a mean CTE [(T.sub.A-50.degree. C.);(T.sub.A+50.degree. C.)] of at
most 1.0.times.10.sup.-6/K is provided.
9. The process as claimed in claim 1, in which the glass-ceramic is
ceramicized in at least two ceramicizing steps.
10. The process as claimed in claim 1, in which the glass-ceramic
is ceramicized from the green body in one ceramicizing step.
11. A glass-ceramic product with a zero crossing of the CTE-T curve
at a temperature T.sub.A.+-.10.degree. C., where T.sub.A is a
temperature from 0 to 100.degree. C.
12. The glass-ceramic product as claimed in claim 11, having a zero
crossing of the CTE-T curve at a temperature T.sub.A.+-.7.degree.
C.
13. The glass-ceramic product as claimed in claim 11, having a zero
crossing of the CTE-T curve at a temperature T.sub.A.+-.50.degree.
C.
14. The glass-ceramic product as claimed in claim 11, in which the
glass-ceramic is from the Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2
system.
15. A substrate for EUV lithography, comprising a glass-ceramic
product with a zero crossing of the CTE-T curve at a temperature
T.sub.A.+-.10.degree. C., where T.sub.A is a temperature from 0 to
100.degree. C.
16. The substrate as claimed in claim 15, in which T.sub.A lies in
a range from 0 to 50.degree. C.
17. The substrate as claimed in claim 16, in which T.sub.A lies in
a range from 19 to 31.degree. C.
18. The substrate as claimed in claim 15, having a zero crossing of
the CTE-T curve at a temperature T.sub.A.+-.3.degree. C.
19. The use of the process as claimed in one of claims 1 to 10 for
producing a glass-ceramic product for metrology, astronomy or
microlithography.
Description
[0001] The present invention relates to a process for producing a
glass-ceramic with a defined zero crossing of the CTE-T curve (CTE:
coefficient of thermal expansion), and to glass-ceramic products
produced using the process according to the invention.
[0002] Transparent glass-ceramics with a low coefficient of thermal
expansion have long been known in the prior art and are described,
for example, in DE 19 02 432 C, U.S. Pat. No. 4,851,372 A and U.S.
Pat. No. 5,591,682 A.
[0003] By way of example, DE 19 02432 C relates to a glass-ceramic
with a composition range belonging to the
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 system, in which composition
changes in the crystalline phase during the annealing do not cause
any significant change to the expansion coefficient of a
glass-ceramic. Specimens which have been ceramicized for a period
of 4 to 100 h at temperatures from 750.degree. C. to 870.degree. C.
are given as an exemplary embodiment. The mean CTEs of the
specimens, which were determined for the temperature intervals
[-30.degree. C.;+20.degree. C.] and [+20.degree. C.;+50.degree.
C.], were determined with an accuracy of
.+-.0.02.times.10.sup.-6/K.
[0004] Glass-ceramics are classified by giving the mean
CTE[t.sub.0;t] in a temperature interval from t.sub.0 to t.
[0005] By way of example, Zerodur.RTM. is divided into the
following three expansion classes, the values indicating the mean
CTE [0.degree. C.;50.degree. C.] in a temperature range from
0.degree. C. to 50.degree. C.:
1 Class 0 0 .+-. 0.02 .times. 10.sup.-6/K Class 1 0 .+-. 0.05
.times. 10.sup.-6/K Class 2 0 .+-. 0.10 .times. 10.sup.-6/K
[0006] However, this specification is no longer sufficient for some
modern applications. By way of example, EUV (Extreme UV)
lithography requires substrates for masks and reflectors which have
a mean CTE in the temperature range from, for example, 19 to
25.degree. C. of less than 0.+-.5 ppb/K and a change in the mean
CTE in this temperature range .DELTA. CTE<6 ppb/K.
[0007] Hitherto, it has been assumed that the CTE of glass-ceramics
cannot be set to more accurate ranges than those described
above.
[0008] Therefore, the object of the present invention was to
provide a process which allows glass-ceramic products which satisfy
the requirements listed above to be produced. Furthermore, it is
intended to provide glass-ceramic products which are able to
satisfy the above specifications.
[0009] The above object is achieved by the embodiments described in
the claims.
[0010] In particular, the invention provides a process for
producing a glass-ceramic which has a zero crossing of the CTE-T
curve at a selected temperature T.sub.A.+-.10.degree. C.,
comprising the steps of providing a glass-ceramic or a green body
of a glass-ceramic and ceramicizing in accordance with a
ceramicizing program, the determination of which comprises the step
of determining the CTE-T curve.
[0011] In the prior art, the ceramicizing of a glass-ceramic is
generally carried out at temperatures at which the mean CTE
substantially does not change. Hitherto, it has been assumed that
it is impossible to control the CTE of a glass-ceramic with any
greater accuracy.
[0012] Surprisingly, it has now been found that the very slight
change in the CTE at such ceramicizing temperatures can quite
easily be used to set the CTE-T curve to a defined zero
crossing.
[0013] Thus, if a glass ceramic material is to be produced for an
application temperature T.sub.A, it is possible to adapt the zero
crossing of the CTE-T curve of a given glass ceramic to this
application temperature by using the process of the present
invention.
[0014] In the figures:
[0015] FIG. 1 shows the influence of the ceramicizing temperature
on the mean CTE [-30.degree. C.;70.degree. C.].
[0016] FIG. 2 shows a set of curves for polynomials which have been
laid through measurement points from .DELTA.l/l.sub.0-T
measurements on various ceramicized Zerodur.RTM. specimens.
[0017] FIGS. 3 to 7 in each case in FIG. a show polynomials which
have been laid through measurement points from .DELTA.l/l.sub.0-T
measurements on Zerodur.RTM. specimens, and in FIG. b show CTE-T
curves which have been obtained by deriving the polynomials from
the respective FIG. a. FIG. 8 shows respective diagrams measured at
a sample of Zerodur.RTM. M.
[0018] The process according to the invention is used to produce
glass-ceramics with specific thermal or temperature-dependent
length-change or expansion properties at a specific application
temperature T.sub.A.
[0019] The term glass-ceramic is understood as meaning inorganic,
nonporous materials with a crystalline phase and a vitreous phase,
the matrix, i.e. the continuous phase, generally being a glass
phase.
[0020] Glass-ceramic material is generally produced as follows:
suitable raw materials are melted, refined, homogenized and then
hot-formed into a glass blank or green body. The "green body" of a
glass-ceramic is understood as meaning a vitreous body which has
been melted from a glass-ceramic composition and can be converted
into a glass-ceramic.
[0021] After the green body has been cooled and annealed, a
heat-treatment is carried out, during which the glass is converted
into a glass-ceramic by controlled volume crystallization. During
this heat treatment, in a first conversion step crystallization
nuclei are formed in the glass. These crystallization nuclei can
either originate from the glass composition and generate
spontaneously or certain compounds such as e.g. TiO.sub.2 or
ZrO.sub.2 in case of Zerodur.RTM. can be added to the glass
composition to act as crystallization nuclei. The term
crystallization nuclei or crystal nuclei is understood as meaning
submicroscopic crystalline aggregates of a characteristic size. In
a second conversion step, if appropriate at a slightly higher
temperature, crystals or crystallites grow from the crystal
nuclei.
[0022] The crystalline phase and the glass phase together provide
glass-ceramics with their particular properties.
[0023] According to the invention, it is preferable to provide a
glass-ceramic with a low mean coefficient of thermal expansion or
CTE.
[0024] In this context, a "low mean CTE" is to be understood as
meaning a value of preferably 0.+-.1.0.times.10.sup.-6/K, more
preferably 0.+-.0.6.times.10.sup.-6/K, which is measured in a
temperature range around the application temperature, in particular
a mean CTE [T.sub.A-100.degree. C.;T.sub.A+100.degree. C.], more
preferably mean CTE [T.sub.A-50.degree. C.;T.sub.A+50.degree.
C.].
[0025] According to the invention, by way of example it is possible
to use glass-ceramics with a structure in which the crystal phase
or crystalline phase has a negative linear thermal expansion, while
that of the vitreous phase or the glass phase is positive. The
particular composition of the base glass of the glass-ceramic, a
defined crystal nucleation and defined crystallization conditions
can then result in a material with an extremely low thermal
expansion.
[0026] Since the crystal phase of a glass-ceramic essentially
determines the expansion characteristics, the expansion coefficient
of the material is dependent on the structural state of the
crystalline phase. The crystal phase of the glass-ceramic is in
turn dependent firstly on the composition and secondly on the
ceramicization conditions.
[0027] In a new composition, it will generally first of all be
necessary to study the way in which the expansion properties are
dependent on the ceramicization conditions in the temperature range
of the subsequent application temperature T.sub.A of the
glass-ceramic. In such a case, the process according to the
invention preferably includes the additional step of compiling a
.DELTA.l/l.sub.0-T set of curves, in order to draw up the
ceramicizing program. A .DELTA.l/l.sub.0-T set of curves is
understood as meaning a series of at least two, and preferably
more, .DELTA.l/l.sub.0-T curves. It is also recommended to compile
a set of curves of this type if the zero crossing of the CTE-T
curve is to be set for an application temperature for which a set
of curves of this type is not currently in existence.
[0028] To draw up a .DELTA.l/l.sub.0-T curve or an expansion curve
or to plot the change in length .DELTA.l/l.sub.0 of a specimen
against the temperature, it is possible to measure the
temperature-dependent change in the length of a specimen from the
starting length l.sub.0 at the starting temperature T.sub.0 to the
length l.sub.t at the temperature t. In doing so, it is preferable
to select small temperature intervals of, for example, 5.degree. C.
or 3.degree. C. to determine a measurement point.
[0029] Such measurements may be carried out, for example, by
dilatometry methods, interferometry methods, for example the
Fabry-Perot method, i.e. evaluating the shift in the resonance peak
of a laser beam which is introduced into the material, or by other
suitable methods.
[0030] The method which is selected to determine the
.DELTA.l/l.sub.0-T measurement points preferably have an accuracy
of preferably at least .+-.0.10 ppm, more preferably of .+-.0.05
ppm.
[0031] Finally, as described below, a polynomial is laid through
the measured values obtained by measurement, the .DELTA.l/l.sub.0-T
measured values. The measured values, together with the selected
polynomial, can be entered into a .DELTA.l/l.sub.0-T diagram. If
.DELTA.l/l.sub.0-T measurements from a plurality of specimens are
entered into a diagram, the result is a set of curves as shown in
FIG. 2.
[0032] FIG. 2 shows a set of curves which has been compiled using
the method described above for differently ceramicized Zerodur.RTM.
specimens. In this case, the application temperature T.sub.A of the
desired glass-ceramic is to lie in the region of room temperature.
Zerodur.RTM. specimens have been pre-ceramicized until they reach a
range in which the mean CTE [0; 50] substantially no longer
changes. Then, a plurality of specimens of this pre-ceramicized
glass-ceramic were each ceramicized at the same ceramicizing
temperature but for different ceramicizing times. Then, the change
in length .DELTA.l/l.sub.0 of the specimen in the temperature range
from -50.degree. C. to 100.degree. C. was determined, and the
values determined were entered into a .DELTA.l/l.sub.0-T diagram
and polynomials which match the measured values were laid through
the measured values.
[0033] To better assess the development of the change in length and
to allow better comparison of the polynomials or curves laid
through the measured values in the range from 0 to 50.degree. C.,
the zero crossing of all the curves was set to 0.degree. C. Curve 1
originates from the specimen with the shortest ceramicizing time,
which increases through curves 2, 3 and 4 up to curve 5. It is
clearly apparent that as the ceramicizing time of the specimen
increases, "the curve rotates counterclockwise about the origin"
and that the curve changes in the range between 0 and 40.degree. C.
from a curve which drops downward toward the right, through a curve
which fluctuates around the zero line, to a curve which rises
upward to the right. Furthermore, the maximum of the curve, which
lies at room temperature, increasingly shifts toward higher
temperatures, and the minimum, which lies around 0.degree. C.,
shifts toward lower temperatures.
[0034] Similar results are produced if the glass-ceramic specimens
are ceramicized for a constant ceramicizing time but at different
ceramicizing temperatures.
[0035] Findings of this nature, taken from the set of curves, are
used to adapt the ceramicizing program. Since the maxima and minima
of the .DELTA.l/l.sub.0-T curve correspond to the zero crossings of
the CTE-T curve, it is now possible to derive from the set of
curves a conclusion as to whether a longer ceramicizing time or
higher a ceramicizing temperature or a shorter ceramicizing time or
a lower ceramicizing temperature is required in order to shift a
defined zero crossing.
[0036] It would also be possible for the process according to the
invention to be carried out only by determining a
.DELTA.l/l.sub.0-T curve and determining the minima and maxima
thereof. However, this corresponds to deriving the curve and is
therefore also covered by the present invention.
[0037] In the case of glass-ceramics, hitherto the mean CTE for a
temperature range has been given and can be determined by means of
the following equation (1):
CTE
[t.sub.0;t]=(1/l.sub.0).times.(l.sub.t-l.sub.0)/(t-t.sub.0)=.DELTA.l/(-
l.sub.0.times..DELTA.) (1)
[0038] where t.sub.0 is the starting temperature, t is the
measurement temperature, l.sub.0 is the specimen length at the
starting temperature t.sub.0, l.sub.t is the specimen length at the
temperature t and .DELTA.l is the corrected change in length of the
specimen for temperature change .DELTA.t.
[0039] To determine a mean CTE, the length of a specimen of a
glass-ceramic is measured at the starting temperature t.sub.0, the
specimen is heated to a second temperature t and the length l.sub.t
is measured at this temperature. The mean CTE [t.sub.0;t] for the
temperature range t.sub.0 to t results from the above formula
(1).
[0040] According to the invention, this conventional method of
determining the mean CTE is unsuitable. The observation of the mean
CTE over a temperature interval means that the true CTE at a
defined temperature is distorted. A CTE-T curve which fluctuates
about the zero line may simulate a low mean CTE, whereas the "true
CTE" at the application temperature may lie outside of the
specifications.
[0041] The "true CTE" at a defined temperature is understood as
meaning the value on a CTE-T curve at this temperature.
[0042] Therefore, according to the invention, the CTE is determined
as a function of the temperature, and the coefficient of
(longitudinal) thermal expansion CTE or .alpha. (CTE=coefficient of
thermal expansion) is defined in accordance with the following
formula (2):
CTE(T)=(1/l.sub.0).times.(.differential.l/.differential.T) (2)
[0043] The process according to the invention comprises the step of
determining the CTE-T curve of a glass-ceramic in order to
determine a ceramicizing program.
[0044] By way of example, a process which comprises the following
steps:
[0045] providing a green body or a pre-ceramicized
glass-ceramic,
[0046] determining .DELTA.l/l.sub.0-T measurement points in a
temperature range T.sub.A of, for example, .+-.100.degree. C.,
.+-.50.degree. C. or .+-.25.degree. C.,
[0047] matching a polynomial to the .DELTA.l/l.sub.0-T measurement
points, and
[0048] deriving the polynomial to obtain a CTE-T curve
[0049] can be selected in order to determine the CTE-T curve of a
glass-ceramic.
[0050] This embodiment is described in more detail below.
[0051] First of all, the .DELTA.l/l.sub.0-T measured values of a
pre-ceramicized glass-ceramic are determined, as described
above.
[0052] Then, according to this embodiment of the present invention,
an n-th degree polynomial, for example a 4th degree polynomial, is
laid through the .DELTA.l/l.sub.0-T measurement points in such a
way that the measurement points and polynomial coincide as
accurately as possible. It is preferable to use a 3rd to 6th degree
polynomial. A 3rd degree polynomial is often already sufficiently
variable to allow it to be matched with sufficient accuracy to the
measured values. Higher degree polynomials, e.g. 8.sup.th degree
polynomials, generally have an "oscillation" which is not generally
appropriate for the measured values.
[0053] Furthermore, it is preferable for the polynomial to be
selected appropriately in particular for the temperature range
T.sub.A.+-.100.degree. C., more preferably T.sub.A.+-.50.degree.
C.
[0054] The CTE-T curve can be determined by deriving the selected
polynomial.
[0055] FIGS. 3b to 8b show the curves in which the CTE is plotted
against the temperature (CTE-T curve) and which have been
determined by the above-described derivation of polynomials or
curves laid through .DELTA.l/l.sub.0-T measurement points as shown
in FIGS. 3a to 8a.
[0056] FIG. 3a shows the .DELTA.l/l.sub.0-T measurement points of
curve 3 from FIG. 2 and the polynomial laid through these
measurement points on a different scale. A derivation of the
polynomial results in the CTE-T curve shown in FIG. 3b, which has
zero crossings of the CTE-T curve at approx. 8.degree. C. and
approx. 31.degree. C.
[0057] If the zero crossing of the CTE-T curve is not yet at the
desired temperature, the ceramicizing program is adapted, for
example by the findings which have been obtained from the set of
curves.
[0058] By way of example, a longer ceramicizing time and/or higher
ceramicizing temperatures or a shorter ceramicizing time and/or
lower temperatures are selected. In certain cases, the zero
crossing of the CTE-curve can also be shifted into the desired
range by further post-ceramicizing.
[0059] FIGS. 4a and 5a show .DELTA.l/l.sub.0-T measurement points
or polynomials laid through these measurement points and the
respective derivations of these polynomials in CTE-T curves in
FIGS. 4b and 5b, which compared to FIG. 3b have zero crossings
which have been shifted as a result of a longer ceramicizing time.
It was possible to shift the zero crossing at lower temperatures
from 8.degree. C. (FIG. 3b) through 4.degree. C. (FIG. 4b) to
1.degree. C. (FIG. 5b). It was possible to shift the zero crossing
of the CTE-T curve at approximately 31.degree. C. (FIG. 3b) through
33.degree. C. (FIG. 4b) to 36.degree. C. (FIG. 5b).
[0060] To obtain a glass-ceramic of the same composition with a
zero crossing at room temperature, in this case it is necessary to
start from a specimen which has been pre-ceramicized to a lesser
degree.
[0061] FIGS. 6a/b shows results of measurements carried out on a
glass-ceramic which is particularly suitable for an application
range around room temperature. In this case, a pre-ceramicized
glass-ceramic which had a mean CTE [0;50] of -0.1.times.10.sup.-6K
and a .DELTA.l/l.sub.0-T curve which approximately corresponded to
the curve 1 from FIG. 1, was used. The .DELTA.l/l.sub.0 measurement
points shown in FIG. 6a were obtained after ceramicizing. After
derivation of the polynomial laid through these measurement points
it was confirmed that the zero crossing of the CTE-T curve lies at
approximately 22.5.degree. C. Furthermore, in the range between 19
and 25.degree. C. the deviation of the CTE from the value 0 is
extremely small, in this temperature range lying between
approximately +4 ppb/K at 19.degree. C. and approximately -2 ppb/K
at 25.degree. C.
[0062] FIGS. 7 and 8 show the effects on the CTE-T curve if
different degree polynomials are laid through the .DELTA.l/l.sub.0
measurement points and these polynomials are then converted into
CTE-T curves by derivation. In FIG. 7, the first zero crossing of
the CTE-T curve at approx. 13.degree. C. shifts by approx.
2.degree. C. to approx. 15.degree. C., the second zero crossing of
the CTE-T curve being at approx. 35.degree. C. for both polynomials
selected. As a result, the zero crossing of the CTE-T curve in FIG.
8 is shifted by only 1.degree. C., from 27.degree. C. to 28.degree.
C. Furthermore, FIG. 8 shows that the present invention is not
limited to Zerodur.RTM., since it relates to a sample of
Zerodur.RTM. M.
[0063] Therefore, even when a slightly different polynomial which
is laid through the .DELTA.l/l.sub.0 measurement points is
selected, the accuracy of a zero crossing of the CTE-T curve of
.+-.10.degree. C., preferably .+-.7.degree. C., more preferably
.+-.5.degree. C., particularly preferably .+-.3.degree. C., can
still be maintained without problems.
[0064] The ceramicizing temperature is dependent on the
glass-ceramic composition which is used in each case. Ceramicizing
temperatures of known glass-ceramics are described in the prior art
and are known to the person skilled in the art (cf. for example
"Low Thermal Expansion Glass Ceramics", Schott Series on Glass and
Glass Ceramics, Science, Technology, and Applications, Hans Bach
(Ed.), Springer-Verlag Berlin Heidelberg, 1995).
[0065] According to the invention, it is preferable, where
Zerodur.RTM. is used, to use ceramicizing temperatures from 700 to
830.degree. C., more preferably 750 to 820.degree. C., in the
process according to the invention.
[0066] FIG. 1 shows the change in the mean CTE at relatively high
ceramicizing temperatures. As can be seen from FIG. 1, as the
ceramicizing time increases, the mean CTE rises steeply at
temperatures above 830.degree. C. At these high temperatures, the
solid solutions with a high quartz content are converted into
keatite solid solutions, which also leads to an opacity in the
previously transparent glass-ceramic and to a significant change in
the mean CTE.
[0067] In the case of Zerodur M.RTM., the preferred ceramicizing
temperatures are slightly lower, preferably in a range from 700 to
830.degree. C., more preferably 700 to 780.degree. C.
[0068] Ceramicizing temperatures of 750 to 840.degree. C., in
particular 770 to 800.degree. C., are preferred for the
glass-ceramic Clearzeram.RTM..
[0069] In the process according to the invention, the glass-ceramic
is preferably ceramicized at a temperature at which the mean CTE in
a range from T.sub.A-50.degree. C. to T.sub.A+50.degree. C., i.e.
the mean CTE [(T.sub.A-50.degree. C.);(T.sub.A+50.degree. C.)],
preferably T.sub.A-25.degree. C. to T.sub.A+25.degree. C., i.e. the
mean CTE [(T.sub.A-25.degree. C.);(T.sub.A+25.degree. C.)],
substantially does not change. In this context, the term "a
temperature at which the mean CTE substantially does not change"
means that the mean CTE of a glass-ceramic before annealing changes
by at most 0.1.times.10.sup.-6/K as a result of annealing at this
temperature for a period of at least 50 h, more preferably at least
100 h.
[0070] In general, it is also possible to use lower temperatures,
provided that there is still little change in the mean CTE at these
temperatures.
[0071] In general, longer ceramicizing times are required at lower
temperatures. If, on the other hand, relatively high ceramicizing
temperatures are selected, lying at or above the maximum
temperature mentioned above, the ceramicizing time is relatively
short and in limit conditions the process is difficult to
control.
[0072] The ceramicizing may be carried out over a period of several
hours to several months.
[0073] According to the invention, the ceramicizing of a green body
may take place either in a plurality of stages or in a single
ceramicizing operation. By way of example, a glass-ceramic which
has already been pre-ceramicized can be ceramicized further in a
second step or a plurality of further steps in order to effect the
fine-adjustment of the zero crossing of the CTE-T curve. However,
if the ceramicizing operation is sufficiently well known, it is
also possible that a single ceramicizing step from the green body
to the glass-ceramic with a set zero crossing of the CTE-T curve
may be sufficient.
[0074] According to one embodiment of the present invention, a
transparent glass-ceramic is used. The transparency means that many
properties of a glass-ceramic of this type, in particular, of
course, its optical properties, can be assessed more
successfully.
[0075] It is preferable for a glass-ceramic from the
Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 system to be provided for the
process according to the invention.
[0076] Transparent glass-ceramics with a low coefficient of thermal
expansion are known in this system, and commercially available
products such as Zerodur.RTM., Zerodur M.RTM. (both from SCHOTT
Glas) and Clearzeram.RTM. (Ohara) may be mentioned by way of
example. These glass-ceramics usually contain approximately 50 to
80% of solid solutions with a high quartz content, also known as
.beta. eucryptite solid solutions, as the main crystal phase. This
crystallization product is a metastable phase which, depending on
the crystallization conditions, changes its composition and/or
structure or is converted into a different crystal phase. The solid
solutions with a high quartz content have a thermal expansion which
is very low or even falls as the temperature rises.
[0077] In some glass-ceramics, such as for example Zerodur.RTM., it
was established that the CTE is dependent on the cooling program.
The CTE may change significantly if the cooling from temperatures
above 320.degree. C. is not carried out at the same cooling rate as
the initial cooling rate. Therefore, if such a glass-ceramic is
used it is preferred to control the cooling rate.
[0078] Furthermore, the present invention relates to a
glass-ceramic product, the CTE-T curve of which has a zero crossing
at a temperature T.sub.A.+-.10.degree. C., preferably .+-.7.degree.
C., more preferably .+-.5.degree. C., particularly preferably
.+-.3.degree. C. T.sub.A denotes the application temperature, i.e.
the temperature to which the glass-ceramic is exposed during
use.
[0079] According to the preferred embodiment of the present
invention, the application temperature T.sub.A of the glass-ceramic
lies in a temperature range from 0C to 100.degree. C.
[0080] The glass-ceramic product according to the invention
preferably comprises a glass-ceramic composition and properties as
described above in connection with the description of the
process.
[0081] According to the invention, the change in length of the
glass-ceramic product according to the invention at application
temperatures preferably results substantially only from reversible
length changes.
[0082] Setting the zero crossing of the CTE-T curve to the
application temperature means that the glass-ceramic product
according to the invention, when used with slight deviations from
the application temperature, has only a minimal true CTE, i.e. a
CTE which differs only slightly from 0, and therefore a minimal
length expansion, which only increases slowly at increasing
distance from the application temperature. However, since such
relatively substantial deviations from the application temperature
do not generally occur in processes with controlled process
conditions, at least while the process is being carried out, for
such applications only the region around the application
temperature is of significance, and the inventive effect brings its
full weight to bear.
[0083] If, on the other hand, only the mean CTE of a glass-ceramic
product is set, the CTE curve in the application range may differ
according to the particular batch, and the true CTE may
disadvantageously be relatively far away from the zero line
precisely at the application temperature.
[0084] The process according to the invention is preferably used to
produce glass-ceramic products which are used in microlithography,
metrology, for example for final dimensions, and/or in astronomy,
for example as substrate for a reflector.
[0085] Depending on the use of the glass-ceramic product according
to the invention, the application temperature T.sub.A may lie in
different temperature ranges. By way of example, for applications
in EUV microlithography and metrology, temperatures in the region
of room temperature are typical, whereas in astronomy even
temperatures well below zero are not unusual.
[0086] Furthermore, the present invention relates to a substrate
for, in particular, EUV lithography, which comprises a
glass-ceramic with a low expansion coefficient, the CTE-T curve of
the glass-ceramic having a zero crossing at a temperature T.sub.A.
The temperature T.sub.A preferably lies in a range from 0 to
50.degree. C., more preferably in a range from 10 to 40.degree. C.,
particularly preferably in a range from 19 to 31.degree. C.
Substrates of this type are preferably used as substrates for
reflectors and/or masks or mask blanks for EUV lithography.
EXAMPLE
[0087] A pre-ceramicized Zerodur.RTM. specimen with a mean
CTE[0;50] of -0.1.times.10.sup.-6/K and approximately the
.DELTA.l/l.sub.0 curve 1 from FIG. 2 was ceramicized for 60 h at a
maximum temperature of 810.degree. C. The result was a
glass-ceramic having the expansion properties shown in FIGS. 3a and
3b.
[0088] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0089] The entire disclosure of all applications, patents and
publications, cited above and below, and of corresponding German
Application No. 101 63 597.4-45, filed Dec. 21, 2001 is hereby
incorporated by reference.
[0090] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention
and, without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *