U.S. patent application number 11/498866 was filed with the patent office on 2007-03-15 for thermal reflow of glass and fused silica body.
Invention is credited to Steven Roy Burdette, Polly Wanda Chu, James Gerard Fagan, Thomas William Hobbs, Sumalee Likitvanichkul, Daniel Raymond Sempolinski, Terry Lee Taft, Michael John Walters.
Application Number | 20070059533 11/498866 |
Document ID | / |
Family ID | 37855537 |
Filed Date | 2007-03-15 |
United States Patent
Application |
20070059533 |
Kind Code |
A1 |
Burdette; Steven Roy ; et
al. |
March 15, 2007 |
Thermal reflow of glass and fused silica body
Abstract
Disclosed are synthetic silica glass body with a birefringence
pattern having low fast axis direction randomness factor and glass
reflow process. The glass reflow process comprises steps of:
providing a glass tube having a notch; and thermally reflowing the
glass tube to form a glass plate. The process can be advantageously
used to produce fused silica glass plate without observable striae
when viewed in the direction of optical axis. Also disclosed are
optical members comprising the fused silica glass body and a
process for reflowing glass cylinders.
Inventors: |
Burdette; Steven Roy;
(Horseheads, NY) ; Chu; Polly Wanda; (Painted
Post, NY) ; Fagan; James Gerard; (Painted Post,
NY) ; Hobbs; Thomas William; (Postdam, NY) ;
Likitvanichkul; Sumalee; (Painted Post, NY) ;
Sempolinski; Daniel Raymond; (Painted Post, NY) ;
Taft; Terry Lee; (Big Flats, NY) ; Walters; Michael
John; (Postdam, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
37855537 |
Appl. No.: |
11/498866 |
Filed: |
August 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60716457 |
Sep 12, 2005 |
|
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Current U.S.
Class: |
428/428 ; 65/102;
65/105; 65/64 |
Current CPC
Class: |
C03B 23/0496 20130101;
C03B 23/04 20130101; C03B 23/051 20130101 |
Class at
Publication: |
428/428 ;
065/102; 065/064; 065/105 |
International
Class: |
B32B 17/06 20060101
B32B017/06; C03B 23/00 20060101 C03B023/00; C03B 33/00 20060101
C03B033/00; C03B 31/00 20060101 C03B031/00 |
Claims
1. A synthetic silica body having an optical axis and a
birefringence pattern as measured in a plane perpendicular to the
optical axis in which fast axis directions of the measured
birefringence have a randomness factor of between -0.50 and
0.50.
2. A synthetic silica body according to claim 1, which is a plate
having two essentially flat and essentially parallel major
surfaces, each major surface having an area of at least 1
cm.sup.2.
3. A synthetic silica body according to claim 1 having an internal
transmission at about 193 nm of at least 99.65% cm.sup.-1.
4. A synthetic silica body according to claim 1 having a low level
of LIWFD.
5. A synthetic silica body according to claim 1 having a
birefringence of less than 5 nm/cm, when measured in a plane
perpendicular to the optical axis.
6. A synthetic silica body according to claim 1, having a fictive
temperature of lower than 1150.degree. C.
7. A synthetic silica body according to claim 1 essentially free of
striae when viewed in the direction of the optical axis.
8. A synthetic silica body according to claim 7 essentially free of
striae when viewed in at least one additional direction
perpendicular to the optical axis.
9. An optical element having an optical axis which is made from the
synthetic silica body according to claim 1, wherein the optical
axis of the optical element is parallel to the optical axis of the
synthetic silica body.
10. An optical element according to claim 9 which is a lens element
for use in lithographic device operating in deep or vacuum UV
wavelength region.
11. A process for making glass plate comprising the following
steps: (I) providing a ready-to-flow notched glass tube having (a)
a longitudinal tube center axis, (b) an identified section between
two cross-sections perpendicular to the tube center axis having a
longitudinal section length L.sub.1; and (c) a notch in the
direction of the tube center axis of the ready-to-flow notched
glass tube through the tube wall; and (II) thermally reflowing the
ready-to-flow notched glass tube thus formed in step (I) at an
elevated temperature to form a glass.
12. A process in accordance with claim 11, wherein in step (II),
the notched side and the notch of the glass tube face upwards and
the un-notched side is placed on the surface of a support.
13. A process in accordance with claim 111, wherein the glass tube
has striae when viewed in the direction of the tube center
axis.
14. A process in accordance with claim 13, wherein the glass tube
has essentially circular striae when viewed in the direction of the
tube center axis.
15. A process in accordance with claim 11, wherein the glass is
consolidated fused silica.
16. A process in accordance with claim 15, wherein the silica glass
is produced by outside vapor deposition.
17. A process in accordance with claim 15, wherein the silica glass
is produced by inside vapor deposition.
18. A process in accordance with claim 15, wherein the glass is
high purity consolidated fused silica and step (II) is conducted in
the presence of a purifying atmosphere comprising a cleansing
gas.
19. A process in accordance with claim 18, wherein the cleansing
gas comprised in the purifying atmosphere is selected from F.sub.2,
Cl.sub.2, Br.sub.2, a halogen-containing compound, and compatible
mixtures thereof.
20. A process in accordance with claim 11, wherein in step (I), the
notch is formed to have a center plane passing through the tube
center axis of the ready-to-flow notched glass tube, and the two
sides of the notch beside the center plane are essentially
symmetric.
21. A process in accordance with claim 11, wherein in step (I), the
notch is formed to have two essentially parallel sides.
22. A process in accordance with claim 11, wherein in step (I), the
notch is formed to have an essentially truncated "V" shape
cross-section when cut by a plane perpendicular to the tube center
axis of the ready-to-flow notched glass tube.
23. A process in accordance with claim 11, wherein in step (I), the
provided ready-to-flow notched glass tube has a cross-section that
is part of a ring-shape defined by an essentially circular outer
boundary having a diameter of OD.sub.1 and an essentially circular
inner boundary having a diameter of ID.sub.1 when cut by a plane
perpendicular to the center axis of the tube.
24. A process in accordance with claim 23, wherein in step (I), the
outer boundary and the inner boundary of the ring-shape are
essentially concentric.
25. A process in accordance with claim 23, wherein in step (I), the
outer boundary and the inner boundary of the ring shape are
essentially eccentric.
26. A process in accordance with claim 25, wherein in step (I), the
notch is formed at the location such that the center plane of the
notch is where the thickness of the wall of the ready-to-flow
notched glass tube is essentially the minimal.
27. A process in accordance with claim 11, wherein in step (II),
the identified section of the ready-to-flow notched glass tube is
formed into a glass plate having two essentially flat major
surfaces, a width of a first major flat surface of L.sub.3, a width
of a second major surface of L.sub.4, L.sub.4.gtoreq.L.sub.3, a
length of both major surfaces of L.sub.2, and a thickness between
the two essentially flat major surfaces of T.
28. A process in accordance with claim 27, wherein
L.sub.1.ltoreq.L.sub.2.ltoreq.2L.sub.1.
29. A process in accordance with claim 27, wherein
L.sub.3.gtoreq.0.5 L.sub.4.
30. A process in accordance with claim 23, wherein in step (II),
the identified section of the ready-to-flow notched glass tube is
formed into a glass plate having two essentially flat major
surfaces, a width of a first major flat surface of L.sub.3, a width
of a second major surface of L.sub.4, L.sub.4.gtoreq.L.sub.3, a
length of both major surfaces of L.sub.2, and a thickness between
the two essentially flat major surfaces of T.
31. A process in accordance with claim 30, wherein
.pi.OD.sub.1-L.sub.arc.ltoreq.L.sub.4.ltoreq.2(.pi.OD.sub.1-L.sub.arc),
where L.sub.arc is the outer arc length of the notch.
32. A process in accordance with claim 30, wherein
L.sub.3.gtoreq.1.0.pi.ID.sub.1.
33. A process in accordance with claim 30, wherein 0.10
(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.45(OD.sub.1-ID.sub.1).
34. A process in accordance with claim 11, wherein: in step (II),
the identified section of the ready-to-flow notched glass tube
forms an identified glass plate having two essentially flat major
surfaces, a width of the first major flat surface of L.sub.3, a
width of a second major surface of L.sub.4, L.sub.4.gtoreq.L.sub.3,
a length of both major surfaces of L.sub.2, and a thickness between
the two essentially flat major surfaces of T; and measured in a
plane perpendicular to the optical axis of the identified glass
plate, the identified glass plate upon edge removal and surface
lapping with a surface area of about L.sub.3L.sub.2 has a
birefringence pattern in which fast axis directions have a
randomness factor between -0.50 and 0.50.
35. A process in accordance with claim 11, wherein step (I)
comprises the following steps: (Ia) providing a precursor glass
tube having (a) a longitudinal tube axis, and (b) an identified
section between two cross-sections perpendicular to the tube axis
having a longitudinal section length L.sub.1; and (Ib) forming a
notch in the direction of the tube axis of the precursor glass tube
through the tube wall, whereby the ready-to-flow notched glass tube
is formed.
36. A process in accordance with claim 35, wherein the glass is
silica and step (Ia) comprises the following steps: (Ia1) forming a
silica soot preform by the OVD process on a mandrel; (Ia2)
consolidating the silica soot preform into fused silica glass
without previously removing the mandrel; and (Ia3) removing the
mandrel to form the precursor glass tube.
37. A process in accordance with claim 35, wherein the glass is
silica and step (Ia) comprises the following steps: (Ia1) forming a
silica soot preform by the OVD process on a mandrel; (Ia2) removing
the mandrel from the soot preform; and (Ia3) consolidating the
silica soot preform into fused silica glass, whereby the precursor
glass tube is formed.
38. A process in accordance with claim 35, wherein the glass is
silica; and step (Ia) comprises the following steps: (Ia1) forming
a silica soot preform by the OVD process on a glass tube mandrel;
(Ia2) consolidating the silica soot preform into fused silica glass
without previously removing the mandrel, whereby the precursor
glass tube is formed.
39. A process in accordance with clam 38 comprising the following
step (III) after step (II): (III) removing the surface part of the
glass plate resulting from the glass tube mandrel.
40. A process in accordance with claim 35, wherein the glass is
silica and step (Ia) comprises the following steps: (Ia1) forming a
silica soot preform by the IVD process on the inner surface of an
outside tube; (Ia2) consolidating the silica soot preform into
fused silica glass without previously removing the outside tube;
and (Ia3) removing the outside tube to form the precursor glass
tube.
41. A process in accordance with claim 35, wherein the glass is
silica and step (Ia) comprises the following steps: (Ia1) forming a
silica soot preform by the IVD process on the inner surface of an
outside tube; (Ia2) removing the outside tube from the soot
preform; and (Ia3) consolidating the silica soot preform into fused
silica glass, whereby the precursor glass tube is formed.
42. A process in accordance with claim 35, wherein the glass is
silica and step (Ia) comprises the following steps: (Ia1) forming a
silica soot preform by the IVD process on the inner surface of an
outside tube; and (Ia2) consolidating the silica soot preform into
fused silica glass without previously removing the outside tube,
whereby the precursor glass tube is formed.
43. A process in accordance with claim 42 comprising the following
step (III) after step (II): (III) removing the surface part of the
glass plate resulting from the outside tube.
44. A process in accordance with claim 35, wherein step (Ia)
comprises the following steps: (I0) providing a precursor glass
cylinder having a precursor cylinder axis, a length L.sub.0 in the
direction of the precursor cylinder axis and a precursor cylinder
outer diameter OD.sub.0; (I1) thermally reflowing, with optional
pressing, the precursor glass cylinder; and (I2) optionally
drilling in a direction essentially parallel to the precursor
cylinder axis to form a cylindrical center cavity, whereby the
precursor glass tube is formed to have a longitudinal tube axis, an
outer diameter OD.sub.1 and a length L.sub.1 in the direction of
the tube axis, where the tube axis is essentially parallel to the
precursor cylinder axis of the precursor glass cylinder,
L.sub.1<L.sub.0, and OD.sub.1>OD.sub.0.
45. A process in accordance with claim 44, wherein
0.3L.sub.0.ltoreq.L.sub.1.ltoreq.0.8L.sub.0.
46. A process in accordance with claim 44, wherein: in step (I0),
the precursor glass cylinder comprises an inner glass cane; said
inner glass cane is located approximately at the center of the
precursor glass cylinder and has a diameter of ID.sub.0; and in
step (I2), the inner glass cane is essentially completely
removed.
47. A process in accordance with claim 44, wherein in step (I0),
the precursor glass cylinder comprises a mandrel in essentially the
central portion.
48. A process in accordance with clam 47, wherein the mandrel is
maintained in place during step (I1), and removed after step
(I1).
49. A process in accordance with claim 48, wherein the dimension of
the mandrel is essentially not changed during step (I1).
50. A process in accordance with claim 47, wherein the mandrel is
inserted into a glass tube.
51. A process in accordance with claim 44, wherein in step (I0),
the precursor glass cylinder comprises an outside tube having
composition and/or properties differing from those of the glass
enclosed in the outside tube.
52. A process in accordance with claim 51 comprising the following
step (III) after step (II): (III) removing the surface part of the
glass plate resulting from the outside tube.
53. A process in accordance with claim 46, wherein after step (I2),
the precursor glass tube has an inner cylindrical cavity with a
diameter ID.sub.1, and OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
54. A process in accordance with claim 44, wherein in step (I0),
the provided precursor glass cylinder has an inner cylindrical
cavity the axis of which is parallel to the precursor cylinder
axis, and the inner cylindrical cavity has a diameter of
ID.sub.0.
55. A process in accordance with claim 54, wherein after step (I2),
the ready-to-flow glass tube has an inner cylindrical cavity with a
diameter ID.sub.1, and OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
56. A process in accordance with claim 11, wherein in step (II),
the reflow is done without external mechanical assistance.
57. A process in accordance with claim 14, wherein after step (II),
the striae are re-oriented to be essentially parallel to the two
major surfaces of the resultant glass plate.
58. A process in accordance with claim 14, wherein after step (II),
when viewed in the direction of the optical axis of the resultant
glass plate, the glass plate is essentially free of striae.
59. A process in accordance with claim 58, wherein after step (II),
when viewed in at least one direction perpendicular to the optical
axis of the resultant glass plate, the glass plate is essentially
free of striae.
60. A process in accordance with claim 58, wherein after step (II),
when viewed in the direction of the center tube axis of the
ready-to-flow glass tube, the resultant glass plate is essentially
free of striae.
61. A process in accordance with claim 11, wherein in step (II),
external forces other than gravity of the ready-to-flow notched
glass tube are exerted on the ready-to-flow notched glass tube to
facilitate the reflow of the glass.
62. A process in accordance with claim 61, wherein in step (II),
the external forces other than gravity of the ready-to-flow notched
glass tube are exerted on the two side surfaces of the notch.
63. A process in accordance with claim 61, wherein in step (II),
the external forces are applied by a plunger to the surfaces of the
inner cavity and/or the side surfaces of the notch.
64. A process in accordance with claim 63, wherein in step (II),
the external forces are applied via an articulating mandrel and a
plunger.
65. A process in accordance with claim 11, wherein step (II)
comprises the following steps: (IIa) placing the ready-to-flow
notched glass-tube on an essentially horizontal longitudinal
mandrel, with the mandrel inserting into the inner cavity of the
tube, and the notch placed facing sideways; (IIb) allowing the
lower part of the notched glass tube to roll out to an essentially
vertical position while restricting the upper part from rolling
out, to result in a partially rolled out glass piece; (IIc) placing
the partially rolled out glass piece on a surface; and (IId)
allowing the partially rolled out glass piece to roll-out on the
surface to form an essentially flat glass plate.
66. A process in accordance with claim 65, wherein in step (IId),
an external force is imposed on the partially rolled-out glass
piece to mechanically assist the roll out of the glass piece.
67. A process in accordance with claim 66, wherein in step (IId),
the external force is imposed via a mandrel.
68. A process in accordance with claim 11, wherein in step (II),
the temperature elevation rate is between 50-600.degree. C./minute
between the annealing point of the glass and the highest
temperature.
69. A process in accordance with claim 68, wherein in step (II),
the temperature elevation rate is between 180-600.degree. C./minute
between the annealing point of the glass and the highest
temperature.
70. A process in accordance with claim 11, wherein in step (II),
the temperature is held for a period of between 10 minutes to 5
hours, at a temperature between the annealing point and the
devitrification range of the glass.
71. A process for reforming a glass cylinder comprising the
following steps: (I.0) providing a precursor glass cylinder having
a precursor cylinder axis, a length L.sub.0 in the direction of the
precursor cylinder axis and a precursor cylinder outer diameter
OD.sub.0; (I.1) thermally reflowing, with optional pressing, the
precursor glass cylinder; and (I.2) optionally drilling in a
direction essentially parallel to the precursor cylinder axis to
form a cylindrical center cavity, whereby a reformed glass cylinder
is formed to have a longitudinal reformed cylinder axis, an outer
diameter OD.sub.1 and a length L.sub.1 in the direction of the
reformed cylinder axis, where the reformed cylinder axis is
essentially parallel to the precursor glass cylinder axis of the
precursor glass cylinder, L.sub.1<L.sub.0, and
OD.sub.1>OD.sub.0.
72. A process in accordance with claim 71, wherein
0.3L.sub.0.ltoreq.L.sub.1.ltoreq.0.8L.sub.0.
73. A process in accordance with claim 71, wherein: in step (I.0),
the precursor glass cylinder comprises an inner glass cane; said
inner glass cane is located approximately at the center of the
precursor glass cylinder and has a diameter of ID.sub.0; and in
step (I.2), the inner glass cane is essentially completely
removed.
74. A process in accordance with claim 71, wherein in step (I.0),
the precursor glass cylinder comprises a mandrel in essentially the
central portion.
75. A process in accordance with clam 74, wherein the mandrel is
maintained in place during step (I.2), and removed after step
(I.2).
76. A process in accordance with claim 75, wherein the dimension of
the mandrel is essentially not changed during step (I.2).
77. A process in accordance with claim 74, wherein the mandrel is
inserted into a glass tube.
78. A process in accordance with claim 71, wherein in step (I.0),
the precursor glass cylinder comprises an outside tube having the
same or differing composition and/or properties.
79. A process in accordance with claim 78 comprising the following
step (III) after step (II): (III) removing the surface part of the
glass plate resulting from the outside tube.
80. A process in accordance with claim 73, wherein after step
(I.2), the precursor glass tube has an inner cylindrical cavity
with a diameter ID.sub.1, and
OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
81. A process in accordance with claim 71, wherein in step (I.0),
the provided precursor glass cylinder has an inner cylindrical
cavity the axis of which is essentially parallel to the precursor
glass cylinder axis, and the inner cylindrical cavity has a
diameter of ID.sub.0.
82. A process in accordance with claim 81, wherein after step
(I.2), the ready-to-flow glass tube has an inner cylindrical cavity
with a diameter ID.sub.1, and
OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority to U.S. Provisional
Patent Application Ser. No. 60/716,457 under 35 U.S.C. .sctn. 119,
filed on Sep. 12, 2005 and entitled "Thermal Reflow of Glass and
Fused Silica Articles," the content of which is relied upon and
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to method of making glass
articles by thermal reflow of glass and fused silica articles made
by such method. In particular, the present invention relates to
thermal reflow of glass tubes, as wall as silica glass bodies thus
produced. The present invention is useful, for example, in
producing high purity fused silica plates from consolidated glass
cylinder produced by using the OVD and IVD processes.
BACKGROUND OF THE INVENTION
[0003] Conventional methods for making plate glass include the
float method, pressing and rolling. These methods have been used
successfully in producing soda-lime glasses and other glasses
having a relatively low softening temperature.
[0004] For glass materials with high softening temperatures, such
as fused silica glass, these methods either cannot be used (such as
the float method using tin bath), or must be adapted to accommodate
the high processing temperatures.
[0005] In many applications, high composition and property
homogeneity of the glass used is required. For example, synthetic
silica glass for use as optical members in precision lithography
tools operating in deep and vacuum UV regions, such as at about 248
nm or 193 nm, is required to have a very high purity, very high
compositional homogeneity and property homogeneity in terms of
concentrations of metal ions and distribution thereof, OH
concentrations and distributions thereof, refractive index and
variation thereof, transmission and variation thereof, laser damage
resistance, birefringence and variation thereof, fictive
temperature and variation thereof, and the like.
[0006] The conventional glass forming techniques mentioned above,
such as the glass plate forming technologies, cannot be easily
adapted for use in making high purity fused silica plate meeting
the stringent compositional and property requirements of the
demanding applications mentioned above.
[0007] High purity synthetic silica glass are typically made via
flame hydrolysis methods, such as outside vapor deposition ("OVD"),
inside vapor deposition ("IVD"), vapor axial deposition ("VAD"),
direct-to-glass methods, and the like. The glasses obtained
directly from these processes tend to have compositional and
property inhomogeneity within the bulk. For example, striae caused
by compositional and/or refractive index variations may exist in
glass cylinders obtained by OVD, IVD and VAD processes. Glasses
made from these processes oftentimes need to be reshaped to a plate
or other configuration before further processing into optical
elements. It is highly desired that such striae be removed or
minimized during such reshaping for demanding applications, at
least in the direction of the use axis of the glass. However,
conventional thermal reflow of the glass cylinder or pressing does
not reduce the striae to a desired level for many demanding
applications. Indeed, direct pressing of an OVD, IVD and VAD fused
silica cylinder can lead to striae present and observable in the
direction of the optical axis of the glass.
[0008] As a result, various methods have been proposed in making
fused silica glass having a high level of homogeneity at least in
the direction of the optical axis. These methods include reshaping
processes and/or homogenization processes. However, these currently
existing approaches are limited in their ability to accomplish the
task.
[0009] For example, Berkey and Moore (U.S. Pat. No. 6,689,516 B2)
identified a means to fabricate plates from OVD blanks, however the
process is limited to thickness up to .about.16 mm. This process
requires use of elaborate fixturing to assist in reshaping
(stretching) the glass with secondary thermal processing to provide
plate straightening. Other approaches employ the use of molds (U.S.
Pat. No. 5,443,607); apply force using various fixture designs
(U.S. Pat. Nos. 4,358,306, 5,443,607, United States Patent
Application Publication Nos. 20030115904A1 and 20030115905A1) to
provide means for homogenizing the fused silica glass via
reorienting, twisting or mixing striae and/or compositional
gradients. The use of these methods either are limited in terms of
the maximum mass, relative effectiveness for striae mixing, or are
elaborate methods which may induce inclusions and other defects in
the glass due to extensive or multiple twisting and kneading
operations on the glass surfaces to get the desired mixing
action.
[0010] Moreover, it has been found that fused silica glass plates
produced by the prior art methods tend to have an undesired level
of randomness factor in terms of the fast axis directions of the
birefringence map.
[0011] Therefore, there remains the need for a process for
reshaping and/or homogenizing glass materials, particularly those
having compositional and/or property variations in the bulk,
wherein the impact of such compositional/property variations are
reduced or minimized. There is also a need for high purity fused
silica glass having a low level of randomness in terms of the fast
axis directions in its birefringence map.
[0012] The present invention satisfies these needs.
SUMMARY OF THE INVENTION
[0013] Accordingly, in a first aspect of the present invention, it
is provided a synthetic silica glass body having an optical axis
and a birefringence pattern measured in a plane perpendicular to
the optical axis in which the fast axis directions of the measured
birefringence pixels have a randomness factor of between -0.50 and
0.50, preferably between -0.40 and 0.40, more preferably between
-0.30 and 0.30. In certain embodiments, the randomness factor is
between -0.20 and 0.20. Preferably, when viewed in the direction of
the optical axis of the silica glass body, it is essentially free
of striae. More preferably, when viewed in at least one direction
perpendicular to the optical axis of the silica body, it is
essentially free of striae. In one embodiment of the glass body of
the present invention, it is a plate having two essentially flat
and essentially parallel major surfaces, each major surface having
an area of at least 1 cm.sup.2, preferably at least 4 cm.sup.2,
more preferably at least 16 cm.sup.2. In certain embodiments, each
of the major surfaces has an area of at least 100 cm.sup.2. In
other embodiments, each of the major surfaces has an area of at
least 225 cm.sup.2, such as about 400 cm.sup.2, 625 cm.sup.2, 900
cm.sup.2, or even larger.
[0014] Preferably, the synthetic silica glass body of the present
invention has a refractive index variation .DELTA.n as measured in
a plane perpendicular to the optical axis, wherein
.DELTA.n.ltoreq.10 ppm, preferably .DELTA.n.ltoreq.5 ppm, more
preferably .DELTA.n.ltoreq.2 ppm, most preferably .DELTA.n.ltoreq.1
ppm.
[0015] Preferably, the synthetic silica glass body of the present
invention has an internal transmission at about 193 m of at least
about 99.65% cm.sup.-2, more preferably at least 99.70% cm.sup.-1,
still more preferably at least 99.75% cm.sup.-1, still more
preferably at least 99.80% cm.sup.-1, most preferably at least
99.85% cm.sup.-1.
[0016] Preferably, the synthetic silica body of the present
invention has a low level of LIWFD.
[0017] Preferably, the synthetic silica body of the present
invention has a birefringence of less than 5 nm/cm, preferably less
than 3 nm/cm, more preferably less than 1 nm/cm, most preferably
less than 0.5 nm/cm, when measured in a plane perpendicular to the
optical axis.
[0018] Preferably, the synthetic silica body of the present
invention has a fictive temperature of lower than 1150.degree. C.,
preferably lower than 1050.degree. C., more preferably lower than
1000.degree. C., most preferably lower than about 900.degree.
C.
[0019] A second aspect of the present invention is an optical
element having an optical axis which is made from the synthetic
silica body is described summarily above and in greater detail
below. Preferably, the optical axis of the optical element is
parallel to the optical axis of the synthetic silica body. In a
preferred embodiment, the optical element is a lens element for use
in lithographic device operating in deep or vacuum UV wavelength
regions, such as about 248 nm, 193 nm and shorter.
[0020] A third aspect of the present invention is a process for
making glass plate, comprising the following steps:
[0021] (I) providing a ready-to-flow notched glass tube having (a)
a longitudinal center axis, and (b) an identified section between
two cross-sections perpendicular to the tube center axis having a
longitudinal section length L.sub.1; and (c) a longitudinal notch
in the direction of the tube center axis of the ready-to-flow
notched glass tube through the tube wall; and
[0022] (II) thermally reflowing the ready-to-flow notched glass
tube at an elevated temperature to form a glass plate. Preferably,
the notch of the ready-to-flow notched glass tube has a center
plane essentially parallel to the tube center axis of the
ready-to-flow notched glass tube. Preferably, in step (II), the
glass plate is formed to have two major surfaces and an optical
axis essentially perpendicular to the two major surfaces.
Preferably, in step (II) of the process of the present invention,
the notched side and the notch of the glass tube face upwards and
the notched side is placed on the surface of a support.
[0023] The process of the present invention is particularly
advantageous in forming glass plates from glass tubes having
striae, such as essentially circular striae when viewed in the
direction of the rube center axis.
[0024] In a preferred embodiment of the process of the present
invention, the glass tube is made of consolidated fused silica
material.
[0025] In a preferred embodiment of the process of the present
invention, the glass is high purity consolidated silica and step
(II) is conducted in the presence of a purifying atmosphere
comprising a cleansing gas. Preferably, the cleansing gas comprised
in the purifying atmosphere is selected from F.sub.2, Cl.sub.2,
Br.sub.2 and halogen-containing compounds, and compatible mixtures
thereof. The halogen-containing compounds may be selected from HF,
HCl, HBr and compounds represented by the general formula
C.sub.aS.sub.bX.sub.c, where X is F, Cl, Br and combinations
thereof, a, b and c are nor-negative integers meeting the valency
requirements of the individual elements.
[0026] The present invention process is particularly advantageous
for thermal reflow of consolidated silica glass cylinders made by
using the soot-to-glass processes, such as the OVD, IVD and VAD
processes, especially those glass cylinders having circular striae
when viewed in the direction of its longitudinal axis.
[0027] In a preferred embodiment of the process of the present
invention, in step (I), the notch is formed to have a center plane
passing through the tube center axis of the ready-to-flow notched
glass tube, and the two sides of the notch beside the center plane
are essentially symmetric around the center plane.
[0028] In a preferred embodiment of the process of the present
invention, in step (I), the notch is formed to have an essentially
rectangular cross-section when cut by a plane perpendicular to the
tube center axis of the ready-to-flow notched glass tube.
[0029] In a preferred embodiment of the process of the present
invention, in step (I), the notch is formed to have an essentially
truncated "V" shape cross-section when cut by a plane perpendicular
to the tube center axis of the ready-to-flow notched glass
tube.
[0030] In a preferred embodiment of the process of the present
invention, in step (I), the provided ready-to-flow notched glass
tube has a cross-section that is part of a ring-shape defined by an
essentially circular outer boundary having a diameter of OD.sub.1
and an essentially circular inner boundary having a diameter of
ID.sub.1 when cut by a plane perpendicular of the tube center axis
of the tube. Preferably, the outer circular boundary and the inner
circular boundary are concentric. In one embodiment, however, the
outer circular boundary and the inner circular boundary are
eccentric. In this latter embodiment, it is preferred that in step
(I), the notch is formed at the location such that the center plane
of the notch is located where the thickness of the wall of the
ready-to-flow notched glass tube is essentially the minimal.
[0031] In yet another preferred embodiment of the process of the
present invention, in step (II), the identified section of the
ready-to-flow notched glass tube is formed into a glass plate
having two essentially flat major surfaces, a width of a first
major flat surface of L.sub.3, a width of a second major surface of
L.sub.4, L.sub.4.gtoreq.L.sub.3, a length of both major surfaces of
L.sub.2, and a thickness between the two essentially flat major
surfaces of T. Preferably, L.sub.1.ltoreq.L.sub.2.ltoreq.2L.sub.1,
more preferably L.sub.1.ltoreq.L.sub.2.ltoreq.1.5L.sub.1, still
more preferably L.sub.1.ltoreq.L.sub.2.ltoreq.1.2L.sub.1. In a
preferred embodiment of the process of the present invention,
L.sub.4.gtoreq.L.sub.3.gtoreq.0.8 L.sub.4, preferably
L.sub.4.gtoreq.L.sub.3.gtoreq.0.9L.sub.4, more preferably
L.sub.4.gtoreq.L.sub.3.gtoreq.0.95L.sub.4.
[0032] In a preferred embodiment of the process of the present
invention, in step (II), the identified section of the
ready-to-flow notched glass tube is formed into a glass plate
having two essentially flat major surfaces, a width of a first
major flat surface of L.sub.3, a width of a second major surface of
L.sub.4, L.sub.4.gtoreq.L.sub.3, a length of both major surfaces of
L.sub.2, and a thickness between the two essentially flat major
surfaces of T. It is preferred that
0.5(.pi.OD.sub.1-L.sub.arc).ltoreq.L.sub.4.ltoreq.2(.pi.OD.sub.1-L.sub.ar-
c), preferably
0.5(.pi.OD.sub.1-L.sub.arc).ltoreq.L.sub.4.ltoreq.1.8(.pi.OD.sub.1-L.sub.-
arc), more preferably
0.7(.pi.OD.sub.1-L.sub.arc).ltoreq.L.sub.4.ltoreq.1.5(.pi.OD.sub.1-L.sub.-
arc), where L.sub.arc is the outer arc length of the notch. It is
also preferred that L.sub.3.gtoreq.1.0.pi.ID.sub.1, more preferably
L.sub.3.gtoreq.1.5.pi.ID.sub.1, still more preferably
L.sub.3.gtoreq.2.pi.ID.sub.1, still more preferably
L.sub.3.gtoreq.3.pi.ID.sub.1. Meanwhile, it is also preferred that
0.10(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.45(OD.sub.1ID.sub.1),
preferably
0.10(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.40(OD.sub.1-ID.sub.1),
more preferably
0.10(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.30(OD.sub.1-ID.sub.1).
[0033] Preferably, in the process of the present invention:
[0034] in step (II), the identified section of the ready-to-flow
notched glass tube forms an identified glass plate having two
essentially flat major surfaces, a width of the first major flat
surface of L.sub.3, a width of a second major surface of L.sub.4,
L.sub.4.gtoreq.L.sub.3, a length of both major surfaces of L.sub.2,
and a thickness between the two essentially flat major surfaces of
T; and
[0035] measured in a plane perpendicular to the optical axis of the
identified glass plate, the identified glass plate upon edge
removal and surface lapping with a surface area of about
L.sub.3L.sub.2 has a birefringence pattern in which the fast axis
directions have a randomness factor of between -0.50 and 0.50,
preferably between -0.40 and 0.40, more preferably between -0.30
and 0.30.
[0036] In one embodiment of the process of the present invention,
step (I) comprises the following steps:
[0037] (Ia) providing a precursor glass tube having (a) a
longitudinal tube center axis, and (b) an identified section
between two cross-sections perpendicular to the tube center axis
having a longitudinal section length L.sub.1; and
[0038] (Ib) forming a notch in a direction parallel to the tube
center axis of the precursor glass tube through the tube wall,
whereby the ready-to-flow notched-glass tube is formed.
[0039] In a preferred embodiment, the glass is silica and step (Ia)
comprises the following steps:
[0040] (Ia1) forming a silica soot preform by the OVD process on a
mandrel;
[0041] (Ia2) consolidating the silica soot preform into fused
silica glass without previously removing the mandrel; and
[0042] (Ia3) removing the mandrel to form the precursor glass
tube.
[0043] In another preferred embodiment, the glass is silica and
step (Ia) comprises the following steps:
[0044] (Ia1) forming a silica soot preform by the OVD process on a
mandrel;
[0045] (Ia2) removing the mandrel from the soot preform; and
[0046] (Ia3) consolidating the silica soot preform into fused
silica glass, whereby the precursor glass tube is formed.
[0047] In yet another preferred embodiment, the glass is silica and
step (Ia) comprises the following steps:
[0048] (Ia1) forming a silica soot preform by the OVD process on a
glass tube mandrel; and
[0049] (Ia2) consolidating the silica soot preform into fused
silica glass without previously removing the mandrel, whereby the
precursor glass tube is formed.
[0050] In this preferred embodiment, the process comprises the
following step (III) after step (II):
[0051] (III) removing the surface part of the glass plate resulting
from the glass tube mandrel.
[0052] According to another preferred embodiment, the glass is
silica and step (Ia) comprises the following steps:
[0053] (Ia1) forming a silica soot preform by the IVD process on
the inner surface of an outside tube;
[0054] (Ia2) consolidating the silica soot preform into fused
silica glass without previously removing the outside tube; and
[0055] (Ia3) removing the outside tube to form the precursor glass
tube.
[0056] According to another preferred embodiment, the glass is
silica and step (Ia) comprises the following steps:
[0057] (Ia1) forming a silica soot preform by the IVD process on
the inner surface of an outside tube;
[0058] (Ia2) removing the outside tube from the soot preform;
and
[0059] (Ia3) consolidating the silica soot preform into fused
silica glass, whereby the precursor glass tube is formed.
[0060] According to yet another preferred embodiment, the glass is
silica and step (Ia) comprises the following steps:
[0061] (Ia1) forming a silica soot preform by the IVD process on
the inner surface of an outside tube; and
[0062] (Ia2) consolidating the silica soot preform into fused
silica glass without previously removing the outside tube, whereby
the precursor glass tube is formed.
[0063] In this preferred embodiment, it is further preferred that
it comprises the following step (III) after step (II):
[0064] (III) removing the surface part of the glass plate resulting
from the outside tube.
[0065] It is further preferred that step (Ia) comprises the
following steps:
[0066] (I0) providing a precursor glass cylinder having a precursor
cylinder axis, a length L.sub.0 in the direction of the precursor
cylinder axis and a precursor cylinder outer diameter OD.sub.0;
[0067] (I1) thermally reflowing in the longitudinal direction of
the precursor glass cylinder, with optional pressing; and
[0068] (I2) optionally drilling in a direction essentially parallel
to the precursor cylinder axis to form a cylindrical inner
cavity,
[0069] whereby the precursor glass tube is formed to have a
longitudinal tube axis, an outer diameter OD.sub.1 and a length
L.sub.1 in the direction of the tube axis, where the tube axis is
essentially parallel to the precursor cylinder axis of the
precursor glass cylinder, L.sub.1<L.sub.0, and
OD.sub.1>OD.sub.0. Preferably, the tube axis is the same as the
precursor cylinder axis of the precursor glass cylinder.
[0070] In the preferred embodiment described above,
0.3L.sub.0.ltoreq.L.sub.1.ltoreq.0.8L.sub.0.
[0071] In the preferred embodiment described above, it is preferred
that:
[0072] in step (I0), the precursor glass cylinder comprises an
inner glass cane; said inner glass cane is located approximately at
the center of the precursor glass cylinder and has a diameter of
ID.sub.0; The glass cane may optionally have the same or a
differing composition and/or properties than the glass surrounding
the inner glass cane; and
[0073] in step (I2), the inner glass cane is essentially completely
removed.
[0074] In an embodiment of the process of the present invention, in
step (I0), the precursor glass cylinder comprises a mandrel in
essentially the central portion. The mandrel may be maintained in
place during step (I2), and removed after step (I2). In one
embodiment, the dimension of the mandrel is essentially not changed
during step (I2). In one embodiment, the mandrel is inserted into a
glass tube. In another embodiment, in step (I0), the precursor
glass cylinder comprises an outside tube having differing
composition and/or properties. In this embodiment, it is further
preferred that the process comprises the following step (III) after
step (II):
[0075] (III) removing the surface part of the glass plate resulting
from the outside tube.
[0076] In this preferred embodiment of the process of the present
invention, it is further preferred that after step (I2), the
ready-to-flow notched glass tube has an inner cylindrical cavity
with a diameter ID.sub.1, and
OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
[0077] In an embodiment of the process of the present invention, in
step (I0), the provided precursor glass cylinder has an inner
cylindrical cavity the axis of which is parallel to the precursor
cylinder axis, and the inner cylindrical cavity has a diameter of
ID.sub.0. Preferably, an inner cylindrical cavity with a diameter
ID.sub.1, and OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1. A mandrel may
be inserted into the inner cylindrical cavity of the precursor
glass cylinder during steps (I) and/or (II).
[0078] In one embodiment of the process of the present invention,
in step (II), the reflow is done without external mechanical
assistance. The external force exerted on the glass tube during
reflow other than those by the vessel in which the reflow is
carried out may include only gravity of the glass tube, or
additional force.
[0079] In one embodiment of the process of the present invention,
external forces other than gravity of the glass tube are exerted on
the ready-to-flow notched glass tube to facilitate the reflow of
the glass. Such external forces other than gravity of the glass
tube may be exerted to the two side surfaces of the ready-to-flow
notched glass tube and/or to the surfaces of the inner cavity
thereof. In a preferred embodiment, the external force is applied
by a plunger to the surfaces of the inner cavity and/or the side
surfaces of the notch. In another preferred embodiment, the
external force is applied via an articulating mandrel and a
plunger.
[0080] In another embodiment of the process of the present
invention, step (II) comprises the following steps:
[0081] (IIa) placing the ready-to-flow notched glass-tube-on an
essentially horizontal longitudinal mandrel, with the mandrel
inserting into the inner cavity of the tube, and the notch placed
facing sideways;
[0082] (IIb) allowing the lower part of the notched glass tube to
roll out to an essentially vertical position while restricting the
upper part from rolling out, to result in a partially rolled out
glass piece;
[0083] (IIc) placing the partially rolled out glass piece on a
surface; and
[0084] (IId) allowing the partially rolled out glass piece to
roll-out on the surface to form an essentially flat glass plate.
Preferably, in step (IId), an external force is imposed on the
partially rolled-out glass piece to mechanically assist the
roll-out of the glass piece. Preferably, the external force is
imposed via a mandrel.
[0085] According to one embodiment of the process of the present
invention, the glass tube has essentially circular striae when
viewed in the direction of the tube center axis, and after step
(II), the striae are re-oriented to be essentially parallel to the
two major surfaces of the resultant glass plate.
[0086] According to one embodiment of the process of the present
invention, the glass tube has essentially circular striae when
viewed in the direction of the tube center axis, and after step
(II), when viewed in the direction of the optical axis of the
resultant glass plate, the glass plate is essentially free of
striae. In addition, in certain embodiments, when viewed in at
least one direction perpendicular to the optical axis of the
resultant glass plate, such as in the direction of the center tube
axis of the ready-to-flow notched glass tube, the glass plate is
essentially free of striae.
[0087] In a preferred embodiment of the process of the present
invention, in step (II), the temperature elevation rate is between
50-600.degree. C./minute, preferably between 180-600.degree.
C./minute between the annealing point of the glass and the highest
temperature. Preferably, in step (II), the temperature is held for
a period of between 10 minutes to 5 hours, preferably between 10
minutes and 3 hours, at a temperature between the annealing point
and the devitrification range of the glass.
[0088] A second aspect of the present invention is a process for
reforming glass cylinders, comprising the following steps:
[0089] (I.0) providing a precursor glass cylinder having a
precursor cylinder axis, a length L.sub.0 in the direction of the
precursor cylinder axis and a precursor cylinder outer diameter
OD.sub.0;
[0090] (I.1) thermally reflowing, with optional pressing, the
precursor glass cylinder; and
[0091] (I.2) optionally drilling in a direction essentially
parallel to the precursor cylinder axis to form a cylindrical
center cavity,
[0092] whereby a reformed glass cylinder is formed to have a
longitudinal reformed cylinder axis, an outer diameter OD.sub.1 and
a length L.sub.1 in the direction of the reformed cylinder axis,
where the reformed cylinder axis is essentially parallel to the
precursor glass cylinder axis of the precursor glass cylinder,
L.sub.1<L.sub.0, and OD.sub.1>OD.sub.0.
[0093] Preferably, in the glass cylinder reforming process of the
present invention, 0.3L.sub.0.ltoreq.L.sub.1.ltoreq.0.8L.sub.0.
[0094] Preferably, in the glass cylinder reforming process of the
present invention:
[0095] in step (I.0), the precursor glass cylinder comprises an
inner glass cane; said inner glass cane is located approximately at
the center of the precursor glass cylinder and has a diameter of
ID.sub.0; and
[0096] in step (I.2), the inner glass cane is essentially
completely removed.
[0097] Preferably, in the glass cylinder reforming process of the
present invention, in step (I.0), the precursor glass cylinder
comprises a mandrel in essentially the central portion. Preferably,
the mandrel is maintained in place during step (I.1), and removed
after step (I.1). Preferably, the dimension of the mandrel is
essentially not changed during step (I.1). The mandrel may be
inserted into a glass tube.
[0098] Preferably, in the glass cylinder reforming process of the
present invention, in step (I.0), the precursor glass cylinder
comprises an outside tube having composition and/or properties
similar to or differing from those of the glass enclosed in the
outside tube. Preferably, this preferred embodiment further
comprises the following step (III) after step (II):
[0099] (III) removing the surface part of the glass plate resulting
from the outside tube. Preferably, in the glass cylinder reforming
process of the present invention, after step (I.2), the precursor
glass tube has an inner cylindrical cavity with a diameter
ID.sub.1, and OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
[0100] Preferably, in the glass cylinder reforming process of the
present invention, in step (I.0), the provided precursor glass
cylinder has an inner cylindrical cavity the axis of which is
parallel to the precursor glass cylinder axis, and the inner
cylindrical cavity has a diameter of ID.sub.0. Preferably, the
ready-to-flow glass tube has an inner cylindrical cavity with a
diameter ID.sub.1, and OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1.
[0101] The present invention process can be applied to thin and
thick wall cross-section cylindrical blanks to attain thick
photolithography lens blanks. No elaborate fixturing or apparatus
is required for glass manipulation in the reshaping process of the
present invention. Additionally, no secondary thermal treatments
are required for plate straightening. This invention also allows
for reorientation of concentric striae and radial compositional
gradients, as seen in OVD cylindrical blanks, to favorable
orientation to attain the required optical properties.
[0102] Surprisingly, the high purity fused silica glass of the
present invention, which can be produced by using the reshaping
method of the present invention, has a low level of fast axis
direction randomness factor in its birefringence pattern.
[0103] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from the
description or recognized by practicing the invention as described
in the written description and claims hereof, as well as the
appended drawings.
[0104] It is to be understood that the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework to understanding the nature and character of the
invention as it is claimed.
[0105] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0106] In the accompanying drawings,
[0107] FIG. 1 is a schematic drawing illustrating steps (I0), (I1)
and (I2) of certain embodiments of the process of the present
invention, wherein a precursor glass tube is formed.
[0108] FIG. 2 is a schematic drawing illustrating step (Ib) of
certain embodiments of the process of the present invention,
wherein a notch is formed in the precursor glass tube to produce
the ready-to-flow notched glass tube.
[0109] FIG. 3 is a schematic drawing illustrating step (II) of
certain embodiments of the process of the present invention,
wherein the identified section of the ready-to-flow notched glass
tube is reflowed to form a glass plate having two essentially flat
major surfaces, a width of a first major surface of L.sub.3, a
width of a second major surface of L.sub.4, and a length of both
major surfaces of L.sub.2.
[0110] FIG. 4 is a schematic drawing illustrating the cross-section
of an embodiment of a precursor glass tube cut by a plane
perpendicular to the longitudinal center tube axis before notch
formation having circular striae in the cross-section.
[0111] FIG. 5 is a schematic drawing illustrating the cross-section
of the embodiment of a ready-to-flow notched glass tube
corresponding to the precursor glass tube of FIG. 4 after
notch-formation, wherein the notch has essentially a rectangular
cross-section when cut by a plane perpendicular to the longitudinal
center axis of the glass tube, and the rectangular cross-section
has a width less than the inner diameter of the glass tube.
[0112] FIG. 6 is a schematic drawing illustrating the cross-section
of the embodiment of a ready-to-flow notched glass tube
corresponding to the precursor glass tube of FIG. 4 after
notch-formation, wherein the notch has essentially a rectangular
cross-section when cut by a plane perpendicular to the longitudinal
center axis of the glass tube, and the rectangular cross-section
has a width substantially equal to the inner diameter of the glass
tube.
[0113] FIG. 7 is a schematic drawing illustrating the cross-section
of the embodiment of a ready-to-flow notched glass tube
corresponding to the precursor glass tube of FIG. 4 after
notch-formation, wherein the notch has essentially a truncated
V-shaped (trapezoidal) cross-section when cut by a plane
perpendicular to the longitudinal center axis of the glass tube,
and the shorter base line of the trapezoidal cross-section has a
width less than the inner diameter of the glass tube.
[0114] FIG. 8 is a schematic drawing illustrating the cross-section
of the embodiment of a ready-to-flow notched glass tube
corresponding to the precursor glass tube of FIG. 4 after
notch-formation, wherein the notch has essentially a trapezoidal
cross-section when cut by a plane perpendicular to the longitudinal
center axis of the glass tube similar to that of FIG. 7, but the
shorter base line of the trapezoidal cross-section in this figure
is longer than that in FIG. 7.
[0115] FIG. 9 is a schematic drawing illustrating the cross-section
of a glass body extracted from the glass plate showed in FIG. 3,
cut by a plane perpendicular to the center axis of the
ready-to-flow notched glass tube. The glass body has a plurality of
striae essentially parallel to each other in the cross-section.
[0116] FIG. 10 is a schematic drawing illustrating a device in
which a glass cylinder is longitudinally reflowed under weight to a
cylinder having shorter length at an elevated temperature.
[0117] FIG. 11 is a schematic drawing illustrating the notched
glass tube of FIG. 7 being reflowed with the mechanical assistance
of external forces exerted via stretching arms to the side surfaces
of the notch.
[0118] FIG. 12 is a schematic drawing illustrating a step of the
roll-out of the notched glass tube of FIG. 5, in which half is
reflowed under gravity, and the other half is restricted from
reflow by mechanical assistance of a mandrel and an upper
fixture.
[0119] FIG. 13 is a schematic drawing illustrating the roll-out of
the half reflowed glass piece showed in FIG. 12 on a slope.
[0120] FIG. 14 is a schematic drawing illustrating the roll-out of
a notched tube approximating the configuration in FIG. 7 via the
mechanical assistance of a hinged articulating mandrel and a
plunger.
[0121] FIG. 15 is a birefringence map (showing directions of the
fast axes only) of a piece of fused silica glass having a
tangential pattern of fast axis direction distribution.
[0122] FIG. 16 is a birefringence map (showing directions of the
fast axes only) of a piece of fused silica glass having a radial
pattern of fast axis direction distribution.
[0123] FIG. 17 is a birefringence map (showing directions of the
fast axes only) of a piece of fused silica glass of the present
invention having a mixed pattern of fast axis direction
distribution.
DETAILED DESCRIPTION OF THE INVENTION
[0124] As used herein, the term "variation of refractive index," or
"refractive index variation," or ".DELTA.n," means the maximal
variation of refractive indices measured in a plane perpendicular
to the optical axis of the glass body or glass optical member along
a predetermined direction by using interferometry at about 633 nm
(He--Ne laser) (with tilt and piston taken out, as indicated
infra). As is typically done by one skilled in the art, when
discussing refractive index variation along a certain direction,
tilt and piston are subtracted. Therefore, the refractive index
variation along a certain direction (such as the direction of the x
axis or they axis of the three-dimension orthogonal coordinate
system illustrated in FIG. 3) in the meaning of the present
application does not include tilt or piston. As indicated below,
typically, the optical axis of a glass optical member, a glass
blank, or a piece of glass material, is selected to be
perpendicular to a plane (a cross-section) in which the measured
refractive index inhomogeneity is the smallest, in order to obtain
a glass member having large clear aperture area. FIG. 3 in the
drawings of the present application schematically illustrates a
glass body of the present invention in a xyz orthogonal coordinate
system. The glass body has an optical axis z. The plane xOy,
perpendicular to axis z, intersects the glass body to obtain a
cross-section of the blank. When measuring refractive index
homogeneity, the sample taken has a uniform thickness. Preferably,
when measured across the cross-section, the variation of refractive
index of the glass body of the present invention in the desired
direction (such as the x direction as illustrated in FIG. 3), with
tilt and piston taken out, is less than 10 ppm, preferably less
than 5 ppm, more preferably less than 2 ppm, still more preferably
less than 1 ppm, most preferably less than 0.5 ppm. Desirably, the
variation of refractive index in both the x and y direction,
measured separately, with tilt and piston taken out, is less than
10 ppm, preferably less than 5 ppm, more preferably less than 2
ppm, still more preferably less than 1 ppm, most preferably less
than 0.5 ppm.
[0125] The birefringence of the glass is measured by a polarimeter
at 633 nm (He--Ne laser) in accordance with methods well
established in the art, using, for example, commercially available
instruments specifically designed for measuring birefringence.
[0126] The silica glass as described in the present application may
be high purity fused silica glass, undoped or doped at various
levels.
[0127] As used herein, the term "low LIWFD" means a laser induced
wavefront distortion, measured at 633 nm, of between -1.0 and 1.0
nm/cm when subjected to 10 billion pulses of a laser operating at
approximately 193 nm having at a fluence of approximately 70
.mu.Jcm.sup.-1 pulse.sup.-1 and a pulse length of about 25 ns.
[0128] The process of the present invention is advantageous for
making high purity fused silica glass plate from silica glass tubes
having circular striae in cross-sections thereof perpendicular to
the tube center axis. However, the process of the present invention
is not limited to fused silica glass tubes. It may be adapted for
reflowing other glass tubes as well. Furthermore, glass tubes
without striae in the tube cross-section, or with non-circular
striae may be reflowed using the process of the present invention
as well. That said, the present invention process is particularly
advantageous for thermal reflow of silica glass tube having
circular striae to produce glass plates essentially free of
observable striae at least in a plane perpendicular to its optical
axis.
[0129] Step (I) of the process of the present invention involves
providing a ready-to-flow notched glass tube having (a) a
longitudinal tube center axis, (b) an identified section between
two cross-sections perpendicular to the tube center axis having a
section length L.sub.1; and (c) a notch in the direction of the
tube center axis of the ready-to-flow notched glass tube through
the tube wall. The ready-to-flow notched glass tube may be provided
and produced as such, or produced from a precursor glass tube
without a notch.
[0130] In an embodiment of the latter case, step (I) comprises the
following steps:
[0131] (Ia) providing a precursor glass tube having (a) a
longitudinal tube axis, and (b) an identified section between two
cross-sections perpendicular to the tube axis having a longitudinal
section length L.sub.1; and
[0132] (Ib) forming a notch in the direction of the tube axis of
the precursor glass tube is through the tube wall, whereby the
ready-to-flow notched glass tube is formed.
[0133] The precursor glass tube concerned in the present
application preferably takes the shape of a cylinder having a
longitudinal cavity therein. The ready-to-flow notched glass tube
concerned in the present application preferably takes the shape of
a part of a cylinder having a longitudinal cavity and a
longitudinal notch. The longitudinal tube axis and/or the
longitudinal center axis as discussed in the present application
are typically, but not limited to, the longitudinal axis of the
outer cylindrical surface of the tube, if the tube has a
cylindrical outer surface. The longitudinal cavity within the tube
is preferably cylindrical as well. It is desired that the
cylindrical cavity and the outer cylindrical surface are
concentric. However, as indicated in the general description supra
and the detailed description of the invention infra, they may be
eccentric as well.
[0134] The precursor glass tube and the ready-to-flow notched glass
tube may be produced by conventional tube-making process, such as
drawing, drilling of a glass rod, and the like. Drawing may be
advantageously used for glasses with a relatively low softening
temperature, such as normal soda-lime glasses, borosilicate
glasses, and the like. For glasses having a high softening
temperature, such as fused silica glass, drawing may be
impractical, in which cases drilling may be advantageously
used.
[0135] In the case of silica glass, particularly high purity
synthetic silica glass, and other high purity glass, the glass may
be produced by known vapor deposition processes, such as outside
vapor deposition ("OVD"), inside vapor deposition ("IVD"), vapor
axial deposition ("VAD") from inorganic silicon precursor
compounds, such as silicon halides, and/or organosilicon precursor
compounds, such as octamethylcyclotetrasiloxane ("OMCTS"), and the
like. The glass may be doped or undoped. These processes may be
plasma assisted as is known in the art. OVD, UVD and VAD are
typically soot-to-glass processes in which soot preforms are first
formed by silica soot particles generated by flame hydrolysis of
the precursor compounds, which are in turn consolidated to form
transparent fused silica glass. In addition, as indicated infra,
sol-gel process may be used for making synthetic silica glass as
well.
[0136] In the case of OVD, silica soot preforms are formed on the
outside surface of an axially rotating mandrel, which can be a
solid core rod, a tube, and the like, made of silica glass or other
materials. The soot preforms may be consolidated prior to the
removal of the mandrel or thereafter. If the consolidation is
performed prior to the removal of the mandrel, the consolidated
silica glass generally has a different composition from that of the
mandrel. Thus the mandrel needs to be removed--usually by drilling,
and the like--to result in a glass tube that can be used as the
precursor glass tube in the process of the present invention. If
the consolidation is performed after the removal of the mandrel,
the consolidated glass directly forms a fused silica glass tube.
These glass tubes, either formed from removing mandrel from
consolidated glass, or from consolidating hollow soot preform with
mandrel previously removed, may be used directly as the precursor
glass tube in the process of the present invention. Alternatively,
in certain situations, as described infra it may be desirable to
further process (such as reflow) the as-consolidated glass with
mandrel remaining in the center before drilling to remove the
mandrel, or the glass tube with mandrel removed, or the glass tube
with a mandrel inserted therein, before the glass tubes are used as
the precursor glass tube in the process of the present invention.
Still alternatively, where a glass tube is used as the mandrel, the
soot preform may be consolidated without removing the mandrel. The
thus formed glass tube with the inner mandrel tube can be used as
the precursor glass tube directly, optionally reflowed, cut to form
the notch, then thermally reflowed to form the glass plate
according to the process of the present invention. The glass tube
mandrel thus forms at least a part of the surface part of the glass
plate produced. The glass plate can then be ground to remove that
surface part to result in a glass plate having essentially
homogeneous composition and property.
[0137] In the case of IVD, silica soot preforms are formed on the
inner surface of an axially rotating tube, which can be made of
silica glass or other materials. The soot preforms may be
consolidated prior to the removal of the outside tube or
thereafter. If the consolidation is performed prior to the removal
of the tube, the consolidated silica glass generally has a
different composition from that of the outside tube. Thus the
outside tube can be removed after consolidation, with or without
further processing (such as further thermal reflow such as, e.g.,
the Squash Process described infra) to form the ready-to-flow
silica glass tube of the present invention. Alternatively, the
outside tube is retained after consolidation, during the formation
of the precursor glass tube, during the formation of the notch and
during the thermal reflow of the notched glass tube. Thus after the
thermal reflow process of the present invention, the outside tube
forms the surface part of the glass plate produced. The glass plate
can then be ground to remove the surface part to result in a glass
plate having essentially homogeneous composition and property. If
the consolidation of the soot preform is performed after the
removal of the outside tube, the consolidated glass forms a fused
silica glass tube having an essentially uniform composition. The
thus obtained glass tubes may be used directly as the precursor
glass tube in the process of the present invention to form a notch
thereon. Alternatively, in certain situations, as described infra
it may be desirable to further process (such as reflow by, e.g.,
the Squash Process described below) the as-consolidated tube before
it is used as the ready-to-flow notched glass tube in the process
of the present invention.
[0138] The VAD silica glass may be processed to form the
ready-to-flow silica glass tube according to the processes
described above in connection with OVD and IVD mutatis
mutandis.
[0139] It has been found that for synthetic silica glass made by
the VAD, OVD and IVD processes, due to variations in the process
conditions during soot deposition, variation in composition in
different layers of the soot preform can occur. Such composition
variation, typically largely circular, can lead to striae upon
consolidation. For the purpose of the present invention, "striae"
mean variations in the bulk in the consolidated glass in
composition and/or physical properties (particularly refractive
index) with magnitudes that are detrimental to the performance of
the glass for its intended purpose. In a given area of a given
plane, striae may appear in a repeated pattern at certain
frequency, or may occur sporadically. Striae, especially those in
the form of refractive index variation, are highly undesirable,
particularly if present in a plane perpendicular to the optical
axis of an optical member. As described supra, visible striae in
planes perpendicular to the optical axis may be present, if glass
plates are formed directly from pressing the cylindrical silica
glass tubes having circular striae in its cross-sections
perpendicular to the tube axis.
[0140] The vapor deposition processes mentioned above were
previously used in the art in producing optical waveguide preforms.
Thus preforms typically have a relatively long length and small
diameter. Thus silica glass tubes directly made from these
waveguide preforms (such as by removing the mandrel) tend to have a
relatively long length and small tube wall thickness. As mentioned
supra, these slim tubes made from the as-consolidated silica glass
may be directly used as the precursor glass tubes in the process of
the present application. However, for a plurality of end
applications of the silica glass, the resulting reflowed glass
plate would not have sufficient width or thickness. For example,
the production of optical blanks for regular photomask substrates
and/or lens elements used in modern photolithography operating at
about 248 and 193 nm by using the thermal reflow process of the
present invention requires the ready-to-flow notched glass tube
have a thicker tube wall and larger tube outer diameter.
[0141] The present inventors have devised a method by which slim
fused silica cylinders or tubes of the dimension of optical
waveguide preforms can be formed into silica glass tubes having
higher tube wall thickness and larger tube outer diameter suitable
for the production of optical blanks for use as regular photomask
and optical element in deep UV and vacuum UV photolithography. This
method is referred to as the "Squash Process" hereinafter. In
general terms, the Squash Process comprises, in step (Ia) of the
process of the present invention mentioned above, the following
steps:
[0142] (I0) providing a precursor glass cylinder having a precursor
cylinder axis, a length L.sub.0 in the direction of the precursor
cylinder axis and a precursor cylinder outer diameter OD.sub.0;
[0143] (I1) thermally reflowing, with optional pressing, the
precursor glass cylinder; and
[0144] (I2) optionally drilling in a direction essentially parallel
to the precursor cylinder axis to form a cylindrical inner
cavity,
[0145] whereby the precursor glass tube is formed to have a
longitudinal tube axis, an outer diameter OD.sub.1 and a length
L.sub.1 in the direction of the tube axis. It is preferred the tube
axis is essentially parallel to or the same as the precursor
cylinder axis of the precursor glass cylinder, L.sub.1<L.sub.0,
and OD.sub.1>OD.sub.0.
[0146] In the preferred embodiment of the Squash Process,
0.3L.sub.0.ltoreq.L.sub.1.ltoreq.0.8L.sub.0. Thus as a result of
the Squash Process, the length of the glass cylinder is
reduced.
[0147] In the Squash Process, it is preferred that:
[0148] in step (I0), the precursor glass cylinder comprises an
inner glass cane having the same or differing composition and/or
properties than the glass surrounding the inner glass cane; said
inner glass cane is located approximately at the center of the
precursor glass cylinder and has a diameter of ID.sub.0;
[0149] in step (I2), the inner glass cane is essentially completely
removed.
[0150] In this preferred embodiment of the Squash Process, it is
further preferred that after step (I2), the precursor glass tube
has an inner cylindrical cavity with a diameter ID.sub.1, and
OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1. Thus the wall thickness of
the precursor glass tube is higher than that of the precursor glass
cylinder if the inner glass cane had been removed from the
precursor glass cylinder.
[0151] In another embodiment of the Squash Process, in step (I0),
the provided precursor glass cylinder has an inner cylindrical
cavity the axis of which is parallel to the precursor cylinder
axis, and the inner cylindrical cavity has a diameter of ID.sub.0.
Preferably, the inner cylindrical cavity has a diameter ID.sub.1,
and OD.sub.0-ID.sub.0<OD.sub.1-ID.sub.1. Thus the wall thickness
of the resulting precursor glass tube is higher than that of the
precursor glass cylinder (which is actually a slimmer tube).
[0152] It has been found that in order to produce glass plates
having a larger thickness, usually higher thickness of the tube
wall of the ready-to-flow notched glass tube is desired.
[0153] FIG. 1 schematically illustrates the steps of an embodiment
of the Squash Process according to the present invention. In this
figure, a precursor glass cylinder A2 is provided in step (I0) from
a cylinder A1. The precursor glass cylinder has an outer diameter
OD.sub.0, a precursor longitudinal cylinder axis and a length
L.sub.0 in the direction of the cylinder axis. In addition, the
precursor glass cylinder A2 has an inner cavity or mandrel having
the shape of a cylinder with an inner diameter of ID.sub.0. In this
figure, it is shown that the precursor glass cylinder A2 are
concentric with the mandrel or inner cavity. In practice, they may
be eccentric. In step (I1), the precursor glass cylinder is
reflowed to form a new cylinder having a shorter length L.sub.1 and
a larger outer diameter OD.sub.1. During the reflow process, the
inner cavity or the mandrel is deformed. Thus in step (I2), the
mandrel is essentially completely drilled out, or the inner cavity
is further drilled to redress the deformation thereof, such that a
new inner cylindrical cavity having a diameter of OD.sub.1 is
formed. Alternatively, if A2 has an inner cavity instead of a
mandrel, a deformable or non-deformable mandrel (made of silica,
graphite, or other materials) may be inserted into the inner cavity
during step (I1), such that at the end of (I1), the mandrel can be
removed, with or without the additional step (I2), to result in the
precursor glass tube. The outer cylindrical surface having a
diameter OD.sub.1 and the inner cylindrical surface having a
diameter ID.sub.1 together with the two end cross-sectional
surfaces define the precursor glass tube A as provided in step (I)
of the process of the present invention. As is obvious from the
figure, because L.sub.1<L.sub.0, the wall thickness of the
precursor glass tube A is larger than that of the precursor glass
cylinder A2.
[0154] FIG. 10 schematically illustrates the cross-sectional view
of a furnace device in which step (I1) of the Squash Process can be
implemented. In this figure, the device 1001 comprises an outer
barrel 1013 in which a crucible 1009, and a sleeve 1003 are placed.
The sleeve is connected to the outer barrel 1013 via supporting
arms 1011. The outer barrel 1013 is supported on ringwall 1015.
Both the sleeve and the crucible are made of refractory materials,
such as purified graphite. The crucible has a depth of H, and an
inner diameter of MD. The precursor glass cylinder having an outer
diameter of OD.sub.0 and a length L.sub.0 is first placed on the
bottom plate of the crucible 1009 and partly inside the sleeve
1003. In this figure it is also shown the optional weight 1005
placed atop the precursor glass cylinder. The furnace is then
heated to a reflow temperature, such as above the softening point
of the glass, where the glass reflows under the influence of its
own gravity and pressed by the weight 1005. The sleeve 1003 guides
the reflow of the glass. At the end of the reflow process, a glass
cylinder having an outer diameter OD.sub.1 and a length L.sub.1 is
obtained, where OD.sub.0<OD.sub.1.ltoreq.MD, and
L.sub.0<L.sub.1.ltoreq.H. The thus formed new cylinder typically
has a center axis of the precursor glass cylinder because of the
use of the guiding sleeve 1003. In addition, if the precursor glass
cylinder has essentially circular striae in cross-sections
perpendicular to the center axis of the precursor glass cylinder,
after the thermal reflow, in the new glass cylinder, the striae
will be maintained largely in circular shape in cross-sections
perpendicular to the center axis of the new cylinder.
[0155] By virtue of the Squash Process, glass plate with larger
width and higher thickness may be produced from slim glass tubes
and rods.
[0156] In step (Ib) of the process of the present invention, a
notch is cut in the precursor glass tube in the direction of,
preferably parallel to, the longitudinal tube center axis of the
precursor glass tube through the tube wall.
[0157] FIG. 4 illustrates the cross-section 401 of the precursor
glass tube according to an embodiment of the process of the present
invention. The precursor glass tube in this figure has an outer
diameter ID.sub.1, an inner cavity having a diameter ID.sub.1, and
a plurality of circular striae 403. FIGS. 5, 6, 7 and 8 illustrate
the geometries of various notches that can be formed into the
precursor glass tube of FIG. 4. FIG. 5 shows a notch 501 having an
essentially rectangular cross-section with a width S<ID.sub.1.
Thus the notch and the inner cavity together form a key-hole
geometry. FIG. 6 shows a notch 601 having an essentially
rectangular cross-section with a width S.apprxeq.ID.sub.1. FIG. 7
shows a trapezoidal (truncated V-shaped) notch 701 having a short
base <ID.sub.1. FIG. 8 shows another trapezoidal notch 801 with
a short base approximating ID.sub.1. As indicated by the dotted
curve lines, the cross-sections of all the notches in these figures
have an outer arc (503, 603, 703 and 803 in FIGS. 5, 6, 7 and 8,
respectively) with a length of L.sub.arc. All these notches can be
produced and used in the process of the present invention.
[0158] Preferably, the notch formed in the wall of the
ready-to-flow notched glass tube has a center plane passing through
the longitudinal tube center axis of the ready-to-flow notched
glass tube, and the two sides of the notch beside the center plane
are essentially symmetric. In such scenario, if the outer cylinder
and the center cylindrical cavity of the ready-to-flow notched
glass tube are concentric, the notch may be formed at any location
of the circumference of the tube wall; if the outer cylinder and
the center cylindrical cavity of the ready-to-flow notched glass
tube are eccentric, the notch is preferably formed at the location
where the center plane of the notch passes the maximal or minimal
thickness, preferably the minimal thickness, of the precursor glass
tube. Thus, it is preferable that the two sides of the notched
ready-to-flow notched glass tube about the center plane of the
notch are symmetric.
[0159] FIG. 2 schematically illustrates the notch-formation step
according to the process of the present invention. The notch
illustrated in this figure corresponds to that of FIG. 5, where the
width of the notch is S, and S<ID.sub.1.
[0160] The notch can be formed by various methods and equipment
known in the art, such as by using wire saw, water jet, band saw,
and the like. Preferably, after cutting the notch, the notched
glass tube is thoroughly cleaned before performing step (III) of
the preset invention. Such cleaning may include acid (HCl, HF, and
the like) washing, solvent washing, Cl.sub.2 treatment at high
temperature, and the like, so that contamination introduced by the
cutting process is eliminated or minimized.
[0161] Step (II) of the process of the present invention comprises
thermally reflowing the ready-to-flow notched tube thus provided in
step (I) at an elevated temperature such that the notched tube
reflows to form a glass plate. The formed glass plate preferably
has two major surfaces and an optical axis essentially
perpendicular to the two major surfaces. Generally this step is
conducted with the notched side and the notch facing upwards and
the un-notched side placed on the surface of a support, such as the
bottom plate of a crucible. It is preferred that the notch is
placed in an essentially vertical position.
[0162] This thermal reflow step (III) is advantageously performed
at above the softening point of the glass. For fused silica glass
whose softening temperature is about 1650.degree. C., this step is
usually carried out at above 1700.degree. C., but below
2000.degree. C., preferably below 1900.degree. C.
[0163] If high purity of the glass and a low metal contamination
are required for the glass, which is the case for high purity
synthetic silica glass for use in deep UV and vacuum UV
lithography, it is desired that the step (II) is performed in a
purifying atmosphere comprising a cleansing gas. The cleansing gas
may be, for example, a halogen, a halogen-containing compound and
compatible mixtures thereof. Such halogen-containing compound may
be selected from HX, C.sub.aS.sub.bX.sub.c and compatible mixtures
thereof, where X is selected from F, Cl and Br, a, b and c are
non-negative integers meeting the valency requirements of the
individual elements.
[0164] FIG. 3 schematically illustrates step (II) of an embodiment
of the process according to the present invention. In this figure,
the ready-to-flow notched glass tube B is reflowed and extended
sideways to form a glass plate C. The plate C is placed in a
three-dimensional orthogonal coordinate system xOyz. The resultant
glass plate C has two essentially flat major surfaces: a smaller
upper surface with a width L.sub.3 (shown above plane xOy) and a
larger lower surface with a width L.sub.4 (shown in plane xOy).
Both surfaces have a length of L.sub.2. The axis z is the optical
axis of the glass plate. The larger surface having an area
L.sub.2-L.sub.4 essentially corresponds to the outer cylindrical
surface of the ready-to-flow notched glass tube B, and the smaller
surface having an area L.sub.2L.sub.3 essentially corresponds to
the inner cylindrical surface of the ready-to-flow notched glass
tube B. The thickness T of the resultant glass plate C corresponds
to the wall thickness 0.5(OD.sub.1-ID.sub.1) of the ready-to-flow
notched glass tube B. The plate having dimension of L.sub.2L.sub.3
T represents the useable plate that can be extracted from the
reflowed glass body. Typically, T<0.5 (OD.sub.1-ID.sub.1).
Typically, L.sub.3>.pi.ID.sub.1, which means that the inner
cylindrical cavity surface is stretched during the reflow process.
FIG. 3 shows the edge portion of the reflowed glass plate as having
a part protruding upwards. In practice, the edges may have a
different configuration, depending on the shape and dimension of
the ready-to-flow notched glass tube, the notch, the reflow
temperature and time.
[0165] If the ready-to-flow notched glass tube has essentially
circular striae such as those illustrated in FIG. 4, in step (II),
such striae are normally reoriented, extended and may be twisted
slightly. FIG. 9 schematically illustrates the cross-section of a
useable glass plate extracted from the plate showed in FIG. 3. This
figure shows the remnant striae in the plate when viewed from the
direction of axis y of the plate in FIG. 3, which are essentially
parallel to each other, but extend in directions essentially
perpendicular to the optical axis z. Thus the circular striae of
the ready-to-flow notched glass tube is reflowed and reoriented in
the glass plate. The overall result is, when viewed in the
direction of the optical axis of the resultant glass plate (axis
z), essentially no striae is observable. Surprisingly, it has also
been found that in certain preferred embodiments, even if the
starting glass tube has circular striae as illustrated in FIG. 4,
the resultant glass plate may still be devoid of striae when viewed
at least in one direction perpendicular to the optical axis of the
plate. It is hypothesized by the present inventors that, in
practice, the reoriented striae may not be strictly parallel to
each other. However, because the tube walls are stretched during
the thermal reflow of the present invention, the dimensions of the
striae are reduced. Moreover, the final striae in the glass plate
may curve and twist slightly, leading to mutual cancellation of
distortion caused by each other. Thus, the overall effect is
reduced striae in the resultant plate and improved optical
performance at least in the direction of the optical axis.
[0166] As a result of step (II), usually
L.sub.1.ltoreq.L.sub.2.ltoreq.2L.sub.1, preferably
L.sub.1.ltoreq.L.sub.2.ltoreq.1.5L.sub.1, more preferably
L.sub.1.ltoreq.L.sub.2.ltoreq.1.2L.sub.1. Thus, at the end of the
thermal reflow process of the present invention, the length of the
ready-to-flow notched glass tube has been extended. However, it is
preferred that the length is not significantly extended, especially
where a high thickness of the final glass plate is desired. As
discussed infra where the thermal reflow of the present invention
is performed without additional external mechanical assistance, the
reflow is essentially the result of the influence of the tube
gravity on the notched glass tube. Even if mechanical assistance is
adopted, it is generally preferred that the overall effect of the
mechanical assistance is similar to the effect of the gravity. It
is generally preferred that the thermal reflow temperature is not
overly high such that the viscosity of the glass becomes so low
that the glass flows freely in all directions. Rather, it is
preferred that the reflow temperature is controlled such that the
movement of the notched glass tube is mostly limited to sideways
roll-out during a desired roll-out time period. Hence the
preference that L.sub.1.ltoreq.L.sub.2.ltoreq.1.5L.sub.1, more
preferably L.sub.1.ltoreq.L.sub.2.ltoreq.1.2L.sub.1.
[0167] According to the process of the preset invention, it is
preferred that in the resultant glass plate,
L.sub.3.ltoreq.0.5L.sub.4, preferably L.sub.3.gtoreq.0.8L.sub.4,
more preferably L.sub.3.gtoreq.0.9L.sub.4, still more preferably
L.sub.3.gtoreq.0.95L.sub.4. As indicated supra the plate having
dimension L.sub.2L.sub.3T represents the useable part for the
intended purpose that can be produced from the reflowed glass plate
at the end of step (II) of the process of the present invention
corresponding to the identified section of the ready-to-flow
notched glass tube provided in step (I) as mentioned above. This
would allow a higher yield of the final useable glass. Typically,
the edge portions (illustrated as enclosed by the dotted edge line
and the side line of the useable rectangular plate, 903 in FIG. 9)
of the reflowed glass plate tend to have less compositional and/or
property homogeneity than those in the flat useable part at least
when viewed in the direction of the optical axis of the plate. Thus
they may need to be sacrificed when extracting the useable part
from the reflowed glass.
[0168] In a preferred embodiment of the process of the present
invention, the ready-to-flow notched glass tube and its inner
cavity are both cylindrical and have a diameter of OD.sub.1 and
ID.sub.1, respectively, and in step (II), the identified section of
the ready-to-flow notched glass tube is formed into a glass plate
having two essentially flat major surfaces, a width of a first
major flat surface of L.sub.3, a width of a second major surface of
L.sub.4, L.sub.4.gtoreq.L.sub.3, a length of both major surfaces of
L.sub.2, and a thickness between the two essentially flat major
surfaces of T. It is preferred in this embodiment that
.pi.OD.sub.1-L.sub.arc.ltoreq.L.sub.4.ltoreq.2(.pi.OD.sub.1-L.sub.arc),
preferably
.pi.OD.sub.1-L.sub.arc.ltoreq.L.sub.4.ltoreq.1.8(.pi.OD.sub.1-L.sub.arc)
more preferably
.pi.OD.sub.1-L.sub.arc.ltoreq.L.sub.4.ltoreq.1.5(.pi.OD.sub.1-L.sub.arc),
where L.sub.arc is the length of the outer arc of the cross-section
of the notch formed on the tube wall. Thus the outer cylindrical
surface of the ready-to-flow notched glass tube is preferably
stretched to a desired level during the reflow process. Typically,
in order to obtain a thicker glass plate, it is desired that the
ratio of L.sub.4/(.pi.OD.sub.1-L.sub.arc) is closer to 1. As
mentioned above, during the thermal reflow process of the present
invention, the inner cavity surface is stretched. Usually, the
smaller the ratio of the diameter of the inner cavity to the
diameter of the outer cylinder, ID.sub.1/OD.sub.1, the higher the
extent to which the inner cavity surface is stretched. Nonetheless,
to obtain a glass plate with larger area, it is preferred that
L.sub.3.gtoreq.1.5.pi.ID.sub.1, more preferably
L.sub.3.gtoreq.2.pi.ID.sub.1, still more preferably
L.sub.3.gtoreq.3.pi.ID.sub.1. Further, as discussed supra, because
of the stretch, the thickness of the resultant glass plate in step
(II), T, tends to be smaller than the wall thickness of the
ready-to-flow notched glass tube in the process of the present
invention. Nonetheless, it is preferred that
0.10(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.45(OD.sub.1-ID.sub.1),
more preferably 0.10
(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.40(OD.sub.1-ID.sub.1), and
still more preferably
0.10(OD.sub.1-ID.sub.1).ltoreq.T.ltoreq.0.30(OD.sub.1-ID.sub.1).
[0169] Crucibles made of purified graphite are preferred for step
(II) if fused silica glass is the material of the ready-to-flow
notched glass tube. Purified porous ceramic felt liner may be used
in conjunction. The ceramic felt liner can be fibrous Zirconia felt
(such as ZYF-100 manufactured by Zircar of Florida NY). The use of
the ceramic felt inhibits reaction between the glass and graphite
at high processing temperatures. This material is seen to be
non-reactive and non-wetting with fused silica as well as allowing
for escape of gaseous species through the felt. The latter feature
avoids entrainment of gases into the glass during reflow and
roll-out. Alternative crucible and liner materials can be employed
depending on furnace environment and degree to which it may react
with and be wetted by fused silica glass. Coating of crucible and
liner materials can be employed to minimize the reaction with fused
silica glass as well as potential for contamination of the glass.
Materials identified to date include refractory metals such as
molybdenum and tungsten, ceramics including stabilized zirconia,
zirconium silicate (Zircon), silicon carbide, alumina, and
crucible/liner coating materials such as boron nitride, yttrium
oxide, and carbon. It should be noted that the use of a rigid liner
or substrate material in contact with the glass during roll out
will also improve the resultant blank homogeneity versus that
attained using the compliant zirconia felt liner. Special wall
design may be employed as well.
[0170] Once positioned in the crucible and loaded into the furnace
the glass is heated to reflow/roll-out temperature to induce
softening and stretching of the glass (i.e., roll-out). For silica
glass, the maximum furnace temperature is on the order of
1700.degree. C. to 1900.degree. C. Current experimental results
indicate that it is desired to raise the furnace temperature to
about 1800.degree. C. to 1850.degree. C. with hold times of up to 1
hour for fused silica glass with nominal .beta.-OH concentrations
up to 500 ppm by weight. Experimentation to date has shown heating
rates of 50.degree. C./hour to 600.degree. C./hour above the glass
anneal point to the maximum temperature useful for roll out. Higher
ramp rates (e.g., 180.degree. C./hour to 600.degree. C./hour) are
seen to be more effective in stretching the center portion of the
glass during roll out. At temperatures between the anneal point and
glass devitrification range a hold at temperature can be employed
to yield more uniform temperature through the glass blank. Present
results were attained using graphite resistively heated furnaces
with inert gas atmospheres of helium or argon employed during
thermal treatment. Pressures of .about.1 to 3 psi
(.about.6.89.times.10.sup.3 to 2.07.times.10.sup.4 Pa) above
atmospheric pressure were maintained during the thermal cycle. The
roll-out process is not seen to be restricted the above type of
furnace design or environment. Alternative furnace types and
atmospheres can be employed for this process so long as materials
used for crucible and liner materials are compatible with glass,
furnace materials and environment.
[0171] The roll out process has been seen to be effective for
silica glass blanks with wall thicknesses between 1'' to 2.5'' (2.5
to 6.3 cm) experimentally. Modeling analysis indicates that notched
blanks with both thinner and thicker wall thickness can be rolled
out. The roll process has also been seen to be scalable in
experimental trials for blank lengths between 4'' to 10'' (10 to 25
cm). No limit seen is presently for the maximum length roll out
possible other than furnace size restrictions.
[0172] A study of the impact of roll out on index homogeneity was
conducted for a sample. This sample was a key-holed notched blank
having a key hole geometry illustrated in FIG. 5. Index homogeneity
measurements in a plane perpendicular to the optical axis, post
grind and anneal indicates a .DELTA.n of <3 ppm over a 127 mm
clear aperture at a final ground blank thickness of 31.8 mm.
Additionally, no micro-striae were observed in the same plane.
[0173] In step (II) of the process of the present invention,
external forces other than gravity of the tube may be exerted on
the ready-to-flow notched glass tube, such as on the two side
surfaces of the notch or to the surface of the inner cavity, to
facilitate the reflow of the glass. Such mechanical assistance of
the roll-out or reflow process can expedite the reflow process or
allow the roll-out to be carried out at a lower temperature. FIG.
11 schematically illustrates a ready-to-flow notched glass tube of
FIG. 7 further equipped with stretching arms 1101. During the
roll-out process, external force F is applied to both notch
surfaces via the arms. Alternative methods for mechanically
assisted roll out are possible as well.
[0174] FIGS. 12 and 13 illustrate an alternative approach to
mechanically assisted roll-out. In FIG. 12, a notched glass tube
having essentially the configuration of FIG. 5 is placed on a
mandrel 1203 inserted through the inner cavity of the tube. The
notch on the glass tube wall is placed sideways. A setter 1201 is
placed atop the upper part of the glass tube above the notch. The
entire set-up is heated to an elevated temperature to allow the
lower part of the tube to roll out to an essentially vertical
position to form the essentially straightened part 1205. In FIG.
13, the partially rolled-out piece of glass is placed on a slope
1301, where the un-rolled-out part of the tube 1207 is allowed to
roll out. Thus at the end of the roll-out process, an essentially
flat glass plate 1309 is formed on the slope. In FIG. 13, a mandrel
1305 is also illustrated. An external force F is applied to the
partially rolled-out glass piece via the mandrel. At the end of the
roll-out process, the mandrel 1305 is placed into a receptive notch
1307 formed on the side wall 1303.
[0175] FIG. 14 illustrates another embodiment of mechanically
assisted roll-out process of the present invention. In this
embodiment, a glass tube having essentially the configuration of
FIG. 7 is rolled out via the assistance of an articulating mandrel
1401 and a plunger 1403. During the initial stage of the roll-out,
the glass tube is essentially pressed open via the assistance of
the mandrel and the plunger. Subsequently, the partially rolled-out
tube having a larger opening is allowed to reflow to substantially
flat as described above. The total roll-out time can be shortened
significantly by using mechanical assistance.
[0176] Surprisingly, the thermal reflow process of the present
invention can result in a glass plate with a birefringence map in
which the fast axis directions of the measured pixels have a low
randomness factor. Thus, preferably, the process of the present
invention is characterized by:
[0177] in step (II), the identified section of the ready-to-flow
notched glass tube forms an identified glass plate having two
essentially flat major surfaces, a width of the first major flat
surface of L.sub.3, a width of a second major surface of L.sub.4,
L.sub.4.gtoreq.L.sub.3, a length of both major surfaces of L.sub.2,
and a thickness between the two essentially flat major surfaces of
T; and
[0178] measured in a plane perpendicular to the optical axis of the
identified glass plate, the identified glass plate with a surface
area of about L.sub.3L.sub.2 upon edge removal, surface lapping and
annealing has a birefringence pattern in which fast axis directions
have a randomness factor of between -0.50 and 0.50, preferably
between -0.40 and 0.40, more preferably between -0.30 and 0.30.
[0179] The glass-plate making process of the present invention has,
inter alia the following advantages:
[0180] (1) The process allows cylindrical fused silica blanks of
thick wall cross section to be used for manufacture of parts with
plate and/or disc-like geometry for use in photolithography lens
applications. The process is scalable.
[0181] (2) The process of the present invention can be carried out
without the use of elaborate fixture in step (II). That is, the
roll-out can be effected via the influence of gravity. However, as
described supra, it is not ruled out that the roll-out is carried
out with mechanical assistance.
[0182] (3) The process of the present invention can provide flat
planar parts which do not require secondary thermal processing
steps for substantial leveling or straightening.
[0183] (4) The process of the present invention is capable of
providing plates of width significantly larger than initial blank
diameter.
[0184] (5) Circular striae in terms of compositional variation
(such as OH concentration variation) and/or property variation
(such as refractive index variation) can be realigned such that
they do not interfere with the optical performance of resultant
glass plate without removing the striae via complex homogenization
and mixing of the glass.
[0185] (6) Provides means to process OVD blanks with center core
removed avoiding the issues related to the index inhomogeneity seen
at the core-overclad interface.
[0186] The Squash Process described summarily and in detail above
for reforming a glass cylinder constitutes a second aspect of the
present invention.
[0187] A third aspect of the present invention is thus a synthetic
silica body having an optical axis and a birefringence pattern as
measured in a plane perpendicular to the optical axis in which fast
axis directions have a randomness factor of between -0.50 and 0.50,
preferably between -0.40 and 0.40, more preferably -0.30 and 0.30,
still more preferably between -0.20 and 0.20. Preferably, the
synthetic silica body is a plate having two essentially flat and
essentially parallel major surfaces, each major surface having an
area of at least 1 cm.sup.2, preferably at least 4 cm.sup.2, more
preferably at least 16 cm.sup.2. In certain embodiments, each of
the major surfaces has an area of at least 100 cm.sup.2. In other
embodiments, each of the major surfaces has an area of at least 225
cm.sup.2, such as about 400 cm.sup.2, 625 cm.sup.2, 900 cm.sup.2,
or even larger. Preferably, the synthetic silica body has a
refractive index variation .DELTA.n as measured in a plane
perpendicular to the optical axis, wherein .DELTA.n.ltoreq.10 ppm,
preferably .DELTA.n.ltoreq.5 ppm, more preferably .DELTA.n.ltoreq.1
ppm, most preferably .DELTA.n.ltoreq.0.5 ppm. Preferably, the
synthetic silica glass body of the present invention has an
internal transmission at about 193 nm of about 99.65% cm.sup.-1,
more preferably at least 99.70% cm.sup.-1, still more preferably at
least 99.75% cm.sup.-1, still more preferably at least 99.80%
cm.sup.-1, most preferably at least 99.85% cm.sup.-1. Preferably,
the synthetic silica body of the present invention has a low level
of LIWFD. Preferably, the synthetic silica body of the present
invention has a birefringence of less than 5 nm/cm, preferably less
than 3 nm/cm, more preferably less than 1 nm/cm, most preferably
less than 0.5 nm/cm, when measured in a plane perpendicular to the
optical axis. Preferably, the synthetic silica body of the present
invention has a fictive temperature of lower than 1150.degree. C.,
preferably lower than 1050.degree. C., more preferably lower than
1000.degree. C., most preferably lower than about 900.degree.
C.
[0188] The birefringence of a glass body, such as a fused silica
blank, is usually analyzed by dividing the glass body aperture into
an array of pixel elements and then using a polarimeter to measure
the magnitude and fast axis direction of each pixel element. The
"randomness" of the birefringence fast axis directions can be
assessed either with a simple visual evaluation of the
birefringence map or through equations which average the direction
of the birefringence over the clear aperture of the glass body. It
has been found that, as shown in FIG. 15, certain silica glass
plates exhibit a tangential pattern, meaning that a majority of the
glass volume has fast birefringence axes which are perpendicular to
radial lines. With this birefringence profile, the direction
vectors produce a "tree ring" pattern. As shown in FIG. 16, certain
other silica glass plates exhibit a radial pattern, meaning that a
majority of the glass volume has fast birefringence axes which are
parallel to any radial line. With this birefringence profile the
direction vectors produce a "star burst" pattern. FIG. 17 shows the
fast birefringence axis pattern of a glass produced using the
roll-out reflow process of the present invention. Its direction
vectors show a mixed pattern in which no particular orientation is
dominant.
[0189] As used herein, the randomness factor of fast axis
directions (FR) is calculated from a birefringence map as follows:
FR = [ cos ( .theta. - .gamma. - sin .function. ( .theta. - .gamma.
) ] N ##EQU1## where:
[0190] .theta. is the angle of the pixel on the measured glass body
in spherical coordinates;
[0191] .gamma. is the orientation angle of the fast axis of the
measured birefringence in the pixel;
[0192] N is the number of pixels measured in the aperture; and
[0193] the operator |x| means the absolute value of x.
[0194] The FR as so defined ranges between -1 and 1. When it is
equal to -1, the fast axis profile is tangential ("tree rings"
pattern). When it is equal to +1, the pattern is radial ("star
burst" pattern). A value of zero represents complete randomness of
the direction of the fast axes of the birefringence in those
measured pixels. As an example, applying this formula to the fast
axis maps given above generates the following values:
TABLE-US-00001 FIG. No. FR Visual Birefringence Pattern 15 -0.88
tangential 16 +0.93 radial 17 -0.36 mixed
[0195] A fourth aspect of the present invention is an optical
element having an optical axis which is made from the synthetic
silica body described supra. Preferably, the optical axis of the
optical element is parallel to the optical axis of the synthetic
silica body. In a preferred embodiment, the optical element is a
lens element for use in lithographic device operating in deep or
vacuum UV wavelength regions, such as about 248 nm, 193 nm and
shorter. In another preferred embodiment, the optical element is a
photomask substrate for use in lithographic devices, such as those
operating in deep or vacuum UV wavelength regions, such as at about
248 nm, 193 nm and shorter. In other embodiments, the optical
element of the present invention can be used in laser generators,
sputter targets, mirrors, optical inspecting devices, and the
like.
[0196] It will be apparent to those skilled in the art that various
modifications and alterations can be made to the present invention
without departing from the scope and spirit of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *