Title: Gas discharge laser chamber improvements
Abstract: A method and apparatus if disclosed which may comprise a high power high repetition rate gas discharge laser UV light source which may comprise: a gas discharge chamber comprising an interior wall comprising a vertical wall and an adjacent bottom wall; a gas circulation fan creating a gas flow path adjacent the interior vertical wall and the adjacent bottom wall; an in-chamber dust trap positioned a region of low gas flow, which may be along an interior wall and may comprise at least one meshed screen, e.g., a plurality of meshed screens, which may comprise at least two different gauge meshed screens. The dust trap may extend along the bottom interior wall of the chamber and/or a vertical portion of the interior wall. The dust trap may comprise a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; and the second meshed screen intermediate the first meshed screen and the interior wall. The chamber may comprise a plurality of dust collecting recesses in at least one of the vertical interior wall and the bottom wall of the chamber which may be selected from a group comprising a one-part recess and a multi-part recess, which may comprise two sections angled with respect to each other. The dust trap may comprise a pressure trap positioned between a portion of a main insulator and an interior wall of the chamber. The chamber may comprise a gas circulating fan comprising a cross-flow fan with a fan cutoff that may comprise a vortex control pocket. The chamber may comprise a preionization mechanism comprising a preionization tub containing a ground rod within an elongated opening in the preionization tube that may comprise a compliant member, an automatic preionization shut-off mechanism, a preionization onset control mechanism and/or a focusing element. The chamber may comprise an elongated baffle plate tha
Patent Number: 7,522,650 Issued on 04/21/2009 to Partlo, et al.
| Inventors: |
Partlo; William N. (Poway, CA), Amada; Yoshiho (San Diego, CA), Carmichael; James A. (Valley Center, CA), Dyer; Timothy S. (Oceanside, CA), Gillespie; Walter D. (Poway, CA), Moosman; Bryan G. (San Marcos, CA), Morton; Richard G. (San Diego, CA), Rettig; Curtis L. (Vista, CA), Strate; Brian D. (San Diego, CA), Steiger; Thomas D. (San Diego, CA), Trintchouk; Fedor (San Diego, CA), Ujazdowski; Richard C. (Poway, CA) |
| Assignee: |
Cymer, Inc.
(San Diego,
CA)
|
| Appl. No.:
|
10/815,387 |
| Filed:
|
March 31, 2004 |
| Current U.S. Class: |
372/59 ; 372/55; 372/58 |
| Current International Class: |
H01S 3/22 (20060101) |
| Field of Search: |
372/59,58
|
References Cited [Referenced By]
U.S. Patent Documents
Foreign Patent Documents
| | | | | |
|
| 20218254 | |
Feb., 2003 | |
DE |
|
| 60187073 | |
Sep., 1985 | |
JP |
|
| 60197073 | |
Sep., 1985 | |
JP |
|
|
Primary Examiner: Nguyen; Dung T
Claims
We claim:
1. A high power high repetition rate gas discharge laser UV light source comprising: a gas discharge chamber comprising an interior wall comprising a vertical wall and an adjacent
bottom wall; a gas circulation fan creating a gas flow path adjacent the interior vertical wall and the adjacent bottom wall; an in-chamber dust trap positioned in a region of low gas flow.
2. The apparatus of claim 1 further comprising: the dust trap is positioned along an interior wall.
3. The apparatus of claim 1 further comprising: the dust trap comprises at least one meshed screen.
4. The apparatus of claim 2 further comprising: the dust trap comprises at least one meshed screen.
5. The apparatus of claim 1 further comprising: the dust trap comprises a plurality of meshed screens.
6. The apparatus of claim 2 further comprising: the dust trap comprises a plurality of meshed screens.
7. The apparatus of claim 3 further comprising: the dust trap comprises a plurality of meshed screens.
8. The apparatus of claim 4 further comprising: the dust trap comprises a plurality of meshed screens.
9. The apparatus of claim 1 further comprising: the dust trap comprises at least two different gauge meshed screens.
10. The apparatus of claim 2 further comprising: the dust trap comprises at least two different gauge meshed screens.
11. The apparatus of claim 3 further comprising: the dust trap comprises at least two different gauge meshed screens.
12. The apparatus of claim 4 further comprising: the dust trap comprises at least two different gauge meshed screens.
13. The apparatus of claim 5 further comprising: the dust trap comprises at least two different gauge meshed screens.
14. The apparatus of claim 6 further comprising: the dust trap comprises at least two different gauge meshed screens.
15. The apparatus of claim 7 further comprising: the dust trap comprises at least two different gauge meshed screens.
16. The apparatus of claim 8 further comprising: the dust trap comprises at least two different gauge meshed screens.
17. The apparatus of claim 1 further comprising: the dust trap extends along the bottom interior wall of the chamber.
18. The apparatus of claim 2 further comprising: the dust trap extends along the bottom interior wall of the chamber.
19. The apparatus of claim 3 further comprising: the dust trap extends along the bottom interior wall of the chamber.
20. The apparatus of claim 4 further comprising: the dust trap extends along the bottom interior wall of the chamber.
21. The apparatus of claim 5 further comprising: the dust trap extends along the bottom interior wall of the chamber.
22. The apparatus of claim 6 further comprising: the dust trap extends along the bottom interior wall of the chamber.
23. The apparatus of claim 7 further comprising: the dust trap extends along the bottom interior wall of the chamber.
24. The apparatus of claim 8 further comprising: the dust trap extends along the bottom interior wall of the chamber.
25. The apparatus of claim 9 further comprising: the dust trap extends along the bottom interior wall of the chamber.
26. The apparatus of claim 10 further comprising: the dust trap extends along the bottom interior wall of the chamber.
27. The apparatus of claim 11 further comprising: the dust trap extends along the bottom interior wall of the chamber.
28. The apparatus of claim 12 further comprising: the dust trap extends along the bottom anterior wall of the chamber.
29. The apparatus of claim 13 further comprising: the dust trap extends along the bottom interior wall of the chamber.
30. The apparatus of claim 14 further comprising: the dust trap extends along the bottom interior wall of the chamber.
31. The apparatus of claim 15 further comprising: the dust trap extends along the bottom interior wall of the chamber.
32. The apparatus of claim 16 further comprising: the dust tap extends along the bottom interior wall of the chamber.
33. The apparatus of claim 17 further comprising: the dust trap extends along a vertical portion of the interior wall.
34. The apparatus of claim 18 further comprising: the dust trap extends along a vertical portion of the interior wall.
35. The apparatus of claim 19 further comprising: the dust trap extends along a vertical portion of the interior wall.
36. The apparatus of claim 20 further comprising: the dust trap extends along a vertical portion of the interior wall.
37. The apparatus of claim 21 further comprising: the dust trap extends along a vertical portion of the interior wall.
38. The apparatus of claim 22 further comprising: the dust trap extends along a vertical portion of the interior wall.
39. The apparatus of claim 23 further comprising: the dust trap extends along a vertical portion of the interior wall.
40. The apparatus of claim 24 further comprising: the dust trap extends along a vertical portion of the interior wall.
41. The apparatus of claim 25 further comprising: the dust trap extends along a vertical portion of the interior wall.
42. The apparatus of claim 26 further comprising: the dust trap extends along a vertical portion of the interior wall.
43. The apparatus of claim 27 further comprising: the dust trap extends along a vertical portion of the interior wall.
44. The apparatus of claim 28 further comprising: the dust trap extends along a vertical portion of the interior wall.
45. The apparatus of claim 29 further comprising: the dust trap extends along a vertical portion of the interior wall.
46. The apparatus of claim 30 further comprising: the dust trap extends along a vertical portion of the interior wall.
47. The apparatus of claim 31 further comprising: the dust trap extends along a vertical portion of the interior wall.
48. The apparatus of claim 32 further comprising: the dust trap extends along a vertical portion of the interior wall.
49. The apparatus of claim 1 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gange; the second meshed screen intermediate the first
meshed screen and the interior wall.
50. The apparatus of claim 2 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
51. The apparatus of claim 3 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
52. The apparatus of claim 4 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
53. The apparatus of claim 5 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
54. The apparatus of claim 6 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
55. The apparatus of claim 7 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
56. The apparatus of claim 8 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
57. The apparatus of claim 9 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
58. The apparatus of claim 10 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
59. The apparatus of claim 11 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
60. The apparatus of claim 12 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
61. The apparatus of claim 13 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
62. The apparatus of claim 14 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
63. The apparatus of claim 15 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
64. The apparatus of claim 16 further comprising: the dust trap comprises: a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; the second meshed screen intermediate the first
meshed screen and the interior wall.
Description
FIELD OF THE INVENTION
The present invention relates to gas discharge lasers, e.g., those used in the production of high power high output pulse repetition rate high stability UV light sources, e.g., DUV light, e.g., for the exposure of integrated circuit photoresists
in integrated circuit lithography manufacturing processes.
BACKGROUND OF THE INVENTION
It is known that in gas discharge lasers, e.g., utilizing fluorine in the laser gas, e.g., KrF, ArF and F.sub.2 gas discharge lasers, there is a great propensity for the production of debris, e.g., in the form of metal fluorides, e.g., due to the
interaction of fluorine with metallic components within the laser gas discharge chamber. This can occur particularly during gas discharge, and e.g., with metal materials in the electrodes between which the electric discharge occurs to cause the gas
discharge, a chemical and electrical phenomenon that generates radiation. Such gas discharge lasers may be used particularly at or about a selected desired center-wavelength, e.g., for KrF gas discharge lasers at about 248 nm and for ArF gas discharge
lasers at about 193 nm. This debris can, over time, plate out on such things a optical components of the laser chamber, e.g., chamber windows, which can cause reduced output power for a number of reasons, e.g., undesired reflection of laser light off of
the optic and/or blockage of transmission of laser light through the optic. This can cause, e.g., the need to operate the laser at, e.g., an undesired elevated discharge voltage, e.g., resulting in reduction in laser chamber lifetime. In addition,
under some conditions depending on fluence levels and wavelength, among other things, the plated debris can cause, e.g., localized high absorption on an optical element, resulting in earlier than normal failure of the optical element under, e.g., DUV
light at high fluence. More importantly, however, dust entrained in the flowing gas can cause, e.g., scatter loss. This phenomena akin to "white-out" on a weather context, can cause the photons generated in a gas discharge between the electrodes of the
gas discharge laser to so scatter that they do not reach the mirrors in the laser resonance cavity in sufficient quantities to cause adequate lasing in the excited gas medium during the discharge. This can be significant enough when the dust content is
high enough, that no lasing occurs at all in a given pulse or pulses. This phenomenon increases in frequency and likelihood as the dust accumulates in the chamber over the live of the chamber, e.g., measured in billions of shots, and eventually can lead
to, or at least be a significant contributor to, what is referred to in the industry as old age syndrome ("OAS"), the onset of which generally requires chamber replacement to maintain, e.g., the required output laser pulse energy (dose), and may also be
impacted by such other requirements as pulse to pulse parameter stability requirements being engendered by increasingly demanding requirements, e.g., from lithography tool makers.
It is known in the art of gas discharge laser systems to provide for a debris/dust trap external to the laser gas discharge chamber, with input and output ports from the chamber and returning to the chamber for chamber gas to flow out of the
chamber, through the debris trap, and back into the chamber. For example, applicants' assignee has sold gas discharge laser systems with a so-called metal fluoride trap ("MFT") having a trap inlet and a trap outlet, e.g., near an output window for
generated laser light, to flush the area of that window with cleaned gas, as shown, e.g., in U.S. Pat. No. 5,018,161, entitled COMPACT EXCIMER LASER, issued to Akins et al. on May 21, 1991, e.g., as also shown in e.g., the 7000 series and XLA series
lasers. Such an external trap may be electrostatic, requiring extra cost and power consumption added to the economics of utilizing such laser systems. Also U.S. Pat. No. 5,373,523, entitled EXCIMER LASER APARATUS, issued to Fujimoto on Dec. 13, 1994
shows an external dust trap on the side of a laser gas discharge chamber. U.S. Pat. No. 6,570,899, entitled GAS LASER DEVICE, issued to Yabu et al. on May 27, 2003, based upon an application Ser. No. 09/648,630, filed on Aug. 28, 2000, illustrates
another form of external debris trap.
These types of external traps are also bulky, and tend to fill and become clogged and require replacement, or potentially allow undetected operation over, e.g., several billion laser output pulses ("shots") of operation with "dirty" gas wherein
unwanted OAS events, e.g., zero or low pulse energy lasing are occurring. In addition, they may not be capable of removing debris from the gas circulating within the laser gas discharge chamber fast enough, e.g., at elevated repetition rates of 4K and
above, and especially at, e.g., the 6K and 8K and above levels, to prevent detrimental effects on the discharge due to debris presence in the gas between the electrodes at the time of discharge, which, e.g., is variable from discharge to discharge with
resultant detrimental effects on such things as bandwidth and wavelength stability, beam shape and spatial coherence stability, etc.
As laser light pulse output repetition rates have increased, along with tighter and tighter controls required on such things as center wavelength, bandwidth and dose and the stability of such characteristics of the laser output light pulses have
become necessary for keeping up with the demands of, e.g., integrated circuit lithography light sources, it has become even more important to effectively, efficiently and quickly remove debris, e.g., metal fluoride dust and the like from the circulating
gas.
Debris removal from the gas, e.g., between discharges in the gas discharge chamber between the electrodes can, e.g., require very high fan motor speeds that both add temperature to the chamber gas and vibrations that can interfere with meeting
laser output light parameter requirements and/or interfere with maintaining stability over time and over different duty cycles and over different output light pulse repetition rates. More debris in the gas can increase rates of deposition of the debris
on optical elements, e.g., chamber windows contributing to reductions in performance and/or failures of the optical elements requiring more frequent replacements that are desirable. For these reasons there is a need for an improved debris removal system
and method for very high repetition rate narrow band gas discharge lasers. According to aspects of an embodiment of the present invention applicants have proposed additional, low cost, easily implemented and very reliable means for debris removal from
the gas circulating within the gas circulation flow path in the laser gas discharge chamber. This also, e.g., extends the life of the MFT, whose function is mostly to maintain a supply of cleaned gas to the chamber window regions.
It is also known in the art of gas discharge laser light sources to utilize preionization of the gas discharge region between gas discharge electrodes that produce the chemical and electrical changes in the gas between the electrodes. Each
discharge of electrical energy between the electrodes causes laser light emission and/or amplification, e.g., in an oscillator resonance cavity or an amplification chamber, e.g., amplifying a narrow banded beam output from an oscillator chamber, e.g., in
a master oscillator, power amplifier ("MOPA") configuration. Preionization may be done, e.g., in lasers sold by applicants' assignee with one or more preionization tubes positioned near the gas discharge region. The preionization tubes emit, via, e.g.,
a corona discharge UV and X-ray radiation which creates electrons via photoionization in the gas between the electrodes assisting the onset of the electric discharge in the gas between the electrodes. Applicants have determined that photoionization in
the gas discharge region is less than ideal because most of the electrons are formed in the region of the preionization tube(s) and not in the gas discharge region. Such spatial nonuniformity of the electron distribution is believed by applicants to
contribute to adverse effects on energy stability, especially early in a burst of laser output pulses. Applicants, therefore, according to aspects of an embodiment of the present invention propose certain methods and apparatus for improvement of
preionization. Applicants propose a number of other improvements for the preionization.
In addition, it is known that acoustic effects in the laser can interfere with proper formation of the discharge in the discharge region, e.g., uniformity in the horizontal or vertical axes of the discharge, which can be caused by a variety of
sources of acoustic wavefronts produced in and transmitted through the gas discharge chamber, including, e.g., from the gas circulation fan. Applicants herein propose ways to mitigate or eliminate most of such harmful effects, e.g., on the shape of the
discharge.
Another problem facing the operation of gas discharge lasers, particularly as the requirements, e.g., for lithography light sources call for ever narrower bandwidths, bandwidth stability and center wavelength stability shot to shot, power (dose)
stability shot to shot or at least over a plurality of shots on average, e.g., within a burst of shots, is in chamber acoustic effects on the repeatability shot to shot. Such requirements for repeatability may, e.g., require essentially exactly the same
gas discharge conditions. In addition to the variability of the gas debris content mentioned above there are other possible sources of variability, e.g. two principal sources of these acoustic variations in the gas discharge chamber, shock waves
generated from the spinning of and, to a degree, vibrations within the gas circulating fan and acoustic waves created by a prior discharge reflecting back to the discharge in time with a subsequent discharge, and, usually also aligned to the longitudinal
axis of the discharge so as to substantially effect the entire length of the subsequent discharge.
According to aspects of an embodiment of the present invention applicants have proposed certain methods and apparatus for the mitigation of these detrimental effects on high repetition rate (e.g., 4 KHz+), high power (e.g., 40 W+), narrow banded,
e.g., <about 0.25 pm bandwidth at full width half max ("FWHM") for ArF and <1.2 pm E95% for ArF and less than about 0.35 pm FWHM and 1.5 pm E95% for KrF, laser light sources. Along with the above are requirements, e.g., for tighter dose stability
requirements, e.g., .+-.about 0.3% for dense lines and less than that for isolated lines, wavelength stability, e.g., .+-.0.1 pm 3.sigma., and bandwidth stability, e.g., about .+-.0.05 pm FWHM 3.sigma., all of which will become even more stringent
requirements as feature sizes ("critical dimensions" "CDs") continue to decrease with resulting decreases in k.sub.1, along with increasing throughput and therefore dose requirements.
SUMMARY OF THE INVENTION
A method and apparatus if disclosed which may comprise a high power high repetition rate gas discharge laser UV light source which may comprise: a gas discharge chamber comprising an interior wall comprising a vertical wall and an adjacent bottom
wall; a gas circulation fan creating a gas flow path adjacent the interior vertical wall and the adjacent bottom wall; an in-chamber dust trap positioned a region of low gas flow, which may be along an interior wall and may comprise at least one meshed
screen, e.g., a plurality of meshed screens, which may comprise at least two different gauge meshed screens. The dust trap may extend along the bottom interior wall of the chamber and/or a vertical portion of the interior wall. The dust trap may
comprise a first meshed screen having a first gauge; a second meshed screen having a second gauge smaller than the first gauge; and the second meshed screen intermediate the first meshed screen and the interior wall. The chamber may comprise a plurality
of dust collecting recesses in at least one of the vertical interior wall and the bottom wall of the chamber which may be selected from a group comprising a one-part recess and a multi-part recess, which may comprise two sections angled with respect to
each other. The dust trap may comprise a pressure trap positioned between a portion of a main insulator and an interior wall of the chamber. The chamber may comprise a gas circulating fan comprising a cross-flow fan with a fan cutoff that may comprise
a vortex control pocket. The chamber may comprise a preionization mechanism comprising a preionization tub containing a ground rod within an elongated opening in the preionization tube that may comprise a compliant member, an automatic preionization
shut-off mechanism, a preionization onset control mechanism and/or a focusing element. The chamber may comprise an elongated baffle plate that may comprise a plurality of pyramidal structures including varying numbers of generally pyramidal elements and
oriented in groups of varying numbers of generally pyramidal elements and oriented along and transverse to the longitudinal axis. Acoustic resonances within the chamber may also be reduced by introducing an artificial jitter into the timing of the laser
discharges varying the inter-pulse period randomly or in a repeating pattern from pulse to pulse within a burst.
BRIEF DESCRIPTION OF TIE DRAWINGS
FIG. 1 shows a cross-sectional partly schematic view of a gas discharge laser chamber according to aspects of an embodiment of the present invention, with the cross-section taken transverse to an optical axis of laser output light beams produced
in the gas discharge laser chamber;
FIGS. 2A-D show aspects of an embodiment of the present invention employing in-chamber debris catching and holding;
FIGS. 3A-C show aspects of another embodiment of the present invention employing in chamber debris catching and holding;
FIG. 4 shows schematically an aspect of an embodiment of the present invention regarding in-chamber debris catching and holding;
FIG. 5 shows schematically another aspect of an embodiment of the present inventions regarding in-chamber debris catching and holding;
FIGS. 6A-C show in greater detail aspects of an embodiment of the present invention shown, e.g., in FIGS. 3A-B;
FIG. 7 shows aspects of an embodiment of the present invention for acoustic resonance mitigation;
FIGS. 8-8B and FIGS. 9 and 10 show aspects of an embodiment of the present invention relating to an improved preionization ground rod construction;
FIGS. 11-13 show aspects of embodiments of the present invention relating to improved preionization and in-chamber debris catching and holding;
FIG. 14 shows a baffle plate according to aspects of an embodiment of the present invention; and,
FIG. 15 shows aspects of an embodiment of the present invention relating to improved preionization.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Turning now to FIG. 1 there is shown a gas discharge laser system gas discharge chamber 20 according to an aspect of an embodiment of the present invention. The chamber 20 may be composed, e.g., of a chamber upper half 22 and a chamber lower
half 24, which may, when connected to each other by suitable means, e.g., by bolting, serve to define a chamber interior 26. The chamber upper half 22 and chamber lower half also define, e.g., a chamber interior vertical wall 28 and the chamber lower
half 24 defines a chamber interior horizontal bottom wall 29.
Also contained within the chamber 26 is, e.g., a gas discharge system comprising two elongated opposing electrodes, cathode 30 and anode 32, defining between them a gas discharge region 34, wherein in response to sufficient voltage being present
across the cathode 34 and anode 32 the gas between the electrodes in the also elongated discharge region 34 conducts and certain chemical and electrical reactions take place in the ionized plasma of the discharge that result in the production of
radiation, e.g., at or near a characteristic center wavelength, that is optically directed along the optical axis of an output laser light pulse generally aligned to the longitudinal axis of the electrodes 30, 32.
Also within the chamber 26 may be, e.g., an anode support 36, which may be made, e.g., of a suitable dielectric material, e.g., a ceramic material, and an anode support bar 38, which may be made of a suitable conductive material, e.g., brass.
the anode may be connected to the chamber upper half 22 through a plurality of current returns 46, with the chamber top 22, along with the chamber bottom 24, e.g. kept as a common voltage, e.g., at ground voltage.
The cathode 32 may, e.g., be connected to an electrical discharge high voltage feed through assembly 40, e.g., by a high voltage feed through 42, which passes through a main insulator 44. The main insulator 44 may keep the cathode electrically
isolated from the chamber upper half 22.
Also within the chamber interior 26 may be, e.g., a preionizer 50, e.g., in the vicinity of the cathode 30. The preionizer 50 may include, e.g., a ground rod 52 that may be made of a suitable conductive material, e.g., brass (shown in more
detail in FIGS. 7, 11 and 12), and may, e.g., be substantially enclosed within the interior of a, e.g., hollow cylindrical preionizer tube 54, which may be made of a suitable dielectric material, e.g., a ceramic. The preionizer 50 may have, e.g., a
cylindrical centerline axis that is generally parallel to the longitudinal centerline axis of the cathode 30 and anode 32 and, therefore, also the discharge region 34. The preionizer 50 may, in operation, e.g., function based upon the difference in
voltage between the cathode 34 and the ground rod 52. In addition the preionizer 50 may operate due to the difference in electrical potential between a conducting shim 56, shown in more detail in FIGS. 11 and 12, which occupies the space between the
preionizer 50 and the main insulator 44 along substantially the length of the preionizer 50, or at least along substantially the entire length of the cathode 30, and which is in electrical contact with the cathode 30.
Also within the gas discharge chamber 26 may be, e.g., a gas circulation system comprising, e.g., a gas circulation fan 60, which may be, e.g., a generally cylindrical crossflow fan 60. The fan 60 serves to move gas within the chamber interior
26, generally in a circular fashion as seen, e.g., in the cross-sectional view of, e.g., FIG. 1, in order to, e.g., remove from the discharge region 34 between successive gas discharges the gas that contains, e.g., ionized particles and debris and
depleted F.sub.2, to replenish the discharge region, e.g., with fresh gas, e.g., containing F.sub.2 before the next successive gas discharge. The gas circulation system may also include a plurality of heat exchangers 70 in the generally circular gas
flow path to remove heat added to the gas, e.g., by the discharges and the operation of the fan 60.
The gas circulation system may also have, e.g., a plurality of curved baffles 80 and a flow directing vane 62, which may serve, e.g., to shape the generally circular gas flow path out of the discharge region 34 toward the heat exchangers 70 and
ultimately the intake of the fan 60 and from the output of the fan 60 to the discharge region 34, respectively.
The chamber 26 may also have a metal fluoride trap 90 as is known in the art, although an aspect of an embodiment of the present invention is to replace the MFT where possible.
Also contained in the chamber interior 26, e.g., along the horizontal bottom interior wall 29 of the bottom 24 of the chamber 20 may be, e.g., a dust trap 100. It will be understood that by dust is meant the various forms of debris, e.g., mostly
metal fluoride material, that circulates with the gas circulation and appears to the naked eye to be dust-like or lint-like.
The dust trap 100 according to an aspect of an embodiment of the present invention may be illustrated by reference to FIGS. 2A-D as an example. As shown in FIGS. 2A-D, the dust trap 100 may be comprised of at least one meshed element, e.g., a
wire mesh 102 having a first gauge, e.g., [.sub.----] and a first pitch, e.g., [.sub.----] and may be comprised of vertical mesh members 102a and horizontal mesh members 102b, e.g., made of wire of, e.g., a suitable metallic material, e.g., brass. Each
wire may have, e.g. a wire gauge, i.e., diameter, e.g., [.sub.----]. The meshed horizontal mesh members 102b and vertical mesh members 102a may form mesh openings, illustrated according to an aspect of an embodiment of the present invention to be
rectilinear, but to be understood to be of any suitable meshed shape, e.g., triangular and/or of square or rectangular shape may be employed.
The dust trap 100 may also comprise, e.g., a second meshed element 104, which may be similar to the first mesh element 102, but may comprise a second gauge and pitch smaller than the first, e.g., [.sub.----] and [.sub.----]. It will also be
understood that the second meshed element 104 may also have horizontal mesh wires 104b and vertical mesh wires 104a having a respective wire gauge or gauges, and forming mesh openings 108 of any possible suitable shape. The first and second meshed
elements 102, 104 may have differing shaped mesh openings. It is also possible according to an aspect of an embodiment of the present invention to have more than two different meshed elements varying, e.g., in gauge, pitch and opening shape and wire
gauge size.
Turning now to FIGS. 2C and D there is illustrated, by way of example, two of the many possible arrangements of meshed elements, e.g., 102,104 according to aspects of an embodiment of the present invention. In FIG. 2C there is shown an
embodiment in which meshed elements 102, 104, e.g., of generally the same thickness, but with differing, e.g., gauge, wire gauge size, opening shape or the like, are layered one on top of the other, e.g., in a four layer embodiment. It will also be
understood that each of the meshed elements 102, 104 as illustrated in FIG. 2C may be different in one or more of the respects noted above, or that a pattern may be repeated, e.g., every third layer, e.g., in a six layer embodiment. That is to say that
the dust trap 100 may comprise, e.g., six layers, with the first three differing from each other e.g., narrowing in opening size and the next three being a repeat of the first three. It will also be under stood that the dust trap 100 could comprise,
e.g., only the first three such layers just described.
Turning to FIG. 2D there is shown aspects of an alternative embodiment according to the present invention. In FIG. 2D there is illustrated, by way of example, an embodiment in which meshed elements, e.g., 102, 104 of differing thicknesses in the
vertical direction (as that direction is shown for illustrative purposes in FIG. 2D) are layered to form the dust trap 100'. It will be understood, that, as with FIG. 2C, the various mesh elements 102,104, may be the same or differ from each other in
aspects noted above apart from the varying thickness. As illustrated in FIG. 2D, the dust trap 100' comprises two alternating thin (104) and thick (102) mesh elements placed on top of an additional thick meshed element 102. It will also be understood
that, for convenience, the thick meshed elements shown in FIG. 2D are illustrated to be the larger meshed elements 102 illustrated in FIG. 2A, but this need not be the case, and one or more of the thicker meshed elements illustrated in FIG. 2D may be of
smaller gauge and/or pitch and/or wire gauge size.
FIGS. 3A-C illustrate an aspect of an embodiment of the present invention wherein dust catcher/collector elements, e.g., 130, 132, 134 may, e.g., be formed in the interior wall portions, e.g., 28, 29 of the chamber 20. According to an aspect of
an embodiment of the present invention, the elements 130, 134 may comprise a first portion 130a, 134a and a second portion 130b, 134b extending in a direction angled from the respective first portion 130a, 134b and the elements 132 may have only a single
portion formed, e.g., in the respective portion, e.g., walls 28, 29 of the chamber 20.
FIG. 3C illustrates schematically the fact that the dust catchers/collectors, e.g., 130, 132 may be randomly sized, positioned and oriented along, e.g., a wall of the chamber 20, e.g., wall 28, e.g., with the flow from top to bottom of FIG. 3C as
illustrated. For example, two piece catcher/collectors 130 may be spaced at differing heights along, e.g., wall 28 (which it will be understood could just as well be, e.g., bottom 29) and having differing widths horizontally (as that direction is
illustrated in FIG. 3C to be horizontal on the page) and can even be oriented, e.g., diagonally. It will also be understood that it is possible according to aspects of an embodiment of the present invention for the illustrated wall, e.g., wall 28 to be
rotated along with the illustrated catcher/collectors 130, 132 so that flow is still from top to bottom of the illustration, as so rotated, but is incident on the openings 130, 132 at a ninety degree (or perhaps, e.g., even a forty-five degree) angle to
that illustrated in FIG. 3C. Also illustrated is the possibility of intermingling the two piece catcher/collectors, e.g., 130 with the single piece ones 132.
FIG. 3B shows an aspect of an embodiment of the present invention in which, e.g., a dust trap 100 may be combined with dust catcher and collector elements, e.g., 132, 134. FIG. 1 illustrates an aspect of an embodiment of the present invention in
which a dust trap 100 rests on (or is suitably attached to), e.g., the horizontally extending (as that direction is shown in FIG. 1 for illustrative purposes) floor 29 of the chamber and FIG. 5 illustrates an aspect of an embodiment of the present
invention where a dust trap 100'' is positioned along both the bottom 28 and vertical side wall 28 of the chamber 20.
In operation the above-described dust collectors/traps have the effect of minimizing the equilibrium particle density of the debris in the gas flow without significantly increasing the impedance within the chamber interior 26 to gas circulation.
This is important because at higher gas discharge repetition rates, e.g., beyond 4 KHz the blower power and heat budget becomes a critical performance issue as well as blower wear, e.g., bearing wear at higher speeds and eventual requirements for blower
replacement due to excessive vibration. The collectors/traps may also be assisted in their operation by the changing gas flow patterns as the fan is cycled through on and off periods and as flow recommences in the on cycle. In operation with the
collectors, e.g., 130, 132, 134 and or traps, e.g., 100, 100', 100'' in and/or along the walls of the chamber, essentially in the boundary flow region, the boundary layer flow, along what would be the otherwise smooth-wall, may, e.g., be disrupted in
such a way that particles of debris (so-called dust) are accelerated into the openings, e.g., 130, 132, 143 in the wall and/or meshed elements in the dust traps, e.g., 100, 100' and 100'', and once inside essentially cannot return to the gas flow inside
the chamber interior 26. For example once inside the meshed elements of the traps 100, 100', 100'', e.g., having entered a mesh opening in the outermost layer, the particle may, e.g., be slowed by eddy currents, or otherwise, and then is further slowed
in each succeeding meshed layer such that the particles then remain within the meshed structure. Applicants also suspect that flow can be created, e.g., around the wire in the mesh, e.g., orthogonal to the flow direction, which also accelerates
particles in the direction of the next succeeding internal mesh layer.
This so-called boundary layer effect is taken advantage of by applicants according to aspects of an embodiment of the present invention. With regard to the dust trap 100,100', 100'', as illustrated very schematically and not to proportion in
FIG. 4, the meshed element, e.g., 102 openings, e.g., 106 presented to the flow 120, e.g., in the boundary layer present an opportunity for debris to precipitate out of the flow as illustrated at 124, pass through, if appropriate, openings, e.g., 108 in
an underlying meshed element layer, and then perhaps be collected in a pocked formed, e.g., by openings, e.g., 106 in a still further underlying meshed element layer, e.g., 102. Applicants believe that the flow in the boundary layer near the boundaries
of the gas circulation path is slow enough to encourage debris to enter the exposed openings, e.g., 106 as illustrated in FIG. 4 and fast enough to also encourage, e.g., eddy currents 122 to form, which further encourages the precipitation 124.
FIGS. 6A-C illustrate the operation dust traps 100-100'' according to aspects of an embodiment of the present invention. It will be understood by those skilled in the art of fluid dynamics that the gas circulation in the generally circular
circulation path referenced above is of relatively high velocities, e.g., measured at points in the cross-section of the circulation path on the order of tens of meters per second, determined, e.g., by the amount of gas flow necessary through the
discharge region to adequately and properly clear the discharge region between shots. More toward the boundaries of the container in which the gas is circulating, e.g., the wall sections 28, 29, due, e.g., to friction of the moving gas along these
boundary portions, the gas flow slows down until its profile at the surface of the boundary element, e.g., interior wall 28 or 29 to zero or almost zero.
FIG. 5 illustrates an aspect of an embodiment of the present invention wherein a dust trap 100'' lies along a wall, e.g., wall 28 and the bottom 29 of the chamber interior 26.
Applicants have found that after relatively short operating times the floating debris being circulated with the laser gas in the gas circulation path essentially all collects in the dust collector, e.g., a dust collector 100 on the floor of the
chamber 20 and does not thereafter significantly migrate or get shocked back into the gas circulation flow path. In the past, applicants believe that dust tended to precipitate to the bottom of the chamber, but that various acoustic and other shock wave
and vibrational disturbances then periodically entrained the dust, or significant quantities of it, back into the gas circulation path. A dust trap, e.g., 100, 100', 100'' according to aspects of an embodiment of the present invention have been found by
applicants to virtually eliminate the reintrainment of dust collected in the dust traps, e.g., 100, 100' and 100'' according to aspects of embodiments of the present invention.
Similarly, the catcher and collector elements, 130, 132 and 134 are believed to work alone or in conjunction with a dust trap 100, 100', 100'', e.g., as shown in FIG. 3B in a similar fashion, e.g., as illustrated in FIGS. 6A-C, wherein flow and
currents 150, e.g., eddy currents from the boundary layer flow 154 and into and within the openings 130, 132 and 134 in the walls, e.g., 28, 29 of the chamber 20, cause the deposition of debris 156 and, especially in the shapes such as 130 and 134,
discourage or prevent its subsequent removal. It will also be understood that the openings 132 (basically grooves in the chamber interior walls, e.g., 28 and 19) may be of other shapes in cross section and the openings 130, 132 and 134 may be of
irregular sized and spacing horizontally and/or vertically. It will also be understood that the openings, e.g., 130, 132, 134 may be formed in a plate separate from the wall(s) e.g., 28, 29 and attached, e.g., by bolting, to the respective wall(s),
e.g., 28, 29. The dust collectors, e.g., 100, 100', 100'' may also be bolted to the respective wall(s), either at the periphery of the meshed elements, e.g., through a frame (not shown) holding the respective meshed element(s) or through the mesh
itself, or both.
Turning now to FIG. 7, there is shown aspects of an embodiment of the present invention wherein an element acting as a cutoff for the fan, e.g., an anode support 36, may have formed therein a pocket 200. The pocket may serve, e.g., to improve
flow efficiency in the chamber 26 by, e.g., improving the control of the fan vortex location allowing a greater pressure drop across the fan for a given volume of flow. This can improve the effectiveness of the replenishment of fresh gas between the
electrodes between shots while at the same time not significantly raising acoustic effects from the fan operation, e.g., generated in the vortex and/or from increased fan vibrations (from, e.g., higher speed) to raise the fan pressure drop without the
just described vortex control.
Also shown in FIG. 7 are additional embodiments of debris traps placed in low gas flow regions, e.g., traps 280, forming trap pockets 282, e.g., in pockets formed between, e.g., the main insulator 42 and the top half 22 of the chamber (also shown
in FIGS. 11 and 12). In this embodiment the dust traps 280 could be, e.g., generally u-shaped honeycombed structures that can fit, e.g., between the main 42 insulator wings as shown in the Figures general along most or all of the longitudinal extension
of the main insulator 42 along the longitudinal axis of the discharge region 34 within the chamber interior 26. Gas pressure and circulation will tend to move debris toward the structures 280 and once the debris passes through the honeycombed openings,
e.g., into the pockets 282 it will be difficult for the debris to be caused to return back through the honeycombed structures 280 to return into the gas flow.
Turning now to FIGS. 8-8B there is shown, partially cut-away, a preionizer ground rod 54 according to aspects of an embodiment of the present invention, which may have, e.g., a ground rod elongated thick section 180 and a ground rod thin section
182 on either end of the thick section 180, which is, e.g., for arc prevention reasons as is known in the art. The ground rod may have, e.g., a terminus section 210, which may serve, e.g., as a connecting pin, which may be, e.g., inserted into a ground
rod mounting fitting, e.g., in the respective chamber wall and held in place, e.g., by a set screw (not shown). One end of the ground rod 54 may have a mounting flange 214. At least one longitudinal flexure member 216 (two are illustrated in FIG. 8)
may be placed at a terminus of the ground rod 54, shaped and sized to fit within the interior of the preionizer ceramic tube 52 (shown, e.g., in FIGS. 7A and B. The flexure member 216 may comprise, e.g., a plurality of slots 218, also illustrated in FIG.
8A and the cross-section of FIG. 8B along the cross-section lines 8B-8B in FIG. 8A. The slots 218 serve to form an accordion-like structure that will compress under compressive stress to the ground rod 54, e.g., due to thermal stressing of the ground
rod 54 or other causes, without also inducing, e.g., bending torque in the ground rod 54, which applicants have found to cause catastrophic preionizer tube failures.
Turning to FIGS. 9 and 10 there is shown an alternative embodiment for a ground rod 54 similar to that shown in FIGS. 8-8B, according to aspects of an embodiment of the present invention in which at least one of the thin sections 260 of the
ground rod 54 may, e.g., be formed into a relatively flexible and elastic member 250 that is sized to also fit within the preionizer tube 52. The flexible member 250 may include, e.g., a plurality of longitudinally extending portions 252, laterally
extending portions 256 and elbow joints 254. It will be understood that this member 250 may be formed by machining the thinned section 260 of the ground rod 54 to a still smaller thickness diameter and bending the still narrowed portion of thinned
section 250 to form the s-shaped bends of the elastic member 250. This flexible section will also allow for compression in the longitudinal direction without inducing bending moment into the ground rod 52.
Turning now to FIG. 11 there is shown a preionization control system 290 according to aspects of an embodiment of the present invention, which may comprise, e.g., a preionization cut-off circuit comprising, e.g., an RC network of resistor 292,
which may be, e.g., a [______] .OMEGA. resistor and a capacitor 294, which may be, e.g., a [______] F capacitor. Applicants have determined that a certain level of preionization in the operation of gas discharge lasers of the type described above in a
burst mode is necessary during the first few pulses in the burst, but that, if maintained at that level during the rest of the burst, actually may be detrimental to proper laser light source operation, e.g., detrimental to producing DUV light as desired,
after a first number of pulses in the burst, e.g., [______] pulses. Therefore, the RC network of the preionizer controller 290 may be timed to charge capacitor 294, which may be connected in parallel with resistor 292 between the preionizer ground rod
54 at 296 and ground, i.e., the potential of the anode 32. The connection of the ground rod 54 to the potential of the anode may be done, e.g., by insulating the ground rod 54 from connection to the chamber body, e.g., chamber top half 22, with which it
is normally in electrical contact in current lasers sold by applicants' assignee and ins
