U.S. patent application number 15/448023 was filed with the patent office on 2018-09-06 for lasing-gas mixture for excimer laser.
The applicant listed for this patent is COHERENT LASERSYSTEMS GMBH & CO. KG. Invention is credited to Igor BRAGIN, Oleg MELNIKOV, Timur MISYURYAEV.
Application Number | 20180254602 15/448023 |
Document ID | / |
Family ID | 61622515 |
Filed Date | 2018-09-06 |
United States Patent
Application |
20180254602 |
Kind Code |
A1 |
BRAGIN; Igor ; et
al. |
September 6, 2018 |
LASING-GAS MIXTURE FOR EXCIMER LASER
Abstract
A xenon chloride (XeCl) excimer laser includes a lasing-gas
mixture including a buffer gas, a noble gas, a halogen-donating
gas, and deuterium. The deuterium is present in a concentration
greater than about 10 parts-per-million.
Inventors: |
BRAGIN; Igor; (Gottingen,
DE) ; MELNIKOV; Oleg; (Gottingen, DE) ;
MISYURYAEV; Timur; (Gottingen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
COHERENT LASERSYSTEMS GMBH & CO. KG |
Gottingen |
|
DE |
|
|
Family ID: |
61622515 |
Appl. No.: |
15/448023 |
Filed: |
March 2, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/2253 20130101;
H01S 3/032 20130101; H01S 3/097 20130101; H01S 3/225 20130101 |
International
Class: |
H01S 3/225 20060101
H01S003/225; H01S 3/032 20060101 H01S003/032; H01S 3/097 20060101
H01S003/097 |
Claims
1. A noble-gas halide excimer laser, comprising: a laser-housing
containing discharge electrodes and a lasing-gas mixture; the
lasing gas mixture including a buffer gas, a noble gas, a
halogen-donating gas, and deuterium; and wherein the deuterium is
present in a concentration greater than about 10
parts-per-million
2. The laser of claim 1, wherein the deuterium concentration is
between about 50 parts-per-million and 100 parts-per-million.
3. The laser of claim 1, wherein the deuterium is present as an
additive to the buffer gas, the noble gas, and the halogen-donating
gas.
4. The laser of claim 1, wherein the deuterium is provided by the
halogen-donating gas.
5. The laser of claim 1, wherein the buffer gas is neon, the noble
gas is xenon, and the halogen-donating gas is hydrogen
chloride.
6. The laser of claim 5, wherein the xenon composition is about 1%
of the total, and the hydrogen chloride composition is about 0.1%
of the total.
7. A method of laser-processing a work piece, comprising:
generating laser-pulses from a noble-gas excimer laser, the laser
having a lasing-gas mixture including a buffer gas, a noble gas, a
halogen-donating gas, and deuterium in a concentration greater than
about 10 parts-per-million; and delivering the laser-pulses to the
workpiece.
8. The method of claim 7, wherein the deuterium concentration is
between about 50 parts-per-million and 100 parts-per-million.
9. The method of claim 7, wherein the deuterium is present as an
additive to the buffer gas, the noble gas, and the halogen-donating
gas.
10. The method of claim 7, wherein the deuterium is provided by the
halogen-donating gas.
11. The method of claim 7, wherein the buffer gas is neon, the
noble gas is xenon, and the halogen-donating gas is hydrogen
chloride.
12. The method of claim 11, wherein the xenon composition is about
1% of the total, and the hydrogen chloride composition is about
0.1% of the total.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates in general to gas-discharge
lasers. The invention relates in particular to lasing-gas mixtures
for excimer gas-discharge lasers
DISCUSSION OF BACKGROUND ART
[0002] Excimer lasers have a capability to generate laser-radiation
having a fundamental wavelength in the ultraviolet (UV) region of
the electromagnetic spectrum. Operation of excimer lasers is based
on an optical transition between different electronically excited
states of noble-gas molecules. Such molecules exist only in an
electronically excited state and accordingly are generally referred
to as "excimers". The noble-gas halide molecules are created in the
excited state by a short and powerful electrical excitation (gas
discharge) of between about 1 nanosecond (ns) and 1000 ns duration.
Relaxation of the excited molecules to the ground state results in
the emission of high-intensity UV light in a laser-resonator, which
delivers the UV light as laser-radiation.
[0003] An excimer laser uses as an active medium (lasing gas) a
noble gas, typically argon (Ar), krypton (Kr), or xenon (Xe), and a
halogen-donating gas, typically hydrogen chloride (HCl) or fluorine
(F.sub.2). These excimer forming gases are low concentration
additives in a neutral buffer gas, typically neon (Ne) or helium
(He). The buffer gas must be at a relatively high pressure, for
example between about 2 atmospheres pressure (Bar) and about 10
Bar, in order to provide a desired impedance matching of the
electrical gas discharge and adequate laser efficiency. The
specific noble-gas and halogen-donating gas determine the
wavelength of the laser-radiation.
[0004] The most powerful state-of-the-art industrial excimer lasers
are based on xenon chloride (XeCl) molecules and generate
laser-radiation having a wavelength of 308 nanometers (nm). Such
excimer lasers can provide 308 nm laser-radiation pulses having a
pulse-energy of about 1000 millijoules (mJ) at a pulse-repetition
frequency (PRF) of about 600 Hertz (Hz). Such a laser is able to
operate continuously over 100 million pulses while maintaining very
high stability of pulse-energy and temporal and spatial optical
parameters.
[0005] In laser-processes requiring greater pulse energy, the
output of two or more such lasers can be combined by suitable
beam-mixing optics and appropriate synchronization of pulse
delivery. Such beam-mixing and synchronization are described in
U.S. Pat. No. 7,408,714 and U.S. Pat. No 8,238,400, respectively,
each thereof assigned to the assignee of the present invention, and
the complete disclosure of each of which is hereby incorporated
herein by reference.
[0006] The duration of continuous operation of the laser is limited
by degradation of the lasing gas and of some components of the
laser, especially windows in a chamber containing the lasing gas
and excitation (discharge) electrodes. Performance degradation can
eventually develop due to chemical and electrical erosion of the
discharge electrodes and other surfaces in the highly-reactive
halogen-containing atmosphere in the chamber. Such erosion leads to
contamination of the lasing gas and the laser chamber windows. As a
result, interruptions of the laser operation are periodically
needed to exchange the lasing gas and to service the laser
windows.
[0007] The lasing-gas mixture typically used in such a laser
consists primarily of Ne as the buffer gas at elevated pressure
(for example about 6 Bar), with small additions of Xe (about 1%)
and HCl (about 0.1%). Composition of the lasing-gas mixture is
optimized in a way that allows an acceptable compromise between
laser-efficiency and the other laser parameters.
[0008] Small additions of hydrogen, for example between about 10
and about 2000 parts-per-million (ppm), are commonly used to help
stabilize excimer laser performance. Such small additions can
significantly extend the lifetime of the lasing gas. However, an
increase of the hydrogen concentration above an optimal
concentration leads to a degradation of the laser performance and
reduction of the laser output energy. A compromise between the
lasing-gas lifetime and the output pulse-energy and stability
determines the optimal concentration of hydrogen. This can depend
on desired output parameters and other features of a particular
laser.
[0009] One unfortunate characteristic of laser-radiation pulses
delivered by an excimer laser is that each pulse is characterized
by a first portion having a certain amplitude followed by a second
portion having about half the amplitude of the first portion with a
minimum amplitude between the first and second portions of the
pulse. This is illustrated in FIG. 1, which is a graph
schematically illustrating pulse amplitude as a function of time
for a 308 nm XeCl excimer laser having a lasing-gas mixture
composition as discussed above. It can be seen that the first
portion of the pulse (portion A) has an amplitude more than twice
that of the second portion of the pulse (portion B).
[0010] This characteristic, while relatively benign for certain
laser-processing operations, such a cutting or ablation of
materials, can be problematical for processes that depend more
critically on the temporal characteristics of pulse energy
delivery. One particular such process is excimer-laser
recrystallization of silicon, which is a process used extensively
in the manufacture of flat panels for large-screen displays. In
this process, there is a particular energy density-per-pulse,
generally referred to as the optimum energy density (OED) that
produces, from an amorphous silicon layer, a poly-crystalline layer
having a relatively uniform grain structure and having a minimum of
defects that could adversely affect production yields of usable
flat panels.
[0011] Problems of the above discussed two-portion pulse delivery
have been substantially minimized by relatively recent developments
in-situ, i.e., on a production line, monitoring of the
recrystallizing process which can be arranged to adjust
pulse-energy at least manually, responsive to optical
characterization of flat panels being recrystallized. Such a
monitoring process and characterization of the silicon
recrystallization itself are described in detail in U.S. Pat. No.
9,335,276, and U.S. Pub. No. 2013/0341310, both assigned to the
assignee of the present invention, and with the complete
disclosures therein are hereby incorporated herein by reference. It
is believed, however, that improvements in the sensitivity and
effectiveness of these processes may be possible if a means could
be found to reduce the difference between the above-described first
and second portions of excimer laser pulses.
SUMMARY OF THE INVENTION
[0012] In one aspect of the present invention, a noble-gas halide
excimer laser, comprises a laser-housing containing discharge
electrodes and a lasing-gas mixture. The lasing gas mixture
includes a buffer gas, a noble gas, a halogen-donating gas, and
deuterium. The deuterium is present in a concentration greater than
about 10 parts-per-million.
[0013] The deuterium may be present as an additive to the buffer
gas, the noble gas, and the halogen-donating gas or may be donated
by the halogen-donating gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and
constitute a part of the specification, schematically illustrate a
preferred embodiment of the present invention, and together with
the general description given above and the detailed description of
the preferred embodiment given below, serve to explain principles
of the present invention.
[0015] FIG. 1 is a graph schematically illustrating amplitude as a
function of time for a pulse delivered by a prior-art XeCl excimer
laser having a lasing gas including a hydrogen additive for
extending the lifetime of the lasing gas.
[0016] FIG. 2 is a graph schematically illustrating amplitude as a
function of time for a pulse delivered by a XeCl excimer laser,
similar to the laser of FIG. 1, but wherein the hydrogen additive
is replaced by a deuterium additive.
[0017] FIG. 3 schematically illustrates a preferred embodiment of
laser processing apparatus in accordance with the present invention
including an excimer laser having a housing supplied with lasing
gas from sources of neon, xenon, hydrogen chloride and deuterium,
the laser delivering a beam of laser pulses via beam-shaping and
projection optics to a workpiece being processed.
DETAILED DESCRIPTION OF THE INVENTION
[0018] A reduction in amplitude difference between first and second
portions of excimer laser pulses is achieved by replacing the
hydrogen additive of prior-art excimer lasing gases with deuterium
(D.sub.2). Deuterium is the closest isotope of hydrogen, but has
slightly different chemical and electrical properties.
[0019] The addition of deuterium instead of hydrogen into the
excimer lasing gas provided improvement of the lasing-gas lifetime
similar to that discussed above. However, an optimal concentration
of deuterium was found to be half the optimal concentration of
hydrogen. The total energy-per-pulse was found to be the same as
that achieved with the prior-art hydrogen additive. The
pulse-to-pulse and long-term pulse-energy stability was similar to
that achieved with the hydrogen additive. The deuterium additive in
this optimal concentration for lifetime and stability, noticeably
and reproducibly reduced the above-discussed amplitude difference
between the first and second portions of the XeCl laser output
pulses.
[0020] FIG. 2 is a graph (solid curve D) schematically illustrating
amplitude as a function of time for a pulse delivered by a XeCl
excimer laser similar to the laser of FIG. 1, but wherein the
hydrogen additive is replaced by a deuterium additive. Dashed curve
H depicts the prior-art pulse of FIG. 1, on the same scale for
comparison.
[0021] It can be seen from a comparison of curves D and H, that the
amplitude of the first portion (A1) of the pulse of curve D is
lower than that of prior-art curve H by about 5%. The amplitude of
the second portion (B1) of curve D is higher than that of curve H
by about 10%. In addition, the temporal width at half-maximum
(TWHM) of the first portion of the pulse of curve D is about 5%
less than that of the pulse of curve H, and the TWHM of the second
portion of the pulse of curve D is about 10% greater than that of
the second portion of the pulse of curve H.
[0022] These differences together amount to a significant reduction
of the difference in energy between the first and second portions
of the pulse of curve D compared with that of curve H. It is
estimated that this reduction could be as much as 20%.
[0023] It should be noted here that the time between the falling
edge of the first portion of the pulse of curve D and the rising
edge of the second portion of the pulse of curve D is less than
that of the pulse of curve H. This difference could be an advantage
for a process in which conditions at the minimum between the first
and second portions of a pulse were below a process threshold.
[0024] The above described results were obtained using a Model
LAMBDA SX E500 XeCl excimer laser available from Coherent Laser
Systems GmbH & Co. KG of Goettingen, Germany. The basic
lasing-gas mixture for curves D and H of FIG. 2 included Ne as the
buffer gas at a pressure of about 6 Bar; Xe at a concentration of
about 1%; and HCl at a concentration of about 0.1%. The electrical
excitation energy-per-pulse in each case was about 25
Joules-per-pulse. The hydrogen additive for prior-art curve H was
at a concentration of 170 ppm. The deuterium additive for curve D
was at a concentration of about 85 ppm.
[0025] A suitable concentration of deuterium for achieving the
above described results is at least 10 ppm and preferably at least
about 50 ppm. Some performance degradation may be encountered if
the concentration exceeds 100 ppm.
[0026] FIG. 3 schematically illustrates a preferred embodiment 10
of laser processing apparatus in accordance with the present
invention. Apparatus 10 includes an excimer laser 12 including an
above-described inventive deuterium-containing lasing-gas mixture.
Laser 12 includes a gas-tight housing 14 filled by the gas mixture.
Located in housing 14 are discharge electrodes 16 and 18.
Reflectors 20 and 22 form a laser-resonator 23 extending through
the housing via windows 24 and 25 therein. Fundamental laser
radiation circulates in resonator 23 as indicated by arrows F. A
beam 26 of laser-radiation is delivered from the resonator via
reflector 22, which is partially transparent to the
laser-radiation.
[0027] Beam 26 is delivered to beam-shaping and projection optics
28 which reform beam 26 into a beam 30 having a cross-section and
parameters appropriate to the laser processing operation. Beam 30,
of course, is a beam of repeated laser pulses 32 (here, only two
shown), with pulses having the above-described double-peak
characteristic. Here it should be noted that as the pulses are
depicted in space, first portion A of a pulse leads second portion
B thereof in the projection direction. Pulses 32 are delivered to a
workpiece 34 being processed. The workpiece may be translated
relative to beam 30, as indicated by arrow M, dependent on the
processing operation.
[0028] In this embodiment of laser-processing apparatus in
accordance with the present invention, laser 12 is assumed to be a
XeCl excimer laser with an inventive gas-mixture composition as
described-above. Lasing-gas is supplied to housing 14 of laser 12
from cylinders 40, 42, 44, and 46, containing respectively neon,
xenon, hydrogen chloride, and deuterium. Gases from the cylinders
are mixed in a manifold 60, in proportions controlled by
regulating-valves 50, 52, 54, and 56. Lasing-gas mixture from
manifold 60 is delivered to housing 14 of laser 12 via a conduit
62, with delivery controlled by a regulating-valve 64.
[0029] It should be noted, here, that only sufficient description
of apparatus is provided for understanding principles of the
present invention. Basics of excimers lasers are well known in the
art and a detailed description thereof is not necessary for
understanding principles of the present invention, and accordingly
is not present herein.
[0030] While the above-presented description of the present
invention describes adding deuterium in elemental form to the
lasing gas of an excimer laser, it is believed that the inventive
deuterium addition could be accomplished by substituting a
deuterium halide for a hydrogen halide as the halogen donating gas.
For example, substituting deuterium chloride for hydrogen chloride.
It is also believed that the advantageous results of the deuterium
addition may be realized in the presence of some hydrogen additive.
Further, while the present invention is described above with
reference to results obtained with a XeCl excimer laser, it is
anticipated that the advantageous results of the deuterium addition
can be realized in other noble-gas-chloride excimer laser types,
for example KrCl excimer lasers.
[0031] In summary, the present invention is described above with
reference to a preferred and other embodiments. The invention is
not limited, however, by the embodiments described herein. Rather,
the invention is limited only by the claims appended hereto.
* * * * *