United States Patent [19]
`
`Miiller-Horsche
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`'
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`lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllIlllllllllll
`5,247,531
`Sep. 21, 1993
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`US005247531A
`[11] Patent Number:
`
`[45] Date of Patent: -
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`[54] APPARATUS FOR PREIONIZING APULSED
`GAS LASER
`
`[75] Inventor: Elmar Miiller-Horsche, Kissing, Fed.
`Rep. of Germany
`
`[73] Assignee:
`
`Lambda Physik
`Forschungsgesellschaft mbH, Fed.
`Rep. of Germany
`
`[21] App]. No.: 849,960
`
`[22] Filed:
`Mar. 12, 1992
`[30]
`Foreign Application Priority Data
`Mar. 15, 1991 [DE] Fed. Rep. of Germany ..... .. 4108474
`
`[51] Int. Cl.5 ml . . . . . . .
`
`. . . . . . . . . .. H015 3/00
`
`[52] US. Cl. ....... ............................. .. 372/38; 372/86;
`372/87
`[58] Field of Search ............................ .. 372/86, 87, 38
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,555,787 11/1985 Cohn et a1. ......................... .. 372/86
`4,718.072 1/1988 Marchetti et a].
`372/86
`4,953.174 8/1990 Eldridge et al. . . . . .
`. . . .. 372/86
`5,090,021 2/1992 Naltatani et al. .................... .. 372/86
`
`FOREIGN PATENT DOCUMENTS
`
`0398330 5/1990 European Pat. Off. .
`60-157280 8/1985 Japan.
`
`OTHER PUBLICATIONS
`“Japanese Journal of Applied Physics”, vol. 29, No. 1,
`Jan. 1990, pp. 95-100 (K. Nakamura et al.).
`Primary Examiner-Léon Scott, Jr.
`Attorney, Agent, or Firm-—Woodard, Emhardt,
`Naughton, Moriarty & McNett
`[57]
`ABSTRACT
`An apparatus for preionization of a pulsed gas laser
`comprises corona preionization electrodes (12, 12’)
`which are each arranged adjacent a respective associ
`ated main electrode (10, 10'). To generate an effective
`preionization with low constructional and circuit ex
`penditure the preionization electrodes (12, 12’) are set
`under voltage with the same high-voltage source (16)
`which also supplies the main electrodes (10, 10') with
`voltage. By means of an inductance (30) a time delay is
`set between the excitation of the preionization elec
`trodes and the triggering of the main discharge between
`the main electrodes (10, 10’).
`
`4 Claims, 2 Drawing Sheets
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`US. Patent
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`Sep. 21, 1993
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`Sheet 1 of 2
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`5,247,531
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`US. Patent
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`Sep. 21, 1993
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`Sheet 2 of 2
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`5,247,531
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`22
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`no.1.
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`1
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`APPARATUS FOR PREIONIZING APULSED GAS
`LASER
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`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The invention relates to an apparatus for preionizing
`a pulse gas laser comprising preionizing electrodes each
`arranged adjacent associated main electrodes in order
`to generate on excitation by means of high voltage
`ultraviolet radiation which prior to a main discharge
`between the main electrodes effects the preionization of
`the gas, the main discharge being triggered by means of
`high-voltage pulses generated by a high-voltage source.
`2. Description of the Prior Art
`Pulsed gas lasers, for example excimer lasers and
`CO2 lasers, can be operated in accordance with the
`prior art by socalled transverse pulsed gas discharges
`(TE gas lasers). This gas discharge (also referred to as
`plasma discharge or main discharge) takes place perpen
`dicularly to the optical axis of the laser. The energy
`necessary for the main discharge is stored in a capacitor
`and transferred to the plasma during the discharge. The
`plasma discharge usually takes place in the laser cham
`ber between main electrodes arranged parallel to the
`optical axis.
`The power and other qualities of the laser depend
`inter alia substantially on the homogeneity of the plasma
`discharge. To ensure the necessary homogeneity of the
`plasma discharge at different pressures of the working
`gas in accordance with the gas mixture, a socalled pre
`ionization is necessary before the plasma discharge
`(main discharge). The preionization of the gas in the
`space between the main electrodes of the laser is carried
`out in particular also to avoid arc discharges.
`In such a preionizing the gas is ionized in the dis
`charge space between the main electrodes to prepare
`for the main discharge, i.e. free electrons are generated
`in the gas. Typically, in such a preionizing relatively
`low electron concentrations (for example 107 elec
`trons/cm3) are generated in the discharge space. In the
`main discharge, which takes place delayed with respect
`to the preionization, the low initial concentration of free
`charges generated during the preionization is multiplied
`in a short time via socalled avalanche processes and by
`ionizing processes in the laser gas electron concentra
`tions of 1014 to 1015 electrons/cm3 are reached.
`In the prior art different methods of preionization are
`known. Usually, ultraviolet radiation is used which is
`obtained for example by spark gaps or by corona dis
`charges.
`7
`Spark preionization apparatuses involve a consider
`able constructional expenditure, requiring in particular
`the introduction of a plurality of insulated high-voltage
`passages in gas-tight manner in the laser chamber on
`both sides along the main electrodes, and furthermore
`spark preionization systems also have the disadvantage
`that by erosion processes in the hot spark plasmas gas
`impurities arise which impair the laser performance and
`in particular the life of the laser.
`Generally, spark preionization systems provide a
`higher electron density in the discharge gas than corona
`preionization apparatuses. However, with corona pre
`ionization apparatuses as well it is possible to achieve a
`. glow discharge sufficient for a pulsed gas laser and of
`good homogeneity, in particular in the case of XeCl
`excimer lasers and CO3 lasers.
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`In the corona preionization apparatus ultraviolet light
`is generated in a gas discharge between a metal and a
`dielectric. This ultraviolet radiation then generates in
`the gas of the discharge space the aforementioned weak
`ionization, i.e. the generation of free electrons referred
`to. Following this preionization a homogeneous gas
`discharge can thenbe triggered between the main elec
`trodes of the laser.
`In a corona preionization apparatus the dielectric
`prevents the formation of spark channels to the preioni
`zation electrodes (which are to be distinguished from
`the main electrodes in a manner known to the person
`skilled’in the art). During the preionization only the
`electrical capacitance formed from the preionization
`electrodes and the dielectric is charged. In spite of rela
`tively low currents an intensive emission of UV light is
`obtained (G. J. Ernst and A. G. Boer, Opt. Commun.
`27, 105, 1978; U. Hasson and H. M. von Bergmann,
`Rev.Sci. Instrum. 50, 59, 1979).
`In such a use of dielectrics in the corona preionization
`sparks are effectively suppressed and thus also the dis
`advantages caused by sparks, in particular erosion pro
`cesses at the electrodes and gas impurities.
`The prior art of corona preionization apparatuses
`contains essentially two types of electrical connection
`of the corona electrode. Either the corona electrode is
`supplied from a separate high-voltage circuit, i.e. the
`corona electrode has its own high-voltage source inde
`pendent of the main electrodes, or the corona electrode
`is connected in simple manner directly to the electrical
`potential of the counter main electrode. This prior art
`will be explained in detail hereinafter with the aid of
`FIGS. 1 to 4.
`FIGS. 1 to 3 show different embodiments of a corona
`preionization apparatus in which the corona electrode is
`connected to the potential of the counter main electrode
`(R. Marchetti and E. Penco, J. Appl. Phys. 56, 3163,
`1984). In known manner, two main electrodes 10, 10'
`are arranged opposite each other in the laser chamber.
`Adjacent the one main electrode 10 preionization elec
`trodes 12, 12’ are arranged. Each of the preionization
`12, 12’ is surrounded by a tubular dielectric (e.g. ce
`ramic) 14, 14'. A high-voltage source known per se is
`designated by the reference numeral 16. The high-volt
`age source 16 charges a storage capacitor 18. Via a
`thyratron the gas discharge is switched in known man
`ner. For this purpose, in known manner a recharge
`inductance 22 (coil) is provided and discharge capaci
`tors C1, C2 are connected in parallel with the main
`discharge taking place between the main electrodes 10,
`10'.
`In accordance with FIG. 1 the preionization elec
`trodes 12, 12' arranged adjacent the one main electrode
`10 are connected to the potential of the counter main
`electrode 10, i.e. the preionization electrodes 12, 12'
`have the potential of the counter main electrode 10’ and
`due to their smaller spacing from the one main electrode
`10 a very high field strength arises between the one
`main electrode 10 and the preionization electrodes 12,
`12' and generates a corona discharge on the dielectric
`tubes 14, 14'. The corona discharge in turn emits UV
`radiation which preionizes the gas between the main
`electrodes 10, 10'.
`FIG. 2 shows a modi?cation of the example of em
`bodiment according to FIG. 1, two preionization elec
`trodes 12, 12' now being arranged near the lower main
`electrode 10’ but being connected to the potential of the
`counter main electrode 10 so that the corona discharge
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`The electrical driving of the preionization electrodes
`effecting a preionization burns close to the main elec
`according to the invention (in a simple modi?cation of
`trode l0’ drawn at the bottom in the Figures.
`the invention a single preionization electrode may also
`In the Figures, corresponding components are pro
`be provided) utilizes the knowledge that the voltage
`vided with the same reference numerals. In FIGS. 2 and
`drop at the deliberately inserted or parasitically present
`3 the high-voltage source 16, the storage capacitor 18,
`inductance in the socalled recharging circuit can be
`the thyratron 20 and the resistor R1 have not been illus
`utilized to apply a voltage to the preionization elec
`trated for the sake of simplicity.
`trodes a short time before application of the high-volt
`A corona preionization apparatus according to the
`age pulse to the main electrodes (with respect to said
`FIGS. 1 and 2 has the advantage that defects in the
`main electrodes) in such a manner that a good preioni
`dielectric (for example small holes and cracks) can lead
`zation is achieved by means of corona discharge. For
`to electrical breakdowns (between the preionization
`during the charging of the discharge capacitors C1, C;
`electrode and the adjacent main electrode) in which the
`a relatively large current change occurs and by con
`energy of the main discharge can be used up and the
`necting the preionization electrode before the induc
`dielectric 14, 14’ can be destroyed. Admittedly, such a
`tance a very rapidly arising potential difference occurs
`consumption of the energy of the main discharge or a
`between the preionization electrodes and the main elec
`destruction of the corona dielectric can be prevented by
`trode and thus a correspondingly intensive corona dis
`a capacitive current limiting by means of the capacitors
`charge.
`'
`C3, C4; however, such a capacitative voltage division
`also leads to a loss of ef?ciency in the transfer of electri
`cal energy to the corona discharge.
`The separate current supply of the preionization elec
`trodes shown in FIG. 4 by means of a separate switch 26
`and a separate high-voltage source 28 (apart from the
`high-voltage source 16) requires a relatively great con
`structional expenditure for the switching elements and
`the synchronization circuits for the time synchroniza
`tion of preionization and main discharge when com
`pared with the socalled automatic or autonomous cir
`cuits according to FIGS. 1, 2 and 3 explained above
`In the journal “JAPANESE JOURNAL OF AP
`PLIED PHYSICS", Vol. 29, No. 1, January 1990, p.
`95-100 (article by K. NAKAMURA et al.) for the pre‘
`ionization an independent separate circuit is provided
`(separate with respect to the voltage supply of the main
`electrodes). This separate preionization circuit consists
`of the elements G3, C3, L3, C5 and L5.
`JP 60-157 280 (A) describes a preionization in which
`the preionization electrodes are driven via an auxiliary
`capacitor. EP 0 398 330 A2 also describes such an .ar
`rangement in which a driving of the preionization elec
`trode is effected via an auxiliary capacitor (number 12 in
`FIG. 4). In these last two systems of the prior art men‘
`tioned the corona electrode is subjected to high voltage
`for an unnecessarily long time and this leads to an in
`creased risk of dielectric breakdown. The prior art ac
`cording to the last three documents mentioned requires
`in each case a complicated structure of the main elec
`trode (either mesh electrodes or electrodes with buried
`corona rods).
`For laser systems with very high repetition rates and
`long lives these known systems have not proved very
`suitable.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIGS. 1-3 show different embodiments of a corona
`preionization apparatus in which the corona electrode is
`connected to the potential of the counter main elec
`trode.
`FIG. 4 shows an embodiment of a corona preioniza
`tion apparatus having a separate current supply for the
`preionization electrodes.
`FIG. 5 shows a circuit for the preionization of a
`pulsed gas laser in which the high voltage for the pre
`ionization electrodes is tapped off in the high-voltage
`pulse direction before an inductance.
`FIG. 6 shows a modi?cation of the example of em
`bodiment according to FIG. 5, the higher voltage for
`the preionization electrodes being tapped off between
`two inductances.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`Hereinafter two examples of embodiment of the in
`vention will be described in detail with the aid of FIGS.
`5 and 6.
`In FIGS. 5 and 6 the components corresponding to
`those according to FIGS. 1 to 4 are provided with
`identical reference numerals. Thus, adjacent respective
`main electrodes 10, 10’ preionization electrodes 12, 12’
`are arranged, the spacing of the preionization electrodes
`12, 12’ from one of the main electrodes being less than
`from the other main electrode. According to the FIGS.
`5 and 6 the preionization electrodes are arranged nearer
`the upper main electrode 10 than the lower main elec
`trode 10'. In a modi?cation of the example of embodi
`ment illustrated it is also possible to provide a single
`preionization electrode 12; the symmetrical arrange
`ment according to the Figures is however preferred.
`Each of the preionization electrodes 12, 12’ is sur
`rounded by a respective tubular dielectric 14, 14'. The
`corona discharge burns on the outer surface of the di
`electric.
`A high-voltage source known per se for generating
`high-voltage pulses is indicated by the reference nu
`meral 16. The thyratron 20 already described above, the
`storage capacitor 18 and the recharging inductance 22
`are omitted in order to concentrate the illustration on
`the essence of the invention.
`Thus, in the circuits according to the invention for
`preionization of a pulsed gas laser as well for the preion
`ization the same energy source is used which also serves
`for the main discharge, as explained with the aid of
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`SUMMARY OF THE INVENTION
`The invention is based on the problem of setting forth
`a simple apparatus for the preionization of a pulsed gas
`laser which achieves a preionization of good quality
`whilst requiring low constructional and circuit expendi
`ture.
`According to the invention this problem is solved in
`that the excitation of the preionization electrodes is
`effected with the same high-voltage source which also
`charges the capacitors of the main electrodes and by
`means of an inductance a time delay is set between the
`excitation of the preionization electrodes and the trig
`gering of the main discharge between the main elec
`trodes.
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`tion. The preionization apparatus according to the in
`FIG. 1, i.e. the high-voltage source 16 and the energy
`vention requires only four high-voltage leadthroughs
`storage means 18 respectively.
`into the laser chamber.
`The essential matter is the provision of an inductance
`The circuit arrangements described above with the
`(coil) 30 in the preionization and main discharge circuit
`aid of FIGS. 5 and 6 for preionization of a pulsed gas
`and the arrangement of the line 32 with which the pre
`laser drive the preionization electrodes 12, 12' utilizing
`ionization electrodes 12, 12' are put under voltage.
`the inherent capacitance of said rod electrodes. Addi
`In the example of embodiment according to FIG. 5
`tional capacitors and switches for operating the preioni
`the high-voltage source 16 generates high-voltage
`pulses with which the discharge capacitors C1, C2 are
`zation are super?uous.
`successively charged. During the charging of the dis
`In addition, the arrangements described ensure an
`only short voltage pulse at the preionization electrodes,
`charge capacitors C1, C2 great current changes and
`corresponding voltage drops occur at the inductance
`this leading on the one hand to an intensive corona
`discharge and on the other to a reduced risk of electrical
`30. According to the invention the preionization elec
`breakdown of the dielectric. The geometry of the main
`trodes 12, 12' are supplied with voltage via a line 32
`discharge electrodes and the preionization electrodes
`which taps the potential off in the charging direction of
`described above with the aid of the Figures and the
`the high-voltage pulses before the inductance 30. Due
`circuit described permit the use of solid pro?le main
`to this voltage tapping in front of the inductance 30, a
`electrodes 10, 10’. Such solid pro?le main electrodes
`very rapidly starting potential difference occurs be
`(i.e. solid electrodes in the form indicated in FIGS. 4
`tween the preionization electrodes 12, 12' and the main
`and 5) have advantages as regards the erosion behav
`electrode 10 adjacent thereto and correspondingly a
`iour and the control of the discharge cross-section.
`corona discharge occurs on the dielectrics 14, 14' sur
`Apart from the advantages referred to above the circuit
`rounding the preionization electrodes 12, 12’. As re
`arrangement according to the invention also permits a
`gards time the corona discharge thus generated coin
`compact structure.
`cides with the voltage rise between the main electrodes
`What is claimed is:
`10, 10’. The inductance 30 is so dimensioned that the
`1. A pulsed gas laser comprising:
`time delay between the starting of the corona discharge
`?rst and second main electrodes which are arranged
`and the main discharge gives a maximum laser power.
`For a given laser system the dimensioning of the capaci
`in a chamber containing a gas;
`?rst and second preionizing electrodes arranged adja
`tances and in particular of the inductance 30 depends on
`cent said main electrodes for generating ultraviolet
`the speci?c line arrangements and the resulting parasitic
`radiation which preionizes said gas between said
`inductances and capacitances and must be determined
`main electrodes prior to a main discharge between
`experimentally for the particular individual case. For a
`said main electrodes;
`great number of excimer laser gas mixtures the time
`a ?rst electrical circuit for generating said ultraviolet
`delay between preionization by means of corona dis
`radiation including said preionizing electrodes and
`charge and subsequent main discharge has proved to be
`an electrical line for applying a voltage to said
`an optimum one for achieving a high laser power and
`preionization electrodes;
`long laser life. Special synchronization circuits are not
`a second electrical circuit including said main elec
`necessary.
`trodes for facilitating said main discharge between
`A further advantage of the circuit arrangement ac-’
`said main electrodes;
`cording to the invention for a preionization resides in
`a switching element for closing both said ?rst electri
`that with increasing charging of the discharge capaci
`cal circuit including said preionizing electrodes and
`tors C1, C2 the potential difference between the preioni
`said second electrical circuit including said main
`zation electrodes and the associated main electrode
`electrodes;
`diminishes. The voltage pulse obtaining between said
`a capacitor common to both said ?rst electrical cir
`electrodes is therefore of relatively short duration so
`cuit and said second electrical circuit which is
`that the risk of a breakdown through the dielectric is
`connected in series with both said first and said
`considerably reduced. As dielectric, in particular A1203
`second electrical circuit such that the charge of
`ceramic or sapphire have proved suitable.
`said capacitor is utilized for both a preionization of
`FIG. 6 shows a modi?cation of the example of em
`said gas and for said main discharge;
`bodiment according to FIG. 5, the inductance 30 being
`a high voltage source for charging said capacitor;
`replaced by two inductances 30a and 30b and the line 32
`and, an inductance connected in series with said
`applying the voltage to the preionization electrodes 12,
`second electrical circuit including said main elec
`12' tapping the voltage between the two inductances
`trodes for setting a time delay between said preioni
`30a, 3012. By suitable variation of the inductances 30a,
`zation and said main discharge.
`30b the voltage pro?le between the preionization elec
`2. The pulsed gas laser of claim 1, wherein said ?rst
`trodes and the main electrode can be set experimentally
`electrical circuit and said second electrical circuit addi
`and optimized for a specific system.
`tionally comprise a second inductance wherein said
`The arrangement described above is very simple in
`electrical line of said ?rst electrical circuit taps the
`construction and circuitry and has proved itself for a
`great number of excimer laser gas mixtures; in particu
`voltage between said two inductances.
`‘
`3. The pulsed gas laser of claim 1, wherein said pre
`lar, with XeCl and KrF laser gas mixtures homogene
`ous glow discharges up to very high pulse repetition
`ionization electrodes are corona electrodes.
`4. The pulsed gas laser of claim 3, wherein said co
`rates of 200 Hz were implementable. The ef?ciency of
`rona electrodes are surrounded by a tubular dielectric.
`the emitted laser radiation was exactly as high as when
`*
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`is
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`using a substantially more complicated spark preioniza
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