United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,247,531
`
`Miiller-Horsche
`
`'
`
`[45] Date of Patent:
`
`.
`
`Sep. 21, 1993
`
`Illlll|ll||||||||||l|l||l||lllll||||||l||||||l||l|||l||||||||||l|l|||ll||||
`US005247531A
`
`[54] APPARATUS FOR PREIONIZING APULSED
`GAS LASER
`
`FOREIGN PATENT DOCUMENTS
`
`[75]
`
`Inventor:
`
`Elmar Miiller-Horsche, Kissing, Fed.
`Rep. of Germany
`
`[73] Assignee:
`
`Lambda Physik
`Forschungsgesellschaft mbl-I, Fed.
`Rep. of Germany
`
`[21] Appl. No.2 849,960
`
`[22] Filed:
`
`Mar. 12, 1992
`
`Foreign Application Priority Data
`[30]
`Mar. 15, 1991 [DE]
`Fed. Rep. of Germany ..... .. 4108474
`
`Int. c1.s .., ........................................... .. H015 3/00
`[51]
`[52] U.S. Cl. ........................................ 372/38; 372/86;
`372/87
`[58] Field of Search ............................ .. 372/86, 87, 38
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`......................... .. 372/86
`4,555,787 ll/1985 Cohn et al.
`.
`4,7l8.072
`l/1988 Marchetti et al.
`372/86
`
`4.953.174
`8/1990 Eldridge at al.
`372/86
`2/1992 Nakatani et al. ................. .. 372/86
`5,090,021
`
`0398330
`60-157280
`
`5/1990 European Pat. Off.
`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
`
`
`
`INTEL 1021
`
`INTEL 1021
`
`

`
`
`
`U.S. PatentU.S. Patent
`
`
`
`Sep. 21, 1993Sep. 21, 1993
`
`
`
`Sheet 1 of 2Sheet 1 of 2
`
`
`
`5,247,5315,247,531
`
`

`
`U.S. Patent
`
`Sep.21, 1993
`
`Sheet 2 of2
`
`5,247,531
`
`2,
`
`FIGJ.
`
`10
`
`

`
`1
`
`5,247,531
`
`APPARATUS FOR PREIONIZING APULSED GAS
`LASER
`
`BACKGROUND OF THE INVENTION
`
`5
`
`2
`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 homogerieous gas
`discharge can thenbe triggered between the main elec-
`trodes of the laser.
`
`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.
`_
`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 CO2 lasers.
`
`l5
`
`20
`
`25
`
`In a corona preionization apparatus the dielectric
`0 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 modification 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
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65
`
`

`
`5,247,531
`
`3
`effecting a preionization burns close to the main elec-
`trode 10’ drawn at the bottom in the Figures.
`In the Figures, corresponding components are pro-
`vided with the same reference numerals. In FIGS. 2 and
`3 the high-voltage source 16, the storage capacitor 18,
`the thyratron 20 and the resistor R1 have not been illus-
`trated for the sake of simplicity.
`A corona preionization apparatus according to the
`FIGS. 1 and 2 has the advantage that defects in the
`dielectric (for example small holes and cracks) can lead
`to electrical breakdowns (between the preionization
`electrode and the adjacent main electrode) in which the
`energy of the main discharge can be used up and the
`dielectric 14, 14' can be destroyed. Admittedly, such a
`consumption of the energy of the main discharge or a
`destruction of the corona dielectric can be prevented by
`a capacitive current limiting by means of the capacitors
`C3, C4; however, such a capacitative voltage division
`also leads to a loss of efficiency 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.
`
`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,
`
`4
`The electrical driving of the preionization electrodes
`according to the invention (in a simple modification of
`the invention a single preionization electrode may also
`be provided) utilizes the knowledge that the voltage
`drop at the deliberately inserted or parasitically present
`inductance in the socalled recharging circuit can be
`utilized to apply a voltage to the preionization elec-
`trodes a short time before application of the high-volt-
`age pulse to the main electrodes (with respect to said
`main electrodes) in such a manner that a good preioni-
`zation is achieved by means of corona discharge. For
`during the charging of the discharge capacitors C1, C2
`a relatively large current change occurs and by con-
`necting the preionization electrode before the induc-
`tance a very rapidly arising potential difference occurs
`between the preionization electrodes and the main elec-
`trode and thus a correspondingly intensive corona dis-
`charge.
`'
`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 modification 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
`
`l0
`
`I5
`
`20
`
`30
`
`35
`
`40
`
`Hereinafter two examples of embodiment of the in-
`vention will be described in detail with the aid of FIGS.
`5 and 6.
`
`45
`
`SO
`
`55
`
`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 modification 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
`
`

`
`5,247,531
`
`6
`tion. The preionization apparatus according to the in-
`vention requires only four high-voltage leadthroughs
`into the laser chamber.
`
`5
`FIG. 1, i.e. the high-voltage source 16 and the energy
`storage means 18 respectively.
`The essential matter is the provision of an inductance
`(coil) 30 in the preionization and main discharge circuit
`and the arrangement of the line 32 with which the pre-
`ionization electrodes 12, 12’ are put under voltage.
`In the example of embodiment according to FIG. 5
`the high-voltage source 16 generates high-voltage
`pulses with which the discharge capacitors C1, C2 are
`successively charged. During the charging of the dis-
`charge capacitors C1, C2 great current changes and
`corresponding voltage drops occur at the inductance
`30. According to the invention the preionization elec-
`trodes 12, 12’ are supplied with voltage via a line 32
`which taps the potential off in the charging direction of
`the high-voltage pulses before the inductance 30. Due
`to this voltage tapping in front of the inductance 30, a
`very rapidly starting potential difference occurs be-
`tween the preionization electrodes 12, 12’ and the main
`electrode 10 adjacent thereto and correspondingly a
`corona discharge occurs on the dielectrics 14, 14’ sur-
`rounding the preionization electrodes 12, 12’. As re-
`gards time the corona discharge thus generated coin-
`cides with the voltage rise between the main electrodes
`10, 10’. The inductance 30 is so dimensioned that the
`time delay between the starting of the corona discharge
`and the main discharge gives a maximum laser power.
`For a given laser system the dimensioning of the capaci-
`tances and in particular of the inductance 30 depends on
`the specific line arrangements and the resulting parasitic
`inductances and capacitances and must be determined
`experimentally for the particular individual case. For a
`great number of excimer laser gas mixtures the time
`delay between preionization by means of corona dis-
`charge and subsequent main discharge has proved to be
`an optimum one for achieving a high laser power and
`long laser life. Special synchronization circuits are not
`necessary.
`A further advantage of the circuit arrangement ac-’
`cording to the invention for a preionization resides in
`that with increasing charging of the discharge capaci-
`tors C1, C2 the potential difference between the preioni-
`zation electrodes and the associated main electrode
`diminishes. The voltage pulse obtaining between said
`electrodes is therefore of relatively short duration so
`that the risk of a breakdown through the dielectric is
`considerably reduced. As dielectric, in particular A1203
`ceramic or sapphire have proved suitable.
`FIG. 6 shows a modification of the example of em-
`bodiment according to FIG. 5, the inductance 30 being
`replaced by two inductances 30a and 30b and the line 32
`applying the voltage to the preionization electrodes 12,
`12' tapping the voltage between the two inductances
`30a, 3012. By suitable variation of the inductances 30:1,
`30b the voltage profile between the preionization elec-
`trodes and the main electrode can be set experimentally
`and optimized for a specific system.
`The arrangement described above is very simple in
`construction and circuitry and has proved itself for a
`great number of excimer laser gas mixtures; in particu-
`lar, with XeCl and KrF laser gas mixtures homogene-
`ous glow discharges up to very high pulse repetition
`rates of 200 Hz were implementable. The efficiency of
`the emitted laser radiation was exactly as high as when
`using a substantially more complicated spark preioniza-
`
`The circuit arrangements described above with the
`aid of FIGS. 5 and 6 for preionization of a pulsed gas
`laser drive the preionization electrodes 12, 12' utilizing
`the inherent capacitance of said rod electrodes. Addi-
`tional capacitors and switches for operating the preioni-
`zation are superfluous.
`In addition, the arrangements described ensure an
`only short voltage pulse at the preionization electrodes,
`this leading on the one hand to an intensive corona
`discharge and on the other to a reduced risk of electrical
`breakdown of the dielectric. The geometry of the main
`discharge electrodes and the preionization electrodes
`described above with the aid of the Figures and the
`circuit described permit the use of solid profile main
`electrodes 10, 10’. Such solid profile main electrodes
`(i.e. solid electrodes in the form indicated in FIGS. 4
`and 5) have advantages as regards the erosion behav-
`iour and the control of the discharge cross-section.
`Apart from the advantages referred to above the circuit
`arrangement according to the invention also permits a
`compact structure.
`What is claimed is:
`1. A pulsed gas laser comprising:
`first and second main electrodes which are arranged
`in a chamber containing a gas;
`first and second preionizing electrodes arranged adja-
`cent said main electrodes for generating ultraviolet
`radiation which preionizes said gas between said
`main electrodes prior to a main discharge between
`said main electrodes;
`a first electrical circuit for generating said ultraviolet
`radiation including said preionizing electrodes and
`an electrical line for applying a voltage to said
`preionization electrodes;
`at second electrical circuit including said main elec-
`trodes for facilitating said main discharge between
`said main electrodes;
`a switching element for closing both said first electri-
`cal circuit including said preionizing electrodes and
`said second electrical circuit including said main
`electrodes;
`a capacitor common to _both said first electrical cir-
`cuit and said second electrical circuit which is
`connected in series with both said first and said
`second electrical circuit such that the charge of
`said capacitor is utilized for both a preionization of
`said gas and for said main discharge;
`a high voltage source for charging said capacitor;
`and, an inductance connected in series with said
`second electrical circuit including said main elec-
`trodes for setting a time delay between said preioni-
`zation and said main discharge.
`2. The pulsed gas laser of claim 1, wherein said first
`electrical circuit and said second electrical circuit addi-
`tionally comprise a second inductance wherein said
`electrical
`line of said first electrical circuit
`taps the
`voltage between said two inductances.
`"
`3. The pulsed gas laser of claim 1, wherein said pre-
`ionization electrodes are corona electrodes.
`4. The pulsed gas laser of claim 3, wherein said co-
`rona electrodes are surrounded by a tubular dielectric.
`it
`It
`III
`1!
`1K
`
`10
`
`l5
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`65