Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/102—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
  • H01S3/104—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the active medium, e.g. by controlling the processes or apparatus for excitation in gas lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/097—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser
  • H01S3/0971—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser transversely excited
  • H01S3/09713—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser transversely excited with auxiliary ionisation, e.g. double discharge excitation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/22—Gases
  • H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
  • H01S3/225—Gases the active gas being polyatomic, i.e. containing two or more atoms comprising an excimer or exciplex

Definitions

• This invention relates to a method for generating a laser beam, and a laser device for practising the method.
• the invention relates to a pulsed, transversely excited gas discharge laser.
• a pulsed, transversely excited gas discharge laser is an excimer laser, such as an XeCl laser.
• Such a laser device generally has the following structure.
• a gas chamber is provided with two oppositely arranged electrodes, to which a voltage can be applied for generating an electric field in the gas chamber, the main direction of which is designated as y-direction.
• a plasma to be generated between those electrodes causes amplification of light by stimulated emission.
• a laser beam can be generated by means of a resonator consisting of mirrors which may also close off the gas chamber. At least one of these mirrors should be partly transparent to the laser light in order that the laser beam can leave the gas chamber.
• the direction of egress of the laser beam is perpendicular to the y-direction and is designated as z-direction.
• the dimension of the laser beam in the y-direction is sometimes referred to as the height.
• the transverse dimension of the laser beam in the direction perpendicular to the y-direction, to be referred to as x-direction, is sometimes referred to as the width.
• An example of such a laser device is described in the publication "A Mew Mode to Excite a Gas-Discharge XeCl Laser" by J.C.M. Timmermans, F.A. van Goor and W.J. Witteman in Applied Physics B, vol. 57 (1993), pp. 441-445.
• the generation of the light-inducing plasma proceeds substantially in three steps.
• the gas is ionised by means of radiation, typically X-rays.
• the electron density is increased through a pre-discharge (breakdown) induced by a relatively short and high voltage pulse.
• the main discharge takes place, whereby a relatively large current flows through the plasma for a relatively long time.
• the laser beam has a profile as uniform as possible in a largest possible area of its cross section.
• the intensities Int(x,y) at different points in the beam are equal to each other to the highest possible extent and so are dependent to the least possible extent on the distances x and y of those points with respect to the main axis of the laser beam, measured perpendicularly to that main axis; and that the beam is defined as sharply as possible at its edge.
• An example of an application where a uniform laser beam profile is desired is the machining of a surface through a shadow mask, for instance in IC technology.
• an equal exposure strength occurs in all points of the surface to be machined, in order that an equal light exposure time results in an equal machining result (such as for instance the burn-off depth) , in particular when the beam is used to machine several products simultaneously via a plurality of juxtaposed identical masks.
• the laser beam profile in the y-direction satisfies the above-mentioned uniformity desire to a sufficient extent. This is due to the sharp boundary provided by the electrodes.
• a first principle is a mechanical principle, and involves the use of specially designed electrodes with a suitable shape (profiling) . It has been found, however, that electrode shapes that could yield a reasonable uniformity of the laser beam profile give rise to instability of the gas discharge. According to this principle, therefore, at best a compromise can be achieved between good uniformity on the one hand and stable gas discharge on the other.
• a further disadvantage of this mechanical principle is that it is relatively complicated and expensive. Furthermore, a disadvantage of this first principle is that it is not applicable in an existing laser device without interventions in that laser device, namely, the replacement of the electrodes.
• a second principle is an optical principle and is concerned with the improvement of the laser beam proper, generated by the laser device itself, by the use of optical means arranged at the output of that laser device.
• An example of this principle is described in the publication "Improvement of the first Kilowatt XeCl laser for different specific applications" by B. Godard, P. Murer, M. Stehle, J. Bonnet and D. Pigache in SPIE vol. 2206, pp. 25-29, and concerns their contribution to the conference on "High-Power Gas and Solid State Lasers” held in Vienna, Austria, from 5 to 8 April 1994.
• this principle cannot be regarded as a way of generating a laser beam which intrinsically has a good uniformity, but only as a way of treating (improving) a laser beam which has a poor uniformity.
• this optical principle can be applied in an existing laser device without interventions in that laser device, but a disadvantage is that the use of the optical correction means is accompanied by losses in beam strength. Further, this optical principle also has the disadvantage that it is relatively complicated and expensive.
• a particular laser beam profile is achieved in a given design of the laser device and/or of the optical correction means. Since that design is a fixed datum, it is not possible during operation of the laser device to modify the beam profile and/or to adjust it to changed operating conditions.
• the object of the invention is to provide a laser beam with a desired laser beam profile via a third principle which is fundamentally different from the above principles and which does not suffer from the disadvantages mentioned.
• the invention is based on the insight that after the pre-discharge, there is disposed between the laser electrodes a plasma with a rather sharp peak in the electron density, and that with the passage of time the electron distribution widens and becomes less concentrated, while the beam profile is determined to a considerable extent by the profile of the electron density at the instant of the main current. Accordingly, on the basis of this insight, in a method and device according to the present invention, a suitable combination is chosen of, on the one hand, the moment of initiation of the main current and, on the other, the time-dependent change in form of the electron density profile.
• the invention provides for the provision of an adjustable and controllable delay between the pre-discharge and the main current.
• the invention provides for the provision of an adjustable and controllable rate at which the electron density profile changes over time. The two variants mentioned can also be combined.
• Fig. 1 diagrammatically shows a side elevation of a laser device according to the invention
• Fig. 2 diagrammatically shows a cross section of that laser device
• Fig. 3 shows an illustrative example of the course of the voltage across and the current through the transversely excited gas discharge in an XeCl laser device according to the inven ion;
• Fig. 4 shows a circuit diagram of an example of an excitation circuit for the XeCl laser according to the invention
• Fig. 4B shows a simplified version of the circuit diagram of
• Fig. 5 is a graph showing the relation between charging voltage and time delay
• Fig. 6 is a graph showing the width profile of the laser beam at different values of the charging voltage
• Fig. 7 is a graph showing the relation between the temperature and the vapour pressure of HC1 and Xe.
• Fig. 8 is a graph showing the width profile of the laser beam at different values of the gas temperature.
• a laser device is generally denoted by the reference numeral l.
• the laser device l comprises a generally tubular gas chamber 2 and an optical axis 3, which defines the z-axis of a rectangular coordinate system.
• first and second mirrors 6 and 7 Disposed perpendicularly to the optical axis 3, at the ends of the gas chamber 2, are first and second mirrors 6 and 7.
• the first mirror 6 is non-transparent, while the second mirror 7 is partly transparent and defines an exit for a laser beam 10 from the gas chamber 2.
• laser electrodes 11, 12 are disposed, which are connected via lines 13, 14 to an electric energy source 15 arranged outside the gas chamber 2.
• a voltage applied between the laser electrodes 11, 12 will generate an electric field E which is substantially mirror-symmetrical and is perpendicular to the optical axis 3.
• the plane of symmetry of the electric field E defines the YZ plane in the rectangular coordinate system referred to.
• the space between the laser electrodes 11, 12 will be designated as discharge space 16.
• one of those electrodes 12 constitutes a wall portion of the gas chamber 2.
• the radiation 21 from the source 20 can reach the discharge space 16 via a window 17 in one electrode 12.
• a window 17 in one electrode 12 For a more detailed description of an example of such a laser device, reference is made to the above-mentioned publication "A New Mode to Excite a Gas-Discharge XeCl Laser" by J.C.M. Timmermans, F.A. van Goor and W.J. Witteman in Applied Physics B, vol. 57 (1993), pp. 441-445.
• the operation of the laser device l is known per se, and therefore will only be summarised briefly here.
• the gas chamber 2 is filled with a suitable gas or gas mixture at a suitable pressure.
• a pulse of ionising radiation 21 is fed to the gas mixture, so that in the discharge space 16 a part of the gas present there will be ionised.
• typically an electron density of about 10 7 cm "3 will be achieved.
• the source 15 delivers a second voltage pulse, also referred to as main pulse, for inducing a gas discharge.
• main pulse a second voltage pulse
• gas atoms and/or gas molecules will be ionised or excited.
• a process of chemical reactions will take place, leading to the formation of excited excimer molecules.
• the excited state is also the laser upper level.
• the gas in the gas chamber 2 is freshened through a gas flow 31 directed along the x-axis, generated, for instance, by a fan 30.
• the discharged gas can be collected in a line 32 and, optionally after cleaning, be returned via a return line 33 to the inlet of the fan 30.
• Fig. 3 shows an illustrative example of the course of the voltage V across the laser electrodes 11, 12 and the current I through the gas discharge as a function of time t.
• the zero point of the time axis is chosen to be the instant when the breakdown mentioned occurs. It clearly appears from this figure that in the illustrated example the main current, whose maximum occurs at about 300 ns, is delayed with respect to that breakdown by a time delay ⁇ t of approximately 100 ns. The occurrence of such a delay is known per se, as appears, for instance, from Fig. 4 of the above-mentioned publication in Applied Physics B, vol. 57 (1993), pp. 441-445.
• the present invention is based on the insight that it is possible to put said time delay ⁇ t to use, and that, in a first realisation mode of the concept of the invention, it is even possible to vary the time delay ⁇ t with relatively simple means and thereby to manipulate the width profile of the laser beam 10 in a useful manner, without adversely affecting the laser action of the laser device 1.
• the concentration of the molecules is greater, for instance by an order of magnitude, than the electron density, there will also occur a spatial redistribution of the electron density because of the fact that the initial spatial distribution of the halogen compounds in excited vibration states is approximately equal to that of the electrons.
• the rate at which that spatial redistribution of the electron density proceeds will be designated as electron redistribution rate.
• a suitable value for the electron redistribution rate it can be effected that even at a fixed value of the time delay ⁇ t a suitable spatial distribution profile of the electron density is present at the moment the main current arises, so that again a laser beam with a suitable beam profile can be generated.
• suitable values for both parameters i.e., both for the time delay ⁇ t and for the electron redistribution rate.
• Fig. 4 shows a schematic diagram of an example of a voltage source 15 capable of delivering two successive pulses at its output, i.e. at the laser electrodes 11, 12, in order to induce two successive discharges in the laser.
• a voltage source 15 capable of delivering two successive pulses at its output, i.e. at the laser electrodes 11, 12, in order to induce two successive discharges in the laser.
• This embodiment is discussed at length in the above-mentioned publication in Applied Physics B, vol. 57 (1993), pp. 441-445.
• Fig. 4B shows a simplified diagram of that excitation circuit, with omission of a few inessential details. The principle of operation, which is described in more detail in the publication mentioned, will now be briefly summarised.
• a main capacitor system C PFN is charged to a charging voltage V PFN , with a charging current flowing via a first magnetically saturable inductor Lc and a second saturable inductor L P . Also, a peaking capacitor C ? is charged to a voltage V P .
• a second switch T s i is closed, whereby, through a resonant charge transfer mechanism, the peaking capacitor Cp is charged to a high voltage which induces the pre-discharge (breakdown) . Thereafter the main discharge is induced from the main capacitor system Cpm, with a time delay ⁇ t occurring between the breakdown and the main discharge in that the second saturable inductor Lp must be brought to saturation first.
• V is the voltage across the second saturable inductor L P
• A is the magnetic surface
• ⁇ B is the change of the magnetic induction.
• the amount of magnetically saturable material in the second saturable inductor Lp is selected for achieving a desired time delay ⁇ t selected in relation to the electron redistribution rate for achieving a desired width profile of the laser intensity.
• setting a desired time delay ⁇ t means making modifications in the current source 15, with the laser device 1 itself remaining unmodified.
• the time delay ⁇ t referred to is settable and controllable in a simple manner by varying the charging voltage V PF1 ⁇ of the main capacitor system Cppn. This can be achieved in a simple manner.by varying the primary voltage HV SUS .
• the charging voltage V PF1T was selected to be 12 kv.
• the laser device 1 used in this experiment is comparable to the device already described in the above-mentioned publication by J.C.M. Timmermans et al in Applied Physics E.
• the window 17 had a width of about 2 cm, the distance between the electrodes 11 and 12 was about 3 cm, and the gas discharge had a z-dimension of about 80 cm.
• Cc ⁇ 5 nF, C? 3 nF, C PFN ⁇ 600 nF.
• the gas chamber 2 was filled with a mixture of HC1, Xe and Ne with a total pressure of about 4.5 bara (4.5- 10 5 Pa), with the partial pressures of HC1 and Xe being maintained equal to, respectively, 0.8 mbar (80 Pa) and 8.4 bar (840 Pa) .
• V PFN is expressed in multiples of V ss , which in this case was about 3.9 kV.
• V ss is meant herein: the voltage across the discharge during the so-called stationary condition which arises if upon the main discharge the loss of electrons resulting from dissociative attachment to HC1 equals the production of electrons resulting from ionisations; the value of V ss is exclusively dependent on the composition and the pressure of the gas and on the electrode gap.
• the inductor Lp can only start to conduct when it has been brought to saturation in the proper direction, which happens when the time integral of the voltage across the inductor becomes equal to or greater than the product of the magnetic surface and the maximum achieved magnetic induction change (see Faraday's law supra) .
• the higher the charging voltage V PF 7.j the sooner that time integral reaches the limiting product mentioned, in other words, the shorter the delay time referred to.
• the present invention provides the important advantage that even during operation of the laser device the width profile of the laser beam can be varied and set in a simple manner by a corresponding variation and setting of that charging voltage, for instance by variation and setting of the primary voltage HV SUS .
• a suitable combination of the time delay ⁇ t and the electron redistribution rate namely, by variation of the electron redistribution rate.
• This variant is based on the insight of the inventors that the electrons formed upon the breakdown are captured by the halogen compounds also formed in the process, and that the number of electrons captured per unit time is dependent on the concentration of those halogen compounds.
• the concept of the invention it is possible in a relatively simple manner to vary and set the concentration of those halogen compounds, as will be described in the following.
• the gas is removed from the gas chamber 2, and preferably returned to the gas chamber 2 via a return line 33.
• the gas before being fed to the gas chamber 2, is brought in heat exchanging contact with a condensation element, also referred to as 'purifier' , which has as a consequence that the partial pressure of the halogen donor, HC1 in the example discussed, corresponds with the vapour pressure thereof at the temperature of that condensation element.
• a condensation element also referred to as 'purifier'
• condensation element Since the nature and construction of such a condensation element are not a subject of the present invention, and a skilled person requires no knowledge thereof for a proper understanding of the present invention, while moreover use can be made of condensation elements which are known per se, they will not be discussed further here.
• T pur of the condensation element is selected and set in such a manner that the halogen donor vapour pressure associated with the temperature set corresponds with the desired halogen donor partial pressure associated with a particular delay time ⁇ t to yield a desired laser beam profile.
• the present invention can be of particularly great use for developing a laser device with a higher repetition frequency than has been possible so far. Thus a repetition frequency of 2 kHz will be attainable. This can be seen as follows.
• the gas in the discharge space 16 will have to be freshened faster. This could in principle be effected by increasing the rate of displacement of the gas, but above particular gas velocities a further increase meets with practical objections.
• An important problem here is, for instance, the occurrence of turbulences in the gas stream.
• the repetition frequency can still be considerably enlarged by reducing the width of the window 17, for instance to 0.5 cm, so that the pre-ionisation is narrowed.
• a consequence is that the profile of the laser beam 10 is changed, and more particularly a narrower window 17 will reduce the uniformity of the beam 10 in that the beam will acquire a rather high intensity at its centre and from there will rapidly become weaker towards the edges, but with the aid of the measures proposed by the invention such changes can be compensated and it is possible in particular to restore the uniformity of the beam profile.
• the invention makes an important contribution to the technical field of laser technology by increasing the usability of a laser. It will be clear to one of ordinary skill in the art that it is possible to change or modify the embodiment of the device according to the invention as represented, without departing from the concept of the invention or the scope of protection. Thus it is for instance possible that the laser device is an exci er laser with a different gas mixture, or a transversely excited gas laser of a different type.
• the KrF excimer laser can be mentioned here.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)
• Laser Surgery Devices (AREA)
• Particle Accelerators (AREA)
• Laser Beam Processing (AREA)

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• 1995
  • 1995-02-02 NL NL9500197A patent/NL9500197A/nl active Search and Examination
• 1996
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  • 1996-02-01 CA CA002186959A patent/CA2186959C/en not_active Expired - Fee Related
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