JP2006203008A - Two-stage laser system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an MOPO-method two-stage laser system suitable for a semiconductor exposure apparatus having low spatial coherence and high energy stability. <P>SOLUTION: The two-stage laser system consists of an oscillation stage laser, and an amplification stage laser 300 which receives the laser light oscillated by the oscillation stage laser as seed light; and then amplifies the seed light and outputs the amplified seed light. The oscillation stage laser and the amplification stage laser 300 each include chambers 30 filled with laser gas. The amplification stage laser 300 comprises a Fabry-Perot etalon stable resonator and a pair of slender discharge electrodes 30a and 30b for discharge excitation which are arranged in parallel so as to face each other inside the laser chamber. The Fabry-Perot etalon stable resonator comprises a rear mirror 36 and a front mirror 37. The rear mirror 36 and/or the front mirror 37 has a positive refractive power in the discharging direction and/or in a direction perpendicular to the discharging direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a two-stage laser system, and more particularly to a two-stage laser system capable of greatly improving output variations.

  In recent years, in an excimer laser and a fluorine molecular laser for semiconductor exposure, high output and ultra-narrow bandwidth have been required at the same time in order to improve the throughput of the exposure machine and realize uniform ultrafine processing.

  In order to increase the output, which is the first requirement, there are a method of increasing the energy per pulse, or a method of increasing the repetition frequency with low pulse energy.

  The ultra-narrow band, which is the second requirement, is achieved by increasing the resolution of a narrow-band module (LNM) that is usually composed of a prism and a grating, or by making the laser pulse longer as described in Patent Document 1. There is a way. However, an increase in the resolution of the LNM or a narrow band due to a long pulse generally leads to a decrease in pulse energy, such as an increase in optical loss. In other words, there is a trade-off between narrowing the band and pulse energy.

  Regarding the repetition frequency increase, a repetition frequency exceeding 4 kHz has a high technical hurdle. For this reason, there is a limit to increasing the output by increasing the repetition frequency while maintaining the ultra-narrow band in one laser.

  Therefore, in order to eliminate the trade-off relationship between ultra-narrow band and pulse energy and satisfy both requirements at the same time, a synchronous laser system (two-stage laser system) using two lasers is proposed in Patent Document 2, for example. Has been. The first oscillation stage laser has a very narrow band spectrum with low pulse energy. In the second amplification stage laser, only the pulse energy is amplified while maintaining the ultra narrow band spectrum of the oscillation stage laser. In this method, since the second amplification stage laser does not include an optical loss portion such as LNM, the laser oscillation efficiency is very high. This synchronous laser system makes it possible to obtain a desired ultra-narrow band spectrum and output. The desired output is the product of the pulse energy and the repetition frequency. For example, the spectrum and output required for the next generation ArF excimer laser are <0.25 pm (FWHM),> 40 W @ 4 kHz. In order to reduce damage to the exposure optical system, it is desirable that the laser pulse has a low peak power in order to reduce damage to the optical system, and a long pulse is required.

  In order to perform good processing with a high yield in the exposure process, it is necessary to stabilize the integrated pulse energy applied to the resist with high accuracy. In the laser system, in order to make the integrated energy within the target range, the charging voltage HV of the next pulse is determined according to the difference between the integrated energy up to the previous pulse and the target integrated energy, and a constant integrated value is obtained. Control is performed as follows.

  As an example of an energy control method in a two-stage laser system, there is an application of Japanese Patent Application No. 2004-203549.

  Here, in order to improve controllability, it is desirable that the energy stability of the laser output when the charging voltage HV is constant is higher.

The present invention to be described later generally relates to a laser apparatus and an exposure apparatus using the same, and relates to a laser apparatus using ArF, F ₂ (fluorine molecules), KrF or the like as a laser medium, and an exposure apparatus using the same. However, the laser apparatus is applied to a laser system composed of two or more lasers. One of them functions as an oscillation stage laser, and the remaining one or more lasers function as amplification stage lasers. This oscillation stage laser is a narrow-band laser equipped with a narrow-band resonator. The present invention to be described later can be applied to a case where the amplification stage laser has a resonator (referred to as injection locked laser or MOPO (Master Oscillator Power Oscillator) method). Is possible.
JP 2000-156367 A JP 2002-151776 A WO2004 / 095661 A1 JP-A-4-101474 JP 2001-111142 A JP-A-2-130990 JP 2003-224320 A

  FIG. 1 shows a configuration diagram of a laser system according to an embodiment to which the present invention described later is applied. 2A is a block diagram of the oscillation chamber and its vicinity, and FIG. 2B is a block diagram of the amplification chamber and its vicinity. FIG. 1 shows a MOPO type two-stage laser apparatus.

  In the two-stage laser apparatus 2, seed light (seed laser light) is generated by the oscillation stage laser (osc) 100 to narrow the band. Then, the seed light is amplified by the amplification stage laser (amp) 300 (hereinafter, symbols added with “osc” and “amp” are related to the oscillation stage laser 100 and the amplification stage laser 300, respectively. To express.). That is, the spectral characteristics of the entire laser system are determined by the spectral characteristics of the laser light output from the oscillation stage laser 100, and the laser output (energy or power) of the laser system itself is determined by the amplification stage laser 300. The laser beam output from the amplification stage laser 300 is input to the exposure apparatus 3, and this laser beam is used for exposure of an exposure target (for example, a wafer).

  The oscillation stage laser 100 includes an oscillation chamber 10, a charger 11, an oscillation high-voltage pulse generator 12, a gas supply / exhaust unit 14, a cooling water supply unit 15, an LNM 16, a front mirror 17, The first monitor module 19 and the discharge detection unit 20 are included.

  The amplification stage laser 300 includes an amplification chamber 30, a charger 31, an amplification high voltage pulse generator 32, a gas supply / exhaust unit 34, a cooling water supply unit 35, a rear mirror 36, an output mirror (front Mirror) 37 and a second monitor module 39.

  Here, the oscillation stage laser 100 and the amplification stage laser 300 will be described. Since the configuration is the same, the oscillation stage laser 100 will be described as a representative of that part.

  Inside the oscillation chamber 10, a pair of electrodes (cathode electrode and anode electrode) 10a, 10b that are separated by a predetermined distance, are parallel to each other in the longitudinal direction, and face the discharge surface are provided. A high voltage pulse is applied to these electrodes 10 a and 10 b by a power source constituted by a charger 11 and an oscillation high voltage pulse generator 12. Then, a discharge is generated between the electrodes 10a and 10b, and the laser gas sealed in the oscillation chamber 10 is excited by this discharge. An example of this power supply is shown in FIG.

  FIG. 3 is a diagram illustrating an example of a circuit configuration inside the power source and the chamber.

  The oscillation high-voltage pulse generator 12 shown in FIG. 3 (a) is a two-stage magnetic pulse compression circuit using three magnetic switches SR1, SR2, SR3 comprising saturable reactors. The magnetic switch SR1 is provided to reduce the switching loss in the solid switch SW and is also called magnetic assist. For example, a semiconductor switching element such as an IGBT is used for the solid switch SW. Instead of using the circuit of FIG. 3A, the circuit of FIG. 3B may be used. FIG. 3A shows a circuit including a step-up transformer Tr1 in addition to the magnetic pulse compression circuit, and FIG. 3B shows an example including a reactor L1 for charging the main capacitor C0 instead of the step-up transformer.

  The circuit configuration and operation will be described below with reference to FIG. Note that the circuit of FIG. 3B is not operated to be boosted by the step-up transformer, and other operations are the same as those of the circuit of FIG. Further, since the configuration and operation of the power supply of the oscillation stage laser 100 and the power supply of the amplification stage laser 300 are the same, description of the power supply of the amplification stage laser 300 is omitted.

  The voltage of the charger 11 is adjusted to a predetermined value HV, and the main capacitor C0 is charged. At this time, the solid switch SW is turned off. When the charging of the main capacitor C0 is completed and the solid switch SW is turned on, the voltage applied to both ends of the solid switch SW is mainly applied to both ends of the magnetic switch SR1. When the time integration value of the charging voltage V0 of the main capacitor C0 applied to both ends of the magnetic switch SR1 reaches a limit value determined by the characteristics of the magnetic switch SR1, the magnetic switch SR1 is saturated and becomes conductive. Then, a current flows through the loop of the main capacitor C0, the magnetic switch SR1, the primary side of the step-up transformer Tr1, and the solid switch SW. At the same time, current flows through the secondary side of the step-up transformer Tr1 and the loop of the capacitor C1, the charge stored in the main capacitor C0 is transferred to the capacitor C1, and the capacitor C1 is charged.

  When the time integration value of the voltage V1 at the capacitor C1 reaches a limit value determined by the characteristics of the magnetic switch SR2, the magnetic switch SR2 is saturated and becomes conductive. Then, a current flows through the loop of the capacitors C1, C2 and magnetic switch SR3, and the electric charge stored in the capacitor C1 is transferred to the capacitor C2, and the capacitor C2 is charged.

  When the time integration value of the voltage V2 at the capacitor C2 reaches a limit value determined by the characteristics of the magnetic switch SR3, the magnetic switch SR3 is saturated and becomes conductive. Then, a current flows through the loop of the capacitor C2, the peaking capacitor Cp, and the magnetic switch SR3, and the electric charge stored in the capacitor C2 is transferred to the peaking capacitor Cp, and the peaking capacitor Cp is charged.

  As shown in FIG. 3A, a preionization means including a first electrode 91, a dielectric tube 92, and a second electrode 93 is provided in the oscillation chamber 10. Corona discharge for preionization occurs on the outer peripheral surface of the dielectric tube 92 starting from a point where the dielectric tube 92 in which the first electrode 91 is inserted and the second electrode 93 are in contact with each other. As the peaking capacitor Cp is charged, the voltage Vp increases. When the voltage Vp reaches a predetermined voltage, corona discharge is generated on the outer peripheral surface of the dielectric tube 92. This corona discharge generates ultraviolet rays on the outer periphery of the dielectric tube 92, and the laser gas between the pair of electrodes 10a and 10b is preionized.

  As the peaking capacitor Cp is further charged, the voltage Vp of the peaking capacitor Cp increases. When this voltage Vp reaches a certain value (breakdown voltage) Vb, the laser gas between the pair of electrodes 10a and 10b is broken down and main discharge is started. This main discharge excites the laser medium. In the case of the oscillation stage laser 100, seed light is generated, and in the case of the amplification stage laser 300 (or amplifier), the injected seed light is amplified. Due to the main discharge, the voltage of the peaking capacitor Cp rapidly decreases, and eventually returns to the state before the start of charging.

  Such a discharge operation is repeatedly performed by the switching operation of the solid-state switch SW, whereby pulse laser oscillation is performed. The switching operation of the solid switch SW is performed based on an external trigger signal. The external controller that sends out this trigger signal is, for example, a synchronous controller 8 described later.

  In this embodiment, the magnetic switches SR2 and SR3 and the capacitors C1 and C2 constitute a two-stage capacitance transfer type circuit. In the capacitance transfer type circuit, if the inductance of each stage is set to decrease as going to the subsequent stage, a pulse compression operation is realized such that the pulse width of the current pulse flowing through each stage is sequentially narrowed. As a result, a short pulse strong discharge is realized between the pair of electrodes 10a and 10b and between the pair of electrodes 30a and 30b.

  Here, returning to FIG. 1, another configuration will be described.

A laser gas supplied from the gas supply / exhaust unit 14 is sealed inside the oscillation chamber 10. The gas supply / exhaust unit 14 is provided with a gas supply system for supplying laser gas into the oscillation chamber 10 and a gas exhaust system for exhausting the laser gas within the oscillation chamber 10. When this laser apparatus is used as a fluorine molecule (F ₂ ) laser, the gas supply / exhaust unit 14 uses fluorine (F ₂ ) gas and a buffer gas made of helium (He), neon (Ne), or the like. Supply to the oscillation chamber 10. When this laser apparatus is used as a KrF excimer laser, the gas supply / exhaust unit 14 is composed of krypton (Kr) gas and fluorine (F ₂ ) gas, helium (He), neon (Ne), and the like. Buffer gas is supplied to the oscillation chamber 10. When this laser apparatus is used as an ArF excimer laser, the gas supply / exhaust unit 14 is composed of argon (Ar) gas and fluorine (F ₂ ) gas, helium (He), neon (Ne), and the like. Buffer gas is supplied to the oscillation chamber 10. The supply and exhaust of each gas are controlled by opening and closing each valve of the gas supply / exhaust unit 14.

  A cross flow fan 10 c is provided inside the oscillation chamber 10. Laser gas is circulated in the chamber by the cross flow fan 10c and sent between the electrodes 10a and 10b.

  Further, a heat exchanger 10 d is provided inside the oscillation chamber 10. The heat exchanger 10d exhausts heat in the oscillation chamber 10 with cooling water. The cooling water is supplied from the cooling water supply unit 15. The supply of the cooling water is controlled by opening and closing the valve of the cooling water supply unit 15.

In the oscillation chamber 10 of FIG. 2A, windows 10e and 10f are provided on the laser beam output portion on the optical axis of the laser beam. The windows 10e and 10f are formed of a material that is transparent to laser light, such as CaF ₂ . The two windows 10e and 10f are arranged with their outer surfaces parallel to each other, installed at a Brewster angle to reduce reflection loss with respect to the laser beam, and the P-polarization direction of the laser beam is shown in FIG. The electrodes 10a and 10b are installed so as to be perpendicular to the discharge direction.

  The pressure sensor Pl monitors the gas pressure in the oscillation chamber 10 and outputs a signal indicating the gas pressure to the utility controller 5. The utility controller 5 generates and outputs a signal for instructing the gas supply / exhaust unit 14 to open and close each valve and its opening degree (or gas flow rate) based on processing described later. Then, since the gas supply / exhaust unit 14 controls the opening and closing of each valve, the gas composition and gas pressure in the oscillation chamber 10 are controlled.

  Laser power varies with gas temperature. Therefore, the temperature sensor Tl monitors the temperature in the oscillation chamber 10 and outputs a signal indicating the temperature to the utility controller 5. The utility controller 5 generates and outputs a signal for instructing the opening and closing of the valve and the opening degree (or the cooling water flow rate) to the cooling water supply unit 15 in order to obtain the desired temperature in the oscillation chamber 10. Then, since the cooling water supply unit 15 controls the opening and closing of the valve, the flow rate of the cooling water supplied to the heat exchanger 10d in the oscillation chamber 10, that is, the amount of exhaust heat is controlled.

  An LNM 16 is provided on the optical axis of the laser light on the window 10e side, outside the oscillation chamber 10, and a front mirror 17 is provided on the optical axis of the laser light on the window 10f side. The LNM 16 includes, for example, an optical element such as a magnifying prism and a grating (diffraction grating) that is a wavelength selection element. The LNM 16 may be configured by an optical element such as an etalon that is a wavelength selection element and a total reflection mirror. The optical element in the LNM 16 and the front mirror 17 constitute a laser resonator.

  The first monitor module 19 monitors laser beam characteristics such as energy, output line width, and center wavelength of the laser beam that has passed through the front mirror 17. The first monitor module 19 generates a signal indicating the center wavelength of the laser light and outputs this signal to the wavelength controller 6. The first monitor module 19 measures the energy of the laser light and outputs a signal indicating this energy to the energy controller 7.

  The configurations and functions of the electrodes 30a and 30b, the cross flow fan 30c, the heat exchanger 30d, and the windows 30e and 30f of the amplification chamber 30 are the same as the configurations and functions of the components of the oscillation chamber 10 described above. Further, a charger 31, an amplifying high voltage pulse generator 32, a gas supply / exhaust unit 34, a cooling water supply unit 35, a second monitor module 39, a pressure sensor P2, and a temperature sensor T2 provided in the amplification stage laser 300 are provided. The configuration and function are the same as the configuration and function of the same elements provided on the above-described oscillation stage laser 100 side.

  On the other hand, the amplification stage laser 300 is provided with a stable resonator described below in place of the resonator made of LNM or the like provided in the oscillation stage laser 100.

  That is, outside the amplification chamber 30, a rear mirror 36 is provided on the optical axis of the laser light on the window 30e side, and an output mirror 37 is provided on the optical axis of the laser light on the window 30f side. The rear mirror 36 and the output mirror 37 are both stable resonators whose reflecting surfaces are formed from a plane. A partial reflection film having a reflectance of about 50% to 95% is deposited on the reflection surface of the rear mirror 36, and a partial reflection film having a reflectance of 10% to 50% is deposited on the reflection surface of the output mirror 37. Yes. In order to prevent multiple reflections inside the mirror, an AR coat is deposited on the back surface of each mirror, and it is desirable that the edge angle is slightly (10 'to 1 °).

  A beam propagation unit 42 including a reflection mirror is provided between the front mirror 17 of the oscillation stage laser 100 and the rear mirror 36 of the amplification stage laser 300.

  The laser light transmitted through the front mirror 17 is guided to the rear mirror 36 by the beam propagation unit 42. Further, a part of the laser light passes through the partial reflection film of the rear mirror 36 and passes through the amplification chamber 30, and a part of the laser light is reflected by the partial reflection film of the output mirror 37. A portion of the laser light reflected by the output mirror 37 passes through the amplification chamber 30 and is partially reflected by the partial reflection film of the rear mirror 36. Further, a part of the laser light reflected by the rear mirror 36 passes through the amplification chamber 30 and is partially transmitted through the partial reflection film of the output mirror 37 to be output. When discharge occurs when the laser light passes through the discharge part of the amplification chamber 30, that is, between the electrodes 30a and 30b, the power of the laser light is amplified.

  Returning to FIG. 1, the wavelength controller 6 receives a signal related to the wavelength detected by the monitor modules 19 and 39. The wavelength controller 6 generates a signal for changing the selection wavelength of a wavelength selection element (grating, etalon, etc.) in the LNM 16 so as to set the center wavelength of the laser light to a desired wavelength, and outputs this signal to the driver 21. The selection wavelength of the wavelength selection element changes, for example, by changing the incident angle of the laser light incident on the wavelength selection element. Based on the received signal, the driver 21 changes the attitude angle of an optical element (for example, a magnifying prism, a total reflection mirror, or a grating) in the LNM 16 so that the incident angle of the laser light incident on the wavelength selection element changes. To control.

  The wavelength selection control of the wavelength selection element is not limited to this. For example, when the wavelength selection element is an air gap etalon, the pressure (nitrogen or the like) in the air gap in the LNM 16 may be controlled, or the gap interval may be controlled.

  An output signal related to the output detected by the monitor modules 19 and 39 is input to the energy controller 7. In addition, as shown in FIG. 1, an output monitor 51 may be provided in the exposure apparatus 3 so that the output signal is directly input to the energy controller 7. Further, the output signal of the output monitor 51 of the exposure apparatus 3 may be input to the controller 52 of the exposure apparatus 3, and the controller 52 may send a signal to the energy controller 7 mounted inside the laser. The energy controller 7 generates a signal indicating the next charging voltages Vosc and Vamp and outputs the signal to the synchronous controller 8 in order to set the pulse energy to a desired value based on processing described later.

  The synchronous controller 8 receives a signal from the energy controller 7 and a signal notifying the start of discharge in each of the chambers 10 and 30 output from the discharge detectors 20 and 40. The synchronous controller 8 controls the charging voltage of the charger 11 based on the signal from the energy controller 7. By the way, if the discharge timing of the oscillation chamber 10 and the discharge timing of the amplification chamber 30 are shifted, the laser light output from the oscillation chamber 10 is not efficiently amplified in the amplification chamber 30. Therefore, the laser beam (seed light) output from the oscillation chamber 10 is discharged in the amplification chamber 30 at a timing when the discharge region (excitation region) between the pair of electrodes 30a and 30b in the amplification chamber 30 is filled. There is a need to.

  Here, the discharge timing of the oscillation stage laser 100 and the amplification stage laser 300 will be described.

  As described above, in order to apply a high voltage pulse having a fast rise time between the electrodes 10a and 10b in the oscillation chamber 10 and between the electrodes 30a and 30b in the amplification chamber 30, magnetic pulses are respectively used. An oscillation high voltage pulse generator 12 having a compression circuit and an amplification high voltage pulse generator 32 are used. In general, the magnetic switches SR2 and SR3 used in the magnetic pulse compression circuit of each of the high voltage pulse generators 12 and 32 are saturable reactors. When energy (voltage pulse) is transferred from the main capacitor C0, the voltage applied to the magnetic switches SR2 and SR3 (V: the charging voltage of the main capacitor C0) and the voltage that is pulse-compressed and transferred by the magnetic switches SR2 and SR3. The value of the product (Vt product) with the pulse transfer time (t) is constant. For example, when the charging voltage of the main capacitor C0 increases, the voltage pulse transfer time (that is, the time during which the magnetic switch is on) is shortened.

  Here, it is necessary to reduce the in-plane low coherence (spatial coherence) in the cross section of the laser beam profile with respect to the two-stage laser apparatus as shown in FIG. 1 (Patent Document 3).

  Furthermore, it is necessary to stabilize the accumulated energy with high accuracy as the throughput of the exposure machine is improved and miniaturized.

  It is necessary to raise the oscillation frequency because of the demand for high throughput. On the other hand, a method using an electrode having a narrow discharge width (1 mm to 4 mm) (hereinafter referred to as an edge electrode) is proposed in Patent Document 4 and the like. By using this method, a region where a discharge product accompanying discharge is generated is narrowed, and a fan flow speed necessary for removing this is reduced. Therefore, it is suitable for high frequency oscillation. However, when such an electrode is used, there is a sudden change in electric field strength in the vicinity of the edge portion. Therefore, the discharge becomes unstable near the edge portion. If the discharge becomes unstable, the generated gain becomes unstable, and as a result, the oscillated / amplified energy becomes unstable.

  Further, from the viewpoint of the load on the optical element of the exposure machine, it is required to extend the oscillated pulse waveform. For this, there is a discharge pulse stretching technique proposed in Patent Document 5 and the like. This makes it possible to extend the laser oscillation pulse width. However, when this technique is used, since the excitation discharge is extended for pulse stretching, the latter half of the discharge tends to become unstable, and the gain contracts to the center of the discharge space as the gain decreases.

  Also, when high repetition oscillation is actually performed, the discharge tends to become unstable due to the remaining discharge product with a lifetime, the remaining acoustic wave accompanying the discharge, and the rise of the electrode surface temperature. It becomes extremely unstable at low gain parts (electrode surface, discharge width edge).

  The above instability of discharge leads to instability of output, and as a result, it is presumed that the accumulated energy is a barrier to stabilization.

  The present invention has been made in view of such problems of the prior art, and its purpose is to take advantage of the high stability, high output efficiency, and narrow line width of the MOPO method, while having low spatial coherence and low energy. To provide a two-stage laser system suitable for a highly stable semiconductor exposure apparatus.

The two-stage laser system of the present invention that achieves the above object comprises an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light and amplifies and outputs the seed light. Both the oscillation stage laser and the amplification stage laser are provided with a chamber filled with laser gas, and the amplification stage laser is disposed in a Fabry-Perot etalon-type stable resonator and is disposed in the laser chamber and is opposed to each other in parallel with each other. A two-stage laser system comprising a pair of long discharge electrodes for
At least one of the rear mirror and the front mirror constituting the Fabry-Perot etalon type stable resonator of the amplification stage laser has a positive refractive power in at least one of the discharge direction and the direction orthogonal to the discharge direction. .

In this case, one of the rear mirror and the front mirror constituting the Fabry-Perot etalon type stable resonator of the amplification stage laser has a positive refracting power in at least one of the discharge direction or the direction orthogonal to the discharge direction, and the resonance of the resonator The radius of curvature in that direction of the mirror having positive refracting power is L
0.001 <L / (curvature radius) <0.77
It is desirable to satisfy

  In that case, it is desirable to satisfy the following expression.

0.001 <L / (curvature radius) <0.20
Alternatively, it is desirable that the following expression is satisfied.

0.001 <L / (radius of curvature) <0.03
Another two-stage laser system of the present invention comprises an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light, amplifies the seed light, and outputs the seed light. Both the laser and the amplification stage laser are provided with a chamber filled with a laser gas. The amplification stage laser is disposed in a Fabry-Perot etalon type stable resonator and a pair of discharge excitations arranged in the laser chamber and facing each other in parallel. In a two-stage laser system comprising a long discharge electrode,
In the Fabry-Perot etalon-type stable resonator of the amplification stage laser, a transmissive optical element having a positive refractive power is disposed at least in one of the discharge direction or the direction orthogonal to the discharge direction. .

Still another two-stage laser system of the present invention comprises an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light and amplifies and outputs the seed light. Both the stage laser and the amplification stage laser have a chamber filled with a laser gas. The amplification stage laser is disposed in a Fabry-Perot etalon-type stable resonator and is disposed in the laser chamber and is opposed to each other in parallel with each other. In a two-stage laser system comprising a pair of long discharge electrodes,
An optical element having a positive refracting power is disposed in at least one of the discharge direction and the direction perpendicular to the discharge direction of the amplification stage laser in an optical path for inputting seed light from the oscillation stage laser to the amplification stage laser. It is characterized by.

In this case, when the resonator length of the Fabry-Perot etalon type stable resonator of the amplification stage laser is L, the refractive power in the direction having the positive refractive power of the optical element is
0 <(refractive power) · L <0.18 (9)
It is desirable to satisfy

Another two-stage laser system of the present invention comprises an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light and amplifies and outputs the seed light. Both the stage laser and the amplification stage laser have a chamber filled with a laser gas. The amplification stage laser is disposed in a Fabry-Perot etalon-type stable resonator and is disposed in the laser chamber and is opposed to each other in parallel with each other. In a two-stage laser system comprising a pair of long discharge electrodes,
A slit member for limiting the beam diameter is disposed in the Fabry-Perot etalon type stable resonator of the amplification stage laser or on the output side of the amplification stage laser at least in one of the discharge direction and the direction orthogonal to the discharge direction. It is characterized by this.

  In the above two-stage laser system, it is desirable that the transverse mode of the oscillation laser light of the oscillation stage laser is multimode, and the amplification stage laser amplifies the multimode laser light while maintaining the multimode.

  In the two-stage laser system of the present invention, in the amplification stage laser, the laser beam is collected in a stable region without using the region where the discharge becomes unstable, and the energy is remarkably stabilized without greatly reducing the output. Improves. By using a transverse multimode oscillation as the oscillation stage laser, a two-stage laser system suitable for a semiconductor exposure apparatus with low spatial coherence and high energy stability can be obtained.

  In the two-stage laser system of the present invention, in order to reduce the spatial coherence, the transverse mode of the oscillation stage laser is set to a multimode, and is input to an amplification stage laser that restricts the transverse mode, and is amplified in the multimode. Thus, it is found that the low coherence performance proposed in Patent Document 3 is not lost, and further, the energy stability is further improved by using a stable resonator as described below for the amplification stage laser. Found that it would be possible.

  In other words, in an amplification stage laser, by collecting laser light in a stable region without using a region where the discharge becomes unstable, energy stability can be significantly improved without greatly reducing the output. It is.

  Similarly, in the discharge of the oscillation stage laser, there is a region where the discharge is unstable, but the slit is disposed, and the energy output from the amplification stage laser is output to the exposure machine, Some fluctuation of the energy of the oscillation stage laser is not a big problem.

  In addition, although it refuses beforehand, in the thing of patent document 6, what uses the concave mirror as a resonator of an amplification stage laser is indicated, but it does not aim at high stabilization of energy, No specific numerical values such as a range of good curvature of the concave mirror are disclosed. Further, the transverse mode of the oscillation stage laser is not multimoded.

  In addition, there are other known examples in which the curvature of the resonator of the amplification stage laser is increased in order to increase the output, but none is intended to stabilize the energy.

  The main part of the MOPO-type two-stage laser apparatus using a stable resonator in the amplification stage laser shown in FIG. 1 will be described below, and a modified part according to the present invention will be described as an example.

  FIG. 4 shows the main part of a two-stage laser device of the MOPO type using a stable resonator composed of a rear mirror 36 and a front mirror (output mirror) 37 which are a plane mirror and an amplification stage laser 300 disclosed in Patent Document 3. FIG. 5 is a plan view of the amplification stage laser (PO) 300 and a plan view (b) of the oscillation stage laser (MO) 100. FIG.

  The oscillation stage laser 100 includes a chamber 10 in which a laser gas is filled in a laser resonator composed of a rear mirror and a front mirror 17 that is also used as an LNM 16 composed of a magnifying prism 16b and a diffraction grating 16a. The front mirror 17 is configured as a partially reflecting mirror. The discharge mirrors 10a and 10b form a gain region by exciting the laser gas therein. Further, slits 10 s for limiting the beam diameter of the output light (seed light) are arranged before and after the chamber 10.

  The amplification stage laser 300 includes a chamber 30 filled with a laser gas in a stable resonator composed of a rear mirror 36 and a front mirror 37, and a discharge electrode 30a that excites the laser gas in the chamber 30 to form a gain region. The rear mirror 36 and the front mirror 37 are configured as partial reflection mirrors.

  In such a configuration, the laser gas in the chamber 10 is excited by the main discharge between the pair of discharge electrodes 10a and 10b of the oscillation stage laser 100, seed light is generated from the oscillation stage laser 100, and the propagation mirrors 42a and 42b are transmitted. Through this, seed light is injected into the amplification stage laser 300. Also in the amplification stage laser 300, the laser gas in the chamber 10 is excited by the main discharge synchronized with the oscillation stage laser 100 between the pair of discharge electrodes 30a and 30b, and the seed light input from the oscillation stage laser 100 becomes the rear mirror 36 and the front mirror. 37 is amplified by passing through the gain region in the chamber 10 while being subjected to multiple reflections in the stable resonator composed of 37, passes through the front mirror 37, and is input to the exposure apparatus 3 or the like as output laser light.

  Now, a first embodiment according to the present invention will be described. FIG. 6 is a plan view of an amplification stage laser (PO) 300 corresponding to FIG. 5A. In this embodiment, the laser resonator of the amplification stage laser 300 is a front mirror (output mirror) 37 made of a plane mirror. The partial reflecting mirror is a concave mirror. In this case, an antireflection coating (antireflection coating) is applied to the incident side of the rear mirror 36, a partial reflection mirror coating is applied to the entire surface near the chamber 30, and an antireflection coating (antireflection coating) is applied to the output side of the front mirror 37. And a partially reflecting mirror coating is applied to the entire concave surface near the chamber 30. Other configurations are the same as those in FIGS. 4 and 5. It should be noted that the partial reflection mirror coating and the non-reflection coating are provided on the rear mirror 36 and the front mirror 37 in the following embodiments unless otherwise specified.

  In the test conducted by the present inventor, by setting the curvature radius of the front mirror 37 to 5 m and 20 m, significant energy stability can be realized as compared with the case where the conventional plane mirror is used for the front mirror 37. The test results are shown in FIGS. The horizontal axis represents the energy E of the output laser beam, and the vertical axis represents the variation (standard deviation) σ.

  By using a concave mirror having a curvature radius of 5 m or 20 m for the front mirror 37, the energy variation σ is substantially halved compared to the case of using a conventional plane mirror. The resonator length at this time was 1.0 m.

  The cause of such a decrease in energy variation is that the laser beam is narrowed by the concave mirror 37 of the laser resonator of the amplification stage laser 300 so that the discharge region end, which is an unstable region of discharge, is no longer used. It is estimated that

  Here, the discharge region end is a side view of a two-stage laser apparatus having the same configuration as that of FIG. 4 in FIG. 9A and FIG. 9B is along a straight line AA ′ of FIG. 9A. As shown in the cross-sectional view, the region around the discharge region X formed between the discharge electrodes 30a and 30b of the amplification stage laser 300, in the vicinity of the surfaces of the electrodes 30a and 30b, and in the discharge width direction of the discharge region X ( There are both end portions of FIG. That is, it is an area surrounding the periphery of the discharge area X in the optical axis direction. In these regions, since the electric field distribution in the discharge operation is non-uniform, the inversion distribution generated by the discharge is also non-uniform. As a result, since the amount of amplification in the region varies for each pulse, it is estimated that the energy stability is deteriorated.

  This unstable discharge region is particularly prominent in the latter half of the pulse. Therefore, it is important not to use this unstable discharge region in the latter half of the pulse, especially in order to suppress the deterioration of the stability of energy dispersion. Presumed.

  In general, a laser beam has a beam waist and a divergence angle. Here, as shown in FIG. 10A, a beam of injection light (seed light) Y from the oscillation stage laser 100 to the amplification stage laser 300. The waist size is W [mm], and the beam divergence angle (full angle) is Δθ [mrad]. In the present invention, as the oscillation stage laser 100, one having a multimode transverse mode is used in order to reduce spatial coherence, and the transverse mode of the injected light Y remains multimode.

The beam width W ′ [mm] when this beam Y propagates A [m] is
W ′ = W + A · Δθ (1)
Is roughly obtained.

  As shown in FIG. 10B, the beam waist size W and its position are determined by arranging a convex lens 301 in the injection light Y immediately after being emitted from the oscillation stage laser 100, and the minimum spot size connected by the convex lens 301. It is possible to estimate from the position based on the imaging relationship of the convex lens 301. In the case of FIG. 10B, the beam waist position is virtually located behind the oscillation stage laser 100.

  As shown in FIG. 4, when a plane mirror-plane mirror is used for the laser resonator of the amplification stage laser 300, there is no refractive power inside the resonator. In the case where the amplification stage laser resonator is reciprocated n times, assuming that the resonator length is L [m], it is substantially equal to propagating 2n · L [m] in total.

  FIG. 11 is a developed view in consideration of the multiple reflection of the injection light Y in the amplification stage laser resonator. In the following, the examination is performed from two viewpoints.

First point:
As described above, it is desirable that the beam does not pass through the region at the end of the discharge region. Therefore, the width of the light beam when the resonator is reciprocated n times is considered.

Here, before entering the amplification stage laser resonator, the refractive power φ [m ⁻¹ ] (= 1 / f [m], where f is the focal length of the lens) at the position L ₀ [m] from the beam waist. Consider the beam flux width W ′ [mm] after placing the lens and then propagating L ₁ [m].

  At this time, it can be simply expressed by using a ray transmission matrix (ray matrix).

  For example, n. 5 (1.5, 2.5,..., 10.5, etc.) The incident position at the position of the front mirror FM (37) when reciprocating, the emission position with respect to the incident angle (x, θ) , The exit angle (x ', θ') is

と表記されるし、
ｎ往復したときのリアミラーＲＭ（３６）位置での出射位置，出射角度（ｘ’，θ’) は、

And
The exit position and exit angle (x ′, θ ′) at the position of the rear mirror RM (36) when reciprocating n times are

と表記される。

It is written.

  Furthermore, the beam at the position after reciprocating n and reflected by the rear mirror RM and passing through the half of the resonator is

となる。

It becomes.

  Now,

としたときの出射位置でのビームの最大幅は、ビームウエストサイズをＷ［ｍｍ］、ビーム拡がり角をΔθ［ｍｒａｄ］としたときに、
Ｍａｘ（Ｗ／２）＝｜Ａn ｜・｜Ｗ／２｜＋｜Ｂn ｜・｜Δθ／２｜
したがって
Ｍａｘ（Ｗ）＝｜Ａn ｜・｜Ｗ｜＋｜Ｂn ｜・｜Δθ｜ ・・・（２）
となる。

The maximum width of the beam at the exit position when the beam waist size is W [mm] and the beam divergence angle is Δθ [mrad],
Max (W / 2) = | An ||| W / 2 | + | Bn | · | Δθ / 2 |
Therefore, Max (W) = | An ||| W | + | Bn | · | Δθ | (2)
It becomes.

  As representative points, focus on the rear mirror RM position (a), the front mirror FM position (b), and the intermediate position (c) of the resonator, and the maximum number of times until the light is emitted from the front mirror FM through the number of round trips The maximum beam width is calculated (FIG. 12).

  FIG. 13 is a development view showing how the maximum light flux changes when the front mirror FM is curved (FIG. 6).

An example of calculation is shown. Considering only the direction perpendicular to the discharge as the incident condition, the initial beam width was 4 mm and the initial divergence (beam divergence angle) was 1 mrad. The resonator length was 1 m, and the width of the maximum beam flux at the resonator mirror and the gain center portion at each reciprocation number when L ₀ = _{0 and} 8.5 reciprocation was estimated. When this increases, the beam tends to expand, vignetting from the discharge space increases, and the gain at the end of the discharge region also increases relatively. The calculation results are shown in FIG. The horizontal axis represents the reciprocal of the radius of curvature R of the front mirror FM, and the vertical axis represents the maximum beam flux width.

  Here, when the light reciprocates 8.5 times in the resonator of L = 1 m, the propagation distance is 17 m, the propagation time is 57 ns, and generally corresponds to the second half of the pulse in consideration of the light emission time from the discharge. .

  As can be seen from the calculation result of FIG. 14, the width of the beam is reduced compared to the case of using a flat front mirror (1 / R = 0) by using a concave mirror having a curvature radius R of 1 m to ∞ m. It is a case where it applies, and it is good to set it as the curvature radius between 1.2m-25m better.

  This characteristic varies slightly depending on the number of round trips taken into consideration, the width of the initial beam light flux, the divergence, etc., but the good region is substantially the same region.

  In addition, the relationship between the propagation distance and beam beam width at a typical curvature (= 1 / R) is shown in FIGS.

  Further, FIG. 16 shows the same characteristics as FIG. 14 when the final round-trip number to be noticed is changed. As can be seen from FIG. 16, the influence of the number of reciprocation points is not so great.

Second point:
The second study is based on ray tracing. Although the first study was very simple, it was calculated properly because it was rough. That is, a predetermined number of times of reciprocation was traced, and the ratio of transmission through a predetermined slit was calculated by changing the curvature of the front mirror FM. The results are shown in FIG. As a premise of this calculation, the vertical and horizontal sizes of the beam waist size are 4 mm × 16 mm, the beam divergence angle is 1 mrad, the resonator length L = 1 m, the distance L ₀ = 1 m from the beam waist to the amplification stage laser resonator, and the vertical and horizontal size is 4 mm. A slit of x16 mm is arranged at a position corresponding to the rear mirror 36 and the front mirror 37.

  In such an arrangement, it can be said that it is desirable that the transmittance is high when a predetermined number of reciprocations are transmitted, from the gist of the present invention that the discharge region end is not passed. The results of FIG. 17 mean that the range of the curvature 1 / R that can be stably used is larger than 0, smaller than 1, and close to 0 in the meantime, as compared with the maximum luminous flux diagram of FIG. It can be seen that both agree.

  Therefore, future studies will proceed more simply by using the first ray transfer matrix.

Now, when considered in terms of the ray transfer matrix, in each parameter of FIG. 13, in the defined W, Δθ, L ₀ , L,
α = Δθ · L / W,
β = L ₀ / L
Considering such a new parameter, the dependence characteristic of the maximum luminous flux on the curvature L / R of the mirror normalized by the resonator length L can be uniquely described. Here, α is related to the beam waist size and the divergence angle and should be called the shape parameter of the incident beam, and β is the beam waist position normalized by the resonator length L. Should be called relative position parameters.

  A typical calculation result is shown in FIG. Here, the vertical axis is not the absolute value of the maximum luminous flux, but is a numerical value normalized as 1 when the maximum luminous flux is minimum. In this case, the range of the standardized curvature L / R of the mirror can be determined so as to increase with respect to the minimum maximum luminous flux.

For example, in the case where the maximum luminous flux is allowed to be double that of when the maximum luminous flux is minimum, a favorable curvature range is determined by the values of α and β as shown in FIG. Considering a typical laser, β = ˜10, α = 0.1-1,
0.001 <L / R <0.77 (3)
It is good to do.

  If the resonator length L = 1 m, it is desirable to use a mirror having a radius of curvature R of 1.3 m to 1000 m.

If this is allowed until the maximum luminous flux is 1.5 times that when the maximum luminous flux is minimum,
0.004 <L / R <0.61 (4)
Is a good range.

  Here, FIG. 20 shows a modification of the amplification stage laser 300 of the first embodiment based on the present invention of FIG. FIG. 20 is a plan view of an amplification stage laser (PO) 300 corresponding to FIG. 5A, and the directions of windows 30e and 30f provided in the chamber 30 are different between the example of FIG. 6 and the example of FIG. In the case of FIG. 6, the rear side window 30e and the front side window 30f are inclined to the opposite side with respect to the optical axis, but in the case of FIG. 6, the rear side window 30e and the front side window 30f are installed. The windows 30f are installed substantially parallel to each other. The same applies to the following embodiments, and the installation angles and inclination directions of the windows 30e and 30f provided in the chamber 30 are arbitrary.

  The direction in which the curvature to be applied to the front mirror 37 of the amplification stage laser 300 is provided is arbitrary. As shown in the plan view of FIG. 21A and the side view of FIG. 21B, a spherical mirror may be used for the front mirror 37 to provide curvature in both the discharge direction and the direction orthogonal to the discharge direction.

  Further, as shown in FIGS. 22 and 23, a cylindrical mirror is used as the front mirror 37, and only the discharge direction (FIG. 22) or only the direction orthogonal to the discharge direction (FIG. 23). In the former case, the beam is prevented from passing through the region where the discharge is unstable near the surfaces of the electrodes 30a and 30b, and in the latter case, the beam is passed through the region where the discharge is unstable at both ends in the discharge width direction. May not pass. The range of the good radius of curvature R at this time is as shown above. Note that, instead of the cylindrical mirror, a toric reflection surface having different curvatures in two orthogonal directions may be used so as to give a difference between the discharge direction and the region that avoids passage in the direction orthogonal to the discharge direction. In any case, the use of an unstable region at the end of the discharge region can be avoided, and the energy stability is greatly improved.

  In the following embodiments, a description will be given with respect to an embodiment in which curvature is given in a representative direction. However, as described above, a spherical mirror (or lens) having the same curvature in two orthogonal directions is used. Alternatively, the curvature may be given only in the discharge direction, or different curvatures may be given in the direction orthogonal to the discharge direction.

  In the embodiment shown in FIGS. 6 and 20, as shown in FIG. 24A, the output laser light 400 extracted from the front mirror 37 is largely spread when reflected by the front mirror 37, and exposure is performed. There may be a problem that the beam is vignetted during propagation to a machine. In order to solve such a problem, as shown in FIG. 24B, it is possible to suppress the spread of the output laser beam 400 by arranging an appropriate correction lens 305 after exiting the front mirror 37. In this lens 305, in order to reduce loss and prevent parasitic oscillation caused by reflected light, a non-reflective coating (anti-reflection coating) is preferably provided on both surfaces.

FIG. 25 shows the difference in emission angle with and without a correction lens under typical conditions. The vertical axis represents the maximum emission angle ratio with respect to the divergence angle of the output laser beam when the amplification stage laser resonator including the plane mirrors of FIGS. 4 and 5 is used and the correction lens is not used. As described above, if this angle becomes too large, a problem occurs and there is an allowable range. For example, when an amplification stage laser resonator composed of a plane mirror can be used up to 5 times, when there is no correction lens, from FIG.
When L / R <0.15 but the correction lens 305 is inserted,
L / R <0.20 (5)
And the allowable range is widened.

If this is acceptable up to 2 times,
L / R <0.03 (6)
It becomes.

If the allowable range is determined from the restriction between the maximum luminous flux in FIG. 18 and the emission angle in FIG. 25, the problem of the divergence angle of the output laser beam can be solved, and the stability of the energy variation which is the object of the present invention is improved. Is possible. Typically, since the output angle is more restrictive, from the upper limit determined from this limit and the lower limit determined from the maximum light flux constraint, for example, from (3) and (5),
0.001 <L / R <0.20 (7)
From (3) and (6),
0.001 <L / R <0.03 (8)
It becomes.

  In any case, by adding a curvature to the front mirror 37 so as to satisfy the formula (7) or the formula (8), the stability of the energy variation can be improved and the spread of the output laser beam can be suppressed. .

  FIG. 26 shows the refractive power (reciprocal of the focal length (m)) necessary for the correction lens 305 as described above.

  24, instead of providing the correction lens 305 on the exit side of the front mirror 37 as shown in FIG. 24, the curvature is provided on the exit surface 306 of the front mirror 37 so as to have a refractive power equivalent to that of the correction lens 305 as shown in FIG. You may make it attach.

  Also, instead of FIG. 6 or FIG. 24, as shown in FIG. 28, a biconvex positive lens or convex plano positive lens 307 having a convex surface facing the rear mirror 36 is prepared as a front mirror 37, and the front mirror 37 of the above example is prepared. Conversely, a non-reflective coating (antireflection coating) may be applied to the incident surface, and a partial reflection mirror coating may be applied to the output surface. Thereby, since some refractive power can be given at the time of emission, some spread correction is possible. Of course, an external correction lens 305 (FIG. 24) may be combined therewith.

  Next, a second embodiment according to the present invention will be described. In this embodiment, as shown in FIG. 29, in the laser resonator of the amplification stage laser 300, the partial reflecting mirror of the rear mirror 36 is a concave surface. The favorable curvature range of the concave surface of the partial reflector is calculated in the same manner as in the first embodiment, and the expressions (3) to (8) can be applied as they are.

  Similarly to the case of FIGS. 22 and 23, a cylindrical mirror is used as the rear mirror 36, and a curvature is given only in the discharge direction (see FIG. 22) or only in the direction orthogonal to the discharge direction (see FIG. 23). In the former case, the beam is prevented from passing through the region where the discharge is unstable near the surfaces of the electrodes 30a and 30b, and in the latter case, the beam is not passed through the region where the discharge is unstable at both ends in the discharge width direction. May be. The range of the good radius of curvature R at this time is as shown above. Note that, instead of the cylindrical mirror, a toric reflection surface having different curvatures in two orthogonal directions may be used so as to give a difference between the discharge direction and the region that avoids passage in the direction orthogonal to the discharge direction. In any case, the use of an unstable region at the end of the discharge region can be avoided, and the energy stability is greatly improved.

  As a modification of FIG. 29, as shown in FIG. 30, the refractive power of the transparent substrate of the rear mirror 36 may be reduced by adding a curvature to the surface 308 of the rear mirror 36 on the seed light injection side. In addition, as shown in FIG. 31, a convex flat positive lens or a biconvex positive lens 309 having a convex surface facing the seed light injection side is prepared as a rear mirror 36, and is partially reflected on the incident surface, contrary to the rear mirror 36 in the above example. A mirror coating may be applied, and a non-reflective coating (antireflection coating) may be applied to the surface on the front mirror 37 side.

  Also in this case, a cylindrical mirror is used as the rear mirror 36, and curvature is given only in the discharge direction (see FIG. 22) or only in the direction orthogonal to the discharge direction (see FIG. 23). In the former case, the electrodes 30a and 30b are used. The beam may be prevented from passing through the region where the discharge is unstable near the surface, and in the latter case, the beam may not be passed through the region where the discharge is unstable at both ends in the discharge width direction. The range of the good radius of curvature R at this time is as shown above. Note that, instead of the cylindrical mirror, a toric reflection surface having different curvatures in two orthogonal directions may be used so as to give a difference between the discharge direction and the region that avoids passage in the direction orthogonal to the discharge direction. In any case, the use of an unstable region at the end of the discharge region can be avoided, and the energy stability is greatly improved.

  Next, a third embodiment according to the present invention will be described. In this embodiment, as shown in FIG. 32, the first and second embodiments are combined. In the laser resonator of the amplification stage laser 300, the front mirror 37 and the rear mirror 36 are curved to form a concave mirror. Is. The combination of the curvatures of the mirrors may be in the range described for the first embodiment and the second embodiment.

  A good range obtained by specific calculation results is shown. However, this calculation was based on the method using the first ray transmission matrix. However, for the sake of simplicity, the g parameter is used. Here, it is a parameter defined by g = 1−L · f / 2 = 1−L / R (see Patent Document 7).

The value of the maximum luminous flux when normalized with the g parameter of the front mirror 37 as gf on the vertical axis and the g parameter of the rear mirror 36 on the horizontal axis as gr, as shown in FIG. FIG. 33 shows the range of the g parameter when is used as a variable. In the case where the maximum luminous flux is allowed to be double that of when the maximum luminous flux is minimum, the range is x2 in FIG. When allowing up to 3 times, the range is x3, and when allowing up to 4 times, the range is x4. This region is known as "stable resonator":
0 ≦ gf · gr ≦ 1
It almost matches.

  Furthermore, among these, the range with good characteristics is the range of x2. For example, if gf = gr = 0.6 within the range, that is, the resonator length L = 1 m, the energy stability is good if the curvature radius is 2.5 m for both the front mirror 37 and the rear mirror 36. A stage laser system is obtained.

  The first embodiment and the second embodiment can be said to be an example of the present embodiment, and ex1 and ex2 shown in FIG. 33 are the desired ranges of the first embodiment and the second embodiment, respectively.

  Further, as shown in FIG. 34, the curvature in the discharge direction is given only to the rear mirror 36 to form a cylindrical concave mirror, and the curvature in the direction perpendicular to the discharge direction is given only to the front mirror 37 to give a cylindrical curvature. The configuration may be a concave mirror.

  If the conditions of the substantially stable resonator are satisfied, as shown in FIG. 35, the front mirror 37 has a relatively stronger curvature of the concave surface and the rear mirror 36 has a relatively weaker curvature of the convex surface (see FIG. 36, or the reverse, as shown in FIG. 36, the rear mirror 36 has a relatively stronger curvature of the concave surface and the front mirror 37 has a relatively weaker curvature of the convex surface. (Corresponding to the position “B” in FIG. 33).

  Next, a fourth embodiment according to the present invention will be described. In the present embodiment, as shown in FIG. 37, the front mirror 37 and the rear mirror 36 are flat surfaces, and a positive lens 310 is inserted into a resonator composed of the plane-plane. May be in the same range as in the first and second embodiments in which a concave mirror is used for the front mirror 37 or the rear mirror 36.

  For example, disposing the lens 310 having the focal length f = 10 m near the rear mirror 36 has substantially the same effect as disposing the rear mirror having the curvature radius R = 10 m in the second embodiment.

  Similarly, disposing the lens 310 with the focal length f = 10 m in the vicinity of the front mirror 37 has substantially the same effect as disposing the front mirror with the curvature radius R = 10 m in the first embodiment.

  Note that it is desirable to apply a non-reflective coating on both surfaces of the positive lens 310 to reduce loss and prevent parasitic oscillation.

  In this embodiment as well, as described above, a refractive power may be applied only in one direction of the discharge direction and the direction orthogonal to the discharge direction using a cylindrical lens.

  Further, this embodiment may be used in combination with the first to third embodiments.

  Next, a modification of the fourth embodiment will be described. In this modified example, as shown in FIG. 38, the positive lens 310 as described above, which is placed inside the amplification stage laser resonator and focuses the maximum light beam, is also used as the windows 30e and 30f provided in the chamber 30. The refractive power of the positive lens 310 may be in the same range as in the first and second embodiments in which a concave mirror is used for the front mirror 37 or the rear mirror 36.

  For example, if the positive lens 310 with the focal length f = 20 m is arranged in both the chamber windows 30e and 30f, the effect is substantially the same as that of arranging the front mirror 37 with the curvature radius R = 10 m in the first embodiment.

  Next, a fifth embodiment according to the present invention will be described. In this embodiment, as shown in FIG. 39, a resonator composed of a rear mirror 36 and a front mirror 37 of a plane mirror is used as the amplification stage laser resonator, but the seed light from the oscillation stage laser 100 is amplified by the amplification stage laser 300. The positive lens 311 is arranged at a position before being injected into the lens.

  FIG. 40 shows the relationship between the maximum luminous flux width (a) and the maximum emission angle (b) with respect to the refractive power of the positive lens 311 when α = 0.1 and β = 10.

  FIG. 41 shows a similar diagram when α = 1 and β = 10.

From this, the region where the maximum luminous flux width is improved is, for example,
0 <(refractive power) · L <0.18 (9)
It is desirable that

  Next, a modification of the fifth embodiment according to the present invention will be described. In this modified example, as shown in FIG. 42, a refractive power may be given by giving a curvature to the propagation mirror 42b or the like arranged before being injected into the amplification stage laser 300. The refractive power applied at that time may be adjusted according to the fifth embodiment.

  As shown in FIG. 43, the substrate of the rear mirror 36 of the amplification stage laser 300 may also be used as the positive lens 311.

  Furthermore, as shown in FIG. 44, the substrate of the front mirror 17 of the oscillation stage laser 100 may also be used as the positive lens 311.

  Furthermore, as shown in FIG. 45, the substrate of the beam sampling mirror 191 for the first monitor module 19 (see FIG. 1) (not shown) may be used as the positive lens 311.

  Of course, as long as the overall refractive power falls within an appropriate range, the above-described FIGS. 39 and 42 to 45 may be used in combination.

  Further, since the beam width is different between the discharge direction and the direction orthogonal to the discharge direction, and the appropriate refractive power is deviated, the refractive power may be applied separately according to each. This can be easily configured by combining the above.

  Next, a sixth embodiment according to the present invention will be described. In this embodiment, as shown in FIG. 46, slits 30s and 30s' are arranged in the amplification stage laser resonator. By blocking the portion where the discharge stability is not good (the end portion of the discharge region) and preventing amplification at that portion, it is possible to minimize the decrease in output and greatly improve the dispersion stability. Note that the slit width at this time is preferably about 2 mm narrower than the discharge gap in the discharge direction (FIG. 47), for example.

  Although the improvement effect is not so remarkable as described above, an appropriate slit 30s may be arranged after the amplification stage laser 300 is emitted, as shown in FIG.

  Although the two-stage laser system of the present invention has been described based on the principle and embodiments, the present invention is not limited to these embodiments and can be variously modified.

1 is a configuration diagram of a two-stage laser system according to an embodiment to which the present invention is applied. FIG. 2A is a configuration diagram (a) in the vicinity of the oscillation chamber of the two-stage laser system in FIG. 1 and a configuration diagram (b) in the vicinity of the amplification chamber. It is a figure which shows one example of the power supply of the 2 stage laser system of FIG. 1, and the circuit structure inside a chamber. FIG. 3 is a side view of a main part of a MOPO type two-stage laser device using a stable resonator composed of a rear mirror and a front mirror as a pre-amplification stage laser of the present invention. FIG. 5A is a plan view of the amplification stage laser of FIG. 4 and a plan view of the oscillation stage laser. It is a top view of the amplification stage laser of the 2 stage laser system of 1st Example based on this invention. It is a figure which shows the test result in the case of this invention which uses the front mirror of an amplification stage laser as a concave mirror (R = 5m), and the case of the prior art example with a plane mirror. It is a figure which shows the test result in the case of this invention which uses a concave mirror (R = 20m) for the front mirror of an amplification stage laser, and the case of the prior art example with a plane mirror. They are a side view (a) and a cross-sectional view (b) of a two-stage laser device for explaining the discharge region end. FIG. 4A is a diagram for explaining a beam waist and a divergence angle of a laser beam, and FIG. 4B is a diagram for explaining a method of detecting a beam waist size and its position. It is the figure expanded and shown in consideration of the multiple reflection of the injection light in an amplification stage laser resonator. It is a figure for showing the representative point for calculating the maximum beam maximum width until it radiate | emits from a front mirror through the number of round-trips which it noticed. It is an expanded view which shows the mode of the change of the maximum light beam at the time of attaching a curvature to a front mirror. It is a figure which shows the calculation result of one example of calculation using a light matrix. It is a figure which shows the relationship between the propagation distance and beam beam width in a typical curvature. It is a figure which shows the characteristic similar to FIG. 14 when changing the frequency | count of the last reciprocation to which attention is paid. It is a figure which shows the calculation result by ray tracing. It is a figure which shows a typical calculation result. It is a figure which shows a favorable curvature range. It is a top view which shows the modification of the amplification stage laser of 1st Example based on this invention of FIG. FIG. 7 is a plan view (a) and a side view (b) showing another modification of the amplification stage laser of the first embodiment based on the present invention of FIG. 6. FIG. 22 is a view similar to FIG. 21 of a modified example in which a curvature is given only in the discharge direction of the front mirror. FIG. 22 is a view similar to FIG. 21 of a modified example in which a curvature is given only in a direction orthogonal to the discharge direction of the front mirror. It is a figure which shows the structure (b) which arrange | positions a correction lens in order to solve the problem (a) which the output laser beam taken out from a front mirror spreads in 1st Example. It is a figure which shows the difference in the output angle with and without the correction lens on typical conditions. It is a figure which shows the refractive power required for a correction lens. FIG. 25 is a plan view of a modified example in which a curvature is given to the exit surface of the front mirror instead of the correction lens of FIG. 24. FIG. 6 is a plan view of a modification in which a biconvex positive lens or a convex flat positive lens is used as a front mirror, a non-reflective coating is applied to the incident surface, and a partial reflective mirror coating is applied to the output surface. It is a top view of the amplification stage laser of the 2 stage laser system of 2nd Example based on this invention. FIG. 30 is a view similar to FIG. 29 of a modified example in which a curvature is given to the surface on the seed light injection side of the rear mirror. FIG. 6 is a plan view of a modified example in which a convex flat positive lens or a biconvex positive lens is used as a rear mirror, and a partial reflection mirror coating is applied to the incident surface and a non-reflection coating is applied to the surface on the front mirror side. It is a top view of the amplification stage laser of the 2 stage laser system of 3rd Example based on this invention. It is a figure which shows the range of g parameter at the time of taking the value of the maximum light beam as a variable when g parameter of a front mirror is set to a vertical axis | shaft and the g parameter of a rear mirror is taken to a horizontal axis. It is the same figure as FIG. 21 of the structure which provided the curvature of the discharge direction only to the rear mirror, and provided the curvature of the direction orthogonal to a discharge direction only to a front mirror. FIG. 33 is a view similar to FIG. 32 of a modified example in which the front mirror has a relatively stronger curvature of the concave surface and the rear mirror has a relatively weak curvature of the convex surface. FIG. 33 is a view similar to FIG. 32 of a modified example in which the rear mirror has a relatively stronger curvature of the concave surface and the front mirror has a relatively weak curvature of the convex surface. It is a top view of the amplification stage laser of the 2 stage laser system of 4th Example based on this invention. FIG. 38 is a view similar to FIG. 37 of a modified example in which a positive lens placed inside an amplification stage laser resonator is also used as a window of a chamber. It is a top view of the amplification stage laser of the 2 stage laser system of 5th Example based on this invention. It is a figure which shows the relationship between the largest light beam width (a) with respect to the refractive power of the positive lens arrange | positioned in the position before inject | pouring into an amplification stage laser, and the largest output angle (b). It is a figure similar to FIG. 40 in another case. It is a side view which shows the modification of 5th Example. It is a top view of the modification which uses the board | substrate of the rear mirror of an amplification stage laser as a positive lens. FIG. 6 is a side view of a modified two-stage laser system in which a front mirror substrate of an oscillation stage laser is also used as a positive lens. It is a side view of the 2 stage laser system of the modification which uses the board | substrate of the beam sampling mirror for a 1st monitor module as a positive lens. It is a top view of the amplification stage laser of the 2 stage laser system of 6th Example based on this invention. It is a side view which shows the modification of 6th Example. It is a side view which shows the modification which arrange | positions a slit after amplification stage laser emission.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 ... Two stage laser apparatus 3 ... Exposure apparatus 5 ... Utility controller 6 ... Wavelength controller 7 ... Energy controller 8 ... Synchronous controller 10 ... Oscillation chamber 10a, 10b ... A pair of electrodes (cathode electrode and anode electrode)
10c ... Cross flow fan 10d ... Heat exchanger 10e, 10f ... Window 10s ... Slit 11 ... Charger 12 ... Oscillation high voltage pulse generator 14 ... Gas supply / exhaust unit 15 ... Cooling water supply unit 16 ... Narrow band module (LNM)
16b ... Magnifying prism 16a ... Diffraction grating 17 ... Front mirror 19 ... First monitor module 20 ... Discharge detector 21 ... Driver 30 ... Amplification chambers 30a, 30b ... A pair of electrodes (cathode electrode and anode electrode)
30c ... Cross flow fan 30d ... Heat exchanger 30e, 10f ... Window 30s, 30s' ... Slit 31 ... Charger 32 ... Oscillation high voltage pulse generator 34 ... Gas supply / exhaust unit 35 ... Cooling water supply unit 36 ... Rear mirror 37 ... Front mirror (output mirror)
39 ... 2nd monitor module 40 ... Discharge detector 42 ... Beam propagation part 42a, 42b ... Propagation mirror 51 ... Output monitor 52 ... Controller 91 ... 1st electrode 92 ... Dielectric tube 93 ... 2nd electrode 100 ... Oscillation stage laser 191 ... Beam sampling mirror 300 ... Amplification stage laser 301 ... Convex lens 305 ... Correction lens 306 ... Front mirror exit surface 307 ... Biconvex positive lens or convex flat positive lens 308 ... Rear mirror seed light injection side surface 309 ... Convex flat positive lens or both Convex positive lens 310 ... positive lens 311 ... positive lens 400 ... output laser light SR1, SR2, SR3 ... magnetic switch SW ... solid switch Tr1 ... step-up transformer C0 ... main capacitor L1 ... reactor C1, C2 ... capacitor Cp ... peaking capacitor Pl ... Pressure sensor Tl ... Temperature sensor P2 ... Pressure sensor T2 ... Temperature Sensor X ... discharge region Y ... injection light (seed light)
FM ... Front mirror RM ... Rear mirror

Claims (9)

It consists of an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light, amplifies the seed light, and outputs it. Both the oscillation stage laser and the amplification stage laser are filled with a laser gas. The amplification stage laser includes a Fabry-Perot etalon-type stable resonator, and a pair of long discharge electrodes for discharge excitation disposed in parallel and facing each other in the laser chamber. In a two-stage laser system,
A two-stage laser characterized in that at least one of a rear mirror and a front mirror constituting a Fabry-Perot etalon type stable resonator of the amplification stage laser has a positive refracting power in at least one of a discharge direction and a direction perpendicular to the discharge direction. system. One of the rear mirror and the front mirror constituting the Fabry-Perot etalon type stable resonator of the amplification stage laser has a positive refractive power in at least one of the discharge direction or the direction orthogonal to the discharge direction, and the resonator length of the resonator is L, the radius of curvature of the mirror having positive refractive power in that direction is
0.001 <L / (curvature radius) <0.77
The two-stage laser system according to claim 1, wherein: 3. The two-stage laser system according to claim 2, wherein the following expression is satisfied.
0.001 <L / (curvature radius) <0.20 3. The two-stage laser system according to claim 2, wherein the following expression is satisfied.
0.001 <L / (radius of curvature) <0.03 It consists of an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light, amplifies the seed light, and outputs it. Both the oscillation stage laser and the amplification stage laser are filled with a laser gas. The amplification stage laser includes a Fabry-Perot etalon-type stable resonator, and a pair of long discharge electrodes for discharge excitation disposed in parallel and facing each other in the laser chamber. In a two-stage laser system,
A two-stage laser having a transmission optical element having a positive refractive power in at least one of a discharge direction or a direction perpendicular to the discharge direction in a Fabry-Perot etalon type stable resonator of the amplification stage laser system. It consists of an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light, amplifies the seed light, and outputs it. Both the oscillation stage laser and the amplification stage laser are filled with a laser gas. The amplification stage laser includes a Fabry-Perot etalon-type stable resonator, and a pair of long discharge electrodes for discharge excitation disposed in parallel and facing each other in the laser chamber. In a two-stage laser system,
An optical element having a positive refracting power is disposed in at least one of the discharge direction and the direction perpendicular to the discharge direction of the amplification stage laser in an optical path for inputting seed light from the oscillation stage laser to the amplification stage laser. A two-stage laser system. When the resonator length of the Fabry-Perot etalon-type stable resonator of the amplification stage laser is L, the refractive power in the direction having the positive refractive power of the optical element is
0 <(refractive power) · L <0.18 (9)
The two-stage laser system according to claim 6, wherein: It consists of an oscillation stage laser and an amplification stage laser that inputs laser light oscillated by the oscillation stage laser as seed light, amplifies the seed light, and outputs it. Both the oscillation stage laser and the amplification stage laser are filled with a laser gas. The amplification stage laser includes a Fabry-Perot etalon-type stable resonator, and a pair of long discharge electrodes for discharge excitation disposed in parallel and facing each other in the laser chamber. In a two-stage laser system,
A slit member for limiting the beam diameter is disposed in the Fabry-Perot etalon type stable resonator of the amplification stage laser or on the output side of the amplification stage laser at least in one of the discharge direction and the direction orthogonal to the discharge direction. A two-stage laser system. The transverse mode of the oscillation laser light of the oscillation stage laser is multimode, and the amplification stage laser amplifies the multimode laser light while maintaining the multimode. The two-stage laser system described.

Images