Patent Description:
In recent years, semiconductor production processes have become capable of producing semiconductor devices with increasingly fine feature sizes, as photolithography has been making rapid progress toward finer fabrication. In the next generation of semiconductor production processes, microfabrication with feature sizes at <NUM> to <NUM>, and further, microfabrication with feature sizes of <NUM> or less will be required. In order to meet the demand for microfabrication with feature sizes of <NUM> or less, for example, an exposure apparatus is needed which combines a system for generating EUV light at a wavelength of approximately <NUM> with a reduced projection reflective optical system.

Three kinds of systems for generating EUV light are known in general, including a Laser Produced Plasma (LPP) type system in which plasma is generated by irradiating a target material with a laser beam, a Discharge Produced Plasma (DPP) type system in which plasma is generated by electric discharge, and a Synchrotron Radiation (SR) type system in which orbital radiation is used to generate plasma.

<NPL>) discloses a method for irradiating a target with a main pulse laser beam and a pre-pulse laser beam, transforming the main pulse laser beam from a first waveform to a second waveform, and amplifying the second waveform.

<NPL>) discloses soft x-ray lasers. A target is firstly irradiated with prepulse the duration of which is <NUM> ps, which produces preplasma. Then the target is irradiated with main pulse the duration of which is <NUM> fs. The contrast ratio between the pedestal and peak of the short pulse (the main pulse) is <NUM> * <NUM>-<NUM>.

<NPL>) discloses a Ti: sapphire laser to irradiate a target with fs pulses.

<CIT> discloses a method for irradiating a target with only one laser pulse beam the duration of which is <NUM> ps.

<CIT> discloses a device and method for producing laser pulses with pulse length shorter than <NUM> ps and peak intensities greater than <NUM><NUM> W/cm<NUM>, wherein a target is capable of releasing a high-energy particle pulse upon irradiation with the laser pulses.

<NPL>) discloses a method for controlling the ASE duration to obtain emission of a proton beam with the maximum energy of <NUM> keV THz radaiation from a thin titanium foil irradiated by a <NUM> fs laser pulse.

The above object is solved by a method for irradiating a target defined in the appended claim <NUM> and by a system for an extreme ultraviolet light source defined in the appended claim <NUM>. Avantageous effects can be achieved by preferred embodiments defined in the dependent claims.

Hereinafter, selected embodiments of this disclosure will be described with reference to the accompanying drawings. Note that a polarizer in this specification may be an example of an optical filter.

Hereinafter, selected embodiments of this disclosure will be described in detail with reference to the accompanying drawings. The embodiments to be described below are merely illustrative in nature and do not limit the scope of this disclosure. Further, configurations and operations described in each embodiment are not all essential in implementing this disclosure. Note that like elements are referenced by like reference numerals and characters, and duplicate descriptions thereof will be omitted herein. The embodiments of this disclosure will be described following the table of contents below.

In certain embodiments of an EUV light generation system to be described below, a target material may be irradiated with a pre-pulse laser beam to thereby be turned into a diffused target, and the diffused target may be irradiated with a main pulse laser beam. Such an EUV light generation system may include a device configured to control a pedestal of the main pulse laser beam. By controlling energy of the pedestal of the main pulse laser beam, energy of EUV light to be generated in the aforementioned EUV light generation system may be controlled.

<FIG> schematically illustrates a configuration of an exemplary LPP type EUV light generation system. An LPP type EUV light generation apparatus <NUM> may be used with at least one laser apparatus <NUM>. Hereinafter, a system that includes the EUV light generation apparatus <NUM> and the laser apparatus <NUM> may be referred to as an EUV light generation system <NUM>. As shown in <FIG> and described in detail below, the EUV light generation system <NUM> may include a chamber <NUM> and a target supply unit. The target supply unit may be a target generator <NUM>. The chamber <NUM> may be sealed airtight. The target supply unit may be mounted onto the chamber <NUM> to, for example, penetrate a wall of the chamber <NUM>. A target material to be supplied by the target supply unit may include, but is not limited to, tin, terbium, gadolinium, lithium, xenon, or any combination thereof.

The chamber <NUM> may have at least one through-hole or opening formed in its wall, and a pulse laser beam <NUM> may travel through the through-hole/opening into the chamber <NUM>. Alternatively, the chamber <NUM> may have a window <NUM>, through which the pulse laser beam <NUM> may travel into the chamber <NUM>. An EUV collector mirror <NUM> having a spheroidal surface may, for example, be provided inside the chamber <NUM>. The EUV collector mirror <NUM> may have a multi-layered reflective film formed on the spheroidal surface thereof. The reflective film may include a molybdenum layer and a silicon layer, which are laminated alternately. The EUV collector mirror <NUM> may have a first focus and a second focus, and may be positioned such that the first focus lies in a plasma generation region <NUM> and the second focus lies in an intermediate focus (IF) region <NUM> defined by the specification of an external apparatus, such as an exposure apparatus <NUM>. The EUV collector mirror <NUM> may have a through-hole <NUM> formed at the center thereof, and the pulse laser beam <NUM> may travel through the through-hole <NUM> toward the plasma generation region <NUM>.

The EUV light generation system <NUM> may further include an EUV light generation controller <NUM> and a target sensor <NUM>. The target sensor <NUM> may have an imaging function and detect at least one of the presence, the trajectory, and the position of a target <NUM>.

Further, the EUV light generation system <NUM> may include a connection part <NUM> for allowing the interior of the chamber <NUM> to be in communication with the interior of the exposure apparatus <NUM>. A wall <NUM> having an aperture may be provided inside the connection part <NUM>, and the wall <NUM> may be positioned such that the second focus of the EUV collector mirror <NUM> lies in the aperture formed in the wall <NUM>.

The EUV light generation system <NUM> may also include a laser beam direction control unit <NUM>, a laser beam focusing mirror <NUM>, and a target collector <NUM> for collecting targets <NUM>. The laser beam direction control unit <NUM> may include an optical element (not separately shown) for defining the direction into which the pulse laser beam <NUM> travels and an actuator (not separately shown) for adjusting the position and the orientation or posture of the optical element.

With continued reference to <FIG>, the pulse laser beam <NUM> outputted from the laser apparatus <NUM> may pass through the laser beam direction control unit <NUM> and be outputted therefrom after having its direction optionally adjusted. The pulse laser beam <NUM> may travel through the window <NUM> and enter the chamber <NUM>. The pulse laser beam <NUM> may travel inside the chamber <NUM> along at least one beam path from the laser apparatus <NUM>, be reflected by the laser beam focusing mirror <NUM>, and strike at least one target <NUM>.

The target supply unit may be configured to output the target(s) <NUM> toward the plasma generation region <NUM> inside the chamber <NUM>. The target <NUM> may be irradiated with at least one pulse of the pulse laser beam <NUM>. Upon being irradiated with the pulse laser beam <NUM>, the target <NUM> may be turned into plasma, and rays of light <NUM> including EUV light <NUM> may be emitted from the plasma. At least the EUV light <NUM> included in the light <NUM> may be reflected selectively by the EUV collector mirror <NUM>. The EUV light <NUM> reflected by the EUV collector mirror <NUM> may travel through the intermediate focus region <NUM> and be outputted to the exposure apparatus <NUM>. Here, the target <NUM> may be irradiated with multiple pulses included in the pulse laser beam <NUM>.

The EUV light generation controller <NUM> may be configured to integrally control the EUV light generation system <NUM>. The EUV light generation controller <NUM> may be configured to process image data of the target <NUM> captured by the target sensor <NUM>. Further, the EUV light generation controller <NUM> may be configured to control at least one of the timing at which the target <NUM> is outputted and the direction into which the target <NUM> is outputted. Furthermore, the EUV light generation controller <NUM> may be configured to control at least one of the timing at which the laser apparatus <NUM> oscillates, the direction in which the pulse laser beam <NUM> travels, and the position at which the pulse laser beam <NUM> is focused. It will be appreciated that the various controls mentioned above are merely examples, and other controls may be added as necessary.

An EUV light generation system according to a first embodiment of this disclosure will now be described in detail with reference to the drawings. In the description to follow, an EUV light generation system 11A in which a target material is irradiated with multiple pulse laser beams will be illustrated as an example.

<FIG> schematically illustrates an exemplary configuration of the EUV light generation system 11A. As shown in <FIG>, the EUV light generation system 11A may include a main pulse laser apparatus 3A, a high-reflection mirror <NUM>, a dichroic mirror <NUM>, a pre-pulse laser apparatus <NUM>, high-reflection mirrors <NUM> and <NUM>, a waveform detection unit <NUM>, a chamber 2A, and an EUV light generation controller 5A.

The main pulse laser apparatus 3A may include a master oscillator <NUM>, a pedestal control device <NUM>, amplifiers <NUM>, <NUM>, and <NUM>, and a laser controller <NUM>. The laser controller <NUM> may be configured to control each of the master oscillator <NUM>, the pedestal control device <NUM>, and the amplifiers <NUM> through <NUM>.

The master oscillator <NUM> may be configured to output a pulse laser beam at a predetermined repetition rate. The pedestal control device <NUM> may be configured to transform a waveform of the pulse laser beam from the master oscillator <NUM>. Each of the amplifiers <NUM> through <NUM> may contain CO<NUM> gas as a gain medium. An amplified pulse laser beam may be outputted from the main pulse laser apparatus 3A as a main pulse laser beam <NUM>. The central wavelength of the main pulse laser beam <NUM> may be about <NUM>.

The high-reflection mirror <NUM> and the dichroic mirror <NUM> may constitute a beam delivery unit. The high-reflection mirror <NUM> may be coated with a film configured to reflect the main pulse laser beam <NUM> with high reflectance. The beam delivery unit may further include an actuator (not separately shown) for adjusting the position and the orientation of the high-reflection mirror <NUM>. The main pulse laser beam <NUM> incident on the high-reflection mirror <NUM> may be reflected toward the dichroic mirror <NUM>.

The pre-pulse laser apparatus <NUM> may be configured to output a pre-pulse laser beam <NUM> at a central wavelength of about <NUM>. The pre-pulse laser apparatus <NUM> may, for example, be a Yttrium Alminum Garnet (YAG) laser apparatus. Pulse duration of the pre-pulse laser beam <NUM> may be about <NUM> ns. The pre-pulse laser beam <NUM> from the pre-pulse laser apparatus <NUM> may be reflected sequentially by the high-reflection mirrors <NUM> and <NUM> and then be incident on the dichroic mirror <NUM>. Each of the high-reflection mirrors <NUM> and <NUM> may be coated with a film configured to reflect the pre-pulse laser beam <NUM> with high reflectance. Further, each of the high-reflection mirrors <NUM> and <NUM> may include an actuator (not separately shown) for adjusting the position and the orientation of the respective high-reflection mirrors <NUM> and <NUM>.

The dichroic mirror <NUM> may serve as a beam axis adjuster for adjusting the beam axes of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> entering the chamber 2A. The dichroic mirror <NUM> may be coated on a first surface thereof with a film configured to reflect the main pulse laser beam <NUM> with high reflectance and transmit the pre-pulse laser beam <NUM> with high transmittance. The dichroic mirror <NUM> may be coated on a second surface thereof with a film configured to transmit the pre-pulse laser beam <NUM> with high transmittance. The dichroic mirror <NUM> may be positioned such that the main pulse laser beam <NUM> is incident on its first surface and the pre-pulse laser beam <NUM> is incident on its second surface. The substrate of the dichroic mirror <NUM> may, for example, be formed of diamond.

The chamber 2A may include a window <NUM>, a laser beam focusing optical system 22A, a target generator <NUM>, a target sensor <NUM>, an EUV collector mirror <NUM>, an energy sensor <NUM>, a beam dump <NUM>, and a connection part <NUM>.

Each of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> which have entered the chamber 2A through the window <NUM> may enter the laser beam focusing optical system 22A. The window <NUM> may be coated with an anti-reflection film. The laser beam focusing optical system 22A may include a laser beam focusing mirror <NUM> and a high-reflection mirror <NUM>. The laser beam focusing optical system 22A may further include a moving plate <NUM>, a plate moving device <NUM>, and mirror holders 71a and 72a. The mirror holder 72a may be provided with an automatic tilt mechanism (not separately shown). The laser beam focusing mirror <NUM> may be an off-axis paraboloidal mirror. The laser beam focusing mirror <NUM> may be fixed to the moving plate <NUM> through the mirror holder 71a. The high-reflection mirror <NUM> may be fixed to the moving plate <NUM> through the mirror holder 72a. The plate moving device <NUM> may be configured to move the laser beam focusing mirror <NUM> and the high-reflection mirror <NUM> along with the moving plate <NUM>.

Each of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> which have entered the laser beam focusing optical system 22A may first be reflected by the laser beam focusing mirror <NUM> toward the high-reflection mirror <NUM>. The high-reflection mirror <NUM> may be positioned to reflect each of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> toward the plasma generation region <NUM>. Then, each of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> may be focused in the plasma generation region <NUM>.

The plate moving device <NUM> may move the moving plate <NUM>, to thereby adjust the focus position of each of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> in the Z-direction. The mirror holder 72a may adjust a tilt angle of the high-reflection mirror <NUM>, to thereby adjust the focus position of each of the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> along the XY plane. The above adjustment may be controlled by the EUV light generation controller 5A, which will be described later.

The target generator <NUM> may be configured to output targets <NUM> toward the plasma generation region <NUM>. The target generator <NUM> may be provided with a two-axis moving device (not separately shown). The two-axis moving device may be configured to move the target generator <NUM>, to thereby adjust the position to which the target <NUM> is supplied.

The target <NUM> that has reached the plasma generation region <NUM> may be sequentially irradiated with the pre-pulse laser beam <NUM> and the main pulse laser beam <NUM>. The pre-pulse laser beam <NUM> and the main pulse laser beam <NUM> may strike the target <NUM> through a through-hole <NUM> formed in the EUV collector mirror <NUM>. Upon being irradiated with the pre-pulse laser beam <NUM>, the target <NUM> may be turned into a diffused target. This diffused target may then be irradiated with the main pulse laser beam <NUM>, to thereby be turned into plasma. Light <NUM> including EUV light <NUM> may be emitted from the plasma.

Part of the pre-pulse laser beam <NUM> and the main pulse laser beam <NUM> that has passed through the plasma generation region <NUM> may be absorbed by the beam dump <NUM>. The beam dump <NUM> may be fixed to the chamber 2A through a support <NUM>.

The energy sensor <NUM> may detect energy of the EUV light <NUM> emitted in the plasma generation region <NUM>. The detected energy may be inputted to the EUV light generation controller 5A.

The waveform detection unit <NUM> may include a beam splitter <NUM>, a focusing lens <NUM>, and a waveform detector <NUM>. The beam splitter <NUM> may reflect a part of the main pulse laser beam <NUM> and transmit the remaining part. The focusing lens <NUM> may be positioned to focus the main pulse laser beam <NUM> reflected by the beam splitter <NUM> on a photosensitive surface of the waveform detector <NUM>. The waveform detector <NUM> may monitor a waveform of the main pulse laser beam <NUM> imaged on the photosensitive surface. Alternatively, a diffusion plate may be provided in place of the focusing lens <NUM>. The waveform detector <NUM> may then monitor a waveform of the main pulse laser beam <NUM> diffused by the diffusion plate. In other embodiments, a plate having a through-hole may be provided in place of the focusing lens <NUM>. The waveform detector <NUM> may then monitor a waveform of the main pulse laser beam <NUM> that has passed through the aforementioned through-hole. In yet another embodiment, the main pulse laser beam <NUM> reflected by the beam splitter <NUM> may be directly incident on the photosensitive surface of the waveform detector <NUM>. A waveform detected by the waveform detector <NUM> may reflect a part of the waveform of the main pulse laser beam <NUM>. The detected waveform may then be inputted to the EUV light generation controller 5A. Here, the waveform detector <NUM> may be configured to detect a change over time in energy of the main pulse laser beam <NUM>, and may be substituted by any suitable energy sensor as long as a value reflecting a waveform of the main pulse laser beam <NUM> can be obtained.

The EUV light generation controller 5A may include an EUV light generation position controller <NUM>, a reference clock generator <NUM>, a target controller <NUM>, a target generation driver <NUM>, a delay circuit <NUM>, and a pedestal controller <NUM>. The EUV light generation position controller <NUM> may be connected to each of the reference clock generator <NUM>, the target controller <NUM>, the pedestal controller <NUM>, and an exposure apparatus controller <NUM>. The EUV light generation position controller <NUM> may further be connected to each of the main pulse laser apparatus 3A and the pre-pulse laser apparatus <NUM> through the delay circuit <NUM>. The target controller <NUM> may be connected to each of the target sensor <NUM> and the target generation driver <NUM>. The target generation driver <NUM> may be connected to the target generator <NUM>. The pedestal controller <NUM> may be connected to the pedestal control device <NUM> of the main pulse laser apparatus 3A and to the energy sensor <NUM>.

The interior of the chamber 2A may be divided into upstream and downstream spaces by a partition <NUM>. The plasma generation region <NUM> may be set in the downstream space. The partition <NUM> may serve to reduce the amount of debris of the target material generated in the downstream space which enters the upstream space. The partition <NUM> may have a through-hole formed therein, through which the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> may travel toward the plasma generation region <NUM>. The partition <NUM> may be positioned such that the through-hole in the partition <NUM> is aligned with the through-hole <NUM> in the EUV collector mirror <NUM>. The EUV collector mirror <NUM> may be fixed to the partition <NUM> through a holder 23a.

An operation of the EUV light generation system 11A shown in <FIG> will now be described. The EUV light generation system 11A may be configured to operate under the control of the EUV light generation controller 5A. The EUV light generation controller 5A may receive a request from the exposure apparatus controller <NUM> regarding a position at which the light <NUM> is to be generated or the plasma generation region <NUM>. The EUV light generation controller 5A may then control each component so that the light <NUM> is generated in an EUV light generation request position indicated by the request from the exposure apparatus controller <NUM>. Alternatively, the EUV light generation controller 5A may control each component so that the EUV light generation request position indicated by the request from the exposure apparatus controller <NUM> coincides with the plasma generation region <NUM>.

The EUV light generation position controller <NUM> may be configured to control the laser beam focusing optical system 22A. The EUV light generation position controller <NUM> may send driving signals respectively to the mirror holder 72a and the plate moving device <NUM>. The mirror holder 72a may control a tilt angle of the high-reflection mirror <NUM> in θx- and θy-directions in accordance with a driving signal from the EUV light generation position controller <NUM>. The plate moving device <NUM> may move the moving plate <NUM> in the Z-direction in accordance with a driving signal from the EUV light generation position controller <NUM>.

The EUV light generation controller 5A may receive an EUV light generation request signal from the exposure apparatus controller <NUM> requesting generation of the EUV light <NUM>. Upon receiving the EUV light generation request signal, the EUV light generation controller 5A may input an EUV light generation request signal to the target controller <NUM>. Upon receiving the EUV light generation request signal, the target controller <NUM> may send an output signal of a target <NUM> to the target generator <NUM>.

The target sensor <NUM> may be configured to detect a position and a timing at which the target <NUM> reaches the plasma generation region <NUM>. Detection results may be inputted to the target controller <NUM>. The target controller <NUM> may control the two-axis moving device (not separately shown) of the target generator <NUM> in accordance with the inputted detection results. Further, the target controller <NUM> may be configured to adjust a delay time in the delay circuit <NUM> in accordance with the inputted detection results. The main pulse laser apparatus 3A and the pre-pulse laser apparatus <NUM> may be configured to respectively output the main pulse laser beam <NUM> and the pre-pulse laser beam <NUM> at timings defined by the delay time set in the delay circuit <NUM>.

A waveform of the main pulse laser beam <NUM> may be detected by the waveform detection unit <NUM>. The waveform detection unit <NUM> may send a detected waveform to the pedestal controller <NUM>. The pedestal controller <NUM> may carry out a feedback-control on the pedestal control device <NUM> of the main pulse laser apparatus 3A in accordance with the inputted waveform of the main pulse laser beam <NUM> under the control of the EUV light generation position controller <NUM>.

Energy of the EUV light <NUM> detected by the energy sensor <NUM> may be inputted to the pedestal controller <NUM>. The pedestal controller <NUM> may carry out a feedback-control on the pedestal control device <NUM> of the main pulse laser apparatus 3A in accordance with the inputted energy of the EUV light <NUM>.

By controlling energy of a pedestal of the main pulse laser beam <NUM>, energy conversion efficiency from the main pulse laser beam <NUM> to the EUV light <NUM> may be improved.

Here, a relationship between a pedestal of a main pulse laser beam and conversion efficiency will be discussed in detail with reference to the drawings. Conversion efficiency is a ratio of energy of emitted EUV light to energy of a pulse laser beam in an LPP type EUV light generation apparatus. <FIG> shows an example of a waveform of a main pulse laser beam having a pedestal. <FIG> shows an example of a relationship between a pedestal ratio and conversion efficiency. <FIG> shows an example of a relationship between pedestal energy and conversion efficiency. Here, a pedestal ratio is a ratio of energy of a pedestal to total energy of a main pulse laser beam.

As shown in <FIG>, a waveform of the main pulse laser beam <NUM> may include a pedestal 31p and a peak portion <NUM>. The pedestal 31p may, for example, rise gradually, and the peak portion <NUM> may rise in approximately <NUM> ns after the rise of the pedestal 31p. Beam intensity of the pedestal 31p may be sufficiently small with respect to beam intensity of the peak portion <NUM>.

As shown in <FIG>, where a pedestal ratio is in a range of <NUM>% to <NUM>%, relatively high conversion efficiency of approximately <NUM>% to <NUM>% is obtained. By varying a pedestal ratio, conversion efficiency may vary. That is, by controlling a pedestal ratio, energy of emitted EUV light may be controlled.

As shown in <FIG>, where energy of a pedestal is in a range of <NUM> mJ to <NUM> mJ, relatively high conversion efficiency of approximately <NUM>% to <NUM>% is obtained. By varying energy of a pedestal, conversion efficiency may vary. That is, by controlling energy of a pedestal, energy of emitted EUV light may be controlled.

An operation of the EUV light generation system 11A according to the first embodiment will now be described in detail with reference to the drawings.

<FIG> is a flowchart showing an example of an overall operation of the pedestal controller according to the first embodiment.

As shown in <FIG>, the pedestal controller <NUM> may stand by until it receives an EUV light generation start signal from the EUV light generation position controller <NUM> (Step S101; NO). Upon receiving an EUV light generation start signal (Step S101; YES), the pedestal controller <NUM> may notify the EUV light generation position controller <NUM> of a start of pedestal control (Step S102). Then, the pedestal controller <NUM> may carry out a pedestal control subroutine to control the pedestal control device <NUM> so that a pedestal ratio or energy of a pedestal of the main pulse laser beam <NUM> is brought to a desired pedestal ratio or energy (Step S103).

When the control of the pedestal control device <NUM> is completed, the pedestal controller <NUM> may notify the EUV light generation position controller <NUM> of the completion (Step S104). Then, the pedestal controller <NUM> may carry out a pedestal stabilization subroutine to stabilize a pedestal of the main pulse laser beam <NUM> (Step S105). Here, the main pulse laser beam <NUM> may be outputted at a predetermined repetition rate.

Thereafter, the pedestal controller <NUM> may carry out an adjustment necessity determination subroutine to determine whether or not the the pedestal needs to be adjusted (Step S106). Subsequently, the pedestal controller <NUM> may determine whether or not the pedestal needs to be adjusted based on a result of the adjustment necessity determination subroutine (Step S107). When the adjustment of the pedestal is needed (Step S107; YES), the pedestal controller <NUM> may return to Step S102 and repeat the subsequent steps. On the other hand, when the adjustment of the pedestal is not needed (Step S107; NO), the pedestal controller <NUM> may then determine whether or not an EUV light generation pause signal has been received (Step S108). When an EUV light generation pause signal has been received (Step S108; YES), the pedestal controller <NUM> may terminate the operation. On the other hand, when an EUV light generation pause signal has not been received (Step S108; NO), the pedestal controller <NUM> may return to Step S105 and repeat the subsequent steps.

With the above-described operation, the main pulse laser beam <NUM> having a pedestal of a desired pedestal ratio or energy may be outputted stably from the main pulse laser apparatus 3A.

Each of the subroutines in the pedestal control flow shown in <FIG> may be carried out using a pedestal ratio (see <FIG>) as a parameter or using pedestal energy (see <FIG>) as a parameter. Subroutines that are carried out using a pedestal ratio as a parameter will first be discussed in detail with reference to the drawings.

<FIG> shows an example of a pedestal control subroutine in Step S103 of <FIG>. <FIG> shows an example of a relationship between a pedestal ratio and conversion efficiency used in the description of the pedestal control subroutine shown in <FIG>.

With reference to <FIG>, in the pedestal control subroutine, the pedestal controller <NUM> may first set "<NUM>" in a variable N (Step S111). Then, the pedestal controller <NUM> may increment the variable N by <NUM> (N=N+<NUM>) (Step S112).

Then, the pedestal controller <NUM> may send a control value P to the pedestal control device <NUM> (Step S113). As described in further detail later, the control value P may, for example, include a value of a voltage to be applied to a Pockels cell, a value indicating a timing at which the aforementioned voltage is applied. When the pedestal control subroutine is carried out for the first time, the smallest or largest control value P=P may be sent as an initial control value to the pedestal control device <NUM>. Thereafter, a control value P=P+(N-<NUM>)·ΔPstp may be sent to the pedestal control device <NUM> for each preset change amount ΔPstp. The control value P may continue to be sent to the pedestal control device <NUM> until the control value P reaches an upper limit or an lower limit (P=P+(k-<NUM>)·ΔPstp) of its measurement range. Here, k may be a natural number and an upper limit of the number of measurement points, and k may be determined in advance through an experiment.

Subsequently, the pedestal controller <NUM> may carry out a pedestal ratio calculation subroutine to calculate a pedestal ratio R (Step S114). Here, a value of the variable N held when the pedestal ratio calculation subroutine is carried out may be used as a parameter in the pedestal ratio calculation subroutine.

Then, the pedestal controller <NUM> may determine whether or not the variable N has reached or exceeded the preset upper limit k (Step S115). When the variable N is smaller than the upper limit k (Step S115; NO), the pedestal controller <NUM> may return to Step S112 and repeat the subsequent steps. On the other hand, when the variable N has reached or exceeded the upper limit k (Step S115; YES), the pedestal controller <NUM> may obtain a lower limit RL and an upper limit RH of a range within which the pedestal ratio R satisfies required conversion efficiency (Step S116). At this time, a pedestal ratio Rc at which the maximum conversion efficiency CE is obtained may also be determined. Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

As Steps S112 through S115 shown in <FIG> are repeated, the k number of pedestal ratios R and the k number of conversion efficiency CE may be obtained. That is, values R1 through Rk and values CE1 through CEk may be obtained. Using these values, a relational curve between the pedestal ratio R and the conversion efficiency CE as shown in <FIG> may be obtained. In <FIG>, a point (R1, CE1) indicates the lower limit of the measurement range, and a point (Rk, CEk) indicates the upper limit of the measurement range. As shown in <FIG>, the conversion efficiency CE may have a peak between the lower limit and the upper limit of the measurement range of the pedestal ratio R. In that case, a pedestal ratio Rc corresponding to the peak in the conversion efficiency CE may be calculated. Further, when the smallest value CEL of the required conversion efficiency CE is set in advance, a range within which a value of the conversion efficiency CE exceeds the smallest value CEL may be set as a control range of the pedestal ratio R. From this control range, the lower limit RL and the upper limit RH of the control range of the pedestal ratio R may be calculated. The relational curve between the pedestal ratio R and the conversion efficiency CE may, for example, be an approximation curve calculated using the least-square approach.

The conversion efficiency CE may not have a peak within a measurement range of the pedestal ratio R. Thus, a pedestal control subroutine in a case where the conversion efficiency CE does not have a peak within the measurement range of the pedestal ratio R will now be discussed. <FIG> shows a first modification of the pedestal control subroutine in Step S103 of <FIG>. <FIG> shows an example of a relationship between pedestal ratio and conversion efficiency used in the description of the pedestal control subroutine shown in <FIG>.

As shown in <FIG>, in the first modification of the pedestal control subroutine, Steps S111 through S115, which are similar to Steps S111 through S115 shown in <FIG>, may be carried out. Detailed description thereof will be omitted here. Thereafter, the pedestal controller <NUM> may obtain the pedestal ratio Rc corresponding to the maximum value of the conversion efficiency CE within the measurement range. Further, the pedestal ratio R at a point where the conversion efficiency CE is at or above the minimum value CEL of the required conversion efficiency CE may be obtained to determine the upper limit RH in the control range (Step S216). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

When the conversion efficiency CE does not have a peak in a measurement range of the pedestal ratio R, Steps S112 through S115 shown in <FIG> may be repeated, and the k number of pedestal ratios R and the k number of conversion efficiency CE may be obtained. That is, values R1 through Rk and values CE1 through CEk may be obtained. Using these values, a relational curve between the pedestal ratio R and the conversion efficiency CE as shown in <FIG> may be obtained. In <FIG>, a point (R1, CE1) indicates the lower limit of the measurement range, and a point (Rk, CEk) indicates the upper limit of the measurement range. As shown in <FIG>, the conversion efficiency CE may monotonically decrease from the lower limit to the upper limit of the measurement range of the pedestal ratio R. In that case, the conversion efficiency CE may be at the highest at the lower limit of the measurement range of the pedestal ratio R. Thus, the pedestal ratio R at the lower limit of the measurement range may be set as an optimal value Rc. Further, when the smallest value CEL of the required conversion efficiency CE is set in advance, a range from the lower limit of the measurement range to a point where a value of the conversion efficiency CE exceeds the smallest value CEL may be set as a control range of the pedestal ratio R. From this control range, the upper limit RH of the control range of the pedestal ratio R may be calculated. The relational curve between the pedestal ratio R and the conversion efficiency CE may, for example, be an approximation curve calculated using the least-square approach.

<FIG> shows an example of a pedestal ratio calculation subroutine in Step S114 of <FIG> and <FIG>. <FIG> shows an example of a relationship between total energy of a main pulse laser beam and energy of a pedestal used in the description of the pedestal ratio calculation subroutine shown in <FIG>.

As shown in <FIG>, in the pedestal ratio calculation subroutine, the pedestal controller <NUM> may first receive a detected waveform of the main pulse laser beam <NUM> from the waveform detection unit <NUM> (Step S121). Then, the pedestal controller <NUM> may receive detected energy Eeuv of the EUV light <NUM> from the energy sensor <NUM> (Step S122).

Subsequently, the pedestal controller <NUM> may calculate total energy Et of a single pulse from the received waveform of the main pulse laser beam <NUM> (Step S123). As shown in <FIG>, the energy Et may be an integrated value of energy Ep of a pedestal and energy Em of a peak portion.

Then, the pedestal controller <NUM> may calculate the energy Ep of the pedestal (Step S124). The energy Ep may be calculated as energy of a portion preceding a rise of the peak portion. Alternatively, the energy Ep may be obtained by subtracting the energy Em of the peak portion from the total energy Et. The rise of the peak portion may be determined based on whether or not the beam intensity has exceeded a predetermined threshold value.

Thereafter, the pedestal controller <NUM> may calculate a pedestal energy ratio Rn, where Rn=Ep/Et, with respect to the total energy Et of the main pulse laser beam <NUM> (Step S125). Here, a value of the variable N held when the processing has moved to the pedestal ratio calculation subroutine may be used as a parameter n. That is, n in the energy ratio Rn may be an ordinal number that is the same as the variable N. Subsequently, the pedestal controller <NUM> may calculate conversion efficiency CEn from the main pulse laser beam <NUM> to the EUV light <NUM> based on the aforementioned energy Eeuv of the EUV light <NUM> and the calculated energy Em of the peak portion (Step S126). Here, n in the conversion efficiency CEn may be an ordinal number that is the same as the variable N. Thereafter, the pedestal controller <NUM> may return to the pedestal control subroutine shown in <FIG> or <FIG>.

In a pedestal stabilization subroutine, the pedestal ratio R may be adjusted accordingly so that the pedestal ratio R approaches the pedestal ratio Rc corresponding to the maximum value of the conversion efficiency CE. <FIG> shows an example of a pedestal stabilization subroutine in Step S105 of <FIG>.

With reference to <FIG>, in the pedestal stabilization subroutine, the pedestal controller <NUM> may stand by until the waveform of the main pulse laser beam <NUM> is detected by the waveform detection unit <NUM> and the energy of the EUV light <NUM> is detected by the energy sensor <NUM> (Step S141; NO). When the waveform of the main pulse laser beam <NUM> and the energy of the EUV light <NUM> are detected (Step S141; YES), the pedestal controller <NUM> may carry out a modification of a pedestal ratio calculation subroutine (Step S142). The modification of the pedestal ratio calculation subroutine may be similar to the pedestal ratio calculation subroutine described with reference to <FIG>.

Then, the pedestal controller <NUM> may calculate a difference ΔR, where ΔR=Rc-R, between the pedestal ratio Rc corresponding to the maximum value of the conversion efficiency CE and the pedestal ratio R obtained in the pedestal ratio calculation subroutine (Step S143). Subsequently, the pedestal controller <NUM> may send a change amount ΔP of the control value to the pedestal control device <NUM> so that the difference ΔR decreases (Step S144). The change amount ΔP may be a preset change amount ΔPstp or a value calculated in accordance with the difference ΔR.

Then, the pedestal controller <NUM> may again carry out the modification of the pedestal ratio calculation subroutine (Step S145). Subsequently, the pedestal controller <NUM> may overwrite the current conversion efficiency CE with the conversion efficiency CE calculated in the modification of the pedestal ratio calculation subroutine (CE=CE). Similarly, the current pedestal ratio R may be overwritten with a newly calculated pedestal ratio R (R=R) (Step S146). The respective values CE and R may, for example, be used in the adjustment necessity determination subroutine in Step S106 of <FIG>. Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

<FIG> shows the modification of the pedestal ratio calculation subroutine. The modification of the pedestal ratio calculation subroutine may be used in the pedestal stabilization subroutine described with reference to <FIG>.

With reference to <FIG>, the modification of the pedestal ratio calculation subroutine may be similar to the pedestal ratio calculation subroutine shown in <FIG>. For simplifying the description, only the operation that differs from that shown in <FIG> will be described below.

In the modification of the pedestal ratio calculation subroutine, in Steps S135 and S136, the variable N may not be referenced. That is, the energy ratio R and the conversion efficiency CE at the time of carrying out the modification of pedestal ratio calculation subroutine may be calculated. Thereafter, the pedestal controller <NUM> may return to the pedestal control subroutine shown in <FIG>.

<FIG> shows an example of an adjustment necessity determination subroutine in Step S106 of <FIG>.

With reference to <FIG>, in the adjustment necessity determination subroutine, the pedestal controller <NUM> may determine whether or not a value set in the pedestal ratio R falls within a range from the lower limit RL inclusive to the upper limit RH inclusive and whether or not a value set in the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S151). When the pedestal ratio R falls within a range from the lower limit RL inclusive to the upper limit RH inclusive and the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S151; YES), the pedestal controller <NUM> may determine that the pedestal does not need adjusting (Step S152). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. On the other hand, when the the pedestal ratio R does not fall within a range from the lower limit RL inclusive to the upper limit RH inclusive or the conversion efficiency CE is smaller than the minimum value CEL (Step S151; NO), the pedestal controller <NUM> may determine that the pedestal need adjusting (Step S153). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. <NUM> Adjustment Necessity Determination Subroutine: First Modification.

When the conversion efficiency CE does not have a peak within a measurement range of the pedestal ratio R, a modification of the adjustment necessity determination subroutine as described below may be carried out. <FIG> shows a first modification of the adjustment necessity determination subroutine in Step S106 of <FIG>.

With reference to <FIG>, in the first modification of the adjustment necessity determination subroutine, the pedestal controller <NUM> may determine whether or not a value set in the pedestal ratio R is equal to or lower than the upper limit RH and whether or not a value set in the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S251). When the pedestal ratio R is equal to or lower than the upper limit RH and the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S251; YES), the pedestal controller <NUM> may determine that the pedestal need not adjusting (Step S252). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. On the other hand, when the the pedestal ratio R exceeds the upper limit RH or the conversion efficiency falls below the minimum value CEL (Step S251; NO), the pedestal controller <NUM> may determine that the pedestal needs adjusting (Step S253). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

A subroutine where pedestal energy Ep is used as a parameter will now be described in detail with reference to the drawings.

<FIG> shows a second modification of the pedestal control subroutine in Step S103 of <FIG>. <FIG> shows an example of a relationship between pedestal energy and conversion efficiency used in the description of the pedestal control subroutine shown in <FIG>.

As shown in <FIG>, in the second modification of the pedestal control subroutine, in which the pedestal energy Ep is used as a parameter, an operation similar to the pedestal control subroutine shown in <FIG> may be carried out. Steps S311 through S315 of <FIG> may correspond to Steps S111 through S115 of <FIG>, and detailed description of Steps S311 through S315 will be omitted here. However, in Step S314, the pedestal controller <NUM> may carry out a pedestal energy calculation subroutine, which will be described later, to calculate the pedestal energy Ep.

As Steps S312 through S315 of <FIG> are repeated, a relational curve between the pedestal energy Ep and the conversion efficiency CE as shown in <FIG> may be obtained. In <FIG>, a point (Ep1, CE1) indicates the lower limit of the measurement range, and a point (Epk, CEk) indicates the upper limit of the measurement range. As shown in <FIG>, the conversion efficiency CE may have a peak between the lower limit and the upper limit of the measurement range of the pedestal energy Ep. In that case, pedestal energy Epc corresponding to the peak of the conversion efficiency CE may be calculated. When the smallest value CEL of the required conversion efficiency CE is set in advance, a range within which a value of the conversion efficiency CE exceeds the smallest value CEL may be set as a control range of the pedestal energy Ep. From this control range, the lower limit EpL and the upper limit EpH of the control range of the pedestal energy Ep may be calculated. The relational curve between the pedestal energy Ep and the conversion efficiency CE may, for example, be an approximation curve calculated using the least-square approach. <NUM> Pedestal Control Subroutine: Third Modification.

The conversion efficiency CE may not have a peak within a measurement range of the pedestal energy Ep. Thus, a pedestal control subroutine in a case where the conversion efficiency CE does not have a peak within the measurement range of the pedestal energy Ep will be discussed below. <FIG> shows a third modification of the pedestal control subroutine in Step S103 of <FIG>. <FIG> shows an example of a relationship between pedestal energy and conversion efficiency used in the description of the pedestal control subroutine shown in <FIG>.

With reference to <FIG>, in a third modification of the pedestal control subroutine, in which the pedestal energy Ep is used as a parameter, Steps S311 through S315, which are similar to Steps S311 through S315 shown in <FIG>, may be carried out. Then, the pedestal controller <NUM> may obtain the pedestal energy Epc corresponding to the maximum value of the conversion efficiency CE within the measurement range. Further, the pedestal controller <NUM> may obtain an upper limit EpH of the pedestal energy Ep at which the required conversion efficiency CE is equal to or higher than the minimum value CEL (Step S416). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

When the conversion efficiency CE does not have a peak within a measurement range of the pedestal energy Ep, by repeating Steps S312 through S315 of <FIG>, a relational curve between the pedestal energy Ep and the conversion efficiency CE as shown in <FIG> may be obtained. In <FIG>, a point (Ep1, CE1) indicates the lower limit of the measurement range, and a point (Epk, CEk) indicates the upper limit of the measurement range. As shown in <FIG>, the conversion efficiency CE may monotonically decrease from the lower limit to the upper limit of the measurement range of the pedestal energy Ep. In that case, the conversion efficiency CE may be highest at the lower limit of the measurement range of the pedestal energy Ep. Thus, the pedestal energy Ep at the lower limit of the measurement range may be set as an optimal value Epc. When the smallest value CEL of the required conversion efficiency CE is set in advance, a range from the lower limit of the measurement range to a point where the value of the conversion efficiency CE exceeds the smallest value CEL may be set as a control range of the pedestal energy Ep. From this control range, the upper limit EpH of the control range of the pedestal energy Ep may be calculated. The relational curve between the pedestal energy Ep and the conversion efficiency CE may, for example, be an approximation curve calculated using the least-square approach.

<FIG> shows an example of a pedestal energy calculation subroutine in Step S314 of <FIG> and <FIG>. Here, a relationship between the total energy of main pulse laser beam and the energy of the pedestal may, for example, be the same as that shown in <FIG>.

With reference to <FIG>, the pedestal energy calculation subroutine of the embodiment shown in <FIG> may include steps that are similar to those in the pedestal ratio calculation subroutine shown in <FIG>. Thus, only the operations of the pedestal energy calculation subroutine of <FIG> that differ from those in the pedestal ratio calculation subroutine shown in <FIG> will be discussed below. Steps S321 through S323 correspond to Steps S121 through S123 in <FIG>, and the description thereof will be omitted here. In Step S324, the pedestal controller <NUM> may calculate pedestal energy Epn of the main pulse laser beam <NUM>. Here, a value of the variable N held when the processing has moved from the pedestal control subroutine may be used as a parameter n. That is, n in the pedestal energy Epn may be an ordinal number that is the same as the variable N.

In Step S325, the pedestal controller <NUM> may calculate energy Em of the peak portion in the waveform of the main pulse laser beam <NUM>. The energy Em may be energy of a portion of the waveform corresponding to a preset duration after the rise of the peak portion. Alternatively, the energy Em may be obtained by subtracting the pedestal energy Epn from the total energy Et of the main pulse laser beam <NUM>. The rise of the peak portion may be determined based on whether or not the beam intensity has exceeded a predetermined threshold value.

Step S326 may be similar to Step S126 shown in <FIG>. Thereafter, the pedestal controller <NUM> may return to the pedestal control subroutine shown in <FIG> or <FIG>.

In a modification of the pedestal stabilization subroutine, the pedestal energy Ep may be adjusted accordingly so that the pedestal energy Ep approaches the optimal value Epc. <FIG> shows the modification of the pedestal stabilization subroutine in Step S105 of <FIG>.

With reference to <FIG>, the modification of the pedestal stabilization subroutine of the embodiment shown in <FIG>, in which the pedestal energy Ep is used as a parameter, may include steps that are similar to those in the pedestal stabilization subroutine shown in <FIG>. Thus, only the operations of the modification of the pedestal stabilization subroutine of <FIG> that differ from those in the pedestal stabilization subroutine shown in <FIG> will be discussed below. Steps S341 and <NUM> may be similar to Steps S141 and S142 of <FIG>. However, in Step <NUM>, a modification of the pedestal energy calculation subroutine described with reference to <FIG> may be carried out.

In Step S343, the pedestal controller <NUM> may calculate a difference ΔEp, where ΔEp=Epc-Ep, the difference between the pedestal energy Epc corresponding to the maximum value of the conversion efficiency CE and the pedestal energy Ep obtained in the modification of the pedestal energy calculation subroutine. Subsequently, the pedestal controller <NUM> may send a change amount ΔP of the control value P to the pedestal control device <NUM> so that the difference ΔEp decreases (Step S344). The change amount ΔP may be a preset change amount ΔPstp or a value calculated in accordance with the difference ΔEp.

Then, the pedestal controller <NUM> may again carry out the modification of the pedestal energy calculation subroutine (Step S345). Thereafter, the pedestal controller <NUM> may overwrite the current conversion efficiency CE with the conversion efficiency CE calculated in the modification of the pedestal energy calculation subroutine (CE=CE). Similarly, the current energy Ep may be overwritten with newly calculated energy Ep (Ep=Ep) (Step S346). The respective values CE and Ep may, for example, be used in the adjustment necessity determination subroutine in Step S106 of <FIG>. Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

<FIG> shows the modification of the pedestal energy calculation subroutine. The modification of the pedestal energy calculation subroutine may be used in the pedestal stabilization subroutine described with reference to <FIG>.

With reference to <FIG>, the modification of the pedestal energy calculation subroutine of the embodiment shown in <FIG> may include steps that are similar to those in the pedestal energy calculation subroutine shown in <FIG>. As such, only the operations of the modification of the pedestal energy calculation subroutine of <FIG> that differ from those in the pedestal energy calculation subroutine shown in <FIG> will be discussed below.

In the modification of the pedestal energy calculation subroutine, in Steps S334 and S336, the variable N may not be referenced. That is, the pedestal energy Ep and the conversion efficiency CE at the time of carrying out the modification of the pedestal energy calculation subroutine may be calculated. Thereafter, the pedestal controller <NUM> may return to the pedestal control subroutine shown in <FIG>.

<FIG> shows a second modification of the adjustment necessity determination subroutine in Step S106 of <FIG>.

With reference to <FIG>, in the second modification of the adjustment necessity determination subroutine, in which the pedestal energy Ep is used as a parameter, the pedestal controller <NUM> may determine whether or not a value set in the pedestal energy Ep falls within a range from the lower limit EpL inclusive to the upper limit EpH inclusive and whether or not a value set in the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S351). When the pedestal energy Ep falls within a range from the lower limit EpL inclusive to the upper limit EpH inclusive and the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S351; YES), the pedestal controller <NUM> may determine that the pedestal need not adjusting (Step S352). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. On the other hand, when the the pedestal energy Ep does not fall within a range from the lower limit EpL inclusive to the upper limit EpH inclusive or the conversion efficiency CE is smaller than the minimum value CEL (Step S351; NO), the pedestal controller <NUM> may determine that the pedestal need adjusting (Step S353). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

When the conversion efficiency CE does not have a peak within a measurement range of the pedestal energy Ep, a third modification of the adjustment necessity determination subroutine described below may be carried out. <FIG> shows the third modification of the adjustment necessity determination subroutine in Step S106 of <FIG>.

With reference to <FIG>, in the third modification of the adjustment necessity determination subroutine, in which the pedestal energy Ep is used as a parameter, the pedestal controller <NUM> may determine whether or not a value set in the pedestal energy Ep is equal to or lower than the upper limit EpH and whether or not a value set in the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S451). When the the pedestal energy Ep is equal to or lower than the upper limit EpH and the conversion efficiency CE is equal to or higher than the minimum value CEL (Step S451; YES), the pedestal controller <NUM> may determine that the pedestal does not need adjusting (Step S452). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. On the other hand, when the the pedestal energy Ep exceeds the upper limit EpH or the conversion efficiency CE falls below the minimum value CEL (Step S451; NO), the pedestal controller <NUM> may determine that the pedestal needs adjusting (Step S453). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

Specific examples of the pedestal control device according to the first embodiment will now be described in detail with reference to the drawings.

<FIG> schematically illustrates an exemplary configuration of a main pulse laser apparatus in which an optical shutter is used as a pedestal control device. As shown in <FIG>, a main pulse laser apparatus 3B may include at least one optical shutter <NUM> serving as the pedestal control device <NUM>. Other configurations may be similar to those of the main pulse laser apparatus 3A shown in <FIG>.

The optical shutter <NUM> may be provided in a beam path of a pulse laser beam from the master oscillator <NUM>. The optical shutter <NUM> may be configured to vary transmittance therethrough in accordance with the control of the pedestal controller <NUM>. The pedestal controller <NUM> may control the transmittance of the optical shutter <NUM> in synchronization with a timing at which the pulse laser beam enters the optical shutter <NUM>. The timing at which the pulse laser beam enters the optical shutter <NUM> may be detected by an optical sensor (not separately shown). The optical sensor may, for example, detect scattered rays of the pulse laser beam outputted from the master oscillator <NUM>.

The optical shutter <NUM> may be provided in a beam path between the master oscillator <NUM> and the amplifier <NUM>. Alternatively, the optical shutter <NUM> may be provided downstream from the amplifier <NUM>.

When the pedestal ratio R of the main pulse laser beam <NUM> is not reduced to a desired value by only a single optical shutter <NUM>, a plurality of optical shutters <NUM> may be used. The plurality of optical shutters <NUM> may be provided in a beam path between the master oscillator <NUM> and the amplifier <NUM>. However, this disclosure is not limited thereto, and the optical shutter(s) <NUM> may be provided in a beam path between the amplifier <NUM> and the amplifier <NUM>, or in a beam path between the amplifier <NUM> and the amplifier <NUM>. Alternatively, the optical shutter(s) <NUM> may be provided downstream from the amplifier <NUM>.

<FIG> show waveforms of a pulse laser beam at respective positions (a) through (c) in <FIG>. As shown in <FIG>, a pulse laser beam having a waveform Ws which includes a pedestal having a relatively high pedestal ratio R or relatively high pedestal energy Ep may be outputted from the master oscillator <NUM>. Then, as shown in <FIG>, the waveform of the pulse laser beam transmitted through the optical shutter <NUM> may be in a shape where the beam intensity of a front portion of the waveform Ws is reduced. This waveform may include a pedestal Wp1, where the pedestal energy Ep is relatively low, and a peak portion Wm1, which is a part of the waveform Ws transmitted through the optical shutter <NUM> with high transmittance. Subsequently, as shown in <FIG>, the waveform of the pulse laser beam amplified in the amplifiers <NUM> through <NUM> may be in a shape where the waveform shown in <FIG> is amplified. Similarly to the waveform shown in <FIG>, the waveform shown in <FIG> may include a pedestal Wp2, where the pedestal energy Ep is relatively low, and a peak portion Wm2, which is a part of the waveform Ws transmitted through the optical shutter <NUM> with high transmittance.

<FIG> schematically illustrates an exemplary configuration of a main pulse laser apparatus, in which an optical shutter and a saturable absorber device are collectively used as a pedestal control device. As shown in <FIG>, a main pulse laser apparatus 3C may include at least one optical shutter <NUM> and at least one saturable absorber device collectively serving as the pedestal control device <NUM>. The optical shutter <NUM> may be similar to the optical shutter <NUM> shown in <FIG>.

The saturable absorber device <NUM> may be a gas cell containing a saturable absorber gas thereinside. The saturable absorber device <NUM> may be configured such that a concentration of the saturable absorber gas thereinside or an optical path length through the saturable absorber gas is adjustable.

The saturable absorber device <NUM> may be provided in a beam path of a pulse laser beam from the master oscillator <NUM>. The saturable absorber device <NUM> may be provided in a beam path between the master oscillator <NUM> and the amplifier <NUM>. Alternatively, the saturable absorber device <NUM> may be provided downstream from the amplifier <NUM>.

The saturable absorber device <NUM> may be provided downstream from the optical shutter <NUM>. With this arrangement, the pedestal energy Ep of the pedestal generated by the optical shutter <NUM> may be adjusted effectively. Depending on the amplification characteristics of the amplifiers <NUM> through <NUM>, the gain of the pedestal may be higher than the gain of the peak portion. In such a case, a desired pedestal ratio may not be obtained solely by the optical shutter <NUM>. Accordingly, the pedestal energy Ep may be reduced by the saturable absorber device <NUM>. Then, a desired pedestal ratio R may be achieved. The saturable absorber device <NUM> may, however, be provided upstream from the optical shutter <NUM>.

<FIG> show waveforms of a pulse laser beam at respective positions (a) through (d) in <FIG>. As shown in <FIG>, a change in the waveform before and after the pulse laser beam from the master oscillator <NUM> passes through the optical shutter <NUM> may be similar to the change in the waveform described with reference to <FIG>. Then, as shown in <FIG>, the waveform of the pulse laser beam that has passed through the saturable absorber device <NUM> may be in a shape where the pedestal energy Ep is further reduced or the pedestal ratio R is further reduced. At this point, energy of a tail portion of the waveform Ws may be reduced by the saturable absorber gas. The waveform at this point may include a pedestal Wp12, where the beam intensity is low, and a peak portion Wm12, where the beam intensity is high. Further, as shown in <FIG>, the waveform of the pulse laser beam amplified in the amplifiers <NUM> through <NUM> may be in a shape where the waveform shown in <FIG> is amplified. Similarly to the waveform shown in <FIG>, this waveform shown in <FIG> may include a pedestal Wp13, where the beam intensity is low, and a peak portion Wm13, where the beam intensity is high.

When a master oscillator includes a Pockels cell, the Pockels cell may be used as a part of a pedestal control device. Hereinafter, a case where the Pockels cell of the master oscillator is used in the pedestal control device will be described with specific examples.

<FIG> schematically illustrates an exemplary configuration of a main pulse laser apparatus, in which a Pockels cell in a master oscillator and a saturable absorber device are collectively used as a pedestal control device. As shown in <FIG>, a main pulse laser apparatus 3D may include a master oscillator <NUM>, a high-reflection mirror <NUM>, and the saturable absorber device <NUM>.

The master oscillator <NUM> may include a resonator formed by high-reflection mirrors <NUM> and <NUM>, an amplification part <NUM>, a polarization beam splitter <NUM>, and a Pockels cell <NUM>. The Pockels cell <NUM> may change the polarization direction of a passing pulse laser beam in accordance with a voltage applied by the laser controller <NUM>. The voltage applied to the Pockels cell <NUM> by the laser controller <NUM> may be controlled by the pedestal controller <NUM>. By adjusting a voltage applied to the Pockels cell <NUM> when a pulse laser beam is outputted from the master oscillator <NUM>, a pulse laser beam having a waveform in a shape where the beam intensity of a front portion of the waveform is reduced may be outputted from the master oscillator <NUM>. That is, a pulse laser beam of which the pedestal ratio R or the pedestal energy Ep has been adjusted may be outputted from the master oscillator <NUM>.

The pulse laser beam outputted from the master oscillator <NUM> may be reflected by the high-reflection mirror <NUM> and enter the saturable absorber device <NUM>. The saturable absorber device <NUM> may be similar to the saturable absorber device <NUM> shown in <FIG>. As the pulse laser beam from the master oscillator <NUM> passes through the saturable absorber device <NUM>, the pedestal energy Ep thereof may be adjusted effectively.

<FIG> show waveforms of a pulse laser beam at respective positions (a) through (c) in <FIG>. As shown in <FIG>, a pulse laser beam having a waveform in which the beam intensity of the front portion is reduced may be outputted from the master oscillator <NUM>. This waveform may include a pedestal Wp21, where the pedestal ratio R or the pedestal energy Ep is relatively low, and a peak portion Wm21, where the energy Em is relatively high. The pedestal Wp21 may, for example, be generated by lowering a voltage applied to the Pockels cell <NUM>. Further, the peak portion Wm21 may, for example, be generated by raising a voltage applied to the Pockels cell <NUM>. When a relatively low voltage is applied to the Pockels cell <NUM>, a change in the polarization direction of the pulse laser beam transmitted through the Pockels cell <NUM> may be small. Thus, the beam intensity of the pulse laser beam reflected by the polarization beam splitter <NUM> may be relatively low. On the other hand, when a relatively high voltage is applied to the Pockels cell <NUM>, a change in the polarization direction of the pulse laser beam transmitted through the Pockels cell <NUM> may be close to <NUM> degrees. Thus, the beam intensity of the pulse laser beam reflected by the polarization beam splitter <NUM> may be relatively high. Then, as shown in <FIG>, the waveform of the pulse laser beam that has passed through the saturable absorber device <NUM> may be in a shape where the pedestal energy Ep is further reduced or the pedestal ratio R is further reduced. This pulse waveform may include a pedestal Wp22, where the beam intensity is low, and a peak portion Wm22, where the beam intensity is high. At this point, energy of a tail portion of the peak portion Wm22 may be reduced by the saturable absorber gas. Further, as shown in <FIG>, the waveform of the pulse laser beam amplified in the amplifiers <NUM> through <NUM> may be in a shape where the waveform shown in <FIG> is amplified. Similarly to the waveform shown in <FIG>, this waveform shown in <FIG> may include a pedestal Wp23, where the beam intensity is low, and a peak portion Wm23, where the beam intensity is high.

<FIG> schematically illustrates an exemplary configuration of a main pulse laser apparatus, in which a Pockels cell in a master oscillator and an optical shutter are collectively used as a pedestal control device. As shown in <FIG>, a main pulse laser apparatus 3E may be similar in configuration to the main pulse laser apparatus 3D shown in <FIG>. However, in the main pulse laser apparatus 3E, the saturable absorber device <NUM> may be replaced by the optical shutter <NUM>. The optical shutter <NUM> may be similar to the optical shutter <NUM> shown in <FIG>. When a pulse laser beam outputted from the master oscillator <NUM> passes through the optical shutter <NUM>, the pedestal energy Ep of the pulse laser beam may be adjusted effectively by adjusting a voltage applied to the optical shutter <NUM>.

A master oscillator of a main pulse laser apparatus may include at least two semiconductor lasers. In that case, at least one of the semiconductor lasers may be used as a pedestal control device.

<FIG> schematically illustrates an exemplary configuration of a main pulse laser apparatus in which a master oscillator includes at least two semiconductor lasers. A main pulse laser apparatus 3F shown in <FIG> may be similar in configuration to the main pulse laser apparatus 3B shown in <FIG>. However, in the main pulse laser apparatus 3F, the master oscillator <NUM> may be replaced by a master oscillator <NUM>. Further, the main pulse laser apparatus 3F may include a regenerative amplifier <NUM>.

The master oscillator <NUM> may include semiconductor lasers <NUM> and <NUM>, and a beam path adjuster <NUM>. Each of the semiconductor lasers <NUM> and <NUM> may, for example, be a quantum cascade laser. The semiconductor lasers <NUM> and <NUM> may be configured to oscillate under the control of the laser controller <NUM>. The beam path adjuster <NUM> may be positioned to adjust the beam paths of the pulse laser beams outputted from the respective semiconductor lasers <NUM> and <NUM> to substantially coincide with each other.

The laser controller <NUM> may, for example, control the semiconductor laser <NUM> to oscillate after the semiconductor laser <NUM> oscillates. In this case, a part of the waveform of the pulse laser beam from the semiconductor laser <NUM> may overlap a part of the waveform of the pulse laser beam from the semiconductor laser <NUM>. In other embodiments, the waveform of the pulse laser beam from the semiconductor laser <NUM> may be temporally separated from the waveform of the pulse laser beam from the semiconductor laser <NUM>.

Energy of the pulse laser beam from the semiconductor laser <NUM> may be substantially smaller than energy of the pulse laser beam from the semiconductor laser <NUM>. When these pulse laser beams are combined such that the pulse laser beam having lower energy preceeds the pulse laser beam having higher energy, a pulse laser beam that substantially includes a pedestal may be outputted from the master oscillator <NUM>.

The pulse laser beam outputted from the master oscillator <NUM> may then be amplified in the regenerative amplifier <NUM>. The amplified pulse laser beam may enter the optical shutter <NUM>. The optical shutter <NUM> shown in <FIG> may be similar to the optical shutter <NUM> shown in <FIG>. Althrough in this example, the optical shutter <NUM> is provided downstream from the regenerative amplifier <NUM>, the optical shutter <NUM> may be provided in a beam path between the master oscillator <NUM> and the regenerative amplifier <NUM>. By adjusting a voltage applied to the optical shutter <NUM> when the pulse laser beam from the master oscillator <NUM> passes through the optical shutter <NUM>, pedestal energy Ep of the pulse laser beam may be adjusted effectively. Here, the optical shutter <NUM> may be omitted when the pedestal energy Ep obtained by adjusting the energy of the pulse laser beam from the semiconductor laser <NUM> is brought to desired pedestal energy even after the pulse laser beam is amplified in the amplifiers <NUM> through <NUM>.

<FIG> show waveforms of a pulse laser beam at respective positions (a) through (d) in <FIG>. As shown in <FIG>, a waveform of the pulse laser beam from the master oscillator <NUM> may, for example, include a waveform Wp31, where the energy is relatively low, and a waveform Wm31, where the energy is relatively high. The waveform Wp31 may, for example, be a waveform of the pulse laser beam from the semiconductor laser <NUM>. The waveform Wm31 may, for example, be a waveform of the pulse laser beam from the semiconductor laser <NUM>. Then, the pulse laser beam from the master oscillator <NUM> may be amplified in the regenerative amplifier <NUM>. In that case, as shown in <FIG>, a waveform of the pulse laser beam from the regenerative amplifier <NUM> may include a waveform Wp32, where the energy is relatively low, and a waveform Wm32, where the energy is relatively high. Subsequently, as shown in <FIG>, a waveform of the pulse laser beam transmitted through the optical shutter <NUM> may include a pedestal Wp33, where the energy of the pulse laser beam from the semiconductor <NUM> is reduced by the optical shutter <NUM> and the pedestal ratio R or the pedestal energy Ep is relative low, and a peak portion Wm33, where the energy Em is relatively high. Further, as shown in <FIG>, a waveform of the pulse laser beam amplified in the amplifiers <NUM> through <NUM> may be in a shape where the waveform shown in <FIG> is amplified. Similarly to the waveform shown in <FIG>, this waveform shown in <FIG> may include a pedestal Wp34, where the pedestal ratio R is relatively low or the pedestal energy Ep is relatively low, and a peak portion Wm34, where the energy Em is relatively high.

In the above-described EUV light generation system, the pedestal control device is adjusted in order to satisfy the required conversion efficiency. However, this disclosure is not limited thereto. For example, energy of the EUV light may be controlled by adjusting the pedestal ratio or the pedestal energy of the main pulse laser beam. Hereinafter, an EUV light generation system configured to control energy of the EUV light by adjusting the pedestal control device will be described in detail as a second embodiment of this disclosure.

The EUV light generation system of the second embodiment may be configured similarly to the EUV light generation system 11A of the first embodiment.

The operation of the EUV light generation system of the second embodiment may be similar to that of the EUV light generation system 11A of the first embodiment. However, in the second embodiment, target energy Eeuvt of the EUV light may be inputted to the EUV light generation controller 5A from an external apparatus, such as the exposure apparatus controller <NUM>. In that case, the EUV light generation controller 5A may control the pedestal control device <NUM> so that the energy of the emitted EUV light is brought to the target energy Eeuvt.

By controlling the pedestal control device <NUM> to adjust the conversion efficiency CE, the energy of the EUV light <NUM> may be controlled. Accordingly, the energy of the EUV light <NUM> may be controlled without largely changing the output power of the main pulse laser apparatus 3A. Thus, variation in a heat load on optical elements provided in a beam path between the main pulse laser apparatus 3A and the plasma generation region <NUM> may be reduced. As a result, these optical elements may be thermally stabilized. Accordingly, focusing performance of the main pulse laser beam <NUM> may be stabilized, and the output stability of the EUV light <NUM> may be improved.

The operation of the EUV light generation system 11A according to the second embodiment may be based on the pedestal ratio R (see <FIG>) or the pedestal energy Ep (see <FIG>). The operation based on the pedestal ratio R will first be discussed in detail with reference to the drawings.

<FIG> is a flowchart showing an example of an overall operation of a pedestal controller according to the second embodiment.

With reference to <FIG>, the pedestal controller <NUM> may first stand by until it receives an target EUV energy Eeuvt of the EUV light <NUM> from an external apparatus, such as the exposure apparatus controller <NUM> (Step S501; NO). Upon receiving a target EUV energy Eeuvt (Step S501; YES), the pedestal controller <NUM> may then stand by until it receives a trigger signal from the EUV light generation position controller <NUM> (Step S502; NO). The trigger signal may be a trigger for generating a single pulse of the EUV light <NUM>. The trigger signals may be inputted to the pedestal controller <NUM> at a predetermined repetition rate while the EUV light <NUM> is to be generated.

Upon receiving the trigger signal (Step S502; YES), the pedestal controller <NUM> may receive EUV energy Eeuv of the EUV light <NUM> detected by the energy sensor <NUM> (Step S503). Subsequently, the pedestal controller <NUM> may carry out a pedestal ratio calculation subroutine to calculate the pedestal ratio R (Step S504). The pedestal ratio calculation subroutine may be similar to the modification of the pedestal ratio calculation subroutine described with reference to <FIG>.

Then, the pedestal controller <NUM> may carry out a pedestal control subroutine to control the pedestal control device <NUM> so that the pedestal of the main pulse laser beam <NUM> achieves a desired pedestal ratio R (Step S505).

Thereafter, the pedestal controller <NUM> may determine whether or not generation of the EUV light <NUM> is to be stopped (Step S506). This determination may, for example, be made in the pedestal control subroutine in Step S505.

When generation of the EUV light <NUM> is to be continued (Step S506; NO), the pedestal controller <NUM> may return to Step S502 and repeat the subsequent steps. On the other hand, when generation of the EUV light <NUM> is to be stopped (Step S506; YES), the pedestal controller <NUM> may terminate the operation.

With the above-described operation, the pedestal ratio R of the main pulse laser beam <NUM> may be adjusted in accordance with the target EUV energy Eeuvt. As a result, the target EUV energy Eeuvt may be achieved without largely changing the output power of the main pulse laser beam <NUM>.

<FIG> shows an example of a pedestal control subroutine in Step S505 of <FIG>. <FIG> shows an example of a relationship between a pedestal ratio and conversion efficiency used in the description of the pedestal control subroutine shown in <FIG>.

With reference to <FIG>, in the pedestal control subroutine, the pedestal controller <NUM> may calculate a difference ΔE, where ΔE=Eeuv-Eeuvt, between the detected EUV energy Eeuv and the target EUV energy Eeuvt (Step S511). Then, the pedestal controller <NUM> may calculate a change amount ΔR of the pedestal ratio R corresponding to the calculated difference ΔE (Step S512). In order to calculate the change amount ΔR from the difference ΔE, a relationship between a pedestal ratio Rn and the total energy Et of the main pulse laser beam to be used to obtain a relationship between the pedestal ratio R and the conversion efficiency CE shown in <FIG> may be used. The relationship between the pedestal ratio Rn and the total energy Et of the main pulse laser beam may be obtained by carrying out the pedestal control subroutine described with reference to <FIG> or <FIG>. Subsequently, the pedestal controller <NUM> may calculate a corrected pedestal ratio R from the current pedestal ratio R and the calculated change amount ΔR, (R=R+ΔR) (Step S513).

Then, the pedestal controller <NUM> may determine whether or not the calculated pedestal ratio R falls within a monotonic decrease region of the conversion efficiency CE in a measurement range of the pedestal ratio R (Step S514). The monotonic decrease region of the conversion efficiency CE may be a region in which the conversion efficiency CE decreases relatively monotonically with respect to the increase in the pedestal ratio R, as shown in <FIG>. The aforementioned determination may be made based on a value of the pedestal ratio R. Here, the relational curve between the pedestal ratio R and the conversion efficiency CE as shown in <FIG> may preferably be obtained in advance. This relationship may be obtained by carrying out the pedestal control subroutine described with reference to <FIG> or <FIG>. In other embodiments, the relationship between the pedestal ratio R and the conversion efficiency CE obtained in advance through an experiment may be stored and referenced accordingly.

When the pedestal ratio R falls within the monotonic decrease region of the conversion efficiency CE (Step S514; YES), the pedestal controller <NUM> may calculate a control value P of the pedestal control device <NUM> to achieve the desired pedestal ratio R (Step S515). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. On the other hand, when the pedestal ratio R does not fall within the monotonic decrease region of the conversion efficiency CE (Step S514; NO), the pedestal controller <NUM> may instruct the EUV light generation controller 5A to terminate the control of the pedestal to control the energy of the EUV light <NUM> (Step S516). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. <NUM> Control Flow Based on Pedestal Energy.

An operation based on pedestal energy will now be described in detail with reference to the drawings.

<FIG> is a flowchart showing an example of an overall operation of a pedestal controller according to a modification of the second embodiment.

The pedestal control flow as shown in <FIG>, in which the pedestal energy Ep is used as a parameter, may include steps that are similar to those of the pedestal control flow described with reference to <FIG>. Thus, only the operations of the pedestal control flow of <FIG> that differ from those in the pedestal control flow shown in <FIG> will be discussed below. Steps S601 through S603 may be similar to Steps S501 through S503 of <FIG>. In Step S604, the pedestal controller <NUM> may carry out a pedestal energy calculation subroutine to calculate the pedestal energy Ep. The pedestal energy calculation subroutine in Step S604 may be similar to the modification of the pedestal energy calculation subroutine described with reference to <FIG>.

Then, the pedestal controller <NUM> may carry out a pedestal control subroutine to control the pedestal control device <NUM> so that a pedestal of the main pulse laser beam <NUM> achieves desired pedestal energy Ep (Step S605).

Step S606 of <FIG> may be similar to Step S506 of <FIG>. However, when the determination is NO in Step S606, the pedestal controller <NUM> may return to Step S602 and repeat the subsequent steps.

With the above-described operation, the pedestal energy Ep of the main pulse laser beam <NUM> may be controlled in accordance with the target EUV energy Eeuvt. As a result, the target EUV energy Eeuvt may be achieved without largely changing the output power of the main pulse laser beam <NUM>.

<FIG> shows an example of a pedestal control subroutine in Step S605 of <FIG>. <FIG> shows an example of a relationship between pedestal energy and conversion efficiency used in the description of the pedestal control subroutine shown in <FIG>.

With reference to <FIG>, in the pedestal control subroutine, the pedestal controller <NUM> may calculate a difference ΔE, where ΔE=Eeuv-Eeuvt, between the detected EUV energy Eeuv and the target EUV energy Eeuvt (Step S611). Then, the pedestal controller <NUM> may calculate a change amount ΔEp of the pedestal energy Ep corresponding to the calculated difference ΔE (Step S612). Subsequently, the pedestal controller <NUM> may calculate corrected pedestal energy Ep, where Ep=Ep+ΔEp, from the current pedestal energy Ep and the calculated change amount ΔEp (Step S613).

Then, the pedestal controller <NUM> may determine whether or not the calculated pedestal energy Ep falls within a monotonic decrease region of the conversion efficiency CE in a measurement range of the pedestal energy Ep (Step S614). The monotonic decrease region of the conversion efficiency CE may be a region in which the conversion efficiency CE decreases relatively monotonically with respect to the increase in the pedestal energy Ep, as shown in <FIG>. The above determination may be made based on a value of the pedestal energy Ep. Here, a relational between the pedestal energy Ep and the conversion efficiency CE as shown in <FIG> may preferably be obtained in advance. This relationship may be obtained by carrying out the pedestal control subroutine described with reference to <FIG> or <FIG>. In other embodiments, a relationship between the pedestal energy Ep and the conversion efficiency CE obtained in advance through an experiment may be stored and referenced accordingly.

When the pedestal energy Ep falls within the monotonic decrease region of the conversion efficiency CE (Step S614; YES), the pedestal controller <NUM> may calculate a control value P of the pedestal control device <NUM> to achieve the desired pedestal energy Ep (Step S615). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>. On the other hand, when the pedestal energy Ep does not fall within the monotonic decrease region of the conversion efficiency CE (Step S614; NO), the pedestal controller <NUM> may instruct the EUV light generation controller 5A to terminate the control of the pedestal to control of the energy of the EUV light <NUM> (Step S616). Thereafter, the pedestal controller <NUM> may return to the operation shown in <FIG>.

The optical shutter of the above-described embodiments will now be described with specific examples.

<FIG> illustrates an exemplary configuration of an optical shutter which includes two polarizers and a Pockels cell. A Pockels cell typically has a responsiveness in the order of a few nanoseconds and is considered to be suitable as an optical shutter in a laser apparatus where high-speed switching is required.

In the configuration shown in <FIG>, a polarizer <NUM> may transmit a Y-polarization component of a laser beam incident thereon and block an X-polarization component thereof by reflecting or absorbing the X-polarization component. On the other hand, a polarizer <NUM> may transmit an X-polarization component of a laser beam incident thereon and block a Y-polarization component thereof by reflecting or absorbing the Y-polarization component. In this way, the polarizers <NUM> and <NUM> may be arranged to transmit different polarization components. In this example, the polarizers <NUM> and <NUM> may be arranged so that the polarization direction of the laser beam transmitted through the polarizer <NUM> differs by <NUM> degrees from the polarization direction of the laser beam transmitted through the polarizer <NUM>.

A high-voltage pulse may be applied to a Pockels cell <NUM> by a high-voltage power source <NUM> under the control of the pedestal controller <NUM>. The Pockels cell <NUM> may, for example, rotate the polarization direction of the entering laser beam while the high-voltage pulse is applied thereto. In this example, a high-voltage pulse at a value for rotating the polarization direction of the entering laser beam incident beam by a predetermined amount may be applied to the Pockels cell <NUM> by the high-voltage power source <NUM>.

When a pulse laser beam L1 containing largely a Y-polarization component outputted from the master oscillator <NUM> enters the optical shutter <NUM> configured as described above, the pulse laser beam L1 may first be incident on the polarizer <NUM>. The polarizer <NUM> may transmit a Y-polarization component of the pulse laser beam L1. The pulse laser beam L1 transmitted through the polarizer <NUM> may then enter the Pockels cell <NUM>.

When a high-voltage pulse is not applied to the Pockels cell <NUM>, the pulse laser beam L1 may be outputted from the Pockels cell <NUM> without having the polarization direction thereof rotated. The outputted pulse laser beam L1 may then be incident on the polarizer <NUM>, and the polarizer <NUM> may block the pulse laser beam L1 in this case. As a result, the pulse laser beam L1 may be blocked by the optical shutter <NUM>.

Meanwhile, when a high-voltage pulse is applied to the Pockels cell <NUM>, the polarization direction of the pulse laser beam L1 entering the Pockels cell <NUM> may be rotated by a predetermined amount. The outputted pulse laser beam L1 may then be incident on the polarizer <NUM>, and the polarizer <NUM> may transmit an X-polarization component of the pulse laser beam L1. As a result, a part of the pulse laser beam L1 may be outputted from the optical shutter <NUM>.

As shown in <FIG>, a pulse laser beam L1 having a pulse duration of about <NUM> ns may enter the optical shutter <NUM>. Then, as shown in <FIG>, a high-voltage pulse of a duration in which a temporal jitter of the pulse laser beam L1 is absorbed may be applied to the Pockels cell <NUM>. For example, when the pulse duration of the pulse laser beam L1 is <NUM> ns, and the temporal jitter is <NUM> ns, the duration of the high-voltage pulse G1 may be approximately <NUM> ns.

Further, the high-voltage pulse G1 may have a step-like pulse shape in which a voltage of a portion where a pedestal Lp is to be generated from the pulse laser beam L1 is low and a voltage of the remaining portion is high to generate a peak portion Lm. By applying a high-voltage pulse having such a pulse shape to the Pockels cell <NUM>, a waveform of the pulse laser beam L1 may be transformed into such a waveform that contains the pedestal Lp and the peak portion Lm, as shown in <FIG>.

In this example, the polarizers <NUM> and <NUM> are arranged such that the polarization direction of the pulse laser beam L1 transmitted through the polarizer <NUM> differs by <NUM> degrees from that of the pulse laser beam L1 transmitted through the polarizer <NUM>. However, this disclosure is not limited thereto. For example, the polarizers <NUM> and <NUM> may be arranged such that the polarization direction of the pulse laser beam L1 transmitted through the polarizer <NUM> may be the same as that of the pulse laser beam L1 transmitted through the polarizer <NUM>. In that case, while a high-voltage pulse is not applied to the Pockels cell <NUM>, the optical shutter <NUM> may transmit the pulse laser beam L1.

<FIG> schematically illustrates an exemplary configuration of an optical shutter of a first modification. In an optical shutter <NUM>-<NUM>, reflective polarizers <NUM> and <NUM> may, for example, be used in place of the transmissive polarizers <NUM> and <NUM>. A polarizer such as an absorbing thin-film reflector (ATFR) may be used for each of the polarizers <NUM> and <NUM>. Even with such a configuration, a similar function to that of the optical shutter <NUM> shown in <FIG> may be achieved. In <FIG>, the high-voltage power source <NUM> is omitted.

<FIG> schematically illustrates an exemplary configuration of an optical shutter of a second modification. As shown in <FIG>, in an optical shutter <NUM>-<NUM>, four reflective polarizers <NUM> through <NUM> may be provided upstream from the Pockels cell <NUM>, and four reflective polarizers <NUM> through <NUM> may be provided downstream from the Pockels cell <NUM>. An ATFR may be used for each of the polarizers <NUM> and <NUM>. The polarizers <NUM> through <NUM> may, for example, be arranged to reflect a Y-polarization component of the pulse laser beam L1 and absorb the other polarization component thereof. The polarizers <NUM> through <NUM> may, for example, be arranged to reflect an X-polarization component of the pulse laser beam L1 and absorb the other polarization component thereof. When a plurality of polarizers that reflects the same polarization component and absorbs the other polarization component is provided upstream and downstream from the Pockels cell <NUM>, respectively, the total absorptance of a polarization component to be absorbed may be increased. Thus, the purity of a given polarization component may be increased.

<FIG> schematically illustrates an exemplary configuration of an optical shutter of a third modification. As shown in <FIG>, an optical shutter <NUM>-<NUM> may include two Pockels cells 503a and 503b. Each of the Pockels cells 503a and 503b may be similar to the Pockels cell <NUM> of <FIG>. The Pockels cells 503a may be provided upstream from the Pockels cell 503b. Reflective polarizers <NUM> and <NUM> provided upstream from the Pockels cell 503a and reflective polarizers <NUM> and <NUM> provided downstream from the Pockels cell 503b may, for example, reflect a Y-polarization component of the pulse laser beam L1 and absorb the other polarization component. Reflective polarizers <NUM> through <NUM> may be provided in a beam path between the Pockels cell 503a and the Pockels cell 503b. The polarizers <NUM> through <NUM> may, for example, reflect a Z-polarization component of the pulse laser beam L1 and absorb the other polarization component. A high-reflection mirror <NUM> may be provided downstream from the Pockel cell 503a and a high-reflection mirror <NUM> may be provided upstream from the Pockels cell 503b. When the plurality of Pockels cells 503a and 503b are used, total absorptance of a polarization component to be absorbed may be increased. Thus, the purity of a certain polarization component may be increased.

<FIG> schematically illustrates an exemplary configuration of an optical shutter of a fourth modification. An optical shutter <NUM>-<NUM> as shown in <FIG> may be similar in a configuration to the optical shutter <NUM>-<NUM> shown in <FIG>, but may differ in that the polarizers <NUM> and <NUM> may respectively be provided with cooling devices <NUM>. A cooling medium supplied from the cooling device <NUM> may flow through a flow channel <NUM> and into an internal flow channel of each of the polarizers <NUM> and <NUM>. Each of the polarizers <NUM> and <NUM> may be provided with an internal flow channel to allow the cooling medium to flow efficiently behind a reflective surface. Cooling the reflective surfaces of the polarizers <NUM> and <NUM> efficiently and in a balanced manner may suppress thermal deformation in the respective reflective surfaces. As a result, the direction and the wavefront of the pulse laser beam L1 transmitted through the optical shutter <NUM>-<NUM> may be stabilized. Note that a cooling device may also be provided on the Pockels cell <NUM> to suppress overheating in the Pockels cell <NUM>.

The saturable absorber device of the above-described embodiments will now be described with specific examples.

A saturable absorber device in which a concentration of a saturable absorber gas can be adjusted will now be described with reference to the drawing. <FIG> schematically illustrates an exemplary configuration of such a saturable absorber device.

As shown in <FIG>, a saturable absorber device 322A may include a saturable absorber gas cell <NUM>, a heat exchanger <NUM>, a gas temperature controller <NUM>, and a saturable absorber gas cell controller <NUM>. The saturable absorber gas cell controller <NUM> may control each component of the saturable absorber device 322A in accordance with a signal from the pedestal controller <NUM>.

The saturable absorber gas cell <NUM> may include windows <NUM> and <NUM>, through which the pulse laser beam L1 may travel. A pressure sensor <NUM> may be connected to the saturable absorber gas cell <NUM> to measure a pressure inside the saturable absorber gas cell <NUM>. The pressure sensor <NUM> may be connected to the saturable absorber gas cell controller <NUM>. The interior of the saturable absorber gas cell <NUM> may be in communication with the heat exchanger <NUM> through a gas pipe <NUM>. A gas pump <NUM> may be provided in the gas pipe <NUM> to allow the saturable absorber gas in the gas pipe <NUM> to circulate between the saturable absorber gas cell <NUM> and the heat exchanger <NUM>. Further, a temperature sensor <NUM> may be provided in the gas pipe <NUM> to detect a temperature of the saturable absorber gas circulating in the gas pipe <NUM>. The gas temperature controller <NUM> may be connected to the temperature sensor <NUM>. The gas temperature controller <NUM> may drive the heat exchanger <NUM> based on a signal from the saturable absorber gas cell controller <NUM>, to thereby control a temperature of the circulating saturable absorber gas.

The saturable absorber gas cell controller <NUM> may be connected to the gas pump <NUM>. The saturable absorber gas cell controller <NUM> may control the number of revolutions of the gas pump <NUM>, to thereby control a flow rate of the saturable absorber gas circulating in the gas pipe <NUM>.

An SF<NUM> gas cylinder <NUM> may be connected to the gas pipe <NUM> through valves <NUM> and <NUM>. Further, a buffer gas cylinder <NUM> may be connected to the gas pipe <NUM> through valves <NUM> and <NUM>. A buffer gas may be N<NUM>, He, or the like. Furthermore, a discharge pump <NUM> may be connected to the gas pipe <NUM> through a valve <NUM>. The valves <NUM>, <NUM>, <NUM>, and <NUM> may be connected to the saturable absorber gas cell controller <NUM>. The discharge pump <NUM> may also be connected to the saturable absorber gas cell controller <NUM>. The saturable absorber gas cell controller <NUM> may appropriately adjust opening of the valves <NUM>, <NUM>, <NUM>, and <NUM> and the number of revolutions of the discharge pump <NUM>, to thereby adjust a gas pressure in the gas pipe <NUM> and a concentration of the saturable absorber gas.

By adusting a concentration of the saturable absorber gas in the saturable absorber gas cell <NUM>, pedestal energy of the pulse laser beam L1 passing through the saturable absorber device 322A may be adjusted.

A saturable absorber device in which an optical path length through a saturable absorber gas can be adjusted will now be described with reference to the drawing. <FIG> schematically illustrates an exemplary configuration of such a saturable absorber device.

As shown in <FIG>, a saturable absorber device 322B may be similar in configuration to the saturable absorber device 322A shown in <FIG>. However, the saturable absorber device 322B of <FIG> may include a saturable absorber gas cell <NUM> in place of the saturable absorber gas cell <NUM>.

As shown in <FIG>, the saturable absorber gas cell <NUM> may include a window <NUM>, through which the pulse laser beam L1 may travel. High-reflection mirrors <NUM> and <NUM> may be provided inside the saturable absorber gas cell <NUM> to bend a beam path of the pulse laser beam L1. The high-reflection mirrors <NUM> and <NUM> may be positioned so that the pulse laser beam L1 entering the saturable absorber gas cell <NUM> through the window <NUM> is reflected sequentially by the high-reflection mirrors <NUM> and <NUM> to be outputted through the window <NUM>.

The high-reflection mirrors <NUM> and <NUM> may be fixed to a moving stage <NUM>. The moving stage <NUM> may be movable along rails <NUM> provided in the saturable absorber gas cell <NUM>. The rails <NUM> may extend in a direction parallel to the travel direction of the pulse laser beam L1.

Further, a moving device <NUM> may be provided in the saturable absorber gas cell <NUM> to move the moving stage <NUM> along the rails <NUM>. The moving device <NUM> may be connected to a driver <NUM>. The pedestal controller <NUM> may control the moving device <NUM> through the driver <NUM> to move the moving stage <NUM>, to thereby adjust an optical path length inside the saturable absorber gas cell <NUM>.

By adusting an optical path length inside the saturable absorber gas cell <NUM>, pedestal energy of the pulse laser beam L1 passing through the saturable absorber device 322B may be adjusted.

In the preceding description, a diffused target may be a target in a state where particles containing at least one of atoms, molecules, clusters, fine droplets of a target material are diffused in a mist or gas form. A diffused target may contain a target material that in part is turned into plasma.

<FIG> shows a target irradiated with a pre-pulse laser beam. As shown in <FIG>, a diffused target <NUM> may be generated when a target <NUM> is irradiated with a pre-pulse laser beam <NUM>. When the spherical or droplet-shaped target <NUM> is irradiated with the pre-pulse laser beam <NUM>, a torus-shaped or disc-shaped diffused target <NUM> may be generated.

Generation process of a diffused target will be described in detail with reference to <FIG>. <FIG> shows a target irradiated with a pre-pulse laser beam, as viewed in a direction perpendicular to the travel direction of the pre-pulse laser beam. <FIG> shows a diffused target generated when a target is irradiated with a pre-pulse laser beam being irradiated with a main pulse laser beam, as viewed in a direction perpendicular to the travel direction of the main pulse laser beam. <FIG> shows a diffused target generated when a target is irradiated with a pre-pulse laser beam being irradiated with a main pulse laser beam, as viewed in the travel direction of the main pulse laser beam.

As shown in <FIG>, when the target <NUM> is irradiated with the pre-pulse laser beam <NUM>, plasma may be generated by laser ablation to a side of the target <NUM> irradiated with the pre-pulse laser beam <NUM>. When the plasma diffuses, a shock wave S1 generated as the reaction to the laser ablation may propagate into the target <NUM>. As a result, the target <NUM> may be broken up, and the diffused target <NUM> may be generated.

As shown in <FIG>, the diffused target <NUM> may typically move with a component in a direction D1 the same as the travel direction of the pre-pulse laser beam <NUM>. The main pulse laser beam <NUM> may be focused to pass through a space including a range within which the diffused target <NUM> moves and/or diffuses. For example, as shown in <FIG>, a diameter Dm of the main pulse laser beam <NUM> in the plasma generation region <NUM> may be larger than a diffusion range Dd of the diffused target <NUM> diffused in a torus-shape or in a disc-shape.

<FIG> shows an example of a relationship between conversion efficiency and a delay time from the irradiation of a target with a pre-pulse laser beam until the irradiation of the target with a main pulse laser beam. <FIG> shows a case where the wavelength of the pre-pulse laser beam <NUM> is <NUM>, the pulse duration is <NUM> ns, and the fluence is <NUM> mJ/cm<NUM>, and where the main pulse laser apparatus 3A is a CO<NUM> laser, the pulse duration of the main pulse laser beam <NUM> is <NUM> ns, and the beam intensity is <NUM>×<NUM><NUM> W/cm<NUM>.

In <FIG>, a line D12 shows a case where the diameter of the target <NUM> is <NUM>, a line D20 shows a case where the diameter of the target <NUM> is <NUM>, a line D30 shows a case where the diameter of the target <NUM> is <NUM>, and a line D40 shows a case where the diameter of the target <NUM> is <NUM>.

When the diameter of the target <NUM> is <NUM>, a delay time for the main pulse laser beam <NUM> with respect to the pre-pulse laser beam <NUM> may be in a range of <NUM> to <NUM>. In other examples, the delay time may be in a range of <NUM> to <NUM>. In yet other examples, the delay time may be in a range of <NUM> to <NUM>.

When the diameter of the target <NUM> is <NUM>, a delay time for the main pulse laser beam <NUM> with respect to the pre-pulse laser beam <NUM> may be in a range of <NUM> to <NUM>. In other examples, the delay time may be in a range of <NUM> to <NUM>. In yet other examples, the delay time may be <NUM>.

A relationship between a fluence of a pre-pulse laser beam and a shape of a diffused target will now be discussed in detail with reference to the drawings.

<FIG> show a shape of a diffused target and plasma observed in a case where a fluence of a pre-pulse laser beam is <NUM> mJ/cm<NUM>. <FIG> shows a case where an elapsed time from irradiation with the pre-pulse laser beam is <NUM>. <FIG> shows a case where an elapsed time from irradiation with the pre-pulse laser beam is <NUM>. <FIG> shows a case where an elapsed time from irradiation with the pre-pulse laser beam is <NUM>. <FIG> shows a case where an elapsed time from irradiation with the pre-pulse laser beam is <NUM>.

Further, <FIG> show a case where Sn is used as the target material, the diameter of the target <NUM> is <NUM>, a YAG laser is used as the pre-pulse laser apparatus <NUM>, and the pulse duration of the pre-pulse laser beam <NUM> is <NUM> ns. In each of <FIG>, the pre-pulse laser beam <NUM> strikes the target <NUM> from the right side of the paper plane.

As shown in <FIG>, <FIG>, and <FIG>, at a point where an elapsed time from irradiation with the pre-pulse laser beam <NUM> is <NUM>, that is, at the time when the target <NUM> is irradiated with the pre-pulse laser beam <NUM>, the diffused target <NUM> was not observed and only plasma <NUM> of the target material was observed.

As shown in <FIG>, when a fluence of the pre-pulse laser beam <NUM> is <NUM> mJ/cm<NUM>, the torus-shaped diffused target <NUM> was observed where an elapsed time from the irradiation with the pre-pulse laser beam <NUM> is in a range of <NUM> to <NUM>. When an elapsed time is equal to or greater than <NUM>, if the spot size Dm of the main pulse laser beam <NUM> is about <NUM>, a large portion of the diffused target <NUM> may be irradiated with the main pulse laser beam <NUM>.

As shown in <FIG> and <FIG>, when a fluence of the pre-pulse laser beam <NUM> is <NUM> mJ/cm<NUM> or <NUM> mJ/cm<NUM>, the disc-shaped diffused target <NUM> was observed where an elapsed time from the irradiation with the pre-pulse laser beam <NUM> is in a range of <NUM> to <NUM>. Further, as the elapsed time increases, the diffusion range of the diffused target <NUM> increased. When a fluence of the pre-pulse laser beam <NUM> is <NUM> mJ/cm<NUM>, the diffusion range of the diffused target <NUM> was greater, compared to the case when a fluence of the pre-pulse laser beam <NUM> is <NUM> mJ/cm<NUM>.

In any of the cases shown in <FIG>, the EUV light <NUM> was generated when the diffused target <NUM> and/or the plasma <NUM> were/was irradiated with the main pulse laser beam <NUM>.

<FIG> illustrates an exemplary configuration of a regenerative amplifier. The regenerative amplifier <NUM> may include a polarization beam splitter <NUM>, a CO<NUM> gas amplification part <NUM>, Pockels cells <NUM> and <NUM>, a quarter-wave plate <NUM>, and resonator mirrors <NUM> and <NUM>.

The polarization beam splitter <NUM> may, for example, be formed of a thin-film polarizer. The polarization beam splitter <NUM> may, for example, reflect an S-polarization component and transmit a P-polarization component of a laser beam incident thereon. When the pulse laser beam L1 that largely contains an S-polarization component with respect to the polarization beam splitter <NUM> enters the regenerative amplifier <NUM>, the pulse laser beam L1 may first be reflected by the polarization beam splitter <NUM>. Thus, the pulse laser beam L1 may be taken into the resonator formed by the resonator mirrors <NUM> and <NUM>. The pulse laser beam L1 taken into in the resonator may be amplified as it passes through the CO<NUM> gas amplification part <NUM>. The pulse laser beam L1 may pass through the Pockels cell <NUM>, to which a voltage is not applied, be transmitted through the quarter-wave plate <NUM>, be reflected by the resonator mirror <NUM>, and again be transmitted through the quarter-wave plate <NUM>. With this configuration, the polarization direction of the pulse laser beam L1 may be rotated by <NUM> degrees.

Thereafter, the pulse laser beam L1 may pass through the Pockels cell <NUM> again, to which a voltage is not applied. At this point, a predetermined voltage may be applied to the Pockels cell <NUM> after the pulse laser beam L1 passes therethrough. The Pockels cell <NUM>, to which a voltage is applied, may give a quarter-wave phase shift to the pulse laser beam L1 passing therethrough. Accordingly, while the predetermined voltage is applied to the Pockels cell <NUM>, the polarization direction of the pulse laser beam L1 incident on the polarization beam splitter <NUM> may largely include a P-polarization component with respect thereto. Thus, the pulse laser beam L1 may be traped in the resonator.

Then, at a timing at which the pulse laser beam L1 is to be outputted, predetermined voltage may be applied to the Pockels cell <NUM>. The pulse laser beam L1 traveling back and forth in the resonator may be transmitted through the polarization beam splitter <NUM> and then be subjected to a quarter-wave phase shift when passing through the Pockels cell <NUM>. Then, the linearly polarized pulse laser beam L1 may be transformed into the circularly polarized pulse laser beam L1. Thereafter, the pulse laser beam L1 may be reflected by the resonator mirror <NUM> and again pass through the Pockels cell <NUM>. Thus, the circularly polarized pulse laser beam L1 may be transformed into the linearly polarized pulse laser beam L1. The pulse laser beam L1 at this point may largely include an S-polarization component with respect to the polarization beam splitter <NUM> and be reflected thereby. Accordingly, the pulse laser beam L1 may be outputted from the regenerative amplifier <NUM>.

The above-described embodiments and the modifications thereof are merely examples for implementing this disclosure, and this disclosure is not limited thereto. Making various modifications according to the specifications or the like is within the scope of the invention as defined by the appended claims. For example, the modifications illustrated for particular ones of the embodiments can be applied to other embodiments as well (including the other embodiments described herein).

Claim 1:
A method for irradiating a target with a main pulse laser beam (<NUM>) and a pre-pulse laser beam (<NUM>) at timings defined by a delay time, comprising:
outputting the main pulse laser beam having a first waveform (Ws),
the first waveform including a first portion and a second portion ,
the first portion is a front portion of the first waveform (Ws),
the second portion (Ws) including a peak having a peak intensity of the main pulse laser beam;
transforming the main pulse laser beam from the first waveform to a second waveform by controlling an optical element (<NUM>) to control transmission of the main pulse laser beam through the optical element (<NUM>),
the second waveform including a transformed first portion (Wp1) and a transformed second portion (Wm1) following the transformed first portion (Wp1),
the transformed first portion (Wp1) including a pedestal,
the transformed second portion (Wm1) including the peak having the peak intensity of the main pulse laser beam;
amplifying the second waveform while maintaining connection between the transformed first portion (Wp1) and the transformed second portion (Wm1), the amplified second waveform including an amplified first portion and an amplified second portion,
the amplified first portion having a maximum energy that is less than the maximum energy of the amplified second portion; and
irradiating the target with the amplified first portion and the amplified second portion;
the method further comprising
controlling a ratio of energy of the amplified first portion to total energy of the amplified first portion and the amplified second portion.