Patent ID: 12230936

DESCRIPTION OF EMBODIMENTS

Hereinafter, a laser amplification device and an extreme ultraviolet light generation apparatus according to embodiments of the present disclosure will be described in detail with reference to the drawings.

First Embodiment

FIG.1is a diagram illustrating a configuration of an extreme ultraviolet light generation apparatus100including a laser amplification device10according to a first embodiment. Extreme ultraviolet light is also referred to as EUV light, and the extreme ultraviolet light generation apparatus is also referred to as the EUV light generation apparatus. The EUV light generation apparatus100is used as a light source of an exposure apparatus, and generates EUV light with a wavelength of 13.5 nm. The EUV light generation apparatus100includes the laser amplification device10and an EUV light generator40.

The laser amplification device10is a master oscillator power amplifier (MOPA) laser amplification device. The laser amplification device10includes a laser oscillator20and a laser amplifier30. The laser oscillator20includes an oscillator including a laser active medium composed of a mixed gas containing carbon dioxide gas (CO2gas), and generates pulsed laser light25. The laser amplifier30amplifies the pulsed laser light25output from the laser oscillator20and outputs an amplified pulsed laser light35to the EUV light generator40. The pulsed laser light25is also referred to as the incoming laser pulse25, and the pulsed laser light35is also referred to as the outgoing laser pulse35. Pulsed laser light is also referred to as a laser pulse.

The EUV light generator40includes a droplet generator41and a collector mirror42. The collector mirror42includes, in a central portion, a transmission part that transmits the pulsed laser light35input from the laser amplifier30. In the EUV light generator40, droplets DL dropped from the droplet generator41are irradiated with the pulsed laser light35input from the laser amplifier30. As the droplets DL, tin (Sn), xenon (Xe), gadolinium (Gd), terbium (Tb), or the like is used. When the droplets DL are irradiated with the pulsed laser light35, the droplets DL are turned into plasma, and EUV light50with a wavelength of 13.5 nm is generated from the droplets DL turned into plasma. The generated EUV light50is concentrated into an intermediate focus (IF) point60by the collector mirror42. Although not illustrated inFIG.1, the EUV light generator40also includes a droplet catcher or the like. In the EUV light generator40, a prepulse may be applied to increase EUV light output.

When the pulse width of the pulsed laser light35applied to the droplets DL is about 5 ns to 30 ns, the extreme ultraviolet light generation apparatus100has an increased efficiency of conversion into EUV light, providing high-power EUV light. It is desirable for the exposure apparatus to which the extreme ultraviolet light generation apparatus100is applied to acquire high-power EUV light exceeding 250 W at the IF point60. The pulsed laser light35exceeding 20 kW is effective for generating high-power EUV light.

Thus, to obtain high-power EUV light, it is desirable for the extreme ultraviolet light generation apparatus100to irradiate the droplets DL as targets with the pulsed laser light35that has a pulse width of 5 ns to 30 ns and has an output of 20 kW or higher.

FIG.2illustrates an example of the pulse width-dependent properties of amplification of the laser amplifier30in the MOPA CO2laser. The horizontal axis inFIG.2indicates a value τp/τrobtained by dividing the pulse width τpof the pulsed laser light25input to the laser amplifier30by the rotational relaxation time τrof CO2molecules. The pulse width τpis represented by the full width at half maximum. The vertical axis inFIG.2indicates normalized extracted energy ηext. The normalized extracted energy ηextis a value obtained by dividing extracted energy extracted from the laser amplifier30by extracted energy when τp>>τr. Hereinafter, the normalized extracted energy ηextis also simply referred to as extracted energy next. τp>>τrindicates that the pulse width τpof the pulsed laser light25is sufficiently larger than the rotational relaxation time τrof CO2molecules. In the properties ofFIG.2, the fluence Einof the pulsed laser light25input to the laser amplifier30is sufficiently larger than the saturation fluence Esof the laser amplifier30.

FIG.2illustrates that the larger the value τp/τr, in other words, the longer the pulse width τpof the pulsed laser light25input to the laser amplifier30is than the rotational relaxation time τr, the larger the extracted energy next, and the larger the energy that can be extracted from the laser amplifier30. At a pressure higher than 20 Torr at which pressure broadening is dominant in the gain spectrum of the active medium of the CO2laser, the rotational relaxation time τris typically about some nanoseconds. Thus, asFIG.2illustrates, when a laser pulse with the pulse width τpof 5 ns to 30 ns effective for generating high-power EUV light is input to the laser amplifier30, the extracted energy next does not become nearly equal to 1, and sufficient energy cannot be extracted from the laser amplifier30. As a result, the amplification factor of the laser amplifier30decreases, decreasing the output of EUV light obtained at the IF point60.

In the first embodiment, by setting the pulse width τpof the pulsed laser light25input to the laser amplifier30to 15 ns to 200 ns, amplification is performed with the effect of the rotational relaxation of CO2molecules reduced, increasing the amplified output of the laser amplifier30.

FIG.3is a diagram illustrating a more detailed configuration example of the extreme ultraviolet light generation apparatus100according to the first embodiment. The extreme ultraviolet light generation apparatus100includes the laser oscillator20that outputs the pulsed laser light25having the pulse width τpof 15 ns to 200 ns, the laser amplifier30that amplifies the pulsed laser light25output from the laser oscillator20and outputs the pulsed laser light35having a pulse width of 5 ns to 30 ns, and the EUV light generator40that irradiates the droplets DL as targets with the pulsed laser light35output from the laser amplifier30to generate EUV light.

InFIG.3, the laser oscillator20is a Q-switched and cavity-dumped oscillator. The laser oscillator20includes a laser active medium21that is a mixed gas containing CO2gas, an electro-optic device22, a polarization beam splitter23, a resonator mirror24, and a resonator mirror26. Pulsed laser light output from the laser oscillator20is transmitted to the laser amplifier30by a mirror27. The laser oscillator20constitutes a resonator with the resonator mirror24and the resonator mirror26. The mixed gas containing the CO2gas may contain gas such as nitrogen (N2), helium (He), carbon monoxide (CO), xenon (Xe), oxygen (O2), and/or hydrogen (H2) in addition to the CO2gas. The laser oscillator20modulates a voltage applied to the electro-optic device22at a high-repetition frequency of, for example, 5 kHz or higher, thereby generating the pulsed laser light25having the pulse width τpof 15 ns to 200 ns by Q-switched and cavity-dumped oscillation. AlthoughFIG.3illustrates an example in which the laser oscillator20performs Q-switched and cavity-dumped oscillation, other pulse oscillation such as Q-switched oscillation may be used. The laser oscillator20may be a pulsed laser oscillator such as a quantum cascade laser that can oscillate at the oscillation wavelength of the CO2laser. Although the electro-optic device22is used inFIG.3, an acousto-optic device may be used to generate the pulsed laser light25having a pulse width of 15 ns to 200 ns.

The laser amplifier30is an amplifier including a mixed gas containing CO2gas as an amplification active medium. AlthoughFIG.3illustrates the laser amplifier30as a single-stage amplifier, a multi-stage laser amplifier in which two or more amplifiers are arranged may be used. The beam diameter of pulsed laser light may be adjusted by a lens, a mirror, or the like in the laser amplifier30to maximize amplified output by the laser amplifier30. The pulsed laser light25having the pulse width τpof 15 ns to 200 ns input to the laser amplifier30is amplified by the laser amplifier30and is output as the pulsed laser light35having a pulse width of 5 ns to 30 ns.

As illustrated inFIG.1, the EUV light generator40includes the droplet generator41and the collector mirror42.

In the configuration according to the first embodiment, the pulse width τpof the pulsed laser light25input to the laser amplifier30is set to 15 ns to 200 ns, so that amplification can be performed with the effect of the rotational relaxation of CO2molecules reduced, and the amplified output of the laser amplifier30can be increased. For the sake of explanation, the following advances the description with the rotational relaxation time τras 1.5 ns, but is not intended to limit the rotational relaxation time τr.

FIG.4is a diagram illustrating a relationship between the pulse width τpof the pulsed laser light25input to the laser amplifier30and the pulse width of the pulsed laser light35output from the laser amplifier30. InFIG.4, the vertical axis represents the pulse width (ns) of the outgoing laser pulse35, and the horizontal axis represents the pulse width τpof the incoming laser pulse25. As described above, when the pulse width of the outgoing laser pulse35is 5 ns to 30 ns, the EUV light generator40is increased in the efficiency of conversion into EUV light. InFIG.4, the region where the pulse width of the outgoing laser pulse35is 5 ns to 30 ns, with which the efficiency of conversion into EUV light is increased, is divided into four regions including a region A, a region B, a region C, and a region D, based on the value of the normalized extracted energy ηextwhen the rotational relaxation time τris 1.5 ns.

The region A is the region where the pulse width τpof the incoming laser pulse25is 5 ns to 15 ns, and is the region where the normalized extracted energy ηext<0.5 as illustrated inFIG.2.

The region B is the region where the pulse width τpof the incoming laser pulse25is 15 ns to 30 ns, and is the region where 0.5<ηext<0.75 as illustrated inFIG.2.

The region C is the region where the pulse width τpof the incoming laser pulse25is 30 ns to 50 ns, and is the region where 0.75<ηext<0.9 as illustrated inFIG.2.

The region D is the region where the pulse width τpof the incoming laser pulse25is 50 ns to 200 ns, and is the region where ηext>0.9 as illustrated inFIG.2.

Thus, the output of the laser amplifier30becomes higher as located toward the right side inFIG.4. The decreasing order of amplified output is the region D, the region C, the region B, and the region A.

In a typical amplification operation, the pulse width of pulsed laser light input to a laser amplifier and the pulse width of pulsed laser light output from the laser amplifier do not greatly change, and thus the pulse width of the pulsed laser light input to the laser amplifier in this case is 5 ns to 30 ns. That is, it can be understood that the typical amplification operation is performed on a broken line K inFIG.4.

In contrast, in the first embodiment, since the pulse width of the pulsed laser light25input to the laser amplifier30is 15 ns to 200 ns, it can be understood that the amplification operation is performed in one of the region B, the region C, and the region D. Since the region B, the region C, and the region D are regions located on the right side of the broken line K, the first embodiment can provide an amplified output higher than the output obtained by the typical amplification operation. Hereinafter, the pulse width τpof the pulsed laser light25input to the laser amplifier30is referred to as the incoming pulse width τp.

In the first embodiment, in the region C and the region D where the incoming pulse width τpis 30 ns to 200 ns, the extracted energy ηext>0.75. In the typical amplification operation, which is performed on the broken line K inFIG.4, the extracted energy ηextdoes not exceed 0.75 since the broken line K is part of the region A and the region B. Thus, when the incoming pulse width τpis 30 ns to 200 ns, an amplified output higher than the output obtained by the typical amplification operation can be obtained.

In the first embodiment, in the region D where the incoming pulse width τpis 50 ns to 200 ns, the extracted energy ηext>0.9. Thus, when the incoming pulse width τpis 50 ns to 200 ns, an amplified output higher than the output obtained by the typical amplification operation can be obtained. In addition, as illustrated inFIG.2, in this region, the extracted energy ηextis saturated for the incoming pulse width τp, and a change in the amplified output becomes smaller relative to a change in the incoming pulse width τp. Therefore, this region is characterized in that the amplification factor is less likely to decrease even when pulse shortening during amplification as described below occurs. Thus, in the region where the incoming pulse width τpis 50 ns to 200 ns, it is possible to perform amplification while minimizing a decrease in the amplification factor.

The incoming pulse width τpmay be set to 15 ns or more with which the extracted energy ηext>0.5. The incoming pulse width τpmay be desirably set to 20 ns or more with which the extracted energy ηext>0.6. The incoming pulse width τpmay be more desirably set to 30 ns or more with which the extracted energy ηext>0.75. The incoming pulse width τpmay be more desirably set to 40 ns or more with which the extracted energy ηext>0.8. The incoming pulse width τpmay be more desirably set to 50 ns or more with which the extracted energy ηext>0.9.

As illustrated inFIG.2, when the incoming pulse width τpis 200 ns or more, the extracted energy next is nearly equal to 1. In this region, even if the incoming pulse width τpis made longer, the value of the extracted energy next does not increase, and a large increase in amplified output cannot be obtained. In addition, the longer the incoming pulse width τp, the more the peak value of the pulsed laser light35output from the laser amplifier30decreases. Thus, to prevent a decrease in laser pulse peak value and obtain high amplified output, the incoming pulse width τpmay be desirably set to 200 ns or less. Details of a decrease in laser pulse peak value will be described later.

Next, the pulse shortening effect of a pulse shape change during amplification in the laser amplifier30will be described.FIG.5illustrates an example of a pulse shape changing process when a super-Gaussian-shaped laser pulse is amplified. Here, attention is paid only to changes in pulse shape, and thus the maximum values of pulse shapes on the vertical axes illustrated inFIG.5are made equal. In actuality, however, the maximum value on the vertical axis increases as amplification proceeds. The horizontal axes t represent time, and the vertical axes I represent intensity. Hereinafter, t is used as a symbol indicating time, and I intensity. When a laser pulse is input to the laser amplifier30, a front portion of the laser pulse first consumes energy stored in the amplification medium. Consequently, the amplification factor of a rear portion of the laser pulse amplified by the remaining energy is lower than the amplification factor of the front portion of the laser pulse. Here, the front portion of the laser pulse refers to a portion entering the laser amplifier30first, and refers to portions of the laser pulse shapes inFIG.5closer to t=0. Since the front portion of the laser pulse consumes energy stored in the laser amplifier30first, the laser pulse is sharpened as the amplification proceeds as illustrated inFIG.5, resulting in the outgoing laser pulse35with a shorter pulse width than the incoming laser pulse25.

By contrast,FIG.6is a diagram illustrating an example of a pulse shape changing process during amplification when the incoming laser pulse25has a typical Gaussian-shaped waveform. In the Gaussian shape, the energy consumption of the laser pulse front portion is smaller than that in the super-Gaussian shape, and thus sharpening of the laser pulse is less likely to occur. As a result, a laser pulse shortening effect like that obtained when the super-Gaussian shape is input cannot be obtained.

As described above, when the super-Gaussian-shaped pulse shape is used as the incoming laser pulse25, by inputting the pulsed laser light25having a pulse width of 15 ns to 200 ns output from the laser oscillator20to the laser amplifier30for amplification, the pulsed laser light35with a pulse width of 5 ns to 30 ns can be output.

FIG.7illustrates an example of the shape of the super-Gaussian-shaped laser pulse. When the time t=t0is the center of symmetry, I0is a maximum intensity, τpis the full width at half maximum, and P is a coefficient, a super-Gaussian function Isg(t) can be expressed as formula (1) as a function of the time t.

[Formula⁢1]Isg(t)=I0⁢exp⁢{-[4⁢ln⁢2P⁢(t-t0)2τp2]P}(1)

In formula (1), when the coefficient P is 1, the super-Gaussian function agrees with a Gaussian function. As illustrated inFIG.7, a rise time τbis defined as a time until the intensity I reaches 0.9 I0from 0.1 I0. In other words, in a pulse time waveform, the time from when the intensity I increases to 10% intensity of the maximum intensity of pulsed light to when the intensity I increases to 90% intensity of the maximum intensity is referred to as the rise time τb. Likewise, the time from when the intensity I decreases to 90% of the maximum intensity of pulsed light to when the intensity I decreases to 10% of the maximum intensity is referred to as a fall time. When the ratio between the full width at half maximum τpof the super-Gaussian shape and the rise time τbis τr=τb/τp, the relationship between the ratio τrand the coefficient P is as illustrated inFIG.8. That is, the ratio τrdecreases as the coefficient P increases. For example, when the ratio τris close to 0.72, the coefficient P is close to 1, which means a Gaussian shape. When the ratio τris close to 0.35, it means that the pulse shape is a super-Gaussian shape with P=2. When the ratio τr<<0.72, P is a value sufficiently larger than 1, which means that the pulse shape is a shape close to a rectangular shape.

When the incoming laser pulse25has any shape other than super-Gaussian, for example, even when the incoming laser pulse25is an asymmetric laser pulse whose rise time and fall time are different, τris also defined as the ratio between the full width at half maximum τpand the rise time τb(τr=τb/τp). In the description of the present disclosure, a super-Gaussian shape whose rise time and fall time are equal is used in the description, which is not intended to limit the pulse shape. Even with a pulse shape of an arbitrary shape such as that of an asymmetric laser pulse, the same effect as that of a symmetric pulse can be obtained.

As illustrated inFIG.6, for a shape close to a Gaussian shape with the ratio τrclose to 0.72, a remarkable pulse shortening effect cannot be obtained. For a shape close to a super-Gaussian shape with the ratio τrsmaller than 0.72, a remarkable pulse shortening effect can be obtained. As the ratio τrdecreases, the pulse shape approaches a rectangular shape, and a greater pulse shortening effect is obtained.

A change in pulse shape resulting in pulse shortening is more obvious when the extracted energy of the laser amplifier30relative to the incoming laser pulse25is larger, in other words, when the amplification factor of the laser amplifier30is larger.FIG.9illustrates an example of changes in pulse shape and changes in pulse intensity in a pulse amplification process. InFIG.9, when the extracted energy from the laser amplifier30relative to the incoming laser pulse25is small, for example, when a laser pulse53illustrated on the left side inFIG.9enters the laser amplifier30to be amplified, a post pedestal55remains in the pulse shape as in a laser pulse54illustrated second from the left inFIG.9or a laser pulse57illustrated in the third diagram from the left inFIG.9. The post pedestal55refers to a pedestal-shaped portion appearing behind a main pulse56. This is because, in the laser pulse54or the laser pulse57, the main pulse56, which is an energy portion extracted from the laser amplifier30, is small, and the contribution of an energy portion of the incoming laser pulse53is large. In contrast, when the amplification factor of the laser amplifier30is high, the main pulse56, which is an energy portion extracted from the laser amplifier30, is large as in a laser pulse58illustrated in the fourth diagram from the left inFIG.9, so that a pulse shape without the post pedestal55can be obtained.

The amplification factor of the laser amplifier30is desirably 1000 times or more to obtain EUV light exceeding 250 W in the EUV light generator40. The reason will be described below. When an EUV light output output from the extreme ultraviolet light generation apparatus100is larger than 250 W, the number of wafers that can be processed by the exposure apparatus per hour is larger than 125 suitable for mass production. Thus, by making the output of the pulsed laser light35output from the laser amplifier30larger than 20 kW to satisfy the EUV light output>250 W, an optimally suited EUV light generation apparatus for the exposure apparatus can be obtained. When the laser oscillator20is a Q-switched and cavity-dumped oscillator, the output thereof is limited to some tens of watts due to the light resistance strength of the electro-optic device22. Since the output of the laser oscillator20is some tens of watts, and for the output of the laser amplifier30, output larger than 20 kW is required, the amplification factor of the laser amplifier30is desirably 1000 times or more. When the laser amplifier30is a multi-stage amplifier, a value obtained by dividing the output of a laser pulse output from the last stage by the output of a laser pulse before entering a first-stage amplifier is desirably 1000 times or more.

The laser oscillator20is not limited to a single wavelength, and may output a laser pulse including two or more wavelengths. A CO2laser that outputs two or more wavelengths is called a multi-line CO2laser. By outputting two or more wavelengths, the effect of a decrease in the amplification factor of the laser amplifier30due to rotational relaxation can be reduced to obtain high amplified output. The two or more wavelengths are desirably transition wavelengths between rotational levels of the CO2laser. For example, in addition to a wavelength of P(20) at which the maximum output is obtained by the CO2laser, a laser pulse of P(16), P(18), P(22), P(24), etc. may be output.

InFIG.3, for the pulsed laser light25of 15 ns to 200 ns input to the laser amplifier30, a super-Gaussian-shaped laser pulse with the ratio τr<0.72 is illustrated. Such a super-Gaussian-shaped laser pulse can be provided by, for example, Q-switched and cavity-dumped oscillation described above. In Q-switched and cavity-dumped oscillation, the pulse width τpof an output laser pulse is approximately equal to 2L/c where L is the resonator length, and c is the speed of light. Thus, the resonator length L can be selected such that 2L/c is 15 ns to 200 ns.FIG.10is an explanatory diagram of super-Gaussian-shaped pulse generation by Q-switched and cavity-dumped oscillation. As illustrated in an upper row ofFIG.10, when the switching time of the electro-optic device22, that is, the time during which loss in the resonator is changed from a low-loss state to a high-loss state is sufficiently shorter than the pulse width τp, a laser pulse output by Q-switched and cavity-dumped oscillation steeply rises, and a super-Gaussian shape with Tr<0.72 is obtained as illustrated in a lower row ofFIG.10.

By contrast, as illustrated in an upper row ofFIG.11, when the switching time of the electro-optic device22is, for example, about half of the pulse width τp, the steep rise of a laser pulse is inhibited by the slow switching of the electro-optic device22. As a result, the laser pulse shape becomes close to a Gaussian shape as illustrated in a lower row ofFIG.11.

As described withFIGS.5and6, the laser pulse shortening effect in the laser amplifier30depends not only on the shape of the incoming laser pulse25but also on its pulse width τp. This is because, as illustrated inFIG.4, the normalized extracted energy ηextvaries depending on the pulse width τpof the incoming laser pulse25. When the extracted energy next is small, energy in the laser amplifier30cannot be sufficiently extracted, so that the sharpening of a front portion of a laser pulse is less likely to occur, and the shortening of the laser pulse is less likely to occur. Thus, as described above, inFIG.4, the order of decreasing likeliness of occurrence of laser pulse shortening is the order of the regions D, C, B, and A. The configuration according to the first embodiment can make the extracted energy ηexthigher than that of the typical amplification operation, and thus can increase the pulse shortening effect obtained.

FIGS.12and13illustrate examples of pulse shortening when laser pulses with different pulse widths enter the laser amplifier30.FIG.12illustrates a case where the pulse width of the incoming laser pulse25is longer than the pulse width of the outgoing laser pulse35. In this case, the outgoing laser pulse35has a shape that rises steeply and falls gently.FIG.13illustrates a case where the pulse width of the incoming laser pulse25is longer than that of the incoming laser pulse25illustrated inFIG.12. Although the outgoing laser pulse35illustrated inFIG.13has a pulse width equivalent to that of the outgoing laser pulse35illustrated inFIG.12, the outgoing laser pulse35illustrated inFIG.13has a longer fall time than the outgoing laser pulse35illustrated inFIG.12. Consequently, when the output of the outgoing laser pulse35illustrated inFIG.12is the same as the output of the outgoing laser pulse35illustrated inFIG.13, the outgoing laser pulse35illustrated inFIG.13has a smaller peak output. As described above, when the pulse width τpof the incoming laser pulse25is larger than 200 ns, the extracted energy next does not change significantly. Thus, even when the pulse width τpof the incoming laser pulse25is made longer than 200 ns, the amplified output does not change significantly. However, when the pulse width τpof the incoming laser pulse25is made longer, the fall time of the outgoing laser pulse35becomes longer. Consequently, when the pulse width τpof the incoming laser pulse25is made longer than 200 ns, the amplified output does not change significantly while the fall time becomes longer, so that the peak output decreases. Therefore, by setting the pulse width τpof the incoming laser pulse25to 200 ns or less, both high amplified output and a pulse with a high peak value can be achieved.

As described above, in the first embodiment, the laser oscillator20emits pulsed laser light with the full width at half maximum of between 15 ns and 200 ns for input to the laser amplifier30, and the laser amplifier30passes the pulsed laser light through the amplifier laser active medium to shorten the pulse width to pulsed laser light with the full width at half maximum of between 5 ns and 30 ns for output. Thus, the first embodiment can generate pulsed laser light that has the optimum full width at half maximum of between 5 ns and 30 ns for the generation of high-power extreme ultraviolet light, and has a high output of 20 kW or higher.

Furthermore, the first embodiment performs amplification operation such that the value τr(=τb/τp) obtained by dividing the full width at half maximum τpof the pulsed laser light25input to the laser amplifier30by the rise time τb, which is the time from when the intensity of the pulsed laser light25input to the laser amplifier30increases to 10% of the maximum intensity to when the intensity increases to 90% of the maximum intensity, becomes smaller than 0.72, and thus can provide a remarkable pulse shortening effect.

Second Embodiment

FIG.14is a diagram illustrating a configuration of an extreme ultraviolet light generation apparatus200including a laser amplification device110according to a second embodiment. InFIG.14, the laser amplification device110includes the laser oscillator20, a transmission optical system120, and the laser amplifier30. The transmission optical system120includes a polarization beam splitter70, an electro-optic device71, a polarization beam splitter72, a beam splitter73, a waveform measurement sensor74, and a mirror75. The electro-optic device71, the polarization beam splitter70, and the polarization beam splitter72function as both an optical isolator and a pulse shape shaper. The waveform measurement sensor74measures the waveform Iinof pulsed laser light before being input to the laser amplifier30.

The operation of the electro-optic device71, the polarization beam splitter70, and the polarization beam splitter72as the optical isolator will be described with reference toFIGS.15,16, and17.FIG.15illustrates going light traveling from the laser oscillator20to the laser amplifier30.FIG.16illustrates return light traveling from the laser amplifier30to the laser oscillator20.FIG.17is a diagram illustrating a time chart of an applied voltage for rotating polarized light 90 degrees in the electro-optic device71and a time chart of a laser pulse output from the laser oscillator20. A region provided with a shade H inFIG.17corresponds to a period during which the applied voltage inFIG.17is on.

The applied voltage for rotating polarized light 90 degrees is applied to the electro-optic device71in synchronization with the repetition frequency of laser pulses output from the laser oscillator20. Then, as illustrated inFIG.15, the going light that has passed through the electro-optic device71rotates 90 degrees from vertical polarized light77to horizontal polarized light78only during the ON period of the applied voltage illustrated in an upper row ofFIG.17. As a result, as illustrated in a lower row ofFIG.17, only a pulse provided with the shade H is cut out and transmitted through the polarization beam splitter72, and the rest is reflected by the polarization beam splitter72.

The return light from the laser amplifier30side is, for example, self-oscillating light of the laser amplifier30etc. The return light from the laser amplifier30side is out of synchronization with the voltage applied to the electro-optic device71illustrated in the upper row ofFIG.17. Thus, even when the return light passes through the electro-optic device71, polarized light does not rotate, so that the horizontal polarized light78is maintained. As a result, as illustrated inFIG.16, the return light from the laser amplifier30side does not pass through the polarization beam splitter70and does not return to the laser oscillator20. Consequently, the laser oscillator20can stably oscillate without being affected by the return light. Using the same principle, the electro-optic device71and the polarization beam splitters70and72can prevent parasitic oscillations that occur between the laser oscillator20and the laser amplifier30. AlthoughFIG.14illustrates the polarization beam splitters70and72, absorbing thin-film reflectors (ATFRs), which are optical elements having the same function, or the like may be used.

FIG.18illustrates the operation of the electro-optic device71, the polarization beam splitter70, and the polarization beam splitter72as the pulse shaper. A region provided with a shade H inFIG.18corresponds to a period during which the applied voltage for rotating polarized light 90 degrees in the electro-optic device71is on, as illustrated inFIG.17. The laser pulse with a pulse width of 15 ns to 200 ns illustrated inFIG.3is illustrated in the super-Gaussian shape with τr<0.72. However, in this super-Gaussian-shaped laser pulse, as illustrated on the left side inFIG.18, a pedestal15may appear before a main pulse14, and a post pulse16or the like may appear after the main pulse14. When the pedestal15or the post pulse16enters the laser amplifier30, consuming energy in the laser amplifier30, amplification of the main pulse14is hindered. Thus, as indicated by the shade H inFIG.18, polarized light of only the main pulse14is rotated 90 degrees and cut out by the electro-optic device71, so that a pulse measured by the waveform measurement sensor74includes only the main pulse14with the pedestal15and the post pulse16removed as illustrated on the right side inFIG.18. In the pulse illustrated on the right side inFIG.18, since the pedestal15and the post pulse16are removed, the pedestal15and the post pulse16do not consume energy in the laser amplifier30, and the amplified output of the main pulse14is increased.

FIG.19illustrates a second example of pulse shaping. A region provided with a shade H inFIG.19corresponds to a period during which the applied voltage for rotating polarized light 90 degrees in the electro-optic device71is on, as illustrated inFIG.17. InFIG.19, by controlling the pulse width of the applied voltage for rotating polarized light 90 degrees in the electro-optic device71, a laser pulse19with a pulse width of 15 ns to 200 ns is cut out from a laser pulse18output from the electro-optic device71. As a result, as described in the first embodiment, the pulse width of a laser pulse output from the laser amplifier30can be set to 5 ns to 30 ns, and EUV light output by the EUV light generator40can be increased. When the cut-out laser pulse has a rectangular shape, the laser amplifier30provides a more remarkable effect of shortening the pulse.

FIG.20illustrates a third example of pulse shaping. A region provided with a shade H inFIG.20corresponds to a period during which the applied voltage for rotating polarized light 90 degrees in the electro-optic device71is on, as illustrated inFIG.17. When the laser oscillator20is made to oscillate by Q-switched oscillation or the like, a Gaussian shape like a laser pulse12illustrated on the left side inFIG.20may be formed. Then, by cutting out only the region provided with the shade H, that is, a rear portion of the laser pulse12by the electro-optic device71, the waveform measurement sensor74measures a laser pulse13as illustrated on the right side inFIG.20. In the laser pulse13, the intensity of a front portion of the pulse is larger than the intensity of a rear portion, and the front portion has a sharpened shape. When the laser pulse13of this shape is passed through the laser amplifier30, the sharpening of the pulse front portion remarkably appears due to amplification, and a more remarkable pulse shortening effect can be obtained.

FIG.21is a diagram illustrating a modification of the extreme ultraviolet light generation apparatus200including the laser amplification device110illustrated inFIG.14. InFIG.21, a beam splitter81, a waveform measurement sensor82, and a mirror99are added to the extreme ultraviolet light generation apparatus200illustrated inFIG.14. Part of a laser pulse output from the laser amplifier30is extracted by the beam splitter81, and its shape is measured by the waveform measurement sensor82. The waveform measurement sensor82measures the waveform Ioutof pulsed laser light output from the laser amplifier30. By monitoring the waveform Iout, it can be determined whether the waveform Ioutis an optimum laser pulse for conversion into EUV light.

A laser pulse illustrated on the left side inFIG.22indicates the shape of a laser pulse input to the laser amplifier30, which is measured by the waveform measurement sensor74. A laser pulse illustrated on the right side inFIG.22indicates an example of the shape of a laser pulse output from the laser amplifier30, which is measured by the waveform measurement sensor82. As illustrated inFIG.22, the laser pulse before being input to the laser amplifier30may have a shape including a post pedestal83in a rear portion of a main pulse after being amplified by the laser amplifier30.

Then, the pulse shape measured by the waveform measurement sensor82is monitored, and the electro-optic device71is adjusted to cut out a laser pulse84as illustrated on the left side inFIG.23so that there is no post pedestal in a post-amplification laser pulse shape as in a laser pulse85illustrated on the right side inFIG.23. By using the waveform measurement sensor82, optimum cutting out of a laser pulse shape as illustrated on the left side inFIG.23can be achieved, and an optimum laser pulse can be input to the EUV light generator40to increase EUV light output generated. Here, adjustment is made so that no post pedestal appears, but the electro-optic device71may be adjusted to cut out a laser pulse such that a laser pulse shape to increase EUV light output is formed.

As described above, the second embodiment controls the period during which the polarization rotation function of the electro-optic device71is on to make the electro-optic device71, the polarization beam splitter70, and the polarization beam splitter72function as the optical isolator, whereby the laser oscillator20can stably oscillate without being affected by return light. Furthermore, by controlling the period during which the polarization rotation function of the electro-optic device71is on, the electro-optic device71, the polarization beam splitter70, and the polarization beam splitter72are made to function as the pulse shaper, whereby the pedestal15or the post pulse16appearing before and after the main pulse14can be removed, and the pulse width can be shortened and shaped, so that the amplification performance in the laser amplifier30can be improved. Moreover, laser pulses input to and output from the laser amplifier30are monitored, and the electro-optic device71is controlled using the monitoring results, so that an optimum laser pulse can be input to the EUV light generator40.

Third Embodiment

FIG.24is a diagram illustrating a configuration of an extreme ultraviolet light generation apparatus300including a laser amplification device130according to a third embodiment. InFIG.24, the laser amplification device130includes the laser oscillator20, the mirror27, and a multipath laser amplifier31. InFIG.24, the laser amplifier30in the extreme ultraviolet light generation apparatus100illustrated inFIG.3is replaced with the multipath laser amplifier31. InFIG.3, when the fluence Einof the incoming laser pulse25is sufficiently larger than the saturation fluence Esof the laser amplifier30, energy stored in the laser amplifier30can be sufficiently extracted, and high amplified output can be obtained. However, when the fluence Einof the incoming laser pulse25is smaller than the saturation fluence Esof the laser amplifier30, energy stored in the laser amplifier30cannot be sufficiently extracted. As a result, the pulse shortening effect cannot be sufficiently obtained. Thus, when the fluence Einof the incoming laser pulse25is smaller than the saturation fluence Esof the laser amplifier30, by using the multipath laser amplifier31as illustrated inFIG.24, energy extraction efficiency can be further increased. As a result, high amplified output can be obtained, and as a result, high EUV light output can be obtained. AlthoughFIG.24illustrates a five-path amplifier in which the number of paths is five, the number of paths may be two or more, and the number of paths is not limited to five.

When the fluence Einof the incoming laser pulse25is smaller than the saturation fluence Esof the laser amplifier30, for example, by optimizing the gas pressure in the laser amplifier30, an optimum pulse shape for higher amplified output and high EUV light output can be obtained. In an extreme ultraviolet light generation apparatus400inFIG.25, a laser amplifier33includes a gas pressure adjustment mechanism32that adjusts the gas pressure of laser gas. The other components are the same as those of the extreme ultraviolet light generation apparatus100illustrated inFIG.3. In the CO2laser, the saturation fluence Esincreases in proportion to the gas pressure N. Thus, when the gas pressure N is lowered, the saturation fluence Esdecreases. Conversely, when the gas pressure N is raised, the saturation fluence Esincreases. When the fluence Einof the incoming pulse is the same, the saturation fluence Escan be adjusted by adjusting the gas pressure N, and energy that can be extracted from the laser amplifier33can be adjusted.

FIG.26is a graph illustrating an example of amplification characteristics when the gas pressure N of the laser amplifier33is changed. Esc) is the saturation fluence at an initial gas pressure, Est is the saturation fluence when the gas pressure is lowered from the initial gas pressure, and Est is the saturation fluence when the gas pressure is raised from the initial gas pressure. As illustrated inFIG.26, when the gas pressure is lowered from the initial gas pressure, Es1<Es0, so that a graph34of the amplification characteristics changes to a graph37. At this time, the saturation characteristics of an amplified output Eoutappear more remarkably relative to an increase in the fluence Einof the incoming pulse. On the other hand, when the gas pressure is raised from the initial gas pressure, Es0<Es2, so that the graph34of the amplification characteristics changes to a graph38. At this time, the saturation characteristics of the amplified output Eoutare obscure relative to an increase in the fluence Einof the incoming pulse and become close to linear characteristics. Thus, when the fluence Einof the incoming laser pulse25is smaller than the saturation fluence Esof the laser amplifier33, and energy stored in the laser amplifier33cannot be sufficiently extracted, for example, by lowering the gas pressure, the saturation fluence Escan be reduced, and the amplified output can be increased. Conversely, when it is desired to lower the amplified output, the gas pressure can be increased. When the gas pressure is changed, impedance may not be matched between a discharge circuit and an amplifier of the laser amplifier33. When there is an impedance mismatch, discharge power that is power used for discharge cannot be effectively used. Thus, when the impedance is matched, the discharge power can be effectively used, and the amplified output can be further improved.

Furthermore, when the gas pressure is decreased, in addition to an increase in the amplified output, the extracted energy from the amplifier is increased, so that the effect that the pulse shortening becomes remarkable is obtained. Moreover, the use of the saturation amplification characteristics as in the graph37allows an improvement in the stability of repetitive pulses output from the laser oscillator20. The following explains that.FIG.27illustrates a case where there are intense fluctuations in repetitive pulses output from the laser oscillator20. When there are such fluctuations, amplification by the laser amplifier whose amplified output is saturated as in the graph37can reduce fluctuations in post-amplification repetitive pulses as illustrated inFIG.28. As a result, pulse stability can be improved. As described above, when the gas pressure is decreased by the gas pressure adjustment mechanism32, three effects of an increase in amplified output, remarkable pulse shortening, and an improvement in pulse stability can be simultaneously obtained. In the present embodiment, when the gas pressure is adjusted in the range of gas pressures at which the rotational relaxation time is some nanoseconds, the above three effects can be expected. The rotational relaxation time is some nanoseconds typically at about 20 Torr to 100 Torr. Thus, the gas pressure can be adjusted in the range of 20 Torr to 100 Torr.

The improvement of the pulse stability of incoming pulses is obtained by the amplification characteristics with a tendency toward saturation. Thus, for example, when the fluence Einof the incoming laser pulse25is sufficiently larger than the saturation fluence Esof the laser amplifier30, the multipath laser amplifier31can also provide the same effect of improving the pulse stability.

As described above, the third embodiment uses the multipath laser amplifier31, and thus can further increase the energy extraction efficiency and can provide high amplified output. In addition, the gas pressure of the laser gas in the laser amplifier33is adjusted to be lowered, so that the saturation fluence Esof the laser amplifier can be reduced, and the amplified output of the laser amplifier can be increased.

Fourth Embodiment

FIG.29is a diagram illustrating an inference apparatus90that infers the pulse shape of the pulsed laser light35output from the laser amplifier30of the laser amplification device110illustrated inFIG.14or21. The inference apparatus90includes a data acquisition unit91and an inference unit92. The data acquisition unit91acquires the pre-amplification laser pulse shape Iinmeasured by the waveform measurement sensor74illustrated inFIG.14or21. The inference unit92infers a laser pulse shape after being amplified by the laser amplifier30, using a learned model storage unit93. That is, by inputting the laser pulse shape Iinacquired by the data acquisition unit91to a learned model stored in the learned model storage unit93, a post-amplification pulse shape Ioifinferred by the inference unit92can be output from the inference unit92. Using the inference apparatus90, the pre-amplification pulse shape may be optimized by the pulse shaper consisting of the electro-optic device71, the polarization beam splitter70, and the polarization beam splitter72illustrated inFIG.14so that the inferred post-amplification pulse shape Ioifbecomes an optimum pulse of 5 ns to 30 ns for EUV light generation. By this optimization, high-power EUV light can be generated.

FIG.30is a flowchart illustrating a processing procedure of the inference apparatus90. In step S1, the data acquisition unit91acquires the pre-amplification pulse shape Iinmeasured by the waveform measurement sensor74. In step S2, the inference unit92inputs the pulse shape Iinto the learned model stored in the learned model storage unit93to obtain the inferred post-amplification pulse shape Ioif. In step S3, the inference unit92outputs the pulse shape Ioifobtained by the learned model to the extreme ultraviolet light generation apparatus200. In step S4, the extreme ultraviolet light generation apparatus200optimizes the post-amplification pulse to laser pulse light having a pulse width of 5 ns to 30 ns, using the inferred pulse shape Ioif. As a result, high-power EUV light can be generated.

FIG.31illustrates a learning apparatus94that generates the learned model used by the inference apparatus90illustrated inFIG.29. The learning apparatus94includes a data acquisition unit95, a model generation unit96, and the learned model storage unit93. In the present embodiment, a case where a learning algorithm used by the model generation unit96is supervised learning is described, but a known algorithm such as unsupervised learning or reinforcement learning may be used. The data acquisition unit95acquires the pre-amplification laser pulse shape Iinmeasured by the waveform measurement sensor74and the post-amplification laser pulse shape Ioutmeasured by the waveform measurement sensor82illustrated inFIG.21.

The model generation unit96learns the output Ioifthat is the post-amplification pulse shape, based on learning data generated based on the laser pulse shape Iinand the laser pulse shape Ioutoutput from the data acquisition unit95. That is, the learned model that estimates the output Ioiffrom the inputs Iinand Ioutis generated. Here, the learning data is data in which the inputs Iinand Ioutare associated with each other.

FIG.32is a flowchart illustrating a processing procedure of the learning apparatus94. In step S10, the data acquisition unit95acquires the inputs Iinand Iout. It is not necessary to acquire the inputs Iinand Ioutat the same time. It is only necessary to acquire the inputs Iinand Ioutin association with each other. Each of them may be acquired at a different timing. In step S11, the model generation unit96learns the output Ioifby so-called supervised learning, according to the learning data created based on the combination of the inputs Iinand Ioutacquired by the data acquisition unit95, to generate the learned model. In step S12, the learned model storage unit93stores the learned model generated by the model generation unit96.

The model generation unit96may learn the output Ioifaccording to learning data created for two or more EUV light generation apparatuses. For example, learning data may be acquired from two or more EUV light generation apparatuses used in the same area, or learning data may be acquired from two or more EUV light generation apparatuses that independently operate in different areas. Further, an EUV light generation apparatus from which to collect learning data may be added to or excluded from those in the middle. Furthermore, a learning apparatus that has learned the output Ioiffor a certain EUV light generation apparatus may be applied to a different EUV light generation apparatus, and the output Ioifmay be relearned and updated for the different EUV light generation apparatus.

Moreover, as the learning algorithm used by the model generation unit96, deep learning to learn extraction of features themselves may be used, and machine learning may be executed according to another known method such as genetic programming, functional logic programming, or a support vector machine.

InFIG.33, the learning apparatus97learns the output Ioifusing an input set98including a plurality of different parameter values and the input Ioutas learning data. The input set98includes the pre-amplification pulse shape Iin, the gas pressure in the laser amplifier30, and the number of paths in the laser amplifier30as parameters. Factors that can change the post-amplification pulse shape described in the first to third elements may be input to the input set98. Factors such as the discharge power of the laser amplifier30and the output of the laser oscillator20may be added to the input set98.

The model generation unit96learns the output Ioif, which is the post-amplification pulse shape, by so-called supervised learning, according to learning data created based on the input set98and the input Ioutacquired by the data acquisition unit95, to generate the learned model. The learned model storage unit93stores the learned model generated by the model generation unit96.

When the learned model learned by the learning apparatus97is applied to the inference apparatus90, the data acquisition unit91illustrated inFIG.29acquires the input set98instead of the input Iin. The inference unit92inFIG.29uses the learned model stored in the learned model storage unit93to infer the pulse shape after being amplified by the laser amplifier30. That is, by inputting the input set98acquired by the data acquisition unit91to the learned model, the post-amplification pulse shape Ioifinferred by the inference unit92can be output from the inference unit92. Factors such as the pre-amplification pulse shape, the gas pressure in the laser amplifier30, and the number of paths in the laser amplifier30may be optimized so that the inferred post-amplification pulse shape Ioifbecomes an optimum pulse of 5 ns to 30 ns for EUV light generation. By this optimization, high-power EUV light can be generated. Furthermore, by using the input set98including a plurality of factors as the input Iin, the plurality of factors can be optimized, and higher-power EUV light can be generated.

The configurations described in the above embodiments show an example of the subject matter of the present disclosure, and can be combined with another known technique. The configurations can be partly omitted or changed without departing from the gist of the present disclosure.

REFERENCE SIGNS LIST

10,110,130laser amplification device;20laser oscillator;21laser active medium;22,71electro-optic device;23,70,72polarization beam splitter;24,26resonator mirror;25pulsed laser light (incoming laser pulse);30,33laser amplifier;31multipath laser amplifier;32gas pressure adjustment mechanism;35pulsed laser light (outgoing laser pulse);40EUV light generator;41droplet generator;42collector mirror;50EUV light;60intermediate focus point;70,72polarization beam splitter;74,82waveform measurement sensor;90inference apparatus;94,97learning apparatus;100,200,300,400extreme ultraviolet light generation apparatus (EUV light generation apparatus);120transmission optical system.