Seed laser system

Abstract

A seed laser system for an EUV radiation source, comprising a laser configured to emit a pulsed laser beam, at least one electro-optic modulator located downstream of the laser, an optical amplifier located downstream of the at least one electro-optic modulator, and further comprising an additional electro-optic modulator located downstream of the optical amplifier.

Description

Technical Field

[0001] Cross - reference to related applications

[0001] This application claims priority to European Application No. 23177360.7, filed on June 5, 2023, the entire content of which is incorporated herein by reference.

[0002]

[0002] The present invention relates to a seed laser system. The seed laser system can form part of a laser system, and thus can form part of a laser - produced plasma (LPP) source. The LPP source can generate extreme ultraviolet (EUV) radiation and can form part of a lithography system.

Background Art

[0003]

[0003] A lithography apparatus is a machine constructed to apply a desired pattern onto a substrate. The lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). The lithography apparatus can project the pattern of a patterning device (e.g., a mask) onto a layer of radiation - sensitive material (resist) provided on the substrate.

[0004]

[0004] To project a pattern onto a substrate, a lithography apparatus may use electromagnetic radiation. The minimum size of the features that can be formed on the substrate is determined by the wavelength of this radiation. A lithography apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 - 20 nm, for example a wavelength of 6.7 nm or 13.5 nm, can be used to form smaller features on the substrate than a lithography apparatus using radiation with a wavelength of 193 nm.

[0005]

[0005] EUV radiation for a lithography apparatus can be generated by a laser - produced plasma (LPP) source. Inside the LPP source, a plasma that emits EUV radiation can be generated by irradiating fuel droplets with a laser beam.

[0006]

[0006] It is desirable that the laser beam used to illuminate the fuel droplets have high power. A seed laser system can be used to generate a pulsed laser beam, which can then be amplified using an optical amplifier. The optical amplifier increases the power of the pulsed laser beam. The amplified pulsed laser beam is then incident on the fuel droplets, thereby generating EUV radiation.

[0007]

[0007] A potential problem is that a small portion of the amplified pulsed laser beam may be reflected from the fuel droplets. The reflected radiation can be amplified by the optical amplifier and may damage the seed laser.

[0008]

[0008] It may be desirable to provide a seed laser that overcomes this problem, or another problem related to the prior art, in a manner not disclosed or suggested in the prior art. [Overview of the project]

[0009]

[0009] According to a first aspect of the present invention, a seed laser system for an EUV radiation source is provided, comprising a laser configured to emit a pulsed laser beam, at least one electro-optic modulator located downstream of the laser, an optical amplifier located downstream of the at least one electro-optic modulator, and further comprising an additional electro-optic modulator located downstream of the optical amplifier.

[0010]

[0010] An electro-optic modulator placed downstream of the optical amplifier has the advantage of protecting the seed laser from radiation that may be back-reflected from droplets of the EUV radiation source. An electro-optic modulator has the advantage of being able to provide this protection with a faster switching time than an acousto-optic modulator (acousto-optic modulators have traditionally been used to provide protection from back-reflected radiation).

[0011]

[0011] The beam path of the seed laser system may include a first path of a pulsed laser beam passing through an optical amplifier and a second path of a pulsed laser beam passing through an optical amplifier.

[0012]

[0012] An additional electro-optic modulator may be positioned after the first pass of the pulsed laser beam through the optical amplifier and before the second pass of the pulsed laser beam through the optical amplifier.

[0013]

[0013] An additional electro-optic modulator is configured to rotate the polarization of the pulsed laser beam before the second pass of the pulsed laser beam through the optical amplifier, so that the polarization of the pulsed laser beam during the second pass through the optical amplifier is approximately orthogonal to the polarization of the pulsed laser beam during the first pass through the optical amplifier.

[0014]

[0014] The polarizing beam splitter may be configured to guide the pulsed laser beam to an additional electro-optic modulator after a first pass of the pulsed laser beam through the optical amplifier, and to guide the pulsed laser beam to a second pass through the optical amplifier after the pulsed laser beam has passed through the additional electro-optic modulator.

[0015]

[0015] An additional electro-optic modulator may be positioned after the second pass of the pulsed laser beam through the optical amplifier.

[0016]

[0016] The acousto-optic modulator and the polarizing rotor may be positioned in the beam path downstream of the first pass of the pulsed laser beam through the optical amplifier and upstream of the second pass of the pulsed laser beam through the optical amplifier.

[0017]

[0017] The polarizing beam splitter may be configured to guide the pulsed laser beam to an acousto-optic modulator after a first pass of the pulsed laser beam through the optical amplifier, and to guide the pulsed laser beam to a second pass through the optical amplifier after the pulsed laser beam has passed through the acousto-optic modulator and the polarizing rotor.

[0018]

[0018] At least one electro-optic modulator may be located downstream of the laser and upstream of the optical amplifier, and may be a pair of electro-optic modulators.

[0019]

[0019] According to a second aspect of the present invention, a laser system is provided which comprises a seed laser system of the first aspect, further comprising a laser beam amplification system comprising a series of optical amplifiers located downstream of the seed laser system.

[0020]

[0020] The laser beam amplification system may include four optical amplifiers.

[0021]

[0021] According to a third aspect of the present invention, a laser-generated plasma radiation source is provided, comprising a fuel emitter operable to provide a fuel target to a plasma-forming region, and a laser system according to a second aspect of the present invention.

[0022]

[0022] According to a fourth aspect of the present invention, a lithography system is provided comprising a laser-generated plasma radiation source according to the third aspect and a lithography apparatus.

[0023]

[0023] A fifth aspect of the present invention provides a method for providing a pulsed laser beam for an EUV radiation source, comprising: emitting a pulsed laser beam from a laser; passing the pulsed laser beam through at least one electro-optic modulator located downstream of the laser; passing the pulsed laser beam through an optical amplifier; and passing the pulsed laser beam through an additional electro-optic modulator located downstream of the optical amplifier.

[0024]

[0024] An electro-optical modulator disposed downstream of the optical amplifier has the advantage of protecting the seed laser from radiation that can be retroreflected from the droplets of the EUV radiation source. The electro-optical modulator has the advantage that it can provide this protection with a switching time faster than that of an acousto-optic modulator (acousto-optic modulators have conventionally been used to provide protection from retroreflected radiation).

[0025]

[0025] The pulsed laser beam can then pass through the optical amplifier a second time.

[0026]

[0026] This method may further include passing the pulsed laser beam through a laser beam amplification system comprising a series of optical amplifiers.

[0027]

[0027] According to a sixth aspect of the invention, there is provided a method of generating EUV radiation, comprising receiving a pulsed laser beam output from a series of optical amplifiers and directing the pulsed laser beam at a fuel target in a plasma formation region.

[0028]

[0028] The features of separate aspects of the invention can be combined.

Brief Description of the Drawings

[0029]

[0029] Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings.

[0030] [Figure 1] A lithography system comprising a lithography apparatus and a radiation source according to an embodiment of the invention is schematically shown. [Figure 2] A laser system according to an embodiment of the invention is schematically shown. [Figure 3] A seed laser system according to an embodiment of the invention is schematically shown. [Figure 4] A seed laser system according to another embodiment of the invention is schematically shown.

Mode for Carrying Out the Invention

[0031]

[0030] Figure 1 shows a lithography system according to one embodiment of the present disclosure. The lithography system comprises a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0032]

[0031] The illumination system IL is configured to adjust the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. Together, the faceted field mirror device 10 and the faceted pupil mirror device 11 provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to the faceted field mirror device 10 and the faceted pupil mirror device 11, or instead of these, the illumination system IL may include other mirrors or devices.

[0033]

[0032] After being adjusted in this manner, the EUV radiation beam B interacts with the patterning device MA. This interaction results in the generation of a patterned EUV radiation beam B'. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image with features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. In Figure 1, the projection system PS is shown to have only two mirrors 13, 14, but the projection system PS may include a different number of mirrors (e.g., 6 or 8 mirrors).

[0034]

[0033] The substrate W may contain a previously formed pattern. In this case, the lithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with respect to the previously formed pattern on the substrate W.

[0035]

[0034] A small amount of gas (e.g., hydrogen) at a relative vacuum, i.e., a pressure considerably lower than atmospheric pressure, may be supplied to the radiation source SO, the illumination system IL, and / or the projection system PS.

[0036]

[0035] The radiation source SO shown in Figure 1 is of a type that may also be called, for example, a laser-generated plasma (LPP) source. The laser system 1 may include, for example, a CO2 laser, and the laser system 1 is configured to store energy via a pulsed laser beam 2 in a fuel, such as tin (Sn), provided by, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel may be used. The fuel may be, for example, a liquid, or it may be, for example, a metal or an alloy. The fuel emitter 3 may include a nozzle configured to guide, for example, a droplet of tin along a trajectory toward the plasma-forming region 4. The laser beam 2 is incident on the tin in the plasma-forming region 4. By storing laser energy in the tin, a tin plasma 7 is created in the plasma-forming region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during the de-excitation and recombination of ions and electrons in this plasma.

[0037]

[0036] The pulsed laser beam 2 incident on the tin in the plasma formation region 4 is sometimes called the main laser beam or main pulsed laser beam. Each individual pulse of this pulsed laser beam 2 is sometimes called a main pulse.

[0038]

[0037] Before the main laser beam 2 is incident on the tin in the plasma-forming region 4, another pre-pulsed laser beam may be incident on the tin. The pre-pulsed laser beam may act to change the shape of the tin in order to increase the conversion efficiency when the main pulse is incident on the tin (then). One or more additional pulses may be used. The pre-pulsed laser beam and one or more additional pulses may be provided, for example, by separate lasers. The pre-pulsed laser beam and one or more additional pulses may travel along one or more beam paths different from the beam path of the main laser beam.

[0039]

[0038] EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 comprises, for example, a per-normal incident radiation collector 5 (sometimes more commonly called a normal incident radiation collector). The collector 5 may have a multilayer mirror structure positioned to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration having two foci. As described below, the first of the foci may be in the plasma-forming region 4, and the second of the foci may be in an intermediate focus 6.

[0040]

[0039] The laser system 1 may be spatially separated from the radiation source SO. In this case, the main laser beam 2 may be delivered from the laser system 1 to the radiation source SO using, for example, a beam delivery system (not shown) including a suitable guide mirror and / or beam expander, and / or other optical systems. The laser system 1, the radiation source SO, and the beam delivery system may be considered together as a radiation system.

[0041]

[0040] The radiation reflected by the collector 5 forms an EUV radiation beam B. The EUV radiation beam B is focused at an intermediate focus 6 to form an image of the plasma present in the plasma formation region 4 at the intermediate focus 6. The image at the intermediate focus 6 functions as a virtual radiation source for the illumination system IL. The radiation source SO is positioned such that the intermediate focus 6 is located at or near the opening 8 of the enclosure structure 9 of the radiation source SO.

[0042]

[0041] A laser system 1 according to one embodiment of the present disclosure comprises a seed laser system 20 and a laser beam amplification system 21, as schematically shown in Figure 2. A beam steering system 26 capable of transporting the resulting main laser beam 2 to a plasma formation region 4 (see Figure 1) is also shown.

[0043]

[0042] The laser beam amplification system 21 comprises four optical amplifiers 22-25. The optical amplifiers 22-25 are sometimes called laser beam amplifiers or optical amplifiers. The optical amplifiers 22-25 are arranged in series so that the first optical amplifier 22 amplifies the laser beam output by the seed laser system 20, the second optical amplifier 23 further amplifies the laser beam output from the first optical amplifier 22, the third optical amplifier 24 further amplifies the amplified laser beam output from the second optical amplifier 23, and the fourth optical amplifier 25 further amplifies the laser beam output from the third optical amplifier 24. Thus, the four optical amplifiers 22-25 apply four amplification stages to the laser beam, and these amplification stages are arranged in series, each increasing the power of the laser beam. The laser beam output from the last, fourth optical amplifier 25 proceeds to the beam steering system 26. The beam steering system 26 directs the amplified laser beam towards the plasma formation region 4 (see Figure 1), where the amplified laser beam is incident on fuel droplets, thereby generating EUV radiation. The amplified laser beam is a pulsed laser beam, sometimes called the main pulsed laser beam.

[0044]

[0043] Figure 2 shows four optical amplifiers 22-25, but a different number of laser amplifiers may be provided. Typically, a series of optical amplifiers may be used.

[0045]

[0044] A seed laser system 20 according to one embodiment of the present disclosure is schematically shown in Figure 3. The seed laser system 20 comprises a laser 30 configured to emit a pulsed laser beam 32. The seed laser system further comprises first and second electro-optic modulators 34, 35 (sometimes referred to as EOMs 34, 35). Pairs of cross-polarization selective filters 34a, b and 35a, b are provided on both sides of each EOM 34, 35. The EOM and associated polarization selective filters together are sometimes referred to as an EOM module. The EOM module transmits radiation only when the EOM is energized (when the EOM is not energized, the pair of cross-polarization selective filters blocks the radiation). The first and second EOM modules 37, 38 are sometimes collectively referred to as an electro-optic modulator pair 36 (or EOM module pair 36).

[0046]

[0045] The pulsed laser beam 32 supplied by the seed laser 30 is linearly polarized. This is schematically shown by the bidirectional arrow 43 as polarization in the plane of Figure 3 (however, in other embodiments, the linear polarization may have a different orientation). The pulsed laser beam 32 has the same polarization after leaving the EOM module pair 36, as indicated by another bidirectional arrow (unlabeled).

[0047]

[0046] The pulsed laser beam 32 is incident on the first polarizing beam splitter 50. The first polarizing beam splitter 50 transmits the polarization of the pulsed laser beam 32.

[0048]

[0047] The optical amplifier 39 is located downstream of the EOM module pair 36 and the first polarized beam splitter 50. The optical amplifier 39 amplifies the pulsed laser beam 32.

[0049]

[0048] A second polarizing beam splitter 52 is positioned downstream of the optical amplifier 39. The second polarizing beam splitter 52 transmits the polarization of the pulsed laser beam 32.

[0050]

[0049] An additional electro-optic modulator 40 (EOM40) is located downstream of the optical amplifier 39 (and downstream of the second polarizing beam splitter 52). A pair of cross-polarization selective filters 40a,b are provided on either side of the additional EOM40. The additional EOM40 and the associated polarization selective filters 40a,b are sometimes referred to as an additional EOM module 41. The additional EOM module 41 rotates the polarization of the pulsed laser beam 32 so that, in this embodiment, it is linearly polarized perpendicular to the plane of Figure 3. This polarization is schematically shown by white and black circles 45.

[0051]

[0050] The pulsed laser beam 32 returns to the second polarizing beam splitter 52. Here, the second polarizing beam splitter 52 reflects the pulsed laser beam 32 (because the polarization of the beam has already rotated). The pulsed laser beam 32 is reflected by the polarizing beam splitter 52 and returns through the optical amplifier 39. Thus, the pulsed laser beam 32 goes through two passes through the optical amplifier 39. The amplified pulsed laser beam is output from the optical amplifier 39.

[0052]

[0051] The amplified pulsed laser beam 32 passes through a second path through the optical amplifier 39 and is then reflected toward the acousto-optic modulator 42 by the first polarizing beam splitter 50.

[0053]

[0052] The pulsed laser beam 32 passes through an acousto-optic modulator (AOM) 42, a polarization-selective filter 44, and a reflection phase delay 46. The reflection phase delay changes the polarization of the pulsed laser beam 32 from linear polarization to circular polarization. In other embodiments, a quarter-wave plate may be used to change the polarization. Typically, a polarization converter that converts linear polarization to circular polarization may be used. The resulting circularly polarized pulsed laser beam 48 proceeds to the laser beam amplification system 21 (see Figure 2).

[0054]

[0053] In the illustrated embodiment, each polarizing beam splitter 50, 52 is positioned such that it transmits the pulsed laser beam 32 when the pulsed laser beam 32 is first incident on the polarizing beam splitter, and reflects the pulsed laser beam 32 when the pulsed laser beam 32 is subsequently incident on the polarizing beam splitter (the polarization of the pulsed laser beam has already rotated). In other embodiments, either or both of the polarizing beam splitters 50, 52 may reflect the pulsed laser beam 32 when the pulsed laser beam 32 is first incident on the polarizing beam splitter. This is applicable to polarizing beam splitters at any position in any embodiment of the present invention.

[0055]

[0054] In conventional seed-pulse laser systems, a seed laser is used to supply a main pulse for EUV emission generation and a pedestal that precedes the main pulse. This pedestal is the low-power portion of the pulse and may, for example, be an extension of the main pulse and precede the main pulse by about 250 ns. The pedestal modifies the fuel droplets so that the main pulse is better absorbed by the fuel droplets. However, in the development of laser system 1, the pedestal is replaced by a separately generated pulse. This separately generated pulse is sometimes called a fuel modification pulse. The fuel modification pulse may have a different wavelength from the main pulse. The fuel modification pulse may have a wavelength of, for example, about 1 μm.

[0056]

[0055] The EOM module pair 36 is configured to shorten the pulses of the pulsed laser beam 32 emitted by the seed laser 30. The seed laser 30 may emit pulses having a duration of, for example, about 300 ns. It may be desirable to supply a main pulse of about 50 ns. The EOM module pair 36 may be configured to reduce the pulse duration to about 50 ns. Each EOM 34, 35 may be energized (switched on) for a period of 50 ns (in this example) while the pulse of radiation passes through the EOM. Each EOM module 37, 38 cuts off radiation outside of the 50 ns period, thereby shortening the pulse. The duration of other pulses may be obtained using the EOM pair 36.

[0057]

[0056] Pedestals supplied by conventional seed pulse laser systems arise because the EOM module pair 36 of the conventional system is not 100% effective. Specifically, each EOM 34, 35 and associated cross-polarization selective filters 34a, b, 35a, b do not have the effect of blocking 100% of radiation when the EOM is not energized. The first EOM module 37 can suppress, for example, about 99% of radiation outside the switch-on period. Therefore, the radiation pulse after the first EOM module 37 may consist of a pulse of about 50 ns and an associated 300 ns low-power pedestal that precedes or is on either side of this pulse. The second EOM module 38 and associated polarizer can suppress this pedestal by a further 99%, for example. Therefore, the EOM pair suppresses 99.99% of the pedestals. However, some influential pedestals may remain. In conventional seed laser systems, pedestals are desirable because they allow for the efficient modification of the fuel. However, as mentioned above, in the development of laser systems, fuel changes may be brought about by a different pulse. As a result, a pedestal for the main pulse is undesirable. Instead, a shortened main pulse without a pedestal is preferable.

[0058]

[0057] Since the EOM module pair 36 does not completely suppress the pedestal, a simple solution is to place an additional EOM module immediately after the EOM module pair. However, the inventors have devised another advantageous method for suppressing the pedestal.

[0059]

[0058] The additional EOM 40 is located downstream of the first pass of the optical amplifier 39, rather than immediately after the EOM pair 36. The three EOMs 34, 35, 40 (and associated cross polarizers) connected in series suppress the pedestal of the pulsed laser beam 32 so that the pedestal has negligibly small power after the additional EOM 40. Since the pedestal has negligibly small power, the pedestal can also have negligibly small power after passing through the laser beam amplification system 21 (see Figure 1). Furthermore, the additional EOM 40 rotates the polarization of the pulsed laser beam 32. This is advantageous because, as described above, polarizing beam splitters 50, 52 are used to provide a second pass through the optical amplifier 39, and then guide the pulsed laser beam to the output of the seed laser system 20. Since the additional EOM 40 provides the necessary polarization rotation, this means that a separate polarization rotation device is not required. This simplifies the configuration of the seed laser system 20.

[0060]

[0059] As described above, the pulsed laser beam 32 supplied by the seed laser 30 is linearly polarized. This is schematically shown by the bidirectional arrow 43 as polarization in the plane of Figure 3. (However, in other embodiments, linear polarization may have a different orientation.) When the first EOM 34 is energized, the first EOM 34 rotates the polarization of the pulsed laser beam 32 by 90°. Thus, after the first EOM 34, the polarization becomes perpendicular to the plane of Figure 3. This is schematically shown by the white and black circles. When the second EOM 35 is energized, the second EOM 35 rotates the polarization of the pulsed laser beam 32 by another 90°. Alternatively, the polarization rotation imparted by the second EOM 35 may reverse the polarization rotation imparted by the first EOM 34. Regardless of the direction of rotation imparted by the second EOM 35, the pulsed laser beam 32 has the same linear polarization as when it entered the EOM module pair 36 when it leaves the EOM module pair 36 (indicated by the bidirectional arrows). When EOMs 34 and 35 are not energized, the cross-polarization filters 34a, b and 35a, b block the pulsed laser beam 32. The blocked radiation is induced into a beam dump (not shown).

[0061]

[0060] The pulsed laser beam 32 has polarization in the plane of Figure 3 as it passes through the first pass through the optical amplifier 39. The pulsed laser beam 32 has polarization in the plane of Figure 3 as it enters the additional EOM module 41. When the additional EOM 40 of the additional EOM module 41 is energized, the additional EOM 40 rotates the polarization of the pulsed laser beam 32 by 90°. The rotated polarization is represented by the white and black circles. If the additional EOM 40 is not energized, the cross-polarization filters 40a,b block the pulsed laser beam 32. The blocked radiation is induced into a beam dump (not shown).

[0062]

[0061] Since the pulsed laser beam 32 now has polarization perpendicular to the polarization during the first pass through the optical amplifier 39, the second polarizing beam splitter 52 can be used to guide the pulsed laser beam into the second pass through the optical amplifier. The first polarizing beam splitter 50 can then guide the pulsed laser beam 32 into the acousto-optic modulator (AOM) 42.

[0063]

[0062] The pulsed laser beam 32 passes through the acousto-optic modulator (AOM) 42. When energized, the AOM 42 guides the pulsed laser beam 32 toward the polarization selective filter 44, and when not energized, it guides the pulsed laser beam toward the beam dump (not shown). The polarization selective filter 44 transmits radiation polarized perpendicular to the plane of Figure 3. Since the pulsed laser beam 32 is polarized perpendicular to the plane of Figure 3, the polarization selective filter 44 transmits the pulsed laser beam 32. The quarter-wave plate 46 imparts circular polarization to the pulsed laser beam 32. Next, the resulting circularly polarized amplified pulsed laser beam 48 proceeds to the laser amplification system 21 (see Figure 2).

[0064]

[0063] As described above, the additional EOM 40 favorably contributes to removing the pedestal from the pulses of the pulsed laser beam 32 and further provides polarization rotation to facilitate the second pass of the pulsed laser beam through the optical amplifier 39. The second pass of the pulsed laser beam 32 through the optical amplifier 39 is desirable because it amplifies the pulsed laser beam more effectively than in the case of a single pass. In other words, (compared to a single pass) it is achieved that the gain is extracted more efficiently from the gain medium. The gain medium may be, for example, CO2 gas.

[0065]

[0064] As described above, the polarization rotation provided by the additional EOM 40 has the advantage of avoiding the need for an additional polarization rotation device (e.g., a half-wave plate). The advantage of using the EOM 40 instead of the AOM is that the pulsed laser beam does not need to be focused onto the EOM. In some cases, it may be necessary to focus the pulsed laser beam onto the AOM in order to achieve the desired switching speed. Such focusing may result in undesirable thermal lensing effects or other undesirable effects. These advantages are obtained because the additional EOM 40 is located downstream of the first pass of the optical amplifier 39 and would not be realized if the additional EOM were located immediately after the EOM pair 36.

[0066]

[0065] The additional EOM 40 is located downstream of the first pass of the optical amplifier 39. This contradicts the understanding of those skilled in the art that an EOM should not be placed downstream of the optical amplifier of the pulsed laser beam 32, because the power of the pulsed laser beam would damage the EOM. The power level of the pulsed laser beam 32 as it passes through the additional EOM 40 may be, for example, about 50W. It contradicts the general understanding of those skilled in the art to place the additional EOM 40 in a location where it will receive a power level of about 50W and where it can reliably operate over a long period of time (e.g., several years). It is especially important that the lithography equipment can reliably operate over a long period of time because interruptions in the operation of the lithography equipment are very costly due to the loss of production when it stops working. The additional EOM 40 can provide reliable operation over a long period of time, contrary to the general understanding of those skilled in the art. The general understanding of those skilled in the art is that an EOM receiving a power level of about 50W is at risk of being damaged. Therefore, the general understanding of those skilled in the art is that an acousto-optic modulator should be used when the modulator needs to handle about 50W of power (acousto-optic modulators are understood to be less likely to be damaged by higher optical power). Contrary to the general understanding of those skilled in the art, the inventors have found that an additional EOM40 can handle more than about 50W of power without being damaged.

[0067]

[0066] As described above, some radiation may be reflected back from the fuel droplets. This back-reflected radiation passes through the laser beam amplification system 21 and can therefore be amplified. The back-reflected laser beam may damage the seed laser 30. As described above, the amplified pulsed laser beam 48 is circularly polarized. The radiation reflected from the fuel droplets has the opposite circular polarization. This means that after passing through the quarter-wave plate 46, the reflected radiation has the opposite linear polarization to the pulsed laser beam 32 (i.e., in the plane of Figure 3). The polarization selective filter 44 transmits only radiation with polarization outside the plane of Figure 3 and therefore blocks the back-reflected radiation. However, a small portion of the radiation (e.g., about 5%) passes through the polarization selective filter 44. This may be because, for example, a small component of the back-reflected radiation has polarization outside the plane of Figure 3.

[0068]

[0067] The AOM42 is de-energized when a small portion of the back-reflected radiation enters it, thus blocking the back-reflected radiation. However, because the operation of the AOM42 is relatively slow, it may transmit a small portion of the back-reflected radiation (the AOM may not be completely de-energized). The back-reflected radiation passing through the AOM42 passes through the optical amplifier 39 and is amplified by this optical amplifier. The additional EOM module 41 blocks this amplified back-reflected radiation. Since polarization rotation is required for radiation transmission to occur, the additional EOM module 41 includes cross polarizers 40a and 40b and transmits radiation only when the EOM is energized. The additional EOM 40 is not energized when back-reflected radiation enters the additional EOM (the operation of the EOM is much faster than the operation of the AOM). Therefore, the additional EOM 40 blocks the back-reflected radiation. Therefore, the back-reflected radiation does not return to the seed laser 30, and does not pass through the second path via the optical amplifier 39.

[0069]

[0068] The additional EOM 40 after the first pass of the optical amplifier 39 provides further advantages compared to providing an AOM instead of the additional EOM. This is because the switching speed of the additional EOM is faster than that of the AOM. If an AOM is provided instead of the additional EOM, the pulsed laser beam 32 needs to be focused into the AOM to accommodate the slow switching speed of the AOM (the switching speed is limited by the time it takes for the acoustically generated grating to pass through the pulsed laser beam). This focusing of the pulsed laser beam can damage the optical components, but this focusing is unnecessary because the additional EOM 40 provides faster switching than the AOM. Thus, potential damage to the optical components due to focusing is avoided.

[0070]

[0069] The additional EOM40 prevents the radiation from passing through the optical amplifier 39 twice when the optical amplifier 39 is not energized. This can prevent self-oscillation in the seed laser system 20. The AOM42 prevents the radiation from passing through the laser beam amplification system 21 and then the optical amplifier 39 when it is not energized. Again, this can prevent self-oscillation.

[0071]

[0070] Figure 4 schematically shows a seed laser system 20 according to another embodiment of the present invention. Similar to the embodiment shown in Figure 3, the seed laser system 20 comprises a seed laser 30 configured to emit a pulsed laser beam 32, an EOM pair 36, and an optical amplifier 39. Features of the embodiment in Figure 4 that correspond to features of the embodiment in Figure 3 are given the same reference numerals and, in some cases, are not described again.

[0072]

[0071] In this embodiment, an acousto-optic modulator (AOM) 60 is located downstream of the first pass of the optical amplifier 39. A polarizing rotor 62 is located downstream of the AOM 60. The polarizing rotor 62 may be, for example, a half-wave plate and is configured to rotate the polarization of the pulsed laser beam 32 by 90°.

[0073]

[0072] An additional EOM 64 is positioned after the second pass of the optical amplifier 39. Cross polarizers 64a,b are provided on either side of this additional EOM. The cross polarizer pair 64a,b and the additional EOM 64 may be referred to as an additional EOM module 66.

[0074]

[0073] A polarization selection filter 44 and a polarization conversion device 46 (for example, a reflection phase delay device 46 or a quarter-wave plate) are located downstream of the EOM module 66.

[0075]

[0074] The operation of the embodiment in Figure 4 is the same as the operation of the embodiment in Figure 3. That is, the pulsed laser beam 32 passes through the EOM module pair 36 that suppress the pedestal. Next, the pulsed laser beam 32 goes through a first path through the optical amplifier 39. After this, the AOM transmits the pulsed laser beam 32 (the AOM transmits the pulsed laser beam 32 when pulses of radiation arrive and blocks the pulsed laser beam 32 at other times). Next, the polarization of the pulsed laser beam 32 is rotated by the polarization rotor 62. The second polarization beam splitter 52 guides the pulsed laser beam 32 through a second path through the optical amplifier 39. Next, the pulsed laser beam 32 is reflected by the first polarization beam splitter 50 and sent to an additional EOM 64. The polarization of the pulsed laser beam 32 is rotated by the additional EOM 64. The pulsed laser beam 32 passes through the polarization selective filter 44 and is converted to circular polarization by the quarter wave plate 46. Next, the circularly polarized amplified pulsed laser beam 48 is output to the laser beam amplification system 21 (see Figure 2).

[0076]

[0075] The additional EOM64 provides a similar function to the additional EOM40 in the embodiment shown in Figure 3. That is, the additional EOM64 removes residual pedestal from the pulses of the pulsed laser beam 32 so that the pedestal can be ignored. The additional EOM64 prevents back-reflected radiation from reaching the optical amplifier 39 more effectively than the AOM in the embodiment of Figure 3. Thus, the loss of gain from the optical amplifier 39 that may occur in the embodiment of Figure 3 can be prevented by the embodiment of Figure 4. A disadvantage of the embodiment of Figure 4 is that the additional EOM64 receives higher optical power than the additional EOM in the embodiment of Figure 3.

[0077]

[0076] The power level that an additional EOM64 receives can be, for example, about 200W or more. Placing an additional EOM64 in a location where it will receive such a power level is contrary to the prior understanding of those skilled in the art for the reasons further explained above. Contrary to the common understanding of those skilled in the art, the inventors have found that an additional EOM64 can handle power of about 200W or more without being damaged.

[0078]

[0077] The AOM60 prevents the radiation from passing through the optical amplifier 39 twice when the optical amplifier 39 is not energized. This can prevent self-oscillation in the seed laser system 20. An additional EOM64 prevents the radiation from passing through the laser beam amplification system 21 and then the optical amplifier 39 when it is not energized. Again, this can prevent self-oscillation.

[0079]

[0078] The specific polarization of the pulsed laser beam 32 in the illustrated embodiment is merely an example. Other linear polarizations may also be used.

[0080]

[0079] The pulse duration of 300 ns for the laser beam emitted by the seed laser 30 is merely an example. The seed laser 30 may emit laser beams with pulses of other durations, such as several hundred nanoseconds.

[0081]

[0080] The seed laser 30 may emit a laser beam having a wavelength of approximately 10 μm. The seed laser 30 may also emit a laser beam having another wavelength, for example, approximately 1 μm.

[0082]

[0081] The 50 ns laser beam pulse duration after shortening by EOM is just an example. The pulse may have a different duration, for example, several tens of nanoseconds.

[0083]

[0082] The polarizing beam splitters 50 and 52 may be polarizing beam splitting cubes. Other polarization-selective beam splitters may also be used. In general, any suitable polarization-selective element may be used.

[0084]

[0083] In the illustrated embodiment of the present invention, an EOM pair is provided before the optical amplifier 39, but in other embodiments, a different number of EOMs may be provided before the optical amplifier. A single EOM may be provided before the optical amplifier.

[0085]

[0084] Embodiments of the present invention may be particularly advantageous when it is desirable to remove the pedestal from the pulses of a pulsed laser beam. However, embodiments of the present invention may also be used when it is desirable to retain the pedestal on the pulses of a pulsed laser beam. For example, in one embodiment, a single EOM may be provided instead of an EOM pair.

[0086]

[0085] While this specification provides specific references to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, guidance patterns and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and the like.

[0087]

[0086] While specific embodiments of the present invention in relation to lithography apparatus are mentioned herein, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses are sometimes commonly referred to as lithography tools. Such lithography tools may operate under vacuum conditions or environmental (non-vacuum) conditions.

[0088]

[0087] Although specific embodiments of the present invention have been described above, it is clear that the present invention can be implemented in forms other than those described above. The above description is intended to be illustrative and not limiting. Accordingly, it will be clear to those skilled in the art that the present invention can be modified as described above without departing from the scope of the claims described below.

[0089]

[0088] Clause 1. A seed laser system for an EUV radiation source, comprising a laser configured to emit a pulsed laser beam, at least one electro-optic modulator located downstream of the laser, and an optical amplifier located downstream of the at least one electro-optic modulator, further comprising an additional electro-optic modulator located downstream of the optical amplifier. 2. The seed laser system according to Clause 1, wherein the beam path of the seed laser system comprises a first path of a pulsed laser beam passing through an optical amplifier and a second path of a pulsed laser beam passing through an optical amplifier. 3. The seed laser system as described in Clause 2, wherein an additional electro-optic modulator is positioned after the first pass of the pulsed laser beam through the optical amplifier and before the second pass of the pulsed laser beam through the optical amplifier. 4. The seed laser system as described in Clause 3, wherein an additional electro-optic modulator is configured to rotate the polarization of the pulsed laser beam before the second pass of the pulsed laser beam through the optical amplifier, so that the polarization of the pulsed laser beam during the second pass through the optical amplifier is approximately orthogonal to the polarization of the pulsed laser beam during the first pass through the optical amplifier. 5. The seed laser system according to Clause 4, wherein the polarizing beam splitter is configured to guide the pulsed laser beam to an additional electro-optic modulator after a first pass of the pulsed laser beam through an optical amplifier, and to guide the pulsed laser beam to a second pass through an optical amplifier after the pulsed laser beam has passed through the additional electro-optic modulator. 6. An additional electro-optic modulator is positioned after the second pass of the pulsed laser beam through the optical amplifier, as described in Clause 2 of the seed laser system. 7. The seed laser system according to Clause 6, wherein the acousto-optic modulator and polarizing rotor are located in the beam path downstream of the first pass of the pulsed laser beam through the optical amplifier and upstream of the second pass of the pulsed laser beam through the optical amplifier. 8. The seed laser system according to Clause 7, wherein the polarizing beam splitter is configured to guide the pulsed laser beam to an acousto-optic modulator after a first pass of the pulsed laser beam through an optical amplifier, and to guide the pulsed laser beam to a second pass through an optical amplifier after the pulsed laser beam has passed through the acousto-optic modulator and the polarizing rotor. 9. The seed laser system according to any one of the clauses 1 to 8, wherein at least one electro-optic modulator located downstream of the laser and upstream of the optical amplifier is a pair of electro-optic modulators. 10. A laser system comprising a seed laser system as described in any of clauses 1 to 9, further comprising a laser beam amplification system comprising a series of optical amplifiers located downstream of the seed laser system. 11. The laser beam amplification system is the laser system described in Clause 10, comprising four optical amplifiers. 12. A fuel emitter capable of providing a fuel target to a plasma formation region, The laser system described in Clause 10 or Clause 11, A laser-generating plasma radiation source equipped with the following features. 13. The laser-generated plasma radiation source described in Clause 12, Lithography equipment, A lithography system equipped with [the following features]. 14. A method for providing a pulsed laser beam for an EUV radiation source, A method comprising: emitting a pulsed laser beam from a laser; passing the pulsed laser beam through at least one electro-optic modulator located downstream of the laser; passing the pulsed laser beam through an optical amplifier; and passing the pulsed laser beam through an additional electro-optic modulator located downstream of the optical amplifier. 15. The method according to clause 14, wherein the pulsed laser beam then passes through the optical amplifier as a second time. 16. The method according to clause 14 or clause 15, further comprising passing a pulsed laser beam through a laser beam amplification system comprising a series of optical amplifiers. 17. A method for generating EUV radiation, comprising receiving a pulsed laser beam output from a series of optical amplifiers described in Clause 16, and guiding the pulsed laser beam to a fuel target in a plasmaforming region.

Claims

1. A seed laser system for an EUV radiation source, comprising: a laser configured to emit a pulsed laser beam; at least one electro-optic modulator located downstream of the laser; an optical amplifier located downstream of the at least one electro-optic modulator; and further comprising an additional electro-optic modulator located downstream of the optical amplifier.

2. The seed laser system according to claim 1, wherein the beam path of the seed laser system comprises a first path of the pulsed laser beam passing through the optical amplifier and a second path of the pulsed laser beam passing through the optical amplifier.

3. The seed laser system according to claim 2, wherein the additional electro-optic modulator is positioned after the first pass of the pulsed laser beam through the optical amplifier and before the second pass of the pulsed laser beam through the optical amplifier.

4. The seed laser system according to claim 3, wherein the additional electro-optic modulator is configured to rotate the polarization of the pulsed laser beam before the second pass of the pulsed laser beam through the optical amplifier, such that the polarization of the pulsed laser beam during the second pass through the optical amplifier is substantially orthogonal to the polarization of the pulsed laser beam during the first pass through the optical amplifier.

5. The seed laser system according to claim 4, wherein the polarizing beam splitter is configured to guide the pulsed laser beam to the additional electro-optic modulator after the first pass of the pulsed laser beam through the optical amplifier, and to guide the pulsed laser beam to the second pass through the optical amplifier after the pulsed laser beam has passed through the additional electro-optic modulator.

6. The seed laser system according to claim 2, wherein the additional electro-optic modulator is positioned after the second pass of the pulsed laser beam through the optical amplifier.

7. The seed laser system according to claim 6, wherein the acousto-optic modulator and the polarization rotor are located in the beam path downstream of the first pass of the pulsed laser beam through the optical amplifier and upstream of the second pass of the pulsed laser beam through the optical amplifier.

8. The seed laser system according to claim 7, wherein the polarizing beam splitter is configured to guide the pulsed laser beam to the acousto-optic modulator after the first pass of the pulsed laser beam through the optical amplifier, and to guide the pulsed laser beam to the second pass through the optical amplifier after the pulsed laser beam has passed through the acousto-optic modulator and the polarizing rotor.

9. The seed laser system according to any one of claims 1 to 8, wherein the at least one electro-optic modulator located downstream of the laser and upstream of the optical amplifier is a pair of electro-optic modulators.

10. A laser system comprising a seed laser system according to any one of claims 1 to 9, further comprising a laser beam amplification system comprising a series of optical amplifiers disposed downstream of the seed laser system.

11. A fuel emitter capable of providing a fuel target to a plasma-forming region, The laser system according to claim 10, A laser-generating plasma radiation source equipped with the following features.

12. A laser-generating plasma radiation source according to claim 11, Lithography equipment, A lithography system equipped with [the following features].

13. A method for providing a pulsed laser beam for an EUV radiation source, A method comprising: emitting a pulsed laser beam from a laser; passing the pulsed laser beam through at least one electro-optic modulator located downstream of the laser; passing the pulsed laser beam through an optical amplifier; and passing the pulsed laser beam through an additional electro-optic modulator located downstream of the optical amplifier.

14. The method according to claim 13, wherein the pulsed laser beam then passes through the optical amplifier a second time, and the method further comprises passing the pulsed laser beam through a laser beam amplification system comprising a series of optical amplifiers.

15. A method for generating EUV radiation, comprising receiving a pulsed laser beam output from the series of optical amplifiers described in claim 14, and guiding the pulsed laser beam to a fuel target in a plasma formation region.

Images