Method and system for triggering seed laser

Abstract

A method of triggering a seed laser, the seed laser being defined by a cavity between a first reflector and a second reflector and comprising a Q-switching element disposed in an optical path of a laser beam between the first reflector and the second reflector, a method of controlling a Q-switching element to generate a laser pulse by switching a transmission parameter of the Q-switching element between a high value and a low value in a cyclic manner includes setting the transmission parameter of the Q-switching element to the high value and gradually reducing the transmission parameter of the Q-switching element from the high value to the low value over time.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to European application 23207836.0, filed on November 6, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a method and a laser system for triggering a seed laser. The seed laser can form part of a laser system, which in turn can form part of a laser-generated plasma (LPP) radiation source. The LPP radiation source can generate extreme ultraviolet (EUV) radiation and can form part of a photolithography system. Background Technology

[0004] A lithography apparatus is a machine configured to apply a desired pattern onto a substrate. Lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithography apparatus can project a pattern at a patterning apparatus (e.g., a mask) onto a radiation-sensitive material (resist) layer disposed on a substrate.

[0005] To project a pattern onto a substrate, a photolithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Compared to a photolithography apparatus using radiation with a wavelength of, for example, 193 nm, a photolithography apparatus using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate.

[0006] EUV radiation used in lithography equipment can be generated by a laser-generated plasma (LPP) radiation source. Within the LPP radiation source, a laser beam can be used to irradiate fuel droplets in order to generate plasma that will emit EUV radiation.

[0007] The desired outcome is a high-power laser beam used to irradiate the fuel droplet. A seed laser system can be used to provide a pulsed laser beam, which is then 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 droplet, thereby generating EUV radiation.

[0008] The seed laser in a seed laser system can be operated via Q-switching to obtain pulsed output. Triggering methods can be used to generate pulsed laser beams with high peak power.

[0009] A potential problem is that triggering the seed laser to generate higher peak power may reduce peak power stability and timing stability.

[0010] It may be desirable to provide a seed laser system and method for triggering a seed laser that overcomes this problem or another problem associated with the prior art in a manner not disclosed or implied in the prior art. Summary of the Invention

[0011] According to a first aspect of the invention, a method for triggering a seed laser is provided, the seed laser being defined by a cavity between a first reflector and a second reflector, and including a Q-switching element disposed between the first reflector and the second reflector in an optical path of a laser beam to generate laser pulses by cyclically switching the transmission parameter of the Q-switching element between high and low values, the method comprising: setting the transmission parameter of the Q-switching element to the high value, and gradually decreasing the transmission parameter of the Q-switching element from the high value to the low value over time.

[0012] Advantageously, gradually decreasing the transmission parameters of the Q-switching element over time can increase the stability of the master laser pulses (both peak power and peak timing). Stability can be considered a measure of the consistency between master laser pulses. Advantageously, gradually decreasing the transmission parameters of the Q-switching element over time can increase the peak power in the seed laser (and the pulse energy after subsequent electro-optic modulator cutting).

[0013] The method may include momentarily or relatively very quickly setting the transmission parameter of the Q-switching element (from the low value) to the high value.

[0014] A seed laser can be used as an EUV radiation source. The first and second reflectors can be mirrors. The second reflector can be an output connector. The first reflector can be a total internal reflection mirror, and the second reflector can be an output mirror (e.g., a half-silvered mirror).

[0015] Q-switching elements can be variable attenuators used to reduce the Q-factor or quality factor of a cavity (or optical resonator). Q-switching elements can be modulators (e.g., acousto-optic modulators).

[0016] The method may also include reducing the transmission parameter of the Q-switching element from the high value to the low value in multiple steps.

[0017] At least one of the multiple steps may include maintaining the transmission parameter of the Q-switching element at an intermediate value between the high value and the low value for a considerable period of time.

[0018] The method may further include reducing the transmission parameter of the Q-switching element from the high value to the low value substantially continuously over time.

[0019] The method may further include reducing the transmission parameter of the Q-switching element from the high value more linearly in time during at least the first half of the inter-pulse cycle.

[0020] The reduction in the transmission parameter of the Q-switching element during the first half of the pulse cycle can be in the range of 40% to 70% of the total difference between the high and low values ​​of the transmission parameter.

[0021] The method may further include reducing the transmission parameter of the Q-switching element from the high value over time at the initial increasing rate before reducing the transmission parameter of the Q-switching element at a decreasing rate compared to the initial increasing rate over time.

[0022] The method may further include instantaneously or relatively very rapidly reducing the transmission parameter of the Q-switching element from the high value to an intermediate value between the high and the low value before gradually reducing the transmission parameter of the Q-switching element to the low value.

[0023] A momentary or relatively very rapid reduction of the transmission parameter of the Q-switching element from the high value to the intermediate value can be considered a decrease or a rapid decrease. The decrease in the transmission parameter of the Q-switching element during the initial rate-increase cycle can be in the range of 10% to 60% or 20% to 40% of the total difference in the transmission parameter from the high to the low value. This timescale of decrease can be as fast as allowed by the Q-switching element.

[0024] This can reduce the amount of follow-up laser light after the main laser pulse. It can also increase the peak value of the main laser pulse.

[0025] The transmission parameter may transition from a high value to a low value, so that multiple visible photons are always present in the cavity mode of the cavity.

[0026] Multiple photons can have power levels at least several orders of magnitude higher than a single photon. The power level over one or more inter-pulse cycles can be at least one of >= ~0.1 mW and >= ~10 mW. The power level over one or more inter-pulse cycles can be at least one of <~10 W and <~30 W. The power level over one or more inter-pulse cycles can range from ~0.1 mW to <~30 W. By continuously decreasing the transmission parameter of the Q-switching element over time (increasing the power of the Q-switching element), high output power during the inter-pulse intervals between the main laser pulses can be prevented.

[0027] The transmission parameter of the Q-switching element can be set based on the power supplied to the Q-switching element, and the method may further include: setting the transmission parameter of the Q-switching element to the high value by setting the power supplied to the Q-switching element to a low power value, and gradually increasing the power supplied to the Q-switching element from the low power value to the high power value over time, and gradually decreasing the transmission parameter of the Q-switching element from the high value to the low value over time.

[0028] The method may include instantaneously or relatively very quickly setting the power to the Q-switching element (from the high power value) to the low power value, and instantaneously or relatively very quickly setting the transmission parameter of the Q-switching element to the high value.

[0029] Power can be RF power. Power can be high-frequency power.

[0030] Low power values ​​can be zero power values.

[0031] The method may further include reducing the transmission parameter of the Q-switching element to the intermediate value by increasing the power to the Q-switching element to an intermediate power value between the low power value and the high power value.

[0032] The method may further include gradually increasing the power to the Q-switching element to the intermediate power value over time, and decreasing the transmission parameter of the Q-switching element to the intermediate value.

[0033] The method may also include using a non-square waveform to trigger the Q-switching element.

[0034] The high value of the transmission parameter of the Q-switching element may be sufficient to trigger the main laser pulse.

[0035] The high value of the transmission parameter of the Q-switching element can be ~96% to 98% of the Q-switching element transmission (e.g., for a CO2 laser).

[0036] The high value of the transmission parameter of the Q-switch element can correspond to the Q-switch element being turned off.

[0037] The low value of the transmission parameter of the Q-switching element is sufficient to prevent pre-laser pulses from occurring.

[0038] The low value of the transmission parameter of the Q-switching element can be ~75% to 95% of the Q-switching element transmission (e.g., for a CO2 laser).

[0039] The method may further include using a first control feedback loop to control the cavity length of the cavity.

[0040] The length of the cavity can be controlled by the piezoelectric actuator of the output connector.

[0041] The first control feedback loop may be based on at least one of the following: the peak power of the main laser pulse and / or the post-laser time, wherein the post-laser time may be the time elapsed between the main laser pulse and the post-laser pulse.

[0042] The first control loop can be configured to maximize the peak power of the main laser pulse or minimize the post-laser time.

[0043] The method may further include using a second control feedback loop to control the scaling of the transmission parameters of the Q-switching element.

[0044] The second control feedback loop can be based on the post-laser time, where the post-laser time can be the time elapsed between the main laser pulse and the post-laser pulse.

[0045] The second control loop can be configured to set the post-laser time to a predetermined value.

[0046] According to a second aspect of the invention, a method for generating EUV radiation is provided, comprising receiving a pulsed laser beam output from a seed laser described above; and directing the pulsed laser beam to a fuel target in a plasma formation region.

[0047] According to a third aspect of the invention, a laser system is provided configured to trigger a seed laser defined by a cavity between a first reflector and a second reflector, and including a Q-switching element disposed between the first reflector and the second reflector in the optical path of a laser beam to generate laser pulses by cyclically switching the transmission parameter of the Q-switching element between high and low values, the system being configured to: set the transmission parameter of the Q-switching element to the high value, and gradually decrease the transmission parameter of the Q-switching element from the high value to the low value over time.

[0048] The laser system may include a Q-switch driver for applying power to the Q-switch element and a control device for controlling the power applied by the Q-switch driver.

[0049] According to a fourth aspect of the invention, a laser-generated plasma radiation source is provided, comprising: a fuel emitter operable to provide a fuel target at a plasma formation region; and a laser system as described above.

[0050] According to a fifth aspect of the present invention, a photolithography system is provided, comprising: a laser-generated plasma radiation source as described above; and a photolithography apparatus.

[0051] According to a sixth aspect of the present invention, a computer program is provided that includes computer-readable instructions configured to cause a processor to perform the methods described above.

[0052] According to a seventh aspect of the present invention, a computer-readable medium is provided, the computer-readable medium carrying a computer program as described above.

[0053] According to an eighth aspect of the present invention, a computer device is provided, comprising: a memory storing processor-readable instructions; and a processor arranged to read and execute the instructions stored in the memory; wherein the processor-readable instructions include instructions arranged to control the computer to perform the methods described above. Attached Figure Description

[0054] Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, wherein:

[0055] Figure 1 A lithography system, including lithography equipment and a radiation source, is schematically depicted.

[0056] Figure 2 A seed laser system according to an embodiment of the present invention is schematically depicted;

[0057] Figure 3 A method for triggering a seed laser for a photolithography system according to an embodiment of the present invention; and

[0058] Figure 4 depicts a graph relating to the simulation of the seed laser of the trigger lithography system according to a previously known method and an embodiment of the present invention. Detailed Implementation

[0059] Figure 1 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a pattern forming apparatus MA (e.g., a mask), a projection system PS, and a substrate stage WT configured to support a substrate W.

[0060] The irradiation system IL is configured to adjust the EUV radiation beam B before it is incident on the pattern forming apparatus MA. The irradiation system IL may include a faceted field mirror assembly 10 and a faceted pupil mirror assembly 11. Together, the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11 provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to or in place of the faceted field mirror assembly 10 and the faceted pupil mirror assembly 11, the irradiation system IL may include other mirrors or devices.

[0061] After such adjustment, the EUV radiation beam B interacts with the patterning apparatus MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. A projection system PS is configured to project the patterned EUV radiation beam B' onto a substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto a substrate W held by a substrate stage WT. The projection system PS may 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 apparatus MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS... Figure 1 The diagram shows only two mirrors 13 and 14, but the projection system PS can include a different number of mirrors (e.g., six or eight mirrors).

[0062] The substrate W may include a previously formed pattern. In this case, the lithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0063] A relative vacuum can be provided in the radiation source SO, the irradiation system IL, and / or the projection system PS, i.e., a small amount of gas (e.g., hydrogen) at a pressure sufficiently below atmospheric pressure.

[0064] Figure 1The radiation source SO shown belongs to the type that can be referred to as a laser-generated plasma (LPP) source, for example. A laser system 1, which may include, for example, a CO2 laser, is arranged to deposit energy via a laser beam 2 onto a fuel, such as tin (Sn), supplied by, for example, a fuel emitter 3. Although tin is mentioned in the following description, any suitable fuel can be used. The fuel can be, for example, in liquid form and can be, for example, a metal or alloy. The fuel emitter 3 may include a nozzle configured to guide tin, for example, in droplet form, along a trajectory toward the plasma formation region 4. The laser beam 2 is incident on the tin at the plasma formation region 4. The laser energy deposited into the tin generates tin plasma 7 at the plasma formation region 4. During the de-excitation and recombination of electrons and ions in the plasma, radiation, including EUV radiation, is emitted from the plasma 7.

[0065] The pulsed laser beam 2 incident on the tin at the plasma formation region 4 can be referred to as the main laser beam or the main pulse laser beam. The individual pulses of this pulsed laser beam 2 can be referred to as the main pulses.

[0066] EUV radiation from the plasma is collected and focused by collector 5. Collector 5 includes, for example, a near-normal incident radiation collector 5 (sometimes referred to as a more general normal incident radiation collector). Collector 5 may have a multi-layered mirror structure arranged to reflect EUV radiation (e.g., EUV radiation with a desired wavelength such as 13.5 nm). Collector 5 may have an elliptical configuration with two foci. The first foci may be located at the plasma formation region 4, and the second foci may be located at the intermediate foci 6, as discussed below.

[0067] Laser system 1 can be spatially separated from radiation source SO. In such a case, laser beam 2 can be transmitted from laser system 1 to radiation source SO by means of a beam delivery system (not shown) including, for example, suitable directional mirrors and / or beam expanders and / or other optical devices. Laser system 1, radiation source SO, and beam delivery system can be considered together as a radiation system.

[0068] The radiation reflected by collector 5 forms an EUV radiation beam B. EUV radiation beam B is focused at intermediate focal point 6 to form an image of the plasma present in plasma formation region 4 at intermediate focal point 6. The image at intermediate focal point 6 serves as a virtual radiation source for irradiating system IL. Radiation source SO is arranged such that intermediate focal point 6 is located at or near opening 8 in the enclosure structure 9 of radiation source SO.

[0069] A seed laser system 20 (or a more general laser system) according to an embodiment of the present invention is schematically depicted in Figure 2The seed laser system 20 includes a seed laser 22 configured to emit a pulsed laser beam 24. The seed laser system 20 may be included in, for example... Figure 1 In the seed laser system 1 depicted in [the diagram], the seed laser system 20 may include a laser beam amplification system (not shown) and a beam steering system (not shown). The laser beam amplification system may include an optical amplifier (also called a laser beam amplifier) ​​to amplify the laser beam output from the seed laser 22, thereby increasing the power of the laser beam. The laser beam output from the optical amplifier is then directed to the plasma formation region 4 (see [the diagram]). Figure 1 The beam steering system involves an amplified laser beam incident on a fuel droplet, thereby generating EUV radiation. The amplified laser beam is a pulsed laser beam and may be referred to as an amplified master pulse laser beam.

[0070] Following the seed laser 22, there exists an electro-optic modulator (EOM) (not shown) or multiple EOMs, which act as a fast switch to cut off a portion of the seed laser pulse with a width smaller than the Q-switching pulse (i.e., the high-power pulsed main laser beam output to generate EUV radiation). Therefore, the peak power is the main seed laser performance parameter that is less important than stability, while the total pulse energy is of lower importance.

[0071] The seed laser 22 includes an elongated amplification (or gain) medium 26, which comprises, for example, CO2 gas to generate laser light. In an embodiment, this gain medium may be a cylindrical sealed discharge tube.

[0072] The total internal reflection mirror 28 and the output mirror 30 are arranged along the axial direction L (optical path of the laser beam) of the gain medium 26 so as to clamp the gain medium 26 in the cavity C.

[0073] Furthermore, an acousto-optic modulator AOM 32, which may be more generally referred to as a Q-switching element, is arranged in the optical path L between the gain medium 26 and the total internal reflection mirror 28.

[0074] AOM 32 is connected to AOM driver 34 (more generally a Q-switch driver), which applies power to AOM 32. The power can be RF power. The power can be high-frequency power. AOM driver 34 is controlled by control device 36, allowing different levels of power to be applied to AOM 32 over time.

[0075] The seed laser 22 is operated via a so-called Q-switch to obtain pulsed output. Using this Q-switch, the quality factor of the cavity C is modulated by the AOM 32. The cavity C surrounding the AOM 32 is aligned such that the zeroth order of the AOM 32 propagates along the direction of the cavity mode (i.e., the optical path L), while other diffraction orders are lost.

[0076] The Q-switching is achieved by positioning AOM 32 inside the optical resonator of the seed laser 22. When AOM 32 is operational (or switched on), the diffracted light leaving the gain medium 26 does not return, and the laser is suppressed (although, as will be explained, a degree of lasering may occur in some cases). This attenuation inside the cavity C corresponds to a decrease in the Q-factor, or quality factor, of the optical resonator. A high Q-factor corresponds to low resonator loss per round trip, and vice versa. Therefore, when used for this purpose, AOM 32 can refer to a "Q-switch" (or a Q-switching element).

[0077] Initially, the laser medium is pumped, while AOM 32 is configured to leak some light out of the cavity (thus creating a low-Q optical resonator). This results in population inversion, but laser operation is typically impossible due to excessive losses at AOM 32. At some point, AOM 32 can rapidly transition from low-Q to high-Q, allowing feedback and optical amplification by stimulated emission to begin. Since a large amount of energy is already stored in gain medium 26, the light intensity in the laser resonator accumulates very rapidly; this also causes the energy stored in gain medium 26 to be depleted almost equally rapidly. The end result is a short pulse of light output from seed laser 22, referred to as the master laser pulse, with a very high peak intensity.

[0078] When AOM 32 is turned on or partially turned on (i.e., power is supplied to AOM 32 but not at full power), some of the light in cavity C is diffracted at AOM 32, and this light is lost from cavity C. When AOM is briefly turned off (i.e., no power is supplied to AOM 32), light cannot diffract from cavity C, and the round-trip transmission through cavity C increases, triggering the main laser pulse.

[0079] AOM 32 can be considered to have a transmission parameter indicating how much light is transmitted through AOM 32 (e.g., expressed as a percentage). When AOM 32 is off, it can then be considered to have a high transmission parameter (e.g., 100% or close to 100% AOM transmission). That is, light passes through AOM 32 without diffraction, or at least most of the light passes through without diffraction. For example, a high transmission parameter can be sufficient to trigger a master laser pulse. This is because, when AOM 32 is off (i.e., with a high transmission parameter), the round-trip loss drops sharply, resulting in a significant build-up of laser power, thus generating the master laser or master laser beam pulse. When AOM 32 is on, the transmission parameter can be set relatively low (i.e., compared to a high transmission parameter). With a low transmission parameter, some light cannot pass through AOM 32, i.e., it diffracts, thus allowing population inversion to increase again.

[0080] Typically, a pulsed laser beam is generated by cyclically switching the transmission parameters of AOM 32 between high and low values. The high values ​​can be as high as practically achievable values. Since AOM 32 is off, there are no diffraction losses at other orders. However, some absorption loss always exists in the AOM crystal, and typical AOM transmission with AOM 32 off can be, for example, ~96% to 98% (for a CO2 laser). In practice, AOM 32 can be completely turned off due to its high transmission parameters. However, more broadly, a high transmission parameter of AOM 32 can be sufficient to trigger the main laser pulse (i.e., AOM 32 can be partially on but at a value higher than the low value). The low value can depend on the state of the seed laser 22. Including absorption loss, typical AOM transmission with AOM 32 on can be, for example, a low value of ~75% to 95% (for a CO2 laser).

[0081] Figure 3 The diagram is used to trigger a seed laser (e.g., Figure 2 Method 100 (for seed lasers).

[0082] In the first step 102, the transmission parameters of AOM 32 are set to a low value by setting the power to AOM 32 to a high power value. The control device 36 controls the power to AOM 32, wherein the power is transmitted by AOM driver 34.

[0083] In the second step 104, the gain medium 26 is pumped to produce population inversion in the gain medium 26.

[0084] In step 106, the transmission parameters of AOM 32 are instantaneously set from a low (e.g., zero) power value to a high value. That is, AOM 32 is turned off. The light energy within the cavity then accumulates very rapidly and depletes the energy stored in the gain medium. This causes the main pulse laser beam 24 to be output from the seed laser, as shown in step 107. In this embodiment, the transmission parameters of AOM 32 are instantaneously (i.e., as quickly as possible) or at least substantially instantaneously set to a high value, but it should be understood that this is not necessary, even if it can be considered optimal. In other embodiments, the transmission parameters of AOM 32 may not be instantaneously set, but can still be set to a high value relatively quickly (e.g., there may be practical limitations to instantaneous setting) to trigger the main laser pulse. The transmission parameters of AOM 32 can be set from a low value to a high value much faster than from a high value to a low value.

[0085] In step 5, 108, the transmission parameters of AOM 32 are gradually set from high to low values ​​over time by gradually increasing the power to AOM 32 from a low power value to a high power value. For example, AOM 32 can be triggered using a non-square RF waveform, and the power can be scaled using this specific AOM RF waveform. AOM 32 can only remain at a high value for a short period of time (e.g., approximately several hundred ns).

[0086] The gradual decrease of the high transmission parameter of AOM 32 to the low transmission parameter of AOM 32 over time can be considered non-instantaneous (or at least substantially non-instantaneous). In other words, it can be considered to be at least one of slow, gradual, and / or non-sudden. Gradual decrease over time can be considered to be in multiple steps (e.g., a relatively large number of relatively small steps over time). These multiple steps may involve maintaining the transmission parameter of AOM 32 at one or more intermediate values ​​between the high and low values. The transmission parameter of AOM 32 may be maintained at one or more intermediate values ​​for a considerable period of time, and / or may be maintained for the same or different durations. In some embodiments, these multiple steps may also be considered to include only two steps (i.e., a step from the high value to the intermediate value, maintained at the intermediate value for a predetermined considerable period of time; and another step from the intermediate value to the low value). In embodiments, the transmission parameter of AOM 32 may be decreased from the high value to the low value continuously or at least substantially continuously over time. That is, the transmission parameters of AOM 32 are continuously decreasing (from high to low values), with little or no noticeable time spent maintaining a specific intermediate value for the transmission parameters of AOM 32. Reducing the transmission parameters of AOM 32 to this intermediate value is achieved by increasing the power to AOM 32 (from a low power value) to an intermediate power value between low and high power values. This power increase can be considered as a gradual increase over time. Furthermore, reducing the transmission parameters of AOM 32 to a low value is achieved by increasing the power to AOM 32 from an intermediate value to a high power value. Again, this power increase can be considered as a gradual increase over time.

[0087] The AOM 32 operates in such a way that after each master laser pulse, a small amount of light is always present in cavity C (i.e., there are at least a few photons in the cavity mode of cavity C, rather than just photons generated by spontaneous emission). In this way, the laser never needs to start from a spontaneous emission event. Advantageously, this can make the peak power and timing of the laser (i.e., the master laser pulse) more stable. Stability can be considered as a measure of consistency between master laser pulses.

[0088] Then, the cycle of steps 106, 107, and 108 is repeated to generate multiple laser pulses over time.

[0089] Although the steps of the method for triggering the seed laser are indicated as first, second, third, etc., this does not necessarily imply a specific required order. While some steps should be performed sequentially (e.g., the fifth step 108, which gradually decreases the transmission parameter of AOM 32 from a high value to a low value over time, must be performed after the third step 106, which instantaneously sets the transmission parameter of AOM 32 to a high value), other steps can be performed in any order. Specifically, the first step 102, which sets the transmission parameter of AOM 32 to a low value, and the second step 104, which pumps the gain medium 26, can be performed in any order, including simultaneously.

[0090] The aforementioned (known) methods for triggering the AOM are different. In such methods, the AOM is mostly partially switched on. That is, the AOM is on, but not at full power. Specifically, the AOM is maintained at a specific (relatively) low level of transmission (relative to the high level of transmission when the AOM is off). Thus, the AOM diffracts some light and also allows some transmission (e.g., about 86% AOM transmission). This causes a portion of the light to be diffracted at the AOM, and this portion of the light is lost from the cavity. Then, the AOM is momentarily switched off (only briefly switched off), thereby increasing the round-trip transmission through the cavity and triggering the main laser pulse. Then, the AOM is momentarily switched on (partially switched on) to achieve the specific (relatively) low level of transmission. For example, the AOM can be triggered using a square RF waveform. After the main laser pulse, the population inversion (gain) in the cavity is at its lowest level and begins to rebuild due to the continuously excited plasma in the cavity. At some point after the main laser pulse, the gain in the cavity recovers sufficiently to exceed the laser threshold, and power begins to be generated again. This causes what is known as pre-lasing (i.e., a lower-power pulsed laser beam emitted before the main laser pulse). Laser emission begins when the round-trip gain > round-trip loss, causing pre-lasing and resulting in a decrease in population inversion, an increase in the power output (Pout) from the laser, and a decrease in the optical energy within the cavity. The timing of this occurrence is determined by (among other factors) the AOM transmission (to zero order) and is actively controlled in a feedback loop that adjusts to the RF power of the AOM, setting the AOM transmission (to zero order) to an "on" state. The feedback loop can be based on or can utilize the pre-lasing time, which is the time elapsed between the pre-lasing pulse and the main laser pulse.

[0091] Transmission through the AOM between main laser pulses can be reduced (by supplying more RF to the AOM), allowing the laser to operate even without pre-lasing. However, while that would result in a main laser pulse with higher peak power, it would also lead to less pulse stability, both in terms of peak power and peak timing. Previously, allowing pre-lasing development was used to remove or at least mitigate this instability.

[0092] Figure 4 shows a graph relating the simulation of the aforementioned triggering method (left) to two examples of the novel triggering method of the present invention (first example in the middle, second example on the right). In the top row, the dashed line (right y-axis) indicates the zero-order transmission through the AOM: 100% means the AOM is completely off, and lower transmission means the AOM is partially on (i.e., the AOM is on, but not at full power). The solid line (left y-axis) shows the power "Pout" from the laser over time. As mentioned, in the aforementioned method, the AOM is only off for a brief moment, and the rise and fall times are primarily limited by the size of the beam on the AOM and the finite velocity of the sound waves in the AOM. In the aforementioned method, it is possible to instantaneously set the transmission parameter of the AOM from a low value to a high value, briefly hold it at the high value, and then instantaneously set it from the high value to a low value. This is illustrated by the dashed line in the top left graph.

[0093] The drawback of the aforementioned triggering method is that it sacrifices peak power (i.e., reduces peak power) in order to obtain improved peak power stability and timing stability. Furthermore, in the aforementioned method, laser emission is accumulated by spontaneous emission within the cavity, and although pre-lasing is allowed to occur between main laser pulses to improve the stability of the main laser pulse (both peak power and peak timing), this still affects the stability of the main laser.

[0094] In the first example of the new method, AOM 32 is switched on more slowly after being switched off (i.e., the transmission parameters of AOM 32 are not instantaneously set to a low value) to be used for the output master laser pulse. This is illustrated by the dashed line in the center top plot. By slowly increasing the power to AOM 32 (i.e., not immediately and continuously adjusting the power to a high power level corresponding to the low transmission parameters of AOM 32), most or all photons in the cavity mode are prevented from being completely expelled from the cavity C. This is shown in the simulated intracavity light energy in the second row of Figure 4. Specifically, it can be seen that the intracavity light energy of the first example of the new method (center bottom plot) is several orders of magnitude higher than that of the aforementioned method (left bottom plot) during the (main) inter-pulse cycle. In some embodiments, it can be crucial to increase the power to AOM 32 more linearly in time during the first half of the inter-pulse cycle (in the simulation, using an error function). The reduction in the transmission parameter of AOM 32 during the first half of the inter-pulse cycle can range from 40% to 70% of the total difference in the transmission parameter from a high value to a low value. It is conceivable to include reducing the transmission parameter of AOM 32 from a high value over time at the initial increasing rate before reducing it at a decreasing rate compared to the initial increasing rate. In some embodiments, the transmission parameter of AOM 32 can be maintained at a high value for a relatively long period (e.g., from about several hundred ns to about μs), and then the transmission parameter of AOM 32 can be reduced (e.g., linearly).

[0095] By gradually increasing the AOM power over time (reducing AOM transmission), high output power is prevented during the inter-pulse intervals (of the main laser). As an example, the power level during the inter-pulse intervals of the new method can have a minimum of ~10 mW. It might be desirable to maintain the output power during the inter-pulse intervals at >= ~0.1 mW, but otherwise as low as possible. The power level during the inter-pulse intervals of the new method can have a maximum of ~30 W (e.g., less than typical pre-lasers). It might be desirable to maintain the output power during the inter-pulse intervals at <~10 W. Equally crucial is how long this power is sustained. In simulations using the aforementioned triggering method, the cavity is completely emptied to the level of a single photon (indicated by the horizontal dashed line in the bottom left plot); however, in the new method, the energy in the cavity is maintained several orders of magnitude above this level. Therefore, by using the new method, the laser does not need to begin from a spontaneous emission event after each pulse. This increases the stability of the output pulses.

[0096] The new method may include providing a so-called post-laser pulse shortly after the main laser pulse. This is illustrated in a center-top plot, which shows a small peak in Pout shortly after the large peak of the main laser. This occurs because some light still remains in the cavity mode of cavity C due to the gradual decrease in AOM transmission.

[0097] When compared with the aforementioned methods, the first example of the new method provides an increase in the peak power of the master laser. This is illustrated by comparing the peak power of the master laser (approximately 700 W) in the central top plot with the peak power of the master laser (approximately 500 W) in the left top plot.

[0098] Advantageously, gradually decreasing the transmission parameters of AOM 32 over time can increase the stability of the master laser pulses (both peak power and peak timing). Stability can be considered a measure of the consistency between master laser pulses. This is because the laser does not need to begin with a spontaneous emission event after each master laser pulse (since a small amount of light in the cavity mode of the cavity is always present in the cavity C), and therefore the contribution of spontaneous emission to variations in peak power and pulse timing will be much lower.

[0099] Advantageously, gradually decreasing the transmission parameters of AOM 32 over time increases the peak power in the seed laser (and the pulse energy after subsequent electro-optic modulator (EOM) cutting). This is because at the beginning of the main laser pulse, the population inversion is high, and the light energy in cavity C is low. This is because the power output is lower in the latter half of the inter-pulse period.

[0100] In the new method, the stability of the main laser pulse has been improved, eliminating the need for a large amount of pre-lasing and thus increasing the peak power. Unlike the previous method, where the light from the previous cycle remained in cavity C, the new method eliminates the need for a large amount of pre-lasing. Due to the absence of pre-lasing (or a very small amount of pre-lasing), the population inversion of the main laser pulse is higher because some of the pre-lasing is not exhausted.

[0101] The second example of the new method is similar to the first example, but with a reduction in the post-laser after the main laser pulse, where the AOM transmission drops rapidly and slightly at the beginning of the inter-pulse cycle. This is illustrated by the dashed line in the top right plot. By doing this, the peak power of the main laser pulse is further increased, as illustrated by comparing the peak power of the main laser in the top right plot (approximately 800 W) with that in the top center plot (approximately 700 W). In the simulation shown, the small, rapid drop is 20% of the total difference in AOM transmission, but it should be understood that this is only an example and other values ​​can be used (e.g., in the range of 10% to 60%). The timescale of this drop can be as fast as allowed by the Q-switching element. This small, rapid drop can be considered the initial rate of increase over time before the transmission parameters of the AOM 32 decrease at a rate of decrease compared to the initial rate of increase.

[0102] In embodiments of the new method, the cavity length of cavity C can be controlled, for example, using a piezoelectric actuator of the output connector (i.e., output mirror 30). A first control feedback loop can be used to control the cavity length of cavity C. This can be based on, for example, the amplitude of the peak power of the main laser or the time difference between the peak power of the main laser and the subsequent laser (this can be referred to as the subsequent laser time). In other words, the subsequent laser time can be considered as the time elapsed between the main laser pulse and the subsequent laser pulse.

[0103] Similar to existing methods, small harmonic modulation of the cavity C can be induced, and the response of peak power or after-laser time can be used for control. In the case of peak power, a first control feedback loop can be set for maximizing peak power (i.e., maximizing the peak power of the main laser pulse), and in the case of after-laser time, a first control feedback loop can be set for minimizing after-laser time (i.e., minimizing after-laser time).

[0104] The second control feedback loop can also be used to control the scaling of the AOM RF waveform. This can also be based on the value of the post-lasing time, which can then become a setting similar to the setting of the pre-lasing time in the aforementioned method. In other words, the second control feedback loop, if desired, can be configured to set the post-lasing time to a specific predetermined value. In some embodiments, the second control feedback loop can be configured to minimize the post-lasing time.

[0105] While specific references can be made to the use of lithography equipment in IC manufacturing within this document, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guiding and testing for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0106] Where circumstances permit, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form that can be read by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. Additionally, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and these actions are actually caused by a computing device, processor, controller, or other means of executing firmware, software, routines, instructions, etc., and may result in the interaction of actuators or other means with the physical world when such operations are performed.

[0107] While specific embodiments of the invention have been described above, it should be understood that the invention can be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the invention as described without departing from the scope of the aspects set forth below.

[0108] aspect

[0109] 1. A method for triggering a seed laser, the seed laser being defined by a cavity between a first reflector and a second reflector, and including a Q-switching element disposed between the first reflector and the second reflector in the optical path of a laser beam to generate laser pulses by cyclically switching the transmission parameters of the Q-switching element between high and low values, the method comprising:

[0110] The transmission parameter of the Q-switching element is set to the high value, and

[0111] Over time, the transmission parameter of the Q-switching element is gradually reduced from the high value to the low value.

[0112] 2. The method according to aspect 1 further includes setting the transmission parameter of the Q-switching element to the high value instantaneously or relatively very quickly.

[0113] 3. The method according to any one of aspects 1 or 2, further comprising reducing the transmission parameter of the Q-switching element from the high value to the low value in multiple steps.

[0114] 4. The method according to aspect 3, wherein at least one of the plurality of steps includes maintaining the transmission parameter of the Q-switching element at an intermediate value between the high value and the low value for a considerable period of time.

[0115] 5. The method according to any one of aspects 1 or 2, further comprising reducing the transmission parameter of the Q-switching element from the high value to the low value substantially continuously over time.

[0116] 6. The method according to any of the foregoing aspects further includes reducing the transmission parameter of the Q-switching element from the high value more linearly in time during at least the first half of the inter-pulse cycle.

[0117] 7. The method according to any of the foregoing aspects further includes reducing the transmission parameter of the Q-switching element from the high value over time at the initial increasing rate before reducing the transmission parameter of the Q-switching element at a decreasing rate compared to the initial increasing rate over time.

[0118] 8. The method according to aspect 7 further comprises: instantaneously or relatively very rapidly reducing the transmission parameter of the Q-switching element from the high value to an intermediate value between the high value and the low value before reducing the transmission parameter of the Q-switching element to the low value.

[0119] 9. The method according to any of the foregoing aspects, wherein the conversion of the transmission parameter from the high value to the low value occurs in such a manner that a plurality of visible photons are always present in the cavity mode of the cavity.

[0120] 10. The method according to any of the foregoing aspects, wherein the transmission parameter of the Q-switching element is set based on the power to the Q-switching element, and the method further comprises:

[0121] By setting the power to the Q-switching element to a low power value, the transmission parameter of the Q-switching element is set to a high value, and

[0122] By gradually increasing the power of the Q-switching element from the low power value to the high power value over time, the transmission parameter of the Q-switching element is gradually decreased from the high value to the low value over time.

[0123] 11. The method according to aspect 10 of reference 3 further includes reducing the transmission parameter of the Q-switching element to the intermediate value by increasing the power to the Q-switching element to an intermediate power value between the low power value and the high power value.

[0124] 12. The method according to aspect 11 further includes reducing the transmission parameter of the Q-switching element to the intermediate value by gradually increasing the power to the Q-switching element to the intermediate power value over time.

[0125] 13. The method according to any of the foregoing aspects further includes using a non-square waveform to trigger the Q-switching element.

[0126] 14. The method according to any of the foregoing aspects, wherein the high value of the transmission parameter of the Q-switching element is sufficient to trigger the main laser pulse.

[0127] 15. The method according to any of the foregoing aspects, wherein the low value of the transmission parameter of the Q-switching element is sufficient to prevent pre-laser pulses from occurring.

[0128] 16. The method according to any of the foregoing aspects further includes using a first control feedback loop to control the cavity length of the cavity.

[0129] 17. The method according to any of the foregoing aspects further includes using a second control feedback loop to control the scaling of the transmission parameters of the Q-switching element.

[0130] 18. A method for generating EUV radiation, comprising receiving a pulsed laser beam output from a seed laser according to aspect 1, and directing the pulsed laser beam at a fuel target in a plasma formation region.

[0131] 19. A laser system configured to trigger a seed laser, the seed laser being defined by a cavity between a first reflector and a second reflector, and the seed laser including a Q-switching element disposed between the first reflector and the second reflector in the optical path of a laser beam to generate laser pulses by cyclically switching the transmission parameters of the Q-switching element between high and low values, the system being configured to:

[0132] The transmission parameter of the Q-switching element is set to the high value, and

[0133] Over time, the transmission parameter of the Q-switching element is gradually reduced from the high value to the low value.

[0134] 20. The laser system according to aspect 19, wherein the laser system includes a Q-switch driver for applying power to the Q-switch element and a control device for controlling the power applied by the Q-switch driver.

[0135] 21. A laser-generated plasma radiation source, comprising:

[0136] A fuel launcher, operable to provide a fuel target at the plasma formation region; and

[0137] The laser system according to aspect 19 or aspect 20.

[0138] 22. A photolithography system, comprising:

[0139] According to aspect 21, a laser-generated plasma radiation source; and

[0140] Photolithography equipment.

[0141] 23. A computer program including computer-readable instructions configured to cause a processor to perform the method according to any one of aspects 1 to 18.

[0142] 24. A computer-readable medium carrying a computer program according to aspect 23.

[0143] 25. A computer device, comprising:

[0144] Memory, which stores processor-readable instructions; and

[0145] A processor, the processor being configured to read and execute instructions stored in the memory;

[0146] The processor-readable instructions include instructions arranged to control the computer to perform the method according to any one of aspects 1 to 18.

[0147] While specific embodiments of the invention have been described above, it should be understood that the invention can be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. A method for triggering a seed laser, the seed laser being defined by a cavity between a first reflector and a second reflector, and including a Q-switching element disposed between the first reflector and the second reflector in the optical path of a laser beam to generate laser pulses by cyclically switching the transmission parameters of the Q-switching element between high and low values, the method comprising: The transmission parameter of the Q-switching element is set to the high value, and Over time, the transmission parameter of the Q-switching element is gradually reduced from the high value to the low value.

2. The method of claim 1, further comprising setting the transmission parameter of the Q-switching element to the high value instantaneously or relatively very rapidly.

3. The method according to any one of claims 1 or 2, further comprising reducing the transmission parameter of the Q-switching element from the high value to the low value in multiple steps. in, At least one of the multiple steps includes maintaining the transmission parameter of the Q-switching element at an intermediate value between the high value and the low value for a considerable period of time.

4. The method according to any one of claims 1 or 2, further comprising: The transmission parameter of the Q-switching element is reduced from the high value to the low value substantially continuously over time; and / or During at least the first half of the inter-pulse cycle, the transmission parameter of the Q-switching element is reduced from the high value more linearly in time.

5. The method according to any preceding claim, further comprising reducing the transmission parameter of the Q-switching element from the high value over time at the initial increasing rate before reducing the transmission parameter of the Q-switching element at a decreasing rate compared to the initial increasing rate over time. The method further includes: Before reducing the transmission parameter of the Q-switching element to the low value, the transmission parameter of the Q-switching element is instantaneously or relatively very quickly reduced from the high value to an intermediate value between the high value and the low value.

6. The method according to any of the preceding claims, wherein, The transmission parameter transitions from the high value to the low value in such a way that multiple visible photons are always present in the cavity mode of the cavity.

7. The method according to any of the preceding claims, wherein, The method further includes setting the transmission parameters of the Q-switching element based on the power supplied to the Q-switching element, and the method also includes: By setting the power to the Q-switching element to a low power value, the transmission parameter of the Q-switching element is set to a high value, and By gradually increasing the power of the Q-switching element from the low power value to the high power value over time, the transmission parameter of the Q-switching element is gradually decreased from the high value to the low value over time.

8. The method of claim 7 when referring to claim 3, further comprising reducing the transmission parameter of the Q-switching element to the intermediate value by: Increase the power to the Q-switching element to an intermediate power value between the low power value and the high power value; and / or Over time, the power of the Q-switching element is gradually increased to the intermediate power value.

9. The method according to any of the preceding claims further comprises using a non-square waveform to trigger the Q-switching element.

10. The method according to any of the preceding claims, wherein, The high value of the transmission parameter of the Q-switching element is sufficient to trigger the main laser pulse.

11. The method according to any of the preceding claims, wherein, The low value of the transmission parameter of the Q-switching element is sufficient to prevent pre-laser pulses from occurring.

12. The method according to any of the preceding claims further comprises using a first control feedback loop to control the cavity length of the cavity.

13. The method according to any of the preceding claims further comprises using a second control feedback loop to control the scaling of the transmission parameter of the Q-switching element.

14. A laser system configured to trigger a seed laser, the seed laser being defined by a cavity between a first reflector and a second reflector, and the seed laser including a Q-switching element disposed between the first reflector and the second reflector in the optical path of a laser beam to generate laser pulses by cyclically switching the transmission parameters of the Q-switching element between high and low values, the system being configured to: The transmission parameter of the Q-switching element is set to the high value, and Over time, the transmission parameter of the Q-switching element is gradually reduced from the high value to the low value; in, The laser system includes a Q-switch driver for applying power to the Q-switch element and a control device for controlling the power applied by the Q-switch driver.

15. A laser-generated plasma radiation source, comprising: A fuel launcher, operable to provide a fuel target at a plasma formation region; and The laser system according to claim 14.

16. A photolithography system, comprising: The laser-generated plasma radiation source according to claim 16; and Photolithography equipment.

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