Device for generating pulsed laser radiation, and method for generating pulsed laser radiation
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- TRUMPF LASERSYSTEMS FOR SEMICONDUCTOR MANUFACTURING SE
- Filing Date
- 2026-01-21
- Publication Date
- 2026-08-06
Smart Images
Figure EP2026051485_06082026_PF_FP_ABST
Abstract
Description
[0001] Device for generating pulsed laser radiation and
[0002] Method for generating pulsed laser radiation
[0003] The present invention relates to a device for generating pulsed laser radiation with a laser radiation source and with a circuit arrangement for providing a pulsed control current for the laser radiation source. The invention further relates to a method for generating pulsed laser radiation with a laser radiation source and with a circuit arrangement that provides a pulsed control current for the laser radiation source.
[0004] The invention can be used in EUV lithography systems, i.e., lithography systems that operate with EUV radiation and enable the imaging of fine and highly precise structures in the production of microchips. EUV radiation refers to the spectral range of electromagnetic radiation between 10 nm and 121 nm. To generate this extreme ultraviolet radiation, a target material is typically placed in a target area. Pulsed laser radiation is amplified by one or more optical amplifiers and focused onto the target material in the target area by a focusing device. This generates a plasma of the target material. The plasma then emits the desired EUV radiation.
[0005] When providing pulsed laser radiation, disruptive effects often occur that reduce the conversion efficiency of EUV generation, i.e., the ratio of the power of the laser radiation used for excitation to the power of the generated EUV radiation. For example, unwanted peaks in laser radiation power with a duration of one or more picoseconds can occur. Overshoot can also occur on the rising and falling edges of the laser radiation power.
[0006] Furthermore, the pulse shape of the pulsed laser radiation is typically only adjustable to a limited extent, meaning that certain pulse shapes enabling high amplification efficiency in the amplification of the laser radiation and high conversion efficiency in the generation of EUV radiation are not readily achievable. Against this background, the challenge arises to enable the provision of pulsed laser radiation for the generation of an EUV-emitting plasma with increased efficiency.
[0007] To solve the problem, a device for generating pulsed laser radiation according to claim 1 is proposed. This device comprises
[0008] a laser radiation source designed as a semiconductor laser, and
[0009] a circuit arrangement for providing a pulsed control current for the laser radiation source,
[0010] wherein the circuit arrangement is configured to provide the pulsed control current for the laser radiation source such that it has at least one control current pulse,
[0011] wherein the circuit arrangement is further configured to provide the pulsed control current such that it increases with a first slope from a first value below a predetermined threshold current strength,
[0012] and that, after reaching the specified threshold current, the pulsed control current initially increases with a second slope that is greater than the first slope, and the slope of the control current then decreases until a maximum value is reached.
[0013] and that the pulsed control current, after reaching the maximum value, falls with a third slope to below the threshold current strength.
[0014] The circuit arrangement of the device according to the invention enables the generation of pulsed laser radiation with a pulse shape which, on the one hand, allows efficient use of an optical amplifier to amplify the laser radiation and, on the other hand, enables efficient excitation of the target material to generate the EUV radiation.
[0015] It has been found that unwanted peaks in laser radiation power lasting one or more picoseconds can be avoided by increasing the control current from an initial value below a predetermined threshold current, for example, zero, with a first, lower slope. Only after reaching the predetermined threshold current does the second slope increase with a greater slope than the first. The initially preferably large slope of the control current decreases until the maximum value is reached. Suppressing the peaks in laser radiation power and increasing the current with a decreasing slope prevents an optical amplifier downstream of the laser radiation source from becoming saturated. Consequently, the amplification efficiency is increased. Furthermore, this enables the efficient generation of EUV radiation.
[0016] Since the control current, after reaching its maximum value, decreases with a third slope until it falls below the threshold current, unwanted oscillation of the radiation power is suppressed. The drop in the control current below the threshold current means that no base power needs to be provided. The power consumption of the laser radiation source and any optical amplifiers is reduced, further increasing efficiency.
[0017] In this context, the lasing threshold is understood to be the current at which the laser operation of the laser radiation source begins, i.e., stimulated emission is the dominant radiation process of the laser radiation source.
[0018] The circuit arrangement is configured such that the first and second slopes of the pulsed control current are each positive, i.e., an increase in current intensity. Furthermore, the circuit arrangement is configured such that the third slope of the pulsed control current is negative, i.e., a decrease in current intensity.
[0019] According to an advantageous embodiment of the invention, the laser radiation source is a laser diode. In a laser diode, a population inversion is generated by electrical pumping. The energy is supplied via the pulsed control current provided by the circuit arrangement. Population inversion occurs when the pulsed control current exceeds the threshold current, such that stimulated emission is the dominant radiation process and the laser diode emits laser radiation. The laser diode preferably comprises a semiconductor material selected from the following group of semiconductor materials: GaAs, GaP, InAs, InP, InGaAsP.
[0020] According to an advantageous embodiment of the invention, the circuit arrangement is configured to provide the pulsed control current for the laser radiation source in such a way that the pulsed control current has several control current pulses that are part of a pulse train with a predetermined period.
[0021] According to an advantageous embodiment of the invention, the circuit arrangement is configured to provide the pulsed control current for the laser radiation source such that the pulsed control current increases linearly with a first slope from the initial value below the threshold current. In this respect, the first slope can remain constant for a certain duration. Alternatively, the first slope can be variable within a predetermined range. In this case, the pulsed control current can increase non-linearly from the initial value below the threshold current, for example, following a logarithmic function.
[0022] According to an advantageous embodiment of the invention, the pulsed control current increases with the first slope for a first duration, wherein the ratio of the first duration to a second duration between reaching the threshold current and reaching the maximum value lies in the range of 0.01 to 0.1. Such a ratio of the aforementioned durations has proven advantageous for suppressing undesirable peaks in laser radiation power.
[0023] According to an advantageous embodiment of the invention, the pulsed control current is provided that it decays for a third duration with a third slope, wherein the ratio of the third duration to a second duration between reaching the threshold current and reaching the maximum value lies in the range of 0.001 to 0.01. Such a ratio of the aforementioned durations has proven advantageous for providing pulsed laser radiation with a steeply decaying edge of the laser radiation power.
[0024] According to an advantageous embodiment of the invention, the pulsed control current is designed to fall to a minimum value with the third slope, where the minimum value is 0 A. In this respect, the laser voltage source can assume a currentless state after reaching the minimum value. This suppresses undesired oscillations of the laser radiation power and undesired peaks of the laser radiation power near the falling edge of the laser pulse.
[0025] According to an advantageous embodiment of the invention, the circuit arrangement comprises a memory device with multiple memory cells and is configured to read the memory cells sequentially to generate a current pulse. This makes it possible to define a desired pulse shape as a sequence of values stored in the memory cells, particularly digital values, and to provide a pulsed control current with this pulse shape by sequentially reading the memory cells. The memory device is preferably designed as a semiconductor memory. Particularly preferably, the memory device is designed as a memory device with sequential access, which enables a particularly long access time. Alternatively, the memory device can be a random access memory (RAM) device.
[0026] According to an advantageous embodiment of the invention, the memory cells contain digital values, and the circuit arrangement includes a digital-to-analog converter (DAC) by which digital values read from the memory device can be converted into the pulsed control current. Preferably, the DAC is connected to an output of the memory device. The DAC can be configured to directly provide a current. Alternatively, the DAC can be configured to provide a voltage with which a driver can be controlled, which is configured to provide the pulsed control current.
[0027] According to an advantageous embodiment of the invention, the circuit arrangement is configured to repeatedly read the memory device to generate multiple control current pulses, which are part of a pulse sequence with a predetermined period. Preferably, the memory content of the memory device defines exactly one control current pulse. Therefore, a sequence of multiple control current pulses can be generated by repeatedly reading the memory device. The period of the pulse sequence can be set by selecting a time interval between individual readouts.
[0028] According to an advantageous embodiment of the invention, the circuit arrangement includes a clock generator configured to provide a clock signal for reading the memory device. The clock generator is preferably configured to provide a clock signal with a frequency in the range of 500 MHz to 2.5 GHz, and particularly preferably in the range of 1 GHz to 2.5 GHz.
[0029] Another object of the invention is an EUV driver laser for generating an EUV radiation-emitting plasma of a target material, with a device for generating pulsed laser radiation as described above. A tin material (Sb) can be used as the target material. Alternatively, target materials comprising xenon (Xe), gold (Au), and / or lithium (Li) can be used. The target material is preferably provided in droplet form. The EUV driver laser according to the present invention is, in principle, configured to generate the EUV radiation-emitting plasma of the target material without prior pre-excitation or pre-conditioning of the target material, while simultaneously achieving high conversion efficiency.To increase the amount of emitted EUV radiation, it can nevertheless be advantageous to first excite the target material with a so-called pre-pulse and then with a so-called main pulse. The pre-pulse and main pulse can originate from independent laser radiation sources or be generated by splitting the laser radiation from a single laser radiation source. Excitation with the pre-pulse causes preconditioning of the target material droplet. For example, the energy input from the pre-pulse can cause the target material droplet to form an approximately disk-like shape, thereby comparatively increasing the surface area of the target material droplet that can be excited by the main pulse. The main pulse then strikes the preconditioned target material droplet, thus exciting it. If the target material is a tin material, this creates a tin plasma.This tin plasma then emits the EUV radiation required for lithography. The EUV driver laser can also provide a so-called rarefaction pulse, which strikes the target material droplet between the pre-pulse and the main pulse. The rarefaction pulse can cause the target material droplet, which has been shaped into a disk by the pre-pulse, to become a cloud of target material, whereby the term "cloud" here is not to be equated with a gaseous or vapor state of the target material. The EUV driver laser can include a device, as described above, for generating pulsed laser radiation, which produces several pre-pulses, main pulses, and rarefaction pulses.Alternatively, the EUV driver laser can have separate devices for generating pulsed laser radiation, each configured to produce a pre-pulse, main pulse, and dilution pulse. The laser radiation source of the device preferably provides laser radiation with a wavelength in the range of 1600 nm to 2300 nm.
[0030] According to an advantageous embodiment of the invention, the EUV driver laser comprises an optical amplifier for amplifying the pulsed laser radiation emitted by the laser radiation source, wherein the optical amplifier is in particular a solid-state amplifier or a semiconductor amplifier. The optical amplifier can be configured as a fiber amplifier or as a crystal amplifier.
[0031] Another object of the invention is an EUV lithography system with an EUV driver laser disclosed above. The EUV lithography system can further comprise a target material generator and an EUV radiation generation chamber. Preferably, the target material generator is a droplet generator through which droplets of a target material can be injected into the EUV radiation generation chamber. To prevent damage to the system due to particle contamination, a high vacuum can be maintained within the EUV radiation generation chamber under a process gas atmosphere, wherein the process gas is preferably hydrogen (H₂) or helium (He).
[0032] Furthermore, to solve the aforementioned problem, a method for generating pulsed laser radiation is proposed.
[0033] with a laser radiation source designed as a semiconductor laser, and
[0034] with a circuit arrangement that provides a pulsed control current for the laser radiation source,
[0035] wherein the circuit arrangement provides the pulsed control current for the laser radiation source such that it has at least one control current pulse,
[0036] wherein the control current increases with a first slope from a first value below a predetermined threshold current strength,
[0037] wherein, after reaching the specified threshold current, the control current initially increases with a second slope that is greater than the first slope, and the slope of the control current then decreases until a maximum value is reached, wherein, after reaching a maximum value, the control current falls with a third slope to below the threshold current.
[0038] The same technical effects and advantages can be achieved with this method as have been explained in connection with the device according to the invention.
[0039] Alternatively or in addition to the advantageous embodiments explained above, the advantageous embodiments and features explained in connection with the device according to the invention can be used in the method.
[0040] Further details and advantages of the invention will be explained below with reference to the exemplary embodiments shown in the figures. These show:
[0041] Fig. 1 shows an embodiment of an EUV lithography system according to an embodiment of the invention in a schematic representation;
[0042] Fig. 2 shows a schematic representation of an embodiment of a device for generating pulsed laser radiation according to an embodiment of the invention; Fig. 3 shows a time course of a control current for the laser radiation source of the device according to Fig. 2; and
[0043] Fig. 4 shows a time course of the voltage or the radiant power of the laser radiation source when controlled with a control current according to Fig.
[0044] 4.
[0045] Figure 1 shows an embodiment of an extreme ultraviolet (EUV) lithography system 101 with an EUV driver laser 100 that supplies laser radiation to a focusing device 3, which focuses the laser radiation 11 onto a target area 40. A target material is arranged in the target area 40, which emits EUV radiation when irradiated with the laser radiation. According to the embodiment, the target area 40 is arranged in an EUV radiation generation chamber 4, in which vacuum conditions prevail. The target material can be, for example, tin, which is provided in droplet form. The tin droplet can be preconditioned by a pre-pulse and, optionally, a dilution pulse, so that the tin droplet expands, vaporizes, ionizes, and / or generates a weak or possibly strong plasma.A main pulse, shortly following the pre-pulse, can convert the essential part of the material affected by the pre-pulse into the plasma state, thereby generating the EUV radiation 42. This EUV radiation 42 is then supplied to an exposure unit 5 of the lithography system 10, in which the EUV radiation 42 can be used to expose semiconductor substrates.
[0046] According to the exemplary embodiment, the EUV driver laser 100 comprises a device 1 by which pulsed laser radiation 11 can be generated. Although only one device 1 for generating pulsed laser radiation is shown here, several devices 1 can be provided to, for example, generate a pre-pulse, optionally a dilution pulse, and a main pulse of the laser radiation 11 separately, each following shortly after the other and focused onto a target surface 40. The pulsed laser radiation 11 generated by the device 1 is amplified by an optical amplifier 2. The optical amplifier 2 is designed as a solid-state amplifier or a semiconductor amplifier.
[0047] Figure 2 shows an embodiment of a device 1 according to the invention for generating pulsed laser radiation 11. The device 1 comprises a laser radiation source 16, which is preferably designed as a laser diode. A circuit arrangement 18 is provided as a further component of the device 1 to supply a pulsed control current for the laser radiation source 16. The circuit arrangement 18 comprises a control unit 12, a storage device 14, a clock generator 13, and a digital-to-analog converter 15. The storage device 14 comprises several memory cells in which digital values are stored that define a pulse shape of the control current. The circuit arrangement, in particular the control unit 12, is configured to read the memory cells of the storage device 14 sequentially in order to provide the stored digital values to the digital-to-analog converter 15 as a data sequence.The digital values are output at a clock signal provided by the clock generator 13. These digital values are fed to the digital-to-analog converter 15 at a controlled rate. The digital-to-analog converter 15 converts the digital values read from the memory device 14 into the control current for the laser radiation source 16.
[0048] To generate a control current pulse, the memory device 14 is read once, so that a corresponding sequence of digital values is transferred to the digital-to-analog converter 15. This provides an analog control current pulse at the input of the laser radiation source 16. The control device 12 of the circuit arrangement 18 is configured to read the memory device 14 multiple times to generate several successive control current pulses, thus enabling the generation of a pulse sequence with a predefined period.
[0049] The laser radiation source 16 is coupled to an optical fiber 17, which directs the laser radiation emitted by the laser radiation source 16 to an optical amplifier 2. The optical amplifier 2 is preferably designed as a solid-state amplifier or a semiconductor amplifier. The optical amplifier 2 can output the amplified laser radiation via a further optical fiber 21.
[0050] Figure 3 shows an exemplary time course of a control current for the laser radiation source 16 of the device 1 according to Figure 2. The digital value I, proportional to the control current and read from the storage device 14, is plotted against time t in nanoseconds (ns).
[0051] The control current for the laser radiation source 16 is provided by the circuit arrangement 18 such that the control current has at least one control current pulse, wherein the control current increases with a first slope from a first value A below a predetermined threshold current, which corresponds to the threshold value T shown in Fig. 3. The duration of this first slope is marked with the reference symbol At1 in Fig. 3. Thus, the increase from the value A, here zero, to the threshold value T is linear.
[0052] After reaching the predetermined threshold current, in Fig. 3 the threshold value T, the control current initially increases with a second slope that is greater than the first. The slope of the control current then decreases until a maximum value M is reached. The time between reaching the threshold current and reaching the maximum value M is marked with the reference symbol At2 in Fig. 3. After reaching the maximum value M, the control current decreases with a third, negative slope until it falls below the threshold current, here to zero. The duration of this third, negative slope is marked with the reference symbol At3 in Fig. 3.
[0053] The control current pulse generated by the circuit arrangement 18 can provide an optical pulse which enables both efficient use of the optical amplifier 2 and efficient excitation of the target material to generate the EU radiation.
[0054] Figure 4 shows an exemplary time course of the laser radiation power of the laser radiation source 16 of the device 1 according to Figure 2, as it can be obtained when excited with the control current explained with reference to Figure 3. The voltage U across the laser radiation source 16, proportional to the laser radiation power, is plotted in millivolts (mV) against time (nanoseconds, ns). The time axis is not synchronized with the time axis in Figure 3.
[0055] It is evident that unwanted peaks in laser radiation power with a duration in the range of one or more picoseconds are suppressed. No unwanted ringing after the falling edge of the optical pulse is observed. Furthermore, the drop in the control current below the threshold current level results in no stimulated emission of laser radiation occurring between two control current pulses.
[0056] 1 Device for generating pulsed laser radiation 2 Optical amplifiers
[0057] 3 Focusing device
[0058] 4 EUV radiation generation chamber
[0059] 5 Exposure unit
[0060] 11 Laser radiation
[0061] 12 Control unit
[0062] 13 Clock generators
[0063] 14 Storage setup
[0064] 15 Digital-to-Analog Converters
[0065] 16 Laser radiation source
[0066] 17 optical fibers
[0067] 18 Circuit arrangement
[0068] 21 optical fiber
[0069] 40 Target area
[0070] 42 EUV radiation
[0071] 100 EUV driver lasers
[0072] 101 EUV lithography system
[0073] A first value
[0074] T threshold current
[0075] I digital value
[0076] t time
Claims
Patent claims:
1. Device (1) for generating pulsed laser radiation (11), with a laser radiation source (16) designed as a semiconductor laser and with a circuit arrangement (18) for providing a pulsed control current for the laser radiation source (16), characterized in that the circuit arrangement (18) is configured to provide the pulsed control current for the laser radiation source (16) such that it has at least one control current pulse, wherein the circuit arrangement (18) is further configured to provide the pulsed control current such that it increases with a first slope from a first value below a predetermined threshold current strength, and that, after reaching the specified threshold current, the pulsed control current initially increases with a second slope that is greater than the first slope, and then the slope of the control current decreases until a maximum value is reached. and that the pulsed control current, after reaching the maximum value, falls with a third slope to below the threshold current strength.
2. Device (1) according to claim 1, characterized in that the laser radiation source (16) is a laser diode.
3. Device (1) according to one of the preceding claims, characterized in that the circuit arrangement (18) is configured to provide the pulsed control current for the laser radiation source (16) such that the pulsed control current has several control current pulses which are part of a pulse train with a predetermined period.
4. Device (1) according to one of the preceding claims, characterized in that the circuit arrangement (18) is configured to provide the pulsed control current for the laser radiation source (16) such that the pulsed control current increases linearly with the first slope from the first value below the threshold current strength.
5. Device (1) according to one of the preceding claims, characterized in that the pulsed control current increases with the first slope for a first duration (At1), wherein a ratio of the first duration (At1) to a second duration (At2) between reaching the threshold current and reaching the maximum value is in the range of 0.01 to 0.
1.
6. Device (1) according to one of the preceding claims, characterized in that the pulsed control current decreases for a third duration (At3) with the third slope, wherein a ratio of the third duration (At3) to a second duration (At2) between reaching the threshold current and reaching the maximum value is in the range of 0.001 to 0.
01.
7. Device (1) according to one of the preceding claims, characterized in that the pulsed control current with the third slope falls to a minimum value, wherein the minimum value is 0 A.
8. Device (1) according to one of the preceding claims, characterized in that the circuit arrangement (18) has a storage device (14) with several storage cells and the circuit arrangement (18) is configured to read the storage cells one after the other to generate a current pulse.
9. Device (1) according to claim 8, characterized in that the memory cells contain digital values and the circuit arrangement (18) has a digital-to-analog converter (15) by which digital values read from the storage device (14) can be converted into the control current.
10. Device (1) according to one of claims 8 or 9, characterized in that the circuit arrangement (18) is configured to read the storage device (14) multiple times to generate several control current pulses which are part of a pulse sequence with a predetermined period.
11. Device (1) according to one of claims 8 to 10, characterized in that the circuit arrangement (18) has a clock generator (13) which is configured to provide a clock for reading the storage device (14).
12. EUV driver laser (100) for generating an EUV radiation (42) emitting plasma of a target material, comprising a device (1) for generating pulsed laser radiation (11) according to any one of the preceding claims.
13. EUV driver laser (100) according to claim 12, characterized by an optical amplifier (2) for amplifying the pulsed laser radiation (11) emitted by the laser radiation source (16), wherein the optical amplifier (2) is in particular a solid-state amplifier or a semiconductor amplifier.
14. EUV lithography system (101) with an EUV driver laser (100) according to claim 13.
15. Method for generating pulsed laser radiation (11), with a laser radiation source (16) designed as a semiconductor laser, and with a circuit arrangement (18) that provides a pulsed control current for the laser radiation source (16), characterized in that the circuit arrangement (18) provides the pulsed control current for the laser radiation source (16) such that it has at least one control current pulse, wherein the control current increases with a first slope from a first value below a predetermined threshold current strength, wherein, after reaching the specified threshold current, the control current initially increases with a second slope that is greater than the first slope, and the slope of the control current then decreases until a maximum value is reached, where, after reaching a maximum value, the control current decreases with a third slope until it falls below the threshold current.