EUV radiation generating method

Abstract

A method of adjusting an amount of extreme ultraviolet (EUV) radiation generated by directing laser pulses onto liquid fuel targets to generate EUV emitting plasma, the laser pulses having a wavelength between 1.6 microns and 2.5 microns, wherein the method comprises adjusting an intensity of the laser pulses, wherein the laser pulses burn fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses burn fully through the liquid fuel targets after the intensity of the laser pulses is adjusted.

Description

EUV RADIATION GENERATING METHODCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of US application 63 / 662,721 which was filed on June 21, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method of generating extreme ultraviolet (EUV) radiation.BACKGROUND

[0003] Light generated by means of a radiation source can be used by exposure apparatuses for semiconductor manufacturing processes. Examples of such exposure apparatuses are a lithographic apparatus, a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (e.g., a photoresist or resist) provided on a substrate. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses EUV radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] An (actinic) mask inspection apparatus is an apparatus that is configured for measuring dimensions or detecting defects in masks or mask blanks. EUV lithography uses reflective mirrors instead of lenses as optics. Mask blanks used in EUV lithography generally have a multilayer structure which functions as a Bragg reflector. The multilayers may be altematingly Molybdenum and Silicon. If a defect exists in this structure, the projected pattern will be deformed in the lithographic process. Therefore, mask inspection to check whether a defect is present is considered a requirement for a massproduction process. EUV mask inspection may be used for several purposes and in several different stages. Firstly, it can be used for the detection of phase defects that may occur in mask blanks. Such phase defects may occur during the manufacturing of the multilayer stack of the mask blank. If undetected, these phase defects are printed on all chips printed with the part of a mask containing the phase defects. Such phase defects may be correctly detected by using the same or similar (13.5nm) actinic EUV wavelength as the lithography tool. Secondly, mask inspection can be used for patterned mask inspection and can be carried out for the quality control of EUV patterned masks. For example,the mask inspection can be used to measure critical dimensions on the mask blank. In addition to phase defects, absorber pattern defects on the surface can be detected. Thirdly, mask inspection can be used for simulating exposure and determining the deterioration of optical contrast of a defect detected in the actinic inspection. Fourthly, the mask inspection can be used for optical proximity correction (OPC) evaluation or during mask repair process so as to improve pattern transfer fidelity. Further, it can be used for inspecting optical contrast after fixing the defect. In addition to the above, mask inspection can also be used to measure small particle / amplitude effects.

[0006] EUV radiation may be produced by a laser produced plasma (LPP) radiation source. Within an LPP radiation source, a laser beam may be used to irradiate fuel droplets so as to produce a plasma which will emit EUV radiation.

[0007] A problem associated with LPP radiation sources is that it may be difficult to control the power of EUV radiation provided by the LPP radiation source.

[0008] It may be desirable to control EUV radiation generation in a manner which is not disclosed or suggested by the prior art.SUMMARY

[0009] According to a first aspect of the invention there is provided a method of adjusting an amount of extreme ultraviolet (EUV) radiation generated by directing laser pulses onto liquid fuel targets to generate EUV emitting plasma, the laser pulses having a wavelength between 1.6 microns and 2.5 microns, wherein the method comprises adjusting an intensity of the laser pulses, wherein the laser pulses burn fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses bum fully through the liquid fuel targets after the intensity of th9e laser pulses is adjusted.

[0010] Advantageously, the method allows adjustment of EUV radiation power whilst avoiding incomplete burning of liquid fuel targets which would cause a build-up of contamination.

[0011] The duration of the laser pulses may also be adjusted.

[0012] The intensity of the laser pulses may be increased when the duration of the laser pulses is decreased.

[0013] The intensity of the laser pulses may be decreased when the duration of the laser pulses is increased.

[0014] The adjustment of the duration of the laser pulses may compensate for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains substantially unchanged.

[0015] The adjustment of the duration of the laser pulses may partially compensate for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains within a desired range.

[0016] The liquid fuel targets may be tin droplets with a diameter of at least 30 microns.

[0017] The adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses may be determined by a controller using a calibration curve or a look-up table.

[0018] The adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses may be determined by a controller using an output from a sensor which measures the power of generated EUV radiation.

[0019] According to a second aspect of the invention there is provided an extreme ultraviolet (EUV) radiation source comprising a nozzle configured to direct liquid fuel targets towards a plasma formation location, and a laser system configured to direct laser pulses having a wavelength between 1.6 microns and 2.5 microns to the plasma formation location, the EUV radiation source further comprising a controller configured to adjust an intensity of the laser pulses, wherein the laser pulses bum fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses burn fully through the liquid fuel targets after the intensity of the laser pulses is adjusted.

[0020] The controller may be configured to also adjust the duration of the laser pulses.

[0021] The controller may be configured to increase the intensity of the laser pulses and decrease the duration of the laser pulses.

[0022] The controller may be configured to decrease the intensity of the laser pulses and increase the duration of the laser pulses.

[0023] The adjustment of the duration of the laser pulses may compensate for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains substantially unchanged.

[0024] The adjustment of the duration of the laser pulses may partially compensate for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains within a desired range.

[0025] The nozzle may be configured to provide liquid fuel targets which are tin droplets with a diameter of at least 30 microns.

[0026] The controller may be configured to determine the adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses using a calibration curve or a look-up table.

[0027] The controller may be configured to determine the adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses using an output from a sensor which measures the power of generated EUV radiation.

[0028] Features of different aspects of the invention may be combined together.

[0029] According to another aspect of the invention, there is provided a lithographic apparatus as disclosed in the first aspect of the invention.

[0030] In a further aspect of the invention , there is presented a method of manufacturing a semiconductor device comprising the steps of receiving a substrate with a photoresist layer and directing radiation from the EUV radiation source as disclosed in the first aspect of the invention, wherein the EUV radiation is generated by directing laser pulses onto liquid fuel targets to generate EUV emitting plasma, the laser pulses having a wavelength between 1.6 microns and 2.5 microns, wherein the method comprises adjusting an intensity of the laser pulses, wherein the laser pulses burn fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses bum fully through the liquid fuel targets after the intensity of the laser pulses is adjusted; wherein the EUVradiation is arranged to transfer a pattern from a mask onto the photoresist layer; and removing a portion of the photoresist layer to form the pattern over the substrate.BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 schematically depicts a lithographic system comprising a lithographic apparatus and further comprising a radiation source according to an embodiment of the invention;Figure 2 schematically depicts a system for (actinic) mask inspection which comprises a radiation source according to an embodiment of the invention;Figure 3 schematically depicts a radiation source according to an embodiment of the invention;Figure 4 is a graph which shows changes of conversion efficiency from laser radiation to EUV radiation provided by the radiation source, as a function of laser pulse intensity; andFigures 5a, 5b, and 5c depict laser pulses having different durations and intensities which may be used by an embodiment of the invention.DETAILED DESCRIPTION

[0032] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic 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 lithographic apparatus LA. The lithographic 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.

[0033] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0034] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning deviceMA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0035] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0036] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0037] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0038] Figure 2 schematically depicts a system MS for (actinic) mask inspection. The mask inspection system MS can be used to identify or inspect defects in a mask to be used in a lithographic process by means of the lithographic apparatus described in figure 1. The mask inspection system MS comprises a radiation source SO, an illumination system ILM, a detection system DS and a mask stage ME.

[0039] The illumination system ILM is configured to condition an EUV radiation beam B before the EUV radiation beam B is incident upon the mask MA. The illumination system ILM may provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system ILM may comprise a plurality of mirrors 22. The illumination system ILM may comprise one or more faceted mirror devices.

[0040] The mask stage ME may be configured to move the mask MA relative to the EUV radiation beam B, so that the EUV radiation beam is incident upon different areas of the mask.

[0041] The detection system DS comprises a detector 20, and may in addition comprise a plurality of mirrors 24. The plurality of mirrors 24 may be configured to collect EUV radiation BR that has been reflected from the mask MA, and form an image of the mask MA on the detector 20 (which may be an imaging array). A processor (not depicted) may receive signals output from the detector 20 and use those signals to look for defects in the mask MA.

[0042] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0043] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0044] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system (if present) may together be considered to be a radiation system.

[0045] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0046] The radiation source SO of the mask inspection system MS may correspond with the radiation source SO depicted in Figure 1. In the same way as depicted in Figure 1, the radiation source SO of the mask inspection system MS may focus the EUV radiation beam B to form an intermediate focus 6.

[0047] Figure 3 schematically depicts a radiation source SO according to an embodiment of the invention. The radiation source SO comprises a fuel generator 3 configured to direct tin droplets to a plasma formation region 4. A tin droplet 34 is depicted at the plasma formation region 4. The fuel generator 3 includes a nozzle 36 from which tin droplets are ejected. The diameter of the tin droplets is determined by the diameter of the nozzle 36. The nozzle includes an actuator (not depicted) which modulates the nozzle 36. The modulation frequency applied to the actuator determines the repetition rate at which tin droplets are ejected from the nozzle 36. Liquid tin is held under pressure in the fuel generator 3. The pressure at which the liquid tin is held determines the velocity of tin droplets ejected from the nozzle 36.

[0048] Figure 3 also depicts a laser system 1 of the radiation source SO. The laser system 1 comprises a laser 30, a seed laser system 33, electro-optic modulators 40, 42, focussing optics 38, and a controller CT. The laser system 1 may include other elements. The electro-optic modulators 40, 42 may beomitted in some embodiments. In other embodiments one or more electro-optic modulators may be provided at other locations.

[0049] The laser 30 may for example be a Tm:YLF laser. The laser 30 is configured to provide a laser beam 2. The laser beam 2 comprises laser pulses which are incident upon tin droplets 34 (one tin droplet is schematically depicted). The laser beam 2 has a wavelength between 1.6 microns and 2.5 microns (e.g. 1.9 microns). The wavelength of the laser beam 2 may be referred to as 2 microns for brevity. One pulse of laser radiation is incident upon each tin droplet. The laser 30 may comprise a gain medium (e.g. Tm:YLF) which is pumped by an optical pump (which may for example comprise one or more solid state lasers or LEDs, or one or more optical fiber lasers). The laser 30 may be electrically pumped. Other examples of lasers that may be used are Tm:YAG and Ho:YAG.

[0050] The seed laser system 33 in this embodiment comprises a seed laser 31 and an electro-optic modulator 40. The seed laser 31 may for example be an electrically pumped laser diode. The seed laser may for example be an optically pumped crystal located between mirrors which define a laser cavity. The seed laser may be a mode locked oscillator in use. Any suitable pulsed laser may be used as the seed laser. The seed laser 31 provides a pulsed seed laser beam 39 which is coupled into the laser 30. The seed laser beam 39 may have a wavelength of 790 nm (or some other wavelength).

[0051] In use, the gain medium of the laser 30 is pumped to provide a population inversion. The seed laser 31 provides a seed laser pulse which enters a gain medium of the laser 30 and causes the laser 30 to emit a laser beam 2. The laser beam 2 is pulsed, with a repetition rate of pulses of the laser beam corresponding with the repetition rate of pulses of the seed laser beam 39 (although the duration of the laser pulses may differ from the duration of the seed laser pulses as explained further below). In some embodiments the laser 30 may be considered to be a laser amplifier.

[0052] The optics 38 may comprise one or more focussing elements (e.g. one or more lenses). The optics 38 may form a waist of the laser beam 2 at or adjacent to the plasma formation region 4. The optics 38 (and / or other optics) may be referred to as optics of the radiation source.

[0053] The controller CT is configured to control a property of the laser beam 2 that is received at the plasma formation location 4.

[0054] The controller CT may for example control the duration of pulses of the laser beam 2. Controlling the duration of pulses of the laser beam 2 is one way in which the power of EUV radiation provided by the LPP source SO can be controlled.

[0055] The laser pulse duration may be controlled by the controller CT in a variety of ways. The laser pulse duration may be controlled by controlling the duration of seed laser pulses of the seed laser beam 39. The controller CT may control the modulator 40 which is configured to control the duration of the seed laser pulses. The modulator 40 may be synchronised to the seed laser 31, and may be configured to reduce the duration of pulses of the seed laser beam 39, for example by removing a leading portion or a trailing portion of the pulses. The modulator 40 may for example be an acousto-optic modulator or an electro-optic modulator. The modulator may be a Pockels cell. The modulator 40 may be referredto as a seed laser beam modulator 40. When the seed laser beam 39 has shorter pulses, shorter pulses are emitted by the laser 30. In some embodiments, the modulator 40 may be located within a laser cavity of the seed laser 31. In other embodiments, the modulator may be located downstream of the seed laser 31.

[0056] The controller CT may control a modulator 42 which is configured to control the duration of pulses of the laser beam 2. The modulator 42 may be synchronised to the seed laser 31 (pulses of the laser beam 2 are also synchronised to the seed laser). The modulator 42 may be configured to reduce the duration of pulses of the laser beam 2, for example by remove a leading portion or a trailing portion of the pulses. The modulator 42 may for example be an acousto-optic modulator or an electro-optic modulator. The modulator 42 may be a Pockels cell. The modulator 42 may be referred to as a laser beam modulator 42. This alternative arrangement may be less efficient than the preceding embodiment. This is because the amount of energy that is removed from the laser beam 2 by the laser beam modulator 42 is much greater than the amount of energy that is removed from the seed laser beam 39 by the seed laser beam modulator 40.

[0057] In a further alternative arrangement (not depicted), the seed laser 31 may comprise a plurality of lasers (e.g. fiber lasers) which have delays relative to each other. The predetermined delays may be configured such the pulses output from the plurality of lasers have a temporal overlap. These pulses may be referred to as sub-pulses for ease of reference. A seed pulse will have a duration that depends upon the relative delays of the sub-pulses. The seed laser pulse can be made shorter by removing a subpulse which is at the front or the rear of the seed laser pulse. More than one sub-pulse may be removed. Removal of a sub-pulse may be achieved by switching off a laser which provides that sub-pulse.

[0058] The above arrangements may change the duration of laser pulses of the laser beam 2 without changing the intensity of the pulses of the laser beam That is, a portion of the front and / or rear of a pulse of the laser beam 2 is removed, but the remainder of the pulse is substantially unchanged.

[0059] The controller CT may for example control the intensity of the pulses of the laser beam 2. Controlling the intensity of pulses of the laser beam 2 is one way in which the power of EUV radiation provided by the LPP source SO can be controlled. The intensity of the pulses of the laser beam 2 may be controlled by the controller CT, by controlling the pump (e.g. optical or electrical pump) which is used to provide the population inversion in the gain medium of the laser 30. Other mechanisms may be used to control the intensity of the pulses of the laser beam 2.

[0060] The effect of controlling the pulse duration and / or the intensity of pulses of the laser beam 2 upon EUV radiation generation is explained further below.

[0061] The tin droplets 34 provided by the fuel generator 3 may be spherical. The tin droplets 34 may have a diameter which is less than the diameter of the laser beam 2 at the plasma formation location 4 (i.e. the location where the laser beam is incident upon the tin droplet). For example, the tin droplet 34 may have a diameter which is less than half of the diameter of the laser beam 2 at the plasma formation region 4.

[0062] The laser beam 2 may for example have a wavelength of at least 1.6 microns. The laser beam 2 may for example have a wavelength of up to 2.5 microns. The laser beam 2 may for example have a wavelength of around 1.9 microns. The gain medium of the laser 30 may be selected to provide a desired wavelength of the laser beam 2. For example, a Tm:YLF laser 30 may provide a laser beam 2 having a wavelength of around 1.9 microns (e.g. 1.88 microns). The gain medium for Tm:YLF may for example be a slab, a disk or a fiber architecture. Other laser gain media may be used.

[0063] A combination of a seed laser diode (e.g. GaAs) emitting a laser beam at a wavelength of 790nm with a Tm:YLF laser emitting a laser beam with a wavelength of 1.88 microns may be particularly advantageous. Laser diodes emitting at 790nm are commercially available with a high power and high efficiency. Tm:YLF has an absorption peak at around 790nm. Fast non-radiative cross-relaxation will populate an upper level of the Tm:YLF, thereby providing particularly effective pumping of the Tm:YLF. Furthermore, the3F4 level of Tm:YLF which emits photons at 1.88 nm has a relatively long lifetime of around 15ms, thereby providing good energy storage and allows efficient multi-pulse extraction (i.e. multiple pulses used to fully extract a population inversion). Other gain media may be used which allow multi-pulse extraction.

[0064] Each tin droplet 34 may receive a single pulse of the laser beam 2 without also receiving any laser pre-pulses. That is, unlike conventional LPP radiation sources, embodiments of the invention do not require pre-pulses. Advantageously, this simplifies the LPP radiation source SO. A pre-pulse is not used to modify the tin droplet to a disk shape before the laser pulse (as is done in prior art LPP radiation sources). Modification of a tin droplet to a disk shape is very sensitive to shock waves generated from vaporization of the preceding droplet. Embodiments of the invention do not use a prepulse and do not require the tin droplet to have a disk shape. Embodiments of the invention are more tolerant to misshapen tin droplets at the plasma formation location, and thus are more tolerant to shock waves generated by a preceding droplet. This means that a separation between successive tin droplets can be reduced (compared with prior art LPP radiation sources). For example, the separation between droplets may be as low as 1mm.

[0065] The single laser pulse converts the tin droplet 34 to EUV emitting plasma. The tin droplet transitions from liquid, to vapour with some liquid. The vapour is ionized to become a plasma. The plasma cools and self-emits light. This continues until all of the liquid tin has been used up, at which point the plasma then extinguishes itself.

[0066] Embodiments of the invention may use tin droplets having a diameter of 30 microns or more, for example 40 microns or more (e.g. up to 50 microns). Embodiments of the invention may use tin droplets having any size. When a laser pulse having a wavelength of between 1.6 microns and 2.5 microns is incident upon a tin fuel droplet with a diameter of 40 microns or more, the amount of EUV radiation which is emitted from the resulting EUV emitting plasma may be lOOmJ or more. This is considerably greater than the amount of EUV radiation emitted by EUV emitting plasma in a conventional LPP source. This relatively large amount of EUV radiation per tin fuel droplet means thatit may not be feasible to adjust a dose of EUV radiation being received by a substrate by selectively omitting tin fuel droplets from EUV radiation generation. That is, selectively omitting a tin fuel droplet from EUV radiation generation will cause a large change (e.g. lOOmJ or more) of the amount of EUV radiation delivered to an exposure area of a substrate. This change may be too coarse for desirable dose control adjustments. In addition, omitting a tin droplet from EUV radiation generation may contribute undesirable debris within the radiation source SO.

[0067] Embodiments of the invention address this issue by adjusting the amount of EUV radiation which is emitted from a tin droplet. This is achieved by adjusting the laser pulse intensity and / or the laser pulse duration for a laser pulse which is incident upon the tin droplet. The adjustment is achieved without relying upon partial burning of a laser pulse through a liquid fuel target (as is explained below).

[0068] The amount of EUV radiation which is generated by a tin fuel droplet has a dependency upon the proportion of the fuel droplet which is converted to EUV emitting plasma. For example, if a laser pulse converts only half of a tin droplet into an EUV emitting plasma, then significantly less EUV radiation is emitted compared with if the laser pulse converts the entire tin droplet into an EUV emitting plasma. As is schematically depicted in Figure 3 the laser beam 2, which consists of a single laser pulse for a given tin droplet, is incident upon the fuel droplet 34 from one side (the left hand side in the depicted embodiment). In the schematic depiction of Figure 3, the laser pulse first converts the left hand side of the tin droplet into EUV emitting plasma. The laser pulse travels through the tin droplet 34 converting the centre of the tin droplet and then the right hand side of the tin droplet to EUV emitting plasma. Eventually, the laser pulse reaches the right hand side of the tin droplet 34, at which point conversion of the tin droplet into EUV emitting plasma is complete. The passage of the laser pulse from one side of the tin droplet to the other side, converting the tin droplet into plasma, may be referred to as the laser pulse burning through the tin droplet. When the laser pulse has reached the opposite side of the tin droplet from the incident side, this may be referred to as the laser beam having fully burned through the tin droplet.

[0069] If it is desired to reduce an amount of EUV radiation incident upon a substrate then this could be achieved by only partially burning through the tin droplet. However, only partially burning through a tin droplet is problematic because a substantial portion of the tin remains in liquid form and is liable to cause contamination of the radiation source SO.

[0070] Embodiments of the invention avoid this issue by adjusting the amount of EUV emitting plasma which is generated whilst at the same time fully burning through the tin droplet. In some instances a small proportion of the tin droplet may remain in liquid form, for example due to some tin being ejected away from the tin droplet in the form of nano / micro droplets which accelerate away from the plasma formation region. Thus, a small amount of contamination may be generated. However, this amount of contamination may generally correspond with an amount of contamination which would ordinarily be generated without adjustments of the amount of EUV radiation being made. By fullyburning through the tin droplet to a side of the droplet which is opposite to a side upon which the laser pulse is incident, excessive contamination is avoided.

[0071] Conversion Efficiency (CE) is a measurement of the energy of EUV radiation emitted by the plasma over a solid angle that is collected by the collector 5 of the radiation source SO, as a function of the energy of the laser pulse which is incident upon the tin droplet. The conversion efficiency may for example be between around 1 and 5%. The conversion efficiency changes as a function of the intensity of the incident laser pulse. In particular, when the intensity of the laser pulse is increased beyond an optimum intensity, the resulting EUV emitting plasma is heated beyond an optimum temperature. As a result although the amount of EUV radiation emitted increases, the increase is not always proportional to the increase of laser pulse intensity.

[0072] Figure 4 is a graph which depicts conversion efficiency (CE) of conversion to EUV radiation of tin droplets, as a function of the intensity of laser pulses which are incident upon the tin droplets. The laser pulses have a wavelength of 1.9 microns. The conversion efficiency is indicated as a percentage of the power of the energy of the laser pulse.

[0073] As is depicted, the conversion efficiency is at a maximum at around 0.8 xlO11W / cm2. In practice, there may be some variation of the laser pulse intensity which gives the maximum conversion efficiency. The variation may arise for example from variation of the spatial profile of the laser pulse, or from variation of the shape of the tin droplet. In Figure 4, two sets of data were obtained at different times, as indicated by disks with different darkness. It can be seen that there is a variation between the conversion efficiency for each set of data. However, the variation is small.

[0074] As can be seen from Figure 4, the laser pulse intensity can be increased to obtain more EUV radiation, but the efficiency with which the EUV radiation is generated is reduced. Similarly, the laser pulse intensity can be reduced to obtain less EUV radiation, and again the efficiency with which EUV radiation is generated is reduced.

[0075] The energy of EUV radiation which is provided from a single tin droplet may be expressed as follows:EUV (mJ) = I * T * CE(I) Equation 1 where EUV (mJ) is the energy of EUV radiation, I is the intensity of the laser pulse, T is the length of the pulse which is incident upon the tin droplet, and CE(I) is conversion efficiency (which as noted above is a function of the intensity of the laser pulse).

[0076] The rate at which tin is burned through (converted to plasma) by a laser beam does not scale linearly with the intensity of the laser pulse, but instead increases approximately as the square root of the laser pulse intensity. Conversion of tin to plasma (burning through the tin) may be referred as mass use. The amount of tin converted to plasma by a laser pulse may be expressed as follows:mass use = \ / I * T, Equation 2 where I is the intensity of the laser pulse which is incident upon the tin droplet and T is the length of the laser pulse. The square root relationship is approximate, and a different rate of tin burn may apply. For example, the rate of tin burn may be up to around 1°6. In general, the rate of tin bum may be Ixwhere x is between 0.5 and 0.6.

[0077] The term “intensity of the laser pulse” is intended to mean the intensity of a portion of the laser pulse which has a substantially constant intensity. Schematic examples of such laser pulses are depicted in Figure 5 (described further below).

[0078] Embodiments of the invention are able to adjust the EUV radiation provided from a tin droplet by adjusting the intensity and duration of the laser pulse which is incident upon the tin droplet.

[0079] A laser pulse which is incident upon a tin droplet may have a nominal intensity of 1 and may have a duration of 200 ns. This is schematically depicted by the laser pulse 60b in Figure 5B. The duration of the rising and falling edges of the laser pulse 60b is not included in the laser pulse duration of 200 ns. The nominal intensity of 1 may be the intensity at which conversion efficiency is at a maximum (e.g. 0.8 W / cm2). This may provide a nominal EUV energy of 1. The duration of the laser pulse may be sufficient to fully bum through the tin droplet. This is schematically indicated by a vertical dashed line at time FB which indicates full bum-through of the tin droplet.

[0080] The intensity of the laser pulse may be doubled, to a nominal intensity of 2. Where this is the case, the conversion efficiency may reduce slightly (e.g. by roughly 5%). The rate at which the laser pulse bums through the tin droplet may be increased by a factor of 1.4 (i. e. V2 ). This means that a shorter pulse will fully bum through the tin droplet (mass use according to Equation 2 remains the same because the tin droplet has the same diameter and the same mass). From Equation 2 it can be determined that a pulse with a duration of 142 ns (200 ns / 1.4) will fully burn through the tin droplet. A laser pulse 60a with a duration of 142 ns and a nominal intensity of 2 is schematically depicted in Figure 5 A. The amount of EUV energy generated from the tin droplet, expressed as a ratio relative to the nominal energy provided by the 200 ns pulse is, according to Equation 1: 2 * (1 / 1.4) * 0.95 = 1.4. Again, the duration of the laser pulse may be sufficient to fully bum through the tin droplet. This is schematically indicated by a vertical dashed line at time FB which indicates full bum-through of the tin droplet.

[0081] As noted above, the rate at which the laser pulse bums through the tin droplet may have a dependency of I06rather than V7. When a dependency of 1°6is applied, the rate at which the laser pulse bums through the tin droplet is increased by a factor of 1.5 (instead of a factor of 1.4). According to Equation 2, a slightly shorter pulse will bum through the tin droplet, the duration of the pulse being 133 ns (200 ns / 1.5). When a pulse having a duration of 133 ns and a nominal intensity of 2 is used, thenthe amount of EUV energy relative to the nominal energy from a 200 ns pulse is 2 * (1 / 1.5) * 0.95v = 1.3.

[0082] In general, when the intensity of the laser pulse is increased, more EUV is generated and a shorter laser pulse may be used. The duration of the shorter pulse may be selected such that the tin droplet is fully burnt through by the laser pulse.

[0083] The intensity of the laser pulse may be halved, to a nominal intensity of 0.5. Where this is the case, the conversion efficiency drops significantly (e.g. by roughly 20%). The rate at which the laser pulse bums through the tin droplet may be reduced by a factor of 0.7 (i. e. V0?5 ). This means that a longer pulse is needed to fully bum through the tin droplet. Specifically, a pulse with a duration of 286 ns (200 ns / 0.7) will fully bum through the tin droplet. A laser pulse 60c with a duration of 286 ns and a nominal intensity of 0.5 is schematically depicted in Figure 5C. The amount of EUV energy generated from the tin droplet, expressed as a ratio relative to the nominal energy provided by the 200 ns pulse is, according to Equation 1: 0.5 * (1 / 0.7) * 0.8 = 0.57.

[0084] The rate at which the laser pulse burns through the tin droplet may have a dependency of I06rather than V7. When a dependency of I06is applied, the rate at which the laser pulse bums through the tin droplet is reduced by a factor of 0.66 (instead of a factor of 0.7). According to Equation 2, a slightly longer pulse is needed to bum through the tin droplet, the duration of the pulse being 303 ns (200 ns / 0.66). When a pulse having a duration of 303 ns and a nominal intensity of 0.5 is used, then the amount of EUV energy relative to the nominal energy from a 200 ns pulse is 0.5 * (1 / 0.66) * 0.8 = 0.61.

[0085] The comparison starting from a 200 ns pulse is merely an example. The same dependencies apply when starting from other pulse durations.

[0086] In general, when the intensity of the laser pulse is reduced, less EUV is generated and a longer laser pulse may be needed. The duration of the longer pulse may be selected such that the tin droplet is fully burnt through by the laser pulse.

[0087] From the above examples, it will be understood that the energy of EUV radiation emitted from a tin droplet can be adjusted between a nominal value of 0.57 and a nominal value of 1.4 by adjusting the laser pulse intensity and the laser pulse duration, whilst ensuring that the tin droplet is fully burned through by the laser pulse. Adjusting the laser pulse intensity and the laser pulse duration allows the energy of EUV emitted from a tin droplet to be adjusted whilst still fully burning through the tin droplet.

[0088] In general, the EUV energy emitted from a tin droplet may be adjusted over a nominal range of 1-2 or more. That is, the EUV energy emitted from a tin droplet may be doubled (or more than doubled) relative to a minimum energy.

[0089] In the above examples, the pulse repetition rate, and the rate at which tin droplets are delivered to the plasma formation location, may be kept constant. Where this is the case, the power of the laser beam changes as set out in the table below:Table 1As may be seen from Table 1, for the adjustments of laser pulse intensity and duration set out above and depicted in Figures 5 A-C, the laser beam power for the high intensity laser pulse will be double the laser beam power for the low intensity laser pulse.

[0090] Optics of the radiation source SO may absorb some heat from the laser beam 2. When the power of the laser beam changes then the amount of heat absorbed from the laser beam will also change. This may cause heating or cooling of optics of the radiation source SO, which in turn may change the optical performance of the optics. The change of the optical performance of the optics may be negligible. However, if the change of the optical performance is significant then the laser pulse duration may be adjusted in order to eliminate the change of optical performance or to bring eliminate the change of optical performance. The table below sets out an example of how this may be achieved:Table 2

[0091] Table 2 illustrates how the laser pulse duration is changed in order to keep the laser beam power constant. The highest laser beam power (nominal 284) occurs when the laser pulse intensity is at its highest (nominal 2). When the laser pulse intensity is at nominal 1, the duration of the laser beam is increased from 200 ns to 284 ns. This provides a nominal laser beam power of 284 ns. Increasing the duration of the laser pulse does not generate an increased amount of EUV radiation. This is because the tin droplet has been fully burnt through at 200 ns, and so no tin droplet is present for the remaining 84 ns.

[0092] When the laser pulse intensity is at nominal 0.5, the duration of the laser beam is increased from 286 ns to 586 ns. This provides a nominal laser beam power of 284 ns. Increasing the durationof the laser pulse does not generate an increased amount of EUV radiation. This is because the tin droplet has been fully burnt through at 286 ns, and so no tin droplet is present for the remaining 200 ns.

[0093] The initial pulse duration of 200 ns is merely an example. Other initial pulse durations may be used. The tin droplet may have a diameter of 40 microns or may have a different diameter. The initial pulse duration may be selected taking into account the diameter of the tin droplet.

[0094] The above examples provide the same laser beam power for each laser pulse intensity. When this approach is taken, the heating of optics of the radiation source SO, and the performance of those optics, may remain constant. In other embodiments, the laser pulse duration for laser pulse intensities below the maximum laser pulse intensity may be increased by an amount which is sufficient to maintain the performance of radiation source optics within a desired range. The pulse duration adjustment required to achieve this may be determined using calibration measurements and / or using modelling.

[0095] Increasing the laser pulse duration beyond the duration required to fully bum through a tin droplet will reduce the conversion efficiency of the radiation source SO. This is because part of the laser pulse is not burning through tin and therefore does not contribute to EUV generation. For this reason, it may be desirable to limit as far as possible increases of laser pulse duration beyond the duration needed to fully bum through a tin droplet (e.g. laser pulse duration increase may be limited to provide a laser pulse duration sufficient to maintain radiation source optics performance within desired boundaries).

[0096] The description set out further above in connection with Figure 3 explains how adjustments of the laser pulse duration and laser pulse intensity can be achieved. The adjustments may be controlled by the controller CT depicted in Figure 3. An input may be provided to the controller, the input indicating a desired EUV power to be provided by the radiation source SO.

[0097] The controller CT may include data which indicates what laser pulse intensity and laser pulse duration is needed to obtain a desired EUV power. The data may be obtained via calibration and / or via modelling. The data may for example comprise a look-up table which relates laser pulse duration and laser pulse power to EUV power. The data may for example comprise a calibration curve which relates laser pulse duration and laser pulse power to EUV power. The calibration curve may provide combinations of laser pulse duration and laser pulse power which will fully bum through the tin droplets.

[0098] The radiation source SO may include a sensor (not depicted) which measures the EUV power. The controller may receive an output from this sensor. The controller may in addition receive an input indicating a desired EUV power. The controller may adjust the laser pulse intensity and laser pulse duration until a desired EUV power is achieved. For example, if the EUV power is lower than the desired EUV power the controller may increase the laser pulse intensity and decrease the laser pulse duration. The adjustments that may be made by the controller may be limited to combinations of laser pulse intensity and laser pulse duration that will fully bum through the tin droplets.

[0099] In an alternative embodiment, the adjustments of EUV power are achieved by changing the laser pulse intensity without changing the laser pulse duration. Where this approach is used, the laser pulse duration is always be sufficiently long to ensure that the tin droplet is fully burned through. Table 3 provides some example values:Table 3As will be understood from Table 2, the laser beam power will change very significantly (e.g. by a factor of 4) if the laser pulse intensity is adjusted without changing the laser pulse duration. Where this is the case, the performance of optics of the radiation source SO may be changed very significantly. In addition, a large amount of laser power is not used to generate EUV radiation (once bum through of the tin droplet has been completed), and as a result the conversion efficiency of this approach is significantly worse than the conversion efficiency achieved when the pulse duration is varied.[000100] Although embodiments of the invention have been described in connection with tin droplets which are spherical, the tin droplets may have some other shape. For example, a tin droplet may be non-spherical or misshapen as a result of forces exerted by vaporization of a preceding tin droplet. For example, the tin droplet may be ellipsoid. Where this is the case, the duration of the laser pulse used to convert the tin droplet to EUV emitting plasma may be adjusted to ensure that the tin droplet is fully burnt through. Calibration measurements, and / or images of tin droplets obtained using an image sensor in the radiation source (SO) may be used to determine the shape and orientation of a non-spherical tin droplet. The duration of the laser pulse may be adjusted accordingly.[000101] The rising and falling edges of a laser pulse may in some embodiments be excluded from modelling which is used to determine laser pulse intensity and laser pulse duration needed to fully bum through a tin droplet. The rising and falling edges may for example have a duration of around 10ns.[000102] A Pockels cell, or other modulator, may be used to change the rising and falling edges of the laser pulses. In one example, making the rising edge longer in duration may make EUV generation less efficient (the plasma may spend significant time at a temperature which is below an optimum temperature for EUV emission). This may reduce the EUV power provided by the radiation source. The laser pulses may be sufficiently long to ensure that the tin droplet is fully burnt through.[000103] The laser pulses may have a top-hat spatial profile. The top-hat spatial profile may have a diameter of at least 80 microns.[000104] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrates) or mask (or other patterning devices). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000105] Where the context allows, 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 readable 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, acoustical, and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.[000106] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method of adjusting an amount of extreme ultraviolet (EUV) radiation generated by directing laser pulses onto liquid fuel targets to generate EUV emitting plasma, the laser pulses having a wavelength between 1.6 microns and 2.5 microns, wherein the method comprises adjusting an intensity of the laser pulses, wherein the laser pulses bum fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses bum fully through the liquid fuel targets after the intensity of the laser pulses is adjusted.

2. The method of claim 1, wherein the duration of the laser pulses is also adjusted.

3. The method of claim 2, wherein when the intensity of the laser pulses is increased the duration of the laser pulses is decreased.

4. The method of claim 2 or claim 3, wherein when the intensity of the laser pulses is decreased the duration of the laser pulses is increased.

5. The method of any of claims 2 to 4, wherein the adjustment of the duration of the laser pulses compensates for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains substantially unchanged.

6. The method of any of claims 2 to 4, wherein the adjustment of the duration of the laser pulses partially compensates for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains within a desired range.

7. The method of any preceding claim, wherein the liquid fuel targets are tin droplets with a diameter of at least 30 microns.

8. The method of any of claims 2 to 7, wherein the adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses is determined by a controller using a calibration curve or a look-up table.

9. The method of any of claims 2 to 7, wherein the adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses is determined by a controller using an output from a sensor which measures the power of generated EUV radiation.

10. An extreme ultraviolet (EUV) radiation source comprising a nozzle configured to direct liquid fuel targets towards a plasma formation location, and a laser system configured to direct laser pulses having a wavelength between 1.6 microns and 2.5 microns to the plasma formation location, the EUV radiation source further comprising a controller configured to adjust an intensity of the laser pulses, wherein the laser pulses bum fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses bum fully through the liquid fuel targets after the intensity of the laser pulses is adjusted.

11. The EUV radiation source of claim 10, wherein the controller is configured to also adjust the duration of the laser pulses.

12. The EUV radiation source of claim 11, wherein the controller is configured to increase the intensity of the laser pulses and decrease the duration of the laser pulses.

13. The EUV radiation source of claim 11 or claim 12, wherein the controller is configured to decrease the intensity of the laser pulses and increase the duration of the laser pulses.

14. The EUV radiation source of any of claims 11 to 13, wherein the adjustment of the duration of the laser pulses compensates for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains substantially unchanged.

15. The EUV radiation source of any of claims 11 to 14, wherein the adjustment of the duration of the laser pulses partially compensates for the adjustment of the intensity of the laser pulses, so that the power of the laser beam remains within a desired range.

16. The EUV radiation source of any of claims 11 to 15, wherein the nozzle is configured to provide liquid fuel targets which are tin droplets with a diameter of at least 30 microns.

17. The EUV radiation source of any of claims 11 to 16, wherein the controller is configured to determine the adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses using a calibration curve or a look-up table.

18. The method of any of claims 11 to 17, wherein the controller is configured to determine the adjustment of the intensity of the laser pulses and the adjustment of the duration of the laser pulses using an output from a sensor which measures the power of generated EUV radiation.

19. A lithographic apparatus comprising the EUV radiation source of any of claims 11 to 17.

20. A method of manufacturing a semiconductor device comprising the steps of:- receiving a substrate with a photoresist layer and directing radiation from the EUV radiation source of any of claims 11 to 17, wherein the EUV radiation is generated by directing laser pulses onto liquid fuel targets to generate EUV emitting plasma, the laser pulses having a wavelength between 1.6 microns and 2.5 microns, wherein the method comprises adjusting an intensity of the laser pulses, wherein the laser pulses bum fully through the liquid fuel targets before the intensity of the laser pulses is adjusted, and wherein the laser pulses bum fully through the liquid fuel targets after the intensity of the laser pulses is adjusted; wherein the EUV radiation is arranged to transfer a pattern from a mask onto the photoresist layer; and- removing a portion of the photoresist layer to form the pattern over the substrate.

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