Target material storage assembly for EUV radiation source

Abstract

An apparatus suitable for storing a molten target material to be irradiated and converted in an extreme ultraviolet radiation source is disclosed in which a reservoir containing the molten target material is composed of molybdenum, such as prepared by using a hot isostatic pressing process such that the material properties of molybdenum are isotropic.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application No. 63 / 441,288, filed on January 26, 2023, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to extreme ultraviolet ("EUV") sources that generate EUV radiation by converting a target material, and more particularly to apparatus and systems for storing the target material. Background Art

[0004] EUV radiation (e.g., electromagnetic radiation having a wavelength of about 50 nm or less, sometimes also called soft X-rays and including light with a wavelength of about 13 nm) is used in photolithography processes to create extremely small features in and on substrates such as silicon wafers.

[0005] Methods for generating EUV radiation include, but are not limited to, those that generate radiation by first converting the physical state of a target material into a plasma state. Plasma is typically generated in a sealed container, such as a vacuum chamber, and monitored using various types of metrology equipment. Target materials include elements with emission lines in the EUV range, such as xenon, lithium, or tin. Immediately prior to introduction into the vacuum chamber, the target material can be solid, liquid, or gas.

[0006] In one known method involving a liquid target material, the necessary plasma is generated by irradiating droplets or clusters of the target material with a drive laser. One technique for generating the droplets involves melting the solid target material and then supplying the liquid target material to a droplet generator. The droplet generator forces the liquid target material under high pressure through a relatively small diameter orifice, such as one with a diameter of about 0.5 μm to about 30 μm, to produce a stream of droplets. The droplet generator directs the droplets toward the primary focal point of a collector optic, where at least some of the droplets are individually irradiated to produce EUV.

[0007] In some systems, the target material is stored in a target material storage system comprising one or more reservoirs. The reservoirs hold the target material in a ready state, ready to be supplied to the droplet generator. The interior of the reservoir is maintained under pressure to force the liquid target material from the reservoir to the droplet generator.

[0008] As mentioned above, tin is a suitable choice of target material. However, hot molten tin is extremely corrosive. Therefore, components of the tin storage system, such as the reservoir, are made of materials that can withstand prolonged exposure to molten tin under pressure. Molybdenum (Mo) is recommended as a choice of reservoir due to its resistance to corrosion when exposed to molten tin. However, care is needed to ensure that the technology used to prepare the storage system does not compromise the mechanical quality of the resulting molybdenum product. Advantageously, the reservoir can be prepared using a material that exhibits corrosion resistance similar to that of molybdenum formed using standard techniques, but using a simplified preparation method suitable for maintaining or improving the mechanical properties of the material.

[0009] It is within this context that the need and advantages of the presently disclosed subject matter arise. Summary of the Invention

[0010] The following is a brief summary that provides a basic understanding of the embodiments. This summary is not intended to be a comprehensive overview of all contemplated embodiments, nor is it intended to identify any element of any embodiment as critical or important. Its sole purpose is to present some concepts related to one or more embodiments as a prelude to the more detailed description presented further below.

[0011] According to one aspect of the present invention, a target material storage system is disclosed that is suitable for storing target material to be irradiated in an extreme ultraviolet radiation source. The target material storage system includes a reservoir that includes substantially isotropic molybdenum. The substantially isotropic molybdenum can be produced using a hot isostatic pressing process. The substantially isotropic molybdenum produced using the hot isostatic pressing process is pure molybdenum. The substantially isotropic molybdenum produced using the hot isostatic pressing process can be unalloyed molybdenum.

[0012] The target material storage system may further include a container cap configured to seal an upper portion of the reservoir, wherein the container cap comprises substantially isotropic molybdenum. The container cap may comprise isotropic molybdenum produced using a hot isostatic pressing process. The container cap may comprise pure isotropic molybdenum produced using a hot isostatic pressing process. The container cap may comprise unalloyed isotropic molybdenum produced using a hot isostatic pressing process.

[0013] The target material storage system may further include a locking screw adapted to secure the container cap to the reservoir, the locking screw comprising isotropic molybdenum. The locking screw may comprise isotropic molybdenum produced using a hot isostatic pressing process. The locking screw may comprise pure isotropic molybdenum produced using a hot isostatic pressing process. The locking screw may comprise unalloyed isotropic molybdenum produced using a hot isostatic pressing process.

[0014] According to another embodiment, a molten tin storage system is disclosed, comprising at least one component in contact with the molten tin, the at least one component comprising isotropic molybdenum. The isotropic molybdenum can be produced using a hot isostatic pressing process. The isotropic molybdenum produced using the hot isostatic pressing process can be unalloyed. The at least one component may include a container defining a chamber suitable for containing the molten tin. The at least one component may also include a container cap configured to seal an upper portion of the reservoir. The at least one component may also include a locking screw adapted to secure the container cap to the reservoir.

[0015] According to another aspect of the embodiment, a source of extreme ultraviolet radiation for semiconductor lithography is disclosed, the source comprising a vacuum chamber, a collector optical device disposed within the vacuum chamber and having a focal point within the chamber, a target material delivery system disposed to dispense droplets of target material to an irradiation site within the chamber at the focal point, and a target material storage system disposed to provide the target material to a droplet generator, the target material storage system comprising a target material reservoir containing isotropic molybdenum. The isotropic molybdenum may be pure and prepared using a hot isostatic pressing process. The target material storage system may further comprise a target material level sensing electrode extending into the target material reservoir and adapted to sense a target material level in the target material reservoir.

[0016] Further embodiments, features, and advantages of the subject matter of the present disclosure, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the presently disclosed subject matter and, together with the description, further serve to explain the principles of the presently disclosed subject matter and to enable a person skilled in the relevant art to make and use the presently disclosed subject matter.

[0018] Figure 1 is a partially schematic functional block diagram of the overall broad concept of a laser produced plasma EUV radiation source system, such as may be incorporated with an aspect of the embodiments.

[0019] Figure 2 is a perspective view of a target material storage system with some hidden elements shown by dashed lines according to one aspect of an embodiment.

[0020] Figure 3 is a cross-sectional view of an upper portion of a target material storage system according to an aspect of an embodiment.

[0021] Figure 4A and Figure 4B is a diagram illustrating the operation of a level sensor in a target material storage system according to an aspect of the embodiment.

[0022] Further features and advantages of the disclosed apparatus, as well as the structure and operation of various embodiments of the disclosed apparatus, are described in detail below with reference to the accompanying drawings. The disclosed apparatus is not limited to the specific embodiments described herein. These embodiments are presented herein for illustrative purposes only. Other embodiments will be apparent to those skilled in the relevant art based on the teachings contained herein. DETAILED DESCRIPTION

[0023] Various embodiments are described below with reference to the accompanying drawings, wherein like reference numerals are used throughout to represent like elements. In the following description, for purposes of explanation, numerous specific details are set forth to facilitate a thorough understanding of the various embodiments. However, it will be apparent that any of the following embodiments may be practiced without employing the specific design details described below, in some or all cases.

[0024] In the following description and claims, the terms "upper," "lower," "upper," "lower," "top," "bottom," "vertical," "horizontal," and similar terms may be used. Unless otherwise specified or clear from the context, these terms are intended to indicate relative orientations only and not any orientation with respect to gravity.

[0025] Figure 1 FIG2 is a schematic diagram of an example of an EUV radiation source, such as a laser-generated plasma EUV radiation source 10. As shown, the EUV radiation source 10 may include a pulsed or continuous laser source 22, such as a pulsed gas discharge CO2 laser source that generates a radiation pulse beam 22b having a wavelength typically below 20 μm, such as in the range of about 11 μm to about 9 μm or less. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and high pulse repetition rate.

[0026] The EUV radiation source 10 also includes a target material delivery system 24 for delivering the target material in the form of droplets or a continuous stream. In this example, the target material is a liquid, but it can also be a solid. The target material can be made of tin or a tin compound, but other materials can also be used. In the depicted system, the target material delivery system 24 introduces droplets 14 of the target material into the interior of the vacuum chamber 26 to an irradiation region 28, where the droplets 14 can be irradiated to produce a plasma. It should be noted that, as used herein, the irradiation region is the region in which irradiation of the target material will occur, and is the irradiation region even when no irradiation actually occurs. The EUV light source 10 also includes a beam focusing and steering system 32.

[0027] In the example shown, the components are arranged so that the droplets 14 travel substantially horizontally relative to gravity. The direction from the laser source 22 to the irradiation region 28, i.e., the nominal propagation direction of the beam 22b, can be referred to as the Z-axis. The path of the droplets 14 from the target material delivery system 24 to the irradiation region 28 can be referred to as the X-axis. Thus, Figure 1 While a system is described in which the droplets 14 travel substantially horizontally, one of ordinary skill in the art will appreciate that other arrangements may be used in which the droplets 14 travel vertically or at some angle relative to gravity between and including 90 degrees (horizontal) and 0 degrees (vertical).

[0028] The EUV radiation source 10 may also include an EUV light source control system 60 and a laser firing control system 65, as well as the beam steering system 32. The EUV radiation source 10 may also include a detector, such as a target position detection system, which may include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of a target droplet, for example, relative to the irradiation area 28, and provide the output to a target position detection feedback system 62. The target position detection feedback system 62 may use the output of the droplet imagers 70 to calculate the target position and trajectory, and thus may calculate the target error.

[0029] Figure 1 The EUV radiation source 10 shown also includes a conditioning laser 23 for generating a conditioning beam 23b. The conditioning beam 23b consists of pulses that prepare the target for heating by the subsequent main drive pulse. The conditioning pulses can change the shape or distribution of the target. They include pulses variously referred to as pre-pulses, pedestal pulses, and rarefaction pulses. A beam steering system 32 is capable of steering the conditioning beam 23b generated by the conditioning laser 23.

[0030] like Figure 1 As shown, the target material delivery system 24 may include a target delivery control system 90. The target delivery control system 90 may adjust the path of the droplets 14 through the irradiation region 28 in response to a signal provided by the system controller 60 (e.g., a target error or some amount derived from the target error). This may be accomplished, for example, by repositioning the point at which the target delivery mechanism 92 releases the droplets 14. The droplet release point may be repositioned, for example, by tilting or moving the target delivery mechanism 92.

[0031] The target delivery mechanism 92 extends into the chamber 26 and is supplied with target material 96 from a target material reservoir 94. The target material reservoir 94 is in fluid communication with the target delivery mechanism 92 via a reservoir target material outlet valve 98a. The reservoir target material outlet valve 98a can be configured as a freeze valve, wherein the flow of molten tin is controlled by allowing / causing molten tin to solidify in the valve to close the valve and allowing / causing solid tin to melt in the valve to open the valve. The target material reservoir 94 is also in fluid communication with a source of liquid target material via a reservoir target material inlet valve 98b. The reservoir target material inlet valve 98b can also be configured as a freeze valve. In some embodiments, there may be only a single target material conduit that allows target material to be added to and removed from the target material reservoir 94 and thus serves as both an inlet and an outlet.

[0032] Liquid target material 96 within target material reservoir 94 is maintained under pressure by gas provided by a gas source (not shown) that is in fluid communication with target material reservoir 94 through reservoir gas inlet valve 98c.

[0033] It should be understood that the EUV radiation source 10 may include more than one target material reservoir 94. The liquid target material 96 within the target material reservoir 94 may be produced by any of several methods, generally involving melting pure solid tin.

[0034] The EUV radiation source 10 also includes a target material trap 80 that captures and holds target material that is not converted by irradiation to limit contamination from such unconverted target material.

[0035] Continue to refer to Figure 1 The EUV radiation source 10 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion is also applicable to other types of optical elements. The collector 30 may be a normal incidence reflector, for example, implemented as a multilayer mirror with an additional thin barrier layer, such as B4C, ZrC, Si3N4, or C, deposited at each layer interface to effectively block thermally induced interlayer diffusion. The collector 30 may be in the form of a prolate spheroid with a central aperture to allow the beam 22b and the conditioned beam 23b to pass through and reach the irradiation region 28. The collector 30 may have a first focus at the irradiation region 28 and a second focus at an intermediate point 40 (also referred to as the intermediate focus 40), at which the EUV radiation may be output from the EUV radiation source 10 and input to, for example, an integrated circuit lithography scanner or stepper 50. The integrated circuit lithography scanner or stepper 50 uses the radiation, for example, using a reticle or mask 54 in a known manner to process a silicon wafer workpiece 52. The silicon wafer workpiece 52 may then undergo additional processing in a known manner to obtain integrated circuit devices.

[0036] Figure 2 FIG2 is a perspective view of a target material storage system according to an aspect of an embodiment, with some hidden components shown in phantom. As can be seen, the target material storage system includes a target material reservoir 94 having walls defining a reservoir chamber 100, which is sealed by a reservoir cap 105 secured by a reservoir cap threaded connector 115, as described in greater detail below. In some embodiments, the target material reservoir 94 is generally cylindrical. Figure 2 The target material storage system further includes a reservoir gas cap 110 having a reservoir gas inlet 120 for supplying gas to the reservoir chamber 100 to place the liquid target material under pressure in the reservoir chamber 100 .

[0037] Figure 2 The target material storage system further includes a reservoir target material inlet 130 and a reservoir target material outlet 140, wherein the reservoir target material inlet 130 is used to add liquid target material to the reservoir chamber 100 and the reservoir target material outlet 140 is used to remove liquid target material from the target material reservoir chamber 100 to supply the target delivery mechanism 92, as described above in conjunction with Figure 1 As described above, in some embodiments, there may be only a single target material conduit that allows target material to be added to and removed from the target material reservoir 94 and thus serves as both an inlet and an outlet.

[0038] Figure 2 The target material storage system also includes a rod heating element 150 for heating the target material reservoir 94. Although Figure 2 Only one rod heating element 150 is shown, but one of ordinary skill in the art will appreciate that, in general, a target material storage system may include more than one heating element.

[0039] As described above, the target material reservoir 94 is made of a material that is resistant to corrosion by exposure to the hot liquid target material 96, such as tin, in the target material reservoir 94. When the target material is tin, one suitable choice of material for the walls of the target material reservoir 94 is molybdenum, such as molybdenum ASTM B387 Type 361 (Molybdenum 361), which is 99.9% molybdenum with trace amounts of potassium, aluminum, silicon, iron, nickel, copper, chromium, sodium, and tungsten. However, when Molybdenum 361 is subjected to various forming or shaping techniques, the resulting mechanical structure can be anisotropic. For example, in some cases, work hardening during rolling / forging can cause the material properties of Molybdenum 361 to vary axially and radially for a generally cylindrical hollow target material reservoir.

[0040] According to one aspect of the embodiment, at least some components of the target material storage system (e.g., the target material reservoir) are made of molybdenum, which is prepared so that the material properties of the molybdenum are substantially isotropic, for example, using a powder metallurgy process, in particular a hot isostatic pressing (HIP) process that causes the molybdenum to be isotropic.

[0041] Here and elsewhere in this specification, the terms "isotropic" and "substantially isotropic" mean that the properties of a material are the same, within + / - 5%, regardless of the orientation in which they are determined.

[0042] HIP is a material preparation method in which the starting powder or preformed shape is subjected to both high temperature and high isostatic pressure. The gas pressure is called isostatic because it is applied uniformly in all directions. HIP can combine high temperature (e.g., up to and exceeding 2200°C) and high isostatic gas pressure (e.g., in the range of 200-500 MPa). The molybdenum starting powder can be prepared in a known manner by concentrating molybdenum disulfide (MoS2) and converting it into impure industrial-grade molybdenum trioxide (MoO3) by roasting. Subsequent purification utilizes the sublimation properties of molybdenum trioxide. See "Production of Refractory Metal Powders" by Neikov et al., Handbook of Non-Rerrous Metal Powders (Second Edition 2019). The molybdenum powder is then compacted and sintered. The resulting blank is then subjected to the HIP process.

[0043] The HIP process does not require any deformation caused by forming processes and is able to produce an isotropic grain structure in the material, resulting in isotropic material properties, and can therefore be used to produce isotropic molybdenum.

[0044] More information on the application of the HIP process to molybdenum can be found in "The Densification of Molybdenum and Molybdenum Alloy Powders Using Hot Isostatic Pressing", Technical Report ARLCB-TR-85025, Accession No. AD-A159886 (August 1985) by Barranco et al. Further information can be found in "Mechanical Properties Of Fine-Grained, Sintered Molybdenum Alloys with Dispersed Particles Developed By Mechanical Alloying", Materials Transactions, Vol. 45, No. 1, pp. 143-148 (2004) by Takida et al., which describes the HIP treatment of pure molybdenum. Suitable balanced molybdenum prepared using the HIP process can be obtained from Plansee SE, Metallwerk-Plansee-Str. 71, 6600 Reutte, Austria.

[0045] In some embodiments, the isostatic molybdenum is pure.Here and elsewhere in this specification, the term "pure" is intended to mean so-called "triple nine" purity, ie, 99.9% purity by weight. For example, the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Codes draft Code Case 3063, "Use of Hot-Isostatically Pressed Pure Isotropic Molybdenum Section VIII, Division 1 and Section VIII, Division 2, Class 1," approved on December 12, 2022, stipulates that isotropic pure molybdenum powder metal parts produced by HIP can be used in ASME Section VIII, Division 1 and Section VIII, Division 2, Class 1 construction, subject to certain additional requirements, including that the process fluid is limited to molten tin and non-flammable fluids. The chemical composition of pure molybdenum referenced in ASME Code Case 3063 is required to conform to the following chemical composition obtained from ASME Code Case 3063:

[0046] element Ingredients (weight) % C, maximum 0.010 O, maximum value 0.0070 N, maximum value 0.0020 Fe, maximum 0.010 Ni, maximum 0.005 SI, max. 0.010 Mo balance

[0047] The molybdenum is "pure" within the meaning of that term in ASME Code Case 3063 and in the specification and claims herein. It is understood that other materials such as tungsten may be present in pure molybdenum at 0.010 weight percent or less, as well as so-called "unavoidable impurities."

[0048] All patent applications, patents, and printed publications cited herein are incorporated by reference in their entirety except where any definition, subject matter disclaimer, or subversion is reached, and unless the incorporated material is inconsistent with the explicit disclosure herein, in which case the language of the disclosure controls.

[0049] In some embodiments, isotropic platinum is unalloyed. As used herein, the term "unalloyed" refers to a material to which no additional material has been intentionally or purposefully added to alter the material properties. Thus, "unalloyed platinum" means platinum to which no other material has been added to alter the material properties of the platinum, and may, for example, include trace amounts of unavoidable impurities. It will be apparent to one of ordinary skill in the art that isotropic platinum may be pure and unalloyed.

[0050] Figure 3 is the upper portion of the target material storage system according to one aspect of the embodiment. As described above, Figure 3 The target material reservoir 94 includes a reservoir chamber 100 sealed by a reservoir container cap 105. According to one aspect of the embodiment, the reservoir container cap 105 is also made of isotropic molybdenum, for example, prepared using a HIP process. The isotropic molybdenum can be pure or unalloyed, or both. The container cap 105 engages a reservoir chamber seal 107 to seal the reservoir chamber 100. The container cap 105 is held in place by a locking screw 114, which engages a container cap threaded connector 115. As shown, in some embodiments, the thread profile of the reservoir cap threaded connector 115 can be trapezoidal to increase strength. According to one aspect of the embodiment, the locking screw 114 is also made of isotropic molybdenum, which can be prepared using a HIP process. The isotropic molybdenum can be pure or unalloyed, or both.

[0051] Figure 3 This portion of the target material storage system also includes a reservoir gas cap 110 having a reservoir gas inlet 120 that allows gas to be introduced into the chamber 100 to pressurize the reservoir chamber 100. The reservoir gas cap 110 is engaged with a gas cap seal 170. Figure 3 The arrangement also includes a target material level sensing electrode 160 as described more fully below.

[0052] Figure 4A and Figure 4B is a diagram illustrating the operating principle of a target material level sensing system according to one aspect of an embodiment. Figure 4AIn the case shown, the amount of liquid target material 96 in the target material reservoir 94 is sufficient to contact the target material level sensing electrode 160. In the embodiment shown, the target material reservoir 94 is oriented substantially vertically with respect to gravity so that the liquid target material 96 collects at the bottom of the reservoir chamber 100, which has a top surface that is substantially horizontal with respect to gravity. The target material sensing electrode 160 is made of a material that is electrically conductive and, when the target material is tin, is resistant to corrosion by liquid target materials such as molybdenum. The target material sensing electrode 160 is electrically connected to the control module 180 via a first connector 190, which applies an input voltage to the target material level sensing electrode 160. The input voltage can be, for example, approximately 24 volts DC. The target material level sensing electrode 160 is also electrically connected to the control module 180 via a line 195, which returns a signal indicative of the sensed voltage. In Figure 4A In the case shown, liquid target material 96 in the target material reservoir completes the electrical connection from the target material level sense electrode 100 to ground (an electrical short), which drives the sense voltage on line 195 downward.

[0053] On the other hand, Figure 4B In the case shown, the height of the liquid target material 96 in the target material reservoir 94 is insufficient to make electrical contact with the target material height sensing electrode 160. Therefore, the target material level sensing electrode 160 is not grounded and its voltage is floating. In this case, the sense voltage on line 195 will be higher. The control module 180 uses the sense voltage returned to it on line 195 to determine whether the amount of liquid target material 96 in the target material reservoir 94 is below a predetermined level and, therefore, needs to be replenished.

[0054] The present disclosure is made with the aid of functional building blocks that illustrate the implementation of specific functions and their relationships. The boundaries of these functional building blocks have been arbitrarily defined herein to facilitate clarity of description. Alternative boundaries may be defined so long as the specified functions are appropriately performed. For example, a control module function may be divided among several systems or at least partially performed by the entire control system.

[0055] The above description includes examples of one or more embodiments. Of course, it is not possible to describe every conceivable combination of parts or methods for the purpose of describing the aforementioned embodiments, but those skilled in the art will recognize that many other combinations and arrangements of various embodiments are possible after being provided with the present invention. Therefore, the described embodiments are intended to include all such replacements, modifications and variations that fall within the spirit and scope of the appended claims. In addition, to the extent that the term "include" is used in the detailed description or claims, such terms are intended to be inclusive in a manner similar to the term "comprising" because "comprising" is interpreted when used as a transition word in the claims. In addition, although the elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural form is encompassed unless explicitly stated or clearly limited to the singular from the context. In addition, all or a portion of any aspect and / or embodiment may be used together with all or a portion of any other aspect and / or embodiment, unless otherwise stated.

[0056] These embodiments can be further described using the following terms:

[0057] 1. A target material storage system adapted to store target material to be irradiated in an extreme ultraviolet radiation source, the target material storage system comprising a reservoir comprising substantially isotropic molybdenum.

[0058] 2. The target material storage system of clause 1, wherein the substantially isotropic molybdenum is produced using a hot isostatic pressing process.

[0059] 3. The target material storage system of clause 2, wherein the substantially isotropic molybdenum produced using a hot isostatic pressing process is pure molybdenum.

[0060] 4. The target material storage system of clause 2, wherein the substantially isotropic molybdenum produced using a hot isostatic pressing process is unalloyed molybdenum.

[0061] 5. The target material storage system of clause 1, further comprising a container cap configured to seal an upper portion of the reservoir, wherein the container cap comprises substantially isotropic molybdenum.

[0062] 6. The target material storage system of clause 5, wherein the container cap comprises isotropic molybdenum produced using a hot isostatic pressing process.

[0063] 7. The target material storage system of clause 5, wherein the container cap comprises pure isotropic molybdenum produced using a hot isostatic pressing process.

[0064] 8. The target material storage system of clause 5, wherein the container cap comprises unalloyed isotropic molybdenum produced using a hot isostatic pressing process.

[0065] 9. The target material storage system of clause 5, further comprising a locking screw adapted to secure the container cap to the reservoir, wherein the locking screw comprises isotropic molybdenum.

[0066] 10. The target material storage system of clause 9, wherein the locking screw comprises isotropic molybdenum produced using a hot isostatic pressing process.

[0067] 11. The target material storage system of clause 9, wherein the locking screw comprises pure isotropic molybdenum produced using a hot isostatic pressing process.

[0068] 12. The target material storage system of clause 9, wherein the locking screw comprises unalloyed isotropic molybdenum produced using a hot isostatic pressing process.

[0069] 13. A molten tin storage system comprising at least one component in contact with the molten tin, the at least one component comprising isotropic molybdenum.

[0070] 14. The molten tin storage system according to clause 13, wherein the isotropic molybdenum is produced using a hot isostatic pressing process.

[0071] 15. The molten tin storage system of clause 13, wherein the isotropic molybdenum produced using a hot isostatic pressing process is unalloyed.

[0072] 16. The molten tin storage system of clause 13, wherein the at least one component comprises a reservoir defining a chamber adapted to contain the molten tin.

[0073] 17. The molten tin storage system according to clause 16, wherein the at least one component further comprises a container cap arranged to seal an upper portion of the reservoir.

[0074] 18. The molten tin storage system of clause 17, wherein the at least one component further comprises a locking screw adapted to secure the container cap to the reservoir.

[0075] 19. A source of extreme ultraviolet radiation for semiconductor lithography, the source comprising:

[0076] vacuum chamber;

[0077] collector optics disposed within the vacuum chamber and having a focal point within the chamber;

[0078] a target material delivery system arranged to dispense droplets of target material to an irradiation site within the chamber at the focal point; and

[0079] A target material storage system is configured to provide the target material to the droplet generator, wherein the target material storage system comprises a target material reservoir comprising isotropic molybdenum.

[0080] 20. The source of clause 19, wherein the isotropic molybdenum is pure and prepared using a hot isostatic pressing process.

[0081] 21. The source of clause 19, wherein the target material storage system further comprises a target material level sensing electrode extending into the target material reservoir and adapted to sense a level of target material in the target material reservoir.

[0082] The implementations described above and other implementations are within the scope of the following claims.

Claims

1. A target material storage system adapted to store target material to be irradiated in an extreme ultraviolet radiation source, the target material storage system comprising a reservoir comprising substantially isotropic molybdenum.

2. The target material storage system of claim 1, wherein the substantially isotropic molybdenum is produced using a hot isostatic pressing process.

3. The target material storage system of claim 2, wherein the substantially isotropic molybdenum produced using a hot isostatic pressing process is pure molybdenum.

4. The target material storage system of claim 2, wherein the substantially isotropic molybdenum produced using a hot isostatic pressing process is unalloyed molybdenum.

5. The target material storage system of claim 1, further comprising a container cap arranged to seal an upper portion of the reservoir, wherein the container cap comprises substantially isotropic molybdenum.

6. The target material storage system of claim 5, wherein the container cap comprises isotropic molybdenum produced using a hot isostatic pressing process.

7. The target material storage system of claim 5, wherein the container cap comprises pure isotropic molybdenum produced using a hot isostatic pressing process.

8. The target material storage system of claim 5, wherein the container cap comprises unalloyed isotropic molybdenum produced using a hot isostatic pressing process.

9. The target material storage system of claim 5, further comprising a locking screw adapted to secure the container cap to the reservoir, wherein the locking screw comprises isotropic molybdenum.

10. The target material storage system of claim 9, wherein the locking screw comprises isotropic molybdenum produced using a hot isostatic pressing process.

11. The target material storage system of claim 9, wherein the locking screw comprises pure isotropic molybdenum produced using a hot isostatic pressing process.

12. The target material storage system of claim 9, wherein the locking screw comprises unalloyed isotropic molybdenum produced using a hot isostatic pressing process.

13. A molten tin storage system comprising at least one component in contact with the molten tin, the at least one component comprising isotropic molybdenum.

14. The molten tin storage system of claim 13, wherein the isotropic molybdenum is produced using a hot isostatic pressing process.

15. The molten tin storage system of claim 13, wherein the isotropic molybdenum produced using a hot isostatic pressing process is unalloyed.

16. The molten tin storage system of claim 13, wherein the at least one component comprises a reservoir defining a cavity adapted to contain the molten tin.

17. The molten tin storage system of claim 16, wherein the at least one component further comprises a container cap arranged to seal an upper portion of the reservoir.

18. The molten tin storage system of claim 17, wherein the at least one component further comprises a locking screw adapted to secure the container cap to the reservoir.

19. An extreme ultraviolet radiation source for semiconductor lithography, the source comprising: vacuum chamber; collector optics disposed within the vacuum chamber and having a focal point within the chamber; a target material delivery system arranged to dispense droplets of target material to an irradiation site within the chamber at the focal point; as well as A target material storage system is configured to provide the target material to the droplet generator, wherein the target material storage system comprises a target material reservoir comprising isotropic molybdenum.

20. The source of claim 19, wherein the isotropic molybdenum is pure and prepared using a hot isostatic pressing process.

21. The source of claim 19, wherein the target material storage system further comprises a target material level sensing electrode extending into the target material reservoir and adapted to sense a level of target material in the target material reservoir.

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