EP4783748A1

EP4783748A1 - Assembly and method for providing a target material ingot

Info

Publication number: EP4783748A1
Application number: EP25154472.2A
Authority: EP
Inventors: Younes AOUAJ; Riëlle DE RUITER; Bart Cornelis Antonius BOVERS; Faridehossadat HAJY AKBARY; Josephus Paulus Augustinus Deeben; Sergiy Alekseyevich YEREMENKO; Sander Catharina Reinier Derks; Joannes Theodorus Valentinus HOOGBOOM
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2025-01-28
Filing date: 2025-01-28
Publication date: 2026-07-29
Also published as: WO2026162278A1

Abstract

An assembly and a method for obtaining a target material ingot for use in a radiation source are disclosed. The assembly comprises a sealable chamber, a heating unit, and a receptacle. The chamber is configured to receive a target material catch bucket of a radiation source. The heating unit is configured to heat the target material contained within the received target material catch bucket to at least a melting temperature of a target material. The receptacle comprises at least one mould cavity arranged to receive a portion of a melted target material from the received target material catch bucket, and the receptacle having a contact surface defining the at least one mould cavity, wherein the contact surface is made of mould material that is inert and/or non-wetting against the target material.

Description

FIELD

The present disclosure relates to an assembly for obtaining a target material ingot for use in a radiation source, a method for obtaining a target material ingot for use in a radiation source, a method for supplying a target material to a radiation source of an EUV exposure apparatus, and a method for generating radiation within a radiation source of an EUV exposure apparatus. Finally, the present disclosure relates to a method of manufacturing a semiconductor device.

BACKGROUND

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.
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.
An overview of the development of laser-produced plasma EUV light source has been published in Chip Vol. 1(3), 2022, 100019.To generate the EUV radiation, a target material ingot of a target material in solid phase is inserted into a target material supply system of the EUV radiation source. The target material is melted to ultimately form target material droplets. The droplets are targeted by a laser source configured to excite molecules of the target material resulting in an EUV light generation. The purity of the target material has to be exceptional to avoid any failure of the EUV radiation source. Thus, the target material has to be purified to satisfy this strict requirement. For example, US10400342B2 discloses a process of providing a high purity tin (Sn) ingot having an extremely low oxygen content to be used as a target material in EUV radiation sources.
However, many of the prior art purifying methods include steps of leaching, electrolytic refining, washing, atmospheric casting and/or vacuum casting requiring use of toxic chemicals and resulting in creation of large amounts of potentially non-environmentally sustainable waste. Additionally, these methods are costly and require long times to be carried out.
Therefore, there is a need to provide an environmentally more sustainable method of forming target material ingots of sufficient purity to be used in an EUV radiation source.

SUMMARY

According to a first aspect of the present disclosure there is provided an assembly for obtaining a target material ingot for use in a radiation source. The assembly comprises a sealable chamber, a heating unit, and a receptacle. The sealable chamber is configured to receive a target material catch bucket of a radiation source, wherein the target material catch bucket is configured to accommodate an amount of a target material. The heating unit is configured to heat the target material contained within the received target material catch bucket to at least a melting temperature of the target material.
The receptacle comprises at least one mould cavity. The at least one mould cavity is arranged to receive a portion of a melted target material from the received target material catch bucket upon melting the target material accumulated in the target material catch bucket. The receptacle has a contact surface defining the at least one mould cavity, wherein the contact surface is made of a mould material that is inert against the target material.
Alternatively, or in addition thereto, the mould material is made of a non-wetting material which is non-wetting against the target material.
The target material catch bucket generally accommodates an amount of unused target material. The unused target material contained within the target material catch bucket still meets requirements of purity to cast a target material ingot that can be used to provide target material to the radiation source. The assembly proposed herein is adapted to maintain the purity level of the target material, so that no purification step is required prior to forming the ingot(s).
As the portion of the target material is transferred into the at least one mould cavity, the intermolecular interactions between the molecular species of the contact surface and the melted target material result in molecular bonds between the two materials such that an amount of molecules forming the contact surface gets attached to the target material ingot thereby contaminating the target material ingot. Using an inert material to form the contact surface this contamination is minimized or completely prevented such that the original purity of the target material is maintained or changed insignificantly.
In the context of the present disclosure, an inert material is a material that does not form intermolecular bonds with the melted target material, or at least, the inert material forms weaker intermolecular bonds (such as van der Waals bonds) with the melted target material than the surface bonds within the contact surface (e.g. covalent bonds, ionic bonds, metallic bonds, etc). Additionally or alternatively, the inert material is any material that does not introduce more than 5 particles per million (ppm) by weight or preferably more than 1 ppm by weight, or even more preferably more than 0.2 ppm by weight of contaminants into a target material ingot formed in the at least one mould cavity having a contact surface made of the mould material.
The heating unit may be arranged anywhere suitable to heat target material present in the target material catch bucket. In an example, if the target material is tin (Sn), the heating unit is configured to heat the received target material catch bucket to at least a melting point of tin which is 232°C.
The radiation source may be a laser-produced plasma (LPP) radiation source comprising a droplet generator. In such a radiation source, the target material catch bucket is configured in a droplet trajectory along which the droplet generator is arranged to release a stream of target material droplets. Any target material that is not transformed into a target material plasma by a source laser beam is captured by and accumulates in the target material catch bucket, while target material this is transformed into the target material plasma will not proceed into the target material catch bucket.
The radiation source may be an EUV radiation source, arranged for generation EUV radiation, particularly a laser-produced plasma EUV radiation source.
It is also noted that the target material may satisfy multiple contaminants levels to be used in the radiation source. These contaminants levels may refer to a total allowable contaminant concentration and/or to an allowable contaminant limit per contaminant species. For example, the target material to be used in an EUV radiation source may not have more than 10 ppm by weight of contaminants in total but at the same time it may not have more than 0.2 ppm by weight of iron. Consequently, the inert material may be defined based on the total allowable contaminant level and/or based on the allowable contaminant level per contaminant species.
The assembly according to the present disclosure may be used to form target material ingots from the target material accumulated in the target material catch bucket of the radiation source. In the target material catch bucket, target material droplets formed when no radiation is generated in the radiation source (so-called off-target droplets or off-droplets) are collected. As the radiation source operates in a sealed, protective atmosphere containing very little to none contaminants, the target material collected in the target material catch bucket is of the same purity as the material from which the target material droplets are generated.
We note that the term "purity" refers to the extent to which the material consists of a single element or compound without significant amounts of other elements, contaminants, or impurities. The purity may be characterized by a percentage of the single element or compound within an amount of a material (e.g. a target material ingot contains 99.999% of tin) or by the impurity (contaminants) concentration (e.g. a target material ingot contains 10 ppm by weight of total impurities, and/or a target material ingot contains 0.2 ppm by weight of iron). These two characterizations are used interchangeably in this disclosure.
Consequently, target material ingots can be cast directly from the collected target material without applying any purification process of the target material, provided that the purity of the target material is maintained within specification, for re-use in the radiation source, during all of the processing and handling of the target material catch bucket until the ingots have been solidified and packaged. The packaging may include wrapping the solidified target material ingots with a polytetrafluoroethylene (PTFE) tape to protect the target material ingots from any further contamination once the target material ingots have been removed from the receptacle. Purification processes, which may be any process containing a step of removing contaminants from received target material to increase purity of the target material, may thus be avoided. For example, such purification process may be any one of the following: leaching, electrolytic refining, and washing.
The assembly according to the present disclosure ensures that the collected target material is not additionally contaminated during the process of forming the target material ingots or, at least, ensures that the contamination (impurity concentration) of the collected target material does not exceed a pre-determined impurity concentration. In a non-limiting example, the pre-determined impurity concentration may be 10 ppm by weight of total impurities. As stated above, additional allowable impurity concentrations may be imposed per impurity species.
The mould material may be a non-wetting material, i.e. a material that prevents wetting of the melted target material to the contact surface. Consequently, the mould material may be chosen to minimize wetting between the melted target material and the contact surface made therefrom. A degree of wetting (i.e., a wettability) is proportional to a contact angle (θ). The contact angle (θ) is an angle at which an interface between a melted target material droplet and an ambient surrounding the at least one mould cavity intersects with an interface between the contact surface and the ambient surrounding the mould cavity.
The mould material may have the contact angle in a range of from 90° to 180°, and preferably in a range of from 120° to 180°, and more preferably in a range of from 135° to 180° such as in a range between 150° and 180° or in a range between 160° and 180°. It is noted that larger the contact angle is, the contamination of the target material ingot becomes smaller, as there are less intermolecular interactions between the contact surface and the target material ingot. Thus, the mould material may be chosen such that the contact angle between the target material and the contact surface made of the mould material is maximized.
The mould material may be provided in a form of a coating over the at least one mould cavity. In such embodiments, the receptacle comprises a first material, and the coating is placed over the first material. The first material has a greater wettability to the melted target material than the mould material.
The mould materials may be different types of ceramics or thin films which are not suitable for forming the receptacle due to their poorer strength and/or moulding properties. In such a case, the receptacle may be made of the first material such as iron or stainless steel and the mould material may be placed over the first material to decrease or prevent the wettability of the receptacle to the melted target material.
The target material may be tin. The tin may have a purity of at least 99.999%. Tin is a material that is readily used in EUV radiation sources as a target material. Particularly, modern laser produced plasma (LPP) EUV radiation sources require that the purity of the tin is at least 99.999% (so called 5N purity). Consequently, prior art facilities for obtaining such a high purity of the tin require additional equipment and units to perform prolonged, extensive, and potentially non-environmentally sustainable method steps to be carried out. Advantageously, the assembly offered by the present disclosure requires fewer units and avoids one or more potentially environmentally unsustainable units.
If the target material is tin, the mould material may include, but is not limited to, one or more selected from: titanium nitride (TiN), silicon carbide (SiC), and diamond-like carbon (DLC). These materials do not form intermolecular bonds with melted tin and themselves result in insignificant additional contamination of the formed target material ingots.
Alternatively, the coating may comprise or be an inchromized layer, preferably a hard inchromized layer. The inchromized layer refers to a surface layer on a material, typically a metal (e.g. stainless steel), that has undergone a chromizing process. Chromizing is a thermochemical surface treatment in which chromium atoms are diffused into the surface of a substrate to form a chromium-rich layer. Preferably, the diffusion layer is a hard inchromized layer due to its high temperature resistance and improved wear resistance, corrosion and oxidation resistance resulting in minimal wettability to tin (as the target material) but also other materials.
The chamber may have a controlled ambient when in a sealed condition.
The assembly may further include a gas controlling unit.
The gas controlling unit may be configured to control a pressure and a gas flow within the chamber such that at least a part of the process of forming the target material ingot may be carried out under the controlled ambient.
In an embodiment, the gas controlling unit may comprise a vacuum pump configured to evacuate the chamber.
The gas controlling unit may comprise a gas inlet configured to allow inflow of a gas mixture into the chamber thereby forming the controlled ambient. In an example, the gas mixture may comprise an inert gas. In another example, gas mixture may be a reducing gas, and preferably a forming gas comprising argon (Ar) and hydrogen (Hz). In such an example, the forming gas consists of around 2% H₂ and a balance of Ar. Around 2% may mean within 20% of 2% (between 1.6% and 2.4%), preferably within 10% of 2% (between 1.8% and 2.2%).
The gas controlling unit as disclosed herein ensures that at least a part of the process of forming the target material ingots is carried out in a protective atmosphere, preventing any degradation of the target material in this process. For example, the reducing gas ensures that the tin does not oxidize within the chamber. Tin oxide is a particular problem for an EUV radiation source as it can block a nozzle of a droplet generator preventing flow of the target material within the EUV radiation source.
The at least one mould cavity may be shaped to cast the target material ingot. For example, the at least one mould cavity may have a shape of a cylinder having a diameter in a range of from 50 mm to 150 mm, in a range of from 95 mm to 105 mm, more preferably of around 100 mm. The cylinder may further have a height in a range of from 50 mm to 200 mm, and preferably in a range of from 80 to 95 mm. More generally, the diameter and the height of the cylinder may be chosen according to dimensions of an insert for the target material ingot of the target material supply system. Consequently, the formed target material ingot may be directly, without any post-processing step, used for the radiation generation in the radiation source. The post-processing may include, for example, cutting larger target material ingots to desired dimensions. However, this step may also introduce contaminants to the formed target material ingot due to interaction of the target material ingot with a cutting tool.
According to a second aspect of the present disclosure, there is provided a method for obtaining a target material ingot for use in a radiation source. The method comprises the steps of:

providing a target material catch bucket containing accumulated unused target material from a radiation source;
melting the provided target material; and
transferring the provided target material from the target material catch bucket directly into a receptacle having at least one ingot-shaped mould cavity to form at least one ingot to be used for generation of radiation.

As explained above, unused target material accumulated in the target material catch bucket may be re-used for forming the target material ingots, for use in the radiation source, without any purification step as it already has a satisfactory purity.
The radiation source may be a laser-produced plasma (LPP) radiation source comprising a droplet generator. In such a radiation source, the target material catch bucket is configured in a droplet path along which the droplet generator is arranged to release a stream of target material droplets. Any target material that is not transformed into a target material plasma by a source laser beam is captured by and accumulates in the target material catch bucket as unused target material.
The radiation source may be an EUV radiation source, arranged for generation EUV radiation, particularly a laser-produced plasma EUV radiation source.
The method may further include a step of solidification of the at least one ingot within the at least one ingot-shaped mould cavity, and preferably, wherein the solidification is carried out under an ambient of a vacuum, an inert gas, and/or a reducing gas. This step ensures that no oxidation or any other form of the degradation of the target material takes place as the target material ingots are formed.
A purity of the target material is at least 99.999%. Such a purity is required for proper functioning of the radiation source. The target material may be tin. The tin may have a purity of at least 99.999%. Tin is a material that is readily used in EUV radiation sources as a target material. Particularly, modern laser produced plasma (LPP) EUV radiation sources require that the purity of the tin is at least 99.999% (so called 5N purity). Consequently, prior art methods for obtaining such a high purity of the tin require additional equipment and units to perform prolonged, extensive, and potentially non-environmentally sustainable method steps to be carried out. Advantageously, present disclosure offers a method for generating high purity ingots without use of such additional time consuming and potentially polluting steps.
In an embodiment, the purity of the provided target material substantially does not change during execution of the method.
In an embodiment, the steps of melting and transferring the melted target material may be carried out under a controlled ambient such as an ambient of a vacuum, an inert gas, and/or a reducing gas. In such embodiment, a pressure may be first reduced to a pressure level in a range of from 1 to 10 mbar, preferably of from 4 to 6 mbar, more preferably of around 5 mbar, to remove any oxygen present in the ambient. After that the reducing gas may be introduced to form the ambient for carrying out the steps of melting and transferring the melted target material. Optionally, the reducing gas may be introduced periodically to prevent oxidation of the target material during execution of the method. To enhance the reducing effect, a temperature may be set to a temperature value in a range of from 20°C to 300°C, preferably around 300°C. Such conditions represent an optimal ambient that ensures that no oxidation or any other form of the degradation of the target material takes place as the target material ingots are formed.
In an embodiment, the method may include a step of providing the receptacle comprising the at least one ingot-shaped mould cavity, wherein the receptacle has a contact surface defining the at least one ingot-shaped mould cavity, wherein the contact surface is made of a mould material that is inert against the melted target material.
Additionally or alternatively, the mould material may be non-wetting against the melted target material.
In an embodiment, the method may be carried out by an assembly according to the first aspect of the present disclosure.
According to a third aspect of the present disclosure there is a method for supplying a target material to a radiation source of an EUV exposure apparatus provided. The method comprises a step of providing a target material ingot obtained using a method according to the second aspect of the present disclosure directly into a target material supply system of a droplet generator of the radiation source. In an embodiment, the method may include a step of melting the provided target material ingot within the target material supply system.
According to a fourth aspect of the present disclosure, there is provided a method for generating a radiation within a radiation source of an EUV exposure apparatus. The method comprises the steps of:

providing a target material ingot obtained using a method according to the second aspect of the present disclosure directly into a target material supply system of a droplet generator of the radiation source;
melting the target material ingot and directing a flow of the melted target material towards a plasma formation region of the radiation source; and
emitting a laser radiation towards the plasma formation region, wherein the laser radiation is configured to be at least partially absorbed by the target material at the plasma formation region thereby generating plasma and the radiation emitted therefrom.

According to a fifth aspect of the present disclosure, there is provided a method of manufacturing a semiconductor device. The method includes the steps of:

receiving a substrate with a photoresist layer,
generating radiation with the method according to the fourth aspect of the present disclosure;
allowing at least part of this radiation via an illumination system into the projection system to generate patterned radiation;
projecting the patterned radiation from the projection system onto the photoresist layer; and subsequently
removing a portion of the photoresist layer to form a corresponding pattern over the substrate.

Preferably, the radiation may be EUV radiation. EUV radiation may have a wavelength within the range of from 4 to 20 nm, for example of 6.7 nm or 13.5 nm.
Further advantages of the present disclosure will become evident in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;
Figure 2 depicts an LPP EUV radiation source;
Figure 3 depicts an assembly for obtaining a target material ingot;
Figure 4 depicts in a schematic view a contact angle between a target material droplet and a contact surface defining a mould cavity; and
Figure 5 depicts in a cross-sectional view a receptacle of assembly for obtaining a target material ingot.

DETAILED DESCRIPTION

The teaching of the present disclosure will be explained in more detail below with second to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not as a restriction of the inventive concept which is to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure.
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.
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 facetted field mirror device 10 and a facetted 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.
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 device MA. 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).
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.
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.
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.
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, which may, for example, include a CO₂ laser, 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 region 4. 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.
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.
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 may together be considered to be a radiation system.
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.
Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a free electron laser (FEL) or a discharge produced plasma (DPP) source may be used to generate EUV radiation.
Fig. 2 illustrates an exemplary embodiment of an LPP EUV radiation source 50. The source 50 may be the radiation source SO as shown in Fig. 1.
The source 50 includes a droplet generator 51 for forming target material droplets 63. The droplet generator 51 includes a target material supply system 54 configured to receive a target material ingot 52. The target material ingot 52 is a solid piece of a highly pure target material. In a non-limiting example, the target material ingot 52 may have a cylindrical shape configured to fit within an insert 60 for the target material ingot of the target material supply system 54. The droplet generator 51 further includes a heating assembly 58 arranged in the proximity of the insert 60 and configured to melt the target material ingot 52 within the insert 60. The melted target material ingot 52 is then directed in the form of target material droplets 63 via a nozzle 62 of the droplet generator 51 towards a plasma formation region 64. As explained above, a laser system 66 is arranged to emit a laser beam 67 towards the plasma formation region 64. As the target material droplet 63 in the plasma formation region interacts with the laser beam 67 a target material plasma is created at the plasma formation region 64. Radiation, including EUV radiation, is emitted from the plasma during the de-excitation and recombination of electrons with ions of the plasma. However, not all generated target material droplets 63 will interact with generated laser beams. Target material droplets 63 that were missed, i.e. not hit by the laser beams (off-target droplets), are collected in a target material catch bucket 68. The source 50 further includes a heated target material vane bucket 70 comprising at least one vane 72. The vane 72 is placed at a distance from the target material catch bucket 68 and is arranged to direct any target material vapor and plasma created in the process of plasma generation into the heated target material vane bucket 70.This target material vapor and plasma is directed via hydrogen flows within the source 50 into vane 72.
The purity of the target material ingot has to be high in order to prevent any failures of the source 50. The target material ingot may not have more impurities than a pre-determined impurity concentration. For example, the pre-determined impurity concentration may be 10 particles per million (ppm) by weight of total impurities. Additionally, each impurity species may have an individual upper limit of allowable concentration. It is also noted that an interior of the source 50 is sealed off from an outside environment to prevent any possible contamination of the source 50 while in operation. An ambient within the source 50 is controlled to prevent any degradation of the target material within the source. For example, presence of oxygen within the source 50 may lead to oxidation of the melted target material in the droplet generator 51 which may lead to a blockage of the nozzle 62 and the source 50 (and hence, a lithographic apparatus) has to be shut down. Consequently, the interior of the source 50 is arranged to prevent contamination of the target material within the source 50 such that the target material collected in the target material catch bucket 68 has the same purity as the target material droplets 63 from which the collected target material originates or the difference between the purity of the collected target material and the purity of the target material droplets 63 differs slightly or insignificantly.
In either case, the purity of the collected target material in the target material catch bucket 68 does not exceed 10 particles per million (ppm) by weight of total impurities. On the other hand, tin vapor and plasma collected in the heated target material vane bucket 70 may have much higher impurity concentration than the original target material ingot 52 (i.e. more than 10 ppm by weight of total impurities).
In an example, the target material ingot 52 may be formed of highly pure tin (Sn). A purity of the target material ingot 52 may be at least 99.999% of Sn. Such a target material ingot 52 may have the individual upper limits per impurity (contamination) species. For example, the target material ingot cannot have more than 0.2 ppm by weight of iron.
Consequently, the collected target material in the target material catch bucket 68 has a satisfactory purity to be used again in the source 50.
According to the present disclosure, target material collected in the target material catch bucket can be used to directly to form target material ingots to be used in an EUV source without a need to purify the target material. As purifying process is costly and involves use of toxic chemicals, disclosed approach offers an environmentally friendly method of obtaining a target material ingot for use in an EUV radiation source such as the radiation source SO, 50 shown in Figs 1 and 2.
A care should be taken that the purity of the collected target material is not diminished in the process of forming target material ingots.
Fig. 3 shows an apparatus 100 for obtaining a target material ingot 111 for use in an EUV radiation source.
The apparatus 100 includes a sealable chamber 102 having an insert 103 for receiving a target material catch bucket 104 of an EUV radiation source such as the radiation source SO, 50 shown in Figs 1 and 2. A heating unit 106 is placed in a proximity of the received target material catch bucket 104. The heating unit 106 may be arranged anywhere suitable to heat target material present in the target material catch bucket 104. For example, the heating unit 106 may be arranged internally within the target material catch bucket 104, or externally thereof. The received target material catch bucket 104 may have an amount of target material 115 accommodated therein. The target material 115 may be tin (Sn) having a purity of 99.999% Sn. The target material catch bucket 104 is sealed to prevent any contamination of the target material 115 during a transfer from the EUV radiation source to the apparatus 100. The target material 115 is in a solid state within the target material catch bucket 104. The heating unit 106 is configured to heat the target material 115 contained within the target material catch bucket 104 and to melt the target material 115 within the target material catch bucket 104.
The assembly 100 further may include a gas controlling unit 108 configured to control a pressure and a gas flow within the chamber 102. Alternatively or additionally, the chamber 102 may have a controlled ambient when in a sealed condition.
The gas controlling unit 108 ensures that an interior of the chamber 102 is at a desired pressure, for example, a relative vacuum, i.e. containing only a small, pre-determined amount of gas. In a non-limiting example, the gas controlling unit 108 may be configured to set a pressure within the chamber 102 to 5 mbar. The gas controlling unit may have a vacuum pump 124 in a fluid connection with the chamber 102 for this purpose.
The gas controlling unit 108 may also be configured to allow inflow of a gas mixture within the chamber 102. This is carried out by, for example, a gas inlet 126 configured to allow inflow of a gas mixture into the chamber 102. Preferably, the gas mixture includes a reducing gas such as a forming gas comprising argon (Ar) and hydrogen (H). Such a gas mixture is desired as it prevents oxidation of the target material 115 during the forming process of the target material ingots 111. The reducing gas may be introduced periodically during the process of forming the target material ingots 111 to purge any contaminants within the chamber 102.
In a non-limiting example, the forming gas may comprise around 98% argon and around 2% hydrogen to allow optimal ambience for carrying out the method and minimize degradation of the target material.
The assembly 100 includes a receptacle 110 comprising at least one mould cavity 114. The receptacle 110 is arranged to receive a portion of the melted target material 115 from the received target material catch bucket 104 into the at least one mould cavity 114 upon melting the target material 115 accommodated in the target material catch bucket 104 by the heating unit 106. This may be carried out by placing the receptacle 110 gravitationally below the target material catch bucket 104.
The assembly 100 may further include a draining unit 121. The draining unit 121 ensures that the melted target material 115 is transferred from the target material catch bucket 104 into the at least one mould cavity 114. In an example shown in Fig. 3, the draining unit 121 is shown in a form of a draining outlet 123 configured to allow a flow of the melted target material 115 into the at least one mould cavity 114. The draining unit 121 may have other forms and may include additional elements. For example, the draining unit 121, may include a displacement unit (not shown) configured to move the draining outlet 121 from one position to another, for example, from a first mould cavity 114a of the at least one mould cavity 114 to a second mould cavity 114b of the at least one mould cavity 114 to allow transfer of the melted target material 115 into both the first and second mould cavities 114a, 114b. In an alternative embodiment, a position of the draining outlet 121 may be fixed and the displacement unit may be configured to displace the receptacle 110 such that a portion of the melted target material 115 is transferred into the first mould cavity 114a and the second mould cavity 114b. In more general, the displacement unit is configured to allow a controlled transfer of a portion of the melted target material 115 into a plurality of mould cavities 114a, 114b.
The receptacle 110 has a contact surface 112 defining the at least one mould cavity 114. The contact surface 112 is a surface of the at least one mould cavity 114 that is in direct contact with the melted target material 115 upon transferring the melted target material into the at least one mould cavity 114.
The inventors have found out that the prior art receptacles introduce additional contaminants when the melted target material is transferred into a mould cavity of the prior art receptacles. In an example, a high purity tin containing 99.999% of Sn has been transferred into a mould cavity of a receptacle according to the prior art having a contact surface made of stainless steel (304 SST). Tin ingots formed in this mould cavity have been found out to have an increased iron (Fe) contamination due to intermolecular interactions between the tin molecules of the tin ingot and iron atoms of the contact surface.
Therefore, the contact surface 112 of the at least one mould cavity 114 is made of a mould material that is inert against the melted target material 115 to prevent contaminating target material ingots 111 as a result of the intermolecular interactions between the melted target material and molecular species of the contact surface. The mould material is a material that does not form intermolecular bonds with the melted target material 115 or form weaker intermolecular bonds (such as van der Waals bonds) with the melted target material than surface bonds within the contact surface 112 (e.g. covalent bonds, ionic bonds, metallic bonds, etc). According to the present disclosure, an inert material is any material that does not introduce more than 5 ppm by weight, or preferably more than 1 ppm by weight of total contaminants into a target material ingot formed in a mould cavity having a contact surface made of this material. Additionally or alternatively, there may be allowable contaminants levels per contaminant species set in place. For example, the inert material may not introduce more than 0.2 ppm by weight of iron (Fe). Consequently, the inert material may be defined based on the total allowable contaminant level and/or based on the allowable contaminant level per contaminant species.
The mould material can be additionally or alternatively described as a non-wetting material, i.e. as a material that prevents wetting of the melted target material to the contact surface.
Consequently, the mould material may be chosen to minimize wetting between the melted target material 115 and the contact surface 112 made of that material. A degree of wetting (wettability) is proportional to a contact angle θ. This angle is shown in Fig. 4. Fig. 4 shows a mould cavity 214 having a contact surface 212 made of the mould material. The contact angle θ is the angle at which the liquid-vapor interface (i.e., an interface between a melted target material droplet 215 and an ambient surrounding the mould cavity 214) meets the solid-liquid interface (i.e., an interface between the contact surface 212 forming a bottom 216 of the mould cavity 214 and the ambient surrounding the mould cavity 214). The mould material has the contact angle in in a range of from 90° to 180°, and more preferably in a range of frrom 120° to 180°, and even more preferably of from 135° to 180°, such as in a range between 150° and 180°, or in a range between 160° and 180°.
Fig. 5 shows a cross-sectional view along a receptacle 310. The receptacle 310 may be the receptacle 110 shown in Fig. 3. The receptacle 310 includes a first structure 330 having at least one mould cavity 314. The receptacle 310 further includes a layer 320 placed over the first structure 330 such that the layer 320 forms a contact surface 312 between the at least one mould cavity 314 and a melted target material to be placed into the at least one mould cavity 314. The layer 320 is made of the mould material. The first structure 330 is made of a first material. The first material may be any material suitable to be used for forming at least one mould cavity 314. For example, the first material may be iron or a stainless steel. The layer 320 may be in a form of a coating covering the entire surface of the first structure 330 as illustrated in Fig. 5. Alternatively, only a surface of the at least one mould cavity 314 may be covered by the layer 320 while a surface of the first structure 330 outside the at least one mould cavity 314 does not have the layer 320, i.e. the coating is not uniformly applied over the entire surface of the first structure 330 but it is only applied to the at least one mould cavity 314. It is noted that the wettability of the layer 320 is significantly lower than the wettability of the first material such that the layer 320 serves as an anti-wetting shield for the first material.
As explained above, the mould material is chosen based on a target material that is used for forming target material ingots. In case of tin (Sn) as the target material, the layer 320 may be, but not limited to, a silicon-carbide (SiC), titanium-nitride (TiN), or diamond-like carbon (DLC).
Alternatively, the layer 320 may be a diffusion layer such as an inchomized layer. The inchromized layer refers to a surface layer on a material, typically a metal (e.g. stainless steel), that has undergone a chromizing process. Chromizing is a thermochemical surface treatment in which chromium atoms are diffused into the surface of a substrate to form a chromium-rich layer. Preferably, the diffusion layer is a hard inchromized layer due to its high temperature resistance and improved wear resistance, corrosion and oxidation resistance resulting in minimal wettability to tin (as the target material) but also other materials.
As shown in Fig. 5, he at least one mould cavity 314 has a shape of a cylinder having a diameter D and a height H. The diameter D and the height H may be chosen according to dimensions of an insert for the target material ingot of the target material supply system. Consequently, the formed target material ingot may be directly, without any post-processing step, used for EUV light generation in the EUV source. The post-processing may include, for example, cutting larger target material ingots to desired dimensions. However, this step may also introduce contaminants to the formed target material ingot due to interaction of the target material ingot with a cutting tool. The diameter D is in a range of from 50 to 150 mm, and preferably of around 100 mm. The height H is in a range of from 50 to 200 mm, and preferably in a range of from 80 to 95 mm.
The assembly 100 as disclosed above ensures that no additional contamination of the target material 115 occurs during the process of formation of the target material ingots. Alternatively or additionally, the assembly 100 ensures that the impurity concentration of the newly formed target material ingots is below the pre-determined impurity concentration such as below 10 ppm by weight. Consequently, the obtained target material ingots may be directly used in an EUV source for an EUV radiation generation without need to apply any non-environmentally sustainable purifying process.
Thus, present disclosure utilizes the fact that the target material collected in the target material catch bucket of an EUV source is already sufficiently pure to be re-used for an EUV radiation generation. The target material ingot for use (or to be re-used) in an EUV radiation source, may be therefore obtained by:

providing a target material catch bucket containing accumulated unused target material from an EUV radiation source;
melting the provided target material; and
transferring the provided target material from the target material catch bucket directly into a receptacle having at least one ingot-shaped mould cavity to form at least one ingot to be used for an EUV radiation generation.

The target material may be a high purity tin (Sn) containing, e.g., at least 99.999% of tin. At least the steps of melting and transferring the provided target material may be carried out under a controlled ambient such as an ambient of a vacuum, an inert gas, and/or a reducing gas to prevent any degradation of the target material during execution of the method. Additionally, the at least one ingot may be then solidified within the at least one ingot-shaped mould cavity preferably under an ambient of a vacuum, an inert gas, and/or a reducing gas. Consequently, the purity of the provided target material substantially does not change during execution of the method. To provide an optimal ambient for carrying out the steps of melting and transferring the provided target material a pressure may be reduced to a value in a range of from 1 to 10 mbar, and preferably of around 5 mbar, prior to introducing the reducing gas. In this manner, the degradation of the target material due to oxidation is minimized. To enhance the effects of reduction, a temperature may be set in a range of from 20°C to 300°C.
Preferably, the method is executed using the assembly shown in Fig. 3.
The formed target material ingot may be then placed directly into a target material supply system of a droplet generator of the EUV radiation source such as the EUV source shown in Fig. 2. The target material ingot is then melted within the target material supply system to form target material droplets or vapor used for generation of the EUV light. A flow of the droplets (or vapor) is then directing towards a plasma formation region of the EUV radiation source and a laser radiation is emitted towards the plasma formation region. As explained above, the laser radiation is configured to be at least partially absorbed by the target material at the plasma formation region thereby generating plasma and the EUV radiation emitted therefrom.
The obtained target material ingots are used in an lithographic apparatus for manufacturing a semiconductor device comprising. The steps to be carried out include:

receiving a substrate with a photoresist layer,
generating radiation as disclosed above;
allowing at least part of this radiation via an illumination system of the lithographic apparatus into the projection system to generate patterned radiation;
projecting the patterned radiation from the projection system onto the photoresist layer; and subsequently
removing a portion of the photoresist layer to form a corresponding pattern over the substrate.

The disclosure is summarized in the following clauses:

1. An assembly for obtaining a target material ingot for use in a radiation source, the assembly comprising:
- a sealable chamber configured to receive a target material catch bucket of a radiation source, wherein the target material catch bucket is configured to accommodate an amount of unused target material;
- a heating unit configured to heat the target material contained within the received target material catch bucket to at least a melting temperature of the target material;
- a receptacle comprising at least one mould cavity, the at least one mould cavity is arranged to receive a portion of a melted target material from the received target material catch bucket upon melting the target material accumulated in the target material catch bucket; and
- the receptacle having a contact surface defining the at least one mould cavity, wherein the contact surface is made of a mould material that is inert against the target material.
2. The assembly according to clause 1, wherein the target material catch bucket accommodates an amount of off-droplet target material.
3. The assembly according to clause 1, wherein the mould material is configured to prevent wetting of the melted target material to the contact surface.
4. An assembly for obtaining a target material ingot for use in a radiation source, the assembly comprising:
- a sealable chamber configured to receive a target material catch bucket of a radiation source, wherein the target material catch bucket is configured to accommodate an amount of unused target material;
- a heating unit configured to heat the target material contained within the received target material catch bucket to at least a melting temperature of the target material;
- a receptacle comprising at least one mould cavity, the at least one mould cavity is arranged to receive a portion of a melted target material from the received target material catch bucket upon melting the target material accommodated in the target material catch bucket; and
- the receptacle having a contact surface defining the at least one mould cavity, wherein the contact surface is made of a mould material that is non-wetting against the target material.
5. The assembly according to clause 3 or 4, wherein a contact angle between the contact surface and the melted target material is in a range of from 90° to 180°, and more preferably in a range of from 120° to 180°, and even more preferably of from 135° to 180°.
6. The assembly according to any one of the preceding clauses, wherein the mould material is provided in a form of a coating over the receptacle.
7. The assembly according to clause 6, wherein the receptacle comprises a first material, and the coating is placed over the first material, and wherein the first material has a greater wettability to the melted target material than the mould material.
8. The assembly according to any one of the preceding clauses, wherein the target material is tin.
9. The assembly according to clause 8, wherein the tin has a purity of at least 99.999%.
10. The assembly according to clause 8 or 9, wherein the mould material comprises one or more selected of the following: titanium nitride, silicon carbide, and diamond-like carbon.
11. The assembly according to any one of clauses 6 to 9, wherein the coating comprises an inchromized layer, preferably a hard inchromized layer.
12. The assembly according to any one of the preceding clauses, wherein the chamber has a controlled ambient when in a sealed condition.
13. The assembly according to clause 12, wherein the assembly includes a gas controlling unit configured to control a pressure and the ambient within the chamber.
14. The assembly according to clause 13, wherein the gas controlling unit comprises a vacuum pump configured to evacuate the chamber.
15. The assembly according clause 13 or 14, wherein the gas controlling unit comprises a gas inlet configured to allow inflow of a gas mixture into the chamber.
16. The assembly according to clause 15, wherein the gas mixture comprises an inert gas.
17. The assembly according to clause 15, wherein the gas mixture is a reducing gas, preferably a forming gas comprising argon and hydrogen.
18. The assembly according to clause 17, wherein the forming gas comprises around 2% hydrogen and a balance of argon.
19. The assembly according to any one of the preceding clauses, wherein the at least one mould cavity is shaped to cast the target material ingot.
20. The assembly according to clause 19, wherein the at least one mould cavity has a shape of a cylinder having a diameter in a range of from 50 mm to 150 mm, preferably in a range of from 95 mm to 105 mm.
21. The assembly according to clause 20, wherein the cylinder further has a height in a range of from 50 mm to 200 mm, preferably in a range of from 80 mm to 95 mm.
22. A method for obtaining a target material ingot for use in a radiation source, the method comprising the steps of:
- providing a target material catch bucket containing accumulated unused target material from a radiation source;
- melting the provided target material; and
- transferring the provided target material from the target material catch bucket directly into a receptacle having at least one ingot-shaped mould cavity to form at least one ingot to be used for generation of radiation.
23. The method according to clause 22, wherein the unused target material is off-droplet target material.
24. The method according to clause 22 or 23, wherein the method includes a step of solidification of the at least one ingot within the at least one ingot-shaped mould cavity, preferably wherein the solidification is carried out under a controlled ambient.
25. The method according to clause 24, wherein the controlled ambient is an ambient of a vacuum, or an inert gas and/or a reducing gas.
26. The method according to any one of clauses 22 to 25, wherein a purity of the target material is at least 99.999%.
27. The method according any one of clauses 22 to 26, wherein the purity of the provided target material substantially does not change during execution of the method.
28. The method according to any one of clauses 22 to 27, wherein the target material consists of tin.
29. The method according to any one of clauses 22 to 28, wherein the steps of melting and transferring are carried out under a controlled ambient.
30. The method according to clause 29, wherein the controlled ambient includes: a vacuum, or an inert gas or preferably a reducing gas.
31. The method according to clause 30, wherein the controlled ambient includes reducing a pressure to a value in a range of from 1 to 10 mbar, and preferably a range of from 4 to 6 mbar, more preferably of around 5 mbar, prior to introducing the reducing gas.
32. The method according to clause 29 or 31, wherein the reducing gas is introduced periodically to prevent oxidation of the target material during execution of the method.
33. The method according to any one of clauses 22 to 32, wherein the receptacle has a contact surface defining the at least one ingot-shaped mould cavity, wherein the contact surface is made of an mould material that is inert against the melted target material.
34. The method according to any one of clauses 22 to 33, wherein the receptacle has a contact surface defining the at least one ingot-shaped mould cavity, wherein the contact surface is made of an mould material that is non-wetting against the melted target material.
35. The method according to any one of clauses 22 to 34, wherein the method is executed using an assembly according to any one of clauses 1 to 21.
36. A method for supplying a target material to a radiation source of an EUV exposure apparatus, wherein the method comprises a step of:
- providing a target material ingot obtained using a method according to any one of clauses 22 to 35 directly into a target material supply system of a droplet generator of the EUV radiation source.
37. The method according to clause 36, wherein the method includes a step of melting the provided target material ingot within the target material supply system.
38. A method for generating a radiation within a radiation source of an EUV exposure apparatus, wherein the method comprises:
- providing a target material ingot obtained using a method according to any one of clauses 22 to 35 directly into a target material supply system of a droplet generator of the radiation source;
- melting the target material ingot and directing a flow of the melted target material towards a plasma formation region of the radiation source; and
- emitting a laser radiation towards the plasma formation region, wherein the laser radiation is configured to be at least partially absorbed by the target material at the plasma formation region thereby generating plasma and the radiation emitted therefrom.
39. A method of manufacturing a semiconductor device comprising the steps of:
- receiving a substrate with a photoresist layer,
- generating radiation with the method of clause 38;
- allowing at least part of this radiation via an illumination system into the projection system to generate patterned radiation;
- projecting the patterned radiation from the projection system onto the photoresist layer; and subsequently
- removing a portion of the photoresist layer to form a corresponding pattern over the substrate.

While specific embodiments of the disclosure 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

An assembly for obtaining a target material ingot for use in a radiation source, the assembly comprising:
- a sealable chamber configured to receive a target material catch bucket of a radiation source, wherein the target material catch bucket is configured to accommodate an amount of unused target material;

- a heating unit configured to heat the target material contained within the received target material catch bucket to at least a melting temperature of the target material;

- a receptacle comprising at least one mould cavity, the at least one mould cavity is arranged to receive a portion of a melted target material from the received target material catch bucket upon melting the target material accumulated in the target material catch bucket; and

- the receptacle having a contact surface defining the at least one mould cavity, wherein the contact surface is made of a mould material that is inert against the target material.
The assembly according to claim 1, wherein the target material catch bucket accommodates an amount of off-droplet target material.
The assembly according to claim 1 or 2, wherein the mould material is configured to prevent wetting of the melted target material to the contact surface.
The assembly according to claim 3, wherein a contact angle between the contact surface and the melted target material is in a range of from 90° to 180°, and more preferably in a range of from 120° to 180°, and even more preferably of from 135° to 180°.
The assembly according to any one of the preceding claims, wherein the mould material is provided in a form of a coating over the receptacle.
The assembly according to any one of the preceding claims, wherein the target material is tin, wherein the tin has a purity of at least 99.999%.
The assembly according to claim 6, wherein the mould material comprises one or more selected of the following: titanium nitride, silicon carbide, and diamond-like carbon.
The assembly according to any one of claims 5 to 7, wherein the coating comprises an inchromized layer, preferably a hard inchromized layer.
The assembly according to any one of the preceding claims, wherein the chamber has a controlled ambient when in a sealed condition.
The assembly according to any one of the preceding claims, wherein the at least one mould cavity is shaped to cast the target material ingot.
A method for obtaining a target material ingot for use in a radiation source, the method comprising the steps of:
- providing a target material catch bucket containing accumulated unused target material from a radiation source;

- melting the provided target material; and

- transferring the provided target material from the target material catch bucket directly into a receptacle having at least one ingot-shaped mould cavity to form at least one ingot to be used for generation of radiation.
The method according to claim 11, wherein the unused target material is off-droplet target material.
The method according to claim 11 or 12, wherein the steps of melting and transferring are carried out under a controlled ambient, and wherein the method includes a step of solidification of the at least one ingot within the at least one ingot-shaped mould cavity, wherein the solidification is carried out under a controlled ambient.
The method according any one of claims 11 to 13, wherein the purity of the provided target material substantially does not change during execution of the method.
A method for supplying a target material to a radiation source of an EUV exposure apparatus, wherein the method comprises a step of:
- providing a target material ingot obtained using a method according to any one of claims 11 to 14 directly into a target material supply system of a droplet generator of an EUV radiation source; and

- melting the provided target material ingot within the target material supply system.