Rotating Target for Extreme Ultraviolet Source with Liquid Metal

Abstract

It is an extreme ultraviolet (EUV) light source and has a vacuum chamber with a rotating target assembly therein. The rotating target assembly has an annular groove with a distal wall relative to the rotation axis. The distal wall has a porous region. With a molten metal layer on the distal wall of the annular groove in the rotating target assembly, the rotating target assembly is rotated to form a target by centrifugal force.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims priority based on U.S. Provisional Patent Application No. 63 / 350,868, filed on June 10, 2022, and incorporates its disclosure herein by reference.

[0002] This disclosure relates to extreme ultraviolet light sources.

Background Art

[0003] In next - generation projection lithography, when mass - producing integrated circuits (ICs) with a structural size of 10 nm or less, extreme ultraviolet (EUV) radiation in the range of 13.5 ± 0.135 nm corresponding to the effective reflection region of a multilayer Mo / Si mirror is used. Controlling the IC to be defect - free is an important part of the metrology process. A general trend in lithographic production is the shift from time - consuming and costly IC inspections in mass production to the analysis of lithographic masks. If there are defects in the mask, they are projected onto a silicon substrate with photoresist and defects appear on the printed chips. The mask in EUV lithography is a Mo / Si mirror, on which a topological pattern formed of a material that absorbs radiation with a wavelength of 13.5 nm is added. The most efficient method for the mask inspection process is to be performed at the same wavelength as the actinic radiation, which is radiation with a wavelength matching that of the lithography operation. Such scanning with radiation having a wavelength of 13.5 nm enables defect detection with a resolution better than 10 nm. Making the lithographic mask defect - free throughout their production and entire operating cycle is a challenge in EUV lithography. The creation of devices for diagnosing lithographic masks and their high - brightness actinic sources is a priority in EUV lithography development.

[0004] As an example of the design, a rotatable target with liquid tin or other metal with a relatively low melting temperature (melting point), such as In, Pb, Ga, Cd, Bi, or Li, or a combination thereof, can be scattered on the inner wall of the rotating drum. This rotatable target can be used as an EUV light source. Due to the interaction between the laser pulse and the liquid metal surface, surface waves are generated for each laser pulse. These waves will interfere with each other due to the high-speed rotation of the drum (e.g., over 1000 rpm) or the high-speed repetition of the laser-generated plasma (e.g., over 10 kHz).

[0005] Due to these surface waves, instability occurs in the position of the liquid metal surface with respect to the focusing spot of the driving laser. As a result, the size of the laser beam at the interaction point with the target fluctuates, and consequently, the laser beam intensity fluctuates. As a result, the in-band conversion efficiency, which is determined by the deviation of the actual laser intensity from the optimal laser intensity, fluctuates, and fluctuations in EUV energy occur for each pulse. Since the brightness of the light source is determined by the ratio of EUV energy to the radiation surface area, it will fluctuate due to the small depth of focus (Rayleigh length) of the focusing lens.

[0006] As indicated by the estimation, the speed of the wave is lower than the linear speed of the drum. Therefore, it is impossible for the wave generated by a single pulse to cause surface disturbance with the next laser pulse. At a high rotation speed (e.g., reaching 200 Hz), the wave propagation until the drum makes a full circle is only 5 ms. The surface disturbance by the wave occurs after one full circle. The interference between waves from multiple pulses may occur, which may cause high-amplitude waves and may be the cause of the instability of EUV energy and luminance.

[0007] The rotating drum has a distal wall (when viewed from the axis of rotation) and may also have a proximal wall. The distal wall is a wall covered with liquid metal. The proximal wall is a wall that reduces the scattering and / or evaporation of liquid metal in the vacuum chamber by laser pulses. When the evaporated liquid metal accumulates on the surface, problems in the operation of the line source may occur. The thickness of the liquid metal in the interaction zone can be several millimeters (e.g., 2 - 3 mm). If the thickness is reduced below the minimum value, the scattering of liquid metal by laser pulses may increase, which is determined by the propagation of shock waves in the liquid metal. Conversely, it may also be necessary to reduce the thickness in order to enhance the effects of friction and viscosity and attenuate the amplitude of the propagating wave.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] Therefore, an improved system and method are required.

Means for Solving the Problems

[0010] In the first embodiment, a system is provided. This system includes a vacuum chamber and a rotating target assembly having an annular groove with a distal wall with respect to the axis of rotation. The rotating target is disposed within the vacuum chamber. A porous region is provided on the distal wall.

[0011] A proximal wall facing the distal wall can be provided on the rotating target assembly, and the annular groove can be formed thereby.

[0012] The system can incorporate a rotating system coupled to its rotating target assembly. The rotating system can be configured to rotate the rotating target assembly about a rotation axis.

[0013] The vacuum chamber can be provided with an incident window and an exit window, or an incident window and an optical system. The proximal wall of the annular groove can be configured such that a line of sight is provided between the distal wall and the incident and exit windows or the optical system during the laser pulse.

[0014] The system can incorporate a laser light source configured to direct a laser beam at the distal wall.

[0015] In the system, molten metal can be disposed in the annular groove. The molten metal can be disposed on the aforementioned porous region.

[0016] The porous region can have holes with a diameter of less than 1 mm.

[0017] The porous region can have a thickness of 1 to 5 mm extending from the distal wall into the annular groove.

[0018] The porous region can be made of titanium, stainless steel, aluminum or molybdenum.

[0019] The thickness of the porous region can vary transversely to the distal wall.

[0020] The second embodiment provides a method. In this method, with a molten metal layer disposed on the distal wall of the annular groove in the rotating target assembly, the rotating target assembly is rotated in a vacuum chamber to form a target by centrifugal force. A porous region is provided on the distal wall. A pulsed laser beam is directed through the incident window of the vacuum chamber. The target on the distal wall is irradiated with the pulsed laser beam. The generated short-wavelength radiation beam is directed from the target.

[0021] The proximal wall of the annular groove can be configured such that a line of sight is provided between the distal wall and both the incident window and the exit window or between the incident window and both the optical system during the aforementioned aiming.

[0022] The molten metal can be disposed on the porous region.

[0023] The porous region can have pores with a diameter of less than 1 mm.

[0024] The porous region can have a thickness of 1 to 5 mm facing into the annular groove.

[0025] The porous region can be made of titanium, stainless steel, aluminum, or molybdenum.

[0026] The porous region can be positioned beneath the surface of the target during the aforementioned rotation.

[0027] The molten metal layer can be made to exhibit a depth greater than the height of the porous region along a direction perpendicular to the axis of rotation of the rotating target assembly during the aforementioned rotation.

[0028] The aforementioned short-wavelength radiation beam can be directed through the exit window of the vacuum chamber or through the optical system within the vacuum chamber.

[0029] For a more complete understanding of the nature and objects of the present disclosure, reference should be made to the following accompanying drawings in conjunction with the detailed description set forth below.

Brief Description of the Drawings

[0030]

Figure 1

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Figure 7

[0031] The subject matter recited in the claims is described by specific embodiments, but other embodiments are also within the technical scope of the present disclosure, including embodiments that do not provide all of the benefits and features described herein. Various structural, logical, processing step, and electronic modifications can be made without departing from the technical scope of the present disclosure. Thus, the technical scope of the present disclosure is defined solely by reference to the appended claims.

[0032] In the present invention disclosure, a laser-produced plasma (LPP) target for an EUV source with a design where the inner surface of a rotating drum is covered with a liquid metal (e.g., tin) is described. In the embodiments of the present disclosure, EUV performance stability (i.e., stability of in-band luminance and energy per pulse of EUV) is improved through reduction of waves generated on the target surface. Since the waves can be attenuated in the design of the present disclosure, a smooth surface for the interaction between the focused laser and the target can be provided after at least one full rotation.

[0033] Figure 1 is a cross-sectional view of system 100. This system has a vacuum chamber 101, and a rotating target assembly 102 is provided within the vacuum chamber 101. The rotating target assembly 102 has an annular groove 109, which is provided with a distal wall 110 and a proximal wall 111 distal and proximal to the rotation axis 104, respectively. The distal wall 110 has a porous region 112. The porous region 112 can extend from the surface of the distal wall 110 into the annular groove 109 (i.e., along the X direction perpendicular to the rotation axis 104) and can have a thickness of 1 to 5 mm. The rotating target assembly 102 can be made of aluminum, titanium, their alloys, or other various materials.

[0034] Depending on the design of the rotating target assembly 102, the proximal wall 111 can be either rotated or stationary. In some embodiments, the proximal wall 111 is omitted, and only the distal wall 110 is provided on the rotating target assembly 102. Even if the proximal wall 111 is not provided, the annular groove 109 can be measured based on a member at the center of the rotating target assembly 102, or alternatively, it can be a circular groove.

[0035] A rotation system 103 is coupled to the rotating target assembly 102. The rotation system 103 rotates the rotating target assembly 102 around the rotation axis 104. With the rotation system 103, rotation can be transmitted to the rotating target assembly 102 using a shaft. The rotation system 103 can be an electric motor or other mechanism.

[0036] The vacuum chamber 101 can be provided with an incident window 107 and an exit window 108. The proximal wall 111 of the annular groove 109 can be configured such that a line of sight is provided between the distal wall 110 and the incident window 107 and the exit window 108 during the laser pulse. The laser light source 105 is configured to direct the laser beam 106 towards the distal wall 110. The liquid metal on the distal wall 110 is the target for the laser beam 106.

[0037] Although shown in FIG. 1 with an exit window 108, the system 100 may also be provided with in-vacuum chamber optical elements for focusing EUV radiation. In this design, the exit window 108 may not be present.

[0038] A molten metal (shown in other figures) is disposed within the annular groove 109. This liquid metal can be tin, other low melting point metals or low melting point alloys. In addition to Sn, those other metals that may be included are In, Pb, Ga, Cd, Bi, Li, or combinations thereof. In one example, the molten metal is disposed on the porous region 112, such as on the surface or within the pores of the porous region 112. By rotating the rotatable target assembly 102 about the axis of rotation 104, the molten metal can be retained on the distal wall 110. Thereby, the direct influence from the laser beam 106 on the porous region 112 can be prevented.

[0039] The porous region 112 can be a sponge-type or sintered metal insert. The porous region 112 can also be formed directly inside or on the surface of the rotatable target assembly 102. The porous region 112 can extend around the entire circumference of the rotatable target assembly 102, or can extend to a part of the periphery of the rotatable target assembly 102. The height (along the Y direction) of the porous region 112 can extend from the base of the annular groove 109 to the top of the molten metal or the top of the distal wall 110 throughout. The height of the porous region 112 can also be made to extend only less than the entire distance from the base of the annular groove 109 to the top of the molten metal or the top of the distal wall 110. The thickness (along the X direction) of the porous region 112 can be uniform or variable across the surface of the annular groove 109.

[0040] The porous region 112 can have holes with a diameter of less than 1 mm. The porous region 112 can be made of titanium, stainless steel, molybdenum, aluminum or other metals or metal alloys. By configuring the thickness of the molten metal layer in the annular groove 109 during rotation (i.e., that along the X direction) to be as small as possible while still being thick enough, the porous region 112 can be prevented from being affected by the laser beam 109. This can help keep the porous region 112 intact. The thickness of the molten metal can be selected experimentally and can be set to 0.5 to 1 mm, corresponding to the depth of the laser crater. Since it is an effective thickness, no splashing occurs due to the interaction between the laser pulse (or the shock wave generated by that pulse) and the liquid metal immersed in the porous region 112. By reducing the thickness of the liquid metal and increasing the roughness of the porous region 112, the attenuation of the wave generated by the laser pulse can be assisted.

[0041] In addition to preventing splashing, the porous region 112 can be made to function as a reservoir for the liquid metal when the liquid metal is cut using the laser beam 106. The liquid metal can be stored in the holes of the porous region 112.

[0042] The rotating target assembly 102 can be disc-shaped. However, the shape of the rotating target assembly 102 can also be wheel-shaped, low-profile polyhedral prism-shaped or other shapes.

[0043] The liquid targets used in the embodiments of the present disclosure, in contrast to solid targets, help ensure the reproducibility of the target surface. This enhances the pulse-to-pulse stability of the emission characteristics of the short-wavelength radiation source. Through the continuous circulation, renewal, and replenishment of the liquid metal, the long-term stability of the short-wavelength radiation source can be achieved. By using a laser-produced plasma of a metal (e.g., tin), the short-wavelength radiation source can surely be made highly bright and highly efficient. This can apply to 13.5 nm, which is the operating wavelength of EUV lithography. In the rotating target assembly 102, debris particle outflow beyond it can be restricted, so that the cleanliness of the short-wavelength radiation source can be improved and the consumption of the target material can be reduced.

[0044] With the laser light source 105, short laser pulses (e.g., 100 ns or less) can be generated. According to an embodiment, the wavelength of the laser can be set to 1 μm to 10 μm. By using a synchronization system in combination with the laser light source 105, the irradiation of the surface of the rotating target assembly 102 can be made along the line of sight. Since a reflected continuous signal of the auxiliary laser radiation modulated by the marker can be detected by the photodetector, the rotation angle of the annular groove 109 when starting the main pulsed laser can be made the same as that brought about by the line of sight connecting the interaction zone and the incident and exit windows 107, 108 via the proximal wall 111.

[0045] According to an example, the fine droplets of the target material passing into the opening of the proximal wall 111 can be reversely discharged into the annular groove 109 under the action of centrifugal force. That is, the plasma-forming material of the target can be prevented from leaving the annular groove 109, and the line source life can be extended without refilling.

[0046] Figure 2 is a cross-sectional view of an embodiment of a portion of the rotating target assembly 102. This embodiment does not have a proximal wall 111. The porous region 112 is a region where the liquid metal 113 has penetrated into a porous material (e.g., sponge or sintered product). The thickness of the liquid metal 113 (i.e., the volume of the liquid metal 113) with respect to the distal wall 110 can be reduced compared to a design without the porous region 112. The liquid metal 113 is held on the distal wall 110 using centrifugal force during the rotation of the rotating target assembly 102.

[0047] Figure 3 is a cross-sectional view of another embodiment of a portion of the rotating target assembly 102. The proximal wall 111 has an incident opening 115 and an exit opening 114 for the laser beam 106 as shown. There is also a cover 116 between the distal wall 110 and the proximal wall 111. The cover 116 can help keep the droplets of the liquid metal 113 within the desired area. In the embodiment of Figure 3, the distal wall 110 is inclined with respect to the proximal wall 111. That is, the distal wall 110 does not meet the base of the rotating target assembly 102 at a perpendicular angle.

[0048] In some cases, waves may propagate within the liquid metal 113 and be reflected at the solid surface, and thus scattering may occur. The wave propagation within the liquid metal 113 is reduced by the porous region 112. The porous region 112 can also prevent the scattering and surface non-uniformity of the liquid metal 113. Due to scattering, micro-droplets may be generated around the EUV and the laser tunnel, and they may gradually clog them. By making the liquid metal 113 have a more uniform distribution (i.e., a uniform surface), the vibration in the rotating target assembly 102 can be reduced. By making the distribution uniform, the vibration of the rotating target assembly 102 is reduced, the position of the target surface with respect to the laser focus spot can be stabilized, and thus the EUV stability can be improved.

[0049] According to the embodiments of the present disclosure, the velocity of the surface wave generated by the interaction between the laser pulse constituting the laser beam 106 and the liquid metal 113 can be estimated based on the Kortweg - de Vries equation that describes the wave on shallow water under the action of the gravitational field. The propagation velocity c = (gh) 1 / 2 is given. In the case of the rotating target assembly 102, the acceleration of free fall is replaced by the centrifugal acceleration V 2 / R. Here, V is the linear velocity of the drum surface having a radius R. In this case, the velocity of the surface wave is c = V(h / R) 1 / 2 . Since c≪V when h≪R, the wave generated by the laser pulse moves out of the focusing zone at a speed V, while the generated wave propagates more slowly. For example, if V = 100 m / s, R = 80 mm, and h = 2 mm, then c = 16 m / s. However, when the rotating target assembly 102 makes a full rotation (for example, at 12000 rpm, it takes 5 milliseconds), unless it is quiet, the wave may hit the laser. The amplitude of the wave can initially be a fraction in the millimeter range (e.g., 0.05 - 0.2 mm), and unless it is attenuated by the porous region 112, it can be maintained as it is or further enhanced like a tsunami.

[0050] FIG. 4 is a cross - sectional view of another embodiment of a part of the rotating target assembly 102. A stationary shield 118 is used within its irradiation zone and can serve as part or all of the proximal wall 111. There is a gap 119 between the rotating target assembly 102 and the stationary shield 118. The incident aperture 115 and the exit aperture 114 can be drilled in the stationary shield 118 and aligned with the laser beam and the EUV optical system. In one example, the incident aperture 115 and the exit aperture 114 are conical. The stationary shield 118 can be separate from the rotating member of the target, and synchronization between the stationary shield and the cover 116 and / or between the stationary shield 118 and the base of the rotating target assembly 102 is not required.

[0051] In some cases, as shown in FIGS. 5 and 6 according to an example, the thickness of the porous region 112 is made to vary transversely to the distal wall 110. Waves can be attenuated using the grooved rotating target assembly 102. Segment 117 extends from the distal wall 110 as shown, thereby forming grooves. Those grooves can be made to a depth such that the scattering interaction between the laser and the liquid metal 113 is avoided. A barrier can be generated by segment 117, and a state can be achieved where there is a liquid metal 113 with a minimum thickness above that segment 117. These waves can be attenuated by this barrier. Since there may be a small thickness of molten metal in the space between the grooves, the waves will pass through surfaces with different depths. In the shallow regions, the waves will slow down and their amplitude will be reduced by viscosity.

[0052] The distribution uniformity of the liquid metal 113 can be provided by proper filling of the rotating target assembly 102. The laser beam 106 can be synchronized with the groove position using an encoder and external triggering.

[0053] The grooves can be filled with the liquid metal 113 to generate a thin layer on top of the segment 117. Since the thickness of the liquid metal 113 on top of the segment 117 with reference to the distal wall 110 can be as small as 0.1 to 0.2 mm or less, surface waves can be efficiently attenuated there. Porosity can generate additional viscous friction.

[0054] According to an embodiment, the segment 117 can be formed within the porous region 112. Since it is a porous material, it can provide the advantage of equalizing the thickness distribution of the liquid metal 113 due to the high rotation speed of the rotating target assembly 102.

[0055] According to the embodiments of the present disclosure, waves generated by the interaction between a laser pulse and a liquid metal surface can be attenuated. As a result, EUV stability on a pulse-to-pulse basis can be improved with respect to both in-band energy and luminance. Since instability related to vibrations and other sources of disturbance is reduced, the thickness distribution of the liquid metal can be made more uniform. As a result, improvements in the target surface position and stabilization of the source luminance are also brought about.

[0056] FIG. 7 is a flowchart of method 200. In 201, with the molten metal layer disposed on the distal wall of the annular groove in the rotating target assembly, the vacuum chamber is rotated within the rotating target assembly to form a target by centrifugal force. The distal wall is provided with a porous region. The molten metal can be located above and inside the porous region. The porous region can have holes with a diameter of less than 1 mm and a thickness of 1 to 5 mm towards the inside of the annular groove.

[0057] In 202, a pulsed laser beam is directed through the entrance window of the vacuum chamber. In 203, the pulsed laser beam is irradiated onto the target (e.g., liquid metal) on the distal wall. In 204, the short-wavelength radiation beam generated by the irradiation is directed through the exit window of the vacuum chamber or an optical system within the vacuum chamber. By appropriately configuring the proximal wall of the annular groove, a line of sight can be provided between the distal wall and both the entrance window and the exit window during the directing. The proximal wall can be arranged as a stationary shield or can have a gap with respect to adjacent rotating members.

[0058] The porous region can be positioned below the surface of the target during rotation. The depth of the molten metal layer can be made greater than the height of the porous region along a direction perpendicular to the axis of rotation of the rotating target assembly during rotation. Wave propagation is reduced, resulting in a more uniform liquid metal distribution.

[0059] To generate a high-temperature laser-produced plasma with a strong optical output belonging to the short-wavelength spectrum ranging from ultraviolet to the soft X-ray band, the laser radiation power density of the laser beam 106 on the target should be 10 10 ~10 12 W / cm 2 and the length of the laser pulse should be set to 100 ns to 0.5 ps.

[0060] Any number of pulsed or modulated lasers can be used to generate the laser beam 106. The laser light source 105 can be of the solid, fiber, disk, or gas discharge type. The average laser radiation power in the laser beam 106 can be in the range of 10 W to about 1 kW or more, and the laser beam 106 can be focused onto a small focal spot on the target, for example, one with a diameter of about 100 μm.

[0061] The laser pulse repetition frequency can be 1 kHz to 10 MHz. By increasing the pulse repetition rate and decreasing the output laser energy within this range, the scattering of debris particles can be reduced.

[0062] The vacuum chamber can be evacuated to less than 10 -5 ~10 -8 bar using an oil-free pumping system, thereby removing gas components such as nitrogen and carbon that can interact with the target material.

[0063] The vacuum chamber can be filled with a buffer gas (e.g., H2, He, or Ar) having a high short-wavelength radiation transmittance to protect the optical system from debris generated by the plasma.

[0064] The liquid metal can be kept in a molten state using an induction heating system configured to keep it within the optimal temperature range through temperature stabilization of the liquid metal.

[0065] Although the present disclosure has been described in connection with one or more specific embodiments, it will be understood that other embodiments of the present disclosure can be made without departing from the technical scope of the present disclosure. That is, the present disclosure is limited only by the appended claims and their reasonable interpretation.

Claims

1. A system comprising: a vacuum chamber; a rotating target assembly having an annular groove with a distal wall distal to a rotation axis; wherein the rotating target is disposed within the vacuum chamber and the distal wall has a porous region.

2. The system of claim 1, wherein the rotating target assembly has a proximal wall on the opposite side of the distal wall, and the annular groove is formed thereby.

3. The system of claim 1, further comprising a rotation system coupled to the rotating target assembly, the rotation system configured to rotate the rotating target assembly about the rotation axis.

4. The system of claim 1, wherein the vacuum chamber has an incident window and an exit window, or an incident window and an optical system.

5. The system of claim 4, wherein the proximal wall of the annular groove is configured such that a line of sight is provided between the distal wall and both the incident window and the exit window, or between the incident window and both the optical system, during a laser pulse.

6. The system of claim 1, further comprising a laser source configured to direct a laser beam at the distal wall.

7. The system of claim 1, further comprising molten metal disposed within the annular groove.

8. The system of claim 7, wherein the molten metal is disposed on the porous region.

9. The system of claim 1, wherein the porous region has pores with a diameter of less than 1 mm.

10. The system of claim 1, wherein the porous region has a thickness of 1 to 5 mm extending from the distal wall into the annular groove.

11. The system of claim 1, wherein the porous region is made of titanium, stainless steel, aluminum, or molybdenum.

12. The system of claim 1, wherein the porous region has a thickness that varies transversely across the distal wall.

13. A method comprising: With the molten metal layer disposed on the distal wall of the annular groove in the rotating target assembly, the rotating target assembly in the vacuum chamber is rotated to form a target by centrifugal force, provided that the distal wall is provided with a porous region, Directing a pulsed laser beam through the entrance window of the vacuum chamber, Irradiating the target on the distal wall with the pulsed laser beam, and Directing the generated short-wavelength radiation beam away from the target, Method.

14. The method according to claim 13, wherein during the directing, a line of sight is provided between the distal wall and both the entrance window and the exit window, or between the entrance window and both the optical system, and the proximal wall of the annular groove is configured.

15. The method according to claim 13, wherein the molten metal is disposed on the porous region.

16. The method according to claim 13, wherein the porous region has holes with a diameter of less than 1 mm.

17. The method according to claim 13, wherein the porous region has a thickness of 1 to 5 mm towards the inside of the annular groove.

18. The method according to claim 13, wherein the porous region is made of titanium, stainless steel, aluminum or molybdenum.

19. The method according to claim 13, wherein during the rotation, the porous region is located below the surface of the target.

20. The method according to claim 13, wherein during the rotation, the molten metal layer has a greater depth than the height of the porous region along a direction perpendicular to the rotation axis of the rotating target assembly.

21. The method according to claim 13, wherein the short-wavelength radiation beam is directed through the exit window of the vacuum chamber or through the optical system in the vacuum chamber.

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