Confocal chromatic metrology for EUV source condition monitoring

Abstract

The light source 300 includes a rotatable drum 302 coated with xenon ice and irradiated with a laser beam to generate a plasma. The drum 302 may also be translatable. The light source 300 further includes a confocal chromatic sensor 200 for measuring the distance from the confocal chromatic sensor 200 to the rotatable drum 302. The confocal chromatic sensor 200 may include a sensor head 206 for focusing light onto the rotatable drum 302 and detecting light reflected from the rotatable drum 302. The sensor head 206 and the rotatable drum 302 may be disposed within a vacuum chamber 304.

Description

[Technical Field]

[0001] The present disclosure relates to metrology for xenon ice coated drums in extreme ultraviolet (EUV) light sources, and more particularly to the use of confocal chromatic sensors for such metrology. [Background technology]

[0002] An EUV light source can include a rotating drum with an outer surface coated with xenon (Xe) ice (i.e., solid Xe). Irradiating the Xe ice with a laser beam forms a plasma that emits EUV light. Optical imaging using a camera can be performed to determine parameters of the Xe ice, such as its thickness and surface quality. The camera images several portions of the Xe ice, and image processing algorithms extract relevant information from those images. However, the extracted information is limited in terms of spatial resolution and bandwidth. Therefore, it is difficult to identify parameters of the Xe ice (e.g., for real-time feedback) using optical imaging. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent Application Publication No. 2017 / 0142818 [Patent Document 2] U.S. Patent Application Publication No. 2017 / 0276615 [Patent Document 3] U.S. Patent Application Publication No. 2009 / 0059232 Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, there is a need for an improved system and method for monitoring Xe ice on a rotating drum in an EUV light source. Such a system and method can involve a confocal chromatic sensor, also known as a confocal sensor. [Means for solving the problem]

[0005] In some embodiments, the light source includes a rotatable drum coated with Xe ice and irradiated with a laser beam to generate a plasma, and the light source also includes a confocal chromatic sensor for measuring the distance from the confocal chromatic sensor to the rotatable drum.

[0006] In some embodiments, a method of operating a light source includes rotating a drum, coating the drum with Xe ice while rotating the drum, and irradiating the drum with a laser beam to generate a plasma while rotating the drum with the Xe ice coated. The method also includes monitoring the drum with a confocal chromatic sensor while irradiating the drum with the laser beam to detect defects in the Xe ice on the drum, and shutting off the laser beam in response to detecting defects in the Xe ice on the drum.

[0007] For a better understanding of the various described implementations, please refer to the following detailed description in conjunction with the following drawings. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a cross-sectional side view of an EUV light source according to some embodiments. [Figure 2] FIG. 1 illustrates a confocal chromatic sensor according to some embodiments. [Figure 3A] FIG. 1 illustrates components of an EUV light source according to some embodiments. [Figure 3B] FIG. 1 illustrates components of an EUV light source according to some embodiments. [Figure 4A]3A-3B and a housing in which the rotatable drum of FIGS. 3A-3B is disposed, according to some embodiments. [Figure 4B] 3A-3B and a housing in which the rotatable drum of FIGS. 3A-3B is disposed, according to some embodiments. [Figure 5] 1 is a graph showing time trace data for Xe ice thickness on a rotatable and translatable drum according to some embodiments. [Figure 6] 10 is a graph illustrating the variation in thickness of Xe ice on a rotatable and translatable drum according to some embodiments. [Figure 7] 10 is a graph illustrating the variation in thickness of Xe ice on a rotatable and translatable drum according to some embodiments. [Figure 8A] 1 is a flow diagram of a method of operating a light source according to some embodiments. [Figure 8B] 1 is a flow diagram of a method of operating a light source according to some embodiments. [Figure 9] 1 is a block diagram of an EUV light source system in accordance with some embodiments. [Figure 10A] 4A-4B and a motorized translation stage with a mounted sensor head, according to some embodiments. FIG. [Figure 10B] 4A-4B and a motorized translation stage with a mounted sensor head, according to some embodiments. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0009] Like reference numbers refer to corresponding parts throughout the drawings and specification.

[0010] Reference will now be made in detail to various embodiments. Examples of the various embodiments are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the various described embodiments. However, it will be apparent to those skilled in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been described in detail as not to unnecessarily obscure aspects of the embodiments.

[0011] FIG. 1 is a cross-sectional side view of an extreme ultraviolet (EUV) light source 100 according to some embodiments. The EUV light source 100 includes a vacuum chamber 102 (e.g., a billet aluminum chamber). A vacuum pump 110 (e.g., a turbo pump) provides a vacuum within the vacuum chamber 102. A laser beam 103 is introduced into the vacuum chamber 102 through a laser objective lens 104 and an associated pellicle. The laser objective lens 104 focuses the laser beam 103 onto the outer surface of a drum 118 (e.g., a copper drum) that is coated with xenon (Xe) ice (i.e., solid Xe). The laser beam 103 illuminates the drum 118 and strikes the Xe ice on the outer surface of the drum 118. When the laser beam 103 strikes the Xe ice on the outer surface of the drum 118, it discharges a plasma, which emits EUV light 105. A mirror 106 collects a portion of the EUV light 105 and directs the collected EUV light 105 to a window 108 in the vacuum chamber 102. The drum 118 is rotated and translated vertically to allow different regions (i.e., portions) of the Xe ice on the outer surface of the drum 118 to be exposed to the laser beam 103. To maintain coverage of the Xe ice, a sprayer 117 can spray Xe onto the outer surface of the drum 118 as it rotates and translates.

[0012] A feedthrough assembly 112 provides liquid nitrogen to the interior of the drum 118 to keep the surface of the drum 118 cool, thus maintaining a coating of Xe ice on the drum 118. The liquid nitrogen boils off and evaporates during operation of the EUV light source 100. The feedthrough assembly 112 exhausts the resulting nitrogen gas from the drum 118.

[0013] The drum 118 is housed within a drum assembly 114, which is coupled to the feed-through assembly 112. The drum assembly 114 also includes a rotation motor 116 that rotates the drum 118. The rotation motor 116 is coupled to the drum 118 independently of the feed-through assembly 112. The drum assembly 114 further includes a sprayer 117. The drum assembly 114 has a water-cooled drum cover that receives water from a water-cooled input 126. Located below the drum assembly 114, and therefore below the drum 118, is a translation motor 120 that linearly translates the drum 118 vertically (i.e., moves the drum 118 up and down). The translation motor 120 is also coupled to the drum 118 independently of the feed-through assembly 112. A corresponding linear stage actuator 124 actuates the translation motor 120. A weight compensation bellows 122 is also located below the drum assembly 114.

[0014] For the EUV light source 100 to operate properly, the Xe ice coating on the drum 118 should be defect-free. For example, the Xe ice should have a minimum thickness everywhere on the drum 118. The minimum thickness should be sufficient to ensure that the laser beam 103 does not strike the exposed outer surface of the drum 118, which could damage the drum 118 and contaminate the vacuum chamber 102. The Xe ice should also have a relatively uniform thickness (e.g., within a specified range of uniformity). To be able to achieve such a Xe ice coating, the exposed outer surface of the drum 118 onto which the Xe will be deposited should be smooth. To determine whether these criteria are met, a confocal chromatic sensor (i.e., a confocal sensor) is used to monitor the drum 118 while it is covered with Xe ice and / or while its exposed outer surface is exposed.

[0015] FIG. 2 illustrates a confocal chromatic sensor 200 according to some embodiments. The confocal chromatic sensor 200 includes a sensor head 206, a controller 202, and an optical fiber 204 coupled between the sensor head 206 and the controller 202. The controller 202 includes circuitry for controlling the sensor head 206 and further includes a light source (e.g., one or more light-emitting diodes (LEDs)) for generating broadband light (e.g., white light) provided to the sensor head 206. The broadband light is transmitted from the controller 202 to the sensor head 206 through the optical fiber 204. The sensor head 206 includes an optical system for dispersing the broadband light, emitting the dispersed light, and focusing the emitted light along an optical axis 220 such that different wavelengths of the emitted light have different focal lengths. In the example of FIG. 2, the dispersed and emitted light includes violet light 208, blue light 210, green light 212, yellow light 214, orange light 216, and red light 218. The focal length of the violet light 208 is shorter than the focal length of the blue light 210, which is shorter than the focal length of the green light 212, which is shorter than the focal length of the yellow light 214, which is shorter than the focal length of the orange light 216, which is shorter than the focal length of the red light 218.

[0016] If an object is positioned in front of the tip of the sensor head 206 (i.e., in front of the end of the sensor head 206 where the dispersed light is emitted) so that it intersects the optical axis 220, the sensor head 206 receives light that is first emitted by the sensor head and then reflected from the object. Based on the wavelength of the received light, the confocal chromatic sensor 200 determines the distance from the sensor head 206 (e.g., tip) to the object. The confocal chromatic sensor 200 can also determine the reflectivity of the object and the roughness (or equivalently, the uniformity) of the portion of the object's surface illuminated by the light emitted from the sensor head 206. The confocal chromatic sensor 200 operates at a sampling rate. In some embodiments, the sampling rate is in the range of 100 Hz to 70 kHz.

[0017] 3A and 3B show components of an EUV light source 300 (e.g., EUV light source 100 of FIG. 1) according to some embodiments. These components include a drum 302 (e.g., drum 118 of FIG. 1) and a confocal chromatic sensor 200 (FIG. 2), which has a sensor head 206, an optical fiber 204, and a controller 202. The drum 302 is rotatable about a central vertical axis to achieve rotational motion 314 and vertically translatable to achieve translational motion 316. In FIG. 3A, the outer surface of the drum 302 is bare and therefore exposed. In FIG. 3B, the outer surface of the drum 302 is coated with Xe ice 316. The Xe ice 316-coated drum 302 can be irradiated with a laser beam (e.g., laser beam 103 of FIG. 1) to generate a plasma that emits EUV light.

[0018] The drum 302 and the sensor head 206 are disposed within a vacuum chamber 304 (e.g., vacuum chamber 102). The controller 202 is disposed outside the vacuum chamber 304 (e.g., outside the vacuum chamber 102) in an atmosphere 308. The optical fiber 204 provides broadband light from the controller 202 to the sensor head 206 and passes through a feedthrough 310 in the wall 306 of the vacuum chamber 304. Accordingly, the optical fiber 204 is disposed partially outside the vacuum chamber 304 and partially inside the vacuum chamber 304.

[0019] 4A and 4B show components of an EUV light source 400, which is an example of an EUV light source 300 (e.g., EUV light source 100 of FIG. 1), according to some embodiments. The components of the EUV light source 400 include the components of the EUV light source 300 and further include a housing 402 that encloses the drum 302. In some embodiments, the housing 402 is part of the drum assembly 114 (FIG. 1). The drum 302 is disposed within the housing 402, while the sensor head 206 is disposed outside the housing 402 (but within the vacuum chamber 304). The housing 402 has a window 404 located between the drum 302 and the sensor head 206. Light 312 from the sensor head 206 travels through the window 404 to the drum 302, and corresponding reflected light from the drum 302 travels through the window 404 to the sensor head 206. The enclosure 402 accommodates a pressure differential between an area inside the enclosure 402 and an area outside the enclosure 402 but inside the vacuum chamber 304. For example, the area inside the enclosure 402 has a higher pressure than the area outside the enclosure 402 but inside the vacuum chamber 304 during operation of the EUV light source 400.

[0020] 10A and 10B show components of an EUV light source 1000, which is an example of an EUV light source 400 (e.g., EUV light source 100 of FIG. 1), according to some embodiments. The components of the EUV light source 1000 include the components of the EUV light source 400 and further include a motorized translation stage 1002 to which the sensor head 206 is attached. The motorized translation stage 1002 is disposed within the vacuum chamber 304. The motorized translation stage 1002 is translatable, such that the sensor head 206 can be translated vertically (e.g., to follow the translational motion 316 of the drum 302) and / or horizontally (i.e., to move the sensor head back and forth toward the drum 302 and window 404). For example, the motorized translation stage 1004 can translate vertically to align the sensor head 206 with the laser spot (e.g., the spot where the laser beam 103 illuminates the Xe ice 316) so that the sensor head 206 monitors the portion of the Xe ice 316 newly illuminated by the laser beam (e.g., the newly illuminated portion rotates from the laser spot into the field of view of the sensor head 206 in less than one rotation of the drum 302). This alignment between the sensor head 206 and the laser spot enables the sensor head 206 to detect defects (e.g., dents) in the Xe ice 316 newly induced by the laser beam. In some embodiments, to achieve this alignment, the motorized translation stage 1004 adjusts the vertical position of the sensor head 206 relative to the vertical position of the drum 302 to compensate for the translational speed of the drum 302 and the relative angular positions of the laser spot and the sensor head 206 around the drum 302. The position of the motorized translation stage 1002 can be measured using an encoder (not shown).

[0021] In some embodiments, the drum 302 has a groove 1004 around its outer surface at a specified vertical position along the drum 302 (e.g., around the center of the drum 302). The groove 1004 can be detected by the confocal chromatic sensor 200 to align the vertical positions of the sensor head 206 and the drum 302. This alignment, based on detection of the groove 1004 by the confocal chromatic sensor 200, is performed, for example, during calibration and / or at the start of operation of the EUV light source 1000.

[0022] In some embodiments, the confocal chromatic sensor 200 measures the distance from the confocal chromatic sensor 200 to the drum 302. A sensor head 206, disposed within a vacuum chamber 304, focuses light 312 onto the drum 302 and detects corresponding light reflected from the drum 302. The light 312 is broadband (e.g., white) light dispersed into different wavelengths (e.g., colors 208-218 in FIG. 2 ). The wavelengths of the light reflected from the drum 302 and detected by the sensor head 206 correspond to and are used to determine the distance from the confocal chromatic sensor 200 (e.g., sensor head 206) to the drum 302. The distance from the confocal chromatic sensor 200 (e.g., sensor head 206) to the drum 302 measured by the confocal chromatic sensor 200 includes a first distance from the confocal chromatic sensor 200 (e.g., sensor head 206) to each portion of the exposed outer surface of the drum 302 before the drum 302 is coated with the Xe ice 316 ( FIGS. 3A and 4A ) and / or a second distance from the confocal chromatic sensor 200 (e.g., sensor head 206) to each portion of the outer surface of the Xe ice 316 when the drum 302 is coated with the Xe ice 316 ( FIGS. 3B and 4B ). The distance from the confocal chromatic sensor 200 (e.g., sensor head 206) to different portions of the exposed outer surface of the drum 302 may vary depending on the surface roughness (i.e., lack of surface uniformity) of the exposed outer surface of the drum 302. The distance from the confocal chromatic sensor 200 (e.g., the sensor head 206) to different portions of the outer surface of the Xe ice 316 may vary due to variations in the thickness of the Xe ice 316 and / or the surface roughness relative to the bare outer surface of the drum 302. The thickness of each portion of the Xe ice 316 can be determined by subtracting the respective first distances from the respective second distances, and the thickness of a particular portion of the Xe ice 316 is equal to the difference between the second distance to that portion and the first distance to that portion. In this manner, variations in the thickness of the Xe ice 316 on the drum 302 are identified.These distances and thicknesses can be calculated by a computer system associated with EUV light source 300 or 400 (eg, the computer system of EUV light source system 900 of FIG. 9).

[0023] The thickness of the Xe ice 316 (e.g., the thickness of each portion of the Xe ice 316) is one example of a parameter for the drum 302 that can be measured using the confocal chromatic sensor 200. Other examples include the roughness of the Xe ice 316 and the reflectivity of the Xe ice 316. On a large (e.g., global) scale for the drum 302, roughness can be measured by measuring a second distance ( FIGS. 3B and 4B ) from the confocal chromatic sensor 200 (e.g., the sensor head 206) to each portion of the outer surface of the Xe ice 316 when the drum 302 is coated with the Xe ice 316. Variations in the second distance provide an indication of the roughness of the Xe ice. On a smaller, local scale, the confocal chromatic sensor 200 can provide an indication of the roughness within a specific portion of the Xe ice 316 (e.g., the specific portion corresponds to a specific measurement sample for the confocal chromatic sensor 200). The confocal chromatic sensor 200 can provide an indication of reflectivity for each portion of the Xe ice 316 (e.g., a particular portion corresponds to a particular measurement sample for the confocal chromatic sensor 200). In this manner, the confocal chromatic sensor 200 can be used to monitor variations and / or absolute values ​​of thickness, roughness, and reflectivity for the Xe ice 316. The roughness and reflectivity values, as well as the distance and thickness, can be calculated by a computer system associated with the EUV light source 300 or 400 (e.g., the computer system of the EUV light source system 900 of FIG. 9).

[0024] Parameters measured using confocal chromatic sensor 200 (e.g., thickness, roughness, and / or reflectivity of Xe ice 316) can be used to provide real-time feedback for controlling operation of EUV light source 400. For example, in response to detecting a defect in Xe ice 316 (e.g., a pit, which is an area in Xe ice 316 that has insufficient thickness and causes excessive roughness), the laser beam irradiating drum 302 (e.g., laser beam 103 in FIG. 1 ) can be shut off (e.g., by turning off the laser generating the laser beam or by removing the laser beam). For example, if the thickness of one or more portions of Xe ice 316 (i.e., one or more thicknesses of one or more respective portions of Xe ice 316) does not meet a threshold (e.g., is less than or equal to a minimum thickness), if the roughness of Xe ice 316 (or one or more portions of Xe ice 316) meets a threshold (e.g., exceeds or equals a maximum roughness), and / or if the reflectivity of one or more portions of Xe ice 316 (i.e., one or more reflectivities of one or more respective portions of Xe ice 316) is not within a specified range (e.g., is less than or equal to a minimum reflectivity, or greater than or equal to a maximum reflectivity, the range being between the minimum and maximum reflectivity), a defect is detected and the laser beam is blocked. Whether to block the laser beam can be determined by a computer system associated with EUV light source 300 or 400 (e.g., the computer system of EUV light source system 900 of FIG. 9 ).

[0025] After the laser beam is interrupted, parameters measured using the confocal chromatic sensor 200 (e.g., the thickness, roughness, and / or reflectivity of the Xe ice 316) can be used to determine whether to resume the laser beam. For example, the laser beam can be resumed in response to determining that the detected defect has been removed (e.g., by regrowth of the Xe ice 316 on the drum 302). For example, the laser beam can be resumed in response to determining that the thickness of the Xe ice 316 (e.g., the thickness of each portion of the Xe ice 316) meets a threshold value (e.g., is greater than or equal to a minimum thickness), that the roughness of the Xe ice 316 (or each portion of the Xe ice 316) does not meet a threshold value (e.g., is less than or equal to a maximum roughness), and / or that the reflectivity of each portion of the Xe ice 316 is within a specified range (e.g., exceeds or equals a minimum reflectivity, is less than or equal to a maximum reflectivity). The decision to restart the laser beam can be made by a computer system associated with EUV light source 300 or 400 (eg, the computer system of EUV light source system 900 of FIG. 9).

[0026] In some embodiments, the drum 302 is licensed for use at least in part by measuring the roughness of its exposed exterior surface. For example, a first distance from the confocal chromatic sensor 200 (e.g., sensor head 206) to each portion of the exposed exterior surface of the drum 302 is measured and its variation is analyzed. The variation in the first distance is an indication of the surface roughness for the exterior surface of the drum 302. Based on the variation in the first distance (e.g., the difference between the maximum and minimum values, the standard deviation, etc.), a determination is made as to whether the drum 302 is suitable for use. If the variation in the first distance does not meet a threshold (e.g., is less than or equal to the maximum variation), the drum 302 is deemed suitable for use and is therefore licensed. In some embodiments, a computer system associated with EUV light source 300 or 400 (e.g., the computer system of EUV light source system 900 of FIG. 9 ) determines that EUV light source 300 or 400 is suitable for use and, based at least in part on this determination (and thus, based at least in part on the roughness of the exposed outer surface of drum 302), authorizes activation of EUV light source 300 or 400. If the variation (and thus, the roughness of the exposed outer surface of the drum) meets a threshold (e.g., exceeds or is equal to or greater than a maximum variation), then drum 302 can be deemed defective.

[0027] In some embodiments, maintenance for the EUV light source 300 or 400 is triggered (e.g., scheduled or required before operation begins or continues) in response to detection of a defect in the Xe ice 316 or on the drum 302.

[0028] Generally, the results of measuring one or more parameters for the drum 302 using the confocal chromatic sensor 200 can be used to improve (eg, optimize) other process parameters for the EUV light source 300 or 400 .

[0029] FIG. 5 is a graph 500 illustrating time trace data 502 for the thickness of the Xe ice 316 on the drum 302, according to some embodiments. The time trace data 502 was generated using a confocal chromatic sensor 200 attached to an EUV light source system having the drum 302. The x-axis of the graph 500 is time measured in seconds, and the y-axis of the graph 500 is the thickness of the Xe ice 316 measured in millimeters. The thickness values ​​for the time trace data 502 were obtained using the confocal chromatic sensor 200 (e.g., as described with respect to FIGS. 3A-3B and 4A-4B). While the thickness of the Xe ice 316 over most of the drum 302 remains stable at just over 0.9 mm, certain portions (i.e., regions) of the Xe ice 316 on the drum 302 have a lower thickness. This particular portion may be a depression in the Xe ice 316. The underthickness is measured repeatedly as the drum 302 rotates and translates, resulting in repeated instances of measured underthickness over time in the time trace data 502. The underthickness drops from approximately 0.8 mm at 6.2 seconds to approximately 0.35 mm at approximately 6.5 seconds, remains stable at that thickness until approximately 7.3 seconds, and then recovers to 0.9 mm just after 7.4 seconds. The underthickness of the Xe ice 316 from approximately 6.3 to 7.4 seconds becomes a defect within the Xe ice 316. Thus, Figure 5 illustrates the formation and subsequent removal of a defect within the Xe ice 316.

[0030] 6 and 7 are graphs 600 and 700, respectively, illustrating the variation in thickness of the Xe ice 316 for a case of the drum 302, according to some embodiments. The data for the graphs 600 and 700 was generated using a confocal chromatic sensor 200 attached to an EUV light source system having the drum 302. The graphs 600 and 700 illustrate the thickness of the Xe ice 316 for different portions of the drum 302, which are defined by their respective rotational positions (in radians) and vertical positions (in millimeters) on the drum 302. Each of the graphs 600 and 700 is for a separate point in time. The rotational position is shown on the x-axis of the graphs 600 and 700, and the vertical position is shown on the y-axis of the graphs 600 and 700. The data for graphs 600 and 700 was obtained using confocal chromatic sensor 200 (e.g., as described with respect to Figures 3A-3B and 4A-4B) while drum 302 was irradiated with a laser beam to generate plasma (and thus generate EUV light).

[0031] The Xe ice 316 in most of the drum 302 has a thickness of about 0.9 mm. However, some portions at the top and bottom of the drum 302 have a lower thickness. Portion 602 at the top of the drum 302 and portion 604 at the bottom of the drum 302 have a thickness as low as about 0.4 mm. These portions 602 and 604 with reduced thickness become defects. In response to detecting these defects (e.g., as described above with respect to FIGS. 3A-3B and 4A-4B), the laser beam can be blocked, allowing the Xe ice 316 to regrow and thus thicken in portions 602 and 604. FIG. 7 shows the result of this regrowth, where the thickness of the Xe ice 316 in portions 602 and 604 (FIG. 6) has increased to the point where it is no longer considered a defect in FIG. 7.

[0032] 8A and 8B show a flow diagram of a method 800 of operating a light source (e.g., EUV light source 100 of FIG. 1, 300 of FIGS. 3A-3B, 400 of FIGS. 4A-4B) according to some embodiments. In method 800, a drum (e.g., drum 118 of FIG. 1, drum 302 of FIGS. 3A-4B) is rotated (802 of FIG. 8A). In some embodiments, the drum is vertically translated while rotating (804). While the drum is rotating (e.g., further vertically translated), it is coated with Xe ice (806). While rotating (e.g., further vertically translated) and coated with Xe ice, the drum is irradiated with a laser beam (e.g., laser beam 103 of FIG. 1) to create a plasma (808).

[0033] A confocal chromatic sensor (e.g., confocal chromatic sensor 200 of FIGS. 2-4B) is used to monitor the drum while irradiating it with a laser beam to detect defects in the Xe on the drum (810). In some embodiments, the confocal chromatic sensor is used to measure the thickness of each portion of the Xe ice on the drum (812) (e.g., as described with respect to FIGS. 3A-3B and 4A-4B). In some embodiments (e.g., in addition to or instead of step 812), the confocal chromatic sensor is used to measure the roughness of the Xe ice on the drum (814). In some embodiments (e.g., in addition to or instead of steps 812 and / or 814), the confocal chromatic sensor is used to measure the reflectivity of each portion of the Xe ice on the drum (i.e., measure the reflectivity of each portion) (816).

[0034] Monitoring of the drum is performed to detect defects in the Xe ice. If no defects are detected (No at 818), monitoring continues (810). However, if a defect is detected (Yes at 818), the laser beam is blocked in response to detecting the defect (820). While the laser beam is blocked, the drum continues to be rotated, translated, and / or coated with Xe ice according to steps 802, 804, and / or 806 to allow the Xe ice to properly regrow on the drum.

[0035] After the laser beam is shut off, a confocal chromatic sensor is used to monitor the drum while it is rotating (e.g., while it is being translated and coated with Xe ice) to detect defects in the Xe ice on the drum (822 in FIG. 8B). In some embodiments, a confocal chromatic sensor is used to measure the thickness of each portion of the Xe ice on the drum (824) (e.g., as described with respect to FIGS. 3A-3B and 4A-4B). In some embodiments (e.g., in addition to or instead of step 824), a confocal chromatic sensor is used to measure the roughness of the Xe ice on the drum (826). In some embodiments (e.g., in addition to or instead of steps 824 and 826), a confocal chromatic sensor is used to measure the reflectivity of each portion of the Xe ice on the drum (i.e., measure the reflectivity of each portion) (828).

[0036] After the laser beam is blocked, monitoring of the drum is performed to determine whether the detected defect has been removed (e.g., due to Xe ice regrowth) and whether any other defects are present (822). If a defect is detected (yes at 830), monitoring of the drum continues (822) while the laser beam is still blocked and the drum is rotated (802), translated (804), and / or coated with Xe ice (806). However, if no defect is detected (no at 830), the laser beam is resumed (832) in response to detecting the absence of a defect in the Xe ice on the drum, and the drum is once again irradiated with the laser beam while being coated with Xe ice (e.g., while the drum is coated with Xe ice (806)) and while rotating (802) and / or translating (804). Operation of method 800 returns to step 810 (FIG. 8A).

[0037] In some embodiments, the drum is mounted on a motorized translation stage (e.g., motorized translation stage 1002 in FIGS. 10A-10B). During monitoring in steps 810 and 822, the motorized translation stage can be translated to align the confocal chromatic sensor with the laser spot where the laser beam illuminates the Xe ice (e.g., to detect new depressions in the Xe ice).

[0038] Thus, method 800 allows for controlling operation of the EUV light source in response to real-time feedback from the confocal chromatic sensor, which provides an indication of the quality of the Xe ice on the drum and allows for a real-time decision to continue operation of the EUV light source or to allow the Xe ice coverage on the drum to rebuild.

[0039] 8 depicts the operations of method 800 in a particular order, the performance of the operations may overlap. For example, steps 802, 804, and / or 806 may be performed throughout method 800, and steps 808 and 810 may be performed simultaneously. Method 800 may include more or fewer operations. Two or more operations may also be combined into a single operation.

[0040] 9 is a block diagram of an EUV light source system 900 according to some embodiments. The system 900 includes an EUV light source 930 (e.g., EUV light source 100 of FIG. 1 , 300 of FIGS. 3A-3B , 400 of FIGS. 4A-4B ), which includes a drum 932 (e.g., drum 118 of FIG. 1 , drum 302 of FIGS. 3A-4B ), a laser 934 (e.g., producing laser beam 103 of FIG. 1 ), and a confocal chromatic sensor 936 (e.g., confocal chromatic sensor 200 of FIGS. 2-4B ). The confocal chromatic sensor 936 can be mounted on a motorized translation stage (e.g., motorized translation stage 1002 of FIGS. 10A-10B ). The system 900 also includes a computer system communicatively coupled to the EUV light source 930. The computer system includes one or more processors 902 (e.g., CPUs), an optional user interface 906, memory 910, and a communication bus 904 interconnecting these components. In some embodiments, the EUV light source 930 is communicatively coupled to the computer system by one or more wired and / or wireless networks. The computer system may further include one or more wired and / or wireless network interfaces for communicating with the EUV light source 930 and / or remote computer systems.

[0041] User interface 906 can include a display 907 and one or more input devices 908 (e.g., a keyboard, a mouse, a touch-sensitive surface of display 907, etc.). Display 907 can display the status of EUV light source 930, including the results of monitoring drum 932 by confocal chromatic sensor 936. For example, display 907 can display graphs similar to graphs 500 (FIG. 5), 600 (FIG. 6), and / or 700 (FIG. 7).

[0042] The memory 910 includes volatile and / or nonvolatile memory. The memory 910 (e.g., nonvolatile memory in the memory 910) includes a non-transitory computer-readable storage medium. The memory 910 optionally includes one or more storage devices located remotely from the processor 902 and / or a non-transitory computer-readable storage medium removably inserted into the computer system of the system 900. The memory 910 (e.g., the non-transitory computer-readable storage medium of the memory 910) includes instructions for implementing the functions described herein (e.g., the functions described with respect to FIGS. 3A-3B and 4A-4B). For example, the memory 910 (e.g., the non-transitory computer-readable storage medium of the memory 910) includes instructions for executing the method 800 (FIGS. 8A-8B).

[0043] In some embodiments, memory 910 (e.g., the non-transitory computer-readable storage medium of memory 910) stores the following modules and data, or a subset or superset thereof: an operating system 912 that handles various basic system services and includes procedures for performing hardware-dependent tasks; a drum control module 914 for controlling the rotation and translation of drum 932; a confocal chromatic sensor control module 916 for controlling and receiving data from confocal chromatic sensor 936 (e.g., controlling the motorized translation stage to which confocal chromatic sensor 936 is attached); a laser beam control module 918 for controlling (e.g., shutting off and restarting) laser 934 and the corresponding laser beam that irradiates Xe ice-coated drum 932; a defect detection module 918 for detecting defects in the Xe ice on drum 932 using data from confocal chromatic sensor 936; and a maintenance module 920 for causing (e.g., scheduling or mandating) maintenance for EUV light source 930.

[0044] Each of the modules stored in memory 910 corresponds to a set of instructions executed by one or more processors 902 to perform one or more functions described herein. Separate modules need not be implemented as separate software programs. Modules and various subsets of modules can be combined or otherwise rearranged. In some embodiments, memory 910 stores a subset or superset of the modules and / or data structures identified above.

[0045] 9 is intended as a functional description of various features that may be present in an EUV light source system, rather than a structural schematic. For example, the functionality of the computer system in EUV light source system 900 may be divided among multiple devices. Alternatively, portions of the modules stored in memory 910 may be stored in one or more other computer systems communicatively coupled to the computer system of EUV light source system 900 via one or more networks.

[0046] The above description has been set forth with reference to specific embodiments for purposes of explanation. However, the exemplary discussion above is not intended to be exhaustive or to limit the claims to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. The embodiments have been selected to best explain the principles underlying the claims and their practical application, thereby enabling those skilled in the art to best utilize the embodiments with various modifications suited to the particular use contemplated.

Claims

1. A rotatable drum coated with xenon (Xe) ice and irradiated with a laser beam to generate plasma, A confocal chromatic sensor for measuring the distance from the confocal chromatic sensor to the rotatable drum, One or more processors, A memory for storing one or more programs to be executed by the one or more processors The program comprises one or more programs, When the rotatable drum is covered with Xe ice, the confocal chromatic sensor is used to detect defects in the Xe ice on the rotatable drum, In response to detecting a defect in the Xe ice on the rotatable drum using the confocal chromatic sensor, the laser beam is shut off. A light source characterized by including instructions for use.

2. A light source according to claim 1, further comprising a vacuum chamber, The rotatable drum is placed inside the vacuum chamber, The light source is characterized in that the confocal chromatic sensor comprises a sensor head positioned within the vacuum chamber to focus light onto the rotatable drum and detect the light reflected from the rotatable drum.

3. A light source according to claim 2, further comprising a controller located outside the vacuum chamber such that the confocal chromatic sensor controls the operation of the confocal chromatic sensor and generates broadband light provided to the sensor head.

4. The light source according to claim 3, further comprising an optical fiber coupled between the sensor head and the controller such that the confocal chromatic sensor provides the broadband light to the sensor head, The vacuum chamber comprises a wall having a feedthrough, The optical fiber passes through the feedthrough. A light source characterized by the following features.

5. The light source according to claim 2, further comprising a housing inside the vacuum chamber, The rotatable drum is arranged inside the housing. The sensor head is positioned on the outside of the housing. The housing comprises a window located between the rotatable drum and the sensor head. A light source characterized by the following features.

6. The light source according to claim 2, wherein the distance from the confocal chromatic sensor to the rotatable drum, as measured by the confocal chromatic sensor, The first distance from the sensor head to each portion of the exposed outer surface of the rotatable drum before the rotatable drum is covered with the Xe ice, and The second distance from the sensor head to each portion of the outer surface of the Xe ice when the rotatable drum is covered with the Xe ice A light source characterized by containing [something].

7. A light source according to claim 1, The instruction for detecting a defect is, Using the aforementioned confocal chromatic sensor, the thickness of each portion of the Xe ice on the rotatable drum is measured, To determine whether the thickness of each portion of the Xe ice on the rotatable drum satisfies a threshold. Includes instructions for, The command for blocking the laser beam includes a command for blocking the laser beam in response to the determination that the thickness of one or more portions of the Xe ice on the rotatable drum does not meet the threshold. A light source characterized by the following features.

8. A light source according to claim 7, further comprising a vacuum chamber, The rotatable drum is placed inside the vacuum chamber, The confocal chromatic sensor includes a sensor head positioned within the vacuum chamber to focus light onto the rotatable drum and detect the light reflected from the rotatable drum. The command for measuring the thickness of each portion of the Xe ice on the rotatable drum is, Before the rotatable drum is covered with the Xe ice, the first distance from the sensor head to each portion of the exposed outer surface of the rotatable drum is measured using the confocal chromatic sensor, When the rotatable drum is covered with the Xe ice, the confocal chromatic sensor is used to measure a second distance from the sensor head to each portion of the outer surface of the Xe ice, Subtracting each first distance from each second distance Includes instructions for A light source characterized by the following features.

9. A light source according to claim 1, The instruction for detecting a defect is, Using the aforementioned confocal chromatic sensor, the roughness of the Xe ice on the rotatable drum is measured, Determining whether the aforementioned roughness meets the threshold. Includes instructions for, The command for blocking the laser beam includes a command for blocking the laser beam in response to the determination that the roughness meets the threshold. A light source characterized by the following features.

10. A light source according to claim 9, The command for determining whether the roughness satisfies the threshold includes a command for identifying a depression in the Xe ice on the rotatable drum using the confocal chromatic sensor, In response to the determination that the roughness satisfies the threshold, the command to block the laser beam includes, in response to the identification of the depression, a command to block the laser beam. A light source characterized by the following features.

11. A light source according to claim 1, The instruction for detecting a defect is, Using the aforementioned confocal chromatic sensor, the reflectance of each portion of the Xe ice on the rotatable drum is measured, To determine whether the reflectivity of each portion of the Xe ice on the rotatable drum is within a specified range. Includes instructions for, The command to block the laser beam includes a command to block the laser beam in response to the determination that the reflectance of one or more portions of the Xe ice on the rotatable drum is not within the specified range. A light source characterized by the following features.

12. A light source according to claim 1, wherein the one or more programs In response to detecting the aforementioned defect, the laser beam is shut off, and then the rotatable drum is monitored for defects in the Xe ice using the confocal chromatic sensor. In response to identifying the absence of defects in the Xe ice on the rotatable drum, the laser beam is restarted. A light source characterized by further including instructions for use.

13. A light source according to claim 12, The command for monitoring the rotatable drum is, Using the aforementioned confocal chromatic sensor, the thickness of each portion of the Xe ice on the rotatable drum is measured, To determine whether the thickness of each portion of the Xe ice on the rotatable drum satisfies a threshold. Includes instructions for, The command for restarting the laser beam includes, at least in part, a command for restarting the laser beam based on the determination that the thickness satisfies the threshold. A light source characterized by the following features.

14. A light source according to claim 12, The command for monitoring the rotatable drum is, Using the aforementioned confocal chromatic sensor, the roughness of the Xe ice on the rotatable drum is measured, Determining whether the aforementioned roughness meets the threshold. Includes instructions for, The command to restart the laser beam includes, at least in part, a command to restart the laser beam based on the determination that the roughness does not meet the threshold. A light source characterized by the following features.

15. A light source according to claim 12, The command for monitoring the rotatable drum is, Using the aforementioned confocal chromatic sensor, the reflectance of each portion of the Xe ice on the rotatable drum is measured, To determine whether the reflectivity of each portion of the Xe ice on the rotatable drum is within a specified range. Includes instructions for, The command for restarting the laser beam includes, at least in part, a command for restarting the laser beam based on a determination that the reflectivity of each portion of the Xe ice on the rotatable drum is within the specified range. A light source characterized by the following features.

16. A light source according to claim 1, wherein one or more programs further include an instruction to trigger maintenance based on the detection of the defect in the Xe ice on the rotatable drum, at least in part.

17. A light source according to claim 1, wherein the one or more programs Before the rotatable drum is covered with the Xe ice, the roughness of the exposed outer surface of the rotatable drum is measured, At least in part, the determination of whether the rotatable drum is suitable for use is based on the roughness. A light source characterized by further including instructions for use.

18. A rotatable drum coated with xenon (Xe) ice and irradiated with a laser beam to generate plasma, A confocal chromatic sensor for measuring the distance from the confocal chromatic sensor to the rotatable drum, Electric translation stage and One or more processors, A memory for storing one or more programs to be executed by the one or more processors Equipped with, The confocal chromatic sensor comprises a sensor head attached to the motorized translation stage, The one or more programs include instructions to translate the motorized translation stage so that the sensor head is aligned with the laser spot on which the laser beam irradiates the Xe ice. A light source characterized by the following features.

19. A light source according to claim 18, The rotatable drum has grooves around its outer surface, The one or more programs further include commands for aligning the vertical position of the rotatable drum and the sensor head based on the detection of the groove by the confocal chromatic sensor. A light source characterized by the following features.

20. A method for operating a light source, Rotating the drum, The drum is rotated while the drum is covered with xenon (Xe) ice, With the drum covered with the Xe ice, the drum is rotated while a laser beam is irradiated onto the drum to generate plasma. While irradiating the drum with the laser beam, the drum is monitored using a confocal chromatic sensor to detect defects in the Xe ice on the drum. In response to detecting a defect in the Xe ice on the drum, the laser beam is shut off. A method characterized by including the following.

21. A method according to claim 20, characterized in that monitoring the drum includes measuring a parameter selected from the group consisting of the thickness of each portion of the Xe ice on the drum, the roughness of the Xe ice on the drum, and the reflectance of each portion of the Xe ice on the drum, using the confocal chromatic sensor.

22. The method according to claim 20, After blocking the laser beam, the drum is rotated while the drum is monitored using the confocal chromatic sensor to detect defects in the Xe ice on the drum. In response to detecting the absence of defects in the Xe ice on the drum, the laser beam is restarted and irradiated onto the drum while the drum is rotating with the Xe ice covering it, thereby generating the plasma. A method characterized by further comprising:

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