Laser and drum control for continuous generation of broadband light

Abstract

The broadband light source includes a rotating drum coated with a plasma forming target material, a rotary actuator configured to rotate the rotating drum, and a rotary encoder connected to the rotating drum. The broadband light source can include a linear actuator configured to axially translate the rotating drum, and a linear encoder connected to the rotating drum. The broadband light source includes a pulsed laser source configured to direct pulsed illumination to a set of spots on the material coated portion of the rotating drum to excite the plasma forming target material and emit broadband light when the drum is actuated. The broadband light source includes a control system. The control system is configured to receive one or more rotational position indicators from the rotary encoder and control triggering of the laser source based on the one or more rotational position indicators from the rotary encoder.

Description

[Technical field]

[0001] The present disclosure relates generally to generating broadband illumination, and more particularly to generating broadband illumination via a pulsed laser and a rotating drum covered with a target material. [Background technology]

[0002] As the demand for lithography-based device structures with smaller and smaller features continues to increase, so does the need for improved illumination sources used to inspect the associated reticles that lithographically print these ever-shrinking devices. One such illumination source includes an extreme ultraviolet (EUV) light source. One method of generating EUV light involves rotating a cylinder coated with a uniformly thick layer of a solid (frozen) plasma-forming target material, such as xenon, and exposing the xenon-coated portion of the cylinder with a pulsed laser suitable for exciting the xenon to generate a plasma. Additionally, the cylinder must be rotated and / or translated to expose fresh areas of solid xenon prior to the next irradiation pulse. As the cylinder rotates, gaseous xenon may be sprayed onto the cool, unilluminated portions of the cylinder's surface, reforming the frozen xenon layer in previously illuminated spots to fill in portions of the xenon ice consumed by the laser over time. A particular spot or zone is not exposed again until a sufficient amount of time has passed, allowing the solid xenon surface to return to its original state. Adjacent spots must be separated by some minimum distance to prevent damage to the cylinder surface.

[0003] Plasma generation is performed at a fixed position, since the associated collection optics must be aligned to a known position and cannot follow a moving plasma source. The need to fix the plasma position precludes the use of a moving illuminator laser spot. This creates challenges in applications involving EUV light sources. To allow for maximum operating time and uniformity of inspection, the inspection tool should have a pulsed EUV light source that is not interrupted, but rather operates at a selected pulse frequency for long periods of time (e.g., months).

[0004] The generation of EUV light using solid xenon on a rotating cylinder has generally been accomplished in two ways. First, a cylinder rotating at a constant speed is moved slowly axially from one end to the other, forming a spiral of spots along the cylinder. When the spiral intersects the top of the usable length of xenon ice, the illuminating laser is shut off or turned off until the vertical movement of the cylinder stops and the direction of movement is reversed. The total length of time between subsequent illuminating laser exposures of the same location must be sufficient to allow the xenon ice to reform. The first method is undesirable as it provides a non-continuous illumination output. In the second method, the cylinder is exposed by the laser source in an uninterrupted pattern, allowing for realignment of the target area on the cylinder surface. In this method, the plasma target may be formed in a first spiral pattern (upward pass) and a returning second spiral pattern (downward pass). The second spiral pattern may be interleaved with the first spiral pattern, which allows for the continuous generation of EUV illumination. Such an approach is described in U.S. Patent No. 8,963,110, which is incorporated herein by reference in its entirety. The second method suffers from the potential for laser crater overlap during the upward pass and the downward return pass. In addition, during the turnaround region (i.e., the transition from the up pass to the down pass), the second method requires first accelerating the cylinder and then decelerating the cylinder to shift the laser column, which makes the motion control relatively complex and also poses mechanical and reliability challenges. The second method requires active control of the rotation of the cylinder and the vertical actuation of the rotating drum. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US Patent Application Publication No. 2017 / 0142817 Summary of the Invention [Problem to be solved by the invention]

[0006] It would therefore be desirable to provide a method and system that addresses the shortcomings of previous solutions identified above. [Means for solving the problem]

[0007] A broadband light source is disclosed according to one or more embodiments of the present disclosure. In an embodiment, the broadband light source includes a rotating drum at least partially coated with a plasma formation target material. In an embodiment, the broadband light source includes a rotary actuator configured to rotate the rotating drum. In an embodiment, the broadband light source includes a linear actuator configured to axially translate the rotating drum. In an embodiment, the broadband light source includes a rotary encoder connected to the rotating drum, the rotary encoder indicating a rotational position of the rotating drum. In an embodiment, the broadband light source includes a linear encoder connected to the rotating drum, the linear encoder indicating an axial position of the rotating drum. In an embodiment, the broadband light source includes a laser source configured to direct pulsed illumination to a set of spots on the material coated portion of the rotating drum to excite the plasma formation target material and emit broadband light when the rotary actuator rotates the rotating drum and the linear actuator axially translates the rotating drum. In an embodiment, the broadband light source includes a control system, the control system configured to receive one or more rotational position indicators from the rotary encoder and control triggering of the laser source based on the one or more rotational position indicators from the rotary encoder. In an embodiment, the broadband light source is integrated into a wafer or mask / reticle inspection system.

[0008] A method of generating broadband light according to one or more embodiments of the present disclosure is disclosed. In an embodiment, the method includes rotating a rotatable drum coated with a plasma-forming target material. In an embodiment, the method includes axially actuating the rotatable drum. In an embodiment, the method includes obtaining rotational position information from a rotary encoder coupled to the rotatable drum. In an embodiment, the method includes triggering a pulsed laser source based on the rotational position information obtained from the rotary encoder to form a first set of target positions for a downstroke of the rotatable drum and a second set of target positions for an upstroke of the rotatable drum offset from the first set of target positions. In an embodiment, the method includes recoating a previously targeted spot of the rotatable drum with the plasma-forming target material.

[0009] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the subject matter of the present disclosure. The description and the drawings together serve to explain the principles of the present disclosure.

[0010] Many advantages of the present disclosure may be better understood by those skilled in the art by reference to the following drawings. [Brief description of the drawings]

[0011] [Figure 1A] FIG. 1 shows a simplified conceptual diagram of a broadband light source for producing a continuous or near-continuous plasma-based lighting output, in accordance with one or more embodiments of the present disclosure. [Figure 1B] FIG. 1 shows a simplified conceptual diagram of a broadband light source for producing a continuous or near-continuous plasma-based lighting output, in accordance with one or more embodiments of the present disclosure. [Diagram 2] FIG. 1 illustrates a block diagram of a control system for controlling one or more characteristics of a broadband source in accordance with one or more embodiments of the present disclosure. [Diagram 3]1 shows a conceptual diagram of a first set of spots and a second set of spots distributed across a surface of a rotating drum of a broadband light source, in accordance with one or more embodiments of the present disclosure. [Figure 4] FIG. 1 illustrates an encoder having a single trigger sequence capable of generating an interlaced set of target spots on the surface of a rotating drum of a broadband light source in accordance with one or more embodiments of the present disclosure. [Diagram 5] 13A-13C show conceptual diagrams of a first set of spots and a second set of spots distributed across a surface of a rotating drum of a broadband light source, in accordance with one or more additional and / or alternative embodiments of the present disclosure. [Figure 6] FIG. 1 illustrates an encoder having two trigger sequences capable of generating an interlaced set of target spots on the surface of a rotating drum of a broadband light source in accordance with one or more embodiments of the present disclosure. [Figure 7] 13A-13C show conceptual diagrams of a first set of spots and a second set of spots distributed across a surface of a rotating drum of a broadband light source, in accordance with one or more additional and / or alternative embodiments of the present disclosure. [Figure 8] FIG. 1 illustrates an encoder having three trigger sequences capable of generating an interlaced set of target spots and transition spots on the surface of a rotating drum of a broadband light source in accordance with one or more embodiments of the present disclosure. [Figure 9] 13A-13C show conceptual diagrams of a first set of spots and a second set of spots distributed across a surface of a rotating drum of a broadband light source, in accordance with one or more additional and / or alternative embodiments of the present disclosure. [Figure 10] 13A-13C show conceptual diagrams of a first set of spots and a second set of spots distributed across a surface of a rotating drum of a broadband light source, in accordance with one or more additional and / or alternative embodiments of the present disclosure. [Figure 11] FIG. 1 illustrates a block diagram of an inspection system incorporating a broadband light source according to one embodiment of the present disclosure. [Figure 12]1 shows a flow diagram illustrating a method for generating continuous or near-continuous broadband light in accordance with one or more embodiments of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Reference will now be made in detail to the disclosed subject matter, which is illustrated in the accompanying drawings.

[0013] 1A-12 generally illustrate embodiments of systems and methods for generating a continuous or near-continuous plasma-based lighting output in accordance with one or more embodiments of the present disclosure.

[0014] Embodiments of the present disclosure are directed to the exposure of a rotating drum (e.g., cylinder) covered with a plasma-forming material (e.g., frozen xenon or carbon dioxide), which can be excited via an illumination source (e.g., a pulsed laser) to emit broadband light (e.g., broadband EUV light or soft x-ray light) from a set of uninterrupted target locations. Embodiments of the present disclosure are directed to the implementation of a rotary encoder to serve as a laser trigger reference. Utilizing a rotary encoder as a laser trigger reference eliminates or at least reduces the overlap of laser craters caused during the upward and downward passes of the laser across the drum. The target material requires some time to recover after being hit with the laser as the drum rotates and translates to provide a new target surface, creating a spiral pattern. The top and bottom of the travel path present the greatest challenge, as the drum must reverse direction without the laser hitting one of the recently affected locations (i.e., craters). The disclosed embodiments solve this problem by i) slightly adjusting the laser frequency at the turnaround location; or ii) adjusting the rotational speed of the cylinder to achieve a similar effect. The disclosed embodiments are directed to shifting the crater spacing by one row per revolution during the turnaround region. The combination of i) using a rotary encoder as a laser trigger reference; ii) shifting the crater spacing allows for a constant illumination laser pulse frequency, easier cylinder control, and fewer mechanical design challenges while improving reliability. In either implementation, the encoder is utilized to ensure that the same target position is not affected twice within too short a time frame. In additional embodiments, the encoder information can also be used to synchronize the laser pulse with the inspection tool's sensor / camera.

[0015] Additional embodiments of the present disclosure provide a reticle inspection system, a wafer inspection system, or a lithography system (or other optical system) that incorporates the plasma-based rotating drum sources described herein.

[0016] 1A and 1B show simplified conceptual diagrams of a broadband light source 100 for generating a continuous or near-continuous plasma-based lighting output in accordance with one or more embodiments of the present disclosure.

[0017] In an embodiment, the broadband source 100 includes a rotating drum 102 adapted for rotation about an axis. For example, the drum 102 may be a cylinder, as shown in FIG. 1A. In other embodiments, the drum 102 includes any cylindrically symmetric shape known in the art. For example, the drum 102 may include, but is not limited to, a cylinder, a cone, a sphere, an ellipsoid, and the like. The drum 102 may also have a composite shape consisting of two or more shapes. For convenience of explanation herein, the broadband source 100 and related embodiments are described in the context of a rotatable or rotating drum 102, as shown in FIG. 1A, although it should be noted that this embodiment should not be construed as a limitation on the scope of the present disclosure.

[0018] In an embodiment, the drum 102 is at least partially coated with a plasma-forming target material 103. The target material 103 may include a solid material disposed on the surface of the drum 102. The plasma-forming target material 103 may include any material known in the art that generates a plasma when excited by an illumination source and subsequently emits broadband light. The broadband light emitted by the plasma may include, but is not limited to, broadband soft x-ray light, broadband EUV light, broadband DUV light, broadband VUV light, broadband UV light, broadband visible light, and / or broadband IR light. For example, the target material 103 may include, but is not limited to, xenon frozen on the surface of the drum 102. It is noted that the use of frozen xenon as the target material 103 may be particularly useful in the context of generating broadband EUV light. As another example, the target material 103 may include, but is not limited to, carbon dioxide frozen on the surface of the drum 102. It is noted that the use of frozen carbon dioxide as the target material 103 may be particularly useful in the context of generating broadband soft x-ray light.

[0019] In an embodiment, the broadband light source 100 includes a pulsed illumination source 104. The pulsed illumination source 104 may include any pulsed or modulated illumination source known in the art. For example, the pulsed illumination source 104 may include, but is not limited to, a pulsed laser. Furthermore, the pulsed illumination source is suitable for initiating and / or sustaining a plasma within the material 103. For example, the pulsed illumination source 104 may include, but is not limited to, one or more infrared (IR) lasers. For example, the pulsed illumination source 104 may include, but is not limited to, one or more CO2 lasers.

[0020] In an embodiment, the broadband source 100 includes one or more actuators configured to rotate and axially actuate the drum 102. In an embodiment, the broadband source 100 includes a rotary actuator 107 configured to rotate the drum 102 about an axis. The rotary actuator 107 can include any rotary actuator known in the art, including, but not limited to, one or more servo motors. Additionally, the broadband source 100 can include a linear actuator configured to translate the drum 102 along an axial direction 111 (e.g., vertical in the example shown in FIG. 1A). It is recognized herein that the disclosure is not limited to the rotary actuator 107 and linear actuator 111 of FIG. 1A. Thus, the description provided above should be construed as merely exemplary. For example, the pulse source 104 may be disposed on an actuation stage (not shown) that provides translation of the beam 105 relative to the drum 102.

[0021] The axial motion provided by the linear actuator 111 and the rotational motion of the drum 102 caused by the rotary actuator 107 allow tracking of the pulsed illumination 105 in sets of spots across the surface of the drum 102. The sets of spots may include a first set of spots arranged in a first spiral pattern and a second set of spots arranged in a second spiral pattern. The first set of spots and the second set of spots are interleaved such that spots from the second set of spots do not overlap with spots from the first set of spots. In this regard, the first set of spots may form a first spiral pattern during rotation and axial acceleration of the drum 102, while the second set of spots forms a return second spiral pattern that avoids overlapping with the first pattern. Examples of such patterns are discussed in more detail herein.

[0022] In an embodiment, the broadband source 100 includes one or more encoders. For example, as shown in FIG. 1B, the system can include a rotary encoder 116. The rotary encoder 116 can be mechanically attached to the rotary actuator 107 (e.g., a servo motor). In this sense, the rotary encoder 116 can be integrated with the rotary actuator 107 and configured to indicate the rotational position of the rotating drum 102, which is coupled to the rotary actuator 107 via a shaft. In an embodiment, the position indicator information from the rotary encoder 116 can be electronically read out from the rotary encoder 116 via the control system 114 (e.g., via electrical wires from the rotary encoder 116). This position indicator information can then be utilized by the control system 114 to control or adjust the movement of the pulsed laser source 104 and / or the rotating drum 102. For example, the position indicator information from the rotary encoder 116 can be used as a triggering criterion for the pulsed laser source 104. As another example, the position indicator information from the rotary encoder 114 may be used to adjust the rotational / linear speed of the drum 102 (e.g., by controlling the rotary actuation 107 and / or the linear actuator 111). In additional embodiments, the position indicator information may be utilized by the control system 114 to synchronize the pulsed illumination source 104 and a detector (e.g., a camera or one or more sensors) that detects illumination from the test sample. In an embodiment, the broadband source 100 may utilize a linear encoder 118 to control certain aspects of the drum 102. The linear encoder 118 may be mechanically attached to the linear actuator 107. In this sense, the linear encoder 118 may be integrated with the linear actuator 111 and configured to indicate an axial (e.g., vertical) position of the rotating drum 102. In an embodiment, the position indicator information from the linear encoder 118 may also be electronically read out from the linear encoder 118 via the control system 114 (e.g., via electrical wires from the linear encoder 118).The position indicator information from the linear encoder 118 may then be utilized by the control system 114 to control or adjust the motion of the pulsed laser source 104 and / or the rotating drum 102. For example, the position indicator information from the linear encoder 118 may be used as a triggering reference for the pulsed laser source 104. As another example, the position indicator information from the linear encoder 118 may be used to adjust the rotational / linear speed of the drum 102 (e.g., by controlling the rotary actuation 107 and / or the linear actuator 111). In additional embodiments, the position indicator information from the linear encoder 118 may be utilized by the control system 114 to synchronize the pulsed laser source 104 and a detector that detects illumination from the test sample. In additional embodiments, the broadband light source 100 may simultaneously implement the rotary encoder 116 and the linear encoder 118 to control the triggering of the pulsed laser source 104 and / or the rotational and linear position of the drum 102, as well as the synchronization of the detector / laser source.

[0023] In an embodiment, the broadband source 100 includes a control system 114. The control system 114 may be communicatively coupled to the rotary and / or linear actuators 107, 111 and / or the pulsed laser source 104. In an embodiment, the control system 114 controls the triggering of the pulsed laser source 104 and / or the actuation along the rotational and / or axial direction of the drum 102. In this regard, the control system 114 may direct the actuators 107, 111 and the drum 102 to trace the pulsed illumination 105 across the surface of the cylinder as it rotates and translates axially in any manner described in this disclosure.

[0024] In an embodiment, the broadband source 100 includes a material source 112. The material source 112 can include a material used to coat the drum 102. For example, the material source 112 can be used to apply a selected material to the surface of the drum 102. In an embodiment, the material source 112 can direct a gas, liquid stream, or spray onto the surface of the drum 102 as the drum 102 rotates and is maintained at a temperature below the freezing point of the selected material. For example, the selected material may include, but is not limited to, xenon, carbon dioxide, and similar materials. For example, in the case of xenon, the drum 102 can be cooled below the xenon freezing point (which is, for example, -11.8°C). The xenon is then applied to the surface of the drum 102, and the xenon freezes onto the surface of the drum 102 as it rotates, forming a solid xenon layer on the surface of the drum 102. In another example, in the case of carbon, the drum 102 can be cooled below the carbon dioxide freezing point (for example, -78°C). Carbon dioxide is then applied to the surface of the drum 102, and as it rotates, the carbon dioxide freezes on the surface of the drum 102, forming a solid carbon dioxide layer on the surface of the drum 102. In an embodiment, the drum 102 can include an internal reservoir for containing a coolant material. For example, the drum 102 can include an internal reservoir that holds a volume of liquid nitrogen that is used to cool the applied xenon or carbon dioxide below the freezing point of the respective material. In an embodiment, the material source 112 is configured to recoat a portion of the rotating drum 102 with the plasma forming material 103. For example, the material source 112 is configured to recoat a previously irradiated portion of the drum 102 with the plasma forming material 103. For example, the material source 112 can recoat a spot that was previously crated by the illumination 105 with a plasma forming material, such as, but not limited to, xenon or carbon dioxide. Additionally, the length of the drum 102 and the vertical speed of the drum 102 can be selected to provide sufficient time for the plasma forming material (e.g., xenon or carbon dioxide) to solidify on the drum 102 before the next irradiation exposure.

[0025] In an embodiment, the broadband light source 100 includes one or more collection optics 106 arranged to collect plasma-based illumination emanating from a plasma formation region on the drum 102. For example, after light is generated by the plasma excited by the pulsed source 104, the light may be collected by the collection optics 106. For example, the collection optics 106 may include any collection or collection optics known in the art. For example, in the case of EUV light generation, the collection optics 106 may include any collection or collection optics known in the art that are compatible with EUV light. In another example, in the case of soft x-ray light generation, the collection optics 106 may include any collection or collection optics known in the art that are compatible with soft x-ray light. In an embodiment, the collection optics 106 may direct and / or focus the illumination 109 emanating from the drum 102 to one or more downstream optical elements. In an embodiment, the collection optics 106 may be configured to focus the illumination emanating from the drum 102 to an intermediate focus 108, as shown in FIG. 1A.

[0026] 2 illustrates a block diagram 200 of the control system 114, according to one or more embodiments of the present disclosure. In an embodiment, the control system 114 includes a motion controller 202, a process controller 210, and a laser controller 212. In addition, the control system 114 may include a computer 214 (or any other GUI-enabled device). In an embodiment, the motion controller 202 directs the rotary actuator 107 and / or the linear actuator 111 to control the rotation and / or linear motion (position, velocity, and acceleration) of the drum 102. In an embodiment, signals from the motion controller 202 may be amplified via a rotary amplifier 204 and a linear amplifier 206 and transmitted to the rotary actuator 107 (e.g., a servo motor) and the linear actuator 111, respectively. The rotary actuator 107 and the linear actuator 111 may then actuate the drum 102 as directed by the motion controller 202. One or more logic elements 208 (e.g., a field programmable gate array (FPGA)) can read position information from a rotary encoder 116 integrated with the rotary actuator 107 and / or a linear encoder 118 integrated with the linear actuator 111. For example, the one or more logic elements 208 can read electrical signals from cables / wires output from the rotary encoder 116 and / or the linear encoder 118. This rotational and linear position information may then be sent to a process controller 212 and / or a laser controller 212. In the case of laser control, the position information from the one or more logic elements can serve as a triggering criterion for the pulsed laser source 104. In this regard, the laser controller 212 may receive the position information from the one or more logic elements and then fire the pulsed laser source 104 based on the position information. For example, in the case of a downstroke of the rotating drum 102, the laser controller 212 may fire the pulsed laser source 104 instantaneously (or nearly instantaneously) until the index of the rotary encoder 116 is read by the one or more logic elements 208.For upstroke targeting, the laser controller 212 may then provide a selected time delay between reading the index and firing the pulsed laser source 104, or may utilize an encoder with a second set of indexes (offset to the first set of indexes). For process control, the process controller 210 may analyze the position information and feed that information back to the motion controller 202, which may then make adjustments to the rotational and / or linear motion of the drum 102 and / or adjustments to the pulsed laser source via the laser controller 212. Additional laser control and / or drum position control scenarios are described further herein.

[0027] FIG. 3 illustrates a conceptual diagram 300 of a first set of spots and a second set of spots distributed across a surface of a drum 102, in accordance with one or more embodiments of the present disclosure. In an embodiment, as the drum 102 rotates, the pulsed illumination source 104 directs pulsed illumination 105 to a first set of spots 302 that traverse the material coated portion of the drum 102 in a first direction 303. Additionally, as the drum 102 continues to rotate, the pulsed illumination source 104 directs pulsed illumination 105 to a second set of spots 304 that traverse the material coated portion of the drum 102 in a second direction 305. In an embodiment, the first set of spots 302 and the second set of spots 304 each form a spiral pattern around the surface of the rotating drum 102, and it should be noted that the surface depicted in FIG. 3 represents a cylindrical surface that spans from −180° to +180° around the axis of the cylinder along the direction of rotation 307. In an embodiment, the drum 102 may be rotated at a selected constant rotational speed (e.g., constant drum RPM) along the rotational direction within the overlap region 306. In an embodiment, the drum 102 may be rotated at a reduced / accelerated rate in the turn-around regions 309, 311.

[0028] FIG. 4 illustrates an encoder 400 having a single trigger sequence capable of generating an interlaced set of target spots 302, 304 on the surface of the drum 102, according to one or more embodiments of the present disclosure. The encoder 400 includes a single set of indices 402. In an embodiment, the encoder 400 provides a control feature to the broadband source 100 to form the first set of spots 302 so as not to overlap with the spots of the second set 204, as shown in FIG. 3. As mentioned above, the indices 402 may be used by the control system 114 to i) serve as a trigger reference for the pulsed laser source 104; ii) adjust the rotational speed and / or linear velocity of the drum 102; and / or iii) synchronize the pulsed laser source 104 and the detector. For example, a read signal from the rotary encoder 400 (e.g., via an electrical wire) can serve as an external trigger to the pulsed laser source 104. During the turn-around region 309, the control system 114 can cause a shift such that the second set of spots 304 of the upstroke 305 are shifted relative to the first set of spots (e.g., shifted by a half cycle). For example, with a single encoder 400, during the downstroke 303, the pulsed laser source 104 may be triggered instantly (or nearly instantly) at each of the indexes 402, and during the upstroke 305, the triggering of the pulsed laser source 104 may be delayed by some fixed delay. This delay shifts the second set of spots 304 relative to the first set of spots 302.

[0029] 5 shows a conceptual diagram 500 of a first set of spots and a second set of spots generated by a two-sequence encoder distributed across the surface of the drum 102 in accordance with one or more additional and / or alternative embodiments of the present disclosure. It should be noted that various features of the arrangement of spots depicted in FIG. 3 should be construed to extend to the arrangement of FIG. 5 unless otherwise indicated.

[0030] FIG. 6 illustrates an encoder 600 having two trigger sequences capable of generating an interlaced set of target spots 502, 504 on the surface of the drum 102, in accordance with one or more embodiments of the present disclosure. It is noted herein that various general features of the encoder 400 should be construed as extending to the encoder 600 and will not be repeated here for brevity. The encoder 600 includes two sets of indexes 602, 604. In this embodiment, the first set of indexes 602 may be used as a trigger criterion for the pulsed laser source 104 during the downstroke, and the second set of indexes 604 may be used as a trigger criterion for the pulsed laser source 104 during the upstroke. For example, during the downstroke 503, the encoder-based indexes 602 can cause the pulsed laser source 104 to generate a laser pulse at a selected frequency. Then, during the turnaround region 509 (moving from the downstroke to the upstroke), the control system 114 can switch the trigger criterion to the second set of indexes 604 for implementation during the upstroke 505. Similarly, during the turnaround region 511 (moving from upstroke to downstroke), the control system 114 can switch the trigger criteria back to the first set of indexes 602 for implementation during the downstroke 503. The switching of the indexes on the encoder 600 corresponds to the occurrence of switching 513a, 513b on the drum 102. The switching from downstroke to upstroke (and vice versa) may be performed over a selected period of time implemented via the logic element 208 of the control system 114. In this embodiment, unlike the single trigger sequence, the broadband source 100 does not require a trigger delay for the upstroke pulse. For example, the pulsed laser source 104 may be triggered instantly (or nearly instantly) for the downstroke index 602 and the upstroke index 604. Due to the interleaving of the individual indices 602, 604 on the encoder 600, the target position of the upstroke 505 is offset relative to the target position of the downstroke 503.

[0031] 7 shows a conceptual diagram 700 of a first set of spots and a second set of spots generated by a three-sequence encoder distributed across the surface of the drum 102, in accordance with one or more additional and / or alternative embodiments of the present disclosure. It should be noted that various features of the arrangement of spots depicted in FIG. 5 should be construed to extend to the arrangement of FIG. 7 unless otherwise indicated.

[0032] FIG. 8 illustrates an encoder 800 having three trigger sequences capable of generating an interlaced set of target spots 702, 704 and transition spots 713 and 715 on the surface of the drum 102, in accordance with one or more embodiments of the present disclosure. It should be noted that various general features of the encoder 600 herein should be construed as extending to the encoder 800 and will not be repeated here for brevity. The encoder 800 includes three sets of indexes 802, 804, 806. In this embodiment, the first set of indexes 802 can be used as a trigger criterion for the pulsed laser source 104 during the downstroke, and the second set of indexes 804 can be used as a trigger criterion for the pulsed laser source 104 during the upstroke. In addition, the third set of indexes 806 can be considered a dynamic encoding system, providing a transition region between the downstroke spots 702 and the upstroke spots 704. In this sense, the third set of indicators 806 can be used to switch the laser crating process during switching of the regions 713 / 715 by changing the frequency of the laser firing from that implemented by the first and second indicators 802, 804, from the trigger caused by the first set of indicators 802 and the trigger caused by the second set of indicators 804. The changed frequency percentage during the transfer between the downstroke spot 702 and the upstroke spot 704 can be controlled by the number of the third set of indexes 806. For example, the change in frequency is about 1% when there are 50 switching shots (i.e., the number of shots in the switching region 713 / 715) and about 0.5% when there are 100 switching shots.

[0033] FIG. 9 illustrates a conceptual diagram 900 of a first set of spots and a second set of spots generated by a three-sequence encoder distributed across the surface of the drum 102, according to one or more additional and / or alternative embodiments of the present disclosure. It should be noted that the various features of the arrangement of spots depicted in FIG. 7 should be interpreted to extend to the arrangement of FIG. 9, unless otherwise indicated. In an embodiment, in the turn-around region 909, 911, the drum 102 may be accelerated (or decelerated) to provide a temporary increase (or decrease) in the rotational speed of the drum 102. For example, in the turn-around region 909, at the end of the downstroke 903, the drum rotational speed (RPM) is increased (or decreased) for a period of time relative to the constant speed region 906, and then decreased (or increased) back to the constant speed of region 906 at the beginning of the upstroke 905. Similarly, in turn back region 911, at the end of upstroke 905, the drum rotational speed (RPM) is increased (or decreased) for a period of time relative to constant speed region 906, and then decreased (or increased) back to constant speed region 906 at the beginning of downstroke 903. In an embodiment, during transition period 913 (or 915), the laser pulse duration may remain the same while the drum rotational speed is increasing. In an embodiment, the dynamic encoding sequence 806 may be implemented in combination with drum acceleration / deceleration to shift between downstroke target spot 902 and upstroke target spot 904 (and vice versa) generated by index 802 and index 804.

[0034] While much of this disclosure focuses on utilizing an encoder index to control the laser firing of a pulsed laser source, it should be noted that the drum acceleration / deceleration scheme of FIG. 9 may be implemented with or without the various encoding sequences of this disclosure. In alternative embodiments, the control system 114 may utilize a laser clock (as opposed to a rotary encoder index) as a triggering reference. In this regard, the laser clock may act as the primary triggering reference for the pulsed laser source 104 and may be used as a reference in controlling the rotation and / or vertical motion of the drum 102.

[0035] FIG. 10 illustrates a conceptual diagram 1000 of a first set of spots and a second set of spots distributed across the surface of the drum 102, according to one or more additional and / or alternative embodiments of the present disclosure. It should be noted that various features of the array of spots depicted in FIG. 9 should be interpreted as extending to the array of FIG. 10, unless otherwise noted. In this embodiment, the first set of spots 1002 and the second set of spots 1004 may be generated via a two-sequence encoder in which the number of indexes in the first set of indexes is different from the number of indexes in the second set of indexes. For example, as shown in FIG. 10, the number of columns associated with the first set of indexes is an even number, and the number of columns associated with the second set of indexes is an odd number. In this example, the laser firing frequency of the pulsed laser source 104, controlled by the first and second indexes of the encoder (not shown), may be a selected frequency. Note that while using an encoder with different numbers of indexes in the first and second sets of indexes ensures that spot 904 of upstroke 1005 will not perfectly coincide with spot 902 of downstroke 1003, this approach cannot guarantee that there will not be some overlap, such as that depicted in region 1013. This corresponds to the intersection of some of the indexes in the first set of indexes with some of the indexes in the second set of indexes.

[0036] FIG. 11 illustrates a block diagram of an inspection system 1100 incorporating a plasma-based illumination source 100, according to one or more embodiments of the present disclosure. In an embodiment, the system 1100 includes an illumination subsystem 1102. The illumination subsystem 1102 can incorporate the broadband light source 100 described throughout this disclosure. In an embodiment, although not shown in FIG. 11, the system 1100 includes a set of illumination optics. In an embodiment, the illuminator optics can direct illumination 109 emanating from the broadband light source 100 to one or more samples 1104 disposed on a sample stage 1106. For example, the one or more samples 1104 may include, but are not limited to, a wafer (e.g., a semiconductor wafer). As another example, the one or more samples 1104 may include, but are not limited to, a reticle or a photomask. In an embodiment, the system 1100 includes one or more detectors 1110. In an embodiment, the system 1100 includes a set of projection optics 1109 suitable for collecting light scattered, reflected, diffracted, or otherwise emanating from the sample and directing the light to one or more detectors (e.g., CCD, TDI-CCD, PMT, etc.). In an embodiment, the system 1100 includes a controller 1112 for receiving and / or analyzing measurements from the detector 1110.

[0037] In an embodiment, the inspection system 1100 is configured as a wafer inspection system or a reticle / photomask inspection system. For example, the inspection system 1100 can include any wafer or reticle / photomask inspection optical architecture known in the art suitable for operating in the EUV spectral range. As another example, the inspection system 1100 can include any wafer or reticle / photomask inspection optical architecture known in the art suitable for operating in the soft x-ray spectral range. It is further recognized that the inspection system 1100 can be configured as an EUV mask or mask blank inspection system. EUV-based mask blank inspection is generally described in U.S. Patent No. 8,711,346 (Stokowski, April 29, 2014), which is incorporated herein by reference in its entirety. EUV-based mask blank inspection is generally described in U.S. Patent Application No. 13 / 417,982 (Xiong et al., March 12, 2012), which is incorporated herein by reference in its entirety. EUV-based reticle inspection is generally described in U.S. patent application Ser. No. 13 / 905,448 (Nasser-Ghodsi et al.), filed May 30, 2013, which is incorporated by reference in its entirety.

[0038] In an embodiment, not shown, the broadband light source 100 described throughout this disclosure may be implemented in an optical lithography system. In an embodiment, the optical lithography system (not shown) may include a set of illumination optics configured to direct output light from the broadband light source 100 to an EUV-compatible lithography mask (e.g., an EUV reflective mask) or a soft x-ray compatible lithography mask. In an embodiment, the optical lithography system includes a set of projection optics configured to receive the illumination reflected from the mask and direct the illumination reflected from the mask to one or more wafers disposed on a wafer stage. The optical lithography system may include any broadband lithography system known in the art. EUV-based lithography is generally described in U.S. Patent Application Serial No. 13 / 419,042 (Wang, March 13, 2012), which is incorporated herein by reference in its entirety.

[0039] FIG. 12 shows a flow diagram illustrating a method 1200 for generating continuous or near-continuous plasma-based lighting according to one embodiment of the present disclosure.

[0040] In step 1202, a rotating drum that is at least partially coated with a plasma-generating target material is rotated. For example, drum 102 may be rotated as shown in FIG. 1A. For example, rotating drum 102 may be rotated at a selected rotational speed by a rotational actuator 107, such as, but not limited to, a servo motor.

[0041] In step 1204, the rotating drum 102 is actuated axially. For example, as shown in FIG. 1A, the rotating drum 102 can be translated along an axial direction (e.g., vertical in FIG. 1A) via a linear actuator 111. A combination of rotational and linear actuation is used to direct a pulsed laser from a pulsed laser source 104 onto the surface of the drum 102 to a first set of spots that traverse the material coated portion of the drum in a first direction in a first spiral pattern, and to a second set of spots that traverse the material coated portion of the drum in a second first direction in a second spiral pattern.

[0042] In step 1206, rotational position information of the drum 102 is obtained. For example, as shown in FIGS. 1A-10, the control system 114 may read one or more electrical signals from a rotary encoder 116 integrated with the rotary actuator 107 (e.g., a servo motor) and connected to the rotating drum 102. In addition, the control system 114 can read one or more electrical signals from a linear encoder 118 integrated with the linear actuator 111 and connected to the rotating drum 102.

[0043] In step 1208, the pulsed laser source is triggered based on the rotational position information obtained from the rotary encoder. For example, the control system 114 may trigger the pulsed laser source 104 based on the rotational position information obtained from the rotary encoder 116 to form a first set of target positions for the downstroke of the rotatable drum 102 and a second set of target positions for the upstroke of the rotatable drum 102, whereby the second set of target positions are offset from the first set of target positions.

[0044] In step 1210, the previously illuminated spots of the rotating drum 102 are recoated with a plasma forming material. For example, as shown in FIG. 1A, after exposure to illumination 105 from a pulsed laser source 104, the material source 112 may recoat the rotating drum 102 with a selected plasma forming material (e.g., xenon or carbon dioxide), allowing the spots of the first and second patterns previously described herein to be re-exposed to illumination 105 on one or more subsequent passes.

[0045] Those skilled in the art will appreciate that there are a variety of vehicles (e.g., hardware, software, and / or firmware) in which the processes and / or systems and / or other technologies described herein may be embodied, and that the preferred medium will vary with the context in which the processes and / or systems and / or other technologies are deployed. In some embodiments, the various steps, functions, and / or operations are performed by one or more of the following: electronic circuits, logic gates, multiplexers, programmable logic devices, FPGAs, ASICs, analog or digital controls / switches, microcontrollers, or computing systems. Computing systems may include, but are not limited to, personal computing systems, mainframe computing systems, workstations, image computers, parallel processors, or any other devices known in the art. In general, the terms "control system" and "controller" in this disclosure are broadly defined to encompass any device having one or more processors that execute program instructions from a memory medium. The memory medium may include, but is not limited to, read-only memory, random access memory, solid-state memory, magnetic or optical disks, or magnetic tapes.

[0046] All of the methods described herein may include storing results of one or more steps of the method embodiments in a storage medium. The results may include any of the results described herein and may be stored in any manner known in the art. The storage medium may include any storage medium described herein or any other suitable storage medium known in the art. After the results are stored, they can be accessed in the storage medium and used by any of the method or system embodiments described herein, formatted for display to a user, used by another software module, method, or system, etc. Furthermore, the results may be stored "permanently," "semi-permanently," temporarily, or for a period of time. For example, the storage medium may be a random access memory (RAM), and the results may not necessarily persist indefinitely in the storage medium.

[0047] While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications and embodiments of the present invention may be made without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the present invention should be limited only by the appended claims.

Claims

1. 1. A broadband light source, comprising: a rotating drum at least partially coated with a plasma forming target material; a rotary actuator configured to rotate the rotary drum; a linear actuator configured to axially translate the rotating drum; a rotary encoder coupled to the rotating drum and configured to indicate a rotational position of the rotating drum, the rotary encoder including at least one of a double sequence encoder and a triple sequence encoder; a linear encoder connected to the rotating drum for indicating an axial position of the rotating drum; a laser source configured to direct pulsed illumination to a set of spots on a material-coated portion of the rotating drum to excite the plasma-forming target material and emit broadband light when the rotary actuator rotates the rotating drum and the linear actuator axially translates the rotating drum; a control system configured to receive one or more rotational position indicators from the rotary encoder and to control triggering of the laser source based on the one or more rotational position indicators from the rotary encoder; Equipped with The double sequence encoder includes two sets of indices, a first set of indices used as triggering criteria for the laser source during a downstroke and a second set of indices used as triggering criteria for the laser source during an upstroke, and the triple sequence encoder includes three sets of indices, a first set of indices used as triggering criteria for the laser source during a downstroke, a second set of indices used as triggering criteria for the laser source during an upstroke, and a third set of indices used as triggering criteria for a broadband light source providing a transition region between a downstroke spot and an upstroke spot.

2. The set of spots is a first set of spots arranged in a spiral pattern; a second set of spots arranged in a spiral pattern, the first set of spots and the second set of spots being interleaved such that spots from the second set of spots do not overlap with spots from the first set of spots; 2. The light source of claim 1, comprising:

3. The light source of claim 1 , wherein the rotary encoder serves as a trigger reference for the laser source.

4. 2. The light source of claim 1, wherein the control system is further configured to control rotation and axial movement of the rotating drum based on at least one of the one or more rotational position indicators from the rotary encoder or one or more axial position indicators from the linear encoder.

5. 3. The light source of claim 2, wherein the control system is configured to direct the rotary actuator to accelerate the rotating drum at a beginning of a turning portion of the rotating drum and to decelerate the rotating drum at an end of a turning portion of the rotating drum to offset the second set of spots such that spots from the second set of spots do not overlap spots from the first set of spots.

6. The light source of claim 1 , wherein the control system is configured to synchronize triggering of the laser source with data acquisition cycles of one or more sensors.

7. The control system includes: a motion controller communicatively coupled to the rotary actuator and the linear actuator, the motion controller configured to control the rotary actuator to control the rotational motion of the rotating drum and to control the linear actuator to control the axial motion of the rotating drum; a laser controller communicatively coupled to the laser source and configured to control triggering of the laser source based on position information from the rotary encoder; a process controller configured to provide position feedback information to the motion controller based on position information from at least one of the rotary encoder or the linear encoder; 2. The light source of claim 1, comprising:

8. The light source of claim 1 , wherein the laser source comprises a pulsed laser source.

9. The light source of claim 1 , further comprising one or more collection optics configured to receive illumination emanating from a plasma generated in response to excitation of the plasma forming target material.

10. 2. The light source of claim 1, wherein the rotating drum is a cylinder.

11. The light source of claim 1 , further comprising a material source configured to recoat previously irradiated portions of the rotating drum with the plasma forming target material.

12. 10. The light source of claim 1, wherein the plasma forming target material comprises at least one of frozen xenon or frozen carbon dioxide.

13. 10. The light source of claim 1, wherein the emitted broadband light comprises at least one of soft x-ray broadband light or EUV broadband light.

14. 1. An inspection system comprising:

1. A lighting subsystem, comprising: a rotating drum at least partially coated with a plasma forming target material; a rotary actuator configured to rotate the rotary drum; a linear actuator configured to axially translate the rotating drum; a rotary encoder coupled to the rotating drum and configured to indicate a rotational position of the rotating drum, the rotary encoder including at least one of a double sequence encoder and a triple sequence encoder; a linear encoder connected to the rotating drum for indicating an axial position of the rotating drum; a laser source configured to direct pulsed illumination to a set of spots on a material-coated portion of the rotating drum to excite the plasma-forming target material and emit broadband light when the rotary actuator rotates the rotating drum and the linear actuator axially translates the rotating drum; a control system configured to receive one or more rotational position indicators from the rotary encoder and to control triggering of the laser source based on the one or more rotational position indicators from the rotary encoder; a lighting subsystem comprising: one or more collection optics configured to collect illumination emanating from a plasma generated in response to excitation of the plasma forming target material; a set of illumination optics configured to direct illumination from the one or more collection optics to one or more samples disposed on a stage; A detector; a set of projection optics configured to receive illumination from a surface of the one or more samples and direct illumination from the one or more samples to the detector; Equipped with The double sequence encoder includes two sets of indexes, a first set of indexes used as trigger criteria for the laser source during a downstroke and a second set of indexes used as trigger criteria for the laser source during an upstroke, and the triple sequence encoder includes three sets of indexes, a first set of indexes used as trigger criteria for the laser source during a downstroke, a second set of indexes used as trigger criteria for the laser source during an upstroke, and a third set of indexes providing a transition area between a downstroke spot and an upstroke spot.

15. The inspection system of claim 14 , wherein the inspection system is configured as a wafer inspection system.

16. The inspection system of claim 14 , wherein the inspection system is configured as a reticle or photomask inspection system.

17. The inspection system of claim 14 , wherein the illumination includes extreme ultraviolet light.

18. The set of spots is a first set of spots arranged in a spiral pattern; a second set of spots arranged in a spiral pattern, the first set of spots and the second set of spots being interleaved such that spots from the second set of spots do not overlap with spots from the first set of spots; 15. The inspection system of claim 14, comprising:

19. The inspection system of claim 14 , wherein the rotary encoder serves as a trigger reference for the laser source.

20. 15. The inspection system of claim 14, wherein the control system is further configured to control rotation and axial movement of the rotating drum based on at least one of the one or more rotational position indicators from the rotary encoder or one or more axial position indicators from the linear encoder.

21. 20. The inspection system of claim 18, wherein the control system is configured to direct the rotary actuator to accelerate the rotating drum at a beginning of a turning portion of the rotating drum and to decelerate the rotating drum at an end of a turning portion of the rotating drum to offset spots from the second set of spots such that spots from the second set of spots do not overlap spots from the first set of spots.

22. The inspection system of claim 14 , wherein the control system is configured to synchronize triggering of the laser source with data acquisition cycles of one or more sensors.

23. The control system includes: a motion controller communicatively coupled to the rotary actuator and the linear actuator, the motion controller configured to control the rotary actuator to control the rotational motion of the rotating drum and to control the linear actuator to control the axial motion of the rotating drum; a laser controller communicatively coupled to the laser source and configured to control triggering of the laser source based on position information from the rotary encoder; a process controller configured to provide position feedback information to the motion controller based on position information from at least one of the rotary encoder or the linear encoder; The inspection system of claim 14 .

24. The inspection system of claim 14 , wherein the laser source comprises a pulsed laser source.

25. The inspection system of claim 14 , further comprising one or more collection optics configured to receive illumination emanating from a plasma generated in response to excitation of the plasma forming target material.

26. 15. The inspection system of claim 14, wherein the rotating drum is a cylinder.

27. The inspection system of claim 14 , further comprising a material source configured to recoat previously irradiated portions of the rotating drum with the plasma-formed target material.

28. The inspection system of claim 14, wherein the plasma forming target material comprises at least one of frozen xenon or frozen carbon dioxide.

29. The inspection system of claim 14 , wherein the emitted broadband light comprises at least one of soft x-ray broadband light or EUV broadband light.

30. 1. A method comprising: rotating a rotating drum coated with a plasma generating target material; axially actuating the rotating drum; obtaining rotational position information from a rotary encoder coupled to the rotating drum, the rotary encoder including at least one of a double sequence encoder and a triple sequence encoder; triggering a pulsed laser source based on rotational position information obtained from the rotary encoder to generate a first set of target positions for a downstroke of the rotating drum and a second set of target positions for an upstroke of the rotating drum that are offset from the first set of target positions; recoating the previously targeted spots of the rotating drum with the plasma-generated target material; the double sequence encoder includes two sets of indices, a first set of indices being used as triggering criteria for the pulsed laser source during a downstroke and a second set of indices being used as triggering criteria for the pulsed laser source during an upstroke; and the triple sequence encoder includes three sets of indices, a first set of indices being used as triggering criteria for the pulsed laser source during a downstroke, a second set of indices being used as triggering criteria for the pulsed laser source during an upstroke, and a third set of indices providing a transition region between a downstroke spot and an upstroke spot.

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