Apparatus and process for reduced deposition in light source

Abstract

A process for generating EUV radiation in an EUV light source includes: during a first period, mis- timing of light pulses in the EUV light source relative to a target material, resulting in a target material hit rate of zero during the first period and no production of EUV light; and, during a transition period following the first period, ramping up the target hit rate in the EUV light source.

Description

APPARATUS AND PROCESS FOR REDUCED DEPOSITION IN LIGHT SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Application No. 63 / 613,549, filed December 21, 2023, titled APPARATUS AND PROCESS FOR REDUCED DEPOSITION IN LIGHT SOURCE, which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to an apparatus and processes for reducing deposition within a source vessel while generating extreme ultraviolet (“EUV”) radiation from a plasma created in the source vessel by the irradiation of targets.BACKGROUND

[0003] Extreme ultraviolet radiation, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in or on substrates such as silicon wafers. Processes for generating EUV radiation include converting a target material to a plasma state. The target material includes at least one element, e.g., xenon, lithium, or tin, or others with one or more emission lines in the EUV portion of the electromagnetic spectrum, or combinations of such elements. The target material can be solid, liquid, or gas. In one such process, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a “source” laser, typically a CO2 laser emitting infrared light at a wavelength at or about 10,600 nanometers (nm), to irradiate with one or more light pulses a target containing one or more EUV lineemitting elements. The plasma is typically produced in a sealed “source vessel” which is typically a vacuum chamber.SUMMARY

[0004] According to an aspect of the present disclosure, a process for generating EUV radiation in an EUV light source is provided, the process including: during an off-droplet period, not hitting targets with light pulses in the EUV light source, resulting in a target hit rate of zero during the off-droplet period and no production of EUV light; and during a transition period following the off-droplet period, ramping up the target hit rate in the EUV light source.

[0005] Implementations can include one or more of the following.

[0006] The process can further include: during the transition period, increasing a flow rate of a gas flow in the EUV light source from a first lower flow rate to a second higher flow rate. The process can further include: beginning during the off-droplet period before the transition period, increasing a flow rate of a gas flow in the EUV light source from a first lower flow rate to a second higher flow rate.Increasing the flow rate of the gas flow can include switching the flow rate stepwise. Increasing the flow rate of the gas during the transition period can include ramping up the flow rate. The process can further include decreasing the flow rate of the gas during a decreasing period, the decreasing period following, including, or immediately preceding a subsequent off-droplet period. Decreasing the flow rate of the gas during the decreasing period can include switching the flow rate stepwise. Decreasing the flow rate of the gas during the decreasing period can include ramping down the flow rate.

[0007] Ramping up the hit rate of targets in the EUV light source can include linearly increasing the hit-rate. Ramping up the hit rate of targets can be performed within a time from 5 to less than 10 milliseconds (ms) or to less than 50 ms.

[0008] In another aspect, a process of reducing deposition in an EUV light source is provided, the process including: during a ramping -up period, ramping up a hit rate of targets in the EUV light source; and adjusting gas flows in the EUV light source during at least a portion of the ramping-up period to reduce or prevent debris overshooting an exhaust opening in the EUV light source. Ramping up the hit rate of the targets during the ramping -up period can prevent or reduce deposition of debris on a chamber wall of the EUV light source.

[0009] Adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source can include increasing a flow rate of a gas flow in the EUV light source. Adjusting gas flows can include: increasing a forward flow from a first flow rate to a second flow rate; maintaining the forward flow at the second flow rate during at least a portion of a time in which the EUV light source is producing EUV light; and decreasing the forward flow from the second flow rate to a third flow rate, wherein the third flow rate is equal to or higher than the first flow rate. Adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source can include adjusting a gas flow toward the exhaust opening in a direction countering or partially countering gas and / or debris flows overshooting the exhaust opening. Adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source can include adjusting a counter flow. Adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source can include adjusting a flow from an obscuration bar.

[0010] In yet another aspect, an apparatus includes: a target delivery system configured to deliver targets into a light source vessel; a source laser configured to deliver light pulses into the light source vessel, the targets including a plasma-producing material that can produce a plasma when hit by the light pulses; and a controller in communication with the source laser, the controller being configured to cause the source laser (1) not to hit targets during an off-droplet period, then (2) to ramp up a hit rate of targets during a transition period.

[0011] Implementations can include one or more of the following.

[0012] The apparatus can further include a gas supply configured to supply gas into the source vessel, and the controller can be in communication with the gas supply and be configured to increasea gas flow rate from the gas supply into the source vessel during at least a portion of the transition period. The controller can be further configured to ramp down a hit rate of targets at the end of the on- droplet period.

[0013] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.DRAWING DESCRIPTION

[0014] FIG. 1A is simplified schematic cross-sectional views of some components of an implementation of an LPP EUV light source.

[0015] FIG. IB is a simplified schematic cross-sectional view of the light source of FIG. 1A, rotated 90 degrees about its axis.

[0016] FIG. 1C is the simplified schematic cross-sectional view of the light source of FIG. IB with some components omitted and others added and showing some gas flow conditions within the source.

[0017] FIGS. 2A and 2B are graphs, over time, of target hit rate and gas flow rate, respectively, in an implementation of processes disclosed herein.

[0018] FIGS. 3 A and 3B are graphs, overtime, of target hit rate and gas flow rate, respectively, in another implementation of processes disclosed herein.

[0019] FIG. 4 is a graph, over time, of target hit rate in yet another implementation of processes disclosed herein.

[0020] FIG. 5 is a simplified schematic cross-sectional view of an LPP EUV light source together with a photolithography apparatus.

[0021] FIG. 6 is a simplified schematic cross-sectional view of a portion of an LPP EUV light source having an annular exhaust opening.DETAILED DESCRIPTION

[0022] FIGS. 1A-1E are simplified schematic cross-sectional views of some components of an implementation of an EUV light source 110. As shown by the reference coordinate axis in the figure, the EUV light source 110 in FIG. 1A is shown in cross section in an x-z plane, with x positive in the leftward direction in the plane of the page, and z positive upward in the plane of the page. The z axis is aligned with an optical axis A of a collector 120 to be described below. As shown by the reference coordinate axes in FIGS. 1B-1E, the EUV light source 110 in FIGS. 1B-1E is shown in cross section in a y-z plane, that is, the EUV light source 110 is rotated 90 degrees from the view shown in FIG. 1A.

[0023] With reference to FIGS. 1A-1E and particularly to FIGS. 1A and IB, the EUV light source 110 includes a source laser 112 for generating a beam 113 of light (for example, laser) pulses and a beam delivery system 126 for delivering the pulsed light beam 113 from the source laser 112 into theinterior 114 of a source vessel 111 to individually irradiate one or more targets 115 as they approach and / or reach an irradiation site 116. The beam delivery system 126 can include a focusing unit (not shown) for focusing the pulsed light beam 113 to focal spot or beam waist at or near the irradiation site 116. The targets 115 travel substantially rightward in the plane of the page (in the negative x direction) from a target delivery system 117a into the interior 114 of the source vessel 111 and toward the irradiation site 116. The source vessel 111 has an interior surface 156 surrounding the interior 114.

[0024] At the irradiation site 116, one or more of the targets 115 individually interact with one or more light pulses (of the light beam 113) to produce plasma 118 that produces EUV light 119. In some embodiments, unused targets 115a (those that are not converted into the plasma 118) are optionally captured in a target trap 117b.

[0025] Light from the plasma 118, positions of the targets 115, and other data can be monitored by one or more metrology devices 150, and signals and / or other information from the one or more metrology devices 150 can be used for control and operation of the EUV light source 110.

[0026] The targets 115 are or include an EUV emitting target material such as, but not necessarily limited to, tin, lithium, xenon, or combinations thereof. The targets 115 can be in the form of liquid droplets, or alternatively can be solid particles or solid particles contained within liquid droplets. For example, the element tin can be presented as a target in the form of pure tin; a tin compound such as SnBr4, SnBr2, SnH : a tin alloy, e.g., tin-gallium alloys, tin-indium alloys; or tin-indium-gallium alloys; or a combination thereof. Note that the term “droplet” is sometimes used to refer only to droplets or targets that have not yet interacted with one or more first light pulses or “pre-pulses” that help to reshape and otherwise transform a droplet to better couple with one or more second light pulses or “main pulses” to form the final plasma from that droplet. Similarly, the term “targets” is sometimes used to refer to only droplets that have interacted with one or more such pre-pulses.However, in this description “droplets” and “targets” will be used interchangeably, and “off-droplet’ ’ or “off-target” will refer to periods or states in which plasma is not being produced (by mis-timing of source laser light pulses relative to droplets or by other means) and “on-droplet” or “on-target” will refer to periods or states in which at least some plasma is being produced by source laser light pulses hitting targets.

[0027] The EUV light source 110 can also include the collector 120. The collector 120 can be a nearnormal incidence collector mirror having the optical axis A and a reflective surface 121. The reflective surface 121 can be in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis), such that the collector 120 has a first or primary focus 122 within or near the irradiation site 116 and a second focus or intermediate focus 123, with the optical axis A defined as a line extending between them. The source vessel 111 of the EUV light source 110 thus encloses at least in part a volume in which, when the EUV light source 110 and source vessel 111 are in use, EUV light is transmitted by the collector 120 from the primary focus 122 to the intermediate focus 123 along the optical axis A. Reflected EUV light 124 from the collector 120 (with only the outer and innermostrays being traced in the figure) can be output from the EUV light source 110 at the intermediate focus 123 and input to a device utilizing the EUV light 124, such as a lithography exposure apparatus (an implementation of which is to discussed below with respect to FIG. 5). The collector 120 is formed with an aperture 125 to allow the light beam 113 of light pulses generated by the source laser 112 to pass through the aperture 125 and reach the irradiation site 116. The aperture 125 creates a shadow or voluminous gap 154 along the optical axis A in the reflected EUV light 124 from the collector 120.

[0028] In order to reflect the EUV light 119, the collector 120 can be in the form of a multi-layer mirror (MLM), with the reflective surface 121 having a graded multilayer coating with alternating layers of molybdenum and silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and / or etch stop layers. Other surface shapes besides the prolate spheroid can also be used for the reflective surface 121. For example, the reflective surface 121 can alternatively be in the form of a parabola rotated about its major axis. In implementations, the reflective surface 121 can be configured to deliver a beam of EUV light 124 having a ring-shaped or other cross section at the intermediate focus 123. In other implementations, the reflective surface 121 can utilize coatings and layers other than or in addition to those described above.

[0029] The collector 120 can be expensive to fabricate. The efficiency of the EUV light source 110 and the brightness or power of the light produced by the EUV light source 110 depend upon the quality of the reflective surface 121 of the collector 120. For these and other reasons, it is desirable to protect the collector 120 from damage to its reflective surface 121.

[0030] However, the collector 120 must be placed within the source vessel 111 and proximate or near to the plasma 118 in order to collect and redirect the EUV light 119 produced by the plasma 118. Structures within the source vessel 111, including the collector 120, may be exposed to high energy ions and / or particles and vapor of or containing target material. The particles of target material and high energy ions and vapor, which are essentially debris and / or byproducts from vaporization or ablation of the targets, can deposit on or otherwise contaminate the collector’s exposed reflective surface 121. Particles of target material and energetic ions and vapor can also cause physical damage and localized heating of the reflective surface 121 of the collector 120.

[0031] A controller or control system or multiple controllers or multiple control systems 199 can receive information 150c from (and optionally provide information 150c to) the one or more metrology devices 150. The controller 199 can be in the form of hardware or software or any combination thereof, and can be in consolidated or distributed form, with wired, optical fiber, or other hard-linked communications, or wireless or other non-hard-linked communications, or combinations thereof. The controller 199 can also provide information 112c, such as control signals, to the source laser 112 (and optionally receive information 112c to the source laser 112). The controller 199 can also provide information 112c, 126c, such as control signals, to the source laser 112 and to the beam delivery system 126 (and optionally receive information 112c, 126c from the source laser 112 and from the beam delivery system 126). The controller 199 can also provide information 117c, such ascontrol signals, to the target delivery system 117 (and optionally receive information 117c from the target delivery system 117).

[0032] The controller 199 can be programmed or operated or instructed or otherwise controlled to synchronize the delivery of targets 115 from the target delivery system 117a and light pulses from the source laser 112 to the irradiation site 116. During operation of the EUV light source 110, there are times during which not every target 115 (or not any target 115) is desired to be irradiated. In the case represented in the diagram of FIG. 1A, some targets 115 are being irradiated, such as the target (not visible in the figure) that was irradiated to create the plasma 118, but some targets 115 are intentionally not being irradiated (by mis-timing the relevant light pulse from the source laser 112), becoming unused targets 115a to be received by the target trap 117b.

[0033] The percentage of targets 115 irradiated or being irradiated within a given interval of time or at or around a given point in time can be referred to as a target hit rate. The hit rate represented in FIG. 1A, as may be seen from the figure, is greater than 0% and less than 100%. During use of the light source to produce a burst of EUV light pulses such as for use by a lithography exposure apparatus, the hit rate during the burst is typically 100% or as close as possible to 100%, as this provides the brightest EUV light and the most time- and resource-efficient operation of the light source 110. Between bursts of pulses, such as during a time required for a lithography exposure apparatus to move a wafer and / or reticle to pattern another area on the wafer, the hit rate is typically 0%, sometimes referred to as being “off-target” or “off-droplet.” An aspect of the present disclosure includes ramping up the hit rate in the EUV source 110 during a transition from an off-target state, or during a transition from no production of EUV light, to production of EUV light (such as at a desired brightness for use in lithography). The hit rate of greater than 0% and less than 100% represented in FIG. 1A thus represents a “snapshot” or a moment in time during such a ramping up.

[0034] A beam blocking structure such as an obscuration bar 127 including an obscuration bar head 130 can be positioned at least in part on the optical axis A of the collector 120. The head 130 can be positioned and sized to fit within the shadow or voluminous gap 154 in the reflected EUV light 124 from the collector 120. For example, the head 130 can have a cross section, taken perpendicular to the optical axis A, which is circular and centered on the optical axis A and matched to the shadow or voluminous gap 154. This geometry prevents the head 130 from blocking any, or any significant part, of the EUV light 124 that is reflected from the collector 120 and directed toward the intermediate focus 123, while simultaneously well-protecting the intermediate focus 123 from direct illumination by the pulsed light beam 113 from the source laser 112, particularly (but not only) during off-target periods.

[0035] In FIGS. IB- IE, the view of the EUV light source 110 is substantially rotated 90 degrees around the optical axis A to show a cross section in a y-z plane. In some embodiments, only the head 130 of the obscuration bar 127 (FIG. 1 A) is visible in this plane, with the rest of the obscuration bar 127 is positioned behind the head 130 of the obscuration bar 127, into the plane of the page. The EUVlight source 110 includes an exhaust port 133 and an associated exhaust opening 155. As shown, the exhaust port 133 is a structure that extends from the source vessel 111 and defines the exhaust opening 155 that is in fluid communication with, and extends out from, the interior 114 of the source vessel 111. Gases and entrained ions, vapor, and debris can be evacuated from the source vessel 111 by one or more vacuum pumps (not shown) through the exhaust opening 155 of the exhaust port 133. The exhaust opening 155 is positioned, measured along the optical axis A, between the collector 120 and the head 130. When in use, the EUV light source 110 can be inclined with respect to gravity in various orientations, not limited to the orientation of gravity being downward in the plane of the figures.

[0036] In FIGS. 1C-1E, various gas flows that can be used in the EUV light source 110 represented in the figures by outline-style arrows. Gas flows such as flows of hydrogen (H2) gas at pressures in the range of about 50 to about 300 Pa can be used within the source vessel 111 as a buffer gas for debris, ion, and vapor control. Given that a vacuum is needed in the interior 114 the source vessel 111 to avoid gas molecules excessively absorbing the EUV light, it would be difficult to protect the collector 120 adequately from target material debris and vapor emanating from the irradiation site 116 without the use of gas flows. Hydrogen (H2) is relatively transparent to EUV radiation having a wavelength of about 13.5 nm.

[0037] H2gas introduced into the source vessel 111 can slow down and guide energetic debris (ions, atoms, and clusters) of target material created by irradiation of targets 115 and irradiation site 116 and by the resulting plasma 118. The debris is slowed down by collisions with the gas molecules. A forward flow 136 of H2 gas at the center aperture 125 of the collector 120 can be used for this purpose. The forward flow 136 can be guided by a tube, a nozzle or an adjustment structure 137 or the like from the aperture 125 at the center of the collector 120 toward the irradiation site 116 at which the plasma 118 is repeatedly created. This direction is counter to a debris trajectory from the irradiation site 116 toward the collector 120, and the forward flow 136 thus serves to prevent or reduce damage to the collector 120 caused by vapor deposition, implantation, and deposition of sputtered target material.

[0038] Deposition on any internal surfaces of the source vessel 111 can shorten the length of service intervals of an EUV source such as the EUV source 110. Growth of such deposits can be reduced by the use of additional gas flows. A lateral flow 139 can be directed along the surface of the collector 120 (from outlets not shown). So-called showerhead flows, in which gas flows through multiple parallel apertures generally perpendicular to the surface to be protected, such as showerhead flow SI and showerhead flow S2, can be provided in areas of the source vessel 111 nearest the collector 120. In additional regions such as regions near the intermediate focus 123, protective gas flows parallel to, or having a component of flow directed parallel to, the surface to be protected can be introduced through apertures aimed in directions having a component along or parallel to the surface to beprotected. For example, gas flows such as gas flows Fl, F2, F3, and F4 can be introduced to protect the interior surface 156 of the source vessel 111 in regions near the intermediate focus 123.

[0039] A counterflow 138 is one or more gas flows used to prevent any material leaving the EUV source 110 in the region of the intermediate focus 123. The counterflow 138 can produce a gas flow from the area of the intermediate focus 123 toward the irradiation site 116.

[0040] During the production of EUV light, a guided flow 140 flowing away from the collector 120 can be formed within the source vessel 111 mainly by the forward flow 136, together with the lateral flow 139 and the showerhead flows SI and S2 (and optionally others not shown). The solid curved lines in FIG. 1C illustrate an example of the boundaries of the guided flow 140. This guided flow 140 helps contain and carry away from the collector 120 materials, including vapor, ions, and micro and nanoparticles, generated from the targets 115 by the production of plasma 118. The forward flow 136 can be controlled or varied by a variable gas supply 198 controlled by the controller 199 as shown in FIGS. 1C-1E, allowing control or variation of the guided flow 140. Alternatively or in addition, the controller 199 can control or vary flow components of the guided flow 140 other than the forward flow 136, such as one or more showerhead flows such as showerhead flows SI, S2, and / or the lateral flow 139.

[0041] An opposing flow 141 moving generally from the intermediate focus 123 toward the collector 120 can be formed mainly by the counterflow 138, together with flows such as flows Fl, F2, F3, and F4, and optionally flows such as sideways and / or downward flows 130f supplied from obscuration bar and / or from the head 130 of the obscuration bar (and optionally others not shown). The dotted curved lines in FIG. 1C illustrate an example of the boundaries of the opposing flow 141.

[0042] Given the low pressures used within the source vessel 111, pressure differentials at the exhaust opening 155 of the exhaust port 133 can be small. But a pressure differential at the exhaust opening 155 produced by vacuum pumping the exhaust port 133, together with a flow momentum balance between the guided flow 140 and the opposing flow 141 at a merging region 142 of the two flows 140, 141, with the merging region 142 being near the exhaust opening 155, can create a stable guided flow of target material byproducts entrained and contained in the guided flow 140 into the exhaust opening 155, without the target-material byproducts substantially contacting any inner surfaces of the source vessel 111. This essentially steady-state flow balance, without target-material byproducts substantially contacting any inner surfaces of the source vessel 111 as shown in FIG. 1C, represents the more or less steady-state flow balance achieved or achievable during the majority of a period of on-droplet operation, when no or very little deposition of target material is deposited on surfaces within the source vessel 111.

[0043] Plasma and EUV light production can be stopped or paused for various reasons. To control the amount of radiation (“exposure dose”) received by a given exposure site on a wafer , the power of the EUV light 119, 124 (FIGS. 1A, IB) produced by an EUV light source such as EUV light source 111 from each light pulse can be detected and the total power for delivered to that site can becalculated in real time. Once a desired exposure dose level has been reached or exceeded, further light pulses can then be immediately mis-timed so that in the source vessel 111, targets 115 are not hit by the light pulses for as long as that exposure site is still positioned for exposure. This results in a sudden cessation of plasma production in the source vessel 111. Stopping and starting of plasma production can also occur during moving from one exposure site on the wafer to the next, or during moving from one wafer to the next.

[0044] Before an essentially steady-state flow pattern as shown in FIG. 1C has developed, but without the ramping of the hit rate of the present disclosure, as mentioned above with respect to FIG. 1 A (and discussed further below), when the light pulses switch from off-droplet to on-droplet at the beginning or the restarting of plasma and EUV light production (at the beginning of an on-droplet period of operation), expanding target material in the form of plasma, vapor, and debris is rather suddenly injected into the guided flow 140 at the irradiation site 116. The sudden injection of expanding target material can briefly widen the guided flow 140 perpendicularly to its flow direction and push the boundary or boundaries of the guided flow 140, together with entrained target material, to the interior surface(s) 156 of the vessel 111. The brief widening of the boundaries of the guided flow 140 can locally disturb or overpower the showerhead flow S 1 and allow target material to contact and deposit on the interior surface 156 of the source vessel 111. By ramping the hit rate (the percentage of droplets hit by the light pulses, or the rate of droplets being hit over time, as discussed above with respect to FIG. 1A) at the transition to EUV light and plasma production (i.e., at the start of “on-droplet” operation), the transition is more gradual and the excessive widening of the guided flow 140 can be prevented or reduced, preventing or reducing deposition of target materials on the interior surface 156 of the vessel 111 of the light source 110.

[0045] During an off-droplet period, light pulses of the light beam 113 (FIG. 1A) can be mis-timed relative to droplets or targets 115 (FIG. 1A) that can continue to be delivered into the vessel 111, such that no targets 115 are hit by the light pulses, and no expanding target material is produced at the irradiation site 116. Without the production of expanding target material at the irradiation site 116, the droplets or targets 115 that continue to be delivered are generally not entrained in the guided flow 140, but simply pass through it. Without the additional mass and momentum of the target material being injected into the guided flow 140, the guided flow 140 can typically have too much momentum and / or mass to be balanced by the flow 141 as in FIG. 1C, resulting in “overshoot” or “breakout” flows from the guided flow 140 that go beyond the exhaust opening 155.

[0046] Breakout flows cause no difficulties during the middle of an off-droplet period. But during the transition from off-droplet to on-droplet, the initial injection of target material can reach beyond the vicinity of the exhaust opening 155 before the steady state flow pattern of FIG. 1C has fully developed, while breakout flows are still present. This can result in deposition of target material on the inner surface 156 of the vessel 111 in areas above the exhaust opening 155.

[0047] Hit rate ramping at the off-droplet to on-droplet transition, although preventing or reducing deposition from broadening of the guided flow 140, can also prolong the time required to develop the steady state flow pattern of FIG. 1C, thus prolonging the duration of breakout flows. For this reason, hit rate ramping at the off-droplet to on-droplet transition is optionally used together with an adjustment to gas flows within in the vessel 111 at or about the same time as the hit rate ramping. For example, the forward flow 136, optionally together with one or more other gas flows contributing to or shaping the guided flow 140, such as the showerhead flows SI and S2 or the lateral flow 139, can be lower during a first off-droplet period and can be increased at or just before the time of hit rate ramping, from the lower flow rate used during the off-droplet period, to a higher flow rate used during on-droplet operation, such that breakout flows are minimized or prevented. This can be achieved, for example, by the controller 199 signaling the variable gas supply 198 (see FIG. 1C) to increase the gas flow delivered to the forward flow 136 (and optionally other flows forming or shaping the guided flow 140 such as the showerhead flows SI, S2) at or just before the hit-rate ramping. At the time of hit-rate ramping or overlapping with hit-rate ramping, as an alternative or as an addition to increasing the forward flow 136 (and optionally other flows contributing to or shaping the guided flow 140), flows such as flows Fl, F2, F3, F4, 130f, and 138 (FIG. 1C) can be adjusted, such as by increasing one or more of the respective flow rate(s), or by providing or temporarily providing one or more of the flows such as the flows 13 Of, to assist in preventing breakout flows during the transition from off- droplet to on-droplet operation. Selected ones or combination of these flows can thereby provide a gas flow toward the exhaust opening 155 in a direction countering or partially countering gas and / or debris flows otherwise overshooting the exhaust opening 155.

[0048] In contrast to a sudden transition at the beginning of on-droplet operation, a sudden transition at the end of on-droplet operation does not typically cause flow disturbances producing deposition in a light source. So a ramp down of the hit rate at the end of on-droplet operation is generally not needed. The resulting sudden cessation of the creation of target products at the irradiation location, however, can in some instances lead to breakout flows 143 (FIG. ID) reappearing while target products are still present in the flow 140, resulting in some target material deposition near and beyond the exhaust opening 155. Decreasing the forward flow 136 (and optionally other flows forming or shaping the guided flow 140) back to the lower flow rate used during the off-droplet period, or to some other flow rate lower than the flow rate during the on-droplet period, at or just before the transition to off-droplet operation, can reduce or prevent such breakout flows during the transition to off-droplet operation.

[0049] FIG. 2A is a graph of percentage hit rate of targets in a light source and FIG. 2B is a related graph of gas flow rates such as gas flow rates of a forward flow and optionally other flows forming or shaping a guided flow in the light source. Time is on the horizontal axis of FIGS. 2A and 2B, and is scaled and aligned the same in both figures. Note that the percentage hit rate can represent a percentage of delivered droplets or targets hit by one or more light pulses to generate plasma relativeto the total droplets delivered into the light source or a similar measure, such as the rate per unit time of droplets or targets hit relative to a maximum hit rate per unit time.

[0050] As seen in FIG. 2A, during a transition period P at the start of on-droplet operation, the hit rate percentage of the light source is ramped up from zero to 100% (or, alternatively, to whatever maximum hit rate is desired). “Ramped” or “ramping” as used herein means a transition with a finite slope overtime and does not apply an instantaneous change from off-droplet (0% hit rate) to 100% hit rate or to some other specific level.

[0051] A lithography exposure apparatus typically does not use the first several pulses of a burst of EUV light, corresponding to the light from the first several targets of on-droplet operation, for exposure or pattern forming on a wafer. The duration of time in which initial pulses of EUV light are not used for pattern exposure can typically be, for example, in the range of 10 to 50 milliseconds (ms), set by manufacturers and / or operators of photolithography apparatuses. The length of period P is chosen to be equal to or less than the duration of time in which initial pulses are not used for pattern exposure so as not to affect the exposure operation. In other words, the length of the transition period P can typically be in the range of greater than zero but less than 10 ms to less than 50 ms. The period P can also be at least about 5 ms to provide sufficient transition time to have a larger possibility of reducing deposition.

[0052] As seen in FIG. 2B, the gas flow rate can also be ramped up during a period Pl from a low first flow rate FR1 used during off-droplet operation to a higher second flow rate FR2 used during production of EUV light or on-droplet operation. The gas flow rate can further be ramped back down to the lower first flow rate FR1 during a second period P2. In some embodiments, an example of which is illustrated by the medium-dashed line segments in FIG. 2B, the gas flow rate is ramped back down to a level, such as third flow rate FR3, higher than the lower first flow rate FR1 but lower than the higher second flow rate FR2. In some embodiments, an example of which is illustrated by the long-dashed line segments in FIG. 2B, the gas flow rate is ramped back down to a level, such as a fourth flow rate FR4, lower than the lower first flow rate FR1. A subsequent change from the third flow rate FR3, or from the fourth flow rate FR4, back to the first flow rate FR1, prior to a next successive transition period P (not shown), can be relatively slower (as shown coming up from the fourth flow rate FR4) or relatively faster (as shown coming down from the third flow rate FR3), or, alternatively, the flow rate ramping in a next successive transition period P (not shown) can simply begin directly from the third flow rate FR3 or from the fourth flow rate FR4. A range of the lower first flow rate FR1 and the higher second flow rate FR2 is from about 70 standard liters per minute (slm) to about 120 slm. In some instances, a flow rate greater than 120 slm reduces an effect of counterflow, resulting a contamination of a scanner. In some instances, a flow rate smaller than 70 slm is not sufficient to alter a trajectory of debris or byproducts. The period Pl can have the same duration as the period P and the same timing, as shown in FIGS. 1A and IB, but can also have different timing, such as beginning earlier, and / or different duration. The period P2 can begin when the hit rate again goes tozero in a subsequent off-droplet period, as shown in FIGS. 1A and IB, but have different timing, such as beginning earlier than the start of the subsequent off-droplet period when the hit rate goes to zero. The period P2 can have the same duration as the transition period P, or as the period Pl, but can also have a duration different from either or both.

[0053] FIGS. 3 A and 3B are similar to 2A and 2B, respectively, but with different timing and / or shapes of the hit rate and flow rate transitions. As shown in FIG. 3 A, the ramp of hit rate at the transition from off-droplet to on-droplet during the transition period P need not be a linear ramp of percentage hit rate overtime, but can instead have increasing slope over time, as shown. Other ramp shapes are also possible. As shown in FIG. 3B, the increase in the gas flow from the lower first flow rate FR1 to the higher second flow rate FR2 need not be gradual, or at least not very gradual — the valve or other control mechanism adjusting the gas flow can be extremely fast acting, if desired, and can be provided with a stepwise transition control signal, for example, such that the increase in flow rate at the mechanism stepwise or is almost instantaneous, with the steepness of the ramp essentially dictated by flow dynamics of associated gas conduits to an interior of the source vessel. As also shown, the beginning of the increase in the gas flow rate (the beginning of period Pl) can precede the beginning of the ramping of the hit rate (the beginning transition period P), and the beginning of the decrease in the gas flow rate (the beginning of period P2) can precede the hit rate going back to zero at the start of a subsequent off-droplet period. As in FIG. 2B above, in FIG. 3B flow rates higher or lower than the first flow rate Fl can also be used at the end of the period P2, if desired.

[0054] FIG. 4 shows a graph of another alternative ramp shape that can be used in a transition period P, namely an “S” curve in which the slope of the percentage hit rate ramp is lower both at the beginning and at the end of the transition period P than at the middle. Note that an S-curve hit rate ramp such as that shown in FIG. 4 can be used with gas flow rates of the types shown in FIGS. 2B and / or 3B. Similarly, a hit rate ramp of the type shown in FIG. 2A can be used with gas flow rates of the type shown in FIG. 3B, and a hit rate ramp of the type shown in FIG. 3A can be used with gas flow rates of the types shown in FIG. 2B.

[0055] FIG. 5 is a diagram showing an implementation of an EUV light source 510 with a lithography exposure apparatus 571. The lithography exposure apparatus 571 receives EUV light 524 produced by the EUV light source 510 and reflects it in one or more illumination mirrors 572 so as to illuminate a reflective pattern or reticle 573. EUV light reflected from the pattern or reticle 573 is further reflected and reduced by one or more reducing mirrors 574 and irradiated on a substrate or wafer 575 (or on one or more photosensitive layers on the substrate or wafer 575, not shown) to allow the formation of patterned structures in or on the substrate or wafer 575. Use of the herein disclosed percentage hit rate ramping, optionally together with the disclosed gas flow adjustments, by preventing or reducing deposition within the light source 510, allows longer service intervals for the light source 510, such that the productivity and value of the light source 510 and the associated lithography exposure apparatus 571 are increased.

[0056] In implementations, gas flow adjustments such as increasing or decreasing a gas flow can include adjusting a flow from zero flow to non-zero flow and / or vice versa. For example, a flow nozzle not used during steady-state operation of the associated EUV light source can be used during transitions from off-droplet to on-droplet and / or on-droplet to off-droplet.

[0057] Note that in implementations of the processes disclosed herein, the exhaust opening of the source vessel of the EUV light source can take other forms than the exhaust opening 155 shown in FIGS. 1B-1E. FIG. 6 shows a schematic cross-sectional view of a portion of a light source 610 (shown in a cross-sectional schematic diagram of only features near the exhaust opening). The light source 610 includes a source vessel 611 enclosing a source vessel interior 614 and including an annular exhaust 633a with an annular exhaust opening 655a including a scrubber 660. (Exhaust is carried away from the annular exhaust 633a within a conduit not visible in the plane of the figure.)

[0058] The aspects and implementations can be further described using the following clauses:1. A process for generating EUV radiation in an EUV light source, the process including: during an off-droplet period, not hitting targets with light pulses in the EUV light source, resulting in a target hit rate of zero during the off-droplet period and no production of EUV light; and during a transition period following the off-droplet period, ramping up the target hit rate in the EUV light source.2. The process of clause 1, further including: during the transition period, increasing a flow rate of a gas flow in the EUV light source from a first lower flow rate to a second higher flow rate.3. The process of clause 1, further including: beginning during the off-droplet period before the transition period, increasing a flow rate of a gas flow in the EUV light source from a first lower flow rate to a second higher flow rate.4. The process of clause 2, wherein increasing the flow rate of the gas flow includes switching the flow rate stepwise.5. The process of clause 2, wherein increasing the flow rate of the gas during the transition period includes ramping up the flow rate.6. The process of clause 2, further including decreasing the flow rate of the gas during a decreasing period, the decreasing period following, including, or immediately preceding a subsequent off-droplet period.7. The process of clause 6, wherein decreasing the flow rate of the gas during the decreasing period includes switching the flow rate stepwise.8. The process of clause 6, wherein decreasing the flow rate of the gas during the decreasing period includes ramping down the flow rate.9. The process of clause 1, wherein ramping up the hit rate of targets in the EUV light source includes linearly increasing the hit-rate.10. The process of clause 1, wherein ramping up the hit rate of targets is performed within a time from 5 to less than 10 milliseconds (ms) or to less than 50 ms.11. A process of reducing deposition in an EUV light source, the process including: during a ramping- up period, ramping up a hit rate of targets in the EUV light source; and adjusting gas flows in the EUV light source during at least a portion of the ramping-up period to reduce or prevent debris overshooting an exhaust opening in the EUV light source.12. The process of clause 11, wherein ramping up the hit rate of the targets during the ramping-up period prevents or reduces deposition of debris on a chamber wall of the EUV light source.13. The process of clause 11, wherein adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source includes increasing a flow rate of a gas flow in the EUV light source.14. The process of clause 11, wherein adjusting gas flows includes: increasing a forward flow from a first flow rate to a second flow rate; maintaining the forward flow at the second flow rate during at least a portion of a time in which the EUV light source is producing EUV light; and decreasing the forward flow from the second flow rate to a third flow rate, wherein the third flow rate is equal to or higher than the first flow rate.15. The process of clause 11, wherein adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source includes adjusting a gas flow toward the exhaust opening in a direction countering or partially countering gas and / or debris flows overshooting the exhaust opening.16. The process of clause 11, wherein adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source includes adjusting a counter flow.17. The process of clause 11, wherein adjusting gas flows in the EUV light source to reduce or prevent debris overshooting an exhaust opening in the EUV light source includes adjusting a flow from an obscuration bar.18. An apparatus including: a target delivery system configured to deliver targets into a light source vessel; a source laser configured to deliver light pulses into the light source vessel, the targets including a plasma-producing material that can produce a plasma when hit by the light pulses; and a controller in communication with the source laser, the controller being configured to cause the source laser (1) not to hit targets during an off-droplet period, then (2) to ramp up a hit rate of targets during a transition period.19. The apparatus of clause 18, further including a gas supply configured to supply gas into the source vessel, wherein the controller is in communication with the gas supply and is configured to increase a gas flow rate from the gas supply into the source vessel during at least a portion of the transition period.20. The apparatus of clause 18, the controller being further configured to ramp down a hit rate of targets at the end of the on-droplet period.

[0059] The above-described implementations and other implementations are within the scope of the following claims.

Claims

CLAIMS1. A process for generating EUV radiation in an EUV light source, the process comprising: during a first period, mis-timing of light pulses in the EUV light source relative to a target material, resulting in a target material hit rate of zero; and during a transition period following the first period, ramping up the target hit rate in the EUV light source.

2. The process of claim 1, further comprising: during the transition period, increasing a flow rate of a gas flow in the EUV light source from a first lower flow rate to a second higher flow rate.

3. The process of claim 1, further comprising: beginning during the first period before the transition period, increasing a flow rate of a gas flow in the EUV light source from a first lower flow rate to a second higher flow rate.

4. The process of claim 2, wherein increasing the flow rate of the gas flow comprises switching the flow rate stepwise.

5. The process of claim 2, wherein increasing the flow rate of the gas during the transition period comprises ramping up the flow rate.

6. The process of claim 2, further comprising decreasing the flow rate of the gas during a decreasing period, the decreasing period following, including, or immediately preceding a subsequent first period.

7. The process of claim 6, wherein decreasing the flow rate of the gas during the decreasing period comprises switching the flow rate stepwise.

8. The process of claim 6, wherein decreasing the flow rate of the gas during the decreasing period comprises ramping down the flow rate.

9. The process of claim 1, wherein ramping up the target material hit rate in the EUV light source comprises linearly increasing the target material hit rate.

10. The process of claim 1, wherein ramping up the target material hit rate is performed within a time from 5 to less than 10 milliseconds (ms) or to less than 50 ms.

11. A process of reducing debris deposition in an EUV light source, the process comprising: during a ramping -up period, ramping up a hit rate of target material in the EUV light source; and adjusting gas flows in the EUV light source during at least a portion of the ramping-up period.

12. The process of claim 11, wherein ramping up the hit rate of the target material during the ramping-up period is configured to prevent or reduce the debris deposition on an inner wall of the EUV light source.

13. The process of claim 11, wherein adjusting gas flows in the EUV light source comprises increasing a flow rate of a gas flow in the EUV light source.

14. The process of claim 11, wherein adjusting gas flows comprises: increasing a forward flow from a first flow rate to a second flow rate; maintaining the forward flow at the second flow rate during at least a portion of a time in which the EUV light source is producing EUV light; and decreasing the forward flow from the second flow rate to a third flow rate, wherein the third flow rate is equal to or higher than the first flow rate.

15. The process of claim 11, wherein adjusting gas flows in the EUV light source comprises adjusting a gas flow toward an exhaust opening in a direction countering or partially countering gas and / or debris flows overshooting the exhaust opening.

16. The process of claim 11, wherein adjusting gas flows in the EUV light source comprises adjusting a counter flow.

17. The process of claim 11, wherein adjusting gas flows in the EUV light source comprises adjusting a flow from an obscuration bar.

18. An apparatus comprising: a target delivery system configured to deliver targets into a light source vessel; a source laser configured to deliver light pulses into the light source vessel, the targets comprising a plasma-producing material that can produce a plasma when hit by the light pulses; and a controller in communication with the source laser, the controller being configured to cause the source laser (1) not to hit targets during a first period, then (2) to ramp up a hit rate of targets during a transition period.

19. The apparatus of claim 18, further comprising a gas supply configured to supply gas into the light source vessel, wherein the controller is in communication with the gas supply and is configured to increase a gas flow rate from the gas supply into the light source vessel during at least a portion of the transition period.

20. The apparatus of claim 18, the controller being further configured to ramp down the hit rate of targets at the end of the second period.

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