JP2018185523A - Method of timing laser beam pulses to regulate extreme ultraviolet light dosing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method to control energy dose output from a laser-produced plasma extreme ultraviolet light system by adjusting timing of fired laser beam pulses.SOLUTION: During stroboscope firing, a laser controller 502 adjusts pulse timing so as to lase droplets until a dose target of EUV has been achieved. Once accumulated EUV reaches the dose target, pulses are timed so as not to lase droplets during the remainder of the packet, and thereby prevent additional EUV light generation during those portions of the packet. In a continuous burst mode, pulses are timed to irradiate droplets until accumulated burst error meets or exceeds a threshold burst error. If accumulated burst error meets or exceeds the threshold burst error, a next pulse is timed not to irradiate a next droplet. Thus, the manipulate pulse timing to obtain a constant desired dose target that can more precisely match downstream dosing requirements.SELECTED DRAWING: Figure 5

Description

[1] The present invention relates generally to laser technology for photolithography, and more specifically to EUV dose control during laser emission.

[2] The semiconductor industry continues to develop lithography techniques that allow printing of smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (sometimes referred to as soft x-ray) is generally defined as electromagnetic radiation having a wavelength between 10 nm and 110 nm. EUV lithography is generally considered to include EUV light with a wavelength in the range of 10-14 nm and is used to produce very small features (eg, features of 32 nm or less) on a substrate such as a silicon wafer. These systems must be extremely reliable and must provide cost-effective throughput and reasonable process tolerances.

[3] Methods for generating EUV light are not necessarily limited, but include one or more elements having one or more emission lines in the EUV range (eg, xenon, lithium, tin, indium, antimony, tellurium). , Aluminum, etc.) to a plasma state. In one such method, often referred to as laser-produced plasma ("LPP"), the required plasma is a droplet, stream, or cluster of material having an element that emits the desired emission line, etc. The target material can be generated by irradiating the target material with a laser beam at the irradiation site.

[4] The element emitting the emission line can be in pure form or in the form of an alloy (eg an alloy that is liquid at the desired temperature), or can be mixed or dispersed with another material such as a liquid. it can. Simultaneous delivery of this target material and laser beam to the desired irradiation location (eg, primary focus) within the LPP EUV radiation source plasma chamber for plasma initiation has several challenges in terms of timing and control. . Specifically, in order to obtain a good plasma by appropriately colliding the laser beam with the target, and thus obtaining a good EUV light, the laser beam is focused at the position where the target material passes and the target material It is necessary to adjust the timing so that it crosses this when it passes this position.

[5] The droplet generator contains the target material and extrudes the target material as a droplet traveling along the primary focus x-axis to intersect the laser beam traveling along the primary focus z-axis. . Ideally, the droplet is targeted to pass through the primary focus. Theoretically, the EUV light output is maximized when the laser beam collides with a droplet at the primary focus. In practice, however, it is extremely difficult to achieve maximum EUV output light through multiple bursts over time. This is because the energy generated by the irradiation of one droplet is randomly different from the energy generated by the irradiation of another droplet.

[6] Thus, maximum EUV light output is sometimes possible but not always achieved. This output variability is problematic for downstream utilization of EUV light. For example, when variable EUV light is used downstream in a lithographic scanner, wafer processing becomes non-uniform, which can compromise quality control of dies cut from the wafer. For this reason, a tradeoff of using non-maximum EUV in exchange for improved reliability may be desirable.

[7] In the stroboscopic pattern, EUV is generated with a short exposure throughout the exposure of the wafer die. Although this burst pattern may be useful for controlling EUV energy dose, what is needed is a reliable generation of an acceptable level of EUV energy output for downstream purposes, ie more accurate EUV energy dose. It is a method to control.

 [8] In one embodiment, a method is provided for adjusting an energy dose generated during strobe firing of an EUV light source configured to generate an energy dose target in one or more packets. This method consists of (a) setting the dose servo value for the current packet by the laser controller, and (b) pulsing the laser beam to irradiate a droplet during the current packet by the laser controller. Adjusting the timing of triggering, (c) detecting the EUV energy generated by the irradiation of the droplet by the sensor, and (d) detecting the EUV energy detected by the laser controller between the current packet. Storing together with EUV energy generated by the irradiation of one or more preceding droplets, and (e) adjusting the accumulated EUV energy in the current packet based on the energy dose target and the accumulated dose error. Steps (b), (c), and (d) are repeated if the dose target is less than Comprising a return, and a shifting the timing of the trigger for the pulse generator of the laser beam so as not to irradiate a separate droplet during (f) by a laser controller, the current packet.

[9] In another embodiment, the method further comprises: (g) calculating a dose error for the current packet by the laser controller; and (h) calculating a dose error for the current packet by the laser controller. Storing together with the dose error for one or more previous packets, and (i) the laser controller creates a new adjusted dose target for the next packet based on the energy dose target and the accumulated dose error. And (j) calculating a new dose servo value for the next packet by the laser controller.

[10] Yet another embodiment is a system for adjusting the energy dose generated during strobe burst firing of an EUV light source configured to generate an energy dose target in one or more packets. . The system includes a drive laser configured to pulse a laser beam when a trigger is received, a sensor configured to detect EUV energy generated by irradiation of a droplet, and a controller, (A) set the dose servo value for the current packet; (b) adjust the trigger timing to pulse the laser beam to irradiate the droplet during the current packet; Accumulates the detected EUV energy generated by the irradiation together with the EUV energy generated by the irradiation of one or more preceding droplets during the current packet; (d) the accumulated EUV in the current packet If the energy is less than the adjusted dose target based on the energy dose target and the accumulated dose error, step (b) and Repeat (c), and a controller configured to shift the timing of the trigger for the pulse generator of the laser beam so as not to irradiate a separate droplet during (e) of the current packet.

[11] In yet another embodiment, the system further comprises: (f) calculating a dose error for the current packet; and (g) determining a dose error for the current packet by one or more preceding. Accumulate along with the dose error for the packet, (h) calculate a new adjusted dose target for the next packet based on the energy dose target and the accumulated dose error, and (i) new for the next packet It is configured to calculate a dose servo value.

[12] A method for adjusting the energy dose generated during a continuous burst mode of an EUV light source, comprising: (a) initiating a burst with a predetermined energy dose target; Adjusting the trigger timing for pulsing the laser beam to irradiate a droplet during (c) detecting the EUV energy generated by the droplet, and (d) detecting by the laser controller Calculating a current dose error for the droplet based on the EUV energy and energy dose target; and (e) calculated for the current dose error and one or more preceding droplets during the burst by the laser controller. Storing a burst error based on a burst error up to the present time; (f) If the burst is not completed and the accumulated burst error is less than the threshold burst error, the steps (b) to (e) are repeated, and (g) the burst is not completed and accumulated. If the burst error is greater than or equal to the threshold burst error, the laser controller shifts the trigger timing for generating a pulse of the laser beam so as not to irradiate the next droplet, and (h) Step (c) until the burst ends. Repeating step (g).

[13] A system for adjusting an energy dose generated during continuous burst firing of an EUV light source configured to generate an energy dose target, wherein the laser beam is pulsed when a trigger is received A drive laser configured as described above, a sensor configured to detect EUV energy generated by the irradiation of a droplet, and a controller, (a) a laser beam so as to irradiate the droplet during a burst (B) calculate the current dose error for the droplet based on the detected EUV energy and energy dose target, and (c) 1 between the current dose error and burst. Accumulate a burst error based on the burst error calculated to date for two or more preceding droplets, ( ) If the burst has not ended and the accumulated burst error is less than the threshold burst error, steps (a) to (c) are repeated, and (e) the burst has not ended and the accumulated burst If the error is greater than or equal to the threshold burst error, the trigger timing for pulse generation of the laser beam is shifted so as not to irradiate the next droplet, and (f) steps (b) to (e) are repeated until the burst ends. And a controller configured as described above.

[14] Fig. 14 illustrates some of the components of a typical LPP EUV system. FIG. 15 is a diagram showing generation of a laser pulse for irradiating a droplet. FIG. 16 is a diagram showing laser pulse generation with mistimed timing to avoid droplet irradiation. [17] A graph showing, over time, energy generated during a laser pulse generation period for irradiating a droplet and during a laser pulse generation period shifted in timing to avoid irradiation of the droplet, according to one embodiment. It is. [18] FIG. 18 is a block diagram illustrating EUV system components involved in EUV light dose control, according to one embodiment. [19] FIG. 19 is a flowchart of a method for controlling strobe EUV dose by laser beam pulse timing adjustment, according to one embodiment. [20] FIG. 20 is a data graph showing the percentage change around an energy dose target achieved in a 2 second burst using laser beam pulse timing adjustment to control EUV dose, according to one embodiment. [21] Fig. 21 shows packet EUV energy (upper frame) and pulse count (lower frame) generated in a 2 second burst using laser beam pulse timing adjustment to control EUV dose according to one embodiment. [22] FIG. 22 is a flowchart of a method for timing a laser beam pulse to control EUV dose during continuous burst emission, according to one embodiment. [23] FIG. 14 illustrates EUV energy (upper frame) and energy dose (lower frame) generated during successive bursts using laser beam pulse timing adjustment to control EUV dose, according to one embodiment.

[24] As described above, the energy (light) output by the EUV system can be used downstream for a number of applications, such as semiconductor lithography. In one typical situation, EUV light can be passed in a strobe burst to a lithography scanner to irradiate a photoresist on a continuous wafer. In laser systems where there is no master oscillator (ie, a “NOMO” system), the energy of such strobe bursts is RF pumped to switch the laser between “on” and “off” states. This is achieved by controlling the power. Thus, the amount of energy delivered to the downstream injection is controlled by this RF power pumping.

[25] MOPA laser systems (ie systems with a master oscillator and power amplifier, with a pre-pulse configuration, including the “MOPA + PP system”) generate higher power output from a pulsed laser source compared to NOMO systems Can be preferred for some downstream applications. However, downstream injection in a MOPA system is less controllable than a NOMO system. This is due to laser startup dynamics (eg, temperature dependent oscillations) of the MOPA system and / or thermal instability of the driving laser components (eg, mirrors and / or lenses) during laser pulse generation. In short, it has been observed that in a MOPA + PP system, an adequate and stable level of EUV cannot be generated for a certain period of time immediately after switching on the RF signal to the power amplifier. Thus, cycling the MOPA + PP laser system between an “on” state and an “off” state is not particularly practical or efficient as a method for controlling EUV injection for downstream applications.

[26] As described herein with reference to various embodiments, the laser activation in question is the continuous pulsing of the laser, ie, keeping the laser system “on” (ie, the RF signal). This can be avoided by keeping the gate in a continuous “on” state. Rather than switching the laser between the “on” and “off” states, the energy output level is controlled by a procedure that adjusts the timing of the laser beam pulses, so that some but not all of the pulses are at the primary focus. It makes it possible to irradiate droplets. By adjusting how many droplets are irradiated by the laser beam pulse, the output energy dose can be maintained at the desired (and stable) dose target level.

[27] More specifically, the drive laser (eg MOPA) is switched “on” to fire a long pulse burst (eg 2 seconds), then briefly switched “off” and then switched “on”. For example, fire a long pulse burst. Within a long burst, the drive laser is timed to fire with a strobe, i.e. continuously fire short minists (or "packets") each having a predetermined number of fast pulses. During each packet, the pulse is timed to irradiate the droplet with a laser beam at the primary focus, thereby generating EUV energy, which continues until an EUV dose target is achieved. Once the EUV energy generated in the packet reaches the dose target, the timing of the pulse firing is adjusted so that the laser beam is not applied to the droplet for the remainder of the packet, thereby adding to this part of the packet Prevents EUV light generation. For each packet (ie, between packets), the injection error calculated from the previous packet (ie, how much the achieved dose differs from the dose target) is used to fine tune the dose target for the next packet.

[28] Alternatively, the drive laser (eg, MOPA) can be timed to fire continuously throughout a long pulse burst (ie, fire in continuous burst mode). During each burst, the pulse is timed to irradiate the droplet with a laser beam at the primary focus, thereby generating EUV energy, which is the dose error accumulated in the burst (ie, of the obtained EUV energy). Continue as long as the deviation from the desired energy dose target is below an acceptable error level. Once the dose error accumulated in a burst (accumulated burst error) is above an acceptable error level, the next pulse is timed so that the laser beam is not applied to the droplet, thereby accumulating burst Reduce errors to an acceptable level. If the burst dose error is at an acceptable level, the next pulse is timed again to irradiate the droplet with the primary focus, thereby generating EUV energy.

[29] Accordingly, the method described herein adjusts the timing of the pulses to achieve the desired dose target. For example, if the pulses are fired at a rate of 50,000 pulses / second and all pulses are fired on a droplet, an average packet power of 35 watts is achieved. However, if the dose target is only 30 watts, the method described herein provides a way to limit the achieved dose to 30 watts even if the pulse rate is 60,000 pulses / second.

FIG. 1 shows some of the components of a typical LPP EUV system 100. A driving laser 101, such as a CO ₂ laser, generates a laser beam 102 that passes through a beam delivery system 103 and a focusing optical component 104. The focusing optical component 104 has a primary focal point 105 at an irradiation location in the LPP EUV radiation source plasma chamber 110. The droplet generator 106 generates and emits a droplet 107 of the appropriate target material, and when the droplet 107 impinges on the laser beam 102 at the irradiated location, it generates a plasma that emits EUV light. The EUV light is collected by an elliptical collector 108 that focuses the EUV light from the plasma at an intermediate focus 109 in order to deliver the generated EUV light to, for example, a lithography system. The intermediate focus 109 is typically in a scanner (not shown) that includes a boat of wafers exposed to EUV light, where a portion of the boat that includes the wafer is illuminated by light passing through the intermediate focus 109. Yes. In some embodiments, there are multiple drive lasers 101 and all of their beams can be focused on the focusing optics 104. In one type of LPP EUV light source, it is possible to use a CO ₂ laser and zinc selenide (ZnSe) lens having an aperture 8 inches to about 6 inches antireflective coating is applied.

[31] The energy output from the LPP EUV system determines how well the laser beam 102 can be focused and can maintain focus on the droplet 107 generated by the droplet generator 106 over time. Based on the various variations. If the droplet is positioned at the primary focus 105 when it collides with the laser beam 102, the EUV system 100 outputs optimal energy. With such droplet positioning, the elliptical collector 108 can collect the maximum amount of EUV light from the generated plasma and deliver it to, for example, a lithography system. A sensor (not shown, eg, a narrow field of view (NF) camera) detects the droplet from the droplet generator 106 through the laser curtain and travels to the primary focus 105 and provides feedback for each droplet as an EUV system. 100. This droplet-by-droplet feedback is used to adjust the droplet generator 106 to realign the droplet 107 to the primary focus 105 (ie, “on-target”).

[32] When firing the drive laser 101 in strobe or continuous burst mode, the EUV system 100 keeps the droplet 107 reasonably on-target using closed loop (per droplet) feedback according to techniques known in the art. To do. However, regardless of how well the droplets are kept on-target, the total energy generated during a packet varies due to random variations in the amount of energy generated by each droplet that is irradiated. It can change. These random variations make it difficult to maintain a constant dose target output. However, maintaining a constant level of output energy is important for downstream purposes. If a constant level of output energy cannot be maintained, the use of output energy downstream, for example in a lithographic scanner, adversely affects the patterning of the silicon wafer.

[33] Adjusting the timing between the arrival of the droplet at the primary focus and the arrival of the laser at the primary focus can ensure that the energy generated during burst firing is maintained at a constant level. This will now be described with reference to FIGS. 2, 3 and 4. FIG. FIGS. 2 and 3 respectively emit pulses that cause the laser to irradiate a droplet (ie, an “on-droplet” pulse) and to avoid irradiating the droplet (“small” The orientation of the droplet 107 during burst firing is shown schematically when timed to be "off-droplet" pulses). FIG. 4 is a graph showing energy generated over time during a laser pulse generation period for irradiating a small droplet and during a laser pulse generation period at which the timing for avoiding the irradiation of the droplet is shifted.

[34] Referring initially to FIG. 2, when the laser is timed to emit a pulse on a droplet (“pulse on droplet”), the pulse of the laser beam 102 is applied to the droplet 107 at the primary focus 105. As a result of the collision, the target material of the droplet 107 is vaporized and a plasma 202 is generated at the primary focus 105. EUV energy emanating from the plasma 202 is collected by the elliptical collector 108 and reflected to the intermediate focus 109 and passed, for example, into the lithography system or used by the lithography system. As shown in FIG. 4, the EUV energy generated during on-droplet pulse generation 401 is generally centered around an average energy value (here about 0.45 mJ), but the energy generated for each droplet is The variation is extremely large due to random fluctuations. Due to this variability, the energy dose obtained from any given packet can leave the desired constant EUV dose target and thus adversely affect downstream operation.

[35] Referring now to FIG. 3, when the timing of the laser pulse is shifted to produce a drop-off pulse ("drop-off pulse generation"), the pulse of the laser beam 102 is a drop and a drop. , The target material of the droplet is not vaporized, and no plasma is generated at the primary focus 105. In the MOPA + PP system, the timing of triggering the pulse generation can be advanced or delayed so that the laser beam 102 passes through the primary focus 105 without impinging on the droplet 107. Thus, as shown in FIG. 4, in the case of the droplet drop pulse generation 402, little or no EUV energy is generated.

[36] Embodiments of the methods described herein for strobe firing determine for each pulse in a packet whether the desired energy dose target of the current packet has been achieved. Thus, after irradiating the droplets in the packet with laser light, the total energy dose for that packet is calculated and compared to the desired energy dose target. If the desired energy dose target is not achieved, the timing for triggering the drive laser for the next pulse is adjusted so that the laser light strikes the next droplet. When the desired energy dose target is achieved, add in the current packet by shifting the timing of triggering the drive laser for the next pulse and deviating the laser light to the next droplet The energy is not generated. Between packets (ie, for each packet), accumulate the calculated dose error from the current packet along with the dose error from the previous packet and “servo” to fine tune the dose target for the next packet servo) ”.

[37] The block diagram of FIG. 5 illustrates components of an EUV system involved in dose control of generated EUV light, according to one embodiment. The laser controller 502 adjusts the trigger for driving the laser 101 to generate a pulse on a small droplet, and generates plasma that emits EUV energy when the droplet is irradiated. The amount of EUV energy collected is sensed by the energy output sensor 501 on a pulse-by-pulse basis and passed to the laser controller 502, which accumulates the total EUV energy generated up to the present time during the current packet. To do. The sensor 501 is a sensor in an LPP EUV radiation source plasma chamber 110 such as an EUV side sensor positioned at 90 degrees with respect to the laser beam 102 or in a scanner that measures energy that has passed through the intermediate focus 109. Sensor. When the accumulated EUV is equal to the dose target or minimally exceeds the dose target, the laser controller 502 shifts the trigger timing for driving the laser 101 so that the drive laser 101 emits a drop-off pulse. Occurs to avoid generating additional EUV energy. The drive laser 101 continues to generate out-of-drop pulses for the remainder of the current packet. When the current packet is complete, the laser controller 502 calculates the dose error for the current packet and accumulates the dose error together with the dose error from the previous packet. Controller 502 then adjusts the dose target based on the accumulated dose error. In the next packet, the accumulated achieved EUV energy is compared against this target.

[38] Embodiments of the laser beam pulse timing adjustment method disclosed herein for strobe pulse generation adjust the average EUV by firing a portion of the pulses in the packet out of droplets. For example, if the pulse energy is increased, the number of pulses fired on the droplet (pulse count) is reduced to maintain the same average EUV. As random fluctuations in the generated EUV energy are better understood over time, the packet size can be adjusted to minimize the time it takes to generate laser light with a drop off.

[39] Referring now to FIG. 6, a flowchart of a method for timing a laser beam pulse to control a strobe EUV dose according to one embodiment is shown. Before starting the following steps, the dose target of EUV energy achieved in each packet of the burst (ie, the set point that adjusts the packet energy) and the packet size (ie, the total number of pulses in each packet) are determined by the user. Enter or determine by system.

[40] The packet size is preferably selected to be the smallest packet size that allows control of the EUV energy dose. If the packet size is too small (eg 1 or 2 droplets), the timing of pulse generation may not be able to be shifted for a sufficient number of droplets to properly control the EUV energy dose. If the packet size is too large (eg 1000 droplets), uncontrollable errors accumulate throughout the packet (eg as shown in FIG. 4), resulting in the amount of EUV generated for downstream injection. It cannot be controlled well. For this reason, the packet size is ideally selected such that the pulse timing adjustment can only be performed on the droplets at the rear of the packet. For example, if an appropriate dose can be achieved with an average of 40 droplets, a packet size of 50 droplets may be appropriate (in this case, pulse timing misalignment may be performed with the last 10 droplets). Just fine).

[41] In step 601, the laser controller 502 sets the dose servo value of the current packet. The dose servo value is an adjustment rate that increases or decreases the dose target as a function of the dose energy generated by the previous packet. That is, the desired dose target is fine-tuned by a dose servo value determined by errors from previous packets (as discussed elsewhere herein). In one embodiment, the dose servo value of the first packet is set to zero.

[42] Once the dose servo value is set, laser pulse firing of the packet can be started. Steps 602 to 607 are performed for each pulse, ie for each pulse of the packet.

[43] In step 602, the laser controller 502 times the trigger for the drive laser 101 to generate an on-droplet pulse so that the laser beam 102 irradiates the drop 107 at the primary focus 105.

[44] In step 603, the sensor 501 detects how much EUV energy has been generated by the irradiation of the droplet 107 in step 602.

[45] In step 604, the laser controller 502 adds the EUV energy detected in step 603 to the total of EUV energy generated since the first pulse of the packet (ie, after step 601) up to the present time. Accumulate.

[46] In step 605, the laser controller 502 determines whether the accumulated EUV energy from step 604 is equal to or slightly greater than the adjusted dose target. The adjusted dose target is the sum of the dose target and the dose servo value of step 601. The stored EUV energy may be slightly larger than the adjusted dose target for various reasons. Various reasons include, for example, the random variation of EUV generated by each irradiated droplet and / or the energy generated by each irradiated droplet (even if there is no random variation) It is not a value. If the accumulated EUV energy is less than the adjusted dose target of step 601, the laser controller 502 returns to step 602 to trigger another on-droplet pulse and repeat steps 603, 604, and 605.

[47] If the accumulated EUV energy is greater than or equal to the adjusted dose target, in step 606, the laser controller 502 shifts the trigger timing to cause the drive laser 101 to generate a drop-off pulse so that the laser beam 102 is primary. Do not irradiate the droplet 107 at the focal point 105. The staggered trigger is relative to the timing of the next trigger that generates the on-drop pulse, i.e., the next trigger that generates the on-drop pulse when the accumulated EUV energy in step 604 is less than the adjusted dose target. The timing can be delayed or advanced in time.

[48] In step 607, the laser controller 502 determines whether or not the packet is completed, that is, whether or not the number of pulses emitted by the drive laser 101 is equal to the packet size. If the laser controller 502 determines that the packet is not complete, the laser controller 502 returns to step 606 to trigger another pulse on a drop drop.

[49] If the laser controller 502 determines that the packet is complete, it executes steps 608 to 611 and another step 601 after which the next packet begins.

[50] In step 608, the laser controller 502 calculates the dose error of the packet. The dose error is defined as the dose target minus the EUV energy accumulated in the packet and is mathematically expressed as:
Dose error _packet = Dose target-Σ EUV _packet

[51] In step 609, the laser controller 502 accumulates the dose error from the packet together with the dose error from the previous packet.

[52] In step 610, the laser controller 502 calculates a new dose servo value using the accumulated dose error calculated in step 609. In one embodiment, the new dose servo value is calculated as follows:
Previous servo value + (gain x accumulated dose error)
Here, the previous servo value is the dose servo value set in step 601. The gain is preferably 1.0. The gain can range between 0.01 and 100.

[53] In step 611, the laser controller 502 resets the accumulated EUV to zero in preparation for the next packet, and returns to step 601 to set a new dose servo value as the dose servo value for the next packet.

[54] What is important is that the packet repeats at a certain frequency. That is, regardless of how many pulses in a packet hit the droplet at the primary focus 105, after firing the number of pulses in a packet, the packet starts at a set time. However, the number of pulses that impact a droplet within a packet varies based on the amount of energy generated by the previous droplet irradiation, so the last pulse that impacts a droplet within a packet is a different packet. There can be various variations between them.

[55] In addition, the packet has a set number of pulses and is not shown in the figure, but when the set number of pulses is reached during the loop from step 602 to 605, a drop out pulse is generated in the laser. The packet can be terminated without the need to stagger the trigger (eg, if the packet's accumulated EUV energy is less than the packet's adjusted dose target). Specifically, if laser controller 502 determines that the packet is complete after accumulating the EUV energy of the packet in step 604 (ie, if the number of pulses emitted by drive laser 101 is equal to the packet size), the laser The controller 502 does not return to step 602 to time another trigger to cause the drive laser 101 to generate an on-droplet pulse, but the next packet begins after executing steps 608-611. Accordingly, the laser controller 502 calculates the packet dose error (step 608), accumulates the dose error from the packet with the dose error from the previous packet (step 609), and stores the accumulated dose error calculated in step 609. To calculate a new dose servo value (step 610), reset the accumulated EUV to zero in preparation for the next packet, and then return to step 601 to return the new dose servo value to the dose servo value for the next packet. (Step 611).

[56] FIGS. 7 and 8 are time alignment graphs showing data generated in 2-second bursts using one embodiment of a laser beam pulse timing adjustment method for controlling EUV dose. FIG. 7 shows the percentage change around the energy dose target achieved in a 2 second burst. As indicated by the percentage dose energy change in the graph centered on the dose target seen in the figure, the packet injection controlled by pulse timing adjustment is within ± 0.5% of the dose target (ie, zero ± 0 in the figure). Well within 5%).

[57] The upper frame of FIG. 8 shows a packet EUV generated in a 2-second burst. As can be seen, the energy is maintained at a dose target (here about 20 mJ) over time and is stably maintained within ± 0.5% of the dose target. The lower pane of FIG. 8 shows the corresponding pulse count in a 2 second burst. Each diamond represents a count of the number of pulses on the droplet within a single packet (“pulse count”). In an exemplary packet EUV energy (upper frame) and packet pulse count (lower frame), a number of on-droplet pulse generations 801 and a number of out-of-droplet pulse generations 802 (and thus a small pulse count) are indicated by arrows. As indicated by the arrows, the number of pulses required to achieve a certain EUV energy may be reduced in response to random variations in the generated EUV energy.

[58] When applied to continuous burst firing, embodiments of the methods described herein provide a dose error (ie, the resulting EUV energy is a desired energy dose) for each droplet for each pulse in each burst. Find the amount that deviates from the target. Dose errors accumulate as the burst progresses. Thus, after applying a laser beam to a single droplet within a burst, the dose error of that droplet is calculated and accumulated along with the dose errors of previous droplets within that burst. If the accumulated dose error in a burst (ie, “accumulated burst error”) is greater than or equal to an acceptable burst error level (ie, “threshold burst error”), the trigger for the drive laser is shifted in the next pulse and the next smaller By making the laser light on the drop off a small drop, no additional energy is generated. Since no additional energy is generated, the dose error for the next droplet is large enough to return the accumulated burst error to an acceptable level (ie, below the threshold burst error). If the accumulated burst error is less than the threshold burst error, the trigger for the next pulsed drive laser is timed to irradiate the laser beam to the next droplet onto the droplet to generate additional EUV energy. .

[59] Referring now to FIG. 9, a flowchart of a method for timing laser beam pulses to control EUV dose during continuous burst emission is shown, according to one embodiment. Prior to starting the following steps, the dose target of EUV energy to achieve within each burst (ie, the set point to adjust the burst energy) and the threshold burst error (ie, acceptable burst error level) are entered by the user. Or determined by the system.

[60] Once the dose target is set, in step 901 burst laser pulse firing can begin. Steps 902 to 908 are performed for each pulse, i.e. for each pulse of the burst.

[61] In step 902, the laser controller 502 times the trigger for the drive laser 101 to generate a drop-on-pulse so that the laser beam 102 irradiates the drop 107 at the primary focus 105.

[62] In step 903, the sensor 501 detects how much EUV energy is generated by the current irradiation of the droplet 107 in step 902.

[63] In step 904, the laser controller 502 calculates the current dose error of the current droplet 107. The current dose error is defined as the EUV energy generated by irradiation of the current droplet 107 (and detected in step 903) minus the dose target and is expressed mathematically as:
Current dose error = EUV _{current droplet} -dose target

[64] In step 905, the laser controller 502 adds the current dose error calculated in step 904 to the total to date of dose errors accumulated since the first pulse of the packet (ie, after step 901). Accumulate burst errors. The current dose error is adjusted by gain, which can range from 0.01 to 100, but is preferably 1. In one embodiment, the accumulated burst error is calculated as follows:
Burst error up to the present time + (gain x current dose error)
Here, the burst error to date is the sum of dose errors accumulated from previous droplets in the burst to date. That is, the burst error up to the present time is the accumulated burst error obtained in step 905 for the previous droplet 107. The burst error so far is set to zero if the current droplet is the first droplet in the burst.

[65] In step 906, the laser controller 502 determines whether the burst has ended. If the laser controller 502 determines that the burst has ended, the laser controller 502 ends the pulse timing adjustment method and / or returns to step 901 to start another burst.

[66] If the laser controller 502 determines in step 906 that the burst has not ended, then in step 907 the laser controller 502 determines whether or not the accumulated burst error in step 905 is greater than or equal to the burst error threshold. . The burst error threshold is entered by the user or determined by the system. The burst error threshold is preferably zero, but may be greater or less than zero.

[67] If the laser controller 502 determines in step 907 that the accumulated burst error is less than the burst error threshold, the laser controller 502 returns to step 902 to trigger the drive laser 101 to generate an on-droplet pulse. The timing is adjusted so that the laser beam 102 irradiates the next droplet 107 at the primary focus 105.

[68] If the laser controller 502 determines in step 907 that the accumulated burst error is greater than or equal to the burst error threshold, then in step 908, the laser controller 502 triggers the drive laser 101 to generate an out-of-drop pulse. Are shifted so that the laser beam 102 does not irradiate the next droplet 107 at the primary focal point 105. A staggered trigger can be fired so that the laser pulse reaches the primary focus earlier or later than the arrival of the droplet.

[69] After shifting the trigger timing to cause the drive laser 101 to generate an out-of-drop pulse for the next drop 107, the laser controller 502 returns to step 903 to determine how much of the current drop 107 is illuminated. It is detected whether EUV energy has occurred, and then in step 904 the current dose error is calculated for the next droplet 107. Since EUV does not occur for the next droplet 107 because the timing of the pulse is shifted, the current dose error calculated for the next droplet 107 is equal in magnitude to the dose target but opposite in sign. For example, if the dose target is 1.75 mJ, the calculated current dose error will be −1.75 mJ, or 100%, and the error around the dose target for the irradiated droplet (typically much more than 40%). (Very small). Thus, if the laser controller 502 accumulates a burst error in step 905 by adding a relatively large current dose error for the next droplet 107 to the current burst error, the accumulated burst error is typically This is reduced compared to the accumulated burst error of the previous droplet 107. Assuming that the logical controller 502 determines in step 906 that the burst has not ended, in step 907 the logical controller 502 determines whether the accumulated burst error is greater than or equal to the burst error threshold. If the laser controller 502 determines that the accumulated burst error is less than the burst error threshold at this point, the laser controller 502 returns to step 902 to time the trigger to cause the drive laser 101 to generate a drop-on-pulse pulse. 9 so that the laser beam 102 irradiates another droplet 107 (which now becomes the current droplet 107) at the primary focus 105, and the process of FIG. 9 repeats from that step. If the laser controller 502 determines that the accumulated burst error is greater than or equal to the burst error threshold, then in step 908, the laser controller 502 shifts the trigger timing to cause the drive laser 101 to generate an out-of-droplet pulse. The laser beam 102 is prevented from irradiating the next droplet 107 at the primary focal point 105, and then the process returns to step 903 again to detect how much EUV energy has been generated. The process of FIG. 9 is then repeated from that point.

[70] In another embodiment, the current dose error in step 904 is defined as the dose target minus the EUV energy generated by irradiation of the current droplet 107 (and detected in step 903). And expressed mathematically as:
Current dose error = Dose target-EUV _{current droplet}

[71] In this embodiment, during the calculation of the accumulated burst error in step 905, the current dose error is adjusted using a negative gain (rather than the positive gain of the above embodiment). The gain can range between -0.01 and -100, but is preferably -1.

[72] Whether this method aspect controls the energy generation by comparing the burst error for each pulse with an acceptable burst error threshold, accumulating the burst error throughout the burst, and shifting the timing of the next pulse. It will be appreciated by those skilled in the art that other embodiments are possible (although not preferred) that are less intuitively satisfying as long as they are internally consistent to achieve the purpose of making such a determination. Let's be recognized. Specifically, the mathematical processing of the current dose error calculation (step 904) and the gain applied to the current dose error in calculating the accumulated burst error (step 905) must be consistent with each other. Rather, it must be consistent with the decision rule result obtained from the comparison of the accumulated burst error with the threshold burst error (step 907).

[73] FIG. 10 illustrates time-aligned EUV energy (upper frame) and energy dose (lower frame) generated during successive bursts using laser beam pulse timing adjustment to control EUV dose, according to one embodiment. Indicates a sliding window. As seen in the upper frame, most pulses were fired on the droplet (eg, on-droplet pulse 1001), but a certain number of pulses around the dose target 1003 (about 1.75 mJ in this figure) Fired out-of-drop to control the error (eg as shown by a pulse generating 0 mJ EUV, such as an out-of-drop pulse 1002). As a result, as shown in the lower frame, a constant injection 1004 was achieved at about 1.75 mJ and was well maintained within ± 0.5% of the dose target 1003 as indicated by reference numeral 1005.

[74] Ideally, if the target conditions are correct and the drive laser has adequate performance, embodiments of the laser beam pulse timing adjustment method described herein may be ± 0.5% of the dose target. It is thought that the dose energy can be maintained in the interior.

[75] Those skilled in the art will appreciate that the timing of the laser pulses can be de-phased by a wide variety of mechanisms known in the art. For example, the drive laser can be fired so that the laser pulse reaches the primary focus earlier or later than the arrival of the droplet. Alternatively, the timing of the system shutter (eg, electro-optic modulator or acousto-optic modulator) can be changed to pass a sufficiently low level of continuous wave light to reduce amplifier seed and system gain. it can. In a preferred embodiment, closing the shutter early advances the laser beam relative to the droplet.

[76] As is known in the art, MOPA + PP laser systems emit both pre-pulses and main pulses. When the laser emits a pulse on a droplet, both the main pulse and the pre-pulse are used to irradiate the droplet with the laser beam. Those skilled in the art will recognize that they are not used to irradiate laser light.

[77] The disclosed methods and apparatus have been described above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. Some aspects of the described methods and apparatus can be readily implemented using configurations other than those described in the above embodiments, or in combination with elements other than those described above.

[78] Furthermore, it will be appreciated that the described methods and apparatus can be implemented in numerous ways, such as as a process, as an apparatus, or as a system. The methods described herein can be implemented by program instructions for instructing a processor to perform such methods. Such instructions are recorded on a computer readable storage medium such as a hard disk drive, floppy disk, optical disk such as a compact disk (CD) or digital versatile disk (DVD), flash memory, or program instructions in a computer network. Are transmitted via an optical communication link or an electronic communication link. It should be noted that the order of the method steps described herein can be varied and still be within the scope of the present disclosure.

[79] It will be appreciated that the examples given here are for illustrative purposes only and can be extended to other embodiments and embodiments using different conventions and techniques. While numerous embodiments are described, there is no intent to limit the present disclosure to the embodiments (multiple embodiments) disclosed herein, and on the contrary, all alternatives, modifications, and equivalents apparent to those skilled in the art. Is intended to be included.

[80] While this specification has described the invention with reference to specific embodiments thereof, those skilled in the art will recognize that the invention is not limited thereto. The various features and aspects of the above-described invention can be used individually or in combination. Further, the present invention may be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the present specification. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive. As used herein, it will be appreciated that the terms “comprising”, “including” and “having” are specifically intended to be read as open-ended terms in the art.

Claims (16)

A method for adjusting the energy dose generated during strobe firing of an EUV light source that generates an energy dose target in one or more packets, comprising:
(A) setting a dose servo value for the current packet by the laser controller;
(B) adjusting the trigger timing to pulse the laser beam by the laser controller to irradiate a droplet during the current packet;
(C) detecting EUV energy generated by irradiation of the droplets;
(D) storing by the laser controller the detected EUV energy together with EUV energy generated by irradiation of one or more preceding droplets during the current packet;
(E) if the accumulated EUV energy in the current packet is less than the adjusted dose target based on the energy dose target and accumulated dose error, steps (b), (c), and ( repeating d);
(F) shifting the trigger timing for pulsing the laser beam so that the laser controller does not irradiate another droplet during the current packet;
A method comprising:   The method of claim 1, wherein the adjusted dose target is equal to the dose target plus the dose servo value. (G) calculating a dose error for the current packet by the laser controller;
(H) storing, by the laser controller, the dose error for the current packet along with the dose error for one or more previous packets;
(I) calculating, by the laser controller, a new adjusted dose target for the next packet based on the energy dose target and the accumulated dose error;
(J) calculating a new dose servo value for the next packet by the laser controller;
(K) repeating steps (a) to (j) for the next packet, wherein the adjusted dose target for the next packet is the new adjusted dose target, and the next packet The dose servo value for is the new dose servo value;
The method of claim 1, further comprising:   The method of claim 3, wherein the dose error for the current packet is equal to the dose target for the current packet minus the accumulated EUV energy for the current packet.   The method of claim 3, wherein the accumulated dose error includes the dose error for the current packet and the dose error for one or more previous packets. A system for adjusting an energy dose generated during a strobe burst launch of an EUV light source that generates an energy dose target within one or more packets, comprising:
A drive laser that pulses the laser beam when a trigger is received;
A sensor for detecting EUV energy generated by irradiation of a droplet;
(A) Set the dose servo value for the current packet,
(B) adjusting the timing of the trigger to pulse the laser beam to irradiate a droplet during the current packet;
(C) storing detected EUV energy generated by irradiation of the droplet together with EUV energy generated by irradiation of one or more preceding droplets during the current packet;
(D) if the accumulated EUV energy in the current packet is less than the adjusted dose target based on the energy dose target and accumulated dose error, repeat steps (b) and (c);
(E) shifting the timing of the trigger to pulse the laser beam so as not to irradiate another droplet during the current packet;
A controller,
A system comprising:   The system of claim 6, wherein the adjusted dose target is equal to the dose target plus the dose servo value. The controller further comprises:
(F) calculating a dose error for the current packet;
(G) storing the dose error for the current packet together with the dose error for one or more previous packets;
(H) calculating a new adjusted dose target for the next packet based on the energy dose target and the accumulated dose error;
(I) calculating a new dose servo value for the next packet;
(J) Repeat steps (a) to (i) for the next packet, the adjusted dose target for the next packet is the new adjusted dose target, and the dose for the next packet The system of claim 6, wherein a servo value is the new dose servo value.   9. The system of claim 8, wherein the dose error for the current packet is equal to the dose target for the current packet minus the accumulated EUV energy for the current packet.   9. The system of claim 8, wherein the accumulated dose error includes the dose error for the current packet and the dose error for one or more previous packets. A method for adjusting the energy dose generated during a continuous burst mode of an EUV light source, comprising:
(A) initiating a burst with a predetermined energy dose target;
(B) adjusting the trigger timing for pulsing the laser beam to irradiate a droplet during the burst by the laser controller;
(C) detecting EUV energy generated by the droplets;
(D) calculating a current dose error for the droplet based on the detected EUV energy and the energy dose target by the laser controller;
(E) accumulating a burst error based on the current dose error and a current burst error calculated for the one or more preceding droplets during the burst by the laser controller;
(F) If the burst has not ended and the accumulated burst error is less than a threshold burst error, repeating steps (b) to (e);
(G) If the burst has not ended and the accumulated burst error is greater than or equal to the threshold burst error, the laser controller causes the laser beam to be pulsed so as not to irradiate the next droplet. Shifting the timing of the trigger;
(H) repeating steps (c) to (g) until the burst ends;
A method comprising:   The method of claim 11, wherein the current dose error is a difference between the sensed EUV energy and the energy dose target. The accumulated burst error is
Burst error up to the present time + (Gain x Dose error)
The method of claim 12, wherein A system for adjusting an energy dose generated during a continuous burst launch of an EUV light source that generates an energy dose target comprising:
A drive laser that pulses the laser beam when a trigger is received;
A sensor for detecting EUV energy generated by irradiation of a droplet;
(A) adjusting the timing of the trigger to pulse the laser beam to irradiate droplets during the burst;
(B) calculating a current dose error for the droplet based on the detected EUV energy and the energy dose target;
(C) accumulating a burst error based on the current dose error and a current burst error calculated for one or more preceding droplets during the burst;
(D) If the burst has not ended and the accumulated burst error is less than a threshold burst error, repeat steps (a) to (c);
(E) If the burst is not completed and the accumulated burst error is greater than or equal to a threshold burst error, the trigger timing for generating the pulse of the laser beam is shifted so as not to irradiate the next droplet. ,
(F) repeating steps (b) to (e) until the burst is completed;
A controller,
A system comprising:   The system of claim 14, wherein the current dose error is a difference between the detected EUV energy and the energy dose target. The accumulated burst error is
Burst error up to the present time + (Gain x Dose error)
The system of claim 15, wherein