JP6654220B2 - Method of adjusting extreme ultraviolet light injection by adjusting timing of laser beam pulse - Google Patents

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 technologies that enable smaller integrated circuit dimensions to be printed. Extreme ultraviolet ("EUV") light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having a wavelength between 10 nm and 110 nm. EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm and is used to create very small features (e.g., features below 32 nm) 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 producing 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 a laser-produced plasma (LPP), the required plasma is a droplet, stream, or cluster of material having the desired emission line emitting element. By irradiating the target material with a laser beam at an irradiation location.

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

[5] The droplet generator contains the target material and extrudes the target material as droplets traveling along the x-axis of the primary focus and intersects the laser beam traveling along the z-axis of the primary focus. . Ideally, the droplet is targeted to pass through the primary focus. When the laser beam hits the droplet at the primary focus, the EUV light output is theoretically at its maximum. However, in practice, it is extremely difficult to achieve maximum EUV output light over multiple bursts over time. This is because the energy generated by irradiation of a certain droplet is randomly different from the energy generated by irradiation of another droplet.

[6] Thus, maximum EUV light output is sometimes possible but not always realized. This output variability is problematic for downstream use of EUV light. For example, if variable EUV light is used downstream in a lithography scanner, processing of the wafer may be non-uniform, which may result in poor quality control of the dies cut from the wafer. For this reason, a trade-off of using a non-maximum EUV may be desirable in exchange for improved reliability.

[7] In a strobe pattern, EUV is generated with a short exposure through the exposure of the wafer die. While this burst pattern may be useful for controlling EUV energy dose, what is needed is to reliably generate an acceptable level of EUV energy output for downstream purposes, ie, to more accurately control EUV energy dose. It is a method of controlling.

 [8] In one embodiment, a method is provided for adjusting an energy dose generated during a strobe firing of an EUV light source configured to generate an energy dose target in one or more packets. The method includes (a) setting a dose servo value for a current packet by a laser controller; and (b) pulsing a laser beam by the laser controller to irradiate a droplet during the current packet. (C) detecting EUV energy generated by the irradiation of the droplet by the sensor; and (d) detecting the EUV energy detected by the laser controller during the current packet. Storing together with EUV energy generated by irradiation of one or more preceding droplets; and (e) adjusting the stored EUV energy in the current packet based on the energy dose target and the stored dose error. If not, repeat steps (b), (c) and (d). 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) storing, by the laser controller, an energy dose target and a new adjusted dose target for the next packet based on the stored dose error. Calculating; 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 an energy dose generated during a 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 upon receiving a trigger, a sensor configured to detect EUV energy generated by irradiation of the droplet, and a controller, (A) setting the dose servo value for the current packet; (b) adjusting the trigger timing to pulse the laser beam to irradiate the droplet during the current packet; Accumulating 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; and (d) storing the accumulated EUV in the current packet. If the energy is less than the energy dose target and the adjusted dose target based on 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 the controller further comprising: (f) calculating a dose error for the current packet, and (g) determining a dose error for the current packet by one or more preceding packets. Storing together with the dose error for the packet, (h) calculating a new adjusted dose target for the next packet based on the energy dose target and the stored dose error, and (i) calculating a new adjusted dose target for the next packet. It is configured to calculate a dose servo value.

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

[13] A system for adjusting the energy dose generated during a continuous burst firing of an EUV light source configured to generate an energy dose target, wherein the laser beam is pulsed upon receiving a trigger. A laser configured to detect EUV energy generated by the irradiation of the droplet, and a controller, comprising: (a) a laser beam configured to irradiate the droplet during a burst. (B) calculate the current dose error for the droplet based on the detected EUV energy and the energy dose target; and (c) calculate the one between the current dose error and the burst. Accumulate a burst error based on the burst error to date calculated for one 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 equal to or greater than the threshold burst error, the timing of the trigger for generating 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.

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 generation of a laser pulse at a timing shifted to avoid irradiation of a droplet (mistimed). [17] A graph showing, over time, the energy generated during a laser pulse generation period for irradiating a droplet and a staggered laser pulse generation period for avoiding the droplet irradiation, according to one embodiment. It is. FIG. 18 is a block diagram illustrating EUV system components involved in EUV light dose control, according to one embodiment. 19 is a flowchart of a method for controlling strobe EUV dose by adjusting laser beam pulse timing, according to one embodiment. [20] Figure 20 is a data graph illustrating a 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 illustrates packet EUV energy (upper box) and pulse count (lower box) generated in a 2 second burst using laser beam pulse timing adjustment to control EUV dose, according to one embodiment. [22] Figure 22 is a flowchart of a method for adjusting EUV dose during continuous burst firing by timing laser beam pulses, according to one embodiment. [23] FIG. 23 illustrates EUV energy (upper box) and energy dose (lower box) generated during a continuous burst using laser beam pulse timing adjustment to control EUV dose, according to one embodiment.

[24] As mentioned above, the energy (light) output by the EUV system can be used downstream for a number of applications, for example, semiconductor lithography. In one typical situation, EUV light can be passed in a strobe burst to a lithography scanner to illuminate photoresist on a continuous wafer. In laser systems where there is no master oscillator (i.e., a "no master oscillator" system), the energy of such a strobe burst is such that the RF pump will switch the laser between "on" and "off" states. Achieved by controlling 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, having a pre-pulse configuration, including the "MOPA + PP system"), generate higher power output from a pulsed laser source than a NOMO system. It is preferred for some downstream applications. However, downstream injection in MOPA systems is less straightforward to control than NOMO systems. This is due to the dynamics of the MOPA system during laser start-up (eg, temperature dependent oscillation) and / or thermal instability of the driving laser components (eg, mirrors and / or lenses) during laser pulse generation. Briefly, it has been observed that in a MOPA + PP system, an appropriate stable level of EUV cannot be produced for a period of time immediately after switching on the RF signal to the power amplifier. For this reason, cycling the MOPA + PP laser system between the "on" and "off" states is not particularly practical or efficient as a way to control EUV injection for downstream applications.

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

[27] More specifically, the drive laser (eg, MOPA) is switched on to emit a long pulse burst (eg, 2 seconds), then briefly switched off, and then switched on. Fire a long pulse burst, and so on. Within long bursts, the drive laser is timed to fire strobed, that is, to continuously fire short mini-bursts (or "packets"), each having a predetermined number of fast pulses. During each packet, the pulses are timed to laser light the droplet at the primary focus, thereby generating EUV energy, and so on until the EUV dose target is achieved. Once the EUV energy generated in the packet reaches the dose target, the timing of the firing of the pulse is adjusted so that the droplet is not lasered during the rest of the packet, thereby adding additional energy in this part of the packet. To prevent EUV light generation. For each packet (ie, between packets), use the injection error calculated from the previous packet (ie, how different the achieved dose is from the dose target) to fine-tune the dose target for the next packet.

[28] Alternatively, the drive laser (eg, MOPA) can be timed to fire continuously during a long pulse burst (ie, fire in continuous burst mode). During each burst, the pulses are timed to irradiate the droplet with laser light at the primary focus, thereby generating EUV energy, which translates into the accumulated dose error within the burst (i.e., the resulting EUV energy). Continue as long as the deviation from the desired energy dose target is below an acceptable error level. Once the accumulated dose error in the burst (accumulated burst error) is above an acceptable error level, the next pulse adjusts the firing timing so that the droplet does not shine laser light, thereby causing the accumulated burst Reduce errors to an acceptable level. If the dose error of the burst is at an acceptable level, the next pulse is timed to re-laser the droplet at the primary focus, thereby generating EUV energy.

[29] Accordingly, the methods described herein adjust 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 the pulses are fired on the 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 at a pulse rate of 60,000 pulses / sec.

FIG. 1 shows some of the components of a typical LPP EUV system 100. A drive laser 101, such as a CO ₂ laser, generates a laser beam 102, which passes through a beam delivery system 103 and focusing optics 104. The focusing optics 104 has a primary focus 105 at the irradiation location in the LPP EUV radiation source plasma chamber 110. The droplet generator 106 generates and emits droplets 107 of a suitable target material, and the droplets 107 generate a plasma that emits EUV light when they collide with the laser beam 102 at the point of irradiation. The EUV light is collected by an elliptical collector 108, which focuses the EUV light from the plasma to an intermediate focus 109 to deliver the generated EUV light to, for example, a lithography system. Intermediate focus 109 is typically within a scanner (not shown) that includes a boat of wafers to be exposed to EUV light, wherein a portion of the boat that includes the wafer is illuminated by light passing through intermediate focus 109. I have. In some embodiments, there are multiple drive lasers 101, all of whose beams may be focused on 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 depends on how well the laser beam 102 can be focused and maintain focus over time on the droplet 107 generated by the droplet generator 106. It fluctuates in various ways. If the droplet is positioned at the primary focal point 105 when colliding with the laser beam 102, the EUV system 100 will output optimal energy. Such droplet positioning allows the elliptical collector 108 to collect the maximum amount of EUV light from the generated plasma and deliver it to, for example, a lithography system. A sensor (not shown, for example, a narrow-field (NF) camera) detects the droplet traveling from the droplet generator 106 through the laser curtain to the primary focus 105 and provides feedback for each droplet to the EUV system. Supply 100. With this drop-by-drop feedback, the drop generator 106 is adjusted to reposition the drop 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 maintains the droplet 107 moderately on-target using closed-loop (per droplet) feedback according to techniques known in the art. I do. However, regardless of how well the droplets are kept on-target, the total energy generated in one packet will vary due to random variations in the amount of energy generated by each irradiated droplet. 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 lithography scanner, will adversely affect 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 ensures 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. 2 and 3 show that the laser emits a pulse to irradiate the droplet (i.e., a pulse "on-droplet") and emits a pulse that avoids irradiating the droplet ("Small"). 5 schematically illustrates the orientation of a droplet 107 during burst firing, when timed ("off-droplet" pulse). FIG. 4 is a graph showing, over time, the energy generated during the laser pulse generation period during which the droplet is irradiated and during the laser pulse generation period in which the timing for avoiding the droplet irradiation is shifted.

[34] Referring first to FIG. 2, if the laser is timed to emit an on-droplet pulse (“pulse-on-droplet generation”), the pulse of the laser beam 102 is applied to the drop 107 at the primary focus 105. Upon 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, for example, to be passed into or used by the lithography system. As shown in FIG. 4, the EUV energy generated during the on-droplet pulse generation 401 is generally concentrated around the average energy value (here, about 0.45 mJ). Extremely large variation due to random variation. Due to this variability, the energy dose obtained from any given packet may be away from the desired constant EUV dose target, thus adversely affecting downstream operation.

Referring now to FIG. 3, if the timing of the laser pulse is shifted so as to generate an off-droplet pulse (“off-droplet pulse generation”), the pulse of the laser beam 102 becomes a droplet and a droplet. , The target material of the droplet is not vaporized, and no plasma is generated at the primary focal point 105. In a MOPA + PP system, the timing to trigger pulse generation can be advanced or delayed so that the laser beam 102 passes through the primary focus 105 without impacting the droplet 107. Thus, as shown in FIG. 4, little or no EUV energy is generated for the drop-off pulse generation 402.

[36] Embodiments of the methods described herein for strobe firing determine, for each pulse in a packet, whether a desired energy dose target for the current packet has been achieved. Therefore, after irradiating the droplet in the packet with the laser light, the total energy dose of the packet is calculated and compared with the desired energy dose target. If the desired energy dose target has not been achieved, the timing for triggering the drive laser for the next pulse is adjusted so that the laser beam falls on the next droplet. When the desired energy dose target has been achieved, the timing of triggering the drive laser for the next pulse is shifted so that the laser beam to the next droplet is out of the droplet and added in the current packet Energy is not generated. Between packets (i.e., on a packet-by-packet basis), the "servo ()" is used to accumulate the calculated dose error from the current packet along with the dose error from the previous packet and 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 controlling the dose of the generated EUV light, according to one embodiment. The laser controller 502 adjusts the timing of the trigger for driving the laser 101 to generate a pulse on the 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 a running total of the total EUV energy occurring during the current packet. I do. The sensor 501 may be a sensor in the 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 passing through the intermediate focus 109. Sensor. When the accumulated EUV equals or slightly exceeds the dose target, the laser controller 502 shifts the timing of the trigger to drive the laser 101 so that the driving laser 101 can generate a drop-off pulse. The generation avoids additional EUV energy generation. The drive laser 101 continues to generate a drop-off pulse for the remainder of the current packet. Upon completion of the current packet, the laser controller 502 calculates the dose error for the current packet and accumulates the dose error along 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 disclosed laser beam pulse timing adjustment method for strobe pulse generation adjust the average EUV by firing some of the pulses in the packet out of the droplet. For example, if the pulse energy increases, the number of pulses (pulse count) fired on the droplet is reduced to maintain the same average EUV. As the random fluctuations in EUV energy that occur over time are better understood, the packet size can be adjusted to minimize the time to generate laser light on a drop off.

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

[40] The packet size is preferably selected to be the minimum packet size that allows control of the EUV energy dose. If the packet size is too small (eg, one or two droplets), it may not be possible to stagger the pulse generation timing for a sufficient number of droplets to properly control the EUV energy dose. If the packet size is too large (e.g., 1000 droplets), uncontrollable errors accumulate throughout the packet (e.g., as shown in FIG. 4), thereby reducing the amount of EUV generated for downstream injection. It cannot be controlled well. For this reason, the packet size is ideally selected so that the pulse timing adjustment can be performed only on the droplets at the back of the packet. For example, if an average dose of 40 droplets is achievable, then a packet size of 50 droplets may be appropriate (in this case, the pulse timing shift is performed on 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 the dose servo values 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, the laser pulse firing of the packet can begin. Steps 602 to 607 are performed for each pulse, that is, for each pulse of the packet.

In step 602, the laser controller 502 adjusts the timing of the trigger for the driving laser 101 so as to generate a pulse on the droplet, so that the laser beam 102 irradiates the droplet 107 at the primary focus 105.

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 running total of EUV generated after the first pulse of the packet (that is, after step 601), thereby obtaining the EUV energy. To accumulate.

[46] At step 605, the laser controller 502 determines whether the stored EUV energy of 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 in step 601. The stored EUV energy may be slightly larger than the adjusted dose target for various reasons. Various reasons include, for example, random variations in EUV generated by each illuminated droplet, and / or constant (even if no random variations) energy generated by each illuminated droplet. Is not a value. If the stored EUV energy is less than the adjusted dose target of step 601, laser controller 502 returns to step 602 to trigger another on-drop pulse and repeat steps 603, 604, and 605.

[47] If the stored 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 an off-droplet pulse so that the laser beam 102 The small droplet 107 is not irradiated at the focal point 105. The off-trigger trigger is relative to the timing of the next trigger to generate an on-drop pulse, i.e., the next trigger to generate an on-drop pulse if the stored EUV energy of step 604 was less than the adjusted dose target. Can be delayed or advanced in time with respect to the timing of.

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

If the laser controller 502 determines that the packet has been completed, it executes steps 608 to 611 and another step 601, after which the next packet starts.

[50] In step 608, the laser controller 502 calculates the dose error of the packet. Dose error is defined as the dose target minus the EUV energy stored 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 the new dose servo value as the dose servo value for the next packet.

[54] It is important that the packets repeat at a constant frequency. That is, regardless of how many pulses in one packet hit the droplet at the primary focus 105, the packet starts at a set time after firing the number of pulses in one packet. However, since the number of pulses that strike a droplet in one packet varies based on the amount of energy generated by previous droplet irradiation, the last pulse that strikes a droplet in one packet is It can vary between.

[55] Further, although not shown in the figure, since the packet has a set number of pulses, when the set number of pulses is reached during the loop of steps 602 to 605, the laser generates an off-droplet pulse. The packet may terminate without having to stagger the trigger to cause the packet to elapse (eg, if the stored EUV energy of the packet is less than the adjusted dose target of the packet). Specifically, if the 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 the drive laser 101 is equal to the packet size), The controller 502 does not return to step 602 to adjust the timing of another trigger to cause the drive laser 101 to generate an on-droplet pulse, but instead executes steps 608 to 611 to start the next packet. Accordingly, the laser controller 502 calculates the dose error of the packet (step 608), accumulates the dose error from the packet together with the dose error from the previous packet (step 609), and stores the accumulated dose error calculated in step 609. Calculate the new dose servo value using step 610 and reset the accumulated EUV to zero in preparation for the next packet and return to step 601 to replace the new dose servo value with 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 centered on 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 the pulse timing adjustment is within ± 0.5% of the dose target (ie ± 0% of zero in the figure). (Within 0.5%).

[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 over time at the dose target (here, about 20 mJ) and is stably maintained within ± 0.5% of the dose target. The lower box 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 the exemplary packet EUV energy (upper box) and packet pulse count (lower box), multiple on-droplet pulse generations 801 and multiple off-droplet pulse generations 802 (and thus small pulse counts) are indicated by arrows. As indicated by the arrows, the number of pulses required to achieve a constant EUV energy may be reduced depending on the random fluctuations in the EUV energy that occurs.

[58] When applied to continuous burst firing, embodiments of the methods described herein provide a dose error (ie, the resulting EUV energy is the desired energy dose) for each droplet for each pulse in each burst. Amount that deviates from the target). Dose errors accumulate as the burst progresses. Therefore, after irradiating one droplet in a burst with a laser beam, the dose error of the droplet is calculated and accumulated together with the dose error of the previous droplet in the burst. If the accumulated dose error of the burst (ie, “accumulated burst error”) is greater than or equal to an acceptable burst error level (ie, “threshold burst error”), the next pulse will be shifted the trigger timing for the drive laser to the next smaller By making the laser light on the droplet out of the droplet, 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 pulse on the drive laser is timed to direct the laser light to the next droplet onto the droplet to generate additional EUV energy. .

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

[60] Once the dose target has been set, in step 901, burst laser pulse firing can begin. Steps 902 to 908 are performed on a pulse-by-pulse basis, ie, for each pulse of the burst.

In step 902, the laser controller 502 adjusts the timing of the trigger for the driving laser 101 so as to generate an on-droplet 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 has been 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 the irradiation of the current droplet 107 (and detected in step 903) minus the dose target, and is mathematically expressed 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 a running total of dose errors accumulated since the first pulse of the packet (ie, after step 901). Accumulates burst errors. The current dose error is adjusted by the gain, which can range from 0.01 to 100, but is preferably one. In one embodiment, the accumulated burst error is calculated as follows.
Up to now burst error + (gain x current dose error)
Here, the burst error up to the present time is the total up to now of the dose errors accumulated from the previous droplets in the burst. That is, the burst error up to the present time is the accumulated burst error obtained in step 905 for the immediately preceding droplet 107. The burst error to date 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 or not 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 the accumulated burst error in step 905 is equal to or greater than 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 driving 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 equal to or larger than the burst error threshold, then in step 908, the laser controller 502 triggers the driving laser 101 to generate a pulse for dropping a droplet. Is shifted so that the laser beam 102 does not irradiate the next small droplet 107 at the primary focal point 105. A staggered trigger can cause the laser pulse to arrive at the primary focus earlier or later than the droplet arrives.

[69] After shifting the trigger timing so that the driving laser 101 generates a pulse for the next droplet 107 so as to cause the driving laser 101 to fall out of the droplet, the laser controller 502 returns to step 903 and determines how much the current droplet 107 irradiates. Detecting if EUV energy has occurred, then calculate the current dose error for the next droplet 107 at step 904. Since EUV does not occur for the next droplet 107 because the pulse timing is shifted, the current dose error calculated for the next droplet 107 is equal in magnitude but opposite in sign to the dose target. 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 illuminated droplet (typically much more than 40%). Extremely small). Thus, if at step 905 the laser controller 502 accumulates the burst error by adding the relatively large current dose error for the next droplet 107 to the burst error to date, the accumulated burst error will typically be This is smaller than the accumulation burst error of the immediately preceding 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 a 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 and adjusts the timing of the trigger to cause the drive laser 101 to generate an on-droplet pulse. The laser beam 102 then illuminates another droplet 107 at the primary focus 105 (now the current droplet 107), and the process of FIG. 9 repeats from that step. If the laser controller 502 determines that the accumulated burst error is equal to or greater than the burst error threshold, then in step 908, the laser controller 502 shifts the trigger timing so that the driving laser 101 generates a pulse for dropping a droplet. , So that the laser beam 102 does not irradiate the next droplet 107 at the primary focus 105, and then return to step 903 again to detect how much EUV energy has been generated. The process of FIG. 9 then repeats from that point.

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

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

[72] An aspect of this method is to compare the burst error for each pulse with an acceptable burst error threshold, accumulate the burst error over the entire burst, and control the energy generation by shifting the timing of the next pulse. It will be appreciated by those skilled in the art that other embodiments with less intuitive satisfaction are possible (although not preferred) as long as they are internally consistent to achieve the purpose of making such a determination. Will be appreciated. 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 mutually consistent. Rather, it must be consistent with the decision rule results 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 box) and energy dose (lower box) generated during a continuous burst using laser beam pulse timing adjustment to control EUV dose, according to one embodiment. Shows a sliding window. As seen in the upper box, most of the pulses were fired on the droplet (eg, on-droplet pulse 1001), but a certain number of pulses were emitted around the dose target 1003 (about 1.75 mJ in this figure). Fired off-drop to control errors (eg, as indicated by a pulse generating 0 mJ EUV, such as off-drop pulse 1002). As a result, as shown in the lower box, 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 conditions of the target are correct and the driving laser has the proper performance, the embodiment of the laser beam pulse timing adjustment method described herein will be able to achieve ± 0.5% of the dose target. It is believed that dose energy can be maintained within.

[75] Those skilled in the art will recognize that laser pulses may be de-timed by a wide variety of mechanisms known in the art. For example, the drive laser can be fired such that the laser pulse reaches the primary focus earlier or later than the droplet arrives. Alternatively, changing the timing of a system shutter (e.g., an electro-optic modulator or an acousto-optic modulator) may allow the passage of low level continuous wave light sufficient to reduce the amplifier seed and system gain. it can. In a preferred embodiment, closing the shutter early causes the laser beam to advance against 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 the droplet, apply the laser beam to the droplet using both the main pulse and the pre-pulse.When the laser emits a pulse outside the droplet, apply both the main pulse and the pre-pulse to the droplet. It will be appreciated by those skilled in the art that it is not used to direct laser light.

[77] The disclosed methods and apparatus have been described above with reference to certain embodiments. Other embodiments will be apparent to those skilled in the art in light of the present 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] Further, it will be appreciated that the methods and apparatus described may be implemented in numerous ways, such as as a process, an apparatus, or a system. The methods described herein can be implemented by program instructions for instructing a processor to perform such methods. Such instructions may be recorded on a computer readable storage medium, such as a hard disk drive, a floppy disk, an optical disk such as a compact disk (CD) or a digital versatile disk (DVD), a flash memory, or may be programmed instructions over a computer network. Is transmitted over an optical or electronic communication link. It should be noted that the order of the steps of the method described herein can be varied and still be within the scope of the present disclosure.

[79] It will be appreciated that the examples provided herein are for illustrative purposes only and can be extended to other implementations and embodiments using different conventions and techniques. Although a number of embodiments are described, there is no intention to limit the disclosure to the embodiment (s) disclosed herein, on the contrary, all alternatives, modifications, and equivalents apparent to those skilled in the art. Is intended to be included.

[80] In this specification, the present invention has been described with reference to specific embodiments, but those skilled in the art will recognize that the present invention is not limited thereto. The various features and aspects of the above-described invention can be used individually or in combination. Moreover, the present invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the present description. Accordingly, the specification and drawings are to be regarded as illustrative instead of limiting. It will be appreciated that, as used herein, the words "comprising,""including," and "having" are specifically intended to be read as open-ended terms in the art.

Claims (6)

A method of adjusting an energy dose generated during a continuous burst mode of an EUV light source, comprising:
(A) initiating a burst having a predetermined energy dose target;
(B) adjusting the timing of a trigger for generating a laser beam pulse so as to irradiate a droplet during the burst by the laser controller;
(C) detecting EUV energy generated by the droplet;
(D) calculating, by the laser controller, a current dose error for the droplet based on the detected EUV energy and the energy dose target;
(E) storing, by the laser controller, a burst error based on the current dose error and a burst error to date calculated for one or more preceding droplets during the burst;
(F) repeating the steps (b) to (e) if the burst has not ended and the accumulated burst error is less than a threshold burst error;
(G) when the burst is not completed and the accumulated burst error is equal to or greater than the threshold burst error, the laser controller causes the laser beam to pulse so as not to irradiate the next droplet. Shifting the timing of the trigger,
(H) repeating steps (c) to (g) until the end of the burst;
A method comprising:   The method of claim 1, wherein the current dose error is a difference between the detected EUV energy and the energy dose target. The accumulated burst error is
Up to now burst error + (gain x dose error)
3. The method according to claim 2, wherein A system for adjusting an energy dose generated during a continuous burst firing of an EUV light source that generates an energy dose target, the system comprising:
A drive laser that generates a laser beam when a trigger is received,
A sensor for detecting EUV energy generated by irradiation of the droplet,
(A) adjusting the timing of the trigger for pulsing the laser beam so as to irradiate a droplet during the burst;
(B) calculating a current dose error for the droplet based on the detected EUV energy and the energy dose target;
(C) storing a burst error based on the current dose error and a burst error to date 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) when the burst is not completed and the accumulated burst error is equal to or greater than a threshold burst error, the timing of the trigger for generating a pulse of the laser beam is shifted so as not to irradiate the next droplet. ,
(F) repeating steps (b) to (e) until the end of the burst;
A controller,
A system comprising:   5. The system of claim 4, wherein the current dose error is a difference between the detected EUV energy and the energy dose target. The accumulated burst error is
Up to now burst error + (gain x dose error)
The system of claim 5, wherein