JP2017529553A - Calibration of photoelectromagnetic sensors in laser light sources - Google Patents

Abstract

A photoelectromagnetic sensor in a laser light source is calibrated. Laser pulses are used to generate EUV light in a laser generated plasma extreme ultraviolet system. In order to determine the energy of the individual laser pulses, the photoelectromagnetic detector is calibrated to the wattmeter using a calibration factor. When measuring a unitary laser beam includes a single wavelength pulse, a calibration factor is calculated based on the burst of pulses. The combined laser beam has a first wavelength main pulse alternating with a second wavelength pre-pulse. In order to calculate the energy of the main pulse in the combined laser beam, the calibration factor calculated for the unitary laser beam of the main pulse is used. A new calibration factor is calculated to calculate the energy of the prepulse in the combined laser beam. When the calculated energy value drifts above a predefined threshold, the calibration factor is recalculated. [Selection] Figure 9

Description

[1] This application relates generally to laser systems, and more specifically to calibration of photoelectromagnetic sensors in a laser light source of a laser produced plasma (LPP) extreme ultraviolet (EUV) system.

[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-rays) is generally defined as electromagnetic radiation having a wavelength between 10 nm and 102 nm. EUV lithography is currently 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 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 generating EUV light are not necessarily limited, but include one or more elements having one or more emission lines in the EUV range, such as xenon, lithium, tin, indium, antimony, Including converting a material having tellurium, aluminum or the like to a plasma state. In one such method, often referred to as laser-produced plasma (“LPP”), the required plasma is a droplet, stream, or stream of material having the desired line-emitting element in the LPP EUV light source plasma chamber. It can be generated by irradiating a target material such as a cluster with a laser beam at an irradiation site.

[4] FIG. 1 illustrates some of the components of a prior art LPP EUV system 100. A laser light source 101, such as a CO ₂ laser, generates a laser beam 102 that passes through a beam delivery system 103 and a focusing optical system 104 (including a lens and a steering mirror). The focusing optical system 104 has a first focal point 105 at an irradiation site in the LPP EUV light source plasma chamber 110. The droplet generator 106 generates a droplet 107 of a suitable target material that generates plasma that emits EUV light when the laser beam 102 strikes the first focal point 105. An elliptical mirror (“collector”) 108 focuses the EUV light from the plasma at a focal spot 109 (also referred to as an intermediate focus position) to deliver the generated EUV light to, for example, a lithography scanner system (not shown). . The focal spot 109 is typically in a scanner (not shown) that includes a wafer that is exposed to EUV light. In some embodiments, there may be multiple laser light sources 101 where all of the beam is concentrated in the focusing optics 104. One type of LPP EUV light source uses a CO ₂ laser and a zinc selenide (ZnSe) lens with an anti-reflective coating and an aperture of about 6 to 8 inches (about 15.24 cm to 20.32 cm). It may be a thing.

[5] The laser light source 101 can be generated in a burst mode in which several light pulses are generated in bursts with some time between bursts. The laser light source 101 can comprise several lasers that produce a pulsed laser beam with well-defined characteristics such as wavelength and / or pulse length. In the laser light source 101, the beam delivery system 103, and the focusing optics 104, separate laser beams can be combined, split, or otherwise manipulated.

[6] Before the laser beam 102 reaches the LPP EUV light source plasma chamber 110, the beam 102 is measured at various points within the laser light source 101, the beam delivery system 103, and / or the focusing optics 104. . Measurements are made using various instruments that measure one or more aspects of the laser beam 102. In some instances, the laser beam 102 can be measured before or after being combined with other generated beams.

[6] However, the instrument may not directly measure certain characteristics of the laser beam 102 or may not be calibrated in such a way as to measure the characteristics of the laser beam 102.

[7] According to an embodiment, the system is configured to measure a laser beam comprising a prepulse and a main pulse separated by a length of time in a laser produced plasma (LPP) extreme ultraviolet (EUV) system. An energy monitor configured to sense the average power of a series of laser pulses over a defined time period; and a portion of the time from the first main pulse during a portion of the defined time period. An opto-electromagnetic (PEM) detector configured to provide a voltage signal indicative of a temporal profile of a first pre-pulse separated by length; an energy monitor; a main pulse calibration factor and a first main pulse And the average power and the first power so as to determine the power of the first main pulse based on the pulse integration of a part of the voltage signal corresponding to Pre-pulse calibration factor based on the power of the first pulse and the integral of a portion of the voltage signal corresponding to the first pre-pulse to determine the power of the first pre-pulse based on the power of the main pulse A calibration module configured to determine a first pulse based on a pre-pulse calibration factor and a pulse integral of a portion of the second voltage signal provided by the PEM detector corresponding to the second pre-pulse. And determining the energy of the second main pulse and determining the energy of the second main pulse based on the main pulse calibration factor and the pulse integration of a portion of the second voltage signal into the second main pulse. A single pulse energy calculation (SPEC) module configured.

[8] The system is configured to calculate the energy of the laser beam over a second defined time period based on the second voltage signal provided by the PEM, the calculated energy of the laser beam; Configured to compare the average power sensed by the power meter over a second defined time period and to instruct the calibration module to update the prepulse calibration factor based on the comparison. A recalibration module can further be provided.

[9] According to an embodiment, the method is to receive a measurement of a laser beam comprising a pre-pulse and a main pulse using an energy monitor in a laser produced plasma (LPP) extreme ultraviolet (EUV) system, wherein the laser The measurement of the beam consists of the average power of a series of laser pulses over a defined time period measured using a power meter and of the prepulses divided by the length of time from the first main pulse of the main pulses. A first voltage signal indicative of a temporal profile of the first prepulse, wherein the first voltage signal is provided by a photoelectromagnetic (PEM) detector; a main pulse calibration factor and a first main pulse Determining the power of the first main pulse based on the integral of a portion of the corresponding first voltage signal; the average power and the first main pulse; Determining the power of the first prepulse based on the power of the first prepulse; based on the power of the first prepulse and the integral of a portion of the first voltage signal corresponding to the first prepulse. Determining a calibration factor; determining the energy of the second prepulse based on the prepulse calibration factor and an integral of a portion of the second voltage signal provided by the PEM detector corresponding to the second prepulse. And determining the energy of the second main pulse based on a main pulse calibration factor and an integral of a portion of the second voltage signal corresponding to the second main pulse.

[10] According to embodiments, the non-transitory computer readable medium has instructions embodied thereon, the instructions using an energy monitor in a laser produced plasma (LPP) extreme ultraviolet (EUV) system. Receiving a measurement of a laser beam comprising a pre-pulse and a main pulse, the measurement of the laser beam comprising the average power of a series of laser pulses over a defined time period measured using a power meter, A first voltage signal indicating a temporal profile of the first pre-pulse of the pre-pulses divided by the length of time from the first main pulse of the first main pulse, wherein the first voltage signal is a photoelectromagnetic (PEM) Provided by the detector; based on a main pulse calibration factor and an integral of a portion of the first voltage signal corresponding to the first main pulse; Determining the power of the main pulse; determining the power of the first prepulse based on the average power and the power of the first main pulse; corresponding to the power of the first prepulse and the first prepulse Determining a pre-pulse calibration factor based on an integral of a portion of the first voltage signal; one of the pre-pulse calibration factor and the second voltage signal provided by the PEM detector corresponding to the second pre-pulse. Determining the energy of the second prepulse based on the integral of the part; and based on the main pulse calibration factor and the integral of the portion of the second voltage signal corresponding to the second main pulse. And determining the energy of the second main pulse can be performed by one or more processors to perform operations.

[11] According to embodiments, the system is configured to measure laser pulses having the same wavelength and occurring in a burst within a laser light source of a laser produced plasma (LPP) extreme ultraviolet (EUV) system. An energy monitor, a power meter configured to measure an average power of a laser pulse over a defined time period and a temporal profile of a burst of laser pulses over at least a portion of the defined time period An opto-electromagnetic (PEM) detector configured to provide a first voltage signal; an energy monitor; configured to determine a calibration factor based on the average power and the first voltage signal A calibration module, wherein the calibration factor is the energy of the burst of laser pulses determined from the average power and the integral of the first voltage signal. A calibration module; and a calibration coefficient and a pulse integral of a second voltage signal provided by a PEM detector indicating a temporal profile of the subsequent pulse, A single pulse energy calculation (SPEC) module configured to determine energy.

[12] The system calculates the energy of the second burst based on a third voltage signal indicative of the second temporal profile of the second burst, and the energy of the second burst is a power meter A recalibration module configured to compare with the second average power sensed by the and to instruct the comparison module to update the calibration factor based on the comparison.

[13] According to an embodiment, the method measures a laser pulse having the same wavelength and generated in a burst using an energy monitor in a laser source of a laser produced plasma (LPP) extreme ultraviolet (EUV) system. Receiving an average power of a laser pulse measured over a defined time period from a wattmeter, and a temporal of a burst of laser pulses sensed during at least a portion of the defined time period Receiving a first voltage signal indicative of a profile from a photoelectromagnetic (PEM) detector; determining a calibration factor based on the average power and the first voltage signal, wherein the calibration factor is The ratio of the energy of the burst of laser pulses determined from the average power and the integration of the first voltage signal; and the calibration factor and the time of the subsequent pulse Profile and integration of the second voltage signal provided by the PEM detector shown, and based on, comprising, determining the energy of the following pulse series of laser pulses.

[14] According to an embodiment, the non-transitory computer readable medium has instructions embodied thereon, wherein the instructions generate laser pulses generated in bursts with the same wavelength by laser generated plasma (LPP). ) Measuring with an energy monitor in the laser source of an extreme ultraviolet (EUV) system, receiving an average power of a laser pulse measured over a defined time period from a power meter; Receiving from a photoelectromagnetic (PEM) detector a first voltage signal indicative of a temporal profile of a burst of laser pulses sensed during at least a portion of the determined time period; average power and first A calibration factor is determined based on a voltage signal of the first pulse of the laser pulse burst energy determined from the average power and the first voltage signal. And a subsequent pulse of a series of laser pulses based on the calibration factor and the integration of the second voltage signal provided by the PEM detector indicating the temporal profile of the subsequent pulse. Can be performed by one or more processors to perform operations including determining the energy of.

[1] FIG. 1 shows a part of an LPP EUV system according to the prior art. [2] FIG. 3 illustrates an energy monitor, according to an exemplary embodiment. [3] FIG. 3 illustrates a burst mode of a laser light source, according to an exemplary embodiment. [4] A graph output by a PEM detector showing the temporal profile of a burst with a main pulse. [5] A graph output by the PEM detector showing the temporal profile of a single main pulse. [6] A graph output by a PEM detector showing the temporal profile of a burst with pre-pulses. [7] A graph output by a PEM detector showing the temporal profile of a single pre-pulse. [8] A diagram showing an example of a graph output by a PEM detector showing a temporal profile of a pre-pulse and a main pulse divided by a length of time. [9] FIG. 9 is a block diagram illustrating a system for measuring the energy of a pulse, according to an exemplary embodiment. [10] FIG. 10 is a flowchart illustrating an exemplary method for measuring the energy of a pulse, in accordance with an exemplary embodiment. [11] A flow chart illustrating an exemplary method for calibrating a photoelectromagnetic (PEM) detector using a power meter for a unitary laser beam. [12] A flow chart illustrating an exemplary method for calibrating a PEM detector using a power meter for a combined laser beam.

[13] In the LPP EUV system, the energy of the laser pulse is calculated at various locations in the laser source, beam delivery system, and / or focusing optics. The sensor used to measure the laser beam in the LPP EUV system does not directly measure the energy of the laser beam pulse. The sensor includes a power meter that provides a measurement of the average power of the pulses generated over a defined time period. The sensor further includes a photoelectromagnetic (PEM) detector that outputs a voltage signal based on infrared (IR) light detected over a limited time period. The voltage signal provides a temporal profile of the individual laser pulses. Using the data collected by the sensor, a calibration factor is calculated for calibrating the PEM detector to the power meter. After calibration, the energy of the pulse can be calculated from the voltage signal provided by the PEM detector.

[14] The laser beam to be measured can comprise a pulse of light of the same wavelength, referred to as a unitary laser beam. The unitary laser beam can comprise either a prepulse with a first wavelength or a main pulse with a second wavelength. In order to determine the energy of the pulse of light in the unitary laser beam, the PEM detector is calibrated to the power meter by calculating a calibration factor for the unitary laser beam. In the case of a unitary laser beam, the calibration factor is a ratio based on measurements received from the power meter and the voltage signal provided by the PEM detector via a burst. After calibration, the energy of the pulse in the unitary laser beam is calculated using the calibration factor and the voltage signal provided by the PEM detector.

[15] In an LPP EUV system, when two unitary laser beams of different wavelengths are combined, the resulting combined laser beam has pulses of alternating wavelengths that are separated in the time domain. In the embodiments described herein, the pulses in the combined laser beam are alternating prepulses of the first wavelength and main pulses of the second wavelength. When calculating the energy of the main pulse in the combined laser beam, a calibration factor calculated from the unitary laser beam of the main pulse is used. Due to the different effects of the optical components in the LPP EUV system between the prepulse and the main pulse in the combined laser beam, another calibration factor for the prepulse in the combined laser beam is calculated. The pre-pulse calibration factor is determined based on the difference from the power due to the main pulse in the laser beam combined with the power measured by the wattmeter. After calibration, the pre-pulse and main pulse energies in the combined laser beam are calculated using the calibration factor and the voltage signal provided by the PEM detector.

[16] FIG. 2 is a diagram of an energy monitor 200 according to an exemplary embodiment comprising a power meter 202 and a PEM detector 208. The energy monitor 200 can receive the laser beam 102 from another component in the laser light source 101 using, for example, a beam splitter. As will be appreciated by those skilled in the art, before the laser beam 102 reaches the energy monitor 200, one or more optical components to pick-off a portion of the laser beam 102 for measurement. Pass through. These optical components can include diamond windows, partial reflectors, or zinc selenide windows. An example of a laser seed module that can include an energy monitor 200 is discussed in US Patent Application No. 2013/0321926, published December 5, 2013, assigned to the assignee of the present application. ing.

[17] The energy monitor 200 is arranged to measure the laser beam 102 at a specific location within the laser light source 101, the beam delivery system 103, or the focusing optics 104. In some embodiments described herein, an energy monitor 200 is arranged to measure a unitary laser beam 102 comprising a laser pulse of light of the same wavelength (eg, pre-pulse or main pulse). I will let you. The laser pulse of light is generated by a single light source, but in other systems it can be generated by multiple light sources. In other embodiments described herein, disposing the energy monitor 200 will cause the energy monitor to measure the combined laser beam 102 generated by two laser light sources with different wavelengths. The laser light source 101, the beam delivery system 103, and the focusing optics 104 can include one or more energy monitors 200.

[18] The laser beam 102 follows an optical path through the energy monitor 200. The laser beam 102 is split by the beam splitter 204 such that the first portion of the laser beam 102 continues along the optical path, while the remaining portion of the laser beam 102 is directed toward the reflector 206. The reflector 206 then sends the remaining portion of the laser beam 102 toward the wattmeter 202.

[19] The power meter 202 is configured to measure the average power of the laser beam 102 over a defined time period. The measurement can span several bursts of the laser beam 102. In some instances, the measurement can span 5, 10, or 20 bursts of the laser beam 102. The defined time period can be a fraction of a second or a few seconds. In some instances, the defined time period is 1 second.

[20] The portion of the laser beam 102 that is not directed to the wattmeter 202 is directed toward a further beam splitter 204. From the beam splitter 204, a first portion of the laser beam is directed toward, for example, further sensors or other optical components (not shown). The remaining part of the laser beam 102 is directed towards the PEM detector 208.

[21] The PEM detector 208 provides a voltage signal indicative of the temporal profile of the laser beam 102. The temporal profile spans at least a portion of the defined time period used by the wattmeter 202. The temporal profile can span at least one burst of the laser beam 102. In order to calculate the energy of the pulses in the combined laser beam, the temporal profile spans the prepulse and the main pulse.

[22] Although only one PEM detector 208 is shown in FIG. 2, additional PEM detectors (not shown) may be included in the energy monitor 200. Further, the laser beam 102 can be modified prior to measurement by the PEM detector 208 using, for example, a lens (not shown) or a diffuser set (not shown). The energy monitor 200 can be enclosed by a casing and attached to a part of the laser light source 101, or can be enclosed inside the laser light source 101.

[23] FIG. 9 is a block diagram of a system 900 for measuring the energy of a pulse according to an exemplary embodiment. The system 900 includes an energy monitor 902, a calibration module 904, a single pulse energy calculation (SPEC) module 906, and an optional recalibration module 908. System 900 can be implemented in a variety of ways known to those skilled in the art including, but not limited to, where a computing device has a processor with access to memory capable of storing executable instructions. is there. A computing device may include one or more input and output components, including components for communicating with other computing devices via a network or other form of communication. System 900 comprises one or more modules embodied in computing logic or executable code.

[24] The energy monitor 902 is configured to receive data relating to laser pulses of the laser beam 102. The energy monitor 902 comprises or communicates electronically with a power meter and a PEM detector. In some instances, the energy monitor 902 is an energy monitor 200 that includes a power meter 202 and a PEM detector 208. The wattmeter is configured to measure the average power of the laser pulse over a defined time period. The defined time period may be, for example, 1 second. The PEM detector is configured to provide a voltage signal indicative of the temporal profile of the laser pulse over at least a portion of the defined time period.

[25] When the energy monitor 200 or 902 is placed in the LPP EUV system 100 to receive the unitary laser beam 102, the calibration module 904 includes a power meter (eg, power meter 202) and a PEM detector (eg, A calibration factor is configured to be determined based on data collected by the PEM detector 208). A calibration factor is calculated for each unitary laser beam. The calibration factor is used to calculate the energy of a single pulse (eg, single main pulse 502 or single prepulse 702) based on PEM detector data collected later. The calibration factor is a ratio determined from the integration of average power and voltage signal.

[26] To determine the calibration factor in the unitary laser beam 102, the burst of pulses is analyzed. FIG. 3 is an illustration 300 of two bursts 302 of laser pulses, in accordance with an exemplary embodiment. The example 300 is a schematic of a temporal profile provided by the PEM detector 208, shown as a voltage that varies as a function of time (measured in milliseconds (ms)).

[27] Each burst 302 has a rising edge 310, peaking 312 and is then shown in the example 300 as a curve that maintains a voltage level 314 that is below the peak level for the length of time until ending at the falling edge 316. It is. Burst 302 has a burst length 304 that begins at a rising edge 310 and ends at a falling edge 316. As described elsewhere herein, when calculating the energy of burst 302 to calculate a calibration factor, PEM detector 208 has a scope window 306 that includes at least burst length 304. The scope window 306 can be extended to capture the time between bursts 302 or shortened to capture only one or two pulses within the burst 302.

[28] The burst period 308 is measured from the rising edge 310 of the first burst 302 to the rising edge 310 of the second burst 302. The burst period 308 can be determined from the repetition rate or “rep rate” of the burst 302, which indicates the number of bursts over a defined time period (eg, 1 second). The burst rep rate can be expressed as a frequency such as 5 hertz (Hz), 10 Hz, or 20 Hz.

[29] FIG. 4 is a graph 400 illustrating a temporal profile provided by the output of the PEM detector 208 of a burst 402 of a unitary laser beam 102 comprising a main pulse. Graph 400 is an actual example of the output shown in FIG. The unitary laser beam 102 is generated by a single light source. In an embodiment, the main pulse has a waveform of 10.59 microns. As shown in the figure, burst 402 lasts approximately 3.5 milliseconds and comprises a predetermined number of laser pulses. According to various embodiments, the burst 402 can comprise a different number of laser pulses based on the burst length. The laser pulse has a width (measured as a length of time) and an amplitude. As part of the calculation of the calibration factor, the calibration module 904 can integrate the pulses of the burst 402 over the length of time shown in the graph 400.

[30] FIG. 6 is a graph 600 output by the PEM detector 208 showing the temporal profile of the burst 602 of the unitary laser beam 102 with prepulses. Graph 600 is an actual example of the output shown in FIG. The unitary laser beam 102 is generated by a single light source, but can be generated by multiple light sources in other systems. In an embodiment, the prepulse has a wavelength of 10.26 microns. As shown in the figure, burst 602 has a duration of approximately 3.5 milliseconds and comprises a predetermined number of laser pulses. According to various embodiments, the burst 602 can comprise a different number of laser pulses based on the burst length. The laser pulse has a width (measured as a length of time) and an amplitude. As part of the calculation of the calibration factor, the calibration module 904 can integrate the pulses of burst 602 over the length of time shown in graph 600.

[31] The calibration factor for the unitary laser beam 102 is calculated similarly for both the unitary beam with the main pulse and the unitary beam with the prepulse. First, the calibration module 904 determines the energy of the burst of laser pulses from the average power. The energy generated during a defined time period during which power was measured over that period is determined as follows.
_Where E _total is the energy of the laser beam 102 over a defined time period, P _measured is a power measurement made by the wattmeter 202, and T _period is a defined time period of the wattmeter 202 (eg, 1 second). From E _total, the amount of energy within a burst with a burst rep rate is calculated as follows.
_Where E _burst is the energy of the burst, E _total is the energy of the unitary laser beam 102 over a defined time period (eg, 1 second), and f _burst is the burst rep rate.

[32] In order to determine the calibration factor K _p, is used following equation.
Therefore,
It is. Where K _p is the calibration factor and V is the PEM detector such that the integral ∫Vdt is the area under the curve of the voltage signal provided by the PEM detector 208 over the length of time of the burst. 208 is a voltage signal received from 208, and E _burst is the energy of the burst determined from the average power data received from wattmeter 202. Calibration factor K _p has units of watts per volt.

[33] The SPEC module 906 is configured to calculate the energy of a single pulse using the calibration factor calculated by the calibration module 904. The SPEC module 906 receives voltage data from the PEM detector 208 comprising a temporal profile of a single pulse (eg, a single main pulse 502 or a single prepulse 702) in the unitary laser beam.

[34] FIG. 5 is a graph 500 output by the PEM detector 208 showing the temporal profile of a single main pulse 502 in the unitary laser beam. A single main pulse 502 can be a pulse in burst 402 or a pulse in a subsequent burst. A single main pulse 502 is captured by reducing the scope window 306 of the PEM detector 208. The main pulse 502 has an amplitude and a width (measured as a length of time). As part of the energy calculation of pulse 502, SPEC module 906 can integrate main pulse 502.

[35] FIG. 7 is a graph 700 output by a PEM detector showing the temporal profile of a single prepulse 702 in the unitary laser beam. A single prepulse 702 is captured by reducing the length of time that the PEM detector 208 measures the laser beam 102 over its length. Prepulse 702 has an amplitude and width (measured as a length of time). As part of the pre-pulse 702 energy calculation, the SPEC module 906 can integrate the pre-pulse 702.

[36] Using a single pulse temporal profile, the energy of a single pulse is calculated according to the following formula:
Where E _pulse is the energy of the pulse, K _p is the calibration factor for the pulse at the wavelength of the pulse being measured, and V is the integral ∫Vdt by the PEM detector 208 over the length of time of the pulse. A voltage signal received from the PEM detector 208 showing the temporal profile of the measured pulse, as is the area under the curve of the provided voltage signal.

[37] The optional recalibration module 908 is configured to determine whether to reconfigure the PEM detector 208. Calibration factors can lose accuracy over time due to instrument drift, instrument degradation, or degradation of the beam splitter from which the laser beam is received. In a unitary laser beam, during subsequent bursts of laser pulses (eg, burst 402 or burst 602), the recalibration module 908 calculates the laser beam 102 using the power meter 202 measurements and data provided by the PEM detector 208. Configured to compare with the measured power. As described herein, this comparison is performed using a 1 second time period. Other time periods may be used, such as burst length 304 or burst period 308, or several burst periods, as will be apparent to those skilled in the art based on the description below.

[38] To calculate the power of the laser beam 102 pulse over the 1 second engine, the calibration factor is used to determine the energy of the pulse over a burst (eg, burst 402 or 602) as follows.
Where K _p is the calibration factor and V is the PEM detector such that the integral ∫Vdt is the area under the curve of the voltage signal provided by the PEM detector 208 over the length of time of the burst. A voltage signal received from 208 and E _burst is the calculated energy of the burst. The sum of the laser pulse energy is used to determine the total energy of the laser beam 102 over a time period as follows.
_Where E _burst is the energy of the burst, E _total is the calculated energy of the laser beam 102 over a defined time period (eg, 1 second), and ΣE _burst is the laser pulse over the defined time period. It is the sum of energy. To determine the power of the pulse, the total energy is divided by the time period (eg 1 second) as follows:
_Where E _total is the calculated energy of the laser beam 102 over a defined time period, P _calculated is the power value calculated from the voltage signal received from the PEM detector 208, and T _period is defined Time period (eg, 1 second).

[39] In order to determine whether to instruct the calibration module 904 to recalculate the calibration factor, the recalibration module 908 may calculate the difference between the calculated power and the measured power. This difference can be expressed as a percentage. To determine whether to recalibrate, the recalibration module 908 can compare this difference to a threshold value. In some instances, if the two power values are more than 15% apart, the recalibration module 908 instructs the calibration module 904 to recalculate the calibration factor. Based on this comparison, the recalibration manager 908 can instruct the calibration module 904 to update the calibration factor by recalculating the calibration factor during a subsequent burst of pulses of the laser beam 102.

[40] When the energy monitor 200 or 902 is placed in the LPP EUV system 100 to receive the combined laser beam 102, the system 900 of FIG. It is further configured to determine a calibration factor used to determine. An energy monitor 902 is placed in the LPP EUV system 100 to measure the combined laser beam 102.

[41] In order to calculate the calibration factor of the prepulse in the combined laser beam 102, the calibration module 904 uses a different set of calculations than that used in calibrating for the unitary laser beam. These calculations use the calibration factor calculated for the main pulse unitary laser beam 102 to determine the portion of power measured by the wattmeter 202 due to the main pulse, and then the remaining portion of the power. Used to determine the calibration factor for the prepulse in the combined laser beam.

[42] FIG. 8 is an exemplary temporal profile 800 of the voltage signal output by the combined pulse PEM detector 208 with a pre-pulse and a main pulse separated by a length of time. The combined laser beam 102 is generated by combining the unitary laser beam 102 into a single laser beam, resulting in alternating burst 602 prepulses and burst 402 main pulses within the combined laser beam 102 burst. Will occur. As shown in FIG. 8, the prepulse 802 is 15 microseconds ahead of the main pulse 804. The laser pulse has a width (measured as a length of time) and an amplitude. Over the length of time shown in graph 800, calibration module 904 and SPEC module 906 can integrate prepulse 802 separately from main pulse 804. Using this integration, a calibration factor for calculating the energy of the prepulse is determined, and the energy of the subsequent prepulse in the combined laser beam is calculated.

[43] The calibration module 904 calculates the power of the main beam of the combined beam based on a portion of the voltage signal provided by the PEM detector 208 that shows the temporal profile of the main pulse within the combined pulse. To do. Using this temporal profile, the energy of the main pulse is calculated according to the following equation:
_Where E _{main pulse} is the energy of the main pulse, K _mp is the calibration factor for the main pulse calculated for the unitary laser beam 102, and V is the integral ∫Vdt over the length of time of the main pulse. A voltage signal received from the PEM detector 208 showing the temporal profile of the main pulse being measured, as is the area under the curve of the voltage signal provided by the PEM detector 208.

[44] The average power of the combined pulses is measured by the wattmeter 202 over a defined time period (eg, 1 second). Based on the energy of the main pulse, the power due to the main pulse over a defined time period is calculated as follows:
_Where E _{main pulse} is the energy of the _{main pulse} , P _{main pulse} is the calculated power of the main pulse, T _period is the defined time period used by the wattmeter 202, and ΣE _{main pulses} is , The total energy of the main pulse that occurs over a defined time period used by the wattmeter 202. To determine the portion of power measured by the wattmeter 202 due to the prepulse, the calibration module 904 determines the difference as follows:
_Where P _pre-pulse is the fraction of power measured by the wattmeter due to the prepulse of the combined pulse over a defined time period, and P _measured is the combined measured by wattmeter 202. P _{main pulse} is the calculated power of the main pulse.

[45] Using the power due to the prepulse, the energy due to the prepulse is determined by:
_Where E _pre-pulse is the total energy of the prepulse over a defined time period used by the power meter 202 and P _pre-pulse is the power resulting from the combined pulse prepulse over the defined time period. Is a portion of the power measured by the meter, and T _period is a defined time _period of power meter 202 (eg, 1 second).

[46] In order to determine the calibration factor of the prepulse in the combined laser beam, the following equation is used:
Where K _pp is the calibration factor of the prepulse in the combined laser beam and V is the curve of the voltage signal provided by the PEM detector 208 over at least part of the time period for which the integral ∫Vdt is defined. Is the voltage signal received from the PEM detector 208, where E _pre-pulse is the total energy of the prepulse over the defined time period used by the wattmeter 202. The calibration factor K _pp has units of watts per volt.

[47] Once the pre-pulse calibration factor in the combined laser beam is determined, the SPEC module 906 determines from the PEM detector 208 that comprises a temporal profile of the pre-pulse and main pulse pairs in the combined laser beam. Receive voltage data. The SPEC module 906 can then determine the energy of the subsequent prepulse using the following equation:
_Where E _pre-pulse is the energy of a single prepulse, K _pp is the calibration factor for the prepulse pulse in the combined laser beam 102, and V is the length of time of the prepulse where the integral ∫Vdt is A voltage signal received from the PEM detector 208 that shows the temporal profile of the prepulse being measured, as is the area under the curve of the voltage signal provided by the PEM detector 208.

[48] The optional recalibration module 908 may further determine whether to recalibrate the PEM detector 208 using the power meter 202 for prepulses in the combined laser beam 102, as described above. it can. For the combined laser beam, recalibration module 908 determines the power of the pulse by summing the energy of the pulses in the combined laser beam over a defined time period corresponding to power meter 202. The recalibration module 908 then compares the power calculated from the sum with the power measured by the wattmeter 202 as described above.

[49] FIG. 10 is a flowchart of an example method 1000 for calculating the energy of a pulse, according to an example embodiment. Method 1000 may be performed by system 900.

[50] At operation 1002, the PEM detector is calibrated using a wattmeter for the beam of the first laser. The first laser can generate a main pulse or a pre-pulse in the unitary laser beam as described above. FIG. 11 is a flowchart of an exemplary method 1100 for calibrating a PEM detector using a power meter to determine the energy of a pulse within a unitary laser beam. Method 1100 may be performed as part of operation 1002 by, for example, energy monitor 200 or 902 and calibration module 904 of system 900.

[51] At operation 1102, a power measurement is received from a power meter (eg, power meter 202). The power measurement shows the average power of the pulses of unitary laser beam 102 over a time period.

[52] At operation 1104, a voltage signal over a length of time is received from a PEM detector (eg, PEM detector 208). This voltage signal is a temporal profile of a burst of pulses of the unitary laser beam 102. The length of time that data is collected by the PEM detector 208 is at least one burst within the time period of the wattmeter 202.

[53] In operation 1106, the calibration factor of the laser beam 102 is calculated. The calibration factor is calculated as described in connection with calibration module 904. The calibration factor can be calculated by the calibration module 904.

[54] If the energy monitor 200 is measuring a unitary laser beam, the method 1000 skips operation 1004 and proceeds to operation 1006. In operation 1006, the energy of the pulse is calculated. The energy of the pulse is calculated, for example, as described in connection with the SPEC module 906 elsewhere herein. In some instances, SPEC module 906 performs operation 1006.

[55] In optional operation 1008, it is determined whether to recalibrate the PEM detector, eg, by recalibration module 908. This determination is performed by comparing the power calculated from the voltage signal provided by the PEM detector with the power measured by the power meter. If it is determined to recalibrate, the method 1000 returns to operation 1002 or, in some instances, returns to operation 1004. If it is determined not to recalibrate, method 1000 returns to operation 1006.

[56] Method 1000 proceeds from operation 1002 to operation 1004 when the laser beam measured by energy monitor 200 is a combined laser beam. The PEM detector is calibrated for the combined beam to determine the energy of the prepulse in the combined beam. The second laser can generate pre-pulses in the burst as described above and is combined with the main pulse in the combined laser beam 102. In operation 1004, a calibration factor is determined for the prepulses in the combined laser beam 102. Since the optical components of the LPP EUV system 100 affect the relationship between the temporal profile of the prepulse and the power measured by the wattmeter 202 after the unitary laser beam 102 is combined, the combined laser The calibration factor for the prepulse in beam 102 is determined separately from the prepulse calibration factor in unitary laser beam 102. The prepulse calibration factor is determined based on the difference between the power measured by the wattmeter and the power due to the main pulse in the laser beam.

[57] FIG. 12 illustrates an exemplary method for calibrating a PEM detector (eg, PEM detector 208) using a wattmeter (eg, wattmeter 202) with a combined laser beam having a prepulse and a main pulse. 12 is a flowchart of 1200. Method 1200 is an exemplary method for performing operation 1004 of method 1000 when the laser beam measured by energy monitor 200 is a combined laser beam. The method 1200 may be performed by the calibration module 904 of the system 900, for example.

[58] In operation 1202, voltage data is received from the PEM detector 208 in the energy monitor 200 or 902. The voltage data is a temporal profile of the combined laser beam 102 as shown in FIG. The length of time that data is collected by the PEM detector 208 is at least part of the time period of the wattmeter 202.

[59] In operation 1204, power due to the main pulse is determined. The power of the main pulse is determined as described in connection with calibration module 904 and SPEC module 906.

[60] In operation 1206, power data is received from the wattmeter 202. The power data indicates the average power of the pulses of the combined laser beam 102 over a time period.

[61] In operation 1208, the power due to the pre-pulses in the combined laser beam 102 is determined. The prepulse power is determined as described in connection with calibration module 904 and SPEC module 906.

[62] In operation 1210, the calibration factor of the prepulse in the combined laser beam 102 is calculated. The calibration factor is calculated as described in connection with calibration module 904.

[63] Proceeding to operation 1006, when the laser beam measured by the energy monitor 200 is a combined laser beam, the respective energy of the main pulse and the pre-pulse in the combined laser beam is described above with respect to operation 1006. As calculated. Using the calibration factor of operation 1002 calculated for the unitary beam of the main pulse, the energy of the main pulse in the combined laser beam is calculated. The calibration factor of operation 1004 is used to calculate the energy of the prepulse in the combined laser beam. When the laser beam measured by energy monitor 200 is a combined laser beam, method 1000 can proceed to optional operation 1008 as described above.

[64] The disclosed method and apparatus have been described above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art from the perspective of this disclosure. Certain aspects of the described methods and apparatus can be readily implemented using configurations other than those described in the above embodiments, or with elements other than those described above. For example, different algorithms and / or logic circuits, and possibly different types of laser beam generation systems, possibly more complex than those described herein, can be used.

[65] It will also be appreciated that the methods and apparatus described 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 to some extent by program instructions to instruct a processor to perform such methods. Such instructions are 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), or a flash memory. In some embodiments, the program instructions may be stored remotely and transmitted over a network via an optical or 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.

[66] It will be appreciated that the given examples are for illustration only and can be extended to other implementations and embodiments using different conventions and techniques. Although several embodiments have been described, it is not intended that the disclosure be limited to the embodiments disclosed herein. On the contrary, it is intended to cover all alternatives, modifications, and equivalents apparent to those skilled in the art.

[67] While the invention has been described with reference to particular embodiments, those skilled in the art will appreciate that the invention is not limited thereto. The various features and aspects of the foregoing invention can be used individually or jointly. Further, the present invention may be utilized in any number of environments and scopes beyond those described herein without departing from the broader spirit and scope of the present specification. The specification and drawings are accordingly to be regarded as illustrative rather than restrictive. It will be understood that the terms “comprising”, “including”, and “having” as used herein are specifically intended to be read as open-ended terms in the art.

Claims (22)

An energy monitor configured to measure a laser beam comprising a prepulse and a main pulse separated by a length of time in a laser produced plasma (LPP) extreme ultraviolet (EUV) system, over a defined time period A wattmeter configured to sense an average power of a series of laser pulses and a first prepulse divided by a length of time from a first main pulse during a portion of the defined time period. An energy monitor comprising: a photoelectromagnetic (PEM) detector configured to provide a voltage signal indicative of a temporal profile;
The average power and the first main power are determined so as to determine the power of the first main pulse based on a main pulse calibration coefficient and a pulse integral of a part of the voltage signal corresponding to the first main pulse. Determining the power of the first prepulse based on the power of the pulse and integrating the power of the first prepulse and a portion of the voltage signal corresponding to the first prepulse. A calibration module configured to determine a prepulse calibration factor based on;
Determining energy of the second prepulse based on the prepulse calibration factor and a pulse integral of a portion of the second voltage signal provided by the PEM detector corresponding to the second prepulse. And configured to determine an energy of a second main pulse based on the main pulse calibration factor and a pulse integral of a portion of the second voltage signal to the second main pulse. A single pulse energy calculation (SPEC) module;
A system comprising:   The recalibration module configured to calculate an energy of the laser beam over a second defined time period based on the second voltage signal provided by the PEM. System.   The recalibration module compares the calculated energy of the laser beam with the average power sensed by the power meter over the second defined time period, and in the comparison The system of claim 2, further configured to instruct the calibration module to update the prepulse calibration factor based thereon.   The system of claim 3, wherein the recalibration module is configured to instruct the calibration module to update a calibration factor for the pre-pulse if the comparison exceeds a threshold.   The calibration module determines the power of the first pre-pulse by subtracting the power due to the main pulse during the defined time period from the average power over the defined time period. The system of claim 1, configured as follows. Receiving a measurement of a laser beam comprising a prepulse and a main pulse using an energy monitor in a laser produced plasma (LPP) extreme ultraviolet (EUV) system, the measurement of the laser beam using a wattmeter The temporal power profile of the first prepulse of the prepulse divided by the average power of a series of laser pulses over a defined time period to be measured and the length of time from the first main pulse of the main pulse. A first voltage signal indicative of, wherein the first voltage signal is provided by a photoelectromagnetic (PEM) detector;
Determining a power of the first main pulse based on a main pulse calibration factor and an integral of a portion of the first voltage signal corresponding to the first main pulse;
Determining the power of the first pre-pulse based on the average power and the power of the first main pulse;
Determining a prepulse calibration factor based on the power of the first prepulse and an integral of a portion of the first voltage signal corresponding to the first prepulse;
Determining an energy of a second prepulse based on the prepulse calibration factor and an integral of a portion of a second voltage signal provided by the PEM detector corresponding to the second prepulse;
Determining an energy of a second main pulse based on the main pulse calibration factor and an integral of a portion of the second voltage signal corresponding to the second main pulse;
Including a method.   The method of claim 6, further comprising calculating energy of the laser beam over a second defined time period based on the second voltage signal provided by the PEM. Comparing the calculated energy of the laser beam with the average power over the second defined time period;
Updating the calibration factor of the prepulse based on the comparison;
The method of claim 7, further comprising:   The method of claim 8, wherein updating the prepulse calibration factor is based on the comparison exceeding a threshold.   Determining the power of the first pre-pulse is performed by subtracting the power due to the main pulse during the defined time period from the average power over the defined time period. The method of claim 6. An energy monitor configured to measure laser pulses having the same wavelength and occurring in a burst within a laser light source of a laser produced plasma (LPP) extreme ultraviolet (EUV) system over a defined time period A power meter configured to measure an average power of the laser pulse and a first voltage signal indicative of a temporal profile of the burst of the laser pulse over at least a portion of the defined time period. An energy monitor comprising a photoelectromagnetic (PEM) detector configured as follows:
A calibration module configured to determine a calibration factor based on the average power and the first voltage signal, wherein the calibration factor is the energy of the burst of the laser pulse determined from the average power And a calibration module that is a ratio of the integral of the first voltage signal;
Based on the calibration factor and a pulse integral of a second voltage signal provided by the PEM detector indicating a temporal profile of subsequent pulses, to determine the energy of the subsequent pulses of the series of laser pulses. A single pulse energy calculation (SPEC) module configured to:
A system comprising:   The recalibration module is further configured to calculate an energy of the second burst based on a third voltage signal indicative of a second temporal profile of a second burst. System.   The recalibration module compares the energy of the second burst with a second average power sensed by the power meter and updates the calibration factor based on the comparison. The system of claim 12, further configured to instruct the comparison module.   The system of claim 13, wherein the recalibration module is configured to instruct the calibration module to update the calibration factor if the comparison exceeds a threshold.   The system of claim 11, wherein the laser pulse is a main pulse.   The system of claim 11, wherein the laser pulse is a pre-pulse. Measuring laser pulses having the same wavelength and generated in a burst with an energy monitor in a laser source of a laser produced plasma (LPP) extreme ultraviolet (EUV) system over a defined time period Receiving a first power signal indicative of a temporal profile of the laser pulse sensed during at least a portion of the defined time period; Receiving from an electromagnetic (PEM) detector;
Determining a calibration factor based on the average power and the first voltage signal, the calibration factor comprising: an energy of the laser pulse determined from the average power and an integral of the first voltage signal; The ratio of
Determining the energy of the subsequent pulse of the series of laser pulses based on the calibration factor and an integration of a second voltage signal provided by the PEM detector that indicates a temporal profile of the pulse;
Including a method.   The method of claim 17, further comprising calculating an energy of the second burst based on a third voltage signal indicative of a temporal profile of the second burst. Comparing the energy of the second burst to a second average power sensed by the power meter;
Updating the calibration factor based on the comparison;
The method of claim 18, further comprising:   The method of claim 19, wherein updating the calibration factor is based on the comparison exceeding a threshold.   The method of claim 17, wherein the laser pulse is a main pulse. The method of claim 17, wherein the laser pulse is a prepulse.