Energy sensors for light beam alignment

An apparatus includes a drive laser system producing an amplified light beam of pulses that travels along a drive axis; a beam delivery system that directs the amplified light beam of pulses toward a target region; a target material delivery system that provides a target mixture containing a target material in the target region; two or more sensors radially separated from a main axis that crosses the target region, the two or more sensors being configured to detect energy of ultraviolet electromagnetic radiation emitted from a plasma state of the target material when the amplified light beam of pulses intersects the target mixture; and a controller that receives the output from the two or more sensors. The controller is configured to estimate a relative radial alignment between the target mixture and the drive axis within the target region based on an analysis of the detected energy.

TECHNICAL FIELD

The disclosed subject matter relates to an apparatus for aligning an amplified light beam from a drive laser system relative to a target material at a target region within an extreme ultraviolet light source.

BACKGROUND

Extreme ultraviolet (“EUV”) light is electromagnetic radiation having wavelengths of around 50 nm or less and is also sometimes referred to as soft x-rays. EUV light can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, for example, xenon, lithium, or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example, in the form of a droplet, stream, or cluster of material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

In some general aspects, a position of an amplified light beam of pulses is adjusted relative to a target material of a target mixture by directing the amplified light beam of pulses along a drive axis toward a target region in which the target mixture is located to thereby convert at least a portion of the target material within the target mixture into a plasma state that emits ultraviolet electromagnetic radiation; detecting the energy of the emitted electromagnetic radiation at two or more locations radially separated from a main axis that crosses the target region; analyzing the detected energy; estimating a relative radial alignment between the target mixture and the drive axis of the amplified light beam within the target region based on the analyzed detected energy; and adjusting a radial alignment of the amplified light beam relative to the target mixture in the target region to thereby adjust the relative radial distance between the target mixture and the drive axis within the target region.

Implementations can include one or more of the following features. For example, the energy of the emitted ultraviolet electromagnetic radiation can be detected by measuring the energy of extreme ultraviolet electromagnetic radiation. The energy of the emitted ultraviolet electromagnetic radiation can be detected by measuring the energy of deep ultraviolet electromagnetic radiation. The emitted ultraviolet electromagnetic radiation can be extreme ultraviolet (EUV) electromagnetic radiation.

The relative radial alignment between the target mixture and the drive axis can be estimated by estimating a radial alignment between the target mixture and the drive axis within the target region.

The radial alignment of the amplified light beam can be adjusted relative to the target mixture by adjusting one or more of a position and an angle of one or more optical elements that steer and move the amplified light beam toward the target mixture within the target region. The one or more of the position and the angle of the one or more optical elements that steer and move the amplified light beam can be adjusted by adjusting one or more of the position and the angle of a curved mirror that redirects the amplified light beam toward the target region.

The energy of the emitted electromagnetic radiation at two or more locations radially separated from the main axis can be detected by measuring the energy of the emitted electromagnetic radiation at four locations radially separated from the main axis.

The method also includes capturing an optical image of a laser beam reflected from the target mixture back toward a drive laser system that supplies the amplified light beam. The relative radial alignment between the target mixture and the drive axis of the amplified light beam within the target region can be estimated at least in part by analyzing the captured image.

The energy of the emitted electromagnetic radiation at two or more locations can be detected by measuring the energy at a rate that is on the order of a pulse repetition rate of the amplified light beam.

The radial alignment of the amplified light beam can be adjusted relative to the target mixture in the target region to thereby reduce the relative radial distance between the target mixture and the drive axis within the target region.

The detected energy can be analyzed by determining a value of a difference between a first total energy of a first set of energies taken at first one or more locations and a second total energy of a second set of energies taken at the second one or more locations, the first one or more locations being distinct from the second one or more locations. The first total energy can be a sum of energies taken at the first one or more locations and the second total energy can be a sum of energies taken at the second one or more locations.

The detected energy can be analyzed by normalizing the difference value by a total energy of all of the energies taken at all of the two or more locations.

The relative radial alignment can be estimated by estimating a radial distance, taken along a first direction that is perpendicular to the main axis, between the target mixture and the drive axis of the amplified light beam within the target region. The relative radial alignment can be estimated by estimating a radial distance, taken along a second direction that is perpendicular to the first direction and to the main axis, between the target mixture and the drive axis of the amplified light beam within the target region.

In another general aspect, an apparatus includes a drive laser system producing an amplified light beam of pulses that travels along a drive axis; a beam delivery system that directs the amplified light beam of pulses toward a target region; a target material delivery system that provides a target mixture containing a target material in the target region; two or more sensors radially separated from a main axis that crosses the target region, the two or more sensors being configured to detect energy of ultraviolet electromagnetic radiation emitted from a plasma state of the target material when the amplified light beam of pulses intersects the target mixture; and a controller that receives the output from the two or more sensors, is configured to analyze the detected energy and estimate a relative radial alignment between the target mixture and the drive axis within the target region based on the analysis, and to output a signal to the beam delivery system to adjust a radial alignment of the amplified light beam relative to the target mixture in the target region to thereby adjust the relative radial distance between the target mixture and the drive axis within the target region.

Implementations can include one or more of the following features. For example, the drive laser system can include one or more optical amplifiers each including a gain medium capable of optically amplifying a desired wavelength at a high gain, an excitation source, and internal optics. The gain medium can include CO2.

The beam delivery system can include a focusing optical element that focuses the amplified light beam to the target region. The target material delivery system can include a nozzle that provides fluid droplets of the target mixture in the target region.

The apparatus can also include a radiation collector that captures and redirects at least a portion of the ultraviolet electromagnetic radiation emitted from the plasma state of the target material when the amplified light beam of pulses intersects the target mixture.

The emitted ultraviolet electromagnetic radiation can include extreme ultraviolet electromagnetic radiation.

The two or more sensors can include at least four sensors that are radially separated from the main axis. Thus, the four sensors can be angularly positioned about the main axis.

At least one of the two or more sensors can be radially separated from the main axis by a distance that is different from a distance that radially separates at least one of the other sensors. All of the two or more sensors can be radially separated from the main axis by the same distance; and thus they can be equidistant from the main axis.

The apparatus can include an imaging device configured to capture an optical image of a laser beam reflected from the target mixture back toward the drive laser system. The controller can also receive the output from the imaging device and can be configured to estimate the relative radial alignment based also on the received output from the imaging device.

The sampling rate of the two or more sensors can be on the order of a pulse repetition rate of the drive laser system.

In another general aspect, a metrology system includes two or more sensors radially separated from a main axis that crosses a target region, the two or more sensors being configured to detect energy of ultraviolet electromagnetic radiation emitted from a plasma state of a target material of a target mixture when an amplified light beam of pulses intersects the target mixture; and a controller that receives the output from the two or more sensors. The controller is configured to analyze the detected energy and estimate a relative radial alignment between the target mixture and the drive axis of the amplified light beam within the target region based on the analysis, and to output a signal to a beam delivery system to adjust a radial alignment of the amplified light beam relative to the target mixture in the target region to thereby adjust the relative radial distance between the target mixture and the drive axis within the target region.

Implementations can include one or more of the following features. For example, the two or more sensors can include at least four sensors that are radially separated from the main axis.

At least one of the two or more sensors can be radially separated from the main axis by a distance that is different from a distance that radially separates at least one of the other sensors.

The metrology system can include an imaging device configured to capture an optical image of a laser beam reflected from the target mixture back toward a drive laser system that produces the amplified light beam. The controller can also receive the output from the imaging device and is configured to estimate the relative radial alignment based also on the received output from the imaging device.

DESCRIPTION

Referring toFIG. 1, an LPP EUV light source100is formed by irradiating a target mixture114at a target region105with an amplified light beam110that travels along a drive axis toward the target mixture114. The drive axis of the amplified light beam110can be considered as the approximate center of the beam110or the general direction that the beam110is traveling because the beam110may be irregularly shaped and/or asymmetrical. The drive axis of the amplified light beam110can be considered the optical axis of the light beam110.

The target region105, which is also referred to as the irradiation site, is within an interior107of a vacuum chamber130. When the amplified light beam110strikes the target mixture114, a target material within the target mixture114is converted into a plasma state that has an element with an emission line in the EUV range. The target mixture114in the plasma state therefore emits EUV radiation, and the EUV radiation is harnessed by a collector mirror135, which can be configured to redirect the emitted EUV radiation toward an intermediate location145, which is also called an intermediate focus.

The created plasma has certain characteristics that depend on the composition of the target material within the target mixture114. These characteristics can include the wavelength of the EUV radiation produced by the plasma, and the type and amount of debris released from the plasma.

The light source100includes two or more sensors170radially separated from a main axis111that is parallel with the z direction of the page. The main axis111crosses the target region105and generally extends along a direction that extends from an aperture140of a collector mirror135toward the target region105. The radial direction is along the plane that is perpendicular to the main axis111in the area of the target region105. Thus, the radial direction extends along the plane defined by the x and y axes and the two or more sensors170are in this plane, which is perpendicular to the main axis111in the area of the target region105. The sensors170are positioned around the main axis111, but they can be at different distances from the main axis111, and they do not need to be equally spaced from each other.

The sensors170are configured to measure energy of the EUV radiation emitted from the plasma state of the target material when the amplified light beam110intersects the target mixture114. In this way, the sensors170are configured to sample differences in energy up and down and left and right around the light beam110to determine the positional relationship between the light beam110and the target region105.

The light source100also includes a master controller155that receives an output from the energy sensors170and performs an analysis based at least in part on this received output to determine the relative alignment between the drive axis of the amplified light beam110and the target mixture114.

Other features of the light source100will be described next before going into greater detail about the energy sensors170and the master controller155.

The light source100includes a target material delivery system125that delivers, controls, and directs the target mixture114in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture114includes the target material such as water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the target material can be tin, which can be pure tin (Sn); a tin compound such as SnBr4, SnBr2, or SnH4; a tin alloy such as a tin-gallium alloy, a tin-indium alloy, a tin-indium-gallium alloy, or any combination of these alloys. The target mixture114can also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture114is made up of only the target material. The target mixture114is delivered by the target material delivery system125into the interior107of the chamber130and to the target region105.

The light source100includes a drive laser system115that produces the amplified light beam110due to a population inversion within the gain medium or mediums of the laser system115. The light source100includes a beam delivery system between the laser system115and the target region105to direct the beam110from the laser system115to the target region105. The beam delivery system includes a beam transport system120and a focus assembly122. The beam transport system120receives the amplified light beam110from the laser system115, and steers and modifies the amplified light beam110as needed and outputs the amplified light beam110to the focus assembly122. The focus assembly122receives the amplified light beam110and focuses the beam110to the target region105. The focus assembly122can also steer the beam110or adjust a position of the beam110relative to the target region105.

In some implementations, the laser system115can include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre-pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system115produces an amplified light beam110due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system115can produce an amplified light beam110that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system115. The term “amplified light beam” encompasses one or more of: light from the laser system115that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system115that is amplified and is also a coherent laser oscillation (and can be referred to as a drive laser beam).

The optical amplifiers in the laser system115can include as a gain medium a filling gas that includes CO2and can amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 1000. Suitable amplifiers and lasers for use in the laser system115can include a pulsed laser device, for example, a pulsed, gas-discharge CO2laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, 10 kW or higher and high pulse repetition rate, for example, 50 kHz or more. The optical amplifiers in the laser system115can also include a cooling system such as water that can be used when operating the laser system115at higher powers.

The collector mirror135includes the aperture140to allow the amplified light beam110to pass through and reach the target region105. The collector mirror135can be, for example, an ellipsoidal mirror that has a primary focus at the target region105and a secondary focus at the intermediate location145(also called an intermediate focus) where the EUV light can be output from the light source100and can be input to, for example, an integrated circuit lithography tool (not shown).

The master controller155is also connected to a laser control system157and a beam control system158. The master controller155can therefore provide a laser position, direction, and timing correction signal to one or more of the laser control system157and the beam control system158. The laser control system157can use the correction signal to control the laser timing circuit. The beam control system158can use the correction signal to control an amplified light beam position and shaping of the beam transport system120to change the location and/or focal power of the beam focal spot within the chamber130.

The light source100can include one or more target or droplet imagers160that provide an output indicative of the position of a droplet, for example, relative to the target region105and provide this output to the master controller155, which can, for example, compute a droplet position and trajectory from which a droplet position error can be computed either on a droplet by droplet basis or on average.

The target material delivery system125includes a target material delivery control system126that is operable, in response to a signal from the master controller155, for example, to modify the release point of the droplets as released by a target material supply apparatus127to correct for errors in the droplets arriving at the desired target region105.

Additionally, the light source100can include one or more photo-detectors165that can be used to look at light reflected from the target mixture114within the target region105. The one or more photo-detectors165can be placed within the chamber130(as shown inFIG. 1) to detect light reflected from the target mixture114from a separate test laser (such as a He—Ne laser directed toward the target region105). In other implementations, the one or more photo-detectors165can be placed near the drive laser system115to detect the amplified light beam or a guide laser beam (from the guide laser175) that is back reflected from the target mixture114.

The light source100can also include a guide laser175that can be used to align various sections of the light source100or to assist in steering the amplified light beam110to the target region105. In connection with the guide laser175, the light source100includes a sampling apparatus124that is placed within the focus assembly122to sample a portion of light from the guide laser175and the amplified light beam110. In other implementations, the sampling apparatus124is placed within the beam transport system120. The sampling apparatus124can include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that can withstand the powers of the guide laser beam and the amplified light beam110. The sampling apparatus124can include an optical sensor that captures images of diagnostic portions of the sampled light, and the optical sensor can output an image signal that can be used by the master controller155for diagnostic purposes. An example of such a sampling apparatus124is found in U.S. Publication No. 2011/0141865, published on Jun. 16, 2011, which is incorporated herein by reference in its entirety.

A metrology system is formed at least in part from the energy sensors170and the master controller155. The metrology system can also include the sampling apparatus124, the target imagers160, and the one or more photo detectors165. The master controller155analyzes the output from the energy sensors170(and can also analyze the output from the target imagers160and the photo detectors165) and uses this information to adjust components within the focus assembly122or the beam transport system120through the beam control system158, as discussed further below.

Thus, in summary, the light source100produces an amplified light beam110that is directed along the drive axis to irradiate the target mixture114at the target region105to convert the target material within the mixture114into plasma that emits light in the EUV range. The amplified light beam110operates at a particular wavelength (that is also referred to as a source wavelength) that is determined based on the design and properties of the laser system115. Additionally, the amplified light beam110can be a laser beam when the target material provides enough feedback back into the laser system115to produce coherent laser light or if the drive laser system115includes suitable optical feedback to form a laser cavity.

Referring toFIG. 2, the light source100includes, in an exemplary implementation, a target region205, a collector mirror235, energy sensors270, and a target material supply apparatus227. In this implementation, the energy sensors270include four energy sensors271,272,273,274. The target material supply apparatus227can produce droplets of the target mixture214in the target region205at a rate of over 10 thousand droplets per second and the droplets of the target mixture214can be traveling at velocities of about 20 msec. The size of the droplets can be about or greater than about 10 μm in width. The collector mirror235includes an aperture240that permits an amplified light beam210from the laser system115to pass through the collector mirror235and intersect the target region205.

In this implementation, the energy sensors270are radially separated from the main axis211(which is parallel with the z direction) and are angularly arranged about the axis. That is, the energy sensors270can be placed in a plane that is perpendicular to the main axis211and placed angularly around the main axis211. Each of the energy sensors270(in particular, sensors271,272,273,274) can be positioned at a radial distance from the main axis211, and the radial distance of a particular sensor (for example, the sensor271) may be distinct from a radial distance of another sensor (for example, any of the sensors272,273,274) from the main axis211. Each energy sensor270can be any sensor that is able to observe and measure energy of electromagnetic radiation in the ultraviolet region. Thus, in some implementations the energy sensors270are photodiodes, and in other implementations, the energy sensors270are photomultiplier tubes.

Prior to use during EUV light production, the energy sensors270are calibrated with a known signal on the main axis211(that is, at the target region205) to determine the relative sensitivity of the energy sensors270. The calibration information is stored and used by the master controller155during the analysis. Because of the calibration, it is not necessary for the energy sensors270to be radially equidistant from the main axis211.

The amplified light beam210is guided toward the target region205to intersect the target material214within the target region205, and the light source100can produce enough EUV radiation if the intersection time and area overlap is great enough. For example, in some implementations, the time during which the amplified light beam210intersects a droplet of the target material214can be between about 1-10 μs. Generally, the drive axis212of the amplified light beam210should be within a certain radial distance from the target region205to produce effective amounts of EUV radiation at the target region205. But, there may be an acceptable range of radial distance within which the drive axis212can be positioned to produce the effective amount of EUV radiation. The light source100can be configured to aim the amplified light beam210toward the target region205. Ultimately though, the alignment of the drive axis212is determined by the master controller155to be that direction and angle of the drive axis212that produces at least a minimum amount of EUV radiation and this alignment may not coincide with the main axis211or a center of the target region205.

Referring toFIG. 3, a metrology system300is used to align the drive axis212relative to the target region205to produce an effective amount of EUV radiation. To this end, the metrology system300includes the energy sensors170(such as, for example, energy sensors270), the output of which are fed into an alignment control module305of the master controller155. The master controller155, in particular, the alignment control module305, performs a procedure, which is discussed below with respect toFIG. 4, to adjust one or more of a position or angle of the drive axis of the amplified light beam110relative to the target region105by sending a signal or signals to the beam control system158to adjust elements within one or more of the beam transport system120and the focus assembly122. The effective amount of EUV radiation can drop substantially for values of offset between the drive axis of the amplified light beam110and the target region205as small as 1 μm. Thus, the metrology system300can be used to make adjustments to the relative radial alignment on the order of 0.1 to 50 μm.

Though not required, the metrology system300can include other components for performing other functions. For example, the metrology system300includes the sampling apparatus124, which outputs an image signal that can be used by an overlap control module310of the master controller155to calculate features of the image signal and send a signal to the beam control system158to tune elements within one or more of the beam transport system120and the focus assembly122, as discussed in greater detail in U.S. Publication No. 2011/0141865.

As another example, the metrology system300includes a laser trigger control module315that receives and analyzes the output from the photo-detectors165and optionally the output from the energy sensors170, and determines how to adjust a timing of the firing of pulses of the amplified light beam110based on the analysis. The laser trigger control module315outputs a signal to the laser control system157, depending on the results of the analysis, to adjust the firing time and rate.

As a further example, the metrology system300includes a droplet position module320that computes a droplet position and trajectory from which a droplet position error can be computed either on a droplet by droplet basis or on average. The droplet position module320thus determines the droplet position error. The output of the module320can thus be fed into the target material delivery control system126, which can use the output to adjust a position or a direction of the target material114within the target region105, or to adjust a timing or rate of target material114output from the target material supply apparatus127. The output of the module320can also be fed into the beam control system158to tune or adjust elements within one or more of the beam transport system120and the focus assembly122, as needed.

Referring toFIG. 4, the metrology system300performs a procedure400for adjusting a radial alignment of the amplified light beam110relative to the target mixture114. After an initial set up of the light source100, the master controller155sends signals to the laser control system157and to the beam control system158to direct the amplified light beam110from the drive laser system115along a drive axis toward the target region105in which the target mixture114is located (step405). At least a portion of the target material within the target mixture114is converted into a plasma state that emits ultraviolet (for example, EUV) electromagnetic radiation.

Next, the energy sensors170detect the energy of the EUV electromagnetic radiation that is emitted from the plasma state of the target material114, and the master controller155receives the output (the sensed energy) from each of the energy sensors170(step410). The master controller155analyzes the sensed energy (step415). In the implementation shown inFIG. 2, the energy sensor271outputs the sensed energy E1, the energy sensor272outputs the sensed energy E2, the energy sensor273outputs the sensed energy E3, and the energy sensor274outputs the sensed energy E4to the master controller155. The master controller155estimates a relative radial alignment RA based on the analyzed sensed energy (step420). In one exemplary implementation, the master controller155estimates a relative radial alignment (RAy) in the y direction based on the following calculation:

Referring also toFIG. 6, an exemplary graph600shows the total energy Etot of the energies taken from all of the energy sensors, where Etot=E1+E2+E3+E4for the implementation shown inFIG. 2, as a function of a position of an element within the beam delivery system taken along the y direction.

Referring also toFIG. 7, an exemplary graph700shows the relative radial alignment RAy between the drive axis of the amplified light beam and the target region as a function of the position of an adjustable element within the beam delivery system taken along the y direction. Because the amplified light beam110interacts with the adjustable element within the beam delivery system, the adjustment of the element causes the amplified light beam to be moved transversely or angularly relative to the target region. The relative radial alignment RAy follows a path that passes through an inflection value705as the element is adjusted along the y direction. The inflection value705indicates the point at which the amplified light beam is generally equidistant in the y direction between the energy sensors271and272and between the energy sensors274and273. As the amplified light beam is offset in the y direction from the equidistant value, the relative radial alignment RAy follows a path away from the inflection value705.

Thus, the RAy signal can be used to determine the offset of the drive axis212of the amplified light beam210from the target region205(which can be represented by the main axis212). For example, as shown inFIG. 5A, the drive axis212is closer to energy sensors271and274, and thus, RAy is greater than the inflection value705, and therefore indicates that the energy signals El and E4from the energy sensors271,274, respectively, are greater than the energy signals E2and E3from the energy sensors272,273, respectively. As another example, as shown inFIG. 5B, the drive axis212is closer to energy sensors272and273, and thus, RAy is less than the inflection value705, and therefore indicates that the energy signals E2and E3are greater than the energy signals El and E4. Referring toFIG. 5C, the drive axis212is generally equidistant from the energy sensors273and274along the y direction and is generally equidistant from the energy sensors271and272along the y direction. Thus, RAy approaches the inflection value705.

If the energy sensors271,272,273,274were perfectly aligned with the y direction and calibrated so that a signal along the main axis211would provide equal energies in each of the energy sensors271,272,273,274, then the inflection value705of RAy would approach 0.

Next, the master controller155adjusts a direction of the amplified light beam relative to the target region105(step425). The master controller155does this by determining how to adjust the position of one or more elements in the beam delivery system to thereby adjust the position and/or angle of the amplified light beam110relative to the target region105. The master controller155then sends a signal to the beam control system158, which adjusts an actuator that is coupled to the one or more elements, which control a position and/or angle of the amplified light beam. In this way, the relative radial distance between the target mixture and the drive axis of the amplified light beam is adjusted. And, because of this, the total energy of the emitted EUV electromagnetic radiation output from the plasma states of the target material can be improved.

For example, the element within the beam delivery system, when adjusted along the y direction, changes a relative alignment between the drive axis212and the target region205(which is represented by the main axis211). The total energy Etot reaches a maximum value for a particular position605of the element. Thus, by adjusting the position of the element in the beam delivery system, relative radial distance between the target mixture and the drive axis of the amplified light beam is also adjusted to thereby increase the EUV radiation emitted by the target material in the plasma state to produce more EUV light from the light source100.

The element or elements that can be adjusted can be one or more of a final focus lens and a mirror within the focus assembly122. An example of such elements and their adjustment can be found in U.S. Publication No. 2011/0140008, published on Jun. 16, 2011, which is incorporated herein by reference in its entirety. In other implementations, the element that can be adjusted can be a final focus curved mirror within the focus assembly122. An example of such an element can be found in U.S. Publication No. 2006/0219957, published on Oct. 5, 2066, which is incorporated herein by reference in its entirety.

As another example, and with reference toFIG. 2, a curved mirror223can be adjusted by translating the mirror223along one or more of the y or x directions or by rotating the mirror223about the x or y directions.

The element that can be adjusted can be a mirror, a curved mirror, a lens, or any other component within the beam transport system120or the focus assembly122. Examples of such elements can be found in the beam transport system described in U.S. Publication No. 2011/0140008.

While the discussion above provides an example for adjustment along the y direction, the relative radial alignment can be adjusted along the x direction or along both the x and y directions. For example, the relative radial alignment RAx along the x direction can be given by the below exemplary equation:

Moreover, there may be other ways to calculate the relative radial alignment in the x or y directions than the ways noted above. The energy sensors271,272,273,274can be placed along different angular positions than the ones shown inFIG. 2, and are not limited to these angular positions. For example, the energy sensors271,272,273,274can be placed as shown inFIG. 8. As few as two energy sensors may be used if only the relative radial alignment along one direction need to be known.

The above described metrology system300enables higher sampling rates than metrology systems that use only optical data to determine alignment of the amplified light beam. For example, the metrology system300can be operated at a rate of one sample (in which the relative radial alignment RA is determined in a sample) per droplet of target mixture at the target region. Moreover, the range and sensitivity of the energy sensors170is greater than the range and sensitivity of prior optical detectors used for determining alignment.

By adjusting the relative radial alignment, the EUV production can be increased and the light source100can be operated with greater efficiency than in prior systems that lack the metrology system300that relies on energy sensors.