Patent Description:
Typically, laser apparatuses generate UV beams by directing a source beam to a non-linear crystal. Within the crystal, the source beam is frequency converted to a higher frequency, producing a shorter wavelength beam. For example, a <NUM> beam can be directed to a non-linear crystal to generate a <NUM> beam (the second harmonic wavelength of the <NUM> beam). For example, a <NUM> beam and a <NUM> beam can generate a <NUM> beam (the third harmonic wavelength of the <NUM> beam). Sometimes multiple crystals are necessary to convert a source beam to a desired UV beam wavelength.

Many processes require very stable UV beam parameters. Semiconductor wafer inspection processes, for example, tolerate less than <NUM> percent beam parameter drift over time. With UV processing, the beam parameter can drift as the non-linear crystal degrades. UV beam parameter drift manifests as unstable beam quality (M<NUM>), unstable axial beam waist location (z<NUM>), and unstable beam waist diameter (2ω<NUM>) (among others).

When source beams are frequency converted using a non-linear crystal, UV beam parameters can degrade due to bulk or surface degradation of the crystal. Bulk degradation can result from photo-assisted modifications of the crystal along the beam path inside the crystal and from related "compaction" of optical material. Bulk degradation causes increased thermal dephasing, and wavefront distortion, related to increasing absorption of the source and UV beams. Surface degradation can result from photo-assisted deposition and decomposition of contaminants of the crystal environment or by gradual destruction of the crystal surface (leading to unwanted wave front distortions or diffraction effects).

Existing solutions to address UV beam degradation include shifting the source beam to a new spot on the crystal when beam parameters hit specification limits (see, e.g., <CIT>). This approach extends crystal life, but disadvantageously leads to step-wise changes in beam parameters after spot shifting, which can impact laser tool performance. This approach does not address unstable beam parameters.

Existing solutions also include continuous shifting of the non-linear crystal relative to the incident beam (see, e.g., <CIT>). This approach can further prolong crystal lifetime but does nothing to address unstable beam parameters.

US patent application with publication number <CIT> discloses a wavelength conversion light source apparatus, including a fundamental wave light source configured to emit a fundamental wave, a nonlinear crystal configured to convert a wavelength of the fundamental wave by being irradiated with the fundamental wave and making the fundamental wave pass therethrough, and a movement unit configured to place the nonlinear crystal thereon and continuously move the nonlinear crystal within a plane, where a phase matching condition is not violated, so that a passage path of the fundamental wave passing through the nonlinear crystal is changed.

US patent application with <CIT> discloses a wavelength conversion apparatus, including a wavelength conversion section including a nonlinear optical crystal, and performing wavelength conversion of incident laser light by allowing the incident laser light to pass through the nonlinear optical crystal; and a relative position control section which, when wavelength conversion is performed by the wavelength conversion section, relatively displaces the incident position of the incident laser light.

Japanese patent application with <CIT> discloses a method for operating a laser light source which monitors damage of a nonlinear crystal used for the laser light source with high accuracy to stably supply a laser beam. The method includes a nonlinear crystal as a wavelength conversion element, and a wavelength converter having a heater which holds the nonlinear crystal at fixed temperature. The method further includes measuring the supply voltage to the heater and determining a damage state of the nonlinear crystal by comparing to a threshold of the supply power.

Although existing solutions prolong crystal lifetime by reducing degradation rates of the crystal, those solutions do nothing to stabilize beam parameters.

The scope of the invention is defined by the independent claims, preferred embodiments are defined by the dependent claims.

This disclosure provides methods and apparatuses which advantageously prolong the crystal lifetime and stabilize beam parameters. In one aspect, a UV laser apparatus includes a non-linear crystal, a laser source, a beam-crystal displacer, a beam parameter monitor, and a laser control unit. The laser source directs a source beam to the non-linear crystal to produce a UV beam and the beam-crystal displacer shifts the non-linear crystal relative to the source beam at a plurality of shift speeds. The beam parameter monitor measures the UV beam and outputs a measurement of a beam parameter. The laser control unit: receives the measurement; determines, based on the measurement, an adjustment in shift speed that steers the beam parameter toward a target value; and outputs the adjustment to the beam-crystal displacer.

In another aspect, a method for controlling a UV beam includes directing a source beam to a non-linear crystal to produce a UV beam; shifting the non-linear crystal relative to the source beam at a first shift speed; measuring a beam parameter of the UV beam; determining, based on the measured beam parameter, a second shift speed that steers the beam parameter toward a target value; and shifting the non-linear crystal relative to the source beam at the second shift speed.

The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate preferred embodiments, and together with the general description given above and the detailed description of the embodiments given below, serve to explain principles of the inventions.

Methods and apparatuses described herein vary a lateral shift speed of a non-linear crystal to steer a UV beam parameter toward a target value. Embodiments described herein are especially suitable for crystals that degrade during UV exposure and then show partial or total recovery without UV exposure. Some embodiments avoid large drift of beam parameters or power loss even at very high UV power. Further advantages may include a smaller initial beam parameter drift after start or restart of the laser, reduced or eliminated beam parameter changes after spot shifts, and very slow long term degradation of the crystal (resulting in much longer crystal life).

Turning now to the drawings, where like features are designated by like reference numerals, <FIG> depicts an embodiment of a UV laser apparatus <NUM>. Laser apparatus <NUM> includes a laser source <NUM>, a non-linear crystal <NUM>, a beam-crystal displacer <NUM>, a beam parameter monitor <NUM>, and a laser control unit <NUM>. Apparatus <NUM> may vary the shift speed of non-linear crystal <NUM> to steer a UV beam parameter toward a target value.

Laser source <NUM> directs source beam <NUM> to non-linear crystal <NUM> to produce UV beam <NUM>. Source beam <NUM> can be continuous or pulsed. Typically, source beam <NUM> is focused in the non-linear crystal <NUM>. In some embodiments, source beam <NUM> includes a spherical or an elliptical cross-section. A typical spherical beam diameter is <NUM> and a typical elliptical beam is <NUM> x <NUM>. In some embodiments, the focused beam diameter is between <NUM> and <NUM>; smaller beam waists may be used for focusing in short crystals and larger beam waists may be used for ultrashort pulsed source laser beams.

In some embodiments, non-linear crystal <NUM> degrades during an exposure to the source beam and at least partially recovers after the exposure to the source beam. Where a non-linear crystal degrades during an exposure to the source beam and at least partially recovers after the exposure to the source beam, there will be a recovery time characteristic that will differ from material to material. Without being limited by a theory, the recovery may be an Arrhenius process that is thermally driven and has a characteristic recovery time. Non-linear crystal <NUM> can be any material sufficient to convert the frequency of source beam <NUM>. Examples for non-linear crystal <NUM> include (but are not limited to) borates, such as cesium lithium borate (CLBO), lithium triborate (LBO), and beta barium borate (BBO). Exemplary wavelengths for UV beam <NUM> include <NUM>, <NUM>, <NUM>, <NUM>, and less than <NUM>. Other wavelengths are contemplated.

Beam-crystal displacer <NUM> shifts non-linear crystal <NUM> relative to source beam <NUM>. In some embodiments, beam-crystal displacer <NUM> is a crystal-shifter having one or more translation stages. In some embodiments, a beam-crystal displacer translates the beam in space while the crystal is stationary.

Beam-crystal displacer <NUM> is able to shift the non-linear crystal <NUM> in at least one direction and at different speeds. In some embodiments, beam-crystal displacer <NUM> shifts a crystal in two directions at different speeds. In some embodiments, beam-crystal displacer <NUM> shifts non-linear crystal continuously. As used herein, continuous shifting includes approximately continuous shifting. For example, beam-crystal displacer <NUM> may be powered such that the crystal is moved in increments with a pause between movements. In some embodiments, beam-crystal displacer <NUM> shifts the crystal step-wise. For example, the crystal is moved to a spot, the crystal pauses at that spot for irradiation, and then moves to the next spot. In some embodiments, the beam spot on non-linear crystal <NUM> may overlap by <NUM>-<NUM>% from spot to spot.

In some embodiments, the source beam <NUM> irradiates a spot for <NUM> seconds to <NUM> minutes and leaves <NUM> seconds to <NUM> hours between irradiations of that spot. In some embodiments, the shift speed is non-zero and preferably varies from <NUM>/s to <NUM>/s and more preferably from <NUM>/s to <NUM>/s. In some embodiments, the cycle time (time between consecutive radiations of the same spot) is preferably <NUM> to <NUM>,<NUM> hours and more preferably <NUM> mins to <NUM>,<NUM> hours. In some embodiments, a source beam traces a closed path of preferably <NUM> to <NUM> in length and more preferably <NUM> to <NUM> in length. One of skill in the art will understand that these values are exemplary and other embodiments may use higher or lower values.

Beam parameter monitor <NUM> measures UV beam <NUM> and outputs (depicted in <FIG> by output connection 108o) a measurement of the beam parameter to laser control unit <NUM>. In some embodiments, the beam parameter is a size or propagation parameter, such as one of beam quality, axial beam waist location, beam divergence, and beam waist diameter. As used herein, a "size or propagation parameter" does not include beam power or beam intensity.

Laser apparatus <NUM> optionally includes a beam splitter <NUM> to divert a portion (laser beam <NUM>) of the UV beam <NUM> to beam parameter monitor <NUM>. In some embodiments, beam parameter monitor <NUM> includes a beam aperture in auxiliary beam <NUM> and a photodiode or some imaging system. The beam parameter monitor <NUM> includes a lens, an aperture, an optional diffuser, and a photo diode. The lens focuses the beam to form a beam caustic. The aperture is located at an appropriate location along the beam caustic; for example, at the beam waist location. The aperture allows transmission of a selected portion of the beam cross section. The optional diffuser is located in the transmitted beam. The photo diode is exposed to the transmitted beam that is optionally diffused. This arrangement can measure changes in the beam caustic, which correlate to a beam parameter change, by evaluating the photo diode signal. This embodiment can be implemented in laser apparatus <NUM> or in a process tool. In some embodiments, a one- or two-dimensional CCD device with corresponding evaluation electronics/software can be applied in lieu of the photo diode. In some embodiments, a portion of the beam is picked off to measure beam parameters (for example, with a commercial beam analyzer).

In some examples useful for the understanding of the invention, beam parameter monitor <NUM> monitors a process property resulting from the UV beam's interaction with a process object. The process property might be a parameter of an object that responds to beam interaction. This arrangement could be utilized as part of a semi inspection tool. These tools typically record stray light, generated by a laser beam focus scanning a patterned or unpatterned wafer or mask. Optics, electronics, and software collect and evaluate the stray light to derive a number of parameters from the complex stray light data. These parameters include data that characterizes the focusing conditions and can be used to derive a figure of merit which contains information about M<NUM>, lateral focus position, or beam focus diameter, as some examples. This figure of merit can be compared to a target value and fed to the laser control unit <NUM> which then determines if and how much the scanning speed needs to be accelerated or decelerated in a closed servo loop.

In some embodiments, beam parameter monitor <NUM> continuously measures UV beam <NUM>. It should be appreciated that continuous measurement includes approximately continuous measurement. Herein, "continuous measurement" means measurement at the maximum duty cycle of the beam parameter monitor.

In some embodiments, beam-crystal displacer <NUM> shifts non-linear crystal <NUM> so that the source beam <NUM> repeatedly traverses a path on the crystal and beam parameter monitor <NUM> measures the UV beam once during each completion of the path. In some embodiments, beam-crystal displacer <NUM> shifts non-linear crystal <NUM> so that the source beam <NUM> repeatedly traverses a path on the crystal and beam crystal displacer adjusts the shift speed at least once while traversing the complete path.

In some embodiments, the beam-crystal displacer shifts the non-linear crystal so that the source beam repeatedly traverses a path and the beam parameter monitor measures the UV beam at a refurbishment interval. Some embodiments include use of a commercial beam analyzer, for example a NanoModeScan from MKS Instruments Inc. of Andover MA, a BeamSquared from MKS Instruments Inc. , of Andover MA, or a ModeMaster from Coherent Inc. of Santa Clara CA. Exemplary beam parameter monitors are disclosed in <CIT> and <CIT>.

Measurement can be performed, for example, at monthly intervals to correct the shifting speed in order to recover the beam parameters to the set point.

Laser control unit <NUM> receives the beam parameter measurement, uses the measurement to determine an adjustment in shift speed that steers the beam parameter toward a target value, and then outputs (depicted by output connection 110o) the adjustment to the beam-crystal displacer <NUM>. In some embodiments, the laser control unit <NUM> compares the beam parameter to a target value and calculates the shifting speed of beam-crystal displacer <NUM> to steer the beam parameter toward the target value. For example, when laser control unit <NUM> determines the beam parameter is above the value, laser control unit <NUM> outputs an increase in shift speed to beam-crystal displacer <NUM>; when laser control unit <NUM> determines the beam parameter is below the value, laser control unit <NUM> outputs a decrease in shift speed to beam-crystal displacer <NUM>. In other embodiments, when laser control unit <NUM> determines the beam parameter is above the value, laser control unit <NUM> outputs a decrease in shift speed to beam-crystal displacer <NUM>; when laser control unit <NUM> determines the beam parameter is below the value, laser control unit <NUM> outputs an increase in shift speed to beam-crystal displacer <NUM>.

In some embodiments, laser control unit <NUM> maintains the beam parameter within a target range. To that end, laser control unit <NUM> may accelerate and retard the shift speed to keep the beam parameter within the target range. For example, if the beam parameter measurement is above the target range (in other words, above an upper threshold of the target range), then laser control unit <NUM> sends a signal to increase the shifting speed (the signal provides, e.g., an incremental increase in shift speed or a set-speed above the current shifting speed); if the beam parameter measurement is below the target range (in other words, below a lower threshold of the target range), then laser control unit <NUM> sends a signal to decrease the shifting speed (the signal provides, e.g., an incremental decrease in shift speed or a set-speed below the current shifting speed).

In some embodiments, laser control unit <NUM> outputs the adjustment in shift speed as a command signal to the beam-crystal displacer <NUM>. In some embodiments, beam parameter monitor <NUM>, laser control unit <NUM>, and beam-crystal displacer <NUM> act together as a servomechanism to achieve the target range.

In some embodiments, the laser control unit sets an initial shifting speed that is slow compared to an expected speed to achieve the target beam parameter. An initial shifting speed can be understood to be a speed of the beam-crystal displacer immediately after the laser is first turned on or a speed of the beam-crystal displacer when the laser is turned on after a prolonged period of off time. The prolonged period may be comparable to or longer than the recovery time of the crystal. In some embodiments, once the beam parameter has settled (after an initial period of beam parameter drift), laser control unit <NUM> may operate as described above: receive the beam parameter measurement, use the measurement to determine an adjustment in shift speed that steers the beam parameter toward a target value, and then output the determined adjustment in shift speed to the beam-crystal displacer <NUM>. The frequency of measurement may be set before-hand, as described elsewhere herein. In some embodiments, the frequency of beam measurement may be dynamic: if the beam parameter measurements show little variation over time, the system may extend the intervals between measurements; if the beam parameter measurements show non-trivial variations in measurements (such as a rapid drop of beam parameter in a given interval), the system may shorten the intervals between measurements. This may advantageously reduce expenditure of system resources (e.g., frequency of beam parameter measurement) for crystals that show little degradation, while focusing such resources on crystals that do show degradation.

In some embodiments, laser control unit <NUM> continuously outputs an adjustment to the beam-crystal displacer <NUM>. In some such embodiments, the continuously output adjustments result from beam parameter monitor <NUM> continuously measuring UV beam <NUM>.

<FIG> depicts an embodiment of a UV laser apparatus <NUM>. Laser apparatus <NUM> includes a laser source <NUM>, non-linear crystals 204a and 204b , a beam parameter monitor <NUM>, a laser control unit <NUM>, beam splitters <NUM> and <NUM>, and power monitor <NUM>. Like features in apparatus <NUM> are designated by like reference numerals as apparatus <NUM> and, for the sake of brevity, are not redescribed here with respect to <FIG>.

Laser source <NUM> includes a laser diode 202a and a laser resonator 202b. Laser diode 202a optically energizes a gain medium in laser resonator 202b. Laser source <NUM> directs source beam <NUM> to non-linear crystals <NUM>.

Laser apparatus <NUM> includes multiple non-linear crystals. The first non-linear crystal 204a is a second harmonic generator (SHG); non-linear crystal 204a takes a <NUM> source beam and converts to a <NUM> beam. The second non- linear crystal 204b is a fourth harmonic generator (FHG); non-linear crystal 204b takes a <NUM> beam and outputs a <NUM> UV laser beam (<NUM>).

<FIG> depicts beam crystal displacer <NUM> moving non-linear crystal 204b. In some embodiments, beam crystal displacer <NUM> is a translation stage. In some embodiments, a beam-crystal displacer shifts multiple non-linear crystals, for example both crystals 204a and 204b. In some embodiments, individual beam-crystal displacers shift each non-linear crystal so that the crystals are shifted independently. This may be advantageous for crystals that degrade at different rates or have different occlusion spots (i.e., spots to be avoided by the source beam).

Laser apparatus <NUM> includes beam parameter monitor <NUM>. Beam splitter <NUM> diverts a portion <NUM> of UV beam <NUM> to the beam parameter monitor <NUM>. The beam parameter monitor <NUM> takes a measurement of a beam parameter (e.g., beam quality) and outputs (via output 208o) the measurement to the laser control unit <NUM>.

Laser control unit <NUM> receives (via output 208o) the beam parameter measurement from the beam parameter monitor <NUM>, uses the measurement to determine an adjustment in shift speed that steers the beam parameter toward a target value, and then outputs (depicted by output connection 210o) the determined adjustment in shift speed to the beam crystal displacer <NUM>. The laser control unit <NUM> also controls the oven temperatures for crystals 204a and 204b to maximize nonlinear conversion efficiency. Laser control unit <NUM> receives a temperature(s) from a thermocouple(s) and adjusts the temperature(s) of the oven(s) accordingly.

Laser apparatus <NUM> is also depicted in a light loop mode by use of UV power monitor <NUM> and laser control unit <NUM>. A beam splitter <NUM> diverts a portion <NUM> to power monitor <NUM> which makes a measurement of the beam power. This measurement is then output to laser control unit <NUM>, which regulates the laser diode 202a current thereby controlling the optical power directed into resonator 202b for energizing the gain medium.

<FIG> depicts an exemplary path <NUM> traversed by a laser beam on non-linear crystal <NUM>. Beam spot <NUM> traverses a rectangular path, including a first horizontal section <NUM>, a first vertical section <NUM>, a second horizontal section <NUM>, and second vertical section <NUM>. In embodiment <NUM>, the vertical sections are <NUM> and the horizontal sections are <NUM>. Note: the beam spot is not drawn to scale.

In some embodiments, a non-linear crystal exhibits both permanent and temporary degradation. In such crystals, when a laser beam traverses a complete path and restarts on that same path, some of the crystal has permanently degraded and the remainder has recovered. In some embodiments, a shifting speed may be adjusted as a path is completed to account for the permanent degradation of the crystal. In further embodiments, the shifting speed is adjusted after the path is first completed to account for permanent degradation of the crystal.

In some embodiments, the beam-crystal displacer shifts a non-linear crystal so that the source beam repeatedly traverses a path (for example, the path depicted in <FIG>) and a time between irradiations of a spot on the path is at least <NUM> times longer than an irradiation time of the spot.

In some embodiments, the beam-crystal displacer shifts the non-linear crystal so that the source beam traverses a limited, smaller area on the crystal. Here, a "limited smaller area" means an area that is relatively small compared to the overall cross-sectional area of the non-linear crystal. Preferably, an area that is less than <NUM>% of the overall cross-sectional area, more preferably, less than <NUM>% of the overall cross-sectional area. This may advantageously reduce large beam parameter fluctuations.

<FIG> depicts an exemplary path <NUM> traversed by a laser beam on non-linear crystal <NUM>. Path <NUM> includes linear sections <NUM>, but instead of the rectangular path in embodiment <NUM>, laser beam spot <NUM> traverses a path <NUM> that covers more of the non-linear crystal <NUM>. When the beam spot <NUM> covers more of the non-linear crystal, the laser can advantageously operate at higher output power and faster shifting speed with the same stable beam parameters. In some embodiments, the shifting speed can be around <NUM>/min. Further, the longer length of path <NUM> may be helpful to avoid scanning of the total closed path in a short period of time, in order to allow a recovery time following exposure to the UV beam.

<FIG> depicts an exemplary path <NUM> traversed by a laser beam (not shown) on non-linear crystal <NUM>. Path <NUM> includes linear sections <NUM>, but traverses a serpentine path. Path <NUM> covers more of the non-linear crystal <NUM> than path <NUM>. In some embodiments, a beam-crystal displacer shifts the non-linear crystal so that the source beam traverses the serpentine path. Path <NUM> may be useful as it avoids large straight portions of path which can cause larger beam parameter drifts. For example, given a path of <NUM> and shifting speed of <NUM>/s, a laser beam will take approximately <NUM> hours to complete the path.

In some embodiments, a path is chosen to avoid occluded or depleted areas of a non-linear crystal.

<FIG> is a graph <NUM> of beam waist diameter versus time, in accordance with an embodiment. Graph <NUM> represents data taken from a UV beam while directing a <NUM> source beam along path <NUM> in <FIG>. The source beam had a fundamental wavelength of <NUM> and power of 15W and was directed to a CLBO NLO crystal to produce a UV beam (SHG) of wavelength <NUM> and power of 3W. The waist of the focused <NUM> source beam, located within the non-linear laser crystal, had an elliptical cross section of <NUM> x <NUM>. The closed path length in embodiment <NUM> is <NUM> and UV beam waist diameter (2ω<NUM>) was measured for a plurality of shift speeds, <NUM>/hr (identified as <NUM> in graph <NUM>), <NUM>/hr (identified as <NUM> in graph <NUM>), and <NUM>/hr (identified as <NUM> in graph <NUM>). The cycle time for these speeds was <NUM>, <NUM>, and <NUM> respectively.

As is known, the beam inside a non-linear crystal is typically a focused beam; the beam is focused to achieve the required intensity for non-linear conversion. Typically, an input beam is focused into a non-linear crystal by a lens and an output beam is collimated by another lens. In Graph <NUM>, the beam waist diameter on the y-axis is a collimated output beam waist diameter.

Graph <NUM> plots the measurements of UV beam waist diameter against the run time for the plurality of shift speeds. Unexpectedly, the inventors discovered that varying shift speed affected certain beam parameters. As can be seen in Graph <NUM>: at <NUM>/hr (identified as <NUM> in graph <NUM>) the beam waist diameter is approximately <NUM>; at <NUM>/hr (identified as <NUM> in graph <NUM>) the beam waist diameter drops to approximately <NUM>; and at <NUM>/hr (identified as <NUM> in graph <NUM>) the beam waist diameter drops further to approximately <NUM>.

As disclosed herein, shifting speeds can affect beam parameters and thus a non-linear crystal's shifting speed can be utilized to control beam parameters. For example, a shifting speed can be changed to steer the beam parameter toward a target value. In some embodiments, the target value is a range and, for example, as a beam parameter degrades below a setpoint (e.g., a lower threshold of the range), the shifting speed can be decelerated to raise the beam parameter above the setpoint. Similarly, if a beam parameter is above a setpoint (e.g., an upper threshold of the range), the shifting speed can be accelerated to raise the beam parameter above the setpoint. These shifts can solely control the beam parameter or can be used in conjunction with active optics that adjust the beam parameter. As described here, shift speed can be used to stabilize the beam parameter within a feedback loop with the shifting speed as the actuating parameter to steer the actual beam parameter toward a target value.

In some embodiments, a target range is a percentage deviation from a target value. For example, a target beam waist diameter and percentage deviation might be pre-set; a measured beam waist diameter that is greater than or less than the target beam waist diameter by the percentage deviation causes an adjustment in the shift speed. A measured beam waist diameter that is within the target beam waist diameter by the percentage deviation does not cause an adjustment in shift speed. The percentage is preferably <NUM>%, more preferably <NUM>%, and most preferably <NUM>%.

As can be seen in graph <NUM>, a small initial drift (<NUM>% or less) occurs after the source beam first begins to irradiate the non-linear crystal. To accommodate this initial drift and reduce its effect on tool performance, the shifting speed can initially be relatively slow until the parameter settles to a stable level. A relatively slow speed may advantageously reduce the difference between the initial beam parameter measurement and the stable beam parameter measurement. Once the beam parameter has stabilized, a laser control unit may shift speed to achieve the desired target value. This may have the result that the shift speed starts at a first speed, the speed is increased as the beam stabilizes after an initial drift, and then the speed is changed (increased or decreased) as to steer the beam toward a target value.

Graph <NUM> depicts noise in the measurement of the beam waist diameter. The noise may represent the active control of the shifting speed, up and down as the servo vacillates around the target beam waist diameter.

<FIG> is a graph <NUM> of irradiation versus time, in accordance with an embodiment. Graph <NUM> depicts an irradiation time (t1) and a recovery time (t2) between irradiations. For example, graph <NUM> may depict the irradiation time and recovery time of a single spot on path <NUM> in <FIG>.

<FIG> is a flow chart of an embodiment of a method <NUM> for controlling a UV beam. Method <NUM> includes directing a source beam to a non-linear crystal to produce a UV beam <NUM>, shifting the non-linear crystal relative to the source beam at a first shift speed <NUM>, measuring a beam parameter of the UV beam <NUM>, determining, based on the measured beam parameter, a second shift speed that steers the beam parameter toward a target value <NUM>; and shifting the non-linear crystal relative to the source beam at the second shift speed <NUM>. In some embodiments, steps <NUM>-<NUM> are optionally repeated (as indicated by the dashed arrow in <FIG>).

In some embodiments, the non-linear crystal degrades during an exposure to the source beam and at least partially recovers after the exposure to the source beam. In some embodiments, shifting the non-linear crystal comprises shifting the non-linear crystal so that the source beam repeatedly traverses a path on the non-linear crystal, wherein a time between irradiations of a spot on the path is preferably at least <NUM> times longer than an irradiation time of the spot.

In some embodiments, the method includes determining whether the measured beam parameter is above the value and, in response to determining the measured beam parameter is above the value, determining a second shift speed faster than the first shift speed.

In some embodiments, the method includes determining whether the measured beam parameter is below the value and, in response to determining the measured beam parameter is below the value, determining a second shift speed slower than the first shift speed.

In some embodiments, the target value of the beam parameter comprises a target range of the beam parameter with an upper threshold and a lower threshold. In embodiments where the target value of the beam parameter comprises a target range, the method may further comprise determining whether the measured beam parameter is above the upper threshold, and in response to determining the measured beam parameter is above the upper threshold, determining a second shift speed faster than the first shift speed. In embodiments where the target value of the beam parameter comprises a target range, the method may further comprise determining whether the measured beam parameter is below the lower threshold, and, in response to determining the measured beam parameter is below the lower threshold, determining a second shift speed slower than the first shift speed.

In some embodiments, shifting the non-linear crystal comprises shifting the non-linear crystal so that the source beam traverses multiple serpentine paths on the non-linear crystal.

In some embodiments, shifting the non-linear crystal comprises shifting the non-linear crystal so that the source beam traverses a limited, smaller area on the crystal. In some embodiments, shifting the non-linear crystal comprises shifting multiple non-linear crystals.

In some embodiments, measuring the beam parameter comprises measuring the UV beam continuously. In some embodiments, the method further includes continuously determining, based on the continuous measurements, additional shift speeds that steer the beam parameter toward the target value, and shifting the non-linear crystal relative to the source beam at the additional shift speeds. In some embodiments, shifting the non-linear crystal comprises shifting the non-linear crystal so that the source beam repeatedly traverses a path on the non-linear crystal, and wherein the shift speed is adjusted at least once while traversing the complete path. For example, once during each completion of the path. In some embodiments, the beam parameter is measured and the speed is adjusted at a refurbishment interval.

In some embodiments, the beam parameter is one of beam quality, axial beam waist location, beam divergence, and beam waist diameter.

In some examples useful for the understanding of the invention, measuring the beam parameter of the UV beam comprises measuring a process property resulting from the UV beam's interaction with a process object.

Claim 1:
A UV laser apparatus (<NUM>) comprising:
a non-linear crystal (<NUM>);
a laser source (<NUM>) configured to direct a source beam (<NUM>) to the non-linear crystal (<NUM>) to produce a UV beam (<NUM>);
a beam-crystal displacer (<NUM>) configured to shift the non-linear crystal (<NUM>) relative to the source beam (<NUM>) at a plurality of shift speeds;
a beam parameter monitor (<NUM>) configured to measure the UV beam (<NUM>) the beam parameter monitor including a lens, an aperture, and a photo diode, the lens is configured to focus the beam to form a beam caustic and the aperture is located along the beam caustic, for example, at a beam waist location, the photo diode being exposed to a selected portion of the beam after passing through the lens and the aperture, with the output of the photo diode measuring changes that correlate to a beam parameter change; and
a laser control unit (<NUM>) configured to
receive the measurement,
determine, based on the measurement, an adjustment in shift speed to steer the monitored parameter toward a target value; and
output the adjustment to the beam-crystal displacer (<NUM>).