Apparatus and method for controlling the focus of a laser beam

Abstract

An apparatus, computer program, computer-readable medium, and method for controlling the focus of a laser beam during a microsweep are disclosed. A laser beam is received by an acousto-optic deflector, and acoustic waves are provided to the acousto-optic deflector. The acoustic waves vary in frequency over time to change the deflection angle of the laser beam over time and thereby achieve a microsweep of the laser beam. Furthermore, as the laser beam passes through the acousto-optic deflector, a rate of change in frequency of the acoustic waves is adapted over the time of the microsweep such that an adapted difference in frequency over the time of the microsweep of the acoustic waves is produced across the width of the laser beam in a direction parallel to the microsweep at the acousto-optic deflector, the adapted difference in frequency over the time of the microsweep being such that a desired focus of the laser beam occurs across the microsweep in a direction parallel to the microsweep.

Description

[Technical Field]

[0001] TECHNICAL FIELD This disclosure relates to acousto-optic deflectors, and more particularly to controlling the focus of a laser beam deflected by an acousto-optic deflector. [Background technology]

[0002] In pattern generation applications such as mask printing, acousto-optic deflectors are often used to deflect one or more light beams when printing a pattern on a photosensitive resist. An acousto-optic deflector includes a transparent material, such as quartz, to which acoustic waves are applied at a frequency that causes the material to compress and dent. When acoustic waves, whose wavelengths are significantly shorter than those of the light beam, pass through the material at an angle that depends on the frequency of the acoustic waves, the material acts as a diffraction grating, deflecting the light beam. Generally, the amount of deflection, or deflection angle, is proportional to the frequency of the acoustic waves. Therefore, by increasing the frequency of the acoustic waves over time, the deflection angle can be increased, thereby sweeping the light beam (referred to herein as a microsweep) over the portion of the photosensitive resist that is exposed according to the desired pattern. Summary of the Invention [Problem to be solved by the invention]

[0003] The inventors have recognized a problem with prior art pattern generators that use acousto-optic deflectors in that the focus of the laser beam in a direction parallel to the microsweep direction varies over the length of the microsweep such that for at least some parts of the microsweep the optical beam is not properly focused in the direction parallel to the microsweep. This results in errors in the printed pattern due to the modulated laser beam of the microsweep. Furthermore, the focus varies even more as the length of the microsweep increases. [Means for solving the problem]

[0004] According to a first aspect, there is provided a method for controlling a focus of a laser beam during a microsweep, the method comprising receiving a laser beam with an acousto-optic deflector and supplying acoustic waves to the acousto-optic deflector, the acoustic waves varying in frequency with time to change the deflection angle of the laser beam over time to achieve a microsweep of the laser beam. The rate of change of the frequency of the acoustic waves is adapted over the time of the microsweep such that as the laser beam passes through the acousto-optic deflector, an adapted difference in frequency over the time of the microsweep of the acoustic waves is produced across the width of the laser beam in a direction parallel to the microsweep, the adapted difference in frequency over the time of the microsweep being such that a desired focus of the laser beam is caused across the microsweep in a direction parallel to the microsweep.

[0005] Here, microsweep is used as a term to describe the sweep of a laser beam along the path of a surface to be exposed (e.g., bearing a photosensitive resist) as the deflection angle of the laser beam is varied by changing the frequency of the acoustic wave passing through an acousto-optic deflector, where the deflection angle is varied over the microsweep from a start angle at the start frequency of the acoustic wave to an end angle at the end frequency of the acoustic wave.

[0006] The difference in frequency of the acoustic waves produced in the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep is caused by the change in frequency of the acoustic waves over time. At any given moment, an acoustic wave that has traveled a long distance through the acousto-optic deflector, i.e., that affects one portion of the width of the laser beam for a long time, will have a different frequency than another acoustic wave that has traveled a short distance through the acousto-optic deflector, i.e., that affects another portion of the width of the laser beam for a short time. Thus, an instantaneous difference in frequency occurs. This difference also occurs in a direction parallel to the microsweep because the deflection angle is changed to achieve a microsweep in a direction parallel to the propagation direction of the acoustic wave.

[0007] The desired focus of the laser beam in a direction parallel to the microsweep refers to the focus of the laser beam in the exposed photosensitive resist in a direction parallel to the microsweep. "Desired" refers to the focus desired for a particular application. Generally, a uniform (constant) focus in the microsweep is desired. Such a focus is sometimes referred to as a precise focus.

[0008] The inventors have realized that changes in the rate of variation of the frequency of the acoustic wave over time will adversely affect the difference in frequency of the acoustic wave across the width of the laser beam in a direction parallel to the microsweep as the laser beam passes through the acousto-optic deflector, and that these frequency differences will adversely affect the focus of the laser beam in a direction parallel to the microsweep. Therefore, the rate of change (increase or decrease) is adapted (calibrated) over the microsweep (i.e., over time) so that the desired focus of the laser beam in a direction parallel to the microsweep can be achieved over the microsweep.

[0009] The rate of change of the frequency of the acoustic wave is preferably adapted over the time of the microsweep such that the adapted difference in frequency over the time of the microsweep is such that the desired focus of the laser beam occurs throughout the microsweep in a direction parallel to the microsweep. By calibrating the change in frequency over time, the desired focus can be achieved for every point or every pixel along the microsweep.

[0010] Furthermore, preferably, the rate of change of the frequency of the acoustic wave is adapted over the time of the microsweep according to a function that produces a difference in the frequency of the acoustic wave in the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep at each instant over the time of the microsweep as the laser beam passes through the acousto-optic deflector, the difference in frequency being such that the desired focus of the laser beam occurs in the direction parallel to the microsweep. Here, function generally refers to the relationship between frequency and time. The function can be, for example, in the form of a table mapping frequency to successive time points (corresponding to successive time intervals) so that the frequency is changed in incremental steps at successive time points and remains constant for the time intervals after each successive time point. The time intervals may be so small that the step function is approximately a continuous function.

[0011] The laser beam is typically modulated over time according to a predetermined pattern used to print the photosensitive resist along the microsweep. By varying the frequency of the acoustic wave over time to achieve the desired focus of the laser beam in a direction parallel to the microsweep, the position nonlinearity of the microsweep (nonlinear microsweep), i.e., the position of the laser beam in the microsweep does not change as a linear function of time, must be taken into account to accurately print the photosensitive resist according to the predetermined pattern.

[0012] In one example, modulation of the laser beam may be adapted to compensate for changes in position (position nonlinearity) within the microsweep due to acoustic waves that change frequency over time to cause a desired focus of the laser beam in a direction parallel to the microsweep. Thus, given a nonlinear microsweep calibrated to provide a desired focus of the laser beam in a direction parallel to the microsweep, a difference (position deviation) relative to a linear microsweep can be derived, and the modulation can be adapted based on the identified difference (position deviation).

[0013] Furthermore, the introduced position nonlinearity (nonlinear sweep) results in a change in the microsweep velocity (i.e., the velocity of the laser beam along the microsweep, i.e., along the surface being exposed), which in turn adversely affects the dose delivered along the sweep. This may be taken into account by adapting the intensity of the laser beam across the microsweep to compensate for the change in microsweep velocity due to acoustic waves that change frequency over time to induce the desired focus of the laser beam in a direction parallel to the microsweep. For example, the intensity may be proportional to the microsweep velocity, i.e., increasing the intensity where the microsweep velocity increases and decreasing the intensity where the microsweep velocity decreases.

[0014] According to a second aspect, there is provided a computer program comprising computer readable instructions which, when executed on a processing device, cause the processing device to perform the method of the first aspect.

[0015] The computer program of the second aspect may further comprise additional features corresponding to the additional features described in relation to the method of the first aspect.

[0016] According to a third aspect, there is provided a computer readable medium having stored thereon a computer program comprising computer readable instructions which, when executed on a processing device, cause the processing device to perform the method of the first aspect.

[0017] The computer-readable medium of the third aspect may further comprise additional features corresponding to the additional features described in relation to the method of the first aspect.

[0018] The computer-readable medium of the third aspect may be, for example, a non-transitory computer-readable medium.

[0019] According to a fourth aspect, there is provided an apparatus for projecting a laser beam onto a surface, such as a photosensitive resist. The apparatus comprises an acousto-optic deflector arranged to receive the laser beam and an acoustic transducer arranged to provide acoustic waves to the acousto-optic deflector. The apparatus further comprises a controller configured to vary the frequency of the acoustic waves over time to vary the deflection angle of the laser beam over time to achieve a microsweep of the laser beam. The controller is further configured to adapt the rate of change of the frequency of the acoustic waves over a time period of the microsweep such that, as the laser beam passes through the acousto-optic deflector, a tailored difference in frequency over a time period of the acoustic waves is produced in the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep. The tailored difference in frequency over a time period of the microsweep is such that a desired focus of the laser beam is produced across the microsweep in a direction parallel to the microsweep.

[0020] The apparatus of the fourth aspect may further comprise additional features corresponding to those described in relation to the method of the first aspect.

[0021] Hereinafter, an embodiment will be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0022] [Figure 1a] FIG. 1a shows a vertical view of a schematic setup in which the method and apparatus of the present disclosure can be implemented. [Figure 1b] FIG. 1b shows a vertical view of a schematic setup in which the method and apparatus of the present disclosure can be implemented. [Figure 2] FIG. 2 shows an illustration of a portion of a photosensitive resist having a representation of a microsweep in a first direction of a second perpendicularly extending strip in which the method and apparatus of the present disclosure can be implemented. [Figure 3] FIG. 3 shows a schematic diagram illustrating an example of an apparatus according to the present disclosure. [Figure 4] FIG. 4 shows a flowchart of an example method according to the present disclosure. [Figure 5a] FIG. 5a shows the focus error along the microsweep of the laser beam in a first direction when calibrated to achieve a linear sweep and when calibrated to achieve a desired focus in a first direction parallel to the microsweep of the laser beam. [Figure 5b] Figure 5b shows the linearity error along the microsweep of the laser beam in a first direction parallel to the microsweep when calibration is performed to achieve a linear sweep and when calibration is performed to achieve a desired focus in the first direction parallel to the microsweep of the laser beam. [Figure 6a] FIG. 6a shows the difference from a linear frequency ramp across the microsweep of a laser beam in a first direction parallel to the microsweep when calibrated to achieve a desired focus in the first direction parallel to the microsweep and when calibrated to achieve a linear sweep. [Figure 6b] Figure 6b shows the difference from a linear frequency ramp across the microsweep of the laser beam in a first direction parallel to the microsweep when calibrated to achieve the desired focus in the first direction parallel to the microsweep and when calibrated to achieve a linear sweep. DETAILED DESCRIPTION OF THE INVENTION

[0023] The drawings are all schematic, and generally show only parts necessary to explain each embodiment, with other parts being omitted or merely suggested.

[0024] The methods and apparatus of the present disclosure may be advantageously implemented in a pattern generator in which at least one laser beam is modulated according to pattern data, and the modulated laser beam is projected onto a photosensitive resist using an acousto-optic deflector.

[0025] 1a and 1b show vertical views of a schematic setup in which the method and apparatus of the present disclosure can be implemented.

[0026] In FIG. 1a, a (modulated) laser beam 110 passes through an acousto-optic deflector 120. An acoustic transducer 125 generates and provides an acoustic wave 127 to the acousto-optic deflector 120. The deflection angle of the laser beam 110 exiting the acousto-optic deflector 120 depends on the frequency of the acoustic wave 127 (and the angle of incidence of the laser beam 110). Generally, the amount of deflection, i.e., the deflection angle, is proportional to the frequency of the acoustic wave. Therefore, by increasing (or decreasing) the frequency of the acoustic wave over time, the deflection angle can be increased (or decreased), thereby causing the laser beam to sweep across the portion of the photosensitive resist where the pattern is to be printed. Such a sweep is referred to as a microsweep in this disclosure. The microsweep is performed in a first direction Y, parallel to the propagation direction of the acoustic wave. Printing is typically performed in the form of pixels, such as 1500 pixels along the microsweep. The pattern is rasterized to generate pattern data defining the laser exposure associated with each pixel.

[0027] The frequency of the acoustic wave can be varied, for example, from 125 to 260 MHz, or from 210 to 420 MHz. The length of the microsweep is typically on the order of 200 μm. It should be noted that the acoustic wave frequency ranges and microsweep lengths are provided as examples only, and any other frequency ranges and sweep lengths can be used as long as the desired microsweep can be achieved, taking into account the characteristics of the laser used to generate the laser beam.

[0028] In addition to deflecting the laser beam 110, the acousto-optic deflector 120 defocuses the laser beam 110 in the first direction Y. This can be seen in FIG. 1 a, where the laser beam 110 diverges after deflection, with the conformal shape of the laser beam 110 in the first direction Y shown by the solid outer line compared to the original shape of the laser beam 110 in the first direction Y shown by the dashed line 137. The acousto-optic deflector 120 is followed by a cylindrical lens 130, which is positioned to focus the laser beam 110 in the first direction Y to compensate for the defocusing of the laser beam in the first direction Y caused by the acousto-optic deflector 120.

[0029] Another lens 140 is positioned to focus the laser beam 110 in a first direction Y of the photosensitive resist.

[0030] FIG. 1b shows a vertical view of the setup of FIG. 1a. The second direction X is perpendicular to the first direction Y. In the second direction X, the laser beam 110 is not deflected when it passes through the acousto-optic deflector. Similarly, in the second direction X, the laser beam 110 is not deflected by the acousto-optic deflector 120. Therefore, the cylindrical lens 130 is positioned so as not to affect the focus of the laser beam in the second direction X. The movement of the laser beam in the second direction X relative to the photosensitive resist may be achieved by moving the photosensitive resist or a substrate on which the photosensitive resist is formed as a thin film in a direction opposite to the second direction X.

[0031] Further, another lens 140 is further arranged to focus the laser light 110 in a second direction X at the photosensitive resist.

[0032] FIG. 2 shows an illustration of a portion of a photosensitive resist displaying micro-sweeps 211, 212, 213, 214, . . . , 225, 226 in a first direction Y along a first strip 230 in a second direction X perpendicular to the first direction Y.

[0033] 2 in the first direction Y may be printed by successive microsweeps of a single laser beam in the first direction Y and moving the photosensitive resist one microsweep width in the opposite direction relative to the second direction X during the time it takes to perform one microsweep. Alternatively, the microsweeps 211, 212, 213, 214, ..., 225, 226 in the first direction Y may be printed by successive microsweeps of n parallel laser beams in the Y direction and intermediate movements of n microsweep widths in the direction opposite to the second direction X. The n parallel laser beams may be created using a beam splitter.

[0034] Once printing has been performed along all microsweeps 211, 212, 213, 214, ..., 225, 226, i.e., along the entire first strip 230, printing proceeds to the second strip 240. This may be achieved by moving the photosensitive resist in a second direction X a first length corresponding to the length of the first strip 230 and moving the photosensitive resist in a direction opposite to or the same as the first direction Y a second length corresponding to the width of the first strip 230, i.e., the length of each microsweep of the microsweeps 211, 212, 213, 214, ..., 225, 226.

[0035] It should be noted that Figure 2 is provided merely to illustrate an example principle for printing a pattern in a photosensitive resist based on micro-sweeps and strips, and the number of micro-sweeps and the relative dimensions of the micro-sweeps and strips are not intended to reflect actual results.

[0036] In the prior art, pattern generators are calibrated to generate linear microsweeps, i.e., microsweeps in which the velocity of the laser beam through the photosensitive resist is constant during the microsweep. To achieve a linear microsweep, the rate of change (increase or decrease) of the frequency of the acoustic wave supplied to the acousto-optic deflector must be adjusted relative to the constant velocity. The inventors have shown that such calibration results in inaccuracies in the printed pattern and that such inaccuracies are due to incorrect focus in the direction parallel to the microsweep, i.e., in the first direction Y as shown in Figures 1a, 1b, and 2. This is illustrated in Figure 5a, which shows the focus error in the first direction Y for a case where calibration is performed to achieve a linear microsweep as indicated by the curve labeled "Flat Linearity."

[0037] FIG. 3 shows a schematic diagram of an example of an apparatus 300 according to the present disclosure. The apparatus 300 comprises an acousto-optic deflector 310 arranged to receive a laser beam and an acoustic transducer 320 arranged to provide acoustic waves to the acousto-optic deflector 310. The acousto-optic deflector 310 and the acoustic transducer 320 are typically provided as a single physical unit, but are shown in FIG. 3 as two separate boxes to account for their logically separate functions. The apparatus further comprises a controller 330 arranged to vary the frequency of the acoustic waves over time to vary the deflection angle of the laser beam over time and thereby achieve a micro-sweep of the laser beam. This is further described in relation to FIG. 1a.

[0038] The inventors have realized that the focus of the laser beam in the direction parallel to the microsweep can be adapted by adapting the difference in frequency of the acoustic waves across the width of the laser beam in the direction parallel to the microsweep as the laser beam passes through the acousto-optic deflector 310. Specifically, the adaptation to achieve the desired focus is performed by adapting the frequency of the acoustic waves across the microsweep relative to a linear increase in the frequency of the acoustic waves across time during the microsweep (hereinafter, a linear frequency ramp). Furthermore, the focus error in the direction parallel to the microsweep is not constant across the microsweep. Therefore, the error cannot be corrected by adapting another fixed lens or an existing fixed lens. Therefore, the controller 330 is further configured to vary the frequency of the acoustic waves across time as the laser beam passes through the acousto-optic deflector, such that the difference in frequency of the acoustic waves at the acousto-optic deflector is across the width of the laser beam in the direction parallel to the microsweep (first direction Y in FIGS. 1a, 1b, and 2). The frequency difference is such that it results in the desired focusing of the laser beam in a direction parallel to the microsweep. Specifically, the rate of change (increase) of the frequency of the acoustic wave is adapted over the time of the microsweep to achieve an adapted frequency difference over the time of the acoustic wave that compensates for the misfocusing of the laser beam over the time of the microsweep so that the desired (precise) focus of the laser beam in a direction parallel to the microsweep can be achieved over the microsweep. Preferably, the desired (precise) focus is constant (uniform) over the microsweep. This is shown in FIG. 5a, which shows the focus error in the first direction Y for a case where calibration is performed to achieve the desired focus as indicated by the curve labeled "Flat Focus." It should be noted that the focus error in the second direction X does not differ significantly between a case where calibration is performed to achieve a linear microsweep and a case where calibration is performed to achieve the desired focus in the first direction Y.

[0039] By increasing the frequency of the acoustic waves supplied from the acoustic transducer 320 to the acousto-optic deflector 310, a difference in the frequency of the acoustic waves is generated in the acousto-optic deflector because acoustic waves of different frequencies travel different distances in the acousto-optic deflector at a given instant. Therefore, a difference in frequency is generated instantaneously between acoustic waves in different parts of the acousto-optic deflector. Therefore, by increasing the rate of change, e.g., by increasing the frequency increase per unit time, the difference in frequency over the same distance increases in the direction parallel to the microsweep in the acousto-optic deflector (first direction Y in FIGS. 1a, 1b, and 2). Correspondingly, by decreasing the rate of change, e.g., by decreasing the frequency increase per unit time, the difference in frequency over the same distance decreases in the direction parallel to the microsweep in the acousto-optic deflector (first direction Y in FIGS. 1a, 1b, and 2).

[0040] The controller 330 is preferably arranged to vary the frequency of the acoustic wave over time such that the difference in frequency causes the desired focusing of the laser beam in a direction parallel to the microsweep throughout the microsweep. To achieve this, the controller 330 may be arranged to adapt the rate of change (increase) of the frequency of the acoustic wave over time such that the difference in frequency associated with any given position along the entire microsweep is such that it causes the desired focusing of the laser beam in a direction parallel to the microsweep at the given position.

[0041] A function may be derived that maps each time point to each frequency. The function produces a difference in the frequency of the acoustic waves in the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep at each time point (instant) over the time of the microsweep as the laser beam passes through the acousto-optic deflector, the difference in frequency being such that the desired focus of the laser beam in the direction parallel to the microsweep is produced. Since the desired focus is achieved at each time point, the desired focus is also achieved throughout the microsweep.

[0042] For example, the frequency of the acoustic wave can be increased stepwise in 20 ns time intervals lasting 1500 times over the microsweep. For each such 20 ns time interval, the focus measurement is used to iteratively determine the frequency increase relative to the previous time interval to identify the frequency increase that achieves the desired focus of the laser beam in a first direction parallel to the microsweep. Thus, a stepwise function of frequency increase is derived to achieve the desired focus. Alternatively, a continuous function for frequency increase can be used.

[0043] Alternatively, a step function can be derived for the difference relative to a linear increase in frequency. For each 20 ns time interval, the difference in frequency relative to the linear increase is iteratively identified using focus measurements to identify the difference in frequency relative to the linear increase that achieves the desired focus of the laser beam in a first direction parallel to the microsweep. Figure 6a shows the difference in Hz from a linear frequency ramp across a microsweep (in this case, 2000 pixels long) when calibration is performed to achieve the desired focus in a first direction parallel to the microsweep. In comparison, Figure 6b shows the difference in Hz from a linear frequency ramp across a microsweep (in this case, the microsweep is 2000 pixels long) when calibration is performed to achieve a linear sweep.

[0044] The apparatus 300 may further comprise a modulator 340 that modulates the laser beam over time according to a predetermined pattern used to print the photosensitive resist along the microsweep. Typically, the predetermined pattern is rasterized to generate pattern data that approximates the predetermined pattern relative to a grid of pixels according to a Cartesian coordinate system.

[0045] For prior art solutions where the pattern generator is calibrated to generate a linear microsweep, i.e. a microsweep where the velocity of the laser beam is constant in the photosensitive resist during the microsweep, the modulation may be performed directly on the pattern data.

[0046] To achieve the desired focus of the laser light in a direction parallel to the microsweep, the frequency of the acoustic wave is varied over time, resulting in a rate of change that varies over time. This variation in rate of change results in a position nonlinearity in the microsweep (nonlinear microsweep), i.e., the position of the laser beam in the photosensitive resist during the microsweep does not change as a linear function of time. To accurately print in the photosensitive resist, such position nonlinearity must be taken into account. Depending on the specific device, the absolute value of the position difference compared to a linear microsweep may be on the order of one pixel or more in length. For example, for the calibration associated with FIG. 5a, which achieves the desired focus in a first direction Y parallel to the microsweep, as shown by the curve labeled "Flat Focus," a position difference occurs compared to a linear microsweep. This is illustrated in FIG. 5b by the curve labeled "Flat Focus," which shows the linearity error when calibration is performed to achieve the desired focus in the first direction Y. The linearity error is the error in distance in nanometers (nm) relative to the linear microsweep along the microsweep, which for FIG. 5b is 250 micrometers (μm). The linearity error in FIG. 5b is approximately 250 nm at most, and a pixel may be, for example, 170 nm long. Thus, the error can be on the order of one or more pixels. This is compensated for so that the error is at most a fraction of a pixel, for example, on the order of one-twentieth of a pixel. For comparison, FIG. 5b also shows the linearity error when calibration is performed to achieve a linear microsweep, as shown by the curve labeled "Flat Linearity," which is close to zero along the entire microsweep.

[0047] The modulator 340 may be controlled such that the modulation of the laser beam compensates for changes in position (position nonlinearity) of the microsweep. For example, consider that the previously determined modulation of the laser beam is based on a calibration for a linear microsweep, i.e., where the position of the laser beam in the microsweep varies as a linear function of time over the length of the microsweep. In this case, compensating for the position nonlinearity can be done by first determining the deviation in position over time of the nonlinear microsweep relative to the linear microsweep, and then adjusting the previously determined modulation for the deviation in position over time to generate a new modulation for the nonlinear microsweep.

[0048] Furthermore, the introduced position nonlinearity (nonlinear sweep) results in a change in the microsweep velocity (velocity of the laser beam along the microsweep), which in turn adversely affects the dose delivered along the sweep. To account for this, the modulator 340 may be controlled by adapting the intensity of the laser beam across the microsweep to compensate for the relative change in microsweep velocity. For example, the intensity may be proportional to the microsweep velocity, i.e., the intensity is increased where the microsweep velocity increases and the intensity is decreased where the microsweep velocity decreases.

[0049] FIG. 4 shows a flowchart of an example method for controlling the focus of a laser beam during a microsweep according to the present disclosure. In the method, a laser beam is received by an acousto-optic deflector (410). Additionally, an acoustic wave is provided to the acousto-optic deflector (420). The acoustic wave varies in frequency over time to change the deflection angle of the laser beam over time and thereby achieve a microsweep of the laser beam. This is further described in relation to FIG. 1a. Specifically, as the laser beam passes through the acousto-optic deflector, the rate of change of the frequency of the acoustic wave is adapted over the time of the microsweep such that an adapted difference in frequency over the time of the acoustic wave is produced across the width of the laser beam in the acousto-optic deflector in a direction parallel to the microsweep (first direction Y in FIGS. 1a, 1b, and 2). The adapted difference in frequency over the time of the microsweep is such that a desired focus of the laser beam is achieved across the microsweep in a direction parallel to the microsweep.

[0050] The laser beam received by the acousto-optic deflector is typically modulated according to a predetermined pattern used to print on the photosensitive resist along the microsweep. Varying the frequency of the acoustic wave over time to achieve the desired focus of the laser beam in a direction parallel to the microsweep results in a positional nonlinearity of the microsweep (nonlinear microsweep), i.e., the position of the laser beam in the microsweep does not change as a linear function of time. Modulation of the laser beam according to the disclosed method may compensate 405 for the positional nonlinearity (positional change) of the microsweep to ensure that the print is precisely located on the photosensitive resist according to the predetermined pattern.

[0051] The examples have been described in terms of increasing the frequency of an acoustic wave to change the deflection angle of the laser beam, thereby achieving a microsweep, since it is preferred to increase the frequency of the acoustic wave, but the principles can also be applied to sweeps achieved by decreasing the frequency of the acoustic wave and sweeping in the opposite direction to that achieved when the frequency is increased.

[0052] The above description of the embodiments should be considered non-limiting. While particular embodiments have been described, it will be apparent to those skilled in the art that various changes, modifications, or alternatives may be made within the scope defined by the appended claims.

[0053] Item List 1. A method for controlling the focus of a laser beam during a microsweep, comprising receiving a laser beam with an acousto-optic deflector; and supplying acoustic waves to the acousto-optic deflector, the acoustic waves varying in frequency with time to change the deflection angle of the laser beam with time to achieve a microsweep of the laser beam, wherein the rate of change of the frequency of the acoustic waves is adapted over the time of the microsweep such that as the laser beam passes through the acousto-optic deflector, a tailored difference in frequency over the time of the microsweep of the acoustic waves is produced across the width of the laser beam in a direction parallel to the microsweep, and the tailored difference in frequency over the time of the microsweep is such that a desired focus of the laser beam is caused across the microsweep in a direction parallel to the microsweep. 2. The method of item 1, wherein the adapted difference in frequency over the time of the microsweep is such as to cause a desired focus of the laser beam in a direction parallel to the microsweep throughout the entire microsweep. 3. A method according to any one of items 1 and 2, wherein the rate of change of the frequency of the acoustic wave is adapted over the time of the microsweep according to a function that produces a difference in the frequency of the acoustic wave in the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep at an instant over the time of the microsweep as the laser beam passes through the acousto-optic deflector, the difference in frequency being such as to cause a desired focus of the laser beam in a direction parallel to the microsweep. 4. The method of any one of items 1 to 3, further comprising modulating the laser beam over time according to a predetermined pattern, wherein the modulation of the laser beam is adapted to compensate for changes in position within the microsweep due to an acoustic wave that changes frequency over time to cause a desired focus of the laser beam in a direction parallel to the microsweep. 5. The method of any one of items 1 to 4, further comprising adapting the intensity of the laser beam across the microsweep to compensate for changes in velocity of the microsweep due to an acoustic wave that changes frequency over time to cause a desired focus of the laser beam in a direction parallel to the microsweep. 6. A computer program comprising computer-readable instructions that, when executed on a processing device, cause the processing device to perform the method according to any one of items 1 to 5. 7. A computer-readable medium storing a computer program comprising computer-readable instructions that, when executed on a processing device, causes the processing device to perform the method according to any one of items 1 to 5. 8. An apparatus for projecting a laser beam onto a surface, comprising: an acousto-optic deflector arranged to receive the laser beam; an acoustic transducer arranged to provide acoustic waves to the acousto-optic deflector; and a controller arranged to vary the frequency of the acoustic waves over time to vary the deflection angle of the laser beam over time to achieve a microsweep of the laser beam, wherein the controller is further arranged to adapt a rate of change of the frequency of the acoustic waves over a time of the microsweep such that as the laser beam passes through the acousto-optic deflector, a tailored difference in frequency over a time of the microsweep of the acoustic waves is produced in the acousto-optic deflector across a width of the laser beam in a direction parallel to the microsweep, wherein the tailored difference in frequency over a time of the microsweep is such that a desired focus of the laser beam is caused across the microsweep in a direction parallel to the microsweep. 9. The apparatus described in item 8, wherein the adapted difference in frequency over the time of the microsweep is such as to cause a desired focus of the laser beam in a direction parallel to the microsweep throughout the microsweep. 10. An apparatus described in any one of items 8 and 9, wherein the control device is further arranged to adapt the rate of change of the frequency of the acoustic wave over the time of the microsweep according to a function that causes a difference in the frequency of the acoustic wave in the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep at an instant over the time of the microsweep as the laser beam passes through the acousto-optic deflector, the difference in frequency being such as to cause a desired focus of the laser beam in a direction parallel to the microsweep. 11. The apparatus of any one of items 8 to 10, further comprising a modulator arranged to modulate the laser beam over time according to a predetermined pattern, the modulation of the laser beam adapted to compensate for changes in position within the microsweep due to an acoustic wave that changes frequency over time to cause a desired focus of the laser beam in a direction parallel to the microsweep. 12. An apparatus according to any one of items 8 to 11, further comprising means for adapting the intensity of the laser beam across the microsweep to compensate for changes in velocity of the microsweep due to an acoustic wave that varies in frequency over time to cause a desired focus of the laser beam in a direction parallel to the microsweep.

Claims

1. 1. A method for controlling a focus of a laser beam during a microsweep, comprising: receiving the laser beam with an acousto-optic deflector; providing acoustic waves to the acousto-optic deflector, the acoustic waves varying in frequency with time to vary the deflection angle of the laser beam with time to achieve the micro-sweep of the laser beam; a rate of change of the frequency of the acoustic wave is adapted to vary over a time period of the microsweep such that, as the laser beam passes through the acousto-optic deflector, a tailored difference in frequency over a time period of the microsweep of the acoustic wave is produced across the width of the laser beam in a direction parallel to the microsweep at the acousto-optic deflector, the tailored difference in frequency over a time period of the microsweep causing a desired focus of the laser beam across the microsweep in a direction parallel to the microsweep; modulating the laser beam over time according to a predetermined pattern, the modulation of the laser beam being adapted to compensate for changes in position within the microsweep due to a rate of change of the frequency of the acoustic wave that changes over time to cause a desired focus of the laser beam in a direction parallel to the microsweep; A method for providing the above.

2. 10. The method of claim 1, wherein the adapted difference in frequency over the time of the microsweep causes a desired focus of the laser beam in a direction parallel to the microsweep throughout the microsweep.

3. 2. The method of claim 1, wherein the rate of change of the frequency of the acoustic wave is adapted over the time of the microsweep according to a function that produces a difference in the frequency of the acoustic wave on the acousto-optic deflector across the width of the laser beam in a direction parallel to the microsweep at an instant over the time of the microsweep as the laser beam passes through the acousto-optic deflector, the difference in frequency causing a desired focus of the laser beam in a direction parallel to the microsweep.

4. 4. The method of claim 1, further comprising adapting the intensity of the laser beam across the microsweep to compensate for changes in velocity of the microsweep due to a rate of change of the frequency of the acoustic wave that changes over time to cause a desired focus of the laser beam in a direction parallel to the microsweep.

5. A computer program comprising computer readable instructions which, when executed on a processing device, cause the processing device to carry out the method of any one of claims 1 to 4.

6. A computer readable medium storing a computer program comprising computer readable instructions which, when executed on a processing device, cause the processing device to perform the method of any one of claims 1 to 4.

7. 1. An apparatus for projecting a laser beam onto a surface, comprising: an acousto-optic deflector positioned to receive the laser beam; an acoustic transducer positioned to provide acoustic waves to the acousto-optic deflector; a controller for varying the frequency of the acoustic wave over time to vary the deflection angle of the laser beam over time to achieve a micro-sweep of the laser beam, a controller further arranged to adapt a rate of change of the frequency of the acoustic wave to vary over a time period of the microsweep such that, as the laser beam passes through the acousto-optic deflector, a tailored difference in frequency over a time period of the acoustic wave is produced across the width of the laser beam in a direction parallel to the microsweep at the acousto-optic deflector, the tailored difference in frequency over a time period of the microsweep causing a desired focus of the laser beam across the microsweep in a direction parallel to the microsweep; and a modulator arranged to modulate the laser beam over time according to a predetermined pattern, the modulation of the laser beam adapted to compensate for changes in position within the microsweep due to a rate of change of the frequency of the acoustic wave that varies with time to cause a desired focus of the laser beam in a direction parallel to the microsweep; An apparatus comprising:

8. 8. The apparatus of claim 7, wherein the adapted difference in frequency over the time of the microsweep causes a desired focus of the laser beam in a direction parallel to the microsweep throughout the microsweep.

9. 8. The apparatus of claim 7, wherein the control device is further arranged to adapt a rate of change of the frequency of the acoustic wave over the time of the microsweep according to a function that causes a difference in frequency of the acoustic wave on the acousto-optic deflector across a width of the laser beam in a direction parallel to the microsweep at an instant over the time of the microsweep as the laser beam passes through the acousto-optic deflector, the difference in frequency causing a desired focus of the laser beam in a direction parallel to the microsweep.

10. 8. The apparatus of claim 7, further comprising means for adapting the intensity of the laser beam across the microsweep to compensate for changes in velocity of the microsweep due to a rate of change of the frequency of the acoustic wave that changes over time to cause a desired focus of the laser beam in a direction parallel to the microsweep.

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