Digital holographic metrology system

Abstract

A digital holographic metrology system is provided that includes a heterodyne light source, an interference optical arrangement, and a sensor arrangement. The heterodyne light source provides a combination laser beam of different corresponding frequencies and wavelengths (e.g., each combination beam can include a corresponding wavelength laser beam and a corresponding frequency-shifted laser beam, which can be orthogonally polarized). The interference optical arrangement utilizes the combination beams to provide an output for imaging a workpiece, the output including an interference beam. The sensor arrangement includes a dichroic component that separates the interference beam for directing to respective time-of-flight sensors. A measurement is determined utilizing output from the time-of-flight sensors (e.g., a measured distance to a surface point on a workpiece can be determined utilizing the output of the time-of-flight sensors, etc.).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to precision metrology, and more specifically to precision workpiece surface measurement devices and systems. BACKGROUND

[0002] Quality control of objects (e.g., workpieces) that include particular surface profiles (e.g., produced by molding and / or machining, etc.) is increasingly demanding in terms of throughput, measurement resolution, and accuracy. Such workpieces can include, but are not limited to, optical components, electronic components, micro-mechanical components, etc. Ideally, such workpieces should be measured / inspected to ensure proper dimensions, functionality, etc. However, measurement tolerances on the order of sub-microns, or even sub-nanometers, can be required to confirm that a workpiece surface has certain application-desired characteristics.

[0003] Various precision metrology systems can be used for workpiece surface measurement and inspection. For example, in some instances, metrology systems that perform such operations can utilize digital holography (e.g., a camera can be utilized to acquire a hologram of a workpiece being processed to determine measurement values, etc.). An important part of such systems and / or other comparable measurement systems is a light source used to illuminate a workpiece being measured (e.g., inaccuracies or inconsistencies in the light source can result in inaccurate measurements, etc.). Signal-to-noise ratio of such systems is also an important factor (e.g., a lower ratio can result in less accurate measurements). It would be desirable to improve or otherwise enhance configurations of such metrology systems (e.g., for measuring and inspecting surfaces of workpieces, etc.). SUMMARY

[0004] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the DETAILED DESCRIPTION. This Summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used to determine the scope of the claimed subject matter.

[0005] A digital holography metrology system is provided that includes a heterodyne light source, an interferometric optical arrangement, and a sensor arrangement. The heterodyne light source includes a multi-wavelength light source and an acousto-optic modulator. The multi-wavelength light source (e.g., a multi-wavelength laser light source) provides at least a first wavelength laser light beam at a first frequency and a second wavelength laser light beam at a second frequency different from the first frequency. The acousto-optic modulator is configured to receive the first wavelength laser light beam and generate a corresponding first frequency-shifted laser light beam that combines with the first wavelength laser light beam as a first combined light beam. The acousto-optic modulator is also configured to receive the second wavelength laser light beam and generate a corresponding second frequency-shifted laser light beam that combines with the second wavelength laser light beam as a second combined light beam (e.g., the second combined light beam and the first combined light beam can be portions of a total combined light beam from the heterodyne light source).

[0006] The interferometric optical arrangement receives the combined light beams from the heterodyne light source and utilizes the combined light beams to provide an output for imaging the workpiece. In various embodiments, the output of the interferometric optical arrangement includes at least a first interference light beam generated based on the first combined light beam from the heterodyne light source and a second interference light beam generated based on the second combined light beam from the heterodyne light source. The sensor arrangement receives the output from the interferometric optical arrangement and includes at least first and second time-of-flight sensors and a first dichroic component. The first dichroic component is configured to separate the first interference light beam from the second interference light beam, the first interference light beam being directed to be received by the first time-of-flight sensor and the second interference light beam being directed to be received by the second time-of-flight sensor. In various embodiments, the digital holographic metrology system further includes a processing portion that receives output from the first and second time-of-flight sensors and utilizes the output to determine at least one measured distance to at least one surface point on the workpiece.

[0007] In various embodiments, the multi-wavelength light source further provides a third wavelength laser light beam of a third frequency different from the first and second frequencies. In such embodiments, the acousto-optic modulator can be further configured to receive the third wavelength laser light beam and generate a corresponding third frequency-shifted laser light beam that is combined with the third wavelength laser light beam into a third combined light beam. The output of the interferometric optical arrangement can further include a corresponding third interference light beam generated based on the third combined light beam from the heterodyne light source. The sensor arrangement can further include a third time-of-flight sensor and a second dichroic component, where the second dichroic component is configured to separate the second interference light beam from the third interference light beam, the second interference light beam being directed to be received by the second time-of-flight sensor and the third interference light beam being directed to be received by the third time-of-flight sensor.

[0008] In various embodiments, the interferometric optical arrangement includes a beam splitter, a reference surface, and an imaging lens portion including an imaging lens. The beam splitter receives the combined light beams from the heterodyne light source and splits the combined light beams into a first portion directed to the workpiece surface and a second portion directed to the reference surface. The reflected first portion and the reflected second portion are received back by the beam splitter and combined and directed as an interference light beam through the imaging lens toward the sensor arrangement.

[0009] In various embodiments, the digital holographic metrology system further includes a timer. The timer provides a signal to the heterodyne light source for operating the acousto-optic modulator (e.g., a timing signal such as a clock signal) and provides a signal to the time-of-flight sensors. In various embodiments, the timer can be included as part of one of the time-of-flight sensors.

[0010] In various embodiments, the digital holographic metrology system is configured to operate in a digital holographic mode and an amplitude modulated continuous wave ("AMCW") mode. In the digital holographic mode, the timer is coupled for operation of the acousto-optic modulator that generates the first and second combined light beams, the processing portion receives output from the first and second time-of-flight sensors, and utilizes the output to determine a first digital holographic mode measured distance (e.g., to a first surface point on the workpiece). In the AMCW mode, the acousto-optic modulator does not generate the first and second combined light beams, and the processing portion utilizes output of at least one of the time-of-flight sensors to determine a first AMCW mode measured distance (e.g., to the first surface point on the workpiece) via homodyne detection. In various embodiments, at least portions of the first AMCW mode measured distance and the first digital holographic mode measured distance are combined to determine a combined measured distance (e.g., to the first surface point on the workpiece).

[0011] In various embodiments, a digital holographic unambiguous range (e.g., absolute measurement range) of the digital holographic mode is greater than a potential distance error of the AMCW mode, and the first AMCW mode measured distance provides an integer multiple of the digital holographic unambiguous range that is combined with the first digital holographic mode measured distance to determine the combined measured distance to the first surface point on the workpiece. In various embodiments, the AMCW mode has an unambiguous range that is at least 50 times greater than the unambiguous range of the digital holographic mode. In various embodiments, the AMCW mode has an unambiguous range that is greater than 500 millimeters, and the digital holographic mode (e.g., when utilizing first, second, and third wavelength laser beams from the light source) has an unambiguous range that is greater than 5 millimeters.

[0012] In various embodiments, a difference between the first wavelength of the first wavelength laser beam and the second wavelength of the second wavelength laser beam is greater than 2% of the first wavelength. In various embodiments, a difference between the first wavelength of the first wavelength laser beam and the second wavelength of the second wavelength laser beam is greater than 10 nanometers.

[0013] A method of operating a digital holographic metrology system is provided, the method including operating a heterodyne light source of the digital holographic metrology system and operating a sensor arrangement. The operation of the heterodyne light source includes operating a multi-wavelength light source and operating an acousto-optic modulator. The multi-wavelength light source is operated to provide at least a first-wavelength laser beam of a first frequency and a second-wavelength laser beam of a second frequency different from the first frequency. The acousto-optic modulator is operated to receive the first-wavelength laser beam and generate a corresponding first frequency-shifted laser beam that combines with the first-wavelength laser beam as a first combined beam. The acousto-optic modulator is further operated to receive the second-wavelength laser beam and generate a corresponding second frequency-shifted laser beam that combines with the second-wavelength laser beam as a second combined beam. An interferometric optical arrangement receives the combined beams from the heterodyne light source and utilizes the combined beams to provide an output for imaging a workpiece. The output of the interferometric optical arrangement includes at least a first interference beam generated based on the first combined beam from the heterodyne light source and a second interference beam generated based on the second combined beam from the heterodyne light source. The sensor arrangement is operated to receive the output from the interferometric optical arrangement. The sensor arrangement includes at least first and second time-of-flight sensors and a first dichroic component. The first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam being directed to be received by the first time-of-flight sensor and the second interference beam being directed to be received by the second time-of-flight sensor. In various embodiments, the method further includes receiving an output from the first and second time-of-flight sensors and utilizing the output to determine at least one measured distance to at least one surface point on the workpiece.

[0014] In various embodiments, the method further includes operating the multi-wavelength light source to further provide a third-wavelength laser beam of a third frequency different from the first and second frequencies. In such embodiments, the acousto-optic modulator is operable to further receive the third-wavelength laser beam and generate a corresponding third frequency-shifted laser beam that combines with the third-wavelength laser beam as a third combined beam. The output of the interferometric optical arrangement can correspondingly further include a third interference beam generated based on the third combined beam from the heterodyne light source. The sensor arrangement can further include a third time-of-flight sensor and a second dichroic component. The second dichroic component can be configured to separate the second interference beam from the third interference beam, the second interference beam being directed to be received by the second time-of-flight sensor and the third interference beam being directed to be received by the third time-of-flight sensor.

[0015] In various embodiments, the method can further include operating a timer to provide a signal (e.g., a timing signal such as a clock signal) to the heterodyne light source for operating the acousto-optic modulator, and providing a signal to the time-of-flight sensors. In various embodiments, the method can further include operating the system in a digital holography mode and an amplitude modulated continuous wave ("AMCW") mode. In the digital holography mode, the timer is coupled for operating the acousto-optic modulator that generates the first and second combined light beams, and the processing portion receives output from the first and second time-of-flight sensors and utilizes the output to determine a first digital holography mode measured distance to the first surface point on the workpiece. In the AMCW mode, the acousto-optic modulator does not generate the first and second combined light beams, and the processing portion utilizes output of at least one of the time-of-flight sensors to determine a first AMCW mode measured distance to the first surface point on the workpiece via homodyne detection. In various embodiments, at least portions of the first AMCW mode measured distance and the first digital holography mode measured distance are combined to determine a combined measured distance to the first surface point on the workpiece. In various embodiments, a digital holography non-ambiguous range of the digital holography mode is greater than a potential distance error of the AMCW mode, and the first AMCW mode measured distance provides an integer multiple of the digital holography non-ambiguous range that is combined with the first digital holography mode measured distance to determine the combined measured distance to the first surface point on the workpiece.

[0016] In various embodiments, a sensor apparatus for a digital holography metrology system is provided. The sensor apparatus includes at least a first time-of-flight sensor, a second time-of-flight sensor, and a first dichroic assembly. The sensor apparatus can receive output from an interferometric optical apparatus, the first dichroic assembly is configured to separate a first interferometric light beam from a second interferometric light beam, and the first interferometric light beam is directed to be received by the first time-of-flight sensor and the second interferometric light beam is directed to be received by the second time-of-flight sensor. In various embodiments, the sensor apparatus can further include a third time-of-flight sensor and a second dichroic assembly. The second dichroic assembly is configured to separate the second interferometric light beam from a third interferometric light beam, the second interferometric light beam is directed to be received by the second time-of-flight sensor and the third interferometric light beam is directed to be received by the third time-of-flight sensor. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a block diagram illustrating various components of a metrology system including a heterodyne light source;

[0018] Figure 2 is a block diagram of a digital holography metrology system including a heterodyne light source similar to Figure 1 ;

[0019] Figures 3A to 3C is a diagram illustrating, for example, Figure 2is a schematic diagram of certain operating principles of a digital holographic metrology system;

[0020] Figure 4 is a schematic diagram of a lookup table for phase unwrapping as part of the operation of a digital holographic metrology system;

[0021] Figure 5 is a schematic diagram illustrating certain values from a lookup table of Figure 4 ;

[0022] Figure 6A and 6B are schematic diagrams illustrating different operating modes of a digital holographic metrology system;

[0023] Figure 7 is a schematic diagram of a heterodyne light source including an imaging system with a single imaging lens and a Wollaston prism;

[0024] Figure 8 is a schematic diagram of a heterodyne light source including an imaging system with two imaging lenses and a Wollaston prism;

[0025] Figure 9 is a schematic diagram of a heterodyne light source including a receiving lens and a birefringent beam displacer;

[0026] Figure 10 is a schematic diagram of a heterodyne light source including a receiving prism and a birefringent beam displacer;

[0027] Figure 11 is a schematic diagram of a heterodyne light source including a spatial filter configuration and a collimating lens;

[0028] Figure 12 is a flowchart diagram illustrating an exemplary embodiment of a routine for operating a heterodyne light source; and

[0029] Figure 13 is a flowchart diagram illustrating an exemplary embodiment of a routine for operating a digital holographic metrology system including a heterodyne light source. DETAILED DESCRIPTION

[0030] Figure 1 is a block diagram showing various components of an embodiment of a metrology system 100. As shown in Figure 1 , the metrology system 100 includes a heterodyne light source 110, an optical arrangement 120, a sensor arrangement 130, a timer 140, and a processing portion 150. In various embodiments, the heterodyne light source 110 can include a light source, an acousto-optic modulator, and source optics (e.g., as will be described below with respect to Figures 7 to 11(e.g., as described in more detail below with respect to FIG. 2). Briefly, the acousto-optic modulator can be configured to receive one or more input wavelength laser beams from the light source and generate one or more corresponding frequency-shifted laser beams (e.g., which can be orthogonally polarized). The source optical arrangement can receive and combine the orthogonally polarized laser beams from the acousto-optic modulator and output a corresponding combined laser beam.

[0031] The optical arrangement 120 (e.g., which can be a measurement optical arrangement) receives the combined output beam from the heterodyne light source 110. The optical arrangement 120 utilizes (e.g., directs) the combined beam as part of a measurement process (e.g., as part of a process to measure / determine at least one measured distance to at least one surface point on the surface 192 of the workpiece, one or more beams can be directed to the surface 192 of the workpiece 190). As an example, in some embodiments (e.g., as will be described in more detail below with respect to FIG. 2), the optical arrangement 120 can be an interferometric optical arrangement that receives the combined beam from the heterodyne light source 110 and utilizes the combined beam to provide an output for imaging the surface 192 of the workpiece (e.g., to an image plane) as part of the measurement process. Figure 2

[0032] The sensor arrangement 130 receives the output from the measurement optical arrangement 120 and includes one or more sensors for sensing the output and providing a corresponding output signal. The timer 140 provides timing signals (e.g., clock signals) for controlling timing, etc. of both the acousto-optic modulator of the heterodyne light source 110 and the sensors of the sensor arrangement 130. The processing portion 150 (e.g., including one or more processors 152) receives the output from the sensor arrangement 130 and utilizes the output (e.g., to determine a measured distance to a surface point on the surface 192 of the workpiece 190).

[0033] ​Those skilled in the art will appreciate that the processing portion 150 (e.g., included or implemented in a computing system or the like) and / or other processing or control systems described or usable with the elements and methods described herein can generally be implemented using any suitable computing system or device, including a distributed or networked computing environment or the like. Such systems or devices can include one or more general- purpose or special-purpose processors (e.g., non-customized or customized devices) that execute software to perform the functions described herein. The software can be stored in memory, such as random access memory (RAM), read only memory (ROM), flash memory, or the like, or a combination of these components. The software can also be stored in one or more storage devices, such as an optical-based disk, a flash device, or any other type of non-volatile storage medium used to store data. The software can include one or more program modules comprising processes, routines, programs, objects, components, data structures, and the like that perform particular tasks or implement particular abstract data types. In a distributed computing environment, the functionality of the program modules can be combined across multiple computing systems or devices, and the functionality of the program modules accessed through service calls in a wired or wireless configuration.

[0034] Figure 2 is a block diagram of a digital holographic metrology system 200 including a heterodyne light source 210 that can perform certain functions similar to the metrology system of Figure 1 It will be appreciated that certain numbered components 2XX of Figure 2 may correspond to and / or have similar operations as similarly numbered counterpart components 1XX of Figure 1 and can be understood in similar ways thereto, unless otherwise described below. This numbering scheme indicating elements having similar designs and / or functions (i.e., unless otherwise stated and / or described) applies to the remaining figures herein as well. As shown in Figure 2 the digital holographic metrology system 200 includes a heterodyne light source 210, an interference optics arrangement 220, a sensor arrangement 230, a timer 240, and a processing portion 250. In various embodiments, the digital holographic metrology system 200 can operate as a heterodyne imaging interferometer.

[0035] In various embodiments, the heterodyne light source 210 includes a coupling to an acousto-optic modulator (e.g., described below with respect to Figures 7 to 11A multi-wavelength light source (e.g., a multi-wavelength laser light source for providing laser light beams of different wavelengths) is described in greater detail with respect to certain examples. In one embodiment, the multi-wavelength laser light source can provide a first wavelength laser light beam of a first frequency vi, an acousto-optic modulator receives the first wavelength laser light beam and generates a corresponding first frequency-shifted laser light beam of frequency vi + AV, which is combined with the first wavelength laser light beam as a first combined light beam. Similarly, the multi-wavelength laser light source can provide a second wavelength laser light beam of a second frequency v2 different from the first frequency (i.e., of a corresponding second wavelength different from the first wavelength), an acousto-optic modulator receives the second wavelength laser light beam and generates a corresponding second frequency-shifted laser light beam of frequency v2 + AV, which is combined with the second wavelength laser light beam as a second combined light beam. Similarly, the multi-wavelength laser light source can provide a third wavelength laser light beam of a third frequency v3 different from the first and second frequencies (i.e., of a corresponding third wavelength different from the first and second wavelengths), an acousto-optic modulator receives the third wavelength laser light beam and generates a corresponding third frequency-shifted laser light beam of frequency v3 + AV, which is combined with the third wavelength laser light beam as a third combined light beam. In various embodiments, the multi-wavelength laser light source can provide additional wavelength laser light beams, with an acousto-optic modulator generating a corresponding additional frequency-shifted laser light beam. In various embodiments, different notations can be used interchangeably herein to represent / correspond to the frequency of a laser light beam (e.g., v, f, w, etc.).

[0036] A timer 240 is coupled to the heterodyne light source 210 (e.g., to an acousto-optic modulator through a driver of the heterodyne light source) to provide a reference clock signal based on which the modulation frequency AV is generated. In various embodiments, the clock signal is provided at the modulation frequency AV (e.g., in certain embodiments, in the range of 40 Mhz to 100 MHz). In various embodiments, the distributed clock signal can be equal to the acoustic wave frequency in the acousto-optic modulator and equal to the frequency shift AV of the light beam (e.g., the photon frequency plus one acoustic frequency phonon). In various alternative embodiments, the distributed clock signal can be any other frequency that is multiplied / divided with additional electronic components to the desired target frequency.

[0037] Interferometric optical arrangement 220 (e.g., which can also be referred to as a measurement optical arrangement) includes beamsplitter 221, quarter waveplates 223 and 226, reference mirror 224 (e.g., which can also be referred to herein as reference surface 224), imaging lens portion 227 (e.g., including an imaging lens such as a camera lens), and polarizer 228. In various embodiments, interferometric optical arrangement 220 is formed and / or operates as a two-path interferometer. Imaging lens portion 227 (e.g., which can include multiple lenses in some embodiments and can be a telecentric arrangement) is provided to image surface 192 of workpiece 190 (e.g., which is being measured and / or otherwise inspected, etc.) to an image plane. In various embodiments, the image plane can be before, on, or after sensors (e.g., sensor TOF-1, sensor TOF-2, etc.) of sensor arrangement 230 (e.g., in various embodiments, the sensors can be spatially arranged such that the optical path length from imaging lens portion 227 to each sensor can be approximately the same). As will be described in greater detail below, in various embodiments each sensor TOF can detect a beat signal at a difference frequency AV and demodulate based on each pixel. In various embodiments, one depth frame can consist of four phase measurements. The recorded hologram can be digitally propagated to an image plane (e.g., after phase unwrapping) of an extracted height map (e.g., indicating measurements of surface points on magnified workpiece surface 192 of workpiece 190).

[0038] Beamsplitter 221 is optically aligned with reference mirror 224 along axis 222. Beamsplitter 221 is also optically aligned with workpiece surface 192 along axis 225 that is substantially perpendicular to axis 222. Quarter waveplate 226 is disposed between beamsplitter 221 and workpiece surface 192, while quarter waveplate 223 is disposed between beamsplitter 221 and reference mirror 224, and imaging lens portion 227 and polarizer 228 are disposed between beamsplitter 221 and sensor arrangement 230 along axis 225. Imaging lens portion 227 and polarizer 228 are optically aligned between beamsplitter 221 and sensor arrangement 230.

[0039] Workpiece 190 to be inspected / measured is positioned relative to digital holographic metrology system 200 such that workpiece surface 192 is imaged by imaging lens portion 227. In certain embodiments, workpiece surface 192 can be imaged onto sensors (e.g., sensor TOF-1, sensor TOF-2, etc.) of sensor arrangement 230, although it will be appreciated that this is not a requirement in digital holographic metrology systems (e.g., in various embodiments, workpiece surface 192 including respective surface points thereon can be imaged to an image plane before or after sensor TOF of sensor arrangement 230).

[0040] In a digital holographic metrology system such as disclosed herein, regardless of where the sensor TOF is located relative to the image plane, the sensor TOF’s measurements of the laser beam can be digitally propagated to the image plane to determine what would occur at the image plane, and that value can be used in the measurement processes described herein. As an example of such numerical propagation, if the amplitude and phase of the laser beam are measured / determined at the measurement plane (e.g., at the sensor TOF), since it is known how the amplitude and phase of the laser beam vary with time and distance, the corresponding value can be numerically propagated (e.g., calculated) as if it occurred at the image plane (e.g., the relative location / distance of the image plane can be known from the characteristics of the imaging lens, etc.). Such features can be particularly useful for measuring certain workpieces / workpiece features (e.g., height variations on a workpiece surface can be large enough that the entire workpiece surface can not fit within a single measurement range of certain systems, but a digital holographic metrology system such as disclosed herein can instead digitally propagate to the image plane to obtain / determine the needed values for measurement, etc.).

[0041] In Figure 2 In examples, each frequency-shifted laser beam is obtained by acousto-optic modulation of a corresponding wavelength laser beam (i.e., in the heterodyne light source 210). However, it should be appreciated that in other embodiments, other techniques can be used to obtain each frequency-shifted laser beam. For example, for each wavelength / frequency-shifted laser beam pair, the laser can be a Zeeman laser having a laser medium provided in a strong axial magnetic field to produce two laser beams having different wavelengths / frequencies. According to the terminology used herein, even if sharing the same beam path, laser beams having different wavelengths / frequencies can in some cases each be referred to as a separate laser beam. If the laser beams share a beam path and / or co-propagate, etc., the combination of such laser beams can in some cases be referred to as a combined beam.

[0042] In operation, the timer 240 provides a clock or reference signal to the drivers of the sensor device 230 and the heterodyne light source 210. The acousto-optic modulator of the heterodyne light source 210 is driven by the driver at a modulation frequency Av. As described above, the multi-wavelength laser source of the heterodyne light source 210 generates laser beams of different frequencies. For example, as noted above, the multi-wavelength laser source can provide a first wavelength laser beam of a first frequency vi. The acousto-optic modulator receives the incoming first wavelength laser beam and produces / generates a corresponding first frequency-shifted laser beam of frequency vi + Av that propagates along the first wavelength laser beam. As a result, the output of the heterodyne light source 210 is a first combined beam relative to the first wavelength laser beam. As will be described in greater detail below, the first combined beam can be detected by the sensor device 230 with an envelope modulated at the difference frequency Av.

[0043] Similarly, the multi-wavelength laser source of the heterodyne light source 210 can provide a second wavelength laser beam at a second frequency v2, the acousto-optic modulator receives the input second wavelength laser beam and produces / generates a corresponding second frequency shifted laser beam at a frequency of v2+Av, and the corresponding second frequency shifted laser beam propagates along the second wavelength laser beam, the corresponding output of the heterodyne light source 210 being a second combined beam. Similarly, the multi-wavelength laser source can provide a third wavelength laser beam at a third frequency v3, the acousto-optic modulator receives the input third wavelength laser beam and produces / generates a corresponding third frequency shifted laser beam at a frequency of v3+Av, and the corresponding third frequency shifted laser beam propagates along the third wavelength laser beam, the corresponding output of the heterodyne light source 210 being a third combined beam. It will be appreciated that the total output of the heterodyne light source 210 can thus include a total combined beam (e.g., which can include the first, second, and third combined beams, etc.). In various embodiments, the multi-wavelength laser source of the heterodyne light source 210 can provide additional wavelength laser beams and the acousto-optic modulator generate corresponding additional frequency shifted laser beams to form corresponding additional combined beams, which can also be included in the total combined beam as output from the heterodyne light source 210.

[0044] In various embodiments, the multi-wavelength laser source of the heterodyne light source 210 can in some cases include separate laser sources for providing laser beams having different wavelengths. In various embodiments, it can be desirable for such separate laser sources (e.g., laser diodes) to have a low temperature sensitivity. In various embodiments utilizing separate laser sources (i.e., laser sources having respective wavelengths), the wavelengths of each laser source can drift in the same direction due to temperature changes, etc., the ratio between the wavelengths can remain relatively stable, which can thus result in a relatively stable measurement accuracy of the system. In various embodiments, the temperature sensitivity of each separate source can be configured to match the corresponding wavelength, which can reduce or eliminate any drift in the ratio of the wavelengths due to temperature. When configuring, for example, the heterodyne light source 210 disclosed herein, in various embodiments, the separate laser sources can be tested (e.g., which can have different values due to manufacturing tolerances, etc.), and a combination of laser sources providing a desired ratio between the wavelengths can be selected / utilized.

[0045] Thus, in various embodiments, different laser sources providing a desired combination of wavelength ratios can be included (e.g., in one particular example embodiment, providing wavelengths of, for example, 633 nm, 687 nm, and 767 nm). In one embodiment, a first laser source included in the multi-wavelength laser source can provide a first wavelength laser beam (e.g., a nominal wavelength of 633 nm in one particular example embodiment). The heterodyne light source 210 can correspondingly produce a first combined beam as described above, including a first wavelength laser beam having a vertical polarization. In one embodiment, a second laser source included in the multi-wavelength laser source can provide a second wavelength laser beam (e.g., a nominal wavelength of 687 nm in one particular example embodiment). The heterodyne light source 210 can correspondingly produce a second combined beam as described above, including a second wavelength laser beam having a vertical polarization. In one embodiment, a third laser source included in the multi-wavelength laser source can provide a third wavelength laser beam (e.g., a nominal wavelength of 767 nm in one particular example embodiment). The heterodyne light source 210 can correspondingly produce a third combined beam as described above, including a third wavelength laser beam having a vertical polarization. a first wavelength laser beam having a first wavelength (e.g., 633 nm) and a wavelength having a horizontal polarization a first frequency-shifted laser beam (e.g., each having a linear polarization). The timer 240 generates a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator of the heterodyne light source 210. As a result, the first combined beam results in a detected beat frequency signal (e.g., 40 MHz).

[0046] Similarly, a second laser source included in the multi-wavelength laser source of the heterodyne light source 210 can provide a second wavelength laser beam (e.g., having a nominal wavelength of 687 nm in one particular example implementation). The heterodyne light source 210 can correspondingly generate a second combined beam as described above, including a second wavelength laser beam having a second wavelength (e.g., 687 nm) and a wavelength having a horizontal polarization a second frequency-shifted laser beam of the second wavelength (e.g., 687 nm) and a wavelength having a horizontal polarization Similarly, a third laser source included in the multi-wavelength laser source can provide a third wavelength laser beam (e.g., having a nominal wavelength of 767 nm in one particular example implementation). The heterodyne light source 210 can correspondingly generate a third combined beam as described above, including a third wavelength laser beam having a third wavelength (e.g., 767 nm) and a wavelength having a horizontal polarization a third frequency-shifted laser beam of the third wavelength (e.g., 767 nm) and a wavelength having a horizontal polarization As noted above, the timer 240 can generate a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator of the heterodyne light source 210. As a result, each of the second and third combined beams generates a detected beat frequency signal at the modulation frequency (e.g., 40 MHz).

[0047] As noted above, during operation of the digital holographic metrology system 200, the heterodyne light source 210 outputs a total combined beam including the combined beams to the interference optics 220. As will be described in greater detail below, the interference optics 220 utilize the combined beams to provide an output for imaging the surface 192 of the workpiece 190 (e.g., to an image plane). For example, as will be described in greater detail below, in one example implementation using three wavelengths, the output of the interference optics 220 can include first, second, and third interference beams. In various implementations, the first interference beam is generated based on the first combined beam from the heterodyne light source, the second interference beam is generated based on the second combined beam from the heterodyne light source, and the third interference beam is generated based on the third combined beam from the heterodyne light source.

[0048] Further to the above example, when the first combined beam is incident on a beamsplitter 221 (e.g., a polarizing beamsplitter) of the interferometric optical arrangement 220, a first reference beam having a frequency vi and a vertical polarization propagates along an axis 222 toward a reference mirror 224, while a first signal beam having a frequency vi + Av and a horizontal polarization propagates along an axis 225 to illuminate a portion of the workpiece surface 192. The first reflected reference beam (i.e., from the reference mirror 224) and the first reflected signal beam (i.e., from the workpiece surface 192) are then combined by the beamsplitter 221, thereby forming a first reflected combined beam that propagates along the axis 222 toward a sensor arrangement 230 via an imaging lens portion 227. A quarter waveplate 226 is used to transform the polarization of the first incident beam having a frequency vi + Av from horizontal to circular, and to transform the polarization of the first reflected signal beam from circular to vertical. Similarly, a quarter waveplate 223 is used to transform the polarization of the first incident beam having a frequency vi from vertical to circular, and to transform the polarization of the first reflected reference beam from circular to horizontal. As a result, the polarizations of the first reflected signal beam and the first reflected reference beam after the first and second quarter waveplates, respectively, are orthogonal, thereby preventing interference.

[0049] The workpiece surface 192 is imaged via the imaging lens portion 227 to an image plane, as will be described in greater detail below. A polarizer 228 is used to control the polarization of the first two reflected beams, allowing them to form a first interference beam. A first dichroic component 231-1 (e.g., a dichroic filter for separating wavelengths) operates to reflect the first interference beam IB-1 to a sensor TOF-1 (e.g., a time-of-flight sensor, which in various embodiments can be a time-of-flight camera, etc.) of the sensor arrangement 230. As will be described in greater detail below, the first dichroic component 231-1 also operates to transmit / pass other interference beams having different wavelengths to continue to be reflected by other dichroic components along the axis 225 to other sensors of the sensor arrangement 230. In various embodiments, the dichroic components as defined herein can each include any optical component that can split a beam into two beams having different wavelengths. As mentioned above, the workpiece surface 192 is imaged via the imaging lens portion 227 to an image plane (e.g., the image plane can be before, on, or after the sensor TOF-1).

[0050] When the second combined beam is incident on beamsplitter 221, the second reference beam having frequency v2and vertical polarization propagates along axis 222 toward reference mirror 224, whereas the second signal beam having frequency v2+ Δv and horizontal polarization propagates along axis 225 to illuminate a portion of workpiece surface 192. The second reflected reference beam and the second reflected signal beam are then combined by beamsplitter 221, thus forming a second reflected combined beam that propagates along axis 225 toward sensor device 230 via imaging lens portion 227. Quarter- waveplate 226 is used to transform the polarization of the second incident beam having frequency v2+ Δv from horizontal to circular, and to transform the polarization of the second reflected signal beam from circular to vertical. Similarly, quarter- waveplate 223 is used to transform the polarization of the second incident beam having frequency v2from vertical to circular, and to transform the polarization of the second reflected reference beam from circular to horizontal. As a result, the polarizations of the second reflected signal beam and the second reflected reference beam after the first and second quarter- waveplates, respectively, are orthogonal, thus preventing interference.

[0051] Workpiece surface 192 is imaged via imaging lens portion 227 to an image plane, as will be described in greater detail below. Polarizer 228 is used to control the polarization of the second two reflected beams, allowing them to form a second interference beam. Second dichroic component 231-2 operates to reflect the second interference beam IB-2 toward sensor TOF-2 of sensor device 230. Similar to how first dichroic component 231-1 reflects first interference beam IB-1 when it transmits / passes through second interference beam IB-2, second dichroic component 231-2 also operates to transmit / pass through other interference beams having different wavelengths to continue to be reflected by other dichroic components along axis 225 toward other sensors of sensor device 230. As mentioned above, workpiece surface 192 is imaged via imaging lens portion 227 to an image plane (e.g., the image plane can be before, on, or after sensor TOF-2).

[0052] It should be appreciated that a similar process can occur for additional combined beams of the system. For example, similar to the process described above, there can be a final combined beam (e.g., numbered the Nth combined beam), there can be a corresponding final reference beam having frequency v N and vertical polarization, and a final signal beam having frequency v N+ Δv and horizontally polarized. The corresponding final reflected reference beam and final reflected signal beam can form a final reflected combined beam, from which a final interference beam can be formed. The workpiece surface 192 can be imaged via the imaging lens portion 227 to an image plane, as will be described in greater detail below. The final dichroic assembly 231-N operates to reflect the final interference beam IB-N toward the sensor TOF-N. As noted above, the workpiece surface 192 is imaged via the imaging lens portion 227 to an image plane (e.g., the image plane can be before, on, or after the sensor TOF-N).

[0053] It will be appreciated that in embodiments where N = 3, the final combined beam can be a third combined beam, or in embodiments where N is greater than 3, there can also be a third combined beam of the system. In either case, there can be a corresponding third reference beam having a frequency v3 and vertical polarization, and a third signal beam having a frequency v3 + Δv and horizontal polarization. The corresponding third reflected reference beam and third reflected signal beam can form a third reflected combined beam, from which a third interference beam can be formed. The third dichroic assembly 231-3 can be operable to reflect the third interference beam IB-3 toward the sensor TOF-3 (i.e., the sensor TOF-N when N = 3). It will be appreciated that the use of dichroic assemblies enables the use of multiple wavelength laser beams in the heterodyne light source 210, the beams are combined in a common beam path, and then separated by the dichroic assemblies for receipt by the sensors TOF (e.g., as part of a related measurement function, etc.).

[0054] More particularly, as noted above, the first dichroic assembly 231-1 is configured to separate the first interference beam IB-1 from the second and third interference beams IB-2 and IB-3, the first interference beam IB-1 being directed (e.g., by the first dichroic assembly 231-1) to be received by the first sensor TOF-1 (e.g., a time-of-flight sensor). The second dichroic assembly 231-2 is configured to separate the second interference beam IB-2 from the third interference beam IB-3, the second interference beam IB-2 being directed (e.g., by the second dichroic assembly 231-2) to be received by the second sensor TOF-2 (e.g., a time-of-flight sensor). The third interference beam IB-3 is directed (e.g., by the third dichroic assembly 231-3) to be received by the third sensor TOF-3 (e.g., a time-of-flight sensor).

[0055] As mentioned above, the total output from the heterodyne light source 210 can be a total combined light beam (e.g., which can include the first, second, and third combined light beams, e.g., in the above example with N = 3). In accordance with the operation as described above, when the total combined light beam from the heterodyne light source 210 is incident on the beam splitter 221, a combined reference light beam including the first, second, and third reference light beams having respective frequencies vi, v2, and v3 and vertical polarization propagates along the axis 222 toward the reference mirror 224 (e.g., the beam combination at the reference mirror 224 can be represented as∑v i which in this example indicates that the combined reference light beam includes reference light beams having frequencies vi, v2, and v3. Also in accordance with the operation as described above, further when the total combined light beam from the heterodyne light source 210 is incident on the beam splitter 221, a combined signal light beam including the first, second, and third signal light beams having respective frequencies vi+Av, v2+Av, and v3+Av and horizontal polarization propagates along the axis 225 to illuminate a portion of the workpiece surface 192 (e.g., the beam combination at the workpiece surface 192 can be represented as∑v i +Av, which in this example indicates that the combined signal light beam includes signal light beams having frequencies vi+Av, v2+Av, and v3+Av.

[0056] In various embodiments, the axis 225 can correspond / be defined as a measurement axis of the digital holographic metrology system 200, and the measurement distance can correspond to a distance from the digital holographic metrology system 200 (e.g., from a designated component or reference point of the digital holographic metrology system 200, such as near or at the quarter waveplate 226, or near or at an end of an enclosure that encloses the interferometric optical arrangement 220, etc.) to a surface point on the workpiece surface 192. In various embodiments, the axis 225 can also or instead correspond / be defined as a z-axis of the digital holographic metrology system 200, and the measurement distance can be referred to as a z-distance or in terms of z-height. In various embodiments, different z-heights of surface points on the workpiece surface 192 (e.g., as part of an overall surface profile or otherwise) can be relative to a reference point and / or to each other according to their relative z-heights, e.g., relative to a reference point on the workpiece surface that can be designated as having a z-height of z = 0. In various embodiments, the z-height of each surface point can correspond / be determined from the respective measurement distance / z-distance from the digital holographic metrology system 200 to the surface point.

[0057] In configurations such as those disclosed herein that utilize dichroic components, in various embodiments, it can be desirable for the wavelengths of the light beams to be sufficiently separated so that the light beams can be effectively separated by the dichroic. In contrast, certain existing systems have utilized light beams having wavelengths that are very close (e.g., having a separation of less than 1 nm, such as to achieve a combined synthetic wavelength to achieve a long absolute measurement range, etc.). In configurations as disclosed herein, it can instead be advantageous to utilize a combination of wavelengths having a relatively larger separation (e.g., a separation of at least 2% or 5% of the shortest wavelength value, or a separation of at least 10 nm or 20 nm, for example) so that the dichroic based on wavelength separation can effectively separate the light beams. One particular example combination of wavelengths that meets such requirements is λ1= 633 nm, λ2= 687 nm, and λ3= 767 nm (e.g., 2% less than 13 nm, and 5% less than 32 nm of the shortest wavelength, each of which is less than the separation between the shortest and longest wavelengths, or the separation between any of the wavelengths in this example, such as a separation of 10 nm or 20 nm).

[0058] For each sensor TOF (e.g., TOF-1, TOF-2...TOF-N) of the sensor arrangement 230, in various embodiments, can be a time-of-flight sensor configured to determine the phase relationship between the modulated signal received at each pixel and the interfering light beam. The sensor TOF outputs a signal that compares the phase of the modulated intensity of the light signal falling on each pixel to an electrical reference signal derived from the modulating frequency of the timer 240 (e.g., which can be used as a common clock source) signal. In various embodiments, the sensor TOF can output quadrature signals (IQ) that compute the phase. In various embodiments, such computations can be performed on the sensor itself (e.g., on a chip / processor on the sensor TOF) or on an associated processor / chip (e.g., provided in the sensor electronics 255 or the host computer 258, etc. of the processing portion 250). Generally, the processing portion 250 receives the output from the sensor TOFs (e.g., TOF-1, TOF-2...TOF-N) of the sensor arrangement 230 and utilizes the output (e.g., to determine a measured distance to a surface point on the surface 192 of the workpiece 190).

[0059] In various embodiments, the timer 240 can be part of or receive timing signals from at least one of the sensor TOFs (e.g., sensor TOF-1). In such embodiments, the phase and frequency of the acousto-optic modulator of the heterodyne light source 210 can be effectively controlled by the sensor TOFs (i.e., by the timers of the sensor TOFs). In various embodiments, each sensor TOF can perform 4 phase measurements with 90 degree offsets within one depth frame to measure amplitude and phase. Each pixel on each sensor TOF can be a combined detection and demodulation photonic mixer device. In various embodiments, the sensor TOFs can be sensitive only to the difference frequency from the measurement signal and the reference arm (i.e., including the reference mirror 224). The sum frequency term can be averaged and removed along with the correlated double sampling with direct current (DC) term.

[0060] In various embodiments, the digital holographic metrology system 200 operates as a phase shift holographic system that measures the phase difference between the workpiece wave and the reference wave (e.g., from the respective reflected signal beam and reflected reference beam) on a per pixel basis. Certain prior systems require a computation step to calculate the hologram from at least 3 interferograms, whereas according to the principles disclosed herein, the computation can be performed in at least near real time (e.g., on the sensor TOF or in a processing portion, etc.). In various embodiments, the measured hologram is digitally propagated to an image plane where a phase unwrapping routine is used to combine the phase information from all wavelengths to reconstruct the surface profile over a relatively long unambiguous range (e.g., an absolute measurement range).

[0061] With respect to the sensor TOF, it should be appreciated that in various embodiments, a time-of-flight sensor (e.g., a time-of-flight camera) can have very good phase resolution that can be used to achieve a large non-ambiguous range (NAR) of the system measurement (e.g., for determining an absolute measurement distance to a surface point on a workpiece, etc.). Generally, the better the resolution of the sensor TOF, the longer the non-ambiguous range (e.g., as decodable paths, etc.) that can be achieved. As defined herein, a time-of-flight sensor is any sensor or configuration configured to sense values indicative of the amplitude and phase of a laser beam. The ratio of the wavelengths used is another important factor with respect to the non-ambiguous range that can be achieved. Thus, in various embodiments, for a given system, it is desirable to determine / utilize a wavelength ratio that results in signals that can be unwrapped / decoded in a manner that enables a large non-ambiguous range.

[0062] Figures 3A to 3C is a schematic diagram illustrating certain operational principles of a digital holographic metrology system 300 that is similar to Figure 2 the digital holographic metrology system 200 of Figures 3A to 3COnly certain portions of the digital holographic metrology system 300 are illustrated in order to simplify the description of the relevant principles of operation. It should be understood that Figures 3A to 3C certain numbered components 3XX of the digital holographic metrology system 300 can correspond to and / or have similar numbering as Figure 1 certain numbered counterpart components 1XX of the interferometric optical arrangement 320 or Figure 2 certain numbered counterpart components 2XX of the sensor arrangement 330, and can be understood in similar fashion thereto unless otherwise described below. As Figure 3A illustrated, the output of the heterodyne light source 310 is received by the interferometric optical arrangement 320. Correspondingly, the output of the interferometric optical arrangement 320 is received by the sensor arrangement 330.

[0063] Similar to the operation described above with respect to Figure 2 the heterodyne light source 330 can provide an output consisting of a collimated light beam having a plurality of base frequencies in one polarization and all frequency-shifted components in an orthogonal polarization state (e.g., provided as a total combined collimated light beam including a first combined light beam, a second combined light beam, a third combined light beam, etc.). Upon incidence of the first combined light beam onto a beamsplitter 321 (e.g., a polarization beamsplitter) of the interferometric optical arrangement 320, a first signal light beam having a frequency fi propagates to illuminate a portion of the workpiece surface 192, while a first reference light beam having an orthogonal polarization and a frequency fi + Af propagates toward a reference surface 324 (e.g., a reference mirror). It should be understood that the light beams used for the signal light beam and the reference light beam in this example are switched relative to the light beams used in the example described above with respect to Figure 2 any configuration can be used in various embodiments. The first reflected signal light beam and the first reflected reference light beam are then combined by the beamsplitter 321, thus forming a first reflected combined light beam that propagates toward the sensor arrangement 330.

[0064] Upon incidence of the second combined light beam onto the beamsplitter 321, a second signal light beam having a frequency f2 propagates to illuminate a portion of the workpiece surface 192, while a second reference light beam having an orthogonal polarization and a frequency f2 + Af propagates toward the reference surface 324. The second reflected signal light beam and the second reflected reference light beam are then combined by the beamsplitter 321, thus forming a second reflected combined light beam that propagates toward the sensor arrangement 330. Similar processes occur upon incidence of the third combined light beam onto the beamsplitter 321, etc. In various embodiments, this type of process can be performed for measuring a reference point RP (e.g., which can be designated as z = 0 or other z height) on the workpiece surface 192 and subsequently for measuring other surface points SP on the workpiece surface, as will be described in more detail below with respect to Figure 3B .

[0065] Similar to the above-described process, the total output of the heterodyne light source 310 can be a total combined light beam (e.g., in the example of N = 3, which can include first, second, and third combined light beams). In accordance with the operations as described above, upon the total combined light beam from the heterodyne light source 310 being incident on the beam splitter 321, a combined signal light beam including the first, second, and third signal light beams having respective frequencies fl, f2, and f3 and vertical polarization propagates along an axis to illuminate a portion of the workpiece surface 192 (e.g., the beam combination at the workpiece surface 192 can be represented as ∑f i ), which in this example indicates that the combined signal light beam includes the signal light beams having frequencies fl, f2, and f3. i In accordance with the operations as described above, further upon the total combined light beam from the heterodyne light source 310 being incident on the beam splitter 321, a combined reference light beam including the first, second, and third reference light beams having respective frequencies fl+Δf, f2+Δf, and f3+Δf and horizontal polarization propagates along an axis toward the reference surface 324 (e.g., the beam combination at the reference surface 324 can be represented as ∑f i +Δf, which in this example indicates that the combined reference light beam includes the reference light beams having frequencies fl+Δf, f2+Δf, and f3+Δf.

[0066] Figure 3B Various example signals related to the above-described process are illustrated, including a first set of example signals 380-1 and a second set of example signals 380-2. The first set of example signals 380-1 includes a first reference light beam signal Rl31 (e.g., having frequency fl+Δf), a first signal light beam signal SB1 (e.g., having frequency fl), a first combined light beam signal CB1 (e.g., modulated at beat frequency Δf), and a first combined light beam phase shift signal CBPS1 (e.g., modulated at beat frequency Δf with an additional phase shift resulting from a path length difference 2Δz). In various embodiments, the first combined light beam signal CB1 can result from measuring a reference point (e.g., reference point RP in Figure 3A ) on the workpiece surface. The first combined light beam phase shift signal CBPS1 can result from subsequently measuring a different surface point (e.g., surface point SP in Figure 3A ) on the workpiece surface. As indicated by Figure 3B , the difference between signals CB1 and CBPS1 corresponds to a first phase shift where c corresponds to the speed of light.

[0067] The second set of example signals 380-2 includes a second reference beam signal RB2 (e.g., having a frequency f2+Δf), a second signal beam signal SB2 (e.g., having a frequency f2), a second combined beam signal CB2 (e.g., modulated with a beat frequency Δf), and a second combined beam phase shift signal CBPS2 (e.g., modulated with a beat frequency Δf, having an additional phase shift caused by the path length difference 2Δz). In various embodiments, the second combined beam signal CB2 may be generated by a reference point on the surface of the workpiece (e.g., ...). Figure 3A The reference point RP in the reference beam is used. The second combined beam phase shift signal CBPS2 can be obtained by subsequently measuring surface points on the workpiece surface (e.g., reference point RP). Figure 3A This is caused by surface points (SP) in the middle. For example... Figure 3B As indicated, the difference between signals CB2 and BCPS2 corresponds to the second phase shift. Where c corresponds to the speed of light. It should be understood that this can be used to determine the third phase shift. A third combination of beams is used to perform a similar process.

[0068] Figure 3C The illustration shows the relationship with Figure 3B Certain components of the digital holographic metrology system 300 related to the indicated phase-shift processing. For example... Figure 3C As shown, the sensor device 330 includes sensors TOF-1, TOF-2, and TOF-3, each sensor including corresponding pixel arrays 335-1, 335-2, and 335-3. In each pixel array, corresponding pixels 336-1, 336-2, and 336-3 are designated for measuring specific surface points on the workpiece surface 192 (e.g., as shown in the image). Figure 3A The surface point SP is shown. In various embodiments, the processing section 350 can receive (e.g., at the sensor electronics 355) information about the phase shift. Data such as (e.g., as described above). Then processing section 350 (e.g., using computer 358) can perform phase expansion (e.g., based on phase shift). (The value of Δz is determined by...). In various embodiments, determining the value of Δz corresponds to determining the measured distance to a surface point (e.g., surface point SP) on surface 192 of workpiece 190. The following is about... Figure 4 and 5 A simplified example of using a lookup table to perform phase expansion (e.g., to determine Δz) is described in more detail.

[0069] Figure 4 This is a schematic diagram of a lookup table 400 used for phase unrolling as part of the operation of a digital holographic metrology system. Figure 4In the example, the first column corresponds to a first sensor TOF-1, which receives a signal corresponding to a laser beam with a first wavelength λ1 (e.g., where λ1 = 500 nm). The first column includes signals corresponding to different possible phases. The value (for example, Figure 4 phase This corresponds to the above regarding Figures 3A to 3C The phase shift mentioned above). Figure 4 In the diagram, the second column corresponds to the second sensor TOF-2, which receives signals corresponding to a laser beam with a second wavelength λ2 (e.g., where λ2 = 800 nm). The second column includes signals corresponding to different possible phases. The third column corresponds to the third sensor TOF-3, which receives signals corresponding to a third wavelength laser beam having a third wavelength λ3 (e.g., where λ3 = 1000 nm). The third column includes values ​​corresponding to different possible phases. The value. As a concrete instance of using lookup table 400, if the phase With values ​​corresponding to 216 degrees, 90 degrees, and 288 degrees respectively, it can be seen that these values ​​correspond to the surface height Δz of the measurement surface point SP at Δz = 0.9 micrometers (e.g., relative to...). Figure 3A The height of the reference point RP on the workpiece surface is shown. The values ​​in the fourth column of Table 400 are shown extending in 0.1-micron steps over the range from surface height Δz = 0 to Δz = 2.0 microns (for a total unambiguous range of 2.0 microns). Such relationships are... Figure 5 This is further illustrated below, which will be described in more detail.

[0070] Figure 5 The illustration comes from Figure 4 A diagram illustrating some values ​​of the lookup table. Figure 5 This includes three graphs, 500A, 500B, and 500C, each indicating the period of the phase value on the Y-axis and plotted relative to the surface height scale included along the X-axis at the top of graph 500A (and also indicating the phase value corresponding to each indicated surface height for each 0.1-micron step on the bottom X-axis of each graph). Figure 5 As shown, graph 500A plots the first phase corresponding to a laser beam having a first wavelength λ1 (i.e., where λ1 = 500 nm). The value is shown as a cycle through 8 periods. Graph 500B plots the second phase corresponding to a laser beam having a second wavelength λ2 (i.e., where λ2 = 800 nm). and the value is shown to cycle through 5 periods. Plot 500C illustrates a third phase and the value of is shown to cycle through 4 periods.

[0071] As shown in Figure 5 , the surface height Δz value depends not only on each phase value, but also on the period into which each phase value falls. As one example, for a surface height Δz = 0.9 microns as noted above, the phase is 0.2 degrees and falls into the fourth period of the corresponding waveform, the phase is 0.5 degrees and falls into the third period of the corresponding waveform, and the phase is 0.8 degrees and falls into the second period of the corresponding waveform. Thus, the phase unwrapping process determines a unique surface height Δz corresponding to the unique combination of phase values (i.e., and which is indicated according to the graphical illustration of Figure 5 ).

[0072] In the example of Figure 4 and Figure 5 , the wavelengths λ1 = 500 nm, λ2 = 800 nm, and λ3 = 1000 nm were chosen to simplify the explanation of the principles of operation as described above, and result in a non- ambiguity range of 2.0 microns in this example. Other combinations of wavelengths (e.g., which can result in longer non-ambiguity ranges) can be selected and utilized in accordance with the principles disclosed herein. For example, a combination of wavelengths λ1 = 633 nm, λ2 = 687 nm, and λ3 = 767 nm, which can be implemented with certain commercially available laser light sources, can result in a non-ambiguity range of approximately 8.82 mm in certain embodiments. In various embodiments, corresponding Δz steps can be selected / utilized based on the degree of precision of the phase measurements (e.g., according to signal-to-noise ratios, etc.).

[0073] In various embodiments, various types of phase unwrapping routines (e.g., for determining surface height Δz values based on unique combinations of phase values) can be utilized. For example, as an alternative or supplement to look-up tables such as described above with respect to Figure 4 and 5 , machine learning processes can be utilized for phase unwrapping. As one example of such phase unwrapping utilizing machine learning, a K-Nearest Neighbor (KNN) regressor can be trained on data for machine learning. The effectiveness of such processing has been demonstrated by training a KNN regressor on clean data without any noise, and then using it to determine relative to phase unwrapping performed on relatively noisy data. Such testing has demonstrated that these processes (e.g., including utilizing machine learning for phase unwrapping, such as utilizing a KNN model) result in high precision for determining measurements.

[0074] It should be appreciated that, for example, the digital holographic metrology systems disclosed herein (e.g., with heterodyne detection using time-of-flight cameras as sensors) can have various advantages over certain existing metrology systems. The heterodyne detection utilized is capable of achieving high signal-to-noise ratios in holography, which is utilized to increase the non-ambiguous range (NAR) as well as the precision of the measurements performed. In addition, long non-ambiguous ranges (e.g., absolute measurement ranges) are achieved using phase unwrapping routines that are not dependent on the synthetic wavelength. Wavelength multiplexing is performed (e.g., can be achieved by utilizing dichroics, which are commercially available at relatively low cost and can be used to separate signal beams having individual wavelengths). Wavelength multiplexing can be utilized to capture all phases for all wavelengths relatively simultaneously within one depth frame to improve environmental robustness, using multiple TOF cameras.

[0075] Such rapid acquisition of measurement data helps to ensure that there is minimal or no movement between the workpiece and the system while the measurement data is being captured (e.g., as opposed to existing systems in which images and / or data are captured individually or otherwise sequentially, thus taking more time and potentially allowing the workpiece to move relative to the system, e.g., due to vibrations, accidental movement, normal travel of the workpiece, etc.). The computational workload is reduced by measuring the phase directly, as opposed to requiring a phase retrieval routine, as compared to existing systems. The configurations disclosed herein can utilize three different wavelengths to achieve similar performance as existing multi-wavelength systems that utilize six different wavelengths, in some embodiments, and thus can result in similar performance at a lower cost. The configurations, e.g., those disclosed herein, can be used for applications such as high-throughput inline metrology sampling / measurement, in various embodiments.

[0076] Figure 6A and 6B are schematic diagrams illustrating different modes of operation of a digital holographic metrology system 600, e.g., as disclosed herein. The digital holographic metrology system 600 is similar to the digital holographic metrology systems 200 and 300 of FIGS. 2A-2C and 3A-3C, unless otherwise described below. In Figure 2 and 3A the digital holographic metrology systems 200 and 300 of FIGS. 2A-2C and 3A-3C, unless otherwise described below. In Figure 6A and 6B only certain portions of the digital holographic metrology system 600 are illustrated in FIGS. 6A-6C, for simplicity, to simplify the description of the relevant operating principles of the different modes. As Figure 6A and 6B indicated, the digital holographic metrology system 600 includes a heterodyne light source 610, an interference optical arrangement 620, a sensor arrangement 630, a timer 640, and a processing portion 650. The heterodyne light source 610 includes a multi-wavelength laser light source 611 and an acousto-optic modulator (AOM) 612 (e.g., similar components are described as included in the heterodyne light source 210 of FIG. 2A). As with the digital holographic metrology systems 200 and 300 of FIGS. 2A-2C and 3A-3C, the digital holographic metrology system 600 is configured to perform a measurement of a workpiece 660 (e.g., a wafer, a reticle, a photomask, etc.) using the heterodyne light source 610, the interference optical arrangement 620, and the sensor arrangement 630. Figure 2 Figure 2 ​One difference of the digital holographic metrology system 200 is that the digital holographic metrology system 600 includes a switch 619 (e.g., included in the heterodyne light source 610 or otherwise) for switching between different modes, as will be described in greater detail below.

[0077] Figure 6A The digital holographic metrology system 600 is illustrated operating in an amplitude modulated continuous wave (AMCW) mode 601A, while Figure 6B The digital holographic metrology system 600 is illustrated operating in a digital holography mode 601B (e.g., with the switch 619 used as part of the switching between modes). As will be described in greater detail below, the AMCW mode 601A can be characterized as providing a relatively coarse scale measurement (e.g., with one or more sensor TOFs to perform certain time of flight measurement operations), while the digital holography mode 601B (e.g., which can operate as described above with respect to Figure 2 and 3A to 3C) can be characterized as providing a relatively fine scale measurement, for which measurements of both modes can be combined to provide a high precision measurement over a relatively large unambiguous range.

[0078] As Figure 6A illustrated, for the AMCW mode 601A, the switch 619 couples the timer 640 (e.g., which provides a clock signal) to the multi-wavelength laser light source 611 (e.g., to a current source of the multi-wavelength laser light source 611) to modulate the intensity of the light. The acousto-optic modulator 612 does not receive a clock signal from the timer 640 and remains off in various embodiments. Light from the multi-wavelength laser light source 611 thus passes through the acousto-optic modulator 612 (e.g., as zero order light). It will be appreciated that, in accordance with the operation in this example, no reference beam is directed by the beam splitter 621 toward the reference mirror 624. Rather, the zero order light passing through the acousto-optic modulator 612 passes through the beam splitter 621 and illuminates the surface 192 of the workpiece 190, which is imaged (e.g., as directed by the beam splitter 621 toward the sensor device 630) by the optical arrangement 620 in accordance with the reflected light. The one or more sensor TOFs of the sensor device 630 (e.g., see Figure 2 ) measure the distance / surface height of surface points on the workpiece surface 192 via homodyne detection. In accordance with such operation of the one or more sensor TOFs, a relatively coarse measurement range can be achieved (e.g., including an unambiguous range of about 1.5 meters in one particular example embodiment, with a potential distance error of about 0.5% at such distances corresponding to about 7.5 mm).

[0079] Accordingly, such operation of the AMCW mode 601A effectively provides a coarse scale measurement, which can be utilized Figure 6Bthe digital holographic mode 601B to provide fine scale measurements that can resolve Figure 6A potential distance errors of the coarse scale measurements, e.g., with an unambiguous range greater than the potential distance errors (e.g., in this example, an unambiguous range greater than 7.5 mm). For the digital holographic mode 601B, the switch 619 couples the timer 640 to the acousto-optic modulator 612, which is turned on, and the digital holographic metrology system 600 operates as described above with respect to Figure 2 and 3A to 3C. In various embodiments, the digital holographic metrology system 600 can alternate between modes 601A and 601B on a frame-by-frame basis to achieve the desired measurements.

[0080] As mentioned above, in one particular example implementation of the configuration of Figure 2 (e.g., the configuration of Figure 6B operates similarly), with a combination of wavelengths λ1=633 nm, λ2=687 nm, and λ3=767 nm (which can be implemented with certain commercially available laser light sources), in certain embodiments, can result in an unambiguous range of approximately 8.82 mm. Note that this is greater than the 7.5 mm potential error for the coarse scale measurements of the AMCW mode 601A in the above example. Thus, the measurements of the two modes in these examples can be combined to achieve a total unambiguous range of approximately 1.5 meters with sub-micron precision. It should be appreciated that the use of the two modes 601A and 601B enables the total unambiguous range to be extended (e.g., to 1.5 meters) without the need to add additional laser light sources. In one specific example, to achieve a similar increase in unambiguous range with only a digital holographic mode, e.g., mode 601B, can require additional laser light sources (e.g., such as adding three additional laser light sources, e.g., for a total of six laser light sources for wavelengths λ1, λ2, λ3, λ4, λ5, and λ6). In contrast to such alternative configurations, the use of the two modes 601A and 601B as shown in Figure 6A and 6B can result in a less complex and lower cost system (e.g., with and implementing measurements from only three laser light sources / wavelengths λ1, λ2, λ3, rather than six, etc.).

[0081] In various embodiments, different techniques can be utilized to combine the measurements of the AMCW mode 601 A and the digital holography mode 601 B. As mentioned above, in various embodiments, the AMCW mode 601 A provides surface height data at a coarser scale, and in some cases can be used primarily to resolve ambiguities that unfold beyond the original holographic range. In one specific example embodiment, a = the non-ambiguous range of measurements in the digital holography mode 601 B, while z AMCW = the surface height measurements of the AMCW mode 601 A, and z Holo = the holographic z height measurements of the digital holography mode 601 B. The term I = floor(z AMCW / a) provides an integer multiple of the non-ambiguous range of the digital holography mode 601 B (e.g., that the measured height is expected to be within the non-ambiguous range of the AMCW mode 601 A). As mentioned above, in one particular example embodiment, a modulation frequency of 100 MHz can correspond to an AMCW non-ambiguous range of 1.5 meters, and the digital holography non-ambiguous range can be greater than the potential distance error (e.g., a non-ambiguous range of 7.5 mm in this example). The final z height can be determined by Az = I-a + z Holo. According to this equation, in one example for determining a combined measured distance to a first surface point on a workpiece, a first AMCW mode measured distance provides an integer multiple of the non-ambiguous range of the digital holography, which is combined with a first digital holography mode measured distance to determine a combined measured distance to the first surface point on the workpiece.

[0082] Figures 7 to 11 Various embodiments of a heterodyne light source (e.g., which can be used as the heterodyne light source 110 of FIG. 1 and / or the heterodyne light source 210 of FIG. 2, etc.) are illustrated. In various embodiments, each of the configurations of FIGS. 6A-6E can be characterized as an inline optical device for co-propagating and cross-polarized beams (e.g., from an acousto-optic modulator for heterodyne interference, etc.). More specifically, each of these embodiments can be characterized as an inline optical device for combining orthogonally polarized output beams (e.g., from an acousto-optic modulator) into a single beam for use as an output of a heterodyne light source. In the inline optical devices disclosed herein, the orthogonally polarized beams are noted to each pass through the same set of components of the source optical device, with the birefringent optical element portion (i.e., including the birefringent optical element) combining the orthogonally polarized output beams into a single beam path as part of a single combined beam. Figure 1 Figure 2 Figures 7 to 10

[0083] ​​​It should be appreciated that the embodiments described below can have certain advantages over certain alternative embodiments. For example, in one alternative embodiment, a fiber coupling can be utilized (e.g., the output of the acousto-optic modulator can be focused by a lens into a fiber (e.g., a polarization maintaining fiber)). However, such a configuration can have high coupling losses due to the following combination: beam shift in the focal plane that results in clipping; beam size / mode field diameter mismatch; and / or limited fiber input numerical aperture (NA). In another alternative embodiment, two polarization beam splitters and reflectors can be utilized, a first beam from the acousto-optic modulator can be directed upward by a first beam splitter to be reflected by a first reflector to a second reflector that reflects the first beam downward back to a second beam splitter that directs the first beam forward, and a second beam from the acousto-optic modulator travels straight through the first and second beam splitters to co-propagate out of the configuration with the first beam. However, such a configuration can have mismatched beam parameters due to unequal path lengths (i.e., first and second beams) and not having a common beam path (i.e., through different optical elements), which can result in a lack of robustness. It should be appreciated that the embodiments described below with respect to Figures 7 to 11 have certain advantages over such configurations.

[0084] Figure 7 is a schematic diagram of a heterodyne light source 710 that includes a laser light source 711, an acousto-optic modulator 712, a driver 713, and source optics 718 (i.e., that includes a receiving optical element portion 714p and a birefringent optical element portion 715p). The receiving optical element portion 714p includes an imaging system with a single imaging lens 714. The birefringent optical element portion 715p includes a Wollaston prism 715.

[0085] In operation, a timer (e.g., timer 140, timer 240, etc.) can provide a timing signal (e.g., a clock / reference signal) to the driver 713. The acousto-optic modulator 712 is then driven by the driver 713 at a modulation frequency Δω. The laser light source 711 can provide a first wavelength laser beam at a first frequency ω1(i.e., with a corresponding first wavelength). The acousto-optic modulator 712 (e.g., a shear wave AOM) receives the incoming first wavelength laser beam and produces / generates a corresponding first frequency shifted laser beam at a frequency ω1+ Δω that is orthogonally polarized with respect to the incoming first wavelength laser beam. The object plane in the acousto-optic modulator 712 is imaged by the imaging lens 714 into the Wollaston prism 715 with a magnification M that matches both the separation angle a of the acousto-optic modulator 712 and the separation angle β of the Wollaston prism 715, where M = tan(a) / tan(β). In accordance with the operational characteristics of the Wollaston prism 715, the output is a first combined beam that includes the combined first wavelength laser beam at the first frequency ω1and the orthogonally polarized first frequency shifted laser beam at the frequency ω1+ Δω.

[0086] The components of the heterodyne source 710 (e.g., including the imaging lens 714 and the Wollaston prism 715) can therefore be characterized as inline optics for co-propagating and cross-polarized beams. More specifically, these components can be characterized as inline optics for combining orthogonally polarized output beams into a single beam from the heterodyne source output. Figure 7 As indicated, the output of the heterodyne light source 710 may be provided to or received by a measuring optical device MOA (e.g., measuring optical devices 120, 220, etc.) for, for example, for measurement operations (e.g., for determining at least one measurement distance to at least one surface point on the workpiece based on a measurement process utilizing a first combined beam).

[0087] Figure 8 This is a schematic diagram of a heterodyne light source 810, which includes a laser light source 811, an acousto-optic modulator 812, a driver 813, and a source optics device 818 (i.e., it includes a receiving optics section 814p and a birefringent optics section 815p). The receiving optics section 814p includes an imaging system with two imaging lenses 814A and 814B. The birefringent optics section 815p includes a Wollaston prism 815. The heterodyne light source 810 and... Figure 9 , 10 The heterodyne light sources 910, 1010, and 1110 will be understood as being related to 11. Figure 7 The heterodyne light source 710 operates similarly, unless otherwise described below. Regarding imaging systems with a single imaging lens 714... Figure 7 Heterodyne light source 710, Figure 8 The main difference of the heterodyne light source 810 is that it includes an imaging system with multiple lenses 814A and 814B, which can provide certain desired operating characteristics for certain applications, as will be described in more detail below.

[0088] In an embodiment where the laser source 811 can be a multi-wavelength laser source, the diffraction angle α of the acousto-optic modulator 812 can depend on the wavelength, for example... Figure 7In some embodiments of the configuration, beams with different wavelengths can thus not co-propagate (e.g., which can result in beam clipping later in the configuration, etc.). To address such issues, the imaging system (i.e., including lenses 814A and 814B) can be configured as a chromatic imaging system, the magnification can be wavelength dependent, and can be matched to the diffraction angle a (i.e., for each wavelength) of the acousto-optic modulator 812 (e.g., such that the laser beams of different wavelengths will co-propagate and / or otherwise not later experience beam clipping in the configuration, etc.). More specifically, according to the configuration, the object plane in the acousto-optic modulator 812 is imaged by the chromatic imaging system (i.e., including imaging lenses 814A and 814B) into the Wollaston prism 815 with a magnification M that varies with each wavelength and is matched to the separation angle a(ω) of the acousto-optic modulator 812 and the separation angle β(ω) of the Wollaston prism 815 for each wavelength, where M = tan(a(ω)) / tan(β(ω)).

[0089] Figure 9 is a schematic diagram of a heterodyne light source 910 that includes a laser light source 911, an acousto-optic modulator 912, a driver 913, and source optics 918 (i.e., which includes a receiving optical element portion 914p and a birefringent optical element portion 915p). The receiving optical element portion 914p includes a receiving lens 914. The birefringent optical element portion 915p includes a birefringent beam displacer 915.

[0090] Similar to the operations described above with respect to Figure 7 the acousto-optic modulator 912 (e.g., a shear wave AOM) receives an input first wavelength laser beam and produces / generates a corresponding first frequency shifted laser beam that is orthogonally polarized with respect to the input first wavelength laser beam. The input lens 914 focuses (e.g., at a focal length f) into the acousto-optic modulator 912 and images to infinity (i.e., the individual beams are collimated). According to the operational characteristics of the birefringent beam displacer 915, the first wavelength laser beam passes through while the first frequency shifted laser beam (i.e., orthogonally polarized) is effectively displaced to be output with the first wavelength laser beam. The output of the birefringent beam displacer 915 is thus a first combined beam that includes the combined first wavelength laser beam and orthogonally polarized first frequency shifted laser beam.

[0091] The components of the heterodyne light source 910 (e.g., including the imaging lens 914 and the birefringent beam displacer 915) can thus be characterized as inline optics for co-propagating and cross-polarized beams. More specifically, these components can be characterized as inline optics for combining orthogonally polarized output beams into a single beam from the heterodyne light source output. As Figure 9As indicated, the output of the heterodyne light source 910 can be provided to / received by the measurement optics MOA (e.g., measurement optics 120, 220, etc.) e.g., for a measurement operation (e.g., for determining at least one measurement distance to at least one surface point on the workpiece based on a measurement process utilizing the first combined light beam).

[0092] Figure 10 is a schematic diagram of a heterodyne light source 1010 that includes a laser light source 1011, an acousto-optic modulator 1012, a driver 1013, and a source optics 1018 (i.e., that includes a receiving optics portion 1014p and a birefringent optics portion 1015p). The receiving optics portion 1014p includes a receiving prism 1014. The birefringent optics portion 1015p includes a birefringent beam displacer 1015. The heterodyne light source 1010 will be understood to operate similarly to the heterodyne light source 910 of Figure 9 Figure 9 Figure 10 The main difference of the heterodyne light source 1010 of

[0093] As mentioned above, Figures 7 to 10 Embodiments of the heterodyne light source 910 of

[0094] It will be appreciated that,​​Figures 7 to 10 Each configured birefringent optical element (e.g., including respective Wollaston prisms 715 and 815 and respective birefringent beam shifters 915 and 1015) of the Figures 7 to 10 configurations can in various embodiments each be characterized as being used in a reverse / backwards direction relative to certain other uses of such components. More specifically, such birefringent optical elements are more typically used to receive a light beam and / or co-propagating light beams along a light path on one side, and then split the light beams apart to be provided as outputs. In contrast to such existing uses, according to the Figures 7 to 10 configurations, respective birefringent optical elements are each used to receive different light beams, and then combine the light beams to be output as part of a single combined light beam (e.g., including co-propagating light beams). As noted above, Figures 7 to 10 Embodiments of the

[0095] Figure 11 is a schematic diagram of a heterodyne light source 1110 including a laser light source 1111, an acousto-optic modulator 1112, a driver 1113, and a source optical arrangement 1118 (i.e., which includes a spatial filtering arrangement 1116 and a collimating lens 1117). In various embodiments, the laser light source 1111 can be a multi-wavelength laser light source (e.g., providing first, second, third, etc. wavelength laser light beams), and the acousto-optic modulator 1112 can correspondingly produce a combined light beam as described herein. In various embodiments, the combined light beam (e.g., including first, second, third, etc. combined light beams) from the acousto-optic modulator 1112 can pass through the spatial filtering arrangement 1116 and the collimating lens 1117 on a path to a measurement optical arrangement MOA. According to the principle of conservation of momentum, different wavelengths can be diffracted at different angles by operation of the acousto-optic modulator 1112. In one embodiment, the spatial filtering arrangement 1116 can include a microscope objective lens 1116A and a pinhole filter element 1116B, the combined light beam from the acousto-optic modulator 1112 can be imaged by the objective lens 1116A onto the pinhole filter element 1116B (e.g., including a 10 pm pinhole) such that diffracted and non-diffracted light beams of different wavelengths overlap, and light exiting the pinhole filter element 1116B is effectively used to provide a point source of light beams (e.g., having a divergence of 0.1 mrad) to the collimating lens 1117. In various embodiments, the collimating lens 1117 can be configured to collimate the light beams from the pinhole filter element 1116B to provide a collimated light beam (e.g., including first, second, third, etc. collimated light beams) to the measurement optical arrangement MOA. In various embodiments, the spatial filtering arrangement 1116 and the collimating lens 1117 can be configured to provide a collimated light beam (e.g., including first, second, third, etc. collimated light beams) to the measurement optical arrangement MOA that is substantially free of different wavelengths of light (e.g., such as different wavelengths of light that are not diffracted by the acousto-optic modulator 1112). Figure 11The indicated available overlap area UA). After the combined beam (e.g., including first, second, third, etc. combined beams) passes through the spatial filter configuration 1116, the collimating lens 1117 operates to collimate the combined beam.

[0096] Typically, multi-wavelength laser sources can be used in Figures 7 to 11 In various configurations of the implementation scheme (e.g., especially as mentioned above) Figure 8 and 11 As described, although it can also be used in other configurations. (Regarding...) Figure 11 The multi-wavelength laser source 1111 provides the following description of its operation; however, it should be understood that this description can also be applied when a multi-wavelength laser source is correspondingly used. Figures 7 to 10 Any configuration of the multi-wavelength laser source 1111. In one example embodiment, the multi-wavelength laser source 1111 can provide a first wavelength laser beam with a first frequency ω1 (i.e., having a corresponding first wavelength), and the acousto-optic modulator 1112 receives the first wavelength laser beam and generates a corresponding first frequency-shifted laser beam with a frequency of ω1+Δω. Similarly, the multi-wavelength laser source 1111 can provide a second wavelength laser beam with a second frequency ω2 (i.e., having a corresponding second wavelength), and the acousto-optic modulator 1112 receives the second wavelength laser beam and generates a corresponding second frequency-shifted laser beam with a frequency of ω2+Δω. Similarly, the multi-wavelength laser source 1111 can provide a third wavelength laser beam with a third frequency ω3 (i.e., having a corresponding third wavelength), and the acousto-optic modulator 1112 receives the third wavelength laser beam and generates a corresponding third frequency-shifted laser beam with a frequency of ω3+Δω. In various embodiments, the multi-wavelength laser source 1111 can provide additional or fewer wavelength laser beams (e.g., where the acousto-optic modulator 1112 generates corresponding additional or fewer frequency-shifted laser beams). For the operation of the acousto-optic modulator 1112, it should be understood that the phase shift Δω on the acoustic wave produces the same amount of optical phase shift Δω on all beams, regardless of wavelength. For such operation, in various embodiments, it may be desirable for the driver 1113 to be a phase-locked radio frequency (RF) driver.

[0097] As mentioned above, a timer is coupled to a driver 1113 of the acousto-optic modulator 1112 to provide a reference clock signal, based on which a modulation frequency Δω is generated in the driver 1113. In one embodiment, the clock signal is provided at a modulation frequency Δω (e.g., in the range of 40 MHz to 100 MHz). The distributed clock signal can be equal to the acoustic wave frequency in the acousto-optic modulator 1112 and equal to the frequency shift Δω of the light beam (e.g., the photon frequency plus an acoustic phonon). In various alternative embodiments, the distributed clock signal can be any other frequency, and then multiplied / divided to the desired target frequency using additional electronic components.

[0098] In this example, each frequency-shifted laser beam is obtained by acousto-optic modulation of a laser beam of corresponding wavelength. Some advantages of this configuration include the availability of multiple wavelengths, all with the same frequency shift, and the possibility of phase-locking; the overall configuration (e.g., relative to some existing configurations) can be relatively less complex and less costly. In various alternative configurations, other methods can be used to obtain each frequency-shifted laser beam. For example, for each source / frequency-shifted laser beam pair, the laser can be a Zeeman laser with a laser medium provided in a strong axial magnetic field to generate two beams with different frequencies. In various other alternative implementations, an electro-optic modulator can be used instead of an acousto-optic modulator, or a combination of multiple dual-wavelength sources (e.g., lasers operating simultaneously at two wavelengths) can be used.

[0099] In various embodiments, the multi-wavelength laser source 1111 may include, in some configurations, separate laser sources for providing laser beams with different wavelengths. For example, different laser sources may be included to provide a desired combination of wavelength ratios (e.g., in a particular example embodiment, wavelengths of, for example, 633 nm, 687 nm, and 767 nm). For example, in one embodiment, a first laser source included in the multi-wavelength laser source 1111 may provide a first wavelength laser beam (e.g., in a particular example embodiment, a nominal wavelength of 633 nm). The heterodyne source 1110 may correspondingly generate the first combined beam as described above, including a first wavelength with vertical polarization. A first wavelength laser beam (e.g., 633 nm) and a wavelength with horizontal polarization A first frequency-shifted laser beam (e.g., each with linear polarization) is generated. A timer generates a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator 1112 (e.g., driver 1113). As a result, the first combined beams result in a detected beat frequency signal (e.g., 40 MHz).

[0100] Similarly, a second laser source included in the multi-wavelength laser source 1111 can provide a second wavelength laser beam (e.g., nominally 687 nm in a particular example embodiment). The heterodyne source 1110 can correspondingly generate a second combined beam as described above, including a second wavelength with vertical polarization. A second wavelength laser beam (e.g., 687nm) and a wavelength with horizontal polarization (e.g., each with linear polarization) a second frequency-shifted laser beam. Similarly, a third laser source included in the multi-wavelength laser source 1111 can provide a third wavelength laser beam (e.g., nominally 767 nm in a particular example embodiment). The heterodyne source 1110 can correspondingly generate a third combined beam as described above, including a third wavelength with vertical polarization. a third wavelength laser beam having a third wavelength (e.g., 767 nm) and a horizontal polarization a third frequency shifted laser beam (e.g., each having a linear polarization). As mentioned above, a timer can generate a clock signal (e.g., 40 MHz) that sets the drive frequency of the acousto-optic modulator 1112 (e.g., driver 1113). As a result, each of the second and third combined beams generates a detected beat frequency signal at the modulation frequency (e.g., 40 MHz). In various embodiments, it can be desirable to use any single laser source having a narrow linewidth (e.g., for having a long coherence length).

[0101] Figure 12 is a flowchart illustrating an exemplary embodiment of a routine 1200 for operating a heterodyne light source. At block 1210, a first light source of the heterodyne light source is operated to provide at least a first wavelength laser beam. At block 1220, an acousto-optic modulator of the heterodyne light source is operated to receive the first wavelength laser beam and generate a corresponding first frequency shifted laser beam (e.g., having a polarization that is orthogonal to a polarization of the first wavelength laser beam). In various embodiments, a source optical arrangement receives and combines the first wavelength laser beam and the first frequency shifted laser beam from the acousto-optic modulator and outputs a corresponding first combined beam. The source optical arrangement includes a receiving optical element portion and a birefringent optical element portion. The receiving optical element portion is configured to receive the first wavelength laser beam and the first frequency shifted laser beam and direct the beams along an optical path toward the birefringent optical element portion. The birefringent optical element portion is configured to receive the first wavelength laser beam and the first frequency shifted laser beam and combine the beams to output the corresponding first combined beam (e.g., to a measurement optical arrangement). At block 1230, at least one measured distance to at least one surface point on a workpiece is determined based on a measurement process utilizing the first combined beam.

[0102] Figure 13 is a flowchart illustrating an exemplary embodiment of a routine 1300 for operating a digital holographic metrology system including a heterodyne light source. At block 1310, a multi-wavelength light source (e.g., a multi-wavelength laser light source) of the heterodyne light source is operated to provide at least a first wavelength laser beam and a second wavelength laser beam. At block 1320, an acousto-optic modulator of the heterodyne light source is operated to: receive the first wavelength laser beam and generate a corresponding first frequency shifted laser beam that combines with the first wavelength laser beam as a first combined beam; and receive the second wavelength laser beam and generate a corresponding second frequency shifted laser beam that combines with the second wavelength laser beam as a second combined beam. An interference optical arrangement receives the combined beams from the heterodyne light source and utilizes the combined beams to provide an output for imaging a workpiece. The output includes at least: a first interference beam generated based on the first combined beam from the heterodyne light source; and a second interference beam generated based on the second combined beam from the heterodyne light source.

[0103] At block 1330, a sensor device is operated to receive an output from the interferometric optical device. The sensor device includes at least first and second time-of-flight sensors and a first dichroic component. The first dichroic component is configured to separate the first and second interference beams, the first interference beam being directed to be received by the first time-of-flight sensor and the second interference beam being directed to be received by the second time-of-flight sensor. At block 1340, the output is received from the first and second time-of-flight sensors and utilized to determine at least one measured distance to at least one surface point on the workpiece.

[0104] While the preferred embodiments of the present disclosure have been illustrated and described, it will be clear to those skilled in the art that many changes can be made in the arrangement and operation of the features illustrated and described without departing from the spirit and scope of the disclosure. Various alternatives to the principles of the disclosure as disclosed herein are possible. In addition, individual embodiments of the above-described arrangements can be combined in further embodiments. All of the U.S. patents referred to in this specification are incorporated by reference herein in their entirety. Aspects of the embodiments can be modified, if necessary, to employ the various patents and applications referred to in this specification to provide yet further embodiments in accordance with the principles of the disclosure.

[0105] These and other changes can be made to the embodiments in light of the above -detailed description. In general, the terms used in the following claims should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled.

Claims

1. A digital holographic measurement system, comprising: Heterodyne light source, the heterodyne light source comprising: A multi-wavelength light source, wherein the multi-wavelength light source is used to provide at least a first wavelength laser beam at a first frequency and a second wavelength laser beam at a second frequency different from the first frequency; an acousto-optic modulator, the acousto-optic modulator being configured to: Receives the first wavelength laser beam and generates a corresponding first frequency-shifted laser beam, wherein the corresponding first frequency-shifted laser beam is combined with the first wavelength laser beam to form a first combined beam; and The second wavelength laser beam is received and a corresponding second frequency-shifted laser beam is generated. The corresponding second frequency-shifted laser beam is combined with the second wavelength laser beam to form a second combined beam. An interferometric optical device receives a first combined beam and a second combined beam from the heterodyne light source and uses the first combined beam and the second combined beam to provide an output for imaging a workpiece, wherein the output includes at least: A first interference beam, the first interference beam being generated based on the first combined beam from the heterodyne source; and The second interference beam is generated based on the second combined beam from the heterodyne source; as well as A sensor device that receives the output from the interferometric optics device, the sensor device comprising at least first and second time-of-flight sensors and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam being guided to be received by the first time-of-flight sensor and the second interference beam being guided to be received by the second time-of-flight sensor.

2. The digital holographic measurement system of claim 1, further comprising a processing unit that receives outputs from the first and second time-of-flight sensors and uses the outputs to determine at least one measurement distance to at least one surface point on the workpiece.

3. The digital holographic measurement system according to claim 1, wherein: The multi-wavelength light source further provides a third wavelength laser beam with a third frequency different from the first and second frequencies; The acousto-optic modulator is further configured to receive the third wavelength laser beam and generate a corresponding third frequency-shifted laser beam, wherein the corresponding third frequency-shifted laser beam and the third wavelength laser beam are combined to form a third combined beam. The output of the interferometric optical device further includes a third interference beam, which is generated based on the third combined beam from the heterodyne source; and The sensor device further includes a third time-of-flight sensor and a second dichroic component, wherein the second dichroic component is configured to separate the second interference beam from the third interference beam, the second interference beam being guided to be received by the second time-of-flight sensor and the third interference beam being guided to be received by the third time-of-flight sensor.

4. The digital holographic metrology system of claim 1, wherein the interferometric optical device comprises a beam splitter, a reference surface, and an imaging lens portion including an imaging lens, the beam splitter receiving the combined beam from the heterodyne light source and splitting the combined beam into a first portion guided to the workpiece surface and a second portion guided to the reference surface, the reflected first portion and the reflected second portion being received back by the beam splitter and combined and guided into the interferometric beam through the imaging lens toward the sensor device.

5. The digital holographic metering system according to claim 1, further comprising a timer, the timer providing: Signals to the heterodyne light source used to operate the acousto-optic modulator; and The signal to the time-of-flight sensor.

6. The digital holographic metering system of claim 5, wherein the timer is included as part of one of the time-of-flight sensors.

7. The digital holographic metrology system of claim 5, wherein the system is configured to operate in the following modes: In a digital holographic mode, during which the timer is coupled to operate the acousto-optic modulator that generates the first and second combined light beams, and a processing section receives outputs from the first and second time-of-flight sensors and uses the outputs to determine a first digital holographic mode measurement distance to a first surface point on the workpiece; and In an amplitude-modulated continuous wave ("AMCW") mode, during which the acousto-optic modulator does not generate the first and second combined beams, and the processing portion uses the output of at least one of the time-of-flight sensors to determine a first AMCW mode measurement distance to the first surface point on the workpiece via zero-difference detection, wherein at least a portion of the first AMCW mode measurement distance and the first digital holographic mode measurement distance are combined to determine a combined measurement distance to the first surface point on the workpiece.

8. The digital holographic measurement system of claim 7, wherein the digital holographic unambiguous range of the digital holographic mode is greater than the potential distance error of the AMCW mode, and the first AMCW mode measurement distance provides an integer multiple of the digital holographic unambiguous range, the integer multiple being combined with the first digital holographic mode measurement distance to determine the combined measurement distance to the first surface point on the workpiece.

9. The digital holographic metering system according to claim 7, wherein the AMCW mode has an unambiguous range at least 50 times larger than the unambiguous range of the digital holographic mode.

10. The digital holographic measurement system of claim 7, wherein the AMCW mode has an unambiguous range of greater than 500 mm, and the digital holographic mode has an unambiguous range of greater than 5 mm when using laser beams of the first, second, and third wavelengths from the light source.

11. The digital holographic metrology system according to claim 1, wherein the difference between the first wavelength of the first wavelength laser beam and the second wavelength of the second wavelength laser beam is greater than 2% of the first wavelength.

12. The digital holographic metrology system according to claim 1, wherein the difference between the first wavelength of the first wavelength laser beam and the second wavelength of the second wavelength laser beam is greater than 10 nanometers.

13. A method for operating a digital holographic metrology system, the method comprising: Operating a heterodyne light source, which includes: Operate a multi-wavelength light source to provide at least a first wavelength laser beam at a first frequency and a second wavelength laser beam at a second frequency different from the first frequency; and Operate the acousto-optic modulator to: Receives the first wavelength laser beam and generates a corresponding first frequency-shifted laser beam, wherein the corresponding first frequency-shifted laser beam is combined with the first wavelength laser beam to form a first combined beam; and The system receives the second wavelength laser beam and generates a corresponding second frequency-shifted laser beam. The corresponding second frequency-shifted laser beam is combined with the second wavelength laser beam to form a second combined beam. The interferometric optical device receives the first combined beam and the second combined beam from the heterodyne light source and uses the first combined beam and the second combined beam to provide an output for imaging the workpiece, wherein the output includes at least: A first interference beam, the first interference beam being generated based on the first combined beam from the heterodyne source; and A second interference beam, the second interference beam being generated based on the second combined beam from the heterodyne source; and The sensor device is operated to receive the output from the interferometric optical device, the sensor device including at least first and second time-of-flight sensors and a first dichroic component, wherein the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam being guided to be received by the first time-of-flight sensor and the second interference beam being guided to be received by the second time-of-flight sensor.

14. The method of claim 13, further comprising receiving output from the first and second time-of-flight sensors and using the output to determine at least one measurement distance to at least one surface point on the workpiece.

15. The method of claim 13, further comprising: Operate the multi-wavelength light source to further provide a third wavelength laser beam at a third frequency different from the first and second frequencies; and The acousto-optic modulator is operated to further receive the third wavelength laser beam and generate a corresponding third frequency-shifted laser beam. The corresponding third frequency-shifted laser beam is combined with the third wavelength laser beam to form a third combined beam. The output of the interferometric optics further includes a third interference beam, which is generated based on the third combined beam from the heterodyne source. The sensor device further includes a third time-of-flight sensor and a second dichroic component, wherein the second dichroic component is configured to separate the second interference beam from the third interference beam, the second interference beam being guided to be received by the second time-of-flight sensor and the third interference beam being guided to be received by the third time-of-flight sensor.

16. The method of claim 13, further comprising operating a timer to provide: Signals to the heterodyne light source used to operate the acousto-optic modulator; and The signal to the time-of-flight sensor.

17. The method of claim 16, further comprising operating the system in the following modes: In a digital holographic mode, during which the timer is coupled to operate the acousto-optic modulator that generates the first and second combined light beams, and a processing section receives outputs from the first and second time-of-flight sensors and uses the outputs to determine a first digital holographic mode measurement distance to a first surface point on the workpiece; and In an amplitude-modulated continuous wave ("AMCW") mode, during which the acousto-optic modulator does not generate the first and second combined beams, and the processing portion uses the output of at least one of the time-of-flight sensors to determine a first AMCW mode measurement distance to the first surface point on the workpiece via zero-difference detection, wherein at least a portion of the first AMCW mode measurement distance and the first digital holographic mode measurement distance are combined to determine a combined measurement distance to the first surface point on the workpiece.

18. The method of claim 17, wherein the digital holographic unambiguous range of the digital holographic mode is greater than the potential distance error of the AMCW mode, and the first AMCW mode measurement distance provides an integer multiple of the digital holographic unambiguous range, the integer multiple being combined with the first digital holographic mode measurement distance to determine the combined measurement distance to the first surface point on the workpiece.

19. A sensor device for a digital holographic metrology system, wherein the digital holographic metrology system comprises: Heterodyne light source, the heterodyne light source comprising: A multi-wavelength light source, wherein the multi-wavelength light source is used to provide at least a first wavelength laser beam at a first frequency and a second wavelength laser beam at a second frequency different from the first frequency; an acousto-optic modulator, the acousto-optic modulator being configured to: Receives the first wavelength laser beam and generates a corresponding first frequency-shifted laser beam, wherein the corresponding first frequency-shifted laser beam is combined with the first wavelength laser beam to form a first combined beam; and The second wavelength laser beam is received and a corresponding second frequency-shifted laser beam is generated. The corresponding second frequency-shifted laser beam is combined with the second wavelength laser beam to form a second combined beam. An interferometric optical device receives a first combined beam and a second combined beam from the heterodyne light source and uses the first combined beam and the second combined beam to provide an output for imaging a workpiece, wherein the output includes at least: A first interference beam, the first interference beam being generated based on the first combined beam from the heterodyne source; and The second interference beam is generated based on the second combined beam from the heterodyne source. The sensor device, the sensor device comprising: First time-of-flight sensor; Second time-of-flight sensor; and First dichroic component, The sensor device receives the output from the interferometric optics, and the first dichroic component is configured to separate the first interference beam from the second interference beam, the first interference beam being guided to be received by the first time-of-flight sensor and the second interference beam being guided to be received by the second time-of-flight sensor.

20. The sensor device according to claim 19, wherein: The multi-wavelength light source further provides a third wavelength laser beam with a third frequency different from the first and second frequencies; The acousto-optic modulator is further configured to receive the third wavelength laser beam and generate a corresponding third frequency-shifted laser beam, wherein the corresponding third frequency-shifted laser beam and the third wavelength laser beam are combined to form a third combined beam. The output of the interferometric optical device further includes a third interference beam, which is generated based on the third combined beam from the heterodyne source; and The sensor device further includes: The third time-of-flight sensor; and The second dichroic component, The second dichroic component is configured to separate the second interference beam from the third interference beam, the second interference beam being guided to be received by the second time-of-flight sensor and the third interference beam being guided to be received by the third time-of-flight sensor.

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