SYSTEM AND METHOD FOR PERFORMING CHARACTERIZATION OF A SAMPLE USING MULTI-WAVEL

Abstract

An optical acoustic measurement device, such as a picosecond laser acoustic measurement device, includes a laser light source for generating pulsed light having a first wavelength and a supercontinuum generator for spectrally broadening the pulsed light. A pump arm receives the pulsed light and generates a pump pulse that irradiates a target sample for inducing a transient perturbation in the target sample. A probe arm receives the pulsed light and generates a probe pulse that irradiates the target sample for generating a reflected probe pulse that is modulated based on the transient perturbation in the target sample. The pump pulse and the probe pulse have different wavelengths or the same wavelength, selected from the spectrally broadened pulsed light, for example, using a filter located before the pump arm and the probe arm or in one of the pump arm or the probe arm. A property of the target sample can be determined based on the reflected probe pulse.

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE Embodiments of the subject matter described herein relate generally to the non-destructive measurement of a sample, and more particularly, to characterization of a sample using optical metrology. [Background technology]

[0002] In the semiconductor and other similar industries, optical metrology devices are often used for non-contact evaluation of substrates during processing. In optical metrology, the sample under test is illuminated with light, for example at a single wavelength or multiple wavelengths. After interacting with the sample, the resulting light is detected and analyzed to determine desired properties of the sample.

[0003] There are many different techniques for measuring the properties of a sample, such as a semiconductor. One such technique is photoacoustic metrology, where an acoustic wave generated in a pump beam reflects a portion of the probe beam that interferes with another portion of the probe beam reflected from a surface interface. Interferometric measurements generated using photoacoustic metrology can provide information about the properties of the sample. Picosecond acoustics (ultrasound), for example, uses a short light pulse as a pump that, when absorbed by the sample, launches a strain pulse into the sample. The strain pulse can be partially reflected at each interface within the sample to generate a reflected pulse that propagates back toward the sample surface. The returning strain pulse can change the optical reflectivity of the structure, which can be measured using a time-delayed probe pulse. Summary of the Invention

[0004] An optical acoustic measurement device, such as a picosecond laser acoustic measurement device, includes a laser light source for generating pulsed light having a first wavelength and a supercontinuum generator for spectrally broadening the pulsed light. The laser light source may generate a single pulsed light beam (in the case of a homodyne configuration) or two pulsed light beams (in the case of a heterodyne configuration). The pump arm receives the pulsed light and generates a pump pulse that irradiates the target sample for inducing a transient perturbation in the target sample. The probe arm receives the pulsed light and generates a probe pulse that irradiates the target sample for generating a reflected probe pulse that is modulated based on the transient perturbation in the target sample. The supercontinuum generator may spectrally broaden the pulsed light in an optical path before the pump arm and the probe arm, or in one of the pump arm or the probe arm. One or more wavelengths may be selected from the spectrally broadened pulsed light such that the pump pulse and the probe pulse may have different wavelengths or the same wavelength. A property of the target sample may be determined based on the reflected probe pulse.

[0005] In one implementation, a photoacoustic measurement device for non-destructive measurement of a target sample includes a laser light source for generating pulsed light having a first wavelength, and a supercontinuum generator for receiving the pulsed light having the first wavelength and spectrally broadening the pulsed light. A pump arm in the photoacoustic measurement device is configured to receive the pulsed light and irradiate the target sample with one or more pump pulses to cause a transient perturbation in a material in the target sample. A probe arm in the photoacoustic measurement device is configured to receive the pulsed light and irradiate the target sample with one or more probe pulses to generate a reflected probe pulse that is modulated based on the transient perturbation in the material. The one or more pump pulses and the one or more probe pulses have different wavelengths or the same wavelength selected from the spectrally broadened pulsed light. The photoacoustic measurement device may include one or more detectors for receiving a reflected probe pulse from the target sample, and at least one processor coupled to the one or more detectors and configured to determine at least one characteristic of the target sample based on the reflected probe pulse.

[0006] In one implementation, a method for non-destructive photoacoustic measurement of a target sample using a photoacoustic measurement device may include generating pulsed light having a first wavelength using a laser light source and spectrally broadening the pulsed light using a supercontinuum generator. Generating one or more pump pulses using the pulsed light in a pump arm, the one or more pump pulses causing transient perturbations in a material within the target sample illuminated by the one or more pump pulses. Generating one or more probe pulses using the pulsed light in a probe arm, the one or more probe pulses generating reflected probe pulses from the target sample illuminated by the one or more pump pulses that are modulated based on the transient perturbations in the material. The one or more pump pulses and the one or more probe pulses have different wavelengths or the same wavelength selected from the spectrally broadened pulsed light. The method may further include detecting the reflected probe pulses from the target sample using one or more detectors and determining at least one property of the target sample based on the reflected probe pulses.

[0007] In one implementation, an optoacoustic measurement device for non-destructive measurement of a target sample may include a laser light source for generating a pulsed light having a first wavelength. The optoacoustic measurement device includes a means for spectrally broadening the pulsed light. A pump arm in the optoacoustic measurement device is configured to receive the pulsed light and to irradiate the target sample with one or more pump pulses to cause a transient perturbation in a material in the target sample. A probe arm in the optoacoustic measurement device is configured to receive the pulsed light and to irradiate the target sample with one or more probe pulses to generate a reflected probe pulse that is modulated based on the transient perturbation in the material. The one or more pump pulses and the one or more probe pulses have different wavelengths or the same wavelength selected from the spectrally broadened pulsed light. The optoacoustic measurement device may include one or more detectors for receiving a reflected probe pulse from the target sample, and at least one processor coupled to the one or more detectors and configured to determine at least one characteristic of the target sample based on the reflected probe pulse. [Brief description of the drawings]

[0008] [Figure 1A] FIG. 1 shows a schematic diagram of an optoacoustic measurement device including a supercontinuum generator for varying the wavelength of one or more of the pump and probe pulses. [Figure 1B] 1 is a graph showing the change in reflectance versus time delay in photoacoustic measurements. [Diagram 2] FIG. 1 shows a schematic diagram of an implementation of an optoacoustic measurement device having a wavelength selector in front of the pump arm and the probe arm, where the pump pulse and the probe pulse have different wavelengths. [Diagram 3] 1 shows a schematic diagram of another implementation of an optoacoustic measurement device having a wavelength selector in front of the pump arm and the probe arm, where the pump pulse and the probe pulse have different wavelengths. [Figure 4]1 shows a schematic diagram of an implementation of an optoacoustic measurement device having a wavelength selector in one of the pump or probe arms, where the pump and probe pulses have different wavelengths. [Diagram 5] 1 shows a schematic diagram of an implementation of an optoacoustic measurement device having a heterodyne configuration with a wavelength selector in one of the pump or probe arms, where the pump and probe pulses have different wavelengths. [Figure 6] 1 shows a schematic diagram of another implementation of an optoacoustic measurement device having a wavelength selector in one of the pump or probe arms, where the pump and probe pulses have different wavelengths, and having a heterodyne configuration. [Figure 7] 1 shows a schematic diagram of another implementation of an optoacoustic measurement device having a wavelength selector in front of the pump and probe arms to select the pump and probe pulses to have different wavelengths or the same wavelength. [Figure 8] 1 shows a schematic diagram of another implementation of an optoacoustic measurement device having a wavelength selector in front of the pump and probe arms to select the pump and probe pulses to have different wavelengths or the same wavelength. [Figure 9] 1 shows a schematic diagram of another implementation of an optoacoustic measurement device having a wavelength selector in front of the pump and probe arms to select the pump and probe pulses to have different wavelengths or the same wavelength. [Figure 10] 1 shows a schematic diagram of another implementation of an optoacoustic measurement device having a wavelength selector in front of the pump and probe arms to select the pump and probe pulses to have different wavelengths or the same wavelength. [Figure 11] 1 shows a schematic diagram of an implementation of an optoacoustic measurement device with a wavelength selector and pulse shaper in the pump arm. [Figure 12] 1 is a flow chart illustrating a method for non-destructive photoacoustic measurement of a target sample using a photoacoustic measurement device as described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] During the manufacture of semiconductors and similar devices, it may be necessary to monitor the manufacturing process by measuring the devices non-destructively. Optical metrology may be employed for non-contact evaluation of samples during processing. For example, photoacoustic metrology, such as picosecond acoustic (ultrasound), may be used to evaluate samples and identify the presence and location of inhomogeneities within the sample or the thickness of various film structures.

[0010] During production, a sample may have different surface layers at different times during production. Furthermore, different samples may have different types of layers. Different materials in these layers may behave differently to different wavelengths of light, for example, different materials may be transparent or opaque to different wavelengths of light. However, photoacoustic measurements, such as picosecond ultrasound, rely on the use of light pulses generated by a laser and are therefore limited to the specific wavelength of the laser, which may be limited as to the different types of materials with which it can be used.

[0011] For example, picosecond ultrasound is well established for metrology of thin metal films, using femtosecond laser pulses to generate acoustic waves that undergo partial reflection at the interface between the metal and the substrate. Another short laser pulse from the same laser is then used to detect the change in reflectivity due to the arrival of the acoustic wave. The efficiency of the signal depends on the absorption of the pump beam for generating strong acoustic waves and the piezoelectric reflectivity of the sample for strong probe beam reflection. The absorption and piezoelectric reflectivity of different metals vary as a function of the laser wavelength. Ideally, ultrafast lasers with a variety of wavelengths are needed for optimal performance for different applications. Efforts have been made to address this by including laser sources that contain second harmonic generators that limit the wavelength selection to only two different fixed wavelengths, or by using OPAs (Optical Parametric Amplifiers) that can generate short laser pulses with a variety of wavelengths, but these are very expensive.

[0012] Thus, as described herein, an optical measurement device performs photoacoustic measurements on a sample using a laser source for generating pulsed light having a first wavelength and a supercontinuum generator for receiving and spectrally broadening the pulsed light. The optical configuration can be customized to demonstrate several different concepts. This technique may not be limited to standard pump-probe measurement setups using mechanical delay lines for variable time delay, but may also be part of a setup using asynchronous optical sampling (ASOPS) for picosecond acoustic measurements.

[0013] For example, in one implementation, the supercontinuum generator may spectrally broaden the pulsed light from a primary laser source, e.g., in the visible and near infrared (NIR), using either the fundamental or second harmonic of the laser. In some implementations, the spectrum may be split into two parts with the help of a dichroic filter, and each spectral component may be used for the pump and probe beams, respectively, thus enabling dual-wavelength picosecond ultrasound measurements. In another implementation, a bandpass filter or an acousto-optic filter for wavelength selection is used before splitting the beam into pump and probe beams.

[0014] In another implementation, the supercontinuum generator can spectrally broaden the pulsed light in the probe beam path (or pump beam path). For example, bandpass or acousto-optic filters can be used to select different wavelengths depending on the sample under study. In this example, the pump wavelength (or probe beam path) will be the same as the wavelength of the master laser source (IR or visible). By allowing the ability to vary the probe wavelength from the visible to the near-infrared spectral range with the help of filters such as acousto-optic filters, the signal can be optimized depending on the piezoelectric reflectivity of the sample.

[0015] In another implementation, the supercontinuum generator may spectrally broaden pulsed light with zero dispersion at two distinct wavelengths (e.g., 775 nm and 945 nm) that are closely spaced apart. In this case, if the pump wavelength happens to be between the two wavelengths (e.g., 800 nm), the generated spectrally broadened light has all the power distributed between the two bands, not as a continuum. The two spectral bands may be above a higher zero dispersion wavelength (e.g., >945 nm) and below a lower zero dispersion wavelength (e.g., <775 nm), which may be due to four-wave mixing and self-phase modulation in the supercontinuum generator. This implementation may be advantageously used by assigning each spectral band to a pump beam and a probe beam, respectively. Using a combination of optical elements such as dichroic mirrors and filter wheels (including special coatings acting as dichroic mirrors or beam splitters or highly reflective mirrors), the photoacoustic measurement device may be configured to include various combinations of pump beam wavelengths and probe beam wavelengths. Furthermore, due to nonlinearities in the supercontinuum generator and the fact that the supercontinuum generator produces zero dispersion at two different wavelengths in two separate spectral bands, the amount of energy provided to each spectral band is determined by the original wavelength of the pump wavelength; therefore, by adjusting the pump wavelength provided to the supercontinuum generator, the ratio of power in the two spectral bands can be adjusted, and the photoacoustic measurement device can be configured to have both IR or visible pump and probe beams.

[0016] In another implementation, whether the supercontinuum generator spectrally broadens the pulsed light before the pump and probe beams are separated or in the probe (or pump) arm, the pump beam can pass through a pulse shaper, such as a spatial light modulator or an acousto-optical modulator. The pulse shaper can be used to change the pulse characteristics by modifying the intensity and phase of the pump pulse. By changing the pulse characteristics, enhancement or suppression of certain acoustic phonon modes can be achieved in some samples. Furthermore, adaptive algorithms can be used to define the optimal pulse shape that results in the desired control over phonon dynamics in the sample under consideration.

[0017] The photoacoustic measurement device may have, for example, a homodyne configuration with a single laser generating pulsed light that is split between a pump arm and a probe arm with a variable delay stage used to generate a delay between the pump and probe pulses. In another example, the photoacoustic measurement configuration may have a heterodyne configuration, in which a light source generates separate pump and probe beams, and the delay between the pump and probe beams is generated based on slightly different frequencies of the pulses in the pump and probe beams.

[0018] 1A shows a schematic diagram of an optoacoustic measurement device 100 that can use a supercontinuum generator to modify the wavelength of one or more of the pump and probe pulses as described herein. The optoacoustic measurement device 100 can be further configured to use a pulse shaper on the pulses in the pump beam. It should be understood that FIG. 1A shows a simplified diagram of the optoacoustic measurement device 100 and that additional optical components, such as lenses, polarizers, wave plates, etc., can be included.

[0019] The photoacoustic measurement device 100 includes a pulsed light source 110 that generates pulsed light 111. The pulsed light source 110 may be, for example, a single wavelength laser or a narrow band laser. The pulsed light source 110 may be a single pulsed laser that generates pulsed light 111 that may have a pulse width ranging from hundreds of femtoseconds to hundreds of picoseconds. In some implementations, the pulsed light 111 may be a single beam that is split between the pump arm 120 and the probe arm 130. In some implementations, the pulsed light 111 may be two separate pulsed beams directed to the pump arm 120 and the probe arm 130, respectively. For example, the pulsed light source 110 may be two separate pulsed lasers that generate separate pulsed beams as pulsed light 111 with slightly different frequency combs. The two separate pulsed lasers can be synchronized to reduce the difference in repetition rate. In another example, the pulsed light source 110 may be a single pulsed laser that generates separate pulsed beams as pulsed light 111 with slightly different frequency combs.

[0020] As shown, the photoacoustic measurement device 100 can include an optical system 115 that directs the pulsed light 111 to the pump arm 120 and the probe arm 130. The optical system 115 can be, for example, a beam splitter that distributes the pulsed light 111 toward the pump arm 120 and the probe arm 130. In another example, the optical system 115 can include one or more filters, mirrors, dichroic mirrors, beam dumps, etc., to distribute the pulsed light 111 toward the pump arm 120 and the probe arm 130. In some implementations, a supercontinuum generator can be located before the optical system 115 (e.g., between the pulsed light source 110 and the optical system 115), and the optical system 115 can select the portion of the pulsed light 111 to distribute to the pump arm 120 and the probe arm 130 such that the pump pulse and the probe pulse incident on the sample can have different wavelengths or the same wavelength. In some implementations, the supercontinuum generator can be located after the optical system 115 (e.g., in the probe arm 130), and the optical system 115 can distribute a desired ratio of the pulsed light 111 to the pump arm 120 and the probe arm 130. In some implementations, the pulsed light source 110 can generate two separate pulsed beams of pulsed light, and the optical system 115 can direct the different pulsed beams to the pump arm 120 and the probe arm 130.

[0021] The pump arm 120 receives at least a portion of the pulsed light 111 and directs a pulsed pump beam 121 to the target sample 102. As shown in FIG. 1A, the pump beam 121 can be directed (e.g., focused) by one or more lenses (not shown) to be normally incident on the target sample 102, although non-normal angles of incidence can be used if desired, including, for example, but not limited to, angles of incidence from normal to 70°. The pump beam 121 generated by the pump arm 120 irradiates the target sample 102 and induces transient perturbations in the material of the target sample 102.

[0022] In some implementations, the pump arm 120 can include a delay stage 122 to increase or decrease the length of the optical path between the pulsed light source 110 and the target sample 102 to control the delay of the pulses in the pump beam 121. The delay stage 122 can vary the optical path length, for example, to control the time delay between illuminating the target sample 102 with each pulse in the pump beam 121 and illuminating the target sample 102 with a corresponding pulse in the probe beam 131.

[0023] The pump arm 120 may further include a pulse shaper 124 that receives the pulsed light 111 and varies at least one of the duration, phase, or both of the pulses of the pulsed light to generate the pump beam 121. The pulse shaper 124 may be, for example, a spatial light modulator or an acousto-optic modulator. As an example, all optical switching (AOS) of magnetic materials is an area of ​​interest due to the demand to control magnetization in devices in a faster and more efficient manner. Study of the interaction of ultrafast lasers with ferromagnetic metals may lead to greater advances in future magnetic data storage devices. The pump beam 121 may be configured to induce magnetization dynamics, for example, using femtosecond pump laser pulses. The photoacoustic measurement device 100 may be used to study light-induced magnetic dynamics. Controlling the shape of the pulse of the pump beam 121 by varying the pulse duration and / or phase of the incident pump beam 121 using the pulse shaper 124 may be advantageously used to determine the dependence of magnetic dynamics in the target sample 102 on the pump pulse characteristics. Furthermore, adaptive algorithms can be used to define and use an optimal pulse shape for the pump beam 121 that provides the desired control over the magnetic dynamics within the target sample 102 under consideration.

[0024] The pump arm 120 may further include a modulator 126 for modulating the amplitude (intensity) of the pump beam 121. For example, the modulator 126 may modulate the amplitude (intensity) of a polarized probe pulse in the pump beam 121. The modulator 126 may be an electro-optic modulator (EOM), an acousto-optic modulator (AOM), a photoelastic modulator (PEM), or a chopper that may modulate the amplitude (intensity) of the pump beam 121, which is advantageous as it allows for measurements with high sensitivity, low noise, and high speed.

[0025] Many other alternative configurations of the pump arm 120 are possible. For example, the pump arm 120 may include only the delay stage 122, the modulator 126, or the pulse shaper 124, or any combination thereof. Further additional optical elements may be present in the pump arm 120. It should be understood that the illustration of the pump arm 120 in FIG. 1A is not intended to be limiting, but rather illustrates one of several exemplary configurations.

[0026] Probe arm 130 receives at least a portion of the pulsed light 111 and directs a probe beam 131, which may include, for example, one or more polarized pulses, to illuminate target sample 102. Probe beam 131 may be directed (e.g., focused) by a lens (not shown) to be obliquely incident on target sample 102, for example, at any angle between 5 and 70 degrees from normal.

[0027] The probe arm 130 may include, for example, one or more polarizing elements 138 (e.g., polarizers and / or wave plates) to generate one or more polarization states and any phase shifts, which may be configurable, e.g., rotatable, to generate a desired polarization state in the probe beam 131. In some implementations, the probe arm 130 may include a delay stage 132 to increase or decrease the length of the optical path between the pulsed light source 110 and the target sample 102 to control the delay of the pulses in the probe beam 131.

[0028] The probe arm 130 may further include a wavelength selector 133 that can receive the pulsed light 111 in the probe arm 130 and select one or more wavelengths to be used in the probe beam 131. The wavelength selector 133 may include, for example, a multi-wavelength generator 134 that can receive a single wavelength or a narrow band of wavelengths in the pulsed light 111 and spectrally broaden the pulsed light 111. The multi-wavelength generator 134 may be, for example, a supercontinuum generator that spectrally broadens the light. In some implementations, the multi-wavelength generator 134 may be a multi-harmonic generator, such as a frequency doubling crystal (DBO), that receives a narrow band of wavelengths and generates a broader band of wavelengths. In some implementations, the multi-wavelength generator 134 may be a photonic crystal fiber that receives a narrow band of wavelengths and generates a broader band of wavelengths. In some implementations, the multi-wavelength generator 134 spectrally broadens the pulsed light 111 to generate a continuous light spectrum. In some implementations, the multi-wavelength generator 134 spectrally broadens the pulsed light 111 to generate multiple discontinuous optical spectral bands. The multi-wavelength generator 134 allows for the ability to increase the wavelength of the probe beam 131, for example, from the visible spectral range to the near-infrared spectral range, which may be continuous or discontinuous wavelengths, and may be filtered to select a specific wavelength or a narrow band of wavelengths for measurement of the target sample 102. For example, in some implementations, the wavelength selector 133 can further include a filter 136, such as an acousto-optic filter, a dichroic mirror, a laser line filter, a notch filter, etc., which, when used with the multi-wavelength generator 134, allows for the ability to select one or more specific wavelengths included in the probe beam 131 from a spectral range from the visible to the near-infrared, for example, used to measure the target sample 102. For example, some sample materials are opaque to some wavelengths and transparent to other wavelengths. In some implementations, the filter 136 can be within one or more components of the optical system 115, such as a dichroic beam splitter.Through the use of a multi-wavelength generator 134 and a filter 136 in the wavelength selector 133, the wavelengths contained in the probe beam 131 may be specifically selected by the photoacoustic measurement device 100 based on the type of material of the target sample 102, thereby enabling measurement of a wide range of different materials.

[0029] The probe arm 130 may include, for example, one or more polarizing elements 138 (e.g., polarizers and / or wave plates) to create one or more polarization states and any phase shifts, which may be configurable, e.g., rotatable, to create a desired polarization state in the probe beam 131.

[0030] The probe arm 130 may further include a modulator 139 for modulating the amplitude and / or phase of the probe beam 131. For example, the modulator 139 may modulate the amplitude (intensity) of the polarized probe pulse in the probe beam 131. The modulator 139 may periodically phase modulate the polarized probe pulse in the probe beam 131. The modulator 139 may be an EOM, AOM, PEM, or rotating compensator that may modulate the phase of the probe beam 131, or an EOM, AOM, PEM, or chopper that may modulate the amplitude of the probe beam 131. For example, the use of an EOM may be advantageous as it allows for measurements with high sensitivity, low noise, and high speed.

[0031] Many other alternative configurations of the probe arm 130 are possible. For example, the probe arm 130 may include only the polarizing element 138, the delay stage 132, the wavelength selector 133 (including one or more of the multi-wavelength generator 134 and the filter 136), the modulator 139, or any combination thereof. Further additional optical elements may be present in the probe arm 130. It should be understood that the illustration of the probe arm 130 in FIG. 1A is not intended to be limiting, but rather illustrates one of several exemplary configurations.

[0032] In some configurations, the probe arm 130 may not include a wavelength selector 133, as shown by the dotted lines, and a wavelength selector 133' may be located before the optical system 115. The wavelength selector 133' may be the same as the wavelength selector 133 described above, including, for example, one or more of a multi-wavelength generator 134 and a filter 136. The wavelength selector 133' may generate, for example, multiple wavelengths, and the optical system 115 may select different wavelengths to direct to the pump arm 120 and the probe arm 130. In another implementation, the wavelength selector 133' may select one or more particular wavelengths, and the optical system 115 directs the selected wavelengths to both the pump arm 120 and the probe arm 130.

[0033] The pump beam 121 and the probe beam 131 interact with the target sample 102, and the probe beam 131 is reflected from the target sample 102 as a reflected beam 141 to a detector arm 140. The detector arm 140 includes one or more detectors, e.g., detectors 144 and / or 146, that receive the reflected probe pulse in the reflected beam 141 from the target sample 102. For surface deflection measurements, for example, both detectors 144 and 146 can be used, and a differential measurement of two signals detected by detectors 144 and 146 can be used. The one or more detectors 146 and / or 144 can be connected to a lock-in amplifier 148 that demodulates a signal from detectors 146 and / or 144 that is generated based on the received reflected probe pulse in the reflected beam 141 from the target sample 102.

[0034] Many other alternative configurations of detector arm 140 are possible. For example, detector arm 140 may include detector 146, detector 144, lock-in amplifier 148, or any combination thereof. Further additional optical elements may be present in detector arm 140. It should be understood that the illustration of detector arm 140 in FIG. 1A is not intended to be limiting, but rather illustrates one of several exemplary configurations.

[0035] The pulsed light source 110, the pump arm 120, the probe arm 130, and the detector arm 140 are connected to and controlled by the controller 150. Additionally, the controller 150 is connected to and can control the stage 104, which holds the target sample 102 and includes an actuator for moving the target sample 102 based on control signals from the controller 150 to position the target sample 102 at a desired measurement position. For example, the stage 104 can be capable of horizontal motion in either Cartesian (i.e., X and Y) coordinates, or polar (i.e., R and θ) coordinates, or some combination of the two. The stage 104 may also be capable of vertical motion along a Z coordinate.

[0036] The controller 150 may further control the operation of a chuck on the stage 104 to hold or release the target specimen 102. It should be understood that the controller 150 may be an embedded or distributed computing device capable of performing necessary calculations, receiving and sending instructions or commands, and receiving, storing, and sending information related to the metrology functions of the system.

[0037] Controller 150 includes one or more processors and may be a workstation, personal computer, central processing unit, or other suitable computer system, or multiple systems. It is understood that controller 150 includes a single processor, multiple separate processors, or multiple concatenated processors, all of which may be referred to interchangeably herein as controller 150, processor 150, at least one processor 150, or one or more processors 150. Controller 150 is preferably included in, connected to, or otherwise associated with photoacoustic measurement device 100.

[0038] The controller 150 may also control the operation of the photoacoustic measurement device 100 and collect and analyze data obtained from the detector arm 140. The controller 150 may analyze the data to determine one or more physical properties of the target sample 102 based on the obtained data. For example, the photoacoustic measurement device 100 may be controlled by the controller 150 to perform photoacoustic measurements. The photoacoustic measurements may be, for example, picosecond ultrasonic measurements that may be used to measure depth-resolved measurements of inhomogeneities in the target sample 102. The photoacoustic measurements may, for example, use a pump beam 121 to generate a transient response in the target sample 102, where a transducer layer, such as a metal layer, in the target sample 102 absorbs the pump pulse energy and launches an acoustic wave perpendicularly into the target sample 102. The propagating acoustic wave interacts with the probe beam 131 via a piezo reflection response and is reflected. Light reflected from the propagating acoustic wave interferes with light reflecting from the top surface of the target sample 102, resulting in a characteristic vibration time evolution signal, i.e., coherent Brillouin scattering. The period of the vibration reveals information about the speed of sound and elastic modulus within the target sample 102. The depth-resolved vibration period may be used to extract the speed of sound and Young's modulus at various depths within the target sample 102, which may be used to provide insight into the presence and location of inhomogeneities or thickness measurements of opaque films.

[0039] The photoacoustic measurement device 100 can perform photoacoustic measurements using the pump arm 120 in conjunction with the probe arm 130. For example, the controller 150 can cause the pump arm 120 to generate a pump beam 121 that is incident on the target sample 102 and generates a transient response in the target sample 102. The pump beam 121 can be amplitude (intensity) modulated using a modulator 126.

[0040] The probe arm 130 generates a probe pulse in a probe beam 131. In some implementations, the probe beam may be amplitude (intensity) modulated by a modulator 139. The probe beam 131 interacts with the target sample 102 after each pump pulse. Photoacoustic measurements may be collected as a function of time delay between the pump beam 121 and the probe beam 131, which may be controlled, for example, by a delay stage 122 in the pump arm 120 and / or a delay stage 132 in the probe arm 130 in a homodyne configuration, or by different frequency combs of separately generated pulsed light beams in a heterodyne configuration.

[0041] Material properties including material type, thickness, and non-uniformity in the target sample 102 can be measured based on the reflected beam 141 by varying the propagation time of the acoustic wave in the target sample 102. The detector arm 140 can detect changes in reflection or surface deformation of the reflected beam 141 using detector 146, for example, with respect to the time delay. FIG. 1B shows a graph illustrating the change in reflectivity ΔR in arbitrary units with respect to the time delay (psec) that may be measured by the photoacoustic measurement device 100, as an example. As an example, the thickness of the target sample 102 may be measured based on the following:

number

[0042] where τ is the echo arrival time and v sound is the speed of sound in the material of the target sample 102.

[0043] Figure 2 is a schematic diagram of a photoacoustic measurement device 200 illustrating one implementation of the photoacoustic measurement device 100 shown in Figure 1A, where the pump and probe pulses incident on the sample 212 have different wavelengths, as indicated by the dashed and dotted lines. In Figure 2, the wavelength selector is in the form of a multi-wavelength generator 234 that receives pulsed light from the pulsed light source 202 and expands the spectrum of the pulsed light. A dichroic beam splitter 208 receives the pulsed light from the multi-wavelength generator 234 and directs the different wavelengths to a pump arm 220 (which can be used as the pump arm 120 of Figure 1A) and a probe arm 230 (which can be used as the probe arm 130 of Figure 1A).

[0044] It should be understood that the photoacoustic measurement device 200 may include components and subsystems in addition to those shown in FIG. 2, such as beam management and conditioning components, such as beam expanders, collimators, and beam power detectors, and focus sensors.

[0045] As shown, a light source 202, such as a pulsed laser in the 510-535 nm range, 50-400 fs, 20-150 MHz, generates a pulsed light beam. Other wavelengths, such as 800 nm or 1020-1070 nm, or other wavelengths can be used, if desired. The light may be directed through intensity control 203, which includes a half wave plate HWP1 and a polarizer P1. The light may pass through a beam expander 204, which may include a series of lenses that expand the beam. The light source 202, intensity control 203, and beam expander 204 may be used as the pulsed light source 110 shown in FIG. 1A.

[0046] The pulsed light from the light source 202 is directed to the beam splitter 208 by the mirror M1. Between the light source 202 and the beam splitter 208 is a wavelength selector in the form of a multi-wavelength generator 234. The multi-wavelength generator 234 receives the pulsed light 111, which is narrow band, and spectrally broadens the pulsed light. The multi-wavelength generator 234 may be, for example, a supercontinuum generator, such as a multiple harmonic generator, for example, a DBO or photonic crystal fiber, which receives a single or narrow band of wavelengths and generates multiple wavelengths in a continuous or non-continuous spectrum of wavelengths for the pulsed light. For example, BBO or LBO crystals from Eksma and PCFs from Thorlabs can be used. The multi-wavelength generator 234 allows the ability to change the wavelengths used in the pump beam or the probe beam, for example, from the visible spectral range to the near infrared spectral range. The beam splitter 208 may be a dichroic beam splitter used to select the wavelengths of pulsed light directed to the pump beam 220 and the wavelengths of pulsed light directed to the probe beam 230. Thus, the multi-wavelength generator 234 and the beam splitter 208 may be used as a wavelength selector 133' that includes the multi-wavelength generator 134 and the filter 136 and the optical system 115 shown in FIG.

[0047] The pump arm 220 includes a variable delay stage 222 shown as including several mirrors M2, M3, M4, and M5, with mirror M4 movable via a piezoelectric motor to change the length of the optical path to control the delay of the pulse in the pump beam generated by the pump arm 220 and to vary the time delay between the pulse in the pump beam and the pulse in the probe beam.

[0048] The pump beam is a KD *The pump beam passes through an EOM 226, such as a P-crystal based EOM. The pump beam may further pass through a polarizer P2 and a half wave plate HWP2, and is directed to the target sample 212 via mirrors M6, M7, M8, a beam splitter 228, and a lens L1. Lens L1 may include one or more reflective or refractive lenses, or a combination thereof. Lens L1 may direct the pump beam to be normally incident on the target sample 212. In some implementations, the pump beam may have a non-normal incidence angle, for example, an incidence angle of normal to 70°.

[0049] The vision system 229 can be focused on the target sample 212 via the beam splitter 228 and lens L1 and can be used to position the target sample 212.

[0050] The probe arm 230 may include a motorized half-wave plate HWP3 in front of a variable delay stage 232, shown as including several mirrors M9, M10, M11, and M12, where mirror M11 is movable via a piezoelectric motor to change the length of the optical path to control the delay of the pulses in the probe beam generated by the probe arm 230, varying the time delay between the pulses in the pump beam and the pulses in the probe beam.

[0051] The probe beam generated by the probe arm 230 is directed to the target sample 212 via mirrors M13, M7, M14 and lens L2, which may include one or more reflective or refractive lenses, or a combination thereof. Lens L2 directs the probe beam to be obliquely incident on the target sample 212.

[0052] Detector arm 140 receives the reflected beam from target sample 212 via lens L3 and mirrors M15 and M16. Lens L3 may include one or more reflective or refractive lenses, or a combination thereof. Detector arm 140 includes detector 244, which may be a Si-based photodetector manufactured by Thorlabs.

[0053] The detector 244 may be connected to a lock-in amplifier 248, which receives a signal from the detector 244 generated based on the reflected probe pulse received in the reflected beam from the target sample 212 and demodulates the signal based on the dc frequency component generated by the light source 202 and, if used, the modulation frequency generated by the EOM 226 in the pump arm 220.

[0054] As shown, the photoacoustic measurement device 200 may further include a stage 214 including a chuck for holding the target sample 212 and an actuator for moving the target sample 212 to a desired positioning system. For example, the stage 214 may be capable of horizontal motion in either Cartesian (i.e., X and Y) or polar (i.e., R and θ) coordinates, or some combination of the two. The stage may also be capable of vertical motion along a Z coordinate.

[0055] The detector 244, e.g., lock-in amplifier 248, as well as other components of the photoacoustic measurement device 200, such as the light source 202, pump variable delay stage 222, EOM 226, probe variable delay stage 232, wavelength selector (multiple wavelength generator 234), stage 214, may be coupled to at least one controller 250, such as a workstation, personal computer, central processing unit, or other suitable computer system, or multiple systems. It should be understood that the controller 250 includes one or more processing units 252, which may be separate or linked processors, and that the controller 250 may be referred to herein as a processor 250, at least one processor 250, one or more processors 250, etc. The controller 250 is preferably included in, connected to, or otherwise associated with the photoacoustic measurement device 200. The controller 250 may control the positioning of the target sample 212, for example, by controlling the movement of a stage 214 on which the target sample 212 is held. The controller 250 may further control the operation of a chuck on the stage 214 to hold or release the target sample 212. The processor 250 may also collect and analyze data obtained from the detector 244. The controller 250 may analyze the data to determine one or more physical properties of the sample, such as based on photoacoustic measurements, as described herein. In some implementations, measured data may be acquired and compared to modeled data, which may be stored in a library or acquired in real time. The parameters of the model are varied and the modeled data and measured data are compared, such as by a linear regression process, and once a good fit is obtained between the modeled data and the measured data, the modeled parameters are determined to be properties of the target sample 212.

[0056] The controller 250 includes at least one processing unit 252 and memory 254, as well as a user interface including, for example, a display 256 and input devices 258. A non-transitory computer usable storage medium 259 having computer readable program code embodied therein may be used by the at least one processor 252 to cause the at least one processor 252 to control the photoacoustic measurement device 200 and perform the measurement functions and analysis described herein. Data structures and software code for automatically implementing one or more operations described in this detailed description may be implemented by one of ordinary skill in the art in view of this disclosure and may be stored, for example, on a computer usable storage medium 259, which may be any device or medium capable of storing code and / or data for use by a computer system such as the processing unit 252. The computer usable storage medium 259 may be, but is not limited to, a disk drive, a magnetic tape, a compact disk, and a flash drive, a magnetic storage device, such as a DVD (digital versatile disk or digital video disk), and an optical storage device. The communication port 257 may also be used to receive instructions stored in the memory 254 and used to program the processor 250 to perform any one or more of the functions described herein, and may represent any type of communication connection, such as the Internet or other computer network. The communication port 257 may further export signals with the measurement results and / or instructions to another system, such as an external process tool, in a feedforward or feedback process to adjust process parameters associated with a manufacturing process step of the specimen based on the measurement results.Additionally, the functionality described herein may be embodied in whole or in part within the circuitry of an application specific integrated circuit (ASIC) or programmable logic device (PLD), and the functionality may be embodied in a computer understandable description language that may be used to create an ASIC or PLD that operates as described herein. Analysis of the data may be stored, for example, in a memory 254 associated with the sample and / or provided to a user, for example, via a display 256, an alarm, a data set, or other output device. Additionally, analysis results may be fed back to process equipment to adjust appropriate patterning steps to compensate for detected processing variations.

[0057] Figure 3 shows a schematic diagram of a photoacoustic measurement device 300, which illustrates an implementation of the photoacoustic measurement device 100 shown in Figure 1A and is similar to the photoacoustic measurement device 200 shown in Figure 2, and similarly designated components may be the same. As shown by the dashed lines in Figure 3, the photoacoustic measurement device 300 generates pump and probe pulses having the same wavelength that are incident on the sample 212.

[0058] In the photoacoustic measurement device 300, a wavelength selector 333 is located before the beam splitter 308. The wavelength selector 333 includes a multi-wavelength generator 334 that receives the pulsed light from the pulsed light source 202 and expands the spectrum of the pulsed light. The multi-wavelength generator 334 may be the same as the multi-wavelength generator 234 shown in FIG. 2. The wavelength selector 333 further includes a filter 336 that can select one or more wavelengths to direct to the beam splitter 308. The filter 336 may be, for example, an acousto-optic filter, a dichroic mirror, a laser line filter, a notch filter, etc., and may be changed to select different wavelengths to direct to the beam splitter 308. The beam splitter 308 may be a non-polarizing beam splitter that separates the pulsed light and directs a portion (e.g., 50%) to the pump arm 220 and another portion (e.g., 50%) to the probe arm 230. A wavelength selector 333 having a multi-wavelength generator 334 and a filter 336 can be used as the wavelength selector 133 shown in Figure 1A. A beam splitter 308 can be used as the optical system 115 shown in Figure 1A.

[0059] Thus, the pump and probe pulses incident on the sample 212 have the same wavelength, which is selected by the wavelength selector 333 .

[0060] FIG. 4 shows a schematic diagram of an optoacoustic measurement device 400, which illustrates an implementation of the optoacoustic measurement device 100 shown in FIG. 1A and is similar to the optoacoustic measurement devices 200, 300 shown in FIGS. 2 and 3, and similarly designated components may be the same.

[0061] As shown in FIG. 4, the wavelength selector 433 is located in the probe arm 230, unlike the photoacoustic measurement devices 100 and 200, which show that the wavelength selection is performed in front of the probe arm 230. Thus, in the photoacoustic measurement device 400, both the pump arm 220 and the probe arm 230 receive the same wavelength of pulsed light from the light source 202 through the beam splitter 308. The wavelength selection is performed only in one arm, shown as the probe arm 230 in FIG. 4 (but may be in the pump arm 220 in some implementations), and the other arm, shown as the pump arm 220 (but may be the probe arm 230 in some implementations), receives the same wavelength of pulsed light generated by the light source 202. Thus, as shown by the dashed lines in FIG. 4, the pump pulse and the probe pulse incident on the sample 212 have different wavelengths controlled by the wavelength selector 433 in the light source 202 and the probe arm 230.

[0062] The wavelength selector 433 in the photoacoustic measurement device 400 may be similar to the wavelength selector 333 in the photoacoustic measurement device 300 shown in FIG. 3. The wavelength selector 433 may include, for example, a multi-wavelength generator 434 that receives pulsed light from the pulsed light source 202 (in the probe arm 230) and expands the spectrum of the pulsed light. The multi-wavelength generator 434 may be the same as the multi-wavelength generator 234 shown in FIG. 2. The wavelength selector 433 further includes a filter 436 that can select one or more wavelengths to be used as the probe pulse. The filter 436 may be the same as the filter 336 shown in FIG. 3. Although FIG. 4 shows the wavelength selector 433 between the probe delay 232 and the mirror M13, it should be understood that the wavelength selector 433 may be in other locations. Furthermore, the components of the wavelength selector 433, for example, the multi-wavelength generator 434 and the filter 436, may be separated. A wavelength selector 433 having a multi-wavelength generator 434 and a filter 436 can be used as the wavelength selector 133 shown in FIG. 1A.

[0063] Thus, the pump pulses produced by pump arm 220 have the same wavelength as those produced by light source 202, but the probe pulses generated by probe arm 230 have one or more different wavelengths selected by wavelength selector 433. Selecting the wavelength to use in the probe pulse can be advantageous, for example, for targets having different piezoelectric reflectivities.

[0064] FIG. 5 shows a schematic diagram of a photoacoustic measurement device 500, which illustrates an implementation of the photoacoustic measurement device 100 shown in FIG. 1A and is similar to the photoacoustic measurement devices 200, 300, 400 shown in FIGS. 2-4, and similarly designated components may be the same.

[0065] As shown in FIG. 5, the photoacoustic measurement device 500 includes a wavelength selector 433 located in the probe arm 230, similar to the photoacoustic measurement device 400 shown in FIG. 4. However, the photoacoustic measurement device 500 shows a heterodyne configuration in which the light source 502 generates separate pump and probe beams using synchronized lasers 502a and 502b. As shown by graph 503, the laser pulses generated by the synchronized lasers 502a and 502b have slightly different frequencies (i.e., repetition rates), which provides a variable delay between the pump and probe pulses. Using the heterodyne configuration, the variable delay stage 222 for controlling the delay between the pump and probe pulses, used in the homodyne configuration shown in FIGS. 2-4, is not necessary and can therefore be omitted from the pump arm 520 as shown in FIG. 5. By using separate pulsed beams for the pump arm 520 and the probe arm 230, the light may be directed through separate intensity controls 203a and 203b, each of which includes half-wave plates HWP1a, HWP1b and polarizers P1a, P1b. The light may also pass through separate beam expanders 204a, 204b, each of which may include a series of lenses to expand the beam. The light source 502, which includes synchronized lasers 502a, 502b with different frequencies, intensity controls 203a, 203b, and beam expanders 204a, 204b, may be used as the pulsed light source 110 shown in FIG. 1A.

[0066] Furthermore, by using separate pulsed beams for the pump arm 520 and the probe arm 230, no beam splitters are required to direct the light from the light source 502 to the pump arm 520 and the probe arm 230. Thus, as shown in Figure 5, the pump light and probe light from the light source 502 may be directed to the pump arm 520 and the probe arm 230 using mirrors M1a, M1b and M2a, M2b, respectively.

[0067] FIG. 6 shows a schematic diagram of photoacoustic measurement device 600, which illustrates an implementation of photoacoustic measurement device 100 shown in FIG. 1A and is similar to photoacoustic measurement devices 200, 300, 400, 500 shown in FIGS. 2-5, and similarly designated components may be the same.

[0068] As shown in FIG. 6, the photoacoustic measurement device 500 includes a wavelength selector 433 located in the probe arm 230, similar to the photoacoustic measurement devices 400 and 500 shown in FIG. 4 and FIG. 5. However, the photoacoustic measurement device 600 shows a heterodyne configuration in which the light source 602 generates separate pump and probe beams with slightly different frequencies (i.e., repetition rates) and provides a variable delay between the pump and probe pulses, as shown by graph 603. The light source 602 generating separate beams with different frequencies, the intensity control 203a, 203b, and the beam expanders 204a, 204b can be used as the pulsed light source 110 shown in FIG. 1A.

[0069] 7-10 show schematic diagrams of an optoacoustic measurement device 700, which illustrates an implementation of the optoacoustic measurement device 100 shown in FIG 1A and is similar to the optoacoustic measurement device 200 shown in FIG 2, and similarly designated components may be the same. Similar to the optoacoustic measurement device 200 shown in FIG 2, the optoacoustic measurement device 700 may include a wavelength selector in the form of a multi-wavelength generator 734 between the pulsed light source 202 and the optics used to direct pulsed light to the pump arm 720 and the probe arm 230.

[0070] The multi-wavelength generator 734 receives the pulsed light from the pulsed light source 202 and expands the pulsed light into a non-continuous spectrum. The multi-wavelength generator 734 may be, for example, a photonic crystal fiber having two zero-dispersion regions with different wavelengths. The zero-dispersion regions may have, for example, wavelengths l1 and l2 close to each other, e.g., 775 nm and 945 nm. The multi-wavelength generator 734 may generate a spectral band around the zero-dispersion region wavelengths.

[0071] The photoacoustic measurement device 700 includes an optical element 715 that can be configured to direct pulsed light of different wavelengths to the pump arm 720 and the probe arm 230, e.g., as shown by dashed lines in Figures 7 and 8, or to direct pulsed light of the same selected wavelength to the pump arm 720 and the probe arm 230, e.g., as shown by dashed lines in Figures 9 and 10. For example, as shown in Figure 7, the optical element 715 may be used to direct a first wavelength of pulsed light (shown by dashed lines) to the pump arm 720 and a second wavelength of pulsed light (shown by dashed lines) to the probe arm 230, and as shown in Figure 8, the optical element 715 may be used to direct the second wavelength of pulsed light (shown by dashed lines) to the pump arm 720 and the first wavelength of pulsed light (shown by dashed lines) to the probe arm 230. 9, optical element 715 may be used to direct a second wavelength of pulsed light (shown in dashed lines) to both pump arm 720 and probe arm 230, and as shown in FIG 8, optical element 715 may be used to direct a first wavelength of pulsed light (shown in dashed lines) to both pump arm 720 and probe arm 230. Thus, multi-wavelength generator 734 and optical element 715 may be used as wavelength selector 133' including multi-wavelength generator 134 and filter 136 and optical system 115 shown in FIG 1A.

[0072] The optical element 715 can include, for example, several filters, such as filter wheels F1, F2, F3, F4, and one or more dichroic elements, such as dichroic mirrors D1, D2, mirrors M2, M2', and movable beam dumps BD1, BD2. The filter wheel can be adjusted to transmit or reflect light of desired wavelengths and direct pulsed light of desired different wavelengths or the same wavelength to the pump arm 720 and the probe arm 230, for example, as shown by the dashed and dot-dash lines in Figures 7-10.

[0073] 2, but may include additional wavelength-sensitive components since different wavelengths of light may be selected to be directed to the pump arm 720. For example, the pump arm 720 may include two sets of EOMs 726a, 726b that may be sensitive to different wavelengths that may be selected to be used by the pump arm 720. Along with the EOMs 726a, 726b, corresponding polarizers P2a, P2b and motorized half-wave plates HWP2a, HWP2b may be used, as well as additional dichroic mirrors D3, D4 and mirrors M6, M6b.

[0074] In addition, the detector arm 740 may be configured to split the reflected beam using, for example, a beam splitter 742 and detect the reflected beam from the target sample 212 using two separate separate detectors, for example, detectors 744 and 746, which may be Si-based photodetectors. Both detectors 746, 744 may be connected to a lock-in amplifier 248. The two detectors 744 and 746 may be used, for example, for surface deflection measurements based on differential measurements of signals detected by detectors 744 and 746. It should be understood that the detector arm 740 may be used in any of the configurations described in Figures 3-6.

[0075] FIG. 11 shows a schematic diagram of an optoacoustic measurement device 1100, which illustrates an implementation of the optoacoustic measurement device 100 shown in FIG. 1A and is similar to the optoacoustic measurement device 200 shown in FIG. 2, and similarly designated components may be the same.

[0076] As shown, the optoacoustic measurement device 1100 can include a pulse shaper 1124 in the pump arm 220. The pulse shaper 1124 receives a pulsed light beam at the pump arm 220 and varies at least one of the duration, phase, or both of the pulses in the pulsed light beam at the pump beam. The pulse shaper 1124 can be, for example, a spatial light modulator or an acousto-optical modulator. For example, a spatial light modulator such as a liquid crystal SLM manufactured by Jenoptik may be used, or a KD modulator manufactured by Conoptics. * An acousto-optic modulator such as a P-crystal based EOM may be used. The pulse shaper 1124 may be used to vary the pulse duration and / or phase of the incident pump beam to determine the dependence of the magnetic dynamics in the target sample 212 on the pump pulse characteristics. Additionally, an adaptive algorithm may be used to define and use an optimal pulse shape of the pump beam that provides the desired control over the magnetic dynamics in the target sample 212 under consideration, for example, by changing the pulse shape of the pump beam until the strongest response is detected. The pulse shaper 1124 may be used as the pulse shaper 124 shown in FIG. 1A. Although the pulse shaper 1124 is shown in a configuration similar to the photoacoustic measurement device 200, it should be understood that the pulse shaper 1124 may be used in any of the photoacoustic measurement devices disclosed herein.

[0077] FIG. 12 is a flowchart 1200 illustrating a method for non-destructive photoacoustic measurement of a target sample using a photoacoustic measurement device, such as the photoacoustic measurement devices shown in FIGS. 1A, 2-11 and described herein.

[0078] In block 1202, the photoacoustic measurement device generates pulsed light having a first wavelength using a laser light source, for example as illustrated by pulsed light source 110, light source 202, 502, or 602 shown herein. The photoacoustic measurement device can include a light source for, for example, a homodyne configuration with a single laser generating pulsed light that is split between a pump arm and a probe arm with a variable delay stage used to generate a delay between the pump and probe pulses, or a heterodyne configuration in which the light source generates separate pump and probe beams (using two synchronized lasers or a single laser) with a delay between the pump and probe that is generated based on slightly different frequencies of the pulses in the pump and probe beams.

[0079] In block 1204, the photoacoustic measurement device spectrally broadens the pulsed light using a supercontinuum generator, for example, as shown in Figures 1A and 2-11 herein. The supercontinuum generator can include a multi-wavelength generator, such as, for example, a multi-harmonic generator (e.g., DBO), which in some implementations may be a photonic crystal fiber that receives a narrow band of wavelengths and generates a wider band of wavelengths, and can broaden the spectrum to a continuous or discontinuous optical spectrum. The means for spectrally broadening the pulsed light can include, for example, any of the multi-wavelength generators 134, 234, 334, 434, or 734, as shown in Figures 1A and 2-11 herein.

[0080] In block 1206, the photoacoustic measurement device generates one or more pump pulses using pulsed light in a pump arm, for example as shown by pump arms 120, 220, 520, and 720 described with reference to Figures 1A and 2-11, where the one or more pump pulses cause transient perturbations in material within the target sample that the one or more pump pulses irradiate. The pump arm may, for example, in some implementations (e.g., in the case of a homodyne configuration) or may not include a variable delay stage for generating a delay between the pump pulse and the probe pulse (e.g., in the case of a heterodyne configuration).

[0081] In block 1208, the photoacoustic measurement device generates one or more probe pulses using pulsed light in the probe arm, for example as shown by the probe arm 130, 230 described with reference to Figures 1A and 2-11, and the one or more probe pulses generate reflected probe pulses modulated based on transient perturbations in the material from a target sample illuminated by the one or more pump pulses, the one or more pump pulses and the one or more probe pulses having different wavelengths or the same wavelengths selected from the spectrally broadened pulsed light by a wavelength selector, which may be a filter, which may be, for example, an AOF, a dichroic mirror or a beam splitter, a laser line filter, a notch filter, etc. For example, the means for selecting different wavelengths from the spectrally broadened pulsed light for one or more pump pulses and one or more probe pulses may be the filter 136, the optical system 115, the dichroic beam splitter 208, the filter 336, or the filter 436 described with reference to Figures 1A and 2 to 11. The wavelength selection of different wavelengths from the spectrally broadened pulsed light for one or more pump pulses and one or more probe pulses may be performed in the optical path prior to the pump arm and the probe arm, or may be performed in either the pump arm or the probe arm, for example as shown in Figures 4 to 6.

[0082] In block 1210, the photoacoustic measurement device detects the reflected probe pulse from the target sample using one or more detectors, as shown, for example, by detector arm 140 using detectors 144 and / or 146, detector arm 240 using detector 244, or detector arm 740 using detectors 744 and 746 described with reference to Figures 1A and 2-11.

[0083] In block 1212, the photoacoustic measurement device determines at least one property of the target sample based on the reflected probe pulse, for example, as described with reference to controller 150 shown in FIG. 1A or controller 250 shown in FIGS. 2-11.

[0084] In some implementations, the pulsed light may be spectrally broadened to include a second wavelength and a third wavelength by a supercontinuum generator in front of the pump arm and the probe arm, and the photoacoustic measurement device may further direct the second wavelength to the pump arm and the third wavelength to the probe arm, for example, as described with reference to FIG. 1A, FIG. 2, FIG. 3, and FIG. 7-FIG. 11. Thus, the one or more pump pulses and the one or more probe pulses may have different wavelengths. The means for directing the second wavelength to the pump arm and the third wavelength to the probe arm may be, for example, the optical system 115, the dichroic beam splitter 208, or the optical element 715, which may include one or more filters, for example, a filter wheel, one or more dichroic elements such as a dichroic mirror or beam splitter, one or more mirrors, etc., as described with reference to FIG. 1A, FIG. 2, FIG. 3, and FIG. 7-FIG. 11.

[0085] In some implementations, the pulsed light may be spectrally broadened by a supercontinuum generator in one of the pump arm and the probe arm, for example, as shown in FIGS. 4-6. In addition, in some implementations, the photoacoustic measurement device may further select a second wavelength from the pulsed light spectrally broadened by the supercontinuum generator using a wavelength-selective filter, for example, as shown in FIGS. 4-6. Thus, the one or more pump pulses and the one or more probe pulses may have different wavelengths. The means for selecting a second wavelength from the pulsed light spectrally broadened by the supercontinuum generator using a wavelength-selective filter may be, for example, a filter such as the filter 136 or 436 described with reference to FIG. 1A or FIGS. 4-6. The filter may be, for example, an AOF, a dichroic mirror or beam splitter, a laser line filter, a notch filter, or the like.

[0086] In some implementations, the pulsed light is spectrally broadened before the pump arm and the probe arm, for example, as described with reference to FIG. 1A, FIG. 2, FIG. 3, and FIG. 7-FIG. 11. The photoacoustic measurement device may further select a second wavelength from the pulsed light using a wavelength-selecting filter, for example, as described with reference to FIG. 1A, FIG. 3, and FIG. 7-FIG. 11, and direct the pulsed light having the second wavelength to the pump arm and the probe arm. Thus, the one or more pump pulses and the one or more probe pulses may have the same wavelength that is different from the first wavelength of the pulsed light generated by the laser source. The means for selecting the second wavelength from the pulsed light may be, for example, a wavelength-selecting filter, such as the wavelength selector 133, the optical system 115, the filter 336, or the optical element 715, described with reference to FIG. 1A or FIG. 4-FIG. 6. The wavelength selector may include, for example, one or more of an AOF, a dichroic mirror or beam splitter, a laser line filter, a notch filter, a beam dump, and the like, for example, as described with reference to FIG. 1A, FIG. 3, and FIG. 7-FIG. 11. The means for directing pulsed light having the second wavelength to the pump arm and the probe arm may be, for example, the optical system 115, the beam splitter 308, or the optical element 715 described with reference to FIG. 1A or FIGS. 4-6.

[0087] In some implementations, the optoacoustic measurement device may further shape at least a portion of the pulsed light to vary at least one of the duration, phase, or both of the one or more pump pulses, for example, using the pulse shaper 1124 described in Figure 11. The means for shaping at least a portion of the pulsed light beam to vary at least one of the duration, phase, or both of the one or more pump pulses may include, for example, a spatial light modulator or an acousto-optic modulator.

[0088] Although the present invention has been illustrated in connection with specific embodiments for illustrative purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the present invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

Claims

1. 1. A photoacoustic measurement device for non-destructive measurement of a target sample, comprising: a laser light source for generating pulsed light having a first wavelength; a supercontinuum generator that receives the pulsed light having the first wavelength and spectrally broadens the pulsed light; a pump arm configured to receive the pulsed light and to irradiate the target sample with one or more pump pulses to induce transient perturbations in a material within the target sample; a probe arm configured to receive the pulsed light and to irradiate the target sample with one or more probe pulses to generate a reflected probe pulse modulated based on the transient perturbation in the material, the one or more pump pulses and the one or more probe pulses having different wavelengths or the same wavelength selected from the spectrally broadened pulsed light; one or more detectors for receiving reflected probe pulses from the target sample; at least one processor coupled to the one or more detectors and configured to determine at least one property of the target sample based on the reflected probe pulses; A photoacoustic measurement device comprising:

2. The photoacoustic measurement device of claim 1 , wherein the laser light source generates a single pulsed light beam or two pulsed light beams having different frequency combs.

3. The photoacoustic measurement device of claim 1 , wherein the supercontinuum generator comprises a photonic crystal fiber.

4. the supercontinuum generator is located before the pump arm and the probe arm and spectrally broadens the pulsed light to include a second wavelength and a third wavelength; The photoacoustic measurement device of claim 1 , further comprising a dichroic beam splitter directing the pulsed light having the second wavelength to the pump arm and directing the pulsed light having the third wavelength to the probe arm.

5. The photoacoustic measurement device of claim 1 , wherein the supercontinuum generator is located in one of the pump arm and the probe arm.

6. The photoacoustic measurement device of claim 5 , further comprising a wavelength-selective filter that receives the pulsed light spectrally broadened by the supercontinuum generator and selects a second wavelength.

7. the supercontinuum generator is located before the pump arm and the probe arm and spectrally broadens the pulsed light to include a second wavelength and a third wavelength; a wavelength-selective filter that receives the pulsed light that is spectrally broadened by the supercontinuum generator and selects the second wavelength; one or more optical elements that direct the pulsed light having the second wavelength to the pump arm and the probe arm; The photoacoustic measurement device of claim 1 further comprising:

8. The photoacoustic measurement device of claim 1 , further comprising a pulse shaper that shapes at least a portion of the pulsed light to vary at least one of a duration, a phase, or both of the one or more pump pulses.

9. The optoacoustic measurement device of claim 8 , wherein the pulse shaper comprises one of a spatial light modulator or an acousto-optic modulator.

10. 1. A method for non-destructive photoacoustic measurement of a target sample using a photoacoustic measurement device, comprising: generating pulsed light having a first wavelength using a laser light source; spectrally broadening the pulsed light using a supercontinuum generator; generating one or more pump pulses using the pulsed light in a pump arm, the one or more pump pulses inducing transient perturbations in a material within the target sample that is irradiated by the one or more pump pulses; generating one or more probe pulses using the pulsed light in a probe arm, the one or more probe pulses producing reflected probe pulses from the target sample illuminated by the one or more pump pulses that are modulated based on the transient perturbations in the material, the one or more pump pulses and the one or more probe pulses having different wavelengths or the same wavelength selected from the spectrally broadened pulsed light; detecting a reflected probe pulse from the target sample with one or more detectors; determining at least one property of the target specimen based on the reflected probe pulse; A method comprising:

11. the pulsed light is spectrally broadened to include a second wavelength and a third wavelength by the supercontinuum generator before the pump arm and the probe arm; The method of claim 10 , further comprising directing the second wavelength to the pump arm and directing the third wavelength to the probe arm.

12. The method of claim 10 , wherein the pulsed light is spectrally broadened by the supercontinuum generator in one of the pump arm and the probe arm.

13. 13. The method of claim 12, further comprising selecting a second wavelength from the pulsed light spectrally broadened by the supercontinuum generator using a wavelength-selective filter.

14. the pulsed light is spectrally broadened before the pump arm and the probe arm; selecting a second wavelength from the pulsed light using a wavelength selection filter; directing the pulsed light having the second wavelength to the pump arm and the probe arm; The method of claim 10 further comprising:

15. 11. The method of claim 10, further comprising shaping at least a portion of the pulsed light to vary at least one of a duration, a phase, or both of the one or more pump pulses.

16. 1. A photoacoustic measurement device for non-destructive measurement of a target sample, comprising: a laser light source for generating pulsed light having a first wavelength; means for spectrally broadening the pulsed light; a pump arm configured to receive the pulsed light and to irradiate the target sample with one or more pump pulses to induce transient perturbations in a material within the target sample; a probe arm configured to receive the pulsed light and to irradiate the target sample with one or more probe pulses to generate a reflected probe pulse modulated based on the transient perturbation in the material, the one or more pump pulses and the one or more probe pulses having different wavelengths or the same wavelength selected from the spectrally broadened pulsed light; one or more detectors for receiving reflected probe pulses from the target sample; at least one processor coupled to the one or more detectors and configured to determine at least one property of the target sample based on the reflected probe pulses; A photoacoustic measurement device comprising:

17. 17. The photoacoustic measurement device of claim 16, wherein the pulsed light is spectrally broadened to include a second wavelength and a third wavelength before the pump arm and the probe arm, and further comprising means for directing the second wavelength to the pump arm and the third wavelength to the probe arm.

18. The photoacoustic measurement device of claim 16 , wherein the pulsed light is spectrally broadened in one of the pump arm and the probe arm.

19. the pulsed light is spectrally broadened before the pump arm and the probe arm; a means for selecting a second wavelength from the pulsed light using a wavelength selection filter; means for directing the pulsed light having the second wavelength to the pump arm and the probe arm; The photoacoustic measurement device of claim 16 further comprising:

20. The photoacoustic measurement device of claim 16 , further comprising means for shaping at least a portion of the pulsed light to vary at least one of a duration, a phase, or both of the one or more pump pulses.

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