Multiple pump-probe coding-decoding for photoacoustic measurements

Abstract

The photoacoustic measurement device is configured to measure or inspect structures within a sample using both vertical and lateral transient perturbations. Multiple probe beams with different incidence positions can detect both vertical and lateral transient perturbations generated by a pump beam, or a single probe beam can detect both vertical and lateral transient perturbations generated by multiple pump beams with different incidence positions. The multiple probe beams or multiple pump beams are modulated with orthogonal waveforms that allow measurements at different positions without interference. The received signals are demodulated based on the orthogonal waveforms to recover the contributions to the received signal associated with each of the multiple probe beams or each of the multiple pump beams.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims priority to U.S. Provisional Application No. 63 / 493,545, filed Mar. 31, 2023, "MULTI PUMP - PROBE ENCODING - DECODING FOR OPTO - ACOUSTIC METROLOGY" and U.S. Non - Provisional Application No. 18 / 612,243, filed Mar. 21, 2024, "MULTI PUMP - PROBE ENCODING - DECODING FOR OPTO - ACOUSTIC METROLOGY". Both of these applications have been assigned to the assignee of this application and are hereby incorporated by reference in their entirety.

[0002] (Field of the Invention) The subject matter described herein generally relates to microscopy and, more particularly, to the use of photoacoustic measurements.

Background Art

[0003] Optical measurement is used to provide non-contact evaluation of samples and is often used during processing in semiconductor and other similar industries. For example, manufacturing processes used in semiconductor and similar industries rely on a series of processing steps to build layers to manufacture a desired device, such as an electronic circuit. These processing steps include the deposition and patterning of material layers, such as insulating layers, polysilicon layers, and metal layers in semiconductor devices. The material layers are typically patterned using a photoresist layer, which is patterned on the material layer using a photomask or reticle. Typically, the photomask has an alignment target or key that is aligned to a reference mark formed on the previous layer on the substrate. Aligning a lithographically defined pattern on top of an underlying pattern, sometimes referred to as an overlay, is essential for the operation of the device in all multilayer patterning process flows. Misalignment between layers or patterns can lead to device failure and is therefore one characteristic that is desirable to evaluate during processing. Another device characteristic essential for the operation of the device is the critical dimension (CD), e.g., line or feature width, line spacing, line height, and sidewall angle. Characterizing the CD is important in ensuring that the device meets its design objectives.

[0004] There are various optical measurement techniques that can be conventionally used to characterize overlay and / or CD or other desired instrument characteristics. For example, conventional imaging techniques may use light of a specific wavelength, such as ultraviolet (UV), visible light, or infrared (IR), to image structures within a sample, including, for example, an underlying structure whose upper layer is aligned. However, in the manufacture of some structures, an optically opaque layer may be present, covering the structure to be measured. Optically opaque materials may be present between target structures, such as in a semi-damascene process flow or after processing of magnetic tunnel junctions (MTJs) in magnetic random-access memory (MRAM), which presents specific challenges to measuring the underlying structure, as well as to alignment and overlay control. For example, the presence of intervening opaque material typically requires extra patterning operations, thereby adding considerable process costs. Therefore, microscopy techniques that can measure opaque and embedded structures are desirable. [Overview of the project]

[0005] A photoacoustic instrument is configured to measure or inspect the structure of a sample using multiple probe beams or multiple pump beams incident at different locations on the sample. The photoacoustic instrument detects vertical and lateral transient perturbations in the sample. For example, the photoacoustic instrument may use multiple probe beams with different incident positions to detect both vertical and lateral transient perturbations generated by a pump beam, or it may use a single probe beam to detect both vertical and lateral transient perturbations generated by multiple pump beams with different incident positions. The multiple probe beams or multiple pump beams can be encoded and decoded, for example, using quadrature waveforms, so that the individual contributions of the multiple probe beams or multiple pump beams to the received signal can be recovered.

[0006] In one embodiment, a method for photoacoustic measurement of a sample includes directing a pump beam containing pump pulses toward the surface of the sample. The pump beam generates vertical transient perturbations and lateral transient perturbations within the sample. The method further includes generating a plurality of probe beams, each probe beam containing a probe pulse, and modulating each of the plurality of probe beams. The plurality of probe beams are directed to different positions on the surface of the sample, and the plurality of probe beams are reflected from the surface of the sample. A first reflected probe beam is modified based on vertical transient perturbations propagating perpendicularly to the surface of the sample, and at least one second reflected probe beam is modified based on lateral transient perturbations propagating along the surface of the sample. The method further includes demodulating the first reflected probe beam and at least one second reflected probe beam. The method further includes determining at least one characteristic of a sample based on vertical transient perturbations obtained from demodulating a first reflected probe beam and on lateral transient perturbations obtained from demodulating at least one second reflected probe beam.

[0007] In one embodiment, a measuring device for photoacoustic measurement of a sample includes a pump arm configured to receive at least a first portion of pulsed light from a light source and direct a pump beam containing pump pulses toward the surface of the sample. The pump beam generates vertical transient perturbations and lateral transient perturbations within the sample. The measuring device further includes a probe arm configured to receive at least a second portion of pulsed light from a light source and direct a plurality of probe beams toward different locations on the surface of the sample, each probe beam containing a probe pulse. The probe arm includes means for modulating each of the plurality of probe beams. The plurality of probe beams are reflected from the surface of the sample, and a first reflected probe beam is modified based on vertical transient perturbations propagating perpendicularly to the surface of the sample, and at least one second reflected probe beam is modified based on lateral transient perturbations propagating along the surface of the sample. The measuring device further includes means for demodulating the first reflected probe beam and at least one second reflected probe beam. The measuring device further includes means for determining at least one characteristic of the sample based on vertical transient perturbations obtained from demodulating a first reflected probe beam and lateral transient perturbations obtained from demodulating at least one second reflected probe beam.

[0008] In one embodiment, a method for photoacoustic measurement of a sample includes generating a plurality of pump beams, each pump beam containing a pump pulse, and modulating each of the plurality of pump beams. The method includes directing the plurality of pump beams to different locations on the surface of the sample, each pump beam exciting transient perturbations within the sample at the corresponding location. The method further includes directing a probe beam containing a probe pulse towards the surface of the sample, and the probe beam is reflected from the surface of the sample. The reflected probe beam is modified based on vertical transient perturbations within the sample propagating perpendicular to the surface of the sample and lateral transient perturbations propagating along the surface of the sample, which are excited by the plurality of pump beams. The method further includes demodulating the reflected probe beam and determining at least one characteristic of the sample based on the vertical and lateral transient perturbations obtained from demodulating the reflected probe beam.

[0009] In one embodiment, a measuring device for photoacoustic measurement of a sample includes a pump arm configured to receive at least a first portion of pulsed light from a light source and direct a plurality of pump beams toward different locations on the surface of the sample, each pump beam containing a pump pulse. The pump arm includes means for modulating each of the plurality of pump beams. Each pump beam excites transient perturbations within the sample at the corresponding location. The measuring device further includes a probe arm configured to receive at least a second portion of pulsed light from a light source and direct a probe beam containing a probe pulse toward the surface of the sample, the probe beam being reflected from the surface of the sample. The reflected probe beam is modified based on vertical transient perturbations within the sample propagating perpendicular to the surface of the sample and lateral transient perturbations propagating along the surface of the sample, which are excited by the plurality of pump beams. The measuring device further includes means for demodulating the reflected probe beam and means for determining at least one characteristic of the sample based on the vertical and lateral transient perturbations obtained from demodulating the reflected probe beam. [Brief explanation of the drawing]

[0010] [Figure 1] For example, a block diagram of a photoacoustic measurement device that uses multiple pump beams or multiple probe beams modulated with orthogonal waveforms is shown. [Figure 2A] The images show side and top views of a sample being measured or inspected by a photoacoustic measurement device using a single pump beam and multiple probe beams with orthogonal waveform modulation, respectively. [Figure 2B] The images show side and top views of a sample being measured or inspected by a photoacoustic measurement device using a single pump beam and multiple probe beams with orthogonal waveform modulation, respectively. [Figure 2C] Figures 2A and 2B are graphs showing the change in reflectance (ΔR) of the demodulated signal generated by the reflected probe beam with respect to time (t). [Figure 3A] The side cross-sectional view and top view of a sample measured or inspected by a photoacoustic measurement device using multiple pump beams with orthogonal waveform modulation and a single probe beam are shown. [Figure 3B] The side cross-sectional view and top view of a sample measured or inspected by a photoacoustic measurement device using multiple pump beams with orthogonal waveform modulation and a single probe beam are shown. [Figure 3C] Figures 3A and 3B are graphs showing the change in reflectance (ΔR) of the demodulated signal generated by the reflected probe beam as a function of time (t). [Figure 4A] An example of a quadrature waveform that can be used to modulate multiple probe beams or multiple pump beams is shown. [Figure 4B] An example of a quadrature waveform that can be used to modulate multiple probe beams or multiple pump beams is shown. [Figure 5A] A schematic diagram of an exemplary photoacoustic measurement system using a single pump beam and multiple probe beams modulated with orthogonal waveforms is shown. [Figure 5B] A schematic diagram of an exemplary photoacoustic measurement system using a single pump beam and multiple probe beams modulated with orthogonal waveforms is shown. [Figure 6] A schematic diagram of an exemplary photoacoustic measurement system using multiple pump beams modulated with orthogonal waveforms and a single probe beam is shown. [Figure 7] This flowchart shows the process of photoacoustic measurement of a sample using multiple probe beams. [Figure 8] This flowchart shows the process of photoacoustic measurement of a sample using multiple pump beams. [Modes for carrying out the invention]

[0011] Photoacoustic measurements, such as picosecond laser acoustic (PLA) measurements, can be used to measure structures on devices such as semiconductor devices and other similar types of equipment. Photoacoustic measurement techniques enable the measurement of opaque devices or devices beneath opaque layers, for example, for the analysis of CDs or overlays. The use of photoacoustic techniques is advantageous because it does not rely on light penetrating opaque layers or structures, but instead generates and detects acoustic waves propagating through optically opaque layers and structures. Photoacoustic techniques can also be used to measure non-opaque and unembedded structures, i.e., top-layer structures.

[0012] Photoacoustic techniques characterize a sample by recording and analyzing its response to the action of a pump beam and the corresponding effect on the reflected probe beam. The pump beam irradiates a target sample, for example, to induce transient perturbations in the target material. The probe beam similarly irradiates the target sample, and the reflected probe beam is modified based on the transient perturbations in the target material. In conventional pump-probe measurement techniques used in photoacoustic measurements, the pump beam and / or probe beam are frequency-modulated, typically with a sinusoidal waveform, and the measurement signal from the reflected probe beam is demodulated to improve the signal-to-noise ratio (SNR).

[0013] However, conventional photoacoustic techniques have limitations. For example, photoacoustic measurement devices are typically configured to measure only bulk-propagated elastic waves, such as vertical transient perturbations, or surface-propagated elastic waves, such as horizontal transient perturbations, depending on the position of the probe beam relative to the pump beam. The single-pump-single-probeam configuration in conventional photoacoustic measurement devices limits throughput due to the one-to-one arrangement of the pump beam and probe beam.

[0014] However, having a one-to-many (or many-to-one) (or many-to-many) arrangement of pump beams and probe beams can be advantageous. For example, the response to a single pump operation can be collected by multiple probe beams strategically positioned to measure both bulk and surface-propagated acoustic waves and their interactions with the structure under study. Similarly, a similar measurement can be performed by collecting the response of a single probe beam to multiple pump beams acting sequentially or simultaneously at selected locations to measure both bulk and surface-propagated acoustic waves and their interactions with the structure under study. Furthermore, a similar measurement can be performed by collecting the responses of multiple probe beams to multiple pump beams acting sequentially or simultaneously at selected locations to measure both bulk and surface-propagated acoustic waves and their interactions with the structure under study. The use of a one-to-many (or many-to-one) (or many-to-many) arrangement of pump beams and probe beams can provide additional useful information related to both bulk and surface properties and improve throughput. For example, triangulation or trilateration may be used to determine the location or overlay of features.

[0015] As described herein, a system for photoacoustic measurement and / or inspection uses a plurality of pump beams and / or a plurality of probe beams, e.g., a plurality of pump beams and a single probe beam, a single pump beam and a plurality of probe beams, or a plurality of pump beams and a plurality of probe beams. The pump beam and the probe beam may irradiate the same position on the sample surface, while the remaining beams irradiate different positions. Signals from one or more probe beams can be collected by a single detector. An encoding / decoding scheme is used to separate the contributions of the plurality of beams to the collected signals. For example, the encoding / decoding scheme can modulate the pump beam and / or the probe beam using orthogonal waveforms such as, but not limited to, sine-cosine pairs at different frequencies, wavelets such as Haar wavelets or Daubechies wavelets, or other orthogonal waveforms. The modulation used can be optimized for a given application and / or the shape of the test structure, as well as the repetition rate, duty cycle, and pulse train synchronization of the pump beam and the probe beam.

[0016] FIG. 1 shows a block diagram of an exemplary photoacoustic measurement device 100 that can use a plurality of pump beams, a plurality of probe beams, or both a plurality of pump beams and a plurality of probe beams, and includes an encoding / decoding scheme in which the pump beam and / or the probe beam is modulated with, e.g., an orthogonal waveform, as described herein.

[0017] The photoacoustic measurement device 100 is shown comprising a pump beam source 120 and probe beam sources 122A and 122B. The pump beam source 120 may include, for example, a laser, which may be referred to herein as an excitation laser. The probe beam sources 122A and 122B may be collectively referred to as the probe beam source 122, which may include at least one laser, which may be referred to herein as a detection laser. In some embodiments, the probe beam sources 122A and 122B may use the same laser and may include a beam splitter for splitting the emitted light into separate probe beams. In addition, in some embodiments, the pump beam source 120 and the probe beam source 122 may use the same laser and may include a beam splitter for splitting the emitted light into separate pump beams and probe beams. The pump beam source 120 and the probe beam source 122 may further include a delay stage (not shown) for increasing or decreasing the optical path length between the laser and the sample 112.

[0018] In the illustration of FIG. 1, the pump beam source 120 can generate a single pump beam 121, while a plurality of probe beams, shown as probe beams 123A and 123B and sometimes referred to as probe beam 123, are generated by the probe beam source 122. Probe beam 123A may be incident on the sample 112 at the same position as the pump beam 121, and probe beam 123B may be incident on the sample 112 at a slightly different position. In some embodiments, the pump beam source 120 can generate a plurality of pump beams shown as pump beam 121 and pump beam 121' (shown in dashed line), and only a single probe beam source 122A can be used to generate a probe beam 123A that is incident on the sample 112 at the same position as the pump beam 121, and the pump beam 121' can be incident on the sample 112 at a slightly different position. In some embodiments, the pump beam source 120 can generate a plurality of pump beams shown as pump beam 121 and pump beam 121', and a plurality of probe beam sources 122A, 122B can be used to generate the probe beams 123A and 123B. In such embodiments, although not necessarily all, some of the pump beams 121, 121' are incident on the sample 112 at different positions, the probe beams 123A and 123B are incident on the sample 112 at different positions, and although not necessarily all, some of the pump beams and probe beams can be paired to be incident on the sample 112 at the same position.

[0019] FIG. 1 shows a plurality of probe beams as including two probe beams 123A and 123B, but it should be understood that the plurality of probe beams may include more probe beams as needed. Further, FIG. 1 shows a plurality of pump beams as including two pump beams 121 and 121', but the plurality of pump beams may include more pump beams as needed.

[0020] The photoacoustic measurement device 100 further includes several modulators, for example, a pump beam modulator 124, and probe beam modulators 125A and 125B. In embodiments having multiple pump beams 121 and 121', a separate pump beam modulator may be used for each pump beam. In some embodiments, the modulators may be electro-optic modulators (EOMs), photoelastic modulators (PEMs), acousto-optic modulators (AOMs), mechanical choppers, etc. The modulators, for example, the probe beam modulators 125A and 125B, are used to encode the probe beams 123A and 123B, for example, with quadrature waveforms, so that the contribution from each probe beam modulator to the detection signal can be separated. The probe beam modulators 125A and 125B may modulate the probe beams 123A and 123B using single-frequency sine and cosine pairs at different frequencies, for example. In other embodiments, probe beam modulators 125A and 125B may modulate probe beams 123A and 123B with orthogonal wavelets, such as but not limited to Haar wavelets or Dobethy wavelets. Other types of orthogonal waveforms, such as multiplexed signals, or orthogonal frequency division multiplexed signals or orthogonal chirp signals, may be used. In some embodiments, pump beam modulator 124 may further modulate the pump beam 121, for example, by pump intensity modulation. For example, if multiple pump beams 121 and 121' are used instead of multiple probe beams 123A and 123B, the pump beams 121 and 121' may be frequency modulated by a separate pump beam modulator.

[0021] The photoacoustic measurement device 100 may include various additional optical elements for guiding the pump beam and probe beam to incident on the sample 112, receiving the reflected probe beam, and detecting the resulting signal. For example, as shown in Figure 1, the photoacoustic measurement device 100 may include a mirror 126 and a beam splitter 127, as well as lenses 134, 136, and 138, and a detector 128. Lenses 134 and 136 may be configured to adjust the spot sizes of the pump beam 121 and probe beam 123 based on a specific target to be measured, and to control the incident positions of the pump beam 121 and probe beam 123. The spot sizes of the respective beams may be similar or different. Additional optical elements such as filters and polarizers may be used, but are not shown in Figure 1.

[0022] Detector 128 receives probe beams 123A and 123B after they have been reflected from the sample 112. In some embodiments, a pump beam 121 may be incident on the sample 112 along the same optical path as the probe beams, and the photoacoustic measuring device 100 may include a beam dump (not shown) to capture light from the pump beam returning from the sample 112. Detector 128 may be, for example, a photodetector or a PIN photodiode. For example, the reflectance of reflected light on the upper surface of the structure 110 changes due to changes in reflectance or surface deformation caused by bulk waves and surface waves. Detector 128 may be configured to receive and demodulate the reflected probe pulses to sense changes in the intensity of the probe beam caused by changes in reflectance and / or interference oscillations. Detector 128 may include or be coupled to a lock-in amplifier 129, which includes a plurality of demodulators 130A and 130B for phase locking to the received signal. Demodulators 130A and 130B correspond, for example, to probe beam modulators 125A and 125B and are used to decode the contribution of each probe beam to the received signal based on the quadrature waveforms of the probe beams 123A and 123B generated by the probe beam modulators 125A and 125B. Demodulators 130A and 130B may be, for example, physical bandpass filters, digital processing, or a combination thereof. In embodiments where multiple pump beams and a single probe beam are used, demodulators 130A and 130B correspond to multiple pump beam modulators and are used to decode the contribution of each pump beam to the received signal based on the quadrature waveforms of the pump beams 121 and 121' generated by the pump beam modulators. When both the pump pulse and the probe pulse are frequency modulated, a combination of frequencies of the received probe beams, such as the sum or difference, can be demodulated.

[0023] The pump pulse and probe pulse may be generated with different delays, and the detector 128 may generate a signal in response to the received reflected probe beam 123 with different time delays between the pump pulse and the probe pulse. Each signal responds, for example, to bulk waves and surface waves. The bulk waves correspond to the arrival of acoustic echoes after reflection from lower layers at different depths within the patterned structure, which can be used to detect the structure and lower layers of the patterned structure. The surface waves correspond to the arrival of surface waves propagating through various structural elements and materials near the surface of the sample, which can be used to detect the structure and materials within the patterned structure. The detector 128 may record changes in the reflectivity or surface deformation of the sample 112 due to bulk waves and surface waves at the incident position of each probe beam 123 and / or pump beam 121 as a function of the time delay between the pump pulse and the probe pulse.

[0024] The photoacoustic measurement device 100 further includes a mechatronics support 140 for a sample 112, the structure 110 of which is part. The mechatronics support 140 is adapted to move the sample 112 relative to the pump beam and probe beam to obtain measurements from a desired position on the sample 112. The device further includes a processing system 132 coupled to the pump beam source 120 and probe beam source 122, the mechatronics support 140, and the detector 128. It should be understood that the processing system 132 may be an embedded or distributed computing device capable of performing calculations, receiving and sending instructions or commands, and receiving, storing and sending information related to the measurement functions of the device.

[0025] In the illustrated embodiment, the pump beam 121 and the probe beam 123 do not share an optical path to or from the sample 112. For example, as shown, the pump beam 121 is incident perpendicularly (or nearly perpendicularly), and the probe beam 123 is incident obliquely. However, in some embodiments, the pump beam 121 may be incident obliquely, and the probe beam 123 may be incident perpendicularly (or nearly perpendicularly). In some embodiments, the pump beam 121 and the probe beam 123 may share an optical path to the sample 112. Therefore, several different configurations may be used in which the optical paths are the same, partially overlapping, adjacent, or coaxial. In some embodiments, the pump beam and the probe beam may be derived from the same light source, such as a pulsed laser. The pump beam source 120 and the probe beam source 122 may be directly controlled to obtain a time interval between pulses of light directed toward the sample 112.

[0026] During operation, the processing system 132 directs a series of light pulses from the pump beam source 120 onto the structure 110. These light pulses are incident on at least one layer within the structure 110 and are at least partially absorbed. The absorption of light causes a transient expansion of the material in the structure 110, which simultaneously produces bulk waves and surface waves depending on the pump beam characteristics. The expansion is short enough to induce what is essentially ultrasound, called bulk ultrasound, which propagates vertically (i.e., perpendicular to the surface of the structure 110), is reflected at each underlying interface in the film stack, and is returned to the upper surface. The expansion further induces ultrasound that propagates horizontally along the surface of the structure 110 and is attenuated or otherwise affected by the structure and various materials within the structure 110, which is called surface ultrasound or transverse (shear) ultrasound.

[0027] In addition to directing the operation of the pump beam source 120, the processing system 132 also directs the operation of the probe beam source 122. The probe beam source 122 directs a series of light pulses incident on the structure 110, and the light pulses are reflected from the uppermost layer of the structure 110 and affected by bulk or surface ultrasound generated within the structure 110 by the pump beam 121. For example, a probe beam 123A incident at or near the incident position of the pump beam 121 is affected by bulk ultrasound traveling vertically within the structure 110, while another probe beam 123B incident at a different location from the pump beam 121 is affected by surface ultrasound traveling horizontally within the structure 110. When multiple pump beams 121 and 121' and a single probe beam 123A are used, the probe beam 123A is incident at or near the incident position of the pump beam 121, but at a different position from the other pump beams 121'. The probe beam 123A is affected by bulk ultrasound that travels vertically within the structure 110 due to the pump beam 121, and by surface ultrasound that travels horizontally within the structure 110 due to the pump beam 121'.

[0028] It should be understood that many optical configurations are possible. In some configurations, the pump beam 121 may be generated by a pulsed laser having a pulse width ranging from several hundred femtoseconds to several picoseconds, and the probe beams 123A and 123B may be coupled to a beam deflection system. For example, delay stages (not shown) may be included in the probe beam sources 122A and 122B (and the pump beam source 120) to increase or decrease the length of the optical path to the structure 110. The delay stages may be controlled by a processing system 132 to obtain a time delay of the optical pulse incident on the object. Many other alternative configurations are also possible. It should be understood that the schematic diagram in Figure 1 is not intended to be limiting, but rather to illustrate one of several exemplary configurations for the purpose of illustrating novel features of this disclosure.

[0029] The processing system 132 is configured to collect and analyze data obtained from the sample 112 by the detector 128, using a lock-in amplifier 129 and multiple demodulators 130A and 130B. The processing system 132 can determine at least one characteristic of the sample 112 using vertical and lateral transient perturbations obtained from demodulating the reflected probe beams 123A and 123B (or, if multiple pump beams 121 and 121' are used, using only the reflected probe beam 123A). For example, the processing system 132 can determine the location and composition of the underlying structure in the sample 112 based on the vertical transient perturbation, and the location and composition of the structure on the surface of the sample 112 based on the lateral transient perturbation. The processing system 132 can detect the structure and composition of the material at different depths or locations on the surface of the sample 112, for example, by changing the known delay between the pump pulse and the probe pulse. In addition, by using three or more probe beams (or pump beams) and a known arrangement of the incident positions of the pump beam and probe beams, the processing system 132 can use triangulation or trilateration of the measurement data for local mapping of the structure within the sample 112.

[0030] Figures 2A and 2B show a side and top view, respectively, of a sample 200 being measured or inspected by a photoacoustic instrument that uses a single pump beam and multiple probe beams and employs quadrature waveform modulation, such as the photoacoustic instrument 100 described in relation to Figure 1. The sample 200 is shown as containing a structure 202 that includes an array of lines in layer 204 above several sublayers 206 and 208. Figures 2A and 2B further show the incidence position of the pump beam 210 by a dotted line at position A, and the incidence positions of the multiple probe beams 212A, 212B, and 212C (sometimes collectively referred to as probe beam 212) by solid lines at different positions A, B, and C. Figures 2A and 2B show the use of three probe beams, in contrast to the two probe beams shown in Figure 1, and it should be understood that additional probe beams may be used if desired. Furthermore, additional pump beams may be used as needed, for example, as described in Figures 3A and 3B; i.e., multiple pump beams and multiple probe beams may be used. The displacement between the incident positions of the pump beam and the probe beam, such as both magnitude and distance, is known and can be used to analyze the resulting signal. Furthermore, while Figures 2A and 2B show that the incident positions of the probe beam are uniformly distributed along a line (e.g., linearly and uniformly distributed along the X-axis), it should be understood that other geometric arrangements of incident positions, such as nonlinear and / or non-uniformly distributed arrangements, can be used and may be desirable depending on the structure under test, for example, which may be advantageous for triangulation or trilateration calculations.

[0031] As shown in Figure 2A, the pump beam 210 is incident at position A. The light pulses of the pump beam 210 are incident on the elements of structure 202 and are absorbed at least partially, causing a transient expansion in the material of structure 202. This transient expansion can simultaneously induce ultrasonic bulk waves and transverse waves (shear waves) within the sample 200, as shown separately in Figures 2A and 2B, respectively. For example, Figure 2A shows bulk waves propagating perpendicularly (along the Z direction) through the bulk material of sample 200, as indicated by the solid arrows, and reflected at each lower layer interface in the film stack of layers 204, 206, and 208, which are returned to the surface of structure 202, as indicated by the dotted arrows. The probe beam 212A incident at position A is also affected by the change in reflectivity or surface deformation of structure 202 due to the returning bulk waves. Probe beams 212B and 212C are incident at different positions than the pump beam 210 and may not be affected by the perpendicularly propagating returning waves. The reflection of acoustic waves by underlying interfaces at various depths within the sample 200 can be resolved by varying the delay between the pulses in the pump beam 210 and the pulses in the probe beam 212A.

[0032] If the pump beam 210 reaches and absorbs the homogeneous layer beneath the structure 202, the energy entering the transverse wave may be very small. However, heterogeneity in the upper part of the sample 200 can create more complex coupled modes combining both bulk (vertical) and transverse components. For example, as shown by the solid arrows in Figure 2B, transient expansion of the material in the structure 202 due to the optical pulse of the pump beam 210 can induce ultrasonic transverse (surface) waves that propagate horizontally (along the X and Y directions) along the surface of the sample 200. Probe beams 212B and 212C, incident at different locations than the pump beam 210, are affected by changes in the reflectivity or surface deformation of the structure 202 due to surface waves propagating through various structural elements and materials of the structure 202. Probe beam 212A is incident at the same location as the pump beam 210. Probe beam 212A may be affected by surface waves generated by the pump beam 210, although in some embodiments, probe beam 212A may not be affected by surface waves generated by the pump beam 210. Surface waves generated by the pump beam 210 and propagating through various materials and structures can be resolved at various distances between the incident position of the pump beam at position A and the incident positions of the probe beams 212B and 212C at positions B and C, and can be resolved by changing the delay between pulses in the pump beam 210 and the probe beams 212B and 212C.

[0033] As shown in Figure 2B, probe beams 212A, 212B, and 212C reflected from positions A, B, and C, respectively, are received by a detector 220 coupled to a lock-in amplifier 222. The lock-in amplifier 222 includes separate demodulators 224A, 224B, and 224C for demodulating the quadrature waveform modulation of the reflected probe beams 212A, 212B, and 212C and for phase-locking to the contribution from each probe beam in the received signal.

[0034] Figure 2C is a graph showing the change in reflectance (ΔR) of the demodulated signals generated by probe beams 212A, 212B, and 212C with respect to the pump-probe delay time (t). Curve 230 represents, for example, the change in reflectance of probe beam 212A due to bulk waves shown in Figure 2A. Curves 232 and 234 represent the changes in reflectance of probe beams 212B and 212C due to surface waves shown in Figure 2B, respectively.

[0035] Figures 3A and 3B show a side section and a top view, respectively, of a sample 300 being measured or inspected by a photoacoustic measuring device, such as the photoacoustic measuring device 100 described in relation to Figure 1. The diagrams of the operation of the photoacoustic measuring device in Figures 3A and 3B are similar to those shown in Figures 2A and 2B, but instead of using a single pump beam and multiple probe beams as shown in Figures 2A and 2B, Figures 3A and 3B show the use of multiple pump beams and a single probe beam. Figures 3A and 3B show the use of three pump beams, in contrast to the two pump beams shown in Figure 1, and it should be understood that additional pump beams can be used if desired. Furthermore, additional probe beams can be used as needed, for example, as described in Figures 2A and 2B, i.e., multiple pump beams and multiple probe beams can be used. The displacement between the incident positions of the pump beams and probe beams, e.g., both magnitude and distance, is known and can be used for the analysis of the resulting signals. Furthermore, while Figures 3A and 3B show that the incident positions of the pump beam are evenly distributed along the line (e.g., linearly and evenly distributed along the X-axis), it should be understood that other geometric arrangements of incident positions, such as nonlinear and / or unevenly distributed arrangements, can be used and may be desirable depending on the structure under test, for example, they may be advantageous for triangulation or trilateration calculations.

[0036] Sample 300, shown in Figures 3A and 3B, like sample 200 shown in Figures 2A and 2B, includes a structure 302 containing an array of lines in layer 304 above several sublayers 306 and 308. Figures 3A and 3B further show the incidence positions of multiple pump beams 310A, 310B, and 310C (sometimes collectively referred to as pump beam 310) with dotted lines at different positions A, B, and C, respectively, and the position of a single probe beam 312 with a solid line at position A. Figures 3A and 3B show the use of three pump beams, in contrast to the two pump beams shown in Figure 1, and it should be understood that additional pump beams can be used if desired. Furthermore, while Figures 3A and 3B show the incidence positions of the pump beams evenly distributed along a line (e.g., along the X-axis), it should be understood that other geometric arrangements of incidence positions, e.g., nonlinear and / or non-uniformly distributed arrangements, can be used and may be desirable depending on the structure under test.

[0037] As shown in Figure 3A, the pump beam 310A is incident at position A. The light pulses of the pump beam 310A are incident on the elements of structure 302 and are at least partially absorbed, causing a transient expansion in the material of structure 302. This transient expansion can simultaneously induce ultrasonic bulk waves and transverse waves (shear waves) within the sample 300, as shown separately in Figures 3A and 3B, respectively. For example, Figure 3A shows bulk waves propagating perpendicularly (along the Z direction) through the bulk material of sample 300, as indicated by the solid arrows, and reflected at each lower layer interface in the film stack of layers 304, 306, and 308, which are returned to the surface of structure 302, as indicated by the dotted arrows. The probe beam 312 incident at position A is also affected by the change in reflectivity or surface deformation of structure 302 due to the returning bulk waves. Pump beams 310B and 310C also propagate perpendicularly (along the Z-direction) through the bulk material of sample 300 and generate ultrasonic bulk waves that are reflected by the underlying interface; however, pump beams 310B and 310C are incident at different locations than probe beam 312, and therefore probe beam 312 may not be affected by the reflected bulk waves generated by pump beams 310B and 310C. The reflection of bulk waves at position A due to the underlying interface at various depths within sample 300 can be resolved by varying the delay between the pulses in pump beam 310A and the pulses in probe beam 312.

[0038] If the pump beams 310A, 310B, and 310C reach and absorb the homogeneous layer beneath structure 302, the energy entering the transverse wave may be very small. However, heterogeneity in the upper part of sample 300 can create more complex coupled modes combining both bulk (vertical) and transverse components. For example, as shown by the solid arrows in Figure 3B, the transient expansion of the material in structure 302 by the optical pulses of pump beams 310B and 310C induces ultrasonic surface waves that propagate horizontally (along the X and Y directions) along the surface of sample 300, as shown by the solid arrows in Figure 3B. The probe beam 312, incident at a different location than the pump beams 310B and 310C, is affected by the changes in reflectivity or surface deformation of structure 302 caused by the surface waves induced by pump beams 310B and 310C, and these surface waves propagate through various structural elements and materials of structure 302. The pump beam 310A also generates ultrasonic surface waves (not shown), but the pump beam 310A is incident at the same location as the probe beam 312. The probe beam 312 may be affected by the surface waves generated by the pump beam 310A, but in some embodiments, the probe beam 312 may not be affected by the surface waves generated by the pump beam 310A. The surface waves generated by the pump beams 310A and 310B and propagating through various materials and structures can be resolved at various distances between the incident positions of the pump beams 310B and 310C at positions B and C, respectively, and the incident position of the probe beam 312 at position A, and can be resolved by changing the delay between pulses in the pump beams 310A, 310B, and probe beam 312.

[0039] As shown in Figure 3B, the probe beam 312 reflected from position A is received by a detector 320 coupled to a lock-in amplifier 322. Each pump beam 310A, 310B, and 310C is modulated over time with a different quadrature waveform (e.g., pump intensity modulation). The sound waves produced from each of the pump beams 310A, 310B, and 310C similarly vary in intensity (amplitude) over time according to their respective quadrature waveforms. The probe beam 312 sensing the sound waves also has a component modulated according to the quadrature waveform, and the resulting signal includes a superposition of the corresponding modulation functions. The lock-in amplifier 322 includes separate demodulators 324A, 324B, and 324C to demodulate the probe beam 312 based on the quadrature waveform modulation of the pump beams 310A, 310B, and 310C and to phase-lock the contribution from each pump beam in the received signal.

[0040] Figure 3C is similar to Figure 2C and shows a graph illustrating the change in reflectivity (ΔR) of the demodulated signal generated by the probe beam 312 with respect to the pump-probe delay time (t). Curve 330 represents the change in reflectivity of the probe beam 312 due to bulk waves generated by, for example, the pump beam 310A shown in Figure 3A. Curves 332 and 334 represent the changes in reflectivity of the probe beam 312 due to surface waves generated by the pump beams 310B and 310C, respectively, shown in Figure 3B.

[0041] If necessary, the operation of a photoacoustic measurement device using a single pump beam and multiple probe beams, as shown in Figures 2A and 2B, may be combined with the operation of a photoacoustic measurement device using multiple pump beams and a single probe beam, as shown in Figures 3A and 3B, for operation using multiple pump beams and multiple probe beams.

[0042] Figures 4A and 4B show examples of quadrature waveforms that may be used to modulate multiple probe beams and / or multiple pump beams, as described herein. Figure 4A shows an exemplary Haar wavelet 400 that may be used to modulate (and demodulate) three beams. Figure 4B shows an example of a Dobethy wavelet 410 that may be used to modulate (and demodulate) three beams.

[0043] Figure 5A shows a schematic diagram of an exemplary photoacoustic measurement device 500 that uses a single pump beam and multiple probe beams modulated with orthogonal waveforms, as described herein. As shown, the light may be generated from a light source 502, such as a 520 nm, 200 fs, 60 MHz laser, but different wavelengths, pulse durations, and repetition rates may be used if desired. The light may be directed through various optical components (not shown) for adjustment, including half-wave plates and polarizers for intensity control, as well as beam expanders. As shown, the beam may be directed to a pump-probe separator 505 that separates the beam to a pump arm 510 and a probe arm 530. As shown, the pump-probe separator may include at least one beam splitter, e.g., unpolarized beam splitters 506 and 508, which provide beams to the pump arm 510 that generates a single pump beam and to the probe arm 530 that generates multiple probe beams.

[0044] In the pump arm 510, the pump beam is shown to be directed by mirror M1 toward a variable delay 512 including mirrors M2, M3, and M4, with mirror M3 being movable to adjust the delay of the pump beam. Mirror M3 may be a retroreflector or mirror coupled to an actuator or voice coil VC having a physical displacement of, for example, about 55 mm or 83.3 ps, to achieve a short, repeatable pump pulse time delay. The pump beam passes through a pump beam modulator 514 to modulate the intensity of the pump beam. The pump beam modulator 514 may be, for example, an electro-optic modulator (EOM), followed by a polarizer and a half-wave plate that can be motorized to rotate. Other intensity modulators may be used if desired. The pump beam is directed toward a focusing unit including a lens L1 by beam steering mirrors, for example mirrors M5 and M6. At least one of mirrors M5 and M6 may be mounted on a piezoelectric motor to adjust the direction of the pump beam. As shown in Figure 5A, the pump beam is directed to enter the sample 501 perpendicularly through lens L1. In some embodiments, the pump beam may be directed to enter the sample 501 obliquely, for example, along the same beam path as the probe beam.

[0045] The probe arm 530 is shown as containing two probe beams. For example, as shown in the figure, a first beam splitter 506 generates a first probe beam, and a second beam splitter 508 generates a second probe beam. It should be understood that additional probe beams can be generated in a similar manner to the first and second probe beams if desired. The first probe beam passes through a first probe beam modulator 532 to modulate its intensity at a first frequency, and the second probe beam passes through a second probe beam modulator 534 to modulate its frequency at a different frequency. The probe beam modulators 532 and 534 may, for example, be EOMs, followed by a polarizer and a half-wave plate, respectively. The probe beam modulators 532 and 534 modulate the first and second probe beams with orthogonal waveforms such as sine-cosine pairs, or Haar wavelets, or Dobethy wavelets, as described herein. The first probe beam is directed by mirror M7 toward a first probe variable delay 536, which may include mirrors M8 and M9, and by mirror M10 toward a beam steering mirror, e.g., mirrors M15, M16, and M17, which directs the first probe beam toward a focusing unit including lens L2. Mirror M9 may be, for example, a retroreflector and may be coupled to an actuator or voice coil to adjust the delay of the first probe beam. The second probe beam is directed by mirror M11 toward a second probe variable delay 538, which may include mirrors M12 and M13, and by mirror M14 toward a beam steering mirror, e.g., mirrors M15, M16, and M17, which directs the second probe beam toward a focusing unit including lens L2. Mirror M13 may be, for example, a retroreflector and may be coupled to an actuator or voice coil to adjust the delay of the second probe beam. At least one of the steering mirrors M15, M16, and M17 may be attached to a piezoelectric motor to adjust the direction of the probe beam.As shown in Figure 5A, the first and second probe beams are directed to enter the sample 501 at an oblique angle through lens L2.

[0046] The incident position of the pump beam is controlled by beam steering mirrors M5, M6 and lens L1, and the incident position of the probe beam is controlled by beam steering mirrors M15, M16, M17 and lens 2. Furthermore, the different incident positions of the first probe beam and the second probe beam can be controlled, for example, by mirrors M10 and M14.

[0047] The variable delay 512 of the pump beam, and the variable delays 536 and 538 of the first and second probe beams, can be operated in absolute or relative (having fixed amplitude and sinusoidal waveform) displacement modes and can be controlled based on the propagation time of the bulk wave generated by the pump beam and the propagation time of the surface wave from the incident position of the pump beam to the incident position of the probe beam.

[0048] The reflected probe beam is received by a focusing optical system, for example, a lens L3 and mirrors M18 and M19. The reflected beam is directed toward a detection unit 550, which may include a lens L4 and a detector 552 coupled to a lock-in amplifier 554. The lock-in amplifier 554 includes several demodulators 556 and 558 for phase-locking to the received signal. Demodulators 556 and 558 decode the contributions from the first probe beam and the second pump beam to the signal received by the detector 552, based on the orthogonal waveforms of the first and second probe beams generated, for example, by a first probe beam modulator 532 and a second probe beam modulator 534, respectively.

[0049] The pulses in the pump beam and probe beam may be generated with different time delays, and the detector 552 may generate multiple signals with different time delays. Each signal generated by the detector 552 corresponds to the arrival of bulk waves from the underlying layers in the patterned structure and the arrival of surface waves propagating through the structure on the surface of the structure.

[0050] Furthermore, as shown in the figures, the sample 501 includes at least one actuator configured to move the sample 501 relative to the optical system of the photoacoustic measurement device 500, or is held on a stage 503 coupled thereto, thereby allowing measurement or inspection of various positions on the sample 501. In the illustrated embodiment, the device may include additional components and subsystems such as beam control and adjustment components, e.g., beam expanders, collimators, polarizers, half-wave plates, as well as beam power detectors and height detectors. Those skilled in the art will understand that modifications of the device shown in Figures 1 and 5A are still suitable for performing the photoacoustic measurement techniques described herein.

[0051] The detection unit 550, for example, the detector 552 coupled to the lock-in amplifier 554, and other components of the photoacoustic measuring device 500, such as the light source 502, variable delays 512, 536, 538, and the stage 503 in which the sample 501 is held, may be coupled to a processing system 570, such as a workstation, personal computer, central processing unit, or other suitable computer system, or a plurality of systems. Demodulation of the signal received by the detector 552 may be performed by analog demodulation or digital demodulation, and for example, the lock-in amplifier 554 may be analog or digital. The digital lock-in amplifier may use, for example, digital signal processing (DSP) or be based on a field programmable logic array (FPGA). In some embodiments, for example, the function of the lock-in amplifier 554 may be performed by the processing system 570. Furthermore, a software-based lock-in amplifier 554 can be used, in which case data acquisition is performed and streamed, for example, to memory in the processing system 570 for analysis in situ, or stored in the form of non-volatile memory for analysis using signal processing algorithms.

[0052] One processor, multiple separate processors, or multiple linked processors may be used, and it should be understood that all of them may be referred to herein as the processing system 570. The processing system 570 is preferably included in, connected to, or otherwise associated with the photoacoustic measuring device 500. The processing system 570 can control the positioning of the sample 501, for example, by controlling the movement of the stage 503 on which the sample 501 is held. For example, the stage 503 may be capable of horizontal movement in Cartesian (i.e., X and Y) coordinates, or polar (i.e., R and θ) coordinates, or any combination of the two. The stage 503 may also be capable of vertical movement along the Z coordinate. The processing system 570 may further control the operation of a chuck on the stage 503 used to hold or release the sample 501. The processing system 570 may also collect and analyze data obtained from the detector 552. The processing system 570 may receive demodulated data, for example, via a lock-in amplifier 554, or a signal from a detector 552, and the processing system 570 may demodulate the signal before analyzing the data. The processing system 570 can analyze the photoacoustic measurement data to determine the properties of the sample 501, including the location and composition of underlying structures within the sample 501 that may be beneath at least one optically opaque layer, using vertical transient perturbations, and the location and composition of structures within the surface of the sample 501, using lateral transient perturbations. Various delays between the pump pulse and the probe pulse can be used by the processing system 570 to detect the structure and composition of the material at different depths or locations on the surface of the sample 501. In addition, by using three or more probe beams (or pump beams) and a known arrangement of the incident positions of the pump beam and probe beam, the processing system 570 can use triangulation or trilateration of the measurement data for local mapping of structures within the sample 501.

[0053] A processing system 570 including at least one processor 572 having memory 574, and a user interface including, for example, a display 576 and an input device 578. A non-temporary computer-readable storage medium 579 having embodied computer-readable program code may be used by the processing system 570 to cause the processing system 570 to control the photoacoustic measuring device 500 and perform functions including the analysis described herein. Data structures and software code, etc., for automatically implementing one or more operations described in embodiments of this invention may be implemented by those skilled in the art taking into account this disclosure and may be stored, for example, on the computer-readable storage medium 579, which may be any device or medium capable of storing code and / or data for use by a computer system such as at least one processor 572. The computer-readable storage medium 579 may be, but is not limited to, a disk drive, magnetic tape, compact disc, and flash drive such as DVD (Digital Multipurpose Disc or Digital Video Disc), magnetic storage devices, and optical storage devices. Communication port 577 may also be used to receive instructions used to program the processing system 570 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 networks. Communication port 577 may, for example, further export signals with 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 the sample manufacturing process steps based on the measurement or inspection results.Furthermore, the functions described herein may be embodied in whole or in part within the circuitry of an application-specific integrated circuit (ASIC), a programmable logic device (PLD), or an FPGA, and the functions may be embodied in a computer-understandable description language that can be used to create an ASIC, PLD, or FPGA that operates as described herein. The results of the data analysis may be stored, for example, in a memory 574 associated with the sample, and / or provided to the user, for example, via a display 576, an alarm, or other output device. Furthermore, the analysis results may be fed back to the process apparatus and appropriate patterning steps may be adjusted to correct any errors detected in the measurement or inspection.

[0054] Figure 5B shows a schematic diagram of the photoacoustic measurement device 500', which is similar to the photoacoustic measurement device 500 and has the same specified elements, but uses a different configuration to generate multiple probe beams.

[0055] As shown in Figure 5B, for example, the photoacoustic measuring device 500' may include a pump-probe separator 505' that uses a beam splitter 508 to separate the beam into a pump arm 510 and a probe arm 530. The first and second probe beams are generated using a second beam splitter 509, shown, for example, as being located before a second probe beam modulator 534 for the second probe beam. The first probe beam is directed by the beam splitter 508 toward a first probe variable delay 536, which may include mirrors M8 and M9, and is shown as being directed by mirror M10 toward beam steering mirrors, e.g., mirrors M15, M16, and M17, which direct the first probe beam toward a focusing unit including a lens L2. Furthermore, a first probe beam modulator 532, which modulates the intensity of the first probe beam at a first frequency, is shown as being located after the first probe variable delay 536.

[0056] Figure 6 shows a schematic diagram of an exemplary photoacoustic measurement device 600 that uses multiple pump beams modulated with orthogonal waveforms and a single probe beam, as described herein. As shown, the light may be generated from a light source 602, such as a 520 nm, 200 fs, 60 MHz laser. The light may be directed through various optical components (not shown) for adjustment, such as a beam expander, as well as for intensity control, which may include half-wave plates and polarizers. As shown, the beam may be directed to a pump-probe separator 605 that separates the beam to a pump arm 610 and a probe arm 630. As shown, the pump-probe separator 605 may be a beam splitter, such as an unpolarized beam splitter 606, which provides the beam to the pump arm 610 that generates multiple pump beams and to the probe arm 630 that generates a single probe beam.

[0057] In the pump arm 610, the pump beam is shown as being directed by mirror M1 toward a variable delay 612 including mirrors M2, M3, and M4, where mirror M3 is movable to adjust the delay of the pump beam. Mirror M3 can be a retroreflector or mirror coupled to an actuator having a physical displacement of, for example, about 65 mm or 83.3 ps to achieve a short, repeatable pump pulse time delay. The pump beam passes through beam splitter 613 to generate a first pump beam and a second pump beam. It should be understood that additional pump beams can be generated in a similar manner to the first and second pump beams, if desired. The first pump beam is directed by mirror M20 and passes through pump beam modulator 614 to modulate its intensity at a first frequency, while the second pump beam passes through second pump beam modulator 616 to modulate its intensity at a different frequency. Pump beam modulators 614 and 616 may be, for example, EOMs, followed by a polarizer and a half-wave plate, respectively. Pump beam modulators 614 and 616 modulate the first and second pump beams with orthogonal waveforms such as sine-cosine pairs, Haar wavelets, or Dobethy wavelets, as described herein. For example, each pump beam may be intensity-modulated over time with an orthogonal waveform. The first and second pump beams are directed toward a focusing unit including a lens L1 by beam steering mirrors, for example, mirrors M5 and M6. At least one of mirrors M5 and M6 may be mounted on a piezoelectric motor to adjust the direction of the first and second pump beams. As shown in Figure 6, the first and second pump beams are guided to be incident perpendicularly or nearly perpendicularly to the sample 601 through the lens L1. In some embodiments, the first and second pump beams may be directed to be incident obliquely to the sample 601, for example, along the same beam path as the probe beam.

[0058] The probe arm 630 is shown as containing a single probe beam that passes through a probe beam modulator 632 to modulate the frequency of the probe beam. The probe beam modulator 632 may, but is not limited to, an EOM, and may be followed by, for example, a polarizer and a half-wave plate. The probe beam is shown to be directed by mirror M7 toward a probe variable delay 636, which may include mirrors M8 and M9, and by mirror M10 toward beam steering mirrors, for example, mirrors M15, M16 and M17, which direct the probe beam toward a focusing unit including lens L2. Mirror M9 may, for example, be a retroreflector and may be coupled to an actuator or voice coil to adjust the delay of the probe beam. At least one of the steering mirrors M15, M16 and M17 may be mounted on a piezoelectric motor to adjust the direction of the probe beam. As shown in Figure 6, the probe beam is directed to be obliquely incident on the sample 601 through lens L2.

[0059] The incident position of the pump beam is controlled by beam steering mirrors M5, M6 and lens L1, and the incident position of the probe beam is controlled by beam steering mirrors M15, M16, M17 and lens 2. Furthermore, the different incident positions of the first pump beam and the second pump beam can be controlled, for example, by mirror M20.

[0060] The variable delay 612 of the pump beam and the variable delay 636 of the probe beam can be operated in absolute or relative (with fixed amplitude and sinusoidal waveform) displacement modes and can be controlled based on the propagation time of the bulk wave generated by the pump beam, as well as the propagation time of the surface wave from the incident position of the pump beam to the incident position of the probe beam. Time zero calibration can be optimized depending on the pump-probe pair.

[0061] The reflected probe beam is received by a focusing optical system, for example, a lens L3 and mirrors M18 and M19. The reflected beam is directed toward a detection unit 650, which may include a lens L4 and a detector 652 coupled to a lock-in amplifier 654. The lock-in amplifier 654 includes several demodulators 656 and 658 for phase locking to the received signal. The sound waves produced from each of the intensity-modulated pump beams change in intensity (amplitude) over time according to the orthogonal waveform of the pump beam. The probe beam sensing the sound waves also has a component that is modulated according to the orthogonal waveform, and the resulting signal includes a superposition of the corresponding modulation functions. Demodulators 656 and 658 decode the contributions of the first and second pump beams to the received signal in detector 652, for example, based on the intensity of the received signal over time and the quadrature waveforms of the first and second pump beams generated by the first pump beam modulator 614 and the second pump beam modulator 616, respectively.

[0062] The pump beam and probe beam pulses may be generated with different time delays, and the detector 652 may generate multiple signals with different time delays. Each signal generated by the detector 652 corresponds to the arrival of bulk waves from the underlying layers within the patterned structure and the arrival of surface waves propagating through the structure within the surface of the structure.

[0063] Furthermore, as shown in the figures, the sample 601 includes at least one actuator configured to move the sample 601 relative to the optical system of the photoacoustic measurement device 600, or is held on a stage 603 coupled thereto, thereby allowing measurement or inspection of various positions on the sample 601. In the illustrated embodiment, the device may include additional components and subsystems such as beam control and adjustment components, e.g., beam expanders, collimators, polarizers, half-wave plates, as well as beam power detectors and height detectors. Those skilled in the art will understand that variations of the device shown in Figures 1 and 6 are still suitable for performing the photoacoustic measurement techniques described herein.

[0064] The detection unit 650, for example, the detector 652 coupled to the lock-in amplifier 654, and other components of the photoacoustic measuring device 600, such as the light source 602, variable delays 612 and 636, and the stage 603 in which the sample 601 is held, may be coupled to the processing system 570, as described with reference to Figures 5A and 5B.

[0065] It should be understood that the photoacoustic measurement device 600 may have other configurations for generating multiple pump beams. For example, similar to the generation of multiple probe beams in Figure 5A, multiple pump beams may be generated using multiple beam splitters in a pump-probe separator. Furthermore, each pump beam may have a separate variable delay, and the pump beam modulator may be positioned in the beam path before or after the variable delay.

[0066] Figure 7 is a flowchart 700 illustrating the process of photoacoustic measurement of a sample using multiple probe beams, as described herein. This process can be performed, for example, using the photoacoustic measurement apparatus 100, 500, or 500' shown in Figure 1, Figure 5A, or Figure 5B, respectively, or the procedure shown in Figures 2A and 2B.

[0067] As shown in the figure, in block 702, the process includes directing a pump beam, including pump pulses, toward the surface of the sample, as described with reference to, for example, the pump beam 121 shown in Figure 1 and the pump beam generated in the pump arm 510 shown in Figures 5A and 5B. The pump beam generates vertical transient perturbations and lateral transient perturbations within the sample, as described with reference to, for example, the pump beam 210 shown in Figures 2A and 2B.

[0068] In block 704, the process includes generating a plurality of probe beams, as described with reference to, for example, the probe beams 123A and 123B shown in Figure 1, the probe beams 212A, 212B, and 212C shown in Figures 2A and 2B, and the beam splitters 506 and 508 for generating probe beams in the probe arm 530 shown in Figure 5A, and the beam splitters 508 and 509 for generating probe beams in the probe arm 530 shown in Figure 5B, each probe beam comprising a probe pulse.

[0069] In block 706, the process includes modulating each of the multiple probe beams, as described with reference, for example, to the probe beams 123A and 123B shown in Figure 1, and the probe beam generated in the probe arm 530 shown in Figures 5A and 5B. Means for modulating each of the multiple probe beams in some embodiments may be, for example, the probe beam modulators 125A and 125B shown in Figure 1, and the probe beam modulators 532 and 534 shown in Figures 5A and 5B, and may be an EOM, PEM, AOM, or mechanical chopper. In some embodiments, the probe beams of the multiple probe beams are modulated with a quadrature waveform. For example, the quadrature waveform may be at least one of a sine-cosine pair, a Haar wavelet, and a Dobethy wavelet, as illustrated, for example, in Figures 4A and 4B.

[0070] In block 708, the process includes directing a plurality of probe beams to different locations on the surface of a sample, as described with reference, for example, to probe beams 123A and 123B shown in Figure 1, probe beams 212A, 212B, and 212C shown in Figures 2A and 2B, and probe beams generated within the probe arm 530 shown in Figures 5A and 5B, wherein the plurality of probe beams are reflected from the surface of the sample, a first reflected probe beam is modified based on a vertical transient perturbation propagating perpendicular to the surface of the sample, and at least one second reflected probe beam is modified based on a lateral transient perturbation propagating along the surface of the sample.

[0071] In block 710, the process includes demodulating a first reflected probe beam and at least one second reflected probe beam, as described with reference to, for example, the detector 128 and lock-in amplifier 129 and demodulators 130A and 130B shown in Figure 1, the detector 220 and lock-in amplifier 222 and demodulators 224A, 224B, and 224C shown in Figure 2B, the curves 230, 232, and 234 shown in Figure 2C, and the detector 552 and lock-in amplifier 554 and demodulators 556 and 558 shown in Figures 5A and 5B. For example, the reflected signal is received by the detector 128 along with contributions from the first reflected probe beam and at least one second reflected probe beam. The contribution from each probe beam in the reflected signal received by the detector is decoded by demodulating the reflected signal based on the modulation of each probe beam. Means for demodulating the first reflected probe beam and at least one second reflected probe beam may include, for example, a physical bandpass filter, digital processing, or a combination thereof, the lock-in amplifier 129 and demodulators 130A and 130B shown in Figure 1, the lock-in amplifier 222 and demodulators 224A, 224B, and 224C shown in Figure 2B, and the lock-in amplifier 554 and demodulators 556 and 558 shown in Figures 5A and 5B.

[0072] In block 712, the process includes determining at least one characteristic of the sample based on vertical transient perturbations obtained from demodulating a first reflected probe beam and on lateral transient perturbations obtained from demodulating at least one second reflected probe beam, as described, for example, with reference to the processing system 132 shown in Figure 1 and the processing system 570 shown in Figures 5A and 5B. For example, at least one characteristic of the sample can be determined using the intensity (amplitude) of vertical and horizontal transient perturbations obtained from contributions from the first reflected probe beam and at least one second reflected probe beam in the reflected signal. Means for determining at least one characteristic of the sample based on vertical transient perturbations obtained from demodulating a first reflected probe beam and on lateral transient perturbations obtained from demodulating at least one second reflected probe beam may be, for example, the processing system 132 shown in Figure 1 and the processing system 570 shown in Figures 5A and 5B.

[0073] In some embodiments, the process may further include varying the delay between the pump pulse and the probe pulse, as described with reference to, for example, the pump beam source 120, probe beam sources 122A and 122B shown in Figure 1, and the variable delays 512, 536, and 538 shown in Figures 5A and 5B. Means for varying the delay between the pump pulse and the probe pulse may be, for example, the variable delays 512, 536, and 538 shown in Figures 5A and 5B. Determining at least one characteristic of the sample may further be based on the delay between the pump pulse and the probe pulse, as described with reference to, for example, the processing system 132 shown in Figure 1, and the processing system 570 shown in Figures 5A and 5B.

[0074] In some embodiments, as illustrated in Figures 2A and 2B, for example, a first reflected probe beam is reflected from a position on the sample that coincides with the incident position of the pump beam, and at least one second reflected probe beam is reflected from at least one position on the sample that is displaced by a known amount from the incident position of the pump beam.

[0075] In some embodiments, the multiple probe beams include at least three probe beams, which are incident on the sample at positions that are linearly distributed and at least one of uniformly distributed positions, as described, for example, in Figures 2A and 2B.

[0076] In some embodiments, the multiple probe beams include at least three probe beams, which are incident on the sample at positions that are nonlinearly distributed and at least one of non-uniformly distributed positions, as described, for example, in Figures 2A and 2B.

[0077] In some embodiments, the method may include using multiple pump beams together with multiple probe beams, as described, for example, with reference to Figure 1 and Figures 2A, 2B, 3A, and 3B. For example, the pump beam may be one of the multiple pump beams, each pump beam containing a pump pulse, and the method may include modulating each of the multiple pump beams to generate a pump beam in the pump arm 610 shown in Figure 6, as described, for example, with reference to the pump beams 121 and 121' shown in Figure 1, the pump beams 310A, 310B, 310C shown in Figures 3A and 3B, and the beam splitters 606 and 613, and generating a pump beam in the pump arm 610 shown in Figure 6. Means for modulating each of the multiple pump beams may be, for example, the pump beam modulator 124 shown in Figure 1, and the pump beam modulators 614 and 616 shown in Figure 6, or they may be EOMs. In some embodiments, the pump beams of the multiple pump beams are modulated with orthogonal waveforms. For example, the orthogonal waveform may be at least one of a sine-cosine pair, a Haar wavelet, and a Dobethy wavelet, as shown, for example, in Figures 4A and 4B. The multiple pump beams may be directed toward different locations on the surface of the sample, and each pump beam excites transient perturbations in the sample at the corresponding location, as described with reference to, for example, the pump beams 121, 121' shown in Figure 1 and the pump beam generated in the pump arm 610 shown in Figure 6, and with reference to the pump beam 310 shown in Figures 3A and 3B. The first reflected probe beam and at least one second reflected probe beam can be further modified based on transient perturbations excited by multiple pump beams, as described with reference, for example, to the probe beam 123A shown in Figure 1, the probe beam 312 shown in Figures 3A and 3B, and the probe beam generated within the probe arm 630 shown in Figure 6.The first reflected probe beam and at least one second reflected probe beam can be demodulated based on the modulation of each probe beam among a plurality of probe beams to determine the contribution from each probe beam, and further based on the modulation of each pump beam among a plurality of pump beams to determine the contribution from each pump beam. Means for demodulating the first reflected probe beam and at least one second reflected probe beam may be, for example, the lock-in amplifier 129 and demodulators 130A and 130B shown in Figure 1, the lock-in amplifier 322 and demodulators 324A, 324B and 324C shown in Figure 2B, and the lock-in amplifier 654 and demodulators 656 and 658 shown in Figure 6. In addition, at least one characteristic of the sample may be determined based on transient perturbations obtained from demodulating the first reflected probe beam and at least one second reflected probe beam, which are excited by the plurality of pump beams. Means for determining at least one characteristic of a sample, based on transient perturbations obtained from demodulating a first reflected probe beam and at least one second reflected probe beam, which are excited by multiple pump beams, may include, for example, the processing system 132 shown in Figure 1, and the processing system 570 shown in Figures 5A and 6.

[0078] Figure 8 is a flowchart 800 illustrating the process of photoacoustic measurement of a sample using multiple pump beams, as described herein. This process can be performed, for example, using the photoacoustic measurement apparatus 100 or 600 shown in Figure 1 or Figure 6, respectively, or the procedure shown in Figures 3A and 3B.

[0079] As shown in the figure, in block 802, the process includes generating a plurality of pump beams, each pump beam including a pump pulse, as described with reference to, for example, pump beams 121 and 121' shown in Figure 1, pump beams 310A, 310B, and 310C shown in Figures 3A and 3B, and beam splitters 606 and 613, and generating a pump beam within the pump arm 610 shown in Figure 6.

[0080] In block 804, the process includes modulating each pump beam in a plurality of pump beams, as described with reference, for example, to the pump beams generated in the pump beams 121 and 121' shown in Figure 1 and the pump beams generated in the pump arm 610 shown in Figure 6. Means for modulating each pump beam in a plurality of pump beams may be, for example, the pump beam modulator 124 shown in Figure 1 and the pump beam modulators 614 and 616 shown in Figure 6, or they may be EOMs. In some embodiments, the pump beams in a plurality of pump beams are modulated with quadrature waveforms. For example, the quadrature waveform may be at least one of a sine-cosine pair, a Haar wavelet, and a Dobethy wavelet, as shown, for example, in Figures 4A and 4B.

[0081] In block 806, the process involves directing multiple pump beams toward different locations on the surface of the sample, each pump beam exciting transient perturbations within the sample at the corresponding location, as described with reference to, for example, pump beams 121, 121' shown in Figure 1 and the pump beam generated within the pump arm 610 shown in Figure 6. Each pump beam also excites transient perturbations within the sample at the corresponding location, as described with reference to, for example, pump beam 310 shown in Figures 3A and 3B.

[0082] In block 808, the process includes directing a probe beam containing a probe pulse toward the surface of the sample, the probe beam being reflected from the surface of the sample, and the reflected probe beam being modified based on vertical transient perturbations within the sample propagating perpendicular to the surface of the sample and lateral transient perturbations propagating along the surface of the sample, which are excited by a plurality of pump beams, as described with reference to probe beam 123A shown in Figure 1, probe beam 312 shown in Figures 3A and 3B, and probe beam generated in probe arm 630 shown in Figure 6.

[0083] In block 810, the process includes demodulating the reflected probe beams, as described with reference to, for example, the detector 128 and lock-in amplifier 129 and demodulators 130A and 130B shown in Figure 1, the detector 320 and lock-in amplifier 322 and demodulators 324A, 324B, and 324C shown in Figure 2B, the curves 330, 332, and 334 shown in Figure 3C, and the detector 652 and lock-in amplifier 654 and demodulators 656 and 658 shown in Figure 6. For example, the reflected signal is received by the detector 128 along with the acoustic signal contribution generated by each pump beam. The contributions generated by each pump beam in the reflected signal received by the detector are decoded by demodulating the reflected signal based on the modulation of each pump beam. Means for demodulating the reflected probe beam may include, for example, the lock-in amplifier 129 and demodulators 130A and 130B shown in Figure 1, the lock-in amplifier 322 and demodulators 324A, 324B, and 324C shown in Figure 2B, and the lock-in amplifier 654 and demodulators 656 and 658 shown in Figure 6.

[0084] In block 812, the process includes determining at least one characteristic of the sample based on vertical and lateral transient perturbations obtained from demodulating the reflected probe beam, as described, for example, with reference to the processing system 132 shown in Figure 1 and the processing system 570 shown in Figures 5A and 6. For example, at least one characteristic of the sample can be determined using the intensity (amplitude) of the vertical and lateral transient perturbations obtained from the contributions generated by each pump beam in the reflected signal. Means for determining at least one characteristic of the sample based on vertical and lateral transient perturbations obtained from demodulating the reflected probe beam may be, for example, the processing system 132 shown in Figure 1 and the processing system 570 shown in Figures 5A and 6.

[0085] In some embodiments, the process may further include varying the delay between the pump pulse and the probe pulse, as described with reference to, for example, the pump beam source 120, the probe beam source 122A shown in Figure 1, and the variable delays 612 and 636 shown in Figure 6. Means for varying the delay between the pump pulse and the probe pulse may be, for example, the variable delays 612 and 636 shown in Figure 6. Determining at least one characteristic of the sample may further be based on the delay between the pump pulse and the probe pulse, as described with reference to, for example, the processing system 132 shown in Figure 1 and the processing system 570 shown in Figure 6.

[0086] In some embodiments, the probe beam is incident on the sample at a position coinciding with the first incident position of one pump beam, and the incident positions of the remaining pump beams among the multiple pump beams are displaced by a known amount from the first position, for example, as described in Figures 3A and 3B. For example, the reflected probe beam may be modified based on the vertical transient perturbation excited at the first position and the lateral transient perturbation excited at the incident positions of the remaining pump beams, for example, as described in Figures 3A and 3B.

[0087] In some embodiments, the multiple pump beams include at least three pump beams, which are incident on the sample at positions that are linearly distributed and at least one of uniformly distributed positions, as described, for example, in Figures 3A and 3B.

[0088] In some embodiments, the multiple pump beams include at least three pump beams, which are incident on the sample at positions that are nonlinearly distributed and at least one of non-uniformly distributed positions, as described, for example, in Figures 3A and 3B.

[0089] Those skilled in the art will understand that the embodiments described herein provide the basis for numerous alternative and modified forms that are considered to fall within the scope of this disclosure. The above description is illustrative and not restrictive. For example, the above examples (or one or more of them) may be used in combination with one another. For example, although this disclosure is described throughout using a single pump beam and multiple probe beams, or multiple pump beams and a single probe beam, combinations of multiple pump beams and multiple probe beams will function similarly in all of the disclosed systems. Other implementations may be used, for example, by those skilled in the art when considering the above description. Also, various features may be grouped together, and fewer features may be used than all features of a particular disclosed implementation. Accordingly, the following embodiments are incorporated herein as examples or implementations, and each embodiment stands on its own as a separate implementation, and such implementations are intended to be combined with one another in various combinations or rearrangements. Accordingly, the intent and scope of the appended claims should not be limited to the above description.

Claims

1. A method for photoacoustic measurement of a sample, A pump beam including a pump pulse, wherein the pump beam generates vertical transient perturbations and lateral transient perturbations within the sample, and the pump beam is directed toward the surface of the sample. The process involves generating multiple probe beams, each of which includes a probe pulse. Modulation of each of the aforementioned plurality of probe beams, Directioning the plurality of probe beams toward different locations on the surface of the sample, wherein the plurality of probe beams are reflected from the surface of the sample, the first reflected probe beam is modified based on the vertical transient perturbation propagating perpendicularly to the surface of the sample, and at least one second reflected probe beam is modified based on the lateral transient perturbation propagating along the surface of the sample. Demodulating the first reflected probe beam and the at least one second reflected probe beam, A method comprising determining at least one characteristic of the sample based on the vertical transient perturbation obtained from demodulating the first reflected probe beam, and based on the lateral transient perturbation obtained from demodulating the at least one second reflected probe beam.

2. The method according to claim 1, wherein one of the plurality of probe beams is modulated with an orthogonal waveform.

3. The method according to claim 2, wherein the orthogonal waveform includes at least one of a sine-cosine pair, a Haar wavelet, and a Dobecy wavelet.

4. Further includes changing the delay between the pump pulse and the probe pulse, The method according to claim 1, wherein determining the at least one characteristic of the sample is further based on the delay between the pump pulse and the probe pulse.

5. The method according to claim 1, wherein the first reflected probe beam is reflected from a position on the sample that coincides with the incident position of the pump beam, and the at least one second reflected probe beam is reflected from at least one position on the sample that is displaced by a known amount from the incident position of the pump beam.

6. The method according to claim 1, wherein the plurality of probe beams include at least three probe beams, and the at least three probe beams are incident on the sample at positions that are linearly distributed and at least one of uniformly distributed positions.

7. The method according to claim 1, wherein the plurality of probe beams include at least three probe beams, and the at least three probe beams are incident on the sample at positions that are nonlinearly distributed and positions that are unevenly distributed.

8. The pump beam is one of a plurality of pump beams, each pump beam includes a pump pulse, and the method is Modulation of each of the aforementioned multiple pump beams, The further includes directing the plurality of pump beams toward different locations on the surface of the sample, wherein each pump beam excites transient perturbations within the sample at the corresponding location. The first reflected probe beam and the at least one second reflected probe beam are modified based on the transient perturbations excited by the plurality of pump beams. Demodulating the first reflected probe beam and the at least one second reflected probe beam is based on the modulation of each probe beam among the plurality of probe beams so as to determine the contribution from each probe beam, and further based on the modulation of each pump beam among the plurality of pump beams so as to determine the contribution from each pump beam. The method according to claim 1, wherein determining the at least one characteristic of the sample is further based on the transient perturbation obtained from being excited by the plurality of pump beams and demodulating the first reflected probe beam and the at least one second reflected probe beam.

9. A measuring device for photoacoustic measurement of a sample, A pump arm is configured to receive at least a first portion of pulsed light from a light source and direct the pump beam toward the surface of the sample, wherein the pump beam includes a pump pulse, and the pump beam generates vertical transient perturbations and lateral transient perturbations within the sample. A probe arm that receives at least a second portion of the pulsed light from the light source and directs a plurality of probe beams, each probe beam containing a probe pulse, to different locations on the surface of the sample, wherein the probe arm comprises means for modulating each of the plurality of probe beams, the plurality of probe beams are reflected from the surface of the sample, a first reflected probe beam is modified based on the vertical transient perturbation propagating perpendicularly to the surface of the sample, and at least one second reflected probe beam is modified based on the lateral transient perturbation propagating along the surface of the sample, Means for demodulating the first reflected probe beam and the at least one second reflected probe beam, A measuring device comprising means for determining at least one characteristic of the sample based on the vertical transient perturbation obtained from demodulating the first reflected probe beam, and based on the lateral transient perturbation obtained from demodulating the at least one second reflected probe beam.

10. The measuring device according to claim 9, wherein one of the plurality of probe beams is modulated with an orthogonal waveform.

11. The measuring device according to claim 10, wherein the orthogonal waveform includes at least one of a sine-cosine pair, a Haar wavelet, and a Dobecy wavelet.

12. The system further comprises means for changing the delay between the pump pulse and the probe pulse, The measuring apparatus according to claim 9, wherein the means for determining the at least one characteristic of the sample is further based on the delay between the pump pulse and the probe pulse.

13. The measuring device according to claim 9, wherein the measuring device comprises a focusing optical system configured to reflect the first reflective probe beam from a position on the sample that coincides with the incident position of the pump beam, and to reflect the at least one second reflective probe beam from at least one position on the sample that is displaced by a known amount from the incident position of the pump beam.

14. The measuring device according to claim 9, wherein the plurality of probe beams include at least three probe beams, and the measuring device comprises a focusing optical system configured to cause the at least three probe beams to be incident on the sample at positions that are at least one of linearly distributed positions and uniformly distributed positions.

15. The measuring device according to claim 9, wherein the plurality of probe beams include at least three probe beams, and the measuring device comprises a focusing optical system configured to cause the at least three probe beams to be incident on the sample at positions that are nonlinearly distributed and positions that are unevenly distributed.

16. The pump beam is one of a plurality of pump beams, the pump arm is further configured to direct the plurality of pump beams toward different locations on the surface of the sample, each pump beam includes a pump pulse, the pump arm includes means for modulating each of the plurality of pump beams, and each pump beam excites a transient perturbation within the sample at a corresponding location. The first reflected probe beam and the at least one second reflected probe beam are modified based on the transient perturbations excited by the plurality of pump beams. The means for demodulating the first reflected probe beam and the at least one second reflected probe beam demodulates based on the modulation of each probe beam among the plurality of probe beams to determine the contribution from each probe beam, and further based on the modulation of each pump beam among the plurality of pump beams to determine the contribution from each pump beam, The measuring apparatus according to claim 9, wherein the means for determining at least one characteristic of the sample determines the at least one characteristic based on the transient perturbation obtained from exciting the plurality of pump beams and demodulating the first reflected probe beam and the at least one second reflected probe beam.

17. A method for photoacoustic measurement of a sample, The process involves generating multiple pump beams, each pump beam containing a pump pulse. Modulation of each of the aforementioned multiple pump beams, The plurality of pump beams are directed toward different positions on the surface of the sample, wherein each pump beam is directed to excite transient perturbations within the sample at the corresponding position. Direction of a probe beam containing a probe pulse toward the surface of the sample, wherein the probe beam is reflected from the surface of the sample, and the reflected probe beam is modified based on vertical transient perturbations within the sample propagating perpendicularly to the surface of the sample and lateral transient perturbations propagating along the surface of the sample, which are excited by the plurality of pump beams. Demodulating the reflected probe beam, A method comprising determining at least one characteristic of the sample based on the vertical transient perturbation and the lateral transient perturbation obtained from demodulating the reflected probe beam.

18. The method according to claim 17, wherein one of the plurality of pump beams is modulated with an orthogonal waveform.

19. The method according to claim 18, wherein the orthogonal waveform includes at least one of a sine-cosine pair, a Haar wavelet, and a Dobecy wavelet.

20. Further includes changing the delay between the pump pulse and the probe pulse, The method of claim 17, wherein determining the at least one characteristic of the sample is further based on the delay between the pump pulse and the probe pulse.

21. The method according to claim 17, wherein the probe beam is incident on the sample at a position that coincides with the first incident position of one pump beam, and the incident positions of the remaining pump beams among the plurality of pump beams are displaced by a known amount from the first position.

22. The method according to claim 21, wherein the reflected probe beam is modified based on the vertical transient perturbation excited at the first position and the lateral transient perturbation excited at the incident position of the remaining pump beam.

23. The method according to claim 17, wherein the plurality of pump beams include at least three pump beams, and the at least three pump beams are incident on the sample at positions that are linearly distributed and at least one of uniformly distributed positions.

24. The method according to claim 17, wherein the plurality of pump beams include at least three pump beams, and the at least three pump beams are incident on the sample at positions that are nonlinearly distributed and positions that are unevenly distributed.

25. A measuring device for photoacoustic measurement of a sample, A pump arm configured to receive at least a first portion of pulsed light from a light source, and to direct a plurality of pump beams, each pump beam containing a pump pulse, toward different positions on the surface of the sample, wherein the pump arm includes means for modulating each of the plurality of pump beams, and each pump beam excites transient perturbations within the sample at the corresponding position, A probe arm configured to receive at least a second portion of the pulsed light from the light source and to direct a probe beam including the probe pulse toward the surface of the sample, wherein the probe beam is reflected from the surface of the sample, and the reflected probe beam is modified based on vertical transient perturbations within the sample propagating perpendicularly to the surface of the sample and lateral transient perturbations propagating along the surface of the sample, which are excited by the plurality of pump beams. Means for demodulating the reflected probe beam, A measuring device comprising means for determining at least one characteristic of the sample based on the vertical transient perturbation and the lateral transient perturbation obtained from demodulating the reflected probe beam.

26. The measuring device according to claim 25, wherein one of the plurality of pump beams is modulated with an orthogonal waveform.

27. The measuring device according to claim 26, wherein the orthogonal waveform includes at least one of a sine-cosine pair, a Haar wavelet, and a Dobecy wavelet.

28. The system further comprises means for changing the delay between the pump pulse and the probe pulse, The measuring apparatus according to claim 25, wherein the means for determining the at least one characteristic of the sample is further based on the delay between the pump pulse and the probe pulse.

29. The measuring device according to claim 25, wherein the measuring device includes a focusing optical system configured to inject the probe beam onto the sample at a first position that coincides with the injection position of one pump beam, and to inject the remaining pump beams of the plurality of pump beams at a position displaced by a known amount from the first position.

30. The measuring apparatus according to claim 29, wherein the reflected probe beam is modified based on the vertical transient perturbation excited at the first position and the lateral transient perturbation excited at the incident position of the remaining pump beam.

31. The measuring device according to claim 25, wherein the plurality of pump beams include at least three pump beams, and the measuring device comprises a focusing optical system configured to cause the at least three pump beams to be incident on the sample at positions that are at least one of linearly distributed positions and uniformly distributed positions.

32. The measuring device according to claim 25, wherein the plurality of pump beams include at least three pump beams, and the measuring device comprises a focusing optical system configured to cause the at least three pump beams to be incident on the sample at positions that are nonlinearly distributed and positions that are unevenly distributed.

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