Photoreflectance method and system

Abstract

A system is presented for use in photoreflectance (PR) measurements of a sample. The system comprises a light source system comprising a pump source assembly and a non-linear medium. The pump source assembly produces: an ultra-short seed pulse exciting the non-linear medium presenting a supercontinuum laser generating a broadband probe pulse propagating along a first path towards the sample to create a broadband focal spot on the sample; and an exciting pump pulse propagating along a second path, inclined with respect to the first path, to create an excitation focal spot on the sample. The light source system defines a delay line to controllably vary a time of arrival of the exciting pump pulse to the sample, thereby inducing a controllably variable pump-probe time delay in real time during a PR measurement session, thereby enabling successive time-domain spectroscopic PR measurements of the sample.

Description

[0001] PHOTOREFLECTANCE METHOD AND SYSTEM

[0002] TECHNOLOGICAL FIELD AND BACKGROUND

[0003] The present disclosure is in the general field of optical metrology and relates in particular to system and methods of photoreflectance.

[0004] Photoreflectance (PR) is a type of pump-probe technique in which the change in a sample’s reflectivity caused by illumination with a pump (first) beam is measured by reflecting a probe (second) beam. PR is non-contact and non-destructive and can be used to characterize semiconductor materials and structures, providing valuable information about the optical and electronic properties of semiconductor materials and interfaces.

[0005] There exist various applications of photoreflectance in semiconductor metrology including the following:

[0006] Bandgap Determination in which PR is used to determine the bandgap energy of semiconductors, i.e., the PR signal (semiconductor's response to probe signal) exhibits characteristic features at the bandgap energy, which can be analyzed to extract this information

[0007] Free Carrier properties (Concentration, mobility, lifetime etc.) and Electric Fields in which PR can provide information about the carrier's concentration and built-in electric fields in semiconductor devices. These parameters affect the refractive index and absorption, influencing the PR signal, as well as being crucial indicators for the device’s electrical performance.

[0008] Interface Quality and Defects - method sensitive to interfaces and used to detect defects, strain, and composition variations at semiconductor interfaces. This makes it valuable for assessing the quality of heterojunctions and other complex structures.

[0009] The existing variants of PR fall into two main categories: Time-domain, where the pump and probe beams are short-pulse, spectrally narrow lasers. The reflectivity at a single wavelength is measured as a function of the relative delay between the pump and probe pulses, which is of the order of picoseconds (ps) to nanoseconds (ns);

[0010] Spectral PR (SPR), where the probe pulse is spectrally broadband, and the spectral reflectivity change is measured at steady state or relatively low temporal time scales (-0-100 kHz range).

[0011] Combined optical metrology (OCD) and PR technique has been developed and is described in US Patent No. 12,392,733 assigned to the assignee of the present application. This technique is aimed at improving the OCD performance in measurements of optical properties of a target sample. The combined measurement system includes a singlechannel or multi-channel OCD set-up utilizing single probe beam or multiple probe beams configured in normal and / or oblique direction(s) to the target sample for measuring the optical properties of the target sample; and includes laser source(s) producing laser beam(s), modulation device(s) turning the laser beam(s) into altematingly modulated laser beam(s) modulating the spectral reflectivity of the target sample, and spectrometer(s) for measuring spectral components of the light beam(s) reflecting off said target sample.

[0012] GENERAL DESCRIPTION

[0013] The present disclosure provides a novel PR-based technique enabling to simultaneously provide spectral and temporal information of a sample. This is challenging because the limitations of available light sources (which are typically of relatively narrow band of output light) require compromise between spectral and temporal information.

[0014] According to the technique of the present disclosure, a broadband probe beam is utilized and both the pump beam and the broadband probe beam are short laser pulses with the time difference between them being controlled, while allowing simultaneous performance of spectroscopic measurements for PR characterization of a sample. Further, the present disclosure makes use of a supercontinuum laser (SCL) to provide the broadband probe beam, and in some embodiments enables use of the SCL's pump beam for the sample's exciting pump pulse.

[0015] More specifically, SCL is a light source that generates a broad spectrum (“supercontinuum”) of output light by passing a narrow-band input laser beam (seed / pump) through a nonlinear medium. The process involves intense, ultrashort input laser pulses (typically from a femtosecond or picosecond laser) that interact with the nonlinear medium, leading to a range of nonlinear optical phenomena such as self-phase modulation, four-wave mixing, and Raman scattering. These interactions significantly broaden the spectrum of the output laser light, creating a supercontinuum that spans from the ultraviolet to the infrared regions of the spectrum.

[0016] In the present disclosure, the output of the SCL may be used as a short, coherent probe pulse in the SPR measurement session. This is due to the following:

[0017] Since an ultra-short (seed / pump) laser pulse seeds the SCL, the output supercontinuum laser pulse is also limited in its temporal extent. Although not as short as the seed pulse, the SCL pulse can still be used as a short probe for SPR measurements. Moreover, the seed (pump) laser, being a short and intense laser pulse, can be used as the pump for exciting the sample in the SPR scheme.

[0018] The SCL beam is spatially coherent and can, in principle, be tightly focused. This feature opens the path to measurements of small targets and tailoring the pump and probe beams in whole, partial, or no overlap on the sample to allow for different spatially resolved measurements.

[0019] Using an SCL in the time-domain SPR scheme (while using separate or the common pump pulse for exciting the sample and the nonlinear medium of the SCL) opens the path to full spectroscopic measurements with large information contents. Schemes such as spectral reflectometry and ellipsometry can be utilized with full spectral range measurements at different pump conditions.

[0020] The PR measurement session includes sequential detection of the spectral responses of the sample to the multiple successive interactions of the exciting pump pulses with the sample each followed by interaction of the broadband probe pulse with the sample, while with a different pump-probe time delay as compared to the other interactions. To this end, the technique of the present disclosure provides a novel PR- based measurement system defining a delay line configured to controllably vary a time of arrival of the exciting pump pulse to the sample, thereby inducing a controllably variable pump-probe time delay in real time during the PR measurement session; and capable of controlling a degree of spatial overlap between the broadband focal spot and the excitation focal spot on the sample during the measurement session.

[0021] Thus, according to a broad aspect of the present disclosure, there is provided a system for use in photoreflectance (PR) measurements of a sample, the system comprising: a light source system comprising a pump source assembly and a non-linear medium, wherein: the pump source assembly is configured and operable to produce an ultra-short seed pulse for exciting the non-linear medium to operate as a supercontinuum laser generating a broadband probe pulse propagating along a first path towards the sample to create a broadband focal spot on the sample, said pump source assembly is configured and operable to produce an exciting pump pulse propagating along a second path, inclined with respect to the first path, to create an excitation focal spot on the sample, said light source system is configured to define a delay line configured to controllably vary a time of arrival of the exciting pump pulse to the sample, thereby inducing a controllably variable pump-probe time delay in real time during a PR measurement session, including multiple successive interactions of the exciting pump pulses with the sample each followed by interaction of the broadband probe pulse with the sample, thereby enabling successive time-domain spectroscopic PR measurements of the sample by detection of a light response of the sample excited by the exciting pump pulse, to the broadband probe pulse; a control system configured and operable to control said variable pump-probe time delay and a degree of spatial overlap between the broadband focal spot and the excitation focal spot on the sample during the measurement session.

[0022] The system may further include a detection system configured and operable to detect the light response of the sample to the broadband probe pulse, and generate data indicative of the light response being detected, thereby enabling extraction of one or more parameters of the sample.

[0023] The one or more parameters extractable from the data indicative of the light response being detected comprises one or more of the following: free charge carrier properties in the sample, band structure of the sample, strain within the sample, defectivity of the sample, interface quality in the sample.

[0024] The detection system may include a spectrometer.

[0025] In some embodiments, one of the first and second paths provides normal incidence of the respective pulse onto the sample's surface, and the other is oriented to provide oblique incidence of the respective pulse onto the sample's surface, such that detection of reflection of the exciting pump pulse from the sample is substantially avoided.

[0026] In some embodiments, the pump source assembly comprises a seed source generating said ultra-short seed pulse for exciting said non-linear medium, and a pump source generating said exciting pump pulse, the seed source and the pump source being controllably operated to provide a controllably variable synchronization between the generation of the exciting pump pulse and the broadband probe pulse, thereby defining said delay line providing said controllably variable pump-probe time delay in real time during the PR measurement session.

[0027] In some embodiments, the pump source assembly comprises an optical seed source configured and operable to generate the ultra-short seed pulse, the light source system further comprising: a beam splitter configured to split the ultra-short seed pulse into an exciting seed pulse propagating to perform said excitation of the non-linear medium and the exciting pump pulse propagating along a path to excite the sample; and the delay line located in said path of the exciting pump pulse, the delay line being configured and operable to induce said controllably variable pump-probe time delay in real time during the PR measurement session.

[0028] The system may further comprise a wavelength tuning assembly configured and operable to controllably vary a wavelength of the exciting pump pulse propagating to the sample. The wavelength tuning assembly may be configured and operable to carry out at least one of parametric down conversion and harmonic generation. The system may further comprise a polarizing assembly to apply polarization control of the broadband probe pulse and its reflection from the sample.

[0029] In some embodiments, the system further comprises a modulation assembly configured and operable to apply first and second modulations of different first and second modulation frequencies, respectively, to the broadband probe pulse and the exciting pump pulse while propagating along said first and second path.

[0030] According to another broad aspect of the present disclosure, it provides a system for performing photoreflectance (PR) measurement of a sample, the system comprising: a supercontinuum laser (SCL) unit comprising an optical seed source configured and operable to generate ultra-short seed pulses and a non-linear medium configured and operable to respond by a broadband probe pulse to excitation by an ultra-short seed pulse; and a beam splitter configured to split the ultra-short seed pulse into an exciting seed pulse propagating to perform said excitation of the non-linear medium of the SCL and an exciting pump pulse propagating along a path to excite the sample; a delay line located in said path of the exciting pump pulse, the delay line being configured and operable to controllably vary a time of arrival of the exciting pump pulse to the sample, thereby inducing a controllably variable pump-probe time delay in real time during a PR measurement session including multiple successive interactions of the exciting pump pulses with the sample each followed by interaction of the broadband probe pulse with the sample, thereby enabling successive time-domain spectroscopic PR measurements of the sample by detection of a light response of the sample excited by the exciting pump pulse, to the broadband probe pulse; a control system configured and operable to control said variable pump-probe time delay and a degree of spatial overlap between regions of interaction of the exciting pump pulses and the broadband probe pulses with the sample during the measurement session.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a system for use in photoreflectance measurements of a sample according to the principles of the present disclosure;

[0033] Fig- 2 shows, by way of a flow diagram, a method of performing a time domain spectral photo-reflective (SPR) measurement session according to the present disclosure;

[0034] Fig- 3 shows a non-limiting example of the system of the present disclosure for use in PR measurements of a sample, where a pump laser excites the sample through a controlled optical channel and generates a supercontinuum, which is used as a light source for a normal incidence spectral reflectometer;

[0035] Fig- 4 shows a non-limiting example of the system of the present disclosure for use in PR measurements of a sample, where a pump laser excites the sample through a controlled optical channel and generates a supercontinuum, which is used as a light source for a spectral ellipsometer; and

[0036] Fig. 5 exemplifies a delay line configuration using moving retroreflector assembly.

[0037] DETAILED DESCRIPTION OF EMBODIMENTS

[0038] Reference is made to Fig. 1 exemplifying, by way of a block diagram, a system 100 of the present disclosure for use in photoreflectance (PR) measurements of a sample. The system 100 includes a light source system 102 and a control system 104, and is associated with a detection system 122.

[0039] The light source system 102 includes a pump source assembly 106 and a nonlinear medium 108. The pump source assembly 106 is configured and operable to produce an ultra-short seed pulse Pl for exciting the nonlinear medium 108 to operate as a supercontinuum laser (SCL) generating a broadband probe pulse P3 propagating along a first path LP1 (free space propagation) towards the sample to create a broadband focal spot on the sample. The pump source assembly 106 is also configured and operable to produce an exciting pump pulse P2 propagating along a second path LP2, inclined with respect to the first path LP1 (free space propagation), to create an excitation focal spot on the sample.

[0040] It should be noted, although not specifically shown here, that the system 100 may also include light directing assembly (e.g., objective optics, deflectors, etc.) to properly direct the exciting pump pulse P2 and the broadband probe pulse P3 to the sample. This will be described more specifically further below.

[0041] The exciting pump pulse P2 is generally created by a short-pulse pump source 107, which can also be used as the ultra-short seed of the SCL, or such ultra-short seed can be a separate pump source 107’. Thus, in some embodiments, the pump source assembly 106 includes a common pump source 107 whose output is used for exciting the sample and for exciting the nonlinear medium 108 of the SCL. In some other embodiments, the pump source assembly 106 includes two separate pump sources 107 and 107' whose outputs are used to excite, respectively, the sample and the nonlinear medium. In this case, the pulse trains of the two sources can be synchronized, by locking the pulse repetition rate of one to the other, or both to some external clock. In case the common pump source is used, its output is split into the ultra-short seed pulse Pl for exciting the non-linear medium and the exciting pump pulse P2 to propagate along the second path LP2.

[0042] The exciting pump pulse P2 propagation in time and space is controlled by the control system 104, and possibly also a pump control assembly 115, to generate the desired excitation characteristics. Specifically, the light source system 102 defines a delay line 110 configured to controllably vary the time of arrival of the exciting pump pulse P2 to the sample, thereby inducing a controllably variable pump-probe P2-P3 time delay in real time during a PR measurement session. Typically, the pump-probe delay is defined by controllably varying, by the control system 104, the exciting pump pulse P2 propagation condition along the optical path LP2 during the measurement session.

[0043] The light source system 102 may also include a spatial overlap controller 112. In many cases, semiconductor targets are defined as small, few micron-sized targets. Therefore, the illumination optics (e.g., objective lens assembly) is properly configured to focus the exciting pump pulse P2 on a small target. The spatial overlap (a degree of such overlap) between the exciting pump pulse P2 and the probe pulse P3 is a parameter that may be of interest during the PR measurements. Therefore the control system 104, and possibly also the spatial overlap controller 112, is / are configured and operable to control the degree of the spatial overlap between the broadband focal spot of the excitation focal spot on the sample during the measurement session. The overlap control can be realized as part of the beam steering mechanism in the optical system, either as manual or motorized degrees of freedom.

[0044] It is noted that the SCL beam is spatially coherent and can, in principle, be tightly focused on the sample. The light source system 102, thus, may include any type of focusing optics along the broadband probe propagation path LP1 (which is not shown in Fig. 1). The focusing feature of the SCL beam / pulse opens the path to time-domain spectroscopic PR measurements of small targets and tailoring the exciting pump pulse and probe pulse in whole, partial, or no overlap on the sample to allow for different spatially resolved measurements with various degrees of spatial overlap.

[0045] The overlap can be controlled by steering the pump beam to change the location where it hits the sample relative to the probe beam. This can be implemented as a static configuration of the optical path, or may be controlled using an active beam steering device such as a motorized mirror, Acousto-Optic Deflector etc.

[0046] The PR measurement session includes multiple successive interactions of the exciting pump pulses P2 with the sample, each followed by interaction of the broadband probe pulse P3 with the sample, thereby enabling successive time-domain spectroscopic PR measurements of the sample by detecting a light response of the sample, excited by the exciting pump pulse P2, to the broadband probe pulse P3.

[0047] The sample excitation is often strongly dependent on the wavelength of the exciting pump pulse P2. Therefore, in some embodiments, the light source system 102 (e.g., the pump control assembly 115) includes a wavelength tuning assembly 114 accommodated in the optical path LP2 and configured and operable to controllably vary a wavelength of the exciting pump pulse P2 propagating to the sample. The wavelength tuning assembly 114 may include wavelength conversion element(s) like parametric down converter(s) and harmonic generation module(s) which may be controlled by the control system 104.

[0048] Sample excitation, especially in patterned structures, is strongly dependent on light polarization. Therefore, polarization optics (polarizers and retarders) can be used to control this property. Specifically, the sample's response to the probe beam / pulse may be dependent on the polarization of probe beam / pulse. Therefore, the light source system 102 may further include a polarizer assembly 116 to apply polarization control of the broadband probe pulse P3 and the sample' response to interaction with the probe pulse P3 (e.g., sample's reflection of the probe pulse). The polarizer assembly 116 may include a polarizer / retarder in the optical path LP1 of broadband probe pulse incidence on the sample and possibly also in a detection path of propagation of the light response of the sample towards the detection system 122.

[0049] In some embodiments, to enhance the signal-to-noise ratio of pump-probe measurements, both the exciting pump pulse P2 and the broadband probe pulse P3 are modulated to facilitate lock-in detection procedures. In these embodiments, the light source system 102 may further include modulation assemblies 120 and 118 configured and operable to apply first and second modulations of (same or) different first and second modulation frequencies, respectively, to the broadband probe pulse P3 and the exciting pump pulse P2 while propagating along the first and second paths, LP1 and LP2. The first and second modulation frequencies may be different from one another to reduce noncommon contribution to the signal.

[0050] The light source system 102 is associated with the detection system 122, which may or may not be part of the system 100. The detection system 122 is configured and operable to detect the light response of the sample, being excited by the pump pulse P2, to the interaction with the broadband probe pulse P3 and to generate measured data indicative of the light response being detected, thereby enabling extraction of one or more parameters of the sample from the measured data. Such parameters may include but not limited to one or more of the following: free charge carrier properties (concentration, lifetime, mobility, diffusivity etc.), band structure, strain, defectivity, interface quality, etc.

[0051] Some non-limiting examples of optical / spectroscopic measurement system utilizing the above-described technique of the present disclosure include: spectral reflectometry, ellipsometry, or interferometry setup based on the SCL broad-band pulse.

[0052] Reference is made to Fig. 2 exemplifying, by way of a flow diagram 200, a method of performing a time domain spectral photo-reflective (SPR) measurement session utilizing the technique of the present disclosure, i.e., utilizing the above describe system 100. A sequence of short-duration laser pulses is provided and directed towards the sample (step 202). These short-duration laser pulses are exciting pump pulses. The durations of the exciting pump pulse may be in the range between a few femtoseconds (1 fs= 10'15s) and a few nanoseconds (1 ns= 10'9s). Also provided is a sequence of broadband probe pulses and directed towards the sample (step 204). As described above, the broadband probe pulses are generated by the SCL. As also described above, the exciting pump pulses and the seed pulses of the SCL may be generated by the same or separate laser sources. The optical characteristics of each exciting pump pulses can be controlled, such as intensity, wavelength, pulse duration, beam profile, angle of incidence, etc. (step 206). Polarization of the probe pulse may be controlled (step 205).

[0053] Controllable variable time delays are applied to the exciting pump pulses (step 208) to provide controllably variable pump-probe delays for multiple pump-probe interactions with the sample. As a result, the sample sequentially interacts with the exciting pump pulses, such that each i-th exciting pump pulse interacts with the sample at time (tep)i (step 210) and the i-th pump-interaction is followed by i-th illuminations of the sample with the probe pulse at time (i)i=(tep)i+(At)i, (At)i being the i-th pump-probe delay (step 212). The interaction of the exciting pump pulse with the sample creates some excitation in the sample that changes its optical properties.

[0054] The excitation can generally have a time dependence - typically increasing rapidly following the pump pulse and then decaying back to equilibrium. The temporal profile of this response (combined with the spectral resolution) carries information on the generation, transport and recombination of free charge carries, which are in turn indicative of many relevant material properties such as carrier mobility, mean free path, lifetime, strain etc.

[0055] Light responses of the sample to the interactions with the broadband probe pulses (e.g., reflections of the probe pulses from the sample) are detected (step 214), each characterizing the temporal state of the sample. Dynamic optical properties of the sample, resulting from the variable pump-probe delays, can be monitored (step 216).

[0056] Various optical probe configurations can be tailored to detect the optical properties of the sample (wavelength, polarization, angle of incidence, etc.). The sample can reflect, transmit, or diffract the probe pulse. Thus, during the PR measurement session, dynamic optical properties of the sample are monitored resulting from the variable pump-probe delay. Suppose the broadband probe pulse is also short-pulsed. In that case, the delay between the exciting pump pulse and the probe pulse can be controlled by a delay stage, allowing for the time-resolved detection of the sample's optical properties and getting insight into the dynamic properties of the excitation.

[0057] The probe pulse spectrum can be tailored to the application. Depending on the materials being studied and the properties of interest, the probe spectrum can be fully broadband or a narrower spectral band (e.g. covering the band gap region of the material) can be selected, as well as one or more discrete wavelengths. This can be accomplished either by changing the probe generation mechanism or by filtering selected regions from a broadband supercontinuum.

[0058] Reference is made to Figs. 3 and 4 showing schematically non-limiting examples of the configuration of the system 100 for use in photoreflectance (PR) measurements of a sample. In both examples, the systems are configured generally similar to the system 100 of Fig. 1, and the same reference numbers are used to indicate the functionally similar elements in all the examples.

[0059] Thus, each of the system exemplified in Figs. 3 and 4 includes a light source system 102 and a control system 104, and is associated with a detection system 122. The light source system 102 includes a pump source assembly 106 and a nonlinear medium 108, and defines a delay line 110 to enable controllably variable pump-probe time delay in real time during the PR measurement session. The pump source assembly 106 is configured and operable to produce an ultra-short seed pulse Pl for exciting the nonlinear medium 108 generating a broadband probe pulse P3 propagating along a first path LP1 towards the sample; and to produce an exciting pump pulse P2 propagating along a second path LP2, inclined with respect to the first path LP1 towards the sample. The delay line 110 is provided in the optical path of the exciting pump pulse propagation to induce the pump-probe delay.

[0060] In this example of Fig. 3, the pump source assembly 106 includes an optical seed source 107 configured and operable to generate the ultra-short seed pulse, and includes a beam splitter BS1 which splits this ultra-short seed pulse into the exciting seed pulse Pl propagating to excite the non-linear medium 108 to thereby generate the broadband probe pulse P3, and into the exciting pump pulse P2 propagating along the path LP2 to excite the sample. The delay line 110 is located in path LP2 of the exciting pump pulse, and is configured and operable to induce the controllably variable pump-probe time delay in real time during the PR measurement session.

[0061] In the non-limiting examples of Figs. 3 and 4, the exciting pump pulse P2 propagates in free space along the optical path LP2, and the delay line 110 configuration is based on a retroreflector assembly 111. The retroreflector assembly is shown here schematically - only two identically symmetrical mirrors are shown.

[0062] The configuration and operation of a retroreflector assembly 111 are known per see', it includes a beam splitter and two identically symmetrical mirrors, and is associated with a linear translation stage (e.g., motorized), translating a mirror or retroreflector changes the optical path length. A pulse is split into two beams, one beam travels through a fixed path and the other travels through a path with a movable mirror; by translating the mirror, the optical path is increased or decreased, introducing a time delay. For example, a 1.5 mm mirror displacement introduces a 10 ps delay.

[0063] Generally, any known suitable implementation of the delay line can be used. Suitable delay line implementations depend on the range of delay to be obtained. Considering the semiconductor metrology discussed in the present disclosure, where relevant time scales are usually on the order of ps-ns, the delay line can be realized by a moving retroreflector assembly. Fig. 5 exemplifies such configuration of the delay line in which a plurality of mirrors are arranged in the optical path LP2 such that the exciting pump pulse P2 successively interacts with these mirror, and some of the mirrors are associated with a linear stage.

[0064] As also shown in Figs. 3 and 4, the system 100 includes the pump control assembly 115 which may include (optical) elements of the spatial overlap controller (112 in Fig. 1) of the exciting pump pulse spot with respect to the broadband probe pulse spot on the sample, and / or wavelength tuning assembly (114 in Fig. 1) and / or modulation assembly (118 in Fig. 1). The exemplified systems also include light directing assemblies including deflectors / mirrors properly redirecting the optical paths.

[0065] In the example of Fig. 3, the probe pulse P3 propagates along the path LP1 providing normal incidence of the probe pulse P3 onto the sample, while the optical path LP2 of the exciting pump pulse propagation provides oblique incidence of the exciting pump pulse P2 onto the sample. The optical scheme of Fig. 3 is a so-called “oblique pump — normal probe” scheme. In this scheme, detection of reflection of the exciting pump pulse P2 from the sample by detector (spectrometer) 122 is substantially avoided. The light focusing assembly includes objective lens assembly OLepin the optical path LP2 of the exciting pump pulse P2 propagation to the sample, and objective lens assembly OLPPin the optical path LP1 of the probe pulse P3 and its reflection P3' propagation to and from the sample. Also, in this example of Fig. 3, the light source system 102 includes a polarizer 116 in the optical path LP1 of propagation of the probe pulse P3 onto the sample and its reflection P3' from the sample (constituting the light response of the sample) to apply polarization control of the broadband probe pulse and its reflection from the sample.

[0066] The optical scheme in the example of Fig. 4 presents a so-called “normal pump — oblique probe” scheme which may be used to perform spectral ellipsometry measurements. More specifically, in this example, the optical path LP2 of the exciting pump pulse propagation is oriented to provide normal incidence of the exciting pump pulse P2 onto the sample, while the probe pulse P3 propagates along the path LP1 oriented to provide oblique incidence of the probe pulse P3 onto the sample. The focusing arrangement includes the objective lens assembly OLepin the exciting pump pulse P2 propagation path LP2, and objective lens assemblies OLPPand OLir for, respectively, focusing the probe pulse P3 onto the sample and collecting the reflection thereof P3' from the sample to be directed to the spectrometer 122. Also, in this example, the system includes polarizers 116 and 117 in the optical path LP1 of the probe pulse propagating towards the objective lens OLPPand in the optical path of the probe pulse reflection P3' being collected by the objective lens OLir. This configuration can be used for timedomain spectral ellipsometry measurements (i.e., measurement of a variation in the change in the polarization state (amplitude ratio and phase shift) between s- and p- polarized reflected light caused by the sample's excitation by the pump pulse). The setup of Fig. 4 provides time-domain spectral ellipsometry, which extends standard ellipsometry to the ultrafast, non-equilibrium regime, enabling measurement of transient optical constants, carrier dynamics, and structural responses with high sensitivity. It should be noted that the objective lens assemblies in the systems of Figs. 3 and 4 might also operate as the spatial overlap controller 112, i.e., they may be controlled to provide a required degree of spatial overlap between the exciting pulse spot and the probe pulse spot on the sample (while with the corresponding time delay between them). As described above and shown in the figures, the detection system 122 is implemented as / includes a spectrometer and enables to perform time-resolved spectral reflectometry measurements of the sample. Such spectral reflectometry measurements are used in optical metrology as a non-contact technique that measures thin film thicknesses, layer structures, and surface properties by analyzing how a sample reflects light across a range of wavelengths. The technique of the present disclosure allows to perform such measurements in time domain, while studying the effects of an exciting pump pulse exciting the sample at varying delays before the broadband probe pulse.

[0067] It should be noted, and already mentioned above, that the technique of the present disclosure is not limited to spectral reflectometry and any other specific detection scheme, and may be used where the sample transmits, or diffracts the probe beam.

Claims

CLAIMS:

1. A system for use in photoreflectance (PR) measurements of a sample, the system comprising: a light source system comprising a pump source assembly and a non-linear medium, wherein: the pump source assembly is configured and operable to produce an ultra- short seed pulse for exciting the non-linear medium to operate as a supercontinuum laser generating a broadband probe pulse propagating along a first path towards the sample to create a broadband focal spot on the sample, said pump source assembly is configured and operable to produce an exciting pump pulse propagating along a second path, inclined with respect to the first path, to create an excitation focal spot on the sample, said light source system is configured to define a delay line configured to controllably vary a time of arrival of the exciting pump pulse to the sample, thereby inducing a controllably variable pump-probe time delay in real time during a PR measurement session, including multiple successive interactions of the exciting pump pulses with the sample each followed by interaction of the broadband probe pulse with the sample, thereby enabling successive time-domain spectroscopic PR measurements of the sample by detection of a light response of the sample excited by the exciting pump pulse, to the broadband probe pulse; a control system configured and operable to control said variable pump-probe time delay and a degree of spatial overlap between the broadband focal spot and the excitation focal spot on the sample during the measurement session.

2. The system according to claim 1, further comprising a detection system configured and operable to detect the light response of the sample to the broadband probe pulse, and generate data indicative of the light response being detected, thereby enabling extraction of one or more parameters of the sample.

3. The system according to claim 2, wherein the one or more parameters extractable from the data indicative of the light response being detected comprises one or more of the following: free charge carrier properties in the sample, band structure of the sample, strain within the sample, defectivity of the sample, interface quality in the sample.

4. The system according to claim 2, wherein the detection system comprises a spectrometer.

5. The system according to claim 1, wherein one of the first and second paths provides normal incidence of the respective pulse onto the sample's surface, and the other is oriented to provide oblique incidence of the respective pulse onto the sample's surface, such that detection of reflection of the exciting pump pulse from the sample is substantially avoided.

6. The system according to claim 1, wherein the pump source assembly comprises a seed source generating said ultra-short seed pulse for exciting said non-linear medium, and a pump source generating said exciting pump pulse, the seed source and the pump source being controllably operated to provide a controllably variable synchronization between the generation of the exciting pump pulse and the broadband probe pulse, thereby defining said delay line providing said controllably variable pump-probe time delay in real time during the PR measurement session.

7. The system according to claim 1, wherein the pump source assembly comprises an optical seed source configured and operable to generate the ultra-short seed pulse, the light source system further comprising: a beam splitter configured to split the ultra-short seed pulse into an exciting seed pulse propagating to perform said excitation of the non-linear medium and the exciting pump pulse propagating along a path to excite the sample; and the delay line located in said path of the exciting pump pulse, the delay line being configured and operable to induce said controllably variable pump-probe time delay in real time during the PR measurement session.

8. The system according to claim 1, further comprising a wavelength tuning assembly configured and operable to controllably vary a wavelength of the exciting pump pulse propagating to the sample.

9. The system according to claim 8, wherein said wavelength tuning assembly is configured and operable to carry out at least one of parametric down conversion and harmonic generation.

10. The system according to claim 1, further comprising a polarizing assembly to apply polarization control of the broadband probe pulse and its reflection from the sample.

11. The system according to claim 1, further comprising a modulation assembly configured and operable to apply first and second modulations of different first and second modulation frequencies, respectively, to the broadband probe pulse and the exciting pump pulse while propagating along said first and second path.

12. A system for performing photoreflectance (PR) measurement of a sample, the system comprising: a supercontinuum laser (SCL) unit comprising an optical seed source configured and operable to generate ultra-short seed pulses and a non-linear medium configured and operable to respond by a broadband probe pulse to excitation by an ultra-short seed pulse; and a beam splitter configured to split the ultra-short seed pulse into an exciting seed pulse propagating to perform said excitation of the non-linear medium of the SCL and an exciting pump pulse propagating along a path to excite the sample; a delay line located in said path of the exciting pump pulse, the delay line being configured and operable to controllably vary a time of arrival of the exciting pump pulse to the sample, thereby inducing a controllably variable pump-probe time delay in real time during a PR measurement session including multiple successive interactions of the exciting pump pulses with the sample each followed by interaction of the broadband probe pulse with the sample, thereby enabling successive time-domain spectroscopic PR measurements of the sample by detection of a light response of the sample excited by the exciting pump pulse, to the broadband probe pulse; a control system configured and operable to control said variable pump-probe time delay and a degree of spatial overlap between regions of interaction of the exciting pump pulses and the broadband probe pulses with the sample during the measurement session.

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