Optical device for surface analysis using reflected light through linear and non-linear optical processes

Abstract

An optical surface analysis device (1) allows for the advantageous measurement, at the same location on a sample surface, of a nonlinear second harmonic generation (SHG) signal and a linear absorption signal in reflectivity, and their correlation to obtain information about the sample surface microstructure. The optical device was designed to integrate nonlinear reflectometry for SHG signal detection, visible reflection-absorption spectroscopy (Vis-RAS), and a visual control device for the sample surface using bright-field microscopy (CF), while maintaining the device's compact size. Abstract figure: Figure 1

Description

Title of the invention: Optical apparatus for surface analysis using reflected light by linear and nonlinear optical processes. Technical field

[0001] This disclosure relates to the field of optical devices for analyzing the surfaces of a sample.

[0002] The present disclosure relates more specifically to an optical device for analyzing the surfaces of a sample from second harmonic generation (SHG) signals and absorption signals in reflectivity. Previous technique

[0003] It is known to use optical techniques based on the interaction between a light beam and a sample to probe the physical and chemical properties of the surface of a sample, and in particular structural defects formed during the manufacture of the sample.

[0004] Ellipsometric imaging and linear reflectometry are based on measuring changes in light during its interaction with the sample to extract surface information. However, since these optical processes are based on exploiting linear optical effects, the extracted information is generally incomplete. These techniques must be coupled with other, more advanced surface characterization methods.

[0005] Chemical analysis techniques such as surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS), and polarization modulation-reflection-absorption infrared spectroscopy (PM-IRRAS) can detect very small quantities of material on surfaces. However, these techniques require a metal plate coupled to a sample to function. The operator must be specifically trained to operate the instruments. Finally, these techniques are relatively complex and time-consuming to implement, and require the collection of a large amount of data to extract surface information, making them poorly suited to the requirements of quality and conformity testing in mass production.

[0006] X-ray (XR) techniques can also be used to diagnose chemical or physical properties. However, they are often destructive and require surface preparation of the sample to be analyzed, which proves difficult.

[0007] Another known technique consists of detecting the non-linear phenomenon called second harmonic generation (SHG) which occurs at the focal point using a light beam to probe the polarizability of a sample. This technique is particularly useful for checking, for example, the quality of the interfaces between two layers during the fabrication of a semiconductor material structure. SHG (Selective Hyperpolarization) is therefore a particularly interesting technique for surface characterization aimed at detecting non-centrosymmetric crystal defects, where symmetry is broken. The very low signal level generally necessitates the use of a powerful laser beam to increase detection sensitivity. However, this technique is usually used with a fixed polarization state. The process can be considered as the combination of two photons to produce a single photon; in other words, the production of a light beam with twice the frequency or half the wavelength of the incident light beam.

[0008] None of the known measurement techniques make it possible to collect both the non-linear SHG signal and the absorption signal in reflection at the same point of the sample and thus to correlate them to extract certain physical and chemical information from the surface of the sample.

[0009] It is therefore desirable to develop an optical measurement device that is quick and easy to implement in order to access a maximum of surface information of the sample, with a resolution on the order of micrometers, and this regardless of the material of the sample.

[0010] The present disclosure also aims to propose a device that allows the use of different modes of analysis on the same sample, in point mode and in mapping mode. Summary

[0011] This disclosure improves the situation.

[0012] An optical measuring device is proposed for characterizing the surface of a sample, comprising - a first source adapted to emit a first incident excitation light beam at a first excitation wavelength (XI); - a second source adapted to emit a second incident excitation light beam at a second excitation wavelength (X2) different from said first excitation wavelength; - a sample holder suitable for receiving the sample; - a projection arm capable of directing the first and second light beams onto a measurement point on the sample surface along a common optical path, said arm comprising a first mirror arranged to receive and reflect said first and second incident light beams onto the sample surface, and a focusing device configured to generate a focal point on the surface of the sample and a polarizing device placed between the first mirror and the focusing device to generate a number of polarization states of the first incident light beam and a linear polarization state of the second incident beam; - a first detection device configured to record a first second harmonic generation (SHG) signal having a detection wavelength equal to half the excitation wavelength associated with the first light beam reflected by a measurement point on the surface of the sample; - a second detection device configured to record a second linear absorption signal in reflectivity associated with the second light beam reflected by a measurement point on the surface of the sample; - a collection arm capable of collecting the first light beam reflected and the second light beam reflected by the surface of the sample and transmitting them respectively to the first detection device and said second detection device; - a data processing module configured to determine structural information from the first non-linear SHG signal and the second linear signal transmitted respectively by the first detection device and the second detection device for the same measurement point on the surface of the sample.

[0013] According to one embodiment, the first mirror can be arranged so that the first incident beam and the second beam are incident on the surface of the sample with an oblique angle of incidence between 65° and 80° relative to the plane of incidence.

[0014] The interaction of the first near-infrared laser beam with the sample surface generates linear and nonlinear optical effects that are collected at double the frequency after reflection. Grazing angle analysis enhances reflection, as reflection coefficients increase with the angle of incidence beyond Brewster's angle for dielectrics, and surface photon-plasmon coupling becomes very efficient for metals and semiconductors.

[0015] According to one embodiment, the device may further comprise: - a first optical device arranged between said first source and the first mirror and configured to project the first laser beam onto the first mirror; - a second optical device arranged between said second source and the first mirror and configured to project the second laser beam onto the first orientation mirror.

[0016] The features described in the following paragraphs may optionally be implemented independently of each other or in combination with each other:

[0017] The first optical device may include a second mirror, a shutter switchable between a passing position allowing the first incident beam to pass and a blocking position allowing the first incident beam to be blocked, a movable density filter capable of varying the power of the first incident light beam between 0.01% and 100% of the total power, and a polarization state controller placed successively on the optical path of the first incident beam between the first source and the first mirror.

[0018] The second optical device may include an optical fiber having a first end coupled to the output of the second source and a second end coupled to a movable collimator, said collimator being able to collimate the second light beam at the output of the optical fiber on the first mirror.

[0019] The polarization device may include: - a first movable polarization module to be placed on the optical path of the first incident beam between the first mirror and the focusing device and configured to generate a number of polarization states of the first incident beam, and - a second movable polarization module to be placed on the optical path of the second incident beam between the first mirror and the focusing device and configured to generate a linear polarization state of the second incident light beam.

[0020] The first polarization module may include a rotating half-wave plate about a propagation optical axis to generate linear polarization at a desired angle and a fixed quarter-wave plate having a fast axis perpendicular to the propagation optical axis of the half-wave to generate elliptic and circular polarization states in association with the half-wave plate.

[0021] The second movable polarization module may include a horizontal linear polarizer capable of polarizing the second incident light beam into a horizontal polarization state “s” and a vertical linear polarizer capable of polarizing the second incident light beam into a vertical polarization state “p”.

[0022] According to one embodiment, the collection arm may be arranged symmetrically to the projection arm with respect to the plane of incidence, said collection arm comprising a second focusing device, a silicon window, a low-pass filter, a rotating polarization state analyzer, and a tilting mirror arranged successively in the optical path of the first and second light beams reflected at the measurement point on the sample surface, said tilting mirror being configured to allow the first light beam reflected by the sample surface to pass towards the first detection device and to deflect the second beam reflected by the surface of the sample towards the second detection device.

[0023] Preferably, the first source is a laser source adapted to emit a first incident excitation light beam at a first excitation wavelength between 1000 nm and 1100 nm, with a pulse between 100 fs and 600 fs, and a frequency between 1 MHz and 100 MHz.

[0024] Preferably, the second source is a tungsten lamp adapted to emit a second incident excitation light beam at a second excitation wavelength between 400 nm and 700 nm.

[0025] In embodiments, the first optical focusing device may include a focusing lens capable of focusing the first light beam and the second light beam into a focal point with a diameter between 2 pm and 50 pm on the surface of the sample.

[0026] According to one embodiment, the apparatus may further include a displacement device coupled to the sample support, said displacement device being configured to move the sample support in a horizontal plane along a first horizontal direction along the X axis, a second horizontal direction along the Y axis, a third vertical direction along the Z axis, so as to move the sample support relative to the focal point to form a plurality of measurement points on the surface of the target sample.

[0027] According to another embodiment, the device may further comprise an imaging system including: - a third light source capable of emitting white light; - a third optical fiber having one end coupled to the output of the third light source; - a collimation mirror capable of collimating the white light exiting the third optical fiber onto a beam splitter; - a focusing lens capable of focusing white light onto the focal point of the first incident light beam and the second incident light beam; - a camera; - said beam splitter being arranged between the collimating mirror and the focusing lens so as to allow white light to pass through in a direction of incidence normal to the surface of the sample and to reflect a part of the white light reflected by the surface of the sample back to the camera in order to form images on the focal point.

[0028] According to some embodiments, the data processing module may include a memory configured to store the spatial coordinates of the sample support or focal point corresponding to each measurement acquisition, by the first detection device and the second detection device of the first non-linear SHG signal and the second linear absorption signal in reflectivity.

[0029] According to some embodiments, the data processing module may include a control unit configured to control the displacement device, an image calculation and construction unit configured to generate images in which each pixel is representative of information from a measurement point of the sample. Brief description of the drawings

[0030] Other features, details and advantages of the invention will become apparent from reading the detailed description below and from analyzing the accompanying drawings, in which: Fig. 1

[0031] [Fig.1] Fig.1 is a schematic representation of an optical device for surface analysis of a sample according to a first embodiment; Fig. 2

[0032] [Fig.2] The [Fig.2] shows an optical device for surface analysis of a sample according to a second embodiment; Fig. 3

[0033] [Fig.3] The [Fig.3] shows a side view of the optical device of the [Fig.2]. Fig. 4

[0034] [Fig.4] Fig.4 shows a schematic view of a clear field viewing area on the focal point of the first incident beam and on the focal point of the second incident light beam. Fig. 5

[0035] [Fig.5] The [Fig.5] is a schematic view showing the optical elements of the optical apparatus of the [Fig.1] used for the second harmonic generation signal measurement technique; Fig. 6

[0036] [Fig.6] The [Fig.6] is a schematic view showing the optical elements of the optical apparatus of the [Fig.1] used for the technique of measuring the linear absorption signal in reflectivity. Fig. 7

[0037] [Fig.7] The [Fig.7] is a schematic view showing the optical elements of the imaging system of the [Fig.2] used for bright-field visualization of the measurement point of the sample surface. Description of the implementation methods

[0038] The drawings and description below contain, essentially, elements of a definite nature. They may therefore not only serve to better understand the present invention, but also contribute to its definition, if necessary.

[0039] In the various figures, the elements designated by the same reference designate identical or similar elements.

[0040] The optical surface analysis device of this disclosure advantageously measures, at a single location on the surface of a sample, both the nonlinear second harmonic generation (SHG) signal and the linear absorption signal in reflectivity, and correlates them to obtain information about the microstructure of the sample surface. The optical device was designed to integrate nonlinear reflectometry for SHG signal detection, visible reflection-absorption spectroscopy (Vis-RAS), and visual inspection of the sample surface by bright-field microscopy (CF), while maintaining the device's compact size.

[0041] Advantageously, the particular design of the device allows for a single projection arm for two incident beams and a single analysis arm, allowing the surface of the sample to be probed by SHG and Vis-RAS at the same point.

[0042] Another advantage is the ability to project the first incident beam onto the surface of the sample so as to be able to measure the SHG signal at grazing incidence, making it possible to increase the amplitude of the reflected SHG signal.

[0043] In this disclosure, the focusing device consists of a single optical lens or a lens assembly with a suitable focal length. The focal length of the focusing lens is chosen in practice according to the desired analysis resolution and the light source used. For example, a mesh can be defined for data acquisition purposes with a spatial resolution between 2 pm and 50 pm, meeting, for instance, the requirements for monitoring defects of micrometer size.

[0044] Reference is now made to [Fig. 1] which illustrates an optical device for surface analysis of a sample 13 according to a first embodiment.

[0045] The operating principle of the optical device 1 consists of directing an incident light beam through a group of optical components towards a study area of ​​the sample so as to excite it at a measurement point within the study area of ​​the material. The interaction of the material with the light beam generates the SHG phenomenon. A group of optical components collects the reflected light beam. The state of the light beam reflected by the sample is then analyzed to extract characteristic information from the light signal at a measurement point on the sample surface. By moving the sample relative to the point By focusing the light beam, a representation of the characteristic data for all measurement points in the predetermined spatial area is generated.

[0046] The proposed optical measuring device can advantageously be used with different types of materials, for example, metals, glasses, dielectric materials, or semiconductor materials. The various parameters used in the measurement are then adjusted according to the target material for each analysis, in particular by selecting the excitation wavelength of the analyzed material and the detection wavelength of the reflected signal.

[0047] To describe in detail the various optical elements of device 1, the optical measuring device 1 comprises - a first source 2 adapted to emit a first incident excitation light beam at a first excitation wavelength XI; - a second source 29 adapted to emit a second incident excitation light beam at a second excitation wavelength (X2) different from said first excitation wavelength; - a sample holder 30 suitable for receiving the sample 13; - a projection arm capable of sending the first light beam and the second light beam onto a measurement point on the surface of the sample 13 along a common optical path, said arm comprising a first mirror 7 arranged to receive said first incident light beam and said second incident light beam and to reflect them towards the surface of the sample 13, a focusing device 15 configured to generate a focal point on the surface of the sample and a polarizing device 8 placed between the first mirror 7 and the focusing device 15 to generate a number of polarization states of the first incident light beam and a linear polarization state of the second incident beam; - a first detection device configured to record a first second harmonic generation (SHG) signal having a detection wavelength equal to half the excitation wavelength associated with the first light beam (XI) reflected by a measurement point on the surface of sample 13; - a second detection device configured to record a second linear absorption signal in reflectivity associated with the second light beam reflected by a measurement point on the surface of sample 13; - a collection arm capable of collecting the first light beam reflected and the second light beam reflected by the surface of the sample and transmitting them respectively to the first detection device and said second detection device; - a data processing module 40 configured to determine surface information from the first non-linear SHG signal and the second linear signal transmitted respectively by the first detection device and the second detection device for the same measurement point of the surface of sample 13.

[0048] In the following description, the light beam is called "incident beam" over the entire path from the light source 2, 29 to the sample 13, and "reflected beam" over the entire path from the sample 13 to the first detection device and to the second detection device.

[0049] The first light source 2 is a laser adapted to emit a first quasi-monochromatic, pulsed femtosecond incident light beam at a first wavelength XL. In the example of [Fig. 1], the excitation wavelength can be 1025 nm. The power of the first light beam is 1.3 W. The pulses can range from 100 fs to 600 fs. The laser 2 emits the laser beam in a direction X.

[0050] The pulse time and frequency of the laser beam are selected so as to maximize the desired nonlinear effects while reducing the risk of damaging the sample, from 1 MHz to 100 MHz.

[0051] According to an alternative, the first source 2 can be a wavelength-tunable source. The wavelength can be between 200 nm and 2000 nm.

[0052] By way of example, the first light source is configured to emit a pulsed light beam of 250 femtoseconds. The use of a pulsed source, specifically in the femtosecond range (10⁻¹⁵ s), makes it possible to deliver a high peak power while maintaining a low average power, thus enhancing nonlinear effects. The pulses are sufficiently spaced in time to ensure complete thermal relaxation of the material, thereby preventing local energy buildup that could damage the sample. Furthermore, advantageously, with a pulse duration of, for example, 250 fs, the instantaneous response of the material originates solely from electrons and is ideally suited to electron spectroscopy by limiting vibrational effects and preventing the propagation of acoustic waves present in the picosecond regime.

[0053] The second light source 29 is a stable light source configured to emit a beam of light covering the visible spectrum. For example, the second light source is a tungsten lamp. The wavelength can be between 400 nm and 700 nm.

[0054] The device includes a first optical device arranged between the first source 2 and the first mirror 7 and configured to project the first incident laser beam onto the first mirror 7.

[0055] The first optical device includes a second mirror 3, a shutter 36 switchable between a passing position allowing the first incident beam to pass through and a blocking position allowing the first incident beam to be blocked, a movable density filter 4 capable of varying the power of the first incident light beam between 0.01% and 100% of the total power, a polarization state controller 5 which are placed successively on the optical path of the first incident beam between the first source 2 and the first mirror 7.

[0056] The second mirror 3 is a perfectly reflective plane mirror, for example for the excitation wavelength X = 1025 nm. It allows the first laser beam, which passes successively through the shutter 36, the density filter 4 and the polarization state controller 5, to be deflected towards the first mirror 7.

[0057] The first mirror 7 and the second mirror 3 are arranged so that the angle of incidence of the laser beam on each of them is approximately 90° in the plane of [Fig.1] (XY), so that the first beam reflected by the first mirror is in the same direction as the first incident beam, namely on [Fig.1], in the X direction. These two mirrors make it possible to arrange the optical elements of the first optical device and the laser 2 in a location chosen according to mechanical space constraints, while allowing the first beam to be routed in the chosen X direction.

[0058] The apparatus 1 includes a second optical device arranged between said second source 29 and the first mirror 7 and configured to project the second incident laser beam onto the first orientation mirror 7.

[0059] The second optical device comprises a first optical fiber 28 having a first end coupled to the output of the second source 29 and a second end coupled to a movable collimator 6. The second incident light beam emitted by the second source 28 is therefore injected into the first optical fiber 28 which carries the incident beam to the collimator 6 which is able to collimate the second light beam at the output of the optical fiber 28 onto the first mirror 7. The second beam reflected by the first mirror 7 is reflected in the same direction as the first incident beam, namely onto [Fig. 1], in the direction X.

[0060] According to one embodiment, the collimator 6 is adapted for achromatic optical fiber up to 780 nm. It has a focal length and numerical aperture adjusted to allow the visible spectrum to remain collimated up to the surface of the sample so that the spot focused on the surface is uniform.

[0061] According to a particularly advantageous embodiment, the first mirror 7 is arranged so as to orient the first incident beam and the second incident beam at an angle of incidence between 65° and 80° with respect to the plane of incidence. This configuration is particularly advantageous for increasing the amplitude of the non-linear reflected SHG signal. An example of this configuration is illustrated in [Fig. 3]. The injection arm 70 of the first incident light beam and the second incident light beam is oriented at a grazing angle of 70° with respect to the plane of incidence, the plane normal to the surface of the sample 13.

[0062] The interaction of the first near-infrared laser beam with the sample surface generates linear and nonlinear optical effects that are collected at twice the frequency after reflection from the plane of incidence. Grazing incidence reflection is enhanced, as the reflection coefficients increase with the angle of incidence beyond Brewster's angle. This principle is notably exploited by absorption measurement techniques in reflectivity, but has not yet been applied to the detection of the nonlinear signal.

[0063] The collection arm 71, which is placed on the optical path to collect the reflected light beam, is arranged symmetrically and complementaryly to the injection arm 70 with respect to the plane of incidence. In [Fig. 3], the collection arm 71 is oriented at -70° with respect to the plane of incidence.

[0064] The polarization device 8 comprises: - a first movable polarization module 10 to be placed on the optical path of the first incident beam between the first mirror 7 and the focusing device 15 and configured to generate a number of polarization states of the first incident beam, and - a second movable polarization module 9 to be placed on the optical path of the second incident beam between the first mirror 7 and the focusing device 15 and configured to generate a linear polarization state of the second incident light beam.

[0065] The first polarization module 10 comprises a rotating half-wave plate 11 about an optical propagation axis to generate linear polarization at a desired angle and a fixed quarter-wave plate 12 having a fast axis perpendicular to the optical propagation axis of the half-wave, along the Y axis.

[0066] The second mirror 7 directs the first incident beam at an angle of 70° to the plane of incidence YZ, which then passes through the zero-order half-wave plate 11 centered at XL. The rotation of the half-wave plate 11, whose angle is denoted W, allows the polarization plane to be rotated 2W around the optical propagation axis to generate any linear polarization at the desired angle. The fixed quarter-wave plate 12 is of zero order centered at XI, whose angle is denoted ¢. Its fast axis lies horizontally along Y. The two optical components 11 and 12 combined allow the generation of all polarization states: horizontal linear (W = 0°), vertical linear (W = 45°), elliptical, right-handed circular, and left-handed circular (W = ± 22.5°).

[0067] According to one embodiment, the first polarization module may include a high-pass filter 14 whose cutoff frequency is, for example, 950 nm, allowing the first incident beam to be spectrally filtered by allowing only infrared to pass through so that no visible light reaches the surface of the sample 13.

[0068] According to one embodiment, the shutter 36, the neutral density filter 4, the rotating half-wave plate 11, and the fixed quarter-wave plate 12 are arranged on a motorized support so as to position them in the optical path of the first incident light beam to measure the SHG signal. When the linear absorption signal measurement mode in reflectivity is used, the high-pass filter 14 is manually removed, and the rotating half-wave plate 11 and the fixed quarter-wave plate 12 are retracted by the motors. The mobility of the shutter 36, the neutral density filter 4, the linear polarizer 9, and the fixed quarter-wave plate 12 is represented respectively by arrows 34, 31, 32, and 33. The half-wave plate 11 is provided with a rotary mechanism 37.

[0069] The second movable polarization module 9 includes a horizontal linear polarizer capable of polarizing the second incident light beam into a horizontal polarization state “s” and a vertical linear polarizer capable of polarizing the second incident light beam into a vertical polarization state “p”.

[0070] The linear polarizer is capable of covering a spectral range from 400 nm to 700 nm which allows the second light beam at the output of the first optical fiber 28 to be polarized.

[0071] At the output of the polarization device, the first incident beam and the second incident beam are focused by the focusing device 15 onto the surface of the sample.

[0072] Sample 13 is associated with an orthonormal coordinate system (X, Y, Z). In the example of [Fig. 1], the Z axis is an axis perpendicular to the analysis surface of the sample, and the (X, Y) plane is parallel to the surface of the sample.

[0073] The sample 13 is placed on a sample support 30 which has two translations (X, Y), one translation (Z), as well as two rotations (0, q>) to ensure fine compensation of the flatness of the sample in the XY plane.

[0074] In the case of using the device to perform a mapping of the sample surface, the device further includes a motorized displacement device 39 coupled to the sample support 30 which can be moved along the two motorized directions X and Y and in the Z direction. The Z direction is used to adjust the focus perfectly to the interface.

[0075] Sample 13 may be, for example, a semiconductor component whose surface must be analyzed to determine whether it contains structural defects or impurities.

[0076] In the event of the presence of such defects at the focal point, an altered SHG signal is emitted.

[0077] In the case where the incident beam is a first incident beam XI, the detection wavelength signal will be observed at X1 / 2 and is an SHG signal.

[0078] In the case where the incident beam is a second incident beam X2, the detection wavelength signal will be observed at X2 and is a linear absorption signal in reflectivity.

[0079] In both cases, a majority of the incident beam is reflected by the specular surface of the sample or by the interface between the sample and the support if the incident beam has passed entirely through the thickness of the sample. Advantageously, the surface of the sample support 30 can optionally be treated with a dichroic coating to increase its surface reflectivity. The reflected beam is collected by the collection arm 71.

[0080] The collecting arm is therefore placed on the optical path of the reflected light beam.

[0081] According to one embodiment, the collection arm in [Fig. 3] comprises a second focusing device 16, a silicon window 17, a low-pass filter 18, a rotating polarization state analyzer 19, and a tilting mirror 20 arranged successively in the optical path of the first and second light beams reflected at the measurement point on the sample surface. The tilting mirror 20 is able to be placed in a first position allowing the first light beam reflected by the sample surface to pass towards the first detection system and in a second position to deflect the second beam reflected by the sample surface towards the second detection system.

[0082] The second focusing device 16 includes a second focal lens that makes the reflected light beam parallel to the propagation axis. The silicon window 17 is adapted to allow the fundamental signal XI to pass through and to reflect the second harmonic generation (SHG) signal to the analyzer 19. The analyzer 19 is a visible-band polarizer mounted on a controlled-rotation stage, allowing the linear polarization states to be filtered according to its rotation. The motorized tilting mirror allows the beam to pass to the first detection system and / or to be deflected to the second detection system.

[0083] The first detection device includes a photomultiplier 24, a depolarizer 22, a bandpass filter 23 centered for example at X1 / 2 (515 nm) with a full width at half height of 40 nm and a third focusing lens 21.

[0084] The photomultiplier 24 is capable of amplifying the reflected SHG signal in order to detect it from a threshold optical power of less than a few pW. This received power depends on the peak power of the laser and the amplification of the detection. Each positive response from the photomultiplier 24 allows optimal determination that the focal point of the first incident beam has interacted with the surface of the sample 13.

[0085] By way of example, the depolarizer 22 and the bandpass filter 23 are added to select the nonlinear signal at 515 nm and uniformly redistribute depolarized light onto the photomultiplier 24. The intensity of the light is then measured by the photomultiplier, which converts the photons it receives from the reflected beam into an electrical signal. The photomultiplier 24 further includes an analog-to-digital converter adapted to convert the signals into digital values. The third focusing lens 21 focuses the reflected light beam onto the photomultiplier 24.

[0086] The second detection device comprises a collimating lens 25, a second optical fiber 26 and a visible spectrometer 27. The second reflected light beam is deflected by the tilting mirror 20 towards the collimating lens 25 which is connected to the spectrometer 27 by means of the second optical fiber 26. The spectrometer 27 records the reflected signal in the visible range.

[0087] The numerical values ​​acquired by the spectrometer 27 and the photomultiplier 24 are then transmitted to a data processing device 40.

[0088] According to one embodiment, the data processing device 40 includes a memory 41 configured to store the spatial coordinates of the sample support or the focal point corresponding to each measurement acquisition, by the first detection device of the first non-linear SHG signal and the second detection device of the second linear absorption signal in reflectivity.

[0089] The data processing module 40 further includes a control unit 42 configured to control the displacement device 39 and the various electromechanical components of the optical apparatus. The control unit 42, the memory 41, and the displacement device 39 are synchronized so as to automatically detect and record the exact positions of the displacement device motor in response to the detection of the first nonlinear SHG signal and the second linear absorption signal in reflectivity.

[0090] The data processing module 40 includes an image processing and construction unit 43 configured to calculate information from the signals transmitted by the two detection devices for each measurement point. By compiling the detection information and associated coordinates of each measurement point, the image processing and construction unit 43 is able to generate a map in which each pixel is representative of information from a measurement point in the sample.

[0091] The construction unit 43 receives information for each measured point whose position is known in the orthonormal coordinate system (XYZ) associated with the sample. Each pixel on the image that represents a measurement point of the sample is associated with at least one piece of information related to the detected signals.

[0092] The construction unit 43 further includes means for subtracting additive noise in order to increase the signal-to-noise ratio.

[0093] Advantageously, the measuring device of [Fig.1] allows, in a sequential manner, to carry out measurements of the nonlinear SHG signal and the absorption signal in reflectivity of the same place of a sample, without increasing the size of the device and the complexity of the measurement process.

[0094] By coupling information from the non-linear signal and information from the linear signal at the same measurement location of the sample, it is possible to offer a more precise and complete analysis of the surface of the analyzed sample than known analysis techniques that allow the detection of only one type of signal, either non-linear or linear.

[0095] By way of example, the nonlinear SHG signal allows the extraction of information concerning the intensity and nonlinear response of absorbing molecules grafted onto a surface (molecular monolayer). The linear absorption signal in reflectivity, both in p and s, allows the surface concentration of the molecules to be estimated, provided their molar extinction coefficient is known. By combining this information, the statistical orientation angle of the absorbing molecules grafted onto a sample surface is estimated.

[0096] With reference to [Fig.2], the optical detection device further includes an imaging system 50 which allows the user to view the probe area in a clear field.

[0097] In this embodiment, the optical apparatus comprises all the elements previously described, represented by the same numerical references as in [Fig. 1]. The optical elements of the bright-field imaging system are placed above the sample holder.

[0098] The imaging system 50 comprises an incoherent light source 51 of the diode type capable of emitting white light, i.e., broad-spectrum light covering the entire visible spectral range, enabling bright-field microscopy imaging. The system also includes a camera 56 capable of recording images.

[0099] The imaging system 50 further includes a third optical fiber 57, a collimation mirror 52, a beam splitter 53 and a lens 54, placed between the source 51 and the sample holder 30. The optical fiber, the mirror and the lens are placed on the optical path of the incident beam.

[0100] The mirror 52 and the lens 54 are arranged with respect to the surface of the sample 13 so that the white light is directed towards the surface of the sample at normal incidence. This arrangement allows visualization of the probing area over a field of 1.5 x 1 mm² while taking into account the size of the optical and mechanical elements of the device and the camera.

[0101] By way of example, the lens 54 is an xlO lens. The beam splitter operates in the spectral range from 350 nm to 1100 nm. The beam splitter 53 is configured so as to allow the white light emitted by the source to pass through in the Z direction, and to reflect a portion of the white light back in the X direction, towards the camera 56 in order to form images, as illustrated in [Fig. 3].

[0102] White light is directed by optical fiber 57 towards collimating mirror 52 to create a wide uniform beam of diameter 1 cm.

[0103] The white light brought by the optical fiber 57 is then reflected by the collimating mirror 52 in the Z direction towards the objective 54 after passing through the beam splitter 53. The objective 54 allows the white light to be focused onto the surface of the sample, at normal incidence.

[0104] Part of the light is then reflected by the surface, in the Z direction, towards the beam splitter 53, after passing through the lens 54. Thus, the reflected light travels the same optical path as the incident light in the opposite direction. The reflected light is redirected by the beam splitter 53 to a focusing lens 55. The reflected light is focused onto the camera to form images.

[0105] The camera 56 is connected to the control unit 40 to display the images taken by the camera. Thus, the user can observe the surface of the sample in real time. In particular, the brightfield imaging system 50 allows the user to identify areas of interest in order to then proceed in a second step to more sensitive analysis techniques with a higher spatial resolution: measuring the nonlinear second harmonic generation signal as a function of power, incident polarization, and output polarization, measuring the absorption signal in reflectivity in s or p polarization.

[0106] The field of view formed by the camera 56 is centered on the focal point of the first light beam and the focal point of the second incident light beam, which are represented by spots labeled 71 and 72, respectively, as illustrated in [Fig. 4]. These spots 71 and 72 represent, respectively, the pixel of the image formed by the reflected SHG signal and the pixel of the image formed by the reflected linear absorption signal. Their diameter is directly related to the spatial resolution of the imaging technique used. The diameter of the image area 70, which is approximately 1.5 x 1 mm², is larger than the focal point of the first beam. bright and at the focal point of the second incident light beam. Camera 56 therefore allows observation of the probing area and recording of images of this same area

[0107] Thanks to the imaging system which allows visualization of the surface of the sample in bright field, it is possible to superimpose the pixels to probe at the same measurement point of the sample.

[0108] Since the positions of the focal points in the X, Y plane are known and determined by the sample holder 30, it is therefore possible to correlate the information extracted from the bright field observation, the SHG signal and that of the absorption by reflection.

[0109] The technique for detecting the nonlinear SHG signal is described below with reference to [Fig. 5], which shows only the elements of the optical apparatus used to detect the nonlinear SHG signal and the optical path of the first incident beam and the first reflected beam. The optical apparatus is used in a reflected SHG signal detection configuration (R-SHG).

[0110] The first source 2, which is here a pulsed infrared laser (1025 nm, 250 fs, 54 MHz), emits a first incident laser beam which has a wavelength of 1025 nm, a pulse of 250 fs and a pulse frequency of 51 MHz.

[0111] The second mirror 3 deflects the first incident beam towards the first mirror 7, passing successively through the shutter 36, the neutral density filter 4, and the Glan-Taylor polarizer 5. The shutter 36 allows the beam to pass through or be blocked on command. The power of the first incident beam is variable using the neutral density filter 31. A Glan-Taylor polarizer (PGT) filters the incident polarization to obtain a 99% linear output polarization.

[0112] The first mirror 7 directs the beam at an angle of 70° to the plane of incidence YZ, which then passes through the rotating zero-order half-wave plate 11 centered at 1030 nm, and the fixed zero-order quarter-wave plate 12 centered at 1030 nm. The rotation of the LDO (angle denoted W) rotates the polarization plane by 2W around the optical propagation axis to generate any linear polarization at the desired angle. The combination of the two optical components 11 and 12 allows for the generation of all polarization states: horizontal linear (W = 0°), vertical linear (W = 45°), elliptical, right-handed circular, and left-handed circular (W = ± 22.5°).

[0113] Upon exiting the fixed quarter-wave plate 12, the first incident beam passes through the high-pass filter 14, the cutoff frequency of which is 950 nm. Thus, the first beam is spectrally filtered so as to allow only infrared to pass through.

[0114] The first incident beam is then focused onto the surface of the sample using the focusing lens 15.

[0115] The interaction of the first near-infrared laser beam with the sample surface generates linear and nonlinear optical effects that are collected at double the frequency after reflection, always at 70° to the plane of incidence. Grazing incidence reflection is enhanced, as the reflection coefficients increase with the angle of incidence beyond the Brewster angle for dielectrics, and surface photon-plasmon coupling for metals and semiconductors becomes very efficient (beyond 60-80°).

[0116] Reflection coefficients increase drastically after Brewster's angle, hence the main interest in performing grazing angle analyses. However, this concept has never yet been applied in second harmonic generation (R-SHG) reflection due to the difficulty of collecting the low intensity of the reflected signal.

[0117] The focal point on the surface of a sample where nonlinear effects will appear is very sensitive to positioning. The present disclosure proposes a perfectly aligned detection system using specifically chosen optical components placed in a thoughtful design, thus optimizing detection.

[0118] The first reflected beam is collected by the collecting arm 71 placed in the optical path of the reflected light, where a second focusing lens 16 makes the light parallel to the propagation axis. The first reflected beam passes successively through the silicon window 17, which reflects the SHG signal towards the photomultiplier tube 24, and the motorized polarizer 19 covering the visible spectrum, which filters the linear polarization states according to its rotation. A final lens 21 is placed in front of the detector to restrict the size of the SHG signal beam before it reaches the depolarizer 22, and then the photomultiplier tube 24.

[0119] In the configuration where the optical device is used to detect the SHG signal, the mirror 20 and the polarization controller 11 are moved so as not to be positioned on the optical path of the first beam.

[0120] Advantageously, the optical device of [Fig.1] and [Fig.2] can be used to operate in two distinct modes of operation, in point mode or in mapping mode for the detection and analysis of the SHG signal.

[0121] In point mode, the sample 13 remains fixed during the measurement at a measurement point. The intensity of the SHG signal (I2“) is measured by varying the incident power (I“) using the neutral density filter 4, the incident polarization angle W using the rotating half-wave plate 11, and the analysis angle <e>using the fixed quarter-wave plate 12.

[0122] Three acquisition methods are derived from these parameters in order to qualify different contrasts: - A first acquisition method consists of measuring and analyzing I2" as a function of power (PWR-Scan) when the incident and analysis polarizations are fixed (angles W and <e>constants) and that the incident power is gradually increased. The result obtained is an R-SHG intensity curve that varies quadratically with respect to the incident power. This provides information on the intensity of the nonlinear response of the material under study. A quantitative response hierarchy is thus established with a reference sample; - A second acquisition mode consists of varying the incident polarization (angle W) when the incident power (I") and the analysis polarization (μ) are constant. This W-Scan is sensitive to symmetry (or to a defect, a symmetry breaking) and to the nature of the material being analyzed, which can be metal, a semiconductor, a dielectric, organized (crystal) or amorphous (glass), etc., by probing the susceptibility components involved in the scan. Furthermore, exploiting this information at a more complex level allows the determination of structural parameters. For example, the degree of isotropy or anisotropy of the surface and the statistical orientation of molecules on the surface for molecular monolayers. - A third acquisition mode consists of varying the analysis polarization (angle μ) when the incident power (I") and the incident polarization (W) are constant. This <e>-Scan allows for the precise characterization of the R-SHG response polarization states through the determination of Stokes parameters, in particular. As a complement to W-Scan, the <e>-Scan can generate unique contrasts and allow for the complete characterization of the ellipsometric response of the reflected beam. Only the absolute left- or right-handed nature of the elliptical (or circular) polarization remains undefined in this case. A more detailed description of the theory associated with the generalized ellipsometric analysis of a reflected SHG signal on a linear and / or nonlinear optical medium is described in the paper by V. Rodriguez and C. Sourisseau, J. Opt. Soc. Am. B 19, 2650 (2002), the paper by V. Rodriguez, J. Chem. Phys. 128, 064707 (2008) and the paper by V. Rodriguez, D. Verreault, F. Adamietz, and A. Kalafatis, ACS Photonics 9, 2510 (2022).

[0123] In R-SHG mapping mode, the sample is spatially displaced while the incident power F, the incident angle W and the analysis angle <e>are constant. The real-time absolute position is determined by the (X,Y) coordinates of the sample holder (Z being constant for focusing on the surface), which is stored in 4L memory. The SHG I2" signal is measured point by point according to user-defined dimensions and steps. The mapping illustrates the degree of surface homogeneity based on the sample's nonlinear response.

[0124] The technique for detecting the linear absorption signal in reflectivity is described below with reference to [Fig. 6], which shows only the elements of the optical apparatus used to detect the linear signal and the optical path of the second incident beam and the second reflected beam. The optical apparatus is used in a reflection-absorption (Vis-RAS) configuration.

[0125] In the Vis-RAS configuration, the second light source 29 emits a second visible light beam which is injected into the optical fiber 28. The achromatic optical fiber collimator 6 collimates the second beam onto the first mirror 7. The second beam then passes through the linear polarizer 9, covering wavelengths from 400 nm to 700 nm, to be polarized to a target polarization state. Here, horizontal "s" and vertical "p" polarizations are considered.

[0126] As in the SHG signal detection configuration, the reflected light is deflected by the silicon window 17 towards the analyzer 19, which filters out the linear component of the incident polarization state. The mirror 20 is placed in the optical path to direct the reflected beam to the focusing lens 25, which connects to the UV-Vis-NIR spectrometer 27 via an optical fiber 26. The reflected signal is collected in the visible range.

[0127] Advantageously, the optical device of [Fig.1] and [Fig.2] can also be used to operate in two distinct modes of operation, in point mode or in mapping mode for the detection and analysis of the linear signal in Vis-RAS configuration.

[0128] In Vis-RAS point mode, the sample remains stationary at all times while the reflected absorption (A) is measured. Analysis of the visible absorbance-reflectance spectrum of the surface is possible when the input polarizations are fixed, specifically in "s" polarization (As) and "p" polarization (Ap). The absorption anisotropies along these two polarizations are measured using this acquisition method at any wavelength in the visible (X) range.

[0129] In Vis-RAS mapping mode, the sample is spatially displaced while the incident and analysis polarization states remain constant and identical, either in "s" or "p" polarization. The real-time absolute position is determined by the (X, Y) coordinates of the sample holder. Reflectance spectra are measured across the entire visible (X) range, point by point, according to user-defined dimensions and steps. The mapping illustrates surface homogeneity based on sample absorption and wavelength (X).

[0130] Thanks to the specific architecture of the optical device of the present disclosure, it is possible to implement two correlated analysis methods since it is possible to probe the same region of the surface by combining R-SHG and Vis-RAS analyses. These sequential bimodal measurements allow the information to be correlated obtained by one or the other of the acquisition methods. As a reminder, in point mode, R-SHG analysis collects the I2" signal according to the parameters I", W, <e>and Vis-RAS analysis collects the surface absorbance according to a fixed polarization, Ap and As, according to the X wavelength. In mapping mode, the analysis of I2", Ap and As is performed spatially.

[0131] The light spot 71, 72 that reaches the surface of the sample represents the pixel of the analysis under consideration and is related to the spatial resolution of the technique, whether R-SHG or Vis-RAS. It is possible to superimpose the R-SHG and Vis-RAS pixels to probe the same location. Furthermore, using the imaging system 50, it is possible to visualize the sample in the bright field. Since the X, Y positions are known and determined by the sample holder 30, it is possible to correlate the mapping information between what is observed in the bright field and what is measured by R-SHG and Vis-RAS.

[0132] The method for this correlative spectroscopy comprises the following steps: - generate a normal incidence white light image of a sample; - generate a grazing incidence image of second harmonic signal; - generate a grazing incidence image of absorption signal in reflectivity; - superimpose the three images thus obtained.

[0133] The three imaging techniques allow for detailed analysis of the sample surface and the acquisition of additional information. Industrial application

[0134] The apparatus of the present disclosure can be applied, in particular, in the field of characterizing the surface of a sample such as a semiconductor component. The information obtained by the apparatus of the present invention makes it possible to determine information on the structure and texture of the observed surface.

[0135] The information obtained from the second harmonic generation (SHG) signal as a function of the incident polarization makes it possible to characterize the degree of isotropy and anisotropy (and texturization and / or contrast) of the surface of the sample.

[0136] The information obtained from the SHG signal as a function of the output polarization of the analyzer makes it possible to characterize, for example, the potential phase shift of the measurement, namely the achiral or chiral character.

[0137] The information obtained from the absorption signal in reflectivity in p or s polarization makes it possible to detect contaminants on the surface of the sample and to potentially estimate the concentration of a given molecule on the surface.

[0138] SHG signal mapping and absorption signal mapping in reflectivity make it possible to illustrate surface homogeneity according to the non-linear response of the sample, and in particular the presence of defects on the surface of the sample.

[0139] The same device is used to probe an area of ​​interest of a component to generate second harmonic signal images and absorption reflection images which constitute complementary information on the structure of the sample, thus providing a more precise and complete analysis.< / e> < / e> < / e> < / e> < / e> < / e>

Claims

1. Demands Optical measuring device (1) for characterizing the surface of a sample (13), comprising - a first source (2) adapted to emit a first incident excitation light beam at a first excitation wavelength (XI); - a second source (29) adapted to emit a second incident excitation light beam at a second excitation wavelength (X2) different from said first excitation wavelength; - a sample holder (30) suitable for receiving the sample (13); - a projection arm capable of sending the first light beam and the second light beam onto a measurement point on the surface of the sample (13) along a common optical path, said arm comprising a first mirror (7) arranged to receive said first incident light beam and said second incident light beam and to reflect them towards the surface of the sample (13), a focusing device (15) configured to generate a focal point on the surface of the sample and a polarizing device (8) placed between the first mirror (7) and the focusing device (15) to generate a number of polarization states of the first incident light beam and a linear polarization state of the second incident beam; - a first detection device configured to record a first second harmonic generation (SHG) signal having a detection wavelength equal to half the excitation wavelength associated with the first light beam reflected by a measurement point on the surface of the sample (13); - a second detection device configured to record a second linear absorption signal in reflectivity associated with the second light beam reflected by a measurement point on the surface of the sample (13); - a collection arm capable of collecting the first light beam reflected and the second light beam reflected by the surface of the sample and transmitting them respectively to the first detection device and said second detection device; - a data processing module (40) configured to determine structural information from the first non-linear SHG signal and the second linear signal transmitted respectively by the first detection device and the second detection device for the same measurement point on the surface of the sample (13).

2. Apparatus according to claim 1, wherein the first mirror (7) is arranged so that the first incident beam and the second beam are incident on the surface of the sample with an oblique angle of incidence between 65° and 80° with respect to the plane of incidence.

3. Apparatus according to claim 1 or 2, further comprising: - a first optical device arranged between said first source (2) and the first mirror (7) and configured to project the first light beam onto the first mirror (7); - a second optical device arranged between said second source (29) and the first mirror (7) and configured to project the second light beam onto the first orientation mirror (7).

4. Apparatus according to claim 3, wherein the first optical device comprises a second mirror (3), a shutter (36) switchable between a passing position allowing the first incident beam to pass and a blocking position allowing the first incident beam to be blocked, a movable density filter (4) capable of varying the power of the first incident light beam between 0.01% and 100% of the total power, and a polarization state controller (5) placed successively on the optical path of the first incident beam between the first source (2) and the first mirror (7).

5. Apparatus according to claim 3 or 4, wherein the second optical device comprises an optical fiber (28) having a first end coupled to the output of the second source (29) and a second end coupled to a movable collimator (6), said collimator being capable of collimating the second light beam at the output of the optical fiber onto the first mirror (7).

6. Apparatus according to any one of claims 1 to 5, wherein said polarization device (8) comprises: - a first movable polarization module (10) to be placed on the optical path of the first incident beam between the first mirror (7) and the focusing device (15) and configured to generate a number of polarization states of the first incident beam, and - a second movable polarization module (9) to be placed on the optical path of the second incident beam between the first mirror (7) and the focusing device (15) and configured to generate a linear polarization state of the second incident light beam.

7. Apparatus according to claim 6, wherein the first polarization module (10) comprises a rotating half-wave plate (11) about an optical propagation axis to generate linear polarization at a desired angle and a fixed quarter-wave plate (12) having a fast axis perpendicular to the optical propagation axis of the half-wave to generate elliptic and circular polarization states in association with the half-wave plate (11).

8. Device according to claim 7, wherein the second movable polarization module (9) comprises a horizontal linear polarizer capable of polarizing the second incident light beam into a horizontal polarization state “s” and a vertical linear polarizer capable of polarizing the second incident light beam into a vertical polarization state “p”.

9. Apparatus according to any one of claims 1 to 8, wherein the collection arm is arranged symmetrically to the projection arm with respect to the plane of incidence, said collection arm comprising a second focusing device (16), a silicon window (17), a low-pass filter (18), a rotating polarization state analyzer (19) and a tilting mirror (20) arranged successively on the optical path of the first light beam and the second light beam reflected at the measurement point of the sample surface, said tilting mirror (20) being configured to allow the first light beam reflected by the sample surface to pass towards the first detection device and to deflect the second beam reflected by the sample surface towards the second detection device.

10. Apparatus according to any one of claims 1 to 9, wherein the first source (2) is a laser source adapted to emit a first incident excitation light beam at a first excitation wavelength between 1000 nm and 1100 nm, with a pulse between 100 fs and 600 fs, and a frequency between 1 MHz and 100 MHz.

11. Apparatus of any one of claims 1 to 10, wherein the second source (29) is a tungsten lamp adapted to emit a second incident excitation light beam at a second excitation wavelength between 400 nm and 700 nm.

12. Apparatus according to any one of claims 1 to 11, wherein the first optical focusing device comprises a focusing lens (15) capable of focusing the first light beam and the second light beam into a focal point of diameter between 2 pm and 50 pm on the surface of the sample.

13. Apparatus according to any one of claims 1 to 12, further comprising a displacement device (39) coupled to the sample holder (30), said displacement device being configured to move the sample holder (30) in a horizontal plane along a first horizontal direction along the X axis, a second horizontal direction along the Y axis, a third vertical direction along the Z axis, so as to move the sample holder (30) relative to the focal point to form a plurality of measurement points on the surface of the target sample.

14. Apparatus according to any one of the preceding claims, further comprising an imaging system (50) including: - a third light source (51) capable of emitting white light; - a third optical fiber (57) having one end coupled to the output of the third light source (51); - a collimating mirror (52) capable of collimating the white light at the output of the third optical fiber onto a beam splitter (53); - a focusing lens (54) capable of focusing the white light onto the focal point of the first incident light beam and the second incident light beam; - a camera (56);- said beam splitter (53) being arranged between the collimating mirror (52) and the focusing lens (54) so ​​as to allow white light to pass through in a direction of incidence normal to the surface of the sample (13) and to reflect a part of the white light reflected by the surface of the sample back to the camera in order to form images on the focal point.;

15. Apparatus according to any one of claims 1 to 14, wherein the data processing module (40) includes a memory (41) configured to store the spatial coordinates of the sample carrier or focal point corresponding to each measurement acquisition, by the first detection device and the second detection device of the first nonlinear SHG signal and the second linear absorption signal in reflectivity.

16. Apparatus according to any one of claims 1 to 15 and claim 13, wherein the data processing module (40) comprises a control unit (42) configured to control the displacement device (39), an image processing and construction unit (43) configured to generate images in which each pixel is representative of information from a measurement point of the sample (13).

Images