CN113777048B - Coaxial ultrafast spectrum ellipsometer and measurement method - Google Patents
Coaxial ultrafast spectrum ellipsometer and measurement method Download PDFInfo
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Abstract
The invention belongs to the technical field of optical measurement and discloses a coaxial ultrafast spectrum ellipsometer and a measurement method. The ellipsometer comprises an illumination light path unit and a spectrum acquisition unit, wherein the illumination light path unit is used for carrying out polarization and phase modulation on a source light beam emitted by a light source to generate a mixed light beam which comprises two components with mutually perpendicular polarization directions and a fixed phase difference, and then the mixed light beam is projected onto the surface of a sample to be detected; the spectrum acquisition unit acquires light beams reflected from the surface of a sample to be detected, and performs polarization demodulation and spectrum dispersion, so that an interference spectrum is obtained; the illumination light path unit is provided with a polarization interference modulation module for forming a mixed light beam of two components having polarization directions perpendicular to each other and having a fixed phase difference. The invention solves the problems of low transverse resolution, low measurement speed and the like of the traditional ellipsometry method, and realizes the ultra-fast ellipsometry of the ultra-thin nano film with small light spot size and wide incidence angle.
Description
Technical Field
The invention belongs to the technical field of optical measurement, and particularly relates to a coaxial ultrafast spectrum ellipsometer and a measurement method.
Background
The information of the structure, the composition, the thickness and the like of a material or a thin layer can be revealed by utilizing the polarization characteristic of an electromagnetic wave vector, and an ellipsometer (ellipsometer for short) is an optical measuring instrument for obtaining the information of a sample to be measured by utilizing the principle. A typical spectroscopic ellipsometer generally comprises an obliquely arranged illumination arm comprising a broadband light source and a polarization state modulator, the illumination arm being configured to be incident on a sample surface at a specific angle of incidence (AOI), and a polarization analysis arm to collect and modulate the light beam reflected from the sample surface to obtain the amount of change in the polarization state of the incident polarized light (including amplitude ratio and phase difference) of the sample to be measured, thereby inverting the relevant information of the sample to be measured. A typical spectroscopic ellipsometer changes the polarization state of a light beam based on a rotating polarizing device (polarizer or compensator), an optical phase modulator, a birefringent liquid crystal system, and obtains the amount of change in polarization state caused by a sample to be measured by analyzing the time series of the acquired intensity spectrum during at least one modulation period. The measurement method based on time polarization modulation has the inherent defects that the measurement accuracy is greatly influenced by the intensity of a light source and the instability of a rotating component, the measurement time is limited by the polarization modulation period and the like, and is difficult to be used for real-time measurement under actual production conditions.
The film preparation process occupies an increasingly important position in the semiconductor and display processes, and in the film preparation process, the real-time accurate measurement of the film thickness and refractive index plays a vital role in improving the process yield; meanwhile, as the measurement target size decreases, measurement techniques and instruments having smaller spot sizes are required; obviously, conventional ellipsometers have been difficult to apply to the above-described scenarios. In order to realize ultra-fast polarization parameter measurement, an interference snapshot ellipsometer based on a double-channel sensing scheme and a channel spectrum snapshot ellipsometer adopting a plurality of wave plates are presented; however, the interference snapshot ellipsometer based on the double-channel sensing scheme has the defects of complex principle, heavy device and higher hardware cost; the channel spectrum snapshot ellipsometer adopting a plurality of thick wave plates has higher original signal spectrum complexity, more complex and time-consuming signal processing, and the available spectrum range is limited by the wave plates. To achieve small spot size measurements, ellipsometry with objective lens placed in front of the sample and coaxial arrangement has been presented, but such instruments often still use conventional polarization modulation methods. In order to solve the problems that the angle configuration of an illumination arm of a traditional ellipsometer is complex, and a plurality of incident angle information cannot be obtained by single measurement, a measuring instrument based on an angle resolution technology or a back focal plane imaging technology is provided. However, there are still few spectroscopic ellipsometry instruments that can comprehensively consider and provide a solution to the above problems.
Disclosure of Invention
Aiming at the defects or improvement demands of the prior art, the invention provides a coaxial ultrafast spectrum ellipsometer and a measurement method, solves the problem of obtaining a plurality of angle information by single measurement, and realizes the ultrafast ellipsometry measurement of the ultrathin nanometer film with small light spot size and wide incidence angle.
To achieve the above object, according to one aspect of the present invention, there is provided a coaxial ultrafast spectroscopic ellipsometer comprising an illumination light path unit and a spectrum acquisition unit, wherein:
the illumination light path unit is used for carrying out polarization and phase modulation on a source light beam emitted by the light source, generating a mixed light beam which comprises two components with mutually perpendicular polarization directions and with a fixed phase difference, and then projecting the mixed light beam onto the surface of a sample to be detected; the spectrum acquisition unit is used for acquiring a light beam reflected from the surface of a sample to be detected, and carrying out polarization demodulation and spectrum dispersion so as to obtain an interference spectrum;
the illumination light path unit is provided with a polarization interference modulation module, the polarization interference modulation module comprises a first non-polarization beam splitter, a second polarizer, a first plane mirror, a third polarizer and a second plane mirror, the first non-polarization beam splitter is used for splitting light into two beams, one beam enters the second polarizer and the first plane mirror arranged behind the second polarizer, the other beam enters the third polarizer and the second plane mirror arranged behind the third polarizer, the polarization axis directions of the second polarizer and the third polarizer are mutually perpendicular, the original path of the two beams returns to the first non-polarization beam splitter after passing through the first plane mirror and the second plane mirror, and the first non-polarization beam splitter merges the two beams into a mixed beam with two components, namely, a stationary phase with two components, wherein the polarization directions of the mixed beam are mutually perpendicular, and the stationary phase is different.
Still preferably, the spectrum acquisition unit includes an objective lens, a first tube lens, a second unpolarized beam splitter, a fourth polarizer, a second tube lens and an imaging spectrometer, wherein the second unpolarized beam splitter is disposed at the rear of the polarization interference modulation module, the mixed beam enters the second unpolarized beam splitter and then enters the first tube lens, the first tube lens is disposed between the second unpolarized beam splitter and the objective lens, and is used for converging the collimated beam onto the back focal plane of the objective lens, the objective lens is disposed above the sample to be measured, and is used for projecting the beam onto the surface of the sample to be measured, the fourth polarized beam splitter is disposed above the second unpolarized beam splitter, and is used for splitting the beam and respectively guiding the beam into the imaging spectrometer and the auxiliary imaging module, the second tube lens is disposed above the second unpolarized beam splitter, and is used for converging the collimated beam onto the slit plane of the imaging spectrometer, and the imaging spectrometer is disposed above the second tube lens, and is used for dispersing the reflected beam to generate the interference spectrum. Further preferably, the focal plane of the first tube lens coincides with the back focal plane of the objective lens, the focal plane of the first tube lens coincides with the focal plane of the second tube lens, and the focal plane of the second tube lens coincides with the slit plane of the imaging spectrometer.
Still preferably, the spectrum acquisition unit is further provided with an auxiliary imaging module, which is used for observing the imaging of the back focal plane of the high numerical aperture objective lens to judge the focusing condition of the sample to be measured, and comprises a third unpolarized beam splitter, a third tube lens and an area array camera, wherein the third unpolarized beam splitter is arranged between the fourth polarizer and the second tube lens, the third tube lens is arranged at the rear of the third unpolarized beam splitter and is used for converging light on the pixel plane of the area array camera, and the area array camera is arranged at the rear of the third tube lens and is used for observing the fourier image of the back focal plane of the objective lens.
Further preferably, a light source, a collimator lens, a first polarizer and a diaphragm are further disposed above the polarization interferometric modulator module in the illumination light path unit, the light source emits a source light beam, the collimator lens is disposed behind the light source for making the divergent light beam into a collimated light beam, the first polarizer is used for converting unpolarized light into linearly polarized light, and the diaphragm is used for limiting the aperture of the light beam.
Further preferably, a first optical shutter and a second optical shutter are disposed in two optical paths of the polarization interferometric modulator module, the first optical shutter being disposed between the first unpolarized beam splitter and the second polarizer, and the second optical shutter being disposed between the first unpolarized beam splitter and the third polarizer.
Further preferably, the polarization axis of the first polarizer is at an angle of 45 degrees to the direction of the imaging spectrometer slit.
Further preferably, the polarization axis of the second polarizer is at an angle of 0 degrees to the direction of the imaging spectrometer slit, and the polarization axis of the third polarizer is at an angle of 90 degrees to the direction of the imaging spectrometer slit
Further preferably, the ellipsometer is further provided with a sample adjusting module, and the sample adjusting module is used for adjusting the relative position between the sample to be measured and the objective lens.
According to another aspect of the present invention, there is provided a method for measuring a coaxial ultrafast spectroscopic ellipsometer as described above, comprising the steps of:
s1, calibrating the coaxial spectrum ellipsometer by using a standard reflector, so as to obtain a light intensity spectrum signal alpha when the coaxial ultrafast spectrum ellipsometer measures the standard reflector 2 、β 2 And I ref By means of I ref Calculating spectral coherence function gamma caused by coaxial ultrafast spectroscopic ellipsometer ref And spectral phase function phi ref ；
S2, placing the sample to be measured on a sample table, and opening a light source and all optical shutters to measure to obtain a light intensity interference spectrum signal I of light reflected back by the sample to be measured sam The method comprises the steps of carrying out a first treatment on the surface of the Analyzing and processing the light intensity interference spectrum signal of the sample to be detected, and combining the instrument calibration result to obtain the polarization state parameters psi and delta of the reflected light of the sample to be detected;
and S4, fitting the polarization state parameter obtained by measurement with a theoretical expression of the polarization state parameter deduced by a theoretical model of the sample to be measured, so as to obtain a film parameter vector of the sample to be measured.
In general, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
1. the invention utilizes the polarization interference modulation module to generate a mixed light beam which simultaneously comprises a p-polarization component and an s-polarization component and has a fixed phase difference, irradiates a sample to be detected by utilizing the mixed light beam and acquires reflected light to obtain a single-frame reflection interference spectrum, thereby being capable of rapidly obtaining the reflection polarization parameter of the sample to be detected by utilizing methods such as Fourier analysis and the like;
2. according to the invention, the angle resolution measurement based on a coaxial system is realized by combining a high-numerical aperture objective lens with a Kohler illumination mode, a two-dimensional spectrum containing reflected light interference spectrum information corresponding to the incident angle of incident light irradiating a sample to be measured can be obtained simultaneously in single measurement, and the ellipsometric amplitude ratio and ellipsometric phase difference information corresponding to all incident angles in the numerical aperture range of the objective lens can be calculated by combining the two-dimensional spectrum with the measurement method;
3. the invention provides a coaxial ultrafast ellipsometer capable of measuring an ultrathin film produced by a large-area module in real time, which does not need a polarization modulation mode based on time sequences, such as a rotating polarization element, an optical phase modulator, a birefringent liquid crystal system and the like, and the polarization parameter of a sample to be measured is accurately obtained only through a single-frame interference spectrum, so that the measurement speed of microsecond level is realized, and ultrafast ellipsometry measurement is realized;
4. the coaxial optical path structure of the high numerical aperture objective lens is introduced, so that the problems of larger light spot size, low transverse resolution, narrow view field, easiness in vibration influence on measurement accuracy and the like of the traditional inclined ellipsometer are avoided while the measurement accuracy is improved;
5. the device provided by the invention has a simple and compact structure and is easy to debug. Meanwhile, the method has great expandability, and can be combined with different measurement objects to perform configuration optimization.
Drawings
FIG. 1 is a schematic diagram of a coaxial ultra-fast spectroscopic ellipsometer according to a preferred embodiment of the present invention;
FIG. 2 is a schematic diagram of a structure and principles of a polarization interferometric modulation module provided in accordance with a preferred embodiment of the present invention;
FIG. 3 is a schematic illustration of the measurement principle of a coaxial ultra-fast spectroscopic ellipsometer provided in accordance with a preferred embodiment of the present invention;
figure 4 is a schematic diagram of BFP surface versus sample surface light incidence angle provided in accordance with a preferred embodiment of the present invention.
The same reference numbers are used throughout the drawings to reference like elements or structures, wherein:
1-light source, 2-collimating lens, 3-first polarizer, 4-diaphragm, 5-first unpolarized beam splitter, 6-first optical shutter, 7-second polarizer, 8-first plane mirror, 9-second optical shutter, 10-third polarizer, 11-second plane mirror, 12-second unpolarized beam splitter, 13-first tube lens, 14-objective lens back focal plane, 15-objective lens, 16-sample to be measured, 17-sample adjustment module, 18-fourth polarizer, 19-third unpolarized beam splitter, 20-second tube lens, 21-imaging spectrometer slit, 22-imaging spectrometer, 23-third tube lens, 24-planar array camera, 100-polarized interferometric modulation module, 101-linearly polarized beam, 102-first sub-beam, 103-second sub-beam, 104-first collimated beam, 105-second collimated beam, 200-auxiliary imaging module, 300-plane of incidence.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, the technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
Referring to fig. 1, 2 and 3, the coaxial ultrafast ellipsometer provided by the invention comprises an illumination light path unit, a sample adjusting module 17, a spectrum acquisition unit and an auxiliary imaging module 200; the illumination light path unit is used for carrying out polarization and phase modulation on a source light beam emitted by an external light source, generating a mixed light beam which contains two components with mutually perpendicular polarization directions and a fixed phase difference, and projecting the mixed light beam to the surface of a sample to be detected; which comprises a light source 1, a collimator lens 2, a first polarizer 3, a diaphragm 4, and a polarizing interferometric modulation module 100 arranged in this order.
The polarization interference modulation module comprises a first unpolarized beam splitter 5, a first optical shutter 6, a second polarizer 7, a first plane mirror 8, a second optical shutter 9, a third polarizer 10 and a second plane mirror 11; the polarization interference modulation module comprises a precise displacement regulating device which is used for driving the first plane reflecting mirror or the second plane reflecting mirror to generate linear displacement and controlling the optical path difference of two beams of light with different polarization states.
The optical elements in the spectrum acquisition unit are coaxially arranged, and are used for acquiring the light beam reflected from the surface of the sample to be detected, carrying out polarization demodulation and spectrum dispersion, so as to obtain an interference spectrum, and the interference spectrum comprises a second unpolarized beam splitter 12, a first tube lens 13, a high numerical aperture objective lens 15, a fourth polarizer 18, a second tube lens 20, an imaging spectrometer 22 and an auxiliary imaging module 200, and the optical axes of the two are arranged together; the reflected light from the sample surface forms a fourier image at the back focal plane of the high numerical aperture objective lens, and forms a frequency domain interference image at the slit plane of the imaging spectrometer by sequentially passing through the first tube lens, the second unpolarized beam splitter, the third polarizer and the second tube lens, and the imaging spectrometer disperses the image spectrum at the slit to form an angle-resolved interference spectrum.
The auxiliary imaging module comprises a third unpolarized beam splitter 19, a third tube lens 23 and an area array camera 24. The second unpolarized beam splitter not only realizes the reflective propagation of the light beam in the illumination light path unit, but also realizes the transmissive propagation in the spectrum acquisition unit.
The light source 1 emits a source light beam, preferably in a spectrally extended wavelength range, for example a broadband halogen illumination light source of 400-800nm. The source beam is collimated by the collimator lens 2 to obtain a collimated beam, and the collimated beam is then transformed into a linearly polarized beam 101 by the first polarizer 3, and the linearly polarized beam 101 enters the polarization interference modulation module 100 after passing through the diaphragm 4. The light source comprises a halogen lamp, an LED lamp, a xenon lamp and other broadband light sources.
Further, the polarization axis of the first polarizer 3 is at an angle of 45 degrees to the direction of the imaging spectrometer slit 21, the direction of the imaging spectrometer slit 21 defining the plane of incidence 300 of the acquired light beam, only light beams within this plane of incidence 300 being acquired by the spectrum acquisition unit.
The linearly polarized light beam 101, after entering the polarization interferometric modulation module 100, is first split into two sub-beams, a first sub-beam 102 and a second sub-beam 103, by the first non-polarizing beam splitter 5. The polarization axis of the second polarizer 7 forms an angle of 0 degrees with the direction of the imaging spectrometer slit 21, the first sub-beam 102 reaches the first plane mirror 8 after passing through the second polarizer 7 and is reflected, and the first collimated beam 104 with the vibration direction parallel to the incident plane 300 is formed after passing through the second polarizer 7 again. The polarization axis of the third polarizer 10 forms an angle of 90 degrees with the direction of the imaging spectrometer slit 21, the second sub-beam 103 reaches the first plane mirror 11 after passing through the second polarizer 10 and is reflected, the second collimated beam 105 with the vibration direction perpendicular to the incident plane 300 is formed after passing through the second polarizer 10 again, and the optical path difference Δz=2 (z 1 -z 2 ). The two collimated light beams, namely the first collimation 104 and the second collimation 105, are combined into a light beam 106 after passing through a first unpolarized beam splitter, and the light beam 106 is composed of two components with mutually perpendicular vibration directions and fixed phase difference, and the two components are respectively a p component and an s component relative to an incident plane 300. The plane of incidence is a plane parallel to the slit direction.
Further, as a non-limiting example, the first plane mirror 8 or the second plane mirror 11 is followed by a precision displacement adjustment device for adjusting the optical path difference between the first collimated light beam 104 and the second collimated light beam 105.
The mixed beam 106 is redirected by the second non-polarizing beam splitter 12 and focused by the first tube lens 13 onto the objective lens back focal plane 14 of the high numerical aperture objective lens 15, and then irradiated to the surface of the sample 16 to be measured by the high numerical aperture objective lens 15.
FIG. 4 schematically shows the relationship between the incidence angle of the light beam from the same point of the back focal plane 14 of the objective lens and the surface of the sample to be measured 19 in the incidence plane 300, and the light beam reflected from the sample to be measured 16 at the same angle will be converged by the same incidence angleFocusing at the same point as the back focal plane 14 of the high numerical aperture objective lens. Thus, the different positions of the line signal selected by the imaging spectrometer slit 21 correspond to different angles of incidence of the light, in particular satisfying the following relationship: θ=sin -1 (d/d max X NA). Where NA is the numerical aperture, d, of the high numerical aperture objective 15 max Is the maximum radius of the spot on the back focal plane 14 of the high numerical aperture objective lens.
In the sample adjustment module 17, the sample 16 to be measured is mounted on a rotary displacement stage by a sample holder, and the rotary displacement stage is mounted on a linear displacement stage. The sample adjusting module is used for adjusting the position of the sample 16 to be measured, so that the front focal plane of the high numerical aperture objective 15 coincides with the surface of the sample to be measured.
The light reflected by the surface of the sample 16 to be measured is collected by the high-numerical aperture objective lens and then imaged in the back focal plane 14 of the high-numerical aperture objective lens, and the fourier image sequentially passes through the first tube lens 13, the second unpolarized beam splitter 12, the fourth polarizer 18 and the second tube lens and then enters the imaging spectrometer slit 21. The high numerical aperture objective lens 15 coincides with the focal plane of the first tube lens 13, the first tube lens 13 and the second tube lens 20 in pairs, and the focal plane of the second tube lens 20 coincides with the high numerical aperture objective lens slit plane 21.
The polarization axis of the fourth polarizer 18 forms an angle of 45 degrees with the incident plane 300, which converts the reflected light beam into linearly polarized light, and the linearly polarized light enters the imaging spectrometer 22 to perform spectral dispersion and then interfere in the spectral dimension, so as to obtain the angle-resolved interference spectrum of the reflected light of the sample to be measured.
Preferably, one example of an imaging spectrometer is the HORIBA company spectrometer iHR or iHR, 350, which can ensure spectral scattering and focusing of the spectrum on the image sensor based on the use of a concave diffraction grating corrected from chromatic aberration.
The auxiliary imaging module 200 is used for observing the high-numerical-aperture objective lens back focal plane imaging so as to judge the focusing condition of the sample to be measured, and the photosensitive chip of the area array camera 24 is positioned above the back focal plane of the third tube lens 23 so as to obtain clear imaging of the high-numerical-aperture objective lens back focal plane 14.
Preferably, the split ratio of the non-polarizing beam splitters 5, 12, 19 is 50:50.
the invention provides a method for measuring parameters of a nano film based on a coaxial ultrafast spectrum ellipsometer. The method specifically comprises the following steps:
s1, placing a standard reflector on a sample table, adjusting a sample adjusting module to realize accurate focusing of the standard reflector, respectively shielding one light path of a polarization interference modulation module by using an optical shutter, and respectively obtaining light intensity spectrum signals alpha of p light by using a spectrum acquisition unit 2 And an s-light only intensity spectrum signal beta 2 ；
S2, opening all shutters, collecting light reflected from a standard reflector through a spectrum collecting module, and performing spectrum dispersion to obtain light intensity interference spectrum signals I under different incident angles ref Analyzing the obtained spectrum to obtain a spectrum coherence function gamma of the spectrum ref And spectral phase function phi ref The calibration procedure described above is only required to be performed once before the measurement is performed;
s3, placing the sample to be measured on a sample table, and measuring according to the same procedure as that of the steps S1 and S2 to obtain a light intensity interference spectrum signal I of light reflected by the sample to be measured sam The method comprises the steps of carrying out a first treatment on the surface of the Analyzing and processing the light intensity interference spectrum signal of the sample to be detected, and combining the instrument calibration result to obtain the polarization state parameters psi and delta of the reflected light of the sample to be detected;
and S4, fitting the polarization state parameters obtained through measurement with theoretical expressions of the polarization state parameters deduced from theoretical models of the samples to be measured, and further obtaining relevant information of the samples to be measured.
Specifically, steps S1 and S2 are calibration and calibration steps, including:
the system is started up, a standard mirror is placed on the sample stage and clamped and fixed, and the sample adjustment module 17 is adjusted so that the surface of the standard mirror is located on the front focal plane of the high numerical aperture objective 15. The light vector of the source beam is defined herein as E in Then:
where k=2pi/λ is the wave number, u/v is the amplitude coefficient of the incident wave vector, and ζ/η is the phase of the wave vibrating in the x-and y-axis directions, respectively.
The source beam enters the polarization interference modulation module 100 through the first polarizer 3 to generate a first collimated beam 104 and a second collimated beam 105, which are p-polarized light and s-polarized light respectively with respect to the incident plane 300, and the light vectors thereof can be expressed as:
wherein P (0)/P (45)/P (90) is Jones matrix of polarizer with polarization axis rotated 0 degree/45 degree/90 degree respectively, r 1 /r 2 The reflection coefficients, z, of the planar mirrors 8 and 9, respectively 1 /z 2 For the optical path lengths of two branches in the polarization interference modulation module, u '/v' and ζ '/η' are the amplitude term and the phase term of the light beam emitted from the polarization interference modulation module, and the jones matrix of the polarizer is specifically expressed as:
the light vector of the mixed light beam 106 exiting the polarization interferometric modulation module 100 can be expressed as:
E out (k)＝E 1 (k)+E 2 (k)
the spectrum acquisition module acquires the light reflected by the standard reflector, if the reflection coefficient of the standard reflector isThe light vector of the light entering the imaging spectrometer can be expressed as:
the intensity of light reaching imaging spectrometer 22 can be expressed as:
wherein i represents an incident angle index, gamma represents a spectral coherence function of the system, phi ref (k) As a spectral phase function of the reference interference spectrum,an additional phase introduced for a standard mirror at the angle of incidence.
The optical shutter shields the branches where the first collimated beam 104 and the second collimated beam 105 are located, respectively, and the light intensity of the light beam received by the imaging spectrometer 22 can be expressed as;
alpha can be obtained from theoretical data of standard reflector 2 And beta 2 After the steps are finished, the fixed spectrum phase function phi of the system can be obtained by utilizing a Fourier analysis method ref (k) And a spectral coherence function gamma (k). The calibration step described above needs to be performed only once before measurement.
The step S3 comprises the following steps:
the sample to be measured is placed on the sample stage, and the light intensity spectrum interference signal of the light reflected back from the sample to be measured is obtained by using the imaging spectrometer 22, which can be expressed as:
wherein, the liquid crystal display device comprises a liquid crystal display device,
an additional phase is induced for the sample to be measured, which phase is related to the angle of incidence.
The phase difference between p light and s light can be calculated by the following formulaAnd amplitude ratio->
The step S4 includes:
the step S3 is performedAnd->And performing least square fitting on the film parameter vector p and a theoretical model established through analysis of the optical characteristics of the film to be tested, wherein a fitting formula is as follows:
wherein N is the number of spectrum dimensional data obtained through experiments, and M is the number of band solving parameters.
In general, the technical scheme provided by the invention is based on the polarization interference technology principle, so that time domain modulation devices such as a moving optical element, an optical phase modulation element and the like are completely avoided from being contained in an optical path, a polarization parameter spectrum can be obtained through single-frame photographing, the ultra-fast measurement speed is realized, and the method can be applied to online monitoring and characterization in a fast reaction process or under production conditions; meanwhile, a vertical objective type coaxial light path structure is adopted, so that the size of a light spot projected to the surface of a sample is reduced, and the transverse resolution of measurement is improved; based on the imaging principle of the back focal plane of the objective lens, the reflection spectrum data under a plurality of incidence angles can be obtained simultaneously under the condition of single measurement, and the measurement efficiency and the inversion solving precision are improved. In addition, the invention provides a system calibration method and a measurement and data processing method, which can simply, conveniently and efficiently acquire the polarization characteristic parameters of the sample to be measured, thereby realizing high-speed parameter extraction.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (8)
1. The coaxial ultrafast spectrum ellipsometer is characterized by comprising an illumination light path unit and a spectrum acquisition unit, wherein:
the illumination light path unit is used for carrying out polarization and phase modulation on a source light beam emitted by the light source (1) to generate a mixed light beam which comprises two components with mutually perpendicular polarization directions and with a fixed phase difference, and then projecting the mixed light beam onto the surface of a sample to be detected; the spectrum acquisition unit is used for acquiring a light beam reflected from the surface of a sample to be detected, and carrying out polarization demodulation and spectrum dispersion so as to obtain an interference spectrum;
the illumination light path unit is provided with a polarization interference modulation module (100), the polarization interference modulation module (100) comprises a first non-polarization beam splitter (5), a second polarizer (7), a first plane mirror (8), a third polarizer (10) and a second plane mirror (11), the first non-polarization beam splitter (5) is used for splitting light into two beams, one beam enters the second polarizer (7) and the first plane mirror (8) arranged behind the second polarizer (7), the other beam enters the third polarizer (10) and the second plane mirror (11) arranged behind the third polarizer (10), the second polarizer (7) is perpendicular to the polarization axis direction of the third polarizer (10), the two beams of light pass through the first plane mirror (8) and the second plane mirror (11) and return to the first non-polarization beam splitter (5) in an original path, and the first non-polarization stationary phase (5) combines the two beams of light into two beams with two polarization components with a perpendicular polarization component;
the spectrum acquisition unit comprises an objective lens (15), a first tube lens (13), a second non-polarized beam splitter (12), a fourth polarizer (18), a second tube lens (20) and an imaging spectrometer (22), wherein the second non-polarized beam splitter (12) is arranged behind the polarization interference modulation module (100), a mixed light beam enters the second non-polarized beam splitter (12) and then enters the first tube lens (13), the first tube lens (13) is arranged between the second non-polarized beam splitter (12) and the objective lens (15) and is used for converging a collimated light beam on the back focal plane of the objective lens (15), the objective lens (15) is arranged above the sample (16) to be measured and is used for projecting the light beam on the surface of the sample (16) to be measured, light emitted from the same point of the back focal plane (14) of the objective lens is projected on the surface of the sample (16) to be measured at the same incident angle, and light reflected back from the sample (16) to be converged on the same point of the back focal plane (14) of the objective lens;
the spectrum acquisition unit is also provided with an auxiliary imaging module (200) for observing imaging of a high-numerical aperture objective lens back focal plane (14) so as to judge focusing conditions of a sample (16) to be detected, and the spectrum acquisition unit comprises a third unpolarized beam splitter (19), a third tube lens (23) and an area array camera (24), wherein the third unpolarized beam splitter (19) is arranged between the fourth polarizer (18) and the second tube lens (20), the third tube lens (23) is arranged behind the third unpolarized beam splitter (19) and is used for converging light on a pixel plane of the area array camera (24), and the area array camera (24) is arranged behind the third tube lens (23) and is used for observing Fourier images of the objective lens back focal plane (14);
the third unpolarized beam splitter (19) is arranged above the second unpolarized beam splitter (12) and used for splitting light, the light is respectively led into the imaging spectrometer (22) and the auxiliary imaging module (200), the second tube lens (20) is arranged above the second unpolarized beam splitter (12) and used for converging the collimated light beam on the slit (21) plane of the imaging spectrometer, and the imaging spectrometer (22) is arranged above the second tube lens (20) and used for performing spectrum dispersion on the reflected light to generate an interference spectrum.
2. A coaxial ultrafast spectroscopic ellipsometer as in claim 1, wherein the focal plane of the first tube lens (13) coincides with the back focal plane of the objective lens (15), the focal plane of the first tube lens (13) coincides with the focal plane of the second tube lens (20), and the focal plane of the second tube lens (20) coincides with the slit plane of the imaging spectrometer (22).
3. A coaxial ultrafast spectroscopic ellipsometer according to claim 1 or 2, wherein a light source (1), a collimator lens (2), a first polarizer (3) and a diaphragm (4) are further arranged above the polarizing interferometric modulator module (100) in the illumination light path unit, the light source (1) emits a source light beam, the collimator lens (2) is arranged behind the light source (1) for converting the diverging light beam into a collimated light beam, the first polarizer (3) is arranged for converting unpolarized light into linearly polarized light, and the diaphragm (4) is arranged for limiting the aperture of the light beam.
4. A coaxial ultrafast spectroscopic ellipsometer according to claim 1, wherein a first optical shutter (6) and a second optical shutter (9) are arranged in two optical paths of the polarization interferometric modulator module (100), the first optical shutter (6) and the second optical shutter (9) are used for shielding the optical paths, the first optical shutter (6) is arranged between the first unpolarized beam splitter (5) and the second polarizer (7), and the second optical shutter (9) is arranged between the first unpolarized beam splitter (5) and the third polarizer (10).
5. A coaxial ultrafast spectroscopic ellipsometer as claimed in claim 3, wherein the polarization axis of the first polarizer (3) is at an angle of 45 degrees to the direction of the imaging spectrometer slit (21), and the polarization axis of the fourth polarizer (18) is parallel to the polarization axis of the first polarizer (3).
6. A coaxial ultrafast spectroscopic ellipsometer as in claim 1, wherein the polarization axis of the second polarizer (7) is at an angle of 0 degrees to the direction of the imaging spectrometer slit (21) and the polarization axis of the third polarizer (10) is at an angle of 90 degrees to the direction of the imaging spectrometer slit (21).
7. A coaxial ultrafast spectroscopic ellipsometer according to claim 1 or 2, wherein a sample adjustment module (17) is further provided in the ellipsometer, the sample adjustment module (17) being adapted to adjust the relative position between the sample (16) to be measured and the objective lens (15).
8. A method of measuring a coaxial ultrafast spectroscopic ellipsometer as recited in any one of claims 1 to 7, comprising the steps of:
s1, placing a standard reflector on a sample table, adjusting a sample adjusting module to realize accurate focusing of the standard reflector, respectively shielding one light path of a polarization interference modulation module by using an optical shutter, and respectively obtaining light intensity spectrum signals alpha of p light by using a spectrum acquisition unit 2 And an s-light only intensity spectrum signal beta 2 ；
S2, opening all shutters, collecting light reflected from a standard reflector through a spectrum collecting module, and performing spectrum dispersion to obtain light intensity interference spectrum signals I under different incident angles ref Analyzing the obtained spectrum to obtain a spectrum coherence function gamma of the spectrum ref And spectral phase function phi ref The calibration procedure described above is only required to be performed once before the measurement is performed;
s3, placing the sample to be measured on a sample table, and measuring according to the same procedure as that of the steps S1 and S2 to obtain a light intensity interference spectrum signal I of light reflected by the sample to be measured sam The method comprises the steps of carrying out a first treatment on the surface of the Analyzing and processing the light intensity interference spectrum signal of the sample to be detected, and combining the instrument calibration result to obtain the polarization state parameters psi and delta of the reflected light of the sample to be detected;
s4, fitting the polarization state parameter obtained by measurement with a theoretical expression of the polarization state parameter deduced by a theoretical model of the sample to be measured, and further obtaining a film parameter vector of the sample to be measured, wherein the calculation relation of the film parameters is as follows:
wherein p is a film parameter vector, N is the number of spectrum dimensional data obtained by experiments, M is the number of band solving parameters, and ψ is the number of band solving parameters i sam Is the ratio of the amplitude of p light to s light, delta i sam Is the phase difference of p-light and s-light, i is the index of the angle of incidence.
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