阅读说明：本技术 一种共轴超快光谱椭偏仪及测量方法 (Coaxial ultrafast spectrum ellipsometer and measurement method ) 是由 王健 翟福琪 彭立华 卢文龙 徐龙 周莉萍 于 2021-08-11 设计创作，主要内容包括：本发明属于光学测量相关技术领域,并公开了一种共轴超快光谱椭偏仪及测量方法。该椭偏仪包括照明光路单元和光谱采集单元,照明光路单元用将光源出射的源光束进行偏振和相位调制,产生包含偏振方向相互垂直并且存在固定相位差的两个分量的混合光束,然后将该混合光束投射到待测样品表面；光谱采集单元采集从待测样品表面反射的光束并进行偏振解调和光谱分散,从而获得干涉光谱；照明光路单元中设置有偏振干涉调制模块,该偏振干涉调制模块用于形成一束偏振方向相互垂直并且存在固定相位差的两个分量的混合光束。通过本发明,解决传统椭偏测量方法横向分辨率低、测量速度慢等问题,实现对超薄纳米薄膜的小光斑尺寸和宽入射角度超快椭偏测量。(The invention belongs to the technical field related to 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 polarizing and phase modulating a source light beam emitted by a light source to generate a mixed light beam containing two components with mutually vertical polarization directions and a fixed phase difference, and then projecting the mixed light beam onto the surface of a sample to be measured; 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 as to obtain an interference spectrum; the illumination light path unit is provided with a polarization interference modulation module which is used for forming a mixed light beam of two components with polarization directions which are perpendicular to each other and a fixed phase difference. By the method, the problems of low transverse resolution, low measurement speed and the like of the traditional ellipsometry measurement method are solved, and the ultra-fast ellipsometry measurement of the small light spot size and the wide incident angle of the ultra-thin nano film is realized.)

1. A coaxial ultrafast spectrum ellipsometer, comprising an illumination light path unit and a spectrum collection 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 containing two components with mutually vertical polarization directions and a fixed phase difference, and then projecting the mixed light beam onto the surface of a sample to be measured; the spectrum acquisition unit is used for acquiring light beams reflected from the surface of a sample to be detected, performing polarization demodulation and spectrum dispersion so as to obtain an interference spectrum;

the illumination light path unit is internally provided with a polarization interference modulation module which 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), wherein 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 arranged behind the second polarizer, the other beam enters the third polarizer (10) and the second plane mirror (11) arranged behind the third polarizer, the polarization axes of the second polarizer (7) and the third polarizer are vertical to each other, and 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 the original path, the first non-polarizing beam splitter (5) combines the two beams into a mixed beam of two components having polarization directions perpendicular to each other and a fixed phase difference.

2. The coaxial ultrafast spectrum ellipsometer of claim 1, wherein the spectrum collection unit comprises an objective lens (15), a first tube lens (13), a second non-polarizing beam splitter (12) disposed behind the polarization interference modulation module, a first tube lens (18) disposed between the second non-polarizing beam splitter and the objective lens for converging the collimated light beam onto a back focal plane of the objective lens, a fourth polarizer (18) disposed above the second non-polarizing beam splitter for projecting the light beam onto the surface of the sample to be measured, a second tube lens (20) disposed between the second non-polarizing beam splitter and the objective lens for splitting the light beam to be introduced into the imaging spectrometer and the auxiliary imaging module, respectively, the second tube lens is arranged above the second non-polarizing beam splitter and used for converging the collimated light beams on a slit plane of an imaging spectrometer, and the imaging spectrometer is arranged above the second tube lens and used for performing spectrum dispersion on the reflected light to generate an interference spectrum.

3. The coaxial ultrafast spectrum ellipsometer of claim 2, wherein 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.

4. The coaxial ultrafast spectrum ellipsometer according to claim 2, wherein the spectrum collection unit further comprises an auxiliary imaging module for observing the image of the back focal plane of the objective lens with high numerical aperture to determine the focusing condition of the sample, the auxiliary imaging module comprises a third non-polarizing beam splitter (19), a third tube lens (23) and an area-array camera (24), the third non-polarizing beam splitter is disposed between the fourth polarizer and the second tube lens, the third tube lens is disposed behind the third non-polarizing beam splitter for converging the light on the pixel plane of the area-array camera, and the area-array camera is disposed behind the third tube lens for observing the fourier image of the back focal plane of the objective lens.

5. A coaxial ultrafast spectrum ellipsometer according to claim 1 or 2, wherein the illumination light path unit further includes a light source (1) disposed above the polarization interference modulation module, the light source emitting a source beam, a collimating lens (2), a first polarizer (3) disposed behind the light source for converting a divergent beam into a bundle of collimating beams, and an optical stop (4) for limiting the aperture of the beam.

6. A coaxial ultrafast spectroscopic ellipsometer according to claim 1, wherein a first optical shutter (6) and a second optical shutter (9) are disposed in the two optical paths of the polarization interferometric modulation module, the first optical shutter (6) and the second optical shutter (9) being used for shielding the optical paths, the first optical shutter being disposed between the first non-polarizing beam splitter and the second polarizer, and the second optical shutter being disposed between the first non-polarizing beam splitter and the third polarizer.

7. A coaxial ultrafast spectroscopic ellipsometer according to claim 5, wherein the polarization axis of the first polarizer (3) is at an angle of 45 degrees to the direction of the imaging spectrometer slit (22), and the polarization axis of the fourth polarizer (18) is parallel to the polarization axis of the first polarizer.

8. A coaxial ultrafast spectroscopic ellipsometer according to claim 2, wherein the polarization axis of the second polarizer (7) is at an angle of 0 degree to the direction of the imaging spectrometer slit (22), and the polarization axis of the third polarizer (10) is at an angle of 90 degrees to the direction of the imaging spectrometer slit (22).

9. The coaxial ultrafast spectroscopic ellipsometer of claim 1 or 2, wherein a sample adjusting module for adjusting a relative position between a sample to be measured and the objective lens is further provided in the ellipsometer.

10. A method of measuring a coaxial ultrafast spectroscopic ellipsometer according to any one of claims 1 to 9, comprising the steps of:

s1, calibrating the coaxial spectrum ellipsometer by using the standard reflector to obtain the light intensity spectrum signal alpha when the coaxial ultrafast spectrum ellipsometer measures the standard reflector²、β²And I_refBy means of I_refCalculating the spectral coherence function gamma caused by a coaxial ultrafast spectroscopic ellipsometer_refAnd spectral phase function phi_ref；

S2, placing the sample to be measured on the sample stage, opening the light source and all optical shutters to measure, and obtaining the light intensity interference spectrum signal I of the light reflected by the sample to be measured_sam(ii) a Analyzing and processing the light intensity interference spectrum signal of the sample to be detected, and combining the calibration result of the instrument to obtain the polarization state parameters psi and delta of the light reflected by the sample to be detected;

and S4, fitting the polarization state parameters obtained by measurement with a theoretical expression of the polarization state parameters deduced from a theoretical model of the sample to be measured, and further obtaining a film parameter vector of the sample to be measured.

Technical Field

The invention belongs to the technical field related to optical measurement, and particularly relates to a coaxial ultrafast spectrum ellipsometer and a measurement method.

Background

The structure, composition, thickness and other information of the material or thin layer can be revealed by utilizing the polarization characteristic of the 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 includes an illumination arm and an analysis arm, which are obliquely arranged, wherein the illumination arm includes a broadband light source and a polarization state modulator, the illumination arm is configured to be incident on a sample surface at a specific incident Angle (AOI), and the analysis arm collects a light beam reflected from the sample surface and performs modulation analysis to obtain a change amount (including an amplitude ratio and a phase difference) of a sample to be measured with respect to a polarization state of the incident polarized light, thereby inverting information about the sample to be measured. A typical spectroscopic ellipsometer changes the polarization state of a light beam based on a rotating polarizer (polarizer or compensator), an optical phase modulator, and a birefringent liquid crystal system, and obtains the polarization state change caused by a sample to be measured by analyzing the time sequence of collected intensity spectra during at least one modulation period. The measuring method based on the time polarization modulation has the inherent defects that the measuring precision is greatly influenced by the intensity of a light source and the instability of a rotating part, the measuring 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 semiconductor and display processes, and the real-time accurate measurement of the thickness and the refractive index of the film plays an important role in improving the process yield in the film preparation process; meanwhile, as the size of the measurement target decreases, measurement techniques and instruments with smaller spot sizes are required; obviously, the conventional ellipsometer has been difficult to apply to the above-described scene. In order to realize ultrafast polarization parameter measurement, an interference snapshot ellipsometer based on a dual-channel sensing scheme and a channel spectrum snapshot ellipsometer adopting a plurality of wave plates are available; however, the interference snapshot ellipsometer based on the dual-channel sensing scheme has the disadvantages of complex principle, heavy device and high hardware cost; the original signal spectrum of the ellipsometer using the channel spectrum snapshot with a plurality of thick wave plates has high complexity, the signal processing is more complicated and time-consuming, and the available spectrum range is limited by the wave plates. In order to realize small spot size measurement, an objective lens is placed in front of a sample and an ellipsometer is arranged coaxially, but the conventional polarization modulation method is still adopted in the ellipsometer. In order to solve the problems that the traditional ellipsometer illumination arm is complex in angle configuration and cannot obtain information of a plurality of incident angles through 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 take the above problems into consideration and provide a solution.

Disclosure of Invention

Aiming at the defects or improvement requirements of the prior art, the invention provides a coaxial ultrafast spectrum ellipsometer and a measurement method, solves the problem of obtaining multiple angle information by single measurement, and realizes the ultrafast ellipsometry for the ultrathin nanometer film with small light spot size and wide incident angle.

To achieve the above object, according to one aspect of the present invention, there is provided a coaxial ultrafast spectroscopic ellipsometer including an illumination light path unit and a spectrum collection 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 containing two components with mutually vertical polarization directions and a fixed phase difference, and then projecting the mixed light beam onto the surface of a sample to be measured; the spectrum acquisition unit is used for acquiring light beams reflected from the surface of a sample to be detected, performing polarization demodulation and spectrum dispersion so as to obtain an interference spectrum;

the illumination light path unit is internally provided with a polarization interference modulation module which comprises a first non-polarization beam splitter, a second polarizer, a first plane mirror, a third polarizer and a second plane mirror, wherein the first non-polarization beam splitter is used for splitting light into two beams, one of which enters the second polarizer and a first planar mirror disposed behind the second polarizer, the other beam enters the third polarizer and a second plane mirror arranged behind the third polarizer, the polarization axes of the second polarizer and the third polarizer are vertical to each other, two beams of light pass through the first plane mirror and the second plane mirror and return to the first non-polarization beam splitter in the original path, the first non-polarizing beam splitter combines the two light beams into a mixed beam of two components with mutually perpendicular polarization directions and a fixed phase difference.

Further preferably, the spectrum collecting unit includes an objective lens, a first tube lens, a second non-polarizing beam splitter, a fourth polarizer, a second tube lens and an imaging spectrometer, the second non-polarizing beam splitter is disposed behind the polarization interference modulation module, the mixed light beam enters the second non-polarizing beam splitter and then enters the first tube lens, the first tube lens is disposed between the second non-polarizing beam splitter and the objective lens and is used for converging the collimated light beam on 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 light beam onto the surface of the sample to be measured, the fourth polarizing beam splitter is disposed above the second non-polarizing beam splitter and is used for splitting the light beam and guiding the light beam to the imaging spectrometer and the auxiliary imaging module respectively, the second tube lens is disposed above the second non-polarizing beam splitter, the imaging spectrometer is arranged above the second tube lens and is used for performing spectral dispersion on the reflected light to generate an interference spectrum. Further preferably, a focal plane of the first tube lens coincides with a back focal plane of the objective lens, a focal plane of the first tube lens coincides with a focal plane of the second tube lens, and a focal plane of the second tube lens coincides with a slit plane of the imaging spectrometer.

Further preferably, the spectrum collection unit is further provided with an auxiliary imaging module for observing the imaging of the rear focal plane of the high-numerical-aperture objective lens so as to judge the focusing condition of the sample to be measured, and the auxiliary imaging module comprises a third non-polarizing beam splitter, a third tube lens and an area array camera, wherein the third non-polarizing beam splitter is arranged between the fourth polarizer and the second tube lens, the third tube lens is arranged behind the third non-polarizing beam splitter and is used for converging light on the pixel plane of the area array camera, and the area array camera is arranged behind the third tube lens and is used for observing the Fourier image of the rear focal plane of the objective lens.

Further preferably, the illumination light path unit further includes a light source, a collimating lens, a first polarizer and a diaphragm, the light source is disposed above the polarization interference modulation module, the light source emits a source light beam, the collimating mirror is disposed behind the light source and is configured to convert the divergent light beam into a collimated light beam bundle, the first polarizer is configured to convert the unpolarized light into linearly polarized light, and the diaphragm is configured to limit an 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 modulation module, the first optical shutter and the second optical shutter are used for shielding the optical paths, the first optical shutter is disposed between the first non-polarization beam splitter and the second polarizer, and the second optical shutter is disposed between the first non-polarization 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 forms an angle of 0 degree with the direction of the slit of the imaging spectrometer, and the polarization axis of the third polarizer forms an angle of 90 degrees with the direction of the slit of the imaging spectrometer

Further preferably, a sample adjusting module is further disposed in the ellipsometer, and the sample adjusting module is configured to adjust a 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 of measuring a coaxial ultrafast spectroscopic ellipsometer as described above, comprising the steps of:

s1, calibrating the coaxial spectrum ellipsometer by using the standard reflector to obtain the light intensity spectrum signal alpha when the coaxial ultrafast spectrum ellipsometer measures the standard reflector²、β²And I_refBy means of I_refCalculating the spectral coherence function gamma caused by a coaxial ultrafast spectroscopic ellipsometer_refAnd spectral phase function phi_ref；

S2, placing the sample to be measured on the sample stage, opening the light source and all optical shutters to measure, and obtaining the light intensity interference spectrum signal I of the light reflected by the sample to be measured_sam(ii) a Analyzing and processing the light intensity interference spectrum signal of the sample to be detected, and combining the calibration result of the instrument to obtain the polarization state parameters psi and delta of the light reflected by the sample to be detected;

and S4, fitting the polarization state parameters obtained by measurement with a theoretical expression of the polarization state parameters deduced from a theoretical model of the sample to be measured, and further obtaining a film parameter vector of the sample to be measured.

Generally, compared with the prior art, the technical scheme of the invention has the following beneficial effects:

1. according to the method, a polarization interference modulation module is utilized to generate a mixed light beam which simultaneously comprises a p-polarization component and an s-polarization component and has a fixed phase difference, the mixed light beam is utilized to irradiate a sample to be detected and collect reflected light to obtain a single-frame reflection interference spectrum, so that the reflection polarization parameters of the sample to be detected can be rapidly obtained by utilizing methods such as Fourier analysis and the like;

2. the invention realizes angle-resolved measurement based on a coaxial system by combining a high-numerical-aperture objective with a Kohler illumination mode, can simultaneously obtain a two-dimensional spectrum containing interference spectrum information of reflected light corresponding to an incident angle of incident light irradiating a sample to be measured in single measurement, and can calculate the ellipsometric amplitude ratio and ellipsometric phase difference information corresponding to all incident angles in the numerical aperture range of the objective by combining the two-dimensional spectrum with the measurement method;

3. the invention provides a coaxial ultrafast ellipsometer capable of measuring ultrathin films produced by large-area modules in real time, which does not need a polarization modulation mode based on time sequence such as a rotating polarization element, an optical phase modulator, a birefringent liquid crystal system and the like, and only obtains the polarization parameters of a sample to be measured by single-frame interference spectrum, has microsecond-level measurement speed and realizes ultrafast ellipsometry;

4. the invention introduces a coaxial optical path structure of the high numerical aperture objective, improves the measurement precision and simultaneously avoids the problems of larger spot size, low transverse resolution, narrow field of view, easy influence of vibration on the measurement precision and the like of the traditional obliquely-arranged ellipsometer;

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 carry out configuration optimization.

Drawings

FIG. 1 is a schematic diagram of a coaxial ultrafast spectroscopic ellipsometer according to a preferred embodiment of the present invention;

FIG. 2 is a schematic diagram of a polarization interferometric modulation module according to a preferred embodiment of the present invention;

FIG. 3 is a schematic diagram of the measurement principle of a coaxial ultrafast spectroscopic ellipsometer provided in accordance with a preferred embodiment of the present invention;

figure 4 is a graphical illustration of BFP plane versus sample surface light angle of incidence provided in accordance with a preferred embodiment of the present invention.

The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:

1-a light source, 2-a collimating lens, 3-a first polarizer, 4-a diaphragm, 5-a first non-polarizing beam splitter, 6-a first optical shutter, 7-a second polarizer, 8-a first plane mirror, 9-a second optical shutter, 10-a third polarizer, 11-a second plane mirror, 12-a second non-polarizing beam splitter, 13-a first tube lens, 14-an objective lens back focal plane, 15-an objective lens, 16-a sample to be measured, 17-a sample adjusting module, 18-a fourth polarizer, 19-a third non-polarizing beam splitter, 20-a second tube lens, 21-an imaging spectrometer, 22-an imaging spectrometer slit, 23-a third tube lens, 24-a plane array camera, 100-a polarization interference modulation module, 101-linearly polarized light beam, 102-first sub-beam, 103-second sub-beam, 104-first collimated light beam, 105-second collimated light beam, 200-auxiliary imaging module, 300-plane of incidence.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

Referring to fig. 1, 2 and 3, the present invention provides a coaxial ultrafast ellipsometer, which includes an illumination light path unit, a sample adjusting module 17, a spectrum collecting 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 containing two components which have mutually vertical polarization directions and fixed phase difference, and projecting the mixed light beam onto the surface of a sample to be measured; the polarization interference modulation device comprises a light source 1, a collimating lens 2, a first polarizer 3, a diaphragm 4 and a polarization interference modulation module 100 which are arranged in sequence.

The polarization interference modulation module comprises a first non-polarization 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 precision displacement regulating and controlling device which is used for driving the first plane reflector or the second plane reflector 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 measured, performing polarization demodulation and spectrum dispersion on the light beam, and thus obtaining an interference spectrum, and the spectrum acquisition unit comprises a second non-polarization 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 21 and an auxiliary imaging module 200 which are arranged coaxially; reflected light from the surface of the sample forms a Fourier image on a focal plane behind the high-numerical-aperture objective lens, and forms a frequency-domain interference image on a slit plane of the imaging spectrometer sequentially through the first tube lens, the second non-polarizing beam splitter, the third polarizer and the second tube lens, and the imaging spectrometer disperses an image spectrum at the slit to form an angle-resolved interference spectrum.

The auxiliary imaging module comprises a third non-polarizing beam splitter 19, a third tube lens 23 and an area-array camera 24. The second non-polarization beam splitter not only realizes the reflective transmission of the light beam in the illumination light path unit, but also realizes the transmissive transmission in the spectrum acquisition unit.

The light source 1 emits a source light beam, preferably in a spectrally spread wavelength range, for example 400-800nm for a broadband halogen illumination source. The source light beam is collimated by the collimating lens 2 to obtain a collimated light beam, the collimated light beam is then changed into a linearly polarized light beam 101 by the first polarizer 3, and the linearly polarized light beam 101 enters the polarization interference modulation module 100 after passing through the diaphragm 4. The light source includes halogen lamp, LED lamp, xenon lamp and other wide band light source.

Further, the polarization axis of the first polarizer 3 is at an angle of 45 degrees to the orientation of the imaging spectrometer slit 22, the orientation of the imaging spectrometer slit 22 defining a plane of incidence 300 of the collected light beam, only light beams in this plane of incidence 300 being collected by the spectrum collection unit.

After entering the polarization interference modulation module 100, the linearly polarized light beam 101 is first split into two sub-beams, a first sub-beam 102 and a second sub-beam 103, by the first non-polarization beam splitter 5. The polarization axis of the second polarizer 7 forms an angle of 0 degree with the direction of the slit 22 of the imaging spectrometer, and the first sub-beam 102 reaches the first plane mirror 8 after passing through the second polarizer 7 and then is reflected, and forms a first collimated beam 104 with the vibration direction parallel to the incident plane 300 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 22, the second sub-beam 103 reaches the first plane mirror 11 after passing through the second polarizer 10 and is reflected, and then forms a second collimated beam 105 with the vibration direction perpendicular to the incident plane 300 after passing through the second polarizer 10 again, and the optical path difference Δ z between the first collimated beam 104 and the second collimated beam 105 is 2(z is 2)₁-z₂). The two collimated light beams, the first collimation 104 and the second collimation 105, are combined into a light beam 106 by a first non-polarizing beam splitter, the light beam 106 is composed of two components with mutually vertical vibration directions and a fixed phase difference, and the two componentsP-component and s-component, respectively, with respect to the plane of incidence 300. The plane of incidence is a plane parallel to the direction of the slit.

Further, as a non-limiting example, a precise displacement adjusting device is installed behind the first plane mirror 8 or the second plane mirror 11 to adjust the optical path difference between the first collimated light beam 104 and the second collimated light beam 105.

The mixed light beam 106 changes direction after passing through the second non-polarizing beam splitter 12 and is focused on the objective back focal plane 14 of the high numerical aperture objective 15 through the first tube lens 13, and then is irradiated on the surface of the sample 16 to be measured through the high numerical aperture objective 15.

Fig. 4 schematically shows the relationship between the incident angles of the light rays on the objective lens back focal plane 14 and the surface of the sample 19 to be measured in the incident plane 300, the light rays emitted from the same point on the objective lens back focal plane 14 will be projected onto the surface of the sample 16 to be measured at the same incident angle, and the light rays reflected from the sample 16 to be measured at the same angle will be converged at the same point on the objective lens back focal plane 14 with high numerical aperture. Thus, different positions of the selected line signal of the imaging spectrometer slit 21 correspond to different angles of incidence of the light, and specifically satisfy the following relationship: theta ═ sin^-1(d/d_maxX NA). Wherein NA is the numerical aperture of the high numerical aperture objective 15, d_maxIs the maximum radius of the spot on the back focal plane 14 of the high numerical aperture objective.

In the sample conditioning 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.

Light reflected by the surface of the sample 16 to be measured is collected by the high numerical aperture objective lens and then forms a Fourier image on the rear focal plane 14 of the high numerical aperture objective lens, and the Fourier image sequentially passes through the first tube lens 13, the second non-polarizing beam splitter 12, the fourth polarizer 18 and the second tube lens and then enters the slit of the imaging spectrometer 22. The focal planes of the high numerical aperture objective 15 and the first tube lens 13, the first tube lens 13 and the second tube lens 20 are overlapped in pairs, and the focal plane of the second tube lens 20 is overlapped with the slit plane 21 of the high numerical aperture objective.

The polarization axis of the fourth polarizer 18 makes an angle of 45 degrees with the incident plane 300, which converts the reflected light beam into linearly polarized light again, and the linearly polarized light enters the imaging spectrometer 22 for spectral dispersion and then interferes in the spectral dimension, thereby obtaining the angle-resolved interference spectrum of the reflected light of the sample to be measured.

Preferably, one example of an imaging spectrometer is the spectrometer iHR320 or iHR350 from HORIBA, which are based on the use of concave diffraction gratings corrected from chromatic aberration to ensure spectral scatter and focus of the spectrum on the image sensor.

The auxiliary imaging module 200 is used for observing the back focal plane imaging of the high-na objective lens to determine the focusing condition of the sample to be measured, and the photosensitive chip of the area array camera 21 is located on the back focal plane of the third tube lens 23 to obtain a clear imaging of the back focal plane 14 of the high-na objective lens.

Preferably, the splitting ratios of the non-polarizing beam splitters 5, 12, 19 are each 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 the standard reflector on the sample stage, adjusting the sample adjusting module to realize the accurate focusing of the standard reflector, respectively shielding one light path of the polarization interference modulation module by the optical shutter, and respectively obtaining the light intensity spectrum signal alpha of only p light by the spectrum collecting unit²And a light intensity spectrum signal beta of only s light²；

S2, opening all shutters, collecting light reflected from the standard reflector through the spectrum collection module, and performing spectrum dispersion to obtain light intensity interference spectrum signals I under different incidence angles_refAnalyzing and processing the acquired spectrum to obtain the spectrum coherence function gamma of the spectrum_refAnd spectral phase function phi_refThe calibration procedure is only required to be performed once before the measurement is performed;

s3, placing the sample to be measured on the sample stage, and measuring according to the same procedures as the steps S1 and S2 to obtain the light intensity interference spectrum signal I of the light reflected by the sample to be measured_sam(ii) a Will be testedAnalyzing and processing the light intensity interference spectrum signal of the sample, and combining the calibration result of the instrument 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 by the measurement with a theoretical expression of the polarization state parameters deduced from a theoretical model of the sample to be detected, and further obtaining the related information of the sample to be detected.

Specifically, steps S1 and S2 are calibration and calibration steps, which include:

and starting the system, placing the standard reflector on the sample table, clamping and fixing the standard reflector, and adjusting the sample adjusting module 17 to enable the surface of the standard reflector to be positioned on the front focal plane of the high-numerical-aperture objective 15. Here, the light vector of the source beam is defined as E_inAnd then:

where k 2 pi/λ is a wave number, u/v is an amplitude coefficient of an incident wave vector, and ξ/η are phases of waves vibrating in the x and y axis directions, respectively.

The source light beam enters the polarization interference modulation module 100 after passing through the first polarizer 3 to generate a first collimated light beam 104 and a second collimated light beam 105, which are p-polarized light and s-polarized light respectively corresponding to the incident plane 300, and the light vectors thereof can be respectively expressed as:

wherein P (0)/P (45)/P (90) is a Jones matrix of a polarizer having a polarization axis rotated by 0 DEG/45 DEG/90 DEG, respectively, and r is₁/r₂The reflection coefficients, z, of the plane mirrors 8 and 9, respectively₁/z₂For the optical path lengths of two branches in the polarization interference modulation module, u '/v' and xi '/eta' are respectively the slave polarization interference modulationAnd (3) making an amplitude term and a phase term of the module emergent light beam, wherein the Jones matrix of the polarizer is specifically represented as follows:

the light vector of the mixed light beam 106 emitted by the polarization interferometric modulation module 100 can be expressed as:

E_out(k)＝E₁(k)+E₂(k)

the spectrum collection module collects 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 light intensity reaching the imaging spectrometer 22 can be expressed as:

where i denotes the angle of incidence index, γ denotes the spectral coherence function of the system, Φ_ref(k) For the spectral phase function of the reference interference spectrum,additional phase introduced for the standard mirror at the angle of incidence.

The optical shutters respectively shield the branches where the first collimated light beam 104 and the second collimated light beam 105 are located, and the light intensity of the light beam received by the imaging spectrometer 22 can be represented as follows;

from the theoretical data of a standard mirror, alpha can be determined²And beta²After the above steps are performed, the fixed spectrum phase function phi of the system can be obtained by using a Fourier analysis method_ref(k) And a spectral coherence function γ (k). The calibration step described above need only be performed once before the measurement.

Step S3 includes:

the sample to be measured is placed on the sample stage, and the imaging spectrometer 22 is used to obtain the light intensity spectrum interference signal of the light reflected back by the sample to be measured, which can be expressed as:

wherein the content of the first and second substances,

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 formulaTo amplitude ratio

Step S4 includes:

obtained in step S3Andand performing least square fitting on a theoretical model established by analyzing the optical characteristics of the film to be measured to obtain a film parameter vector p, wherein the fitting formula is as follows:

wherein, N is the number of the spectrum dimensional data obtained by the experiment, and M is the number of the band solving parameters.

In general, the technical scheme provided by the invention is based on the principle of polarization interference technology, completely avoids time domain modulation devices such as a moving optical element and an optical phase modulation element in a light path, can obtain a polarization parameter spectrum through single-frame photography, has ultra-fast measurement speed, and can be applied to online monitoring and characterization in a rapid reaction process or under production conditions; meanwhile, a vertical objective lens type coaxial optical path structure is adopted, so that the size of a light spot projected on the surface of a sample is reduced, and the transverse resolution of measurement is improved; based on the imaging principle of the rear 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 obtain the polarization characteristic parameters of the sample to be detected, thereby realizing high-speed parameter extraction.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.