CN102589692A - Vertical incidence broadband polarization spectrometer for splitting optical fiber bundle and optical measurement system - Google Patents
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Abstract
The invention provides a vertical incidence broadband polarization spectrometer for splitting an optical fiber bundle and an optical measurement system. The vertical incidence broadband polarization spectrometer comprises a light source, the optical fiber bundle, a light detector and a polarization unit, wherein the optical fiber bundle comprises an incident optical fiber sub-bundle and an emergent optical fiber sub-bundle; the incident optical fiber sub-bundle is provided with a first port group and a second port group; the emergent optical fiber sub-bundle is provided with a third port group and a fourth port group; and the second port group of the incident optical fiber sub-bundle and the third port group of the emergent optical fiber sub-bundle are on a same cross section. According to the vertical incidence broadband polarization spectrometer and the optical measurement system disclosed by the invention, by the adoption of the optical fiber bundle splitting and the addition of a polarization element, the luminous flux efficiency of the spectrometer is improved and the control capability to polarization lights is enhanced.
Description
Technical Field
The invention relates to the technical field of optical measurement, in particular to a vertical incidence broadband polarization spectrometer for splitting optical fiber bundles and an optical measurement system.
Background
With the rapid development of the semiconductor industry, the optical measurement technology is used to rapidly and accurately detect the thickness, material characteristics and three-dimensional morphology of the periodic structure of the semiconductor film, which is a key link for controlling the production process and improving the productivity, and is mainly applied to the industries including film structures, such as integrated circuits, flat panel displays, hard disks, solar cells and the like. The films made of different materials and different structures have different reflectivities at different wavelengths for incident light with different polarization states, and the reflection spectrum of the film has uniqueness. Today's advanced thin film and three-dimensional structure measuring devices, such as ellipsometers and Optical Critical Dimension (OCD) meters, are required to satisfy as wide a spectral measurement capability as possible to increase the measurement accuracy, typically 190nm to 1000 nm. In the case of known film structure parameters, the film reflectance spectrum can be calculated by a mathematical model. When unknown structural parameters exist, such as film thickness, film optical constants, surface three-dimensional structure and the like, the unknown structural parameters can be obtained by fitting measurement and simulating and calculating a spectrum through regression analysis. The measurement apparatus is generally divided into an optical system that is normally incident with respect to the sample surface and an optical system that is obliquely incident with respect to the sample surface. The vertical incidence optical system can be integrated with other process equipment due to the more compact structure, so that the integration of production and measurement and real-time monitoring are realized. In the prior art, an optical system of a vertical incidence spectrometer mainly separates a detection light beam from a sample reflected light beam through a light splitter, so that the sample reflected light beam cannot reversely return to a light source and is independently incident to a detector. FIG. 1 is a prior art spectrometer that utilizes a beam splitter for beam splitting. As shown in fig. 1, in the spectrometer, divergent light emitted from a light source 101 passes through a lens 102, is incident and transmitted in parallel through a beam splitter 103, and is focused on the surface of a sample 105 after being converged by a lens 104; after reflected light on the surface of the sample 105 is reflected by the lens 104, the reflected light is vertically incident to the beam splitter 103; after being reflected, the light is converged by the lens 106 and then enters the detector 107, and the reflection spectrum of the surface of the sample is obtained.
The main problems with spectrometers using optical splitters are: 1) the luminous flux is low, and in the whole measuring process, a light beam is transmitted and reflected once by the same light splitter from a light source and enters a detector. The maximum light flux ratio which can be achieved is 25 percent, namely, the light splitter has 50 percent of each of the transmittance and the reflectivity; 2) if high-quality light spots and a wide spectral range are simultaneously realized, the system complexity is high and the cost is high. The detailed analysis is as follows:
A) in the case of a beam splitter, the beam splitter is used at 45 ° to the main beam of the light, as shown in US6900900B 2. The disadvantages of this structure are: under the condition of broadband light beam transmission, when the light beam is a parallel light beam, chromatic aberration is generated; this problem can be corrected for chromatic aberration by providing an identical dichroic sheet, but adds complexity to the system and reduces luminous flux. Another type of beam splitter based on a beam-splitting foil is a spot beam splitter (Polka-dot Beamsplitter) (e.g., US 5450240, edmund optics spot beam splitter) or a spot beam splitter with a thickness of only 100 microns (e.g., US6525884B2), which is structurally characterized by: the reflected light beam can realize a wide spectrum (including a deep ultraviolet range) and has no dispersion per se; but the periodic structure of the surface lattice can cause diffraction light spots, thereby greatly influencing the measurement accuracy;
B) in the case of a beam splitter prism (as in US6181427B1), the disadvantages are: the spectral prism is difficult to realize wide-spectrum light splitting at the same time and is generally divided into three areas of 400-700nm, 700-1100nm and 1100-1600nm, so that the measurement spectral range is limited;
C) under the condition that the light splitter is a polarization light splitting prism, the transmitted light/reflected light is in a fixed polarization direction, and the polarization state is changed by rotating the polarization light splitting prism, or rotating a sample or additionally arranging a polarizer, so that the realization is very complicated;
D) when the light splitter is a thin film light splitter (Pellie Beamsplitter), the structure of the light splitter has the defects that the thickness of a thin film is only 2 micrometers, the light splitter is greatly influenced by the environment, is extremely easy to damage, cannot clean the surface and has high cost; and the film has absorption in the ultraviolet band.
The ability of the spectrometer to control polarization defines the scope of application of the spectrometer, particularly for anisotropic samples, e.g., samples of three-dimensional periodic structures. The method comprises the steps of calculating phase characteristics by measuring the reflection spectrum of polarized light on the surface of a sample, fitting a numerical simulation result, and measuring the Critical Dimension (CD) and the three-dimensional appearance of a periodic pattern on the surface of the sample and the film thickness and the optical constant of a multilayer material. The spectrometer for realizing the critical dimension measurement requires controlling the polarization state of a light beam in the optical signal acquisition process, so that a sample can be accurately measured.
In implementing the present invention, the inventor has recognized that the prior art has the following drawbacks: the spectrometer using the conventional spectrometer has low luminous flux efficiency and lacks polarization control capability.
Disclosure of Invention
Technical problem to be solved
Aiming at the problems in the prior art, the invention provides a vertical incidence broadband polarization spectrometer and an optical measurement system for splitting an optical fiber bundle, so as to improve the luminous flux efficiency of the spectrometer and improve the control capability of the spectrometer on polarization.
(II) technical scheme
The invention discloses a vertical incidence broadband polarization spectrometer for splitting optical fiber bundles. The spectrometer comprises: the device comprises a light source, an optical fiber bundle, a polarization unit and a light detector. The optical fiber bundle comprises an incident optical fiber sub-bundle and an emergent optical fiber sub-bundle, and the incident optical fiber sub-bundle and the emergent optical fiber sub-bundle both comprise at least one optical fiber or optical fiber core; the incident fiber sub-bundle has a first port group and a second port group, the exit fiber sub-bundle has a third port group and a fourth port group, and the second port group of the incident fiber sub-bundle and the third port group of the exit fiber sub-bundle are on the same cross section. The incident optical fiber sub-beam is used for guiding the detection light emitted by the light source incident from the first port group, emergent from the second port group and incident to the surface of the sample after passing through the polarization unit; and the emergent fiber sub-beam is used for guiding the reflected light of the sample surface incident from the third port group after passing through the polarization unit, and is emergent from the fourth port to enter the light detector. The polarization unit is positioned between the same cross section and the sample and is used for enabling the detection light emitted from the second port group to form polarized light; and the reflected light from the sample surface is analyzed.
Preferably, in the vertical incidence broadband polarization spectrometer for splitting the optical fiber bundle, the incident optical fiber sub-bundle is a predetermined optical fiber of the reflection/back scattering optical fiber bundle, and the emergent optical fiber sub-bundle is an optical fiber outside the predetermined optical fiber of the reflection/back scattering optical fiber bundle. The optical fiber bundle consists of a central optical fiber and a plurality of branch optical fibers surrounding the central optical fiber, the circle centers of the plurality of branch optical fibers on the same cross section are positioned on a concentric ring of the central optical fiber, and the ring is equally divided. The central optical fiber is used as an incident optical fiber sub-beam, and the plurality of branch optical fibers are used as emergent optical fiber sub-beams; or the central optical fiber is used as an emergent optical fiber sub-beam, and the plurality of branch optical fibers are used as incident optical fiber sub-beams.
Preferably, the normal incidence broadband polarization spectrometer for splitting the optical fiber bundle of the present invention may further comprise: polarization unit rotation control means. The polarization unit rotation control device is used for controlling the polarization direction of the polarization unit.
Preferably, the normal incidence broadband polarization spectrometer for splitting the optical fiber bundle of the present invention may further comprise: and (4) a diaphragm. The diaphragm is positioned between the polarizing unit and the sample and is used for preventing e light generated after passing through the polarizing unit from being incident on the surface of the sample and/or preventing reflected light of the e light from being reflected back to the polarizing unit.
Preferably, the normal incidence broadband polarization spectrometer for splitting the optical fiber bundle of the present invention may further comprise: a light focusing assembly. The light-gathering component is positioned between the same cross section and the sample and used for gathering the detection light emitted from the second port group to the surface of the sample after passing through the polarizing unit and gathering the reflection light on the surface of the sample to the third port group after passing through the polarizing unit. The light condensing assembly includes: a first condensing unit and a second condensing unit. The first light gathering unit is positioned between the same cross section and the polarization unit and is used for gathering the detection light incident from the first port group to form incident approximately parallel light to the polarization unit; and the emergent linearly polarized light after passing through the polarization unit is converged to the third port group. The polarization unit is used for polarizing the detection light emitted from the second port group into incident linear polarized light; and is used to analyze the reflected light from the sample surface as outgoing linearly polarized light. A second condensing unit, located between the polarizing unit and the sample, for condensing the incident linearly polarized light to the sample surface; and the reflected light from the sample surface is converged to form approximately parallel light to be emitted to the polarizing unit. Preferably, the first condensing unit is a first focusing lens, and the second condensing unit is a second focusing lens. The first focusing lens and/or the second focusing lens is a correcting three-lens assembly, a tri-cemented lens or a bi-cemented lens.
(III) advantageous effects
The vertical incidence broadband polarization spectrometer for splitting the optical fiber bundle adopts the optical fiber bundle for splitting, so that the luminous flux efficiency is greatly improved compared with a spectrometer adopting a light splitter in the prior art. Meanwhile, the vertical incidence broadband polarization spectrometer for fiber bundle light splitting is additionally provided with a polarization element, and parameters of polarization states of a sample in two orthogonal directions can be obtained through measurement, so that optical parameters of a sample material can be calculated, and a three-dimensional morphology structure of the sample can be analyzed.
Drawings
FIG. 1 is a prior art spectrometer that utilizes a beam splitter for beam splitting;
FIG. 2 is a schematic diagram of a normal incidence broadband polarization spectrometer for splitting a fiber bundle according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a cross-section of each port of a preferred seven-core fiber bundle for a normal incidence broadband polarization spectrometer for splitting a fiber bundle according to an embodiment of the present invention;
FIG. 4 is a diagram of an optical path of a normal incidence broadband polarization spectrometer for splitting a fiber bundle according to an embodiment of the present invention;
FIG. 5 is a diagram of an optical path of a vertical-incidence broadband polarization spectrometer with a single lens focusing, where an image plane moves as a reference in a defocused state according to an embodiment of the present invention;
FIG. 6 is a diagram of an optical path of a vertical-incidence broadband polarization spectrometer with a fiber bundle split according to an embodiment of the present invention in a defocused state with an object plane moving with reference to a single lens when focusing;
FIG. 7 is a diagram of an optical path in a defocused state with reference to an object plane moving when two lenses of a vertical incidence broadband polarization spectrometer for splitting optical fiber bundles focus according to an embodiment of the present invention;
FIG. 8 is a simulated diagram of the ZEMAX luminous flux in a defocused state under an ideal lens condition of the vertical incidence broadband polarization spectrometer for fiber bundle splitting according to the embodiment of the invention;
FIG. 9 shows simulation results of ZEMAX software when EdmundOptics optics are used in a normal incidence broadband polarization spectrometer for fiber bundle splitting according to an embodiment of the present invention;
fig. 10 is a schematic diagram of adjusting the arrangement of the optical fiber ports of the outgoing optical fiber sub-beam according to the shape of the spectrometer slit in the vertical incidence broadband polarization spectrometer for splitting an optical fiber beam according to the embodiment of the present invention;
FIG. 11 is an optical schematic of a Rochon prism polarizer according to an embodiment of the present invention;
FIG. 12 is a diagram illustrating a structure of a front trench of a silicon wafer according to an embodiment of the present invention;
FIG. 13 is a graph of absolute reflectance spectra of TE and TM in an absolute reflectance measurement in accordance with an embodiment of the present invention;
FIG. 14 is a spectrum of the TM/TE reflectivity ratio of TE and TM and the phase difference between TM and TE in an ellipsometry method according to an embodiment of the present invention.
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 specific embodiments and the accompanying drawings.
In one exemplary embodiment of the present invention, a normal incidence broadband polarization spectrometer for splitting a fiber bundle includes: the device comprises a light source, an optical fiber bundle, a polarization unit and a light detector. The optical fiber bundle comprises an incident optical fiber sub-bundle and an emergent optical fiber sub-bundle, and the incident optical fiber sub-bundle and the emergent optical fiber sub-bundle both comprise at least one optical fiber or optical fiber core; the incident fiber sub-bundle has a first port group and a second port group, the exit fiber sub-bundle has a third port group and a fourth port group, and the second port group of the incident fiber sub-bundle and the third port group of the exit fiber sub-bundle are on the same cross section. The incident optical fiber sub-beam is used for guiding the detection light emitted by the light source incident from the first port group, emergent from the second port group and incident to the surface of the sample after passing through the polarization unit; and the emergent fiber sub-beam is used for guiding the reflected light of the sample surface incident from the third port group after passing through the polarization unit, and is emergent from the fourth port to enter the light detector. The polarization unit is positioned between the same cross section and the sample and is used for polarizing the detection light emitted from the second port group; and the reflected light from the sample surface is analyzed.
In the traditional spectrometer adopting the optical splitter, under the condition of not considering the influence of a polarization unit, in the whole measurement process, a light beam is transmitted and reflected once by the same optical splitter from a light source and enters a detector, and the maximum light flux ratio which can be reached is 25 percent, namely the optical splitter has 50 percent of transmittance and reflectivity respectively; meanwhile, if high-quality light spots and a wider spectral range are realized, the system complexity is higher and the cost is higher. Through simulation calculation, the light flux efficiency of the vertical incidence broadband polarization spectrometer for splitting the optical fiber bundle can reach 50 percent under the condition of not considering the influence of the polarization unit, and is far higher than that of the traditional spectrometer adopting the optical splitter. Meanwhile, the vertical incidence broadband polarization spectrometer for fiber bundle light splitting is additionally provided with the polarization element, and the parameters of the polarization state of the sample in two orthogonal directions can be obtained through measurement, so that the optical parameters of the sample material can be calculated, the three-dimensional morphology structure of the sample can be analyzed, and the application range of the spectrometer is greatly expanded.
In a further embodiment of the present invention, a normal incidence broadband polarization spectrometer for splitting a fiber bundle, comprises: the device comprises a light source, an optical fiber bundle, a light detector, a first light-gathering unit, a second light-gathering unit and a polarizer. The optical fiber bundle comprises an incident optical fiber sub-bundle and an emergent optical fiber sub-bundle. The incoming fiber sub-beam and the outgoing fiber sub-beam each comprise at least one optical fiber or optical fiber core. The first light-condensing unit and the second light-condensing unit are positioned between the optical fiber bundle and the sample, and the light beams are approximately parallel light to propagate between the first light-condensing unit and the second light-condensing unit. The polarizer is positioned between the first light gathering unit and the second light gathering unit, the incident optical fiber sub-beam is provided with a first port group and a second port group, the emergent optical fiber sub-beam is provided with a third port group and a fourth port group, and the second port group of the incident optical fiber sub-beam and the third port group of the emergent optical fiber sub-beam are on the same cross section. The incident optical fiber sub-beam is used for guiding the detection light emitted by the light source incident from the first port group, is emitted from the second port group, and is vertically incident to the surface of the sample through the first light-gathering unit, the polarizer and the second light-gathering unit; and the emergent fiber sub-beam is used for guiding the detection light incident from the third port group to be reflected by the surface of the sample, then the reflected light passing through the second light-condensing unit, the polarizer and the first light-condensing unit is emergent from the fourth port, and the detection light is incident to the light detector. Preferably, the first condensing unit and the second condensing unit are both focusing lenses.
In a preferred embodiment of the present invention, the predetermined optical fiber of the reflection/back-scattering optical fiber bundle is used as the incident optical fiber sub-bundle, and the optical fiber outside the predetermined optical fiber of the reflection/back-scattering optical fiber bundle is used as the emergent optical fiber sub-bundle. Optimally, the optical fiber bundle consists of a central optical fiber and a plurality of branch optical fibers surrounding the central optical fiber, the circle centers of the plurality of branch optical fibers on the same cross section are positioned on a concentric ring of the central optical fiber and equally divide the ring, the central optical fiber is used as an incident optical fiber sub-bundle, and the plurality of branch optical fibers are used as an emergent optical fiber sub-bundle; or the central optical fiber is used as an emergent optical fiber sub-beam, and the plurality of branch optical fibers are used as incident optical fiber sub-beams.
In the present invention, the light source may be a light source including multiple wavelengths. In particular, the spectrum of the light source may be in the vacuum ultraviolet to near infrared light range, i.e. in the wavelength range of 190nm to 1100 nm. The light source may be a xenon lamp, a deuterium lamp, a tungsten lamp, a halogen lamp, a mercury lamp, a composite broadband light source containing a deuterium lamp and a tungsten lamp, a composite broadband light source containing a tungsten lamp and a halogen lamp, a composite broadband light source containing a mercury lamp and a xenon lamp, or a composite broadband light source containing a deuterium tungsten halogen, and typically the light beam of such light sources is natural light. Examples of such sources include Oceanoptics, HPX-2000, HL-2000 and DH2000, and Hamamtsu, L11034, L8706, L9841 and L10290. The light source can also be natural light formed by converting partially polarized light or polarized light by using a depolarizer. For example, the depolarizer can be a Lyot depolarizer (U.S. patent No. 6667805). The detector may be a spectrometer, in particular a spectrometer comprising a grating, a mirror, and a Charge Coupled Device (CCD) or a photodiode array (PDA), for example an Ocean OpticsQE65000 spectrometer or B&W Teck Cypher H spectrometer. In the present invention, the polarizer may be a thin film polarizer, GlanA Thompson prism polarizer, a Rochon prism polarizer, a Glan Taylor prism polarizer, and a Glan laser polarizer. In particular, the polarizer is preferably a Rochon prism polarizer, and the material thereof is preferably magnesium fluoride (MgF)2). Preferably, the normal incidence broadband polarization spectrometer for splitting the fiber bundle may further include: a polarizer rotation control device. The polarizer rotation control device is used for controlling the polarization direction of the polarizer. Preferably, the normal incidence broadband polarization spectrometer for splitting the fiber bundle may further include: and (4) a diaphragm. The diaphragm is positioned between the polarizing unit and the sample and is used for preventing e light generated after passing through the polarizing unit from being incident on the surface of the sample and/or preventing reflected light of the e light from being reflected back to the polarizing unit.
The present invention will be described in detail below with reference to specific embodiments as examples.
Example one
FIG. 2 is a schematic diagram of a normal incidence broadband polarization spectrometer for splitting a fiber bundle according to an embodiment of the present invention. As shown in fig. 2, the normal incidence broadband polarization spectrometer for splitting a fiber bundle includes: a light source 202, a detector 203, a fiber bundle 204, a sample 205, a focusing lens 206, a polarizer 207, and a focusing lens 208. The key technical scheme comprises the following steps:
the optical fiber bundle 204 is divided into three end points, the end point a includes all the paths of the optical fiber bundle 204, that is, includes a second port group of the incident optical fiber sub-bundle and a third port group of the emergent optical fiber sub-bundle, the end point B includes a first port group of the incident optical fiber sub-bundle and the end point C includes a fourth port group of the emergent optical fiber sub-bundle, the light source 202 is connected with the optical fiber bundle end point B, the emitted detection light beam is diverged and incident to the focusing lens 206 through the first port group of the incident optical fiber sub-bundle at the end a via the end point B, and is converged to form a parallel light beam via the focusing lens 206; the parallel light beams form linearly polarized light after passing through a polarizer 207; the linearly polarized light is converged by the focusing lens 208 and is incident perpendicularly to the surface of the sample 205. The reflected light beam is converged by a focusing lens 208, a polarizer 207 and a focusing lens 206 and then enters an endpoint A of the optical fiber bundle 204; part of the sample reflected light finally enters the detector 203 through the third port group of the exit fiber sub-beam at the end point a and the fourth port group of the exit fiber sub-beam at the end point C.
In this embodiment, the optical fiber bundle 204 may be a reflection/back scattering optical fiber bundle. FIG. 3 is a cross-sectional view of each port of a preferred seven-core fiber bundle for a normal incidence broadband polarization spectrometer for splitting a fiber bundle according to an embodiment of the present invention. As shown in fig. 3, the optical fiber is composed of three fiber ports. The optical fiber bundle light paths are divided into two groups; a group of dotted optical fiber cores including six optical fiber cores, and a group of solid optical fiber cores including one optical fiber core; two separate light paths are formed, respectively, an incident path and a reflection path being formed in the present embodiment. At the end A of the optical fiber bundle, two optical paths share one optical fiber interface; at the end B and the end C of the optical fiber, the two optical paths are respectively independent optical fiber interfaces. In this embodiment, port B is connected to the light source and port C is connected to the detector. Such optical fibers, for example, Oceanoptics QR230-7-XSR/BX, have a fiber core diameter of 230 μm and a spectral range of 180 nm and 900 nm. In this embodiment, the light-gathering unit may also be integrated into the incident sub-fiber and/or the exit sub-fiber.
FIG. 4 is an optical path diagram of a normal incidence broadband polarization spectrometer for splitting a fiber bundle according to an embodiment of the present invention. The port A is located at the focus of the focusing lens 206, the central optical fiber in the port A, that is, the optical fiber connected with the end B can be regarded as a point light source, and divergent light forms a parallel light beam after being converged by the focusing lens 206; the parallel light beams are formed into linearly polarized light by the polarizer 207; the normal incidence to the surface of the sample 205 is converged by the focusing lens 208. As shown in fig. 4, if the sample is located at the image plane 205' corresponding to the port a, the reflected light beam from the sample surface passes through the focusing lens 208, the polarizer 207, and the focusing lens 206, and most of the reflected light beam returns to the central fiber port of the fiber bundle, and cannot be incident on the six fiber cores distributed around the central fiber. In this case, by fine tuning the distance between the sample plane and the focusing lens 208, to the out-of-focus position 205, part of the reflected light of the sample surface will be coupled into the six fiber cores distributed around the central fiber in port a, and eventually incident to the detector.
The light propagation relationship in the defocus state is given below. In the derivation process, the situation of a single focusing lens is taken as a basis, and the situation is expanded to the situation of a double focusing lens or a multi-focusing lens. In the case of a single focusing lens. The light propagation relation of the defocused state is divided into two conditions according to an adjusting method: (1) the sample plane moves with the image plane as a reference when focusing, and (2) the fiber plane moves with the object plane as a reference when focusing.
Fig. 5 is an optical path diagram of an out-of-focus state in which the sample plane moves with reference to the image plane at the time of focusing in the case of a single focusing lens. As shown in fig. 5, if the light ray of the central exit angle θ of the optical fiber is reflected by the optical system to a point having a horizontal distance d from the exit point, in case 1) the sample plane moves by a distance h with the image plane at the time of focusing as a reference, it can be obtained by the following steps:
s·tanθ＝t·tanθ1 (2)
wherein f is the focal length of the lens, the distance s between the object plane and the focusing lens, t is the image distance when the object distance is s, h is the distance between the sample and the image plane, d is the offset distance between the intersection point and the center of the reflected light beam and the incident plane (object plane), and theta is the emergent angle of the A-end light beam.
The formula (1), (2), (3) and (4) can be used to obtain,
FIG. 6 is a diagram of the optical path of the optical fiber in the out-of-focus state when the plane of incidence of the optical fiber is shifted with reference to the object plane at the time of focusing in the case of a single focusing lens. In case 2) the sample plane is moved h with reference to the image plane at focusing, which can be derived from case 1) a similar procedure:
in the case of a double-focus lens or a multi-focus lens, it is contemplated that the focusing lens group may be converted to a single equivalent focusing lens, as shown in FIG. 7, having a focal length of
Where d is the distance between the focusing lenses and f1、f2Is the focal length of two focusing lenses
The object distance s is the distance between the object and the main surface of the object, and the distance X between the main surface and the focusing lens f1 closest to the objectHIs composed of
In the present invention, the beam incident on the polarizer 207 should be kept as parallel as possible; thus, the exit end of the bundle 204 is preferably located at the focal length of the lens f1, i.e., defocusing is preferably adjusted by adjusting the image plane position; obtained by substituting the equations (7) and (8) into the equation (5),
wherein f is2The focal length of the lens f2, h the distance between the sample and the image plane, and theta the exit angle of the A-end light beam. According to the cross-sectional structure of the seven-fiber core reflection/back scattering fiber bundle, the luminous flux calculation simulation is carried out on the condition that the diameter of the fiber core is 200 microns. Considering an ideal lens with f1 focal length of 100mm and f2 focal length of 50mm, i.e., ignoring chromatic aberration and aberration, ZEMAX light flux simulation calculation in the out-of-focus state was performed.
Fig. 8 is a simulation diagram of the ZEMAX luminous flux in the out-of-focus state of the normal incidence broadband polarization spectrometer for splitting a fiber bundle according to the embodiment of the invention under the ideal double-lens condition. As shown in fig. 8, when the outgoing light intensity of the central optical fiber is 100%, the light intensity in the annular region with the radius of 100 micrometers to 300 micrometers is 71.429% except the area occupied by the core of the incident light. The annular region contains six fiber cores occupying 3/4 area ratio of the annular area; therefore, the final luminous flux is about 53.6%; much higher than optical systems using beam splitters (as in simulation 1 listed in table one).
The defocus method of the simulation 1 comprises the following steps: the end of the optical fiber bundle 204A is fixed, the end of the optical fiber bundle 204A is held at the focal point of the lens f1, and the position h of the sample surface is adjusted to 0.1 mm. In simulation 2, only the distance between the two focusing lenses was changed from 50mm to 25 mm, and the total luminous flux was not affected, which can be concluded from equation (9). In simulation 3, the focal lengths of the focusing lens f1 and the focusing lens f2 were changed relative to simulation 1 and simulation 2, respectively, and the end position and the image distance of the optical fiber bundle 204A were changed accordingly, that is, the magnification ratio of the light spot was changed; the luminous flux remained almost unchanged from the cases in simulation 1 and simulation 2. The method can be used for adjusting the spot size of the detection beam and realizing the measurement of the sample with non-uniformity or limited sampling area. In simulation 4, the exit of the six-core fiber bundle is realized by exchanging the connection between the port B and the port C of the fiber bundle 204; in this case, the coupling efficiency of the single fiber core to the central fiber core is 11%, and assuming that the light flux of each fiber core is equal to that of the single fiber core connected light source when 6 fibers are connected to the light source, the total efficiency of 11% × 6 is 66%. Table 1 is a table of defocus simulation results of a vertical-incidence broadband polarization spectrometer for splitting optical fiber bundles according to an embodiment of the present invention.
TABLE 1 Table of defocus simulation results of vertical incidence broadband polarization spectrometer for fiber bundle splitting
| Simulation of | Structure of the product | f1 focal length | f2 focal length | Distance d | h | Overall efficiency | Cross sectional efficiency of optical fiber |
| 1 | 1 core → 6 core | 100mm | 50mm | 50mm | 0.1mm | 71％ | 54％ |
| 2 | 1 core → 6 core | 100mm | 50mm | 25mm | 0.1mm | 71％ | 54％ |
| 3 | 1 core → 6 core | 175mm | 35mm | 50mm | 0.1mm | 75％ | 56％ |
| 4 | 6 core → 1 core | 175mm | 35mm | 50mm | 0.1mm | 66％ | 66％ |
As can be seen from table 1, the luminous flux efficiency of the optical path system portion in the vertical incidence broadband polarization spectrometer for splitting optical fiber bundles according to the present invention is greater than that of the prior art, that is, the vertical incidence broadband polarization spectrometer realized by using the optical splitter. Assuming that the splitting ratio of the splitter is x, the optical path luminous flux efficiency is (1-x) x, and when x is 50%, the maximum value is 25%. Are all lower than the technical scheme of the invention. From the above description, it can be seen that a simple way to adjust the system is to adjust the surface height of the sample 205 individually so that the detector reaches the strongest signal. In addition, the size of the facula can be adjusted by setting different object distance and image distance ratios, and the measurement of a limited area (small facula) is realized.
In the case of a broad spectrum light source, the lens causes chromatic dispersion; in the case of normal incidence, accuracy is affected, although accuracy of the measurement results is not affected. In practice, a lens group with better dispersion correction capability can be selected to realize the effect of maximum correction of different focal lengths caused by dispersion. Lenses are, for example, a corrective three-piece mirror assembly, a triplexer lens or a doubler lens. An example of an ultraviolet to infrared corrected three-piece mirror assembly is now the Edmund Optics, NT64-837 and NT 64-840.
FIG. 9 shows the simulation results of ZEMAX software when EdmundOptics optics are used in a normal incidence broadband polarization spectrometer for fiber bundle splitting according to an embodiment of the present invention. In fig. 9, a shows the optical efficiency of each of the three wavelength bands 230nm, 500nm and 900nm at a distance 293mm from the surface of the sample, and considering that the average luminous flux efficiency of the three wavelength bands can reach 39.1%, which is still higher than the 25% optical efficiency of the case of using the spectroscope. When the distances from the optical fibers to the surface of the sample are different, the luminous fluxes of all the wave bands are different, and the overall measurement effect can be optimized by adjusting the distances from the optical fibers to the surface of the sample according to the spectral distribution of the light intensity of the light source. Under individual conditions, the spectrum area can be divided into a plurality of sections according to the dispersion condition, and each section respectively takes different defocus distances for measurement to achieve the best measurement result; usually 190nm-1000nm split into two sections is sufficient to meet the measurement requirements. As shown in fig. 9b, the optical efficiency at 230nm wavelength is greatly improved and the signals at 500nm and 900nm wavelength are very low in the case of 294mm distance from the fiber to the sample surface.
Preferably, a plurality of ports of a plurality of branch optical fibers of the outgoing optical fiber sub-beam of the vertical incidence broadband polarization spectrometer for splitting the optical fiber bundle are arranged in a shape corresponding to the shape of the light inlet of the optical detector, so as to improve the lighting efficiency of the optical detector. In this embodiment, when the port C of the optical fiber bundle 204 is connected to the spectrometer, the multiple optical fiber cores at the port C of the optical fiber bundle 204 can be arranged in a line according to the structure of the slit of the spectrometer, so as to obtain higher optical coupling efficiency. Fig. 10 is a schematic diagram of adjusting the arrangement of the optical fiber ports of the outgoing optical fiber sub-beam according to the shape of the spectrometer slit in the vertical incidence broadband polarization spectrometer for splitting an optical fiber beam according to the embodiment of the present invention.
As the polarizer used in the present invention, a rochon prism polarizer RP as shown in fig. 11 can be used. The Rochon prism polarizer can be made of MgF2a-BBO, calcite, YVO4Or quartz. The Rochon prism polarizer utilizes a birefringent crystal (the refractive indexes of o light and e light are different) to enable two beams of polarized light in the orthogonal direction of an incident beam to form a certain included angle to be emitted when passing through the cross surface of the Rochon prism, wherein the o light and the incident direction are kept consistent,and exits in a linearly polarized light state. With MgF2The Rochon prism polarizer of material can reach a spectral range of 130-7000 nm. Since different materials have different refractive indexes of o light and e light, the included angle of the o light and the e light in the transmitted light is also different. For example, for MgF2Or quartz, the angle between o-light and e-light is 1 to 2 degrees, however, for a-BBO or YVO4The included angle can reach 8 to 14 degrees. This angle also depends in part on the tangential angle θ of the Rochon prism and the length of the polarizer. When the probe beam is transmitted through the polarizer, o light is perpendicularly incident to the sample S, and e light is obliquely incident to the sample S; while the reflected beam of e-light may enter the polarizer optical aperture range, its reflected beam of e-light may likewise be reflected to the polarizer and then enter the detector. For the polarizer with larger e-light deflection angle, the reflected light of the e-light on the sample surface is not easy to re-enter the polarizer. To improve the measurement accuracy and avoid the influence of the reflected light of e-light, a stop D (as shown in fig. 11) may be provided above the sample surface at a position where the o-light is separated from the e-light to avoid the e-light from being incident on the sample surface or the reflected light being reflected back to the polarizer.
In the above-mentioned normal incidence broadband polarization spectrometer for fiber bundle splitting, the polarization characteristics of the light beam can be kept unchanged during the propagation process between the polarizer and the sample, and the change of the sample to the polarization state can be correctly measured by a proper measuring method, thereby calculating various properties of the sample itself.
Through the above-mentioned vertical incidence broadband polarization spectrometer for fiber bundle splitting, two measurement methods can be implemented:
(1) absolute reflectance measurements measure the absolute reflectance of two polarization states of a sample in orthogonal directions. To measure the reflectance of a sample, the following should be done:
a. measuring dark value I of spectrometerd；
b. Measuring the reflectivity of a reference sample, e.g. a bare silicon wafer, and obtaining a spectral value Ir；
c. Measuring the sample and obtaining a value I;
thus, the reflectance of a particular sample is:
R＝(I-Id)/(Ir-Id)×R(ref)
where R (ref) is the absolute reflectivity of the reference sample. R (ref) can be obtained from other measurements or calculated from characteristics of a reference sample, typically the reflectivity of bare silicon.
For example, in a one-dimensional grating structure, as shown in fig. 12, two orthogonal polarization directions are respectively defined as a direction TM perpendicular to the linear structure and a direction TE parallel to the linear structure. When the period p is 100nm, the line width w is 50 nm, and the trench depth t is 50 nm, the reflectivity is as shown in fig. 13, where the dotted line is the reflectivity in the TE polarization direction, and the solid line is the reflectivity in the TM polarization direction.
(2) Elliptical polarization measurement method: the vertical incidence spectrometer for splitting the optical fiber bundle is equivalently an ellipsometer with a polarizer-sample-analyzer (PSA) structure, wherein the analyzer and the polarizer are the same polarizer. Ellipsometry can measure the reflectivity ratio of TM/TE in two polarization states and the phase difference caused by the sample on TM and TE. The specific measurement principle can be referred to book HANDBOOK OF ELLIPSOMETRY, Harland G. Tompkins, 2005; spectroscopic electrophoresis Principles and Applications, Hiroyuki Fujiwara, 2007; the principle formulas illustrated in U.S. patent No.7115858B1 and U.S. patent No.7330259B2 the following are only briefly described. The total transmittance is given by the jones matrix,
i(out)＝JAR(A)JSR(-P)i(in)，
wherein
Is a Jones matrix reflected by the sample, and x and y are two orthogonal polarization directions;
when A ═ P, for rxy+ryxIn the case of 0 (see, for example, Li life, j.opt.soc.am.a17, 881 (2000)), the above equation can be simplified as:
wherein,
a0、a2、a4the Fourier coefficients can be calculated according to Fourier expansion or linear fitting. Δ is the phase difference between x-and y-polarized light due to reflection from the sample, RxxIs the reflectivity, rxxTan phi ═ r for the reflection constantxx/ryyL. Finally, the process is carried out in a batch,
the detailed operation of ellipsometry consists of the following three main steps: 1) due to the presence of the rotating system, the system needs to be calibrated to exclude measured light intensity deviations caused by the rotation of the polarizer. The calibration method is to use a standard uniform sample, such as a silicon wafer, to measure the light intensity of the uniform sample at different polarizer angles; theoretically, the light intensities should be identical; the variation relation of the light intensity and the angle can be used as a reference value, and the light intensity influence of the system at different polarizer angles is removed through the ratio. Specifically, the reflected light intensity spectrum of the silicon wafer at each angle is recorded every time the polarizer rotates by 1 degree, and all 360-degree scanning is completed, and the data are stored as reference values. 2) During measurement, the reflected light intensity of each angle is compared with a reference value, and a relative true value of the light intensity at each angle is obtained. In the elliptical polarization measurement, the calculated value is the relative ratio of the intensity and the phase of the optical signal, not the absolute intensity, and does not influence the data processing and the measurement result. 3) The TM/TE reflectance ratio and the spectrum of the phase difference between TM and TE are calculated. Taking the structure shown in fig. 12 as an example, the spectrum is shown in fig. 14.
After the absolute reflectivity of TE and TM or the amplitude ratio and the phase difference of TM/TE are obtained through measurement, the critical dimension, the three-dimensional appearance and the film thickness and the optical constant of the multilayer material of the periodic pattern on the surface of the sample can be measured through comparison with a numerical simulation result and numerical regression calculation. In this case, the normal incidence broadband polarization spectrometer for fiber bundle splitting may further comprise a calculation unit for calculating optical constants of the sample material and/or for analyzing critical dimension characteristics or three-dimensional topography of the periodic microstructure of the sample material by mathematical model calculation and curve regression fitting of the reflectivity.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (17)
1. A normal incidence broadband polarization spectrometer for splitting a fiber bundle, comprising: a light source, a fiber bundle, a polarization unit and a light detector,
the optical fiber bundle comprises an incident optical fiber sub-bundle and an emergent optical fiber sub-bundle, and the incident optical fiber sub-bundle and the emergent optical fiber sub-bundle both comprise at least one optical fiber or optical fiber core; the incident fiber sub-beam has a first port group and a second port group, the exit fiber sub-beam has a third port group and a fourth port group, and the second port group of the incident fiber sub-beam and the third port group of the exit fiber sub-beam are on the same cross section;
the incident optical fiber sub-beam is used for guiding the detection light emitted by the light source and incident from the first port group, emitting from the second port group, and then incident on the surface of the sample after passing through the polarization unit; the emergent fiber sub-beam is used for guiding the reflected light of the sample surface incident from the third port group after passing through the polarization unit, emergent from the fourth port and incident to the light detector;
the polarization unit is positioned between the same cross section and the sample and is used for enabling the detection light emitted from the second port group to form polarized light; and analyzing the reflected light from the sample surface.
2. The fiber bundle split normal incidence broadband polarization spectrometer of claim 1, wherein: the incident optical fiber sub-beam is a preset optical fiber of the reflection/back scattering optical fiber beam, and the emergent optical fiber sub-beam is an optical fiber outside the preset optical fiber of the reflection/back scattering optical fiber beam.
3. The fiber bundle splitting normal incidence broadband polarization spectrometer of claim 2, wherein:
the optical fiber bundle consists of a central optical fiber and a plurality of branch optical fibers surrounding the central optical fiber, the circle centers of the plurality of branch optical fibers on the same cross section are positioned on a concentric ring of the central optical fiber, and the ring is equally divided;
the central optical fiber is used as an incident optical fiber sub-beam, and the branch optical fibers are used as emergent optical fiber sub-beams; or the central optical fiber is used as an emergent optical fiber sub-beam, and the branch optical fibers are used as incident optical fiber sub-beams.
4. The fiber bundle split normal incidence broadband polarization spectrometer of claim 3, wherein: when the plurality of branch optical fibers are used as emergent optical fiber sub-beams, the fourth port groups of the plurality of branch optical fibers are arranged into shapes corresponding to the shapes of the light inlets of the optical detectors.
5. The fiber bundle split normal incidence broadband polarization spectrometer of claim 1, further comprising: a polarization unit rotation control device for controlling the rotation of the polarization unit,
the polarization unit rotation control device is used for controlling the polarization direction of the polarization unit.
6. The fiber bundle split normal incidence broadband polarization spectrometer of claim 5, further comprising: the light diaphragm is arranged on the light guide plate,
the diaphragm is positioned between the polarizing unit and the sample and is used for preventing e light generated after passing through the polarizing unit from being incident on the surface of the sample and/or preventing reflected light of the e light from being reflected back to the polarizing unit.
7. The normal-incidence broadband polarization spectrometer of the fiber bundle split according to claim 5, characterized in that the polarizing unit is a thin film polarizer, a Glan Thompson prism polarizer, a Rochon prism polarizer, a Glan Taylor prism polarizer or a Glan laser polarizer.
8. The normal incidence broadband polarization spectrometer for splitting of fiber bundles according to claim 7, wherein the polarization unit is a Rochon prism polarizer made of one of the following materials:
MgF2a-BBO, calcite, YVO4Or quartz.
9. The fiber bundle split normal incidence broadband polarization spectrometer of claim 1, further comprising: the light-gathering component is arranged on the light-gathering component,
the light-gathering component is positioned between the same cross section and the sample and is used for gathering the detection light emitted from the second port group to the surface of the sample after passing through the polarizing unit, and gathering the reflection light on the surface of the sample to the third port group after passing through the polarizing unit.
10. The fiber bundle split normal incidence broadband polarization spectrometer of claim 9, wherein the light focusing assembly comprises: a first condensing unit and a second condensing unit;
the first light gathering unit is positioned between the same cross section and the polarization unit and is used for gathering the detection light incident from the first port group to form incident approximately parallel light to the polarization unit; and converging the outgoing linearly polarized light after passing through the polarization unit to the third port group;
the polarization unit is used for polarizing the detection light emitted from the second port group into the incident linearly polarized light; and for analyzing said reflected light from said sample surface into said outgoing linearly polarized light;
the second light-condensing unit is positioned between the polarizing unit and the sample and is used for condensing the incident linearly polarized light to the surface of the sample; and the reflected light of the sample surface is converged to form emergent approximately parallel light to the polarizing unit.
11. The fiber bundle split normal incidence broadband polarization spectrometer of claim 10, wherein the first focusing unit is a first focusing lens and the second focusing unit is a second focusing lens.
12. The fiber bundle split normal incidence broadband polarization spectrometer of claim 11, wherein the first focusing lens and/or the second focusing lens is a corrective three-piece mirror assembly, a triplexer lens or a doublet lens.
13. The fiber bundle split normal incidence broadband polarization spectrometer of claim 11, wherein: the second port group is positioned at the focal position of the first focusing lens, and when the sample plane moves by taking the image plane as a reference, the relation is satisfied:
wherein the image plane is the total image plane of the first and second focusing lenses, f2The focal length of the second focusing lens is h is the distance between the sample and the image plane, d is the offset distance between the exit point of the detection light positioned in the second port group and the exit point in the exit plane after the exit light beam is reflected by the sample, and theta is the angle of the detection light exiting from the second port group.
14. The normal incidence broadband polarization spectrometer for fiber bundle splitting according to any one of claims 1-13, wherein: the light detector is a spectrometer.
15. The fiber bundle split normal incidence broadband polarization spectrometer of claim 14, further comprising:
and the calculating unit is used for calculating the optical constant and the film thickness of the sample material and/or analyzing the critical scale characteristic or the three-dimensional morphology of the sample with the periodic structure by comparing mathematical model calculation and curve regression fitting according to the absolute reflectivity of the sample in the TE and TM directions and/or the amplitude ratio and the phase difference of the sample in the TE and TM directions, which are measured by the spectrometer.
16. The fiber bundle split normal incidence broadband polarization spectrometer of any one of claims 1-13, wherein the light source is a light source comprising multiple wavelengths.
17. An optical measurement system comprising a normal incidence broadband polarization spectrometer splitting the fiber bundle of any of claims 1-16.
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| CN108286939A (en) * | 2018-02-10 | 2018-07-17 | 北京工业大学 | A kind of laser traces measurement optical system Energy Analysis for High based on ZEMAX emulation |
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