CN103776537B - A kind of measurement mechanism of polarised light stokes parameter and optimization method thereof - Google Patents

Abstract

The invention discloses a polarized light Stokes parameter measuring device and an optimization method thereof. The measuring device includes a beam splitter and wave plates introduced respectively on two splitting paths of the beam splitter, a splitting device, two photoelectric detector. After the incident polarized light passes through the beam splitter, two beams of polarized light with different polarization states are generated, and the two beams of polarized light are vertically incident on the two wave plates, and through the phase modulation of the wave plate, two beams with new polarization states are generated. light beam; the other two beam splitters convert the two beams of polarized light into four beams of polarized light, which are perpendicularly incident on the four photodetectors to generate corresponding current signals and adjust the azimuth angles of the two wave plates to the best azimuth angles , which can optimize the instrument matrix. After the instrument matrix is obtained by calibration, the current signal caused by the light to be measured is calculated and solved, and the real-time measurement of the Stokes parameters of the incident light can be realized. The structure and optimization method of the invention are simple, easy to implement, high in measurement accuracy and stronger in adaptability.

Description

A Measuring Device and Optimizing Method for Stokes Parameter of Polarized Light

technical field

The invention belongs to the technical field of optical measurement and metrology, and in particular relates to a measuring device and an optimization method for polarized light Stokes parameters.

Background technique

The accurate and fast measurement of the Stokes parameter of the beam polarization state has wide application significance in industry, military affairs and scientific research. Polarized Stokes parametric ellipsometer is a typical application. It has the characteristics of fast measurement speed and high measurement accuracy. It has important applications in many fields such as optical measurement, thin film and material property research, and photolithographic imaging. The devices for measuring the Tox parameters of the polarization state of light mainly include: the photometric measurement device of the rotating element and the optical polarization measurement device of the sub-amplitude (DOAP). At present, more than ten kinds of amplitude-divided light polarization measurement devices with different structures have been developed at home and abroad, mainly including coated beam splitter type, four-detector type, metal grating type, liquid crystal type and optical fiber type, etc. In 1982, the American scholar R.M.A.Azzam designed the first device (DOAP) to measure the polarization of light using the amplitude division method. It does not have any rotating parts or modulators and has a simple structure. But its structure has not been optimized. Actual research shows that there are certain requirements for the design parameters of the coated beamsplitter in DOAP, so that the system can be optimized, the measurement accuracy is the highest, the stability is the best, and the sample adaptability is the strongest. However, it is not easy to obtain a coating beam splitter that meets the requirements. It is necessary to accurately design and control the coating process and parameters. Aiming at the problems existing in the device for measuring light polarization by the amplitude division method, on the basis of this structure, the present invention introduces two wave plates with phase modulation function, constructs an amplitude-divided light polarization measurement device based on phase modulation, and proposes a system Optimization methods for instrument matrices.

Contents of the invention

In order to solve the problems existing in the prior art, the object of the present invention is to provide a measuring device and optimization method for the Stokes parameter of sub-amplitude polarized light, which not only avoids special and complicated coating or etching processes, but also improves the polarization The reliability, measurement accuracy and adaptability of optical parameters, the specific technical solutions are as follows.

A device for measuring Stokes parameters of polarized light, which includes a beam splitter that divides incident light into transmitted light and reflected light, and sequentially includes a first wave plate for phase modulation in the transmitted light path, and a device for realizing polarization splitting The first light splitting device and the first photodetector and the second photodetector respectively receive the two beams of light split by the first light splitting device; the reflected light path also includes a second wave plate for phase modulation in order to realize polarization The second light-splitting device for splitting light and the third photodetector and the fourth photodetector that respectively receive the two beams of light split by the second light-splitting device; the light intensity signal output ends of the four detectors are connected by the data acquisition card and Electronic computer connection for data processing.

In the above-mentioned measuring device for the Stokes parameter of polarized light, the transmitted light and the reflected light are two beams of polarized light with different polarization states, the transmitted light is vertically incident on the first wave plate, and the reflected light is vertically incident on the second wave plate , the first beam-splitting device and the second beam-splitting device further convert the two beams of polarized light with different polarization states into four beams of polarized light, and the four beams of polarized light generate corresponding current signals after passing through the four detectors; The azimuth angles of the first wave plate and the second wave plate are adjustable. Adjusting the azimuth angles of the two wave plates to the optimum azimuth angle can optimize the instrument matrix. After the instrument matrix is obtained through calibration, the Calculate and solve the current signal of the incident light to realize the real-time measurement of the Stokes parameters of the incident light.

Theoretically, there are many combinations of the phase retardation of the first wave plate and the second wave plate, the typical ones are: the combination of the first wave plate and the second wave plate adopts half wave plate and quarter wave plate Combine each other or use a combination of half-wave plate and quarter-wave plate, half-wave plate and half-wave plate, quarter-wave plate and quarter-wave plate, or A combination of a quarter-wave plate and its wave plates with different phase delays, a combination of a half-wave plate and its wave plates with different phase delays, etc.

The optimization method of the measuring device of the above-mentioned polarized light Stokes parameter can adopt the actual measurement method, and the specific process is: under the conditions that the optical path structure of the measuring device is determined and each optical element is selected, by adjusting the first wave plate, the second The azimuth angle of the wave plate is optimized, that is, by actually measuring the determinant size of the instrument matrix of the measuring device under different azimuth angles, the relationship curve between the size of the matrix determinant and the azimuth angle of the wave plate is obtained to determine the corresponding value when the instrument matrix is the largest. The optimal azimuth angle of the wave plate realizes the optimization of the instrument matrix of the measuring device.

The optimization method of the measuring device of the Stokes parameter of the above-mentioned polarized light can adopt the simulation method, and the specific process is: use an optical measuring instrument or equipment to actually measure the ellipsometric parameters of all the light splitting elements in the optical path of the measuring device, and obtain a single The matrix expression of the spectroscopic element, and calculate the instrument matrix of the measuring device, carry out numerical simulation and analysis on the instrument matrix of the measuring device and its determinant, and obtain the optimal azimuth angle of the wave plate corresponding to the maximum instrument matrix, and realize the instrumentation of the measuring device Matrix optimization. In this method, by fixing the azimuth angle of one of the two wave plates, the numerical simulation method is used to calculate the change curve of the instrument matrix determinant with the azimuth angle of the other wave plate, and the abscissa corresponding to the maximum value on the curve is the other wave plate The best azimuth angle of the slice.

In the above optimization method, the Stokes parameter of the incident light is set as , Let T and R represent the transmission matrix and reflection matrix of the beam splitter (1), respectively, , is the transmission matrix and reflection matrix of the first light splitting device (4), , is the transmission matrix and reflection matrix of the second light splitting device (5), is the photoelectric conversion coefficient of the photodetector, , are the transmission matrices of the first wave plate and the second wave plate in the transmitted optical path and the reflected optical path, respectively, then the current signals generated by the four photodetectors are expressed as:

First detector:

Second detector:

Third detector:

Fourth detector: ,

Let D represent the instrument matrix of the entire measuring device, and I represent the current matrix formed by the current signals of each photodetector, then

(1),

The instrument matrix D is calculated by the calibration method, if there is an invertible matrix in the instrument matrix D , then the Stokes vector of the ray to be measured is

(2),

The instrument matrix of the measuring device is known, and the Stokes vector of any light is determined by the detected electrical signal vector;

Let ABS=

ABS= (3).

Among them, det means to find the determinant of the matrix, and the absolute value ABS of the determinant is maximized by adjusting the azimuth angle of the wave plate to realize the optimization of the instrument matrix.

When the linear independence of each row of the instrument matrix D is greater, the instrument matrix will be more optimized, and the measurement accuracy of the polarization state will be greatly improved. In this case, the ABS corresponding to the determinant is the largest mathematically, so choose the appropriate optical element , making appropriate azimuth adjustments to maximize the absolute value of the determinant, ABS, can achieve the optimization of the instrument matrix.

Compared with the prior art, the present invention has the following advantages:

(1) The method of the present invention can realize the optimization of the equipment matrix. The optimization method and process are simple, reliable and easy to implement.

(2) The device of the present invention has a simple structure, does not require any special processing and coating process, and the required components are easy to obtain, and the optical path is convenient to adjust, and the parameter settings can be changed at any time according to various selected spectroscopic devices.

(3) The device of the present invention measures polarized light Stokes parameters with high precision, more stable and reliable results, and stronger adaptability; not only is the cost cheap, but also can be miniaturized, and is suitable for various actual measurement systems, such as ellipsometers, etc. .

Description of drawings

Fig. 1 is the optical path structure and schematic diagram of the measuring device of the present invention;

Fig. 2 is a schematic diagram of the optical path structure of a specific embodiment of the measuring device of the present invention;

Fig. 3 is a schematic diagram of the structure and optical path of the Stokes ellipsometer.

detailed description

In order to understand the present invention better, below in conjunction with accompanying drawing and implementation example---the application of this device in Stokes ellipsometer, further illustrate the optimization method and process of device structure of the present invention and instrument matrix thereof, but the present invention Implementation and protection are not limited to this.

As shown in accompanying drawing 1, it is an optical path structure and schematic diagram of a measuring device for polarized light Stokes parameters, and the incident light is usually divided into two beams of light after passing through the beam splitter 1, that is, transmitted light and reflected light; A wave plate (the first wave plate 2 and the second wave plate 3) for phase modulation is respectively introduced into the transmitted optical path and the reflected optical path; the first wave plate 2 and the first optical splitter 4 used for phase modulation are placed in sequence in the transmitted optical path , the first photodetector 6, the second photodetector 7; the reflected optical path is similar to the transmitted optical path, and the second wave plate 3, the second spectroscopic device 5, the third photodetector 8, and the fourth optical path for phase modulation are placed in sequence. Photodetector 9. The azimuth angle of the wave plate is closely related to the ellipsometric parameters of the used optical splitting device. The incident light passes through the beam splitter and the wave plate, splits the light again, and then enters the four detectors. The light intensity signal output by the detectors is transmitted to the electronic computer for data processing through the data acquisition card. The instrument matrix of the measuring device can be optimized by adjusting the azimuth angle of the wave plate. The measurement or calibration of the instrument matrix can be realized by using multiple (four or more) polarized light beams with known Stokes parameters to be sequentially incident into the measurement device.

The spectroscopic devices (1, 4, 5) used in the present invention have more options, such as ordinary coated spectroscopic plane mirrors, polarized coated spectroscopic prisms, Wollaston prisms, triangular prisms and wedge prisms. The wave plate can choose devices made of materials such as mica, quartz lamps or liquid crystals. The phase delays of the first wave plate 2 and the second wave plate 3 can be selected differently, and there are many possible combinations. The combination of common half-wave plate and quarter-wave plate can also be used to optimize the instrument matrix. It is also possible to use a combination of a half wave plate (first wave plate 2) and a quarter wave plate (second wave plate 3), a combination of a half wave plate and a half wave plate, a quarter wave plate A combination of a wave plate and a quarter wave plate, or a combination of a quarter wave plate and wave plates with different phase delays, a combination of a half wave plate and wave plates with different phase delays, etc.

It can be seen from the aforementioned optimization method that after the spectroscopic device and detector are selected, is a certain value, the main parameters affecting ABS are only , , after the wave plate phase delay is determined, , The parameters of are mainly determined by the azimuth angles of the two wave plates, so the optimization degree of the instrument matrix depends on the value of the azimuth angles of the wave plates.

Through the above analysis, the measuring device of the present invention can adopt the following two optimization methods.

Optimization method 1:

Determine the best azimuth angle of the wave plate by actual measurement: After selecting the spectroscopic device and the wave plate, change the azimuth angle by rotating the wave plate, and measure and record the size of the ABS of the instrument matrix under different azimuth angles. ABS can be measured and calculated by E-P method, four-point in-situ calibration method or multi-point calibration method. The azimuth corresponding to the maximum value of ABS is the best azimuth of the wave plate.

Optimization method 2:

Determining the best azimuth angle of the wave plate by simulation method: After selecting the type of the optical splitting device and the wave plate, measure the ellipsometric parameters of the optical splitting device (1, 4, 5) with an extinction ellipsometer, and compare it with the retardation of the wave plate The parameters are brought into formula (3) to derive the expression of ABS. Fix the azimuth angle of one of the wave plates, and use the numerical simulation method to calculate the change curve of ABS with the azimuth angle of the other wave plate. The abscissa corresponding to the maximum value on the curve is the optimal azimuth angle of the wave plate.

In accompanying drawing 2, the beam splitter 1 adopts the ordinary coated dichroic prism, the first wave plate 2 is approximately a half wave plate, the second wave plate 3 is approximately a quarter wave plate, and the first light splitting device 4 adopts Wollaston prism, the second light splitting device 5 also adopts Wollaston prism, 6, 7, 8, 9 are photodetectors, and the first wave plate rotator 10, the second wave plate rotator 11 are also included in the figure , Prism holder (12, 13, 14), 15 is the incident light hole, 16 is the reference hole. Among them, the ordinary coated beam splitting prism adopts the GCC-401021 model produced by Daheng Optoelectronics Company. The Wollaston prism adopts the GCC-402032 model produced by Daheng Optoelectronics Company. The photodetectors (6, 7, 8, 9) are photovoltaic detectors, and the PIN-13DP photodiodes produced by AnOSI Systems Company of the United States are selected. The first wave plate 2 and the second wave plate 3 can be selected as a half wave plate and a quarter wave plate made of materials such as mica or quartz, respectively. The accuracy of the first wave plate rotator 10 and the second wave plate rotator 11 is 2°. The diameters of the incident light hole 15 and the reference hole 16 are about 2-3 mm. In accompanying drawing 2, all devices are placed in a small black box 17, the box is airtight, the inner surface of the box is black, and the size of the black box is about 15*7*13 (cm) , There are openings at the front and rear of the box.

Fig. 3 is a device diagram of Stokes ellipsometer instrument matrix measurement and sample measurement, including: a helium-neon laser 18, a polarizer 19, a sample stage 20, a standard film or a sample film 21. The wavelength of the laser is 632.8nm, the polarization angle of the polarizer is adjustable, and the rotation accuracy is 0.5°. The sample stage is a controllable lifting table, and its height and horizontal inclination angle can be adjusted.

The device provided by this embodiment example can be used to measure the Stokes parameter of polarized light and the refractive index and thickness of the thin film sample, wherein the assembly and measurement process of the measuring device is realized through the following steps:

(1) Beam splitter 1, that is, the installation of the beam splitter prism: the laser beam is incident vertically into the box from the incident light hole 15, exits from the reference hole 16, and is put into the beam splitter prism 1. The emitted light is also emitted from the reference hole, and it is fixed with the prism holder 12, and the position of the emitted light point of the reflected light path on the box is marked S.

(2) Installation of the Wollaston prism (the first beam splitting device 4 and the second beam splitting device 5): similar to the installation of the beam splitting prism, after putting the first Wollaston prism in, adjust its position so that the transmitted light path The outgoing light can pass through the reference hole smoothly. Similarly, after the second Wollaston prism is placed, the exit point of the reflected light path is still located at S. After determining the position of the prisms, fix the two prisms respectively with the fixing brackets (13, 14).

(3) Use the simulation method to find the best azimuth angle of the wave plate. The specific steps to determine and install the best azimuth angle of the wave plate are as follows:

Step 1: Use the HST type extinction ellipsometer to measure the transmission ellipsometric parameters and reflection ellipsometric parameters of the coated beamsplitter prism, and the transmission matrices of the first Wollaston prism and the second Wollaston prism are known.

The second step: the selection of the wave plate. The measured phase delays of the first wave plate 2 and the second wave plate 3 are 84.7° and 162.2°, respectively.

Step 3: Bring the ellipsometric parameters of each prism and the phase delay of the wave plate into the formula (3) for simulation analysis, and obtain multiple sets of optimal azimuth angles, and select one of them, that is, the first wave plate 2 and the second wave plate The optimal azimuth angles of wave plate 3 are (180°, 225°).

Step 4: Install the first wave plate 2 and the second wave plate 3 into the wave plate rotating frame (10, 11) respectively, rotate to the best azimuth position and then fix them.

The fifth step: the installation of the detector. The positions of the four photodetectors are respectively adjusted so that the four incident light spots are respectively located at the central positions of the four detectors, and the detectors are connected with the data acquisition card.

(4) Measurement and calibration of instrument matrix. A four-point calibration method is used to measure the size of the instrument matrix in the optimal state of the system. Use SiO ₂ standard film sheets with thicknesses of 57.3nm., 199.3nm, 86.0nm, and 9.5nm and a refractive index of 1.46 for calibration. After the laser is preheated for half an hour, the four standard film sheets are sequentially Placed on the sample stage 20, the helium-neon laser emits laser light that passes through the polarizer, that is, the 19-polarizer (located at an azimuth angle of 45°), and then enters the sample with linearly polarized light at an incident angle of 70°, and the light reflected by the sample Measure the four beams of light, and use the labview automatic acquisition program to record the collected four current signals accordingly . Block the incident light hole 15 to obtain a set of detector dark current signals ,but , solve the instrument matrix D according to the formula (1), and save it.

So far, the optimization of the Stokes ellipsometry instrument system has been completed and the calibration of the instrument matrix of the device has been realized. For example, a film sample with unknown parameters is placed on the sample stage, and the collected current signal is substituted into formula (3), and the Stokes parameter S of the polarized light reflected by the sample is calculated, and then the refraction of the film is calculated according to the ellipsometric equation Parameters such as rate and thickness.

Table 1 shows the measurement results obtained by using the Stokes ellipsometer developed by the device and instrument matrix optimization method of the present invention. Experiments show that the system has good experimental stability, high measurement accuracy and strong sample adaptability.

Table 1 Experimental measurement results of Peking University standard film sheet

standard sample no d (nm) 1 Thin film (n=1.46, d=100nm) 79.01° 41.42° 1.463±0.003 100.1±0.2

The patent of the present invention has multiple specific embodiments. Therefore, as long as two wave plates are respectively introduced into the two beam paths in the measuring device of the split-amplitude polarized light Stokes parameters, the optimized technical scheme and measuring device of the system instrument matrix can be realized by rotating the azimuth angle of the wave plate , all belong to the scope of protection of this patent.

Claims (4)

1. a measurement mechanism for polarised light stokes parameter, is characterized in that comprising incident light is divided into transmitted light and reflectionThe beam splitter of light, also comprises the first wave plate of modulating mutually for position, the first light splitting that realizes polarization spectro successively at transmitted light pathDevice and receive respectively the first photodetector and second photodetector of the two-beam after the first light-splitting device light splitting; ?On reflected light path, also comprise successively the second wave plate of modulating mutually for position, realize the second light-splitting device of polarization spectro and connect respectivelyReceive the 3rd photodetector and the 4th photodetector of the two-beam after the second light-splitting device light splitting; Four photodetectorsLight intensity signal output by data collecting card be connected for the electronic computer that data are processed; The first wave plate positionThe combination of the bit phase delay amount of phase retardation amount and the second wave plate comprises: 1/2nd wave plates and quarter-wave plate combination, four pointsOne of wave plate and 1/2nd wave plate combined, 1/2nd wave plates and 1/2nd wave plate combined, quarter-wave plate and four pointsOne of not coordination of the wave plate combined of wave plate combined, quarter-wave plate bit phase delay amounts different from it or 1/2nd wave plates and itsThe wave plate combined of phase retardation amount; Described transmitted light is the different polarised light of two bundle polarization states with reverberation, transmitted light vertical incidenceTo the first wave plate, reverberation impinges perpendicularly on the second wave plate, described in the first light-splitting device, the second light-splitting device are further incited somebody to actionThe different polarised light of two bundle polarization states becomes four bundle polarised lights, and four bundle polarised lights produce corresponding electricity after four photodetectorsStream signal; The azimuth of described the first wave plate, the second wave plate is adjustable, regulates the azimuth of two blocks of wave plates to optimum azimuth, energyMake instrument matrix optimization, solve after instrument matrix by calibration, will treat that the caused current signal of photometry calculates to askSeparate, realize the real-time measurement of incident light stokes parameter.

2. for optimizing the method for a kind of measurement mechanism of polarised light stokes parameter described in claim 1, it is characterized in thatAdopt measurement method, detailed process is: under the condition that light channel structure is determined and each optical element is selected of measurement mechanism, by adjustingSave the azimuth of the first wave plate, the second wave plate and realize optimization, by the instrument of measurement mechanism under actual measurement different orientationsDeterminant of a matrix size, the size of acquisition matrix determinant and the azimuthal relation curve of wave plate and then definite when instrument matrixCorresponding wave plate optimum azimuth when maximum, realizes the instrument matrix optimization of measurement mechanism; Wherein fix in two blocks of wave platesThe azimuth of one, adopts method for numerical simulation, and computing equipment matrix determinant is with the azimuthal change curve of another piece wave plate,On curve, the corresponding abscissa of maximum is the optimum azimuth of another piece wave plate.

3. for optimizing the method for a kind of measurement mechanism of polarised light stokes parameter described in claim 1, it is characterized in thatAdopt simulation, detailed process is: utilize optical gauge or equipment, and all in the light path of measurement mechanism described in actual measurementThe ellipsometric parameter of beam splitter, obtains the matrix expression of single beam splitter, and the instrument matrix of computation and measurement device, to surveyingInstrument matrix and the determinant thereof of amount device carry out numerical simulation and analysis, and corresponding wave plate is when the most maximum to obtain instrument matrixGood azimuth, realizes the optimization of measurement mechanism instrument matrix.

4. according to method described in claim 2 or 3, it is characterized in that:

If the stokes parameter of incident light is S (S₀，S₁，S₂，S₃), make T, R represent respectively transmission matrix and the reflection of beam splitterMatrix, T₁、R₁Be transmission matrix and the reflection matrix of the first light-splitting device, T₂、R₂Be that the transmission matrix of the second light-splitting device is with anti-Penetrate matrix, d₁For the photoelectric conversion factors of photodetector, M_t、M_rBe respectively the first wave plate in transmitted light path and reflected light pathWith the transmission matrix of the second wave plate, the current signal that four photodetectors produce is expressed as:

The first photodetector: I₀＝d₁T₁M_tTS

The second photodetector: I₁＝d₁R₁M_tTS

The 3rd photodetector: I₂＝d₁R₂M_rRS

The 4th photodetector: I₃＝d₁T₂M_rRS，

Make D represent the instrument matrix of whole measurement mechanism, I represents the current matrix that each photodetector currents signal forms,?

I＝D·S(1)，

Instrument matrix D is obtained by calibrating method, if instrument matrix D exists invertible matrix D^-1, the stoke of light so to be measuredThis vector is

S＝D^-1·I(2)，

The instrument matrix of known measurement mechanism, determines the Stokes vector of any light by the signal of telecommunication vector of surveying;

Make ABS=|det (D) |

ABS＝f(T，R，T₁，R₁，T₂，R₂，d₁，M_r，M_t)(3)，

Wherein, det represents Matrix Calculating determinant, by adjust to make determinant absolute value ABS to maximize to wave plate azimuth,Realize the optimization of instrument matrix.