A terahertz mueller matrix ellipsometer and a method of measuring a thin film sample

Abstract

The application discloses a terahertz Mueller matrix ellipsometer and a measuring method for a thin film sample, and belongs to the technical field of ellipsometric measurement. The application discloses a terahertz Mueller matrix ellipsometer, the included angle between the polarizing support and the vertical direction is θ0 degrees, the terahertz emission source, the first collimating lens, the first hollow rotating motor and the first focusing lens are sequentially arranged on the polarizing support and gradually close to the sample placing table, the second collimating lens, the second hollow rotating motor, the second focusing lens and the terahertz detector are sequentially arranged on the detecting support and gradually far away from the sample placing table, the first wave plate and the second wave plate are single-frequency quarter-wave plates and are respectively fixedly arranged in the corresponding hollow rotating motor, the signal acquisition and processing system records the light intensity signals received by the terahertz detector under different angles, and the Mueller matrix of the measured thin film sample is obtained according to the multiple sets of light intensity signal data, and the thickness and the refractive index of the measured thin film sample are obtained through inversion.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ellipsometry, and more particularly, to a terahertz Mueller matrix ellipsometer and a measurement method for a thin film sample. BACKGROUND

[0002] Terahertz wave (THz wave for short) refers to a section of electromagnetic wave with a frequency of 0.1 THz-10 THz (wavelength of 30 μm-3 mm), which is located between the infrared wave and the microwave wave band. The terahertz wave has important application prospects in safety inspection, nondestructive testing, medical diagnosis and the like. The terahertz wave has good penetration for many dielectric materials, and thus can be used to detect internal information of an object, which is of great significance to safety inspection and engineering detection. Moreover, the low energy of the terahertz wave makes it possible to achieve nondestructive testing, and the safety is much higher than that of electromagnetic waves in other wave bands. Therefore, the terahertz wave is used in an ellipsometric thickness measurement system, which does not damage the sample for thickness measurement, and it can precisely measure the thickness of a millimeter level and a hundred micron level thin film, which is not possible for visible light.

[0003] Ellipsometry is an optical method for studying the phenomena and characteristics occurring in the interface between two media or a thin film, and its principle is to use the polarization transformation of a polarized light beam when the light beam is reflected or projected on the interface or the thin film. The application range of ellipsometry is very wide, such as semiconductors, optical thin films, circular crystals, metals, dielectric thin films, glass (or coated film), laser mirrors, large-area optical films, organic thin films, etc., and it can also be used for measurement of dielectric, amorphous semiconductor, polymer thin film, real-time monitoring during thin film growth, etc. After being combined with a computer, it has the advantages of manual change of incident angle, real-time measurement, fast data acquisition, etc. After the polarized light is reflected by the thin film system, the change in the polarization state is related to the thickness and refractive index of the thin film, and thus as long as the change in the polarization state is measured, the film thickness and refractive index can be determined by using a computer program for multiple approximations.

[0004] At present, the research on Mueller ellipsometry is mainly concentrated in the visible light wave band, and the measurement range is about hundreds of nanometers, which cannot measure samples with a thickness of hundreds of microns. Moreover, visible light cannot penetrate most dielectric materials. Due to the unique wavelength range of the terahertz wave, the terahertz ellipsometer can measure samples with a thickness of hundreds of microns and most samples containing dielectric materials. However, since there is no complex frequency wave plate that can be used in the terahertz wave band, there is no mature terahertz Mueller matrix ellipsometer reported at present.

[0005] Therefore, the existing Mueller matrix ellipsometer cannot be applied to the terahertz wave band, and the sample detection range is limited. SUMMARY

[0006] In view of the defects of the prior art, the present application aims to provide a terahertz Mueller matrix ellipsometer and a method for measuring a thin film sample, so as to solve the problem that the existing Mueller matrix ellipsometer cannot be applied to the terahertz wave band and the sample detection range is limited.

[0007] To achieve the above-mentioned purpose, in a first aspect, the present application provides a terahertz Mueller matrix ellipsometer, comprising:

[0008] a polarizing support and an analyzing support, wherein the angle between the polarizing support and the analyzing support and the vertical direction is θ0 degrees;

[0009] a sample placement table horizontally placed between the polarizing support and the analyzing support;

[0010] a terahertz emission source, a first collimating lens, a first hollow rotary motor and a first focusing lens arranged along the polarizing support and gradually approaching the sample placement table;

[0011] a first wave plate, which is a single-frequency quarter wave plate and is fixedly arranged in the first hollow rotary motor;

[0012] a second collimating lens, a second hollow rotary motor, a second focusing lens and a terahertz detector arranged along the analyzing support and gradually moving away from the sample placement table;

[0013] a second wave plate, which is a single-frequency quarter wave plate and is fixedly arranged in the second hollow rotary motor;

[0014] a motor controller connected to the first hollow rotary motor and the second hollow rotary motor, for simultaneously controlling the rotation angles of the first hollow rotary motor and the second hollow rotary motor;

[0015] and a signal acquisition and processing system connected to the terahertz detector;

[0016] In operation, the terahertz linearly polarized wave beam emitted by the terahertz emission source is collimated by the first collimating lens, changes the polarization state after passing through the first wave plate in the first hollow rotary motor, converges to the thin film sample to be measured on the sample placement table after passing through the first focusing lens and is reflected; after reflection, the polarization state of the terahertz wave beam changes; the reflected terahertz wave beam is collimated by the second collimating lens, changes the polarization state again after passing through the second wave plate in the second hollow rotary motor, converges after passing through the second focusing lens and is received by the terahertz detector; the signal acquisition and processing system receives the light intensity signals at different angles through the terahertz detector, and obtains the Mueller matrix of the thin film sample to be measured according to a plurality of groups of light intensity signal data, and inversely obtains the thickness and the refractive index of the thin film sample to be measured.

[0017] Optionally, the motor controller adopts a stepper motor controller; the motor controller adjusts the ratio between the angle of the first wave plate in the first hollow rotary motor and the angle of the second wave plate in the second hollow rotary motor, and the ratio ranges from 1:5 to 1:1.

[0018] Optionally, the detection frequency range of the terahertz detector is 0.22 THz-0.33 THz; the terahertz emission source is used to emit a terahertz linearly polarized beam with a frequency of 0.3 THz.

[0019] Optionally, the first wave plate and the second wave plate are both single-frequency quarter-wave plates with a center frequency of 0.3 THz, which are used to produce a quarter-wavelength phase delay of the linearly polarized terahertz beam with a frequency of 0.3 THz, so as to obtain a terahertz wave with full polarization state.

[0020] Optionally, the value of θ0 ranges from 30 degrees to 60 degrees.

[0021] Optionally, the signal acquisition and processing system comprises a lock-in amplifier and a computer.

[0022] The lock-in amplifier is used to receive the voltage signal converted by the terahertz detector from the light intensity signal, and to read and display the voltage signal.

[0023] The computer is connected to the lock-in amplifier and the motor controller respectively, the computer is used to synchronously control the first hollow rotary motor and the second hollow rotary motor through the motor controller, and the computer is also used to read the voltage data from the lock-in amplifier, record the voltage data in one-to-one correspondence with the angle information of the first hollow rotary motor and the second hollow rotary motor, obtain the light intensity signal at different angles, input the obtained light intensity signal at different angles into the Mueller matrix algorithm, obtain 16 elements of the sample Mueller matrix by using the full-polarization incident Mueller matrix algorithm, and then inversely obtain the thickness and the refractive index of the measured thin film sample.

[0024] Optionally, the terahertz emission source comprises an emission source, a first horn antenna, a power supply and an electrical modulator, the power supply is used to supply power to the emission source, the emission source is used to emit a terahertz linearly polarized beam, the first horn antenna is used to improve the output power of the terahertz linearly polarized beam, and the electrical modulator is used to modulate the waveform of the terahertz linearly polarized beam.

[0025] The terahertz detector comprises a second horn antenna and a Schottky diode, the second horn antenna is used to amplify the power of the received light intensity signal, and the Schottky diode is used to convert incident photons into photoelectrons and generate an electric current to detect the change in light intensity.

[0026] In the second aspect, the application further provides a measurement method of a thin film sample, which is applied to the terahertz Mueller matrix ellipsometer as described in any one of the first aspect.

[0027] starting the terahertz Mueller matrix ellipsometer;

[0028] zeroing the first hollow rotary motor and the second hollow rotary motor through the motor controller and synchronously controlling the rotation angles of the first hollow rotary motor and the second hollow rotary motor;

[0029] acquiring the thickness and the refractive index of the measured film sample calculated from the signal acquisition processing system.

[0030] Compared with the prior art, the above technical scheme conceived by the present application can achieve the following beneficial effects:

[0031] 1. The present application provides a terahertz Mueller matrix ellipsometer, which uses a single-frequency wave plate to generate a single-frequency full-polarization terahertz wave band incident into the terahertz Mueller matrix ellipsometer, adopts a self-designed optical path structure, and uses a "collimation-focusing-collimation-focusing" optical path structure to expand the wave band application range of the measurement device to the terahertz wave band; two single-frequency quarter-wave plates are adopted to perform 1 / 4 wavelength phase delay on polarized light by using the birefringence phenomenon, which is simpler than the "polarizer-compensator-compensator-analyzer" structure of the visible light Mueller matrix ellipsometer; the angles of the two quarter-wave plates are changed by rotating them, the measurement light intensity under different angle configurations of the two wave plates is measured, the 16 elements of the sample Mueller matrix are obtained by using the full-polarization incident Mueller matrix algorithm, the measured exit light intensity values are substituted into the constructed system transfer function, and the sample Mueller matrix is fitted through the system transfer function, and then the thickness and optical properties of the sample are inversely obtained from the sample Mueller matrix. Compared with the existing complex-frequency Mueller matrix ellipsometer that can only use visible light, the single-frequency Mueller matrix ellipsometer of the present application has higher precision and wider application range, and can detect samples with a thickness of hundreds of microns.

[0032] 2. The present application provides a terahertz Mueller matrix ellipsometer, which acquires the Mueller matrix of the sample by fitting the exit light intensity under different angle configurations of the two wave plates, while the visible light Mueller matrix ellipsometer acquires the spectral data under different speed ratios of the two rotating compensators, so the acquisition method of the present application is simpler. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a structural schematic diagram of a terahertz Mueller matrix ellipsometer provided by an embodiment of the present application;

[0034] In the above figure, the same reference signs have the same meaning, wherein 1, sample placement table, 2, terahertz emission source, 3, terahertz detector, 4, first collimating lens, 5, first hollow rotating motor, 6, first wave plate, 7, first focusing lens, 8, second collimating lens, 9, second hollow rotating motor, 10, second wave plate, 11, second focusing lens, 12, motor controller, 13, lock-in amplifier, 14, computer. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0036] The content involved in the above examples is described below in combination with a preferred embodiment.

[0037] As shown in the drawings, a terahertz Mueller matrix ellipsometer comprises: Figure 1 a polarizing support and an analyzing support; the included angle between the polarizing support and the vertical direction and the included angle between the analyzing support and the vertical direction are θ0 degrees;

[0038] a sample placement table 1 horizontally placed between the polarizing support and the analyzing support;

[0039] a terahertz emission source 2, a first collimating lens 4, a first hollow rotating motor 5 and a first focusing lens 7 arranged along the polarizing support and gradually approaching the sample placement table 1;

[0040] a first wave plate 6, which is a single-frequency quarter wave plate and is fixedly arranged in the first hollow rotating motor 5;

[0041] a second collimating lens 8, a second hollow rotating motor 9, a second focusing lens 11 and a terahertz detector 3 arranged along the analyzing support and gradually away from the sample placement table;

[0042] a second wave plate 10, which is a single-frequency quarter wave plate and is fixedly arranged in the second hollow rotating motor 9;

[0043] a motor controller 12 connected with the first hollow rotating motor 5 and the second hollow rotating motor 9, for simultaneously controlling the rotation angles of the first hollow rotating motor 5 and the second hollow rotating motor 9;

[0044] and a signal acquisition and processing system connected with the terahertz detector 3;

[0045]

[0046] ​During operation, the terahertz linearly polarized beam emitted by the terahertz emission source 2 is collimated by the first collimating lens 4, changes its polarization state after passing through the first waveplate 6 in the first hollow rotating motor 5, and is converged by the first focusing lens 7 onto the thin film sample to be tested on the sample placement stage 1 and reflected. After reflection, the polarization state of the terahertz beam changes. The reflected terahertz beam is collimated by the second collimating lens 8, changes its polarization state again after passing through the second waveplate 10 in the second hollow rotating motor 9, and is converged by the second focusing lens 11 and received by the terahertz detector 3. The signal acquisition and processing system receives light intensity signals at different angles through the terahertz detector 3, and obtains the Mueller matrix of the thin film sample to be tested based on multiple sets of light intensity signal data, and inverts to obtain the thickness and refractive index of the thin film sample to be tested.

[0047] like Figure 1 As shown, the sample placement stage 1 is horizontally positioned between the polarization support and the polarization analyzer for placing the thin film sample. Both the polarization support and the polarization analyzer are at an angle of θ0 degrees to the vertical direction, and their vertical height increases with the direction away from the sample placement stage 1. The value of θ0 ranges from 30 to 60 degrees. Based on multiple experimental results, an angle with smaller experimental measurement error was selected for setting; in this embodiment, 45 degrees or 60 degrees is used. The polarization support includes: a terahertz emission source 2 positioned at the rightmost end, and from right to left, a first collimating lens 4, a first hollow rotating motor 5, a first waveplate 6, and a first focusing lens 7, wherein the first waveplate 6 is fixedly installed in the first hollow rotating motor 5; the terahertz wave is incident from the polarization support onto the thin film sample to be tested on the sample placement stage 1, and after reflection from the thin film sample, it enters the polarization analyzer; the polarization analyzer includes: from right to left, a second collimating lens 8, a second hollow rotating motor 9, a second waveplate 10, and a second focusing lens 11, and finally, a terahertz detector 3 positioned at the leftmost end, wherein the second waveplate 10 is fixedly installed in the second hollow rotating motor 9. The first waveplate 6 and the second waveplate 10 employ two single-frequency quarter-wave plates, utilizing birefringence to delay the polarized light by 1 / 4 wavelength, compensating for the phase delay of the terahertz beam and changing its polarization state; when the angle between the electric field vector of the incident linearly polarized beam and the principal plane of the quarter-wave plate is 45°, the outgoing beam is circularly polarized. The motor controller 12 simultaneously controls the rotation of the first hollow rotary motor 5 and the second hollow rotary motor 9, thereby driving the waveplates within them to rotate. Using two single-frequency quarter-wave plates, without changing the polarization state of the incident light, all matrix elements of the Mueller matrix of the sample under test can be measured. Optionally, the terahertz emission source includes an emission source, a first horn antenna, a power supply, and an electrical modulator. The power supply is used to power the emission source, the emission source is used to emit a terahertz linearly polarized beam, the first horn antenna is used to increase the output power of the terahertz linearly polarized beam, and the electrical modulator is used to modulate the waveform of the terahertz linearly polarized beam.

[0048] The terahertz detector comprises a second horn antenna and a Schottky diode, the second horn antenna is used to amplify the power of the received light intensity signal, and the Schottky diode is used to convert incident photons into photoelectrons and generate current to detect changes in light intensity.

[0049] The terahertz emission source emits a terahertz linearly polarized beam with a frequency of 0.3 THz. The frequency of the terahertz emission source is the same as the center frequency of the first wave plate 6 and the second wave plate 10. The output power is improved through the first horn antenna. The continuous terahertz beam is modulated into a time-domain rectangular wave signal with a frequency of 0.3 THz by the electric modulator. The terahertz beam emitted by the terahertz emission source is a linearly polarized beam, and the optical wave frequency can be 0.2-0.4 THz. The beam waist radius of the terahertz emission source used in the experimental instrument is 2.3 mm, the optical wave frequency is 0.3 THz, and the power is about 200 milliwatts. The first wave plate 6 and the second wave plate 10 are both single-frequency quarter-wave plates with a center frequency of 0.3 THz, which are used to produce a quarter-wave phase delay of the 0.3 THz linearly polarized terahertz beam, so as to obtain a fully polarized terahertz wave. The detection frequency range of the terahertz detector is 0.22 THz-0.33 THz, which meets the requirement of detecting the terahertz beam. Further, in order to make the energy received by the detector more, both the two collimating lenses and the two focusing lenses are optical lenses made of TPX material, with a transmittance of 90%-95% in the 0.3 THz frequency band, which can make the detection device achieve the greatest terahertz wave collection efficiency and the detection effect is more accurate.

[0050] Optionally, the motor controller 12 adopts a stepper motor controller; the ratio range between the angle of the first wave plate 6 in the first hollow rotating motor 5 and the angle of the second wave plate 10 in the second hollow rotating motor 9 adjusted by the motor controller 12 is 1:5 to 1:1.

[0051] The motor controller 12 controls the angles of rotation of the first hollow rotating motor 5 and the second hollow rotating motor 9 to be different each time, so as to form light intensity signals at different angles. In order to obtain sufficient light intensity signals at different angles, the angle ratio cannot be too large, and a suitable angle ratio is selected as 1:5 to 1:1 according to experimental errors. In the embodiment of the present application, the experimental results obtained by adopting an angle ratio of 2:3 are better, and the stepper motor controller controls the first hollow rotating motor 5 to rotate 2° and controls the second hollow rotating motor 9 to rotate 3° each time. By changing the angle configuration of the two compensators (the first wave plate 6 and the second wave plate 10), the measured light intensity at different angle configurations is measured.

[0052] Optionally, the signal acquisition and processing system comprises a lock-in amplifier 13 and a computer 14.

[0053] The lock-in amplifier 13 is used to receive the voltage signal converted by the terahertz detector 3 from the light intensity signal, and to read and display;

[0054] The computer 14 is connected to the lock-in amplifier 13 and the motor controller 12, respectively, and is used to synchronously control the first hollow rotating motor 5 and the second hollow rotating motor 9 through the motor controller 12, and is also used to read the voltage data from the lock-in amplifier 13 and record the corresponding angle information of the first hollow rotating motor 5 and the second hollow rotating motor 9 to obtain the light intensity signal at different angles, and to input the obtained light intensity signal at different angles into the Mueller matrix algorithm to obtain 16 elements of the sample Mueller matrix by using the full polarization incident Mueller matrix algorithm, and to inversely obtain the thickness and refractive index of the measured thin film sample.

[0055] The computer 14 pre-constructs the system transfer function of the Mueller matrix, obtains different function values according to the light intensity values at different angles, and the Mueller matrix is a 4*4 matrix containing 16 matrix elements. The computer 14 inputs the obtained light intensity signal at different angles into the Mueller matrix algorithm to obtain 16 elements of the sample Mueller matrix by using the full polarization incident Mueller matrix algorithm, and inversely obtains the thickness and refractive index of the measured thin film sample.

[0056] Based on the above-mentioned terahertz Mueller matrix ellipsometer, the following measurement method is used to measure the parameters of the thin film sample. A measurement method of a thin film sample, comprising:

[0057] Starting the terahertz Mueller matrix ellipsometer;

[0058] Zeroing the first hollow rotating motor and the second hollow rotating motor through the motor controller, and synchronously controlling the rotation angles of the first hollow rotating motor and the second hollow rotating motor;

[0059] Obtaining the calculated thickness and refractive index of the measured thin film sample from the signal acquisition and processing system.

[0060] The method specifically comprises the following steps:

[0061] (1) The terahertz emission source 2 emits a terahertz linearly polarized beam with a frequency of 300 Hz and a beam waist radius of 2.3 mm.

[0062] (2) The terahertz linearly polarized beam is collimated by the collimating lens 4 in the polarization support, then changes the polarization state after passing through the first wave plate 6, and is converged to the preset sample placement table 1 after passing through the first focusing lens 7 to irradiate the measured thin film sample.

[0063] (3) The terahertz beam reflected by the sample film to be measured changes the polarization state, is collimated by the second collimating lens 8, changes the polarization state again after passing through the second wave plate 9, and is focused into the terahertz detector 3 by the second focusing lens 11.

[0064] (4) The light intensity signal received by the terahertz detector 3 is converted into an amplified voltage signal, and then transmitted to the lock-in amplifier 13 for reading and display.

[0065] (5) The computer 14 controls the motor controller 12 to zero the first hollow rotating motor 5 and the second hollow rotating motor 9, and sets the angle at this time to 0°; the computer 14 synchronously controls the angles of the first hollow rotating motor 5 and the second hollow rotating motor 9, and rotates the angle of the first hollow rotating motor 5 by 2° and the angle of the second hollow rotating motor 9 by 3° each time.

[0066] (6) The computer 14 reads the data from the lock-in amplifier 13, and records the light intensity signals at different angles corresponding to the angle information of the first hollow rotating motor 5 and the second hollow rotating motor 9.

[0067] (7) The motor controller 12 moves the first hollow rotating motor 5 to 2° and the second hollow rotating motor 9 to 3°, and then repeats step (6).

[0068] (8) The motor controller 12 moves the first hollow rotating motor 5 to 4° and the second hollow rotating motor 9 to 6°, and then repeats step (6).

[0069] (9) The motor controller 12 moves the first hollow rotating motor 5 to 6° and the second hollow rotating motor 9 to 9°, and then repeats step (6).

[0070] (10) In this way, finally, the first hollow rotating motor 5 is moved to 198° and the second hollow rotating motor 9 is moved to 297°, and then step (6) is repeated; a total of 100 groups of data are obtained.

[0071] (11) The 100 groups of data obtained are introduced into the Mueller matrix algorithm, and the Mueller matrix of the film sample can be obtained, and a multi-beam interference model of the sample is constructed, and the film thickness and optical properties of the sample to be measured, i.e. the thickness and refractive index of the sample to be measured, are obtained by inversion iteration.

[0072] In this embodiment, 100 groups of data can construct 100 groups of equations about 16 elements, and the 100 equation groups are solved to obtain the analytical solution of the 16 elements. Other groups of data greater than 16 groups can also be selected to construct equations, and the 16 elements of the Mueller matrix can be solved.

[0073] Through the above steps, the film sample thickness and refractive index measured by the terahertz Mueller matrix ellipsometer can be obtained.

[0074] The embodiment of the present application generates a single frequency full polarization terahertz wave band incident terahertz Mueller matrix ellipsometer by using a single frequency wave plate, adopts a self-designed optical path structure, uses a "collimation-focusing-collimation-focusing" optical path structure, expands the wave band application range of the measuring device to the terahertz wave band, adopts two quarter-wave plates, uses the birefringence phenomenon to perform 1 / 4 wavelength phase delay on the polarized light, collects the outgoing light intensity of the two wave plates under different angle configurations, obtains 16 elements of the sample Mueller matrix by using the full polarization incident Mueller matrix algorithm, and inversely obtains the thickness and optical characteristics of the sample from the sample Mueller matrix. The technical problem that the existing Mueller matrix ellipsometer cannot be applied to the terahertz wave band and the sample detection range is limited is solved, and the constructed terahertz Mueller matrix ellipsometer has the beneficial effects of higher precision and wider application range.

[0075] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present application, and is not intended to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A terahertz Mueller matrix ellipsometer, characterized by, The application relates to a thin film sample thickness and refractive index measuring device. The device comprises a polarizing bracket and an analyzing bracket, the included angle between the polarizing bracket and the analyzing bracket and the vertical direction is theta0 degrees; A sample placing table is horizontally placed between the polarizing bracket and the analyzing bracket; A terahertz emission source, a first collimating lens, a first hollow rotating motor and a first focusing lens are arranged along the polarizing bracket and gradually approach the sample placing table; A first wave plate is a single-frequency quarter wave plate and is fixedly arranged in the first hollow rotating motor; A second collimating lens, a second hollow rotating motor, a second focusing lens and a terahertz detector are arranged along the analyzing bracket and gradually move away from the sample placing table; A second wave plate is a single-frequency quarter wave plate and is fixedly arranged in the second hollow rotating motor; A motor controller is connected with the first hollow rotating motor and the second hollow rotating motor and is used for simultaneously controlling the rotating angles of the first hollow rotating motor and the second hollow rotating motor; A signal acquisition and processing system is connected with the terahertz detector; During operation, a terahertz linear polarized wave beam emitted by the terahertz emission source is collimated by the first collimating lens, the polarization state of the terahertz linear polarized wave beam is changed after the terahertz linear polarized wave beam passes through the first wave plate in the first hollow rotating motor, the terahertz linear polarized wave beam is converged on a to-be-measured thin film sample on the sample placing table by the first focusing lens and is reflected; after the reflection, the polarization state of the terahertz wave beam is changed; the reflected terahertz wave beam is collimated by the second collimating lens, the polarization state of the reflected terahertz wave beam is changed again after the reflected terahertz wave beam passes through the second wave plate in the second hollow rotating motor, the reflected terahertz wave beam is converged and is received by the terahertz detector; the signal acquisition and processing system receives light intensity signals under different angles through the terahertz detector, obtains a Mueller matrix of the to-be-measured thin film sample according to a plurality of groups of light intensity signal data, and inversely obtains the thickness and the refractive index of the to-be-measured thin film sample.

2. The terahertz Mueller matrix ellipsometer of claim 1, wherein, The motor controller is a stepping motor controller; the ratio between the angle of the first wave plate in the first hollow rotating motor and the angle of the second wave plate in the second hollow rotating motor is 1:5 to 1:

1.

3. The terahertz Mueller matrix ellipsometer of claim 1, wherein, The detection frequency range of the terahertz detector is 0.22THz to 0.33THz; the terahertz emission source is used for emitting a terahertz linear polarized wave beam with a frequency of 0.3THz.

4. The terahertz Mueller matrix ellipsometer of claim 3, wherein, The first wave plate and the second wave plate are single-frequency quarter wave plates with a center frequency of 0.3THz, are used for generating a quarter wavelength phase delay of the 0.3THz linear polarized terahertz wave beam, and are used for obtaining a full polarization state terahertz wave.

5. The terahertz Mueller matrix ellipsometer of claim 1, wherein, The value range of theta0 is 30 degrees to 60 degrees.

6. The terahertz Mueller matrix ellipsometer of claim 1, wherein, The signal acquisition and processing system comprises a lock-in amplifier and a computer; The lock-in amplifier is used for receiving voltage signals converted by the terahertz detector from the light intensity signals, and is used for reading and displaying. The computer is connected with the lock-in amplifier and the motor controller respectively, the computer is used for synchronous control of the first hollow rotary motor and the second hollow rotary motor through the motor controller, and the computer is also used for reading voltage data from the lock-in amplifier and recording the voltage data corresponding to the angle information of the first hollow rotary motor and the second hollow rotary motor one by one to obtain the light intensity signals at different angles, introducing the obtained light intensity signals at different angles into a Mueller matrix algorithm, obtaining 16 elements of a sample Mueller matrix by using a full polarization incident Mueller matrix algorithm, and inversely obtaining the thickness and the refractive index of the measured thin film sample.

7. The terahertz Mueller matrix ellipsometer of claim 1, wherein, The terahertz emission source comprises an emission source, a first horn antenna, a power supply and an electrical modulator, the power supply is used for supplying power to the emission source, the emission source is used for emitting a terahertz linearly polarized wave beam, the first horn antenna is used for improving the output power of the terahertz linearly polarized wave beam, and the electrical modulator is used for modulating the waveform of the terahertz linearly polarized wave beam; The terahertz detector comprises a second horn antenna and a Schottky diode, the second horn antenna is used for amplifying the power of the received light intensity signal, and the Schottky diode is used for converting incident photons into photoelectrons and generating an electric current to detect the change of light intensity.

8. A method of measuring a thin film sample using a terahertz Mueller matrix ellipsometer as claimed in any one of claims 1 to 7, characterized in that, It comprises: starting the terahertz Mueller matrix ellipsometer; zeroing the first hollow rotary motor and the second hollow rotary motor through the motor controller and synchronously controlling the rotation angles of the first hollow rotary motor and the second hollow rotary motor; obtaining the thickness and the refractive index of the measured thin film sample calculated by the signal acquisition and processing system.

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