CN116255910B - Self-mixing metrology type displacement measurement device and method based on self-traceable grating - Google Patents

Abstract

The invention relates to a self-mixing metering type displacement measuring device and method based on a self-tracing grating, wherein the device comprises the following components: the laser light source is used for emitting laser of a single longitudinal mode; an optical element for adjusting the degree of convergence of the emitted laser light; the optical attenuator is used for attenuating the light intensity of the laser; the self-tracing grating generates first-order diffraction light according to the attenuated laser and adjusts the angle to enable the first-order diffraction light to return along an original path; the displacement generator is used for enabling the self-tracing grating to generate displacement; the photoelectric detector is used for detecting self-mixing signals of a self-mixing interference light field in the self-mixing laser light source; and the signal processor is used for carrying out filtering and signal normalization pretreatment on the self-mixed signal, obtaining a phase value through a phase unwrapping method, and obtaining a displacement value according to the phase value and the inversion of the pitch value of the self-tracing grating. Compared with the prior art, the invention has the advantages of direct traceability, no need of external calibration, accurate measurement and the like.

Description

Self-mixing metrology type displacement measurement device and method based on self-traceable grating

Technical Field

The present invention relates to the field of precision displacement measurement, and in particular to a self-mixing metrology type displacement measurement device and method based on a self-traceable grating.

Background technique

Precision displacement measurement technology is the basis of advanced nano-manufacturing and processing technology. The precision and accuracy of displacement measurement determine the precision limit of processing and manufacturing technology.

Traditional precision displacement measurement technology is mainly based on laser interferometers or grating interferometers. Laser interferometers mainly use Michelson interference structure, which has the characteristics of large range and high precision, so they are widely used in the verification and calibration of precision processing equipment such as precision machine tools and lithography machines. The measurement value of the laser interferometer is based on the wavelength and is easily affected by environmental disturbances; while the grating interferometer uses the grating period as the measurement basis and has better anti-interference ability. Therefore, grating interferometers are used as displacement measurement devices on the world's most advanced ASML extreme ultraviolet lithography machine. However, both traditional laser interferometers and grating interferometers require a large number of peripheral beam splitters, reflectors, gratings and other precision optical components, which makes this type of precision displacement measurement equipment expensive and large in size, and is limited to use in high-precision fields.

In the past two decades, a high-precision, low-cost precision displacement measurement technology has been developed, namely laser self-mixing interferometry technology. Laser self-mixing interferometry uses the principle of laser feedback interference. It is fed back to the laser resonant cavity through the external cavity and interferes with the original laser in the resonant cavity to generate a power-modulated self-mixing interference signal. If the phase change of the external feedback element is caused by displacement, the corresponding displacement measurement information can be obtained by demodulating the phase information of the self-mixing interference signal. Since laser self-mixing interferometry has the characteristics of not requiring additional external optical elements except for the feedback element, it realizes a low-cost, easy-to-integrate, and high-precision precision displacement measurement method. It has been widely used in nano-level precision displacement measurement, absolute displacement measurement, micro-vibration measurement, flow velocity measurement, etc. According to the type of feedback element, it can be divided into self-mixing laser interferometer and self-mixing grating interferometer. The self-mixing laser interferometer uses a reflector or a scattering surface as a target, and its displacement measurement value is based on the laser wavelength. However, due to the existence of the feedback effect, the laser wavelength will change accordingly with the change of the feedback phase and cannot remain constant, resulting in the displacement measurement value being difficult to accurately trace and having a large error, usually tens of nanometers. The measurement value of the self-mixing grating interferometer is based on the grating period, but the commonly used holographic grating or ruled grating itself does not have the self-traceability feature, and its period needs to be calibrated with the help of other metrological nanometer length measurement equipment such as atomic force microscope. The traditional self-mixing measurement scheme cannot be directly traced, which limits the precision and accuracy of the self-mixing measurement and its application in the field of high-precision nanometer measurement.

A self-traceable grating is an atomic deposition grating prepared by laser convergence atomic deposition technology (i.e., atomic lithography technology). It has the characteristics of high line density and self-traceability of period. During the preparation process of the self-traceable grating, the frequency of the laser standing wave field needs to be matched with the transition frequency of the specific energy level of the atom. The period of the atomic deposition grating finally formed is half of the period of the laser standing wave field, and it is directly traced back to the wavelength corresponding to the atomic transition energy level. Since the period of the self-traceable grating has the characteristics of self-traceability, no additional calibration is required, and its preparation process is formed in one step, its period has extremely high accuracy. It has been proved both theoretically and experimentally that the pitch error value of the self-traceable grating does not exceed 0.01nm. The line density of the one-dimensional chromium atomic deposition grating is as high as 4700nm line/mm, and it has been approved as a national first-level nanometer length standard material by a national authority, with a certificate number of GBW 13982. The period of the self-traceable grating is on the order of hundreds of nanometers. It can be combined with a laser in the visible light band to realize a metrological self-mixing grating interferometer with self-traceability and no calibration required, thereby realizing a laser self-mixing nano-displacement measurement method with metrological characteristics.

Based on this, the application of self-traceable gratings to solve the problem that traditional self-mixing displacement measurement solutions cannot be directly traced and need to be calibrated by other metrological equipment is of great research significance.

Summary of the invention

The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a self-mixing metrology type displacement measurement device and method based on a self-traceable grating.

The purpose of the present invention can be achieved by the following technical solutions:

A self-mixing metrology displacement measuring device based on a self-traceable grating, the device comprising:

A laser light source, used to achieve single longitudinal mode laser emission through temperature and current control;

An optical element, connected to the laser light source, and used to adjust the convergence degree of the laser light emitted by the laser light source;

An optical attenuator, connected to the optical element, is used to attenuate the intensity of the laser light so that the intensity of the returned light is less than 10% of the intensity of the incident laser light;

The self-traceable grating is placed on the same horizontal line as the laser light source, and is used to generate first-order diffracted light and reflected light according to the attenuated laser light, and adjust the angle so that the first-order diffracted light returns to the laser light source along the incident direction of the laser to generate self-mixing interference, and provide a displacement measurement benchmark with pitch traceability characteristics for self-mixing measurement;

A displacement generator is fixed to the self-traceable grating and is used to apply a periodic voltage driving signal to cause the self-traceable grating to displace along a direction without changing the laser incident angle;

A photodetector connected to the laser light source is used to detect an AC signal of a photocurrent of a self-mixing interference light field formed by self-mixing interference in the laser light source, namely, a self-mixing signal;

The signal processor is connected to the photodetector and is used to receive the self-mixing signal and perform normalized preprocessing on the self-mixing signal, obtain the phase value by the phase unwrapping method, and obtain the displacement value by inverting the phase value and the pitch value of the self-traceable grating.

Furthermore, the laser light source is a laser diode light source with a linear cavity structure.

Furthermore, the optical element includes an aspheric lens and a micro-displacement adjustment frame for mounting the aspheric lens.

Furthermore, the self-traceable grating is an atomic deposition grating prepared by one-dimensional laser focusing atomic deposition technology.

Furthermore, the one-dimensional laser convergence atomic deposition technology is specifically: using a one-dimensional laser standing wave field orthogonal to the direction of the atomic beam as a mask to form a periodic arrangement of the atomic deposition structure.

Furthermore, the frequency of the one-dimensional laser standing wave field corresponds to the frequency of a certain transition energy level of the atom, and the frequency difference between the two does not exceed 1 GHz.

Furthermore, the grating cross section of the self-traceable grating is a planar Gaussian or sinusoidal grating with a pitch error of ±0.1 nm.

A method for using the self-mixing metrology displacement measuring device based on self-traceable grating, the method comprising the following steps:

1) The laser light source emits a single longitudinal mode laser, and the optical element adjusts the convergence degree of the laser so that the self-traceable grating produces the best diffraction efficiency;

2) The optical attenuator attenuates the laser to a suitable optical power and then incidents it onto the self-traceable grating surface at a Littrow angle to generate first-order diffraction light and reflected light. The first-order diffraction light returns to the laser light source along the incident direction of the laser.

3) The first-order diffracted light forms self-mixing interference with the laser light emitted from the laser light source, and after interference, a self-mixing interference light field is formed;

4) Move the shifter in a direction that does not change the incident angle of the laser, so that the first-order diffraction light produces a phase shift;

5) The photoelectric detector detects the self-mixing signal of the self-mixing interference light field with phase shift, and sends it to the signal processing unit;

6) The signal processing unit performs preprocessing of low-pass filtering, DC component removal and normalization on the received self-mixing signal, obtains the phase value of the self-mixing signal using a phase unwrapping algorithm, and obtains the displacement value by inverting the phase value and the pitch value of the self-traceable grating.

Furthermore, the Littrow angle is calculated using the following formula:

arcsin(λ/2d)

Where d is the pitch of the self-traceable grating and λ is the central wavelength of the laser.

Furthermore, the self-mixing signal has the following form after normalization processing:

cos(ωτ+2πΔx/d)

Wherein, ω is the laser frequency, τ is the round trip time of light in the external cavity, Δx is the displacement generated by the displacement generator, and d is the pitch value of the self-traceable grating;

The displacement values are:

in, is the phase value of the self-mixing signal, and d is the pitch value of the self-traceable grating.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention directly traces the displacement measurement value of the self-mixing interferometer to the pitch value of the self-traceable grating, thereby achieving high-precision nano-displacement measurement while also having the advantages of self-calibration, self-calibration, and self-traceability of the displacement measurement value, solving the problem that the traditional self-mixing displacement measurement scheme cannot be directly traced and needs other metrological equipment for calibration, thereby improving the accuracy of the self-mixing displacement measurement value.

2. The self-traceable grating of the present invention is an atomic deposition grating prepared by one-dimensional laser convergence atomic deposition technology. It has a small period uncertainty and does not require additional calibration once prepared. As a result, the displacement measurement value of the metrological laser self-mixing interferometer based on the self-traceable grating does not require external calibration, but can be directly traced back to the pitch value of the self-traceable grating and indirectly traced back to the transition energy level constant of the atom, thereby realizing the value transfer of nanometer length measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a measuring device of the present invention;

FIG2 is a schematic diagram of the self-traceable grating preparation process of the present invention;

FIG3 is a scanning image of the self-traceable grating of the present invention under an atomic force microscope;

Figure 4 is a schematic diagram of the characteristics of the laser self-mixing interference signal based on the self-traceable grating of the present invention; wherein Figure (a) represents the sinusoidal displacement signal given by the displacement generator, and Figure (b) represents the laser self-mixing signal under sinusoidal displacement.

The numbers in the figure are as follows:

1. Laser light source, 2. Optical element, 3. Optical attenuator, 4. Self-traceable grating, 5. Displacement generator, 6. Photodetector, 7. Signal processing unit.

Detailed ways

The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

Example

As shown in FIG1 , a self-mixing metrology displacement measuring device based on a self-traceable grating comprises:

The laser light source 1 is a laser diode light source with a linear cavity structure, and is used to realize single longitudinal mode laser emission through temperature and current control; the wavelength of the laser light source 1 is less than 2 times the period of the self-traceable grating 4, and a 405nm laser light source 1 is used in this embodiment;

The optical element 2 is connected to the laser light source 1, and includes an aspheric lens and a micro-displacement adjustment frame for mounting the aspheric lens, and is used to reduce the divergence angle of the laser light emitted by the laser light source 1, so that the laser light emitted by the laser light source 1 covers the surface of the self-tracing grating 4 to generate the maximum first-order diffraction efficiency;

The optical attenuator 3 is connected to the optical element 2 and is used to attenuate the intensity of the laser light so that the intensity of the returned light is less than 10% of the intensity of the incident laser light. If the feedback intensity is not high, the optical attenuator 3 may not be used.

The self-traceable grating 4 is placed on the same horizontal line as the laser light source 1. It is the core element for realizing self-mixing metrology displacement measurement. It is a chromium atom deposition grating prepared by one-dimensional laser convergence atom deposition technology. It is used to generate first-order diffraction light and reflected light according to the attenuated laser, and adjust the angle so that the first-order diffraction light returns along the incident direction of the laser. The grating cross section of the self-traceable grating 4 is a planar Gaussian or sinusoidal morphology. The pitch value of the self-traceable grating 4 used in the present invention is 212.8±0.1nm.

The displacement generator 5 is fixed to the self-traceable grating 4 and is used to apply a periodic voltage driving signal to displace the self-traceable grating 4 in a direction that does not change the incident angle of the laser, which will cause the first-order diffraction light to produce a phase shift due to the Doppler effect of the light;

The photodetector 6 is connected to the laser light source 1 and is used to detect an AC signal of a photocurrent of a self-mixing interference light field formed by self-mixing interference in the laser light source 1, that is, a self-mixing signal;

The signal processor 7 is connected to the photodetector 6, and is used to receive the self-mixing signal and perform normalization preprocessing on the self-mixing signal, obtain the phase value by the phase unwrapping method, and obtain the displacement value by inverting the phase value and the pitch value of the self-traceable grating 4.

The one-dimensional laser convergence atomic deposition technology is specifically as follows: after the atomic powder is heated to a certain temperature, an atomic beam is ejected by leakage, the ejected atomic beam is first collimated by a slit and laser Doppler cooling, and the collimated atomic beam passes through a one-dimensional laser standing wave field and interacts with it. The frequency of the one-dimensional laser standing wave field is close to the resonance energy level of the atom. The case where the frequency of the one-dimensional laser standing wave field is less than the resonance energy level of the atom is red detuned, and vice versa is blue detuned. For a one-dimensional laser standing wave field with red detuning, the atomic beam will focus on the maximum light intensity of the one-dimensional laser standing wave field, that is, the position of the antinode, and vice versa, it will focus on the node. Placing the substrate at a position no more than 20% near the center of the one-dimensional laser standing wave field can make the atoms form a laser convergence atomic deposition grating according to the period of the one-dimensional laser standing wave field. The pitch value of the grating is λ/2, and λ is the frequency of the laser standing wave field. The wavelength of the laser standing wave field is traced back to the wavelength of the atomic transition energy level, and the pitch value of the formed atomic deposition grating is also traced back to the wavelength corresponding to the atomic transition energy level, which can be used as a self-traceable grating 4. For the chromium atomic deposition grating, the frequency of the one-dimensional laser standing wave field corresponds to the chromium atomic transition energy level ⁷ S ₃ → ⁷ P ₃ ⁰ , and the frequency difference between the two does not exceed 1 GHz.

A method for using the self-mixing metrology displacement measuring device based on self-traceable grating, the method comprising the following steps:

1) The laser light source 1 emits a single longitudinal mode laser, and the optical element 2 adjusts the convergence degree of the laser to produce the best diffraction efficiency;

2) After the laser is attenuated to a suitable optical power by the optical attenuator 3, it is incident on the surface of the self-traceable grating 4 at the Littrow angle to generate first-order diffraction light and reflected light. The first-order diffraction light returns to the resonant cavity of the laser light source 1 along the incident direction of the laser. The Littrow angle is calculated by the following formula:

arcsin(λ/2d)

Wherein, d is the pitch value of the self-traceable grating 4, and λ is the central wavelength of the laser. The corresponding Littrow angle in this embodiment is 72.12 degrees;

3) The first-order diffracted light forms self-mixing interference with the laser light emitted from the laser light source 1, and after interference, a self-mixing interference light field is formed;

4) moving the shifter 5 in a direction that does not change the incident angle of the laser, so that the first-order diffraction light produces a phase shift;

5) The photodetector 6 detects the self-mixing signal of the self-mixing interference light field with phase shift and sends it to the signal processing unit; the self-mixing signal has the following form after normalization processing:

cos(ωτ+2πΔx/d)

Wherein, ω is the laser frequency, τ is the round trip time of light in the external cavity, Δx is the displacement generated by the displacement generator 5, and d is the pitch value of the self-traceable grating 4;

6) The signal processing unit 7 performs low-pass filtering, DC component removal and normalization preprocessing on the received self-mixing signal, and uses a phase unwrapping method to obtain the phase value of the self-mixing signal. Specifically, the wrapped phase can be obtained by applying the inverse cosine method to the preprocessed self-mixing signal. Then, the maximum and minimum search algorithm is applied to the preprocessed self-mixing signal to obtain the maximum and minimum values in the signal. The local maximum and minimum points are found by setting the threshold value in the maximum and minimum values, which are the turning points R. The turning point is the point where the grating movement direction changes. The turning points of the local maximum and local minimum of the self-mixing signal must appear alternately. The number of maximum points between two adjacent turning points is represented by N, and the unwrapping phase between the two turning points is It can be expressed as:

The unwrapping phase between the two turning points in the next section The result is:

Among them, n＝(0,1,2,3…2N), k＝(0,1,2,3…N), k represents the serial number of the extreme point, and n increases by one value every time it passes an extreme point.

By alternately applying the above two phase unwrapping formulas in sequence, the phase unwrapping result of the entire self-mixing signal can be obtained, and the displacement value can be inverted according to the phase value and the pitch value of the self-traceable grating 4.

The displacement values are:

in, is the unwrapping phase value of the self-mixing signal, and d is the pitch value of the self-traceable grating 4.

Since the pitch value of the self-traceable grating 4 is determined to be 212.8 nm, which is traced back to the energy level transition frequency of the chromium atom, the pitch value of the self-traceable grating is traced back based on the displacement value of the self-traceable grating 4, and no additional calibration is required.

As shown in FIG2 , the self-traceable grating preparation process of the present invention is specifically as follows: Generally, a crucible filled with chromium powder is heated to between 1550°C and 1650°C in a vacuum environment to make it reach a sublimation state, forming a metal atom beam. Then, the chromium atom beam is collimated by a slit and laser cooling, and the collimated Cr atom beam passes through a one-dimensional laser standing wave field orthogonal to it, and is deposited on a template under the action of a dipole force to form a one-dimensional deposition grating structure. The wavelength of the one-dimensional laser standing wave field is 425.553nm, _and the corresponding resonant transition energy level of the Cr atom is ^7S3 → ^7P40 , _and the frequency of the ^one -dimensional laser standing wave field is adjusted to the positive detuning (+250MHz) or negative detuning (-250MHz) position of the center frequency corresponding to the resonant energy level. Therefore, the pitch of the formed one-dimensional chromium (Cr) atom deposition grating is half of the wavelength of the one-dimensional laser standing wave field used, which is 212.8nm. In addition, during the preparation process, the cutting ratio of the lateral displacement of the sample of the one-dimensional chromium atomic deposition grating in the one-dimensional laser standing wave field is limited to 50±10%. The substrate is generally made of silicon or indium phosphide. Since the one-dimensional chromium atomic deposition grating prepared by laser convergence atomic deposition technology needs to form a structure, the wavelength of the one-dimensional laser standing wave field must correspond to the wavelength of the atomic transition energy level, and the wavelength of the formed one-dimensional chromium atomic deposition grating is exactly equal to half of the wavelength of the one-dimensional laser standing wave field. Therefore, the one-dimensional chromium atomic deposition grating is a self-traceable grating with metrological characteristics4.

As shown in FIG3 , the image of the self-traceable grating 4 scanned under an atomic force microscope has a uniform period distribution.

As shown in Figure 4, the laser self-mixing interference signal characteristics based on the self-traceable grating 4 of the present invention are demonstrated. The displacement generator 5 shown adopts a sinusoidal displacement drive mode, and the corresponding photocurrent signal is noise-free and has been de-DCed and normalized. The density of the stripes represents the speed of the displacement of the displacement generator 5, and each stripe spacing corresponds to the pitch value of the self-traceable grating 4, i.e., 212.8nm.

The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims (6)

1. A self-mixing metrology displacement measuring device based on a self-traceable grating, characterized in that the device comprises: A laser light source (1) for achieving single longitudinal mode laser emission through temperature and current control; An optical element (2) connected to the laser light source (1) and used to adjust the convergence degree of laser light emitted by the laser light source (1); An optical attenuator (3) is connected to the optical element (2) and is used to attenuate the intensity of the laser light so that the intensity of the returned light is less than 10% of the intensity of the incident laser light; A self-traceable grating (4) is placed on the same horizontal line as the laser light source (1), and is used to generate first-order diffracted light and reflected light according to the attenuated laser light, and adjust the angle so that the first-order diffracted light returns to the laser light source (1) along the incident direction of the laser light to generate self-mixing interference, and provide a displacement measurement benchmark with pitch traceability characteristics for self-mixing measurement; The self-traceable grating (4) is an atomic deposition grating prepared by one-dimensional laser convergence atomic deposition technology, using a one-dimensional laser standing wave field orthogonal to the atomic beam direction as a mask to form a periodic arrangement of the atomic deposition structure, and the frequency difference between the two does not exceed 1 GHz; The frequency of the one-dimensional laser standing wave field corresponds to the frequency of a certain transition energy level of the atom; the grating cross section of the self-traceable grating (4) is a planar Gaussian or sinusoidal morphology, and the pitch error value is ±0.1nm; A displacement generator (5) is fixed to the self-traceable grating (4) and is used to apply a periodic voltage driving signal to cause the self-traceable grating (4) to generate a displacement in a direction that does not change the laser incident angle; A photodetector (6) is connected to the laser light source (1) and is used to detect an AC signal of a photocurrent of a self-mixing interference light field formed in the laser light source (1) due to self-mixing interference, i.e., a self-mixing signal; The signal processor (7) is connected to the photodetector (6) and is used for receiving the self-mixing signal, performing normalization preprocessing on the self-mixing signal, obtaining a phase value by a phase unwrapping method, and obtaining a displacement value by inversion based on the phase value and the pitch value of the self-traceable grating (4). 2. According to the self-mixing metrology type displacement measurement device based on self-traceable grating according to claim 1, it is characterized in that the laser light source (1) is a laser diode light source with a linear cavity structure. 3. According to the self-mixing metrology displacement measuring device based on self-traceable grating in claim 1, it is characterized in that the optical element (2) comprises an aspheric lens and a micro-displacement adjustment frame for mounting the aspheric lens. 4. A method for using the self-mixing metrology displacement measuring device based on self-traceable grating according to claim 1, characterized in that the method comprises the following steps: 1) The laser light source (1) emits a single longitudinal mode laser, and the optical element (2) adjusts the convergence degree of the laser so that the self-traceable grating (4) produces the best diffraction efficiency; 2) The optical attenuator (3) attenuates the laser light to a suitable optical power and then incidents the laser light onto the surface of the self-traceable grating (4) at a Littrow angle to generate first-order diffracted light and reflected light. The first-order diffracted light returns to the laser light source (1) along the incident direction of the laser light; 3) The first-order diffracted light forms self-mixing interference with the laser light emitted from the laser light source (1), and after interference, a self-mixing interference light field is formed; 4) moving the shifter (5) in a direction that does not change the incident angle of the laser, so that the first-order diffraction light produces a phase shift; 5) a photodetector (6) detects a self-mixing signal of the self-mixing interference light field with phase shift, and sends the signal to a signal processing unit; 6) The signal processing unit (7) performs preprocessing of low-pass filtering, removing DC components and normalization on the received self-mixing signal, obtains the phase value of the self-mixing signal using a phase unwrapping algorithm, and obtains the displacement value by inversion based on the phase value and the pitch value of the self-traceable grating (4). 5. The method according to claim 4, characterized in that the Littrow angle is calculated using the following formula: arcsin(λ/2d) Wherein, d is the pitch value of the self-traceable grating (4), and λ is the central wavelength of the laser. 6. The method according to claim 4, characterized in that the self-mixing signal has the following form after normalization processing: cos(ωτ+2πΔx/d) Wherein, ω is the laser frequency, τ is the round trip time of light in the external cavity, Δx is the displacement generated by the displacement generator (5), and d is the pitch value of the self-traceable grating (4); The displacement values are: in, is the phase value of the self-mixing signal, and d is the pitch value of the self-traceable grating (4).