CN116222400A

CN116222400A - Metal film thickness measuring device and method

Info

Publication number: CN116222400A
Application number: CN202310169850.3A
Authority: CN
Inventors: 宋有建; 胡春光; 董佳琦; 胡明列
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2023-02-27
Filing date: 2023-02-27
Publication date: 2023-06-06

Abstract

The invention discloses a metal film thickness measuring device and a metal film thickness measuring method, wherein in the device, a light source module emits a femtosecond laser beam, and the light splitting module is divided into two parts of pump light and detection light, wherein after the pump light sequentially passes through a pump light adjusting module and a pump light transmission module, a part of pump light enters a detection sample after passing through a detection light transmission receiving module, and a part of pump light enters an optical power detector. The detection light sequentially passes through the delay module and the detection light transmission and reception module and then enters the detection sample, the detection light reflected by the sample to be detected is received and transmitted to the photoelectric detector through the detection light transmission and reception module, and the detection light transmission and reception module also filters the pump light reflected by the sample to be detected. Furthermore, the pump light and the detection light can be collinearly incident to the sample to be measured by the measuring device and the measuring method. The device can realize a compact space light path or an all-fiber structure.

Description

Metal film thickness measuring device and method

Technical Field

The invention relates to the technical field of optical measurement, in particular to a metal film thickness measuring device and method.

Background

In various industrial applications today, the properties of a single material often are difficult to meet the actual requirements, and the material properties can be improved by plating the surface of the substrate with a metal film. Different metal films have different use scenes, and the thickness and uniformity of the film coating layer can obviously influence the mechanical, optical and chemical properties of materials, so that the thickness measurement of the metal film is very important. Among these measurement methods, the non-destructive and non-contact method is more significant due to its wide application range.

Unlike traditional laser ultrasonic thickness measurement, the femtosecond laser ultrasonic thickness measurement system uses femtosecond or picosecond laser as a light source, and simultaneously uses ultrashort pulse for detection, thereby greatly improving the detection resolution. The thickness measurement can be carried out on the ultra-thin metal film with the resolution of more than 1nm.

In the existing femtosecond laser thickness measurement system, the whole system is huge, detection light and pumping light are usually incident to a sample to be measured at different angles, and because two beams of light are not overlapped in space, the whole structure is complex, and when the sample changes, the whole light path needs to be adjusted, so that the operation is complex, and the miniaturization and commercialization of the device are not facilitated.

Disclosure of Invention

The invention provides a metal film thickness measuring device and a metal film thickness measuring method, which are used for realizing the same-angle incidence of detection light and pumping light, simplifying the structure of a measuring system and reducing the whole volume.

According to an aspect of the present invention, there is provided a metal film thickness measuring apparatus including:

the device comprises a light source module, a beam splitting module, a delay module, a detection light transmission and reception module, a pump light adjustment module, a pump light transmission module, an optical power detector, a photoelectric detector and a control module;

the light source module is used for emitting laser beams; the beam splitting module is positioned in the light path of the laser beam transmission and is used for splitting the laser beam into a detection beam and a pumping beam; the detection light transmission and reception module is positioned in the light path of the detection light beam transmission, and is used for transmitting the detection light beam to a sample to be detected, receiving the reflected light beam of the detection light beam reflected by the sample to be detected and transmitting the reflected light beam of the detection light beam to the photoelectric detector;

the pump light adjusting module is positioned in the optical path of the pump light beam transmission and is used for adjusting the optical parameters of the pump light beam; the pump light transmission module is positioned at the downstream of the pump light adjustment module and is used for splitting the pump light beam into first pump light and second pump light, transmitting the first pump light to the sample to be tested and transmitting the second pump light to the optical power detector; the detection light transmission and reception module is also used for filtering the reflected light beam of the first pump light reflected by the sample to be detected; the incidence angle of the first pump light to the sample to be detected is the same as the incidence angle of the probe light to the sample to be detected;

The delay module is positioned in the light path between the light splitting module and the detection light transmission and reception module, or in the light path between the pump light adjustment module and the pump light transmission module, or in the light path between the light splitting module and the pump light adjustment module, and is used for delaying the transmission of the detection light beam or the pump light beam;

the control module is respectively and electrically connected with the optical power detector, the photoelectric detector and the delay module, and is used for adjusting the delay amount of the delay module, adjusting the light splitting ratio of the light splitting module based on the intensity of the pumping light beam detected by the optical power detector and the intensity of the detecting light beam detected by the photoelectric detector, and measuring the metal film thickness based on the reflected light beam of the detecting light beam detected by the photoelectric detector.

According to one embodiment of the invention, the spectroscopic module comprises: the device comprises a half-wave plate and a first polarization splitting prism, wherein the half-wave plate is used for splitting the laser beam into a detection beam and a pump beam, and the first polarization splitting prism is used for transmitting the detection beam and reflecting the pump beam.

According to an embodiment of the present invention, the probe light transmission reception module includes: the device comprises a first polarization splitting prism, a quarter wave plate, a dichroic mirror and a lens, wherein the first polarization splitting prism is used for transmitting a probe beam to the quarter wave plate, the quarter wave plate is used for adjusting the polarization state of the probe beam, the dichroic mirror is used for transmitting the probe beam with the adjusted polarization state to the lens, and the lens is used for focusing the probe beam with the adjusted polarization state to the sample to be tested;

the sample to be detected reflects the detection light beam to form a detection reflected light beam and transmits the detection reflected light beam to the dichroic mirror through the lens, the dichroic mirror is used for transmitting the detection reflected light beam to the quarter wave plate, the quarter wave plate is used for adjusting the polarization state of the detection reflected light beam, and the second polarization splitting prism is used for reflecting the detection reflected light beam with the adjusted polarization state to the photoelectric detector.

According to one embodiment of the invention, the pump light transmission module is a non-polarizing beam splitter prism.

According to one embodiment of the invention, the beam splitting module is a first wavelength division multiplexer, and is used for splitting the laser beam into a probe beam and a pump beam.

According to an embodiment of the present invention, the probe light transmission reception module includes: the optical attenuator, the optical fiber circulator, the second wavelength division multiplexer and the optical fiber coupler are connected in sequence; and the third end of the optical fiber circulator is connected with the photoelectric detector.

According to one embodiment of the present invention, the pump light transmission module includes an optical splitter, a first output end of the optical splitter is connected to a third end of the second wavelength division multiplexer, and a second output end of the optical splitter is connected to the optical power detector.

According to one embodiment of the present invention, the probe optical transmission and reception module includes a dense wavelength division multiplexer, a second optical fiber circulator, a third wavelength division multiplexer, and a second optical fiber coupler, and the pump optical transmission module further includes a second optical splitter;

one end of the intensive wavelength division multiplexer is connected with one end of the optical attenuator, the second end of the intensive wavelength division multiplexer is connected with one end of the optical fiber circulator, and the third end of the intensive wavelength division multiplexer is connected with one end of the second optical fiber circulator; the second end of the second optical fiber circulator is connected with one end of the third wavelength division multiplexer, the second end of the third wavelength division multiplexer is connected with one end of the second optical fiber coupler, the third end of the second optical fiber circulator is connected with the second photoelectric detector, the third end of the third wavelength division multiplexer is connected with one end of the second optical divider, the second end of the second optical divider is connected with one end of the optical fiber coupler, and the third end of the second optical divider is connected with one end of the optical divider.

According to an embodiment of the invention, the pump light adjustment module comprises: the system comprises a light intensity modulator and an optical frequency doubling and shaping module, wherein the light intensity modulator modulates the intensity of pump light at a fixed frequency through a signal generator, and the optical frequency doubling and shaping module comprises a nonlinear crystal.

According to an embodiment of the invention, the pump light adjustment module comprises: the system comprises a light intensity modulator and an optical frequency doubling and shaping module, wherein the light intensity modulator modulates the intensity of pump light at a fixed frequency through a signal generator.

According to one embodiment of the present invention, the delay module includes an optical fiber electric delay line, the pump light beam or the probe light beam is incident on one end of the optical fiber electric delay line, and is transmitted to the probe light transmission receiving module or the pump light adjustment module or the pump light transmission module from the other end of the optical fiber electric delay line.

According to one embodiment of the present invention, further comprising: the phase-locked amplifier is respectively and electrically connected with the control module, the signal generator and the photoelectric detector, and is used for receiving the synchronous signal sent by the signal generator and the detection signal of the photoelectric detector, extracting the influence result of the ultrasonic wave excited by the pumping light beam on the detection light beam based on the synchronous signal and the detection signal and sending the influence result to the control module.

According to another aspect of the present invention, there is provided a metal film thickness measuring method, implemented based on the metal film thickness measuring apparatus provided in any one of the embodiments of the present invention, including:

acquiring a detection signal light spot of the photoelectric detector, and adjusting a pitch angle of a sample table for placing the sample to be detected according to the size of the detection signal light spot so as to enable the size of the detection signal light spot to reach a preset size;

obtaining a detection value of the optical power detector, controlling the pump light adjusting module to adjust optical parameters of the pump light, and/or adjusting the output laser intensity of the light source module to enable the detection value to reach a preset value;

according to the ratio of the intensity of the detection signal light spot to the intensity of the detection value, adjusting the light splitting ratio of the light splitting module to enable the ratio of the intensity of the detection signal light spot to the intensity of the detection value to reach a preset ratio;

controlling the sample stage to move so that the probe beam and the pump beam scan the sample to be detected;

acquiring a curve formed by a detection signal of the photoelectric detector and a synchronization signal of the pump light adjusting module along with time;

and calculating the metal film thickness of the sample to be measured based on the curve.

According to one embodiment of the present invention, the calculating the metal film thickness of the sample to be measured based on the curve includes:

intercepting and removing a first peak value from left to right along a time axis in the curve to form a pretreatment curve;

adopting quadratic function fitting of a least square method to remove background noise of the pretreatment curve and form a background noise-free curve;

smoothing the background noise-free curve by adopting a Savitzky-Golay filtering method to form a filtering curve;

carrying out peak searching depending on local maximum value on the filtering curve to obtain a series of peak positions to form a peak curve;

performing fast Fourier transform on the peak value curve to obtain a frequency domain curve;

and calculating the metal film thickness of the sample to be measured according to the frequency domain curve.

In summary, according to the apparatus and method for measuring a thickness of a metal film provided by the embodiments of the present invention, the apparatus includes: the device comprises a light source module, a beam splitting module, a delay module, a detection light transmission and reception module, a pump light adjustment module, a pump light transmission module, an optical power detector, a photoelectric detector and a control module; the light source module is used for emitting laser beams; the beam splitting module is positioned in the light path of the laser beam transmission and is used for splitting the laser beam into a detection beam and a pumping beam; the detection light transmission and reception module is positioned in the light path of the detection light beam transmission and is used for transmitting the detection light beam to the sample to be detected, receiving the reflected light beam of the detection light beam reflected by the sample to be detected and transmitting the reflected light beam of the detection light beam to the photoelectric detector; the pump light adjusting module is positioned in the optical path of the pump light beam transmission and is used for adjusting the optical parameters of the pump light beam; the pump light transmission module is positioned at the downstream of the pump light adjustment module and is used for splitting the pump light beam into first pump light and second pump light, transmitting the first pump light to the sample to be tested and transmitting the second pump light to the optical power detector; the detection light transmission and reception module is also used for filtering the reflected light beam of the first pump light reflected by the sample to be detected; the incidence angle of the first pump light to the sample to be detected is the same as the incidence angle of the probe light to the sample to be detected; the delay module is positioned in the light path between the light splitting module and the detection light transmission receiving module, or in the light path between the pump light adjusting module and the pump light transmission module, or in the light path between the light splitting module and the pump light adjusting module, and is used for delaying the transmission of the detection light beam or the pump light beam; the control module is respectively and electrically connected with the optical power detector, the photoelectric detector and the delay module, and is used for adjusting the delay amount of the delay module, adjusting the splitting ratio of the splitting module based on the intensity of the pumping beam detected by the optical power detector and the intensity of the detection beam detected by the photoelectric detector and measuring the metal film thickness based on the reflected beam of the detection beam detected by the photoelectric detector. Furthermore, the pump light and the detection light can be collinearly incident to the sample to be measured through the measuring device and the measuring method, the system adopts the fiber laser as a light source according to a designed space light path or a fiber structure, and the whole measuring system can be integrated and miniaturized by matching with an electrical structure and software processing.

It should be understood that the description in this section is not intended to identify key or critical features of the embodiments of the invention or to delineate the scope of the invention. Other features of the present invention will become apparent from the description that follows.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly described below, and it is apparent that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

FIG. 1 is a schematic diagram of a metal film thickness measuring apparatus according to an embodiment of the present invention;

FIG. 2 is a schematic view showing a structure of a metal film thickness measuring apparatus according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of a metal film thickness measuring apparatus according to another embodiment of the present invention;

FIG. 4 is a schematic diagram showing selection of pump light and probe light in a metal film thickness measuring device according to an embodiment of the present invention;

FIG. 5 is a schematic view showing a structure of a metal film thickness measuring apparatus according to still another embodiment of the present invention;

FIG. 6 is a schematic diagram showing selection of pump light and probe light in a metal film thickness measuring apparatus according to another embodiment of the present invention;

FIG. 7 is a flow chart of a metal film thickness measurement method according to an embodiment of the present invention;

FIG. 8 is an idealized graph of a metal film thickness measurement method according to an embodiment of the present invention;

FIG. 9 is a graph showing the actual thickness of a metal film according to an embodiment of the present invention;

FIG. 10 is a raw graph of a metal film thickness measurement method according to an embodiment of the present invention;

FIG. 11 is a graph showing pretreatment of a metal film thickness measurement method according to an embodiment of the present invention;

FIG. 12 is a graph of background noise free for a metal film thickness measurement method according to an embodiment of the present invention;

FIG. 13 is a filtering graph of a metal film thickness measurement method according to an embodiment of the present invention;

FIG. 14 is a graph showing peaks in a metal film thickness measurement method according to an embodiment of the present invention;

fig. 15 is a frequency domain plot of a metal film thickness measurement method according to an embodiment of the present invention.

Detailed Description

In order that those skilled in the art will better understand the present invention, a technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in which it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.

It should be noted that the terms "first," "second," and the like in the description and the claims of the present invention and the above figures are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate such that the embodiments of the invention described herein may be implemented in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.

In 1986, c.thomsen et al, bronzing university, first proposed using a pump-probe based femtosecond ultrasonic thickness probe system and experimentally verified with As2Te3, ni, etc. After that, researchers perform thickness measurement on different materials such as ALN, cr, gaN, pt by using a femtosecond ultrasonic technology, and also provide improved modes such as optical distortion detection and double phase lock detection to improve the signal to noise ratio of measurement, and research on the dynamic process of the method.

In recent years, as the practical requirement for detecting nano-films increases, the research on the system is gradually increasing. In 2015, osamu Matsuda et al, university of North sea, japan, performed detailed theoretical analysis on problems such as ultrasonic generation and ultrasonic detection in femtosecond ultrasound. In 2020, michal Kobecki et al, university of Duotemond industry, germany, compared a small mode-locked semiconductor laser with a large Ti-Sa laser, similar experimental results were obtained on 112nm aluminum films, demonstrating the potential for femto-second ultrasound system miniaturization. Rahman Md Mahmudur et al, university of European damine in 2021, utilized the femtosecond pulse pumping detection technique to realize on-line monitoring of the thickness of a thermally evaporated aluminum film and to realize thickness measurement of the aluminum film of 32nm-388 nm. In the same year, JUSTINAS PUPEIKIS et al, the Federal academy of regulations, zuishi, switzerland, applied asynchronous scanning of a free-running single-cavity double-comb system to a pump detection system, replaced the previous mechanically delayed scanning, and shortened the detection time of the system. Liu Peipei et al, korean scientific and technical institute, applied optical distortion technology to metal film thickness detection, realized the measurement of gold thickness. In 2022, yi Kiyoon et al of the group realized measurement of 50nm gold film thickness through a collecting and sending mode in the state that the sample moved at 20mm/s, and proved the application potential of the system.

2021, the detection method of films disclosed in the Yangtze river store (CN 202111412733.2) used similar principles, but the description thereof remained only at the principle level. A laser ultrasonic measurement system and method (CN 113281265A) suitable for wide film thickness range samples is also disclosed in the university of Huazhong science and technology, which is also used to test semiconductor films, but only the design of the optical path is mentioned. Both paths are conventional spatially separated.

Therefore, the invention provides the metal film thickness measuring device and the metal film thickness measuring method, so that the pumping light and the detection light can be collinearly incident to the sample to be measured, the volume of the measuring device is reduced, and the measuring device is miniaturized.

Fig. 1 is a schematic structural diagram of a metal film thickness measuring device according to an embodiment of the present invention. As shown in fig. 1, the metal film thickness measuring apparatus includes:

a light source module 101, a spectroscopic module 102, a delay module 103, a probe light transmission and reception module 104, a pump light adjustment module 105, a pump light transmission module 106, an optical power detector 107, a photodetector 108, and a control module 109;

the light source module 101 is used for emitting laser beams; the beam splitting module 102 is located in the optical path of the laser beam transmission and is used for splitting the laser beam into a probe beam and a pump beam; the probe light transmission and reception module 104 is located in the optical path of the probe light transmission, and is used for transmitting the probe light to the sample 110 to be tested, and is also used for receiving the reflected light of the probe light reflected by the sample 110 to be tested, and transmitting the reflected light of the probe light to the photodetector 108;

The pump light adjusting module 105 is located in the optical path of the pump light beam transmission and is used for adjusting the optical parameters of the pump light beam; the pump light transmission module 106 is located at the downstream of the pump light adjustment module 105, and is configured to split the pump light beam into a first pump light beam and a second pump light beam, and is further configured to transmit the first pump light beam to the sample 110 to be tested, and is further configured to transmit the second pump light beam to the optical power detector 107; the probe light transmission receiving module 104 is further configured to filter a reflected beam of the first pump light reflected by the sample to be tested 110; the incident angle of the first pump light to the sample 110 to be measured is the same as the incident angle of the probe light to the sample 110 to be measured;

the delay module 103 is located in the optical path between the beam splitting module 102 and the probe light transmission receiving module 104, or in the optical path between the pump light adjusting module 105 and the pump light transmission module 106, or in the optical path between the beam splitting module 102 and the pump light adjusting module 105, and is used for delaying the transmission of the probe light beam or the pump light beam;

the control module 109 is electrically connected to the optical power detector 107, the photo detector 108, and the delay module 103, and the control module 109 is used for adjusting the delay amount of the delay module 103, adjusting the splitting ratio of the splitting module 102 based on the pump beam intensity detected by the optical power detector 107 and the detection beam intensity detected by the photo detector 108, and performing metal film thickness measurement based on the reflected beam of the detection beam detected by the photo detector 108.

It can be understood that referring to fig. 1, the light source module 101 may be an optical fiber femtosecond laser, and emits a femtosecond laser beam, and the beam splitting module 102 divides the beam into two parts of pump light and probe light, wherein after the pump light sequentially passes through the pump light adjusting module 105 and the pump light transmitting module 106, a part of pump light enters the probe sample 110 after passing through the probe light transmitting and receiving module 104, and a part of pump light enters the optical power detector 107. The probe light sequentially passes through the delay module 103 and the probe light transmission and reception module 104, then enters the probe sample 110, and the probe light reflected by the sample 110 to be tested is received and transmitted to the photoelectric detector 108 through the probe light transmission and reception module 104, wherein the probe light transmission and reception module 104 also filters the pump light reflected by the sample 110 to be tested.

It should be noted that, a beam of strong ultrashort pulse laser is focused by a lens and is incident to a point on the surface of the sample 110 to be measured, the metal film absorbs the laser, the light energy is absorbed into heat energy, the ultrasonic wave is generated due to the thermal strain of the crystal lattice, and the highest frequency is the vertical surface ultrasonic wave. The vertical surface ultrasonic wave propagates within the metal film layer at a certain speed, and when it is transmitted to the interface between the metal film and the substrate, the acoustic wave is transmitted and reflected at the interface due to the difference in acoustic impedance between the two substances, and a part of energy escapes to the substrate while another part continues to be transmitted inside the metal film. When the sound wave is transmitted to the upper surface of the metal film again, the surface reflectivity is changed due to elastic stress, and simultaneously, tiny deformation occurs. The probe light is processed by a delay line so that it has an adjustable delay with respect to the pump light. The weaker detection light is focused at the same point, and then the delay time is adjusted, when the delay time is just equal to the integral multiple of the ultrasonic round trip time, the detection light is affected by the transmission of the sound wave to the surface, and the change is detected by the photoelectric detector 108, so that the thickness of the sample 110 to be detected can be obtained by using the flight time of the sound wave pulse.

The probe light and the pump light passing through the probe light transmitting and receiving module 104 are both incident on the sample 110 to be tested at the same angle, wherein the angle may be perpendicular to the incident of the sample 110 to be tested (for implementation, refer to the following examples). The pump light adjusting module 105 may adjust optical parameters such as the frequency of the pump light, so that the frequencies of the pump light and the probe light are different, and the pump light and the probe light may be incident to the sample 110 under test at the same angle.

The delay module 103 shown in fig. 1 is located in the probe optical path, and may also be located in the pump optical path, and is only illustrated here as an example, and is not a limitation of the present invention (the following embodiments are also illustrated by taking the delay module located in the probe optical path as an example). The control module 109 may adjust the delay time of the delay module 103.

It should be further noted that, the control module 109 may finally obtain the influence of the ultrasonic signal in the sample 110 to be tested excited by the pumping light on the signal of the detection light according to the signal of the detection light detected by the photodetector 108, so that the thickness of the metal film may be calculated according to the time interval of the influence of the ultrasonic signal and the propagation speed of the ultrasonic signal in the sample 110 to be tested.

In addition, the control module 109 may also adjust the split ratio of the detection light path and the pump light path according to the detection signals of the photodetector 108 and the optical power detector 107. The light splitting ratio between the detection light path and the pumping light path is generally 1:20, so that after detecting the detection signals of the photoelectric detector 108 and the optical power detector 107, the light splitting ratio of the light splitting module 102 can be adjusted to more accurately detect the thickness of the metal film.

The metal film thickness measuring apparatus according to the present invention will be described below by way of specific examples.

Example 1

Fig. 2 is a schematic diagram of a metal film thickness measuring apparatus according to an embodiment of the present invention. As shown in fig. 2, the spectroscopic module 102 includes: a half-wave plate 1021 and a first polarization splitting prism 1022, the half-wave plate 1021 is used for splitting the laser beam into the probe beam and the pump beam, and the first polarization splitting prism 1022 is used for transmitting the probe beam and reflecting the pump beam. The probe light transmission reception module 104 includes: the second polarization splitting prism 1041, the quarter-wave plate 1042, the dichroic mirror 1043 and the lens 1044, the second polarization splitting prism 1041 is used for transmitting the detection beam to the quarter-wave plate 1042, the quarter-wave plate 1042 is used for adjusting the polarization state of the detection beam, the dichroic mirror 1043 is used for transmitting the detection beam with the adjusted polarization state to the lens 1044, and the lens 1044 is used for focusing the detection beam with the adjusted polarization state to the sample 110 to be tested; the sample 110 to be measured reflects the probe beam to form a probe reflected beam and transmits the probe reflected beam to the dichroic mirror 1043 through the lens 1044, the dichroic mirror 1043 is used for transmitting the probe reflected beam to the quarter wave plate 1042, the quarter wave plate 1042 is used for adjusting the polarization state of the probe reflected beam, and the second polarization splitting prism 1041 is used for reflecting the probe reflected beam with the adjusted polarization state to the photodetector 108. The pump light transmission module 106 is a non-polarizing beam splitter prism. The pump light adjustment module 105 includes: the optical intensity modulator 1051 and the optical frequency doubling and shaping module 1052, the optical intensity modulator 1051 modulates the intensity of the pump light at a fixed frequency through the signal generator, and the optical frequency doubling and shaping module 1052 includes a nonlinear crystal. The metal film thickness measuring apparatus further includes: a mirror 111. The delay module 103 includes an optical fiber electric delay line, and the probe beam is incident to one end of the optical fiber electric delay line and is transmitted to the probe light transmission receiving module 104 from the other end of the optical fiber electric delay line.

Specifically, with continued reference to fig. 2, the light source module 101 emits laser light, the laser light is divided into first polarized light and second polarized light by the half-wave plate 1021, the first polarized light and the second polarized light pass through the first polarization splitting prism 1022, the first polarization splitting prism 1022 transmits the first polarized light, reflects the second polarized light, the first polarized light is incident to the optical fiber electric delay line, the optical fiber electric delay line continuously reflects the first polarized light to the second polarization splitting prism 1041, the second polarization splitting prism 1041 transmits the first polarized light to the quarter-wave plate 1042, delays the phase of the first polarized light, and finally vertically enters the sample 110 to be detected by the dichroic mirror 1043 and the lens 1044, the sample 110 to be detected reflects the first polarized light, and sequentially passes through the lens 1044, the dichroic mirror 1043 and the quarter-wave plate 1042, and delays the phase of the first polarized light again, so that the first polarized light at this time is 90 degrees different from the phase of the first polarized light at the time of incidence, and the returned first polarized light can be reflected to the detector 108 by the second polarization splitting prism 1041.

The second polarized light is reflected by the reflecting mirror 111, enters the light intensity modulator 1051 and the optical frequency doubling and shaping module 1052, modulates the light intensity by the light intensity modulator 1051, and adjusts the frequency wavelength of the second polarized light by the optical frequency doubling and shaping module 1052, and is reflected by the non-polarized beam splitter prism 106 to the dichroic mirror 1043, is reflected by the dichroic mirror 1043 to the lens 1044, finally vertically enters the sample 110 to be tested, and excites the ultrasonic wave in the sample 110 to be tested. In addition, the second polarized light is transmitted to the optical power detector 107 through the unpolarized beam splitter prism 106.

It should be noted that, the plurality of delay lines in the delay module 103 may adjust the distance through the control module 109, and further adjust the optical path length of the first polarized light relative to the second polarized light, so as to delay the time when the first polarized light reaches the sample 110 to be measured. In addition, the duty ratio of the first polarized light and the second polarized light may be distributed by adjusting the angle of the half-wave plate 1021 in the spectroscopic module 102 with respect to the laser beam. The light intensity adjuster 1051 may be a chopper, an electro-optic modulator, or an acousto-optic modulator, and may shift the detection signal to a high frequency region where noise is low by applying sinusoidal or square wave intensity modulation of a certain frequency to the pump light. The modulation frequency of the chopper is generally in the order of kHz, and the acousto-optic modulator and the electro-optic modulator can reach the order of MHz, so that a better modulation effect is realized. The optical frequency doubling and shaping module 1052 is used for realizing the frequency doubling process through lens group focusing and nonlinear crystal, and the wavelength of the pump light is changed into half of the original wavelength after passing through the optical frequency doubling and shaping module 1052. Through wavelength division, the pump light can be in common path with the detection light and normally incident to the surface of the sample, so that the whole system is easy to adjust, and the maximum detection efficiency is ensured. Meanwhile, the wavelength division is high, and interference of stray light is avoided. The module performs beam shaping on the pump light by utilizing the lens group at the same time of frequency multiplication, collimates the pump light and enables the light spot size of the pump light to be larger than that of the signal light. The reflection-transmission spectroscopic ratio of the non-polarizing beam-splitting prism 106 is preferably 9:1. The preferred parameters of the light source module 101 are that the central wavelength is 1030nm or 1550nm, the light power is more than 1W, the pulse width is less than 1ps, the light source can be realized by utilizing an ytterbium-doped or erbium-doped fiber laser, compared with a common titanium precious stone laser, the fiber laser has small volume, and meanwhile, the wave band devices are more and the interference of visible light is avoided. The sample 110 to be measured may be placed on a two-dimensional displacement platform and have two degrees of freedom of adjustment, yaw and pitch.

Before measurement, the splitting ratio of the splitting module 102 may be adjusted according to the ratio of the photodetector 108 to the optical power detector 107 so that the ratio of the pump light to the probe light reaches 20:1, and then the delay module 103 is adjusted so that the delay time of the probe light is an integer multiple of the round trip time of the ultrasonic wave. The light intensity modulator 1051 is then adjusted by the signal generator to make the frequencies of the pump light and the probe light different, and the wavelength of the pump light is adjusted by the optical frequency doubling and shaping module 1052, so that the pump light and the probe light can be incident to the sample 110 to be tested at the same angle, and the pump light and the probe light do not interfere. The final lock-in amplifier extracts the detection signal of the photodetector 108 and the synchronization signal of the signal generator to the light intensity modulator 1051 to analyze the influence of the ultrasonic signal on the detection signal of the photodetector 108, and the control module 109 calculates the metal film thickness based on the influence.

Thus, by changing the frequency and wavelength of the pump light, the pump light and the probe light do not interfere, so that the sample 110 to be measured can be incident from the same angle, and the output and the reception of the probe light are realized through the same optical path, the output of the pump light is also integrated in the output optical path of the probe light, and further, the whole optical path structure is relatively miniaturized.

Example two

Fig. 3 is a schematic structural diagram of a metal film thickness measuring apparatus according to another embodiment of the present invention. As shown in fig. 3, in the metal film thickness measuring apparatus, the beam splitting module 102 is a first wavelength division multiplexer 1045, and is used for splitting laser beams into a probe beam and a pump beam. The probe light transmission reception module 104 includes: an optical attenuator 112, an optical fiber circulator 1046, a second wavelength division multiplexer 1047, and an optical fiber coupler 1048 connected in this order; a third terminal of the optical fiber circulator 1046 is connected to the photodetector 108. The pump light transmission module 106 is an optical splitter, a first output end of the optical splitter is connected to the third end of the second wavelength division multiplexer 1047, and a second output end of the optical splitter is connected to the optical power detector 107. The pump light adjustment module 105 includes a light intensity modulator 1051 that modulates the intensity of the pump light at a fixed frequency through a signal generator. The delay module 103 is composed of an optical fiber electric delay line or an optical fiber stretcher, both of which occupy small volumes, and the optical fiber delay line can adjust the length of the optical fiber in a large range, but has low adjustment speed. The length of the optical fiber is changed by stretching the piezoelectric ceramic, and the scanning speed is high. The probe beam enters one end of the delay line, and enters the probe light transmission receiving module 104 from the other end after being delayed for a period of time.

Therefore, the embodiment realizes the construction of the metal film thickness measuring device through the all-fiber structure. The optical path transmission principle is basically the same as that of the first embodiment. The difference is that in this embodiment, the original pulse is divided into the pump pulse and the probe pulse by the first wavelength division multiplexer 1045 by utilizing the characteristic of the ultra-short pulse spectrum width. The pump and probe light shown in fig. 4 respectively select spectral components on both sides of the center wavelength, ensuring high isolation. The optical frequency doubling module is replaced, the system structure is simplified, and the light utilization rate is improved. Then the attenuator 112 adjusts the spectral ratio of the pump light to the probe light to be more than 20:1. The probe light enters from the first end of the optical fiber circulator 1046, exits from the second end, combines the two paths of light by using the second wavelength division multiplexer 1047, and then makes the two paths of light incident on the surface of the sample 110 through the optical fiber coupler 1048, and changes the distance between the two paths of light to adjust the light path. The probe light is reflected from the sample, returns to the fiber coupler 1048 and the second wavelength division multiplexer 1047, finally enters the second end of the fiber circulator 1046, and enters the photodetector 108 from the third end.

Because the system adopts the design of multi-wavelength common transmission, parallel multipoint measurement can be realized. As shown in fig. 5, an intensive wavelength division multiplexer 10491 and a second optical splitter 1061 are added on the basis of the second example, in which 2 paths of simultaneous measurement are shown, and the number of the second example can be increased or decreased as required. As shown in fig. 6, the dense wavelength division multiplexer 10491 finely divides the original probe light into two channels with different wavelengths, and then enters the optical fiber circulator 1046 and the second optical fiber circulator 10461, respectively. The second optical splitter 1061 divides the pump light energy into two channels (1:1). The first path of the pump light and the probe light are combined by the second wavelength division multiplexer 1047, and the pump light and the first path of the probe light are incident to the sample through the optical fiber coupler 1048 and then return to the optical fiber coupler 1048 and the optical fiber circulator 1046, and enter the photodetector 108 for detection. Similarly, the pump light and the second path of the probe light are combined by the third wavelength division multiplexer 10471, are incident on the sample through the second optical fiber coupler 10481, return to the second optical fiber coupler 10481 and the second optical fiber circulator 10461, and enter the second photodetector 1081 for detection. The control module 109 is also connected to a second photodetector 1081, whereby a multipoint measurement of the system is achieved.

The three embodiments described above all include: the phase-locked amplifier and the signal generator are used for example, and the phase-locked amplifier is electrically connected with the control module 109, the signal generator and the photo detector 108 respectively (the principle of the multi-point measurement is the same as that of the single point, reference is made to the single point, and details are not repeated), and the phase-locked amplifier is used for receiving the synchronous signal sent by the signal generator and the detection signal of the photo detector 108, extracting the influence result of the ultrasonic wave excited by the pumping beam on the detection beam based on the synchronous signal and the detection signal, and sending the influence result to the control module 109. For the control module 109 to calculate the metal film thickness.

That is, a modulation signal of a fixed frequency is given to the intensity modulation module by the signal generator so that the pump light is intensity-modulated. Meanwhile, the signal generator inputs the synchronous signal of the signal to the reference end of the phase-locked amplifier to transmit the modulation frequency information. The photodetector 108 converts the optical signal of the detection light into an electrical signal and transmits the electrical signal to the input terminal of the lock-in amplifier. The lock-in amplifier then extracts the influence of the ultrasonic wave excited by the pump light on the probe light by using the photoelectric signal and the synchronization signal, and sends the value to the control module 109 for storage. Meanwhile, parameters such as the integration time, the measurement range and the like of the lock-in amplifier are controlled by a control module according to actual conditions. In order to obtain signals with different time delays, the delay module 102 is further controlled in real time by the control module 109, different delay amounts are adjusted according to requirements, and meanwhile, position information is recorded, and each displacement amount corresponds to an output value of a lock-in amplifier.

In summary, the optical path structure is designed into the internal core optical path and the peripheral optical path, the core optical path is kept unchanged after the construction is completed, the peripheral optical path (the light source module, the sample to be detected, the photoelectric detector and the optical power detector) can be adjusted according to the sample and the actual requirement, the whole system is convenient to use and maintain, and the whole system can be better combined with the film manufacturing device to realize on-line monitoring. Meanwhile, the whole light path has only two directions, the light path alignment difficulty is obviously reduced by virtue of the design of a collinear system, the excitation and detection efficiency is higher, and the automatic adjustment of different samples can be realized. Compact structure and small occupied volume. In the second embodiment, an all-fiber structure is provided, so that the system volume is further reduced, and the overall integration level is improved. And through the arrangement of the signal generator, the lock-in amplifier, the control module and other electrical devices, the metal film thickness measurement is realized by matching with a related software calculation method.

Fig. 7 is a flowchart of a metal film thickness measurement method according to an embodiment of the present invention. The method is implemented based on the metal film thickness measuring device provided by any embodiment of the present invention, as shown in fig. 7, and includes:

S101, obtaining a detection signal light spot of a photoelectric detector, and adjusting a pitch angle of a sample stage for placing a sample to be detected according to the size of the detection signal light spot to enable the size of the detection signal light spot to reach a preset size;

when the size of the detected signal light spot reaches the maximum, the detection light path is indicated to complete primary collimation.

S102, acquiring a detection value of an optical power detector, controlling a pump light adjusting module to adjust optical parameters of pump light, and/or adjusting output laser intensity of a light source module to enable the detection value to reach a preset value; wherein the preset value is generally 9:1.

S103, according to the ratio of the intensity of the detection signal light spot to the intensity of the detection value, adjusting the light splitting ratio of the light splitting module to enable the ratio of the intensity of the detection signal light spot to the intensity of the detection value to reach a preset ratio;

the predetermined ratio may be a value greater than 20:0.1.

S104, controlling the sample platform to move so that the detection beam and the pumping beam scan the sample to be detected;

s105, acquiring a curve formed by a detection signal of a photoelectric detector and a synchronization signal of a pump light adjusting module along with time;

s106, calculating the thickness of the metal film of the sample to be measured based on the curve.

In detail, firstly, the sample to be measured is placed, then the pitching deflection of the sample stage is regulated, and when the output value of the photoelectric detector is maximum, the primary collimation of the light path is completed. After the preliminary adjustment of the light path is completed, the light path is finely adjusted, on one hand, the light intensity of the two paths is adjusted, and on the other hand, the light intensity ratio of the two paths is adjusted. Since the whole system has only two directions, the system can be automatically adjusted according to the output value without considering the alignment problem. The specific scheme is that the output light intensity of the light source module and the position of a displacement platform for placing a lens in the optical frequency doubling and shaping module are regulated firstly (the efficiency of regulating the optical frequency doubling) so that the value of the optical power detector is a rated value. And then, the rotation angle of the half wave plate is regulated, the two-path light splitting ratio is calculated according to the values of the optical power detector and the photoelectric detector, and the pumping light intensity is generally: the detected light intensity was adjusted to 20: more than 1. And finally, adjusting the position of a displacement platform of the sample to be measured to ensure that the sizes of light spots striking the sample to be measured are different, and finishing adjustment when the output value of the lock-in amplifier is maximum. The whole process can be automated by using a motor and a collection card.

In this process, the non-collinear system cannot be realized because the adjustment of the position of the sample affects both paths of light at the same time, and in addition, the positions where the two paths of light of different samples strike are different, so that alignment needs to be performed again at the end, and the collinear system is not affected for different samples.

The sample stage on which the sample is placed in the optical path and the size of the front aperture stop of the photodetector should then also be adjusted according to the measured sample. The sample stage is then subjected to a rapid rough scan, if the curve trend is correct for the next step, otherwise readjusted. And then, carrying out fine scanning under different time delays by utilizing a control acquisition program to obtain the output value of the phase-locked amplifier under each time delay, finally obtaining a result curve, and calculating the thickness of the metal film layer of the sample through data processing.

Fig. 8 is a result of an ideal pump detection process in the metal film thickness measurement method of the embodiment of the present invention. The rate of change of the reflected light intensity is the transient signal of the detected light at different time delays when the pump light is present. This variation is generally 10 ^-4 -10 ^-6 Magnitude. As can be seen from the figure, at a delay less than zero, the value is essentially unchanged since there is no pump light at this time. The variation value is at a maximum at this time, since the pump pulse and the signal pulse substantially overlap in the time domain, in the vicinity of the delay equal to zero. Then, as the delay increases, the pump light and the probe light are gradually separated in time, and the variation value gradually decreases due to thermal relaxation. However, due to the presence of the ultrasonic wave, the modulation of the refractive index of the ultrasonic wave causes abrupt change of the reflected light intensity every time the ultrasonic wave is reflected to the surface of the sample to be measured, and the two peaks in fig. 8 are the abrupt change caused by the ultrasonic wave. Knowing the ultrasonic velocity of the specific material, the thickness information of the sample to be measured can be obtained by using the flight time of the two echoes. Because the ultrasonic wave is transmitted back and forth in the metal film layer to be measured, the thickness formula is

Wherein the time interval between the two peaks is the propagation velocity of the ultrasonic wave within the material.

FIG. 9 is an actual measurement result for a 50nm gold film. During operation, the obtained curves have noise interference due to environmental changes, noise of lasers and electronic devices, and the like, and the signal to noise ratio is low, so that subsequent software processing is required to more accurately determine the thickness of the material to be measured.

Thus, S106 calculates the metal film thickness of the sample to be measured based on the curve, including:

intercepting a first peak value from left to right along a time axis in the removal curve (shown in fig. 10) to form a pretreatment curve (shown in fig. 11);

a quadratic function fitting of a least square method is adopted to remove the background noise of the preprocessing curve and form a curve without the background noise (as shown in figure 12);

smoothing the background noise-free curve by adopting a Savitzky-Golay filtering method to form a filtering curve (shown in figure 13);

peak searching is carried out on the filtering curve depending on local maximum values, a series of peak positions are obtained, and a peak curve is formed (shown in fig. 14);

performing fast Fourier transform on the peak curve to obtain a frequency domain curve (shown in FIG. 15);

That is, fig. 10 is an original data curve, and it is difficult to obtain a peak position due to the influence of noise, and an accurate thickness value cannot be obtained. On the basis of the raw data, the data after the first peak is first truncated to obtain fig. 11, which is mainly to eliminate the influence on peak extraction due to the excessive variation of the first peak data. As can be seen from fig. 11, the data overall has a downward trend due to the thermal background noise, which is equivalent to low frequency noise. Therefore, a quadratic function fitting based on a least square method is adopted, and the background trend is removed, so that fig. 12 is obtained. The individual peaks can be seen from fig. 12, but peak discrimination is affected due to limited collection points and noise limitations. Therefore, the data is smoothed by adopting a Savitzky-Golay filtering method, and the influence of high-frequency noise on the data is filtered, so that the graph 13 is obtained. On the basis of fig. 13, peak finding depending on the local maximum is performed, resulting in a series of peak positions, as shown in fig. 14. In fig. 14, the difference between the abscissas of adjacent peaks is the time interval shown in fig. 8, and a plurality of sets of results can be obtained due to the large number of peaks. The frequency domain curve obtained by performing fast fourier transform on the whole data is shown in fig. 15, the peak corresponding frequency f is taken as the center frequency, and finally, the film thickness d=v/2 f is obtained, which is equivalent to averaging a plurality of groups of results, and the obtained results are more accurate. Therefore, the signal-to-noise ratio of measurement is improved, and the measurement result is more accurate.

In summary, according to the apparatus and method for measuring a thickness of a metal film provided by the embodiments of the present invention, the apparatus includes: the device comprises a light source module, a beam splitting module, a delay module, a detection light transmission and reception module, a pump light adjustment module, a pump light transmission module, an optical power detector, a photoelectric detector and a control module; the light source module is used for emitting laser beams; the beam splitting module is positioned in the light path of the laser beam transmission and is used for splitting the laser beam into a detection beam and a pumping beam; the detection light transmission and reception module is positioned in the light path of the detection light beam transmission and is used for transmitting the detection light beam to the sample to be detected, receiving the reflected light beam of the detection light beam reflected by the sample to be detected and transmitting the reflected light beam of the detection light beam to the photoelectric detector; the pump light adjusting module is positioned in the optical path of the pump light beam transmission and is used for adjusting the optical parameters of the pump light beam; the pump light transmission module is positioned at the downstream of the pump light adjustment module and is used for splitting the pump light beam into first pump light and second pump light, transmitting the first pump light to the sample to be tested and transmitting the second pump light to the optical power detector; the detection light transmission and reception module is also used for filtering the reflected light beam of the first pump light reflected by the sample to be detected; the incidence angle of the first pump light to the sample to be detected is the same as the incidence angle of the probe light to the sample to be detected; the delay module is positioned in the light path between the light splitting module and the detection light transmission receiving module, or in the light path between the pump light adjusting module and the pump light transmission module, or in the light path between the light splitting module and the pump light adjusting module, and is used for delaying the transmission of the detection light beam or the pump light beam; the control module is respectively and electrically connected with the optical power detector, the photoelectric detector and the delay module, and is used for adjusting the delay amount of the delay module, adjusting the splitting ratio of the splitting module based on the intensity of the pumping beam detected by the optical power detector and the intensity of the detection beam detected by the photoelectric detector and measuring the metal film thickness based on the reflected beam of the detection beam detected by the photoelectric detector. Furthermore, by the measuring device and the measuring method, the pump light and the detection light can be collinearly incident to the sample to be measured, and the whole measuring structure is miniaturized.

It should be appreciated that various forms of the flows shown above may be used to reorder, add, or delete steps. For example, the steps described in the present invention may be performed in parallel, sequentially, or in a different order, so long as the desired results of the technical solution of the present invention are achieved, and the present invention is not limited herein.

The above embodiments do not limit the scope of the present invention. It will be apparent to those skilled in the art that various modifications, combinations, sub-combinations and alternatives are possible, depending on design requirements and other factors. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of the present invention.

Claims

1. A metal film thickness measuring apparatus, characterized by comprising:

2. The metal film thickness measuring apparatus according to claim 1, wherein the spectroscopic module includes: the device comprises a half-wave plate and a first polarization splitting prism, wherein the half-wave plate is used for splitting the laser beam into a detection beam and a pump beam, and the first polarization splitting prism is used for transmitting the detection beam and reflecting the pump beam.

3. The metal film thickness measuring apparatus according to claim 2, wherein the probe light transmission-reception module includes: the device comprises a first polarization splitting prism, a quarter wave plate, a dichroic mirror and a lens, wherein the first polarization splitting prism is used for transmitting a probe beam to the quarter wave plate, the quarter wave plate is used for adjusting the polarization state of the probe beam, the dichroic mirror is used for transmitting the probe beam with the adjusted polarization state to the lens, and the lens is used for focusing the probe beam with the adjusted polarization state to the sample to be tested;

4. The apparatus according to claim 1, wherein the beam splitting module is a first wavelength division multiplexer for splitting the laser beam into a probe beam and a pump beam.

5. The metal film thickness measuring apparatus according to claim 4, wherein the probe light transmission-reception module includes: the optical attenuator, the optical fiber circulator, the second wavelength division multiplexer and the optical fiber coupler are connected in sequence; and the third end of the optical fiber circulator is connected with the photoelectric detector.

6. The apparatus according to claim 5, wherein the pump light transmission module comprises an optical splitter, a first output terminal of the optical splitter is connected to a third terminal of the second wavelength division multiplexer, and a second output terminal of the optical splitter is connected to the optical power detector.

7. The apparatus according to claim 6, wherein the probe light transmission/reception module includes a dense wavelength division multiplexer, a second optical fiber circulator, a third wavelength division multiplexer, and a second optical fiber coupler, and the pump light transmission module further includes a second optical splitter;

8. The metal film thickness measuring apparatus according to claim 1, wherein the pump light adjusting module includes: the system comprises a light intensity modulator and an optical frequency doubling and shaping module, wherein the light intensity modulator modulates the intensity of pump light at a fixed frequency through a signal generator, and the optical frequency doubling and shaping module comprises a nonlinear crystal.

9. The apparatus according to claim 1, wherein the delay module includes an optical fiber electric delay line, the pump light beam or the probe light beam is incident to one end of the optical fiber electric delay line, and is transmitted to the probe light transmission reception module or the pump light adjustment module or the pump light transmission module from the other end of the optical fiber electric delay line.

10. A metal film thickness measuring method, characterized by being realized based on the metal film thickness measuring apparatus according to any one of claims 1 to 9, comprising:

11. The method according to claim 10, wherein the calculating the metal film thickness of the sample to be measured based on the curve includes: