CN114216858B - Method for detecting film - Google Patents

Method for detecting film Download PDF

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Publication number
CN114216858B
CN114216858B CN202111414461.XA CN202111414461A CN114216858B CN 114216858 B CN114216858 B CN 114216858B CN 202111414461 A CN202111414461 A CN 202111414461A CN 114216858 B CN114216858 B CN 114216858B
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echo
film
signal
arrival time
ultrasonic wave
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CN114216858A (en
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李辉
王凌
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Yangtze Memory Technologies Co Ltd
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Yangtze Memory Technologies Co Ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Length Measuring Devices Characterised By Use Of Acoustic Means (AREA)
  • Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)

Abstract

The embodiment of the application discloses a detection method of a film, which comprises the following steps: providing a laser pulse excitation signal to the first surface of the film to be detected so as to excite ultrasonic waves in the film to be detected; the film to be tested comprises a first surface and a second surface which are opposite; acquiring a signal spectrum of ultrasonic wave transmitted in a film to be measured; determining the signal amplitude of at least one echo of the ultrasonic wave and the arrival time of the echo on the first surface according to the signal spectrum, and the attenuation coefficient of the signal amplitude of a plurality of echoes generated by the ultrasonic wave along with the arrival time of the echo; wherein, the echo is the wave that the ultrasonic wave returns to the first surface after being reflected by the second surface; determining the average sound velocity of ultrasonic wave propagating in the film to be tested according to the signal amplitude of at least one echo, the arrival time of the echo and the attenuation coefficient; and determining the thickness of the film to be measured according to the average sound velocity and the arrival time of the echo.

Description

Method for detecting film
Technical Field
The application relates to the technical field of semiconductors, in particular to a detection method of a film.
Background
In the fabrication of semiconductor devices, such as three-dimensional memories, it is often necessary to form thin film layers on a wafer, including mask layers, metal thin film layers, etch stop layers, dielectric layers, and the like. Physical parameters such as thickness, density, and hardness of the thin film layer have an important impact on the formation of the semiconductor structure, and the performance of the resulting semiconductor device. For example, the thickness and hardness of the mask layer affects the pattern accuracy and precision of the semiconductor structure, and the thickness and density of the metal thin film layer affects the line resistance of the metal interconnect layer. Therefore, in the process of manufacturing semiconductor devices, it is important to detect physical parameters such as thickness, density, hardness, etc. of the thin film layer.
Disclosure of Invention
In view of the above, an embodiment of the present application provides a method for detecting a thin film, including:
providing a laser pulse excitation signal to a first surface of a film to be measured so as to excite ultrasonic waves in the film to be measured; wherein the film to be tested comprises a first surface and a second surface which are opposite;
acquiring a signal spectrum of the ultrasonic wave transmitted in the film to be measured;
determining the signal amplitude of at least one echo of the ultrasonic wave and the arrival time of the echo to the first surface according to the signal spectrum, and the attenuation coefficient of the signal amplitudes of a plurality of echoes generated by the ultrasonic wave along with the arrival time of the echo; wherein the echo is a wave of the ultrasonic wave which is reflected by the second surface and returns to the first surface;
determining the average sound velocity of the ultrasonic wave transmitted in the film to be detected according to the signal amplitude of the at least one echo, the arrival time of the echo and the attenuation coefficient;
and determining the thickness of the film to be detected according to the average sound velocity and the arrival time of the echo.
In some embodiments, the determining the average sound velocity of the ultrasonic wave propagating in the film to be measured according to the signal amplitude and the echo arrival time of the at least one echo and the attenuation coefficient includes:
Establishing a first linear regression equation taking the average sound velocity as a dependent variable, the signal amplitude and the echo arrival time of the at least one echo, and the attenuation coefficient as an independent variable;
substituting the signal amplitude and the echo arrival time of the at least one echo of the film to be detected and the attenuation coefficient into the first linear regression equation to calculate the average sound velocity of the ultrasonic wave propagating in the film to be detected.
In some embodiments, the establishing a first linear regression equation having the average speed of sound as a function of the signal amplitude and echo arrival time of the at least one echo, and the attenuation coefficient as an argument, comprises:
establishing a first linear regression model taking the average sound velocity as a dependent variable, the signal amplitude and the echo arrival time of the at least one echo, and the attenuation coefficient as an independent variable;
providing laser pulse excitation signals to a first surface of a plurality of calibration films of different densities and/or thicknesses to excite ultrasound waves in the calibration films; acquiring a plurality of calibration signal spectrums of the ultrasonic wave transmitted in a plurality of the calibration films;
determining the signal amplitude and the echo arrival time of at least one echo of ultrasonic wave propagating in each calibration film according to a plurality of calibration signal spectrums, and the attenuation coefficient of the signal amplitude of a plurality of echoes generated by the ultrasonic wave along with the echo arrival time;
Acquiring the thickness of a plurality of calibration films, and determining the sound velocity of the ultrasonic wave transmitted in each calibration film according to the thickness of each calibration film and the arrival time of the echo;
and determining a first regression coefficient of the first linear regression model according to the sound velocity, the signal amplitude and the echo arrival time of at least one echo corresponding to each calibration film and the attenuation coefficient to obtain the first linear regression equation.
In some embodiments, the acquiring a signal spectrum of the ultrasonic wave propagating in the film to be measured includes:
providing a laser pulse detection signal to the first surface after a delay of a preset time period from the moment of providing the laser pulse excitation signal, wherein the laser pulse detection signal and the laser pulse excitation signal are emitted to the first surface at different angles;
detecting a reflected signal of the laser pulse detection signal reflected from the first surface;
and determining the signal spectrum of the ultrasonic wave propagating in the film to be detected according to the signal spectrum of the reflected signal.
In some embodiments, the determining, according to the signal spectrum, attenuation coefficients of signal amplitudes of a plurality of echoes generated by the ultrasonic wave with arrival time of the echoes includes:
Determining the signal amplitude of a primary echo generated by the ultrasonic wave, the echo arrival time of the primary echo, the signal amplitude of a secondary echo and the echo arrival time of the secondary echo according to the signal spectrum;
and determining the attenuation coefficient according to the signal amplitude of the primary echo, the echo arrival time of the primary echo, the signal amplitude of the secondary echo and the echo arrival time of the secondary echo.
In some embodiments, after the step of acquiring a signal spectrum of the ultrasonic wave propagating in the film under test, the method further comprises:
determining the time when at least two wave peaks and/or wave troughs of a surface wave propagating in a film body with a preset depth below the first surface in the ultrasonic wave reach the first surface according to the signal spectrum; the preset depth is smaller than the thickness from the first surface to the second surface of the film to be detected;
determining the surface sound velocity of the surface wave propagating in a film body with a preset depth below the first surface according to the time when at least two wave peaks and/or wave troughs of the surface wave reach the first surface;
calculating a ratio of the surface sound velocity to the average sound velocity; wherein the ratio characterizes the physical properties of the film to be measured.
In some embodiments, the determining the surface sound velocity of the ultrasonic wave propagating on the surface layer of the film to be measured according to the time when at least two peaks and/or troughs of the surface wave reach the first surface includes:
establishing a second linear regression equation taking the surface sound velocity as a dependent variable and the time when at least two peaks and/or troughs of the surface wave reach the first surface as an independent variable;
substituting the time when at least two wave peaks and/or wave troughs of the surface wave of the film to be measured reach the first surface into the second linear regression equation, and calculating the surface sound velocity of the ultrasonic wave propagating in the surface layer of the film to be measured.
In some embodiments, the establishing a second linear regression equation with the surface acoustic speed as a function of the time at which at least two peaks and/or troughs of the surface wave reach the first surface as an independent variable comprises:
establishing a second linear regression model taking the surface sound velocity as a dependent variable and the time when at least two peaks and/or troughs of the surface wave reach the first surface as an independent variable;
providing laser pulse detection signals to a first surface of a plurality of calibration films of different densities and/or thicknesses to excite ultrasound waves in the calibration films; acquiring a plurality of calibration signal spectrums of the ultrasonic wave transmitted in a plurality of the calibration films;
Determining the echo arrival time of an echo propagating in each calibration film and the arrival time of at least two peaks and/or troughs of a surface wave at the first surface according to a plurality of calibration signal spectrums;
acquiring the thickness of a plurality of calibration films, and determining the sound velocity of the ultrasonic wave in each calibration film according to the thickness of each calibration film and the arrival time of the echo;
and determining a second regression coefficient of the second linear regression model according to the sound velocity corresponding to each calibration film and the time when at least two wave peaks and/or wave troughs of the surface wave reach the first surface, so as to obtain a second linear regression equation.
In some embodiments, the times at which the two peaks and/or troughs reach the first surface include a first time of arrival at which the first trough reaches the first surface, and a second time of arrival at which the first peak reaches the first surface.
In some embodiments, the thin film under test comprises a carbon film.
In some embodiments, the laser pulse excitation signal comprises a femtosecond laser beam.
According to the film detection method provided by the embodiment of the application, the average sound velocity of ultrasonic waves in the film to be detected is determined by utilizing echo parameters (such as the signal amplitude of echo, the arrival time of echo and the attenuation coefficient of the signal amplitude of a plurality of echoes along with the arrival time), and then the thickness of the film to be detected is determined according to the average sound velocity and the arrival time of echo. Since the propagation path of the echo includes the thickness of the whole film, the average sound velocity calculated by the echo parameter is more accurate, and the determined film thickness is also more accurate.
Further, the sound velocity V is related to the physical properties of the solid material, such as the elastic modulus E, the density ρ, and the like, so that some parameters (such as the average sound velocity) determined by the present application can also represent the physical properties of the film to be measured, such as the density, the hardness, and the like.
Drawings
FIG. 1 is a schematic flow chart of a method for detecting a thin film according to an embodiment of the present application;
FIG. 2 is a schematic diagram of a pulse laser measurement system according to an embodiment of the present application;
FIG. 3 is an enlarged schematic view of the wafer of FIG. 2;
FIG. 4 is a schematic diagram of the measuring principle of the pulsed laser measuring system shown in FIG. 2;
FIG. 5 is a schematic diagram of signal spectrum of ultrasonic wave propagating in a film to be measured according to an embodiment of the present application;
FIG. 6 is a partial schematic view of signal spectra of a plurality of calibration films of different densities provided by an embodiment of the present application;
FIG. 7 is a line-fitted plot of the speed of sound of ultrasonic wave propagation in the plurality of calibration films shown in FIG. 6 versus the baseline slope in the signal spectra of the plurality of calibration films;
fig. 8 is a flow chart of another method for detecting a thin film according to an embodiment of the present application.
Reference numerals illustrate:
10: pulsed laser measurement system, 11: laser emitting device, 12: spectroscope, 13: optical path delay device, 14: first mirror 15: lens, 16: wafer, 161: a film to be measured; 17: detection device, 18: an optical path adjusting device; 191: first calibration film, 192: second calibration film, 193: third alignment film, 194: a fourth calibration film;
20: laser pulse excitation beam, 30: laser pulse probe beam, 40: ultrasonic waves;
401: surface wave, 402: primary echo, 403: secondary echo, 404: a base line.
Detailed Description
The technical scheme of the application is further elaborated below by referring to the drawings in the specification and the specific embodiments.
In the description of the present application, it should be understood that the terms "length," "width," "depth," "upper," "lower," "outer," and the like indicate orientations or positional relationships based on the orientation or positional relationships shown in the drawings, and are merely for convenience in describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the present application.
The deposition of thin film layers on wafers is one of the most commonly used processes in semiconductor manufacturing, and therefore, it is critical to detect and monitor parameters such as thickness, density, etc. of thin films, which directly affect the performance and yield of semiconductor devices.
Currently, in the field of semiconductor technology, common methods for measuring film thickness include optical measurement and weighing. Optical measurement refers to the step-like film growth on a control wafer using the same program, and the thickness of the film on the control wafer is measured by a near infrared optical machine to characterize the thickness of the film on the product wafer. However, the optical measurement method has a limited measurement range, and is generally capable of accurately measuring a thin film having a thickness of 2 μm or less. In semiconductor manufacturing processes, the thickness of the thin film is often greater than 2 μm, for example, a carbon film having a thickness of 1 μm to 5 μm is used. And, as the number of stacked layers increases, thicker carbon films may be used later. When thicker films are measured, the result obtained by the optical measurement method has deviation; and the film thickness on the control wafer can only reflect the film thickness of the product wafer near the control wafer, but cannot reflect the thickness difference of the films at different positions on the product wafer.
The weighing method is to weigh the total weight of the film, and the thickness interval is reversely deduced by the weight, so that the measurement accuracy is greatly influenced by the film density, and the weighing method can only obtain the whole average density of the film and cannot reflect the thickness difference and the thickness change trend of different positions.
In addition, in the field of semiconductor technology, the characteristics of film density or hardness are characterized by using the N-K value of the film, N refers to the refractive index of the film, and K refers to the extinction coefficient of the film. The refractive index and extinction coefficient of a film are related to the density, hardness, residual stress, etc. of the film, and thus, the physical properties of the film can be characterized by the N-K value. N-K values were measured using a near infrared optical bench. The near infrared optical machine commonly used in the industry can only obtain optical signals within 500nm below the surface of the film, and the obtained N-K value can only represent the physical properties of materials within 500nm below the surface of the film. For films with a thickness greater than 500nm, if the film growth process varies, only the physical properties of the material within 500nm below the surface are obtained and cannot be reflected in the physical properties across the thickness of the film.
In view of this, the present application proposes a method of detecting a thin film by acquiring a signal spectrum of ultrasonic waves propagating in the thin film, determining the thickness of the thin film based on wave parameters in the signal spectrum, etc., and characterizing and monitoring the properties of the thin film.
Fig. 1 is a schematic flow chart of a method for detecting a thin film according to an embodiment of the present application. Referring to fig. 1, the method comprises the steps of:
s100: providing a laser pulse excitation signal to the first surface of the film to be detected so as to excite ultrasonic waves in the film to be detected; the film to be tested comprises a first surface and a second surface which are opposite;
s200: acquiring a signal spectrum of ultrasonic wave transmitted in a film to be measured;
s300: determining the signal amplitude of at least one echo of the ultrasonic wave and the arrival time of the echo on the first surface according to the signal spectrum, and the attenuation coefficient of the signal amplitude of a plurality of echoes generated by the ultrasonic wave along with the arrival time of the echo; wherein, the echo is the wave that the ultrasonic wave returns to the first surface after being reflected by the second surface;
s400: determining the average sound velocity of ultrasonic wave propagating in the film to be tested according to the signal amplitude of at least one echo, the arrival time of the echo and the attenuation coefficient;
s500: and determining the thickness of the film to be measured according to the average sound velocity and the arrival time of the echo.
In step S100, a laser pulse excitation signal is provided to the first surface of the thin film to be measured, and the laser pulse excitation signal excites ultrasonic waves in the thin film to be measured. The laser pulse excitation signal may comprise a femtosecond laser beam, for example.
The mechanism by which laser excites ultrasound in a solid can be summarized as: when the pulse laser beam is incident on the solid surface, part of the laser energy is absorbed by the solid and converted into heat energy, and the irradiated area of the solid surface is rapidly heated, so that the local area is rapidly thermally expanded to generate stress and strain, and ultrasonic waves are excited.
By adopting laser excitation ultrasonic wave, nondestructive detection can be performed by controlling the laser energy of the excitation ultrasonic wave to be low and only elastically deforming the solid surface. Therefore, the film detection method can be directly carried out on the film of the product wafer, the product wafer is not damaged, the measurement result directly reflects the property of the film on the product wafer, and the measurement result is accurate; in addition, the control chip can be saved, the cost is reduced, the process steps are reduced, and the efficiency is improved.
The film detection method of the application does not limit the composition components of the film to be detected. The film to be tested can detect the property by using the film detection method provided by the application as long as the film to be tested can excite ultrasonic waves under the action of laser. In various embodiments of the present application, the thin film to be tested includes, but is not limited to, a carbon film, a metal film, a high-K dielectric layer, an organic mask layer, and the like.
In addition, in this step, the film to be measured is often not a single film but a film layer attached to the substrate. In the technical field of semiconductors applicable to the film detection method, the wafer comprises a substrate and a film to be detected arranged on the substrate, wherein the first surface of the film to be detected is a surface far away from the substrate, and the second surface is a surface close to the substrate.
In step S200, a signal spectrum of the ultrasonic wave propagating in the film to be measured is acquired.
The embodiment of the application does not limit how to acquire the signal spectrum of the ultrasonic wave propagating in the film. In some embodiments, the signal spectrum of the ultrasonic wave propagating in the film is obtained by optical deflection techniques.
In one embodiment of the present application, the step S200 includes:
s210, starting from the moment of providing a laser pulse excitation signal, providing a laser pulse detection signal to the first surface after delaying for a preset time period, wherein the laser pulse detection signal and the laser pulse excitation signal are emitted to the first surface at different angles;
s220, detecting a reflection signal of the laser pulse detection signal reflected from the first surface;
s230, determining the signal spectrum of the ultrasonic wave transmitted in the film to be detected according to the signal spectrum of the reflected signal.
In order to more clearly describe the method for acquiring the signal spectrum of the ultrasonic wave propagating in the film to be measured in the embodiment of the application, a measurement system for executing the method for acquiring the signal spectrum of the ultrasonic wave propagating in the film to be measured is described. Fig. 2 is a schematic structural diagram of a pulse laser measurement system according to an embodiment of the present application, and fig. 3 is an enlarged schematic diagram of a wafer in fig. 2.
Referring to fig. 2 and 3, the pulsed laser measurement system 10 includes a laser emitting device 11, a beam splitter 12, an optical path delay device 13, a first reflecting mirror 14, a lens 15, and a detecting device 17.
The laser pulse beam emitted by the laser emitting device 11 is divided into a laser pulse excitation beam 20 and a laser pulse detection beam 30 after passing through the spectroscope 12, and the laser pulse excitation beam 20 is used for generating ultrasonic waves 40 in a film 161 to be tested of the wafer 16; the energy of the laser pulse detection beam 30 is much smaller than that of the laser pulse excitation beam 20, and no ultrasonic wave is generated in the thin film 161 to be measured, and the laser pulse detection beam 30 is used for detecting the ultrasonic wave 40 generated by the laser pulse excitation beam 20.
In other embodiments of the present application, two laser emitting devices 11 may be provided, and the two laser emitting devices 11 emit the laser pulse excitation beam 20 and the laser pulse detection beam 30, respectively.
The laser pulse excitation beam 20 is incident to the first surface of the film 161 to be measured after passing through the lens 15, and part of energy of the laser pulse excitation beam 20 is absorbed by the film to be measured and converted into heat energy, so that the irradiated area of the first surface is instantaneously heated to cause vibration to generate ultrasonic waves 40; the unabsorbed laser pulse excitation beam reflects off the first surface.
After passing through the optical path delay device 13, the first reflecting mirror 14 and the lens 15, the laser pulse detection beam 30 is incident on the first surface of the thin film 161 to be measured at a different angle from the laser pulse excitation beam 20, so that the laser pulse detection beam 30 reflected from the first surface can be incident on the detection device 17. It is emphasized that the laser pulse excitation beam 20 reflected from the first surface does not impinge on the detection means 17.
The optical path delay means 13 delays the laser pulse detection beam 30 to the first surface after a preset time period Δt with respect to the laser pulse excitation 20 beam, and the delay time period can be changed by adjusting the optical path delay means 13. In the embodiment of the present application, the optical path delay device 13 includes a delay line composed of a set of mirrors, and the delay time length is continuously adjusted by moving the delay line. For example, the delay line length is 0.15mm, and the preset duration Δt=1 psec (picosecond).
The detection means 17 receives a reflected beam of the laser pulse detection beam 30 reflected from the first surface.
In addition, the pulse laser measurement system 10 according to the embodiment of the present application further includes an optical path adjustment device 18, and the optical path adjustment device 18 is disposed on the optical path of the laser pulse excitation beam 20. The optical path adjusting device 18 is configured to delay the time for the laser pulse excitation beam 20 to reach the first surface of the film 161 to be measured, so that the laser pulse detection beam 30 can reach the first surface in advance with respect to the laser pulse excitation beam 20, or the laser pulse excitation beam and the laser pulse detection beam reach the first surface at the same time, so that the delay time period can start from zero.
Fig. 4 is a schematic diagram of the measuring principle of the pulse laser measuring system shown in fig. 2. The principle of acquiring a signal spectrum of ultrasonic waves propagating in a film to be measured according to the present embodiment is described below with reference to fig. 4.
In the aforementioned step S100, the laser pulse excitation beam 20 is provided to the first surface of the thin film 161 to be measured, the laser pulse excitation beam 20 generates the ultrasonic wave 40 on the first surface, and the ultrasonic wave 40 forms multiple reflections on the first surface and the second surface of the thin film 161 to be measured.
In step S210, the laser pulse detection beam 30 is supplied to the first surface after a delay of a preset period Δt from the time when the laser pulse excitation beam 20 is supplied. The reflected beam of the laser pulse detection beam 30 from the first surface is used to detect a change in the light propagation characteristics of the first surface, which is detected by the laser pulse detection beam 30 due to the ultrasonic wave 40 generated by the laser pulse excitation beam 20.
Referring to fig. 4, when the laser pulse detection beam 30 is delayed relative to the laser pulse excitation beam 20 by a first predetermined period Δt 1 When reaching the first surface, since the ultrasonic wave 40 generated by the laser pulse excitation beam 20 also propagates in the thin film 161 to be measured, the reflected beam of the laser pulse detection beam 30 from the first surface is not affected by the ultrasonic wave 40.
When the laser pulse detecting beam 30 is delayed relative to the laser pulse exciting beam 20 by a second predetermined period of time Δt 2 When reaching the first surface, if the ultrasonic wave 40 generated by the laser pulse excitation beam 20 reaches the first surface just at this time, the reflected beam of the laser pulse detection beam 30 from the first surface changes due to the surface vibration caused by the ultrasonic wave 40 reaching the first surface, which is represented in the signal spectrum as a signal amplitude change (e.g., signal amplitude enhancement).
With continued reference to fig. 4, when the laser pulse probe beam 30 is delayed relative to the laser pulse excitation beam 20 by a third predetermined period Δt 3 When reaching the first surface, if the peak of the ultrasonic wave 40 generated by the laser pulse excitation beam 20 reaches the first surface just at this time, a large vibration of the first surface is caused, so that the reflected beam of the laser pulse detection beam 30 from the first surface is greatly changed, which is represented by a peak or trough of the signal amplitude in the signal spectrum.
While the laser pulse detection beam 30 is delayed by a fourth predetermined time period Δt with respect to the laser pulse excitation beam 20 4 Upon reaching the first surface, due to the ultrasonic wave 40 passing through the firstAfter being reflected by one surface, the laser pulse probe beam 30 propagates back into the film 161 to be measured, and thus the reflected beam from the first surface is again not affected by the ultrasonic wave.
Thus, the laser pulse excitation beam 20 and the laser pulse detection beam 30 are continuously supplied to the first surface of the thin film 161 to be measured, constituting a laser pulse excitation signal and an excitation pulse detection signal; and continuously obtaining a reflected beam of the laser pulse probe beam 30 reflected from the first surface, constituting a reflected signal and forming a reflected signal spectrum. From the signal spectrum of the reflected signal, the signal spectrum of the ultrasonic wave propagating in the thin film 161 to be measured can be determined.
In the embodiment of the present application, the thickness of the film 161 to be measured is typically 1 μm to 20 μm, and the time for which the ultrasonic wave 40 propagates in the film of the micrometer scale is very short, typically only several tens of picoseconds. Thus, both the laser pulse excitation beam 20 and the laser pulse detection beam 30 may employ a femtosecond laser beam, and the delay time of the laser pulse detection beam 30 with respect to the laser pulse excitation beam 20 is in the order of femtosecond. Meanwhile, the pulse interval time of the laser pulse excitation signal is in the order of microseconds or milliseconds, and thus each laser pulse detection beam 30 can detect only the ultrasonic wave 40 generated by the laser pulse excitation beam 20 emitted last before it. For example, in some embodiments of the present application, a laser pulse excitation beam and a laser pulse detection beam with a pulse time of 200fsec (femtosecond), a frequency of 85MHz, and a wavelength of 522nm are used to obtain a signal spectrum of the propagation of the ultrasonic wave in the thin film to be measured.
FIG. 5 is a schematic diagram of signal spectrum of ultrasonic wave propagating in a film to be measured obtained according to an embodiment of the present application. Before interpreting the signal spectrum, it is also to be noted that: the laser excites ultrasonic waves, and can excite ultrasonic waves of various modes including longitudinal waves and surface waves in the film at a time.
The longitudinal wave propagates from the surface of the film along the thickness direction of the film, and when reaching the second surface of the film, part of the longitudinal wave is reflected and enters the film again to continue to propagate back to the first surface; part of the longitudinal wave returning to the first surface is reflected again to enter the film for propagation, and returns to the first surface after reaching the second surface. In this way, the longitudinal wave propagates between the two surfaces of the film a plurality of times until it is completely absorbed or scattered by the material. The energy of the surface wave is mainly concentrated in a wavelength range area near the surface, the surface wave is sensitive to the surface property of the film, and the surface wave is suitable for the characterization of the surface property of the film.
The signal spectrum of the ultrasonic wave propagating in the film to be measured, which is obtained by the embodiment of the application, comprises a plurality of echoes of the longitudinal wave and surface waves.
The echoes of the ultrasonic waves described later in the present application include echoes of longitudinal waves. The propagation mode of the echo can be seen that the propagation path of the echo comprises the thickness of the whole film, is sensitive to the property of the film along the thickness direction, and is suitable for representing the property of the film along the thickness direction and measuring the thickness of the film.
Referring to fig. 5, in the signal spectrum shown in fig. 5, the abscissa indicates the time of arrival of the ultrasonic wave at the first surface, and the ordinate indicates the signal amplitude. The signal spectrum shown in fig. 5 includes a surface wave 401, a primary echo 402, and a secondary echo 403.
The primary echo 402 is a wave that returns to the first surface after the ultrasonic wave is reflected by the second surface of the film to be measured for the first time, and the secondary echo 403 is a wave that returns to the first surface after the ultrasonic wave is reflected by the second surface of the film to be measured for the second time.
Although only the primary echo 402 and the secondary echo 403 are shown in fig. 5, the actual signal spectrum may include three echoes, four echoes, and the like.
In step S300, a signal amplitude of at least one echo of the ultrasonic wave and an echo arrival time of the echo reaching the first surface are determined according to the signal spectrum, and attenuation coefficients of signal amplitudes of a plurality of echoes generated by the ultrasonic wave along with the echo arrival time are determined.
It will be appreciated that although there are a plurality of echoes generated by the propagation of the ultrasonic wave in the film, the signal intensities of the primary echo 402 and the secondary echo 403 are large, and the signal intensities of the three echoes and the following echoes are small, so that the acquisition is difficult. Therefore, in the embodiment of the present application, determining, in step S300, the signal amplitude of at least one echo of the ultrasonic wave and the arrival time of the echo to reach the first surface according to the signal spectrum specifically includes:
And determining the signal amplitude of the primary echo generated by the ultrasonic wave and the arrival time of the echo of the primary echo to the first surface according to the signal spectrum.
In order to facilitate the reading of the signal amplitude and the arrival time of the echo in the signal spectrum, in the embodiment of the present application, the signal amplitude H is set to be the signal wave height value of the adjacent wave crest and wave trough of the echo. The echo arrival time T is set as the arrival time of the first trough or the first peak of the echo to the first surface.
When the at least one echo is the primary echo 402, see fig. 5, the signal amplitude H of the at least one echo 1 The echo arrival time T is the signal wave height value of the adjacent wave crest and wave trough of the primary echo 402 1 The arrival time of the first trough of the primary echo 402.
It should be noted that, in some embodiments of the present application, the signal amplitude H may also be the signal peak of the first trough or the first peak of the echo. The echo arrival time T may also be the time at which the wave front of the echo arrives at the first surface.
In addition, the present application is not limited to the specific manner of determining the attenuation coefficient of the signal amplitude of the plurality of echoes generated by the ultrasonic wave with the arrival time of the echoes in this step. In some embodiments of the present application, determining, in step S300, attenuation coefficients of signal amplitudes of a plurality of echoes generated by an ultrasonic wave with arrival time of the echoes according to signal spectra specifically includes:
S310, determining the signal amplitude of a primary echo generated by ultrasonic waves, the echo arrival time of the primary echo, the signal amplitude of a secondary echo and the echo arrival time of the secondary echo according to the signal spectrum;
s320: and determining an attenuation coefficient according to the signal amplitude of the primary echo, the echo arrival time of the primary echo, the signal amplitude of the secondary echo and the echo arrival time of the secondary echo.
In step S310, since the signal amplitudes of the first two echoes are larger, the error of the measured signal amplitudes is smaller, and the accuracy is higher, so that the accuracy of the attenuation coefficient can be improved.
Needs to be as followsIllustratively, when the signal amplitude H of the primary echo 402 is 1 The echo arrival time T of the primary echo 402 is the signal wave height value of the adjacent wave crest and wave trough of the primary echo 402 1 When the arrival time of the first trough of the primary echo 402 is the same, then correspondingly the signal amplitude H of the secondary echo 403 2 The echo arrival time T of the secondary echo 403 is the signal wave height value of the adjacent wave crest and wave trough of the secondary echo 403 2 Is the arrival time of the first trough of the secondary echo 403.
In step S320, the attenuation coefficient k can be determined by the formula (signal amplitude of the secondary echo 403-signal amplitude of the primary echo 402)/(echo arrival time of the secondary echo 403-echo arrival time of the primary echo 402).
In implementation, the embodiment of the application provides another way to determine the attenuation coefficient k by fully analyzing and understanding the signal spectrum. Referring to fig. 5, the signal spectrum includes not only a surface wave 401, a primary echo 402, and a secondary echo 403, but also a base line 404 formed by a reflected signal of a laser pulse probe signal from the first surface when an ultrasonic wave does not reach the first surface, the base line 404 connecting a signal amplitude of a wave tail of a surface wave or a preceding echo and a signal amplitude of a wave front of a following echo. It can be understood through the above-mentioned signal spectrum acquisition mode that the signal spectrum acquired by the present application is not a waveform diagram of the propagation of the ultrasonic wave in the film to be measured, but a wave signal of a plurality of echoes generated by the propagation of the ultrasonic wave in the film to be measured. Thus, when an adjacent subsequent echo is traveling a distance of two film thicknesses more than the previous echo, the signal amplitude of the subsequent echo is less than the signal amplitude of the previous echo, resulting in a gradual decrease in the baseline 404 between the adjacent two echoes with echo arrival time. Based on this, embodiments of the present application propose to characterize the attenuation coefficient k by the slope of the baseline 404. Specifically, in step S300, determining, according to the signal spectrum, attenuation coefficients of signal amplitudes of a plurality of echoes generated by the ultrasonic wave with arrival times of the echoes, includes:
From the signal spectrum, a slope of a baseline between two echoes in the signal spectrum is determined, the slope being used to characterize an attenuation coefficient of signal amplitudes of a plurality of echoes produced by the ultrasound wave over echo arrival time.
In the signal spectrum, the slope of the base line between any two adjacent echoes is substantially the same, and therefore, the attenuation coefficient can also be characterized by the slope of the base line between any two echoes. The attenuation coefficient is preferably determined by the baseline between the primary echo 402 and the secondary echo 403, because only the first two echoes need to be acquired, and all the echoes do not need to be acquired, so that the time for acquiring the signal spectrum can be saved. Furthermore, the present application has experimentally demonstrated that it is feasible to determine the speed of sound by characterizing the attenuation coefficient by the slope of the baseline between the two echoes, as will be discussed later in the acquisition of the speed of sound for the calibration film. Here, it should be further explained that the values of the attenuation coefficients determined by the signal amplitudes of the two echoes and the arrival times of the echoes may be slightly deviated from the values of the attenuation coefficients determined by the slope of the baseline between the two echoes, but in actual use, the attenuation coefficients are determined in the same manner as long as the same manner is used for different films to be measured and calibration films described later, and the average sound velocity obtained by the two manners of determining the attenuation coefficients and the thickness of the films to be measured are the same.
In step S400, an average sound velocity of the ultrasonic wave propagating in the film to be measured is determined according to the signal amplitude and the echo arrival time of at least one echo, and the attenuation coefficient.
The wave velocity u propagating in the solid can be represented by the formulaObtained, wherein E is the elastic modulus of the solid material and ρ is the density of the solid material. From this, it is understood that the sound velocity V of the ultrasonic wave propagating in the film to be measured is related to the density ρ and the elastic modulus E of the film to be measured, and can be expressed as v=f (E, ρ).
When a wave propagates in a solid, the intensity of the wave gradually attenuates due to scattering and absorption of the wave by the material, the amplitude A of the wave gradually attenuates, and the attenuation dA of the amplitude and the initial amplitude A of the wave 0 The relation between the thickness dx of the solid and the absorption coefficient alpha of the material is-da=alphaA 0 dx, the absorption coefficient α of the material is related to the density ρ of the material, the elastic modulus E (which can be understood as hardness), and the frequency f of the wave. From this, it can be seen that when the ultrasonic wave propagates in the film to be measured, the attenuation dH of the signal amplitude is equal to the thickness THK of the film, the density ρ and the elastic modulus E of the film to be measured, and the signal amplitude H of the initial ultrasonic wave 0 Related to the frequency F, dh=f (E, ρ, THK, H) 0 F). The attenuation dH of the signal amplitude can be determined by the signal amplitude H of at least one echo 1 And echo arrival time T 1 And the attenuation coefficient k of the signal amplitude of a plurality of echoes generated by the ultrasonic wave in the film to be detected along with the arrival time of the echoes.
The propagation time of an ultrasonic wave in a solid is related to the propagation distance and the speed of sound. From this, the echo arrival time T is related to the thickness THK of the film to be measured and the sound velocity V of the ultrasonic wave propagating through the film to be measured, and can be expressed as t=f (E, ρ, THK).
In other embodiments of the application, the relationship of sound velocity to signal amplitude and echo arrival time of at least two echoes may be established theoretically by taking an exact formula. However, the difficulty of determining such a relationship by theory is great, and many severe constraints often exist in the theoretical formula, which makes it difficult to apply the theoretical formula to engineering practice. Therefore, the embodiment of the application provides that a mathematical fitting model is established, and coefficients of the mathematical fitting model are determined through actually measured data, so that a relation applicable to reality is obtained.
As described above, the attenuation dH of the signal amplitude of the echo is related to the density ρ and the elastic modulus E of the film to be measured; the echo arrival time T is related to the density ρ and the elastic modulus E, so that the density and the elastic modulus of the film to be measured can be reversely deduced from the attenuation of the echo signal amplitude and the echo arrival time, which can be expressed as ρ=f (dH, T), e=f (dH, T), and the sound velocity V is related to the density ρ and the elastic modulus E of the film to be measured, whereby v=f (dH, T) can be obtained. While the attenuation of the signal amplitude of the echo can be performed by using the signal amplitude H of at least one echo 1 And echo arrival time T 1 And the signal amplitude of multiple echoes generated by ultrasonic waves in the film to be tested is along with the arrival of the echoesThe attenuation coefficient k between them. Therefore, in the embodiment of the application, the average sound velocity V of the ultrasonic wave transmitted in the film to be measured is established a And signal amplitude H of at least one echo 1 And echo arrival time T 1 Fitting equations with the attenuation coefficient k are possible.
In this step, an average sound velocity V is established a Signal amplitude H with at least one echo 1 And echo arrival time T 1 Fitting equation V for attenuation coefficient k a ＝F(H 1 ，T 1 And k), substituting the signal amplitude and the echo arrival time of at least one echo obtained in the step S300 and the attenuation coefficient into a fitting equation to calculate the average sound velocity of the ultrasonic wave propagating in the film to be measured.
In step S500, the thickness of the film to be measured is determined according to the average sound velocity determined in step S400 and the echo arrival time.
In this step, the formula for determining the thickness of the film to be measured based on the average sound velocity and the echo arrival time is
Wherein T is 1 Is the echo arrival time of the primary echo 402.
When the arrival time of the secondary echo 403 is adopted, the denominator in equation (1) should be 4; similarly, when using the arrival time of three echoes, the denominator in equation (1) should be 6.
Substituting the average sound velocity of the ultrasonic wave transmitted in the film to be measured and the arrival time of the primary echo obtained in the step S400 into the formula (1) to calculate the thickness of the film to be measured.
According to the film detection method provided by the embodiment of the application, the average sound velocity of ultrasonic waves in the film to be detected is determined by utilizing echo parameters (such as the signal amplitude of echo, the arrival time of echo and the attenuation coefficient of the signal amplitude of a plurality of echoes along with the arrival time), and the thickness of the film to be detected is further determined according to the average sound velocity and the arrival time of echo. Since the propagation path of the echo includes the thickness of the whole film, the average sound velocity calculated by the echo parameter is more accurate, and the determined film thickness is also more accurate. Further, according to the above relation between the sound velocity V and the elastic modulus E and density ρ of the solid material, some parameters (such as average sound velocity) determined by the present application may further represent the physical properties of the film to be measured, such as density and hardness.
The embodiment of the application has the average sound velocity V for the ultrasonic wave to propagate in the film to be measured a And signal amplitude H of at least one echo 1 And echo arrival time T 1 And the specific form of the fitting equation of the attenuation coefficient k is not limited. In the embodiment of the application, the inventor provides an average sound velocity V of ultrasonic wave transmitted in the film to be measured through theoretical analysis and measured data verification a And signal amplitude H of at least one echo 1 And echo arrival time T 1 And a fitting equation for the attenuation coefficient k. Specifically, step S400 includes:
s410: establishing a first linear regression equation taking the average sound velocity as a dependent variable, the signal amplitude and the echo arrival time of at least one echo, and the attenuation coefficient as independent variables;
s420, substituting the signal amplitude and the echo arrival time of at least one echo of the film to be measured and the attenuation coefficient into a first linear regression equation, and calculating the average sound velocity of the ultrasonic wave transmitted in the film to be measured.
In step S410, a first linear regression equation is established with the average sound velocity as a dependent variable, the signal amplitude and the echo arrival time of at least one echo, and the attenuation coefficient as an independent variable.
The first linear regression equation is:
wherein V is a Is the average sound velocity beta 0 、β 1 、β 2 A first regression coefficient that is a first linear regression model; h 1 And T 1 Signal amplitude and echo arrival time of an echo respectivelyK is the attenuation coefficient of the signal amplitude of a plurality of echoes generated by ultrasonic waves in the film to be measured along with the arrival time of the echoes.
In the first linear regression model, the ratio of the signal amplitude to the echo arrival time is taken as an independent variable, because the echo arrival time is avoided being used twice when the thickness of the film to be measured is calculated; and in order to weaken the influence of the thickness of the film to be detected on the fitting average sound velocity by adopting the signal amplitude and the echo arrival time, the interference factor of the first linear regression equation is eliminated.
In addition, as described above, the signal amplitude and echo arrival time of at least one echo are obtained, and the attenuation coefficient is used to fit the average sound velocity; the signal intensity of the primary echo 402 is larger, so that the echo parameters are easy to read. Therefore, in the embodiment of the application, the signal amplitude and the echo arrival time of only one echo are selected, and the attenuation coefficient establishes a first linear regression equation.
To obtain a first regression coefficient in the first linear regression equation. In some embodiments of the present application, a plurality of calibration films of different densities and/or different thicknesses are provided, and the sound velocity is determined by obtaining the measured thickness of the calibration film and the arrival time of the echo in the signal spectrum; and determining a first regression coefficient by the sound velocity, the signal amplitude of at least one echo in the signal spectrum, the arrival time of the echo and the attenuation coefficient. Specifically, step S410 includes:
s411: establishing a first linear regression model taking the average sound velocity as a dependent variable, the signal amplitude and the echo arrival time of at least one echo, and the attenuation coefficient as independent variables;
s412: providing laser pulse excitation signals to a first surface of a plurality of calibration films of different densities and/or thicknesses to excite ultrasonic waves in the calibration films; acquiring a plurality of calibration signal spectrums transmitted by ultrasonic waves in a plurality of calibration films;
S413: determining the signal amplitude and the echo arrival time of at least one echo transmitted by the ultrasonic wave in each calibration film according to the plurality of calibration signal spectrums, and the attenuation coefficient of the signal amplitude of a plurality of echoes generated by the ultrasonic wave along with the echo arrival time;
s414: acquiring the thickness of a plurality of calibration films, and determining the sound velocity of ultrasonic waves in each calibration film according to the thickness of each calibration film and the arrival time of echo;
s415: and determining a first regression coefficient of the first linear regression model by the sound velocity corresponding to each calibration film, the signal amplitude of at least one echo, the echo arrival time and the attenuation coefficient to obtain a first linear regression equation.
In step S412, providing laser pulse excitation signals to the first surfaces of the plurality of calibration films of different densities and/or thicknesses to excite ultrasonic waves in the calibration films; and acquiring a plurality of calibration signal spectra of the ultrasonic wave propagating in the plurality of calibration films.
In this step, the laser pulse excitation signals with the same power are supplied to the first surfaces of the calibration films with different densities and/or thicknesses to eliminate the signal amplitude H of the initial ultrasonic wave 0 And the influence of the frequency f of the ultrasonic wave on the attenuation dH of the signal amplitude, so that the first linear regression equation is more accurate. Meanwhile, when the thickness of the film to be measured is calculated by adopting the first linear regression equation, the signal spectrum of the ultrasonic wave in the film to be measured is obtained by adopting the laser pulse excitation signal with the same power, so that the thickness of the film to be measured is accurately obtained.
In some embodiments of the application, the thickness ranges of the plurality of calibration films and the thickness ranges of the films to be measured are substantially the same.
In some embodiments of the present application, the preparation parameters of the plurality of calibration films of different densities and/or thicknesses include common preparation parameters of the film to be tested. Thus, the fitted first linear regression equation can be more accurate.
The number of the plurality of calibration films of different densities and/or thicknesses is not less than 10. In some embodiments, the number of calibration films is not less than 20.
It should be noted that a plurality of calibration films of different densities and/or thicknesses may be formed on the same wafer or on different wafers.
In step S413, when determining the signal amplitude and the echo arrival time of at least one echo propagating in each calibration film according to the plurality of calibration signal spectrums, the wave parameters of the same echo in each calibration film should be obtained, for example, the signal amplitude and the echo arrival time of a primary echo in one calibration film should be obtained, and then the signal amplitude and the echo arrival time of a primary echo in other calibration films should be obtained.
When the attenuation coefficients of the signal amplitudes of the plurality of echoes generated by the ultrasonic wave with the arrival time of the echoes are obtained according to the plurality of calibration films, the attenuation coefficients should be obtained in the same manner. If the attenuation coefficient is determined by the signal amplitudes and echo arrival times of two echoes in one calibration film, then the attenuation coefficient should also be determined by the signal amplitudes and echo arrival times of two echoes in the other calibration film. If the attenuation coefficient is determined by the slope of the baseline between two echoes in one calibration film, then the attenuation coefficient should also be determined by the slope of the baseline between the same two echoes in the other calibration film. In the embodiment of the application, the slope of the base line between the two echoes can be automatically read out through software, so that the attenuation coefficient is determined to be a better choice through the slope of the base line between the two echoes.
In step S414, the thicknesses of the plurality of calibration films are obtained, and the sound velocity of the ultrasonic wave in each calibration film is determined according to the thickness of each calibration film and the arrival time of the echo.
In this step, the method for obtaining the thickness of the plurality of calibration films is not limited
In one embodiment of the present application, the step of obtaining the measured thicknesses of the plurality of calibration films includes:
s4141: cutting the calibration film along a direction perpendicular to the surface of the film to be measured to expose the cross section of the calibration film;
s4142: acquiring a cross-sectional image of the calibration film by adopting a scanning electron microscope;
s4143: and determining the thickness of the calibration film according to the sectional image.
In some embodiments of the present application, the incident position of the laser pulse excitation signal on the first surface of the calibration film is selected to perform cutting, so as to obtain the thickness of the film to be calibrated at the position, so that the first linear equation is more accurate.
The step of determining the thickness of the calibration film according to the sectional image comprises measuring the thickness value of the section of the calibration film on the sectional image, and obtaining the thickness of the calibration film according to the scaling.
In this step, after obtaining the measured thickness of the calibration film and the arrival time of the echo, the sound velocity of the ultrasonic wave propagating in the calibration film can be calculated according to the above formula (1).
In the embodiment of the present application, after obtaining the slope of the baseline 404 between the two echoes in step S413, and actually obtaining the thickness of the calibration film and determining the sound velocity of the ultrasonic wave propagating in the calibration film in step S414, the feasibility of using the slope of the baseline 404 between the two echoes to characterize the attenuation coefficient and obtain the sound velocity of the ultrasonic wave propagating in the film is verified. Referring to fig. 6, fig. 6 is a partial view of signal spectra of four calibration films of different densities near a single echo provided by an embodiment of the present application. Fig. 7 is a line-fitted plot of the speed of sound of ultrasonic waves propagating in the four calibration films shown in fig. 6 versus the baseline slope in the signal spectrum corresponding to the four calibration films.
In fig. 6, the linear fitting equation of the base line 404 between the secondary echoes 403 of the primary echo 402 of the first calibration film 191 is y= -21.277x+50615, and the determination coefficient R is 2 = 0.9594; the linear fitting equation of the base line 404 between the secondary echoes 403 of the primary echo 402 of the second calibration film 192 is y= -20.711x+48733, the coefficient R is determined 2 = 0.9531; the linear fitting equation of the base line 404 between the secondary echoes 403 of the primary echo 402 of the third calibration film 193 is y= -19.579x+43254, the decision coefficient R 2 = 0.9398; the linear fitting equation of the base line 404 between the secondary echoes 403 of the primary echo 402 of the fourth calibration film 194 is y= -18.768x+38481, the decision coefficient R 2 = 0.9321. The closer the determination coefficient is to 1, the closer the line segment is to a straight line. By FIG. 6, a first school is obtainedThe slopes between the two echoes of the quasi-thin film to the fourth calibration thin film are-21.277, -20.711, -19.579, -18.768, respectively. Fitting graphs are plotted according to the slope between two echoes of the first to fourth calibration films and the sound velocity at which the ultrasonic waves propagate in the four calibration films, as shown in fig. 7. As can be seen from fig. 7, the slope of the baseline between the two echoes and the sound velocity satisfy the unitary linear regression equation y= -0.205x-20.084, and the determination coefficient R 2 =0.9945, it can be seen that it is feasible to determine the speed of sound by characterizing the attenuation coefficient with the slope of the baseline between the two echoes.
In step S415, the sound velocity corresponding to each calibration film, the signal amplitude and the echo arrival time of at least one echo, and the attenuation coefficient are substituted into a first linear regression model, and a first regression coefficient beta is calculated by a least square method 0 、β 1 、β 2 A first linear regression equation is obtained.
After the first linear regression equation is obtained in step S410, in step S420, the signal amplitude and the echo arrival time of at least one echo of the film to be measured and the attenuation coefficient are substituted into the first linear regression equation, so that the average sound velocity of the ultrasonic wave propagating in the film to be measured can be calculated.
It should be noted that, when determining the signal amplitude and the echo arrival time of at least one echo of the film to be measured, and the attenuation coefficient, the same echo and the same manner of determining the attenuation coefficient as those when determining the first linear regression equation by the calibration film should be selected to ensure that the calculation result is accurate. If the signal amplitude and the echo arrival time of the primary echo in the plurality of calibration films are obtained and a first linear regression equation is established according to the slope of the base line between the two echoes, then when calculating the average sound velocity of the ultrasonic wave in the film to be measured, the signal amplitude and the echo arrival time of the primary echo of the film to be measured and the slope of the base line between the two echoes should be selected to determine the average sound velocity.
In the embodiment of the application, the thickness of the film to be measured can be accurately obtained by comparing the measured thickness of the film to be measured with the thickness of the film to be measured determined by adopting the method for determining the film thickness provided by the embodiment of the application; meanwhile, the first linear regression equation of the average sound velocity, the signal amplitude of at least one echo, the arrival time of the echo and the attenuation coefficient provided by the embodiment of the application is proved, so that the average sound velocity of the ultrasonic wave transmitted in the film to be measured can be accurately obtained. Therefore, the average sound velocity can be used for more accurately representing the properties of the film to be measured such as density, hardness and the like in the thickness direction.
The embodiment of the application also provides a film detection method for characterizing and monitoring the properties of the film through the wave parameters. Fig. 8 is a schematic flow chart of another thin film detection method provided by the application. Referring to fig. 8, the method for detecting a thin film provided by the embodiment of the application includes:
s100: providing a laser pulse excitation signal to the first surface of the film to be detected so as to excite ultrasonic waves in the film to be detected; the film to be tested comprises a first surface and a second surface which are opposite;
s200: acquiring a signal spectrum of ultrasonic wave transmitted in a film to be measured;
s300: determining the signal amplitude of at least one echo of the ultrasonic wave and the arrival time of the echo on the first surface according to the signal spectrum, and the attenuation coefficient of the signal amplitude of a plurality of echoes generated by the ultrasonic wave along with the arrival time of the echo; wherein the echo is the wave of the ultrasonic wave which is reflected by the second surface and returns to the first surface;
s400: determining the average sound velocity of ultrasonic wave propagating in the film to be tested according to the signal amplitude of at least one echo, the arrival time of the echo and the attenuation coefficient;
s600: according to the signal spectrum, determining the time when at least two wave peaks and/or wave troughs of a surface wave propagating in a film body with a preset depth below the first surface in ultrasonic waves reach the first surface; the preset depth is smaller than the thickness from the first surface to the second surface of the film to be detected;
S700: according to the time when at least two wave crests and/or wave troughs of the surface wave reach the first surface, determining the surface sound velocity of the surface wave propagating in the film body with preset depth below the first surface;
s800: calculating a ratio of the surface sound velocity to the average sound velocity; wherein the ratio characterizes the physical properties of the film to be measured.
Steps S100 to S400 are as described above, and are not described here again.
In step S600, a time for at least two peaks and/or troughs of a surface wave propagating in a film body at a preset depth below a first surface to reach the first surface is determined from a signal spectrum.
As previously described, the surface wave propagates within the film body below the surface of the film by a predetermined depth, and thus the surface wave can reflect the physical properties of the film body below the surface of the film by a predetermined depth. In the embodiment of the application, the surface speed of the surface wave is used for representing the physical properties of the film body in a certain depth below the surface of the film to be measured. The preset depth is smaller than the thickness from the first surface to the second surface of the film to be measured.
In addition, as shown in fig. 5, the first trough and the first peak of the surface wave 401 have larger signal amplitudes and are easy to read, and therefore, in step S600, the time P at which the two peaks and/or troughs reach the first surface 1 And P 2 Including a first arrival time at which the first trough arrives at the first surface and a second arrival time at which the first peak arrives at the first surface.
In step S700, a surface acoustic velocity of the surface wave propagating in the film body at a preset depth below the first surface is determined according to a time when at least two peaks and/or troughs of the surface wave reach the first surface.
From the equation v=λf for the wave velocity V, the wavelength λ, and the frequency f, it is known that the wave velocity and the wavelength are positively correlated on the premise that the frequency of the ultrasonic wave is known. In the signal spectrum shown in fig. 5, the time that adjacent peaks and troughs of the surface wave 401 reach the first surface can be considered as half the wavelength. It is therefore possible to determine the surface acoustic speed at which the surface wave propagates in the surface layer of the film to be measured using the time at which the peak and/or trough of the surface wave reach the first surface. Because the frequency of the ultrasonic wave is difficult to obtain in the formula, the embodiment of the application provides the surface acoustic velocity V for establishing the propagation of the surface wave in the film to be measured t And (3) withTime P at which at least two peaks and/or troughs of a surface wave reach the first surface 1 And P 2 Fitting equation V of (2) t ＝F(P 1 ，P 2 )。
Substituting the time at which at least two peaks and/or troughs of the surface wave acquired in step S600 reach the first surface into a fitting equation V t ＝F(P 1 ，P 2 ) The average sound velocity of the ultrasonic wave propagating in the film to be measured can be calculated.
In step S800, a ratio of the surface sound velocity to the average sound velocity is calculated, the ratio characterizing the physical properties for the film to be measured.
The formula for the ratio can be expressed as:
Ratio＝V t /V a (3)
in other embodiments of the application, the formula for the Ratio can also be expressed as ratio=v a /V t 。
In the formula (3), the surface sound velocity is obtained by step S700, and the average sound velocity is obtained by step S400.
It will be appreciated that in theory, it is desirable to obtain films with uniform properties, but in practical processes, the film preparation parameters are numerous. The change of each parameter can influence the growth process of the film, so that the film has property layering along the thickness direction, wherein the density of the lower layer is higher and the density of the upper layer is lower; or the hardness of the lower layer is larger, and the hardness of the upper layer is smaller. However, the existing characterization methods, such as N-K value characterization methods, are unable to characterize the change of the physical properties of the thin film in the thickness direction.
In the embodiment of the application, a method for representing and monitoring the properties of a film through the Ratio of the surface sound velocity to the average sound velocity is provided. The density, hardness and other physical properties of the film body within a certain depth below the surface of the film by the surface acoustic velocity reaction, while the average acoustic velocity reaction is the overall density, hardness and other physical properties of the film in the thickness direction. Therefore, when the film growth process changes, the change can lead to the change of the surface sound velocity and the change of the average sound velocity, so that the ratio of the surface sound velocity to the average sound velocity is changed, and the property of the film can be represented by detecting the surface sound velocity, the average sound velocity and the ratio of the surface sound velocity to the average sound velocity, and the growth process of the film can be monitored.
In practical application, the uniformity of the film to be detected can be detected by obtaining the ratio of the surface sound velocity to the average sound velocity at different positions of the same film to be detected. The consistency of the films to be tested can be monitored by obtaining the ratio of the surface sound velocity to the average sound velocity of the films to be tested. When the production line is abnormal, the abnormal preparation parameters can be reversely pushed through the surface sound velocity, the average sound velocity and the ratio of the surface sound velocity to the average sound velocity, so that the problem solving speed is increased.
Surface acoustic velocity V for surface wave propagation in film under test t Time P to reach the first surface with at least two peaks and/or troughs of the surface wave 1 And P 2 The application is not limited by the specific form of fitting equation. In the embodiment of the present application, the inventor proposes a fitting equation through theoretical analysis and data analysis, specifically, step S700 includes:
s710, establishing a second linear regression equation taking the surface sound velocity as a dependent variable and the time when at least two wave peaks and/or wave troughs of the surface wave reach the first surface as the independent variable;
s720, substituting the time when at least two peaks and/or troughs of the surface wave of the film to be measured reach the first surface into a second linear regression equation, and calculating the surface sound velocity of the ultrasonic wave propagating in the surface layer of the film to be measured.
In step S710, a second linear regression equation is established with the surface acoustic velocity as a dependent variable and the time for at least two peaks and/or troughs of the surface wave to reach the first surface as an independent variable.
The second linear regression equation is:
V t ＝F(P 1 ，P 2 )＝θ 0 +θ 1 P 1 +θ 2 P 2 (3)
where Vt is the average sound velocity, θ 0 、θ 1 、θ 2 A second regression coefficient that is a second linear regression model; p1 and P2 are tablesThe time at which at least two peaks and/or troughs of the surface wave reach said first surface.
As previously described, obtaining the time at which at least two peaks and/or troughs of the surface wave reach the first surface can fit the surface acoustic velocity; the signal strength of the first wave crest and the first wave trough of the surface wave is high, and the arrival time of the first wave crest and the first wave trough reaching the first surface is easy to read, so that in the embodiment of the application, the arrival time of the two wave crests and/or wave troughs of the surface wave reaching the first surface is selected to establish a second linear regression equation.
To obtain a second regression coefficient in the second linear regression equation. In some embodiments of the present application, a plurality of calibration films of different densities and/or different thicknesses are provided, and the sound velocity is determined by obtaining the measured thickness of the calibration film and the arrival time of the echo in the signal spectrum; and determining a first regression coefficient by the sound velocity and the arrival time of at least two peaks and/or troughs of the surface wave in the signal spectrum to the first surface. Specifically, step S710 includes:
S711, establishing a second linear regression model taking the surface sound velocity as a dependent variable and the time when at least two wave peaks and/or wave troughs of the surface wave reach the first surface as the independent variable;
s712, providing laser pulse detection signals to the first surfaces of the calibration films with different densities and/or thicknesses so as to excite ultrasonic waves in the calibration films; acquiring a plurality of calibration signal spectrums transmitted by ultrasonic waves in a plurality of calibration films;
s713, determining the echo arrival time of the echo of the ultrasonic wave propagating in each calibration film and the arrival time of at least two wave crests and/or wave troughs of the surface wave to the first surface according to the plurality of calibration signal spectrums;
s714, acquiring the thickness of a plurality of calibration films, and determining the sound velocity of ultrasonic waves in each calibration film according to the thickness of each calibration film and the arrival time of echo;
s715, substituting the sound velocity corresponding to each calibration film and the time when at least two wave peaks and/or wave troughs of the surface wave reach the first surface into a second linear regression model, and calculating a regression coefficient to obtain a second linear regression equation.
It should be noted that, in the embodiment of the present application, when determining the second regression coefficient of the second linear regression equation, the sound velocity of the calibration film used is not the surface sound velocity of the calibration film, but the average sound velocity. The reason for this is: the surface sound velocity of the calibration film is difficult to obtain; and after the second linear regression equation is determined by adopting the average sound velocity of the calibration film, although the obtained surface sound velocity of the film to be measured is different from the actual surface sound velocity, the property of the film is not affected by using the surface sound velocity, and the property of the film is not affected by using the ratio of the surface sound velocity to the average sound velocity. Because, in practical application, the properties of different positions of the same film to be tested are the same, and the properties of different films to be tested in the same batch are the same, the surface sound velocity of different positions of the same film to be tested, the ratio of the surface sound velocity to the average sound velocity are the same, and the ratio of the surface sound velocity to the average sound velocity are the same. While when the surface sound velocity obtained by the second linear regression equation characterizes the properties of the same film at different locations or different film properties, the characterization criteria are the same, and therefore, even if the surface sound velocity obtained by the second linear regression equation is different from the actual surface sound velocity, the properties of the film characterized by it are not affected.
That is, the present application establishes the second linear regression equation not for obtaining the surface acoustic velocity of the surface wave but for reflecting the physical properties of the film body within a certain depth below the surface of the film by the surface acoustic velocity. And further, the change condition of the film to be measured in the growth process is represented by the physical property change and the tissue change of the reaction film along the thickness direction through the ratio of the surface sound velocity to the average sound velocity. Therefore, the application adopts the average sound velocity of the calibration film to determine the second linear regression equation, which does not influence the characterization of the film and can reduce the workload.
In step S712, laser pulse excitation signals with the same power are provided to the first surfaces of the calibration films with different densities and/or thicknesses, so as to eliminate the influence of the frequency of the ultrasonic wave on the second linear regression equation of the surface sound velocity, and make the second linear regression equation more accurate. It should be noted that, when the second linear regression equation is used to calculate the surface sound velocity of the film to be measured, the laser pulse excitation signal with the same power should be used to obtain the signal spectrum of the ultrasonic wave in the film to be measured.
In step S713, the same time for the peaks and/or troughs of the surface wave to reach the first surface should be obtained for each calibration film. If the arrival time of the first trough and the first peak of the surface wave in one calibration film is obtained, the arrival time of the first trough and the first peak of the surface wave in the other calibration film should also be obtained. Similarly, the echo arrival time of the same echo in each calibration film should be obtained. If the echo arrival time of the primary echo in one calibration film is acquired, the echo arrival time of the primary echo in the other calibration film should also be acquired.
Based on the above analysis, the method for determining the sound velocity of the ultrasonic wave propagating in the calibration film in step S714 of the embodiment of the present application is the same as the method for determining the sound velocity of the ultrasonic wave propagating in the calibration film in step S414, and therefore will not be described in detail.
In step S715, the sound velocity corresponding to each calibration film and the time when at least two peaks and/or troughs of the surface wave reach the first surface are substituted into the second linear regression model, and the regression coefficient θ is calculated by the least square method 0 、θ 1 、θ 2 And obtaining a second linear regression equation. The linear regression equation is typically established using software (e.g., SPSS, matlab).
In addition, in some other embodiments of the present application, after step S500, the method further includes:
s900: and obtaining preparation parameters when the film to be measured is formed, and establishing a first corresponding relation table among the average sound velocity, the thickness of the film to be measured and the preparation parameters.
In some embodiments of the present application, after step S800, further includes:
s1000: and obtaining preparation parameters when the film to be detected is formed, and establishing a second corresponding relation table between the ratio of the surface sound velocity to the average sound velocity and the preparation parameters.
Thus, the production can be guided through the first corresponding relation table and the second corresponding relation table; and when the film to be detected is abnormal, the reasons for the abnormality are conveniently checked.
In some embodiments of the present application, there is also provided a method for preparing a thin film, including:
acquiring the thickness of a film to be prepared, the ratio of the surface sound velocity to the average sound velocity, and the first relation corresponding table and the second relation corresponding table;
determining preparation parameters;
and forming the film to be prepared by adopting the preparation parameters.
The preparation parameters are determined through the first corresponding relation table among the average sound velocity, the thickness of the film to be detected and the preparation parameters and the second corresponding relation table among the ratio of the surface sound velocity to the average sound velocity and the preparation parameters, so that the controllability of the film preparation process can be improved, production anomalies can be reduced, and the production efficiency can be improved.
According to the film detection method provided by the application, on one hand, the thickness of the film to be detected is determined by the echo parameters of the echo generated in the film to be detected by the ultrasonic wave, and the thickness of the film determined by the echo parameters is more accurate because the propagation path of the echo is the thickness of the whole film to be detected. On the other hand, the property of the film to be measured is represented by the surface sound velocity of the surface wave propagating on the surface layer of the film to be measured, the average sound velocity of the echo propagating in the film to be measured and the ratio of the surface sound velocity to the average sound velocity, so that the change of the film to be measured in the growth process can be reflected.
The detection method of the film provided by the embodiment of the application has no damage to the film, can directly test the film of the product wafer, and has accurate result and convenient operation. When the signal spectrum of the ultrasonic wave propagating in the film is obtained through the laser pulse excitation signal and the laser pulse detection signal, the signal spectrum obtained within +/-2 ℃ of temperature change is not influenced by the ambient temperature; the method is not influenced by the water vapor adsorbed by the film, so that the detection result is stable and accurate. The film detection method provided by the application has the advantages of high detection efficiency, capability of realizing mass rapid measurement of film properties at different positions of the wafer, capability of detecting the ultra-short pulse laser used, long service life of up to 6 years, stable output and low detection cost.
The present application is not limited to the specific embodiments, and any person skilled in the art can easily think about the changes or substitutions within the technical scope of the present application, and the scope of the present application is covered by the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims (9)

1. A method for detecting a thin film, comprising:
Providing a laser pulse excitation signal to a first surface of a film to be measured so as to excite ultrasonic waves in the film to be measured; wherein the film to be tested comprises a first surface and a second surface which are opposite;
acquiring a signal spectrum of the ultrasonic wave transmitted in the film to be measured;
determining the signal amplitude of at least one echo of the ultrasonic wave and the arrival time of the echo to the first surface according to the signal spectrum, and the attenuation coefficient of the signal amplitudes of a plurality of echoes generated by the ultrasonic wave along with the arrival time of the echo; wherein the echo is a wave of the ultrasonic wave which is reflected by the second surface and returns to the first surface;
determining the average sound velocity of the ultrasonic wave transmitted in the film to be detected according to the signal amplitude of the at least one echo, the arrival time of the echo and the attenuation coefficient;
determining the thickness of the film to be detected according to the average sound velocity and the echo arrival time;
the determining, according to the signal amplitude and the echo arrival time of the at least one echo, and the attenuation coefficient, an average sound velocity of the ultrasonic wave propagating in the film to be measured, includes:
Establishing a first linear regression equation taking the average sound velocity as a dependent variable, wherein the ratio of the signal amplitude of the at least one echo to the arrival time of the echo and the attenuation coefficient are independent variables;
substituting the ratio of the signal amplitude of the at least one echo to the arrival time of the echo of the film to be detected and the attenuation coefficient into the first linear regression equation to calculate the average sound velocity of the ultrasonic wave propagating in the film to be detected.
2. The method according to claim 1, wherein the establishing a first linear regression equation having the average sound velocity as a dependent variable, the ratio of the signal amplitude of the at least one echo to the echo arrival time, and the attenuation coefficient as an independent variable, comprises:
establishing a first linear regression model taking the average sound velocity as a dependent variable, wherein the ratio of the signal amplitude of the at least one echo to the arrival time of the echo and the attenuation coefficient are independent variables;
providing laser pulse excitation signals to a first surface of a plurality of calibration films of different densities and/or thicknesses to excite ultrasound waves in the calibration films; acquiring a plurality of calibration signal spectrums of the ultrasonic wave transmitted in a plurality of the calibration films;
Determining the signal amplitude and the echo arrival time of at least one echo of ultrasonic wave propagating in each calibration film according to a plurality of calibration signal spectrums, and the attenuation coefficient of the signal amplitude of a plurality of echoes generated by the ultrasonic wave along with the echo arrival time;
acquiring the thickness of a plurality of calibration films, and determining the sound velocity of the ultrasonic wave transmitted in each calibration film according to the thickness of each calibration film and the arrival time of the echo;
and determining a first regression coefficient of the first linear regression model according to the sound velocity, the ratio of the signal amplitude of the at least one echo to the arrival time of the echo and the attenuation coefficient corresponding to each calibration film, so as to obtain the first linear regression equation.
3. The method for detecting a thin film according to claim 1, wherein the acquiring a signal spectrum of the ultrasonic wave propagating in the thin film to be detected comprises:
providing a laser pulse detection signal to the first surface after a delay of a preset time period from the moment of providing the laser pulse excitation signal, wherein the laser pulse detection signal and the laser pulse excitation signal are emitted to the first surface at different angles;
Detecting a reflected signal of the laser pulse detection signal reflected from the first surface;
and determining the signal spectrum of the ultrasonic wave propagating in the film to be detected according to the signal spectrum of the reflected signal.
4. The method according to claim 1, wherein determining the attenuation coefficient of the signal amplitudes of the plurality of echoes generated by the ultrasonic wave with the arrival time of the echoes based on the signal spectrum comprises:
determining the signal amplitude of a primary echo generated by the ultrasonic wave, the echo arrival time of the primary echo, the signal amplitude of a secondary echo and the echo arrival time of the secondary echo according to the signal spectrum;
and determining the attenuation coefficient according to the signal amplitude of the primary echo, the echo arrival time of the primary echo, the signal amplitude of the secondary echo and the echo arrival time of the secondary echo.
5. The method of detecting a thin film according to claim 1, wherein after the step of acquiring a signal spectrum of the ultrasonic wave propagating in the thin film to be detected, the method further comprises:
determining at least two times of a surface wave propagating in a film body with a preset depth below the first surface in the ultrasonic wave according to the signal spectrum, wherein the times are the times when the wave crest and/or the wave trough of the surface wave reach the first surface; the preset depth is smaller than the thickness from the first surface to the second surface of the film to be detected;
According to at least two times of the surface waves, determining the surface sound velocity of the surface waves propagating in the film body with a preset depth below the first surface;
calculating a ratio of the surface sound velocity to the average sound velocity; wherein the ratio characterizes uniformity and/or consistency of the film to be tested;
the determining, according to at least two of the times of the surface waves, a surface acoustic velocity at which the surface waves propagate within the film body at a preset depth below the first surface, includes:
establishing a second linear regression equation taking the surface sound velocity as a dependent variable and at least two times of the surface wave as independent variables;
substituting at least two times of the surface waves of the film to be measured into the second linear regression equation, and calculating to obtain the surface sound velocity of the ultrasonic wave propagating in the surface layer of the film to be measured.
6. The method according to claim 5, wherein said establishing a second linear regression equation having said surface acoustic velocity as a dependent variable and at least two of said times of said surface wave as independent variables comprises:
establishing a second linear regression model taking the surface sound velocity as a dependent variable and at least two times of the surface wave as independent variables;
Providing laser pulse excitation signals to a first surface of a plurality of calibration films of different densities and/or thicknesses to excite ultrasound waves in the calibration films; acquiring a plurality of calibration signal spectrums of the ultrasonic wave transmitted in a plurality of the calibration films;
determining echo arrival times of echoes of the ultrasonic wave propagating in each of the calibration films and at least two of the times of the surface waves from a plurality of calibration signal spectra;
acquiring the thickness of a plurality of calibration films, and determining the sound velocity of the ultrasonic wave in each calibration film according to the thickness of each calibration film and the arrival time of the echo;
and determining a second regression coefficient of the second linear regression model according to the sound velocity corresponding to each calibration film and at least two times of the surface waves to obtain a second linear regression equation.
7. The method of claim 5, wherein the two times comprise a first arrival time of a first trough to the first surface and a second arrival time of a first peak to the first surface.
8. The method according to claim 1, wherein the thin film to be measured includes a carbon film.
9. The method of claim 1, wherein the laser pulse excitation signal comprises a femtosecond laser beam.
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