CN103673904A - Laser-scanning thermal wave imaging film thickness measuring method - Google Patents
Laser-scanning thermal wave imaging film thickness measuring method Download PDFInfo
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- CN103673904A CN103673904A CN201310742687.1A CN201310742687A CN103673904A CN 103673904 A CN103673904 A CN 103673904A CN 201310742687 A CN201310742687 A CN 201310742687A CN 103673904 A CN103673904 A CN 103673904A
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Abstract
The invention relates to a laser-scanning thermal wave imaging film thickness measuring method. High-power linear laser beams are adopted to perform quick scanning on the surfaces of test pieces, different thermal excitation times are set for different positions on the surfaces of the test pieces in the scanning direction, a thermal imager is adopted to simultaneously record thermal wave signals at the different positions, a curve changing with the times of thermal wave signals of the test pieces can be obtained and then is fitted with a theoretical model so as to obtain the thickness of the test pieces. By adopting the technical scheme, high-frame-frequency acquisition is performed in a laser-scanning mode to achieve thickness measurement of a film layer.
Description
Technical Field
The invention relates to a film thickness measuring method based on a thermal wave imaging technology, in particular to a laser scanning thermal wave imaging technology, belonging to the technical field of infrared nondestructive testing.
Background
With the rapid development of science and technology, the application of coatings and films is more and more extensive, and the industrial world puts higher requirements on the measurement and quality control of film thickness, such as the requirement of on-line, dynamic, non-contact and real-time detection and the like. These require more advanced techniques and detection means. The existing methods for detecting the thickness of the film layer mainly comprise a probe method and an optical method, but the methods cannot completely meet the requirements of modern industry on film thickness measurement, for example, the probe method belongs to contact type detection and is not suitable for many application occasions, most of the optical methods require that a test piece is a transparent medium and cannot effectively detect some non-transparent test pieces such as a coating and a paint layer, and therefore some advanced detection technologies need to be adopted.
The thermal wave imaging technology is a nondestructive testing means developed recently, and the basic principle is that firstly, a thermal excitation source is adopted to heat the surface of a test piece, a generated thermal pulse is transmitted to the interior of the test piece, when a thermal wave encounters a defect in the test piece or the thermal impedance changes, a part of thermal energy is reflected back to the surface of the test piece, a certain temperature distribution is generated on the surface of the test piece, and the thermal wave changes along with time. The infrared thermal imager is used for continuously acquiring information of temperature change along with time from the surface of the test piece, and then the modern image information processing technology is used for acquiring, processing and analyzing thermal wave signals, so that the thickness of the test piece is measured. Compared with the traditional nondestructive detection means, such as ultrasonic wave, eddy current, X-ray and other technologies, the infrared thermal wave imaging technology has unique advantages, such as non-contact, large-area imaging, sensitivity to thermal properties and the like, so that the requirements of modern industry on the thickness of the detection film layer can be met.
For the detection of a thin film layer, particularly a high-thermal-conductivity material film layer, the thermal excitation time needs to be short because the thermal wave signal changes rapidly, otherwise, the thermal excitation is not finished when the echo of the thermal wave reaches the surface of the test piece, and the detection precision is affected. Detection of rapidly changing thermal wave signals requires two problems to be solved, high energy short pulse thermal excitation and high speed image acquisition. Aiming at the problem of high-energy short-pulse thermal excitation, the products in the foreign market all adopt a high-energy flash lamp as a pulse thermal excitation source. However, such high energy flash lamps have many limitations, such as limited total energy, poor repeatability, beam divergence, inability to operate at long distances, and the like. Aiming at the problem of high-speed image acquisition, only a thermal imager with a high frame frequency function is adopted at present. Such thermal imagers are expensive and the resolution of the output image decreases significantly as the frame rate increases. The above-mentioned drawbacks of the prior art limit the accuracy of film thickness measurement.
Disclosure of Invention
The invention aims to provide a film thickness measuring method based on a laser scanning thermal wave imaging technology, aiming at the defects of the existing film thickness measuring technology. The laser has stable output power and uniform energy distribution, so the laser can be well applied to the detection of the film thickness. The invention adopts a high-power continuous laser as a thermal excitation source, controls the shape of a light spot by a light beam shaping device, rapidly scans the surface of a test piece by a light beam deflection device, records the distribution of thermal wave signals on the surface of a film layer by a thermal infrared imager, and then fits with a theoretical model, thereby deducing the thickness of the film layer.
The laser scanning thermal wave imaging technology can effectively solve two problems of short pulse thermal excitation and high frame frequency acquisition. Firstly, when high-power laser continuously output is rapidly scanned on the surface of a film layer, the time of irradiation of the film layer by the laser can be regarded as a short pulse for any fixed point of the film layer, and because thermal excitation of the surface of a test piece does not occur at the same moment but has a continuously changing delay in the scanning direction during scanning, pixels along the scanning direction of the laser in an image collected by a thermal imager have different thermal wave signals, the signals are arranged according to spatial positions, namely a change curve of the thermal wave signal of the test piece along with time, namely the distribution of the thermal wave signals in space represents the change of the thermal wave signal of the test piece along with time, and the equivalent sampling period is the time of the laser beam scanning two pixels, so that the change of the sampling frequency can be achieved by changing the laser scanning speed. In practical detection, the laser scanning speed mainly depends on the thermal conductivity of the test piece and the thickness of the film layer.
According to the principle, the method comprises the following steps when the thickness of the film layer is measured:
a. selecting laser power and scanning speed according to the characteristics of the film layer of the tested piece;
b. rapidly scanning the surface of the film layer by adopting a linear laser beam, and simultaneously acquiring a thermal wave image of the film layer of the tested piece by adopting a thermal infrared imager;
c. selecting pixel values along the scanning direction to obtain a thermal wave signal time-varying curve of the film layer of the tested piece;
d. and fitting the time-varying curve with the temperature with a theoretical model corresponding to the film layer to obtain the film layer thickness of the tested piece.
Drawings
FIG. 1 is a block diagram of a laser scanning thermal wave imaging system for use in the method of the present invention;
FIG. 2 is a schematic diagram showing the variation of thermal wave signals with time at different points on the surface of a test piece;
FIG. 3 is a graph of thermal wave signal versus time for films of different thicknesses;
FIG. 4 shows the results of an experiment performed according to the method of the present invention;
fig. 5 is a schematic diagram of a laser two-dimensional scanning mode.
Detailed Description
In order that the features of the invention may be better understood, the invention will now be further described with reference to the following specific drawings and examples.
When linear laser scanning is adopted for thermal excitation, the temperature field in the test piece can be approximate to the solving problem of a two-dimensional heat conduction equation, and the thickness of the film layer can be derived through the change of the temperature field on the surface of the test piece. For an infinite homogeneous medium, the excited thermal wave field can be simplified to the following thermal conduction equation as a linear homogeneous laser beam parallel to the surface of the test piece is scanned:
wherein,
the temperature of the position where the distance from the laser scanning starting point in the material at the time t is x and the depth is z, z = 0 represents the surface of the test piece, q
Represents a laser-scanned thermally-activated source function, scanned along the x-positive coordinate at a velocity v, where q is a constant, is the amount of heat applied per unit area,
is the thermal conductivity. Density of
The product of the specific heat and the specific heat c is the bulk heat capacity of the dielectric material, and the thermal diffusion coefficient of the measured material is
In general, the thermal diffusivity may be considered constant for a particular test piece. The heat conduction equation can be solved by numerical values, and the result is relatively complicated and will not be described herein.
FIG. 1 is a schematic diagram of a system for measuring film thickness by laser scanning thermal wave imaging. The high-power continuous laser 27 scans the surface of the test piece 25 through the light beam deflection device 22, the laser beam 23 has a linear light spot 24, the thermal infrared imager 26 records the temperature change of the surface of the test piece, the data acquisition unit 28 acquires a thermal wave image of the thermal infrared imager 26, and a curve of the temperature field of the surface of the test piece 25 changing along with time is obtained through data processing. The beam shaping device 21 is used to adjust the shape of the spot 24 to accommodate measurements of different layers.
For thinner films, the change in the thermal wave signal occurs in a shorter time. The thermal image sequence of the surface of the test piece is recorded through the thermal infrared imager, the shortest acquisition time depends on the frame frequency of the thermal infrared imager, the frame frequency of the conventional thermal imager is 30-60Hz, the frame frequency period is 17-33 milliseconds, and the detection of the thickness of a thin film layer is difficult. Thermal imagers with high frame rate capability are commonly used for this purpose, but such thermal imagers are expensive and the resolution of the output image drops significantly as the frame rate increases.
When the laser beam is scanned, the thermal excitation time of the surface of the test piece is not simultaneous, fig. 2 shows a curve of the thermal wave signal along with the time in the laser scanning direction, the peak is at the position of the laser beam, and the shape of the curve 16 is related to the parameters of the film layer, the substrate material of the test piece 25, the scanning speed of the laser beam 23 and the like. For a uniform substrate, when the film thickness is different, the thermal wave signal generated by laser scanning has different time-varying curves, and fig. 3 shows the time-varying curve of the thermal wave signal of the film with different thickness obtained by numerical calculation according to the theoretical model, where the film thickness is from 10um to 100 um. It can be seen that the film layers with different thicknesses have different temperature curves, so that the thickness of the film layer can be known according to the change of the curves. The method of the invention is to fit the actually measured time-temperature variation curve of the test piece with a corresponding theoretical model to obtain the film thickness of each point of the tested piece.
As the laser beam 23 scans across the surface of the test piece 25, the thermal wave signal at each point along the scan direction is progressively reduced from the thermal excitation occurrence time, while the signal amplitude is progressively increased. If the thermal imager 26 is of the staring type, the integration of all pixels occurs at the same time, so that the signal at different points in the acquired image at the pixels will have an increasing trend in the scanning direction, with the maximum occurring at the position of the laser beam, which is the curve of the thermal wave signal of the test piece 25 over time. That is, the variation of the thermal wave signal in the time domain is represented as a variation of the spatial distribution, and the time of the projection of the laser beam 23 on the thermal imager 26 sweeping one pixel is the sampling period, so that the sampling frequency can be very high as the scanning speed is increased.
To further understand the principle of the laser scanning thermal wave imaging film thickness measurement technique, as a simple example, if the scanning speed of the laser beam projected onto the thermal imager chip is 500 pixels/second, this is the equivalent sampling frequency, i.e. the sampling time interval is 2 ms. If the thermal wave signal in the image substantially disappears after 200 pixels, the thermal wave curve has about 200 valid data points, and the thickness of the film layer can be well fitted.
Fig. 4 shows a measured result, where the test pieces are 6 films with different thicknesses, from 50 um to 300 um, and the films with different thicknesses have different thermal wave signal attenuation speeds from the image.
In summary, the film thickness measuring method adopted by the invention comprises the following steps:
a. selecting the output power and scanning speed of the laser 27 according to the characteristics of the film layer of the tested piece 25;
b. rapidly scanning the surface of the film layer by using a linear laser beam 24, and simultaneously acquiring a thermal wave image of the film layer of a tested piece 25 by using a thermal infrared imager 26;
c. selecting pixel values in the collected thermal wave image along the scanning direction to obtain a thermal wave signal time-varying curve of a film layer of a tested piece;
d. and fitting the time-varying curve with the temperature with a theoretical model corresponding to the film layer to obtain the film layer thickness of the tested piece.
If a progressive scanning thermal imager is adopted, the equivalent sampling frequency is determined by the speed difference between laser scanning and thermal imager scanning, wherein the laser scanning speed refers to the projection moving speed of a laser beam on a thermal imager chip.
The linear spot 24 is used in the above steps, and the method is simplest in terms of structure and scanning control. The system may also employ a spot 42 of a point shape as shown in fig. 5. The beam deflecting means 22 performs two-dimensional scanning, which has an advantage that the laser intensity distribution in the image is uniform, but the mechanical structure becomes complicated and bulky. Meanwhile, the problem of high-speed scanning of the laser beam is solved, for example, line scanning is performed by adopting a multi-reflecting-surface rotating mirror.
Some of the variables in the theoretical model are usually calibrated by measuring a standard test piece, which has the same characteristics as the piece to be tested, while the thickness of the film layer has been previously determined by other means.
For some transparent films, the laser 27 may be selected to have a different wavelength such that absorption of the laser energy occurs primarily at the surface of the film.
The foregoing description of the invention is illustrative rather than limiting and it is intended that the following claims be interpreted as covering all alterations, modifications and equivalents as fall within the true scope of the invention.
Claims (4)
1. A method for measuring the film thickness by laser scanning thermal wave imaging is characterized by comprising the following steps:
selecting the power of a laser (27) and the scanning speed of a beam deflection device (22) according to the characteristics of the film layer of the tested piece;
the laser beam (23) rapidly scans the surface of the tested piece (25) through the beam deflection device (22), and simultaneously, a thermal infrared imager (26) is adopted to collect a thermal wave image of a film layer of the tested piece (25);
selecting a plurality of pixel values in the thermal wave image along the scanning direction, and sequentially arranging the pixel values to obtain a thermal wave signal change curve of the film layer of the tested piece (25);
and fitting the thermal wave signal change curve with the theoretical model corresponding to the film layer to obtain the film layer thickness of the tested piece (25).
2. The film thickness measuring method of laser scanning thermal wave imaging according to claim 1, wherein the laser beam (23) has a linear uniform spot (24), and the scanning deflection device (22) performs one-dimensional scanning.
3. The film thickness measuring method of laser scanning thermal wave imaging according to claim 1, wherein the laser beam (23) has a point-shaped uniform spot (24), and the scanning deflection device (22) performs two-dimensional scanning.
4. The method for measuring the film thickness through laser scanning thermal wave imaging according to claim 1, wherein the parameters in the theoretical model are calibrated by a standard test piece with known thickness.
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Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN105571549A (en) * | 2015-12-10 | 2016-05-11 | 南京诺威尔光电系统有限公司 | Nondestructive test method of heat wave imaging of cylindrical surface coating |
| CN106077956A (en) * | 2016-06-28 | 2016-11-09 | 深圳英诺激光科技有限公司 | A kind of remove thin film or the laser processing of coating and equipment |
| CN106705911A (en) * | 2015-11-14 | 2017-05-24 | 南京诺威尔光电系统有限公司 | Thermal wave imaging film layer thickness detection system and method |
| CN108801194A (en) * | 2018-08-02 | 2018-11-13 | 广州市盛华实业有限公司 | A kind of non-contact powder wet film thickness gauge |
| CN110073171A (en) * | 2017-11-24 | 2019-07-30 | 韩国科学技术院 | Method for performing visual measurement on thickness distribution of paint film and apparatus therefor |
| CN112037126A (en) * | 2020-07-02 | 2020-12-04 | 电子科技大学 | Image synthesis method for detecting object surface crack defects based on laser scanning method |
| CN117288103A (en) * | 2023-09-20 | 2023-12-26 | 广州泽亨实业有限公司 | Method and system for measuring film thickness of coating |
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2013
- 2013-12-30 CN CN201310742687.1A patent/CN103673904A/en active Pending
Cited By (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106705911A (en) * | 2015-11-14 | 2017-05-24 | 南京诺威尔光电系统有限公司 | Thermal wave imaging film layer thickness detection system and method |
| CN105571549A (en) * | 2015-12-10 | 2016-05-11 | 南京诺威尔光电系统有限公司 | Nondestructive test method of heat wave imaging of cylindrical surface coating |
| CN106077956A (en) * | 2016-06-28 | 2016-11-09 | 深圳英诺激光科技有限公司 | A kind of remove thin film or the laser processing of coating and equipment |
| CN106077956B (en) * | 2016-06-28 | 2018-02-23 | 英诺激光科技股份有限公司 | A kind of laser processing and equipment for removing film or coating |
| CN110073171A (en) * | 2017-11-24 | 2019-07-30 | 韩国科学技术院 | Method for performing visual measurement on thickness distribution of paint film and apparatus therefor |
| CN108801194A (en) * | 2018-08-02 | 2018-11-13 | 广州市盛华实业有限公司 | A kind of non-contact powder wet film thickness gauge |
| CN112037126A (en) * | 2020-07-02 | 2020-12-04 | 电子科技大学 | Image synthesis method for detecting object surface crack defects based on laser scanning method |
| CN112037126B (en) * | 2020-07-02 | 2022-11-22 | 电子科技大学 | An Image Synthesis Method Based on Laser Scanning Method to Detect Surface Crack Defects |
| CN117288103A (en) * | 2023-09-20 | 2023-12-26 | 广州泽亨实业有限公司 | Method and system for measuring film thickness of coating |
| CN117288103B (en) * | 2023-09-20 | 2024-06-07 | 广州泽亨实业有限公司 | Method and system for measuring film thickness of coating |
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Address after: 210046 Nanjing economic and Technological Development Zone, Jiangsu, Chi Chi Road, Hing Chi 6-3 science and Technology Park, building A, floor 10 Applicant after: Nanjing Nuoweier Photoelectric System Co., Ltd. Address before: 501 room 8, No. 210038, Heng Fei Road, Nanjing economic and Technological Development Zone, Nanjing, Jiangsu Applicant before: Nanjing Nuoweier Photoelectric System Co., Ltd. |
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Application publication date: 20140326 |