Method for measurement of surface profile

FIELD: measurement of surface profiles.

SUBSTANCE: the method consists in obtaining of a set of interferograms of the surface under examination at scanning of it by a low-coherent radiation source and recreation of the original profile of the surface under examination with the aid of them. At obtaining of each main interferogram an additional interferogram is read off at a shift of the bearing surface by a fractional part of the wavelength, after that the signal of the additional interferogram is subtracted from the signal of the main interferogram and a differential interferogram is obtained, and the original profile of the surface under examination is recreated from the obtained differential interferograms.

EFFECT: enhanced quality of interferograms due to localization of the zone of interference on the surface under examination.

4 cl, 6 dwg

 

At the present time to detect surface defects, the most critical parts used contactless control methods, in particular optical profilometers, allowing to obtain information about the depth of defects micron level.

There is a method to determine the depth of the defects on the surface of the object, namely, that illuminate the object nemonokhromaticheskogo beam of light, see the system of interference fringes in the region of the defect, and then move the object along the axis of the illuminated light beam, measure the distance between the old and the new provisions of the adjacent interference fringes and the value of this distance determines the depth of the defect (see A.S. USSR №1442817, CL G 01 B 9/02, 11/30, 1988).

The main disadvantage of this method is its low productivity is related to the fact that to carry out the measurement of displacement of the interference bands and determining the magnitude of movement of the depth of the defect is only possible in manual mode under the microscope. This is actually the operator of a small field of view of the microscope must view the entire surface of the part and determine any defects.

Closest to the claimed technical solution is the method of control of surface defects based on low-coherence interferometry (see ANGEWANDTE OPTIK, ANNUAL REPORT 1992, p. 12, "Coherence radar as a accurate measuring tool on rough, opalescent surfaces").

The method includes obtaining a set of interferograms surface when scanning its low-coherence radiation source and restore them the original profile of the test surface. Unlike traditional laser interferometry, in this case, due to the short coherence length is used polychromatic light source, interoperation field, the depth of the object has dimensions of the order of several microns, which effectively discriminates information corresponding to different cross-sections of three-dimensional object, and thus to restore his profile.

The main disadvantage of this method is the difficulty allocation of zones of interference, because of the irregularity of the scattering properties of the surface of the object, because it is almost impossible to unambiguously interpret whether it is a zone of interference, whether uneven scattering of light.

In addition, the accuracy of the measurement depends on the unambiguous definition of the boundaries of the zones of interference.

The technical task of the present invention is to eliminate this drawback, namely improving the quality of interferograms by improving the accuracy of localization of interference zones on the test surface.

The specified task in the measurement of p is ofile surface, including obtaining a set of interferograms surface when scanning its low-coherence radiation source and restore them the original profile of the test surface, is achieved by obtaining each primary interferogram remove additional interferogram shear bearing surface on the fractional part of the wavelength, then the signal from the main interferogram subtract the signal of the interferogram to obtain the differential interferogram, obtained by differential interferograms to restore the original profile of the test surface.

The specified execution method by obtaining the differential interferogram to exclude those parts of the surface, where there are scattered radiation from the surface and there is no zone of interference, because the shift of the reference surface does not lead to changes in ambient light, but changes the phase of the interference pattern, and thus to changes in the differential interferogram in the zones of interference.

It is advisable to perform a get interferogram of the test surface, for example, by using a Michelson interferometer, mainly the shoulder which establish the bearing surface with the possibility of its longitudinal movement, at the same time as light up the El using partially coherent radiation. Partially coherent radiation allows to localize the zone of interference within the coherence length of the radiation.

Advantageous additional interferogram off shift support surface a quarter of a wavelength effective radiation illuminator. This is because when you shift a quarter wavelength phase of the interference pattern is shifted by a half wavelength, i.e. is out of phase with the main signal that gives the maximum amplitude of the differential signal, and respectively receive the maximum contrast when restoring the image.

Promising as the illuminator to use a white light source such as an incandescent lamp, which has a minimum coherence length, therefore, has a maximum localization of interference zones.

The inventive method allows to precisely localize the zone of interference, and thus to restore with high accuracy original profile of the test surface that has no analogues in optical microscopy, and therefore meets the criterion of “inventive step”.

Figure 1 shows a drawing of a device illustrating an implementation of the method.

Figure 2 shows an illustration explaining the principle of obtaining the differential interferogram shear abutment shoulder on the quarter of the length of the ox is s devices: (a) - the original signal, b) - the inverse signal is a differential signal. The intensity profiles corresponding to these signals.

Figure 3 shows a drawing explaining the principle of obtaining the differential interferogram when approaching the abutment shoulder on one-eighth wavelength of the device: (a) original signal, b) - the inverse signal is a differential signal. The intensity profiles corresponding to these signals.

Figure 4 shows an illustration explaining the principle of obtaining the differential interferogram when removing the supporting shoulder on one-eighth wavelength of the device: (a) original signal, b) - the inverse signal is a differential signal. The intensity profiles corresponding to these signals.

Figure 5 shows: (a) primary and b) - additional interferograms taken with a shift of a quarter wavelength, and C) differential interferogram, obtained by subtracting the additional interferogram from the principal.

Figure 6 shows: (a) the set of differential interferograms at different displacements of the object (step scan 5 μm), b) is the result of the recovery of the three-dimensional profile image and b) contour map of depth.

The device for implementing the inventive method includes: an illuminator 1, lens 2, the semi-transparent mirror 3, a lens 4, 5 and 6; the measured volume of the t 7; the reference object 8 and the CCD camera 9.

The device operates as follows.

The illuminator 1 through the lens 2 light semi-transparent mirror 3 parallel beam of light, half of the radiation which passes through the lens 4 and illuminates the object to be measured 7, and the other half of the radiation passes through the lens 5 and illuminates the reference object 8. Reflected from the measured object 7 of the light through the lens 4 falls on a semi-transparent mirror 3 and is partially reflected from it, through the lens 6 reaches the entrance of the CCD camera 9. There also receives light reflected from the reference object 8 through the lens 5, the semi-transparent mirror 3 and the lens 6. Thus receive the main signal of the interferogram. After that, the reference object 8 is shifted by the fractional part of the wavelength, for example a quarter, and the measurement is repeated. While the CCD camera 9 captures additional interferogram. Subtracting from the signal of the main interferogram signal additional interferogram receive a differential signal of the differential interferogram, which will allow you to isolate only those areas that have changed signal when the phase change of the wavelength, i.e. in those areas where there is a true interference pattern.

Example 1. To obtain the interferogram of the surface topography (pigv) used main (5A) and will complement the function (5B) interferogram, shot with a shift of a quarter wavelength, and the differential interferogram, obtained by subtracting the additional interferogram from the principal.

Example 2. To obtain the interferogram of the surface topography (figb) used a set of differential interferograms at different displacements of the object (step scan 5 μm) (figa).

Thus, the inventive method allows high accuracy to localize the zone of interference and restore with high accuracy original profile of the test surface.

1. A method of measuring the surface profile, comprising obtaining a set of interferograms surface when scanning its low-coherence radiation source and restore them the original profile of the test surface, wherein obtaining each primary interferogram remove additional interferogram shear bearing surface on the fractional part of the wavelength, then the signal from the main interferogram subtract the signal of the interferogram to obtain the differential interferogram, obtained by differential interferograms to restore the original profile of the test surface.

2. The method according to claim 1, characterized in that the receiving interferogram of the test surface is raissadat, for example, using a Michelson interferometer, mainly the shoulder which establish the bearing surface with the possibility of longitudinal movement, the illuminator uses partially coherent radiation.

3. The method according to claim 1, characterized in that the additional interferogram is removed by sliding the supporting surface a quarter of a wavelength effective radiation of the light source.

4. The method according to claim 2, characterized in that the illuminator uses a white light source, for example, the incandescent.



 

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