DE102014016710A1 - Method and device for performing an optical inspection of a sample surface - Google Patents

Abstract

Disclosed is a method for performing an optical inspection of a sample surface, wherein at least a portion of the sample surface is exposed to radiation at an angle of incidence and reflected or transmitted radiation is recorded, characterized in that the reflected or transmitted radiation at least partially recorded and the angle of incidence in the course the inspection is varied with the focal plane of the record being set outside the sample surface. Furthermore, an apparatus for carrying out the method is disclosed.

Description

The invention relates to a method for carrying out an optical inspection of a sample surface according to the preamble of patent claim 1, as well as a device according to the preamble of patent claim 5.

Optical, imaging measurement techniques of surface extended objects aim at a characterization of surfaces. In particular, the change in the local reflectivity and / or the height extent of the respective surface is utilized in order to carry out quality controls, for example. Typical examples are rough surfaces, printing plates, surfaces with traces of processing, but also surfaces with transparent coatings, in particular laminated printed products.

In addition to methods in which the surface directly -. B. on an electronic camera - is displayed, the result is then evaluated by means of digital image processing, so-called scattered light measurement methods have established. In this case, the scattering pattern or the far field distribution of the surface to be examined over the hemisphere or parts thereof is detected. This happens i. a. purely in terms of performance, either by means of goniometer arrangements or with imaging arrangements. The far field distribution is in principle linked via a Fourier transformation with the reflection function of the surface. A conclusion on the surface itself is not possible, since the scattering distribution is absorbed only in terms of power and therefore the phase information is lost, but a conclusion can be drawn on the autocorrelation function of the reflection function of the surface. Since the latter i. a. is complex in nature and leads to a substantial phase modulation with corresponding upper spectra, the uniqueness of square magnitude formation (power measurement) is the less given.

Such scatter images mainly contain information about preferred directions and periodicities of the surface being examined. Next they are used for so-called Appearance measurements, they continue z. B. used for surface modeling in virtualreality systems. In terms of quality control they are z. B. useful if derived from them parameters, such as the higher-order moments, as features for classification method, z. Based on a discriminant analysis.

Due to the properties of the Fourier transformation, information about locally high-frequency components of the surface function can be well taken from the scattering image, while the low-frequency components can rather be taken from the actual image of the surface. A transfer of the two into each other is not possible because of the missing phase information. The DE 10 2013 007 932.2 shows, for example, a scattered light measuring device, with both the microscopic image of the surface, and their scattering pattern can be recorded in order to gain information from both areas complementary can. It turns out, however, that there are other states between the original image and the scatter image that may be important for the investigation of certain surface properties.

There are also known imaging methods in which not the object itself, but its shadow is displayed to certain effects, such. B. to hide high self emission of the object and the visualization of streaks in the surrounding medium. The shadow can be caught on an intermediate screen, but can also be recorded directly, as an aerial image, by means of an imaging system.

In particular, in the area remote from the object, a geometric shadow is produced in parallel illumination in transmitted light. This area of the geometric shadow can not necessarily be delimited exactly, it is ultimately distinguished by the fact that the diffraction pattern of the edges of the object does not exceed the spatial extent of the object.

From the far field perspective, it is a field occupancy existing at a certain level, which is generated by the illuminated object plane and ultimately - in the sense of Huygen's principle - itself generates the far field distribution. Compared to the field assignment of the original diffractive aperture, it is characterized by additional phase terms, which depend on the distance from the original aperture; it results arithmetically by evaluating the diffraction integral for the near field with non-simplified Green's functions, i. H. without application of far field approximation. One could also call it the result of an incomplete Fourier transformation, if one considers the transition from the original aperture to the far field distribution as a Fourier transformation.

Due to the additional phase terms and the still strongly distance-dependent superimposition of the Green's functions or elementary waves of neighboring object points, phase or transit time differences, as they arise due to density and / or refractive index change of the surrounding medium, are particularly easily visible in the shadow.

However, this measurement method is limited to transmitted light examinations. investigations Surfaces of opaque objects are therefore not possible. In particular, surfaces with transparent layers, such as those caused by lacquers and laminated layers, but also transparent impurities on surfaces such. B. oils and fats u. Like., And printed circuit boards or printed circuits can thus not be analyzed in the manner described.

A known device 1 for optical imaging measurement in an xz plane is, for example, in 1 outlined. This is an optical recording device 2 , For example, a camera, at a receiving angle β on a flat surface extended sample surface 4 directed. The sample surface 4 has, for example, bumps or flaws 6 indicating a change in height or a change in the reflectivity of the sample surface 4 among other things. With the help of the camera 2 can thus the sample surface 4 be characterized. The sample surface 4 may be, for example, a printing plate, a surface with processing marks, but also a surface with transparent coatings, such as a laminated surface.

The object of the invention is to provide a method for the large-area inspection of surfaces, in particular for the visualization of deviations from target surfaces, which eliminates the aforementioned disadvantages. It is another object of the invention to provide a device for carrying out the method.

This object is achieved by a method having the features of patent claim 1, and by a device having the features of patent claim 5.

In a method for performing an optical inspection of a sample surface, at least a portion of the sample surface is exposed to radiation at an angle of incidence. In this case, a reflected or transmitted radiation is recorded. According to the invention, the reflected or transmitted radiation is at least partially recorded and the angle of incidence varies during the course of the inspection. The focal plane of the recording is outside the sample surface 4 set.

The method can be operated both in transmitted light and in reflected light mode. In particular, the reflected radiation is examined here. Here, a portion of the reflected radiation is observed which is spaced from the sample surface but not yet in the far-field region. Optimally, the area to be examined, in the form of a focal plane, should have the lowest possible depth of field. By changing the angle of incidence, the structure of the sample surface is irradiated from different angles. For example, deviations from the regular sample surface in the form of scratches or dirt also create shadows. At different angles of incidence, the shadows of deviations from the target sample surface also change their shape. Based on this law, for example, the sample surface can be inspected for defects. The angle of incidence can be varied for example by a movable radiation source. Another way to change the angle of incidence is given by multiple radiation sources, which can be controlled sequentially.

According to one embodiment of the method, the sample surface is exposed to convergent or divergent or parallel radiation. As a result, different properties of the sample surface can be made visible, since the angles of incidence also change depending on the type of radiation.

In a further embodiment of the method, the reflected or transmitted radiation is recorded at a shallow depth of field. Depending on the distance of the focal plane from the sample surface, different information of the sample surface can be measured. With a high depth of field, many different pieces of information are recorded simultaneously, so that only the lowest possible depth of field allows extracting the desired examination areas.

According to another embodiment of the method, the sample surface is not recorded. In particular, this results in the recording of such information that causes the incident light shadow. Thus, the errors and / or irregularities on the sample surface in the recorded data come to the fore.

According to a further embodiment, the reflected or transmitted radiation is recorded with a variable focal plane. By varying the focal plane of the recording device, different regions of the sample surface can be analyzed.

An apparatus for carrying out the method for carrying out a planar optical inspection of a sample surface has at least one radiation source for irradiating a sample at an angle and a recording device for receiving a reflected or transmitted radiation. According to the invention, the reflected or transmitted radiation is at least can be recorded area by area and the angle can be varied during the inspection. In this case, according to the invention, a receiving device has a focal plane which lies outside the sample surface. Here, in particular, the reflected or transmitted shadow is considered, which may be located in the area between the sample surface and the far field. Therefore, the focal plane of the recording device is placed on this area in order to obtain the information necessary for the incident or transmitted light shadow.

According to one embodiment of the device, the angle of incidence is varied over at least one locally movable radiation source. In order to visualize possible contamination or defects in the sample surface, the angle of incidence can be varied. The defects of the sample surface cause a shadow that changes shape at different angles of incidence. Preferably, the angle of incidence is varied by means of a mobile radiation source which is movable along a straight path.

In a further embodiment of the device, the angle of incidence is varied over a plurality of locally offset radiation sources. A variation of the angle of incidence is also possible by different, spatially offset radiation sources are controlled sequentially. This reduces the number of moving parts.

According to a further embodiment of the device, the receiving device has a variable focal plane. This allows information from certain areas of the sample surface to be extracted and viewed separately, especially at a shallow depth of field.

In a further embodiment of the device, the recording device forwards the recorded data to an evaluation device. This allows the recorded data to be analyzed to analyze the surface and thus draw conclusions about, for example, the quality of a printed circuit.

In the following, preferred embodiments of the invention are explained in more detail with reference to greatly simplified schematic illustrations. Show it:

1 a schematic representation of a known device or a known method,

2 a schematic representation of the device according to the invention according to a first embodiment,

3 a schematic representation of the device according to the invention according to a second embodiment,

4 a schematic representation of the device according to the invention according to a third embodiment,

5 a detailed view of the device according to the invention according to the third embodiment, and

6 and 7 Detailed views of the device according to the invention according to a fourth and fifth embodiment.

In 2 is a schematic representation of a device according to the invention 8th for performing an optical inspection of a sample surface 4 according to a first embodiment. This is the section to be examined 10 the sample surface 4 preferably with a parallel, but at least with a homocentric beam 12 illuminated at a defined angle of incidence α. The beam 12 is from a radiation source 14 generated, which is a monochromatic laser according to this embodiment. In addition, the beam 12 before hitting the sample surface 4 be worked up. In this embodiment, the beam becomes 12 through a collimator lens 16 directed. A resulting near field distribution or an incident light shadow 18 arises at the angle of the direct reflection direction -α. In the simplest case, the incident light shadow 18 at a distance 19 from the sample surface 4 by means of an intermediate screen, the near field or shadow image 18 be recorded, which then by means of a recording device 2 is shown.

The cradle 2 consists of an optical system 20 For example, a 4f system and a CCD sensor 22 a camera. Advantageous is the use of the 4f system 20 because of its telecentricity. It allows the distance 19 of the incident light shadow 18 to vary without losing the magnification of the incident light shade 18 to change as such. This allows different information about the sample surface 4 be obtained, while maintaining a constant magnification.

Furthermore, it is possible with appropriate dimensioning, the near field directly on a camera sensor 22 catch. Basically, the incident light shade 18 also as a defocused image of the sample surface 4 be understood.

In 3 is a schematic representation of a device according to the invention 8th to Performing an optical inspection of a sample surface 4 according to a second embodiment.

In particular, the second embodiment differs from the first embodiment in the processing of the reflected radiation. Here, the image of the incident shadow takes place 18 via a single lens system 24 , The magnification remains with a change in the distance 19 but then not constant. This device 8th is particularly easy to realize technically.

4 shows a schematic representation of a device according to the invention 8th for performing an optical inspection of a sample surface 4 according to a third embodiment.

In particular, the third embodiment differs from the previously presented by a variation of the angle of incidence α by a small amount γ. This creates a shift of the shadow image 18 and thus a local scan along the height extension γ of the sample surface 4 , The displacement can be either by means of several spatially offset radiation sources 14 by being switched through, for example, sequentially or by a mobile radiation source 14 be generated. The variation of the angle of incidence α results in a shift of the incident-light shadow 18 on the xy plane, by the cradle 2 can be recorded.

5 clarified in a detail view of the sample surface 4 the change of the angle of incidence α of the device 8th according to the third embodiment. Because of the radiation source 14 is offset locally, the angle of incidence changes by the angular amount γ. Thus, the angle -α, -α-γ of the reflected radiation changes 26 , As a result of this effect, the incident light shade changes 18 its shape and / or its position with a change in the angle of incidence α. Because the incident light shadow 18 For example, unevenness or faulty surface areas 6 caused, these can 6 be detected by a variation of the angle of incidence α particularly simple.

In the 6 and 7 are detail views of sample surfaces 6 shown with a converging radiation 12 ( 6 ) or with a divergent radiation 12 ( 7 ) are acted upon according to a fourth and fifth embodiment of the device 8th for performing an optical inspection of a sample surface 4 , In particular, by the deviation of the incident radiation 12 from the parallel shape, a change in the scale of the incident shadow 18 opposite the original sample surface 4 about the change of a divergence angle μ of the radiation source 14 be caused. As a result, for example, the scale change in, for example, single-lens imaging systems in a change in the distance from the sample surface 4 be compensated.

In the embodiments described here, it is assumed that a focal plane of the recording device 2 in the area of the light shadow 18 lies.

Disclosed is a method for performing an optical inspection of a sample surface, wherein at least a portion of the sample surface is exposed to radiation at an angle of incidence and reflected or transmitted radiation is recorded, characterized in that the reflected or transmitted radiation at least partially recorded and the angle of incidence in the course the inspection is varied with the focal plane of the record being set outside the sample surface. Furthermore, an apparatus for carrying out the method is disclosed.

LIST OF REFERENCE NUMBERS

1

Device according to the prior art

2

cradle

4

sample surface

6

Defects / irregularities

8th

Device according to the invention

10

Section of the sample surface

12

Radiation beam / radiation

14

radiation source

16

collimator lens

18

Auflichtschatten

19

Distance of the reflected light shadow

20

optical system

22

CCD sensor / camera

24

Single lens system

26

reflected radiation

α

angle of incidence

-α

angle of reflection

β

acceptance angle

γ

Changing an angle

μ

Divergence angle of the radiation source

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102013007932 [0005]

Claims (10)

Method for performing an optical inspection of a sample surface ( 4 ), whereby at least one section ( 10 ) of the sample surface ( 4 ) with radiation ( 12 ) is acted upon at an angle of incidence (α) and reflected or transmitted radiation ( 26 ), characterized in that the reflected or transmitted radiation ( 26 ) is at least partially recorded and the angle of incidence (α) is varied in the course of the inspection, wherein a focal plane of the recording outside the sample surface ( 4 ) is set. The method of claim 1, wherein the sample surface ( 4 ) with convergent or divergent or parallel radiation ( 12 ) is applied. Method according to claim 1 or 2, wherein the reflected or transmitted radiation ( 26 ) is recorded with a shallow depth of field. Method according to one of claims 1, 2 or 3, wherein the sample surface ( 4 ) is not recorded. Method according to one of claims 1 to 4, wherein the reflected or transmitted radiation ( 26 ) is recorded with variable focal plane. contraption 8th for carrying out the method for carrying out an optical inspection of a sample surface ( 4 ) according to claims 1 to 5, with at least one radiation source ( 14 ) for irradiating a sample surface ( 4 ) at an angle of incidence (α) and with a receiving device ( 2 ) for receiving a reflected or transmitted radiation ( 26 ), wherein the reflected or transmitted radiation ( 26 ) can be recorded at least in regions and the angle of incidence (α) can be varied during the inspection, wherein the receiving device ( 2 ) a focal plane outside the sample surfaces ( 4 ) having. Apparatus according to claim 6, wherein the angle of incidence (α) via at least one locally movable radiation source ( 14 ) is varied. Apparatus according to claim 6 or 7, wherein the angle of incidence (α) over a plurality of spatially offset radiation sources 14 is varied. Device according to one of claims 6 to 8, wherein the receiving device ( 2 ) has a variable focal plane. Device according to one of claims 6 to 9, wherein the receiving device ( 2 ) passes the recorded data to an evaluation device.

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