WO2004057297A1

WO2004057297A1 - Method and apparatus for automatic optical inspection

Info

Publication number: WO2004057297A1
Application number: PCT/DK2003/000910
Authority: WO
Inventors: Preben Knipp HJØRNET
Original assignee: Lk A/S
Priority date: 2002-12-19
Filing date: 2003-12-19
Publication date: 2004-07-08
Also published as: AU2003287923A1

Abstract

The invention relates to a method and an apparatus for the detection of defects (13) in or on the surface of an object with a high degree of accuracy. This is obtained y an apparatus comprising image forming means (2, 3, 4, 6) for generating an image of the defect (13), first (1) and second (18) light sources and filter means (14), preferably of the Schlieren type. The defects (13) can be detected without moving the optical parts of the apparatus and the object relative to each other by using two light sources (1, 18) to generate an image on light sensitive elements (CCD array (7)).

Description

METHOD AND APPARATUS FOR AUTOMATIC OPTICAL

INSPECTION

The present invention relates to the field of optical methods for inspecting objects so as to detect for defects and to provide a basis for determining a defect category.

Systems are known which can automatically inspect objects for defects in mass production lines. Such objects may be optically transmissive, or partly transmissive, such as high quality lenses or transparent panels for display screens etc. It may also be optically reflecting objects where a smooth sur- face is crucial to the overall quality of the object. Compared to visual inspection by humans automatic systems can provide a fast, low cost, and yet reliable quality check for determining whether a produced object fulfils desired quality criteria or if it should be discarded due to defects.

In some cases it not only important to determine if an object should be dis- carded or accepted. It may also be crucial to determine which type of defects that are present in an object determined to be defect. The reason for this is that the detected type of defects may contain important information regarding the production line. An early detection of the defect type of a series of object can possibly lead to correction of various parameters of the production proc- ess before a large series of defect objects are produced. Therefore, a precise and reliable inspection system can be used not only as a quality control but also as an active part of a production control system in order to save production loss.

In order to be able to reliably distinguish between several defect categories a combination of at least two different optical inspection techniques is well known, such as a dark field and a light field. The optical techniques are selected so as to provide different optical signatures associated with different types of defects. By combining the different signatures it is possible to extract further information for categorising defects. US 6,201,600 describes two different embodiments each capable of applying three different optical techniques: a dark field, a light field, and a "fine pattern mask" field. The two embodiments are capable of inspecting optically transmissive objects. In one embodiment (fig. 10) each technique is applied in separate optical set-ups each having a light source, an optical mask, and a camera. The object is subsequently moved between the optical set-ups and an image associated with each optical technique is generated. In another embodiment (fig. 16) images with the three optical techniques are subsequently generated using one light source, and one camera. The different op- tical techniques are applied by mechanically bringing different optical masks into position between the light source and the object.

Both embodiments described in US 6,201 ,600 suffer from the disadvantage that physical movement of the set-up or the object is involved between subsequent generations of the separate images generated with each optical technique. Hereby, important precision is lost since the physical movement will result in imperfect alignment of the images. When combining the images for generating a total image of the defects the misalignment will result in a poor resolution. The methods described in US 6,201 ,600 therefore fail to detect small defects of the object under test.

It may be seen as an object of the present invention to provide a method for optically inspecting and detecting defects in an object. The method must be able to precisely detect even small defects of the object under test. At the same time the method must be fast and reliable so as to fulfil demands for application in mass production lines.

According to a first aspect of the present invention, the object is complied with by providing an apparatus for imaging signatures associated with a defect in an object, the apparatus comprising image forming means for generating an image of at least that part of the object having the defect, first and second light sources for illuminating at least that part of the object having the defect, and filter means positioned between the object and the image forming means, the filter means causing a first image of the defect to be generated on the image forming means when using light from the first light source, and causing a second image of the defect to be generated on the image forming means when using light from the second light source, wherein the first and second images provide different signatures associated with the defect.

The image forming means, the first and second light sources and the filter means may be fixed relative to each other during generation of the first and the second images. The object may have a fixed position during generation of the first and the second images. The filter means comprises a first and a second zone, the first and second zones having different optical properties. In one embodiment the first zone transmits light originating from the first and the second light source, and the second zone reflects a majority of light originating from the first light source, and transmits light originating from the sec- ond light source. In another embodiment the first zone transmits light originating from the first light source and reflects a majority of light originating from the second light source, and wherein the second zone reflects a majority of light originating from the first light source, and transmits light originating from the second light source. The filter means may be a schlieren type filter.

Schlieren is a well known imaging method which can be implemented as a transmission-mode arrangement or as a reflection-mode arrangement. In the following description only the optical components for obtaining transmission mode will be shown in detail but it is well known that a reflection-mode arrangement can be obtained by folding the transmission system about the op- tical plane.

A useful Schlieren system does not simply consist of the optics. It may also comprise digital frame grabbing, timing circuitry for controlling the state of the device in coincidence with the image acquisition, staging and navigation, and overall computer control of the data acquisition and processing. These well known components will only be mentioned sporadically in the following description since these are well known to the man skilled in the art.

The first and second light sources may be laser light sources. The laser light sources may comprise means for providing a collimated light field. The laser light sources may emit substantially monochromatic light around two centre wavelengths. The two centre wavelengths are preferably separated by at least 20 nm. The laser light sources may emit light within the wavelength range 480-930 nm.

The image forming means may comprise a two-dimensional array of light sensitive elements. The two-dimensional array of light sensitive elements may comprise a CCD-array or a CMOS-array. The two-dimensional array comprises at least 300000 light sensitive elements. The two-dimensional array may form part of a smart camera, wherein the two-dimensional array comprises at least 300000 light sensitive elements.

In an embodiment at least one of the first and second light sources illuminate the defect of the object at substantially normal incidence. At least one of the first and second light sources may illuminate the defect of the object at an incident angle smaller than 90 degrees. The apparatus may further comprise means for generating at least a first and a second collimated light beam, wherein the first light beam has a first angle of incidence with respect to the object, wherein the second light beam has a second angle of incidence with respect to the object, and wherein the first and second angles of incidence are different. A difference between the first and second angles of incidence may be at least 1 degree. The light beams may be generated using one or more optical diffractive elements and one or more primary light sources.

According to a second aspect of the present invention the object is complied with by providing a method for obtaining signatures associated with a defect in an object, the method comprising the steps of applying a first optical detection technique so as to obtain a first signature of the defect on an image forming means, a second optical detection technique so as to obtain a second and different signature of the defect on the image forming means, wherein the obtaining of the first and second signatures are provided by use of filter means positioned in a light path between the object and the image forming means. The obtaining of the first and second signatures may be provided by a fixedly positioned filter means.

A more detailed description of the invention and preferred embodiments is given below with reference to the accompanying figures, in which

figure 1 shows an optical inspection set-up with one light source and a dark field mask with a object under inspection,

figure 2 shows the set-up from figure 1 with a defect object,

figure 3 shows an optical inspection set-up with one light source and a light field mask with an object under inspection,

figure 4 shows the set-up from figure 3 with a defect object,

figure 5 shows an object under inspection in an optical inspection set-up with two light sources for applying a dark field and a light field mask,

figure 6 shows the set-up from figure 5 with a defect object.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

The method according to the present invention concerns generation of images of an object under test so that defects appear clearly in relation to an image of an object free from defects. Images of different types of defects are generated differently, thus making reliable inspection of objects possible as well as classification of defects. In principle, the method can be carried out by an operator analysing the image generated, or by a series of algorithms in a computer performing the control. Alternatively, a combination of operator control and automatic control is possible.

The embodiments described in the following are suited to inspect optically refractive objects, but the principles are valid also for optically reflecting objects as well. The types of defects that can be detected are such as scratches, holes or dents on the surface, or of inhomogeneities in the object such as air pockets or foreign bodies. The described embodiment is especially suited for inspection of display windows for cellular phones. The described embodiment allows localisation of defects with sizes of the order 50 μm.

The preferred embodiment involves applying light from two different light sources. The light from the two sources is transmitted through the object to be inspected. Via an optical system part of the refracted light generates a 2D (two-dimensional) image on a 2D optical detector or camera. The optical detector generates an electrical signal in response to the image and the electri- cal signal is connected to a computer. In the computer, the signal from the optical detector indicates a measurement of the spatial distribution of the light intensity that passes through to the detector. The computer processes this distribution information using a set of filtering algorithms ensuring a greater contrast between significant defects and insignificant background variations. This way, an "electronic image" is generated with clearly indicated defects and unambiguous signatures. This electronic image can be reproduced on a monitor if an operator is to carry out the inspection. If inspection is to be carried out automatically, the electronic image in the computer is processed using a set of analysis algorithms that automatically analyse the electronic im- age of defect signatures defined. If a special type of camera known as a smart camera is used as the optical detector the need for the computer to process the image data can be relaxed because a smart camera has its own processing power.

For the unambiguous signature of each defect category, it is essential that the image is generated using a single optical detector only. By applying different optical techniques and generating an image of the object with each of these techniques important signatures of defects in the object can be extracted. In order to be able to extract the information regarding different defect categories the images generated with each optical technique are mapped together so as to form a combined image. Ideally the spatial match between the different images should be perfect. In other words, for all images to be combined to one, the information in each pixel must correspond to exactly the same position of the object. Otherwise the information is blurred and signatures associated with very small defects will be lost. Therefore, the resolution of the detection method is determined by the mapping tolerance.

In practice this mapping tolerance is determined by a number of factors relating to the position of the object and all optical elements of the detection setup during generation of the different images. Perfect mapping, and therefore maximum spatial resolution, can only be obtained if all positions are fixed during the inspection period where the different images are obtained. It is essential that the object to be inspected is in the same position during the inspection period. It is also essential that all positions of mechanically move- able objects forming part of the optical path in the set-up are maintained during the inspection period. Elements forming part of the optical path include: image detector, light sources, mirrors, optical masks, optical filters and all other elements forming part of the system that interacts with the light in the path between the source and the detector. In special set-ups this could also include optical diffractive or holographic elements. When used for practical, industrial purposes, background variations will occur rather frequently over time, for example, in the form of small vibrations in the object subjected to inspection, or in the components forming the complete measurement set-up. In practise, these background variations will often cre- ate blurred signatures of defects, unless consecutive images are recorded so quickly that variations are prevented before they occur. In order to reduce disturbing vibrations generated by the inspection set-up itself the number of moving or rotating objects must be reduced, of course especially with respect to objects relating to the optical path.

If two or more separate image generations are to be used to make a total image, the object to be inspected or the components forming the optical system must not be moved in-between the two or more acquisitions. This is also crucial in order to be able to acquire more images sufficiently fast. In case of moving objects in the optical paths the mechanical speed of the necessary movements would limit the possible inspection speed. In addition, every moving object of the set-up would create vibrations that would result in blurred images.

The preferred method requires that a minimum of two optical phenomena are used to provide a separate signature of a given defect. In one or more of the optical phenomena, some of the defects occur with blank signatures, i.e. they do not stand out from the background. When a combination of the various optical phenomena is used, even a blank signature will contain information to help make the combined signature unambiguous and more reliable. This unambiguous signature can, for example, be generated using a dark field set- up as the first detection method, as shown in figure 1 , and a light field set-up as the second detection method, as shown in figure 3. First, each separate set-up is described with reference to figure 1-4. Hereafter, a preferred embodiment is described with reference to figures 5-6. In figure 1 an optically transmissive object 10 under test is shown in a dark field set-up. By means of a parabolic mirror 3, a light source 1 such as a laser with optical equipment 2 directs a collimated light field with homogenous intensity distribution towards an object 10 to be inspected. If the object 10 has no defects, the light field refracted through the object will be more or less deflected. The refracted light field is reflected via a parabolic mirror 4 towards the detector 7. The detector is provided with optical equipment 6 generating images. A schlieren aperture 5 is positioned in the focal point of the mirror between the optical detector equipment 6 and the parabolic mirror 4. If the object 10 has no defects, no light from the source will pass through to the detector 7 since the entire light field hits the non-refractive zone 8 of the schlieren aperture 5.

In figure 2 the same set-up as in figure 1 is shown. The difference is that the object 10 to be inspected in figure 2 has a defect 13. Some of the light from the source 1 that hits the defect 13 in the object 10 is deflected via the mirror 4 in such a way that it hits the refractive zone 9 of the schlieren aperture 5. This light passes through to the detector 7, thereby generating a signature of the defect 13.

In figure 3 an optically transmissive object 10 under test is shown in a light field set-up. By means of a parabolic mirror 3, a light source 1 such as a laser with optical equipment 2 directs a collimated light field with homogenous intensity distribution towards an object 10 to be inspected. If the object 10 has no defects, the light field refracted through the object will be more or less deflected. The refracted light field is reflected via a parabolic mirror 4 towards the detector 7. The detector is provided with optical equipment 6 generating images. An aperture 5 is placed in the focal point of the mirror between the optical detector equipment 6 and the parabolic mirror 4. If the object 10 has no defects, some of the light from the source will pass through to the detector 7. How much depends on the size of the zone 11. For an object free of de- fects 10, this will result in homogenous light intensity distribution on the detector 7.

In figure 4 the same set-up as in figure 3 is shown. The difference is that the object 10 to be inspected in figure 4 has a defect 13. Some of the light from the light source 1 that hits the defect 13 in the object 10 is deflected via the mirror 4 in such a way that it misses the refractive zone 11 of the aperture 5. This light does not pass through to the detector 7, thereby generating a signature of the defect 13.

Regarding the total signature of the relevant defect 13 shown in figures 2 and 4, the defect 13 appears in one of the detection methods as a bright spot against a dark background, while in the other detection method it appears as a dark spot against a bright background. Different types of defects appear with different total signatures.

Figures 1 and 3 show two different detection methods in separate set-ups, i.e. each with its own detector and the object to be inspected in two separate positions. For the reasons mentioned above it is inconvenient to use separate set-ups for the mapping of different signatures of the same defect.

Figure 5 shows an object 10 in a set-up capable of generating separate images dark field and a light field by the use of a first and a second light source 1 ,18 and one detector 7. By combining the aperture 5 in figure 1 having optical zones 8 and 9 with the aperture 5 in figure 3 having optical zones 11 and 12 into the aperture 14 in figure 5 having optical zones 15 and 16 in such a way that zone 16 becomes reflective of light from source 1 and refractive for light from source 18 while zone 15 becomes refractive for light from source 1 and reflective for light from source 18. In this way, a first image can be generated based on the method indicated in figures 1 and 2, followed by a second image based on the method indicated in figures 3 and 4, simply by ensuring that the light sources 1 ,18 are switched on alternately. Thus, the same detector is used to generate both images without moving the object to be in- spected. In addition, any kind of physical or mechanical movement of the equipment forming part of the test set-up is avoided. The optical element 17 is refractive for light from source 1 and it is reflective for light from source 18.

Figure 6 shows the same set-up as in figure 5. The difference is that the ob- ject 10 to be inspected in figure 6 has a defect 13. A first image is generated having light source 1 switched on and light source 18 switched off. A light ray that hits the defect 13 is deflected in such a way that it hits the refractive zone 15 for that source and thereby forming a bright signature of the defect 13 in the first image. A second image is generated having light source 18 switched on and light source 1 switched off. A light ray that hits the defect 13 is deflected in such a way that it hits the reflective zone 15 for that source and thereby forming a dark signature of the defect 13 in the second image.

Since there is no need for changing position of either object or test set-up the best possibility for aligning the separate images generated by each optical technique is provided. It is not necessary to physically restore the positions of objects and part of the set-up between generation of the separate images. Switching between the first light source 1 and the second light source 18 may be performed purely by electrical means and thus avoiding any mechanically moving parts that could generate disturbing vibrations. Therefore, it is possi- ble to obtain a high degree of precision in detecting and categorising defects in the object under test. In addition, since no mechanical changes are necessary the separate images can be generated quickly after each other. This is crucial for mass production lines and it also ensures that there is only a minimum of possibility that external factors could influence the relative posi- tion of the test object and the optical parts of the test set-up.

The sources 1 ,18 can, for example, be lasers emitting light of different wavelengths with a minimum difference of 20 nm.

The described set-ups are examples only of how a system can be implemented.

Claims

1. Apparatus for imaging signatures associated with a defect in or on an object, the apparatus comprising

- image forming means for generating an image of at least that part of the object having the defect,

- first and second light sources for illuminating at least that part of the object having the defect, and

- filter means positioned between the object and the image forming means, the filter means causing a first image of the defect to be gen- erated on the image forming means when using light from the first light source, and causing a second image of the defect to be generated on the image forming means when using light from the second light source, wherein the first and second images provide different signatures associated with the defect.

2. Apparatus according to claim 1 , wherein the image forming means, the first and second light sources and the filter means are fixed relative to each other during generation of the first and the second images.

3. Apparatus according to claim 1 , wherein the object has a fixed position during generation of the first and the second images.

4. Apparatus according to claim 1 , wherein the filter means comprises a first and a second zone, the first and second zones having different optical properties.

5. Apparatus according to claim 4, wherein the first zone transmits light originating from the first and the second light source, and wherein the second zone reflects a majority of light originating from the first light source, and transmits light originating from the second light source.

6. Apparatus according to claim 4, wherein the first zone transmits light originating from the first light source and reflects a majority of light originating from the second light source, and wherein the second zone reflects a majority of light originating from the first light source, and transmits light originating from the second light source.

7. Apparatus according to claim 1 , wherein the filter means is a schlieren type filter.

8. Apparatus according to any of the preceding claims, wherein the apparatus comprises optical means for producing transmission-mode images of the defect.

9. Apparatus according to any of claims 1 -7, wherein the apparatus comprises optical means for producing reflection-mode images of the defect.

10. Apparatus according to claim 1 , wherein the first and second light sources are laser light sources.

11. Apparatus according to claim 10, wherein the laser light sources comprise means for providing a collimated light field.

12. Apparatus according to any of claim 10, wherein the laser light sources emit substantially monochromatic light around two centre wavelengths.

13. Apparatus according to claim 12, wherein the two centre wavelengths are separated by at least 20 nm.

14. Apparatus according to any of claim 10, wherein the laser light sources emit light within the wavelength range 480-930 nm.

15. Apparatus according to claim 1 , wherein the image forming means comprises a two-dimensional array of light sensitive elements.

16. Apparatus according to claim 15, wherein the two-dimensional array of light sensitive elements comprises a CCD-array or a CMOS-array.

17. Apparatus according to claim 16, wherein the two-dimensional array comprises at least 300000 light sensitive elements.

18. Apparatus according to claim 15, wherein the two-dimensional array forms part of a smart camera, and wherein the two-dimensional array comprises at least 300000 light sensitive elements.

19. Apparatus according to claim 1 , wherein at least one of the first and second light sources illuminate the defect of the object at substantially normal incidence.

20. Apparatus according to claim 1 , wherein at least one of the first and second light sources illuminate the defect of the object at an incident angle smaller than 90 degrees.

21. Apparatus according to claim 1 , further comprising means for generating at least a first and a second collimated light beam, wherein the first light beam has a first angle of incidence with respect to the object, wherein the second light beam has a second angle of incidence with respect to the object, and wherein the first and second angles of incidence are different.

22. Apparatus according to claim 21 , wherein a difference between the first and second angles of incidence is at least 1 degree.

23. Apparatus according to claim 21 or 22, wherein the light beams are generated using one or more optical diffractive elements and one or more primary light sources.

24. Method for obtaining signatures associated with a defect in or on an ob- ject, the method comprising the steps of applying - a first optical detection technique so as to obtain a first signature of the defect on an image forming means,

- a second optical detection technique so as to obtain a second and different signature of the defect on the image forming means,

wherein the obtaining of the first and second signatures are provided by use of filter means positioned in a light path between the object and the image forming means.

25. Method according to claim 24, wherein the obtaining of the first and second signatures are provided by a fixedly positioned filter means.