Patent Application
20050280807
Priority is claimed to German patent application 10 2004 029 012.1, the entire disclosure of which is hereby incorporated by reference herein.
The invention concerns a method for inspecting a wafer, in particular for examining edge bead removal, an optical image of the region to be inspected being acquired. The invention further concerns a corresponding system having an optical detector for acquiring an optical image of the region to be inspected. Lastly, the invention concerns a computer program and a computer program product for implementing the inspection method.
In semiconductor production, wafers are coated with layers such as photoresist and often also anti-reflection layers. This is generally done by applying a predetermined amount of the substance to be applied onto the rotating wafer disk, on which the substance becomes uniformly distributed. With this method, slightly more substance (photoresist) becomes deposited in the edge region of the wafer than in the middle of the wafer. An “edge bead” is thereby formed. An edge bead of this kind can result, in later wafer processing steps, in detachment of portions of the edge bead and thus in contamination of production machinery, and in the creation of defects on the wafer.
To eliminate these effects, an edge bead removal (EBR) is performed. Edge bead removal can be accomplished in wet-chemical and/or optical fashion. For wet-chemical removal, a suitable solvent is sprayed onto the edge of the wafer; for optical edge bead removal, the edge is exposed in controlled fashion and the exposed region is subsequently removed in the development process.
Edge bead removal defects can result from inaccurate alignment of the corresponding bead removal apparatuses relative to the wafer. Further defect sources include inaccurate alignment of the illumination device relative to the wafer during exposure of the photoresist. Edge bead removal defects can cause the bead-removed wafer edge to be too narrow or too wide, or result in an eccentric profile of that edge. Insufficient edge bead removal can result in contamination during subsequent wafer processing; excessive edge bead removal, on the other hand, can cause an increase in wastage due to a decrease in the usable wafer area. In both cases, the productivity of the production process is reduced. It is therefore necessary to be able to draw conclusions as to the edge bead removal width. It is of interest to inspect this after each wafer production step, i.e. after each application of a photoresist layer with subsequent edge bead removal.
DE 102 32 781 A1 refers to a known device by means of which a wafer is illuminated in bright-field fashion and scanned with a camera. The images obtained are then examined via image processing in order to make the edge bead removal visible. It becomes apparent in this context that depending on the process step, the acquired images show a wide variety of edges, deriving from various process steps, on the wafer surface in the edge region of the wafer. The edges differ from one another in terms of color or grayscale, partially intersect and overlap one another, and in some cases also modify the profile of the color or grayscale value. It has therefore hitherto been considered difficult or even impossible to detect edge bead removal automatically using this kind of image processing.
DE 102 32 781 A1 therefore proposes a system comprising an incident illumination source and an imaging device for inspection of a wafer surface, the illumination device being rotated through a suitable angle out of the bright-field illumination setting, in such a way that an observation of the wafer surface in dark-field mode occurs. This allows particularly effective inspection of, for the most part, small structures that are characterized by a small elevation difference as compared with the background.
A disadvantage that has emerged in the context of this inspection method, however, is that here again a clearly delimited edge often is not visible, and edges from previous process steps greatly reduce the detectability of the edge bead removal.
It is therefore an object of the present invention to provide a method and a system for inspecting a wafer, in particular for examining edge bead removal, in which context an optical image of the region to be inspected is to be acquired by means of an optical detector, and the structures being examined are to become clearly evident.
According to the present invention, a first optical image is acquired prior to the application of a layer onto the wafer, and a second optical image after the at least partial removal of that layer; and that the imaged region of the wafer surface is inspected by comparing the first and the second image. The layer to be applied is usually, in practice, a photoresist layer (resulting from application of photoresist material) or an antireflection layer. Without limitation as to generality, the discussion below will refer predominantly to a photoresist layer on which the aforementioned edge bead removal is performed. The invention is also valid, however, for layers of other kinds that are at least partially removed, such as anti-reflection layers.
The comparison according to the present invention between the first and the second image makes it possible to eliminate features of previous processes. As a result, structures that derive from previous process steps can no longer have an interfering effect. The comparison of the two images is performed by suitable image processing, which works out the difference between the images. This can be done in simple fashion, for example, by creating a difference image.
Taking the example of the photoresist layer with subsequent edge bead removal, in the method according to the present invention the first optical image is acquired before application of the photoresist layer (i.e. before application of the resist droplet onto the rotating wafer). This image then shows the actual state of the wafer surface resulting from the previous process steps. Depending on the type of edge bead removal (wet-chemical and/or optical), the second optical image is acquired either immediately after edge bead removal or only after development of the photoresist layer. In the latter case, the second image shows the relief resulting from development of the photoresist, including edge bead removal. The structures present in the first image are also (at least partially) visible in the second image. A comparison of the two images thus allows these structures remaining behind from the previous process steps to be eliminated. In the simplest case, a difference image is created for this purpose, but weighted differentiation or other known image processing methods can also be performed in order to make the differences between the two images maximally recognizable.
It has been found that with the method according to the present invention, the edge bead removal, i.e. the distance from the wafer edge over which material has been removed, can be determined better than previously. From a comparison of the two images, the edge width and possibly further variables of interest, such as tolerance, eccentricity, etc., can be quickly determined by image processing. It is useful to store only the resulting image (for example, the difference image) or, in order to reduce data volume even further, only the specific resulting values. In problem cases that are difficult to evaluate, provision can also be made, for example, to store both images in order to enable later visual re-examination.
The method according to the present invention can operate with the known illumination modes, in which context the optical images can be acquired in bright-field mode, dark-field mode, or combined bright- and dark-field mode.
The region to be inspected can be scanned by an optical detector (linear or matrix detector). One-shots (imaging of the entire wafer in one image) are also useful. The optical resolution in this context must be adapted to the desired resolution of the regions to be detected (edge bead removal width).
In the case of edge bead removal inspection, a linear camera (e.g. CCD or CMOS), which acquires images of the edge region of the wafer that is rotating beneath the linear camera, proves advantageous. The image frequency of the linear camera and the rotation frequency of the wafer must be suitably coordinated with one another. With an arrangement of this kind, the edge bead removal width tolerance that must be complied with can be determined with sufficient resolution.
Various combinations of illumination and detection modes are suitable for the method according to the present invention. The region to be inspected (or even the entire wafer) can be illuminated monochromatically or polychromatically. The optical detector can constitute a monochrome or color camera. In this context, polychromatic illumination does not necessarily require a color camera; instead, a monochrome camera that is spectrally sensitive at least to a region of the polychromatic illumination can also be used. In general, it is possible to work with incident bright-field illumination.
Because the number of structures imaged with incident dark-field illumination is small as compared with the corresponding bright-field illumination, the use of such illumination must be carefully considered. In suitable cases, only the structures to be examined remain visible after evaluation of the images. A bright-field illumination could additionally be supplemented with a dark-field illumination in order to emphasize particular properties.
Components of an apparatus described in DE 102 32 781 A1 are usable, in principle, for the method according to the present invention. Reference is explicitly made to the aforesaid Unexamined Application regarding the properties and mode of operation of such an apparatus.
According to the present invention, a wafer inspection system for inspecting a wafer, in particular for examining edge bead removal, is equipped with an optical detector for acquiring an optical image of the region to be inspected, and with a data readout device for reading out the image data furnished by the optical detector, having a computer unit connected to the data readout device for comparing acquired images of the region to be inspected. It is sufficient and advantageous if the system comprises a single optical detector that acquires a first image prior to application of a layer onto the wafer, and a second image after at least partial removal of that layer. The computer unit compares the acquired images and thus makes possible inspection according to the present invention of the imaged region of the wafer surface.
The use of a single optical detector ensures that the first and the second image can be acquired without calibration problems. The system according to the present invention can usefully be integrated into the wafer manufacturing process, so that the processed wafer passes through the inspection system once prior to application of, for example, the photoresist layer, and then, for example, after development of the photoresist.
The computer unit advantageously undertakes not only comparison of the image data, but at the same time determination of measurement variables of interest, such as edge width, tolerance, etc.
A computer program having program code means is advantageously executable in the aforesaid computer unit of the inspection system in order to perform the inspection method according to the present invention. The computer program advantageously comprises for that purpose an image processing module that works out the difference between two optical images in a manner optimal for the present wafer inspection process. The computer program furthermore comprises means, such as a pattern recognition module, for extracting the data of interest from the resulting comparison or difference image. In the case of edge bead removal inspection, the data to be extracted include the width of the edge, the average deviation thereof (tolerance), the eccentricity of the bead-removed edge on the round wafer, etc. The data that are ascertained can be stored in suitable form; it may be useful, in the event that tolerances that are to be complied with are exceeded, if appropriate warning signals or notifications thereof are issued.
The computer program can be stored on suitable data media, such as EEPROMs or flash memories, but also CD-ROMs, diskettes, or hard drives. Downloading of the computer program via internal or publicly usable networks is also possible and known.
The invention will be explained below in more detail with reference to exemplifying embodiments depicted in the drawings.
System 1 depicted here makes possible incident illumination in both bright- and dark-field modes. For that purpose, incident illumination device 5 is directed onto the region to be inspected of wafer edge 23 of wafer 2. Light travels via a light source 7 and a light-guiding bundle 6 into illumination device 5, which is arranged at an inclination with respect to the surface of wafer 2. Imaging device 9 is arranged on a displaceable support element 8 by means of a support rail 15. The axes of imaging device 9 and illumination device 5 are drawn with dashed lines, and intersect at the surface of wafer edge 23.
If an inspection of the wafer edge in bright-field mode is to occur, illumination device 5 is then arranged with respect to imaging device 9 in such a way that the axes (drawn with dashed lines) of the two devices 9 and 5 lie, at the intersection of the two axes, in a common plane with the wafer normal line that is perpendicular to the surface of wafer 2.
On the other hand, a dark-field observation can also be performed with system 1 by rotating illumination device 5 out of the aforementioned plane through an angle, so that the axis of illumination device 5 no longer lies in the plane spanned by the aforesaid wafer normal line and the axis of imaging device 9.
Lastly, a combined bright- and dark-field observation is also possible with system 1 depicted here, for example by the fact that a bright-field observation is performed with illumination device 5, and one or more additional illumination devices are provided for additional dark-field observation.
In inspection system 1 depicted here, wafer 2 rests on a receiving device 3 that retains wafer 2 by vacuum suction. The necessary vacuum is conveyed to receiving device 3 via a vacuum line 4.
Some aspects of the system depicted here in
By appropriate selection of the angles of the axes of illumination device 5 and imaging device 9 with respect to the wafer normal line, and optionally by adjusting a dark-field angle, the user can adapt the bright-field and dark-field illumination to the property that is to be inspected, so that the structure to be examined can be optimally imaged.
It should be mentioned that other illumination and imaging axes can also be implemented with the system according to
It is useful to integrate system 1 depicted here into the production process of wafer 2. According to the present invention, wafer 2 passes through inspection system 1 twice for each processing cycle. The (possibly pre-processed) wafer is brought into inspection system 1 for the first time prior to the next processing step, and there an optical image (of the wafer edge, in this exemplifying embodiment) is acquired. The usual processing of the wafer then occurs, by application of resist to the wafer; curing of the resist; edge bead removal (EBR), usually by wet-chemical EBR and then, depending on the production process, by optical EBR (OEBR); then exposure of the photoresist in the stepper; and lastly development of the photoresist layer to form the desired relief on the wafer surface. Further steps (implantation, evaporative deposition of metal layers, etc.) can follow, in which context the wafer usually passes through the aforesaid processing steps several times. An examination of edge bead removal occurs in each cycle, according to the present invention, prior to application of the photoresist layer and after edge bead removal, i.e. advantageously after development of the photoresist layer.
The processed wafer is accordingly conveyed a second time, usefully after development of the photoresist layer, to inspection system 1 in order to acquire a second optical image of the wafer edge. If the wafer is recoated with resist immediately thereafter, this acquired image can serve once again as the first image in the next processing cycle.
The image data of the first and the second image are conveyed via data line 16 to data readout device 17 having computer unit 18. There a comparison is made of the first and the second image by image processing; in this exemplifying embodiment, the width of the edge bead removal at wafer edge 23 is to be represented as accurately as possible. The comparison according to the present invention of the images is accomplished most easily by differentiation, in which context a weighting of the image data of the first and second images may be advisable.
First optical image 25 (
The invention presented here makes possible rapid throughput of examined wafers within their production process with a minimal space requirement. Instead of imaging device 9 described above, X/Y scans or one-shots (images of the entire wafer) can also be used.
The best results for examination of edge bead removal have been obtained with an incident bright-field illumination, the use of color images being advantageous. Either a color camera is needed for this, or illumination occurs sequentially at different wavelengths.
Possible radiation sources are, among others, fiber illumination, LEDs, fluorescent lamps, halogen lamps, metal vapor lamps, flash lamps, or lasers. The spectrum of the radiation can be polychromatic or monochromatic. The spectral range can also lie, depending on the sensitivity of imaging device 9, in the visual, infrared, or even UV region. Photodiodes or linear or matrix cameras, which in turn can be configured as monochromatic or color cameras, e.g. in the form of CCD or CMOS cameras, are suitable as optical detector 9.
For optimum management of the data sets in data readout unit 17, it may be useful to store only the results of the inspection and to discard the actual image data. If no results are to be calculated with the available images, or if the tolerances to be complied with are exceeded, in individual cases the various optical images can be stored for subsequent visual examination.
Further exemplifying embodiments will be presented below; it should be noted in this context that the statements made above regarding the types of radiation sources, optical detectors, and illumination to be used are also valid for the exemplifying embodiments below. What will be addressed in particular below, therefore, are differences between the exemplifying embodiments below and the one discussed above.
Optical images 25 and 26 can be acquired, for example, in a system such as the one depicted in
First optical image 25 (
Second optical image 26 (
A comparison image 27 is depicted in
Further alternatives to the wafer inspection system depicted in
In contrast to the system of
In addition to bright-field illumination device 13, which can correspond to incident illumination device 5 depicted in
With wafer inspection system 1 depicted in
Dark-field illumination device 14 in
In system 1 according to
Lastly,
Regarding the advantages of combined bright- and dark-field observation, reference is made once again to the statements above. The particular concrete configuration that is selected depends principally on the structures being examined and the quality of the resulting images and the comparison image.
It should be noted once again that the individual components of the systems shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 029 012.1 | Jun 2004 | DE | national |
