Flawless wafer measurement from Cognex

Citation

(2005), "Flawless wafer measurement from Cognex", Sensor Review, Vol. 25 No. 2. https://doi.org/10.1108/sr.2005.08725baf.001

Flawless wafer measurement from Cognex

Flawless wafer measurement from Cognex

Keywords: Machine vision, Inspection

Structural measurement of wafers by means of image processing guarantees customer satisfaction due to highly reliable algorithms.

Applied metrological control systems with exceptionally accurate technology have become necessary today due to the rapidly growing precision demands of the micro-electronic industry. Handling critical dimensions (CD) in the measurement of component structures has become the “daily bread” of MueTec, Automatisierte Mikroskopie und Messtechnik GmbH, located at both Munich and Alholming in Germany. The company develops, manufactures and supplies systems for the automatic measurement of line widths for the semiconductor industries. In addition, the company supplies research institutes working with micro-mechanically produced minute structures on a wide variety of substrata, and for the manufacturers of such products. The high-performance- measuring systems of MueTec impose a binding obligation upon the applied image processing systems to adhere to extreme requirements. After having previously used developments of their own, the enterprise is now equipping all its measuring facilities with vision technology from Cognex.

Conducted as a dimensional check under the powerful magnification of microscopes, the width measurement of the extremely small structures on wafers is a very demanding task. Many problems are encountered in image processing due to magnification and when operating in a range of a few nanometers or less, repeatability can become an issue.

The challenges experienced are in the surface and material qualities of the wafers, depth of focus from the centre of the image to its edge, lighting and contrast. The measuring facilities are, of course, designed and intended for clean room conditions. It is furthermore, a prime objective to ensure that in the automatic mode, the operational capacity/throughput of the customized measuring and inspection facilities are, as profitable as possible, because the semiconductor industry has to calculate costs and prices using a scathing pen.

MueTec must comply with the customer's requirements and this applies to all three product lines: mask measurement, measurement of wafers and micro-mechanical structures. Since the company was founded in 1991, and also later on during the close cooperation with Leica Microsystems Semiconductor (1995), the business has applied an internally developed image processing software that was created to meet the specific requirements of the company. However it soon became apparent, that in the extremely dynamic technological semiconductor market, this required an increased effort for further development of this image processing software. The image processing system was no longer able to meet the requirements, because it had increasingly become necessary to integrate other characteristics, such as highly sensitive methods of defect control. That is why the company decided to take a close look at the software packages offered on the market, and at the hardware of various suppliers of vision systems.

Trademark of reliability

Considering the 20 to 30 measuring facilities delivered by MueTec every year, the decision in favour of a vision- system was of strategic importance. The objective was not only to master very specific image processing functions with extreme safety, the integration of further features called for a supplier with a great deal of experience in the technological application of high-end image processing algorithms.

The algorithms are based on the Cognex PatMax^® technology software package for PC-based vision-systems. With this vision software, Cognex has set milestones in new process- technologies in industrial image processing. In the past, the image processing systems had been based on the so-called gray scale/half-tone correlation for the localisation of objects or edges. In this analytical method, the half-tones of the image pixels are compared with the reference object, and the position is then calculated from there. This method rapidly reaches the limits of its capacity, when utmost precision is demanded, and when influences such as rotation, scaling or varying light- and contrast-conditions are to be dealt with.

MueTec has been incorporating the PatMax technology from Cognex into its measuring and inspection systems since 2002. This also applies to the new measuring systems MueTec 3000 NT (see Plate 1), which are equipped with an automatic (wafer) sample feeder, and which are oriented on the best possible operational capacity. The hardware of the MVS 8000 high- performance framegrabber series of Cognex is used in the measuring unit's PC for real-time image processing. The comfortable user-interface of the MueTec vision tool based on PatMax is regarded as a great advantage with its customer-friendly design. The customer in the semiconductor industry thereby gets an individually configured, easy-to- handle user interface with simple and safe programming, which meets his specific needs and requirements: the adaptation to a new job can be tested and conducted very quickly in reliable small steps.

Plate 1 The respective recipe is determined in the new automatic measuring unit 3000 NT, based on the bar code read previously using the In-Sight ID Reader vision sensor

Reliable edge identification of the software may be seen in the application of the high-performance measuring unit 3000 NT, even in defocused mode. If the distance of focus adjustment of the microscope objective is changed, then other edges will come into focus, and the contrast issue will intensify the problem. In such problem cases, the algorithms will prove their effectiveness, and they will deliver reliable results. Finding structures on the substratum and the identification of their base orientation is one of the main tasks of PatMax in this application. This is done by exact measurement of two points on the X- and Y-axes and extremely precise calculation of the resulting angle. The data of the codes of every single wafer are read in an automatic process, thereby providing important parameters for further measurement and inspection. Here, another working partnership with Cognex has developed because this task is performed by the compact vision sensor In-Sight 1700 Wafer ID Reader, which is an independent standalone system unit with extraordinary reading capabilities and comes equipped with an integrated illumination.

Complex vision-abilities

Unlike the gray scale/half-tone correlation, the patented methods of PatMax use the basic geometric structures of objects in a three-stage procedure. First the most important individual features of the object image, such as edges, measurements, shapes, angles, curves and shades are identified separately. The spatial relations between these central features of the trained image are compared with the real-time image. Then the position of the object is specifically determined with precision by employing the analysis of the geometric information of the features and of their spatial relation as well. Features with low contrast, such as contours, can therefore be recognised at a considerably higher level of reliability, precision and speed.

On the basis of a partial contour, PatMax will for example immediately recognise where to find further features despite distortion, displacement or covering. It is no longer necessary to conduct a linear analysis of the complete plotted image. This simplifies the identification of features, and it makes the vision system fast, flexible and extremely stable. PatMax also permits access to very high resolutions all the way down to the sub-pixel region, and it provides for reliable angle determinations on objects with a precision of 0.02 degrees. The vision tool is thus invariant with regard to location, orientation and changes of scale of the object. The influences of changing conditions of light and contrast are eliminated by simultaneous examination of the contour and the structure of the object image.

A whole range of other vision software packages from Cognex are based on this vision technology, for example PatInspect™ and PatFlex™. New improvements of the algorithms for the localisation and orientation of patterns even permit the detection of hardly visible objects in granular or pale images. These and other enhancements are integrated in the new vision-tools of PatMax XLC (eXtremly Low Contrast) and PatMax SA (Scene Alignment). Completely new is the fact that PatMax is presently also available on the new high-performance vision sensors In- Sight 5100, 5400 and 3400. The geometry oriented mode of operation of PatMax is also quite an important aspect for simple programming of very demanding and sophisticated image processing tasks, and it minimises the manual interventions of the operational staff. All these aspects have contributed to the fact that PatMax has become the most frequently used software for wafer orientation in the production of semiconductors around the world.