Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
1999-02-02
2003-04-15
Mehta, Bhavesh (Department: 2725)
Image analysis
Applications
Manufacturing or product inspection
C250S491100, C250S559300, C356S399000, C356S501000, C382S144000, C702S127000, C702S151000, C716S030000
Reexamination Certificate
active
06549648
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of German Application No. 198 25 829.1, filed Jun. 10, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for determining the position of a structural element on a substrate, wherein the substrate is mounted on a measuring stage. The measuring stage is displaceable in an interferometrically measurable fashion in a measuring plane relative to a reference point. The structural element is imaged on a detector array by an imaging system that has its optical axis perpendicular to the measuring plane. The pixels of the detector array are arranged in rows and columns parallel to the axes of an X/Y coordinate system associated with the substrate. The position of the structural element is defined by the distance of one edge of the structural element relative to the reference point, and the position of the edge is determined on the detector array by evaluating an intensity profile of the edge image that is perpendicular to the edge direction and is derived from pixels located in a defined measuring window of the detector array.
A metrology system suitable for performing the method, such as Leica's commercially available LMS IPRO® measurement system is described, for example, in the text of the paper “Pattern Placement Metrology for Mask Making,” by Dr. Carola Bläsing, delivered in the Education Program of Semicon Genf on Mar. 31, 1998, the disclosure of which is expressly incorporated by reference herein.
The structural elements (structures) to be measured consist in particular of opaque or transparent areas (such as patterns) on mask surfaces or structures on wafers or reticles used in semiconductor manufacturing. The positions of the edges of the structural elements are measured in a coordinate system defined on the mask (mask coordinate system). The mask is mounted in the measuring machine on a measuring stage (such as an X/Y stage) that is displaceable in a measuring plane. The measuring stage is displaceable relative to a reference point in an interferometrically measurable fashion, with the position of the mask coordinate system being aligned using alignment marks relative to a machine coordinate system of the measuring machine. Usually, the contact point of the optical axis of the imaging system on the mask is used as the reference point.
A structural range to be measured, following a suitable displacement of the measuring stage, is imaged (enlarged by the imaging system) on a detector array of a CCD camera. The pixels of the CCD camera are arranged in rows and columns parallel to the aligned axes of the mask coordinate system. Conventionally, the edges of the structural elements to be measured are likewise parallel or perpendicular to the axes of the mask coordinate system and hence also to the rows and columns of the detector array. The edge position that results from the evaluation of the image of the structural element taken by the detector array is provided by the interpolated pixel rows or pixel columns on which the edge lies relative to the reference point. The detector array is generally aligned such that the center of its camera screen image lies on the optical axis of the imaging system, so that this center (screen origin) is used as the reference point.
The image of the structural element is evaluated using image analysis process methods. A specific array range is selected for the measurement with the aid of a rectangular measurement window (sometimes referred to as a measurement field) defined and generated by software. The measurement window is placed on a portion of the image of the structural element to be measured. As a result of the resolution and imaging quality of the imaging system, the degree of contrast of the edge image varies. The best contrast is set with the aid of a TV autofocusing system. An average value is formed from the intensities of the pixels that lie in a row or column parallel to the edge of the structural element within a measuring window. Perpendicular to the edge, this produces an intensity profile of the edge image over one pixel row or pixel column. The position of the edge is defined by the 50% level value of this intensity profile.
The structural elements to be measured have different widths and lengths. In order to specify the position of a structural element on the mask (such as the pattern placement), the edge lengths that are parallel to one another are frequently measured and the center line (or midpoint) between the two edges is given as the position. In a structural element that can be measured by the width and length within the measuring window formed by the detector array or in two intersecting structural elements detected using two measuring windows, the position of the structure is defined by the coordinates of the intersection of the two center lines through the windows.
To an increasing degree, structural elements used in designing semiconductor circuits no longer extend parallel or perpendicular to the mask coordinate system. The image that results from the detector array representing these non-orthogonal structural elements is therefore likewise no longer parallel to or perpendicular to the pixel rows and pixel columns of the detector array.
There is therefore needed a system and method for measuring structural elements having angles other than parallel or perpendicular to the coordinate system (so-called “non-orthogonal” elements).
By rotating the CCD camera or the measuring stage, these structural elements can again be aligned orthogonally to the pixel rows and columns of the detector array. The rotational angle can be measured and hence the position of the edge or the position of the structural element can be calculated back to the non-rotated coordinate system so that the measuring machine for orthogonal and non-orthogonal structural elements can perform measurements directly using the same evaluation method and can output the measurement results in a comparable form.
One disadvantage of this method is the high mechanical cost required for precise mounting when rotating the CCD camera or the measuring stage. In addition, the rotation itself and the alignment with the edge requires additional time expenditures which lengthens the time required for measurements. With the ever increasing structural density and number of structural elements to be checked by measurement, however, limiting the amount of processing time has steadily grown in importance.
Hence, the goal of the invention is to provide a method and measuring machine that can be used on structural elements aligned in any direction while not requiring mechanical changes in the measurement process within a measuring field.
This goal is achieved according to the present invention by providing a method in which a rectangular measuring window is produced and aligned with a boundary line parallel to the edge direction, the direction of the boundary line of the measuring window that is perpendicular to the edge direction is determined in the coordinate system of the detector array by its rotational angle &thgr;, and in the case of the boundary lines of the measuring window that are not orthogonal to the rows or columns of the detector array, a virtual array is formed whose fields lie in rows and columns parallel to the boundary lines of the measuring window. Intensity values are assigned to the fields that are determined by a weighted evaluation of the intensities of the pixels in the detector array, each of the pixels being covered by a field. The position P
IPC
of the edge is determined from its distance to the reference point, using the intensity values associated with the fields, and the position P of the edge in the coordinate system of the substrate is given as a function of the rotational angle &thgr;, the position P
IPC
, and the x,y position of the reference point in the coordinate system of the substrate by the equation: P=P
IPC
+L, wherein L=x·cos &thgr;+y·sin &thgr;.
For automatic alignment o
Desire Gregory
Leica Microsystems Wetzlar GmbH
Mehta Bhavesh
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