German Patent DE 198 17 714 discloses a method for measuring the position of structures on a mask surface, wherein the mask is positioned in an image-evaluating coordinate measuring device on a measurement table displaceable in a direction perpendicular to the optical axis of an imaging measurement system in an interferometrically measurable way. A mask coordinate system associated with the mask is aligned relative to a measuring device coordinate system with the help of alignment marks. The desired position of the structures is given in the mask coordinate system. A coordinate position of two outer edges of the mask, which are perpendicular to each other, is measured in the mask coordinate system. For this purpose, the distances from two structures to an outer edge and from one structure to the other outer edge are determined.
German published patent application DE 198 25 829 discloses a method for determining the position P of a structural element non-orthogonal to the coordinate axes (x, y) of a substrate. The structural element is imaged onto a detector array of a CCD camera including a reference point. With the help of a measurement window rotated at an angle Θ with respect to the coordinate system, the position of an edge of the structural element relative to the reference point is determined. The position L of the reference point relative to the origin of the coordinate system is determined based on the angle Θ and the current measurement table coordinates, so that the position P is obtained. Determination of centrality is not included.
A measuring device for performing such a method is described with its basic elements in the lecture script “Pattern Placement Metrology for Mask Making,” Dr. C. Bläsing, Semicon Geneva, Education Program, issued on Mar. 31, 1998. The measuring device particularly serves for controlling the quality of masks for the semiconductor production. The quality of the mask is gaining more and more importance in type production. The specifications for the positions of the structures (pattern) from one mask to the other are becoming tighter and tighter. The measuring device described in the lecture script may measure the position of the structures relative to defined alignment marks, which define the mask coordinate system, with an accuracy of typically less than 10 nm. With the help of these alignment marks, the masks may be aligned in the stepper for the projection on wafer surfaces. Errors made in this process are directly added to the error budget of the lithography process. The mask is aligned in the stepper such that the respective alignment marks are positioned exactly one on top of the other during exposure. However, the steppers only have a certain area in which the mask may be shifted and/or rotated for physical alignment. The centrality of the masks is defined by the position of the pattern on the mask and thus determines the central position of the pattern (for the exposure of the wafer) within the mask.
German published patent application DE 10 2004 055 250 A1 discloses the inspection of a wafer, wherein a wafer-to-wafer comparison is restricted to predetermined compared regions selectable by the user. Only areas of a wafer are compared. This does not include the determination of coordinates and/or relative positions within predetermined coordinate systems.
European Patent EP 0 105 611 B1 discloses a device for inspecting structures applied to a mask for the semiconductor production. The patent is directed at the measurement of the structures on the mask rather than at the actual alignment.
U.S. Pat. No. 4,586,822 discloses a method for inspecting a mask for the production of a semiconductor device. The alignment marks on the mask are used to align the mask and the wafer such that structures may be exposed in an exact position with respect to each other. The result of the exposure on the wafer is also inspected to find defects and/or foreign particles on the mask. This method is not beneficial to achieving a high throughput in mask measuring.
U.S. Pat. No. 4,388,386 discloses a method with which insufficient matching of a mask set may be reduced to a minimum. Each mask of the set is compared to a known standard. Again, determination of centrality is not included. The position of structures with respect to the edges of the mask is not determined.
As specifications for all components become tighter and tighter, the position of the structures relative to the outer edge of the mask has turned into an important quality criterion of the mask. The exact rectangularity of the mask is no longer a negligible quality criterion either. Previously, the outer edges of the masks were assumed to meet at a right angle. As the accuracy of the coordinate measuring machine increases, the exact rectangularity and/or deviations from rectangularity where the outer edges meet also have come to play an important role.
The rectangularity of the outer edges and/or the knowledge about deviations of the outer edges from rectangularity play an important role because, in the lithography device (such as e-beam or laser lithography), the mask is normally contacted in three places to obtain a reproduced position. The three points define the two outer edges of the mask. The outer edges of the masks are assumed to be at a right angle with respect to each other. If this is not the case or if the non-rectangularity of the outer edges influences the result of the lithography, it is necessary to know about the rectangularity or non-rectangularity of the outer edges. Only in this way these edges may be used as reference for the pattern generated on the mask by the structures.
However, each new chip generation has increased requirements regarding accuracy and measurement throughput. The accuracy achievable by manual measurement using a conventional microscope is no longer sufficient. In addition, the manual measurement requires a lot of time for the alignment in the measuring device, the locating of the structures and the actual measuring. Therefore it is particularly important that measuring the structures on the surface of a mask and determining the rectangularity of the outer edges with respect to each other may be performed in one measuring device. If this is the case, the mask does no longer have to be transported from one measuring device to the other, which ultimately represents an error source for damage to the mask. Furthermore, the time required for temperature adaptation of the mask may be avoided if the measurement takes place in a single device.