Process for inspecting objects showing patterns with dimensional tolerances and reject criteria varying with the locations of said patterns and apparatus and circuits for carrying out said process
First Claim
1. A process for inspecting objects showing patterns with dimensional tolerances and reject criteria varying in accordance with the location of said patterns, of the type including the comparision of image Iref of a reference object with image Iexa of an object to be inspected, the latter being liable to show geometrical defects with respect to the reference object, characterized in that in includes the following steps;
- Step 1--elaborating binary electronic images IREF and IEXA from images Iref and Iexa,Step 2--defining a structuring element B for each image point (pixel) the size of said structuring element being able to vary for each pixel in function of predetermined data and adjusting IREF to the maximum and minimum dimensional tolerances by expanding and eroding said image by structuring element B, which provides (IREF)max and (IREF)min,Step 3--forming images of the "spreading" and "lack" type defects by respectively carrying out the following logical operations;
space="preserve" listing-type="equation">[(I.sub.REF).sub.max OR I.sub.EXA ] EXCL. OR (I.sub.REF).sub.max and
space="preserve" listing-type="equation">[(I.sub.REF).sub.min OR I.sub.EXA ] EXCL. OR I.sub.EXA'"'"' andStep 4--measuring the size of the defects on the defect images and comparing said defects with respect to reject criteria which define, for each pixel, the dimensions of the defects which can be accepted according to predetermined data.
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Abstract
First of all, this invention relates to a process based on the comparison of the image of a reference object Iref with the image of an object to be inspected Iexa. Images Iref and Iexa are picked-up, sampled, discretized and thresheld to produce electronic or binary images IREF and IEXA, respectively. These images are cleaned, then centered, IREF is adjusted to the minimum dimensional tolerances and becomes (IREF)min. For each image point (pixel), the adjustment is performed by using a structuring element of variable size, the size of which is controlled by a bus from the data contained in a memory unit which take the location of the pixel on the reference object, into account. (IREF)min is compared to IEXA which ensures the following function:
[(I.sub.REF).sub.min OR I.sub.EXA ]EXCL. OR I.sub.EXA.
The resulting image is the image of the "lack" type defects (which does not appear in Iexa with respect to Iref). Each defect is studied in a defect analysis unit which computes the dimensions of the defect and compares these dimensions with the maximum allowed size of the defect in this location. For this purpose, this circuit uses the reject criteria for this location, which are contained in a memory unit. The processing operations (adjustment and analysis) and the production of the corresponding data contained in the memory, are synchronized by count and delay circuits. The binary result (accepted/rejected) is available on a line. The defects of the "spreading" type are processed in the same way. The logic level of the line which is connected to a computer, determines the final decision (accepted/rejected). This invention also relates to an apparatus and circuits for carrying out the described process. This invention can be more particularly used in manufacturing semiconductors (masks, modules, chips, . . . ).
84 Citations
25 Claims
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1. A process for inspecting objects showing patterns with dimensional tolerances and reject criteria varying in accordance with the location of said patterns, of the type including the comparision of image Iref of a reference object with image Iexa of an object to be inspected, the latter being liable to show geometrical defects with respect to the reference object, characterized in that in includes the following steps;
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Step 1--elaborating binary electronic images IREF and IEXA from images Iref and Iexa, Step 2--defining a structuring element B for each image point (pixel) the size of said structuring element being able to vary for each pixel in function of predetermined data and adjusting IREF to the maximum and minimum dimensional tolerances by expanding and eroding said image by structuring element B, which provides (IREF)max and (IREF)min, Step 3--forming images of the "spreading" and "lack" type defects by respectively carrying out the following logical operations;
space="preserve" listing-type="equation">[(I.sub.REF).sub.max OR I.sub.EXA ] EXCL. OR (I.sub.REF).sub.max and
space="preserve" listing-type="equation">[(I.sub.REF).sub.min OR I.sub.EXA ] EXCL. OR I.sub.EXA'"'"' andStep 4--measuring the size of the defects on the defect images and comparing said defects with respect to reject criteria which define, for each pixel, the dimensions of the defects which can be accepted according to predetermined data. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
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17. An apparatus for inspecting objects showing patterns with dimensional tolerances and reject criteria varying with the location of said patterns, of the type comparing image Iref of a reference object with image Iexa of an object to be inspected liable to show geometrical defects with respect to the reference object, characterized in that it includes:
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means for picking-up at least Iexa, means for elaborating binary electronic images IREF and IEXA, processing means including means for adjusting IREF by expanding and eroding it to the maximum and minimum tolerances, by a structuring element B the size of which can vary for each pixel, in function of predetermined data, compare means for forming images of the defects of the "spreading" and "lack" types by respectively carrying out the following logic operations;
space="preserve" listing-type="equation">[(I.sub.REF).sub.max OR I.sub.EXA ] EXCL. OR (I.sub.REF).sub.max and
space="preserve" listing-type="equation">[(I.sub.REF).sub.min OR I.sub.EXA ] EXCL. OR I.sub.EXA, andmeans for analysing defects, including means for measuring the size of the defects and means for comparing said defects with reject criteria defining for each pixel, the defect dimensions which can be accepted, in accordance with predetermined data. - View Dependent Claims (18, 19, 20, 21, 22)
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23. A circuit for adjusting an electronic reference signal IREF to minimum dimensional tolerances by using a structural element of circular shape and variable size, formed of a plurality of concentric rings characterized in that it includes:
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means for storing all the pixels of IREF constituting said structuring element, into memory cells, a plurality of AND gates respectively forming each of said rings, the inputs of each AND gate being constituted by the contents of the cells corresponding to the constitutive pixels of the ring under consideration, decoding means for selecting the number and rank of the rings to be considered, and an AND gate for integrating said selected rings so that signal (IREF)min is obtained at the output of said gate. - View Dependent Claims (24)
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25. A circuit for adjusting an electronic reference signal IREF to maximum dimensional tolerances by using a structuring element of circular shape and variable size, comprised of a plurality of concentric rings, characterized in that it includes:
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means for storing all the pixels of (IREF) constituting said structuring element into memory cells, a plurality of AND gates respectively constituting each of said rings, the inputs of each AND gate constituted by the contents of the cells corresponding to the constitutive pixels of the ring under consideration, decoding means for selecting the number and rank of the rings to be considered, and an AND gate (69) followed by an invert circuit (71) for integrating said selected rings so that signal (IREF)max is obtained at the output of this gate.
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Specification