Patent for License

Straight line defect detection tool

The technology has particular applicability for in-line inspection of reticles with submicron design features.

Overview

A method and apparatus for inspecting the straightness of edges of features on a reticle is provided wherein the feature edges are analyzed for local defects as small groups of pixels, rather than pixel-by-pixel. In one embodiment, the reticle is imaged to produce pixels having a gray level and associated with an x and a y coordinate. A small array of pixels, such as a 2.times.2 array, that lays on an edge of the target feature is identified, a plane is calculated based on the x coordinate, the y coordinate and the gray level of each of the pixels in the array, and an angle .alpha. between a reference line and a line at the intersection of the calculated plane and a reference plane is calculated. Several other edge-straddling arrays adjacent to the first array are identified, their respective angles .alpha. are calculated, and all the .alpha.'s are compared. If the .alpha.'s are substantially equal, it is determined that the target feature edge is straight, and if the .alpha.'s are not substantially equal, it is determined that the target feature edge is curved. A corresponding reference feature image is analyzed in the same way as the target feature image, and a defect is determined to exist in the target feature when the target feature edge is curved and the reference feature edge is straight.

SUMMARY OF THE TECHNOLOGY

An advantage of the present technology is the ability to reliably detect defects in straight line features without increasing inspection time.

The foregoing and other advantages are achieved in part by a method of inspecting a target feature formed on a surface, the method comprising imaging the target feature to produce a matrix of pixels representative of the target feature, each pixel having a gray level and associated with an x and a y coordinate corresponding to its respective location on the surface. A small array of pixels, such as a 2.times.2 array, that lays on an edge of the target feature is identified, a plane is calculated based on the x coordinate, the y coordinate and the gray level of each of the pixels in the array, and an angle .alpha. between a reference line (such as the positive x coordinate axis) and a line at the intersection of the calculated plane and a reference plane is calculated. Several other arrays adjacent to the first array and laying on the target feature edge are identified; e.g., about four other 2.times.2 arrays, and their respective angles .alpha. are calculated. The angles .alpha. are then compared. If the .alpha.'s are substantially equal, it is determined that the target feature edge is straight. On the other hand, if the .alpha.'s are not substantially equal, it is determined that the target feature edge is curved. A corresponding reference feature image is analyzed in the same way as the target feature image, and a defect is determined to exist in the target feature when the target feature edge is curved and the reference feature edge is straight, indicating an unwanted bite or a bump on the target feature edge.