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Method for reticle inspection using aerial imaging    

A techniques for inspecting reticles that are used in fabricating microelectronic devices through a microphotolithographic process.

Overview

A reticle inspection system for inspecting reticles can be used as an incoming inspection tool, and as a periodic and pre-exposure inspection tool. Mask shops can use it as an inspection tool compatible to their customers, and as a printable error detection tool. The inventive system detects two kinds of defects: (1) line width errors in the printed image; (2) surface defects. The line width errors are detected on the die area. The detection is performed by acquiring the image of the reticle under the same optical conditions as the exposure conditions, (i.e. wavelength, numerical aperture, sigma, and illumination aperture type) and by comparing multiple dies to find errors in the line width. Surface defects are detected all over the reticle. The detection of surface defects is performed by acquiring transmission and dark-field reflection images of the reticle and using the combined information to detect particles, and other surface defects.


Summary of Tehcnology
It is one feature of the technology to provide an inspection system that provides more complete information on the properties of the photolithographic mask. Particularly, it is a feature of the technology to provide a mask inspection system capable of detecting errors in the image that the mask would actually print on the photoresist during exposure.
Another feature of the technology to provide a mask inspection system capable of detecting the presence of other mask defects, particularly surface defects.
It is yet another feature of the technology to provide a mask inspection system that provides speedy and reliable identification of the above mask defects.
To implement the above features and achieve the advantages of the technology, there is provided a method for inspecting a multiple die reticle that is used with an optical exposure system under a set of exposure conditions, the multiple die reticle including at least a first and a second die. A plurality of images of the reticle are acquired using transmitted light under the indicated exposure conditions. The plurality of images of the reticle includes images of the first die and images of the second die. Each of the plurality of images of the reticle corresponds to a different focal condition. The images of the first die and the second die are used to detect variations in line width in the first die.
An apparatus is provided which can be used to practice the inventive method. The apparatus includes a scanner for acquiring a plurality of images of the multiple die reticle under the set of exposure conditions. The plurality of images includes images of the first die and images of the second die. The apparatus also includes an image processing module for detecting variations in line width of the first die by comparing the images of the first die and the images of the second die.
An apparatus is provided including: a laser light source; a homogenizer and a transmission light illumination means for illuminating the reticle; and an optical system for producing a plurality of magnified images of the reticle under the set of exposure conditions, the optical system having variable illuminating and imaging apertures for reproducing the indicated set of exposure conditions. The plurality of acquired images of the reticle includes images of the first die and images of the second die. The inventive apparatus also includes an image acquisition module for acquiring the plurality of magnified images of the reticle; and an image processing module for analyzing a condition of the reticle by comparing the images of the first die and the images of the second die. The apparatus also includes an optical system for acquiring dark field reflection images.

Patent Summary

U.S. Patent Classes & Classifications Covered in this Patent:

Class 430: Radiation Imagery Chemistry: Process, Composition, Or Product Thereof

This is the generic class for: (1) Forming the likeness of an object, or an instrumented or discernible phenomenon, in a chemically defined receiver or in a receiver wherein radiation produces a chemical reaction, by use of radiation. (2) Finishing an image by chemical processing regardless how formed. (3) A radiation sensitive receiver, composition, or product disclosed solely for radiation imagery chemistry, and process of making same. (4) A nonradiation sensitive-receiver, composition, or product. (5) An imaged product by a process or employing a receiver, composition, or product.

Subclass 30: INCLUDING CONTROL FEATURE RESPONSIVE TO A TEST OR MEASUREMENT

Class 356: Optics: Measuring And Testing

Methods and apparatus (1) for analyzing light to measure or test its characteristics, such as intensity, color and polarization; (2) for determining the optical or nonoptical properties of materials or articles by noting, as by inspection, measurement, or test the effect produced by the materials or articles on light associated therewith; and (3) for measuring the dimensions of structures or the spatial relationships such as distances or angle bearings of spaced points by comparison of the respective properties (usually direction or spatial position) of the light from these points or by comparison of the properties of these lights with some scale or standard. The light analyzing includes or is for spectroscopy, interference, polarization, beam direction or pattern, focal position of a light source, shade or color, and photometers. The material or article properties determined are or involve crystal or gem examination, material strain analysis, blood analysis, optical pyrometers, egg candling, cutting blade sharpness, oil testing, document verification, flatness, lens or reflector testing, refraction testing, monitoring moving webs or fabrics, light transmission or absorption, light reflection, inspection for flaws or imperfections in materials, and thread counting. The dimensioning and spatial relationship determination includes triangulation by a light beam, contour plotting, range or height finders, motion stopping, velocity or velocity/height measuring, sighting where the optical element or reticle moves with the sighted object, particle size determination, particle light scattering, electrophoresis, angle measuring or axial alignment, mensuration or configuration comparison, alignment in a lateral direction, and fiducial instruments.

Subclass 237.1: INSPECTION OF FLAWS OR IMPURITIES

Class 382: Image Analysis

This is the generic class for apparatus and corresponding methods for the automated analysis of an image or recognition of a pattern. Included herein are systems that transform an image for the purpose of (a) enhancing its visual quality prior to recognition, (b) locating and registering the image relative to a sensor or stored prototype, or reducing the amount of image data by discarding irrelevant data, and (c) measuring significant characteristics of the image.

Subclass 144: Mask inspection (e.g., semiconductor photomask)