This invention relates to a scanning electron microscope (SEM) which can observe minute objects on the surface of a specimen.
A scanning electron microscope (SEM) scans a specimen with electron beams, detects secondary electrons emitted from the specimen as the result of bombardment by electron beams, and displays a secondary electron image representing the scanned objects on a display screen. This technology is also used for observation of minute structures in semiconductor manufacturing fields. Recently, as semiconductor devices have quickly become smaller and smaller, objects and defects on specimens have also become much smaller. They are too small to be searched and detected by conventional optical object/defect investigating devices and the like. Their resolutions have almost reached the limits of searches and observations of objects and defects. In production of such minute semiconductor devices, foreign objects and defects of micro sizes on silicone wafers may cause errors and problems. Further each silicone wafer has a lot of objects and defects to be observed (e.g. some tens to some hundreds. For detailed investigation of these objects and defects, the semiconductor manufacturers have been longing for means for automatic observation and investigation which combine an Auto Defect Review (ADR) function by a scanning electron microscope (SEM) and an Auto Defect Classification (ADC) function which automatically classifies objects and defects detected by the ADR function.
Usually to observe such minute objects and defects, the SEM takes the steps of locating objects and defects on each wafer in advance by another inspection system, searching and observing them according to the coordinate data of their positions. However, substantially, there is a slight difference between the coordinates system of the inspection system and the coordinates system of the SEM and this difference (error) is one of the factors which prevent automation of investigation of objects and defects by the SEM. Usually, the SEM magnifies the secondary electron image of objects and defects of submicron sizes by at least 5000 times to display it on the screen of the SEM. Naturally, the size of the SEM screen is limited and the area you can investigate at a time is also limited. Therefore, if the positional data of an object or defect obtained by the inspection system contains a large error, the image of the object or defect may not be In the SEM screen. For example, you can observe an area of only 40 μm square at a time on the SEM screen of 200 mm square at a magnification of ×5000. If the object/defect coordinate data obtained by the inspection system contains an error of ±20 μm or more, the image of the object/defect is not in the screen area and you cannot find it.
One of methods to solve the above problem comprises the steps of obtaining coordinate data of objects or defects of known sizes on a test specimen by an inspection system, obtaining the coordinate data of the objects or defects on the test specimen in the SEM coordinates system, determining a coordinate converting expression to minimize their coordinate difference, and using this expression for fine positional adjustment. However, if the correction is made by objects or defects which are selected at random, the correction values may be various and the result of correction will not always be assured. To solve such as problem, Japanese Non-examined Patent Publications H11-167893 (1999) (titled “Scanning electron microscope”) discloses a method of automatically re-calculating a coordinate converting expression using only objects or defects close to a new position on a wafer when the wafer is moved. This method is expected to give a high accuracy of correcting coordinates because objects or defects which are close to each other have a very similar coordinate error.
However, there is no means to check whether the coordinates are corrected to the expected frequency of occurrence of objects (assuming that all objects on the screen can be detected by the ADR). If the frequency of occurrence was low in an actual automatic measurement, the operator had to increase points of measurement for fine adjustment, increase the accuracy of correction of coordinates, and measure again. If an error of some tens micrometers is corrected down to some micrometers, objects and defects can be caught for observation. However, for accurate classification of objects and defects, images of a higher magnification are required. A typical manual detection of an object or defect after correction of coordinates by a fine adjustment comprises the steps of observing the specimen for an object or defect at a low magnification, moving the object or defect to the center of the screen, and increasing the magnification. An automatic object/defect detecting method comprises the following steps:
Measuring the locations of an object or defect on a wafer in advance by the other inspection system, moving the stage to the position of the object or defect according to the coordinate data obtained by the measurement, taking an image of the area including the object or defect at a preset low magnification (for searching), comparing this image by a normal pattern image which was obtained at the same magnification in advance, and thus detecting the object or defect. For investigation of objects or defects on a patterned wafer, the above detecting method further comprises the steps of taking an image of a pattern element (called “die”) next to the pattern element including this object or defect at the same coordinates, comparing these images, moving the stage until the object or defect comes to the center of the screen, and shooting the object or defect at a preset high magnification.