Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-10-30
2003-11-04
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S096000, C250S311000, C250S492200, C356S456000, C382S149000
Reexamination Certificate
active
06642726
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods and apparatus for detecting electrical defects in a semiconductor device or test structure having a plurality of features that are specifically designed to produce varying voltage potentials during a voltage contrast inspection. More particularly, it relates to voltage contrast techniques for detecting open and short type defects within the features of the circuit or test structure.
A voltage contrast inspection of a test structure is accomplished with a scanning electron microscope. The voltage contrast technique operates on the basis that potential differences in the various locations of a sample under examination cause differences in secondary electron emission intensities when the sample is the target of an electron beam. The potential state of the scanned area is acquired as a voltage contrast image such that a low potential portion of, for example, a wiring pattern might be displayed as bright (intensity of the secondary electron emission is high) and a high potential portion might be displayed as dark (lower intensity secondary electron emission). Alternatively, the system may be configured such that a low potential portion might be displayed as dark and a high potential portion might be displayed as bright.
A secondary electron detector is used to measure the intensity of the secondary electron emission that originates from the path swept by the scanning electron beam. Images may then be generated from these electron emissions. A defective portion can be identified from the potential state or appearance of the portion under inspection. The portion under inspection is typically designed to produce a particular potential and resulting brightness level in an image during the voltage contrast test. Hence, when the scanned portion's potential and resulting image appearance differs significantly from the expected result, the scanned portion is classified a defect.
Several inventive test structures designed by the present assignee are disclosed in copending U.S. patent application Ser. No. 09/648,093 (Attorney Docket No. KLA1P016A) by Satya et al., filed Aug. 25, 2000, which application is incorporated herein in its entirety. In one embodiment, a test structure is designed to have alternating high and low potential conductive lines during a voltage contrast inspection. In one inspection application, the low potential lines are at ground potential, while the high potential lines are at a floating potential. However, if a line that is meant to remain floating shorts to an adjacent grounded line, both lines will now produce a low potential during a voltage contrast inspection. If there is an open defect present within a line that is supposedly coupled to ground, this open will cause a portion of the line to be left at a floating potential to thereby produce a high potential during the voltage contrast inspection. Both open and short defects causes two adjacent lines to have a same potential during the voltage inspection.
In theory, defects within the above described test structure may be found by comparing images of portions of the test structure that are designed to have identical appearances during voltage contrast inspection. Thus, any difference between two supposedly identical imaged portions can be classified as an electrical defect. However, certain physical defects within the conductive line may introduce “nuisance” defects into the inspection process. That is, physical defects may be inadvertently counted as electrical defects. In one voltage contrast technique, a first set of imaged adjacent conductive lines are subtracted from a second set of imaged adjacent lines. The imaged sets are selected so that they will be identical if they contain no defects. Thus, if the subtraction operation results in a nonzero value, imaged sets are flagged as having one or more defects. Unfortunately, this subtraction procedure may result in physical defects being flagged, as well as electrical defects.
The total defect count which results from a voltage contrast inspection on specially designed test chips can run into the thousands for a complete wafer. Manually reviewing and classifying these defects to isolate the electrical defects from the physical defects is very time consuming and tedious. Additionally, manual classification is inherently subject to human errors.
Accordingly, there is a need for improved apparatus and methods for detecting electrical defects in semiconductor device and test structures without having to manually filter physical defects from the inspection results.
SUMMARY OF THE INVENTION
Accordingly, mechanisms are provided for automatically filtering out physical defects from electrical defects during a voltage contrast inspection of a test structure on a semiconductor device. In general terms, the test structure is designed to include a plurality of features that will charge to specific voltage potentials when scanned with an electron beam during a voltage contrast inspection. Images of the scanned features are generated, and the relative brightness level of each feature depends on the corresponding potential of each feature during the inspection. That is, some features are expected to appear dark, and other features are expected to appear bright. If there is no defect present in the scanned feature, the corresponding image will have the expected number of bright and dark features. However, if there is a defect present, the number of dark and bright features within the generated image will not match expected results.
The test structure may also be designed so have substantially identical pairs of features. In one implementation, each pair includes a feature that is expected to appear bright and another feature that is expected to appear dark within the generated image. Thus, an image may be generated for each set of features. The generated images are then subtracted from each other to determine whether any defects are present. Alternatively, an image of an inspected set of features may be compared to a reference image constructed from a design database. A subtraction that results in a defect is then analyzed to determine whether it is an electrical defect or a physical defect.
In one embodiment, the aspect ratio of each scanned feature is known. If the defect has substantially the same aspect ratio as the known aspect ratio, it is determined to be an electrical defect. Otherwise, it is determined to be a physical defect. Other mechanisms are described for locating the found electrical defect and filtering out the location of physical defects during this process. Additionally, mechanisms are described for calibrating the inspection process based on a test structure having a plurality of known defects.
In one embodiment, a method of detecting defects in a plurality of features on a semiconductor device is disclosed. Each of the features is designed to have a particular potential when scanned with an electron beam. A first and second image of a first and second set of feature portions, respectively, are provided. The first and second images are generated as a result of scanning an electron beam over the first and second set of feature portions, and the first set of feature portions are designed to be substantially identical to the second set of feature portions when there is no defect present. The first image is subtracted from a second image to generate a difference image. When the difference image has a significantly sized defect that represents a difference between the first set of feature portions and the second set of feature portions, it is determined whether the defect is an electrical defect or a physical defect. In one implementation, this determination is accomplished by determining that the defect indicates an electrical defect when the defect has about a same aspect ratio as a one of the scanned feature portions. When the defect has a different aspect ratio than a one of the scanned feature portions, it is determined that the defect indicates a physical defect.
In a specific implementat
Lewis Isabella T.
Verma Gaurav
Weiner Kurt H.
Beyer Weaver & Thomas LLP
Deb Anjan K.
KLA-Tencor Corporation
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