This invention generally relates to measuring submicron structures or shapes; and more specifically, the invention relates to measuring such shapes from electron beam images of the shapes. Even more specifically, the present invention relates to a measurement procedure and system particularly well suited for determining the relative position of edges of submicron structures or shapes formed by photolithography on semiconductor wafers.
As photolithography K factors continue to decrease and complex reticle enhancement techniques are employed, the printed shapes appearing on the wafer can vary dramatically from the design in both shape and location. The traditional metrics of line width and overlay, although still useful, often are inadequate for predicting many effects on product. A means of accurately measuring wafer images in a full two dimensions is required for a variety of applications including fully understanding effects of imaging, etching, and other wafer processing on product performance. Another especially important application of two-dimensional shape metrology is for use in the calibration of shape prediction models that are just becoming widely offered in the semiconductor industry.
Little has been done in the area of full two-dimensional submicron shape metrology. Early two dimensional submicron measurements are generally achieved with a top down SEM. Metrology SEMs are generally one-dimensional instruments, that scan in a single direction that is approximately normal to the edge to be detected. An edge detection algorithm is then applied to this signal. Early attempts at two-dimensional metrology with SEM's consist of multiple one-dimensional measurements (e.g. to evaluate a bar, a scan along the width will be followed by a different scan along the length). This treatment of shapes is generally inadequate for many applications in terms of quantity of data and it is customary for the scans to be independent (i.e. the two scans are not connected by a coordinate system with a single origin).
An object of this invention is to improve methods and systems for measuring submicron shapes.
Another object of the present invention is to use an image of a submicron shape to identify relative position of edges of that shape.
These and other objectives are attained with a method and apparatus for extracting two-dimensional image shapes from image data on a pixel array. The method comprises the steps of selecting intensity vs. pixel information in at least one direction in the vicinity of an edge of the image shape, and recognizing scans with sufficient contrast as containing edge information. Acceptable scans are subjected to an edge detection algorithm and the edge location is detected. The locus of points generated from applying edge detection at multiple points around the image define the two-dimensional shape of the image.
Any suitable edge detection algorithm may be used. That algorithm may be a user defined edge detection algorithm tailored for the application, and, for example, a threshold edge detection algorithm may be employed. Also, in a preferred embodiment, the selecting step includes the step of selecting intensity vs. pixel information in at least four directions, and the plurality of directions are angularly spaced apart at least about 45 degrees. With one embodiment, one of these directions may be normal to an approximate edge location.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
The method being disclosed makes use of a two-dimensional SEM image which consists of a regular grid of detector signal values that have a one-to-one correspondence with the position of the electron beam. The detector signal is related to the number of electrons collected by one or more detectors as a function of beam location on the sample under investigation.
The method disclosed herein makes use of the fact that the detector signal peaks in the vicinity of an edge, as shown in FIG. 3. Since the edge is at a higher intensity than the rest of the background a relatively high constant intensity contour gives an approximate estimate of the edge location, as shown in contour plot of FIG. 2. Using this approximate edge location as a starting point, a line or scan of pixel intensities can be selected that passes through the approximate edge pixel location in any of many directions.
It is possible to make use of the normal to the approximate edge location and interpolation to produce intensity vs. position for lines that pass through the approximate edge at very close to normal. However, in practice it is found that by selecting intensity vs. pixel location for four directions, the edge can be reliably detected. Four scans ensures that a scan will be no more than 22.5 degrees off from the normal to the edge. More or less scans would also, generally, produce acceptable results.
By applying this four-scan method to all pixels near the edge of the shape of interest, it is possible to produce a full two-dimensional image shape where each point on the edge shares a common origin. The result is shown in FIG. 9. In particular, to obtain the result shown in
Although the technique disclosed herein was developed for SEM metrology, it applies equally well to other detection devices, such as optical, AFM, and SFM.
The data processing needed to perform this invention may be done on any suitable computer, and
While it is apparent that the invention herein disclosed is well calculated to fulfill the objects stated above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.
Number | Name | Date | Kind |
---|---|---|---|
5099521 | Kosaka | Mar 1992 | A |
5272763 | Maruyama et al. | Dec 1993 | A |
5398292 | Aoyama | Mar 1995 | A |
5487116 | Nakano et al. | Jan 1996 | A |
5491759 | Nagao et al. | Feb 1996 | A |
5825914 | Tsuboi et al. | Oct 1998 | A |
5914784 | Ausschnitt et al. | Jun 1999 | A |
20030095710 | Tessadro | May 2003 | A1 |
Number | Date | Country |
---|---|---|
57-104371 | Jun 1982 | JP |
61-88440 | May 1986 | JP |
62-105006 | May 1987 | JP |
03-081602 | Apr 1991 | JP |
05-035872 | Feb 1993 | JP |
05-307609 | Nov 1993 | JP |
06-348836 | Dec 1994 | JP |
08-136236 | May 1996 | JP |
8-136236 | May 1996 | JP |
10-065911 | Mar 1998 | JP |
11-201919 | Jul 1999 | JP |
11-237231 | Aug 1999 | JP |