OPTICAL METROLOGY OF SINGLE
CROSS-REFERENCE TO RELATED
This application is a continuation application of U.S. patent application Ser. No. 10/175,207 filed Jun. 18, 2002 now U.S. Pat. No. 6,775,015.
1. Field of the Invention
The present invention relates to wafer metrology, and more particularly to optical metrology of single features.
2. Related Art
In semiconductor manufacturing, periodic gratings are typically utilized for quality assurance. For example, one typical use of such periodic gratings includes fabricating a periodic grating in proximity to a semiconductor chip. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
The profile of a periodic grating can be determined using optical metrology. In general, optical metrology involves directing an incident beam at the periodic grating, and measuring the resulting diffraction beam. However, in conventional optical metrology, multiple periods of the periodic grating are typically illuminated. Thus, the determined profile for the periodic grating is more of an average representation of the illuminated periods rather than of an individual period.
In an exemplary embodiment, the profile of a single feature formed on a wafer can be determined by obtaining an optical signature of the single feature using a beam of light focused on the single feature. The obtained optical signature can then be compared to a set of simulated optical signatures, where each simulated optical signature corresponds to a hypothetical profile of the single feature and is modeled based on the hypothetical profile.
DESCRIPTION OF DRAWING FIGURES
The present invention can be best understood by reference to the following description taken in conjunction with the accompanying drawing figures, in which like parts may be referred to by like numerals:
FIG. 1 depicts an exemplary optical metrology system;
FIG. 2 depicts an exemplary source;
FIG. 3 depicts an exemplary detector;
FIG. 4 depicts another exemplary detector;
FIG. 5 depicts a graph of various exemplary optical signatures;
FIG. 6 depicts an exemplary source and detector; FIGS. 7-A and 7-B depict a source and detector pair with pupil stops;
FIGS. 8-A and 8-B depict a source and detector pair with pupil stops;
FIG. 9A depicts an exemplary periodic pattern; FIGS. 9B and 9C depict exemplary diffraction matrices of the exemplary pattern depicted in FIG. 9A;
FIG. 10A depicts an exemplary periodic pattern; and
FIGS. 10B and IOC depict exemplary diffraction matrices of the exemplary pattern depicted in FIG. 10A.
The following description sets forth numerous specific configurations, parameters, and the like. It should be recognized, however, that such description is not intended as a limitation on the scope of the present invention, but is 10 instead provided as a description of exemplary embodiments.
With reference to FIG. 1, an optical-metrology system 100 can be used to determine the profile of periodic grating 102 formed on wafer 104. As described earlier, periodic
15 grating 102 can be formed in test areas on wafer 104. For example, periodic grating 102 can be formed adjacent to a device formed on wafer 104. Alternatively, periodic grating 102 can be formed in an area of the device that does not interfere with the operation of the device or along scribe
20 lines on wafer 104.
As depicted in FIG. 1, optical-metrology system 100 can include an electromagnetic source 106 and a detector 112. Periodic grating 102 is illuminated by an incident beam 108 from source 106. In the present exemplary embodiment, incident beam 108 is directed onto periodic grating 102 at an angle of incidence 9, with respect to normal n of periodic grating 102. Diffracted beam 110 leaves at an angle of Qd with respect to normal n and is received by detector 112.
30 To determine the profile of periodic grating 102, opticalmetrology system 100 includes a processing module 114, which converts diffracted beam 110 received by detector 112 into a diffraction signal (i.e., a measured-diffraction signal). Processing module 114 then compares the measured-diffrac
35 tion signal to simulated-diffraction signals stored in a library 116. Each simulated-diffraction signal in library 116 can be associated with a hypothetical profile. Thus, when a match is made between the measured-diffraction signal and one of the simulated-diffraction signals in library 116, the hypo
40 thetical profile associated with the matching simulateddiffraction signal can be presumed to represent the actual profile of periodic grating 102.
As described above, in conventional optical metrology, multiple periods of periodic grating 102 are typically illu
45 minated and thus the determined profile for periodic grating 102 is based on an average representation of the illuminated periods. As described below, in one exemplary embodiment, optical-metrology system 100 can be used to determine the profile of a single period of periodic grating 102. Moreover,
50 optical-metrology system 100 can be used to determine the profile of various types of single features formed on wafer 104, such as a line, space, contact hole, dot, and the like.
More particularly, source 106 can be configured to generate a beam to use in determining the profile of a single
55 feature formed on wafer 104. With reference to FIG. 2, in one exemplary embodiment, source 106 can include a light source 202, a collimator 204, and a focusing lens 206. In the present exemplary embodiment, to determine the profile of a single feature formed on wafer 104, focusing lens 206 is
60 configured to have a numerical aperture of greater than X/2d, where X corresponds to the wavelength of the light being used and d corresponds to the distance between the feature of interest and an adjacent feature. It should be noted that focusing lens 206 can be custom made or adapted from
65 various existing types of lenses, such as compact-disc pickup lens, microscope objectives, monomode optical fiber, and the like.