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APPARATUS AND METHOD FOR THE
MEASUREMENT OF DIFFRACTING
FIELD OF THE INVENTION 5
This invention relates in general to metrology devices and in particular to metrology devices that may be used to measure diffracting structures.
It is desirable to measure circuit structures and other types of structures, e.g., resist structures, during the production of integrated circuits. Optical metrology tools are particularly 15 well suited for measuring microelectronic structures because they are nondestructive, accurate, repeatable, fast, and inexpensive. Often different metrology tools are required to measure different structures or parameters on a wafer. For example, certain structures on a wafer act as diffraction 20 gratings, which conventionally require a different metrology tool, e.g. critical dimension-scanning electron microscopy (CD-SEM), than is used to measure planar thin films.
One tool that is sometimes used to measure diffracting 25 structures is a scatterometer. Scatterometry is an angleresolved measurement and characterization of light scattered from a structure. Scatterometry is discussed in detail in U.S. Ser. No. 09/036,557, filed Mar. 6, 1998, now U.S. Pat. No. 6,483,580 Bl, which is assigned to KLA-Tencor Corpora- 3Q tion, which has an International Publication No. WO 99/45340, dated Sep. 10, 1999, and which is incorporated herein by reference.
U.S. Ser. No. 09/036,557 discloses the use of a spectroscopic ellipsometer to measure the diffracting structure. The 35 sampling beam is incident on the sample at an oblique angle. The incident light of the spectroscopic ellipsometer is polarized to provide a beam in the TE mode (S-polarized) when the incidence plane of the beam is perpendicular to the grating of the diffracting structure or to provide a beam in 40 the TM mode (P-polarized) when the incidence plane of the beam is parallel to the grating. Aligning the incident radiation with the grating of the diffracting structure unfortunately is difficult, particularly where the wafer stage is an r-0 stage. With an r-9 stage, the entire metrology apparatus 45 must be rotated to properly align the incident radiation with the grating. U.S. Ser. No. 09/036,557 discloses a dedicated scatterometer instrument that uses a spectroscopic ellipsometer with non-normal incident light and that is used in a scatterometer mode. 50
In addition, U.S. Ser. No. 09/036,557 teaches that a reference database is generated using optical modeling. The reference database is simplified by measuring the film thickness and optical indices of film underlying the diffracting structure. Thus, prior to ellipsometrically measuring the 55 diffraction grating, a measurement of the underlying film is performed. A broadband ellipsometric measurement is then made at a single polarization orientation, and the reference database is consulted to determine the structure of the diffraction grating. As can be seen, even though the size of 60 the database is reduced by measuring the film thickness and optical indices of the underlying film, this process still requires the generation of a relatively large database. Further, the sample or metrology device must be moved and refocused to measure the underlying film, i.e., without the 65 diffracting structure, and the diffracting structure itself, which is time intensive.
Thus, what is needed is an optical metrology tool to quickly and accurately measure diffraction gratings, as well as other non-diffracting structures, and that may be used with various wafer stages, including X, Y, Z, 9 stages, as well as stages capable of r-9 movement only.
A normal incidence reflectometer uses normally incident broadband radiation to measure one or more parameters of a diffracting structure. A rotatable analyzer/polarizer is used to analyze the diffracted radiation that is reflected off the diffracting structure. Relative rotation of the rotatable analyzer/polarizer with respect to the diffracting structure permits analysis of the diffracted radiation at multiple polarity orientations. The analyzer/polarizer is a single unit, which advantageously reduces cost and simplifies operation. A spectograph detects the intensity of the spectral components at different polarity orientations. Because the normal incidence reflectometer, in accordance with the present invention, uses normally incident radiation and an analyzer that rotates relative to the diffracting structure, or vice-versa, the orientation of the grating of the diffracting structure does not affect the accuracy of the measurement. Consequently, different types of sample stages, including X, Y, and Z, as well as r-9 type stages may be used. Further, the normal incidence reflectometer advantageously does not require that the polarization orientation of the incident light be aligned with the grating of the diffraction structure.
One aspect of the present invention is directed towards an apparatus for measuring one or more parameters of a diffracting structure on a sample, the apparatus includes a radiation source that emits broadband radiation, a polarizing element that polarizes the radiation, which is then normally incident on the diffracting structure. At least one of the polarizing element and the diffracting structure are rotatable such that a plurality of polarization orientations of the polarizing element with respect to the diffracting structure may be achieved. The light is reflected off the diffracting structure, passes through the polarizing element and received by a spectrograph that detects the intensity of spectral components of said polarized beam at different polarization orientations of the polarizing element with respect to the diffracting structure. Thus, multiple orientations of the polarization of the reflected light may be received by the spectrograph.
Another aspect of the present invention includes an apparatus for measuring one or more parameters of a diffracting structure on a sample, the apparatus includes a radiation source that emits broadband radiation that is normally incident on the diffracting structure, a polarizing element that is in the beam path of the radiation, an r-9 sample stage that holds the sample with the diffracting structure, and a spectrograph that detects the intensity of spectral components of radiation reflected off said diffracting structure. The polarizing element is positioned such that the radiation passes through the polarizing element toward said sample, the radiation is reflected off the diffracting structure on the sample, the reflected radiation passes through the polarizing element, and the polarizing element is rotatable to produce a relative rotation between said polarizing element and said diffracting structure. The spectrograph detects the intensity of spectral components of the reflected radiation after passing through the polarizing element at a plurality of polarization orientations between the polarizing element and the diffracting structure.