News Column

Patent Application Titled "Techniques for Matching Spectra" Published Online

June 5, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors Shrestha, Kiran Lall (San Jose, CA); Swedek, Boguslaw A. (Cupertino, CA); David, Jeffrey Drue (San Jose, CA); Lee, Harry Q. (Los Altos, CA), filed on November 12, 2012, was made available online on May 22, 2014.

No assignee for this patent application has been made.

Reporters obtained the following quote from the background information supplied by the inventors: "An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. A variety of fabrication processes require planarization of a layer on the substrate. For example, for certain applications, e.g., polishing of a metal layer to form vias, plugs, and lines in the trenches of a patterned layer, an overlying layer is planarized until the top surface of a patterned layer is exposed. In other applications, e.g., planarization of a dielectric layer for photolithography, an overlying layer is polished until a desired thickness remains over the underlying layer.

"Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. Abrasive polishing slurry is typically supplied to the surface of the polishing pad.

"One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness, or when a desired amount of material has been removed. Variations in the slurry distribution, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations, as well as variations in the initial thickness of the substrate layer, cause variations in the time needed to reach the polishing endpoint. Therefore, determining the polishing endpoint merely as a function of polishing time can lead to within-wafer non-uniformity (WIWNU) and wafer-to-wafer non-uniformity (WTWNU).

"In some systems, a substrate is optically monitored in-situ during polishing, e.g., through a window in the polishing pad. However, existing optical monitoring techniques may not satisfy increasing demands of semiconductor device manufacturers."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "In some in-situ monitoring processes, a spectrum is measured from a substrate. The spectrum is compared to a plurality of reference spectra from a library, and the best matching reference spectrum is identified. One technique for identifying the best matching reference spectrum is to identify the spectrum with the least difference using a sum of squares algorithm. However, the sum of squares technique is computationally heavy, e.g., requires a large number of calculations. The greater the computational load per reference spectrum, the fewer reference spectra that can be compared to the measured spectra while maintaining an acceptable real-time identification of the best matching reference spectrum. However, more efficient algorithms are possible for identifying a matching reference spectrum.

"In one aspect, a method of controlling processing of a substrate includes processing a substrate, measuring a spectrum reflected from the substrate with a spectrographic monitoring system to generate a measured spectrum, for each partition of a plurality of partitions of the measured spectrum, computing a partition value based on the measured spectrum within the partition to generate a plurality of partition values, for each reference spectrum signature of a plurality of reference spectrum signatures, determining a membership function for each partition, for each partition, computing a membership value based on the membership function for the partition and the partition value for the partition to generate a plurality of groups of membership values with each group of the plurality of groups associated with a reference spectrum signature, selecting a best matching reference spectrum signature from the plurality of reference spectra signatures based on the plurality of groups of membership values, determining a characterizing value associated with the best matching reference spectrum signature, and adjusting processing of the substrate or a subsequent substrate based on the characterizing value.

"Implementations can include on or more of the following features. Computing the partition value may include computing an average value of the measured spectrum within the partition. Selecting a best matching reference spectrum signature may include, for each group, computing a total value of the membership values in the group to provide a plurality of totals values, and selecting a reference spectrum signature associated with a group having the largest total value of the plurality of total values. Selecting a best matching reference spectrum signature may include, for each group, determining a second-lowest value of the group of values to provide a plurality of second-lowest values, and selecting a reference spectrum signature associated with a group having the largest second-lowest value of the plurality of second-lowest values. Each reference spectrum signature may include a plurality of code values, and each code value of the plurality of code values may be associated with one of the plurality of partitions. Determining the membership function for each partition may include determining the code value in the reference spectrum signature associated with the partition, and determining the membership function associated with the code value. The plurality of membership functions may include a first triangular function having a first maximum value at a midpoint of a first intensity range and a second function having a second maximum value at a midpoint of a second intensity range, and the first triangular function and the second triangular function may have an equal value at a boundary between the first intensity range and the second intensity range. The plurality of reference spectrum signatures may be computed based on a plurality of reference spectra. Computing the plurality of reference spectrum signatures may include computing an average reference spectrum, and the average reference spectrum may be an average of the plurality of reference spectra. Boundary wavelengths between the partitions based on the average reference spectrum may be determined. Determining boundary wavelengths may include computing wavelengths of a plurality of inflection points in the average reference spectrum, and computing boundary wavelengths that are a fraction of a distance between adjacent inflection points. Boundary intensities between a plurality of intensity zones may be determined based on the average reference spectrum. A second partition value may be computed for each partition of the average reference spectrum, and determining boundary intensities may include selecting at least some of the second partition values. Determining boundary intensities may include calculating an average of the second partition values. For partition of each reference spectrum, a third partition value may be calculated based on the reference spectrum within the partition, and one of a plurality of unique code values may be selected based on comparing the third partition value to the intensity zones. Processing the substrate may include polishing the substrate. Measuring the spectrum may include monitoring the substrate with an in-situ spectrographic monitoring system. A sequence of measured spectra may be generated, the characterizing value for each measured spectrum of the sequence of measured spectra may be determined to generate a sequence of characterizing values, and a polishing endpoint or an adjustment to a polishing parameter may be computed based on the sequence of characterizing values.

"In another aspect, a non-transitory computer program product, tangibly embodied in a machine readable storage device, includes instructions to carry out the method.

"Implementations may optionally include one or more of the following advantages. Fewer computations are required per reference spectrum, permitting the best matching reference spectrum to be identified more quickly or permitting the number of reference spectra that are compared to be increased while maintaining an acceptable real-time identification. Reliability of the endpoint system to detect a desired polishing endpoint can be improved, and within-wafer and wafer-to-wafer thickness non-uniformity (WIWNU and WTWNU) can be reduced.

"The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 illustrates a schematic cross-sectional view of an example of a polishing apparatus.

"FIG. 2 illustrates a measured spectrum from the in-situ optical monitoring system.

"FIG. 3 illustrates a library of reference spectra.

"FIG. 4 illustrates computing an average reference spectrum from a library of reference spectra.

"FIGS. 5A-5C illustrate partitioning the average reference spectrum.

"FIG. 6 illustrates determine intensity zones for the average reference spectrum.

"FIG. 7 illustrates a plurality of membership functions.

"FIG. 8 illustrates a sequence of values generated by the in-situ optical monitoring system.

"FIG. 9 illustrates a sequence of values having a user-input function fit to the sequence of values.

"Like reference numbers and designations in the various drawings indicate like elements."

For more information, see this patent application: Shrestha, Kiran Lall; Swedek, Boguslaw A.; David, Jeffrey Drue; Lee, Harry Q. Techniques for Matching Spectra. Filed November 12, 2012 and posted May 22, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=2546&p=51&f=G&l=50&d=PG01&S1=20140515.PD.&OS=PD/20140515&RS=PD/20140515

Keywords for this news article include: Patents.

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Source: Politics & Government Week


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