News Column

Patent Issued for Spectraphic Monitoring Based on Pre-Screening of Theoretical Library

September 3, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventor David, Jeffrey Drue (San Jose, CA), filed on February 27, 2013, was published online on August 19, 2014.

The assignee for this patent, patent number 8808059, is Applied Materials, Inc. (Santa Clara, CA).

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. One fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. A conductive filler layer, for example, can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs, and lines that provide conductive paths between thin film circuits on the substrate. For other applications, such as oxide polishing, the filler layer is planarized until a predetermined thickness is left over the non-planar surface. In addition, planarization of the substrate surface is usually required for photolithography.

"Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier 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. A polishing liquid, such as a slurry with abrasive particles, 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 initial thickness of the substrate layer, the slurry composition, 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 cause variations in the time needed to reach the polishing endpoint. Therefore, it may not be possible to determine the polishing endpoint merely as a function of polishing time.

"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, VerticalNews editors also obtained the inventor's summary information for this patent: "In some optical monitoring processes, a measured spectrum is compared to a library of reference spectra to find the best matching reference spectrum. One technique to build a library of reference spectra is to calculate a reference spectrum based on an optical model of the thin film stack on the substrate. The size of the library of reference spectra grows rapidly as the number of layers, the number of variable parameters for each layer, or the number of increments each variable spans increase. The larger the library, the more memory needed to hold the library and the higher the processing load to search the library. A larger library may also produce multiple possible matches, causing lower reliability or requiring other techniques to sift the possible matches and select an acceptable match.

"Another optical monitoring technique is to fit a function, e.g., the optical model, to the measured spectra. However, a complex optical model with a large the number of variable input parameters can suffer similar problems of computational load and possible false fitting of the optical parameters.

"The region of a parameter space that should be searched, or the likely seed values to use when fitting the optical model, can be limited by identifying local minima in the parameter space. The local minima can be found by generating model spectra from the optical model across a wide range for each input parameter, and comparing the model spectra to a spectrum from a test substrate.

"In one aspect, a method of controlling a polishing operation includes storing an optical model for a layer stack having a plurality of layers. The optical model has a plurality of input parameters, the plurality of input parameters defining a parameter space. A plurality of model spectra are generated by calculating a model spectrum using the optical model for each of a first plurality of different points in the parameter space. A test spectrum of a test substrate is measured. For each model spectrum of the plurality of model spectra, the test spectrum is compared to the model spectrum to determine a difference value, thereby generating a plurality of difference values. A plurality of minima in the plurality of difference values are determined. For at least one local minimum of the plurality of minima, a plurality of reference spectra is determined. The plurality of reference spectra for each local minimum are generated using the optical model with a second plurality of different points in the parameter space clustered at a point in the parameter space corresponding to the local minimum. A spectrum of a substrate is measured with an in-sequence or in-situ monitoring system to provide a measured spectrum. A best matching reference spectrum from the plurality of reference spectra that provides a best match to the measured spectrum is determined. The first value associated with the best matching reference spectrum is determined. The substrate is polished with the polishing apparatus. A polishing endpoint or a polishing parameter of the polishing apparatus is adjusted based on the first value associated with the best matching reference spectrum.

"Implementations may include one or more of the following features. Data defining a first plurality of first values for a first parameter of the plurality of input parameters and a second plurality of second values for a second parameter of the plurality of input parameters may be stored. Generating the plurality of model spectra may includes calculating the model spectrum for each combination of a first value of first plurality and a second value of the second plurality. The data may include, for each parameter of the plurality of parameters, a range and an increment or a number of increments. Determining the plurality of minima may include dividing the parameter space into a plurality of sections and determining a minimum for each section. The first plurality of different points may be distributed in the parameter space at a uniform first spacing and the plurality of sections may cover the parameter space at a uniform second spacing greater than the first spacing. The second plurality of different points may be uniformly distributed in the parameter space at the first spacing. The second plurality of different points may be a subset of the first plurality of different points. The second plurality of different points may be distributed in the parameter space at spacing different from the first spacing. The second plurality of different points may be centered around the point in the parameter space corresponding to the local minimum. Data defining a third plurality of third values for a first parameter of the plurality of input parameters and a fourth plurality of fourth values for a second parameter of the plurality of input parameters may be stored. Generating the plurality of reference spectra may include calculating the reference spectrum for each combination of a third value of third plurality and a fourth value of the fourth plurality. The data may include, for each parameter of the plurality of parameters, a range and an increment. Comparing the test spectrum to the model spectrum may include calculating a sum of squares difference, a sum of absolute differences, or a cross-correlation between the test spectrum and the model spectrum.

"In another aspect, a method of controlling a polishing operation includes storing an optical model for a layer stack having a plurality of layers. The optical model has a plurality of input parameters, the plurality of input parameters defining a parameter space. A plurality of model spectra are generated by calculating a model spectrum using the optical model for each of a first plurality of different points in the parameter space. A test spectrum of a test substrate is measured. For each model spectrum of the plurality of model spectra, the test spectrum is compared to the model spectrum to determine a difference value, thereby generating a plurality of difference values. A plurality of minima in the plurality of difference values is determined. A spectrum of a substrate is measured with an in-sequence or in-situ monitoring system to provide a measured spectrum. The optical model is fit to the measured spectrum. The fitting including finding a first value of a first parameter of the plurality of input parameters that provides a minimum difference between an output spectrum of the optical model and the measured spectrum, the fitting including using a point in the parameter space corresponding to a local minimum from the plurality of minima as a seed value in searching for the minimum difference. The substrate is polished with the polishing apparatus. A polishing endpoint or a polishing parameter of the polishing apparatus is adjusted based on the first value.

"Certain implementations can include one or more of the following advantages. The region of a parameter space that should be searched, either by fitting of an optical model or by searching a library of reference spectra, can be reduced, thus reducing computational load and decreasing the likelihood of false fits or matches. The parameter space to be searched can still span the likely range of variations of incoming substrate, enabling the algorithm to remain reliable when there is variation in underlying parameters. Thus, reliability of an endpoint system to detect a desired polishing endpoint may be improved, and within-wafer and wafer-to-wafer thickness non-uniformity (WIWNU and WTWNU) may be reduced."

For more information, see this patent: David, Jeffrey Drue. Spectraphic Monitoring Based on Pre-Screening of Theoretical Library. U.S. Patent Number 8808059, filed February 27, 2013, and published online on August 19, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8808059.PN.&OS=PN/8808059RS=PN/8808059

Keywords for this news article include: Applied Materials Inc.

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Source: Journal of Engineering


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