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Patent Issued for Polishing Monitoring Method, Polishing Method, and Polishing Monitoring Apparatus

July 23, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Kobayashi, Yoichi (Tokyo, JP), filed on October 9, 2013, was published online on July 8, 2014.

The patent's assignee for patent number 8773670 is Ebara Corporation (Tokyo, JP).

News editors obtained the following quote from the background information supplied by the inventors: "The present invention relates to a method of monitoring progress of polishing of a substrate, such as a semiconductor wafer, and more particularly to a method of monitoring progress of substrate polishing based on a change in spectrum obtained from reflected light from the substrate and determining a polishing end point.

"The present invention also relates to a polishing monitoring apparatus for performing such a polishing monitoring method.

"The present invention further relates to a polishing method for a substrate using such a polishing monitoring method.

"In fabrication processes of a semiconductor device, several kinds of materials are repeatedly deposited in the form of film on a silicon wafer to form a multilayer structure. It is important for forming such a multilayer structure to planarize a surface of a top layer. Chemical mechanical polishing (CMP) is widely used as one of techniques for achieving such planarization.

"The chemical mechanical polishing (CMP) is performed by a polishing apparatus. The polishing apparatus of this type typically includes a polishing table supporting a polishing pad thereon, a top ring for holding a substrate (a wafer with a film formed thereon), and a polishing liquid supply mechanism for supplying a polishing liquid onto the polishing pad. Polishing of a substrate is performed as follows. The top ring presses a surface of the substrate against the polishing pad, while the polishing liquid supply mechanism supplies the polishing liquid onto the polishing pad. In this state, the top ring and the polishing table are rotated independently to provide relative movement between the substrate and the polishing pad, thereby polishing the film that forms the surface of the substrate.

"Generally, the polishing apparatus has a polishing end point detection device. An optical polishing end point detection device is one example of such a polishing end point detection device. This device is configured to direct light to the surface of the substrate and to determine a polishing end point based on spectrum of the light reflected from the substrate. For example, a Japanese laid-open patent publication No. 2004-154928 discloses a method in which intensity of the reflected light is processed in order to remove noise components to create characteristic value and the polishing end point is determined based on a distinctive point (i.e., a local maximum point or local minimum point) of temporal variation in the characteristic value.

"The spectrum is an arrangement of the light intensity in the order of wavelength. The characteristic value created from the spectrum varies periodically with polishing time, as shown in FIG. 1, and the local maximum point and the local minimum point appear alternately. This phenomenon is due to interference between light waves. Specifically, the light, directed to the substrate, is reflected off an interface between a medium and the film and an interface between the film and a layer beneath the film. The light waves from these interfaces interfere with each other. The manner of interference between the light waves varies depending on the thickness of the film (i.e., a length of an optical path). Therefore, the intensity of the reflected light from the substrate varies periodically in accordance with the thickness of the film. The intensity of the light can also be expressed as a relative value, such as a reflectance or a relative reflectance.

"The above-described optical polishing end point detection device counts the number of distinctive points (i.e., the local maximum points or local minimum points) of the temporal variation in the characteristic value during polishing and monitors the polishing progress based on the number of distinctive points. The polishing process is terminated when a predetermined period of time has elapsed from a point of time when the number of distinctive points has reached a predetermined value.

"There is also a method of determining the polishing end point by comparing spectrum obtained during polishing with reference spectrum that is prepared in advance, as disclosed in a Japanese laid-open patent publication No. 2009-505847. In this method, the spectrum at each point of time during polishing is compared with the reference spectrum. A point of time when a difference between both spectra satisfies a condition of a target difference is determined to be the polishing end point. The reference spectrum is prepared in advance by polishing a sample substrate of the same type as the substrate to be polished.

"A plurality of spectra including the reference spectrum, which are obtained during polishing of the sample substrate, are associated with index values that are correlated with polishing time and rotational speed of the polishing table. The spectra thus obtained are stored as library. Therefore, by comparing spectrum obtained during polishing of another substrate with the spectrum in the library, a polished state of the substrate at each point of time during polishing can be expressed by the index value. This index value can be defined as an index that indicates a film thickness of the substrate relatively or indirectly.

"However, in an actual substrate, multiple interconnect layers with different interconnect patterns and multiple dielectric films of different types are piled up to form multilayer interconnect structure. The optical sensor detects the light reflected from a lower dielectric film through a non-interconnect portion of an upper dielectric film. Consequently, if there are variations in thickness of the lower dielectric film and optical constant between substrates, the spectrum is affected by these variations. As a result, the above-described method cannot accurately measure the thickness of the upper film (i.e., the film to be polished) and it is therefore difficult to accurately monitor the progress of polishing. Furthermore, the variations in thickness of the lower dielectric film and optical constant would result in differences in the detected polishing end point between the substrates."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide a method and an apparatus capable of accurately monitoring progress of polishing and capable of detecting an accurate polishing end point. It is also an object of the present invention to provide a polishing method for a substrate using such a polishing monitoring method.

"One aspect of the present invention is to provide a method of monitoring progress of polishing of a substrate having a film. The method includes: directing light to the substrate during polishing of the substrate; receiving reflected light from the substrate; measuring intensity of the reflected light at each wavelength; producing spectrum indicating relationship between intensity and wavelength from measured values of the intensity; calculating an amount of change in the spectrum per predetermined time; integrating the amount of change in the spectrum with respect to polishing time to obtain an amount of cumulative change in the spectrum; and monitoring the progress of polishing of the substrate based on the amount of cumulative change in the spectrum.

"In a preferred aspect of the present invention, the amount of change in the spectrum is a magnitude of relative change between two spectra produced at two different points of time.

"In a preferred aspect of the present invention, the magnitude of relative change is root mean square of difference in the intensity between the two spectra in a predetermined wavelength range.

"In a preferred aspect of the present invention, the magnitude of relative change is an average of absolute values of difference in the intensity between the two spectra in a predetermined wavelength range.

"In a preferred aspect of the present invention, the amount of change in the spectrum is a rate of change in the spectrum obtained by dividing the magnitude of relative change by a time interval between the two points of time.

"In a preferred aspect of the present invention, the amount of change in the spectrum has a plus or minus sign.

"In a preferred aspect of the present invention, the spectrum is a spectrum indicating relationship between wavelength and normalized intensity that is obtained by dividing the intensity at each wavelength by an average of the intensity in a predetermined wavelength range.

"In a preferred aspect of the present invention, the substrate is a second substrate to be polished after a first substrate having the same structure, and the method further includes: directing light to the first substrate during polishing of the first substrate; receiving reflected light from the first substrate; measuring intensity of the reflected light at each wavelength; producing reference spectrum indicating relationship between intensity and wavelength from measured values of the intensity; calculating an amount of change in the reference spectrum per predetermined time; integrating the amount of change in the reference spectrum with respect to polishing time to obtain an amount of cumulative change in the reference spectrum; and converting the amount of cumulative change in the spectrum with respect to the second substrate into an amount of removed film of the second substrate based on the amount of cumulative change in the reference spectrum, an initial film thickness of the first substrate, and a final film thickness of the first substrate.

"In a preferred aspect of the present invention, the method further includes: obtaining an initial film thickness of the second substrate; and converting the amount of removed film into a film thickness of the second substrate by subtracting the amount of removed film from the initial film thickness of the second substrate.

"In a preferred aspect of the present invention, the polishing of the substrate is polishing of the substrate for adjusting a height of metal interconnects formed in the substrate.

"In a preferred aspect of the present invention, the substrate has the film, a barrier layer formed on the film, and the metal interconnects formed in the film; and the method further comprises determining a removal point of the barrier layer based on the amount of change in the spectrum.

"In a preferred aspect of the present invention, the substrate has a surface having steps formed thereon, and the calculating of the amount of cumulative change in the spectrum is started from a point of time when the steps are removed.

"In a preferred aspect of the present invention, the method further includes determining a polishing end point of the substrate based on the amount of cumulative change in the spectrum.

"Another aspect of the present invention is to provide a method of polishing a substrate having a film. The method includes: polishing the substrate by providing sliding contact between the substrate and a polishing pad; directing light to the substrate during polishing of the substrate; receiving reflected light from the substrate; measuring intensity of the reflected light at each wavelength; producing spectrum indicating relationship between intensity and wavelength from measured values of the intensity; calculating an amount of change in the spectrum per predetermined time; integrating the amount of change in the spectrum with respect to polishing time to obtain an amount of cumulative change in the spectrum; and monitoring progress of polishing of the substrate based on the amount of cumulative change in the spectrum.

"Still another aspect of the present invention is to provide a polishing monitoring apparatus including: a light-applying unit configured to direct light to the substrate during polishing of the substrate; a light-receiving unit configured to receive reflected light from the substrate; a spectroscope configured to measure intensity of the reflected light at each wavelength; and a processing device configured to process measurement data from the spectroscope, wherein the processing device is configured to produce spectrum indicating relationship between intensity and wavelength from measured values of the intensity, calculate an amount of change in the spectrum per predetermined time, integrate the amount of change in the spectrum with respect to polishing time to obtain an amount of cumulative change in the spectrum, and monitor the progress of polishing of the substrate based on the amount of cumulative change in the spectrum.

"Still another aspect of the present invention is to provide a program for enabling a computer to execute the steps of: producing spectrum indicating relationship between intensity and wavelength of reflected light from a substrate; calculating an amount of change in the spectrum per predetermined time; integrating the amount of change in the spectrum with respect to polishing time to obtain an amount of cumulative change in the spectrum; and monitoring progress of polishing of the substrate based on the amount of cumulative change in the spectrum.

"Because the progress of polishing is monitored based on the amount of change in the spectrum in its entirety, the present invention can be applied to polishing of various substrates. In particular, even in the case where the wavelengths of the local extremum points of the spectrum do not change greatly because of a small amount of polishing and because of multilayer transparent films having greatly different refractive indexes (e.g., like a polishing process for adjusting the height of copper interconnects), the method according to the present invention can accurately obtain the change in thickness of a film to be polished. Because the amount of cumulative change in the spectrum, which is obtained by integrating (or adding up) the amount of change in the spectrum, corresponds to the amount of polishing (i.e., the amount of film removed or the change in film thickness), the polishing end point can be detected accurately without being affected by the variation in thickness of the underlying film between substrates. Moreover, even if the substrate has a complicated multilayer structure, the amount of cumulative change in the spectrum increases monotonically during polishing, basically. Therefore, it is easy to grasp the progress of substrate polishing from the amount of cumulative change in the spectrum. That is, by simply comparing the amount of cumulative change in the spectrum with a predetermined target value or threshold value, the polishing end point can be detected easily."

For additional information on this patent, see: Kobayashi, Yoichi. Polishing Monitoring Method, Polishing Method, and Polishing Monitoring Apparatus. U.S. Patent Number 8773670, filed October 9, 2013, and published online on July 8, 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=8773670.PN.&OS=PN/8773670RS=PN/8773670

Keywords for this news article include: Electronics, Semiconductor, Ebara Corporation.

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Source: Electronics Newsweekly


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