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  Eddy current sensing of metal removal for chemical mechanical polishingUSPTO Application #: 20060009128Title: Eddy current sensing of metal removal for chemical mechanical polishing Abstract: A sensor for monitoring a conductive film in a substrate during chemical mechanical polishing generates an alternating magnetic field that impinges a substrate and induces eddy currents. The sensor can have a core, a first coil wound around a first portion of the core and a second coil wound around a second portion of the core. The sensor can be positioned on a side of the polishing surface opposite the substrate. The sensor can detect a phase difference between a drive signal and a measured signal. (end of abstract) Agent: Fish & Richardson P.C. - Minneapolis, MN, US Inventors: Hiroji Hanawa, Nils Johansson, Boguslaw Swedek, Manoocher Birang USPTO Applicaton #: 20060009128 - Class: 451005000 (USPTO) Related Patent Categories: Abrading, Precision Device Or Process - Or With Condition Responsive Control, Computer Controlled The Patent Description & Claims data below is from USPTO Patent Application 20060009128. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation application and claims the benefit of priority under 35 U.S.C. Section 120 of U.S. application Ser. No. 10/447,165, filed May 27, 2003, which is a divisional of U.S. application Ser. No. 09/574,008, filed on May 19, 2000, now U.S. Pat. No. 6,924,641. The disclosure of each prior application is considered part of and is incorporated by reference in the disclosure of this application. BACKGROUND [0002] The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for monitoring a metal layer during chemical mechanical polishing. [0003] 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 until the non-planar surface is exposed. For example, a conductive filler layer can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. The filler layer is then polished until the raised pattern of the insulative layer is exposed. 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. In addition, planarization is needed to planarize the substrate surface for photolithography. [0004] 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 placed against a rotating polishing disk pad or belt pad. The polishing pad can be either a "standard" pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad. [0005] 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. Overpolishing (removing too much) of a conductive layer or film leads to increased circuit resistance. On the other hand, underpolishing (removing too little) of a conductive layer leads to electrical shorting. 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, the polishing endpoint cannot be determined merely as a function of polishing time. [0006] One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate can be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time-consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable. [0007] More recently, in-situ monitoring of the substrate has been performed, e.g., with optical or capacitance sensors, in order to detect the polishing endpoint. Other proposed endpoint detection techniques have involved measurements of friction, motor current, slurry chemistry, acoustics and conductivity. One detection technique that has been considered is to induce an eddy current in the metal layer and measure the change in the eddy current as the metal layer is removed. Unfortunately, the proposed eddy current sensing techniques typically require complex electronics. In addition, the sensors are positioned on the backside of the substrate. Since the magnetic field of the sensor extends toward the platen, special shielding is needed to prevent the metal platen from interfering with the eddy current measurements. SUMMARY [0008] In one aspect, the invention is directed to a sensor for monitoring a conductive film in a substrate. The sensor has a core positionable in proximity to the substrate, a first coil wound around a first portion of the core, an oscillator electrically coupled to the first coil to induce an alternating current in the first coil and generate an alternating magnetic field in proximity to the substrate, and a second coil wound around a second portion of the core. A capacitor is electrically coupled to the second coil, and an amplifier is electrically coupled to the second coil and the capacitor to generate an output signal. [0009] Implementations of the invention may include one or more of the following features. The oscillator may induce an alternating current with a frequency selected to provide a resonant frequency when the substrate is not in proximity to the core. The core may consist essentially of ferrite, and may includes two prongs and a connecting portion between the two prongs. The first coil may be wound around the connecting portion, and the second coil may be wound around at least one of the two prongs. The second coil and the capacitor may be connected in parallel. The sensor may be positioned on a side of a polishing pad opposite the substrate. The polishing pad may includes an upper layer and a lower layer, and an aperture may be formed in at least a portion of the lower layer adjacent the core. A computer may receive the output signal. [0010] In another aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a polishing pad, a carrier to hold a substrate against a first side of the polishing surface, an eddy current sensor, and a motor coupled to at least one of the polishing pad and carrier head for generating relative motion therebetween. The sensor includes at least one inductor positioned on a second side of the polishing pad opposite the substrate, an oscillator electrically coupled to the at least one inductor to induce an alternating current in the coil and generate an alternating magnetic field, and a capacitor electrically coupled to the at least one inductor. [0011] Implementations of the invention may include one or more of the following features. A platen may support the polishing pad, and the at least one inductor may be positioned in a recess in a top surface of the platen. The platen may rotates, and a position sensor may determine an angular position of the platen and a controller to sample data from the eddy current sensor when the at least one inductor is positioned adjacent the substrate. A recess may be formed in the second side of the polishing pad. The polishing pad may include a cover layer on the first side of the polishing pad and a backing layer on the second side of the polishing pad, and the recess may be formed by removing a portion of the backing layer. The eddy current sensor may include a core having two poles positioned adjacent the recess in the polishing pad, and the at least one inductor is wound around a first portion of the core. The eddy current sensor may include a core, and the at least one inductor may include a first inductor wound around a first portion of the core and a second inductor wound around a second portion of the core. The oscillator may be electrically coupled to the first coil to induce an alternating current in the first coil. The capacitor may be electrically coupled to the second coil. The oscillator may induce an alternating current with a frequency selected to provide a resonant frequency when the substrate is not in proximity to the core. An endpoint detection system may receive an output signal from the eddy current sensor. The endpoint detection system may be configured to signal a polishing endpoint if the output signal exceeds a predetermined threshold. [0012] In another aspect, the invention may be directed to a method of monitoring a thickness of a conductive layer in a substrate during a polishing operation. In the method, a substrate is positioned on a first side of a polishing surface, and an alternating magnetic field is generated from an inductor positioned on a second side of the polishing surface opposite the substrate. The magnetic field extends through the polishing surface to induce eddy currents in the conductive layer. A change in the alternating magnetic field caused by a change in the thickness of the conductive layer is detected. [0013] Implementations of the invention may include one or more of the following features A first coil may be driven with an oscillator at a first frequency. The first frequency may be a resonant frequency when the substrate is not in proximity to the magnetic field. The alternating magnetic field may be sensed with a second coil. The second coil may be connected in parallel with a capacitor. The first coil may be wound around a first portion of a core, and the second coil may be wound around a second portion of the core. When the inductor is adjacent the substrate may be determined. The inductor may be driven with a first signal, and a second signal may be generated from the alternating magnetic field. A change in amplitude in the second signal may be determined. A change in a phase difference between the first signal and the second signal may be determined. [0014] In another aspect, the invention is directed to a method of chemical mechanical polishing. In the method, a substrate having a conductive layer is positioned on a first side of a polishing surface. An alternating magnetic field is generated from an inductor positioned on a second side of the polishing surface opposite the substrate. The magnetic field extends through the polishing surface to induce eddy currents in the conductive layer. Relative motion is created between the substrate and the polishing surface to polish the conductive layer. The eddy currents in the substrate are sensed, and polishing is halted when the sensed eddy currents exhibit an endpoint criteria. [0015] Implementations of the invention may include one or more of the following features. The endpoint criteria may be the eddy currents passing a threshold strength or leveling off. [0016] In another aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a polishing pad with a polishing surface, a carrier to hold a substrate against the polishing surface, a motor coupled to at least one of the polishing pad and carrier head for generating relative motion therebetween, and a conductive layer thickness monitoring system. The conductive layer thickness monitoring system including at least one inductor, a current source that generates a drive signal, the current source electrically coupled to the at least one inductor to induce an alternating current in the at least one inductor and generate an alternating magnetic field, sense circuitry including a capacitor electrically coupled to the at least one inductor to sense the alternating magnetic field and generate a sense signal, and phase comparison circuitry coupled to the current source and the sense circuitry to measure a phase difference between the sense signal and the drive signal. [0017] Implementations of the invention may include one or more of the following features. At least one first gate, e.g., an XOR gate, may convert sinusoidal signals from the inductor and the oscillator into first and second square-wave signals. A comparator, e.g., an XOR gate, may compare the first square-wave signal to the second square-wave signal to generate a third square-wave signal. A filter may convert the third square-wave signal into differential signal having an amplitude proportional to the phase difference between the first and second square wave signals. The phase comparison circuitry may generate a signal with a duty cycle proportional to the phase difference. [0018] In another aspect, the invention may be directed to a method of monitoring a thickness of a conductive layer on a substrate during a chemical mechanical polishing operation. In the method, a coil is energized with a first signal to generate an alternating magnetic field. The alternating magnetic field induces eddy currents in a conductive layer of the substrate. The alternating magnetic field is measured and a second signal is generated indicative of the magnetic field. Te first and second signals are compared to determine a phase difference therebetween. [0019] Implementations of the invention can include zero or more of the following possible advantages. The endpoint detector can sense the polishing endpoint of a metal layer in-situ. The magnetic field apparatus for the endpoint detector can be embedded in the platen below a polishing pad. The magnetic field apparatus can be protected from polishing environment, e.g., corrosive slurry. The endpoint detector need not use complex electronics. Polishing can be stopped with reasonable accuracy. Overpolishing and underpolishing substrate can be reduced, thereby improving yield and throughput. [0020] Other features and advantages of the invention will become apparent from the following description, including the drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading... Full patent description for Eddy current sensing of metal removal for chemical mechanical polishing Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Eddy current sensing of metal removal for chemical mechanical polishing patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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