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Researchers Submit Patent Application, "Substrate Holding Apparatus and Polishing Apparatus", for Approval

June 4, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventor SHINOZAKI, Hiroyuki (Tokyo, JP), filed on November 14, 2013, was made available online on May 22, 2014.

The patent's assignee is Ebara Corporation.

News editors obtained the following quote from the background information supplied by the inventors: "The present invention relates to a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against a polishing pad (polishing surface), and more particularly to a substrate holding apparatus for holding a substrate such as a semiconductor wafer in a polishing apparatus for polishing and planarizing the substrate. Further, the present invention relates to a polishing apparatus having such substrate holding apparatus.

"In recent years, high integration and high density in semiconductor device demands smaller and smaller wiring patterns or interconnections and also more and more interconnection layers. Multilayer interconnections in smaller circuits result in greater steps which reflect surface irregularities on lower interconnection layers. An increase in the number of interconnection layers makes film coating performance (step coverage) poor over stepped configurations of thin films. Therefore, better multilayer interconnections need to have the improved step coverage and proper surface planarization. Further, since the depth of focus of a photolithographic optical system is smaller with miniaturization of a photolithographic process, a surface of the semiconductor device needs to be planarized such that irregular steps on the surface of the semiconductor device will fall within the depth of focus.

"Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). In the chemical mechanical polishing, while a polishing liquid containing abrasive particles such as silica (SiO.sub.2) therein is supplied onto a polishing surface of a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface and polished by using the polishing apparatus.

"The polishing apparatus which performs the above-mentioned CMP process includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding apparatus, which is referred to as a top ring or a polishing head, for holding a substrate such as a semiconductor wafer. When the substrate is polished with such a polishing apparatus, the substrate is held and pressed against the polishing surface of the polishing pad under a predetermined pressure by the substrate holding apparatus. At this time, the polishing table and the substrate holding apparatus are moved relative to each other to bring the substrate into sliding contact with the polishing surface, so that the surface of the substrate is polished to a flat mirror finish.

"If a relative pressing force produced between the substrate being polished and the polishing surface of the polishing pad is not uniform over the entire surface of the substrate, then the substrate is insufficiently or excessively polished depending on the pressing force applied to each area of the substrate. Therefore, it has been attempted that a substrate-holding surface of the substrate holding apparatus is formed by an elastic membrane made of an elastic material such as rubber, and a plurality of pressure chambers to which a pressurized fluid is supplied are formed at the reverse side of the elastic membrane and a fluid pressure such as air pressure is applied to the pressure chambers to uniform the pressing force applied to the substrate over the entire surface of the substrate.

"In the above polishing apparatus, when the substrates are polished using a polishing pad made of synthetic resin, the polishing pad is progressively worn each time it is dressed and with the passage of polishing time. In order to keep the surface pressure distribution unchanged on the substrate held by the top ring, it is necessary to keep the distance between the top ring and the polishing pad constant during polishing.

"When a product substrate is processed, the top ring is moved by a servomotor to a contact position (height), between a substrate held by the top ring and the polishing pad, determined beforehand by an operation referred to as pad search which is a function or action for determining a reference height position for polishing, and the product substrate is polished at the contact position (height) in a positioning control state. In some cases, the top ring is lifted from the contact position (height) to a certain height corresponding to a clearance between the polishing pad and the membrane, and the product substrate is polished in a positioning control state.

"In the operation of pad search for determining the above contact position, for example, if the contact position is determined by simply measuring a distance to the polishing pad by a distance measuring device or the like, it may cause considerable error because the polishing pad is made of an elastic material and has a concavo-convex surface. Therefore, it has been customary that the top ring is lowered from a lifted position onto the surface of the polishing pad to detect a contact force against the polishing pad, thereby determining the contact position. The contact force is detected by monitoring an output torque (output current) of the servomotor of a positioning mechanism for lifting and lowering the top ring.

"The function or action referred to as the pad search is a function for determining the contact position (height) between the substrate held by the top ring and the polishing pad from a certain reference height (for example, a transfer height in a horizontal direction) of the top ring. When performing this function, a torque limit value which has been obtained beforehand is set, and the top ring is stopped at the position at which the top ring is brought into contact with the polishing pad and the output torque of the servomotor becomes the preset torque limit value, and then this position is set as a reference position (height) for polishing.

"The pad search operation will be described further in detail below. The torque limit value for the pad search is determined beforehand. The method for determining the torque limit value is as follows: For example, first, an initial value of the torque limit value is set from a design value. Then, a load cell is placed on the polishing table to which the polishing pad is attached, and the top ring is lowered until the set torque limit value (initial value) is reached, and a lowering thrust force of the top ring shaft is measured by the load cell to determine the relationship between the torque limit value and the thrust force. Specifically, if the measured value of the thrust force is larger than the reference (design) thrust force (value having a certain range), the torque limit value is made smaller, and the same measurement is performed again. Conversely, if the measured value of the thrust force is smaller than the reference (design) thrust force (value having a certain range), the torque limit value is made larger, and the same measurement is repeated, whereby the torque limit value which falls within the range of the reference (design) thrust force (value having a certain range) is determined (searched). The torque limit value thus determined is set as a torque limit value for the pad search.

"The inventors of the present invention have conducted various experiments and obtained the following knowledge. Specifically, in the polishing apparatus described above, a loss torque of the vertical movement mechanism for vertically moving the top ring is decreased by operation history, compared to startup operation. Because the limit value is set for the motor torque of the vertical movement mechanism to determine the position at which the top ring is brought into contact with the surface of the polishing pad, the change of the loss torque of the vertical movement mechanism has an effect on a thrust force for pressing the top ring against the surface of the polishing pad. When the loss torque of the vertical movement mechanism is reduced, the thrust force for pressing the top ring against the surface of the polishing pad is increased by an amount corresponding to the reduction, thus pressing the top ring against the polishing pad with more powerful force. Consequently, if the pad search is conducted at a predetermined period determined by the number of processed substrates, the wear amount of a retainer ring, or the like, the top ring is pressed excessively. Therefore, a gap between the surface of the polishing pad and the elastic membrane provided in the top ring for pressing the substrate cannot be kept constant. As a result, the polishing process condition varies, causing adverse effect on a process performance, such as poor uniformity of the surface, being polished, of the substrate."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventor's summary information for this patent application: "Based on the above knowledge obtained from various experiments, the present invention has been made. It is therefore an object of the present invention to provide a substrate holding apparatus which can detect a time-dependent change of loss torque of a vertical movement mechanism for moving a top ring vertically to correct a torque limit value, used as a reference for a pad search, by using the detected time-dependent change of the loss torque, thereby obtaining an accurate height of a surface of a polishing pad at the time of the pad search.

"Another object of the present invention is to provide a polishing apparatus having such substrate holding apparatus.

"In order to achieve the above object, according to one aspect of the present invention, there is provided a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against a polishing pad, the substrate holding apparatus comprising: a top ring configured to hold the substrate and press the substrate against the polishing pad; a vertical movement mechanism configured to vertically move the top ring; a torque detector configured to detect a torque of the vertical movement mechanism when the top ring is being lowered or being lifted by the vertical movement mechanism; and a controller in which a torque of the vertical movement mechanism when the top ring is brought into contact with a surface of the polishing pad at the time of a pad search is preset as a torque limit value, the pad search being defined as a process in which the top ring is lowered and brought into contact with the surface of the polishing pad; wherein the controller calculates a torque correction amount from the torque detected by the torque detector and a preset reference value, and corrects the torque limit value by using the torque correction amount.

"According to the present invention, a torque of the vertical movement mechanism when the top ring is brought into contact with the surface of the polishing pad, at the time of the pad search in which the top ring is lowered and brought into contact with the surface of the polishing pad, is preset as the torque limit value. A torque of the vertical movement mechanism is detected when the top ring is being lowered or being lifted at the time of the pad search or the substrate processing, then a torque correction amount is calculated from the detected torque and the reference value determined by the torque of the vertical movement mechanism at the time of the preceding pad search, the preceding substrate polishing process, or the like. Then, the preset torque limit value is corrected by using the torque correction amount.

"According to the present invention, the torque of the vertical movement mechanism for the top ring is detected at the time of the preceding pad search or the preceding substrate polishing process, and the detected torque is set as the reference value. The torque of the vertical movement mechanism for the top ring at the time of a subsequent pad search or a subsequent substrate polishing process is detected, and a time-dependent change of loss torque of the vertical movement mechanism is detected by comparing the detected torque and the reference value. The time-dependent change of loss torque is determined as a torque correction amount, then the torque limit value for the pad search is corrected by using the torque correction amount. Accordingly, even if the loss torque of the vertical movement mechanism for the top ring is changed with the passage of time, the height of the surface of the polishing pad can be detected by substantially the same pressing force (thrust force) at the time of the pad search as an initial startup of the polishing apparatus.

"In a preferred aspect of the present invention, the reference value is determined from a torque of the vertical movement mechanism at the time of the preceding pad search.

"According to the present invention, the torque of the vertical movement mechanism is detected when the top ring is being lowered or being lifted, at the time of the preceding pad search, and the detected torque is set as the reference value.

"In a preferred aspect of the present invention, the reference value is determined from a torque of the vertical movement mechanism at the time of the preceding substrate polishing process.

"According to the present invention, the torque of the vertical movement mechanism is detected when the top ring is being lowered or being lifted, at the time of the preceding substrate polishing process, and the detected torque is set as the reference value.

"In a preferred aspect of the present invention, the reference value is a center value of a threshold having a predetermined range with an upper limit and a lower limit.

"In a preferred aspect of the present invention, the center value of the threshold is an average torque when the top ring is being lowered or lifted at a constant velocity.

"According to the present invention, an average torque of the vertical movement mechanism when the top ring is being moved at a constant velocity corresponds to a torque in a stable condition where an acceleration of the movement of the top ring is zero. Thus, the torque corresponds to a sum of a mechanical loss of the vertical movement mechanism and a gravity load (mass). The torque during lowering of the top ring corresponds to a value obtained by subtracting the gravity load from the mechanical loss. The torque during lifting of the top ring corresponds to a value obtained by adding the mechanical loss to the gravity load. Since the gravity load is estimated as a constant value, a time-dependent change of the mechanical loss can be detected by monitoring the torque when the top ring is being lifted or being lowered. Therefore, the reference value should be an average torque at a constant velocity movement when the top ring is being lowered or being lifted by the vertical movement mechanism in the operation for initially determining the torque limit value. The reference value, i.e. the center value of the threshold, can be an accurate value with less error. At this time, in the case where an average torque during lowering of the top ring is used as the reference value, the time-dependent change of mechanical loss is obtained by monitoring the average torque when the top ring is being lowered in subsequent operations.

"In a preferred aspect of the present invention, the torque correction amount is a difference between the torque detected by the torque detector and the reference value.

"According to the present invention, in the case where the torque detected by the torque detector becomes smaller than the preset reference value, a difference between the detected torque and the reference value is subtracted from the torque limit value which has been used for the pad search. The reason of subtraction is that the mechanical loss of the vertical movement mechanism is decreased to reduce the torque needed for moving the top ring, and thus a thrust force for pressing the top ring against the surface of the polishing pad is increased if the torque limit value for pad search remains the same. On the other hand, in the case where the torque detected by the torque detector becomes larger than the preset reference value, the difference between the detected torque and the reference value is added to the torque limit value which has been used for the pad search.

"In a preferred aspect of the present invention, the torque detector detects the torque when the top ring is being lowered or lifted at a constant velocity.

"According to the present invention, since the torque of the vertical movement mechanism when the top ring is being moved at a constant velocity corresponds to a torque in a stable condition where an acceleration of the movement of the top ring is zero, the torque detector can detect an accurate torque without error.

"According to another aspect of the present invention, there is provided a polishing apparatus for polishing a substrate, comprising: a polishing table having a polishing pad; and a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against the polishing pad; the substrate holding apparatus comprising: a top ring configured to hold the substrate and press the substrate against the polishing pad; a vertical movement mechanism configured to vertically move the top ring; a torque detector configured to detect a torque of the vertical movement mechanism when the top ring is being lowered or being lifted by the vertical movement mechanism; and a controller in which a torque of the vertical movement mechanism when the top ring is brought into contact with a surface of the polishing pad at the time of a pad search is preset as a torque limit value, the pad search being defined as a process in which the top ring is lowered and brought into contact with the surface of the polishing pad; wherein the controller calculates a torque correction amount from the torque detected by the torque detector and a preset reference value, and corrects the torque limit value by using the torque correction amount.

"According to the present invention, since the substrate holding apparatus has a function to correct the torque limit value for detecting that the top ring is brought into contact with the surface of the polishing pad at the time of the pad search by the top ring, the time-dependent change of loss torque of the vertical movement mechanism for the top ring can be corrected. Accordingly, even if the loss torque of the vertical movement mechanism for the top ring is changed with the passage of time, the height of the surface of the polishing pad can be detected at the time of the pad search by substantially the same pressing force (thrust force) as the initial startup of the polishing apparatus. As a result, a chemical mechanical polishing apparatus in which a gap between the surface of the polishing pad and the elastic membrane of the top ring, having an important role in a polishing process, can be highly-reproducibly controlled, can be provided.

"According to still another aspect of the present invention, there is provided a polishing method for polishing a substrate by holding the substrate and pressing the substrate against a polishing pad on a polishing table by a top ring, comprising: preset a torque limit value of a vertical movement mechanism for vertically moving the top ring; detecting a torque of the vertical movement mechanism by lowering or lifting the top ring; calculating a torque collection amount from the detected torque and a preset reference value; collecting the torque limit value by using the torque collection amount; and lowering the top ring until the collected torque limit value is reached, and polishing the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is a schematic view showing an entire structure of a polishing apparatus according to an embodiment of the present invention."

For additional information on this patent application, see: SHINOZAKI, Hiroyuki. Substrate Holding Apparatus and Polishing Apparatus. Filed November 14, 2013 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=2380&p=48&f=G&l=50&d=PG01&S1=20140515.PD.&OS=PD/20140515&RS=PD/20140515

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

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Source: Electronics Newsweekly


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