US20070243796A1

US20070243796A1 - Method and apparatus for polishing a wafer with a higher in-plane uniformity

Info

Publication number: US20070243796A1
Application number: US11/734,622
Authority: US
Inventors: Koji Torii
Original assignee: Elpida Memory Inc
Current assignee: Micron Memory Japan Ltd
Priority date: 2006-04-13
Filing date: 2007-04-12
Publication date: 2007-10-18
Also published as: JP2007287787A

Abstract

A wear thickness for a retainer ring is measured at a specified timing. The height of the retainer ring or the polishing condition of the polishing apparatus is changed in an amount greater than or equal to the wear thickness of the retainer ring. This compensates for the wear thickness of the retainer ring which degrades the within wafer uniformity of polishing rate due to a change in the bottom surface of the retainer ring.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method and an apparatus for polishing a wafer. More specifically, the present invention relates to improvement of a chemical-mechanical polishing (CMP) apparatus for polishing a semiconductor wafer in a process for manufacturing a semiconductor device.
2. Description of the Related Art
Along with development of a finer design rule in the semiconductor fabrication process, a higher degree of flatness is requested for the surface of a semiconductor wafer in each step of the process for manufacturing semiconductor devices.
For achieving a flat surface of the film on the wafer, attempts have been made to satisfy the requested degree of flatness by improving the flatness of the surface of the as-deposited film itself, and by using a reflow technique such as annealing coated films including a SOG (spin on glass) film and a BPSG (borophospho-silicate glass) film at a high temperature. The CMP technique, which directly polishes the surface of the semiconductor wafer, has been increasingly used since the design rule for the semiconductor device was settled at as low as 0.35 micrometers or smaller. The CMP process is generally effective for eliminating the step difference on the surface of the semiconductor wafer; however, causes a relatively poor within wafer uniformity in a larger wafer area after the polishing. In recent years, semiconductor devices are fabricated on a larger-diameter wafer having a diameter of 300 mm, for example. This necessitates a further improvement of the within wafer uniformity. It is therefore difficult to achieve a requested within wafer uniformity only by using a commercially available CMP apparatus.
In general, the polishing process of a conventional technique lo includes the steps of: holding a semiconductor wafer on a polishing pad of a polishing head, grounding a retainer ring of the polishing head while pressing the retainer ring with a specified pressure, applying a specific pressure onto the wafer via an air bag, rotating the polishing pad with respect to the wafer, and supplying an abrasive from an abrasive nozzle.
In the above polishing process, the surface of the wafer is polished by means of the basic operations including applying the pressure, applying the rotational motion, and supplying the abrasive. During the polishing, the periphery of the wafer is in contact with a boundary area in which the polishing pad is applied with a specific pressure. It is generally known that the pressing force is uneven on the peripheral region of the wafer that is located within several millimeters from the boundary area due to the reaction of a downward deformation of the polishing pad.
Use of the retainer ring can shift the unevenly pressed region from the bottom surface of the periphery of the semiconductor wafer toward the bottom surface of the retainer ring. Changing the pressure applied to the retainer ring can change the polishing speed profile in the vicinity of the periphery of the semiconductor wafer.
As a problem in the polishing of the semiconductor wafer, the retainer ring, which is made of resin in general, wears down after a number of wafers are polished, whereby the polishing speed profile varies in the vicinity of the periphery of the semiconductor wafer even if the same polishing condition is used. The change of polishing characteristics due to wearing of the retainer ring will be described hereinafter with reference to the accompanying drawings.
FIG. 5 schematically shows the polishing head of a typical CMP apparatus. The head 31 of the CMP apparatus is a principal component that presses the semiconductor wafer 19 against the polishing pad 20 to effect a polishing operation. In most cases, the retainer ring 10 is positively grounded on the polishing pad 20. A retainer ring 10 may not be grounded in another type of polishing apparatus such as manufactured before the early 1990s or depending on the type of objects to be polished. The retainer ring 10 has a function of guiding the semiconductor wafer 19 as an object to be polished and adjusting the uniformity in the vicinity of the periphery of the semiconductor wafer 19.
General polishing operations include the steps of grounding the retainer ring 10 of the polishing head 31, which holds the semiconductor wafer 19 on the polishing pad 20 with a specified pressure, and applying a pressure onto the semiconductor wafer 19 via elastic membranes 18 a, 18 b, 18 c. Further, the polishing pad 20 is rotated and an abrasive is supplied from an abrasive nozzle 21 during the rotation. In this way, the surface of the semiconductor wafer 19 is polished by the basic operations including the steps of applying the pressure, applying the rotational motion, and supplying the abrasive.
FIG. 16 schematically shows the configuration and operation of the retainer ring 10 during the polishing, wherein the structure of the peripheral portion of the polishing head 31 is shown in the top figure, and the pressure applied to the polishing pad 20 is plotted in the bottom figure against the radial position of the polishing pad 20. The membranes encircled by the retainer ring 10 include a disk-shaped first membrane 18 c which occupies most of the effective pressing surface for the semiconductor wafer 19, an annular second membrane 18 b disposed on the outer periphery of the first membrane 18 c, and an annular third membrane 18 a disposed between the second membrane 18 b and the annular retainer ring 10. These membranes 18 a, 18 b, 18 c are pressed against the surface of the semiconductor wafer 19 by a membrane support 22.
A constant pressure is applied onto the semiconductor wafer 19 via the membranes 18 a, 18 b, 18 c. The periphery of the semiconductor wafer 19 is located on the boundary area at which the pressure is applied onto the polishing pad 20. As understood from the bottom figure of FIG. 16, it is generally known that the pressure applied to the peripheral area of the semiconductor wafer 19 as a reaction to the downward deformation of the polishing pad 20 is not uniform, the peripheral area residing within several millimeters from the boundary of the membrane 18 c. The use of the retainer ring 10 can shift the uneven pressure area from the bottom surface of the periphery of the semiconductor wafer 19 to the bottom surface of the retainer ring 10, as shown in the bottom figure. Changing the pressure applied onto the retainer ring 10 allows the polishing speed profile in the vicinity of the periphery of the semiconductor wafer 19 to be changed.
Several patent publications describe changing of the polishing characteristics based on wear of the retainer ring 10. The following describes outlines of those patent publications and the relationship between the technique of the present invention and the techniques of these patent publications.
Patent Publication JP-2001-25962A describes a technique for efficiently detecting the ting for replacing the retainer ring which cannot provide the intended polishing characteristic. However, this invention does not have the object or configuration for controlling the polishing characteristics by the retainer ring.
Patent Publication JP-2003-25217A describes a structure for adjusting the height of the retainer ring. However, this invention does not teach the way or extent of the adjustment for adjusting the height of the retainer ring.
Patent Publication JP-2003-273047A describes a polishing apparatus including means for measuring the height of the retainer ring, means for correcting the height, and means for repairing the uneven wear thickness of the retainer ring. In this invention, the retainer ring is repaired by grinding the non-wearing portion of the retainer ring for achieving an even wear thickness, and accordingly, the retainer ring will have a shorter lifetime due to the grinding repair of the retainer ring.
The problems encountered due to the wear of the retainer ring in the conventional CMP apparatus will be further described hereinafter. FIG. 17 shows the principal part of the polishing head which holds the wafer 19 in the polishing apparatus. Although there are several types of the polishing head, the polishing head shown in FIG. 17 has a support (membrane support) 22 that supports the single membrane 18 and is fixed onto the polishing head. Since the periphery of the semiconductor wafer 19 is supported in this structure, after the retainer ring 10 wears to change the relative distance with respect to the membrane support 22, the elastic body of the membrane 18 changes the reactive force in the vicinity of the worn periphery of the retainer ring 10. The degree of the change in the reactive force is larger at the portion nearer to the retainer ring 10, whereby the effective pressure applied to the wafer 19 is likely to change in the wear portion of the retainer ring 10.
FIG. 18 shows the relationship between the wear thickness of the retainer ring, which is represented by the depth of a groove formed on the retainer ring, and the range of variation in the height of the wafer surface polished using the CMP process. Although the range of variation in the height of the wafer surface partly depends on the initial uniformity of the wafer surface before the polishing, the improvement of the range of variation is also achieved by the within wafer uniformity of polishing rate of the polishing device, and thus this figure is considered to show the correlation between the wear thickness of the retainer ring and the within wafer uniformity of the wafer surface. The solid line in FIG. 18 represents the data thus acquired, and the dotted line will be described later. As will be understood from the solid line, as the wear of the retainer ring 10 proceeds the within wafer uniformity is degraded on the polished surface of the semiconductor wafer.
It is to be noted that, the retainer ring generally has a wear thickness (or wear amount) which is approximately 0.1 to 0.5 mm after polishing 1000 wafers, depending on the material of the retainer ring. It has been the general practice that the retainer ring is discarded after it has an approximately 0.5- to 1.5-mm wear thickness. There is a trade-off between the polishing performance which is affected by the change of the polishing characteristics of the retainer ring due to the wear and the polishing cost caused by replacement of the worn retainer ring.
In consideration for the above situation, the inventor conducted an experiment in which the polishing condition was modified by changing the height of the retainer ring in an amount corresponding to the wear thickness of the retainer ring. The modification acted so as to suppress the change in the within wafer uniformity of polishing rate; however, could not restore the original within wafer uniformity, as shown by the broken line depicted in FIG. 18. As a result of analyzing the cause of the difference, it was found that a change in the shape of the retainer ring, as well as the change of the thickness, had a significant effect thereon.
Along with the progress of the polishing operation, the retainer ring has an increased wear thickness and changes the shape thereof so that the bottom of the ring periphery is curved. The increase of the wear thickness and the change of the shape of the retainer ring change is the effective pressure applied onto the portion of the wafer in the vicinity of the retainer ring. The two factors, i.e., the wear thickness and the shape change affect, in the same direction and also synergistically, the effective pressure applied onto the wafer. As the polishing operation advances, these two factors change the pressure applied onto the periphery of the wafer even under the same polishing condition in the direction along which the pressure effectively and strongly acts. Thus, if the polishing condition is optimized for a new retainer ring, the within wafer uniformity of polishing rate will gradually degrades along with the progress of the polishing time.
In view of the above, it is an object of the present invention to provide a method and an apparatus for polishing a semiconductor wafer in a semiconductor manufacturing process, which is capable of polishing the semiconductor wafer with a better within wafer uniformity of polishing rate and increasing the lifetime of the retainer ring by reducing the change in the polishing characteristic of the retainer ring caused by the wear thickness and the shape change of the retainer ring.
The present invention provides, in a first aspect thereof, a method for polishing an object wafer by using a polishing apparatus including a retainer ring for guiding the object wafer in an in-plane direction of the object wafer, a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer, and a polishing pad for polishing a top surface of the object wafer, the method including the steps of: obtaining a wear thickness of the retainer ring at a specific timing during or after polishing the object wafer in a first polishing condition; calculating a correction amount of a polishing condition based on the obtained wear thickness; and polishing the object wafer in a second polishing condition obtained by adding the calculated correction amount to the first polishing condition.
The present invention provides, in a second aspect thereof, a method for polishing an object wafer by using a polishing apparatus including a retainer ring for guiding the object wafer in an in-plane direction of the object wafer, a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer, and a polishing pad for polishing a top surface of the object wafer, the method including the steps of: obtaining a wear thickness of the retainer ring at a specific timing during or after polishing the object wafer by using the retainer ring located at a first height; calculating a correction amount of a height of the retainer ring based on the obtained wear amount, the calculated correction amount being larger than a correction amount corresponding to the obtained wear thickness; and polishing the object wafer by using the retainer ring located at a second height obtained by adding the calculated correction amount to the first height.
The present invention provides, in a third aspect thereof, a polishing apparatus including: a retainer ring for guiding an object wafer in an in-plane direction of the object wafer; a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer; a polishing pad for polishing a top surface of the object wafer: a wear thickness determination unit for determining a wear thickness of the retainer ring at a specific timing during or after polishing the object wafer in a first polishing condition; a calculation unit for calculating a correction amount of the polishing condition based on the determined wear thickness; and a polishing unit for polishing the object wafer in a second polishing condition obtained by adding the calculated correction amount to the first polishing condition.
The present invention provides, in a fourth aspect thereof, a polishing apparatus including: a retainer ring for guiding an object wafer in an in-plane direction of the object wafer, a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer, a polishing pad for polishing a top surface of the object wafer; a wear thickness determination unit for determining a wear thickness of the retainer ring at a specific timing after or during polishing the object wafer in a first polishing condition; a calculation unit for calculating a correction amount of the retainer ring based on the determined wear thickness, the calculated correction amount being is larger than a correction amount corresponding to the determined wear thickness; and a polishing unit for polishing the object wafer in a second polishing condition obtained by adding the calculated correction amount to the first polishing condition.
In accordance with the first and third aspects of the present invention, the wear thickness of the retainer ring is compensated for by the change of the process condition determined based the wear thickness and the original process condition. This allows a suitable compensation superior to the conventional technique which merely changes the height of the retainer ring by an amount corresponding to the wear thickness.
In accordance with the second and fourth aspects of the present invention and the preferred embodiment of the first and third aspect of the present invention, the polishing apparatus uses a polishing condition which is larger than the polishing condition corresponding to the wear thickness of the retainer ring. This provides a superior compensation for the wear thickness of the retainer ring, substantially allowing restoration of the original polishing performance achieved by the retainer ring.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a polishing apparatus, according to a first embodiment of the present invention, disposed in a semiconductor manufacturing system;

FIG. 2 is a sectional view of a measurement unit for measuring the thickness of the retainer ring in the polishing apparatus shown in FIG. 1;

FIG. 3 is a block diagram of the polishing apparatus shown in FIG. 1;

FIG. 4 is a flowchart showing the operation of the polishing apparatus of FIG. 3;

FIG. 5 is a sectional view of a typical polishing apparatus;

FIG. 6 is a graph showing the relationship obtained between the thickness of the retainer ring in the first embodiment and the within wafer uniformity of polishing rate;

FIG. 7 is a block diagram of a polishing apparatus according to a second embodiment of the present invention;

FIG. 8 is a sectional view of the peripheral portion of the polishing apparatus of FIG. 7;

FIG. 9 is a flowchart showing the operation of the polishing apparatus of FIG. 7;

FIG. 10 is a graph showing the relationship obtained between the thickness of the retainer ring in the second embodiment and the within wafer uniformity of polishing rate;

FIG. 11 is a block diagram of a polishing apparatus according to a third embodiment of the present invention;

FIG. 12 is a flowchart of the operation of the polishing apparatus of FIG. 11;

FIG. 13 is a block diagram of a polishing apparatus according to a fourth embodiment of the present invention;

FIG. 14 is a flowchart of the operation of the polishing apparatus of FIG. 13;

FIG. 15 is a flowchart showing the operation of a polishing apparatus according to a fifth embodiment of the present invention;

FIG. 16 is a sectional view of the peripheral portion of a conventional polishing apparatus, accompanied by a pressure distribution profile in the polishing apparatus;

FIG. 17 is a sectional view of the peripheral portion of the conventional polishing apparatus, illustrating the problem thereof; and

FIG. 18 is a graph showing the relationship between the wear thickness of the retainer ring in the conventional polishing apparatus and the range of variation in the height of wafer surface polished by the polishing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention is more specifically described with reference to accompanying drawings, wherein similar constituent elements are designated by similar reference numerals throughout the drawings.
FIG. 1 is a top plan view of the principal part of a polishing apparatus, according to a first embodiment of the present invention, disposed in a semiconductor device manufacturing system. The polishing apparatus includes: a polishing head 31 for holding and pressing a semiconductor wafer to be polished; a cross member 32 for applying a swivel movement to the polishing head 31; a platen 33 including a polishing pad and applies a rotational movement thereto for polishing; a dresser 38 for dressing the surface of the polishing pad 31; a load/unload unit 34 for loading/unloading the semiconductor wafer on the polishing head 31; a carriage robot 35 for transferring the semiconductor wafer to/from the polishing apparatus; and a cleaning unit 36.
The polishing apparatus includes, in addition to those general constituent members, a wear measurement unit for measuring the wear thickness of the retainer ring as by measuring the depth of a groove formed on the bottom surface of the retainer ring 10 for the purpose of measurement. The wear measurement unit includes, for example, a laser displacement meter not shown in the figure and an associated sensor 37. The sensor 37 is provided in the vicinity of or inside the load/unload unit 34, or may be provided between two of platens 33 if the polishing apparatus includes a plurality of platens.
Above the laser displacement meter and sensor 37, there may be provided a roof member for protecting the laser displacement meter and sensor 37 against the, droplet of slurry or pure water. The polishing apparatus may include, instead of the laser displacement meter and sensor 37, a thickness measurement unit for directly measuring the current thickness of the retainer ring 10 or vertical position (height) thereof.
FIG. 2 is a side view showing the principal part of the wear measurement unit in the polishing apparatus shown in FIG. 1. The laser displacement meter and the sensor 37 are provided on the load/unload unit 34 disposed below the retainer ring 10 for guiding the wafer. The sensor 37 includes a laser device 37 a for emitting laser onto the bottom surface of the retainer ring 10 or bottom of the groove 11 formed on the bottom surface of the retainer ring 10, and a photodiode 37 b for receiving the laser reflected from the bottom surface of the retainer ring 10 or bottom of the groove 11. The laser displacement meter measures the depth of the groove 11, i.e., the step difference between the bottom surface of the retainer ring 10 and the bottom of the groove 11 based on the time length of the interval between emission of the laser and receipt of the reflected laser.
Instead of the above, it may be preferable in some cases to provide a CCD camera on the side of the polishing head in the polishing apparatus for detecting the wear thickness of the retainer ring 10. The wear thickness may be measured from the change of the thickness or height of the retainer ring 10 by means of visual recognition.
FIG. 3 shows the functional block diagram of the polishing apparatus of FIG. 1. The polishing apparatus includes: a polishing controller 12 that controls a polishing unit 16 including the polishing head as described above; a calculation unit 13 that calculates the amount of change in the thickness of the retainer ring 10 based on the measured thickness of the retainer ring 10 and then calculates an optimum polishing condition based on the change of the thickness of the retainer ring by using a predetermined approximation formula; a storage unit 14 that stores calculation values; a pressure control unit 15 that reflects the calculated value on the pressure condition of the polishing apparatus and changes the pressure to the optimum pressure; an instruction unit 17 that controls those units and supplies an instruction to the polishing unit 16 including the polishing head, and the wear measurement unit 40 as described above. The approximation formula used in the calculation unit 13 is derived from an optimum polishing condition corresponding to each of the wear thickness of the worn retainer ring obtained in advance.
In the above configuration, the polishing apparatus operates so that the wear measurement unit 40 detects a change in the thickness of the retainer ring, the calculation unit 13 calculates the optimum polishing condition based on the change in the thickness; and the instruction unit 17 receives the calculated result and executes the polishing operation based on the calculated optimum polishing condition by way of the polishing unit 16. This makes it possible to provide the polishing apparatus that effectively compensates or negates the defect of the worn retainer ring and ensures a stable polishing characteristic.
A method used in the polishing apparatus of the present embodiment will be described with reference to FIGS. 1 through 5. FIG. 4 shows a flowchart of the processing performed by the polishing apparatus of the present embodiment. After starting a polishing process, the instruction unit 17 first checks a flag to judge whether or not the polishing head or retainer ring is in a state of immediately after replacement i.e., the retainer ring 10 is a new one (Step S1). If the judgment is affirmative, the wear measurement unit 40 measures the thickness of the retainer ring, i.e., the depth of the groove to determine the wear thickness of the retainer ring (Step S3). If the judgment is negative at Step S1, the instruction unit 17 determines whether or not the number of polished wafers exceeds a specific number N determined in advance to reach a lot end (Step S2). If it is judged that the number of polished wafer exceeds N, i.e., the lot end is found, the process advances to Step S3, wherein the wear thickness of the retainer ring 10 is determined based on the measured thickness.
The wear thickness obtained at Step S3 is delivered to the calculation unit 13 (Step S4). Based on the thickness of the retainer ring or the depth of groove, the calculation unit 13 performs a calculation using the approximation formula for obtaining the optimum polishing condition (Step S5). The calculation unit 13 may calculate the wear thickness based on the thickness measured by the thickness measurement unit 40 and then calculates the optimum condition based thereon. The optimum polishing condition obtained by the calculation unit 13 is set and stored in the storage unit 14 (Step S6), and used thereafter for the polishing operation (Step S7). It is then judged whether or not the polishing operation is finished (Step S8). If the result of judgment at Step S8 is negative, the polishing process returns to Step S2 to iterate the above processings.
In the cycle between Step S2 and Step S8, the instruction unit 17 reads the optimum polishing condition provided by the calculation unit 13 from the storage unit 14 (Step S9), until it is judged that the predetermined number (N) of wafers is treated, i.e., the lot end of the wafers is found at Step S2. In this cycle, the polishing unit 16 performs the polishing operation using the optimum polishing condition (Steps S6 to S8).
If it is judged at Step S2 that the number of polished wafers exceeds N, i.e., the lot end is found, the instruction unit 17 allows the wear measurement unit 40 to measure the wear thickness of the retainer ring (Step S3) and transfer the measured wear thickness to the calculation unit 13, for determining the optimum polishing condition. The calculation unit 13 calculates the optimum polishing condition at Step S5, and delivers the calculated optimum polishing condition to the polishing unit 16, which polishes the wafer by using the optimum polishing condition.
For example, the pressure applied by the retainer ring 10 is changed from 10 PSI to 9.8 PSI at Step S6 to change the polishing condition. The polishing apparatus corrects the current polishing condition by using a correction amount calculated based on the wear amount of the retainer ring, and polishes the succeeding wafers or succeeding lot of wafers while using the new polishing condition. There may be a case where the polishing is finished before the number of processed wafers reaches the specified number N. In such a case, the next polishing operation starts after the polishing apparatus checks the flag for the retainer ring (Step S1), and follows the above-described procedure.
The following describes in detail the polishing operation and the measurement of the wear thickness of the retainer ring 10 according to the present embodiment. With reference to FIG. 1, the carriage robot 35 transports a semiconductor wafer from an interface unit 39 of the semiconductor manufacturing system. The load/unload unit 34 allows the polishing head 31 to hold the semiconductor wafer by air suction. The semiconductor wafer is placed onto the platen 33 and is supplied with abrasive. The semiconductor wafer is then contacted by the polishing pad attached onto the rotating platen 33. With reference to FIG. 5, the retainer ring 10 is applied with a suitable pressure to press the wafer 19 via the membranes 18 a, 18 b, 18 c, which are divided into three pieces and pressed to the wafer 19 with specific pressures.
Generally, the dresser 38 is used to treat the surface of the polishing pad 20 after a number of wafers are polished using the polishing apparatus. After the polishing operation is finished, pure water is used to rinse the polishing pad 20, polishing head 31 and wafer 19 The wafer 19 is then removed from the surface of the polishing pad 20 and is moved to the load/unload unit 34. The carriage robot 35 transports the wafer 19 to the cleaning unit 36 for cleaning. The wafer 19 is then received in the FOUP of the interface unit 39 of the semiconductor manufacturing system.
There has been described a general flow of the process heretofore. The process includes the step of measuring the wear thickness of the retainer ring after the semiconductor wafer 19 is removed therefrom. The process may include, before the measurement, the steps of rotating the polishing head 31 at a higher speed to eliminate the water attached to the retainer ring 10, and additionally providing an air blow to the retainer ring 10 for drying.
As shown in FIG. 2, the retainer ring 10 is moved to be positioned above the sensor 37. The sensor 37 of the laser displacement meter measures the height of the retainer ring 10 from a reference position. Since the retainer ring 10 is provided with the groove 11 in this embodiment, the polishing head is rotated at a lower speed during measuring the depth of the groove 11. For achieving a desired accuracy in the measurement, the thickness of the retainer ring 10 or the groove depth should be formed with a specified accuracy, with the relative differences therebetween being limited to a specified range. One or more of a plurality of polishing conditions can be changed based on the wear thickness of the retainer ring 10.
It is possible to select a retainer pressure applied from the retainer ring 10 and select a membrane pressure applied from the membrane such as 18 a, to apply the combination of pressures in the vicinity of the edge of the semiconductor wafer 19 for achieving a suitable pressure. As described before, the data of pressure dependency of the retainer ring and the data of pressure dependency in the vicinity of the wafer edge should be obtained for different wear thicknesses in advance, to formulate the approximation formula. The amount of change used in the actual case is determined based on the approximation formula thus obtained.
In the polishing apparatus and polishing method of the above embodiment, the wear thickness of the retainer ring is measured, as described above, the characteristic change in the polishing process depending on the wear thickness of the retainer ring is predicted, and the characteristic change is compensated for based on the prediction, for example, by changing one or more of the polishing condition.
The polishing apparatus of the present embodiment suppresses the change in the polishing characteristic accompanied by the wear thickness of the retainer ring caused by the polishing operation, thereby improving the product yield of the semiconductor device. At the same time, the polishing apparatus improves the production efficiency and reduces the cost as compared to the conventional technique.
FIG. 6 shows the improvement of the within wafer uniformity provided by the polishing apparatus of the present embodiment in comparison with the conventional technique. The abscissa represents the thickness of the retainer ring whereas the ordinate represents the range of variation on the polished wafer surface, for showing the within wafer uniformity thereof. Curve I shows a substantially flat within wafer uniformity with respect to different thickness of the retainer ring by correcting the polishing condition according to the present embodiment, whereas curve II shows a larger range of variation along with the reduction of the thickness of the retainer ring thereby revealing the degraded within wafer uniformity. It is confirmed from FIG. 6 that the polishing apparatus of the above embodiment achieves a significant improvement in the within wafer uniformity of polishing rate without depending the wear thickness of the retainer ring.
Now, a polishing apparatus according to a second embodiment of the present invention will be described with reference to FIGS. 7 through 9. FIG. 7 is a block diagram of the polishing apparatus of the present embodiment. The polishing apparatus includes as the principal constituent members: a wear measurement unit 40 for measuring the wear thickness of the retainer ring; a calculation unit 13 for calculating the optimum height of the retainer ring; a storage unit 14 for storing the calculated optimum height; a polishing unit 16; a polishing controller 12; a pressure control unit 15; and an instruction unit 17 for controlling these units and supplying an instruction to the polishing unit 16. The polishing apparatus includes, in addition to the principal constituent members: a retainer height control unit 25 for adjusting the height of the retainer ring; and a retainer height changer 24 for changing the height of the retainer ring.
The retainer height control unit 25 acquires correlation data between the wear thickness of the retainer ring and the height correction amount by examining the operation of the polishing unit 16, to formulate the approximation formula for calculating the optimum correction amount. The optimum correction amount is used for controlling the polishing condition in the polishing unit 16. The optimum correction amount thus obtained is used in the polishing unit 16 for correcting the height of the retainer ring, for example, by 0.6 mm against a wear thickness of 0.5 mm, by 1.3 mm against a wear thickness of 1.0 mm, and by 2.0 mm against a wear thickness of 1.5 mm. The polishing characteristic can approximate to an initial state of the retainer ring by applying a correction height larger than a height corresponding to the wear thickness. The correction height calculated based on the approximation formula is delivered to the instruction unit 17.
FIG. 8 exemplifies a mechanism for changing the height of the retainer ring, showing the principal part of the polishing head. A pulse motor 26 is provided for changing the distance between the retainer ring 10 and a head carrier 29, driving together a slide unit 28 and a ball screw with respect to the head carrier 29. The pulse motor 26 rotates the ball screw to thereby vertically shift the retainer ring 10 relative to the head carrier 29. A retainer fixing membrane 27 is provided between the head carrier 29 and the retainer ring 10 to fix the position of the retainer ring 10 during the polishing.
FIG. 9 is a flowchart showing the operation of the polishing apparatus according to the present embodiment. After issuing an instruction to start polishing the instruction unit 17 checks a flag indicating whether or not the polishing head or retainer ring is in a state of immediately after replacement. If the judgment is affirmative, it is also judged whether or not the number of wafers polished heretofore is smaller than or equal to N after the replacement (Step S12). If either the judgment is affirmative, the wear measurement unit 40 measures the height of the retainer ring or the depth of the groove (Step S13), and delivers the measured value (Step S14).
Based on the measurement of the height of the retainer ring or the depth of the groove, the calculation unit 13 calculates the optimum correction amount for the height of the retainer ring while using the approximation formula, and stores the calculated optimum correction amount (Step S15). The height of the retainer ring 10 is then adjusted based on the calculated optimum correction amount (Step S16). It is to be noted that, for a new retainer ring manufactured by a regular press, the height correction is not necessary and an ordinary process condition is used in most cases.
The polishing unit 16 repeatedly performs the polishing operation (Step S17), based on the stored optimum height correction amount, up to the specific number of wafers. If it is judged that the polishing operation is not finished (Step S18), the process returns to Step S12. If it is judged at Step S12 that the number of polished wafers is smaller than or equal to N, i.e., the lot end is not reached, the instruction unit 17 receives the optimum height correction amount from the storage unit 14 at Step S19, controls the polishing operation based on the stored correction amount (Steps S16 and S17), and returns to Step S12 if it is judged at Step S18 that the polishing is not finished.
If it is judged at Step S12 that the number of polished wafers reaches the specified number N to detect the lot end, the polishing unit polishes the succeeding wafers and later lots by applying the height correction amount greater than or equal to the height correction amount corresponding to the wear thickness of the retainer ring 10. More specifically, the wear measurement unit 40 measures the wear thickness of the retainer ring (Step S13) and transfers the measured wear thickness to the calculation unit (Step S14).
The calculation unit 40 calculates the optimum height correction amount based on the measured wear thickness by using the approximation formula The polishing unit 16 adjusts the height of the retainer ring 10 to the optimum height (Step S16). It is to be noted that if the polishing is finished before the number of polished wafers reaches the specified number N, and thereafter the polishing apparatus starts for polishing of wafers, the polishing apparatus checks the flag for the retainer ring 10 and follows the above-described flow of the process.
FIG. 10 shows the within wafer uniformity of polishing rate, obtained in the second embodiment, in terms of the range of variation on the polished surface which is plotted against the thickness of the retainer ring. In FIG. 10, the within wafer uniformity is improved by applying the correction amount larger than the correction amount corresponding to the wear thickness at curve I according to the present embodiment. Curve II and III respectively show, as a comparative example, the within wafer uniformity of polishing rate in the case of applying no height correction to the retainer ring, and the within wafer uniformity of polishing rate in the case of applying the height correction amount corresponding to the wear thickness. As understood from FIG. 10, the present embodiment remarkably compensates for the degradation of the within wafer uniformity that cannot be compensated for by the correction amount corresponding to the wear thickness of the retainer ring.
A third embodiment of the present invention will be described hereinafter. In the present embodiment, the wear thickness of the retainer ring is not monitored, and instead calculated and estimated based on the polishing condition and the cumulative polishing time length. The polishing apparatus acquires in advance the correlation data between the cumulative polishing time length and the wear thickness under a plurality of specific polishing conditions, to formulate the approximation formula. The polishing apparatus performs calculation using the approximation formula and changes the lo polishing condition similarly to the first embodiment.
If a plurality of process conditions are used for the polishing operation by changing process parameters such as a polishing pressure and a rotational speed of the platen, the coefficients of the wear thickness caused by respective process conditions are separately obtained in advance, whereby the total wear thickness is estimated by adding the wear thicknesses caused by respective process conditions.
For example, assuming that a, b, and c are coefficients in the respective process conditions for the wear thickness per unit time length, the total wear thickness W is calculated by the following formula:
W=aX+bY+cZ,
wherein X, Y and Z are time lengths of the respective process conditions in the polishing operation actually conducted. Based on the estimated wear thickness of the retainer ring, the optimum process condition is selected similarly to the first embodiment.
FIG. 11 shows the configuration of the polishing apparatus according to the third embodiment. The polishing apparatus includes: a polishing controller 12; a calculation unit 13; a storage unit 14; a pressure control unit 15; a polishing unit 16; and an instruction unit 17. FIG. 12 is a flowchart showing the process performed in the polishing apparatus. After issuing an instruction to start polishing, the instruction unit 17 checks the flag indicating whether or not the polishing head or the retainer ring 10 is in a state of immediately after replacement (Step S21). If the judgment is affirmative, the polishing unit 16 performs polishing in accordance with the initially-settled basic condition (Steps S29 and S27). Thereafter, until the wear thickness estimated based on the approximation formula reaches the threshold M to detect the lot end (Step S22), the instruction unit 17 reads basic condition from the storage unit 14 (Step S30). The polishing unit 16 repeatedly performs the polishing operation using the basic condition as the optimum polishing condition (Steps S26 and S27).
If it is judged at Step S22 that the estimated cumulative wear thickness reaches the threshold M to detect the lot end, the estimated cumulative wear thickness of the retainer ring is transferred to the calculation unit 13 for calculation of the optimum polishing condition. The calculation unit 13 calculates the optimum polishing condition based on the estimated wear thickness (Step S25). The calculated condition is settled as the optimum polishing condition (Step S26). The polishing unit 16 polishes the succeeding wafers by using the new optimum condition corresponding to the estimated wear thickness of the retainer ring 10 (Step S27). If the process terminates before the number of processed wafers reaches a specified number, and thereafter polishing is to be restarted, the instruction unit 17 checks the flag for the retainer ring and follows the above-described flow of process.
The polishing apparatus of the present embodiment does not require special hardware such as a sensor or a mechanism for adjusting the height of the retainer ring. The embodiment can provide the advantage of the present invention only by modifying the software and can improve the cost efficiency.
A polishing apparatus according to a fourth embodiment of the present invention is such that the polishing apparatus approximates the wear thickness of the retainer ring similarly to the third embodiment. However, differently from the third embodiment, the polishing unit in the fourth embodiment changes the height of the retainer ring similarly to the second embodiment.
FIG. 13 is a block diagram showing the configuration of the polishing apparatus of the fourth embodiment. The polishing apparatus includes: a polishing controller 12; a calculation unit 13; a storage unit 14; a pressure control unit 15; a polishing unit 16; an instruction unit 17; a retainer height changer 24, and a retainer height control unit. FIG. 14 is a flowchart showing the process performed in is the polishing apparatus of FIG. 13. In this embodiment, the height of the retainer ring 10 is changed by the retainer height control unit 25, and the actual change of the retainer height is effected by the retainer height changer 2, instead of employing the different polishing condition at Steps S29 and S26 in FIG. 12. The other steps in FIG. 14 are similar to those in FIG. 12. The embodiment can provide the advantage of the present invention while using a simpler configuration than the second embodiment.
FIG. 15 is a flowchart showing a process performed in a polishing apparatus according to a fifth embodiment of the present invention. The polishing apparatus of the first embodiment is obtained by combining the features of the first and second embodiments. In the present embodiment, the process includes the steps of measuring the wear thickness of the retainer ring at a specified timing (Step S43), correcting the height of the retainer ring, and correcting the polishing condition (Step S46). Upon compensating for the wear thickness of the retainer ring, the height correction is directed to cancelling the wear thickness itself from the initial thickness, for example, and correction of the polishing condition such as a polishing pressure is changed in an amount corresponding to a change in the shape of the retainer ring in addition to the height correction. The ratio of the height correction amount for correcting the wear thickness to the amount of condition correction for correcting the shape change can be selected as desired.
A polishing apparatus according to sixth embodiment of the present invention will be described hereinafter. The polishing apparatus of the present embodiment is similar to the conventional polishing apparatus in terms of the hardware configuration. In the present embodiment, the polishing head is removed from the polishing apparatus at a regular interval, and is measured in the thickness of the retainer ring to actually measure the wear thickness. The wear thickness measured in this measurement is compensated for by using one or more out of a plurality of shims having different thicknesses. The one or more of the shims having different thicknesses is inserted between the retainer ring and the polishing head 31 such as shown in FIG. 5.
The present embodiment applies a correction amount greater than the correction amount corresponding to the decrease in the thickness of the retainer ring 10, and compensates for the wear thickness and the shape change of the retainer ring. That is, the shim to be inserted is thicker than the wear thickness. For example, a 0.6-mm-tick shim is inserted against a wear thickness of 0.5 mm on the retainer ring; a 1.3 mm-thick shim against a wear thickness of 1.0 mm; and a 2.0-mm-thick shim against a wear thickness of 1.5 mm. The embodiment uses the conventional polishing apparatus and modifies only the polishing process to stabilize the polishing characteristics.
As described heretofore, the method or apparatus of the present invention may have the embodiments as listed below.
The correction amount may include a first correction amount corresponding to the obtained wear thickness and a second correction amount corresponding to shape change of the retainer ring.
The fit polishing condition may include a first height of the retainer ring, the calculating step calculates a height correction amount corresponding to the obtained wear thickness, and the polishing step uses a second height of the retainer ring obtained by adding the height correction amount to the first height of the retainer ring.
The height correction amount may correspond to a change in a thickness of the obtained wear thickness.
The calculating step or unit may calculate the correction amount by using an approximation formula.
The wear thickness measurement unit may measure a depth of a groove formed on a surface of the retainer ring in contact with the polishing pad.
The wear thickness obtaining step may estimate the wear thickness based on a cumulative time length for polishing by the retainer ring and a polishing condition used in the polishing for the cumulative time length.
Since the above embodiments are described only for examples, the present invention is not limited to the above embodiments and various modifications or alterations can be easily made therefrom by those skilled in the art without departing from the scope of the present invention.

Claims

1. A method for polishing an object wafer by using a polishing apparatus including a retainer ring for guiding the object wafer in an in-plane direction of the object wafer, a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer, and a polishing pad for polishing a top surface of the object wafer, said method comprising the steps of:

obtaining a wear thickness of the retainer ring at a specific timing during or after polishing the object wafer in a first polishing condition;

calculating a correction amount of a polishing condition based on said obtained wear thickness; and

polishing the object wafer in a second polishing condition obtained by adding said calculated correction amount to said first polishing condition.

2. The method according to claim 1, wherein said correction amount includes a first correction amount corresponding to said obtained wear thickness and a second correction amount corresponding to shape change of the retainer ring.

3. The method according to claim 1, wherein said first polishing condition includes a first height of the retainer ring, said calculating step calculates a height correction amount corresponding to said obtained wear thickness, and said polishing step uses a second height of the retainer ring obtained by adding said height correction amount to said first height of the retainer ring.

4. The method according to claim 3, wherein said height correction amount corresponds to a change in a thickness of said obtained wear thickness.

5. The method according to claim 1, wherein said calculating step calculates said correction amount by using an approximation formula.

6. The method according to claim 1, wherein said wear thickness obtaining step measures a depth of a groove formed on a surface of the retainer ring in contact with said polishing pad.

7. The method according to claim 1, wherein said wear thickness obtaining step estimates said wear thickness based on a cumulative time length for polishing by the retainer ring and a polishing condition used in the polishing for the cumulative time length.

8. A method for polishing an object wafer by using a polishing apparatus including a retainer ring for guiding the object wafer in an in-plane direction of the object wafer, a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer, and a polishing pad for polishing a top surface of the object wafer, said method comprising the steps of:

obtaining a wear thickness of the retainer ring at a specific timing during or after polishing the object wafer by using the retainer ring located at a first height;

calculating a correction amount of a height of the retainer ring based on said obtained wear amount, the calculated correction amount being larger than a correction amount corresponding to said obtained wear thickness; and

polishing the object wafer by using the retainer ring located at a second height obtained by adding said calculated correction amount to said first height.

9. The method according to claim 8, wherein said calculating step calculates said correction amount by using an approximation formula.

10. The method according to claim 8, wherein said wear thickness obtaining step measures a depth of a groove formed on a surface of the retainer ring in contact with said polishing pad.

11. The method according to claim 8, wherein said wear thickness obtaining step estimates said wear thickness based on a time length for polishing the wafer and a polishing condition used at least in said first polishing condition.

12. A polishing apparatus comprising:

a retainer ring for guiding an object wafer in an in-plane direction of the object wafer;

a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer;

a polishing pad for polishing a top surface of the object wafer:

a wear thickness determination unit for determining a wear thickness of the retainer ring at a specific timing during or after polishing the object wafer in a first polishing condition;

a calculation unit for calculating a correction amount of the polishing condition based on said determined wear thickness; and

a polishing unit for polishing the object wafer in a second polishing condition obtained by adding said calculated correction amount to said first polishing condition.

13. The polishing apparatus according to claim 12, wherein said first polishing condition includes a first height of the retainer ring, said calculation unit calculates a height correction amount based on said obtained wear thickness, and said polishing controller polishes the object wafer by using a second height of the retainer ring obtained by adding said height correction amount to said first height of the retainer ring.

14. The polishing apparatus according to claim 13, wherein said height correction amount includes a first correction amount corresponding to said obtained wear thickness and a second correction amount corresponding to shape change of the retainer ring.

15. A polishing apparatus comprising:

a retainer ring for guiding an object wafer in an in-plane direction of the object wafer,

a pressure member for pressing a bottom surface of the object wafer in a thickness direction of the object wafer,

a polishing pad for polishing a top surface of the object wafer;

a wear thickness determination unit for determining a wear thickness of the retainer ring at a specific timing after or during polishing the object wafer in a first polishing condition;

a calculation unit for calculating a correction amount of the retainer ring, which is larger than a correction amount corresponding to said determined wear thickness; and