Patent Grant
8900033
The present invention relates to a wafer polishing method and, in particular to a method of simultaneously polishing both surfaces of wafers by rotating carriers retaining the wafers between an upper polishing plate and a lower polishing plate provided with a polishing pad on each surface.
For a process of polishing silicon wafers, a polishing method of forcing an upper polishing plate and a lower polishing plate that are provided with a polishing pad on their surface against wafers retained by carriers and moving the wafers between the upper polishing plate and the lower polishing plate that are rotated, thereby simultaneously polishing both surfaces of the wafers has been generally employed. The carriers each have a circular hole for retaining the wafers, and have a smaller thickness than the wafers. In this polishing method, a polishing solution containing abrasive grains has been generally used. This polishing method has been used because surfaces and rear surfaces of a plurality of wafers can be simultaneously polished and accordingly productivity and flatness of the surfaces and the rear surfaces of the wafers can be improved.
The above polishing method using carriers and a polishing solution containing abrasive grains however has a problem in that the carriers are worn by the abrasive grains contained in the polishing solution, and the thicknesses of the carriers vary, which would lead to thickness variation of the wafers after polished.
Patent Document 1 discloses carriers for a double-side polishing apparatus capable of preventing the carriers from wearing at the time of simultaneously polishing both surfaces of wafers. However, the carriers for a double-side polishing apparatus in Patent Document 1 are carriers each made by coating the surface of a resin matrix with a DLC (diamond-like carbon) film. Therefore, the production cost is extremely high as compared with metal carriers, carriers each having a metal plate of which surface and rear surface are coated with resin, and resin carriers, which are commonly used. This has been a problem.
In cases where a polishing solution free of abrasive grains is used as a polishing solution for simultaneously polishing both surfaces of wafers, whichever of the following carriers may be used to prevent the carriers from wearing: metal carriers, carriers each having a metal plate of which surface and rear surface are coated with resin, and resin carriers. However, as polishing proceeds, the difference between the thickness of the carriers and the thickness of the wafers becomes small, and polishing pads contact the carriers to cause the carrier to vibrate, which results in undesired sounds generated from the carriers. This has been another problem.
When polishing is performed in a state where carriers continue to vibrate, not only the problem of undesired sounds being generated from the carriers arises, but the variation of the wafer thickness that should be reduced by polishing would conversely increase as a result of the variation in the contact of polishing pads with both surfaces of the wafers, which is yet another problem. Therefore, the time point at which the carriers begin to vibrate should be ensured to be detected, and polishing is required to be terminated at the time point.
Patent Document 2 discloses a method of simultaneously polishing both surfaces of wafers by moving the wafers retained by carriers between upper and lower polishing plates that are rotated. In the polishing method, from vibrations generated from the polishing plates accompanying wafer polishing, one or a plurality of specific frequencies at which the vibration level changes reflecting the progression of polishing are selected in advance, and the change in the vibration level at the specific frequencies is detected in polishing, so that the termination point of polishing is estimated from the change in the vibration level.
Further, in other polishing methods in which polishing is terminated with the use of polishing sounds generated when wafers are polished, polishing sounds generated in polishing of wafers are detected, and the termination point of polishing is determined based on a change in the polishing sounds, for example as disclosed in Patent Documents 3 and 4. In accordance with this method, the termination point of polishing can be determined during the progress of a polishing process using a relatively simple system configuration.
Patent Document 1: Japanese Patent Application Publication No. 2006-303136
Patent Document 2: Japanese Patent Application Publication No. 2005-252000
Patent Document 3: Japanese Patent Application Publication No. H6-45299
Patent Document 4: Japanese Patent Application Publication No. 2001-15467
However, in the invention of Patent Document 2, in fact, a vibration sensor provided on an upper surface of an upper polishing plate detects not only vibrations of polishing plates but also the total vibrations including vibrations of the polishing plates, vibrations of wafers, and vibrations of carriers. Therefore, noise is mixed in the detected specific frequencies at which the vibration level changes; as a result, the change in the vibration level cannot be accurately measured. Thus, the termination point of polishing cannot be accurately detected, which would lead to further variations in the thickness of the wafers polished.
Further, in the inventions of Patent Documents 3 and 4, while the termination point can be quantitatively determined, cases of polishing using carriers are not considered. Thus, the thickness variation of wafers after polished due to wear of the carriers still remains as a problem to be solved.
An object of the present invention is to provide a method of polishing silicon wafers, capable of suppressing undesired sounds generated from carriers and reducing the thickness variation of the wafers after polished.
In order to solve the above problems, the inventors of the present invention have made various studies on a wafer polishing method including the steps of supplying a polishing solution to a surface of a pair of polishing pads positioned above and below a carrier which has a circular hole for retaining a wafer and is thinner than the wafer; and sliding the polishing pads relatively to the carrier, thereby simultaneously polishing both surfaces of the wafer retained in the carrier. As a result, the inventors found the following facts. The carrier generates information such as vibrations or sounds when the difference between the thickness of the carrier and the thickness of the wafer reaches a specific value. And by detecting the information, the thickness of the wafer can be determined. Furthermore, the termination of polishing based on the determined thickness allows the polishing amount of the wafer to be stably controlled; therefore, undesired sounds generated from the carrier can be effectively suppressed, and a wafer after polished with reduced thickness variation can be obtained.
In order to achieve the above object, the present invention primarily includes the following components.
(1) A wafer polishing method comprising the steps of: supplying a polishing solution to a surface of a pair of polishing pads positioned above and below a carrier which has a circular hole for retaining a wafer and is thinner than the wafer; and sliding the polishing pads relatively to the carrier, thereby simultaneously polishing both surfaces of the wafer retained in the carrier. Information sourced from the carrier when a difference between a thickness of the carrier and a thickness of the wafer reaches a predetermined value, is detected to calculate the thickness of the wafer and thereby terminate polishing.
(2) The wafer polishing method according to (1) above, wherein the termination of polishing is performed at a termination point of polishing determined by extracting a first signal and a second signal that are signals of two setup frequency ranges from the information obtained from the carrier, and detecting the termination point of polishing where an intensity of the first signal is higher than a predetermined value and an intensity of the second signal is lower than a predetermined value.
(3) The wafer polishing method according to (1) above, wherein the polishing solution is a polishing solution free of abrasive grains.
(4) The wafer polishing method according to (1) above, wherein the information sourced from the carrier is sounds and/or vibrations.
(5) The wafer polishing method according to (1) above, wherein the difference between the thickness of the carrier and the thickness of the wafer at a time when the information is sourced, is in the range of 0.1 μm to 20 μm.
(6) The wafer polishing method according to (1) above, wherein a pressure applied to the polishing pads in the direction perpendicular to the surfaces of the wafer is in the range of 100 g/cm2 to 300 g/cm2.
(7) The wafer polishing method according to (1) above, wherein the information sourced from the carrier is detected with detectors provided for each carrier.
(8) The wafer polishing method according to (7) above, wherein the detector is provided on a mechanism for holding the carrier.
(9) The wafer polishing method according to (8) above, wherein the detector is provided on external gears for moving the carrier.
(10) The wafer polishing method according to (4) above, wherein the detection of the sounds from the carrier is performed by detecting sounds having a frequency of 10 Hz to 1000 Hz.
(11) The wafer polishing method according to (1) above, wherein the polishing solution contains a specific high molecular weight component.
(12) The wafer polishing method according to (2) above, wherein the extracting the signals from the information is calculation of a power spectra of digital data obtained by A/D converting the detected information and of a mean value or a maximum value of signal intensities of power spectra corresponding to the setup frequency ranges.
The present invention can provide a polishing method capable of suppressing undesired sounds generated from carriers and further reducing thickness variation of wafers after polished.
a) and 3(b) are graphs showing the intensity of information generated from carriers. Specifically,
A wafer polishing method of the present invention will be described with reference to the drawings.
A wafer polishing method of the present invention is a method in which wafers 20 are polished by supplying a polishing solution (not shown) to surfaces 30a of a pair of polishing pads 30 positioned above and below carriers 10 each having a circular hole 11 for retaining the wafers 20, the carriers 10 being thinner than the wafers 20; and sliding the polishing pads 30 relatively to the carriers 10, thereby simultaneously polishing both surfaces of the wafers 20 retained in the carriers 10 as shown in
The method is characterized in that information sourced from the carriers 10 when a difference between the thickness of the carriers 10 and the thickness of the wafers 20 reaches a predetermined value, is detected to calculate the thickness of the wafers 20, and thereby terminate polishing.
With the above structure, as shown in
a) and 3(b) are graphs showing the intensity of information (herein referring to sounds and vibrations) for both cases where carriers are and are not generating information (sounds and vibrations).
Further, the termination of polishing is preferably performed at a termination point of polishing determined by acquiring the information sourced from the carriers 10 when the difference between the thickness of the carriers 10 and the thickness of the wafers 20 reaches a predetermined value, and extracting a first signal and a second signal that are signals of two setup frequency ranges from the information obtained, to detect the termination point of polishing where the intensity of the first signal is higher than a predetermined value and the intensity of the second signal is lower than a predetermined value.
The first signal and the second signal extracted from the information refer to two signals of different frequency ranges that are extracted, after acquisition of the information sourced from the carriers 10, from the acquired information by a specific process. In the present invention, a signal of high frequency is referred to as a first signal, while a signal of low frequency is referred to as a second signal. Note that the frequencies of the first signal and the second signal are not particularly limited and may be any value as long as the termination of polishing can be controlled by a combination of the first signal and the second signal.
On the other hand,
The information sourced from the carriers 10 is not particularly limited and may be, for example, sounds, vibrations, or heat generated from the carriers as long as the thickness TW of the wafers can be determined based on the information. However, the information is preferably sounds and/or vibrations generated from the carriers 10 because the termination point of polishing the wafers 20 can be easily and reliably detected.
Note that the method of extracting signals from the information is not particularly limited as long as the above-mentioned first signal and second signal can be effectively extracted with the method. For example, since desired signal data can be obtained relatively easily, it is preferable to perform the signal extraction by calculating power spectra of digital data obtained by A/D converting the detected information, and by calculating a mean value or a maximum value of signal intensities of power spectra corresponding to the setup frequency ranges. And if necessary, only specified frequencies may be A/D converted by filtering the information, or the peak hold processing may be conducted after such filtering.
Further, since the polishing amount can be more accurately controlled, a difference between the thickness TC of the carriers 10 and the thickness TW of the wafers 20 (TC-TW) at the time when information is sourced from the carriers 10, is preferably in the range of 0.1 μm to 20 μm although depending on the rotation speed of the polishing pads 30, pressure, and the type of the polishing solution as well.
The information (sounds or vibrations and the like) sourced from the carriers 10 can be detected, for example with detectors 40 provided adjacent to the carriers 10 as shown in
When the information to be detected is sounds, their frequencies are preferably in the range of 10 Hz to 1000 Hz. The frequency range is expected to be suitable for finding the termination point of polishing; therefore, if polishing is terminated with sounds outside the frequency range, desired wafer thickness would not be achieved.
Further, the frequency range of the first signal is more preferably in the range of 200 Hz to 1000 Hz, while the frequency range of the second signal is more preferably in the range of 10 Hz to 200 Hz. The first signal and the second signal of these wavelength ranges make it possible to ensure determination of the termination point of polishing when undesired sounds are generated from the carriers 10 because their signal intensities greatly differ from one another.
Meanwhile, when vibrations generated from the carriers 10 are detected, as shown in
Further, the detected sounds and/or vibrations generated from the carriers 10 are converted into signals, and the signals are sent to a control device 60 as shown in
The carriers 10 used in a polishing method of the present invention are, as shown in
The polishing pads 30 are members for sliding the wafers 20 being interposed between the upper and lower polishing pads to simultaneously polish the surfaces of the wafers 20, as shown in
A polishing solution used in a polishing method of the present invention is preferably a polishing solution free of abrasive grains. Use of a polishing solution containing abrasive grains makes it difficult to detect information from the carriers 10 that is required for the termination of polishing. Here, the “polishing solution free of abrasive grains” is polishing solution in which abrasive grain components may be contained but such abrasive grain components are not positively added.
The type of the polishing solution is not particularly limited. For example, an alkaline aqueous solution with pH controlled to the range of 8 to 14 is preferably used. With a pH of less than 8, the etching action is weak to reduce the polishing rate; on the other hand, with a pH of more than 14, handling of the polishing solution becomes difficult. The alkaline aqueous solution may be an aqueous ammonia solution, or an alkaline hydroxide solution such as potassium hydroxide or sodium hydroxide, or an alkaline carbonate solution. Alternatively, an aqueous solution of hydrazine or amines can be used. It is preferable to use an aqueous solution of amines in particular in terms of increasing the polishing rate. Further, the polishing solution preferably contains a specific high molecular weight component, since undesired sounds (squeals) generated from the carriers 10 can be further reduced. The type of the high molecular weight component is not particularly limited, but hydroxyethyl cellulose or polyethylene glycol is preferably used. In particular, hydroxyethyl cellulose, which has a relatively high molecular weight, can efficiently reduce the coefficient of friction between the polishing pads 30 and the carriers 10 due to the bearing action between the polishing pads 30 and the carriers 10. As a result, defects in wafers after polished can be reduced. Further, the concentration of the high molecular weight component added to the polishing solution is preferably in the range of 0.01 ppm to 1000 ppm. If the concentration of the high molecular weight component is less than 0.01 ppm, the friction coefficient in polishing is high, which would cause defects in the surface of the wafers after polished. On the other hand, if the concentration is more than 1000 ppm, the polishing rate is significantly reduced, and considerable time is required for mirror polishing.
In terms of removing metal ions contained in the polishing solution, a chelating agent is preferably added to the polishing solution. The chelating agent may be any substance without particular limitation as long as it has a chelating effect on metal ions. For example, a phosphonic acid type chelating agent, an aminocarboxylic acid type chelating agent, or the like can be used. Considering the solubility in an alkaline aqueous solution, an aminocarboxylic acid type chelating agent is preferable. In addition, considering chelating effect on heavy metal ions, an aminocarboxylic acid salt such as ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA) is more preferable. The concentration of the chelating agent is preferably in the range of 0.1 ppm to 1000 ppm, which allows metal ions such as Cu, Zn, Fe, Cr, Ni, and Al to be trapped.
Further, in a polishing method of the present invention, the polishing pads 30 are slid relatively to the carriers 10, thereby polishing surfaces of the silicon wafers 20. The sliding method is not necessarily limited in particular. The sliding may be performed by moving only the polishing pads 30 or by moving only the carriers 10; alternatively, the sliding may be performed by relatively moving both the polishing pads 30 and the carriers 10.
The pressure applied to the polishing pads 30 in the direction perpendicular to the surfaces of the wafers 20 is preferably in the range of 100 g/cm2 to 300 g/cm2. When the pressure is less than 100 g/cm2, the pressure is excessively low, so that the information sourced from the carriers 10, such as sounds and vibrations would not be sufficiently obtained. On the other hand, when the pressure is more than 300 g/cm2, the excessively high pressure makes sounds and vibrations of the carriers 10 be easily generated; thus, the polishing termination point of the wafers 20 would not be accurately ascertained.
Note that the foregoing descriptions merely illustrate some examples of embodiments of the present invention and various modifications can be made thereto within the scope of claims.
In Example 1, as shown in
The polishing conditions (the type of the polishing solution, whether the polishing solution contains a high molecular weight component or not, and the detection method of a polishing termination point) are shown in Table 1. Further, the silicon wafers were polished five times in total (25 sheets).
In Example 2, silicon wafers (25 sheets) were polished by a similar method to Example 1 except for that the sounds generated from the carriers 10 were acquired by a sound input microphone 41 provided adjacent to the carriers 10, and a first signal (480 Hz) and a second signal (100 Hz) were extracted from the sounds obtained.
The polishing conditions (the type of the polishing solution, whether the polishing solution contains a high molecular weight component or not, and the detection method of a polishing termination point) are shown in Table 1.
In Comparative Example 1, silicon wafers (25 sheets) were polished under similar conditions to Example 1 except for using a polishing solution containing abrasive grains (a potassium hydroxide aqueous solution containing colloidal silica as free abrasive grains).
The polishing conditions (the type of the polishing solution, whether the polishing solution contains a high molecular weight component or not, and the detection method of a polishing termination point) are shown in Table 1.
In Comparative Example 2, silicon wafers (25 sheets) were polished under similar conditions to Example 1 except for that polishing was controlled not using the information (sounds or vibrations) sourced from the carriers but using a setup time.
The polishing conditions (the type of the polishing solution, whether the polishing solution contains a high molecular weight component or not, and the detection method of a polishing termination point) are shown in Table 1.
(1) Wear of Carriers
The thickness TC of the carriers in Example 1 and Comparative Example 1 was measured before polishing, after a predetermined time lapse from the beginning of the polishing, and after the polishing. The wear level of the carriers was evaluated by plotting the relationship between the polishing time and the thickness TC of the carriers. The results are shown in
(2) Thickness variation of Wafers
The thickness profiles of the silicon wafers obtained in Examples and Comparative Examples were measured using WaferSight manufactured by KLA-Tencor Corporation, and the thickness variation of the wafers was calculated. Thus, the mean value of the variation was evaluated. The evaluation results are as shown in Table 1, in which smaller values are better.
Table 1 and
The present invention can provide a polishing method capable of suppressing undesired sounds generated from carriers and further reducing thickness variation of silicon wafers after polished.
10: Carrier
11: Circular Hole
20: Wafer/Silicon Wafer
30: Polishing Pad
40: Detector
41: Sound Input Microphone
42: Vibration Detector
50: External Gear
60: Control Device
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-273255 | Dec 2009 | JP | national |
| 2009-276517 | Dec 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/071777 | 11/30/2010 | WO | 00 | 5/23/2012 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2011/068236 | 6/9/2011 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3093937 | Balamuth et al. | Jun 1963 | A |
| 3097458 | Richmond | Jul 1963 | A |
| 3348338 | Messerschmidt | Oct 1967 | A |
| 3579922 | Mandonas et al. | May 1971 | A |
| 4197676 | Sauerland | Apr 1980 | A |
| 4272924 | Masuko et al. | Jun 1981 | A |
| 5099614 | Arai et al. | Mar 1992 | A |
| 5222329 | Yu | Jun 1993 | A |
| 5876265 | Kojima | Mar 1999 | A |
| 5882245 | Popovich et al. | Mar 1999 | A |
| 5941759 | Kitajima et al. | Aug 1999 | A |
| 6074275 | Yashiki et al. | Jun 2000 | A |
| 6155908 | Hakomori | Dec 2000 | A |
| 6210259 | Malkin et al. | Apr 2001 | B1 |
| 6431953 | Carter et al. | Aug 2002 | B1 |
| 6709981 | Grabbe et al. | Mar 2004 | B2 |
| 7137867 | Nagayama et al. | Nov 2006 | B2 |
| 7147541 | Nagayama et al. | Dec 2006 | B2 |
| 8113913 | Pietsch et al. | Feb 2012 | B2 |
| 8125654 | Benvegnu et al. | Feb 2012 | B2 |
| 8221190 | Tokura et al. | Jul 2012 | B2 |
| 8267741 | Kimura et al. | Sep 2012 | B2 |
| 20020052064 | Grabbe et al. | May 2002 | A1 |
| 20020065024 | Fitzgerald et al. | May 2002 | A1 |
| 20040192171 | Koike | Sep 2004 | A1 |
| 20060194511 | Nagayama et al. | Aug 2006 | A1 |
| 20060194512 | Nagayama et al. | Aug 2006 | A1 |
| 20080233840 | Pietsch et al. | Sep 2008 | A1 |
| 20110064971 | Kimura et al. | Mar 2011 | A1 |
| 20120252319 | Fujihira | Oct 2012 | A1 |
| 20130017763 | Takaishi et al. | Jan 2013 | A1 |
| 20130109180 | Tanimoto et al. | May 2013 | A1 |
| Number | Date | Country |
|---|---|---|
| H0645299 | Jun 1994 | JP |
| 9-139369 | May 1997 | JP |
| H09139369 | May 1997 | JP |
| 9-150367 | Jun 1997 | JP |
| H09150367 | Jun 1997 | JP |
| 2001-007063 | Jan 2001 | JP |
| 2001015467 | Jan 2001 | JP |
| 2005501410 | Jan 2005 | JP |
| 2005252000 | Sep 2005 | JP |
| 2005252000 | Sep 2005 | JP |
| 2006303136 | Nov 2006 | JP |
| 546724 | Aug 2003 | TW |
| 200940693 | Oct 2009 | TW |
| 2003019627 | Mar 2003 | WO |
| WO-2009041697 | Apr 2009 | WO |
| Entry |
|---|
| Office Action in corresponding German application No. 11 2010 004 635.2, dated May 26, 2014 (8 pages). |
| Number | Date | Country | |
|---|---|---|---|
| 20130017763 A1 | Jan 2013 | US |
