Method and apparatus for polishing a thin plate

Abstract: A method and an apparatus, for polishing a thin plate (1) comprises the steps of: rotating a holding plate (12) on one surface (12a) of which the thin plate (1) is held; and bringing the thin plate (1) into contact with a polishing pad (7) mounted on a surface of a rotating turn table (5). However, prior to this polishing step the said one surface (12a) of the holding plate (12) is polished using the said polishing pad (7). Preferably the said one surface (12a) is also made of a material having the same polishing properties as the thin plate (1).

Description: The present invention relates to a technique for polishing a thin plate, in particular, to a polishing method which can polish a thin plate such as a semiconductor wafer (hereinafter, it may be simply referred to wafer"), with a high flatness, and to a polishing apparatus. In a process for fabricating a semiconductor integrated circuit, with the requirement of larger scale integration of recent semiconductor devices, the design rule for fabricating a device has a tendency to become finer and the depth of focus required for a stepper becomes shallower. Therefore, the requirement for the flatness of a semiconductor wafer which is the objective material becomes larger. Hereinafter, the term "flatness" means a uniformity with respect to variations in thickness of the wafer because use of a vacuum-chuck to hold the wafer in the stepper reforms a deformation of the wafer to some extent. Generally, polishing of a wafer is carried out by pressing the wafer against a polishing pad while giving a relative motion between the wafer and the polishing pad and supplying an abrasive slurry. For example, such a polishing apparatus for a thin plate has a structure shown in FIG. 7. In FIG. 7, the reference numeral 1 denotes a wafer which is a thin plate. The wafer 1 was processed to have upper and lower surfaces precisely parallel to each other and a uniform thickness, by slicing a silicon (Si) single crystal ingot by using an inner diameter saw slicing machine, thereafter by carrying out a lapping step and the like. A wafer or a plurality of wafers 1 are held on the lower surface of a holding plate 2. Both surfaces of the holding plate 2 are processed to have flat planes precisely parallel to each other. On the upper surface of the holding plate 2, a top ring 3 is provided as a loading member for giving a pressing force. The top ring 3 is attached to the lower end of a rotating shaft 4 and is driven to rotate at a predetermined rotational speed by a driving mechanism (not shown) which is attached to the rotating shaft 4 while giving a predetermined pressing force downward. Under the holding plate 2, a turn table 5 having an approximate disc-shape is horizontally disposed in parallel with the lower surface of the holding plate 2. The turn table 5 is rotatable on a rotating shaft 6 which is attached to the central portion of the lower surface of the turn table 5, at a constant rotational speed. The upper surface of the turn table 5 is processed to have a precise flat plane and a polishing pad 7 is adhered thereon. The polishing pad 7 is made of unwoven cloth of a visco-elastic body having a suitable toughness and flexibility. An abrasive slurry 8 including abrasive grains of colloidal silica is supplied onto the central portion of the upper surface of the polishing pad 7 by a pump 9. The reference numeral 10 denotes a drain receiver for receiving the used abrasive slurry and the reference numeral 11 denotes an abrasive slurry tank. In such a polishing device, the wafer 1 is held to adhere on the lower surface of the holding plate 2 by using vacuum suction, wax-adhesion, or the like, and the temperature of the surface of the polishing pad 7 on the surface of the wafer 1 and the polishing condition, e.g., a relative speed between the wafer 1 and the polishing pad 7 and the like, are kept constant, in order to obtain a desired flatness of the wafer 1. Because the wafer 1 is sandwiched between the holding plate 2 and the turn table 5 through the elastic polishing pad 7, the portion of the wafer having a larger thickness receives a larger contact pressure than that of the portion having a smaller thickness, so that the portion having a larger thickness is preferentially polished. As the result, the surface of the wafer 1 becomes flat gradually as the processing progresses. Further, in order to obtain a higher flatness, it is required to make the surfaces of the holding plate 2 and the turn table 5 flat, to make the thickness of the polishing pad 7 uniform, to remove the heat of the surface of the polishing pad 7 caused by friction by flowing a cooling water through the inside of the turn table 5, and the like. When the wafer 1 is polished by the above-described conventional method, removal of Si is carried out by the so-called mechano-chemical function which compounds a mechanical function and a chemical function. Therefore, there are a lot of factors which have an effect on flatness of the wafer. For example, they are the surface roughness, the characteristics of the surface of the polishing pad 7 such as a loading condition due to the removed material by polishing or the like, the characteristics of the body of the polishing pad 7 such as the thickness, the viscoelasticity or the like, the surface temperature of the polishing pad 7 which closely relates to a frictional heat generated by rubbing the wafer 1 with the polishing pad 7, the degree of the abrasive slurry 8 reaching the surface of the wafer 1, and the like. The load applied to the polishing pad 7 varies with the position in the polishing pad 7 and is not uniform, for reason of its structure. The polishing pad 7 is influenced by the applied load. For example, at a portion thereof to which a larger load is applied, the surface roughness becomes higher (flatter) by friction caused by rubbing motion, much loading due to products by polishing are caused, and the polishing pad 7 is apt to become thin and hard by the applied pressure. That is, the ability of the polishing pad 7 to remove the material changes (ordinarily, it is degraded) and becomes non-uniform with respect to the portion of the polishing pad 7. The degree of such non-uniformity varies with time. There is also a problem that the cooling of the turn table 5 causes thermal deformation thereof. That is, even if conditions to improve the flatness of the wafer 1 were established, practically, there was a limitation of flatness which could be obtained for the wafer 1 by thermal deformation of the turn table 5, wear of the polishing pad 7, non-uniformity of creep deformation, and change of them with the passage of time. An attempt to improve flatness of wafers produced by the above mentioned polishing system is disclosed in EP-A-0672499 wherein the holding plate is composed of two lightweight, high rigidity plates superimposed in tight adhesion. The present invention was developed in view of the above-described problems. Therefore, an object of the present invention is to provide a thin pate polishing method and a thin plate polishing apparatus, which can obtain a thin plate having a high flatness easily. That is, in accordance with a first aspect of the present invention, a method of polishing a thin plate, comprises the steps of: rotating a holding plate on one surface of which the thin plate is held; and, bringing the thin plate held on the rotating holding plate into contact with a polishing pad mounted on a surface of a rotating turn table, to polish the thin plate; characterised in that prior to holding the thin plate on the holding plate the one surface of the holding plate is polished by using the said polishing pad. The thin plate such as a wafer or the like, to which the present invention is applied, has a good flexibility to a small amount of deformation because of its very small thickness. Therefore, even if the surface of the holding plate is bent a little with respect to the flat plane, the thin plate can be easily adhered to the surface of the holding plate. Because the thin plate is processed while it is sandwiched between the holding plate and the polishing pad, it is considered that the relationship between their shapes is very important and therefore a shape of the holding plate which can make the thin plate flat must exist. From such a viewpoint, the inventors performed a processing (holding plate polishing) of the holding plate by using the polishing pad for polishing the thin plate, as described above. In order to carry out such holding plate polishing, it is preferable to perform processing under the same polishing condition, for example, polishing pressure, relative speed between the holding plate and the polishing pad, type of the abrasive slurry, temperature and the like, as that of the thin plate-processing. More preferably, the surface for holding the thin plate, of the holding plate is made of the same material as the thin plate, and the step of polishing the holding surface for holding the thin plate, of the holding plate is carried out under the same condition as that of the thin plate. The thin plate may be one selected from the group consisting of an Si wafer, an SiO 2 mask, and an SOI wafer. The thin plate may comprise a semiconductor substrate, a wiring of such materials as metal, metal silicide, polysilicon or the like formed on the substrate, and an insulating film deposited on the wiring and the substrate, and the method is performed for planarization of the insulating film of the thin plate. In accordance with another aspect of the present invention, polishing apparatus for polishing a silicon wafer, comprises: a rotatable holding plate for holding the silicon wafer on one surface of it; and, a rotatable turn table having a polishing pad on it for polishing the silicon wafer by bringing the rotating silicon wafer into contact with the rotating polishing pad; characterised in that the said one surface of the holding plate for holding the silicon wafer is made of a material which has substantially the same polishing properties as the silicon wafer. Preferably, a plurality of through holes for vacuum suction are formed in the holding plate in order to hold to adhere the thin plate on one surface thereof. The reason for having such a structure is as follows. If the holding surface for holding the thin plate is made of a material hard to process such as a ceramics, e.g., Al 2 O 3 , because it cannot be polished by using an apparatus for polishing a thin plate, another processing apparatus have to be used. Even if the holding surface of the holding plate is made of a material easy to polish, the material of the holding surface having a property to be polished differently from that of the thin plate is not desirable for making the thin plate flat because the shape of the holding surface formed by using the polishing pad comes to be different from that of the thin plate by using the same polishing pad. On the contrary, according to the polishing apparatus of the present invention, because the properties to be polished, of the holding surface and the thin plate are the same or similar to each other, the holding surface can be polished by the same polishing apparatus as one for the thin plate, and it is possible to polish the thin plate with high flatness since the holding surface and the thin plate can be made to have approximately the same surface shapes by the polishing pad. In order to provide the holding plate having such a surface for holding the thin plate, one surface of the holding plate, for holding to adhere the thin plate may comprise fine powder of material which is polished in approximately the same degree as the thin plate when the one surface is polished under the same polishing condition as that of the thin plate, and binding resin material for binding the fine powder material. The surface of the holding plate, for holding to adhere the thin plate, comprising Si or Si fine powder bound with a resin material is preferable for polishing an Si wafer. The holding plate may comprise a surface member for holding to adhere the thin plate on one surface thereof, which is made of a material which is polished in approximately the same degree as the thin plate when the one surface is polished under the same polishing condition as that of the thin plate, and a plate body attached to the surface member. The surface member may be made of Si single crystal and the plate body is made of Al 2 O 3 . The surface member may also comprise fine powder of Si or CaCO 3 , and binding resin material for binding the fine powder. The binding resin material may be an epoxy resin and the plate body is made of Al 2 O 3 , and a plurality of through holes for vacuum suction are formed through the surface member and the plate body. According to the polishing apparatus having such a structure, it is possible to easily form the through holes for vacuum suction in the epoxy resin layer by supplying compressed air with a high temperature through through holes formed in the plate body to make through holes in the soft epoxy layer and thereafter to harden it. As described above, according to the method for polishing a thin plate of the present invention, because the holding surface of the holding plate is polished by using the polishing pad for polishing the thin plate before the thin plate is held to adhere on the holding surface of the holding plate, it is possible to obtain a thin plate having a very high flatness easily. Because the holding surface of the holding plate, for holding to adhere the thin plate is made of a material which is polished in approximately the same degree as the thin plate when the holding surface is polished under the same polishing condition as that of the thin plate, it is possible to process the holding surface for holding the thin plate in a desired shape easily by the apparatus for polishing a thin plate, and to obtain a thin plate having a high flatness. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein; FIGS. 1A to 1C are vertical sectional views showing steps of the polishing method according to a first embodiment of the present invention; FIG. 2 is a vertical sectional view of the principal portion of the polishing apparatus according to a first embodiment of the present invention; FIG. 3 is a vertical sectional view of the principal portion of the polishing apparatus according to a second embodiment of the present invention; FIG. 4 is a vertical sectional view of the principal portion of the polishing apparatus according to a third embodiment of the present invention; FIG. 5 is a vertical sectional view of a useful example to which the present invention can be applied; FIG. 6 is a vertical sectional view of another useful example to which the present invention can be applied; and FIG. 7 is a schematic side view of an example of a apparatus for polishing a thin plate. Hereinafter, embodiments of the polishing method and the polishing apparatus, according to the present invention will be explained. A whole system of the polishing apparatus according to an embodiment of the present invention is shown in FIG. 7. A first embodiment of the holding plate 2 in this figure is shown in FIG. 2. In FIG. 2, the holding plate 12 is made of silicon (Si) single crystal which is the same material as the wafer (thin plate) 1. In the holding plate, a plurality of through holes 13 which is communicated with a vacuum pump through a vacuum passage are formed vertically. The wafer 1 is held to adhere on a holding surface 12a for holding a thin plate, of the holding plate 12 by vacuum suction through the through holes 13. In order to polish the wafer with a higher flatness, first, the holding plate 12 is set on the top ring 3 of the polishing apparatus without holding the wafer 1 on the holding plate 12 by vacuum suction, as shown in FIG. 1A. The lower surface (holding surface) 12a of the holding plate 12 is brought into contact with a polishing pad 7 mounted on the upper surface of the rotating turn table 5, with applying a predetermined pressure, so that the holding surface 12a is polished under the same condition as one for polishing the wafer 1, as shown in FIG. 1B. Such a polishing is carried out while rotating the holding plate 12 through the top ring 3 and rotating the polishing pad 7 through the rotating turn table 5, and supplying the abrasive slurry 8 to the polishing pad 7, as shown in FIG. 7. As the result, the holding surface 12a has a shape like one to which the upper surface of the polishing pad 7 is transferred. Thereafter, the wafer 1 is held to adhere on the holding surface 12a of the set holding plate 12 by vacuum suction and the lower surface of the wafer 1 is polished by the polishing pad 7, in the same manner and under the same polishing condition, as polishing for the holding surface 12a of the holding plate 12, as shown in FIG. 1C. FIGS. 1A to 1C show polishing steps in the case of the upper surface of the polishing pad 7 curved a little. According to the embodiment, because the whole holding plate 12 is made of the same material as the wafer 1, it is possible to polish the holding plate 12 by using the polishing pad 7 for polishing the wafer 1 easily. By polishing both the holding plate 12 and the wafer 1 by using the same polishing pad 7, it is possible to secure a higher flatness of the wafer 1 even if the condition or the like of the polishing pad 7 was changed with the passage of time. A polishing apparatus according to the second embodiment of the present invention is shown in FIG. 3. In FIG. 3, the holding plate 14 comprises a surface member 14b for adhering to hold the wafer (thin plate) 1 on the lower surface (holding surface) 14a thereof and a plate body 14c which is adhered to the surface member 14b by an epoxy adhesive or the like. The surface member 14b is made of a material which can be polished in approximately the same degree as the thin plate 1 when the lower surface 14a is polished under the same polishing condition as that of the thin plate 1, e.g., a silicon single crystal or the like. Therefore, when the thin plate 1 is a silicon wafer, preferably, the surface member 14b is also made of a silicon single crystal, and the plate body 14c is made of, for example, Al 2 O 3 or the like. In the holding plate 14, a plurality of through holes 15 which pass through the surface member 14b and the plate body 14c, are formed for holding to secure the wafer 1 on the holding surface 14a of the surface member 14b by vacuum suction. In the second embodiment, in order to polish the wafer 1 with a higher flatness, the holding surface 14a of the surface member 14b is also polished by using the polishing pad 7 for polishing the wafer 1. Thereafter, the wafer 1 is held to adhere on the holding surface 14a of the set holding plate 14 by vacuum suction and the lower surface of the wafer 1 is polished by the polishing pad 7, in the same manner and under the same polishing condition, as the preceding polishing step for the holding surface 14a, like the first embodiment. According to this embodiment, it is possible to provide approximately the same function and the same advantageous effects as the first embodiment. A polishing apparatus according to the third embodiment of the present invention is shown in FIG. 4. In FIG. 4, the holding plate 16 comprises a surface member 16b for adhering to hold the wafer 1 on the lower surface 16a thereof and a plate body 16c which is adhered to the surface member 16b. The plate body 16c is made of Al 2 O 3 or the like. The surface member 16b comprises Si fine powder having a particle diameter of 20-100 µm and binding resin material, e.g., epoxy resin, for binding the Si fine powder. In the holding plate 16, a plurality of through holes 17 which pass through the surface member 16b and the plate body 16c, are formed for holding to secure the wafer 1 on the holding surface 16a of the surface member 16b by vacuum suction. In order to manufacture the holding plate 16, first, epoxy resin containing the above-described Si fine powder is coated directly to a surface of the plate body 16c which has a plurality of through holes 17 for vacuum suction formed therein, with a predetermined thickness, and the surface of the epoxy coating layer for the surface member 16b is made flat by a knife edge. Thereafter, through holes 17b are formed to communicate with the through holes 17 of the holding plate 16c, in the coating layer 16b by supplying compressed air through the through holes 17 of the holding plate 16c. The epoxy coating layer 16b having the formed through holes 17b is hardened by making the temperature of the supplied air higher than the hardening temperature of epoxy resin. According to such a method, it is possible to form the through holes 17b very easily. Thereafter, a lapping treatment is performed to the lower surface to be the holding surface 16a, of the hardened coating layer 16b to make it flatter. In the third embodiment, in order to polish the wafer 1 with a higher flatness, the holding surface 16a of the surface member 16b is also polished by using the polishing pad 7 for polishing the wafer 1. Thereafter, the wafer 1 is held to adhere on the holding surface 16a of the set holding plate 16 by vacuum suction and the lower surface of the wafer 1 is polished by the polishing pad 7, in the same manner and under the same polishing condition, as polishing for the holding surface 16a, like the first embodiment. According to this embodiment, it is possible to provide approximately the same function and the same advantageous effects as the first embodiment. Instead of forming the surface member for the thin plate by binding the Si fine powder with an epoxy binder, it may form the surface member by binding fine powder of a material which is polished in approximately the same degree as the thin plate when the surface thereof is polished under the same polishing condition as that of the thin plate, for example, of CaCO 3 , with a binder of thermosetting resin such as epoxy or the like. According to use of such a surface member, it is possible to improve the polishing speed in comparison with use of only a thermosetting resin. For example, when polishing was carried out under a polishing condition including a polishing pressure of 500 g/cm 2 , a relative speed of 113 m/min, and abrasive slurry containing abrasive grains of colloidal silica, the polishing efficiency in the use of only an epoxy resin was 0.18 µm/min, however, the polishing efficiency in the use of an epoxy resin containing CaCO 3 fine powder of 30% (W/N) having a particle diameter of 20-50 µm was 0.75 µm/min. This value is approximately equivalent to 0.90 µm/min for silicon and is therefore suitable for polishing of a silicon wafer. The present invention can be applied for polishing for an Si wafer, an SiO 2 mask, an SOI wafer and the like. FIG. 5 shows an SOI wafer 18 which comprises an Si substrate 19 having a thickness of 600-800 µm and an Si layer 21 having a thickness of 1±0.3 µm formed on the Si substrate 19 through an SiO 2 layer 20 having a thickness of 0.5 µm. In the SOI wafer 18, the Si layer 21 requires a high uniformity of the thickness. Therefore, the present invention is very useful for making the thickness of the Si substrate 19 uniform, and for making the thickness of the Si layer 21 uniform after bonding the Si layer 21 to the Si substrate 19 through the SiO 2 layer 20. The present invention is useful for planarization for a device, that is, for removing steps formed on the surface of SiO 2 film which is deposited by CVD or the like to cover wiring of such materials as metal, metal silicide, polysilicon or the like provided on an Si substrate. FIG. 6 shows a useful application of the present invention for such a processing. In FIG. 6, the surface member 22b for holding a thin plate 1, of the holding plate 22 is made of SiO 2 which is the same material as the object to be polished. The plate body 22c of the holding plate 22 is made of Al 2 O 3 or the like. In the holding plate 22, a plurality of through holes 23 which pass through the surface member 22b and the plate body 22c, are formed for holding to secure the wafer 1 on the holding surface 22a of the surface member 22b by vacuum suction. A wafer 24 comprises an Si substrate 25, wiring 26 having a thickness of about 1 µm which is disposed on the lower surface of the Si substrate 25, and an SiO 2 film 27 formed thereon to cover the wiring 26 and the Si substrate 25. The SiO 2 film 27 has a high uniformity of thickness in the whole region of the wafer 24, however, minute steps, i.e., unevenness having a thickness of about 1 µm, are formed on the lower surface thereof at positions corresponding to the wiring 26. Therefore, the polishing process for the SiO 2 film 27 requires the removal of the steps with keeping a high uniformity of thickness in the whole region of the wafer 24. In order to meet the requirement, the present invention can be applied. That is, the holding surface 22a of the surface member 22b is polished by using the polishing pad 7 for polishing the SiO 2 film 27 under the same polishing condition as polishing for the SiO 2 film 27. Thereafter, the SiO 2 film 27 is polished by the polishing pad 7, in the same manner and under the same polishing condition, as the preceding polishing step for the holding surface 22a, so that it is possible to obtain a wafer 24 having a high flatness. In the above-described embodiments, only the cases that the portion to be polished of the wafer is made of silicon or SiO 2 , are explained. However, it is of course that the present invention is not limited to these cases. Basically, the present invention has no limitation with respect to the material to be polished, for example, it can be applied to a polishing for various types of compound semiconductor substrates, a general polishing for a thin plate other than a wafer, and the like.

Claims: A method of polishing a thin plate (1), comprising the steps of: rotating a holding plate (12) on one surface of which the thin plate (1) is held; and, bringing the thin plate (1) held on the rotating holding plate (12) into contact with a polishing pad (7) mounted on a surface of a rotating turn table, to polish the thin plate (1); characterised in that prior to holding the thin plate (1) on the holding plate (12) the one surface of the holding plate (12) is polished by using the said polishing pad (7). A method of polishing according to claim 1; wherein the step of polishing for the one surface of the holding plate (12) is carried out under substantially the same conditions as that of polishing the thin plate (1). A method of polishing according to claim 1 or 2; wherein the one surface of the holding plate (12) is made of a material with substantially the same polishing properties as the thin plate (1). A method of polishing according to any one of the preceding claims, wherein the thin plate (1) is an Si wafer, an SiO 2 mask, an SOI wafer, or a semiconductor substrate with wiring formed on it and an insulating film deposited on the wiring and the substrate, and the method is performed for planarization of the insulating film. A polishing apparatus for polishing a silicon wafer (1), comprising: a rotatable holding plate (12) for holding the silicon wafer (1) on one surface (12a) of it; and, a rotatable turn table (5) having a polishing pad (7) on it for polishing the silicon wafer (1) by bringing the rotating silicon wafer (1) into contact with the rotating polishing pad (7); characterised in that the said one surface (12a) of the holding plate (12) for holding the silicon wafer (1) is made of a material which has substantially the same polishing properties as the silicon wafer (1). A polishing apparatus according to claim 5, wherein at least the said one surface (12a) of the holding member is made of or includes substantially the same material as the silicon wafer (1). A polishing apparatus according to claim 6, wherein the said one surface (12a) of the holding plate (12) is made entirely from silicon or from a mixture of silicon powder and a binding resin. A polishing apparatus according to any one of claims 5 to 7, wherein the holding plate (12) comprises a surface member (14b,16b) including the said one surface (12a) which is made of a material which has substantially the same polishing properties as the silicon wafer (1), and a plate body (14c, 16c) attached to the surface member (14b,16b). A polishing apparatus according to claim 8, wherein the surface member (14b,16b) is made of silicon single crystal or a fine powder of Si or CaCO 3 , and binding resin material such as an epoxy resin for binding the fine powder, and the plate body (14c, 16c) is made of Al 2 O 3 . A polishing apparatus according to any one of claims 5 to 9, wherein a plurality of through holes (13,15,17,17b) for vacuum suction are formed in the holding plate (12).

Inventors: MORITA KOJI, TAKAKU TSUTOMU, TANAKA KOICHI, TSUCHIYA TOSHIHIRO, MORITA, KOJI, TAKAKU, TSUTOMU, TANAKA, KOICHI, TSUCHIYA, TOSHIHIRO, Morita, Koji, c/o SEH Shirakawa R &amp; D Center, Takaku, Tsutomu, c/o SEH Shirakawa R &amp; D Center, Tanaka, Koichi, c/o SEH Shirakawa R &amp; D Center, Tsuchiya, Toshihiro, SEH Shirakawa R &amp; D Center
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI CO., LTD.
Application year: 1997
Main classification: B24B37
IPC codes: H01L21, B24B37