Method for cutting a wafer hard to cut

Abstract: A method for cutting a wafer hard to cut into sections comprising steps of scribing the wafer so as to form a groove along a cutting line, drilling bores on the grooves at proper intervals and breaking the wafer along the groove.

Description: Background of the Invention Field of the invention The present invention relates to a method for cutting a wafer hard to cut, and more specifically to a method for cutting a wafer hard to cut such as a SrTiO 3 single crystal wafer etc. Description of related art Most of crystalline wafers of inorganic materials such as semiconductor wafers, oxide wafers etc. which are utilized for electric devices are very hard. In order to divide or cut the wafers, a method utilizing cleavage which is one of crystallographic characteristics, or a method utilizing a diamond tool such as a diamond cutter or a dicing saw are generally used. For example, on one silicone wafer which is utilized for manufacturing integrated circuits, a lot of semiconductor integrated circuits are simultaneously prepared and they are divided into individual integrated circuits by using diamond cutter. A thin film of an oxide superconductor recently advanced in study is often deposited on an oxide single crystal wafer such as an MgO single crystal wafer or a SrTiO 3 single crystal wafer etc. Since, high quality oxide superconductor thin films are rather easily deposited on these oxide single crystal wafers. Of course, devices which utilize oxide superconductor thin films are often prepared on the above oxide wafers. When an oxide superconductor thin film is deposited on the above oxide wafer, the wafer may be divided into sections after the oxide superconductor thin film is deposited. The wafer on which an oxide superconductor thin film is deposited is usually divided by using a diamond cutter, not using a dicing saw. Since, dicing saw cuts the wafer with lubrication with water or oil, so that the oxide superconductor thin film is contaminated and degraded. An MgO single crystal wafer can be easily cut and divide into sections by a diamond cutter. However, owing to crystallographic characteristics of SrTiO 3 , it is difficult to cut a SrTiO 3 single crystal wafer by a diamond cutter. If a SrTiO 3 single crystal wafer is tried to cut by a diamond cutter, it breaks in pieces. Therefore, in the prior art, it is difficult to divide an oxide superconductor thin film into desired sections, if it is deposited on a SrTiO 3 single crystal wafer. In particular, if fine superconducting devices are prepared on a SrTiO 3 single crystal wafer, it is almost impossible to cut the SrTiO 3 single crystal wafer without damaging the fine superconducting devices. JP-A-02 179 708 discloses a method for breaking a semiconductor wafer by scribing and drilling. This method allows to reduce the stress during breaking, but is time-consuming and needs very precise apparatus and operation. DE-A-2 121 455 also discloses a method for breaking a semiconductor wafer by scribing and drilling. This method is analogous to the method of the hereabove mentionned document. US-A-3 453 097 discloses a method for cutting glass which is an amorphous material. Amorphous materials are isotropic and thus differ from semiconductor crystal wafers. DD-A-248 544 discloses a method where a crystal silicon wafer receives a groove and the wafer is submitted to an homogeneous temperature field to break the wafer. With this method, relatively high stresses would appear in the crystal with risk of inducing unhomogeneous breaking line or splittering the wafer. Summary of the Invention Accordingly, it is an object of the present invention to provide a method for cutting a wafer hard to cut, which have overcome the above mentioned defects of the conventional ones. Another object of the present invention is to provide a method for cutting a wafer hard to cut, on which an oxide superconductor thin film is formed, which have overcome the above mentioned defects of the conventional ones. The above and other objects of the present invention are achieved by a method for breaking an oxide single-crystal wafer (1) wherein the wafer (1) comprises an oxide superconductor film (2) deposited on its surface, comprising the steps of scribing the wafer (1) so as to form a groove (3) along a cutting line, and applying a bending moment to the wafer (1), characterized in that the groove (3) is scribed into a (100) crystal surface of the wafer (1) and that the wafer is locally heated in the groove (3). In this method, the groove is preferably scribed so as to have a depth equal to 10 to 60 % of the thickness of the wafer. If the groove is too shallow, it is difficult to cut the wafer into the required sections. If the groove is too deep, the wafer may break. In one preferred embodiment, the wafer is a SrTiO 3 wafer and is heated to a temperature of 800 to 1000°C. If the wafer heated to a temperature lower than 800°C, the wafer can not cut sharply. If the wafer is heated to a temperature higher than 1000°C, the crystalline of the wafer may be degraded. In addition, if a thin film, for example an oxide superconductor thin film, is formed on the wafer, mutual diffusion may occur between the thin film and the wafer, when the wafer is heated to a temperature higher than 1000°C. In accordance with the present invention, the wafer is preferably heated for 30 seconds to 5 minutes. If the heating duration is shorter than 30 seconds, the wafer can not be cut sharply. However, if the heating duration is too long, the crystalline of the wafer may be degraded and mutual diffusion may occur between a thin film formed on the wafer and the wafer. The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. Brief Description of the Drawings Figures 1A to 1E are schematic perspective views for illustrating a first embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which an oxide superconductor thin film is deposited; Figures 2A and 2B are schematic perspective views for illustrating featured steps of a second embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which an oxide superconductor thin film is deposited; Figure 3A is a schematic view of a SrTiO 3 wafer on which an oxide superconductor thin film is deposited, which will be cut by the method in accordance with the present invention; and Figure 3B is a schematic view of a SrTiO 3 wafer on which superconducting field effect devices are prepared, which will be divided into individual devices by the method in accordance with the present invention. Description of the Preferred embodiments Embodiment 1 Referring to Figures 1A to 1E, a first embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which an oxide superconductor thin film is deposited will be described. As shown in Figure 1A, a SrTiO 3 (100) wafer 1 having dimensions 15 mm × 15 mm and a thickness of 0.5 mm is prepared. Then, as shown in Figure 1B, a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 having a thickness of 200 nanometers is formed on the (100) surface of the SrTiO 3 wafer 1 by a MBE (Molecular Beam Epitaxy). A condition of forming the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 by an MBE is as follows: Molecular beam source Y: 1250°C and its crucible temperature Ba: 600°C Cu: 1040°C Wafer temperature 700°C Pressure 1 × 10 -5 Torr Thereafter, as shown in Figure 1C, the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 and the SrTiO 3 wafer 1 is scribed by using a diamond cutter so as to form a groove 3 across the SrTiO 3 wafer 1 having the maximum width of 0.35 mm and a depth of 0.1 mm. Then, as shown in Figure 1D, bores 4 having a diameter of 1 mm and a thickness of 0.25 mm are drilled in the SrTiO 3 wafer 1 at intervals of 3 to 4 mm on the groove 3 by using a diamond drill 40. As mentioned above, the bores preferably have a diameter of on the order of 1 mm and are drilled at intervals 3 to 4 mm. If the diameter of the bore is small, a margin to cut can be reduced. However, it is difficult to drill a bore having a small diameter and it is necessary to drill more bores. The depth of the bore is preferably 10 to 60 % of the thickness of the wafer. If the bore is too shallow, it is difficult to cut the wafer into the required sections. If the bore is too deep or penetrates the wafer, the wafer may break. Finally, one side of the SrTiO 3 wafer 1 with respect to the groove 3 is hold by a clamp and broken into two pieces along the groove 3, as shown in Figure 1E. After the SrTiO 3 wafer 1 is cut into two pieces, the superconducting characteristics of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 are not affected. As explained above, if a SrTiO 3 wafer on which an oxide superconductor thin film is deposited is cut into sections in accordance with the embodiment of the method of the present invention, the SrTiO 3 wafer can be easily divided into desired sections. In addition, individual sections of the oxide superconductor thin film are not degraded and the superconducting characteristics are not affected. Therefore, it becomes possible to prepare a lot of oxide superconductor thin films which are formed by the same conditions and have the same characteristics so that study of oxide superconductors can be advanced efficiently. Embodiment 2 Referring to Figures 2A and 2B, a second embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which an oxide superconductor thin film is deposited will be described. In this second embodiment, the same processings as those shown in Figures 1A to 1C are performed. After the groove 3 are scribed on the SrTiO 3 wafer 1, a heating wire 5 of Pt having a diameter of 0.3 mm is inserted into groove 3, as shown in Figure 2A. The depth of the groove is preferably 10 to 60 % of the thickness of the wafer. If the groove is too shallow, it is difficult to cut the wafer into the required sections. If the groove is too deep, the wafer may break. Then, direct current is supplied to the heating wire 5 by a power supply 50 so that a portion of the SrTiO 3 wafer 1 near the groove 3 is locally heated to a temperature of 1000°C for four minutes. The temperature to which the wafer is heated is preferably 800 to 1000°C and the duration of the heating is preferably 30 seconds to 5 minutes. For this purpose, the heating wire is preferably formed of a material hard to be oxidized such as Pt, Pt-Rh alloy. After the SrTiO 3 wafer 1 is cooled to the room temperature, one side of the SrTiO 3 wafer 1 with respect to the groove 3 is hold by a clamp and broken into two pieces along the groove 3, as shown in Figure 2B. The superconducting characteristics of the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 are not affected, even after the SrTiO 3 wafer 1 is cut into two pieces. As explained above, if a SrTiO 3 wafer on which an oxide superconductor thin film is deposited is cut into sections in accordance with the second embodiment of the method of the present invention, the SrTiO 3 wafer can be easily divided into desired sections. In addition, individual sections of the oxide superconductor thin film are not degraded and the superconducting characteristics are not affected. Therefore, it becomes possible to prepare a lot of oxide superconductor thin films which are formed by the same conditions and have the same characteristics so that study of oxide superconductors can be advanced efficiently. Embodiment 3 Referring to Figure 3A, a third embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which an oxide superconductor thin film is deposited will be described. In this third embodiment, a SrTiO 3 wafer 1 having dimensions 15 mm × 15 mm and a thickness of 0.5 mm, on which a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 is deposited is cut along cutting lines 31 to 33 into sections 21 to 24 by the same method as the Embodiment 1. At first, the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 having a thickness of 200 nanometers is formed under the same condition as the Embodiment 1. Then, the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 and the SrTiO 3 wafer 1 are scribed by using a diamond cutter so as to form grooves 31 to 33 similar to the groove 3 of the Embodiment 1. Thereafter, bores having a diameter of 1 mm and a thickness of 0.25 mm are drilled in the SrTiO 3 wafer 1 at intervals of 3 to 4 mm on the grooves 31 to 33 by using a diamond drill. Finally, one side of the SrTiO 3 wafer 1 with respect to one of the grooves 31 to 33 is hold by a clamp and broken into two pieces along the groove, and this process is repeated three times so that the SrTiO 3 wafer 1 is divided into sections 21 to 24. The Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin films of the sections 21 to 24 were evaluated by XPS (X-ray photoelectron spectroscopy), measuring the critical temperature and the critical current density, ICP (inductively coupled plasma) and SEM (scanning electron microscope). It became clear that the oxide superconductor thin films were of good condition. Embodiment 4 Referring to Figure 3A again, a forth embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which an oxide superconductor thin film is deposited will be described. In this forth embodiment, a SrTiO 3 wafer 1 having dimensions 15 mm x 15 mm and a thickness of 1 mm, on which a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 is deposited is cut along cutting lines 31 to 33 into sections 21 to 24 by the same method as the Embodiment 2. At first, the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 having a thickness of 200 nanometers is formed under the same condition as the Embodiment 1. Then, the Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film 2 and the SrTiO 3 wafer 1 is scribed by using a diamond cutter so as to form grooves 31 to 33 having a depth of 0.3 mm. Thereafter, heating wires similar to the heating wire 5 of the Embodiment 2 are inserted into grooves 31 to 33. Then, direct current is supplied to the heating wires so that portions of the SrTiO 3 wafer 1 near the grooves 31 to 33 are locally heated to a temperature of 900°C for five minutes. After the SrTiO 3 wafer 1 is cooled to the room temperature, one side of the SrTiO 3 wafer 1 with respect to one of the grooves 31 to 33 is hold by a clamp and broken into two pieces along the groove, and this process is repeated three times so that the SrTiO 3 wafer 1 is divided into sections 21 to 24. The Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin films of the sections 21 to 24 were evaluated by XPS, measuring the critical temperature and the critical current density, ICP and SEM. It became clear that the oxide superconductor thin films were of good condition. Embodiment 5 Referring to Figure 3B, a fifth embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which a lot of superconducting field effect devices (abbreviated as super-FET hereinafter) utilizing a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film are formed will be described. In this fifth embodiment, a SrTiO 3 wafer 1 having dimensions 15 mm × 15 mm and a thickness of 0.7 mm, on one surface 20 of which a lot of super-FETs utilizing a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film are formed is cut along cutting lines 301 to 314 and 401 to 414 into individual superconducting field effect devices by the same method as the Embodiment 1. At first, a lot of super-FETs are manufactured on the surface 20 of the SrTiO 3 wafer 1. The structure of the super-FET and the process for manufacturing the super-FET are disclosed by U. S. Patent No. 5,236,896 issued to NAKAMURA et al. Then, the surface 20 of the SrTiO 3 wafer 1 is scribed by using a diamond cutter so as to form grooves 301 to 314 and 401 to 414 similar to the groove 3 of the Embodiment 1. Thereafter, bores having a diameter of 1 mm and a thickness of 0.35 mm are drilled in the SrTiO 3 wafer 1 at intervals of 3 to 4 mm on the grooves 301 to 314 and 401 to 414 by using a diamond drill. Finally, one side of the SrTiO 3 wafer 1 with respect to one of the grooves 301 to 314 and 401 to 414 is hold by a clamp and broken into two pieces along the groove, and this process is repeated so that the super-FET are divided into individual devices. Each of the divided super-FETs was evaluated, so that it became clear that it was of good condition and its properties were not spoiled. Embodiment 6 Referring to Figure 3B again, a sixth embodiment of the method in accordance with the present invention for cutting a SrTiO 3 wafer on which a lot of super-FET utilizing a Y 1 Ba 2 U 3 O 7-δ oxide superconductor thin film are formed will be described. In this sixth embodiment, a SrTiO 3 wafer 1 having dimensions 15 mm × 15 mm and a thickness of 0.7 mm, on one surface 20 of which a lot of super-FETs utilizing a Y 1 Ba 2 Cu 3 O 7-δ oxide superconductor thin film are formed is cut along cutting lines 301 to 314 and 401 to 414 into individual superconducting field effect devices by the same method as the Embodiment 2. The surface 20 of the SrTiO 3 wafer 1, on which a lot of super-FET are manufactured similarly to that of the Embodiment 5 is scribed by using a diamond cutter so as to form grooves 301 to 314 and 401 to 414 having a thickness of on the order of 0.35 mm. Thereafter, heating wires similar to the heating wire 5 of the Embodiment 2 are inserted into grooves 301 to 314 and 401 to 414. Then, direct current is supplied to the heating wires so that portions of the SrTiO 3 wafer 1 near the grooves 301 to 314 and 401 to 414 are locally heated to a temperature of 800°C for four minutes. After the SrTiO 3 wafer 1 is cooled to the room temperature, one side of the SrTiO 3 wafer 1 with respect to one of the grooves 301 to 314 and 401 to 414 is hold by a clamp and broken into two pieces along the groove, and this process is repeated so that the super-FET are divided into individual devices. Each of the divided super-FETs was evaluated, so that it became clear that it was of good condition and its properties were not spoiled. In the above mentioned embodiments, the oxide superconductor thin film can be formed of not only the Y-Ba-Cu-O compound oxide superconductor material, but also a high-T c (high critical temperature) oxide superconductor material, particularly a high-T c copper-oxide type compound oxide superconductor material, for example a Bi-Sr-Ca-Cu-O compound oxide superconductor material, and a Tl-Ba-Ca-Cu-O compound oxide superconductor material. In addition, the present invention can be applied to cutting not only SrTiO 3 wafer but also a wafer hard to cut such as a wafer of a material which can not be cloven, an amorphous wafer, etc. The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the illustrated structures but converts and modifications may be made within the scope of the appended claims.

Claims: A method for breaking an oxide single crystal wafer (1) wherein the wafer (1) comprises an oxide superconductor film (2) deposited on its surface, comprising the steps of scribing the wafer (1) so as to form a groove (3) along a cutting line, and applying a bending moment to the wafer (1), characterized in that the groove (3) is scribed into a (100) crystal surface of the wafer (1) and that the wafer is locally heated in the groove (3). A method as claimed in Claim 1, wherein the groove (3) has a depth equal to 10 to 60 % of the thickness of the wafer. A method as claimed in anyone of Claims 1 or 2, wherein the portion of the wafer (1) is locally heated to a temperature of 800 to 1000°C. A method as claimed in anyone of Claims 1 to 3, wherein the wafer (1) is heated for 30 seconds to 5 minutes. A method as claimed in anyone of Claims 1 to 4, wherein the wafer (1) is heated by a heating wire. A method as claimed in anyone of Claims 1 to 5, wherein the wafer (1) is a SrTiO 3 wafer. A method as claimed in anyone of Claims 1 to 6, wherein the method further comprises the step of cooling the wafer (1) to room temperature between the heating step and the breaking step. A method as claimed in anyone of claims 1 to 7, wherein the oxide superconductor film (2) is Y 1 Ba 2 Cu 3 O 7-δ . A method as claimed in anyone of Claims 1 to 8, wherein the oxide superconductor film (2) has a thickness of approximately 200 nanometers.

Inventors: NAKAMURA TAKAO, NAKAMURA, TAKAO, Nakamura, Takao, c/o Osaka Works Sumitomo El.Ind.
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1993
Main classification: B28D5
IPC codes: B28D5, H01L21, H01L39