Apparatus for separating a composite member using fluid jet

Abstract: Apparatus is provided to separate a composite member consisting of a plurality of bonded members (1 and 2) without destructing or damaging it, a fluid (7) is jetted against the composite member from a nozzle (8) to separate it into a plurality of members (11 and 12) at a position (3) different from a bonding position (14). The apparatus is provided with a holder for supporting and positioning the composite member, and a jet or jets for jetting fluid against the edge surface of the composite member.

Description: The formation of a single crystal Si semiconductor layer on an insulating surface of a substrate is widely known as a semiconductor on insulator (SOI) technique, and many efforts have been made to research this technique because devices produced using the SOI technique have many advantages that cannot be achieved by bulk Si substrates used to fabricate normal Si integrated circuits. The use of the SOI technique provides the following advantages: (1) The dielectric separation can be easily made to attain high integration. (2) Radiation resistance is excellent. (3) The stray capacity is reduced to attain high speed. (4) The well formation process can be omitted. (5) Latch-up can be prevented. (6) The thickness can be reduced to provide a fully depleted field effect transistor. To achieve the many advantages of the device, methods for forming SOI structures have been researched for decades. One of such known methods is SOS (silicon on sapphire) in which Si is heteroepitaxially formed by CVD (chemical vapor deposition) on a single crystal sapphire substrate. This technique has been successful as the maturest SOI technique, but its applications are limited by a large amount of crystal defects due to the misalignment of lattices in the interface between an Si layer and a sapphire substrate, by the mixture of aluminum from the sapphire substrate into the Si layer, and in particular, by the high costs of the substrate and the still insufficient the enlargement of area of the device. More recently, an attempt has been made to implement an SOI structure without the sapphire substrate. This attempt can be roughly classified into the following two methods. 1. After the surface of an Si single crystal substrate is oxidized, a window is made in the oxidized film to expose a part of the surface of the Si substrate, and this part is used as a seed to allow a horizontal epitaxial growth to form an Si single crystal layer on the SiO 2 (in this case, an Si layer is deposited on SiO 2 ). 2. The Si single crystal substrate is used as an active layer and SiO 2 is formed under this layer (this method does not require an Si layer to be deposited). Known means for realizing the above method 1 include a method for allowing the direct horizontal epitaxial growth of single crystal layer Si using CVD, a method of depositing amorphous Si and allowing its horizontal epitaxial growth in a solid phase by thermal treatment, a method of irradiating an amorphous or polycrystal Si layer with converging energy beams such as electron or laser beams, and allowing a single crystal layer to grow on SiO 2 by means of melting recrystallization, and a method of using a bar-like heater to scan a molten area in such a way that the scanning trace appears like a band (zone melting recrystallization). Although these methods have both advantages and disadvantages, they still have many problems in terms of their controllability, productivity, uniformity, and quality and none of them have been put to industrially practical use. For example, the CVD method requires sacrificial oxidization to provide flat films. The solid phase growth method provides poor crystallinity. The beam anneal method has problems in terms of the time required for converging-beam scanning, and control of the superposition of beams, and focusing. Among them, the zone melting recrystallization method is maturest and has been used to produce relatively-large-scale integrated circuits on an experimental basis, but it still causes a large amount of crystal defects such as sub-grains to remain in the device, thereby failing to fabricate minor-carrier devices and to provide sufficiently excellent crystals. The above method 2 that does not use the Si substrate as a seed for epitaxial growth includes the following four methods. (1) An oxide film is formed on an Si single crystal substrate with a V-shaped groove etched anisotropically in its surface, a polycrystal Si layer is deposited on the oxide film so as to be as thick as the Si substrate, and then an Si single crystal region surrounded by the V-shaped groove so as to be separated dielectricly is formed on the thick polycrystal Si layer by polishing from the rear surface of the Si substrate. This method provides excellent crystallinity but the steps for depositing polycrystal Si by a thickness of several hundred microns and polishing the single crystal Si substrate from its rear surface to leave only the separated Si active layer have problems in terms of controllability and productivity. (2) SIMOX (Separation by Ion-Implemented Oxygen) that forms an SiO 2 layer in an Si single crystal substrate by means of oxygen ion implantation and that is the presently maturest technique due to its excellent compatibility with the Si process. To form an SiO 2 layer, however, 10 18 ions/cm 2 or more of oxygen ions must be implanted, resulting in the need for a large amount of time for the implantation, thereby leading to reduced productivity. In addition, the costs of wafers are high. Furthermore, this method cause a large amount of crystal defects to remain in the device and does not industrially provide a sufficient quality to fabricate minor-carrier devices. (3) A method for forming an SOI structure by dielectric separation through the oxidization of porous Si. In this method, an N-type Si layer is formed like an island on a surface of a P-type Si single crystal substrate by proton-ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) or by epitaxial growth and patterning. Only the P-type Si substrate is made porous by an anodization method using an HF solution in such a way that the porous region surrounds the Si island from the surface, and the N-type Si island is then oxidized at a high speed for dielectric separation. In this method, the separating Si region is determined prior to the device step, thereby limiting the degree of freedom of device design. (4) A method for forming an SOI structure using thermal treatment or an adhesive to bond an Si monocrystal substrate on a different Si single crystal substrate that is thermally oxidized is attracting attention. This method requires an active layer for a device to be formed as a uniformly thin film. That is, the thickness of a several-hundred-micron-thick Si single crystal substrate must be reduced to the order of micron or less. The following two methods can be used to provide a thinner film. 1) Thickness reduction by polishing 2) Thickness reduction by selective etching The polishing in 1) cannot provide uniformly thin films easily. In particular, if the thickness is reduced to the order of submicron, the thickness variation will be several tens %, resulting in a serious problem for providing uniformity. The difficulty in achieving uniformity further increases with increasing size of the substrate. In addition, although the etching in 2) is supposed to be effective in providing uniform thin films, it has the following problems. The selection ratio is at most 10 2 and is insufficient. The surface obtained after etching is bad. The crystallinity of the SOI layer is bad due to the use of ion implantation or epitaxial or heteroepitaxial growth on a high concentration B-doped Si layer. A semiconductor substrate formed by bonding requires two substrates, one of which is mostly uselessly removed and disposed of through polishing and etching, thereby wasting limited global resources. Thus, SOI with bonding presently has many problems in terms of its controllability, uniformity, and costs. In addition, generally due to the disorder of the crystal structure of a light-transmissive substrate represented by glass, a thin film Si layer deposited on the substrate can only form an amorphous layer or a polycrystal layer based on the disorder of substrates, and therefore high-performance devices cannot be produced. This is because since amorphous structure of the substrate is amorphous, an excellent single crystal layer cannot be obtained by simply depositing an Si layer. The light-transmissive substrate is important in producing a contact sensor or a projection liquid-crystal image display device that is a light-receiving element. Not only the improvement of pixels but also a high-performance drive element are required to attain higher density, higher resolution, and finer definition of the pixels in the sensor or display device. Thus, to provide elements on the light-transmissive substrate, a single crystal layer of an excellent crystallinity is required. Among such SOI substrate production methods, the method of forming a non-single-crystal semiconductor layer on a porous layer and transferring the layer onto a supporting substrate via an insulating layer as disclosed in Japanese Patent Application Laid-Open No. 5-21338 is very excellent due to the uniform thickness of the SOI layer, its capability of maintaining the crystal-defect density of the SOI layer at a low level easily, the flatness of the surface of the SOI layer, no need for an expensive apparatus of a special specification for fabrication, and the capability of using the same apparatus for various SOI film thicknesses ranging from about several 100 Angstrom to 10 micron. Furthermore, by combining the above method with the method disclosed in Japanese Patent Application Laid-Open No. 7-302889, that is, by forming a nonporous single crystal semiconductor layer on a porous layer formed on a first substrate, bonding the nonporous single crystal layer onto a second substrate via an insulating layer, separating the first substrate and the second substrate by the porous layer without destruction, and smoothing the surface of the first substrate and forming porous layer again for reuse, the first substrate can be used many times. This method can significantly reduce production costs and simplify the production steps. There are several methods for separating the bonded substrates mutually to divide into the first substrate and the second substrate without destruction-For example, one of them is to pull the substrate in a direction vertical to the bonded surface. Another method is to apply a shearing stress in parallel with the bonded surface (for example, moving the substrates in the opposite directions within planes in parallel with the bonded surface or rotating the substrates in the circumferentially opposite directions). A pressure can be applied to the bonded surface in the vertical direction. Furthermore, a wave energy such as ultrasonic waves can be applied to the separation region. A peeling member (for example, a sharp blade such as a knife) can also be inserted into the separation region in parallel with the bonded surface from the side of the bonded substrates. Furthermore, the expansion energy of a material infiltrated into the porous layer that functions as the separation region may be used. The porous layer functioning as the separation region may also be thermally oxidized from the side of the bonded substrates to expand the volume of this layer. The porous layer functioning as the separation region may also be selectively etched from the side of the bonded substrates to separate the substrates. Finally, a layer formed by ion implantation to provide microcavities may be used as the separation region and the substrates may then be irradiated with laser beams from the normal direction of the bonded surface to heat the separation region containing the microcavity for separation. However, these methods for separating the two bonded substrates mutually are ideally excellent, but all of them are not suitable for the production of semiconductor substrates. One of the difficulties is that the bonded semiconductor substrates are generally shaped like discs and have a small thickness, for example, 0.5 to 1.0 mm and that the bonded portion has few relatively large recesses on which a jig can be caught. Thus, a method of catching on an orientation flat portion of each substrate a jig having a recessed portion that fits the orientation flat portion and rotating the substrates in parallel with the bonded surface, or a method of catching the jig on a small recessed portion made in the bonded portion in the side of the bonded substrates to peel them are limited. The pressure-based separation requires a very large pressure, thereby forcing the size of the apparatus to be increased. In the wave energy method, the wave irradiation method must be substantially improved to irradiate the bonded substrates with wave energy efficiently, and immediately after separation, the separated substrates may partly contact and damage each other. In the separation from the side, the substrates may be bent to allow only their sides to be peeled, with their central portions remaining unseparated. In the method of inserting the peeling member into the separation region from the side of the bonded substrates, the insertion of the peeling member may damage the bonded surface between the substrates due to the friction of the peeling member and the substrates. One solution for avoiding these problems is to reduce the mechanical strength of the separation region appropriately. This method, however, may increase the possibility that the separation region is destroyed by an external impact prior to the bonding of the substrates. In such a case, part of the destroyed separation region may become particles and contaminate the inside of the production apparatus. Although the conventional separation methods have the major advantages, they still have the above problems. The present invention is intended to provide an improved separation method and apparatus that can separate bonded substrates mutually without the separated substrates being damaged,and that is unlikely to destroy the separation region prior to the step of separating the bonded substrates even when an external force is applied thereto, thereby preventing the production apparatus from being contaminated with particles. A composite member having a plurality of members as mutually bonded is to be separated into a plurality of members at positions different from the bonded position (separation region) of the plurality of members by jetting a fluid against the composite member. The present invention provides a separation apparatus comprising a first holder for rotatably holding a first surface of a disc-like composite member having a plurality of mutually bonded members; a second holder for rotatably holding a second surface of the disc-like composite member; synchronising means for allowing the first and second holders during rotation to synchronise mutually; and a nozzle that is to jet a fluid against the edge surface of the composite member, whilst it is rotating, in order to separate the composite member into a plurality of members. The present invention further provides a method of separating a disc-like composite member, having a plurality of mutually bonded members, into a plurality of members, which method is performed by steps of: placing the composite member in the separation apparatus according to the present invention and holding the first and second face surfaces of the composite member by said first holder and said second holder of said apparatus, respectively; rotating said first holder and said second holder in mutual synchronisation; and jetting a fluid from the nozzle of said apparatus against the edge surface of the composite member thereby to separate the composite member into a plurality of members. With respect to the separation method of the present invention, the composite member may be any member having a separation region inside, whereas with respect to the application of the separation method according to the present invention to a semiconductor substrate production method, it must have the following structure. A major example of the composite member is bonded substrates by bonding a first substrate and a second substrate, the first substrate being a semiconductor substrate in which a separation region is formed as a layer in a portion located deeper than its surface and in parallel therewith and in which the surface and the portion shallower than it has no separation region. In the semiconductor substrate production method, the members obtained after separation are not the same as the first and second substrates prior to bonding. The separation region is located at a position different from the bonding interface (junction surface) between the first and second substrates. In the separation step, the substrates must be separated by the separation region located at a position different from the bonding interface. Thus, the separation region is adapted to be mechanically weaker than the bonding interface so that the separation region is destroyed before the bonding interface. Thus, when the separation region is destroyed, a portion of the surface side of the first substrate which has a predetermined thickness is separated from the first substrate while remaining bonded on the second substrate, thereby transferring the portion to the second substrate. The separation region may be a porous layer formed by the anodization method or a layer formed by ion implantation to provide microcavities. These layers have a large amount of microcavities. This region may also be a heteroepitaxial layer in which distortion and defects are concentrated in crystal lattices. The separation region may also be multiple layers of different structures. For example, it may consist of multiple porous layers having different porosities or a porous layer of a porosity changing in the direction perpendicular to the layers, as required. The layer transferred from the first substrate to the second substrate by, for example, separating the composite member comprising the first and second substrates bonded together with each other via the insulating layer is used as a semiconductor layer (an SOI layer) on the insulating layer to fabricate a semiconductor device. Jet of a fluid that can be used for the separation can be conducted by a so-called water jet method that injects a flow of high-pressure water through a nozzle. Instead of water, this fluid may be an organic solvent such as alcohol, an acid such as hydrofluoric or nitric acid, an alkali such as potassium hydroxide, or a liquid capable of selectively etching the separation region. A fluid consisting essentially of an abrasive particle-free liquid is preferable. Furthermore, a fluid consisting of a gas such as air, a nitrogen gas, carbon dioxide, or a rare gas may be used. A fluid consisting of a gas or plasma that can etch the separation region may also be used. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A, 1B and 1C are schematic views illustrating a method for separating a composite member; FIGS. 2A and 2B are schematic views illustrating an example of a method for separating the composite member using a fluid; FIG. 3 is a top view of an embodiment of the separation apparatus according to this invention; FIG. 4 is a side view of the separation apparatus shown in FIG. 3; FIG. 5 is a schematic view showing a state of separating the composite member; FIG. 6 is a sectional view of the separation apparatus shown in FIG. 4, in its standby state; FIG. 7 is a sectional view of the separation apparatus shown in FIG. 4, in its substrate-holding state; FIG. 8 is a sectional view of the separation apparatus shown in FIG. 4 , in its separating-operation starting state; and FIG. 9 is a sectional view of the separation apparatus shown in FIG. 4, in its separating-operation ending state. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1A to 1C are schematic views illustrating a method of separating a composite member. FIG. 1A shows a state prior to the bonding of a first member 1 and a second member 2. The first member 1 has inside a separation region 3 which is a separation position of this member. The separation region 3 shaped like a layer has a lower mechanical strength than a layer region 5 located on the side of a bonding surface 4a. The two members 1 and 2 are bonded such that the bonding surface 4a is faced to a bonding surface 4b in order to form a disc-like composite member having a bonding interface 14, as shown in FIG. 1B. A fluid 7 is jetted from a nozzle 8 toward the end of the separation region 3 located on the side (end surface) 6 of the composite member. The separation region 3 against which the fluid 7 is jetted is removed or collapsed. Thus, the composite member is separated into two members 11 and 12 at the separation region 3, as shown in FIG. 1C. The layer region 5 is not present on a separation surface 13a of the separated member 11, and a layer region 5 has been transferred onto a bonding surface 4b of the original second member 2 so as to expose a separated surface 13b. Thus, a member having the thin layer region 5 on the second member 2 is obtained. By forming the second member 2 and layer region 5 by using different materials, a member having a heterogeneous bonding can be produced easily. Specific examples of such materials include conductors, semiconductors, and insulators, and two of which are selected to form the second member 2 and the layer region 5. In particular, silicon, quartz, glass, or silicon having an insulating film formed on its surface is preferably used as the second member. A semiconductor material such as silicon, silicon germanium, silicon carbide, gallium arsenide, or indium phosphorus is preferably used as the layer region. The layer region of such a material may partially include an thin insulating layer. The most preferred composite member that is separated into at least two is obtained by bonding two semiconductor substrates, or one semiconductor substrate and one insulating substrate and is called bonded substrates or bonded wafers. Separating such a composite member provides a semiconductor substrate of an excellent SOI structure. Prior to bonding, the separation region is desirably formed inside a substrate along the bonding surface. The separation region may be fragile enough to allow the composite member to be separated into two at the separation region by the jetted fluid and to prevent damage to other regions other than the separation region. Specifically, it can be made fragile by containing a plurality of microcavities inside the separation region or implanting heterogeneous ions to cause strain. The microcavity is formed of pores of a porous body or bubbles generated by ion implantation, as described below. The separation region is preferably 0.1 to 900 µm and more preferably 0.1 to 10 µm. The flow of a fluid used to execute separation can be implemented by jetting the fluid through a nozzle. A method for converting the injected flow into thin beams at a high speed and a high pressure may be the water jet method using water as the fluid such as that introduced in "Water Jet" Vol. 1, No. 1, p. 4. In the water jet that can be used, high-pressure water at several-thousand kgf/cm 2 (1 kgf = 9·8 N) pressurized by a high-pressure pump is jetted through a thin nozzle and can cut or process ceramics, metal, concrete, resin, rubber, or wood (an abrasive material such as SiO 2 grains is added to water if the material is hard), remove a paint film from a surface layer, or wash the surface of a member. The water jet has been mainly used to remove a part of the material, as described above. That is, water jet cutting has been carried out to remove a cut edge from a main member, and the removal of the paint film and the washing of the member surface has been executed to remove unwanted portions. If the water jet is used to form the flow of a fluid, it can be jetted toward the bonding interface on the side (end surface) of bonded substrates to remove at least a part of the separation region from the side. In this case, the water jet is jetted against the separation region exposed on the side of the bonded substrates and against a part of the first and second substrates in the vicinity of the separation region. Then, the separation region of a low mechanical strength is removed or destroyed by the water jet to separate the composite member into two substrates without damage to each substrate. Even if the separation region is not exposed on the side but is instead covered with a certain thin layer for any reason, the water jet may be used to remove the layer covering the separation region on the side and then to remove the separation region exposed from the side. Although not often used in the prior art, the water jet may be jetted against a small recessed portion on the side of two bonded chamfered substrates, that is, on their circumference to penetrate and extend microcavities or pores in the separation region of a fragile structure to separate the bonded substrates. This operation is not intended to perform for cutting or removal, so little chips occur from the separation region and the composite member can be separated without the need for abrasive particles or damage to surfaces obtained by separation, even if the material of the separation region cannot be removed by the water jet. This is not a cutting or polishing effect but a kind of wedge effect provided by the fluid. Thus, this is very effective if there is a recessed or narrow gap on the side of the bonded substrates and the jetting force of the water jet is applied in a direction in which the substrates are peeled off at the separation region. To obtain a sufficient effect, the side of the bonded substrates is preferably recessed rather than protruding. FIGS. 2A and 2B show this effect. In FIGS. 2A and 2B, 901 and 911 indicate first substrates, 902 and 912 second substrates, 903 and 913 separation regions, 904 and 914 semiconductor layers, 905 and 915 insulating layers, 906 and 916 bonding interfaces, 907 a jet of a fluid, and 908 and 918 the directions of forces applied to the substrates by the fluid. FIG. 2A conceptually shows the direction of a force applied to the substrates by the water jet when the side of the end of the bonded substrates is recessed. The force is applied in a direction in which the recessed portion is extended, that is, in a direction in which the bonded substrates are peeled off. On the contrary, FIG. 2B conceptually shows the direction of a force applied to the substrates by the water jet when the side of the end of the bonded substrates is protruding. In this case, a force is not applied in the direction in which the bonded substrates are peeled off, so the substrates cannot be separated mutually unless a part of the separation region can be initially removed. Even if the separation region is not exposed on the side but is instead covered with a certain thin layer for any reason, a sufficient separation effect can be obtained when the side of the bonded substrates is recessed as described above because a force is applied in the direction in which the vicinity of the separation region is extended to destroy the thin layer covering the separation region on the side and then to extend and destroy the separation region. To efficiently receive the flow of the water jet, the aperture width of the recessed portion is desirably equal to or larger than the diameter of the water jet. When the present invention is applied to manufacture a semiconductor substrate, since the thickness of the first and second substrates is less than 1.0 mm, the thickness of the bonded substrates, that is, of the composite member is less than 2.0 mm. Since the aperture width of the recessed portion is normally about half this value, the diameter of the water jet is preferably 1.0 mm or less. Actually, a water jet of about 0.1 mm diameter can be put to practical use. The nozzle jetting the fluid may have any shape including a circle. A long slit-like nozzle can also be used. By jetting the fluid through such a nozzle, thin band-like flows can be formed. Various jet conditions of the water jet can be selected arbitrarily depending on the type of the separation region or the shape of the side of the bonded substrates. For example, the pressure of the jet and its scanning speed, the diameter of the nozzle (approximately equal to the diameter of the water jet) and its shape, the distance between the nozzle and the separation region, and the flow rate of the fluid are important parameters. In an actual separation step, separation can be achieved by scanning the nozzle along the bonded surface while jetting the water jet from a direction in parallel with the bonding surface or fixing the water jet while moving the bonded substrates in parallel. In addition, the water jet may be scanned so as to draw a fan around the nozzle, or the bonded substrates may be rotated around the position of the fixed nozzle as a rotational center if, as is often the case, the bonded substrates are shaped like discs such as wafers with orientation flats or notches. Furthermore, the water jet may be jetted against the separation region from an angled direction as required instead of placing the nozzle in the same plane as the bonded interface. The scanning of the water jet is not limited to these methods but may be carried out by any other method as required. Since the water jet has a very small diameter and the injection direction is almost parallel with the surface of the substrate, vector resolution shows that a high pressure of several-thousand kgf/cm 2 is rarely applied to the substrates. Since the water jet applies a force of only several hundred grams to the bonded substrates except for the separation region, the substrates are prevented from being destroyed. Instead of water, an organic solvent such as alcohol, acid such as hydrofluoric or nitric acid, or alkali such as potassium hydroxide, or a liquid that can selectively etch the separation region may be used. Furthermore, a gas such as air, nitrogen gas, carbon dioxide gas, or rare gas may be used as fluid. A gas or plasma that can etch the separation region may also be used. As water to be used for a composite member separation method introduced into the process of producing a semiconductor substrate, pure water with a minimized amount of an impurity metal and particles, and ultrapure water are desirably used, but the substrates may be washed and the impurity metal and particles are removed after separation using the water jet, due to the perfect low-temperature process. In particular, the fluid is preferably free of abrasive particles so as not to leave unwanted scratches in the substrates. A semiconductor substrate separated as described above can be used to fabricate a semiconductor device and to form a single crystal semiconductor layer on the insulating layer into a fine structure instead of an electronic device. FIGS. 3 and 4 are a top and a side view showing an embodiment of the separation apparatus for a composite member according to the present invention. This separation apparatus has a rotation synchronization mechanism and can rotate a first holder for holding a first surface of the composite member and a second holder for holding a second surface of the composite member, at the same angular speed in the same direction. When a rotational drive force is applied to only one surface of the composite member or synchronization such as that described above is not provided, the following phenomenon is likely to occur. Immediately before a wafer that is a composite member is completely separated over the entire wafer, there is a moment at which a very small region, which is finally separated, remains unseparated somewhere on the separation surface. The following two separation modes can be assumed depending on a position of this very small unseparated region. A first mode is a case in which the unseparated region remains almost at the center of the separation surface and a second mode is a case in which it remains in an area other than the center. FIG. 5 conceptually shows these modes. The first mode occurs if separation progresses uniformly from the circumference of the wafer toward its center or if the strength of the vicinity of the center of the separation surface is high. In this case, if a rotational drive force is applied to only one of the holders 21 of one side of the wafer, this rotation causes the very small unseparated region to be twisted off and separated. The second separation mode occurs if during the initial step of fluid jetting, a crack extends over the radius of the wafer or longer from a certain circumferential portion resulting in quick separation or if the strength of areas other than the vicinity of the center of the separation surface is high. In this case, if a rotational drive force is applied to only one of the holders 21 of one side of the wafer, this rotation causes shearing stress , thereby causing the very small unseparated region to be separated. This is because the opposite holder 22 is subjected to no independent drive force and is only rotated through the wafer, causing a slight force to be effected in a direction in which the rotation of the holder 22 is stopped even if softly the holder 22 is held by a bearing. Such torsion or shear causes complicated forces in directions other than the vertical one to be applied to the separation surface, resulting in the unwanted separation of an area other than the separation surface. Thus, when the wafer is separated while being rotated and if the wafer is rotationally driven without allowing both sides of it to synchronize mutually, separation may occur from a surface other than a desired separation surface or the wafer or an active layer may be damaged. These phenomena may significantly reduce the yield. A motor support 36 for supporting a motor 32 that can control the speed and a pair of shaft supports 37 and 38 for rotatably supporting a motor shaft 31 are fixed on a supporting stand 40. Furthermore, a first holder support 33 for rotatably supporting the holder 21 and a second holder support 34 for rotatably supporting the holder 22 are fixed on the supporting stand 40. A timing pulley 29 mounted on the motor shaft 31 and a timing pulley 25 mounted at the rear end of a rotating shaft 23 of the holder 21 are connected together in such a way as to rotate in the same direction by means of a timing belt 27. Likewise, a timing pulley 30 mounted on the motor shaft 31 and a timing pulley 26 mounted at the rear end of a rotating shaft 24 of the holder 22 are connected together in such a way as to rotate in the same direction by means of a timing belt 28. The pulleys 25 and 26 have the same driving radius, and the pulleys 29 and 30 have the same driving radius. The timing belts 27 and 28 are the same. A drive force from the motor 32 is transmitted from the shaft 31 to the holders 21 and 22 via the pulleys and belts in order to rotate the holders 21 and 22 at the same angular speed in the same direction with the same timing. In FIG. 4, 60 is a jet nozzle that jets a fluid and 61 is a shutter. For clarity, the illustration of the nozzle and shutter are simplified. The nozzle 60 is fixed on the supporting stand 40 using a fixation jig (not shown), and a wafer positioning member 35 is provided on the supporting stand 40 so as to be aligned with the nozzle 60. FIG. 6 is a partially sectional view of the holder of the separation apparatus before it holds the wafer 20. The holder 21 or 22 is an assembly of a holding section 45a or 46a that actually sucks and holds a wafer; a fixation section 45b or 46b that rotates the holding section 45a or 46a together with the rotating shaft 23 or 24; and detents 41 and 42 or 43 and 44. Using a tube 52 and a pressurized gas passed through a pressurizing passage 56, the holding section 45a can move against a compression spring (a coil spring 47) in a direction in which it leaves the rotating shaft 23 (rightward in the figure). An opening op is provided near the center of the holding section 45a and is in communication with a pressure reducing passage 55 in the rotating shaft. Using a vacuum pump (not shown) connected to the opening op via a pressure reducing tube 51, vacuum is drawn into the opening op to reduce the atmospheric pressure. The holder 21 or 22 is moved forward (rightward in the figure) by having its holder section 45a that directly sucks the wafer, guided by the rotating shaft 23, as shown in FIG. 6, and using the pressure of air introduced from the pressurizing tube 52. The holder 21 or 22 is moved backward (leftward in the figure) by the compression spring 47. The holding section 45a rotates with the rotating shaft 23 using the detents 41 and 42. Basically, the holder 22 is specularly symmetrical with the holder 21 and has the same mechanism as it. To allow the bonded wafer 20 and nozzle 60 to be always set at specified positions when the bonded wafer 20 is positioned and held on the holder 22, pressure is controlled and adjusted so that a stronger force is applied to the holder 21 than to the holder 22 during a forward operation, while a stronger force is applied to the holder 22 than to the holder 21 during a backward operation. The usage of this apparatus, that is, an embodiment of the method for separating a composite member according to this invention is described below. The bonded wafer 20 is set so as to fit on a notch in a positioning stand 35, as shown in FIG. 6. Pressurized air is then introduced to cause the holding section 45a to advance, thereby allowing the holder 21 to suck and hold the wafer, as shown in FIG. 7. The holder 21 can fit the bonded wafer 20 on the notch in the positioning stand 35 to hold the center of the bonded wafer 20. When the bonded wafer 20 is held in an accurate position, the nozzle 60 is located perpendicularly to the top of the bonded wafer 20 and the distance between the bonded wafer 20 and the nozzle 60 is 10 to 30 mm. The holding section 46a of the holder 22 is moved forward (leftward in the figure) to suck and hold the bonded wafer 20, and the feeding of pressurized air of the holding section 46a is stopped. The bonded wafer 20 is stopped due to a force acting rightward in the figure which is an combination of a force effected by the compression spring and a vacuum suction force. The force effected by the compression spring does not exceed the force required by the holding section 46a to suck the bonded wafer 20, so the vacuum destruction of the inside of the pressure reducing passage 55 or 57 does not occur, which may in turn eliminate the suction force to cause the wafer 20 to fall. A fluid is then fed from a pump 62 to the nozzle 60 for a specified period of time until the jetted fluid is stabilized. Once the fluid has been stabilized, the shutter 61 is opened to jet the high-pressure fluid from the nozzle 60 against the thickness-wise center of the bonded wafer 20. At this point, the speed controller motor 32 is rotated to rotate the holders 21 and 22 in synchronism in order to rotate the wafer 20. By jetting the high-pressure fluid against the thickness-wise center of the wafer 20, the high-pressure fluid also enters the separation region to extend the bonded wafer 20, thereby finally separating it into two. Since the high-pressure fluid is applied uniformly against the bonded wafer 20 and the holders 21 and 22 each apply a force in a direction in which the bonded wafer 20 is drawn, as described above, separated pieces further leave each other and are prevented from sliding. In addition, in the wafer supporting means shown in FIGS. 6 to 9, the wafer is supported while being subjected to a force by the holders 21 and 22 in a direction in which the holders move backward from the wafer, but the holders 21 and 22 may effect a force in a forward direction and this pressure may be used to hold the wafer. In this case, the high-pressure fluid also advances while extending the bonded wafer 20 to create a small gap, thereby finally causing the wafer to be separated into two. In this method, if the holders 21 and 22 do not synchronize mutually, the bonding surfaces of the separated pieces damage each other due to sliding, whereas if the holders rotate in synchronism, no damage occurs. Furthermore, when a force is applied in a direction in which the holders 21 and 22 move backward, the wafer 20 is pulled to move backward during separation by the holders 21 and 22 and there may occur a difference in the amount of displacement between a separated portion and an unseparated portion to unbalance the bonded wafer 20, thereby causing a crack when the high-pressure fluid is jetted. If, however, a force is applied to the holders 21 and 22 in a direction in which they move forward, the bonded wafer 20 will maintain balance to enable the wafer to be separated stably. A high- or atmospheric-pressure fluid can be injected against the entirely separated wafer to effect a force in a direction in which it moves backward in order to break the surface tension of intervening water, thereby separating it into two completely. (Example) A disc-like P-type single crystal Si wafer having a resistivity of 0.01 Ω·cm was prepared as a first Si substrate and had its surface subjected to anodization in an HF solution. The anodization conditions are listed below. Current density: 7 (mA·cm -2 ) Anodization solution: HF: H 2 O: C 2 H 5 OH = 1 : 1 : 1 Time: 11 (minute) Thickness of the porous Si layer: 12 (µm) This wafer was oxidized in an oxygen atmosphere at 400°C for one hour. The oxidization caused the inner wall of the pores in the porous Si layer to be covered with a thermally oxidized film. The surface of the porous Si layer was treated with hydrofluoric acid to remove only the oxidized film on the surface of the porous Si layer while leaving the oxidized film on the inner wall of the pores, and the CVD was then used to allow single crystal Si to epitaxially grow by 0.3 µm on the porous Si layer. The growth conditions are listed below. Source gas: SiH 2 Cl 2 /H 2 Gas flow rate: 0.5/180 l/min. Gas pressure: (80 Torr) 10.7 kPa Temperature: 950°C Growth speed: 0.3 µm/min. Furthermore, a 200 nm thick oxide film (an SiO 2 layer) was formed on the epitaxial Si layer as an insulating layer, using thermal oxidation. Besides the first substrate formed in this manner, a disc-like Si wafer was prepared as a second Si substrate. The surface of the second Si substrate was placed on the surface of the SiO 2 layer of the first Si substrate to contact them mutually. These substrates were then subjected to thermal treatment at 1180°C for 5 minutes for bonding. Next, preparations were made to separate the composite member consisting of the bonded wafer using the apparatuses shown in FIGS. 3, 4, and 6 to 9. The wafer, which is the composite member, was located so as to stand in the vertical direction while fitting on the notch in the positioning base 35. Pressurized air was supplied from the tubes 52 and 54 to the pressurizing passage 56, and the holding sections 45a and 46a were moved forward to the front and rear surfaces of the wafer, respectively, in order to abut the front and rear surfaces of the wafer with the holding surface of the holding sections 45a and 46a each having an opening op, respectively, as shown in FIG. 7. Using the tubes 51 and 53, the wafer was sucked and fixed to the holding sections 45a and 46a. The supply of pressurized air was stopped and tension was supplied to the wafer in the opposite normal directions of the front and rear surfaces of the wafer using the springs 47 and 48. With the shutter 61 closed, pure water without abrasive grains was fed forcefully from the pump 62 to the nozzle of 0.15 mm diameter and the pump 62 was operated to inject water at a pressure of about 200 kgf/cm 2 . The positioning base 35 was moved to its standby position, and the power to the motor 32 was turned on to transmit rotational drive force via the shaft 31 and belts 27 and 28 in order to rotate the holders 21 and 22. Since the wafer was sucked by the holding sections 45a and 46a, it started to rotate simultaneously with the holders 21 and 22 at the same angular speed in the same direction. The shutter 61 was opened to inject the water jet against the separation portion in the side of the wafer, as shown in FIG. 8. Water from the water jet apparatus entered the pores in the separation portion to separate the wafer around the porous layer that is the separation portion. As the injection of the water jet and the rotation of the wafer continue, the gap formed by separation gradually grew from the periphery of the wafer toward its rotational center and the wafer could be finally separated as shown in FIG. 9. Since the wafer was subjected to forces in the directions shown arrows TA and TB in FIG. 9 , the wafer was separated as shown in FIG. 9, simultaneously with the final separation of the rotational center of the wafer. Subsequently, the forced feeding of water was stopped and the separated wafer was removed from the holding sections 45a and 46a. Subsequently, the remaining porous Si layer transferred to the second substrate was selectively etched by being stirred using a mixture of 49% hydrofluoric acid and 30% hydrogen peroxide solution. The transferred single crystal Si layer under the porous layer remained without being etched, whereas the porous Si layer was entirely removed by selective etching using the single crystal Si layer as an etch stop material, thereby exposing the thin single crystal Si layer. Thus, a first SOI substrate having the single crystal Si layer of 0.2 µm thickness on the Si oxide film of the second substrate was obtained. The single crystal Si layer was not affected by the selective etching of the porous Si layer. When 100 points of the overall surface of the single crystal Si layer formed were measured for thickness, the value obtained was 201 nm ± 2 nm. An observation of the cross section by the transmission electron microscope indicated that new crystal defects did not occur in the Si layer and that excellent crystallinity was maintained. Thermal treatment was further carried out in hydrogen at 1100°C for 50 minutes and the surface roughness was evaluated using the interatomic force microscope. The mean square roughness of a 50-µm square region was about 0.2 nm. In addition, the porous Si layer remaining on the first substrate was selectively etched by being stirred using a mixture of 49% hydrofluoric acid and 30% hydrogen peroxide solution. Subsequently, surface treatment such as polishing was carried out. The first substrate, which had been polished, was again subjected to anodization to form a porous Si layer and nonporous single crystal Si was allowed to grow thereon. The surface of the nonporous single crystal Si layer, which had grown epitaxially, was oxidized. Then, the surface of a separately prepared Si wafer that was a third substrate was bonded on the oxidized surface of the single crystal Si layer of the first substrate. The conditions for the above process were the same as those for the first bonded-wafer production. The wafer was again separated in the same manner as in the first separation method described above to obtain a second SOI substrate having the single crystal Si layer on the insulating surface of the third substrate. The above process was repeated to recycle the first substrate in order to fabricate a third and a fourth SOI substrates. As described above, this invention enables a composite member having a separation region inside to be separated into a plurality of smaller members around the separation region without damaging or destructing those portions other than the separation region. Therefore, this invention enables semiconductor substrates with higher quality than the conventional ones to be fabricated easily and reliably with a high yield.

Claims: A separation apparatus comprising a first holder (21) for rotatably holding a first surface of a disc-like composite member (20) having a plurality of mutually bonded members; a second holder (22) for rotatably holding a second surface of the disc-like composite member; synchronising means (25 to 31) for allowing the first and second holders during rotation to synchronise mutually; and a nozzle (60) that is for jetting a fluid against the edge surface of the composite member, whilst it is rotating, in order to separate the composite member into a plurality of members. An apparatus according to claim 1 having means for setting the position of the nozzle (8) so that the fluid is jetted against a separation position (3,903,913) of the composite member different from the bonding position (14,906,916) of the composite member. A method of separating a disc-like composite member (20), having a plurality of mutually bonded members, into a plurality of members, which method is performed by steps of: placing the composite member in the apparatus of claim 1 and holding the first and second face surfaces of the composite member by said first holder (21) and said second holder (22) of said apparatus, respectively; rotating said first holder and said second holder in mutual synchronisation; and jetting a fluid from the nozzle (60) of said apparatus against the edge surface of the composite member thereby to separate the composite member into a plurality of members. A method according to claim 3 including a step of setting the position of the nozzle so that the fluid is jetted against a separation position (3,903,913) of the composite member different from the bonding position (14,906,916) of the composite member. A method according to claim 3 wherein the composite member is one having a recessed portion provided in its edge surface, said step of setting the position of the nozzle being performed so as to ensure that the fluid is jetted against the bottom of the recessed portion.

Inventors: OHMI KAZUAKI, SAKAGUCHI KIYOFUMI, YANAGITA KAZUTAKA, YONEHARA TAKAO, OHMI, KAZUAKI, SAKAGUCHI, KIYOFUMI, YANAGITA, KAZUTAKA, YONEHARA, TAKAO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: H01L21
IPC codes: H01L21