Method of preparing a semiconductor substrate

Abstract: A semiconductor substrate comprises a plurality of substrates to be bonded, wherein a bond promotion layer into which silicon atoms are implanted is provided in an interface between the substrates to be bonded, and the substrates are bonded to each other with the interposition of the bond promotion layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for preparing a semiconductor substrate obtained by bonding which semiconductor substrate can be applied to a micro-machine technique, a derivative separation technique, an SOI technique, sensors, high-power elements, a high-frequency strip integrated circuit technique for communication, and the like. Related Background Art In recent years, it has been actively researched and developed to apply, to a micro-machine mechanism, a sensor, an SOI (silicon on insulator) and the like, a technique called "wafer direct bonding" or "Van der Waals bonding" in which two substrates are bonded to each other without interposing any adhesive. This situations can be seen in detail in literature, for example, "The first international symposium on semiconductor wafer bonding science, technology, and applications, Extended Abstract of Fall meeting of Electrochemical Society, Phoenis, Arizona, October 13-17, 1991, pp 674-749". Heretofore, an important problem in this technical field is to inhibit the generation of voids (pores generated in a bonded interface) by a suitable means. With regard to the generation of the voids, various causes are considered. The main cause can be presumed to be that oxygen atoms and hydroxyl groups which cover the surfaces of substrates to be bonded are dehydrated and condensed to become water vapor, and the water vapor collects and comes to the voids. The thus generated voids can be reduced by a further high-temperature heat treatment to diffuse the water vapor. However, it is very difficult to decrease the voids which are caused by dust stuck on the surfaces of the substrates, roughness of wounds on the surfaces, contaminants on the surfaces, captured gases, or the warpage of the substrates. In the case that a silicon substrate is bonded to an insulating substrate which is made of a material such as glass other than silicon and which functions as a supporting member, after the two substrates have been bonded to each other, a heat treatment is carried out at about 1,000°C in order to strengthen the bonding between these substrates. At this time, however, there occur problems such as the warpage of the bonded substrates, the cracks of the substrates and the separation of both the substrates from each other owing to different thermal expansion coefficients of both the substrates. An example is also present in which a substrate synthesized from a material having a thermal expansion coefficient close to that of silicon is used as the supporting substrate, but as known, such a material is poor in heat resistance and cannot withstand a heat treatment for the reinforcement of the bonding and a process temperature for the formation of devices. DE-OS-38 29 906 discloses a method of producing semiconductor devices by direct bonding (SDB). The method comprises the provision of two substrates having flat surfaces in which the respective surfaces of these two substrates are roughened and at least one of the two roughened surfaces is ion-implanted to create a crystal lattice defect in the surfaces and are then bonded together by contact of the two surfaces treated in the above manner. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for preparing a semiconductor substrate in which the generation of fine voids attributed to roughness on the surfaces of substrates to be bonded is inhibited. Another object of the present invention is to provide a method for preparing a semiconductor substrate in which the bonding force of a bonding interface is increased so much as to overcome stress caused by thermal expansion, in the case that a silicon single crystal film is formed on a substrate having a thermal expansion coefficient different from that of silicon. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are schematic sectional views for explaining one example of the method of the present invention. Fig. 2 is a schematic sectional view for explaining another example of the method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first aspect of the present invention is directed to a method of preparing a semiconductor substrate comprising a plurality of substrates to be bonded to each other via an insulating layer wherein a siliconatom-implanted bond promotion layer is provided in a surface region of said insulating layer in an interface between the substrates to be bonded. The second aspect of the present invention is directed to a method of preparing a semiconductor substrate by bonding a plurality of substrates, said method comprising: a step of forming an insulating layer on the surface of a first substrate as a substrate to be bonded, a step of implanting silicon atoms into the surface of the insulating layer to form a bond promotion layer thereon, a step of contacting a second substrate to be bonded to the bond promotion layer on the first substrate, and a step of heating the contacted substrates to strougly boundand unify them. According to the method of preparing the semiconductor substrate of the present invention, the silicon atoms are implanted into the interface between the bonded substrates, so that bonding force properties to a certain temperature of the interface are changed and increased. In consequence, it is possible to prevent the formation of unbonded regions in the bonding interface and the generation of voids owing to the unbonded regions which are the most important problems in a wafer bonding technique. As a result of intensive investigation by the present inventor, it has been found that when insulating materials typified by silicon oxides are bonded to each other, silicon atoms in the uppermost surfaces of the insulating materials are terminated with oxygen atoms and so they are extremely stable and less reactive, and the bonding force between the insulating materials is weaker than when highly reactive silicon substrates are bonded to each other. Thus, the present invention employs a method in which the silicon atoms capable of changing the surface physical properties of the insulating layer are slightly implanted preferably as accelerated charged particles under a good control only into the uppermost portion of a thermally oxidized film maintaining flatness at an atomic level. By the employment of this method, silicon is implanted into the uppermost surface of the above-mentioned stable oxidized layer in which the silicon atoms are terminated with oxygen atoms and unbonded hands are not present, to cut the bonding hands which are terminated with the oxygen atoms. In consequence, unbonded hands are formed to activate the surface of the insulating layer. As a result, the bonding force of the bonding interface can be increased. As described above, according to the present invention, a bond promotion layer having an extremely excellent flatness and a large bonding force can be formed, whereby a semiconductor substrate which is free from voids in a bonding interface can be prepared. Now, the present invention will be described with reference to Figs. 1A and 1B and Fig. 2 in accordance with examples. Embodiment 1 Figs. 1A and 1B are schematic sectional views illustrating Embodiment 1 of the present invention. In the drawings, a first substrate 1 and a second substrate 4 are composed of a single element semiconductor such as silicon or germanium, a compound semiconductor such as gallium arsenide or indium phosphorus, or a transparent insulating material, for example, glass such as quartz. In the first place, a flat insulating layer 2 is formed on the surface of the first substrate 1. A thermally oxidized film is optimum for silicon. When the flatness of the thermally oxidized film is quantitatively measured by an interatomic force microscope, it is apparent that an extremely flat surface having a roughness of about several nanometers is formed. In the case that a stable thermally oxidized layer is not obtained on the surface as in the case of the compound semiconductor, it is formed by a chemical vapor deposition, particularly a vacuum deposition, or a sputtering process. Next, impurities which change surface physical properties of the oxide and which promote a bonding force are added to the uppermost surface (about 100 nm or less) of the insulating layer 2. In the case that the insulating layer 2 is silicon oxide, silicon is desirable. Furthermore, as the technique of the addition, there is used an implanting method of accelerated charged particles, that is, an ion implanting method. In consequence, a bond promotion layer 3 is formed on the surface of the insulating layer 2 without impairing its flatness (Fig. 1A). Next, as another substrate, a second substrate 4 is prepared, and both the substrates are carefully washed, dried, bonded to each other with the interposition of the above-mentioned bond promotion layer, and then heated to strongly bond to each other (Fig. 1B). According to the preparation method of this embodiment, the flatness of the bonded surfaces is originally good, but by providing the bond promotion layer of the present invention, such extremely fine voids as are caused by steps comparable to an atomic surface can be eliminated. Embodiment 2 Fig. 2 is a schematic sectional view illustrating another embodiment of the present invention. In this embodiment, a second substrate 4 is also provided with a bond promotion layer 6, as in the bond promotion layer 3 on the first substrate 1 described in Embodiment 1, and both the substrates are then bonded to each other. In this connection, reference numerals 2 and 5 are insulating layers of the respective substrates. By this constitution, an effect of eliminating voids attributed to the further large surface roughness can be obtained. Next, typical examples of the present invention will be described in more detail. Example 1 In the first place, a 12.5 cm (5-inch) p-type (100) single-crystal silicon substrate (0.1-0.2 Ωcm) was prepared, and a thermally oxidized film having a thickness of 0.5 µm was formed on the surface of the substrate. Next, silicon was added to the surface of the thermally oxidized film as thick as 0.1 µm at 1 × 10 18 atom·cm -3 by ion implantation. This substrate was washed with a mixed solution of hydrochloric acid, aqueous hydrogen peroxide and water, dried, and then bonded to a 12.5 cm (5-inch) single-crystal silicon substrate washed in the same manner at room temperature. The thus bonded substrates were thermally treated at 1,000°C for 20 minutes in nitrogen to further strengthen the bonding. The thus bonded substrates of this example were observed by X-ray topography, and as a result, any voids were not observed. According to a tensile strength test, a bonding strength of 10 8 N/m 2 (1,000 kgf/cm 2 ) or more was confirmed. The first substrate was thinned to a thickness of 1 µm by grinding and polishing, and the detection of fine voids (10 µm or less) was attempted by an optical microscope, but any voids could be detected. Example 2 In the first place, a p-type (100) silicon single-crystal substrate having a resistivity of 0.01 Ω·cm was prepared, and an epitaxial layer having a thickness of 1.5 µm was formed. The surface was oxidized as thick as 0.5 µm in the same manner as in Example 1, and silicon was ion-implanted at 1 × 10 17 atom·cm -3 into the thermally oxidized layer as thick as 0.1 µm from the surface of the layer. As a second substrate, there was prepared a silicon substrate which was thermally oxidized as thick as 1 µm and into which silicon was implanted on the surface thereof at 1 × 10 18 atom·cm -3 , and both the substrates were then washed with a mixed solution of hydrochloric acid, aqueous hydrogen peroxide and water, dried, and then bonded to each other at room temperature. The bonded substrates were further subjected to a heat treatment at 1,100°C for 60 minutes in nitrogen. The thus bonded substrates were observed by X-ray topography, and as a result, any voids were not observed. According to a tensile strength test, a bonding strength of 1.2·10 8 N/m 2 (1,200 kgf/cm 2 ) or more was confirmed. Example 3 In the first place, a P-type (100) silicon single-crystal substrate having a resistivity of 0.01 Ω·cm was anodized on the surface thereof to form a porous layer, and an epitaxial layer having a thickness of 1.5 µm was formed. Next, the surface was oxidized as thick as 0.1 µm, and a silicon oxide layer having a thickness of 0.5 µm was then deposited by a chemical vapor deposition. Furthermore, silicon was ion-implanted into the oxidized layer as thick as 0.1 µm from the surface of the layer at 2 × 10 17 atom·cm -3 . As a second substrate, there was prepared a fused quartz substrate into which silicon was implanted as thick as 0.1 µm from its surface at 1 × 10 18 atom·cm -3 , and both the substrates were then washed with a mixed solution of hydrochloric acid, aqueous hydrogen peroxide and water, dried, and then bonded to each other at room temperature. The bonded substrates were further subjected to a heat treatment at 400°C for 6 hours in nitrogen. The bonded substrates were ground from the side of the silicon substrate by polishing and selective chemical etching for the porous portion to obtain a single-crystal silicon thin layer on the quartz glass. The substrates of this example were observed by an optical microscope and X-ray topography, and as a result, any voids were not observed. According to a tensile strength test, a bonding strength of 500 kgf/cm 2 or more was confirmed. As described above, according to the present invention, in a semiconductor substrate obtained by a bonding method, an impurity such as silicon is introduced into an insulating layer surface of a bonded interface, whereby bonding force characteristics to a certain temperature in the interface can be changed and increased. Therefore, a bond promotion layer having an excellent flatness and a large bonding force can be formed. In consequence, it is possible to prevent the formation of unbonded regions in the bonding interface and the generation of voids owing to the unbonded regions which are the most important problems in a wafer bonding technique. Thus, according to the present invention, the generation of the fine voids attributed to roughness on the surfaces of the substrates to be bonded can be effectively inhibited, and also in the case that the substrates having different thermal expansion coefficients are bonded under the control of stress caused by thermal expansion, they can be strongly bonded.

Claims: A method of preparing a semiconductor substrate by bonding a plurality of substrates, said method comprising: a step of forming an insulating layer (2) on the surface of a first substrate (1) as a substrate to be bonded, a step of implanting silicon atoms into the.surface of the insulating layer to form a bond promotion layer (3) thereon, a step of contacting a second substrate (4) to be bonded to the bond promotion layer (3) at a first bonding strength on the first substrate (1), and a step of heating the contacted substrates (1, 4) to bond them at a second bonding strength greater than the first bonding strength. The method of preparing a semiconductor substrate according to claim 1, wherein said method further comprises: a step of forming a second insulating layer (5) on the surface of the second substrate (4), a step of implanting an impurity for increasing a bonding force into the surface of the second insulating layer (5) to form a bond promotion layer (6) thereon, and a step of bonding the bond promotion layer (6) on the second substrate (4) to the bond promotion layer (3) on the first substrate (1). The method of preparing a semiconductor substrate according to claim 1 wherein the silicon atoms are implanted as accelerated charged particles. The method of preparing a semiconductor substrate according to claim 1 wherein the silicon atoms are implanted via a gaseous phase. The method of preparing a semiconductor substrate according to claim 1, wherein the substrates are composed of a single element, a compound semiconductor or a glass. A method of preparing a semiconductor substrate according to claim 1, said first substrate is prepared by forming an epitaxial layer on a surface of a silicon monocrystalline substrate. A method of preparing a semiconductor substrate according to claim 1, wherein said first substrate is a substrate having a non-porous monocrystalline silicon layer disposed on a surface of a monocrystalline silicon substrate sandwiching a porous layer between the monocrystalline silicon substrate and the non-porous monocrystalline silicon layer. A method of preparing a semiconductor substrate according to claim 7, further comprising a step of removing said monocrystalline silicon substrate and said porous layer from a member comprising said first and second substrates bonded together. A method of preparing a semiconductor substrate according to claim 1, wherein said insulating layer is a silicon oxide layer. A method of preparing a semiconductor substrate according to claim 1, wherein said step of contacting is performed at room temperature. A method of preparing a semiconductor substrate according to claim 1, wherein said step of heating is performed in nitrogen. A method of preparing a semiconductor substrate according to claim 1, wherein said bond promotion layer is formed in a region within 100 nm from the surface of said insulating layer.

Inventors: YONEHARA TAKAO, YONEHARA, TAKAO, YONEHARA, TAKAO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L21
IPC codes: H01L27, H01L21