Process for preparing a semiconductor substrate

Abstract: A process for preparing a semiconductor substrate comprises bringing a first substrate provided with at least one of boron and phosphorus on the surface of an insulating layer formed on the surface of the substrate in contact with a second substrate, and integrating both of the substrates by a heat treatment.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for preparing a semiconductor substrate by bonding. The present invention is suitably applied particularly to micromachine techniques, dielectric isolation techniques, SOI techniques, sensors, high power devices, communication high frequency band integrated circuit techniques, etc. Related Background Art Formation of a monocrystalline Si semiconductor layer on an insulator is widely known as a silicon-on-insulator (SOI) technique and has been extensively studied because devices based on utilization of the SOI technique have many advantages that have not been obtained in case of bulk Si substrates for preparing ordinary Si integrated circuits. That is, the following advantages can be obtained by utilizing the SOI technique: 1. Easy dielectric isolation with a possibility of higher level integration, 2. Distinguished resistance to radiation, 3. Reduced floating capacity with a possibility of higher speed, 4. Omission of well formation step, 5. Prevention of latch-up, 6. Possibility to form a fully deplated, field effect transistor by thin film formation, etc. To obtain the above-mentioned many advantages of device characteristics, processes for forming the SOI structure have been studied for these more than ten years. The results are summarized, for example, in the following literature: Special Issue: "Single-crystal silicon on non-single-crystal insulators", edited by G.W. Cullen, Journal of Crystal Growth, Volume 63, No. 3, pp 429-590 (1983). Among many SOI techniques, SOS (silicon-on-sapphire) formed by heteroepitaxial growth of a silicon film on a monocrystalline sapphire substrate by CVD (chemical vapor phase deposition) method was disclosed in the past as a research example advanced to such a level as to form practical integrated circuits, using monocrystal as a silocon layer and regarded as a successful, matured SOI technique. However, its wide application was interrupted by the occurrence of many crystal defects due to lattice mismatching at the interface between the Si layer and the underlayer sapphire substrate, by diffusion of aluminum into the Si layer from the sapphire substrate, and largely by a high cost of the substrate and delay in response to the formation of larger area. Recently, some attempts have been made to form an SOI structure without using the sapphire substrate. The attempts can be classified into the following three major groups. (1) After the surface oxidation of a silicon monocrystalline substrate, windows are made in a part of the oxide film to partially expose the silicon substrate, and a silicon monocrystalline layer is formed on the SiO 2 by epitaxial growth of silicon in the lateral direction, while utilizing the exposed silicon substrate as a seed. (2) A silicon monocrystalline substrate itself is utilized as an active layer and a SiO 2 embedded layer is formed below by some means. (3) After the silicon substrate is bonded to an insulating substrate, the silicon substrate is polished or ground or etched to leave the monocrystalline layer having a desired thickness. Basically three processes are available as a means of realizing the above group (1): a process for direct epitaxial growth of a monocrystalline silicon layer in the lateral direction by CVD (gas phase process); a process for deposition of amorphous silicon and successive epitaxial growth in the lateral direction in the solid phase by heat treatment (solid phase process); and a process for irradiation of an amorphous or polycrystalline silicon layer with a converged energy beam such as an electron beam, a laser beam, etc. to make a monocrystalline silicon layer grow on SiO 2 through melting and recrystallization or scanning a melt zone by a rod heater bandwise (zone melting recrystallization)(liquid phase process). These processes have advantages and disadvantages together, and still have many problems in the controllability, productivity, uniformity and quality, and thus have been hardly commercially used yet. For example, the CVD process requires grinding technique of good controllability or sacrificial oxidation to form a flat thin film, whereas the solid phase growth process suffers from poor crystallinity. The beam anneal process has problems in the treating time by scanning with the converged beam, the state of overlapped beams, focus adjustment, etc. Among them, the zone melting recrystallization process is a most matured process, by which relatively large integrated circuits have been made on trial, but there remain still many crystal defects such as quasigrain boundaries, etc. and thus minority carrier devices have not yet been made. The technique now advanced by research and development by so many research organizations as a means of realizing the above group (2) is a process for forming a SiO 2 layer by oxgen ion implantation into a silicon monocrystalline substrate, called SIMOX (separation by ion implanted oxygen). This technique is now the most matured one because of good matching with the silicon process. However, in order to form the SiO 2 layer, oxygen ions must be implanted at least at 10 18 ions/cm 2 , and the implantation time is very long and the productivity is not so high. The wafer cost is also high. Furthermore, there remain many crystal defects. That is, no commercially satisfactory quality for making minority carrier devices can be obtained. Besides SIMOX a process for forming SOI structure by dielectric isolation on the basis of oxidation of porous silicon is known. The process comprises forming an n-type silicon layer of island shape or the surface of a p-type silicon monocrystalline substrate by proton ion implantation (Imai et al: J. crystal Growth, Vol. 63, 547 (1983)) or by epitaxial growth and patterning, then making only the p-type silicon substrate porous by anodization in a HF solution so as to surround the silicon islands from the surface, and then. dielectrically isolating the n-type silicon island by accelerated oxidation. This process has such a problem that since the isolated silicon is determined before a device step the degree of freedom in a device design is limited. As a means of realizing the above group (3), there is a technique of making the mirror surfaces of two wafers where an insulating film is formed on at least one of the mirror surfaces by oxidation, etc. to tightly adhere to each other, subjecting the wafers to a heat treatment to strengthen the bonding at the tightly adhered interface, and then grinding or etching the bonded wafers from one side thereof thereby leaving a silicon monocrystalline thin film having a desired thickness on the insulating film. This technique is called "bonding SOI", and there are many problems to be solved in this technique. The most important problem is in a step of making the silicon substrate into a uniformly thin film. That is, a silicon substrate usually having a thickness of several hundred pm must be ground or etched uniformly to a thickness of several µm or less than 1 µm. This is technically very difficult from the viewpoint of controllability and uniformness. This technique has a possibility to provide a monocrystalline thin film of best quality in the SOI techniques, but has not been used in the commercial production because of the difficulty in the film thickness control. Another important problem is a generation of a stress due to a difference in the coefficient of thermal expansion between two substrates, where an insulating substrate other than the silicon substrate is used as a support member. That is, there is substantially no problem when a silicon substrate is used as a support member (that is, bonding of two silicon substrates), but when other insulating substrate than silicon, for example, glass, is used as a support member, the substrates are sometimes warped, while keeping the bonding, or cracked or separated from each other due to a difference in the coefficient of thermal expansion in the heat treatment step at about 1,000°C for strengthening the interface bonding after the two substrates are bonded together. There are cases of synthesizing materials having a similar coefficient of thermal expansion to that of silicon and using them as a support substrate, but to our knowledge such synthetic materials have a poor heat resistance and cannot endure the heat treatment for strengthening the bonding or the process temperature for forming devices. Beside these problems, how to suppress the generation of voids (empty spaces formed at the bonding interface) is also a problem. There are many theories on the cause for generation of such voids. According to one theory, oxygen atoms or hydroxy groups covering the surfaces of bonded substrates mainly undergo dehydration and condensation to generate water vapors, and the generated water vapors get together to form voids. The thus generated voids can be made to disappear by further heat treatment at a higher temperature, thereby diffusing the vapors. Surface unevenness caused by dusts, scars etc. on the surface of the substrate will positively generate voids, and the voids generated by the surface unevenness of the substrate are very difficult to make to disappear. As disclosed, for example, in 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, the application of substrate bonding technique has been regarded as important in the field of not only SOI, but also micromachine structures, sensors, etc. and a substrate bonding technique capable of suppressing the generation of voids, and releasing the stresses generated at the bonding of substrates having different coefficients of thermal expansion, thereby preventing warpings has been keenly desired. As described above, the technique capable of providing SOI substrates good enough to prepare electronic devices of improved characteristics with a high productivity has not been established yet. The US-A-4,771,016 describes a method of forming a high quality silicon on insulator semiconductor device using wafer bonding. The method comprises the steps of bringing a first silica glass substrate provided with boron and phosphorous by chemical vapor deposition on the surface of an insulating layer formed on the surface of the substrate in contact with a second substrate and integrating both of the substrates by a heat treatment. The JP-A-55 016 464 describes a method of forming a wafer for a semiconductor device comprising the step of providing a phosphorous glass layer on the surface of a first silicon substrate, providing a phosphorous glass layer on a separately prepared silicon substrate, and heat-bonding both silicon substrates to each other with the phosphorous glass layers opposing to and contacting with each other. SUMMARY OF THE INVENTION An object of the present invention is to provide a process for preparing a semiconductor substrate, where generation of fine voids due to the surface unevenness of bonded substrates is suppressed when an SOI substrate of high characteristics is prepared by bonding. This object can be achieved according to the present invention by providing a process as recited in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1, 2 are schematic views showing one example of the process for preparing a semiconductor substrate according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, there is provided a process for preparing a semiconductor substrate according to claim 1. This process for preparing a semiconductor substrate may comprise bringing a first substrate and a second substrate each provided with at least one of boron and phosphorus on the surfaces of heat-softening insulating oxide layers formed on the surfaces of the substrates in contact with each other so that the heat-softening insulating oxide layers are in contact with each other, then heating the substrates, thereby integrating the substrates. In the present process for preparing a semiconductor substrate, the viscosity characteristics with respect to temperatures at the bonding interface are changed by introducing an impurity to the region near the bonding interface, and microscopic unevenness existing at the bonding interface is filled up by promoted viscous movement of substances in the region near the bonding interface, thereby making the unevenness to decrease or disappear completely. That is, unbonded regions appearing at the bonding interface and the consequent voids, which are the most important problem in the wafer bonding technology, can be made to decrease or disappear completely for the first time. Reflow of boron-phosphorus glass is known as one of the flattening procedures in the semiconductor integrated circuit production technique (R.A. Levy, K. Nassau: Solid State Technology, Japanese Edition, pp 69 - 71, November issue, 1986). Recently, Imai disclosed that voids can be reduced on the substrates having surface level differences by the presence of boron-phosphorus glass (Japanese Journal of Applied Physics, Vol. 30, No. 6, June, 1991, pp 1154 - 1157). However, our extensive researches revealed that the surface of the glass deposited by chemical vapor deposition process, as disclosed in the above-mentioned report, had considerable unevenness and could not be used for bonding as such. Furthermore, the remaining of a large amount of boron and phosphorus, which serve as acceptors and donors, in the region near the bonding interface is a serious problem in the production of electronic devices, optical devices and micromachines. In the present invention, an impurity capable of changing the viscosity of an insulating layer is imparted as a small amount of accelerated charged particles with a good controllability only to the surface of the thermally oxidized film maintaining a surface flatness of atomic level to form a viscous flow-promoting intermediate layer with a very high flatness, and the void-free bonding step as desired can be established for the first time. When other materials than silicon, for example, quartz, is used as a second substrate serving as a support member, the entire substrates are considerably warped or the silicon film is peeled off or broken due to a difference in the coefficient of thermal expansion from the silicon substrate, and thus it is quite difficult to bond substrates of different materials to each other and subject the bonded substrates to heat treatment. However, when there is a viscous flow-promoting intermediate layer between the two materials and the intermediate layer is softened by heat treatment, the stresses generated between the different materials are released and warping, etc. of the substrates can be largely controlled. Thus, bonding of different materials to each other can be made possible. Preferable concentration of boron or phosphorus to be imparted to the surface of an insulating layer according to the present invention is not less than about 10 % of B 2 O 3 or not more than about 20 % of P 2 O 5 in SiO 2 in case of diffusion. By the addition at that concentration, the surface of the insulating layer can be softened within the process temperature (1100°C). In case of the presence of P 2 O 5 or B 2 O 3 at the above-mentioned concentrations, softening starts at about 700°C. When imparting of boron or phosphorus is carried out by ion implantation, preferable amount of implanted ions is 1 x 10 20 cm -3 to 5 x 10 21 cm -3 . The present invention will be described in detail below, referring to the embodiments and the drawings. Embodiment 1 In Fig. 1, a first substrate 100 and a second substrate 110 are composed of a single element semiconductor such as silicon, germanium, etc., a compound semiconductor such as gallium-arsenic, indium-phosphorus, etc., or glass such as transparent insulating quartz, etc. At first, a flat insulating layer 102 is formed on the surface of the first substrate. In case that the first substrate is composed of silicon, the most suitable insulating layer 102 is a thermally oxidized film, and quantitative determination of the flatness by an interatomic force microscope reveals that a very flat surface having an unevenness of a few nanometers in the thickness direction is formed. However, as has been mentioned before, referring to the prior art, there are very fine unbonded regions at the bonding interface due to various causes, and it is very hard to obtain completely bonded interface characteristics. Main causes for incomplete bonding interface, or unbonded regions, occurrences of voids in the region near the interface seem to be local surface stainings, scars, level differences of crystal face, warping, etc., but have not been determined yet. When stable and thermally oxidized layer is not obtained at the interface like a compound semiconductor, the insulating layer is formed by chemical vapor deposition, particularly vacuum deposition or by sputtering. Then, an impurity capable of promoting a viscous flow of the oxide is added to the surface of the insulating layer 102, preferably down to about 100 nm maximum. In case that the insulating layer is composed of silicon oxide, boron or phosphorus are added thereto as an impurity. The addition is made by implantation of accelerated charged particles, that is, ion implanting. In this manner, a viscous flow-promoting layer 103 can be formed on the surface of the insulating layer without deteriorating the flatness of the insulating layer. The first substrate 100 and a second substrate 110 are carefully washed, dried and firmly bonded each other while heating the bonded substrates at least to a temperature where the viscosity of the viscous flow-promoting layer 103 is lowered. Though the flatness is very high, very fine voids due to the steps equivalent to the atomic surface can be made disappear by the viscous flow of the viscous flow-promoting layer. Embodiment 2 According to an embodiment of the present invention shown in Fig. 2, insulating layers 202 and 212 are provided on a first substrate 200 and a second substrate 210, respectively, and viscous flow-promoting layers 203 and 213 are also provided on the surfaces of the insulating layers 202 and 212, respectively where even voids due to much larger surface unevenness can be made effectively to disappear. Example 1 A thermally oxidized film having a thickness of 0.5 µm was formed on the surface of a p-type (100) monocrystalline silicon substrate, 12.7 cm (5 inches) in diameter, and 0.1 to 0.2 Ω·cm in resistivity as a first substrate, and a viscous flow-promoting layer was formed in the surface layer to a depth of 0.1 µm from the surface of the thermally oxide film by ion implantation of boron and phosphorus onto the surface of the thermally oxidized film. The viscous flow-promoting layer contained 1 x 10 20 atoms/cm 3 each of boron and phosphorus. Then, the first substrate was washed with a liquid mixture of hydrochloric acid, hydrogen peroxide and water and dried, and was tightly bonded to a monocrystalline silicon substrate as a second substrate, 12.7 cm (5 inches) in diameter, washed in the same manner as above, at room temperature. Then, the bonded substrates were heat treated in a nitrogen atmosphere at 1000°C for 20 minutes. Observation of the bonded substrates by an X-ray topography revealed that there were no voids, and the tensile strength test showed that the bonding strength was 981 bar (1000 kgf/cm 2 ) or higher. The first substrate of the bonded substrates was ground and polished to a thin film having a thickness of 1 µm. When five voids (less than 10 µm in diameter) were detected with an optical microscope, no such voids were not found at all. Example 2 An epitaxial layer was formed on a p-type (100) silicon monocrystalline substrate having a resistivity of 0.01 Ω·cm as a first substrate to a thickness of 1.5 µm. The surface of the epitaxial layer was thermally oxidized to a depth of 0.5 µm in the same manner as in Example 1, and then 1 x 10 21 atoms/cm 3 of phosphorus was ion-implanted into the surface region of the thermally oxidized layer to a depth of 0.1 µm. As a second substrate, a silicon substrate provided with a thermally oxidized film having a thickness of 1 µm by thermally oxidation and a boron-implanted surface region containing 1 x 10 20 atoms/cm 3 of boron on the surface of the thermally oxided film is formed. Both substrates were washed with a liquid mixture of hydrochloric acid, hydrogen peroxide and water, dried and tightly bonded to each other at room temperature. Then, the bonded substrate was heat treated in a nitrogen atmosphere at 1100°C for 60 minutes. Observation of the bonded substrates revealed that there were no voids, and the tensile strength test revealed that the bonding strength was 1177 bar (1200 kgf/cm 2 ) or higher. Example 3 An epitaxial layer was formed on a p-type (100) silicon monocrystalline substrate having a resistivity of 0.01 Ω·cm as a first substrate to a thickness of 1.5 µm. The surface of the epitaxial layer was thermally oxidized to a depth of 0.5 µm, and then 1 x 10 21 atoms/cm 3 each of boron and phosphorus was ion-implanted into the surface region of the thermally oxidized layer to a depth of 0.1 µm. As a second substrate, a fused quartz substrate ion-implanted with 1 x 10 20 atoms/cm 3 each of boron and phosphorus into the surface region to a depth of 0.1 µm and then both substrates were washed with a liquid mixture of hydrochloric acid, hydrogen peroxide and water, dried and tightly bonded to each other at room temperature. Then, the bonded substrate was heat treated in a nitrogen atmosphere at 200°C for 6 hours. Observation of the bonded substrates revealed that there were no voids, and the tensile strength test revealed that the bonding strength was 490 bar (500 kgf/cm 2 ) or higher. A process for preparing a semiconductor substrate comprises bringing a first substrate provided with at least one of boron and phosphorus on the surface of an insulating layer formed on the surface of the substrate in contact with a second substrate, and integrating both of the substrates by a heat treatment, wherein the boron or the phosphorous is provided as accelerated charged particles.

Claims: A process of preparing a semiconductor substrate which comprises bringing a first substrate (100; 200) provided with at least one of boron and phosphorus on the surface of a heat-softening insulating oxide layer (102; 202, 212) formed on the surface of the first substrate in contact with a second substrate (110; 210), wherein the boron or the phosphorus is provided on the surface of an insulating oxide layer (102; 202, 212) as accelerated, charged particles, whereby said insulating oxide layer becomes said heat-softening insulating oxide layer (102; 202, 212) and integrating both of the substrates (100, 110; 200, 210) by a heat treatment. A process according to claim 1, wherein each of the first (100; 200) and second substrate (110; 210) is provided with at least one of boron and phosphorus on the surface of the heat-softening insulating oxide layer (102; 202, 212), and which comprises bringing the first (100; 200) and the second substrate (110; 210) in contact with each other such that the heat-softening insulating oxide layers (102; 202, 212) are in contact with each other. A process according to claim 1 or 2, wherein the first substrate (100; 200) and the second substrate (110; 210) are formed of a single element semiconductor material of a compound semiconductor material or one of the first substrate and the second substrate is glass.

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