Method for forming single crystal layer

Abstract: A crystal forming method comprises disposing, on a surface of a substrate or in recessed portion formed in the substrate having a surface with a low nucleation density, primary seed having a sufficient small volume to singly aggregate and a rectangular prismatic or cubic shape in which all the sides and the bottom are surrounded by an insulator in contact therewith; performing heat treatment for aggregating the primary seed to form monocrystalline seed crystal having controlled plane orientation and in-plane orientation; and selectively growing monocrystal by crystal growth treatment using the seed crystal as starting point.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of forming a crystal, and particularly to a method of forming a crystal by selectively growing monocrystal on a substrate having a surface with a low nucleation density. The method of forming a crystal of the present invention is preferably used for electronic devices such as semiconductor integrated circuits, optical integrated circuits and the like, optical devices, magnetic devices, piezoelectric devices, surface acoustic devices and the like. Related Background Art Monocrystal films used for semiconductor devices and optical devices are generally formed by epitaxially growing monocrystal on a monocrystalline substrate. For example, it is known that Si, Ge or GaAs is epitaxially grown on a Si monocrystalline substrate (silicon wafer) from a liquid phase, a vapor phase or a solid phase thereof. It is also known that a GaAs or GaAlAs monocrystal is epitaxially grown on a GaAs monocrystalline substrate. The semiconductor films formed by the above methods are used for producing semiconductor devices, integrated circuits, light emitting devices such as semiconductor lasers, LED and the like. On the other hand, very high speed transistors which use two-dimensional gases and superlattice elements which use quantum wells have recently actively been researched and developed. These devices can be realized by high precision epitaxial techniques such as MBE (molecular beam epitaxy) using super high vacuum, MOCVD (metal organic chemical vapor deposition) and the like. Such epitaxial growth on a monocrystalline substrate requires conformity in lattice constant and thermal expansion coefficient between the monocrystalline material of the substrate and the epitaxially grown layer. For example, although an Si monocrystalline film can be epitaxially grown on sapphire used as an insulator monocrystalline substrate, the crystal lattice defects caused by the differenced hetween the lattice constants at the interface and the diffusion of the aluminum component of sapphire into the epitaxial layer are problems in application to electronic devices and circuits. As described above, it is known that the conventional methods of forming a monocrystalline film by epitaxial growth are significantly dependent on the substrate material used. Mathews et al. investigated combinations of substrate materials and epitaxially grown layers (EPITAXIAL GROWTH, Academic Press, New York, 1975, edited by J.W. Mathews). Presently, the size of a substrate is about 15.24 cm (6 inches) in the case of an Si wafer and the size of a GaAs or sapphire substrate is slightly increased. In addition, since a monocrystalline substrate is produced at high cost, the cost per chip is increased. The conventional methods of forming a monocrystalline layer which enables the formation of excellent devices have the problem that the kinds of materials for the substrate are limited to a very narrow range. On the other hand, there have recently been active research and development of three-dimensional integrated circuits in which a semiconductor device is laminated on a substrate in the direction of a normal line thereof so that an increase in integration and diversification of the function can be achieved. In addition, large area semiconductor devices such as solar cells, liquid crystal pixel switching transistors and the like in which elements are arrayed on inexpensive glass are increasingly actively researched and developed year after year. The both types of devices commonly require the technique of forming a semiconductor film on an amorphous insulator and then forming electronic devices such as transistors or the like thereon. particularly, there is demand for the technique of forming high-quality monocrystalline semiconductors on an amorphous insulator. In general, when a thin film is deposited on an amorphous insulator substrate composed of SiO 2 , the crystal structure of the deposited film is amorphous or polycrystalline due to the absence of long-distance order in the substrate material. The terms "an amorphous film" represent a film assuming the state order is held within a short range such as a distance between adjacent atoms but is not held at a longer distance, and the terms "a polycrystalline film" represent a film comprising monocrystalline grains having no particular crystal orientation and gathering with grain boundaries which separate the grains from each other. For example, Si is deposited on SiO 2 by a CVD process, amorphous silicon is formed at a deposition temperature of about 600 °C or less, and polycrystalline silicon comprising grains having a size of several tens to several hundreds nanometer (several hundreds to several thousands Å) at a deposition temperature of 600 °C or more. However, the grain size of the polycrystalline silicon formed is significantly dependent on the formation conditions. Further, an amorphous or polycrystalline film is molten and solidified by using an energy beam generated from a laser or a rod heater to form a polycrystalline film comprising grains having a large size of several microns or millimeters (Single Crystal Silicon on Non-Single Crystal Insulators, Journal of Crystal Growth Vol. 63, No. 3, 1983 edited by G.W. Cullen). As a result of measurement of electron mobilities of transistors formed on the thin films formed by the above method and having various crystal structures, the mobility of amorphous silicon was ∼ 0.1 cm 2 /V·sec, the mobility of polycrystalline silicon comprising grains having a size of several tens nm (several hundreds Å) was 1 to 10 cm 2 /V·sec, and the mobility of polycrystalline silicon formed by melting and solidification and comprising large grains was substantially the same as that of monocrystalline silicon. These results show that the electrical properties of the device formed in a monocrystalline region within a crystal grain are significantly different from those the device formed on a region including a grain boundary. Namely, the film deposited on an amorphous substrate has an amorphous or polycrystalline structure, and the device formed on the deposited film has poor properties, as compared with the device formed on a monocrystalline layer. The application of such a device formed on a deposited film is limited to a simple switching element, a solar cell, a photoelectric conversion element or the like. Methods of depositing a crystal layer on an amorphous substrate are roughly divided into the following two methods. One method is a method in which an amorphous insulator (for example, SiO 2 ) is coated on a monocrystal (for example, Si) used as a substrate and then partially removed to expose the surface of the ground monocrystal, and a monocrystalline region is then formed on the amorphous insulator layer by horizontal epitaxial growth from a vapor phase, solid phase or liquid phase using the exposed ground monocrystal surface as a seed crystal. The other method is a method of growing a crystal thin film directly on a substrate without using a monocrystal as a substrate. As described above, since the surface of an amorphous substrate has no long-distance order, as the surface of a monocrystalline substrate, but holds short-range order only, the deposited thin film at best has a polycrystal structure in which grain boundaries are randomly present. In addition, since both the long-distance order and anisotropy defining crystal orientation (orientation in the normal direction of the substrate and in the surface) are absent in the surface of an amorphous substrate, the crystal orientation of a layer formed on the substrate cannot be controlled. Namely, a problem of deposition of a monocrystal on an amorphous substrate is to establish a technique of controlling boundary positions and crystal orientation. A description will now be given of conventional methods of controlling boundary positions and crystal orientation and the problems thereof. In regard to the control of boundary positions, EP244081A1 publication discloses that boundary positions can be determined by artificially previously defining nucleation positions. In this technique, for example, Si 3 N 4 is localized on SiO 2 , Si monocrystal are grown from Si 3 N 4 serving as nucleation sites, and grain boundaries are formed by collision between the crystals grown from adjacent nucleation sites, thereby determining boundary positions. Another technique is also proposed in which a non-monocrystalline material serving as primary seed is previously patterned in stead of nuclei which are spontaneously produced at nucleation positions, the material is changed to monocrystal serving as seed crystals by employing aggregation (Japanese Patent laid-Open No. 1-132117). The plane orientation (crystal orientation in the direction vertical to the substrate) of the crystals grown by the above techniques is determined by the factors such as stabilization of interfacial energy at the amorphous interface with the substrate, stabilization of free surface energy, relaxation of internal stress and so on during the generation of nuclei or aggregation of primary seeds. However, it is difficult to obtain complete orientation in a single direction, and main plane orientation and other orientations are frequently mixed. Particularly, one in-plane crystal orientation is not determined because the nucleation surfaces or aggregation surfaces are amorphous and do not have anisotropy. On the other hand, H. I. Smith first showed that anisotropy caused by irregularity is artificially provided on the surface of an amorphous substrate by lithography so that the crystal orientation of KC1 deposited on the substrate can be controlled, and named this technique "graphoepitaxy" (H. I. Smith and D. C. Flanders, Applied Physics Letters Vol. 32, pp349, 1978) (H. I. Smith, U.S. Patent No. 4,333,792, 1982). It was later confirmed that an artificial relief pattern formed on the surface of a substrate affects the crystal orientation in the growth of grains in a Ge thin film (T. Yonehara, H. I. Smith, C. V. Thompson and J. E. Palmer, Applied Physics Letters, Vol. 45, pp. 631, 1984) and in the initial growth of Sn (L. S. Darken and D. H. Lowndere, Applied Physics Letters Vol. 40, pp. 954, 1982). Although it was found that graphoepitaxy has an effect on the orientation of each of separate KC1 or Sn crystals deposited in an initial stage, there have also been reports of continuous layers, e.g., an Si layer formed by laser annealing crystal growth after deposition (M. W. Ceis, D. A. Flanders and H. I. Smith, Applied Physics Letters, Vol. 35, pp. 71, 1979) and a Ge layer formed by solid phase growth (T. Yonehara, H. I. Smith, C. V. Thompson and J. E. Palmer, Applied Physics Letters, Vol. 45, pp. 631, 1984). However, in the cases of Si, Ge layers, although orientation is controlled to some extent, a crystal group is arranged in a mosaic pattern, and grain boundaries are randomly present between crystals having slightly different crystal orientations. It is thus impossible to obtain monocrystal uniformly arranged in a large area. The reason for this is that the three-dimensional crystal orientations of respective crystals are not completely the same, and the nucleation positions cannot be controlled by a surface relief pattern. EP-A-0 365 166 discloses a crystal forming method comprising providing a concavity or recess having an n-gonal shape such as an octagonal planar shape and having a non-nucleation surface in a substrate, forming a nucleation surface in the center of the recess, and then performing a crystal growth treatment for forming a single crystal in the n-gonal shaped concavity or recess. EP-A-0 366 276 discloses a crystal forming method comprising applying, on a substrate having a primary seed comprising a non-single crystalline material having a surface area fine enough to be agglomeratable into a single-crystalline body by heat treatment and a non-nucleation surface with smaller nucleation density than said primary seed, a heat treatment for agglomerating said primary seed to form a single crystalline seed in an atmosphere containing hydrogen gas at a temperature lower than the temperature at which said primary seed is melted, and applying a crystal growth treatment, thereby permitting a single crystal to grow with said seed as the origination point. EP-A-0 284 441 discloses a crystal forming method comprising forming a SiO 2 film as a non-nucleation surface on a substrate, then ion implanting Se ions into the photoresist-masked SiO 2 film so as to form nucleation surfaces at the non-masked portions where Se ions are implanted, and subsequently growing a CdSe crystal thereon as CdSe monocrystals starting from the nucleation surfaces. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of forming a crystal which is capable of growing monocrystal having grain boundaries at controlled positions and the same plane orientation and in-plane orientation. It is another object of the present invention to provide a method which is capable of uniformly and simply forming monocrystal having a single orientation in a large area without producing a plurality of orientations other than main orientation. These objects are solved by the crystal forming method as set out in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a schematic drawing showing an example of a primary seed used in a method of forming a crystal of the present invention; Figs. 1B to 1D are schematic drawings respectively showing the shapes of insulators surrounding primary seed; Figs. 2A to 2D are schematic drawings showing an example of methods of forming crystals of the present invention; Figs. 3A to 3C are schematic drawings showing an example of processes of forming a primary seed in a method of forming a crystal of the present invention; Figs. 4A to 4D are schematic drawings showing another example of processes of forming a primary seed in a method of forming a crystal of the present invention; Fig. 5A is a schematic drawing showing an example of a primary seed in a method of forming a crystal of the present invention; Figs. 5B to 5D are schematic drawings respectively showing the shapes of insulators surrounding a primary seed; Figs. 6A to 6D are schematic drawings showing an example of processes in a method of forming a crystal of the present invention; Figs. 7A to 7C and Figs. 8A to 8D are schematic drawings respectively showing examples of processes in a method of forming a crystal of the present invention. The method of forming crystals of the present invention enables the selective growth of monocrystal from monocrystalline seed crystal serving as starting point and having controlled plane orientation and in-plane orientation and thus enables the formation of monocrystal having controlled grain boundaries and uniform plane orientation and in-plane orientation on an insulating substrate. The present invention also permits a primary seed of which has a rectangular prismatic or cubic shape having surfaces in contact with an insulator and which is changed to monocrystal by melting and solidification to be changed from non-monocrystal to monocrystal having uniform plane orientation and in-plane orientation. When a semiconductor material is selectively grown by using the monocrystal as seed crystal, SOI devices can be formed with controlled boundary positions and uniform plane orientation and in-plane orientation. The present invention is capable of completely controlling the boundary positions (number) and orientation, which cause deviations in properties of SOI device, and thus forming device with high uniformity. In addition, since melting and solidification on the three vertical surfaces at an angular portion of recessed portion in the substrate has good effects on the plane orientation and in-plane orientation of seed crystals, the present invention is capable of uniformly and simply forming monocrystal over a large area with a single orientation without a plurality of orientations other than main orientation. Further, since the three vertical surfaces at an angular portions of recessed portion in the substrate have good functions as equivalent surfaces to the stable orientation structure of the seed crystal formed by aggregation, the present invention is capable of uniformly and simply forming monocrystal over a large area with a single orientation without a plurality of orientations other than main orientation. The plane orientation of a crystal formed by aggregation or melting and solidification is determined so that the free surface energy of the crystal and the interfacial energy at the interface with the substrate are minimum, as described above. For example, when polycrystalline Si is aggregated on SiO 2 , since the free surface energy of Si is minimum on the (111) plane, the plane orientation is produced along the (111) plane. On the other hand, since the Si-SiO 2 interfacial energy is stable on the (100) plane, the interfacial energy is conformed to the (100) plane. In addition, since polycrystalline Si generally has (110) orientation depending upon the deposition temperature, the factor of the initial state also has an influence on the orientation. As a result, Si aggregated on SiO 2 is relatively strongly oriented on the (111) plane, with having (110) orientation with a ratio of about 1/2 to 1/3 of (111) orientation in terms of the reflection intensity in X-ray diffraction. In X-ray diffraction, weak peaks of (100), (311), (331), (422) and so on are also observed. Possible reasons for domination of (111) orientation are that aggregated Si comprises semispherical crystal and has a surface area significantly greater than the area of the interface, and that the stabilization of free surface energy among the factors for determining orientation is the most important factor. However, since the above-described many factors have influence on orientation, it is very difficult to completely control a single orientation even by changing the material of the substrate surface or aggregation conditions. Much more, it is substantially impossible to control the in-plane orientation by aggregation on a two-dimensional surface. The inventors thus conceived an arrangement in which the stabilization of interfacial energy is a main factor which determines orientation for controlling a single orientation and then an arrangement in which in-plane orientation is further controlled. Both arrangements can be satisfied by the arrangement of the present invention which is obtained by aggregating a primary seed having a rectangular prismatic or cubic shape formed so as to be surrounded by an insulator in contact with the sides and the bottom thereof and having a small volume sufficient to singly aggregate. A typical example of primary seed is shown in Fig. 1A. However, the rectangular prism (or cube) shown in the drawing is surrounded by an insulator in contact with all the surfaces thereof excepting one surface (the upper surface of the xy plane). Although the stabilization of interfacial energy during aggregation is affected by the bottom of an primary seed and only one of the surfaces of an insulator in conventional methods, in the crystal forming method of the present invention, the five surfaces contribute to the stabilization of the interfacial energy and respectively intersect the adjacent surfaces. The method of the present invention thus has the advantage that the five surfaces have the functions as equivalent surfaces to the stable orientation structure of a crystal formed by aggregation. In addition, since the exposed area is smaller than that in the conventional methods, the method of present invention has the effect of decreasing the influence of stabilization of interfacial energy on changes in the structure during aggregation. As a result, the control of orientation (including in-plane orientation) which is produced in graphoepitaxy or growth of Ge grain which takes plane in a surface relief structure can be reproduced in an aggregation system comprising primary seeds. Embodiments of the present invention are described below with reference to Figs. 1A to 1D and Figs. 2A to 2D. Embodiment A description is first made of the shape and size of a primary seed which is the most important in the present invention. Although the shape of primary seed must be a rectangular prismatic or cubic shape, as described above, strictly speaking, the objects of the present invention can be achieved even if the shape of the primary seed is not a completely rectangular prismatic or cubic shape. Conversely, if primary seed has a completely rectangular prismatic shape, it is difficult to provide a monocrystalline seed with specified orientation in the direction vertical to a surface of an insulator without deviation of several second degrees even by aggregating the primary seed surrounded by the insulator. If the crystals grown from such seed crystals collide with each other, grain boundaries are invariably produced. In the present invention, it is therefore sufficient that the position of grain boundaries is controlled, and no deviation occurs in the properties of devices due to deviations in orientation during the production of devices. Namely, deviations from design values which are caused by sagging or over etching produced at the pattern edge in a photolithographic process or etching process, i.e., an angle of several degrees or less, is allowable. The terms "a rectangular prismatic or cubic primary seed surrounded by an insulator in contact with the side walls and the bottom thereof" represent, for example, the shape shown in Figs. 1B, 1C or 1D. Figs. 1B and 1C show so-called "buried type" shapes. These buried type shapes are formed by partially patterning the surface of an insulator substrate 102 with squares or rectangles and etching the surface to form rectangular prismatic or cubic space and respectively burying the primary seed in the space formed. In this case, a primary seed 101 to be buried may have a shape which causes the exposed surface to be at the same level as that of the surface of the insulator, i.e., which makes the whole surface flat, as shown in Fig. 1B, or a shape which causes the exposed surface to be at a level lower than that of the insulator surface, i.e., which has a height (the c value shown in Fig. 1A) smaller than the etching depth. On the other hand, Fig. 1D shows "a side-wall formation type". This type is formed by patterning a primary seed in a rectangular prismatic or cubic region on an insulator substrate or a substrate having an insulator surface layer and then forming an insulator on the side walls of the primary seed with leaving the upper surface so as to surround the primary seed. In any one of the cases, the material of the side walls need not to be the same as the material of the substrate 102. For example, in the case of the buried type, a rectangular prismatic or cubic space is formed in the substrate, and an insulating film is formed in the space formed so that the surfaces of the insulating film are used as insulator side walls. The size of the primary speed is described below. In the shape shown in Fig. 1A, the primary seed has a, b values which are preferably a, b ≤ 2 µm (both values may be the same, a = b), more preferably 1 µm or less, and most preferably about 0.5 µm. This is because, if the a, b values exceed 2 µm, the primary seed is not singly aggregated but aggregated in a form divided into a plurality of portions. Whether the seed crystal is divided or not significantly depends upon the thickness of the film, i.e., the c value, and if the aspect ratio between the value a or b and the value c is high, the seed crystal is easily divided. The reason for determining the optimum a, b values of about 0.5 µm is that if the a , b values are set to be 0.5 µm or less, a general semiconductor photoprocess cannot be easily applied to the primary seed. On the other hand, the thickness (= c ) of the primary seed is preferably 0.05 to 0.5 µm, and more preferably 0.1 to 0.4 µm. The optimum thickness is determined by the values a and b . Although the smaller the thickness is, the more easily aggregation takes place, if the thickness is less than 0.05 µm, the values a , b must be decreased for singly aggregating the primary seed. This causes the problems that a general photoprocess cannot be easily applied, as described above, and that the effect of the side walls for controlling orientation is decreased. If the value c is greater than 0.5µm, aggregation does not easily take place. The optimum value of the thickness is therefore within the range of 0.5 > c > 0.05 (µm). Examples of materials that can be used for the primary seed to be aggregated include semiconductor elements such as Si, Ge, Sn and the like; metals such as Au, Ag, Cu, Pt, Pd and the like; alloys; compounds and mixtures thereof. An amorphous insulator such as SiO 2 , Si x N y , SiON or the like is used as an insulator which forms non-nucleation surfaces. The composition of the insulator which forms the non-nucleation surfaces need not to be the same as that of the insulator in the portions contacting with the primary seed. The process in an embodiment of the present invention is described below with reference to Figs. 2A to 2D. As shown in Fig. 2A, a substance used for primary seeds 201a, 201b is buried in small recessed portions 203a, 203b in an insulator substrate 202 or a substrate having a surface layer covered with an insulator. Each of the recessed portions 203a, 230b has a rectangular prismatic or cubic form. As described above, the recessed portions 203a, 203b need not to be formed in the substrate, and the side walls of each of the primary sees 201a, 201b may be surrounded by an insulator in contact therewith. Although Si, Ge or the like is used as a material for the primary seeds 201a, 201b, as described above, a polycrystal or an amorphous material may be used. As shown in Fig. 2B, the primary seeds 201a, 201b are then subjected to heat treatment in an atmosphere of hydrogen so that aggregation takes place to form seed crystals 204a, 204b having uniform plane orientation and in-plane orientation. Although the conditions of heat treatment depend upon the material use for the primary seeds and the volume thereof, when the material for the primary seeds is a semiconductor element such as Si, Ge or the like, the temperature is generally about 700 to 1100 °C. Although the pressure may be atmospheric pressure, heat treatment is preferably performed under reduced pressure (several hundreds Pa to 26.6 · 10 4 Pa [several Torr to 200 Torr]) so that aggregation easily takes place. Doping of the primary seeds with large quantities of impurities (phosphorus, boron, arsenic or the like) has the effect of decreasing the aggregation starting temperature. As shown in Fig. 2C, monocrystals are selectively grown from the monocrystalline seed crystals formed by aggregation and serving as centers. The crystal growth is effected by the CVD process. For example, when Si crystals are grown, the growth can be achieved at a temperature of 800 to 1200 °C by using as a source gas a silane type gas such as SiH 4 , Si 2 H 6 or the like, a chlorosilane type gas such as SiH 2 Cl, SiHCl 3 , SiCl 4 or the like, or a fluorosilane gas such as SiH 2 F 2 , SiF 4 or the like, which source gas is mixed with etching gas such as HCl or the like in H 2 dilution gas. A grain boundary 206 is produced between the crystal 205a grown from the seed crystal 204a and the crystal 205b similarly grown from the adjacent seed crystal 204b at an intermediate position between the adjacent seed crystals which are first patterned. If the two crystals 205a, 205b originally have the completely same orientation including in-plane orientation, a continuous crystal film is formed without any grain boundaries. However, in fact, there are slight deviations in crystal orientation between crystals, grain boundaries are formed in most cases. Each of the grown crystals 205a, 205b is a monocrystal and thus has a crystal face (facet) peculiar to the crystal. When electronic devices are formed on the grown crystals, therefore, the crystals are polished to form a flat surface as occasion demands, as shown in Fig. 2D. Another embodiment of the present invention is described below. The crystal forming method comprises disposing monocrystalline seed crystal on a substrate, and growing monocrystal using the seed crystal as starting point, wherein recessed portion having at least an angular portion where three surfaces comprising the bottom and two sides vertically contact with each other is formed in the substrate, a primary seed is disposed in the angular portions so as to contact with the three surfaces and then subjected to heat treatment for singly aggregating the primary seed to form monocrystalline seed crystal, and monocrystal is then grown using the seed crystal as starting point. In the present invention, the inventor conceived a structure for controlling the in-plane orientation of each grain and the positions of the grain boundaries produced by slight deviation in orientation. In the control of orientation, it was first confirmed that the orientation vertical to the plane of the substrate is controlled to predetermined orientation by melting and solidifying the film material. Since no information about in-plane orientation is provided, information about the control of horizontal orientation is provided by forming the side walls (surfaces) vertical to the plane of the substrate. The side walls are formed so that four surfaces are vertical to each other, i.e., the portion surrounded by the surfaces forms a rectangular prism or a cube, thereby providing information about orientation control from the four directions. In addition, since the rectangular prism or cube formed must comprise a monocrystal containing no grain boundary during melting and solidification, and since the walls must affect the whole primary seed having a rectangular prismatic or cubic shape, the rectangular prism or cube must be designed so as to have an appropriate volume. Typical vaues of the size of the rectangular prism or cube are described below. The position of grain boundary can be controlled by patterning primary seed having a rectangular prismatic or cubic shape at desired positions. Namely, when primary seeds are patterned at desired two positions, crystals are respectively grown from the primary seeds and collide with each other at just an intermediate position between the two points to form a grain boundary. For example, when primary seeds are patterned at lattice points at distances ℓ, grain boundaries are formed at intermediate positions between the respective lattice points. As a result, square crystals each having sides ℓ are regularly arranged in a lattice form. Although it is thought that when crystals having completely uniform plane orientation and in-plane orientation collide with each other, no grain boundary is generated, grain boundaries are in fact produced because it is difficult to form and grow seed crystals without deviation of only 1 second degree in the direction vertical to the substrate and the in-plane direction thereof. In short, it is sufficient that the positions of grain boundaries are controlled, and no deviation is produced in the properties of devices due to deviation in orientation during the production of the devices. Slags produced at the pattern edges or deviations from the design values, which are caused by over etching, in a usual photolithographic process or etching process, i.e., angles of several degrees or less, are allowable. A description will now be made of the method of forming a crystal film having controlled plane orientation and in-plane orientation on an insulating substrate using the seed crystals formed on the basis of the above-described idea. The shape and volume of a primary seed are described below. As shown in Fig. 5A, a primary seed has a rectangular prismatic shape (or a cubic shape (a = b = c)). This is because when a certain orientation is preferential in one surface of a rectangular prism or cube, orientations equivalent to that orientation are preferential in all the other surfaces thereof. The volume of a primary seed must be sufficiently small to generate no grain boundary in the primary seed during melting and solidification, and the area of the sides are sufficinetly large to stably determine in-plane orientation, as described above. Specifically, the value a of the primary seed shown in Fig. 5A is preferably within the range of 0.1 µm to 5 µm, and more preferably within the range of 0.5 µm to 2 µm, assuming that a = b. The optimum value depends upon the materials for the primary seed and the substrate. The minimum value is determined by the limit value of precision of the photolithographic method or the minimum area which permits the stabilization energy at the interface with the substrate to control orientation. The maximum value is experimentally determined so that no grain boundary is produced during solidification. However, this maximum value is related to the value of the height c of the sides. The value of the height c of the sides of the rectangular prism or cube is preferably within the range of 0.1 µm to 2 µm, and more preferably within the range of 0.3 µm to 1 µm. The optimum value depends upon the values a and b and the material for the primary seed. The minimum value is determined to be a minimum value which allows the sides to affect the determination of in-plane orientation. The maximum value is determined to be a value which does not cause the formation of grain boundary during solidification. All the surfaces of a primary seed must be covered with an insulator, as shown in Figs. 5B, 5C and 5D. Figs. 5B and 5C respectively show "buried types" of primary seeds. In the buried types, the surface of an insulator substrate 601 is partially patterned with squares or rectangles and then etched to form rectangular prismatic or cubic shapes in the substrate, a primary seed 602 is respectively buried in the spaces, and an insulating film is then formed thereon. In this case, the upper surfaces of the primary seed may be at the same level as that of the surface of the insulator substrate 602, i.e., the whole surface becomes flat, as shown in Fig. 5B, or the upper surfaces of the primary seed may be lower than that of the insulator surface, i.e., the height c of each primary seed may be smaller than the etching depth, as shown in Fig. 5C. On the other hand, Fig. 5D shows "a side wall formation type". In this type, the primary seed 602 is first patterned in rectangular prism on the insulator substrate 601 or the substrate having a surface layer comprising an insulator. If the primary seed 602 consists of a material which can stably form an oxide, the exposed surface may be oxidized, or an insulator may be deposited thereon by CVD or the like. In any one of the types, the material of the side walls need not to be the same as that of the substrate 601. For example, in the buried types, after rectangular prismatic or cubic space is formed in the substrate an insulator film may be formed in the space, the primary seed 602 may be buried in the space, and the insulating film 603 may be then formed thereon. In all the types shown in Figs. 5B, 5C and 5D, the upper surface of the primary seed is covered with the insulating film 603 consisting of an insulator for the purpose of preventing the aggregation or volatalization of the primary seed during melting. The insulating film 603 may be formed by oxidizing the primary seed or depositing an insulator by CVD or the like. Examples of materials that can be used for the primary seed melted include semiconductor elements such as Si, Ge, Sn and the like; metals such as Au, Ag, Cu, Pt, Pd and the like; alloys; compounds; and mixtures thereof. SiO 2 , Si x N y , SiON or the like can be used as the insulator (substrate) which forms the non-nucleation surfaces, as well as the insulator which covers the upper surface. The processes in this embodiment of the present invention are described below with reference to Fig. 6. Rectangular prismatic primary seed 702 is formed on the surface of a substrate 701 having an insulator surface, as described above with reference to Figs. 5A to 5D. The whole surface including the upper surface of the primary seed 702 (the upper surface being covered with an insulating film 703) is then covered with an insulator (Fig. 6A). The substrate 701 is then heated to a temperature higher than the melting point of the primary seed 702. The heating is performed by a method using an energy beam such as a laser beam, an electron beam or the like or a lamp. When all the primary seeds on the substrate are melted, the heating is stopped, and the melted primary seeds are then solidified. The insulating film 703 consisting of an insulator which covers the upper surfaces of the primary seeds is then removed so that monocrystalline seed crystals 705a, 705b having controlled in-plane orientation are exposed to air (Fig. 6B). The seed crystals 705a, 705b exposed are then selectively grown by the selective CVD process. For example, if it is desired to grow Si, a chlorosilane type gas such as SiCl 4 , SiHCl 3 , SiH 2 Cl 2 or the like, a silane type gas such as SiH 4 , Si 2 H 6 or the like, a fluorosilane type gas such as SiF 4 , SiH 2 F 2 or the like can be used as a source gas. At the same time, HCl gas or HF gas which has the etching function is used together with H 2 carrier gas. Although the temperature and pressure of the crystal growth depend upon the types of the gases used and the composition of the insulator substrate, the temperature is within the range of about 800 to 1200°C, and the pressure is within the range of several hundreds Pa to 3.32 · 10 4 Pa (several Torr to 250 Torr). The grown crystal 706a is grown while surrounded by a facet peculiar to a monocrystal and collides with the crystal 706b grown from the adjacent seed crystal 705b to form a grain boundary near the center between the two seed crystals 705a, 705b (Fig. 6C). When it is desired to form an electron device on each of the grown crystals, if required, the crystals may be polished to form monocrystalline thin films 707a, 707b. In this case, since the position of the grain boundary 708 is known, if a device is formed on a single island, quality which is the same as or higher than that of the device formed on a monocrystal can be obtained. Examples of the present invention are described below with reference to the drawings. Reference Example 1 Figs. 3A to 3C are sectional views showing the steps of forming primary seeds in a Reference Example 1 of the crystal forming method. Since the steps of aggregating a primary seed to form a seed crystal and growing a monocrystal using the seed crystal as starting point are the same as those described above with reference to Figs. 2B to 2D, the steps are described below with reference to Figs. 2B to 2D using the same reference characters. As shown in Fig. 3A, a 4 inch Si wafer was prepared as a substrate, and the surface of the wafer was oxidized to a thickness of 1 µm to form a SiO 2 layer 302. The surface of the SiO 2 layer 302 was then partially patterned with square by a usual photoprocess, and the square portions were then etched to an intermediate position of the SiO 2 layer 302 by RIE (reactive ion etching). In this case, the sides a and b of each square were 0.8 µm, and the etching depth c was 0.4 µm. A plurality of the same recessed portions were formed in a matrix with intervals ℓ = 60 µm (only recessed portions 303a, 303b are shown in Fig. 3A). As shown in Fig. 3B, polycrystalline Si 304 was then deposited to a thickness of 0.4 µm on the substrate by the LPCVD process. Phosphorous ( 31 P + ) was implanted into the polycrystalline Si 304 by ion implantation so that the concentration was 8 x 10 20 cm -3 . A resist 305 was then coated on the polycrystalline Si 304 doped with phosphorous so that the surface was flattened. As shown in Fig. 3C, both the polycrystalline layer Si 304 and the resist were then etched back by RIE under the conditions that the etching rate of the polycrystalline Si was the same as that of the resist until the surface of the SiO 2 layer 302 was exposed so that polycrystalline Si 304a and 304b remained only in the recessed portions. The polycrystalline Si 304a and 304b were used as primary seeds. The etching back conditions were that OFPR5000™ was used as the resist, and RIE was effected by using a gas mixture C 2 F 6 /O 2 (80/35 sccm) at a pressure of 50 pa. with output of 2 kW. The thus-formed polycrystalline Si 304a and 304b respectively filled in the rectangular prismatic recessed portions were then subjected to heat treatment at a pressure of 1.33·10 4 Pa (100 Torr) and a temperature of 1050°C for 10 minutes in an atmosphere of hydrogen, as shown in Fig. 2B. As a result, the polycrystalline Si 304a and 304b were aggregated in a solid phase and changed to monocrystals 204a and 204b, respectively, each having a semispherical surface. Si crystals were then selectively grown from the Si monocrystalline Si 204a and 204b which were formed by aggregation and which served as seed crystals, as shown in Fig. 2C. The crystal growth was carried out at a temperature of 1030°C and a pressure of 1.06·10 4 Pa (80 Torr) for a growing time of 90 minutes under the conditions that the substrate was placed in a gas mixture of SiH 2 Cl 2 as a source gas, HCl as an additive gas for etching and H 2 as a carrier gas with a ratio of 0.53 : 1.6 : 100 (l/min). As a result, when a Si monocrystal 205a was grown to a width of 30 µm in the horizontal direction, it collided with the adjacent monocrystal 205b to form a grain boundary 206. However, all the crystals had appearances with substantially the same facet shape and direction. The thus-obtained crystals were oriented in the (100) direction vertical to the substrate. X-ray diffraction measurement showed no other orientations. As a result of EPC (Electron Channeling Pattern) analysis of in-plane orientation, deviations are held within ± 5° from orientation equivalent to (100) orientation in the patterning direction of the seed crystal. The thus-formed crystals were then flattened for forming devices such as transistors on the crystals, as shown in Fig. 2D. The flattening was performed by the same polishing process as that for polishing a usual Si wafer in which rough polishing and finish polishing were made in turn by using a weak alkaline solution in which abrasive grains were suspended until the residual thickness was 1 µm. Example 1 This example is described with reference to Figs. 4A to 4D. Since the steps of aggregating a primary seed to form a seed crystal and growing a monocrystal using the seed crystal as starting point are the same as those described above with reference to Figs. 2B to 2D, the steps are described below with reference to Figs. 2B to 2D using the same reference characters. Polycrystalline Si 402 was first deposited to a thickness of 0.1 µm on a fused quartz substrate 401 having a diameter of 10.16 cm (4 inches) by the LPCVD process, as shown in Fig. 4A, and a Si 3 N 4 film 403 was then deposited to a thickness of 0.05 µm thereon by the LPCVD process, as shown in Fig. 4A. Island-like regions of 1 x 1 µm were then patterned at intervals of 50 µm, and the Si 3 N 4 film and the polycrystalline Si film were then etched by RIE so that the island-like regions remained, as shown in Fig. 4B. Polycrystalline Si 402a and 402b with Si 3 N 4 film caps 403a and 403b separated in the form of islands were then oxidized as such, as shown in Fig. 4C. As a result, only the peripheral portion of each of the square polycrystalline Si 402a and 402b was oxidized so that the polycrystalline Si 402a and 402b were surrounded by SiO 2 walls 404a and 404b, respectively, as viewed from above the substrate. Only the Si 3 N 4 layers 403a and 403b on the polycrystalline Si 402 were then etched by using hot phosphoric acid (H 3 PO 4 ), as shown in Fig. 4D. The polycrystalline Si 204a and 204b were then aggregated to selectively grow in accordance with the same steps as shown in Figs. 2A to 2D under the same conditions as those used in Reference Example 1. The flattening is similarly carried out. Reference Example 2 This Example is described below with reference to Figs. 7A to 7C and 8A to 8D. As shown in Fig. 7A, the surface of a substrate 801 comprising a 10.16 cm (4 inch) Si wafer was first oxidized to a thickness of 1 µm to form an SiO 2 layer 802 thereon. A square was then patterned on a portion of the surface of the SiO 2 layer 802 by the usual photoprocess and then etched to an intermediate position of the SiO 2 layer by RIE (reactive ion etching). In this Example, the length of one side of the square (a = b) was 0.8 µm, and the depth c of the etched portion was 0.4 µm. A plurality of the same recessed portions 803 as that formed by the above method were formed in a matrix with intervals 1 of 60 µm. As shown in Fig. 7B, a polycrystal Si 804 was then deposited to a thickness of 0.4 µm on the substrate by the LPCVD process. A resist 805 was further coated on the polycrystalline Si 804 so that the surface become flat. As shown in Fig. 7C, both the resist 805 and the polycrystalline Si 804 were then etched back by RIE under the conditions (described below) that the etching rate of the resist 805 was the same as that of the polycrystalline Si 804 until the surface of the SiO 2 layer 802 was exposed so that the polycrystalline Si 804a remained only in the recessed portion 803. The etching back was performed by using resist OFPR5000™, and RIE was performed by using a gas mixture of C 2 F 6 and O 2 (80/35 sccm) at a pressure of 50 pa. and output of 2 kW. As shown in Fig. 6A, an SiO 2 layer 703 was then deposited to a thickness of 0.6 µm on the flat surface by the atmospheric CVD process. The surface of the substrate was then heated to a set temperature of 1420°C by applying infrared light thereto (from the side of the polycrystalline Si) using a lamp heating apparatus. After the temperature was kept for 30 seconds, heating was stopped. This heating method enabled the formation of the state wherein only the surface polycrystalline Si 702 was melted, without melting the substrate Si. As shown in Fig. 6B, the SiO 2 layer 703 of 0.6 µm used as a cap layer was then removed by using buffered hydrofluoric acid so that the surfaces of monocrystalized Si 705a and 705b were exposed. As shown in Fig. 6C, Si monocrystals were selectively grown using the monocrystalized Si 705a and 705b as seed crystals. The crystal growth was performed at a temperature of 1030°C and a pressure of 1.06·10 4 Pa (80 Torr) for a time of 100 minutes under the condition that the substrate was placed in a gas mixture containing SiH 2 Cl 2 as a source gas, HCl as an etching added gas and H 2 as a carrier gas with a ratio of 0.53 : 2.0 : 100 (l/min). The thus-grown crystals 706a and 706b collided with each other at an intermediate position between the seed crystals 705a and 705b to form a grain boundary 708. All the crystals had the appearances with the same facet shape and facet direction. The thus-obtained crystals had (100) orientation vertical to the substrate and showed no other orientation in X-ray diffractometry. As a result of analysis of in-plane orientation by ECP (Electron Channeling Pattern), deviations were within ±5° from orientation equivalent to the (100) orientation in the patterning direction of the seed crystal. As shown in Fig. 6D, the thus-formed crystals were then flattened for forming devices such as transistors or the like thereon. The flattening was performed until the thickness of the crystals 707a and 707b was 1 µm by the completely same process for polishing general Si wafer in which rough polishing and finish polishing were effected in turn by using a weak alkaline solution in which abrasive grains were suspended. When an n-MOS transistor was formed on each of the flattened crystals by a usual semiconductor process, there were substantially no deviation in the electrical properties of the transistors formed, and the electrical properties and deviations were substantially the same as those of the n-MOS transistors simultaneously formed on a Si wafer. Example 2 This Example is described with reference to Figs. 8A to 8D and 6A to 6D. As shown in Fig. 8A, an Si polycrystal 902 was deposited to a thickness of 0.1 µm on a fused quartz substrate 901 with a diameter of 10.16 cm (4 inch) by the LPCVD process, and an Si 3 N 4 film was further deposited to a thickness of 0.05 µm by the LPCVD process. As shown in Fig. 8B, island-like regions of 1 x 1 mm 2 were patterned at intervals of 50 µm in the Si pclycrystal film 902 and the Si 3 N 4 film 903, all the portions of the Si 3 N 4 film 903 and the Si polycrystal film other than island-like region being etched by RIE. As shown in Fig. 8C, the separate island-like polycrystalline Si regions 902a with the Si 3 N 4 cap films 903a were then oxidized to a depth of 0.1 µm. As viewed from above the substrate, the square polycrystalline Si 902a was oxidized only in the peripheral portion thereof and surrounded by the SiO 2 wall 904, the Si 3 N 4 cap film 903a and the ground fused quartz substrate 901. The substrate in this state was then heated by applying infrared light thereto from both sides thereto at a set temperature of 1500°C for 60 seconds using a lamp heating apparatus. Although the temperature of the fused quartz substrate 901 itself was slightly increased to about 400°C because substantilly no infrared light was absorbed by the substrate 901, the Si film 902a which absorbed a large quantity of infrared light was melted at a temperature higher than the melting point thereof. As shown in Fig. 8D, after the melted Si had been solidified to a monocrystal in the off state of the lamp heating apparatus used, the Si 3 N 4 cap layers 903a were etched by using hot phosphoric acid (H 3 PO 4 , 300°C). The SiO 2 side wall 904 of the monocrystal Si 905 was then etched by using buffered hydrofluoric acid. As shown in Figs. 6C to 6D, the monocrystalized Si was selectively grown by the same process as that employed in Reference Example 2 using as seed crystals the monocrystal Si 905. The conditions of crystal growth were as follows: SiH 2 Cl 2 : HCl : H 2 = 0.53 : 1.6 : 100 (l/min), 1030°C, 800 Torr, 75 minutes The thus-obtained crystal had controlled orientation, like the crystal obtained by the method in Reference Example 2. The n-MOS transistors formed on the crystal had the same characteristics as those of the transistors formed in Reference Example 2.

Claims: A crystal forming method, which comprises: (a) forming a semiconductor material layer or a metal material layer on a substrate having an insulating surface; (b) forming an insulating layer on said semiconductor material layer or said metal material layer; (c) partially removing the semiconductor material layer or the metal material layer and the insulating layer previously formed thereon to leave a semiconductor material region or a metal material region which can become a primary seed to singly aggregate or to singly melt and solidify; (d) forming an insulating region at the peripheral portion of the semiconductor material region or the metal material region to cover the semiconductor material region or the metal material region with an insulating material; (e) removing the insulating top layer existing on the semiconductor material region or the metal material region; (f) performing heat treatment for aggregating or for melting and solidifying the semiconductor material region or the metal material region as the primary seed to form a monocrystalline seed crystal having controlled plane orientation; and (g) selectively growing monocrystal by a gas phase crystal growth method using the seed crystal as a starting point. The crystal forming method according to claim 1, wherein the monocrystal is grown at a temperature of 800°C to 1200°C. The crystal forming method according to claim 1, wherein said substrate has an amorphous insulating surface. The crystal forming method according to claim 1, wherein the temperature of said heat treatment is lower than the melting point of said primary seed. The crystal forming method according to claim 1, wherein said heat treatment is performed in an atmosphere of hydrogen. The crystal forming method according to any one of the claims 1 to 5, wherein said heat treatment is to form a monocrystal having a controlled plane orientation and in-plane orientation. The crystal forming method according to claim 6, wherein said monocrystal formed by said heat treatment has an (100) orientation vertical to the substrate. The crystal forming method according to claim 1, wherein the semiconductor material layer or the metal material layer and the insulating layer are partially removed to leave island-like regions of the semiconductor material or metal material patterned at intervals on the substrate. The crystal forming method according to claim 1, wherein said insulating region is formed by oxidizing the peripheral portion of the semiconductor material region or the metal material region. The crystal forming method according to claim 9, wherein the oxidization is performed so that the semiconductor material region or the metal material region is surrounded by walls of the insulating region. The crystal forming method according to claim 1, wherein the heat treatment for melting and solidifying is carried out by applying infrared light for melting, and subsequently stopping the application of infrared light for solidifying.

Inventors: YAMAGATA KENJI, YAMAGATA, KENJI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: C30B25
IPC codes: H01L21, C30B25