FIELD OF THE INVENTION This invention is in the field of crystals manufacturing techniques and relates to a method for growing single crystals.

BACKGROUND OF THE INVENTION Crystals due to their specific geometry pattern and physical properties are widely used in electronics, in particular semiconductor devices, such as transistors, diodes, thyristors, photodiodes, and integrated circuits. Crystals are characterized by a regular manner arrangement of their atoms or molecules, the values of certain physical properties depending on the direction in which they are measured. When formed freely, a crystalline mass is bounded by plane surfaces (faces) intersecting at definite angles.

To obtain certain desired physical properties (working parameters) of a crystal-based electronic device, it is typically required to utilize a mono-crystal structure in such a device. The commonly used technique for growing mono-crystals on substrates is the epitaxy, according to which a thin crystalline layer is grown on a substrate by means of depositing atoms of the layer material onto the substrate at high temperature, and the crystalline orientation of the deposited layer is the same as that of the substrate. In other words, to obtain a crystalline layer of silicon, for example the atoms of silicon should be deposited onto a silicon substrate.

Various techniques for growing polysilicon films on Si02 have been developed. One of the known techniques utilizes the deposition of an ultra-thin microcrystalline-Si seed layer onto a Si02 substrate. During the deposition of a poly-Si thin film, this intermediate film provides nucleation sites at which poly-Si film growth can be initiated. This is disclosed in the following article :"Low Temperature (450°C) Poly-Si Thin Film Deposition on SiO2 and Glass Using a Microcrystalline Si Seed Layer", David M. Wolfe et al., North Carolina State University, Dept. of Physics, Ralelgh, NC; Material Research Society (MRS), Spring Meeting, 1997.

Another technique is disclosed in the article"Selective NucleationASolid Phase Epitaxy as a Low-Temperature Route to large-Grained Si and Ge Films on Glass", Harry A. Atwater et al., California Institute of Technology, Dept. of Applied Physics, Pasadena, CA; Material Research Society (MRS), Spring Meeting, 1997. Here, selective nucleation is based on a thin film reaction between a patterned array of deposited metal (typically In) nucleation sites and an amorphous semiconductor film.

The use of laser-induced crystallization of amorphous silicon on a Si02 substrate is disclosed in the following publication :"The Recrystallization Depth Control of the Excimer-Laser-Recrystallized Poly-Crystalline Silicon Film", Kee-Chan Park et al., Seoul Nat. Univ., Scool of Electrical Engeneering, Seoul, Korea; Material Research Society (MRS), Spring Meeting, 1999.

According to yet another technique, thermal annealing of amorphous silicon film on Al-coated substrate is used. This is disclosed in the article"Rapid Thermal Annealing Crystallization of High Rate Deposited Amorphous Silicon Films Enhanced by Al Coating on Substrate", Kuixun Lin et al., Amorphous Semiconductor Laboratory, Shantou Univ., Shantou, Guangdong, P. H. China; Material Research Society (MRS), Spring Meeting, 1999; and The use of an intermediate porous-Si02 layer on Si02 substrate is disclosed in the following article :"Low-Temperature Preparation of Poly-Silicon Thin-Films Having Giant Grains", Wen-Chang Yeh et al., Tokyo Inst. Of Tech., Dept. of

Physical Electronics, Tokyo, Japan; Material Research Society (MRS), Spring Meeting, 1999. The spin-on-glass porous Si02 film was used as the interlayer since it is heat-tolerant, and Si-film was crystallized on it by a single shot of KrF excimer-laser light pulse.

SUMMARY OF THE INVENTION There is a need in the art to facilitate the manufacture of single-crystal (mono-crystal), as well as the manufacture of a poly-silicon thin film, by providing a novel method of growing crystals.

The main idea of the present invention consists of growing one or more mono-crystal structures on an amorphous substrate (e. g., plastic, glass, etc.). This is implemented by fabricating suitably shaped microenvironment for depositing therein a material to be crystallized. The deposition process may utilize Liquid Phase Epitaxy, Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), e. g., Molecular Beam Epitaxy (MBE), or suitable variations of these techniques.

The suitably shaped microenvironment according to the invention actually presents a support groove or crater formed in the amorphous substrate for receiving the deposited material for a corresponding single-crystal structure. The special shape of the crater enables to obtain the single-crystal structure. To this end, the crater is shaped like a cone having a substantially V-or U-shaped cross-section.

The base of such a cone-like crater is located at the surface of the amorphous substrate, and its tip-like portion is located within the interior of the substrate.

The term"tip portion"used herein signifies that end portion of the crater whose diameter is smaller than that of the opposite end portion, and which is located within the interior of the substrate.

To fabricate the crater, lithography-and-etching technique can be used. After the crystallization of the deposited material, the upper surface of the resulting structure (i. e., substrate with the deposited material in the crater or craters) is polished (by chemical or plasma techniques).

There is thus provided according to one aspect of the present invention, a method of manufacturing at least one single-crystal structure, the method comprising the steps of : (i) providing a substrate layer of an amorphous material; (ii) fabricating at least one crater in the substrate, wherein said at least one crater extends from the surface of the substrate towards its interior, with a cross sectional area of the crater being continuously reduced from its upper portion located at the surface of the substrate towards its lower portion located in the interior of the substrate; and (iii) depositing a material to be crystallized into said at least one crater, and allowing the growth of the at least one single-crystal inside said at least one crater.

The above design of the crater may have a substantially U-shaped cross section, or a substantially V-shaped cross section having a tip-like lower end portion. Preferably, the surface of the structure obtained in step (iii) is then planarized. Thereafter, a three-dimensional device containing at least a pair of vertically arranged single-crystal structures can be obtained by repeating steps (i) to (iii) on top of the lowermost planarized structure.

Preferably, an array (e. g., two-dimensional) of spaced-apart craters is fabricated in the substrate and filled with the deposited material to form an array of spaced-apart single-crystal structures.

By allowing the so-obtained regularly arranged single-crystal structure to grow outside the craters, a layer (film) of poly-crystalline material can be fabricated on the surface of the substrate.

Thus, the present invention, according to another of its aspects, provides a method of manufacturing a poly-crystalline film on the surface of an amorphous substrate.

According to yet another aspect of the present invention, there is provided a single-crystal structure, wherein the structure is grown in a crater made in an amorphous substrate, the crater extending from the surface of the substrate towards

its interior, such that a cross sectional area of the crater is continuously reduced from its upper portion located at the surface of the substrate towards its lower portion located in the interior of the substrate.

According to yet another aspect of the present invention, there is provided a structure comprising an amorphous substrate and an array of spaced-apart single-crystal structures formed in said substrate, each of the single-crystal structures being grown in a crater made in said amorphous substrate, the crater extending from the surface of the substrate towards its interior, such that a cross sectional area of the crater is continuously reduced from its upper portion located at the surface of the substrate towards its lower portion located in the interior of the substrate.

According to yet another aspect of the present invention, there is provided a device comprising at least two vertically arranged structures each constructed as described above.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Figs. 1A and 1B illustrates one possible example of a crater design used for growing a single-crystal therein; Figs. 2A and 2B illustrates another example of the crater design used for growing a single-crystal therein; Figs. 3A and 3B illustrates yet another example of the crater design used for growing a single-crystal therein; Figs. 4 and 5 schematically illustrate simultaneous manufacture of an array of single-crystal structures; Fig. 6 illustrates the principles of manufacturing a multi-layer device utilizing the structure of Figs. 4 and 5; and

Fig. 7 illustrates how the present invention can be used for manufacturing a poly-crystalline film on the surface of an amorphous substrate DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The present invention provides a technique for growing a single-crystal structure in a crater made in an amorphous substrate. Referring to Figs. 1A and 1B, there are shown cross-sectional and top views of a crater 1 according to one possible example of the invention. The crater 1 is made in amorphous substrate 2 (e. g., Si02), by lithography-and-etching technique, and is in the form of a cone having a V-shaped cross section. Generally, the crater 1 extends from the surface 2a of the substrate 2 such that a cross sectional area of the crater is continuously reduced from its upper portion la located at the surface of the substrate towards its lower portion lb located in the interior of the substrate. Thus, in the present example, dl>d2. In this specific example of V-shaped cross section, the crater 1 has a tip-end.

Referring to Figs. 2A and 2B there is illustrated another example of a crater 10 made in the substrate 2, wherein the crater has a U-shaped cross section with the upper and lower diameters dl and d2, respectively, such that dl>d2.

Figs. 3A and 3B illustrate yet another example of a crater 20, which also has a U-shaped cross-section, and a similar relation between its upper and lower diameters di and d2, respectively.

It should be noted although not specifically shown, that the crater may be shaped like a pyramid having a polygon-geometry base.

Fig. 4 illustrates a structure 30 composed of the amorphous substrate 2, which is formed with a two-dimensional array of spaced-apart single-crystal structures, generally at 32. Each structure 32 is a crater formed in the substrate 2 and containing a material to be crystallized.

As better shown in Fig. 5, to manufacture the structures 32, the surface of the substrate is patterned (e. g., by lithography-and-etching) to create the craters 34.

Then, a Liquid Phase Epitaxy, CVD, PVD (e. g., MBE), etc. is applied to deposit

the material to be crystallized 36 (e. g. semiconductor, piezoelectric, etc.) into the craters 34. Considering CVD or PVD processes, the deposition can be carried out at substantially room temperature with the appropriate vacuum conditions.

For example, by the technique of the present invention, a single silicon crystal can be grown inside the crater with the lower portion of a 0. 5um-diameter during 0.5 hour.

The deposition procedure is typically followed by a polishing procedure (such as Chemical Mechanical Planarization or Ion Planarization) so as to remove the residuals of the deposited material from the substrate surface within the spaces between the craters.

Turning now to Fig. 6, there is illustrated how a three dimensional array of single-crystals can be fabricated using the structure of Figs. 4 and 5. To this end, the surface of the previously obtained structure 30 is polished. Then, a new substrate layer 40 is deposited thereon, and is processed in a manner described above to fabricate a further two-dimensional array of spaced-apart single-crystal structures, generally at 42.

Reference is now made to Fig. 7, illustrating a structure 50 composed of the amorphous substrate 2 formed with the array of spaced-apart craters 1, and a poly-crystalline film 52 on the surface of the substrate. Each crater 1 presents a center of nucleation. The structure 50 is manufactured in the following manner. The material to be crystallized is deposited into the craters as described above, and grows throughout the craters so as to form a thin layer on the surface of the substrate. The so-obtained poly-crystalline film has a regular structure of its single-crystals.

Those skilled in the art will readily appreciate that various modifications and changes can be applied to the preferred embodiments of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims. For example, the substrate, in which crater or craters are formed, can be made of any amorphous material. The shape of the crater may be a cone, a so-called truncated cone having, respectively, a V-shaped or U-shaped cross

section. Alternatively, the crater may be shaped like a pyramid. The material to be deposited may be any atomic or molecular crystallizable material that can be used for fabricating semi-or super-conductive materials, piezoelectric materials, as well as thermo-electric and optically active materials.