Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1998-11-24
2002-08-06
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S501000, C438S478000, C438S097000
Reexamination Certificate
active
06429035
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of growing a silicon crystal in a liquid phase. A silicon crystal produced by the method of the present invention can be used in silicon devices having a large area such as solar cells and picture element driving circuits for liquid crystal display devices.
2. Related Background Art
Solar cells are prevailing as electric power sources which are systematically linked with driving power sources for various kinds of appliances and commercial line power. It is desirable to manufacture solar cells at low cost. For example, it is desired to produce solar cells on inexpensive substrates at a low cost. Silicon is generally used as a semiconductor for composing solar cells. Single crystalline silicon is extremely excellent from a viewpoint of efficiency of converting light energy into electric power, that is, photoelectric conversion efficiency. From the viewpoints of enlargement of area and reduction of manufacturing cost, on the other hand, amorphous silicon is advantageous. In recent years, polycrystalline silicon has been used for the purpose of obtaining a cost as low as that of amorphous silicon and a photoelectric conversion efficiency as high as that of single crystalline silicon.
However, it cannot be said that the expensive crystalline materials are sufficiently utilized by a method which is conventionally adopted to manufacture silicon devices using single crystalline silicon or polycrystalline silicon since the method is configured to slice a lump crystal to form plate-like substrates and is hardly capable of preparing substrates which have thicknesses of 0.3 mm or smaller, thereby allowing the substrates to have thicknesses larger than a thickness (20 &mgr;m to 50 &mgr;m) generally required to absorb incident rays. Furthermore, there has recently been proposed the spin method of forming a silicon sheet by flowing drops of melted silicon into a template. However, a silicon sheet formed by this method has a quality insufficient for use as a semiconductor and cannot provide a photoelectric conversion efficiency which is so high as that in the case of using a general crystalline silicon.
There has been proposed and actually applied to trial production of a solar cell under the circumstances described above, an idea of growing on an inexpensive substrate a silicon crystal of a good quality until it has a required and sufficient thickness and forming an active region (for example, a photoelectric conversion region) thereon. Moreover, there has been proposed an idea of growing a silicon crystal epitaxially on a substrate of a good quality and then peeling off the silicon crystal and reusing the substrate.
On a premise that large area devices such as solar cells are to be produced in mass, however, it is not so easy to grow a silicon crystal until it has a thickness required for absorbing incident rays. A silicon crystal of a good quality is generally grown by the thermal CVD method of thermally decomposing a raw material gas such as silane chloride. In order to grow a single crystal at a high rate on the order of 1 &mgr;m/minute in particular, it is typical to use the so-called epitaxial growing furnace. However, such a growing furnace is not only unsuited to mass production since it can treat 10 wafers at most at one batch, but also requires a high raw material cost since it utilizes a raw material gas at a low efficiency. Though it is possible to treat 100 or more wafers at one batch by utilizing the so-called low pressure CVD furnace, this furnace also provides a crystal insufficient in quality and allows the crystal to grow at a rate only on the order of 0.01 &mgr;m/minute, thereby being low in productivity.
As another method of growing a silicon crystal, there is known a liquid phase growing method of supersaturating a liquid metal solution in which silicon is dissolved and allowing a crystal to deposit from the solution onto a substrate. This liquid phase growing method is capable of growing a crystal of a high quality at a high rate on the order of 1 &mgr;m/minute and treating 100 or more wafers at one batch, thereby being suited to mass production. However, the liquid phase growing method is not generally used for growing silicon and has some technical problems to be solved though it widely prevails as a method of growing compound semiconductors.
One important problem lies in selection of a metal which is to be used as a solvent. It is desirable that a metal to be used for this purpose has a solubility for silicon which is as high as possible and can hardly be incorporated into deposited silicon. Furthermore, a metal having a lower melting point and a lower vapor pressure can be handled easier. Tin is used most generally as a solvent for silicon. Tin can be handled relatively easily since it has a low melting point and a relatively high solubility for silicon. It has been considered that tin is a preferable solvent since tin and silicon belong to Group IV of the periodic table, and tin is inactive as a dopant even when it is incorporated into deposited silicon.
However, the inventors have recently found that tin is incorporated into silicon in a relatively large amount when growth conditions (in particular, a growth temperature) are inadequate, thereby deforming a lattice of a silicon crystal and adversely affecting electric characteristics of a semiconductor probably due to the atomic size of tin which is very different from that of silicon though they are atoms belonging to Group IV. From this viewpoint, there is posed a doubt in the aptitude of tin as a solvent which is used to grow a crystal for a solar cell with high efficiency.
In addition to tin, elements such as gallium, indium and aluminum which belong to Group III can be mentioned as metals which are usable as solvents. Gallium and indium, in particular, having a low melting point can be handled easily. Since gallium is extremely expensive, indium is hopeful for use as a practical melt. However, indium posed a problem which is described later in control by introducing dopant a conductivity type of a silicon crystal which is grown using an indium melt. There are known examples wherein gallium is used as p-type dopant in combination with an indium melt (G. F. Zheng et al.: Solar Energy Materials and Solar Cells. 40 (1996) 231-238). Though gallium is usable at relatively low concentrations, it cannot be used for doping at high concentrations since a solid of gallium can be dissolved into silicon at concentrations within a relatively low solubility and is extremely expensive. On the other hand, examples which use n-type dopants in combination with indium melts are disclosed by Japanese Patent Application Laid-Open Nos. 9-183695 and 9-183696.
Boron and aluminum are generally used as p-type dopants, whereas phosphorus and arsenic are often used as n-type dopants. It is therefore conceivable to use these dopants for growing silicon crystals in liquid phase with the indium melt. In practice, however, problems were posed in conductivity types or reproducibility of conductivities of grown silicon crystals in certain cases. Furthermore, it is feared that a metal of Group III such as indium which is originally active by itself as a dopant may control a crystal to a strong p-type when incorporated into silicon and may be incapable of controlling it to p
−
-type or n-type.
The problems described above make it still impossible to judge whether or not the liquid phase method has a true aptitude for growth of silicon crystals on scales of mass production and whether or not solar cells utilizing thin films of silicon crystals have practical utility.
Thin films of silicon crystals are also used as devices for driving picture elements of liquid crystal displays and so on. Progress made in mass communication media have produced increasing demands for a display having a larger screen and capable of more minutely driving at a higher speed. Though the TFTs (thin film transistors) of amorphous silicon have hitherto been utilized as a d
Iwane Masaaki
Nakagawa Katsumi
Nishida Shoji
Ukiyo Noritaka
Fitzpatrick ,Cella, Harper & Scinto
Mulpuri Savitri
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