RIBBON-TO-RIBBON CONVERSION SYSTEM METHOD AND APPARATUS

Abstract: A system, method and apparatus for converting polycrystalline material, e.g., silicon in ribbon form, to macrocrystalline material in ribbon form, be means of a moving molten zone (20) requiring a desired or optimum melt shape, wherein a substantial portion of the melt energy is supplied by noncoherent radiant energy (12) focused on the molten zone, and the melt shape is controlled by supplying a precisely controllable beam of energy from a second supplemental source such as a laser beam or electron beam (16), and also means by which the sections of the molten zone can be monitored and the supply of supplemental energy to the sections can be modulated.

Description: RIBBON-TO-RIBBON CONVERSION SYSTEM METHOD AND APPARATUS Background of the Invention This invention relates to a system, method and apparatus for converting polycrystalline material in ribbon form to macrocrystalline material in ribbon form. In the manufacture of semiconductor devices such as transistors, diodes, integrated circuits, photovoltaic cells and the like, the semiconductor industry uses large quantities of semiconductor material in the form of thin wafers or sheets. To replace the more expensive con¬ ventional methods of producing semiconductor sheets by first growing a single crystal semiconductor ingot and processing the ingot into a plurality of thin sheets of desired thickness and surface finish, alternative tech¬ niques have been developed for the production of thin sheets of semiconductor materials suitable for the production of large area semiconductor devices, such as, for example, solar cells-. The techniques of particular interest relate to the ribbon-to-ribbon (RTR) conversion process for producing high quality substrates usable in solar cell fabrication as well as in the production of a variety of other semiconductor products. The RTR process uses a scanned, narrow beam of energy impinging on a polycrystalline ribbon to create a molten zone of a desired shape in the ribbon and to induce crystal growth as the ribbon is translated past the energy beam. Thus, a polycrystalline ribbon is transformed into a macro¬ crystalline ribbon by the process of localized melting and subsequent recrystallization, wherein a macrocrys¬ talline structure is defined as one in which the crystals are of sufficiently large size that the grain boundaries contribute littl performance degradation to the semi¬ conductor device afe a whole. In the prior art a narrow beam of energy performed the line melting function, and furnaces on either side of the molten zone provided for control of temperature profiles during heating and cooling of the ribbon 'in a way that limited thermal stresses in the ribbon to levels low enough to minimize ribbon deformation and crystal defect formation. In this configuration, the laser used for melting had to provide a large fraction of the energy lost by radiation from the neighborhood of the molten zone. Thus a high power laser was needed. The RTR conversion process requires a precisely controlled power source in order to establish a molten zone of the desired shape and, simultaneously, to permit rapid growth, e.g., 4 c /min. , of wide ribbons, e.g., 7 cm. This was accom¬ plished by modulating the intensity of the laser beam in synchronism with the scanning mirror used to sweep the beam back and forth across the ribbon; intensity was in¬ creased or decreased to widen or narrow the melt at dis¬ crete intervals across the ribbon. The state of the art in respect to ribbon-to-ribbon technology is set forth in a paper entitled "Recent Advances in Silicon Sheet Growth by the Ribbon-to-Ribbon (RTR) Process", K. R. Sanaa, R.W. Gurtler, R. N. Legge, R. J. Ellis, and I. A. Lesk, delivered to the Photovolta¬ ic Specialists Conference proceedings, Orlando, Florida on November 15, 1981, and an article entitled "Silicon Ribbon Growth Using Scanned Lasers, by Asian Baghdadi, R. James Ellis, and Richard . Gurtler, published in Applied Optics, volume 19, page 909, et seq. , on March 15, 1980. The RTR process is suitable for the production of such macrocrystalline substrates but there is a continuing need for reducing the cost of the RTR process to make it more competitive with alternative processes and to realize its full economic potential. Accordingly a need has existed for a system embodying apparatus and method of making the RTR process more efficient and economical. It is therefore an object of this invention to provide an improvement in the RTR process for conversion of polycrystalline material to macrocrystalline material. It is a further object of this invention to provide an improved RTR process which requires less power in terms of cost than prior systems, apparatus and methods for converting polycrystalline material to macrocrys¬ talline material. It is- yet another object of this invention to provide an improved RTR process in which the melt shape can be precisely controlled in the presence of the savings in overall energy costs. Yet another object of the invention is to achieve a substantial reduction in capital cost of the equipment required for the RTR conversion process. It is still another ' object of this invention to achieve the goals set forth in the foregoing objects in a rigid edge mode semiconductor conversion process. Summary of the Invention From a reading of the specification hereafter it will be seen that these and other objects are attained in this invention wherein a system for converting polycrystalline semiconductor material to macrocrys¬ talline semiconductor material in a ribbon-to-ribbon process includes means for transporting the semiconductor material ribbon through the conversion steps, a growth furnace, means for producing noncoherent radiant energy, means for focusing said radiant energy on a zone of the polycrystalline semiconductor ribbon material to be melted, laser or electron beam means for producing a scanning and correcting beam of energy to be directed onto said polycrystalline semiconductor ribbon molten zone, and means for modulating the power of said laser or electron beam energy as directed to various areas of the molten zone, to achieve precise control of the melt shape. The noncoherent radiant energy in the system may be supplied by various means including a tungsten-halogen lamp. The correcting beam of energy in the system may be provided by various precisely controllable means including laser and electron beams. An outstanding advantage of the system is that the laser power may be furnished by a low power laser, defined with reference to processing a 7 cm wide ribbon as 600 + 200 watts, which costs less and consumes considerably less power to operate than does the standard prior art high power (>1 kw) laser used on the same width ribbon. Wider ribbons will require proportionately higher wattages in each case. A preferred laser apparatus is the carbon dioxide laser. The system uses a method for converting the poly¬ crystalline ribbon of semiconductor material to the macrocrystalline state wherein the energy impinging on the ribbon forms a molten zone therein and the ribbon is in motion relative to the impinging energy, which causes the molten zone to move along the length of the ribbon. The improvement in this method comprises the steps of supplying part of the energy to form the molten zone from a first source of focused, noncoherent radiant energy and then supplying a beam of correcting energy from a second precisely controllable source of energy such as laser beam or electron beam energy, to precisely control the melt shape. In the preferred practice of this method the bulk of the energy required to establish the molten zone is supplied by the focused, noncoherent radiant energy, which is inexpensive as well as ' relatively more efficient in terms of power utilization, and the laser or electron beam energy is used to control the total energy supplied to predetermined values in various parts of the molten zone. The source of the radiant energy used in the method may be tungsten-halogen lamps, which are preferred and the laser means may be a low power, carbon dioxide laser, which is also preferred. The apparatus for use in the system may be described as the combination of a growth chamber for reception and treatment of polycrystalline semiconductor material in ribbon form, noncoherent radiant heating means, means for focusing the energy output of the said radiant heating means on the ribbon at a desired place and laser or electron energy means adapted to supply supplemental energy to complement the energy supplied by said radiant heating means; whereby the desired melt shape is achieved in the polycrystalline semiconductor ribbon material being processed. This invention finds particularly beneficial appli¬ cation to silicon as the semiconductor material. Brief Description of the Drawings Turning now to the drawings in which a presently preferred embodiment of this invention is illustrated: Fig. 1 is a schematic representation of the apparatus used in a preferred subcombination of the system; Fig. 2 is a schematic representation of the system of this invention; Fig. 3 is a schematic diagram of a desired melt shape for achieving recrystallized ribbon with uniform thickness, in a particularly preferred application of the teachings of this invention; Fig. 4 illustrates a preferred intensity profile from the lamp on the ribbon plane; and Fig. 5 shows an alternative, preferred embodiment of the means for providing focused, noncoherent, radiant energy. Detailed Description of the Preferred Embodiments Fig. 1 is a schematic representation of apparatus for use in the system of this invention. It consists of a growth chamber 1 fabricated preferably with transparent silica (quartz) for growth atmosphere control, a set of pre-heaters 2 and post-heaters 4 to control the tempera¬ ture of the polycrystalline portion 6 and recrystallized portion 8 of ribbon 10 for controlling thermal stresses; a fine filament (about 1 mm) tungsten-halogen lamp 12, a focussing elliptical reflector 14, and a scanned laser beam 16. The presently preferred tungsten-halogen lamp, for example, is model Q1500 T3/CL lamp manufactured by General Electric (GE) , which is a 1500 lamp with a filament diameter of about 1mm and a light emitting length of about 16 cm. The focussed, noncoherent radiation from the tungsten-halogen lamp 12 and the elliptical reflector 14 provide as much constant energy as is needed for heating the ribbon to a temperature close to its melting point, while allowing the scanning laser beam 16 to melt and control the size and shape of the molten zone 20 by beam power modulation. Fig. 2 illustrates the system of the invention which includes, in a presently preferred embodiment, a means for viewing the molten zone and recrystallization process and a means for controlling the laser beam power during the scan to obtain the desired melt shape. The process involves translating the polycrystalline silicon ribbon 10 through the pre-heater 2 area to the line source, energizing the tungsten-halogen lamp 12, supplying heat to . raise the temperature of the ribbon close to the melting point, energizing the laser 24, adjusting the laser power at different regions (segments) of the scan to supply the energy needed to obtain the desired ' melt shape, and translating the ribbon 10 through the post- heater area to control cooling of the recrystallized (macrocrystalline) ribbon 10. The laser beam 16 which is about 20-25 mm in diameter is focused by a Zn Se plano-convex lens 26 to a spot size of about 1 mm on the ribbon 10. A Gallium Arsenide (Ga As) optic 28 placed between the growth chamber 1 and the scanning mirror 30, at a suitable angle, allows reflection of visible light from the molten zone 20 into a closed circuit TV camera 32 / while allowing the 10.6 micron C0 2 laser beam 16 to go through the zinc-selenide window 29 into the growth chamber. The movement of the scanning mirror 30 is responsive to the beam scanner motor 31 which produces a signal 33. The dual trace oscilloscope 35 indicates the position of the scanning mirror on the monitor. The TV camera 32 sends a signal 37 that reproduces the image of the melt shape 40 on the TV monitor 39. By these means the condition of the ribbon during processing can be closely monitored. The scan mirror position 34 and the laser power modulation wave form 36 are synchronized to control the laser energy impinging at different segments of the molten zone 20 during the scan. By viewing the existing melt shape 40 in the TV monitor and adjusting the laser power modulation waveform 36 by means of the segment power-- controls 38 the melt shape can be precisely controlled. EXAMPLE Using the apparatus shown in Fig. 2, 0.025 cm thick 7 cm wide ribbons were recrystallized in a rigid edge mode with about 550-600 watts of laser power at 2.5 cm per minute. The radiant lamp power was set at 1300 watts. When the laser beam alone was used (i.e., radiant lamp power was zero) as in the prior art method, the required laser power was found to be in the range of 1100-1200 watts. Thus there is about a 50% reduction in the size of the laser with the practice of this invention compared to prior art methods. Considering that C0 2 lasers have about 10% wall plug-to-beam efficiency, saving of 600 watts of laser power implies saving 6000 watts of electrical energy at the expense of 1300 watts for the radiant lamp. While any melt shape can be obtained using the apparatus shown in Fig. 2, Fig. 3 shows a schematic diagram of a preferred melt shape used for achieving recrystallized ribbons 8 with a uniform thickness. This melt shape has a uniform height across most of its width, and a slightly larger melt height at the ends. Ribbon growth can be horizontal or vertical as shown in Figs. 1 and 2. The radiant lamp 12 and reflector 14 can be placed at an angle as shown in Fig. 4 to adjust the energy . distribution. This may have an advantage in high speed ribbon growth by providing a steeper intensity (temperature) profile at the recrystallizing (growth) solid-liquid interface of the melt. This steeper temperature profile 44 is conducive to larger latent heat of fusion dissipation associated with ribbon growth at higher speeds. It may be advantageous to use more than one radiant lamp and reflector combination as shown in Fig. 5. This dual line source configuration can provide more noncoherent radiant energy than can be obtained with a single line source, if needed. In this specification examples of presently preferred embodiments of the invention have been disclosed. These are meant to be representative, and not limiting of the scope of the invention, as it will be readily apparent to persons skilled in the art that modifications and changes within the fair scope of the disclosure may be made. What is claimed is:

Claims: 1. In a method for converting polycrystalline rib¬ bon to macrocrystalline ribbon wherein energy impinging on said polycrystalline ribbon forms a molten zone having a desired melt shape, and said ribbon is in motion relative to said impinging energy thereby causing said molten zone to move along said ribbon, the improvement which comprises the steps of supplying part of the en¬ ergy for the creation of the molten zone from a first source of focused, noncoherent radiant energy, and sup¬ plying supplemental energy from a second, precisely con¬ trollable, source of energy to modulate the total energy required for said melt shape. 2. The method of claim 1 wherein the bulk of the energy required to establish said molten zone is supplied by the focused, noncoherent radiant energy and said second source of energy is used to modulate the total energy supplied to the molten zone. 3. The method of claim 1 wherein said radiant non¬ coherent energy is produced by means of a tungsten- halogen lamp. 4. The method of claim 2 wherein said noncoherent radiant energy is produced by means of a tungsten-halogen lamp. 5. The method of claim 1 wherein the supplemental energy means is a carbon dioxide laser. 6. The method of claim 2 wherein the supplemental energy means is a carbon dioxide laser. 7. The method of claim 3 wherein the supplemental energy means is a carbon dioxide laser. 8. The method of claim 1 wherein the supplemental energy means is an electron beam. 9. The method of claim 2 wherein the supplemental energy means is an electron beam. 10. The method of claim 3 wherein the supplemental energy means is an electron beam. 11. Apparatus for use in a semiconductor ribbon-to- ribbon conversion process wherein a moving molten zone is created in a polycrystalline ribbon, which recrystal- lizes with macrocrystalline grains, said apparatus comprising: (a) a growth chamber for reception and treatment of polycrystalline ribbon of semiconductor material; (b) noncoherent radiant heating means; (c) means for focusing the energy output of said noncoherent radiant heating means on the ribbon at the molten zone; (d) precisely controllable beam means adapted to supply supplemental energy to the molten zone whereby a desired melt shape in said polycrystalline ribbon to be processed is achieved. 12. The apparatus of claim 11 wherein said radiant energy means is a tungsten-halogen lamp. 13. The apparatus of claim 11 wherein said precisely controllable beam means is a carbon dioxide laser. 14. The apparatus of claim 12 wherein said precisely controllable beam means is a carbon dioxide laser. 15. The apparatus of claim 11 wherein said precisely controllable beam means is an electron beam. 16. The apparatus of claim 12 wherein said precisely controllable beam means is an electron beam. 17. A system for use in the ribbon-to-ribbon process for converting polycrystalline to macrocrystal¬ line semiconductor material, comprising:. (a) a growth furnace; (b) means for moving the ribbon through said furnace; (σ) a source of noncoherent radiant energy associated with said furnace; (d) means for focusing the radiant heat on a line across the path of said moving ribbon; (e) supplemental energy means selected from laser beam and electron beam means; (f) modulation means for precisely controlling the amount of supplemental energy delivered to various sections along the line across the path of the moving ribbon to create and maintain a molten zone that has a desired configuration, and that moves relative the ribbon. 18. The system of claim 17 wherein the radiant energy source is a tungsten-halogen lamp. 19. The system of claim 17 wherein the laser means is a carbon dioxide laser. 20. The system of claim 18 wherein the laser means is a carbon dioxide laser. 21. The system of claim 17 wherein said modulation means comprises signal means for showing the condition of various sections in the molten zone and means for adjusting the intensity of said beam energy in the various sections of the molten zone. 22. The system of claim 21 wherein said signal means is a monitor having a ' TV screen that produces an image- of the melt shape responsive to a camera trained on the molten zone and an oscilloscope responsive to the movement of a scanning mirror, and said adjustment means is a segment power controller.

Inventors: SARMA KALLURI, SARMA, KALLURI
Applicants: SOLAVOLT INT, SOLAVOLT INTERNATIONAL
Application year: 1986
Main classification: C30B13
IPC codes: C30B13