GB2139916A - EFG Apparatus - Google Patents

Abstract

An apparatus for and method of simultaneously growing a plurality of crystalline bodies is disclosed. The apparatus comprises capillary die member means comprising (a) a like plurality of separate spaced-apart die top sections arranged relative to one another substantially in a plane so as to prescribe a substantially closed geometric shape and (b) capillary means for transferring melt material to each of die top sections so that each of the bodies can be respectively grown from a separate one of the die top sections. The method of the present invention comprises the method of simultaneously growing a plurality of bodies of a crystalline material, each body being of a selected shape which forms a portion of a closed hollow body, from a like plurality of separate die-top sections of capillary die member means replenished with melt of the crystalline material as the bodies are grown from the die top sections. The method comprises the step of growing the bodies parallel but spaced from one another so as to substantially form the hollow body. <IMAGE>

Description

SPECIFICATION Apparatus for and method of growing crystalline bodies The present invention relates generally to crystal growth and in particular, to the fabrication of bodies of crystalline materials from a liquid melt for use in forming solar cells and other solid state devices.

Various techniques are now known for growing crystalline bodies from a liquid melt. One such technique which has proven to be quite successful in growing such bodies is the edge-defined film-fed growth process, usually referred to in abbreviated form as the "EFG" Process. This process is described in U.S. Patent No. 3,591,348, issued to Harold E.

LaBelle, Jr., as well as many subsequently issued patents. By this process, it is possible to grow crystalline bodies of silicon, or other material such as alpha-alumina (sapphire), spinel, chrysoberyl, barium titanate, lithium niobate, and yttrium aluminium garnet.

Crystalline bodies have been grown in such diverse shapes as rods, hollow tubes, and flat ribbons.

The hollow tubes have included various crosssectional shapes including circular, polygonal, and oval cross-sectional shapes. See, for example, U.S.

Patent No. 3,687,633, issued to Harold E. LaBelle, Jr., for apparatus for growing rods, circular crosssectioned tubed and ribbons, while U.S. Patent 4,036,666, issued to Abraham I. Mlavsky, shows tubular bodies of oval cross-sectional shape.

In greater detail, the EFG Process utilizes a "crucible-die" assembly which typically includes a crucible member for containing the melt material at a temperature above the melting point of the material, and a capillary die member partially disposed in the crucible member. The capillary die member includes one or more passageways of capillary dimensions providing fluid communication between the melt in the crucible member and the top surface of the capillary die member. When growing crystalline material from this crucible-die assembly a seed crystal is first brought into contact with the die top to allow sufficient material to melt at the die top and into the portions of the capillary-dimensioned passageways above the melt in the crucible member.

The seed crystal is then pulled up at a constant rate from the die top. As the seed crystal is pulled the liquid melt at the die top, i.e., the meniscus, between the die top and the solidified crystalline body being formed is continuously rep!enished by drawing the material by capillary action from the pool of melt disposed in the crucible member below the die top through the capillary passageways of the die top member. The shape of the crystalline body grown from the die member is determined by the external or edge configuration of the top end surface of the die member, i.e., the top edge delimiting the area of the die face wetted by the melt.For example, a hollow cylindrical crystalline body may be grown by providing the top end of the die top with a hole of the same shape as the cross-section of the hollow portion of the body since the liquid feed film does not discriminate between outside edges and inside edges of the die top of such die members; provided however, that the hole in the die member is made large enough so that surface tension will not cause the film around the hole to fill in over the hole.

The thickness of each crystalline body grown in accordance with this process is a function of the temperature at the die top, as well as the speed at which the body is pulled from the die top. By way of example and not limitation, a typical temperature of the die top when growing silicon is about 1450"C, while a typical pulling speed is about 0.75 to 1.5 inches/minute.

Initially, solar cells were commonly fabricated in substantially flat ribbon form. Ribbons employed in solar cells must be substantially monocrystalline, uniform in size and shape and substantially free of crystal defects. One problem, however, encountered in growing substantially flat ribbon bodies is that temperature gradients present across the die top can result in uneven growth, and can produce undesirable ingrown stresses within the body as it solidifies.

U.S. Patent 4,036,666, issued to Harold E. LaBelle, describes a relatively inexpensive technique of producing semiconductor grade ribbon of, for example, silicon, by first growing a tube of the semi-conductor material having a flat oval cross-section. The tube is then sliced lengthwise to remove the curved side sections so as to provide discrete substantially flat ribbons. Preferably, the outer surface of the flat oval cross-sectioned body is first coated with a conventional photoresist material such as a polymethylmethacrylate positive resist material. Then, the portions of the resist layer covering the broad side wall sections are exposed to a narrow beam of light so that at each side wall section two straight and narrow longitudinally extending areas of the photoresist material are exposed and thereby altered to a different molecular weight polymer.The tube is then immersed in a preferential solvent or etchant, such as methyl isobutyl with the result that the unexposed portions of the resist material remain intact, while the exposed areas are dissolved away to expose two narrow line portions of each of the side wall sections. A silicon etchant, such as potassium hydroxide, is then applied to the tube so as to divide the tube along its exposed areas. The photovoltaic junctions can then be formed in the resulting ribbon shaped bodies.

In U.S. Patent 4,095,329, issued to Kramadhati Venkata Ravi, discloses another technique of inexpensively producing semiconductor grade silicon ribbon-like bodies. A large tubular body of the semiconductor material is first grown in accordance with the EFG Process. A photovoltaic junction is formed in the tubular body and then the tubular body is etch cut into its individual sections. A principal advantage of growing tubes and subsequently etching the tube into ribbon or ribbon-like bodies is that economies of scale are achieved by essentially growing multiple ribbons simultaneously.Further, it presumably reduces the problems of edge defects found in individual ribbons grown directly from a die member, which are believed due to the shape of the liquid/solid interface at the ribbon edges when the ribbon is being grown, or the accumulation adjacent the ribbon edges of impurities present in the melt. These edge defects are objectionable and the ribbons directly grown must be further processed to remove the defects before they can be used. However, growing the tubular body and subsequently chemically etching the body to form ribbons or ribbon-like bodies creates the problem of requiring the chemical etchantto be applied in a very controlled manner.

Accordingly, an object of the present invention is to provide an apparatus for and a technique of simultaneously growing a plurality of substantially monocrystalline semiconductor bodies having some of the advantages of growing a hollow body of the same material and subsequently cutting the hollow body lengthwise into plural sections.

According to the present invention an apparatus for use in a system for simultaneously growing a plurality of crystal line bodies each of a selected shape, comprises a capillary die member having a like plurality of separate spaced-apart die top sections arranged relative to one another substantially in a plane so as to prescribe a substantially closed geometric shape and capillary means for transferring melt material to each of die top sections so that each of the bodies is respectively growable from a separate one of the die top sections.

The method of the present invention is of the type for simultaneously growing a plurality of bodies of a crystalline material, each body being of a selected shape which forms a portion of a closed hollow body, from a like plurality of separate die-top sections of capillary die member means replenished with melt of the crystalline material as the bodies are grown from the die top sections. The method comprises the step of growing the bodies parallel but spaced from one another so as to substantially resemble the hollow body.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and the apparatus possessing the construction, combination of elements, and arrange ment of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects ofthe present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein: Figure 1 is a side elevational view, in cross-section of one embodiment of the crucible and die assembly of the present invention, disposed in a heat susceptor; Figure 2 is a cross-sectional view, taken along line 2-2 of Figure 1; Figure 3 is an enlarged side elevational crosssectional fragmentary view, taken along line 3-3 in Figure 2; Figure 4 is a top plan view of another embodiment of the present invention; Figure 5 is a top plan view of yet another embodiment of the crucible and die member of the present invention, disposed in a heat susceptor; Figure 6 is a cross-sectional view taken along line 6-6 of Figure 5;; Figure 7 is a cross-sectional view taken along line 7-7 of Figure 5; Figure 8 is a cross-sectional view of still another embodiment of the present invention.

Crucible and die assemblies made in accordance with the present invention can be used in producing monocrystalline bodies of a flat ribbon-shaped configuration or a curved ribbon-shaped configuration, the latter being hereafter referred to as "ribbonoids". The material of which the crucible die assemblies are constructed, are largely dependent on the type of monocrystalline material grown from the die top. For example, for growing silicon the parts of the crucible die assembly shown are preferably made of graphite, although other materials have been suggested for at least portions of the assembly. For convenience, the following detailed description of the invention is directed to crucible die assemblies for growing substantially monocrystalline bodies of silicon, although it is not intended that the invention be so limited.

Referring to Figures 1-3, the assembly for simul taneouslygrowing a plurality of silicon bodies preferably although not necessarily includes a cylindrical heat susceptor 20, preferably made of molybdenum or graphite. Susceptor 20 is open at its top end and includes a bottom wall 22 and a cylindrical side wall 24. An annular cavity 26 is provided on the inner wall of side wall 24 for purposes which will be more evident hereinafter.

Disposed within susceptor 20 is a cylindrical crucible and die assembly, indicated generally at 30.

As shown, crucible and die assembly 30 comprises two discrete members 32 and 34, which for descriptive purposes shall be respectively referred to as the crucible member and the liner member. Crucible member 32 is formed as a single integrally formed cylindrical cup having a bottom wall 36 and a cylindrical side wall 38. The polygonal crosssectional shape of the crucible and die assembly 30 provides edges 39 on the outer surface of the wall 38. These edges contact the inner surface of the wall 24 of the susceptor 20 to provide a snug fit. Wall 38 includes a plurality of spaced-apart top sections 40 circumferentially spaced around the assembly so as to substantially prescribe a closed geometric shape.

Each top section 40 has a tapered top edge 42.

Crucible member 32 is preferably dimensioned to snuggly nest within the suseceptor 20 with each top edge 40 extending above and being free from the top of the susceptor 20.

The outer cylindrical surface 44 of the liner member 34 is formed with vertically oriented ribs 46 which snugly engage the inside surface of the crucible member so as to create a fluid passageway 48 between the inside surface of the cylindrical wall 38 of crucible member 32 and the outer surface of the member 34 between adjacent ribs 44. Each passageway 48 is dimensioned within capillary proportions so that melt material can be drawn up each passageway by capillary action in a manner well-known in the art. The liner member also includes a plurality of spaced-apart top sections 50 corresponding to and opposing the top sections 40 of the crucible member 32.The top sections 50 are each positioned between a pair of adjacent ribs 46 and each are tapered toward the top end 52. Aflange 54 is provided around the interior wall of the liner member for supporting a heat shield and melt cover (neigher being shown) in a well-known manner.

The crucible member 32 and liner member 34 are held in concentric relation and the top sections 40 and 50 are held in opposing relation by a pluraity of rivets or pins 56 which extend through suitable openings formed through the side wall 38 of crucible member 32 and through liner member 34. As shown, cavity 26 of the susceptor 20 is adjacent pins 56 so as to provide a clear space for the collection of melt that may leak past any of the pins 56.

The tapered or bevelled top ends 42 and 52 of each pair of opposing top sections 40 and 50 form a pair of parallel end edges providing a gap 58 of capillary dimension therebetween. The top ends may be knife-edges or they may have a predetermined width. The edges may be disposed in the same plane or they may be displaced from one another. As shown in Figure 1, with the liner member 34 secured in place, the bottom edge 60 of the liner member is positioned just above the inner surface of the bottom wall 36 of the crucible member 32 to allow for the passage of melt material therebetween. Alternatively, or additionally, one or more openings can be formed in the liner member to permit melt to flow through each passageway 48 through the capillary gap 58.

The cross-sectional shape prescribed by the pairs of die top sections 40 and 50 can be polygonal as shown in Figures 1-3, or circular as shown in Figure 4, or any other closed geometric shape, such as oval.

Where an assembly having a polygonal crosssection is used, such as shown in Figures 1-3, the individual bodies grown will be flat ribbons, while the circular cross-section of the assembly 30A of Figure 4 provides ribbonoids.

The assembly, thus described, is identical to that described in U.S. Patent No. 4,230,674 except that a plurality of die tops (each formed by a pair opposing top sections 40 and 50 of the respective crucible member and liner member) are provided, with each pair of adjacent die tops being separated by a gap 70. Each gap 70 is dimensioned to be sufficiently wide and deep so that an insufficient amount of melt will be present between die tops to allow crystalline bodies to be simultaneously, but separately grown from each die top.Typical dimensions, which are believed to be satisfactory when growing silicon from a graphite assembly such as shown at 30, are a die top having tapered top sections 40 and 50 each having a height of about 120 mils, a thickness of about 180 mils below the tapered ends 42 and 52, and a thickness of about 3 mils across the top edge of each end 42 and 52. Gap 58 is about 36 mils wide between the sections 40 and 50, and each gap 70 being at least 100 mils wide and at least 30 mils deep, although these dimensions can vary.

When growing crystalline bodies, melt is provided in the crucible at a temperature about 30"C above the melting point of the crystalline material being grown. A seed of the crystalline material is brought into contact with each pair of die top sections 40 and 50 allowing sufficient amount of material to melt in each of the gaps 58 and passageways 48. The seeds are then simultaneously pulled away from the die top at a substantially constant speed, e.g., at about 1.0 inches per minute. Since the gaps 70 are dimensioned to prevent sufficient melt material from accumulating between each pair of die tops the bodies will be grown separately. As the bodies are pulled, they will be pulled parallel to one another so as to resemble a hollow body of the cross-sectional shape of the crucible and die assembly.Each body will thus be of a cross-sectional dimension conforming to the cross-sectional shape provided by the die top and gap of each pair of die top sections. The bodies grown from a polygonal cross-sectional shape assembly shown in Figures 1-3 will each be a flat ribbon, and those grown from the circular cross-sectional shaped assembly shown in Figure 4 will each be a curved ribbonoid.

It should be appreciated that various modifications can be made to the embodiments shown in Figures 1-4 without departing from the scope of the invention. For example, as shown in Figues 5-7, the single discrete die member described in U.S. Patent 4,230,674 can be modified in accordance with the present invention. The modified single discrete die member 80, shown in Figures 5-7, is in the form of a right-angle cylinder having a flat bottom 82 and cylindrical side wall 84, integrally formed together as a single element. The member 80 can have any closed geometric shape, such as the polygon shown, or a circle or oval. Side wall 84 has an outer surface dimensioned to snuggly fit within the susceptor 20, shown in Figure 6.The top of the member 80 is provided with a plurality of inner die top sections 86 and a corresponding plurality of outer die top sections 88 respectively opposing the inner die top sections 86 and spaced by the gap 80 of capillary dimension. Slots 92 are formed in the lower portion of the inner cylindrical wall of the member 80. These slots are also of capillary dimensions and are in fluid communication with each gap 90 so that melt provided within the member 80 can be drawn by slots 92, to the gap 90 and pulled from the die top formed by each pair of inner and outer top sections 86 and 88.

As shown in Figure 8, member 80 shown in Figues 5-7 is modified by omitting the bottom of the capillary die member 100, so that the cylindrical element is open at its bottom end. The member 100 is snuggly disposed within a cylindrical quartz cup-shaped container 102 made for example of quartz and having a bottom 104 and side wall 106 such that the top end sections extend above and are free with respect to side wall 106. The container 102 is in turn snuggly received within susceptor 20.

Further, while each group of the inner die top sections and the outer die top sections are shown in the drawings as integrally formed together, each pair of inner and outer die top sections can be defined by the upper end of a separate die member, with each die. member being disposed in a crucible member so as to define a section of a closed hollow body. For example, as shown in Figure 8, each flat wall section of the member 100 can be a separate die member with the individual die members being arranged and mounted in the polygonal configuration shown so that the crystalline bodies grown from the die top sections will remain parallel and collectively as a substantially closed hollow body.

The assemblies thus shown provide an improved technique and apparatus for simultaneously growing substantially monocrystalline bodies from a common pool of melt, having some of the advantages (such as economies of scale) of growing large hollow bodies and subsequently cutting the body lengthwise into sections. By growing the bodies separately, the subsequent cutting step and its inherent problems are omitted.

Claims (17)

1. An apparatus for use in a system for simultaneously growing a plurality of crystalline bodies each of a selected shape, comprising a capillary die member having a like plurality of separate spacedapart die top sections arranged relative to one another substantially in a plane so as to prescribe a substantially closed geometric shape and capillary mans for transferring melt material to each of die top sections so that each of the bodies is respectively growable from a separate one of the die top sections.

2. An apparatus as claimed in claim 1, in which the closed geometric shape is a circle and the die top sections are each an arc of the circle so that each of the crystalline bodies is a ribbonoid.

3. An apparatus as claimed in claim 1, in which the closed geometric shape is a polygon and the die top sections are each respectively disposed on a different one of the sides of the polygon so that each of the crystalline bodies is a substantially flat ribbon.

4. An apparatus as claimed in claim 1, in which the capillary die member includes a like plurality of notches, each notch being disposed between and separating contiguous ones of the die top sections and being dimensioned so that insufficient melt will accumulate in each notch and crystalline bodies grown from contiguous die top sections will remain separate from one another.

5. An apparatus as claimed in claim 4, in which each notch is at least approximately 30 mils deep and at least approximately 100 mils wide.

6. An apparatus as claimed in claim 1, in which a crucible is provided for containing a common pool of melt material for the die top sections and for receiving at least a portion of the capillary die member so that the capillary means is in fluid communication with melt contained in the crucible.

7. An apparatus as claimed in claim 6, in which the crucible and the capillary die member are formed so that at least a part of the capillary die member is an integral portion of the crucible and the crucible is an essential portion of the capillary die member.

8. An apparatus as claimed in claim 7, in which the crucible is open at its top end, closed at its bottom end and has a side wall defining an interior space for containing melt material, the die top sections each include two parallel top edge surfaces spaced apart by a gap of capillary dimensions and the side wall of the crucible has an upper end defining at least one of the top edge surfaces of each of the die top sections.

9. An apparatus as claimed in claim 8, in which the capillary die member comprises a liner including a side wall having an upper end defining the other of the top edge surfaces of each of the die top sections.

10. An apparatus as claimed in claim 9, in which the crucible and the liner are integrally formed as a discrete member.

11. An apparatus as claimed in claim 10, in which the crucible includes at least one passageway of capillary dimension formed in the side wall thereof.

12. An apparatus as claimed in claim 11, in which the passageway comprises a slot formed on the interior surface of the side wall of the crucible.

13. An apparatus as claimed in claim 9, in which the crucible and the liner are discrete members, means being provided for securing the crucible and the liner together so that the top edge surfaces provided by the upper end of the side wall of the crucible are respectively positioned with respect to the corresponding top edge surfaces defined by the upper end of the liner so as to form the die top sections.

14. An apparatus as claimed in claim 1, in which the capillary die member includes a like plurality of capillary die members, each of the capillary die members defining at their upper ends a corresponding die top section, the capillary die members being positioned relative to one another so that the die top sections prescribe the substantially closed geometric shape.

15. A method of simultaneously growing a plurality of bodies of a crystalline material, each body being of a selected shape which forms a portion of a closed hollow body, from a like plurality of separate die-top sections of a capillary die member replenished with melt of the crystalline material as the bodies are grown from the die top sections, the method comprising the step of growing the bodies parallel but spaced from one another so as to substantially form the closed hollow body.

16. An apparatus for use in a system for simul- taneously growing a plurality of crystalline bodies each of a selected shape constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

17. A method of simultaneously growing a plurality of bodies of crystalline materials substantially as herein described.