FR2512847A1 - Forming single crystal silicon ribbons - by pulling at small angle over edge of meniscus attachment member - Google Patents

Abstract

Crystalline ribbon (14) is formed by heating a melt in a rimmed crucible (10), placing a seed crystal (34) on the melt surface and pulling the seed from the melt at a low positive acute angle relative the melt surface at a speed commensurate with the longitudinal growth of the ribbon. Separate site of attachment (20) is provided inwardly of the crucible rim for a meniscus formed by the melt from the lower face of the ribbon as it leaves the surface of the melt to reduce spillage and freezing. The growth rate of the ribbon is passively adjusted to changes in the speed of movement of the ribbon by drawing the growing end of the ribbon from the surface of the melt above sub surface stabiliser (22) having an uppermost forward portion arranged closer to the melt surface than the lowermost rearward portion thereof, the crystal growing portion of the melt is thermally isolated from the remaining portion. Produces a high quality semiconductor ribbon of uniform dimensions without any cutting and waste of material, at a high rate of production.

Description

Method and apparatus for manufacturing crystal ribbons
The invention relates, in general, to the manufacture of a monocrystalline ribbon and, more particularly, to a process for the continuous production of silicon crystal ribbons as well as to an apparatus for implementing the method.

Monocrystalline semiconductor wafers for various electronic applications have so far been typically obtained in the form of rods by slowly pulling up a seed crystal from a mass of molten silicon. The rod, once formed, is then cut into thin slices which can also be cut into smaller semiconductor components. This process is very time consuming and very expensive due to the fact that much of the material is lost during the cutting operation.

In order to eliminate some of the problems inherent in vertical pulling techniques, considerable effort has been made to be able to directly manufacture the semiconductor material from a ribbon, thus eliminating the waste of material and time inherent in cutting the sliced stems. A thin monocrystalline ribbon, once formed, can then be notched in the form of the desired design and the various elements thus formed detached from the ribbon for their use as components.

While the thin ribbon could be pulled directly through a slit in a die in a vertical direction over a bath of molten material, the process is extremely slow. More recently, efforts have been made to draw a ribbon horizontally from the surface of a bath of molten semiconductor material, which technique provides a larger area of solid / liquid crystal formation interface and, consequently, is more productive than vertical crystal growth techniques. However, the techniques and apparatuses used until now for the growth of semiconductor monocrystalline ribbons using the horizontal pulling process have not been satisfactory from the point of view of the quality, thickness and width which must be uniform and the operating stability.

It is therefore an object of the invention to provide improvements to the methods and apparatus for the continuous formation of crystalline semiconductor tapes.

It is another object of the invention to provide a method and an apparatus for manufacturing high quality semiconductor tapes with a high production yield and having uniform dimensions.

The invention provides a method for continuously growing a ribbon of crystalline semiconductor material, characterized in that a bath of molten semiconductor material is formed, a crystal seed is placed on the surface of the bath, the seed in a direction forming a slight angle from the surface of the bath and over the edge of a meniscus attachment wall, at a speed corresponding to the formation of the solid crystal ribbon and the - below the surface of the bath the speed of crystal formation in width and thickness.

The invention also provides an apparatus suitable for use in the continuous manufacture of crystalline semiconductor tapes, containing a relatively shallow vessel capable of containing a mass of molten semiconductor material, a scraper disposed below the surface of the molten material. , its upper edge being arranged close to the surface and suitable for forming a meniscus drawn up to the underside of the ribbon pulled at an angle of small value above the edge of the wall of attachment of the meniscus. A leading edge stabilizer or advancing edge is disposed below the surface of the bath and below the leading edge of the solid tape to automatically compensate for changes in drawing speed, growth speed, temperature melt and other factors.Lateral stabilizers are provided below the bath surface on each side of the tape to adjust the width of the tape and a thermal impedance is mounted below the tape interface and liquid material to regulate thermal convection.

The invention is described in more detail below with reference to the accompanying drawings, in which FIG. 1 is a sectional side view of an apparatus for growing a crystalline ribbon according to the invention; Figure 2 is a view of part of Figure 1, on a larger scale; Figure 3 is a top plan view; Figure 4 is a cross-sectional view along line 4-4 of Figure 3; Figure 5 is a plan view, somewhat schematic, showing the growth characteristics of a ribbon formed in accordance with the invention; Figure 6 is a top plan view of a variant of the invention; Figure 7 is a cross-sectional view along line 7-7 of Figure 6; Figure 8 is a plan view of another variant of the invention, Figure 9 is a cross-sectional view along line 9-9 of Figure 8; Figure 10 is a perspective view of the embodiment according to Figure 8, with parts broken away; Figure 11 is a sectional view showing a detail of Figure 1, on a larger scale; and Figure 12 is a side elevational view, in section, of another variant of the invention.

According to the invention, a continuous ribbon is formed by placing a seed crystal on the upper surface of a bath of molten silicon, the temperature at the surface being adjusted to produce solidification. The seed is then drawn from the surface of the bath at a low positive acute angle above the edge of the upper edge of a wall at a speed proportional to the growth rate of the advancing edge of the crystal, in one direction. opposite to the draw direction. The silicon bath is kept shallow and at a relatively constant level to ensure that the advancing (or advancing) edge of the ribbon at the solid / liquid interface remains stable. The growth of the crystal is stabilized by automatically increasing or decreasing the rate of advance of the growing edge of the ribbon by passing the advancing edge of the ribbon over a stabilizer disposed below the surface of the bath.

The ribbon is also moved between lateral stabilizers arranged below the surface of the bath which regulate the width of the ribbon.

Referring now to the drawings and more particularly to FIG. 1, there is seen an apparatus for implementing the method described above for continuously pulling a crystalline ribbon from a silicon melt in accordance with the invention. The apparatus is generally organized around a crucible 10 defining a shallow tank intended to contain an amount of molten silicon 12 from the surface from which a ribbon 14 of crystalline silicon is drawn. The tank is very small depth compared to the devices of the prior art and greatly reduces the unfavorable effect of the convection currents usually present in the deep vessels normally used in installations for forming crystals by pulling action. The configuration of the crystal growth apparatus according to the invention is of the type allowing the growing region of the ribbon to be extracted from the molten mass contained in the container.

The crucible 10 is preferably made of a material resistant to high temperatures and which will not react with molten silicon. To this end, quartz has been shown to provide satisfactory results. The crucible of the embodiment shown is of generally rectangular shape at comprises a bottom wall 16 and side walls of low height edge 18 which contain the molten silicon 12. Inside the crucible itself are mounted several passive elements which contribute to the correct formation of the tape 14.

These elements include a vertical wall 20, a leading edge stabilizer 22, a thermal impedance 24, and a pair of side stabilizers 26 and 28. The active components of the system include a heat source 30 typically arranged in or at below the bottom wall 16.

The apparatus also includes a germ support and pulling mechanism 32 suitable for grasping a crystalline germ 34, for holding it on the surface of the bath while the ribbon 14 is being formed and for pulling the germ and the ribbon 14 at a constant speed. in a direction opposite to the growth direction of the crystal.

The seed pulling mechanism 32 is arranged so as to pull the ribbon 14 at a slightly positive acute angle, type 50, above the horizontal and at a speed substantially equal to the speed of crystal formation on the surface. of the bath.

In the preferred embodiment of the invention, the ribbon 14 is pulled over the upper edge of the wall 20. The wall 20 is typically formed of quartz and, in the preferred embodiment, is constituted by a rigid vertical element the upper end of which forms an edge 36 situated more or less in the same plane as the upper edge of a side wall 18, as best seen in FIG. 2.

The ribbon 14 is pulled over the edge of the wall at an angle of small value from the surface of the bath and forms behind the wall a raised meniscus 38 between the ribbon and the level of the liquid above the molten silicon. The leading edge of the ribbon where crystal formation takes place moves in a direction opposite to the pulling direction.

Pulling the ribbon at a low value positive angle from the surface of the bath provides a number of beneficial results. This eliminates the need to fill the crucible with a slight surplus, which would be required if the ribbon were to be pulled along an exactly horizontal path. It has been found to be extremely difficult to maintain the melt at a level which would allow the ribbon to be drawn horizontally from a crucible without the melt overflowing. To this was added the problem that the ribbon stuck (by freezing) to the edge of the crucible.

By pulling the ribbon at a slight upward angle, these problems are overcome. The leading edge where the crystal forms is shaped in an arc where it meets the surface of the melt, as seen in Figures 3 and 5. The shape of the leading edge is determined by the thermal field just below and in front of the leading edge, which field is determined by the temperature gradients introduced by the stabilizers. This shape of the leading edge of the crystal contributes to a better quality of ribbon for the reason that any imperfection which could develop during the formation of the crystal will grow towards the lateral edges of the ribbon rather than in length following the ribbon as it would form if the growth edge was straight
For example, in FIG. 5, a crystal growth defect is indicated at 40 and this defect will generally move along the path indicated at 42 towards a lateral edge of the ribbon 14 while the growth of the ribbon takes place as represented by the edges in advance dotted lines. Thus, any defect that may occur has a minimal and self-correcting effect on the tape.

The ribbon 14, as shown in FIG. 4, has a planar lower surface and a planar upper surface as a whole but slightly rounded at the edges in cross section. In general, the thickness of the tape partially depends on the length of which the tape is in contact with the melt and the speed at which it grows downward.

In accordance with the invention and as best represented in FIG. 12, all of the molten material is separated from the growth region of the crystal by being contained in a reservoir 42 so that the crucible 10 is maintained with a very shallow depth of molten material from which the ribbon 14 is formed.

Various filling systems of molten material can be used in connection with the invention and, while FIG. 12 illustrates such a system, other systems are available and can also be used. In Figure 12, the crucible 10 is mounted in a housing 44 comprising an outer housing 46, preferably made of quartz. The crucible serves as a reservoir 42 for the molten silicon 50 to replenish the tank 10. The housing 46 comprises a positioning plate 52 arranged across the upper part of the housing and which serves to provide a pressure seal for the pressure defined by the housing and which supports a supply tube 54 between the pressure chamber and the tank 10. The lower end of the tube 54 extends below the surface of the melt 50 and ends at above the upper part of the crucible 48. The upper end of the tube 44 is curved so as to extend above the upper part of the vessel 10 to form an outlet hopper through which the molten silicon is transferred from the reservoir 42 to the tank 10. The chamber formed in the housing can be pressurized by means of pressurized gas supplied by a support tube 56 and slots 58 which extend along the internal walls of the bowl and are coincident with passages 60 communicating with the tube 56. An auxiliary heating device 62 can be arranged near the upper end of the supply tube 54 to keep the silicon supplied in fusion and a radiation protection screen 64 can be provided at the above the heating device as shown.

The use of a shallow tank from which the ribbon is grown eliminates some of the destabilizing characteristics inherent in deep crucibles, particularly with regard to the convection currents present in deep molten siliaium baths.

The profile of the center of the ribbon is slightly curved and this occurs naturally since the increasing thickness of the silicon causes a slight reduction in the rate of dissipation of the heat of fusion. This can be adjusted by placing a variable heat sink over the solid surface or by modifying the heat supply to the mass of molten silicon.

The aim desired in carrying out the invention is to maximize the distance between the tape advancing edge and the point at which the full thickness of tape is obtained.

This will have the effect of maximizing the surface area of the solid / liquid interface, resulting in a maximization of the growth rate and / or a minimization of the temperature gradients through which the solidified sillicon material must pass. This will result in reduced stresses in the plastic flow range resulting in improved quality of the ribbon material.

In addition to the system requirements determined by the large solid / liquid interface desired, there are other conditions which are determined by certain desirable operating characteristics. These conditions include full control of the edge. This is done in two stages. First, the problem of convective heat transfer is limited by the fact that solidification takes place in a shallow tank. Second, two techniques for maintaining the thermal gradient are shown. In the first case, as shown in Figures 6 and 7, side heating elements 66 provide a totally variable heat supply to the edges of the solidifying tape. In the second technique shown in FIGS. 8 to 10, the bottom of the tank is thicker under the zone in which crystallization takes place and defines a ramp 68. A bottom heating element 70 provides heat equally to the bottom of the tank and, due to the variation in the thickness of the bottom, the growth region is surrounded by warmer zones which will limit the width of the ribbon. Adding heat shields to the edges can enhance this effect.

With regard to operation, this system is very simple. There will be a slight increase in width for an increase in the growth rate, since the total temperature of the tank will be reduced due to the rate of solidification. Obviously, heating elements can be added if necessary.

The second particularity of the device lies in the fact that the growing tape is prevented from freezing on the apparatus. In the present case, the solidification zone is surrounded by a small hot zone which contributes to the adjustment of the width of the ribbon. The edge of the wall above which the ribbon is pulled is almost covered by liquid silicon. The temperature should be adjusted to allow a very small amount to melt back into the tank at this point rather than further solidification. In practice, the bottom heating element can be made of two separately controllable elements to provide suitable thermal insulation although the bottom of the tank can appropriately provide the desired temperature profiles. In the event of freezing, the scraper is surrounded by a hot region of liquid to easily provide a melt back in order to eliminate the possibility of a flow of silicon during a melt after a frozen condition.

In operation, after the tank 10 has been filled with molten silicon 12, the temperature is dropped until an "island" of solid silicon begins to form. The settings for the heating elements and heat shields for future cycles are based on the shape of the island.

The temperature of the mass is then carefully raised until the island is just molten. The silicon seed 34 is then inserted so as to touch the melt in the region where the tube was formed. As the edge of the ribbon grows away from the wall 20, the ribbon 14 is pulled over the wall. The drawing speed of the drawing mechanism 32 is increased as the temperature of the tank is reduced, which maintains the growth edge of the crystal in the region where the initial island appeared. Allowing the crystal growth zone to extend beyond this zone will indicate that the hot protection zones are approaching freezing temperature and that the temperature of the vessel should be slightly increased.

The stabilizer 22 is positioned behind the wall 20, and extends perpendicularly upwards from the bottom wall 16 and across the tank. The upper surface of the stabilizer can be tilted downward back and forth, its upper edge being disposed slightly below the surface of the melt and below the leading edge of the tape where it meets the surface of the molten mass. More complex contours can also be used. The function of the stabilizer is to compensate for small changes in the tape's drawing speed as it is pulled out of the bath. In a state of constant draft and in stable temperature conditions, no problem arises. However, if the traction speed S increases by a value as, then the leading edge of the ribbon moves forward, that is to say.

to the right as seen in FIG. 11, by the distance A X such that A T decreases from A T1 to Q T2 enough to increase the speed of growth of the leading edge of A S.

The stabilizer thus responds to changes in traction speed, growth speed, melting temperature, radiant power, etc. The speed of advance of the ribbon is equal to S = fl (1 / Q T) in which AT = = f2 (AX), that is to say S = f1 (f2 (AX)).

Thus, the leading edge remains stable within a range of drawing speed and temperature. Likewise, using the lateral stabilizers 26 and 28, the width of the strip remains substantially stable. Within the stability range of the feed edge, the drawing speed controls only the thickness of the tape in addition to productivity.

Although the invention has been described with reference to the particular embodiments illustrated, numerous modifications can be made to it, as will be apparent to specialists. For example, while the invention has been basically described with reference to the formation of crystalline silicon ribbons, it can be advantageously used for the formation of ribbons of a wide variety of other crystalline materials which are prepared directly from the molten mass Such materials can be conductive, non-conductive or semiconductor and can have various applications. For example, sapphire panes can be made by this process or bubble memory materials, for example gadolinium, gallium and garnet can be made in the form of a ribbon.

Claims (10)

Claims. 1. A method of manufacturing a crystalline ribbon from a melt, characterized in that it comprises the following steps a) heating a melt in a crucible (10); b) a crystalline seed is placed on the surface of said melt; c) adjusting the temperature of the melt to allow solidification of the melt at a free edge of the seed to form a ribbon (14) while drawing the seed from the melt in a direction forming a positive acute angle with respect to at the surface of the melt at a speed proportional to the longitudinal growth of the tape; d) providing a separate attachment site (20) for a meniscus (38) formed by the melt from the underside of the tape as it leaves the surface of the melt to reduce effusion and freezing of the ribbon: e) the speed of growth of the ribbon is adjusted to the modifications of the speed of displacement of the ribbon by pulling the growing end of said ribbon from the surface of the melt over a stabilizer (22) upper front part is closer to the surface of the melt than an upper rear part. 2. Method according to claim 1, characterized in that the melt is formed in a relatively shallow bath from which said strip is drawn. 3. Method according to claim 1, characterized in that one adjusts the temperature of the melt selectively inside the melt below the two lateral edges of the ribbon to adjust the width of the ribbon. 4. Method according to claim 1, characterized in that the melt replenishes the crucible to maintain a substantially constant level. 5. Method according to claim 1, characterized in that the melt is silicon. 6. Method according to claim 1, characterized in that the growing end of the ribbon is adjusted with regard to its shape and its growth rate separately from the other portions of the ribbon. 7. Apparatus for manufacturing a crystalline ribbon from a melt, characterized in that it comprises: a) a crucible (10) relatively shallow suitable for containing a certain amount of melt; b) heating means (30) functionally associated with the crucible to regulate the temperature of the melt; c) means (32) for germ support and draw functionally associated with the crucible (10) for drawing a crystalline germ (34) from the surface of the melt in the form of a ribbon (14) in a direction forming a small positive angle relative to the surface of the melt at a longitudinal speed proportional to the speed of solidification of the melt at the end of the strip; d) meniscus attachment means (20, 36) mounted on the crucible (10) between the surface of the melt and the underside of the tape to provide a stable bottom edge for attachment of a meniscus (38) formed by the melt; and e) means (22) for stabilizing the growth edge mounted in the crucible (10) below the solidifying end of said ribbon to selectively increase and decrease the growth rate of the ribbon in correspondence with the speed of said means draw. 8. Apparatus according to claim 7, characterized in that the stabilization means comprise a vertical element (22) mounted on the bottom (16) of the crucible (10) and having a shaped upper face whose highest front part is disposed closer to the surface of the melt than the lowest rear part. 9. Apparatus according to claim 7, characterized in that it comprises lateral stabilization means (26, 28) arranged in the crucible below the lateral edges of the ribbon to adjust the width of the ribbon. 10. Apparatus according to claim 7, characterized in that the meniscus attachment means are constituted by a vertical element (20) ending in a rectilinear edge (36) horizontal extending transversely below the path of the tape to provide a stable place of meniscus attachment.