US 3031275 A
Description (OCR text may contain errors)
- April 24, 1962 w. sHocKLEY 3,031,275
PROCESS FOR GROWING SINGLE CRYSTALS A TTORNEYS April 24, 1962 w. sHocKl-EY 3,031,275
PROCESS FOR GROWING SINGLE CRYSTALS Filed Feb. 20, 1959 2 Sheets-Sheet 2 FIG. 4
P /5 /fy /NCREASING TEM Uf C EAS/VG TEMP f f 4 i? 1/ f/ f/ FIG. 5
WILLIAM SHOCKLEY HEAT INVENTOR.
ATTORNEYS nited States Patent ti-Office 3,631,275 Patented Apr. 24, 1962 3,031,275 PROCESS FOR GROWING SINGLE CRYSTALS William Shockley, 23466 Corta Via, Los Altos, Calif. Filed Feb. 20, 1959, Ser. No. 794,608 17 Claims. (Cl. 23--301) This invention relates generally to a process for the growing of single crystals and more particul-arly to a process for growing single crystal plates, ribbons, sheets or the like by supporting the same on molten material.
With the increasing development of solid state devices such as transistors, rectiers, diodes, solar batteries, thermistors, transducers, ferrites and others in electronics and related elds, there is an ever increasing need for precise, -thin crystals in the form of plates, discs, flakes or even films. Presently, such shapes are prepared by slicing a single crystal rod followed by tedious, time consuming grinding and lapping operations to obtain the precise dimensions required. These operations usually result in a large percentage of the original crystal being wasted. In certain instances, machine operations cause undesirable impurities to penetrate the crystal plates and render them worthless for their intended use.
It is a general object of the present invention to provide a process for reproducibly and reliably growing thin single crystal plates, ribbons and the like.
It is another object of the present invention to provide a process of growing thin single crystals by supporting the growing crystal on a molten material.
It is another object of the present invention to provide a process for growing thin single crystals (a geometrical configuration with one dimension small in relation to the other two) employing a molten material immiscible over a wide range of temperatures with the crystalline material to be grown, the molten material having a higher specific gravity than the crystalline material so that throughout the process, the liquid as well as the solid phase of the crystalline material will float on the molten material.
It is another object of the present invention to provide a process for growing single crystals -by supporting the same on a molten material whereby the crystals are formed with minimum mechanical stresses.
It is a further object of the present invention to provide a process for producing thin crystals from imperfect `single or polycrystalline sheets, ribbons, bars, rods or from powdered crystalline material.
It is a Ifurther object of the present invention to provide a process for purifying relatively small sheets, bars, etc., of crystalline material.
These and other objects of the invention will become more clearly apparent from the following description when read in conjunction with the accompanying drawmg.
Referring to the drawing:
FIGURE l is a sectional elevation View showing a furnace suitable for carrying out the process of the invention;
FIGURE 2 is a sectional view taken along the line 2 2 of FIGURE l;
FIGUREl 3 shows a portion of the furnace of the type shown in FIGURE 1 in which powdered crystalline material is fed into the melting zone;
I FIGURE 4 shows another configuration of a furnace suitable for carrying out the invention;
FIGURE 5 is a sectional view taken along the line 5-5 of FIGURE 4;
FIGURE `6 shows an isothermal plot of the temperalture distribution in the furnace; and
FIGURE 7 shows an enlarged portion of a modification of the furnace of vFIGURE 4 in which impurities tend to be removed from the surface of the molten material by convective forces in the molten material.
Referring to FIGURES l and 2, the furnace includes a refractory tube 11 which may, for example, be circular or rectangular in cross-section. The tube is supported -by refractory mounts 12 carried on a support surface 13. Means (not shown) are provided for levelling the furnace by moving the support surface. The refractory tube and materials within the same may be electrically heated by resistance or induction heaters. schematically illustrated are inductive heating coils 14 surrounding the refractory material. The center portion of the refractory tube may be shielded y16. Suitable temperature controls and spacing of the heating coils is employed to form a higher temperature in the center portion than in the remainder of the furnace whereby material such as silicon which is being `formed into thin plates, sheets, ribbons, or the like in 'accordance with the present invention, becomes molten.
A graphite crucible or boat 17 filled with a molten material which is immiscible with the crystals being grown is disposed in the refractory tube. For example, in the growing of silicon crystals, the molten material may, for example, be lead. The molten material extends above the surface of the crucible as indicated at 18. This can be achieved by adjusting the volume of lead and the volume of the crucible whereby when the molten material is heated, it expands and extends above the surface but is contained within the crucible as a convex meniscus -by surface tension forces.
Preferably, the crucible 17 is placed within a larger graphite boat 19 which will catch any molten material which spills over the sides of crucible 17. This is purely a protective measure for the refractory tube and in certain instances may not be necessary.
The ends of the refractory tubes may be connected to suitable means for causing the flow of gases through the tubes to provide different atmospheres within the refractory tube. For example, nitrogen, helium, argon or other inert gases may be caused to flow through the tube, or, if desired, oxidizing gases may be caused to flow therethrough.
In accordance with the embodiment of the invention, an imperfect single crystal or a polycrystalline rod or bar 21 of crystalline material is fed onto the molten material 18. It may be fed by passing over a plurality of idler rollers 2v2 and spaced feed rollers 3` which serve to drive the material inwardly onto the surface of the molten material 18.
Illustrated at the opposite end of the furnace is a thin single crystal ribbon 24 which is drawn from the furnace by means of the spaced rollers 26. The portion of the ribbon within the furnace is floated or supported on the molten material as it is withdrawn from the furnace region. The crystal is continuously formed in the high temperature region and withdrawn by the rollers. VIn essence, raw material is fed from one end of the furnace along the molten material into a region of the furnace maintained at a temperature higher than its melting point where the material melts and is floated as a molten film on the surface of the molten material with which it is irnmiscible. As the sheet is drawn from the other end, a relatively pure single crystal isv continuously formed on the end extending into the high temperature region.
It is observed that the process not only provides means for growing relatively thin sheets of material, but also tends to purify the silicon since, in essence, zone refining is being carried out as the crystal is grown from the molten crystalline material supported by the molten material.
To start a process of this character, a seed is inserted on a pulling means and the seed is moved inwardly to the high temperature region. The seed then serves to cause nucleation of crystalline material. The rod is withdrawn outwardly and the material continues to grow in the form of a relatively thin single crystal plate. The thickness of the plate is determined by the temperature of the molten zone, and the rate at which the crystal is withdrawn. As crystalline material is supplied, it melts and recrystallizes into the growing crystal.
The temperature gradient within the furnace is such that at the ends of the furnace the temperature is near the melting point of the supporting material, for example, if lead is employed, 327 C. with increasing temperature as one proceeds from the ends of the furnace towards the center. As described, close control of temperature by Well known means is maintained at the center of the crucible. The temperature is maintained slightly above the melting point of the material from which the crystal is being grown, as for example, l420i2 C. for the growth of silicon plates, ribbons and the like.
It is observed that the molten material has a higher specific gravity than either the solid crystalline material or the molten crystalline material so that both the liquid and solid phases of the crystalline material from which the single crystal to be withdrawn is floated. 'This allows thin crystals to form on the surface and the amount of mechanical stress to which the crystals are subjected is minimal.
In certain instances, it rnay be desirable to gro-w a single crystal from powdered crystalline material. In such event, rather than feeding a rod Z1 onto the molten vmaterial into the melting zone, it is possible to feed the crystalline material through a suitable tube 31 (FIGURE 3) into the high temperature (melting) zone wherein the crystalline material melts and recrystallizes on the growing crystal as it is withdrawn from the melting zone.
In order to reduce convection currents, it is desirable 'to employ a relatively thin layer of supporting molten material. Referring to FIGURE 4, a relatively thin supporting material 18a is illustrated. As a result of the thinness, small thermal pressures are built up and relatively small convection currents are set up within the material. Convection occurring within the molten material might introduce dislocations in the crystal growing from the high temperature region. Convective gas currents should be minimized by making the space between the lead surface and the furnace walls as small as possible as illustrated by the space 32 in FIGURE 4. Another advantage which is gained is the reduction of the power required to maintain the temperature.
In the particular case of a lead-silicon system, certain additional refinements may be warranted in the furnace construction. These refinements are illustrated in FIG- URE 4. Vertical as well as horizontal temperature gradients may be produced by the distribution of heat in the heater coils. The isothermal lines 33 are of the general form shown in FIGURE 6. The small dimensions of the furnace and the temperature gradient serve to control the flow of lead vapor. At the melting point of silicon, lead has a vapor pressure in the order of 100 mm. of mercury. As a result of this high vapor pressure, it is possible for lead to vaporize from the hotter region of the furnace, condense on the surface of the colder region, and form drops which may fall on the silicon film and fracture it or interfere with growing crystal.
This may be prevented in .part by producing an opposing gas iiow which lio-ws inwardly from the ends to the high temperature regions so as to prevent the high vapor ressure at the high temperature regions of the furnace from reaching temperature regions where condensation might occur.
This effect may be enhanced by employing gas liow inwardly from the two ends. The lead vapor must then iiow against the current for an appreciable distance to find asurface on which to condense. In FIGURE 4, an outlet is shown at the center of the furnace for gases.
Thus, gas from the ends will iiow upward in the tube 34 and the lead vapor will condense in the portion 36.
Means are provided for replenishing the lead as it is vaporized. For example, an opening 37 may be formed in the furnace wall and lead in the form of a wire 3S, pellets or the like may be dropped into the molten pool to maintain the volume. Alternatively, the condensed vapors 39 may be directed back into the furnace to replenish the lead which has been vaporized. In such instance, the additional amount of lead 38 required will be minimized.
It may be necessary to provide for keeping the lead surface as clean as possile in the crystal molten region. This is particularly true in the neighborhood in which the crystal solidities. By applying heat to a localized area where the crystal solidiiies as indicated in FIGURE 7, convective currents in the lead can be set up as indicated by the lines 41. Here, a depression is formed in the Crucible to allow for the development of higher hydrostatic pressure in the lead. The localized heating is provided at the center of the depression so that an upward iiow of molten lead is produced which rises to the surface in the region where the silicon is molten and spreads out in both directions as indicated by the arrows 41. This flow tends to carry oxides or other impurity materials which may form on the surface of the lead toward the sides so that silicon, as it solidiies, is solidifying on a clean lead surface. It is evident that this means may also be utilized to provide means for cleaning the lead surface and a side arm might be produced on the furnace extending some distance laterally so that any surface matter can be carried to the side and be eliminated.
It should be noted that thin plates can be grown in high temperature gradients without thermal stresses because it is possible to provide a temperature distribution which leads to a conformal transformation in two dimensions. (See, for example, Electricity and Magnetism, by I. H. Jeans, Cambridge University Press, 1927, page 265.) A conformal transformation defines a twodimensional temperature distribution where the temperature T is given by requiring that the length of the edge 0f the crystallographic unit cell at temperature T at a given point is proportional to the quantity p defined in the reference on page 264.
It should also be noted that thin sheets provide easy escape for vacancies or interstitial atoms which may be present at high temperatures. If these cannot escape, they may produce vacancy platelets or dislocation loops as discussed, for example by I. C. Fisher in Dislocations and Mechanical Propetries of Crystals, John Wiley and Sons, 1957, page 513. Thus, more perfect crystals may vbe grown in thin plates than in large cylinders.
Impurities, such as lead in silicon, are often less soluble as the temperature is lowered. For the system described above, these impurities have an easy opportunity to escape into the molten support.
It is evident that the princples of this invention can Ibe extended to the case in which the molten support is moved with the silicon as in the case of Zone refining in a crucible.
It is also evident that the method can be extended to cases in which the supported material tends to spread to undcsirably thin layers due to surface tension effects by controlling the rate of feed and the temperature distribution so that solidiiication occurs when the layer has spread to the desired thickness.
The method of growing single crystal plates is not restricted to the silicon lead system. Some other possible systems characterized by having phase diagrams with two substantially immiscible liquid phases are nickel plates on silver, aluminum plates on cadmium, aluminum plates on indium, aluminum plates on potassium, aluminum plates on lead, aluminum plates on thallium, cobalt on lead, gallium on mercury, etc.
Although information on ternary phase diagrams is available only to a limited degree, it is evident that possible systems exist in this case. Thus, the three-iive coinpound semi-conductors, such as are listed in the article by Jenny in the .Tune 1958 issue of the Proc. I.R.E., have melting points high compared to lead and frequently higher than the constituent elements. This suggests that the molten three-live compound will have limited solubility in molten lead.
Molten salts may also be used as supports and crystals of quite different types may be grown in such systems.
1. A process for growing thin plates of a crystalline semiconductor material which comprises the steps of forming a support of molten second material, said second material being immiscible with the crystalline material and having a specilic gravity higher than said crystalline material at both the liquid and solid phases of the crystalline material, forming a region of predetermined extent in said second material which is at a temperature higher than the melting point of the crystalline material, supplying crystalline material to said region whereby it melts and is supported on the surface of the second material, growing a crystal from said crystalline material in the higher temperature region, and supporting the grown crystal on the second molten material.
2. A process as in claim 1 in which the crystalline material is moved on the surface of the second material past the region of higher temperature.
3. A process as in claim 1 wherein the region of higher temperature is progressively moved along the second material.
4. A process for treating crystalline semiconductor material which comprises the steps of forming a support of molten material, said molten material being immiscible with the crystalline material being treated and having a speciic gravity higher than said crystalline at both the liquid and solid phases of the crystalline material, forming a region of predetermined extent in said molten material which is at a temperature higher than the melting point of said crystalline material, supplying crystalline material to said region whereby it melts and is supported on the surface of the molten material, growing a crystal from said molten crystalline material, supporting said grown crystal on the supporting molten material, and withdrawing the grown crystal along the surface of the molten material.
5. A process as in claim 4 wherein the crystalline material supplied to said region is powdered crystalline material.
6. A process as in claim 4 wherein the crystalline material ysupplied to said region is solid crystalline material.
7. A process for treating crystalline semiconductor material which comprises the steps of forming a support of molten material, said molten material being immiscible with the crystalline material and having a specific gravity higher than said crystalline material at both the liquid and solid phases of the crystalline material, forming a region of predetermined extent in said liquid which region is at a temperature higher than the melting point of said crystalline material, supporting elongated crystalline material on said molten material and feeding the same along the molten material to said region whereby the crystalline material is progressively melted at said region, growing -a crystal from said molten crystalline material, withdrawing said crystal on said molten material whereby the grown crystal is supported on the surface of the molten material as it is drawn from the region.
8. A process as in claim 7 wherein said crystalline material being supplied is stressed polycrystalline material and wherein the material withdrawn is stress free polycrystalline material.
9. A process as in claim 7 wherein the material supplied is stressed polycrystalline material and wherein the material withdrawn is single crystal material in the form of a ribbon or sheet.
10. A process as in claim 7 wherein the material supplied is impure polycrystalline material and wherein the material withdrawn is single crystal relatively pure material.
1l. A process for growing single crystal silicon ribbons or sheets which comprises the steps of forming a pool of molten lead, forming a region of predetermined extent on said molten pool which is at a temperature higher than the melting point of silicon, supplying silicon material to said region whereby it melts and is supported on the surface of the molten lead, growing a crystal from said molten silicon in said region, and withdrawing and supporting said grown crystal on the molten lead.
l2. A process as in claim ll wherein the silicon supplied is powdered crystalline silicon.
13. A process as in claim 1l wherein the silicon supplied is solid silicon.
14. A process for growing thin plates of a crystalline semiconductor material comprising the steps of placing a molten support of a second material in a crucible, forming a convex meniscus, said second material being immiscible with the crystalline material and having a surface tension sufficient to support said crystalline material in both the liquid and solid state of said crystalline material, forming a region of predetermined extent in said second material which is at a temperature higher than the melting point of the crystalline material, supplying the crystalline material to said region whereby it melts and is supported on the surface of the second material, growing a crystal from said crystalline material in the high temperature region, and supporting the grown crystal on the second material.
15. A process for growing single crystal plates of a crystalline semiconductor material comprising the steps of forming a pool of a molten second material, forming a rst region of predetermined extent on said pool which is at a temperature higher than the melting point of said crystalline material, supplying said crystalline material to said rst region whereby it melts, forming a second region on said pool which is at a temperature lower than the melting point of said crystalline material, forming conduction currents in said molten material at said iirst region, growing a crystal of said crystalline material in said iirst region, and withdrawing said crystal to said second region whereby it solidiles. v
16. A process as defined in claim 15 wherein said iirst material is a powdered crystalline material.
17. A process as defined in claim 15 wherein said iirst material is a solid crystalline material.
References Cited in the le of this patent UNITED STATES PATENTS 789,911 Hitchcock May 16, 1905 2,739,088 Pfann Mar. 20, 1956 2,872,299 Celmer Feb. 3, 1959 FOREIGN PATENTS 567,339 Belgium 1957 769,692 Great Britain Mar. 13, 1957 OTHER REFERENCES Pfann: Zone Melting, 1958, pp. 93-95.