US3128154A - Process for producing crystalline silicon over a substrate and removal therefrom - Google Patents

Description

April 7, 1964 K. E. BEAN ETAL.

y PRocEss FOR UCING CRYSTALLINE SILICON EFROM PROD Filed Dec. 19, 1958 OVER A SUBSTRATE AND REMOVAL THER United States Patent O PRCESS FR PRODUCING CRYSTALLEIE SlLl- CGN OVER A SUBSTRATE AND REMOVAL Medcalf, Miami, Okla., Cincinnati,

This invention relates to a method for producing crystalline silicon of a type adapted to be directly introduced into zone refining equipment. More particularly, it relates to a method for obtaining large pieces of crystalline silicon by deposition on a hot filament comprised of a metal other than silicon in such a manner that the silicon is readily removable in integral piece form from the fiament on which it has been deposited.

Semiconductor devices, which today are a primary outlet for elemental silicon, require silicon of the highest purity. Such standards of purity are best met, it has been found, by final purification of the semi-pure product with zone refining methods. In the recent past, semi-pure crystalline silicon in the bar form adapted for the utilization of zone refining purification methods has occasionally been produced by vapor phase deposition on a slender, elongated filament, or core, which itself is made of silicon, so as to act as a seed crystal and so as to become an integral part of the deposit thereon. Silicon filaments also have been obtained by pulling out a filament from a molten mass of the metal; this method is inherently both tedious and expensive. While both methods have the advantage that the bar subseqently deposited on the filament is relatively free of core impurities, and each provides a bar or rod shape which is adapted for direct introduction into refining equipment of the floating zone type, the use of silicon as a filament upon which deposition of silicon is effected is very expensive as to initial and to operating costs and the technique has not found favor despite its theoretical advantages.

Alternatively, crystalline silicon has been deposited on filaments of metals other Such filaments are often three or four feet in length and of small diameter. When the deposition on the filament is completed, the silicon must be cracked away from the bar before final purification of the metal may begin. The diiculty characteristic of this method is that one cannot simply remove the filament or core by pulling it longitudinally out of the silicon sheathing, because the filament inevitably breaks under the strain. The difculty in removal of the filament stems not from an especial affinity between the silicon and the filament material but rather from the fact that microscopic protuberances over the surface of the filament project into the superimposed layer of metal deposited thereon so as to prevent sliding movement necessary to disengage the one from the other by pull alone. For this reason, it has been necessary either to break the deposit away from the filament in small pieces which at the same time destroy or may destroy the filament, or to latch the bar and filament for an excessively long time, perhaps several weeks, in a strong acidic bath of, for example, hydrofluoric acid. In the practice of the former method, in which the bar is broken up to facilitate removal of the filament, the small pieces f silicon must be remelted and cast into bar form before final purification may begin, while in the latter method, complete removal of the filament is not always effected, and even if it is, the long leaching process causes the produce to become contaminated with acidic solution. Because of these concomitant difficulties, neither of these methods has been entirely suitable. Due to the increasing demand for silicon of high purity for use in semi-conthan silicon, such as tantalum.`

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ductor and other devices, and the increasing application of the fioating zone purification process in industry, it is evident that a method of production for silicon is highly desirable in which the silicon, after production, is adapted to be directly introduced into a floating zone, without prior melting, cutting or leaching.

A principal objective of the present invention has been to provide a method for deposition of silicon crystal bars on metallic filaments whereby the bars may be removed intact from the filament without becoming contaminated to an appreciable extent in the process. Further objectives have been to provide a method which coincidentally preserves the filament, so that it is available for reuse, and which eliminates the need for time-consuming leaching procedures.

We have discovered that crystalline silicon of bar shape shape which is readily removable from the filament on which it is deposited may be produced by first coating the filament as a step of the reduction process itself with a layer of silicon of very fine particle structure or of amorphous structure, and subsequently depositing the crystalline silicon on top of the powdery substrate so formed.

Otherwise expressed, we have discovered that silicon may be caused to deposit upon a hot filament in an amorphous or finely divided particle form as well as in the form of a solid crystalline material, and that deposition of either of such forms selectively may be caused to occur by control of the operating conditions under which the deposition is effected. More specifically, we have discovered that if a layer or substrate of amorphous silicon is caused first to be deposited upon the filament, after` which the silicon is caused to be deposited in the solid or continuous metallic state, then the latter deposit may be stripped longitudinally from the filament as an intact tube or hollow billet by reason of the mobility which is afforded by the intervening layer of amorphous silicon. Where the filament is eighteen inches to two feet or so in length, which is a length suitable for practical operations, the deposited billet of crystalline silicon readily may be pulled manually from the filament, thereby providing a billet which is physically intact and which, as a single piece, may be charged directly into a fioating zone unit or other zone refining equipment either alone or with other pieces.

The amorphous silicon, constituting the interlayer between the crystalline or solid state silicon and the filament, apparently facilitates removal of the latter `from the former by reason of its chalk-like or powdery property. It is believed that the discrete particles, though generally adherent to the filament, either are, or readily become, mobile if a longitudinal pull is exerted on the filament relative to the layer of pure crystalline material. This mobility of the 4amorphous layer may occur as a rolling motion of the particles between the filament and Ithe solid metallic layer of silicon surrounding it, or it may be that the amorphous silicon coating simply constitutes a layer which prevents microscopic protucerances on the surface of 4the filament from forming a locking engagement with the crystalline silicon deposit over the layer and which is easily ruptured by reason of its amorphous nature or by reason of the fine particles of which it is composed. The amorphous layer need only be very thin to accomplish lthe desired result. The crystalline deposit need only be thick enough to be capable of withstanding the longitudinal pull to separate it from the filament, though i in the preferred practice of the invention, the deposition of crystalline silicon is continued until a billet of substantial thickness is built up thereby to increase the productivity of the hot wire equipment. Upon separation from the billet, the filament is unharmed and may be re-used repeatedly.

The 'deposition of silicon `upon a hot wire as produced through thermal decomposition of a 4Vaporous silicon-containing compound, such as `silicon tetrachloride in the presence of a suitable gas atmosphere, is well-known in the art. In the practice of the present invention, silicon deposition in either the amorphous state or continuous crystalline state generally is governed by operating conditions, and the production and deposition of amorphous silicon is facilitated when a silane such as the trichlorosilane (SiHCl3) is employed as the vapor source of silicon. In general, at lower filament temperatures (which of course must be high enough to effect thermal decomposition), formation of the silicon in the amorphous state is favored, while higher filament temperatures favor the formation of a continuous crystalline layer or deposit. -By way of explaining the process in this respect, consider, Ifor example, the behavior of a typical halogen-containing silane, trichlorosilane, SiHCl3. This compound is a liquid at room temperature but boils at 33 C. It has the property of undergoing thermal dissociation at relatively low temperatures and atmospheric pressure. In the dissociation, what apparently occurs is a molecular rearrangement in which the hydrogen atom acts as a reducing agent:

Cl Cl The (SiCl2) is electronically incomplete, silicon normal' In the preferred method of practicing this invention the filament is made of tantalum. The reason lfor this preference is that because of the intimate contact of the filament and silicon, some of the material of which the filament is made generally is transferred to the crystal as an impurity, and tantalum, having a distribution coe-fficient of 1x10-S, can be removed from the crystal by the floating zone technique more readily than can other metallic elements. Otherwise expressed, tantalum is used because silicon can so easily be cleansed of it. However, the practice of the invention is not limited to the use of tantalum filaments and the use of other metallic elements such as molybdenum, tungsten, and similar metals which are non-reactive with the gaseous silicon source are intended to be included within the scope of this improvement in the art.

The precise extent, if any, to which the filament metal itself chemically enters into the chemical reduction of the silane is not known. Tantalum is inert or at least relatively unreactive to dry gaseous silanes and on that basis it would not be expected to act other than asa heat source and as the core on which the deposit forms. It may be, however, that the initial coating which forms on the tantalum filament is not elemental silicon but rather is tantalum silicide, resulting from the direct union of the two elements:

Ta-i-Si-TaSi which in `some manner is then reduced. The process takes place in the presence of hydrogen and helium gas, as will subsequently be explained, and the hydrogen may be the reducing agent. In any event, the substrate protects the filament from being eroded by the hydrogen, which otherwise might attack it by formation of the hydride, TaHil.

It is also to be noted that while the substrate is referred to herein and previously has been identified as amorphous silicon, it may be a mixture of substances rather than a pure compound, and some crystalline silicate may indeed be present in this layer. On that account when, hereinafter, the substrate is referred to as amorphous silicon, it will be understood that this is intended as a generic term used in contradistinction to the term crystalline silicon andencompasses both line particle silicon alone and a mixture of fine particle and amorphous silicon.

The substrate may be visually recognized, provided the filament is not red hot, by its yellowish, greenish or reddish color as contrasted with ythe characteristic silvery color of crystalline silicon. `(Of course, if the filament is energized, the entire substrate glows red at the operating tempera-ture.) Because of the characteristic difference in color, simply by permitting the filament to cool during the deposition process one may know whether conditions inside the reactor are promoting the formation of crystalline or of amorphous silicon.

The use of trichlorosilane as the silicon supplier in the reaction is preferred because `of the ease with which amorphous silicon may be deposited from it, and because it is a readily' available source of silicon which is amenable to hot Wire deposition, but those `skilled in the art readily will understand that `other suitable :silanes may be employed as silicon source materials in .accordance with the teachings of this invention.

The following detailed description of the invention is best understood in reference to the accompanying drawing in Iwhich FIGURE 1 is a schematic diagram 4of typical apparatus adapted for the practice of the invention.

The arrangement shown in FIGURE 1 essentially comprises a reactor 1 and a boiler 2r. The reactor is an elongated tubular structure having sealed top and bottom ends. A slender filament 31` of tantalum extends` between the two ends, so that it is essentially parallel to the axis of the reactor. Externally, the two ends yof the filament are connected to a conventional source of power whereby the filament may be heated through its internal electrical resistance toa tempenature of about 1100 C. The power source shown is a generator 4, connected to the filament through a switch 5 and a rheostat 6. However, the specific details of the power source `are not limited to the embodiment shown and any conventional means may be used.

The boiler 2 is an enclosed vessel `suitable for containing the silicon-supplying compound 7. The boiler resides in a constant temperature bath 8 by means of which the temperature of the compound may be accurately regulated.

We have empirically determined that the depositionof amorphous silicon is carried out most effectively in the presence of certain other gases :such as hydrogen and helium. These gases, in specified amounts and during specified periods, are admitted to the boiler, to be mixed with the silicon compound, land/ or to the reactor. TheyI have a singularly beneficial effect on the deposition of the amorphous layer although just precisely why this should be so Iis not yet known. A tank of hydrogen gas under pressure is shown at 9. A pressure line 10 leads from the tank 9 through a yconventional flow meter 11, which measures the rate of flow in volumetric units of gas per unit time, to a three-way valve 12. By means of the valve '12 the hydrogen flow from the tank may be directed so las to be shut off entirely, admitted only to the boiler through line 13, admitted both to the boiler through line 13 `and concurrently to the reactor through line 14, or admitted only to fthe reactor through line 14. A second tank 15 containing an inert gas such as helium or argon, is connected in similiar fashion to both the boiler and the reactor, through a flow meter 16, l-ine 17, valve 18, boiler inlet line 19', and reactor inlet line '14. The lower ends of the boiler inlet lines 13` yand 19 depend below the surface of the liquid silane 7, so that when gas is admitted to the boiler, it will bubble -up through the silane and thereby become dissolved in it. To maintain a steady pressure in the boiler, an outlet line 20 leads after it has passed through the boiler.

.to drive all the :air

reactor, thereby preventing from the boiler to a three-way valve 21 by which the boiler gases may be bled off, as desired, through line 22. Through line 23` the boiler outlet line 20 may, by turning the valve 2li appropriately, be opened to the reactor inlet line 14, so that silane vapor may be permitted to enter the reactor from the boiler. So that a steady pressure may be maintained in the reactor, the reactor is provided with a blow-off line 24.

=It must be noted that the foregoing 'apparatus is in no way specified as a limitation on the scope of this invention. It is merely an arrangement representative of the type with which the deposition may be carried out, and is included so that the process itself may be better understood.

rFollowing are examples of the specific process of this invention with the utilization of different hydrogen-containing silanes:

Example 1 A. quantity of trichlorosilane 7 is introduced into the boiler 2, sufficient in amount to submerge lthe lower ends of the boiler inlet lines 13 and 19. The constant ternperature bath -8 is adjusted `so as to maintain the boiler and trichlorosilane in it at a uniform temperature of approximately C., at which temperature the silane is lin a liquid phase. Hydrogen gas from supply tank 9 is admitted through valve 12 first to Ithe boiler, valve 21 being opened so that the gas flows through the reactor By means of the ow meter 11, the rate of fiow is maintained at 50 cubic feet per hour for lapproximately ten minutes, or for a period `of time sufficient to outgas the apparatus, that is, from it, so that the atmosphere will be essentially hydrogen.

At the completion of the outgassing, .the hydrogen ow is shut ofi from the boiler and is by-passed into the reactor through valve 12, valve 21 being closed to retain the hydrogen atmosphere in the boiler. The reactor is outgassed with hydrogen at 50 cubic -feet per hour for approximately one hour 'and ytwenty minutes. Hydrogen fiow Iis shut off at valve 12 and 'the reactor is then outgassed with a fiow of helium from tank 15 at the rate of 25 cubic feet per hour, admitted through valve 18, vfor 30 minutes. A condition `of steady-state flow through the reactor is permitted by the reactor outlet line 24, which vents the admitted gas after its passage through the the interial pressure from r-ising.

After the total period of outgassing, the boiler outlet valve 21 is opened so that trichlorosilane vapors which have accumulated in the boiler through vaporization of the silane are admitted to the reactor through line 14. The tantalum filament 3 is resistively heated to ra temperature of approximately l050 C., the helium flow continuing as before. The power to heat the filament is drawn from the generator 4, switch 5 being closed. The rheostat 16 may be calibrated so as to directly indicate filament temperature, yor -the temperature may be determined b-y means of a thermocouple located near the filament or by the use of an optical pyrometer. In regard to the latter means, it is of course necessary that the reactor have a window for viewing the filament. At a later stage of the process, during deposition itself, the provision of a window in the reactor is independently advantageous because through it one may observe, the power to the filament and permitting it to cool from its red-hot condition, the color of the deposit, and thereby` know whether the deposit is amorphous or crystalline silicon. The window is preferably of clear fused quartz.

Deposition of amorphous silicon begins at the time the filament is heated. Silane vapors, at a temperature of 0 C., and mixed with helium, fiow into the reactor, and as the silane molecules come into contact with the hot wire, they are given sufficient energy to cause them to dissociate in the manner previously described, the resultant elemental silicon being deposited on the hot wire.

after switching off At these temperatures, silicon is a solid so that the deposit is not vaporized by the heat. The substrate is heat conductive and itself acts as the heat source supplying energy of decomposition to subsequent sane molecules, becoming red-hot at the operating temperature. So long as the current to the filament is temporarily shut off, the amorphous silicon may be recognized through the quartz window by its characteristic yellowish, golden or reddish color. The reactor walls, being at a much lower temperature because of the helium blanket which carries the heat away as it leaves the reactor via the outlet line 24, do not have sufficient heat energy to cause the silane to decompose and consequently they are not covered with amorphous silicon but at the same time are warm enough to prevent the silane vapor from condensing on them; the window therefore remains clear and unclouded.

When the filament has been heated to this temperature for approximately fifteen minutes, hydrogen iioW is again started, being admitted to the reactor only at a rate of 50 cubic feet per hour. Helium flow continues at a constant rate, both gases by-passing the boiler to the reactor for two minutes. After that period, the lines 13 and 1 9 inletting to the boiler are opened at valves 12 and 18 respectively and valve 21 is opened to connect the boiler outlet to the reactor, both hydrogen and helium flow being directed to fiow first through the boiler and then through the reactor for a period of ten minutes. Next, helium flow, at the same rate, is directed through the by-pass valve 18 to the reactor, while hydrogen continues to enter the reactor after passing through the boiler. The boiler temperature is gradually brought up to a temperature of 27 C. by means of the constant temperature bath over a one hour period. Helium flow is then shut off entirely at valve 18 and conditions are maintained at these levels. When the temperature of the silane vapor is raised to 27 C. in the presence of hydrogen, the deposition of amorphous silicon on the Wire ceases and the deposition of crystalline silicon begins.

The process is continued under these conditions until the coating builds up to the desired thickness; for example, deposition over a period of about eleven hours builds up a billet of approximately one-fourth inch wall `thickness, as indicated at 25 in FIGURE l. At the end of this time, the filament temperature is gradually reduced over a period of one hour, during which time the boiler is cooled to about 0 C., while the hydrogen fiow is maintained through the reactor. After reaching room temperature, the crystal bar is removed from the reactor, still on the lament. The filament may be pulled away from the bar merely with manual pull by gripping the bar with the hand and the filament with a pair of pliers. The bar is now ready to be refined without further treatment.

*' The product of the above-described process is a crystalline silicon bar of size ranging up to 46 inches in length -and of a diameter up to, say, 1/2, displaying, upon refining, excellent electrical properties. It should be pointed out that the apparatus may be modified to permit the location of a plurality of filaments in the reactor, all obtaining power from the same source if desired. This results in a greater production but the process is in all respects otherwise similar.

The above steps are not intended to be limiting procedures but are presented as the general preferred method of practicing our invention to attain satisfactory results.

It will be seen that the deposition of the amorphous layer in the above example is accomplished by maintaining the silane boiler at a relatively low temperature together with the presence in the reactor of a mixed hydrogen-helium stream. The crystalline silicon deposit, on the other hand, is provided by a higher boiler operating temperature, i.e. 27 C., with a stream of hydrogen alone in the reactor. Argon or any other inert gas might be used rather than helium in any of the places where helium has been called for.

Example 2 An example of this invention when tribromosilane, SiHBr3, and argon gas are used in place of trichlorosilane and helium is as follows: The tribromosilane having been introduced into the boiler, the reactor and boiler are outgassed with hydrogen at 30 cubic feet per hour for ten minutes, the boiler being maintained at C. The tribromosilane is a less volatile compound than that previously referred to, and boils at a temperature of 109 C. at atmospheric pressure. The hydrogen flow to the boiler is shut off and the reactor alone is outgassed for another hour and twenty minutes. Hydrogen liow is then stopped entirely and the reactor is outgassed with helium for one-half hour at a rate of cubic feet per hour. After this the filament is heated up to the operating temperature of 1050 C., argon flow continuing, for fifteen minutes and the deposition of amorphous silicon begins. The hydrogen flow is again started at 40 cubic feet per hour and is directed entirely into the reactor, along with the argon, for two minutes. The lines 13 and 19 to the boiler are then opened and the argon and hydrogen are allowed to fiow through the boiler as well as into the reactor. After fifteen minutes, the argon is by-passed directly to the reactor. The boiler is now gradually brought to an operating temperature of approximately 90 C. by means of the constant temperature bath. After hydrogen has passed through the boiler for one hour, the argon iiow is stopped and the duration of the run is maintained at these conditions while the crystalline silicon deposits on the hot wire. The run is continued for twelve hours, at the end of which the filament temperature is gradually reduced over a two hour span, and the boiler temperature is reduced to 0 C. After the filament reaches room temperature, the crystal is removed from the reactor and stripped intact from the filaments and is then ready for Zone refining.

Having described our invention, we claim:

1. A process for preparing crystalline silicone comprising the steps of depositing a substrate of powdery silicon in the form of fine discrete particles on a filament of a material other than silicon, and then depositing crystalline silicon over said substrate.

2. A process for preparing crystalline silicon comprising the steps of depositing on a filament a substrate of powdery silicon in the form of tine discrete particles produced by the thermal decomposition of a silane, depositing crystalline silicon over said substrate, and then pulling the crystalline silicon deposit longitudinally from the iilament.

3. A process for producing crystalline silicon in which a layer of amorphous silicon is deposited as a substrate and crystalline silicon is deposited over said amorphous substrate asa subsequent step of said process, whereby the removal of the crystalline silicon from the material on which it is deposited is facilitated by the intermediacy of said substrate.

4. A process for preparing crystalline silicon in bar form, comprising the steps of mixing vaporized trichlorosilane wtih hydrogen and helium gases at a temperature of approximately `0 C., flowing said mixture of gases over a tantalum filament heated to a temperature of about l050 C., whereby said trichlorosilane is caused to thermally decompose producing a deposit of amorphous elemental silicon on said, filament, then flowing a mixture of trichlorosilane and hydrogen at a temperature of approximately 27 over said heated filament, thereby depositing crystalline silicone over said amorphous silicon, and subsequently stripping said crystalline silicon from said filament by pulling said filament longitudinally from said silicon.

5. The method of facilitating the removal of a billet of crystalline silicon from a filament on which said billet has been deposited by thermal decomposition of a siliconcontaining vapor, said method comprising initially depositing a layer of amorphous silicon on said iilament prior to the deposition of said billet of crystalline silicon whereby said amorphous silicon permits said billet to be easily removed from said filament.

6. The method of facilitating the removal of a billet of crystalline silicon from a filament upon which said billet is deposited by the decomposition of a silicon-containing gas at an elevated temperature, said method cornprising initially depositing a layer of amorphous silicon on said filament prior to the deposition` thereon of crystalline silicon, the deposition of said amorphous silicon Ybeing effected by the decomposition of the silicon-containlwhich improvement comprises initially adding heat energy to a vaporized silane by said hot wire to effect deposivtion of amorphous silicon on said hot wire by thermal decomposition ofV said silane and thenl depositing crystalline silicon over said amorphous silicon.

l1. A method of preparing crystalline silicon comprising the steps of depositing a layer of amorhpous silicon onto a body and thereafter depositing crystalline silicone onto said layer of amorphous silicon.

12. The method comprising, depositing an initial powdery vehicular layer of silicon in the form of fine discrete particles onto a body and thereafter depositing silicon in solid continuous form over said layer.

References Cited in the file of this patent FOREIGN PATENTS Great Britain Feb. `2,9, 1956 OTHER REFERENCES FIAT Final Report 789, Experiments to Produce Ductile Silicon, April 3, 1946, 5 pages.

Sangster et al.: Journal of the Electrochemical Society (1957), vol. 104, No. 5, pp. 317-319.

Szekely: Journal of the Electrochemical Society (1957), V61. 104, No. 11, pp. 663-667.

Pfann: Zone Melting, John Wiley-and Sons, Inc., 1958, p. 89.

Claims (1)

1. 2. A PROCESS FOR PREPARING CRYSTALLINE SILICON COMPRISING THE STEPS OF DEPOSITING ON A FILAMENT A SUBSTRATE OF POWDERY SILICON IN THE FORM OF FINE DISCRETE PARTICLES PRODUCED BY THE THERMAL DECOMPOSITION OF A SILANE, DEPOSITING CRYSTALLINE SILICON OVER SAID SUBSTRATE, AND THEN PULLING THE CRYSTALLINE SILICON DEPOSIT LONGITUDINALLY FROM THE FILAMENT.

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