US3540871A

US3540871A - Method for maintaining the uniformity of vapor grown polycrystalline silicon

Info

Publication number: US3540871A
Application number: US698366A
Authority: US
Inventors: Lawrence D Dyer
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1967-12-15
Filing date: 1967-12-15
Publication date: 1970-11-17
Anticipated expiration: 1987-11-17

Description

3,540,871 METHOD FOR MAINTAINING THE UNI- FORMITY F VAPOR GROWN POLY- CRYSTALLINE SILICON Lawrence D. Dyer, Dallas, T ex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware N0 Drawing. Filed Dec. 15, 1967, Ser. No. 698,366 Int. Cl. C03c 3/22 US. CI. 65-31 Claims ABSTRACT OF THE DISCLOSURE In the production of polycrystalline silicon by the deposition of silicon from a gaseous stream of a halosilicon compound and hydrogen onto a silicon substrate being maintained at an elevated temperature, there will on occasion develop an area of single crystal silicon. Growth of the single crystal area may be terminated and polycrystalline growth initiated by increasing the concentration of the halosilicon compound in the gaseous stream by at least 50 mole percent for a period of about five minutes and then returning the concentration of the halosilicon compound back to its previous level. Alternatively, the single crystal growth may be inhibited and the growth of polycrystalline material initiated by introducing an oxygen impurity into the gaseous stream of the halosilicon compound and hydrogen. By decreasing the temperature of the substrate upon which the silicon is being deposited by about 100 C. for a period of about five minutes, the development of a single crystal silicon can also be terminated and polycrystal silicon growth initiated. Polycrystalline growth can also be initiated on a single crystal surface by etching the surface at about 900 C. with a hydrogen and hydrogen chloride gas stream.

This invention relates to the vapor deposition of silicon on a silicon substrate, and more particularly to the production of polycrystalline silicon bodies which are to be further processed for use as semiconductor materials and the like.

The vapor reduction of halosilicon compounds onto a hot silicon filament to obtain semiconductor grade silicon is a process well known in the semiconductor industry. One of the problems of such growth is that deposition can sometimes be substantially in the form of a single crystal or consist of large grains which do not have sufficient perfection to be useful either as a sliceable single crystal rod or as a fusible polycrystalline rod.

The present invention permits the production of uniform polycrystalline material by providing a method for inhibiting the growth of single crystal material and initiating the growth of polycrystalline material thereby eliminating the problems encountered with a material which is only partially polycrystalline. The present invention may be generally described as a method for inhibiting single crystal silicon growth and initiating polycrystalline growth by increasing the concentration of the halosilicon compound in the gaseous stream from which the silicon is being deposited upon a substrate. The concentration of the halosilicon is increased at least 50 mole percent and maintained at that level for a period of about five minutes before being lowered back to the previous level.

The invention also contemplates the conversion of single crystal silicon areas to polycrystalline form by introducing an oxygen impurity into a gaseous stream of the halosilicon compound and hydrogen.

In yet another aspect, the invention includes a method for terminating single crystal silicon development and promoting polycrystalline growth by temporarily lowering States Patent the temperature of the substrate upon which the silicon is being deposited by about C.

In still another aspect of the present invention polycrystalline growth is initiated on a single crystal surface by etching the surface with a hydrogen and hydrogen chloride gas stream at about 900 C.

To be more specific, reference is here made to Examples I through VIII which relate to the termination of single crystal development by increasing the concentration of the halosilicon compound; to Examples IX through XIV which relate to the introduction of an oxygen impurity into the gaseous stream to terminate the single crystal silicon development; to Examples XV and XVI which relate to terminating single crystal silicon development by lowering the temperature of the silicon substrate; and to Example XVII which relates to etching the single crystal surface at a temperature of about 900 C. with a hydrogen and hydrogen chloride gas to initiate polycrystalline growth.

EXAMPLE I This example serves to demonstrate that polycrystalline growth is promoted by utilizing a high concentration of trichlorosilane, hereinafter TCS, in the gaseous stream of hydrogen and TCS from which silicon is being deposited.

An 8 inch single crystal silicon filament approximately /4 inch in diameter was suspended between graphite electrodes in an approximately 2 inch I.D. cylindrical quartz enclosure having end plates fitted with gas inlet and exhaust ports. The conventional quartz enclosure was purged with hydrogen at the rate of 10 liters per minute while the silicon filament was elevated to a temperature of 1300 C. by passing an electric current through the filament in a conventional manner. Purge of the enclosure was continued for ten minutes after the filament had reached 1300 C. The gas flow rate through the enclosure was then brought to 11 liters per minute by introducing 1 liter per minute of hydrogen chloride gas into the hydrogen stream before the hydrogen stream entered the enclosure. The hydrogen and hydrogen chloride gas stream was circulated through the enclosure for 15 minutes to etch from the surface of the filament any impurities which may have adhered thereto during preparation of the filament from a melt or the like and mounting of the filament in the enclosure. At the end of the 15 minute etching period, the temperature of the filament was lowered, over a few minute period, to 1175 C. after which the hydrogen chloride flow was terminated. After terminating the hydrogen chloride flow, the hydrogen flow rate was reduced to 9 liters per minute and 1 liter per minute of TCS was added to the gaseous stream of hydrogen. The hydrogen and TCS were circulated through the enclosure for a period of 20 minutes, after which the rod formed by deposition of silicon from the TCS and hydrogen stream onto the filament was removed for inspection.

The single crystal filament was covered with defects which initiate polycrystalline growth. More than 1,000 twin defects per inch were formed during the above deposition on the six inches of the rod exposed to gases, one inch at either end of the rod being contained within the electrodes. No single inch of the rod contained less than 1,000 defects.

EXAMPLE II This example, like Example I, serves to demonstrate that polycrystalline growth is promoted by utilizing a high concentration of TCS in the gaseous stream of hydrogen and trichlorosilane.

The procedure of Example I was followed through etching of the silicon filament and after etching the temperature of the filament was dropped to 1250 C. rather than 1175 C. After the temperature of the filament had been lowered and the flow of hydrogen chloride gas terminated, a gaseous stream containing hydrogen and TCS, the latter of which comprised 20 mole percent of the total mixture, was circulated through the enclosure at the rate of 20 liters per minute. After several minutes during which silicon was deposited from the TCS and hydrogen stream onto the silicon filament, the filament was cooled and found to be covered with more than 1,000 twin defects per inch for that portion of the filament exposed to the gases. As in Example I, no single inch of the rod contained less than 1,000 defects.

EXAMPLE III This example, like Examples I and II, serves to demon strate that polycrystalline growth is fostered by a high TCS concentration.

The procedure of Example II was followed, except the silicon was deposited at a temperature of 1275 C. rather than a temperature of 1250 C. The resulting silicon rod was found to contain over 1,000 twin defects per inch for that portion of the rod exposed to the gaseous stream of hydrogen and TCS.

EXAMPLE IV This example demonstrates that a lower TCS concentration in the hydrogen and TCS stream from which slicon is deposited will not initiate polycrystalline growth in the manner described in the preceding Examples I, II and III.

The procedure of Example I was repeated exactly, with the single exception that the TCS and hydrogen stream contained only mole percent TCS and that the resulting rod formed by the deposition of silicon on the filament contained only 191 twin defects over one inch of the rod chosen for a defect count.

By contrasting the results of Example IV with those detailed in Examples I-III, it can be seen that by increasing the concentration of the halosilicon compound in the gaseous stream from which silicon is being deposited, the production of polycrystalline material is promoted since defectsgreatly increase over the single crytal surface with increasing halosilicon concentration. The halosilicon concentration can also be increased by using a more volatile halosilicon compound in conjunction with TCS, this will be most particularly pointed out by the following examples.

EXAMPLE V This example points out how few defects will be formed by using a low TCS concentration in the hydrogen and TCS stream.

As in Example I, an 8 inch single crystal filament approximately 4 inch in diameter was suspended between conventional electrodes in a quartz enclosure having an approximately 2 inch I.D. Approximately 10 liters per minute of hydrogen were circulated through the quartz enclosure While the temperature of the filament was elevated to a temperature of 1325 C. by passing the current to the filament in a conventional manner. While the filament was being elevated in temperature and for a period of 11 minutes thereafter, 10 liters per minute of hydrogen were circulated through the enclosure. Upon expiration of the 11 minute period, 2 liters per minute of hydrogen chloride gas were introduced into the 10 liter per minute hydrogen stream prior to its entry into the reactor bringing the total gas flow through the reactor to 12 liters per minute. The hydrogen chloride and hydrogen mixture was circulated over the filament for one minute, after which the hydrogen chloride flow was terminated. Ten liters per minute of hydrogen continued to flow over the filament for an additional 2 minutes, at the end of which 1 liter of hydrogen chloride was introduced into the hydrogen stream bringing the total gas load to 11 liters per minute. The gaseous mixture con taining 10 mole percent hydrogen chloride (based on the hydrogen) was passed over the filament for a period of 15 minutes. The hydrogen chloride fiow was terminated for a period of 1 minute during which 10 liters per minute of relatively pure hydrogen continuted to circulate over the filament. After the enclosure had been purged with the pure hydrogen, 2 liters per mintue of hydrogen chloride were again introduced into the stream bringing the total gas fiow in the enclosure to 12 liters per minute, which flow rate was maintained for 1 minute.

After the filament had been etched as described above, the hydrogen chloride concentration was reduced to 1 liter per minute over a 1 minute period. The hydrogen flow rate was then, over about a 1 minute period, in-

creased to 17 liters per minute, and over a 5 minute period, a hydrogen stream containing trichlorosilane was gradually added to the first hydrogen stream to bring the total flow rate to 30 liters per minute, 4.2%, of which was trichlorosilane, and 1 liter per minute of which was hydrogen chloride. After 1 minute at the 30- liter per minute flow rate, the hydrogen chloride flow rate was gradually reduced to bring the hydrogen chloride concentration of 3.5%. After the expiration of about 1 minute, the temperature of the filament was lowered to 1215 C. and the silicon was deposited on the filament for a period 1 hour. The TCS passing through the enclosure is added to the hydrogen stream by bubbling a carrier stream of hydrogen through a bottle of liquid TCS to permit the hydrogen to become saturated with the TCS vapor, all of which is known to those skilled in the art. The TCS used in the procedure described above contained only 0.01% dichlorosilane as a result of which the rod formed by depositing silicon on the filament contained only 32 twin defects. The defects appeared on the {211} faces which developed on the rod formed during the deposition. The starting filament had its axis oriented in the [111] direction.

EXAMPLE VI This example serves the same purpose as Example V.

The procedure of Example V was repeated, except silicon was deposited on the filament for a period of 1 hour and 25 minutes. The resulting rod contained only 18 twin defects on the {211} faces of the rod formed by the deposition. The liquid TCS used in the bubbler, as above, contained only 0.01% dichlorosilane.

EXAMPLE VH This example demonstrates that the concentration of the halosilicon compound can be raised by using a trichlorosilane with a relatively high concentration of dichlorosilane. Increasing the halosilicon concentration serves to initiate polycrystalline growth.

The procedure of Example V was followed through the etching steps. After the filament had been etched, as described in Example V, the hydrogen chloride concentration was reduced to 1 liter per minute over a 1 minute period. The hydrogen flow rate was then increased and TCS introduced into the enclosure in a hydrogen carrier stream bringing the total gas flow through the enclosure to 20 liters per minute, 2 mole percent of which was trichlorosilane and 3.5 mole percent of which was hydrogen chloride. The filament received silicon for a period of minutes, after which the rod formed by the deposition was removed and found to contain more than 600 twin defects on the {211} faces.

In this instance, the liquid TCS contained in the bubbler through which a portion of the hydrogen stream was directed to pick up the TCS contained 1.32% dichlorosilane which increased the number of defects.

EXAMPLE VIII This example further supports the results demonstrated in Example VII.

The procedure of Example Vll was repeated, except the filament received silicon for a period of 60 minutes, rather than 65 minutes. The rod formed by the deposition was found to contain more than 1,000 twin defects over the {211} faces. Here, as in Example VII, the trichlorosilane used in the bubbler contained 1.32% dichlorosilane. As will be noticed in contrasting Examples V and VI with Examples VII and VIII, the addition of the dichlorosilane, which has a lower boiling point than that of TCS, to the gaseous stream passing through the enclosure increases the halosilicon concentration promoting polycrystalline growth since it encourages formation of twin defects over the single crystal filament.

The formation of single crystal silicon material can also be spoiled by the addition to the enclosure of oxygen impurities, as explained below in more detail.

EXAMPLE IX This example demonstrates that defects leading to polycrystalline growth may be formed by introducing oxygen impurities into the reactor through outgassing of the reactor walls.

The procedure of Example V was repeated, except that before the filament was elevated in temperature to 1325 C., and while hydrogen at the rate of liters per minute was being circulated through the enclosure, the quartz vessel was outgassed. The outgassing was conducted over a five minute period by heating the outer surface of the quartz enclosure with a gas oxygen torch to drive any oxygen or water vapor contained in the quartz out of the quartz into the enclosure.

The etching and deposition steps of Example V were followed and the rod resulting from the deposition contained over 300 twin defects.

EXAMPLE X This example serves to demonstrate that oxygen impurities may be introduced into the reactor by outgassing of the quartz reactor wall under high temperatures.

The procedure of Example V was repeated, except the quartz enclosure was wrapped during the etching and deposition steps with a microquartz wool material /8 inch thick. The microquartz insulation caused the temperature on the wall of the quartz enclosure to reach 714 C. which is greater than twice the normal temperature of 300 C. Deposition on the silicon filament was carried out for minutes, and the rod resulting from the deposition was found to contain more than 300 twin defects due to outgassing of oxygen impurities from the quartz wall of the quartz vessel under the high temperatures to which the wall was subjected during the etching and deposition steps.

The oxygen impurity may be added to the gas stream prior to its entry into the reactor vessel, as demonstrated in the following Examples XI through XIV.

EXAMPLE XI duced into the reactor through a carrier gas of helium containing oxygen.

As in the previous examples, an 8 inch single crystal filament was suspended between electrodes within a quartz enclosure having an ID. of approximately 2 inches. The enclosure was elevated to 1300 C. while the enclosure was being flushed with a 10 liters per minute gaseous mixture containing mole percent helium and 50 mole percent hydrogen. Then, for about 30 seconds after the rod has been elevated to a temperature of 1300" C., the hydrogen flow rate was increased from 5 liters per minute to 10 liters per minute while the helium flow rate was being decreased from 5 liters per minute to 0 liters per minute. The 10 liters per minute stream of hydrogen was circulated through the enclosure for a period of 680 seconds, then 2 liters per minute of hydrogen chloride gas was added to the hydrogen stream bringing the total gas flow to 12 liters per minute. The hydrogen chloride and hydrogen stream was circulated through the enclosure for one minute. After the expiration of the one minute period, 1 liter per minute TCS was added to the hydrogen chloride and hydrogen stream bringing the total gas flow to 13 liters per minute. After addition of the TCS, the hydrogen chloride flow rate over a period of about 1 minute was decreased to about 500 cc. for one minute, and silicon was deposited upon the element for a period of one hour and thirtythree minutes. The rod formed by the deposition was found to contain more than 300 twin defects over the {211} faces. Rods grown under identical conditions, but without the addition of helium to the initial hydrogen purge were found to contain no more than 21 defects.

The helium used above contained oxygen in the form of water vapor on the order of 10 p.p.m.

EXAMPLE XII This example, like Example XI, is directed to the introduction of oxygen into the reaction through use of an oxygen containing helium gas.

As in the above examples, an 8 inch single crystal silicon filament was suspended between graphite electrodes in a cylindrical quartz enclosure. The enclosure was purged with hydrogen while elevating the temperature of the filament to 1300 C. After the filament had reached 1300 C., two liters per minute of hydrogen were circulated through the enclosure for a period of 680 seconds. Upon termination of the hydrogen treatment, the initial 2 liters per minute of hydrogen chloride gas were circulated through the enclosure bringing the total gas fiow to 12 liters per minute. The hydrogen chloride and hydrogen gases were circulated through the enclosure for 1 minute. Following the hydrogen chloride and hydrogen treatment of the filament, the hydrogen chloride gas flow was terminated and a stream comprising hydrogen and TCS was circulated through the enclosure at the rate of 20 liters per minute. A mole percent hydrogen and 5 mole per-cent gas stream was circulated through the enclosure for twenty minutes at the end of which period a gas stream of helium was circulated through the enclosure at the rate of 10 liters per minute for ten minutes. The fiow of the hydrogen and TCS gas stream was then resumed for an additional thirty minute period. The rod developed from the deposition of silicon on the filament contained more than 300 twin defects.

EXAMPLE XIII This example uses helium to introduce the oxygen impurity during etching of a single crystal filament.

As above, an 8 inch single crystal silicon filament suspended within a quartz enclosure. The enclosure was purged with hydrogen at the rate of 10 liters per minute while the temperature of the filament was elevated to 1300 C. After the filament had reached 1300 C., a gaseous stream comprising 95 mole percent helium and 5 mole percent hydrogen chloride circulated through the enclosure for a period of ten minutes. At the end of the ten minute period, the temperature of the filament was decreased to 1250" C. After termination of the helium and hydrogen chloride etch treatment, a gaseous stream comprising 95 mole percent hydrogen and 5 mole percent trichlorosilane was circulated through the enclosure at the rate of 20 liters per minute for minutes. The rod resulting from the deposition of silicon contained more than 300 twin defects.

EXAMPLE XIV This example, like Example XIII, introduces the oxygen impurity during the etching procedure.

The procedure of Example XIII was repeated, except the concentration of the helium and hydrogen chloride stream was modified and a stream comprising 90 mole percent helium and 10 mole percent hydrogen chloride was used. The deposition period was limited to 30 minutes and the rod formed during the deposition had more than 300' twin defects.

The helium used in Examples XIIXIV contained a high water vapor content which resulted in defects being formed on the single crystal surface. The defects, as explained above, initiate polycrystalline growth.

EXAMPLE XV This example demonstrates that polycrystalline growth may be initiated by low temperature deposition.

As above, an 8 inch single crystal silicon filament approximately fli inch in diameter was suspended between electrodes positioned within a quartz enclosure. The temperature of the filament, after purging the enclosure with hydrogen, was raised to about 1325 C. by passing a current to the filament. After the filament had reached 1325 C., 10 liters per minute of hydrogen were circulated through the enclosure for a period of 11 minutes, following which 2 liters per minute of hydrogen chloride gas were introduced into the hydrogen stream bringing the total flow through the enclosure to 12 liters per minute. The hydrogen chloride and hydrogen mixture were circulated over the filaments for 1 minute, after which the hydrogen chloride flow was terminated. Ten liters per minute of hydrogen continued to flow over the filament for an additional 2 minutes, at the end of which 1 liter of hydrogen chloride was introduced into the hydrogen stream bringing the total gas fiow to 11 liters per minute. The gaseous mixture containing a 1:10 hydrogen chloride to hydrogen ratio was passed over the filament for a period of 15 minutes.

After the filament had been etched as described above, a gas stream comprising hydrogen and TCS was added to the hydrogen and hydrogen chloride gas stream bringing the total flow rate to 2.0 liters per minute. The combined stream contained 2.25 mole percent trichlorosilane. After one minute at the liter per minute flow rate, the hydrogen chloride flow rate was lowered to 500 cc. per minute over a one minute period. After expiration of another minute, the temperature of the filament was lowered to 1100 C. over a two minute period. The gaseous stream of hydrogen, hydrogen chloride and trichlorosilane was circulated over the filament for a period of 10 minutes. At the end of this period, the hydrogen chloride flow rate was reduced to 250 cc. per minute for an additional 90 minutes during which time silicon was deposited upon the silicon filament. The rod formed from the above described deposition contained more than 1,000 twin defects over its surface.

EXAMPLE XVI This example points out that the low temperature deposition of Example XV resulted in the defects formed on the single crystal filament used in Example XV.

The procedure of Example XV was repeated, except the deposition temperature was 1200 C. rather than 1150 C. and the deposition period was extended to 4 hours.

The rod produced by the deposition described above contained only 3 twin defects.

As will be evident from examination of Examples XV and XVI, deposition at a lower temperature will create defects leading to polycrystalling growth.

EXAMPLE XVII This example demonstrates that polycrystalline growth may be initiated by etching a single crystal surface with hydrogen chloride gas at a low temperature.

The procedure of Example V was repeated through the etching procedure.

After etching of the filament, the hydrogen chloride concentration was reduced to 1 liter per minute over about a 1 minute period. The hydrogen flow rate was then, over about a 1 minute period, increased to 17 liters per minute, and over a 5 minute period, a hydrogen stream containing TCS was gradually added to the first hydrogen stream to bring the total flow to 30 liters per minute, 4% of which was TCS and 1 liter per minute of which was TCS. The hydrogen chloride concentration was then brought to 4% of the total stream and the temperature of the filament reduced to 1250' C. at which temperature silicon was deposited on the filament for 10 hours with no noticeable defects being formed.

After the 10 hour deposition period, the TCS concentration was dropped to 0%. The temperature of the rod was lowered to 900 C. and the hydrogen chloride concentration increased to 2.5 fold for a period of 5 minutes. Then the concentration of the hydrogen chloride was decreased to its original level, the temperature of the rod was elevated back to 1250 C. and the TCS concentration returned to 4%. After depositing silicon on the rod for 30 minutes many defects were observed over the surface. The temperature of the rod can be lowered to a temperature between about 600 C. and 950 C. during the high concentration hydrogen etch to produce the defects which as explained above lead to development of polycrystalline material.

As will be clear from consideration of Examples I-VII, the concentration of the halosilicon compound may be increased in a variety of ways. In addition to the manner described in Examples I through VII, the flow rate of the hydrogen can be decreased in the hydrogen-TCS stream thus increasing the concentration of the halosilicon compound. The effective concentration of TCS can also be increased by increasing the reactor pressure. For example, by increasing the reactor pressure from substantially atmospheric to a pressure of 4 or 5 p.s.i.g., the polycrystalline growth can be initiated since the effective TCS concentration, that is the number of TCS molecules per cubic inch, is increased. The concentration may be increased for long periods of time to produce defects before returning to lower concentrations, though five minutes is usually sufiicient.

As will also be clear from examination of Examples IX through XIV, oxygen impurities, such as pure oxygen, water vapor, carbon dioxide, or sulfur dioxide and ozone may be introduced intothe gas stream or brought into contact with the filament in a variety of ways other than those described. For example, relatively pure oxygen in minute quantities may be added to the hydrogen stream during purge of the enclosure, or may be added to the hydrogen and TCS stream during deposition.

Polycrystalline material may also be formed by lowering the temperature of filament during deposition, as is clear from examination of Examples XV and XVI. The temperature need be decreased in most instances only for a period of five minutes though it may be reduced for longer periods, if desired.

While rather specific terms have been used to describe several embodiments of the present invention, they are not intended nor should they be construed to limitation upon the inventions to define the following claims.

I claim:

1. In the production of polycrystalline silicon by the deposition of silicon from a gaseous stream of a halosilicon compound and hydrogen onto a silicon substrate being maintained at an elevated temperature of 950- 1300" C., during which deposition single crystal silicon begins to develop, an improved method of terminating single crystal silicon development and promoting polycrystalline growth which comprises:

(a) increasing the concentration of the halosilicon compound in the gaseous stream by about mole percent for a period of about five minutes; and

(b) then lowering the concentration of the halosilicon compound back to its previous level.

2. The method of claim 1, wherein the halosilicon is trichlorosilane which initially comprises about 5 mole percent of the gaseous stream and the concentration of the trichlorosilane is increased in step (a) to between about 8 and 10 mole percent.

3. The method of claim 2, wherein the hydrogen flow rate is not altered during step (a).

4. In the production of polycrystalline silicon by the deposition of silicon from a gaseous stream of a halosilicon compound and hydrogen onto a silicon substrate being maintained at an elevated temperature of 950- 1300 C., during which deposition single crystal silicon begins to develop, an improved method of terminating the single crystal silicon development and promoting polycrystalline growth, which comprises:

(a) introducing into the gaseous stream an oxygen impurity in minute quantities.

5. The method of claim 4, wherein the oxygen impurity is selected from the class consisting of: pure oxygen, water vapor, carbon dioxide, sulfur dioxide and ozone.

6. The method of claim 5, wherein the oxygen containing impurity is introduced in an inert carrier gas.

7. In the production of polycrystalline silicon from a gaseous stream of a halosilicon compound and hydrogen onto a silicon substrate being maintained at an elevated temperature of 950-1300 C., during which production the deposition of polycrystalline silicon. begins to convert to the deposition of single crystal silicon, an improved method of returning the single crystal silicon to polycrystalline form, which comprises:

(a) lowering the temperature of the substrate about 100 C. or more for a period of about five minutes; and

(b) then returning the temperature of the substrate to the initial temperature at which deposition was being conducted.

8. The method of claim 1, wherein the initial deposition temperature is about 1200" C. and the temperature is lowered in step (a) to about 1150 C.

9. In the production of polycrystalline silicon by the deposition of silicon from a gaseous stream of a halosilicon compound and hydrogen onto a silicon substrate being maintained at a temperature above 950 C. but below 1300 C., during which deposition single crystal silicon begins to develop, an improved method of terminating the single crystal silicon development and promoting polycrystalline growth, which comprises:

(a) terminating the flow of the halosilicon compound;

(b) reducing ,the temperature of the substrate to a temperature between about 600 C. and 950 C.; and

(c) etching the substrate with a stream containing hydrogen and hydrogen chloride.

((1) returning the temperature of the substrate to the deposition temperature after etching with the hydrogen and hydrogen chloride stream.

10. The method of claim 9, wherein the halosilicon compound is trichlorosilane, the temperature during the etching step (c) is about 900 C. and the concentration of the hydrogen chloride in the gas stream of hydrogen and hydrogen chloride is about 10 mole percent.

References Cited UNITED STATES PATENTS 3,021,198 2/1962 Rummel 23 223.5 3,171,755 3/1965 Reuschel et a1. 117 229 XR FOREIGN PATENTS 1,084,580 9/1967 Great Britain.

S. LEON BASHORE, Primary Examiner I. H, HARMAN, Assistant Examiner US. Cl. X.R.