US3119778A - Method and apparatus for crystal growth - Google Patents

Description

Jan. 28, 1964 N. E. HAMILTON 3,119,778

METHOD AND APPARATUS FOR caysm. GROWTH Filed Jan. 20, 1959 INVENTOR.

NOBLE E. HAMILTON BY v iwm' ATTORNEY United States Patent Ofiflce 4 Patented Jan. 28, 1954 3,119,778 METHOD AND AZKARATUS FOR CRYSTAL GRUWTH Noble E. Hamilton, Belmont, Mass, assignor to Clevite Corporation, Cleveland, @hio, a corporation of Ohio Filed Jan. 2% 1959, Ser. No. 787,939 6 Claims. (U. 252-4526) This invention relates to methods and apparatus for growing single crystals from the melt, particularly single crystals of semiconductor materials such as germanium and silicon.

The growth of single crystals from the melt by the Czochralslci method and variations thereof (e.g., the Stockbarger method) is well known in the art. Basically this method involves fusing the material to be crystallized, bringing a small single crystal seed into contact with the surface of the melt, and slowly withdrawing the seed so as to allow the melt to freeze progressively onto the seed forming a single crystal of bar-like configuration. Depending on the material being crystallized and its end use, the method may be carried on in controlled atmospheres. The effect of the rate of pulling, melt temperatures, and other pro ess parameters on the crystal produced are known and subject to adjustment to suit the needs of the particular situation. The basic technique also is varied, as by rotating the crystal relative to the melt, establishing a vertical temperature gradient across the freezing zone, etc.

Where the method is applied to grow crystals for use in semiconductor devices, the known methods and apparatus for carrying them out have certain disadvantages with limit to their usefulness.

in the manufacture of semiconductor devices it is important to use semiconductor crystals which have a re sistivity falling within a given range, as this affects the variation of electrical characteristics in the completed articles.

Unfortunately, crystals grown from the melt by the conventional methods alluded to above arecharacterized by a resistivity gradient caused by uniform variation in the concentration of impurities incorporated into the crystal as will now be explained.

In growing crystals for semiconductors the melt of highly purified semiconductor materials, e.g., germanium or silicon, is doped with donor or acceptor impurities which give the crystallized material the desired type of conductivity and effect other important properties including resistivity. The concentration of the impurit incorporated in the crystal is a function or" the segregation constant; the physical laws governing segregation are wellknown and well-understood. However, inasmuch as the volume of the melt diminishes, as the crystal pulling proceeds, the impurity concentration in the melt and, therefore, that in the crystal varies continuously. Consequently when crystal growth is completed, a large fraction of its length falls outside the limits of the permissible range of variation in the doping concentration and must be reprocessed.

Comparatively recently, the now well-known zone leveling process was developed which, by maintmning a substantially constant volume of melt -(i.e., molten zone volume), increases the yield of crystal within the permissible range of variation. Zone leveling, however, also has its disadvantages: (1) the crystal must be contained With n and freezes in a :boat which exerts restraint on the crystal; (2) the lack of complete radial symmetry complicates the problem controlling thermal gradient in the newly grown crystal as required to secure a high degree of crystal perfection and (3) there are resistivity variations transverse to the longitudinal (horizontal) axis of the crystal due to the directional convection currents in the molten zone near the crystals growing surface.

Moreover, both basic types of crystal growing techniques are batch-type processes and, consequently, are possessed of the usual inefficiencies characteristic of such processes.

The present invention contemplates a method and apparatus for growing single crystals from the melt which avoid the disadvantages of both the Czochralski-type and zone-leveling type processes. The method contemplated comprises preparing a bar of the material to be crystallized, disposing the bar in a substantially vertical position, establishing a molten zone adjacent the upper end of the bar, supporting the molten zone, and crystallizing material from the molten zone while simultaneously continuing to support the zone and moving the zone progressively downwardly relative to the bar so as to maintain the volume of the zone substantially constant.

The apparatus contemplated by the invention comprises a crucible member having an open bore in its bottom adapted to slidably receive, with a close tolerance fit, a bar of the material to be crystallized. Means are provided for supporting a bar of the material in a vertical position in alignment with the bore. A heat-generating element laterally surrounds the crucible and means are provided for moving the crucible member and heatinggenerating element, in unison, vertically downward relative to the bar so as to establish a constant-volume molten zone contained within the crucible. Means are provided also for pulling a crystal vertically upward from the crucible member.

The fundamental object of the present invention is to provide a novel method and apparatus for growing single crystals from the melt which avoid at least one of the disadvantages common to the prior art methods and apparatus mentioned above.

More particularly, it is an object of the present invention to provide a method and apparatus for growing from a melt single crystals which are characterized by a high degree of perfection and uniformity.

Another object is to provide methods and apparatus for growing from a melt doped single crystals of semiconductor materials which have substantially uniform resistivity throughout.

A further object is the provision of a novel continuous process for growing single crystals from a melt and apparatus for carrying out such process.

These and additional objects of the present invention and the manner of their accomplishment Will be apparent from the following description and subjoined claims taken in conjunction with the annexed drawing in which the single figure is a vertical axial section of apparatus in accordance with the invention.

The initial step of the method of the invention involves the preparation of a bar of the material to be crystallized. Where the crystal is to be used for semiconductor devices, the material must be of high purity and, preferably, would be prepared by zone-refining in a manner well known in the art. This yields a generally bar-shaped ingot which is then machined, cast, or otherwise formed into an elongated symmetrical shape, most conveniently that of a cylindrical rod or bar.

To facilitate operation as a continuous process, a number of such bars are prepared and have means. permitting them to be coupled together end-to-e'nd. Thus, for eX-- ample, each bar may be provided with male threads at one end and female threads at the other as hereinafter more fully explained.

A bar of the material then is disposed and supported 3 ably supported and contained so as to prevent flow of the molten phase down the bar. The volume and configuration of the molten zone are determined by the axial extent of the region heated and the nature and construction of the structure employed for supporting and containing the molten zone.

Assuming that the material being crystallized is intended for semiconductor devices, significant doping impurities are now introduced into the molten zone which, while supported and contained, is moved slowly downward relative to the bar; simultaneously, a crystal is pulled from the molten zone. It is pointed out that relative movement between the bar and the molten zone may be accomplished by downward movement of the zone, upward movement of the bar or a combination of both.

The crystal may be pulled in a conventional manner, viz., by bringing a imonocrystalline seed body of the material into contact with the free surface of the molten zone and then slowly moving the seed vertically upward away from the surface of the melt. The rate of withdrawal would vary greatly depending upon the material, the cross-section of the crystal, etc.; for a germanium crystal, a typical withdrawal rate of about 1 to 4 inches per hour is satisfactory. In addition rotation of the crystal about its vertical axis is beneficial, for example, at a rate of 5 to 20 rpm.

The rate at which the molten zone is moved downward along the bar depends upon the rate of crystal growth, being adjusted to maintain a substantially constant volume of molten phase. Relative movement between the bar and the grown crystal may be very slight or nil, as where the cross-sectional areas of the bar and grown crystal are equal.

in a continuous process, before the molten zone arrives at the bottom of the bar, an additional bar is coupled on so that operation may continue without interruption. To the same end, the pulled crystal may be broken off above the melt when it reaches a satisfactory length and the impurities in the molten zone replenished as necessary to maintain a substantially constant concentration.

Where necessitated by the material being processed, the molten zone may be disposed within an enclosure which is evacuated or filled with an inert gas or with a gas or vapor having a desired chemical effect upon the melt. Thus, the enclosure may be filled with argon, an oxidizing gas (e.g., O reducing gas (e.g., H or a vapor of a significant impurity to be incorporated in the crystal.

Apparatus for carrying out the principal steps of the method will now be described with continued reference to the drawing, wherein the apparatus as a whole is designated by numeral 10. It comprises a crucible member 12 laterally surrounded by a heat-generating element 14 and disposed within an enclosure 16.

Crucible member 12 takes the general form of a hollow cylinder having a stepped bore. The upper end of the crucible member is formed with a cup-shaped cavity 18 which serves to support and contain the molten zone 20. The lower end of crucible member 12 contains an open bore 22 coaxial with and extending into the bottom of cavity 18.

Bore 22 is dimensioned to receive with a close-tolerance fit a bar 24 of the material to be crystallized.

The clearance between bore 22 and bar 24 is great enough to allow the bar to slide easily in the bore yet small enough to prevent flow of molten metal from zone 20 into the clearance and down the bar.

Just below the shoulder 26 at the juncture of cavity 20 and bore 22, the latter is spanned by a tapered pin 28 removably fitting into suitably-shaped aligned radial bores in the sidewalls of the crucible member. With pin 28 in place, entry of bar 24 into cavity "18 is obstructed so that the crucible member can slide down over the bar until the pin engages the end of the bar. This is the normal relative disposition of the bar and crucible member at the start of operation.

Heat-generating element 14 may be any suitable device for heating the region of cavity 18 which it surrounds. In the illustrated embodiment element 14 is an induction coil and is connected to a source of high frequency A.-C., shown schematically at 39. The region of cavity 18 enveloped by coil 14 extends from the bottom of the cavity, i.e., shoulder 26 to a point at a substantial distance below the top of the cavity. The coil may be mounted on the crucible member or otherwise arranged so that its position relative to the crucible is maintained constant.

From the structure thus far described it will be seen that crucible member 12 can slide down over bar 24 which melts as it passes into cavity 18 heated by element 14.

Enclosure 16 completely envelops crucible 12 and its associated components. Inlet and outlet nipples 32 and 34 respectively enable evacuating the enclosure and introduction of inert gases or gases of desired chemical activity as necessary or desired.

Coaxially aligned with bar 24 is an aperture 36 in the bottom of enclosure 16. For continuous operation, a bar 24 is fed into the apparatus by connecting it to the lower end of bar 24. This may be accomplished by a threaded connection as shown at 33 or by any other suitable means. For example, end to end connection may be more simply made by matched reassembly of an irregular fracture or butting square ends with or without a small cylindrical insert or key to keep the bar centers aligned. Since bars virtually balance on end a rigid joint is not necessary.

Packing 40 in aperture 36 forms a gas-tight fit around bar 24 and a pair of opposed contoured rollers 42, 42' guide, support and slowly feed bar 24' into the system.

In substantial alignment with aperture 36, the top of enclosure 16 contains a similar aperture 44, sealed by packing 46 through which a single crystal 48 may be pulled from melt 20. A seed 50 for starting the crystal is secured to a suitable holder 54 suspended from a line 52 running over pulleys 56, 56'. A pair of opposed drive rollers 58, 58, frictionally engage the sides of the grown crystal once it projects above opening 44. Thus, after the crystal has been withdrawn to this stage, line 54 can be disconnected and its function taken over by drive rollers 58, 58. This permits portions of the grown crystal to be broken 01f as they reach the desired length without disrupting the growing operation.

If required, the apparatus is provided with a supply tube 60, which extends through the upper wall of enclosure 16 and is directed into cavity 18. Tube 60 enables the addition of doping impurities to the melt, as necessary, to maintain a substantially constant concentration.

Movement of crucible member 20 down along bar 24 preferably is gravity-controlled, i.e., the weight of the crucible member causes it to move downwardly as the upper end of bar 24 melts in the region of pin 28. If desired external weights, not shown, may be attached to the crucible member in any suitable manner.

The operation of the apparatus is as follows: bar 24 of the material to be crystallized, having first been shaped to the proper dimensions and configuration, is inserted upwardly between feed rollers 42, 42, into bore 22, and into engagement with pin 28 so that crucible member 12 is supported by the bar. It will be understood that enclosure 16 necessarily would have a removable closure, not shown, permitting access to its interior. The current to induction coil 14 is turned on and impurities introduced into the molten zone. As crucible member 12 moves downwardly, the volume of molten zone 20 increases up to the desired value. At this point, crystal seed 50 is brought into contact with the upper surface of the melt so as to start crystallization. Thus, as the molten zone progresses downwardly along bar 24, crystal 48 grows from the upper surface of melt 20, the melting at the bottom of the molten zone and freezing at the top taking place at substantially the same rate, i.e., crystal 48 is Withdrawn at the same rate as bar 24 is melted, both lgy virtue of the downward progress of crucible mern When the crucible member approaches the lower end of bar 24, a new bar .24 is coupled on. Then, crystal 48 is pulled up, by means of holder 52 and line 54 and/ or rollers 58, 58' simultaneously as bars 24 and 24- are moved upwardly so that molten zone 24 crucible 12, and coils 13 are raised as a unit to a point near the top of enclosure 16.

The process is repeated when crucible member has traversed the length of bar 24' and is approaching the bottom of enclosure 16.

If desired rollers 58, 58' and 42, 42' or equivalent means may be continuously operated soas to move bars 24, 24. and crystal 48- upwardly at the same rate as crucible member 12 moves downwardly. Moreover, crucible member 12 and heating element 14- may be stationary with respect to enclosure 16 and the charge bar 24 and crystal 4'8 moved through the enclosure.

At the end of a run, the heating current is turned off and, after cooling, pin 28 is withdrawn to permit removal of the crucible member from the bar. During the process, the atmosphere within enclosure 16 is controlled as permitted by nipples 32 and 34 and impurities introduced into the melt, as necessary, through feed tube 6%.

The apparatus described can also be operated to perform a batch-type process. In this case the charge bar 24 may be irnrnovably disposed within enclosure 16 and crucible member 12 slipped onto the upper end of the bar. Heating element 14, which may be located outside of enclosure 16 if desired, is then positioned so that it surrounds crucible member 12 and the power turned on. When molten zone 20 is established, growth of the crystal 4% is initiated as previously explained and heat generating element 14 moved slowly downward. The started crystal 48 is maintained substantially stationary relative to enclosure 16 and charge bar 24. As heating element 14 moves downwardly, it is automatically followed by crucible member 12 which is supported on the solid phase of charge bar 24 by pin 28. When the crucible member reaches the bottom of the 'bar 24, the power is turned off, the crystal removed and the apparatus reloaded for another run.

It will be appreciated from .the foregoing description that the apparatus disclosed aiTords great simplicity in construction and ease of manipulation. In certain forms no mechanical means are necessary for movement of crucible member 12. Moreover, where the crucible member is not driven or mechanically linked to heating element 14, the position of the crucible member relative to the heart element acts as a sensitive indicator of error in the power level of the heating element. If desired, changes in this relative position can be detected and fed back for automatic regulation of the power supply. This can be accomplished by mechanical linkages to switches, optical or magnetic means or, where induction heating is used, the shape of the crucible may be designed to reflect a change in relative position as a change in inductive coupling.

Feed bar 24 usually would have about the same crosssection as grown crystal 48, particularly in batch operation; consequently, there is little or no relative movement between the charge bar and crystal thus enabling crystals of 3 to 6 feet or more in length to be grown without the bulk, complexity, head room requirements and cost of mechanical mechanisms necessary to provide the long pull, rotation, and rigidity essential to conventional crystal pulling apparatus.

While there have been described what at present are believed to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. Apparatus for growing single crystals from a melt comprising: an open crucible having a through bore in its bottom adapted to slidably receive and, in service, receiving, with a close tolerance fit, a bar of the material to be crystallized; a heat-generating element laterally surrounding only a portion of said crucible which is above said bore; means for fixedly mounting a bar of material to be crystallized in a substantially vertical position; means partially obstructing said bore whereby said crucible can be supported on a bar of said material, said bar constituting the sole support for said crucible; means for moving said heat-generating element vertically downward at a controlled rate so as to progressively melt a bar of such material; and means for supporting a crystal grown vertically upward from said crucible.

2. A method of growing single crystals of a fusible solid material from a melt, using an open crucible having a through bore in its bottom, comprising: providing a rod of the material to be crystallized having a cross-section making a close tolerance sliding fit in said bore; disposing said rod in a substantially vertical attitude; providing a partial obstruction in said bore sufficient to preclude passage of said bar therethrough in the solid state; placing the crucible in an upright position atop the bar with the upper end of the bar inserted into the bore up to the obstruction therein; fusing an axial section of said bar within said bore adjacent said obstruction, enabling passage of the fused bar material beyond the obstruction to form, and supply material to, a melt in said crucible; progressively advancing the melting interface of said axial section along said bar toward its lower end, with concomitant passage of said bar into said bore and continuous replenishment of said melt; and pulling a crystal from the free liquid surface of said melt.

3. A method according to claim 2 for growing, in a continuous process, a doped single crystal of semiconductor material containing significant impurities, comprising the further steps of: adding significant impurities to the melt in said crucible; repetitively extending the length of said bar by periodic attachment of an additional bar of said material to the lower end thereof; adding additional quantities of said impurities to said melt as necessary to maintain a substantially constant concentration thereof; and periodically detaching portions of the grown crystal.

4. Apparatus for growing single crystals from a melt, comprising: means for supporting a bar of the material to be crystallized in a substantially vertical position; an open crucible having a through bore in its bottom adapted to slidably receive and, in service, receiving, with a close tolerance fit, a bar of said material supported by said means; means normally partially obstructing said bore to preclude passage of said bar therethrough in the solid state, said bar constituting the sole support of said crucible; a heat-generating element, laterally surrounding an upper portion only of said crucible, adapted to fuse a proximate section of said bar permitting progressive passage of the bar through said bore to form and supply material to a melt within the crucible, said heat-generating element being axially displaceable downwardly relative to said bar of material, with concomitant movement of said crucible along said bar as enabled by progressive melting and passage of said bar by said obstructing means and through said bore; and means for supporting a crystal grown vertically upward from the free liquid surface of said melt.

5. Apparatus for growing a single crystal from a melt including a crucible member having an upwardly-open cavity and means forming an elongate open-ended bore of smaller cross-section than, and extending coaxially downward from the bottom of, said cavity; means partially obstructing said bore at a location proximate said cavity and sufiiciently remote from the lower end of the bore to enable said crucible member to be supported entirely by means of a solid charge body inserted upwardly, with a close References Cited in the file of this patent UNITED STATES PATENTS 2,768,074 Staufier Oct. 23, 1956 Buehler Oct. 30, Erneis May 21, Mortimer Oct. 8, Kniepkamp Mar. 3, Rusler June 30, Schweickert July 7, Matare July 28, Kelemen May 23,

OTHER REFERENCES Keck: Review of Sci. Inst, vol. 25, #4, pages 331 to 334, April 1954.

Claims (1)

1. APPARATUS FOR GROWING SINGLE CRYSTALS FROM A MELT COMPRISING: AN OPEN CRUCIBLE HAVING A THROUGH BORE IN ITS BOTTOM ADAPTED TO SLIDABLY RECEIVE AND, IN SERVICE, RECEIVING, WITH A CLOSE TOLERANCE FIT, A BAR OF THE MATERIAL TO BE CRYSTALLIZED; A HEAT-GENERATING ELEMENT LATERALLY SURROUNDING ONLY A PORTION OF SAID CRUCIBLE WHICH IS ABOVE SAID BORE; MEANS FOR FIXEDLY MOUNTING A BAR OF MATERIAL TO BE CRYSTALLIZED IN A SUBSTANTIALLY VERTICAL POSITION; MEANS PARTIALLY OBSTRUCTING SAID BORE WHEREBY SAID CRUCIBLE CAN BE SUPPORTED ON A BAR OF SAID MATERIAL, SAID BAR CONSTITUTING THE SOLE SUPPORT FOR SAID CRUCIBLE; MEANS FOR MOVING SAID HEAT-GENERATING ELEMENT VERTICALLY DOWNWARD AT A CONTROLLED RATE SO AS TO PROGRESSIVELY MELT A BAR OF SUCH MATERIAL; AND MEANS FOR SUPPORTING A CRYSTAL GROWN VERTICALLY UPWARD FROM SAID CRUCIBLE.

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