WO2000052233A1

WO2000052233A1 - Apparatus for growing single crystals

Info

Publication number: WO2000052233A1
Application number: PCT/IL2000/000120
Authority: WO
Inventors: Yuri Gelfgat; Herman Branover; Shaul Lesin; Mark Tsirlin
Original assignee: Solmecs Israel Ltd
Current assignee: Solmecs Israel Ltd
Priority date: 1999-03-04
Filing date: 2000-02-28
Publication date: 2000-09-08
Anticipated expiration: 2001-09-04
Also published as: AU2936700A; IL128827A0; KR20010113724A

Abstract

The invention provides apparatus for single crystal growth, including a chamber housing a heatable crucible having a bottom wall and side walls and being rotatable about an axis, means for feeding raw material into the crucible, and further including a seed crystal mounted on a movable rod along the axis, the apparatus including an annular heater disposed in the crucible about the axis and above the bottom wall so as to leave a gap between them for melted crystal material flow from a first heating zone defined by the inner surface of the side walls, the outer surface of the annular heater and the peripheral area of the bottom wall, and a second heating zone defined by the inner surface of the annular heater and the central area of the bottom wall.

Description

APPARATUS FOR GROWING SINGLE CRYSTALS Technical Field

The present invention relates to apparatus for the production of large single crystals by the Czochralski method. More particularly, the present invention is concerned with the production of high-quality, single crystals of silicon, gallium arsenide and other semiconductor materials. Background Art

Conventional apparatus for single crystal growth by the Czochralski method comprises an air-tight chamber with a crucible, which as a rule is made of quartz, mounted along its axis and surrounded by coaxial heaters and heat screens. The crucible is symmetrically arranged on a special support, commonly made of graphite, fixed on a shaft capable of rotating and moving in a vertical direction. A rod with a mono-crystalline seed, fixed at its lower end and fitted with means for its rotation and vertical displacement, is located above the crucible along the common vertical symmetrical axis.

Devices of such a type operate as follows: Starting material, for instance, silicon in the solid state, is placed into a crucible and then melted by heaters and heated to a temperature somewhat exceeding the silicon melting point. A seed crystal is immersed into the melt and then pulled upwards, the single crystal formed is grown to the required size, and a cylindrical single crystal is grown at the subsequent upward displacement of the rod. The crystal and the crucible are conventionally rotated in opposite directions, and the top surface of the melt in the crucible is maintained at a constant level.

In the process of melting the starting material and the subsequent growth of a single crystal, a continuous heat loss takes place in the melt, primarily through its free surface, the growing crystal and the crucible bottom. These heat losses should be compensated by an increase in the heat output of resistive heaters surrounding the crucible. An increase in the heaters' output leads to a rather intense heating of the crucible walls, considerably above the melting point of a semiconductor material. This technologically inevitable considerable heating of the crucible walls is one of the main drawbacks of conventional growth apparatus, especially when growing single crystals more than 200 mm in diameter, since the larger the diameter of a crystal and, respectively, of the crucible, the greater are the heat losses and the overheating of the crucible walls.

An increase in the temperature of the crucible walls causes their enhanced dissolution in the melt, resulting in the incorporation of an elevated amount of oxygen and unfavorable background impurities from the crucible wall material into the melt and further into the single crystal.

Another undesirable drawback of the overheating of the crucible walls consists in the appearance of island-like crystobalite structures. These islands break away from the crucible walls and may be transferred by convective flows to the crystallization front in the form of fine particles with a high probability degree, thus disturbing the dislocation-free process of single crystal growth. It is therefore very difficult to preserve a dislocation-free growth mode over the entire length of a large-diameter single crystal. The length-to-diameter ratio of single crystals grown under such conditions is respectively rather small, which considerably decreases the productivity of the process.

A further unfavorable result of the overheating of the crucible walls up to rather high temperatures, is the large temperature gradient between the crystallization front of a growing crystal and the crucible walls. Such a gradient causes the appearance of intense, uncontrollable, thermo- convective flows in the melt, leading to local fluctuations of temperature and a concentration of doping and background impurities, and resulting in an inhomogeneous distribution of said impurities, particularly, oxygen, in a single crystal grown.

In addition, temperature fluctuations in the melt enhance thermal stresses arising in a single crystal during the process of its growth, increasing the risk of dislocation generation on the phase boundary and its further propagation into the dislocation-free portion of a ready single crystal to a depth approximately equal to the diameter of the single crystal. As a result, the length of a high-quality single crystal more than 200 mm in diameter, which is limited for the above reasons, is further reduced, thus decreasing the productivity of the growth apparatus.

To decrease the adverse effect of convective flows and intense temperature fluctuations in the melt on the quality of single crystals, alternative apparatus are known, equipped with steady magnetic field systems. In such an apparatus, a crucible with the melt is located within the zone of an axial (aligned with growth axis), transverse (normal to growth axis) or axial-radial (cusp-type) steady magnetic field, produced by magnetic systems fixed either outside or inside the growth apparatus chamber. Steady magnetic fields suppress velocity and temperature fluctuations in the melt and reduce the intensity of convective flows, thus reducing crucible wall erosion and the extent of oxygen incorporation into the melt. The drawbacks of these approaches are as follows:

- a need for applying magnetic fields with rather high induction values (>0.4÷0.5T) that are hard to realize, due to the large dimensions of single crystals to be grown and, respectively, of growth apparatus;

- high power consumption needed to generate the required magnetic fields;

- the large dimensions of magnetic systems;

- inadmissibly high radial temperature gradients in a crucible, since the suppression of convective flows reduces the intensity of the melt agitation and leads to an increase in temperature gradients between the crucible wall and a single crystal.

Apparatus for single crystal growth by the Czochralski method, equipped with systems of a rotating magnetic field, are also known. In such apparatus, an inductor of a rotating magnetic field mounted around a crucible with the melt, either outside or inside the chamber, affects the melt and sets it in rotation around the crucible axis. Melt rotation stabilizes convective flows in a crucible, but this requires such high melt rotation rates that segregation effects appear. As a result, these approaches do not allow one to produce single crystals of large diameters with a reduced oxygen content and a homogeneous distribution of doping and background impurities over the radius and length of a mono-crystalline ingot.

To increase the length of the grown single crystals to a diameter of more than 200 mm in diameter, i.e., to raise the productivity of the growth process, apparatus operating on the principle of a continuous Czochralski process (CCZ) were proposed. Such apparatus are equipped with means for feeding additional raw material directly into a crucible with the melt during the process of single crystal growth. To prevent any disturbances of the growth process, the crucible is divided by a special coaxial annual insert into two zones: a central zone from which a single crystal is pulled, and a peripheral one, where additional raw material is added. The annular insert has small holes for feeding the melt from the peripheral zone into the central one.

However, such apparatus does not ensure either the required quality of single crystals grown or a high process productivity, since it has a number of drawbacks, especially while growing single crystals of large diameters exceeding 200 mm. These drawbacks are:

- the annular insert, which as a rule is made of quartz, hinders the heating of the starting material in the central zone of the crucible, which prolongs the process of its melting, reducing productivity, and requires an increase in heater output, resulting in considerable overheating of the crucible walls;

- the overheating of crucible walls considerably in excess of the melting point of semiconductor material leads to an elevated oxygen content in the crystal and disturbance of the dislocation-free growth process;

- adding additional raw material to the peripheral zone of a crucible leads, in the absence of intense melt mixing, to abrupt fluctuations of its temperature which adversely affect the quality of a final single crystal;

- the presence of an annular insert in a crucible and a single heater disposed around the crucible, makes the process of melt heating in the central region highly inertial and hinders the generation of the temperature gradient between the crystal and the annular insert which is required for the growth process. In addition, the uncontrollable thermo-convective flows arising in such cases in the central zone of a crucible, do not provide necessary directions of flow that will ensure minimal oxygen content in the produced single crystals.

The above examples of conventional apparatus for single crystal growth by the Czochralski method indicate that known designs of such apparatus do not ensure the production of high-quality single crystals having a diameter above 200 mm, along with high growth process productivity and reliability.

As a prototype of the basic features of the present invention, one can refer to U.S. Patent No. 5,279,798, entitled "Silicon Single Crystal Manufacturing Apparatus." All of the drawbacks described above are inherent to the prototype. In addition, in the design of this patent there is no possibility of additional melt mixing, either in the peripheral zone for intensified melting of additionally fed material, or in the central zone for establishing mixing flows in the required directions and for melt homogenizing in the sub-crystal zone. Disclosure of the Invention

It is therefore an object of the present invention to enable the growth of high-quality, large-diameter, single crystals, while increasing the productivity of the growth process and achieving a reliable, continuous, growth process.

It is a further object of the present invention to provide high quality, single crystals with low oxygen content and elevated homogeneity of dopants and background impurities distribution over the radius and the length of a mono-crystalline ingot, without dislocations, for instance, in silicon single crystals, or with reduced dislocations, for instance, in gallium arsenide single crystals.

In accordance with the present invention, there is therefore provided apparatus for single crystal growth, including a chamber housing a heatable crucible having a bottom wall and side walls and being rotatable about an axis, means for feeding raw material into said crucible, and further including a seed crystal mounted on a movable rod along said axis, said apparatus comprising an annular heater disposed in said crucible about said axis and above said bottom wall so as to leave a gap between them for melted crystal material flow from a first heating zone defined by the inner surface of said side walls, the outer surface of said annular heater and the peripheral area of said bottom wall, and a second heating zone defined by the inner surface of said annular heater and the central area of said bottom wall. Brief Description of the Drawings

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Fig. 1 is a schematic, cross-sectional view of an apparatus for single crystal growth according to the present invention; Fig. 2 is a schematic, cross-sectional view of the embodiment of Fig. 1 having a heater equipped with electromagnetic field concentrators; Fig. 3 is a schematic, cross-sectional view of a modification of the apparatus of Fig. 1, and Fig. 4 is a schematic cross-sectional view of a still further modification of the apparatus of Fig. 1. Description of Preferred Embodiments

The apparatus for single crystal growth according to the present invention as illustrated in Fig. 1 includes an air-tight growth chamber 1 housing a quartz crucible 2 for retaining a melt 3 of semiconductor material, e.g., silicon. The crucible 2 is installed on a graphite support 4. which is, in turn, fixed to a shaft 5. The shaft 5 is equipped with mechanisms (not shown) for its rotation and vertical displacement, as indicated by arrows. Heater 6 and heat shields 7 disposed between the heater and the chamber, are mounted around the crucible 2. An annular channel 8, open from above and closed from below, is mounted inside the crucible 2 coaxially to the latter, so as to retain a gap d between its lower end and the bottom of crucible 2 for a hydraulic connection between the peripheral P and central C zones of the crucible 2. The annular channel 8 is fixed by several holders 9, disposed symmetrically over the crucible's circumference. One end of each holder 9 is fixed to the support 4, whereas the second end is connected with the annular channel 8 in such a way that the annular channel 8 can be vertically displaced, either together with the crucible 2, or independently. The annular channel 8 can be made of insulating material, e.g., quartz, or conducting material, e.g., graphite; iridium, depending on the physio-chemical properties of the melt, from which a single crystal is pulled. Heater 10, having flexible current leads 1 1, is disposed inside annular channel 8.

The heater 10 provides the possibility of creating a longitudinal temperature gradient along its axis. For this purpose, and a heat shield 12 is disposed between the heater 10 and the inner wall of annular channel 8. Per-se known means, preferably an inductor 13 for agitating the melt and accelerating its melting, made as an inductor of an alternating (travelling, rotating or pulsating) magnetic field connected with a corresponding power source, is mounted around the chamber 1. The chamber 1 is equipped from above with means 14 for feeding additional amounts of raw material to the crucible 2 for realization of the continuous growth process of a single crystal 15. The single crystal 15 is pulled from the melt to a seed crystal 16 affixed on a rod 17 connected with mechanisms (not shown) for its rotation and vertical displacement in the directions indicated by arrows. The heater 6 disposed around the crucible 2, and the heater 10 disposed inside the annular channel 8, are connected, respectively, to AC or DC power sources (not shown). Depending on specific conditions and requirements, they can be connected in circuits of a travelling, rotating or pulsating magnetic field. In another possible embodiment, heaters can be simultaneously connected to DC and AC sources. Optionally, heaters can be equipped with means for varying the frequency of their supply current.

The presence in the proposed apparatus of the external and internal heaters 6 and 10 makes it possible to more flexibly control the field of temperature in the silicon melt. Thus, at some proportions of capacities being introduced into the system by the said heating contours, a temperature gradient may be provided which is directed from the device axis to its periphery. This direction of the temperature gradient may provide the formation of a hard silicon crust, the width of which may be controlled, on the wall of the crucible 2. In doing so, the silicon mono-crystal grows from the silicon crucible (i.e., scale melting takes place). This excludes the undesirable effect of admixture transfer, mainly of oxygen, from the walls of the external crucible 2. In addition to a spectacular decrease of oxygen transfer into the silicon melt, growth with scale melting provides an increase in the service lifetime of the crucible. In other words, it is possible to increase the duration of the process without terminating the process and reloading the crucible. In doing so, the duration of one growth cycle is determined exclusively by the dynamics of admixture accumulation in the silicon melt.

In order to ensure the process of the growth of a single crystal with a neck (see Fig. 2), the heater 10 can be equipped with an electromagnetic field concentrator 18 connected, respectively, with a HF power source.

Referring to Fig. 3, to grow single crystals under a flux layer on the melt surface in zones C and P, flux layers 19 and 20 can be built up, the thickness of these layers being essentially different in each zone: from a zero value of thickness in layer 19 to a thickness of twice the growing crystal diameter in layer 20.

In Fig. 4, there is illustrated a further possible modification of the subject apparatus in which the annular channel 8 is mounted on an apertured ring 21 so as to allow melt 3 to flow therethrough from the peripheral zone P to the central zone C^*.

The apparatus operates as follows: The crucible 2, disposed in the growth chamber 1, is charged with raw material, for instance, with polycrystalline silicon. Then the heaters 6 and 10 are switched on, and the heat released ensures heating and melting of the raw material. Here, the presence of an additional heater 10 in the annular channel 8 considerably intensifies the process of silicon heating and melting, especially at large crucible diameters. This makes it possible not to overheat the walls of the crucible 2 and of the annular channel 8 much above the melting temperature of silicon. As a result, quartz wall erosion and, respectively, oxygen incorporation into the melt 3, are considerably reduced.

For further acceleration of the process of silicon melting and formation of a homogeneous melt, an inductor 13 of alternating (travelling, rotating or pulsating) magnetic field is connected. It excites electromagnetic forces in the melt, causing the appearance therein of intense mixing flows. A similar effect is achieved by supplying alternating current to the heaters 6 and 10 and connecting them in circuits of a travelling, rotating or pulsating electromagnetic field.

At a combined operation of the heaters 6 and 10 and the inductor 13, the raw material melting time is considerably reduced, despite a slight overheating of the walls of the crucible and of the annular channel, which time reduction increases apparatus productivity.

After melting and homogenizing the raw material for growth of a single crystal 15, a seed crystal 16 affixed on rod 17 is lowered into the C zone of the melt 3. First, the seed crystal 16 and the crucible 2 are set in rotation. Since the annular channel 8 is fixed at the support 4, it rotates in synchronism with the crucible 2, thus ensuring a homogeneous temperature distribution within zone C, from which the single crystal is pulled. When the seed crystal 16, whose temperature is below the melt temperature, touches the melt, a single crystal 15 starts crystallizing on the former, which is grown up to the required size and then pulled upward by the mechanism of vertical displacement of the rod 17. In the growth process of the single crystal 15, the operation mode of the heaters 10 is maintained in such a way that, taking into account the heat shield 12, the temperature of the inner wall of the annular channel 8 is somewhat lower than the temperature of the outer wall of the annular channel 8, whereas convective flows formed in the melt due to the vertical temperature gradient in the zone C are directed in the zone below the crystal in such a way that the supply to the growing crystal is realized by surface flows of the melt directed radially from the crucible wall to its center. Intense oxygen evaporation takes place in said surface flows, so that oxygen is incorporated into the single crystal in minimum amounts.

The formation of a scale silicon layer on the internal wall of the crucible 2 is implemented at a relative decrease of the capacity of heater 6 and at a corresponding increase of the capacity of heater 10. The width of the scale layer is adjusted by trial -and- error, at the expense of changing the proportions of the capacities' values. The formation of a temperature gradient directed from the annular channel 8 to the surface of the crucible 2 creates a convective silicon flow to the peripheral area of the melt. In addition, this gradient decreases the probability of oxygen atoms being washed out from the annular channel 8 and getting into the growth area. At the same time, the process of oxygen transfer from the walls of crucible 2 into the silicon melt ceases, the temperature of crucible 2 decreases, and its service lifetime increases correspondingly.

Radial and vertical gradients of temperature, intensity and direction of mixing flows in the vicinity of the phase boundary required for the production of high-quality single crystals, as well as conditions of dislocation-free growth, are realized by supplying the heater 10 with alternating (travelling, rotating or pulsating) and/or direct current, or with their combinations. The necessary optimum operation mode of the heaters 10 is selected experimentally for each standard size of a single crystal to be grown.

When growing a single crystal with a neck, for instance, in order to produce silicon single crystals with a rather low oxygen content (<2÷3xl0¹⁷ at/cm³), the WO 00/52233 PCT/ILOO/OO 120

1 1 heater 10 is equipped with an electromagnetic field concentrator 18 and connected to an HF source of alternating current. Under the action of the HF electromagnetic field concentrated in the vicinity of the phase boundary of the growing crystal (Fig. 2), a constriction of its diameter (neck) is formed, and then the crystal is grown again up to the necessary dimensions. The appearance of a neck causes oxygen evacuation by evaporation out of the narrow constriction zone, and further growth of the single crystal proceeds without any contact with the solid quartz walls. The absence of contact with the walls ensures the production of dislocation-free silicon single crystals with a very low oxygen content (<2÷3χ l0¹⁷ at/cm³).

In the course of the growth and pulling up of single crystal 15, the level of the melt 3 in the crucible 2 is lowered. To keep the level of the melt unchanged, ensure stationary conditions of the crystal formation process, and increase the length of the crystal grown, thus increasing apparatus productivity, additional polycrystalline raw material is fed to the peripheral zone P of the crucible by a special feeder 14, for instance, in the form of powder or granules. Its amount corresponds to the amount of melt consumed for the single crystal growth. When growing single crystals of large diameters (>200 mm), the application of the continuous mode of single crystal growth acquires primary importance, since the crucible volume is always limited.

When additional raw material is fed to zone P of crucible 2, agitating flows excited by the inductor 13 rapidly mix it into the bulk of the melt and promote its accelerated melting. The heaters 6 and 10 still make it possible to implement this process at an insignificant overheating (15÷20°C) of the walls of crucible 2 and of annular channel 8, without breaking the optimum conditions of single crystal growth in zone C.

To grow single crystals under a flux layer, e.g., gallium arsenide single crystals, a flux layer, e.g., B₂0₃, can be built up on the melt surface. To prevent dislocation generation in the ready single crystal at the expense of reducing temperature gradients in the latter, level 20 of the flux surrounding the crystal in zone C considerably exceeds level 19 of the flux in zone P and equals 0.1 ÷2 diameters of the single crystal. A sufficiently high level 20 of the flux heated by the heater 10 ensures a gradual, smooth temperature decrease in a solid single crystal, preventing its cracking and dislocation generation. A longitudinal temperature gradient of heater 10 contributes to the appearance of convective flows in the flux, thus promoting a smooth temperature decrease in a mono-crystalline ingot and respectively reducing the probability of dislocation generation.

Thus, the apparatus for single crystal growth by the Czochralski method according to the invention makes it possible:

- to grow single crystals of large diameters (>200 mm) without significant overheating of the crucible walls and, consequently, with a low oxygen and background impurities content, and at the same time, in the absence of conditions for both dislocation generation in the crystal and accelerated wash-out of the crucible walls;

- to ensure a homogeneous distribution of dopants and background impurities over the radius and the length of a growing crystal;

- to ensure a continuous single crystal production process, without disturbing its optimum growth conditions, by feeding additional material to the crucible;

- to ensure accelerated mixing and melting of the material fed to the crucible, with subsequent rapid homogenization of the melt;

- to grow a single crystal with a neck in the growth process, which ensures the production of single crystals with a very low oxygen content;

- to control oxygen content in single crystals by means of electromagnetic action on the melt exerted by the heaters and the inductor surrounding the chamber and by means of creating a silicon scale layer on the surface of the main crucible; and

- to grow single crystals under a flux layer, creating different flux layer levels in the central and peripheral zones of the crucible, thus decreasing the probability of dislocation generation in a grown single crystal. It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. Apparatus for single crystal growth, including a chamber housing a heatable crucible having a bottom wall and side walls and being rotatable about an axis, means for feeding raw material into said crucible, and further including a seed crystal mounted on a movable rod along said axis, said apparatus comprising: an annular heater disposed in said crucible about said axis and above said bottom wall so as to leave a gap between them for melted crystal material flow from a first heating zone defined by the inner surface of said side walls, the outer surface of said annular heater and the peripheral area of said bottom wall, and a second heating zone defined by the inner surface of said annular heater and the central area of said bottom wall.

2. The apparatus as claimed in claim 1, wherein said annular heater is configured as an open-top, annular channel, housing at least one heating element.

3. The apparatus as claimed in claim 2, wherein said annular heater further comprises a heat shield disposed within said channel between said heating element and the inner wall of the channel.

4. The apparatus as claimed in claim 1, wherein said annular heater is coupled to said crucible by means of one or more holders.

5. The apparatus as claimed in claim 1, wherein said annular heater is mounted on the bottom wall of said crucible by means of an apertured ring.

6. The apparatus as claimed in claim 1 , wherein said means for feeding raw material into said crucible feeds the raw material into said first zone.

7. The apparatus as claimed in claim 2, wherein the heater in said annular channel is arranged to produce a temperature gradient in the direction of said axis.

8. The apparatus as claimed in claim 2, wherein the heater in said annular channel is arranged to produce a temperature gradient peφendicular to said axis in the direction from the annular channel to the crucible.

9. The apparatus as claimed in claim 1, further comprising electromagnetic field focusing concentrators for focusing an electromagnetic field towards the crystallization front, for forming a crystal with a neck portion.

10. The apparatus as claimed in claim 1 , wherein said heater is also connected to an alternating power supply, for effecting the flow of melted material in said second zone.

11. The apparatus as claimed in claim 1 , wherein said annular heater extends to a level higher than the plane of the side walls of said crucible, enabling the forming of a layer of melt flux above said second zone of a height greater than the height of the melt flux on top of said first zone.

12. The apparatus as claimed in claim 1, further comprising electromagnetic field producing means disposed adjacent, or affixed, to said chamber, for effecting rapid mixing and melting of the raw material in said first zone.