WO1997036822A1

WO1997036822A1 - Method and apparatus for growing extended crystals

Info

Publication number: WO1997036822A1
Application number: PCT/IL1997/000112
Authority: WO
Inventors: Moshe Zarudi; Izida Tchernina; Alexander Katsen
Original assignee: ELMATEC Ltd
Current assignee: ELMATEC Ltd
Priority date: 1996-04-02
Filing date: 1997-03-28
Publication date: 1997-10-09
Anticipated expiration: 1998-10-02
Also published as: IL117770A0; AU2041897A

Abstract

A method and apparatus for producing an extended crystal formed of a semiconductor material by depositing on a surface of an initial rod formed of a seed crystal particles of the semiconductor material from a reaction gas containing the latter. The initial rod is heated by supplying a conductive current passing therethrough. At least one parameter of the growing crystal is measured so as to determine as to whether or not this parameter is of a predetermined value thereof. Upon detecting that this at least one parameter is of the predetermined value, disconnecting the supply of the conductive current and providing a further heating of the growing crystal of a substantially uniform temperature distribution within a whole volume of the growing crystal. The further heating is provided by eddy currents produced within the growing crystal. The eddy currents are induced by an alternating magnetic field of a predetermined frequency so as to provide a skin effect in the growing crystal. A weight force of the growing crystal may then be reduced by means of a travelling magnetic field created in a manner to provide a movement of the waves thereof in a direction opposite to the direction of the weight force.

Description

Method and apparatus for growing extended crystals

FIELD OF THE INVENTION

The present invention relates to the technology of growing crystals in general, and particularly of producing an extended crystal rod of a semiconductor material by depositing on a surface thereof the semicon- ductor material from an appropriate reaction gas containing the same.

BACKGROUND OF THE INVENTION

There is a great variety of systems for producing a pure polycrystal silicon in the form of extended rod, for example by using so- called "Siemens method". Thus, an initial rod of seed silicon crystal is placed into a heat-insulated, closed vessel and an appropriate reaction silicon containing gas, such as Si_nH_mCl_p, passes through the vessel. A conductive current of commercial frequency is passed through the seed rod by simply connecting the latter by its opposite ends to a suitable power source, which causes a heating of the seed rod. Thus, a deposition of silicon particles on a heated surface of the seed rod is provided. It is appreciated that the higher the surface temperature of the rod, the more particles deposited thereon. Therefore, an increase of the surface temperature is one of the main factors defining a considerable increase of a deposition rate and, thereby, speeding up the production.

However, since the heating of the crystal rod is carried out in the above described manner, this results in a current density within a central region of the crystal, i.e. along its main axis, be of a significantly higher value than in a periphery region. This, in turn, leads to increase of heating and, therefore, electro-conductivity of that central region due to such a property of semiconductor materials as direct dependence between their electro-conductivity and temperature. Hence, continuous heating leads to a further increase of the current density within the central part, and an appearance of considerable inhomogeneity of a radial temperature distribu¬ tion. Thus, a maximum value of the surface temperature of the crystal rod which can be achieved is limited by the risk of heating of its central region up to the melting temperature of the semiconductor material and, therefore, breaking off of the crystal. As specifically indicated in U.S. Patent No. 4,426,408, by that reason the heating is successively continued up to the diameter of the rod being 50 mm only. Thereafter, the surface temperature of the crystal rod should be decreased. Hence, in order to produce the silicon polycrystal rod of 150-200 mm diameter employing the above described technology, the surface temperature should not exceed 80% of the silicon melting temperature. It is thus evident that such conventional approach to the problem of growing extended crystals leads to such limitations as diameter of the crystal rod and temperature of the process.

SUMMARY OF THE INVENTION

It is a major object of the present invention to overcome the above listed and other disadvantages of the conventional systems and provide a method and an apparatus for producing an extended crystal formed of a semiconductor material grown on an initial rod formed of a seed crystal by depositing on the surface thereof particles of the semiconductor material from a reaction gas containing the latter. - 3 -

It is thus provided according to one broad aspect of the present invention a method of producing an extended crystal formed of a semicon¬ ductor material by depositing on a surface of an initial rod formed of a seed crystal particles of the semiconductor material from a reaction gas containing the latter, the method comprising the steps of:

(a) initially heating the initial rod by supplying a conductive current for passing therethrough;

(b) measuring at least one parameter of the growing crystal for determining as to whether or not said at least one parameter is of a predetermined value thereof;

(c) upon detecting said predetermined value of said at least one parameter:

(i) disconnecting the supply of the conductive current; and

(ii) providing a further heating of the growing crystal of a substantially uniform temperature distribution within a whole volume of the growing crystal.

The further heating includes producing eddy currents within the growing crystal. The eddy currents are induced by an alternating magnetic field of a predetermined frequency f„ so as to provide a skin effect in the growing crystal.

The at least one measured parameter is, preferably, a radius R of the growing crystal. The disconnecting of the supply of the conductive current is effected at once the radius R of the growing crystal becomes of about 25 mm. Preferably, the measuring of the radius R is carried out by suitable sensor means, which may be a tenso-sensor for measuring a weight force P of the growing crystal. The frequency of the alternating magnetic field fa is previously determined according to the following relationship:

wherein:

Kp is a coefficient characterizing a mathematical connection between the wight force P and the radius R;

K_μ is a coefficient characterizing magnetic properties of the semicon- ductor material;

K_Δ is a skin-effect coefficient determined as a ratio Δ/R, wherein Δ is a depth of penetration of the alternating magnetic field into the growing crystal;

ØTT) is an electro-conductivity of the semiconductor material as a function of temperature T.

The method may further comprise the step of reducing the weight force P of the growing crystal. Preferably, this is effected by means of creating a travelling magnetic field so as to provide a movement of waves thereof in a direction opposite to the direction of the weight force P of the growing crystal. A frequency of the travelling magnetic field ft should be less than the frequency of the alternating magnetic field f_n.

According to another aspect of the present invention there is provided an apparatus for producing an extended crystal formed of a semiconductor material grown on an initial rod formed of a seed crystal, the apparatus comprising:

- a heat insulated vessel for mounting therein the extended crystal;

- a conductive current supply source for supplying the conductive current to pass through the crystal;

- sensor means for measuring at least one parameter of the crystal and generating data representative thereof;

- control means coupled to the sensor means for processing the data representative of said at least one measured parameter so as to deter¬ mine whether or nor said parameter has reached a predetermined value thereof; - an additional heat source selectively actuated by the control means for heating the crystal so as to provide a substantially uniform temperature distribution within a whole volume of the growing crystal Preferably, the apparatus further includes a compensation means for reducing the weight force P of the growing crystal

More specifically, the present invention is used for producing silicon polycrystals and is, therefore, described below with respect to this application

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how the same may be carried out in practice, a preferred embodiment will now be described, by way of example only, with reference to the accompanying drawings, in which Fig. 1 is a schematic cross-section view of an apparatus according to the invention,

Fig. 2 is a schematic cross-section of the apparatus of Fig 1 taken along line A-A, and

Figs. 3a-3b illustrate a flow diagram of the principal steps of operation of the apparatus of Fig 1

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to Fig 1, there is illustrated an apparatus, generally designated 1, comprising a cylindπcally-shaped vessel 4 which is heat- insulated The vessel 4 is closed by an upper cover 2 and a lower cover 9 at its opposite ends Two pairs of openings 2a and 9a are provided in the covers 2 and 9, respectively An extended rod 3 formed of a seed silicon crystal, typically of 5-10 mm in its diameter, is axially mounted inside the vessel 4 in a manner to be supported by its opposite ends in a pair of clamps 8a and 8b, which are freely movable relative to the vessel 4 along an axis thereof. An appropriate power source 11 is coupled to the rod 3 in a conventional manner through a power regulator 12 so as to supply a conductive current of the commercial frequency passing through the rod 3. The conductive current thus provides a heating of the rod 3. Two gaseous reagents, Si_nH_mCl_p and H₂, are injected into the vessel 4 through the openings 9a and, when passing through a clearance 6 provided between a surface of the rod 3 and an inner surface of the vessel 4, react in a manner to educe pure silicon Si, which, in turn, is continuously deposited on the heated rod 3. This results in growing of the crystal rod 3 into a crystal 3a of a larger diameter. Resulted reaction products are ejected from the vessel 4 through the openings 2a. All these components and their functional features are known per se and, therefore, need not be more specifically described, except to note that this is carried out at surface temperature of the grown crystal 3a of about 1100-1150°C corresponding to the diameter of the crystal about 50 mm.

One of the essential features of the present invention is provision of an inductor 7 coupled to a power source 19 of alternating currents. The alternating currents create an alternating magnetic field (AMF), which, in turn, induces eddy currents within the growing crystal 3a. The inductor 7 is configured like a spiral formed of a plurality of spaced turns 22, and mounted in a manner to encompass the vessel 4 along its length, which is better shown in Fig. 2.

Another essential feature of the present invention is provision of an inductor 5 which is coupled to a power source 14 for creating a travelling magnetic field (TMF) and is mounted in a manner to encompass the inductor 7 along its length and, thereby, the crystal 3a. The inductor 5 is similarly comprised of spaced turns 21 so as to form a multiphase winding which is placed in a multiradial magnetic circuit 20. The turns 21 and 22 of the inductors 5 and 7, respectively, are formed of electro-conductive and water- cooled tubes as self-explanatory shown in Fig. 1.

Further provided is an optic sensor 15 of a known type mounted for measuring a surface temperature of the growing crystal 3a. For the purpose, the sensor 15 has an imaging unit 16 which protrudes through the spaces between the turns 21 and 22 of the inductors 5 and 7 and is capable of observing the surface of the crystal 3a through a transparent window 4' made in the vessel 4. The sensor 15 is electrically connected to the power source 19 through a processing unit 17 and a temperature regulator 18.

As further illustrated in Fig. 1, the lower supporting clamp 8b is supported on a tenso-electric sensor 10. The sensor 10 may also be of any known type adapted for continuously measuring a weight force P by which the growing crystal 3a presses onto the sensor 10, and generating data representative thereof. By that reason the supporting clamps 8a and 8b are made vertically movable relative to the vessel 4, as specifically indicated above. Interconnected between the sensor 10 and the processing means 17 is a regulator 13, which is further coupled to the power source 14 of the inductor 5.

It should be understood that the AMF of a certain frequency fa induced by the inductor 7 causes heating of the growing crystal 3a and, therefore, leads to such an electro-conductivity of the silicon and such a radius of the growing crystal 3a that a skin effect appears. As known, the skin effect is characterized by the following factor:

κ_ - (l)

^A R

where Δ is a depth of penetration of a wave of the AMF into the crystal 3a, and R is a current value of the crystal's radius. For example, these factors

could be evaluated like K_ ≤ - . The depth of penetration Δ is, in turn,

defined as follows:

where K_μ is a coefficient characterizing magnetic properties of the crystal material, that is silicon Si in the present example; f_a is a frequency of alternation of the AMF, and σiT) is the electro-conductivity of the crystal material (Si) being a function of its temperature T.

At initial stage of operation of the apparatus 1, the crystal 3a is considered to be of relatively small diameters such as 5-10 mm to 50 mm. Therefore, the surface temperature T of the crystal 3a is kept to be the same within its volume. The radius R of the crystal 3a is calculated by the processing means 17 which are inputted by the data representative of the measured weight force P of the crystal 3a coming from the sensor 10. Thus,

P = π -R² -h -p -g (³)

where: π = 3,14159; h is a height of the crystal 3a; p is a mass density of the crystal; and g is the acceleration of gravity If K = ιτ -Λ - p -g then:

R² = - (4)

^K _P

Implementation ofthe appropriate mathematical conversions using the above formulas (1) to (4) enables to obtain a resulting dependence between the AMF frequency f_a, the crystal's weight force P and the electro- conductivity σ(T), as follows:

Turning now to Fig. 3, a flow diagram of the principal steps of operation of the apparatus 1, utilizing physical sense of the above mathe¬ matical dependence (5), is illustrated. In the process of growing the crystal 3a its weight force P is continuously measured by the tenso-sensor 10. When the weight force P reaches a predetermined value thereof correspond¬ ing to a certain value of the crystal's radius R, which is appropriately detected by the processor 17, the supply of the conductive current is discontinued, namely the power source 11 is switched off. Then, the PCML97/00112

- 9 -

inductor 7 is actuated. The inductor 7 creates the AMF of such a frequency f_a that the clearly expressed skin effect takes place. In other words, heating sources of the crystal 3a are thus moved from its central region to the surface region thereof. The frequency f_a is found to be of 10kHz and higher. It should be noted that in order to realize the method of the present invention there is no need to resort to a high frequency heating, that decreases an expenditure level.

Thus, the surface temperature T of the crystal 3a increases from

Ti = 0.8T,πett to Ti « 0.95Tmeit. This results in significant increase of deposition rate, which can be approximately determined by the known

K{T_) Arrenius Law as a ratio of rate constants of the reaction , where:

wherein A(T) « const; R_g = 8.134 Joul/(Mol*K) is gas constant per mol; E is an energy of activation. Assuming £ = (45 -- 115) ^• 10³ Joul/Mol, as is known from the prior art, for example "Chemical Encyclopedia", Moscow, 1983, and considering T2 « 1650 K; 7, =0.8 ^■T_me_ = 136QK, it is found that:

It becomes clear from the estimation (7) that the rate of silicon particles deposition is increased by no less than 2, while the surface temperature T of the crystal 3a increases from 1360K to 1650K, or 1087°C to 1377°C.

It will be readily understood that the larger the diameter of the crystal 3a, the more the weight force P. Therefore, at this stage of higher temperatures T and higher weight force P, a potential danger of breaking off of the crystal 3a appears. Thus, when the diameter of the growing crystal 3a reaches its critical value, which is 150-200 mm as specifically indicated above, the inductor 5 is actuated. Obviously, the inductor 5 may be actuated earlier, simultaneously with the actuation of the inductor 7. Further, the weight force P and the temperature T are concurrently measured by the tenso-sensor 10 and the optic sensor 15, respectively. This process enables to produce crystals of 300 mm and more in diameter.

Returning back to Fig. 1, there is shown that the inductor 5, when actuated, creates an electromagnetic force F directed against the crystal's weight force P. Direction of movement of the TMF waves coincides with the direction of the force F, while its wave length λ equals to double distance, τ, between those turns 21 of the inductor 5 which contain conductors of the same phase and in which currents are of opposite directions. Thus, the wave length λ is determined as follows: λ =2-τ (8)

It will be readily understood that since the inductor 5 and the crystal 3a form together an unclosed electromagnetic system which elements, the inductor and the crystal, are of limited lengths, undesirable end effects appear influencing on the creation of the electromagnetic force. One of the most effective and simplest ways to reduce such 'negative' influence consists in choosing such a relation between the wavelength λ and the crystal's height h, wherein:

h ≥i - λ (9)

In contrast to the AMF which acts on the surface region of the growing crystal 3a, the TMF should, practically, act on the whole volume of the crystal 3a. If a frequency f, of the TMF is appropriately previously chosen, or changed during the process when the depth of penetration of TMF into the crystal 3a approaches to the radius R of the crystal 3a, the above formula (2) allows to conclude the following:

Considering the electro-conductivity of the silicon heated to near-melting temperature, and the penetration depth of the electromagnetic wave be equal to the crystal's radius, the frequency of the TMF is found to be of about 2-5 kHz.

It is appreciated that such a location of the inductor 7 to be between the inductor 5 and the crystal 3a does not provide an obstacle for penetration of the TMF into the crystal 3a. Indeed, a total value of an electromotive force created by the TMF in all the turns 22 of the inductor 7 is equal to zero at any given time. Additionally, there is no considerable influence of the AMF on to the TMF due to the fact that the inductor 7 functionally acts like a long solenoid. A magnetic field of such solenoid is concentrated inside thereof, whilst being insignificantly small outside thereof, since lines of an outside magnetic flux are very far-between.

It should also be noted that in the process of heating the crystal 3a in the manner described above, namely providing such frequencies fa and ft of the AMF and TMF, respectively, a contribution of the TMF is negligibly small in comparison to that of the AMF. Therefore, the TMF has practically no influence on the radial temperature distribution within the crystal 3a.

Hence, at any given time, there exists such a set of the surface temperature value and the radius of the growing crystal 3a {T; R} defined by the above process of growing crystal 3a, that the process of deposition proceeds, on the one hand, at a maximum rate thereof and, on the other hand, without damaging a mechanical stability of the crystal 3a. Further¬ more, if a deviation of either the surface temperature, or the radius occurs, values of the frequency f, and the magnetic induction of the AMF are regulated in such a manner as to decrease the possible deviation up to zero. For the purpose, the surface temperature T is measured by the optic sensor 15 and the data representative thereof is transmitted into the processing unit 17 as described above. The sensor 10, in turn, measures the difference between the weight force P and the electromagnetic force F created by the TMF, as follows:

where F_m =K_c -P is measured by the sensor 10, K_c being a desirable degree of a weight compensation force.

The degree of compensation K_c is variable from 0 to 1. Thus, when K_c = 0, the weight force P is full compensated, whilst being not compensated at all when K_c = 1. At the initial stage of operation of the apparatus 1, the inductor 5 is in its non-operative mode, and, therefore, F = 0; K,. = 1, and the sensor 10 thus actually measures the real weight of the growing crystal 3a.

Using the well known mathematical method of par-axial approximation, the following dependence is obtained:

F = K_z f_l -σ .T) -R* 'l² (12)

where K_z is a coefficient which value depends on parameters of the magnetic circuit 20 and is, therefore, constant during the operation of the apparatus 1; I is a current in the inductor 5. Considering the above formulas (4), (11) and (12), the following balance of forces is obtained:

K 'f -c(T) l² 'R -K -R² +F =0 (13)

It will be readily understood that by measuring such values as f„ T, Fm and I in the manner described above and computing the formula (13) by the processing unit 17, the current value of the crystal's radius R is provided. Thus, the values of I, T and R are determined and transmitted to the power source 19 of the TMF through the regulator 18, which provides appropriate changes of frequency and/or current in the inductor 7. As a result, respective parameters of the AMF are changed accordingly, which, in turn, enables to obtain the required set {T; R}. The TMF parameters are changed in accordance with the deviation of the product K_c -P from the given one, by means of the sensor 10, the processing unit 17, the regulator 13, the power source 14 and the inductor 5.

Those skilled in the art will readily appreciate that various modifications may be applied to the preferred embodiment as described above without departing from its scope defined by the appended claims. For example, the optic sensor 15 may be replaced by another kind of sensing means adapted for measuring the surface temperature of the crystal. Additionally, the apparatus 1 may be easily modified to be employed for producing a silicon monocrystal from the polycrystal thereof due to the provision of the TMF inductor 5 reducing the weight force of the crystal in a non-contact manner and, thereby, eliminating one of the major problems of conventional systems of the kind associated with undesirable increase of crystal's weight force.

Claims

WHAT IS CLAIMED IS:

1. A method of producing an extended crystal formed of a semiconductor material by depositing on a surface of an initial rod formed of a seed crystal particles of the semiconductor material from a reaction gas containing the latter, the method comprising the steps of

(a) initially heating the initial rod by supplying a conductive current passing therethrough;

(c) upon detecting said predetermined value of said at least one parameter:

(i) disconnecting the supply of the conductive current; and (ii) providing further heating of the growing crystal of a substan- tially uniform temperature distribution within a whole volume of the growing crystal.

2. The method according to Claim 1, wherein said further heating includes producing eddy currents within the growing crystal, the eddy currents being induced by an alternating magnetic field of a predetermined frequency fa so as to provide a skin effect in the growing crystal.

3. The method according to Claim 1, wherein said at least one measured parameter is a radius R of the growing crystal.

4. The method according to Claim 3, wherein said predetermined value of the radius R is about 25 mm.

5. The method according to Claim 1, wherein said at least one measured parameter is a surface temperature T of the growing crystal.

6. The method according to Claim 3, wherein said measuring of the radius R is effected by a sensor means.

7. The method according to Claim 6, wherein said sensing means is adapted for measuring a weight force P of the growing crystal.

8. The method according to Claim 5, wherein the temperature is measured by means of an optic sensor.

9. The method according to Claim 1, wherein said predetermined value of the frequency fa of the alternating magnetic field is determined according to the following relationship:

wherein:

K_p is a coefficient characterizing a mathematical connection between the weight force P and the radius R; K_μ is a coefficient characterizing magnetic properties of the semicon¬ ductor material;

K_Δ is a skin-effect coefficient determined as a ratio Δ/R, wherein Δ is a depth of penetration of the alternating magnetic field into the growing crystal; σ(T) is an electro-conductivity of the semiconductor material as a function of the temperature T.

10. The method according to Claim 2, wherein said further heating is effected with the frequency f_a of a constant value.

11. The method according to Claim 1, further comprising the step of: (d) reducing the weight force P of the growing crystal.

12. The method according to Claim 11, wherein said reducing includes creating a travelling magnetic field in a manner to provide a movement of waves thereof in a direction opposite to a direction of the weight force P of the growing crystal.

13. The method according to Claim 12, wherein said travelling magnetic field is characterized by at least two waves thereof.

14. The method according to Claim 12, wherein a frequency of the travelling magnetic field ft is less the frequency of the alternating magnetic field fa.

15. An apparatus for producing an extended crystal formed of a semiconductor material grown on an initial rod formed of a seed crystal, the apparatus comprising:

- a heat insulated vessel for mounting therein the extended crystal; - a conductive current supply source for supplying the conductive current to pass through the crystal;

- control means coupled to the sensor means for processing the data representative of said at least one measured parameter so as to deter¬ mine whether or not said parameter has reached a predetermined value thereof;

- an additional heating source selectively actuated by the control means for heating the crystal so as to provide a substantially uniform temperature distribution within a whole volume of the growing crystal.

16. The apparatus according to Claim 15, wherein said additional heating source comprises an alternating magnetic field generator for inducing eddy currents within the growing crystal, said alternating magnetic field being of a predetermined frequency f_n so as to provide a skin effect in the growing crystal.

17. The apparatus according to Claim 15, wherein said at least one measured parameter is a radius R of the crystal.

18. The apparatus according to Claim 17, wherein said predetermined value of the radius R is about 25 mm.

19. The apparatus according to Claim 15, wherein said at least one measured parameter is a surface temperature T of the crystal.

20. The apparatus according to Claim 15, wherein said sensor means includes a tenso-sensor for measuring a weight force P of the crystal.

21. The apparatus according to Claim 19, wherein said sensor means comprises an optic sensor having an imaging means.

22. The apparatus according to Claim 16, wherein said regulating means are adapted for estimating the value of fa according to the following relationship:

wherein:

K_p is a coefficient characterizing a mathematical connection between the weight force P and the radius R;

K_μ is a coefficient characterizing magnetic properties of the semicon¬ ductor material; K_Δ is a skin-effect coefficient determined as a ratio Δ/R, wherein Δ is a depth of penetration of the alternating magnetic field into the growing crystal; σ(T) is an electro-conductivity of the semiconductor material as a function of the temperature T.

23. The apparatus according to Claim 16, wherein said further heating is effected with the frequency f_a of a constant value.

24. The apparatus according to Claim 15, further comprising:

- compensation means for reducing the weight force P of the crystal.

25. The apparatus according to Claim 24, wherein said compensation means comprises a power source for creating a travelling magnetic field which waves moves in a direction opposite to a direction of the weight force P of the crystal.

26. The apparatus according to Claim 25, wherein said travelling magnetic field is characterized by at least two waves thereof.

27. The apparatus according to Claim 24, wherein a frequency of the travelling magnetic field ft is less than the frequency of the alternating magnetic field fa.

28. The apparatus according to Claim 15, wherein said extended semiconductor crystal is a polycrystal produced by deposition on a surface thereof of particles of the semiconductor from a gaseous phase.

29. The apparatus according to Claim 24, wherein said extended semiconductor crystal is a monocrystal produced from a polycrystal thereof.

FIG.l