Abstract:
There is provided a heating element and an arrangement of heating elements, respectively, for heating crucibles, in particular for LEC devices for growing semiconductor single crystals, with a tulip-shaped bottom heater ( 20 ) being built such that the heater legs of the main heater ( 40 ) positioned thereabove can barely be guided towards the bottom. Such arrangement of bottom and main heater enables multi-heater arrangements without having to interfere with a lateral insulation ( 18 ) which, thus, need not be cut out or pierced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heating element for heating crucibles as disclosed in claims  1 ,  7  and  17  as well as to an arrangement of heating elements for heating crucibles as disclosed in claim  13 . In particular, these heating elements can be used for heating a crucible when growing semiconductor crystals such as silicon, germanium and gallium arsenide. 
     2. Prior Art 
     A known device for growing semiconductor crystals using the Liquid Encapsulated Czochralski (LEC) process is schematically shown in FIG.  7 . Such device is known for example from Ishiki, M., Recent Development of Bulk Crystal Growth, Research Signpost 1998, pages 63-64 (ISBN: 81-86481-58-3). A crucible  1  for receiving a molten mass  2  of semiconducting material is arranged on a crucible holder  3  within a cooled recipient  4 . The molten semiconductor  2  is covered with a molten mass of boron oxide  12  to prevent volatile components (e.g. As in GaAs melts) from escaping by evaporation. To that end, the steam pressure in the recipient  4  has to be greater than the steam pressure of the volatile component (e.g. &gt;2 bars in the case of GaAs). For heating the crucible  1 , there is provided an arrangement of heating elements consisting of a lower heating element  100  surrounding the crucible  1  at its bottom and in a wall region adjacent to the bottom, and an upper heating element  200  provided above the lower heating element  100  and surrounding the upper region of the crucible wall. The lower heating element  100  is connected via power leads  110  to electrodes  120  provided at the bottom of the recipient and the upper heating element  200  is connected via power leads  210  to electrodes  220  which are also provided at the bottom of the recipient. A crystal nucleus  5  is connected to a pulling device  6  fed through the cover of the recipient  4  and arranged above the crucible  1 . Also, a growing crystal  7  being pulled from the moltem mass  2  is connected to the pulling device  6  via said crystal nucleus  5 . A drive is provided for rotating the crucible holder  3  and the crystal nucleus  5  in opposite directions during the growing process. The arrangement of heating elements  100 ,  200  is thermally insulated by an annular insulating pipe  8  surrounding the heating elements and consisting of a thermally insulating material. Only a small gap is provided between the insulating pipe  8  and the heating elements  100 ,  200  in order to prevent excessive gas convection which, due to the high pressure within the recipient, may result in large temperature fluctuations in particular with semiconductor materials having a highly volatile component such as gallium arsenide or indium phosphide. 
     U.S. Pat. No. 4,533,822 discloses a tulip-shaped heating element used as a bottom heater, which heating element is formed as a jacket-type hollow body having an upper edge and a lower edge, said hollow body comprising a first hollow cylindrical portion having a first cross section and a second portion adjacent thereto and having a tapering cross section, for surrounding a bottom region of a crucible. 
     DE 44 23 196 A1 discloses a hollow cylindrical heating element  200  for heating crucibles, having an upper edge and a lower edge, and comprising at least one power lead extending from said lower edge as an extension of the cylinder wall of said hollow body. 
     However, a combination of the known tulip-shaped heating element  100  and the hollow cylindrical heating element  200  in the vertical arrangement shown in FIG. 7 requires an interruption of the tubular thermal insulation with the insulating pipe  8  at least at two openings  9 ,  10  for feeding through the power leads  210  to the upper heating element  200 . Through these openings  9 ,  10 , a cold gas may flow in from the space surrounded by the cooled recipient  4 , thus affecting crystal growth. 
     The vertical arrangement of two heating elements as shown in FIG. 7 enables variable adjustment of the axial temperature gradient within the inner space. However, an abrupt thermal transition at the gap  11  between the lower heating element  100  and the upper heating element  200  causes a disturbance of the axial temperature distribution in this region as well as unfavorable mechanical stresses in the growing single crystal, reducing the yield of single crystal material. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a heating element and an arrangement of heating elements for heating a crucible, in particular for growing semiconductor single crystals, preventing disturbances in the desired temperature distribution in the region of the molten material and the growing crystal and improving its yield. 
     This object is achieved by a heating element according to claims  1 ,  7  or  17  and by an arrangement of heating elements according to claim  13 . Further improvements of the invention are presented in the dependent claims. 
     The invention will be better understood from the following description and figures of preferred embodiments which are presented for illustrative purposes only and are not to be construed as limiting the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a device according to the invention for growing semiconductor crystals. 
     FIG. 2 a  is a cross sectional view of a first embodiment of a heating element according to the invention. 
     FIG. 2 b  is a top view of the heating element of FIG. 2 a.    
     FIG. 3 a  is a cross sectional view of a second embodiment of a heating element according to the invention. 
     FIG. 3 b  is a top view of the heating element of FIG. 3 a.    
     FIG. 4 a  is a cross sectional view of the arrangement of heating elements according to the invention. 
     FIG. 4 b  is a section along A—A of FIG. 4 a  across the arrangement of heating elements according to the invention. 
     FIG. 5 is a part of a section across a further arrangement of heating elements according to the invention. 
     FIG. 6 a  is an enlarged cutout illustration of a meander of a further embodiment of a heating element according to the invention. 
     FIG. 6 b  is a schematical cutout view of an arrangement of two heating elements according to the invention. 
     FIG. 6 c  is a schematical cutout illustration of a further embodiment of the arrangement of two heating elements according to the invention. 
     FIG. 7 is a schematical illustration of a known device for growing semiconductor crystals. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A first heating element  20  according to a first embodiment of the invention is shown in FIGS. 2 a  and  2   b  and  1 . The heating element  20  is formed as a jacket-type hollow body having an upper edge  21  and a lower edge  22 . The jacket-type hollow body comprises a first hollow cylindrical portion  23  with its inner diameter dimensioned such that the crucible holder  3  with the crucible  1  can be received in it. Adjacent to the first portion  23 , a second portion  24  extends with the cross section of its heating element conically tapering and which ends in an adjacent third portion  25  having a smaller cross section than the first section  23 . Thus, the heating element  20  is formed as a tulip-shape. Slits  26  starting from the upper edge  21  and spaced at regular circumferential intervals extend coaxially to the center axis M of the heating element across the first portion  23  and all the way into the second portion  24 . Slits  26 ′ in staggered relationship thereto and starting from the lower edge  22  extend coaxially with respect to the center axis M through the third portion  25  and the second portion  24  all the way into the first portion  23 . The slits  26 ,  26 ′ are equally spaced, separating the hollow body into individual meander-shaped segments  27  connected to each other. Each meander-shaped segment  27  is separated into a left-hand side and a right-hand side half portion  27   a,    27   b  of the meander, which half portions are connected to each other by a meander turning bend  27   c.  In the region of the lower edge  22 , two power leads  30 ,  30 ′ are provided through which the heating element  20  is connected to electrodes  31 ,  31 ′ provided at the recipient bottom. Two meander-shaped segments  28 ,  28 ′ diametrically opposite to each other are shortened in such a manner that they extend only from the third portion  25  all the way to the border line between the second portion  24  and the first portion  23 . Thus, two cutouts  29  and  29 ′ or gaps opposite to each other and having a width equal to a meander-shaped segment are formed in the hollow cylindrical portion  23 , in which cutouts or gaps power leads for a second heating element described below are arranged. 
     FIGS. 3 a  and  3   b  show a second heating element according to a first embodiment. The heating element  40  is formed as a cylindrical hollow body having an upper edge  41  and a lower edge  42 . The cross section of the heating element  40  corresponds to the cross section of the hollow cylindrical first portion  23  of the first heating element  20 . At equal intervals, in alternating relationship and starting from the upper edge  41  and the lower edge  42 , respectively, coaxially arranged slits  43  and  43 ′, respectively, extend all the way to a predefined distance from the lower edge  41  and the upper edge  42 , respectively. The slits  43 ,  43 ′ separate the hollow body into meander-shaped segments  47 . At two locations circumferentially and diametrically opposite to each other, each of a pair of meander-shaped segments  48 ,  48 ′ is connected to a power lead  50 ,  50 ′ having a width corresponding to the width of the meander-shaped segment  48 ,  48 ′ and extending from the lower edge  42  as an extension of the meander-shaped segment  48 ,  48 ′ along a length such that, when arranged in the crystal growing installation, the heating element  40  with the power leads  50 ,  50 ′ engages the gaps  29 ,  29 ′ of the heating element  20  and can be connected to the electrodes  51 ,  51 ′ of an electrical power source, arranged at the recipient bottom. Preferably, the power lead  50 ,  50 ′ is formed as an extension of the meander-shaped segment  48 ,  48 ′. 
     Both the heating element  20  and the heating element  40  are preferably made from graphite. 
     Referring to FIGS. 4 a  and  4   b,  the arrangement of heating elements according to the invention consists of a combination of the first heating element  20  and the second heating element  40 . In operation, heating element  40  is arranged above heating element  20  such that the power leads  50 ,  50 ′ of the second heating element  40  almost exactly engage the cutouts  29 ,  29 ′ of the first heating element  20 . The length of the power leads of the second heating element has been chosen such that, when arranged on top of each other, both heating elements may be connected to the electrodes  51 ,  51 ′ of an electrical power source, with each of the electrodes being provided at the recipient bottom. The power leads  50 ,  50 ′ have a width, thickness and resistivity dimensioned such that during operation, their heating power roughly corresponds to the heating power of an adjacent meander-shaped segment  27  of the first heating element  20 . 
     Referring to a further embodiment of the heating elements illustrated in FIGS. 5 and 6 a  through  6   c,  the upper edge  21  of the first, lower heating element  20  and the lower edge  42  of the upper, second heating element  40  are serrated. Each meander turning bend  27   c  and  47   c,  respectively, is formed as a triangle and the meander-shaped segments  27  of the lower heating element  20  are arranged with respect to the meander-shaped segments  47  of the upper heating element  40  in a position turned by an azimutal angle corresponding to a meander-shaped segment, such that the heating elements  20  and  40 , when arranged on top of each other, are in serrated or meshing engagement. 
     The geometrical parameters determining the depth of serration V of a triangular serration are illustrated in FIGS. 6 a  through  6   c,  where a represents a truncation of the triangle tip; b is the meander spacing, i.e. the width of the above slits  26 ,  26 ′ and  43 ,  43 ′, respectively; c corresponds to the width of one half  27   a,    27   b,    47   a,    47   b  of a meander-shaped segment and half the triangle base, respectively; e is the distance between the meander turning point and the triangle base; and d is the height of the shape-determining triangle. In the case where the smallest distance of the heaters  20 ,  40  among each other equals the meander spacing b and the shape-determining geometry is a triangle, the depth of serration V is given by 
     
       
           V=d− ( b/c )·{square root over ( c   2   -d   2 +L )}− ad/ 2 c    
       
     
     Preferably, 
     
       
         
           c=e+d  
         
       
     
     in order to guarantee that power dissipation throughout the heating region is as homogeneous as possible. 
     All the edges that are created are rounded to reduce breakability. 
     The form of the serrated edges according to the invention is not limited to the heating elements  20  and  40  with the power lead of the upper heating element  40  engaging a gap of the lower heating element  20 . Even regular heating elements where a power lead for the upper heater is passed through the insulating pipe, may be formed with such serration. Also, the serration may be provided at the upper and at the lower edge of each heating element. 
     The arrangement of the heating elements  20  and  40  according to the invention for growing a semiconductor single-crystal using the LEC method is shown in FIG.  1 . The apparatus corresponds to the one described with regard to FIG. 7, and identical elements are given the same reference numerals and their description will not be repeated. Unlike the apparatus according to FIG. 7, the apparatus according to FIG. 1 comprises the tulip-shaped first heating element  20  as a bottom heater having its power leads  30 ,  30 ′ connected to electrodes  31 ,  31 ′ arranged at the bottom of the recipient. Above the first heating element  20 , the second heating element  40  coaxially aligned thereto is arranged and engages the cutouts  29 ,  29 ′ of the lower heating element  20 . The power leads  50 ,  50 ′ are connected to electrodes  51 ,  51 ′ provided at the bottom of recipient  4 . Thus, it is not necessary to provide openings in the insulation pipe  18  for feeding the power leads to the second heating element  40 , and the insulation pipe  18  is formed with a closed wall. Since no parts protrude towards the outside from the second heating element  40 , the insulation pipe  18  may be arranged with a minimum distance to the heating element. As a result, heat losses are minimized and parasitic gas convection as well as temperature variations associated to it in the region of the crucible  1  can almost be ruled out. 
     The apparatus according to FIG. 1 further comprises a control for independently controlling the heating elements  20 ,  40 . The control is formed such that the heating power of the above heating element in the region of the power leads  50 ,  50 ′ corresponds to the heating power of an adjacent meander-shaped segment  27  of the heating element  20  located thereunder. 
     During the operation of the apparatus according to FIG. 1, a molten mass  2  of the semiconductor material is first produced in the crucible, using the heating elements  20 ,  40 . The crystal nucleus  5  is submerged into the molten mass  2  and the the formation of the single crystal starts on it. During the process, the crystal nucleus  5  is rotated by the pulling device  6  while the crucible holder  3  together with the crucible  1  and the molten mass  2  contained in it are rotated in the opposite direction of the crystal nucleus. At the beginning of the single-crystal growth the crystal nucleus submerged into the molten mass is kept as cold as possible to prevent volatile components from evaporating and destroying the nucleus, while at a later stage of the growth the crystal piece which has already been grown is kept as warm as possible to minimize thermal stresses. The temperatures required therefor are adjusted primarily by the upper region of the heating element arrangement, i.e. by the heating element  40 . In the region of the molten mass, the temperature is kept very stable using the lower portion of the heating element arrangement, i.e. by adjusting the heating element  20 . Then, the slowly growing crystal  7  is being pulled out in the known fashion using the pulling device  6 . As the region of the power leads  50 ,  50 ′ of the upper heater  40  also acts as a heating region in the lower heater  20 , a uniform azimutal temperature distribution will be maintained by appropriate dimensioning. In the vertical direction at the transition between the two heating elements  20  and  40 , the mutual serration of the two heating elements enables a continuous thermal junction/transition. Due to the meshingly engaging meander-shaped segments, a local azimutal heat exchange is achieved, thus transforming the axial inhomogeneities of the temperature field in the conventional arrangement into azimutal inhomogeneities according to the invention. These, however, are largely balanced by the crucible rotation. 
     The invention has the advantage that the assembly of the heaters is simple and safe, that heaters with a smallest possible maximum diameter and space-saving lateral insulation pipes may be used, thus making good use of the available space, and that minimal convection spaces exist in the heating region. Especially with very large oven arrangements suitable for receiving crucibles which are 28 cm and above in diameter, the invention results in reduced material costs with regard to the graphite parts for the heating elements as well as more safety during the process.