US 3381114 A
Description (OCR text may contain errors)
April 30, 1968 sHo NAKANUMAl 3,381,114
DEVICE FOR MANUFACTURING EPITAXIAL CRYSTALS Filed Dec. 18, 1964 2 Sheets-Sheet l 7 25 fwm EEE- 1.. /E 1 (3M-fa Page er V 'W J a @wm 4g' L45/ 4f/I7l V/IVAV//l i//AVAVAV/j V/l- TIES.
April 30, 1968 sHo NAKANUMA 3,381,114
DEVICE FOR MANUFACTURING EPITAXIAL CRYSTALS Filed Dec. 18, 1954 2 sheets-sheet 2 3,381,114 Patented Apr. 30, 1968 ABSTRACT F THE DHSCLOSURE This invention teaches an apparatus for use in producing crystals preferably of the epitaxial type which enables the mas-s production of such crystals having uniform quality and uniform operating characteristics. The apparatus is comprised of a housing having first and second halves which have cylindrical mating sides to form a substantially cylindrical-shape reaction chamber therein. A first half of the chamber is provided with inlet ports arranged in symmetrical fashion for the introduction of gaseous material in an extremely uniform fashion within the chamber, which gaseous materials are employed in the epitaxial growth process. The remaining half of the housing is provided with outlet ports likewise arranged in a symmetrical fashion for removing exhausting gases from the chamber. A gas trap is provided between the outlet ports and the exterior of the housing.
A novel heating element is provided within the reaction chamber and is formed of a plurality of spiral shaped heating sections spiralling outwardly from a central point of the heating element. Each of the sections are substantially identical in configuration and have substantially continually decreasing cross-sections from the center of each section outward to assure uniform heat level within the reaction chamber during the growth process. Preferably three sections form the heating element with the outward ends thereof being coupled to suitable connections of a three-phase power system. The center points of each of the spiral sections are electrically joined at the center of the heating element. While a delta-type three-phase system may be employed, it is likewise advantageous to utilize a Y-type three-phase power system having its center point grounded and electrically connected to the center point of the heating element. The heating element has a first surface which is substantially flat for the purpose of positioning and supporting crystal substrates used in the growth process. The symmetrical aspect of the housing and its reaction chamber assures the production of crystals having uniform operating characteristics.
The instant invention relates to crystal manufacture and more particularly to apparatus for producing crystals preferably of the epitaxial type which apparatus permits the mass production of epitaxial wafers all being of uniform quality and having uniform operating characteristics.
The widespread use of devices of the semiconductor type such as, for example, transistors, diodes, rectitiers and the like, have placed extremely large demands for epitaxial type wafers, which demands have become so great that the epitaxial crystals available must be produced in large quantities and even more importantly must have superior operating characteristics, which characteristics are uniform among the devices produced. While great emphasis has been placed upon the production of epitaxial crystals through mass production techniques, and while it has been quite practical to manufacture epitaxial crystals in accordance with conventional techniques, an extreme amount of difculties have been experienced in those at- 4tempts to manufacture epitaxial wafers in large quantities wherein the wafers produced have extremely uniform quality.
The instant invention provides a novel apparatus and method for producing epitaxial crystals of high uniform quality through mass production techniques by providing a chamber of unique designs in which such crystals are formed.
The instant invention is comprised of a substantially metal housing which defines a chamber therein for receiving a quartz disc and a plurality of wafer-like crystal substrates for the epitaxial growth to take place. The chamber is a substantially circular or symmetric container, having symmetrically located inlets communicating with associated nozzles through manifolds and capillary tubes for the purpose of introducing the necessary gaseous materials employed during the growth process. A heater element is positioned beneath the quartz disc and is so designed as to provide constant heat in a uniform manner within the entire chamber. The heating element is preferably formed of a suitable carbon material physically arranged so as to define three substantially concentric spirals all of which lie substantially in a plane and which are energized by a three-phase power source, The thickness of the heater assembly, in its crosssection, resembles a convex lens structure and is so designed as to generate extremely uniform heat throughout the entire chamber region. The physical configuration of the heater assembly, coupled with the fact that it is powered by a three-phase source, provides an extremely efficient heating apparatus in which the temperature throughout the entire chamber is substantially constant. The nozzles through which the gaseous material is introduced into the chamber are also arranged in a symmetrical manner so as to be evenly distributed within the chamber, thereby cooperating with the heater assembly to yield epitaxially grown crystals, all of which have substantially identical characteristics.
The heater element may preferably be designed by first setting out an equilateral triangle and locating its center of gravity. The sides of the triangle are then extended outwardly so as to effectively extend as radii from the center of gravity point. A circular arc may then be drawn from one vertex of the triangle so as to circumscribe approximately one-third of a circumference so as to intersect at the next extended side of a triangle which lies approximately away from the firs-t radial line. Each succeeding arc of a third of a circle may be drawn in a like manner until the complete spiral is drawn. The convex shape of the spiral heating element is established by increasing the crosssectional area towards the center of the spiral relative to the cross-sectional area near the periphery of the spiral element so that the entire heater element generates even heat over the entire surface of the heater element. The use of a heater element energized by a three-phase power source is advantageous since the load presented to the power source is more uniform and hence more efiicient.
By providing symmetrical disposition for the outlets which introduce the gaseous mixtures into the chamber, a very uniform feeding of the gaseous mixtures results, thereby ultimately resulting in the production of epitaxially grown crystals having extremely uniform characteristics.
It is therefore one object of the instant invention to provide novel means for producing epitaxially grown crystals in large quantities wherein the crystals so grown have extremely uniform characteristics.
A further object of the instant invention i's to provide apparatus for epitaxially growing a large number of crystals, which apparatus employs a spiral heater element energized by a three-phase power source to provide extremely uniform heating within the chamber in which the crystals are grown.
Another object of the instant invention is to provide apparatus for epitaxially growing a large number of crystals, which apparatus employs a spiral heater element energized by a threeaphase power source to provide extremely uniform heating within the chamber in which the crystals are grown wherein the heater element has a convex configuration in order to provide uniform heating over the entire surface of the heater element.
Still another object of the instant invention is to provide apparatus for epitaxially growing a large number of crystals, which apparatus employs .a spiral heater element energized by a three-phase power source to provide extremely uniform heating within the chamber in which the crystals are grown, wherein the spiral heater element is symmetrical about its central axis in order to provide extremely uniform heat for the epitaxial growth process.
Still yanother lobject of the instant invention is to provide novel apparatus for epitaxially growing crystals comprised of a chamber having gas inlet ports arranged symmetrically about the chamber to provide uniform ow of gases into the chamber in order to yield epitaxially grown crystals having extremely uniform characteristics.
These and other objects will become apparent when reading the accompanying description and drawings in which;
FIGURE 1 is a cross-sectional view of an apparatus employed for the purpose of epitaxially growing crystals in mass quantities and which is designed in accordance with the principles of the instant invention.
FIGURE 2a shows the top view of the heater element employed in the apparatus of FIGURE 1.
FIGURE 2b is a cro'ss-section of the hea'ter element Iof FIGURE 2a taken along the line A-A.
FIGURE 3a is a top View of a heater element known to the prior art.
FIGURE 3b is a cross-section of the heater element of FIGURE 3a taken along the line B-B'.
Referring now to the drawings, FIGURE 1 shows apparatus, 10, employed for the purpose of ep'itaxially growing crystals and which is designed in accordance with the principles 'of the instant invention. The apparatus 16 is comprised of a metallic-acidJproof housing, generally ycomprised of an upper half, 11, and a lower h'alf, 12. The two-housing portions are suitably fastened together such as shown at 12a and 12b, around the periphery of the housing, but cannot be clearly seen from FIGURE 1. It should be understood that the housing comprised of upper and lower halves 1-f1 and 12, respectively, when viewed from a top view would have a generally circular configuration.
Spaced slightly inward from the periphery of the upper and lower halves 11 and 12 is a suitable gasket 12e, seated within a groove 12d in lower half 12 in order to provide an air-tight chamber 12e, which is dened by the housing upper and lower portions 1-1 and 12.
The lower housing portion 12 provides a marginal ledge 13 which should be understood to be substantially circular, upon which ledge is supported a quartz disc 14. The lower housing portion 12 is further provided with suitable openings (only two of which are shown) 114 and 15, for receiving the electrical terminals 16 and 17 for providing electrical connections between the heater element power source and the heater element, to be more fully de'scribed. Since a three-phase power source is used for energization of the heater element, it should be understood that three such openings of the type of openings 14 'and 15 should be provided. Each terminal 16 and 17 is insulated from the housing lower portion 12 by the insulating support means 19 and 20, respectively. Each support means is provided with suitable resilient O-ring structures 21 and 22, respectively, for hermetically sealing the interior of the housing from the inliuence of the exterior region surrounding the housing.
The heater element 23 is physically secured to and supported by the electrical terminals 16, 17 and 18 (it being considered that the terminal 1S lies'immediately behind the terminal 17) so as to lie immediately beneath the quartz disc 14. The center of the heater element 23 is supported by a metallic support member 2'4, which electrically connects the center of the heater element to the housing lower portion 12. In addition to electrically connecting the center of the heater element to the housing lower portion 12, member 24 further provides support for the heater element so as t-o prevent any sagging of the heater element, thereby keeping its .spacing between its upper surface and the lower surface of the quartz disc 14 relatively constant.
The heater element 2-3 is energized by a suitable threephase power source 25, which may, for example, be coupled to the heater element through 'a transformer means 26 having its secondary or output terminals 26a connected to the terminals 16, 17 and 18, respectively, and -having its terminal at ground potential 2611, electric'ally connected to the housing lower portion 12 which, in turn, is connected to the center o-f heater element 2-3 through metallic support member 24. While the use of a Y-type connection is suggested herein, it should be noted that a three-phase delta connection may be used, if desired. The heater element 23, when so energized, provides a suitable level of heat within .the chamber 12 with the heater element preferably being formed of carbon. In .the case where a delta three-phase connection is employed, thus making it unnecessary to ground the lcenter point of the heater element, support 24 may be formed of a suitable insulating material to prevent the eater element from sagging in the center thereo-f with the insulator material being 'su-ch as to be insensitive to the heat generated by the heater element.
'Ihe quartz ldisc 14, which is supported by the ledge 13 of lower housing portion 12, in turn supports a plurality of single crystal substrates 27 which are arranged in a concentric manner upon quartz disc 14. In order that the temperature within chamber -12 be clearly determined and regulated, the housing upper portion 11 is provided with a .suitable opening 2S which, in turn, is hermetically sealed by a quartz window 29 which is centrally disposed relative to the housing upper portion. The temperature within chamber 12 is detected by means of an optical pyrometer 30, the output of which yis taken across its output terminals 30a and is impressed upon vthe control input terminal of the heater element power source 25, for the purpose of automatically controlling the temperature by virtue of controlling the heater current injected into the heater element 2:3.
In order to epitaxially grow crystals within the chamber 12 the container upper portion 11 4is provided with a plurality of gas inlet means 31a-31c which receive vaporized silicon tetrachloride and hydrogen gas and introduce these mixtures into the reaction chamber 12 by means of the annular-shaped manifolds 32a-32e, respectively. While it cannot be specifically seen from FIGURE l, it should be understood that each of the manifolds 32a-32C has a substantially annular or toroidal shape in conformity with the substantially circular symmetry desired from the overall apparatus.
Each of the manifolds 32a-32o has a plurality of capillary tubes 33 extending downwardly from the annular manifolds to provide passage for the gaseous mixtures from the manifolds to the reaction chamber 12. While it i should be understood that each manifold is provided with a fairly substantial number of capillary tubes uniformly `arranged around the annular manifold, FIGURE l shows only two such capillary tubes for each of the manifolds. For example, `the outermost manifold 32e isshown as having two capillary tubes 33C. The intermediate manifold B2b is shown as having two capillary tubes 33h, and in a like manner the innermost annular-shaped manifold 32a is shown as having two such capillary tubes 33a, respectively. Each of the capillary tubes opens to form an associated nozzle 34 in order to distribute the gaseous mixtures in the manner shown by the arrows 35.
As one preferred method for growing such epitaxial crystals, the vaporized silicon tetrachloride and hydrogen gas mixture decomposes over the silicon single crystal substrates 27, which have preferably beenheated to a temperature of approximately 1250 C., causing the silicon` to be extricated, which results in the ep-itaxial crystal growth upon the substrates 10.
The gaseous mixture within reaction chamber 12 is preferably exhausted from the chamber by means of a plurality of symmetrically arranged exhaust tubes (only two of which are shown in FIGURE 1) 36 and 37, which tubes communicate from the reaction chamber 12 to a gas trap 38 in which the gases are collected so as to be ultimately exhausted or removed from the trap outlet 39.
As has been previously described, it is extremely important that the reaction conditions for each substrate be equal in order to produce epitaxial crystals having a high degree o-f uniformity in such a mass production apparatus. This requires that each substrate be heated to substantially identical temperatures and secondly, that the flow of the reacting gas mixtures be extremely uniform and symmetrical throughout the chamber. In order to achieve uniform heating of all the crystal substrates, this requires the provision of uniform heating over an extremely large area with a high degree of symmetry. In order to achieve this requirement, it becomes necessary to have a heated area which is as close to being circular as possible. This is accomplished by providing a heater having a spiral ooniiguration such as is shown in FIGURES 2a and 2b. The heater element 23 shown therein is a substantially spiral arrangement comprised of three individual spirally arranged metallic sections 40, 41 and 42, respectively, with each of the spiral sections being separated from the adn jacent spiral section by a substantially constant distance D. Each spiral section is provided with a suitable aperture 40o-42a, respectively, for suitable connection to the electrical terminals 16, 17 and 18, respectively, shown in FIG- URE l. Any suitable electrical fastening means may be employed for physically and electrically connecting heater element 23 to the electrical terminals 16-18.
The individual spiral segments 40-42 are both physically and electrically joined at their innermost ends 4Gb-42h, respectively, which define an equilateral triangle 43, having its center of gravity at 44 from which it can clearly be seen that the spiral segments are very symmetric about the point 44. It should be understood that the equilateral triangle 43 is not an opening, but is an extension of the spiral section inner ends, being integrally formed with each section so as to electrically connect these sections at their inner ends. Thus, the symmetrical arrangement of the heater element 23 very readily lends itself to connection to a three-phase source of a Y-type configuration with the center of the Y-type configuration being grounded and connected to the center section 43 of the heater element 23 and with .the three arms of the Y-coniiguration being connected across the outer ends of the spiral segments 40-42, respectively.
Considering a sectional view of the heater element 23 of FIGURE 2a, which sectional view is shown in FIGURE 2b, it can be seen that the heater element has a configuration substantially analogous to a convex lens cross-section with the thickness at the ends being T1 and increasing toward the center to a thickness T2 which is somewhat greater than the thickness T1. Since the spiral heater element 23 will in general, generate more heat near the central portion thereof, by controlling the cross-sectional area of the spiral sections from the outer ends toward the center thereof, it is thereby possible to regulate the heating gradient along each section so that the outermost crosssectional areas, being less than the innermost cross-sectional areas, will generate more heat thereby yielding an overall effect of a substantially constant temperature level being present over the entire sunface of the heater element 23. This result is possible due to the fact that the heat generated by a conductive element is related to the cross-sectional area of the heater element.
While the preferred embodiment of the heater element of the instant invention is a substantially circular-shaped spiral arrangement having a convex-lens-like cross-section, it should be understood that a spiral heater element having a rectangul-ar cross-section may be employed which greatly facilitates production of heater elements, but which occurs at a sacrifice to the heating characteristics of the heater element.
FIGURES 3a and 3b show the conventional carbon heater element 4S of the prior art which is comprised of a single phase heater section 46, having a substantially square-shaped periphery and arranged in a regular, serpentine fashion, in the manner shown, and having suitable openings 47 and 48 at the extreme ends thereof for connection to a single-phase power source. The slots 49 are provided to form the serpentine configuration for the heater element. The convex lens-like cross-section, shown in FIGURE 2b, yields a higher current density near the periphery of the heater element 23 than that obtained in the central portion, thus providing a large heating area having a substantially high degree of symmetry and a substantially uniform temperature distribution far superior to that achieved through the prior art heater element 45.
It becomes apparent from the manufacturing point of view, as well as from the performance characteristics that a substantially circular-shaped housing is superior to a rectangular or square-shaped housing. Exhaustive experimentation in which substantially identical circular housings were provided with one being provided with a three-phase heater element as shown in FIGURE 2 and a second being provided with a single phase heater element as shown in FIGURE 3, being operated to perform the epitaxial growth operations. From the geometric viewpoint, the heating area of heater element 23 is approximately 1.5 times that of the heating area of element 4S. The area with uniform temperature is approximately two times greater in the heating element 23 over the heating element 45, thereby accommodating substantially two times as many crystal substrates 27 in an apparatus employing heater element 23 as opposed to an apparatus employing a heater element 45 within a circular container, such as the container formed from the upper and lower portions 11 and 12, shown in FIGURE l.
The manner in which a spiral type heater 43 may be formed is as follows:
Firstly, an equilateral triangle having the vertices 50, 51 and 52, is drawn, which equilateral triangle has its center of gravity at 44. The sides 50-52, 52-51 and 51-50 are extended outwardly in the radial direction. Substantially one-third of a circle with a suitable radius is then drawn about the vertex 50 in the region defined by the extended lines 51-50 and 50-52, while another third of a circle is drawn about the vertex 52, with a radius measured substantially from vertex 52 to the inner section between the iirst circle segment and the extended line 50-52 and in the area defined by the extended lines 50-5Z and 52-51. By continuously repeating this process one member of the three spiral members is drawn. Other sets of spirals can be drawn in a like manner. Another major advantage of the heater element 23 of FIGURES 2a and 2b is that a three-phase load provides a more uniform load to a power source than does a single-phase load.
The second major objective of the inventive apparatus, beingthe achievement of extremely uniform and symmetrical flow of the reacting gas mixtures, is achieved by the substantially symmetrical arrangement of both the annular manifolds and their accompanying capillary tubes and nozzles, as well as the symmetrical arrangement of the exhaust tubings so that the general flow of gases from both input to output is extremely uniform and symmetrical.
The above apparatus satisfied every necessary condition to enable mass production of epitaxial crystals yielding extremely substantial increased production quantities, while at the same time yielding crystals having extremely uniform characteristics.
Although there has been described a preferred embodiment of this novel invention, many variations and moditications will now lbe apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.
The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:
1. A heater element for use in apparatus employed in the manufacture of epitaxial crystals, said heater element comprising first, second and third individual metallic heater sections, each of said heater sections having a spiral configuration being symmetrical about a singie point; a first surface of each of said first, second and third sections all lying within a plane; the inner ends of said sections being electrically connected; the outer ends of said sections having terminals for connection to a suitable three phase power source; the sides of adjacent heater sections being separated from one another by a constant predetermined distance to form three elongated spaces; each of said elongated spaces having a spiral configuration formed by connecting one-third of a circumference of a circle which portions are successively drawn about an associated center point successively selected from three vertices of an equilateral triangle formed at the center of a heating element and having its center of gravity at said single point; the width of each heater section being constant over its entire length and being equal to the length of a side of said equilateral triangle; the width of said elongated spaces being equal and being substantially less than the width of said heater sections; a substantially circular-shaped housing enclosing said heater element; means for introducing gaseous mixtures, employed in the epitaxial manufacturing process, into said housing in a constant uniform manner over the region adjacent the planar surface of said heater element.
2. The heater element of claim 1 wherein the first surface of each heater element is a substantially fiat surface for supporting said crystal substrates; and a substantially convex opposing surface being provided to form a substantially convex cross-section to provide uniform heat over the entire surface area of said heater element.
3. Apparatus for the manufacture of epitaxial crystals comprising a metallic housing defining a reaction charnber therein; said housing being substantially circular; one half of said housing having a plurality of annular manifolds being concentric to one another; a plurality of inlet tubes exterior to said housing connected to an associated manifold; each of said manifolds having a plurality of openings symmetrically arranged about the associated manifold communicating between the manifold and the reaction chamber; the remaining half of said housing being provided with outlet means for exhausting gases from said reaction chamber; a substantially circular heating element positioned within said reaction chamber for providing a uniform temperature level across the entire chambe for heating crystal substrates supported by said heating element.
4. The apparatus of claim 3 wherein each manifold opening is provided with a capillary tube and a nozzle for symmetrically and uniformly guiding gaseous mixtures into said reaction chamber.
5. The apparatus of claim 3 wherein said outlet means is comprised of a plurality of exhaust tubes symmetrically arranged about said remaining housing portion for guiding gaseous mixtures out of said reaction chamber to maintain a continuous even flow of gases within said reaction chamber; a gas trap being coupled between said exhaust tubes and the exterior of said housing.
`6. Apparatus for the manufacture of epitaxial crystals comprising a metallic housing defining a reaction chamber therein; said housing being substantially circular; the upper half of said housing having a plurality of annular manifolds being concentric to one another; a plurality of inlet tubes exterior to said housing connected to an associated manifold; each of said manifolds having a plurality of openings symmetrically arranged about the .associated manifold communicating between the manifold and the reaction chamber; a heater element positioned in said reaction chamber for use in uniformly heating crystal substrates used in the manufacture of epitaxial crystals, said heater element comprising first, second and third individual heater sections, each of said heater sections having a spiral configuration being symmetrical about a single point; the cross-section of each of said spiral sections continuously decreasing from the center outward to pro- Vide uniform heat across the chamber; said first, second and third sections all lying substantially within a plane, the inner ends of said sections being electrically connected; the outer ends of said sections having terminals for connection to a suitable three phase power source.
7. The device of claim 1 wherein the three-phase power source is a delta-connected system; each of said terminals being respectively connected to one phase of said delta-connected three-phase system.
8. The device of claim 1 wherein the three-phase power source is a Y-connected three-phase system; the center point of the Y-connected three-phase system being electrically connected to the center of said heating element and being electrically grounded; said terminals each being respectively connected to one of the phases of said Y- connected three-phase system.
References Cited UNITED STATES PATENTS 563,032 6/1896 Hadaway 338--218 X 1,638,857 8/1927 Keene 13-24 1,988,845 1/1935 Jewett 13--24 X 2,282,226 5/ 1942 Hoop 13--24 2,596,327 5/1952 Cox et al. 338-217 X 3,146,123 8/1964 Bischoff 117--106 3,151,006 9/ 1964 Grabmaier et al 148--174 3,208,888 9/1965 Zeigler et al. 148-175 3,222,217 12/ 1965 Grabmaier 11S-49.5 X
FOREIGN PATENTS 361,960 11/1931 Great Britain. 425,232 3/ 1935 Great Britain. 256,198 12/ 1948 Switzerland.
RICHARD M. WOOD, Primary Examiner.
C. L. ALBRITTON, Assistant Examiner.