US 3776703 A
Silicon monocrystals having a 1-0-0 crystallographic orientation are prepared using a crystal growing apparatus of the float zone type having a heating coil and upper and lower focusing coils, wherein the silicon feed bar is held upright and moved downward as the monocrystal rod is formed. After the stem of the monocrystal has been grown, the lower focusing coil is lowered slightly, thus producing a condition more conducive to the formation of the 1-0-0 crystalline structure.
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Description (OCR text may contain errors)
0 United States Patent 1191 1111 3,776,703 Valant et al. I 5] Dec. 4, 1973 [5 METHOD OF GROWING 1-0-0 3,261,722 7/1966 Keller et al. 23 301 ORIENTATION IIIGII PERFECTION 3,265,470 8/1966 Keller SINGLE CRYSTAL SILICON BY 3,310,384 3/1967 Keller 23/301 ADJUSTING A FOCUS COIL FOREIGN PATENTS OR APPLICATIONS 75 Inventors; G MIchaeI vaIant Gal-land; Hoyt 829,422 3/1960 Great Britain 23/301 Tex.
 Assignee: Texas Instruments Incorporated,
 Filed: Nov. 30, 1970  Appl. No.: 93,617
 US. Cl. 23/301 SP, 23/273 SP  Int. Cl B01j 17/18  Field of Search 23/301 SP, 273 SP  References Cited UNITED STATES PATENTS 2,961,305 11/1960 Dash 23/301 3,275,417 9/1966 23/301 2,905,798 9/1959 23/301 3,271,118 9/1966 23/301 3,194,691 7/1965 23/301 3,447,902 6/1969 Benedict et a] 23/301 Randuff Guffey, McKinney, both of Primary ExaminerNorman Yudkoff Assistant Examiner-R. T. Foster Attorney.lames 0. Dixon, Andrew M. Hassell, Harold Levine, Melvin Sharp, Gary C. Honeycutt, John E. Vandigriff, Henry T. Olsen and Michael A. Sileo, Jr.
 ABSTRACT Silicon monocrystals having a l-O-O crystallographic orientation are prepared using a crystal growing apparatus of the float zone type having a heating coil and upper and lower focusing coils, wherein the silicon feed bar is held upright and moved downward as the monocrystal rod is formed. After the stem of the monocrystal has been grown, the lower focusing coil is lowered slightly, thus producing a condition more conducive to the formation of the l-0-0 crystalline structure.
11 Claims, 2 Drawing Figures MOVABLE FOCUS COIL ALLOWING l-O-O CRYSTAL GROWTH PATENTED DEC 4 I975 MOVABLE FOCUS COIL ALLOWING l-O-O CRYSTAL GROWTH I/VVE/VTORS Gary M/chae/ Va/am Hoyf Ranauff Gaffey W/T/VESS 1 METHOD OF GROWINQ I-OQQRJJQNTATION HIGH PERFECTION SINGLE CRYSTAL SILICON BY ADJUSTING A FOCUS COIL This invention relates generally to the growing of single crystal, or monocrystal, silicon rods for use insemiconductor component manufacturing, and more partic ularly to a float zone method of growing monocrystal silicon in the l--0 crystallographic orientation.
Among the various methods that can be used to 'grow monocrystals for use in semiconductors, the two more commonly used methods are the float zone method and the crucible method. According to the crucible method, a seed crystal is dipped into a crucible containing molten silicon and a silicon rod of controlled diameter is pulled therefrom. Some recognized defects of the crucible method include a high content of oxygen and other impurites, and the low resistivity of the crystals grown thereby.
The float zone method basically comprises contacting a molten end of a seed crystal with a molten end of a polycrystalline feed bar and gradually passing a molten zone through the length of the bar by means of passing the bar through a heating coil. As the molten zone passes along the bar, it resolidifies as a monocrystal. The general float zone process is disclosed in Keller US. Pat. No. 2,992,3ll. An improved float zone process whereby substantially dislocation-free monocrystals can be produced is described in Hunt US. Pat. No. 3,397,042. The present invention achieves the advantages of the Hunt process in a method of growing sili-Q and repeatability heretofore unknown.
Most monocrystals presently used in the semiconductor manufacturing ndustrie wnin h-. .9r; a tation. Recently it has been fournd that crystals grown in the 10-0 orientation possess certain desirable char:
con monocrystals in the I- O-OorientatiQr acteristics for semiconductor applications, including superior etching characteristics, easier and cleaner breaking into slices, and superior electrical breakdown characteristics. This has resulted in an increasing demand for. ys and a n2d.. 9 -@im2 2y q method of producing such crystals.
The initial factorindetermining the orientation of the grown crystal is the orientation of-the seed crystal. But it is more difficult to grow silicon monocrystals in the HM) Orientation than n, thsmlil r imiwaat least partly because materials whichcrystallize in a cubic lattice, as silicon does, inherently attempt to do so in the I'If]. orientation. Despite this, 1-0-0 crystals can be readily grown by the crucible method, but such crystals are subject to defects resulting from the method used, as previously mentioned herein. Previous attempts to grow l-0-0 crystals by the float 59 method, and thus obtain characteristic higher quality, have been unsuccessful. The inherent reluctance of the crystal lattice o eminfil ;1:9:99ristttfifi j sulted in dislocation densities which are unsatisfactory for most semiconductor applications.
It is an object of the invention to provide a process of growing l-O-O monocrystals according to afloat zone technique.
It is a further object of the invention to provide a process of growing l-O-O monocrystals which are essen tially free from dislocations and are of generally high quality for use in semiconductor components.
An advantage of the invention is that it can be practiced upon existing float zone apparatus or with only slight modifications thereto.
Accordingly, the present invention comprises a float zone method of growing high quality l-O-O monocrystals for use in semiconductor components. After the growing process has begun, the position of a lower focusing coil is adjusted, changing the dynamic forces interacting in the molten zone and thus producing a condit ion more conductive to the formation of a crystal latt ce ut .1 LQQE EMQQQE For a more complete understanding of the present invention, reference should now be had to the following detailed description in conjunction with the accompanying drawings in which: I
FIG. 1 is an elevational view, partially in cross section, of a crystal growing apparatus suitable for practicing the invention; and
FIG. 2 is a side view of a crystal during the growth process, revealing the molten zone and the characteristi terrn nal. li tisaa r a slit asssp a j t It should be recognized at the outset that the crystallographic orientations referred to herein as l-l-l and :2-9 ar sletsxtit das as 9 th a m s qui sally accepted Miller lndices. For a discussion of crystal orientation including Miller lndices, reference may be hadto Wood, Crystal Orientation Manual, (Columbia University Press, 1963).
Materials which crystallize into acubic lattice-inherently tend to do so in the 1 1-1 orientation. It isthjs phenomenon which causes all snowflakes and ice crystals to form a hexagonal crystal lattice. It is this natural EY fi1iEQELQF2WiH the r 1tati9n.th h e r etofore made it impossible to grow dislocation-free 1 0-0 cyrstals with the repeatability required to make production feasible using a float zone technique.
Referring to FIG. 1, a suitable float zone apparatus for practicing the method of the invention is shown. A substantially round polycrystalline silicon feed bar 11, which is tapered at the lower end, isv attached at the upper end by a holding mechanism 13. A silicon seed crystal 15 is attached at the lower end thereof by a seed holder l7, which is mounted on a spin shaft 19. Spin mechanism 21 couples to spin shaft 19 and is used to rotate the spin shaft as a crystal rod is being grown.
The seed crystal and feed bar 11 are extended within a quartz tube. 23. The quartz tube 23 is sealably mounted at each end thereof such that the entire crystal growing process takes place within a closed chamber 25. During the growth process an inert gas, such as argon, is circulated through chamber 25, as by entering under slight pressure at channel 27 and exiting at channel 29.
Spin mechanism 21 and holding mechanism 13 are coupled to a traversing mechanism (not shown) contained in housing 33, as by frame 35. This enables the feed bar 11 and seed crystal 15 to move downward at the same rate, thusproducing a monocry'stal of substantially the same diameter as the feed bar. Spin shaft 19 protrudes through spin mechanism 21 and is movable in the vertical direction with respect to said spin mechanism. This enables the seed crystal 15 to be initially located in the proper position with respect to heating coil 37 and feed bar 11, and enables the movement of the seed crystal at a faster rate than the feed bar so as to properly grow the stem region of a monocrystal.
Radio frequency heating coil 37 maintains a molten zone in the feed bar during the growth process; a feed bar and a newly grown monocrystal interface at this molten zone. An additional heating source, such as hydrogen torch 39, must be supplied to initially heat the feed bar so that heating coil 37 can electromagnetically couple thereinto. Upper focusing coil 41 and lower focusing coil 43 affect the electromagnetic field produced by heating coil 37 so as to control the size and shape or the molten zone. Heating coil 37 is fixedly mounted to the crystal growing apparatus, thus enabling the molten zone to traverse the length of the feed bar by means of moving said feed bar through the electromagnetic field produced by said heating coil. Focusing coils 41 and 43 are adjustably mounted to the crystal growing apparatus such that they can be ad justed upward and downward with respect to heating coil 37 an independently from the feed bar.
The parameters available for controlling the crystal growth process on apparatus of the type described.
above include such factors as the vertical rate of movement of the feed bar and seed crystal and the heating coil power setting. Other factors which affect the process include the rate of spin of the seed crystal, the rate of argon flow through the chamber, the frequency of the power source, and the position of the focusing coils with respect to the molten zone. In a float zone crystal growing process the primary parameter used to control the process is the power setting, which determines the temperature of the molten zone. The positions of the focusing coils are dictated by the conditions required for proper growth of the stem and shoulder regions of the crystal with the temperature being varied as required. After the shoulder region has been grown, the power setting will typically only be varied slightly around a nominal value required to maintain the temperature of the molten zone at on the order of 1,420" C.
The upper and lower focusing coils are initially located at the positions required to properly control the molten zone during growth of the stem and shoulder regions, such positions being dictated by operator experience. For apparatus equipped to grow crystals on the order of 1.25 inches in diameter, the upper focusing coil is typically located on the order of 0.55 inch above the heating coil and the lower coil is typically located approximately 0.35 inch below the heating coil, wherein the above distances are measured between nearest surfaces as opposed to center-to-center. If a [-1-] crystal is toBe grawmifiaeisiifig coils arel e ft in these positions throughout the growth process. But as already mentioned herein, previous attempts to grow 1 0 crystals li ave resulted in unsatisfactorily high dislocation densities.
After the seed crystal is affixed to the seed holder and the feed bar affixed to the holding mechanism, the traversing mechanism is moved vertically so that lower end of the feed bar is only slightly above the plane of the upper surface of the heating coil. The spin shaft is then adjusted vertically such that the top end of the seed crystal is only slightly below the lower surface of said heating coil. Then the argon flow is started, typically at a rate of 4.0 liters per minute. The hydrogen torch is then ignited to heat the feed bar and the heating coil power is turned on. If the feed bar is less than 1.0 ohm-cm in resistivity, it can probably be heated by the coil without the necessity of preheating by the torch.
Thr radio frequency generator that supplies power to the heating coil is typically initially set to 1.8 amps and to a frequency of 3.2 to 3.4 megahertz. As the tip of the bar begins to melt, the generator setting may need to be gradually adjusted to obtain the proper conditions for growth of the stem. After equilibrium is reached with approximately 0.25 inch of the tip of the feed bar in the molten state, the seed crystal is moved upward to make contact with the feed bar, and then immediately moved back downward approximately 0.125 inch, with the melt adhering thereto by means of surface tension. At this point the spin mechanism is energized to rotate the seed crystal, typically at a rate of 22 rpm. As the silicon melt on the tip of the bar begins to melt the endof the seed crystal, the power setting is usually reduced to prevent the molten silicon from running down the sides of the seed.
The traversing mechanism is then energized to begin the downward movement of the feed bar and seed crystal, typically at a rate of 6 inches per hour for a 1.25 inch diameter crystal. As the melt begins to solidify on the seed, the spin shaft is pulled downward, thus moving the seed crystal with respect to the feed bar in order to grow the stem region of the crystal. As the stem is being grown, the power setting is reduced as required. As the stem nears the desired length, the lower focusing coil is lowered by approximately 0.20 inch. When the coil is lowered, a noticeable downward sag of the molten zone appears. At this point downward movement of the spin shaft with respect to the feed bar is discontinued. The continued downward movement of the traversing mechanism passes the feed bar through the heating coil causing the molten zone to traverse the length of the feed bar.
The process is further explained with reference to FIG. 2 which shows a crystal rod during the growth process. The process beginswith a pure l-O-O seed crystal 51 having no dislocations. As contact is made between the seed crystal 51 and the molten tip of feed bar 53, dislocations result. These dislocations must be eliminated during growth of stem 55. Dislocations in silicon propagate in the 1-l-0 direction and therefore gradually move to the surface as a l-l-l or l-0-0 crystal is.
being grown. The stem 55 must be grown long enough and narrow enough to allow the dislocations to grow out to the surface and disappear. A typical desirable diameter for the stem is 0.10 inch. After dislocations have disappeared from the stem, the lower focusing coil is lowered on the order of 0.20 inch and downward movement of the seed with respect to the feed bar is ceased. The taper of the feed bar allows proper transition of diameter from that of the seed to that of the feed bar, as in shoulder region 59 in FIG. 2.
The line 61 shown on the crystal of FIG. 2 denotes one of four slight ridges which characteristically appear on the surface of a 1-0-0 crystH These ridges are formed by the intersection of three crystallographic planes at the-crystal surface, and are spaced equally apart around the crystal. The ridges begin to appear during growth of the stem, and as the shoulder region of the crystal is being grown each ridge widens into a planar surface 63 and then narrows back into a ridge. These planar surfaces furnish a criteria of the quality of the crystal at an early stage during the growth process. Each of the planar surfaces should be smooth and should begin and end at approximately the same point circumferentially around the crystal. The continuity of the ridges also furnishes a criteria of the quality of the crystal, as breaks in a ridge indicate the presence of dislocations.
Referring again to FIG. 2, as the polycrystalline feed bar 53 is passed through a heating coil a molten zone 67 is produced. The molten material solidifies and forms a monocrystal rod 69. Two distinctive regions of molten zone 67 appear. The upper region 71 is the zone of transition from the solid to the molten state. The uneven appearance is caused by the uneven melting of the feed bar from the outer surface inward. The lower region 73 is in the molten state.
As disclosed in the Hunt process previously mentioned herein, the use of one or more focusing coils enables control over the size and shape of the molten zone, thus enabling the production of dislocation-free crystals. According to the method of the present invention, a lower focusing coil is initially located in an upper position to enable proper growth of a stern. Then, as dislocations disappear from the stem, the lower focusing coil is moved to a lower position, allow ing the molten zone to bulge downward, producing a condition more conducive to the growth of a 1-0-0 crystal. lf thelower focusing coil were initially located in this lower position, the stem could not be properly grown and the dislocations which result from contact of the seed crystal with the.feed bar could not be removed The present invention enables the growth of 1 0 0 sihgle crystal silicon having a dislocation deiisifi.
.below 500 dislocations per square centimeter with no slippage or lineage as defined by ASTM Standard No. F47-68. t
The invention has been described with reference to the growi ng of T125 inch diam etercPystal sofl apparatus of the type shown in FIG. 1, and typical parameter values have been enumerated. This mode of description is not to be interpreted by way of limitation. The actual parameter values will depend to some extent upon operator preference and upon the particular apparatus used. Theparameter values disclosed in Hunt US. Pat. No. 3,397,042 previously referred to herein, are generally applicable tothe present invention with the exception that a generator frequency of 3.2 to 3.4 megahertz is preferred.
Although the invention has been described with reference to the growth of silicon crystals, it is equally-applicable to growth of single crystals of other materials, such as germanium. Even though many of the problems associated with the growth of silicon crystals are not encountered in the growthof single crystals of such other materials, and thus high quality dislocation-free .crystals of such materials can be produced by known methods, the present invention could be utilized if desired.
It should be recognized that crystal growth is a highly operator-dependent process, requiring specific knowledge of the effect of varying the different parameters during the growth process. However, it is anticipated that those skilled in the art will appreciate the significance of the present invention.
a. enclosinga crystal feed bar and a l-O-O single crystal seed in a sealed chamber;
b. heating said feed bar sufficiently for radio frequency energy to inductively couple thereinto;
c. establishing with an induction coil an inductively coupled field through said chamber, said indictively coupled field being limited on the lower boundary thereof by a focusing coil said focusing coil being spaced below the induction coil an appropriate distance for growth of a 1-1-1 disloca-i tion free single crystal;
d. following an inert gas through said chamber;
e. contacting said crystal seed with a molten end of said feed bar; f. rotating said crystal seed while vertically moving said crystal seed and said feed bar through said inductively coupled field, with said seed crystal initially being moved at a faster rate than said feed bar to grow a crystal stem;
g. changing the position of said focusing coil with respect to said inductively coupled field after dislocations have disappeared from the stem to produce a condition more conductive to the formation of a l-0-0 crystal; and
h. continuing the movement of said crystal seed and said feed bar to cause a molten zone to traverse the length of said feed bar. v
2. The method of claim 1 wherein said monocrystal is a silicon monocrystal.
3. A. method of growing a dislocation-free 1 0-0 monocrystal, comprising: i v
a. enclosing a ,crystal feed bar and a l-0-0 single crystalseed in a sealed chamber with said feed bar extending downwardly and said seed crystal extending upwardly;
b.,heating'said feed bar sufficiently for radio frequency-energy to inductively couple thereinto;
c. establishing with an induction coil an inductively coupled field through said chamber, said inductively coupled field being limited on the lower boundary thereof by a lower focusing coil and on the upper boundary thereof by an upper focusing coil said lower focusing coil being spaced below the induction coil an appropriate distance for growth of a l-l-l dislocation free single crystal;
d. flowingan inert gas through said chamber;
e. contacting said crystal seed with a molten zone initially located at the lower end of said feed bar;
f. rotating said crystal seed and beginning thedownward movement of said feed bar and said crystal seed, initially moving said crystalseed downward faster than said feed bar to grow a crystal stem;
g. lowering the position of said lower focusing coil with respect to said inductively coupled field, after dislocations have dissapeared from the stem, to a position which produces a condition more conductive to the formation of a l-O-O crystal; and
h. continuing the downward movement of said crystal seed and said feed bar to cause said molten zone to traverse the length of said feed bar.
4. The method of claim 3 wherein said monocrystal is a silicon monocrystal.
5. The method of claim 4 wherein said lower focusing coil 'is lowered by approximately 0.20 inch.
6. The method of claim 5 wherein the frequency of said inductively coupled field is approximately 3.2 to 3.4 megahertz.
7. The method of claim 6 wherein said crystal seed is rotated at a rate of approximately 22 revolutions per minute.
8. The method of claim 7 wherein the continued downward movement of said crystal seed and said feed bar is approximately 6 inches per hour.
9. The method of claim 8 wherein said inert gas is argon and the rate of flow through said chamber is approximately 4 liters per minute.
10. The method of claim 3 wherein said lower focusing coil is lowered in position sufficiently to allow the molten material in said molten zone to sag noticeably downward without running down the sides of said monocrystal.
11. The method of claim 4 wherein the diameter of the crystal to be grown is about 1.25 inches, said upper focusing coil is located approximately 0.55 inch above a heating coil which establishes said inductively coupled field, and said lower focusing is initially located approximately 0.35 inch below said heating coil and is lowered to a position approximately 0.55 inch belo said heating coil.
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