Method of continuously growing thin strip crystals
US 3124489 A
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March 10, 1 F. L. VOGEL, JR., ETAL METHOD OF CONTINUOUSLY GROWING THIN STRIP CRYSTALS Filed May 2, 1960 W 5k 5 0 N6 EWE 8 WLM MWN F W1 4 a w 4 n n m a U E u 7T7 Ri N W a i z a a t a 3 a .41 4 02/0. M 2 I ll, 0 0 Y i nwh! n 4 3 4 ,2 89 3 Z 3 w w. R W
in the use of the large thick crystals.
United States Patent 3,124,489 METHGD 0F CGNTINUGUSLY GROWING THHJ ST CRYSTALS Ferdinand L. Vogel, Jan, Glen Gardner, and Eric F. Cave, Somerville, NJZ, assignors to Radio Corporation of America, a corporation of Delaware Filed May 2, 1960, Ser. No. 25,933 Claims. (Cl. 14$1.6)
This invention relates to the growing of single crystal substances in the for-tn of thin strips. More particularly, it relates to the growing of single crystals of a substance having relatively high surface tension, such as germanium, in ribbon form.
Up to the present, single crystals intended to be used in the production of semiconductor devices, such as transis-tors and diodes, have been grown in the form of relatively thick ingots, one inch or more in diameter and of a length up to 12 inches or more. Emphasis has been placed on growing such crystals with closely controlled resistivities, a relatively low number of lattice imperfections, and a high degree of purity except for those impurities intentionally added for the purpose of controlling the conductivity type of the product. A high degree of perfection has been achieved in growing these crystals, and semiconductor device production has been based on the successful making of relatively large crystals which may weigh up to several pounds.
Despite the successful use of large single crystals in manufacturing semiconductordevitxes, it has been reco nized that there are a number of disadvantages inherent One of these disadvantages is that in order to prepare (from these crysfeels the very small semiconductor pellets used in individual devices, a number of costly mechanical operations are required. First of all, the resistivity of the crystal must be measured along its length since resistivity usually varies from one end to the other. Then, sections one or two inches in length are cut from the crystal, each section having approximately uniform resistivity. The
ends of the crystals often are not usable and must be reprocessed and used again for the making of additional crystals since the semiconductor is expensive and waste cannot be discarded and thrown away.
Each of the slices must then be ground flat and etched to a'th-ickness approaching the thickness of the tiny pellets that will be used in the devices. After reducing the thickness of the slice, his then diced into individual pellets whcih are further etched to the exact thickness desired.
All of these mechanical operations result in wasting a much'larger percentage of the crystal than is used and,
although the waste material is mostly recovered and reused, the recovery process adds appreciably to the cost of making the finished devim. It has long been recognized that considerable savings in the cost of manufacturing semiconductor devices could be achieved if the crystal could be grown in the form of a thin ribbon, the thickness of which was approximately that which would be used in the pellet that becomes apart of the device. Such acrystal could be merely divided into sections, each one of which could become a part of a finished device with little or no further proc essing of the semiconductor material. Thus, all of the waste presently encountered in cutting and polishing operations could be completely eliminated and much hand labor could be bypassed.
3,124,48d 'Patented Mar. 10, 1964 Although many previous attempts have been made to grow semiconductor single crystals in long thin strips, the working out of a practical method has proved to be very difficult. This, in spite of the tact that techniques have been known for a number of years for growing long crystals of some substances. For example, zinc single crystals have been grown in the form of thin wires by the Czochralski-Gomperz method. This has involved floating a mica die on the surface of a quantity of molten zinc, dipping a nucleating means such as the end of a copper wire, or a seed crystal, suitably oriented, into the molten zinc, pulling a column of the molten material above the surface of the die, and then directing a cooling stream of an inert gas at a selected level above the die to cause solidification and growth of the crystal.
The above-described method has been successful for the growth of at least zinc crystals having a thin transverse crosssection diameter. nique was applied to the attempted growth of germanium crystals, it was unsuccessful. The very high surface tension of molten germanium was such that when a die having a relatively small opening was floated on the surface of molten germanium, the germanium would not rise within the die aperture. Consequently, any attempt to dip a seed crystal or any other nucleating means down into the die aperture in order to start pulling a long strip usually resulted in the material solidifying within the die aperture and the die stuck to the crystal and was pulled up from the surface of the molten germanium. Attempts to circumvent this by pulling a column of liquid up through the die and solidifying the crystal at some distance above the die, as has been successful in growing With the present invention, however, the difficulties have been overcome and it is now possible to grow very thin ribbon-like crystals of germanium of almost any desired length. a
One object of the present invention is to provide a method of growing single crystals of germanium in thin ribbon form.
Another object of the invention is to provide an improved method of growing thin, ribbon-like crystals of a substance where the substance is a material having relatively high surface tension.
A further object of the invention is to provide an improved method of growing long, thin crystals of germanium suitable for economical use in manufacturing semiconductor devices. I i
These and other objects of the invention .will be further explained in the following description including the drawing of which:
FIGURE 1 is a partially cross-section elevation view of one [form of apparatus suitable for carrying out the method of the present invention,
FIGURE 2 is a plan view of part of the apparatus of FIGURE 1, and
FIGURE 3 is a magnified detail view of part of the apparatus of FIGURE 1.
A feature of the present invention is a method of preparing a strip,having predetermined transverse crosssection area and shape, of single crystal material. The method comprises, in brief, first, maintaining a melt of the material, and applying pressure to the melt such that a portion of it is extruded into a restrictive passage or die having a particular transverse cross-section area and shape.
However, when the same tech-.
3 to cause the molten material to solidify at a level within the die passage, closely adjacent to the exit thereof. As the single crystal forms attached to the seed crystal,.the
seed crystal is withdrawn at a predetermined rate in order to grow a continuous strip of single crystal attached to the seed. The cooling stream of gas is preferably directed against both sides of the emergent strip in a particular manner and the rate of pulling relative to the rate of supplying molten material to the die is adjusted so as to obtain the desired cross-section area and shape, and pressure is continuously applied to the melt to continuously extrude additional material into the die passage to replace the material growing into the crystal.
Referring now to the drawing, in which like parts are indicated with the same numerals in the various figures thereof, a specific example of a method of growing a germanium crystal in accordance with the invention will now be given.
Apparatus for carrying out the method of the invention may comprise, as shoyn in the drawing, a heated reservoir for containing a quantity of molten material and including a die or restricted passageway at one end thereof, means for extruding molten material at a desired rate into the passageway, means for solidifying the molten material as it emerges from the die, and means for supporting and withdrawingthe ribbon-like crystal which is being grown.
The means for containing the molten material comprises a carbon cylinder 2 supported in an upright position on a pedestal 4 which, in turn, rests on a base plate 5. At the upper end of the cylinder is a carbon die 6 having a rectangular shaped passage 8, transverse dimensions of which are .156" in length by .006" in width. The upper exterior portion of the cylinder 2 is threaded and a threaded cap member 9 is provided to hold the die 6 in place on the top of the cylinder. The cylinder reservoir has an internal diameter of /8. The carbon cylinder and its contents are heated by means of an RF heating coil 10, which surrounds at least the upper portion of the cylinder.
Means for extruding the molten material into the die passage comprises a carbon piston 12 fitting snugly within the bore of the carbon cylinder 2. Driving means for the piston comprises a drive shaft 14- the upper end of which abuts the lower end of the piston and the lower portion of which is threaded or provided with a rack. The upper portion of the drive shaft extends'through a sleeve 16 in the base plate 5. Meshing with the threaded portion of the drive shaft is a worm, or pinion, gear 18 driven through cogs 2t? and 22 from a variable speed motor (not shown). Rotation of the worm gear in one direction of the other causes corresponding upward or downward movement of the drive shaft.
Means for solidifying the molten material as it leaves the die comprises two gas jets 24 and 26 having openings disposed on either side of the exit from the die passage 8.
These openings are axially perpendicular to the longitudinal dimension of the die passage and their diameters are preferably appreciably less than this longitudinal dimension. The jets are connected through a pipe 28 to a gas supply (not shown).
Means for withdrawing the completed crystal comprises two rollers 3t and 32 spaced a short distance above the die exit with means for driving the rollers at a desired speed. This means comprises a shaft 33, the lower end of which is connected to the worm gear-18 and the upper end of whichis connected through gears (not shown) to at least the roller 30. There may be additional means (not shown) for resiliently urging the rollers toward each other. The rollers may be provided with resilient surfaces.
In order to better control the atmosphere surrounding the growing apparatus, the apparatus is contained within closure member which may comprise a piece of rubberlike plastic 38 having a slit 39 therein to permit the exit of the crystal ribbon.
Operation of the above apparatus for growing a crystal of germanium is as follows. A quantity of germanium 4th having the desired type of conductivity and purity is brought to a molten condition within the cylinder 2 by application of power to the RF heating coil 10. The molten germanium is brought to a temperature slightly above its melting point. The top of the piston 12, at the beginning of the run, is just entering the lower end of the cylinder. The drive shaft 14 is then advanced by rotation of the worm gear 18 so that the shaft moves upwardly a short distance, which, in turn, moves the piston upwardly a correspondingly short distance and extrudes some of the germanium up into the die passage 8 so that it is about even with the top exit of the passage.
One edge of a long, thin, flat seed crystal, oriented in a predeterminedmanner, is then lowered by hand so that the lower edge just touches the liquid within the die passage. The upper portion of the seed is engaged between the rollers 3t and 32. Gas pressure is meanwhile turned on so that a stream of relatively inert gas (nitrogen, for example), at room temperature, emerges from the jets 24 and 26. This cooling gas withdraws heat rapidly from the seed crystal and prevents the seed from melting and dropping off into the molten germanium. The cooling gas also causes the molten germanium to start solidifying attached to the seed crystal and, as soon as this action begins, the rollers 30 and 32 are rotated slowly so that the seed begins to rise away from the die and a crystal of germanium 42 continuously grows attached to the lower end of the seed.
The streams of cooling nitrogen emerging from the jets 24 and 26 are directed at the centers of the opposing faces of the crystal ribbon 42 at a level slightly above the exit of die passage 8. The center of the strip is thus cooled more rapidly than the edges. This causes solidification of the growing crystal to occur at a slightly lower level in the central portion of the crystal ribbon than at the edges, and the liquid-solid interface 44 of the growing crystal is thus caused to be concave downwardly in the transverse longitudinal dimension of the die and ribbon. Because of the shape of this interface, any tendency toward unwanted nucleation at the edges of the ribbon is inhibited. Spurious crystals which may start to grow will grow outward and will be halted immediately at the edges of the ribbon. Also, as shown in the drawing, the liquid-solid interface 44 (FIG. 3) is concave downwardly in the thickness dimension of the ribbon.
In the example illustrated in the drawing, the growing crystal takes the same transverse cross-sectional shape and area as the exit of the die passage. At the same time the crystal is being grown by continuously withdrawing germanium from the die slot, upward motion is continuously imparted to the piston 12 at a rate calculated to just keep the liquid level very close to the upper exit of the die. By means of the drive shaft 33 connecting the worm gear 18 and the roller 30, the rate of feed of the molten germanium to the die and the rate of travel of the ribbon 42 can be coordinated as desired. The exact point at which the liquid solidifies must be very closely controlled by careful balance between the rate of feeding. the molten germanium to the die, the rate of withdrawal of the crystal, and the rate of heat removal from the crystal. If the rate of heat removal is too slow, the liquid level will rise above the die exit and the cross-sectional transverse shape will become deformed due to surface tension acting on the liquid column. If rate of heat removal is too rapid, the liquid-solid interface will descend too far within the die passage, the crystal will begin to solidify at too low a level and will jam in the die.
grow narrower ribbons with the same apparatus. This can be done by increasing the pulling speed slightly with out correspondingly increasing the rate of feed of the molten material to the die. This action causes the emerging ribbon to become stretched out, so to speak, so that it has a longitudinal transverse dimension somewhat less than that of the die passage exit.
With the apparatus shown, crystals have been grown as rapidly as one inch per minute and it is believed possible to ,increase this rate considerably, if desired.
Crystal can be grown continuously until the germanium in the reservoir of the cylinder is exhausted. The crystal ribbon which is grown is quite flexible and can be wound on a reel, if desired. The ribbon can be made quite uniform in cross-section by careful control of the various factors which have been described.
The length of ribbon which is produced can be increased merely by increasing the size of the cylinder bore.
The ribbon crystal which is grown is similar in all respects to the finished material used for semiconductor devices by cutting slices from large ingots except that it has the further advantage of much more uniform resistivity. Because of the rapidity of growth of the crystal, the segregation constants of the impurities that are present have little effect on the proportion of the impurity remaining in the liquid as opposed to the amount going into the solid.
Although the die in the example had a width of .006, this width can be decreased still further and the die can be made of other materials, such as quartz, which are capable of taking a higher polish and thereby producing smoother surfaced crystals, as Well as Wearing better.
What is claimed is:
1. A method of preparing a ribbon, having predetermined transverse cross-sectional area and shape, of single crystal material, comprising maintaining a melt of said material, applying pressure to said melt such that a portion thereof flows into a restrictive passage having an exit opening of approximately said area and shape, contacting a nucleating means to said portion, withdrawing said nucleating means at a predetermined rate while directing a cooling stream of gas which is inert with respect to said material, adjacent the exit of said passage so as to maintain the liquid level at the exit of said passage and to cause said molten material to solidify Within said passage and form a continuous, emerging, single crystal ribbon having said predetermined transverse cross-section area and shape and attached to said nucleating means, and continuing to apply pressure to said melt to supply molten material to said passage at the same rate said ribbon is solidifying.
2. A method of preparinga ribbon of single crystal substance comprising maintaining a melt of said substance, extruding a portion of said melt into a restrictive passage of elongated rectangular transverse cross section, contacting an edge of a thin, flat seed crystal to said portion, maintaining the temperature at said passage sufficiently low to cause said molten substance to solidify within said passage at the exit thereof and form a single crystal attached to said seed crystal, and withdrawing said seed crystal at a predetermined rate so as to grow said attached crystal in ribbon form while continuing to extrude said melt into said passage at a rate corresponding to said predetermined rate, and maintaining the interface between said melt and said crystal within said passage.
3. A method of preparing a single crystal of germanium in relatively thin, elongated form of a desired trans verse cross-sectional area and shape comprising maintaining a melt of said germanium, applying pressure to said melt such that a portion thereof flows into a restrictive passage having said desired I cross-sectional area and shape, contacting a nucleating means to said portion, maintainingthe temperature at said passage at a level to cause said molten germanium to solidify within said passage at the exit thereof and form a single crystal attached to said nucleating means, and withdrawing said nucleating means at a predetermined rate so as to grow said attached crystal with said desired transverse cross-sectional area and shape while continuing to cause said melt to flow into said passage at a rate sufiicient to maintain said predetermined rate of crystal growth, and maintaining the interface between said melt and said crystal within said passage.
4. A method of preparing a ribbon, having predetermined transverse cross-sectional area and shape,'of single crystal material, comprising maintaining a melt of said material, applying pressure to said melt such that a portion thereof flows into a restrictive passage having an exit opening of approximately said area and shape, contacting a nucleating means to said portion, withdrawing said nucleating means at a predetermined rate while directing a cooling stream of gas inert with respect to said material, adjacent the exit of said passage so as to cause said molten material to solidify within said passage at the exit thereof and form a continuous, emerging, single crystal ribbon attached to said nucleating means, said gas being directed at opposing faces of said ribbon such that said ribbon has said predetermined transverse crosssectional area and shape, and continuing to apply pressure to said melt to supply molten material to said passage at the same rate said ribbon is solidifying.
5. A method according to claim 4 in which said gas is directed in streams having a diameter appreciably less than the width of said faces whereby heat is withdrawn from the central portions of said faces more rapidly than from the edges thereof.
References Cited in the file of this patent UNITED STATES PATENTS 2,782,473 Brennan Feb. 26, 1957 2,809,136 Mortimer Oct. 8, 1957 2,876,147 Kniepkamp et al Mar. 3, 1959 2,889,240 Rosi June 2, 1959 2,927,008 Shockley Mar. 1, 1960 3,002,824 Francois Oct. 3, 1961