Radiantly heated crystal growing furnace

Abstract

Crystal growing furnace having an externally mounted traveling radiant heat source with high intensity lamps for melting materials within the furnace chamber. The radiant heat energy passes through the chamber wall without appreciable absorption, and air cooling means is provided for cooling the lamps and chamber walls. A viewing port permits direct observation of the molten zone and the freezing interface, and the contours of the molten zone and interface are controlled by the position and intensity of the lamps.

Description

BACKGROUND OF THE INVENTION

This invention pertains generally to the preparation of semiconductor materials and more particularly to a furnace for compounding, zone refining and growing crystals of high purity semiconductor compounds.

The invention has particular application in the production of large, high purity, high prefection, low cost gallium arsenide (GaAs) crystals and other compounds containing elements from columns III and V of the periodic table, commonly known as III-V compounds, such as GaP, InAs, InAsP, and the like. The invention has similar application in the preparation and purification of other like compounds having at least one metallic constituent and at least one voltatile constituent.

Heretofore, single crystals of III-V compounds, such as GaAs, and other like compounds have commonly been prepared by the Czochralski method. According to this method, a charge of the compound from which the crystal is to be made is melted in a crucible by RF induction heating. As seed crystal, i.e., a small, highly perfect, oriented crystal of the desired compound, is dipped into the melt, then rotated and very slowly withdrawn from the melt. If the termperature is properly maintained, the seed grows as a single, oriented crystal as it is withdrawn. This method of growing single crystals is sometimes referred to as "crystal pulling." When arsenic is an element of the compound, as it is in GaAs, it is necessary to use boron oxide or other suitable means over the melt to contain the arsenic.

In the past, there have been some attempts to use RF and resistance heated zone refining furnaces in the compounding and purification of GaAs and other like crystals. In these furnaces, a hot zone is moved in a horizontal direction through a polycrystalline ingot to melt the material and form a single crystal behind the molten zone. These attempts have been unsatisfactory in certain respects. In some it has not been possible to observe the molten zone and freezing front, and in others it has not been possible to maintain a proper thermal gradient in the furnace to provide a desired shape of freezing interface. Also, in prior art resistance heated devices, the furnace walls are heated to substantially the same temperatures as the materials in the furnaces, and the temperatures at which the furnaces can operate are limited by the melting point of the walls. For example, the melting point of GaAs is 1,240.degree.C, whereas the quartz which is commonly used in furnace walls has a softening point on the order of 1200.degree.C. It is difficult to cool the walls without also cooling the heating coils in resistance heated furnaces.

SUMMARY AND OBJECTS OF THE INVENTION

The invention pertains to a crystal growing furnace having a traveling radiant heat source for melting different zones of a material in the furnace chamber as the heat source is moved. The heat source is located outside the chamber, and the wave length of the radiant heat energy and the material of which the chamber walls are made are such that the heat energy passes through the walls without being absorbed so that the walls remain relatively cool and unheated by this energy. The heat source comprises a plurality of individually controlled high intensity lamps which are spaced about the periphery of the chamber. The molten zone is visible at all times, and the contour of the freezing interface can be controlled as desried by the lamps. Air passageways are provided in the heat source for cooling the lamps and the chamber walls.

It is in general an object of the invention to provide a new and improved furnace for processing semiconductor materials.

Another object of the invention is to provide a furnace of the above character which is particularly suitable for compounding, zone refining and growing crystals of high purity III-V compounds and other like compounds having at least one metallic constituent and at least one volatile constituent having a vapor pressure on the order of one atmosphere at the melting point of the compound.

Another object of the invention is to provide a furnace of the above character which has a traveling radiant heat source for melting desired portions of a material within the furnace.

Another object of the invention is to provide a chamber of the above character in which the chabmer wall remains cool and essentially unheated.

Additional objects and features of the invention will be apparent from the following description in which the preferred embodiment is set forth in detial in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section view, largely schematic, of one embodiment of the invention.

FIG. 2 is an enlarged vertical section view taken in the plane of line 2--2 in FIG. 1.

FIG. 3 illustrates one embodiment of a control system for adjusting the amount of heat produced by the lamps to provide a desired contour for the freezing interface.

FIG. 4 is a top view of the molten zone and freezing interface produced by the apparatus of FIGS. 1-3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It should be understood that the furnace is shown in generally schematic fashion in the drawings and that it is intended to be mounted on a suitable support structure (not shown), together with electrical power sources and other attendant apparatus. For purposes of clarity of illustration, only those portions of the furnace necessary to illustrate the invented concepts disclosed herein have been shown in the drawings. It will be understood that those portions of the furnace illustrated are intended to be supported on the aforementioned support structure in any suitable fashion.

The furnace includes a generally cylindrical elongated tube 11 defining an axially extending chamber 12 therein. The tube has a closed end 11a and an open end 11b, and it extends in a generally horizontal direction. The axis of the tube is preferably inclined relative to the horizontal, with the open end being slightly higher than the closed end. In the preferred embodiment, the angle of inclination is on the order of 1.5.degree. , and this inclination assures that the melt produced by the furnace will not draw material toward the open end of the tube.

Tube 11 is fabricated of a material which is transparent to light and also transparent to radiant heat energy of short wave length. In its preferred form, the tube is fabricated of quartz and is transparent to radiant heat energy having a wave length on the order of one micron.

Means is provided for heating a volatile material 13, such as arsenic, toward the closed end of the tube. This means includes a heater 14 which is disposed coaxially of the tube at its closed end. This heater comprises an electrically energized heating coil 14a and an insulative housing 14b. Coil 14b is preferably fabricated of a material such as nichrome wire, and it is connected to a source of electrical energy in a known manner. A cup-shaped shield 16 is loosely disposed in the tube to decrease the direct exposure of the arsenic to the heat emanting from the hot boat. The shield is preferably fabricated of quartz, and its outer diameter is less than the inner diameter of the tube so that vapor from the volatile material can pass around the shield to the remainder of chamber 12. A thermocouple 17 is provided for monitoring the temperature at the closed end of the tube, and other temperature sensors (not shown) are provided for monitoring the temperature in other portions of the tube.

A boat or crucible 18 is removably disposed in chamber 12 between shield 16 and the open end of the tube for holding semiconductor materials to be procssed in the furnace. The boat is illustrated holding an ingot 19 having solid zones 19a and 19b and a molten zone 19c intermediate the solid zones. The boat is fabricated of a material such as quartz, graphite, boron nitride, carbon, glassy carbon, and the like.

Means is provided for heating the interior of chamber 12 and the material in boat 18 to a temperature below the melting point of the material in the boat. This means includes an elongated heating coil 21 disposed in a fixed position coaxialy of tube 11. As illustrated, this heating coil is longer than the boat. It is fabricated of a wire such as super Kanthal, and it has leads 21a and 21b for connection to a power source.

A removable seal assembly 24 is provided for closing the open end of tube 11. This assembly is preferably of the type described in co-pending application Ser. No. 381,421, filed of even date and assigned to the assignee herein. It includes an annular flange 26 which is bonded to the end of tube 11 to form an integral structure. A seal ring 27 is disposed between flange 26 and an end cap 28. A force transmitting ring 29 engages the outer side of the end cap, and clamping rings 31 and 32 engage flange 26 and ring 29, respectively. Draw bolts 33 and nuts 34 provide means for drawing the clamping rings together to compress seal ring 27 between flange 26 and end cap 28. As is discussed more fully in the aforereferenced co-pending application, force transmitting ring 29 and draw bolts 33 are selected to have thermal properties such that ring 29 expands more than the bolts as temperature increases. A vacuum fitting 35 is carried by the end cap to provide means for evacuating the chamber when the tube is sealed.

A heater 37 is provided for heating the closed end of tube 11 to prevent the occurrence of cold spots within the furance which would attract the vapor of a volatile constituent such as arsenic. In the preferred embodiment, heater 37 is generally similar to heater 36 and includes an electrically energized heating coil 37a mounted in an insulative housing 37b.

A traveling radiant heater assembly 38 disposed externally of tube 11 and movable axially thereof for producing a molten zone, such as zone 19c, in the material in boat 18. This assembly includes a lamp holder comprising a generally annular block disposed co-axially of the tube. This block is preferably fabricated of a reflective material such as aluminum, and its inner wall 39a is polished to provide a highly reflective surface to permit maximum utilization of the heat generated by the assembly.

A plurality of high intensity lamps 41 are carried by the lamp holder and spaced peripherally of tube 11. In the preferred embodiment, the lamps are high intensity tungsten filament lamps having a transparent quartz envelope and a halogen gas contained therein. The lamps produce and transmit radiant energy of short wave length, preferably on the order of one micron. As illustrated, the lamps are axially elongated, and the axes of the lamps are substantially parallel to the axis of chamber 12. The lamps are mounted in axially extending parabolic recesses 42 which are formed in inner wall 39a of the block and highly polished to direct the heat energy produced by the lamps toward the material in the boat. The lamps are mounted in sockets 43, and conventional electrical connections (not shown) are made to the lamps through the sockets.

It should be noted that lamps 41 are substantially shorter in length than boat 18, and consequently only a limited portion of the material in the boat is melted by the lamps. The shape of the molten zone 19c and the freezing interface 44, i.e the interface between the molten zone 19c and the solid zone 19a formed behind the molten zone as it moves through the material in the boat, can be controlled by the placement of the lamps and the degree to which they are energized. A viewing port or window 46 is formed in lamp holder block 39 to permit direct observation of the molten zone and freezing interface.

As illustrated in FIG. 3, lamp controls 47a-47g are provided for adjusting the amount of heat produced by the peripherally spaced lamps. In this figure, the lamps are designated 41a-41g, and separate control is provided for each lamp. This arrangement permits the amount of heat produced by each lamp to be adjusted independently, and adjustments can be made for variations among the lamps. Alternatively, if desired, some of the controls can be ganged together, or some of the lamps can be adjusted by a single control. In the preferred embodiment, the lamps are more heavily concentrated toward the bottom of the boat, and the controls are adjusted so that the molten zone is hotter and longer at the bottom than the top and freezing interface 44 is vertically inclined as illustrated in FIG. 1. Also, in the preferred embodiment, the relative intensities of the lamps are adjusted in such manner that the molten zone is wider at the sides than in the middle, as viewed from the top and illustrated in FIG. 4. Consequently, the crystal formed behind molten zone 19c grows outwardly from the central portion of the molten zone toward the sides and bottom of the boat, preventing spurious nucleation from the boat.

Means is provided for circulating air around lamps 41 and the side wall of tube 11 to cool the same. This means includes a plenum chamber 48 formed in block 39. An air inlet 49 communicates with the plenum chamber and is adapted for connection to a source of cooling air. Air passageways 50 extend from the plenum chamber to the recesses 42 in which the lamps are mounted. Thus, cooling air introduced through inlet 49 is forced to flow through plenum chamber 48, through passageways 50, and around lamps 41 to tube 11. The air discharged at the ends of the block through the opening between the outer wall and the inner wall of the block.

Means is provided for moving heater assembly 38 axially of chamber 12. This means includes a rotatably mounted feed screw 51 which threadedly engages lamp holder block 39. The feed screw is driven by a motor 52 at a speed and in a direction controlled by a motor control 53.

Operation and use of the furnace to produce a polycrystalline ingot of a compound such as GaAs can now be described. By way of example, let it be assumed that a GaAs ingot is to be produced from a charge consisting of 250 grams of gallium and 280 grams of arsenic. Before the furnace is used, tube 11, shield 16, boat 18, and end cap 28 are cleaned carefully to prevent contamination of the product. One suitable method of cleaning these quartz parts consists of etching the parts in aqua regia for 15 minutes, washing them in high purity water, then etching them with a solution consisting of 50 per cent hydrofluoric acid and 50 per cent water for 15 minutes, washing them in high purity water again, and then drying them.

The arsenic is placed in the closed end of tube 11, and shield 16 is inserted into the tube and positioned as shown in FIG. 1. The gallium is placed in the boat which is then placed in the tube, approximately in the position shown in FIG. 1. Seal assembly 24 is attached to the open end of the tube, and the tube is placed in heating coil 21 so that the boat is generally centered in the coil and the arsenic is outside the coil. A vacuum pump is connected to vacuum fitting 35, the chamber is evacuated to 10.sup.-.sup.5 mm of Hg, and the chamber is sealed. Heating coil 21 is energized, and the charge is baked at a temperature on the order of 700.degree.-800.degree.C for 4 hours.

Radiant heater assembly 38 is positioned at the left end of boat 19, and heater 21 is adjusted as necessary to bring the temperature at the right end of the boat to 700.degree.C. Lamps 41 are then turned on and adjusted to bring the left end of the boat to a temperature of 1000.degree.C. Heater 14 is now turned on at a rate on the order of 100.degree.C per minute to raise the temperature of the arsenic to 610.degree.C. Lamps 41 are now turned up to raise the temperature at the left end of the boat to 1280.degree.C, and motor 52 is set to drive heater assembly 38 at a rate of 4 inches per hour. Two reaction passes are made, that is the heater assembly is moved past the boat twice, then the lamps are turned off. Heater 14 is then turned down to 400.degree.C for 10 minutes, following which heaters 14 and 21 are turned off. After the furnace has cooled, seal assembly 24 is opened, the boat is removed from the tube, and the polycrystalline GaAs ingot is removed from the boat. The quartz parts are then cleaned again in the manner described above.

The furnace can also be used to produce single crystals. To do so, the quartz parts are cleaned, and the arsenic and shield are placed in the tube as above. A seed crystal which fills the cross-section of the boat is placed in the left end of the boat, and the remainder of the boat is charged with gallium. The boat is placed in the tube, the tube is sealed, and the chamber is evacuated. With heaters 14 and 21 turned off, traveling heater 38 is passed over the boat at a temperature of about 900.degree.C in order to bake out the gallium. Heater 21 is then brought up to about 650.degree.-700.degree.C, and heater 14 is brought up to 610.degree.C. The traveling heater is then turned up to 1300.degree.C, and it is moved toward the seed end of the boat without melting the seed. This heater is then moved to the right to react the gallium and arsenic, producing a polycrystal ingot as above. The heater is returned to the seed end and then passed again down the boat to zone refine the polycrystalline ingot. The the block is returned to the seed end and moved into the seed so that about one-half inch of the seed melts. The block is then moved toward the right end of the boat at a rate on the order of 1 inch per hour, and a single crystal is formed behind the melt.

The invention has a number of features and advantages. For example, light use of radiant heat and air cooling maintain the quartz tube safely below its softening point even near the molten zone. The liquid-solid interface can be shaped by controlling the circumferential temperature gadient produced by the lamps around the molten zone. The removable seal permits the tube to be opened and closed easily. The tube can be reused, whereas in some prior art furnaces the tubes were sealed with a torch and had to be broken open and discarded. The molten zone can be viewed directly, and the length of the zone can be monitored and used to control light intensity. Accurate seeding can be done because the seed is visible, and the traveling heater can produce ingots many times its own length.

It is apparent from the foregoing that a new and improved crystal growing furnace has been provided. While only the presently prefered embodiment has been described, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

Claims

I claim:

1. In a furnace for processing semiconductor materials:

A. means defining an axially extending chamber having a wall fabricated of a material which is transparent to heat energy of a predetermined wavelength;

B. boat mean disposed within the chamber for holding a semiconductor material;

C. traveling heat source means comprising a generally annular heating element holder movable axially of the chamber and having a plurality of inwardly facing axially extending recesses, a plurality of axially extending radiant heating elements mounted in the recesses and elements spaced about the inner periphery of the heating element holder for delivering radiant heat energy of the predetermined wavelength to an axially limited portion of the material held by the boat means to melt the same, the walls of said recesses serving to reflect heat energy toward the material in the boat, and air passageways communicating with the recess in the heating element holder for directing air to the lamps and chamber wall to cool the same; means for moving the traveling heat source means axially of the chamber; and

E. the relative amounts of heat delivered by the heating elements being adustable whereby the distribution of heat peripherally of the chamber can be set to provide desired molten zone and freezing front contours in the material held by the boat means.

2. The furance of claim 1 in which the radiant heating elements are high intensity lamps.

3. In a furnace for preparing a semiconductor compound having at least one metallic constituent and at least one volatile constituent:

A. a substantially horizontal elongated tube defining an axially extending chamber;

B. a boat disposed within the chamber for holding the compound and/or its constituents;

c. a fixed heating coil disposed coaxially of the chamber and outside the tube for heating materials with the chamber;

D.

heater means disposed coaxially of the chamber and outside the tube for delivering radiant heat energy of a predetermined wavelength to a portion of the material in the boat to melt the same, said traveling heater means including `1. a lamp holder movable axially of the chamber, said lamp holder comprising an annular body having a plurality of radially spaced inwardly facing axially extending recesses formed therein, the walls of said recesses serving to reflect heat energy toward the material in the boat, and air passageways in the body communicating with the recesses, and

2. a plurality of axially extending high intensity lamps mounted in the axially extending recesses and spaced peripherally about the tube;

E. means for moving the traveling heater means axially along said chamber; and

F. control means for adjusting the relative amounts of heat energy produced by the lamps whereby the distribution of heat peripherally of the tube can be ajdusted to provide desired molten zone and freezing front contrours in the material in the boat.

4. The furnace of claim 3 in which the lamp holder comprises an annular body having

A. a plurality of inwardly facing axially extending recesses in which the lamps are mounted, the walls of said recesses serving to reflect heat energy toward the material in the boat,

B. air passageways in the body communicating with the recesses, and

C. a window formed in the body to permit the melted portion of the material to be viewed externally of the chamber.

5. The furnace of claim 3 in which one end of the tube is closed by a removable closure and sealed by a reusable seal.