The present invention generally relates to a heater arrangement for a furnace and, more specifically, to a heater arrangement for a crystal growth furnace suitable for producing a high volume of crystals.
Furnaces for the production of crystals, such as single crystals of calcium fluoride, typically have a crucible which is loaded with a seed and/or starting material. A heater (or heaters) is arranged about the crucible to produce a temperature gradient to grow the crystals in the crucible. Growth is obtained by varying power to the heater according to an established power-temperature relationship to obtain the desired thermal environment.
The thermal gradient obtained is critical to growing a single crystal rather than a polycrystalline structures. Additionally, the quality of a single crystal is believed to be primarily affected by the applied thermal gradient. Present furnaces for the production of macrocrystals, therefore, have elaborate and complex heaters and/or controllers for controlling the heaters to obtain the desired thermal environment. These complex devices are expensive to produce and complicated to operate and maintain.
Further, high quality calcium fluoride crystals are of great need in the semiconductor manufacturing industry. Commercial success in growing large, monocrystalline ingots from calcium fluoride, has been limited to ingots have a diameter of 15 inches or less.
In high volume production of crystals it is important to obtain the desired thermal environment and is also important to get consistent temperature environments. In addition, large diameter single-crystal growth requires increased precision and consistency in control of temperature environment. Accordingly, there is a need in the art for a crystal growth furnace which is simple to produce and operate, produces desired thermal environments for growing crystals, and produces consistent thermal environments for growing a high volume of crystals and for growing crystals of a large diameter.
It was found, particularly when growing large crystals in the order of 28xe2x80x3 diameters, that the outer circumference of the crystal experienced different temperatures than the central area. As a result, defects could occur in the outer few inches of the circumference of the crystals. Such defects would include small bubbles, reduced height, inclusions, etc.
In order to provide more uniform growth of large crystals, this invention includes a heater extension or supplemental heating unit. It includes a disk that is generally xe2x80x9cwasher-shaped,xe2x80x9d having an opening in its center. However, it could be solid and not have a center opening. Dimensions may vary widely depending on the size of the crucible and the primary heater. This disk is located below the bottom heater and above the bottom heater insulation. Whatever the dimensions, the disk should extend beyond the diameters of the bottom heater, crucible and crucible insulation.
The second part of the heater extension or supplemental heating unit is a cylinder connected to or near the disk. The cylinder extends upwardly from the bottom disk to a height which is sufficient to overcome the cooled edge effect.
In all cases, the dimensions and material of the supplemental heating unit may be varied to enhance the main heater""s effectiveness in maintaining a constant temperature at the edges of the crucible and the crystal inside of it.
If desired, a nearly identical heater extension could be used on top of the crucible. The upper cylinder would have to be attached firmly to the upper disk by a fastener or adhesive of some type since gravity would be trying to separate them. However, the upper disk could simply be laid on top of the crucible without otherwise fastening it. Alternatively, it could be spaced from the crucible top or top wall.
Alternately, the supplemental heating units could be active heaters. As such, they would have a power source which could be varied to provide more precise control of the heat at the edge of the crucible. In this embodiment, different amounts of power could be supplied to the supplemental disk and cylinder to direct or xe2x80x9cvectorxe2x80x9d heat more upwardly or inwardly depending on the needs of the crystal being grown. This latter feature gives control over crystal growth and cooling not heretofore know. Such a powered supplemental heating unit could be placed at the top and bottom edges of the crucible.
A crystal growth station including a cylindrical crucible having a bottom wall, side wall and top wall. The bottom wall and side wall form a bottom edge. The top wall and side wall form a top edge. A bottom resistance heater is mounted below the crucible to provide heat to the crucible. A top resistance heater is positioned to heat the top of the crucible. At least one power source is attached to the top and bottom heater. A first supplemental heating unit is positioned around the bottom edge of the crucible. The first supplemental heating unit may or may not be supplied with a power source to heat it independently of the top and bottom heaters.
If the first supplemental heating unit is not heated independently, then it is formed of a first cylinder around the lower portion of the crucible side wall. A first disk which is washer shaped is attached to the first cylinder and extends under at least part of the crucible bottom wall. The first supplemental heating unit is, in general, wedge shaped and goes around the bottom edge of the crucible. At the bottom edge, the first supplemental heating unit is heated by radiation from the bottom heater. The heat is conducted from the first disk to the first cylinder. As a heated unit, it holds the temperature of the bottom edge more uniform in a heating and cooling process. A second similar shaped supplemental heater may be used at the top edge of the crucible. The unpowered supplemental heaters may be a solid heat conductive material.
Alternately, the supplemental heating units may be resistance heaters and have independently variable power sources. In this case the first cylinder and the first disk are made of resistance heater material and have slits therein to form current paths. The cylinder and disk will usually be separated from one another and may be independently controllable to balance the heat at the top and/or bottom edge of the crucible and its contents.
The present invention provides a furnace for growing crystals which overcomes at least some of the above-noted problems of the prior art. According to the present invention, a heater arrangement for a crystal growth furnace includes a plurality of individual growth stations each having a crucible. The crystal growth furnace also includes a first heater matrix having at least two resistance heaters electrically connected in series or parallel. Each of the individual growth stations has at least one of the resistance heaters of the first heater matrix associated therewith and located near the crucible. By connecting the resistance heaters of separate growth stations in this manner, the temperatures produced by the resistance heaters in the separate growth stations are fixed at the same temperature for a given power level when the resistance heaters are connected to a single power source.
According to another aspect of the present invention, a heater arrangement for growing crystals includes a plurality of individual growth stations each having a crucible. The heater arrangement also includes a first heater matrix and a second heater matrix separate from the first heater matrix. Each heater matrix preferably includes at least two legs electrically connected in parallel with each of the legs having at least two resistance heaters electrically connected in series. Each of the individual growth stations has at least one of the resistance heaters of the first heater matrix and at least one of the resistance heaters of the second heater matrix associated therewith. By having two separate heater matrices, the temperatures produced by the resistance heaters in a large quantity of separate growth stations can be fixed at the same temperature for a given power level yet the temperature gradient formed in each of the growth stations can be varied when each heater matrix is connected to a separate power source. Preferably, the resistance heaters within the first heater matrix are located above the crucibles and provide a homogeneous temperature across the top of the crucibles and the resistance heaters within the second heater matrix are located below the crucibles and provide a temperature gradient across the bottom of the crucibles.