FIELD OF THE INVENTION
This invention relates to the use of a reusable crucible for silicon ingot growth.
BACKGROUND OF THE INVENTION
Silicon ingots are typically produced by one of three methods: (1) pulling an ingot from melt, (e.g., the Czochralski process); (2) solidifying a melt in a crucible by directional solidification techniques; or (3) pouring a melt from a crucible into a mold using casting techniques.
In a crystal pulling process, such as the Czochralski method, a clear fused silica crucible (commonly called a quartz crucible) is used for melting silicon. The ingot is pulled from the melt such that there is contact between the melt and the crucible, but no contact between the solidified silicon ingot and the crucible.
In a directional solidification process, silicon is typically melted in the crucible and directionally solidified in the same crucible. In this case, there is contact between the silicon melt and the crucible, as well as contact between the solidified silicon ingot and the crucible.
In a casting process, the silicon is melted in a crucible and the melt is then poured from the crucible into a mold, where the silicon solidifies. In this case, there is contact between the melt and the crucible, the melt and the mold (although minimal), and the mold and the solidified ingot.
Casting useful and acceptable silicon ingots can be difficult due to the characteristics of silicon in its solid and molten forms. One problem that arises is that impurities from the container can contaminate the silicon melt because of the high reactivity of the silicon melt. Furthermore, the solidifying melt can adhere to the container walls, causing an ingot and/or a container to crack during the cooling process because of differences in respective thermal expansion coefficients. Protective layers of compacted Si3N4, SiO2, Si, and graphite powder, or of a low-melting encapsulant, are used to reduce such adherence.
Various methods of silicon ingot casting have been proposed in the past to attempt to overcome these obstacles. For example, molds have been lined/coated with Si3N4 powder. When the silicon melt has been poured into an adequately heated mold, a reaction between the mold wall and the molten silicon is minimized.
Historically, clear fused silica crucibles have been favored for crystal pulling processes. Clear fused silica can be used because there is no contact between the ingot and the crucible. In addition, the reaction between the melt (Si) and the crucible (SiO2)
is minimized by keeping the pressure in the chamber above approximately 20 torr so formation of the reaction product, SiO, is minimized.
Silica remains the material of choice for crucible and mold applications because it is readily available in high purity form, and because the reaction product of Si and SiO2 is a gaseous phase and removable from the heat zone, thereby minimizing the contamination that can occur during the production of silicon ingots.
However, when clear fused silica crucibles was initially used for directional solidification or as molds for casting techniques, the SiOx (0≦x≦2) phase was formed between the ingot and the crucible/mold, resulting in a tenacious bond between the ingot and the crucible/mold. During the cooling process, this tenacious bond caused the ingot to crack. Thus, while clear fused silica crucibles can be used for melting silicon, they are not appropriate for use in directional solidification processes or as molds for casting (without a coating) if the desired result is crack-free silicon ingots.
Schmid et al. U.S. Pat. No. 4,218,418, describes a silica crucible with a graded structure that can be utilized as a crucible for producing crack-free silicon ingots because the crucible delaminates during the cooling process. Other approaches, such as using protective coatings of Si3N4 powder on the inner walls of quartz or sintered silica crucibles, have also been described for production of crack-free silicon ingots. In all such cases, the crucible/mold is sacrificed to produce a crack-free silicon ingot. Therefore, although these crucibles can be used to produce crack-free silicon ingots, they can only be used in production once, thereby adding significantly to the cost of silicon ingot production. During production of a silicon ingot, high temperatures cause the initial amorphous or glass phase silica to transform, at least partially, to the crystalline phase. This transformation reduces the reliability of the silica crucible to contain molten silicon during reuse.
A typical processing cycle for producing multicrystalline silicon ingots using a graded silica crucible involves the following procedure. The square cross-section crucible is supported with graphite plates at the bottom and sides of the crucible. Polycrystalline silicon meltstock, with an appropriate amount of dopant to achieve the desired resistivity in the ingot, is loaded into the crucible and the crucible is placed on the heat extraction system in the heat zone of a Crystal Systems, Inc. Heat Exchanger Method (HEM) furnace designed for directional solidification of silicon ingots. The furnace chamber is evacuated and heat is applied with a graphite resistance heater. The temperature in the heat zone is increased to approximately 1500° C. and maintained at this temperature until the entire silicon is in a molten stage. The chamber is maintained at approximately 0.1 torr pressure.
The furnace temperature is slowly decreased until the heat zone is less than approximately 100 above the melting point of silicon. During this decrease of furnace temperature, heat is extracted with a heat extraction system so that directional solidification of molten silicon is achieved from the bottom of the crucible towards the top surface of the melt with a slightly convex solid-liquid interface. The movement of the solid-liquid interface from the bottom of the crucible to the top surface of the melt is approximately 1-3 cm per hour. After complete solidification of silicon has been achieved in the crucible, the furnace temperature is decreased and the silicon ingot is cooled in the furnace within the crucible. The crucible delaminates, thereby preventing the ingot from cracking. The delamination causes the ingot to be rough and an allowance of nearly an inch is required to fully clean up the surface.
The requirements for the use of silica as a mold are less stringent than those for its use as a crucible. Molten silicon is poured from a crucible into a mold which is maintained at a temperature just below the melting point of silicon (1412° C.) so that a chilled solid layer is formed on the inner walls of the mold. Solidification of the ingot is achieved in a controlled manner; therefore, in the case of a mold, (i) the highest temperature used is 1412° C.; (ii) there is essentially no contact between the silicon melt and the mold; and (iii) the solidification is rapid so that the time the mold has to contain the molten silicon is short. Mold materials that have been used to produce crack-free silicon ingots include mullite, Si3N4 and graphite. Applying a protective coating to these molds makes it easier to remove the ingot and increases the reusability of the molds.
The requirements for the use of silica as a crucible, by contrast, are quite stringent. A crucible for use in silicon ingot production (i) must withstand significantly higher temperatures than 1412° C. so that the entire silicon meltstock can be melted; (ii) is initially in contact with the solid meltstock and with the molten silicon after it has melted; (iii) has to withstand longer solidification times; and (iv) is in contact with the solid silicon. In other words, the crucible must withstand much higher temperatures and for a significantly longer period of time. Any protective coating put on the inner walls of the crucible has to withstand similar conditions and retain its integrity through any movement of the melt during meltdown and solidification.
In an effort to reduce the costs of producing crack-free silicon ingots, two reusable crucibles have been developed. Graphite with liners, such as Si3N4, and/or encapsulant has been used as a reusable crucible. The liner and/or encapsulant is used to prevent the contact of the silicon ingot with the crucible which minimizes the cracking problem. However, use of the encapsulant adversely affects dimensional control of the ingot and removal from the crucible. The dimensional control problem occurs because the thickness of the encapsulant layer is non-uniform. This non-uniformity leads to variation in the ingot size and therefore, poor utilization of usable standard size bars. In order to remove the ingot from the encapsulated ingot, the crucible is placed upside down after solidification and re-heated to a temperature at which the encapsulant will melt. After the encapsulant has melted, the ingot can be removed from the crucible.
SUMMARY OF THE INVENTION
The present invention is directed to a reliable and reusable crucible for use in directional solidification of multicrystalline silicon ingots and in casting of silicon ingots.
Embodiments of the invention include use of a silicon nitride crucible coated with a crucible release coating for use in directional solidification/ingot casting of multicrystalline silicon ingots. The crucible is preferably made of reaction bonded silicon nitride, but can also be isopressed silicon nitride. The coating prevents the crucible from direct contact with the silicon melt when used for solidification of multicrystalline silicon ingots. After removing the silicon ingot, the release coating is easily removed and the crucible can be repeatedly recoated and reused.
Other features and advantages will become apparent from the following detailed description and claims.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to use of silicon nitride, preferably reaction bonded silicon nitride (RBSN) or isopressed silicon nitride, in reusable crucibles for silicon ingot growth formation with directional solidification. The crucible is coated with a release coating that adheres to the crucible and acts as a buffer between the crucible and the ingot so that the ingot can be removed from the crucible and the crucible can be reused after it is coated again.
The RBSN crucible can also be used as a reusable mold. In addition, one of skill will understand that it is possible to use the crucible as a mold for creating one or more crack-free silicon ingots, to then use the crucible as a reusable crucible before switching back to using the crucible as a mold. Other variations are also contemplated.
In one embodiment, the crucible release coating is Si3N4. The Si3N4 coating also acts as a barrier to the incorporation of impurities in the silicon melt from the crucible during processing. Other materials that can be used for a release coating include, but are not limited to, SiO2, mixture of SiO2 and Si3N4, SiC, graphite wool/cloth, and silica wool/cloth.
The ingots that are produced have flat sides with good uniformity, so the material utilization is high, and less is wasted around the sides than in some other processes.
The crucible is preferably designed to have an expansion coefficient that is low and is similar to that of the silicon. Silica, by contrast, has a much lower coefficient than silicon nitride. In the case of RBSN, additives (such as a grog) can be provided to provide a lower density in the silicon nitride and thereby RBSN will be more resilient to expansion changes.
The crucible is preferably designed with an approximate 2°-5° vertical taper for the side walls which range from about 10 mm to about 25 mm thick for ease of removal of the ingot after directional solidification.
For the times and temperatures required for use of crucibles, Si3N4 has a high vapor pressure, so it is desirable to develop a processing cycle that is compatible with the crucible. The temperature and pressure parameters should be designed to substantially prevent silicon from wicking up the inner surface of the crucible, and to prevent decomposition of the coating. The modified processing cycles described in the following examples were developed for use in HEM graphite resistance heated furnace. If another type of furnace, such as an induction heated furnace, were used, optimal temperature and pressure parameters could be somewhat different. One of skill in the art could determine an efficient temperature and pressure to use in their particular furnace.