The present invention is related to the field of silica crucibles, and more specifically to a silica crucible having a multi-layer wall in which one or more of the wall layers are doped with aluminum.
Silicon wafers used in semiconductor industries are made from ingots of single-crystal silicon. Such ingots generally are manufactured by one of two processes: the Czochralski (CZ) process and the floating zone (FZ) process. Among those, the CZ-process is more widely used for mass production of single-crystal ingots.
In the CZ-process, metallic silicon is charged in a silica glass crucible housed within a susceptor located in a crystal growth chamber. The charge is then heated by a heater surrounding the susceptor to melt the charged silicon. The susceptor typically is rotated during this procedure. A single silicon crystal is pulled from the silicon melt at or near the melting temperature of silicon.
For higher ingot productivity, a more rapid CZ-process is desirable. However, accelerating the crystal pulling rate beyond a certain rate results in improper silicon crystal structures. Many trials are done to shorten the “melt down” period by increasing heating power. Similarly, in the production of large-diameter wafers, the amount of silicon charge and the melt-down time are increased and more intense energy is input. The total process time is much longer than that for small-diameter ingots.
This harsh melt-down procedure increases the rate of crucible inner surface roughening. Compressing the meltdown period also negatively affects the rigidity of silica crucible. Silica glass is not hard enough to prevent sagging of the side wall in harsher melt down processes. A more dimensionally stable crucible is desired.
At operating temperatures, the inner surface of a silica crucible reacts with the silicon melt. In many cases, it is the inner layer of the crucible that undergoes a change in morphology. More exactly, the inner surface roughens after prolonged operation in the CZ-process. This roughening can cause a loss of crystal structure of the pulled ingot. Inner surface roughening renders the crucible unfit for use in silicon ingot manufacture.
Additionally, the inner layer of a silica glass crucible can be dissolved into the silicon melt during the CZ-process. Silicon and oxygen, the main components of a silica crucible, are not deleterious to the silicon melt. However, impurities in the inner layer of the crucible can be transferred to the silicon melt during this process. To keep the silicon melt free from such impurities, a crucible is required to be extremely pure or to be insoluble by the silicon melt.
A standard method for making a silica glass crucible is disclosed in U.S. Pat. No. 4,935,046. Quartz grain is supplied in a rotating mold in a crucible shape. The grain is then heated by an electric arc to fuse the inner part of the formed grain, leaving the outside grain unfused. During fusion, additional grain is supplied to the inside surface of the crucible. Quartz grain is melted and piled up as a transparent inner layer, while the formed grain is fused rather promptly to make an translucent silica glass substrate. The resultant crucible has a wall comprising a transparent inner layer and an translucent outer layer having a rough outer surface, which is the interface between fused grain and unfused grain.
One crucible is known to reduce the dissolution of the inner surface of the crucible. U.S. Pat. Nos. 5,976,247 and 5,980,629 disclose the creation of a “devitrified” layer inside of a crucible to prevent particulate generation at the silica-melt interface. The devitrified layer is reported to dissolve uniformly. Here, the devitrified layer means a crystallized silica layer, which the present inventors found to dissolve more slowly in the silicon melt than does amorphous silica. The above references claim alkaline-earth elements as a devitrification promoter, with barium recited as an example.
One of the present inventors filed Japanese Patent 3100836 (laid open Tokukai Hei8-2932), teaching an inner layer containing from 100-2000 ppm aluminum and 0.5-1 mm in thickness. The inner layer crystallizes in the CZ-process, so dissolution is suppressed and the dimensional stability of the crucible is improved.
It is known in the ingot manufacturing industry that circular patterns (“rosettes”) are observed on the crucible surface contacting the silicon melt. Examples are shown in U.S. Pat. No. 4,935,046, FIGS. 6A-6B. The ring is referred to in U.S. Pat. No. 4,935,046 as crystobalite. This phenomenon was investigated and determined to be a rosette surrounded by crystobalite.
The crystobalite ring is normally decorated with brown deposit when cooled down after a CZ-process use. It is hypothesized that the brown deposit is either silicon mono-oxide or colloidal silicon. The center of the rosette has a rough surface that is either not covered by crystobalite or covered by a very thin crystobalite layer. The outside of the rosette is the original silica glass surface, which has retained its original smoothness.
As CZ-process time continues, rosettes grow and the surfaces of the rosette centers become rough. Further, the rosettes merge and the rough surface area is increased. The smooth virgin surface decreases and finally disappears. When a major portion of the inner surface of the crucible is covered by a rough surface, the pulled silicon crystal loses its crystalline structure. Such a roughened crucible is unsuitable for ingot manufacture and silicon crystal pulling using a roughened crucible must be ceased to avoid manufacture of substandard ingots.
A method to reduce roughening of the crucible inner surface is disclosed in U.S. Pat. No. 4,935,046. The reference further mentions that growth of crystobalite is suppressed, as the result of applying the method. By applying the layer-by-layer deposition method as taught by this reference, however, suppression of crystobalite is insufficient and roughening still proceeds to a significant extent. “Devitrification” of the outer layer of a crucible is disclosed in U.S. Pat. Nos. 5,976,247 and 5,980,629. By coating a crucible with barium-containing chemicals, the outside of the crucible is “devitrified”, i.e., crystallized, when used in a CZ-process. This crystallized layer reinforces the crucible at operating temperatures and prevents sagging of the crucible side wall.
By using barium as a crystallization promoter, the crystallized layer grows as CZ-process time elapses. The silica glass experiences a large volume change when it crystallizes, creating stress at each of the interfaces of the glassy phase and crystalline phase. This stress is relieved by micro-scale deformation of the crucible. If the crystalline layer thickness exceeds a certain level, the crucible is prone to cracks and possible leakage of the silicon melt. Even if the amount of barium-doped material is carefully optimized to the running conditions, the crucible nevertheless occasionally experiences cracking toward the end of a CZ-process run.
Japanese Patent P2000-247778A discloses a three-layer crucible. The layers are a transparent synthetic silica inner layer, a synthetic silica or natural quartz glass middle layer, and an aluminum-doped silica outer layer. The optimum range for aluminum concentration in the outer layer is reported to be 50-120 ppm. The best mode taught in this reference has an approximately 3 mm thick outer layer doped with aluminum at about 75 ppm.
The doped aluminum outer layer helps to prevent sagging of crucible. However, the inner layer of this crucible is still prone to uncontrolled rosette formation and growth during a CZ-process.
A long-life crucible is therefore desirable, especially a large-diameter CZ-process crucible. Specifically, the side wall of the crucible should be able to maintain its structural integrity without warping, and the inner surface of the wall should resist roughening during a CZ-process.