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Research from Taiwans Chin-Yi University of Technology has revealed a novel heater design in the Czochralski silicon crystal growth process that can control and decrease oxygen concentration without incurring the costs associated with other methods, such as installing magnets or using alternative crucible materials.
Researchers at Taiwans National Chin-Yi University of Technology proposed a novel heater design for Czochralski silicon crystal growth equipment to control and decrease oxygen concentration.
The method was simulated and validated, finding that the optimal design allowed oxygen reduction to 6 parts per million (Ppm) of atoms.
For multi- and monocrystalline silicon photovoltaic applications, oxygen is a key impurity. For example, it can cause the formation of silicon oxide, which increases the hardness of the crystals, which can complicate further processing.
“Our findings indicate that certain oxygen defects reduce the lifetime of the material and increase recombination activity at dislocations,” Amir Reza Ansari Dezfoli, first author of the research, explains to pv magazine .
There are several ways to address these types of problems. “In our study, we focused on controlling, mainly reducing, the oxygen impurity by modifying the heater design in the Czochralski (CZ) extractor,” Dezfoli said, noting that an oxygen reduction of 6 Ppm atoms could be achieved “simply by altering” the heater design configuration.
“It was particularly interesting to find that a low-cost modification to the heater could significantly impact oxygen impurity control, demonstrating how a simple adjustment can have a significant effect,” he added.
The study first investigated the heater design and the distribution of heat sources using simulations, and then tested it in an experimental setup to validate it. The simulation was based on the growth of CZ crystals on a silicon ingot of 200 mm diameter and 700 mm length, with four different heater designs. The heaters were made of graphite and the lengths of the upper section varied between 500 mm and 200 mm.
For validation, the experimental setup used a heater height (H1) of 500 mm. Fourier transform infrared spectroscopy (FTIR) data analysis was used to measure oxygen concentrations in the crystals along the axial direction at the ingot centerline. Heat transfer and impurity transport, heater power, maximum crystal front deflection, and oxygen concentration measurements were performed. Temperature profiles within the heater as well as the impact on the silicon melt and crystal were analyzed.
The validation comparison confirmed a “high agreement between simulation and experimental data.”
Concluding that heater design "significantly influences temperature distributions and melt patterns, affecting oxygen distribution and its transport mechanisms," the paper listed the effects of varying the length of H1, noting that it was clear that controlling temperature profiles and melt flow patterns "significantly" affected oxygen distribution.
The group is now preparing to present the first discrete model to simulate the formation of crystalline-derived particles (COP) and the formation of void defects during the CZ process. “It will be the first model capable of operating in a similar way to laser particle counting in the silicon wafer production industry, opening up new possibilities for simulation-based quality control,” says Dezfoli.
The proposed technology was presented in “ Engineering insights into heater design for oxygen reduction in CZ silicon growth,” published in Case Studies in Thermal Engineering. |
