*3.1. Temperature Distribution*

Although a temperature of around 1300 K is required for polysilicon deposition, the furnace wall maintains a relatively low temperature due to the existence of the interlayer cooling water outside the furnace. Figure 4 shows the temperature distribution inside the furnace with different designs. The temperature of the furnace wall is typically somewhere between 400–460 K, and the mid-upper part has a higher temperature, which corresponds to the higher temperature of silicon rods. In contrast, design C has a lower furnace wall temperature and better distribution uniformity than design B, which is related to the heat consumption of the larger flow of inlet gases. In order to understand the temperature distribution in the furnace cavity, four horizontal sections with different heights (from bottom to top, they are 0.5 m, 1.1 m, 1.7 m and 2.3 m, respectively) and a symmetrical vertical section (passing through two nozzles and the center of the base plate) in the reduction furnace are displayed. Compared with simply increasing the flow velocity, a larger flow rate is conducive to reducing the temperature in the furnace cavity to a certain extent, and its temperature distribution is relatively uniform. The results imply that design C can provide similar uniform deposition conditions to those of the original scheme. Although design B improves the gas inlet flow velocity, the overheated area at the top of the furnace cavity is large and concentrated in the middle and upper parts of the silicon rod, which may lead to an intense gas-phase reaction and the formation of too much silicon dust or powder. In actual production, excessive silicon dust may lead to a short circuit between electrodes or silicon rods, which increases the risk of production process termination and greatly reduces the output.

The heat inside the whole furnace comes from the resistance heating of the silicon rods; the heat is mainly concentrated on the surface of the silicon rod, and the current density in the center of the silicon rod is almost zero [23]. The surface of the silicon rods is where the reaction takes place directly, so the heat transfer state on the surface of the silicon rod deserves attention. The temperature distribution along the rod surface was obtained after the heat transfer was simulated, as shown in Figure 5. The surface temperature of the silicon rods is around 1360–1400 K, but the temperature at the upper part of the silicon rod is particularly high. In the actual process, these areas correspond to areas where a popcorn-shaped structure is most likely to be produced [6]. According to the Arrhenius formula, the reaction rate constant is exponentially related to temperature. The higher surface temperature of the silicon rod greatly increases the reaction deposition rate in this area, resulting in the phenomenon of popcorn-shaped polysilicon near the bridge part of the silicon rods in the actual process, which directly leads to a reduction in the proportion of dense or qualified materials. The rod surface temperature in design B and design C is generally higher, which means a faster deposition rate and greater preparation efficiency. Meanwhile, the temperature at the silicon rod bridge in designs B and C is not very prominent compared with that in design A, which indicates that the temperature uniformity in design B and design C is improved. The temperature measurement at a height of 2 m for the outer-ring silicon rods shows that the difference between the simulated value and the measured value is equivalent to the error range of the temperature meter, which suggests that the calculated result is reliable.

**Figure 4.** Temperature distribution of furnaces. (**a**) Design A; (**b**) design B; (**c**) design C.

**Figure 5.** Temperature distribution along rod surface. (**a**) Design A; (**b**) design B; (**c**) design C.

A higher temperature in the deposition area will make the surface unstable and produce popcorn polysilicon. The design of the furnace structure means that the silicon rod is easily overheated at the rod bridge part, because more Joule heat is formed at this part due to the concentration of current density. Local overheating of the silicon rod will cause too intense a reaction to form unqualified polysilicon material. In addition, as the feed gas enters from the base plate inlet nozzles, the raw material is gradually heated during its rising process. At the same time, the decrease in the gas flow rate forms a gas stagnation zone, which leads to the failure of the timely replenishment of reaction gas and timely removal of tail mixed gas, and also promotes the heat concentrated in this area. Therefore, in order to improve the utilization rate of raw materials and reduce energy consumption, it is necessary to ensure the uniformity of deposition conditions on the surface of the silicon rods, reduce the overheating area and then improve the reaction rate, especially in the upper part of the reactor. In view of this, the gas flow rate and the thickness of the boundary layer on the surface of the silicon rods in different schemes are analyzed in the following content.
