vacuum system in straight pull single crystal furnace

According to the most commonly used silicon single crystal Czochralski process, the vacuum system is generally equipped with slide valve vacuum pump. However, with the increasing demand for large diameter and heavily doped silicon single crystal, the requirements for single crystal furnace equipment are becoming higher and higher, and the disadvantages of using slide valve vacuum pump are increasingly obvious. In order to improve the production efficiency, reduce the cost, and ensure the consistency and reliability of device parameters, we all hope that Czochralski single crystal equipment with large diameter, highly automatic equipment control, and excellent stability and reliability. In order to improve the internal quality of single crystal or the special requirements of some parameters, the water ring vacuum pump is applied to the Czochralski single crystal furnace equipment by reforming the vacuum system to improve the overall performance of the straight pull single crystal furnace.

A straight pull single crystal furnace includes a furnace body, a furnace body includes a furnace seat and a furnace cavity, the furnace cavity is arranged on the furnace seat, the furnace body is provided with a heating device and a direct drawing forming device, the heating device includes a quartz crucible, a graphite crucible, a heater and a graphite electrode, the graphite crucible is wrapped on the outer side of the quartz crucible, the heater is arranged on the outer side of the graphite crucible, and the graphite electrode is arranged at the bottom end of the heater The pull forming device includes a seed crystal shaft and a crucible shaft arranged vertically. The seed crystal shaft is arranged above the quartz crucible, the crucible shaft is arranged at the bottom of the quartz crucible, the seed crystal shaft driving device connected with the seed crystal shaft is arranged above the seed crystal shaft, and the crucible shaft driving device is arranged at the bottom of the crucible shaft.

The slide valve vacuum pump is a mechanical vacuum pump which uses oil to keep the seal. Because there is a certain gap between the slide valve ring and the pump cavity, the two do not contact and can pump out the gas containing tiny dust. In the normal growth process of single crystal, silicon oxide and impurity volatiles produced by high temperature will be removed by slide valve pump. Most of these impurities will enter the pump chamber and mix with vacuum pump oil. As the growth time of silicon single crystal is relatively long, especially the growth time of large diameter silicon single crystal is longer, with the increase of time, the impurities in the vacuum pump oil also accumulate more and more, so that the viscosity of vacuum pump oil is also increasing, resulting in the increase of vacuum pump load and the decrease of pumping speed, which will seriously cause the vacuum pump can not work normally.

Therefore, the vacuum pump oil needs to be replaced regularly, which can also extend the life of the vacuum pump. But frequent replacement of vacuum pump oil not only has high maintenance cost, but also can’t be realized in a process of crystal drawing. In particular, when the silicon single crystal is heavily doped with arsenic and other highly toxic elements, a part of arsenic and other highly toxic elements are incorporated into the vacuum pump oil, which makes the maintenance of the vacuum pump more difficult. The treatment of vacuum pump oil containing arsenic and other highly toxic doping elements is a very difficult matter.

In the application of vacuum system in straight pull single crystal furnace, the water ring vacuum pump can be used as the main pump, and the slide valve vacuum pump can be used as the main pump when the bypass vacuum ball valve is closed to meet the needs of different crystal pulling processes.

When the water ring vacuum pump is used as the main pump, the silicon oxide and impurity volatiles produced by normal crystal pulling are pumped into the water ring vacuum pump and dissolved in the water of the water ring vacuum pump. The water of the water ring vacuum pump is continuously updated to ensure the cleanness of the water, so that the water ring vacuum pump can work normally, and the working performance of the water ring vacuum pump will not be affected due to the long crystallization time Maintain normal pumping speed to ensure stable vacuum in furnace. The discharged water can also be reused after treatment, which is not only convenient for maintenance but also low cost. Especially when the silicon crystal doped with highly toxic elements such as arsenic is pulled heavily, it is easier to separate and collect the highly toxic doped elements such as arsenic into the water, so as to reduce the impact on the environment.