Keywords: Czochralski method; Large diameter single crystal; Vacuum stability; Air flow control
1 Introduction
The rapid development of semiconductor technology promotes the development of silicon single crystal growth technology in the direction of large diameter. At present, the large-scale production of large-diameter Czochralski single crystal manufacturing in China has just started, and many technologies are still in the exploration stage. The growth of dislocation-free large-diameter single crystals requires a high-stability growth environment. This makes some factors that undermine the stability of single crystal growth have little influence on the original small-diameter single crystal growth, but the negative influence on the large-diameter single crystal growth is increasingly apparent.
In the process of Czochralski single crystal growth, the airflow in the furnace runs through the single crystal growth zone from top to bottom, taking away the silicon oxide and impurity volatiles produced by high temperature in time. Therefore, it has become an important subject to improve equipment and increase crystallization rate in the field of semiconductor material manufacturing to keep the vacuum value in the single crystal furnace stable, and at the same time make the protective gas have a reasonable airflow direction and quickly take away impurities.
2 stability control of vacuum degree
High-purity argon is injected from the top of the single crystal furnace, and the gas is pumped out from the bottom by a vacuum pump. The vacuum value in the furnace keeps dynamic balance (generally around 20 Torr). However, due to various external factors, this balance is often destroyed, which makes the vacuum value change in a large range, especially in the growth of large-diameter single crystals.
2. 1 Factors affecting vacuum instability
First of all, in general equipment, the inlet flow of argon is controlled by a rotameter. The rotameter controls the gas flow by changing the size of the vent hole. Its disadvantage is that the air flow must change with the change of inlet pressure. In actual production, the pressure of gas source will inevitably be affected by the ambient temperature and the amount of argon in the storage tank.
Secondly, the vacuum pump is the power equipment for vacuumizing. In the process of crystal pulling, impurities and silicon oxide volatilized at high temperature in the furnace will be absorbed into the vacuum pump oil and mixed with the pump oil. With the increase of working time, the viscosity of vacuum pump oil will increase continuously, which will lead to the decrease of vacuum pump efficiency. To a certain extent, the vacuum pump must replace the pump oil regularly. In addition, the temperature of vacuum pump oil is also a factor affecting the efficiency of vacuum pump.
2.2 improvement scheme
In view of the two problems mentioned above, we start with argon gas intake system. In order to ensure a constant inlet velocity, we use a mass flow controller (MFC) instead of a rotameter. The mass flow controller can accurately measure and control the gas flow, and its measurement technology is based on an American patent (U.S. Patents 4,464,932 and 4,679,585). The mass flow controller detects the "mass flow" of gas, which is only affected by three characteristics of gas itself (heat capacity, density and molecular structure). For a certain gas, the above three parameters are all certain. Therefore, the measurement accuracy of MFC is not affected by external factors such as gas temperature and pressure. In the range of 20 ~ 20~200SLPM, the control accuracy can be higher than 65438 0.0% and the response time is less than 2s.
Secondly, consider the control of vacuum pumping speed. We added a stepping butterfly valve to the pipeline between the single crystal furnace and the vacuum pump. The purpose of adopting stepped butterfly valve is to adjust the vacuum pumping speed by changing the aperture of the air pumping channel. This is a closed-loop control system. The vacuum pressure in the furnace is detected in real time by digital vacuum meter, and the vacuum value is compared with the set vacuum value. When the vacuum value in the furnace is high, gradually open the step butterfly valve to increase the pumping speed and reduce the vacuum value to the set value. On the other hand, if the vacuum value in the furnace is low, turn down the step butterfly valve to reduce the pumping speed. With such a closed-loop system, the vacuum value in the single crystal furnace can be very stable and the interference of external factors can be avoided.
3 the best control of airflow
In the process of single crystal growth, a large amount of silicon oxide (mainly composed of SiO, but also a small amount of SiO _ 2, which is yellow smoke), impurity volatiles and volatile gases will be produced by objects in the furnace such as silicon melt and timely crucible due to high temperature. These gas and dust particles float around the single crystal growth interface. When the argon flow rate decreases, it can be clearly seen that there is thick smoke above the silicon melt, commonly known as "smoke". Argon gas passes through the single crystal growth zone from top to bottom, taking away gas and dust impurities. Sometimes, SiO particles may be adsorbed on the growth interface of single crystal, causing dislocation in the atomic crystal direction of the growing single crystal, which makes the growth of single crystal fail, commonly known as "bud breaking" and reduces the crystallization rate.
Because of the large diameter of single crystal, more polysilicon is needed, and a larger diameter timely crucible is used. Naturally, when large-diameter single crystals grow, more gas and dust impurities will be produced, which will increase the dislocation probability. Therefore, large-diameter single crystals need to remove gas and dust impurities more quickly.
3. 1 air inlet modification
In order to take away the volatile gas dust as soon as possible, the argon flow rate must be large enough. The argon flow rate of large diameter single crystal is generally 60- 100SLPM. Especially for the heavily doped single crystal which is difficult to crystallize, due to the large doping amount and volatile matter, a large argon flow rate is needed. It is worth noting that atmospheric flow will produce high-speed airflow at the air inlet of the furnace top and form irregular airflow vortex around the airflow. In the process of crystal pulling, the single crystal hangs on the steel cable in the form of a flexible shaft, and the high-speed airflow will burst into a whirlwind, which will make the steel cable and the single crystal shake back and forth and become unstable, greatly increasing the possibility of dislocation and bud breakage of the single crystal growth.
In order to avoid this situation, we improved the shape of argon inlet. The improved air inlet is like an annular shower head. From the original air inlet to a plurality of microporous air inlets, the air flow direction diverges outward. In this way, the total aperture of the air inlet is unchanged, which ensures a large air intake and makes the air flow relatively moderate and dispersed.
3.2 Reasonable airflow direction
Large air volume does not mean that the effect of taking away particulate dust is good, but a reasonable airflow direction is a more important factor. When argon gas passes through the single crystal growth zone, because the liquid level of silicon melt is lower than the upper edge of the timely crucible, and the surface of the melt is recessed into the crucible, most of the airflow will directly flow from the outside of the crucible wall to the lower part of the furnace body, and only a small amount of airflow will enter the timely crucible, so the efficiency of taking away gas, dust and impurities naturally decreases. This situation is more serious when the melt in the crucible is shallow. In order to avoid this situation, when drawing large-diameter high-quality single crystal, the gas guide hood technology is adopted to make the gas flow reasonably in the furnace body and take away the impurity gas dust more effectively.
It is very important to use the gas guide cover for the growth of large diameter single crystals. The air guide cover can be oriented by airflow and has different shapes and designs to achieve different functions. This is a basic air guide cover. First, argon enters the single crystal growth zone downwards, and the gas flow is directly introduced into the crucible from the cylindrical gas guide cover, and the lower edge of the gas guide cover penetrates into the crucible, which directly acts on the gas and dust impurities near the single crystal growth surface. Then, due to the guiding effect of the inner wall of the crucible, the gas diffuses on the surface of the melt, rises with the inner wall of the crucible, and finally flows from the outer side of the crucible to the lower part of the furnace body.
4 conclusion
Among the improvement measures introduced above, the use and design of the gas guide cover is very important for the growth of large-diameter single crystals, which can greatly improve the crystallization rate of single crystal growth. Because the production of large diameter single crystal in China has just started, the research on gas guide cover is still in its infancy. In foreign countries, gas guide covers have been widely used for the growth of large-diameter single crystals. The maturity of a set of gas guide covers requires semiconductor material manufacturing companies to pay a lot of time and money for repeated testing and improvement. Therefore, major semiconductor companies have their own patented technology for the development of gas guide cover technology, which is kept secret from each other and generally refuses to visit and exchange. For this reason, this paper only makes a theoretical analysis on the technology of air guide cover.
There are many factors to be considered in the design of gas guide cover, such as the influence of gas guide cover on the quality indexes such as oxygen content and carbon content of single crystal; Proper clearance between the gas guide hood and the objects in the furnace; How to repair the installation; The influence on the visual field of the main observation window and the side observation window diameter detection equipment after installation; It is also used in conjunction with the air guide ring installed above the heater to achieve better results and so on. With the progress of domestic large-diameter single crystal growth technology, it is of great significance to study and explore these aspects.