(l) The vacuum is too high and the vacuum gauge has an indication (ie <133Pa), but the ultimate vacuum pressure cannot be pumped to the original set pressure. There may be three phenomena in this situation: The first is that there is a slow leak in the vacuum system, and the system fails to detect leaks.
Cause and solution: O-ring of each interface is not clean or the joint nut is not tightened. What we often do is that the large O-ring of the furnace door is not installed or the O-ring is aging and deformed after a period of use, which affects the sealing performance. At this time, it can be solved by replacing the O-ring;
(2) The second case is that the vacuum degree indication is abnormal, but the system leak detection is acceptable, and the leakage rate is less than 0.3Pa/min.
Reason and solution: It is very likely that the pressure sensing part is blocked, and it can be eliminated by cleaning it. Of course, the possibility of a decrease in the pumping rate of the mechanical pump cannot be excluded.
(3) The third phenomenon is that the vacuum cannot be drawn up after the furnace tube is cleaned.
Reason and solution: It may be a cold trap (particle trap), the bellows is not completely dried after cleaning, and the remaining water is slowly vaporized after the mechanical pump is vacuumed. Therefore, it is required that the bellows and cold trap should be dried in an oven after cleaning, and the temperature of the oven should be kept at 80-100°C.
B. When the mechanical pump is vacuuming, the vacuum gauge has no indication (that is, the pressure is less than 133Pa).
Cause and solution: It may be that the furnace door is not completely closed when entering and exiting the boat. At this time, a "creaking" sound can be heard at the furnace mouth. This phenomenon can be eliminated after closing the furnace door.
C. The deposition rate becomes slower. Under normal circumstances, when the furnace temperature, gas flow, and working pressure are constant, the reaction rate remains basically stable, but in actual work, we found that the deposition rate sometimes slows down significantly. Check that the gas flow, furnace temperature, and working pressure are all normal. This phenomenon is especially common in the SiH2C12+NH3 system.
Reasons and solutions: The flow of SiH2C12 into the reaction system is reduced. Due to excessive NH3, it is sometimes difficult to see from the working pressure. Disassemble the SiH2C12 flowmeter control valve (solenoid valve) or sensing part, you can see a partial blockage from the small hole, and the rate can be restored after cleaning.
D. No deposition. This phenomenon sometimes occurs in the deposition process of polysilicon thin films. Because the furnace temperature is required to be 550-620℃ during polysilicon deposition, sometimes the heating power switch is tripped due to overcurrent when the furnace heats up, and the furnace temperature has dropped during actual deposition. When the temperature is below 500°C, the gas flow is abnormal and the deposition cannot be achieved.
In order to keep the CVD equipment operating normally for a long time, in addition to requiring the operator to operate and use the equipment correctly, taking some effective methods to maintain the equipment is the key to protecting the normal operation of the CVD equipment. For this reason, the following measures can be taken:
(l) Keep the vacuum system of the CVD equipment in a vacuum state as much as possible. According to the requirements of the deposition process, the CVD equipment used process gas and reaction generated by-products are generally highly corrosive. in case
After the Si3N4 film is deposited, if the reaction residual gas is not exhausted in time, it is likely to corrode the gas interface, bellows, vacuum gauge, vacuum pump, etc. Therefore, after the Si3N4 film is deposited, the pumping time of the vacuum pump should be extended as much as possible to remove the residual gas as much as possible. When not in use for a few days, turn on the vacuum pump once to pump once to improve the equipment's intact rate.
(2) Fill with nitrogen in time. After the process is completed, the quartz boat in the furnace tube is equipped with the deposited silicon wafers. It must be stored in an environment filled with nitrogen to prevent oxidation and corrosion, so that it can be better preserved.
(3) Regularly replace the vacuum pump oil. Although the vacuum system used is equipped with a vacuum pump oil filter, it can keep the pump oil clean for a long time. However, for a reaction system like SiH2C12+NH3, the byproduct NH4CI white powder is generated at the end of the furnace and both ends of the furnace mouth during the reaction. The deposition of NH4CI on the bellows and vacuum tube walls at the back of the reaction tube is quite serious. It and HCI and other harmful impurities are in danger of being pumped to the vacuum pump at any time. These particles continue to enter the vacuum pump in small quantities, thicken the pump oil, block the pump oil filter system, and affect the normal operation of the equipment. Therefore, the vacuum pump oil must be replaced regularly.
(4) Regularly check and repair the solenoid valve. In order to prevent the vacuum pump oil from returning to the vacuum pipeline, a solenoid valve is installed on the vacuum pump. The common fault of the solenoid valve is that a small amount of NH4CI powder in the solenoid valve reduces the sealing performance of the O-ring seal in the solenoid valve, resulting in There is a slow leak in the vacuum system. The vacuum system of the entire equipment cannot maintain a vacuum state after shutdown. Only regular maintenance of the solenoid valve and replacement of the O-ring inside the solenoid valve after cleaning can the CVD equipment be vacuumed after the film deposition is completed. The system is under vacuum.
(5) Clean the quartz tube regularly. During the deposition process, a thin film is deposited on the quartz tube and the quartz boat. After each process, the quartz boat should be moved appropriately in place to prevent the quartz boat and the quartz tube from sticking. With the increase of process production, the film deposited on the quartz tube and quartz boat becomes thicker and thicker. After reaching a certain level, the quartz tube has cracked silicon, which increases the load-bearing capacity of the entire quartz tube, exceeding its maximum load, and a quartz tube sinks to a certain extent (the original two quartz tubes are on the same horizontal plane). on). As a result, the silicon wafers on the quartz boat also sink together, deviating from the center of the furnace tube, causing uneven film deposition. Therefore, regular cleaning can ensure the stability and repeatability of the process, thereby ensuring product quality