Clean air-conditioning method for high-precision control of medium temperature and humidity in semiconductor factory (dust-free workshop)

2019-05-21 11:20:36   Editor: jushong shares    0

In recent years, with the development of microelectronics industries such as semiconductors and display panels, the production process has gradually increased the requirements for temperature, humidity and cleanliness of rooms. In some semiconductor manufacturing plants and laboratories, there are clean rooms with a temperature accuracy of ±0.1 °C, a humidity accuracy of ±3%, and a cleanliness class of 5. For the air conditioning treatment solutions of these semiconductor dust-free workshops, we have adopted the MAU+FFU+DCC (two-stage) air conditioning system. The secondary DCC chilled water supply temperature is not much different from the room temperature and the fine temperature difference of the return water is small. The temperature and humidity of the clean room are controlled with high precision, energy saving and high control precision.


1 Air conditioning plan introduction


A clean constant temperature and humidity room in a microelectronics laboratory project in Beijing, with an area of 30 m2 and a net high demand of 3.5 m under the ceiling. Two electron beam lithography process equipments are placed in the clean constant temperature and humidity room. The heat output of each equipment is 800W, the process air volume is 300 m3/h, and the auxiliary equipment of the process equipment is placed under the interlayer. The total heat generation is 7489.5W. The ambient temperature of the clean constant temperature and humidity room is 23 °C ± 1 °C, the humidity is 43% ± 5%, the cleanliness is 5, the particle size is 0.1 μm, the internal temperature is 23 °C ± 0.1 °C, and the humidity is 43% ± 3%. The cleanliness is 5 grades, the control particle size is 0.1 μm, and the indoor pressure is ≥5 Pa relative to the peripheral clean room. In order to ensure the temperature, humidity and cleanliness requirements of clean constant temperature and humidity, MAU+FFU+DCC (two-stage) air conditioning system is adopted. The schematic diagram of the air conditioner in the clean constant temperature and humidity room is shown in Figure 1.


The air temperature and humidity point of the MAU system is the same as that of the indoor. The humidity of the room is controlled by changing the temperature of the air supply dew point. At the same time, to ensure the indoor humidity control requirements, the MAU air volume takes 12 times/h and the air volume + the positive air volume maximum. The MAU system ensures the positive pressure in the room by adjusting the electric regulating damper on the new air duct and the return air duct; the air frequency is controlled by the high-efficiency differential pressure at the end of the MAU unit to ensure a constant air volume sent to the room. The MAU unit section is equipped with a middle, high and high grade filter section, hot water (45 ° C ~ 35 ° C) heating section, first stage cold section (12 ° C ~ 19), secondary table cold section (5 ° C ~ 12 ° C), electric heating Humidification section, fan section, equalizing section, electric heating and reheating section, as shown in Figure 2. The FFU system controls indoor cleanliness, filtration efficiency MPPS efficiency ≥99.9995% @0.12μm, and PTFE filter material to reduce FFU running resistance. In order to ensure the indoor temperature accuracy and cleanliness control, the FFU design section wind speed is 0.40 m/s, and the arrangement rate is 100%. The air volume of the FFU system is greater than the air volume of the room 12 times/h, and the air supply temperature difference is less than 1 °C, which satisfies the requirements of the air exchange frequency and the air supply temperature difference in the constant temperature and humidity room.



The first-stage DCC is located in the lower mezzanine to eliminate the heat generated by the larger heat-generating equipment in the lower mezzanine. The temperature is controlled by the temperature of 12°C~19°C, and the electric two-way regulating valve of the first-class DCC air-conditioning pipe is controlled by the temperature sensor after the first-stage DCC. The temperature before the secondary DCC is stabilized, which provides a prerequisite for high-precision control of indoor temperature.


The secondary DCC is located in the upper static pressure box and is arranged in sections. According to the electric temperature three-way regulating valve of the corresponding secondary DCC air-conditioning pipe controlled by the indoor temperature sensor in each area, different air supply temperatures are obtained, which are used to eliminate the heat generation in different areas of the room and meet the requirements of indoor high-precision temperature control. The secondary DCC uses 12 °C medium temperature water supply and 22 °C backwater for constant temperature and humidity mixing. The water supply temperature after mixing is 20 °C. An electric two-way regulating valve is arranged on the middle temperature water return pipe, and a check valve is arranged between the return water pipes, and the electric two-way regulating valve is controlled by a temperature sensor disposed on the 20°C medium temperature water supply pipe after the water mixing The opening degree, at the same time, the mixing water pump is used with a set to ensure that the water supply temperature of the mixed water temperature is constant. That is to say, the secondary DCC chilled water supply temperature is not much different from the room temperature and the fine temperature difference of the return water is small to meet the indoor temperature control requirements. The diagram of the secondary DCC mixing system is shown in Figure 3.


2 Compared with traditional precision air conditioner


In the past, the traditional constant temperature and humidity air conditioning system was first mixed with fresh air and return air, and then cooled and dehumidified, and dew point temperature control plus reheat mode was implemented. This type of air conditioning method is largely offset by cold heat, resulting in wasted energy. In the MAU+FFU+DCC air conditioning mode, the MAU system treats the fresh air separately, the MAU air volume controls the indoor positive pressure, the MAU air supply parameter controls the humidity, and the DCC controls the indoor temperature. The simple decoupling means is used to achieve independent control of temperature and humidity. The terminal DCC adopts a fine adjustment method in which the temperature of the chilled water supply is not much different from the room temperature and the small temperature difference between the water supply and the return water is small. In addition to the MAU fresh air reheating, the reheating is not used in the return air system, avoiding the traditional constant temperature and humidity air conditioner. The phenomenon of cold heat offset has achieved better energy saving effect. At the same time, the control accuracy of the electric water valve is higher than that of the electric heating and reheating, which satisfies the indoor temperature and humidity control accuracy.


3 DCC dry surface cooler calibration calculation


According to the load of the room, the clean constant temperature and humidity room is partitioned, and the DCC dry surface cooler is arranged in the static pressure box and the constant temperature and humidity interval in the six sections. By comparing the performance parameters of the DCC dry-cooler calibration parameters with the demand performance parameters, the DCC dry-cooler calibration parameters of a certain manufacturer can greatly meet the performance parameters of the demand, that is, the performance parameters of the designed DCC dry-cooler can be Satisfied production production needs.


4 airflow simulation


The airflow simulation is carried out between clean constant temperature and humidity. The temperature distribution of the room at 1m height is shown in Figure 4. The airflow distribution is shown in Figure 5. The positive pressure distribution is shown in Figure 6. Through the analysis of the simulation results of the clean constant temperature and humidity, it can be seen that: (1) the temperature distribution of the whole field without equipment heating is uniform, almost no temperature gradient; (2) the temperature gradient exists in the area where the equipment is heated, which can satisfy the process. For the temperature gradient requirements, the fluctuation range of the temperature and humidity of the process should be controlled within the range of accuracy requirements; (3) there will be disturbances in the airflow of the equipment, and the airflow in the process room is vertical, and there is no serious drift. Meet the room cleanliness requirements; (4) The positive pressure of the process room and the lower mezzanine is greater than 5Pa relative to the peripheral clean room, which can meet the room's requirements for positive pressure.



5 Conclusion


(1) For the design of clean air conditioner with high temperature and humidity control, it is necessary to understand the specific requirements of the process for the control of clean constant temperature and humidity; reduce the influence of the load inside and outside the room on the temperature, humidity, cleanliness and positive pressure of the room; clean constant temperature and humidity The peripheral protection structure cannot be set, the temperature and humidity of the peripheral environment are as stable as possible; the heat generated by the equipment in the clean constant temperature and humidity room is reduced as much as possible, and the equipment with large heat generation and low control precision is removed from the clean constant temperature and humidity room; The volatility of the heat makes the clean constant temperature and humidity run smoothly in the steady state.


(2) MAU+FFU+DCC air-conditioning system is applied in clean air-conditioning with high temperature and humidity control. The temperature, humidity and cleanliness are controlled independently. The end uses electric water valve fine adjustment instead of the end air duct electric heating fine adjustment. The phenomenon of cold and heat offset is reduced, and the control precision is higher than that of the end duct electric heating fine adjustment mode.


(3) From the results of DCC selection and airflow simulation, it can be seen that the DCC equipment selection can meet the production requirements, and the MAU+FFU+DCC air conditioning system can be used as a clean air conditioning method with high temperature and humidity control. Various indicators required between constant temperature and humidity.


(4) Temperature accuracy requirements ± 0.1 ° C, humidity accuracy requirements ± 3% and clean level 5 clean + constant temperature and humidity room, you can use MAU + FFU + DCC (two-stage) air conditioning system, using two-stage DCC chilled water supply The temperature is not much different from the room temperature and the fine temperature difference of the return water is small to meet the high temperature control requirements of the clean room, which is energy-saving and has high control precision.