Research on Improving the Removal Efficiency of exhaust gases from a Thermal Oxidizer
|關鍵字:||電熱式氧化處理器;矽甲烷;三氟化氮;electirc thermal oxidizer;SiH4;NF3|
|摘要:||矽甲烷(SiH4)為半導體工業CVD(CVD, Chemical Vapor Deposition)製程中所使用的重要氣體之一。在CVD機台中，部份SiH4氣體會氧化成二氧化矽(SiO2)微粒而附著於機台內壁上。若SiO2微粒累積在機台內壁，將會使晶圓表面受到汙染，造成產品良率降低。因此常使用全氟化物( PFCs，Perfluorinated Compounds )清潔CVD製程反應機台。目前在大部份半導體廠中是以三氟化氮(NF3)氣體取代傳統之C2F6氣體。本研究以電腦數值模擬配合實驗值比對之方式，研究提升電熱式氧化腔之廢氣去除效率，除了NF3之處理效率外，本研究也進行SiH4尾氣在電熱氧化腔之處理效率研究。
研究方法為先模擬舊型電熱氧化腔之流場和溫度場。結果發現以紊流模式High Reynolds Number model並考量自然對流效應所模擬出之溫度值和實測值最為接近。由廢氣進氣口和燃燒空氣進氣口分別通入32.85和31.89 L/min的空氣，在加熱棒表面溫度為800 oC時，模擬結果和實驗值誤差約為正負100oC。當確定流場和溫度場的準確性後，再加入汙染氣體濃度場之計算，以研究不同進氣流量和不同加熱棒之表面溫度對NF3及SiH4去除效率之影響。固定流量為50 L/min且濃度為5000ppm的NF3，另外再通入20 L/min的燃燒空氣，在加熱棒溫度為900、1000和1100oC時，電熱氧化腔對NF3之去除效率模擬值分別為22.74%、32.72%和50.06%，實際測得NF3去除效率為31%、42%和59%。NF3去除效率實驗值和模擬值均會隨加熱溫度的增加而提升。此舊型電熱氧化腔對NF3去除效果偏低。舊型電熱氧化腔對SiH4去除效果則很好。固定流量為50 L/min且濃度為18900ppm的SiH4，另外再通入31.89 L/min的燃燒空氣，在加熱棒溫度為800oC時，模擬之SiH4去除效率為100%，和去除效率實驗值99.7%十分接近。
由舊型電熱氧化腔之溫度場分佈和去除效率的關係中發現，增加熱源可使腔體內部高溫區分佈範圍更均勻，有利於NF3之熱解反應。本研究設計之新型電熱氧化腔在腔體壁面上加上加熱設備，除了能讓氣體溫度更高，同時也建立出腔體壁面溫度高於腔體內部氣體溫度之溫度梯度。因此可預見在處理易氧化氣體如SiH4時，氧化物SiO2微粒不會附著在腔體壁面上。由新型電熱氧化腔廢氣處理效率之模擬結果中發現，固定NF3流量為120 L/min且濃度為5000ppm，另外通入30 L/min的燃燒空氣，在加熱溫度設定為800、900和1000oC時， NF3之去除效率模擬值分別為75%、81.36%和100%。在加熱溫度為1000oC且氣體處理量高達120 L/min時，NF3之去除效率高達100%。新型電熱氧化腔方面，固定SiH4流量為120 L/min且濃度為5000ppm，另外通入30 L/min的燃燒空氣，在加熱溫度設定為800、900、1000和1100oC時，SiH4之去除效率模擬值均達100%。新型電熱氧化腔對SiH4和NF3都有不錯的去除效果。|
Silane (SiH4) is widely used as feedstock gas for chemical-vapor-deposition (CVD) process in semiconductor industry and it can be easily oxidized to form silicon oxide (SiO2) particles. Deposited SiO2 accumulate on the wall and form some polymers. The chamber is required to be clean before the next process. Nitrogen Trifluoride (NF3) is used to clean the CVD chamber in the production process of semiconductor. NF3 subjected to microwave plasma will dissociate into free fluorine to react with silica (SiO2). To remove hazardous exhaust gases, local scrubbers are usually installed after the process tool. In this study, the removal efficiency of exhaust gases from a thermal oxidizer was simulated, and compared it with experimental data. The objective of this work was to design a high removal efficiency thermal oxidizer for process applications. The numerical solutions of flow field and temperature field were compared to the experimental data to evaluate the accuracy of the predictions. It was found that the predictions obtained using the High Reynolds Number Model and considering the effect of natural convection were in reasonable agreement with experimental data. When temperature of heater was set at 800oC, exhaust gas flow of 32.85 L/min and chamber air of 31.89 L/min were injected in to the thermal oxidizer, and the inaccuracy was 100oC. After confirming the accuracy of flow and temperature fields, concentration of exhaust gas was calculated. The destruction and removal efficiency (DRE) of NF3 and SiH4 were investigated by changing gas flow rate and heating temperature. NF3 gas flow rate of 50 L/min with a concentration of 5000 ppm and chamber air of 20 L/min were injected into the thermal oxidizer. When heating temperature were 900, 1000 and 1100oC, the simulated DRE values of NF3 were 22.74%, 32.72% and 50.06%, and the measured DRE values were 31%、42% and 59%, respectively. Both of the DRE values of simulated result and experimental data were increased with heating temperature. SiH4 gas flow rate of 50 L/min with a concentration of 18900 ppm and chamber air of 50 L/min were injected into the thermal oxidizer. When heating temperature was 800oC, the simulated DRE value of SiH4 was 100% and close to the measured DRE values of 99.7%. A heating device on the inner wall of chamber was added in new design of thermal oxidizer to increase gas temperature and make it more uniform inside chamber. In high temperature environment, pyrolysis reaction of NF3 can be more complete. Because of heating device on the inner wall of chamber, the inward gas velocity through the chamber produces a unique clean boundary effect at the surface, channelling the SiO2 formed into the centre of the combustor chamber before passing into the wet scrubber section below. Simulated results were shown that the performance of new design thermal oxidizer was much better than the old one. NF3 gas flow rate of 120 L/min with a concentration of 5000 ppm and chamber air of 30 L/min were injected into the thermal oxidizer. When heating temperature were 800, 900 and 1000oC, the simulated DRE values of NF3 were 75%, 81.36% and 100%, respectively. Simulated DRE values of SiH4 were all 100% under different inlet conditions.