整合式薄膜系統回收CMP廢水之研究

Abstract

化學機械研磨(chemical mechanical polishing, CMP)為半導體製程其中之一環，所產生之廢水含有奈米研磨砥粒及超純水，大量表面帶負電性之膠體顆粒穩定懸浮於廢水中造成高濁度，雖有處理上之困難但也具有水回收價值。近年薄膜程序開始被用於CMP廢水的處理及回收上，其中又以超過濾(ultra-filtration, UF)能攔阻去除奈米研磨砥粒及產生高品質濾液被廣泛應用，亦有合併各種前處理或電過濾等發展，但是目前各廠水回收成本仍偏高，因此若能將水中固體物回收再利用能更具經濟價值，減少污泥處置費。 本研究利用一兩段式薄膜過濾程序，先以掃流式UF薄膜過濾處理氧化層研磨(oxide-CMP)廢水，令薄膜濃縮液不斷循環迴流過濾而提升濃度，再以截流式微過濾(microfiltration, MF)薄膜過濾廢水濃縮液截留固體物。除了解各項操作變因對過濾效能之影響，並研究同時CMP廢水處理、回收薄膜濾液及研磨砥粒之可行性，期盼發展為一環境友善之CMP廢水處理回收技術。 研究結果顯示，在掃流式UF處理oxide-CMP廢水之濾程中以30 kDa之UF polyvinylidine fluoride (PVDF)膜有較佳的產水通量。在1-2 bar較低之操作壓力下，較能減緩薄膜通量之衰減，較高之操作壓力雖有較好之初始通量，但通量衰減快且進入穩定期後之產水通量較低；掃流速度之提升能加強水流剪力，對於薄膜表面積垢物的累積有抑制作用，越高之掃流速度過率有越高之平均通量。因此掃流式薄膜濃縮過濾應之操作以低操作壓力、高掃流速度為主，可以讓薄膜能更長時間連續操作，減少清洗頻率。在薄膜清洗方面，低壓高流速的操作情況下不可逆積垢情形不嚴重但仍會隨著操作次數的增加導致通量回復率降低，FE-SEM圖顯示反洗後仍然有研磨砥粒及結垢沉積於薄膜表面，若欲以化學清洗應以鹼液為主。UF薄膜對處理CMP廢水有品質較佳之濾液，對水中之濁度去除率可達99%以上，但對導電度、溶解矽及溶解性有機物等物質去除率較差。 截流過濾探討不同進流水濃度對過濾效能之影響，廢水經過濃縮後不僅體積減量、回收濾液，結果亦顯示隨著濃縮液濃度提升單位時間內能留阻較多之固體物量，故先濃縮後過濾能提升過濾效率，減少反洗次數。所使用之MF薄膜中以孔徑1 μm、親水改質之polytetrafluoroethylene (PTFE)薄膜在濃縮液濁度達到2740 NTU時有最高之固體物留阻效率，其濾液產量較高但濾液水質、通量回復率較差；孔徑0.3 μm之PVDF薄膜雖然在濃縮液濃度為2000 NTU附近即達到最高之留阻效率，但具有相對良好的濾液水質及較高之通量回復率。截流過濾自濃縮液中回收之濾餅，經EDS分析其組成元素幾乎為矽跟氧，顯示回收之固體物有相當高之純度，將濾餅再分散於水中其界達電位、粒徑分佈並無太大改變，仍具有-40 mV左右之高負電荷及奈米粒徑。 本研究顯示以兩道薄膜程序同時處理廢水、回收濾液及研磨砥粒是可行的，期望能藉由回收研磨砥粒之效益與減少污泥處置費用來沖銷目前較高的水回收成本。

Chemical mechanical polishing (CMP) is a part of semiconductor manufacturing process, which produces wastewater that contains nano-sized abrasives and ultra-pure water. Due to the large amount of negatively charged colloids suspended in the CMP wastewater, the turbidity is very high in CMP wastewater. In recent years, membrane processes begin to be applied for treatment and recycling of CMP wastewater. Ultra-filtration (UF) membrane is widely used because it can remove nano-sized particles and produce high quality effluent. There are also many membrane processes like combination of pretreatment in membrane unit or electro-filtration, but the cost of wastewater recycling is still high. If the SiO2 abrasives could be recycled and reused, it has economical profit and reduces the cost of sludge disposal. This study used a 2-step membrane filtration process as an integrated membrane system to treat oxide-CMP wastewater. The wastewater was first treated by a cross-flow UF membrane unit and the concentrate was continuously recycled to increase the concentration. Then the concentrate was filtered by a dead-end microfiltration (MF) module to separate solids from wastewater. The purposes of this study are to investigate the effect of the operational conditions on the filtration efficiency and to investigate the possibility of simultaneously recycling the effluent and the high-pure CMP abrasives. The results showed that in cross-flow UF test, the 30 kDa polyvinylidine fluoride (PVDF) membrane had better performance than 30 kDa polystyrene. Operation under lower driving pressure at 1-2 bar has lower flux decline than the higher one at 3 bar, which had higher initial flux but had lower flux in stable period. Increasing of cross-flow velocity not only had higher average flux but also increased the shear force that cloud decrease the accumulation of foulants. Therefore, in the cross-flow filtration, operated under low driving pressure and high cross-flow velocity could extend the operational period that could reduce the frequency of backwash. In these conditions, the flux recovery still declined because of the irreversible fouling. The FE-SEM images showed that there were still some abrasives and scaling deposits on the membrane surface after ultrasonic cleaning. As a result, the alkali needed to be used in chemical cleaning. Better effluent quality was obtained in UF membrane. The removal of turbidity could reach over 99%, but the removal of conductivity, dissolved silica and dissolved organics were poor. In dead-end filtration test, more solids could be recovered due to the high concentrate CMP wastewater concentrated by the UF membrane. The concentrated CMP wastewater resulted in reducing volume, increasing the efficiency of dead-end filtration and decreasing the backwash frequency. The polytetrafluoroethylene (PTFE) MF membrane with 1 μm pore size and hydrophilic surface modification had the highest solid retention efficiency with feed turbidity of 2740 NTU, but had lower permeate quality and flux recovery. On the other hand, the PVDF MF membrane with 0.3 μm pore size had highest solid retention efficiency with feed turbidity around 2000 NTU, and the permeate quality and flux recovery were better than the PTFE membrane. The dead-end filtration recovered the solids as cake formed on membrane. EDS spectrum showed that the components of recovered solids were almost Si and O, which means the purity of recovered solids was high. After redispersing the solids in DI water, it still had high negative surface charge around -40 mV and similar size distribution compared with the raw oxide-CMP wastewater. The study showed that simultaneously recycling effluent and solids is feasible by using the integrated membrane system. We expect that the recovered abrasives and the saving of sludge disposal can offset the high cost of wastewater recycling.