The Study of Cleaning Methods for Dynamic Random Access Memory (DRAM) Capacitor
Ng, Wei Yeeng
|關鍵字:||清洗方式;電容;cleaning method;capacitor;60nm;DRAM;EKC265;Hydroxylamine;rotation speed;chemical|
|摘要:||堆疊式電容為動態隨機存取記憶體(DRAM)的關鍵組件。為增加電容值，目前製程大多利用高選擇比之氫氟酸將氮化鈦(TiN)下電極周圍之介電質做濕式等向性回蝕刻(Crown Wet Etching Back Process)，以期達到雙倍表面積的覆蓋率(Surface Area Coverage)之目的。回蝕刻製程後易產生許多副產物與雜質並會重新附著於晶圓表面，進一步影響到製程良率。研究中藉由應用材料公司(Applied Material)所發展之缺陷量測機台做掃描式電子顯微鏡(SEM)外觀分析及X光能量分散光譜(EDS)元素分析後得知副產物成分以碳(Carbon)、矽(Silicon)、鈦(Titanium)及氧(Oxygen)為主。本研究主要目的為找出一個適合高深寬比電容結構的清洗方式，期能去除電容製程後所產生之副產物並避免電容結構倒塌且維持電容間的電子特性。研究方向包含選取適當機台(單片處理機台或批次噴灑式處理機台)及不同種類pH值之混合酸液(EKC6800、REZI38、EKC265、ELM C30、EcoPeeler)。決定出最適當的機台與酸液後，將進一步探討處理時間與旋轉速度對雜質及副產物去除能力之影響。
研究結果顯示之最佳條件為兼具製程容忍度大且雜質去除率最高的條件：酸液EKC265搭配批次噴灑式處理機台。含氰胺(HDA)之化學混合溶劑EKC265，使雜質與基板之間的界面電位(zeta potential)處於同極性而互相排斥，進而提升雜質去除率。氰胺為強氧化還原劑，使金屬氧化物質還原成可溶於異丙酮(IPA)之螯合物。此外選擇應用於EKC265處理時間則不宜過久，若處理時間太長Titanium將被過度回蝕刻，結構易倒塌；然則太短雜質去除率有限，多種不同處理時間經搭配並選擇適當之旋轉速度後所得最佳化條件，可使雜質去除率達到80%。旋轉速度低於650rpm 以下則雜質去除率呈線性，但於650rpm以上去除率則下降；主要原因為化學反應效果於650rpm以上被限制全由物理力量主導所致。
In a modern stacked-capacitor DRAM device, the structure of the storage capacitor can be thought of as a vertical cylinder made of titanium nitride (TiN). To maximize the capacitance of the cell in high-performance DRAM devices, the dielectric material surrounding the storage nodes needs to be removed during the fabrication process to increase the surface area. This process is known as the crown capacitor wet etch process. However, the wafer surface is highly contaminated by particle defects during the dielectric material removal process. Using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS), these particulate defects are shown to be mostly silicon-rich, titanium-rich, carbon-rich and oxygen rich. Particulate defects that appeared on top of the capacitor are the major yield killers for the DRAM device. This study will focus on choosing a suitable tool (between the batch type spray tool and the single wafer tool); and chemical (EKC6800, REZI38, EKC265, ELMC30 and EcoPeeler) with a high residue and particles removal ability. The best optimized cleaning recipe, which includes process time and rotation speed, is developed to provide a sufficient process window for this process. Also, the results of this work are discussed and analyzed. The batch type process spray tool in combination with the hydroxylamine (HDA) based chemical: EKC265 with high pH, is proved to be efficient in removing particles and residues of high aspect ratio capacitor structures in this work. Zeta potential in the alkaline solution (EKC265) is negative, thus it tends to repel the particles on specimen surface which is also negative-charged. In addition, the combination of HDA and an organic amine form a strong reducing complex solution which can reduced insoluble metal oxide into a lower oxidation state and subsequently chelated with the ligand to form a more soluble metal complex. Particle count reduces proportionally with chemical treatment time. However, there is a risk of pattern collapse with a long treatment time due to the increment of titanium etch amount. Rotation speed also plays an important rule for particle removal. A higher rotation speed implies stronger external momentum to detach particles. But, it will also limit the efficiency of the chemical reaction. Thus, a balance between chemical reaction and physical reaction has to be considered. The final optimal condition proves to be significant to yield improvement and is demonstrated to be robust and able to be implemented for 60nm generation DRAM.