多孔性低介電奈米群集二氧化矽薄膜結構與性質關係之探討

Abstract

本研究探討奈米群集二氧化矽多孔性低介電薄膜(k ~2.8 to 2.0) 由控制母體TEOS/MTMS比率以及結構抑制劑(起孔洞劑) TPAOH之含量對於結構性質之關係。研究著重在結構之因素，例如：化學以及分子鍵結、孔隙率、孔洞形狀、大小、以及孔洞之分部。並且探討以上結構因素對於電性、機械性質、以及熱應力之影響。 NCS2~NCS5多孔性低介電薄膜具有介電常數介於κ=2.3~2.8。在二氧化矽(TEOS)之母體內加入了適當的甲基起始物(MTMS)。孔隙率因此隨著TPAOH的增加而提升，NCS2~NCS5具有孔隙率<27%。因此NCS2~NCS5具有Si-O-Si很好的交聯程度而且孔隙率的增加並尚未破壞整個Si-O-Si交聯的情形，所以最終還是可達到很好的機械強度(E~10GPa)。孔洞的形貌是為圓形，且具有很窄的孔洞大小分佈並且孔洞大小皆集中在0.5 nm。 為了更降低NCS多孔性低介電薄膜的κ=2.0，因此 NCS1具有較高的孔隙率 (38.6%)、Si-O-Si交聯結構較鬆散、以及具有較低極性Si-CH3之鍵結，以上因素皆有利於降低介電常數。NCS1具有大量的MTMS (Si-CH3)比起TEOS (Si-O-Si)。儘管得到最低之κ值，相對而言其機械強度也受到損害(E=2.76GPa)。Si-O-Si交聯結構較鬆散之母體中具有高孔隙率38.6%的孔洞，則是高孔隙率以及鬆散之母體無法撐住來自加熱過程中之熱應力而倒塌，原先圓形的孔洞也因此變成橢圓形(din-plane = 3.3 nm dout-of-plane = 1.9 nm)。 總結而言，控制母體的Si-O-Si交聯程度(TEOS/MTMS) 以及孔隙率 (TPAOH) 是有利於增加最終的機械強度，加入適當的較低極性的 Si-CH3 鍵結和孔隙率，亦可降低介電常數且不破壞薄膜的機械強度。最後，NCS2 奈米群集二氧化矽薄膜是具有最好的薄模性質 k=2.3、E=9.2GPa 且有利於將來多孔性低介電材料薄膜進行銅鑲嵌後段製程整合的主要選擇之一。

The structure-property relationship of nano-clustering silica (NCS) porous low-k films (k ~2.8 to 2.0) was investigated by controlling silica matrix TEOS/MTMS ratio and structure directing agent or also acted as pore generator TPAOH concentration. Emphasize had been laid on structure parameters such as the chemical and molecular bonding, porosity, pore size, pore shape and pore size distribution and the influence on ultimate electrical, mechanical, thermal stress properties. NCS2 to NCS5 porous low-k film have κ-value ranging from κ =2.3 to κ =2.8. The silica matrix was composed by the addition of MTMS (Si-CH3) in balance with TEOS (Si-O-Si). Therefore high modulus porous low-k film was obtained (E~10GPa). NCS2 through NCS5 film possessed spherical pore shape. The accommodation of Si-CH3 was equilibrated by the addition of Si-O precursor, in the agreement that the Si-O-Si linkage of NCS low-k film still preserved in the present of Si-CH3 groups. By adding more TPAOH, the porosity increased from NCS5 to NCS2 (ρ<27%) without causing much deterioration on mechanical modulus. Pore size was well controlled in the order of d<10nm with most population in the d=0.5nm. In order to push κ-value <2.3, NCS1 porous low-k film was proposed with κ=2.0, that was obtained from less cross-link structure and higher porosity. The silica matrix was composed by higher content of MTMS (Si-CH3) instead of TEOS. Moreover, the incorporation of high methyl content and high porosity (ρ=38%) to NCS1 significantly reduced the κ value. But the modulus substantially dropped to E=2.76GPa. Nevertheless, its Si-O-Si cross-linking was not rigid enough and also the higher porosity cause matrix could not sustain the thermal stress, therefore collapsed to form more stable structure, and formed elliptic cylindrical pore shape (din-plane=3.3nm dout-of-plane=1.9nm). Overall, the degree of Si-O-Si cross-link (TEOS/MTMS ratio) and porosity (TPAOH ratio) must be controlled in order to obtain high mechanical modulus NCS film with low dielectric constant. Among these NCS films, NCS2 with k=2.3 and E=9.2 GPa, which possessed optimal ratio of Q [(SiO)4Si] to T [(SiO)3Si(CH3)] group, would be the best ILD candidate materials meeting the requirements of device integration.