低介電常數聚亞醯胺薄膜之化學機械研磨研究

Abstract

當IC技術微縮至深次微米時，多層導體的遲滯現象(RC delay)，對於決定積體電路的工作速度，扮演著愈趨重要的角色。為降低RC delay，提高積體電路的工作速度，更低電阻的金屬新材料與低介電常數的絕緣材料正受到廣泛的研究。聚亞醯胺是一種低介電常數的聚合物，它的介電常數值比一般以二氧化矽為主體的聚合物還低。另外，它的良好的絕緣電性，熱穩定性與機械強度都有很好的表現。由於在深次微米 IC 產業上，微影技術上聚焦深度（DOF）變得更小，進而對平坦化的考量更加嚴苛，目前唯有化學機械研磨這種方式能夠達到全面性的平坦化。化學機械研磨平坦化的過程中，聚亞醯胺的薄膜以及研磨液的組成對移除速率與移除的不均勻度以及薄膜表面的損壞程度，有相當大的影響。有機聚合物的聚亞醯胺化學機械研磨與一般的二氧化矽機械研磨機制並不相同，因為聚亞醯胺表面的疏水性與一般二氧化矽親水性不同，而大大限制了表面化學反應的速率。 本研究中，在聚亞醯胺的聚合程度與研磨液的組成反映出化學機械研磨的現象中，其彼此之間的關係與影響，正是本實驗研究的目的。本研究結果顯示，聚合程度與化學機械研磨的移除率有很大的相關性。當聚合程度越高，化學機械研磨的移除率就越低。聚亞醯胺聚合的程度可以由實驗中烘烤的溫度來決定，溫度從一百七十五度到兩百七十五度。聚合的程度可以由傅力葉轉換IR吸收光譜的吸收峰值來計算。由實驗中知道，超過兩百度時，聚合程度已達到百分之六十，而化學機械研磨的速率也因此急遽下降。化學機械研磨的水解反應與研磨液中的氫氧根離子濃度對移除速率有極大的影響。當氫氧根離子濃度越濃，則化學機械研磨的移除率越高。而實驗中加入不同碳數的氫氧化四級銨，由於提高研磨表面的潤濕性，因此可以增加有機聚合物聚亞醯胺的化學機械研磨速率，但另方面卻抑制了二氧化矽移除速率，正可藉此調整製程所需的選擇率。聚亞醯胺表面的潤濕性對移除速率也有極大的影響，當其接觸角越小則潤濕性越好，即會有較好的移除速率，由實驗發現，不同碳數的氫氧化四級銨可以提升不同的潤濕性。此外，聚亞醯胺研磨後的表面粗糙程度 (0.5 到 0.7nm) 比未研磨前 (0.8nm) 亦有相對較小的粗糙值，顯示化學機械研磨使得聚亞醯胺表面更加平坦。 另外，我們也發現化學機械研磨之後的電性相較於研磨前約略相同，在一百七十五度烘烤時所製備的聚亞醯胺薄膜其研磨後的漏電流 (5%10-11 A/cm2) 甚至表現比未研磨的聚亞醯胺薄膜 (1%10-10 A/cm2) 更小。顯示出研磨液中的移動離子，例如鉀離子，鈉離子等，並不至於影響研磨後聚亞醯胺薄膜的電性表現，反而因為聚亞醯胺的研磨促使在聚亞醯胺中的溶劑揮發更完全而使得薄膜聚合程度更好，電性表現更佳。研磨後的聚亞醯胺介電常數值約在2.8至2.9。

As IC manufacturing shrinking down to deep sub-micron region, interconnect RC delay would take the major part of total signal propagation delay. New materials of low electrical resistance conductors and low dielectric constant insulators are thus being extensively investigated for the reduction of interconnect RC delay. Polyimide (PI) is one promising candidate of the low-k polymers whose dielectric constants are lower than that of silicon dioxide, not only for its good electrical insulating properties, but for the benefits from its excellent thermal stability and mechanical strength. Owing to the severe depth of focus (DOF) budget in deep sub-micron lithography, chemical- mechanical polishing (CMP) is the only enabling technique known to achieve global planarization. Polishing performance, such as removal rates, removal non-uniformity and surface damage would be strongly dependent upon the films being polished and slurry formulations. Unlike inorganic silicon dioxide polishing, polyimide is one kind of organic polymers and hydrolysis reactions would be inhibited on its hydrophobic surface. In this study, both the polymerization chemistry of PI and slurry formulation were explored for their influences on polishing performance. Our preliminary results showed that removal rates of PI would be heavily dependent upon the imidization degree of PI. The higher degree of imidization of PI would normally lead to lower polish rate. Before full curing, the polyimide films with various degrees of imidization by controlling baking temperature ranged from 175℃ to 275℃, were polished. In the curing process of baking polyimide, an imidization degree of ~60% would be obtained by baking at 200℃ for 25 min. The hydrolysis reaction of polyimide, which contributes mainly to the total removal rates, would be enhanced by adding hydroxide ions in the aqueous solution. Tetra-alkyl-amonium cations, such as tetra-methyl-amonium hydroxide (TMAH), added in the slurry, which acted as a surfactant on the PI surface, would improve wettability and then the interaction between the PI surface and the aqueous slurry. The removal rates of PI were found to be increased by using the TMAH-added slurries owing to improved surface wettability of polyimide. The electrical characteristics of the polyimide films being polished were investigated to evaluate the influences of the moisture adsorption and surface damage. After polishing, both the leakage current density and the dielectric constant of the polyimide film were found not to degrade and were about 5%10-11 A/cm2 and 2.8-2.9, respectively. The surface roughness of the polished polyimide film, Rq = 0.5 nm - 0.7 nm, was better than that of the unpolished one, Rq = 0.8 nm, and no water adsorption peak was found in the FTIR spectra. It implies that polyimide CMP could be feasible for the planarization and etch-back applications.