以電漿輔助化學氣相沈積低介電常數之含氟氧化物薄膜的化學機械研磨技術與材料特性研究

Abstract

先進的半導體元件內連線技術是左右元件最後的良率與可靠度的重要 因素。在朝向設計法則縮小及堆積密度增加的趨勢下，驅使了多層內連線 金屬化系統的研究發展。當內連導線尺寸縮小及彼此距離靠近時，導線與 栓塞的電阻和以二氧化矽為基底的內金屬介電質的電容會限制元件傳輸速 度進一步的增加。所以，將具有低介電常數的介電質和電阻較低、抗電遷 移性較佳的金屬導體引入多層內連線系統之中，藉以消減深次微米製程的 電阻- 電容時間延遲(RC time delay)，就成為非常重要的事情。 在本論文中，一種低介電常數的介電質材料，即利用電漿輔助式化學氣相 沈積法製造的氟氧化矽薄膜，即為我們研究評估其在超大型積體電路技術 中的可行性。本論文中首先探討的是氟氧化矽薄膜的物理及化學特性。這 些薄膜的形成是利用六氟化二碳(C2F6)作為氟的來源摻入二氧化矽之中。 當氟摻入後，對二氧化矽薄膜的介電常數、反射率、化學鍵結型態、熱穩 定性和表面型態都會有重大的影響。氟氧化矽薄膜的介電常數在矽-氟鍵 結 濃度高達百分之8.9時，可降至3.2。傅立葉轉換紅外線(FTIR)光譜則 顯示出當氟含量越高時，會增強薄膜的吸水性。另外我們也發現，在作熱 處理時，薄膜的吸水性與釋氣性則是影響氟氧化矽薄膜穩定性和可靠度的 決定性因素。 全面性平坦化是 多層內連線金屬化最基本的一個要求。而化學機械研磨(CMP)製程則 是已 被公認達到全面性平坦化唯一最可行的方法。本論文則針對氟氧化矽薄膜 ，探討其對化學機械研磨製程的可行性。結果顯示，氟氧化矽薄膜的微硬 度、彈性係數以及鍵結型態會跟化學機械研磨的蝕刻率有關連。我們發現 在不改變化學變因和機械參數下，化學機械研磨的蝕刻率會跟氟氧化矽薄 膜內所含的氟含量成正比，亦即和氟氧化矽薄膜的硬度(或 是彈性係數) 成反比。此外，氟氧化矽薄膜在以NH4F為緩衝液的HF溶液中的化學濕蝕刻 率 與化學機械研磨速率有顯著的一致關係。 Advanced semiconductor device interconnects are the pivotal component governing the final device yield and reliability. The trend toward shrinking design rules and increased interconnect packing density have driven the development of multilevel interconnect system. As interconnect lines shrink and move closer together, the resistance of conductor lines and plugs and the capacitance of SiO2-based inter-metal dielectrics limit further increase in clock speed. The incorporation of low permittivity (low-k) dielectrics and lower-resistance, electromigration-immune conductors into the interconnect scheme has become imperative to further reduce the RC delay in the deep submicron regime. One of these low-k dielectrics, fluorine-doped silicon dioxide films which have been fabricated by dual frequency plasma enhance chemical vapor deposition (PECVD), are evaluated the feasibility of this technology for ultra-large integrated (ULSI) processes. In this study, the physical and chemical properties of fluorine- doped silicon dioxide (SiOF) films are evaluated. These films were deposited by TEOS-PECVD technique with C2F6 as the fluorine source. The effects of fluorinedoping on dielectric constant, refractive index, chemical bonding, thermal stability and surface morphology of the SiO2 will be assessed. The relative dielectric constant of SiOF films can be lowered to 3.2 with Si-F bonding concentration 8.9 at.% in our experiments. Fourier Transform infrared (FTIR) spectra provide evidence for enhanced moisture absorption of films with higherfluorine content. Moisture absorption and desorption during heat treatments are found to be the critical factor that influences the stability and reliability of SiOF films. One of the basic requirements for the utilization of this multilevel metallization scheme is to achieve the global planarization. Chemical- mechanical polishing(CMP) process has been proven to be the most promising method to accomplish global planarization. In this thesis, results of chemicalmechanical polishing of SiOF thin films were studied. Nanohardness, elastic modulus, and bonding structure of fluorinated silicon dioxides are characterized in order to evaluate their correlations with CMP performance. We find out that under fixed chemistry and mechanical parameters, the CMP removal rate increases significantly with increasing fluorine content in the oxides due to the lower hardness and elastic modulus in the SiOF films. In addition, the greater slope of the removal rate versus pressure line for SiOF films compared with undoped oxides signifies the enhanced chemical erosion during CMP of fluorinated oxides, in agreement with the wet etching results which etched by BOE solutions.