微波陣列式電子迴旋共振電漿系統之研究與應用

Abstract

本論文乃研發微波陣列式電子迴旋共振電漿系統，以產生大面積均勻電漿。本研究以2×2微波陣列輸入系統與永久磁鐵建構大面積電子迴旋共振電漿源，2.45GHz微波經由2×2微波陣列輸入系統在微波窗下方150mm處產生均勻微波場型，而環繞真空艙建置的永久磁鐵也在相同位置產生520×520mm2方形面積的均勻875高斯磁場，以產生電子迴旋共振電漿。為量測500×500mm2面積範圍的電子迴旋共振氮氣電漿特性，實驗中使用蘭牟爾電漿探針進行電子密度量測，在微波窗下方360mm處量得電子密度最佳均勻度達±7.7%。 在產生大面積均勻電子迴旋共振電漿的要素中，磁場是非常關鍵的因素，因此本研究深入探討磁場分佈對產生均勻電子迴旋共振電漿的影響。在電子迴旋共振區域與下游靠近基板區域建構均勻磁場對於電子迴旋共振電漿的均勻度有很大的幫助，此外，這兩個區域間的磁場變化也是非常重要，由於週邊區域的微波場比較弱，中央區域的的微波場相對較強，所以週邊區域採用可提供較高電漿解離效率的磁場分佈，中央區域則是採用較低電漿解離效率的磁場分佈，兩區之間電漿解離效率的差異正好有互補效果，使電漿分佈更均勻。因此，更大的微波輸入陣列如4×4或8×8搭配針對微波場設計的大面積磁場，可有效擴大微波陣列式電子迴旋共振均勻電漿的面積。 為展示電子迴旋共振電漿技術的應用，本論文提出電子迴旋共振電漿化學氣相沉積製程的兩個應用例子。第一是以電子迴旋共振電漿化學氣相沉積設備在二氧化矽基板上沉積氫化微晶矽薄膜，探討各種製程條件對沉積氫化微晶矽的影響，在特定氫釋比與真空壓力下，沉積速率可達13.7Å/sec。 第二個應用例是以電子迴旋共振電漿化學氣相沉積設備在矽晶圓上沉積碳化矽薄膜，實驗中使用異丁烷、甲烷與矽甲烷等反應氣體，搭配氫氣或氦氣做為載氣進行電漿鍍膜。沉積之碳化矽薄膜經傅立葉轉換紅外線光譜儀檢測，結果顯示反應氣體與載氣對於碳化矽薄膜特性有很大影響，其中異丁烷可解離產生大量CH3原子團，對於成長碳化矽有很大幫助。在以甲烷為反應氣體的情況下，使用氦氣做為載氣，其相對於氫氣較重的原子質量與穩定的電漿特性有利於反應氣體的解離與基板表面的成核反應，可促進碳化矽薄膜的成長。研究結果顯示，選用適當的反應氣體與載氣是改善電子迴旋共振電漿化學氣相沉積碳化矽薄膜之組成與微結構的有效方法。

The purpose of this thesis is to develop a microwave arrays electron cyclotron resonance(ECR) plasma system to produce large-area uniform plasma. This study employs a 2×2 array microwave inputs system and permanent magnets to generate large-area ECR plasma. The uniform microwave field was produced by launching a 2.45GHz microwave through 2×2 array microwave inputs system. The permanent magnets are arranged around the vacuum chamber to create a uniform 875 Gauss magnetic field in a square geometry with dimensions of 520×520 mm2 coincides with uniform microwave field. Therefore the ECR plasma was created at 150 mm below the microwave windows. The characteristics of the ECR nitrogen plasma over 500×500mm2 in area was obtained by a Langmuir probe. A uniform ECR plasma with the electron density fluctuation uniformity of ±7.7% was achieved at 360 mm below the microwave windows. Generating large-area uniform ECR plasma hinges on several factors, among which the magnetic field is very crucial. The influence of magnetic field distribution on the generation of uniform ECR plasma has been studied. Uniform magnetic field distributions of the ECR zone and the downstream zone near the substrate are beneficial to the production of uniform ECR plasma. In addition, the transition of the magnetic field between ECR zone and the downstream zone near the substrate is also crucial. The magnetic field distribution correspond to the peripheral part of the ECR downstream zone is designed to be either flat profile or mirror type to yield higher ionization efficiencies. On the other hand, the ionization efficiency of the central part is lower due to magnetic field distribution of divergence type. Higher ionization efficiencies can compensate the weaker microwave field at peripheral part of the vacuum chamber, vice versa at central part of the vacuum chamber. Thus the uniformity of plasma is improved. With a well-designed, large-area magnetic field, the idea of generating uniform electron cyclotron resonance plasma can be expanded to a larger area through a larger array of microwave input system, for example, a 4×4 or 8×8 array. In order to demonstrate the utility of ECR plasma, two practical example of electron cyclotron resonance chemical vapor deposition(ECR-CVD) were executed in the thesis. At first, hydrogenated microcrystalline silicon (μc-Si:H) films were prepared on SiO2 substrate using a ECR-CVD system. The effects of hydrogen dilution ratio and pressure of ECR-CVD on the deposition rate of microcrystalline silicon thin films were studied. By combining the hydrogen dilution and pressure conditions, a high deposition rate of 13.7Å/sec were achieved in the μc-Si:H film growth process. Secondly, the formation of silicon carbide (SiC) films on silicon wafer substrates using ECR-CVD system has been investigated. Reactant gases of C4H10, CH4, and SiH4 have been employed to deposit SiC films with carrier gases of H2 or He. The as-grown SiC films were examined with Fourier transform infrared spectroscopy. The results revealed that the influences of both reactant gases and carrier gases on the characteristics of the SiC films were crucial. The dissociation of C4H10 is beneficial to the production of CH3 radicals to enhance the growth of SiC films on silicon. In contrast to H2, the heavy mass and stable plasma of He are favorable for the decomposition of reactant gases and nucleation reaction on the substrate, thus promoting the formation of SiC. This study suggests proper selection of reactant gases and carrier gases is an effective method to improve the composition and microstructure of SiC thin films grown by ECR-CVD.