直交實驗計畫法分析Si-C-N在矽晶蝕刻槽內和平面上之沉積

Abstract

摘 要 本研究用直交實驗計畫法，研究在矽晶蝕刻槽內及矽晶平面上，沉積三元Si-C-N膜之合成製程。沉積在矽晶上的膜是利用微波電漿沉積系統(Microwave plasma-enhanced chemical vapor deposition， MWCVD)，以CH4，N2，及外加的矽為原料。矽晶蝕刻槽的製備是採用微影技術配合濕式蝕刻製成。試片的分析是使用掃描式電子顯微鏡( Scanning electron microscope， SEM)及原子力顯微鏡( Atomic force microscope， AFM)來觀察表面型貌與粗糙度，用電子能譜化學分析儀( Electron spectroscopy for chemical analyzer， ESCA)做成份定量，X光繞射儀( X-ray diffraction， XRD)是用來觀察結晶結構，用陰極激發光儀( Cathodoluminescence， CL)來量測能隙，在機械性質的量測上則是使用奈米級壓痕儀( Nano-indentation system)。由實驗的結果，得到的結論可以分為以下四大部份: (A).在矽晶平面的沉積方面:較小的壓力配合較高的甲烷濃度可以使得最大晶粒表面積增加。較小的微波功率可以使得CH4/N2的解離比率降低;因此提高了薄膜中的(氮/碳)的原子比例。在沉積薄膜的初期，矽的傳輸路徑較短，外加矽源的消耗較少，因此沉積速率是沉積時間的遞減函數，且膜的表面Si-N鍵比Si-C鍵容易形成，因此在沉積的初期(氮/碳)的原子比例也會比較高。結果也顯示膜的晶體結構較接近α-SiCN的結構，並且薄膜經CL光譜儀量出來的能隙為3.94 eV(λmax=315 nm)，能隙的半高寬為0.96 eV(或78nm)。薄膜的視硬度可達48.15GPa，比TiC或TiN的被覆層還硬。 (B). 比較沉積在矽晶平面上及蝕刻槽內之不同: 就直交實驗的分析指標中，例如薄膜最大晶粒表面積，(氮/碳)原子比及Si%，而言，在平面矽晶上和蝕刻槽內之沉積，其主要參數對分析指標的影響，顯示了相同的方向，僅管貢獻率不同。結果同時也證實了，矽平面上的薄膜有較大的晶粒、低的碳含量、大部份具較高的(氮/碳)原子比、以及較好的結晶性。薄膜在矽晶蝕刻槽內的能隙為4.00 eV (λmax=310 nm)，半高寬為 0.73 eV(或60 nm)，但比起平面上沉積的薄膜，能隙很接近，但強度高出甚多。 (C). 就薄膜沉積在矽晶平面上計畫一與計畫二之比較:在最大晶粒表面積的比較上，影響計畫一的主要參數為氣壓和甲烷濃度的交互作用，沉積時間無關;影響計畫二的主要參數則為沉積時間。 (D).在薄膜與矽的選擇性蝕刻方面:以7.86M KOH溶液蝕刻薄膜以及矽基材180分鐘，發現KOH和薄膜並沒有明顯的反應，而此時矽已經被蝕刻了250μm。此表示7.86M KOH溶液可以作為Si-C-N材料相對於矽晶片的選擇性蝕刻溶液。

Abstract An orthogonal experimental analysis method was used to study the deposition of the ternary Si-C-N films on the etched trenches and the flat surface of Si wafer. The films were deposited on Si wafer by a microwave plasma chemical vapor deposition (MWCVD) system with CH4, N2 and additional Si source as raw the materials. The trenches in Si wafer were fabricated by the photolithography combined with wet etching process. Morphologies and surface roughness, compositions, crystal structure, bandgaps and mechanical properties of the deposited films were examined by SEM and AFM, ESCA, XRD, CL spectroscopy and nano-indentation system, respectively. The experimental results was described as follows: (A). Deposition on flat Si surface: A lower system pressure combined with a higher CH4 concentreation can result in an increase in surface area of the largest grain. A lower microwave power gives rise to an decrease in CH4/N2 dissociation rate ratio hence favors a higher N/C atomic ratio in the films. At the beginning of the deposition process, Si transfer path is shorter and depletion of the addition Si source is less; therefore, the deposition rate is a decreasing function of the deposition time, and it favors a higher N/C atomic ratio in the films by considering a greater tendency to form Si-N bonding than Si-C bonding. The results also show that the crystal structure of the films is similar to that of α-Si3N4 type structure; and the bandgaps determined by CL spectra are around 3.94 eV (λ=315nm) with FWHM 0.96 eV( or 78 nm). The apparent hardness of the films determined by nano-indentation technique is as high as 48.15 GPa, which is much higher than that of TiC or TiN coatings. (B). The differences in deposition on flat and trench Si surface: On the orthogonal experimental analysis indexes, such as the surface area of the largest grain, N/C atomic ratio and Si%, effects of the main parameters on the analysis indexes for the films deposited on flat or trench Si surfaces appear to be in the same direction, in spite of differences in contribution values. The results also demonstrate that the films on flat Si surface prossess larger grains, lower carbon contant, almost higher N/C atomic ratio and better crystallinity than that on trench Si surface. The bandgaps of the films deposited on trench surfaces are around 4.00 eV (λ=310 nm) with FWHM 0.73 eV(or 60nm), which are close to each other but much stronger than that on flat surface of Si wafer. (C). The differences in deposition on flat Si surface for Plan 1 and Plan 2: On the index of surface area of the largest grain in the film, the main parameter is the interaction between pressure and CH4 concentration, and is nothing to do with the deposition time for deposition time greater than 6 hours , i.e. Plan 1. In contrast, for the deposition time below 2 hours, i.e. Plan 2, the main parameter is the deposition time. (D). On selective etching of the films and Si wafer: The results indicate that no significant interaction between 7.86M KOH solution and the film can be detected after etching for 180 min, and the corresponding etched depth for Si can go up to 250μm. This signifies that 7.86M KOH solution can be used as the selective etchant for Si-C-N material and Si wafer.