氟化非晶質碳膜之性質

Abstract

本研究是用四氟化碳(CF4)及乙炔(C2H2)為前驅物，以射頻輔助電子迴旋共振化學氣相蒸鍍法來沉鍍非晶質氟化碳(a-C:F)膜。鍍膜時所使用的氣體流量比R從0.90至0.98，其中R = CF4/( CF4+C2H2)；而成膜後的退火是在300℃純氮氣氣氛下進行。霍氏紅外線光譜儀(FTIR)及X光光電子光譜儀(XPS)被用來鑑定非晶質氟化碳膜的化學健結及氟含量；而高解析電子能量損失光譜儀(HREELS)被用來檢測電子的結構。較高的四氟化碳氣體流量比會產生較多的線性sp3結構，因而使得非晶質氟化碳膜更平滑及更柔軟。 本研究製作出R = 0.98成膜及退火後介電常數能達1.5及介電強度超過35 MV/cm的非晶質氟化碳膜。而奈米孔(nanovoid)及sp2/sp3鍵結比例可被用來討論及闡述關於電性量測時觀察到膜的巨大電流跳躍及電傳導現象。 成膜及退火過後的氟化非晶質碳膜的光學性質與類鑽石膜 (a-C:H)性質相仿，其中R = 0.98非晶質氟化碳膜擁有較類鑽石膜(0.07 奈秒)長的0.34 微秒的螢光壽命及較類鑽石膜(~2.5 eV)寬的2.75 eV的光能隙。同時，非晶質氟化碳膜的短載子壽命會使得螢光光譜產生藍偏的現象，此現象與非晶質矽(a-Si:H)的性質相似。 退火會改變非晶質氟化碳膜的結構及組成，非晶質氟化碳膜的機械及物理性質是隨著氟化碳的鍵結及電子結構的改變而變化。sp3鍵結的含量隨著氟化非晶質碳膜的氣體流量比 ( R )氟含量增加而增加，sp3鍵結的增加會使得膜的漏電流減小、介電常數下降、能帶變寬………等等，然而這些sp3鍵結經鑑定結果絕大份是以練狀型態出現，故會使得氟化非晶質碳膜質地更平滑而且柔軟。此外，氟化非晶質膜氟含量的增加則有助於懸空鍵結(dangling bond)密度的降低。

Fluorinated amorphous carbon (a-C:F) films were deposited by RF bias assisted microwave plasma electron cyclotron resonance chemical vapor deposition (ECR-CVD), using tetrafluoromethane (CF4) and acetylene (C2H2) as precursors. The deposition process was performed at flow ratios from R = 0.90 to R = 0.98, where R = CF4/( CF4+C2H2). The samples were annealed at 300℃ for 30 min. in a N2 atmosphere. Both Fourier transform infrared absorption spectrophotometer (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to observe the a-C:F film chemical bond and fluorine concentration respectively, and a high resolution electron energy loss spectrometer (HREELS) was applied to detect the electronic structure. The higher CF4 flow ratio generated more sp3 linear structures, and it made the a-C:F film smoother and softer. A low dielectric constant of around 1.5 and high dielectric strength beyond 35 MV/cm were obtained in the as-deposited and after annealing conditions, respectively. The huge current surge and electrical conductivity of the films were discussed on the basis of the nano-voids and sp2/sp3 bonding fractions as derived from the above observations. The optical properties of a-C:F films are similar to those of a-C:H films. The 0.34 μsec photoluminescence lifetime of R = 0.98 a-C:F film is longer than that of diamond-like carbon films. The ~ 2.75 eV optical band gap of the R = 0.98 a-C:F film is wider than diamond-like carbon films. The short carrier lifetime in the a-C:F film made the PL peak blue-shift. Such condition is similar to a-Si:H films. The annealing changed both the structure and composition of the a-C:F film. The type of fluorocarbon bond and electronic structure characterized the a-C:F film mechanical and physical properties. The density of sp3 bonds is positively proportional to the gas flow ratio in ECR-CVD and fluorinate concentration of a-C:F films. The increase of sp3 bonds lowers the leakage current as well as the dielectric constant of the a-C:F films. At the same time the increase of sp3 bonds helps to generate the optical band gap. However, as most of the sp3 bonds are consisted in chain structure, they make the a-C:F films smoother and softer. The sp3 bonds help to decrease the density of the dangling bonds in a-C:F films.