氦气对 MPCVD 沉积金刚石的影响

Effect of Helium on Diamond Films Deposited Using Microwave PCVD

为了确定添加氦气对微波等离子体化学气相沉积(MPCVD)金刚石膜的影响,采用发射光谱法(OES)在线诊断了CH_4-H_2-He等离子体的发射光谱特性,研究了He对等离子体内基团空间分布的影响;并利用扫描电子显微镜(SEM)和拉曼(Raman)光谱对不同He体积分数下沉积出的金刚石膜进行了表征。结果表明:随着He体积分数的增加,等离子体内H_α,H_β,H_γ,CH和C_2基团的谱线强度均呈上升趋势,其中H_α基团的谱线强度增加最大。光谱空间诊断发现He的加入导致等离子体中各基团的空间分布均匀性变差,造成沉积出的金刚石膜厚度极不均匀。沉积速率测试表明,He的加入导致碳源基团相对浓度增加,有利于提高薄膜的沉积速率,当He体积分数由0 vol.%增加至4.7 vol.%时,沉积速率提高了24%。SEM测试结果表明,随着He体积分数的增加,金刚石膜表面形貌由(111)晶面取向向晶面取向混杂转变,孪晶生长明显。高He(4.7 vol.%)体积分数下由于C_2基团的相对浓度较高,导致二次形核密度增加。此外,由于基片台受到等离子体的刻蚀和溅射作用,导致薄膜沉积过程中引入了金属杂质原子。二次形核和杂质原子的存在使得孪晶大量的产生,薄膜呈现出压应力。

Optical emission spectroscopy （OES）was used to in situ diagnose the CH4-H2-He plasma in order to know the effect of helium on the diamond growth by microwave plasma chemical vapor deposition （MPCVD）.The spatial distribution of radicals in the plasma as a function of helium addition was studied.The diamond films deposited in different helium volume fraction were investigated using scanning electron microscope （SEM）and Raman spectroscopy.The results show that the spectra intensity of radicals of Hα,Hβ,Hγ,CH and C2 increases with the increasing of helium volume fraction,especially,that of radical Hα has the most improvement.The spectrum space diagnosis results show that the uniformity of C2 ,CH radicals in the plasma tends to poor due to the helium addition and resulted in a different thickness along the radial direction The measurement of deposition rate shows that the addition of helium is useful for the improvement of the growth rate of diamond films,due to relative concentration of carbon radicals was increased.The deposition rate increases by 24% when the volume fraction of He was increased from 0 vol.% to 4.7 vol.%.The micrographs of SEM reveal that with the increasing of helium volume fraction,the diamond films’ crystallite orientation changes from （111 ）to disorder and a twins growth becomes obvious.The secondary nucleation density during growth increases because the high relatively concentration of C2 radicals under higher helium volume fraction （4.7vol.%）.In addition,the substrate was etched and sputtered by the plasma,which introduced metallic atoms into the plasma during the deposition of diamond films.Eventually,the existing of secondary nucleation and impurity atoms lead to the appearance of twins and results in the compressive dress.