製程參數對鑽石薄膜光學性質之影響

Abstract

本實驗採用微波電漿化學氣相沉積法,以CH4和H2作為反應氣体原料,在F 型(100)矽基上板上沉積多晶鑽石薄膜。其後以化學蝕刻除去基材,使之成 為鑽石自持膜,膜厚約為2∼10μｍo藉由FTIR的量測,分析薄膜的紅外線光 學穿透及反射性,並探討製程參數對鑽石膜品質及其光學性質的影響,以尋 求改善其光學穿透率的方法o 沉積膜是以SEM,RAMAN,X-RAY繞射法分分析 其特性 。實驗結果顯示,隨著甲烷濃度,沉積時間,微波功率,氣氛壓力的 改變 ,鑽石膜的品質,厚度,表面粗糙度亦隨之不同,因而影響其紅外線穿 透性o其中,甲烷濃度和沉積時間對表面粗糙度的影響最為明顯,因此對紅 外線穿透性影響最大o 在沉積條件為: 甲烷濃度=0.4% ; 沉積時間=24 hr ; 微波功率=200W (690∼710 C) ; P=20 torr 所沉積之鑽石膜 (2.1 μｍ)品質最好且IR(5000∼500cm-1)平均穿透率最好,約高於60% 。本實 驗並嘗試以二階段式成長法沉積鑽石薄膜,亦即於短時間內先通入高甲烷 濃度氣氛 (2∼10分,甲烷/氫氣=2.0%) ,而後再降低甲烷濃度 (甲烷/氫 氣=0.4%) 以便沉積鑽石膜 。其目地在於不嚴重影響鑽石膜的品質條件 下,藉第一階段沉積來提高成核密度,使得第二階段成長晶粒較為細小的鑽 石膜 。結果顯示,二階段式成長法確能降低薄膜之表面粗糙度,進而改善 其穿透率 ; 但是,若第一階段沉積之含非晶質與石墨層太厚, 亦會造成較 明顯之吸收效應而降低其紅外線光學穿透性 。 Polycrystalline diamond films were deposited on the p-type 100) silicon substrate by a microwave plasma-assisted chemical vapor deposition (MPCVD) system with a mixture of mathane and hydrogen as the source gases. The free standing diamond films of 2∼10μｍ in thickness were also prepared by etching off the Si substrate. The FTIR spectrometer was used to determine the infrared optical transmission and reflection properties of the films. Effects of process variables on the film quality and their optical properties were also investigated in order to improve the optical transmittance of the films. The films were also charactterized be SEM, RAMAN spectrometry and X-Ray diffractometry. A two-step process was also investigated to evaluated its possibility in improving the optical transmittance of the film. It consists of feeding the gases with a higher methane concen- tration for a short period of time( e.g., 2.0vol %, 2∼5 minutes) , and then reducing to a lower concentration (e.g., 0.4%) to deposited diamond film. The first step of this method is for promoting the nucleation density, so the deposition of diamond inporvement in IR transmittance for the film deposited by the two-step method. However , the film thickness after the first step deposition should be minized in order to reduce its infrared absorption effect, because it consists of amorphous carbon and graphite.