偏壓輔助化學氣相沉積均勻鑽石-成核和成長之研究

Abstract

本實驗以偏壓輔助化學氣相沉積鑽石之均勻成核和成長的研究，主要分成三個部分來探討。其中第一部份，描述我們設計一個Mo質的上電極，去改善微波電漿和輝光放電(二次電漿)的分佈，進而能沉積分佈均勻的鑽石膜在Si(100)基材上。第二部分則是藉此上電極的輔助，來探討不同製程參數在偏壓階段時對鑽石成核的影響，結果可發現高方向性的單晶奈米級鑽石。第三部分，藉由改善偏壓時，鑽石會隨空間變化而不均勻沉積的問題。我們研究鑽石在初始成核階段時，形成進展的過程，進而能澄清鑽石在Si(100)的成核機制。 在第一部分中，利用微波電漿化學氣相沉積法(MPCVD)，使用偏壓輔助成核的方法(BEN)來輔助鑽石成核，以氫氣和甲烷為氣體源，來合成鑽石在1 x 1 cm2 Si(100)基材上。藉由使用自行設計的圓頂狀 Mo質上電極，可改善鑽石沉積分佈的均勻性，包含鑽石的密度、尺寸和形貌等。分析技術方面，應用了掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)和Raman光譜儀等方式，結果發現，在偏壓期間，若是使用圓頂狀上電極，以及在成長階段時通入2%的甲烷，可以得到<100>織構的高品質鑽石薄膜，且其為一相當平整的表面。 第二部份的討論則著重於，在偏壓階段，我們藉由上電極輔助時，各種不同製程參數對於鑽石成核的影響。沉積的核種經由SEM的分析，可知所有不同條件試片，都可沉積高密度且均勻分佈的圓錐物。我們還了解到，這些圓錐物的密度與參數的相互關係：若初步碳氫加熱階段的時間越長，則密度越高；圓錐物密度還隨著在偏壓階段時的甲烷濃度、偏壓時間、電漿功率增加而增加，但會隨著偏壓增高而降低。TEM更進一步顯示出圓錐物的結構，乃是由Si cone、SiC和daimond，由下自上組合而成，且SiC和Si cone 一定有磊晶的關係。經由使用更短的偏壓時間來作成核研究，可發現大量具方向性的單晶奈米鑽石磊晶在SiC上。長時間的偏壓後，會發現到多晶鑽石的形成，推測可能是因為二次成核所造成的。 最後一部份探討的是，經由MPCVD並輔以直流偏壓時，鑽石在Si(100)上的成核機制。此處，我們比較不同偏壓時間下的結果(20s – 4 min)，同時也發現到鑽石核種的形成過程。SEM的結果顯示出，在初期的偏壓階段中，有相當高密度的圓錐物形成。偏壓後期，則是有很多的樹狀物形成在基材上。TEM的結果更顯示出，無論是錐狀或是樹狀物，其結構都是最下面為Si cone晶體，SiC的磊晶則成長其上，並且形成一個V型火山口，diamond則成長其中。 最後對錐狀物結構的形成過程和表面型態之發展，也利用SEM和TEM來觀察研究。

The dissertation is divided into three main parts. The first part, we designed a Mo anode which improved the distribution of microwave plasma and dc glow discharge to deposit uniform diamond films on Si substrates. In the second part, we discuss the effect of the different parameters for the nucleation of diamond with this anode during bias stage. As a result, oriented diamond nuclei as single crystals can be found. In the last part, we investigated the evolution of diamond crystallites formed in the early bias stage. We have clarified the mechanism for the diamond nucleation on Si (100) without the effect of spatial variation of diamond during the bias stage.

In the first part of the dissertation, diamond films on 1 x 1 cm2 Si (100) substrates were synthesized by microwave plasma chemical vapor deposition (MPCVD) using a mixture of methane and hydrogen gases. The bias-enhanced nucleation method was used to avoid any mechanical pretreatments. Distribution of deposited diamond crystallites in terms of density, size, and morphology has been significantly improved over all the Si substrate surface area by using a novel designed Mo anode. Films were characterized from the center to the edges of substrates using scanning electron microscopy, transmission electron microscopy, and Raman analysis. The results also show that uniform diamond films can be obtained by a short bias nucleation period using a dome-shaped Mo anode. Using 2% CH4 in the growth stage, high-quality diamond films in the <100> texture can be obtained with relatively smooth surfaces.

In the second part of the dissertation, we show the effect of the different manufacturing conditions for the nucleation of diamond during bias stage using this dome-shaped Mo anode. The deposits were characterized by scanning electron microscopy (SEM), and transmission electron microscopy (TEM). SEM observations show that there is a high density of cone-like particles uniformly deposited on the surface of the substrate in all of the different deposition conditions. The density of cone-like particles was increased with time in the heating stage using hydro-carbon plasma and increased with methane concentration, bias time, and power in the bias stage. TEM reveals that each cone-like particle is actually composed of Si conic crystal covered with diamond. Between Si and diamond, a thin layer of cubic SiC is found in epitaxy with Si. Furthermore, the oriented diamond nuclei as single crystals with facets can form on self-formed Si cones through epitaxial SiC within a short bias period of 60 s. After a longer bias time, it has been observed that polycrystalline diamonds formed as a result of secondary nucleation.

In the final part of the dissertation, we studied a nucleation process of diamond on Si (100) substrates performed by bias-enhanced microwave plasma chemical vapor deposition (MPCVD). Deposition for different bias time (20 s– 4 min) has been demonstrated from CH4-H2 gas mixtures and a crystal phase formation process is found. SEM shows that there is a high density of cone-like particles in the very early stage and tree-like particles were uniformly deposited on the surface of the substrate in the later stage of bias application. TEM reveals that the cone-like and tree-like structures are actually composed of Si cone covered with epitaxial cubic SiC of a volcano shape which is topped with diamond. The evolution of the morphology of the conic structure has been studied using SEM and TEM.