Graduation Year

2011

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Mechanical Engineering

Major Professor

Ashok Kumar, Ph.D.

Committee Member

Muhammad Rahman, Ph.D.

Committee Member

Garrett Matthews, Ph.D.

Committee Member

Frank Pyrtle ІІІ, Ph.D.

Committee Member

Jing Wang, Ph.D.

Committee Member

Rasim Guldiken, Ph.D

Keywords

carbon, MPECVD, HFCVD, I-V characteristics, CMUTs

Abstract

The studies covered in this dissertation concentrate on the various forms of diamond films synthesized by chemical vapor deposition (CVD) method, including microwave CVD and hot filament CVD. According to crystallinity and grain size, a variety of diamond forms primarily including microcrystalline (most commonly referred to as polycrystalline) and nanocrystalline diamond films, diamond-like carbon (DLC) films were successfully synthesized. The as-grown diamond films were optimized by changing deposition pressure, volume of reactant gas hydrogen (H2) and carrier gas argon (Ar) in order to get high-quality diamond films with a smooth surface, low roughness, preferred growth orientation and high sp3 bond contents, etc. The characterization of diamond films was carried out by metrological and analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectroscopy. The results of characterization served as feedback to optimize experimental parameters, so as to improve the quality of diamond films. A good understanding of the diamond film properties such as mechanical, electrical, optical and biological properties, which are determined by the qualities of diamond films, is necessary for the selection of diamond films for different applications. The nanocrystalline diamond nanowires grown by a combination of vapor-liquid-solid (VLS) method and CVD method in two stages, and the graphene grown on silicon substrate with nickel catalytic thin film by single CVD method were also investigated in a touch-on level.

Microwave plasma enhanced chemical vapor deposition (MPECVD) polycrystalline diamond films were deposited with mixed gas CH4 and H2 at flow rate ratio 1:100 by changing the H2 volume from 100sccm to 3000sccm. SEM micrographs revealed that the samples of 100sccm and 500sccm H2 had agglomerates of a cauliflowerlike surface, whereas samples with 1000sccm to 3000sccm H2 had a faceted surface; the size of the faceted crystallites ranged from 200nm to 1500nm. Raman spectra indicated that the samples of 100sccm H2 contain a certain amount of graphitic phase, whereas samples of more than 1000sccm H2 had a concentration of crystalline diamond. The XRD patterns highly exhibited the crystallinity of deposited diamond with largely (111) and (220) plane; (220) plane accounted for three times more than (111) plane in the whole deposited surface.

MPECVD nanocrystalline diamond films displayed a nano peak at 1140cm-1 in the Raman spectrum, and showed 100nm global particles on the surface in SEM images, presented (111) and (220) planes with a low ratio of 1:2 in the XRD pattern. Threedimensional AFM images provided consistent grain size with that of the SEM images, and 94nm average roughness in the 10μm×10μm scan area.

Diamond-like carbon (DLC) did not exhibit any sharp peaks in the Raman spectrum, and only showed broad bumps in the position of D band and G band, illustrating that there was no crystalline structure formed on the surface. SEM disclosed a rough surface with scattered particles embedded in the tiny boundary-like ditches.

The morphologies of nanocrystalline diamond films changed from scattered global particles to texture with the Ar flow rate varying from 170sccm to 200sccm under 120HPa constant pressure. XRD pattern displayed similar height of (111) and (220) plane. In another condition, keeping the Ar flow rate at 180 sccm constantly, a sample of 110HPa pressure presented a smoother surface, and the (111) plane was the primary structure of surface with the (220) plane dematerialized and a (311) plane generated.

Nitrogen-doped nanocrystalline diamond films, with 5%, 10%, 15% N2 flow rate variation, were investigated, and it turned out with the increase of N2, the nano peak in the Raman spectra dematerialized, the XRD pattern revealed that the (111) peak became the major component rather than the (220) peak; a sample of only 10% N2 displayed texture structure.

15% N2 doped nanocrystalline diamond films grown on SiO2 demonstrated a smooth surface, high growth rate, and high (111) peak, indicating the SiO2 layer changed the surface electron density and, therefore, changed the quality of the diamond film.

The surface residual stress evaluated by the Raman shift showed that HFCVD polycrystalline diamond film had tensile stress at the corner and compressive stress in the center, whereas MPECVD polycrystalline diamond film had compressive stress on the whole surface. The roughness of MPECVD was higher than that of HFCVD.

A capacitive micromachined ultrasonic transducer (CMUT) with a diamond membrane was successfully fabricated by overcoming the challenges such as diamond window etching, two-stage diamond depositing, and vacuumed cavity formation.

Download

Share

COinS