W. S. Huang

Surface acoustic waves on nanocrystalline diamond

Abstract Surface acoustic wave (SAW) devices based on polycrystalline diamond have recently achieved success as microwave filters. This is due in part to the large acoustic wavelength of diamond at microwave frequencies, a consequence of its high surface wave velocity, and the resulting ability to use photolithography for transducer fabrication. Since nanocrystalline diamond has a smooth surface and is elastically isotropic, it may offer considerable advantages over thick films of polycrystalline diamond. We have studied the propagation of surface waves on nanocrystalline diamond prepared by microwave plasma chemical vapor deposition (CVD) on silicon substrates. Films were synthesized on 75-mm Si wafers using input gas mixtures consisting of Ar with 1% CH4 and 0–4% H2. The deposition parameters studied included pressure, 2.45 GHz microwave power, and total gas flow rate. Film thicknesses up to 23 μm were produced. SAW transducers were fabricated by photolithography on as-grown nanocrystalline diamond surfaces covered with a 1–3 μm overlayer of oriented polycrystalline piezoelectric ZnO prepared by reactive dc sputtering. The device response was analyzed with frequency and time domain methods. The resonant frequencies of the devices agree with the results of numerical solutions for sound propagation in layered media. Several surface acoustic modes exist at frequencies between 0.5 and 1 GHz that exhibit appreciable dispersion. We have propagated surface waves in nanocrystalline diamond over distances varying from 0.1 to 3 mm with low attenuation. For a film with mean grain size of approximately 30 nm, the SAW velocity is similar to test devices on thick polycrystalline diamond. We conclude that nanocrystalline diamond is a highly attractive substrate material for SAW devices, possessing the high sound velocity of diamond but requiring less materials processing.

Plasma diagnostic measurements of argon-hydrogen-methane discharges used for ultra-nanocrystalline diamond deposition in a microwave CVD system

Summary form only given, as follows. Argon-methane and argon-methane-hydrogen discharges used in a microwave CVD system for the deposition of diamond films are investigated in this study. These discharges have been used to produce small grain size diamond films, also known has ultra-nanocrystalline diamond. To further understand the plasma deposition process experimental measurements of the discharges have been undertaken. Specifically, the objective of this study is to determine the relationship of the deposition results versus plasma species concentrations and temperatures. The deposition discharges being studied are at pressures of 80-160 Torr with 0-3% methane and 0-10% hydrogen in an argon discharge. The plasma discharges studied are excited using a 17.8 cm diameter microwave resonant cavity reactor. The discharges are excited using 2.45 GHz microwave power in a discharge chamber with 12.5 cm diameter. Experimental characterizations include OES measurements of selected atomic hydrogen, C/sub 2/, and CH emission lines, and OES measurements of gas temperature in the discharge. Changes in the input gas composition are observed to produce substantial variations in these OES measurements. The measurements are conducted as functions of the discharges quartz wall temperature (100-200 C) and the substrate temperature (400-900 C). The discharge experimental measurement results are also correlated with the properties of the diamond films deposited.

The deposition of ultra nanocrystalline diamond films using a Ar/H/sub 2//CH/sub 4/ microwave discharge

Summary form only given. During the past ten years, plasma assisted chemical deposition of diamond thin films has been extensively investigated by many research groups. These experimental investigations have shown that diamond films can be synthesized from heated or ionized H/sub 2//CH/sub 4/ gas mixtures. The synthesized films result in a columnar growth texture with rough surface; i.e. the films are polycrystalline. This surface roughness usually prevents these films from being used in many wear and cutting tool applications. Thus, it is desirable to develop new methods to synthesize small crystalline and smooth diamond films. This investigation presents the experimental results concerned with the development of a new method of synthesizing smooth, ultra-nanocrystalline films. Building upon the experimental results of Gruen (Zhou et al., 1998), microwave plasma assisted film deposition is investigated using carbon containing Ar plasma. Experiments are performed with a MSU developed microwave plasma reactor. In this investigation the dominant input gas is argon. CH/sub 4/ concentrations are less than 3% and H/sub 2/ concentrations are limited to less than 6% of the total input gases.

High rate diamond deposition using a microwave discharge

Summary form only given. Presently, plasma assisted synthesis of diamond films often employs a microwave discharge/reactor for the source of the radical species. Typical deposition takes place using varying CH/sub 4//H/sub 2//CO/sub n/ gas mixtures over a 20-100 Torr pressure regime and with a CW, 2.45 GHz input power of less than 3 kW. While excellent diamond films are synthesized under these conditions, the linear deposition rates are usually limited to only a few /spl mu/m/hr. Since diamond synthesis costs are still high, it is desirable to deposit high quality diamond films with higher deposition rates. One method to increase the diamond deposition rate is to increase the deposition pressure and discharge absorbed power density. This paper will present the results of an experimental investigation exploring microwave plasma synthesis at the 100-200 Torr pressure and 2.5-6 kW input power regimes. This work, which builds upon earlier reported work (Kuo et al., 1992, Kahler, 1997), utilizes a microwave reactor developed for operation at higher pressure/power regimes.

Investigation in the effect of nitrogen incorporation in heteroepitaxial diamond film growth, texture, morphology, and crystalline quality

Summary form only given. It has been shown that even small concentrations of nitrogen have a substantial effect on the growth of CVD diamond films, while larger concentrations may result in degraded and rough growth. In the present study we show the effect of nitrogen concentration on the growth, texture, morphology, and crystalline quality of CVD diamond films. All films are heteroepitaxially grown on a 3 inch silicon substrate and are scratch-seeded with a seeding density of 10/sup 9//cm/sup 2/. A computer controlled 5 inch discharge 2.45GHz microwave plasma reactor with ultra-high vacuum gas handling systems combined with optical emission spectroscopy allows exact control of the gas phase concentrations and reactor input variables. Nitrogen gas phase concentration is varied from l0ppm to over 1000ppm and is controlled by the measurement of CN bands in the optical emission spectrum of the plasma. Pressure ranges from 38-50 Torr, methane/hydrogen from 0.5-2%, and flow rate from 50-200 sccm. Growth and morphology are carefully determined using SEM while texture is obtained using XRD. Crystalline quality is measured by determining the FWHM of the diamond Raman peak at 1332/cm using unpolarized micro-Raman spectroscopy. Detailed growth, texture, morphology, and crystalline quality maps are given to show the sequential effect of increasing nitrogen versus all other reactor input variables.