Recent developments in the science and technology of diamond deposition have, surprisingly, almost kept pace with the expectations generated by the popular press. Diamond of optical, i.e. gem, quality has been deposited on silica based glasses. In addition, free standing, optical quality films have been prepared by numerous laboratories in both the U.S. and Japan. Despite the rapid progress of recent months several goals have proved elusive. These include such important ones as uniform, high density (~108 cm-2 or greater) nucleation without surface damage and commercially feasible rates of deposition (~0.1 to 1 μm/hr) at substrate temperatures below 500°C. It has been found that the "quality" of diamond deposited at low substrate temperatures can be equivalent to that obtained at more conventional deposition temperatures of 800° to 1000° C. Nevertheless, the achievements which have been accomplished show significant promise for the future.
Diamond films by chemical vapor deposition have good potential for optical application only if transparent films can be produced. Thin diamond films were deposited on silicon, lead glass, MgO, fused silica and soda-lime silica at low temperatures (<500°C) by microwave plasma enhanced chemical vapor deposition. Low temperatures were achieved either by lowering microwave powers and pressures or by remoting the plasma. The deposited films were either white and transluscent or highly transparent depending on the different gas mixtures being used. The effect of gas composition on diamond formation will be discussed. Raman peak shifts of 2 to 8 cm-1 were observed due to the strain in the film. The exact temperature of the growth surface is uncertain as measurements were made only of the substrate bulk. Estimated temperatures were reported by careful calibration.
A 13.56 MHz inductively coupled plasma system has been used to deposit diamond from CH4/O2 in H2, CO in H2' and CH4/CO2 in H2 plasmas. Depositions were made on substrates of silicon and quartz with surfaces modified by scratching, or coated with Diamond-Like Carbon (DLC), or left unmodified. The C atom to 0 atom ratio was systematically varied as well as the total concentration. Polycrystalline films, faceted particles and spherical particles were deposited. We found that a C:0 ratio of 1:1 with a total concentration between 2% and 3% in hydrogen resulted in large area polycrystalline films for the deposition parameters investigated.
We developed a high-speed diamond synthesis technique called DIA-JET process that uses a DC plasma jet. Diamond is made by spraying the plasma jet (generated from the DC arc discharge) onto a water-cooled substrate. After examining the film with X-ray diffraction, Vickers hardness, and Raman scattering, we found that the quality approximated that of natural diamond. This paper describes the properties of the plasma jet and the synthesized diamond.
Reported here is the use of a unique laser plasma source able to deposit thin films of amorphic diamond at practical rates of growth. The beam from a pulsed Nd-YAG Laser is focused at very high power densities of 1012 W/cm2 onto graphite feedstock in an ultrahigh vacuum environment. The resulting plasma ejects carbon ions able to migrate to the plane of deposition. In this way diamond-like coatings have been applied to silicon, gold, germanium, glass, and plastic. No seeding or heating of the substrate is needed and substrate temperatures seem to remain at ambient room values during processing. Progress in the characterization of this material will be reported.
We have deposited thin amorphous carbon films on various substrate materials by irradiating a high purity (5N) graphite target in vacuum with pulsed light at 248 nm from a KrF excimer laser. The films have been characterized by Raman scattering, AES, SEM and chemical inertness tests. The films deposited on silicon are smooth and partially transparent. The preliminary Raman spectra show no evidence of crystalline grains (diamond or graphite) however the films deposited on silicon and quartz survived etching by a solution of HF:HNO3 (1:1). We will report on the substrate temperature dependence of laser deposited films and the variation of film quality on substrate material.
In order to understand the complicated chemical and physical processes that occur during the deposition of hard face coatings such as diamond, experiments that are remote, nonintrusive and sensitive to critical chemical species have been performed. Coherent anti-Stokes Raman spectroscopy (CARS) has been used to measure temperature and detect species such as methane and acetylene under low pressure, CVD environments. Results of these experiments for both an rf PACVD and heated-filament apparatus are described. In addition, these results and literature studies are interpreted using modeling (kinetic and equilibrium) calculations. Intepretations of the experimental results confirm the importance of high concentrations of hydrogen atoms, suggest that (hydrocarbon) radical species play a negligible role, and support proposals that in the presence of reactive hydrogen atoms virtually any hydrocarbon (or hydrocarbon oxygenate) can lead to diamond growth. The results in other laboratories on diamond deposition in acetylene/oxygen flames strongly support the first of these interpretations. In order to understand the competive process of soot/amorphous carbon formation, the equilibrium analysis of Stein and Fahr has been extended to low pressure, diamond forming conditions. This study indicates that a thermodynamic barrier exists to the growth of polyaromatic hydrocarbons at temperatures above 1300 to 1400K, pressures of 25 torr and hydrogen/acetylene ratios of 200.
Many different process and deposition equipment technologies have been developed worldwide for the deposition of thin diamond films. The diamond films that result from these technologies are, in general, polycrystalline and differ primarily in nucleation density, growth rate, crystallite size, non-diamond carbon content, etc. This paper reviews, briefly, the primary technologies and their resulting films. The primary focus of this paper is, however, the presentation of a methodology for the determination of the relative quality of diamond materials, both bulk and film. The methodology presented utilizes a combination of measurements from the mirage effect (Thermal Wave Technique) and the raman effect. This paper reviews, briefly, both techniques and their resulting data. Results are presented that show an excellent correlation between the thermal diffusivity and the graphitic content of thin diamond films.
The superior thermal conductivity of single crystal diamond (20 W/cm/K at room temperature for type IIa diamond) makes diamond desirable for many applications requiring the dissipation of heat. Several experimental methods have been used to determine whether chemical vapor deposited (CVD) diamond has a comparable thermal conductivity. Values as high as 10 W/cm/K have been measured. In this paper we discuss the use of photothermal radiometry to measure the thermal diffusivity and conductivity of CVD diamond.
We have developed a laser pulse technique to measure the thermal diffusivity of diamond films deposited on a silicon substrate. The effective thermal diffusivity of diamond film on silicon was measured by observing the phase and amplitude of the cyclic thermal waves generated by the laser pulses. An analytical model is developed to calculate the effective in-plane (face-parallel) diffusivity of a two layer system. The model is used to reduce the effective thermal diffusivity of the diamond/silicon sample to a value for the thermal diffusivity and conductivity of the diamond film. Phase and amplitude measurements give similar results. The thermal conductivity of the films is found to be better than that of type la natural diamond.
Using acoustic microscopy, we have examined polycrystalline diamond films grown by the CVD process on silicon substrates. This technique enables nondestructive characterization of these films with regard to their grain size, stress patterns and boundary anomalies between the film and the substrate. We have used a scanning acoustic microscope (SAM) to characterize several diamond films grown on silicon substrates at Crystallume and at ASTEX Co. The film thicknesses ranged from 1 μm to 100 μm with nominal grain sizes from 0.3 to 8 μm. Results will be presented showing various microstructural features in the films.
A multichannel-detection Raman microprobe, with laser-excitation at 514.5 nm, is employed in the rapid characterization of the microstructural and compositional perfection of diamond deposits prepared by the hot-filament chemical vapor deposition (CVD) method. Examined are single microcrystals of CVD diamond and polycrystalline thin diamond films deposited on silicon ((111) Si) and polycrystalline mullite (3Al2O3.2SiO2) substrates. Reported are the results from a series of films grown under constant deposition conditions of substrate temperature (750 °C), gas pressure (40 torr), and gas flow rate (52 sccm), but employing varying gas compositions with CH4:H2 ratios of 0.1 to 1.0 percent. The analysis focuses on the Raman range from 800 to 2000 cm-1 to establish the purity of the diamond phase based on the observation of characteristic carbon (i.e., sp3, sp2, and sp bonding) signatures and the level of the spectral background. A second spectral range from 5600 to 6200 cm-1 (Raman shift) is examined to monitor the presence of a photoluminescence (PL) band centered at 738 nm (1.68 eV) attributed to a lattice vacancy in diamond. The spectra characterizing the laser-excited optical emissions are correlated with the structure and morphology of these depositions established by several other characterization techniques. The conclusions drawn with respect to structural imperfections of the diamond phase are related to the deposition parameters employed and the resulting nucleation and growth processes involved.
We have employed a variety of in-situ optical diagnostic probes to analyze the filament-assisted diamond deposition gas-phase environment. In this paper, our studies using infrared diode laser absorption and multiphoton ionization spectroscopies are summarized. Emphasis is placed on a recently developed technique for the detection of gas-phase atomic hydrogen using third harmonic generation. The results are discussed in terms of the growth environment, the role of atomic hydrogen, and the carbon deposition mechanism.
Diamond film deposition processes are of great interest because of their potential use for the formation of both protective as well as anti-reflective coatings on the surfaces of optical elements. Conventional plasma-assisted chemical vapor deposition diamond coating processes are not ideal for use on optical components because of the high processing temperatures required, and difficulties faced in nucleating films on most optical substrate materials. A unique dual ion beam deposition technique has been developed which now makes possible deposition of diamond films on a wide variety of optical elements. The new DIOND process operates at temperatures below 150 aegrees Farenheit, and has been used to nucleate and grow both diamondlike carbon and diamond films on a wide variety of optical :taterials including borosilicate glass, quartz glass, plastic, ZnS, ZnSe, Si, and Ge.
Diamondlike films synthesized from liquid and gaseous sources have been reviewed and a novel process of synthesis from a solid (non-graphitic) geomorph has been reported for the first time. The compositions of the gaseous precursors have been identified by mass spectrometry. The diamondlike features have been studied by a combined analyses of microhardness, electrical resistivity, optical transparency over a wide wavelength range and chemical inertness. Laser crystallisation studies were carried out using a Nd : YAG laser. The formation of diamond was corroborated by Laser Raman scattering analysis and x-ray photoelectron spectroscopy. The effect of additive gases on the film properties, viz., stress, transparency and adhesion, critical for optical applications have been studied and an optimized parameter space established. The process parameters were utilized to generate hard, low-stress films over infrared (m) optical components.
The hemispherical transmittance of free standing films (1-20 microns thick) of polycrystalline diamond grown with a filament assisted chemical vapor deposition (FACVD) system and an oxygen-acetylene torch has been measured. Measurements were performed in the infrared (2-16 microns) with a Fourier Transform Infrared Spectrophotometer (FTIR) equipped with a diffuse gold integrating sphere and in the ultraviolet, visible and near infrared (0.20 - 2.5 microns) by a dispersive spectrophotometer used with an integrating sphere attachment. For FACVD films of approximately 1 micron thickness grown with a small amount of oxygen in the chamber, strong interference effects are observed in both spectral regions, and the total transmittance was above 60% in the visible and MR. The best films grown in the oxygen-acetylene flame show a sharp band edge at 220-222 nm, and a transmittance at long wavelengths (> 16 microns) which approaches 70%, for film thicknesses of 10-20 microns. These features are comparable to type IIA natural diamond. The optical transparency of the flame grown films is sufficient to read newsprint when held next to the text, however the large grain size (2-5 microns) and rough surfaces introduce sufficient scatter to blur the image of the text as the film and text are separated.
In the microwave plasma deposition (PECVD) of diamond from methane, the variables available for controlling the microstructure of the resulting films are the plasma composition and density, the substrate surface properties, and the temperature. It has been demonstrated that the competition between nucleation site formation and the rate of reactive plasma etching is the critical feature in the development of the film microstructure. Through reducing the deposition temperature and enhancing the etching rates of sp2 carbon, fine grained optical quality diamond films have been produced.
It has been shown that the optical transmission of microwave-plasma CVD produced diamond films is a strong function of the surface roughness. Polycrystalline diamond films consisting of pm-size crystallites and deposited at relatively high substrate temperatures (~700-1000 °C) have significant surface roughness with values ranging from a few hundred Angstroms to a few thousand Angstroms. The discrepancy in the transmission spectra between single crystal type IIA natural diamond and polycrystalline diamond films can be fully accounted for by optical scattering and difference in the thicknesses of the two types of samples. For sufficiently long wavelengths in the infrared, the optical scattering is predominantly due to surface roughness. Our data show that for diamond films with surface roughness values of about 500 Å or less, the infrared transmission of such films at wavelengths about 5μm and greater is comparable to that of single crystal type IIA natural diamond.
Optically active defects in diamond films grown by the hot-filament chemical vapor deposition method were investigated by cathodoluminescence (CL) imaging and spectroscopy in a scanning electron microscope. A set of films grown on silicon substrates at deposition temperatures (Td) from 600°C to 850° C was studied. The spatial resolution of the CL images was approximately 0.2 to 0.5 μm; CL spectra were measured with wavelength resolution 0.4 nm in the wavelength range 350 to 900 nm.
Cathodoluminescence, absorption and Raman spectra of diamond particles and polycrystalline films prepared by a microwave plasma method from gaseous mixtures of methane and hydrogen have been studied. The results are discussed in relation to the effect of the methane concentration on the optical properties of the diamond material produced. Absorption edge at 5.5eV was observed for the polished film prepared at 0.3% methane concentration, and for particles prepared at 1% methane concentration containing water vapor. Edge emission in the cathodoluminescence spectra was observed for particles obtained at methane concentrations from 0.5 to 2%, while the emission was not observed in the films obtained at the same conditions. The edge emission became weaker with increasing methane concentration. The results indicate that the particles prepared at lower methane concentrations are high quality diamond similar to type IIa diamond, the purest form of natural diamond, while the films are heavily strained.
Infrared spectra were obtained from diamond films grown on Si wafers by microwave plasma assisted chemical vapor deposition. The transmission of a 3-μm thick film varied from near 60% at 5000 cm-1 to near 73% at 1000 cm-1. Infrared spectra were fit with simulated spectra to determine values for thickness, index of refraction, and surface roughness or thickness variation. Impurity peaks at 2925-2830 cm-1 and 3325 cm-1 were assigned to sp3 carbon CH2 or CH groups and sp carbon CH groups, respectively.
Results of room-temperature optical studies on Ã¢â‚¬â€?10 micron free-standing diamond films are reported. The films were grown on Si(100) substrates by hot filament-assisted chemical vapor deposition (CVD) from a methane/hydrogen mixture. The as-grown, free surface of the film exhibits a surface roughness of scale 0.2-5 microns, depending on the methane/hydrogen ratio of the growth gas mixture, which introduces significant optical scattering losses for frequencies greater than 0.5 eV. Reflection and transmission spectra in the range 0.01-10 eV were collected for films grown in different methane/hydrogen mixtures. Below the threshold for interband adsorption, the film behaves approximately as a thin parallel plate of refractive index 2.4, with the rough free surface leading to increasingly larger loss of specular transmission/reflection with decreasing wavelength (λ). For λ>s, where s is the average scale of the surface roughness, distinct interference maxima are observed, and the data in this region can be analyzed to determine the refractive index and film thickness. Structure associated with absorption from one-, and multi-phonon processes and chemisorbed hydrogen are also observed. Near 5.3 eV the onset of interband adsorption is observed, in good agreement with the value of the indirect bandgap in crystalline type IIa diamond. The films are found to exhibit optical properties similar to that of bulk diamond. However, the surface roughness must be better controlled by the deposition process, if these CVD films are to be used in applications as protective, high refractive index optical coatings.
We have demonstrated for the first time that in diamonds some defects correlated with the presence of intense H-related infrared absorptions, also absorb in the visible range. These diamonds exhibit two types of optical absorption spectra, resulting in a browniqh yellow or gray color. We believe that the 1405 and 3107 cm-1 H-related infrared absorptions are not simply due to C-H vibrations, but rather to stretching modes of complexes involving hydrogen and nitrogen in all cases. These types of H-rich brownish yellow and gray diamonds - or sectors of particular stones - are probably the result of cuboid growth.
Free-standing diamond films are prepared by CVD technique to examine their properties directly. The products have a variety of shapes such as plates, tubes and curved diaphragms. Coefficients of thermal expansion (GTE) of the tube are similar to the values of a bulk diamond in the range from 40°C to 500°C. It is found that polished diamond film has uniform infrared transmission ranging from 500cm-1 to 4000cm-1. A speaker diaphragm will be a good application for free-standing diamond film.
Diamond possesses a number of interesting properties that make it an ideal candidate for a variety of x-ray optical applications. Most notable are the high x-ray transmission, exceptional mechanical properties and low diffusion rate of diamond films. One application that has been made possible recently with plasma-enhanced chemical vapor deposition of diamond films is an ultra-thin x-ray detector window for elemental analysis. This paper discusses the advantages of using diamond films for this application and what developments were necessary in order to meet performance specifications. We will also discuss qualification testing of the x-ray window and suggest other areas of application which may benefit from this new technology.
The influence of argon ion and CO2 laser irradiation on the structure properties of diamond-like carbon films is reported. The structure ratio of sp2/sp3-CH bond in the films increases as the argon ion irradiates while it decreases as the CO2 laser irradiates , which show that the argon ion irradiation is main for breaking the C-H bond while the laser irradiation is a kind of process of thermal annealing.
Hydrogenated amorphous carbon films were prepared by rf glow discharge decomposition of pure methane at various deposition conditions. The structure and properties of the films have been investigated in respect to the growth rate, conductivity, hardness and bonding structure as a function of substrate temperature, rf power and total gas pressure. The bonding structure has been determined by infrared absorption and electron energy loss spectrum measurements. It is found that a diamond-like film with a hardness of 2200 kg/mm2 can be deposited under the optimum deposition condition. The properties of the films used as a protective coating has also been investigated.