Taras V. Kononenko

Direct observation of laser-induced crystallization of a-C : H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm2. At fluences below 100 mJ/cm2 an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (≈400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm2. The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.

Laser treatment of tribological DLC films

The friction and wear reduction in applications that allow only a minimal use of liquid lubricants is done with solid lubricant films or with protective coatings, such as diamond-like carbon (DLC). Further improvements are possible if the geometries of the contact surfaces are modified in a controlled way, as we have already demonstrated it for TiN and TiCN. In this work, the possibilities to generate patterned DLC coated low wear tribological surfaces by means of laser processing were investigated. In the first approach, a two step method was used: steel substrates were laser patterned and subsequently DLC films were deposited on them. The second considered approach was the laser processing of coated surfaces. DLC films were irradiated with laser pulses of different durations and energy densities (100 fs, 800 nm, <4 J/cm2; 150 ns, 1064 nm, <10 J/cm2) and the treated spots were examined using optical microscopy, SEM, AFM and Raman spectroscopy. The graphitisation of a-C:H films under both fs- and ns- regimes was shown as well as a film-peeling phenomenon during the ablation process. Microstructured and DLC coated surfaces obtained in the former approach were used for preliminary tribological tests (oscillation-friction-wear method). The results showed that the friction coefficient did not increase, as compared with the unstructured and DLC coated surfaces, and that the structure pores trapped the debris particles produced when the DLC film eventually broke.

Optical monitoring of nucleation and growth of diamond films

Early stages of diamond film deposition on molybdenum substrates using dc arc discharge in CH4/H2 gas mixtures were studied by in situ measurements of optical reflectivity of growing film. Ultrafine diamond grit of ≊200 A size was used for seeding to increase nucleation density up to 2×109 cm−2 and to produce smooth thin films. Evolution of He‐Ne laser beam reflection at 0.63 μm wavelength is described in terms of Mie scattering by nonabsorbing dielectric spheres in the case of nucleated film and of light interference in the system of continuous diamond film on a metal substrate. During the deposition process the growth rate passes through a minimum at the moment when a minimum roughness is supposed to be achieved.

Ablation of CVD diamond with nanosecond laser pulses of UV-IR range

Etch rates of CVD diamond upon irradiation by nanosecond (5‐9 ns) pulses at three diVerent wavelengths 1078, 539 and 270 nm at laser fluences in the range 1‐1000 J/cm2 were measured. A Nd:YAP laser system operated at first, second and fourth harmonics was used in the ablation experiments. Both shallow (<15 microns) and through holes were etched in a 95-mm thick free-standing diamond film grown by microwave plasma CVD. The ablation rate was found to be wavelength-independent, this result being ascribed to surface blackening caused by amorphization/graphitization as confirmed by Raman analysis. The maximum etch rate approached 600 nm/pulse. The etch rate depended on the crater depth, which was ascribed to the eVect of laser‐plasma interaction inside the deep channel. The possibility of cutting trenches of high aspect ratio has been demonstrated. In a separate experiment, a batch of thin diamond films diVering in thermal conductivity (k=2‐5 W/cmK ) was ablated with a KrF excimer laser (l=248 nm). No dependence of ablation rate on film quality was observed, which could be explained assuming grain boundaries to be the main source of thermal resistance.

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Abstract Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.

Catalytic interaction of Fe, Ni and Pt with diamond films: patterning applications

Abstract Diamond films were patterned using thin films of Fe, Ni and Pt for catalytic etching at temperatures of 850–950°C in an H2 atmosphere. The diamond etching mechanism involves carbon dissolution in metal, diffusional transport to the metal-gas interface and carbon desorption in the form of methane. Iron films have shown the highest catalytic activity of the metals examined, providing etch rates up to 8 μm min−1. An effect of catalyst deactivation was observed and assigned to graphite-like carbon accumulation on the outer surface of the metal. Several methods for the patterning of iron films are described.

Hole formation process in laser deep drilling with short and ultrashort pulses

The drilling process in different materials (diamond, steel, ceramics and PMMA) was studied for a large range of pulse lengths from about 100 fs to 10 ns using different approaches. In transparent materials the penetration process was visualized with high-speed video analysis and microscopy. The drilling rate as well as the relation between processing energy density and ablation threshold were determined in situ. The penetration of the laser beam inside the channel and the influence of laser-ignited plasma were investigated by transmission measurements. Mechanisms of energy coupling and heat losses were examined by applying simple analytical calculations. Proposals for the basic understanding of the drilling process are presented.

Plasma effects during ablation and drilling using pulsed solid-state lasers

Plasma and vapor plumes generated by ultrashort laser pulses have been studied by various optical methods for both single pulse ablation as well as high-repetition rate drilling. Time-resolved shadow and resonance absorption photographs enable to determine the plume and vapor expansion behavior and, by means of an analytical shock wave model, allow to estimate an energy balance that can be refined by plasma transmission measurements. The results furthermore suggest that several types of laser-induced plasmas can be distinguished according to their origin: the material vapor plasma originating at the ablated surface even at moderate intensities, a breakdown plasma at increased power densities occurring in cold vapor or dust particles left from previous ablations during repetitively-pulsed processing and, finally, the optical breakdown in the pure atmosphere at high intensities. The latter also gives rise to nonlinear scattering phenomena resulting in a strong redistribution of the energy density in the beam profile.

Lithographic application of diamond-like carbon films

Abstract A combined laser-plasma-etching technique has been investigated for utilizing diamond-like carbon (DLC) films as resists in lithography. The technique is based on the selective enhancement of O2 plasma etch rates of DLC films due to graphitization in the regions exposed to excimer laser radiation. The patterns delineated in the DLC resist by laser and oxygen plasma exposure have been transferred into the underlying silicon substrate by fluorocarbon reactive ion etching with (RIE) the patterned DLC film acting as an in situ mask. Using this scheme, space-line patterns 5 μm wide were made on silicon substrates to demonstrate the feasibility of the proposed laser-RIE process for semiconductor patterning. The laser fluence required for graphitization is ~100 mJ cm−2, the O2 plasma etch rates of the graphitized regions are higher by a factor of ~3.5 and the contrast (γ) for the DLC resist is 1.1. The use of DLC for submicron resolution warrants further investigation.

Excimer laser etching of diamond-like carbon films: spalling effect

Abstract Etching of diamond-like amorphous hydrogenated carbon films by 20 ns pulses of a KrF excimer laser has been investigated. In addition to previously reported etching mechanisms realized via either carbon oxidation or vaporization, a new etching mode based on a spalling effect was observed. It was found that there is a narrow window in laser fluences in which the etch rate of the carbon films examined exceeds by nearly two orders of magnitude the etch rates measured at fluences beyond this interval. The effect of anomalously fast etching occurs below a threshold for normal laser ablation (vaporization), and is ascribed to stress-induced lift-off of the material in the form of 30–60 nm thick macroscopic sheets. Very smooth surface relief in the etched crater and a reduced degree of film graphitization are characteristic features of this new etching mode.