Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring

This paper reports tribological properties of diamond-like carbon (DLC) films nanostructured by femtosecond (fs) laser ablation. The nanostructure was formed in an area of more than 15 mm × 15 mm on the DLC surface, using a precise target-scan system developed for the fs-laser processing. The frictional properties of the DLC film are greatly improved by coating a MoS₂ layer on the nanostructured surface, while the friction coefficient can be increased by surface texturing of the nanostructured zone in a net-like patterning. The results demonstrate that the tribological properties of a DLC surface can be controlled using fs-laser-induced nanostructuring.     

Sub-wavelength Ripple Formation on Silicon Induced by Femtosecond Laser Radiation

Periodic microstructures on silicon bulk axe formed by the irradiation of the femtosecond laser with the laser wavelength of 800 nm and the pulse length of 130 fs. We investigate the surface periodic ripple structures produced by femtosecond laser treatment. The effects of feedrate of sample, v, on laser-induced surface topography are studied. We find that the femtosecond laser produce periodic ripples of the sub-micron level on silicon surface. At the same time, we realize the optimal conditions to produce these surface structures. When choosing NA = 0.3, and v = 2000μm/s or 3000μm/s, we find a series of periodic-structure ripples where the spacing is about 120 nm and the width is about 45nm. The experimental results indicate that femtosecond laser treatment can produce line arrays on the sub-micron level, which is a positive factor for fabricating grating and other optical applications in nanoscales.     

Practical uses of femtosecond laser micro-materials processing

We describe several approaches to basic femtosecond machining and materials processing that should lead to practical applications. Included are results on high-throughput deep hole drilling in glasses in ambient air, and precision high-speed micron-scale surface modification of composite materials and chalcogenide glasses. Ablation of soda-lime silicate glass and PbO lead-silicate is studied under three different sets of exposure conditions, for which both the wavelength and pulse duration are varied. Ablation rates are measured below and above the air ionization threshold. The differences observed are explained in terms of self-channeling in the ablated hole. Fabrication of practical devices such as waveguides and gratings is demonstrated in chalcogenide glass. Received: 11 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-407/8233-570, E-mail: mrichard@mail.ucf.edu     

Ablation of polymer foils for backside opening of thin film layered flexible polymer substrates

For increasing the packing density of electronic devices and enabling 3D wiring, new concepts of interconnection for flexible circuit boards are required. The backside wiring is one innovative concept which, however, requires interconnections from the back to the front side by means of vias.Results on backside opening of polymer foils for exposing a thin metal film deposited at the front side are presented. For the experiments, a thin polyimide foil covered with a thin molybdenum metal film was used. By using mask projection of a pulsed UV-laser beam (248 nm, 20 ns) polymer foil was ablated. The laser ablation process must be adjusted in the manner to avoid damage of the thin metal film, to prevent cones formation at laser ablation, but still enabling the clean ablation of the polymer. The influence of process parameters on the backside opening is discussed and compared with theoretical estimations of the laser-induced temperatures. Using a two-step ablation process applying first high fluences to ablate the main part of the foil and finishing with low laser fluence turns out to be advantageous. This backside opening (BSO) can be used to perform an electrical contact from the backside.     

Angular laser cleaning for effective removal of particles. from a solid surface

Laser removal of small particles from a metal surface is carried out by changing the incident angle of the laser beam. It has been found that a dramatic improvement of cleaning efficiency in terms of area and energy is observed when using the laser at glancing angle of incidence as compared to perpendicular. Furthermore substrate damage is greatly reduced and probably eliminated at glancing angles. The process mechanism is discussed by considering the adhesion and the laser-induced cleaning forces for different incident angles. It is shown that there are different laser–matter interactions operating. Received: 25 April 2000 / Accepted: 9 May 2000 / Published online: 5 October 2000     

Transmission and scanning electron microscopy studies of single femtosecond- laser-pulse ablation of silicon

The final state of the material resulting from laser irradiation of silicon using 130 fs pulses at 790 nm was studied using a number of techniques including scanning and transmission electron microscopies, as well as atomic force microscopy. Structural details and the level of damage to the nearby solid following irradiation were characterized and are discussed in the context of recent dynamical studies. Received: 28 September 2001 / Accepted: 3 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Department of Engineering Physics, McMaster University, Hamilton, Ontario, L8S 4M1, Canada RID="**" ID="**"Corresponding author. Fax: +1-905/521-2773, E-mail: borowia@mcmaster.ca RID="***" ID="***"Present address: Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK RID="****" ID="****"Department of Materials Science and the CEMD, McMaster University, Hamilton, Ontario, L8S 4M1, Canada RID="*****" ID="*****"Departments of Engineering Physics, and Physics and Astronomy, and the CEMD, McMaster University, Hamilton, Ontario, L8S 4M1, Canada     

Laser ablation and micropatterning of thin TiN coatings

Laser ablation of thin TiN films deposited on steel substrates has been studied under wide-range variation of irradiation conditions (pulsewidth, wavelength, energy density and spot size). It has been demonstrated that both picosecond (150–300 ps) and nanosecond (5–9 ns) laser pulses were suitable for controllable ablation and microstructuring of a 1-μm-thick TiN film unlike longer 150-ns pulses. The ablation rate was found to be practically independent of the wavelength (270–1078 nm) and pulsewidth (150 ps–9 ns), but it increased substantially when the size of a laser spot was reduced from 15–60 μm to 3 μm. The laser ablation technique was applied to produce microstructures in the thin TiN films consisting of microcraters with a typical size of 3–5 μm in diameter and depth less than 1 μm. Tests of lubricated sliding of the laser-structured TiN films against a steel ball showed that the durability of lubricated sliding increased by 25% as compared to that of the original TiN film. Received: 28 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Laser removal of copper particles from silicon wafers using UV, visible and IR radiation

Laser removal of small copper particles from silicon wafer surfaces was carried out using Nd:YAG laser radiation from near-infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm). It has been found that both 266 nm and 532 nm are successful in removing the particles from the surface whereas 1064 nm was shown to be ineffective in the removal of particles. The damage-threshold laser fluence at 266 nm was much higher than other wavelengths which provides a much wider regime for safe cleaning of the surface without causing any substrate damage. The cleaning efficiency was increased with a shorter wavelength. The effect of laser wavelength in the removal process is discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning forces for the three wavelengths. Received: 31 May 2000 / Accepted: 14 July 2000 / Published online: 20 June 2001     

Femtosecond ablation of ultrahard materials

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm² fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 20 March 2002     

Photochemical effects in the UV laser ablation of polymers: Implications for laser restoration of painted artworks

0 ∝t^1/2. The best results are expected for a circular top-hat beam shape. Received: 15 January 1999 / Accepted: 18 January 1999     

Ablation with a single micropatterned KrF laser pulse: quantitative evidence of transient liquid microflow driven by the plume pressure gradient at the surface of polyesters

A microscopic flow of a transient liquid film produced by KrF laser ablation is evidenced on targets of PET and PEN. Experiments were done by using single pulses of the excimer laser beam micropatterned with the aid of submicron projection optics and grating masks. The samples of various crystalline states, ablated with a grating-forming beam (period Λ=3.7 μm), were precisely measured by atomic force microscopy, in order to evidence any deviation from the ablation behavior predicted by the current theory (combination of ablation curve and beam profile). This was confirmed by comparing various behaviors dependent on the polymer nature (PC, PET and PEN). PC is a normally ablating polymer in the sense that the ablated profile can be predicted with previous theory neglecting liquid-flow effects. This case is called ‘dry’ ablation and PC is used as a reference material. But, for some particular samples like crystalline PET, it is revealed that during ablation a film of transient liquid, composed of various components, which are discussed, can flow under the transient action of the gradient of the pressure of the ablation plume and resolidify at the border of the spot after the end of the pulse. This mechanism is further supported by a hydrodynamics theoretical model in which a laser-induced viscosity drop and the gradient of the plume pressure play an important role. The volume of displaced liquid increases with fluence (0.5 to 2 J/cm²) and satisfactory quantitative agreement is obtained with the present model. The same experiment done on the same PET polymer but prepared in the amorphous state does not show microflow, and such an amorphous sample behaves like the reference PC (‘dry’ ablation). The reasons for this surprising result are discussed. Received: 31 October 2002 / Accepted: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Present address: ST Microelectronics, Crolles, France RID="**" ID="**"Corresponding author. Fax: +33-556/84-6645, E-mail: s.lazare@lpcm.u-bordeaux1.fr     

X-ray emission from femtosecond laser micromachining

We characterize the spectral properties of X-rays generated from selected metal and semiconductor targets when 120-fs laser pulses are focused to intensities of∼10¹⁴–3×10¹⁵ W/cm² during laser micromachining in air. High fluxes of multi-keV-energy X-rays could be obtained with 280-μJ pulses at a 1 kHz repetition rate. The yield and spectral composition of the X-rays are found to depend sensitively on the processing conditions, and thus the X-ray emission is expected to be a novel indicator of optimal laser machining. Received: 17 July 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001     

Femtosecond laser patterning of Ta0.1W0.9Ox/ITO thin film stack

Selective laser patterning of thin films in a multilayered structure is an emerging technology for process development and fabrication of optoelectronics and microelectronics devices. In this work, femtosecond laser patterning of electrochromic Ta_0.1W_0.9O_x film coated on ITO glass has been studied to understand the selective removal mechanism and to determine the optimal parameters for patterning process. A 775 nm Ti:sapphire laser with a pulse duration of 150 fs operating at 1 kHz was used to irradiate the thin film stacks with variations in process parameters such as laser fluence, feedrate and numerical aperture of objective lens. The surface morphologies of the laser irradiated regions have been examined using a scanning electron microscopy and an optical surface profiler. Morphological analysis indicates that the mechanism responsible for the removal of Ta_0.1W_0.9O_x thin films from the ITO glass is a combination of blistering and explosive fracture induced by abrupt thermal expansion. Although the pattern quality is divided into partial removal, complete removal, and ITO film damage, the ITO film surface is slightly melted even at the complete removal condition. Optimal process window, which results in complete removal of Ta_0.1W_0.9O_x thin film without ablation damage in the ITO layer, have been established. From this study, it is found that focusing lens with longer focal length is preferable for damage-free pattern generation and shorter machining time.     

Adhesion measurement of thin films by a modified laser spallation technique: theoretical analysis and experimental investigation

Laser driven shocks can lead to a dynamic failure, called film spallation. Here, we use a modified laser spallation set-up to measure the dynamic adhesion of thin films and we propose a novel diagnostic technology. Based on correlation theory, new spallation criteria for characterizing the progressive damage at the interface between the film and the substrate are established, such as interface delamination, film spallation and film expulsion. With the help of the theory, the degree of damage and the dimension of damage (i.e. fracture), such as the minimum width of delamination radius, the thickness of the film etc., are estimated. Experiments are carried out on epoxy/stainless steel and epoxy/Al, and the experimental results show that their dynamic bonding strengths are about 25 MPa and 20 MPa, respectively. The detailed results, analyses and discussions are presented in this paper. Received: 6 February 2001 / Accepted: 3 December 2001 / Published online: 11 February 2002     

Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

The intensity dependence of the total and specific yields of positive ions desorbed from SrF₂ under 193 nm and 308 nm excimer-laser irradiation has been investigated by the time-of-flight method. The following positive ion species have been detected: F⁺, Sr⁺, Sr⁺⁺, SrF⁺⁺ and SrF ₂ ⁺ . The Sr⁺ and SrF⁺ emission yields are found to increase as E ⁿ, where E represents the laser energy per pulse. The exponent n is related to defect-initiated neutral particle emission and gas-phase ionization. The influence of surface damage on this power dependence is investigated. The F⁺ emission yield showed a quite different behaviour compared to that of the Sr⁺ and SrF⁺ emission. At both wavelengths the total positive ion emission yields saturate at a certain laser energy. In the saturation regime the SrF⁺ emission vanishes and alternative emission of F⁺ and Sr⁺ was observed at both wavelengths, but the total emission yield in the saturation regime (F⁺ + Sr⁺) remained constant. A Scanning Electron Microscope (SEM) was used to investigate the damage spots after laser irradiation for thermal effects.     

Laser-induced desorption of Na-dimers

We find that Na-dimers are desorbed in a thermal process if rough Na surfaces are irradiated with pulsed laser light of λ=532 nm. In contrast, for light of λ=355 nm, Na₂ can be detached in a non-thermal reaction at low laser fluences. This is concluded from the kinetic energy distributions of the dimers determined by time-of-flight measurements using a second laser at λ=248 nm for photoionization. The transition from non-thermal to thermal desorption at large fluences of the laser light can also be identified. Received: 23 July 1996 / Accepted: 26 August 1996     

Ultrashort pulse laser ablation of silicon: an MD simulation study

Received: 30 July 1997/Accepted: 5 August 1997     

Time-resolved measurements during backside dry etching of fused silica

The laser-induced backside dry etching (LIBDE) investigated in this study makes use of a thin metal film deposited at the backside of a transparent sample to achieve etching of the sample surface. For the time-resolved measurements at LIBDE fused silica samples coated with 125 nm tin were used and the reflected and the transmitted laser intensities were recorded with a temporal resolution of about 1 ns during the etching with a ∼30 ns KrF excimer laser pulse. The laser beam absorption as well as characteristic changes of the reflection of the target surface was calculated in dependence on the laser fluence in the range of 250-2500 mJ/cm² and the pulse number from the temporal variations of the reflection and the transmission. The decrease of the time of a characteristic drop in the reflectivity, which can be explained by the ablation of the metal film, correlates with the developed thermal model. However, the very high absorption after the film ablation probably results in very high temperatures near the surface and presumably in the formation of an absorbing plasma. This plasma may contribute to the etching and the surface modification of the substrate. After the first pulse a remaining absorption of the sample was measured that can be discussed by the redeposition of portions of the ablated metal film or can come from the surface modification in the fused silica sample. These near-surface modifications permit laser etching with the second laser pulse, too.     

Femtosecond laser micromilling of Si wafers

Femtosecond laser micromilling of silicon is investigated using a regeneratively amplified 775 nm Ti:Sapphire laser with a pulse duration of 150 fs operating at 1 kHz repetition rate. The morphological observation and topological analysis of craters fabricated by single-shot laser irradiation indicated that the material removal is thermal in nature and there are two distinct ablation regimes of low fluence and higher fluence with logarithmical relations between the ablation depth and the laser fluence. Crater patterns were categorized into four characteristic groups and their formation mechanisms were investigated. Femtosecond laser micromilling of pockets in silicon was performed. The effect of process parameters such as pulse energy, translation speed, and the number of passes on the material removal rate and the formation of cone-shaped microstructures were investigated. The results indicate that the microstructuring mechanism has a strong dependence on the polarization, the number of passes and laser fluence. The optimal laser fluence range for Si micromilling was found to be 2-8 J/cm² and the milling efficiency attains its maximum between 10 and 20 J/cm².