Femtosecond Pulsed Laser Micromachining Research Articles

Surface damage and threshold determination of Ge–As–Se glasses in femtosecond pulsed laser micromachining

The manuscript titled “Surface damage and threshold determination of Ge–As–Se glasses in femtosecond pulsed laser micromachining” contains one mistake. This erratum acknowledges the error. Figure 9 was incorrect in the published article.1 Below is the correct version:

Bench scale glass-to-glass bonding for microfluidic prototyping

Microfluidics, an increasingly ubiquitous technology platform, has been extensively utilized in assorted research areas. Commonly, microfluidic devices are fabricated using cheap and convenient elastomers such as poly(dimethylsiloxane) (PDMS). However, despite the popularity of these materials, their disadvantages such like deformation under moderate pressure, chemical incompatibility, and surface heterogeneity have been widely recognized as impediments to expanding the utility of microfluidics. Glass-based microfluidic devices, on the other hand, exhibit desirable properties including rigidity, chemically inertness, and surface chemistry homogeneity. That the universal adoption of glass-based microfluidics has not yet been achieved is largely attributable to the difficulties in device fabrication and bonding, which usually require large capital investment. Therefore, in this work, we have developed a bench-scale glass-to-glass bonding protocol that allows the automated bonding of glass microfluidic devices within 6 h via a commercially available furnace. The quality of the bonds was inspected comprehensively in terms of bonding strength, channel deformation and reliability. Additionally, femtosecond pulsed laser micromachining was employed to rapidly engrave channels on a glass substrate with arbitrary-triangular in this case-cross-section. Bonded glass microfluidic devices with machined channels have been used to verify calculated capillary entry pressures. This combination of fast laser micromachining that produces arbitrary cross-sectioned microstructures and convenient bench-scale glass bonding protocol will facilitate a broad range of micro-scale applications.

Surface damage and threshold determination of Ge–As–Se glasses in femtosecond pulsed laser micromachining

AbstractGe–As–Se chalcogenide glasses were prepared and the surface damage characteristics under femtosecond laser irradiation were experimentally investigated. Femtosecond laser beams with different power intensities, focusing depths, pulse repetition rates, and pulse numbers were employed to determine the damage thresholds of the samples. The surface morphologies and feature sizes of the damage craters were studied with regard to laser energy density to evaluate the damage circumstances of the glasses. The generation of the surface damage was also recorded by using different numbers of pulses. Results showed that the damage area increased monotonically with the laser power at low energy density and tended to saturation when reaching a critical value. According to the linear circumstances of laser energy density on the damage crater area, the average damage thresholds of the Ge–As–Se glasses with different repetition rates and pulse numbers were obtained. Results showed that the damage threshold decreased with the increase in pulse repetition rate and number of pulse.

Femtosecond-pulsed laser micromachining and optical damage by an erbium-doped fiber-laser system

Micromachining by ablation using a femtosecond erbium-doped fiber-laser system was studied. The system consisted of a passively mode-locked fiber-laser oscillator using a nonlinear-amplifying loop mirror, a pulse extractor, and a final amplifier. The output had a pulse width of 240fs and a peak power of 25kW at a repetition rate of 10kHz. A 6-μm-wide ablation trace was obtained by focusing on an amorphous carbon film. For direct amplification without a pulse extractor, the repetition rate was 12MHz, the peak power decreased to 5.1kW, and the average power was 40mW. For direct amplification, the ablation trace of the amorphous carbon film was as large as 16μm wide. However, direct amplification was suitable for micromachining of plastics: cutting of polyvinyl chloride film with a width of 4μm was performed. The optical damage was observed at a facet of a fiber-optic connector after an operation for 60min by pulse extraction and amplification, whereas no damage was observed for direct amplification.

Fabrication of diffraction grating in polydimethylsiloxane using femtosecond-pulsed laser micromachining

This study investigates a new process for fabricating a diffraction grating in a flexible polydimethylsiloxane (PDMS) sheet by using a high-intensity femtosecond-pulsed laser (FS-laser). Unlike soft-lithography with the bonding technique, FS-laser direct writing provides a unique micromachining method for fabricating an embedded diffraction grating (inside a PDMS substrate) with laser pulses of visible light. This approach is a nonlinear mask-less process for rapid prototyping. The wavelength of the FS-laser beam is frequency-doubled to 517 nm. The repetition rate and pulse width of the FS-laser system are 100 kHz and 350 fs, respectively. An embedded PDMS diffraction grating is successfully demonstrated based on a calculated optical phase shift structure.

Femtosecond-pulsed laser micromachining of a 4H–SiC wafer for MEMS pressure sensor diaphragms and via holes

The challenging issues in conventional microfabrication of SiC pressure sensor diaphragms from bulk wafers are low etch rates, thicker (>40 µm) diaphragms, low spatial resolutions, rough surfaces and substantial contamination. In via hole drilling of SiC, the critical concern is the low drilling speed (nm per minute). In this work, femtosecond (fs)-pulsed laser ablation was conducted to overcome some of these deficiencies. Circular diaphragms (0.5 to 1 mm) by trepanning mode and via holes (30–50 µm) by percussion drilling mode were micromachined in 250 µm thick 4H–SiC single crystals using an 800 nm wavelength, 120 fs, 1 mJ Ti:sapphire laser. Pulse energy, number of pulses and scan rate were varied to obtain a high etch rate and high quality features. Results showed that the etch rates are 2–10 µm per pulse, diaphragm thicknesses are 20–200 µm, surface roughness is 1–2 µm Ra and via hole drilling speeds are up to 25 µm per second. The etch depth control was well within ± 1%. High aspect ratio features with excellent spatial resolutions were obtained due to the absence of thermal damages such as a recast layer and contamination. Thus, femtosecond-pulsed laser ablation by virtue of its unique characteristics such as multiphoton ionization and the absence of lattice heating offers high speed, precision and accuracy in micromachining 4H–SiC wafers.

Small fatigue crack growth in metallic materials: A model and its application to engineering alloys

Characterization of the growth behavior of small fatigue cracks is important for materials used in structurally demanding applications such as aircraft turbine discs and some automotive engine components. Here, we present a general, dislocation-based fracture mechanics approach to predict the growth rate of small fatigue cracks in metallic materials. The applicability of the model to the small fatigue crack growth behavior of four engineering alloys was examined. Small fatigue cracks were initiated and propagated, in a controlled manner, from micronotches fabricated by femtosecond pulsed laser micromachining. The results suggest that a methodology consisting of crack-tip damage accumulation and fracture provides a common framework to estimate the fatigue crack propagation lifetime of structural materials.

Ultrafast laser micromachining of 3C-SiC thin films for MEMS device fabrication

Femtosecond pulsed laser (800 nm, 120 fs) micromachining of thin films of 3C-SiC (β-SiC) semiconductor deposited on silicon substrate was investigated as a function of pulse energy (0.5 μJ to 750 μJ). The purpose is to establish suitable laser parametric regime for the fabrication of high accuracy, high spatial resolution and thin diaphragms for high-temperature MEMS pressure sensor applications. Etch rate, ablation threshold and quality of micromachined features were evaluated. The governing ablation mechanisms, such as thermal vaporization, phase explosion, Coulomb explosion and photomechanical fragmentation, were correlated with the effects of pulse energy. The results show that the etch rate is higher and the ablation threshold is lower than those obtained with nanosecond pulsed excimer laser ablation, suggesting femtosecond laser’s potential for rapid manufacturing. In addition, the etch rates were substantially higher than those achievable in various reactive ion and electrochemical etching methods. Excellent quality of machined features with little collateral thermal damage was obtained in the pulse energy range (1–10 μJ). The leading material removal mechanisms under these conditions were photomechanical fragmentation, ultrafast melting and vaporization. At very low pulse energies (<1 μJ), nanoscale material removal has occurred with the formation of nanoparticles that is attributed to Coulomb explosion mechanism. The effect of assist gas on the process performance at low and high energy fluences is also presented.

Femtosecond pulsed laser micromachining of glass substrates with application to microfluidic devices.

We describe a technique for surface and subsurface micromachining of glass substrates by using tightly focused femtosecond laser pulses at a wavelength of 1660 nm. A salient feature of pulsed laser micromachining is its ability to drill subsurface tunnels into glass substrates. To demonstrate a potential application of this micromachining technique, we fabricate simple microfluidic structures on a glass plate. The use of a cover plate that seals the device by making point-to-point contact with the flat surface of the substrate is necessary to prevent the evaporation of liquids in open channels and chambers. Methods for protecting and sealing the micromachined structures for microfluidic applications are discussed.

Femtosecond pulsed laser micromachining of single crystalline 3C–SiC structures based on a laser-induced defect-activation process

A femtosecond pulsed Ti:sapphire laser with a pulse width of 120 fs, a wavelength of 800 nm and a repetition rate of 1 kHz was employed for direct write patterning of single crystalline 3C–SiC thin films deposited on Si substrates. The ablation mechanism of SiC was investigated as a function of pulse energy. At high pulse energies (>1 µJ), ablation occurred via thermally dominated processes such as melting, boiling and vaporizing of single crystalline SiC. At low pulse energies, the ablation mechanism involved a defect-activation process that included the accumulation of defects, formation of nano-particles and vaporization of crystal boundaries, which contributed to well-defined and debris-free patterns in 3C–SiC thin films. The interactions between femtosecond laser pulses and the intrinsic lattice defects in epitaxially grown 3C–SiC films led to the generation of nano-particles. Micromechanical structures such as micromotor rotors and lateral resonators were patterned into 3C–SiC films using the defect-activation ablation mechanism.

High-energy femtosecond pulsed laser micromachining of thin film deposited silicon in self-focused air medium

The ultrashort pulsed laser ablation of silicon wafers deposited with 1 μm thin film of SiC was performed in air medium using a 1 mJ, 120 fs, 800 nm Ti:sapphire laser, the objective being to determine the self-focusing capability of air to produce nanostructures. The effects of pulse energy, the number of pulses, pulse repetition rate and x–y translation speed on the size, shape, and thermal damage of holes and grooves were evaluated. In addition, a 200 ns pulsed, 1064 nm Nd:yttritium–aluminum–garnet laser was used to process grooves and the results were compared against those processed by the femtosecond pulses. Results show that holes as small as 1 μm were produced with femtosecond pulses using the self-action property of air. Despite the capability of air medium to produce tiny holes of a few micron diameters, the shape of the hole is highly asymmetrical due to the beam profile distortion. The critical laser power for nonlinear effects of air to become effective was calculated as 40 MW. However, both the beneficial and deleterious effects of air were not found when the peak power of laser beam was under 2 GW. A comparison of femtosecond pulsed laser ablation with nanosecond pulsed laser ablation demonstrated the advantages of small size, minimal thermal damage and cleanliness for ultrashort pulses.

Femtosecond pulsed laser ablation with spatial filtering

With the rise in demand for miniaturized features with better acute edge acuity and negligible thermal damage zone, one of the key vital areas lies in the refinement of the quality of the laser beam itself. Spatial filter is routinely used in optical micromachining systems to smoothen the Gaussian profile of the machining spot in order to obtain a feature of the desired quality. However, its profile smoothening effect has never been investigated for femtosecond pulsed laser micromachining process since the extremely high peak power of femtosecond pulses will cause damage on the filtering aperture of spatial filter. During the development of an acousto-optical micromachining system using femtosecond pulses, we found that if the damage of the filtering aperture can be circumvented, spatial filter can improve the machining quality of femtosecond pulse ablation, especially when ablation is conducted at low-fluency range (just above the ablation threshold fluency). In this paper, we investigate and demonstrate both the improvement and potential that beam refinement can bring about. In our experiment, a series of test patterns were ablated with a 400 nm second-harmonic Ti:Sapphire femtosecond laser of 150 fs duration at varying pulse energy ranging from 31 to 39 nJ. The specimen used in the experiment is a platinum- (Pt) sputtered coating of 100 nm thickness on a quartz substrate. The results show a significant improvement in the constancy of the shape as well as the size of ablated feature, revealing an improved beam profile and beam energy distribution due to spatial filtering.