Analysis of materials modifications caused by UV laser micro drilling of via holes in AlGaN/GaN transistors on SiC

Pulsed UV laser drilling can be applied to fabricate vertical electrical interconnects (vias) for AlGaN/GaN high electron mobility transistor devices on single-crystalline silicon carbide (SiC) substrate. Through-wafer micro holes with a diameter of 50-100 μm were formed in 400 μm thick bulk 4H-SiC by a frequency-tripled solid-state laser (355 nm) with a pulse width of ≤30 ns and a focal spot size of ∼15 μm. The impact of laser machining on the material system in the vicinity of micro holes was investigated by means of micro-Raman spectroscopy and transmission electron microscopy. After removing the loosely deposited debris by etching in buffered hydrofluoric acid, a layer of <4 μm resolidified material remains at the side walls of the holes. The thickness of the resolidified layer depends on the vertical distance to the hole entry as observed by scanning electron microscopy. Micro-Raman spectra indicate a change of internal strain due to laser drilling and evidence the formation of nanocrystalline silicon (Si). Microstructure analysis of the vias’ side walls using cross sectional TEM reveals altered degree of crystallinity in SiC. Layers of heavily disturbed SiC, and nanocrystalline Si are formed by laser irradiation. The layers are separated by 50-100 nm thick interface regions. No evidence of extended defects, micro cracking or crystal damage was found beneath the resolidified layer. The precision of UV laser micro ablation of SiC using nanosecond pulses is not limited by laser-induced extended crystal defects.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

Controlled process for polymer micromachining using designed pulse trains of a UV solid state laser

A flexible workstation equipped with a solid state laser operating at 266 nm wavelength was used to machine holes in polyethylene terephthalate, polyimide and polycarbonate. An optical pulse picker was employed to reduce the high repetition rates of the laser, while a breakthrough sensor was used to avoid over-drilling of through holes. For each material, different repetition rates and designed pulse trains were tested to improve feature quality and process efficiency. Although the three polymers had very different reactions at this wavelength they all showed an improvement in feature quality with decreasing repetition rate due to a reduction in thermal effects. Up to 10 kHz the average depth per pulse remained unchanged and afterwards a slight increase was observed but this was accompanied by large uncertainties. Bursts of pulses at 40 kHz inserted inside the low repetition rate pulse train reduced the drilling time and the amount of debris redeposited without affecting the feature quality. It was found that a number of cleaning pulses after perforation eliminates the heat affected zone around exits. Holes with entrance diameters below 20 μm and exit diameters as small as 2 μm were obtained with high repeatability.     

A calibrated atomic force microscope using an orthogonal scanner and a calibrated laser interferometer

A compact and two-dimensional atomic force microscope (AFM) using an orthogonal sample scanner, a calibrated homodyne laser interferometer and a commercial AFM head was developed for use in the nano-metrology field. The x and y position of the sample with respect to the tip are acquired by using the laser interferometer in the open-loop state, when each z data point of the AFM head is taken. The sample scanner, which has a motion amplifying mechanism was designed to move a sample up to 100 μm × 100 μm in orthogonal way, which means less crosstalk between axes. Moreover, the rotational errors between axes are measured to ensure the accuracy of the calibrated AFM within the full scanning range. The conventional homodyne laser interferometer was used to measure the x and y displacements of the sample and compensated via an X-ray interferometer to reduce the nonlinearity of the optical interferometer. The repeatability of the calibrated AFM was measured to sub-nanometers within a few hundred nanometers scanning range.     

Surface and transport studies on La0.7Ba0.3MnO3:SnO2 bilayer

We report on study of morphology, optical contrast and transport characteristics of La_0.7Ba_0.3MnO₃ (LBMO) manganite thin films bilayered with SnO₂ on Si (0 0 1) substrate, synthesized using pulsed laser deposition system. X-ray diffraction study reveals that both LBMO and SnO₂ show polycrystalline growth over the substrate. Atomic force microscopy shows interesting pyramidal structures of LBMO of size ∼2 μm × 1 μm × 0.1 μm. On the other hand, SnO₂ grows in the form of close packed cylindrical clusters of ∼200 nm radius. Near-field optical microscopy (NSOM) study using 532 nm laser reveal that optical NSOM output intensity in LBMO is four times less than SnO₂ signal. Transport characterizations show that this bilayer configuration exhibit non-linear current-voltage characteristics from 300 upto 50 K. The nature becomes linear below this temperature. The results project the system as a promising candidate in non-conventional device category in the area of spintronics.     

Fiber laser annealing of nanocrystalline PZT thick film prepared by aerosol deposition

Nanocrystalline PZT thick films (1 mm square and over 10 μm thick) directly deposited onto stainless-steel substrates (PZT/SUS) by aerosol deposition (AD) technique and then annealed using focused laser beam with a fiber laser to suppress thermal damage to the back sides of the PZT/SUS and substrate near the film edge and to retain the dielectric and/or ferroelectric properties of the PZT/SUS. Compared with CO₂ laser annealing, fiber laser annealing suppressed thermal damage to the substrate. Compared with PZT/SUS annealed at 600 °C using an electric furnace, PZT/SUS annealed at 600 °C using a fiber laser showed superior properties, namely, dielectric constant ? > 1200 at a frequency of 100 Hz, remanent polarization P_r > 30 μC/cm², and coercive field strength E_c < 50 kV/cm at a frequency of 10 Hz. Furthermore, the grain growth for the PZT/SUS formed by AD technique and annealed by fiber laser irradiation was occurred within the laser spot size.     

Electro-optic Q-switched intracavity optical parametric oscillator at 1.53 μm based on KTiOAsO4

An eye-safe, high peak power optical parameter oscillator (OPO) intracavity pumped by electro-optic Q-switched Nd:YAG laser is presented. This OPO is based on a 20 mm length KTiOAsO₄ crystal with non-critical phase matching (θ = 90°, ?=0°) cut. An aperture ∅3 mm acted as limiting diaphragm to get good beam quality of pumping laser. The output energy of 25 mJ at the signal wavelength 1.53 μm was obtained with repetition rate of 1 Hz. The highest peak power intensity was up to 88 MW/cm² with pulse width of 4 ns. Without diaphragm, the maximum output energy of 90 mJ was achieved with area of light spot (∅6 mm) four times larger, but the peak power intensity was lower.     

Micro-Raman investigation of stress distribution in laser drilled via structures

Through-wafer vertical electrical interconnects (vias) with diameters varied from 15 to 80 μm were formed on Si substrates using a UV diode-pumped solid state laser (355 nm). Micro-Raman spectroscopy was employed for the investigation of stress and structural changes induced in silicon within the heat-affected zone due to laser machining. A maximum stress of ∼300 MPa, as a result of laser drilling, was observed close to the via edge. It was found that the stress decays within a distance of 1-3 μm from the via’s side-wall and that the laser machining did not lead to the formation of amorphous silicon around the via structures.     

Q-switched Nd lasers pumped directly into the F3/2 emitting level

The influence of the direct pumping into the ⁴F_3/2 emitting level on the output characteristics of continuous-wave (CW) pumped, passively or actively (acoustooptic, AO) Q-switched Nd lasers is discussed. In case of passive Q-switching by Cr⁴⁺:YAG saturable absorber (SA) crystal, the change of pumping wavelength from 0.81 μm into the highly-absorbing ⁴F_5/2 level to 0.88 μm into the ⁴F_3/2 level of Nd does not modify the energy of the Q-switch pulse, but increases the pulse repetition rate and the laser average power for the same absorbed pump power. This is demonstrated with 0.81 and 0.88 μm CW laser diode-pumped Nd:YAG and Nd-vanadate lasers with average output power in the watt-level range at 1.06 μm. The effect is explained by the control of passive Q-switching by the intracavity photon flux that is influenced by the pump wavelength and by the initial transmission of the SA crystal. On the other hand, it is discussed and experimentally proved that due to the possibility to control externally the frequency of switching, in case of the AO Q-switched Nd laser the change of the pump wavelength from 0.81 to 0.88 μm increases the pulse energy for a fixed frequency, leading to a corresponding increase of the average laser power.     

Passively Q-switched laser performance of c-cut Nd:Gd0.63Y0.37VO4 crystal

Passively Q-switched c-cut Nd:Gd_0.63Y_0.37VO₄ laser performance at 1.06 μm was demonstrated with Cr⁴⁺:YAG as saturable absorbers for the first time to our knowledge. This c-cut mixed crystal was found to have large energy storage capacity. The shortest pulse width, largest pulse energy, and highest peak power were obtained to be 6.6 ns, 201.7 μJ, and 30.6 kW, respectively.     

Two-photon recording of microholograms in photopolymer materials with new cationic thioxanthone photoinitiators

We have demonstrated two-photon induced recording of the microholograms at an arbitrary point within thick (∼100 μm) photopolymer material using photoinitiators on a basis of new cationic thioxanthone derivatives. Such material provides high values of refractive index change Δn = 4.8 × 10⁻³, and holographic recording sensitivity S = 1.2 cm/J. A nanosecond laser pulse at a wavelength of 532 nm was used for recording. For the selective on the depth reading of the microholograms the method of collinear heterodyning was applied.     

Anisotropy of two-photon absorption in gallium selenide at 1064 nm

The Bridgman method is used to grow single crystals of ε-GaSe. The two-photon absorption (TPA) coefficient β was measured for especially un-doped crystals at room temperature for the ordinary o-ray (β_⊥) and extraordinary e-ray (β_∥) using single-wavelength excitation by a Nd:YVO₄ laser at 1.064 μm with a pulse duration of 10 ps and a repetition rate of 81 MHz. No large anisotropy was observed for the TPA coefficients between the two geometries. The TPA coefficients were found to be β_⊥ = 1.07 × 10⁻⁹ cm/W and β_∥ = 1.88 × 10⁻⁹ cm/W. It was found that doping slightly increases the TPA coefficient. The value of the TPA for crystals doped with 0.5 at.% of Tl is β_⊥ = 7.56 × 10⁻⁹ cm/W.     

Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers

Ultra-short pulsed laser ablation and micromachining of n-type, 4H-SiC wafer was performed using a 1552 nm wavelength, 2 ps pulse, 5 μJ pulse energy erbium-doped fiber laser with an objective of rapid etching of diaphragms for pressure sensors. Ablation rate, studied as a function of energy fluence, reached a maximum of 20 nm per pulse at 10 mJ/cm², which is much higher than that achievable by the femtosecond laser for the equivalent energy fluence. Ablation threshold was determined as 2 mJ/cm². Scanning electron microscope images supported the Coulomb explosion (CE) mechanism by revealing very fine particulates, smooth surfaces and absence of thermal effects including melt layer formation. It is hypothesized that defect-activated absorption and multiphoton absorption mechanisms gave rise to a charge density in the surface layers required for CE and enabled material expulsion in the form of nanoparticles. Trenches and holes micromachined by the picosecond laser exhibited clean and smooth edges and non-thermal ablation mode for pulse repetition rates less than 250 kHz. However carbonaceous material and recast layer were noted in the machined region when the pulse repetition rate was increased 500 kHz that could be attributed to the interaction between air plasma and micro/nanoparticles. A comparison with femtosecond pulsed lasers shows the promise that picosecond lasers are more efficient and cost effective tools for creating sensor diaphragms and via holes in 4H-SiC.     

Injection-seeded Tm:YAG laser at room temperature

The solid-state, tunable, narrowband, high pulse energy and high reliability lasers are attractive source for LIDAR system. In this paper, we demonstrated a diode pumped injection-seeded 2 μm Tm:YAG laser. By inserting two F-P etalons into the laser cavity, linear-polarized single-frequency seed-laser was achieved at a wavelength of 2013 nm, with a maximum output power of 60 mW. Long-term and short-term frequency stability for the seed-laser were 1.27 × 10^− 7 and 97 Hz/μs, respectively. High power Q-switched laser was operated using a bowtie cavity, the bidirectional output of which was favorable for the injection-seeded. After injecting the seed-laser to the power-laser, single-frequency, nearly transform-limited pulsed 2 μm laser was obtained. As much as 2.0 mJ output energy was achieved at an operating repetition rate of 15 Hz, with a pulse width of 356.2 ns.     

Coating formation by plasma electrolytic oxidation on ZC71/SiC/12p-T6 magnesium metal matrix composite

Plasma electrolytic oxidation (PEO) of a ZC71/SiC/12p-T6 magnesium metal matrix composite (MMC) is investigated in relation to coating growth and corrosion behaviour. PEO treatment was undertaken at 350 mA cm⁻² (rms) and 50 Hz with a square waveform in stirred 0.05 M Na₂SiO₃.5H₂O/0.1 M KOH electrolyte. The findings revealed thick, dense oxide coatings, with an average hardness of 3.4 GPa, formed at an average rate of ∼1 μm min⁻¹ for treatment times up to 100 min and ∼0.2 μm min⁻¹ for later times. The coatings are composed mainly of MgO and Mg₂SiO₄, with an increased silicon content in the outer regions, constituting <10% of the coating thickness. SiC particles are incorporated into the coating, with formation of a silicon-rich layer at the particle/coating interface due to exposure to high temperatures during coating formation. The distribution of the particles in the coating indicated growth of new oxide at the metal/coating interface. The corrosion rate of the MMC in 3.5% NaCl is reduced by approximately two orders of magnitude by the PEO treatment.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.     

Femtosecond laser embedded grating micromachining of flexible PDMS plates

We report on the femtosecond laser micromachining of photo-induced embedded diffraction grating in flexible Poly (Dimethly Siloxane) (PDMS) plates using a high-intensity femtosecond (130 fs) Ti: sapphire laser (λ_p = 800 nm). The refractive index modifications with diameters ranging from 2 μm to 5 μm were photo-induced after the irradiation with peak intensities of more than 1 × 10¹¹ W/cm². The graded refractive index profile was fabricated to be a symmetric around from the center of the point at which femtosecond laser was focused. The maximum refractive index change (Δn) was estimated to be 2 × 10⁻³. By the X-Y-Z scanning of sample, the embedded diffraction grating in PDMS plate was fabricated successfully using a femtosecond laser.     

Subwavelength ripple formation induced by tightly focused femtosecond laser radiation

Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, t_p = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO₃. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented.     

Photocarrier transport in iron-doped potassium lithium tantalate niobate studied by time-of-flight measurement

The photocarrier mobility of Fe 0.03 wt%-doped potassium lithium tantalate niobate (K_0.95Li_0.05Ta_0.61Nb_0.39O₃) was investigated by time-of-flight (TOF) measurement. The longitudinal photocarrier response due to pulsed excitation leads to values of the drift mobility of μ_h = 1.45 × 10⁻² cm²/V s for holes, μ_e = 0.325 × 10⁻² cm²/V s for electrons, and a value for the range of holes (μτ)_h = 4.38 × 10⁻⁵ cm²/V at room temperature and at low field 3 KV/cm. The response time of holes and electrons (or the relaxation time) is determined to be 3.02 × 10⁻³ s and 3.74 × 10⁻³ s, respectively. The mobility of holes strongly depends on the field strength, and is observed to decrease with increasing bias field.     

Pure UV photoluminescence from ZnO thin film by thermal retardation and using an amorphous SiO2 buffer layer

ZnO/SiO₂ thin films were fabricated on Si substrates by E-beam evaporation with thermal retardation. The as-prepared films were annealed for 2 h every 100 °C in the temperature range 400-800 °C under ambient air. The structural and optical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL). The XRD analysis indicated that all ZnO thin films had a highly preferred orientation with the c-axis perpendicular to the substrate. From AFM images (AFM scan size is 1 μm×1 μm), the RMS roughnesses of the films were 3.82, 5.18, 3.65, 3.40 and 13.2 nm, respectively. PL measurements indicated that UV luminescence at only 374 nm was observed for all samples. The optical quality of the ZnO film was increased by thermal retardation and by using an amorphous SiO₂ buffer layer.