Ultraviolet Nanosecond Pulse Laser Research Articles

Gradient-Pattern Micro-Grooved Wicks Fabricated by the Ultraviolet Nanosecond Laser Method and Their Enhanced Capillary Performance.

Capillary-gradient wicks can achieve fast or directional liquid transport, but they face fabrication challenges by traditional methods in terms of precise patterns. Laser processing is a potential solution due to its high pattern accuracy, but there are a few studies on laser-processed capillary-gradient wicks. In this paper, capillary step-gradient micro-grooved wicks (CSMWs) were fabricated by an ultraviolet nanosecond pulsed laser, and their capillary performance was studied experimentally. The CSMWs could be divided into three regions with a decreasing capillary radius. The equilibrium rising height of the CSMWs was enhanced by 124% compared to the non-gradient parallel wick. Different from the classical Lucas-Washburn model describing a uniform non-gradient wick, secondary capillary acceleration was observed in the negative gradient direction of the CSMWs. With the increase in laser power and the decrease in scanning speed, the capillary performance was promoted, and the optimal laser processing parameters were 4 W-10 mm/s. The laser-enhanced capillary performance was attributed to the improved hydrophilicity and reduced capillary radius, which resulted from the increased surface roughness, protrusion morphology, and deep-narrow V-shaped grooves induced by the high energy density of the laser. Our study demonstrates that ultraviolet pulsed laser processing is a highly efficient and low-cost method for fabricating high-performance capillary gradient wicks.

Investigation of through micropatterns fabrication on C194 copper foil by ultraviolet nanosecond pulsed laser microdrilling

While the lead frames of C194 copper alloy widely used in IC packages contain high density of through micropatterns, the method for realizing high precision manufacturing of the micropatterns on copper foil with thickness down to a few hundred micrometers is highly demanded. In the present work, we demonstrate the feasibility of manufacturing high density through micropatterns on C194 copper foil with high precision and high uniformity by using ultraviolet nanosecond pulsed laser microdrilling. Specifically, the absorption coefficient of C194 copper foil for 355 nm wavelength is experimentally derived, based on which the sublimation enthalpy of the material is calibrated by iteratively comparing predicted crater morphology by finite element simulations with experimental data. Then the multi-pulse laser ablation mechanisms of C194 copper foil are investigated jointly by finite element simulations and experiments, which derive the quantitative correlation of ablation crater depth with laser ablation parameters. Finally, high density arrays of through micropatterns with high dimensional accuracy and high uniformity are fabricated on C194 copper foil with a thickness of 100 μm by the proposed ultraviolet nanosecond pulsed laser microdrilling with theoretically predicted repeat scanning number. Furthermore, the effect of pickling on surface quality and resistance of the manufactured copper lead frame is addressed. This work provides a feasible method for manufacturing copper alloy lead frames with high precision through micropatterns in an environmentally friendly manner.

Capillary-Driven Boiling Heat Transfer on Superwetting Microgrooves.

Boiling can transfer a vast amount of heat and thereby is widely used for cooling advanced systems with high power density. However, the capillary force of most existing wicks is insufficient to surpass the liquid replenishing resistance for high-efficient boiling. Herein, we report a new microgroove wick on high-conductive copper substrates that was constructed via ultraviolet nanosecond pulsed laser milling. The phase explosion, combined with melting and resolidification effects of laser milling induces dense microcavities with sizes around several micrometers on the microgroove surface. The hierarchical microstructures significantly improve the wettability of the microgroove wicks to obtain strong capillary and meanwhile provide abundant effective nucleation sites. The boiling heat transfer in a visualized flat heat pipe shows that the new wicks enable sustainable liquid replenishing even under antigravity conditions, thus resulting in maximum 33-fold improvement of equivalent thermal conductivity when compared with the copper base. This research provides both scientific and technical bases for the design and manufacture of high-performance phase change cooling devices.

Growth of highly oriented graphite by ultraviolet nanosecond pulsed laser ablation of monocrystalline diamond

Inducing highly oriented graphite layer as buried conductive media in diamond by laser ablation is crucial for promoting the electronic functionalities of structured monocrystalline diamond. In the present work, we fabricate surface microgrooves with high precision and limited heat affected zone on monocrystalline diamond by low cost ultraviolet nanosecond pulsed laser micromachining. In particular, fruitful highly oriented graphite layer is formed between top transformed amorphous layer and bottom pristine diamond, with visible regular boundaries among them. Raman spectroscopy and high-resolution transmission electron microscopy characterization are jointly utilized to identify the phase transformation-induced microstructure evolution in subsurface of ablated diamond. Furthermore, molecular dynamics simulations are performed to elucidate the underlying ablation mechanisms, as well as predict the dynamic ablation process of nanosecond pulsed laser micromachining of diamond. Finally, the effect of laser fluence on the machined surface morphology and the phase transformation characteristics of the ablated diamond is addressed.

Surface micro-texture on sapphire fabricated by laser ablation trajectory regulation

Fabrication of the surface micro-texture on the C-plane sapphire is undertaken by a 355 nm Ultraviolet (UV) pulsed laser. The surface micro-textures of sapphire with different laser scanning line spacing ranging from 10 μm to 100 μm are obtained, where the selection range of scanning line spacing is controlled in the range of the groove width and plasma width to obtain a surface of high Peak-Valley (PV) value. A reasonable processing order is proposed to manufacture different types of surface micro-textures on sapphire by laser ablation trajectory regulation. In the multiple-passes laser ablation of sapphire by the UV nanosecond pulsed laser, the scanning lines in each direction is treated as once scanning. On this basis, the multiple processing can be carried out to avoid the influence of the subsequent scanning on the machined groove. In addition, the effect of different scanning line spacing on the PV value is quantified and different types of surface micro-textures on sapphire, including the squares, the rhombuses and the hexagons, are successfully fabricated, which can be applied in the friction reduction or anti-reflection field.

Investigation of Elemental Composition Change by Laser Ablation of a Rare‐Earth Containing Material

Rare earth doped materials, in the form of nanoparticles, have attracted extensive attention in photonic and biomedical applications. Separation of rare earth elements in a clean way has been also topic of great interest. Laser ablation has been actively examined to convert the rare earth containing bulk materials into functional nanoparticles as a potentially clean, efficient production method. However, elemental composition change during laser ablation based rare earth containing nanoparticle conversion process has not been studied extensively despite its significance. In this study, an ultraviolet nanosecond pulsed laser is used to ablate an erbium containing glass material with a major aim of tracking elemental composition in collected nanoparticles. The effect of laser fluence and laser beam spot is experimentally investigated by morphological, optical, spectroscopic, and elemental analyses. The results indicates the careful choice of laser parameters for the laser ablation can either preserve the elemental composition of bulk materials for applications dealing with rare earth containing materials or separate rare earth elements from bulk materials.

Experimental Study on Formation of the Microstructure on Copper Film Using Ultraviolet Nanosecond Laser

In this paper, the ablated microstructures on copper film affected by ultraviolet nanosecond pulse laser are presented. The experimental system was consisted of two lasers, optics and controlling electronics. A 3000mW, 355nm Q-switched ultraviolet lasers was used to the micro-polishing experiments in the work. The repetition rate of the ultraviolet pulse laser is from single-shot to 100kHz, and the pulse width is less than 40ns. The sample used in experiment is copper film (200 nm) sputtered on glass. A series of experiments at different laser parameters and speed of work platform are done. The ablating experiments are also carried out on focusing and defocusing application in the laser direct writer. The results were analyzed.

Laser micro-polishing for metallic surface using UV nano-second pulse laser and CW laser

During laser micro-polishing, the controllable laser energy density is a key technology to improve the surface roughness. In this paper, we have investigated a method which controls the energy density of ultraviolet (UV) pulse laser in real time with the control of continuous wave (CW) laser spot size in laser micro-polishing for metallic surface. The experimental and analytical considerations of several influence factors such as laser spot size, fusion zone, and focal offset were investigated. In addition, using a laser micro-polishing system manufactured with this method, the laser micro-polishing experiments on the two different surface shapes of stainless steel 316L were conducted. For the inclined or curved surface, the surface roughness improvements of up to 56.4 and 57.3 % were respectively obtained, and the analysis of the results were discussed.

Innovative through-silicon-via formation approach for wafer-level packaging applications

Through-silicon via (TSV) is an emerging technology for three-dimensional integrated circuit, system-in-packaging and wafer-level packaging applications. Among several available TSV formation methods, Bosch deep reactive ion etching (DRIE) is widely used because it enables the fabrication of TSVs with almost any diameter, from the submicrometer level to hundreds of micrometers. However, the high cost of Bosch DRIE makes it uneconomical for industrial production. We present a novel wafer-level TSV formation approach that is effective and cost-efficient. The proposed method integrates a diode-pumped solid-state ultraviolet nanosecond pulsed laser and rapid wet chemical etching. The former is effective in drilling through 400 µm thick silicon wafers and the latter is used for removing the unwanted heat-affected zone, recast layer and debris left after drilling. Experimental results demonstrate that the combined approach effectively eliminates the unwanted material formed by nanosecond laser pulses. Furthermore, this approach has a significant cost advantage over Bosch DRIE. In summary, the proposed approach affords superior TSV quality, higher TSV throughput and lower cost of process ownership than Bosch DRIE. These advantages could provide the necessary impetus for rapid commercialization of the several high-density fabrication methodologies that depend on TSVs.

Realization of nanosecond pulse laser micromachining system

In this article a new nanosecond pulse laser micromachining system has been designed, which consists of laser, mechanical and optical structure, three-dimensional (3D) work platform, and control system. 355nm ultraviolet nanosecond pulse laser, with 40ns pulse duration, is chosen as the light source. The repetition frequency can be set from single shot to 100kHz. Mechanical and optical structures are designed for laser beam focusing. Computer software resolves the graphics. Moreover, control system based on digital signal processing and field programmable gate array is designed to drive the 3D work platform. With the designed system, some processing parameters have been analyzed and optimized in experiments. Some measures are also taken to improve the microfabrication quality and minimize the splashes of the heat-affected zone. Finally, some microstructures are fabricated by the designed nanosecond pulse laser micromachining system. The width of processing line is about 40μm with less splashes.

Study of Nanosecond Pulse Laser Micro-Fabrication

We study some microstructures fabricated by designed 355nm ultraviolet nanosecond pulse laser micromachining system, which consists of LASER, mechanical and optical structure, 3D work platform, and control system. Nanosecond pulse LASER, with 40ns pulse duration, is chosen as the light source. Mechanical and optical structure is designed for laser beam focusing. Computer software resolves the graphics. And control system based on DSP and FPGA is designed to drive the 3D work platform. Raster sensor is used to measure the real distance that the platform moves. Using this system, some experiments are analyzed and some parameters are optimized. Some microstructures are also fabricated on single crystal silicon wafer and organic glass. The lines of fabricated graphics are 20-40um wide and smooth, with less effect of heat.

Vacuum-Ultraviolet Light Sources and Their Applications for Processings. Laser-induced Etching of Chlorinated Silicon Surfaces.

Measurements of excitation spectra of photoetching of chlorinated Si(111)7×7 surfaces stimulated by ultraviolet nanosecond pulse laser light revealed that there are two distinct spectral peaks, one (α-peak) being located near 290nm and another (β-peak) at 245 nm. The α-peak exhibits a strong supralinear dependence on light intensity above laser fluence of∼100mJ/cm2 pulse, while the β-peak grows linearly with light intensity. The red shift of the α-peak with increasing intensity coincides with the shift of laser abration, which strongly suggests that the origin of the α-peak photoetching is thermal etching enhanced by surface heating due to interband transition in the substrate. The dependence on polarization of the incident light suggests that the β-peak photoetching originates in excitation of hot carriers in the substrate which are captured resonantly by the anti-bonding states of the adsorbate chlorides resulting in destabilization of the molecules.

Effects of vibrational excitation in photostimulated desorption

Excitation of vibrational and rotational degrees of freedom, which occurs in the desorption process of NO and CO molecules from Pt(111) and Pt(001) surfaces with ultraviolet nanosecond-pulsed laser irradiation, is investigated by resonance-enhanced multiphoton ionization detection. Desorbed NO molecules are highly excited in vibration and rotation, whereas CO desorbed from Pt is rotationally cold and vibrationally excited. On the basis of the substrate-mediated desorption mechanism, the population in the first vibrational excited state and the rotational energy distribution can be consistently interpreted. In particular, we propose a simple scheme interpreting the rotational energy distribution.

Ultraviolet-laser-induced desorption of CO and NO from Pt surfaces

Desorption of CO and NO molecules chemisorbed on Pt(001) and Pt(111) surfaces at 80 K induced by ultraviolet nanosecond-pulsed laser irradiation has been studied by a resonance-enhanced multiphoton ionization technique. Desorption is not thermally driven, but induced by electronic excitation. CO desorption from Pt(111) and NO desorption from both Pt surfaces are found to be single-photon processes, while CO desorption from Pt(001) is a three-photon process. The translational, rotational, and vibrational temperatures of desorbed NO from Pt(001) are independent of pump-laser wavelength in the energy range 3.5–6.4 eV. For NO from Pt(111), on the other hand, the internal-energy distribution is strongly dependent on pump-laser wavelength. On the basis of these results, the desorption mechanism induced by valence-electron excitation for molecules chemisorbed on metals has been discussed in connection with the unoccupied electronic structures of the adsorbate and the metal.