Laser-induced Chemical Etching Research Articles

Controllable Si micro-structuring by temporally modulated single-shot femtosecond pulse lithography

Micro-structured silicon surface plays a significant role in the electronics industry. Crystalline and amorphous phases of silicon, which have significantly different physical properties, can be transformed into each other using femtosecond laser, and assisted chemical etching enables the easy fabrication of various micro-structures. However, efficient and controllable fabrication of micro-cylinders, micro-rings, and micro-ring grooves on silicon surfaces still remains a challenge. Here, we propose a temporally modulated single-shot femtosecond pulse lithography technology, combining laser-induced silicon modification and chemical etching. In this technology, the occurrence of recrystallization allows for the flexible manipulation of the shape of the modified area, transitioning from circular to annular shapes by adjusting the laser fluence. This corresponds to the formation of micro-cylinders and micro-rings after etching. In the case of temporally shaped pulses, with an increasing pulse delay, the micro-rings formed after etching gradually transform into micro-ring grooves, as the recrystallization area created by the first sub-pulse is reacted by the second sub-pulse. Due to the characteristics of the single shot, the technology, when combined with the flying punch method, can be used for the high-efficiency fabrication of large-area silicon surface micro-structure arrays.

Femtosecond laser-assisted fabrication of piezoelectrically actuated crystalline quartz-based MEMS resonators

A novel technology for the precise fabrication of quartz resonators for MEMS applications is introduced. This approach is based on the laser-induced chemical etching of quartz. The main processing steps include femtosecond UV laser treatment of a Cr-Au-coated Z-cut alpha quartz wafer, followed by wet etching. The laser-patterned Cr-Au coating serves as an etch mask and is used to form electrodes for piezoelectric actuation. This fabrication approach does not alter the quartz’s crystalline structure or its piezo-electric properties. The formation of defects, which is common in laser micromachined quartz, is prevented by optimized process parameters and by controlling the temporal behavior of the laser-matter interactions. The process does not involve any lithography and allows for high geometric design flexibility. Several configurations of piezoelectrically actuated beam-type resonators were fabricated using relatively mild wet etching conditions, and their functionality was experimentally demonstrated. The devices are distinguished from prior efforts by the reduced surface roughness and improved wall profiles of the fabricated quartz structures.

Development of 3-D Wafer Level Packaging for SAW Filters Using Thin Glass Capping Technology

The introduction of 5G mobile communication created the need to support a large number of radio frequency (RF) bands. This required high performance and reduced size for microacoustic filters. Based on these requirements, a new three-dimensional wafer-level packaging (3-D WLP) solution based on the predecessors’ developed technology was presented to resolve reliability issues for surface acoustic wave (SAW) filter packages with large cavities. In the 3-D WLP, vertical interconnects were realized using through glass vias (TGVs). To enable such a process scheme, thin glass caps with TGVs were formed through laser-induced chemical etching. Then, glass capping dies were bonded on the corresponding LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (LT) device wafer to generate a cavity structure. Partial filling of the vias on the pad was used, and a metal trace was formed on the glass. Ball grid arrays (BGAs) were formed on top of the glass. Finally, an SAW filter WLP was fabricated. This glass capping packaging avoids the outgassing problem of conventional thin-film acoustic packaging at high temperatures, which would cause the contamination of the interdigital transducers (IDTs). In addition, a numerical study based on the 3-D finite element (FE) model has been conducted to analyze the reliability of the package. The built model was applied to perform parametric studies on the effects of package geometry, material properties, and the processing conditions on reliability. Based on the above studies, the optimized SAW filter WLP successfully passed the reliability test of pre-con level 3 and temperature cycling test (TCT, −40 °C ~ 125 °C).

Nd:YAG laser rod manufactured by femtosecond laser-induced chemical etching

We present a cylindrical rod of single-crystal Nd:YAG fabricated from a bulk crystal using femtosecond laser-induced preferential etching. The rod is pumped at 808 nm, and the laser characteristics at 1064 nm emission and the thermal stability are investigated. The slope efficiency was determined with a maximum optical-to-optical efficiency of 7.9%±0.29% and a FWHM linewidth of 299 ± 63 pm. The etched rod shows parameters consistent with existing Nd:YAG gain crystals. This fabrication technology will find use in composite micro-optical devices where microfluidics, active and passive optics, and structures can be etched out of many different materials and combined into a single device.

Three-Dimensional Laser Printing of Macro-Scale Glass Objects at a Micro-Scale Resolution.

Three-dimensional (3D) printing has allowed for the production of geometrically complex 3D objects with extreme flexibility, which is currently undergoing rapid expansion in terms of materials, functionalities, as well as areas of application. When attempting to print 3D microstructures in glass, femtosecond laser-induced chemical etching (FLICE)—which is a subtractive 3D printing technique—has proved itself a powerful approach. Here, we demonstrate the fabrication of macro-scale 3D glass objects of large heights up to ~3.8 cm with an identical lateral and longitudinal feature size of ~20 μm. The remarkable accomplishment is achieved by revealing an unexplored regime in the interaction of ultrafast laser pulses with fused silica, which results in depth-insensitive focusing of the laser pulses inside fused silica.

Machining of optical micro-mold by laser-induced chemical etching

Triangular grating has an important application in modern optical field. The large-scale production of such components can be realized by injection molding process, among which the fabrication of optical molds is the most important technology. This paper proposed to adopt femtosecond laser-induced chemical etching to make a micro-mold based on triangular diffraction gratings. Based on theoretical analysis and processing experiments, we studied the feasibility applied to manufacture the microstructure of optical mold.

Chemical etching mechanisms and crater morphologies pre-irradiated by temporally decreasing pulse trains of femtosecond laser

We report the influence of temporally decreasing pulse trains on femtosecond laser-induced chemical etching (FLICE) of fused silica. A systematic comparison of the unshaped pulse and decreasing pulse trains of femtosecond laser for FLICE was conducted, and the differences were interpreted using a plasma model. The results revealed that the decreasing pulse trains not only affected the etching efficiency but also affected the morphology of the etched crater. When an etched crater was pre-irradiated by decreasing pulse trains, it presented a funnel-like shape at the early stage of the etching process, which contrasted with the one pre-irradiated by unshaped pulse. At the later stage of the etching process, the funnel-like shape gradually disappeared, and the crater increased in size. Compared with the unshaped pulse under the same processing conditions, the decreasing pulse trains enhanced the etched crater volume by approximately 18 times. Theoretical calculations based on the plasma model indicated that the free-electron density generated using the unshaped pulse was much higher than that generated by the decreasing pulse trains in skin layer of the sample during the first few hundred femtoseconds. The high free-electron density increased the reflectivity in skin layer of the sample; thus, the tail part of the incident pulse was strongly reflected. Consequently, the laser energy deposition into the fused silica sample decreased, eventually led to a low etching efficiency.

Printing of complex free-standing microstructures via laser-induced forward transfer (LIFT) of pure metal thin films

A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach.

Closed-loop quality control system for laser chemical machining in metal micro-production

Small geometrical features in the scale of 10 to 400 μm can be produced on metallic workpieces with the laser-induced chemical etching process at low costs. However, the interactions among the subsystems of the laser chemical etching process, the mechanical positioning unit, the etchant pump, the laser source, and among the removal paths hinder the precise determination of the required process parameters for producing the desired geometry. For this reason, a closed-loop quality control is designed to compensate the deviations of quality features of a single removal and also of a workpiece produced by a sequence of removals. The developed control approach is based on inverse process models, which are used for a feed-forward control. Finally, a closed-loop control is achieved by designing an adaptive controller for the non-linear multi-inputs-multi-outputs process. The closed-loop control is realized as a production-discrete control by using a post-production measurement, and applied for producing a micro forming tool in the shape of a rectangular die. As a result, the laser chemical etching process is stabilized and, thus, the desired geometrical quality features of the workpiece are obtained with a reduced shape deviation of 2.4 μm.

Formation of rectangular channels in fused silica by laser-induced chemical etching

In this paper, we report results of our research on the formation of micro-channels with the rectangular cross section inside bulk fused silica. The selective etching of channels was performed by the technique called the femtosecond laser induced chemical etching. Hydrofluoric acid was used as an agent for selective removal of laser-modified regions in fused silica samples. The method of the channel cross section control based on multiple scanning is proposed. The effect of scanning speed, laser pulse energy, and polarization on the etching selectivity was investigated and discussed.

Highly sensitive TGA diagnosis of thermal behaviour of laser-deposited materials

Thermal gravimetric analysis (TGA) combining the use of digital microbalances, gas-chromatography and mass spectroscopy is demonstrated as a highly sensitive technique for the determination of thermal behaviour of polymeric materials obtained in very low quantities in laser-induced chemical processes. The article is a brief review on some authors’ TGA studies of materials prepared by laser ablation of polymers, laser-induced chemical vapour deposition and laser back-side etching in the gas phase. The TGA-diagnosed materials are (i) chemical vapour-deposited N-containing polyoxocarbosilane and Fe/polyoxocarbosilane composites, (ii) crosslinked polymers ablatively deposited from pure and metal particles-loaded poly(vinyl acetate), poly(vinyl chloride-co-vinyl acetate) (PVACVC) and poly[oxy(tetramethyldisilane-1,2-diyl)] (POTMD), and (iii) carbon/polyoxocarbosilane composites obtained through gas-phase etching process.

Laser-induced etching of CrO doped GaAs and wavelength dependent photoluminescence

The laser-induced chemical etching mechanism of CrO doped GaAs (1 0 0) substrate in 40% HF-H 2O solution is explained by the generation of e–h pairs through defect states in the presence of sub-bandgap photon illumination by using a Nd:YAG Laser ( λ∼1.06 μm). The central feature of the laser etching technique is pits initiation by surface defects. The etched GaAs samples are characterised by photoluminescence (PL) spectroscopy. The PL spectrum shows the formation of GaAs nanostructures and chemical reaction products. The shape of the PL band (1.3–1.8 eV) obtained from the top surface depends on the penetration depth of excitation wavelength. An estimate of the nanocrystallites sizes present in the laser etched layer is obtained by quantum confinement model.

Microscale Patterning of Mesoscopic PZN-PT Single Crystal Films by Crystal ion Slicing and Laser-Induced Etching

Advances in the fabrication of solid-solution single crystal relaxor ferroelectrics have made it possible to produce highly efficient piezoelectric crystals, and have attracted renewed interest in the use of these crystals for a new generation of piezoelectric transducers, actuators and sensors. Of particular interest is their incorporation into micro-electromechanical systems (MEMS). In this paper we report on the laser-induced wet chemical etching of lead zinc niobate-lead titanate (PZN-PT) in hydrochloric acid (HCl). Argon-ion laser radiation at power levels up to 4 W is focused to a spot diameter of about 15μm and results in the chemical etching of grooves at patterning speeds up to 5μm/sec. Crystal ion slicing, an ion-implant-based film separation technique, is used in combination with laser etching to form 5 to 10μm-thick patterned and freestanding films for incorporation into micro-electromechanical devices.

Etching of crystalline Si in Cl2 atmosphere by means of an optical fiber tip

We report experiments on laser-induced chemical etching of silicon in chlorine atmosphere using a near-field optical configuration. Crystalline (100) Si surfaces were locally illuminated in 300 mbar Cl2 through a tapered fiber tip. In most of the experiments, we used UV argon ion laser lines around 350 nm for illumination. The etched samples were analyzed by means of atomic force microscopy. Patterns with a width of 140 nm at full width half maximum and a vertical etch rate of the order of 15 nm/s have been achieved. Comparison of etching with a visible wavelength at intensities that cannot cause surface melting shows that the etching is primarily a photophysical process.

Femtosecond laser-induced formation of spikes on silicon

We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF6. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes.

Thermochemical laser etching of stainless steel and titanium in liquids

Laser-induced wet chemical etching of stainless steel 304 and Ti in phosphoric acid, sulfuric acid and aqueous KOH has been investigated using cw Ar and cw Nd : YAG lasers. Two different phases of laser-induced etching of Ti in phosphoric acid were found by electrochemical investigations. Laser-enhanced electrochemical dissolution of stainless steel was observed in the passivation and transpas sivation region. In the latter case, laser heating accelerates the metal dissolution. In the passivation region, laser heating results in a breakthrough of the passivation layer near the Flade potential. By multiple scanning microstructures of high quality and an aspect ratio > 10 have been produced. By EDX analysis the laser-etching process was found to be practically free of chemical residues on etched surfaces.

Nd:YAG-laser-induced wet chemical etching of titanium and stainless steel

Abstract Laser-induced wet chemical etching of titanium and stainless steel foils in phosphoric and sulphuric acid was investigated. Focussing of cw-Nd:YAG laser radiation at power levels up to 7 W to a focal spot diameter of about 25 μm resulted in chemical etching of grooves at speeds up to 90 μm s −1 and cutting of foils at velocities of 10–50 μm s −1 . Aspect ratios up to 2 were obtained. Fabrication of free-standing microstructures is demonstrated.

Estimation of the capabilities of maskless micropatterning by laser-induced chemical etching

A three-dimensional mathematical model of laser-induced chemical etching of polysilicon films in chlorine atmosphere is presented. The model takes into account both photochemical and thermochemical mechanisms. A numerical solution of a simple process model and consequences for spatial resolution limits of the mask-less laser-induced chemical etching are presented.

Estimation of the capabilities of maskless micropatterning by laser-induced chemical etching

Estimation of the capabilities of maskless micropatterning by laser-induced chemical etching

Model of laser-induced chemical etching of silicon in chlorine atmosphere

A mathematical model for the chemical etching of silicon in a chlorine atmosphere induced by laser irradiation is described. The model takes into account: dissociation of molecules having absorbed radiant energy into chlorine atoms and their diffusion onto the substrate surface, generation of photocarriers in the silicon substrate, kinetics of chlorine atom chemisorption on the silicon surface, chemical reaction of chemisorbed chlorine atoms with silicon atoms, and desorption of reaction products. The obtained results are compared with experimental data.