Laser-assisted Chemical Etching Research Articles

Attomolar Sensing Based on Liquid Interface-Assisted Surface-Enhanced Raman Scattering in Microfluidic Chip by Femtosecond Laser Processing.

Surface-enhanced Raman scattering (SERS) is a multidisciplinary trace analysis technique based on plasmonic effects. The development of SERS microfluidic chips has been exploited extensively in recent times impacting on applications in diverse fields. However, despite much progress, the excitation of label-free molecules is extremely challenging when analyte concentrations are lower than 1 nM because of the blinking SERS effect. In this paper, a novel analytical strategy which can achieve detection limits at an attomolar level is proposed. This performance improvement is due to the use of a glass microfluidic chip that features an analyte air-solution interface which forms on the SERS substrate in the microfluidic channel, whereby the analyte molecules aggregate locally at the interface during the measurement, hence the term liquid interface-assisted SERS (LI-SERS). The microfluidic chips are fabricated using hybrid femtosecond (fs) laser processing consisting of fs laser-assisted chemical etching, selective metallization, and metal surface nanostructuring. The novel LI-SERS technique can achieve an analytical enhancement factor of 1.5 × 1014, providing a detection limit below 10-17 M (<10 aM). The mechanism for the extraordinary enhancement afforded by LI-SERS is attributed to Marangoni convection induced by the photothermal effect.

Fabrication of Chalcogenide Glass Based Hexagonal Gapless Microlens Arrays via Combining Femtosecond Laser Assist Chemical Etching and Precision Glass Molding Processes.

Chalcogenide glasses (ChGs) are emerging as critical infrared (IR)-enabled materials in advanced IR optical systems by the wealth of their transparency in the key wide infrared (IR) transmission window. However, fabrication of ChG-based integrated micro-optical components in an efficient and economical way remains a huge challenge. In this paper, a 3D close-packed hexagonal microlens array (MLA) possessing over 6000 convex hexagonal micro-lenslets with the size of tens of micrometers within a footprint of 10 mm × 10 mm on a Ge20Sb15Se65 ChG surface was successfully fabricated via a precise thermal-mechanical molding process. The master mold of ChG MLA was firstly fabricated by a femtosecond laser-assisted chemical etching process and then transferred on to the surface of the ChG via a precision thermo-mechanical molding process, which resulted in a convex MLA. The morphology, imaging and focusing performances of the as-prepared ChG MLA were investigated and demonstrated the advancement of the method. Meanwhile, the IR transmittance and x-ray diffraction image of the ChG MLAs were measured to verify the structural and compositional stability of the ChG under the given molding conditions. The combined results proved a new route to mass production of miniaturized gapless ChG MLAs for advanced infrared micro-optics.

Photoresist-free, laser-assisted chemical etching process for long-period fiber grating.

In this study, we propose a photoresist-free, laser-assisted wet chemical etching process used to control the grating depth of a long-period fiber grating (LPFG) termed laser-assisted etching LPFG (LLPFG). This process can considerably reduce production time, while the photoresist-free laser etching allows tiny long-period notches to be etched on the fiber surface, distinguishing the etching rate of the process from that of standard wet etching processes. The LLPFG, which has a period of 610 µm, was scanned using a KrF excimer laser. The results showed a resonant-attenuation wavelength of 1551 nm through a fiber diameter of 60 µm and a grating depth of 26 µm and to being a green process due to the photoresist-free etching.

Positive- and negative-tone structuring of crystalline silicon by laser-assisted chemical etching.

We demonstrate a structuring method for crystalline silicon using nanosecond laser internal irradiation followed by chemical etching. We show a dramatic dependence of the etch rate on the laser-writing speed. Enhanced isotropic etch rates of silicon by laser-induced internal damage were recently demonstrated with strong acids, but our results add the possibility to obtain reduced etch rates leading to different topographies. Material analyses indicate the possibility to efficiently produce high-aspect ratio channels, thanks to laser-induced porosities, as well as silicon micro-bumps due to highly stressed regions. This holds promises for fabricating microfluidic, photovoltaic, and micro-electromechanical systems.

Polygonal pits on silicon surfaces that are created by laser-assisted chemical etching

Laser-assisted chemical etching was conducted for creating periodic textures on silicon surfaces. Silicon plates with the (111) surface orientation were immersed in an aqueous solution of potassium hydroxide, and a pulsed laser beam (532 nm wavelength, 5 ns duration, 10 pulse/s) was irradiated on their surface to promote anisotropic etching. The laser beam was patterned by using a glass capillary plate that contained a hexagonal array of micropores (10μm diameter, 12μm period). The focused beam projected the hexagonal image on the silicon surface, creating bright spots of 4μm period. During the laser irradiation process of 3 min, both laser-induced ablation and chemical etching took place at these bright spots. After stop of laser irradiation, the chemical etching progressed further, and consequently, a periodic array of triangular or hexagonal pits emerged on the silicon surface. The direction of the triangular pits changed by rotation of the silicon plate. When a silicon plate with the (100) surface orientation was used, diamond or rectangular pits were created on its surface. The mechanism of this polygonal texturing was explained by using the normal and intersecting vectors of the (100), (110), and (111) planes that exhibited different etching rates.

Transmission electron microscopy investigation of crystalline silicon surface irradiated by femtosecond laser pulses in different background atmospheres

This article aims to obtain structural and compositional characteristics of a crystalline silicon surface irradiated by femtosecond laser pulses in SF6, N2, air, and vacuum background atmospheres by performing transmission electron microscopy observation of 〈110〉 cross-sectional specimens. Conical microstructures covered with defective outer layers were formed in SF6 gas. The elemental sulfur dopants in the surface microstructure, which located in close proximity to defects, were mainly concentrated at the tip region of the microcones, and about several hundred nanometers thick. In N2 atmosphere, the defects produced regularly on the silicon surface were of the same types with those formed in SF6 gas and confirmed to be stacking faults and overlapped twins. Furthermore, silicon crystalline grains with different orientations were observed on the silicon surface irradiated in N2, air, and vacuum atmospheres. Especially, β-Si3N4 crystalline grains were found to be formed in N2 and air as chemical products when elemental nitrogen exists, and the SiO2 amorphous phase was formed in air by the oxidation effect. Based on these experimental results, the relevant interaction mechanisms between pulsed laser and crystalline silicon were suggested to be mainly attributed to laser-assisted chemical etching and laser ablation, i.e., if volatile silicon compounds can be produced in a reactive gas atmosphere (e.g., SF6), the strong laser-assisted chemical etching dominates over the laser irradiation process. Otherwise, laser ablation is the dominant mechanism such as in N2, air, and vacuum.

Laser-Assisted Chemical Polishing of Silicon (112) Wafers

Pulsed laser-assisted chemical etching (PLACE) offers an advanced, novel substrate preparation method for molecular beam epitaxy (MBE) growth of mercury cadmium telluride on silicon (112) wafers. By controlling the laser fluence, the chemical etch process is refined into a final polish step. PLACE offers surface roughness on the order of chemical mechanical polishing standards and has been verified by 488-nm Raman and high-resolution x-ray diffraction as causing no surface or subsurface damage. To the contrary, experiments show that using PLACE not only alters the surface chemically but also removes subsurface damage through recrystallization reaching micron depths. The process occurs in a modular vacuum chamber that could conceivably be transferred between tools so that vacuum is not broken between polishing and MBE deposition. PLACE can achieve ultra-high-purity and fine dimensional control since it is a dry process relying on pyrolytic vapor-phase reactions initiated, and constrained, by a pulsed laser. Since the process is a function of laser fluence and optics, it is imminently scalable to 6-inch wafer sizes and beyond.

Femtosecond laser fabrication of high reflectivity micromirrors

High-quality freestanding micromirrors consisting of 40 dielectric layers on silicon have been fabricated by ultrashort-pulse laser ablation in combination with laser-assisted wet chemical etching. Backside material removal enables direct access to both faces of the dielectric coating. The amplitude reflectance of the micromirrors has been determined by Fabry–Pérot interferometry; a finesse in excess of 8900±700, corresponding to a reflectivity exceeding 99.95%, has been found. The mechanical quality factor, Q, of the microresonators, measured at 20 K, is determined to be between 5000 and 6000.

Microchanneled Chirped Fiber Bragg Grating Formed by Femtosecond Laser-Aided Chemical Etching for Refractive Index and Temperature Measurements

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this work, a microchanneled chirped fiber Bragg grating (MCFBG) is proposed and fabricated through the femtosecond laser-assisted chemical etching. The microchannel (<emphasis emphasistype="italic"><formula formulatype="inline"> <tex Notation="TeX">$\sim$</tex></formula></emphasis>550 <emphasis emphasistype="italic"><formula formulatype="inline"><tex Notation="TeX">$\mu$</tex> </formula></emphasis>m) gives access to the external index liquid, thus inducing refractive index (RI) sensitivity to the structure. In the experiment, the transmission bands induced by the reduced effective index in the microchannel region were used to sense the surrounding RI and temperature changes. The experimental results show good agreement with the theoretical analysis. The proposed MCFBG offers enhanced RI sensitivity without degrading the robustness of the device showing good application potential as bio-chemical sensors. </para>

Photo-selective chemical etching of InAs and GaSb to manufacture microscopic mirrors

1 IntroductionPhotochemical processing of materials, particularly insemiconductors, has been a relatively well-known techniqueused for some time with relatively few applications [1].However, devices demanding new processing techniques,suchasMEMS[2],Leds[3]andlaserswithintegratedlensesor gratings, etc. have renewed interest in this technique.The technique can be described as follows [4]: a semi-conductor is immersed in an electrolyte, forming a spacecharge region at the solution–semiconductor interface; at thesame time the semiconductor is illuminated with light (nor-mally a laser) of energy greater than the material’s band gap,creating electron-hole pairs. The electric field due to bandbending at the semiconductor surface attracts one type ofcarrier to the surface and repels the other type of carriertowardsthebulk.Normally,inn-typematerial,theholesgotothesurfaceandtheelectronstothebulk.Inthiscasetheexcessholes increase the oxidation state of the surface atoms,increasingtheetchingrate.Ifthereactionrateislimitedbythesupplyofphoto-generatedholesatthesurface,theetchingratecan be adjusted simply by changing the light intensity.Since the technique uses a beam of monochromaticphotons (laser) to locally control reactive interactions nearthe solid surface, it is particularly useful to performlocalized material deposition (writing), solid doping,alloying and/or, as in our case, material removing (etching)without masking assistance [5]. Due to these unique char-acteristics Laser-Assisted Chemical Etching (LACE) offersadvantageous distinctiveness that makes it preferred toother materials processing techniques [6].Of particular interest in this work is the photochemicaletching of semiconductors, also called LACE. This techniquehas been already used successfully to produce channels withapproximately parabolic cross section along GaAs substrates,creating a refraction index negative gradient inside a Graded-Index Separate Confinement Heterostructure (GRIN-SCH)laser structure. This effect was used to deviate the higher orderlateral modes out of the laser str ucture keeping only the centralone[7].Inthatworkthefeasibilityofanewtypeofhigh-powercoherent semiconductor laser with lateral emission was dem-onstrated. As far as we know this was the first time LACE wasused successfully to build structures in laser diodes thatsimultaneously improved their power output and coherence.In this work we show the results of applying LACE ton-type GaSb substrates to create cavities with nearly par-abolic cross sectioned profiles that could be used as micromirrors or micro lenses to concentrate or deviate lightbeams emitted from a surface emitting laser diode. Aparabolic profile of the mirror is best suited to obtain singlemode operation in an unstable resonator [8, 9].GaSb is an interesting material for the production oflasers and photodetectors in the near and mid infraredregion. However, the use of GaSb requires the developmentof some processing techniques such as etching andpolishing. Chemical processing of GaSb is particularly

Ultrafast-laser-assisted chemical restructuring of silicon and germanium surfaces

This article reports a comparative study on texturing in silicon and germanium surfaces after exposure to femtosecond laser irradiation in the gaseous environments of sulfur hexafluoride (SF 6) and hydrogen chloride (HCl). The surface texturing results from the combined effect of laser-assisted chemical etching and laser ablation. Optimized processing conditions have produced features on the order of nanometers in size. We demonstrate for the first time that regular conical pillars can be formed in Ge and that HCl can be used to form regular conical pillars in Si.

Nanometer-Scale Laser Direct-Write Using Near-Field Optics

AbstractThis article summarizes research on laser-based processing and structuring of materials at the nanoscale using optical near-field schemes. Both apertureless and tapered fiber near-field scanning optical microscope probes can deliver highly confined irradiation at sufficiently high intensities to cause morphological and structural changes in materials at the nanometer level. The energy emitted by the probes and the absorption within the target material are predicted by carrying out calculations of the near-field electromagnetic distribution. The effects of shrinking laser beam dimensions compete with the energy diffusion into the target material. Experimental results have shown well-controlled subtractive material modification with minimum feature size in the neighborhood of 10 nm. Precise patterning can be achieved via laser-assisted chemical etching. Control of the nucleation of nanostructures via rapid melting and crystallization is demonstrated. The article concludes with an outlook to applications.

Periodic nanostructuring of Er∕Yb-codoped IOG1 phosphate glass by using ultraviolet laser-assisted selective chemical etching

The patterning of submicron period (≈500nm) Bragg reflectors in the Er∕Yb-codoped IOG1 Schott, phosphate glass is demonstrated. A high yield patterning technique is presented, wherein high volume damage is induced into the glass matrix by exposure to intense ultraviolet 213nm, 150ps Nd:YAG laser radiation and, subsequently, a chemical development in potassium hydroxide (KOH)∕ethylenediamine tetra-acetic acid (EDTA) aqueous solution selectively etches the exposed areas. The electronic changes induced by the 213nm ultraviolet irradiation are examined by employing spectrophotometric measurements, while an estimation of the refractive index changes recorded is provided by applying Kramers-Kronig transformation to the absorption change data. In addition, real time diffraction efficiency measurements were obtained during the formation of the volume damage grating. After the exposure, the growth of the relief grating pattern in time was measured at fixed time intervals and the dependence of the grating depth on the etching time and exposure conditions is presented. The gratings fabricated are examined by atomic and scanning electron microscopies to reveal the relief topology of the structures. Gratings with average depth of 120nm and excellent surface quality were fabricated by exposing the IOG1 phosphate glass to 36 000 pulses of 208mJ∕cm2 energy density, followed by developing in the KOH∕EDTA agent for 6min.

The effect of laser light propagation through a self-induced inhomogeneous process gas on temperature dependent laser-assisted chemical etching

High resolution and precise surface morphology is a primary goal of laser-assisted chemical etching (LACE) for microfabrication. However, the interaction of the laser light with the process gas can change the intensity incident to the target surface due to self-induced, thermally developed inhomogeneities within the gas. The exponential relationship between LACE rates and surface temperature means that very small changes in intensity, and thus surface temperature distribution, has a very large effect on the etch profile. This paper models both the inhomogeneous light–process gas interaction and the etching to predict the regimes in time and absorption path length where this effect needs to be considered. Numerical calculations of LACE microfabrication of borosilicate glass in a sulfur hexafluoride process gas with a 10.6 μm wavelength laser beam are given that show how the surface morphology of the glass wafer is changed by the inhomogeneous interaction.

Laser-assisted chemical etching for embedded microchannels and overhanging microstructures on Si/SiO2 substrates

The feasibility of fabricating buried (embedded or subsurface) microstructures between silicon and silicon oxide using laser-assisted chemical etching (LACE) in chlorine atmosphere is demonstrated in this article. An associated LACE ablation model and sample calculations based on one-dimensional thermal analysis and chemical kinetics are presented. A strategy for optimizing the laser process for 3D micromachining is outlined. Potential applications of such fabricated microstructures include microfluidic MEMS for transducers and electronic cooling.

Subwavelength-resolvable focused non-Gaussian beam shaped with a binary diffractive optical element

The diffraction-limited spot size limits the optical disk storage capacity and microscopic resolution. We describe a technique to shape a focused Gaussian beam into a superresolving beam by using a diffractive optical element fabricated by laser-assisted chemical etching. The focused shaped beam has a smaller width and a longer depth of focus than a similarly focused Gaussian beam. Using the diffraction-limited shaped beam along with threshold writing, we achieved a written pit size of less than 0.33 microm at a 695-nm laser wavelength, compared with a 0.7-microm focused Gaussian spot size (full width at e(-2) of the peak) with the same focusing lens. The energy conversion efficiency for the beam shaping was approximately 81%.

High-efficiency flat-top beam shaper fabricated by a nonlithographic technique

We report a compact flat-top beam shaper fabricated by laser-assisted chemical etching upon an ion-exchanged high-energy- beam-sensitive glass. The beam shaper combines a phase filter element and a focusing lens element into a single diffractive optical element with a quasi-continuous surface relief profile. The fabrication takes advantage of the laser generation of gray-level patterns and one-step etching that are effective especially for such beam shaper fabrication. The demon- strated beam shaper with good flatness and edge features has an en- ergy efficiency of 82%. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)02202-3)

Effects of a quartz beam-guide on high-precision laser-assisted etching of Al2O3 ceramics

Abstract Laser-assisted chemical etching of ceramic materials has been attracting continuous attention as a laser microfabrication technique with relatively low power levels. In this study, in order to improve the accuracy of etching depth and diameter, the application of a quartz beam-guide to the pulsed Nd:YAG laser-assisted etching process is proposed. By using this technique, possibilities of high-precision hole drilling with high aspect ratio and uniform surface etching in the radial direction of the beam using a beam-homogenizing effect of the beam-guide have been investigated for Al 2 O 3 ceramics immersed in H 3 PO 4 solution at room temperature. The results revealed that: (1) the distance between the beam-guide surface and the workpiece surface dominates the surface etching rate; (2) by using the beam-guide, the beam-homogenizing effect in the radial direction of the beam at the surface is confirmed; and (3) possibilities of uniform surface etching and high-precision hole drilling are demonstrated by applying the beam-homogenizing effects of the beam-guide.

Multilevel diffractive microlens fabrication by one-step laser-assisted chemical etching upon high-energy-beam sensitive glass

A new technique of laser-assisted single-step chemical etching for diffractive microlens fabrication upon high-energy-beam sensitive glass is reported. Laser direct writing with calibrated writing parameters results in gray-level mask patterns upon the ion-exchanged layer of the glass. The transmittance-dependent chemical etching upon the glass is then effectively utilized to yield suitable surface relief structures for multiple-phase-level diffractive optical elements. The one-step nonphotolithographic fabrication technique has been successfully applied for the realization of an eight-phase-level diffractive microlens.

Fine patterning of diamond films by laser-assisted chemical etching in oxygen

Abstract We report on the use of low power (~2 W) continuous wave Ar+ laser for etching (engraving) of diamond films via a laserinduced local oxidation (burning) reaction. Smooth fine-grained diamond films of about 10 μm thickness have been grown on Mo and Si substrates in a d.c. arc discharge using CH 4 H 2 gas mixtures. The sharply focused laser beam was scanned under computer control over the film surface causing fast diamond oxidation in air or pure O2 atmosphere. The dependence of etch rate on laser power, beam scanning velocity and film quality was examined. Various grooves and holes of a few micrometres in size have been produced with etch rate of tens of micrometres per second.