Maximum Etch Depth Research Articles

Fabrication of Homogeneous Nanoporous Structure on 4H-/6H-SiC Wafer Surface via Efficient and Eco-Friendly Electrolytic Plasma-Assisted Chemical Etching.

Nanoporous single-crystal silicon carbide (SiC) is widely used in various applications such as protein dialysis, as a catalyst support, and in photoanodes for photoelectrochemical water splitting. However, the fabrication of nano-structured SiC is challenging owing to its extreme chemical and mechanical stability. This study demonstrates a highly-efficient, open-circuit electrolytic plasma-assisted chemical etching (EPACE) method without aggressive fluorine-containing reactants. The EPACE method enables the nano-structuring of SiC via a plasma-enveloped microtool traversing over the target material in an electrolyte bath. Through process design, EPACE readily produces a uniform nanoporous layer on a 4H-SiC wafer in KOH aqueous solution, with adjustable pore diameters in the range 40-130nm. Plasma diagnosis by optical emission spectrometry (OES) and surface microanalysis reveal that EPACE realizes a nanoporous structure by electrolytic plasma-assisted oxidation and subsequent thermochemical reduction of an oxide. An increase in voltage or a decrease in etch gap intensifies the plasma and improves the etching efficiency. The maximum etch rate and depth reach 540nm min-1 and 10µm, respectively, demonstrating the significant potential of the approach as a time-saving and sustainable nanofabrication method for industrial applications. Further, the effectiveness of the fabricated SiC nanoporous structure for application in photoelectrochemical water splitting is demonstrated.

Study of metal-assisted chemical etching of silicon as an alternative to dry etching for the development of vertical comb-drives

Metal-assisted chemical etching (MaCEtch) has recently emerged as a promising technique to etch anisotropic nano- and microstructures in silicon by metal catalysts. It is an economical wet chemical etching method, which can be a good alternative to deep-reactive ion etching (DRIE) process in terms of verticality and etch depth. In the present study, gold is used as a metal catalyst and deposited using physical vapour deposition. It has already been demonstrated that (100) p-type Si wafer can be etched with vertical and smooth side walls. Effects of varying concentrations of etchant constituents and various other parameters, that is, porosity of deposited Au, surface contaminants, oxide formation, metal catalyst, etching time, role of surface tension of additives on the etch depth and surface defects are studied and discussed in detail. By increasing the hydrofluoric acid (HF) concentration from 7.5 M to 10 M, lateral etching is reduced and the microstructure’s width is increased from 17 µm to 18 µm. Porous defects are suppressed by decreasing the hydrogen peroxide (H2O2) concentration from 1.5 M to 1 M. On increasing the etching time from 30 min to 60 min, the microstructures are over-etched laterally. Smoother side walls are fabricated by using the low-surface-tension additive ethanol. The maximum etch depth of 2.6 µm is achieved for Au catalyst in 30 min. The results are encouraging and useful for the development of vertical comb-drives and Micro-Electro-Mechanical Systems (MEMS).

Generation of high-quality circular Airy beams in laser resonator

A scheme for forming high-quality circular Airy beams inside the laser resonator is presented theoretically. The desired circular Airy beam can be generated when the common reflective mirror is replaced by a designed diffractive optical element. The mode generated in the proposed cavity can be stimulated by using the so-called eigenvector method. The calculated results show that the parameters of the beams can be controlled by changing the phase distribution of the diffractive optical element. The loss of the generated mode is very low, which is close to that of the fundamental Gaussian mode. The purity of the generated mode is very high, which is much better than that from the phase-only encoding method in Fourier space. The phase distribution of the diffractive optical element needs designing for a fixed resonator length. In practice, the real resonator length may not be equal to the designed resonator length. Thus, the influence of the alignment error of the resonator length is discussed in detail. The results show that the diffraction loss of the proposed system is still very small even when the error reaches up to 2 mm. Meanwhile, the purity of the generated mode decreases little. Then, the influence of etching depth errors and the decenter of the reflective mirrors are discussed in detail. Here we assume that the fluctuations are randomly distributed. The value of the maximum fluctuation is used to represent the etching depth error degree. The results show that the diffraction loss of the proposed system is more sensitive to production error, and the purity of the generated mode is more sensitive to alignment error. Thus, we estimate that the maximum etching depth error should be less than six percent of the wavelength, and the vertical distance between the centers of the two reflective mirrors should be less than 7 μm if one wants to obtain high-quality CAB with high efficiency. The requirements for precision are acceptable for existing microfabrication and operation technologies.

Increasing etching depth of sapphire nanostructures using multilayer etching mask

In this study, the etching of sapphire nanostructures in inductively coupled plasma reactive ion etching using a multilayer etch mask is studied. The goals are to increase the etching depth and enable the fabrication of higher aspect ratio nanostructures in sapphire, which is traditionally difficult to micromachine. The etching rates and chemistry of different masking materials are examined for better understanding of the etching process. The etching of sapphire nanostructures is then studied using single and multilayer masks with Cl2-based chemistry. The fabrication results show that using the multilayer mask is an effective method for sapphire nanostructure fabrication, increasing the maximum etching depth from 25 to 230 nm for a ninefold improvement. To further validate the optical properties of fabricated sapphire nanostructures, the antireflection effects have been characterized. This work indicates that applying the multilayer mask can increase the etching depth of sapphire nanostructures, which can find applications in thin-film optics, optoelectronic devices, and composite windows.

Superlens-enhanced laser interference lithography

A one-step lithography method based on a superlens is proposed to fabricate diffraction-unlimited metallic patterns. By controlling the material parameters and the distribution of the impinging energy, various phenomena, such as periodic nanonetworks, ultrathin nanowires (sub-50-nm feature size), and variable-sized nanoparticles (ranging from sub-10 nm to several hundreds of nanometers), are fabricated using a 1,064-nm nanosecond laser. The evolution pathway of such phenomena is explained by the dewetting process of metallic films. The direct-writing performance of a transparent material with a superlens is studied, and the maximum etching depth of Si gratings can reach 2 µm under a single laser pulse, with fine profiles.

Precise Blaze Angle Adjustment of Echelle Grating by Self-Shadowing Rotating Mask

Angular precision of 0.1° for the blaze angle of echelle gratings is an important technical requirement for spectral instruments with echelle gratings. This paper proposes a precise adjustment method for the blaze angle of echelle gratings that are etched or deposited by self-shadowing rotating mask. The blaze angle will decrease or increase when the grating is rotated in beams for etching or deposition, respectively. The blaze angle can be controlled by adjusting the etching or deposition conditions. For the echelle grating with nominal blaze angle of 63.0° and groove density of 52.7 g/mm, the blaze angle was decreased by 0.12° with a maximum etching depth of 19 nm on the blaze facet. Additionally, the blaze angle adjustment precision will be better than 0.02° if the maximum etch depth on the blaze facet is less than 3 nm.

Removal and characterization of focused-ion-beam-induced damaged layer on single crystal diamond surface and application to multiple depth patterning

In this research, we investigated both the etching characteristics of the non-diamond phase induced by focused ion beam (FIB) irradiation and the patterning method of shallow multiple depth structures. FIB irradiation on the diamond surface induces a non-diamond phase, and this area can be selectively etched by heating in air. The dependence of the shape on the FIB and heating conditions was investigated. Process temperatures equal to or lower than 600°C were effective for structure fabrication in terms of the fine shape of the structure. The removal depth can be controlled by varying the ion fluence and ion energy. The process temperature and heating time were important factors for determining the depth range and etch rate. The shape of the structure was affected by the morphology of the FIB-induced non-diamond phase, and the maximum etching depth was dependent on the defect density induced by FIB irradiation. Concave multiple-depth structures with depths of several tens of nanometres were fabricated based on these results, indicating the possible use of this method for shallow multiple depth patterning on diamond surfaces.

Laser-induced front side etching of fused silica with femtosecond laser radiation using thin metal layers

Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation (λ=780nm, Δtp=150fs, f=1kHz). The etching process of fused silica is studied in dependence on the laser fluence, the laser pulse numbers as well as metal absorber layer thicknesses. Especially the influence of the varied parameters on the surface morphology, the etching depth, and the surface modification is presented and discussed. The etched trenches were analyzed with white light interferometry. A fluence window from 0.5 to 2.5J/cm2 for the LIFE process of fused silica with a single laser pulse using a metal layer with a thickness from 5 to 50nm was found. The measured maximum etching depth of approximately 100nm suggests that the LIFE process is appropriated for precision machining.The attempt to simulate the fs laser LIFE process by a thermodynamic model was moderate successfully and exhibits significant differences within the experimental results.

Silver (Ag) as a novel masking material in glass etching for microfluidics applications

In this paper, we report the use of a single masking film for deep glass etching in hydrofluoric acid (HF). Thin film silver (Ag) is the key masking material in this work enabling a simple and low cost fabrication of microfluidic structures. The Ag film was deposited by evaporation and etched in a diluted nitric acid and de-ionized water solution at a ratio of 1:3. Surface morphology for different thicknesses of Ag film and its correlation to the maximum achievable etch depth is analyzed. AFM results shows low roughness values (<5 nm), indicating the Ag films are of smooth surface. With a 100 nm Ag film, a 220 μm etch depth in borosilicate glass substrates were produced and by further thickening the Ag to 300 nm, etch depths exceeding 300 μm were successfully achieved. SEM images show that thinner Ag films are of finer grains, potentially a source for pinholes formation where rapid penetration of HF along the grain boundaries peels off the Ag film from the glass surface. However, the Ag film was found not to react with HF. The process was demonstrated in the fabrication of cavities for integration with other microfluidic devices.

Impact of Temperature on Thick Photoresist Lithography Process of Glass Microfluidic Chip

The positive photoresist AZ4620 was used for thick photoresist lithography of glass microfluidic chip. Different test temperature of soft baking, post exposure baking, hard baking, impacted the surface quality of photoresist layer, the adhesion of photoresist and glass matrix, the photoresist tolerance time, the etching rate and the maximum etching depth. The results show that the soft-bake temperature of 80°C improves the adhesion of the photoresist and the glass substrate well; PEB (post exposure bake) temperature of 60~70°Chelps to the quality of graphics after development; hard bake temperature of 80~90°Ccan conducive to a larger etching depth; through the controlling of heating temperature and mixing rate, it effectively improves the etching rate, increases the etching depth as well. It has been applied to the fast and high-volume production of a certain microfluidic detection chips.

Development of Multiple-Step SOI DRIE Process for Superior Notch Reduction at Buried Oxide

A novel two step etch process using the Bosch-etch mechanism to prevent notching on an SOI wafer is presented. The first etch step is used to attain the maximum etch depth with high etch rate and stop before the buried oxide (BOX). Followed by the second etch step with lower etch rate and tuned to soft land on the BOX to etch the remaining depth. In addition to that it is tailored to also provide a tapered etch profile which is beneficial in reducing the notch if over etching occurs.

Surface Etching of MgO : LiNbO3 Crystal by Ti Metal for Integrated Optics

Surface etching of a MgO-doped crystal is demonstrated by annealing a sandwich structure formed by at in a wet atmosphere. The etching depth, raised Ti layer height and surface roughness were studied as a function of initial Ti film thickness, annealing temperature, and duration. The results show that the maximum etching depth can reach 260 nm while the surface roughness is . The features of the raised Ti layer height and surface roughness of the etched surface and raised Ti layer are discussed on the basis of the Ti diffusion mechanism. Combined studies of X-ray single-crystal diffraction, Raman scattering, energy-dispersive X-ray, waveguiding characteristics, surface refractive index, and secondary-ion mass spectrometry show that the etched surface not only retains the crystalline phase and the original crystal composition but is also Ti-free. An etching mechanism based upon an electrochemical reaction process is proposed. The MgO doping effect on the etching rate and surface roughness is discussed and explained in terms of the MgO doping effect on Ti solubility. A comparison of the and surface etching is made. Finally, the present method is compared with other etching techniques and its possible application is suggested.

A simple three-dimensional computer simulation tool for predicting femtosecond laser micromachined structures

Laser micromachining is a powerful technique that is commonly used in microfabrication. Typically such laser systems are computer controlled, and the required microstructure is fabricated by multiple machining iterations, where the control program is modified until the required structure is achieved. Here a simple method of simulating an approximation of the surface topography that will result from a computer numerically controlled (CNC) laser machining program is presented for glass and silicon. The simulation allows rapid prediction of the etch depths, workpiece contours, resulting structuring and machining artifacts. It was found that the simulated surface topology closely matched that of the structures machined in each material (4% of the maximum etch depth for borosilicate, 7% for sodalime and 9% for silicon). A non-planar microfluidic channel system with through-substrate ports was produced in both types of glass and silicon. The design took 12 min to simulate and 74 min to machine, and the simulation was accurate to within 4 µm of the machined glass substrate.

Application of highly charged Ar ion beams to ion beam lithography

Ion beam lithography (IBL) is a very useful tool for the fabrication of nano-structures. In order to expand the applicability of IBL by using highly charged ions (HCI), Ar1+ and Ar9+ ions were irradiated onto spin-on-glass (SOG) through a stencil mask. The step structure was successfully fabricated on SOG by the chemical etching after the irradiation. The depth of etching of the SOG using the Ar9+ ions increases linearly with dose and was greater than the etching depth obtained using Ar1+ ions. The maximum etching depth of the SOG using Ar9+ ions was larger than the range calculated by the SRIM code. The results show the effectiveness of HCI for IBL and indicate the contribution of the damage created by indirect irradiation processes.

Stress control in masking layers for deep wet micromachining of Pyrex glass

In this paper, we report three etching masking technologies of Cr/Au, Cr/Cu and PECVD amorphous silicon developed for Pyrex glass micromachining in hydrofluoric acid solution. Our study reveals that the residual stress, especially the tensile stress, in the mask layers is responsible for the pinholes on the glass surface and notch defects on the edge formed at the etch edges of glass due to the breakage of highly stressed mask layers during the etching process. The Cr/Au metal mask can achieve a glass etch depth up to 100 μm, along with a number of pinholes and notch defects on the edge due to the high tensile residual stress in the Cr/Au layer. The Cr/Cu metal masking layer improves the glass etch quality by the reduced residual stress. Detailed studies have been done using the amorphous silicon film as a glass etch mask. The PECVD process and the subsequent annealing process have been optimized to reduce the compressive residual stress in the amorphous silicon layer. The maximum etch depth in the glass can be as high as 200 μm and almost without pinholes and notch defects on the edge. To our knowledge, this is the best result reported in the literature so far.

Optimization of an amorphous silicon mask PECVD process for deep wet etching of Pyrex glass

Silicon is well known as an inert material in hydrofluoric acid and can be used during wet etching of glass as a mask with good results. In this paper, we report on the optimization of a PECVD amorphous silicon layer as etch mask for deep Pyrex glass micromachining in hydrofluoric acid solution. Our study reveals that the residual stress, especially the tensile stress, in the amorphous silicon masking layer is responsible for the defects generated during the etching process. The PECVD process and the subsequent annealing process have been optimized to reduce the compressive residual stress in the amorphous silicon layer. The maximum etch depth of glass achieved is as high as 300 μm.

Size distribution of droplets in film prepared by pulsed laser ablation

Volume and size distribution of droplets in oxide films were investigated for a variety of laser fluences, wavelengths and target temperatures. Effects of both the ambient gas pressure and the substrate temperature on the droplet formation was investigated. It was found that the increase in the ratio of the droplet volume to the film volume V d/ V f caused by increasing the maximum etching depth of target per laser shot d e ( V d/ V f∝ d e 3, as previously reported) due to changes in the deposition conditions other than the target temperature is mainly attributed to an increase in the formation of large droplets. It was also found that the correlation V d/ V f∝ d e 3 is also valid for elevated target-temperatures, and the increase in the target temperature enhances the formation of small droplets compared with large ones. The effect of cooling of droplets after the ejection from the target due to the ambient pressure and/or the substrate temperature is discussed.

Laser-induced dry etching of integrated InP microlenses

Maskless laser dry etching of monolithically integrated InP microlenses for infrared-optical coupling between optoelectronic devices and a glass fiber at 1.53 μm wavelentgth is reported. The process is based on the projection of a conventional chromium-on-quartz reticle on to the InP wafer surface using a pulsed 248 nm KrF excimer laser. Etching proceeds with a fluence-dependent rate in a chlorine atmosphere as described in detail before. The lens shape is defined by a rotationally symmetric fluence distribution with smooth radial variation in the image plane of the reticle. The demagnified image only represents the mean local transmission of the reticle since the dimensions of chromium structures on the reticle are chosen so small that they are not optically resolved on the wafer surface. A typical lens has 50 μm diameter, 5-10 μm maximum etch depth at the edge, and needs 3000 laser pulses within 150s for fabrication. Upscaling is possible by etching an array of lenses simultaneously in an expanded field of view and by increasing the pulse repetition rate. The lens surfaces are smooth and deviations from the nominal design are below 50 mm(λ/10). The coupling losses to single-mode fibers were measured with a laser diode on a separate chip to be 4 ± 1 dB.

Improved electron-beam patterning of Si with self-assembled monolayers

A set of processes has been explored that enhances the utility of self-assembled-monolayer electron-beam resists for patterning silicon. A self-assembled monolayer resist of octadecylsiloxane was exposed using a scanning electron microscope with a 20 keV beam energy and dose of 320 μC/cm2. After the patterned monolayer was developed using ultraviolet light and ozone, it served as a wet etch mask for the underlying native oxide. Linewidths of ∼30 nm were etched in silicon using the patterned oxide as a reactive ion etch mask. The maximum etch depth achieved in silicon was 90 nm using this process.

Plasma etching with self-assembled monolayer masks for nanostructure fabrication

Self-assembled monolayers of octadecylthiol on GaAs and octadecylsiloxanes on titanium, aluminum, and silicon have been used as electron beam resists for plasma etching into the substrates. An electron cyclotron resonance source was used to excite a low-pressure, high-density plasma with low ion energy to achieve high selectivity with the thin masking layers. Patterning of monolayers on GaAs usually produced a negative tone, but etching of the metal surfaces resulted in positive tone patterning. The maximum etch depth into the GaAs was ∼100 nm using the negative tone process. Lines have been etched into Ti with linewidths down to ∼20 nm. The negative tone process can be explained by the cross linking of the monolayer under high-dose electron beam exposure; however, the positive tone process must rely on contrast either from different etching characteristics of the oxides or different structural arrangements of the different SAMs.