Fabrication of Nanoscale Step Height Structure Using Atomic Layer Deposition Combined with Wet Etching

The current techniques used for the fabrication of nanosteps are normally done by layer growth and then ion beam thinning. There are also extra films grown on the step surfaces in order to reduce the roughness. So the whole process is time consuming. In this paper, a nanoscale step height structure is fabricated by atomic layer deposition （ALD） and wet etching techniques. According to the traceable of the step height value, the fabrication process is controllable. Because ALD technology can grow a variety of materials, aluminum oxide （Al2O3） is used to fabricate the nanostep. There are three steps of Al2O3 in this structure including 8 nm, 18 nm and 44 inn. The thickness of Al2O3 film and the height of the step are measured by anellipsometer. The experimental results show that the thickness of Al2O3 film is consistent with the height of the step. The height of the step is measured by AFM. The measurement results show that the height is related to the number of cycles of ALD and the wet etching time. The bottom and the sidewall surface roughness are related to the wet etching time. The step height is calibrated by Physikaliseh-Technische Bundesanstalt （PTB） and the results were 7.5±1.5 nm, 15.5±2.0 nm and 41.8±2.1 nm, respectively. This research provides a method for the fabrication of step height at nanoscale and the nanostep fabricated is potential used for standard references.

Improvement in a-plane GaN crystalline quality using wet etching method

Nonpolar （1120） GaN films are grown on the etched a-plane GaN substrates via metalorganic vapor phase epitaxy. High-resolution X-ray diffraction analysis shows great decreases in the full width at half maximum of the samples grown on etched substrates compared with those of the sample without etching, both on-axis and off-axis, indicating the reduced dislocation densities and improved crystalline quality of these samples. The spatial mapping of the E2 （high） phonon mode demonstrates the smaller line width with a black background in the wing region, which testifies the reduced dislocation densities and enhanced crystalline quality of the epitaxial lateral overgrowth areas. Raman scattering spectra of the E2 （high） peaks exhibit in-plane compressive stress for all the overgrowth samples, and the E2 （high） peaks of samples grown on etched substrates shift toward the lower frequency range, indicating the relaxations of in-plane stress in these GaN films. Furthermore, room temperature photoluminescence measurement demonstrates a significant decrease in the yellow-band emission intensity of a-plane GaN grown on etched templates, which also illustrates the better optical properties of these samples.

Fabrication of Diamond Microstructures by Using Dry and Wet Etching Methods

Diamond films have great potential for micro-electro-mechanical system（MEMS） application.For device realization,precise patterning of diamond films at micrometer scale is indispensable.In this paper,simple and facile methods will be demonstrated for smart patterning of diamond films,in which two etching techniques,i.e.,plasma dry etching and chemical wet etching（including isotropic-etching and anisotropic-etching） have been developed for obtaining diamond microstructures with different morphology demands.Free-standing diamond micro-gears and micro-combs were achieved as examples by using the experimental procedures.It is confirmed that as-designed diamond structures with a straight side wall and a distinct boundary can be fabricated effectively and efficiently by using such methods.

Wet etching and passivation of GaSb-based very long wavelength infrared detectors

The etching and passivation processes of very long wavelength infrared(VLWIR)detector based on the InAs/GaSb/AlSb type-II superlattice have been studied.By studying the effect of each component in the citric acid solution(citric acid,phosphoric acid,hydrogen peroxide,deionized water),the best solution ratio is obtained.After comparing different passivation materials such as sulfide+SiO_(2),Al_(2)O_(3),Si_(3)N_(4) and SU8,it is found that SU8 passivation can reduce the dark current of the device to a greater degree.Combining this wet etching and SU8 passivation,the of VLWIR detector with a mesa diameter of 500μm is about 3.6Ω·cm^(2) at 77 K.

Physical analysis of normally-off ALD Al_(2)O_(3)/GaN MOSFET with different substrates using self-terminating thermal oxidation-assisted wet etching technique

Based on the self-terminating thermal oxidation-assisted wet etching technique,two kinds of enhancement mode Al_(2)O_(3)/GaN MOSFETs(metal-oxide-semiconductor field-effect transistors)separately with sapphire substrate and Si sub-strate are prepared.It is found that the performance of sapphire substrate device is better than that of silicon substrate.Comparing these two devices,the maximum drain current of sapphire substrate device(401 mA/mm)is 1.76 times that of silicon substrate device(228 mA/mm),and the field-effect mobility(μ_(FEmax))of sapphire substrate device(176 cm^(2)/V·s)is 1.83 times that of silicon substrate device(96 cm^(2)/V·s).The conductive resistance of silicon substrate device is 21.2Ω-mm,while that of sapphire substrate device is only 15.2Ω·mm,which is 61%that of silicon substrate device.The significant difference in performance between sapphire substrate and Si substrate is related to the differences in interface and border trap near Al_(2)O_(3)/GaN interface.Experimental studies show that(i)interface/border trap density in the sapphire substrate device is one order of magnitude lower than in the Si substrate device,(ii)Both the border traps in Al_(2)O_(3) dielectric near Al_(2)O_(3)/GaN and the interface traps in Al_(2)O_(3)/GaN interface have a significantly effect on device channel mobility,and(iii)the properties of gallium nitride materials on different substrates are different due to wet etching.The research results in this work provide a reference for further optimizing the performances of silicon substrate devices.

Etching of quartz crystals in liquid phase environment:A review

Quartz crystals are the most widely used material in resonant sensors,owing to their excellent piezoelectric and mechanical properties.With the development of portable and wearable devices,higher processing efficiency and geometrical precision are required.Wet etching has been proven to be the most efficient etching method for large-scale production of quartz devices,and many wet etching approaches have been developed over the years.However,until now,there has been no systematic review of quartz crystal etching in liquid phase environments.Therefore,this article provides a comprehensive review of the development of wet etching processes and the achievements of the latest research in thisfield,covering conventional wet etching,additive etching,laser-induced backside wet etching,electrochemical etching,and electrochemical discharge machining.For each technique,a brief overview of its characteristics is provided,associated problems are described,and possible solutions are discussed.This review should provide an essential reference and guidance for the future development of processing strategies for the manufacture of quartz crystal devices.

Paired interference 3-dB coupler based on SOI rib waveguides with anisotropic chemical wet etching

A 3-dB paired interference （PI） optical coupler in silicon-on-insulator （SOI） based on rib waveguides with trapezoidal cross section was designed with simulation by a modified finite-difference beam propagation method （FD-BPM） and fabricated by potassium hydroxide （KOH） anisotropic chemical wet etching. Theoretically, tolerances of width, length, and port distance are more than 1, 100, and 1 μm, respectively. Smooth interface was obtained with the propagation loss of 1.1 dB/cm at the wavelength of 1.55 μm. The coupler has a good uniformity of 0.2 dB and low excess loss of less than 2 dB.

Wet etching characteristics of a HfSiON high-k dielectric in HF-based solutions

The wet etching properties ofa HfSiON high-k dielectric in HF-based solutions are investigated. HF-based solutions are the most promising wet chemistries for the removal of HfSiON, and etch selectivity of HF-based solutions can be improved by the addition of an acid and/or an alcohol to the HF solution. Due to densification during annealing, the etch rate of HfSiON annealed at 900℃ for 30 s is significantly reduced compared with as-deposited HfSiON in HF-based solutions. After the HfSiON film has been completely removed by HF-based solutions, it is not possible to etch the interfacial layer and the etched surface does not have a hydrophobic nature, since N diffuses to the interface layer or Si substrate formation of SiN bonds that dissolves very slowly in HF-based solutions. Existing Si-N bonds at the interface between the new high-k dielectric deposit and the Si substrate may degrade the carrier mobility due to Coulomb scattering. In addition, we show that N2 plasma treatment before wet etching is not very effective in increasing the wet etch rate for a thin HfSiON film in our case.

A novel oxidation-based wet etching method for AlGaN/GaN heterostructures

A novel wet etching method for AlGaN/GaN heterojunction structures is proposed using thermal oxidation f ollowed by wet etching in KOH solution.It is found that an AlGaN/GaN heterostructure after high temperature oxidation above 700℃could be etched off in a homothermal（70℃） KOH solution while the KOH solution had no etching effects on the region of the AlGaN/GaN heterostructure protected by a SiO_2 layer during the oxidation process.A groove structure with 150 nm step depth on an AlGaN/GaN heterostructure was formed after 8 h thermal oxidation at 900℃followed by 30 min treatment in 70℃KOH solution.As the oxidation time increases,the etching depth approaches saturation and the roughness of the etched surface becomes much better.The physical mechanism of this phenomenon is also discussed.

Investigation of chlorine-based etchants in wet and dry etching technology for an InP planar Gunn diode

Mesa etching technology is considerably important in the Gunn diode fabrication process. In this paper we fabricate InP Gunn diodes with two different kinds of chlorine-based etchants for the mesa etching for comparative study. We use two chlorine-based etchants, one is HCl-based solution （HC1/H3PO4）, and the other is Cl2-based gas mixture by utilizing inductively coupled plasma system （ICP）. The results show that the wet etching （HCl-based） offers low cost and approximately vertical sidewall, whilst ICP system （C12-based） offers an excellent and uniform vertical sidewall, and the over-etching is tiny on the top and the bottom of mesa. And the fabricated mesas of Gunn diodes have average etching rates of 0.6 p.m/min and 1.2 pm/min, respectively. The measured data show that the current of Gunn diode by wet etching is lower than that by ICP, and the former has a higher threshold voltage. It provides a low-cost and reliable method which is potentially applied to the fabrication of chip terahertz sources.

Etching Behavior of GaN/GaAs(001) Epilayers Grown by MOVPE

Wet etching characteristics of cubic GaN (c GaN) thin films grown on GaAs(001) by metalorganic vapor phase epitaxy (MOVPE) are investigated.The samples are etched in HCl,H 3PO 4,KOH aqueous solutions,and molten KOH at temperatures in the range of 90～300℃.It is found that different solution produces different etch figure on the surfaces of a sample.KOH based solutions produce rectangular pits rather than square pits.The etch pits elongate in 1 0] direction,indicating asymmetric etching behavior in the two orthogonal <110> directions.An explanation based on relative reactivity of the various crystallographic planes is employed to interpret qualitatively the asymmetric etching behavior.In addition,it is found that KOH aqueous solution would be more suitable than molten KOH and the two acids for the evaluation of stacking faults in c GaN epilayers. direction,indicating asymmetric etching behavior in the two orthogonal <110> directions.An explanation based on relative reactivity of the various crystallographic planes is employed to interpret qualitatively the asymmetric etching behavior.In addition,it is found that KOH aqueous solution would be more suitable than molten KOH and the two acids for the evaluation of stacking faults in c GaN epilayers.

Enhanced etching of silicon didioxide guided by carbon nanotubes in HF solution

This paper describes a new method to create nanoscale SiO2 pits or channels using single-walled carbon nanotubes （SWNTs） in an HF solution at room temperature within a few seconds. Using aligned SWNT arrays, a pattern of nanoscale SiO2 channels can be prepared. The nanoscale SiO2 patterns can also be created on the surface of three- dimensional （3D） SiO2 substrate and even the nanoscale trenches can be constructed with arbitrary shapes. A possible mechanism for this enhanced etching of SiO2 has been qualitatively analysed using defects in SWNTs, combined with H3O＋ electric double layers around SWNTs in an HF solution.

Fabrication of Silicon Crystal-Facet-Dependent Nanostructures by Electron-Beam Lithography

Silicon crystal-facet-dependent nanostructures have been successfully fabricated on a （100）-oriented silicon-oninsulator wafer using electron-beam lithography and the silicon anisotropic wet etching technique. This technique takes advantage of the large difference in etching properties for different crystallographic planes in alkaline solution. The minimum size of the trapezoidal top for those Si nanostructures can be reduced to less than 10nm. Scanning electron microscopy （SEM） and atomic force microscopy （AFM） observations indicate that the etched nanostructures have controllable shapes and smooth surfaces.

Electrical characteristics of a vertical light emitting diode with n-type contacts on a selectively wet-etching roughened surface

Low resistance and thermally stable n-type contacts to N-polar GaN are essentially important for vertical light emitting diodes （VLEDs）. The electrical characteristics of VLEDs with n-type contacts on a roughened and flat N-polar surface have been compared. VLEDs with contacts deposited on a roughened surface exhibit lower leakage currents yet a higher operating voltage. Based on this, a new scheme by depositing metallization contacts on a selectively wet-etching roughened surface has been developed. Excellent electrical and optical characteristics have been achieved with this method. An aging test further confirmed its stability.

Fabrication of ridge waveguide of 808 nm GaAs-based laser diodes by wet chemical etching

The fabrication of ridge waveguide of 808 nm GaAs-based laser diodes by wet chemical etching is investigated. The etching behavior of GaAs, InGaP and A1GalnP in various solutions is evaluated. As a result, the etching solutions simultaneously corroding InGaP and A1GalnP layers are searched successfully. Effects of etching time and the concentration of mixtures on etching depth and the geometrical shape of ridge are analyzed. It is found that under proper conditions, appropriate etching depth and smooth surfaces can be obtained and the steep degree of pattern can be accepted, especially for wide ridge waveguide laser diodes.

Rear-surface light intensification caused by a Hertzian-conical crack in 355-nm silica optics

Theoretical studies show that a Hertzian-conical crack can be considered to be composed of double cone faces for simplicity. In the present study, the three-dimensional finite-difference time-domain method is employed to quantify the electric-field distribution within the subsurface in the presence of such a defect under normal incidence irradiation. Both impurities （inside the crack） and the chemical etching have been investigated. The results show that the maximum electric field amplitude │E│max is 9.57374 V/m when the relative dielectric constant of transparent impurity equals 8.5. And the near-field modulation will be improved if the crack is filled with the remainder polishing powders or water vapor/drops. Meanwhile, the laser-induced initial damage moves to the glass-air surface. In the etched section, the magnitude of intensification is strongly dependent on the inclination angle θ. There will be a highest modulation when θ is around π/6, and the maximum value of IEImax is 18.57314 V/m. When θ ranges from π/θ to π/4, the light intensity enhancement factor can easily be larger than 100, and the modulation follows a decreasing trend. On the other hand, the modulation curves become smooth when θ〉π/4 or θ 〈 π/8.

Self-aligned-gate AlGaN/GaN heterostructure field-effect transistor with titanium nitride gate

Self-aligned-gate heterostructure field-effect transistor（HFET） is fabricated using a wet-etching method.Titanium nitride（TiN） is one kind of thermal stable material which can be used as the gate electrode.A Ti/Au cap layer is fixed on the gate and acts as an etching mask.Then the T-shaped gate is automatically formed through over-etching the TiN layer in 30% H2O2 solution at 95 ℃.After treating the ohmic region with an inductively coupled plasma（ICP） method,an Al layer is sputtered as an ohmic electrode.The ohmic contact resistance is approximately 0.3 Ω·mm after annealing at a low-temperature of 575 ℃ in N2 ambient for 1 min.The TiN gate leakage current is only 10^-8 A after the low-temperature ohmic process.The access region length of the self-aligned-gate（SAG） HFET was reduced from 2 μm to 0.3 μm compared with that of the gate-first HFET.The output current density and transconductance of the device which has the same gate length and width are also increased.

Fabrication and Performance of Novel RF Spiral Inductors on Silicon

This paper discusses fabrication and performance of novel circular spiral inductors on silicon. The substrate materials underneath the inductor coil are removed by wet etching process. In the fabrication process, fine polishing of the photoresist is used to simplify the processes and ensure perfect contact between the seed layer and the top of pillars. Dry etching technique is used to remove the seed layer. The results show that Q-factor of the inductor is greatly improved by removing silicon underneath the inductor coil. The spiral inductor with line width of 50 μm has a peak Q-factor of 10 for the inductance of 2.5 nH at frequency of 1 GHz, and the resonance frequency of the inductor is about 8.5 GHz. For the inductor of conductor width 80 μm, the peak Q-factor increases to about 17 for inductance of 1.5 nH in the frequency range of 0.05 -3.00 GHz.

High Yield Transfer of Clean Large-Area Epitaxial Oxide Thin Films

In this work,we have developed a new method for manipulating and transferring up to 5 mm×10 mm epitaxial oxide thin films.The method involves fixing a PET frame onto a PMMA attachment film,enabling transfer of epitaxial films lifted-off by wet chemical etching of a Sr3Al2O6 sacrificial layer.The crystallinity,surface morphology,continuity,and purity of the films are all preserved in the transfer process.We demonstrate the applicability of our method for three different film compositions and structures of thickness~100 nm.Furthermore,we show that by using epitaxial nanocomposite films,lift-off yield is improved by~50% compared to plain epitaxial films and we ascribe this effect to the higher fracture toughness of the composites.This work shows important steps towards large-scale perovskite thin-film-based electronic device applications.

Design and Fabrication of 2-DOF Micromirror Array Based on Electro-Thermal Actuators

With surface- and bulk-micromachining, an 8 × 8 mirrors array is designed, fabricated and tested, which is based on electro-thermal actuators and can be addressed individually. The micromirror is square shaped, 4-corner-actuated. Its dimension is 200 μm × 200 μm. The substrate below it is caved away to ensure a tilt at an angle as large as possible. To protect the etch-sensitive features on the front side of the wafer undamaged during wet deep silicon etch on the backside, the wax protective coating process is used. Mirror actuated by powering an alternative pair of heaters will tilt in 2-DOF. If its 4 cantilevers/heaters are powered synchronously, it will move in a piston mode. The effective arrays are more than 80% out of the three finished wafers. When the ramp voltage frequency applied to a pair of neighboring cantilevers is 5 Hz at 10 V, the average tilting angle can be ± 8°.