Vertical Etching Research Articles

Fabrication of ultra-high-aspect-ratio nano-walls and nano-structures on silicon substrates

Realization of high-aspect-ratio vertical features on silicon substrates is a crucial step in the fabrication process of microelectromechanical systems devices. Therefore deep reactive ion etching (DRIE) plays an important role in the fabrication of those structures. Recently many efforts have been made in order to propose the novel DRIE processes in order to achieve higher aspect ratios; however, they suffer from complexity and high sensitivity to process parameters. Herein, the authors report a high-aspect-ratio vertical etching of silicon by employment of proposed low temperature SiO2 deposition on Si substrate, using of H2 and O2 gases. The process employs multiple sequences of alternating steps of etching by a fluorine donating gas such as SF6 and sidewall passivation by SiO2. Etch rates reach up to 0.8 μm min−1 and values of aspect ratio higher than 100 are readily achievable by this process. Furthermore, this process also allows for controlled fabrication of nano-wall and high-aspect-ratio nano-wall tube as well as three-dimensional silicon structures.

Precision Wet Etching of ZnO Using Buffer Solutions

Zinc oxide (ZnO) is a metal oxide semiconductor of interest for a wide range of electronic and optoelectronic device applications. Many devices require etching of ZnO structures and there have been many investigations of ZnO wet-etching processes. However, most reported etches have problems with reproducibility and especially control of vertical and lateral (or undercut) etching uniformity. In this work, we report new buffer solutions that provide controlled vertical and lateral etching of ZnO. We compare the ZnO etch characteristics of buffer etchants to a previously reported ammonium chloride etch. Using buffer etchants of suitable composition we find reaction-rate-limited, uniform, reproducible etching, and similar vertical and lateral etch rates.

Anisotropic Etching of Pyramidal Silica Reliefs with Metal Masks and Hydrofluoric Acid.

This work describes the fabrication of anisotropically etched, faceted pyramidal structures in amorphous layers of silicon dioxide or glass. Anisotropic and crystal-oriented etching of silicon is well known. Anisotropic etching behavior in completely amorphous layers of silicon dioxide in combination with purely isotropic hydrofluoric acid as etchant is an unexpected phenomenon. The work presents practical exploitations of this new process for self-perfecting pyramidal structures. It can be used for textured silica or glass surfaces. The reason for the observed anisotropy, leading to enhanced lateral etch rates, is the presence of thin metal layers. The lateral etch rate under the metal significantly exceeds the vertical etch rate of the non-metallized area by a factor of about 6-43 for liquid and 59 for vapor-based processes. The ratio between lateral and vertical etch rate, thus the sidewall inclination, can be controlled by etchant concentration and selected metal. The described process allows for direct fabrication of shallow angle pyramids, which for example can enhance the coupling efficiency of light emitting diodes or solar cells, can be exploited for producing dedicated silicon dioxide atomic force microscopy tips with a radius in the 50nm range, or can potentially be used for surface plasmonics.

Performance improvement of ultraviolet-A multiple quantum wells using a vertical oriented nanoporous GaN underlayer

Well-aligned, lateral and vertical oriented nanoporous GaN was fabricated using the electrochemical etching procedure and its influence on the optical characteristics of ultraviolet-A multiple quantum well (MQW) structure was investigated. We used a MQW structure with a V-defect and n-Al0.1Ga0.9 N layer, which greatly improved the uniformity of vertical electrochemical etching. Compared to the as-grown MQW structure, the lateral and vertical oriented nanoporous MQW structures have 3.8-fold and 8.1-fold photoluminescence intensity enhancement and the full width at half maximum has been narrowed from 18.4 nm to 7.9 nm and 2.8 nm, respectively. The vertical oriented nanoporous MQW structure has a rectangular far-field emission pattern with uniform forward light distribution and the view angle of 85% intensity is 50°. This study provides an effective method for improving the light output and controlling the emission angle of GaN based light emitting devices, as well as a method for preparing well-aligned nanopores in semiconductors.

Causes of anisotropy in thermal atomic layer etching of nanostructures

In this work, the authors have investigated the dependence of the anisotropy level in an atomic layer etching (ALE) process of Al2O3 on form factor constraints when the ALE process involves etching in non-line-of-sight locations beneath a silicon nitride mask. In the experiments described here, thermal etching of Al2O3 without the use of any direction-inducing plasma components was explored utilizing the well characterized hydrogen fluoride/dimethyl-aluminum-chloride atomic layer etching process. The degree of anisotropy was quantified by measuring the ratio of lateral etch rate of this process in comparison to the vertical etch rate as a function of process step time inside 60 nm holes of aluminum oxide. Inside these holes, the authors determined that the horizontal etch rates slowed to an amount of 19% compared to the vertical rate when short process times were used. For process times operating in the saturation mode of the ALE process, horizontal etch rates per cycle could be sped up to 71% of the vertical rate but never reached parity with the latter. The authors propose a simple mechanism for explaining the anisotropy dependence on process step time and applied a reduced-order algorithm to model it. In this model, the authors introduced fitting parameters for surface modification depths and reaction times to match the experimentally found etch results. Conclusions could be drawn regarding topological hindrance or tortuosity for reactants to reach surfaces in shaded areas under the mask and for reaction by-products to escape from these locations and the impact on etch rate. In addition, the authors recognize that this mechanism could explain the unwanted depth dependence of the etch rate per cycle in high aspect ratio structures.

Fabrication of High-Aspect-Ratio Microwire Electrodes Using a Vertical Liquid Membrane Electrochemical Etching Method

A vertical liquid membrane electrochemical etching method is proposed for the fabrication of high-aspect-ratio microwire electrodes. The potential and current density distributions are simulated using COMSOL software. The simulation results show that the unique potential distribution generated by the proposed method, combined with reciprocating linear motion of the wire electrode, can be used to fabricate microwire electrodes with large aspect ratio. These simulation results are verified experimentally. Different concentrations of electrolyte result in different reaction rates and thus different distributions of the diffusion layer and different bubble flows, resulting in different morphologies of the prepared wire electrodes. The effect of electrolyte concentrations is studied experimentally. At low electrolyte concentrations, the products of electrolysis are removed from the processing area under the action of gravity, and uniform electrodes with high aspect ratio can be prepared. At high electrolyte concentrations, intense bubble production destroys the diffusion layer around the electrode and leads to a dynamic equilibrium state, allowing the preparation of uniform wire electrodes in a rough machining process. The proposed method can prepare wire electrodes with different initial diameters. In this paper, uniform wire electrodes of length 1900 μm, diameter 7 μm, and aspect ratio 270 are fabricated by the proposed method from a tungsten wire of initial diameter 50 μm.

Fabrication of Fe-based metal glass microelectrodes by a vertical liquid membrane electrochemical etching method.

The excellent physical and chemical properties of metallic glasses and the absence of crystal defects make them ideal materials for the preparation of microelectrodes. A vertical liquid membrane electrochemical etching method is proposed here for the fabrication of microelectrodes from the Fe-based metallic glass Fe77.5B15Si7.5. With this method, insoluble electrolysis products are kept away from the processing area, and the nonuniformity of the diffusion layer that occurs in traditional transverse liquid membrane electrochemical etching is reduced. Simulations of the potential distribution and the current density distribution are carried out in COMSOL software, and the simulation results are verified by experiments. The passivation characteristics of the Fe77.5B15Si7.5 metallic glass in H2SO4 and H3PO4 electrolytes are studied. The effects of processing voltage on electrode morphology and surface quality are studied experimentally. Micropins with good tip size can be prepared in the active dissolution stage in 0.1M H2SO4 solution, and high-aspect-ratio cylindrical microtool electrodes with high surface quality can be fabricated in the transpassivation stage in 80% H3PO4 electrolyte with high machining stability.

Metal assisted chemical etching of silicon in the gas phase: a nanofabrication platform for X-ray optics.

High aspect ratio nanostructuring requires high precision pattern transfer with highly directional etching. In this work, we demonstrate the fabrication of structures with ultra-high aspect ratios (up to 10 000 : 1) in the nanoscale regime (down to 10 nm) by platinum assisted chemical etching of silicon in the gas phase. The etching gas is created by a vapour of water diluted hydrofluoric acid and a continuous air flow, which works both as an oxidizer and as a gas carrier for reactive species. The high reactivity of platinum as a catalyst and the formation of platinum silicide to improve the stability of the catalyst pattern allow a controlled etching. The method has been successfully applied to produce straight nanowires with section size in the range of 10-100 nm and length of hundreds of micrometres, and X-ray optical elements with feature sizes down to 10 nm and etching depth in the range of tens of micrometres. This work opens the possibility of a low cost etching method for stiction-sensitive nanostructures and a large range of applications where silicon high aspect ratio nanostructures and high precision of pattern transfer are required.

Deep, vertical etching for GaAs using inductively coupled plasma/reactive ion etching

Deep etched structures in GaAs have many applications in optoelectronics and MEMS devices. These applications often require an anisotropic etch profile with smooth sidewalls as well as a high etch rate and high aspect ratio. Developing an etch process that demonstrates high selectivity for the etch mask is critical as etch times for deep grooves can be protracted due to the effect of RIE lag. In this work, the authors describe etching deep, vertical grooves in GaAs using Inductively Coupled Plasma/Reactive Ion Etching. The effects of RF power, pressure, and gas composition on mask selectivity, etch rate, and anisotropy are discussed. Using a SiO2 etch mask and Cl2 as the main etchant gas, grooves with a vertical sidewall and depths of >120 μm (aspect ratio of 9) have been achieved.

Optical characteristics of porous silicon photonic crystals prepared on the back surface of silicon wafers

Surface-enhanced Raman scattering sensors and optical detection devices often incorporate rough surfaces or porous Si photonic crystals to enhance optical absorption and localized surface plasmon resonances. This study reports a porous Si photonic crystal structure fabricated on the backside of a crystalline Si substrate by room-temperature electrochemical etching. Scanning electron microscopy showed that the rough surface did not affect the vertical etching rate, but influenced the porosity and pore size distribution. X-ray diffraction analysis showed broader and lower-intensity peaks from the sample prepared on the back side, indicating that the backside porous Si had strong light-scattering effects and lattice contraction, which led to greater oxidation because of its higher porosity. A reflectance of ∼1% was obtained over a broad wavelength range (400–700 nm) and the optical reflectance suppression mechanism was analyzed. These optical characteristics showed backside porous Si photonic crystals have significant potential utility in photovoltaic and photonics applications.

Microscale pattern etch of 4H–SiC by inductively coupled plasma

SiC is commonly used in the field of high temperature, high frequency, high power and radiation-resistant semiconductor devices and ultraviolet photodetectors due to its excellent photoelectric characteristics. Plasma etching is one of the key processes in the fabrication of SiC devices. However, it is difficult to obtain a smooth and vertical etch profile due to the subtrench effect when the conventional inductively coupled plasma (ICP) is used to etch SiC with small-size patterns (such as 1 μm holes or trenches), thus affecting the subsequent process and device performance. To study microscale pattern etch of 4H–SiC, we used different mask and etch gas systems for ICP etching. The etch profile was characterized using SEM. When the chlorine gas was used, there was no subtrench but a very inclined angle between the SiC bottom and sidewalls, which is hard to improve. When we used photoresist (PR) mask–HBr system, sidewall deposition happened. SiO2 mask–HBr/Ar system could help to etch different angles but has a limitation angle of up to 80°. Compared to above systems, the etching profile was much more vertical when HBr/SF6/O2 was used. After many rounds of process debugging, we obtain a good etching profile with flat and vertical sidewalls, no subtrench in the bottom and no damage of the SiO2 mask. Our work shows a good way of microscale pattern SiC etching by ICP.

(Invited) Characterization of Isotropic Thermal ALE of Oxide Films in Nanometer-Size Structures

Isotropic atomic layer etching (ALE) is increasingly becoming an option and even a necessity for modern chip manufacturing. Especially for application such as 3D-NAND memory, the ability to perform isotropic etches will be a critical addition to the established suite of reactive ion etching. In this work, we have characterized the reaction of aluminum oxide (Al2O3) via the vapor-based DMAC ligand exchange mechanism in nanometer-size structures in which the oxide is buried below a silicon nitride mask. In the oxide, we measured vertical and horizontal etch rates and found that the etch showed some residual anisotropy even though all elements of the etch itself (flow, no plasma) were none-directional. We applied a simple Monte-Carlo simulation to show that anisotropy is caused by a delay in the onset of the horizontal etch beneath the mask and by molecular scattering effects in the vapor phase above the nano-structures. We are contrasting these results with experimental findings in structures without an anisotropy of the etch.

Dual nanotransfer printing for complementary plasmonic biosensors

One of the main challenges in the widespread utilization of localized plasmon resonance-based biosensors is the fabrication of large-area and low-cost plasmonic nanostructures. In this work, we fabricated large-area and low-cost complementary plasmonic biosensors such as nanohole and nanodisk arrays using dual nanotransfer printing (NTP) with a single metal deposition and a single reusable mold. The suspended nanohole arrays and the suspended nanodisk arrays were fabricated using the subsequent dry etching process. We confirmed a maximum enhancement in bulk sensitivity in experiments and simulations by controlling the vertical and lateral etching depths of the dielectric layer underneath the gold (Au) nanohole and nanodisk arrays. Furthermore, we show that the surface sensitivity evaluated by atomic layer deposition of aluminum oxide increased because appropriate vertical and lateral etching depths allow the target analyte to access the additional near-field formed at the bottom of the Au nanostructure. The dual NTP method provides a practical solution for the realization of large-area and low-cost label-free plasmonic biosensing systems, with a reduction in complexity and cost of the fabrication process of complementary plasmonic structures and metasurfaces.

III–V Heterojunction Platform for Electrically Reconfigurable Dielectric Metasurfaces

Achieving an electrically tunable phased array optical antenna surface has been a principal challenge in the field of metasurfaces. In this Letter, we demonstrate a device platform for achieving reconfigurable control over the resonant wavelength of a subwavelength optical antenna through free-carrier injection. We engineer and grow, using molecular beam epitaxy, a heterostructure of In1–xAlxAs/InAs/AlyGa1–ySb layers designed to achieve large amplitude and phase modulation of light by maximizing the refractive index change in regions of resonant field enhancement The p-i-n layers are grown on a heavily doped n-InAs layer which forms a reflecting substrate to confine the Mie resonances within the nanowires of the index tunable layers. We outline the fabrication process developed to form such tunable metasurface elements using a four-step projection lithography process and a self-aligned vertical dry etch. We experimentally demonstrate the operation of an electrically reconfigurable optical antenna element ...

Light-output enhancement of InGaN light emitting diodes regrown on nanoporous distributed Bragg reflector substrates

Utilising our novel wafer-scale electrochemical porosification approach which proceeds through the top surface by means of nanoscale vertical etching pathways, we have prepared full 2 inch wafers containing alternating solid GaN and nanoporous GaN (NP-GaN) layers that form distributed Bragg reflectors (DBRs), and have regrown InGaN-based light emitting diode (LED) heterostructures on these wafers. The NP-GaN DBR wafer is epi-ready and exhibits a peak reflectance of 95% at 420 nm prior to growth of the LED heterostructure. We observe a 1.8× enhancement in peak intensity of LED electroluminescence from processed devices, and delayed onset of efficiency droop with increased injection current.

Improved Performance in GeSn/SiGeSn TFET by Hetero-Line Architecture With Staggered Tunneling Junction

Vertical tunneling FET utilizing hetero-line architecture with GeSn/SiGeSn staggered tunneling junction (TJ) is designed and theoretically characterized. Utilizing vertical spacer etching and selective growth techniques, p+ GeSn source/n+ SiGeSn pockets line tunneling can be realized, which strengthens the boosting effect of staggered TJ. It is demonstrated that, in addition to reducing the tunneling barrier by increasing Sn composition, the device performance can be improved by optimizing length and thickness of n+ SiGeSn pocket.

High-Efficient Solar Cells Textured by Cu/Ag-Cocatalyzed Chemical Etching on Diamond Wire Sawing Multicrystalline Silicon.

We reported a novel texture method through one-step Cu/Ag-cocatalyzed chemical etching which can be widely used in the photovoltaic industry because of its simple and low-cost process. The etching mechanism of an inverted rectangular pyramid is the cooperation of Ag-catalyzed vertical etching and Cu-catalyzed lateral etching. In our texture method, neither saw damage removal nor post-treatment is needed. During the etching process, the digging holes by Ag-catalyzed etching and enlarging holes by Cu-catalyzed etching completed at the same step. Benefiting from the excellent light-trapping and passivation effect of the inverted rectangular pyramid, diamond wire sawing multicrystalline silicon (mc-Si) Al-BSF solar cells with a super high efficiency of 19.49% had been obtained.

Micro-Reflector Integrated Multichannel μLED Optogenetic Neurostimulator With Enhanced Intensity

This paper reports the fabrication and characterization of an optical neuro-stimulator array that consists of 32-channel microscale light-emitting diodes (µ-LEDs) coupled with microscale reflectors for intensity enhancement. The hemi-spherical micro-reflector is able to collect the rear side emission of LED while also acting as a collimator to focus the diverged LED light, aiming towards driving power minimization through light intensity increase, for wireless neuro-stimulator applications. The micro-reflector was constructed by wet etching of silicon followed by aluminum coating as the reflective mirror. The reflective cavity was filled with polydimethylsiloxane (PDMS) that acts as the planarization polymer to facilitate device integration with the µ-LED chip. Deviation of hemi-spherical geometry cavities due to the uneven lateral and vertical etching rate was shown, and the surface morphology was characterized experimentally. Optical intensity enhancement was studied in both simulation and experiments, demonstrating that the micro-reflector enables 65% intensity enhancement. The reflector-coupled LED had an operating temperature increase of <1oC, well within the ANSI/AAMI safety limit for biomedical implants. The potential of the stimulator for use in optogenetics was validated by in-vitro experiments.

Role of Ag-catalyst morphology and molarity of AgNO3 on the size control of Si nanowires produced by metal-assisted chemical etching

In this paper we demonstrate a controlled fabrication of vertically aligned silicon nanowires (Si NWs) on n-type (100)-oriented and p-type (111)-oriented Si wafers by metal-assisted chemical etching (MaCE) method. The etching of Si-wafer samples covered with Ag nanoparticles as catalyst agents was carried out in HF/H2O2/H2O solution of various volumetric ratios. We show that molar concentration of AgNO3, Ag-nanoparticles deposition time, and the etching time determine the shape and size of produced Si NWs. Vertically aligned tall Si NWs can be produced by immersing the n-Si(100) wafer with pre-deposited from AgNO3 [22mM]/HF [5M]-solution silver nanoparticles in the etchant HF/H2O2/H2O (4:1:4) for different time. When the duration of Ag-deposition increased from 15 to 30 s in a solution 22 mM AgNO3/5 M HF, the etching rate of Ag-loaded n-Si(100) samples was found to be the same (∼ 1.6 µm/min). For 120 s Ag-deposited samples, the etching rate decreased to 560 nm/min. So, the height and density of Si NWs can be accurately controlled by varying the etching time as the vertical etching rate remained constant for 15 and 30 s Ag-deposited samples. Also we show that in case of p-Si(111), highly structured array of Si NWs can be produced only after a 30 min etching of the p-Si(111) wafer covered with Ag deposited from AgNO3 [22mM]/HF [5M]-solution in an etchant HF/H2O2/H2O (4:1:8).

Porous AlGaN-Based Ultraviolet Distributed Bragg Reflectors.

Utilising dislocation-related vertical etching channels in gallium nitride, we have previously demonstrated a simple electrochemical etching (ECE) process that can create layered porous GaN structures to form distributed Bragg reflectors for visible light at wafer scale. Here, we apply the same ECE process to realise AlGaN-based ultraviolet distributed Bragg reflectors (DBRs). These are of interest because they could provide a pathway to non-absorbing UV reflectors to enhance the performance of UV LEDs, which currently have extremely low efficiency. We have demonstrated porous AlGaN-based UV DBRs with a peak reflectance of 89% at 324 nm. The uniformity of these devices is currently low, as the as-grown material has a high density of V-pits and these alter the etching process. However, our results indicate that if the material growth is optimised, the ECE process will be useful for the fabrication of UV reflectors.