Fast Atom Beam Etching Research Articles

GaN microring waveguide resonators bonded to silicon substrate by a two-step polymer process.

Using a polymer bonding technique, GaN microring waveguide resonators were fabricated on a Si substrate for future hybrid integration of GaN and Si photonic devices. The designed GaN microring consisted of a rib waveguide having a core of 510nm in thickness, 1000nm in width, and a clad of 240nm in thickness. A GaN crystalline layer of 1000nm in thickness was grown on a Si(111) substrate by metal organic chemical vapor deposition using a buffer layer of 300nm in thickness for the compensation of lattice constant mismatch between GaN and Si crystals. The GaN/Si wafer was bonded to a Si(100) wafer by a two-step polymer process to prevent it from trapping air bubbles. The bonded GaN layer was thinned from the backside by a fast atom beam etching to remove the buffer layer and to generate the rib waveguides. The transmission characteristics of the GaN microring waveguide resonators were measured. The losses of the straight waveguides were measured to be 4.0±1.7 dB/mm around a wavelength of 1.55μm. The microring radii ranged from 30 to 60μm, where the measured free-spectral ranges varied from 2.58 to 5.30nm. The quality factors of the microring waveguide resonators were from 1710 to 2820.

Fabrication of freestanding nanoscale gratings on silicon-on-insulator wafer

We report a bottom-up process for the fabrication of freestanding nanoscale gratings on silicon-on-insulator (SOI) wafer. Freestanding membrane devices suffer deflection due to the residual stress of the buried oxide layer of SOI wafer. The deflection will affect the device shape and result in the fracture problem for devices fabricated on thin silicon membrane. The bottom-up process is developed to overcome the fabrication issue for thin silicon membrane gratings. The silicon handle layer is removed through back wafer etching of silicon, where the buried oxide layer acts as an etch stop layer. The grating structures are then defined on thin silicon device layer by electron beam lithography and generated by fast atom beam etching. The grating structures are finally released in vapor HF to form the freestanding nanoscale gratings. The freestanding linear/circular gratings, 1,500-nm period grating with the grating width of 200- and 850-nm period grating with the grating width of 100 nm, are successfully achieved on 260-nm silicon device layer.

Design and Fabrication of Diffractive Light-Collecting Microoptical Device with 1D and 2D Lamellar Grating Structures

This paper presents the optimal design method of diffractive light-collecting microoptical device and its fabrication method by E-beam lithography, fast atom beam etching, and hot-embossing processes. The light-collecting device proposed in the paper is comprised of 9 (3 × 3) blocks of optical elements: 4 blocks of 1D lamellar grating structures, 4 blocks of 2D lamellar grating structures, and a single block of nonpatterned element at the center, which acts for lens to be able to collect the diffracted and transmitted lights from the lamellar grating structures into the focus area. The overall size of the light-collecting device is 300 × 300 μm2, and the size of each block was practically designed as 100 × 100 μm2. The performance of 1D and 2D lamellar grating structures was characterized in terms of diffraction efficiency and diffraction angle using a rigorous coupled-wave analysis (RCWA) method, and those geometric parameters, depth, pitch, and orientation, were optimized to achieve a high light-collecting efficiency. The master molds for the optimized structures were fabricated on Si substrate by E-beam lithography and fast atom beam etching processes. The 100 μm thick patterned polymethyl methacrylate (PMMA) film was then replicated by a hot-embossing process. As a result, the patterned PMMA film collected 63.0% more incident light than a nonpatterned one.

Characterization of bonding interface prepared by room-temperature Si wafer direct bonding using the surface smoothing effect of a Ne fast atom beam

We performed room-temperature Si wafer direct bonding by surface activation using a Ne fast atom beam (FAB). The bonding energy between Si wafers prepared by Ne FAB etching is equivalent to that of the bulk materials. We also investigated the surface smoothing effect by Ne FAB etching. A rough Si surface with a surface roughness of 0.40nm rms was etched by Ne FAB, and its surface roughness was decreased down to 0.17nm rms. For a Si surface etched at a depth of 30nm by the Ne FAB, the 1D-PSD (1-dimensional power spectrum density) amplitude was decreased in the range of spatial frequencies >10μm−1. Although the Si wafers with surface roughness of 0.4nm rms could not be bonded, they were successfully bonded after Ne FAB smoothing of etching depths over 5nm, and a bonding energy equivalent to that of the bulk materials was attained. We succeeded in dramatically decreasing the local strain at the bonding interface by the Ne FAB smoothing effect.

Single and multiple optical switches that use freestanding silicon nanowire waveguide couplers

Silicon photonic devices consisting of nanowire waveguides are a promising technology for on-chip integration in future optical telecommunication and interconnection systems based on silicon-large scale integration fabrication. However, the accommodation of variable optical components on a chip remains challenging due to the small size of microchips. In this study, we investigated the characteristics of a microelectromechanical silicon nanowire waveguide switch with a gap-variable coupler. Due to its capacitive operation, the proposed waveguide switch consumed negligible power relative to switches that use a thermo-optical effect and carrier injection. The proposed switch was characterized using analyses based on coupled-mode theory for rectangular waveguides, as well as a simulation using the finite difference time domain method. A 2×2 single switch with an improved configuration and a 2×6 multiple switch composed of the 2×2 switches was designed and fabricated by a combination of electron beam lithography, fast-atom beam etching and hydrofluoric acid vapor sacrificial etching. The properties of the switches were measured and evaluated at a wavelength of 1.55 µm. A new miniature silicon optical switch requires far less electrical power than schemes employing thermo-optic effects or carrier injection. The device, developed by Yuta Akihama and Kazuhiro Hane from Tohoku University in Japan, relies on optical coupling between two arms in a 2×2 silicon waveguide coupler. By allowing one of the arms to move and connecting it to an electrostatic comb-drive actuator, an electrical voltage can be used to vary the gap between the waveguide arms and thus control the amount of coupling. The result is an efficient switch that operates at the wavelength of 1.55 µm and has a footprint of 100 µm×100 µm. As well as demonstrating a single 2×2 switch, the researchers also cascaded several devices together to construct a 2×6 switch.

III‐nitride grown on freestanding GaN nanostructures

AbstractWe report here the epitaxial growth of III‐nitride on the freestanding GaN nanostructures by molecular beam epitaxy growth. Various GaN nanostructures are defined by electron beam lithography and realized on GaN‐on‐silicon substrate by fast atom beam etching. Silicon substrate beneath GaN nanostructures is removed from the backside to form the freestanding GaN slab, and the epitaxial growth of III‐nitride by MBE is performed on the prepared GaN template. The selective growth takes place with the assistance of GaN nanostructures and generates hexagonal III‐nitride pyramids. Thin epitaxial structures, depending on the shape and the size of GaN nanostructure, can produce the promising optical performance. This work opens the way to combine silicon micromachining with the epitaxial growth of III‐nitride by MBE on GaN‐on‐silicon substrate for further integrated optics (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaN/GaN quantum wells grown on freestanding HfO2 photonic crystals

AbstractWe report here epitaxial growth of InGaN/GaN quantum wells (QWs) on suspended HfO2 photonic crystal structures by molecular beam epitaxy (MBE). Suspended HfO2 photonic crystals are achieved on silicon substrate by combining film evaporation, electron beam lithography, fast atom beam etching of HfO2 film with dry etching of silicon substrate. InGaN/GaN QWs are subsequently grown on suspended HfO2 photonic crystal structures by MBE technique, and freestanding HfO2 photonic crystal slab can sustain complicated stress change during MBE growth. The reflectance and photoluminescence measurements are performed for epitaxial III‐nitride structures (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single layer silicon photonic crystal slab

We present here the fabrication and characterization of single layer silicon photonic crystal mirror on a silicon-on-insulator wafer. By a combination of electron beam lithography, fast atom beam etching with deep reactive ion etching, silicon photonic crystal slabs are achieved on 260nm freestanding silicon membrane and sandwiched with air on the top and bottom. Their high refractive index contrasts enable photonic crystal slabs function as dielectric mirrors for externally incident light. The optical performances of fabricated photonic crystal slabs can be tuned by varying the width of separation grooves or the air-hole size, which represents a significant advantage of offering various approaches for optical response control.

III-Nitride grating grown on freestanding HfO2 gratings

We report here the epitaxial growth of III-nitride material on freestanding HfO2 gratings by molecular beam epitaxy. Freestanding HfO2 gratings are fabricated by combining film evaporation, electron beam lithography, and fast atom beam etching of an HfO2 film by a front-side silicon process. The 60-μm long HfO2 grating beam can sustain the stress change during the epitaxial growth of a III-nitride material. Grating structures locally change the growth condition and vary indium composition in the InGaN/GaN quantum wells and thus, the photoluminescence spectra of epitaxial III-nitride grating are tuned. Guided mode resonances are experimentally demonstrated in fabricated III-nitride gratings, opening the possibility to achieve the interaction between the excited light and the grating structure through guided mode resonance.PACS: 78.55.Cr; 81.65.Cf; 81.15.Hi.

Freestanding HfO2 grating fabricated by fast atom beam etching

We report here the fabrication of freestanding HfO2 grating by combining fast atom beam etching (FAB) of HfO2 film with dry etching of silicon substrate. HfO2 film is deposited onto silicon substrate by electron beam evaporator. The grating patterns are then defined by electron beam lithography and transferred to HfO2 film by FAB etching. The silicon substrate beneath the HfO2 grating region is removed to make the HfO2 grating suspend in space. Period- and polarization-dependent optical responses of fabricated HfO2 gratings are experimentally characterized in the reflectance measurements. The simple process is feasible for fabricating freestanding HfO2 grating that is a potential candidate for single layer dielectric reflector.PACS: 73.40.Ty; 42.70.Qs; 81.65.Cf.

Lateral epitaxial overgrowth of GaN on a patterned GaN-on-silicon substrate by molecular beam epitaxy

We report here the lateral epitaxial overgrowth (LEO) of GaN on a patterned GaN-on-silicon substrate by molecular beam epitaxy (MBE) growth with radio frequency nitrogen plasma as a gas source. Two kinds of GaN nanostructures are defined by electron beam lithography and realized on a GaN substrate by fast atom beam etching. The epitaxial growth of GaN by MBE is performed on the prepared GaN template, and the selective growth of GaN takes place with the assistance of GaN nanostructures. The LEO of GaN produces novel GaN epitaxial structures which are dependent on the shape and the size of the processed GaN nanostructures. Periodic GaN hexagonal pyramids are generated inside the air holes, and GaN epitaxial strips with triangular section are formed in the grating region. This work provides a promising way for producing novel GaN-based devices by the LEO of GaN using the MBE technique.

Comb-drive GaN micro-mirror on a GaN-on-silicon platform

We report here a double-sided process for the fabrication of a comb-drive GaN micro-mirror on a GaN-on-silicon platform. A silicon substrate is first patterned from the backside and removed by deep reactive ion etching, resulting in totally suspended GaN slabs. GaN microstructures including the torsion bars, movable combs and mirror plate are then defined on a freestanding GaN slab by the backside alignment technique and generated by fast atom beam etching with Cl2 gas. Although the fabricated comb-drive GaN micro-mirrors are deflected by the residual stress in GaN thin films, they can operate on a high resistivity silicon substrate without introducing any additional isolation layer. The optical rotation angles are experimentally characterized in the rotation experiments. This work opens the possibility of producing GaN optical micro-electro-mechanical-system (MEMS) devices on a GaN-on-silicon platform.

Patterned growth of InGaN/GaN quantum wells on freestanding GaN grating by molecular beam epitaxy

We report here the epitaxial growth of InGaN/GaN quantum wells on freestanding GaN gratings by molecular beam epitaxy (MBE). Various GaN gratings are defined by electron beam lithography and realized on GaN-on-silicon substrate by fast atom beam etching. Silicon substrate beneath GaN grating region is removed from the backside to form freestanding GaN gratings, and the patterned growth is subsequently performed on the prepared GaN template by MBE. The selective growth takes place with the assistance of nanoscale GaN gratings and depends on the grating period P and the grating width W. Importantly, coalescences between two side facets are realized to generate epitaxial gratings with triangular section. Thin epitaxial gratings produce the promising photoluminescence performance. This work provides a feasible way for further GaN-based integrated optics devices by a combination of GaN micromachining and epitaxial growth on a GaN-on-silicon substrate.PACS81.05.Ea; 81.65.Cf; 81.15.Hi.

Fabrication and characterization of subwavelength nanostructures on freestanding GaN slab

We develop a novel way to fabricate subwavelength nanostructures on the freestanding GaN slab using a GaN-on-silicon system by combining self-assemble technique and backside thinning method. Silicon substrate beneath the GaN slab is removed by bulk silicon micromachining, generating the freestanding GaN slab and eliminating silicon absorption of the emitted light. Fast atom beam (FAB) etching is conducted to thin the freestanding GaN slab from the backside, reducing the number of confined modes inside the GaN slab. With self-assembled silica nanospheres acting as an etching mask, subwavelength nanostructures are realized on the GaN surface by FAB etching. The reflection losses at the GaN interfaces are thus suppressed. When the InGaN/GaN multiple quantum wells (MQWs) active layers are excited, the light extraction efficiency is significantly improved for the freestanding nanostructured GaN slab. This work provides a very practical approach to fabricate freestanding nanostructures on the GaN-on-silicon system for further improving the light extraction efficiency.

Fabrication and characterization of freestanding circular GaN gratings

It's of significant interest to combine freestanding nanostructure with active gallium nitride (GaN) material for surface-emitting optoelectronic application. By utilizing bulk micromachining of silicon, we demonstrate here a promising way to fabricate freestanding GaN nanostructures using a GaN-on-silicon system. The well-defined nanoscale circular GaN gratings are realized by fast-atom beam (FAB) etching, and the freestanding GaN gratings are obtained by removing silicon substrate using deep reactive ion etching (DRIE). The freestanding GaN slab is thinned from the backside by FAB etching to reduce the confined modes inside the GaN slab. The measured microphotoluminescence (micro-PL) spectra experimentally demonstrate significant enhancements in peak intensity and integrated intensity by introducing freestanding circular grating. This work represents an important step in combining GaN-based active material with freestanding nanostructures for further increasing light-extraction efficiency.

Freestanding circular GaN grating fabricated by fast-atom beam etching

We report here a top-down process for fabricating a freestanding circular GaN grating. The circular gratings are defined by electron-beam lithography and realized by fast-atom beam (FAB) etching. The silicon substrate below the GaN grating region is completely removed to make the circular grating suspended in space. The optical responses of the fabricated GaN gratings are characterized in reflectance measurements. The polarization-independent responses of circular gratings are experimentally demonstrated, corresponding well with the theoretical prediction. This work represents an important step in combining GaN-based material with freestanding nanostructures.

Design and Fabrication of Structural Color Filters with Polymer-Based Guided-Mode Resonant Gratings by Nanoimprint Lithography

We investigate structural color filters theoretically and experimentally using polymer-based guided-mode resonant gratings fabricated by nanoimprint lithography. Each grating area is 200×200 µm2, which is a suitable pixel size for displays and multichannel detectors. In the mold fabrication, electron beam lithography and fast atom beam etching are used. The grating periods are 600, 350, and 300 nm for the red, green, and blue filters, respectively. Reflectivity is measured as a function of wavelength. The wavelength at peak reflectivity is proportional to the grating period as expected from the calculated results. At a wavelength of 572.0 nm, maximum reflectivity of 58.3% is obtained for the 400-nm-period grating with a bandwidth of 17.5 nm.

Fabrication and characterization of nanoscale resonant gratings on thin silicon membrane

We report the design and fabrication of nanoscale resonant gratings which is of interest for narrow bandwidth filtering application. The linear/circular grating structures, of which the grating width is 200nm and the grating height is 260nm, are generated on silicon-on-insulator wafer. Nanoscale gratings are fabricated on the silicon device layer by a combination of electron beam lithography and fast atom beam etching. The silicon handle layer under grating region is removed by deep reactive ion etching, and the buried oxide layer is kept. The reflectance measurements are performed to characterize the optical response of fabricated freestanding nanoscale gratings. The resonant behavior of linear gratings agrees with the theoretical predication, and the polarization-independent responses of circular gratings are also experimentally demonstrated.

Pitch-variable blazed grating consisting of freestanding silicon beams

Theoretical analysis is presented for a pitch-variable blazed grating which consists of freestanding silicon beams. The pitch-variable blazed grating is implemented by combining silicon-on-insulator (SOI) technology with microelectronicmechanical system (MEMS) technology. The whole device is fabricated on a 10 microm silicon device layer to guarantee sufficient stiffness. The 4-level blazed surface profile is realized by combining a two-mask process with fast atom beam etching. Electrostatic combdrive microactuators with double folded springs are proposed to stretch the freestanding grating beams. In association with reactive ion etching and vapor HF release, the freestanding grating beams and the microactuators are obtained, and a Cr/Au film is deposited onto the blazed grating surfaces by a protective mask process to improve the diffracted power. Mechanical response and diffraction efficiency of fabricated devices are characterized and the experimental results indicate that the fabricated 4- level blazed gratings extend both the tuning range and the diffraction efficiency of the 1st diffraction order of present MEMS diffraction gratings.

Design and fabrication of freestanding pitch-variable blazed gratings on a silicon-on-insulator wafer

This paper presents the design, fabrication and characterization of tunable blazed gratings by combining microactuators. A two-mask process is used to define a four-level blazed grating on silicon-on-insulator wafers, and high lateral resolution is achieved by fast atom beam etching. Electrostatic comb drive actuators with double-folded springs are proposed to translate the freestanding grating beams. In association with reactive ion etching and vapor HF release, the freestanding grating beams and the actuator structures are obtained on a 10 µm thick silicon device layer; the vapor HF release is used to remove the buried oxide layer, successfully relieving the residual stress and preventing sticking problems. The changes in the grating period are tested by applying the voltage to the microactuators and the optical experiments are carried out to characterize the diffraction properties of the fabricated grating. These experimental results show good agreement with theoretical prediction.