Chemically Assisted Ion Beam Etching Research Articles

Reactive ion beam figuring of optical aluminium surfaces

Ultra-smooth and arbitrarily shaped reflective optics are necessary for further progress in EUV/XUV lithography, x-ray and synchrotron technology. As one of the most important technological mirror optic materials, aluminium behaves in a rather difficult way in ultra-precision machining with such standard techniques as diamond-turning and subsequent ion beam figuring (IBF). In particular, in the latter, a strong surface roughening is obtained. Hence, up to now it has not been possible to attain the surface qualities required for UV or just visible spectral range applications. To overcome the limitations mainly caused by the aluminium alloy structural and compositional conditions, a reactive ion beam machining process using oxygen process gas is evaluated. To clarify the principle differences in the effect of oxygen gas contrary to oxygen ions on aluminium surface machining, we firstly focus on chemical-assisted ion beam etching (CAIBE) and reactive ion beam etching (RIBE) experiments in a phenomenological manner. Then, the optimum process route will be explored within a more quantitative analysis applying the concept of power spectral density (PSD) for a sophisticated treatment of the surface topography. Eventually, the surface composition is examined by means of dynamic secondary ion mass spectrometry (SIMS) suggesting a characteristic model scheme for the chemical modification of the aluminium surface during oxygen ion beam machining. Monte Carlo simulations were applied to achieve a more detailed process conception.

ChemInform Abstract: Good Selectivity Between a NiCr Mask and GaAs and AlGaAs by Chemically Assisted Ion Beam Etching with Cl2 Gas.

The authors report highly selective etching rates between a Ni-Cr mask and GaAs and AlGaAs layers as well as highly anisotropic profiles in these materials at a nanometer scale using the Ni-Cr mask and chemical assisted ion beam etching. The etching rates of Ni-Cr, GaAs, and AlGaAs depend on Cl2 gas flux. ion beam energy, and ion beam current density. Selectivities of the order of 40:1 and 8:1 between GaAs, respectively. Al0.29Ga0.71As and Ni0.7Cr0.3, were obtained at an ion beam energy of 500 eV, beam current density of 0.2 mA/cm2, and a Cl2 flow of 4.2 ml/min

Next generation 15 µm terahertz antennas: mesa-structuring of InGaAs/InAlAs photoconductive layers

Mesa-structuring of InGaAs/InAlAs photoconductive layers is performed employing a chemical assisted ion beam etching (CAIBE) process. Terahertz photoconductive antennas for 1.5 microm operation are fabricated and evaluated in a time domain spectrometer. Order-of-magnitude improvements versus planar antennas are demonstrated in terms of emitter power, dark current and receiver sensitivity.

Design and fabrication of high-efficiency fibre couplers for nanophotonic devices

A dual waveguide heterostructure device that couples light from single mode optical fibres into nanophotonic devices with an overall modelled efficiency of 81% has been investigated. A four-stage taper design for the ridge waveguide which allows for a total device length of 610 μm is described. The fabrication of the ridge waveguide using chemically-assisted ion-beam etching has been optimised.

Deep ridge GaN cw‐laser diodes

AbstractThe processing of homoepitaxial grown GaN based laser diodes by means of chemical assisted ion beam etching (CAIBE) is discussed. These laser diodes, emitting in the blue/UV spectral region, have been processed as buried ridge wave guide structures. Both, the influence of process parameters during etching such as the angle of incidence of the ions and gas fluxes as well as the impact of the ridge depth on the device performance will be analysed. Steep and smooth sidewalls of the ridge have been achieved by optimising the process parameters and by etching the ridge through the active region, cw‐lasing has been achieved. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of out-of-plane micromirrors in silicon-on-insulator planar waveguides

Emerging optical waveguide spectrometers, sensors, and biochips require the simultaneous acquisition of numerous optical signals. Aligning many tens or hundreds of optical fibers to waveguides is a formidable and costly exercise in packaging. We propose a data acquisition scheme using arrays of micromirrors to redirect light propagating in each waveguide out of the device surface, where all the beams can be collected by a single imaging array. Micromirrors, which are the key elements in this data acquisition scheme, were fabricated in 2μm thick silicon-on-insulator waveguides. Chemically assisted ion beam etching (CAIBE) was used to obtain inclined mirror facets, benefiting from the unique capability of CAIBE to tilt the sample relative to the ion beam. Finite difference time domain (FDTD) simulations of an ideal micromirror oriented at 45° to the propagation direction predict a >96% out-coupling efficiency (loss of 0.14dB) and a negligible polarization dependent loss (PDL). The measured total insertion loss for the fabricated waveguide and mirror was −5dB, including fiber-to-waveguide coupling loss, waveguide propagation loss, and mirror loss due to fabrication errors and diffraction, with a PDL of 0.7dB. The dependence of micromirror performance on facet angle and etch depth was studied by FDTD simulations. The influence of CAIBE etching chemistry and selectivity in obtaining optimum mirror parameters is discussed.

Influence of chlorine on etched sidewalls in chemically assisted ion beam etching with SU-8 as mask

A thin layer of SU-8 submicron pattern produced by holographic lithography is used as the dry etch mask in a chemically assisted ion beam etching (CAIBE) system. The effect of the chlorine gas flow rate on etched sidewalls is investigated; by matching the lateral etch rate and the deposition rate, etching selectivity of up to 7:1 is achieved, rendering smooth vertical sidewalls and damage-free upper portions for the etched structure.

256×256 focal plane array midwavelength infrared camera based on InAs/GaSb short-period superlattices

An infrared camera based on a 256×256 focal plane array (FPA) for the second atmospheric window (3–5 µm) has been realized for the first time with InAs/GaSb short period superlattices (SLs). The SL detector structure with a broken gap type-II band alignment was grown by molecular beam epitaxy on GaSb substrates. Effective bandgap and strain in the superlattice were adjusted by varying the thickness of the InAs and GaSb layers and the controlled formation of InSb-like bonds at the interfaces. The FPAs were processed in a full wafer process using optical lithography, chemical-assisted ion beam etching, and conventional metallization technology. The FPAs were flip-chip bonded using indium solder bumps with a read-out integrated circuit and mounted into an integrated detector cooler assembly. The FPAs with a cut-off wavelength of 5.4 µm exhibit quantum efficiencies of 30% and detectivity values exceeding 1013 Jones at T=77 K. A noise equivalent temperature difference (NETD) of 11.1 mK was measured for an integration time of 5 ms using f/2 optics. The NETD scales inversely proportional to the square root of the integration time between 5 ms and 1 ms, revealing background limited performance. Excellent thermal images with low NETD values and a very good modulation transfer function demonstrate the high potential of this material system for the fabrication of future thermal imaging systems.

Roughness and damage of a GaAs surface after chemically assisted ion beam etching with Cl 2/Ar +

The surface roughness and the near surface damage of semiconductor structures after etching are important criteria for applications in the field of opto- and micro-electronics. The chemically assisted ion beam etching (CAIBE) causes a surface roughness higher than that for ion beam etching (rms < 0.2 nm) but lower than that for chemical dry etching (rms > 5 nm). Compared to the roughness an inverse behaviour is observed for the damage of the near surface region. Physical processes like sputtering, implantation or diffusion cause heavy damage. The higher the chemical assistance of the physical ion etching, achieved at a high chlorine pressure or a high surface temperature, the higher the etch rate and the lower the damage. For the CAIBE process an average damage depth of about 200 nm was measured. The damage was evaluated by photoluminescence spectroscopy measurements on a multi quantum well structure of Al 0.35Ga 0.65As/GaAs after bevel etching of the surface to improve the depth resolution of the damage measurements.

Fabrication of two-dimensional InP-based photonic crystals by chlorine based chemically assisted ion beam etching

Two-dimensional photonic crystals (PhCs) were etched into InP/(Ga,In)(As,P) planar waveguides using chlorine-based chemical assisted ion beam etching (CAIBE). The processed PhCs were optically characterized by measuring transmission through simple slabs and one-dimensional cavities. The optical performances inside the photonic band gap are much better compared to both previously reported CAIBE results and results obtained with other etching methods. In particular, we measured a record quality factor of 310 for one-dimensional cavities fabricated in this material system.

Fabrication of hyperboloid-drum structure for electrically pumped laser of sub-micrometer to nanometer diameter active areas

This article proposes a method of fabricating a hyperboloid-drum structure, which can be applied to develop an electrically pumped laser with an active-layer diameter as small as 100 nm, while maintaining a wide contact area for easy electrical connection. The structure is fabricated using conventional photolithography and the chemically assisted ion-beam etching (CAIBE) process with an Ar:Cl2:BCl3=5:2:3 gas mixture. The process conditions for CAIBE and the size of etch mask are adjusted to obtain the required structure with a height of ∼8 μm, an active layer diameter of 0.1–2.0 μm, and a contact diameter larger than 4 μm. It is demonstrated that a laser device with an active layer diameter as small as 600 nm and a contact layer diameter of ∼5 μm can also be fabricated with the proposed method.

Fabrication of optical structures using SU-8 photoresist and chemically assisted ion beam etching

We develop a method for the fabrication of optical structures in GaAs substrates using UV holographic lithography in SU-8 resist, pro- cessed to fabricate a mask, followed by chemically assisted ion-beam etching (CAIBE). The technique is based on simple processing steps without procedures of mask transfer, enabling easy fabrication of optical structures. A predevelopment relief behavior is investigated to optimize the processing parameter to form an etching mask in SU-8. By adjusting both exposure dose and time in the postexposure bake (PEB), an SU-8 mask with a flexible duty cycle and high profile quality can be easily produced. Furthermore, an optical structure with a rectangular shaped profile and a 1-mm period in a GaAs substrate is produced by optimizing the processing parameters during the CAIBE process. © 2003 Society of

Development of a chemically assisted micro-beam etching system for three-dimensional microanalysis

A chemically assisted micro-beam etching system for 3D microanalysis was designed. Using chemically assisted ion beam etching (CAIBE) method with FIB shave-off scanning, about several hundred micrometers clean cross-section will be acquired in a few hours. We use focused ion beam (FIB) and electron beam (EB) as micro-beams, halogen or halide mainly as reactive gases. The apparatus was manufactured based on this concept. We found that the FIB, Q-MS and SED worked as expected. The instrumentation has been completed.

Fabrication and characterization of dry and wet etched InGaAs/InGaAsP/InP long wavelength semiconductor lasers

This paper presents fabrication and characterization of ridge waveguide InGaAs/InGaAsP/InP lasers with an operating wavelength of 1.5m using reactive ion etching (RIE), chemically assisted ion beam etching (CAIBE) and wet etching techniques. Characterization results of the lasers with 2m-wide waveguides are given of the two etching methods comparatively using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), L-I-V (Light-current-voltage) and spectral measurement techniques. Additionally, a comparison of wet and RIE etched lasers with 20m-wide waveguide is also discussed. Highly smooth (2.1+0.4nm rms surface roughness) and vertical (~90o) structures are obtained using RIE, in which the 2m-wide fabricated devices exhibit better performance over the CAIBE etched ones.

High-Brightness Unstable-Resonator Lasers Fabricated with Improved Dry-Etching Technology for Ultra-Smooth Laser Facets

In this article, we present the fabrication and investigation of high-brightness semiconductor lasers. Details about the epitaxial growth together with results of broad-area lasers are shown. For the fabrication of the dry-etched laser mirrors, a multilayer etch mask together with an optimized chemically-assisted ion-beam etching (CAIBE) process have been developed leading to ultra-smooth laser facets. With this technique, unstable-resonator lasers with curved mirrors have been fabricated. The devices were tested in pulsed and continuous wave operation, exhibiting high optical output powers. Corrected far fields were measured and the corresponding virtual source size was calculated to be in the range of 2 µm leading to a brightness of 60 MW/(cm2 sr). This was achieved in cw operation using only the main lobe intensity of the uncoated single-facet output power.

Deeply etched two-dimensional photonic crystals fabricated on GaAs/AlGaAs slab waveguides by using chemically assisted ion beam etching

High quality two-dimensional photonic crystals were fabricated by electron-beam lithography and a combination of reactive ion etching (RIE) and chemically assisted ion beam etching (CAIBE) in GaAs/AlGaAs slab waveguides. With optimized parameters, etch depths of more than 2 μm were achieved. This technique was used to fabricate three-dimensionally confined optical microcavities defined by a slab waveguide and a two-dimensional photonic crystal mirror structure. The optical quality of the deeply etched structures were tested by optical spectroscopy. The Q-factor of the cavity mode is dependent on the structural quality and depth of the holes in the photonic crystals. Due to the large etch depth and smooth surfaces, high quality planar microcavities could be realized with Q-factors of about 1000.

Dry Etching of Photonic Crystals in InP Based Materials

Photonic crystal (PC) etching in InP/GaInAsP using two different processes, namely Ar/CH4/H2 based Reactive Ion Etching (RIE) and Ar/Cl2 based Chemically Assisted Ion Beam Etching (CAIBE), is investigated in detail and the results are compared. Our goal was to identify the limits of the processes and to optimize process parameters for PC etching. With Ar/CH4/H2 RIE, we obtained PC holes with smooth profiles. However, the etch depth depends strongly on the hole diameter; the smaller the hole diameter, the smaller is the obtained hole depth. This together with the obtained hole profiles indicates the presence of an etch-limiting mechanism and is attributed to inefficient removal of etch-products. In the case of Ar/Cl2 CAIBE, we find that both shape and depth of the holes, depend on sample temperature, Cl2 flow and etching duration. By optimizing the process parameters, we show that it is possible to balance the physical and chemical components in the etch process. We demonstrate that Ar/Cl2 CAIBE is a promising process for PC etching in InP. With this process, we can obtain sufficiently deep holes (2.3–2.5 μm) even for hole diameters as small as 220 nm.

Low-Electron-Energy Cathodoluminescence Study Of Polishing And Etching Effects On The Optical Properties Of Bulk Single-Crystal Gallium Nitride

ABSTRACTLow electron energy cathodoluminescence (LEECL) was used to examine polishing-induced damage in a bulk high-pressure grown GaN single-crystal platelet. The Ga-polarity face of the platelet was mechanically polished; chemically-assisted ion-beam etching (CAIBE) to a depth of 200 nm was performed on a portion of this face. Low-temperature (15 K) CL spectra of the polished-only and polished+CAIBE regions of the Ga-face were taken at 2.8 kV, 5.4 kV, and 10.6 kV (corresponding to average electron penetration depths of 19 nm, 56 nm, and 170 nm). The low-temperature CL spectrum of the unpolished, N-polarity face was taken at 10.6 kV. In the near-band-edge region, all the CL spectra from the Ga-polarity face show a narrow peak near 3.47 eV, ascribed to donor-bound exciton recombination, and several overlapping peaks at lower energy (3.1 eV to 3.4 eV), ascribed to defect-related levels or to donor-acceptor pair recombination. Functional curve-fitting analysis enabled deconvolution of the spectra into the sum of an asymmetric peak (the donor-bound exciton) and several symmetric Gaussian peaks (the lower energy, defect-related or donor-acceptor peaks). The linewidth of the donor-bound exciton peak decreased with increasing penetration depth, and also decreased on going from the polished-only to the polished+CAIBE region. The relative intensity of a defect-related peak at ≈3.325 eV showed a similar decreasing trend with increasing penetration depth or with CAIBE treatment. The LEECL results suggest that the thickness of the polishing damage layer is approximately 400 nm; the 200 nm CAIBE step is thus insufficient to completely remove the damage.

Effect of Various Pre-Treatments on Ti/Al/Ti/Au Ohmic Contacts for AlGaN/GaN HFET Devices

The effect of a number of different treatments on the contact resistance for Ti/Al/Ti/Au (30/80/120/55 nm) contacts to Al0.3Ga0.7N (20 nm)/GaN HFET structures were investigated. The structures were grown by MBE. Neither preannealing of the surface at temperatures up to 1100 °C in N2, nor etching of the surface using chemically assisted ion beam etching (CAIBE) was effective in reducing the contact resistance. Both light CAIBE etching (≈5 nm) to partially remove the AlGaN layer, and deeper etching (≈50 nm and 100 nm) to remove the AlGaN and contact the GaN channel layer directly, resulted in a substantial increase in the measured contact resistance. The only treatment that was found to decrease the contact resistance was a brief wet etch in K2S2O8/KOH solution under 254 nm UV irradiation. A resist stabilization treatment was developed so that the process was self-aligned. The contact resistance in Ω mm was decreased by a factor of 1.7 for the optimum etch time. Scanning electron microscopy showed that the etching resulted in the formation of discrete holes perforating the AlGaN layer.

Effect of Various Pre-Treatments on Ti/Al/Ti/Au Ohmic Contacts for AlGaN/GaN HFET Devices

The effect of a number of different treatments on the contact resistance for Ti/Al/Ti/Au (30/80/120/ 55 nm) contacts to Al 0.3 Ga 0.7 N (20 nm)/GaN HFET structures were investigated. The structures were grown by MBE. Neither preannealing of the surface at temperatures up to 1100 °C in N 2 , nor etching of the surface using chemically assisted ion beam etching (CAIBE) was effective in reducing the contact resistance. Both light CAIBE etching (5 nm) to partially remove the AlGaN layer, and deeper etching (50 nm and 100 nm) to remove the AlGaN and contact the GaN channel layer directly, resulted in a substantial increase in the measured contact resistance. The only treatment that was found to decrease the contact resistance was a brief wet etch in K 2 S 2 O 8 /KOH solution under 254 nm UV irradiation. A resist stabilization treatment was developed so that the process was self-aligned. The contact resistance in Ω mm was decreased by a factor of 1.7 for the optimum etch time. Scanning electron microscopy showed that the etching resulted in the formation of discrete holes perforating the AlGaN layer.