Submicron Period Research Articles

Sub-micron periodic structuring of sapphire by laser induced backside wet etching technique

The periodic structuring of sapphire, by using laser induced backside wet etching technique (LIBWE) and 266 nm, 150 ps Nd:YAG laser radiation, is demonstrated here for first time. Sub-micron period Bragg reflectors are successfully patterned in sapphire wafers using modest energy densities and number of pulses. The gratings are characterized using diffraction efficiency measurements, AFM, and SEM. Issues related to the ablation process and to the phase mask holography are presented and discussed. The experimental results presented depict that the applied method is capable to produce relief structures of depth as deep as 100 nm, while maintaining high resolutions, close to the thermal diffusion length of the material corresponding to the ultrashort pulse duration.

Fabrication of a close-packed hemispherical submicron lens array and its application in photolithography

The fabrication and characteristics of close-packed lens arrays with a feature size close to the optical diffraction limit are presented in this study. By controlling the size of the submicron nickel rods and the time for reactive dry etching, the hemispherical lens array with a submicron period is made directly on a borosilicate glass. Finite-difference time-domain calculations and optical near-field measurements show that such a lens array can generate a subwavelength optical spot array near the glass surface. Moreover, the spot array periodically appears in the propagation direction. Using this novel optical property, we propose a photolithographic method for the mass-production of multilayer hexagonal structures with a period of 500nm.

Periodic nanostructuring of Er∕Yb-codoped IOG1 phosphate glass by using ultraviolet laser-assisted selective chemical etching

The patterning of submicron period (≈500nm) Bragg reflectors in the Er∕Yb-codoped IOG1 Schott, phosphate glass is demonstrated. A high yield patterning technique is presented, wherein high volume damage is induced into the glass matrix by exposure to intense ultraviolet 213nm, 150ps Nd:YAG laser radiation and, subsequently, a chemical development in potassium hydroxide (KOH)∕ethylenediamine tetra-acetic acid (EDTA) aqueous solution selectively etches the exposed areas. The electronic changes induced by the 213nm ultraviolet irradiation are examined by employing spectrophotometric measurements, while an estimation of the refractive index changes recorded is provided by applying Kramers-Kronig transformation to the absorption change data. In addition, real time diffraction efficiency measurements were obtained during the formation of the volume damage grating. After the exposure, the growth of the relief grating pattern in time was measured at fixed time intervals and the dependence of the grating depth on the etching time and exposure conditions is presented. The gratings fabricated are examined by atomic and scanning electron microscopies to reveal the relief topology of the structures. Gratings with average depth of 120nm and excellent surface quality were fabricated by exposing the IOG1 phosphate glass to 36 000 pulses of 208mJ∕cm2 energy density, followed by developing in the KOH∕EDTA agent for 6min.

Metal 1D micro(nano) self-organized wires on semiconductor surface: Preparation, topology, and optical properties

Preparation of selectively deposited wire-like 1D arrays of metal nanoparticles on patterned semiconductor surfaces is reported. A quasi-grating microrelief on the GaAs and InP (100) surfaces has been obtained by anisotropic etching in the mixture HF:H2SO4:H2O2 (2:2:1) and concentrated HCl, respectively. According to the AFM and SEM data, the quasi-periodic microrelief is a set of V-like grooves oriented along the [011] crystallographic direction, with a submicron period and a length over 10 μm. The gold coating has been grown by photostimulated chemical deposition. The surfaces with the quasi-periodic microrelief have been used as site templates for nanoparticle deposition. The EDX data show that gold particles are predominantly located at the microrelief tops and form a system of near-parallel wires. The metal-semiconductor system reflectance and the metal wire-like ensemble transmittance have been numerically simulated in the region of SPP excitation. The generalized effective-medium approximation has been invoked with regard to the gold coating anisotropy which results from the different electron scattering in the wire-parallel and perpendicular directions.

Measurement of period of interference patterns with sub-micron period

Measurement of period of interference pattern is desirable, while tuning the mirror interferometer for changing photo-written Bragg wavelengths during inscription of fiber Bragg gratings. The article presents the application of simple method, which uses a single lens, for the on-line measurement of the period of interference pattern with sub-micron wide period along with experimental results. Sub-micron period interference fringes are created using a mirror–beam splitter combination and biprism and a source at visible wavelengths. Analysis of experiments on magnifying, recording and measurement of period of these interference patterns using a CCD camera are presented and results are discussed. Its extension to magnifying and recording of sub-micron period fringes in UV region is discussed.

Sub-micron period grating structures in Ta2O5 thin oxide films patterned using UV laser post-exposure chemically assisted selective etching

A high-resolution and low-damage method for patterning relief structures in thin Ta2O5 films by chemically assisted UV laser selective etching is presented. The method is based on the initial exposure of the Ta2O5 films to pulsed UV radiation (quadrupled Nd:YAG laser at 266 nm) at fluences below the ablation threshold, for the creation of volume damage in the exposed areas. Subsequent immersion of the exposed sample in a KOH solution results in selective etching of the UV-exposed areas, developing relief structures of high quality. Interferometric exposure was used for the patterning of such gratings with periods of the order of 500 nm in films with a thickness of 100 and 500 nm. The behaviour of the patterning process is studied using diffraction efficiency measurements and AFM scans. Diffraction efficiency increases by a factor of ≈63, compared to the undeveloped structure, were obtained for gratings exposed with 1000 pulses of 30 mJ/cm2 energy density, which were developed in a KOH solution. The etching method presented is being applied to the fabrication of gratings in optical waveguides.

Excimer laser inscribed submicron period relief gratings in InOx films and overlaid waveguides

Relief gratings of submicron periodicity are patterned in InOx thin oxide films using 248 nm interferometric excimer laser ablation. The ablation process is studied in terms of grating depth versus exposure conditions, using optical diffraction efficiency measurements. Real time monitoring of grating growth and film resistivity during grating writing are also presented. To study the exact grating morphology, scanning electron microscopy and atomic force microscopy microscans of the machined structures are performed. A discussion on the ablation behavior of InOx thin films is given in accordance with the experimental data obtained. Relief gratings are patterned in InOx thin films overlaid on ion-exchanged channel waveguides, and reflection spectra are also reported.

Formation of two-dimensional periodic nanostructures on fused quartz, polyimide, and polycrystalline diamond by pulsed four-wave interference laser modification

Atomic force microscopy is used to examine the topography of submicron periodic structures formed on the surfaces of synthetic polycrystalline diamond and polyimide films. The films are deposited on fused quartz substrates by four-wave interference modification using a pulsed 308-nm UV XeCl excimer laser. It is demonstrated that a two-dimensional periodic relief with a submicron period can be formed on the diamond surface directly by laser evaporation in the absence of a photoresist. Depending on the exposure, two mechanisms of polyimide film modification are observed. At exposures less than 100 mJ/cm2, the relief is formed due to swelling at the positions of interference maxima. At exposures greater than 100 mJ/cm2, holes are formed in the films. A periodic relief on the fused quartz surface is formed by using a UV photoresist exposed to pulsed interference laser radiation and subsequent Ar ion etching.

Phase masks fabricated by interferometric lithography for working in 248 nm wavelength

We demonstrate that phase masks can be generated by using interferometric lithography along with the Taguchi method as an optimization tool for processing phase masks. The optimized fabrication process, allows phase masks of sub-micron period, centimeter long, with the zero-order intensity suppressed down to 8%. For the demonstration of its practicality, one optimized phase mask with 1.08 μm period and 5% zero-order diffraction efficiency is shown capable of fabricating fiber Bragg gratings with 7 dB transmission loss at 1.563 μm wavelength. Furthermore, another 0.44 μm period phase mask is used to produce a photoresist pattern with halved period.

Fabrication of sub-micron period diffraction gratings in self-processing sol–gel glasses

We demonstrated a single-step method for fabricating sub-micron period surface relief gratings. The process consists of illuminating an acrylate-modified silica–titania sol–gel glass through a phase mask with an excimer laser at 193 nm. Since the material is self-processing, no further etching step is necessary to reveal the smooth sinusoidal profile. A Corrugation period of 0.56 μm and groove depth of 115 nm is reported. The resistance of the gratings to temperature or solvents makes them competitive for various applications in micro-optics.

Fabrication and Surface Treatment of Semiconductor Photonic Crystals by Plasma Process.

Semiconductor photonic crystals are expected to realize the ultimate control of spontaneous emission, so-called Purcell effect. However, there are two important issues. One is a fine dry etching process for a high aspect ratio over 10 in submicron period structures. The other is a surface treatment for the reduction in surface recombination. With regard to these issues we have studied InP-based materials, since they have a low surface recombination velocity νs. We developed the etching process using the inductively coupled plasma. The surface recombination was evaluated through the lifetime measurement of photoluminescence at a wavelength over 1 µm using the phase resolved spectroscopy. We investigated various surface treatments and found that the CH4 plasma irradiation is effective for realizing crucial νs of 1×103 cm/s order. The SIMS analysis suggests that it is due to the electronic insulation near the surface by the shallow implantation of C.

Optical properties of three-dimensional photonic crystals based on III–V semiconductors at infrared to near-infrared wavelengths

The optical properties of three-dimensional photonic crystals based on III–V semiconductors are investigated. The crystals are constructed by stacking GaAs (or InP) stripes with a wafer-fusion technique to form an asymmetric face-centered-cubic structure. It is shown that a crystal with eight-stacked layers (two units), whose period is 4 μm, has a considerable band-gap effect (∼30 dB attenuation) in the transmission spectrum at infrared wavelengths (5–10 μm), and the band gap is observed independently of the incident angles. Then, a crystal with four-stacked layers with a submicron period is constructed, and a clear band gap (attenuation up to 10 dB) is successfully observed at near-infrared wavelengths (1–1.5 μm).

Neutral atom imaging (invited) (abstract)

The space plasma diagnostic technique of neutral atom imaging has its origins in the charge exchange analyzers developed for magnetic fusion. In both the space and laboratory applications, the objective is to detect and measure the energy spectrum of energetic neutrals escaping from a confined plasma. From an experimental standpoint, the technical challenges are similar: detect neutral atoms in the presence of intense charged particle and radiation backgrounds. With sufficient information about the spatial distribution of the neutrals and the plasma ions, it is possible to determine the ion temperature along the instrument’s line-of-sight from the neutral atom energy spectrum. However, the push towards remote imaging of plasmas in the space community has led to the development of multiple line-of-sight neutral atom detectors. These instruments are essentially neutral atom cameras. While deconvolving the image information into ion temperature or ion density spatial profiles is problematic, neutral atom imagers can provide a unique perspective on the large-scale dynamics and structure of complex plasma systems such as planetary magnetospheres. The current status of space-based neutral atom imaging will be reviewed, including data from current missions and expectations for the IMAGE mission to be launched in early 2000. The IMAGE instrument will utilize a new technique involving submicron period, free-standing, transmission gratings that block ultraviolet light while permitting the passage of neutral atoms. It is possible to apply the techniques of neutral atom imaging to laboratory plasmas and a conceptual design for a three-dimensional, divertor neutral atom imager will be discussed.

Excimer laser ablation of polymers and glasses for grating fabrication

In this paper we describe studies carried out related to grating formation for optoelectronics applications using 248 nm and 193 nm excimer lasers. These include basic studies of UV laser induced incubation and ablation of various glasses, and an evaluation of laser-induced-periodic-surface-structures (LIPSS) and UV holographic techniques based on a phase mask for forming gratings on polymers and in fibres. Modelling and experiments show that the use of excimer laser illuminated phase masks in a contact printing mode or Talbot reimaging arrangement provide a particularly effective approach to forming submicron period gratings.

A pixellated grating array using photoelectrochemical etching on a GaAs waveguide

We have demonstrated and evaluated a grating array outcoupler fabricated by photoelectrochemical (PEC) etching, a manufacturable and practical approach for fabrication of grating-based III-V semiconductor waveguide devices. An array of submicron period gratings was etched into photolithographically delineated areas in a single PEC step. The fabricated devices are: 10-/spl mu/m wide rib waveguides with 0.35-/spl mu/m first-order outcoupling gratings; and 10-/spl mu/m wide rib waveguides with 10 /spl mu/m/spl times/10 /spl mu/m pixellated areas of gratings. Device characterization demonstrates the effectiveness of outcoupling grating fabrication using PEC and that the pixellated grating outcoupler is an effective and simple means of generating an optical beam array. >

Characterization of a single-layer quantum wire structure grown directly on a submicron grating

A single AlGaAs/GaAs quantum well (QW)/quantum wire (QWR) structure was grown by metalorganic vapor phase epitaxy on a submicron period grating of V grooves. High resolution transmission electron microscopy studies of the sample identifies three regions of the QW; between the grooves, approximately 3.5 nm thick and oriented along (100), on the sidewalls of the V groove slightly thinner and oriented along {111}. At the bottom of the groove an approximately 70 nm wide crescent shaped region forms a QWR. In addition, a vertical quantum well (VQW) extends from the bottom of each V groove in the GaAs substrate to the surface. The luminescence spectra of the sample are dominated by a peak originating in the QW, with additional peaks of the QWR, the VQW and the AlGaAs barrier. The striped nature of the sample is revealed in the top view cathodoluminescence (CL) images of all four peaks. In side view CL images, the QWR emission appears spot like, whereas the emission of the VQW is elongated in the direction perpendicular to the surface. Photoluminescence excitation spectroscopy reveals that the main source for excitation in the QWR comes from the VQW, even though a small contribution comes from the QW.

Application of Selective Area MOVPE for DFB Gratings with Modulated Coupling Coefficient

ABSTRACTA new application of selective area metalorganic vapour phase epitaxy is demonstrated: fabrication of distributed feedback (DFB) gratings with sub-micron period and modulated coupling coefficient Kc. A SiO2 mask layer defines the InGaAs(P) grating pattern as well as the lateral variation in growth rate and, hence, thickness of the grating. In this way K can be varied along the laser cavity in a controlled and reproducible manner.

Selective-area MOVPE for InP-based optoelectronic components

Selective-area MOVPE of InP-based semiconductor material is discussed. As an example of its applicability, the use of the technique to make distributed feedback gratings with sub-micron period and modulated coupling coefficient k is described. The grating pattern and the lateral variation in growth rate of the InGaAs(P) material are defined and controlled by a SiO2 mask layer.

Maskless Single-Sided Wet Etching Process for the Fabrication of Ultra-Low Distortion Polyimide Membranes

AbstractA process for volume production of ultra-low distortion (<200 ppm), thin polyimide membranes on silicon wafers was developed using the RotoEtch™ dynamic fluid confinement tool. A critical advantage of the process is that it exposes the sample to the etching solution over a selected area on one side only, without contacting, wetting, or otherwise contaminating the front surface. This unique feature allows the etching away of a circular portion (over 40 mm diameter) of the backside of a patterned silicon wafer to form a freestanding thin polyimide membrane (<1 μim thick). The polyimide film is patterned prior to wet etching with a sub-micron period grating (200 nm period). The resulting distortion of the grating on the freestanding membrane is less than 200 ppm over the entire membrane area. This process seems ideally suited for instances—like the one above—when immersion, contacting, or contamination of one side of the sample would be impossible or impractical. It also allows backetching finished micro-structures that would otherwise be disturbed or destroyed by immersion in the fluid. Finally, it speeds up the fabrication of freestanding films since it does not require masking or any other form of front-side protection or backside lithographic steps. In this paper we report on a silicon through-etch process based on an HF:HNO3 acid mixture which typically forms membranes in only 10–20 minutes. Since polyimide easily distorts due to excessive heat or mechanical strain, the etching process needs to be carefully controlled. This process is also ideal for forming large membranes of other HF:HNO3-inert materials such as silicon carbide or diamond.

Real-time latent image monitoring during holographic fabrication of submicron diffraction gratings

Real-time monitoring of latent images in photoresist during the holographic exposure of submicron period diffraction gratings is reported. The first-order diffraction efficiency is recorded as a function of time for different resist thicknesses and polarizations. By monitoring the diffraction efficiency in real time, the process can be controlled to compensate for variations in exposure dose, resist thickness, and the optical properties of underlying materials. It is demonstrated that latent image monitoring (LIM) can be performed in real time and that the diffraction efficiency correlates well with the linewidth. To interpret diffraction efficiencies with changes in resist thickness and polarization, rigorous coupled wave analysis combined with a commercial software package to simulate the real-time data was used. However, the data show evidence for slow changes in the composition of the resist during exposure that are consistent with chemical intermediates reacting with water in the film. The commercial exposure simulation tool cannot account for these phenomena, which complicate the application of LIM to real-time exposure control.