Large area, 38 nm half-pitch grating fabrication by using atomic spacer lithography from aluminum wire grids

We wrapped 150 nm period aluminum wire grid polarizer (WGP) with AlSiOx by using atomic layer deposition at 250 degrees C. The nanometer precision coating defined the spacer to double the spatial frequency of the 100 mm diameter grating fabricated by using a legacy immersion holography setup at 351 nm wavelength. Half-pitch grating of approximately 38 nm was demonstrated with good pattern uniformity, excellent repeatability, and a wide processing window. We believe 10 nm half-pitch grating over even larger areas are viable, overcoming one major hurdle to commercialize nanoimprint.     

铝栅极降低氧化铟锡薄膜表面电阻

大面积钙钛矿电池组件效率一直比小面积电池最高效率低10%左右,其中透明导电氧化物(TCO)电极的横向电阻是构成串联电阻增加的主要因素。本文采用湿法刻蚀的方法在超白玻璃上制备了深达1-2 μm的凹槽,横向宽度最小为10 μm,随后蒸镀金属铝,射频溅射沉积氧化铟锡(ITO)薄膜,通过调控刻蚀各参数,得到良好的铝栅极形状。对比分析有无铝栅极的ITO薄膜,结果表明,有铝栅极的ITO薄膜相较无铝栅极的ITO薄膜,平均透过率下降约3.5%,横向电阻从17.4Ω降至2.4Ω,得到明显改善。并可通过调控刻蚀厚度,可使横向电阻进一步降低,从而显著提升ITO薄膜在太阳能电池领域里作为透明导电电极时横向电子收集能力,对以玻璃为基底的薄膜太阳能电池大面积产业化应用提供了可能。     

Aluminum nitride grating couplers

Grating couplers in sputtered aluminum nitride, a piezoelectric material with low loss in the C band, are demonstrated. Gratings and a waveguide micromachined on a silicon wafer with 600 nm minimum feature size were defined in a single lithography step without partial etching. Silicon dioxide (SiO(2)) was used for cladding layers. Peak coupling efficiency of -6.6 dB and a 1 dB bandwidth of 60 nm have been measured. This demonstration of wire waveguides and wideband grating couplers in a material that also has piezoelectric and elasto-optic properties will enable new functions for integrated photonics and optomechanics.     

Influence of surface roughness on the polarimetric characteristics of a wire-grid grating polarizer

The influence of surface roughness on the polarimetric performance of a wire-grid polarizer (WGP) is numerically investigated using rigorous coupled-wave analysis over 100 random surface realizations. Surface roughness is modeled with a Gaussian surface, represented by two independent parameters: surface height deviation and correlation length of a profile. The results show that WGP performance can suffer from significant degradation as well as increased deviation with surface roughness, although the extent varies with specific parameters. The influence of roughness was also examined with respect to grating period as a WGP parameter and incident light properties, such as wavelength and angle.     

Narrowband multichannel filters and integrated optical filter arrays

We propose and demonstrate three approaches to achieve narrowband multichannel filters. These are multiple heterostructures with defects, guided-mode resonance (GMR) Brewster filters with multiple channels, and integrated narrow bandpass filter arrays. Transmission studies for multiple heterostructures with defects are presented. We show that the enlargement of the forbidden band and multiple-channel filtering can be reached simultaneously with these configurations. GMR Brewster filters with multiple channels can be obtained with a single-layer grating. The same properties can be obtained by use of double-layer structures that consist of a homogeneous layer and a grating with equal refractive index. We developed a combinatorial etching technique that has 32 elements on a single substrate with which to fabricate integrated narrow bandpass filters. Single- and double-chamber integrated optical filter arrays were fabricated by use of this etching technique. These narrowband multichannel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.     

Application of wire-grid polarizers to projection displays

We have measured the optical properties of wire-grid polarizers (WGPs). The implications of these results to the application of WGPs in projectors that use reflective light valves are discussed. In particular, the brightness and the contrast ratio of the projection system are investigated as functions of the angle of incidence of the light beam onto the WGP. It was found that the optimal incident angle is dependent on the physical design of the wire grids. In the sample that we describe, the optimal incident angle was 35 degrees instead of 45 degrees. At the optimal incident angle, both the transmission and the reflection extinction ratios can be quite good. However, WGPs suffer from the drawback of free-carrier absorption by the metal grid.     

Sputtered Ni-Cr-based resistors terminated on sputtered aluminum and screeen-printed Pt-Ag conductors

The standard technology to fabricate thin film resistors is to deposit a layer of resistive material, usually by evaporation or sputtering, followed by thin layers of nickel and gold. Gold plating is then used to augment the thickness of the gold layer, typically to 1 μm. The three-layer structure is then selectively etched to yield resistors with terminations suitable for wire bonding and soldering. We have used a somewhat unconventional approach in which a film 1–2 μm thick of aluminum is first sputtered onto 99.5% Al₂O₃ substrates, then etched to produce the desired conductor pattern. Isotropic (wet) etching is used to produce sloping edges to contact better the Ni-Cr based resistor material which is sputter deposited and patterned using a lift-off technique.We have also combined the thin and thick film technologies by terminating our thin film resistors on Pt-Ag conductors which are screen printed onto the substrate prior to sputter deposition of the resistors.Aluminum terminations can be wire bonded, or, when soldering is required, coated with sputtered or plated nickel. When a layer of suitable adhesion promoter is sputtered between aluminum and nickel, adhesion of aluminum to nickel is greatly improved.The use of a sputtering target made of an Ni-Cr-based alloy has enabled us to obtain resistors in the 50 Ω/□ range which can be mass produced with near-zero temperature coefficient of resistance (TCR). Such resistors show stability within better than 0.02% after 1000 h at 398 K under load. Also reported are resistance spread and shift, due to annealing, as a function of aspect ratio (ratio between length and width of resistor) for both aluminum and Pt-Ag terminations, as well as TCR as a function of annealing temperature.     

M‐shaped Grating by Nanoimprinting: A Replicable,Large‐Area,Highly Active Plasmonic Surface‐Enhanced Raman Scattering Substrate with Nanogaps

Plasmonic nanostructures separated by nanogaps enable strong electromagnetic‐field confinement on the nanoscale for enhancing light‐matter interactions, which are in great demand in many applications such as surface‐enhanced Raman scattering (SERS). A simple M‐shaped nanograting with narrow V‐shaped grooves is proposed. Both theoretical and experimental studies reveal that the electromagnetic field on the surface of the M grating can be pronouncedly enhanced over that of a grating without such grooves, due to field localization in the nanogaps formed by the narrow V grooves. A technique based on room‐temperature nanoimprinting lithography and anisotropic reactive‐ion etching is developed to fabricate this device, which is cost‐effective, reliable, and suitable for fabricating large‐area nanostructures. As a demonstration of the potential application of this device, the M grating is used as a SERS substrate for probing Rhodamine 6G molecules. Experimentally, an average SERS enhancement factor as high as 5×10⁸ has been achieved, which verifies the greatly enhanced light–matter interaction on the surface of the M grating over that of traditional SERS surfaces.     

Interference grating fabrication in spin-coated As2S3 films

The optical properties and solubility of spin-coated As₂S₃ films significantly change after silver is introduced by photodiffusion. The different types of holographic grating that can be fabricated in spin-coated Ag/As₂S₃ double film structures by photodiffusion and subsequent selective etching are described in this paper. The silver diffusion occurs under the influence of visible monochromatic light and the grating pattern is produced by a holographic arrangement.     

Stacked subwavelength gratings as circular polarization filters

We have stacked subwavelength gratings (SWGs) on a single substrate to create a compact, integrated circular polarization filter. The SWGs consist of a wire grid polarizer and a broadband form-birefringent quarter-wave plate (QWP). Rigorous coupled-wave analysis was used to design the QWP for operation over the 3.5-5.0-mum wavelength range. The fabricated silicon broadband QWP exhibited a phase retardance of 82-97 degrees across this wavelength range. Two stacked structures are presented, each with a different wire grid polarizer fabricated on an organic planarization layer (SU-8) that is deposited on a QWP grating. Transmittance measurements of the first structure when illuminated with nominally right- and left-circularly polarized light indicate a circular extinction ratio (CER) limited by the low linear extinction ratio of the polarizer. Use of a wire grid polarizer with a higher extinct ratio led to a stacked SWG structure that demonstrated CERs of 10-45 across the 3.5-5.0-mum wavelength range.     

Comparison of mechanically ruled versus holographically varied line-spacing gratings for a soft-x-ray flat-field spectrograph

Recent progress in the design of aspheric wave-front recording systems has permitted the manufacture of holographic gratings with highly variable groove densities that are suitable for flat-field spectrographs. A holographic grating thus recorded was processed to produce a laminar profile by use of reactive-ion etching. Measurements are reported of the absolute diffraction efficiency of this grating and of a comparable mechanically ruled grating. It is found that the holographic grating is much more effective in suppressing the higher orders. The spectral resolution was determined by use of a carbon Kalpha x-ray generator and a spectrograph with an imaging detector. The spectral resolution of the holographic grating was approximately 3 times worse than that of the ruled grating.     

Micropolarizer fabricated from CaCO3 by anisotropic wet etching

A micropolarizer is fabricated on a birefringent crystal, calcite, by anisotropic wet etching. This device consists of a v-groove grating on calcite, covered by an index-matching material. The grating is fabricated by acid wet etching. When its acid concentration and stirring speed are altered, the etching mechanism can be controlled within the surface-reaction regime. This results in anisotropic etching, which produces a v-groove grating on a calcite surface. This v-groove grating can be fabricated to have a period as small as 2 microm. To the best of our knowledge, this type of v-groove calcite grating is reported for the first time. Although the transmission efficiency of this device is wavelength dependent, a broadband micropolarizer can be made by gluing together two devices with periods of 12 and 16 microm.     

Tungsten wire grid polarizer for applications in the DUV spectral range

In this paper, we present a broadband wire grid polarizer with a spectral working range down to a wavelength of 193 nm. Tungsten is chosen as grating material because it provides a high extinction ratio and transmission compared with other common grating materials. The fabrication of the grating with 100 nm period was accomplished using a spatial frequency doubling approach based on ultrafast electron beam lithography and a sophisticated deposition technique. At a wavelength of 193 nm, a transmission of about 44% and an extinction ratio of 20 was measured.     

表面和亚表面特性对多层膜光栅成像对比度的影响

基于Si/SiO2材料对制备出名义节距为50 nm的多层膜光栅,重点分析了多层膜光栅研磨抛光过程中的亚表面损伤和湿法刻蚀均匀性问题。并利用原子力显微镜（AFM）和透射电子显微镜（TEM）对多层膜的截面粗糙度和刻蚀光栅结果进行了测量和分析。测量结果显示:多层膜光栅制备过程中的截面粗糙度降低和刻蚀均匀性的提高,有助于TEM测量获得均一的高成像对比度多层膜光栅图像。     

Nanopatterned silicon emitters of a solar cell fabricated by anodic aluminum oxide masks

We investigated the nanopattern transferring process by a template of anodic aluminum oxide and the formation of a nanoporous aluminum oxide layer on a Si solar cell by the anodization process of Al thin films. The anodization process provided a template to transfer the nanopattern onto the Si surface. The small-sized nanoporous alumina template was attached to be covered on the textured surface and played the role of etching mask in the F-based dry etching process. Furthermore, we deposited an Al thin film onto the Si surface and the subsequent anodization process was performed. The alumina formulated on the deposited Al thin film did not show the array of nanoporous structure and no nanopatterns were transferred onto the surface. The large-areal alumina deposited on the Si surface showed enhanced photo-absorption in the ultraviolet spectral region of 243 nm, but increased the photo-reflectance in the visible and infrared spectral regions when compared to the Si-bare sample.     

基于相移布喇格光纤光栅的折射率传感实验研究

利用图解法求解光纤特征方程,获得腐蚀过程和传感过程中光纤光栅的光谱特性,设计了光纤光栅的部分腐蚀方案,采用氢氟酸溶液腐蚀制作相移布喇格光纤光栅。对光纤光栅进行部分腐蚀,将形成相移光纤光栅。根据相移光纤光栅的特性,讨论了利用反射光光强解调的方式,该方式具有结构简单且对温度不敏感的优点。     

Single-shot fabrication of waveguide ternary gratings with narrow-band optical response

Single-shot fabrication of three sets of nano-scale grating structures with different periods is reported, which are constructed on a glass substrate coated with a waveguide layer made of 200-nm-thick indium tin oxide (ITO). Multiple waveguide resonance modes are observed in the visible spectral range with a bandwidth as narrow as 10 nm. Angle-resolved tuning properties of these resonance modes enable simultaneous three-color optical response of the nanodevice to cover the whole visible spectrum. This implies very simple methods for the potential design and realization of flexible optoelectronic devices.     

Optical monitoring of the end point in thin film plasma etching

An in-process optical technique is described for accurately monitoring the end point in plasma etching processes. A grating pattern is lithographed somewhere in the film to be etched. The grating modulation decreases as the film is etched out and the process may be monitored by measuring the diffraction of a low power He-Ne laser beam aimed at the grating. The etching end point is accurately detected by the disappearance of all diffracted orders. The laser beam does not need to be directed at normal incidence and so any available plasma etching equipment is suitable. The detection is carried out with low cost photovoltaic detectors but simple visual inspection is satisfactory also.Comparative experimental results are presented.     

Spatial patterning of colloidal nanoparticle-based thin film by a combinative technique of layer-by-layer self-assembly and lithography

A novel approach to generating clear patterns of different types of nanoparticles is presented in this paper. Nanoassembly in the vertical direction was combined with planar micropatterning. This provides industrial applications of a popular layer-by-layer method to produce multilayers of polymers, nanoparticles, and proteins organized on the nanometer scale. A thin film of organic polystyrene spheres was first coated on the pretreated silicon wafer with layer-by-layer self-assembly. Then a layer of aluminium was deposited on the thin film. A layer of positive photoresist was spun on the surface of aluminum and then illuminated with UV light. The exposed parts of the resist were removed and windows were opened above the aluminum. The subsequent etching removed exposed aluminium and left a polystyrene thin film in the open windows. Oxygen plasma was employed to remove the polystyrene thin film on the bottom. Eventually, aluminum and photoresist were removed and only the desired pattern remained. This approach was also employed for the patterning of the silica nanoparticle thin film, a widely used material in various applications. In this case, wet etching was demonstrated to etch silica particles. A scanning electron microscope was used to produce the image of the pattern.     

F2-laser-machined submicrometer gratings in thin dielectric films for resonant grating waveguide applications

Surface-relief gratings with submicrometer modulation periods were ablated by F2-laser radiation in thin metal-oxide films to produce resonant grating waveguide structures. For 150 nm films of Nb2O5, grating amplitudes in the range of 5-50 nm could be reproducibly excised with a controlled exposure of a laser energy density and a number of pulses within a narrow processing window. Resonant coupling of 800 nm ultrashort pulsed laser light into the resulting grating waveguide structure is verified with reflection and transmission spectra and satisfactorily modeled by coupled-mode theory. The laser-fabricated grating waveguides are attractive for high damage threshold reflectors and biosensor applications.