Analog micro-optics fabrication by use of a two-dimensional binary phase-grating mask

The utilization of phase-mask technology for the fabrication of an analog micro-optics profile with a thick photoresist was investigated. A two-dimensional phase-grating mask with pi phase depth can produce a desired analog variation of exposure intensity, which allows one to vary the thickness of an analog photoresist after its exposure by a photolithographic stepper and development of the photoresist. A two-dimensional phase-grating mask of square pixels was simulated, designed, and fabricated. The fabrication of analog micro-optics in a thick SPR-220 photoresist by use of this phase mask was also demonstrated.     

Dry etching characteristics of GaN using Cl2/BCl3 inductively coupled plasmas

ICP power/RF power, operating pressure, and Cl₂/BCl₃ gas mixing ratio are altered to investigate the effect of input process parameters on the etch characteristics of GaN films. The etch selectivity of GaN over SiO₂ and photoresist is studied. Although higher ICP/RF power can obtain higher GaN/photoresist etch selectivity, it can result in faceting of sidewall and weird sidewall profile due to photoresist mask erosion. Etch rates of GaN and SiO₂ decrease with the increase of operating pressure, and etch selectivity of GaN over SiO₂ increases with the increasing operating pressure at fixed ICP/RF power and mixture component. The highest etch selectivity of GaN over SiO₂ is 7.92, and an almost vertical etch profile having an etch rate of GaN close to 845.3 nm/min can be achieved. The surface morphology and root-mean-square roughness of the etched GaN under different etching conditions are evaluated by atomic force microscopy. The plasma-induced damage of GaN is analyzed using photoluminescence (PL) measurements. The optimized etching process, used for mesa formation during the LED fabrication, is presented. The periodic pattern can be transferred into GaN using a combination of Cl₂/BCl₃ plasma chemistry and hard mask SiO₂. Patterning of the sapphire substrate for fabricating LED with improved extraction efficiency is also possible using the same plasma chemistry.     

Physical bounds of metallic nanofingers obtained by mechano-chemical atomic force microscope nanolithography

To obtain metallic nanofingers applicable in surface acoustic wave (SAW) sensors, a mechano-chemical atomic force microscope (AFM) nanolithography on a metallic thin film (50 nm in thickness)/piezoelectric substrate covered by a spin-coated polymeric mask layer (50-60 nm in thickness) was implemented. The effective shape of cross-section of the before and after etching grooves have been determined by using the AFM tip deconvolution surface analysis, structure factor, and power spectral density analyses. The wet-etching process improved the shape and aspect ratio (height/width) of the grooves and also smoothed the surface within them. We have shown that the relaxed surface tension of the polymeric mask layer resulted in a down limitation in width and length of the lithographed nanofingers. The surface tension of the mask layer can be changed by altering the initial concentration of the polymer in the deposition process. As the surface tension reduced, the down limitation decreased. In fact, an extrapolation of the analyzed statistical data has indicated that by decreasing the surface tension from 39 to 10 nN/nm, the minimum obtainable width and length of the metallic nanofingers was changed from about 55 nm and 2 μm to 15 nm and 0.44 μm, respectively. Using the extrapolation’s results, we have shown that the future SAW sensors buildable by this nanolithography method possess a practical bound in their synchronous frequency (∼58 GHz), mass sensitivity (∼6125 MHz-mm²/ng), and the limit of mass resolution (∼4.88 × 10⁻¹⁰ ng/mm²).     

Fabrication of Nanoimprint Stamp Using Interference Lithography

Interference lithography is used to fabricate a nanoimprint stamp, which is a key step for nanoimprint lithography. A layer of chromium in thickness of about 20 nm is deposited on the newly cleaned fused silica substrate by thermal evaporation, and a layer of positive resist in thickness of 150nm is spun on the chromium layer. Some patterns, including lines, holes and pillars, are observed on the photoresist film by exposing the resist to interference patterns and they are then transferred to the chromium layer by wet etching. Fused silica stamps are fabricated by reactive ion etching with CHF3/O2 as etchants using the chromium layer as etch mask. An atomic force microscope is used to analyse the pattern transfer in each step. The results show that regular hole patterns of fused silica, with average full width 143nm at half maximum （FWHM）, average hole depth of 76nm and spacing of 450nm, have been fabricated. The exposure method is fast, inexpensive and applicable for fabrication of nanoimprint stamps with large areas.     

利用准谐频叉指换能器实现GHz高频SAW滤波器

本文利用准谐频设计方法,在常规光刻工艺水平上,成功地实现了ＧＨＺ级声表面波（ＳＡＷ）器件,准谐频叉指换能器（ＱＨＩＤＴ）的优点为：其指条宽与指间隙可以比一般的单指换能器宽,克服了单指换能器内指间的多次反射,并且又较好地解决了普通谐频叉指换能器基频抑制不好的缺点,本文给出了制作在ＳＴ石英基片上,工作频率１１２８ＭＨＺ的声表面波滤波器的实验结果。     

Electromagnetic modeling of near-field phase-shifting contact lithography with broadband ultraviolet illumination

Near-field phase-shifting contact lithography is modeled to characterize electromagnetic absorption in a photoresist layer with one face in contact with a quartz binary phase-shift mask. The broadband ultraviolet illumination is represented as a frequency-spectrum of normally incident plane waves. A rigorous coupled-wave analysis is carried out to determine the absorption spectrum of the photoresist layer. The specific absorption rate in the photoresist layer is calculated and examined in relation to the geometric parameters. Columnar features in the photoresist layer are of higher quality on broadband illumination in contrast to monochromatic illumination, in conformity with some recent experimental results. Feature resolution and profile are noticeably affected by the depth of the grooves in the phase-shift mask. Ideally, the feature linewidth can be less than about 100 nm for broadband illumination in the transverse-magnetic mode. These conclusions are subject to modification by the photochemistry-wavelength characteristics of the photoresist.     

Demagnifying far field focusing spot to deep subwavelength scale by truncated hyperlens for nanolithography

A truncated hyperlens composed of pairs of metal-dielectric cylindrical multilayers is proposed to demagnify the diffraction limited spot to achieve deep subwavelength imaging. The diffraction limited spot is focused by far field converging cylindrical wave. Numerical simulations demonstrate that full width at half maximum (FWHM) of the image down to 32 nm (∼λ/11) can be achieved from object (∼λ/3) by 365 nm light illumination. It is discussed that the influence of size and focusing shift of the spot on those of the demagnification image on photoresist. It is also demonstrated that multi-objects can be demagnified and projected on the photoresist.     

Quantitative depth profiling of photoacid generators in photoresist materials by near-edge X-ray absorption fine structure spectroscopy

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to quantify the surface composition and depth profiling of photoacid generators in thin film photoresist materials by varying the entrance-grid bias of a partial electron yield detector. By considering model compositional profiles, NEXAFS distinguishes the surface molar excess within the top 6 nm from the bulk. A surface enriched system, triphenylsulfonium perfluorooctanesulfonate, is contrasted with a perfluorobutanesulfonate photoacid generator, which displays an appreciable surface profile within a 6 nm segregation length scale. These results, while applied to 193-nm photoresist materials, highlight a general approach to quantify NEXAFS partial electron yield data.     

Simulation of Birefringence and Absorption in InGaAs-GaAs Multiple Quantum Well Acoustooptic Modulators Operating Near 980 nm

This paper presents the simulation of surface acoustic wave (SAW)-induced absorption coefficient and refractive index change in InGaAs-GaAs multiple quantum well (MQW) structures operating near 980 nm. The exciton problem is solved in two dimensional momentum space to include the non-axial effect due to strain induced valance band mixing and nonparabolicity. The optical absorption coefficient and refractive index changes near the band gap in the MQWs are calculated as a function of SAW power.     

High-efficiency multilayer-coated ion-beam-etched blazed grating in the extreme-ultraviolet wavelength region

We describe a simple method to fabricate blazed gratings used in the extreme ultraviolet wavelength region. The method uses an argon and oxygen mixed-gas ion beam to directly etch the grating substrate through a rectangular profile photoresist grating mask. With this method the etched grating groove profile can be well controlled. An Mo/Si multilayer-coated specimen with a blaze angle of 1.9 degrees was fabricated and measured. At an incident angle of 10 degrees and a wavelength of 13.62 nm, the diffraction efficiency of the negative second order reaches 36.2%.     

全息光栅实时显影监测曲线的理论模拟

在特定的工艺条件下,光刻胶的非线性效应非常显著;合理地利用非线性效应,能够制作出近似矩形的全息光栅掩模。为了分析清楚非线性效应对光栅沟槽成形的作用机理,有必要建立一个显影理论模型。模拟得到的光栅轮廓和实验样片的扫描电子显微镜照片结果很吻合。根据这个模型,进而使用光栅严格理论,得到其主要特征与实时监测实验曲线一致的理论模拟监测曲线。理论分析和实验证实,该模型基本表征了工艺条件对光栅沟槽形状的影响,并揭示了光刻胶呈现显著非线性效应时,必然对应着明暗条纹中心位置之间的显影刻蚀速率相差很大。这个模型为全息光栅的工艺研究提供了一个有效的理论分析工具。     

嵌入压电基底蜂窝状声子晶体声表面波带隙特性

提出了将部分镍圆柱体镶入128°YX-LiNbO₃基底的二维蜂窝状压电型声子晶体。用有限元法结合低反射边界条件计算并分析了该结构的能带结构与传输损失。结果显示:与倒圆锥体结构相比,镶入型结构具有更低的声表面波(SAW)带隙,原因是将部分柱体嵌入到基底中增加了共振体的质量。通过分析带隙边缘处振动本征模态结合传输损失发现,第一SAW带隙是由局域共振机理打开,第二SAW带隙同时具有Bragg散射特性和局域共振特性。应用质量弹簧振动系统估算了第一SAW带隙的起始频率,结果为82.98 MHz。此外,讨论了柱体几何参数对带隙特性的影响。该研究为微米尺度SAW声子晶体的带隙调控奠定了一定基础。      

1.5μm波导声光可调滤光器研究

对１．５μｍ波导型声光ＴＥ－ＴＭ模式转换光滤波器进行了研究。采用了可降低驱动功率的表面声波导结构。在１．５￣１．６μｍ波段实现了可调滤光，通带宽度小于２ｎｍ，达到９７￥以上转换率所需的射频驱动功率约８０ｍＷ（１９ｄＢｍ），器件总插入损耗约９ｄＢ。     

Bach Fabrication of Microlens at the end of Optical Fiber using Self-photolithgraphy and Etching Techniques

This paper describes a simple batch process for fabrication of microlens and microlens array at the end of an optical fiber or an optical fiber bundle using self-photolithography and etching techniques. A photoresist micro-cylinder was exactly formed at the core of the fiber end by exposing an UV light from the other end of the fiber and conventional development, rinse processes. A photoresist microlens was formed by thermal reflowing of the fiber at 170°C for 1 h. A measurement of transmissivity showed that the fabricated photoresist microlens is applicable for a wavelength that is longer than 450 nm. Alternatively, a glass microlens was fabricated at the core of the fiber by dry etching with an SF₆ gas using the photoresist microlens as a mask. The focusing of the lensed fiber was confirmed and simulation work showed that the lensed fiber could focus the light with a beam spot of 2 μm, numerical aperture (NA) of 0.285 and a depth of focus of 16 μm.     

Development of a SAW duplexer with one-chip isolation circuits

We propose new structures of one-chip type SAW duplexers where Tx and Rx SAW ladder filters as well as isolation networks are fabricated together on a single 36 degrees LiTaO(3) piezoelectric substrate. The new SAW duplexer can overcome the difficulty in fabrication of conventional SAW duplexers while providing the performance matching that of conventional duplexers. Validity of the structure is verified through numerical simulation and experiments.     

Three-dimensional electromagnetic band-gap structure containing TiO2 fabricated by rapid-prototyping

Three-dimensional (3D) diamond structure electromagnetic band-gap (EBG) structures containing TiO₂ fabricated by rapid-prototyping (RP) technique were investigated. The simulations based on finite element method (FEM) were employed to model the band structures. The influence of aspect ratio on the band gap width was studied. The optimal band gap width EBGs were fabricated and investigated experimentally. Gel-casting together with RP technique were used in the fabrication. TiO₂ gel was cast into the diamond structure molds fabricated by RP method to obtain the green EBG structures. The transmission characteristics of the EBG structures were measured by transmission/reflection (T/R) methods using a vector network analyzer. Complete band-gap was observed in the transmission characteristics in the frequency from 11 GHz to 12 GHz, which agreed well with the simulation results.     

Ti-sapphire femtosecond pulse pumped laser emission from all-plastic organic waveguide with distributed feedback resonator

We demonstrated the spectrally narrowed laser emission from all-plastic organic waveguide with the distributed feedback (DFB) resonator pumped by Ti-sapphire femtosecond laser. We fabricated the DFB resonator on a surface of a photoresist polymer using an interference of laser beams. All-plastic waveguide with organic dyes dispersed in poly(N-vinylcarbazole) and polystyrene matrix was spin-coated on a DFB resonator. Very narrow full width at half maximum (FWHM) of 0.15 nm in emission wavelength was observed, whereas an excitation laser of Ti-sapphire femtosecond laser has broad FWHM of 14 nm.     

Filtering performance improvement in V-doped ZnO/diamond surface acoustic wave filters

High-frequency surface acoustic wave (SAW) filters using undoped and V-doped ZnO films were fabricated on diamond. Compared with their counterparts, the SAW filters using V-doped ZnO films have higher electromechanical coupling coefficient of ∼2.9% and lower insertion loss. The filtering performance improvement is considered to be due to the ferroelectricity in V-doped ZnO films and the resultant high piezoresponse (∼110 pm/V), which is one order of magnitude larger than that of undoped ZnO films. In addition, more perfect (0 0 2) preferred orientation, better uniform grains and smoother surface of V-doped ZnO films also contribute to the filtering performance improvement.     

Fabrication of high‐aspect‐ratio Fresnel zone plates by e‐beam lithography and electroplating

The fabrication of gold Fresnel zone plates, by a combination of e‐beam lithography and electrodeposition, with a 30 nm outermost zone width and a 450 nm‐thick structure is described. The e‐beam lithography process was implemented with a careful evaluation of applied dosage, tests of different bake‐out temperatures and durations for the photoresist, and the use of a developer without methylisobutylketone. Electrodeposition with a pulsed current mode and with a specially designed apparatus produced the desired high‐aspect‐ratio nanostructures. The fabricated zone plates were examined by electron microscopy and their performances were assessed using a transmission X‐ray microscope. The results specifically demonstrated an image resolution of 40 nm.     

全息光栅光刻胶掩模槽形演化及其规律研究

为了控制全息光栅光刻胶掩模槽形,运用曝光模型和显影模型模拟了掩模的槽形形成过程和变化规律。实验对比了不同曝光量,不同显影浓度,不同空频条件下的掩模槽形,特别是占宽比(光栅齿宽度与光栅周期的比)的情况。实验结果表明,实际槽形与模拟槽形很接近。模拟和实验均发现,在大曝光量、高浓度显影、高空频的光栅条件下所得槽形占宽比较小,槽深主要由原始胶厚决定。