SELF-ORGANIZED FORMATION OF HEXAGONAL NANOPORE ARRAYS IN ANODIC ALUMINA

Conditions for a self-organized formation of ordered hexagonal structure in anodic alumina were investigated, using oxalic or sulphuric acid as an electrolyte. Highly-ordered nanopore arrays with pore densities of 9×10⁹-6.5×10¹⁰cm^-2 and high aspect ratios over 3000 were fabricated by a two-step anodization process. The array exhibits characteristics analogous to a two-dimensional polycrystalline structure of a few micrometres in size. The interpore distance can be controlled by changing the electrolyte and/or the applied voltage. The formation mechanism of ordered arrays is consistent with a previously proposed mechanical stress model, i.e., the repulsive forces between neighbouring pores at the metal/oxide interface promote the formation of hexagonally ordered pores during the oxidation process.     

Large-area In2O3 ordered pore arrays and their photoluminescence properties

Large-area In₂O₃ ordered pore arrays were prepared on glass and silicon substrates by the sol–gel technique based on colloidal monolayer spheres. The morphologies of such arrays are determined by precursor concentration used and colloidal sphere size, and are thus controllable. It has been shown that the formed ordered pore arrays consist of In₂O₃ polycrystallites. The photoluminescence measurement of the In₂O₃ ordered pore arrays shows that there is a strong photoluminescence band in the blue-green region centered around 465 nm, which does not exist in the bulk materials. Further experiments reveal that this peak originates from the oxygen deficiencies in In₂O₃ skeletons. This polydomain ordered pore-structured array could be of great potential for Si-based integrated nanophotonics and optoelectronic devices of the next generation, in addition to new gas sensors. PACS  01.30.-y; 78.20.-e; 78.55.-m; 78.55.Et     

Fabrication of high-aspect-ratio silicon nanopillar arrays with the conventional reactive ion etching technique

We fabricate silicon nanopillar arrays with pillar diameters smaller than 200 nm by using the conventional reactive ion etching (RIE) technique and nickel masks. We use the ratio between the lateral and vertical etching rates as an estimate of the etching anisotropy. The dependence of this ratio on the rf power, the chamber pressure, and the gas mixture is investigated systematically to achieve the largest etching anisotropy. Using the optimized etching parameters in the RIE process, we demonstrate silicon pillars with smooth surface, vertical sidewalls, and aspect ratios higher than 20. In addition, we employ dilute aqua regia to treat the pillars and shrink the diameters to 70 nm. The pillar height remains ∼2500 nm after the treatment. PACS 52.77.Bn; 81.65.Cf; 85.40.Hp     

Reversible creation of nanostructures between identical or different species of materials

In this study, accurate nanostructures with various aspect ratios are created on several types of material. This work is highly applicable to the energy, optical, and nano-bio fields, for example. A silicon (Si) nano-mold is preserved using the method described, and target nanostructures are replicated reversibly and unlimitedly to or from various hard and soft materials. It is also verified that various materials can be applied to the substrates. The results confirm that the target nanostructures are successfully created in precise straight line structures and circle structures with various aspect ratios, including extremely high aspect ratios of 1:18. It is suggested that the optimal replicating and demolding process of nanostructures with high aspect ratios, which are the most problematic, could be controlled by means of the surface energy between the functional materials. Relevant numerical and analytical studies are also performed. It is possible to expand the applicability of the nanostructured mold by adopting various backing materials, including rounded substrates. The scope of the applications is extended further by transferring the nanostructures between different species of materials including metallic materials as well as identical species.     

Micromachined ultrasonic transducers and arrays based on piezoelectric thick film

This paper reports on the design, fabrication, and characterization of diaphragm-type piezoelectric micromachined ultrasonic transducer (pMUT) and arrays. A combination of piezoelectric composite thick film techniques and silicon micromachining has been proven to be a promising approach for a batch production of pMUTs, especially pMUT transmitter arrays for ultrasound radiation. In this paper, some important issues related to the pMUT element design and micromachining processes are addressed. Thanks to the well-developed processing technology, pMUTs and arrays operating at different resonance frequencies by dimension variation have been successfully fabricated with a high yield for possible mass manufacturing. The characterization of piezoelectric composite thick film will be briefly reported. The performance of the prototype devices has been characterized in terms of vibration modes, dependency of the resonance frequency on bias voltage, nonlinearity, electromechanical coupling efficiency, equivalent circuit, output sound pressure level and directivity of a two-dimensional pMUT array. PACS 85.50.-n; 85.85.+j     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

Direct writing of microtunnels using proton beam micromachining

The production of high aspect ratio microstructures is a potential growth area. The combination of deep X-ray lithography with electroforming and micromolding (i.e. LIGA) is one of the main techniques used to produce 3D microstructures. The new technique of proton micromachining employs focused MeV protons in a direct write process which is complementary to LIGA. During ion exposure of positive photoresist like PMMA, scission of molecular chains occurs. These degraded polymer chains are removed by the developer. The aim of this paper is to investigate the capabilities of proton micromachining as a lithographic technique. We show the realization of sub-surface channels, or microtunnels, which have been fabricated in only one exposure and without cutting or resurfacing the material. Using our Van-de-Graaff accelerator, the resist (PMMA) has been exposed with high-energy protons (2.5 MeV). The range of charged particles in matter is well-defined and depends on the energy. Therefore, it is possible to obtain a dose which is sufficient to develop the bottom part of the ion paths but not the top part. Thus, by selecting the energy and the exposure time, a big variety of microtunnels can be realized.     

The study of the solution concentration influencing on laser-induced electrochemical etching silicon

The laser electrochemical etching process, which combines the laser direct etching process and the electrochemical etching process, is a compound etching technique. In order to further understand the solution concentration influencing on the laser-induced electrochemical etching of silicon; a 248 nm excimer laser as a light source and KOH solution as an electrolyte were adopted in this study. The experiments of micromachining silicon by laser-induced electrochemical etching were carried out. On the basis of the experiments results, the solution concentration influencing on the etching rates in the process of laser electrochemical etching of silicon was researched. The reasons of the etching phenomena were analyzed in detail. The experimental results indicate that the solution concentration influencing on the etching process is mainly rooted in the absorption of different concentration solutions to laser. In general, less absorption and low solution concentration are good for the etching role in the process of laser electrochemical etching.     

大面积高深宽比硅微通道板阵列制作

利用光辅助电化学刻蚀方法,在厚度为425μm的5英寸硅片上,制作成深宽比达50以上的微通道板阵列结构.理论分析了影响微孔阵列形貌形成的关键因素,并结合实验条件,通过调整刻蚀电压值和根据莱曼模型修正实验电流值得到理想的孔壁形貌.结果表明,相比于目前在硅基上制作高深宽微结构的几种技术,光辅助电化学刻蚀方法能够实现孔壁光滑、面积大和深宽比高的微通道板阵列结构的低成本制作.     

Development of x-ray scintillator functioning also as an analyser grating used in grating-based x-ray differential phase contrast imaging

In order to push the grating-based phase contrast imaging system to be used in hospital and laboratories,this paper designs and develops a novel structure of x-ray scintillator functioning also as an analyser grating,which has been proposed for grating-based x-ray differential phase contrast imaging. According to this design,the scintillator should have a periodical structure in one dimension with the pitch equaling the period of self-image of the phase grating at the Talbot distance,where one half of the pitch is pixellated and is made of x-ray sensitive fluorescent material,such as CsI(Tl),and the remaining part of the pitch is made of x-ray insensitive material,such as silicon. To realize the design,a deep pore array with a high aspect ratio and specially designed grating pattern are successfully manufactured on 5 inch silicon wafer by the photo-assisted electrochemical etching method. The related other problems,such as oxidation-caused geometrical distortion,the filling of CsI(Tl) into deep pores and the removal of inside bubbles,have been overcome. Its pixel size,depth and grating pitch are 3 μm×7.5 μm,150 μm and 3 μm,respectively. The microstructure of the scintillator has been examined microscopically and macroscopically by scanning electron microscope and x-ray resolution chart testing,respectively. The preliminary measurements have shown that the proposed scintillator,also functioning as an analyser grating,has been successfully designed and developed.     

ns脉冲激光对K9玻璃的破坏实验

采用高速PIN光电探测器和高带宽的数字存储示波器,实时检测透射光脉冲和散射光脉冲的变化特征,并将之用作材料破坏的光学判据,测量得到K9玻璃在1.06μm纳秒脉冲激光作用下的能量损伤阈值约18mJ,相应的能量密度阈值为1.0kJ/cm2。通过分析透射光脉冲和散射光脉冲的特征,给出了材料的破坏时刻,并推断出K9玻璃所能承受的极限光强为1015W/m2。研究了能量透过率与泵浦能量的关系,并初步探讨了透明材料的破坏机理。结果表明:在多纵模激光的作用下,透明光学材料破坏是电离击穿与自聚焦效应综合作用的结果。     

一种简单高效的制备硅纳米孔阵结构的方法

本文介绍了一种简单高效的制备硅纳米孔阵结构的方法. 利用激光干涉光刻技术, 结合干法和湿法刻蚀工艺, 直接将光刻胶点阵刻蚀为硅纳米孔阵结构, 省去了图形反转工艺中的金属蒸镀和光刻胶剥离等必要步骤, 在2英寸的硅 (001) 衬底上制备了高度有序的二维纳米孔阵结构. 利用干法刻蚀产生的氟碳有机聚合物作为湿法刻蚀的掩膜, 以及在干法刻蚀时对样品进行轻微的过刻蚀, 使SiO₂点阵图形下形成一层很薄的硅台面, 是本方法的两个关键工艺步骤. 扫描电子显微镜图片结果表明制备的孔阵图形大小均匀, 尺寸可控, 孔阵周期为450 nm, 方孔大小为200–280 nm. 关键词： 激光干涉光刻 纳米阵列 刻蚀 氟碳有机聚合物     

Hybrid pulsed laser deposition and Si-surface-micromachining process for integrated TiC coatings in moving MEMS

Titanium carbide (TiC) is one of the preferred coatings for improving the performance of macroscopic moving mechanical components due to its established wear-resistance. Pulsed laser deposition (PLD) is an excellent method for depositing TiC, because unlike any other deposition process for TiC, PLD offers the capability of producing high-quality films even at room temperature. Using a modified PLD technique, especially designed for the deposition of particulate-free films, TiC coatings have been deposited at room temperature on silicon (Si) and on several types of thin films typically employed for fabricating microelectromechanical systems (MEMS). Our results demonstrate that TiC coatings also offer a high wear-resistance to Si surfaces, which in turn has led to our application of TiC to “moving” Si MEMS devices. The performance of moving Si MEMS devices is limited by their poor operational lifetimes, which have been attributed to the excessive wear at sliding Si interfaces. The work presented here describes a hybrid process, whereby PLD is used in conjunction with a user-friendly Si surface micromachining scheme for inserting wear-resistant TiC coatings between critical sliding Si interfaces in MEMS devices. This paper describes the properties of PLD-TiC for MEMS and the hybrid PLD-surface micromachining process for the integration of TiC coatings into Si MEMS. Received: 23 January 2003 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-310/563-7614, E-mail: gouri.radhakrishnan@aero.org     

Enhanced Field Emission from Large-Area Arrays of W18O49 Pencil-Like Nanostructure

Field enhancement and field screening are two major factors affecting field emission performance of arrays of quasi one-dimensional nanostructures. We have observed enhanced field emission from large-area arrays of W₁₈O₄₉ pencil-like nanostructure due to both the effects of high aspect ratio and enlarged spacing between neighboring nanostructures. These arrays may be grown on silicon substrates by the multi-step thermal evaporation process. The spacing of nanotip-to-nanotip between neighboring nanostructures may be increased by adjusting the growth temperature. The arrays are observed to have a typical turn-on field as low as about 1.26 MV/m and a threshold field as low as about 3.39 MV/m, resulting in increasing field enhancement and decreasing field screening effect.     

Si nanopillar arrays with nanocrystals produced by template-induced growth at room temperature

Well-aligned and closely-packed silicon nanopillar (SNP) arrays are fabricated by using a simple method with magnetron sputtering of Si on a porous anodic alumina (PAA) template at room temperature. The SNPs are formed by selective growth on the top of the PAA pore walls. The growth mechanism analysis indicates that the structure of the SNPs can be modulated by the pore spacing of the PAA and the sputtering process and is independent of the wall width of the PAA. Moreover, nanocrystals are identified by using transmission electron microscopy in the as-deposited SNP samples, which are related to the heat isolation structure of the SNPs. The Raman focus depth profile reveals a high crystallization ratio on the surface.     

Inherent diode isolation in programmable metallization cell resistive memory elements

The feasibility of a storage element with inherent rectifying or isolation properties for use in passive memory arrays has been demonstrated using a programmable metallization cell structure with a doped (n-type) silicon electrode. The Cu/Cu–SiO₂/n-Si cell used in this study switches via the formation of a nanoscale Cu filament in the Cu–SiO₂ film which results in the creation of a Cu/n-Si Schottky contact with soft reverse breakdown characteristics. The reverse bias leakage current in the on-state diode is dependent on the programming current employed as this influences the area of the electrodeposit and hence the area of the Cu/n-Si junction. The programming current also controls the on-state resistance of the device, allowing multi-level cell (MLC) operation, in which discrete resistance levels are used to represent multiple logical bits in each physical cell. The Cu/Cu–SiO₂/n-Si elements with heavily doped silicon electrodes were readily erasable at voltage less than −5 V which allows them to be re-programmed. Lightly doped silicon electrode devices were not able to be erased due to their very high reverse breakdown voltage but exhibited extremely low leakage current levels potentially allowing them to be used in low energy one-time programmable arrays.     

不同气氛下飞秒激光诱导硅表面微结构

利用钛宝石飞秒激光脉冲对单晶硅在SF6、空气和真空环境中进行了累积脉冲辐照,研究了硅表面微结构的演化。在SF6气氛中,在激光辐照的初始阶段,硅表面形成了1维的波纹结构,随着辐照脉冲数的增加,波纹结构演化成了2维凹凸结构。累积600个脉冲后,硅表面产生了准规则排列且具有大纵横比的锥形尖峰结构。该结构呈现高度相对较低、锥形尖端小球不明显的特征,分析认为主要与环境气压的大小有关。对比空气、SF6和真空中的微结构发现,尖峰的数密度依次减小;SF6中形成的尖峰高度最大,其次为真空,再次为空气。研究结果表明,真空、SF6和空气3种环境下微结构的形成及表面形貌主要由激光烧蚀、化学刻蚀和氧化决定。     

Influence of successive electron and laser irradiation on the photoluminescence of porous silicon

The influence of electron irradiation on the light-emitting properties of p-and n-type porous silicon prepared by electrochemical etching is investigated. The dose and energy dependences of the electron-stimulated quenching of the photoluminescence (PL) are determined. It is shown that electron treatment of a porous silicon surface followed by prolonged storage in air can be used to stabilize the PL. The excitation of photoluminescence by a UV laser acting on sections of porous silicon samples subjected to preliminary electron treatment is discovered for the first time. The influence of the electron energy and the power of the laser beam on this process is investigated. The results presented are attributed to variation in the number of radiative recombination centers as a result of the dissociation and restoration of hydrogen-containing groups on the pore surface. Zh. Tekh. Fiz. 68, 58–63 (March 1998)     

Silicon microlens structures fabricated by scanning-probe gray-scale oxidation

We report on the micromachining of silicon microlens structures by use of scanning-probe gray-scale anodic oxidation along with dry anisotropic etching. Convex, concave, and arbitrarily shaped silicon microlenses with diameters as small as 2 microm are demonstrated. We also confirm the high fidelity of pattern transfer between the probe-induced oxides and the etched silicon microlens structures. Besides the flexibility, the important features of scanning-probe gray-scale anodic oxidation are small pixel size and pitch (of the order of tens of nanometers), an unlimited number of gray-scale levels, and the possibility of creating arbitrarily designed microlens structures with exquisite precision and resolution. With this approach, refractive, diffractive, and hybrid microlens arrays can be developed to create innovative optical components.     

Silver nanowires electrodeposited into nanoporous templates: Study of the influence of sizes on crystallinity and structural properties

In this work, results on the study of the influence of silver nanowire dimensions on the crystallinity and structural properties are presented. Silver nanowire arrays with high aspect ratios were prepared in the hollow structures of nanoporous templates using potentiostatic electrodeposition. Two types of material were employed as a template: commercial porous anodic aluminum oxide (with a mean pore diameter of 180 nm) and track-etched polycarbonate membranes (with a mean pore diameter of 15, 30 and 80 nm). Characterization of the silver nanowires has been done by EDS, XRD, TEM and electron diffraction. The degree of preferred crystallographic orientation (along the (1 1 1), (2 0 0) or (2 2 0) crystallographic planes) and the crystallite size of the silver nanowires as a function of template pore diameter are given and discussed.