Black silicon with high density and high aspect ratio nanowhiskers

The physical properties of black silicon (b-Si) formed on Si wafers by reactive ion etching in chlorine plasma are reported in an attempt to clarify the formation mechanism and the origin of the observed optical and electrical phenomena, which are promising for a variety of applications. The b-Si consisting of high density and high aspect ratio sub-micron length whiskers or pillars with tip diameters of well under 3 nm exhibits strong photoluminescence (PL) both in the visible and the infrared, which is interpreted in conjunction with defects, confinement effects and near band-edge emission. Structural analysis indicates that the whiskers are all crystalline and encapsulated by a thin Si oxide layer. The infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder induced LO-TO optical mode coupling can be an effective tool in assessing the structural quality of the b-Si. The same phonons are likely coupled to electrons in visible region PL transitions. Field emission properties of these nanoscopic features are demonstrated indicating the influence of the tip shape on the emission. Overall properties are discussed in terms of the surface morphology of the nanowhiskers.     

Fabrication of p-type silicon nanowire arrays with a high aspect ratio using electrochemical and alkaline etching

We report on the fabrication of silicon nanostructures with a high aspect ratio that were created using a combination of electrochemical etching and alkaline etching. With this technique, we were able to fabricate nano- and/or micro-wire structures that are perfectly periodic over large areas of 3.14 cm2. After porous silicon was created by electrochemical etching, the effect of post-alkaline etching was investigated to determine how changes in the etching time, solution concentration and temperature of the etchant influenced the silicon morphology. As a result, periodic silicon wire arrays with good vertical alignment were obtained, and these arrays had a width of less than 500 nm and/or a high aspect ratio of more than 20.     

Fabrication of high aspect ratio metal nanotips by nanosecond pulse laser melting

The authors have developed an approach to fabricate sharp and high aspect ratio metal tips using nanosecond pulse laser melting. A quartz wafer covered with a thin chromium (Cr) film was placed on top of a second wafer with a sub-micrometer gap between them and the Cr film facing the second wafer. Then an excimer laser pulse (308?nm wavelength, 20?ns pulse duration) was shone from the back of the quartz wafer and melted the Cr film momentarily (several hundred nanoseconds). It is found that the molten Cr films can self-form discrete metal pillars connecting the two wafers. After separating the two wafers, nanotips were formed at the broken pillar necks. The sharpest tip achieved has an apex diameter 10?nm and height 180?nm. The self-formation of Cr pillars between the two wafers was attributed to the attractive electrostatic force caused by the work function difference of two wafers that were in close proximity. This technique could be extended to other metals, and a periodic uniform tip array could be obtained by pre-patterning the metal into identical isolated mesas and precisely controlling the gap between the two wafers.     

Fabrication of very high aspect ratio metal nanowires by a self-propulsion mechanism

A novel synthesis method of very high aspect ratio metal nanowires is described. The synthesis utilizes a nanoporous membrane as a template and self-electrophoresis as a directed force that continuously push formed nanowires out of the pores in a rate that is identical to the rate of their elongation. As a result, while the pores of membranes are only 6 microm long, the formed nanowires could be more than 100 microm long.     

Fabrication of high aspect ratio zirconia nanotube arrays by anodization of zirconium foils

The zirconia nanotube arrays with diameter of about 130 nm, a length of up to 190 μm and aspect ratios of more than 1400 were prepared by anodizing a zirconium foil in mixture of formamide and glycerol (volume ratio = 1:1) containing 1 wt.% NH₄F and 3 wt.% H₂O. The as-prepared nanotube arrays were amorphous zirconia. Monoclinic and tetragonal zirconia coexisted when annealed at 400 °C and 600 °C, while monoclinic zirconia was obtained at 800 °C. The ZrO₂ nanotubes retained their shape after heating up to 800 °C. The lower dissolving rate of zirconia in organic electrolytes might be the main reason for fabrication of zirconia nanotube arrays with high aspect ratio.     

深高宽比微结构的干法刻蚀

介绍了深高宽比微结构在干法刻蚀过程中遇到的刻蚀滞后、刻蚀中止、侧壁弯曲和开槽效应等与传统器件刻蚀不同的现象,讨论了制作高深宽比结构所需的关键技术和检测手段.     

碳纳米管场发射阴极的制备及其场发射特性

采用电泳法将碳纳米管组装到电化学淀积的银台阵列上作为场发射阴极并研究了它的场发射特性.场发射特性测试结果表明:该阴极具有优异的场发射特性,开启电场为2.8V/μm,在应用电场为5.5V/μm时,发射电流密度达到1.7mA/cm2.具有优异的发射性能的原因可以归结到银台的边缘和银台类山状的表面增强了碳纳米管的场致电子发射.该阴极制备工艺简单、发射特性优异,且容易实现大面积制备,可以应用到大面积场发射显示器件中.     

A sub-atmospheric chemical vapor deposition process for deposition of oxide liner in high aspect ratio through silicon vias

The formation of a Through Silicon Via (TSV) includes a deep Si trench etching and the formation of an insulating layer along the high-aspect-ratio trench and the filling of a conductive material into the via hole. The isolation of the filling conductor from the silicon substrate becomes more important for higher frequencies due to the high coupling of the signal to the silicon. The importance of the oxide thickness on the via wall isolation can be verified using electromagnetic field simulators. To satisfy the needs on the Silicon dioxide deposition, a sub-atmospheric chemical vapor deposition (SA-CVD) process has been developed to deposit an isolation oxide to the walls of deep silicon trenches. The technique provides excellent step coverage of the 100 microm depth silicon trenches with the high aspect ratio of 20 and more. The developed technique allows covering the deep silicon trenches by oxide and makes the high isolation of TSVs from silicon substrate feasible which is the key factor for the performance of TSVs for mm-wave 3D packaging.     

Profile simulation of high aspect ratio contact etch

A semi-empirical profile simulator was employed to better understand fundamental mechanisms of feature evolution in a high aspect ratio contact plasma etch process. Simulation results showed that the net deposition rate of polymer on sidewall defined the necking and surface scattering of ions from the secondary facet caused the formation of bowing. As neutral depositor flux was increased, the resulting profile showed a monotonic increase in necking. In contrast, the extent of bowing showed a maximum, such that minimal bowing was obtained at low and at high depositor fluxes. Primary faceting of photo resist showed only a small influence on the SiO₂ etch profile.     

Introduction of zirconium oxide in a hardmask concept for highly selective patterning of scaled high aspect ratio trenches in silicon

The fabrication of high aspect ratio silicon trenches (critical dimension < 100 nm, aspect ratio > 10:1) by dry etch processing has proven to be a challenge mainly due to limited etch selectivity of conventional hardmask materials to Si. Moreover, for future technology nodes the hardmask thickness will be limited by the thickness of the photoresist. This work focuses on a concept to enable the usage of very thin resist layers (< 100 nm) for patterning of silicon trenches by the integration of an unconventional hardmask stack consisting of SiO₂ and ZrO₂. Deposition of such material films has been investigated, as well as e-beam lithography exposure and finally pattern transfer by dry etching. Using this hardmask stack and 100 nm thin resist, the fabrication of 35 nm wide trenches with an aspect ratio of ~ 20:1 is demonstrated revealing a very high selectivity (> 100:1) of the ZrO₂ layer to Si during the deep silicon etch. A silicon etch rate > 1.5 μm/min was achieved. The ZrO₂ layer itself provides the main selectivity improvements of the final hardmask stack.     

A systematic study of DRIE process for high aspect ratio microstructuring

Various MEMS devices like Accelerometers, Resonators, RF- Filters, Micropumps, Microvalves, Microdispensers and Microthrusters are produced by removing the bulk of the substrate materials. Fabrications of such Microsystems requires the ability to engineer precise three-dimensional structures in the silicon substrate. Fabrication of MEMS faces multiple technological challenges before it can become a commercially viable technology. One key fabrication process required is the deep silicon etching for forming high aspect ratio structures. There is an increasing interest in the use of dry plasma etching for this application because of its anisotropic etching behavior, high etch speed, good uniformity and profile control, high aspect ratio capabilities without having any undesired secondary effects i.e. RIE lags, Loading, microloading, loosing of anisotropic nature of etching as aspect ratio increases, micro-grass and even etch stalling. Developing a DRIE micro-machining process requires a thorough understanding of all plasma parameters, which can affect a silicon etching process and their use to suppress the secondary effects. In this paper our intention is to investigate the influence of etching gas flow, etching gas pressure, passivation gas pressure, ICP coil power, Platen power and etch and passivation time sequence on etch rate and side wall profile. Parameter ramping is a powerful technique used to achieve the requirements of high aspect ratio microstructures (HARMS) for MEMS applications by having high etch rate with good profile/CD control. The results presented here can be used to rationally vary processing parameters in order to meet the microstructural requirements for a particular application.     

Selective epitaxial silicon growth in high aspect ratio contact on 70 nm node flash memory

Selective epitaxial growth (SEG) silicon as sacrificial layer is proposed to circumvent junction leakage (I_jun) of bit line contact due to silicon substrate loss by high aspect ratio contact etching and spontaneous salicide formation. The result indicates that the appropriate SEG silicon in contact area significantly reduces I_jun about three orders compared with no SEG silicon. In addition, the SEG method provides acceptable Kelvin contact resistance. Furthermore, during salicide formation, the consumed ratio of titanium to silicon is 0.7. It is confirmed that the feasible approach not only prevents from I_jun deterioration but also adjusts contact resistance as well.     

Fabrication of KSr2Nb5O15 particles with high aspect ratio by two-step molten salt synthesis

KSr₂Nb₅O₁₅ (KSN) particles with high aspect ratios (>50) were synthesized by a two-step molten salt synthesis from two kinds of seeds. The effect of KSN seed on the particles size and morphology was investigated, and the crystal growth mechanism was discussed. The results show that only seeds with (0 0 1) surface act as a site for further growth. KSN particles synthesized by the two-step MSS preserved the shape of seed and had a high aspect ratio. When the rod-like seed prepared by a molten salt synthesis was used, the aspect ratio of obtained particles increased to a maximum value (40–60 μm in length and 0.5–1 μm in diameter) at the seed content of 10 wt%, then decreased with the increasing of seed content. The product is an ideal template for fabricating fiber with tungsten bronze structure by a combination of the co-extrusion and template grain growth process.     

Fabrication and characterization of plasmonic nanorods with high aspect ratios

Metallic nanostructures with high aspect ratios are important for developing devices in photonics and integrated optics. However, fabricating well-aligned plasmonic arrays is challenging due to the difficulties of etching metals. In this work, we investigate the feasibility of constructing high aspect ratio nanorods with desired shapes and controllable geometric parameters using direct focused ion beam etching. The whole fabrication process only involves a metal-deposition step and a single milling of designed patterns. Detailed characterizations of the fabricated devices are also experimentally demonstrated.     

Metallic nanowire-graphene hybrid nanostructures for highly flexible field emission devices

We report a simple but efficient method to prepare metallic nanowire-graphene (MN-G) hybrid nanostructures at a low temperature and show its application to the fabrication of flexible field emission devices. In this method, a graphene layer was transferred onto an anodic alumina oxide template, and vertically aligned Au nanowires were grown on the graphene surface via electrodeposition method. As a proof of concept, we demonstrated the fabrication of flexible field emission devices, where the MN-G hybrid nanostructures and another graphene layer on PDMS substrates were utilized as a cathode and an anode for highly flexible devices, respectively. Our field emission device exhibited stable and high field emission currents even when bent down to the radius of curvature of 25 mm. This MN-G hybrid nanostructure should prove tremendous flexibility for various applications such as bio-chemical sensors, field emission devices, pressure sensors and battery electrodes.     

高深宽比微结构深度测量技术的研究进展

高深宽比孔/槽微结构现广泛应用于微机电系统（MEMS）与三维集成电路（3D-IC）等领域,是微纳器件的基础性工艺结构。随着器件微型化与功能化的发展需求,孔/槽微结构的深宽比不断提升。深度作为重要参数对器件加工工艺、器件性能有直接影响,微孔/槽结构深度的精确测量具有重要意义,但测量方法面临巨大挑战,成为测量领域的难题之一。针对这一问题,按照非光学和光学测量方式将测量方法分为两大类,介绍了扫描电子显微镜、扫描探针术、白光显微干涉技术、共焦显微技术和反射光谱技术等测量方法的工作原理,在微孔/槽深度测量方面的研究现状,尝试从中总结每种测量方法的优缺点,最后,讨论了未来高深宽比微结构深度测量发展趋势以及研究重点,为之后高深宽比微结构深度的测量技术研究提供帮助。     

Conformal grid generation for high aspect ratio simply and doubly connected regions

Conformal co-ordinate transformations are used to map rectangular computational domains onto arbitrary simply and doubly connected regions with smooth boundaries. The efficient numerical schemes of Wegmann involving the solution of the inverse boundary correspondence function problems associated with the mapping of the unit disc or circular annulus onto simply or doubly connected domains, respectively, are employed. The numerical implementation of these schemes is emphasized. Examples are generated for regions with elliptic inner and outer boundaries. Additional examples are used to demonstrate the accuracy and convergence of the schemes and their practical limitations. The techniques are found to converge well if holomorphic functions are used to describe the boundaries. The use of preconditioning maps is also discussed.     

Fabrication of carbon nanoflakes by RF sputtering for field emission applications

In this paper, an ultra thin sheet-like carbon nanostructure, carbon nanoflake (CNF), has been effectively fabricated by RF sputtering on Si substrate without any catalyst or special substrate pre-treatment. The CNFs were chosen to be the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The effect of deposition parameters such as substrate temperature, gas flow ratio and RF power on the field emission properties is discussed in detail. The sheet-like structures with thickness of about 10 nm or less stand on edge on the substrate and have a defective graphite structure. The field emission properties of the sample deposited at the optimum deposition conditions are turn-on field of 5.5 V/μm and current density of 1.4 mA/cm² at 11 V/μm. Considering the inexpensive manufacturing cost, lower synthesis temperature and ease of large-area preparation, the CNFs with low turn-on field deposited by RF sputtering might have a potential application in field emission devices.     

Fabrication and optical characterization of light trapping silicon nanopore and nanoscrew devices

We have fabricated nanotextured Si substrates that exhibit controllable optical reflection intensities and colors. Si nanopore has a photon trapping nanostructure but has abrupt changes in the index of refraction displaying a darkened specular reflection. Nanoscrew Si shows graded refractive-index photon trapping structures that enable diffuse reflection to be as low as 2.2% over the visible wavelengths. By tuning the 3D nanoscale silicon structure, the optical reflection peak wavelength and intensity are changed in the wavelength range of 300-800?nm, making the surface have different reflectivity and apparent colors. The relation between the surface optical properties with the spatial features of the photon trapping nanostructures is examined. Integration of photon trapping structures with planar Si structure on the same substrate is also demonstrated. The tunable photon trapping silicon structures have potential applications in enhancing the performance of semiconductor photoelectric devices.     

Development of FBG probes for dimensional metrology with micro parts of high aspect ratio

The development is described of fiber Bragg grating (FBG) probes with single or four cores using spheres fixed onto optical fibers comprising Bragg gratings in their cores. Simulations and preliminary experiments were conducted to evaluate the performance of these FBG probes for dimensional metrology with micro parts of high aspect ratio. Simulation and experimental results indicate that a one-dimensional FBG probe with single core can be used to achieve an axial resolution of 100 nm at an aspect ratio of 15:1. A three-dimensional FBG probe with four cores can be used to achieve a theoretical axial resolution as high as that of a FBG probe with single core, with a capability of decoupling two-dimensional radial displacements with a resolution of 13 nm.