Role of etch products in polysilicon etching in a high-density chlorine discharge

For low-pressure, high-density plasma systems, etch products can play a significant role in affecting plasma parameters such a.s species concentration and electron temperature. The residence time of etch products in the chamber can he long, hence depleting the concentration of the reactants, and leading to a decrease in etch rate. We use a spatially averaged global model including both gas phase and surface chemistry to study Cl₂ etching of polvsilicon. Etch products leaving the wafer surface are assioned to he SiCL₂ and SiCl₄. These species can be fragmented and ionized by collisions with energetic electrons, generating neutral and charged SiCl, products (x=0–4). Two limiting cases of the etch mechanism are found. an ion flux-limited regime and a neutral reactant-limited regime. The high degree of dissociation in high-density plasmas leads to the formation of elemental silicon, which can deposit on the chamber walls and wafer surface. We include surface models for both the wall and the wafer to better understand the role of etch products as a function of flowrate, pressure, and input pwer. A phenomenological model for the surface chemistry is based on available experimental data. We consider the two limiting conditions of nonreactive and reactive walls. These models are perfectly reflective walls, where all silicon-containing species are reflected; and reactive walls, which act as reactive sites for the formation of SiCl₂ and SiCl₄ etch products. The two limiting conditions give significantly different results. A decrease in the absolute atomic silicon density and a weaker dependence of etch rate on flowrate are observed for the reactive wall.     

Reactive species generated during wet chemical etching of silicon in HF/HNO3 mixtures

The role of intermediate species generated during wet chemical etching of silicon in a HF-rich HF/HNO3 mixture was studied by spectroscopic and analytical methods at 1 degrees C. The intermediate N2O3 was identified by its cobalt blue color and the characteristic features in its UV-vis and Raman spectra. Furthermore, a complex N(III) species (3NO+.NO3-) denoted as [N4O6(2+)] is observed in these solutions. The time-dependent decay of the N(III) intermediates, mainly by their oxidation at the liquid-air interface, serves as a precondition for the study of the etch rate as function of the intermediate concentration measured by Raman spectroscopy. From a linear relationship between etch rate and [N4O6(2+)] concentration, NO+ is considered to be a reactive species in the rate-limiting step. This step is attributed to the oxidation of permanent existing Si-H bonds at the silicon surface by the reactive NO+ species. N2O3 serves as a reservoir for the generation of NO+ leading to a complete coverage of the silicon surface with reactive species at high intermediate concentrations. As long as this condition is valid (plateau region), the etch rate is constant and yields a smooth silicon surface upon completion of the etching. If the N2O3 concentration is insufficient to ensure a coverage of the Si surface by NO+, the etch rate decreases linearly with the N2O3 concentration and results in a roughening of the etched silicon surface (slope region).     

SEM–EDX and SAM–AES Investigations on Rochow Contact Masses

Appropriate Rochow contact masses have been investigated by the spatial resolution techniques SEM–EDX and SAM–AES. The results gave evidence of the existence and the catalytic action of (X-ray)-amorphous copper–silicon (Cu–Si) surface species, i.e. extremely highly dispersed particles or two-dimensional species. The well-known Rochow promoter zinc seems to act as a moderator rather than as a real accelerator. It ensures a stable rate for the reaction by neutralizing the detrimental action of silicon impurities. The silicon impurities make the whole of the silicon surface reactive and in this way cause a general blockade of the silicon surface by inactive copper species. Zinc localizes the reaction. The silicon surface remains partly free, and active Cu–Si surface species can be formed by lateral diffusion of copper onto the silicon surface that is still free. © 1997 by John Wiley & Sons, Ltd.     

Study of reactive ion etching of Si and SiO2 for CFxCl4−x gases

A parametric study of the etching of Si and SiO₂ by reactive ion etching (RIE) was carried out to gain a better understanding of the etching mechanisms. The following fluorocarbons (FCs) were used in order to study the effect of the F-to-Cl atom ratio in the parent molecule to the plasma and the etching properties: CF₄, CF₃Cl, CF₂Cl₂, and CFCl₃ (FC-14, FC-13, FC-12, and FC-11 respectively). The Si etch rate uniformity across the wafer as a function of the temperature of the wafer and the Si load, the optical emission as a function of the temperature of the load, the etch rate of SiO₂ as a function of the sheath voltage, and the mass spectra for each of the FCs were measured. The temperature of the wafer and that of the surrounding Si load strongly influence the etch rate of Si, the uniformity of etching, and the optical emission of F, Cl, and CF₂. The activation energy for the etching reaction of Si during CF₄ RIE was measured. The etch rate of Si depends more strongly on the gas composition than on the sheath voltage; it seems to be dominated by ion-assisted chemical etching. The etching of photoresist shifted from chemical etching to ion-assisted chemical etching as a function of the F-to-Cl ratio and the sheath voltage. The etch rate of SiO₂ depended more strongly on the sheath voltage than on the F-to-Cl ratio.     

Etching of Silicon Nitride in CCl2F2, CHF3, SiF4, and SF6 Reactive Plasma: A Comparative Study

Silicon nitride is an important material layer in various types of microelectronic devices. Because of continuous integration of devices, patterning of this layer requires a highly selective and anisotropic etching process. Reactive ion etching is one of the most simple and popular plasma processes. The present work is an experimental analysis of primary etch characteristics in reactive ion etching of silicon nitride using chlorine- and/or fluorine-based organic and inorganic chemistries (CCl ₂ F ₂+O ₂ , CHF ₃+O ₂ , SiF ₄ +O₂, SF₆+O ₂ , and SF ₆+He) in order to obtain a simultaneous etch selectivity against polysilicon and silicon dioxide. A recipe, in CCl ₂ F ₂ /O ₂ plasma chemistry, which provides acceptable etch characteristics, along with a reasonable simultaneous selectivity against polysilicon and silicon dioxide, has been formulated.     

Dry etching of copper phthalocyanine thin films: effects on morphology and surface stoichiometry

We investigate the evolution of copper phthalocyanine thin films as they are etched with argon plasma. Significant morphological changes occur as a result of the ion bombardment; a planar surface quickly becomes an array of nanopillars which are less than 20 nm in diameter. The changes in morphology are independent of plasma power, which controls the etch rate only. Analysis by X-ray photoelectron spectroscopy shows that surface concentrations of copper and oxygen increase with etch time, while carbon and nitrogen are depleted. Despite these changes in surface stoichiometry, we observe no effect on the work function. The absorbance and X-ray diffraction spectra show no changes other than the peaks diminishing with etch time. These findings have important implications for organic photovoltaic devices which seek nanopillar thin films of metal phthalocyanine materials as an optimal structure.     

Facile fabrication of 2-dimensional arrays of sub-10 nm single crystalline Si nanopillars using nanoparticle masks

A simple procedure for the fabrication of sub-10 nm scale Si nanopillars in a 2-D array using reactive ion etching with 8 nm Co nanoparticles as etch masks is demonstrated. The obtained Si nanopillars are single crystalline tapered pillar structures of 5 nm (top) x 8 nm (bottom) with a density of approximately 4 x 10(10) pillars cm(-2) on the substrate, similar to the density of Co nanoparticles distributed before the ion etching process. The uniform spatial distribution of the Si nanopillars can also be patterned into desired positions. Our fabrication method is straightforward and requires mild process conditions, which can be extended to patterned 2-D arrays of various Si nanostructures.     

Plasma etched polymer microelectrochemical systems

This paper presents a novel technique based on plasma etching for the mass production of polymer microchip devices. The method consists of the patterning of a photo-resist by a high resolution printer on a foil composed of three layers (5 microm copper/50 microm polyimide/5 microm copper). After this step, both copper layers are chemically etched in order to serve as a contact mask on the polyimide surface so as to produce the desired microstructure pattern. The foil is placed into a reactive plasma chamber in order to etch the exposed polyimide by means of an oxidizing plasma. The method enables holes, lines or larger areas to be etched, thereby generating either microholes, microchannels or electrodes in the plastic material. The copper can then be chemically removed or further patterned to produce conductive pads which are further electroplated with gold. The microchannel is then covered with a polyethylene terephthalate/polyethylene (PET/PE) lamination. The strength of this technology is that access holes for the fluid inlet and outlet, as well as gold coated electrodes can be fabricated without post-processing in a batch process. Demonstration of the application of such microelectrochemical systems is shown here by voltammetric detection inside a 60 nL microchannel, which presents the special feature of linear depletion of the analytes in the direction parallel to the microchannel.     

A one-step etching method to produce gold nanoparticle coated silicon microwells and microchannels

Gold (Au) nanoparticles (NPs) have large surface areas and novel optical properties and can be readily functionalized using thiol-based chemistry; hence, they are useful in bioanalytical chemistry. Here, we describe a one-step, plasma-etching process that results in the spontaneous formation of Au NP coated recessed microstructures in silicon (Si). Mechanistically, the plasma etch rate of Si was enhanced in the vicinity of 10-100 nm thick Au patterns resulting in the formation of microwells or microchannels uniformly coated with 20-30 nm sized Au NPs. The methodology provides versatility in the types of microstructures that can be formed by varying the shape and dimensions of the Au patterns and the etch time. We also describe selective binding of antibodies to Au NP coated Si microwells using thiol-based surface modification.     

Effect of Plasma and Control Parameters on SiC Etching in a C2F6 Plasma

Effects of radio frequency (RF) source power (plasma density) on silicon carbide etching are examined with variations in RF bias power, pressure, O₂ fraction, and gap spacing. The etching was conducted in a C₂F₆ inductively coupled plasma. Depending on parameters or plasma conditions, the etch rate varied quite differently. When the source power was varied, the bias power (ion energy) was strongly involved in determining the relative variation in the etch rate. Complex interactions between the parameters were ascertained by means of a predictive model. The model was obtained by using a neural network in conjunction with a 2⁵ full factorial experiment. Model behaviors were consistent with experimental ones. By correlating the etch rate to the DC bias, it was identified that the source power effect on the etch rate is significantly enhanced as the DC bias is maintained at relatively low values.     

Low temperature plasma for the preparation of crater walls for compositional depth profiling of thin inorganic multilayers

An indirect, compositional depth profiling of an inorganic multilayer system using a helium low temperature plasma (LTP) containing 0.2% (v/v) SF₆ was evaluated. A model multilayer system consisting of four 10 nm layers of silicon separated by four 50 nm layers of tungsten was plasma‐etched for (10, 20, 30) s at substrate temperatures of (50, 75, and 100) °C to obtain crater walls with exposed silicon layers that were then visualized using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) to determine plasma‐etching conditions that produced optimum depth resolutions. At a substrate temperature of 100 °C and an etch time of 10 s, the FWHM of the second, third, and fourth Si layers were (6.4, 10.9, and 12.5) nm, respectively, while the 1/e decay lengths were (2.5, 3.7, and 3.9) nm, matching those obtained from a SIMS depth profile. Though artifacts remain that contribute to degraded depth resolutions, a few experimental parameters have been identified that could be used to reduce their contributions. Further studies are needed, but as long as the artifacts can be controlled, plasma etching was found to be an effective method for preparing samples for compositional depth profiling of both organic and inorganic films, which could pave the way for an indirect depth profile analysis of inorganic–organic hybrid structures that have recently evolved into innovative next‐generation materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Design of experiment for optimization of plasma-polymerized octafluorocyclobutane coating on very high aspect ratio silicon molds

As-fabricated deep reactive ion etched (DRIE) silicon mold with very high aspect ratio (>10) feature patterns is unsuitable for poly(dimethylsiloxane) (PDMS) replication because of the strong interaction between the Si surface and the replica and the corrugated mold sidewalls. The silicon mold can be conveniently passivated via plasma polymerization of octafluorocyclobutane (C4F8), which is also employed in the DRIE process itself, to enable the mold to be used repeatedly. To optimize the passivation conditions, we have undertaken a Box-Behnken experimental design on the basis of three passivation process parameters (plasma power, C4F8 flow rate, and deposition time). The measured responses were fluorinated film thickness, demolding status/success, demolding force, and fluorine/carbon ratio on the fifth replica surface. The optimal passivation process conditions were predicted to be an input power of 195 W, a C4F8 flow rate of 57 sccm, and a deposition time of 364 s; these were verified experimentally to have high accuracy. Demolding success requires medium-deposited film thickness (66-91 nm), and the thickness of the deposited films correlated strongly with deposition time. At moderate to high ranges, increased plasma power or gas flow rate promoted polymerization over reactive etching of the film. It was also found that small quantities of the fluorinated surface were transferred from the Si mold to the PDMS at each replication, entailing progressive wear of the fluorinated layer.     

锂离子电池Si/RGO@PANI三明治纳米结构负极材料的制备与电化学性能

以石墨烯和纳米硅颗粒为起始原料,苯胺为单体,植酸为掺杂剂,过硫酸铵为氧化剂(引发剂),通过超声波的作用成功原位合成了具有三明治纳米结构的Si/RGO@PANI锂离子电池负极材料。石墨烯片层与导电聚苯胺与纳米硅颗粒构成的夹心结构可形成有效的导电网络,且具有优异的结构稳定性,能够有效缓解硅在嵌锂/脱锂过程中产生的巨大体积效应,表现出良好的循环性能和倍率性能。电化学性能测试表明,这种Si/RGO@PANI三明治纳米结构复合材料适合作为一种优良的锂离子电池负极材料。     

锂离子电池Si/RGO@PANI三明治纳米结构负极材料的制备与电化学性能

以石墨烯和纳米硅颗粒为起始原料,苯胺为单体,植酸为掺杂剂,过硫酸铵为氧化剂（引发剂）,通过超声波的作用成功原位合成了具有三明治纳米结构的Si/RGO@PANI锂离子电池负极材料。石墨烯片层与导电聚苯胺与纳米硅颗粒构成的夹心结构可形成有效的导电网络,且具有优异的结构稳定性,能够有效缓解硅在嵌锂/脱锂过程中产生的巨大体积效应,表现出良好的循环性能和倍率性能。电化学性能测试表明,这种Si/RGO@PANI三明治纳米结构复合材料适合作为一种优良的锂离子电池负极材料。     

A thermodynamic model of deposition by etching-enhanced reactive sputtering

Deposition by etching-enhanced reactive sputtering (DEERS) is believed to be a three-step process: plasma etching of a sputtering target, transport of volatile etch products to a substrate, followed by conversion of etch products adsorbed on the substrate to form a desired film material. While there are undoubtedly kinetic factors involved in this process, results of a thermodynamic analysis of the above process as a sequence of two chemical equilibrium reactors (target and substrate) correlates well with available experiments on oxide deposition and with optimum ratios of etchant to oxidant gases.     

Chlorination-methylation of the hydrogen-terminated silicon(111) surface can induce a stacking fault in the presence of etch pits

Recently, we reported STM images of the methylated Si(111) surface [prepared through chlorination-alkylation of the Si(111)-H surface] taken at 4.7 K, indicating that the torsion angle of the methyl group with respect to the subsurface silicon layer is phi = 23 +/- 3 degrees . Repulsions between H atoms in adjacent methyl groups are minimized at 30 degrees , while repulsions between H atoms and second layer Si atoms are minimized at 60 degrees . The experimental result of 23 degrees is surprising because it suggests a tendency of the methyl group toward the eclipsed configuration (0 degrees ) rather than staggered (60 degrees ). In contrast, extensive fully periodic quantum mechanical Density Functional Theory studies of this surface give an equilibrium torsion angle of 37.5 degrees , indicating a tendency toward the staggered configuration. This discrepancy can be resolved by showing that the CH3 on the step edges and etch pits interacts repulsively with the CH3 on the surface terraces unless a stacking fault is introduced between the first and second silicon layers of the Si(111)-CH3 surface terraces. We propose that this could occur during the chlorination-alkylation of the Si(111)-H surface. This stacking fault model predicted phi = 22.5 degrees measured with respect to the bulk (corresponding to phi = 37.5 degrees with respect to the second layer Si atoms). This model can be tested by measuring the orientation of the CH3 within the etch pits, which should have phi = 37.5 degrees , or by making a surface without etch pits, which should have phi = 37.5 degrees .     

酸浸蚀Al-Si合金制备锂离子电池高性能多孔硅负极材料

本文首次提出利用酸浸蚀Si-Al（含Al 80%）合金粉末的方法制备多孔硅材料. 分析表明制得的多孔硅材料为晶体,并具有由纳米颗粒结集成的海绵状多孔结构,其粒径约20 μm,比表面102.7 m²·g^-1. 多孔硅电极按多孔硅:导电碳:粘结剂 = 1:1:1（by mass）涂成. 在添加15%氟化碳酸乙烯酯（FEC）的1 mol·L^-1 LiPF₆/EC + DMC（1:1,by volume）电解液,在100 mA·g^-1电流密度充放电,多孔硅电极的首次放电比容量2072 mAh·g^-1 Si. 经237次充放电循环后,其放电容量仍可保持在1431 mAh·g^-1 Si,显示了相当高的充放电稳定性. 这归因于其海绵状多孔结构有足够的微空间以承受充电过程中硅的急剧膨胀. 硅微粒的纳米尺寸有利于锂在Li-Si合金中的扩散. 纳米硅微粒可牢固地联成一整体,不易因膨胀、收缩而粉化断裂. 这种构筑多孔硅负极材料的新方法操作简便、成本低廉,有着很好的应用前景.     

Interaction of alkali salt promoters and silicon impurities in Rochow contact masses

Cesium chloride- and rubidium chloride-promoted Rochow contact masses based on both technical-grade and highly pure silicon have been investigated in the Rochow reaction and by REM/EDX surface analysis. The alkali-salt promoters seem to act analogously to the well-known zinc promoter, by localizing the reaction to distinct reactive areas and keeping free the surface area for the reaction, probably for the formation of catalytically active Cu–Si surface species. The alkali salts exhibited their promoting action only in combination with the impurities within the technical-grade silicon. Otherwise, they acted as blocking poisons. The promoter action of alkali chlorides in contact masses based on technical-grade silicon is possibly connected with the formation of salt melts, containing alkali chlorides and impurities. These melts, analogously to zinc chloride, could dissolve oxidic impurities from the silicon surface which otherwise would enhance the blocking of potentially active surface by extensive copper deposition. © 1998 John Wiley & Sons, Ltd.     

Novel fabrication process using nanoporous anodic aluminum oxidation and MEMS technologies for gas detection

A class of nanoporous TiO₂ gas sensors processed by novel anodic aluminum oxidation (AAO) of Al thin films and microelectromechnical systems (MEMS) techniques are presented. To enhance the sensitivity and reduce the sensing dimensions of a gas sensor, a nanoporous surface of the gas-sensitive material on the sensor is required. These sensors can be implemented on silicon or silicon dioxide substrate featuring a thin membrane of micro-hotplate structure featuring micro-heaters, thermometers and electrodes, and thus operate as chemoresistive devices. Combining the AAO method with dry-etch process, a homogeneous and nanoporous SiO₂ surface of the sensor can be effectively configured by modulating various hole diameters and depth, hence replacing conventional photolithography and electrochemical etch. The process integration including AAO, reactive ion etch (RIE) and microfabrication is mainly developed and a feasibility study of PVD TiO₂ thin film deposition upon the porous device is also provided. TiO₂ thin films deposited on the nanoporous surface are investigated and compared with non-porous TiO₂ films. It is encouraging that our fabrication process is able to provide relatively high surface area to enhance sensitivity of the sensor without additional doping steps. Our promising experimental results have revealed these miniature and cost-effective devices are not only compatible, but applicable to smart bio-chemical sensors of next generation.     

二次包覆法制备煤沥青基硅/碳复合物及其锂离子电池性能

本文采用市售纳米硅为硅源,以软化点低、得碳率高、价格便宜的煤沥青作为碳源,通过两步包覆法制备了煤沥青基硅/碳(Si/C/C)复合物,并研究其作为锂离子电池负极材料的电化学性能。 结果表明,所得复合物的粒径在300~350 nm间,Si纳米粒子被C包覆并相互连结成C-Si-C网络结构,其中Si含量为27%的硅/碳复合物(Si/C/C-27%)作为锂电池电极材料表现了良好的储锂性能。 在0.1 A/g的小电流密度下,Si/C/C-27%的放电比容量为1281 mA·h/g;在3 A/g的大电流密度下,其放电比容量仍能保持在582 mA·h/g,表现了良好的倍率性能。Si/C/C-27%在2 A/g的电流密度下经过100次的循环后其比容量保持率为76.61%,表现了良好的循环稳定性。 相比于煤沥青基碳的一次包覆所得的硅/碳复合材料(Si/C),Si/C/C有效提高了Si纳米粒子的导电性并抑制了其在嵌锂和脱锂过程中的体积膨胀。 本文提出的二次包覆的新方法为制备具有优异电化学性能的锂离子电池负极材料提供了新的研究思路。