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.     

Anisotropic etching of silicon in hydrazine

This paper contains a detailed discussion of the practical issues related to the anisotropic etching of single crystal silicon using a 5050 hydrazinewater solution. Characteristics of the etchant, etching reactor design, etch procedures, safety precautions, etch rate data for typical samples and appropriate etch-masks are among the topic discussed. The etching process is carried out in a atmospheric reflux reactor, continuously purged with nitrogen. The etch rate of (100) silicon at 115°C in this hydrazine solution is nearly 3 μm/min, which is much higher than that of ethylenediaminepyrocatecholwater (EDP) solutions. Silicon dioxide, silicon nitride and most metallic thin films, except aluminium, can be used to mask the etching process. The etch rate is reduced significantly in highly-boron-doped silicon; a boron concentration of 1.5 × 10²⁰ cm⁻³ practically stops the etch. The use of the hydrazine solution for micromachining thin silicon diaphragms, cantilevers and fibers is demonstrated.     

A study of the reaction of N+ with O2: experimental quantification of NO+(a 3Sigma+) production (298-500 K) and computational study of the overall reaction pathways

The product branching ratios for NO+(X 1Sigma+) and NO+(a 3Sigma+) produced from the reaction of N+ with O2 have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO+(a) at both temperatures, despite the fact that the reaction to form NO+(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N+ + O2 reaction show that the reaction from N+(3P) + O2(3Sigma(g)) reactants starts with an efficient early stage charge transfer to the N(2D) + O2+(X 2Pi) channel, which gives rise to the O2+(X 2Pi) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO+ and O+ products. Pathways to produce NO+(a 3Sigma+) are found to be less favorable than pathways leading to the major product NO+(X 1Sigma+). Production of N(2D) has implications for the concentration of NO in the mesosphere.     

Heterogeneous chemistry of the NO3 free radical and N2O5 on decane flame soot at ambient temperature: reaction products and kinetics

The interaction of NO3 free radical and N2O5 with laboratory flame soot was investigated in a Knudsen flow reactor at T = 298 K equipped with beam-sampling mass spectrometry and in situ REMPI detection of NO2 and NO. Decane (C10H22) has been used as a fuel in a co-flow device for the generation of gray and black soot from a rich and a lean diffusion flame, respectively. The gas-phase reaction products of NO3 reacting with gray soot were NO, N2O5, HONO, and HNO3 with HONO being absent on black soot. The major loss of NO3 is adsorption on gray and black soot at yields of 65 and 59%, respectively, and the main gas-phase reaction product is N2O5 owing to heterogeneous recombination of NO3 with NO2 and NO according to NO3 + {C} --> NO + products. HONO was quantitatively accounted for by the interaction of NO2 with gray soot in agreement with previous work. Product N2O5 was generated through heterogeneous recombination of NO3 with excess NO2, and the small quantity of HNO3 was explained by heterogeneous hydrolysis of N2O5. The reaction products of N2O5 on both types of soot were equimolar amounts of NO and NO2, which suggest the reaction N2O5 + {C} --> N2O3(ads) + products with N2O3(ads) decomposing into NO + NO2. The initial and steady-state uptake coefficients gamma 0 and gamma ss of both NO3 and N2O5 based on the geometric surface area continuously increase with decreasing concentration at a concentration threshold for both types of soot. gamma ss of NO3 extrapolated to [NO3] --> 0 is independent of the type of soot and is 0.33 +/- 0.06 whereas gamma ss for [N2O5] --> 0 is (2.7 +/- 1.0) x 10(-2) and (5.2 +/- 0.2) x 10(-2) for gray and black soot, respectively. Above the concentration threshold of both NO3 and N2O5, gamma ss is independent of concentration with gamma ss(NO3) = 5.0 x 10(-2) and gamma ss(N2O5) = 5.0 x 10(-3). The inverse concentration dependence of gamma below the concentration threshold reveals a complex reaction mechanism for both NO3 and N2O5. The atmospheric significance of these results is briefly discussed.     

Catalytic amplification of the soft lithographic patterning of Si. Nonelectrochemical orthogonal fabrication of photoluminescent porous Si pixel arrays

Photoluminescent, porous silicon pixel arrays were fabricated via a Pt-promoted wet etching of p-type Si(100) using a 1:1:1 EtOH/HF/H2O2 solution. The pixels were fabricated with micrometer-scale design rules on a silicon substrate that had been modified with an octadecyltrichlorosilane (OTS) monolayer patterned using microcontact printing. The printed OTS layer serves as an orthogonal resist template for the deposition of a Pt(0) complex, which preferentially deposits metal species in areas not covered with OTS. The Pt centers generate a localized oxidative dissolution process that pits the Si in the Pt-coated regions, resulting in the formation of a porous silicon microstructure that luminesces around 580 nm upon illumination with a UV source. Scanning electron microscopy and fluorescence microscopy images of the fabricated porous silicon structures showed that features in the size range of approximately 10-150 microm, and possibly smaller, can be generated by this catalytically amplified soft lithographic patterning method. Importantly, the OTS acts as an etch mask, so that, even with significant hole transport, etching is confined to areas coated with the Pt(0) complex.     

NO在氧化铝负载的Pd催化剂上吸附的TPD-MS研究

消除汽车尾气中的氮氧化物(NOx)对保护大气环境有着重要意义.为了除去NOx,已经进行了许多卓有成效的研究,例如NOx在分子筛上的直接分解和催化还原,在贵金属三效催化剂上的还原等.     

Mo／ZSM-5催化剂上NO催化还原反应机理与动力学研究

采用浸渍法制备了Mo/ZSM-5催化剂样品,并以氨为还原剂对其NO选择性催化还原活性、以及NO转化的反应速率进行测定.结果表明,在Mo/ZSM-5催化剂上不会发生NO氧化成NO2的反应,也没有N2O生成,然而有少量的NO分解反应发生.在氧气存在条件下,Mo/ZSM-5催化剂上NO-NH3-O2的SCR反应遵循LH机理.NO、O2和NH3首先吸附在Mo/ZSM-5表面,吸附态NO物种与吸附NH3物种直接反应生成氮气,气相氧的作用是加强NO吸附、补充催化剂表面吸附氧物种.并由此推导出NO转化的速率方程式,分别计算和模拟了在不同O2浓度、NO浓度和反应温度条件下NO的反应速率rNO值及其变化关系.结果表明,理论模拟值能够与实验值很好地吻合,所推测的机理能够很好地描述Mo-ZSM-5催化剂上NO选择性催化还原行为.     

In Process Citation

Knowing the values of the equilibrium constants corresponding to the reactions N(2)O(4) right harpoon over left harpoon 2NO(2) and N(2)O(4) right harpoon over left harpoon NO(+) + NO(3)(-) in sulpholane, we have undertaken the electrochemical study of N(2)O(4) by means of linear and cyclic voltammetry at the platinum electrode. The N(2)O(4) species undergoes one oxidation step N(2)O(4) right harpoon over left harpoon 2NO(2) right harpoon over left harpoon 2NO(2)(+) + 2e and two reduction steps NO(2) + N(2)O(4) + e(-)right harpoon over left harpoon N(2)O(3) + NO(3)(-) (1st wave), followed by 3N(2)O(4) + 2e(-) right harpoon over left harpoon 2N(2)O(3) + 2NO(3)(-), N(2)O(4) + e(-) right harpoon over left harpoon NO + NO(3)(-), 2N(2)O(3) + e(-) right harpoon over left harpoon 3NO + NO(3)(-) (2nd wave). The redox properties of N(2)O(4) are complicated by trace quantities of water because of the formation of the electroactive species N(2)O(3), HNO(3) and HNO(2) according to N(2)O(4) + H(2)O right harpoon over left harpoon HNO(2) + HNO(3) and N(2)O(4) + HNO(2) right harpoon over left harpoon N(2)O(3) + HNO(3). The standard potentials of the couples concerned have been evaluated and are discussed. sont discutés et évalués.     

Formation of the surface NO during N2O interaction at low temperature with iron-containing ZSM-5

Interaction of N2O at low temperatures (473-603 K) with Fe-ZSM-5 zeolites (Fe, 0.01-2.1 wt %) activated by steaming and/or thermal treatment in He at 1323 K was studied by the transient response method and temperature-programmed desorption (TPD). Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of NO adsorbed at room temperature as a probe molecule indicated heterogeneity of surface Fe(II) sites. The most intensive bands were found at 1878 and 1891 cm(-1), characteristic of two types mononitrosyl species assigned to Fe2+(NO) involved in bi- and oligonuclear species. Fast loading of atomic oxygen from N2O on the surface and slower formation of adsorbed NO species were observed. The initial rate of adsorbed NO formation was linearly dependent on the concentration of active Fe sites assigned to bi- and oligonuclear species, evolving oxygen in the TPD at around 630-670 K. The maximal coverage of a zeolite surface by NO was estimated from the TPD of NO at approximately 700 K. This allowed the simulation of the dynamics of the adsorbed NO formation at 523 K, which was consistent with the experiments. The adsorbed NO facilitated the atomic oxygen recombination/desorption, the rate determining step during N2O decomposition to O2 and N2, taking place at temperatures > or =563 K.     

Inclined N2 desorption in a steady-state N2O + CO reaction on Pd(110)

The angular and velocity distributions of desorbing products were analyzed in the course of a catalyzed N2O + CO reaction on Pd(110). The reaction proceeded steadily above 450 K, and the N2 desorption merely collimated sharply along 45 degrees off the surface normal toward the [001] direction. It is proposed that this peculiar N2 desorption is induced by the decomposition of adsorbed N2O oriented along the [001] direction. On the basis of the observation of similar inclined N2 desorption in both NO + CO and N2O + CO reactions, the N2 formation via the intermediate N2Oa dissociation was confirmed in catalytic NO reduction.     

Two‐Dimensional Spatial Resolution of Concentration Profiles in Catalytic Reactors by Planar Laser‐Induced Fluorescence: NO Reduction over Diesel Oxidation Catalysts

Planar laser‐induced fluorescence (PLIF) enables noninvasive in situ investigations of catalytic flow reactors. The method is based on the selective detection of two‐dimensional absolute concentration maps of conversion‐relevant species in the surrounding gas phase inside a catalytic channel. Exemplarily, the catalytic reduction of NO with hydrogen (2 NO+5 H₂→2 H₂O+2 NH₃) is investigated over a Pt/Al₂O₃ coated diesel oxidation catalyst by NO PLIF inside an optically accessible channel reactor. Quenching‐corrected 2D concentration maps of the NO fluorescence above the catalytic surface are obtained under both, nonreactive and reactive conditions. The impact of varying feed concentration, temperature, and flow velocities on NO concentration profiles are investigated in steady state. The technique presented has a high potential for a better understanding of interactions of mass transfer and surface kinetics in heterogeneously catalyzed gas‐phase reactions.     

One-electron reduction of aqueous nitric oxide: a mechanistic revision

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (3)NO(-) and singlet (1)HNO, respectively, which further react with NO to produce the N(2)O(2)(-) radical, albeit with the very different specific rates, k((3)NO(-) + NO) = (3.0 +/- 0.8) x 10(9) and k((1)HNO + NO) = (5.8 +/- 0.2) x 10(6) M(-)(1) s(-)(1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (3)NO(-) and (1)HNO under all experimentally accessible conditions. As a result, (3)NO(-) and (1)HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N(2)O(2)(-) radical extremely rapidly acquires another NO, k(N(2)O(2)(-) + NO) = (5.4 +/- 1.4) x 10(9) M(-)(1) s(-)(1), producing the closed-shell N(3)O(3)(-) anion, which unimolecularly decays to the final N(2)O + NO(2)(-) products with a rate constant of approximately 300 s(-)(1). Contrary to the previous belief, N(2)O(2)(-) is stable with respect to NO elimination, and so is N(3)O(3)(-). The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N(3)O(3)(-) anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M(-)(1) cm(-)(1)). The spectra previously assigned to the NO(-) anion and to the N(2)O(2)(-) radical are due, in fact, to a mixture of species (mainly N(2)O(2)(-) and N(3)O(3)(-)) and to the N(3)O(3)(-) anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (3)NO(-) and (1)HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN(2)O(3)(-), in NO-containing solutions.     

Oxidation studies of dipositive actinide ions, An2+ (An = Th, U, Np, Pu, Am) in the gas phase: synthesis and characterization of the isolated uranyl, neptunyl, and plutonyl ions UO2(2+)(g), NpO2(2+)(g), and PuO2(2+)(g)

Reactions of atomic and ligated dipositive actinide ions, An2+, AnO2+, AnOH2+, and AnO2(2+) (An = Th, U, Np, Pu, Am) were systematically studied by Fourier transform ion cyclotron resonance mass spectrometry. Kinetics were measured for reactions with the oxidants, N2O, C2H4O (ethylene oxide), H2O, O2, CO2, NO, and CH2O. Each of the five An2+ ions reacted with one or more of these oxidants to produce AnO2+, and reacted with H2O to produce AnOH2+. The measured pseudo-first-order reaction rate constants, k, revealed disparate reaction efficiencies, k/k(COL): Th2+ was generally the most reactive and Am2+ the least. Whereas each oxidant reacted with Th2+ to give ThO2+, only C2H4O oxidized Am2+ to AmO2+. The other An2+ exhibited intermediate reactivities. Based on the oxidation reactions, bond energies and formation enthalpies were derived for the AnO2+, as were second ionization energies for the monoxides, IE[AnO+]. The bare dipositive actinyl ions, UO2(2+), NpO2(2+), and PuO2(2+), were produced from the oxidation of the corresponding AnO2+ by N2O, and by O2 in the cases of UO2+ and NpO2+. Thermodynamic properties were derived for these three actinyls, including enthalpies of formation and electron affinities. It is concluded that bare UO2(2+), NpO2(2+), and PuO2(2+) are thermodynamically stable toward Coulomb dissociation to [AnO+ + O+] or [An+ + O2+]. It is predicted that bare AmO2(2+) is thermodynamically stable. In accord with the expected instability of Th(VI), ThO(2+) was not oxidized to ThO2(2+) by any of the seven oxidants. The gas-phase results are compared with the aqueous thermochemistry. Hydration enthalpies were derived here for uranyl and plutonyl; our deltaH(hyd)[UO2(2+)] is substantially more negative than the previously reported value, but is essentially the same as our deltaH(hyd)[PuO2(2+)].     

优化金属辅助法腐蚀液组分制备多孔硅

金属辅助化学腐蚀法可以在无外加电路的条件下,在40%HF/30%H2O2/乙醇的混合溶液中完成多孔硅的制备,该方法简单快速。本文研究了金属辅助法腐蚀液体系各组分(HF、H2O2、乙醇)含量对多孔硅表面的SiHx成分和多孔层结构的影响,根据Si-H和Si-O的红外吸收峰强度的变化曲线优化了腐蚀液体系中各组分含量。在腐蚀液各组分体积比为V40%HF∶V30%H2O2∶V乙醇=2∶2∶1和腐蚀时间为4 min的条件下制备了形貌均匀、化学活性(SiHx成分)和多孔结构稳定性较好的多孔硅,并对金属辅助法与阳极蚀刻法制得的两种多孔硅进行比较,结果显示金属辅助法制备的多孔硅的化学活性和稳定性在后续的生物技术应用中具有明显的优越性。     

Selective catalytic reduction of NO by propane over CoO(x)/Al2O3: an investigation of the surface reactions using in situ infrared spectroscopy

The partial oxidation of propane and the mechanism of the selective catalytic reduction (SCR) of NO by C3H8 over CoO(x)/Al2O3 catalysts were investigated using in situ infrared spectroscopy. Emphases are placed on the formation and reactivity of surface oxygenates during the SCR reaction. The SCR reaction starts with partial oxidation of propane to adsorbed acetate and formate. Impregnation of cobalt onto alumina greatly enhanced this reaction. The as-formed acetate acts as an efficient reductant for NO reduction. Surface nitrates (nitrites) are also reactive to propane and to oxygenates generated from C3H8 + O2 reaction. Surface -NCO species are formed over CoO(x)/Al2O3 catalysts. These nitrogen containing organic species are believed to be the direct intermediates in the final formation of N2. On the basis of these investigations, a proposed reaction mechanism explains the formation and roles of all intermediates detected by IR spectroscopy in this study.     

Hydrogen bubbles and formation of nanoporous silicon during electrochemical etching

Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etched nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of heavily n‐doped (R = 0.05–0.5 Ω·cm) silicon 〈100〉 single crystals at a current density of 10 mA cm^?2 in buffer oxide etch (BOE) electrolyte solution, defect craters containing textured nanopores were observed to occur in ring‐shaped patterns. The defect craters apparently originate at the hydrogen/BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density and high BOE viscosity allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current‐carrying Si? OH species are most likely responsible for the widening of the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the few nanopores formed at the ring edges are isolated from each other. Overall features observed in the photoluminescence intensity by XPS strongly emphasize the role of surface oxide that influences these properties. Copyright © 2005 John Wiley & Sons, Ltd.     

SiO2—Al2O3和MgO催化剂上吸附CH3NO2的TPD和IR研究

用TPD和IR方法研究了CH_3NO_2在典型固体酸SiO_2-Al_2O_3和固体碱MgO催化剂上的吸附分解。结果表明,在SiO_2-Al_2O_3表面CH_3NO_2吸附转化为表面甲酰胺物种,后者在高温下分解为CO_2和NH_3。在MgO表面CH_3NO_2吸附形成多种表面化学物种,它们在升温过程中脱附,并通过表面亚硝基甲烷物种分解为NO、C_2H_4、C_2H_6和N_2O.讨论了CH_3NO_2分解过程中表面酸、碱中心的作用。     

SO2对NO催化氧化过程的影响V.NiO/γ-Al2O3上SO2的作用机理

采用程序升温脱附(TPD)及漫反射原位红外光谱(DRIFT)技术分析比较了SO2存 在前后，NO—02反应气体在NiO／γ—Al2O3催化剂上所形成吸附物种的变化情况， 发现SO2能促使硝酸盐物种在低温分解并释放出NO2，而且耐热稳定的硝酸盐物种也 比单纯NO-02吸附时多．室温时催化剂表面上的SO2以弱吸附物种为主，特征红外吸 收峰位于1324cm^-1附近，温度升高后表面硫酸盐物种数量增多．关联SO2气氛中 NO2的生成规律后得出，类似于铅室反应中间体的多分子吸附物种[NO2(SO3)x]是产 生N02的活性物种，由SO2在载体或催化剂表面弱碱位吸附后吸引气相NO所产生，解 离O^-起到稳定活性物种和补充弱碱位的作用．同时该物种也是毒性物质SO4^2-的 前驱体，当KO氧化反应发生后催化剂的失活也开始了．     

In situ infrared spectroscopic studies on the mechanism of the selective catalytic reduction of NO by C3H8 over Ga2O3/Al2O3: high efficiency of the reducing agent

The partial oxidation of propane and the mechanism of selective catalytic reduction (SCR) of NO by propane over Ga2O3/Al2O3 in excess of O2 have been investigated using in situ Fourier transform infrared spectroscopy. An optimized Ga2O3/Al2O3 catalyst shows high activity and efficiency of the reducing agent propane (100% conversion of NO at 623 K, GHSV: 10,000 h(-1)). One molecule of propane converts more than 4 NO molecules to N2. The reaction starts with the partial oxidation of C3H8 by O2 and carboxylates (acetate, formate) are formed on the catalyst surface above 573 K. This oxidation represents the rate-determining step of the SCR reaction. These surface carboxylates represent a dominating intermediate and (easily) react with (adsorbed) NO forming nitrogen-containing organic species. The latter are proposed to react with NO to form N2. Total oxidation of propane was enhanced at temperatures above 773 K leading to decreased reductant efficiency. Surface nitrite and nitrate species can also be observed, but they were found to be spectators only. This could be concluded from the electron balance (conversion of propane relative to NO) and from the relative rates of the single reaction steps. On the basis of these investigations and stoichiometric calculations, a conclusive reaction mechanism is proposed.