Wet Chemical Etching Of Silicon Research Articles

Wet chemical etching of silicon for conically textured surfaces

Wet chemical etching of silicon for conically textured surfaces

About determining reliable etching rates and the role of temperature in kinetic experiments on acidic wet chemical etching of silicon.

A characteristic of the wet chemical etching of silicon in concentrated HF-HNO3 and HF-HNO3-H2SiF6 mixtures is the release of a high reaction heat, without its numerical value being known. This liberated heat can lead to a significant temperature increase during the etching process, especially when the volume of etching solution provided is low. A noticeable increase in temperature not only leads to an increase in the etching rate, it simultaneously changes the concentrations of dissolved nitrogen oxides (e.g. NO, N2O4 and N2O3) and intermediary species (HNO2), resulting in a change in the overall reaction process. The same parameters also influence the experimental determination of the etching rate. Further factors affecting the determination of the etching rate are transport phenomena as a result of the wafer positioning in the reaction medium and the surface properties of the Si used. As a result, etching rates determined from the mass difference of a silicon sample before and after etching are highly uncertain. This work describes a new method for the valid determination of etching rates using turnover-time curves that are calculated from the time-dependent temperature increase in the etching solution during the dissolution process. If only a slight increase in temperature is caused by the choice of proper reaction conditions, bulk etching rates representative for the etching mixture are obtained. Based on these investigations, the activation energy of Si etching was determined as a function of the concentration of the reactive species in the initial reaction step, the undissociated nitric acid (HNO3, undiss). Based on a total of 111 investigated etching mixtures, a process enthalpy for acidic etching of Si was determined for the first time from the calculated adiabatic temperature increases. With a value of -(739 ± 52) kJ mol-1, the determined enthalpy underlines the strongly exothermic character of the reaction.

Kinetic studies on acidic wet chemical etching of silicon in binary and ternary mixtures of HF, HNO3 and H2SiF6.

The etching of silicon with mixtures of hydrofluoric acid (HF), nitric acid (HNO3) and hexafluorosilicic acid (H2SiF6) proceeds in a complex reaction scenario consisting of interacting side reactions. Almost no other dissolution reaction is so massively dependent on the reaction conditions that influence the etching rate and the mechanism of the individual reactions. Extensive studies of the reaction rate of silicon etching in binary and ternary acid mixtures have allowed the transition point between the reaction-controlled and diffusion-controlled reaction regimes to be determined as a function of the composition of the etching mixture. It was verified that the reaction mechanism for binary and ternary mixtures does not differ and only the lower water content in ternary mixtures favours an enhanced formation of the reactive N(III) intermediate HNO2 in side reactions. Based on the exact knowledge of the point of mechanism change, determination of the reaction rate under quasi-isothermal conditions in the bulk etching range allows, for the first time, deriving formal kinetic terms from the kinetic data to describe the dissolution rate in both the reaction-controlled and diffusion-controlled regimes. The formal kinetic terms were designed both for the kinetically correct quasi-isothermal approach to the dissolution rate in the Si bulk and for the application-oriented approach that includes induction phases and temperature increases in the considered dissolution period as well as influences of the surface properties. Moreover, by using the water content of the etching mixtures as a proxy variable, uniform calculation of the etching rates in HF/HNO3 as well as in HF/HNO3/H2SiF6 mixtures in the entire composition range of the application can be formulated.

Comprehensive stoichiometric studies on the reaction of silicon in HF/HNO3 and HF/HNO3/H2SiF6 mixtures.

The stoichiometry of the wet chemical etching of silicon in concentrated binary and ternary mixtures of HF, HNO3 and H2SiF6 was comprehensively investigated. A complete quantification of both dissolved and gaseous reaction products was carried out for a variety of different acid mixtures. It could be shown that the total nitric acid consumption is directly determined by the concentration of undissociated HNO3 in the mixture and can be attributed to the consumption in subsequent reactions with increasing concentration. Furthermore, a critical minimum concentration of undissociated HNO3 of q(HNO3, undiss) ≥ 0.35 mol kg-1 could be determined, which is required to start the reaction at 20 °C with agitation, irrespective of the composition of the mixture (binary/tertiary). The simultaneous determination of the nitrogen oxides in the gas phase supports the theory that NO is the only direct reduction product of HNO3 in the reaction with Si. Furthermore, the amount of formed hydrogen is determined by both the HF and the HNO3 concentration in the mixture. For binary mixtures, the H2 formation can be quantitatively described as a function of the concentration of HNO3, HF and H2O. The most important finding from comparative investigations between binary and ternary mixtures is that the overall reaction is largely determined by the formation of the reactive intermediate HNO2 as a result of complex reaction pathways. Both the formation and the accumulation of this intermediate are determined by the water content of the etching mixture. The consumption of HNO3 and also the formation of the reaction products NOx and H2 can therefore be functionally described on the basis of the H2O content in the etching mixture, regardless of a binary or ternary mixture.

On the Operation Modes of Dual-Gate Reconfigurable Nanowire Transistors

We investigate the operation of dual-gate reconfigurable field-effect transistor (RFET) in the programgate at drain (PGAD) and program-gate at source (PGAS) configurations. To this end, dual-gate silicon nanowire (SiNW) FETs are fabricated based on anisotropic wet chemical silicon etching and nickel silicidation yielding silicide-SiNW Schottky junctions at source and drain. Whereas in PGAD-configuration ambipolar operation is suppressed, switching is deteriorated due to the injection through a Schottky-barrier. Operating the RFET in PGAS configuration yields a switching behavior close to a conventional MOSFET. This, howewer, needs to be traded off against strongly non-linear output characteristics for small bias.

Nano-finishing of MEMS-based platforms for optimum optical sensing

The surface finish of the microelectromechanical systems substrate, particularly the ones that are deployed in chip-based optofluidic systems, is of utmost importance, and the overall surface finish helps in preventing light scattering and associated losses. The proposed system is made up of a microchannel with a coating on its interior which acts as a cladding layer and possesses an air-like refractive index. The water-based solutions with refractive indices higher than the coating, when confined within such channels, act as waveguides with a refractive index difference which would allow grazing incidence at the solution film interface. The microchannel is fabricated over a piece of Si along <100> direction with the help of lithography and wet etching technique. After wet chemical etching of silicon, multiple pyramidal hillocks with overall large surface roughness is observed which are not appropriate for loss-free light transmission and cause various optical losses. So the overall optimization of surface roughness created by the etching processes is critical from an optical standpoint. Roughness in the microchannel surface mainly arises due to wet etching through tetra methyl alcohol, potassium hydroxide, potassium ferricyanide and isopropyl alcohol. In this work, we have obtained surface finish upto ~1.33 nm at an etch rate of 141 nm/min which is obtained by tweaking the composition of the participating reagents in the etchants and also the etching temperature. The surface roughness obtained is quite small in comparison to the wavelength range of the visible spectrum and thus losses were greatly avoided. The low refractive index films over silicon substrate were characterized by field emission scanning electron microscopy, energy dispersive analysis of X-ray, atomic force microscopy, 3D optical profilometry and ellipsometry. The transmission results show that transmission loss was reduced by 27.42% for the coated samples with 33 nm surface roughness as compared to surface with 250 nm roughness.

Anodic oxidations: Excellent process durability and surface passivation for high efficiency silicon solar cells

We investigate the versatility of anodically grown silicon dioxide (SiO2) films in the context of process durability and exceptional surface passivation for high efficiency (>23%) silicon solar cell architectures. We show that a room temperature anodic oxidation can achieve a thickness of ~70 nm within ~30 min, comparable to the growth rate of a thermal oxide at 1000 °C. We demonstrate that anodic SiO2 films can mask against wet chemical silicon etching and high temperature phosphorus diffusions, thereby permitting a low thermal budget method to form patterned structures. We investigate the saturation current density J0 of anodic SiO2/silicon nitride stacks on phosphorus diffused and undiffused silicon and show that a J0 of <10 fA cm−2 can be achieved in both cases. Finally, to showcase the anodic SiO2 films on a device level, we employed the anodic SiO2/silicon nitride stack to passivate the rear surface of an interdigitated back contact solar cell, achieving an efficiency of 23.8%.

A revised model of silicon oxidation during the dissolution of silicon in HF/HNO3 mixtures.

The stoichiometry of wet chemical etching of silicon in concentrated HF/HNO3 mixtures was investigated. The formation of nitrogen species enriched in the etching mixture and their reactivity during the etching process was studied. The main focus of the investigations was the comprehensive quantification of the gaseous reaction products using mass spectrometry. Whereas previously it could only be speculated that nitrogen was a product, its formation was detected for the first time. The formation of hydrogen, N2, N2O and NH4+ showed a dependence on the etching bath volume used, which indicates the formation of nitrogen compounds by side reactions. Simultaneously, the ratio of the nitrogen oxides, NO and NO2, formed decreases with increasing etching bath volume, while nitric acid consumption increases, so that the formation of NO2 could also be identified as a side reaction. Based on the stoichiometries obtained, a new reaction scheme for the reduction of nitric acid during etching in HF/HNO3 mixtures and an electron balance for the oxidation of silicon is presented.

Ferromagnetic nickel silicide nanowires for isolating primary CD4+ T lymphocytes

Direct CD4+ T lymphocytes were separated from whole mouse splenocytes using 1-dimensional ferromagnetic nickel silicide nanowires (NiSi NWs). NiSi NWs were prepared by silver-assisted wet chemical etching of silicon and subsequent deposition and annealing of Ni. This method exhibits a separation efficiency of ∼93.5%, which is comparable to that of the state-of-the-art superparamagnetic bead-based cell capture (∼96.8%). Furthermore, this research shows potential for separation of other lymphocytes, B, natural killer and natural killer T cells, and even rare tumor cells simply by changing the biotin-conjugated antibodies.

Two-Dimensional Terahertz Photonic Crystals Fabricated by Wet Chemical Etching of Silicon

The fabrication of two-dimensional terahertz photonic crystals using wet chemical etching of high-resistivity silicon is described. Wet-etched photonic crystals with a sloped sidewall were fabricated in 16 × 10 square lattices with a period of 1430 μm and a height of 150 μm, and were bonded between two parallel Au-coated silicon wafers for tight confinement in the vertical direction. The formation of a photonic band gap at a frequency of about 0.1 THz with a width of 14 GHz is demonstrated by the experimental results and the numerical simulations of the transmission frequency spectrum for the transverse magnetic mode.

Application of KOH Anisotropic Etching in the Fabrication of MEMS Devices

It is known that the wet chemical etching of silicon in alkaline solution has attracted wide attention due to its advantages such as lower cost, simpler setup, higher rate, smoother surface at micro level, higher degree of anisotropy, and lower pollution. In this paper, the key processes of fabricating vacuum microelectronic accelerometer and slits are presented. The cone curvature radius of the silicon tip arrays less than 30nm was fabricated with wet anisotropic etching of silicon in 33wt. % KOH solution at 70°C added potassium iodine (KI) and Iodine (I2) as additive and the cone aspect ratio was about 0.7. Smooth surface after etching in 33wt. %KOH solution added isopropyl alcohol (IPA) at 80°C was obtained and lateral etching was less than 5um after etching several hours for etching depth over 400um. Scalar slits with bottom width 25um and depth 500um were attained. A constant etch rate lead to precise and reproducible production. The test result reveals that the process to a specific occasion can reach practical requirements.

Simulating anisotropic etching of silicon in any etchant: evolutionary algorithm for the calibration of the continuous cellular automaton

An evolutionary algorithm is presented for the automated calibration of the continuous cellular automaton for the simulation of isotropic and anisotropic wet chemical etching of silicon in as many as 31 widely different and technologically relevant etchants, including KOH, KOH+IPA, TMAH and TMAH+Triton, in various concentrations and temperatures. Based on state-of-the-art evolutionary operators, we implement a robust algorithm for the simultaneous optimization of roughly 150 microscopic removal rates based on the minimization of a cost function with four quantitative error measures, including (i) the error between simulated and experimental macroscopic etch rates for numerous surface orientations all over the unit sphere, (ii) the error due to underetching asymmetries and floor corrugation features observed in simulated silicon samples masked using a circular pattern, (iii) the error associated with departures from a step-flow-based hierarchy in the values of the microscopic removal rates, and (iv) the error associated with deviations from a step-flow-based clustering of the microscopic removal rates. For the first time, we present the calibration and successful simulation of two technologically relevant CMOS compatible etchants, namely TMAH and, especially, TMAH+Triton, providing several comparisons between simulated and experimental MEMS structures based on multi-step etching in these etchants.

Faster and exact implementation of the continuous cellular automaton for anisotropic etching simulations

The current success of the continuous cellular automata for the simulation of anisotropic wet chemical etching of silicon in microengineering applications is based on a relatively fast, approximate, constant time stepping implementation (CTS), whose accuracy against the exact algorithm—a computationally slow, variable time stepping implementation (VTS)—has not been previously analyzed in detail. In this study we show that the CTS implementation can generate moderately wrong etch rates and overall etching fronts, thus justifying the presentation of a novel, exact reformulation of the VTS implementation based on a new state variable, referred to as the predicted removal time (PRT), and the use of a self-balanced binary search tree that enables storage and efficient access to the PRT values in each time step in order to quickly remove the corresponding surface atom/s. The proposed PRT method reduces the simulation cost of the exact implementation from to without introducing any model simplifications. This enables more precise simulations (only limited by numerical precision errors) with affordable computational times that are similar to the less precise CTS implementation and even faster for low reactivity systems.

Quantitative comparison of simulated and experimental silver crystal formation at the interface of silver thick film contacts on n-type silicon

Abstract We present results from a study of the nanostructure of silver thick film contact interfaces on n-type Si-(111) and Si- (100). At the interface of such contacts silver crystals grow in pits, which form during contact formation. They carry the current across the interface and hence determine the contact resistance, which is an efficiency limiting parameter of silicon solar cells. The size and shape of the silver crystals is governed by the pits in the silicon surface, because the crystals only emerge in these pits. Consequently, being able to predict pit characteristics in dependence of contact processing parameters will enable the prediction of the crystal size and coverage, which influences the contact resistance. In the present work, we investigate these pits experimentally by scanning electron microscopy. We are the first to simulate the pit formation at a silver thick film contact interface based on the removal probability of silicon surface atoms. For this purpose, an existing model, which was originally designed to describe the mechanism of wet chemical etching of silicon, is modified to match our interface conditions. Our simulations lead to a consistent and quantitative correct description of all experimental data. The simulations enable to predict pit formation for arbitrary contact formation process parameters like temperature or duration for silver thick film contacts on n-type silicon.

Analysis of gaseous reaction products of wet chemical silicon etching by conventional direct current glow discharge optical emission spectrometry (DC-GD-OES)

Major parts of the mechanism of the wet chemical etching of Si using mixtures of nitric and hydrofluoric acid are still in question. One of them is the issue about the formation of H and its relevance in the oxidation of Si. In the present work H evolution during acid etching of Si was estimated as a function of the HF/HNO3 mixing ratio and the temperature. A commercial GD-OES instrument with an unchanged Grimm type glow discharge chamber was used to determine the amount of formed H. A small fraction of the reaction gases, originated by etching of Si in a PTFE apparatus under Ar atmosphere, was fed by a constant Ar flow via a drying column into the discharge chamber. The gases were excited by the plasma of a continuously DC sputtered Fe sample. Numerous elements like Si, N, O, H, and even B from the B-doped Si were detected. However, only the H emissions at 121.5 nm and 656.3 nm were used for the estimation of the released H. The obtained results show that H is preferentially generated in HNO3-poor etch solutions (HNO3 < 40% (v/v)). In the range from −10 to 35 °C the temperature of the etch solution shows a negligible influence on the H generation, but affects the amount of nitrous gases. This indicates that particularly in HNO3-poor etch solutions the oxidation of Si proceeds to a considerable extent via the formation of H parallel to the already known pathway via the reduction of HNO3.

Integrated freestanding single-crystal silicon nanowires: conductivity and surfacetreatment

Integrated freestanding single-crystal silicon nanowires with typical dimension of100 nm × 100 nm × 5 µm are fabricated by conventional 1:1 optical lithography and wet chemical silicon etching. Thefabrication procedure can lead to wafer-scale integration of silicon nanowires in arrays. Themeasured electrical transport characteristics of the silicon nanowires covered with/withoutSiO2 support a model of Fermi level pinning near the conduction band. TheI–V curves of the nanowires reveal a current carrier polarity reversal depending on Si–SiO2 and Si–H bonds on the nanowire surfaces.

Octree-based, GPU implementation of a continuous cellular automaton for the simulation of complex, evolving surfaces

Presently, dynamic surface-based models are required to contain increasingly larger numbers of points and to propagate them over longer time periods. For large numbers of surface points, the octree data structure can be used as a balance between low memory occupation and relatively rapid access to the stored data. For evolution rules that depend on neighborhood states, extended simulation periods can be obtained by using simplified atomistic propagation models, such as the Cellular Automata (CA). This method, however, has an intrinsic parallel updating nature and the corresponding simulations are highly inefficient when performed on classical Central Processing Units (CPUs), which are designed for the sequential execution of tasks. In this paper, a series of guidelines is presented for the efficient adaptation of octree-based, CA simulations of complex, evolving surfaces into massively parallel computing hardware. A Graphics Processing Unit (GPU) is used as a cost-efficient example of the parallel architectures. For the actual simulations, we consider the surface propagation during anisotropic wet chemical etching of silicon as a computationally challenging process with a wide-spread use in microengineering applications. A continuous CA model that is intrinsically parallel in nature is used for the time evolution. Our study strongly indicates that parallel computations of dynamically evolving surfaces simulated using CA methods are significantly benefited by the incorporation of octrees as support data structures, substantially decreasing the overall computational time and memory usage.

New insights into acidic wet chemical silicon etching by HF/H 2O–NOHSO 4–H 2SO 4 solutions

Acidic wet chemical etching of crystalline silicon has been examined by utilization of HF–NOHSO 4–H 2SO 4 mixtures. In light of our previous studies the effects of nitrosyl ion concentrations on etching rates were studied time- and temperature resolved. The reactivity of crystalline silicon surfaces in HF/H 2SO 4 solutions is determined by NO +-ion concentrations at the silicon/electrolyte interface, measured by ion chromatography. Quantitative solution analysis proofed accumulation of ammonium ions and indicated the conversion of NO + as limiting for the overall etching process. Direct participation in the rate-limiting step was confirmed by calculation of activation energies. Increasing NO +-ion contents cause transition from reaction ( E A=55 kJ mol −1) to diffusion controlled ( E A=10 kJ mol −1) etching procedures. In combination with time and concentration dependent studies of produced structures a convenient regime for selective texturing or polishing polycrystalline silicon surfaces is reported. Qualitative analysis by 19F-NMR and Raman spectroscopy identified SiF 5 −/HF 2 − complexes as well as elementary hydrogen (H 2) as hitherto unknown products of silicon dissolution reactions in HF–NOHSO 4–H 2SO 4 mixtures. Based on our findings a strategy for fundamental investigations of relevant reaction pathways is presented and discussed with regard to reported mechanistic concepts.

Realization of Vertical and Zigzag Single Crystalline Silicon Nanowire Architectures

Silicon nanowire (SiNW) ensembles with vertical and zigzag architectures have been realized using wet chemical etching of bulk silicon wafers (p-Si(111) and p-Si(100)) with a mask of silver nanoparticles that are deposited by wet electroless deposition. The etching of SiNWs is based on subsequent treatments in chemical solutions such as 0.02 M aqueous solutions of silver nitrate (AgNO3) followed by 5 M hydrofluoric acid and 30% hydrogen peroxide (H2O2). The etching of the Si wafers is mediated by the reduction of silver on the silicon surface and in parallel by the oxidation of Si thereby forming SiO2 which is dissolved in the HF surroundings. The morphology of the starting silver (Ag) layer/Ag nanoparticles that form during processing on the Si wafer surfaces strongly influences the morphology of the SiNW ensembles and homogeneity of the etch profile. Our observations suggest that the Ag layer/Ag nanoparticles not only catalyze the wet chemical etching of silicon but also strongly catalyze the decomposit...

Wet chemical etching of silicon {1 1 1}: Etch pit analysis by the Lichtfigur method

The Lichtfigur method is introduced to determine the average pit morphology of etched silicon surfaces. It is an effective method in surface analysis complementary to optical microscopy. The surface morphology of silicon {1 1 1} after wet chemical etching in aqueous KOH solution has been investigated by this technique. The morphology does not change significantly in pure KOH solutions as a function of time. In solutions containing additive isopropanol, however, the etch pits evolve from circular to triangular, with a final shape that depends on the isopropanol concentration. It has been experimentally proven that the IPA additive does not participate in the etching reaction and no etching occurs in the absence of water, which implies that OH − only serves as a catalyst.