Macropore Etching Research Articles

(Invited) Scratch Controled Electrochemical Production of Macropores in Silicon

Electrochemical etching of macropores in Si can be done either randomly (wild pores) or lithographically defined. When the points to be etched are not defined, the etching process starts at defects, or statistically distributed on the surface, just limited by the space-charge-region created in the semiconductor as effect of the electric field applied [1]. Ordered pores are usually accomplished by lithography plus a preliminary etching step for the formation of inverted pyramids [2]. At the pits of the pyramids the electric field is more intense during the electrochemical etching process, causing that the probability to etch pores is higher at those points. Defining the positions where pores are etched is necessary for different applications, where order is important. However, this makes the process to produce pores, which is not necessarily clean, more expensive with the need clean-room appliances (masking layer deposition and lithography).In order to simplify the production of ordered micropores, in this work it is proposed to take advantage of the increased probability of surface defects to act as nucleation points of growing pores. An electronic grade single side polished p-type Si wafer, with resistivity of 20 Ωcm was used as starting material. The electrolyte was a 10 % (v/v) HF solution in DMF. The wafer was scratched with fine sand paper previous to the etching process. Electrochemical etching was performed aplying a current density of 20 mA/cm^2 for 10 min. Macropores grew mainly along the scratches (see Figure). The spacing between pores depends on the distances between scratches. The regions without scratches have a lower probability to be etched. However, if the defects introduced by the sand paper are cured by a short chemical etching process in KOH, the probability to electrochemically etch those sections is further increased, in comparison with the sections with scratches and no KOH treatment. With the observations, it is straight to think that it is possible to do deffect engingineering to tune the electrochemical etching of micropores in Si.[1] E. Quiroga-González, H. Föll, book chapter "Chapter 2: Fundamentals of Silicon Porosification via Electrochemical Etching", in "Porous Silicon: Formation and Properties, Volume 1", editor G. Korotcenkov, CRC Press. (2015). ISBN: 9781482264548.[2] E. Quiroga-González, E. Ossei-Wusu, J. Carstensen, H. Föll, J. Electrochem. Soc. 158(11) E119 (2011). Figure 1

Correction to: Generation and Auto-Revealing of Dislocations in Si During Macropore Etching

On page 5116, last paragraph in the XRD Data section, there is an error. The corrected sentence is as follows:

Generation and Auto-Revealing of Dislocations in Si During Macropore Etching

The influence of the variations in the photoelectrochemical etching regimes of macropores in n-Si (within the limits of V. Lehmann optimums) on the quality of structures was studied. At some regimes, generation of screw (cylindrical or spiral) dislocations during the etching process under normal conditions was observed. First, dislocations are formed, and then macropores are etched, mainly on near-surface traces of dislocations. We call this process “auto-revealing”. The values of residual lattice distortions due to deformation of the silicon plate during the etching process were determined by x-ray diffraction on a structure obtained in the acceptable boundary regime.

Quasi-Zero-Voltage Controlled Etching of Macropores in n-Type Silicon

Here, we report for the first time on the controlled and accurate photoelectrochemical etching of macropores in n-type silicon in hydrofluoric acid-based electrolytes at anodic voltages below the electropolishing peak value down to quasi-zero Volt. Our results break a new ground on the controlled electrochemical dissolution of n-type silicon by further improving flexibility of this technology and pushing it towards commercial microelectronic applications. In the last few years the electrochemical micromachining (ECM) technology, which is based on the photoelectrochemical etching (PECE) of n-type silicon, is increasingly emerging as a promising silicon microstructuring technique alternative to standard both dry and wet etching tools1. A number of high aspect-ratio three-dimensional microstructures and microsystems have been reported over the past 10 years with applications ranging from automotive and biomedics to microelectronics and nanomedicine. The golden rule for the controlled PECE of macropores in low doped n-type silicon states to conduct the etching at anodic voltage values (Vetch ) above the electropolishing voltage value VPS (typically of a few Volts)2-5. Nonethless, even at Vetch >VPS there are a few constraints that limits the feasibility of macropores with arbitrary size and pitch. Specifically, it has been suggested that the maximum diameter of stable pores (about 10 μm for silicon of a few Ωcm) depends on the resistivity of the silicon substrate3,5. Moreover, the maximum pore pitch (about 20 μm for silicon of a few Ωcm) appears to be roughly limited to twice the depletion region at the silicon/electrolyte interface3. The starting material is a (100) oriented, n-type silicon wafer, with a resistivity of 3÷8 Ω∙cm and thickness of 750 μm, provided with a thermally grown 200-nm-thick silicon dioxide layer. A square lattice of holes with side of 4 μm and pitch of 20 μm was defined on the front surface of the silicon wafer by standard photolithography followed by buffered hydrofluoric acid (BHF) etching of the silicon dioxide and potassium hydroxide etching of silicon through a photoresist mask. Patterned wafers were electrochemically etched for 6000 s under back-side illumination (250 W halogen lamp) by using an aqueous solution of HF (concentration of 5% by vol). The current density/voltage (J/V) curve of the n-type silicon-electrolyte system under investigation has an electropolishing peak (VPS , JPS) at JPS =70.0 mA/cm2 (± 0.5 mA/cm2) and VPS =1.2 V. The same initial current density value Jetch0 =4.8 mA/cm2 was used for all the experiments. The Jetch value was then varied over time to ensure the etching of macropores with constant diameter over depth1,2,4. The anodic voltage values Vetch was varied among the different experiments from 0 to 2 V. Eventually, the electrochemically etched wafers were rinsed in ethanol and dried at 60 °C for 10 min. For each Jetch-Vetch parameter pair were performed 3 experiments, at least. Notice that, three of the Vetch values (i.e. 0, 0.4 and 0.8 V) are below the electropolishing peak value VPS and one is above (i.e. 2 V). Macropores etched at Vetch >VPS (i.e. 2 V) show a significant lateral branching and, in turn, a major roughness on the inner surface. This was somehow expected as both pitch (about 20 μm) and size (about 10 μm) are at close to the maximum features controlled by PECE according to the state-of-the-art literature3. On the contrary, the etching at Vetch <VPS led to very exciting and totally unexpected results, with the formation of regular macropores array without missing pores at any of tested voltages. Moreover, pore branching was significantly reduced as the anodic voltage was diminished below VPS and it completely disappeared at 0 V. A quantitative analysis of average values of etched macropores in terms of length and size points out that as the anodic voltage is reduced below the electropolishing value VPS regular macropores with reduced roughness, increased size, and shorter length are etched. We argue that the lower anodic voltage enables clustering of oxide patches at the tip of macropores over larger areas, compared to higher anodic voltages, thus slowing down the etching process in the vertical direction and increasing, in turn, the macropore diameter. Bassu, S. Surdo, L. M. Strambini, G. Barillaro, Advanced Functional Materials, 2012, 22 (6), 1222-1228.Föll, M. Christophersen, J. Carstensen, G. Hasse, Materials Science and Engineering R, 2002, 39(4), 93-142.Lehmann, Electrochemistry of Silicon: Instrumentation, Science, Materials, and Applications, Wiley-VCH, ISBN: 3-527-29321-3, Weinheim, 2002.Barillaro, F. Pieri, Journal of Applied Physics, 2005, 97, 116105-3.Barillaro, L.M. Strambini, Electrochemistry Communications, 2010, 12, 1314-1317.

Quasi-Zero-Voltage Controlled Etching of Macropores in n-Type Silicon

Here, we report an experimental study about the controlled etching of macropores in n-type silicon electrodes at quasi-zero anodic voltage using hydrofluoric acid-based photo-electrochemical etching (P-ECE). This breaks a new ground on the controlled electrochemical dissolution of n-type silicon, for which the use of anodic voltage above the electropolishing peaks (VPS , JPS) has been considered to be a golden rule for the etching control, so far. Remarkably, our experimental results clearly show that it is possible to control the etching of macropores at anodic voltages well below (close to zero Volt) the electropolishing peak and that the use of low anodic voltages is beneficial for the fabrication of regular macropores with large diameter (i.e. 10 μm) and spatial pitch (i.e. 20 μm).

FFT-Impedance Spectroscopy for Silicon Microwire Anodes: Identifying Temperature and Diffusional Changes

Li-ion batteries replace nowadays many devices for different applications like in automotive industries, in smart phones, medicine, aerospace and in other fields. Those applications/fields are very different and many different requirements have to be fulfilled like fast charging, high capacity, long term stability, cost effective production and temperature stability. For a wide application range, a battery has to perform under different temperatures and environmental conditions and nevertheless have a stable and high capacity. The silicon microwire anodes exhibit a high stable capacity of 3150 mAh/g for 300 cycles with a high amount of active material (1.35 mg/cm2) at ambient temperatures. Those anodes are fabricated based on different dry and wet chemical etching processes as pre-structuring of the p-doped Silicon wafer. The main step in the micro-structuring of the silicon microwires lies in the electrochemical macropore etching. By applying a specific current-time profile, macropores of specific diameter and lengths are etched deep in the pre-structured p-doped silicon. Such a profile could be changed according to the needs of the application; changing the lengths and the shape of the pores. In this paper, wire lengths of 36 µm up to 75 µm with a constant thickness of 1.4 µm as well as wire thicknesses between 1.2 µm up to 1.8 µm with a length of 60 µm. During the additional over-etching step, the interstices between the pores are etched away until the wires remain free-standing. In a two-step process, a copper current collector is deposited galvanically on top and in between of the wires. With this approach a good mechanical stability between the silicon wires and the current collector is provided and reduces the lateral volume expansion in only one direction. In a layer-transfer process, the anode could be separated from the silicon wafer. Compared to other silicon anode systems, this approach manages without any conductive additives and with a high amount of active material. The anodes are tested in half cells with metallic Lithium as counter and reference electrode with standard anode electrolyte LP30, provided by BASF, with lithiumhexafluorophosphate (LiPF6) as lithium salt. In order to test the temperature influence on the electrochemical performance of the silicon microwire anodes, this study focuses on the properties inside the anode recorded via in-situ FFT-impedance spectroscopy. In a special set-up the half-cells are heated and recorded between 20 °C up to 75 °C. This technique is a linear response analysis, where a small senoidal signal is superimposed on the dc operation point. In comparison to the standard electrochemical impedance spectroscopy (EIS), perturbations of different frequencies can be applied at the same time between 5 Hz up to 20kHz [1, 2]. The resulting three time constants and resistances can be interpreted as different electrochemical processes inside the battery anodes. Those measurements are correlated with the cyclic voltammetry measurements which are performed in a voltage regime between 1 V and 20 mV for five cycles. In previous studies [3] the size dependency of the wire dimension on the lithiation/delithiation potential was shown at ambient temperatures. Here, it showed that the charge transfer mechanism into the silicon wires of different thicknesses a “radial diffusion” process dominates. This study illustrates the diffusional changes of the properties and its influence on the dimension if temperature is impeded on the anodes. Figure 1a shows results of the time-dependent resistances and time constants for two different temperatures. Figure 1b shows a photo of the type of anodes used for this study. Figure 1 c shows an example of curve of the cycling performance of the silicon wire anodes at different temperatures with SEM images of the microwires after the tests. This study focuses on the in-situFFT-Impedance spectroscopy to monitor the temperature influence on diffusional and charge transfer processes. The recorded time constants shown in Figure 1a at different temperatures show, that the charge transfer process is much slower at high temperatures showing the slow diffusional processes due to side effects. Those results are used for process optimizations to achieve long-term stable anodes under cycling temperatures. Figure 1: Results of electrochemical characterization a) in-situ-FFTImpedance spectroscopy at various temperatures for 75 µm long wires, b) an example of silicon wire anodes, c) cycling experiments at different temperatures. [1] J. Carstensen, H. Föll, A. Cojocaru, and M. Leisner, Phys. Status Solidi C6(7), 1629 (2009). [2] J. Carstensen and H. Föll, ECS Trans.25(3), 11 (2009). [3] S. Noehren, E. Quiroga-Gonzalez, J. Carstensen, and H. Foell, J.Electrochem.Soc. 163(6), A1 (2016). Figure 1

Electrochemical macroporous silicon etching with current compensation

The depth-dependent changes of macroporous etching in aqueous hydrofluoric (HF) solutions are addressed. Macroporous etching of n-type silicon in HF acid produces high aspect ratio structures. However, an irregular etch profile can emerge as the pore depth increases, in particular branched regions emerging from vertical pores. The root cause of this non-ideal etch profile is investigated, focusing on the dark current component of electrochemical etching. The dark current during etch progression was simulated, and it was found that increased dark current resulted from increasing the interfacial contact between the electrolyte and silicon surface. A software-mediated photocurrent compensation method is developed that periodically samples the dark current component of the etch and adjusts the overall etch current, achieving stable pore formation.

Scalable processing and capacity of Si microwire array anodes for Li ion batteries.

Si microwire array anodes have been prepared by an economical, microelectronics compatible method based on macropore etching. In the present report, evidence of the scalability of the process and the areal capacity of the anodes is presented. The anodes exhibit record areal capacities for Si-based anodes. The gravimetric capacity of longer anodes is comparable to the one of shorter anodes at moderate lithiation/delithiation rates. The diffusion limitation of the lithium ions through the electrolyte in depth among the wires is the limiting factor for cycling longer wires at high rates.PACS82.47.Aa; 82.45.Vp; 81.16.-c

Fast Macropore Etching via Large Current and High HF Concentration on p-Type Silicon

Etching rate of macropores silicon is important to the application of porous silicon for massive production. In this study, we demonstrate a fast macropore etching on p-type silicon. HF-containing electrolytes were modified with organic solvent, dimethylformamide (DMF), and 1-2 μm macropores with nice quality were fast produced on p-type silicon substrate. The effects of HF concentration and current densities on the macropore etching rate have been investigated. By applying large current densities and HF concentration, an etching rate as high as 1900 μm/h (approx 32 μm/min) was achieved in our work, along with rather straight and smooth sidewalls.

Fast Macropore Etching Via Large Current and High HF Concentration On p-Type Silicon

not Available.

Copper electrodeposition into macroporous silicon arrays for through silicon via applications

The present paper deals with the formation of high conductivity through silicon via from macroporous silicon arrays. The through wafer macropores were first etched by anodization into a hydrofluoric acid - ethanol mixture. The conditions of straight and ordered macropore etching were studied. The high aspect ratio (18) and high density via (above 10^5/cm^2) were then filled by copper using an optimized potentiostatic technique involving a specific electrolyte with additives. The copper micro-wires were observed by SEM whereas XRD analysis enabled the determination of the average grain size.

Good cycling performance of high-density arrays of Si microwires as anodes for Li ion batteries

Optimized Si wire anodes have been prepared by macropore etching in p-Si and subsequent chemical/galvanic processing. The anodes have been optimized with respect to capacity, cycling performance and processing costs by fine-tuning the wire geometry. The optimized anodes have wires with ≈1μm in diameter and length of 70μm; they are as densely spaced as possible regarding volume expansion and electrolyte penetration requirements. A stable capacity of 3150mAh/g for 100 cycles has been reached during cycling to 75% of the maximum capacity. The operational range of the anodes is 0.11–0.7V. Optimal charging conditions were established by cyclic voltammetry measurements and battery cycling tests.

Silicon nanowires made via macropore etching for superior Li ion batteries

AbstractThe future of electro mobility depends critically on substantially improved Li ion batteries. Si as anode material has a more than tenfold higher capacity as compared to the standard graphite anode, but needs to be nanostructured to avoid fracture. It is shown that macropore etching combined with suitable follow‐up processes allows to produce nanowire arrays with optimized geometries. First tests of these anodes showed very promising results with respect to prime battery parameters like capacity and capacity losses during cycling. In particular, a first test battery showed superior performance for more than 60 cycles in comparison to an otherwise identical battery with a graphite anode. Critical processes like galvanic Cu deposition at the nanowire bottom can be avoided by using an optimized pore etching process that produces complex pore diameter profiles as a function of depth, allowing easy separation of the nanowire layer from the Si substrate and processing the nanowire surface area. In total, the production of Si nanowire anodes using this improved process should allow mass production at competitive costs. magnified imageSi nanowire array for use as a high‐capacity anode in a Li ion battery.

Si nanowire arrays as anodes in Li ion batteries

AbstractSi nanowires can incorporate large amounts of Li without fracturing and are thus prime candidates for anodes in Li ion batteries. Anodes made from Si nanowires offer a specific capacity per gram more then 10 times larger than the present graphite standard. It is shown how optimized Si nanowire arrays embedded in Cu can be produced in a relatively simple way employing macropore etching in Si followed by chemical etching and Cu galvanic deposition. First tests of these arrays in half‐cells and batteries demonstrated a substantially increased capacity, small irreversible losses and cycle stability. In particular more than 60 charge/discharge cycles could be realized without loss of capacity. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photo-electrochemical etching of macro-pores in silicon with grooves as etch seeds

Role of linear nucleation centres (surface grooves) in formation of macro-pores has been studied for n-type Si. It has been found that linear nucleation centres, contrary to the case of point nucleation pits, initiate formation of semi-regular pore pattern: along the grooves the pores are arranged randomly as a result of self organization process, across the grooves they follow the prescribed period a. The average distance between the pores along a groove line depends on a, and decreases as a increases. At the optimal value of etching current density and seed period a, the distinct pores can merge, thus forming a periodic structure of one-dimensional photonic crystal. Optical properties of grooved structures with merged pores have been investigated by means of Fourier infrared reflection micro-spectroscopy in the spectral range from 1.45 μm to 16.5 μm. Reflectance spectra reveal presence of photonic band gaps which positions in the middle IR range are in a good agreement with calculations by transfer matrix method. The fabrication approach developed is promising for application in silicon-based photonics.

New modes of FFT impedance spectroscopy applied to semiconductor pore etching and materials characterization

AbstractA systematic approach for the application of fast Fourier transform (FFT) impedance spectroscopy to semiconductor photo‐electrochemistry is given. In particular, photo‐impedance spectroscopy in two novel modes is used in conjunction with conventional current–voltage impedance spectroscopy, and applied to some aspects of pore etching in InP and Si as well as to the characterization of multicrystalline Si or other solar cell material. The technique, applicable in situ with high time resolution, employs optimized hardware and software, extensive mathematical modeling of carrier transport, and the numerical implementation of fast parameter extraction algorithms. The complete system with a fully integrated impedance spectrometer delivers a wealth of new data including key parameters of pore etching. Examples given include the in situ tracking of the fast switch‐over between pore growth modes in InP as well as following a pore etching process over long time scales in Si. FFT impedance spectroscopy in the novel back side illumination mode, in combination with the conventional mode, allows not only some active control of macropore etching in n‐type Si by supplying crucial real‐time data like the valence of the dissolution process and the pore depth, but also provides new fundamental insights into the electrochemical processes occurring at the (porous) electrode. FFT photo‐impedance spectroscopy in the front side illumination mode used in conjunction with a scanned laser beam allows fast local measurements of all semiconductor parameters relevant for solar cell applications, like doping concentration, minority carrier lifetime, diffusion constant, or surface recombination velocity with high spatial resolution; this is demonstrated for large multicrystalline Si wafers. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

In-Situ Assessment of Macropore Growth in Low-Doped n-Type Silicon

Understanding pore growth in semiconductors in detail needs some in-situ information about the processes occurring in a growing pore. It is shown that dual-mode in-situ FFT impedance spectroscopy can provide useful data for macropore etching in n-type Si under back side illumination. A new illumination mode is used alternating with the conventional current-voltage mode, and data evaluation is based on an advanced model developed for this kind of pore etching. The principles of the technique and the basics of the model are introduced and illustrated by a number of examples, some of which show new kinds of pore growth modes.

Electrochemical etching of macropores in silicon with grooved etch seeds

Specific features of macropore formation in n-Si with grooved etch seeds on the surface have been studied. In contrast to point nucleation centers, linear centers form a half-ordered lattice of pores: in the course of the self-organization process, pores randomly originate along the grooves and follow a prescribed period a across the grooves. The average distance between the pores along the groove line depends on a and decreases as a grows. By choosing the optimal a and etching photocurrent density, separate pores can be merged into grooves to give rise to a periodic structure of trenches with vertical walls. Some aspects of the pore-pore interaction for different surface profiles and the formation of zones free from pore nucleation are considered.

Nonuniformity of the macropore etching front in thin n-Si wafers

A factor responsible for the appearance of a dome-shaped front of macropore etching in thin silicon wafers during their photoelectrochemical etching in a cell with a ring anode has been established. It is shown that the formation of a dome-shaped front is related to a radial voltage drop in the substrate, which leads to a decrease in the current density in the central part of the etched wafer. The effect of applied voltage on the shape of the front has been studied. We suggest monitoring of the development of nonuniformity in the course of etching using a change (i) in the temporal variation of the adjusted intensity of illumination and (ii) in the current-voltage characteristic. Various methods intended to provide deep uniform macropores are considered.

Random and Ordered Macropore Formation in p-Type Silicon

Random and ordered macropore formation in p-type silicon have been studied experimentally. We found that the density of macropores in a random macropore nucleation regime is linearly depending on the samples doping concentration. Macropore etching rate depends linearly on the current density up to the critical current density value; a saturation in the etching rate takes place when this value is exceeded. The so-called proximity effect was discovered for the ordered macropore formation regime. This phenomenon, and large pore diameters and pore wall thickness, and their variations have been observed in the experiments which can be accounted for in the frame of a simple model of the current localization at the pore bottom. An analysis of the experimental data based on the at present time existing theoretical models has been done. This analysis leads to the conclusion that none of these models can describe the existing experimental data including data of this work. It is clear that new experimental data are needed to come to a universal model of the electrochemical macropore formation. © 2001 The Electrochemical Society. All rights reserved.