Electrochemical Etching Of N-type Silicon Research Articles

Surface structure and wettability tuning of porous silicon films by capillary-driven surface texturing under different current densities of electrochemical etching

Silicon-based biomimetic surfaces with special wettabilities are difficult to be fabricated without lithographic techniques. In this report, superhydrophobic surfaces of porous silicon (PSi) with adjustable papillary microstructures have been prepared simply by electrochemical etching of n-type silicon, capillary-driven surface texturing (CDST) of PSi films and surface modification. According to the results of FE-SEM and Drop Shape Analyzer, surface morphologies (from 2D pore structures to 3D papillary microstructures) and corresponding wettabilities (Contact angles, CAs: 134.8°-158.9°; Sliding angles, SAs: 180°-1°) of PSi films can be adjusted efficiently by current density (<20 mA cm−2). The sizes and distribution of inverted-pyramid structures (IPSs) at the interfaces and symbiotic macropores in their centers are essential for such an adjustment. Considering the excellent biocompatibility of PSi, Such surfaces will be beneficial to the silicon-based biosensors, microarrays and microfluidics.

Controlling the Electrochemical Etching of Pores with High Aspect Ratio at the Submicrometer Scale in Silicon

In recent years, anodic dissolution of silicon in HF (hydrofluoric acid)-based electrolytes has been largely used for the preparation of ordered and random forms of porous silicon [1,2], from both p-type and n-type materials. In particular, randomly-distributed meso and nanopores have been prepared from p-type silicon electrochemically etched in dark at high HF concentrations [2]; ordered macropores and complex microstructures [3] have been fabricated by back-side illumination electrochemical etching of n-type silicon at low HF concentrations. Here we report for the first time on the controlled etching at high anodic voltage (up to 40V) of two-dimensional (2D) arrays of ordered pores with sub-micrometric diameter (down to 800 nm) at high depths (up to 40µm), yielding a maximum aspect ratio (AR) of 50, using low-doped (resistivity 3-8 ٟcm) n-type silicon. Back-side illumination etching of silicon at low HF concentration (5% by vol. in H2O) is used for the silicon etching, starting from a pre-patterned silicon surface featuring a 2D lattice of holes with size of 1µm and spacing of 1.8µm. The electrochemical etching of the above-described patterned silicon at low anodic voltages (<3V), which is the standard condition for back-side illumination etching, does not allow to obtain a uniform array of pores with constant diameter and same depth, according to both literature and experimental data obtained in this work. Holes generated at the back-side surface (by illumination) diffuse through bulk silicon and are collected at the silicon/electrolyte interface in correspondence of surface defects (either native or pre-patterned), which are known to act as seed-points for pore growth due to the hole-focusing effect arising from the higher electric field established at their tip. At low Vetch values (<3 V) hole-focusing effect has been already proven to be highly effective when diameter and spacing of defects/pores are greater than a few microns (>2 μm) [3]. Nonetheless, when diameter and spacing of defects/pores is below 2μm, low Vetchvalues (<3 V) lead to an unstable and non-uniform pore growth, where some of the pores stop growing and others pores continue their growth though uncontrolled both in depth and diameter. We argue that such a mechanism is due to overlapping (partially at least) of the depletion regions between adjacent pores, which leads to defocusing of the electric field lines in correspondence of surface defects and, in turn, to an uncontrolled etching. In this work we show that increasing the Vetch value (from 15 up to 40V) has beneficial effects on the controlled etching of pores with sub-micrometric diameter and spacing below 2μm. Experimental data show that as the anodic voltage is increased over 15V, electric field lines narrow more and more around the defect/pore tips, thus improving the hole-focusing effect and enabling, in turn, the uniform etching of pores with constant diameter and same depth. In fact, the number of missing pores (pores that stop their growth at a given depth) significantly reduces as the anodic voltage approaches 15V and for arrays etched at Vetch=35V no missing pores are visible. According to the literature [4], our experimental results also show that a Jetch threshold value also exists setting the minimum porosity (P) value (Pmin about 15% for this work) versus pore density over which stable growth of pores can be achieved at high anodic voltages. For a given Vetch value, as Jetch increases, the array porosity and, in turn, the pore diameter increases. On the other hand, for a given Jetch value, as the anodic voltage increases, the diameter of etched pores reduces and depth increases. For instance, etching of our patterned silicon with Jetch=13.4 mAcm-2 (P=17%) and Vetch=35V leads to the formation of ordered arrays of sub-micrometric pores with diameter of about 800nm and depth of about 40μm. Building on these results, controlled electrochemical etching of ordered meso and nanopores and structures in low-doped n-type silicon is envisaged. If successful, this would result to be game-changing in the silicon micro and nanofabrication arena, with applications (though not limited) to nanoelectronics, nanoelectromechanical systems (NEMS), and nanomedicine.

Electrochemical Preparation of In-Silicon Hierarchical Networks of Regular Out-Of-Plane Macropores Interconnected by Secondary In-Plane Pores Through Controlled Inhibition of Breakdown Effects

In-silicon hierarchical networks of ordered out-of-plane macropores interconnected by high-density secondary in-plane pores are prepared by controlled electrochemical etching of n-type silicon in HF:H2O2 electrolyte through the synergistic use of back-side illumination, avalanche breakdown anodization voltage, and high-oxidizing-power chemical. Preparation of the hierarchical networks of pores is enabled by controlled inhibition of breakdown effects at high anodization voltages (in the breakdown region) through back-side illumination of the silicon electrode. Inhibition of breakdown effects in pre-patterned silicon electrodes etched under galvanostatic condition at high anodization voltages is used to simultaneously control formation of out-of-plane macropores by regulating the photogenerated etching current density flowing at the pore tips and enable in-plane branching of out-of-plane pores by increasing leakage current at the lateral pore surface through high voltage effects. Further, high-oxidizing-power chemical, namely H2O2, is used to increase density, length, and diameter of branching and, in turn, enable interconnection of out-of-plane macropores with in-plane secondary pores.

Photocarrier Promoted Pore and Inverted Pyramid Formation in Porous Silicon Films on n-Type Silicon

Porous silicon (PS) films with photocarrier-created pores (PCPs) in the films and symbiotic inverted pyramids (SIPs) at the interfaces were prepared by electrochemical etching of n-type silicon in HF ethanol solution. The effect of back side illumination, doping level of silicon, current density and HF concentration on pore and interface morphology of PS films were investigated. SEM results demonstrated that Photocarriers, generated by back side illumination, was essential to the formation of PCPs and SIPs. The moderately doped n-type silicon with P doping level in 1.1 × 1017−2 × 1016 was suitable for PCP formation. The sizes of SIPs and the quantity of PCPs at their centers could be adjusted by current density when it was smaller than 25 mA cm−2, in which the formation of PCPs competed with the growth of PS film for photocarriers and the increase of current density enhanced the growth of PS film. The formation of SIPs with four crystal planes approaching energy-stable (111) planes was believed to balance these two competitive behaviors. Since the PCPs were actually caused by photocarrier-promoted chemical etching, the HF concentration suitable for PCP formation was smaller than 20%.

AES and XPS investigations of the surface layers of porous silicon with Fe, Co, and Ni embedded pores

A Composite material of porous silicon with pore embedded d-metals por-Si(Me) can be used for fabricating memory cells. Por-Si layers have been produced by electrochemical etching of n-type silicon (100) according to the conventional process. Fe, Co, and Ni galvanic deposition in por-Si has been performed using aqueous solutions of corresponding sulfate salts. The Auger profiles of the obtained por-Si(Fe), por-Si(Co), por-Si(Ni) nanocomposites have shown that its surface layers (up to 40 nm thick) contained about 10% Fe and no more than 1% Co and Ni. These facts confirm data that Co and Ni, unlike Fe, penetrate deep into the silicon pores. The value of the magnetic moment for the nanocrystalline Ni atom incorporated into the por-Si(Ni) has been estimated based on the dependence of the relative intensity of the maxima for the 3s multiplet splitting of the X-ray phase spectra on the number of uncoupled d-electrons in systems with 3d metals. The obtained value ∼2.4 μB exceeds the atomic magnetic moment in bulk metallic Ni.

Macroporous silicon: A versatile material for 3D structure fabrication

This work focuses on the fabrication of three-dimensional (3D) microstructures by electrochemical etching of n-type silicon. We exploit the possibility of modulating the pore diameter along the growing direction to produce quasi-3D structures which exhibit periodicity also in third dimension. The fabricated structures are further processed to convert them into truly 3D networks consisting of interconnected silicon veins embedded in air. In particular, we show how a variety of complex 3D networks can be obtained starting from a simple initial pore arrangement, e.g. square or hexagonal. These 3D structures can be easily fabricated with very high precision and uniformity on a large scale. They constitute ideal hosts for sensing applications as well as perfect molds for casting 3D structures of unusual materials.

Tunable ferroelectric photonic crystals based on porous silicon templates infiltrated by sodium nitrite

Ferroelectric photonic band-gap (PBG) structures are fabricated by the infiltration of sodium nitrite into the nanoporous silicon templates composed by the electrochemical etching of n-type silicon. Reversible spectral tuning of the PBG and of the microcavity mode up to 15nm is attained in the temperature interval from 50to165°C due to the temperature changes of the refractive index of sodium nitrite in nanoporous silicon matrix. Optical second-harmonic generation measurements prove the ferroelectric state of the composed nanocomposite PBG structures.

Bi-stable pore size during electrochemical etching of n-type silicon during a thermal ramp

We have investigated the change in size and density of pores during electrochemical etching of n-type (5 Ωcm) silicon under backside illumination and subjected to a thermal ramp. The pore structure was allowed to self organize, and for the parameters reported here this results in macropores with diameters in the ∼ μm order of magnitude range. As the etching progressed under constant current conditions, the electrolyte and the sample heated slowly up in the temperature range 20–60 °C. The resulting pore structure was obtained by scanning electron microscope examination of cross sections of cleaved samples. The temperature ramp caused the pore diameters and pore densities to change abruptly rather than continuously. The change can thus phenomenologically be described as a transition between two stable pore size configurations; bi-stable switching or phase transition. This phenomenon is observed for a range of parameters yielding macropores: current densities between 2–20 mA/cm 2 and varying light intensity. The transition is between known configurations.

Fabrication of silicon oxide microneedles from macroporous silicon

This paper presents a novel technique for silicon dioxide (SiO 2) microneedle fabrication. Microneedles are hollow microcapillaries with tip diameters in the range of micrometers. They can be used in the fabrication of microsyringes. These structures can be of high interest in medical and biological applications, such as DNA injection, antibody manipulation and drug delivery, and cell manipulation. Fabrication process is based on electrochemical etching of n-type silicon in hydrofluoric acid (HF) solutions. Basic process flow and etching conditions that ensure a stable pore growth are described. These conditions also determine the geometry of the resulting microneedle structure. Microneedle arrays of different dimensions can be fabricated in a single run on the same wafer. In this work, microneedle arrays with pore diameters ranging from 2 to 5 μm, pore lengths from 30 to 140 μm and wall thicknesses in the range of 70–110 nm are reported.

Formation of ordered pore arrays at the nanoscale by electrochemical etching of n-type silicon

Electrochemical etching has been studied to structure n-type silicon substrates at the nanoscale. In this work, well-ordered pore arrays with diameters in the range of 150–500 nm and depths up to 50 μm have been fabricated. The pores were successfully formed by anodic etching in (100)-oriented n-type silicon wafers of low-resistivity, typically 1 Ω cm , using aqueous hydrofluoric acid solutions. The lithographic step was performed in a thermally grown oxide using a stepper and dry oxide etching technique. Two types of oxide openings and pitch sizes were tested. The smallest oxide opening realised at this stage was 0.5 μm for a pitch of 1 μm. Stable pore formation was obtained and the smallest pore size obtained was about 200 nm with an aspect ratio close to 100.

超小型宇宙機用気液分離システムを目指した撥水ナノ加工技術(O.S.5 宇宙環境の高度利用を目指して,OS5:宇宙環境の高度利用を目指して)

A novel technology to fabricate hydrophobic surface in microchannel was developed by using electrochemical etching of N-type silicon. It is found that pores of diameter of a few micrometer are formed in a high density on both bottom and side walls of microchannels. In order to maintain stable wettability, thin deposition of noble metal is also tested and contact angle as high as 155 degree is accomplished. This technique is simple to fabricate and effective for the microchannel-based fluidic systems for the future micro space systems.

Electrochemical Etching of n-Type Silicon Based on Carrier Injection from a Back Side p-n Junction

A technique for electrochemical etching of n-type silicon in aqueous hydrofluoric acid is presented. This technique differs from photoelectrochemical etching because the holes (positive carriers) ...

Preparation of porous n-type silicon sample plates for desorption/ionization on silicon mass spectrometry (DIOS-MS)

This study focuses on porous silicon (pSi) fabrication methods and properties for desorption ionization on silicon mass spectrometry (DIOS-MS). PSi was prepared using electrochemical etching of n-type silicon in HF-ethanol solution. Porous areas were defined by a double-sided illumination arrangement: front-side porous areas were masked by a stencil mask, eliminating the need for standard photolithography, and backside illumination was used for the backside ohmic contact. Backside illumination improved the uniformity of the porosified areas. Porosification conditions, surface derivatizations and storage conditions were explored to optimize pSi area, pore size and pore depth. Chemical derivatization of the pSi surfaces improved the DIOS-MS performance providing better ionization efficiency and signal stability with lower laser energy. Droplet spreading and drying patterns on pSi were also examined. Pore sizes of 50-200 nm were found to be optimal for droplet evaporation and pore filling with the sample liquid, as measured by DIOS efficiency. With DIOS, significantly better detection sensitivity was obtained (e.g. 150 fmol for midazolam) than with desorption ionization from a standard MALDI steel plate without matrix addition (30 pmol for midazolam). Also the noise that disturbs the detection of low-molecular weight compounds at m/z < 500 with MALDI could be clearly reduced with DIOS. Low background MS spectra and good detection sensitivity at the 100-150 fmol level for pharmaceutical compounds were achieved with DIOS-MS.

Plasma etched initial pits for electrochemically etched macroporoussilicon structures

Macroporous silicon structures of different shapes were preparedby electrochemical etching of n-type silicon in diluted HF underillumination. Depths reaching 80 µm and 40:1 aspect ratios wereachieved. A PECVD amorphous silicon layer was used as a masking layer.Undoped amorphous silicon was found to be quite resistant to the HFetching solution. The initial pits were prepared by reactive ion etching(RIE) instead of the conventional anisotropic alkaline wet etching. Theadvantages of RIE-made initial pits were demonstrated by electrochemicallyetched Fresnel lenses. Electrochemically etched complex trenches withcurved shapes have been demonstrated for the first time. Illuminationmodulation was employed to fabricate arrays of macropores with variablediameters and free-standing macroporous film.

Formation of wide and deep pores in silicon by electrochemical etching

Electrochemical etching of n-type silicon in hydrofluoric acid electrolyte is now well known as a technique for micro- or macroporous silicon formation. It is commonly admitted that the width of pores can extend over four orders of magnitude, from 2 nm to 20 μm. In this study the feasibility of using this technique to form larger pores is demonstrated. The use of a water–ethanol solvent mixture (1:1) is shown to modify the electrochemistry of silicon dissolution and pore formation. The formation of stable wide pores requires adjustment of the etching current during the pore formation as a function of the evolution of the current–voltage curve with etching time. An array of 42-μm wide pores with 2-μm wall thickness and 200-μm depth were etched using this method. The feasibility to etch pores up to 100 μm in width is also presented. The results enable to conclude that the electrochemical etching of n-type silicon could be used to form vertical structures, without restrictions concerning the wall spacing. This provides a useful tool for micro-machining.

Photonic Crystals Based onMacroporous Silicon

AbstractThe formation of macropore arrays with high aspect ratios by electrochemical etching of n-type silicon in hydrofluoric acid is a well established technique. By using standard photolithograpy the geometry of the array can be controlled with high precision. This enables us to fabricate twodimensional photonic crystals for the infrared regime. The calculated photonic band structure of the crystal corresponds well with the transmission observed experimentally. Furthermore first defect structures like waveguides and cavities have been realized.

The limits of macropore array fabrication

The formation of pore arrays with high aspect ratios by electrochemical etching of n-type silicon in hydrofluoric acid is a well established technique. The macropore morphology depends sensitively on the anodization conditions such as current density, etching time, HF concentration, temperature and bias as well as on substrate properties such as doping density and orientation. The limits of feasible pore geometries and pore patterns and their dependence on formation conditions will be discussed in this work.