Macro and microsurface morphology reconstructions during laser-induced etching of silicon

Surface morphologies of the laser-etched silicon were studied as a function of the laser power densities. Scanning electron microscope (SEM) results show that different kind of microstructures develop. Pores like structures are formed at low laser power density and pillar like structures are obtained at higher laser power density. It is the etching rate, which is responsible for the surface morphology reconstructions. Etching rate was found to be a function of the laser power density. Atomic force microscope (AFM) results reveal that macro and microsurface morphology reconstructions take place simultaneously as a result of increasing etching rate. Macrosurface morphology reconstruction takes place on the silicon wafer surface and the microsurface morphology reconstruction takes place inside the pore wall.     

Influence of pre-surface treatment on the morphology of silicon nanowires fabricated by metal-assisted etching

Herein we demonstrate an improved metal-assisted etching method to achieve highly dense and uniform silicon nanowire arrays. A pre-surface treatment was applied on a silicon wafer before the process of metal-assisted etching in silver nitrate and hydrogen fluoride solution. The treatment made silver ion continuously reduce on silver nuclei adherence on the silicon surface, leading to formation of dense silver nanoparticles. Silver nanoparticles acting as local redox centers cause the formation of dense silicon nanowire arrays. In contrast, an H-terminated silicon surface made silver ion reduce uniformly on the silicon surface to form silver flakes. The silicon nanowires fabricated with a pre-surface treatment reveals higher density than those fabricated without a pre-surface treatment. The volume fraction improves from 18 to 38%. This improvement reduces the solar-weighted reflectance to as low as 3.3% for silicon nanowires with a length of only 0.87 μm. In comparison, the silicon nanowires fabricated without a pre-surface treatment have to be as long as 1.812 μm to achieve the same reflectance.     

准分子激光电化学刻蚀金属的研究

为了探寻准分子激光电化学刻蚀工艺的特性,采用功率密度大的248nm准分子激光聚焦照射浸在溶液中的金属表面,实现了一种激光电化学刻蚀复合工艺。在实验的基础上,通过对激光电化学刻蚀金属和硅的基本形貌进行比较和分析,研究了该工艺的工艺特性。研究结果表明,该复合工艺为激光直接刻蚀和激光热诱导电化学刻蚀。其中激光热诱导电化学刻蚀是通过激光的光热效应和由激光诱导的冲击波来实现对腐蚀液和材料的冲击、微搅拌等作用的。     

Effect of temperature and silicon resistivity on the elaboration of silicon nanowires by electroless etching

The morphology of silicon nanowire (SiNW) layers formed by Ag-assisted electroless etching in HF/H₂O₂ solution was studied. Prior to the etching, the Ag nanoparticles were deposited on p-type Si(1 0 0) wafers by electroless metal deposition (EMD) in HF/AgNO₃ solution at room temperature. The effect of etching temperature and silicon resistivity on the formation process of nanowires was studied. The secondary ion mass spectra (SIMS) technique is used to study the penetration of silver in the etched layers. The morphology of etched layers was investigated by scanning electron microscope (SEM).     

Nanoporous silicon fabricated at different illumination and electrochemical conditions

The properties of porous silicon prepared at different illumination and electrochemical conditions were studied. The preparation procedure was based on the electrochemical etching in HF containing electrolyte. In the dissolution of n-type silicon, an external source of light is necessary to obtain a sufficient holes flux density. Here, illumination was applied from the backside of the wafer (the side not immersed in the electrolyte), from topside (the side immersed in the electrolyte), and for the highly doped silicon, etching proceeds without illumination. The electrolyte contains HF in the range 2–50 wt%. The highest current density flows with topside assisted illumination. Backside illumination and etching in the dark resulted in a reduction in the current density. In the dark the current density significantly increased at a higher anodic bias. These conditions gave rise to pores formation with a diameter from 20 nm up to 3 μm. The smallest pore size was obtained for highly doped n-Si (111) wafers, etched without illumination. The present paper confirms the possibility of porous silicon formation in the dark and with backside illumination, these being alternative methods for topside assisted illumination etching methods.     

Surface and bulk structural properties of nanostructured porous silicon prepared by electrochemical etching at different etching time

Nanostructured porous silicon (NPSi) is versatile nanomaterials, and attractive area in device application after visible luminescence was observed from NPSi by Canham (1990). NPSi has been prepared by electrochemical techniques with silicon wafer as a based material. The electrolyte solution consists of ethanol and hydrofluoric acid at volume ratio of 1:1. The etching time was varied while other preparation parameters were fixed to produce different porosity of NPSi samples. The structural properties of samples were measured using field emission scanning electron microscope and Raman spectrometer. The surface structural study has shown the surface roughness increase at inertial stage but decrease gradually with longer etching time. However, nanostructured surface was decreased with increasing of etching time. From side view measurement, the nanopillar of NPSi becomes smaller size while increase of etching time. The crystallinity of PSi is observed by Raman scattering varied with different etching time. The photoluminescence measurement will be carried out to study the correlation between optical and structural properties.     

Biological functionalization and patterning of porous silicon prepared by Pt-assisted chemical etching

Porous silicon fabricated via Pt-assisted chemical etching of p-type Si (1 0 0) in 1:1:1 EtOH/HF/H₂O₂ solution possesses a longer durability in air and in aqueous media than anodized one, which is advantageous for biomedical applications. Its surface SiHx (x = 1 and 2) species can react with 10-undecylenic acid completely under microwave irradiation, and subsequent derivatizations of the end carboxylic acid result in affinity capture of proteins. We applied two approaches to produce protein microarrays: photolithography and spotting. The former provides a homogeneous microarray with a very low fluorescence background, while the latter presents an inhomogeneous microarray with a high noise background.     

深振幅Al调制靶的化学腐蚀制备工艺研究

 介绍了用于惯性约束聚变分解实验的铝调制靶的制备。以半导体光刻工艺结合化学腐蚀工艺在铝箔表面引入周期为50 μm的条槽图形,研究腐蚀条件对腐蚀速率的影响;采用光学显微镜、扫描电镜和台阶仪对图形形貌和样品表面成分进行测量和分析,获得厚度在32 μm左右、腐蚀深度达到20 μm的铝调制靶。     

Silica-based microcavity fabricated by wet etching

Silica whispering gallery mode(WGM) microcavities were fabricated by the buffered oxide etcher and potassium hydroxide wet etching technique without any subsequent chemical or laser treatments. The silicon pedestal underneath was an octagonal pyramid, thus providing a pointed connection area with the top silica microdisk while weakly influencing the resonance modes. The sidewalls of our microdisks were wedge shaped, which was believed to be an advantage for the mode confinement. Efficient coupling from and to the 60 μm diameter microdisk structure was achieved using tapered optical fibres, exhibiting a quality factor of 1.5×10~4 near a wavelength of 1550 nm. Many resonance modes were observed, and double transverse electric modes were identified by theoretical calculations. The quality factor of the microdisks was also analysed to deduce the cavity roughness. The wet etching technique provides a more convenient avenue to fabricate WGM microdisks than conventional fabrication methods.     

酸蚀深度对熔石英三倍频激光损伤阈值的影响

 采用干涉仪和台阶仪测试蚀刻深度随时间的变化,结合材料去除速率测量,研究了HF酸蚀液对熔石英表面蚀刻的影响。测试了蚀刻后损伤阈值和表面粗糙度的变化。研究表明,熔石英表面重沉积层厚度约16 nm,亚表面缺陷层大于106 nm;重沉积层去除后损伤阈值增大,随亚表面缺陷层暴露其阈值先降低后又增加,最后趋于稳定;然而,随蚀刻时间的增加,其表面粗糙度增大。分析表明,蚀刻到200 nm能有效地提高熔石英的低损伤阈值,有利于降低初始损伤点数量和提高熔石英表面的机械强度。     

Porous silicon layers prepared by electrochemical etching for application in silicon thin film solar cells

In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores.     

Aluminum thin film enhanced IR nanosecond laser-induced frontside etching of transparent materials

Laser processing of glass is of significant commercial interest for microfabrication of precision optical engineering devices. In this work, a laser ablation enhancement mechanism for microstructuring of glass materials is presented. The method consists of depositing a thin film of aluminum on the front surface of the glass material to be etched. The laser beam modifies the glass material by being incident on this front-side. The influence of ablation fluence in the nanosecond regime, in combination with the deposition of the aluminum layer of various thicknesses, is investigated by determining the ablation threshold for different glass materials including soda-lime, borosilicate, fused silica and sapphire. Experiments are performed using single laser pulse per shot in an air environment. The best enhancement in terms of threshold fluence reduction is obtained for a 16 nm thick aluminum layer where a reduction of two orders of magnitude in the ablation threshold fluence is observed for all the glass samples investigated in this work.     

Optical properties of one-dimensional photonic crystals fabricated by photo-electrochemical etching of silicon

The optical properties of one-dimensional photonic crystals (1D PCs), fabricated by electrochemical etching of periodic wall arrays on n-type (100) Si substrates, are investigated in this study. Several 1D PCs were fabricated with lattice periods varying from 4 to 7 μm and with trench depths in the range 160–210 μm. In-plane reflection spectra of the photonic structures at different depths were registered over a wide spectral range of 1.5–15 μm using Fourier Transform Infra-Red (FTIR) micro-spectroscopy. Some of the features observed in the reflection spectra of the structures investigated are believed to be as a result of interface roughness. A corrugated side-wall surface, an artifact of the fabrication technique, results in the degradation of optical reflection characteristics, principally mainly in the near IR spectral range, and the emergence of optical anisotropy. As a result of the periodicity, modulation of the reflection spectra, that is, the difference between the maxima and minima of the interference fringes, reached a value of 95% in the mid-infrared. The optical properties of the structures investigated indicate that they show promise for microphotonics applications.     

The study of the solution concentration influencing on laser-induced electrochemical etching silicon

The laser electrochemical etching process, which combines the laser direct etching process and the electrochemical etching process, is a compound etching technique. In order to further understand the solution concentration influencing on the laser-induced electrochemical etching of silicon; a 248 nm excimer laser as a light source and KOH solution as an electrolyte were adopted in this study. The experiments of micromachining silicon by laser-induced electrochemical etching were carried out. On the basis of the experiments results, the solution concentration influencing on the etching rates in the process of laser electrochemical etching of silicon was researched. The reasons of the etching phenomena were analyzed in detail. The experimental results indicate that the solution concentration influencing on the etching process is mainly rooted in the absorption of different concentration solutions to laser. In general, less absorption and low solution concentration are good for the etching role in the process of laser electrochemical etching.     

Preparation of a nanopatterned surface of bonded silicon wafers using electrochemical thinning and chemical etching: A scanning tunnel microscopy investigation

Sacrificial anodic oxidation is used to thin silicon wafer bonding substrates. Chemical solutions, sensitive to the periodic strain field present in the upper ultra-thin silicon layer, are employed for selective etching. Subsequent scanning tunnel microscopy observations reveal a square array of trenches corresponding to the buried screw dislocation network initially formed at the bonding interface. The influence of the initial thickness and the annealing of the ultra-thin film on roughness and trench depth of the nanopatterned substrates are examined. Germanium growth experiments are performed in order to show the self-organization character of resulting structured surfaces.     

激光诱导直接化学光刻

本文利用Ar~+激光倍频257.3nm进行扫描蚀刻Si、GaAs和Zn片,获得的最小光刻线宽为1.2μm,给出蚀刻深度与光强/扫描速度的关系曲线等参量,首次利用光刻技术估计卤素自由基的扩散系数、自由路程以及激光聚焦光斑大小,以及观察到阈值现象。     

Regularities of the photoluminescence of porous silicon after chemical etching in HF

The effect of chemical treatment of porous silicon samples by HF on its photoluminescence and its evolution with time in sample aging in air is investigated. It is shown that the effect of HF on the luminescence parameters depends on the duration of the treatment and the initial photoluminescence intensity of the sample. It is found that chemical etching in HF accelerates the growth of the total luminescence intensity in aging of the sample in air. The evolution of the photoluminescence spectrum in aging of the sample in air after chemical etching can be explained within the framework of the quantum-size model of the luminescence of porous silicon. Presented at the Fall Meeting of the Material Research Society, December 1–5, 1997, Boston, USA Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 423–427, May–June, 1999.     

Electrical and optical properties of nanowires based solar cell with radial p-n junction

In our studies the absorption, transmittance and reflectance spectra for periodic nanostructures with different parameters were calculated by the FDTD (Finite-Difference Time-Domain) method. It is shown that the proportion of reflected light in periodic structures is smaller than in case of thin films. The experimental results showed the light reflectance in the spectral range of 400–900 nm lower than 1% and it was significantly lower in comparison with surface texturing by pyramids or porous silicon.Silicon nanowires on p-type Si substrate were formed by the Metal-Assisted Chemical Etching method (MacEtch). At solar cells with radial p-n junction formation the thermal diffusion of phosphorus has been used at 790 °C. Such low temperature ensures the formation of an ultra-shallow p-n junction. Investigation of the photoelectrical properties of solar cells was carried out under light illumination with an intensity of 100 mW/cm². The obtained parameters of NWs' solar cell were I_sc = 22 mA/cm², U_oc = 0.62 V, FF = 0.51 for an overall efficiency η = 7%. The relatively low efficiency of obtained SiNWs solar cells is attributed to the excessive surface recombination at high surface areas of SiNWs and high series resistance.     

Microstructural and magnetic properties of CoPt nanowires

In this work, the magnetic and microstructural properties of CoPt nanowires are presented as a function of the electrolyte pH and current density during electrodeposition into anodized alumina templates. CoPt nanowires of high aspect ratio have been prepared using electrolyte pH values in the range from 2 to 6. The as-made samples exhibit a face centered cubic (fcc) structure with soft magnetic properties which transform into the face centered tetragonal (fct) L10 phase after thermal treatment. Different pH values of the electrolyte during electrodeposition lead to significantly different microstructures and, therefore, different magnetic properties. The CoPt nanowires prepared at high pH value are composed of fcc nanorods of about 25 nm in length. Thermal annealing of these samples leads to a preferred (0 0 1) orientation (along the direction perpendicular to the direction of nanowires) which increases with annealing time. On the other hand, the CoPt nanowires prepared at lower pH value are composed of uniform fcc nanograins with the size ∼2−3 nm. Magnetization curves for the later sample are virtually identical in both directions indicating an isotropic behavior.     

Aligned silicon nanowires with fine‐tunable tilting angles by metal‐assisted chemical etching on off‐cut wafers

The morphology control of aligned silicon nanowires (SiNWs) is highly desirable as SiNWs demonstrated high prospect in a variety of applications. Besides the control over length, shape and distribution of aligned SiNWs, the fine‐tuning of tilting angles thereof also attracted intense interest. Up to now, only several discrete tilting angles have been reported. In this Letter, the ability to fine‐tune the tilting angle of SiNWs is demonstrated and the range that can be achieved is identified. Our technique employs the anisotropic characteristic of the etching process using custom‐produced off‐cut Si wafers of various orientations as substrates. With this technique, a uniquely favoured etching direction can result and the tilting angle can be precisely controlled. Tilted SiNWs with tilting angles from 0° to 50° relative to the wafer normal were obtained. The mechanism of the tilting angle manipulation is also discussed. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)