Patterned Silicon Surface Research Articles

Enhancement of optical emission and ion currents in a laser produced silicon plasma by femtosecond laser-induced periodic surface structuring

Laser-induced periodic surface structuring is a powerful technique for modifying the surface properties of solids. Using ultrashort pulses from a Ti:sapphire laser (800 nm, 100 fs), we have fabricated nanoscale order laser-induced periodic surface structures (LIPSSs) on a silicon (100) surface. The LIPSS patterns are found to be strongly dependent on the laser pulse energy, state of polarization, number of shots delivered on the target, and the ambient pressure. The role of laser wavelength is studied by using the second harmonic output (400 nm) also for fabrication. Large area (5 × 4 mm2) surface structuring has been carried out at atmospheric pressure after optimizing the laser parameters at 800 nm. The patterned silicon surface shows a substantial reduction in the reflection of incident light over a wide range of wavelengths, in comparison to plain silicon. Moreover, when used for laser induced breakdown spectroscopy experiments, enhanced optical emission and ion current have been observed from the patterned surface. This indicates that the coupling of laser energy to the surface is enhanced in the case of patterned silicon. While spectral lines emitted by doubly ionized silicon have been observed from the patterned surface, the plain surface gives lines from singly ionized silicon only. A discernible enhancement has been measured in both optical (50%–90%) and electrical (34%) signals from the plasma generated on the patterned surface. These results confirm that LIPSS is a versatile method for enhancing the coupling of laser energy onto irradiated solid surfaces.

Pre‐patterned silicon substrates for the growth of III–V nanostructures

AbstractThis paper reviews the recent progresses obtained by direct growth of III–V semiconductor quantum dots (QDs) on pre‐patterned and flat silicon substrates. This combination allows us to study in detail the growth mechanisms of III–V materials on silicon substrates. For the flat surfaces, we concentrate on basic growth studies addressing mainly morphological properties of QD‐like structures with a main emphasis on surface preparation and growth parameters. For the pre‐patterned substrates, we report the optimization of electron beam lithography and dry etching processes to fabricate sub‐100 nm holes in pre‐patterned Si (100) substrates with controlled size, shape, and periodicity. The pre‐patterned silicon substrates underwent thorough ex situ chemical and in situ cleaning processes before the molecular beam epitaxy (MBE) growth. Finally, the MBE growth sequence of QDs on patterned silicon surface has shown highly selective formation of localized dome like nanostructures in patterned holes with 1 µm period. magnified imageA 3D AFM image of a single nanohole in silicon substrate with diameter and depth of about 70 nm.

Tailoring the wettability of patterned silicon surfaces with dual-scale pillars: From hydrophilicity to superhydrophobicity

Wettability tailoring of patterned silicon surface has great potential in fields producing integrated circuits, solar cells, sensors, detectors, and micro/nano electromechanical systems. The present paper presents a convenient yet effective method of combining reactive ion etching and catalyzed etching to prepare silicon surface with micro–nano dual-scale pillars. The experimental results indicate that the hydrophilic surface transformed to a superhydrophobic surface when micro–nano dual-scale pillars were formed. The surface preserved superhydrophobicity even when the geometric parameters of the micropillars were changed. Overhangs of water drops on steep micro–nano dual-scale pillars result in superhydrophobicity. This method offers a new way for tailoring the wettability of patterned silicon surfaces.

Microtribology friction measurements on microtextured silicon

This paper describes the results of microtribology sliding experiments that were carried out on a range of micropatterned silicon surfaces. The objective of this work was to identify critical pattern features that control friction and fluid interaction on surfaces under a broad range of contact conditions and develop models to explain these relationships. These scratch tests were carried out in air, and with deionised water. Tests were carried out under a range of test loads and speeds, from low to high. It was found that in the dry tests, the measured friction was higher on the patterned surfaces than on the non-patterned surface. For the liquid experiments, there was a reduction in friction in some cases, dependent on the pattern and conditions used, indicating that the liquid acted as a lubricant. The greatest reduction in friction was obtained with a herringbone pattern aligned perpendicular to the direction of travel of the probe. However, for higher load conditions, high friction coefficients were measured for many of the patterns. The surfaces of the tested silicon samples were examined with optical microscopy and in high resolution scanning electron microscopy so that the interactions between the probe and the sample could be determined. It was found that the high friction values that had been observed in many experiments were correlated with considerable damage to the structure of the patterned silicon surface.

Ge Quantum Dot Memory Structure with Laterally Ordered Highly Dense Arrays of Ge Dots

This work was devoted to the development of a Ge quantum dot memory structure of a MOSFET type with laterally ordered Ge quantum dots within the gate dielectric stack. Lateral ordering of the Ge dots was achieved by the combination of the following technological steps: (a) use of a focused ion beam (FIB) to create ordered two-dimensional arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunneling oxide of homogeneous thickness on the patterned silicon surface, and (e) self-assembly of the two-dimensional arrays of Ge dots on the patterned tunneling oxide. The charging properties of the obtained memory structure were characterized by electrical measurements. Charging of the Ge quantum dot layer by electrons injected from the substrate resulted in a large shift in the capacitance-voltage curves of the MOS structure. Charges were stored in deep traps in the charging layer, and consequently the erasing process was difficult, resulting in a limited memory window. The advantages of controlled positioning of the quantum dots in the charging layer will be discussed.

Formation and ordering of Ge nanocrystals on SiO 2 using FIB nanolithography

We first investigate the spontaneous formation of nanometric and highly dense (∼3×10 12 cm −2) Ge droplets on SiO 2/Si(0 0 1) by scanning tunnelling microscopy and spectroscopy. Ge dots are grown using a two-step process: first, Ge deposition on the clean SiO 2 surface at room temperature (RT) and second, thermal annealing at 500 °C. Ge dots appear free of germanium oxides. Then we show the ordering of Ge nanocrystals (NCs) on SiO 2/Si(0 0 1) substrates patterned by focused ion beam (FIB). Lateral ordering of the ultra-dense Ge nanodots was achieved by the combination of the following technological steps: (a) use of a FIB to create ordered two-dimensional (2D) arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunnelling oxide of homogeneous thickness on the patterned silicon surface and (e) self-assembly of the 2D arrays of Ge dots on the patterned tunnelling oxide. We show that Ge NCs are ordered in the FIB holes whatever is their dimension/density.

In situ observation of biomolecules patterned on a PEG-modified Si surface by scanning probe lithography

A Si(100) wafer was modified with methoxy-poly(ethylene glycol) (M-PEG silane) via a self-assembly technique and nano-/micro-sized patterns were then fabricated by scanning probe lithography. The protrusive silicon dioxide pattern was more reactive compared to the non-patterned area, i.e. the PEG deposited area. To demonstrate the feasibility of the submicron patterning of protein based on the anodic oxidation of the Si surface by atomic force microscopy (AFM), streptavidin labelled with Au-colloidal particle and non-labelled streptavidin were site-selectively immobilized on the patterned areas. The streptavidin-patterned surface produced by these procedures can be utilized for the detection of biotinylated materials, such as an antibody and an antigen. A patterned silicon surface is the basis of biosensing devices, in which the patterned areas serve as sensing elements that directly interact with bioanalytes, while the background of the substrate remains passive to the deposition of analytes, thus resulting in a high signal-to-noise ratio.

Surface Nanopatterning by Organic/Inorganic Self‐Assembly and Selective Local Functionalization

Patterning silicon surfaces by well-ordered nanoperforated TiO(2) layers is achieved through a simple block copolymer-assisted liquid deposition technique followed by direct thermal treatment. The crater diameters are tuned between 10 and 50 nm, depending on the molecular weight of the surfactant patterning agent. The formation mechanism of such systems is discussed. The present nanopatterned heterogeneous surfaces (nanoholes with SiO(2) bottom surfaces and TiO(2) walls) are selectively functionalized with two different perfluorinated organic groups. The accessibility to the substrate surface through such nanoholes and the selective distribution of both functions on the surface is evidenced by water-contact-angle measurements, which satisfactorily verify the Cassie relationship. Such systems have wide application prospects in varied fields such as nanotechnology and engineering.

Nanotube surface arrays: weaving, bending, and assembling on patterned silicon.

We report the fabrication of ordered arrays of oriented and bent carbon nanotube on a patterned silicon surface with a micron scale spacing extending over millimeter size surface areas. We suggest that the patterning is controlled by the hydrodynamic behavior of a fluid front and orientation and bending mechanisms are facilitated by the pinned carbon nanotubes trapped by the liquid-solid-vapor contact line. The bending of the pinned nanotubes occurs along the shrinking receding front of the drying microdroplets. The formation of stratified microfluidic layers is vital for stimulating periodic instabilities of the contact line.

A New Method of Preparing Monolayers on Silicon and Patterning Silicon Surfaces by Scribing in the Presence of Reactive Species

Here we describe a new and simple method for preparing alkyl monolayers on silicon, which consists of mechanically scribing oxide-coated silicon while it is wet with 1-alkenes or 1-alkynes (neat or in inert solvents) under ambient conditions. X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, wetting data, and stability tests suggest covalent bonding of unsaturated species to exposed silicon surfaces. Enclosures (hydrophobic corrals) made by scribing silicon that is wet with unsaturated hydrophobic species hold droplets of water and liquids with substantially lower surface tensions. Wetting tests suggest that 1-alkynes make better hydrophobic corrals than 1-alkenes, and theoretical results suggest it should be more difficult for alkyl chains of chemisorbed 1-alkenes to pack than those of 1-alkynes. Underivatized interior regions of hydrophobic corrals are functionalized with polyelectrolyte multilayers. Theoretical energies for water and methanol droplets (gravitational and surface) in hydrophobic corrals are calculated, and a model of failure of liquid droplets in hydrophobic corrals is presented.

Micro-structure Transformation of Silicon: A Newly Developed Transformation Technology for Patterning Silicon Surfaces using the Surface Migration of Silicon Atoms by Hydrogen Annealing

The micro-structure transformation of silicon (MSTS), which is a transformation technology for patterning silicon surfaces by hydrogen annealing, is presented for the first time. The transformation was controlled by the parameters of annealing pressure as well as annealing time and temperature. Voids of sub-micrometer regime size can be intentionally formed in the silicon substrates by making use of transformation. Electrical characteristics, such as the reliability of the thin dielectrics formed in the deep trenches, were improved with the aid of the MSTS process, due to the flattening of the inside surface of the trenches and the rounding of the corners. The mechanism of the transformation by MSTS was studied by means of molecular dynamics, which clearly shows the migration of silicon atoms on the surface. MSTS is a promising technology for the fabrication of future integrated circuits in silicon.

Polymer Thin Films on Patterned Si Surfaces

The propagation of roughness from a patterned silicon surface by polymer thin films was measured as a function of film thickness, time, and surface interaction using atomic force microscopy and synchrotron X-ray reflection. In the presence of an interacting surface, the decay length of the surface modulation was much longer than that observed in simple liquids. By measuring the time dependence of the surface corrugation amplitude, we were able to extract a surface diffusion coefficient by applying a modified version of the Mullins theory for surface diffusion in crystalline solids. The measured diffusion coefficients were an order of magnitude smaller than in the bulk, and scaled as 1/M3/2, in agreement with previous SIMS results. The results are interpreted in terms of surface interactions confining polymer chains over distances larger than the radii of gyration.

Diamond membranes with controlled porosity

Porous diamond membranes were fabricated using a method based on growing diamond on a patterned silicon surface. Protrusions on the silicon surface acted as molds for forming the pores on the diamond film. The membrane described here was 10 μm thick, 15 mm in diameter, had a pore size of about 60 μm in diameter and a nominal pore density of 1.56 mm −2 . The method is highly versatile and a wide variety of pore surface configurations can be formed; the technique used allows a wide parameter range. These kinds of membranes could have application as heat exchangers and as filtering devices.

Study of porosity in permeable diamond membranes

As reported previously, porous diamond membranes can be fabricated using a method based on growing diamond on a patterned silicon surface. Protrusions on the silicon surface act as molds for forming the pores on the diamond films. This method leads to a non-negligible pore size non-uniformity. The present work is concerned with identifying the process that accounts for this, so as to minimize its effects. The approach developed here is based on the digital processing of the scanning electron microscope (SEM) images obtained from selected pores in five different membranes.