Electrochemical Etching In Hydrofluoric Acid Research Articles

Superwicking on Nanoporous Micropillared Surfaces.

Engineering surfaces with excellent wicking properties is of critical importance to a wide range of applications. Here, we report a facile method to create superhydrophilic nanoporous micropillared surfaces of silicon and their applicability to superwicking. Nanopores with a good control of the pore depth are realized over the entire surface of three-dimensional micropillar structures by electrochemical etching in hydrofluoric acid. After rinsing in hydrogen peroxide, the nanoporous micropillared surface shows superhydrophilicity with the superwicking effect. The entire spreading process of a water droplet on the superhydrophilic nanoporous micropillared surface is completed in less than 50 ms, with an average velocity of 91.2 mm/s, which is significantly faster than the other wicking surfaces reported. Owing to the presence of nanopores on the micropillar array, the wicking dynamics is distinct from the surfaces decorated only by micropillar arrays. The spreading dynamics of a water droplet shows two distinct processes simultaneously, including the capillary penetration between micropillars and the capillary imbibition into the nanopore's interior. The wicking dynamics can be described by the two stages separated by the time when the contact line starts to recede. The transition between the two wicking regimes is due to the increasing effect of the imbibition of the bulk droplet by the nanopores. While a similar transition of the wicking dynamics is shown on the surfaces with different pore depths, the nanopore structure with a greater depth causes a greater amount of imbibition to slow down the spreading and promote superwicking.

Three dimensional integration of waveguides in bulk silicon

Abstract We demonstrated the ability to achieve three-dimensional (3D) vertical stacking of waveguides in bulk silicon using proton beam irradiation with electrochemical etching in hydrofluoric acid. With low fluence irradiation, the ion beam acts to selectively stop etching only at the end-of-range, resulting in a silicon core that is embedded in porous silicon cladding. By using the well-defined ion path, we are able to build a 2 and 3 level waveguide systems with single and double energy irradiation through a patterned mask respectively. This means more devices can be packed densely on a single chip for greater functionality and economic returns.

Controlled intensity emission from patterned porous silicon using focused proton beam irradiation

We have fabricated light emitting porous silicon micropatterns with controlled emission intensity. This has been achieved by direct write irradiation in heavily doped p-type silicon (0.02Ωcm) using a 2MeV proton beam, focused to a spot size of 200nm. After electrochemical etching in hydrofluoric acid, enhanced photoluminescence is observed from the irradiated regions. The intensity of light emission is proportional to the dose of the proton beam, so the PL intensity of the micropattern can be tuned and varied between adjacent regions on a single substrate. This behavior is in contrast to previous ion beam patterning of p-type silicon, as light is preferentially created as opposed to quenched at the irradiated regions.

Modification of vapor sensitivity in ellipsometric gas sensing by copper deposition in porous silicon

The sensitivities of porous silicon layers modified by copper deposition to low vapor concentrations of methanol, ethanol and 2-propanol are studied with spectroscopic ellipsometry. The porous silicon layers are prepared with electrochemical etching in hydrofluoric acid, and copper deposition is done in aqueous CuSO 4. The ellipsometric vapor sensitivities, in terms of the ellipsometric spectral shifts due to gas exposures, of these samples and their oxides are studied and compared. It is shown that ellipsometric vapor sensitivities of porous silicon layers to alcohols are improved by copper deposition. It is also found that the oxidation of copper deposited in porous silicon layers shows improved selectivity to methanol.

Pillar structures with a sub-micron space fabricated by macroporous-based micromachining

Abstract This paper reports a new macroporous-based micromachining technique. Macroporous silicon is formed by electrochemical etching in hydrofluoric acid. This etching technique produces straight pores with a high aspect ratio. In this work, the technique is extended to create oxide pillars. The pillars are made by thermal oxidation, followed by an etch-back of the silicon. The SiO 2 pillar structures have a 200 nm space in a channel formed on a silicon substrate. The space between the SiO 2 pillars is less than the lithographic resolution. These structures can be used for DNA separation.

Fabrication of a beam-mass structure using single-step electrochemical etching for micro structures (SEEMS)

This paper presents a new wet etching technique formicromachining called single-step electrochemical etching for microstructures (SEEMS), which is based on electrochemical etching inhydrofluoric acid. During the etching, the shape of the etched structurecan be controlled by changing the light intensity. Several problemsinvolved in the SEEMS process had to be solved. The two main problems are,firstly, that over-etching can be seen at the clamping point of the freestanding beam and, secondly, it is difficult to remove large areas. Theseproblems can be considerably reduced by an improved mask layout and aperforated mass supported by a single cantilever can be achieved. Inaddition, a new initial pit formation is demonstrated to make structureswhich are free from crystal orientation of siliconsubstrate.

Initial pits for electrochemical etching in hydrofluoric acid

This paper reports on a characterization of structures fabricated by electrochemical etching in hydrofluoric acid (HF) using different types of initial pits. An initial pit has been thought to require a sharp tip in order to collect electronic holes generated by illumination. Therefore, the initial pits have been formed by etching in a KOH solution, which suffers from crystal orientation dependence. We successfully demonstrate the structures fabricated by the electrochemical etching with the initial pits, which have flat or round base. These pits are formed by isotropic wet etching or reactive ion etching (RIE), which are free from crystal orientation of silicon substrate. Furthermore, regular hole patterns can be achieved without initial pits. This makes the electrochemical etching process more simple. The electric field in the silicon is calculated and the calculated results support the experimental results.

Realisation of membranes for atomic beam collimator by macropore micromachining technique (MMT)

The macropore formation in n-type silicon by electrochemical etching in hydrofluoric acid is an established technique. In this work we present the realisation and application of macroporous technology to realise a suspended membrane of 300 μm with pore of 5 μm right through the membrane. Double electrochemical reactor with a double electrolitical contact has been used. The necessary holes injection for the chemical dissolution is provided by illuminating the back contact of the wafer by halogen lamp. The membrane will be employed as an atomic beam collimator in an experiment of molecular and atomic spectroscopy. Other possible applications range from gas sensor to biomedical application.

Fabrication of free standing structure using single step electrochemical etching in hydrofluoric acid

This paper presents a new technique of micromachining using single step electrochemical etching in hydrofluoric acid (HF). The electrochemical etching in HF is known as a technique for porous silicon formation. This etching technique is applied to fabricate 3-D structures in single crystal silicon by a combination of anisotropic and isotropic modes. The diameter of the pore, or the width of the trench, can be controlled by the current density. First, vertical walls are formed and after desired depth is obtained, current density is increased by adjusting the light intensity. The width of the trenches is increased under the structures without affecting the width of existing trenches. The connection of the trenches can be achieved and free standing beams obtained with only one mask. The free standing beams with height, width and length of 40 μm, 2 μm and 250 μm, respectively, have been made of single crystal silicon.

Porous Silicon as a Sacrificial Material for Microstructures Fabrication

AbstractPorous silicon (PS) is generated by electrochemical etching in hydrofluoric acid (HF). Recently porous silicon has been applied to micromachining and micro-devices as an alternate material, this material being used as a sacrificial layer. This technology competes with conventional techniques like surface and bulk micromachining regarding its speed, simplicity and reduced costs. A wide range of microstructures and free-standing structures can be fabricated with a large freedom of design in relation to the isotropic behavior of the etching. A sacrificial layer may be realized fast over varying thickness (PS formation rate 45 μm/h compared to silicon bulk micromachining rate 20 μm/h for KOH etching).This contribution is devoted to the materials aspects of patterning and processing: we will show how basic microstructures (trenches, polysilicon cantilevers, polysilicon free-standing membranes) may be fabricated using a very simple process based on a single photolithography. The important points are the choice of the mask, porous silicon properties as a function of its formation parameters and the choice of the solution removing the sacrificial layer. The morphology and porosity of the porous silicon layers are indeed mainly determined by the electrolyte composition and by the current density for a given substrate type. Optimized conditions (HF 15% and 80 mA/cm2) lead us to an appropriate porous silicon. Finally the applicability of this technology for various microsensors will be underlined.

High aspect ratio submicron silicon pillars fabricated by photoassisted electrochemical etching and oxidation

A technique for fabricating submicron free-standing silicon pillars has been developed. The silicon pillars have a high packing density, and aspect ratios over 50:1 can easily be achieved. Photoassisted electrochemical etching in hydrofluoric acid is used to etch deep macropores in n-type silicon wafers which have been patterned by standard photolithography. The regular macropores can be used for fabricating photonic band-gap structures. The bulk silicon remaining between the close-packed macropores is oxidized. Free-standing pillars are then formed by subsequently wet etching the silicon dioxide. The pillars are the initial structures for forming quantum wires using further oxidation and etch steps.