Porous Silicon Membranes Research Articles

A Micromachined Glucose Monitoring Sensor with Diffusion Control

Considering the long-term, flexible blood glucose monitoring demand, a sensor monitoring system for interstitial fluid (ISF) ultrafiltration sampling and on-line monitoring of blood glucose is presented. In this paper, the glucose sensor chip used in this system will be introduced in detailed. This sensor chip was developed by MEMS technique and it has the advantages of less ISF consumption, smaller structures and easier integration. Otherwise, the silicon glucose sensor chip is provided with diffusion control of the analyte through a porous silicon membrane into a silicon etched cavity containing the agarose immobilised enzyme. First, glucose monitoring principle of this sensor system will be intrudouced briefly. Then, this paper focuses on the design and fabrication of the key component, which is the MEMS sensor chip with diffusion control. Finally, experiments were carried out, and results show that the sensor chips signal response time at increasing glucose concentrations is about 5s; The linear range is large enough to cover the required broad area of blood glucose (0.2 ~ 20 mmol / L), the sensitivity is 9.76 nA/mmol.L-1, and the correlation coefficient is 0.9954. In addition, experiments results of sensor chip with different pore membrane were compared with each other. We can see that different measuring range and sensitivity can be obtained, which agrees with the theoretical analysis.

Highly Porous Silicon Membranes Fabricated from Silicon Nitride/Silicon Stacks

Nanopore formation in silicon films has previously been demonstrated using rapid thermal crystallization of ultrathin (15 nm) amorphous Si films sandwiched between nm-thick SiO2 layers. In this work, the silicon dioxide barrier layers are replaced with silicon nitride, resulting in nanoporous silicon films with unprecedented pore density and novel morphology. Four different thin film stack systems including silicon nitride/silicon/silicon nitride (NSN), silicon dioxide/silicon/silicon nitride (OSN), silicon nitride/silicon/silicon dioxide (NSO), and silicon dioxide/silicon/silicon dioxide (OSO) are tested under different annealing temperatures. Generally the pore size, pore density, and porosity positively correlate with the annealing temperature for all four systems. The NSN system yields substantially higher porosity and pore density than the OSO system, with the OSN and NSO stack characteristics fallings between these extremes. The higher porosity of the Si membrane in the NSN stack is primarily due to the pore formation enhancement in the Si film. It is hypothesized that this could result from the interfacial energy difference between the silicon/silicon nitride and silicon/silicon dioxide, which influences the Si crystallization process.

Self detachment of free-standing porous silicon membranes in moderately doped n-type silicon

In this article we describe a reliable etching method to fabricate porous silicon free-standing membranes (FSMs) based on a self detachment of the porous layer in moderately doped n-type silicon substrates. We found that stable growth of smooth and straight pores is restricted to a narrow range of etching conditions and, unlike p-type substrates, the lift-off of the membrane is a self-limited process that does not require a large burst of current. The detachment of the porous membrane is independent of the structure of the already porosified layer, meaning that the average pore diameter can be tuned from nano to macro size within the same membrane. We also demonstrate that, despite their limited thickness, FSMs are quite robust and can sustained further processing. Thus, the etching receipt we are proposing here extends the range of sensors and filters that can be fabricated using porous silicon technology.

High-aspect-ratio nanoporous membranes made by reactive ion etching and e-beam and interference lithography

Nanoporous membranes engineered to mimic natural filtration systems can be used in "smart" implantable drug delivery systems, hemodialysis membranes, bio-artificial organs, and other novel nano-enabled medical devices. Conventional membranes exhibit several limitations, including broad pore size distributions and low pore densities. To overcome these problems, lithographic approaches were used to develop porous silicon, silicon nitride, ultrananocrystalline diamond (UNCD), and polymer film membranes. Here we report processing of high porosity, high-aspect-ratio membranes by two techniques: UNCD fabricated by reactive ion etching after e-beam lithography and epoxy fabricated by interference lithography.

Electrochemically deposited and etched membranes with precisely sized micropores for biological fluids microfiltration

This paper presents simple and economical, yet reliable techniques to fabricate a micro-fluidic filter for MEMS lab-on-chip (LoC) applications. The microporous filter is a crucial component in a MEMS LoC system. Microsized components and contaminants in biological fluids are selectively filtered using copper and silicon membranes with precisely controlled microsized pores. Two techniques were explored in microporous membrane fabrication, namely copper electroplating and electrochemical etching (ECE) of silicon. In the first technique, a copper membrane with evenly distributed micropores was fabricated by electroplating the copper layer on the silicon nitride membrane, which was later removed to leave the freestanding microporous membrane structure. The second approach involves the thinning of bulk silicon down to a few micrometers thick using KOH and etching the resulting silicon membrane in 5% HF by ECE to create micropores. Upon testing with nanoparticles of various sizes, it was observed that electroplated copper membrane passes nanoparticles up to 200 nm wide, while porous silicon membrane passes nanoparticles up to 380 nm in size. Due to process compatibility, simplicity, and low-cost fabrication, electroplated copper and porous silicon membranes enable synchronized microfilter fabrication and integration into the MEMS LoC system.

Transport in Surface Passivated Porous Silicon Membranes

Permeable porous silicon membranes for ultra- and nanofiltration are realized by combination of microfabrication methods and self-organized nanostructuring. The material properties of native and surface passivated membranes are investigated for different substrate doping and fabrication conditions. Transport mechanisms in the membranes are investigated by permeability measurements for both gases and liquids. A transport model using an electric analog circuit as equivalent model is proposed for simulation of gas transport in nanostructured membranes. Adsorption and desorption isotherms are used to analyze the surface interaction between porous surface and adsorbate. As independent measure, AFM based distance-force measurements have been performed to investigate the adhesion forces as function of the surface properties. The influence of surface passivation on transport processes is evaluated.

Porous silicon membrane for investigation of transmembrane proteins

This article presents the assembly and signal transduction of an artificial biological membrane suspended on a thin porous silicon template. The electrochemically-fabricated porous silicon membrane has average pore diameters in the range 0.50–2μm and dimensions of about 200×200×3μm3 and may be batch fabricated in large arrays for combinatorial testing. Biological membranes may be deposited on one or both sides of this template are fully accessible for studies using electrochemical impedance spectroscopy. Initial results using a two probe impedance measurement clearly show a significant impedance change between the porous silicon structure and the lipid bilayer. Furthermore, there is a clear reduction in the impedance of lipid bilayer when fused with a transmembrane ion channel protein. The photoluminescence and biodegradability properties of porous silicon in addition to lower cost and ease of fabrication make it superior over e-beam patterned silicon structures used in previous works, and thus suitable for in vivo monitoring.

Dry adhesive bonding of nanoporous inorganic membranes to microfluidic devices using the OSTE(+) dual-cure polymer

We present two transfer bonding schemes for incorporating fragile nanoporous inorganic membranes into microdevices. Such membranes are finding increasing use in microfluidics, due to their precisely controllable nanostructure. Both schemes rely on a novel dual-cure dry adhesive bonding method, enabled by a new polymer formulation: OSTE(+), which can form bonds at room temperature. OSTE(+) is a novel dual-cure ternary monomer system containing epoxy. After the first cure, the OSTE(+) is soft and suitable for bonding, while during the second cure it stiffens and obtains a Young’s modulus of 1.2 GPa. The ability of the epoxy to react with almost any dry surface provides a very versatile fabrication method. We demonstrate the transfer bonding of porous silicon and porous alumina membranes to polymeric microfluidic chips molded into OSTE(+), and of porous alumina membranes to microstructured silicon wafers, by using the OSTE(+) as a thin bonding layer. We discuss the OSTE(+) dual-cure mechanism, describe the device fabrication and evaluate the bond strength and membrane flow properties after bonding. The membranes bonded to OSTE(+) chips delaminate at 520 kPa, and the membranes bonded to silicon delaminate at 750 kPa, well above typical maximum pressures applied to microfluidic circuits. Furthermore, no change in the membrane flow resistance was observed after bonding.

Porous Silicon in a Semiconductor Manufacturing Environment

In the Bosch-proprietary “advanced porous silicon membrane process”, porous silicon (PSi) is used for the first time in high-volume industrial production of microelectromechanical devices. Nanoporous silicon acts as an auxiliary layer during manufacturing of monolithically integrated pressure sensors in a mixed-signal IC process supplemented by microelectromechanical-systems-specific steps in the front end of line. In this paper, the technical design and performance of a fully automated production tool capable of high-volume fabrication of PSi under the specific constraints of a semiconductor manufacturing environment are discussed. The process requires stringent control on the PSi layer thickness, uniformity, porosity, and morphology. The impact of chamber and electrode geometry, the electrolyte flow, and the mode of current coupling into the wafer back side on the uniformity of the PSi layer is addressed. The need for a well-defined PSi morphology demands high reproducibility and stability of the electrolyte composition, particularly with respect to the hydrofluoric acid concentration.

Effect of LiBr pore-filling on morphological, optical and electrical properties of porous silicon membrane

This study describes the developmental properties of PS membrane derived from the pore-filling with aqueous LiBr solutions. A complete process is described and the effect of pore-filling with Lithium bromide (LiBr) on PS membrane structural, optical and electrical properties is investigated. The microstructural and surface morphology of the elaborated samples were characterized mainly by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The AFM shows an increase of the nanoparticles dimension compared to the initial dimension of the PS membranes nanostructure. This result shows that LiBr covers the nanoparticles of PS membranes, which was confirmed by photoluminescence study. A modification of PS membranes morphology accompanied with significant decrease of reflectivity. The rise of the conductivity is significant after pore-filling with LiBr solution. The conductivities of porous silicon membrane are 0.021S/cm before pore-filling and 0.019S/cm after pore-filling with LiBr solution. This suggests the choice of LiBr as filler to improve the electrical conductivity of porous silicon membrane.

A study on the surface modification of a specialized porous silicon membrane

The surface modification of a porous silicon membrane is presented in this paper. The base porous silicon membrane is fabricated by anodization and has a range of pore sizes and porosities. In order to change its wettability, the surface of the porous silicon membrane is modified using silane-octadecyl-trichlorosilane. The average contact angle of the modified porous silicon membrane against water exceeds 126°. This modified membrane is incorporated to form part of the anode plate in a novel passive micro direct methanol fuel cell. Its use leads to a decrease in methanol crossover and an increase in the operating time.

Quantitative Imaging of Ion Transport through Single Nanopores by High-Resolution Scanning Electrochemical Microscopy

Here we report on the unprecedentedly high resolution imaging of ion transport through single nanopores by scanning electrochemical microscopy (SECM). The quantitative SECM image of single nanopores allows for the determination of their structural properties, including their density, shape, and size, which are essential for understanding the permeability of the entire nanoporous membrane. Nanoscale spatial resolution was achieved by scanning a 17 nm radius pipet tip at a distance as low as 1.3 nm from a highly porous nanocrystalline silicon membrane in order to obtain the peak current response controlled by the nanopore-mediated diffusional transport of tetrabutylammonium ions to the nanopipet-supported liquid-liquid interface. A 280 nm × 500 nm image resolved 13 nanopores, which corresponds to a high density of 93 nanopores/μm(2). A finite element simulation of the SECM image was performed to assess quantitatively the spatial resolution limited by the tip diameter in resolving two adjacent pores and to determine the actual size of a nanopore, which was approximated as an elliptical cylinder with a depth of 30 nm and major and minor axes of 53 and 41 nm, respectively. These structural parameters were consistent with those determined by transmission electron microscopy, thereby confirming the reliability of quantitative SECM imaging at the nanoscale level.

Anodized Macroporous Silicon Anode for Integration of Lithium-Ion Batteries on Chips

Macroporous silicon membranes were investigated as anodes for potential integration of lithium-ion batteries on chips. Free-standing and partially etched porous silicon membranes were fabricated from a p-type silicon wafer by using electrochemical anodization. Free-standing porous membranes were achieved with high aspect ratio of 40, average pore size of 1 μm, porosity around ~70%, and wall thickness of less than 0.1 μm. Lithium storage capacity was quantified by galvanostatic discharge/charge measurements, showing specific capacity of 1425 mAh/g with Coulombic efficiency of 81% in the first discharge/charge cycle. The discharge/charge capacity was reduced as the charge rate increased, and the capacity fading was much less pronounced at high charge rate. Morphological analyses showed an increase of surface roughness and reduction of porosity after lithiation/delithiation cycling.

Transport in Surface Passivated Porous Silicon Membranes

not Available.

Structural and Optical Properties Study of Porous Silicon Membrane Filled with Lithium Bromide

not Available.

Phase-Sensitive Detection for Optical Sensing With Porous Silicon

We report on a photonic sensor with an ultralow limit of detection (LoD) based on a phase interrogation readout scheme together with an anisotropic porous silicon (PSi) membrane. First, the fabrication of porous free-standing membranes from medium doped (100) surface oriented silicon, with pore diameters suitable for the infiltration of biomolecules, around 50 nm, is reported. Then, the phase interrogation scheme for characterizing the PSi membranes is presented whose results show that while volumetric sensitivity increases with the membrane thickness, the resolution in the birefringence measurements decrease dramatically due to depolarization effects. The best LoD was found to be equal to 6.25 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> RIU, from the thinnest 10-μm-thick membrane. Finally, the thermooptic coefficient of the 10 μm membrane was measured in an aqueous environment and shown to be equal to 8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> rad/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C.

Ballistic and non-ballistic gas flow through ultrathin nanopores

We show that ultrathin porous nanocrystalline silicon membranes exhibit gas permeance that is several orders of magnitude higher than other membranes. Using these membranes, gas flow obeying Knudsen diffusion has been studied in pores with lengths and diameters in the tens of nanometers regime. The components of the flow due to ballistic transport and transport after reflection from the pore walls were separated and quantified as a function of pore diameter. These results were obtained in pores made in silicon. We demonstrate that changing the pore interior to carbon leads to flow enhancement resulting from a change in the nature of molecule–pore wall interactions. This result confirms previously published flow enhancement results obtained in carbon nanotubes.

Transport mechanisms in nanostructured porous silicon layers for sensor and filter applications

Abstract Mechanisms such as diffusion, adsorption, desorption and capillary condensation of gases or fluidics in porous materials play an important role when using these materials for sensing or filtration. Transport processes were investigated in freestanding porous silicon (PS) membranes for liquids and gases. Transport models in nanostructured PS membranes are analyzed considering several types of transport mechanisms for different liquids and gases. As a new analytical approach, transport of liquids (organic and aqueous solutions) and vapors was studied optically in PS optical multilayers. Adsorption in the meso- and microporous materials occurs rather fast (typical adsorption time of 5 s). Natural desorption time at ambient conditions depends on the vapor pressure of the investigated adsorbates and can be reduced using heating or purging. Adsorption and desorption isotherms are measured to investigate interactions between sample surface and adsorbate as a function of surface treatment and to evaluate specific surface area and pore size distribution of porous material.

Monocrystalline thin-film waferlevel encapsulation of microsystems using porous silicon

A new technology for thin-film MEMS encapsulation based on a monocrystalline silicon membrane and interfacial bonding is presented. The thickness of the membrane, used here, is 36μm. It is produced on a dedicated wafer using a further development of the “Advanced Porous Silicon Membrane (APSM)” Process. The membrane wafer is attached on waferlevel to a MEMS wafer by glass-frit bonding. Before and during bonding the membrane is mechanically connected to its substrate by vertical anchors. Finally, the substrate is detached by cracking the anchors. Detaching the substrate is optimized by a variation of the lateral and the vertical anchor geometries. The new encapsulation technology enables a very low sensor height by a hermetically sealed monocrystalline MEMS-Cap while using well known technologies from mass production. Hence, a cost-efficient all-purpose thin-film encapsulation is presented. We demonstrate the new encapsulation technology by a capped absolute pressure sensor.

Nanoporous silicon membrane for fuel cells realized by electrochemical etching

In this work we propose a simple method to realize nanoporous silicon proton exchange membranes for fuel cells. The electrochemical etching allows in a single step, the grooving of the membrane and the realization of double porous silicon (PS) layer.We have studied the impact of the different electrochemical conditions: current density, the electrolyte concentration (ethanoic HF) and the anodization time leading to optimal thickness (of 50–90μm) and nanoporosity for silicon microstructures. The experimental techniques employed for surface studies and depths of groove in silicon are mainly scanning electronic microscopy (SEM) and atomic force microscopy (AFM) which give information of the silicon microstructures. Photoluminescence (PL) measurements of porous silicon membrane were carried out in order to investigate the optical properties of the PS.