M.C. Acero

The use of ferrofluids in micromechanics

An introduction to ferrofluids in MEMS applications is presented. Ferrofluids are fluids with magnetic properties. By applying a magnetic field, the balance of forces within the ferrofluid is varied so that the magnetic fluid can move or apply pressure. These capabilities can be used in the field of the microsystems. In this work, we have obtained the typical values of pressure which can be expected from a ferrofluid. Using a piezoresistive pressure sensor, a pressure of 0.04 bar has been obtained.

Design of a modular micropump based on anodic bonding

A simple and reliable technology for the fabrication of micromachined micropumps is presented. The assembling of different wafers to produce valves and cavities is usually the critical step regarding final yield. Our technology uses exclusively the well known anodic bonding technique for this purpose. The prospective performance of the devices has been evaluated by finite element methods and system level simulations.

A study of the undercutting characteristics in the TMAH-IPA system

A new etching system of tetramethyl ammonium hydroxide-2-propanol (TMAH-IPA) is suggested. The undercutting ratio for TMAH at 80 degrees C is about 7, much higher than for KOH etchants. The addition of IPA in the TMAH system maintains the main features of TMAH and reduces the undercutting ratio by a factor of 2-3.

Synthesis of SiC Microstructures in Si Technology by High Dose Carbon Implantation: Etch‐Stop Properties

The use of high dose carbon ion implantation in Si for the production of membranes and microstructures is investigated. Si wafers were implanted with carbon doses of 10{sup 17} and 5 {times} 10{sup 17} cm{sup {minus}2}, at an energy of 300 keV and a temperature of 500 C. The structural analysis of these samples revealed the formation of a highly stable buried layer of crystalline {beta}-SiC precipitates aligned with the Si matrix. The etch-stop properties of this layer have been investigated using tetramethyl-ammonium hydroxide as etchant solution. Secondary ion mass spectrometry measurements performed on the etched samples have allowed an estimate of the minimum dose needed for obtaining an etch-stop layer to a value in the range 2 to 3 {times} 10{sup 17} ions/cm{sup 2}. This behavior has been explained assuming the existence of a percolation process in a SiC/Si binary system. Finally, very thin crystalline membranes and self-standing structures with average surface roughness in the range 6 to 7 nm have been obtained.

Test microstructures for measurement of SiC thin film mechanical properties

In this work, test microstructures for SiC film mechanical property measurements by beam bending using an atomic force microscope are presented. Crystalline 300 nm thick -SiC layers obtained by high temperature multiple C implantation into Si have been used. The low residual stress level in the layers along with the high stiffness and excellent etch-stop properties of SiC allowed the fabrication of free standing microstructures using standard Si bulk micromachining techniques. This demonstrates the potential of SiC as an alternative to Si for MEMS applications.

β-SiC on SiO2 Formed by ION Implantation and Bonding for Micromechanics Applications

β-SiC on SiO2 multilayer structures have been fabricated by ion implantation into Si substrates and thermal bonding. This process involves three steps: i) multiple energy C+ implants into Si, to obtain a broad buried β-SiC layer, ii) selective oxidation of the top Si layer, and iii) bonding and etch-back of Si. These are processes compatible with Si processing technology, and permit high crystalline quality β-SiC films on SiO2 to be formed without using expensive bulk SiC or Silicon-On-Insulator wafers. The structures have been characterised after the different process steps mainly by Fourier Transform Infrared Spectroscopy, X-Ray Photoelectron Spectroscopy, Secondary Ion Mass Spectroscopy and Atomic Force Microscopy. The analysis of samples processed after the different steps has allowed the key parameters for fabricating high quality structures for electronic devices and sensors applications to be defined.

Anisotropic etch-stop properties of nitrogen-implanted silicon

Abstract An analysis of the anisotropic etch-stop properties of buried layers obtained by sub-stoichiometric nitrogen implantation into silicon and annealing has been performed as a function of the processing parameters. The aim of this work is to determine the minimum implantation dose needed for the formation of etch-stop layers stable under high-temperature thermal processing, needed in smart-sensor Si technology, using tetramethyl ammonium hydroxide (TMAH) as etchant in a way that is compatible with CMOS. The results obtained show this value to be in the range (2–4) × 1017 N cm−2 for an effective N+ implantation energy of 75 keV. After annealing, etching of the N-rich buried layer occurs through localized pitch discontinuities determined by the implantation and annealing processes.

Etch‐Stop Behavior of Buried Layers Formed by Substoichiometric Nitrogen Ion Implantation into Silicon

In this work the etch-stop behavior of buried layers formed by substoichiometric nitrogen ion implantation into silicon is studied as a function of the processing parameters, the implantation dose and temperature, and the presence of capping layers during implantation. Etching characteristics have been probed using tetramethylammonium hydroxide or KOH solutions for different times up to 6 h. Results show that, after annealing, the minimum dose required for the formation of an efficient etch-stop layer is about 4 {times} 10{sup 17} cm{sup {minus}2}, for an implantation energy of 75 keV. This is defined as a layer with an efficient etch selectivity in relation to Si of s {ge} 100. For larger implantation doses efficient etch selectivities larger than 100 are obtained. However, for these doses a considerable density of pits is observed in the etch-stop layer. These are related to the presence of nitrogen poor Si regions in the buried layer after annealing, due to a partial separation of silicon and silicon nitride phases during the annealing process. The influence of this separation of phases as well as nitrogen gettering in the buried layer on the etch-stop behavior is discussed as a function of the processing parameters.

Comparison between Al 2 O 3 thin films grown by ALD using H 2 O or O 3 as oxidant source

Alumina (Al<inf>2</inf>O<inf>3</inf>) thin films have been deposited on silicon substrates by atomic layer deposition at 200°C using TMA as Al precursor and H<inf>2</inf>O or O<inf>3</inf> as oxygen precursor. The growth rate has been found to be lower for ozone-based processes as compared to H<inf>2</inf>O. The electrical characterization of the deposited layers has shown that when using O<inf>3</inf> the films exhibit larger defect densities as compared to those grown using H<inf>2</inf>O, although these show larger trapping. A post deposition anneal process at 650°C has been shown to lower the defect densities, being this annealing more efficient for O<inf>3</inf>-grown layers. The effect of post-metallization annealing in forming gas is also investigated.

Carrier transport and storage in Si3N4 for metal-nitride-oxide-semiconductor memory applications

Abstract The carrier transport and storage properties in the nitride of metal-nitride-oxide-semiconductor (MNOS) capacitors have been studied by combining repeated voltage ramp stress and constant voltage stress measurements. From the positive flat band voltage shift that results after bias application to the samples, it is clear that electrons dominate in the transport current and trapping. However, it is shown that, whereas at intermediate and low fields electrons are the single species involved in the capture-emission processes in the nitride, at higher electric fields a certain mechanism responsible for positive charge accumulation in the oxide-nitride interface region is also evident. By fitting theoretical results describing electron trapping in the nitride to the experimental flat band voltage shift dependence on bias, the density of electron traps in the nitride and the trapping efficiency have been estimated to be 3.7 × 1018 cm−3 and 10−2s cm2 respectively. We have also compared the experimental flat band voltage shift dependence on current obtained in constant current stressed MNOS capacitors with theoretical values derived from the fitted parameters. Quite good agreement has been found (except at low stress levels where leakage currents through the parallel resistance of the current source become significant), thus supporting the validity of the proposed model.