Mihaela Ilie

Adhesion between PMMA mask layer and silicon wafer in KOH aqueous solution

The adhesion of PMMA layers on silicon wafer has been studied in order to protect the front side of the silicon wafer while etching the backside in KOH aqueous solution. Pre and post-bake treatment have been performed, different primers have been used to optimise the superficial and interfacial tension of both mask layer and substrate. An adherent layer has been obtained and its behaviour has been explained based on the polar and nonpolar interactions across the interface. Keywords: PMMA, silicon, KOH, work of adhesion, surface energy, interfacial tension, superficial tension, contact angle.

Optimized deep wet etching of borosilicate glass through Cr-Au-resist mask

In order to obtain specific channels and reservoirs in glass for analytic systems, the structuring of borosilicate glass has been studied. We used wet etching in HF diluted solution for etching channels up to 150 μm depth. A mask obtained by successively wet etching of previously evaporated Au and Cr layers has been used. A thick SJR 5740 type resist has been spun-on in order to accomplish the optical transfer of the pattern. A normal underetching not larger than the depth, has been obtained when adding a small amount of nitric acid, and using an appropriate annealing process after metal deposition. Neither pinholes nor cracks have been noticed after getting an etching depth of 180 μm. Double side etching has been performed for penetrating the glass. The dependence of the etching rate vs. both HF and HNO3 concentration is outlined together with the etched surface quality.

Developing a miniaturized continuous flow electrochemical cell for biosensor applications

The development of a miniaturized electrochemical cell for biosensor application regards both the structuring of an array of electrodes in a fluidic chamber and their connections to the control & processing unit The sensitivity of the chrono-amperometric measurement performed with the cell is increased by: (a) integrating the reference electrode on the same chip with the counter- and working- electrodes, (b) designing a specific pattern of the gold electrodes and (c) serially distributing them along the pipeline reservoir. Borosilicate glass is used as substrate for the electrodes, allowing, due to its transparency, an accurate and easy pad to pad alignment of the up-side-down chip versus a PCB soldered on a standard DIL 40 socket. This alignment is necessary to accomplish the elastomer-based-solderless electric contact, between chip and PCB. The solderless contact significantly improves both reliability and signal processing accuracy. The reservoir and its cover are micromachined out of silicone rubber respectively photosensitive glass in order to easy disassemble the fluidic chamber without any damage. Both thickness and elasticity of the photosensitive glass rend the device less brittle. A plug-in -plug-flow device with improved characteristics has been obtained with a modular structure that allows further extension of the number of electrodes.

Addressable electrodes microarray for biological analysis system

Multidisciplinary efforts, combining microfabrication, chemistry and molecular biology, have been recently focused on the development of large electrode arrays loaded with oligonucleotide probe to allow rapid analysis of nucleic acid samples. Different micromachining techniques can be used for obtaining the inlet, outlet and main reservoirs for the analyte. In the present work silicon wafers are used as substrates for the microarrays, patterned by means of direct writing or optical lithography. Three methods are developed in order to obtain reservoirs with depths ranging from 5 microns to 200 microns, allowing an analyte volume in the range of 1 nl to 1 ml: reactive ion etching of a polyimide layer, wet anisotropic etching of silicon, respectively deep wet isotropic etching of the glass cover. The glass cover is bonded at low temperature, using spin-on glass as adhesive and ensures a protection of the analyte, as well as a rapid entering of the analyte in the reservoirs, increasing thus the speed of the analysis. A custom laser induced fluorescence set-up is used in order to perform the analysis. The fluorescent DNA molecules are concentrated and localized during an observation time of 60 seconds, proving the functionality of the device.

Engineering a continuous flow electrochemical micro-cell for biosensor applications: new achievements

The functional tests of a previously obtained continuous flow micro-cell revealed a rather low value of the sensitivity of the chronoamperometric measurements as well as a poor reliability of both fluidic and electric connections. The engineering of the micro-cell regards the improvement of the following characteristics: (1) integrating the reference electrode on the chip, (2) designing a specific pattern of the electrodes and (3) serially distributing them along the pipeline reservoir. Thus, a plug-flow device is obtained and the preliminary tests confirm its functionality together with an improved sensitivity. The use of photosensitive glass (Foturan®) for obtaining the reservoir and its vertical openings allowed an improvement of the transversal shape and thus of the fluid flow. Better connections are achieved by the development of more reliable and rigid glass fluidic connections as well as of flexible elastomer-based solderless electric connections combined with both a standard DIL40 socket and standard flat cable connectors.

Silicate-based spin-on-glass used as adhesive in micromachining

The silicate spin-on-glass (SOG) assisted low temperature bonding of different types of glasses on silicon and silicon compounds substrates is widely used in micromachining of analytical devices. Two silicate spin-on glasses (SOG), potassium silicate KASIL 2130 and sodium silicate N/N CLEAR, both of them from PQ Inc., are used. Previous experiments have revealed the formation of clusters and voids in the cured glass layer, that diminishes the bonding quality. A quantitatively analysis of the bonding process in terms of work of adhesion and interfacial tensions enabled us to identify the hot points of the bonding process: the wettability of the surfaces to be bonded, the appropriate concentration of the soluble glass, the adhesion of the spin-on-glass on these surfaces in both liquid and solid state, the spun-on-glass curing process. To overcome these hot points appropriate technological steps are added to the bonding process: O2 plasma and hot HNO3 exposure of glass/silicon respectively silicon nitride surfaces, one minute delay of spinning after sog-deposition on the substrate, increasing up to 125°C the annealing temperature of the spun-on-glass. Smooth, uniform, reproducible glass layers, are obtained and the dependency of their thickness (ranging from 100 Å to 5000 Å) versus silicate concentration of the soluble glass is determined. In order to explain the clusters and voids formation, successively observations of the cured layer after the annealing treatment and after room temperature storage are performed, and show that room temperature storage of non-completely cured silicate layers leads to the formation of clusters. The effect of the concentration of the soluble silicates is qualitatively analyzed, by means of optical microscopy, showing that silicate solutions having 2-3% of wt. are suitable for bonding applications with best results when the obtained glass layer is thin enough.

Steps in miniaturizing analytical systems

The completion of the DNA sequence of several genomes, including the human one, has opened completely new scientific and technological frontiers. The huge amount of genetic information available requires the development of faster and cheaper analytical tools. This can be possible by miniaturising the analytical system itself and by the development of proper analytical procedures, involving fluidic processes. A precise genetic identifying technique is hybridization, that can be accomplished in an array format on very small bidimensional surfaces. In order to automate the fluidic process involved in the DNA hybridization, three micromachining techniques are approached by the authors team, for obtaining reservoirs with volumes ranging from 1nl to 2μl using different materials as polyimide, silicon and glass. Several configurations were proposed targeting a turbulence free fluid flow. A qualitatively fluid flow study was performed and the influence of the reservoir shape was revealed. One obtained device was tested in a Laser Induced Fluorescence detection set-up.

Strategy to determine the constant of elasticity for static micromechanical structures with given shape

The aim of this work was to determine the constant of elasticity for a static silicon micromechanical structure with a given shape, having its width and length in the hundredths of micrometers range and its thickness in the micrometer domain, subject to torsion. This is done by experimentally studying the torsion vibration of the structure. On this purpose, test structures have been manufactured, with thickness varying in a certain domain. Special alignment marks have been used in order to align the structures with respect to the crystallographic directions of the silicon. The structures have been activated with acoustic waves. The resonance frequency in the torsion mode has been measured by means of an optical set-up. Successive measurements and etch-thinnings of the structures provided the dependency of the resonant frequency on the structure thickness. A theoretical formula expressing the resonance frequency fr in terms of the shear modulus G, thickness e, and damping coefficient γ has been fitted with the experimental points in order to obtain the values of G and g. The proportionality factor k between the activation force and the angular or linear displacement has been caluclated in terms of mechanical engineering. Considerations regarding the bending phenomenon complete the strategy to determine the constant of elasticity for arbitrary structures.