This paper presents a micro reactor, which consists of a permeable membrane fabricated by silicon micro machining technology. The fabrication process is a combination of anisotropic silicon etching (wet etching and dry etching) and porous silicon technology. To avoid a reaction chamber with a high dead volume, we have realised a permeable membrane in conjunction with porous silicon to achieve a high surface to volume ratio, impregnated with palladium or platinum. For the activation of the heterogeneous reaction on the surface of the catalytic material a heating element around the permeable membrane, which is thermally decoupled with a porous silicon well of the surrounding bulk material is realised. The gas flows through the membrane and reacts during the passing time. The reaction time for gases depends on the membrane thickness and the active surface of the porous silicon. The application is the integration into a gas analysing system combined with a gas sensor array, a gas chromatographic system, microvalves, and calibration units mounted all on a pneumatic motherboard.
Presently two porous silicon formation technologies are published: the anodization into an electro chemical cell and stain etch without external current into a hydrofluoric acid/nitride acid solution. For anodization an external current is necessary in order to achieve porous silicon thicknesses up to 100 micrometers . Stain etch is an electroless process, and the porous layer thickness sis limited to a few micrometers. A novel porous silicon formation technique that combines the advantages of thick layer anodization and electroless stain etch will be shown. A current generated by a galvanic element of silicon and a precision metal on the backside of a silicon wafer in a hydrofluoric acid (HF)/hydrogen peroxide (H2O2)/ethanol electrolyte is utilized in order to generate porous silicon. In this case the silicon operates as anode and the metal as cathode for current generation. This current is similar to the external current needed for anodization. Beside the standard porous silicon etch solution HF and ethanol to oxidizing agent H2O2 is used to support the etch process and to generate a higher etch rate. Etch rate control is given by concentration of etching solution and metalization. Different kinds of metalizations and etching solutions were investigated. This novel technology enables to generate stable porous silicon layers of e.g. 80 micrometers within 10 minutes without an external current. This can be the first efficient way for porous silicon batch processing. Detailed process parameters and characterization will be presented.