Double Sided Porous Silicon on Patterned Substrates for Thermal Effect Microsystems
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Double Sided Porous Silicon on Patterned Substrates for Thermal Effect Microsystems
S. Périchon, V. Lysenko, B. Remaki, D. Barbier Laboratoire de Physique de la Matière, INSA de Lyon, Bât. 502, 20 avenue A. Einstein, 69621 Villeurbanne Cedex, France. ABSTRACT Application of porous silicon in thermal microsystem structures often requires the formation of deep localized porous silicon layers. The most commonly used method to prepare the porous layers is the dc anodic etching of monocrystalline silicon in a hydrofluoric acid (HF) based electrolyte. However inhomogeneity of the nanocrystallite size along the layer depth due to the decrease of HF concentration within the pores as well as the poor uniformity of the porous layer thickness limit the elaboration of deep porous layers. Thus we propose an original pulsed anodisation technique, using a double tank etching cell that allows localized porous silicon layers formation throughout the whole wafer thickness. Furthermore a selective double sided pulsed anodisation of silicon was performed on patterned silicon substrates. Porous silicon is formed in pre-determined parts of the wafer using composite polysilicon-silicon nitride masking layers. Technological solutions to get rid of porous layer thickness inhomogeneity due to non uniform current density distribution are discussed. Finally a toric porous silicon layer, crossing the whole silicon wafer, surrounding a 20 mm diameter monocrystalline silicon cylinder was successfully achieved ensuring a new approach of thermal insulation for thermal effect microsystems.
INTRODUCTION On the basis of facilities and knowledge from microelectronics, silicon is one of the major materials for microsensor and actuator applications. Sensor and electronics combined on the same chip would be possible and lead to mass production of microsystems. During the last thirty years, new materials and technologies were elaborated to realize high level microsystems with specific properties. Porous silicon is one of these materials [1-3]. Porous silicon is obtained from monocrystalline silicon by electrochemical anodisation in an HF-based solution. Regarding the initial substrate doping, three morphologies can be achieved so called nano-, meso- and micro-porous silicon. The interconnected network of silicon crystallites constituting the layer confers this material astonishing properties [4-5]. For thermal effect microsystems which require reliable thermal insulation of the heating and reference elements and high mechanical stability, porous silicon can be used in two different ways. First considering the high chemical reactivity of porous silicon resulting from its huge specific surface (200 to 800 m²/cm3 [6]), it is successfully used for frontside bulk silicon micromachining. Porous silicon is used as a sacrificial layer that can be removed by chemicals such as KOH leaving smooth surface and sidewalls [2,7-8]. However the resulting suspended microstructures are fragile (membrane thickness lower than 10 µm) and stressed. Secondly nanoand meso-porous
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