Authors: A. Tricoli, M. Graf, F. Mayer, S. Kühne, A. Hierlemann and S.E. Pratsinis
Affilation: Swiss Federal Institute of Technology (ETH Zürich ), Switzerland
Pages: 287 - 289
Keywords: nanostructured film, tin dioxide, gas sensor, annealing, direct deposition
A wafer-level, CMOS-compatible, dry process for in-situ synthesis, deposition and annealing of nanostructured, porous metal-oxide layers for gas sensors is presented. First, lace-like, nanostructured layers of pure or Pt-doped SnO2 featuring high porosity (98%) are self-assembled on a wafer hosting an array of microsensors by flame spray pyrolysis (FSP). Second, these layers are in-situ flame-annealed resulting in cauliflower-like layers with strong particle cohesion and adhesion to the substrate (which is highly desirable for subsequent dicing and micromachining), while preserving layer uniformity, crystal phase and size. Microstructuring of these layers down to 100 μm diameter is achieved by placing selected shadow masks in front of the nanoparticle-laden FSP hot jet close to the wafer surface. Multiple FSP deposition-annealing cycles drastically improved the mechanical stability of these layers as determined quantitatively by N2 or H2O jet impinging tests. In addition, these cycles reduced the layer’s electric resistance by two orders of magnitude without crystallite growth. The sensor performance has been assessed for CO and ethanol (EtOH), so concentrations down to the low ppm level have been detected for both conventional-type and microfabricated sensors.