ACTIVATING TECHNOLOGY OF SNO2 LAYERS BY METAL PARTICLES FROM ULTRATHIN METAL-FILMS

The selectivity and sensitivity of gas sensitive semiconductor materials can be improved by doping the resistor material with catalytically active dopants. Recent work deals with the activating technology of SnO2 sensors by metal particles originating from a sputtered ultrathin metal layer. Ultrathin metal films are sputtered on top of the SnO2 layer (0.5 W/cm2 r.f. power density in argon at 0.1 Pa pressure, 0-60 s sputtering time, resulting in 0-8 nm layer thickness). The sputtered samples were heated up to 600 K in the atmosphere until the end of agglomeration of the ultrathin metal film. The sheet resistivity of the Pd-SnO2 layer is 10-100 OMEGA before agglomeration, and 10(6)-10(8) OMEGA after the heat treatment. Before the heat treatment the sheet resistivity is determined by the continuous Pd layer only, after the agglomeration process the resistivity is controlled by the SnO2 layer and the metal-semiconductor Schottky barriers. SIMS and XPS results clearly show the decrease in surface coverage of the activator after agglomeration, and the presence of small amounts of oxidized metal phase on the surface. The agglomerated Pd layer is the best activator for H-2 sensors: the sensitivity increases by three to four orders of magnitude after this procedure, compared to the sensitivity of the simple SnO2 layer. Ultrathin metal films from other materials (Ag, Pt, Au) behave similarly to the Pd layers. An agglomerated silver layer is a very good activator for H2S. Gold and platinum are also effective for CO and H-2, respectively.