328 Sensors zyxwvutsrqponmlkjihgfedcbaZYX and Actuators B. 15-16 (1993) 328-333 Activating technology of SnO, layers by metal particles from ultrathin metal films J. Mizsei Technical University of Budapest, Department of Electron Devices, H- 15221 Budapest (Hungary ) Abstract 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 SnO, sensors by metal particles originating from a sputtered ultrathin metal layer. Ultrathin metal films are sputtered on top of the SnO, layer (0.5 W/cm2 r.f. power density in argon at 0.1 Pa pressure, O-60 s sputtering time, resulting in O-8 nm layer thickness). The sputtered sampleswere heated up to 600K in the atmosphereuntil the end of agglomeration of the ultrathin metal film. The sheet resistivity of the Pd-SnO, layer is IO-100 R before agglomeration, and 106-10’ Cl 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 SnO, layer and the metal-semicon- ductor 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, sensors: the sensitivity increases by three to four orders of magnitude after this procedure, compared to the sensitivity of the simple SnO, 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 H,S. Gold and platinum are also effective for CO and H,, respectively. IntToduction One of the main disadvantages of the solid state semiconductor gas sensors is the limited selectivity. The selectivity and sensitivity can be improved by doping the sensor material with catalytically active dopants. The doping technology is more or less simple in the case of the thick film sensor technology; the activator material is usually mixed with the material to be printed ]1,21. It is not easy to control the amount of doping material in the case of thin films. Thin SnO, sensor layers with evaporated Pd deposits has been investi- gated by AES and XPS [3,4]. Lee and Chung used the dipping method for Pt doping their sensor layers [5]. The basic idea of the recent work is that the amount of doping material can be well controlled by sputtering time and power density, and the suitable surface mor- phology can be reached by appropriate heat treatment, which causes agglomeration of the sputtered activator layer. The agglomeration of metal films (formation of particles from continuous layers) has been discussed generally by Maissel [6], in connection with silver and gold by Sharma [7], and Romanowski and Kepin- ski [8]. Experimental R.f.-sputtered, SnO, semiconductor gas sensor films were used for the experiments. The development of the SnO, sensor layer technology has been discussed earlier [9]; the main sputtering parameters are: 1.3 W/cm* 13.56 MHz r.f. power density, tin-dioxide disc target, 90% argon 10%oxygen at 0.1 Pa pressure, 20 min sputter time. Thermally oxidized silicon slices, glass, ceramic and Al plates were used as substrates. Ultrathin (up to 10 nm) metal films (Pd, Pt, Au, Ag) were sputtered as activa- tors onto the top of the SnOz layer and, in some cases, onto the clean substrates (0.5 W/cm2 13.56 MHz r.f. power density, metal disc target, argon, 0.1 Pa pressure, 0- 100 s sputter time). All layer thicknesses were measured by a Talystep instrument. Before thickness measurement the ultrathin metal layer was structured by the inverse technique, i.e. the ultrathin Pd layer was sputtered onto a patterned photoresist layer. The photoresist was removed by ace- tone: 4nm or thicker Pd layers formed mechanically continuous flakes in the liquid acetone. The sputtered samples were heated up to 600 K in the atmosphere until the end of agglomeration of the ultra- thin metal film. This process could be accelerated by blowing H, gas onto the hot surface. 0925-4005/93/$6.00 @ 1993 - Elsevier Sequoia. All rights reserved