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Structure and ammonia sensitivity of thin platinum or iridium gates in metal-oxide-silicon capacitors

1989; Elsevier BV; Volume: 177; Issue: 1-2 Linguagem: Inglês

10.1016/0040-6090(89)90558-0

1879-2731

Anita Lloyd Spetz, Ulf Helmersson, F. Enquist, M. Armgarth, I. Lundström,

Catalytic Processes in Materials Science

Metal-oxide-semiconductor (MOS) structures with thin discontinuous platinum or iridium gates have a strong sensitivity to ammonia gas. Surface potential changes caused by NH3-derived species adsorbed on the metal grains are assumed to be capacitively coupled to the semiconductor surface through the cracks in the metal film. This causes a negative shift along the voltage axis of the capacitance-voltage curve of the MOS capacitor. The structure of the platinum or iridium film is thus of crucial importance for the response to NH3. Transmission electron microscopy (TEM) studies were therefore performed with specially prepared substrates which enabled thin metal films on silicon dioxide to be studied by TEM without any further treatment of the films. TEM micrographs of platinum films showed that the metal coverage and crack density corresponded well to the NH3 sensitivity of the films. Iridium films with a reproducible film structure were made through evaporation of about 10 nm of iridium at a pressure below 2 × 10-7 Torr at room temperature. Iridium films prepared in that way exhibited a very good NH3 response. Heat treatments of platinum and iridium films were shown to influence the structure of the metal film. H2 and NH3 treatments (in synthetic air) initiated structural changes at lower temperatures. For platinum films a change in film structure was always coupled with a decrease in the speed of response to NH3. For metal films with very large grains the surface potential change due to NH3 did not couple under the whole metal grain, which provides strong support for the proposed model of NH3 sensitivity.