Reactive IrO 2 sputtering in reducing/oxidizing atmospheres
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An Ir metal target was reactively rf sputtered in a planar magnetron source to develop indium oxide deposition conditions. Gas blends of hydrogen, oxygen, and argon were used to provide competitive control over the reduction/oxidation characteristics of the sputter plasma. Optical emission spectroscopy allowed direct observation of hydrogen, oxygen, and iridium atomic peaks and OH molecular bands. Each of the twelve gas flow conditions could be clearly defined as either reducing or oxidizing by plasma emission spectroscopy. A given plasma reduction/oxidation state can be maintained over a wide range of gas flow conditions by coordinated adjustment of hydrogen and oxygen flows. The electrochemical properties of the iridium oxide films change dramatically in the vicinity of the reduction/oxidation plasma transition.
I. INTRODUCTION The plasma reduction-oxidation (redox) conditions obtained while reactively sputtering a metal target are determined by a combination of conditions. While not an equilibrium process, steady-state sputtering is influenced by factors that would determine thermodynamic equilibrium. In particular, the redox condition of the sputter plasma will be strongly influenced by chemical composition (a combination of target composition and inlet gas blend), total pressure (chamber pressure), and plasma temperature (related to power). Unfortunately, the redox state of the sputter plasma is often inadvertently affected by transient evolution of species from the target surface or chamber walls. Undesirable variations in film and target properties resulting from plasma transients can be avoided by actively setting the redox conditions. Effective control is most readily implemented by including two competing species, one oxidizing and the other reducing, in the chamber gas inlet mixture. Traditionally, water vapor additions have been applied successfully to both reactive1-2 and nominally unreactive3"5 sputtering of electrically important oxides. The morphological and electrochemical characteristics of reactively sputtered iridium oxide films are quite sensitive to deposition conditions.1'2 Traditionally, water vapor additions have been employed to obtain the desired film properties.1'2 Investigators have observed beneficial effects of H2O vapor3 or H 2 gas4'5 additions in providing spatially and temporally stable sputter deposition of in situ superconducting YBa2Cu3O7_^ (YBCO) films. The generation of additional atomic oxygen has been suggested as a possible mechanism.4 Deposition of IrO 2 films by reactively sputtering an Ir metal target in an H 2 /O 2 environment provides an 328 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 2, Feb 1995 Downloaded: 07 Apr 2015
excellent opportunity to characterize a redox process. Reactively sputtered iridium oxide thin films are employed in several electrochemical applications such as charge injection electrodes6 for neural stimulation and optical switching layers7 in electrochromic devices. Equilibrium processes involving metal oxidation are often describe
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