Stable thin film resistors using double layer structure
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H. J. Lee, E. Ma, and W. A. Anderson Department of Electrical and Computer Engineering, State University of New York at Buffalo, Buffalo, New York 14260
F. M. Collins Ohmtek, Inc., 2160 Liberty Drive, Niagara Falls, New York 14304 (Received 10 June 1994; accepted 27 February 1995)
Highly stable bilayer thin film resistors, which consist of an underlying layer of tantalum nitride and of a capping layer of ruthenium oxide, were developed by taking advantage of the desired characteristics of two different materials in a single system. The resistors fabricated in such a way were highly stable under power loading or thermal cycling. Resistors with one digit temperature coefficient of resistance (TCR) could be easily controlled by the layer thickness ratio of the tantalum nitride to the ruthenium oxide and the ex situ annealing temperature or duration. Auger electron spectroscopy depth profile on the thin films indicates that the ruthenium oxide layer is well defined for the as-deposited form. Nevertheless, interdiffusion takes place after thermal treatment of the bilayer which is used to tune the temperature coefficient of resistance and to stabilize the resistance of the resistors.
I. INTRODUCTION Microelectronics has brought in a revolutionary change in the realization of electronic circuits and systems by making them smaller, faster, cheaper, and more reliable. A resistor is one of the most frequently used elements in discrete and in hybrid integrated circuits. Thick film resistors were extensively investigated in the past.1"3 However, about 20% variation of resistivity in thick film resistors is unavoidable due to the distribution of thickness of the resistor and unstable firing conditions. It is obvious that thin film resistors (TFR's) are superior to the thick film resistors in cost, reliability, compactness, uniformity, resistivity, and size. In particular, TFR's are becoming more and more important in precision circuitries including industrial, medical, military, and space applications. TFR's, as an important element in precision network, must have the proper resistance, be stable with time, and be stable with temperature from about - 2 0 °C to above 135 °C. The commercially available TFR's are mainly nichrome (NiCr) and tantalum nitride (Ta2N), although some other structures, such as NiCr/Al/NiCr with nitrogen-doped NiCr films4 and the NiCrSi system,5 have also been investigated. The main drawback of the NiCr system is its degradation during aging.6 An increase in the oxide film, mostly of the chromium oxides, results in an unstable nature for NiCr.4 Experimental work has proven that the Ta2N system provides better stability in terms of degradation with time and high temperature J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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operation.6"8 Depending on the partial pressure of nitrogen, sputtered tantalum films with maximum stability and good reproducibility are obtained in the transition region from Ta2N to TaN (plateau region), giving a temperature coefficien
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