Stable Thin Film Resistors for Amorphous Silicon Integrated Circuits
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STABLE THIN FILM RESISTORS FOR AMORPHOUS SILICON INTEGRATED CIRCUITS
HUGO STEEMERS and RICHARD WEISFIELD Xerox Palo Alto Research Center, 3333 Coyote Hill Rd, Palo Alto, CA 94304
ABSTRACT Recent progress in the range of applications of amorphous silicon (a-Si:H) for large area electronics has led to a better understanding of the requirements for stable circuit operation. In particular, the application of doped a-Si:H asa material for thin film resistors has highlighted the instability of this material both due to its thermal history and its relatively high activation energy. In this paper, these limitations are addressed and alternative materials studied. The properties of thin film resistor materials for a-Si:H integrated circuits are driven by the requirements of long term stability, a high sheet resistance, a weak temperature dependence, and processing compatibility with a-Si:H. Cermet films are a promising material as the nature of the transport results in a low activation energy, though the sheet resistance of these materials is usually limited to 10 - 50 kohms/square. In this paper, the suitability of rf sputtered Cr/SiO 2 cermet films for thin film resistors is addressed. Its electrical properties are studied as a function of target composition and its structure investigated by transmission electron microscopy. A sheet resistance of 40 Mohms/square with an 0.05 eV activation energy for a 500A thick film is demonstrated. The carrier transport is interpreted in terms of a tunneling model of electrons between Cr islands.
INTRODUCTION Recent progress in the range of applications of amorphous silicon to thin film microelectronics has generated a need for a stable thin film resistor material. The requirements for the material include a sheet resistance of greater than 10 Mohms/sq, long term stability, weak temperature dependence (typically less than 0.5%/°C) and process compatibility with a-Si: H. n-type a-Si:H is an obvious candidate as its sheet resistance is in the appropriate range and is already used as a contact or device material. However, it has two disadvantages; 1) The activation energy of highly doped n-type a-Si is about 0.18 eV [1]. This is equivalent to 2%/°C and can necessitate accurate, and potentially expensive, temperature control of the microelectronics circuit. 2) Due to the equilibration of the defect structure in doped amorphous silicon when cooled after deposition, the conductivity of n-type a-Si:H decreases with time [2]. Depending on its thermal history, the decay time can be up to several years. In figure 1 the room temperature sheet resistance of 1% PH3 doped aSi:H is plotted as a function of time. After deposition at 230C, the material was cooled to room temperature over a period of several hours. Figure 1 shows a 60% increase in sheet resistance in 4 months. The constraints of a low activation energy and a high sheet resistance suggests that a material with an alternative transport mechanism to that found in amorphous Mat. Res. Soc. Symp. Proc. Vol. 118. ý 1988 Materials Research So
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