Thermal Stability and Electrical Properties of Ag(Al) Metallization
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Thermal Stability and Electrical Properties of Ag(Al) Metallization Hyunchul C. Kim1, N. David Theodore2, James W. Mayer1, and Terry L. Alford1 1 Department of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287, U.S.A. 2 Digital DNATM Labs., Motorola Inc., 2100 E. Elliot Rd. MD-EL622, Tempe, AZ, 85284, U.S.A. ABSTRACT The thermal stability and electrical resistivity of Ag(Al) alloy thin films on SiO2 are investigated and compared to pure Ag thin films by performing various analyses: Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), transmission electron microscopy (TEM), and four-point probe. The susceptibility to agglomeration of Ag on SiO2 layer is a drawback of Ag metallization. Ag(Al) thin films show good thermal stability on SiO2 layer without any diffusion barrier. The films are stable up to 600 oC for 1 hour in vacuum. Electrical resistivity of as-deposited Ag (5 at % Al) thin film is slightly higher than that of pure Ag thin film. However, the resistivity of Ag(Al) samples annealed at high temperatures (up to 600 oC for 1 hour in vacuum) remains constant due to the improvement of thermal stability (large reduction of agglomeration). This finding can impact metallization for thin film transistors (TFT) for displays, including flexible displays, and high-speed electronics due to lower resistivity value compared to Cu thin film. INTRODUCTION Silver has been considered as a potential interconnect material for ultralarge-scale integration (ULSI) technology due to its lower bulk electrical resistivity (1.57 µΩ-cm at room temperature) when compared with other interconnect materials (Al 2.7 µΩ-cm and Cu 1.7 µΩcm)[1-3] since lower resistivity interconnects can reduce RC delays and power consumption. Also, silver has been attractive for display technologies due to low absorption in the visible ranges and low electrical resistivity[4,5]. However, it has been reported that Ag film on most substrates, including SiO2, is reshaped and disrupted at high temperature. Eventually, the electrical resistance of the Ag film is increased such that the Ag’s function of a conductor is destroyed[1]. This phenomenon is called agglomeration of polycrystalline films. Agglomeration of thin films is a mass transport process that occurs at high temperatures in order to reduce the total system energy[6,7]. This phenomenon is initiated by grain boundary grooving and is followed by void, hillock, and island formation[8]. Changes in surface morphology of thin films increase electrical resistivity of thin films by enhancing scattering of conduction electrons through the films[1]. Therefore, the enhancement of thermal stability of Ag films by minimizing agglomeration has been considered as a reliability issue for Ag metallization. Several techniques have been suggested for the improvement of thermal stability of Ag film, including plasma treatment of low-k dielectrics[3] and the use of diffusion barriers[9]. Ag can be a good candidate of future interconnects if the drawback of weak ther
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