Superconducting characteristics and the thermal stability of tungsten-based amorphous thin films
- PDF / 818,580 Bytes
- 8 Pages / 612 x 828 pts Page_size
- 75 Downloads / 217 Views
Superconducting characteristics and the thermal stability of sputtered, tungsten-based, amorphous thin films are investigated. Electronic properties and crystalline structures are analyzed as a function of the metalloid content in the films. It is well known that the superconducting Tc of a bulk crystalline tungsten is 0.01 K, which is one of the lowest transition temperatures among the superconducting metals. We have found that the W film containing 5 to 70 at. % metalloids exhibits a great enhancement in Tc. In the region of 15 to 35 at. % metalloids, the Tc shows the maximum of 5.0 K, and the transition from normal to superconducting state occurs very sharply. SEM observation together with x-ray diffraction analysis indicates that these films are amorphous in structure. The electrical resistivity is about 150 /xil-cm, and shows little temperature dependence from Tc to 300 K. In addition, the W - S i amorphous superconductor is thermally very stable after annealing at 700 °C, but the W - G e amorphous alloy crystallizes at 600 °C.
I. INTRODUCTION Refractory metals have been used for gate and interconnect metals in silicon (Si) ultralarge scale integration (ULSI). Currently, tungsten (W) and tungsten silicide (WSi2) are investigated intensively because of their low electrical resistivity and high resistance to stressmigration and electro-migration.1 Our previous work2 showed a substantial increase in the superconducting transition temperature (Tc) of the W films prepared by low-pressure chemical vapor deposition (LPCVD) using tungsten hexafluoride (WF6) and silane (SiH4). The measured Tc 4.4 K was considerably higher than the crystalline Tc of 0.01 K.3 This is considered to be attributed to the amorphous structure of the films that appears as a result of Si impurities. This paper reports on the superconducting characteristics based on further intensive experiments and on the thermal stability in sputtered, W-based, amorphous thin films. Several authors have previously reported superconductivity enhancement in W films and W-based metallic alloy films. This enhancement can be classified into three cases: formation of the /3-W phase, intermetallic compounds (silicides), and amorphous W. For the /3-W phase, superconductivity enhancement was observed in W films due to a presence of some structure other than the a - W (bcc structure). J c 's as high as 3.35 K4 and 3.2 K5 have been reported in the /3-W phase. The /3-W phase (A15 structure) is known to be a metastable phase of W, and this structure favors a high Tc. On the other hand, Petroff and Reed6 reported the absence of superconductivity down to 1.4 K in the /3-W phase. The superconductivity of the /3-W phase is not well understood because it is not only difficult to obtain J. Mater. Res., Vol. 7, No. 4, Apr 1992
http://journals.cambridge.org
Downloaded: 12 Mar 2015
a pure /3-W phase, but also a phase transformation from /3-W to a - W is irreversible. For the intermetallic compounds, the superconducting behavior was examined in compounds of the W alloy system. T
Data Loading...
