engleski

Observation of amorphous tungsten in "bulk" thin films deposited by magnetron sputtering technique

Studies in past forty years have shown that metal thin films such as Fe, Al, Cr, Pd, Ni, Mo, Ta, Nb, V, Zr and W also, prepared by low-temperature condensation, exhibit an amorphous-like structure. However, the substrate should be kept on a liquid helium temperature and the thickness of the films should not excess 15 nm (vanadium: 30 nm!) in the most favourable case. The crystallization temperature was found to be very low, of the order of 20 K. It should be mentioned that refractory metals are very attractive as a gate electrode material in comparison to silicon, because of having much high melting point and excellent thermal stability. We have investigated the structural properties of the dc-magnetron sputtered thin W films (about 300 nm thick), which appeared to be, depending on Ar gas pressure, substrate material, substrate temperature and deposition duration, of a single a- or b-tungsten, or of the two phase a+b-tungsten. In a particular deposition experiments the Ar gas pressure was varied in the interval from 0.7 to 2.8 Pa. However, after increasing the Ar pressure to 3.5 Pa, we obtained a broad maximum on X-ray diffraction pattern, typical for the amorphous structure, which was an unexpected result. Subsequent examination showed that amorphous film appeared regardless to deposition substrate (glass, quartz, Si, sapphire), was very stable and did not crystallize up to 800 K. Also the variation of the electrical resistivity with temperature up to 500 K showed a decrease of resistivity, which is behaviour typical for an amorphous conductor. In comparison to the previously known results, we obtained the amorphous phase in relatively very thick films (300 nm in comparison to 15 nm) and the phase is very stable. It should be mentioned that it was reported in 1998 [K. M. Chang et al., J. Appl. Phys. 37 (1988) 4933-4937] that a 250 nm thick amorphous-like tungsten film was obtained by the chemical vapour deposition at room temperature, using WF6 gas through SiH4 reduction. However, in order to deposit an amorphous tungsten film, very high SiH4 flow rate was used, which caused higher concentration of Si atoms in W films. So it is possible that these films were not pure tungsten films at all but some kind of amorphous W-Si phase with a low Si concentration.

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