Mengdi Yang

Comparison of tungsten films grown by CVD and hot-wire assisted atomic layer deposition in a cold-wall reactor

In this work, the authors developed hot-wire assisted atomic layer deposition (HWALD) to deposit tungsten (W) with a tungsten filament heated up to 1700–2000 C. Atomic hydrogen (at-H) was generated by dissociation of molecular hydrogen (H2), which reacted with WF6 at the substrate to deposit W. The growth behavior was monitored in real time by an in situ spectroscopic ellipsometer. In this work, the authors compare samples with tungsten grown by either HWALD or chemical vapor deposition (CVD) in terms of growth kinetics and properties. For CVD, the samples were made in a mixture of WF6 and molecular or atomic hydrogen. Resistivity of the WF6-H2 CVD layers was 20 lXcm, whereas for the WF6-at-H-CVD layers, it was 28 lXcm. Interestingly, the resistivity was as high as 100 lXcm for the HWALD films, although the tungsten films were 99% pure according to x-ray photoelectron spectroscopy. X-ray diffraction reveals that the HWALD W was crystallized as b-W, whereas both CVD films were in the a-W phase.

Hot-wire assisted atomic layer deposition of Tungsten films

This thesis aims to establish a novel technique of atomic layer deposition (ALD) for the future ultra-large-scale integration (ULSI) of microelectronics. We developed a hot-wire assisted ALD (HWALD), where a heated tungsten (W) filament is utilized instead of a plasma to generate radicals. HWALD is expected to be another candidate for deposition in future ULSI technology. Particularly, this thesis focuses on the application of HWALD for W deposition by providing sequential pulses of atomic hydrogen (at-H) and WF6. This thesis demonstrates the results of HWALD W in the cold-/hot-wall reactor. In the cold-wall reactor, β-phase W of high resistivity was obtained, whereas the α-phase W of low resistivity was obtained in the hot-wall reactor. The α-phase W possessed a low resistivity of 15 µΩ•cm. Furthermore, a uniform and conformal coverage of HWALD W on high aspect ratio structures (up to an aspect ratio of 36). Moreover, an inherent area-selective HWALD of W was proposed. The nucleation and growth of HWALD W on various substrates were studied. No nucleation was found on a thermally-grown SiO2 surface nor on (ALD-grown) TiN and Al2O3 surfaces. On the contrary, HWALD W could be successfully deposited on W and Co surfaces. Due to the nucleation delays on different surfaces, an area-selective HWALD W process was achieved on W/SiO2 and Co/SiO2 patterned surfaces.

Electrical test structures for verifying continuity of ultra-thin insulating and conducting films

In this work, electrical characterization on insulating aluminium nitride (AlN) and conducting tungsten (W) films was performed using dedicated test structures, in order to determine the thickness at which the films reached continuity. A discontinuous-to-continuous transformation of the AlN layer (occurring around 11 nm) resulted in a transition from ohmic to non-ohmic current conduction, in addition to drastically reduced current density levels. For similar transformation of the W layer (occurring between 2–3 nm) the reverse transition was observed, which was accompanied by a rapid convergence of the film resistivity to the bulk value. The electrical analysis of film continuity was complemented optically by in-situ monitoring of the film growth and its closure, with the spectroscopic ellipsometry (SE) technique.