A 1 eV neutral atomic fluorine beam has been shown to produce etch rates in silicon as high as 1 μm/min. Using a CaF2 resist layer we fabricated 120 μm deep by 1 μm wide trenches (aspect ratio 120:1) in silicon with little sidewall taper (slopes of about 1000:1) or aspect-ratio dependent etching effects. Achieving such anisotropic etching suggests that the scattered species do not contribute significantly to sidewall etching under the conditions of this experiment. We estimate that the ultimate depth attainable for a 1 μm wide trench is about 250 μm and that the critical parameter for attaining a trench of a certain depth is the aspect ratio. Our observations and analysis suggest that this etching technique can be used to fabricate trenches on a nanoscale level while maintaining high aspect ratios of 100 or greater.

Angular distributions are measured for individually resolved ν′, j′ states of HF produced by F + H 2 f HF(ν′ ) 1, j′) + H and F + H 2 f HF(ν′)2, j′) + H reactive collisions in a crossed-beams scattering apparatus. Simultaneous resolution of the HF vibrational and rotational states is achieved spectroscopically for the first time, using laser excitation in conjunction with bolometric detection. The technique is sensitive to population differences between ν′ ) 1, j′ and ν′ ) 2, j′ -1 states optically coupled by specific P 2 (j′) lines of a vibrotational chemical laser. The measurements are greatly facilitated by the development of a new high-temperature atomic fluorine beam source, which exhibits excellent stability, very high intensity, and narrow velocity distributions. Features common to individual product rotational states are as follows: strong backward scattering into ν′ ) 2, j′; weaker backward scattering into ν′ ) 1, j′; and heretofore unobserved scattering into ν′ ) 1, j′ in the forward hemisphere. These angular distributions agree qualitatively with predictions from fully three-dimensional exact quantum reactive scattering calculations (Castillo et al., J. Chem. Phys. 1996, 104, 6531) that were conducted on an accurate potential energy surface (Stark and Werner, J. Chem. Phys. 1996, 104, 6515). However, quasi-classical calculations conducted on the same potential energy surface do not produce any substantial forward-scattered HF in ν′ ) 1 (Aoiz et al., Chem. Phys. Lett. 1994, 223, 215), suggesting that its appearance in the forward hemisphere may be a quantum effect. The quantum theoretical cross-sections also suggest that the forward ν′ ) 1 products arise almost entirely from H 2 reactants initially in j ) 1.

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