Teresa A. Moore

Hyperthermal neutral beam etching

A pulsed beam of hyperthermal fluorine atoms with an average translational energy of 4.8 eV has been used to demonstrate anisotropic etching of Si. For 1.4 Hz operation, a room-temperature etch rate of 300 A/min for Si(100) has been measured at a distance of 30 cm from the source. A 14% undercutting for room-temperature etching of Novolac-masked Si features was achieved under single-collision conditions, with no detectable mask erosion. Translational energy and angular distributions of scattered fluorine atoms during steady-state etching of Si by a normal-incidence, collimated beam demonstrate that unreacted F atoms can scatter inelastically, retaining a significant fraction of their initial kinetic energies. The observed undercutting can be explained by secondary impingement of these high-energy F atoms, which are more reactive upon interaction with the sidewalls than would be expected if they desorbed from the surface at thermal energies after full accommodation. Time-of-flight distributions of volatile reaction products were also collected, and they show evidence for a dominant nonthermal reaction mechanism of the incident atoms with the surface in addition to a thermal reaction channel.

Primary and secondary dissociation pathways in the ultraviolet photolysis of Cl2O

The photodissociation of dichlorine monoxide (Cl_2O) at 308, 248, and 193 nm was studied by photofragment translational energy spectroscopy. The primary channel upon excitation at 308 and 248 nm was Cl–O bond fission with production of ClO+Cl. A fraction of the ClO photoproducts also underwent spontaneous secondary dissociation at 248 nm. The center-of-mass translational energy distribution for the ClO+Cl channel at 248 nm appeared to be bimodal with a high energy component that was similar in shape to the 308 nm distribution and a second, low energy component with a maximum close to the threshold for the 2Cl+O(3P) channel. Observation of a bimodal distribution suggests that two pathways with different dissociation dynamics lead to ClO+Cl products. The high product internal energy of the second component raises the possibility that ClO is formed in a previously unobserved spin-excited state a 4∑−. Following excitation at 193 nm, a concerted dissociation pathway leading to Cl_2+O was observed in addition to primary Cl–O bond breakage. In both processes, most of the diatomic photofragments were formed with sufficient internal energy that they spontaneously dissociated. The time-of-flight distributions of the Cl_2+O products suggest that these fragments are formed in two different channels Cl_2(3II)+O(3P) and Cl_2(X1∑)+O(1D).

Direct Observation of CIO from Chlorine Nitrate Photolysis

Chlorine nitrate photolysis has been investigated with the use of a molecular beam technique. Excitation at both 248 and 193 nanometers led to photodissociation by two pathways, CIONO2 → CIO + NO2 and CIONO2 → Cl + NO3, with comparable yields. This experiment provides a direct measurement of the CIO product channel and consequently raises the possibility of an analogous channel in CIO dimer photolysis. Photodissociation of the CIO dimer is a critical step in the catalytic cycle that is presumed to dominate polar stratospheric ozone destruction. A substantial yield of CIO would reduce the efficiency of this cycle.

Photodissociation of Cl2O at 248 and 308 nm

Molecular beam studies of Cl_2O photolysis at 248 and 308 nm have been repeated and the analysis refined. At 248 nm, three distinct dissociation pathways that led to Cl+ClO products were resolved. At 308 nm, the angular distribution was slightly more isotropic than previously reported, leaving open the possibility that Cl_2O excited at 308 nm lives longer than a rotational period.

Photodissociation of ClONO2 at 193 and 248 nm

Abstract The photodissociation of chlorine nitrate (ClONO2) at 193 and 248 nm was investigated by photofragment translational energy spectroscopy. Only two primary dissociation channels, Cl + NO3 and ClO + NO2, were observed. The branching ratios between Cl and CIO photoproducts were found to be ΦClO : ΦClO = 0.66:0.34 at 193 nm and 0.54:0.46 at 248 nm, with uncertainties of ±0.08 in the relative yields. The yield of CIO products is a lower bound, because a fraction of primary ClO fragments underwent secondary photodissociation. No compelling evidence was found for photodissociation via a ClONO + O channel at either wavelength. Center-of-mass total translational energy and angular distributions were obtained for each observed primary channel. In the analysis of the 248 nm data, the secondary photodissociation of CIO was modeled in order to permit deconvolution of all contributions to the signal at m/z= 35 (Cl+). All channels had positive anisotropy parameters, indicating that dissociation was prompt (occurring in less than a rotational period) and that parallel transitions were excited. The similarity of the ClO + NO2 translational energy distributions (

Dissociation dynamics of ClONO2 and relative Cl and ClO product yields following photoexcitation at 308 nm

Chlorine nitrate photolysis at 308 nm has been investigated with a molecular beam technique. Two primary decomposition pathways, leading to Cl + NO3 and ClO + NO2, were observed. The branching ratio between these two respective channels was determined to be 0.67 ± 0.06 : 0.33 ± 0.06. This ratio is an upper limit because some of the ClO photoproducts may have undergone secondary photodissociation. The angular distributions of the photoproducts with respect to the direction of polarization of the exciting light were anisotropic. The anisotropy parameters were β= 0.5 ± 0.2 for the Cl + NO3 channel and β= 1.1 ± 0.2 for the ClO + NO2 channel, indicating that dissociation of ClONO2 by either pathway occurs within a rotational period. Weak signal at mass-to-charge ratios of 35 and 51, arising from products with laboratory velocities close to the beam velocity, was observed. While this signal could result from statistical dissociation channels with a total relative yield of 0.07 or less, it is more likely attributable to products from ClO secondary photodissociation or from dissociation of clusters.