Etching of Si through a thick condensed XeF2 layer

Etching of silicon by XeF2 is studied in a multiple-beam setup. Below 150 K XeF2 condenses and forms a layer on the silicon, which blocks the etching. Upon ion bombardment, this layer is removed and etching will resume. As a function of the layer thickness, the various removal mechanisms of the layer are studied. For a thick condensed layer it is found that 1 keV Ar+ ions sputter the condensed layer with a yield of 160 XeF2 molecules per ion for 1 keV Ar+ ions and 280 for 2 keV ions. For thinner layers (below 9 nm for 1 keV ions), this sputter rate by ions decreases significantly. Here, the removal is mainly due to consumption of XeF2 by etching at the bottom of the layer. This consumption rate reaches a maximum for a layer thickness of about 5 nm. In the steady-state situation, the layer thickness is further decreased, resulting in a smaller consumption and etch rate. Here, sputtering is the most important removal mechanism for the deposited XeF2 layer. From this, it is concluded that a pulsed ion beam should be used in cryogenic etching to obtain the highest etch rate

Roughening during XeF2 etching of Si(100) through interface layers : H:Si(100) and a-Si/Si(100)

Real-time spectroscopic ellipsometry has been applied in situ in an Ar+ Xe F2 beam-etching experiment to study the roughening of Si(100) etched by Xe F2 at room temperature. The role of initial surface conditions has been examined. For the etching of hydrogen-terminated (H:)Si(100), the roughness evolution as a function of Xe F2 dose can be characterized by an initially fast roughening phase followed by a slower, final roughening phase. Similar behavior is observed when etching through an amorphous silicon (a-Si) layer on top of crystalline Si(100) bulk as obtained by sputter cleaning of Si(100) substrates. These observations can be explained as follows. Both H termination and a-Si lead to patch formation on the surface where etching is impeded and hence, high aspect-ratio etch pits develop. The quantitative differences in roughening can then be attributed to the duration and timing of the influence of the H-terminated and a-Si patches on the etch process until H-bonded Si surface atoms or a-Si are totally removed from the surface. Surface area increase due to the roughening can therefore be held responsible for observed trends and differences in etch rates, reaction layer thickness, and composition as a function of etch time. © 2009 American Vacuum Society

Ion-radical synergy in HfO2 etching studied in a beam experiment

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Si/XeF2 etching: Temperature dependence

The temperature dependence of the Si(100)/XeF2 etch reaction is studied quantitatively in a molecular beam setup. At a sample temperature of 150 K the reaction probability reaches unity initially, after which the XeF2 condenses on the surface and blocks the etching process. For increasing temperatures the XeF2 reaction probability initially decreases from 100% at 150 K down to 20% around 400 K, but for temperatures above 600 K it increases again up to 45% at 900 K. In a simple reaction scheme the high etch rate at low temperatures is explained by a XeF2-precursor, with an activation energy for desorption of 32±4 meV. Furthermore the increased etch rate at high temperatures is explained by the desorption of SiF2 with an activation energy of 260±30 meV. The steady-state fluorine content of the SiFx reaction layer, measured using thermal desorption spectroscopy, reaches a maximum of 5.5 monolayers at 300 K. For increasing temperatures it decreases to a submonolayer coverage above 700 K. The temperature dependence of the formation of the reaction layer is described well by including the XeF2-precursor in a previously developed adsorption model. © 1996 American Vacuum Societ

Long-range diatomic s + p potentials of heavy rare gases

We examine the long-range part of the rare-gas diatomic potentials that connect to the R{(n-1)p5ns}+R{(n-1)p5np} atomic states in the separated atom limit (n=3, 4, 5, and 6 for Ne, Ar, Kr, and Xe, respectively). We obtain our potentials by diagonalization of a Hamiltonian matrix containing the atomic energies and the electric dipole-dipole interaction, with experimentally determined parameters (atomic energies, lifetimes, transition wavelengths, and branching ratios) as input. Our numerical studies focus on Ne and Kr in this paper, but apply in principle to all other rare gases lacking hyperfine structure. These diatomic potentials are essential for applications in which homonuclear rare-gas pairs interact at large internuclear separations, greater than about 20 Bohr radii. Among such applications are the study of cold atomic collisions and photoassociative spectroscopy

Time-of-flight analysis of neutrals from a plasma as a diagnostic for the ion velocity distribution

In a plasma energy is transferred from ions to neutrals by elastic collisions and charge exchange. By measuring the velocity distribution of the fast neutrals with a time-of-flight spectrometer, one gets very detailed information on the ion velocity distributio

Critical collisional opacity in a Bose-Einstein condensate

Summary form only given. In a Bose-Einstein condensate, due to the very low temperature, the s-wave scattering length can be used as a measure for the strength of the atom-atom interaction. Under typical experimental conditions, this interaction is weak and hence can be treated in terms of a mean field. However, when scattering length is large or the density is high, the mean field approximation breaks down. In this collisional (hydrodynamic) regime, effects of the interactions such as quantum depletion or shifts in the frequencies of the elementary excitations become large. It is therefore of great interest to study condensates close to or in the collisional regime. It has been demonstrated in recent experiments that the scattering length and thus the interactions among the atoms can be tuned by means of a Feshbach resonance (Inouye et al, 1998; Courteille et al., 1998; Vuleti et al., 1999; Cornish et al., 2000). In the vicinity of Feshbach resonances, however, the increase of the cross-section for elastic collisions is accompanied by a dramatic increase of particle losses. In this paper we report on the observation of anomalous losses from a 87Rb condensate with a high column density in the absence of an inelastic scattering resonance

Vibrational state distribution of 2-Na^+ ions created in ultracold collisions

The vibrational distribution P(v) of 2-Na^+ ions created in ultracold collisions in a magneto-optical trap has been deter- mined. Only two vibrational states with v = 2 and 3 are popu- lated and we find P(2)=0.29±0.02 and P(3)=0.71±0.02. The results provide conclusive evidence that the ionization mech- anism is photo-associative autoionization,and not photo- associative photoionization and will form a fundamental test for the theoretical description of the process