Hcw Herman Beijerinck

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 depen...

State selected total penning ionisation cross sections for the systems Ne*(3P0, 3P2) + Ar, Kr, Xe and N2 in the energy range 0.06 < E0(eV) < 8.0

Abstract The velocity dependence and absolute values of the total ionisation cross sections of Ar, Kr, Xe and N 2 by metastable Ne*( 3 P 0 ) and Ne*( 3 P 2 ) atoms have been measured in a crossed beam scattering experiment. State selection of the beam of metastable atoms has been performed by optical pumping with a cw dye laser. Our technique, which uses the UV photons released in the radiative decay following the laser excitation to measure the density of metastable atoms in the scattering centre, is very insensitive to details of the process of optical pumping. Systematic errors in detection efficiencies of the metastable atoms are largely eliminated in this approach. We have analysed our experimental data in terms of an optical potential, using a least-squares method to determine the potential parameters. For the real part we use an ion—atom Morse—Morse—spline—van der Waals potential ( V 0 ( r ) as proposed by Siska. The well area for V 0 ( r ) ⩽ 0.1 eV is left unmodified. The energy dependence of the cross sections for the Ne*-rare gas systems points unambiguously to a pronounced “kink” in the repulsive branch at 0.1–0.2 eV. For the imaginary part, with the usual exponential behaviour, we have to introduce a saturation to a constant value at small internuclear distances r r Im with r Im in the range 2.1 r Im (A) 3 P 2 )—Ar, Kr, Xe systems. For the Ne*( 3 P 0 systems additional modifications of the well area are necessary. By calibrating the density—length product of the secondary beam and using the available detection efficiencies for ions and UV photons of the spiraltron detector we have also determined absolute values of the total ionisation cross section. In the thermal energy range (0.06–0.16 eV) they are in fair agreement with the rate constants for the quenching of metastable atoms as measured by Brom in flowing afterglows.

Ionisation of small molecules by state-selected Ne* (3P0, 3P2) metastable atoms in the 0.06 < E < 6 eV energy range

Abstract The velocity dependence and absolute values of the total ionisation cross section for the molecules H2, N2, O2, NO, CO, N2O, CO2, and CH4 by metastable Ne* (3P0) and Ne* (3P2) atoms at collision energies ranging from 0.06 to 6.0 eV have been measured in a crossed beam experiment. State selection of the two metastable states of Ne* was obtained by optical pumping with a cw dye laser. We observe a strongly different velocity dependence at collision energies below about 1 eV for the ionisation cross section of the systems Ne*H2, N2, CO, and CH4, and the systems Ne*O2, NO, CO2, and N2O, respectively. The first group shows an increasing cross section in this energy range, similar to the Ne*Ar system, while the second group shows a very flat behaviour. This behaviour correlates with the difference in character (π or σb) of the orbital of the electron that is removed from the target molecule. For the molecules H2, N2, CO, and CH4 an electron from a σb orbital is removed from the molecule, whereas for O2, NO, N2O, and CO2 an outer π-ortibal electron is involved. For the systems Ne* (3P0, 3P2)H2 we have derived the imaginary part of the optical potential by assuming a real potential similar to the theoretically calculated ground state NaH2 potential of Botschwina et al. The resonance width Γ(r) as a function of the internuclear distance r shows a saturation at small r (r

Reaction layer dynamics in ion-assisted Si/XeF2 etching: Temperature dependence

We study the dynamics of the reaction layer during Ar+ ion-assisted Si etching by XeF2 in the temperature range T=150–800 K. Depending on temperature, the etch rate can be enhanced a factor of 8 by ion bombardment. The dynamics are studied with ion-pulse measurements on a time scale of 1–100 s in a molecular beam setup. A reaction layer with a submonolayer fluorine coverage and dangling bonds is found to be formed on the Si(100) surface during ion bombardment. The dangling bond concentration increases with ion flux and is independent of temperature in the range 150–600 K. Chemisorption on these dangling bonds results in a higher reaction probability of XeF2. The temperature dependence of the reaction probability of XeF2 is fully determined by the temperature dependence of the XeF2 precursor state. A simple model gives a very good description of the reaction probability as a function of both temperature and ion flux. The model description of the behavior of the precursor concentration as a function of ion ...

ION-ASSISTED SI/XEF2 ETCHING : TEMPERATURE DEPENDENCE IN THE RANGE 100-1000 K

The Ar+‐ion enhanced Si(100)/XeF2 reaction is studied in a multiple beam setup for silicon temperatures from 100 K up to 1000 K. The XeF2 flux is 2.7 monolayers/s and the Ar+ flux 0.033 monolayers/s at an energy of 1000 eV. Both the XeF2 consumption and the SiFx production are measured by mass spectrometry. The enhancement of the etch rate peaks around 250 K as is observed in both the XeF2 and SiFx signals. The gradual decline above 250 K is attributed to a diminished surface fluorination and XeF2 precursor concentration. The dropoff below 250 K is presumably caused by sputtering of the XeF2 precursor, as is concluded from the temperature dependence of the XeF+/XeF2+ signal ratio. Around 175 K this decrease is so strong that the ions seem to no longer enhance, but rather reduce, the etch rate. Below 150 K the ions are driving the etch process. In this range the spontaneous process is blocked by XeF2 condensation, but the ion‐assisted process continues due to sputtering or dissociation of the condensate.

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.

Atom lithography of Fe

Direct write atom lithography is a technique in which nearly resonant light is used to pattern an atom beam. Nanostructures are formed when the patterned beam falls onto a substrate. We have applied this lithography scheme to a ferromagnetic element, using a 372nm laser light standing wave to pattern a beam of iron atoms. In this proof-of-principle experiment, we have deposited a grid of 50-nm-wide lines 186nm apart. These ultraregular, large-scale, ferromagnetic wire arrays may generate exciting new developments in the fields of spintronics and nanomagnetics.

Silicon etch rate enhancement by traces of metal

We report the effect of nickel and tungsten contamination on the etch behavior of silicon. This is studied in a molecular beam setup, where silicon is etched by XeF2 and Ar+ ions. The etch process is directly monitored by the SiF4 reaction products which leave the surface. The effect of contamination appears very pronounced after the ion beam is switched off: it leads to a temporary enhancement of the spontaneous etch rate on a time scale of 500 s. With traces of contamination on the order of 0.01 ML, the etch rate may be enhanced by a factor of 2 for W and somewhat less for Ni. It is concluded that the contamination moves into the silicon by diffusion to vacancies created by the Ar+ ions. For 1 keV Ar+ ions the contamination moves to a depth of 25 A, comparable to the penetration depth of the ions. After etching a 170 A thick layer, the catalytic effect of contamination is reduced to less than 5%. A simple model, which describes the measured effect of contamination very well, indicates that only 3% of th...

Avalanches in a Bose-Einstein condensate.

Collisional avalanches are identified to be responsible for an 8-fold increase of the initial loss rate of a large (87)Rb condensate. We show that the collisional opacity of an ultracold gas exhibits a critical value. When exceeded, losses due to inelastic collisions are substantially enhanced. Under these circumstances, reaching the hydrodynamic regime in conventional Bose-Einstein condensation experiments is highly questionable.

Excitation transfer collisions of metastable Ar*, Kr* and Xe* atoms with N2 (X, ν = 0 ): collision energy dependence of excitation cross sections and product vibrational distribution of N2(C 3Πu, ν′ = 0, 1, 2, 3)

Abstract We report the collision energy dependence of the cross sections for excitation of N2 (C 3Πu) to individual vibrational levels in collisions of N2(X, ν=O) with Ar* and Kr* obtained in crossed beam experiments. The range of collision energies covered is 0.08 1.5 e V is observed. The resulting cross section indicates that the excitation probability for N2(C) by Xe* (3P0) is comparable to that for Ar* and Kr*.