G. N. A. van Veen

The excitation of the umbrella mode of CH3 and CD3 formed from photodissociation of CH3I and CD3I at 248 nm

Molecular beams of CH3I and CD3I are photodissociated by 248 nm light. Dissociation takes place into CH3 (CD3) and II = I(2P32)] or I*[= I(2P12)]. The quantum yields for I* formation are respectively 0.71 and 0.81. The CH3 and CD3 fragments are found to be vibrationally excited in the umbrella mode. Both distributions for CH3 peak at υ = 2. The distribution in the I* channel equals that at 266 nm, found by other authors, in sharp contrast with earlier theoretical predictions by Shapiro and Bersohn. The distribution for CH3+I is broader and contains vibrations at least up to υ = 7. In the case of CD3I the vibrational distribution of the CD3 fragments peaks at υ = 3 for the I* channel and around υ = 5 in the I channel. Simple models are developed which give a fair description of the excitation of the umbrella mode of the CH3 and the CD3 radicals in both the I and I* channels. The angular distributions of the CH3 and CD3 fragments exhibit a predominantly parallel character in both channels, the I signal being slightly less anisotropic than the I* distribution. The anisotropy parameters for the CH3 fragments are β(I) = 0.72±0.02 and β(I*) = 0.76±0.02. For the CD3 fragments β(I) = 0.77±0.07 and β(I*) = 0.83±0.04. The main absorption is due to the 3Q0 ← 1A1 transition. The I signal is almost exclusively due to curve crossing between the 3Q0 and the 1Q states. Additionally, accurate values for the CI bond strength in CH3I and CD3I are obtained. These values are respectively 2.30±0.01 and 2.33±0.01 eV.

Photofragmentation of CH3Br in the A band

Abstract CH 3 Br is photodissociated in the first continuum. Dissociation takes place into ground state CH 3 and Br [ = Br( 2 P 3 2 ] or Br* [ = Br(*P 1 2 )]. Time of flight and angular distributions of the CH 3 fragments are measured. The Br*/Br ratios upon excitation at 222 and 193 nm are found to be 1.00 and 0.20 respectively. The anisotropy parameters at these wavelengths are β = 0.28±0.04 and β = −0.23±0.02, respectively. The total absorption cross section is decomposed into partial absorption cross sections of the 1 Q, 3 Q 0 and 3 Q 1 states. It appears that excitation at 222 nm takes place to the 3 Q 0 and 3 Q 1 states whereas at 193 nm the 1 Q and 3 Q 0 states are excited. Contrary to CH 3 I, the adiabatic curve crossing between the 3 Q 0 and the 1 Q states in Ch 3 Br is not important. The dissociation energy of the CBr bond is determined to be D 0 (CH 3 Br) = 2.87±0.02 eV.

Predissociation of specific vibrational states in CH3I upon excitation around 193.3 nm

Abstract A molecular beam of CH 3 I is photodissociated with laser light around 193.e nm. Excitation takes place to a bound state that predissociates into ground state CH 3 and I( 2 P 1 2 0 ). The CH 3 fragments appear to be vibrationally excited in the umbrella mode. The distribution is bimodal: it peaks at v =2 and v =5,6. The anisotropy parameter for the angular distribution of the CH 3 fragments is found to be β = −0.36±0.05 indicating that the fragments result from a perpendicular transition. A simple Franck-Condon model gives a fair description of the observed vibrational excitation in the CH 3 fragments.

Near threshold sputtering of Si and SiO2 in a Cl2 environment

Si sputtering yields and Si to SiO2 etch rate ratios have been determined by measuring the depth of the etched craters after Ar+ ion bombardment. The experiments have been performed with energies down to 50 eV both with and without Cl2. Surprisingly high Si sputtering yields are obtained in a Cl2 environment by low‐energy Ar+ ions. Hence, the influence of Cl2 on the Si sputtering mechanism is much larger for low ion energies than for high ion energies. Whereas the Si sputtering yield is enhanced by the presence of Cl2, the SiO2 sputtering yield is hardly affected. Therefore, large differences in the etch rate (high selectivities) between Si and SiO2 are obtained at low ion energies.

Laser‐induced etching of Si with chlorine

Photo‐induced dry etching of silicon with chlorine is studied by measuring mass spectra and time‐of‐flight (TOF) distributions of the particles desorbed from a chlorinated target during irradiation with 308‐ and 248‐nm photons. The detected masses are Si, SiCl, SiCl2, and SiCl3. The measured TOF spectra can be fitted with Maxwell–Boltzmann‐like distributions. The temperatures obtained by these fits depend on laser power and chlorine pressure. A higher laser power or gas pressure results in a higher temperature. Activation energies for desorbing Cl, SiCl, and SiCl2 are obtained. Possible mechanisms to explain the results will be discussed. Etching of rough silicon is much more efficient than the etching of polished silicon. The maximum etch rate obtained is 30 A per laser pulse. No difference is found between p‐ and n‐type silicon.

Critical current as a function of temperature in thin YBa2Cu3O7−δ films

The critical current Ic of several triode‐sputtered and laser‐ablated thin YBa2Cu3O7−δ films is investigated over the temperature range of 5–85 K. Near the critical temperature Tc it is found that Ic∝(1−T/Tc)γ with γ≂1.5–2. The value of γ depends very sensitively on the value determined for Tc. The low‐temperature data, for temperatures below about 0.6Tc, can be well analyzed in terms of a proximity‐effect model. The effective barrier thickness dN between superconducting parts of the samples is of the order of a few nanometers for all samples studied.The critical current Ic of several triode‐sputtered and laser‐ablated thin YBa2Cu3O7−δ films is investigated over the temperature range of 5–85 K. Near the critical temperature Tc it is found that Ic∝(1−T/Tc)γ with γ≂1.5–2. The value of γ depends very sensitively on the value determined for Tc. The low‐temperature data, for temperatures below about 0.6Tc, can be well analyzed in terms of a proximity‐effect model. The effective barrier thickness dN between superconducting parts of the samples is of the order of a few nanometers for all samples studied.

Etching of silicon by SF6 induced by ion bombardment

Abstract Etching of silicon by SF6 induced by keV Ar+ ions has been investigated as a function of target temperature and flux of SF6 molecules. The emitted species have been identified by mass spectrometry and their energy distributions have been determined by time-of-flight measurements. The results indicate that new products are formed, predominantly by reactions of silicon with F atoms formed upon dissociation of SF6 molecules. The effective binding energies are obtained for SiFx (x = 1−4) and SiS. It can be concluded that the newly formed products are present in an amorphized modified top layer of the silicon, and are subsequently sputtered by a collision-cascade-like mechanism. For SiFx-compounds evaporation is only important for SiF4 molecules.

A time‐of‐flight study of the neutral species produced by nanosecond laser etching of CuCl at 308 nm

A time‐of‐flight (TOF) study of the particles leaving a CuCl target after irradiation by 15 ns laser pulses at 308 nm is performed. It is shown that the ejected species are Cl, Cu, CuCl, Cu2Cl, Cu2Cl2, and Cu3Cl3. The majority of the products consists of CuCl. The TOF spectra can be fitted by the sum of two contributions: a Maxwell–Boltzmann (MB) and a Gaussian‐type (G) distribution. The MB distribution has a temperature of T=6000 K for all masses. The average energy and the standard deviation in the energy of the G contributions are typical for every individual product. The results strongly suggest that the MB contribution is due to a single photon‐induced process, whereas the G contribution originates from a multiphoton and/or a multistep process.

A time-of-flight study on the nanosecond laser induced etching of Cu with Cl2 at 308 nm

Chemical etching of Cu is studied using Cl2 and a ns pulsed UV laser at 308 nm. At Cl2 pressures in the range of 10−6–10−4mbar and a laser fluence up to 0.82 J/cm2 the velocity distributions of the ejected species are determined. CuCl and Cu3Cl3 are the main products. The time-of-flight spectra of these particles can be fitted with Maxwell-Boltzmann distributions at high temperatures viz. 1750<T<6000 K. Starting with a clean Cu sample the system evolves to a steady state situation in which a considerable amount of Cl has diffused into the bulk. The chlorinated Cu layer has a pronounced influence on the coupling of the laser beam into the substrate, thereby determining the amount of particles desorbed and their time-of-flight distributions. A model is presented to explain the results.

The influence of substrate material and annealing procedure on the properties of superconducting thin films

Thin layers YBa2Cu3O7−x. are deposited by a laser ablation technique using a pulsed excimer laser operating at 308 nm. The influence of the substrate material and the annealing procedure on the superconducting behaviour of the 123 film and the reactions between the film and the substrate are studied by resistance, X-ray patterns and TEM measurements. The best results are obtained for deposition on (100) SrTiO3 substrates. The resistance of the 1 μm thick film shows a metallic behaviour, an onset in superconductivity at a temperature of 90 K, and has zero resistance at 86 K. The 123 material has a preferential oriented c-axis perpendicular to the surface plane.