H. Störi

Surface excitation probability of medium energy electrons in metals and semiconductors

Abstract Reflection energy electron loss spectra (REELS) have been measured for several metals and semiconductors (Be, Al, Si, V, Fe, Co, Ni, Cu, Ge, Mo, Pd, Te, Ta, W, Au, Pb) in the medium energy range (150–3400 eV) for normal incidence and an emission direction of 60° with respect to the surface normal. The ratio of the number of electrons that induced a surface excitation to the intensity of the elastic peak was extracted from each spectrum providing the so-called surface excitation parameter (SEP). This quantity is equal to the average number of surface excitations an electron experiences when it crosses the surface once. For the nearly free-electron materials the results agree reasonably with free-electron theory while significant deviations are observed for the other materials. In all cases the SEP is found to be inversely proportional to the speed of the probing particle. It is generally found that the surface excitation parameter decreases with the generalized plasmon energy. A simple predictive formula to estimate the surface excitation parameter for medium energy electrons entering or leaving an arbitrary material is proposed.

Elastic electron reflection for determination of the inelastic mean free path of medium energy electrons in 24 elemental solids for energies between 50 and 3400 eV

Abstract Elastic electron backscattering coefficients have been measured for 24 elemental solids (Ag, Al, Au, Be, Bi, C, Co, Cu, Fe, Ge, Mg, Mn, Mo, Ni, Ta, Te, Ti, Pb, Pd, Pt, Si, V, W and Zn) for energies between 50 and 3400 eV. Values for the electron inelastic mean free path (IMFP) were derived from the experimental intensities on the basis of a simple physical model that accounts for bulk elastic and inelastic scattering only. The internal consistency of the data set and the evaluation procedure is satisfactory for energies >150 eV. For energies ≥200 eV the IMFP values were found to be well described by the Bethe equation for inelastic scattering, as found earlier by analysis of optical scattering data with the aid of the linear response theory [Surf. Interface Anal. 21 (1994) 165]. We also compare the parameters β (being related to the total dipole matrix elements for inelastic scattering) and γ (determining the details of the energy dependence of the IMFP) that appear in the Bethe equation with corresponding results based on optical data. Both parameters were found to agree well with the earlier analysis by Tanuma et al. [Surf. Interface Anal. 21 (1994) 165] within the statistical accuracy of the present analysis. Experimental data on the recoil energy in elastic scattering were also analyzed and compared with detailed results of Monte Carlo simulations of this phenomenon. The recoil energies are correctly predicted to within 10% by the single deflection model based on the Rutherford cross section.

Studies of photon induced gas desorption using synchrotron radiation

Abstract In view of finalizing the design of the vacuum system of the Large Electron and Positron Storage Ring (LEP) we have studied synchrotron radiation induced neutral gas desorption. A 3 m section of an aluminum vacuum chamber has been exposed to the photon beam emerging from the electron storage ring DCI in Orsay, under conditions closely simulating the environment in a particle acceletor. In order of importance the gases desorbed were H 2 , CO 2 , CO and CH 4 with H 2 O practically absent. Under the experimental conditions of an unbaked chamber and 11 mrad glancing incidence of the photons, the initial molecular desorption yields for these gases were typically 0.5, 8 × 10 −2 , 2 × 10 −2 and 8 × 10 −3 molecules per photon respectively. These values could be reduced by about 1 to 2 orders of magnitude during continued photon exposure and most cases without evidence that this ‘beam cleaning action’ would be limited. After exposure to air and pumpdown of the previously cleaned chamber, we observe a significant memory effect. The dependence of the photon desorption on the angle of incidence has been studied down to a glancing angle of 11 mrad showing a definite deviation from the previously assumed 1/sin o scaling. The implications of the results in terms of the expected beam-gas lifetime in LEP are discussed.

Electron inelastic mean free path measured by elastic peak electron spectroscopy for 24 solids between 50 and 3400 eV

Abstract Elastic electron backscattering coefficients have been measured for 24 elemental solids (Ag, Al, Au, Be, Bi, C, Co, Cu, Fe, Ge, Mg, Mn, Mo, Ni, Ta, Te, Ti, Pb, Pd, Pt, Si, V, W, Zn) for energies between 50 and 3400 eV. Values for the electron inelastic mean free path (IMFP) were derived from the experimental intensities on the basis of a simple physical model for elastic backscattering that accounts for bulk elastic and inelastic scattering only. For energies ⩾200 eV the IMFP values were found to be well described by the Bethe equation for inelastic scattering, as found earlier by analysis of optical scattering data with the aid of linear response theory (Surf. Interf. Anal. 21 (1994) 165). The parameters β and γ appearing in the Bethe equation are compared with corresponding results derived from optical data. Both, the parameter β , that is related to the total dipole matrix element for inelastic scattering, as well as the parameter γ , that describes the energy dependence of the IMFP, were found to coincide with the results based on optical data within the experimental accuracy of our analysis. Since the present analysis of electron scattering data is entirely free of linear response theory, the present results may be regarded as an independent test of the previous analysis based on optical scattering data.

Radio‐frequency sputter deposition of boron nitride based thin films

Thin films (≊2 μm) of boron nitride, titanium boron nitride, and titanium aluminum boron nitride have been grown on molybdenum, niobium, and cemented carbide substrates employing nonreactive as well as reactive rf magnetron sputter deposition from either a BN, a TiN‐BN, or a TiN–AlN–BN target. Substrates have been rf biased, with dc potentials up to −200 V. By means of nonreactive sputtering mixed‐phase structures with dominant phases B48B2N2 (using a BN target), or B48B2N2 and hexagonal Ti–B–N (using a TiN–BN or a TiN–AlN–BN target) are formed. Reactive deposition leads to the existence of hexagonal BN in all deposition modes. In the cases of Ti–B–N and Ti–Al–B–N films this phase is accompanied by fcc Ti–B–N. SEM cross sections revealed very fine grained to fracture‐amorphous film structures. Hardness measurements gave the following maximum HV 0.02 values: B–N films 2800, Ti–B–N films 2750, and Ti–Al–B–N films 1650.

Plasma‐assisted chemical vapor deposition of titanium nitride in a capacitively coupled radio‐frequency discharge

A new type of plasma reactor for the plasma‐assisted chemical vapor deposition (PACVD) of titanium nitride (TiN) was designed and built. The reactor uses a rf discharge operating at a pressure ∼1 mbar and at a frequency of 13.56 MHz. The dependence of various properties of the coating produced on the deposition parameters is studied in detail. Besides a determination of the thickness of the coating different analytical techniques such as Auger electron spectroscopy (AES), x‐ray diffraction (XRD), and scanning electron microscopy (SEM) have been used to characterize the coating. In order to investigate mechanical properties of the deposit, Vickers hardness and critical load have been determined. TiN coatings of excellent quality have been deposited on all surfaces of objects of complex geometry. The quality of these coatings is equivalent to the quality obtained by classical CVD and physical vapor deposition (PVD) techniques. The deposition temperature of 500 °C permits the coating of hardened steel tools.

Sputter deposition of decorative boride coatings

Abstract Decorative hard coatings have to meet a variety of requirements such as attractive colour as well as high wear and corrosion resistance. After a review of PVD coatings used for decorative metal-finishing, an account is given of studies to extend the range of useful coating materials. Within the scope of the search for new colours several borides such as zirconium diboride ZrB2, lanthanum hexaboride LaB6 and zirconium dodecaboride ZrB12 were evaluated using magnetron sputtering. The nitrogen flow in a partially reactive process was used as an additional deposition parameter to influence film composition and properties. The interrelationships between structure, chemical composition and mechanical as well as optical properties were investigated. By means of non-reactive sputtering, crystalline films containing the ZrB2 or the LaB6 phase were formed. In the case of sputtering from the ZrB12 target, the ZrB2 phase was accompanied by a highly disordered boron or boron-rich phase. The incorporation of nitrogen leads to increasing lattice distortion and amorphous film growth at high concentrations. Among the coating types with interesting properties are nitrogen containing films sputtered reactively from ZrB2 targets that had an anthracite coloured surface and outstanding corrosion resistance. Violet coloured coatings deposited non-reactively from LaB6 targets are also potential candidates for decorative applications.

A Monte Carlo study of the angular dependence of the depth distribution function of signal electrons in electron spectroscopies

The emission angle dependence of the depth distribution function of signal electrons in electron spectroscopies has been studied by means of a Monte Carlo simulation in which elastic scattering plays a significant role. Calculations were made for Si, InP, and In substrates at different emission angles (0°, and 40° to 80° off normal). The calculated depth distribution of signal electrons showed significant deviations from exponential attenuation at the more oblique emission angles. In consequence, the ‘‘attenuation length’’ of signal electrons loses its status of a material constant. A new empirical depth distribution function is proposed which consists of the sum of two exponential terms. This function describes the calculated Monte Carlo depth distribution as a function of the emission angle. The first term of this depth distribution function represents the contribution of those electrons which follow parallel straight‐line paths, whereas the second term accounts for a randomization of electron direction...

Angular dependence of the surface excitation probability for medium energy electrons backscattered from Al and Si surfaces

Reflection electron energy loss spectra have been measured for medium energy electrons backscattered from Al and Si surfaces. Angular distributions were obtained for emission angles between 15° and 90° with respect to the surface normal as well as for incidence angles in the same range. The surface excitation parameter (SEP), i.e., the average number of surface excitations an electron experiences when it crosses a surface once, was extracted from each spectrum by fitting the raw data to theory and determining the ratio of the surface loss peak to the elastic peak intensity. No difference in the SEP for incoming and outgoing electrons could be detected in the data. The SEP was found to depend linearly on the time an electron spends in the vicinity of the surface. Both the energy and angular dependence of the SEP can be accurately described by free-electron theory when the electron momentum is rescaled by a material-dependent parameter. The value of the scaling parameter is given for Al and Si so that the S...

Study of the ignition behavior of a pulsed dc discharge used for plasma-assisted chemical-vapor deposition

An investigation of a pulsed dc discharge used for plasma-assisted chemical-vapor deposition of titanium nitride has been performed in order to understand the nonuniformity of quality and thickness of the deposited films. The experiments have been performed using a gateable image intensifier to study the temporal and spatial light intensity of the discharge. Additionally, a single Langmuir probe has been used to study the temporal evolution of the plasma potential and the charged particle density. The influence of varying parameters like geometry, gas composition, voltage, pressure, duration of the pulses, etc., on the spreading of the plasma has been investigated. Our experiments reveal that in the presence of electronegative species like TiCl4, which is a source gas for the production of titanium nitride, the spreading of the discharge along the substrates is slow, reaching some parts of the reactor with substantial delay. The result is a nonuniform plasma power density in front of the cathode as well as a spatially varying exposure time of the surface to the plasma. These effects are the reason for the inhomogeneity of the deposited films. The experiments revealed that the slow spreading of the discharge is accompanied with low local cathode fall voltages. The problems with the slow spreading of the discharge can be solved by measures which increase the conductivity of the plasma at the beginning of the pulses. The effectiveness of such measures has been studied.An investigation of a pulsed dc discharge used for plasma-assisted chemical-vapor deposition of titanium nitride has been performed in order to understand the nonuniformity of quality and thickness of the deposited films. The experiments have been performed using a gateable image intensifier to study the temporal and spatial light intensity of the discharge. Additionally, a single Langmuir probe has been used to study the temporal evolution of the plasma potential and the charged particle density. The influence of varying parameters like geometry, gas composition, voltage, pressure, duration of the pulses, etc., on the spreading of the plasma has been investigated. Our experiments reveal that in the presence of electronegative species like TiCl4, which is a source gas for the production of titanium nitride, the spreading of the discharge along the substrates is slow, reaching some parts of the reactor with substantial delay. The result is a nonuniform plasma power density in front of the cathode as well a...