Characteristic photon emission from ion bombarded silicon and silica surfaces

Ion bombardment with 50 keV inert gas and reactive gas ions has been used to study the photon emission from excited radiating atoms and ions in the immediate vicinity of the surface of both silicon and silica glass targets (SiO₂).     

The polarization of light emitted from sputtered Mg atoms

The polarization fraction of light emitted from Mg atoms excited during bombardment of the Mg polycrystalline target by low-energy Ne⁺ ions has been measured. Taking into account the results, a possible mechanism of excitation of sputtered atoms is suggested.     

A comparison of radiation from sputtered atoms with radiation from projectiles scattered on oxidized magnesium at varying angle of incidence

The intensity of light emitted from sputtered atoms and neutralized, scattered primary ions, excited during 4 keV Ne⁺ and Ar⁺ bombardment of oxidized magnesium has been measured as a function of the incidence angle. It was found that the photon yield of sputtered atoms increases with the angle of incidence much more rapidly than the theoretical sputtering yield and the photon yield of scattered projectiles. In order to explain the experimental results a numerical approach was made based on the following assumptions: (1) the sputtered atom can be excited when it crosses the surface after getting the momentum from the collision cascade; (2) at oblique incidence the sputtered excited atom can be directly emitted after a gentle collision between the incident ion and the surface atom; (3) the neutralized primary ion can only be excited in a violent collision with the surface atom.     

On the mechanism of cluster emission in sputtering

The intensity and the energy distribution of Si⁺_n cluster ions emitted from clean silicon have been measured for different target orientations as a function of the primary ion energy (3–30 keV) and the projectile mass (noble gas ion bombardment). The results favour the idea that clusters are emitted as such rather than being produced by vacuum recombination of individually emitted atoms and ions.     

Surface analysis by ion-electron spectroscopy; Silicon target

The energy spectra of secondary elections emitted from a Si(111) surface due to bombardment by 6 keV He⁺ and O⁺₂ ions have been examined. The fine structure in the spectra is explained on the basis of a novel mechanism of creation of Auger electrons at the surface. There are two stages of interaction between incoming ions and the substrate via adsorbed atoms. In the first stage, due to a level promotion mechanism, vacancies in the adsorbed atoms are created. In the second stage, Auger neutralization processes accompanied by the emission of electrons from a solid with characteristic energies take place. These electrons provide a good indication of the degree of coverage of the silicon surface with contaminant atoms. The energy losses of escaping electrons are also discussed.     

Influence of ion bombardment induced radiation damages in the crystal lattice on the angular regularlties in ion scattering

The paper examines the angular, spatial and energy distribution of ions scattered by various faces of silicon and germanium single crystals. It is shown that, due to the orderly arrangement of atoms in the crystal lattice, the orientation effects in the angular, spatial and energy distributions are observed at temperatures exceeding the annealing temperature of ion bombardment induced radiation damage. At high temperatures of the sample a reduction of the anisotropy in the angular, spatial and energy distributions of scattered ions was observed which is due to the changing transparency of the crystal resulting from the thermal vibrations of atoms in the lattice.

Results of investigations testify to the possibility of using the orientation dependences of phenomena occurring during the interaction of ions with crystals for determining the annealing temperature of ion bombardment damage and for studying the kinetics of the process of ion bombardment. It is shown that further studies along the same lines as conducted by the authors of this paper could develop a technique for the quantitative estimation of ion bombardment damage in crystals and also in thin epitaxial films.     

Chemical sputtering,a discussion of mechanisms

Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called “physical sputtering”. If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorp-tion of particles, the process could be classified as “chemical sputtering”. There are a number of molecules such as CH₄, CF₄, CF₃H, CF₃CI, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain absorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena: however, few systematic quantitative investigations of the subject have been completed. In this paper we will review the evidence for chemical sputtering and discuss mechanisms based on experimental information obtained for the chemical sputtering of silicon and SiO₂ under argon ion bombardment in the presence of a molecular beam of XeF2. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as SiF₄ (gas) and the rest leaves as silicon atoms or SiFx radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of XeF₂ flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for SiO₂ is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, SiO₂ is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces SiF₄, and the SiF4 is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the SiF₄ molecule.     

Mechanism of low-temperature ion-bombardment-activated surface self-diffusion

Elementary events of low-temperature surface erosion induced by the bombardment with accelerated helium atoms and ions were studied at the atomic level. It is established that the regular arrangement of surface atoms is disturbed due to the release of energy of formation of interstitial atoms emerging at the surface and to the expenditure of part of this energy on the formation of surface defects in excited states. The adatom excitation energy allowing the short-range diffusion processes was determined experimentally.     

Effect of inelastic and elastic energy losses of Xe ions on the evolution of hydrogen blisters in silicon

We analyze the effect of irradiation by heavy ions on the formation of blisters on the silicon surface preliminarily ion-doped with hydrogen. An attempt is made at differentiating inelastic and elastic processes of interaction between ions and Si atoms using bombardment of the sample with high-energy charged particles through a bent absorbing filter by varying the radiation doses and the energy of bombarding Xe ions. It is found that irrespective of specific ionization energy losses of heavy ions, the blister formation is completely suppressed in the zone of the inelastic interaction during postradiation annealing. Conversely, stimulated development of hydrogen porosity takes place at the same time in the zone of elastic interaction, which is manifested in the form of blisters and flaking.     

Charge exchange features of excited sputtered particle production

Abstract

The results are presented which have been obtained by studying the angular dependence of photon and ion emission and the spatial energy distribution of excited silicon ions produced under ion bombardment of monocrystalline and amorphized silicon surface. The experimental data are discussed which have been obtained mostly by the coincidence method. The theoretical concepts making it possible to account for the observed behavioural features are also discussed.     

Coherent emission of a photon by many atoms

The theory ofWeisskopf andWigner of the natural line width is extended to a case where many atoms at given positions interact with one common quantized radiation field. At timet=0, one quantum of radiation energy is stored by the atoms, which however does not mean necessarily that exactly one of the atoms must be excited. We study the process of the creation of a photon in dependence on the positions of the atoms and on the state of the system of atoms att=0. The dimensions of the emitted wave train and the decay time of this excited atomic state depend sensitively on these parameters.     

Observation of an underlying relativistic effect in the valence bands of Pt

We have measured the photoelectron spin polarization emitted by unpolarized UV radiation from the valence-bands of the well ordered Pt(0 0 1)-(5 × 1) surface and the disordered surface destroyed by Ar ions bombardment. Almost identical spin polarizations have been observed in both cases. This observation suggests that the electron spin polarization in photoemission caused by unpolarized light is determined by a short-range order of atoms. This finding has an obvious implication that the electron spin polarization in photoemission caused by unpolarized light can be used to study the bulk electronic structure of the nonmagnetic materials.     

Three-channel electronic instrument for the measurement of metastable level excitation functions

A three-channel electronic instrument for the measurement of the excitation function f₁₂(U₁) of forbidden transitions is described. The instrument consists of two electron guns in antiparallel arrangement and a mobile metallic detector. It is shown that in order to determine f₁₂(U¹), it is sufficient to measure either the current from collisions of the second kind of excited atoms with slow electrons, the current of secondary electrons being emitted during the bombardment of the detector surface by metastable atoms, or the radiation intensity of the spectral line emitted during step-by-step excitation. Formulas are also proposed for the determination of the absolute magnitudes of the effective cross sections of step-by-step excitation and collisions of the second kind.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 61–65, December, 1977.The author wishes to thank N. I. Petrov for valuable assistance in the preparation of the instrument.     

Influence of the radiationless transitions on an intensity of the optical radiation emitted by excited atoms ejected from solids by an ion beam

The excited atoms of the target material are ejected during an ion bombardment of solids. These atoms belong to one of two velocity groups — fast or slow. The fast atoms arise in binary collisions of bombarding ions with target atoms and the slow ones are knocked out as a result of a sputtering process. Excited atoms flying off the surface intersect the solid-vacuum boundary and can transfer their excitation energy in the radiationless transitions mainly of the resonance ionization type. The probability of this process depends strongly on the electronic energy level structure of solids and a velocity of ejected atoms. On this statement our method of the electronic energy level structure of solids study is based by means of investigation of excited atoms velocity spectrum. On the results of paper [4] we remark that in their experimental conditions the surface of the lithium target was apparently strongly oxidized. Using our method and results of paper [4] we can estimate the energy width of the conduction band of Li₂O to 0.4 eV. In general the cascade corrections to the mean life times of excited atoms may be important and one can take them into account. The detailed analysis of the influence of the cascade corrections to the mean life times of upper excited states of some TiI lines (λλ 5210 Å, 5064 Å, 4682 Å, 3981 Å, 4533 Å, 4856 Å) was carried out. It was found that in the case when our method was applied to determination of the work function of metallic titanium the cascade corrections either are negligible or not necessary.     

126Xeq+轰击Al表面产生的原子和离子光谱线

报道了高电荷态离子^126Xe^q (6≤q≤30)入射到固体Al表面产生的200～1000nm波段的发射光谱的实验结果。实验表明,在弱束流(nA量级)高电荷态的情况下,通过入射离子与固体靶的相互作用可有效地产生原子和离子的复杂组态间跃迁所形成的可见光波段的特征谱线,而且当入射离子的电荷剥离数超过一临界值后(对Al,q=26),谱线相对强度突然显著增强。根据经典过垒模型COB(The classic over-barrier model),在入射离子的动能较小(～1keV／u)的条件下．高电荷态离子与表面相互作用过程中电子的俘获或转移起着非常重要的作用,通过提高入射离子的电荷态可增强入射离子俘获电子的能力．显著增强激发粒子的光谱线的强度。     

Organic contaminants removal by oxygen ECR plasma

Recently electron cyclotron resonance (ECR) plasma have been explored for wafer cleaning applications, since it is known to do less damage to silicon surface than conventional plasma. Organic contaminants removal efficiency and plasma radiation damage of the ECR plasma cleaning have been investigated. In oxygen ECR plasma cleaning, the plasma exposure time needed to remove the organic contaminants on the silicon surface down to the detection limit is 40 s, but the one to reach the lowest surface roughness is 10 s. The leakage current level of the MOS capacitor made using the Si substrate exposed to oxygen ECR plasma for 40 s is 8 × 10⁻⁹ A. The optimum exposure time determined by considering the contaminants removal efficiency and the plasma radiation damage (or the leakage current level) is 40 s. Organic contaminants seem to be removed through both sputter-off mechanism by oxygen ion bombardment and evaporation mechanism by chemical reactions with excited oxygen atoms.     

Photon emission under bombardment of ruby by ions and electrons of medium energies

The characteristics of optical radiation produced under bombardment by ions (1–15 keV) and electrons (100–1000 eV) of a ruby surface are studied. Two broad bands with maxima at 330 and 450 nm attributed to defects of the crystal lattice are discovered in the spectral region of 200 to 800 nm. Characteristic radiation related to radiative relaxation of excited Cr³⁺ ions in the near-surface region of the crystal is observed. The dependence of the radiation intensity on the energy and current density of the bombarding particles is obtained experimentally. The degree of radiation polarization is determined.     

X-ray spontaneous emission control by 1-dimensional photonic bandgap structure

The possibility of controlling the X-ray spontaneous emission of atoms embedded in a 1-dimensional photonic bandgap structure by the so-called Purcell effect, is studied. Calculations of the spontaneously emitted power are presented from Fermi’s golden rule in the framework of the Wigner-time approach extended to absorbing media. Numerical simulations are compared to experimental results for the case of the K emission from silicon atoms excited by electrons within a Mo/Si multilayer Bragg reflector. The inhibition or enhancement of X-ray emission from such structures appear to be feasible.     

Spectroscopic diagnostics of the laser erosion plasma of an AgGaS<Subscript>2</Subscript> polycrystalline target

Results are presented from experimental studies of the radiation emitted from a plasma produced in vacuum after irradiating a polycrystalline target by 1.06-μm laser radiation with an intensity of (3–5)×10⁸ W/cm². Plasma radiation from regions located at distances of 1 and 7 mm from the target is analyzed. It is shown that the main contribution to the plasma radiation in the 220–600 nm spectral range is made by transitions from the excited states of single-charged Ag⁺ and S⁺ ions. The atomic component of plasma radiation is represented by intense spectral lines corresponding to transitions from the Rydberg states of Ag and Ga atoms, whereas no resonance lines of these atoms are observed.     

Chirping in light emitted by an assembly of two-level atoms

It is shown quantum mechanically that Lamb shift in radiation emitted by an assembly of two-level atoms contained in a small volume and in high excitation is opposite to that for its weak excitation and to that for a single excited atom. This leads to chirping in emitted radiation.