Electronic sputtering of solid nitrogen and oxygen by keV hydrogen ions has been studied at two low-temperature setups. The yield of the sputtered particles has been determined in the energy regime 4–10 keV for H+, H 2+ and H3+ ions. The yield for oxygen is more than a factor of two larger than that for nitrogen. The energy distributions of the sputtered N2 and O2 molecules were measured for hydrogen ions in this energy regime as well. The yields from both solids turn out to depend on the sum of the stopping power of all atoms in the ion. The yield increases as a quadratic function of the stopping power for oxygen, but slightly slower for nitrogen. The energy distributions do not exhibit strong features, but are similar to those published earlier for electron sputtering.

VUV light of 118.2 nm (10.49 eV) is produced by frequency tripling the third harmonic of a Nd:YAG laser (354.6 nm) in a Xe cell. With these photons time-of-flight mass spectra of n-hexane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane and cyclohexane are obtained by single photon ionization. The simple set-up for production of 118.2 nm photons, the necessity of separating the 354.6 nm photons and the obtained mass spectra are discussed. The fragmentation of hexane as a function of its temperature is compared with the predictions from quasi-equilibrium theory. The model shows that for n-hexane less than 50% of the difference between photon energy and ionization energy is deposited in the molecular ion.

Solid N2 has been bombarded by 0.5 keV electrons. Mass spectrometry and time of flight measurements were performed for the sputtered neutral particles. The dominant component is molecular N2, but also N-atoms and a small number of N3- and N4-molecules were observed. The energy distribution of N2 and N indicates that the sputtering is caused by non-radiative transitions that release up to more than 6 eV. The important non-radiative transitions in solid N2 are discussed.

Sputtering of Si in a Cl2 environment by Ar+ and Xe+ ions with energies down to 75 eV has been investigated. Mass spectra and time-of-flight distributions of the sputtered species have been measured. Under 75-eV Ar+-ion bombardment of the Si target, SiCl, SiCl2, SiCl3, and/or SiCl4 are sputtered. When increasing the ion energy the SiCl4 contribution decreases in comparison with SiCl. This is caused by the fact that the newly formed Si-Cl compounds are sputtered at a high rate compared to the rate of SiCl4 formation. Time-of-flight distributions indicate that under 100-eV Ar+-ion bombardment the species are not sputtered by a collision-cascade mechanism. The spectra can be fitted by Maxwell–Boltzmann distributions at a high (>2000 K) temperature. Increasing the Ar+-ion energy to approximately 250 eV the time-of-flight spectra of the sputtered species change from Maxwell–Boltzmann-like into spectra as expected for a collision-cascade mechanism. For low-energy Xe+ ion bombardment the sputtered species also show Maxwell–Boltzmann time-of-flight distributions. The change from Maxwell–Boltzmann to collision-cascade distributions occurs at higher ion energies than for Ar+-ion bombardment. The results obtained for low ion energies are discussed in terms of evaporation from an ion-induced hot spot.

An impulsive collision model describing the vibrational, rotational and translational energy of sputtered diatomic molecules is presented. A pre-existing molecule at the surface undergoes a single collision with a subsurface particle and is consequently ejected from the solid. Repulsive potential interactions and an impulsive approximation of the collision determine energy transfer to both atoms of the molecule and thus the partition over internal and kinetic motion. Model calculations of the vibrational, rotational and kinetic energy distributions yield a good agreement with recent experimental data of sputtered diatomic sulfur molecules. Almost all of the qualitative features are reproduced by the model calculations.

Recently energy spectra of reaction products emitted during Ar+ ion bombardment induced etching of Si by SF6 condensed on top of it have been reported [8]. The Si(SF6; Ar+) system differs from all others studied to date in that multilayer adsorption of reactive gas occurred during the experiments. This is shown here to induce an additional ejection mechanism. The matrix from which reaction products are finally emitted changes from a “SF6-mixed-into-Si” into a “Si-mixed-into-SF6” type and finally into pure SF6 when the SF6 flux is increased or the substrate temperature is lowered. The profound changes observed in the kinetic energy distributions of specific emitted products upon variation of experimental conditions are reinterpreted and attributed largely to an alteration of the thickness of the condensed SF6 “buffer” layer.

S2 molecules are sputtered by keV ion bombardment from sulfur. With a laser induced fluorescence technique the vibrational and rotational energy distributions are measured. With an additional time-of-flight method a velocity selection is attained. This enables us to determine the internal energy of the S2 as a function of the molecular velocity. The shape of the population distribution is found to resemble Boltzmann behaviour, yielding effective “temperatures” for vibration and rotation. The vibrational energy is almost independent of the velocity, Tvip ∼− 1500 K. However, the average rotational excitation energy increases with both molecular velocity and vibrational quantum number. Approximated by Boltzmann behaviour, the “temperature” Trot varies from 300 to 1600 K. Sputtering with He+ instead of Ar+ ions gives identical results except for a higher ground vibrational level population. The results are compared with a double and single collision model of molecule sputtering. It is concluded that the single collision mechanism cannot explain the results. The following qualitative features of the energy partition are correctly predicted with the double collision model: the vibrational population for high molecular kinetic energies, the rotational populations of the ground vibrational state, increasing rotational excitation with vibrational quantum number and increasing rotational excitation with molecular velocity.

This paper concerns sputtering of molecules by keV ions and by 0.1–1 keV electrons. The emphasis is on sputtering of molecules initially not present in the solid substrate. Mass spectra, kinetic energies and internal energies of the sputtered species will be discussed.

SiO2 has been bombarded by 3 keV Ar+ ions under simultaneous exposure to a thermal beam of Cl2 or XeF2. Mass spectra and time-of-flight distributions of the sputtered species have been measured. It is observed that during XeF2 exposure the sputtering yield of SiO2 is enhanced with a factor of 2.3. After ionizing the neutral ejected Si species, we have detected SiF+x (x=0–4) and SiOF+y ( y=0–2). The kinetic energy distributions of these particles indicate that the newly formed species have been bound loosely to the lattice and that they are sputtered predominantly by a collision cascade mechanism. The results are explained by assuming that adsorbed F atoms are mixed into subsurface layers in which Si–F bond formation takes place. Hardly any chemical enhancement of the sputtering yield of SiO2 is observed when Cl2 is added. In this case the Si species are detected as SiCl+x and SiOCl+y (x, y=0–2). The chlorine peak (Cl+) is an order of magnitude higher. The kinetic energy distribution of Cl shows that most of the chlorine is in the lattice chemically unbound. From the measurements it is concluded that the heat of formation determines that in the collision cascade Si–F bonds are formed while Si–Cl bonds are not. Differences in the enhancement of the sputtering yield between silicon and SiO2 can also be explained by differences in heat of formation.

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.

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 Å per laser pulse. No difference is found between p- and n-type silicon.

A Si substrate is bombarded by 3-keV ${\mathrm{Ar}}^{+}$ ions. From time-of-flight spectra of resputtered Ar neutrals at various target temperatures, we conclude that Ar-bubble formation takes place in the amorphized-Si top layer. The bubbles form and open during etching. The average kinetic energy of the Ar atoms is in agreement with the calculated average potential energy of the Ar atoms inside the bubbles.

Condensed layers ot methane at 20 K have been bombarded by 6–8 keV Ar+, He+ and H2+ ions. Mass spectra and Kinetic energy distributions of neutral species sputtered from these layers have been measured. We have found sputtered species with masses up to 72 amu and thus with at least 5 carbon atoms. In addition to this an involatile residue was formed. Analysis by pyrolysis mass spectrometry showed this residue to contain species with masses up to at least 170 amu which therefore contain at least 12 carbon atoms. The kinetic energy distributions of sputtered methane molecules lie between those of a Maxwell-Boltzmann distribution and a collision cascade. Higher values are reached for Ar+ than for the light ions. From these observations we conclude: for both light and heavy ions radicals are formed, which combine to new molecules. These exothermic reactions produce heat which causes desorption. The high energy tail for bombardment with argon ions shows that part of the sputtering is caused by momentum transfer.

Recent results obtained in our laboratory on sputtering of silicon and SiO2 are summarized. In the experiments the target is exposed simultaneously to a reactive gas and to bombardment by either keV ions, keV electrons or UV photons. Results have been obtained by mass spectrometry and time of flight (TOF) measurements of the sputtered species. From these it is concluded that etching of silicon by the three types of bombardment is caused by three different mechanisms. 3 keV Ar+ ion-induced sputtering of silicon and SiO2 in a Cl2 or in a XeF2 atmosphere is explained by a collision cascade mechanism. It is suggested that in the cascade adsorbed halogen atoms are mixed into a subsurface layer. High sputtering yields are caused by formation of weakly bound SiXx (X=F, Cl; x=1−4) species, which are ejected by momentum transfer in the collision cascade. It is argued that the amount of formed silicon halides depends on the exothermicity of the chemical reactions. 1 keV electron-induced etching of silicon by SF6 has been investigated. The mass spectrum of the sputtered species indicates that during etching SiF4 is formed and ejected. From the TOF measurement it is concluded that besides evaporation at substrate temperature (100 K) also relatively hot molecules are ejected. Etching of silicon in a Cl2 atmosphere by laser pulses of 308 and 248 nm photons has been studied. The TOF spectra of the ejected species (Si, SiCl and SiCl2) are explained by assuming that in the laser pulse the near surface region is heated to 1400–4000 K. Evaporation at these temperatures leads to a high etch rate. Surprisingly, the translational temperature of the species depends on the amount of Cl atoms adsorbed at the surface.

Experiments show that silicon at 100 K can be etched by 1 keV electrons, when simultaneously exposed to a beam of thermal (300 K) SF6. A yield has been found of approximately 0.4 Si atoms per incoming electron. The etch mechanism has been investigated by mass spectrometry and time-of-flight measurements of the ejected species. The results indicate that during etching SiF4 is formed, which subsequently evaporates. In addition, species are sputtered from the target with relatively high energies. Mechanisms for the formation of these energetic particles are discussed.

Condensed layers of methane at 20 K. have been bombarded by electrons with an energy of 265 and 515 eV. The neutral sputtered flux was analyzed for mass and kinetic energy. Besides CH4 also larger molecules are sputtered. During this process an involatile residue is formed. The kinetic energy distributions for methane, obtained from the times of flight, fit a Maxwell-Boltzmann distribution curve with T = 170 K. These results are explained by exothermic chemical processes which are initiated by the electrons and which lead to spots with a higher temperature. Large molecules which do not desorb are subjected to chemical reactions which ultimately lead to a nonvolatile residue.