D.J. Oostra

Reactive sputtering of simple condensed gases by keV ions II: Mass spectra

Abstract Condensed gas layers of H2O, NH3 and CO at 15–20 K have been bombarded by 6 keV H+2 and 3 keV He+ and Ar+ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H2O frozen on top of the CO-residue H2CO was detected.

Reactive sputtering of simple condensed gases by kev ions III: Kinetic energy distributions

Abstract Condensed gas layers of H2O, NH3 and CO have been bombarded by 3–6 keV H+, H+2, He+ and Ar+ ions. Kinetic energy distributions of the sputtered neutral particles have been measured using a time-of-flight technique. The mechanism of ejection of the sputtered products in nearly all cases appears to involve momentum transfer in the final steps, as is inferred from the collision cascade-like energy spectra. The effective surface binding energies observed are much lower than the sublimation energies. Nevertheless most sputtered species have energies far in excess of those expected for evaporation at the ambient temperature. Only H2 from NH3 shows evaporation as the main desorption mechanism under irradiation by light ions.

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.

Mechanisms of sputtering of Si in a Cl2 environment by ions with energies down to 75 eV

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 s...

Sputtering of SiO2 in a XeF2 and in a Cl2 atmosphere

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 ...

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.

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.

Emission of large molecules from methane by ion bombardment

Abstract 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.

Ion‐assisted etching of silicon by SF6

Sputtering of silicon by 3‐keV Ar+ ions in the presence of a molecular SF6 beam has been investigated by mass spectrometry and time‐of‐flight measurements. At temperatures below 100 K chemical reactions are induced between the silicon and an adsorbed layer of SF6 under ion bombardment leading to newly formed molecular products. The main products formed are SiFx’ (x=0–4) and a small amount of SiSFy compounds (y=0–2). The kinetic energies of these molecular species are for the major part in the 0.1‐eV region which excludes evaporation at substrate temperature to be a dominant mechanism for erosion. An effective sputtering yield for silicon of approximately 15 is obtained.

Sputtering of solid nitrogen and oxygen by keV hydrogen ions

Abstract 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.