The etching of Si(100) is studied quantitatively in a molecular beam setup. After exposing the silicon surface to XeF2 doses between 102 and 104 monolayers (MLs) of XeF2, thermal desorption spectroscopy is used to study the SiFx content of the reaction layer. Large values of the fluorine content (up to 30 ML) are observed. These are explained by surface roughening due to the etching process, which increases the effective surface by up to a factor of 6 after 105 ML of XeF2, corresponding to 104 ML of etched Si. This implies a porous or canyon-like structure. The dose dependence of the fluorine content is not single exponential; it increases rapidly for the first 102 ML and much more slowly thereafter, saturating after approximately 104 ML. This behaviour is described by the rapid formation of a monolayer of SiFx species in the initial regime, followed by the slow formation of a steady state multilayer of SiF–SiF2–SiF3 and SiF2–SiF3 chains. The etch rate shows a similar dose dependence, increasing from 12% of the incident flux in the monolayer regime to 20% in the multilayer regime. Finally, it is shown that fluorine diffusion into the silicon bulk is not an important process in the reaction layer formation.

An ultrahigh vacuum multiple-beam setup has been designed to study surface reactions that are of importance in plasma etch processes. The setup consists mainly of five beams (reactive neutrals, ions, electrons, CFx radicals, and photons) all of which can be focused on the same sample area, and a quadrupole mass spectrometer detecting only molecules desorbing from this area. Data are reported on spontaneous etching for the Si(100)/XeF2 system. Both the incident flux of XeF2 on the sample and the desorbing fluxes of SiFx products and nonused XeF2 were measured quantitatively. The reaction of XeF2, the production of SiFx species and the accumulation of fluorine on the silicon surface were studied as a function of temperature (300–900 K) for XeF2 fluxes of 0.24 and 1.04 monolayers/s. For the reaction probability of XeF2 an exponential increase with temperature from 11% at 300 K up to 50% at 900 K was found. The main product was found to be SiF4 at 300 K gradually becoming replaced by SiF2 at temperatures higher than 600 K. A remarkable result is the accumulation of a rather thick SiFx reaction layer on the etched surface. From thermal desorption spectra the fluorine content was calculated to be 38 monolayers at 300 K decreasing rapidly for higher temperatures. This suggests that fluorine diffusion into the silicon bulk might be of importance in the etching process.

To investigate the reaction layer dynamics of ion-assisted etching we have measured the time response of product formation in an ultrahigh vacuum beam-surface experiment on a time scale ranging from 100 μs to 1000 s. Both the step function response and the delta function response are investigated. For the latter the pseudorandom cross correlation method is used. The system investigated is the classic Si(100)/XeF2/Ar+ example, at low flux conditions of 0.6 ML/s XeF2 and 0.04 ML/s Ar+ ions at 1 keV energy. We observe a consistent picture of the fourfold action of the bombarding ions. First, on a 1 ms time scale and shorter, the release of tightly bound intermediate radical species such as SiF and SiF2 by physical sputtering, i.e., by momentum of the impinging ions, is the main effect. Second, on a time scale of 40 ms, we observe ion-enhanced formation of SiF4, most likely by the influence of ion bombardment on a rate limiting step in the reaction chain such as the formation of SiF4 from SiF3. Third, on a time scale of 4 s, there is a redistribution of intermediate SiFx products in the reaction chain, both in depth profile and in absolute density. Finally, on an even longer time scale, detectable after some 10 s at an ion bombardment of 0.04 ML/s, the production of vacancies and/or broken bonds as reactive sites deep in the substrate becomes important, as observed by a long-term enhanced etch rate after switching off the ion beam. The results are consistent with a model that for low temperatures (T<600 K) the disproportionation reaction, 2 SiF3→SiF4+SiF2, is the rate limiting step, while at high temperatures (T≳700 K) the reaction step leading from SiF2 to SiF3 plays this role.

The Ar ϩ ion-enhanced Si͑100͒/XeF 2 reaction at 300 K is studied quantitatively in a molecular beam setup. Measurements are done for XeF 2 -fluxes from 0.1 up to 3.4 monolayer/s, Ar ϩ -fluxes from 8 ϫ10 Ϫ4 up to 8ϫ10 Ϫ2 monolayers/s and Ar ϩ -energies of 500, 1000 en 2000 eV. Both the XeF 2 consumption and the SiF x production are measured by mass spectrometry. It is concluded that physical and chemical sputtering are the only significant ion-induced mechanisms: damage-enhanced etching and enhanced spontaneous etching can be neglected. The flux dependence of the etch process is found to be solely determined by the ratio of the ion over the neutral flux. This behavior is described by a simple kinetic model. From the energy dependence it is concluded that both the physical and the chemical sputtering contributions scale with the square root of the ion energy. The ion-enhanced Si/XeF 2 reaction is most efficient for ion/neutral ratios of 0.1 and higher: 80% of the XeF 2 is then consumed in the reaction process versus 10% during spontaneous etching. It is concluded from the mass spectrometer signals that under these conditions between 35% and 61% of the SiF x products are released by physical sputtering and the remainder desorbs by chemical sputtering.

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