Experimental and modeling study of O and Cl atoms surface recombination reactions in O2 and Cl2 plasmas

Abstract

Abstract:In low-pressure plasmas commonly used in materials processing, plasma-wall inter actions play a crucial role in the evolution of the plasma properties both over time and across large-area wafers. We have recently studied the heterogeneous recombination of O and Cl atoms on reactor walls in O 2 and Cl 2 plasmas through both experiments and modeling. The Langmuir-Hinshelwood (i.e., delayed) recombination was investigated using a "spinning-wall" technique in which a portion of the substrate surface is periodically exposed to an inductively coupled plasma and to a differentially pumped chamber where either Auger electron spectroscopy (AES) or line-of-sight mass spectrometry (MS) is used to detect surface and desorbing species. In this paper, a review of the various effects driving the O and Cl atoms recombination dynamics on anodized aluminum (AA) and stainless steel (SS) surfaces is presented. It is shown that recombination probabilities, γ, can vary following plasma exposure due to surface conditioning. In Cl 2 plasmas, γ was also found to depend on the Cl-toCl 2 number density ratio, a mechanism ascribed to a competition for adsorption sites between Cl and Cl 2 . We have also determined the recombination rates of Cl atoms in Cl 2 high-density plasmas sustained by electromagnetic surface waves by comparing the measured degrees of dissociation of Cl 2 to those predicted by an isothermal fluid model. For a reactor with large SS and quartz surfaces exposed to the plasma, γ values and their dependence on the Cl-toCl 2 number density ratio were consistent with those obtained from the rotating substrate technique. Similar values were obtained for plasmas sustained in a quartz discharge tube. It is expected that for plasmas sustained in or adjacent to a silica tube or plate, the Cl atoms recombination coefficient becomes independent of chamber wall material due to reactor seasoning, producing a silicon-oxychloride layer.

Deterministic and Monte Carlo methods for simulation of plasma‐surface interactions

“…This assumption is not valid for highly catalytic surfaces or reactive molecular species. For instance, in chlorine plasmas sustained in stainless steel and anodized aluminium vessels, neutral Cl 2 molecules can get adsorbed on surfaces and therefore enter in competition for adsorption sites with atomic species. The competitive adsorption of atomic and molecular species is not considered in this paper, but it can be readily modeled in the deterministic approach by including additional processes in the above reaction set.…”

“…From the modeling perspective, several possibilities exist to account for such distributions. In deterministic models, surface sites may be divided into groups according to their properties, e.g., the adsorption energy, and then a rate balance equation is formulated to follow the evolution of the fractional coverage in each individual group. A similar approach was implemented in KMC simulations of NO oxidation by adsorbed oxygen atoms on Pyrex surface, with each group of O ads treated as a different type of species characterized by a specific set of reaction rates.…”

“…In previous sections, a simple model system with only one type of chemisorption sites was investigated. Recently, it was demonstrated that atoms adsorbed on real disordered surfaces are usually characterized by distributions of adsorption energy and reactivity. As it was discussed in Section 2.2.1, the deterministic treatment is not well‐suited for modeling such complex systems while the Kinetic Monte Carlo approach can handle arbitrary distributions of active sites with only minor modifications of the basic code.…”

Investigation of the radially resolved oxygen dissociation degree and local mean electron energy in oxygen plasmas in contact with different surface materials

“…Furthermore, the wafer material itself and the condition of the other wall materials in the reactor affect the absolute number densities of reactive species due to wall loss processes, the probability for which is dependent on many factors. [12][13][14][15][16][17][18][19][20][21][22][23][24][25] This makes it particularly difficult to accurately control the densities of highly reactive species, such as atomic oxygen, in industrial processing applications. Previous investigations have sought insight into these processes by measuring the density of atomic oxygen and its surface recombination probability for various materials using twophoton absorption laser-induced fluorescence (TALIF), 12,[26][27][28][29][30] optical emission spectroscopy (OES), 21,22,[31][32][33][34] and VUV absorption spectroscopy.…”

Photo-assisted etching of silicon in chlorine- and bromine-containing plasmas

“…(Cl emission from dissociative excitation of Cl 2 cannot be neglected when n Cl2 ) n Cl ) Therefore, it appears that the Cl number density is unaffected by the presence of the window. Since Cl atoms have a low recombination coefficient on dielectrics such as quartz [45][46][47] and probably MgF 2 , this is not surprising.…”