Etch products desorption

Product desorption is a crucial step in the etch process. A free radical can react rapidly with a solid surface, but unless the product species has a reasonable vapor pressure so that desorption occurs, no etching takes place. For instance, when an aluminum surface is exposed to fluorine atoms, the atoms adsorb and react to form AIF3. However, the vapor pressure of AIF3 is 1 torr at 1240 C thus etching is precluded at ambient temperatures. 

Fig. 12. Molecular representation of primary processes occurring in plasma etching. The basic steps are etchant generation (step I), diffusion to the surface (2), adsorption (3), reaction on the surface to form product (4), product desorption (5), and product diffusion into the flowing gas (6). The surface processes can be strongly affected by energetic ions bombarding the surface. After [111.

A similar kinetic analysis clarities the other specific etching mechanisms and can be applied for sputtering, as well as for ion energy-driven etching limited by the formation and desorption of etch product, which takes place in the case of silicon etching by F atoms (Lieberman Lichtenberg, 1994). 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

As was suggested in a previous paper dthe steady state etching of solid material by exposure to gas phase particles with or without a plasma is usually described by the following sequence of steps (1) nondissociative adsorption of gas phase species at the surface of the solid being etched (2) dissociation of this absorbed gas (i.e., dissociative chemisorption) (3) reaction between adsorbed radicals and the solid surface to form an adsorbed product molecule, e.g., SiF fads) (4) desorption of the product molecule into the gas phase and (5) the removal of nonreactive residue (e.g., carbon) from the surface. 

Due to the multiple desorption products, the surface mechanism of adsorption and desorption of Cl etching seems to be quite complex. More theoretical studies are expected. 

Due to the multiple desorption products, the etching of the surface with halogen appears to be quite complex. A multi-step reaction mechanism has been suggested to account for the SiCl2 desorption species. In the case of fluorine atom adsorption, F atom abstraction and dissociative chemisorption mechanisms have been suggested. In order to account for the complex surface reactions, more studies are needed. 

As a result of numerous studies a basic understanding of the mechanism of electron impact induced desorption was achieved in the past decade In contrast, few papers have been published on the electron impact induced chemical evaporation. Besides the above-mentioned etching of Si and some other materials by XeFj only the C/H2-, TiC/Hz-, TiB2/H2- and the Si/Hj-systems have been investigated. In the case of silicon etching by hydrogen the effect of the electron impact cannot be considered as a purely catalytic one since the reaction product, silane, is thermodynamically unstable with respect to solid silicon and Hj. Nevertheless, it provides some insight into the mechanism of such processes and, therefore, we shall discuss it briefly. 

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