SF ₆ Plasma Etching of Silicon: Evidence of Sequential Multilayer Fluorine Adsorption

Abstract

It is shown, from the diffusion model for fluorine-based plasma etching of silicon, how the anisotropy evolution, as a function of the partial pressure of atomic fluorine, may account for adsorption and desorption mechanisms on silicon surfaces. In addition to the anisotropy-isotropy transition, sequential multilayer adsorption is demonstrated experimentally. Etching mechanisms are discussed in the light of the observed results.

Surface diffusion model accounting for the temperature dependence of tungsten etching characteristics in a SF6 magnetoplasma

“…͑i͒ the ion bombardment induced etching is not a temperature activated process and is, thus, substrate temperature independent; ͑ii͒ the lateral etching ͑beneath the mask͒ is a purely spontaneous chemical process and its kinetics being temperature activated slows down with decreasing T s ; ͑iii͒ the vertical etch rate of Si in SF 6 plasmas is proportional to the sole concentration of fluorine atoms in the gas phase, [37][38][39][40] i.e., independent of the other parameters of the plasma-surface interaction, namely ion bombardment energy, 41 current density, 42 and substrate temperature, 35 provided the reaction products (SiF 4 on Si 2 F 6 ) are still volatile at this temperature.…”

Parametric study of the etching of SiO2 in SF6 plasmas: Modeling of the etching kinetics and validation

“…However, this Si-like overlayer on SiO 2 where silicon is linked to oxygen ͑Si-O back bonds͒ is different from a Si layer on bulk silicon ͑Si-Si back bonds͒; ͑d͒ there is no surface diffusion of oxygen adatoms: the activation barrier for oxygen diffusion is much greater than thermal energy at room temperature; ͑e͒ as a consequence of ͑d͒, there are no adsorption sites on surfaces free of ion bombardment ͑lateral walls͒; ͑f͒ as a consequence of ͑d͒ and ͑e͒, at room temperature, empty adsorption sites are available for fluorine adsorption only on surfaces submitted to ion bombardment; ͑g͒ as a consequence of ͑d͒ to ͑f͒, lateral etching of SiO 2 cannot occur at or below room temperature; ͑h͒ with fluorine, the etching mechanisms of the fraction of the Si-like monolayer on SiO 2 are similar to those of bulk Si; ͑i͒ as a consequence of ͑h͒, the etch rate of the Si overlayer can include two components, i.e., the ion bombardment induced etching and spontaneous etching; ͑j͒ as for bulk Si, the spontaneous etching of the Si overlayer by fluorine can only occur above a fluorine threshold coverage t ( F Ͼ t ): 23,24,26-28 ͑k͒ the value of the threshold coverage is t ϭ3/4 23,24,[26][27][28] if the saturation density 0 of adsorption sites for fluorine corresponds to a complete SiF 2 monolayer ͑or t ϭ1/2 if 0 corresponds to a complete SiF 3 monolayer͒. However, this Si-like overlayer on SiO 2 where silicon is linked to oxygen ͑Si-O back bonds͒ is different from a Si layer on bulk silicon ͑Si-Si back bonds͒; ͑d͒ there is no surface diffusion of oxygen adatoms: the activation barrier for oxygen diffusion is much greater than thermal energy at room temperature; ͑e͒ as a consequence of ͑d͒, there are no adsorption sites on surfaces free of ion bombardment ͑lateral walls͒; ͑f͒ as a consequence of ͑d͒ and ͑e͒, at room temperature, empty adsorption sites are available for fluorine adsorption only on surfaces submitted to ion bombardment; ͑g͒ as a consequence of ͑d͒ to ͑f͒, lateral etching of SiO 2 cannot occur at or below room temperature; ͑h͒ with fluorine, the etching mechanisms of the fraction of the Si-like monolayer on SiO 2 are similar to those of bulk Si; ͑i͒ as a consequence of ͑h͒, the etch rate of the Si overlayer can include two components, i.e., the ion bombardment induced etching and spontaneous etching; ͑j͒ as for bulk Si, the spontaneous etching of the Si overlayer by fluorine can only occur above a fluorine threshold coverage t ( F Ͼ t ): 23,24,26-28 ͑k͒ the value of the threshold coverage is t ϭ3/4 23,24,[26][27][28] if the saturation density 0 of adsorption sites for fluorine corresponds to a complete SiF 2 monolayer ͑or t ϭ1/2 if 0 corresponds to a complete SiF 3 monolayer͒.…”

“…͑14͒ and ͑15͒ is the addition of a spontaneous etching component 23,24,[26][27][28] to the ion induced component, when Ͼ t . ͑14͒ and ͑15͒ is the addition of a spontaneous etching component 23,24,[26][27][28] to the ion induced component, when Ͼ t .…”

“…The second term of the right hand side of this equation corresponds to the spontaneous etching component, 23,24,[26][27][28] where is the adsorption lifetime of fluorine adatoms before their spontaneous desorption as part of the reaction product SiF 4 . 23,29,30 If R is the activation energy for the associative desorption of SiF 4 , the adsorption lifetime of fluorine adatoms, which is proportional to the inverse of their reaction probability, can be simply expressed as 23,29,30 …”

Distributed electron cyclotron resonance in silicon processing: Epitaxy and etching

“…The steplike variation of anisotropy (Fig.6, top) is simply the result of the sequential ordering within each successive sublayer. 25 Perfect anisotropy is obtained in plasmas by lowering the fluorine coverage below the threshold value 0,. This can be done either by lowering the atomic fluorine concentration in the plasma or by increasing the desorption rate induced by ion bombardment.…”