Effect of alternating Ar and SF₆/C₄F₈ gas flow in Si nano-structure plasma etching

Abstract

Si is very reactive to normal plasma etchants such as fluorine (F) based chemicals and the reactions are inherently isotropic. To fabricate small and/or high aspect ratio nanoscale structures in Si, an anisotropic etching process is necessary. SF₆ combined with C₄F₈ has been demonstrated as a good gas combination for anisotropic Si etching. In this study, Ar gas was introduced into the etching chamber to improve Si etching rate. In addition to mixing Ar with F etching gases directly, an alternating Ar and F gas flow process is proposed. It is interesting to see that not only Si etching rate but also etching selectivity are improved by alternating Ar bombardment and SF₆/C₄F₈ etching steps. The Si etching rate is determined by the Ar treatment step in this new alternating Ar and F two step process.

Introduction

Plasma is a gas that contains equal numbers of positive and negative charges, as well as neutral atoms, radicals, and molecules. The reactive and high energy species in plasma etch away materials such as resist, dielectrics, or metals by physical and chemical interactions. For normal etching processes, the plasma species adsorb on the materials surface, there is a chemical reaction and/or physical bombardment, and then the final products desorb from the surface. Plasma processes have been widely used to etch substrates in many industries. The advantage of plasma etching is its accurate profile control capability. Small and high aspect ratio structures have been successfully demonstrated by plasma etching technology. The applications include semiconductor fabrication, micro-electromechanical systems (MEMS), magnetic storage, flat panel displays, opto-electronic devices, polymer surface modification and so on [1], [2], [3], [4], [5].

Silicon has been widely used in semiconductor device fabrication, photovoltaic manufacturing, microelectronics and MEMS industries. Si can be etched by both wet chemicals and dry plasma. However, Si is very reactive to normal plasma etchants such as F based chemicals and the reactions are inherently isotropic. This isotropic profile prevents the miniaturization of Si structures down to nanometer scale and the integration of Si based components to achieve better performance. To fabricate a small and/or high aspect ratio structures in Si, side-wall protection is necessary [6], [7]. Several gas combinations, such as SF₆/O₂ [6] and SF₆/C₄F₈ [7] have been used to protect Si sidewall during the trench etching. In addition to add the passivation layer at room temperature, cryogenic wafer cooling is also used to condense the polymer protection layer on the sidewall [8]. Since a side wall protection is required in F based Si etching, Cl and Br gases were used for etching Si directly [4]. Recently, nano-scale Si structures have been etched successfully by BCl₃/Cl₂/Ar/O₂ [9], HBr [10] and the other similar gas combinations. Other than reactive etching gases, neutral beam, such as Ar neutral species in plasma, also has been used to etch nano-Si structures recently [11]. The Si etching profile and etching rate have also been studied in detail [12].

In this study, C₄F₈ was mixed with SF₆ etching gas as a side wall protection gas to control the Si etching profile. Ar was introduced as the ion source to improve the etching rate. It has been demonstrated that the etching rate is a synergism between the fluxes of ions and neutrals. The total etch rate is greater with both ions and neutrals than with either alone [12]. In the past, the ions were added with neutrals in the same process step. Here, we separate the ions and the neutrals in different steps. An alternating Ar physical bombardment and F chemical etching process was proposed and the impact of the separated Ar ions on the etching rate was studied. It was found that by alternating Ar physical bombardment and F chemical etching steps, not only Si etching rate but also etching selectivity were improved.