Effect of alternating Ar and SF6/C4F8 gas flow in Si nano-structure plasma etching
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.
Section snippets
Experimental
SF6 combined with C4F8 is one of the demonstrated Si etching gas combination, in which C4F8 was used to protect Si sidewall while SF6 was used to etch Si [13]. During etching, the sample is, in general, negatively biased with respect to the plasma so that positive ions directly strike the bottom surface only by means of an external power supply. With direct ion bombardment on the bottom Si surface, SF6 removes the bottom polymer coating and chemically reacts with Si to generate a vertical
Results and discussion
A good chemical combination is important to achieve anisotropic etching. In this paper, C4F8 was introduced to protect the Si sidewall while SF6 was used to etch Si. C4F8 plasma free radicals can form a polymer which creates a protective barrier that blocks chemical attack on the side of the features. Two mechanisms have been proposed to explain this side wall protection. The first mechanism is that the coating plasma species induces the growth of a protective film, whose composition and
Conclusions
In conclusion, smooth and straight Si sidewall etching profile has been achieved by using SF6, C4F8 and Ar plasmas. Ar plays an important role for etching profile, etching rate and selectivity. By alternating Ar bombardment and SF6/C4F8 etching steps, the concentration of F chemical etchant in each etching step can stay constant while the efficiency of the Ar in physical bombardment step can be improved. The impact of the Ar bombardment step on Si etching rate is significant and can increase
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