Plasma etching selectivity ratio

Etch selectivity between two materials is defined as the ratio between their etching rates at identical plasma conditions. High selectivity is usually referred in etching and related to a high etching-rate ratio between chemically different materials or between the etched and the underlying layer. 

High etch rates and selectivity can be achieved by judicious selection of feed gases to a plasma reactor. The atomic and radical species formed by electron impact dissociation depend largely on feed gas composition, and the intrinsic etch rates measured in the absence of a plasma (i.e., downstream etching) provide a useful indicator of chemical selectivity in the presence of a plasma. For example, the ratio of (100) silicon (34) to thermal oxide (Si02) (37) etching by F atoms is 41 1 at room temperature. As etch rates generally follow an Arrhenius type dependence on substrate temperature. 

All three processes can occur simultaneously when a surface is exposed to a plasma. The relative importance of one mechanism over the others depends on the material system (surface and gas), the ratio of neutral to ion flux, and the ion energy. In general, conditions are selected such that ion assisted chemistry dominates. In cases that chemical etching dominates, a mechanism for wall passivation (Section... 

Selectivity (S) During the etching process, the mask layer and the substrate are both immersed in the etching media which can be a wet chemical etching bath or plasma. Although the etchant mainly reacts with the substrate, the mask layer is also attacked. The relationship between the etching of the substrate and the mask layer is the selectivity S which is given as the ratio of the etch rate of the... 

RIE equipment is usually parallel plate reactors in which both plasma and DC bias voltage are created using one capacitively coupled plasma (CCP) source [5]. The wafers are placed on top of the electrode which is typically in direct contact with plasma glow (Fig. 3). This ensures a reasonable etch rate but the controllability of etching is difficult. A major downside of this type of reactor is that the wafers are exposed to heavy ion bombardment because plasma generation and bias voltage are coupled. The etch rate of silicon is typically 0.1 - 1 (xm/min and selectivity against the photoresist mask is usually below 5 1. Anisotropy is based on the directionality of ion bombardment and aspect ratios are usually limited to 2 1. 

Subsequently, the opened resist mask obtained can be used to etch the underlying substrate. In the case of sihcon or silica, plasmas with fluorine (mainly CF4 and SFg gases), bromine (H Br) or chlorine (CI2, HCl) can be used. Usable aspect ratios are in general larger than 1, that is, the feature height exceeds the line width. To achieve this, imprinted resists have to exhibit the correct selectivity to the underlying material this means that the etch rate of the resist in the plasma has to be sufficiently small compared with the etch rate of the substrate. 

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