Etching and polymerization

In order to achieve selective oxide and nitride etching, additives to F-source plasmas are chosen to make a F-deficient chemical environment. These include H2, C2H4 and CH4 which are quite efficient F scavengers. The amount of additive necessary remains more an art than a science because oxide and nitride selectivity requires operation in an environment very close to the demarcation between etching and polymerization shown in Figure 10. In fact in some cases (57,59) polymer deposition on Si occurs... 

A brief description of the key plasma diagnostic techniques, which have been especially useful in delineating the gas phase processes in fluorocarbon plasmas, will be given followed by an extensive discussion of plasma etching and polymerization mechanisms. 

Experimental Methods Used in the Study of Etching and Polymerizing Plasmas... 

Fig. 2.4. Etching and polymerization dependence as a function of pressure in a CF4 plasma. Pressure varied by the different techni iues involving flow rate and residence time...

Fig. 3.10. Schematic representation of the boundary region between etching and polymerization as influenced by the fluorine-to-carbon ratio of the etching species and the bias applied to the surface of interest (see text for discussion)...

We have already inferred several methods by which a transition from etching to polymerization might be accomplished. These involved the effective lowering of the F/C ratio in the system by, for example, the addition of Hj or C2F to a CF, plasma or even the effective removal of fluorine by conversion to stable volatile fluorides in the etching process. The transition in the reverse direction (from polymerization to etching) may also be achieved by, for example, for example, the addition of Oj to a CjF plasma to effectively remove carbon as CO. In much of the work to be discussed below, CjFg is employed in the system whose F/C stoichiometry allows convenient rates of both etching and polymerization to be maintained. 

The F/C ratio model accounts for the fact that for carbon-containing gases etching and polymerization occur simultaneously. The process that dominates depends upon etch gas stoichiometry, reactive-gas additions, amount of material to be etched, and electrode potential and upon how these factors affect the F/C ratio. For instance, as described in Figure 8, the F/ C ratio of the etchant gas determines whether etching or polymerization is favored. If the primary etchant species for silicon (F atoms) is consumed either by a loading effect or by reaction with hydrogen to form HF, the F/... 

The Incorporation of metals Into polymer films produced by plasma techniques Is an attractive prospect since It can be envisaged that careful choice of the metal and organic phases, and close control of the overall composition of the product would greatly extend the scope of these plasma polymerized materials In, for example, electrical, magnetic and optical applications. In a previous paper (1) we have outlined a convenient method for the preparation of such materials derived from fluorinated monomers by simultaneous chemical plasma etching and polymerization in the same system. 

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