present in the plasma environment are a result of the response of the initial surface to the incident species profile. It is likely that certain specific surfaces may be produced only in the plasma environment. Because of this characteristic it is not possible to draw on databases that were developed for other technological fields, as is possible in the area of electron impact processes, for example. If information is available, e.g. thermochemical data for solids in the 1985 JANAF thermochemical tables,1 its use may be questionable, at least for direct ion-surface interactions, because of departures from thermodynamic equilibrium due to ion bombardment.

An important review of the fluorine-silicon system and to a lesser extent of other systems has been published by Winters and Coburn.2 They present their survey as a summary of "the status of this virtually unexplored field of surface chemistry." The fluorine-silicon model system can serve as a prototype of the complexities encountered in plasma-surface chemistry.

Not all information available on the important key applications has been reviewed in a similar fashion. Surface processes related to poly-Si etching have at least been studied by several investigators using C12 and HBr gases.3 For SiO2 etching in high-density fluorocarbon plasmas many studies exist, but little surface chemistry work has been performed.4 The surface processes related to silicon dioxide PECVD have been studied by attenuated-total reflectance IR.5

Techniques for Improving the Database

Approach

A two-fold experimental approach appears most promising in improving the database in this field. First, detailed measurements on actual plasma processing systems need to be made. Second, controlled investigations of the different surface science aspects of the plasma etching or deposition process need to be made in an ultrahigh-vacuum (UHV) apparatus using well-controlled and well-characterized beams of different plasma species at the relevant energies. These can interact with the substrate one at a time, two at a time, and so on. The measurements performed under each approach are listed in Table 4.1.

TABLE 4.1 Goals of a Two-Fold Experimental Approach Including Measurements in Actual Plasma Processing Systems and in Ultrahigh-Vacuum (UHV) Reactive Beams

Plasma Studies

UHV Beam Studies

Measurement of incident species flux decomposition

Production of well-characterized and "clean" beams of

 

 

 

  • Realistic ions and

Measurement of loss rates of species

 

 

  • Realistic neutral radicals

Characterization of surfaces in situ

 

 

  • With realistic energies

Characterization of products

Measurement of interactions with pristine and realistic surfaces

Characterization of effects associated with three-dimensional structures

Determination of

 

 

 

  • Etching or deposition rates,

Verification of technological figures of merit

 

 

  • Reaction probabilities,

 

 

 

  • Products and their energy content, and

 

 

 

  • Composition of the surface reaction layer

Measurements On Realistic Plasma Reactors

Incident Flux and Desorbing Flux Analysis

Regarding the incident flux, we would like to know the identities and energies of species incident on the surface; how these species interact with the surface and with each other; the importance of angular effects; scaling with process parameters; and reaction probabilities.



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