Kinetics thermal etching

C2.18.3.1 EXPERIMENTAL STUDIES OF THERMAL ETCHING REACTION KINETICS AND DYNAMICS... 

Leroux and Raub (10) performed a detailed study of the etching of silver and silver alloys in oxygen and hydrogen at elevated temperatures. They were the first to propose that thermal etching is kinetically controlled. Specifically, they suggested that faceting results from differences in the rate of evaporation from different planes and that evaporation rates are influenced by adsorbed species. 

There was also a great deal of modeling of transport processes in support of the kinetic model of thermal etching. The basis of these models is that differences in chemical potential lead to mass transport via a number of mechanisms. It is important to note that these models treat the surface as a continuum and do not involve atomic-level mechanisms. 

Srinivasan etal.,64 in a phenomenological development, split the etch rate into thermal and photochemical components and used zeroth-order kinetics to calculate the thermal contribution to the etch rate. An averaged time-independent temperature that is proportional to the incident fluence was used to determine the kinetic rate constant. The photochemical component of the etch rate was modeled using, as previously discussed, a Beer s law relationship. The etch depth per pulse is expressed, according to this model, in the form... 

As described above, the limiting flatness of the surface is related to the smoothness of the Si/Si02 interface before etching and the kinetics of step recession. Thermal oxidation, as opposed to chemical treatment (e.g., RCA clean), results in a smoother interface region so that subsequent treatment of Si(lll) in buffered HF... 

For example, an RF field of lOV/cm will cause an ion displacement of only 0.002 mm while the mean free path, due to thermal motion, is about 0.2 mm. The only real effects of radio frequency electrical fields are in the dark spaces over each electrode and they are symmetrical for both electrodes if those electrodes have the same area. Therefore, in the absence of a bias voltage on the parts, the ions have very little kinetic energy and there is almost no ion-etching of the type used in some semiconductor equipment... 

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