Heat transfer in a microelectronics plasma reactor

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Plasma etching has became an indispensible technique for microelectronics device manufacturing. However, its application is often limited by the wafer temperature increase due to the ion bombardment, which may exceed the susceptor temperature by as much as 200 °C when the semiconductor wafer is loosely lying on the cooled susceptor. The aim of this study is to perform an accurate evaluation of heat transfer in an industrial etching reactor, and in particular to point out the critical role of the susceptor/substrate interface in this problem of damageable overheating. The required experimental precision, necessary to reach this goal, is based on in situ temperature monitoring via fluoroptometry, and on electrical characterization of the plasma to investigate the effective heating power dissipated on the substrate. It is thus shown that the only wafer heating source to be considered is the real electrical rf power dissipated in the plasma, and that the heated substrate evacuates the main part of this power to the cooled susceptor, via thermal conduction through the residual gas present in the interface between these two elements. A non-negligible part of energy can also be lost via thermal radiation. The thermal conduction through the interface is shown to be strongly dependent on the gas pressure, and the quasilinear rate obtained is evidence for the molecular regime of the gas molecules in this region. The type of gas also strongly affects the interface heat flow, and in a quite uncommon way: an effect of thermal conductance inversion between two different types of gases when the pressure decreases is observed. A criterion is proposed to evaluate a priori and in a qualitative point of view whether a given couple of gases may exhibit this effect, which consequences are very important from a technological point of view.