Spontaneous Reactions in Plasma Etching

Chemical or so-called “spontaneous” etching is the result of the interaction of reactive free radicals with the surface. Free radicals are electrically neutral species that have incomplete outer shells such as CF3 and F.

The mechanism of chemical etching consists of three elementary steps:
- adsorption of reactive species on the surface and dissociation if it is a molecule
- etch products formation (chemical reaction)
- etch products desorption

In a spontaneous etch process, these steps proceed without the need for activation by ion bombardment. Examples for spontaneous etch reactions are given in slide 1.

A classical example of a chemical reaction which is relevant for plasma etching is the reaction of XeF2 with silicon. Coburn and Winters studied this system and could provide experimental evidence that Ar bombardment enhances the etch rate in this system. This was the proof for the concept of ion enhanced chemical etching which is the foundation of plasma etching. The etch mechanism of XeF2 is actually quite complex and very dependent on the surface temperature as shown on slide 2.

Another way to generate free fluorine without inhibitors is to mix CF4 and O2 in the appropriate ratio (slide 3). The etch rate in the experiment by Mogab, Adams and Flamm peaked for an O2 percentage of 10 to 20 % in the gas feed.

Slide 4 describes etch rate effects for etching silicon in a SF6 plasma. The silicon etch rate is driven by radical concentration in the gas phase. A higher source power leads to deeper SF6 dissociation and more free fluorine. A higher SF6 flow reduces the concentration of reaction products in the gas phase. For very high source power / SF6 flow combinations, the etch rate saturates, indicating that surface processes are becoming rate limiting.

Slide 5 illustrates the importance of the chemical aspect of plasma etching on etch rate selectivities. The silicon to nitride selectivity for a SF6 plasma is shown as a function of Vdc. The lower Vdc, the more chemical the etch. The results show that nitride requires ion bombardment to be etched in a fluorine based plasma while silicon etches spontaneously. Therefore very high selectivities can be obtained by reducing the ion component of the etch. Decoupled ICP or ECR plasma sources and downstream reactors are suited for this application.

The most significant application of chemical etch processes in advanced logic device manufacturing is the so called resist trim process (slide 6). In-situ XPS studies show the presence of a perturbed layer on the sidewall of the resist. It’s thickness depends on the bias power, i.e. ion energy. The thickness of the layer also depends on the microloading conditions, i.e. is different for dense and isolated features. The trim rate slows down for thicker perturbed layers. Bias power, among others, can be used to adjust the dense/iso effect of the trim process.

More plasma etch fundamentals …