Vol. 62, No.
Printed in Great Britain.
Mechanisms of silicon etching in fluorine- and
Daniel. L. Flamm
Department of Electrical Engineering, University of California, Berkeley, CA
Silicon can be etched in fluorine- and chlorine- containing plasmas in many ways.
This article discusses some of the basic chemical and physical phenomena which play a role and
more complicated interactions and side effects found in commercial process equipment.
Circuit patterns are transferred to silicon by exposing surfaces of this material to the species formed in fluorine and
chlorine-containing plasmas. The physics and chemistry associated with these processes have been studied for many
years, and in broad form they are understood.
The elementary interactions of free halogen and halogen-containing species with silicon are discussed below, and in turn
they are connected to the complex phenomena and chemical feeds which have been developed for use in semiconductor
Phenomenological mechanisms of etching
In general, we may divide etching mechanisms into the four basic phenomenological categories shown in Figure 1,
sputtering, chemical etching, ion-enhanced energetic mechanisms and ion-enhanced inhibitor processes (refs.
The four phenomenological etching mechanisms: I. sputtering, 11. chemical etching,
111. ion- enhanced energetic etching
ion-enhanced inhibitor etching.
Briefly, sputter mechanisms include those in which material is mechanically ejected from a surface by the energy and
momentum transferred in energetic ion bombardment. Virtually any material can be sputtered if the ion energy is high
and the pressure is
enough for ejected matter to be thrown across the reactor with few collisions. Consequently,
pressures on the order of a millitorr or below are required for efficient sputter etching, since the apparatus dimensions
are typically some centimeters, and the mean free path of low energy neutral species is about 5/pcm, (p is the pressure
in millitorr). Sputtering is unselective and generally slow.
The remaining three mechanisms, described in the following, are fundamentally different from sputter removal in at
least two ways: a)
reactions are central to the the etching process and b) the substrate is converted into
gaseous products. Hence, unlike sputter etching, the other mechanism do not depend on a long mean free
path, nor is the amount of material removed sharply limited by the ion current to the surface. A third characteristic of
sputtering is the geometrical facets which are produced. While a discussion is beyond the scope of this article, briefly,
sputter removal is at a maximum when the surface is inclined about 40" to
to ion trajectories; thus sputter-etched
planes tend to form along this angle to the surface.