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Source: SEMICONDUCTOR MANUFACTURING HANDBOOK

CHAPTER 12

PLASMA ETCHING

Shouliang Lai
Unaxis USA, Inc.
St. Petersburg, Florida

12.1 INTRODUCTION

Plasma is an assembly of ions, electrons, and neutrals in which the motion of particles is dominated
1
by electromagnetic interaction. Plasma as a whole is electrically neutral. Since the 1960s plasma has
been employed in etching processes to replace wet chemical etching in semiconductor manufactur-
ing. 2,3 The driving force is that modern semiconductor manufacturing depends on mass replication
of tightly controlled micron-sized features, and plasma etching is the only technology capable of
doing the job efficiently and effectively. Advanced plasma etching technology is capable of faithfully
transferring submicron patterns, satisfactorily controlling process uniformity, readily adapting to
wide varieties of materials, and effectively reducing chemical wastes and pollution to the environ-
ment. In the last decades, plasma etching was stimulated by and greatly contributed to the progress
of semiconductor technology.

12.1.1 Plasma Fundamentals
Plasma can be described by various parameters including plasma density, temperature, pressure, fre-
quency, sheath thickness, Debye length, skin depth, and so forth. A number of excellent books dis-
cuss plasma physics at great length and in rigorous detail. 4–6 Also, some books cover specific plasma
topics related to plasma processing. 7–9 Here, only a simplified picture is provided to illustrate the
aspects of plasma relevant to etching processes.
Plasma can be started by supplying external dc or radio frequency (RF) power to pairing elec-
trodes in an evacuated chamber. Gas ionization is initiated and amplified by electron/molecule col-
lisions. With the ongoing input of RF energy and gases into the chamber, the electron/molecule
collisions continue to generate ions, electrons, and neutrals. Exposed surfaces within the chamber
absorb and neutralize these species. After a transient period, a steady state is reached so that the gen-
eration and loss processes are balanced. At this point, the discharge is characterized by a bulk glow
region and dark sheath regions, as illustrated in Fig. 12.1a.
The bulk region is semineutral, with nearly equal number of negative and positive particles. The
sheath region is depleted with charged particles. Particles drift to the boundary of the bulk region in
a random thermal motion and sweep across the sheath region. Due to the difference in mobility, more
electrons than ions move toward the chamber wall initially until a steady state is reached, at which
equal fluxes of ions and electrons are swept across the plasma sheath. As such, the chamber wall
has a negative potential V relative to the floating potential of the plasma V . Hence, there is an electri-
f p
cal field across the sheath, as illustrated in Fig. 12.1b. This electrical field exerts a dc bias to accelerate

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