Abstract:
Capacitively coupled plasma (CCP) has been widely used in plasma assisted
processing such as plasma etching, deposition and other surface treatments in microelectronics
industries. High plasma density and ion bombarding energy, which are
crucial in these applications, particularly in etching, cannot be controlled independently
in traditional single frequency (SF) driven CCP. Dual frequency (DF) CCP is already in
use in the fabrication industries for the new generation ultra-large-scale integrated
(ULSI) circuits. But it is reported that the quality of separate control of ion energy and
flux in DF discharge is limited due to the coupling of two frequencies and effect of
secondary electrons. It is not very far that more frequencies will be used simultaneously
in the CCP in order to achieve better control etch parameters. An analytical model of
sheath parameters for a multi-frequency driven collisional CCP has been developed
considering time dependent ion density and electric field within the sheath. Sheath
parameters for DF and triple frequency (TF) case have been determined and studied
against different operating parameters, e.g., chamber pressure, plasma density, RF
frequencies, phase differences etc. The determined values of the sheath parameters are
compared with some published DF and TF models.
In the analyses for DF and TF case different properties of sheath width and
potential have been observed. The oscillating behaviour of the parameters increases as
the number of RF source frequencies increases. More oscillation in the sheath
parameters increases the ion-neutral collisions within the sheath resulting in a reduced
ion bombarding energy at the electrode. The oscillation pattern of the parameters
depend on the ratio of RF frequencies. Compared with collisional time independent
models this model estimates lower values of the sheath parameters. Lower values of the
parameters is another reflection of the lower ion energy. The sheath parameters show
inverse relationship with both pressure and plasma density. The parameters also show
smaller values at higher pressure ranges when compared with a collisionless time
dependent model, as collisionality is not considered in that model. It has been found that
proper choice of phase difference may facilitate to have better control over the sheath
properties.
As this model considers the instantaneous ion motion and the instantaneous
electric field, so this model will estimate the sheath parameters of CCP more accurately
when multi-frequency source is used. This will facilitate the plasma users to have better
control on the etch properties.
