Biomedical Engineering Reference
In-Depth Information
operates at a frequency of 13.56 MHz, which is above the ion plasma frequency but below the
electron plasma frequency. Therefore, electrons respond to the varying electric fi eld, but ions expe-
rience only the average fi eld. The pressure during discharge is between 10
-3
and 100 Torr [1,2]. The
plasma density in RF glow discharge in low pressure (10
-3
to 1 Torr) varies from 10
9
to 10
11
/cm
3
,
whereas that in medium pressure (1-100 Torr) can reach 10
12
/cm
3
[3]. The low-pressure discharges
have charge exchange mean free paths typically longer than the sheath dimensions at the substrate.
Besides, the electron temperature of low-pressure discharge is usually in the range of 1-5 eV, and
the electron energy distribution functions are close to Maxwellian. In addition, the ion temperature
(0.1-0.5 eV) is somewhat higher than the gas temperature (0.03-0.1 eV) but much less than the
electron temperature. The plasma potential of the RF discharge is typically in the range of 10-30 V
above the wall potential and is proportional to the electron temperature [4].
19.1.1.2 Glow Discharge
A glow discharge is triggered by two common ways: thermionic fi lament glow discharge and pulsed
high-voltage glow discharge. A simple thermionic discharge based on the electron emission from a
hot cathode produces the desired plasma density by ionization of the background gas. Plasma gen-
eration requires a suitable selection of the cathode that is directly heated by a fl oating power supply
that passes suffi cient current through the fi lament to heat the cathode resistively to the emissive
temperature. Usually, refractory metals are selected as the fi lament cathode as they are simple to
fabricate and have suffi ciently low evaporation rates and sputtering yields to provide a reasonable
lifetime [5]. Thermionic discharges typically utilize electron currents that range from 1 A to several
kilamperes depending on the types of gases used, plasma density, and plasma size. In addition,
the voltage applied between the cathode and anode can be lower than the ionization energy and is
commonly in the range of 25-100 V where the maximum of the ionization cross-section occurs for
most gases.
In pulsed high-voltage glow discharge, the pulsed voltage serves the dual purpose of generat-
ing the plasma and accelerating the ions in the sheath to the substrate. The method is very versatile
since it can be used practically for any electrode geometry and gases. The pulsed high voltage pro-
duces a sudden electric fi eld between the substrate (cathode) and the surrounding system (anode).
Free electrons that are emitted from the substrate are accelerated and collide with the gas molecules
or the anode. Ions produced during the collisions continually strike the cathode to generate second-
ary electrons, which are accelerated back into the discharge volume. This discharge mechanism is
described by the Paschen curves in gas discharge physics restricted by the minimum pressure of the
process gas for a given geometry, voltage amplitude, and nature of the gas [6].
19.1.1.3
Cathodic Arc Discharge
In general, a cathodic arc plasma source is composed of two parts: plasma production unit and mac-
roparticle fi lter. An arc discharge is characterized by a relatively high current (tens or hundreds of
amperes) and a relatively low voltage (10-80 V) between the cathode and the anode. The vacuum
arc is sustained by materials originating from the cathode. Before the arc is established, the atomic
(molecular) density is not high enough to sustain the electrical discharge. When a feedback mecha-
nism is established in which a very small area of the cathode surface is heated by electron emission,
more particles are ejected as illustrated in Figure 19.2. The cathodic discharge is characterized by
an ensemble of luminous cathode spots that move in a rapid and chaotic manner across the surface
[7,8]. The plasma expands in all directions from the cathode spots toward the anode and vacuum
chamber walls.
The cathode spot is small (10
-
8
to 10
-
4
m in diameter), but an intense plasma with a current
density of 10
6
-10
12
/A
2
can be produced. The spot velocity is determined by factors that include the
nature of the cathode, residual vacuum, and presence of the external magnetic fi eld. The cathode
erosion rate depends upon the state of the surface, and cathode spot characteristics change as the
