Biomedical Engineering Reference
In-Depth Information
polymerization, the transformation of low-molecular-weight molecules (monomers) into high-
molecular-weight molecules (polymers) occurs with the assistance of energetic plasma species such
as electrons, ions, and radicals. Polymer formation in plasma polymerization encompasses plasma
activation of monomers to form radicals, recombination of the formed radicals, and reactivation of
the recombined molecules. Plasma polymers do not comprise repeating monomer units but, instead,
complicated units containing cross-linked, fragmented, and rearranged units from the monomers as
well as a number of functional terminals.
Dual plasma deposition is a novel technology derived from PIII. In this process, gas and metal
plasmas are simultaneously generated typically using a RF glow discharge source and a vacuum
arc plasma source, respectively, or by direct introduction of the gas into the vicinity of the dis-
charge area of the vacuum arc plasma source. Dual plasma deposition has many advantages as a
thin-fi lm technique. The obvious advantage is that a fi lm composed of several elements (gaseous
and metallic) with various compositions can be fabricated in the same instrument without breaking
the vacuum. Several functional thin fi lms with desirable properties can be synthesized by proper
control of the plasma parameters and deposition modes. For instance, the technique can be used to
fabricate optoelectronic materials such as ZnO, autocatalytic and biocompatible materials such as
TiO
2
, high dielectric constant materials such as ZrO
2
and TaO, apochromatic materials such as WO,
electronic materials such as AlN, as well as hard-coating materials including TiN and metal-doped
diamond-like carbon (DLC).
19.2.4 PIII
VERSUS
C
ONVENTIONAL
B
EAM
-L
INE
I
ON
I
MPLANTATION
Conventional ion implantation is a line-of-sight process. During ion implantation, atoms or mol-
ecules are ionized and accelerated by an electrostatic fi eld into a solid. In this way, a myriad of
combinations of ions and substrates are possible, such as nitrogen into iron, boron into silicon,
silicon into silicon, tellurium into gallium arsenide, and so on. The acceleration energy can be
between a few hundred electron volts and several million electron volts. The ion penetration depth
depends upon not only the energy, but also the mass of the ions and atomic mass of the solid [25].
Ions are extracted from the plasma by an extraction system, accelerated as a collimated beam to
high energy, and then used to bombard the samples. In the semiconductor industry, a mass fi lter is
added to obtain an ion beam consisting of a single ion species. The ion beam typically has a small
cross-sectional area, and so either beam or sample rastering must be performed to achieve uniform
implantation into a large sample. If the sample is nonplanar, sample rotation is required to implant
all the surfaces. This manipulation adds complexity and in many cases casts a limit on the size of
the workpieces that can be implanted in a cost-effective manner.
Since beam extraction optics, focusing optics, scanning, masking, and target manipulation are
absent in PIII, the instrumentation is substantially cheaper and simpler than a conventional beam-
line ion implanter. The smaller equipment footprint also bodes well for the space-conscious clean-
room environment in the semiconductor industry. In PIII, the specimen is placed directly in the
plasma and biased to high negative potential. Ions bombard normally to the entire surface of the
sample with good conformality as long as the dimensions of the plasma sheath remain small com-
pared with the sizes of the specimen features. The retained dose problem is also mitigated in PIII
because ion acceleration occurs mostly perpendicular to the surface. Therefore, PIII circumvents
the line-of-sight problem of conventional ion implantation and also alleviates the retained dose
problem [26,27].
19.2.5 A
PPLICATIONS
OF
PIII
From the industrial viewpoint, PIII is attractive because of the simple instrumentation, low cost,
and small footprint. The high throughput or effi ciency of PIII is another big advantage because
all surfaces are implanted simultaneously, and the implantation time is independent of the sample
