Nano-manufacturing Technologies for Biodevices: Interacting at a Molecular Scale - Advanced Design and Manufacturing Technologies for Biomedical Devices - page 244

13.4 Chemical Vapour Deposition Processes

A standard chemical vapour deposition process obtains a coating through the
deposition on a substrate of a chemical product generated from a gaseous reaction.
Other volatile by-products are usually produced in this reaction and are removed
from the reaction chamber. On other occasions the chemical reaction between the
gases introduced into the chamber and the substrate material is encouraged, so that
the product of reaction can be then condensed on that surface (Smith 1995 ; Albella
et al. 2003 ; Albella 2006 ).
By using these processes, thin layer deposits of micrometric thicknesses (and
even nanometric) can be obtained, including details of microcrystalline, polycrys-
talline, amorphous and epitaxial structures. A wide range of materials are usually
deposited in this way, such as silicon, fi bres, nanofi bres and carbon nanotubes, sili-
con dioxide, tungsten carbide and various oxides and nitrates with a high surface
hardness.
However, the consistency and adherence of the thin layer depends to a large
extent on the compatibility of the substrate and the surface deposit. To encourage
this compatibility, multilayer structures are often used in which the transition from
substrate to the required fi nal coating includes several intermediate layers for a bet-
ter transition with fewer residual stresses that can cause the appearance of cracks in
many thin coatings (Bunshah 1994 ; Glocker and Shah 2002 ).
Operating pressures for standard CVD equipment range from atmospheric
(APCVD) to high vacuum (UHVCVD) and on occasions aerosols and plasmas are
used, so as to favour the chemical reaction or to focus it on the substrate zone for a
more effective process. All this increases the complexity of the related systems
whose installation and maintenance costs are usually high.
Although the CVD and PVD systems mentioned are expensive, there are many
different suppliers who offer the chance to subcontract their thin-fi lm deposition
services for coating different materials, including metallic, ceramic and polymeric
substrates. This can be very convenient in projects linked to the development of a
novel implant or of a special material for medical applications, in case it might be
interesting to compare the mechanical, chemical and biological behaviour of pos-
sible surface coatings, before choosing the most suitable substrate-coating combi-
nation for the end application.
A good association (and related website) for locating different services and
information on technological supply and demand in the surface coating sector is
“The Society of Vacuum Coaters” ( www.svc.org ) website. There is also detailed
information on teaching resources, seminars and specifi c conferences and con-
gresses, where specialised information on these tools can be found. It has free
resources on vacuum generation techniques, surface and thin-fi lm characterisa-
tion and matters related to the preparation of substrate, deposition using differ-
ent technologies and the major surface coating and multilayer structure
applications.