Due to the fast development of microelectronics, quite an important number
of the chemical aspects of component formation has been occulted. In partic
ular just a few studies are related to thin film formation in the scope of
chemical reactions and modelisation. Thermodynamics itself has been impleme
nted rather ponctually and in an incomplete way. One of the main reasons fo
r this is that thin films are considered as standingly non equilibrium syst
ems and that, thermodynamics, as a macroscopic science, has mainly been con
sidered as inadequate. Thus, we have decided to reconsider the problem of t
hin films formation from a strictly chemical point of view. Using a simple
system (formation of silicon nitride on monocrystalline silicon), the chemi
cal reactions have been investigated. Classical thermodynamics, though inac
curate and approximate for such systems is necessary to develop the first s
teps of a thermodynamical model describing the deposit formation. Using a c
omputer program helping a global treatment of several chemical reactions co
nfirms the conclusions obtained by classical thermodynamics. We have discus
sed the insufficiency of the classical model, and shown that, with respect
to what was experimentally observed, it was nevertheless globally valid, an
d that the nitride formation should be regarded as a superposition of class
ical chemical reactions that occurred in "close to the equilibrium" states
and globally following the classical laws of thermodynamics. Thus, we propo
se a model of formation for the first mono layer of Si3N4 on monocrystallin
e silicon which is in total agreement to experimental observations.