Page 94 - Sami Franssila Introduction to Microfabrication

7

Thin-film Growth and Structure

In this chapter, we deal with deposition processes electrochemical and chemical potential differences are
and the resulting film structures. Interface stability and the main driving forces.
sharpness, grain size, texture, stress and other film Transport of material from the source to the wafer
properties are dependent on film deposition processes, can be directional or diffuse. With directional deposition
but they depend on preceding and subsequent process reactor geometry, the wafer position and the structures
steps too. Structures already made on the wafer set on the wafer determine the flux that can be easily mod-
various limitations on the processing conditions. Now, elled. Evaporation and molecular beam epitaxy (MBE)
we will also consider deposition on non-planar surfaces, are examples of directional, line-of-sight deposition sys-
which introduces new considerations. tems. With diffuse transport, the arrival of the deposit-
ing specie is usually difficult to model, as in mass-
transport limited regime of chemical vapour deposition
7.1 GENERAL FEATURES OF THIN-FILM (CVD).
PROCESSES Film deposition on the substrate surface is a sum of
many factors. In the first approximation, the deposition
The general features of thin-film deposition pro- is independent of the substrate (this distinguishes the
cesses are visualized in Figure 7.1. Thin-film deposi- deposition from growth processes such as thermal oxi-
tion involves thermal physics, fluid dynamics, plasma
physics, gas-phase chemistry, surface chemistry, solid- dation and epitaxy, which are intimately coupled with
state physics and materials science. We must deal with the substrate). But the surfaces do interact with the depo-
source materials (sputtering targets, precursor chemi- sition processes via available chemical bonds, contam-
cals, electrolyte compositions), we must address the ination and crystallography. An important parameter is
the sticking coefficient, or the probability that an imping-
transport of source material to the substrate (in high
vacuum, low vacuum, atmospheric pressure or liquid), ing particle will remain on the surface. A high-sticking
coefficient means that the particle will come to rest at
and we have to understand surface processes (adsorp-
the point of impingement, and a low-sticking coefficient
tion, reaction, desorption, ion-bombardment induced
effects). Characterization of films entails dozens of means that only the energetically favourable attached
specie will stick, and the others will desorb. Sticking
techniques ranging from optical to nuclear, electri-
cal to mechanical. This multidisciplinarity leads to a coefficients range from 0.001 to 1, and they are gener-
great number of phenomena and models that must be ally lower for CVD processes than for physical vapour
taken into account, both in experimental work and in deposition (PVD).
simulation. Even if no annealing is done immediately after film
There are a few basic methods of source excita- deposition, the films will experience thermal treatments
tion and their different configurations. Thermal acti- during subsequent processing. Thermal loads from these
vation can be either resistive, photothermal or elec- treatments can be considerable, and they affect many
tron beam–induced, and laser or ion beams can be film properties, such as grain size, resistivity and
used. Plasma sources range from simple DC-diodes stress. Film surfaces and interfaces will be modified
to microwave, helical and inductive configurations. In during these anneal steps by diffusion, dissolution or
the liquid phase, the choices are less numerous, and chemical reactions.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)