Page 58 - An Introduction to Microelectromechanical Systems Engineering

Basic Process Tools 37

provide better-quality films and slightly higher deposition rates (5–100 nm/min),
but the deposition system is more complex, requiring water cooling of the target and
shielding from x-rays generated when the energetic electrons strike the target. Fur-
thermore, radiation that penetrates the surface of the silicon substrate during the
deposition process can damage the crystal and degrade the characteristics of
electronic circuits.
Evaporation is a directional deposition process from a relatively small source.
This results in the majority of material particles being deposited at a specific angle
to the substrate, causing poor step coverage and leaving corners and sidewalls
exposed. This is generally an undesirable effect if thin film continuity is desired (e.g.,
when the metal is an electrical interconnect). Rotating the substrate to face the
source at different angles during deposition reduces the effect. In some cases, how-
ever, shadowing can be used deliberately to selectively deposit material on one side
of a step or a trench but not the other.
Thin films deposited by evaporation tend to exhibit tensile stress, increasing
with higher material melting point. Evaporated niobium and platinum films, for
example, can have tensile stress in excess of 1 GPa, sufficient to cause curling of the
wafer or even peeling. As with sputtering, an adhesion layer must be used with
many metals.

Chemical-Vapor Deposition
Chemical-vapor deposition (CVD) works on the principle of initiating a surface
chemical reaction in a controlled atmosphere, resulting in the deposition of a
reacted species on a heated substrate. In contrast to sputtering, CVD is a high-
temperature process, usually performed above 300ºC. The field of CVD has grown
substantially, driven by the demand within the semiconductor industry for high-
quality, thin dielectric and metal films for multilayer electrical interconnects. Com-
mon thin films deposited by CVD include polysilicon, silicon oxides and nitrides,
tungsten, titanium and tantalum as well as their nitrides, and, most recently, copper
and low-permittivity dielectric insulators (ε < 3). The latter two are becoming
r
workhorse materials for very-high-speed electrical interconnects in integrated cir-
cuits. The deposition of polysilicon, silicon oxides, and nitrides is routine within the
MEMS industry.
Chemical vapor deposition processes are categorized as atmospheric-pressure
(referred to as APCVD), or low-pressure (LPCVD), or plasma-enhanced (PECVD), 2
which also encompasses high-density plasma (HDP-CVD). APCVD and LPCVD
methods operate at rather elevated temperatures (400º–800ºC). In PECVD and
HDP-CVD, the substrate temperature is typically near 300ºC, though the plasma
deposition of silicon nitrides at room temperature is feasible. The effect of deposi-
tion parameters on the characteristics of the thin film is significant, especially for
silicon oxides and nitrides. Substrate temperature, gas flows, presence of dopants,

2. Energetic electrons excited in a high-frequency electromagnetic field collide with gas molecules to form ions
and reactive neutral species. The mixture of electrons, ions, and neutrals is called plasma and constitutes a
phase of matter distinct from solids, liquids, or gases. Plasma-phase operation increases the density of ions
and neutral species that can participate in a chemical reaction, be it deposition or etching, and thus can
accelerate the reaction rate.