PROCESS DESCRIPTION
P
ROCESS
M
ANUAL
4.3-1
4.3 Silicondioxide (LTO SiO
2
) from SiH
4
and O
2
4.3.1
Basic configuration
LPCVD Low Temperature Oxide (LTO)
SiH
4
, O
2
, (PH
3
and/or TMB optional)
Injectors are used for SiH
4
+PH
3
+TMB, and for O
2
Flat temperature of 425
o
C
Quartz caged cassettes
4.3.2
Description
4.3.2.1 Purpose
Low Temperature Oxide (LTO) is mainly used as a passivation layer over devices, which
already have metal contacts. Aluminum is widely used but with melting point of roughly
600
o
C a passivation layer deposition technique with a deposition temperature higher than the
aluminum melting point is not possible.
LTO is used to create a passivation layer while maintaining the aluminum contact integrity.
The low temperature reduces the electrical and mechanical properties of the deposited SiO
2
film, which makes it typically unsuitable for electrical applications.
4.3.2.2 Chemicals
Oxygen is the basic gas of every combustion reaction and therefore leaks can be dangerous
near hot areas.
SiH
4
is a pyrogenic gas, which means it will burn spontaneously when it comes into contact
with O
2
or air. It is also toxic and should be handled accordingly, although generally it will
burn before it gets toxic.
4.3.2.3 Process
Due to the pyrophoric nature of SiH4 combining SiH
4
and O
2
to do a process requires a
distinct handling procedure. This includes reduction of the process pressure and applying a
specific gas distribution system.
The deposition of LTO from SiH
4
and O
2
is a very physical process. This means, the
deposition rate strongly depends on the gas flows and diffusion properties. The gas flow is
mainly affected by the hardware configuration and not so much by the temperature. The
result is, that the deposition rate and thickness uniformity do not react much on temperature
changes, but do react on hardware changes such as cage and injector design, position of these
in view of each other, injector alignment and wafer alignment.
Additionally, as with most LPCVD processes, the pressure will also affect the process results.
The chemical reaction of a basic LTO process is as follows:
SiH
4
+ O
2
Æ
SiO
2
+ 2 H
2
