Chemistry Reference
In-Depth Information
sition tends to depend on the type and doping level of silicon, because deposition
involving charge exchange with the surface depends on the electronic properties of the
substrate.
461
In addition to electrochemical deposition, metals may also precipitate onto
the silicon surface in the form of an oxide. According to Ohmi
et al
.
220
metals such as
A
, Cr, and Fe, which have a larger enthalpy of oxide formation than Si, tend to be oxi-
dized more easily than Si and tend to be included as particles in the silicon oxide.
Cleaning of metallic impurities on a silicon surface requires the oxidation and
dissolution of the metal atoms. Most metals have a standard potential that is much
higher than that of silicon. To oxidize the metal atoms the electrode potential, at least
in the local area of the deposits, has to be raised to the level of the reversible potential
of the metal in the solution. For example, in SC1 solution the potential of silicon is
at which the silicon surface is passivated (it is
l
without
This
solution the species with reversible potential less than tend
to be oxidized on the surface. Passivation is also responsible for the slow etch rate in
SC1 solution, for only about 30-40 material is removed during the RCA cleaning.
454
The fact that the silicon surface is passivated serves three purposes: (1) It allows the
surface to be anodically polarized which is required for oxidizing of metal impurities,
(2) it prevents much etching of the substrate during the cleaning process, and (3) it
helps to physically dislodge the metal atoms from the surface by forming an oxide layer
between the metal atoms and the silicon surface.
The surface condition—whether a native oxide is present or not—has a signifi-
cant effect on bonding of iron and copper deposited on the surface and thus the clean-
ing efficiency.
453
Each metal has its own characteristic chemical state.
454
Metal impurity
such as Fe tends to be present on the silicon surface oxide layer in the form of oxidized
Fe(III), whereas Cu tends to bond directly to Si atoms in the elemental state. The former
can be cleaned by etching away the oxide whereas the latter can only be removed by
a redox reaction.
Experimental results have shown that after RCA cleaning, the number of parti-
cles and zinc concentration on a silicon wafer are drastically reduced, while copper and
iron concentrations remain similar to the as-received condition.
131,493,665
With SC1 solu-
tion it is not easy to reduce surface heavy metals such as Fe below
means that in SC
1
Modi-
fied SC1 solutions are used to remove the Fe, Cu, and Ni to below
Dilute
solutions of
are found to be more effective than RCA
454
at removing surface metal impurities such as Ca, Ni, Zn, and Al.
Treatment with a
mixture of and diluted HF, which removes about 30 nm silicon, after RCA clean-
ing reduces significantly Fe, Ca, and Mg surface concentration.
631
SC1 cleaning
followed by a very dilute acid of produces better results regarding
removal of metallic impurities and particles.
465
Using ultrapure water to prepare the
RCA cleaning solutions leads to better results.
131
Cleaning with ozonized water and
peroxide to remove particles and metals is shown to produce a clean surface with Cu
concentration below
7.8.2. Defect Etching
Defect etching refers to the etching process that preferentially attacks the strained
bonds of defects within a crystal. It is a simple and fast method of determining the
