Etch
is able to remove unreacted Pt without requiring Pt complexation decreases the effective Pt
nitric acid
[16]
. These HCl based chemistries have oxidant potential. In HPM solutions, chlorine is
been implemented for 65nm generation devices the complexant species and H2O2 or Cl2 are the
with good selectivity to the metal silicide. For oxidant species. The mechanism of Pt dissolution
the 45nm and 32nm generations, however, the in high temperature SPM mixture is not well
selectivity of HCl based mixtures is reduced due understood.
to the presence of metal rich silicide left by the It may be possible that Pt is dissolved
lower first anneal temperatures used at these through a thermally activated process, possibly
nodes. through a radical pathway, to form Pt
++
or PtO
++
.
Standard SPM processes are commonly Another possibility is the formation of platinum
used at or below 150 ºC in a recirculated sulphate. Further studies are underway to better
immersion bath system or with 95 ºC pre heated understand the exact mechanism of Pt residue
Issue III 2009
sulfuric acid for fresh dispense point of use removal in the high temperature SPM chemical
square4
mixed spray system. At these temperatures the mixtures.
.com
SPM mixture is unable to effectively remove
unreacted Pt residues. It was found, however, Parasitic Oxidation Issues
that higher temperature SPM processes carried with Cl Based Chemistries
out in a batch spray system are indeed able to Since the introduction of platinum in Nickel
remove the Pt residues and are also highly films, new selective wet etch chemistries have
selective to the metal rich silicide formed by the been introduced to also remove the excess
lower first anneal temperature. Pre heating the of Pt. As described above, noble metals, such
oasiasemiconductor
sulfuric acid to 150 ºC before point of use as Pt, are commonly dissolved by aggressive
.eur
mixing with hydrogen peroxide leads to a solutions containing hydrochloric acid such as
www
chemical temperature of close to 180 ºC on the Aqua Regia (HNO3 – HCl mix) or Hydrochloric
wafer surface, which is sufficient to remove the Peroxide Mixture (HPM: HCl/H2O2).
19
finely divided Pt residue remaining after the This chemistry efficiently removes
unreacted Ni metal has been removed. Pt stringers and residues. When the
Pt complexation chemical reactions have Ni silicide formation temperature is reduced,
been already described in detail
[17]
. In a forming a Ni rich silicide phase, however, a
complexant solution, the thermodynamic new parasitic phenomenon appears. This
equilibrium is modified and Pt reacts with HCl to parasitic phenomenon has detrimental
form various chloride ion complexes such as effects on silicide morphology and
Pt(Cl)
2-
4 or Pt(Cl)
2-
6 . Since dissolved chlorine (Cl2) silicide electrical properties.
and HCl have significant concentrations, a The TEM cross section shown in
soluble hexachloroplatinic acid is formed. The Figure 3 displays a silicide oxide
simplified chemical equation is given by: bilayer film on top of the polysilicon
gate, correlated to a dramatic
Pt + 2 HCl + 2 Cl2 b2leftb2rightH2PtCl6 increase of the polysilicon line
resistance. The footprint of the
silicide is preserved, but most of it is
replaced by this oxide.
This phenomenon appears as a
catastrophic oxidation as soon as both HCl
and H2O2 are present in the solution
independent of the HPM chemistry time and
HCl/H2O2 ratio. Additional experiments have
shown that despite a first selective etch with a
non oxidant chemistry, applying the HPM
solution to silicide samples causes oxidation of
the Ni2Si phase.
This parasitic phenomenon is not observed
Figure 4 - Measured temperature rise when with conventional Ni selective etching solutions
room temperature 30 wt% hydrogen peroxide is such as the sulfuric acid hydrogen peroxide
mixed with 100 ml of room temperature 96 wt% mixture (SPM: H2SO4/H2O2) used with standard
sulfuric acid process times and temperature conditions, but
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