Etch
substrate (Figure 2), engineers have introduced
Pt
[5] [6]
, optimized surface preparation
[7]
and
implemented low temperature silicide phase
formation. Ni migration, however, still requires
careful control.
Voltage contrast e-beam inspection
[8]
is
used to detect abnormal device leakage. This
process captures the number of electrical
defects as a function of silicidation process
conditions. A lower RTA1 temperature and
thicker deposited Ni(Pt) layer result in better
silicide stability and lower defect density.
At low RTA1 temperatures, a variety of Ni
rich silicide phases can be formed (Ni3Si, Ni2Si,
Ni3Si2…)
[9]
. The challenge is to form the right
phase to prevent NiSi2 formation and/or
abnormal Ni migration. At temperatures below
the NiSi2 nucleation temperature Ni3Si2 typically
Figure 2 - Ni abnormal unreacted metal remaining on top of field oxide transforms to NiSi2 without forming NiSi
[10]
. By
lateral migration and spacer dielectric must be removed with a accurately controlling the RTA1 process
under spacer and highly selective blanket etching process. The conditions, the formation of Ni2Si, the desired
Poly-Si gate selective metal etching step is usually a heated phase for optimal silicide stability, predominates.
wet chemical process. After selective removal of
the unreacted metal a second, higher Etch Process Challenges
18
temperature anneal step is carried out in order The challenge in selective etching lies in
to set the final desired phase and structure of successfully removing the TiN capping layer and
www
the silicide layer. Conditions of the pre clean, all unreacted metal without attacking the metal
.eur metal deposition, annealing, and selective etch silicide layer, while minimally etching the field
oasiasemiconductor
processes are all highly dependent on the metal oxide and sidewall spacers. The common wet
and on the details of device integration, such as etchant used to selectively remove TiN and Ti
junction depth and thermal budget. during Ti salicide formation is a mixture of
ammonium hydroxide, hydrogen peroxide, and
Need for Platinum water, referred to as “standard clean 1”, “SC1”
While platinum has been widely used for silicide or “APM”
[11], [12]
. For Co salicide formation,
.com
formation in various applications, mainstream lower pH etchants, such as a mixture of
CMOS manufacturing initially favored titanium hydrochloric acid, hydrogen peroxide, and water,
for salicide processes in the 0.25 micron and commonly referred to as “standard clean 2” or
square4
Issue III 2009
0.18 micron generations
[1]
, followed by Co in “SC2” or “HPM”
[3], [13]
, are used. A mixture of
the 0.13 micron and 90nm generations
[2]
. Nickel sulfuric acid and hydrogen peroxide known as
was first introduced by some manufacturers at “piranha” or “SPM” has also been used.
the 90nm CMOS node, and at 65nm and 45nm Unreacted Ni is easily removed with SPM with
an alloy of Ni and Pt is being used. Current high selectivity to NiSi
[14]
. All these etchants are
volume manufacturing typically uses 5 percent able to achieve high selectivity to the metal
Pt, while 10 percent Pt or more is being silicide, but selectivity is also dependent on the
evaluated in R&D for future generations
[3]
. All metal species and anneal conditions.
the work discussed in this article uses 5 percent The addition of even a few percent of Pt to
Figure 3 – 10-nm-thick Pt, unless otherwise noted. Ni films creates a challenge for the common
Ni(Pt) layer salicide selective etching mixtures described above.
process: RTA1<300°C Challenges Related to Silicide These common mixtures cannot remove
+ HPM + RTA2 a) Integration unreacted Pt and leave dispersed Pt residues
TEM cross section of Nickel silicide is widely used in advanced devices across the wafer surface. Aqua regia, a mixture
transistor after for its low formation temperature, resistivity and of hydrochloric acid, nitric acid, and water, can
selective wet etch. b) Si consumption
[4]
. The physical stability of nickel remove unreacted Ni and Pt
[15]
. A more
EFTEM compositional silicide with thermal budgets is an important concentrated version of the SC2 mixture in a
colour map issue. To avoid abnormal Ni migration in the Si fresh dispense, point of use mixing configuration
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