As a manufacturer of specialty materials, POCO’s materials research
and development teams are constantly Introduction In advanced DRAM
manufacturing, gate length and spacing continue to shrink while
performance requirements become more stringent. Film stack composition
imposes additional complexity, because the film stack consists of
two different materials. Polycide (tungsten silicide [WSix] on poly)
is currently the mainstream gate electrode material in DRAM fabrication,
while tungsten (W) on poly or directly on gate oxide are targeted
for next-generation technology. WSix and W are favored, because
their low resistivity reduces the resistive current delay of the
gate transistor, thereby increasing device speed.
Critical dimension (CD) bias uniformity of < 3 nm is commonly required
for gate lengths of approximately 130 nm with a 1- to-1 line-to-space
(LS) ratio. Yet as the LS ratio approaches 1, it becomes extremely
difficult to minimize profile microloading between dense and isolated
lines. Other challenges include HM selectivity, profile loading,
selectivity to gate oxide, and CD bias uniformity.
To maximize HM selectivity, CF4 flow and bias power must be decreased
in the WSix ME. To clear all WSix, a so-called overetch step (WSix
OE) is introduced. To minimize profile microloading, the WSix OE
percentage must be high enough that, after the OE, the WSix profile
on dense lines is not too tapered compared to that on isolated lines.
In turn, CD bias uniformity improves as profile microloading between
dense and isolated lines decreases and profile loading improves.
These issues were addressed in developing a process that produced
the requisite profile and selectivities to achieve CD bias uniformity
of < 3 nm (3ó).
A baseline process was established on 200 mm wafers to achieve
good WSix and poly profile, and CD bias uniformity of < 3 nm (3ó).
This process was then scaled up and fine-tuned to achieve the same
performance on 300 mm wafers. The work was performed using a high
conductance polysilicon etch chamber equipped with a dual-coil,
tunable plasma source and advanced gas distribution system. The
former facilitates control of plasma density uniformity. The latter
permits control of pressure and gas flow throughout the chamber
to adjust distribution of etchants and their byproducts and, thereby,
influence etch rate behavior.
200 mm Process Development
The process required a uniform CD bias across the wafer, high selectivity
to the nitride HM, and a slightly tapered WSix profile, the latter
to be rendered vertical by a subsequent wet clean process. The initial
process produced a vertical WSix profile and tapered poly profile,
CD bias uniformity of approximately 5 nm (3ó), and insufficient
HM selectivity. The HM selectivity and WSix profile were optimized
using process chemistry, total flow, and source and bias power levels.
HM selectivity is most affected by changes in CF4 flow and bias
power in the WSix ME step as these two parameters have the greatest
effect on the HM etch rate. Figure 1 shows the effect of reducing
CF4 flow. HM remaining improved by 5 percent, but the WSix profile
also became more tapered. Three parameters were adjusted in an attempt
to produce a more vertical profile. First the total flow and ratio
of source power to bias power were increased.
However, the profile became too undercut (Figure 2). Increasing
N2 flow resolved the problem by providing sidewall passivation without
affecting HM selectivity. As can be seen in Figure 3, reducing the
ratio of source power to bias power and increasing N2 flow produced
a vertical WSix profile and good HM selectivity. The final 200 mm
process resulted in good WSix and poly profiles with > 2000A HM
remaining and CD bias uniformity of < 3 nm (3ƒÐ) (Figure 4).
300 mm Process Development
Using a known methodology, the 200 mm process was scaled to serve
as the starting recipe for 300 mm development. The scaling maintained
pressure and gas flow, but increased source power by approximately
30 percent and bias power by 50 to 60 percent for the ME and SL
steps. Because the OE could not scale up directly, the baseline
process for the 300 mm chamber was used.
This phase of development emphasized gate oxide selectivity in
the SL step, improved poly profile loading, and CD bias tuning.
Originally, the SL step had a threesecond extension after endpoint
to remedy a tapered poly profile. However, the thickness of gate
oxide remaining measured at the low end of the specified range.
The extension was therefore eliminated, but the SL step itself needed
fine-tuning to avoid increased tapering of the HeO2-sensitive poly
profile. Higher HeO2 flow improved gate oxide selectivity but produced
more tapered profiles. Substantially increasing the flow of Cl2,
however, restored the vertical profile (Figure 5).
The primary cause of CD nonuniformity is profile microloading between
center and edge of the wafer. Typical poly SL and OE steps result
in profiles that are more vertical at the center of the wafer than
at the edge. To minimize this profile loading, the poly etch rate
non-uniformity for all three steps had to be optimized. In the DPS
II poly etcher, etch rate nonuniformity can be easily tuned. The
dualcoil source power supply allows independent control of RF power
to each coil, enabling uniform RF power deposition on the wafer
under a wide range of process conditions. By adjusting the coil
ratio (outer coil current to inner coil current) plasma uniformity
can be adjusted. Using the tunable gas nozzle, relative gas flows
to the center and edge of wafer can be adjusted to control byproduct
distribution and, hence, relative etch rates and passivation.
Increasing the coil ratio and reducing the TGN ratio (volume of
outer to inner flow), improved ME etch rate nonuniformity from 5
percent (1ó) to 2.5 percent (1ó) (Figure 6). To optimize OE etch
rate non-uniformity, helium flow was increased to reduce the etch
rate at the wafer center. However, this created slightly tapered
poly profiles at both center and edge of the wafer, although less
so at the center (Figure 6). Optimizing the SL etch rate non-uniformity
remedied this effect. Typically, the SL etch rate pattern is slow
at the center and edge of the wafer and fast in the area between.
Because the profile was more tapered at the wafer’s edge, the process
was tuned to etch faster there. Coil ratio was raised and the TGN
ratio in the SL step reduced to create a center slow, edge fast
pattern that produced vertical poly profiles at both center and
edge. Once profile microloading was minimized, CD bias uniformity
improved. Figure 7 shows CD bias and uniformity across the wafer
before and after etch rate optimization. Before optimization, CD
bias trended upward towards the edge of wafer and uniformity was
approximately 5 nm (3ó). After optimization, the CD bias trend became
more random, CD bias uniformity improved to less than 3 nm (3ó),
and dense/iso CD microloading also improved.
CD Bias Tuning
CD bias could be adjusted by modifying the WSix and/or poly profile
through both the ME and SL. In the ME, profile – hence CD bias –
was most sensitive to N2 flow (Figure 8a). Alternatively, pressure,
source power, bias power, and HeO2 flow in the SL could influence
CD bias (Figure 8b). The percentage OE after the WSix ME step also
affected CD bias. CD loss grew with the OE percentage (Figure 8c),
changing by more than 7 nm as OE percentage increased from 30 to
40 percent. This was because the taper remaining in the WSix profile
after ME endpoint, especially in the dense area, was eliminated
during the OE. Poly film is also etched during WSix OE. Therefore,
the amount of poly remaining after WSix OE also depends on the OE
percentage. The more poly remaining, the longer it takes to reach
SL endpoint. In this work, SL endpoint time of approximately 16
seconds yielded optimal profile and CD bias.
By fine-tuning the ME, SL, and OE steps, an HM polycide gate etch
process was developed to etch polycide gate wafers for advanced
DRAM fabrication at 130 nm and below. The process produced good
HM and gate oxide selectivities, and the requisite slightly tapered
WSix profiles, good vertical poly profiles, and CD bias uniformity
of < 3 nm (3ó) (Figure 9). The WSix was etched with CF4/Cl2/N2 chemistry
to endpoint plus an OE percentage; remaining poly film was etched
to endpoint with a HBr/Cl2/HeO2 SL step. Poly residue was removed
with a high-selectivity HBr/HeO2/He OE.