Formed by NH3 Nitridation of Chemical Oxide and Reoxidation with O
2A nitrogen distribution profile across the 25Å and 70 Å oxynitride gate dielectric
revealed by secondary ion mass spectrometry (SIMS) are shown in Fig. 2-3.
Apparently high nitrogen concentration with a peak located at the dielectric surface is
observed. Such high nitrogen concentration is more helpful in resisting the boron
penetration of the gate dielectric from the P+ polysilicon electrode. The low nitrogen
concentration at the interface also improves reliability [63], [64]. Figure 2-4 shows the
C-V shift of P+-gated samples for the ultrathin oxynitride (23 Å) and conventional
oxide (30 Å) with annealing at 900 for various times (60, 90 and 120 min). ℃
Comparing oxynitride with conventional oxide, it is apparent that the C-V curve shift
in oxynitride is relatively smaller than that in conventional oxide. This implies that
boron penetration is highly impeded by this oxynitride film. Meanwhile, the oxidation
rates for the wafers, which have an existing oxide film grown by the conventional
method or our nitrided-chemical oxide, are compared in Fig. 2-5. The results show
that the oxidation rate of the nitrided-chemical oxide is much smaller than that of the
conventional oxide. At this point, this implies that the nitrided-chemical oxide
provides a wider process window to well control gate oxide thickness if ultrathin
oxide is needed.
The measured gate current-voltage (I-V) of the P+-gated MOSFET with EOT
20Å oxynitride films are shown in Fig.2-6. We can observe that gate leakage current is
very small and balance when the device operates in inversion region. When the
process of chemical oxide formation proceeds, the clean oxide inhibits defect
formation because it does not require a high-temperature process. This means that
chemical oxide as a starting oxide is essential in this process. Fig. 2-7 shows the
thickness of oxynitride is thinner than oxide. As oxide thickness shrinkage down to
the direct-tunneling region, tunneling current will strongly dependent on the oxide
thickness. if significant reduction of leakage currents is obtained. Because, effective
nitrogen incorporation from NH3 nitridation the chemical oxide film increases
dielectric constant, which allows physically thicker film with EOT to suppress the
direct tunneling leakage current.
For further study, the hysteresis in the C-V characteristics of the P+-gated
sample with 20 Å oxynitride film was also evaluated. The data are shown in Fig.2-8.
The figure shows that there is no hysteresis found in the high-frequency C-V curve,
indicating that the film has a very low bulk or interface trap density. Fig.2-9 shows
that the dominant conduction mechanisms in this oxynitride are direct tunneling and
F-N tunneling. Frenkel-Poole conduction mechanism does not exist, because Ig-Vg
curve is weak temperature dependence. We believe the limited of traps in this
oxynitride is probably responsible for this, because the F-P conduction requires a high
density of traps.
Fig. 2-10(a) shows a family of drain current curves of an pMOSFET transistor
of oxynitride and oxide. We can find that the oxynitirde has better driving capability
than oxide at the same overdrive (Vov). While Fig.2-10(b) shows the corresponding
transconductance and current at Vd=-0.1V characteristics. The oxynitride device
exhibits high drain current and high transcondance(110uS). The mobility of oxynitride
compared to conventional oxide shows at Fig.2-11.
Fig 2-12 shows the Vth-roll-off characteristics. When channel length is less than
1um, Vth-roll-off phenomenon is more serious. Fig 2-13 indicates the sub-threshold
swing of devices with different channel length. And we find that sub-threshold swing
is almost the same in different channel length. The Vth Weibull distribution in
oxynitride and conventional oxide shows in Fig.2-14. It shows that oxynitride Vth has
uniform distribution.
2-4 Summary
From basic electrical characteristics, we can get better performance for
oxynitrdie. And we have a special process to achieve the high quality oxynitride. We
have proposed an approach for forming high quality ultrathin oxynitride films with a
nonuniform nitrogen content distribution, which has high content at the surface and
low content at the interface between oxynitride and the silicon substrate, without
using extra equipment or gas and which is totally compatible with current
semiconductor fabrication technology. Oxynitride has lower leakage current than
thermal oxide at the same EOT (equivalent oxide thickness), and have better driving
capability, excellent suppression of boron diffusion. The direct tunneling and FN
tunneling dominate the current transport in the oxynitride film, which weakens the
temperature dependence. And we can control the growth of film precisely. There are
many advantage for oxynitride we adopt. So we prove this process of oxynitride is a
good candidate in the future.
Fig.2-1 Experimental Flow.
Source/Drain Activation : 950C for20 sec Source/Drain implant
0 5 10 15 20 25
-3 -2 -1 0 1 2 3 Å) with different annealing times.
75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450
Growth time for dry oxidation (s) Conventional dry oxidation Oxynitride process
Fig.2-5 Growth time for dry oxidation versus physical thickness of oxide for
conventional dry oxidation and oxynitride processes.
4 2 0 -2 -4
Fig.2-6 Gate leakage current density versus gate bias for fresh p-channel devices
at room temperature.
-2 -1 0 1 2
Fig2-8 Hystersis characteristics of oxynitride film with EOT=20A°.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Gate Current Density Jg((((A/cm 2))))
VG(V)
0.0 -0.5 -1.0 -1.5 -2.0
Fig.2-10 (a) Id-Vd characteristics (b) Gm and Id-Vg characteristic of oxynitride
for nMOSFET.
0.0 0.1 0.2 0.3 0.4 0.5 0
20 40 60 80 100 120 140 160 180
Ueff(cm2 /Vs)
Eeff(MV/cm)
Oxide Oxynitride Universal
Fig.2-11 Compared electron mobility with oxynitride and oxide measured by split-CV method.
0.4 0.5 0.6 0.7 0.8 0.9 1.0
Fig.2-12 Vth roll off characteristic of oxynitride and conventional oxide.
0.5 0.6 0.7 0.8 0.9 1.0
Fig.2-13 Subthreshold swing of devices with different channel length.
0.25 0.30 0.35 0.40 0.45 0.50 -4
-3 -2 -1 0 1 2 3
V
th(V)
ln [- ln (1 -F )]
Conventional Oxide Oxynitride
Fig.2-14 Vth Weibull distribution in oxynitride and conventional oxide.