Reliability of Ultrathin JVD Silicon Nitride
MNSFETs Under High Field Stressing
K. N. ManjulaRani, V. Ramgopal Rao and J. Vasi
Department of Electrical Engineering, Indian Institute of Technology, Bombay Mumbai 400 076 INDIA
email: rani@ee . iitb.ac . in
Abstract- In this paper, we study the reliability of n-channel MetalNitride-Silicon FETs fabricated using ultrathin Jet Vapor Depusiled (JVD)
Silicon Nitride gate dielectric under constant voltage stressing. Due to the
s t m . shills in thmshold voltage and tramconductance as well as interface
slate generation are observed. Our study shows that dogradation is polarity dependent. MNSFETs show lower degradation when the applied strrps
voltage is positive. We have also compared the performance of MNSFETs
with conventional MOSFETs under identical s t i e s mnditiam. Under p i tive stmsing, MNSFETs clearly outperform L e MOSFETs hut under negative stressing MNSFETs show mom degradation.
I. INTRODUCTION
Alternate high-K gate dielectrics are needed to replace conventional SiOl in MOS transistors. Some of the alternative gate
materials that have k e n investigated are silicon nitride, and tantalum, hafnium and zirconium oxides. However, the degradation and reliability of high-K dielectrics need to he explored in
depth. Jet Vapor Deposited (JVD) Si3N4 has shown promise
as an alternate dielectric with improved hot carrier performance
as compared to conventional MOSFETs [I]. It has been shown
to be resistant to boron penetration [2]. Conventional MOSFETs with ultrathin SiOz gate dielectric have shown increased
interface state generation under high-field stressing causing gm
and Vth shifts. In MOSFETs, the degradation and hence the
charge-to-breakdown Q B D is known to he polarity dependent
[3]-[7]. In this paper, we study the polarity dependent reliability of n-channel MNSFETs and compare performance of MNSFETs with MOSFETs under equivalent stress fields.
Fig. 1. Re-suess Id-Vgsand gm charactenstics of MNSFET and
MOSFET
data. The charge pumping current was measured by applying
a trapezoidal waveform of lMHz frequency to the gate-with a
rise and fall time of 25011s. N,, was calculated from the charge
pumping current.
IV. RESULTS
11. DEVICES
The devices used in this study are n-channel, n+ poly-Si gate
transistors fabricated using an identical CMOS process except
the gate deposition process. One set of transistors has Si3Na as
the gate dielectric deposited using the JVD process[2] followed
by annealing at 800°C for 25 min in Nz (referred to as MNSFETs). The second set of transistors has SiOz grown at 800°C
followed by an in-situ anneal in Nz (referred as MOSFETs). The
MNSFETs have an Equivalent Oxide Thickness (EOT) of 3.lnm
and the MOSFETs have a gate oxide thickness of 3.9nm. The
transistors have W x L = l o x 0.18pm2.
111. EXPERIMENT
A. Prestress characteristics of MNSFETs and MOSFETs
The pre-stress Id-V,., gm-Vgaand charge pumping characteristics of MNSFET and MOSFET are compared first. The prestress Id-V,. and gm-Vgacharacteristics of the transistors are
shown in Fig 1. Both g, and Id are normalized with CO=.The
threshold voltage of MOSFET and MNSFET obtained from the
maximum slope of Id-Vda characteristics is 0.43 and 0.47V respectively. The pre-stress transconductance g,,, of MOSFETs is
comparable to that of MNSFETs. The prestress I, is shown
in Fig. 2. The average pre-stress N;, is of the order of 12x 10'ocm-z in MOSFETs and ahout4-6x 10" cm-2 in MNSFETs. The MNSFETs and the MOSFETs have comparable prestress characteristics except for the interface state density N i t
which is slightly higher in the case of MNSFETs.
The transistors were subjected to high-field stressing with
constant voltage applied to the gate with the source, drain and
substrate grounded. The stress was interrupted periodically to
monitor the changes in the gate threshold voltage Vlh, transcon- B. Post-stress performance of MNSFETs and MOSFETs
ductance gm and Nil. The values of Vth and gm were obtained
The performance of MNSFETs and conventional MOSFETs
from the Id-Vgs characteristics measured at a drain bias of 50 is illustrated under identical equivalent positive stress of +12 and
mV. The peak transconductance was calculated from the Id-VgB +14MV/cm. -AN,t defined as ( N ; l - N ; t , ~
is)plotted in Fig. 3.
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Fig. 2. Pre-suess I,, plotted w
. a function of Vios.
Fig. 3. Comparision of effect of positive Stress 6 elds on N t in conventionnl MOSFETsandJVDMNSFETs. AN;t = (.N,,-Nit,o) i s
plotted as a funtion of lime.
Ag,,, which is defined as I(gmoz-gmoz,o)lis shown in Fig. 4. It is
clear that for positive fields, MNSFETs show better performance
compared to MOSFET’s. Next, we compare the performance
of MNSFETs with MOSFETs under negative stress fields equal
to -11 and -1ZMVlcm. The change in Nit, A n i t is plotted in
Fig. 5 . The change in g, is shown in Fig. 6. We can see that
for negative stress fields, MNSFETs do not always outperform
MOSFETs.
C. Polarity dependence ojpost-stress MNSFETs
The stress voltage polarity dependence in MNSFETs is illustrated for effective fields of +lZMV/cm and -12MVkm respectively. The charge pumping current Icp and drain current Id-vgb
is illustrated in Figs. 7, 8, 9 and 10 respectively. The charge
pumping current for positive stress is lower than that for negative
stress by a factor of 4. The change in threshold voltage is also
lower for positive stress. The normalized increase in Nit defined
as (Nit-Nit,o)/Nit,o. transconductance ((gmoz-gmaz,O)(igmoz,o
and threshold voltage (&h-&h,O)/&h,O are plotted as a function
of stress time in Fig. II . We can see that compared to gate positive stressing, gate negative stressing produces higher Nit, g,
and V t h shifts.
V. DISCUSSION
With varying stress time, ANi, and Ag, show power law
(crt”) dependence for both MNSFETs and MOSFETs for both
positive and negative stress. For positive stress (inversion), the
exponent value n for MNSFETs varies between 0.35-0.38 for
AN,, and 0.3-0.4 for Ag, which are similar to the n value
(0.3) observed in p-MNSFETs subjected to constant voltage FN
stress under inversion [8]. Our results show that under gatepositive stress (inversion), MNSFETs show lesser ANjt degradation
than MOSFETs which has n value of 0.53. We can see that both
the magnitude and rate of degradation is smaller for MNSFETs
for positive stress. In case of MNSFETs, values of n for A&, is
about 0.3-0.4 which is almost same as n values of ANil. In case
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Fig. 4. Camparision of effect of positive stress fields on & in MOSFETs and I V D MNSFETs. Ay, = l(ymo,-ymor,o)l i s plotted
as a funtion of time.
of MOSFETs, values of n are about 0.4-0.46, slightly lower than
that of ANil. Compared to MOSFETs, MNSFETs show lower
magnitude and slightly lower values of n. Fig 3 and Fig 4 clearly
illustrates that JVD nitrides perform better than MOSFETs under positive gate voltage stress. The threshold voltage Vth is
an important parameter in the operation of a transistor. Both
interface states and bulk trapping-contribute to the variation in
the threshold voltage. We have separated the two contributions
and plotted AVit and AV,& Figs. 12 and 13 respectively.
AKt shows power law dependence for both MNSFETs as well
as MOSFETs. In case of MNSFETs, AVOt.inirially increases
rapidly and saturates suggesting that there are bulk traps which
are filled up during stressing. This behaviour rules out bulk trap
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J V O MNSFET, E = + l Z M V i c m
10X0.18L"
t0,=3.?n m ( E 0 T )
20.10-
0.0
-0.5
0.0
0.5
1.0
1.5
V,~(VOltS)
Fig. 5. Comparision of effect of negative suess fields on Nt in conventional MOSFETs and IVD MNSFETs. A N ( t = ( N i t - N i t , ~is)
plotted a a funtian of time.
Fig. 7. I,, platted a a function of stress time for a JVD MNSFET
stressed at E=+12 MVIcm.
'or0
0
JVO MNSFET(E--IIMVlsm)
MOSFET (E=-l?MVlcm)
JVD MNSFET (Es-1IMVlcm)
MOSFET (E--lIMVlcm)
4 5
0.0
0.5
1.0
1.5
Fig. 8. I,, plotted as a function of stress time far a JVD MNSFET
stressed at E=-I2 MVIcm.
Fig. 6. Comparision of effect ofnegative mess fields on &, in MOSFETs and JVD MNSFETs. Agm= l(gmaz-g.,,.,=,o)l is plaaed,as
a funtion of time.
generation. In case of MOSFETs, the initial bulk trap density
is lower. With stress, MOSFETs show bulk trapping which increases with time, indicating that there is trap generation.
Under negative stress, the magnitude 4 N i t is higher for
MNSFETs than MOSFETs. The exponent is about (0.3) for
nitrides compared to MOSFETs (0.4). The exponent value of
4g, is 0.3 for both MNSFETs and MOSFETs but the magnitude is lower for nitrides: Under negative voltage stress, MOSFETs show positive charge trapping while MNSFETs show net
negative trapped charges. The degradation of Vth is higher in
MNSFETs than M O S E T s as illustrated by Fig. 14 and Fig. 15
respectively resulting in degraded performance of MNSFETs.
MOSFETs also show polarity dependent degradation as
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shown in Figs. 3, 4, 5 and 6. Here also we see that the damage due to negative stress is higher compared to the positive
stress. Polarity dependent degradation in ultrathin MOSFETs is
attributed to the defects in the gate/dielectric interface[4], presence of a structural transition layer at the substrate/dielectric interface[5], or due to the difference in the electron energy dissipated at the anode which in turn depends on the anode Fermi
level [6], [7].According to the anode Fermi-level model, damage is maximum for p-type anodes where the electron impact
generates hot holes. The defect generation rate is higher when
the density of holes is larger in the anode [9]. We have used
the AH1 model to explain the polarity dependent degradation in
MNSFETs. The band diagram of MNSFET in accumulation and
inversion is shown in Fig. 16. MNSFETs show lesser 4 N i t and
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J V D MNSFET. E=+1PMVlcm
10''
1.,=3
lnm(E0T)
Tlms(aes0nds)
Id-V,, plotted as a function of stress time for a JVD MNSFETs stressed BI E=+12MVkm.
Fig. 9.
Fig.
I I.
Pal8rity dependence of degradation of max. nansconducE=llZlMV/cm. Normalized transconduclance l ( ~ ~ . . - ~ ~ ~ = , o ) plotted
l / ~ ~ as
~ a= function
, ~
of time.
tance in JVD MNSFETs at
JVD MNSFET.E=-l ZMVIcm
ta,=3.1 nm(E0T)
Fig.
IO. Id-Vgaplotted as a function of stress time for a JVD MNSFETs stressed at E=- I2MVIcm.
Fig. 12. Effect of positive smss fields on AV;, in conventional MOS
4g, degradation than MOSFETs under positive stressiing. This
may be due to the higher bond breaking energy of Si-N bonds.
However, under negative stress 4Nit and AKh shifts are higher
than oxides which may be due to the increased hot-bole flux due
to the smaller hole barrier height in nitrides (1.9eV as compared
to 4.9eV). If an electron arrives with an energy of 3eV, impact
ionizes to create a hole with a maximum energy of 3-1.1=1.9eV
comparable to the SiNfSi barrier of 1.9eV. Compared to this,
hole flux in SiOz/Si would be smaller where barrier height for
holes is 4.9eV.
VI. CONCLUSION
MNSFETs perform better compared to the MOSFET's under positive bias which is important for n-channel devices. The
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FETs and JVD MNSFETs plotted as a funtion of time.
nitrides show smaller Nit generation compared to the oxides under identical fields. The transconductance degradation is lower
compared to MOSFETs. Also there is no bulk trap generation in
MNSFETs and hence threshold voltage shifts are also smaller.
We can therefore conclude that the JVD MNSFETs can easily out perform MOSFETs under gate positive condition, that is
when the transistor is in inversion. However the degradation of
Vth in MNSFETs is larger compared to MOSFETs under negative stress fields, that 'is when-the surface is in accumulati&
although transconductance degradation is lower. By controlling
the initial bulk trap density, JVD MNSFETs can be expected to
outperform MOSFETs.
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0
0
0
/
I
Fig. 13. Effect of positive mess fields an Vet in conventional MOSFETs and JVD MNSFETs plotted as B funtion of time.
Fig. 15. Effect of negative stress fields on AV,{ in conventional
MOSFETs and JVD MNSFETs plotted as a funtian of time.
Gate
I
i
Si
Fig. 14. Effect ofnegative stress 6 eldson AV;, inconventional MOSFETs and JVD MNSFETs plotted as a funtian of time.
Fig. 16. Band diagram of a NMOS-MNS transistor under a) Accumulation and
hilnvsnion.
VII. ACKNOWLEDGEMENTS
We would like to thank Prof. J. C. S. Woo, UCLA for the
.p, Ma, Yale university for JVD deposition.
samples and
151
[6]
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