Impact of Substrate Bias on p-MOSFET Negative Bias Temperature Instability
P. Bharath Kumaf, T. R. Dalei, D.Varghese, D.Saha, S . Mahapatra and M. A. Ala"
Department of Electrical Engineering, Indian Institute of Technology Bombay, 400076, lndia
'School of Electrical and Computer Engineering, Purdue University, West Lafayette, JN,USA
'Phone: +9 1-22-25764483, Fax: +91-22-25723707, Email: bharath@ee.iitb.ac.in
"TRODUCTION
Negative Bias Temperature Tnstability (NBTI) of p-MOSFET is
an important reliability issue for digital [ I ] as well as analog [2]
CMOS circuits. Till date, characterization [3-91 and modeling [IO121 efforts to analyze NBTI mechanism involve devices stressed
with zero substrate bias (ITB).However, many circuits utilize nonzero V, to vary the device threshold voltage (VT), (e.g., for dual VT
CMOS,standby leakage reduction, etc.) [ 13-161. This paper aims to
systematically study NET1 for VB>OV stress, which to the best of our
knowledge has not been done so far. It is shown that NBTI increases
for VB>OV stress. This i s attributed to enhanced interface (") and
bulk (Nor) trap generation due to impact ionization and hot-hole
(HH) generation. The role of gate bias (VG), VB,temperature (T) and
oxide thickness (TPHY)is studied. This work would help all efforts in
determining (i) reliability budget for any operating VB, (ii) proper
choice of stress VB during accelerated aging tests, and (iii) suitable
TCAD and SPICE models.
RESULTS AND DISCUSSION
Experiments were performed on p-channel non-nitrided gate
oxide MOSFETs having TpHyof 20A0 through %A0. Fig. 1 shows
AVT (t) for stress at different VB but identical oxide field (Eox). AVT
increases with VB and shows a power law in time whose exponent n
also increases, seriously affecting extrapolated NBTI lifetime. Fig. 2
shows the impact of T on AVT (t) for stress with and without VB. For
VB=OV stress, long-time AVr increase seen at high T is due to faster
H2 diffusion in poly, as explained elsewhere [6,10]. Enhanced AVT is
observed both at low and high T for VB>OV stress. However, AVT
enhancement at high T shows up at a later time and has smaller n
than that at RT.Therefore unlike VB-OV stress (conventional NBTI),
the worst-case degradation for V+OV stress occurs at lower T.
Fig.3 shows ANIT(t) for stress under different VB but identical
Eox, measured by charge pumping at HOOkHz. ANir increases with
VB, shows power law in time, and the exponent n increases with Vg.
For VB=OV, n for ANITand AVT are identical. It is well known that
ANoT= 0 for such stress conditions and hence AVTE ANIT[6,7]. For
V+OV stress, Cox/q. AVT (and corresponding n) is larger than ANIT
(and corresponding n), which implies the presence Of ANoT [ 5 ] . As a
proof, Fig. 4 shows V+OV stress induced enhanced ANIT and AVT
(differential increase from VB=OV) and ANOT(=Cox/q. AV* - ANIT)
as a function o f stress time. High-VG SILC, which tracks mid-oxide
AN~T
[17], is also shown. VpDV stress induced AVT enhancement is
larger than that of ANITand is due to ANOT, as substantiated by the
presence of SILC. Note, VB>OVstress induced enhanced ANIT, ANoT
(SILC) and hence enhanced AV, show t" (n-0.5) time dependence.
Fig. 5 and Fig. 6 show VB>OVstress induced enhanced AVT and
ANJTrespectively for a wide range of stress conditions. The n 0.5
power law is always observed, so is the fact that enhanced AVT is
greater than enhanced ANrr (because of AN,,). Enhancement in AVT
and ANITincreases with increase in Eox and VB and decrease in
TPHY.
Moreover, enhanced AVT and ANITis found to reduce at higher
T (not plotted in this paper).
V+OV stress causes HH generation due to impact ionization by
electrons tunneling from gate as shown in Fig. 7 [5]. The observed
Y T
dependence of enhanced AVT and ANrron VG(Eox), VB, T ~ Hand
can be explained by amount of HH generation, as tabulated in Fig. 7.
As a proof, Fig. 8 plots enhanced AVT and ANlr versus Quantum
-
Yield of HH generation for different stress Eox and VB (TPHYand T
constant). Excellent correlation justifies that hot-holes create ANOT
[IS], enhanced ANITand hence enhanced AVTfor V+OV stress.
Fig. 9 shows buildup and recovery of AVT and ANI^ during and
after stress with and without V,. Identical AVT and ANn recovery is
observed for VB=OV stress since AN07 is negligible. AVTrecovery is
larger than ANITrecovery for vB=2v stress due to detrapping of
holes trapped in generated NOT.Fractional recovery in both ANn and
AVTis lower for V+-OV compared to VB=OV stress.
Fig. 10 and Fig.1 I show ANlT generation (for VB=OV and VB>OV
stress) and recovery respectively for various stress VB and post-stress
VG.ANITbuildup during stress increases with stress VB (Fig. lo), but
the magnitude of recovery (generated remaining) is identical for all
stress VB (identical to that for VB=OV stress). The amount of ANIT
recovery for both VB=OV and VB>OVstress is weakly dependent on
post-stress VG (Fig.] 1). The above results clearly show that VB>OV
stress induced enhanced ANIT does not recover.
NIT buildup during VB=OV stress is believed due to broken Si-H
bonds with subsequent release and diffusion of neutral H2[5,11,12].
Nor buildup during V+OV stress is believed due to broken 5-0
bonds at the oxide bulk due to hot hole injection [I 8,191. While ANIT
recovers after the stress is removed due to reformation of Si-H bonds
[7,11,12], no known mechanism exists for the recovery of ANoT. We
postulate that due to their non-recoverable nature, enhanced ANrrfor
VB>OV stress is due to broken Si-0 bonds at the Si-Si01 interface.
We realize that enhanced ANITduring VB>OV stress can also be due
to broken Si-H bonds with subsequent release and diffusion of H+
[ 121. However the lack of recovery, and more importantly, its weak
dependence on post-stress VG(Fig.] 1) suggests that & release is not
likely to be involved in creating additional ANlTfor V,>OV stress.
~
CONCLUSlONS
To summarize, NBTI of p-MOSFETs is studied under presence
of V,, It is shown that NET1 increases due to increased Nlr and N O T
generation for V+OV stress, and is correlated to hot-hole generation
under such condition. ANOTand enhanced ANIT follow a power law
in time with n 0.5, increase with increase in Eox and Vg, strongly
increase at lower TpHY, and decrease at higher T. For V+OV stress,
both AVT and ANIT show lower fractional recovery while AVT shows
higher absolute recovery than ANrr compared to VB=OV stress. Due
to its differences with conventional NBTI, p-MOSFET degradation
under VB>OV stress needs careful attention.
-
REFERENCES
[ I ] V. Reddy et. al., IWS, p.248, 2002. [2] C. Schlunder et. al.,
IRPS, p.5, 2003. [3] K. Uwasawa et. al., IEDM, p.87 I , 1995. [4] N.
Kimizuka et. al., VLSI, p.73, 1999. [5] S. Mahapatra et. al., IEDM,
p.505, 2002. 161 S. Mahapatra et. al., IEDM, p.337, 2003. [7] S.
Rangan et. al., IEDM,p.341, 2003. [3] A. Krishnan et. al., IEDM,
p.349,2003.[9] S. Tsujikawa et. al., IRPS, p.28, 2004. [IO] M. Alam
et. al., IWGI, p. 10, 2001. [1 I] M. Alam, IEDM, p.345, 2003. [12] S .
Chakravarthi et. al., IRPS, p.273, 2004. [I31 A. Keshavarzi et. al.,
Int. Symp. LPED, p.207, 2001. [I41 S. M. Martin et. al., Int. Conf.
CAD,p.721, 2002. [I51 J. Tschanz et. al, ISSCC, p.422, 2002. [16]
S. Narendra et. al., ISSCC, p.270, 2002. [I71 M. Alam, TED, p.226,
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IEDM, p.179, 1998.
IEEE 05CH37616 43d Annual International Reliability
0-7803-8803-81051920.00 02005 IEEE
700
Physics
Symposium,San Jose,
Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on December 31, 2008 at 01:39 from IEEE Xplore. Restrictions
apply.
2005
7x10.'
t
A 2.0, 0.36,0.32
V,o/): slope
0.0;0.22
U
-L
2Io - ~
Figure 1. Time evolution of ~v~for stress at
different VB(Eox constant).
t
.
. . . .....I
,
. . . , . .I
. . ...,.
io3
io2
10'
1
,
io'
io4
10'
10'
stress time (5)
IO'
Figure 2. Time evolution of AVTfor Ve=OV
and Ve=2V stress at different T.
stress time (s)
Of
Figure 3. Time evolution
different Ve (Eox constant).
10.2
IO'
1o4
O'I
IO*
for stress at
lo4
10'
stress time ( 5 )
Figure 6. Time evolution of enhanced ANITfor
Ve'OV stress under different Eox,Ve and TPW.
E,& 8.8 - 10 MVlun
V,: 1 . 5 - Z S V
+
Figure 7. p-MOSFET energy band diagram
in inversion under high VE showing impact
ionization. Dependence of HH generation on
experimental parameters is also shown.
open: recovered at next 1Ks
-
rr
E
20
t
2
I
.
,
.
,
.
,
.
,
,
,
0.10 0.15 0.20 0.25 0.30 0.35
Quantum yield = IsD / f,
NITand VT to QY of HH generation.
1000 2000 3000 4000
stress / recovery time (s)
Figure 9. Generation and recovery of ANl,
and AVT for stress with and without VE.
0
V,(V):stress/post-stress
18
0 : -2.4/0.0
O
15 & A
Q
V
: -2.7/0.0
: -2.4/1.0
+ b *
14 N
?
6
0.0 0.5 2.0 1.5 2.0 2.5
.+
128
5
6-
&
4
$
H
4
open: recovered at next 1Ks
Figure 11. Generation and recovery of ANIT 4 1 4
2for different post-stress V
.,
0 '
Stress V, (V)
4 A v,=~.ov
W
1
-. hashed: generated at 1Ks
10-•
?
'
0
Stress: VC=-3.0b
l6
v
-
1
0 v,:o.ov
Figure I O . Generation and recovery
of ANITfor stress at different VB.
-2.55/0.0
T=27'C, T,=22A0
"
0
4
3
Ei
B
e
T=27OC, TM=26A0
Post-stress VB=OV
"
"
Post-stress
701
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VG (V)
"