Material Dependence of NBTI Physical Mechanism in Silicon Oxynitride (SiON) p-MOSFETs: A Comprehensive Study by Ultra-Fast On-The-Fly (UF-OTF) IDLIN Technique

Abstract

An ultra-fast on-the-fly (UF-OTF) I_DLIN technique having 1 mus resolution is developed and used to study gate insulator process dependence of NBTI in silicon oxynitride (SiON) p- MOSFETs. The nitrogen density at the Si-SiON interface and the thickness of SiON layer are shown to impact temperature, time, and field dependencies of NBTI. The plausible material dependence of NBTI physical mechanism is explored.

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Material Dependence of NBTI Physical Mechanism in Silicon Oxynitride (SiON) p-MOSFETs: A
Comprehensive Study by Ultra-Fast On-The-Fly (UF-OTF) IDLIN Technique

E. N. Kumar, V. D. Maheta, S. Purawat, A. E. Islam1, C. Olsen2, K. Ahmed2, M. A. Alam1 and S. Mahapatra
Department of Electrical Engineering, IIT Bombay, India (Email: souvik@ee.iitb.ac.in, Ph:+91-22-25720408, Fax:+91-22-25723707)
1School of EECS, Purdue University, W. Lafayette, IN, USA, 2Applied Materials, Santa Clara, CA, USA

ABSTRACT

An Ultra-Fast On-The-Fly (UF-OTF) IDLIN technique having
1µs resolution is developed and used to study gate insulator
process dependence of NBTI in Silicon Oxynitride (SiON) p-
MOSFETs. The Nitrogen density at the Si-SiON interface and
the thickness of SiON layer are shown to impact temperature,
time, and field dependencies of NBTI. The plausible material
dependence of NBTI physical mechanism is explored.

INTRODUCTION AND BACKGROUND

Negative Bias Temperature Instability (NBTI) is a
serious reliability issue for SiON p-MOSFETs [1-8]. It is
important to understand the physical mechanism of NBTI,
i.e., whether it is dominated by generation of interface traps
(∆NIT) [3,4] or by hole trapping in pre-existing traps (∆Nh)
[1,2,5,6], to develop proper models [5-7,9-12] to extrapolate
stress data (high VG, short time) to operating (low VG, long
time) condition. Using Ultra-Fast Stress-Measure-Stress (UF-
SMS) scheme [7], it has recently been suggested that NBTI is
purely a ∆Nh related effect. Therefore, conclusions based on
relatively slower, conventional On-The-Fly (C-OTF) method
[13]; i.e., ∆NIT dominates NBTI in plasma nitrided oxides
(PNO), and both ∆NIT and ∆Nh contribute in thicker thermal
nitrided oxides (TNO) [8], need to be re-verified. As reliable
lifetime extrapolation depends on the mechanics of ∆Nh and/
or ∆NIT dependence of NBT degradation, it is important to
reconsider all older results by ultra-fast measurements.
In this work, an UF-OTF scheme (see Fig.1) is used to
study the time, temperature (T) and field (EOX) dependence of
NBTI in SiON p-MOSFETs having different N profile, N
density and film thickness (see Table-1). It is shown that the
Si-SiON interfacial N density and SiON thickness determine
the time exponent (n), T activation (EA) and EOX-dependence
(Γ) of NBTI, re-verifying previous conclusions [8]. Plausible
process dependence of ∆NIT and ∆Nh contribution to overall
NBTI is suggested. Such material dependence of NBTI has
not been appreciated by recent modeling attempts [5-7,9-12],
and must be considered for reliable lifetime estimation.

TIME DEPENDENCE

Under identical stress EOX and T, TNO (in spite having
lower total N dose) shows much larger IDLIN degradation than
PNO (Fig.2). The extracted degradation (Fig.3) for TNO is
not only larger, but also is significantly impacted by t0 delay
(time between application of stress VG and ID0 measurement
[14]). Note, extracted degradation (Fig.3) is related, but not
exactly equal to VT shift since mobility degradation is not
separated [15]. In a log-log plot (Fig.2), TNO clearly shows
much lower n (long time stress, for t>10s) than PNO for all t0
(Fig.4). Though n increases with higher t0 as expected [14],
for a given t0 it remains constant for a large range of stress VG
and T (Fig.5), and indicates the robustness of the underlying
physical mechanism that governs time dependence of NBTI.
For t0 range of 1µs to ~100µs, the observed variation in n is
small (Fig.4) and well within the error bar caused by noise in
ID0 measurement. As n saturates with reduction in t0, a faster
(t0<1µs) OTF is not likely to produce much different values
than reported with t0=1µs. Furthermore, lower n obtained for
UF-OTF results in longer extrapolated lifetime than C-OTF
(not shown), especially in PNO devices where the impact of
t0 on long-time degradation magnitude is small (Fig.3, LHS).
TNO shows (PNO does not show) log time dependence
when plotted in a semi-log scale (Fig.6), as reported also in
[2]. However, such log time dependence is not observed for
thinner TNO and PNO (not shown). Even for thick TNO, the
non-uniqueness of log-time dependent slope (as stress VG and
T are varied, not shown) makes it difficult to use it as reliable
time extrapolation scheme. However when degradation is
plotted as power-law time dependence, the robustness of n for
long stress (t>10s) holds for a wide range of stress VG and T
(n actually reduces slightly, by less than 0.01 for additional 2
decades in stress time due to reduction in stress EOX [14]).
This is true for a wide variety of devices studied (see Table-I)
and makes power-law time dependence physically justified
for extrapolation to end-of-life.

TEMPERATURE AND BIAS DEPENDENCE

PNO shows clear T dependence for the entire duration
of stress (Fig.7, LHS). TNO shows larger overall degradation
than PNO for all T, negligible T dependence at early stress
time (up to t~1-10ms), and weaker T dependence (compared
to PNO) at longer stress time (Fig.7, RHS). Note, the overall
difference in long-time degradation between TNO and PNO
can be attributed to a large extent to the early, T independent
degradation for TNO. For both PNO and TNO, long time T
dependence follows Arrhenius activation, as is apparent from
the T independence of n (Fig.5, LHS) [3]. Note that such T
independence of n has been observed for all devices used in
this study (not shown), irrespective of N dose, device type or
EOT (as described in Table-I).
TNO shows higher degradation magnitude over a wide
range of stress EOX, but much lower Γ compared to PNO
(Fig.8). For all EOX, the difference in degradation magnitude
as t0 is varied (fixed t-stress) is much larger (the difference is
more apparent at shorter stress time) for TNO compared to
PNO. However for both PNO and TNO, Γ is independent of
t0 and stress time. Note, lower Γ for TNO results in higher
degradation magnitude and lower lifetime as extrapolation is
done from stress to operating EOX (not shown).

PROCESS DEPENDENCE

Si-SiON interfacial N density is much larger for TNO
compared to PNO for a particular total N dose [16,17]. The
significantly different magnitude, time exponent, EOX and T

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dependence of degradation (see Figs.2-8) when PNO (D4) is
compared to TNO (D7) can be attributed to differences in N
density at Si-SiON interface (as XPS thicknesses are similar).
This is verified with observed reduction in n, EA and Γ as Si-
SiON interfacial N density is increased, i.e., when D3 (PNO)
is compared to and D5 (TNO+PNO) and D6 (TNO) having
similar XPS thickness (Fig.9). It is interesting to note that in
spite of having drastically different N profile (Fig.10, top)
and very different N density at the SiON-poly-Si interface,
similar Si-SiON interfacial N density for D5 and D6 results in
nearly identical NBTI magnitude (not shown), n, EA and Γ.
This shows that N density at SiON-poly-Si interface does not
play a significant role in NBTI. Therefore, though D4 has
higher total N dose and higher N density at the SiON-poly-Si
interface compared to D7, higher N density at the Si-SiON
interface for the later results in higher NBTI (see Figs.3,6,7).
For PNO, NBTI magnitude increases (not shown), while
n, EA and Γ reduces as total N dose is increased (D1 to D2),
or as total N dose is increased while XPS thickness is reduced
(D4 to D2), consistent with increase in Si-SiON interfacial N
density. However, note that the above process changes caused
a drastic increase in atomic N% (16% for D4, 22% for D1 but
41% for D2) and significant increase in Si-SiON interfacial N
density. For PNO having small N dose, Γ reduces slightly but
n and EA remain constant as XPS thickness is reduced (D4 to
D1). For TNO, n and EA increases but Γ remains constant as
XPS thickness is reduced at constant N dose (D7 to D6).

PHYSICAL MECHANISM

It is believed that NBTI is due to donor like ∆NIT [3,4]
and/or ∆Nh [1,2]. Tunneling of inversion layer holes (depends
on hole density, tunneling barrier and EOX) to Si−H bonds at
the Si-SiON interface and N-related trap sites in SiON bulk
respectively results in ∆NIT and ∆Nh (illustrated in Fig.10,
bottom). ∆NIT shows power law time dependence and strong
T activation [3,4,8,11]. Nature of diffusion species (released
from broken Si−) determines n. Classical diffusion suggests
n=0.5 for H+, =0.25 for H0, =0.16 for H2 (T independent) [10,
18]. Dispersive diffusion suggests n≤0.5 for H+, ≤0.25 for H0,
≤0.16 for H2 with T dependent n [12,19]. ∆Nh shows log time
dependence and weak T activation [1,2]. The N density at Si-
SiON interface impacts hole tunneling barrier [11] and Si−H
bond strength [20] and therefore both ∆NIT and ∆Nh. The N
density at SiON bulk governs N-related trap sites and only
∆Nh. However, ∆Nh is more efficient near Si-SiON interface
especially for thicker SiON due to higher tunnel in (from
substrate) and lower tunnel out (to poly-Si) probabilities (see
Fig.10, bottom). Generation of both NIT (stronger time and T
dependence) and Nh (weaker time and T dependence) would
reduce the n and EA of overall ∆VT during NBT stress [8].
Unless N dose is high, NBTI in PNO (devices treated
with proper Post Nitridation Anneal; have lower N density at
Si-SiON interface) is likely dominated by ∆NIT, as evident
from clear T activation for entire stress duration (Fig.7, LHS),
relatively higher and thickness independent n and EA (Fig.9;
D1,D4). However as n is independent of T (Fig.5, RHS), the
slightly lower n (~0.12) than predicted by basic theory (with
H2) is unlikely due to strong dispersive diffusion [12,19] and
needs attention. Increase in total N dose results in higher N
density at (or near) Si-SiON interface (D2>D3>D1,D4), and
resultant reduction in n and EA is possibly due to additional
∆Nh contribution (though ∆NIT also increases [11,20]). As Si-
SiON interfacial N density is very high for TNO (D6,D7) and
TNO+PNO (D5), the contribution due to ∆Nh is significant
and large reduction is observed in n and EA (see Fig.9). In
spite of similar interfacial N density for D6 and D7, lower
tunnel out probability (see Fig.10, bottom) and higher charge
trapping volume for the later cause higher ∆Nh and lower n
and EA. Unlike D4, NBTI in D7 is likely dominated by ∆Nh
as evident from very high, T independent degradation at early
stress time (Fig.7, RHS), log time dependence (Fig.6, RHS)
[1,2] and very low n and EA for long time (t>10s) stress (see
Fig.9). However, the time constant and T (in)dependence of
∆Nh needs careful attention to explain the T independence of
n at longer stress time. Finally, Si-SiON interfacial N density
similarly influences the EOX dependence of ∆NIT and ∆Nh by
influencing the hole tunneling barrier, and hence Γ (though
reduces for higher N density) is independent of t0 and stress
time (see Fig.8).

CONCLUSION

Using an UF-OTF technique, NBTI is studied in SiON
p-MOSFETs having wide range of N density, N profile and
SiON thickness. Measured NBTI parameters (n, Γ and EA)
show strong dependence on Si-SiON interfacial N density
and film thickness. Experimental results are explained by
process dependence of relative ∆NIT and ∆Nh contribution to
overall NBTI. In general, material dependence results from
UF-OTF are consistent with that obtained earlier by C-OTF.

References:

[1] V. Huard et al., p.40, IRPS 2004
[2] M. Denais et al., p.109, IEDM 2004
[3] D. Varghese et al., p.684, IEDM 2005
[4] A. T. Krishnan et al., p.688, IEDM 2005
[5] T. Yang et al., p.92, VLSI 2005
[6] H. Reisinger et al., p.448, IRPS 2006
[7] C. Shen et al., p. 12.5.1, IEDM 2006
[8] S. Mahapatra et al., p.1, IRPS 2007
[9] M. A Alam, p.345, IEDM 2003
[10] S. Chakravarthi, p.273, IRPS 2004
[11] A. E. Islam et al., p.12.4.1, IEDM 2006
[12] T. Grasser et al., p.268, IRPS 2007
[13] S. Rangan et al., p.341, IEDM 2003
[14] A. E. Islam et al., APL, v.90, 083505, 2007
[15] A. E. Islam et al., IEDM 2007
[16] J. R. Shallenberger et. al., JVST-A, v.17, p.1086, 1999.
[17] S. Rauf et. al., JAP, v.98, 024305, 2005.
[18] M. A.Alam, NBTI Tutorial, IRPS 2006
[19] B. Kaczer et al., p.381, IRPS 2005
[20] S. S. Tan et. al., SSDM, p.70, 2003.

Page 3

D Type Base N XPS EOT
1 PNO 15 2.8 18.5 14.0
2 PNO 15 5.8 21.0 12.3
3 PNO 20 5.3 23.2 15.6
4 PNO 25 3.1 28.1 23.5
5
TNO+
PNO
20
0.8+
5.1
22.8 13.1
6 TNO 20 0.8 21.1 18.5
7 TNO 25 0.8 26.1 22.0
DCPS
SMU
DSO
IVC
PGU
Trigger DSO & SMU, D at DCPS

Start ID sampling

Trigger PGU, apply VG pulse

Capture ID transient (DSO)

Switch D from DCPS to SMU

Continue ID capture (SMU)
Fig.1. Ultra-Fast On-The-Fly (UF-OTF) IDLIN setup and measurement sequence
during NBTI stress. Initial IDLIN transient (1µs to 30ms) is captured using IV
Converter-DSO at S, with DC Power Supply at D. Long time IDLIN transient is
captured using SMU at D. Use of DCPS helps prevent RC related issues that
affect IDLIN transients in early time. IV converter is set for a gain of 103 – 104.
10
-7
10
-5
10
-3
10
-1
10
1
10
3
0.90
0.92
0.94
0.96
0.98
1.00
1.02
TNO (8E14 cm
-2)
EOT=22A
O
IDLIN (normalized)
stress time (s)
EOX ~ 8.5 MV / cm
T = 1250C
PNO (3E15 cm
-2)
EOT=23.5A
O
Fig.2. Captured IDLIN transients for 9 decades
of stress time for PNO (D4) and TNO (D7)
devices. A 10 point adjacent averaging is
done to smooth as measured data.
10
-6
10
-3
10
0
10
3
3x10
-3
10
-2
10
-1
PNO
EOX ~ 8.5 MV / cm
T = 1250C
t0 delay:
1µs
1ms
30ms
∆ID/ID0*(VG-VT0) (V)
stress time (s)
10
-6
10
stress time (s)
-3
10
0
10
3
3x10
-3
10
-2
10
-1

TNO
t0 delay:
1µs
1ms
30ms
EOX ~ 8.5 MV / cm
T = 125
0C
Fig.3. Extracted time evolution of degradation from IDLIN transient of Fig.2, using
ID0 obtained at different time after application of stress VG (t0 delay) for PNO
(LHS) and TNO (RHS) devices.
10
-6
10
-5
10
-4
10
-3
10
-2
0.04
0.06
0.08
0.10
0.12
0.14
0.16
PNO
TNO
t0 delay (s)
time exponent (t=10-1000s)
T=125
EOX=8.5MV/cm
OC
Fig.4. Extracted power-law time exponent (linear
fit from 10s to 1000s) for degradation calculated
from Fig.2 as a function of t0 delay for PNO and
TNO devices.
2.22.42.62.83.03.2
0.04
0.06
0.08
0.10
0.12
0.14
0.16
T=125
OC
PNO
TNO
stress -VG (V)
time exponent (t=10-1000s)
t0 delay:1µs 1ms
Fig.5. Extracted power-law time exponent (linear fit from 10s to 1000s) as a
function of stress VG (LHS) and T (RHS) for t0 delay of 1µs and 1ms for PNO and
TNO devices (identical to Fig.2). Lines are guide to the eye. Maximum error in
time exponent due to noise induced scatter in ID0 is ± 0.005.
2550
Temperature (
75100 125 150
OC)
0.04
0.06
0.08
0.10
0.12
0.14
0.16
t0 delay:1µs1ms
EOX ~ 8.5 MV / cm
TNO
PNO
Table-I. Process details of devices used.
Starting oxide (Base), XPS thickness and EOT
are in AO, Nitrogen dose (N) in 1015 cm-2. PNO:
Plasma Nitrided Oxide, TNO: Thermal Nitrided
Oxide. D4 and D7 are used in Figs. 2 through
Fig.8.

Page 4

Fig.6. Time evolution of degradation for different stress VG plotted in a semi-log
scale for PNO (LHS) and TNO (RHS) devices (identical to Fig.2). Measurements
were performed with t0 of 1µs.
10
-6
10
stress time (s)
-3
10
0
10
3
0.00
0.02
0.04
0.06
0.08
0.10
PNO
-VG (V)
2.5
2.9
3.3
T=125
OC
-∆ID / ID0 * [VG - VT0] (V)
10
-6
10
stress time (s)
-3
10
0
10
3
0.00
0.05
0.10
0.15
0.20
0.25
TNO
T=125
OC
-VG (V)
3.3
2.9
2.5
10
-6
10
stress time (s)
-3
10
0
10
3
3x10
-3
10
-2
10
-1
EA (t:10-1000s) ~ 0.08eV
PNO
T (
0C)
125
85
55
-∆ ID / ID0 * [VG - VT0] (V)
EOX ~ 8.5MV/cm
10
-6
10
stress time (s)
-3
10
0
10
3
3x10
-3
10
-2
10
-1
EA (t:10-1000s) ~ 0.04eV
TNO
T (
0C)
125
85
55
EOX ~ 8.5 MV / cm
Fig.7. Time evolution of degradation for PNO (LHS) and TNO (RHS) devices
(identical to Fig.2) at different stress T, measured for t0 delay of 1µs. Estimated
error in EA is approximately ± 0.005eV.
6.5 7.0 7.5 8.0 8.5 9.0 9.5
EOX (MV / cm)
4x10
-3
10
-2
10
-1
slope (Γ) = 0.6
PNO
t0 delay:
1µs
1ms
T=125
OC
1000s stress
1s stress
-∆ID/ID0*[VG - VT0] (V)
6.5 7.0 7.5 8.0 8.5 9.0 9.5
EOX (MV / cm)
Fig.8. EOX dependence of degradation for
PNO and TNO devices (identical to Fig.2)
measured after 1s and 1000s of stress, for t0
delay of 1µs and 1ms. Reported Γ is in
cm/MV, with maximum error of approximately
± 0.02 cm/MV.
4x10
-3
10
-2
10
-1
-∆ID/ID0 * [VG - VT0] (V)
TNO
slope (Γ) = 0.32
T=125
OC
t0 delay:
1µs
1ms
1s stress
1000s stress
Fig.9. Material dependence (Table-I) of NBTI parameters: (LHS) power-law time
exponent (linear fit from 10s to 1000s) of degradation, activation energy (t:10-
1000s) and (RHS) slope for field dependence (Γ). Obtained n reduces by less than
0.01 for additional 2 decades of stress time. Maximum error in n is ± 0.005, in EA is
± 0.005eV, and in Γ is ± 0.02 cm/MV.
Fig.10. (Top) Schematic N profile for devices in
Table-I, and (Bottom) plausible NBTI physical
mechanism. Dashed lines towards LHS and
RHS respectively denote Si-SiON and SiON-
poly-Si interfaces. A and B denote hole trap
positions near substrate and near gate.
SiH
p
A
B
n-Si
SiON
p+-poly
Tunneling barrier (T.B.)
PNO
TNO+PNO
TNO
N (log scale)
1234567
0.02
0.04
0.06
0.08
0.10
0.12
n, EA (eV)
Device (Table-I)
n
EA
12
Device (Table-I)
34567
0.2
0.3
0.4
0.5
0.6
Γ (cm/MV)