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IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 2, FEBRUARY 2009
A Comparative NBTI Study of HfO2, HfSiOx, and
SiON p-MOSFETs Using UF-OTF IDLIN Technique
Shweta Deora, Vrajesh Dineshchandra Maheta, Gennadi Bersuker, Christopher Olsen, Khaled Z. Ahmed,
Raj Jammy, and Souvik Mahapatra
Abstract—The time, temperature, and oxide-field dependence
of negative-bias temperature instability is studied in HfO2 /TiN,
HfSiOx /TiN, and SiON/poly-Si p-MOSFETs using ultrafast
on-the-fly IDLIN technique capable of providing measured degradation from very short (approximately microseconds) to long
stress time. Similar to rapid thermal nitrided oxide (RTNO) SiON,
HfO2 devices show very high temperature-independent degradation at short (submilliseconds) stress time, not observed for plasma
nitrided oxide (PNO) SiON and HfSiOx devices. HfSiOx shows
lower overall degradation, higher long-time power-law exponent,
field acceleration, and temperature activation as compared to
HfO2 , which are similar to the differences between PNO and
RTNO SiON devices, respectively. The difference between HfSiOx
and HfO2 can be attributed to differences in N density in the SiO2
IL of these devices.
Index Terms—Activation energy, field acceleration, high-k dielectric, hole trapping, interface traps, negative-bias temperature
instability (NBTI), plasma oxynitride, p-MOSFET, thermal oxynitride, time exponent.
I. I NTRODUCTION
N
EGATIVE-BIAS temperature instability (NBTI), resulting in shifts in device parameters (linear drain current
IDLIN , threshold voltage VT , etc.) is a serious reliability concern in p-MOSFETs having Si-oxynitride (SiON) [1]–[4] and
high-k [5]–[8] gate dielectrics. It is now well known that defects
generated during NBT stress recovers after stress is removed,
and conventional stress–measure–stress (SMS) methods suffer
from recovery-related artifacts that result in incorrect degradation magnitude, time, and temperature (T ) dependence of
NBTI [9], [10]. To overcome this issue, ultrafast (UF) SMS
[11] and UF on-the-fly (UF-OTF) IDLIN [4] techniques were
developed. UF-SMS provides direct estimation of VT shift,
although it cannot be used to measure degradation at very short
stress time. UF-OTF can be used to determine degradation from
very short (approximately microseconds) to long stress time,
although it requires postprocessing to convert IDLIN degradation to VT shift as discussed in [12]. However, while NBTI in
SiON p-MOSFETs has been studied using UF-SMS or UF-OTF
techniques [4], [11], [13], to the best of our knowledge, most
Manuscript received August 29, 2008. First published December 12, 2008;
current version published January 28, 2009. The review of this letter was
arranged by Editor A. Chatterjee. IIT Bombay acknowledges financial support
from SRC/GRC.
S. Deora, V. D. Maheta, and S. Mahapatra are with the Department of Electrical Engineering, Indian Institute of Technology Bombay, Mumbai 400076,
India (e-mail: [email protected]).
G. Bersuker and R. Jammy are with SEMATECH, Austin, TX 78741 USA.
C. Olsen and K. Ahmed are with Applied Materials, Inc., Santa Clara,
CA 94085 USA.
Digital Object Identifier 10.1109/LED.2008.2009235
studies of NBTI in high-k p-MOSFETs [5]–[8] were done using
conventional SMS and calls for a reevaluation by UF methods,
particularly to probe the degradation at early stress time and to
have recovery-artifact-free degradation at longer stress time.
In this letter, NBTI stress-induced degradation in HfSiOx
and HfO2 p-MOSFETs with SiO2 (N) interlayer (IL) and TiN
gate is studied using the UF-OTF IDLIN method [4]. The time
and T dependence of degradation at short (submillisecond)
stress time is shown to be significantly different for HfO2
as compared to HfSiOx devices. For longer stress time, the
power-law time exponent (n), T activation (EA ), and IL field
(EOX ) dependence are also shown to be different for HfO2 as
compared to HfSiOx stacks. It is shown that time, T , and EOX
dependence of NBTI of HfSiOx and HfO2 are similar to that of
plasma nitrided oxide (PNO) and rapid thermal nitrided oxide
(RTNO) SiON/poly-Si devices, respectively. The differences in
NBTI between HfO2 and HfSiOx stacks can be attributed to
the difference in N density in the SiO2 IL [16], [17]. This letter
shows that NBTI is strongly influenced by the IL quality, and
N content in IL plays a very important role, consistent with
earlier reports [8], [15], [18].
II. R ESULTS AND D ISCUSSION
Experiments were performed on fully processed pMOSFETs with 2- and 3-nm-thick HfSiOx and HfO2 layers
on 1-nm-thick SiO2 (N) IL and TiN metal gate. The high-k
layers were deposited using ALD on thermally grown SiO2
and was followed by postdeposition anneal (PDA) in NH3 . For
comparison, PNO SiON devices (identical starting base oxide
thickness but different N dose that results in different Si/SiON
interfacial N density) and RTNO SiON device (having highest
interfacial N density) were also studied. The details of UF-OTF
measurement setup has been presented elsewhere [4].
Extracted degradation from measured IDLIN transients,
ΔV = −ΔIDLIN /IDLIN0 ∗ (VGSTR − VT 0 ) [2], is shown in
Fig. 1 for HfSiOx and HfO2 devices for two different stress
T and time-zero (t0 ) delay. Note that ΔV is proportional to
but different from ΔVT (obtained using SMS) as mobility
degradation is not taken into account [12]1 ; VGSTR is stress
bias; VT 0 is prestress VT ; IDLIN0 is the peak IDLIN ; ΔIDLIN is
degradation in IDLIN from IDLIN0 ; and t0 delay implies delay
between application of VGSTR and measurement of IDLIN0 [4].
1 As described in [12], mobility correction can be done on devices having
negligible hole trapping and results in parallel shifts in degradation versus time
curve in a log–log plot, i.e., it impacts degradation magnitude but does not
impact power-law time exponent n.
0741-3106/$25.00 © 2009 IEEE
DEORA et al.: COMPARATIVE NBTI STUDY OF HfO2 , HfSiOx , AND SiON p-MOSFETs
Fig. 1. Time evolution of measured ΔV for (left-hand side) HfSiOx and
(right-hand side) HfO2 at different temperature and t0 delay under identical
VG stress.
Fig. 2. Power-law time exponent (linear fit from 10 to 1000 s) as a function of
EOX for (a) 2- and 3-nm-thick HfO2 and HfSiOx and (b) PNO (with different
N dose resulting in different N density) and RTNO (with highest interfacial
N density) under constant t0 delay and temperature.
Note that HfO2 shows very high degradation at short stress
time (submilliseconds) as compared to HfSiOx when measured
using t0 = 1 μs. It is also important to note that this large
short-time degradation seen for HfO2 is T independent, while
HfSiOx shows clear T dependence. Such differences between
HfO2 and HfSiOx have been observed for a wide range of stress
EOX and T , which cannot be captured by using conventional
SMS [5] or conventional OTF (having t0 of 1 ms) [2], [3]
measurements, and hence, such differences between HfSiOx
and HfO2 stacks were never reported before. Reduction in
measured degradation at higher t0 (due to noncapture of IDLIN
degradation at short stress time) is larger for HfO2 as compared
to HfSiOx , which is more prominent at shorter stress time, with
stronger reduction seen when t0 delay is increased from 1 μs to
1 ms (shown) than from 1 ms to 10 ms (not explicitly shown).
The time evolution, impact of t0 delay, and stress T for HfO2
are similar to RTNO, while HfSiOx shows similar behavior as
PNO SiON devices [13].
It is evident as shown in Fig. 1 that the time evolution of
ΔV does not show a single power-law dependence for the
entire (short to long) stress duration. However, the long-time
(t > 1 s) degradation shows power-law dependence, with ΔV
153
Fig. 3. T dependence of ΔV at t-stress of 100 s for (a) 2- and
3-nm-thick HfO2 and HfSiOx and (b) PNO(with different N dose resulting in
different N density) and RTNO (with highest interfacial N density) at constant
t0 delay and VG /EOX stress.
Fig. 4. EOX dependence of ΔV at t-stress of 100 s for (a) 2- and 3-nm-thick
HfO2 and HfSiOx and (b) for PNO (with different N dose resulting in different
N density) and RTNO (with highest interfacial N density) at constant t0 delay
and T stress.
asymptotically converging to a “final” power-law exponent.
Fig. 2 shows extracted time exponent (n) for t-stress > 10 s
at different stress EOX for HfO2 and HfSiOx devices having
different high-k thicknesses and for PNO (with different interfacial N density) and RTNO devices. Note that, although
n is different for different devices, it is independent of stress
VG (EOX ), suggesting absence of trap generation in high-k
or SiON bulk during NBT stress [14]. Although not explicitly
shown, n was found to be independent of stress T as well for all
the devices used in this letter. The EOX and T independence of
n validates the power-law time dependence of NBTI at long
stress time. HfSiOx shows higher n as compared to HfO2 ,
although n is insensitive to high-k thickness. Reduction in
n is seen for PNO with increase in interfacial N density,
while RTNO device having highest interfacial N density shows
lowest n.
Fig. 3 shows measured ΔV at fixed stress time but different
stress T for HfO2 and HfSiOx devices with different high-k
thicknesses and for PNO (with different interfacial N density)
and RTNO devices. Obtained EA , although not sensitive to
high-k thickness, is larger for HfSiOx as compared to HfO2 .
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IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 2, FEBRUARY 2009
Reduction in EA is seen for PNO with increase in interfacial
N density, while RTNO device having highest interfacial N
density shows lowest EA . Note that n and EA show similar
process dependence, which has been discussed in detail for
SiON devices in [13]. Fig. 4 shows measured ΔV at fixed
stress time as a function of EOX in IL for HfO2 and HfSiOx
devices with different high-k thicknesses and as a function of
EOX for PNO (with different interfacial N density) and RTNO
devices. The EOX -dependent slope (Γ) is lower for HfO2 as
compared to HfSiOx , although Γ is independent of high-k layer
thickness for HfO2 but increases with high-k layer thickness for
HfSiOx . Reduction in Γ is seen for PNO as interfacial N density
(proportional to N%) is increased, while RTNO device having
lowest N% have highest interfacial N density and, therefore,
shows lowest Γ [4], [13].
We now explain the possible physical mechanism behind
the difference in HfSiOx and HfO2 devices.2 As Γ is a strong
indicator of interfacial N density (i.e., Γ is higher for lower
N density and vice versa, see [13] for details), N density in
SiO2 IL after identical high-k PDA is higher for HfO2 as
compared to HfSiOx . As both 2- and 3-nm HfSiOx devices
underwent similar PDA, higher Γ (implying lower N) seen for
higher thickness implies better N blocking capability of the
HfSiOx layer. HfO2 on the other hand is a poor N blocker, as
reported in [16] and [17], which results in higher N content
in the IL and lower Γ. Presence (in RTNO and HfO2 ) or
absence (in PNO and HfSiOx ) of large T -independent shorttime degradation (Fig. 1) clearly suggest presence or absence
of large hole trapping (ΔVh ) component in N-related traps [4],
[11], particularly at early stress time. As ΔVh saturates fast
and has weaker T activation [3]–[5], [11], [12], it causes lower
n (Fig. 2) and EA (Fig. 3) for ΔV (= ΔVIT + ΔVh ) when
added to generated interface traps (ΔVIT ) that show power-law
time dependence and stronger T activation [2], [3]. Relatively
high n and EA for HfSiOx implies lower ΔVh due to lower
N density in SiO2 IL, similar to that observed for PNO SiON
devices [13].
III. C ONCLUSION
The use of UF-OTF technique has helped identify a very
important difference between HfO2 and HfSiOx stacks. Similar to RTNO SiON, HfO2 shows very high T -independent
degradation at short stress time. This is not seen for HfSiOx
that behave similar to PNO SiON films. Similar to differences
between RTNO and PNO SiON devices, HfO2 shows higher
overall degradation, lower EA , and lower EOX -dependent slope
when compared to HfSiOx stacks. This is uniquely attributed to
higher N density in SiO2 IL for HfO2 as compared to HfSiOx
that results after similar PDA. The importance of SiO2 IL in
governing high-k NBTI is consistent with published reports.
2 An alternative explanation relates the difference between HfO and HfSiO
x
2
to differences in amount of Hf diffusion and oxygen deficiency in the IL [15].
The change in IL is more profound in HfO2 .
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