Analysis of anomalous traps measured by charge pumping technique in HfO2/metal
gate n-channel metal-oxide-semiconductor field-effect transistors
Szu-Han Ho, Ting-Chang Chang, Ying-shin Lu, Wen-Hung Lo, Ching-En Chen, Jyun-Yu Tsai, Hua-Mao Chen,
Chi-Wei Wu, Hung-Ping Luo, Guan-Ru Liu, Tseung-Yuen Tseng, Osbert Cheng, Cheng-Tung Huang, and
Simon M. Sze
Citation: Applied Physics Letters 101, 233509 (2012); doi: 10.1063/1.4769444
View online: http://dx.doi.org/10.1063/1.4769444
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/23?ver=pdfcov
Published by the AIP Publishing
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Analysis of anomalous traps measured by charge pumping technique in
HfO
2/metal gate n-channel metal-oxide-semiconductor field-effect transistors
Szu-Han Ho,1Ting-Chang Chang,1,2,a)Ying-shin Lu,2Wen-Hung Lo,2Ching-En Chen,1
Jyun-Yu Tsai,2Hua-Mao Chen,3Chi-Wei Wu,1Hung-Ping Luo,1Guan-Ru Liu,2
Tseung-Yuen Tseng,1Osbert Cheng,4Cheng-Tung Huang,4and Simon M. Sze1,2,5
1
Department of Electronics Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 2
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 3
Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
4
Device Department, United Microelectronics Corporation, Tainan Science Park, Taiwan 5
Department of Electronics Engineering, Stanford University, Stanford, California 94305, USA
(Received 20 September 2012; accepted 16 November 2012; published online 7 December 2012) This letter investigates anomalous traps measured by charge pumping technique in high voltage
in HfO2/metal gate n-channel metal-oxide-semiconductor field-effect transistors. N-Vhigh level
characteristic curves with different duty ratios indicate that the electron discharge time dominates the value of N for extra traps. By fitting ln (N (tbase level¼ 2.5ls)-N (tbase level))-Dtbase level at
different temperatures and computing the equation t¼ s0 exp (ae,SiO2dSiO2þ ae,HfO2dHfO2,trap),
results show that these extra traps measured by the charge pumping technique at high voltage can be attributed to high-k bulk shallow traps.VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4769444]
With the scaling down of metal-oxide semiconductor
field-effect transistors (MOSFETs), conventional SiO2-based
dielectric is only a few atomic layers thick, causing gate cur-rent to rise, power dissipation to increase, and performance to degrade. In addition, conventional SiO2-based dielectrics
have approached their physical limits. Consequently, replac-ing SiO2-based dielectrics with high-k based dielectrics is a
valid solution to these problems. Furthermore, high-k/metal gates can be integrated with techniques such as silicon on insulator (SOI),1–3 strained-silicon,4,5 and multi-gate to improve device characteristics. As recommended in the International Technology Roadmap for Semiconductors, Hf-based dielectrics have been heavily studied to replace
SiO2-based dielectrics in recent years.6–9 However, with
changes in Hf-based dielectrics, many measurement ques must be corrected, especially charge pumping techni-ques. For instance, with a decrease in frequency, charge pumping current (Icp) decreases in conventional SiO2-based
dielectrics since carriers have enough time to discharge from interface shallow traps. Conversely, with a decrease in
fre-quency, Icp increases in Hf-based dielectrics since carriers
have enough time to tunnel into high-k bulk traps.10Charge
pumping techniques play an important role to inspect defect. Thus, this study mainly focuses on anomalous traps meas-ured by the charge pumping technique at high voltage, with
the devices used in this study HfO2dielectrics n-MOSFETs.
The causes of the anomalous traps are explained in this letter.
The HfO2/metal gate n-MOSFETs used in this study
were fabricated by the gate first process. First, a high quality 1 nm-thick thermal oxide was grown as an interfacial layer.
Second, 3 nm of HfO2 dielectrics were sequentially
depos-ited by atomic layer deposition. Third, 10 nm of TixN1xwas
deposited by radio frequency physical vapor deposition, because metal gates can eliminate gate depletion and resist
remote phonon scattering.11,12 Next, poly-Si was deposited
as a low resistance gate electrode. Finally, the dopant
activa-tion was performed at 1025C. The n-MOSFETs are
meas-ured by the charge pumping technique with different duty ratios at different temperatures. A pulse train with
low-voltage of 0.6 V, high-voltage from 0 V to 1.8 V, and
fre-quency of 200 kHz was applied on the gate terminal. Ig-Vg
transfer curves were measured with the source, drain, and
body terminals all grounded, with Vg given from 0 V to
1.8 V. Then through body floating (BF), source/drain floating (SDF), and source/drain/body all grounded (SDB) process, the current path and carrier polarity can be confirmed. Next, the Ig-Vgcurve is fitted by Frenkel-Poole current and
tunnel-ing current. All experimental curves were measured ustunnel-ing an Agilent B1500 semiconductor parameter analyzer.
Figure1shows the N-Vhigh levelcharacteristic curves at
different duty ratios. N is the number of traps, and duty ratio¼ (thigh level/tcycle). Clearly, N-Vhigh level characteristic
curves are the same in the Vhigh level< 1.1 V with an increase
in duty ratio. This information implies that interface traps detected by the charge pumping technique are not dependent on tbase level. This is because the time for electrons in the
interface traps to recombine with holes is very short. Hence, the numbers of interface traps measured by Icpare not
sensi-tive to duty ratio. On the contrary, N decreases with a rise in duty ratio in Vhigh level> 1.1 V. Furthermore, only interface
traps can be measured with a duty ratio value of 98%. In other words, extra traps nearly disappear. The detrap time (tbase level) of electron dominates the value of N such that N
becomes smaller with a decrease in detrap time. This demon-strates that electrons need time to discharge. Thus, it is nec-essary to know the relationship between N and the detrap time (tbase level) in Vhigh level> 1.1 V. The inset of the Fig.1
a)
Electronic mail: tcchang@mail.phys.nsysu.edu.tw.
shows Isub-Vg curves with source, drain, and body all
grounded. It can be first observed that body current is small. In addition, N (Icp) is dependent on the detrap time. Hence,
these results mean that N measured by the charge pumping technique is not caused by gate leakage current, but rather high-k bulk traps that have been detected, as shown in the
energy band diagram of Fig.2.
The inset of Figure2shows N-tbaselevelcurve at 30C, for
Vg¼ Vt(0.67 V)þ 1.1 V, as shown by the dotted red line in
Fig. 1. Since N can also represent the numbers of electrons
discharged from high-k bulk traps, N (tbase level¼ 2.5ls)-N
(tbase level) is the number of electrons still charged in the
high-k bulk traps at tbase level, an important parameter. Figure2
shows ln (N (tbase level¼ 2.5ls) N (tbase level)) Dtbase level
curves fitted from the inset of Figure2. Dtbase levelis tbase level
tbase level(0.1ls). Dtbase levelis the time for electrons to
dis-charge from traps. Clearly, fitting these curves can be accom-plished with straight lines even for different temperatures. In addition, slopes are also similar at these temperatures. The
discharge equation can be described by dQ(t)/dt¼ DQ(t)/
sp¼ epDQ(t), DQ(t)¼ DQ(0)exp(ept),
13
where ep is the
escape probability and spis the average escape time. Thus,
slope is indicated by epor 1/spwith epnot dependent on
tem-perature. Hence, epmay be the tunneling probability. The
av-erage value of the slope at different temperatures (maverage) is
1.534 106, and sp,averageis 6.52 107(s). Now the value of
tunneling distance can be determined by using sp,averageand
can verify that the traps are actually in the high-k bulk. The relationship between tunneling time and distance can be approximated by t¼ s0 exp (aex), ae¼ 2(2meq/0/h
2
)0.5,14,15 where s0 is an electron tunneling characteristic time, me is
electron effective mass for SiO2, and q/0is an effective
tun-neling barrier height. However, because electrons are
tunnel-ing through two layers, SiO2 and HfO2, this equation can
be described by t¼ s0 exp (ae,SiO2dSiO2þ ae,HfO2dHfO2,trap),
ae,SiO2¼ 2(2 me,SiO2q/0,SiO2/h 2
)0.5, and ae,HfO2¼ 2(2me, HfO2q/0,HfO2/h
2
)0.5, where dSiO2 is the thickness of SiO2,
dHfO2,trap is the distance from traps to interlayer between
SiO2 and HfO2, me,SiO2and me,HfO2 are effective mass in
SiO2 and HfO2, respectively, and q/0,SiO2 and q/0,HfO2
are effective tunneling barrier height in SiO2 and HfO2,
respectively. s0, me,SiO2, and me,HfO2 can be obtained from
other research.14,16,17 Thus, only one parameter (/0,HfO2) is
unknown.
The inset in Figure 3(a) shows Ig-Vg characteristic
curves with BF, SDF, and SDB for distinguishing gate cur-rent at 30C. Clearly, the Ig-Vgcharacteristic curve in BF is
similar to that in SDB, and the Ig-Vgcharacteristic curve in
SDF is much smaller than either. These results indicate that electrons transfer from source/drain to the gate, rather than holes transferring from gate to body. Clearly, section A indi-cates the tunneling current in Fig.3(b), from Vg¼ 0.35 V to
Vg¼ 0.75 V, while section B is Frenkel-Poole current, shown
in the inset of Fig. 3(d), from Vg¼ 1.1 V to Vg¼ 1.8 V.
uB¼ 0.49 eV can be obtained by fitting the Frenkel-Poole
mechanism in the inset in Fig. 3(d).18–20 Figure3(c) shows the N-Vhigh levelcharacteristic curves at different duty ratios.
It can be observed that when Vg< 1.1 V, N is interface traps
(Nit) only. On the contrary, when Vg> 1.1 V, N is both
high-k bulhigh-k shallow traps (Nhkst) and Nit. A comparison of Fig.
3(c)with Fig.3(a)shows that N is only Nitwhen gate current
is tunneling current and Frenkel-Poole current is very small.
Conversely, N is both Nit and Nhkst when gate current is
Frenkel-Poole current. This indicates that bulk traps charging electrons via the Frenkel-Poole mechanism and the traps
dis-charging electrons through Icpmay be the same. In order to
confirm this theory, uB¼ /0,HfO2¼ 0.49 eV is substituted
into the equation t¼ s0exp (ae,SiO2dSiO2þ ae,HfO2dHfO2,trap),
with ae,SiO2¼ 2(2 me, SiO2q/0, SiO2/h 2
)0.5, and ae,HfO2
¼ 2(2 me,HfO2q/0,HfO2/h2)0.5, where me, SiO2 is 0.95m0,
me,HfO2 is 0.03m0, s0¼ 6.6 1014(s), dSiO2 is 10 A˚ , and
/0,SiO2 ¼ 1.6 eV þ /0,HfO2. Finally, it can be acquired that
dHfO2,trapis 13 A˚ . This is a reasonable value. While Vg
tran-sits from Vhigh levelto Vbase level, electrons in the high-k bulk
shallow traps near the gate and substrate discharge to gate and source/drain, respectively. Hence, only traps in the mid-dle of the high-k bulk shallow traps can be measured by the charge pumping technique. In addition, the falling time is
1 107(s), which matches the time at duty ratio of 98%,
FIG. 1. N-Vhigh levelcharacteristic curves at different duty ratios by charge pumping measurement. Inset shows Isub-Vg curve with source, drain, and body all grounded.
FIG. 2. ln (N (tbase level¼ 2.5 ls) N (tbase level)) - Dtbaselevelcurves at differ-ent temperatures at Vg¼ Vtþ 1.1 V. Inset shows N-tbaselevelcurve in 30C at Vg¼ Vt(0.67 V)þ 1.1 V. The energy band diagram shows Icpis caused by bulk trap, not gate leakage current.
that of 1 10 7(s). This implies that electrons in the mid-dle of the high-k bulk shallow traps have no time to tunnel to the substrate in the accumulation area. Thus, only interface traps can be measured by Icpat a duty ratio value of 98%.
Combining the results above, the energy band diagram of the model for charge pumping measurement with
anoma-lous traps can be acquired, as shown in Fig.4. Figures4(a)
and 4(b) show the energy band diagram when pulses are
applied to gate with the charge pumping technique at the high and base levels, respectively. When 1.1 V > Vhigh level
> Vt, gate current is tunneling-path dominated, leading to
high-k bulk shallow traps not charging electrons. Electrons merely charge to interface traps, as shown in Fig.4(a). Sub-sequently, holes recombine with electrons in the interface
traps at Vbase level, as shown in Fig. 4(b). Thus, Icp only
detects Nit. On the contrary, when Vg> 1.1 V, the gate
current is dominated by the Frenkel-Poole mechanism, caus-ing high-k bulk shallow traps to charge electrons. Next, elec-trons charge in interface traps and high-k bulk shallow traps,
as shown in Fig.4(c). Then holes recombine with electrons
in the interface traps at Vbase level, and electrons discharge
from high-k bulk shallow traps to the body by the tunneling
mechanism. Therefore, Icpmeasures not only interface traps
but also high-k bulk shallow traps.
In summary, N-Vhigh levelcharacteristic curves are nearly
the same in value for Vhigh level< 1.1 V with a rise in duty
ra-tio. However, N decreases with an increase in duty ratio for Vhigh level> 1.1 V. This indicates that the electron discharge
FIG. 3. (a) Ig-Vgcharacteristic curves with SDB. Inset shows Ig-Vg characteristic curves with BF, SDF, and SDB. (b) Gate current in section A is fitted by tunneling model. (c) N-Vhigh levelcharacteristic curves with different duty ratios for charge pump-ing measurement. (d) Gate current in sec-tion B is fitted by Frenkel-Poole model.
FIG. 4. The energy band diagram of high-k/ metal gate MOSFETs with charge pumping measurement (a) in Vhigh level and (b) in Vbase level, when Vhigh level< 1.1 V. The energy band diagram of high-k/metal gate MOSFETs with charge pumping measurement (c) in Vhigh leveland (d) in Vbase level, while Vhigh level > 1.1 V.
time dominates the value of N. In addition, the values of
ep obtained by the slope of ln (N (tbase level¼ 2.5 ls)-N
(tbase level))-Dtbase levelare independent of temperature. Hence,
electrons discharge from high-k bulk shallow traps via the tunneling mechanism. The distance of traps can be acquired by the equation t¼ s0 exp (ae,SiO2dSiO2 þ ae,HfO2dHfO2,trap)
and /0,HfO2¼ 0.49 eV, and /0,HfO2can be obtained from
fit-ting the gate current with the Frenkel-Poole mechanism. From this, dHfO2,trapcan be calculated to be 13 A˚ , a
reasona-ble value. This result is proof that traps are actually located in the high-k shallow bulk. This study shows that anomalous
traps measured by the charge pumping technique in a HfO2/
metal gate at high gate voltage can be attributed to high-k bulk shallow traps.
Part of this work was performed at United Microelec-tronics Corporation. The work was supported by the National Science Council under Contract No. NSC 101-2120-M-110-002.
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