Investigation of extra traps measured by charge pumping technique in high voltage
zone in p-channel metal-oxide-semiconductor field-effect transistors with HfO2/metal
gate stacks
Szu-Han Ho, Ting-Chang Chang, Bin-Wei Wang, Ying-Shin Lu, Wen-Hung Lo, Ching-En Chen, Jyun-Yu Tsai,
Hua-Mao Chen, Guan-Ru Liu, Tseung-Yuen Tseng, Osbert Cheng, Cheng-Tung Huang, and Xi-Xin Cao
Citation: Applied Physics Letters 102, 012106 (2013); doi: 10.1063/1.4773914
View online: http://dx.doi.org/10.1063/1.4773914
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/1?ver=pdfcov
Published by the AIP Publishing
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Investigation of extra traps measured by charge pumping technique in high
voltage zone in p-channel metal-oxide-semiconductor field-effect transistors
with HfO
2/metal gate stacks
Szu-Han Ho,1Ting-Chang Chang,2,a)Bin-Wei Wang,3Ying-Shin Lu,2Wen-Hung Lo,2 Ching-En Chen,1Jyun-Yu Tsai,2Hua-Mao Chen,4Guan-Ru Liu,2Tseung-Yuen Tseng,1 Osbert Cheng,5Cheng-Tung Huang,5and Xi-Xin Cao3
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 Electronics Engineering, Peking University, Beijing 100871, China 4
Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
5
Device Department, United Microelectronics Corporation, Tainan Science Park, Taiwan
(Received 9 November 2012; accepted 17 December 2012; published online 8 January 2013) This letter investigates extra traps measured by charge pumping technique in the high voltage zone in p-channel metal-oxide-semiconductor field-effect transistors with HfO2/metal gate
stacks. N-Vhigh levelcharacteristic curves with different duty ratios show that the hole discharge
time (tbase level) dominates the value of extra traps. By fitting ln (N (tbase level¼ 1ls) N
(tbase level)) Dtbase level at different temperatures and computing the equation t¼ s0 exp
(ah,SiO2dSiO2þ ah,HfO2dHfO2,trap), the results show that these extra traps measured by the charge
pumping technique at high voltage zone can be attributed to high-k bulk shallow traps.VC 2013
American Institute of Physics. [http://dx.doi.org/10.1063/1.4773914]
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. Besides, conventional SiO2-based dielectrics
have approached their physical limits. Hence, replacing SiO2-based dielectrics with high-k based dielectrics is a valid
solution to these problems. In addition, 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 de-vice characteristics. As recommended in the International Technology Roadmap for Semiconductors, Hf-based dielec-trics have been heavily studied to replace SiO2-based
dielec-trics in recent years.6–9However, with changes in Hf-based dielectrics, many measurement techniques must be cor-rected, especially charge pumping techniques. For instance, with a decrease in frequency, charge pumping current (Icp)
decreases in conventional SiO2-based dielectrics since
car-riers have enough time to discharge from interface shallow traps. Conversely, with a decrease in frequency, Icpincreases
in Hf-based dielectrics since carriers have enough time to tunnel into high-k bulk traps.10 Charge pumping techniques play an important role in inspecting defects. Thus, this study mainly focuses on extra traps measured by the charge pump-ing technique at high voltage, with the devices used in this study HfO2 dielectric p-channel MOSFETs (p-MOSFETs).
The causes of the extra traps are explained in this letter. The HfO2/metal gate p-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 p-MOSFETs were meas-ured by the charge pumping technique with different duty ratios at different temperatures. A pulse train with low-voltage of 1 V, high-low-voltage from 0 V to 1.29 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
1.29 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 con-firmed. Next, the Ig-Vg curve is fitted by Frenkel-Poole
mechanism and tunneling mechanism. All experimental curves were measured using an Agilent B1500 semiconduc-tor 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 at Vhigh level<j0:8Vj with an increase
in duty ratio. This is because the time for holes in the inter-face traps to recombine with electrons is very short. Hence, the number of interface traps measured by Icpis not sensitive
to duty ratio. On the contrary, N decreases with a rise in duty ratio for Vhigh level>j0:8Vj. Furthermore, only interface
traps can be measured with a duty ratio value of 97.5%. In other words, extra traps nearly disappear. The detrap time (tbase level) of holes dominates the value of N such that N
becomes smaller with a decrease in the detrap time. This result demonstrates that holes need time to discharge. Thus, it is necessary to know the relationship between N and the detrap time (tbase level) for Vhigh level>j0:8Vj. The inset of
a)
Electronic mail: [email protected].
0003-6951/2013/102(1)/012106/4/$30.00 102, 012106-1 VC2013 American Institute of Physics
Fig. 1shows I – Vhigh level curves with source, drain, and
body all grounded. It can be first observed that body current (Ib) is much smaller than Icp. In addition, N (Icp) is dependent
on the detrap time. Hence, these results indicate that N meas-ured by the charge pumping technique is caused by high-k bulk traps rather than gate leakage current.
The inset of Figure2shows the N-tbase levelcurve at 30C,
for Vg¼ Vt 0.7 V, shown by the dotted red line in Fig. 1.
Since N can also represent the number of holes discharged from high-k bulk traps, N (tbase level¼ 1 ls) N (tbase level) is the
num-ber of holes still charged in the high-k bulk traps at tbase level, an
important parameter. Figure 2 shows ln (N (tbase level
¼ 1 ls) N (tbase level)) Dtbase levelcurves fitted from the inset
of Figure2. Dtbase levelis tbase level tbase level(125 ns). Dtbase level
is the time for holes to discharge from traps. Clearly, fitting these curves can be accomplished with straight lines even for different temperatures (30C–90C). In addition, slopes are also similar at these temperatures. The discharge equation can be described by dQ(t)/dt¼ DQ(t)/sp ¼ epDQ(t),
DQ(t)¼ DQ(0)exp(ept),13where epis the escape probability,
and spis the average escape time. Thus, slope is indicated by
epor 1/spwith epnot dependent on temperature. Hence, epis
the tunneling probability. The average value of the slope at dif-ferent temperatures (maverage) is 4.76 10
6
, and sp,average is
2.1 107 (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 s0exp (ahx),
ah¼ 2(2mhq/0/h 2
)0.5,14,15 where s0is hole tunneling
charac-teristic time, mh is hole effective mass for SiO2, and q/0 is
an effective tunneling barrier height. However, because holes are tunneling through two layers, SiO2and HfO2, this
equation can be described by t¼ s0 exp (ah,SiO2dSiO2
þ ah,HfO2dHfO2,trap), ah,SiO2¼ 2(2 mh, SiO2q/0, SiO2 /h 2
)0.5, and ah, HfO2¼ 2(2 mh, HfO2q/0,HfO2/h
2
)0.5, where dSiO2is the
thick-ness of SiO2, dHfO2,trapis the distance from traps to interlayer
between SiO2and HfO2, mh, SiO2and mh, HfO2are 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. Values for s0, mh, SiO2,and mh, HfO2can be
obtained from other research.14,16,17Thus, only one parameter (/0,HfO2) is unknown.
The inset in Figure3(a)shows Ig-Vgcharacteristic curves
with BF, SDF, and SDB for distinguishing gate current 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 holes transfer from source/drain to the gate, rather than electrons transferring from gate to body. Clearly, section A indicates the tunneling current in Fig. 3(b), from Vg¼ 0.26 V to Vg¼ 0.42 V,
while section B is Frenkel-Poole current, shown in the inset of Fig.3(d), from Vg¼ 0.94 V to Vg¼ 1.29 V. The parameter
uB¼ 0.244 eV can be obtained by fitting the Frenkel-Poole
mechanism in the inset in Fig.3(d).18–20Figure3(c)shows the N-Vhigh levelcharacteristic curves at different duty ratios. When
Vg<j0:8Vj, N is interface traps (Nit) only. On the contrary,
when Vg>j0:8Vj, N is both high-k bulk 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 Nitand Nhkst
when gate current is Frenkel-Poole current. This indicates that bulk traps charging holes via the Frenkel-Poole mechanism and the traps discharging holes through Icpmay be the same.
In order to confirm this theory, uB ¼ /0,HfO2 ¼ 0.244 eV is
substituted into the equation t¼ s0exp (ah,SiO2dSiO2
þ ah,HfO2dHfO2,trap), with ah,SiO2 ¼ 2(2 mh, SiO2q/0, SiO2/h 2
)0.5, and ah, HfO2¼ 2(2 mh, HfO2q/0,HfO2/h
2
)0.5, where mh, SiO2 is
0.32m0, mh, HfO2is 0.85m0 1.28 m0, s0¼ 6.6 1014 (s),
dSiO2is 10 A˚ , and /0, SiO2¼ 1.4 eV þ /0,HfO2. Finally, dHfO2,trap
is calculated as 13.2 A˚ –16.2 A˚. This is a reasonable value. While Vg transits from Vhigh level to Vbase level, holes in the
high-k bulk shallow traps near the gate and substrate discharge to gate and source/drain, respectively. Hence, only traps in the middle of the high-k bulk shallow traps can be measured by the charge pumping technique. In addition, the falling time is 1.25 107(s), which matches the hole discharge time at duty ratio of 97.5%, that of 1.25 107(s). This implies that holes in the middle 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 97.5%.
FIG. 1. N-Vhigh levelcharacteristic curves at different duty ratios by charge pumping measurement. Inset shows I-Vhigh levelcurve with source, drain and body all grounded.
FIG. 2. ln (N (tbase level¼ 1 ls) N (tbase level)) Dtbase levelcurves at differ-ent temperatures at Vg¼ Vt 0.7 V. Inset shows N-tbase levelcurve at 30C for Vg¼ Vt 0.7 V.
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 the gate with the charge pumping technique at high and base levels, respectively. When j0:8Vj > Vhigh level> Vt, gate current is tunneling-path dominated,
leading to high-k bulk shallow traps not charging holes. Holes merely charge to interface traps, as shown in Fig.4(a). Subsequently, holes recombine with electrons in the inter-face traps at Vbase level, as shown in Fig. 4(b). Thus, Icponly
detects Nit. On the contrary, when Vg>j0:8Vj, the gate
current is dominated by the Frenkel-Poole mechanism, caus-ing high-k bulk shallow traps to charge holes. Next, holes 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 holes discharge from
high-k bulk shallow traps to the body by the tunneling mech-anism. Therefore, Icpmeasures not only interface traps, Nit,
but also high-k bulk shallow traps.
In summary, N-Vhigh level characteristic curves are
nearly the same in value for Vhigh level<j0:8Vj with a rise
FIG. 3. (a) Ig-Vg characteristic curves with SDB. Inset shows Ig-Vg character-istic curves with BF, SDF, and SDB. (b) Gate current in section A is fitted by tunneling model. (c) N-Vhigh level char-acteristic curves with different duty ratios for charge pumping measurement. (d) Gate current in section B is fitted by Frenkel-Poole model.
FIG. 4. The energy band diagram of high-k/metal gate MOSFETs with charge pumping measurement (a) at Vhigh level and (b) at Vbase level, where Vhigh level<j0:8Vj. The energy band diagram of high-k/metal gate MOSFETs with charge pumping measurement (c) at Vhigh leveland (d) at Vbase level, where Vhigh level>j0:8Vj.
in duty ratio. However, N decreases with an increase in duty ratio for Vhigh level>j0:8Vj. This indicates that the
dis-charge time dominates the value of N. In addition, the values of epobtained by the slope of ln (N (tbase level¼ 1 ls) N
(tbase level))Dtbase level are independent of temperature.
Hence, holes discharge from high-k bulk shallow traps via the tunneling mechanism. The distance of traps can be acquired by the equation t¼ s0 exp (ah,SiO2dSiO2
þ ah,HfO2dHfO2,trap) with /0,HfO2¼ 0.244 eV and /0,HfO2
obtained from fitting the gate current with the Frenkel-Poole mechanism. From this, dHfO2,trap can be calculated as
13.2 A˚ –16.2 A˚, a reasonable value. This result is proof that traps are actually located in the high-k shallow bulk. This study shows that extra traps measured by the charge pump-ing technique in a HfO2/metal gate p-MOSEFTs 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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