IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 3, MARCH 2012 303
Impact of Mechanical Strain on GIFBE in PD SOI
p-MOSFETs as Indicated From NBTI Degradation
Wen-hung Lo, Ting-Chang Chang, Chih-Hao Dai, Wan-Lin Chung, Ching-En Chen,
Szu-Han Ho, Osbert Cheng, and Cheng Tung Huang
Abstract—This letter investigates the impact of mechanical
strain on gate-induced floating-body effect in partially depleted silicon-on-insulator p-channel metal–oxide–semiconductor field-effect transistors. The strained FB device has less NBTI degrada-tion than unstrained devices. This behavior can be attributed to the fact that more electron accumulation induced by strain effect reduces the electric oxide field significantly during NBTI stress. Analysis of the body current (IB) under source/drain grounded
and floating operation indicates an increase in the anode electron injection and electron tunneling from conduction band which occur at the partial n+poly-Si gate and Si substrate, respectively. This phenomenon can be attributed to the bandgap narrowing which has been induced by the strain effect.
Index Terms—Gate-induced floating-body effect (GIFBE),
neg-ative bias temperature instability (NBTI), silicon-on-insulator (SOI) MOSFETs, strained silicon.
I. INTRODUCTION
S
ILICON-on-insulator (SOI) CMOS devices are attractive for switching application because of their high speed, lower power dissipation, and ability to restrain latch-up. However, as devices are scaled down below 90 nm, it becomes critical to realize the high drive current due to the degradation of carrier mobility caused by the required increase in channel doping [1]. Because of that, the use of a strained silicon technique offers an alternative method to enhance the SOI performance. In recent years, negative bias temperature instability (NBTI) has become a critical challenge due to the continuous shrinking of gate oxide thicknesses. Several studies have indicated that process-induced strain results in more serious NBTI degradation [2]–[4]. This is because forming gas may increase the number of Si–H bonds and induce nitrogen incorporation, both of which aggravate NBTI [2], [5], [6]. In contrast, significant gateManuscript received November 7, 2011; accepted November 21, 2011. Date of publication January 23, 2012; date of current version February 23, 2012. The review of this letter was arranged by Editor M. Östling.
W.-h. Lo and W.-L. Chung are with the Department of Physics and the Institute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan.
T.-C. Chang is with the Department of Physics, the Department of Photonics, and the Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan (e-mail: tcchang@ mail.phys.nsysu.edu.tw).
C.-H. Dai is with the Department of Photonics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan.
C.-E. Chen and S.-H. Ho are with the Department of Electronics Engineer-ing, National Chiao Tung University, Hsinchu 300, Taiwan.
O. Cheng and C. T. Huang are with the Device Department, United Micro-electronics Corporation, Hsinchu 300, Taiwan.
Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2011.2177956
leakage current has been reported to induce a new floating-body effect in SOI devices, called gate-induced floating-floating-body effect (GIFBE) [7], [8], which could reduce NBTI degradation because of the lower oxide electric field during stress [9]–[11]. However, the influence of the strain effect on GIFBE has not yet been studied. Therefore, this letter investigates the impact of strain on GIFBE by analysis of NBTI behavior. Mechanical strain was used in this letter to avoid the forming gas process issues and therefore study the pure strain effect on GIFBE [12]–[14].
II. EXPERIMENT
Using 65-nm SOI CMOS technology, partially depleted SOI p-channel metal–oxide–semiconductor field-effect transistors (PD SOI p-MOSFETs) with p+ poly-Si gate are produced in a T-gate structure. The SOI film and buried oxide thicknesses are 75 and 145 nm, respectively. The gate oxide has a thickness of 12 Å, and the channel doping concentration is about 3× 1018 cm−3. The channel is along the 110 direction on the (100) substrate. In this letter, devices have channel lengths (L) of 0.2 μm and widths (W ) of 1.0 μm. A stress gate voltage of
−1.8 V + VT was applied to the gate electrode at 125◦C, while
the source and drain were grounded. The threshold voltage (VT) was defined as the maximum of transconductor (Gmmax)
in the linear region. All electrical characteristics were measured using an Agilent B1500 semiconductor parameter analyzer.
III. DISCUSSION ANDRESULT
The ID–VG and Gm–VG characteristics are shown in the
inset of Fig. 1(a). After uniaxial compressive strain, the device shows better performance. This phenomenon indicates that our proposed strain is valid on the device due to a smaller effective mass and longer scattering lifetime [15]. Based on this mechanical strain, Fig. 1(a) shows the NBTI-induced Vthshift for the FB device before and after compressive strain. The Vth shift for the FB device is less than that for the GB device. This phenomenon can be attributed to the electron accumulation in the body, reducing the oxide electric field during NBTI stress [9]–[11]. However, it is worth noting that, when applying strain to the FB devices, the NBTI degradation becomes less serious than in FB and GB conditions without strain. This is because mechanical strain induces more electron accumulation in the FB, resulting in the significant lowering of the oxide electric field during NBTI stress. To further examine the physical origin of the degraded reliability resulting from the strain, the thermal activation energy (Ea) of NBTI was evaluated, shown
in Fig. 1(b). Several studies indicated that changes in the NBTI
304 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 3, MARCH 2012
Fig. 1. (a) Time evolution of threshold voltage shift in SOI PD p-MOSFETs with unstrained and strained FB devices, in addition to unstrained GB device. The inset shows the I–V curve of device before and after compressive strain. (b) Temperature dependence of threshold voltage shift of devices that are compressively strained and unstrained, with activation energy extracted from the power law with time under NBTI. The inset shows an illustration of bending sample with mechanical compressive strain.
mechanism can be observed by the changes in Ea of NBTI
[5], [6].
However, Fig. 1(b) shows a nearly unchanged Ea of NBTI
with strain (∼0.11 eV). This suggests that the strain does not change the NBTI mechanism itself but only changes the amount of interface traps. These results evidence that the different levels of NBTI degradation in Fig. 1(a) can be purely attributed to the difference in oxide electric field induced by the amount of electron accumulation in the FB.
As described previously, the FB device has a decrease of electric field which is associated with electron accumulation in the body during NBTI. Furthermore, the amount of electron accumulation in the FB can be determined from measurement of the body current (IB) under GB condition. Therefore, to
real-ize the influence of mechanical strain on NBTI under FB condi-tion, the body current both with and without mechanical strain was measured and shown in Fig. 2. It can be seen that there is indeed an increase in IB under mechanical strain, further
confirming that the strain induces more electron accumulation in the FB, resulting in less NBTI degradation for the FB device. In addition, previous studies have indicated that the IG can
be effectively suppressed by applying compressive strain [16]. This slight reduction of gate current is due to strain-induced change in the valence band offset between Si and the SiO2 gate dielectric. Accordingly, the degree of NBTI degradation
Fig. 2. Body current with S/D grounded for GB devices that are compres-sively strained and unstrained. The inset shows the gate current composed by source and drain current for GB devices that are compressively strained and unstrained.
Fig. 3. IB–VG curves with floating S/D for strained and unstrained GB
devices. The inset shows the schematic diagram of ECB from the partial n+
poly-Si gate for an ultrathin gate oxide in PD SOI p-MOSFETs and a cross section of the device.
is closely related to the IG magnitude, which is consistent
with the probability of reaction of the holes and Si–H bonds [17]. Experimental results indicate that the GB device has an improvement of about 5.3% in NBTI reliability after strain (not shown here). However, there is a significant improvement in NBTI reliability of about 22.8% for the FB device. Therefore, this additional improvement can be attributed to the lowering of the oxide electric field by more electron accumulation.
The two stages of increase in IB, as shown in Fig. 2,
can be attributed to the electron tunneling from conduction band (ECB) and anode electron injection (AEI) mechanisms, respectively [9]. The ECB tunneling is induced by the partial n+ poly-Si of the body-tied device, as shown in the inset of Fig. 3. The AEI is associated with the inversion layer supplied by the source/drain (S/D). The holes tunnel from the inversion layer and generate hot electrons by impact ionization in the poly-Si depletion region, resulting in the second increase of IB.
Fig. 2 shows that there is an increase in both stages of IBunder
the strain condition. To clearly examine the influence of strain effect on the first stage of IB, ECB tunneling before and after
strain was measured under floating S/D operation and shown in Fig. 3. There is an increase in IB, with gate bias increasing
LO et al.: IMPACT OF MECHANICAL STRAIN ON GIFBE IN PD SOI p-MOSFETs 305
Fig. 4. Comparison of effective body current versus gate voltage for compres-sive strained and unstrained devices. The inset shows the cross section and the schematic diagram of AEI mechanism under strain.
beyond the VG of −0.4 V. In addition, the difference of IB
between unstrained and strained devices becomes significant with increasing VG. Electrons tunneling from the poly-Si gate
still can generate hot holes by impact ionization in the depletion region of an n-type substrate. This suggests that the first stage of
IBcannot be purely attributed to ECB tunneling but still reflects
the contribution of hot holes by impact ionization in the n-type substrate, as shown in the inset of Fig. 3. Therefore, we suggest that the increase of IB in Fig. 3 is mainly due to the bandgap
narrowing of the n-type substrate induced by the mechanical strain effect. This phenomenon reduces the threshold energy to generate electron–hole pairs by impact ionization, resulting in the first stage increase of IB after strain. Therefore, with an
increase in gate bias, the difference of IBbefore and after strain
gradually becomes clear, as shown in Fig. 3. However, in Fig. 2, the second stage of IBalso increases under mechanical strain, a
phenomenon which may be related to the first stage increase of
IB. To clearly distinguish the contribution of the strain effect on
the second stage of IB, the pure AEI-induced electron current
(effective IB) was also obtained, as shown in Fig. 4, by
subtract-ing the ECB tunnelsubtract-ing component from the second increase of
IB. Based on the AEI model, the IBgenerally decreases as the IG decreases. However, the strain effect increases the effective IB, particularly over VG=−1 V. This unusual phenomenon is
caused by the bandgap narrowing induced by the mechanical strain effect. Because the impact ionization rate is exponential to the energy bandgap, the narrowing bandgap causes more additional electron generation in the poly-Si depletion region by impact ionization even if IGdecreases.
IV. CONCLUSION
This letter has investigated the influence of mechanical strain on GIFBE from NBTI degradation in PD SOI p-MOSFETs. It is found that the strained device under FB operation exhibits much less NBTI degradation. This is because the higher electron accumulation induced by strain effect has reduced the electric oxide field during NBTI stress. These electrons are generated by the ECB and AEI mechanisms. Based on our systematical analysis, strain effect induces an increase in two stages of IB.
This can be attributed to bandgap narrowing in n−substrate and p+poly-Si gate, respectively.
ACKNOWLEDGMENT
Part of this work was performed at United Microelectronics Corporation.
REFERENCES
[1] T. Ghani, K. Mistry, P. Packan, S. Thompson, M. Stealer, and S. Tyagi, “Scaling challenges and device design requirements for high performance sub-50 nm gate length planar CMOS transistors,” in VLSI Symp. Tech
Dig,, 2000, pp. 174–175.
[2] H. S. Rhee, H. Lee, T. Ueno, D. S. Shin, S. H. Lee, Y. Kim, A. Samoilov, P. O. Hansson, M. Kim, H. S. Kim, and N. I. Lee, “Negative bias tempera-ture instability of carrier-transport enhanced pMOSFET with performance boosters,” in IEDM Tech. Dig., 2005, pp. 692–695.
[3] G. Thareja, F. Z. J. Lee, A. V. Y. Thean, V. Vartanian, and B. Y. Nguyen, “NBTI reliability of strained SOI MOSFETs,” in Proc. ISTFA, 2006, pp. 423–425.
[4] Y. W. Jeon, D. H. Ka, C. G. Yu, W. J. Cho, M. S. Islam, and J. T. Park, “NBTI and hot carrier effect of SOI p-MOSFETs fabricated in strained Si SOI wafer,” Microelectron. Reliab., vol. 49, no. 9–11, pp. 994–997, Sep.–Nov. 2009.
[5] Y. Mitani, M. Nagamine, H. Satake, and A. Toriumi, “NBTI mechanism in ultra-thin gate dielectric - nitrogen-originated mechanism in SiON,” in
IEDM Tech. Dig., 2002, pp. 509–512.
[6] N. Kimizuka, K. Yamaguchi, K. Imai, C. T. Liu, R. C. Keller, and T. Horiuchi, “NBTI enhancement by nitrogen incorporation into ultrathin gate oxide for 0.10-μm gate CMOS generation,” in VLSI Symp. Tech. Dig., 2000, pp. 92–93.
[7] J. Pretet, T. Matsumoto, T. Poiroux, S. Cristoloveanu, R. Gwoziecki, C. Raynaud, A. Roveda, and H. Brut, “New mechanism of body charging in partially depleted SOI-MOSFETs with ultra-thin gate oxides,” in Proc.
ESSDERC, 2002, pp. 515–518.
[8] M. Casse, J. Pretet, S. Cristoloveanu, T. Poiroux, C. Fenouillet-Beranger, F. Fmleux, C. Raynaud, and G. Reimbold, “Gate-induced floating-body effect in fully-depleted SOI MOSFETs with tunneling oxide and back-gate biasing,” Solid State Electron., vol. 48, no. 7, pp. 1243–1247, Jul. 2004.
[9] C. H. Dai, T.-C. Chang, A.-K. Chu, Y.-J. Kuo, F.-Y. Jian, W.-H. Lo, S.-H. Ho, C.-E. Chen, W.-L. Chung, J.-M. Shih, G. Xia, O. Cheng, and C.-T. Huang, “On the origin of gate-induced floating-body effect in PD SOI p-MOSFETs,” IEEE Electron Device Lett., vol. 32, no. 7, pp. 847– 849, Jul. 2011.
[10] R. Mishra, D. E. Ioannou, S. Mitra, and R. Gauthier, “Effect of floating-body and stress bias on NBTI and HCI on 65-nm SOI pMOSFETs,” IEEE Electron Device Lett., vol. 29, no. 3, pp. 262–264, Mar. 2008.
[11] J. Zhang, A. Marathe, K. Taylor, E. Zhao, and B. En, “New findings of NBTI in partially depleted SOI transistors with ultra-thin gate dielectrics,” in Proc. Int. Reliab. Phys. Symp., 2004, pp. 687–688.
[12] Y. J. Kuo, T. C. Chang, P. H. Yeh, S. C. Chen, C. H. Dai, C. H. Chao, T. F. Young, O. Cheng, and C. T. Huang, “Substrate current enhance-ment in 65 nm metal–oxide–silicon field-effect transistor under external mechanical stress,” Thin Solid Films, vol. 517, no. 5, pp. 1715–1718, Jan. 2009.
[13] C. H. Dai, T. C. Chang, A. K. Chu, Y. J. Kuo, S. C. Chen, C. T. Tsai, W. H. Lo, S. H. Ho, G. Xia, O. Cheng, and C. T. Huang, “Enhanced gate-induced floating-body effect in PD SOI MOSFET under external mechanical strain,” Surf. Coatings Technol., vol. 205, no. 5, pp. 1470– 1474, Nov. 2010.
[14] C. S. Lin, Y.-C. Chen, T.-C. Chang, F.-Y. Jian, W.-C. Hsu, Y.-J. Kuo, C.-H. Dai, T.-C. Chen, W.-H. Lo, T.-Y. Hsieh, and J.-M. Shih, “NBTI degradation in LTPS TFTs under mechanical tensile strain,” IEEE
Elec-tron Device Lett., vol. 32, no. 7, pp. 907–909, Jul. 2011.
[15] C. W. Liu, S. Maikap, and C. Y. Yu, “Mobility-enhancement tech-nologies,” IEEE Circuits Devices Mag., vol. 21, no. 3, pp. 21–36, May/Jun. 2005.
[16] X. Yang, J. Lim, G. Sun, K. Wu, T. Nishida, and S. E. Thompson, “Strain-induced changes in the gate tunneling currents in p-channel metal–oxide–semiconductor field-effect transistors,” Appl. Phys. Lett., vol. 88, no. 5, p. 052 108, Jan. 2006.
[17] T. Irisawa, T. Numata, E. Toyoda, N. Hirashita, T. Tezuka, N. Sugiyama, and S. I. Takagi, “Physical understanding of strain-induced modulation of gate oxide reliability in MOSFETs,” IEEE Trans. Electron Devices, vol. 55, no. 11, pp. 3159–3166, Nov. 2008.
