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Hyun Chul Sagong, Kyong Taek Lee, Seung-Ho Hong, Hyun-Sik Choi, Gil-Bok Choi, ... Seung-Hyun Song, Min-Sang Park, Jae Chul Kim, and Yoon-Ha Jeong.
RF and Hot Carrier Effects in Metal gate/high-k Dielectric nMOSFETs at Cryogenic Temperature Hyun Chul Sagong, Kyong Taek Lee, Seung-Ho Hong, Hyun-Sik Choi, Gil-Bok Choi, Rock-Hyun Baek, Seung-Hyun Song, Min-Sang Park, Jae Chul Kim, and Yoon-Ha Jeong Dept. of Electronic and Electrical Engineering, Pohang University of Science and Technology POSTECH Pohang, Korea Phone : +82-54-279-2897, [email protected]

Sung-Woo Jung National Center for Nanomaterials Technology (NCNT) Pohang, Korea

Chang Yong Kang SEMATECH Austin, USA dielectric gate stacks were processed by atomic layer deposition (ALD) of 3 nm HfO2 on interfacial layer (IL) of 1 nm thickness and then ALD TiN electrode stack was deposited. The equivalent oxide thickness (EOT) was 1.24 nm. For comparison, a control n-poly Si/SiO2 device was prepared and its EOT was 2 nm.

Abstract—We investigate RF performances and hot carrier effects of nMOSFETs at cryogenic temperature. RF performances of HfO2 dielectric nMOSFET at 77 K are improved more than those of SiO2 dielectric nMOSFET although DC performances are improved similarly. The nMOSFET with HfO2 dielectric has 127.4 GHz fT and 75.4 GHz fmax at 77 K. In hot carrier injection measurement, gm of HfO2 nMOSFET at 77 K is degraded more than 300 K although Vth shift is less. The cause of gm reduction is discussed related to the trapping. Keyword-RF; HfO2; hot carrier effect; high-k; cryogenic; low temperature

I.

INTRODUCTION

Low temperature operation of MOSFETs has received greater attention in cryogenic electronics because MOSFETs at cryogenic temperature showed improvement of device performance [1]. Analysis of MOSFETs at cryogenic temperature is needed in a variety of applications for space exploration, high-sensitivity cooled sensors, semiconductorsuperconductor hybrid systems, and very low noise receiver front-ends for radio astronomy [2]. However, the RF characteristics and hot carrier effects of MOSFETs at cryogenic temperature are still unexplored research area. Furthermore, RF and hot carrier study at cryogenic temperature may provide deeper insight of understanding for metal gate/high-k dielectric devices. In this work, the RF performance and hot carrier induced degradation of metal gate/high-k nMOSFETs are characterized in the 77 K - 300 K temperature range. II. EXPERIMENTS We used nMOSFETs with a 100-nm length and a finger width of 5-μm with 2 fingers. The SiO2 gate nMOSFETs were fabricated using conventional process flow. The high-k

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Recently, for a short channel devices with a very thin gate dielectrics, the worse case of hot carrier degradations was reported to occur at the Vgs = Vds condition rather than Vds/2, so Vgs = Vds = Vth + 1.6 V stress condition is used for hot carrier injection (HCI) measurement [3]. Vth shift by PBTI is measured to compare with HCI at the Vgs = Vth + 1.6 V and Vds = 0 V condition. S-parameters were measured for analysis of RF characteristics in frequency range from 500 MHz to 40 GHz by Anritsu 37397C vector network analyzer (VNA). LRRM calibration can be used for both reflection and transmission measurements for calibration of cable connected with VNA, and all systematic error terms are measured and removed. Open-short de-embedding (two step de-embedding) was performed for removing pad parasitic components. Agilent IC-CAP program was used for extracting parameters related to DC and RF characteristics. III. RESULTS AND DISCUSSION The drain current is increased as the temperature is dropped from 300K to 77K as shown in Figure 1. Both the HfO2 dielectric nMOSFET and the SiO2 dielectric nMOSFET show increase of drain current at low temperature and Ids increment looks saturated. Temperature reduction allows a substantial increase of the carrier mobility and saturation velocity, so the drain current was increased at 77 K compared to 300 K [4]. With decreasing the temperature, sub-threshold swing (SS) is decreased for the both HfO2 dielectric and SiO2

IEEE CFP09RPS-CDR 47th Annual International Reliability Physics Symposium, Montreal, 2009

W = 5 μm x 2 fingers L = 100 nm Vgs = 1.2 V

gm x EOT (mS x nm)

ΔIds (mA)

6.0

HfO2

SiO2 HfO2

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12 50

300

nMOSFET W = 5 μm x 2 fingers L = 100 nm

SiO2 HfO2

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Temperature (K)

Temperature (K) Figure 1. Ids increment between Vds = 1.2 V and Vds = 1.2 V of nMOSFET at various temperatures. Ids increment at 77 K is enhanced.

Figure 3. Effect of temperature on transconductance (gm) in nMOSFET with different gate dielectric.

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10

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-1

300 K 200 K 100 K 77 K

10

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-3

SiO 2 W = 5 μ m x 2 fingers L = 100 nm

0.2

0.4

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Vgs (V)

0.8

V gs (V)

1.0

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1.2

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Temperature (K) Figure 4. Effect of temperature on current gain cutoff frequency (fT) in nMOSFET with different gate dielectric.

dielectric in Figure 2. The transconductance (gm) is important factor to determine the current gain cutoff frequency (fT) in equation (1). The transconductance multiplied by EOT is plotted to eliminate the effect of EOT for evaluating transconductance in Figure 3. In case of both HfO2 and SiO2, the transconductance at 77 K is increased due to reduced phonon scattering compared to 300 K by about 11.3 %, which means that the effective mobility at low temperature increases in a similar way regardless of dielectric material as shown in Figure 3. The current gain cutoff frequency also increases with decreasing temperature as shown in Figure 4. The nMOSFETs are biased at Vgs = 1.1 V and Vds = 1.2 V for measurement of RF characteristics. Equation of current gain cutoff frequency is expressed as gm 2πCgg

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1.2

Figure 2. Ids - Vgs curves in log scale of nMOSFET at various temperatures. Drain voltage is biased at 50 mV.

fT =

nMOSFET W = 5 μm x 2 fingers L = 100 nm

SiO2 HfO2

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0

-2

10

Ids (m A )

Ids (mA)

HfO2 W = 5 μm x 2 fingers L = 100 nm

fT (GHz)

-1

10

300 K 200 K 100 K 77 K

(1)

negligible with the measurement temperature as shown in Table I. Additionally, reduction of current gain cutoff frequency under 100 K in SiO2 dielectric nMOSFET is considered due to carrier freeze-out [5]. Table I.

Temperature (K)

Parameters of small signal equivalent circuit at Vgs = 1.1 V and Vds = 1.2 V nMOSFET with HfO2 dielectric Rg (Ω)

Cgg (fF)

Cgd (fF)

Rg (Ω)

Cgg (fF)

Cgd (fF)

77

70.866

14.689

5.7048

108.09

14.402

4.1246

300

111.70

18.180

4.3810

124.95

14.261

3.5363

Equation of maximum oscillation frequency (fmax) is expressed as f max ≅

where Cgg is gate capacitance. The transconductance works as a dominant factor of the RF performance improvement because the charge of gate capacitance is almost

nMOSFET with SiO2 dielectric

fT 8πR g C gd

(2)

where Rg and Cgd is gate resistance and gate-to-drain capacitance, respectively. Gate resistance reduces with

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ΔVth (mV)

105

90

nMOSFET W = 5 μm x 2 fingers L = 100 nm

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nMOSFET W = 5 μm x 2 fingers L = 100 nm

ΔVth (mV)

fmax (GHz)

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45

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Temperature (K) (b)

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HCI stress

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77 K

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300 K

-15

HfO2 HCI

(b)

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5 After 1000sec stress After 1000sec relaxation

0

Relaxation 1000

-10

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(a)

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Open : 77 K Solid : 300 K

SiO2 HfO2

-15

HfO2 PBTI

After 1000sec stress After 1000sec relaxation

0

Figure 5. Effect of temperature on (a) gate resistance (Rg) and (b) maximum oscillation frequency (fmax) in nMOSFET with different gate dielectric.

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Figure 7. PBTI is measured at Vgs = Vth +1.6 V and Vds=0 in HfO2 nMOSFET to compare with HCI stress.

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Stress time (sec)

(a) SiO2 HfO2

Relaxation

Δgmmax (%)

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Open : 77 K Solid : 300 K

HCI PBTI

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Rg ( Ω)

HfO2

SiO2 HfO2

77 K

300 K

10

2000

Figure 8. Vth and gm shift during PBTI (a) and HCI (b) stresses on HfO2 devices. In HCI stress, gm degradation at 77 K is related with interface degradation.

Stress time (sec) Figure 6. Vth shift during 1000 sec hot carrier stress on nMOSFET at Vstress = Vgs = Vds = Vth+ 1.6 V. Relaxation during 1000 sec at Vrelaxation = -1 V.

During hot carrier injection (HCI) stress, the HfO2 dielectric nMOSFET is degraded more than SiO2 dielectric as shown in Figure 6. Large Vth shift in HfO2 dielectric nMOSFET is observed because both hot carriers and cold carriers impact HfO2 dielectric devices due to bulk traps in the high-k layer [6]. The cold carrier portion of Vth shift is recovered during relaxation time, which is confirmed from PBTI measurement (Figure 7).

decreasing temperature while gate-to-drain capacitance (Cgd) is almost identical (Table I). The resistance of TiN metal gate exhibits greater temperature dependence than that of poly-Si (Figure 5 (a)). Thereby, the HfO2 sample shows greater temperature dependence of fmax (Figure 5 (b)). This is mainly caused by the gate resistance considering that the temperature dependence of fT was similar.

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In case of PBTI, Vth shift is attributed to charging by gate bias and gm degradation is less severe than degradation of HCI [7]. For the HCI at 77 K, less Vth shift is observed and is attributed to carrier freezing-out of hot carriers. Comparing with Vth shift from PBTI measurement, this freeze-out effect becomes severe for HCI (figure 7). The cause is attributed to more energetically activated hot carriers easily losing their energy at 77 K even though the phonon scattering also decreases. Therefore, we can guess that hot carriers at 77 K impact more the interfacial layer (IL) than bulk layer. Effects of the hot carriers to the IL are also analyzed in Figure 8. In HCI measurement, Vth shift by relaxation at 77 K is smaller, but gm degradation is larger compared to 300 K. It can be assumed that gm degradation of HfO2 device at 77 K is related with interface degradation. After all, interface degradation by hot carrier injection at 77 K causes gm degradation.

ACKNOWLEDGMENT This work was partially supported by the BK21 program and the National Center for Nanomaterials Technology (NCNT) in Korea. REFERENCES [1]

[2]

[3]

[4]

IV. CONCLUSION In this paper, RF characteristics and hot carrier effects of 100-nm nMOSFET at low temperature were investigated. The nMOSFET with HfO2 dielectric and the nMOSFET with SiO2 dielectric were compared in DC and RF performances. In both cases, DC performances (on current, off current, and subthreshold swing) were improved at 77 K. The nMOSFET with HfO2 dielectric had 127.4 GHz fT and 75.4 GHz fmax at 77 K while the nMOSFET with SiO2 dielectric had 101.7 GHz fT and 39.7 GHz fmax at 77 K. In reliability measurements related to hot carrier, we could consider that the gm degradation of HfO2 nMOSFET devices at liquid nitrogen temperature was dominated by the interface degradation compared with 300 K. In this case bulk trapping at 77 K is reduced due to carrier freeze-out.

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[5]

[6]

[7]

S. H. Hong, G. B. Choi, R. H. Baek, H. S. Kang, S. W. Jung, and Y. H. Jeong, “Low-Temperature Performance of Nanoscale MOSFET for Deep-Space RF Applications,” in IEEE Electron Device Lett., 2008, pp. 775-777. S. Venkataraman, B. Banerjee, C. H. Lee, J. Laskar, and J. D. Cressler, “Cryogenic small signal operation of 0.18 μm MOSFETs,” in Proc. Silicon Monolithic Integr. Circuits RF Sys., 2007, pp. 52-55. E. Li, E. Rosenbaum, J. Tao, G.C-F. Yeap, M-R. Lin, and P. Fang, “Hot carrier effects in nMOSFETs in 0.1μm CMOS Technology”, 1999 IRPS, pp. 253-258. G. Ghibaudo, and F. Balestra, “Low temperature characterization of silicon CMOS devices,” in Proc. 20th Int. Conf. Microelectron., Sep. 12-14, 1995, vol. 2, pp. 613-622. F. Balestra, and G. Ghibaudo, “Brief review of the MOS device physics for low temperature electronics,” Solid State Electron., vol. 37, no. 12, pp. 1967-1975, Dec. 1994. J. H. Sim, L. Byoung Hun, C. Rino, S. Seung-Chul, and G. Bersuker, "Hot carrier degradation of HfSiON gate dielectrics with TiN electrode," IEEE Transactions on Device and Materials Reliability, vol. 5, pp. 177-182, 2005. K. T. Lee, C. Y. Kang, O.S. Yoo, R. Choi, B. H. Lee, J. C. Lee, H.-D. Lee, and Y. H. Jeong, “PBTI associated high temperature hot carrier degradation of nMOSFETs with metal gate/high-k dielectrics,” IEEE Electron Device Lett., vol. 29, no. 4, pp. 389-391, Apr. 2008.