672 IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 6, JUNE 2009 Impact of Uniaxial Strain on Low-Frequency Noise in Nanoscale PMOSFETs Jack J.-Y. Kuo, Student Member, IEEE, William P.-N. Chen, and Pin Su, Member, IEEE Abstract—This letter investigates the low-frequency noise char- acteristics and reports a new mechanism for uniaxial strained PMOSFETs. Through a comparison of the input-referred noise and the trap density of the gate dielectric/semiconductor inter- face between co-processed strained and unstrained devices, it is found that the tunneling attenuation length for channel carriers penetrating into the gate dielectric is reduced by uniaxial strain. The reduced tunneling attenuation length may result in smaller input-referred noise, which represents an intrinsic advantage of low-frequency noise performance stemming from process-induced strain. Index Terms—Interface state, low-frequency noise, process- induced strain, trap density, tunneling attenuation length, uniaxial strained PMOSFET. I. I NTRODUCTION L OW-FREQUENCY noise is becoming a concern for con- tinuously down-scaled CMOS devices because increased low-frequency noise in these nanoscale transistors may limit the functionality of analog, mixed-signal, and RF circuits. As strained silicon is widely used in state-of-the-art CMOS technologies to enable the mobility scaling [1], [2], the low- frequency noise performance for strained devices is particularly important. Several studies regarding this topic have been carried out in the past [4], [5]. For example, Giusi et al. [3] have reported that the low-frequency noise of strained PMOSFETs with HfO 2 gate dielectric could be degraded due to worse gate dielectric quality when processing the SiGe source/drain. The work of Simoen et al. [4] and Ueno et al. [5] revealed that the low-frequency noise performance of strained devices with SiON gate dielectric may be preserved. While the low- frequency noise performance is determined by the device fabri- cation processes [3]–[6], whether there exists any other intrinsic stress effect on low-frequency noise is still not clear and merits further investigation. In this letter, through an in-depth com- parison between co-processed strained and unstrained devices regarding the low-frequency noise characteristics, we report a new mechanism for uniaxial strained PMOSFETs. Manuscript received March 11, 2009. First published May 12, 2009; current version published May 27, 2009. This work was supported in part by the National Science Council of Taiwan under Contract NSC97-2221-E-009-162 and in part by the Ministry of Education in Taiwan under the ATU Program. The review of this letter was arranged by Editor S. Kawamura. The authors are with the Department of Electronics Engineering, Na- tional Chiao Tung University, Hsinchu 30050, Taiwan (e-mail: jack.ee93g@ nctu.edu.tw; williamchen.ee93g@nctu.edu.tw; pinsu@faculty.nctu.edu.tw). 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.2009.2020069 Fig. 1. Drain current noise spectral density S Id for devices with Lgate = 65 nm at |V d | = 0.05 V and |Vgst | = 0.2 V showing typical 1/f r noise type with r close to one. II. DEVICES Co-processed uniaxial strained and unstrained PMOSFETs are investigated in this letter [7], [8]. Strained and unstrained devices with channel direction 110 and poly/SiO 2 gate stack were fabricated on (100) silicon substrate. The strained de- vice features compressive uniaxial Contact Etch Stop Layer (CESL) and SiGe source/drain. For the transistors with gate length L gate = 65 nm, the saturation drain current (I dsat ) of the strained device is improved by more than 100% as com- pared with its control counterpart. The low-frequency noise measurements were carried out using the BTA9812 [3], [9] measurement system. III. RESULTS AND DISCUSSION The drain current noise spectral densities (S Id ) for strained and unstrained devices biased at gate overdrive |V gst | = 0.2 V are shown in Fig. 1. The spectra show typical 1/f r noise type with the frequency index γ close to one. Fig. 2 shows the normalized noise spectral density, S Id /I 2 d , as well as (g m /I d ) 2 versus drain current I d for strained and unstrained devices at frequency f = 10 Hz. It can be seen that the S Id /I 2 d shows a fairly good proportionality with (g m /I d ) 2 , indicating a carrier- number-fluctuation origin [10], [11]. Fig. 3 shows the input-referred voltage spectral density (S Vg = S Id /g 2 m ) as a function of V gst taken from the average of ten devices. It can be seen that the S Vg for strained and 0741-3106/$25.00 © 2009 IEEE