Insight Into Electron Traps and Their Energy Distribution Under Positive Bias Temperature Stress and Hot Carrier Aging

Abstract

Abstract-The access transistor of SRAM can suffer both Positive Bias Temperature Instability (PBTI) and Hot Carrier Aging (HCA) during operation. The understanding of electron traps (ETs) is still incomplete and there is little information on their similarity and differences under these two stress modes. The key objective of this paper is to investigate ETs in terms of energy distribution, charging and discharging properties, and generation. We found that both PBTI and HCA can charge ETs which center at 1.4eV below conduction band (Ec) of high-k (HK) dielectric, agreeing with theoretical calculation. For the first time, clear evidences are presented that HCA generates new ETs, which do not exist when stressed by PBTI. When charged, the generated ETs' peak is 0.2eV deeper than that of pre-existing ETs. In contrast with the power law kinetics for charging the pre-existing ETs, filling the generated ETs saturates in seconds, even under an operation bias of 0.9 V. ET generation shortens device lifetime and must be included in modelling HCA. A cyclic and anti-neutralization ETs model (CAM) is proposed to explain PBTI and HCA degradation, which consists of pre-existing cyclic electron traps (PCET), generated cyclic electron traps (GCET), and anti-neutralization electron traps (ANET).

Interaction between hot carrier aging and PBTI degradation in nMOSFETs: Characterization, modelling and lifetime prediction

“…At flat band condition, they are located at 1.4, 1.6 and more than 1.8 eV below Ec of high-k (Ec_HK), respectively. This is supported by the measured ETs energy distributions (Fig.8) [12], where a lower HCA bias generates relative less GCETs. CETs (PCET & GCET) can be repeatedly charged/discharged at certain low ±Vg biases (albeit a more negative bias is needed for discharging GCET), but AND cannot be discharged once generated.…”

“…Unlike PCET, GCET can only be generated by HCA stress, not by PBTI [12]. Modelling by extrapolation requires constant time exponent 'n', but Fig.13a&c shows 'n' reducing for higher charging Vg.…”

“…Simply adding degradations of HCA and PBTI together have been proven invalid [7]. We proposed a unified Cyclic Anti-neutralization Model (CAM) framework [12] for HCA and PBTI, but their interaction has not been investigated so far.…”

An assessment of RTN-induced threshold voltage jitter

“…The instabilities of modern MOSFETs have a number of sources: bias temperature instabilities (BTI) [1][2][3][4][5][6][7], hot carrier ageing (HCA) [8][9][10], and random telegraph noise [11][12][13][14][15][16][17]. To increase the instabilities and make them measurable, it is a common practice to use voltage-acceleration [1][2][3][4][5][6][7][8][9][10]. This works well for both BTI and HCA and the threshold voltage shift, ΔVth, has been reliably measured at pre-specified time by the measure-stress-measure (MSM) technique [1][2][3][4][5][6][7][8][9][10].…”

“…To increase the instabilities and make them measurable, it is a common practice to use voltage-acceleration [1][2][3][4][5][6][7][8][9][10]. This works well for both BTI and HCA and the threshold voltage shift, ΔVth, has been reliably measured at pre-specified time by the measure-stress-measure (MSM) technique [1][2][3][4][5][6][7][8][9][10]. The MSM technique, however, is inapplicable for the RTN-induced jitter in Vth because of two reasons.…”