深次微米元件內熱載子效應所造成汲極漏電流特性退化之研究

  



Hot Carrier Stress Induced Drain Leakage Current

Degradation in Deep Submicron MOSFET’s



NSC 88-2215-E-009-042

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E_mail:wang@jekyll.ee.nctu.edu.tw




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Abstract

Drain leakage current at zero gate bias has been recognized as a major reliability issue in deep submicron MOSFET’s. In this project, we investigated various drain leakage current mechanisms. Hot carrier stress induced drain leakage current degradation has been modeled and characterized. The dependence of drain leakage current on gate oxide thickness (from 30Å to 80 Å), supply

voltage (from 1.5V to 5.0V) and operation temperature (from room temperature to 100 Y ) has been investigated. In modeling, various drain leakage current components including drain-to-source subthreshold current, band-to-band tunneling current and trap enhanced drain leakage current are taken into account. Our result shows that interface trap-assisted leakage current may become a dominant drain leakage mechanism as supply voltage is reduced. In addition, a two-stage drain leakage current degradation was observed in relatively thick oxide nMOSFET’s.

Keywords: drain leakage degradation mechanism, interface trap, hot carrier, oxide thickness dependence
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"Oxide Thickness Dependence of Hot Carrier Stress Induced Drain Leakage Current Degradation in Thin-Oxide n-MOSFET's,"1998 SSDM.

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"Voltage Scaling and Temperature Effects on Drain leakage Current Degradation in a Hot Carrier Stressed

Current Degradation in Thin-Oxide

n-MOSFET's,"IEEE TED, p.1877,

IBB Is IBB ∆ITF Gh ∆ISRH ∆ITAT ∆ITF Te Te Gh Ge Ge Th Th ∆ITF Ge Gh

Fig.1 Illustration of subthreshold leakage current (IS) and band-to-band tunneling current (IBB)

Fig.2 Illustration of various trap-assisted carrier transition process

Fig.3 Measured drain leakage current characteristics before and after hot carrier 10-13 10-12 10-11 10-10 10-9 10-8 1.0 1.5 2.0 2.5 3.0 3.5 Drain Current(Amp) Supply Voltage (V) pre-stress post-stress T=353K T=292K Vgs=0V --- post-stress pre-stress

Table 1 Modeling of major drain leakage paths in a stressed MOSFET. Et andEl denote the total field and the lateral field. εt stands for the interface trap energy. fv, ft and fc are the electron occupation factors in the valence band, trap states and the conduction band. ∆L is the width of the interface trap region. Other variables have their usual definitions.

Trap-independent mechanisms: Trap-assisted mechanisms: Temperature-dependent parameters: Field-dependent parameters: IBB(band-to-band) = AEt2exp(-B/Et) IS(drain-to-source) = Ioexp( q nkTVgs) Ge = vthσn[niexp(

ε

t -

ε

_i kT )-ns(1-ft)] Gh = vthσp[niexp(

ε

i -

ε

_t kT )-psft] Th = (1 - ft) - (1 - f_τ v) h τh = τoexp[8π h(2mp) 1/2(

ε

t-

ε

v) 3/2 3qEl ] τe = τoexp[8π h(2mn) 1/2(

ε

c-

ε

t)3/2 3qEt ] Te = ft_τ-fc e

ε

g(T) =

ε

g(0) - αT2/(T+β) ni = NcNvexp[-

ε

g(T) 2kT ] ∆ITAT = qW ∆L bandgap ∆Nit(x,

ε

) TeTh Ge+Ted

ε

dx ∆ITF = qW ∆L bandgap ∆Nit(x,

ε

)TeGh+ThGe Ge+ Te d

ε

dx ∆ISRH = qW ∆L bandgap ∆Nit(x,

ε

) GeGh Ge+Ted

ε

dx ∆Id = ∆ISRH + ∆ITF + ∆ITAT

10-13 10-12 10-11 10-10 10-9 10-8 1.3 1.7 2.1 2.5 2.9 3.3 Drain Current(Amp) measurement calculation Supply Voltage (V) T=292K ∆Id IBB IS 10-13 10-12 10-11 10-10 10-9 10-8 1.3 1.7 2.1 2.5 2.9 3.3 Drain Current(Amp) measurement calculation Supply Voltage (V) T=353K ∆Id IBB IS 10-13 10-12 10-11 10-10 10-9 1.3 1.7 2.1 2.5 2.9 3.3 Drain Current(Amp) Supply Voltage (V) ITAT I TF I SRH ∆I_d T=292K Vgs=0V 100 101 102 101 102 103 104 1st stage 2nd stage Stress Time (s) tox = 30? tox = 53? Nit Qox meas. 0.20 ln(I) ∝ t 0.4 I ∝ t Vgs=0V & Vds=2.5V

Normalized Drain Leakage Current

Fig.5 Various zero gate bias drain leakage current components from measurement and calculation (T=292K)

Fig.6 Various zero gate bias drain leakage current components from measurement and calculation (T=353K)

Fig.7 Calculation of various interface trap-assisted drain leakage current at T=292K

Fig.8 Normalized drain leakage currents versus stress time in the different gate oxide thickenss n-MOSFET's 0 20 40 60 80 100 120 1.3 1.7 2.1 2.5 2.9 3.3

Drain Leakage Current Enhancement Factor

T=292K

T=353K

Supply Voltage (V)

Fig.4 The ratio of the post-stress drain leakage current to the pre-stress drain leakage current versus supply voltage

10-10

10-9 10-8

101 102 103 104

Charge Pumping Current (Amp) _{Stress time(sec)} tox=30?

tox=53?

Fig.9 Charge pumping current versus stress time in different gate oxide thickenss n-MOSFET's