IMPACT OF GENERATION CENTERS ON THE RETENTION 1T-FBRAM

Transcription

1 IMPACT OF GENERATION CENTERS ON THE RETENTION TIME IN 1T-FBRAM M. AOULAICHE, CH. CAILLAT* E. SIMOEN, G. GROESENEKEN AND M. JURCZAK IMEC, KAPELDREEF 75, B 3001 LEUVEN, BELGIUM * MICRON TECHNOLOGYBELGIUM, KAPELDREEF 75, 3001 LEUVEN, BELGIUM

2 INTRODUCTION 1T/1C DRAM 1T WL 1T DRAM 1T WL 1C Charge storage BL Charge storage BL compatible process with CMOS Scalable High density 2

3 OUTLINE I. Sample and experimental conditions II. Generation lifetime III. Field enhancement (trap assisted tunneling) IV. Band-to-band tunneling V. Retention time and distribution simulations VI. Conclusions IMEC 2012 MARC AOULAICHE 3

4 DEVICE PROCESSING UTBOX SOI: BOX=10nm and t Si =50nm STI formation Ground Plane doping Gate stack: 5nm SiO 2 / 5nm TiN+poly Spacer Selective Epitaxial Growth L=60nm and W=1µm T=85C, unless specified S/D implants and RTA Standard BEOL 4

5 PROGRAMMING SCHEME V[V] W rite 1 read read V D V G W rite t[n s] Holes generation by II BJT current on Holes removed from the source BJT current off 5

6 OUTLINE I. Sample and experimental conditions II. Generation lifetime III. Field enhancement (trap assisted tunneling) IV. Band-to-band tunneling V. Retention time and distribution simulations VI. Conclusions IMEC 2012 MARC AOULAICHE 6

7 RETENTION TIME MEASUREMENTS Current [µa/µm] E E E E E-03 Holding time[s] The retention time is due to the state-0 degradation by hole generation during the holding C.P[%] T=85 o C L=60nm Retention time [s] The retention time is distributed over 1 decade 7

8 GENERATION MECHANISMS Vertical view lateral view Surface generation VG=-2.5V Surface generation VB=+3V Q FG E C BBT TAT E C Q BG BOX Bulk generation Q Si 5nmSiO 2 Source E V Body Drain E V 10nmSiO 2 IMEC

9 With respect to SRH Bulk generation BULK / SURFACE GENERATION ( Etrap Ei ) = + ( Ei E τ g τ h. exp τ e.exp K. T K. T τ = e / h σ e / h 1. ν.n th trap Surface generation t K. T τ s = 2. Se. S Surface velocity h trap ) g[s] τ g Eit Ei Ei Eit si. Se.exp + Sh.exp S e / h = σe / h. νth. Nit K. T cm cm Et E V Key parameters N T : density of traps E T : trap energy level σ: Capture cross section 1.1 E C 9

10 GENERATION LIFETIME Bulk generation Effective generation lifetime τ eff = τ τ g g. τ s + τ s τeff f [s] Surface generation Effective generation lifetime E[eV] 10

11 MODEL EQUATIONS Continuity equation (assuming no diffusion and no generation due to external source) N h t N h N h, i = N h, eq τ ( t ) = N h, eq V body Nh ( t) ( t) t = Nh, eq. 1 exp τeff q t =. N h, equ. C. 1 exp Si τeff Vbody[V] ( ) Isense t t 1 Time [s] ( V body ( t) ) q. ( V body ( t) ) q. V S V + D = α. I F 0 exp 1 I exp R 0 1 K. T K. T 11

12 RETENTION TIME measurements simulations IBJT( t, 0.62) IBJT( t, 0.74) Ret 1 Ret t, t Ret 0, Ret 0, 20 With reasonable density of defects (10 15 cm -3,10 16 cm -3 ) longer retention times are obtained by simulations 12

13 OUTLINE I. Sample and experimental conditions II. Generation lifetime III. Field enhancement (trap assisted tunneling) IV. Band-to-band tunneling V. Retention time and distribution simulations VI. Conclusions IMEC 2012 MARC AOULAICHE 13

14 FIELD ENHANCEMENT Field enhancement factor impact on the generation lifetime 0.01 Field en nhancement factor Hurks model Poole Frenkel Dirac E max [V/cm] τ g = χ F τ h + Γ coul τg_pf( 0.62, Fmax) τ g [s] ( E T E.exp K B. T i ) + χ E Fmax max [V/cm] F τ e + Γ coul ( E i E.exp K B. T Coulombic field-enhancement term T ) Poole-Frenkel thermal emission enhancement factor 14

15 FIELD ENHANCEMENT Trap assisted tunneling Barrier lowering for a Coulombic well Barrier lowerin ng [ev] 0.2 Elow( Fmax) E( Emax ) = q. E π. ε. ε 0 max si E max [MV/cm] 0.1 Fmax For extension less junction E=0.18eV E T =E C -E A + E

16 RETENTION TIME FITTING The simulated retention time is reduced by the field enhancement 16

17 OUTLINE I. Sample and experimental conditions II. Generation lifetime III. Field enhancement (trap assisted tunneling) IV. Band-to-band tunneling V. Retention time and distribution simulations VI. Conclusions IMEC 2012 MARC AOULAICHE 17

18 BAND-TO-BAND TUNNELING MODEL EQUATIONS Generation Rate by BBT [1/s.cm 3 ] 1e25 1e20 1e15 1e10 1e5 1e0 ATLAS simulation Source V G,hold =-2.9V V G,hold =-1.9V Channel V G,HOLD -2.9 V -1.9 V Body Drain Source 35nm Lenght [nm] BBT Drain The tunneling generation rate : A = q RBBT 2 h 2m * 2 1/ 2. EG B = A. E γ exp E and B = E is the maximum field 2 π * 3/ 2 m. E 2 q. h G. Body potential change due to the generated holes Q( t) V Si ( t) = = C Si R BBT C Si. V. t. IMEC 2012 MARC AOULAICHE V=L dep xw dep xt Si 18

19 OUTLINE I. Sample and experimental conditions II. Generation lifetime III. Field enhancement (trap assisted tunneling) IV. Band-to-band tunneling V. Retention time and distribution simulations VI. Conclusions IMEC 2012 MARC AOULAICHE 19

20 LOW AND HIGH FIELD MECHANISMS ion time [s] Retenti Surface generation at the front interface N it =10 10 cm [cm -3 ] [cm -3 ] TAT [cm -3 ] BBT E[MV/cm] Field enhan ncement factor TAT Poole Frenkel E max [V/cm] E max [V/cm] BBT N it close to the junctions contribute to TAT TAT is the limiting mechanism for the retention time 20

21 RETENTION TIME FOR A SINGLE TRAP VERSUS E T Retention n time[ms] Temperature activation Single trapn T ~1.2x10 15 cm -3 E ~0.62eV A E ~0.56eV A /K.T[eV -1 ] time[s] Retention BBT limit at 0.7MV/cm E [ev] T E T variation from device to device 1 trap with a distribution in its energy level can induce a large retention time distribution 21

22 RETENTION TIME DISTRIBUTION Numb ber of traps E V Trap energy level [ev] Distribution of the defects density and their energy level in the Si band gap is assumed E C C..P[%] T=85 o C Simulated Measured Retention time [s] The retention time distribution is reproduced 22

23 OUTLINE I. Sample and experimental conditions II. Generation lifetime III. Field enhancement (trap assisted tunneling) IV. Band-to-band tunneling V. Retention time and distribution simulations VI. Conclusions IMEC 2012 MARC AOULAICHE 23

24 CONCLUSIONS TATisthedominantmechanismfortheretentiontime at low field. The origin of wide retention time distribution has been correlated with the distribution of G-R center in the Si band gap. Single defect with the Si midgap energy level can generate a leakage path affecting strongly the cell retention time. This can explain also the wide retention distribution. Tight control of such defects poses extreme challenge for the manufacturing of FBRAM IMEC 2012 MARC AOULAICHE 24

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