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Click to edit Master title style STT-RAM Circuit Design Column Circuitry Simulation (IBM 45nm SOI) Fengbo Ren Summary Use floating body device as access transistor won’t degrade effective TMR as long as we perform pre-charge in reading operation. The bipolar current (due to the floating body) will appear at the beginning of writing operation. It should be not enough to be destructive (12uA/um, last for 80 ps). Min. cell size can be achieved is 17F2 (F=0.095 um). For min. cell size, I_W(P->AP)=645 uA, I_W(AP->P)=415 uA. More write current than 65-nm for the same cell size. Use NMOS-only-MUX results in 15-20% write current degradation. Still working on how to do reliable short pulse reading. 2 Body contacted (BC) v.s. floating body (FB) device Choosing device for access transistor – Best available BC device has 2x worse drive strength as compared to FB device, it also has body contact making it really big. FET Name Regular-Vt floating body NFET Analog body contacted NFET Idsat (uA/um) 1229 687 Area (for same I) 1 5 – In the interest of area, it is better to use FB device, but FB device has variation on RMOS depending upon Vbody (can range from 0-0.65V in our design), the following slide will analyze how this will affect the effective TMR. 3 Effective TMR Some definition TMR = (RAP-RP)/RP Effective RP (RP_Eff) = RP+RMOS1 Effective RAP (RAP_Eff) = RAP+RMOS2 Effective TMR = (RAP_Eff-RP_Eff)/RP_Eff _BC: body connected case _FB: floating body case Effective TMR (%) – – – – – – 100 Effective TMR when TMR=125% 90 120 80 100 70 80 60 60 50 40 40 20 30 0 50 20 40 30 Best case in reading 20 2 Cell Size (F ) 0 500 1000 1500 2000 Rp (Ohm) – Vbody >> 0 and are same when reading RP and RAP – RP_Eff_FB < RP_Eff_BC, RAP_Eff_FB < RAP_Eff_BC, Worst case in reading – Vbody = 0 when reading RP, Vbody >> 0 when reading RAP – RP_Eff_FB = RP_Eff_BC, RAP_Eff_FB < RAP_Eff_BC 4 Effective TMR when body is floating Best case improve effective TMR a little bit Worst case degrade effective TMR by 5-6% For our reading circuit, BL and SL are always pre-charged to the same voltage level. So, Vbody should always be the same (somewhere between VDD and VSS) regardless of MTJ resistance. Therefore, by performing pre-charge, we are always in the best case, which means using FB device won’t degrade TMR in our design. 5 Bipolar Current (FB device) When g = 0, d = 1, s = 1-> 0, since body is floating (Vbody>>0), we have bipolar current (Ibipolar). In our design, this current will be seen at the beginning of writing operation. Bipolar current at the beginning of writing operation Ibipolar has a peak of 12 uA/um and last for 80 ps, which should be not enough to accidentally flip MTJs. In our design we have 128 WLs, the peak of Ibipolar_total will be around 1.5 mA, a high but short current pulse. 6 Min. Cell Size – W = 434 nm – L = 40 nm Cell size: – 0.154 um2 – 17 F2 ● Feature size: 0.095 um From 65 -> 45nm – Metal pitch ● 0.2 -> 0.19 um (M4) ● 0.2 -> 0.14 um (M1) Note: This is the min. cell size can be achieved without violating design rule. 0.405 um 0.38 um Transistor size: – Transistor pitch (D/S shared) ● 0.5 -> 0.38 7 Cell Size v.s. Write Current Boosted VDDW Rp = 700 Ohm, TMR = 125% Boost up VDDW, VWL BL – dual VWL (VWL_P , VWL_AP) S’ VBIAS 2 17 Transistor W (um) IWP (uA) IWAP (uA) S Cell Size (F , F=0.095 um (C1)) 20 25 30 35 Boosted dual VWL 40 0.43 0.57 0.81 1.05 1.29 1.52 645 415 789 452 982 492 1100 513 1220 535 1270 547 “1” “0” S’ S SL VBIAS Recall 65-nm Cell Size (F2) 35 40 27.75 30 45 50 Transistor W (um) 0.75 0.84 1.04 1.24 1.44 1.64 IWP (uA) 723 779 884 972 1040 1090 IWAP (uA) 410 422 443 458 471 480 8 Using NMOS only in MUX Mux size W – NMOS: W – PMOS: 2W Using NMOS only in MUX will result in 15-20% write current degradation for P>AP, 10-15% degradation for AP->P. The main reason causes the degradation is that the VGS of the NMOS that close to VDD will be very low during the write operation (Shown in green in the bottom Fig. ), in which case we need a PMOS. So, simply increasing the size of NMOS won’t help. 9 Read Normal X-INV based reading – Monte Carlo, TMR=125% Rp (Ω) 500 750 1000 Read time (ps) 60~160 80~160 120~270 Still working on how to do reliable short pulse reading. – No positive result yet. 10