Redox-Active Molecular Flash Memory for On-Chip Memory

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1 Redox-Active Molecular Flash Memory for On-Chip Memory By Hao Zhu Electrical and Computer Engineering George Mason University, Fairfax, VA

2 Outline Introduction Molecule attachment method & characterizations Molecular charge-trapping memory Ferrocene molecule for fast and reliable NVM (Ru) 2 molecule for multi-bit NVM Molecular Flash memory Self-aligned SiNW FETs Integration of redox molecules in Flash memory Summary

3 Introduction: non-volatile memory On-chip memory in central process unit (CPU) Dynamic random access memory (DRAM) Static RAM (SRAM) cache memory Occupy large floor space Consume high operation power Volatile Non-volatile memory as the on-chip memory in CPUs

4 Non-volatile memory Non-volatile memory (NVM) - Speed - Reliability - Integration density - Manufacture cost Not suitable for use as primary storage or on-chip memory Next generation NVM - Flash memory - Ferroelectric RAM - Phase-change RAM - Resistive RAM - Magnetoresistive RAM

5 Flash memory Floating-Gate NVM Control Gate Inter Ploy Dielectric Charge-trapping NVM Simple structure Better scalability Low power Less sensitivity to the SILC Floating Gate Tunnel Oxide N + N + p-si substrate Charge Trapping NVM Silicon-oxidenitride-oxidesilicon (SONOS) Nitride-base read-only memory (NROM) Nanocrystal memory (NCM)

6 SONOS charge-trapping NVM Thinner tunnel oxide for faster P/E speed poor retention and stability Thicker blocking oxide to suppress leakage current larger operation voltage SONOS-like NVM Novel charge-trapping mediums: New high-k materials Organic semiconductor Redox-active molecules Control Gate Blocking Layer Charge Trapping Layer Tunnel Oxide N + N + p-si substrate

7 Introduction: molecular NVM Redox-active molecules as charge trapping medium Better reliability endure more than P/E cycles Intrinsic redox centers provides naturally distinct charged states Lower operation voltage Higher speed Higher integration density Simple and low-cost process Redox molecules in solid-state flash memory CMOS compatible Embedded molecules for better stability Regular electronics characterization metrologies Gate Oxide SiO 2 Si

8 Strategies to enhance memory density More devices per unit of volume by using 3D integration (complicated and high-cost process); Embedded nanocrystal (controlling of size and density) Multiple dielectric charge storage layers (stack engineering and cell size) Redox-active molecules with multiple redox centers

9 Molecule attachment Self-assembled monolayer (SAM) on H-Si or SiO 2 surfaces Simple and low-cost attaching process: Solution of molecules in dichloromethane; Wafer soaking or dropping droplets of solution; ~ 100 o C in inert environment. Different molecules with different linker

10 Molecule attachment characterizations Ι [ na ] SAM on SiO 2 20 SAM on Si Surface coverage of cm -2 and cm -2 for SAM on Si and SiO 2 SAM survives the deposition of ALD Al 2 O 3 Ι [ na ] 0-10 V V [V] 0.2 V/s to 4 V/s Scan rates (from inside) 0.2 V/s, 0.5 V/s, 1 V/s, 2 V/s and 4 V/s Intensity [ a.u. ] Intensity [ a.u. ] (a) Ferrocene on H-Si Ferrocene on SiO 2 Fe 2p 3/2 Fe 2p 1/ Binding Energy [ ev ] (b) Fe 2p 1/2 Fe 2p 3/2 5 nm Al 2 O 3 /Ferrocene/SiO 2 /Si Binding Energy [ ev ]

11 Molecular charge-trapping NVM MAFOS Gate Al 2 O 3 SiO 2 Si C [ pf ] Forward ±1 V ±3 V ±5 V ±7 V ±9 V C [ pf ] Gate Bias Gate Bias Backward MAFOS MAFS MAOS MAS V FB MAFOS MAFS MAOS MAS Voltage Pulse Width 500 µs P/E Voltage V 2 3 = = + FB q n TAl O T q n C ε ε ε ε 2 3 linker 0 Al O 0 linker Charging density was calculated as cm -2

12 Program V FB MAFOS MAFS V g =±10 V P/E time [ s ] Excellent endurance: Intrinsic redox behavior of the Ferrocene molecule good gate stack interfaces Molecular charge-trapping NVM V FB Erase V FB V FB Time [ s ] Program: 10 V, 500 µs (a) MAFOS MAFS Erase: -10 V, 500 µs MAFOS MAFS Number of P/E Cycles V FB Good retention compared with MAFS (b) ±10 V, 50 µs ±10 V, 100 µs ±10 V, 500 µs MAFOS Number of P/E Cycles

13 Molecules with multiple redox states Work in progress: Attachment characterization: XPS, CyV, FTIR; Planar memory devices. Intensity [ a.u. ] Intensity Ru 3p 1/2 Ru 3p 3/2 Intensity Binding Energy [ ev ] Binding Energy [ ev ] (Ru) 2 on SiO 2 Reference SiO 2 C 1s Ru 3d 5/2 O 1s Ru 3p C 1s / Ru 3d Si 2s Binding Energy [ ev ] Si 2p Br 3d Ι [ µa ] SAM on Thermal SiO 2 Scan Rate: 4 V/s 2 V/s 1 V/s 0.5 V/s V

14 Si Nanowire FET Platform Self-aligned gate-surrounding SiNW FET Patterned Au catalyst Si Nanowire Source/Drain Gate Al 2 O 3

15 Si Nanowire FET Platform -Ι DS [µa] V DS [V] High ON/OFF current ratio Small subthreshold slope Small leakage current V GS [V] Ι DS [A] -Ι DS [ A ] φ t -V DS [V] V DS =-5 V V DS =-0.10 V V DS =-0.15 V V GS V GS [V] -3.0 Strong inversion Moderate inversion Weak inversion Leakageaffected region

16 Molecular Flash based on SiNW FET Redox-active ferrocene attaching on SiO 2 tunneling layer Large memory window High On/Off ratio Negligible memory window in reference sample (without molecule layer) -Ι DS [ µa ] 8 6 -Ι DS [ A ] ±4 V ±6 V ±8 V ±10 V ±12 V Forward Backward V DS = -50 mv V GS Backward 0.12 ±4 V V 0.10 DS = -50 mv ±6 V 8 ±8 V ±10 V 6 Backward 4 Forward 2 Reference Sample V GS 4 ±4 V ±6 V ±8 V 2 Forward ±10 V ±12 V V DS = -50 mv I DS [ µa ] V GS

17 Molecular flash based on SiNW FET Program V Th ± 10 V P/E ± 8 V P/E ± 6 V P/E P/E time [ s ] Erase V Th V Th Voltage pulse width 500 µs Ferrocene-attached Reference P/E Voltage Fast P/E speed Effective charge separation by SiO 2 tunneling barrier

18 Molecular NVM based on SiNW FET (a) 0.4 (b) V Th P/E by 500 µs, ±10 V pulse P/E by 100 µs, ±10 V pulse 10 years V Th µs, ±10 V P/E pulse 100 µs, ±10 V P/E pulse Time [ s ] Number of P/E Cycles Better data retention due to optimized tunneling SiO 2 Behave well after 10 9 P/E cycles

19 Summary Redox-active molecules were implemented for advanced flash memory devices with excellent endurance (10 9 P/E cycles), and are very attractive for future on-chip memory applications; Selective molecules with simple structure and multiple redox states for Flash memory with better performance; Multi-bit storage in molecular Flash memory can be realized by employing multiple redox states molecules.

20 Thank you!