Redox-Active Molecular Flash Memory for On-Chip Memory By Hao Zhu Electrical and Computer Engineering George Mason University, Fairfax, VA 2013.10.24
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
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
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
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)
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
Introduction: molecular NVM Redox-active molecules as charge trapping medium Better reliability endure more than 10 12 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
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
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
Molecule attachment characterizations Ι [ na ] 30 20 10 0-10 -20 SAM on SiO 2 20 SAM on Si 10-0.5 0.5 1.0 Surface coverage of 5.23 10 13 cm -2 and 3.14 10 13 cm -2 for SAM on Si and SiO 2 SAM survives the deposition of ALD Al 2 O 3 Ι [ na ] 0-10 V -20-0.25 0 0.25 0.50 0.75 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 1/2 740 730 720 710 700 Binding Energy [ ev ] (b) Fe 2p 1/2 Fe 2p 3/2 5 nm Al 2 O 3 /Ferrocene/SiO 2 /Si 740 730 720 710 700 Binding Energy [ ev ]
Molecular charge-trapping NVM MAFOS Gate Al 2 O 3 SiO 2 Si C [ pf ] 32 28 24 20 Forward 16 12 8 ±1 V ±3 V ±5 V ±7 V ±9 V 4-12 -9-6 -3 0 3 6 9 12 C [ pf ] 60 50 40 30 20 10 Gate Bias 0-6 -4-2 0 2 4 6 Gate Bias Backward MAFOS MAFS MAOS MAS V FB 3 2 1 0-1 -2 MAFOS MAFS MAOS MAS Voltage Pulse Width 500 µs -30-20 -10 0 10 20 30 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 4.41 10 12 cm -2
Program V FB 2.0 1.5 1.0 0.5 MAFOS MAFS V g =±10 V 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 P/E time [ s ] Excellent endurance: Intrinsic redox behavior of the Ferrocene molecule good gate stack interfaces Molecular charge-trapping NVM -0.2-0.4-0.6-0.8-1.0-1.2-1.4 V FB Erase V FB 1.0 0.8 0.6 0.4 0.2-0.2-0.4 V FB 0.6 0.4 0.2-0.2-0.4-0.6 10 0 10 1 10 2 10 3 10 4 10 5 10 6 Time [ s ] Program: 10 V, 500 µs (a) MAFOS MAFS Erase: -10 V, 500 µs MAFOS MAFS -0.6 10-1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 Number of P/E Cycles V FB Good retention compared with MAFS 0.8 0.6 0.4 0.2-0.2-0.4 (b) ±10 V, 50 µs ±10 V, 100 µs ±10 V, 500 µs MAFOS -0.6 10-1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 Number of P/E Cycles
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 490 480 470 460 300 290 280 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 1200 1000 800 600 400 200 0 Binding Energy [ ev ] Si 2p Br 3d Ι [ µa ] 0.2 0.1-0.1-0.2-0.3 SAM on Thermal SiO 2 Scan Rate: 4 V/s 2 V/s 1 V/s 0.5 V/s -2-1 0 1 2 V
Si Nanowire FET Platform Self-aligned gate-surrounding SiNW FET Patterned Au catalyst Si Nanowire Source/Drain Gate Al 2 O 3
Si Nanowire FET Platform -Ι DS [µa] 14 12 10 8 6 4 2 0 0.5 1.0 1.5 2.0 -V DS [V] High ON/OFF current ratio Small subthreshold slope Small leakage current V GS [V] -3.0-2.8-2.6-2.4-2.2 2.0 -Ι DS [A] -Ι DS [ A ] 10-5 10-6 10-7 10-8 10-9 10-10 10-11 0.5 1.0 1.5 2.0 10-7 10-8 10-9 10-10 10-11 10-12 10-13 3φ t -V DS [V] V DS =-5 V V DS =-0.10 V V DS =-0.15 V 10-14 -3-2 -1 0 1 2 V GS V GS [V] -3.0 Strong inversion -1.0-0.8-0.6-0.4-0.2 Moderate inversion Weak inversion Leakageaffected region
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 ] 10-7 10-8 10-9 10-10 ±4 V ±6 V ±8 V ±10 V ±12 V 10-11 Forward Backward 10-12 10-13 V DS = -50 mv 10-14 -10-5 0 5 10 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 0-10 -8-6 -4-2 0 V GS 4 ±4 V ±6 V ±8 V 2 Forward ±10 V ±12 V V DS = -50 mv 0-12 -10-8 -6-4 -2 0 2 -I DS [ µa ] V GS
Molecular flash based on SiNW FET Program V Th 1.2 1.0 0.8 0.6 0.4 0.2 ± 10 V P/E ± 8 V P/E ± 6 V P/E 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 10 1 P/E time [ s ] 0.2-0.2-0.4-0.6-0.8-1.0-1.2-1.4 Erase V Th V Th 2.0 1.5 1.0 0.5-0.5-1.0-1.5-2.0 Voltage pulse width 500 µs Ferrocene-attached Reference -30-20 -10 0 10 20 30 P/E Voltage Fast P/E speed Effective charge separation by SiO 2 tunneling barrier
Molecular NVM based on SiNW FET (a) 0.4 (b) 0.4 0.2 0.2 V Th -0.2-0.4 P/E by 500 µs, ±10 V pulse P/E by 100 µs, ±10 V pulse 10 years V Th -0.2-0.4 500 µs, ±10 V P/E pulse 100 µs, ±10 V P/E pulse 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 Time [ s ] 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 Number of P/E Cycles Better data retention due to optimized tunneling SiO 2 Behave well after 10 9 P/E cycles
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.
Thank you! 2013.09.20