EECS598 Non-Volatile Storage

1. EECS598Non-Volatile Storage Jerry Kao jckao@umich.edu Electrical Engineering & Computer Science Department The University of Michigan, Ann Arbor

2. Ali Sheikholeslami, and P. Glenn Gulak A Survey of Circuit Innovations in FerroelectricRandom-Access Memories

3. FRAM Structure • Motives for FRAM: short programming time and low power consumption. • Easily integration in a SoC. • Research are done in following three areas: material processing, modeling, circuit design.

4. FRAM Comparison • FRAM is superior in term of write-access time and overall power consumption. • Target application: contactless smart card, and digital camera • Also hoping to be part of the mobile device market. • This paper focused on the six innovative circuit techniques.

5. Ferromagnetic Cores Background • Main technology prior to the 1950’s. • a current the x-access and y-access wire magnetized in a “0” or “1” direction. • Read access consists of a write access followed by sensing. • Writing the wrong data will induce a large current. • write the data stored in sense amp back to cell after write access.

6. Ferroelectric Capacitors Background • Name was adopted to convey similarity in the hysteresis loop. • Key concept: spontaneous polarization: a displacement that is inherent to the cycstal structure and does not disappear in absence of electric field. • Popular matieral is lead zirconate titanate (PZT), perovskites. • At 0V, the cell has two possible states.

7. Techniques to Reduce Voltage Disturbance • Novel material process to make the loop more square like. • Add the access transistor to each cell. (1T-1C) • Access transistor OFF • FE cap disconnect from bit line (BL) • Access transistor ON • FE cap is connected to BL and can be read or write from plate line (PL). • voltage boosted VDD is applied to WL.

8. Step-Sensing Approach Timing Diagram • Step PL before sensing. • BL precharge to 0V • turn on WL resulting in a capacitor divider consisting CFE and CBL between PL and ground. • Raise PL to VDD. • Sense the voltage on BL, Vx. • Sense amp restore the original data in the cell.

9. Pulse-Sensing Approach • pulse PL before sense amp. • has a smaller common mode voltage. • step-sensing approach is preferred due to higher cm voltage.

10. Reference Voltage Generation • Reference voltage between V0 and V1 is need to do the comparison. • V0 and V1 are not exact and are process and time dependent. • Two type of ferroelectric imperfections: • Relaxation: a partial loss of remanent charge in a µs if cap is not access for a period of time. → V1↓ or V0↑ • Imprint: the tendency of a cell to prefer one state over the other if it stay in that state for a long period of time. → shift in V1, V0, and VREF. • A variable reference is need to track the process Variation.

11. One Oversized Reference Capacitor per Column • Two additional cells in each column (1C’/BL). • CREF is sized larger than CFE so that VREF is midway between V0 and V1. • When WL0 and RWL0 or WL1 and RWL1 are turned on at the same time, and the sense amp amplify the difference between BL and /BL. • Reset transistor are added to reduce a voltage build up in the CREF. • VREF tuning achieves using adjustable CREF, adjustable RPL, or adjustable voltage reference generator.

12. Two Half-Sized Reference Cap per Column • also call (2 ×0.5C/BL) • Generate VREF=(V0+V1)/2 • CREF1 and CREF0 are half of the size of CFE. • In this case, VREF is going to be slightly larger than (V0+V1)/2. • CREF1 and CREF0 fatigues faster than CFE.

13. Two Full-Sized Reference Cap per Two Columns • also called (2C/2BL). • CREF1 = CREF0 = CFE • BL1 has V1 and BL2 has V0 before EQ turn ON. • After EQ turn ON, VBL1=VBL2=(V0+V1)/2 • At the end, a “0” and “1” must be restored in CREF0 and CREF1 by pulsing RPL thru transistor driven by RP.

14. Adding Reference Cells to Rows • also called (2C/WL) • fatigue the reference voltage circuit less. • reference generated by shorting RBL and /RBL. • need to add Cext to balance cap due to RBL.

15. A Self-Reference Fully Differential Arch. • also called (2T-2C) • Two CFEs store opposite values. • twice the voltage difference between BL and /BL. • only used in lower density memory.

16. Summary • 2T-2C is the most robust, but has density issue. • among 1T-1C, 2C/2BL and 2C/WL schemes have superior sensing complexity and fatigue immunity, respectively.

17. Ferroelectric Memory Architecture • adopted folded bitline architecture to reduce the bitline mismatch. • constant PL architecture is desired since PL is slow to move. • Two disadvantages: • A refresh is required. • voltage range across CFE is smaller.

18. Wordline-Parallel Plateline • also called (WL//PL) • PL is parallel to WL • a row of cells are access at the same time. • If PL is shared between two row, un-accessed row can be disturbed. • When disturbed, “0” is reinforced, and “1” might be flipped.

19. Bitline-Parallel Plateline • also called (BL//PL) • only a single cell can be selected. • absorb the y-decoder and reduce the power significantly. • PL activation can disturb all the cells in the column.

20. Segmented Plateline • also called (Segmented PL) • Break the PL into local segments. • faster PL than WL//PL • no disturbance to non-selected cell compared to BL//PL.

21. Merged Wordline/Plateline (ML) Architecture • Since WL and PL are parallel, people though of ways to merge them. • either two 1C-1T cells or one 2C-2T cell. • write “0” into C1 and “1” into C2. • four phase operations: • BLn=0V and BLn+1=VDD • ML1 and ML2 set to VDD, forcing “0” into C1. • ML1 pulled down to ground, leaving “0” in C1, and forcing “1” into C2. • ML1 pull to VDD and ML2 are pull to ground forcing “1” into C1 if BLn were at VDD. • Faster read access time. • same read/write time • higher density write access read access

22. Nondriven Plateline Architecture • also called Nondriven Plateline(NDP) • Constant voltage on PL reduce read/write access time. • PL=VDD/2 • read operation • BL1=BL2=0V • activate WL • VDD/2 used to switch the cap storing “1”. Good for SrBi2Ta2O9 • Sense amp restore the value by holding BL1=BL2. • Write operation is done similar to read operation except that BL is hold at VDD or 0V.

23. Bitline-Driven Architecture • PL=0V • full VDD when read, and no refresh on VDD/2 • Shaded circuit precharge BL and /BL to VDD or 0V before activating the WL. • PL is only pulsed after sensing. • This reduce the read access time, but not read cycle time. • Performance can be improved if combined with segmented PL.

24. Dual-Mode Ferroelectric Memories • limited the switching of CFE during the power down and power up mode to reduce the fatigue problem. • During power shutdown: • STO is turn on. • PL is pulsed, writing data to CFE • STO pull to ground, ready for power off. • During power on sequence:

25. Transpolarizer-Based Architectures • two CFE connected in opposite direction. • Simpler reference voltage since (V1+V0)/2 always equal to VDD/2. • Although it is a 1T-2C structure, the C is smaller than 1T-1C to get small signal level on BL. • Read operation with t4 and t5 doing write back.

26. Cross-Point Array of Ferroelectric Gain Cells • Memory architecture without PL and destructive read. • consist of array of gain cells. • two caps form a capacitor divider, and the transistor amplify the result. • In standby, WL=BL=VDD • In read, precharge BL to VDD and lower WL slightly. BL with cell storing “0” would have a larger current than BL with cell storing “1”.

27. Chain FRAM (NAND Architecture) • similar to NAND flash. • in unit of cell block. • A cell block is terminated by a BL and PL on each end. • In standby, all WL=VDD. • in active operation, WLx=0V and raising Block-Select(BS). other WL remain high allowing BL voltage and PL voltage to reach the selected cells. • Increase the number of cell in cell block increase density but reduce readout delay. • 1024 cells per bit line and 16 cells per cell block reduces area by 63%.

28. Architecture Summary

29. Future Trends • Progress in density, access time, and SoC integration can be assumed. • 62kb and 256kb has been achieved with 1Mb expected. • Access time hasn’t improved, but can be through circuit innovation. • It is easier to integrate FRAM to SoC compare to EEPROM.

30. Gaurav Mathur, Peter Desnoyers, Deepak Ganesan, Prashant Shenoy UltraLowPower Data Storage for Sensor Networks

31. Motivation • What is the most energy-efficient storage platform for the sensor networks, and what is the implication on sensor network design? • Results • Parallel NAND flash is 100X more energy-efficient storage compared to other flash memories and the radio on MicaZ.

32. Background • NOR flash is less dense than NAND and uses more energy for erase and programming, but provides random read access time less than 100ns. • NAND flash has significantly higher starting latency, but can stream subsequently read bytes at high speed since it is always page-oriented. • Writes are “one-way.” Need to erase before the next write. A microcontroller is used to translate the disk like operation to NAND interface, which also increase power consumption. This takes care of erasure, page remapping, ECC, and wear leveling.

33. Flash Energy Consumption • measured on Mica mote with 10Ω resistor with 3.3V supply • Toshiba NAND is 21X more efficient than Telos NOR.

34. Affect of Size of Data on Energy Consumption • read operation has a smaller energy overhead compared to write operation. • having a write buffer can amortizes the fix cost over a larger number of data bytes.

35. Idle Current • NOR and NAND device are smaller between 2µA and 5µA, which is smaller than mote CPU’s 5µA and 15µA or self discharge current of AA battery of 10µA. • NOR and NAND device has idle current that is 17X smaller than MMC.

36. Summary • parallel NAND flash is the most energy efficient storage for sensor network. • A desired device would have the performance of a parallel NAND and the pin count of a serial NAND flash. • ECC is better handle using the microcontroller during idle cycle.

37. Implication on Sensor Systems • Compare energy consumption of flash to CPU, radio. • writing a byte in flash is 11X more expensive than computation. • radio transmission of a byte is 200X over write access, and 500X over read access. • Suggested that storage energy should be part of the trade-off. • Applications that benefit • In-network Query Process. • Use of History • Network-level compression • Custody Transfer

38. Re-thinking Sensor Net Design • Sensor network service involve three operation: computation, storage and communication. • characterize those operations by two parameters: frequency and magnitude. • Model using a sensor service emulator.

39. Impact on Communication Service • NAND flash provides significant energy gain for batch size greater than 128 bytes. • In 1% duty cycles, it achieves 3.8 times less energy/byte with batch size of 512 bytes and 58 times improvement for a batch size of 65kbytes. • The 7.5% duty cycle has smaller preamble resulting in less fix energy cost per packet.

40. Impact on Data Aggregation • effect of compression on energy consumption. • Three type of compression: lossless encoding, lossy encoding, feature extraction. • use a benchmark wavelet compression scheme optimized for floating pointless operation with computation complexity of 60N. • Conclude that 10X energy consumption saving for using of data aggregation.

41. Conclusion • parallel NAND flash has 100 fold more energy efficient than serial NOR flash. • This observation has implication for sensor network design. • Data shows that communication and data aggregation achieves at least an order of magnitude energy reduction.

42. Dmitri B. Strukov, Gregory S. Snider, Duncan R. Stewart & R. Stanley Williams The missing memristor found

43. The four fundamental two terminal circuit elements

44. Operation