Non-MOSFET Based Memory Alex Rodriguez-Triana Terence Frederick April 21, 2008

Outline MOSFET Based RAM Memory Problems with MOSFET Memory DRAM, SRAM, FLASH Problems with MOSFET Memory Scaling Alternative Memory MRAM FeRAM PCRAM Summary

History of MOSFET Memory Concept goes back to the 1960s People were speculative BJT was more advanced and faster Leakage current They were attractive Simple Processing Layout Advantages Leads to high-density integrated circuits

History of MOSFET Memory SRAM were proposed six MOSFET’s per cell SRAM began to be used in the mid-70s DRAM patented in 1968 1 MOSFET, 1 Capacitor First commercial DRAM 1971 by Intel

Dynamic RAM Most common type of RAM memory Arranged in a square array one capacitor and transistor per cell Stores one bit per cell Recharging/Refreshing : capacitors lose their charge Rows: Word Lines Columns: Bit Lines

Advantages/Disadvantages of DRAM Cost Small 1T & 1C vs. 6T for SRAM Number of Read/Write Cycles > 10^15 Disadvantages Slow Need to refresh Volatile Data is lost when memory is not powered

Static RAM Memory cell uses flip-flop to store bit Requires 6 transistors Each bit is stored on 4 transistors that form two inverters Two other transistors control the access to a cell during read and write operations This storage cell has two stable states 0 and 1

Advantages/Disadvantages of SRAM Performance better than DRAM Faster Less Power Hungry Number of Read/Write Cycles > 10^15 Disadvantages Cost More than DRAM Volatile Data is lost when memory is not powered

FLASH Memory Invented by Dr. Fujio Masuoka at Toshiba in 1984 Stores information in an array of memory cells made from floating-gate transistors Single-Level Cell Devices - each cell stores only one bit

Advantages/Disadvantages of FLASH Cost Non-Volatile Does not lose information when the power is off Low Power Fast Erase Large blocks rather than one word at a time Disadvantages Number of Read/Write Cycles ~ 10^6 Slow Write Entire block must be read, word updated, then entire block written back

Future of MOSFET Memory Current memory technologies are nearing the end Main issue with MOSFET RAMs Scalability Designers put more components onto each chip Width of the smallest features is shrinking 130 nm in 2000 to 45 nm today Existing memory technologies will be good for several more generations Unlikely to make the transition to 22 nm (scheduled for 2011) or 16 nm (2018) New types of technologies MRAM, FeRAM, PCRAM

MOSFET Scaling Late 1990s Reasons for smaller MOSFETs Scaling resulted in great improvement in MOSFET circuit operation Reasons for smaller MOSFETs Same functionality in a smaller area Reduces cost per chip Smaller ICs allow for more chips on a wafer Fab costs for wafer are relatively fixed

MOSFET Scaling Problems when scaling too small Slower chip speed Greater delay due to interconnects Operational problems Higher sub-threshold, increased gate-oxide and junction leakage, lower transconductance, heat production, and process variation Simulation Difficult to predict what the final device will look like Modeling of physical processes Microscopic variations in structure due to the probabilistic nature of atomic processes require statistical predictions

Alternative Technologies Magnetic RAM (MRAM) Ferroelectric RAM (FeRAM) Phase Change RAM (PCRAM)

Magnetoresistive RAM Under development since the 1990s Data is stored by magnetic storage elements Formed from two ferromagnetic plates Plates can hold a magnetic field Polarization doesn’t leak away with time like charge Less wear since switching states doesn’t involve movement of electron or atoms One plates is a permanent magnet Set to a certain polarity Second plate’s field will change to match that of an external field A memory device is built from a grid of "cells"

4MB MRAM 1st commercial available MRAM Based on 1T and 1 magnetic tunnel junction Isolates read and write path Separates programming components from the sense circuit Improved performance

Read and Write of MRAM Read Write Current is passed through the bit resistance of the bit is sensed Write Current is passed through the programming lines Induced magnetic field is created at the junction, which the writable plate picks up

MRAM Cell works in a toggling mode Same direction Opposite direction Low resistance state (0) Opposite direction High resistance state (1)

MRAM in Embedded Systems Inserted late in the SC fabrication process Low temperature Compatible with CMOS processing Consolidate multiple MRAM into one highly reliable NVRAM Less complexity High performance RD/WR

Advantages/Disadvantages of MRAM Non-volatile Does not require programming sequences or block erasing Very fast RD/WR and unlimited endurance Simple device Architecture and easy software development Due to easy write and overwrite Disadvantages Scalability of magnetic domain? Might have the same problems as a transistor Disturbance of neighboring cells when put close together Leads to false writes High power needed to write

Ferroelectric RAM Borrows concepts from DRAM most popular design follows the 1T1C design concept similar/same write process write accomplished by applying charge that is stored in capacitor Similarity to Floating Gate Design 1T design Also reminiscent of MRAM focuses on ferroelectric properties, whereas MRAM techniques often focus on ferromagnetic properties both characteristics take form of hysteresis loop

Structure 1T type 1T1C type Similar to normal transistor Identical to floating gate design where floating gate is ferroelectric material 1T1C type ferroelectric material serves ONLY as capacitor

“Recent Progress in Ferroelectric Memory Technology” by Hiroshi Ishiwara

Introduction Two major focuses in the paper developing a better material to deal with leakage currents in 1T1C FeRAM replace some Fe in lattice with Mn Improve upon 1T FeRAM design create MFIS-FET Introduce a new 1T2C FeRAM design

Results I

1T2C Design 2 Ferroelectric capacitors of the same size connected to the gate of the transistor capacitors polarized opposite the gate Good performance non-destructive data reads good data retention time high on/off current ratio

Advantages/Disadvantages of FeRAM lower power usage faster write speed greater number of rewrites already being mass-produced Disadvantages still more research to be done on reliability (i.e. high NRE cost) only applicable to a small niche

“Study of Phase Change Random Access Memory (PCRAM) at the Nano-Scale” by R. Zhao, L.P. Shi, W.J. Wang, H.X. Yang, H.K. Lee, K.G. Lim, E.G. Yeo, E.K. Chua and T.C. Chong

Introduction RAM based on floating-gate design (i.e. Flash memory) will soon meet physical limitations interpoly tunneling intercell crosstalk Flash memory is the most prevalent non-volatile memory on the market a viable option must be found soon PCRAM may be that option

Fabrication/Design “Bybrid” process used to etch the layers Electronic Beam Lithography (EBL) Optical Lithography Electrodes made of TiW Dielectric is common SiO2 Phase Change material is Ge2Sb5Te2 Feature size refers to contact between PC and bottom electrode

How it Works Unique Phase Change material has two states Crystalline state has low resistance and represents a stored ‘1’ Amorphous state has high resistance and represents a stored ‘0’ To change bit from 1 to 0 (i.e. RESET), a relatively high voltage is applied for a short time such that the compound melts but is not able to recrystallize To change bit from 0 to 1 (i.e. SET), a lower voltage is applied for a longer time so that compound can crystallize

Simulation Pulse generator created to produce short (<10ns) signal Known resistance placed in circuit Voltages measured to determine drop across resistor Current into PCRAM approximately (V1-V2)/Rload Cells with feature sizes ranging from 40 to 200 nm created same wafer used

Results

Advantages/Disadvantages of PCRAM great scalability fast for both reads and writes low current required to program Disadvantages as of yet, only in the research phase still limited read/write accesses (108)

Summary SRAM DRAM FLASH MRAM FeRAM PCRAM Read Speed Fast Medium   SRAM DRAM FLASH MRAM FeRAM PCRAM Read Speed Fast Medium Write Speed Slow Non-Volatile No Yes Endurance Infinite Limited Low Voltage 35