PROGRAMMABLE METALLIZATION CELL -- A NON-VOLATILE MEMORY TECHNIQUE S.Pooja sri lakshmi K.Poornima firstname.lastname@example.org email@example.com 3 / 4 ECE 3 / 4 ECE from VIGNAN’S ENGINEERING COLLEGE Vadlamudi To GAYATRI VIDYA PARISAD COLLEGE OF ENGINEERING Visakhapatnam Abstract Programmable Metallization Cell (PMC) memory utilizes electrochemical control of nanoscale quantities of metal in thin films of solid electrolyte. A silver or copper layer and an inert electrode formed in contact with a Ag+- or Cu2+- containing electrolyte film creates a device in which information is stored using large non-volatile resistance change caused by the reduction of the metal ions. Key attributes are low voltage, low current, rapid write and erase, good retention and endurance, and the ability for the storage cells to be physically scaled to a few tens of nm. This paper describes the principle of operation of PMC. Keywords — electro deposition ; non -volatile memory; resistance change; solid electrolyte. Contents: 1. Introduction 2. What is PCM? 3. Key Benefits 4. How PCM works? 5. Information storage 6. Technology Integration 7. Scalability 8. Summary of PMC characteristics 9. Conclusions 10. Bibliography Introduction Ø The electrochemical redistribution of nanoscale quantities of metal in structures containing solid electrolytes is the basis of Programmable Metallization Cell (PMC) technology. Ø Applications in electronics, MEMS, micro fluidics, optics… • We can use this effect to form and dissolve conducting pathways on command so that we have an electronic switch that can be closed and opened rapidly using small voltages. • Such switches have potential applications in non-volatile memory and programmable logic devices. What is PMCm? The Programmable Metallization Cell is a simple and elegant structure that operates as very effective non-volatile memory (NVM). The size and production process is compatible with the technology used to fabricate the smallest transistors. The mechanism that defines memory behavior is a proprietary Axon process and uses a thin amorphous film with 2 metal contacts. It makes use of a little-known feature of some amorphous materials that they can incorporate relatively large amounts of metal and behave as solid electrolytes. Under appropriate bias conditions, the metal ions in the electrolyte has to be reduced to form a conducting pathway through the material but the process can easily be reversed to recreate the insulating amorphous layer. Solid electrolytes: Solid electrolytes behave like liquid electrolytes… e e - - M M M M M + M M M M M M + O M + M M Mobile Mobil M M +M + M +M + + + + R + + + M ions e ions + M + e - Liquid Lateral/coplanar Vertical Ions move under the influence of an electric field and electrochemical reactions are possible. cathode (conductor): M+ + e- ® M reduction anode (with excess M): M ® M+ + e- oxidation The process is characterized by controlled ion motion and in recent years, this field of science called Solid State Ionics has been receiving increasing attention. Similar to the way in which the behavior of electrons in semiconductors has been exploited to create solid-state electronics and ultimately the microelectronics industry, ion mobility and the associated electrochemical phenomena in solid-state materials form the basis for revolutionary products and perhaps entirely new industries based on the concept of integrated ionics. The realization of the PMCm has been greatly assisted by the convergence and maturity of a number of contributing technologies: Ø Ultra-small devices where the ion motion required to bring about the desired electrical effect occurs in nanoseconds. Ø Barrier metallurgy, which allows hitherto unacceptable metal combinations to be used in, integrated circuits. Ø Understanding of solid-state ionics has been helped by the widespread demand for better battery performance. Ø When these background developments are applied to PMCm structures, the result is a radical new functional capability for electronic systems. Key Benefits: PMCm has a number of unique attributes that make it a highly attractive component for future systems on silicon: Ø It operates at low voltages of the order of 0.3 V. Ø High speed write and erase operations are the assets of PMCm. These can be performed within 30ns. Ø The energy required for changing the state is less than 1 pJ. Ø Physical scalability to tens of nm. Ø Easy integration is possible with the help of IC logic circuitry. It operates as a low refresh-rate DRAM or as a true non-volatile memory with high endurance (based on the programming mode). These features define a class of devices that are essential for projected electronics systems and which will be difficult to realize using developed versions of today's circuits. How PMCm Works? Silver can be dissolved in chalcogenide glasses up to many tens of atomic percent to form ternary compounds that act as high ion mobility solid electrolytes. Forming electrodes in contact with a layer of such a solid electrolyte, an anode which has oxidizable silver and an inert cathode, creates a device that has an intrinsically high resistance but which can be quickly switched to a low resistance state. Inner Workings - Nanostructure Oxidizable electrode Inert electrode <10 nm Glassy insulator <2 nm Superionic region At an applied bias of a few hundred mV in stacked thin-film structures, the silver ions are reduced at the cathode and the silver in the anode oxidized. The result of this electrochemical reaction is the rapid formation of a stable conducting electro deposit extending from cathode to anode. Inner Workings – Switching Oxidizable electrode + Io Inert electrode n Iprog = s nA - mA Glassy insulator VT = 0.25 V El Super ionic region e ct Electrodeposited metal - ro n s The electro deposit acts as a conducting link between the electrodes and hence the resistance of the device can be altered by many orders of magnitude via this non-volatile electrically stimulated deposition process. A reverse bias will cause dispersion of the link, returning the device to a high resistance state and this write-erase cycle may be repeated many tens of millions of times per second. This reversible switching effect and the associated large change in resistance of the device is the basis of the Programmable Metallization Cell memory (PMCm) technology. Information storage Ø We store information as reduced metal in the solid electrolyte. Ø Write in “forward bias” to inject and reduce ions. – A few thousand electrodeposited atoms is fC charge range. – Ion mobility as high as 10-3 cm2/V.s and internal field around 105 V/cm leads to electron deposit growth rates of 1nm/nsec. Ø Erase in “reverse bias” to remove excess metal. – Decrease concentration by oxidation of electro deposit. Ø Read options involve detection of amount of reduced metal in electrolyte. – Resistance change is large and easy to detect. – Other sensing options also exist. Communication and Computing Applications Are Converging • Cell phones are adding more computer- like functions: – Graphics, photos, video – Games, sounds, music – E-mail and text messaging – Web page viewing – Java apps • Handheld devices are adding wireless connectivity: – BlueTooth – TCP/IP • All these application need big, non- volatile memories • Consumers want speed, small size, and long battery life! Technology Integration Today's computer chips use very dense arrays of logic transistors interconnected by up to 7 layers of metal tracks. Two key features of this technology are the introduction of barrier layers that allow good conductors such as copper (or silver) to be used. These metal layers also have to be coupled selectively using metal plugs which fill small vias (or pores) in the inter-metal dielectric layers. By substituting an amorphous layer for a metal plug, we create a PMCm or a selectively switchable connection between metal layers. This can either be used to configure the logic configuration or as part of a local memory array where the transistors in the underlying silicon provide the drive and sense circuitry. By making a very small change in the total chip fabrication process, PMCm technology minimizes its cost and leverages the rapid advances that have been made in mainstream silicon processing but at the same time offering radically new system solution possibilities. The Market Opportunity for Memory is Huge Flash Content for Digital Cell Phones 1,200 200 180 1,000 Avg. Mbit Flash Content Millions of Cell Phones Cell Phones 160 Avg. Mbit Flash 800 140 120 600 100 80 400 60 200 40 20 0 0 2001 2002 2003 2004 2005 Scalability: Over the past 30 years, computing power for a given cost has increased at a rate of about 30% per year. This benefit is largely due to continuous technology development that has allowed the critical dimensions of transistors to be steadily reduced from 6 microns in 1971 to 0.13 microns today. The size reduction increases speed and it has been achieved without a pro-rata increase in cost. The reduction in transistor size is often called Moore's Law and it is now used as an international benchmark of industry progress. The International Technology Roadmap for Semiconductors (ITRS) is a formal representation of the anticipated developments in semiconductor technology. The next 4 years are mapped out in some detail and approximate projections are made to 2014. It therefore provides a series of performance criteria that can be used to assess new and evolutionary technology on an objective and quantitative way. Since the dimensions of the PMCm are compatible with those of transistors, the technology evolution implied by the roadmap is directly applicable. More significantly, some roadblocks are also avoided. Power dissipation in small devices is a major problem and to reduce its impact, supply voltage levels have been systematically reduced in line with device dimensions. The prospect of operating current memory designs below 0.6 volts in 2014 is daunting but the PMCm does not face this limitation. The electrochemical dissolution process operates at potentials below 0.5 volts so the scalability requirement for future technology is satisfied. In this respect alone, the PMCm is unique among future memory contenders. Additional benefits come from scalability of other performance measures such as speed and power. Summary of PMC characteristics Ø Scalable – Nanostructured materials allow small physical device size. – Scalable voltage, current, power, and energy! Ø Flexible functionality – Non-volatile, fast, good endurance. Ø Manufacturable? – Simple structure and simple process. – BEOL additive with copper protocol. – Devices do not consume silicon real estate. – Potentially removes the barrier between memory and logic. – Opens up EPLD applications space. Conclusions: This paper has described the principle of operation of Programmable Metallization Cell memory. Low resistance is of considerable use in non-memory applications, such as interconnection elements in programmable logic. Devices based on Ag- or Cu-doped WO3 also show great promise as memory devices. The main advantage of this approach, particularly with Cu, is that materials that are already in use in advanced integrated circuits can be “converted” into embedded memory elements at very low cost. The low resistivity of the electrodeposits means that even a nanoscale link, in the order of a few tens of nm in diameter, bridging an electrolyte a few tens of nm thick, will result in an on resistance in the order of 10 - 100 k.. An electro deposit a small means that the entire device can also be shrunk to nanoscale dimensions without compromising its operating characteristics. This physical scalability, combined with low voltage and current operation, suggests that extremely high storage edensities will be possible. The other benefit of forming a small volume electro deposit is that it takes little charge to do so – as little as a few thousand deposited atoms will result in a stable low resistance link and this will require around 1 fC of charge. The charge required to switch a PMC element to a non-volatile low resistance state is therefore comparable to or lower than the charge required for each refresh cycle in a typical DRAM. Bibliography:  M.N. Kozicki, M. Park, and M. Mitkova, “Nanoscale memory elements based on solid state electrolytes,” IEEE Trans. Nanotechnology.  M. Mitkova, M. N. Kozicki, H. Kim, T. Alford, “Local Structure Resulting From Photo- and Thermal Diffusion of Ag in Ge-Se Thin Films,” J. Non-Cryst.  M. Mitkova, M.N. Kozicki, H.C. Kim, and T.L. Alford, “Thermal and Photodiffusion of Ag in S-Rich Ge-S amorphous films,” Thin Solid Films.  www.asu.edu - arizon state university.
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