electronics by lakshmidhar



                         -- A NON-VOLATILE MEMORY TECHNIQUE

   S.Pooja sri lakshmi                               K.Poornima

poojasrilakshmi@rediffmail.com               poornima_kanagala@rediffmail.com

      3 / 4 ECE                                     3 / 4 ECE







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.

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
   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
                                                               O      M
                                                                    M M           Mobile
   Mobil         M M +M                            +   M
                                              +        +
                                                               R    +   +
   e ions           +  M
                       +                                                      e
                   Liquid             Lateral/coplanar             Vertical
Ions move under the influence of an electric field and electrochemical reactions are
       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
                  Inert electrode       n               Iprog =
                                        s              nA - mA
                  Glassy insulator

                                     VT = 0.25 V         El
                  Super ionic region                     e
                  Electrodeposited metal
                                                   -     ro

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
                                                    –   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

                                                                                        Avg. Mbit Flash Content
  Millions of Cell Phones

                                           Cell Phones                            160
                                           Avg. Mbit Flash
                             800                                                  140
                             600                                                  100
                             400                                                  60
                             200                                                  40
                               0                                                  0
                                    2001         2002        2003   2004   2005

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.

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.


[1] M.N. Kozicki, M. Park, and M. Mitkova, “Nanoscale memory elements
based on solid state electrolytes,” IEEE Trans. Nanotechnology.
[2] 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.
[3] 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
[4] www.asu.edu - arizon state university.

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