03 Oct 2008
Country: Korea
New metal oxide memory
There are many companies, research centers and universities working on printed photovoltaics, transistors and displays but very few work on the memory that will be essential in most of the envisaged applications of printed electronics. It is therefore interesting to see what is coming along as memory compatible with today's silicon chip electronics because these new technologies can lead to printed, flexible versions at lower cost, though not necessarily with high storage capacity or high speed, at least initially. In this respect the new metal oxide memory is of interest to practitioners of printed electronics. Here is the story:
Two layer stack
Samsung developed a two-layer stack in 2007. The problem was making sure that the nickel oxide layer used in the device survived the high temperatures called for in producing the upper metal matrix. "The nickel oxide can be grown under room-temperature conditions," claims M J Lee, an engineer with Samsung Semiconductor. "Our results show that, during operation, there is no interference between the layers."
However IET magazine reports some difficulties. It reports that Ugo Russo of Milan Polytechnic says: "The big problem is the relatively high current needed to perform the reset. It suggests that Joule heating plays a role in the reset operation."
The IET says that the problem is similar to that of phase-change memories, which also rely on the heat generated by electrical current passing through the material to switch. This kind of power is bad news for battery-powered systems.
"We investigated if we can, in some way, reduce the current. As we reduce the diameter of the oxide filament, the current reduces. That reduces both the electrical and thermal resistance of the filament," Russo claims. "The question is how to control the size of the filament," reports the IET, adding that Katsuhisa Aratani of Sony says one approach that reduces switching current is to apply a negative voltage to the bottom electrode and use pulses of current. However, he says a further problem is storage time: "The memory is better for fast switching compared with other non-volatile memories but it is not good for retention. Something needs to be added to improve the retention time."
Because the reset process seems to rely partly on heat, it is possible for the memory to reset itself slowly at room temperature. Work with materials allowed Sony to push up the retention time to around ten years at room temperature based on tests at 130°C.
"It is promising for a next-generation non-volatile memory," says Aratani, who reckons it will scale down well to the 10nm generation at least.
Before resistive RAM can make it to market, it will have to go through many more tests and manufacturability work. These are the areas that have tripped up earlier candidates for the perfect memory. So you will probably have to wait a while longer before you can kiss goodbye to DRAM and flash says the IET. But IDTechEx thinks printing memory made of nickel oxide or other oxides to create flexible electronics will be a different ball game.
Researchers have found that metal oxides can provide non volatile memory. They store data and keep it after the power supply has been disconnected. Sony has an experimental inorganic memory that relies on ions forming conductive filaments.
Samsung Semiconductor sees this so called resistive RAM as an alternative to flash memory: "The flash market grows remarkably each year but the floating-gate technology is reaching its fundamental limit of density. The NAND flash will reach its scaling limit around the 30nm or 20nm process node. Resistive RAM is a strong candidate for the next-generation memory."
Advancing FRAM
Ferroelectric RAM (FRAM) is non-volatile and used in RFID and power meters but it is proving difficult to get cost down further and memory capacity up beyond one megabit. One development route is to develop ferroelectric memory where Thin Film Electronics of Sweden is in the lead and 100Mbit is possible.
Metal oxide resistive RAM
The big advantage of the new resistive RAM is also its relative simplicity. By contrast the old DRAM needs a capacitor which, to keep the stored charge high calls for exotic shapes. Conventional flash must have extra layers built into a tiny transistor.
Simplicity
Resistive RAM is a capacitor like structure. It puts a layer of oxide at the intersection between two metal strips. If the oxide layer passes a current that is read as a '1' - if it acts as a resistor, then the read circuitry sees that as a '0'. The resistance ratio in Samsung's experimental memory is more than 100.
Resistive RAM switches in a similar way to phase-change or magnetoresistive memory - by passing a high current through it. Resistive memory has fast switching time better than flash. It takes less than 10ns to set a bit and about 10ns to reset it.
Because the RAM relies on the ability to measure changes in resistance, multilevel memories, similar to the technique used in NAND flash, are feasible. Building multilayer memories may be just a matter of adding pairs of metal layers and circuitry to address each layer.
For more attend Printed Electronics Asia 2008 or Printed Electronics USA 2008.
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Article by Dr Peter Harrop
Telephone: 0 1256 862163
Email: p.harrop@IDTechEx.com
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