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LC: ISi has come up with what might be described as a true breakthrough for memory technology. Can you give us an idea what Z-RAM is all about and why it is such a hot thing right now?

J.L.: Traditional embedded memory technologies are based on a six transistor cell, or even a single transistor, single capacitor structure. As CMOS technology evolves below 100 nm, new memory devices are being considered as DRAM, SRAM and/or Flash replacement. Emerging memory devices include Magnetic RAM (MRAM), Phase Change Memories, nano floating gate memories, single or few electron memories and molecular memories. All these new memory concepts are, however, based on the integration of new and exotic materials in a baseline CMOS process and require a multi device approach (1 transistor plus 1 other device). Introducing such memory concepts implies therefore two breakthroughs: a concept breakthrough and a materials or technology breakthrough.

By harnessing the Floating Body (FB) effect of Silicon On Insulator devices, Innovative Silicon (ISi) has taped out megabit designs for a true capacitor-less, single transistor DRAM - named Z-RAM™ (Zero Capacitance DRAM) - which is capable of doubling memory density when compared to existing embedded DRAM technology (and achieving five times the density of current embedded SRAM), yet requires no exotic materials, no extra mask steps and no new physics.

Illustration from: Zero Capacitor Embedded Memory Technology addresses dual requirements of die size and scalability
(Dr. Pierre Fazan, CTO, Innovative Silicon)

LC: Can we take this a little slower and walk through this in a step by step fashion? What you say makes it sound like the key feature of Z-RAM is a problem that others have been trying to solve for decades and ISi has actually turned it into a feature.

J.L.: No problem. The biggest advantage of Silicon-on-Insulator is the effect that it electrically isolates all devices such as transistors and capacitors. This leads to the so-called floating body effect of SOI, meaning that the body of the transistor itself maintains the charge because it cannot drain. From a classic electrical engineering standpoint, this results in all kinds of anomalies. An example is the “kink effect”, which used to be a main problem since it can lead to data corruption because the floating body effect of the pass gate transistor can mask the state of the memory cell. Solutions were, amongst others, fully depleted devices, meaning that the insulator layer was significantly thinner than the channel depletion width.

Bottom line here is that the transistor itself may not be able to get rid of a charge in an SoI device and that causes competition of one signal (stored in the capacitor) and one pseudo-signal (generated by the FB effect of the transistor). If there are two competing signals, it is necessary to get rid of one of them, for example by trying to implement the above mentioned fully-depleted devices.

The ZRAM approach, on the other hand, solves this problem by getting rid of the storage capacitor. Essentially, what that means is getting rid of the fundamental memory structure that holds the data in the form of electric charges. On the other hand, there is really no need for the capacitor anymore since the floating body effect can perfectly substitute for the capacitor. In other words, instead of reversing the insulation of SoI to get rid of the capacitance, why not turning a problem into a feature, use the capacitance and then get rid of the now redundant storage capacitor?

LC: Sounds like a really intriguing idea, essentially a classic example of Occam’s Razor. Who are the brains behind this strike of genius? Can you give us a bit of the background and how the team found together? Also, everybody loves some anecdotes, is there any story behind this invention?

J.L.: As you know Pierre Fazan, our founder and CTO, worked for Micron in Boise, Idaho for 8 years and they have more than 150 patents with his name on. When he returned to Switzerland for personal reasons, he met up with Serguei Okhonin, our co-founder and Chief Scientist, and talked about how difficult it would be for the DRAM industry to keep scaling the capacitor in the bit cell. High K dielectrics had their own problems, aspect ratios of the capacitor structure were extreme already, roughening had been used to increase area already…… One day, over (several) glasses of wine, while they were discussing this, Serguei explained that the best solution would be not to have a capacitor and use the floating body effect on an SOI transistor. After discussing more, they founded ISi.

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