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Micron patent reminiscent of IBM's latent image memory

13 Mar 2014  | Ron Neale

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Around December last year, when Micron was withdrawing the last of its phase change memory (PCM) products from their standard products list, they were issued with a PCM related patent, US No. 8605490 B2. It combines the speed advantages of the performance of an SRAM with the non-volatility of a PCM, the holy grail of a unified memory. It is remarkable that the inventor cited on the patent is Richard Fackenthal, who was also the lead author on the recent Micron-Sony 16Gb ReRAM paper at ISSCC 2014. This may be interpreted by some as confirmation of a new direction of emphasis from PCM to ReRAM for Micron.

While the patent examiner's references cited more recent activity, the operation of device on which the new patent is based has a history and prior art that can be traced back to an invention by IBM called latent image memory.

The principles of the latent image device are relatively easy to understand. If you construct a perfect cross-coupled SRAM (perfect as is in every aspect of its structure, width, thickness and resistance of conductors, gain, or capacitance of the transistors, etc.) and then gradually apply the power by increasing the applied voltage, then the data contents of the memory will come up in what appears to be a completely random state, as might be expected. However, with power applied slowly, that same memory array will always come into full operation with the same initial data content, a latent image. In fact, the data content of the memory will be determined by the smallest of differences in the resistance of the conductors in the two sides of the bi-stable element that makes up the cross-coupled SRAM bit. Once the memory array is fully powered then it can be operated in a normal manner as a high speed SRAM, the small biasing differences will be ignored.

In order to avoid the normal as fabricated randomness of the latent data, the IBM developers deliberately made one of the conductors in one of the branches of the SRAM circuit slightly narrower or modified other components in one branch of the circuit. This predetermined the latent image and, in doing so, realized the possibility of creating a non-volatile memory (in effect a read-only memory, ROM), hidden or latent beneath a normally operating SRAM without compromising the normal operation of the SRAM. They called this a latent image memory (LIM).

I believe that while the Micron work is commendable (including the free-running bistable oscillator where the resistance of the PCM controls the frequency), an opportunity to create a memory that is able to learn has been missed. (This is discussed in more detail in the later paragraphs of this article.) In IBM's original work on their latent image memory, the narrowing of a conductor was all that was necessary by way of a resistance change to provide the bias that would determine the latent state of the memory. For me, that fact raises the intriguing possibility that it should be possible to use the principles of the latent image memory to create a memory that can learn. Such a memory would not require the phase-change material, or other materials for that matter, to be switched, but would employ changes that accumulate with time and exposure to changes in its local environment, e.g. thermal, electrical, or both.

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