000-M96 exam
In a IBM 000-M96 PCM cell, where a phase-change material is deposited between a top and a bottom electrode, phase change can controllably be induced by applying voltage or current pulses of different strengths. These heat up the material and when distinct temperature thresholds are reached cause the material to change from crystalline to amorphous or vice versa.
We apply a voltage pulse based on the deviation from the desired level and then measure the resistance. If the desired level of resistance is not achieved, we apply another voltage pulse and measure again – until we achieve the exact level.
Despite using the iterative process, the scientists achieved a worst-case write latency of about 10 microseconds, which represents a 100x performance increase over even the most advanced Flash memory on the market today.
For demonstrating reliable read-out of data bits, the scientists needed to tackle the problem of resistance drift. The modulation coding technique is based on the fact that, on average, the relative order of programmed cells with different resistance levels does not change due to drift.
Because of IBM 000-M96 structural relaxation of the atoms in the amorphous state, the resistance increases over time after the phase change, eventually causing errors in the read-out. To overcome that issue, the IBM scientists applied an advanced modulation coding technique that is inherently drift-tolerant.
Using that technique, the IBM scientists were able to mitigate drift and demonstrate long- term retention of bits stored in a subarray of 200,000 cells of their PCM test chip, fabricated in 90-nanometer CMOS technology.
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