IBM achieves storage memory breakthrough
Scientists at IBM have demonstrated reliably storing 3 bits of data per cell using a memory technology known as phase-change memory (PCM).
The current memory landscape spans from DRAM to hard disk drives to flash.
In the past several years, PCM has attracted the industry’s attention as a potential universal memory technology based on its combination of read/write speed, endurance, non-volatility, and density.
PCM doesn’t lose data when powered off, unlike DRAM, and the technology can endure at least 10 million write cycles. The average USB flash stick tops out at 3,000 write cycles.
The research breakthrough provides fast and easy storage, which will benefit mobile devices and the Internet of Things.
How PCM Works
PCM materials exhibit two stable states, the amorphous (without a clearly defined structure) and crystalline (with structure) phases, of low and high electrical conductivity, respectively.
To store a ‘0’ or a ‘1’, known as bits, on a PCM cell, a high or medium electrical current is applied to the material.
A ‘0’ can be programmed to be written in the amorphous phase or a ‘1’ in the crystalline phase, or vice versa.
Then to read the bit back, a low voltage is applied. This is how re-writable Blue-ray Discs store videos.
IBM scientists are presenting, for the first time, successfully storing 3 bits per cell in a 64k-cell array at elevated temperatures and after 1 million endurance cycles.
To achieve multi-bit storage IBM scientists have developed two innovative enabling technologies: a set of drift-immune cell-state metrics and drift-tolerant coding and detection schemes.
More specifically, the new cell-state metrics measure a physical property of the PCM cell that remains stable over time, and are thus insensitive to drift, which affects the stability of the cell’s electrical conductivity with time.
To provide additional robustness of the stored data in a cell over ambient temperature fluctuations a novel coding and detection scheme is employed.
This scheme adaptively modifies the level thresholds that are used to detect the cell’s stored data so that they follow variations due to temperature change.
As a result, the cell state can be read reliably over long time periods after the memory is programmed, thus offering non-volatility.
The experimental multi-bit PCM chip used by IBM scientists is connected to a standard integrated circuit board. The chip consists of a 2 × 2 Mcell array with a 4- bank interleaved architecture.
The memory array size is 2 × 1000 μm × 800 μm. The PCM cells are based on doped-chalcogenide alloy and were integrated into the prototype chip serving as a characterization vehicle in 90 nm CMOS baseline technology.
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