IBM introduces the world's first PCM memory with three bits per cell

The specialists of our company at the annual IEEE International Memory Workshop thematic forum in Paris presented a working memory prototype based on phase transition (Phase-change memory, PCM). In each memory cell is stored on three bits of data. The test sample was created on the 90-nm CMOS process and is presented as an array of 32 Mbps.

According to the developers, such a memory is very promising, since it is able to withstand several million write cycles. At the same time, the usual flash-memory - no more than 3000 rewriting cycles. The speed of the PCM memory is approximately equal to the speed of the RAM. If the development can be put into mass production, it will allow us to obtain a universal memory, moreover, in the near future. Well, now the company plans to use PCM chips for SSD, as well as buffer memory for SSDs with NAND flash as a basis.

Interestingly, PCM memory is not new in itself. But this development has not yet become widespread due to its main problem - low recording density. A three-bit PCM cell solves this problem. Earlier, Intel and Micron were trying to solve the problem of low PCM write density, which presented a multi-layered 3D structure in the form of 3D XPoint memory. Yes, this is also a solution to the problem, but three-bit memory is cheaper and somewhat easier to manufacture. In addition, a single-layer structure is more reliable than a multi-layered one, and in the latter case, mass production is fraught with the risk of obtaining a large number of defective elements.
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How PCM Memory Works

Compounds that are used in this type of memory have two physical states — amorphous (without clearly selected crystalline structure) and crystalline (where this structure is clearly visible). The transition from one state to another occurs when an electrical current is applied to the connection. The amorphous state with high resistance is used to represent binary 0, and the crystalline state with low resistance is 1.

In order to store in memory 0 or 1, you must use an electrical signal (strong and medium for 0 and 1, respectively). In order to read a bit of information, you need to use a low voltage. The volume of the substance transferred into the memory cell from the initial amorphous state to the crystalline state strongly depends on the applied current strength. Reading from such a memory occurs with the use of small currents. To write 3 bits in one cell, you need to use a rather complex 8-level signal.

Previously, as mentioned above, various teams of researchers sought to save one bit of information in a memory cell. But now the specialists of our company managed to keep three bits in the cell.


The effect of temperature on PCM memory

The new design uses two approaches. The first is the development of a reference signal to determine the level of entry in the cell. The second is the use of special programming methods for recording levels. Previously, PCM memory, in addition to a certain complexity of working with it, had another problem - exposure to temperature. Because of this, over time, the level of the entry in the cell “floated”, the information became poorly readable. Now, our specialists managed to solve this problem, and the temperature no longer presents such a serious problem as before.

Speech about the commercial use of technology is not yet, but in the near future, perhaps, a new type of PCM-memory will become the standard. Initially, we plan to release several test samples, and then to start production of commercial systems.