New Technology Phase Change Memory

Introduction

My Dear friends this article discussed molecular memory, phase change memory, holographic memory, and magneto-resistive RAM. In this article I thought I’d revisit that article and expand a bit on the section discussing phase change memory. This will also give me a chance to introduce some advances in the technology recently announced by IBM.

Background

To understand the new advances made by IBM researchers you’ll need to know the basic science behind phase change memory and the challenges associated with it. Phase change memory is really very similar to the technology used in CD-R and DVD-R technologies.
In optical disc writing, a laser changes the opacity of small regions on the disc. The laser can make the small region more opaque or less opaque. It does this by changing the material from an amorphous state to a crystalline state or from a crystalline state to an amorphous state. When a disc reader then reads the reflection from the disc the reader will be able to determine if the region is in an amorphous or a crystalline state thus reading either a logical ‘1’ or a logical ‘0’.
Likewise, phase change memory also changes a small region from an amorphous state to a crystalline state and back again. In phase change memory implementations though, this change is done via an electrical current instead of a laser. More precisely, it is the heat that can change the state the optical disc and phase change memory implementations merely change how that heat is created/distributed. In my previous article Memory Storage, I highlighted the scientific similarities of resistivity to opacity, which even includes reflection!
Phase change memory systems can determine whether a region, often called a cell, is in an amorphous or crystalline state by measuring its electrical resistivity. This leads to one of the major advantages of a phase change memory system; the resistivity levels of amorphous and crystalline states are drastically different. This allows us to implement different levels of crystallization so that each region can have not just two states but potentially several states. This type of storage where one region can have multiple states is referred to as MultiLevel Cell (MLC) storage.

Advantages

Since phase change memory implementations are quite expensive to produce it is believed that a significant implementation of MLC storage will allow for very large amounts of storage and justify the high price of the technology in a competitive market. Of course, MLC storage isn’t unique to phase change memory implementations; it’s also seen in flash based memory. In flash based memory storage solutions it’s common to see four states supported per cell. This provides two bits of storage for each cell - as opposed to one bit per sell for Single Level Cell (SLC) storage.

Figure 1: attributes of phase change memory (PCM) compared to other types (Courtesty of hothardware.com)

Other than MLC, what are some other advantages of phase change memory solutions? Well, like flash, there are no moving parts which increase longevity and usability. Also, since the cell states are changed with heat via an electric pulse, there is no danger in having small magnets in close proximity to the memory. Also, these electrical pulses can change the states very quickly which can result in significantly faster write times when compared with flash.

Challenges

There are some drawbacks to MLC storage. Since there are more states stored in a single cell there is less of a difference between these states which can result in a higher bit error rate (BER). In flash based memory storage solutions this can be adequately handled via an increased role for error correction modules. The increased error correcting could potentially slow down an application, but this is the trade off for getting basically double the storage in the physical area.
In phase change memory the effect of having four states per cell is even more dramatic than in the flash based implementations. This is due to the cells having what’s known as a resistance drift. A resistance drift describe the effect of the resistance level of the different states chaning - or drifting - over time.
So you may be asking - what’s the big deal about this resistance drift? Can’t we just store a known value in some cells and measure the drift over time, then compensate for the drift when reading from the other cells? Yes, we could, if the drift was uniform across the cells. It’s not. The resistance drift is somewhat erratic and cannot be compensated for in this manner.