The phases, they are a-changin’
According to Numonyx (recently acquired by Micron) and Samsung, the times may indeed be a-changin’ when it comes to memory system design. Just like theoretical physicists have been in search of a unified field theory to explain everything from gravity to electromagnetics to nuclear forces, memory designers have been in search of the ultimate memory that can do it all – fast access, non-volatility, high density, low cost, and low power consumption. Using phase change memory technology licensed from Ovonyx, Numonyx has developed the P5 and P8 phase change memories and offers them in both serial(P5Q/P8Q) and parallel(P8P) interface versions using 90nm lithography. The other major competitor in the PCM space, Samsung, has a 60nm 512Mb PCM available, also using Ovonyx technology.
Ovonyx’ first granted US patent is 6,314,014: Programmable Resistance Memory Arrays with Reference Cells. This first disclosure of a phase change memory array still has 9 and a half years of life and forms part of the IP Ovonyx has successfully licensed out to Samsung and Numonyx. USP 6,869,883 introduced the idea of forming a vertical heater under a planar layer of phase change material, and this patent still has over 12 years of life left. USP 6,914,255 replaces the transistor or diode access device with an ovonic threshold switch, itself made of chalcogenide material. This decoupling of the selector from crystalline silicon erases the limit imposed by planar geometry, enabling stacked arrays for double or quad density. What we see in the Omneo P8P device itself is USP 6,989,580 disclosing PNP bipolar access elements. It’s clear that Tyler Lowery at Ovonyx and the gang at ST Micro have a very strong IP position and stand to make some royalties should the PCM market ever take off. More info is available in UBM TechInsights’ Patent Landscape Report.
How does phase change memory work? It’s based on the chalcogenide alloy Germanium Antimony Telluride, Ge2Sb2Te5. Originally discovered by Stanford Ovshinsky(who is the namesake of the term “ovonic” as in “ovonic threshold switch” and later the company Ovonyx), this material can be reversibly melted, solidified into an amorphous state, and crystallized into a polycrystalline state to modulate its resistivity. The polycrystalline state is about 20 times more conductive than the amorphous state, allowing a sense amplifier to distinguish easily between the two states. The actual change of phase is carried out by placing a joule heating element (think light bulb filament) in series with and adjacent to the GST material. Current pulses heat the GST, and their power and duration can be selected either to melt (amorphize) or crystallize the GST.
SEM cross-section in the bitline direction of Omneo PCM cells
So why is this better than Flash or DRAM? PCM has a number of interesting advantages.
- It is bit-alterable, meaning individual cells can be altered without first erasing an entire block and rewriting the block with that one value changed. This eliminates a huge amount of system overhead relative to flash.
- The write throughput is also faster than NOR or even NAND, perhaps because the phase change is fast and self-limiting, whereas the channel hot electron programming of flash is not self-limiting and must be done slowly with lots of monitoring to guard against over-program.
- PCM matches the read latency of NOR flash and the read bandwidth of DRAM. Combine this with its obvious nonvolatility, and it can therefore be used as code execution memory.
- PCM write cycle endurance is at 1 million cycles, which beats most other NVM available, and as an added bonus, it is radiation hard. Ionizing radiation can disturb the fragile balance of charge in DRAM and SRAM, but it cannot create the concentrated thermal energy required to change phase bits.
- PCM scaling doesn’t seem to be limited to a first order by the chalcogenide/heater system itself. Current bipolar-accessed products are going to have to change to another type of access element to achieve useful densities in the multi-gigabit range. Ovonyx has already provided a possible solution in the ovonic threshold switch, which was used to demonstrate a stackable 3-D PCM array.
What this all means is that for certain systems, PCM can act as the code storage, code execution, and working memory as long as nonvolatility of working memory is valued over the write performance of DRAM for that particular application. My first thought is using PCM as firmware storage medium for devices such as Blu-Ray players which expect the end user to update the firmware regularly. With high cycle endurance, a Wi-fi enabled PCM Blu-Ray player could update its own firmware over your 802.11N network every day and never run into trouble.
What about power? Joule heating is not one of nature’s most efficient methods of getting the phonons bouncing. PCM loses the power race with DRAM and NAND by only about half an order of magnitude according to a Numonyx white paper on PCM in new memory usage models. However, power consumption can be shaved by fine tuning the volume of GST utilized in the phase change and the resistivity swing required to differentiate between states.
Another wrinkle is that the process of lead-free soldering can heat the PCM array to the point of crystallizing all the bits to a “1” state. Preprogrammed PCM chips would be erased as soon as they were soldered into a system. System designers must therefore provide a facility for in-system programming. And UBM TechInsights, if we want to read contents of PCM dies, must do so with packages still attached to circuit boards, having never seen a hotplate or a heat gun. This rad-hard technology is not heat-hard.
Samsung, as a maker of mobile handsets, already has designed their PCM into models slated for release later this year. Micron’s acquisition of Numonyx places it into competition with Samsung in the PCM arena to add to NAND flash and SDRAM. This rivalry may drive the development and adoption of PCM as much as the merits of the technology itself. In spite of it all, the king of storage is still the venerable floating gate, and the king of floating gate memories is the NAND flash. PCM is currently best suited as a drop-in replacement for NOR until its densities get up to NAND levels. As long as PCM’s market is limited to this type of segment, it could still turn out to be “just a phase.”