While on the topic of novel memories, we might as well continue down this path, this time with a transistor-less memory cell designed for non-volatile memory products. Before delving into the memory cell details and the associated physics, a little info about the company, namely Unity Semiconductor, might be of interest. Based out of Sunnyvale, CA and founded in 2002, Unity Semiconductor emerged from stealth mode just about a month ago as it announced that it raised additional $22 million in Series C funding. This brings to total amount of funding that the company has received to a very respectable $75 million over the last few years, proving that an interesting idea in the semiconductor space will attract some serious money. The company was founded by Darrell Rinerson, Ed Ward and David Bostwick, all of whom seem to have had stints at some of the major semiconductor companies, including Micron and AMD. It is fairly unusual for a company to wait this long before existing stealth-mode, but just like with the device physics that took time to calculate, Unity Semi has also been calculative on how to penetrate the market and maintain control of their new conductive metal oxide and ionic motion (CMOx) technology. For this purpose, the company has amassed a war chest of patents before existing stealth-mode. How Many? How about in excess of 60 granted and another 90 applications currently pending, so feel free to knock yourself out examining them.
As mentioned before, the technology is targeted at the non-volatile memory (NVM) market which the company beliefs will grow to more than $25 billion by 2013. Currently, this market is being served by NAND flash technology; however there is a concern in the industry that NAND technology scaling might be reaching its limits. Hence, the recent spur in the development of new technologies that are to replace NAND flash including: resistive ram (RRAM), ferroelectric RAM (FeRAM), phase-change memory (PCM), magneto-resistive ram (MRAM), multi-chip packaging (MCP), and finally 3D-memory. Unity Semi briefly discusses the cons for each of these technologies in section 7 of this document. The company intends to tape-out a 64-gigabit product in the first half of 2010 and ramp up to volume production in the first half of 2011.
The figure above shows a cross section of the CMOx technology. On a high level, through the application of a high electric field across the tunnel oxide (TO) layer, the charge buildup in the tunnel oxide can be controlled which leads to a change in the trap-assisted tunnel current through the oxide, or in other words a change in the device resistance. If that just went over your head, you’re not the only one for sure; thankfully Unity Semiconductor has a presentation that discusses all of this in detail, including oxygen ion mobility and the associated memory effect. This is a very good presentation indeed and a detailed paper is included as well, so it is well worth your time. The company claims that the current difference between the two resistive states is approximately 10x and that the program and erase times are in the 1us range at +/-3V. Compared to flash NAND technology, Unity Semi is claiming a 4x density improvement and a 5 – 10x write speed improvement.
Overall, Unity’s technology is interesting and impressive at the same time, which explains why the company has been able to raise this much capital. The BEOL approach to commercialization, where the memory layer with CMOx technology is deposited on top of a regular CMOS logic layer by a secondary fab seems possible as well, although it will require a good partnership with an integrated device manufacturer (IDM). At the same time, NAND flash technology should not be written off too quickly. For years, people have predicted the end of transistor scaling due to various reasons, and yet the scaling continues and if you talk any of the major semiconductor players you will find them to be optimistic about being able to continue this trend for several process nodes to come. From published material it seems that the program vs. erase current is predicted to scale with device size, but it will be interesting to observe whether Unity Semi can maintain the 10x difference, especially as the technology is moved to next process nodes. On the flip-side, because of the BEOL approach, the company does not need to necessarily move to the latest and greatest process, since the CMOS logic layer is able to use a different process than the CMOx memory layer, which is a very nice option to have.