Thanks for the LNV Memory
Larry Morrell – The most important criterion is overall cost. A Flash process will cost at least 30% more to manufacture the wafer over standard logic NVM. But the designer needs to remember that the cost will be born for the entire chip, not just the part of the chip that is memory. So the larger the chip, the higher the Flash premium (at the chip cost level). However, if the design requires a large bit count and tens of thousands of cycles over the life of the product, Flash is ideal.
The second criterion is availability of the Flash process. Flash requires additional development time for the additional manufacturing steps and so lags the standard logic processes by at least 2 generations. Flash has only recently become broadly available in 0.18 um, while advanced digital designs are now done routinely in 90 nm – two generations more advanced.
David Sowards – In general, the main criteria are density of the memory required along with cost. If large amounts of NVM are required, in the 1Mb or greater range, then embedded Flash is typically chosen. For smaller amounts of memory in the 64Kb-or-less range, embedded Flash would be cost prohibitive since there is a significant adder to the wafer cost for the additional masks. Thus LNVM is the better choice there. An additional consideration is the relative amount of NVM to the entire SOC. For the particular LNVM type of technology, a multitude of factors should be considered. But for the greatest risk reduction, one should choose a technology which has been proven to be as reliable as any traditional NVM technologies.
4) Please explain how various metrics play into the argument for LNVM?
Charles Hsu –
* Robustness (with respect to temp, radiation, trauma, stress) – Same and better than embedded Flash due to process simplicity.
* Retention – Same and better than embedded Flash due to lower endurance application
* Cost – Much lower than embedded Flash due to 7-to-9 masking layers less.
* Performance – Better than embedded Flash, because lower power and higher speed are achievable.
* Security – OTP has better security than embedded Flash.
* Test – Same or less cost due to the reduced testing time.
Larry Morrell – There are many different types of logic NVM, so the answers are varied. Flash is multi-time programmable (MTP) and uses a floating-gate approach to store charge to determine a memory state. Some logic NVM technologies use the same approach – storing charge on a floating gate – and so would have very similar technical characteristics to Flash relative to retention, temperature, radiation and voltage stress.
Other technologies are one-time programmable (OTP) and use either a fuse or anti-fuse approach. In the fuse approach, a circuit element is damaged on purpose to create an open circuit. This can be done electrically by putting more current through a conductor (i.e., a poly line) than it can handle and thermally destroying the conductor. Other techniques involve cutting metal lines with a laser after the wafer is manufactured, but before it is packaged.
The anti-fuse uses the approach of creating a short circuit. This is typically done by applying a high voltage across a gate and deliberately breaking down the gate oxide.
Both fuse and anti-fuse have the attribute of making physical changes in the device which are hard to validate at the time of the event. Since some annealing can occur at high temperature operation, this could cause “repairs.” That is, the “blown” poly has a property where current can flow through a fuse that wasn’t completely destroyed and in the case of anti-fuse, soft breakdowns can be mistaken for hard breakdowns and could behave differently over operating temperature. Because of the reliability issues with these technologies, some users apply a voting technique, where three fuses are used and they “vote” on the state of the bit. This error correction works for most applications.
David Sowards –
* Robustness (w.r.t. temp, radiation, trauma, stress) – This is, of course, driven by the end application. For those applications that require high reliability under stress, then a technology like NOVeA is an excellent choice because of its proven floating gate reliability over many years of production.
* Retention – Any particular LNVM technology should be able to support the same 10-year data retention as Flash
* Cost – As stated above, LNVM is more cost effective for relatively small amounts of NVM (< 64Kb).
* Performance – Since LNVM is an embedded solution, it allows for high performance due to the fact that you do not have package parasitics such as inductance and capacitance that exist with a discrete solution.
* Security – Since LNVM is embedded in the SOC, it is a much more secure solution than discrete approaches such as EEPROM. With a discrete approach, the data is exposed as it crosses pin boundaries between the NVM and the controller and thus it becomes a security risk. With LNVM the data is embedded and does not cross pin boundaries, which allows for a much more secure solution.
* Test – If a multi-time programmable LNVM is chosen, then you get the benefit of 100 percent verification at test. This is important in that it greatly reduces the likelihood of field failures. If a one-time programmable option is chosen then you do not know if the memory has weak cells and you can get field failures which are very expensive.
5) Please briefly articulate the advantages/disadvantages of one-time versus multi-time programmable LNVM (OTP versus MTP).
Charles Hsu – OTP has 1) smaller size, 2) less IP area, 3) better retention, 4) fast development cycles, 5) lower cost, and 6) higher data. Of course, the disadvantage is that OTP can only be written once.
Larry Morrell – This is mostly an application-specific question. If the use of the product requires a potential for frequent changes to the memory, then multi-time programming is needed. Also, if the user’s reliability requirement necessitates 100% testing of the bits to be programmed, usually multi-time programming is needed even if the end application doesn’t require frequent changes to the data. On the plus side for OTP, it is usually smaller than MTP since it doesn’t have all the erase/rewrite circuitry on board.
David Sowards – For applications that truly require only one-time programmable and require larger amounts of memory, in the 1Mb range, one-time programmable will generally be more cost effective. One drawback of OTP is that it can not be tested. If the memory is shipped to the end application unprogrammed there is no way to test the memory if it is OTP. With MTP, however, you could test it prior to shipping to the end application and thus you know you have a good memory.
MTP can be used in a greater number of security applications because of the fact that it can be reprogrammed. In the event that somebody gets to the encryption key that may be stored in the NVM, you can reprogram the key and thus you are no longer exposed. With OTP you can not reprogram it and thus if a hacker gets the key, the system is no longer secure. A greater number of security and digital rights management applications are turning to MTP for this reason.
6) What is the average data retention today for an LNVM product? Should we expect that to improve over the next several years?
Charles Hsu – More than 10 years, which is satisfactory for consumer electronics applications.
Larry Morrell – Current technology allows for OTP and MTP to be specified from 10-to-50 years for nominal data retention. These figures are based on life testing which, in some cases, takes years to collect. The technology currently deployed may well have longer retention, but there just haven’t been enough years of retention testing performed to make that claim. So the specs may change in the future as more data becomes available.