Nand Flash for SSD

Intel's flash chips

MLC NAND Flash applications take up the lion's share of the market with MLC to SLC in a proportion of 99% to 1%. The two Flash types differ most in the cost, read/write endurance and read/write characteristics. In the case of Toshiba, the wafer used is exclusive to SLC Flash production. When it comes to MLC wafer production, exclusive MLC wafers are stacked in manufacturing process. Same-sized wafers produce much more MLC than SLC dies resulting into a wide cost gap. However, some wafer makers use identical wafers to produce SLC and MLC dies with difference lying only in single or multiple read/write settings. Although the cost of SLC chips produced in this way is still less than that of Toshiba products, this SLC output keeps in a relatively low volume and thus the SLC chip sells at a high price.

Compared with the SLC chip, the MLC chip has higher reading speed due to the multi-channel technology. However, MLC has a slower writing speed, which needs a good controller chip to improve with its algorithm, memory cache and delayed writing. Currently, the MLC SSD is already able to keep up with the SLC SSD in writing speed.

So what's the positioning of SLC SSDs? MLC to SLC is 99 to 1 in the NAND Flash market while MLC to SLC is 90 to 10 in the SSD market. It is because SLC chips, which are more reliable in data processing, have much less data checking and correcting (ECC) cases than MLC chips. Moreover, SLC chips are better than MLC chips in terms of the read/write endurance, meaning a longer service life of the SLC SSD.

For these reasons, the SLC SSD is mainly used in military, aerospace and industrial computer fields where its high price can be afforded. For servers and workstations, the SLC SSD is also a good choice. However, this doesn't mean that MLC SSDs are not suitable for servers and workstations. Instead, due to the MLC cost reduction, new controller chip algorithm, performance improvement, adoption of multi-channel technology and DRAM memory cache technology, the high read/write speed SATA 3 SSD also fits for servers. At the same price, the MLC SSD has more storage than the SLC SSD and also meets performance requirements.

2X nm & 3X nm

In the whole list of products for DRAMeXchange SSD testing lineup, me products indicate the NAND Flash process level. Generally, the process with smaller nanometers produces more energy-saving chips. In the case of the same number of transistors, the smaller the wafer is, the cheaper the chip costs.

Nevertheless, NAND Flash chip manufacturing process differs from other semiconductor processes in one point: because of its physical characteristics, the Flash chip with more advanced nanometer technology needs more storage units for charge and discharge activity in the same space and thus the latency for data reading and writing lasts longer and the error rate is higher in chip's data processing. In this case, the controller chip is required to provide better ECC capability.

Take the 25nm MLC NAND Flash chip by certain semiconductor giant for instance: its latency is 1200ms, longer than 900ms of the 32nm chip by the same fab. If the SSD is made of 32 nm chips, it will run faster than 25nm SSD. In fact, between Q1 and Q2 2011, some popular NAND Flash chips are in such tight supply that they sell higher on average than common Flash chips.

Instead of performance pursuit, some semiconductor manufacturers tend to be conservative. The specification of their 2X nm Flash chips reads 1600 ms in latency so the SSD product with their chips will show lower performance. The SSD cannot be adjusted for overclock or overspeed as one likes unless the controller chipmaker works together with the original Flash chipmaker for this purpose. It often occurs to wafer fabs with fixed Flash chip outlets especially semiconductor giant with important customers like Apple.