Introduction

The history of the flash memories is closely linked to the history of the digital camera. The first compact flash data carrier with a capacity of 4MB was presented by SanDisk in 1994. In 1998, Sony presented the first memory stick and in 2000, IBM presented the first USB stick with a capacity of 8 MB. Hints of the SSD subject have been an issue within the electronic sector since 1999, but only in 2005 Samsung prepared the market for SSD with the introduction of the 1.8“ and 2.5“ SSD. Due to the decrease in pricing (partly caused by “shrinks”) and the opening of the mass market, e.g., as a substitute for a HDD, the SSD has been predicted a great future.

Differences within the NAND Technology

In general, we can distinguish between two different types of NAND flash, the SLC and the MLC technology:

SLC = single level cells. SLC are memory cells consisting of NAND flash, in which one bit can be stored per memory cell.

MLC = multi level cells.

MLC means more bits of data (2 or 3 or 4bit) can be stored in the same cell.
This means:
* Higher writing density
* Higher access time
* Cheaper price / flash
* Higher error rate

Therefore, there are advantages and disadvantages to both technologies: The SLC technology has a faster read/write cycle, but is more expensive whereas the MCL can store more bits on one cell, but is slower and more vulnerable.

Technology: Erase-Write cycle

The erasing procedure deeply meshes with the cell and its oxid layer. The cell’s condition, i.e., the stored electrons within in the floating-gate, has to be modified with a high (negative) voltage. Before you can re-write a cell, its content has to be deleted. This is why you call it an Erase-Write cycle.

When being deleted, electrons go through the oxid layer (due to the high voltage, this has to be done block by block). This means, each deleting process damages the oxid layer a bit more (= makes it thinner). In the end, the cell is destroyed.

The electrons are no longer “imprisoned” within the floating gate, i.e., the cell changes its state and the information which was stored in the cell (1 bit) is lost. This is the reason why manufacturers only mention a certain life cycle (or Read-Write cycle) in the data sheets.

For SLC, this means approximately 100,000 cycles, for MLC it’s 10,000 cycles and a 10-year data preservation. These values go for a temperature of 25°C most of the time.

The block by block deleting causes differences of speed between reading and writing (e.g., read: 100MB, write: “only” 80 MB). These values apply to the whole SSD, not to only one single chip.

The defect of one single cell does not destroy a flash memory. Failures of single cells are recognized by an error detection program and are recorded in a secure area, the so-called “spare area”.

A so-called “Wear-Levelling” procedure ensures a long lifetime of the flash memory: read-write cycles do not always start at the address 0, but are equally distributed within the whole storage area of a chip.

Due to the smaller structures another problem might occur: the deleting process could influence the block next to the affected one which would mean data loss.

Error-free data preservation is another important parameter of a flash cell. This is a minimum period of time guaranteed by the manufacturer in which a stored information stays error-free / accurate, i.e., in which there will be no data loss.

But this means, the exact number of read-write cycles of NAND flash memories cannot be determined.

Limits of the shrink process

The semi-pervious isolated layer of flash semiconductors, which is supposed the be very thin, is quite critical around the floating gate. Approximately 10 nanometers (nm) are considered as the lower limit nowadays. This means, it is very difficult to produce structures below 43 nm. The isolated layer is supposed to admit the electrons with little effort (low write voltage) and to keep them up to 10 years without loss. When deleting before re-writing, the electrons should be removed with little effort as well (low erase voltage). At the moment, the flash manufacturers move around the range of 43 Ö 45 nm. To be able to shrink even more and to keep the electrons (= the stored info), scientists and manufacturers have to find new materials. We cannot simply expect the flash prices to decrease continuously. Many difficulties have to be smoothed out so that the SSD can set out to conquer the world.

I personally think, scientists will find solutions to this matter Ö and go as far as the organic memory. When I graduated in 1983, the general opinion was that the limit for silicon structures could be 100nm and that this should have been achieved by the year 2000.

Life cycle of a flash memory, lifecycle of a card / SSD

This topic / question (How long can I use a flash card, an USB stick or SSD? How long is its life cycle? Will my data be stored safely or will I lose important files, documents, pictures or videos?) gains more and more importance.

This has always been an issue in the flash card business and is now coming up again in the course of the introduction and mass application of the SSD, in particular re. industrial customers.

We have to discuss the important components of flash cards, USB sticks or SSD separately: This means, the topics should be: which flash and which controller is used? Which measures have been implied into the controller by the manufacturer (in the so-called firmware)? Which steps can the manufacturer of a flash card / SSD take?

This is partly a question of what is feasible re. the component and re. the firmware / software programming, i.e., how much effort can I put in this and still offer an economically sensible solution?

As discussed above, there is a certain lifecycle mentioned for the flash cell Ö a lifecycle considered to be certain by the flash manufacturer. The flash cell itself might be able to stand more read-write cycles (as already proved in various tests and studies). E.g., instead of the 100K cycles mentioned in the datasheet, a flash cell has been tested up to 300K, 500K and more.

MSC Vertriebs GmbH will soon offer a tool to display a tendency re. the lifecycle of compact flash cards. For this, the writing intensity, the erase counter and the spare blocks will be taken into account. This is not easy considering the above mentioned set of problems and not yet having discussed the future application of the flash card / SSD. The application is a very important topic when we bear in mind the various different application possibilities such as notebooks, industrial PCs, raid-systems, PDAs, navigation systems, MP3 players, medical engineering, cameras, video devices, servers, military applications, automotive accessories, etc.

If a flash is mostly read and seldom re-written, the lifecycle question will scarcely pose itself, only data storage is an issue when temperature (of storing) is relevant.

But if a flash cell is continuously re-written, we have to take into account which controller, which firmware, which wear-leveling, which bad-block management, which temperature, which flash and how many Bit ECC revisions have been used and how many spare blocks have been reserved. The ECC information, for example, differs in revision / correction from manufacturer to manufacturer from 2bit to 4bit to 6bit to 7bit.

Depending on the manufacturer, 2% of the total memory capacity of an SSD are reserved as spare block, for example. The bad block management can determine damaged cells and replace them by spare ones Ö as long as there are spare ones left. Some SSD manufacturers already implied the S.M.A.R.T. function (known from HDD) “onboard”. SMART stands for Self-Monitoring, Analysis and Reporting Technology. This means, we are talking about error and limit monitoring, temperature monitoring, state scans, operating hours counter, etc.

How does SMART work?

SSD monitors and analyzes specific parameters and deduces imminent error conditions from these results.

If there is the danger of an upcoming error, the SMART status will report it.

Flash cards, USB sticks and SSD

MSC offers the whole range of flash technology for the market and for the user. This means, before offering a flash card, a USB stick or an SSD, you have to talk about the requirements of the user and the corresponding application. To facilitate this, MSC has designed its own internal questionnaire which should clarify the following aspects in advance:

Is it a consumer (= standard device) or an industrial application?

Read and write speed, access time, MTBF, temperature range and the budget help to find and recommend the best suitable card / USB / SSD Ö they are also indicators for the price of the component. MSC can design, develop and offer the SD card, the USB stick and the compact flash card according to the customer’s specifications. MSC closely cooperates with Hyperstone, a controller manufacturer, and Samsung (= market leader in the flash area). The Samsung flash quality as well as Hyperstone’s flexibility and experience in the controller segment ensure an optimized interaction between the components and MSC as card manufacturer.

This is a typical requirement for so-called OEM versions. MSC can also take care of the programming, the labelling and the packaging.

At the moment, the following types are available:

SD: 256MB to 2GB

SDHC: 4GB to 16GB

Capacities smaller than 256MB are available as OEM versions upon agreement with the customer Ö and in adequate quantities. MSC can offer components by the following manufacturers: SanDisk, Samsung, Kingmax, PQI, TRS-STAR and MSC’s own brands.

Regarding compact flash cards, MSC can offer SanDisk, PQI and TRS-STAR. MSC’s attention is particularly turned to the industrial requirements, i.e., the cards can be offered in extended temperature range and industrial grade (à for specific applications).

MSC can offer USB sticks by SanDisk, Kingmax, PQI, AddOn, AENEON and TRS Star.

For SSD, MSC deals with and offers the following manufacturers: Samsung, SanDisk, STEC und Mtron.

At the moment, SSDs are available as 16GB to 64GB versions, 128MB soon to come. In many cases, the price might not yet be acceptable, but this is going to change within the next one or two years. And in addition, more and more SSD will be available with newer MLC-flash and a better controller. This will be the breakthrough in the mass market.

SSD is very impressive re. access time: it is much faster than mechanical hard disks and has a more robust stability. Most of the SSD are offered as a 2.5”-version (there are also 1.8” and 3.5” available). The installation of such hard disks in 2.5” hard drives does not cause any problems. The assembly in a 3.5” hard drive can be realized by a simple fitting frame.

Flash products can also stand a higher range of operating temperature than mechanical hard disks. The latest flash components have a converted life cycle of about 100 years (2 million hours for SSD vs. 300K hours for HDD).

Example: Comparison of an SSD 32GB and a HDD

	SSD	HDD
Performance in idle periods	0.1 watt	0.8 watt
Access Performance	0.5 watt	2.0 watt
Access Time	0.1 ms	2 ms
Shock operation	1500 G	350 G
Noise emission	0 db	29 db
Weight	54 g	102 g
Read	120 MByte	50 MByte
Write	100 MByte	50 MByte

When we talk about an SSD, we have to be aware of the fact that there are various different assembled versions: SLC or MLC, differences in controllers, firmware, layout, structure, ECC, temperature range MTBF (Mean operating Time Between Failures). Therefore, there can be enormous price differences for the same capacity. E.g.: 16GB SSD with MLC and a cheap controller (read: 30MB, write: 20MB) would cost around 160 €, whereas a 16GB SSD with SLC and a better controller (read: 100MB, write: 80MB) could cost 300 €.

Trend/Tendency

According to iSupply, the flash market was worth approximately 13.9 billion $ in 2007 and is supposed to grow to 15.2 billion $ in 2008. In 2009 we will talk about 128GB (MLC) USB sticks, 32GB Compact Flash Cards (SLC), 32GB SD cards and 256GB SSD. The SSD market might grow to more than 10 billion $ till 2012 according to analysts. Its main share will be mobile devices (as already mentioned above).

Dipl.Ing.(FH) Robert Herth
MSC Vertriebs GmbH