Both use NAND technologies

Single Level cell (SLC) and Multi Level Cell (MLC) are two common NAND flash technologies in use today. As we know SLC is more expansive while MLC has more capacity in the same size. Here is a quick matrix in comparison.

  SLC MLC e-MLC
Price High low Medium
Endurance high low Medium
Performance high low Medium
Capacity less high Medium

 

It is important to understand what makes up a Flash cell before explaining the variation between SLC and MLC Flash

SLC cell

SLC device can store a single bit of data in one cell. That corresponds to a “0” or a “1”, which allows an SLC cell to store only two different program states.

MLC cell

An MLC device can store two bits of data in a single cell. That corresponds to 4 different program states: “00”, “01”, “10”, and “11”. 

e-MLC cell

e-MLC stands for Enterprise MLC , some MLC suppliers have been able to make modifications to increase the endurance of their MLC devices. They have found that by slowing down the programming time (t prog) endurance can be increased from only 3,000-5,000 read/writes to as many as 30,000—but at the cost of performance.

Slowing down the programming reduces the generation of trap sites and damage, which improves cell reliability. This type of device is called e-MLC NAND.

In this article, we will only focus on SLC and MLC only.

SLC vs MLC major difference, performance, capacity

For a given technology node an SLC or MLC device will have the same Vt voltage range. The key difference
is that a SLC cell is divided into two states and an MLC cell is divided into four.

For MLC, since the delta between each level has decreased, the sensitivity between each level increased. Thus, more rigidly controlled programming is needed to manipulate a more precise amount of charge stored on the floating gate. In order for a Flash cell to be considered MLC technology, the cell must exhibit two characteristics:

1. Precise charge placement
2. Precise charge sensing

Thus, MLC Flash works the same way as SLC Flash. The threshold voltage Vt, is used to manipulate the state of the Flash. The amount of charge on the floating gate is what determines the threshold voltage.

This difference in Vt voltage ranges for each state causes the MLC device to be slightly slower and less reliable than the SLC device. However, the benefit of MLC over SLC is you get double the density of storage on the same amount of silicon.

Factors affect endurance

The number of PE cycles, or endurance, a device can withstand is determined by two factors:

NAND technology ground rules

Flash technologies have shrinking ground rules just like all semiconductors technologies. The problem with flash components is that each time data is programmed and erased in a cell there is small amount of damage to the storage structure, creating trap sites. After many program/erase (PE) cycles on the cells, electrons get trapped changing/shifting the Vt distributions. The Vt distributions get larger, shift and start to overlap. This Vt overlap causes read errors which, in most cases, are correctable with Error Correction Code (ECC). However, after some finite number of PE cycles, error rates are too high to be correctable and the data is no longer good.

Cell structure/layout (SLC or MLC)

As discussed in SLC vs MLS different section, generally speaking, MLC cell is slower and less reliable due to less delta between states. However some MLC suppliers have been able to make modifications to increase the endurance of their MLC devices. They have found that by slowing down the programming time (t prog) endurance can be increased from only 3,000-5,000 read/writes to as many as 30,000—but at the cost of performance.

Slowing down the programming reduces the generation of trap sites and damage, which improves cell reliability. This type of device is called e-MLC NAND.

Solid-State Drives(SLC and MLC)

Solid state drives are mainly made up of NAND flash memory(SLC or MLC) and a controller. The first generations of solid-state drives (SSDs) were generally made with SLC technologies. As the need for additional capacity grew, SSD suppliers introduced MLC-based SSDs. This allowed for double the density for almost the same cost as the SLC-based drives.

Overprovisoning

Almost all SSDs have spare or extra NAND flash memory above and beyond its stated capacity. This is called overprovisioning. That means a 320GB drive may actually have 420GB (or more) of raw NAND, so that as memory cells wear out, others are there to take their place. The controller controls the writing and the erasing of data to the flash. Most or all controllers are designed to maximize the life of the NAND flash PE cycles. This is done primarily with wear-leveling algorithms. Wear leveling is the equal distribution of data writes/erases across the available NAND (including spares). This assures that both active and spare NAND devices do not wear out prematurely.
The bottom line is that the SSDs will receive write commands, but the controller will manage the program/erase cycles of the NAND.

Other methods to maximizing SLC/MLC cells

SSD suppliers have other methods of maximizing memory cells besides wear leveling, such as compression of data and caching, which can also be used to minimize PE cycles on the NAND devices. Minimizing the number of PE cycles by consolidating individual SSD writes extends the life of the SSDs and NAND flash devices, and is therefore extremely beneficial for the lower PE cycles on MLC devices.

However, SLC-based SSDs will always have a better endurance performance than a MLC-based drive for a given technology node.

SLC SSD and MLC SSD specification  comparison

Now that the differences between SLC and MLC have been explained, let’s compare their specifications to help further make a distinction between the two grades.

  SLC MLC
Density 16Mbit 32Mbit/64Mbit
Read speed 100ns 120/150ns
Block Size 64k 128k
Architecture x8 x8/x16
Endurance 100,000 cycles 10,000 cycles

The endurance of SLC Flash is 10x more than MLC Flash. The endurance of MLC Flash decreases due to enhanced degradation of Si. This is a main reason why SLC Flash is considered industrial grade Flash and MLC Flash is considered consumer grade Flash.

So, as you can see SLC Flash is good in commercial and industrial applications that require high performance and long-term reliability.

While MLC Flash is good for consumer applications that do not require long term reliability such as consumer grade USB Flash drives, portable media players, and Compact Flash cards.

The read speeds between SLC and MLC are comparable. Reading the level of the Flash cell compares the threshold voltage using a voltage comparator. Thus, the architecture change does not affect sensing. In general, the read speeds of Flash are determined by which controller is used.

SSD Life time measurement

There are several ways to calculate SSD lifetime. One of the simplest ways is total data written to the NAND flash. One SSD supplier has established baseline maximums for total data/bytes written. The total number of bytes written to the flash will be equal to the total bytes written to the SSD drive plus housekeeping overhead. This overhead, as we mentioned before, is the controller writes for data refreshes and block management, which increase the total number of bytes written to the NAND Flash.
Present solutions allow for a total of 4 petabytes (1015) written (PBW) for the MLC technologies being manufactured today (320GB drive). As one would expect SLC drives have a much higher PBW. The PBW is 75PB for a similar 320GB drive made with SLC. The PBW scales linearly as drive capacity increases or decreases.
The main message here is that SSDs have a finite lifetime based on total bytes written to the NAND flash. Most MLC-based storage solutions have a one- to three-year warranty. However, it is critical to understand the use conditions because a MLC based SSD could have a lifetime of three years or less if excessive writes are performed. On the other hand, SLC based solutions have greater than a three-year life even under heavy PE cycles.

SSD Selection

The three most critical parameters are total expected

terabytes written daily (TBW or 1012)
write/read ratio
target lifetime of the drive (in years)

Thecombination of these parameters should guide the user to the proper capacity and NAND flash
technology. If the time and effort is put into the selection process, there should be no unexpected
wear-out of the SSD.

Operating Temperature Industrial Commercial

Higher temperatures cause more leakage in the cells. Combined with the increased sensitivity required to differentiate between the levels, this leakage will cause the sensors to read the wrong level. As a result, the operating temperature of MLC spans only the commercial range. Leakage is not significant in SLC Flash
and thus, it can operate in an industrial temperature range.

Qualities of SLC and MLC
it is important to evaluate what type of Flash memory your system needs. If performance and durability are
essential for your system, SLC Flash incorporated. If low cost and high density are essential, MLC Flash is the right choice.