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Exploring split-gate thin-film storage

Posted: 17 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:split-gate thin-film storage? NVM? ferroelectric RAM?

Non-volatile memory (NVM) plays a vital role in a broad variety of systems ranging from mobile computing platforms to embedded systems. Flash memory has dominated this space for a number of years, with gate sizes continually shrinking to allow it to meet performance demands. There are limits to minimum gate size, however, forcing the industry to see alternatives. An emerging nanotechnology-enabled approach, with a particular focus on a recent development called split-gate thin-film storage (SG-TFS), provides a promising alternative. Let's take a step back and look at the super category of NVM technologies, and then explore the world of flash.

There are memory technologies that cannot retain the stored information when the product is not powered; hence, they are called volatile memory technologies. NVM is a semiconductor memory technology that can retain the stored information even when it is not powered; hence it's called non-volatile. Examples of NVM include read-only memory (ROM), flash memory, ferroelectric RAM (FRAM), and most types of magnetic computer storage devices.

In most electronic systems, there are two types of memory technologies used: A primary storage technology (volatile), and a secondary storage technology (NVM; figure 1).

Figure 1: This simple lighting module MCU uses two types of memory: primary (volatile) and secondary (NVM).

The most popular type of primary storage technology today is called static random access memory (SRAM), which is a volatile form of memory technology, hence when the system is shut down, anything contained in SRAM is lost. NVM is typically used for secondary and long-term storage, when retention of the stored information is needed during power-down. If retention of the information is not required, for example due to the presence of an external memory drive, dynamic RAM (DRAM) is often used. This discussion will focus on the former case, in which NVM is required for secondary storage.

The SRAM primary storage provides read and write times close to the clock period of the system's processor, and thus is used where minimal latency is required. The NVM typically requires several clock cycles to read or write, but it is chosen for secondary storage because it retains its contents during power-down and provides minimal cost per bit; hence, many systems use a moderate amount of RAM in combination with a larger amount of NVM (typically flash memory) to optimize both cost and performance.

There are a few technologies under development that are more comparable to SRAM, or at least to DRAM, in terms of performance while also meeting the non-volatility requirement for the secondary memory . These include magnetoresistive RAM (MRAM), phase-change memory (PCM), and resistive RAM (RRAM). While these technologies show good potential, they do not typically match the sub-5-ns read/write times needed to fully replace SRAM, and have not yet achieved the maturity and cost-per-bit to fulfill the NVM secondary memory requirement. There is also ferroelectric RAM (FRAM), which has shown good advantages over conventional NVM concerning endurance, write time and write power, but again FRAM does not even approach SRAM in read/write performance, and has been difficult to scale beyond the 130-nm node, or to use in applications requiring temperatures higher than 85�C. There is thus an ongoing need for innovative solutions that will allow continued scaling of flash memory as the choice for secondary storage.

Types of NVM technologies
Some of the major consumers of NVM technologies are micro-controller units (MCUs), which are small single-chip computers containing one or multiple processor cores, different kinds NVM memories, and programmable input and output peripherals. If we confine our discussion to conventional semiconductor-based NVMs in use today in typical MCUs, the simplest NVM is read-only memory (ROM), which uses a pre-programmed MOSFET and contact/via to store a zero or one data level. The programming is done during manufacture of the product, and thus cannot be altered. Electrically eraseable programmable ROM (EEPROM) allows the data to be erased and re-written in the application.

Flash memory is simply a special version of EEPROM that requires the erase operation to be performed on large portions of the memory at one time, in order to greatly reduce the chip area required per bit. Flash memory thus can be written one byte or word at a time, but erase operations will affect a large portion, called a sector or block. The sector or block size ranges typically from 1 KB to 256 KB. Contents are changed by erasing a sector, then writing fresh data to one or more locationshence, the name "flash," since the contents of a whole memory array or memory block are erased in a single step.

To avoid the cost of external memories, MCUs come with on-chip program memory. Originally, these were EPROMs that had a window on the top of the device, where the program memory could be erased by ultraviolet light, and then reprogrammed (this was also called a burn cycle). Since the 1990s, EPROM has been replaced by EEPROM and flash, which are easier to use and cheaper to package. Today MCUs include SRAM for data storage, as well as NVM/flash for program storage, and even EEPROM or EEPROM emulation space, using flash technology (figure 2).

Figure 2: MCUs use multiple types of memory. This entry-level MCU from the Kinetis line, for example, includes 32 KB to 128 KB flash, up to 16 KB of SRAM, and the equivalent of up to 2 KB of user-segmentable byte write/erase EEPROM for data tables/system data.

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