Product category: Memory Devices and Modules
News Release from: Freescale Semiconductor
Edited by the Electronicstalk Editorial Team on 1 April 2003

Silicon nanocrystals point
to Flash replacement

Motorola has demonstrated the world's first 4Mbit memory device based on silicon nanocrystals

The fully functional 4Mbit test chip represents a major milestone in the search for successors to floating gate-based Flash memories, which many believe will not continue to scale to smaller geometries. These advances could lead to memories that are smaller, more reliable and more energy-efficient than floating gate-based Flash memories.

Silicon nanocrystal memories are part of an advanced class of memory techniques called thin-film storage.

Motorola has developed techniques designed to help simplify the manufacture of these memories.

Using traditional deposition equipment, researchers at Motorola's DigitalDNAT Laboratories, deposited silicon nanocrystals resembling 50-angstrom diameter spheres between two layers of oxide.

The silicon spheres are engineered to hold and prevent lateral movement of charge to other isolated nanocrystals.

This is expected to increase reliability and scalability because a single oxide defect does not lead to complete charge loss as in a conventional floating-gate nonvolatile memory.

"Silicon nanocrystal-based memories have the potential to be an evolutionary replacement for conventional flash memory, which is widely used in automobiles, appliances, wireless devices and industrial controls", said Joe Mogab, Vice President, Advanced Products Research and Development Lab, Motorola's Semiconductor Products Sector.

"As the first company to demonstrate a 4Mbit device, we're one step ahead in meeting the future needs of the embedded, nonvolatile memory market.

These encouraging results suggest that memories based on silicon nanocrystals could be cost-effectively manufactured with today's equipment".

Floating gate-based embedded Flash memory is the workhorse of embedded nonvolatile memories.

Flash memory technology allows manufacturers to store both software code and data.

This flexibility allows manufacturers to reprogram microcontrollers several times in the development cycle, or store data while operating the controller in the field.

It also makes it easy to adjust to fast-changing market demands or correct software problems remotely in the field.

However, as the industry begins manufacturing at smaller geometries - 90nm and smaller - manufacturing floating-gate-based Flash becomes impractical.

At those dimensions, the chip area spent on the 9-12V high-voltage transistors needed to write and erase the Flash becomes too expensive.

Further, engineers cannot reduce the high voltage in floating-gate based flash without compromising reliability, at the risk of memory failures and loss of data.

Motorola is also an industry leader in magnetoresistive random access memory (MRAM).

MRAM has the potential to be a truly revolutionary memory technology, combining the best attributes of the three major memory types onto a "single" chip - the density of eDRAM, the speed of eSRAM and the nonvolatility of Flash.

Motorola built the test array on 200mm wafers using its 90nm process in the Dan Noble Centre at its east-side campus in Austin, Texas.

The key challenge Motorola researchers overcame is getting the nanocrystals to grow repeatedly to consistent size and density.

If the nanocrystals are too small or too dispersed, then the memory device will not hold sufficient charge density.

The proper charge density is what allows the memory to detect "on/off" states or 0s and 1s.

If the nanocrystals are too large or too dense, the electrons may move either to other nanocrystals or leak through defects in the tunnel oxide beneath the nanocrystals.

By experimenting with different process chemicals and modifying conditions such as temperature, pressure and time, Motorola developed a method to repeatedly grow the nanocrystals with existing semiconductor equipment.

Researchers may now focus on reducing the die size and tightening the technology specifications in order to be ready for potential products in 2004.

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