New Freescale ARM-M4 and ColdFire-based 32-bit microcontrollers feature on-chip nanocrystal non-volatile memories

June’s Microprocessor Report carries an article written by Editor-in-Chief Jim Turley that describes two new 32-bit microcontroller families from Freescale (formerly Motorola Semiconductor)--one family dubbed Kinetis featuring an ARM Cortex-M4 processor core and the other a revamped ColdFire processor architecture dubbed ColdFire+. Both microcontroller families feature a non-volatile, “Flash-like” memory technology dubbed FlexMemory that’s based on nanocrystal silicon dots rather than on polycrystalline silicon floating gates. Although these microcontrollers are the first to feature Freescale’s nanocrystal memory technology, the company has been developing this semiconductor memory technology since 2003 and it first discussed the technology at the 2005 International Electron Devices Meeting (IEDM). Nanocrystal memories have been discussed in the literature at least as far back as 1995, when IBM researchers at the Watson Research Center discussed the idea in a theoretical IEDM paper with some experimental results.

Flash-like nanocrystal memories offer several benefits over conventional Flash memories. In particular, the distributed nature of the electron storage in a planar, "thin-film" array of hemispherical silicon nanoparticles means that a gate-oxide defect will not drain the entire floating gate in a memory-storage transistor; only the nanoparticle(s) touching the defect are affected. Each nanoparticle holds only a “handful” of electrons, so loss of any one nanocrystal or even a few nanocrystals from leakage will not kill the memory cell. Consequently, nanocrystal memory cells are expected to be more robust than conventional Flash memory cells. Second, silicon nanocrystal fabrication employs a CVD (chemical vapor deposition) process and does not require the complexities of a dual-poly process so there may be some manufacturing-cost savings.

Freescale’s nanocrystals are hemispheres of silicon measuring roughly 10 to 15 nm in diameter and they’re spaced roughly 7 to 8 nm apart. They’re deposited in one planar layer within the gate-to-channel oxide, which must be formed in two steps: one thin layer on top of the channel that serves as the substrate for the nanocrystals and then a thicker covering layer that insulates the nanocrystals from each other and from the transistor’s gate. These geometries result in a storage distribution of approximately 200 nanocrystals per bit in the 90nm process technology Freescale uses to fabricate the new ARM- and ColdFire-based microcontroller families, so the loss of a few nanocrystals to oxide defects will not affect transistor thresholds very much. At the 65nm and 45nm process nodes, which FreeScale currently cannot build, there would be fewer than 100 such nanocrystals per bit, but that’s still a sufficient number to create a memory cell with robust characteristics.

However, the robustness of the nanocrystal memory is not the only aspect that has caused Freescale to employ the technology in these new microcontroller families. In total, the nanocrystals forming the floating gate of the FlexMemory cell contain less mass than a conventional Flash floating gate, so the nanocrystals require less tunneling write current. As a result, nanocrystal memory requires less programming current, cutting the amount of power needed to write a bit in the memory. In addition, the nanocrystal FlexMemory write time is approximately 100 microseconds, which is about 10x faster than Flash memory. That too is due to the lower “mass” of the nanocrystal “floating gate” and is a highly desirable feature. In fact, Freescale exploits the extra speed by allowing the processor to treat the nanocrystal memory like Flash EEPROM or, through emulation, embedded EEPROM. The emulation ability is derived from FlexMemory’s faster speed and write/erase durability.

FlexMemory may appear in Freescale’s older microcontroller families such as the 56800 DSP-centric and the PowerPC-based MPC5xx microcontroller families. In addition, Freescale appears to be interested in licensing the nanocrystal memory technology, although the company has yet to announce a licensee.