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News Article

ST beats soft errors

STMicroelectronics says that it has a new semiconductor technology that virtually eliminates "soft errors" caused by the earth's low intensity background radiation.
STMicroelectronics says that it has a new semiconductor technology that virtually eliminates "soft errors" caused by the earth's low intensity background radiation.

ST's patented solution redesigns the structure of the basic SRAM memory cell used in systems-on-chip (SOCs), while not significantly increasing the silicon area occupied by the integrated circuit (IC).

The company says that the technology does not incur significant cost or performance penalties. The new rSRAM technology was developed at the company's central R&D site in Crolles, France.

The modification ST has made to the standard SRAM memory cell adds additional capacitors that are integrated into the cell structure in the vertical dimension. The effect of these capacitors is to increase the amount of charge required to "flip" a memory cell, thus reducing the number of soft errors in any given time interval.

ST has fabricated test chips in a 120nm technology - ST’s next technology generation to enter mass production - using the new rSRAM cells. The test vehicles were subjected to aggressive bombardment with high levels of artificial radiation and measuring the resulting soft error rate. These tests (performed at the Los Alamos Neutron Science Centre) have confirmed that ST's rSRAM cells are around 250 times more resistant to soft errors than conventional SRAM cells - the cells are completely immune to alpha particles and almost completely immune to neutron-induced errors.

ST's test chips included rSRAM blocks, standard SRAM, and standard SRAM supported by additional error correction circuitry, as well as logic (non-memory) circuits, allowing the soft error rates (SER) of all of them to be measured under identical conditions. All tests were fully compliant with the JEDEC JESD89 standard.

The measured failure rate for ST's rSRAM is less than 10 FIT per Mbit at 1.2V and the devices are fully neutron-immune (0 FIT) at 1.32V - 1FIT (Failure in Time) means 1 failure per 1bn hours of operation.

The "soft error" problem originates from cosmic rays or from the radioactive elements that occur in all materials. Such particles can disrupt the operation of silicon chips and the electronic equipment that depends on them. The smaller the transistors are made, the more susceptible they become to the effects of stray ionising particles.

Within the chips, information is usually stored inside transistors as electrical charges. A nuclear particle such as a neutron or an alpha particle passing through the silicon chip can change the charge stored on a nearby transistor. Changes to the charge caused by an ionising particle give rise to "soft errors" - the chip is not physically damaged but may temporarily contain erroneous data. This soft error may cause the device to malfunction temporarily, even though subsequent testing will show a perfectly working device.

While the shrinking individual transistors in the embedded SRAM are becoming increasingly vulnerable to background radiation, the amount of embedded SRAM in chips is also increasing - currently it accounts for more than 50% of the transistors in a typical silicon chip. According to the ITRS (International Technology Roadmap for Semiconductors), this percentage is expected to reach 90% by the end of the decade. Embedded SRAM is used to store program code and data for higher performance.

"With the technologies that are in volume production today, soft errors do not usually cause many serious problems," says Dr Jean-Pierre Schoellkopf, director, Advanced Design and Tools, in ST's central R&D group. "The issue for electronic equipment manufacturers is that the continuing trend towards smaller transistors and larger on-chip SRAM memories, which they need in order to maintain their market momentum, make it inevitable that worst-case scenarios such as computer crashes or lost or misrouted data would happen with increasing frequency."

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