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The Effects of Repeated Refresh Cycles on the Oxide Integrity of EEPROM Memories at High Temperature.
Keywords: Oxide wearout, Data Retention, EEPROM
The data retention of stored-charge based memories, such as Flash and EEPROMs, decreases with increasing temperature. This can be compensated for by periodically re-writing the data, though the total number of erase-write cycles is limited by the oxide integrity. An alternate approach is to use a refresh cycle consisting of a write cycle without a prior erase cycle. This is a viable approach, providing that the refresh cycle induces less damage than an erase-write cycle. This paper studies the effects of repeated refresh cycles on oxide integrity in a high temperature environment and makes comparisons to the damage caused by erase-write cycles. The experiment consisted of running a large number of refresh cycles on a selected byte. The oxide integrity was checked by performing repeated erase-write cycles on that byte until the oxide failed. Other bytes that were not refreshed were used as controls. Data was collected from multiple parts, with different numbers of refresh cycles, and at temperatures ranging from 25C to 175C. The experiment was conducted on microcontrollers containing embedded EEPROM memories. The microcontrollers were programmed to test and measure their own memories, and to report the results to an external controller. This greatly simplified the hardware setup, since only six wires were required to operate and test the memories of 77 microcontrollers. It also allowed data collection from the microcontrollers while the experiment was in progress. Data is presented showing that refresh cycles do not have a significant impact on oxide integrity. This result shows that the refresh cycle approach will provide a mechanism to extend data retention at high temperatures without affecting device reliability. The data also showed a bimodal distribution of the number of erase-write cycles necessary to damage the oxide. Although the cause of this distribution is not yet understood, it may provide an additional means to significantly improve EEPROM reliability through appropriate screening.
Lynn Reed, President
Austin, Texas

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