The present invention relates to a data carrier system including a ferroelectric memory.
A ferroelectric memory is an excellent non-volatile memory, operating at high speed, consuming little electricity, and having a long repeatability life, and is thus regarded as having great potential in the multimedia society. In recent years, multimedia products with built-in ferroelectric memories have been flooding the market, and the number of such products is increasing every year. Data carriers are one of those products.
However, when currently available ferroelectric memories are left to stand in high temperature environments, the data retention characteristics thereof deteriorate, which may results in data loss. The data retention characteristics of a ferroelectric memory might therefore deteriorate in a high temperature treatment process, such as a process for forming an IC, a process for mounting onto a semiconductor chip, and a reflow process.
Conventionally, in order to restore the degraded data retention characteristics, a reader/writer reads data from the ferroelectric memory properly and writes the data back into the ferroelectric memory, thereby refreshing the ferroelectric memory (see, for example, FIGS. 2 and 4 in page 5 in Japanese Laid-Open Publication No. 2003-76953).
Nevertheless, the refresh operation according to the above-described conventional technique cannot be practically performed in high temperature environments in which the temperature is as high as the Curie temperature, because data to be refreshed has already been lost from the ferroelectric memory in such high temperature environments. Therefore, the operating temperature condition for the conventional data carrier system is limited to below the temperature at which the polarization can maintained enough to be refreshed.
Furthermore, when a ferroelectric memory is left to stand in high temperature environments, imprint degradation, which makes rewriting of data difficult, occurs to cause decrease in the reliability. With the conventional technique described above, the problem of imprint degradation cannot be addressed sufficiently.