Continued from New Storage Device Potentially Store Data for 1,000 Years (1)

The self capacitance of a bit line changes depending on the existence and number of metal nanodots. Therefore, data are written (metal nanodots are formed) after coding the data so that the existence of a metal nanodot ("0" and "1") is equalized.

However, if it is simply equalized, the coding ratio becomes low. For example, four bits of data are coded with a coding ratio of about 38%. This time, the group improved coding ratio to about 88% with a coding method that performs equalization by using dummy lines, too (a dummy line is located on each side of a word line for data recording).

The self capacitance of a bit line changes depending on the existence and number of metal nanodots. Therefore, data are written after coding the data so that the existence of a metal nanodot ("0" and "1") is equalized.

Potential areal density equivalent to HDD

To verify the new technology, the research group made a test chip with four recording layers by using a 180nm CMOS process technology. Its areal density is equivalent to 10 Gbits/(inch)2. The areal density can be improved if a more advanced process technology is applied.

For example, if a 14nm process technology is used to increase the number of recording layers to seven, the areal density will exceed about 1 Tbit/(inch)2 in theory, which is equivalent to the areal density of existing HDD products. If a 28nm process technology is applied to increase the number of recording layers to six, the recording density will be 0.4 Tbits/(inch)2.

The reading speed of the test chip is 40 kbits per second, which can be further improved, according to the group. When the long-term data retaining capability of the test chip was measured in an accelerated test, the result showed that data can be read out even after 1,000 years.

The areal density can be further improved by applying a more advanced process technology.

When the long-term data retaining capability of the test chip was measured in an accelerated test, the result showed that data can be read out even after 1,000 years.