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).
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.