US Patent 7164646 - Scanning Probe Memory Cell With >2 Storage States

http://www.freepatentsonline.com/7164646.pdf

Conventional memory states are digitally stored in magnetic or optical media in terms of binary values (i.e. 0 or 1.) Scanning probe/Atomic force microscopy have added phase change material to the types of media on which digital data can be written, allowing for bit sizes on the nanoscale and greater memory densities. However, these scanning probe memory devices still typically use binary storage. This patent from Hewlett Packard teaches using the scanning probe tip to form multilevel (>2) data storage to further increase memory densities. Claim 1 reads:

1. A storage device comprising: a storage medium having plural storage cells; and a probe to scan across a surface of the storage medium to program the storage cells, wherein the probe is adapted to selectively program each storage cell to one of more than two storage states, the probe to selectively program each storage cell to have a dent having one of plural depths that represent at least two of the storage states, and wherein the probe is operable to contact a surface of the storage medium to read a storage state of at least one storage cell during a read operation.