An improved method for nanocrystal placement for the floating gate of a flash memory cell, using a protein-mediated self-assembly approach, was described at IEDM by Shan Tang, U. of Texas, Austin. A template formed by a chaperonin protein lattice can be used to place nanocrystals of different types in a regular array at high density from a colloidal suspension, according to Tang and her coworkers.
Chaperonins are large multimeric structures with two stacked rings having a central cavity into which proteins bind. The interior of the cavity is hydrophobic, so that nanocrystals combined with hydrophobic molecules can be trapped inside cavities of some 4.6nm diameter with 4.5nm walls for the protein used in the experiments. The chaperonins can be self-assembled into a crystal lattice on a silicon surface through noncovalent interactions. Experiments showed that cavity size can be varied to provide a potential nanocrystal-size filter using magnesium or potassium ions or adenosine triphosphate (ATP), Tang explained. After the nanocrystals are uniformly distributed at high density, the protein is removed by annealing.
Nanocrystals or quantum dots between the control and tunnel oxide in flash memory cells are being explored because they promise to greatly extend retention time, while avoiding leakage from any weak spot across the tunnel oxide (electrons are stored at specific sites, rather than across a film). They also might operate at lower power and at higher speed with longer lifetimes.
The experiments with lead selenide (PbSe) and cobalt (Co) nanocrystals were done with chaperonin 60 (GroEL), the most studied chaperonin protein, with Co showing the best storage retention. This was expected, since a metal provides a higher density of states so that more electrons can be stored at each site. Small bottles of the protein are readily available commercially, according to Tang. The authors conclude that flash memories could be fabricated with the protein-mediated self-assembly process for floating gates using any existing nanocrystals. - B.H.