Static information storage and retrieval – Systems using particular element – Molecular or atomic
Patent
1990-08-17
1994-05-31
Dixon, Joseph L.
Static information storage and retrieval
Systems using particular element
Molecular or atomic
250306, 250307, G11C 1100, G11C 1300, G01N 2300, G21K 700
Patent
active
053175336
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The invention pertains to the field of scanning tunneling microscopes, and more particularly, to the field of integrated versions of same.
Scanning tunneling microscopes were first invented by a team of researchers from IBM (Binnig and Rohrer). The basic concept of a scanning tunneling microscope is to place a very sharp, conducting tip having tip dimensions on the order of the size of 1 atom in diameter close to a conductive surface. If the tip is brought very close to a conductive surface, i.e., within the space of the diameters of several atoms, (approximately within 5 angstroms), a tunneling current flows between the tip and the surface. That is, the probability density function of electrons for atoms in the tip overlaps in space the probability density function of electrons for atoms on the surface. As a result, tunneling occurs in the form of electron current flow between the tip and the surface if a suitable bias voltage between these two conductors is applied.
The magnitude of the tunneling current is exponentially dependent upon the distance between the tip and the surface. If the distance between the tip and the surface increases by only 1 angstrom, the current is reduced by a factor of 10. Typically, 100 millivolts of bias voltage will provide 1 nanoampere of current for a tip to sample spacing of a few angstroms.
This tunneling current phenomenon can be used to take an image of the surface. To do this, the tip must be placed very close to the surface and must be moved in raster scan-like fashion across the surface while maintaining the relative distance between the tip and the surface. The tip must be moved up and down to follow the contour of the surface to maintain a relatively constant distance between the highest point on the surface and the tip. This distance must be accurately maintained to be within the tunneling overlap distance to maintain constant current flow. As the tip is scanned across the contour of the surface, an image of the surface contour may be built up by keeping track of the movements of the tip. Typically this process of tracking the tip movement is done by keeping track of the voltage applied across a piezoelectric transducer which moves the tip to maintain the constant distance between the tip and the surface. Typically the apparatus that controls the tip distance monitors the current flowing between the tip and the surface and controls a mechanical system to move the tip in such a manner as to stabilize the current flowing between the tip and the surface at some steady state value. Thus, changes in the current will result in changes in the distance between the tip and the surface so as to counteract the changes in the current and stabilize it at a steady state value. Thus, changes in the drive signals to the tip movement mechanism track changes in the surface contour as the height of the tip above the surface is adjusted to maintain constant current.
2. Prior Art:
A collection of papers defining the state of the art in scanning tunneling microscopy is published in the IBM Journal of Research and Development, Vol. 30, No. 4, pages 353-440, July 1986. In an article entitled "Scanning Tunneling Microscopy" by Binnig and Rohrer at pages 355-369 of that journal, a scanning tunneling microscope is depicted in FIG. 2 using a piezoelectric tripod. This tripod consists of 3 piezoelectric rods of material joined at a junction; each rod expands and contracts along one of 3 Cartesian coordinate axes. The tip is mounted at the junction of the 3 rods. The tip is brought into proximity of the surface by a rough positioner. Thereafter the piezoelectric tripods are used to scan the tip across the surface to develop an image of that surface. The collection of papers in the IBM Journal of Research and Development shows scanning tunneling microscopy as being done with large scale apparatus.
One reference teaches an integrated form of a scanning tunneling microscope. This reference is European Patent Application Publication No. 01943
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patent: 4575822 (1986-03-01), Quate
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patent: 4831614 (1989-05-01), Duerig et al.
"Atomic Force Microscope" in Physical Review Letters by Binnig et al vol. 56, No. 9, Mar. 86, pp. 930-933.
"Bimorph-driven x-y-z translation stage for scanned image microscopy"; Review of Scientific Instruments 58(4); Apr. 1987; pp. 567-570; J. R. Matey et al.
Albrecht Thomas R.
Quate Calvin F.
Zdeblick Mark J.
Dixon Joseph L.
The Board of Trustees of the Leland Stanford University
Whitfield Michael A.
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