IBM’s experimental “millipede” memory chip is being touted as a breakthrough in the fields of both
computer memory andnanotechnology. Announced in 2002, the chip combines the older
techniques of MEMs (micro-electro-mechanical systems)with an Atomic Force Microscope (AFM)
and is being developed at IBM’s Zurich research laboratory by a team that includesGerd
Binnig (inventor of the AFM) and engineer Peter Vettiger.
An animated view of the Millipede nanomechanical storage device illustrates how an individual tip creates an
indentation in a polymer surface (bottom) and how a large number of such tips are operated in parallel (top).
Courtesy of IBM Zurich Research Laboratory. Unauthorized use not permitted.
The millipede chip grew out of the fact that AFMs probe tips are just a few atoms wide; so narrow,
in fact, that they are used to “feel” the surface of individual atoms or push them around. IBM
engineers realized that the tips could also be used to indent a suitable surface, making it possible
to inscribe digital data. Using a thin sheet of plastic, the probe tips are heated and pressed into
the surface for a fraction of a second, leaving a tiny indentation just a few nanometers wide.
These indentations can be very closely spaced, allowing a great deal of information to be written
on a small surface area. The surface can also be “erased” by reinserting the heated probe tip, in
effect melting the pit surface, which then springs back to its original shape. IBM engineers then
designed an array of many probe tips, each mounted on MEMs-like structures that could move
them up-and-down to write and erase, and also side to side to access various points on the
plastic surface. The moveable cantilever structure is etched out of a layer of silicon so thin (just
20 nanometers or so) and thus is very flexible. The prototype chip demonstrated in 2002 had
1,024 tips, resembling the “thousand legs” of the millipede insect, and researchers estimate that a
much more complex version of the chip could store up to a trillion bits on a square inch of plastic.
The technology is not unlike the earliest days of computing, when digital information was written
and read in the form of holes punched in paper cards; of course in the 1950s those cards were
much larger, but the principle was similar.
The second-generation chip was slated to be demonstrated in 2003 and IBM announced that it
would become a part of cell phones and PDAs in 2005. However, by 2005 IBM was still in the
prototype stage, demonstrating an improved millipede chip early that year and saying that regular
production was still two years away. However, in the interim, engineers have made significant
strides, increasing the number of probe tips to 4,000 and providing a terabyte of storage on a
single chip. If they succeed, the millipede may make a huge impact on the memory chip
industry...or, perhaps, a very, very small impact.
Dynamic random access memory (DRAM) is a type of random access memory that stores
each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak
charge, the information eventually fades unless the capacitor charge is refreshed periodically.
Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and
other static memory.
The main memory (the "RAM") in personal computers is Dynamic RAM (DRAM), as is the "RAM"
of home game consoles (Playstation, Xbox
360 and Wii), laptop, notebook and workstation computers.
The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are
required per bit, compared to six transistors in SRAM. This allows DRAM to reach very
high density. Unlike flash memory, it is volatile memory (cf. non-volatile memory), since it loses its
data when the power supply is removed. The transistors and capacitors used are extremely
small—millions can fit on a single memory chip.
Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution
type of scanning probe microscopy, with demonstrated resolution of fractions of ananometer,
more than 1000 times better than the optical diffraction limit. The AFM is one of the foremost
tools for imaging, measuring, and manipulating matter at the nanoscale. The information is
gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate
tiny but accurate and precise movements on (electronic) command enable the very precise
scanning. In some variations, electric potentials can also be scanned using conducting
cantilevers.
The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the
specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of
curvature on the order of nanometers. When the tip is brought into proximity of a sample
surface, forces between the tip and the sample lead to a deflection of the cantilever according
to Hooke's law. Depending on the situation, forces that are measured in AFM include mechanical
contact force, van der Waals forces,capillary forces, chemical bonding, electrostatic forces,
magnetic forces (see magnetic force microscope, MFM), Casimir forces, solvation forces, etc.
Typically, the deflection is measured using alaser spot reflected from the top surface of the
cantilever into an array of photodiodes.