Toys for Dots
Improved Nanodots for Data Storage
Every now and again, a sentiment emerges from a collection of varied sources that is inserted into a folder titled “Common Sense.” Micromanagement is one such term that burst into popular jargon along with phrases like paradigm shift and thinking outside the box. Rather than appearing in boardroom presentations, micromanagement is often heard around the water cooler, where it is used as a pejorative term by employees desiring more autonomy. In a society still evolving from a mechanistic industrial revolution, I view this term as the positive result of effective employee education and training. While upper management shepherds the big picture, the boots on the ground are best equipped to manage the particulars.
This approach works wonders in intelligent human systems, but often fails when applied to mindless machines. A misplaced semicolon or misspelled variable name can wreak havoc on the most sophisticated computer code. Attention to detail and vigorous micromanagement are prized expertise in physical and computer scientists. An example of these skills appearing earlier this year deals with the topic of magnetic data storage. Modern computer memory, known as random access memory (RAM), is available in two popular flavors. Dynamic RAM (DRAM) is based on electronic capacitors having charges that must be refreshed periodically, and static RAM (SRAM) that uses electronic transistors that maintain their value once written. While both forms of RAM are very fast, they lose their values when power is removed. Slower, but longer-term, non-volatile memory is often provided by magnetic storage media commonly in the form of a hard disk drive (HDD). Pioneered by IBM in the mid-1950s, the first IBM-350 HDD contained 50 24-inch disks coated with ferromagnetic iron oxide, and had a storage capacity of 4.4 megabytes. Individual bits were recorded by magnetizing a section of the disk in a particular direction through exposure to a strong magnetic field. After the magnetic field was removed, each ferromagnetic bit would maintain its orientation, and could be measured by an additional sensor. Today’s HDDs boast capacities of several hundred gigabytes in form factors you can hold in your hand, but they are based on the same fundamental design of the IBM-350. These drives use thin films of cobalt (Co) alloy instead of iron oxide and are formatted into specific regions of tracks, sectors, and bits at the factory. Improvements in storage capacity are achieved by shrinking the area required for each bit and increasing the sensitivity of the read sensors. However, at the sub-micrometer level, the magnetic fields of adjacent bits begin to interfere, and this presents a technological obstacle to further miniaturization.
One solution to this problem is to use electron beam lithography to fabricate individual stacks of Co alloy, known as nanodots. These nanodots are typically less than 100 nm in diameter, are tens of nanometers tall, and are patterned on silicon (Si) wafers in rectangular arrays with controlled spacing between the dots. Cobalt-palladium (Co/Pd) nanodots can be magnetized perpendicular to the plane of the Si base, much like an upright bar magnet. Sensing whether the magnetized dot is displaying its north or south magnetic pole to the read sensor provides a sharp signal, indicating a bit value of 1 or 0 that is isolated from neighboring dots. Unfortunately, initial studies of nanodots revealed a large variation in magnetic field strength needed to flip each dot’s magnetic orientation. Known as the switching field distribution (SFD), this limits how close the dots can be packed. If some dots require stronger field strength, they could fail to switch, or the increased strength could affect neighboring dots that switch more easily. Some scientists suggested the polycrystalline nature of traditional thin films was the source, while others pointed to variations in the lithography process.
Researchers at the National Institute of Standards and Technology (NIST) in Boulder, CO, along with colleagues at the University of Arizona, have fabricated various nanodot arrays using strictly controlled methods in an attempt to find the source of the large SFDs. Polycrystalline Co/Pd nanodots were fabricated using dc magnetron sputtering and single-crystal Co/Pd nanodots were grown using the more complicated process of molecular beam epitaxy (MBE). They found that the single-crystal nanodots exhibited similar SFDs as compared to the polycrystalline versions, suggesting grain boundaries and orientations were not a major contributor. However, their major discovery is that the SFD can be reduced significantly with the addition of Tantalum (Ta), a material whose main use is in the production of electronic capacitors, as a layer inserted between the nanodot and the Si wafer. Additional studies at this fundamental level can lead to the production of magnetic storage media that have densities greater than 1 terabit/in², enabling a new generation of storage devices. As our nanotechnology continues to increase in sophistication, we will know our electronic micromanagement is effective when our intelligent electronic systems begin to complain about “nanomanagment.”
This material originally appeared as a Contributed Editorial in Scientific Computing 24:5 April 2007, pg. 12.
William L. Weaver is an Associate Professor in the Department of Integrated Science, Business, and Technology at La Salle University in Philadelphia, PA USA. He holds a B.S. Degree with Double Majors in Chemistry and Physics and earned his Ph.D. in Analytical Chemistry with expertise in Ultrafast LASER Spectroscopy. He teaches, writes, and speaks on the application of Systems Thinking to the development of New Products and Innovation.
