One Giant Leap
Measuring the small with precision
The great philosopher, Glinda, the Good Witch of the North, who is quoted in the 1900 text, The Wonderful Wizard of Oz, offered the truism “It’s always best to start at the beginning.” Consequently, on their journey to a four-year undergraduate degree, the first discussion I have with our new freshman students is the question of “How Many?” We explore the width of corpulent king’s thumbs to create the “inch” and the standardization of other monarchal body parts to get digits, nails, palms, hands, shaftments, spans, feet, cubits, yards and fathoms. The standardization of precise measurement tools enabled the concept of distributed manufacturing, wherein individual components of massive and complex creations could be fabricated in parallel anywhere on the globe and assembled at a speed that was inconceivable prior to the industrial revolution.
Some 200 years later, we find ourselves in the opening acts of revolutions in information and biotechnology. Rather than focusing on the industrial “big” of colossal factories and buildings, these technologies have their foundations in the “small.” Tiny, nanometer-sized electronic components form complex integrated circuits while nanometer-sized proteins compose living systems and medicines. The micrometer precision of high-quality manufacturing is now 1000 times larger than the nanoscale precision required by these nascent technologies and is recognized by The National Nanotechnology Initiative (NNI), located at www.nano.gov, as one of the “Grand Challenges” that must be addressed. Much like the molded plastic rulers we used in grade school that have ridges representing 1/16 of an inch on one side and 1 mm on the other, standardized rulers are invaluable to the latest technologies. However, these ridges need to be a million times closer.
At these tiny separations, light is not simply reflected by the ridges. Rather, the ridge spacing is similar to the wavelength of visible light resulting in destructive interference that causes the photons to diffract from the surface. The result is that different colors of light appear to reflect from the surface at different angles in a pattern of dark and light regions and has been the basis of optical diffraction gratings since the early 1800s. Instead of creating reflective ridges, it is easier to create non-reflective grooves by scratching the reflective surface with a suitably hard material, such as diamond.
Problems arise when the machine tool is created to manufacture the ruled grating since grating quality degrades if the grooves are not perfectly parallel and the spacing is not exact over the entire surface area. In 1880, Albert Michelson invented the optical “interferometer” that permitted two single beams of light to interfere when they were overlapped with a precision on the order of the wavelength, a few hundred nanometers. Using the interferometer to measure position accurately is the basis for a device known as a displacement measuring interferometer (DMI). MIT Professor George Harrison equipped his grating machine with a DMI in 1955 to provide feedback on the position of the diamond tool, dramatically increasing the precision of ruled gratings and this has been the state-of-the-art for the past 50 years.
The mechanical process is very slow and the diamond tool must be replaced after carving 15 km of grooves. A modern 12-in diameter, 400-nm grating requires a groove distance of over 230 km and would take months to complete while requiring the minimization of environmental noise in the machine and the laser-based DMI. To address these problems, MIT Research Scientist Mark Schattenburg has led his team to explore the utility of interference lithography (IL). In this process, two coherent laser beams interfere to create an interference pattern of alternating constructive and destructive fringes that are used to expose a photosensitive material. The process contains no moving parts and the entire grating is exposed simultaneously. Unfortunately, the ultimate size of the grating is limited as the interference pattern is circular and only a small portion can be used to expose nearly-linear grooves.
The Schattenburg group has invented a hybrid mechanical/IL machine called the “Nanoruler.” A small, but linear IL region containing hundreds of fringes is scanned over the surface of a 12-in diameter photosensitive substrate to create the same 400-nm grating in 20-minutes, greatly reducing the time window over which noise must be minimized. This scanning beam interference lithography (SBIL) device must operate in a vibration-free environment that is temperature-controlled to 5 millidegrees C, but is capable of producing large 300-mm diameter gratings down to 150-nm grove spacing with a precision on the order of 1 nm in less than a half an hour. Modern all-optical DMIs utilize bulky, high-finesse laser systems complete with their own noise environment. Linear optical encoders that read ridges like our plastic mm rulers are less noisy and currently have a precision of a couple micrometers. A DMI based on the precise gratings made by the Nanoruler would be inexpensive and equivalent to having a linear optical encoder with a precision on the order of 1 nanometer. As SBIL and the Nanoruler continue to mature, one of the Grand Challenges has been met and developments can begin in several new industries.
Bill Weaver is an assistant professor in the Integrated Science, Business and Technology Program at La Salle University. He may be contacted at email@example.com.