As they eliminate tiny air bubbles that form when liquid droplets are molded into intricate circuits, a Princeton-led team is dissolving a sizable obstacle to the mass production of smaller, cheaper microchips.
Led by Stephen Chou, the Joseph C. Elgin Professor of Engineering at Princeton, the team worked to troubleshoot one form of nanoimprint lithography, a revolutionary method invented by Chou in the 1990s. Nanoimprint uses a nanometer-scale mold to pattern computer chips and other nanostructures, and is in marked contrast to conventional methods that use beams of light, electrons or ions to carve designs onto devices.
This technique allows for the creation of circuits and devices with features that are not much longer than a billionth of a meter, or nanometer -- more than 10 times smaller than is possible in today's mass-produced chips, yet more than 10 times cheaper. Because of its unique capabilities and reasonable cost, nanoimprinting is a key solution to the future manufacturing of computer chips and a broad range of nanodevices for use in optics, magnetic data storage and biotechnology, among other disciplines.
In dispensing-based nanoimprinting, liquid droplets on the surface of a silicon wafer are pressed into a pattern, which quickly hardens to form the desired circuitry. This technique is more attractive to manufacturers than some other forms of nanoimprinting because it does not need to be done in an expensive vacuum chamber. However, the widespread use of the technique has been hindered by the formation of gas bubbles that distort the intended pattern.
"This is an important step because to benefit from the technology of nanoimprinting you need to be able to use it in mass manufacturing at low cost," Chou said. The team's findings are reported today (Jan. 17, 2007) in the journal Nanotechnology.
In a series of experimental and theoretical studies, Chou and his colleagues studied the factors that cause air bubbles to form and explored ways to eliminate the sub-millimeter-sized scourges. By increasing the imprinting pressure or using liquids that have higher air solubility, they were able to dramatically increase the likelihood that the bubbles would dissolve in the liquid before it hardened.
The work was supported in part by the Office of Naval Research and the Defense Advanced Research Projects Agency.
The research team also includes Princeton electrical engineers Xiaogan Liang and Zengli Fu as well as Hua Tan of the Monmouth Junction-based Nanonex Corporation, founded by Chou in 1999.
Liang, Tan, Fu and Chou. Air bubble formation and dissolution in dispensing nanoimprint lithography. Jan. 17, 2007. Nanotechnology. doi:10.1088/0957-4484/18/2/025303.
We report an experimental and theoretical study of two most critical yet still to-be-answered issues in dispensing-based nanoimprint lithography (D-NIL): air bubble formation and absorption, and discuss their impact on NIL yield and throughput. Using real-time observation via video, we found two different mechanisms for air bubble formation (feature pinning and multi-droplet encircling), and studied the dynamic behavior of the air absorption and air bubble shrinking under different conditions. Furthermore, we developed theoretical models and simulation programs of the air absorption and bubble shrinking based on molecular diffusion theory and hydrodynamics. We compared these models with experiments, and found excellent agreement. Our study shows that the key factors that affect the air dissolution time (and hence the air bubble shrinking time) are air bubble initial size, imprinting pressure, air solubility, and resist residue layer thickness. One of our key conclusions from the study, which has significant practical importance, is that although the air in a bubble can be completely dissolved in a resist liquid as long as the bubble is smaller than a certain size, the air absorption time might be too long for the dispensing-NIL operating in atmosphere or poor vacuum to have a necessary throughput in mass manufacturing.