Avoiding a Fatal Attraction
Alexander E. Braun, Senior Editor -- Semiconductor International, 12/19/2007 7:00:00 AM
Before high-throughput multi-step manufacturing of nanoscale devices can be realized, it will be necessary to achieve reproducible alignment and registration between the template containing the nanoscale elements and the substrate onto which those elements are to be transferred. Researchers in the Center for High-Rate Nanomanufacturing (CHN, Boston), sponsored by the National Science Foundation (NSF, Arlington, Va.), are working on methods to reproducibly align these two surfaces. One of these methods uses “molecular guides,” where the template is brought into close proximity (~10 nm) using conventional positioning devices, such as stepper motors and piezoelectric actuators, and the final, most delicate alignment step is accomplished through the use of intermolecular attraction forces between molecules adsorbed on matching regions of the two surfaces.
CHN researchers from the University of Massachusetts Lowell (Lowell, Mass.), Northeastern University (Boston) and the University of New Hampshire (Durham, N.H.) are exploring the use of self-assembled monolayers, which can be oriented and organized on gold and silicon surfaces to serve as molecular guides. Chemistry professor James Whitten and his students at the University of Massachusetts Lowell are attempting to achieve attractive forces by using molecules that attract each other via hydrogen bonding, dipole-dipole interactions or ion-dipole interactions. An atomic force microscope (AFM) is being used to measure the nature and range of attraction between two monolayer-covered surfaces. “By measuring the ‘jump-in’ and adhesive forces as a function of distance between a functionalized AFM tip and surface,” Whitten said, “we’re optimizing the types of functional groups used to achieve long-range, yet reversible attraction.” Examples of molecular systems that are being investigated include fluorinated alkanethiols with strong dipole moments and alkanethiols terminated in protonated amines. The goal is to achieve matching patches on the substrate and template that would be functionalized with molecules that would attract each other, such that when they are brought close enough (within 10 nm), the intermolecular forces take over and line up the substrate and template.
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| Transfer of a nanoelement (black sphere) from a template to a substrate, with molecular guides used for alignment and registration. After the substrate and template are brought into close proximity by external positioning devices, intermolecular attractions between matching patches are used for the surfaces’ final alignment. Once the nanoelement has been transferred, the surfaces are pulled apart. (Source: Prof. James Whitten) |
“We’re doing very fundamental studies,” Whitten said. “We’re not actually using a substrate and template. Instead, what we’re trying to do is identify the molecules that could be used for these intermolecular attractions, and understand the attractive and pull-off forces when two molecules are brought into close proximity. By functionalizing the AFM’s tip, we can bring it down to surfaces in close proximity and measure force as a function of distance. We’re using gold-coated tips that we’ve functionalized with alkanethiols and exploring a variety of different functional groups at the end of the alkanethiols that are both on the tip and substrate.”
There is a downside to achieving the sought-after long-range attraction. It cannot be too attractive, because otherwise it becomes impossible to separate the elements and reuse the guides. “We have been working with molecules that have a strong dipole-dipole interaction, as well as exploring hydrogen bonding interactions,” Whitten said, adding that they are energetically pursuing ion-dipole interaction. “Some of the molecules we’ve been dealing with are fluorinated alkanethiols that would have a strong dipole pointing out from the surface. Then, on the other surface, we would have something such as an ammonium ion [NH4+].”
Although the CHN researchers have been able to achieve strong attraction with other systems, none have yet proven to be fully reversible. “We have done some work in solutions, some in air,” Whitten said. “As of now, we have achieved the kind of long-range attraction that we need with that system. In another system, we’ve been able to find a strong adhesive force. When the two surfaces are brought into close proximity, there is an attractive regime and, once they’re together, there’s strong adhesive force. This was accomplished by using a silicon surface with an alkanethiol on top that bonds to a mating gold region. But that surface, it turns out, has some irreversibility.”
The effort continues to isolate a chemistry with a strong long-range attraction, which is also reversible. While the work so far has used commercially available molecules, it appears likely that some very special ones may have to be synthesized. “We’re not giving up,” Whitten said. “We know that it is doable.”
