Building electronics bottom-up
January 16, 2013
Currently, modern electronics are primarily fabricated by etching the smooth surface of a starting material — say, a wafer of silicon, using micro- or nanolithography to establish a pattern on it. This top-down method might involve a prefabricated template, such as a photomask, to establish the pattern.
But the approach is becoming more and more challenging, because reducing the size of the features on the requisite templates is getting extremely expensive as engineers work their way further down the nanoscale.
“Going from 500 to sub-30 nanometers is cost prohibitive for large-scale production,” said Tang, an assistant professor in the department of chemistry and biochemistry in USC’s College of Arts and Sciences.
The bottom-up approach
As a chemist, Tang uses a bottom-up approach: he works with the individual molecules that go onto a surface, coaxing them to self-arrange into the patterns needed. One established method of doing this involves block copolymers, in which a polymer (large molecule) chain is made up of two or more sections of different types of monomers (building block of polymers) .
If the different block sections are properly designed, the blocks will self-aggregate when placed on a surface, and the aggregation can be harnessed to create desirable patterns on the nanoscale without the need for any templates.
Di-block copolymers of poly(ethylene oxide) and polystyrene, for example, have been used to construct highly ordered arrays of perpendicular cylinders of nanoscale materials. Solvent evaporation, or annealing, of these polymers on surfaces exerts an external directional field that can enhance the patterning process and create nearly defect-free arrays.
Tang’s team has fabricated nanoparticles of pure, crystalline iron oxide with controlled size and spacing on silicon wafers. Incorporating metals into nanoscale designs is crucial for fabricating electronic devices, and Tang’s method is a step forward for the field because it simplifies the process.*
The technique is a promising addition to the available tools for addressing the chronic need to decrease the size of electronic components. “The industry won’t replace top-down methods,” Tang said, “but they plan to use bottom-up together with the existing top-down methods soon.”
* Ferrocene is covalently bonded to the block copolymer; there is no need for a complexation step to add a metal-containing compound to the surface — a burdensome requirement of most previous methods. Moreover, their technique is a step beyond related polymer systems that contain covalent ferrocenylsilane linkages, in which removal of the organic components leaves behind silicon oxide as an impurity in the metal oxide.
There’s versatility in the technique as well. They use a ferrocene-containing polymer, which they convert into the inorganic iron oxide. “But if we replace the ferrocene in the polymer with carbon precursor, we could make a perpendicular carbon nanorod, which would have a lot of potential uses,” Tang said. “Or we can incorporate a semi-conducting polymer, like polythiophene, which would be very useful in solar cell applications.”