Ancient and Modern Printing Technologies Combine to Solve a Stubborn Nano-Fabrication Problem.
By Tom Imerito
In the half-century since Nobel Laureate, Richard Feynman gave his landmark, 1959 lecture, “There’s Plenty of Room at the Bottom,” the field of nanotechnology has come a long way. As Professor Feynman prophesied, microscopes 100 times more powerful than those available then have become a reality. Today, scientists can pick up individual atoms, put them exactly where they want, and take pictures to prove they did it.
Nevertheless, the grand challenge of scaling bottom-up manufacturing to commercial production levels has remained elusive.
On one hand, picking and placing individual atoms one by one has proven to be an inordinately tedious and time-consuming task, while on the other hand, faster processes, such as direct stamping, typically result in stray atoms diffusing beyond their intended boundaries. Tradeoffs between speed and precision are mutually obstructive to the success of either approach.
In response to this persistent challenge, Professors Anne Andrews and Paul Weiss along with a team of interdisciplinary colleagues at UCLA’s California NanoSystems Institute are taking an elegantly pragmatic approach to solving nano-manufacturing’s stubborn speed-versus-precision conundrum. Using an eclectic mix of old and new technologies, the Andrews/Weiss team has invented a method of printing nanoscale particles in precise patterns. The new method is called Subtractive Patterning by Chemical Liftoff Lithography (CLL), and as the name suggests, the technology employs subtractive, top-down fabrication methods in order to arrive at a starting point for bottom-up manufacturing.
By merging the physical principles of 1,800 year-old block printing with those of 200 year-old stone lithography, 50 year-old microlithography and 20 year-old electron beam lithography, CLL provides a way to construct a precise array of atoms from a nanoscale layer of atoms, called a self-assembled monolayer (SAM). When complete, the process results in consistently reproducible nano-patterns at speeds vastly higher than pick-and-place methods.
Although the process employs the principles of both intaglio (direct stamp) and lithographic (offset) printing, it is called lithography in deference to its use of chemically tuned, hydrophilic (water-attracting) molecular ink.
Like all printing methods, CLL employs three essential components: 1) a stamp or plate, 2) a substrate or paper, and 3) ink or paste. However, unlike conventional printing, CLL stamps and substrates are treated to induce an affinity for a chemically tuned ink.
To begin, CLL stamps are made of a type of organic silicone polymer known as PMDS (polydimethylsiloxane). Stamps are etched negatively to the shape of the desired pattern. To cut large features in the stamp, Andrews and Weiss use conventional etch-resist lithography processes. For smaller features they use electron-beam lithography. When the stamp is subjected to an oxygen plasma (a supersonic jet of electrically charged oxygen gas) the PDMA surface becomes attracted to the hydroxyl or amine end molecules of the CLL ink.
The substrate is composed of a self-assembled monolayer (SAM) of gold. Gold’s well understood chemistry along with its affinity for sulfur make it a preferred material for this stage of technology development.
CLL inks are engineered to form a molecular strand or tether with terminating molecules that gravitate toward and affix themselves preferentially to both the stamp and the substrate. The inks are chemically designed as a hydrocarbon chain with either a hydroxyl (OH) or amine (NH2) molecule at one end, and a sulfur atom at the other.
When a one-molecule-thick layer of CLL ink is applied to the gold substrate, the sulfur ends of the molecular tethers affix themselves to the gold surface, leaving the other ends exposed. When the plasma-activated stamp is brought into contact with the molecular ink, the exposed hydroxyl or amine molecules bind to the stamp surface. When the stamp is pulled away, the strength of the bonds between the molecular tether and its bonds with both the the stamp and the top atomic layer of the gold substrate cause the weaker bonds between the gold atoms in SAM to break, thereby removing the molecular tether along with a single atomic layer of gold. The result is a nano-precise, positive impression of the negatively etched stamp.
The process is sufficiently robust to reliably imprint precise nanoscale features across areas as large as centimeters, which encompasses a length-accuracy range of eight orders of magnitude. Features as small as 20 nanometers are achievable.
In further work, Andrews and Weiss have added inorganic materials to previously removed negative areas of a printed CLL pattern. The possibilities for sequential addition, subtraction and buildup of nanoscale materials, both organic and inorganic, appear to be almost endless.
Now that we have a practical starting point for Professor Feynman’s proverbial bottom, perhaps the time has come to start building from there.