merry christmas from infiniflux!

Hey internet!

Hope you have a great and restful holiday break! The above image is a scanning tunnelling micrograph (STM) of a Si (111) wafer passivated with hydrogen under ultrahigh vacuum. The pattern of blackish dots arises due to the STM tip tunnelling current through the unfilled antibonding states of the surface atoms. Translated into layspeak, those little black dots are fairly good representations of individual Si atoms. Boo-yah. Let's see Hallmark do that.

In fact, the image above is about 100 nm x 100 nm, which is around 1.6 trillion times smaller than a 5 inch square Christmas card.

The white dots spelling out my very special christmas message (it helps if you squint)[1] were created in the surface by running the STM tip over the pattern with a high bias, ripping off hydrogen as it travels and leaving a dangling bond. This forms a midgap state, which is easier to pass current through; therefore the dangling bond shows up as a white dot on the image. This sort of patterning is much more useful than just making cheesy holiday cheer, and helps scientists to better understand the electronic properties of surfaces and interfaces. Such patterns may allow them to precisely create atomic wires for use in technologies such as spintronics. So cool!

The image was made by my friend and fellow grad student Marco, who is studying with Dr. Robert Wolkow at the University of Alberta. Marco is officially an honorary infinifluxer for life, an esteemed and exclusive club to which I am very happy to welcome him into. You too can join this club if you discover an element and name it after me. Act now in the next ten minutes and we'll throw in a complimentary Slap Chop. Operators are standing by.

Your Canadian tax dollars at work!
Joel

[1] I would have tried to write a more inclusive holiday message, but it's a huge pain in the rear to write large patterns. Sorry to all fans of Hanukkah, Kwanzaa and Festivus. We can still have an airing of the grievances if you want.
[1a] The largeish white blobs are gunk that tends to fall off of the tip when you are patterning. I definitely gained a lot of respect for people that work with STMs for their research in doing this. There are umpteen millions of different ways for things to go wrong when you're trying to manipulate atoms.

Nanolithography by block copolymers

Just saw a pretty neat talk on lithography using block co-polymers by Paul Nealey of the University of Wisconsin:


(above: a figure from Nealey's recent Science paper)


Block copolymers are cool for lithiography because you can take a copolymer, put it on your substrate, and it will magically and spontaneously form into a pattern! If you can figure out what polymers and conditions to use, you can control the pattern, including the size and type of features. In comparison, other techniques require complicated steps such as multiple mask/etches or fancy pants equipment like an e-beam.

The way it works is that you take two monomers that don't want to be near each other, like polystyrene (nonpolar) and polymethylacrylate (polar), and chemically bond them to make a block co-polymer. It's like making boys and girls in grade school dance together; they ain't having none of those cooties, so they position themselves as far away as possible. In the case of copolymers, you can get features such as dots, cylinders or lamellae (fancy word for strips), and depending on the molecular weight of the polymers, you can control the size down to the nanoscale.


Reprinted by permission from Nature: Nealey et. al, Nature 424 (6947), pp. 411-414.

What Nealey has done that's pretty cool, is found a way to force the cootie-separation (techical term) in an incredibly regular and repeating fashion. He accomplishes this by pre-patterning the substrate[1] to increase difference in the surface energy between the two monomers. Some of the examples he gave showed perfect lamellae of 25 nm width and 250 nm height stretching out for hundreds of microns (almost millimeters!).

The size of features of block co-polymers is smaller than other conventional patterning techniques, and therefore this lithography approach has tons of interest from the semiconductor industry. Nealey even stated that a collaboration he is doing with an industrial partner could lead to block copolymer lithography being used in hard drives with storage densities of 1 TB / sq. inch!

Really cool talk on an awesome-sauce field. Hope to hear more on this!

References:
1. R. Ruiz, H. Kang, F. A. Detcheverry, E. Dobisz, D. S. Kercher, T. R. Albrecht, J. J. de Pablo, P. F. Nealey; "Density Multiplication and improved lithography by directed block copolymer assembly,"Science, 321(5891), 936-939 (2008).

2. S. O. Kim, H. H. Solak, M. P. Stoykovich, N. J. Ferrier, J. J. de Pablo, P. F. Nealey; "Epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates," Nature, 424(6947), 411-414 (2003).




[1] The pre-patterning step might seem to render all of this excitement baloney. Why go to all the effort of applying the polymer to the surface when you already have to pattern it with something else? The answer is that Nealey doesn't have to pattern the whole surface; just enough of it to tip the surface energy balance in favour of the repeating, regular pattern. In the Science paper referenced above, his group achieved a "density multiplication" of 4 times.