The squirrel in question, created by Alberto Guerrero '11
World's Smallest Black Squirrel?
Alberto Guerrero ’11 has created what is possibly the world’s smallest image of a black squirrel. Just how small is it? You could fit 1000 of his squirrels head-to-tail on the head of a pin. Roughly 100 of them could sit on top of on a grain of sand. 40 could stand inside the width of a human hair. And, “four squirrels could line up and use a red blood cell as a raft,” according to Guerrero. These squirrels are so small that they can only be measured on what is called the nanoscale, where the basic units are the nanometer (one millionth of a millimeter) and the micron (one thousandth of a millimeter).
So just how did Guerrero create these images of fluffy-tailed rodents that are just two to three microns wide? The physics major used a process known as electron beam lithography. It’s a method in which he begins by drawing a pattern (in this case, a black squirrel) using a computer-aided design program. Using the computer to control a scanning electron microscope (SEM), he then “writes” the pattern using the SEM’s electron beam on a silicon chip that has been covered with a plastic called resist, that is sensitive to electrons. Then he “develops” the image by removing the exposed regions of the resist.
“It’s like the film in a camera,” explains Guerrero. “You took the image, but until you develop it you don’t really have anything, just an exposed piece of film. So we have to develop it. We take the sample out, put it in some chemicals, and the chemicals scoop away the areas that we drew on. When we hit it with electrons, we weaken the bonds in the desired pattern. So what you see with our squirrel is a scooped out outline of the squirrel. If you were a little tiny human being standing on the outside, it would like a Grand Canyon in the shape of the border around the squirrel. And you’d be on standing on the canyon rim of resist.”
This academic year has been the first in Haverford’s history when its students have been able to use this technology on campus. The SEM, which makes electron beam lithography possible, was purchased last summer as part of the $1 million National Science Fund grant that also brought a confocal microscope, a florescence-activated cell sorting system and a transmission electron microscope to KINSC.
“We have had a collaboration with Dr. Charlie Johnson at the University of Pennsylvania, who has been doing the electron beam lithography for us,” says Walter Smith, professor of physics. “But the ability to do it ourselves will greatly increase the throughput of samples that we can make and allow us better flexibility with the different types of samples that we can make.”
Haverford is one of only two colleges of its size that has access to this technology, which is usually the purview of graduate students at large research universities. “We’ve joined a rather elite club,” says Guerrero. “Undergrads don’t get to do this.”
Guerrero not only gets to work with the scanning electron microscope—it is the basis for his senior thesis—but as part of his summer research for Professor Smith last year, he also helped set it up. He singlehandedly set up the computer, hooked it up to the microscope and then spent months learning how to optimize the electron beam and the writing process, working together with fellow Smith group member Lee Muller ‘13. “Alberto is really the guy on this instrument, at this point, who knows it better than anybody else at Haverford,” says Smith.
“Walter gave me the opportunity to be the head of the SEM,” says Guerrero. “So whenever there was an email from a professor or a post-doc or a student who needed training, he forwarded them to me. He was just like, ‘Take care of this.’ It was very cool… I get to be the manager of a $100,000 SEM, and I’m an undergrad.”
As for the future of the SEM, Guerrero hopes that his successor will push the machine ever farther, and try to draw smaller and smaller lines with it. (Right now the smallest line he can reliably get is 50 nanometers, which is roughly the width inside the squirrel’s paw.) Smith hopes that other bi-co professors will make use of the SEM in their own research, and is incorporating the technology into his Advanced Physics Lab coursework so that more students will get to try their hands at electron beam lithography. But, says Smith, “We’re still exploring all the possibilities.”