“I never was so small as this before, never!”
Lewis Carroll, Alice’s Adventures in Wonderland
Electron-beam Lithography: Printing in Minuscule Scale
Our minuscule book is the smallest reproduction of Alice’s Adventures in Wonderland in the world. Alice would rightly exclaim, “I never was so small as this before, never!” But how small is small and how can such a minute size be achieved? Read on to find out about tiny units of measurement, and about the cutting edge of nanotechnology: electron-beam lithography.
We can all more or less visualize a meter: roughly the width of a doorway, five steps up a staircase, the height of a five-year-old, one long step of an adult man, etc.
Now let’s make this 1000 times smaller: that’s a millimetre. This is getting harder now. Still, most of us will have a clear visual sense of a millimetre: roughly the thickness of a credit card or library card, or about 10 sheets of paper on top of each other.
Shall we try to do this once more? Make the millimetre 1000 times smaller. That’s a micron, the abbreviation for micrometre. This is getting really difficult now. This is a size we cannot see with a naked eye. A human hair has a diameter of 50-70 microns, and you can see that. But one micron is much smaller than that. Smaller than a particle of pollen (30-50 microns), smaller than one red blood cell (5-10 microns), smaller than one tiny particle of a spider’s web (2-3 microns), and the human eye can’t see any of those. In order to comprehend the size of one micron, we need to imagine it.
And if imagining a micron may be a bit of a stretch, what if we took this even further, and imagined making a micron 1000 times smaller? That would give us a nanometer. This is definitely the stuff of the imagination. The nanometer is real, of course, it exists, indeed, nano-sized particles are part of what make up everything around us. But we need the imagination to conceive and understand such a tiny, tiny scale.
Here are two videos that explain further the size of a nanometer.
If you are a young reader, you will probably enjoy most this one:
If you are a grown-up, you may find this one more engaging:
So where does our tiny Alice book sit in this impressive array of ever-diminishing sizes? It’s smaller than a millimeter, but bigger than a micron. It measures exactly 85-60 microns, that is, around the same size as the diameter of a human hair!
The technology we used to print this tiny book is called electron-beam Lithography (or e-beam lithography). But what is it, and how does it work?
Let’s begin at the beginning. “Lithography” is a word of Greek origin, which combines Greek λίθος (stone) + γράφειν (to write). It literally means “writing on stone”. It actually brings to mind ancient ways of writing, before papyrus, velum, or paper, when inscriptions would be chiseled on stone.
But lithography is a much more recent technology, though already over 200 years old! It was invented in Germany in the late 18th century and it works on the principle that water and grease repel each other.
In traditional lithography, the lithographer makes marks (texts or images) in a greasy medium on a flat printing surface (stone). The surface is then dampened with water, but the water only settles on the unmarked areas, as the greasy areas repel it. Then the lithographer rolls over the surface a roll which is covered in greasy printing ink. The ink now “sticks” only to the drawn marks, the water repelling it from the rest of the surface. Finally, the ink is transferred to a sheet of paper by running paper and the printing surface together through a press.
Now electron-beam lithography works in a similar way, but a tiny scale. Instead of stone, the surface on which our book is printed is a tiny silicon chip, on which a “resist” is applied by a process called “spin-coating.” Spin coating involves covering the chip with the liquid resist and literally spinning it very fast (about 3000 rpm) until the liquid is just a thin film covering the chip. The resist is then baked until hard, before being scanned with a focused beam of electrons, which creates tiny shapes, in our case the letters that make up the words and sentences of Alice’s Adventures in Wonderland.
A resist is sensitive to exposure to the electron beam. It is actually a polymer*, a long chain of molecules that does not dissolve very easily, as the long polymer chains are tangled together and strongly cling to each other. The electron beam has enough energy to break-up these long polymer chains into shorter ones, making them more easily dissolved. By scanning the electron beam across the resist, we can make the desired patterns – in our case, the letters that make up the book.
(* The polymer we use is called poly(methyl methacrylate), but you may know it as acrylic or Perspex).
The resist is then developed, which means that it is placed in a liquid that dissolves away the broken polymer chains that have been exposed to the electron beam. Now we have letter-shaped holes in the resist, so the next step is to fill them with gold. Gold is deposited everywhere on the resist, but the unwanted portions are removed when we finally peel-off the unexposed resist. The last step is to clean the resist: the golden letters only now remain.
Follow the steps of how electron-beam lithography created our miniature book in the video below:
You can see below some images of how the tiny Alice’s Adventures in Wonderland book looks under a scanning electron microscope. From left to right you can see:
- An overview image showing all 78 pages of the book
- An image of just one page
- A close-up image showing the detail of the lettering