Dateline: April 22, 2001
The electron is a most convenient particle (or wave) for microscopy and lithography, not to mention electronic devices such as your computer. The mass of this subatomic particle is so small that only a negative exponent can do it justice: about 10-27 g. That is about one billionth of a billionth of a billionth of a gram. See what I mean? Too small for words. Compared to this super-small mass, it has a quantized negative charge of 1 e (elementary charge = 1.6*10-19 Coulomb). As this charge is exactly the same for all electrons, it is possible to precisely predict and control the movement of electrons. This tendency for very small objects such as electrons to be identical to one another is one of the motivating factors behind nanotechnology. That is, unlike components used in larger scale manufacturing, there need not be an inspector to check the mass and charge of each electron in a nanodevice.
The small and uniform mass to charge ratio of an electron makes its momentum extremely sensitive to electric fields, which exert a force on charged particles. Such fields can be used to accelerate, direct, and focus electrons in the form of an electron beam. Scanning an electron beam over a surface, and measuring its interactions with the surface is a method known as Scanning Electron Microscopy (SEM), which allows nanoscale resolution of conducting surfaces. Furthermore, if a surface is chosen that reacts in response to exposure to electrons then an electron beam can be used as a writing tool for the rapid patterning of surfaces with ultra-high resolution, a method known as Electron Beam Lithography (EBL).
Now, consider the wave properties of this beam. The de Broglie wavelength of a particle can be related to its momentum by the relatively simple formula l = h/p, where l is the wavelength, h is Plank's constant and p is the momentum of the particle. Electrons differ from photons in that their mass is much larger, which results in a larger momentum and smaller wavelength. While a photon has a wavelength much larger than the size of one atom, the wavelength of an electron is far smaller than an atom. Thus, unlike optical microscopy or lithography, the far-field diffraction limit is not a barrier to the resolution of electron microscopy or lithography. Rather, other difficulties must be (and are being) overcome, such as our ability to focus the beam on a spot so small as a single atom. In Feynman's founding speech of 1959, he explained how electron microscopy might one day see individual atoms:
"I would like to try and impress upon you while I am talking about all of these things on a small scale, the importance of improving the electron microscope by a hundred times. It is not impossible; it is not against the laws of diffraction of the electron. The wave length of the electron in such a microscope is only 1/20 of an angstrom. So it should be possible to see the individual atoms."
It is remarkable how prophetic Feynman was in his speech, seeing as this exact feat was was accomplished 22 years later with the invention of Scanning Tunneling Microscopy (STM). STM is indeed invaluable for determining atomic precision within conducting samples and even selectively positioning individual atoms. However, even with its sub-atomic resolution, STM relies on slow mechanical actuators and is not currently useful for industrial nanolithography. Thus, it is still entirely possible that continued improvements in the focusing of electron beams (or other particle beams) might result in more rapid atomic resolution.
