Feynman's Nano, Now -2

This is an excerpt from the same Feynman's famous lecture mentioned in the earlier posts. This is about the how part of capturing all the volumes of the encyclopedia on a pin head.

"The next question is: How do we write it? We have no standard technique to do this now. But let me argue that it is not as difficult as it first appears to be. We can reverse the lenses of the electron microscope in order to demagnify as well as magnify. A source of ions, sent through the microscope lenses in reverse, could be focused to a very small spot. We could write with that spot like we write in a TV cathode ray oscilloscope, by going across in lines, and having an adjustment which determines the amount of material which is going to be deposited as we scan in lines.


This method might be very slow because of space charge limitations. There will be more rapid methods. We could first make, perhaps by some photo process, a screen which has holes in it in the form of the letters. Then we would strike an arc behind the holes and draw metallic ions through the holes; then we could again use our system of lenses and make a small image in the form of ions, which would deposit the metal on the pin.

A simpler way might be this (though I am not sure it would work): We take light and, through an optical microscope running backwards, we focus it onto a very small photoelectric screen. Then electrons come away from the screen where the light is shining. These electrons are focused down in size by the electron microscope lenses to impinge directly upon the surface of the metal. Will such a beam etch away the metal if it is run long enough? I don't know. If it doesn't work for a metal surface, it must be possible to find some surface with which to coat the original pin so that, where the electrons bombard, a change is made which we could recognize later."

Forgive me, for capturing such a big excerpt, which makes you comment whether it is a full article itself. But without this entire excerpt, the following post would not be meaningful.

Before stating this, Feynman tries to bring about some analogical printing methods like plastic molding, embossing etc.,. which were known in those days and used for classical printing. But this excerpt, especially, is very important to be reviewed, beacuse it is amazing to know that someone could explain, five decades back, what is called photolithography today, in such a lucid manner. Just that, the electron beam is replaced by UV light and the possibile surface is replaced with positive or negative photoresist material. Know more about photolitugraphy, here. Though this photolithography technique is not used for printing books on a nanoscale, it is still a possibility. Currently, this technique is used in Integrated Circuits or chip fabrication. All that comments above are my first thoguhts about the excerpt, before I could think a little more.

Wonder what? There is a photomask involved in photolithography, which contains the pattern to be etched in the photoresist layer. This is what confines the UV rays to a specific nano dimension on to the photoresist. Without this, UV rays can not be used to etch small dimensions as in microelectronic circuits. So, there should be more better method available currently, which could be used to manufacture this mask, without using a smaller mask. This thought and a few search in the world wide web guided me to such a method. That is called as "Electron beam lithography".

When I studied more and more, it was amazing to know that the actual process is almost same as Feynman's imagination and the limitations too, were. Here is the Wiki link to know more about the process. For those, who could not spend enough time to go through the link completely, here are the excerpts,

What?
"Electron beam lithography is the practice of scanning a beam of electrons in a patterned fashion across a surface covered with a film and of selectively removing either exposed or non-exposed regions of the resist"

How?
"Electron beam lithography systems used in commercial applications are dedicated e-beam writing systems that are very expensive (>4M USD). For research applications, it is very common to convert an electron microscope into an electron beam lithography system using a relatively low cost accessory (<100K USD)."

"Lower resolution systems can use thermionic sources, which are usually formed from LaB6. However, systems with higher resolution requirements need to use field electron emission sources, such as heated W/ZrO2 for lower energy spread and enhanced brightness."

Advantage:
"The primary advantage of electron beam lithography is that it is one of the ways to beat the diffraction limit of light and make features in the nanometer regime."

Limitations:
"The key limitation of electron beam lithography is throughput, i.e., the very long time it takes to expose an entire silicon wafer or glass substrate. A long exposure time leaves the user vulnerable to beam drift or instability which may occur during the exposure."

"With today's electron optics, electron beam widths can routinely go down to a few nm. This is limited mainly by aberrations and space charge."


Is it not the same as Feynman imagined?
After so much study, I feel that feynman's imagination had, in someway, driven the research and technology in this front.

Please don't empty your amazement with this post, as there are some more to come and strike you...