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Feynman’s Path to Nanotech (part 4)

MEMS

Another reason the Feynman Path may not have been tried is the perception that a machine-based approach has been tried in the form of MEMS, and that standard machine designs do not work at this scale and below due to stiction.

MEMS are in fact crippled by this phenomenon, which is a essentially an increase of friction with decreasing scale. This is, however, to a great extent because MEMS have very poor tolerances: “… in traditional machining, relative tolerances of 10-6 are becoming standard, whereas in the integrated circuit industry, a 10-2 relative tolerance is considered good. The definition of precision machining, with relative tolerances of 10-4, actually excludes micromachining!” A full machining and manipulation capability at the microscale would allow lapping, polishing, and other surface improvement techniques, which photolithography-based MEMS does not.

Feynman understood the key importance of precision:

If you work through a pantograph, even today, you can get much more than a factor of four in even one step. But you can’t work directly through a pantograph which makes a smaller pantograph which then makes a smaller pantograph–because of the looseness of the holes and the irregularities of construction. …

At each stage, it is necessary to improve the precision of the apparatus. If, for instance, having made a small lathe with a pantograph, we find its lead screw irregular–more irregular than the large-scale one–we could lap the lead screw against breakable nuts that you can reverse in the usual way back and forth until this lead screw is, at its scale, as accurate as our original lead screws, at our scale.

We can make flats by rubbing unflat surfaces in triplicates together—in three pairs—and the flats then become flatter than the thing you started with. Thus, it is not impossible to improve precision on a small scale by the correct operations. So, when we build this stuff, it is necessary at each step to improve the accuracy of the equipment by working for awhile down there, making accurate lead screws, Johansen blocks, and all the other materials which we use in accurate machine work at the higher level. We have to stop at each level and manufacture all the stuff to go to the next level—a very long and very difficult program.

We know, of course, that it is possible to create more precise machines using less precise ones, in some theoretical sense, since all our industrial base got built somehow in a chain of machines that stretches back to the days when blacksmiths shaped tools by beating them with hammers. Those of you who are avid enough amateur astronomers know that you can grind your own telescope mirrors by hand to a precision much, much higher than your hand motions can actually achieve.

In other words, the Feynman Path does not envision simply building a simplistic factory which can spit out another factory at quarter-scale and quarter tolerance as if it were stamping out consumer goods. It will require attention and craftmanship at each level, and indeed probably significant experimentation and the development of new techniques in many cases.

But — and this is a mantra I intend to repeat often throughout this exposition — at each stage we will have the full fabrication and assembly capability we need to do experiments, build instruments, and perform novel techniques. It is difficult to overstress how valuable it will be to maintain this capability, taken for granted at the macroscale, at each step to the nanoscale.

2 Responses to “Feynman’s Path to Nanotech (part 4)”

  1. Michael Kuntzman Says:

    Right now photolithography is at the 45nm node, and the next nodes are under development. Assuming a tolerance of 10^-1 or better, that makes errors on the order of 5nm or less. This will obviously improve as time goes by, at Moore’s law rates.

    For a 500um device, where the typical part features would probably be on the order of 5-50 um, that gives us tolerances of 10^-3 – 10^-4 or better, even without surface improvement.

    So would I be correct to expect that for a 500um device, stiction should not be a major concern?

  2. J. Storrs Hall Says:

    Whether stiction proves to be a major concern depends on a lot of details. One major one is does the material you’re working in get smooth with wear, or undergo major erosion? Good tolerance (actually, low surface roughness) makes the friction go down but the stickiness is always there. The key is making surfaces slippery enough to slide even with the stickiness. A 10^-4 tolerance should make standard designs work, e.g. wheels with axles and bearings. Bearings can run dry as far as heat is concerned but not if wear is an issue.
    Watches etc use jeweled bearings, which just means sleeve bearings in hard covalent crystal solids. It seems likely we could make spheres by various methods and use them in ball bearings. The key is being able to pick them up and put them in the race!

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