>My gut reaction is that selecting materials and etchants is probably easy for
>crude structures, with tighter and tighter constraints as the requirements for
>smoothness in the final tweezers are tightened.
Right. For a first cut, most any selection would do. If there is a group willing to _develop_ nanotechnology this could well be _the_ way to start. This would be the first tool in the toolbox. Maybe it would stay usefull for a long period as a way to move small workpieces around.....
In an absolutely ideal system,
>you would want to get the tines of the tweezers bounded by atomically flat
>surfaces, both vertically and horizontally. This is conceivable, but VERY
hard.As nearly as I can tell, the vertical flatness would be set by
>a) The uniformity of the initial deposition, basically how well the epitaxy
> works
>b) How selective an etchant one can get
A silicon wafer should be a ideal starting point as far as flatness goes. If you start worrying about lattice matching, the game could gain complexity _very_ quickely.
>The horizontal flatness would depend on the lithography step. Normally, both
>the resist exposure technique (eg e-beam exposure of poly methyl methacrylate)
>and the pattern transfer technique (eg ion milling) will introduce molecular
>level noise into the pattern. If we put yet another requirement on the metal
>and structural materials: That they both be crystals, aligned in the same
>direction, and that they both be anisotropically etchable, then we might be
>able to make the lateral patterning leave smooth edges. This would also
>require that the boundaries of the tines be oriented in the right
>crystallographic direction.
I would like to avoid the necessity of masks completely. With some development masks could well be a benefit, but for now I see them as a problem. I see the basic process as putting down layers with ( eventually with MBE ) and doing whatever annealing is necessary and then cutting one trench with whatever method is convienient. Removing the material between the tines through this trench then cutting the tines to shape.
It sounds like you don't think E-beam or ion milling is up to the job. I thought that the processes that are used for minimum geometry things like microwave mesfet gates would be in the ball park.
> By using this
>technique, well-defined tips and lines in the nanometer range have
>been fabricated. The shape of these mesa structures is basically
>determined by the physical growth mechanism. The sidewalls are
> ^^^^^^^^^^^^^^^^^
(emphasis added)
>well-defined crystallographic planes. This technique offers a
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>convenient tool for the study of one- and zero-dimensional electronic
>systems. (Author abstract) 14 Refs.
>Proceedings of the 15th Dry Process Symposium (DPS 1993)
>
>Admittedly this is an additive, rather than subtractive process, but if it
>produces reproducible tip structures...
I agree, If It Works, use it whenever convienient. I don't know how this could be extended to arbitrary shapes, but I don't see any laws against 90 degree wide tweezer tips....
Chris