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April 21, 2011

Intrinsic top-down unmanufacturability using current microelectronic fabrication at 3 nanometers

Although small structures can be fabricated by deposition, lithography and etching, in some cases their intrinsic variability precludes their use as elements in useful arrays. Manufacture is a proper subset of fabrication. We show that structures with 3 nanometer design rules can be fabricated but not manufactured in a top-down approach—they do not have the reproducibility to give a satisfactory yield to a pre-ordained specification. It is also shown that the transition from manufacturability to intrinsic unmanufacturability takes place at nearer 7 nm design rules.

A 6 nm array of 3 nm diameter features is intrinsically unmanufacturable using the most modern tools of microelectronic fabrication, because of the intolerable level of feature-to-feature fluctuations and the inability to address or read out from individual sites on the array.

4 page article

NBF NOTE- going to full molecular nanotechnology would not have this limitation.



Even if an alternative fabrication technique, other than those described above, could result in a perfect 6 nm pitch array of 3 nm quantum dots (such as some as yet unknown method for perfectly crystallizing a perfect and single layer of 2 functionalized quantum dot structures made in the gas phase and then subject to a strict mass selection), one would not be able to integrate the elements of the array for the purposes of addressing or reading out. Any such wires for contacting and passing current on the 3 nm scale would be subject to the same variability and consequent low yield as the quantum pillars. The transport would be well into the disordered regime and tunnelling of electrons between adjacent wires would be uncontrolled, resulting in uncontrolled and uncontrollable parasitic interference.

Given that 3 nm design rule arrays are intrinsically unmanufacturable using a top-down process, what are the wider implications of this result?

(1) Current manufacture achieves 6σ yield, i.e. the prespecified range performance must encompass the ±3σ range of the properties of the produced artefacts. At 3 nm design rules we struggle to achieve even 2σ yield.

(2) In practice there are many experimental examples that show intrinsic variability above the level imposed by the statistics of small numbers. It is important to note that, in scaling for silicon CMOS technology, the limits are at a larger size, as it is the failure to define a capacitor structure and gate a channel at about 10–15 nm which prevents further scaling in manufacture. In the case of single tunnel barriers in III–V semiconductor alloys, the precise positions of (say) Al and Ga atoms within an AlxGa1−xAs interface layer can introduce large variability. Splitgate transistors have a much greater variability in the threshold voltage because of the precise disposition of a few dopant atoms in the electron supply layer

(3) Recent work on quantum wires at 7 nm design rules shows that the conditions on uniformity within structures can be achieved that just reaches the levels required for silicon integrated circuit wiring. Since we have established that 3 nm design rules cannot be manufactured with current fabrication techniques, the interface between manufacturability and unmanufacturability must be somewhere in between.

(4) All electronic and optoelectronic devices in highvolume low-cost production are also accompanied by a comprehensive suite of simulation tools that allow both right-first-time design of new products and reverse engineering for quality control. At this level of nanostructure, made by deposition, lithography and etching, there are no such simulation capabilities, and the intrinsic level of variability precludes their development

(5) Many results in nanoscience can be shown to be intrinsically unmanufacturable in terms of ideas for applications in electronic or optoelectronic components, and so will remain as scientific curiosities.

(6) Arrays of quantum dots, single-electron tunnel junction transistors, split-gate transistors, carbon nanotubes, etc, can always be used for their aggregate or averaged properties, but not as elements in any form of pixellated array.

(7) Any precision optical metamaterials that rely on feature accuracy and precision at less than ∼5 nm will also be unmanufacturable: the main structural features in these metamaterials are at about 150 nm, so this means that accuracy to 2–3% cannot be achieved.

(8) None of these findings preclude the use of such small structures as one-offs subject to individual biasing.

(9) The comments above have focused on electronic and optical properties, but they can be broadened to cover the variability of electromechanical properties, or indeed any other properties that might be used in sensing applications.

(10) Pace Feynman and his 1959 lecture that initiated nanotechnology as we know it today, there is plenty of room at the bottom for fabrication, but rather less for
manufacture!
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