Nanoassembler

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Nanoassembler

Release status: Concept

Description	Central page for researching construction methods for complex, multi-material structures with nanoscale patterns at a reasonable rate
License	GPL
Author	User:Sapient cogbag
Contributors	User:Sapient cogbag
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This is the page for developing viable technology for comparatively rapid production of complex, multi-material devices with patterning and structure on the nanoscale at a reasonable rate. The most immediate goal is the production of full-capability semiconductor devices of at least a similar order-of-magnitude scale to that of modern semiconductor fabrication techniques, plus full 3D structure, though with significantly different processes of course.

Challenges (General)

Nanomanufacturing & Semiconductor Manufacturing poses very difficult challenges in a single-user-accessible or near-tabletop scale environment.

Construction Rate

Existing, top-down semiconductor manufacturing involves multiple very complex steps for producing even a single patterned layer. Each layer is only a few nanometres thick. The complex steps also take a long time and each require different environments, so the substrate has to be moved between multiple chambers for construction.

To make nanoassembly practical for home-ish users, the construction rate must be accelerated and have much simpler environmental requirements so the objects or devices being produced don't need to be moved around to different construction chambers for production. Furthermore, it must be possible to produce more "bulk" structures (without nanometre-scale patterning) at a very accelerated rate, as one of the major points of nanoassembly is the embedding of circuitry and other nanostructures within larger material. Ideally, construction rate would correspond roughly to the patterning complexity of the current layers.

An important aspect of this is that we can't continue current 3D printing techniques down to the nanoscale. Most 3D printers right now are based on a rasterisation like system - either a single mobile head (for deposition-like printers), or using crossed lasers point-by-point (like in resin printers). As the feature size of printers becomes smaller and smaller, these sorts of systems decrease in rate in a cubic fashion (unless the speed of "rasterising" a specific point increases sufficiently). If a current printer-style system takes several hours to print a 30x30x30cm³ region at 100micrometre resolution, then scaling each of those dimensions by 10,000 (for a 10nm resolution) would result in several hours for a 3x3x3micrometre³ region, which is ridiculously tiny, much smaller than human hair (which has width on the order of 50 micrometres).

Chemical Access and Handling

A lot of chemicals used in current silicon-based semiconductor manufacturing are very difficult or actively unsafe to handle.

For example, most p-type doping is done using Borane gas, which is highly toxic. Any nanoassembly process designed for 3D-printing-like usage needs to use chemicals that are at least somewhat safe to handle by someone who's not got massive amounts of chemistry experience - any highly toxic reagants must be produced only ephemerally and only in trace amounts to be immediately consumed by the process, from safer raw materials.

This also applies to any kind of liquid-based process. Modern semiconductor manufacturing is moving away from liquid processes towards more gas, plasma, vacuum, or ion-based processes for a reason - liquids are very difficult to handle and keep pure and if we end up using them we need to ensure that it's only ever in microscopic amounts at a time, or that we always need the liquid rather than having to switch it out in the process, because that would likely be unacceptably slow and have the same issues as semiconductor manufacturers currently experience.

Ingredient Purity & Environmental Cleanliness

Much of modern semiconductor and nanoscale assembly requires extremely high purity of materials and extremely clean environments - even micron-scale contamination is unacceptable. For any nanomanufacturing system, this is likely to at least partially be the case, so building a tabletop nanoassembler requires embedding mechanisms to maintain and produce that kind of environment. Furthermore, the general expense of current equipment to make these sorts of environments means that it is of somewhat high importance that we can produce most of the parts needed for this from our system.

The cleanliness requirement also adds significant complexity to the construction of new nanoassemblers from parts produced by other ones. In particular, it's very desirable to figure out a system to encase the most sensitive parts (for example, a massively parallel, nanostructured toolhead) such that they can be inserted into a new machine without being exposed much to the environment. Furthermore, it's a good idea to be able to combine parts in a near-sealed fashion. That is, we need to figure out a way to combine certain produced parts with particularly sensitive surfaces while reducing or eliminating that sensitive surface's exposure to atmosphere, in a way that is still user friendly.

My proposal for this is producing parts inside enclosures which have structured weak points to allow slotting together, which then enables the inner parts to be slotted together, but we have a lot more work to do to design such a thing.

Ingredient purity is another problem. Modern semiconductor manufacture requires highly pure silicon substrate for best performance (though it can work with less pure substrate). Most important is a lack of contamination with things that affect the electronic band structure too much, and current semiconductor manufacturing processes require highly monocrystalline silicon as a prerequisite for the best possible results. For dopants, purity is only slightly less important (because you are already adding far less of that into the semiconductor and hence contaminants are even further rarified), but purity is still pretty important.

My suggestion for dopant purity is to include something like an electrostatics-based or even magnetic mass spectrometer to act as a filter. Importantly, it doesn't need to be super precise, just enough to filter out unwanted elements. However, it may also be acceptable to start by working with less pure ingredients as long as they are reasonably pure.

Masks

Current semiconductor processes are mostly oriented around production lines - and require the production and use of highly specialised masks, which are then used thousands of times to produce the patterning on a semiconductor's surface to then apply a single process to it in that pattern. This is then repeated in many different steps, using different processes and masks.

Masks are pretty much the antithesis of 3D printing and rapidly reconfigurable production systems. Any system we design must involve maskless processes and enable direct patterning without the intermediary production of another "thing" with that pattern on it.

Massively Variable Processes

Modern semiconductor manufacturing involves many processes with extremely distinct environments and behaviour - some might require vacuum, some are additive, some are subtractive/etching (and in fact most mix additive and subtractive processes), some require liquids while some require large or small scale environments, etc.

This is an absolute nightmare for the kind of nanoassembly machine that we may envision. Any kind of process that requires changes to the "bulk" environment (e.g. a whole chamber increasing or decreasing in temperature or pressure at rapid speed), rather than occurring ephemerally and locally (e.g. a laser heating one spot for a very short amount of time), is unacceptable if it's required for producing any *single* patterned material or a *single* layer, and takes more than a few seconds.

What we need is a *uniform* (or close to uniform) process to assemble more layers of distinct material, which is directly controllable. Having a large variety of processes doesn't work. Ideally, such a process would not require any kind of vaccuum or high temperature either.

Substrate Flatness

Building semiconductor chips usually starts with a layer of highly pure silicon of monocrystalline structure and near-atomically-smooth surface, which is pretty much required in order for the components built on and above that surface to actually connect with each other. Even micrometre-scale bumps - which may be almost invisible to us - are like a mountain compared to the sorts of nanometre scale features we want.

This means we need to design a process - and a somewhat-flat surface - that can be built upon or etched away (e.g. with lasers) to get something close to atomically flat. Furthermore, we need to construct our own substrate on top of this in a reasonably removable way, before our assembler can then start constructing nanometre-scale components on top.

Precision Assembly

Even once we devise a concrete method for combining nanoassembler components together by hand that avoids contaminating any nanostructured surfaces, we still need to deal with the issue of combining components with nanostructured surfaces to nanometre scale precision - or more specifically, I suspect trying to align components like that to be nearly impossible (though it may be doable with nanometre-scale fractal joints, if you can avoid them shattering).

Instead, I strongly suggest that we prefer embedding methods of precisely measuring (electronically) the misalignments of components to nanometre scale and then using whatever our nanometre-precise positioning system is, and the way we transfer designs for construction, to compensate for these microscopic misalignments.

Precise Motion & Positioning

Another difficulty is precise positioning. Current nanometre scale positioning systems are expensive, inaccessible and focused on XYZ style, pointlike motion - though piezoelectric positioning does exist. I would strongly suggest that, once we are capable of reasonably small scale manufacture with semiconductors, we use this capability to build some kind of nanopositioned electrostatic motor system. To compensate for imprecise combination of positioning components, they should contain an embedded system to calculate their offset at each joint.

I have some ideas for that related to having essentially a very large number of adders and combining that with voltage offsets or calculated resistance due to nanoscale misalignment to derive accurate absolute positioning numbers, but haven't fleshed it out yet.

Another aspect of positioning is accounting for external vibrations that could disrupt any nanoscale structure. This may be accounted for with either active or passive damping - perhaps using electrostatics to tune the system to damp any common frequencies of vibration in the environment.

Depending on material choices, we may need to account for temperature-based expansion - however, the production of doped semiconductor materials generally implies the ability to create Peltier-based solid state cooling devices (even if not super efficient), and temperature issues can hence be avoided by using such devices to keep any super-precise tools' macro-scale dimensions consistent by pumping out excess heat or pumping in heat if it gets too cold.

Bootstrap Path

Something very desirable for any nanoassembly solution is being able to come up with a reasonably inexpensive bootstrap path from current printing and construction technology. Furthermore, it's pretty vital that - for feature sizes larger than a couple of nanometres - the system can construct components for assemblers of at least slightly higher precision than itself. This is to avoid a slow loss of precision and correctness as you start replicating and producing any nanostructured parts.

Developing Technology to Actually Do This

This is obviously a very daunting task. Progress, however, can and has been made.

For ease of terminology and referring to relevant technologies, I propose the notion of a *pathway* for integrated circuit and semiconductor manufacturing, which refers to a somewhat fuzzy collection of things like the substrate to use and how it can be constructed, what sort of dopants it might involve (and the type of techniques involved in created doped semiconductor material), the kinds of chemicals and environments that need to be made, etc.

Traditional Silicon Path

The "Traditional Silicon Path" is what I would call the technology path most focused on replicating existing semiconductor manufacturing but in a tabletop/3D-printer-like environment. This would primarily focus on using, for example, highly localised environmental changes for things like etching, rather than liquid/wet processes, as well as some mechanism to dope the silicon using localised high temperatures rather than turning everything into a furnace.

However, I think this is unlikely to be a successful path. In particular, without masks and without photoresist (which has the same issues as any wet process), the primary way to get resolution anywhere close to modern chips is via direct electron-induced sputter deposition and sputter doping, or ion beam doping. Both of these have pretty big problems. Furthermore, the process doesn't allow for growing the bulk silicon (even atomically thin silicon layers require the spontaneously flammable and toxic gas Silane (SiH4)), so the hypothetical assembler would need an entirely different toolset and process for that too.

Plastic-Graphene Path

One area that has been somewhat explored is that certain types of plastic - like Kepton, which can likely be produced in thin film layers by using the two soluble reagents that are used to create it plus an atomiser - turn into graphene when hit with specific amounts or types of light.

I've explored this path to some degree, and it does make a fairly decent potential process to sit alongside current 3D printing technology due to the ease of handling the substrate reagents for creating thin-film Kepton layers.

However, I have concerns about doping (though laser doping presents a potential pathway here, but the use of optics severely restricts minimum feature size and it makes doping a secondary non-additive process), and it is once again a secondary process on top of existing 3D printing tech, though one much more compatible with it than any hypothetical Silicon process would be. I may post some of my work on this, and people who've made more progress on this front I'd encourage you to add it :)

Diamond Direct-Doped Growth

The path I've mostly been working on lately. This is a path based on growing diamond-like material, with the precursor gases pre-mixed with dopant in external chambers for direct deposition of doped material. There are various processes that I've researched that provide potential routes to do chemical vapour deposition of diamond without intense heat/furnace stuff (mostly, plasma and electric field related things), and generalised directed deposition of material via CVD has been demonstrated if by a slightly different mechanism than my various ideas.

I intend to flesh out this section the most with my progress on developing ideas over the next few days, as a lot of the processes also permit accelerated growth of diamond when you care less about fine-grained features, which means it is probably possible to build semi-bulk structures with the same process components as the fine-grained structures.

Furthermore, the bulk ingredients and feedstock are chemicals like methane (which we could even use electrocatalysis to synthesise from CO2 and water if we discover a process for that), and growth of diamond with boron doping has been demonstrated with the relatively non-toxic boron oxide (though we'd likely use an intermediate mixing process to create small amounts of feedstock gas with different dopant concentrations).

Path - Diamond Direct-Doped Growth

This section contains information on various potential mechanisms and technology for growing diamond-like material with dopants.

Growth Mechanisms

Existing common methods to grow simple (nanostructure-free) diamond films for scientific purposes primarily involve the use of Chemical Vapour Deposition of various forms, often involving microwave assistance or other forms of energisation of the relevant gases. There are other, less relevant methods (like high-pressure diamond construction) that are used for commercial synthesis, but the primary mechanism we care about are ones that can extend a surface in a controlled manner.

Typically, for diamond growth, the CVD involves a hydrocarbon gas of some kind plus hydrogen (a large amount) which supposedly avoids the formation of graphene (though there is some questions on this, in some science papers I'm not allowed to link yet - might not be necessary for what we're doing).

Existing CVD Methods

The wikipedia page on CVD is a good resource on various existing CVD methods. There are older and newer methods - older ones tend to require more heat overall, while newer ones often use plasmas, microwaves, or arcing to energise the CVD gases with less heat. All these methods - as exist right now - are designed for uniform, bulk construction tasks. That is, they construct layers of uniform composition.

These methods have to solve several problems they have to solve:

• Strip the terminator structures off the surface of a material - crystalline materials like diamond and silicon don't terminate in a free radical (carbon/silicon/etc. atom with one bond not made), so a CVD process either has to strip the other atoms (usually hydrogen, oxygen, or some combo) off the surface before the reaction or otherwise account for them before new deposition can occur.
• Prevent the formation of graphene - at the low-to-atmospheric pressures that CVD often occurs at, graphene is technically a more stable chemical state than diamonds. This means that a diamond growth process usually has to have some way to reduce the probability of the production of graphene/graphite layers. In typical CVD, this is accomplished by some amount of hydrogen (though there are some questions on the level of necessity here)
• Prevent contamination from the containment chamber in the produced material - for any process needed for our purposes, this is a much less significant issue because we need to create conditions on a micro/nano scale and this already implies containment, especially if the tools used are made of the same material as that which is being grown. Though a related issue is avoiding corrosion or other breakdown of the structures used to grow the resultant material.
• Substrate heating - a lot of methods require heating of the growth substrate. This is undesirable for any methods we pull from.

Our Growth Methods

The growth mechanisms we use must solve most of the above problems, but they also require a couple other specific properties:

• Variable growth rate - While for very small scale, high-detail structures it's acceptable for a growth mechanism to only do a few nanometres per second, being able to adjust the rate to be capable of producing more coarsely-structured regions faster is a necessary quality. This is required so any mechanism we provide is capable of producing at least some amount of bulk material (not necessarily massive blocks, but at least a few centimetres thickness in a non-absurd timescale). It may be acceptable to produce somewhat degraded material (e.g. mixed graphite/diamond rather than pure diamond) in such cases, but it's much better if such a thing can be avoided.
• Directability - Being able to direct the growth is extremely important, as otherwise it's of no use for producing nanoscale structures. This directability must also be of sub-constructor resolution (i.e. can be used to construct devices of more fine resolution than was used to build the constructor)
• Non-Extreme Environmental Conditions - Bulk environmental conditions should not need to be extreme (in a "leaky" way e.g. via heat conduction or vacuum) for it to function. It shouldn't require any super exotic materials.
• Reasonably power efficient - Shouldn't require megawatts of power to build 1cm² of material continuously.

Proposed Surface Termination/Passivation Stripping Methods

Any growth mechanism needs a way to strip the diamond surface of it's passivation/termination-atom layer, to enable reaction with the Carbon structure below. In the ideal case, the mechanism itself does this "automatically". However, should this not be quite so doable, there are some potential methods:

• Argon/other noble gas - this method is used in Low Energy Plasma-Enhanced Chemical Vapour Deposition (see the wikipedia page for details) - this knocks off the surface passivation/termination layer with minimal damage.
• Electric field catalysis of hydrogen removal - this method is based on some obscure papers from 1999 (doi:10.1016/S0039-6028(99)00377-5), which compute the electric fields at the surface to initiate the dissolution of C-H bonds.
• UV-induced bond dissolution - Identify the bond energy for the passivation/termination layer, and shoot ultraviolet radiation with appropriate photon energy/wavelength to induce the bond to split into free radicals. This is the same way CFCs would destroy ozone in the atmosphere (they'd split a Chlorine-carbon bond and the Chlorine free radicals would induce ozone dissolution by bonding to the component oxygens more permanently).

Proposed Substrate Conductivity Mechanism

The primary proposed mechanism for nanostructured deposition relies on the existence of an electric field between the nanodeposition nozzle array and the diamond growth substrate. Furthermore, this substrate must be conductive during construction to prevent rapid charge buildup and the end of the electric field between the construction plate and the actual growth surface. To do this, there are some possible methods.

• Direct Avalanche Breakdown - In this proposed mechanism, a controlled avalanche breakdown is initiated in the substrate directly (with a resistor to prevent everything melting). This renders the substrate conductive. To avoid large voltages, an array of alternating electrodes could be used instead of one on either end. Once a material is in avalanche breakdown state, that can be maintained with much lower voltages, and it has an absurdly low resistance, so the material ends up having minimal internal electric field, which means it is usable as a minimal-resistance ground).
• Indirect Photoconductivity via External Avalanche Breakdown - Instead of directly supplying current through the substrate, this method proposes using a square array of the same diamond material just on the outsides of the substrate - then causing avalanche breakdown within that. By pushing sufficient voltage across it (probably resonant A/C to avoid extreme energy dissipation), this should make it flouresce at the right wavelength with photons that can push electrons in the substrate up into the conduction band via direct photon absorption (with no need for a phonon/thermal transition even though diamond is an indirect semiconductor).

Proposed Mechanism - Electroguided & Electrocatalyzed Ion Vapour Deposition

This is the primary mechanism I've been researching and analyzing the most. It seems to have a very high potential capability, though simulation and modelling needs to be performed to determine the practical parameters.

The basic principle is as follows:

• Hold some pre-doped growth gases at a strong negative voltage, charging them with excess electrons (to become negative ions). The technical details for this are complex but existing and fairly accessible methods of doing this exist (e.g. via thermionic emission, though we want to avoid any grounding and build up a high density, highly-ionised, low-energy cold gas in-atmosphere - avoiding the stripping of electrons from molecules, which would cause positive ions to be deposited on chamber walls - essentially, we want a non-neutral plasma of negative ions).
• Direct the gas using electric fields from surfaces at various degrees of negative voltage, down to a large array of nanoscale electrically-charged and controlled nozzles. These nozzles are gated by nanoscale electrodes, which enables the control of when and where and how a given amount of gas is held above the substrate. More details will be clear in a basic diagram to be uploaded soon.
• Hold the growth substrate (diamond) at ground voltage. Methods for making diamond conductive are discussed below (as it's an insulator).
• The strong electric field at the diamond's surface should aid in catalysing the spontaneous dissociation of any terminating atoms and enable reaction - this is the most experimental part of this concept, as the papers on this are fairly obscure and computational and from 1999.
• By holding the ionised gas in controlled positions and gating it on and off, you can control exactly where growth happens.
• As the surface grows, the array of nanodeposition nozzles moves upward (or the surface plate moves downward) at a rate appropriate to avoid one bump on the surface from causing exponential growth increase and an eventual short-out.

In terms of mechanics, probably the closest concept that exists today (on a macro scale) is Low-Energy Plasma-Enhanced Chemical Vapor Deposition. This typically relies on a plasma of a noble gas (like Argon) to create free radicals and ease the deposition reactions instead. The noble gas also deals with the surface terminator structure problem automatically.

The proposed method also carries aspects of the use of CVD where the gas energisation comes from electrical arcing. As, essentially, the entire process goes from a negative electrode in the source gases, down through a complex system of electrically-charged nanofluidic control mechanisms, through a very small gap to the surface, where the excess electrons (plus ions) get pulled down through the substrate being grown as the ion bonds to the surface.

It may also be the case that it's necessary to flip the polarity - either because it may be easier to create a positively charged plasma, or because of the requirements of the electric field direction to sufficiently increase the probability of the disassociation of hydrogen from the diamond surface. If these are conflicting, there are several other options to deal with the surface termination layer, as detailed above.

Either way, the control of this method then goes to the manipulation of electric fields within the gap between the hypothetical construction array and the grounding substrate growth surface. This can occur in one of several modes:

• Sub-cell resolution - In this case, the desire is to grow a structure with resolution smaller than a single constructor head nozzle "cell". To do this, my proposal is that structures of this sophistication are built in 9 stages for each layer, where the relevant matrix of construction cells has only one of the 9 positions active in each 3x3 grid at a time. The other surrounding 8 are gated closed and their voltages are held at high magnitudes and used to compact the area that the ions from the central constructor nozzle cell are guided to on the growth surface. By varying the precise voltage, it becomes possible to grow structures at very tiny spots. This, naturally, requires modelling and simulation to account from field distortions on any existing grown potrusions.
• Cell resolution - In this case, the desire is to grow within a single cell, fully. This provides some problems with beam shaping, that may require it to occur using the same mechanism as the sub-cell resolution mechanism (in particular, as the beam shape is likely to be circular, a square cell would not be covered fully without some redirectio).
• Supercell resolution - For bulk growth of material, the idea here is to open the floodgates and release more of the ionised gas through a whole collection of cells simultaneously. There are several potential ways to reduce the circle shaping issue - moving the substrate back and forth, or altering the hypothetical nozzle to add smaller beam diverters.

There is also the control aspect of gating the rate of current and ionised gas flow to increase or decrease the growth rate. Furthemore, it may be possible to create nozzles which allow both a cylindrical beam shape and a square beam shape (for instance adding a second electrode in a square shape on the bottom of the nozzle that can be enabled or disabled to squeeze the beam).

It should also be noted that while this system does entail the use of ions, the term "beam" is a slight misnomer. The ions here are not intended for acceleration to high velocities as in ion-bombardment doping mechanisms. The means of bonding with the existing surface is not implantation but chemical reaction - the ionic status is primarily to facilitate chemical reactions as well as provide a guidance mechanism via electric fields, to precisely control the location of reactions (and hence growth) while mitigating beam dispersion due to electrostatic repulsion. This does entail some acceleration - to get ionised gas out of the nozzle and into the area between the growth surface and the nozzle array - but not the kind of hyper-acceleration involved with ion beam deposition.

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