Monday, July 11

Printing Semiconductors

Our economy depends not only on mass scale popularity driven production, public education, solid work ethics and a healthy respect for experimentation but also on hobbiest activities where the future designers and decision makers learn from experience how things can actually be made to work.

So that, I think, means I should be leveraging the "3d printing" technology efforts and getting into semiconductor printing. Most of the relevant patents there have now expired, and there's quite a lot of solid precursor work dating from the early part of the 20th century.

Basically, it's taking the technologies behind the television tube (and other technologies - scanning electron microscopes, mass spectrometers) and using them to develop hobby scale circuitry. Or, that is my hope. (Now let's see if I can motivate myself enough to carry this through the obvious problems...)

Inspirations include:

Ben Krasnow's home built electron microscope

Charles Moore's OKAD vlsi design system

And hopefully I can get enough done to engage interested people at nova-labs

So, first off, that electron microscope is possibly simpler than what an initial build of the printer would be:

First, you need to have some way of isolating problems. For a reprap printer, the human eyeball is good enough, but for things like semiconductors you need something that can see on the microscopic scale, and the electron microscope is a plausible first approximation for what you need there.

Second, you need to be boiling off the materials that you are going to be depositing, and that means things that will need to be replaced over time.

And, of course, there's the need for the semiconductor blanks to be printed on. Almost certainly this will be silicon for any first efforts. So that's another cost item and something else where things can go wrong. (How do I check for problems on the surface of the blanks? If there are fingerprints, or other defects, how do I find a part of the surface which can still be used? Is there some way of cleaning or polishing away minor defects? What would that take?)

For dopants, I imagine initial efforts would focus on boron (p-type) and phosphorus and arsenic (n-type), and since I really don't know what I'm doing here, I'll guess that I'll need a vacuum of 10e-8 torr (though maybe that is too taxing for an initial effort) and dopant concentrations between 1% and 0.001%. (But what temperatures do I need, for these things? What voltages? How should focusing work?)

Also, reading various references, the temperatures involved for the wafer itself might range quite widely. suggests 450..650 C for a variety of semiconductor substrate types, but another reference suggested temperatures as high as 850C..900C. And the melting point of silicon is slightly above 1400C.

Meanwhile, suggests that tightly controlled temperature ranges are important for repeatable results (which, in turn, are important for the results of destructive testing to be useful). [There's also a considerable body of knowledge to absorb, relating to various models of how semiconductors form and perform - but I think that having some practical examples to work with will help significantly in understanding that material. In fact, that's a significant part of the point of this exercise.]

That said: repeatability is *not* a priority for the initial implementation. For the initial implementation we need a relatively minimal goal. The point of building this thing in the first place is to get practical experience, turning it into something that can be useful of others is going to need that experience.

So, ok, honestly, the first thing to try to do is get a diode working. This is the absolute simplest thing that can demonstrate viability. (How do I test that I am putting it together right? How do I affix electrodes onto the result?)

After that would be a transistor, then perhaps a logic gate and then a binary counter. Another path gets into analog amplification. And if I can even get close to 1970's integrated circuit capabilities, I'll be happy that I understand those fundamental concepts.

For a source of interested people, I'm thinking makerspace community, educational community and maybe eventually business people (but ramping things up for popular use would require engagement with people who are very used to doing other things and with priorities which conflict with my own, so that's something for much later).

I spoke with one person at nova-labs, and he suggested I might need an x-ray license. So I spent some time trying to research virginia and federal (nuclear regulatory commission) regulations, and I currently think that I will not need a license. First off, though, I do not have enough specifics to even know what would be needed. But I am not planning on using this on people, and I'm not going to even approach 1e6 electron volts.

Though, also, there's a bit of an assumption there (I'm going to be working with volts, maybe 5 KV, and I think the implication with the electron volt system of measurement is that we are talking about the amount of energy imparted to one particle - so that's a mix of temperature and voltage and whatever else, and it all gets a bit fuzzy how much exactly is involved, especially when you start thinking about the energy which exists as mass - in principle though, the electron volt regulations are about difference from rest state, and are not about regulating mass itself).

Anyways... as long as the energies are low enough, no licensing should be needed. If licensing does wind up being needed, because of potential radiation hazards then I guess I would need to figure out which licenses would be needed from which agency. But, for now, this is looking more like a hand waving issue than a real issue.

Finally, there's the issue of how to install the connector pins that we traditionally use to hook up with integrated circuits. Conceptually, we might be able to just have some pads which we connect to, and that is something to play with. But "soldering" titanium wires using (for example) molybdenum deposition might be the place to start. This will apparently destroy the crystal structure underneath it (where it deposits), and it seems like perhaps something which has a cooler vapor phase than molybdenum might be the right place to start (lead? tin+lead? tin? ... probably tin, but apparently since lead atoms are so big they can be a carrier for a doping material and the lead will just sit on the surface and not become a part of the crystal...)

In fact, this is such a basic issue that probably the right place to start is first hooking two wires to a silicon wafer and verifying that they can conduct and that the thing is mechanically sound. Once that is doable, try making a semiconductor resistor. And only once that is possible does it make sense to try to make a diode. (And, then, after that: a transistor, some logic gates, a flip/flop, a counter, and be reviewing to make sure the techniques are documented, with some stream-of-thought blogging for things that might be useful memory jogs, later. Somewhere in this, repeatability and, thus, temperature control and high vacuum starts mattering. And, hypothetically, after this has been done it would make sense to bring in design tools, such as OKAD.)

(fifth draft of this page)

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