Today and tomorrow, we're reporting on presentations at an important conference on Productive Nanosystems: Launching the Technology Roadmap. Chris Phoenix is providing live blog coverage for us...
Third talk, John Randall, Zyvex: A completely different approach. Zyvex was founded to create atomically precise manufacturing on the way to productive nanosystems. In other words, building precise structures using big machines rather than nanoscale tools.
- APM is valuable.
- Digital matter is "an advantage ripe to be exploited." (I've been saying this for a long time - it's a fundamental advantage of molecular manufacturing.)
- Self-assembly is powerful but limited.
- Brute-force top-down engineering is not always elegant but it works.
Goal: Produce 3D rigid covalent structures with top-down control direct from CAD (computerized blueprint). This is the result of improvements in ultra-precision manufacturing, but it'll take a change in mindset. (Current manufacturing still treats matter as jelly-like and infinitely divisible.)
They've found commercial applications for even very limited initial capabilities.
Putting atoms where you want them: Eigler's creation of the "IBM" logo made use of atoms dropping into minimum-energy positions. (This is a reference back to the digital theme.)
Wilson Ho did molecular pick and place, creating covalent bonds. (There have been a variety of scanning probe chemistry demonstrations.
Mechanosynthesis has issues: You have to pick up the part, verify you have it, transfer it, verify you've done that. They've looked at tool tip reactions; they think that existing tools are adequate to deposit dimers on diamond surface at room temperature. Although this is theoretically exciting, there are practical problems, including how to synthesize the tool tip. So they took a different approach...
Atomic layer deposition builds amorphous materials; atomic layer epitaxy (ALE) builds crystalline materials. Start with a protected (passivated) surface: every available bond has a hydrogen atom. If you deprotect the surface, removing the hydrogen, then you can deposit a layer of atoms. If you choose the right precursor gas, you add only one monolayer which is protected as it's added. Then you can deprotect and add exactly one more layer of atoms. There are a number of precursor gases available. There are literally hundreds of systems to grow things with atomic precision in one dimension.
Now, if you combine this with the ability to deprotect the surface in selected locations... With a scanning tunneling microscope, you can remove single hydrogen atoms with atomic precision. Several groups have demonstrated this. This is "the limit of a thin resist" - a monolayer of hydrogen.
If you do this layer by layer, you can build 3D structures. Prof. Joe Lyding at University of Illinois has done repeated desorption/deposition. He's probably created amorphous, not crystalline, but it does show patterning.
Differences from mechanosynthesis:
- Building blocks don't have to be captured by the tool tip.
- The tool tip can be used to inspect both deprotection and assembly.
- You can do large areas (fast) or atomic resolution, depending on mode.
- This is a very general technique.
- All you need is an atomic-resolution STM tip - don't need anything else with atomic resolution.
You can't make large, reentrant, or releasable structures. However, there are some useful products. They aren't interested in a laboratory demonstration; they want manufacturing.
You need an atomically precise, invariant tip. ALIS has built such a tip. A reproducible atomic structure at the end of a tungsten wire. There are several other possibilities. Note that the tip never has to touch the surface, so it should last quite a while without damage.
He wants a parallel array of SPMs for higher throughput. They think they can get sub-nanometer closed loop X-Y position control with integrated electronics, using CMOS MEMS processes.
They're trying to develop a dual-material process, silicon and germanium, so that you can make releasable structures. (They think they can deal with lattice mismatch.)
One possible product is a nano-imprint template. They expect atomically precise tools to be the most valuable product. They expect to enable productive nanosystem factories.
Question: Hydrogen migrates at normal temperatures. Is that compatible with the deposition technologies? A: We believe (after careful study) that the hydrogen is stable on a silicon surface, up to 200-300 degrees C. We think we can get epitaxy to work in that window. Cryogenic temperatures are not necessary. You do get motion on a single dimer, but no long-range motion.
Question (from Drexler): There's a big divide in molecular technologies is between processes where parts go together due to fit or reactivity, and those where the resulting pattern is due to mechanical control. Conceptually, your approach comes under mechanosynthesis. About error rate: If you have a mis-removal, can you put a hydrogen back where it should be? And how can you correct errors in silicon deposition? A: ALE balances errors: It relies on mobility of silicon on unpassivated surfaces. This may not work on small surfaces. We don't know what error rates will be in small areas. But at least we'll have a way to inspect. We don't have a generic way of removing silicon or putting down hydrogen. We may be able to deposit hydrogen in an area and then go back and clean it up.
Q: Have you looked at atomically precise *doped* structures? A: You'll hear Michelle Simmons talk about putting down phosphorous atoms exactly where she wants them. So yes, we can create structures with controlled doping. Again, the reaction is generic. We think there's a wide range of heterostructures you can make.