LED Fabrication (Take 2) & Vacuum Technology

So when I previously wrote about LED fabrication, I talked about doping. It is, in theory, one way to create the desired material properties to turn electrical current into light. However, the efficiency of an LED depends on how crystalline (how orderly or consistent) the device is. And doping is like…..well….shooting bullets into a nice bale of hay. It’s not very crystalline. It’s a great way to make electronic gates when you don’t have to worry about high current flow, because you are more worried about pushing a force verses an electron through the device.

So instead of shooting atoms into a material, we would rather coat them in thin layers. And this is typically done by a process called MOVPE (or metal-organic vapor phase epitaxy), or just VPE for short. They can also use MOCVD (metal-organic chemical vapor deposition), which is kind of similar. And there are many other theoretical and experimental ways to layer atoms, but there are only a few, well, feasible of creating an LED for 5 cents.

So this is the basics behind MOVPE. Taking a material that usually wants to stay a solid (the metal “stuff”), we combine it with an additional molecule (an organic compound) which makes the MO material more volatile (basically a vapor phase). A lot of times it’s ammonium (NH4+, but anything goes as long as it works.

The vapor coats just about everything in a thin layer of the metal-organic substance (wafer, pipes, chamber, you name it). And the fancy word for atom-layering is “epitaxy.” So there’s the five parts to that massive scientific acronym. Change the vapor pressure, temperature, flow rate; and the growth rates are altered in non-linear, but predictable ways.

You only need 2 layers to create some sort of LED, but that’s not exciting enough. For, example, modern chips create a mirror underneath the LED. And when I mean “underneath, they grow the LED, sputter metal/oxide/something on top, and flip the chip around. You can’t grow anything “underneath” a crystal layer without breaking it apart, so there are a lot of fancy fab tricks that we utilize.

The whole topic of high-vacuum processing is interesting by itself. When the only atoms in the chamber are the ones of interest (metal/oxides) without “air” in the way, they behave very differently than what we are used to. It’s like the difference between working with water and sand (if that makes sense) They don’t interact with much besides the vacuum chamber itself and the most important part, what we put in it that vacuum chamber.

We even have to create vacuum pumps in a very different way to reach sub-atmosphere vacuum pressures, because we aren’t actually “sucking” at that point. Hyper fast turbines with hundreds of fragile blades, ones which would rip themselves to shreds in normal pressures, force single atoms through numerous rebounds. Cryogenic pumps cool down atoms to bring them out of the vapor phase. Non-volatile oils, which don’t evaporate even under extremely low pressures, “flow” in circular paths to catch unsuspecting gas molecules.

And once you have removed all the unwanted “junk” from your system, then you can have some real fun. Wondrous things can occur when you don’t have “gas” in the way. Cathode ray tube technology still exists in many forms. Nothing like glitter when you bombard an electron layer into a sample of pure gold. Or utilize it to raster scan a sample to create your ever-so-popular “Scanning Electron Microscope” (SEM) images.

Or you can add some pure gas, and excite it with some electromagnetic radiation. You don’t know fab unless you have seen the eerie purple glow of Argon as it eats away at your target sample. And specific chemicals will selectively eat specific materials. Oxygen eats plastic; chloroform eats specific semiconductor compounds; and sulfur hexafluoride (which is a lot of fluorine)……… yeah, I’ve seen that used too. Or if you take SEM technology, except you replace the electrons with actual atoms (Gallium, for example), you can physically mill into your sample. Just like this 10 micron flower that I did last year.

FIB_Flower

Is there a practical reason why I made the flower….not really. But it made for a very unique Mother’s Day gift, even if she doesn’t truly understand how I made it.

But it’s one of the few ways to truly create sub-micron structures out of nothing. You can’t just take the tiniest drill bit and “mill away.” Yes it’s slow, but by making a “master,” you can copy the sample into “daughter” wafers significantly faster. Stamping family seals into wax is significantly easier that making the wooden/rubber stamp itself. But you still need to make the stamp. That’s nano-imprint lithography for you in a nutshell.

How do I wrap this up……

While reading and studying the theory behind modern-day applicable science is interesting (while also putting you to sleep), it doesn’t even come close to physically experiencing these ideas put into practice. Even the “mistakes” are memorable, like my partially reflective mirror of a glass slide (not enough metal), or my wasted efforts in vacuum-based “Reactive Ion Etching” (lots of glowing gas). All while watching my advisor’s money going down the drain….kind of. But you don’t know unless you try, and that’s why we call it research!

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Antimatter [ Book Review]

I’ll just get straight to the point. As much hype there is about utilizing anti-matter for weapons, energy, and whatever obscure science-fiction application you’ve seen in the media, I don’t ever see it being feasible in my lifetime, my great-great-grandchild’s lifetime, or almost never. That is, unless we create large chunks of anti-carbon (or any element that packs well together, maybe a metal) and confine it in a vacuum chamber utilizing inverse space-time warpage (or anti-gravity, which (currently) doesn’t exist).

If you are lost, I’ll try to explain.

Mass is made up of atoms. Atoms are made up of quarks. Quarks can come in a few forms, but the stable ones are made with up & down quarks (unlike your ever-short lived top & bottom quarks). But that’s not really that important.

However, there is something “unique” about all quarks, so that there can be a “mirror-like” version of it, which can be called an anti-quark. And with anti-quarks, they can form anti-protons and anti-neutrons [Note: you can never truly isolate a quark, and likewise, you will never separate anti-quarks from an anti-proton]. Anti-electrons also exist, but we call them positrons (not protons). Basically any form that exhibits some sort of mass can come in an anti-form. And that’s why we don’t have anti-photons (just out-of-phase ones used in interference and holograms).

So when a proton and an anti-proton come together, they kind of cancel each other out and release mass-less energy (like two cars in driving opposite directions crash into each other, except the cars disappear and the gas fire is more like a mushroom cloud of doom…..).

This is the only (known) way we can convert pure mass into pure energy (as described by E = mc2). If you want to turn a golf ball into a bomb, you must find a similar amount of antimatter for it to touch with. But we can’t!

We can make anti-particles, that is true. It’s only been a couple decades since we created the first anti-proton on Earth, but we can only make them roughly 1 particle at a time. And if you are familiar with Avogadro’s number, it would take us many Earth-lifetimes to accumulate enough antimatter to do anything useful.

And then we have to store it. Since anti-protons react with every-day protons (which are in EVERYTHING), they can’t be stored by traditional means. So the only way to store these obscure particles is through indirect means, of the four fundamental forces of nature. Weak force is essentially nuclear decay, so that’s not possible. As for the strong force, we can’t even create an atom with 200 protons, let alone 120 to achieve the predicted “island of stability.” And I’m still waiting for anti-gravity, which is another rejection. What’s left is…..electromagnetics.

If something has a charge (+ or -), it can rotate in a circular motion within a magnetic field (aka, the Lorentz Force). We can actually confine a small amount of electrons in a vacuum. If the vessel is complicated enough, we can also do the same thing with protons, which are significantly heavier. And since antimatter is the mirror-like image of these particles, we have been able to do the same with the anti-matter forms as well. What’s also cool, is that we can actually cool down anti-protons in an electron cloud. They are mirror images; they just interact without exploding.

But…..how do you confine a gram of electrons into a small space. The amount of electron-electron repulsion is overwhelming. You could neutralize the electrons by combining them with anti-protons, creating anti-hydrogen, but the Lorentz Force is no longer applicable; they just run into the wall of the vacuum chamber (and we tried, believe me).

And that’s my brief description of why antimatter is just a buzz word. Even I was kind of disappointed after reading the book.

The book itself, Antimatter by Frank Close, is the same author that wrote Neutrinos. And I liked both books. Being a book on particle physics, there isn’t a lot to discuss once the academic descriptions are scraped from the topic. Unlike the topic of Neutrinos, ther is a lot more history and ethic topics that rise up in the work, which helps stretch the book to its 150 page length.

And as you can imagine, it also talks about …..(….wait for it….)….the universe!

Didn’t see that one coming, now did you!

Neutrinos may hold the key to why we only have matter (and not anti-matter). Physicists believe that during the big bang, there was an equal amount of matter and anti-matter. But since we don’t see any forms of anti-matter in the universe, we must make up theories why! The current “theory” involves majorons, but I’ll call them superneutrinos for kicks.

The big bang theory called for a very short period of “exotic particle” interaction, which involved unique transformations and particle decay. The unique thing about superneutrinos is that when they are created, they only come into existence as one type of particle. There is no particle-antiparticle formation, just a coin flip. And the odds there being an equal number heads and tails is zero, so the scale will be slightly lopsided in one random direction. Once the “coins have been flipped” and the universe has cooled enough to stabilize, allowing the head particles to cancel out the tail particles, there will be some left over; the “few” extra pieces to a LEGO set. And those extra pieces are what we call matter today.

If anything, I wish it went into a bit more detail on the many weird ways one creates an anti-particle (unless, there really aren’t that many ways to do it). The one thing that I wish they talked about more was on “space.” The physical nothingness that supposedly is swarming with particle-antiparticle creations and annihilations at the sub-atomic scale. You know, the reason why we have Hawking radiation!

Overall, a good read. I wouldn’t say its Mom-proof, but easily accessible for someone with a light chemistry knowledge (knowing physics II will definitely help with the “confinement” visualization).