Re: Patterning, overlay and EUV
Gahhh, I've started this explanation about 3 times now and my brain keeps screwing me over, talking about all those little details that are completely unimportant. I apologise in advance for any spelling or grammar errors, I'm typing this on a way too old laptop without a spellchecker and I'm kinda tired of proofreading after starting again for the third time.
So, lets try again.
Lets start by thinking about what makes lithography itself hard.
First thing is the size. Computer chip feature sizes are measured in nanometers (shortened to nm). That's 1000ths of 1000ths of a milimeter. That's hard to even comprehend. Take your own hair. Grows at leisurely pace of 0,3 mm a day. That's 3,47 nm per second! By the time you have read this sentence your hair has grown more than the distance between 2 features on an Intel processor.
Quick term explanation: The silicon bit of a computer chip is produced many at a time on/from a wafer. A slice of mono-crystaline silicon. Usually circular, 300 mm diameter and 0,775mm thick)
What we are trying to achieve is to project a pattern of lines onto a resist covered wafer. Easy squeezy you'd say, all you need is a fancy slide projector. At the very core that is sort of what an litho machine is. It shines a bundle of light through a slide (called a reticle) and then shrinks that image with some lenses and projects it onto a wafer.
The earliest version of litho machines were called steppers/repeaters. Project an image of the whole die, move the wafer a bit, expose another die, etc. All well and good, but at some point someone decided they wanted to do it faster. And faster. And faster still. At some point, stopping the wafer every time to expose an image starts to take time. So a clever guy came up with the scanner. Project a slit of light, move the reticle underneath it and then move the wafer the other way simultaneously. Now you can keep the reticle going back and forth and the wafer moving in a constant SSSSS pattern. Result, you can expose even faster. Modern DUV tools are now producing over 100 wafers per hour. That means that in under a minute a 300 mm diameter wafer is completely filled with exposed dies (something like 6x6 or 10x10 mm) and ejected again. You can imagine that getting that scan synchronised between the wafer and reticle is extremely important. With ever smaller structures the accuracy also needs to get better and better. The chucks holding the wafer move at incredible speeds. Accelerations over 100Gs and top speeds over 25 m/s. And the reticle has to keep up.
So how accurate does this alignment have to be? Well, given the feature size and the speed of the chuck the alignment has to be well below 1 nm. (and keep in mind we are talking about 2 physically completely separate items not mechanically connected in any way). So if we were to scale that up, it's like flying 2 jumbo jets at 700 km/h within 0,003 mm of each other. Again, and again and again, scan after scan.
The projection itself is also a challenge. Remember that demonstration with the laser and the fine grating of parallel lines, creating a diffraction pattern? All those lines an a reticle do the same thing. What you get is not a clean image, what you get is a diffraction. So the lens system has to eliminate all the diffraction orders expect the ones you want.
Then there is the matter of vibration. You're trying to project something very very small, very, very accurately. No matter how thick you make your foundations, vibrations are going to happen. From earth tremors, trucks moving outside or fat Mike from Accounting walking down the hall. So you have to keep those out. Usual solution for those in any industry is air bearing and voicecoils, but you can imagine that all shaking a wafer and reticle around business is going to cause a fair bit of shaking in and off itself. ALL that needs to be compensated.
Further bit of trouble is temperature. Materials shrink or expand as they cool down or heat up. Silicon is no different. But what happens if I have a wafer that is cold on one side and hotter on the other, expose a die and then let the wafer change temperature? Well, simply put the next layer you expose on top is going to end up in entirely the wrong place and you have a very expensive bit of useless silicon on your hands.
Now here come a few of the challenges in combining all of these things. We want to expose wafers faster. Pumping more light onto that wafer means the resist hardens faster, so I can move the chuck faster. Move the chuck faster and I get more vibrations (not to mention I still have to make sure the damn thing doesn't move relative to my chuck under all that load). More light also means more heat generated on the wafer, so now I have to cool it more. Faster exposure also means I have to get the wafer into and out of the machine faster. At 100 wafers an hour, that's a load and unload every 36 seconds.
Last thing I want to touch on is overlay. Overlay is the accuracy of alignment between the different layers of a chip. All those different layers have to connect together so every time you expose a new layer, it has to be accurately positioned relative to the previous layer(s). But to process that layer you have to remove the wafer from the machine, do a whole lot of processing to it and then feed it back in. Then the new layer has to be within a few nanometers of the old one. Time and time again. This is achieved using special alignment marks. They are exposed and etched in the first layer. Problem then is that you still have 30 or 35 or maybe even more layers worth of exposing and etching and processing to do. You can't redo those alignment marks because you can't be sure you get them in exactly the same place but you still have to keep track of a mark that keeps fading and fading and fading into oblivion. (And somehow they manage to do this)
So now lets look at EUV. All the fun we talked about previously with some added bonus hurdles.
First off is how do you get the light? EUV is a weird sort of photon. Not quite Xray, not quite UV any more. Some bright spark somewhere found out you get these photons at a nice usable 13,5 nm wavelength if you convert very pure tin into a plasma by blasting it with a lot of energy. Like a CO2 laser or a pulse of high voltage. Aside from the fact that air would make creating and maintaining this plasma difficult EUV light does not travel through air very far. If your screen where emitting EUV light right now it wouldn't even reach your eyes. So you replace the air right? With what? Not many gasses are transparent to EUV. One of the few that does that you COULD use is pure hydrogen. Good luck with that, I'll be WAY over there taking shelter if you ever try this. SO you remove all the air and do it in a vacuum. Seal the whole thing in a nice sturdy jar, pump it down and blast away... (I'll get back to why this is not easy in a minute)
But how do you get the light from that plasma onto the wafer? You have to somehow focus it. Big problem number 2, EUV light doesn't really do lenses, or mirrors for that matter. There is no known lens material that is transparent to EUV, has a usable refraction index and is economically viable for production. That leaves mirrors. The standard single surface mirror we all know doesn't cut it. It just doesn't do anything. EUV can be reflected by a so called multi surface mirror. Lots and lots of layers of alternating material. It's still not a perfect bounce though, only a part of the light gets reflected. I'll get back to this in a minute
Only, how do you shoot tin with a laser and form a plasma? What happens to the tin after that? How do you direct the light? So here comes the next challenge. Several crazy and/or smart people have gotten involved in the matter. Cymer (US) and Gigaphoton both went with the laser produced plasma method, shooting droplets of tin with a high power CO2 laser. Xtreme (Germany) went with a high voltage method.
In the end I believe Xtreme didn't quite make it and Gigaphoton is still working on it. Cymer got acquired by ASML and their source seems to be the main option right now. Having only ever been up close and personal with the Cymer system I'll focus on that one here.
So how does a LPP EUV source work? Well, take a big vacuum pot. Shoot tiny, tiny (micrometer size) droplets of tin across it with a high pressure gas and use a very accurate targeting system to blast each droplet with a CO2 laser beam as it passes. The droplet superheats, explodes, produced a tiny bit of tin plasma giving off EUV light and a lot of tin debris. Catch any un-hit droplets on the other end, let the debris condense on the walls, where it'll flow down to a collection drain where you can then pump it up again to repeat the process.
Then you stick a nice shiny multilayer mirror behind it to focus the light and bobs your uncle... right? One problem with multi-layer mirrors. They don't really like tin. Or plasma, fingerprints, carbohydrates, moisture, acetone, getting hit with EUV light (yes, really. Though only little bit), etc, etc. So you have to get this mirror really close to the tin without actually getting it into the tin. Then you have to catch ALL the tin debris flying around before it can hit any of the other mirrors you need to project that light on the wafer. How this is done unfortunately starts veering into NDA territory so I'm going to leave it here.