Comment by seanalltogether
2 days ago
The thing I didn't understand after watching that video was why you need such an exotic solution to produce EUV light. We can make lights no problem in the visible spectrum, we can make xray machines easily enough that every doctors office can afford one, what is it specifically about those wavelengths that are so tricky.
The efficiency of X-ray tubes is proportional to voltage, and is about 1% at 100kV voltage. This is the ballpark for the garden variety Xray machines. But the wavelength of interest for lithography corresponds to the voltage of only about 100V, so the efficiency would be 10 parts per million.
The source in the ASML machine produces something like 300-500W of light. With an Xray tube this would then require an electron beam with 50 MW of power. When focused into a microscopic dot on the target this would not work for any duration of time. Even if it did, the cooling and getting rid of unwanted wavelengths would have been very difficult.
A light bulb does not work because it is not hot enough. I suppose some kind of RF driven plasma could be hot enough, but considering that the source needs to be microscopic in size for focusing reasons, it is not clear how one could focus the RF energy on it without also ruining the hardware.
So, they use a microscopic plasma discharge which is heated by the focused laser. It "only" requires a few hundred kilowatts of electricity to power and cool the source itself.
The issue isn't in generating short wavelength light, it's in focusing it accurately enough to print a pattern with trillions of nanoscale features with few defects. We can't really use lenses since every material we could use is opaque to high energy photons so we need to use mirrors, which still absorb a lot of the light energy hitting them. Now this only explains why we need all the crazy stuff that asml puts in it's EUV machines to use near x-ray light, but not why they don't use x-ray or higher energy photons. I believe the answer to this is just that the mirrors they can use for EUV are unacceptably bad for anything higher, but I'm not sure
Photoresist too. XRays are really good at passing through matter, which is a bit of a problem when the whole goal is for them to be absorbed by a 100 nanometer thick film. They tend to ionize stuff, which is actually a mechanism for resist development, but XRay energies are high enough that the reactions become less predictable. They can knock electrons into neighboring resist regions or even knock them out of the material altogether.
It really is the specific wavelength. Higher or lower is easier. But euv has tricky properties which make it feasible for Lithography (although just barely it you have a look at the optics) but hard to produce with high intensities.
Specifically, what makes x-rays easy to generate are these: https://en.wikipedia.org/wiki/Characteristic_X-ray In essence, smashing electrons into atoms allows you to ionize the inner shell of an atom and when an electron drops down from an outer shell, the excess energy is shed as high-energy photons. This constrains the energy range of X-ray tubes ("smash electron into metal") to wavelengths well below 13.5nm.
(These emission lines are also what is being used in x-ray spectroscopy to identify elements)
You can also generate broad spectrum bremsstrahlung radiation easily, this is widely used for medical X-rays.
Any source to this? I am hearing this for the first time.
ITs easy to make X-rays, you just hit a metal target with electrons: https://en.wikipedia.org/wiki/X-ray_tube
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There is such a thing as X-ray lithography, but it comes with significant challenges that make it not really worth it compared to EUV.
I'd like to hear more about these challenges
As I understand it, primarly because due to the high energy level of x-rays, light x-ray interacts very differently with materials[1]. Primarily they get absorbed, so very difficult to make mirrors or lenses, which are crucial for litography to redirect and focus the light on a specific miniscule point on the wafer.
The primary method is to rely grazing angle reflection, but that per definition only allows you a tiny deflection at a time, nothing like a parabolic mirror or whatnot.
[1]: https://en.wikipedia.org/wiki/X-ray_optics
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Stochastic effects become a bigger and bigger problem. At some point (EUV) a single photon has enough energy to ionize atoms, causing a cascade that causes effects to bloom outside of the illumination spot.
There are no normal x-ray mirrors. The only way to focus them is to use special grazing mirrors where the x-rays hit them almost parallel to the surface.
https://science.gsfc.nasa.gov/662/instruments/mirrorlab/xopt...