# How does photolithography acheive finer resolution than the wavelength? **URL:** \ **Category:** Factual Questions\ **Created:** [July 24, 2022, 6:37pm UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484 "2022-07-24T18:37:58Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [July 24, 2022, 6:37pm UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/1 "2022-07-24T18:37:58Z")
Thinking about silicon wafers here. Back in the dark ages when I had physics classes at university, it seemed to be an article of faith that you can’t accurately image anything smaller than the wavelength of the light you are using to observe it. Which would seem to imply that you can’t accurately project a smaller image either. As far as I can tell, the current tech uses 13 nm extreme ultraviolet for etching, but the chip feature sizes I have heard are quite a bit smaller (3 or even 2 nm have been mentioned). How does this work? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [July 24, 2022, 8:13pm UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/2 "2022-07-24T20:13:16Z")
I got the impression somebody someplace created interference patterns to create some simple sort of photolithography. This would have created repeating structures, though their geometry wouldn’t have been arbitrary, it would have had to follow some shape. Could this be it? I’m embarrassed to say I don’t actually know or remember whether interference patterns can be smaller than the wavelengths used to create them – though I should. Optics 201 was a long time ago. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [July 25, 2022, 12:20am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/3 "2022-07-25T00:20:03Z")
> [@xtenkfarpl](#): > > but the chip feature sizes I have heard are quite a bit smaller (3 or even 2 nm have been mentioned). One way to achieve those tiny sizes is to lie about it. > **[TSMC’s 7nm, 5nm, and 3nm “are just numbers… it doesn’t matter what the number...](https://www.pcgamesn.com/amd/tsmc-7nm-5nm-and-3nm-are-just-numbers)** > > "These numbers are just numbers. They're like models in a car - it’s like BMW 5-series or Mazda 6." ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 25, 2022, 2:44am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/4 "2022-07-25T02:44:39Z")
There are many different techniques at play. One of them is double-patterning (as well as triple patterning, etc.). Imagine that you tune your photoresist and exposure time so that only those parts that are near the peak of the sine wave are cured. This leaves you lines that are much narrower than your wavelength, but the minimum spacing between lines is still the wavelength. However, you can then shift your mask by a fraction of a wavelength, and create lines that are at a different offset. Do it enough times and you can fill in the large gaps, leaving lines at a much closer spacing than the wavelength. Another trick is called immersion lithography. The idea is simple: the photoresist is put under a film of water. Since the speed of light is slower with a higher index of refraction, the effective wavelength goes down. That makes a typical 193 nm laser operate as if it were 134 nm. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [July 25, 2022, 2:58am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/5 "2022-07-25T02:58:56Z")
> [@Dr.Strangelove](#): > > There are many different techniques at play. This page goes into many of the techniques you mention: [Deep UV Photolithography](https://www.newport.com/n/deep-uv-photolithography) ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [July 25, 2022, 4:23pm UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/6 "2022-07-25T16:23:18Z")
> [@Dr.Strangelove](#): > > “Another trick is called immersion lithography. The idea is simple: the photoresist is put under a film of water. Since the speed of light is slower with a higher index of refraction, the effective wavelength goes down. That makes a typical 193 nm laser operate as if it were 134 nm.” I wonder if anyone has tried a film of something with a higher refractive index than water? Seems like an obvious idea? ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [July 26, 2022, 2:40am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/7 "2022-07-26T02:40:16Z")
[quote=“xtenkfarpl, post:6, topic:968484, full:true”] > [@Dr.Strangelove](#): > > I wonder if anyone has tried a film of something with a higher refractive index than water? Seems like an obvious idea? Wouldn’t this something also have to be transparent to work? As well as non-corrosive, non-reactive with the photo-resistive ink, etc. That might limit the liquids that can be used. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 26, 2022, 2:58am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/8 "2022-07-26T02:58:25Z")
I agree that it seems like an obvious idea, but I haven’t heard of anything except ultrapure water being used. There are definitely a number of significant constraints. Cost and compatibility with other processing steps will be a big one. I did find this patent: > **[US8007986B2 - Immersion lithography fluids - Google Patents](https://patents.google.com/patent/US8007986B2/en)** > > Suitable additives that may be added into immersion fluids, immersion fluids comprising at least one carrier medium selected from an aqueous fluid, a non-aqueous fluid, and mixtures thereof, and immersions fluids comprising at least one carrier... > Still other immersion fluids that have been considered for 15 nm immersion lithography are KRYTOX™ and perfluoropolyether (PFPE). Searching for these, I can find some mentions of them in the literature, but no examples of them being used. I think that ultimately, the materials problem just gets too hard/expensive, and you end up better off going with EUV (extreme ultraviolet, using a 13.5 nm laser). There are numerous challenges there as well, but it’s a “reset” in terms of tech maturity, and further improvements are relatively easier. ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [July 26, 2022, 7:38pm UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/9 "2022-07-26T19:38:10Z")
Right, I’m sure there are a lot of constraints. And I’m not enough of an inorganic chemist to make any actual suggestions. There are probably quite a lot of bright and well informed scientists looking into this, since it is, after all, a very obvious idea. ---
**Author:** ![peccavi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peccavi/32/388_2.png) [@peccavi](https://boards.straightdope.com/u/peccavi)\ **Post date:** [July 26, 2022, 11:11pm UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/10 "2022-07-26T23:11:34Z")
To be clear, the CMOS technology nodes started out as straightforward measures of something on the wafer, usually the gate length. Using standard transistor designs, gate length could be used to predict the number of transistors one could stuff into a square mm of chip area (Moore’s Law!). Somewhere below a micron, they started to represent “effective” dimensions. Once they got below about 22 nm, the transistors became vertical (FinFets) so they simply didn’t represent a physical dimension at all. They also had to abandon Deep UV along the way in favor of EUV. The node numbers aren’t made up or imaginary, though. They more or less represent the dimension required in the straightforward calculation to stuff the number of transistors they can actually stuff into a square mm of chip area. However, they aren’t marketing hype. The customers for these technology nodes are spending increasing amounts to design these chips ( ~$100M at 5nm, with mask sets costing $10-15M alone) so they are not going to be persuaded by a new number alone. Their ROI is predicted on the cost of a wafer ($17K at 7 nm) , how many chips they can get off a wafer, and the volume of chip sales that will cover the design costs. Here are a couple of interesting reads that will give you some insight into the extreme technoogy applications that are taking place and the associated costs: > **[The Dark Side Of The Semiconductor Design Renaissance – Fixed Costs Soaring...](https://www.semianalysis.com/p/the-dark-side-of-the-semiconductor?utm_source=substack&utm_medium=email&utm_content=share)** > > As the semiconductor design renaissance flourishes, fixed costs and risk soar. Semiconductors will march forward, but how many dead bodies will litter the ground? > **[TSMC Details 5 nm](https://fuse.wikichip.org/news/3398/tsmc-details-5-nm/)** > > TSMC details its 5-nanometer node for mobile and HPC applications. The process features the industry's highest density transistors with a high-mobility channel and highest-density SRAM cells. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [July 27, 2022, 12:29am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/11 "2022-07-27T00:29:51Z")
The machines to make the latest chip are insanely high-tech: > **[EUV techology beakthrough thanks to master laser builder](https://www.trumpf.com/en_US/newsroom/stories/euv-techology-beakthrough-thanks-to-master-laser-builder/)** > > Michael Kösters and his team developed the High Power Seed Module, a key component of the world’s most powerful pulsed industrial laser. With this achievement, they have contributed decisively to the commercial use of EUV technology and the... > Weighing more than 10 metric tons, it consists of 450,000 individual parts. As the system’s light source, this laser generates plasma with a temperature of 220,000°C, which is 30 to 40 times hotter than temperatures on the surface of the sun. The giant laser is aimed at a stream of tin droplets inside the lithography system, where it strikes and flattens 50,000 of these tiny droplets every second. Actually, the giant laser takes two shots at each of these tin droplets, with the second hit transforming the flattened droplet into plasma that emits the precious EUV light. > **[Extreme ultraviolet lithography](https://en.wikipedia.org/wiki/Extreme_ultraviolet_lithography)** > > Extreme ultraviolet lithography (also known as EUV or EUVL) is an optical lithography technology used in semiconductor device fabrication to make integrated circuits (ICs). It uses extreme ultraviolet (EUV) wavelengths near 13.5 nm, using a laser-pulsed tin (Sn) droplet plasma (Sn ions in the ionic states from Sn IX to Sn XIV give photon emission spectral peaks around 13.5 nm from 4p64dn - 4p54dn+1 + 4dn-14f ionic state transitions.), to produce a pattern by using a reflective photomask to exp As... That machine is like the Manhattan Project multiplied by the Apollo program. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 27, 2022, 12:37am UTC](https://boards.straightdope.com/t/how-does-photolithography-acheive-finer-resolution-than-the-wavelength/968484/12 "2022-07-27T00:37:54Z")
It was surprising to me to learn about their Rube Goldberg laser system, as opposed to using a free-electron laser, but I don’t know what the natural limits are. I did find this from 2010: [13.5-nm FREE-ELECTRON LASER FOR EUV LITHOGRAPHY](https://accelconf.web.cern.ch/fel2010/papers/tupa28.pdf) They propose a 40x20 m facility, which is rather large. It seems like there’s a practical lower limit to the size, which might be too big. The FEL achieves a whopping 0.1% efficiency and 5 kW beam power, compared to the hot tin approach with 0.02% efficiency and 200 W beam power.