Re: Patterning, overlay and EUV
Part 2 due to character limit:
Back to the mirrors. So now we have light. It's been nicely collected and concentrated by a collector mirror in the light source and now we have to project it onto a wafer. Remember that slide projector analogy for DUV tools? A DUV reticle is transparent and light is passed THROUGH the reticle to create the imagine in the light beam. Now look back at the bit about the lenses. That also applies to reticles. The solution is the same as the mirrors, you create a reticle that is basically a multi-layer mirror with the pattern etched into it and then reflect the light beam off of it. ASML has not come up with a way of doing this that requires less than (I believe, I don't know the exact number) 14 mirrors. The problem is that these mirrors are not 100% reflective. They only reflect about 79% of the light. Not a problem if you have just one mirror and plenty of light power. But you only get maybe 120 or 130 watt of EUV light from the source. That means that at the end of the line you have 120*0,79^14=4,4 watt! reaching the wafer. Not a whole lot.
Then there is the wafer positioning. On the DUV tool we could use airbearings for eliminating shaking and moving around. Air bearings in a vacuum are not possible. Running into the NDA territory again I'm just going to say this involves magnets. Lots and lots of magnets.
Then there is the challenge of building all this. The traditional method for building very clean (ultra high) vacuum systems is to build all of it out of nice sturdy non porous materials like stainless steel, make sure everything is resistant to a little heat and then bake the whole system at 120 degrees for a few days while sucking out all the contamination. This is not an option if you have a vacuum vessel the size of a luxury saloon car with lots and lots and lots of electronics crammed into it. You can bake it out before you put everything in, but not after. ASML had to define a whole new category of vacuum cleanliness for this called Ultra Clean Vacuum. Modules are built clean and kept clean because any contamination put in at assembly can no longer be removed after assembly is done. You'd think this is easy but let me just give a small example of what this entails for me, the technician.
First off I'm in an ISO grade 6 cleanroom, fully suited up. Anti-static coveralls, hood, socks, gloves and shoes and surgical mask. Before I even touch ANYTHING on the machine I have to put nitrile gloves over the anti-static ones (possibility of sweat seeping through and leaving stains). Then I clean the area around where I'm going to be working with cleanroom wipes soaked in isopropyl alcohol. Then put down a piece of ultra clean plastic (doubled up) between which I can then place/prepare my vacuum tools and parts I'll be needing. Only then is it time to open the hatch I'll be working through. Before I touch anything that will be touching something inside the vacuum chamber (like tools, my own hands or parts) I put on ANOTHER pair of nitrile gloves OVER the first pair, taking special care not to touch the palm or fingers. Once I wear those gloves I can work in the vacuum parts. If I touch anything, like support myself on the edge of the chamber, or scratch my nose, or just idly leave my hand hanging by my side and touching my coveralls I have to put on a fresh pair of those second gloves. You can imagine this gets tedious. Especially since parts come nicely packaged in plastic, but that packaging is NOT vacuum clean. So I can use 8 pairs of gloves JUST for swapping a part with 3 bolts. I've had days where 2 of us went through an entire pack of gloves (50 pairs) in a single day. Anything cleaned for vacuum can never touch anything that is not.
As this is getting quite long I'll just leave some of the engineering challenges to a single line and let you figure it out.
The multi layer mirrors are extremely accurate and flat, blown up to the size of Germany the largest bump would be a few millimetres
Holding a wafer down in air is easy, just use suction. How do you do this in a vacuum where suction doesn't work?
How do you get wafers into and out of the vacuum, positioned accurately enough that you can then expose them?
How do you make things move in a vacuum if you can't use grease (normal vacuum grease contains Fluorine, which wreaks havoc on the mirrors) and steel parts touching directly will instantly cold-weld and fuse together?
How do you keep the inside of the system clean?
How do you keep the wafer temperature stable in a vacuum? Any water leak inside the vacuum could potentially destroy millions of dollars of equipment.
Then also keep in mind that these systems are VERY complex. You can't just drop a box on your customers doorstep, hand them the manual and tell them to have at it. Even basic operation requires month of training. Basic maintenance and troubleshooting adds another few months, etc, etc. All that training has to be prepared and thought up by someone. Any problems encountered that the customer cannot solve must be escalated to a support team, who must be able to escalate to even brighter minds. Once a machine is in the field, someone needs to think about what parts are spares and can be needed in the field, then make sure they are available. Someone must write the manuals and instructions. Someone must build the control software that keeps everything running. There are hundreds of thousands of tasks involved with this type of machine that many people don't even start to contemplate.
All of this put together means its quite an achievement ASML has got systems running at customer sites. A system that solves all of those engineering hurdles (and then some, with more still to go) that is barely larger than a 60 ft shipping container. Keep in mind that many said that EUV was simply impossible. Time will tell if all of this works out or if it was a swing and a miss. The shrink will have to stop at some point.