A friend who is one of only a handful of lens designers for modern FABs (IC fabrication plants) recently wrote up the following - It explains some of the enormous complexity of IC photo-lithography - amazing stuff really... read and enjoy...
Nick ICs are produced by depositing layers of material onto a silicon substrate, > coating it with photo-resist, exposing the resist in a thing like a > glorified slide projector, then developing it and etching away the original > material (or implanting the whole thing in an ion implanter to dope the > exposed area of silicon). Anyone who's had a go at making their own PCBs > will understand the principal. It's the detail that's astonishing. First of > all, as mentioned above, the smallest feature printed on the silicon can be > as little as 40nm across. Lets get that in perspective. A human hair (the > universal indicator of smallness in the same way that football pitches and > double-decker buses are the universal bigness indicators) is about 80 > microns in diameter, so one micron is one 80th of a hair. 40nm is four > hundredths of one micron, so about 1/2000th of a hair. Features that small > have to be printed with perfect definition across a field up to 30mm > square. Since the silicon substrate (or wafer, as they're known) we're > talking about is up to 300mm in diameter, a grid of exposures is made, with > the wafer being moved on a stage under the lens from step to step (hence > stepper) until the whole wafer is covered. > > That's the easy bit. > > Chips are made up from up to thirty layers of material, each one with its > own pattern, which of course has to be aligned to the one below to an > accuracy of about .01 microns. Think about that. The wafer is 12" in > diameter, and is sitting on a stage made of quartz about 15mm thick. That > in turn sits on piezo feet that keep the image in focus (depth of field is > around 1 micron). This whole assembly weighs about thirty kilos, and has to > be aligned under the lens, focussed, exposed, then moved to the next image, > aligned again to .01 micron, focussed and exposed in a cycle that takes > around one second. To achieve this, the stage sits on an air cushion on top > of a lump of granite that weighs around half a ton, and is driven in x and > y by a couple of hefty great linear motors, position being measured by > laser interferometers. This one-second cycle covers a wafer in about thirty > five shots, so a wafer goes through in about 45 seconds, hour after hour, > day after day. Astonishing. > > The current production lenses use 193nm deep UV . They consist of a lens > about 1.5m long made up of 30-35 elements made from Calcium Fluoride and > Fused Silica , up to 220mm diameter. Essentially it's like a giant > microscope objective working in reverse, taking mask designs at 5x final > scale and reducing them onto an active chip area of about 30x30mm. > > Originally , lenses at 193nm were operating at NA of 0.5 ( = F/1 ) in air, > and diffraction-limited. That gave about 90-100nm linewidth, I think. Since > then, they have pushed the designs to an NA of over 0.9 in air, then with > water immersion between the lens and wafer, to an effective NA of 1.3 - > which is how the 40-50nm linewidths have been achieved. > > The lens element surfaces have to be finished to an regularity of about > 1/100th of a wave of light or better, in the visible. This requires > conventional polishing followed by cycles of measurement and > ion-beam-figuring ( I believe ) to finish. > > The lens elements are mounted into Invar ( ~zero expansion) cells and > assembled as a stack, one lens at a time, using optical monitoring of the > lens to get it centred. The Invar cells are diamond-machined to about 0.5 > micron parallelism. > > These lenses produce by far the most 'information' in one shot, of any > optical systems made in any field. If you wanted to capture the available > detail from this lens using a digital sensor, you would need to use about > 430 Giga-pixels - I know as I calculated this recently for interest, after > seeing an impressive 2G-pix image assembled from shots above Everest base > camp. > > Alternatively, consider that if the chip was enlarged to 400x400m , the > line structures would be at the 0.5mm level . > > Very few people understand the extraordinary technology that goes into the > common processor chips that are in their computers > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/eda3cb3d-20fd-4711-8907-b28155854522%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
