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How are the lenses and other apparatus required for projecting a miniaturized version of the circuit using photo lithographic technique for IC manfuacture made?
Its known that even the tiniest spec of dust can ruin an entire chip so do you make sure the lens is perfectly clean and devoid of any distortions or scratches that may ruin the projection?
Actually, for the most part dust on the lens surface won't produce intolerable results at the wafer. At the lens surface the image being projected is not in focus, so a dust particle will not produce an image of the particle at the wafer.
The everyday example of this used to be called the "screen door effect", although don't bother to Google it - the meaning of the term has changed. It used to refer to the fact that a screen door can be optically opaque from a distance, but transparent up close. Put your nose up against a screen door and you can't see the screen pattern.
What you do see is a slight blurriness. This means that cleanliness is, as the saying goes, next to godliness, particularly in clean rooms. So a resist projector will undergo scrupulous cleaning before it is installed - it's just that a single speck on the lens surface is unlikely to cause significant issues.
Lenses are indeed a very important part of lithography machines. Also not only one lens is used but an array of different lenses to get the best results.
However, except for optics there is a gigantic complicated system built around it with hardware and software compensation of everything you can imagine like temperature, pressure and form of the wafers.
Lecture 16 – Introduction to Optical Lithography might give an insight to what you are looking for in more detail.
As others have pointed out, dust on the lens is not a big deal because it's far from the focal point. It's also usual that this kind of stuff goes on in a cleanroom.
However the lens does have to be made to very tight specs. If we are talking about laser scanning the type of lens is an f-theta lens (meaning that they focus the laser onto a flat plane), and they are usually telecentric (meaning that although the beam enters the lens at an angle it comes out perpendicular to the target, regardless of position.
It happens that I have been working on this type of system recently (not semiconductors but similar technology) and the lens quality turns out to be critical. Factors that do matter are having a focal length which remains constant, and focusing to a consistently round spot, across the field of view. That's what you pay the real money for.
If you are interested in the lens design, here is a nice explanation.
