Lee - Re: "how do "they" position the stepper so that each successive mask is perfectly positioned to properly overlay the last layer."
Superb question and EXTREMELY relevant.
Each mask/reticle prints on to the surface of a wafer (in photoresist) not only the circuit patterns but also ALIGNMENT patterns (and test structures, as well).
These alignment patterns are generally (but not always) printed in the scribe line - the "vacant" areas between adjacent die sites.
The actual patterns vary from one layer to another, from one semiconductor manufacture to another, and from one type of stepper to another.
Each subsequent layer projects the previous layers alignment pattern back through the reduction lens (which actually enlarges it) and merges this previous pattern with a corresponding new pattern on the reticle that is being printed. This is all done automatically by the stepper machine. The two patterns (etched pattern on wafer plus the new pattern on reticle) are electronically imaged and compared for alignment with a variety of image processing techniques using image detectors and enhancers.
The misalignment between the wafer pattern and corresponding pattern on the new reticle is computed and the wafer stage is minutely adjusted (x, y and theta) to bring the two images into proper alignment. Once the overall "misalignment" is re-computed to be within tolerance (let's assume +/- 0.02 microns) the reticle pattern is then "Printed" (i.e., projected with appopriate light source) onto the photoresist on top of the wafer surface.
The wafer is then mechanically indexed via the x-y stage to the next die site and the fine/critical alignment is repeated again automatically by the austomatic alignment features of the wafer stepper or stepper/scanner.
There are some expensive, exotic optics and electronics that go into these machines that account for a large portion of the multi-million dollar price tag of the steppers.
Also, the technology development engineers must "engineer" the proper design of the alignmnet targets (crosses, verniers, etc.) and they have to insure that the new printed and etched alignment marks are visible with appopriate contrast when the next layer is subsequently imaged and printed .
What appears on the "final product" is a series of alignment marks in one thin film or another (oxide, polysilicon, metal, etc.) with another alignment mark right on top of the corresponding prior one.
Sometimes, one particular layer will have several alignment targets imaged, printed and etched and the next 2, 3 or 4 layers will be aligned to the SAME prior layer in order to align these subsequent layers to one earlier laye, therby preventing accumulation of misalignment errors (run -out).
It all works!
Paul |
