Discussion
In the process of photolithography, an optical stepper is used for the exposure of the wafers. Write the correct order of various processes starting from the light source to the die on wafer in an optical stepper.
(1) Illumination diffuser
(2) Condenser lens
(3) Reficle
(4) Interferometer
Answer: B
The order is (1), (2), (3), (4): illumination diffuser, condenser lens, reticle, interferometer.
In an optical stepper, light from the source first passes through the illumination diffuser, which spreads it uniformly. The condenser lens then collects the light and directs it evenly onto the reticle (printed 'Reficle' in the paper), the mask carrying the pattern of one or a few dies. Projection optics image the reticle onto the wafer, whose stage position is measured by a laser interferometer so that each die is exposed in exactly the right place.
The other options put the reticle before the condenser, or start with the interferometer, which belongs to the wafer stage at the end of the path, not at the light source.
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