Final answer:
The 25 μm gap in a proximity aligner setup references the distance between the mask and the substrate, which affects the resolution and accuracy of the pattern projected onto the substrate during photolithography.
Step-by-step explanation:
The gap of 25 μm in the context of a proximity aligner used to expose 1 μm apertures typically refers to the distance between the mask and the substrate. In a proximity aligner, this gap is the physical separation over which the mask pattern is projected onto the photoresist-coated substrate without the mask actually touching the substrate. The gap is crucial for the process, as it influences the resolution and accuracy of the pattern transfer due to diffraction effects, but it does not refer to the distance between apertures, the thickness of the aligner, or the aperture size directly.
The gap of 25 um refers to the distance between apertures in the context of a proximity aligner. A proximity aligner is a device used for exposing apertures, and in this case, the size of the apertures being exposed is 1 um. The 25 um gap refers to the spacing between these apertures.