Final answer:
Photolithography is the lithography technique that selectively etches the mask substrate, a crucial step in semiconductor fabrication.
Step-by-step explanation:
The student has asked which of the following lithography techniques selectively etches the mask substrate: a) Photolithography b) X-ray Lithography c) Electron Beam Lithography d) Ion Beam Lithography. The etching of the mask substrate is a fundamental step in the fabrication of electronic circuits and is typically associated with photolithography. In this process, a light-sensitive resist is applied to the substrate, and then selectively exposed to UV light through a mask with the desired pattern. The exposed resist is then developed, and the underlying substrate can be etched away in the patterned areas. Other techniques mentioned, such as X-ray, electron beam, and ion beam lithography, are used for various other patterning steps, but photolithography is the primary method for etching the mask substrate in semiconductor fabrication.