You may wish to take a look at this slide (from the low-resolution 'How to Make a Transistor' presentation at Applied Materials):
Oxidation converts the silicon into silicon oxide (a.k.a. oxide), at very high temperatures. As this is a slow process, it's used when you don't need a thick oxide (at most a few low hundreds of nm), but do need one which is high quality (smooth, crystalline, free of voids and pinholes). Creating the gate insulator of the MOSFET transistors that comprise CMOS is what this process is ideal for.
Since it happens at very high temperatures (which might melt metal layers, or migrating things you don't want migrated, or start crystallizing things you don't want crystallized), it isn't typically used beyond the early steps.
Thermal oxidation isn't the only way to make oxide however, so you'll usually see other oxide (like the plugs that insulate adjacent transistors from one another) deposited via some CVD (chemical vapour deposition) process.
Lastly, it's lithography, not photography. Hope this helps!