I could be wrong, but I think optical processors would have serious diffraction problems with a 7 nm feature size, since the wavelength of blue light is somewhere around 300 nm.

As far as fabrication, one problem is that obviously you aren't using visible light to etch features on your wafers. The x-rays must be fun to work with... Not to mention, your photoresist would have to resist x-rays. Getting x-ray-resisting photoresist on and off your wafer must be tricky. Since 7 nm is about the size of several atoms, your wafer probably needs to be almost perfectly pure, which can't be easy, either.