Laurel M. Sheppard, Contributing Editor
Calcium fluoride springs out of the IR to open a window for deep-UV semiconductor lithography.
Calcium fluoride (CaF2) is something of a prima donna in the world of optical lens materials. Even where it occurs naturally as the mineral fluorite, the material is prized for its broad range of rich colors. CaF2 crystals transmit light between 130 nm and 10 µm and have played bit parts in microscope and camera lenses since before World War I. CaF2 is used today in infrared applications such as spectroscopic accessories, prisms and lenses. Doped CaF2 is also used as a gamma ray scintillator.
But the material's career took off more recently when talent scouts from around the world discovered its gift for transmitting excimer laser energy at the 193- and 157-nm wavelengths critical to photolithography advancement in semiconductor manufacturing. Suddenly CaF2 attained stardom, and with it came new demands on material properties, supply and performance.
But as demand for CaF2 rapidly grows, so does its reputation for being difficult to work with. Growing the crystals is a slow and exacting process: Impurities can result in performance problems and rapid degradation. Also, grinding and polishing CaF2 blanks demands more time and technology fused silica, the industry standby. But until the next hot young star comes along, CaF2's command of the lithography stage will continue to grow.