Optical Old-Timer Finds a New Niche

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.