Generally, UV filters offered by U.S. manufacturers have poor bandshape and low transmission. In this chapter, I design UV filters with high transmission (>40%) and deep blocking (>6 OD). This newer approach is based on my previously published work. The filters use hafnium oxide and magnesium fluoride or silicon dioxide dielectrics, and aluminum is the selected metal; the substrate is quartz, and the designs are matched to air. The dispersive indices used are based on previous work and also the literature (see Chapter 2.) HfO2 and MgF2 or SiO2 are deposited by e-beam deposition on substrates at ~80˚C. The highest reflection metal films utilize thermal deposition. To apply the aluminum layers and obtain good optical constants, a vacuum in the 6-10 Torr range and metal deposition rates of 2-4 nm/sec are proposed. The temperature of the substrate is lowered by cooling the filter in situ for one hour in a gas atmosphere after depositing the initial dielectric layers; this process may be necessary even if IAD is used to stabilize the films (the substrate becomes warm). Following the metal layers, the balance of layers is deposited at high temperature and/or with IAD.
High-density blocked filters with 10-nm bandwidth are designed in this chapter. The thickness of the dielectric layers are in quarterwave units; the metal layers are specified in nanometers. The back surface requires a low-reflection coating. Other bandwidths can also be achieved with the method that is used.
10.2 Metal Portion Starting Design
The starting design is a 15-layer 4M (four-metal layer) enhanced filter using MgF2. The design is developed from ideas presented at OIC in 2001 in Banff.
Filter design: 13nmAl 1.42L 30nmAl 1.45L 25nmAl 1.54L 20nmAl 0.78L H 2L HLHLH yields a 10-nm bandpass with 30%T for a design wavelength of 250 nm. Blocking is six optical densities starting at ~360 nm.
This design is derived from 11 layers: 13nmAl 1.42L 30nmAl 1.45L 25nmAl 1.54L 20nmAl 1.78L HLH; an augmented filter. An augmented filter design is shown in Fig. 10.1; the bandwidth is 18 nm. While I was technical director (1971 - 1990) at MicroCoatings Inc., my staff fabricated many filters with these structures.