Characterization of Physically Vapor Deposited AF2400 Thin Films
- PDF / 925,730 Bytes
- 7 Pages / 414.72 x 648 pts Page_size
- 21 Downloads / 221 Views
ABSTRACT
Anti-reflective optical coatings made with Teflon AF2400 had the highest laser damage thresholds recorded for physical vapor deposited coatings at the Lawrence Livermore National Laboratory damage facility. Physical vapor deposited layers of Teflon AF2400, a perfluorinated amorphous polymer, maintained the bulk optical properties of a high transmittance from 200 nm to 1200 nm, and a low refractive index. In addition, the refractive index can be intentionally reduced by control of two common deposition parameters, deposition rate and substrate temperature. Scanning electron microscopy and nuclear magnetic resonance observations indicated that morphological changes caused the variations in the refractive index rather than compositional changes. The coatings adhered to fused silica and silicon wafers under normal laboratory handling conditions.
INTRODUCTION
Teflon AF2400 is an attractive candidate for high performance optical coatings. The bulk properties of this particular perfluorinated amorphous polymer show a high transmittance range from 200 nm to 2000 nm, and a low refractive index, n. In fact, the bulk n of 1.29 for AF2400 is lower than any dielectric compound currently used for optical coatings. Single layer coatings can be deposited using either a spin or dip solution technique. In these cases, a solution of AF2400 dissolved in an expensive (hundreds of dollars per gallon) fluorinated solvent are required. 1,2,3 Recent work has shown that this material can be thermally evaporated 4 ,5 to make corrosion barriers for extra-terrestrial equipment 6 and as a possible insulator for submicron electronic devices. 7 If the viability of physical vapor deposition (PVD) AF2400 optical coatings can be demonstrated, the material should find easy acceptance in the optical coatings community. Also, anti-reflective coatings produced by a solution process had the highest damage thresholds ever recorded at 1.06 gm. If the PVD process does not increase the absorption of the material during evaporation, the resulting coatings should also retain the low absorption required for high laser damage thresholds.
EXPERIMENTAL SET-UP
Figure 1 shows the chemical constituents of AF2400. Resistance heating was used to evaporate the Teflon AF2400 in a stainless steel vacuum chamber. The pump stack consisted of a 731 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
diffusion pump, a liquid N 2 trap, and a mechanical roughing pump. The substrates were heated with quartz-halogen lamps controlled manually with a 120 VAC variable transformer. The chamber was backfilled with 02 to 0.66 Pa (5 x 10-3 Torr) for a glow discharge cleaning. The discharge bias was applied between the Al ring and the substrate platen for five minutes.
- (Cý
Fig. 1. Chemical make-up of Teflon AF2400. Teflon AF2400 is a copolymer of tetrafluoroethylene and 2,2-bistrifluoromethyl-4,5-difluoro1,3-dioxole. The approximate ratio of a:b is 1:4.
-CF ) (2aCFI -CFI 0
0 C
CF
CF 3
A Box-Behnkem experimental strategy 8 was used to examine the
Data Loading...
