ANGLE-DEPENDENT DIP-COATING TECHNIQUE (ADDC) AN IMPROVED METHOD FOR THE PRODUCTION OF OPTICAL FILTERS

The production of high quality interference filters by a sol-gel coati ng process (e.g., dip-coating) with large areas of flat or shaped surf aces and with tubes has been developed. The angle dependent dip-coatin g process (ADDC) is an improved dip-coating technique method for the p roduction of optical filters. In contrast to conventional dip-coating (DC), the substrate is withdrawn from the coating solution at an angle of inclination, that offers advantages for the production of optical interference tilters. Thereby, the two surfaces of the substrate are c oated with different film thicknesses. To gain full control over the i ndividual layer thicknesses, reaching from 20 nm to 160 nm, it is nece ssary to vary the following process parameters: lifting speed (upsilon ) of the substrate, angle of inclination (phi) (Fig. 2c) and concentra tion of the dipping solution (c). An ADDC long pass filter produced in 10 coating steps reaches the same optical performance as a convention al DC filter made in Ih steps. The possibility to improve quality demo nstrate a second example: a commercial DC beamsplitter can be improved with respect to the flatness of transmission and reflection curves wh en being produced in four steps by ADDC instead of right steps by DC. Furthermore, ADDC offers the possibility to fabricate even narrow band pass filters, which are naturally difficult to obtain by conventional DC. (C) 1997 Elsevier Science B.V.