Figuring by Multilayer Deposition

We have begun using an additive technique to correct optical surface figures. This poses a problem in that thin films tend to roughen with increasing thickness. Thus, thin-film deposition can be used for corrections on the order of 10 nm or less, but cannot be used for larger corrections, or, say, making aspheric corrections to high-quality spherical optics.

To make larger corrections we are using the technique of multilayer deposition. By alternating very thin (~10 nm or less) layers of the appropriate materials and ion polishing layers that are susceptible to roughening, the roughening process can be stopped in each layer before it starts. The techniques of producing smooth multilayers with tightly controlled thicknesses is mature thanks to the advent of extreme ultraviolet (EUV) normal-incidence reflectors. Thickness control and smoothness that meet the demands of advanced optical applications have been demonstrated for thicknesses in the 1000 nm range.

The Automated Production Technology Division of the Manufacturing Engineering Laboratory measured the figure of a test optic and extracted the radially-symmetric deviations from the desired figure (the black curve of Fig. 1). Using electron-beam evaporation with ion polishing, we made a number of test depositions through a stationary mask onto rotating test substrates and measured the thickness as a function of radius using x-ray diffraction. When the correction was of the desired profile, we determined the number of layers required, with the result shown in Fig. 1.

Figure 1. The height h of the surface of a test optic as a function of radius r before (black dashed line) and after (red line) deposition of a corrective multilayer. Only radially-symmetric deviations from a "perfect" surface are included.