Reactive Ion Etching of Fused Silica

Figure 7: Fabricating holograms with reactive ion etching.

We employ reactive ion etching to create binary holograms in 1-mm thick substrates of polished fused silica, a transparent medium with an index of refraction of $n = 1.456$ at the wavelength of our trapping laser, $\lambda = 532$ nm. Obtaining a phase shift of $\pi$ radians requires a feature depth of $\lambda/(2n-2) = 583$ nm. The fabrication process has three main steps: creating a photomask, transferring this pattern to an etch mask covering the silica, and then etching to a precise depth, Fig. 7.

High contrast, high resolution film can be used to create masks for many of the trapping patterns we have investigated. We begin by laser printing the calculated phase profile as a binary image, with black pixels representing a relative phase shift of $\pi$ radians, and white representing 0 radians. This image is photoreduced to the actual dimensions of the hologram. Each of our holograms covers a square whose width, $2f = 3.24$ mm, matches the laser beam's diameter at B$^\ast$ in Fig. 1. Holograms involving finer linewidths were vectorized before processing with commercial mask writers at the National Nanofabrication Facility.

We next create an etch mask on the surface of the fused-silica substrate. First the surface is protected with a 25 nm layer of chromium and a 1.76  layer of positive photoresist, Fig. 7(a). The photomask is placed in contact with the photoresist, and the entire sample is exposed to UV radiation, Fig. 7(b). The photomask is removed and exposed regions of the photoresist are dissolved away, revealing parts of the chromium layer, Fig. 7(c). Finally, the exposed chromium is removed with an acid wash, exposing the sections of silica to be etched, Fig. 7(d).

Unprotected regions of the silica are susceptible to attack by fluoride ions. Reactive ion etching provides a controlled exposure to ions generated by RF dissociation of a mixture of oxygen and carbon tetrafluoride. These reactive ions rapidly oxidize the organic photoresist, but are halted by the layer of metallic chromium. The unprotected regions of the silica surface continue to be removed at a rate of about 0.5 nm/sec, Fig. 7(e), until the etched regions reach the desired depth. As the final step, we remove the remaining chromium to reveal a precisely textured fused-silica surface, Fig. 7(f).

The etching process could be repeated with different photomasks to produce a more nuanced pattern [14]. $N$ such steps would yield $2^N$ gradations of phase delay. Each step, however, would require planarizing and polishing the previously etched pattern, recoating the surface, and precisely aligning the new photomask over the existing pattern before etching. Not only is this is time consuming, it is not necessary for many applications.