Low-Surface-Energy Photoresist as a Medium for Optical Replication

In these applications, a master surface relief is created by exposing photoresist through a suitably designed grayscaleintensity mask. Development of the resist produces a profiled surface according to the mask transmittance and exposure. Components can be replicated from this master by contact molding; this requires a release layer to insure the master and replicant can be separated without damage to either. Typically, this release layer is an evaporated metal-glucose-metal sandwich structure that allows separation without loss in modulation-function fidelity.1 Ideally, the on-demand separation between the surfaces occurs at the glucose layer, and by reevaporating the glucose and a single metal layer, the master is ready for repeated use. This is akin to polymer injection molding, where the metal mold (“master”) is also reused for thousands of injection operations before wear requires replacement. In practice, however, the metal-glucose-metal release layer disintegrates unevenly or adheres to the master. To save the master for reuse, the release-layer residues must be entirely removed and then redeposited. The former requires chemical etching, which tends to wear the photoresist master, significantly reducing its useful life. In some cases, the production of new-exposure masters or several masters at once can be inexpensive; however, for optical elements of large diameter, such as required by inertial-confinement-fusion lasers, this can require many, costly-to-prepare, precision-polished master substrates. The development of sixty, 30-cm-diam distributed phase plates for OMEGA necessitated the search for a simplified approach to a deterministic master-replicant release operation in optical replication. Since surface energy determines the adhesion of contacting surfaces, the solution lies in the development of a photoresist that has a low surface energy, thereby obviating the need for a release layer. In this article, we describe a first step toward developing, by environmentally benign methods,2 a resist that has a low surface energy and maintains the desired photosensitivity. In its first implementation, the combination of photoresist and on-demand, mechanical release action in a single medium is realized as a guest-host version. Since this technique is compatible with current technology, it has the potential for widespread acceptance.