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We present a method to fabricate high-quality and environmentally rugged monolithic diffractive optical elements (DOE's). Analog direct-write e-beam lithography was used to produce analog resist profiles that were transferred into their substrates by the use of chemically assisted ion-beam etching (CAIBE) in one single etching step. An iterative method was used to compensate for the proximity effect caused by electron scattering in the resist and from the substrate during the e-beam exposure. Slope-dependent differential etch rates that occur during the transfer process were characterized and compensated for. Finally, the DOE was divided into regions with different period ranges, and the exposure dosages were set to achieve even and accurate etch depths in the final element. The presented fabrication method will increase manufacturability and reduce processing time, which will result in a general cost reduction per element.

Mechanical interlocking provides a simple and effective means of improving adhesion between dissimilar materials in micro-electro-mechanical systems (MEMS). Following successful implementation in hybrid Si-polymer systems (Larsson, Syms and Wojcik 2005 J. Micromech. Microeng. 15 2074–82), it was established that maximum interface strengthening does not necessarily rely on the presence of overhang between interlocking lobes. Instead, careful design of the lobe profile is advised in order to balance the opposing actions of physical restraint and lobe pull-out and to obtain optimal interface strength. When an interlocked interface is immersed in aggressive liquid media, however, the situation is clearer: chemical bonds are degraded or completely destroyed and lobe overhang provides the only source of physical restraint. Generating overhanging features in Si substrates is possible through reactive ion etching (RIE), but in the case of glass, the situation is more problematic. A straightforward, robust process is now described that extends mechanical interlocking to generic MEMS substrates, avoiding the need for RIE. By using inexpensive and established processes such as electroplating and wet etching, interlocking features with an overhanging profile are generated in glass substrates. Peel tests on cured strips of SU-8 confirm an increase in average peel strength by a factor of 3.5, compared with strips peeled from smooth substrates. The method can readily be applied to a number of substrates, including Si, providing a low-cost route towards attaining mechanical interlocking.

A method for improving adhesion between polymer films on Si substrates is described involving mechanical interlocking. Isotropic reactive ion etching (RIE) is applied to form pits in the Si substrate which fill upon application of a polymer via spinning. After curing, interpenetrating polymer lobes form a robust bond with the Si substrate. Adhesion improvements over films on smooth substrates are demonstrated qualitatively through prolonged immersion in heated 40% potassium hydroxide (KOH) solution, and quantitatively through peel tests. Strips of SU-8 on smooth substrates separate completely within minutes of immersion in KOH, whereas mechanically interlocked strips remain attached throughout. Tests reveal significant improvements to peel resistance for strips with interpenetrating lobes, due to a combination of crack deflection and physical restraint at the bond interface. Surprisingly, lobes with a vertical profile offer better resistance to strip peeling compared with lobes having significant overhang. Stress concentrations at sharp bends in the latter raise local stresses to levels promoting failure in SU-8, limiting the nominal load-carrying capacity of the interpenetrating interface. For maximum peel resistance, the lobe profile needs to be optimized such that maximum pull-out stresses are close to the failure stress of SU-8.