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In recent work [K. Srinivasan, P. E. Barclay, O. Painter, J. Chen, A. Y. Cho, and C. Gmachl, Appl. Phys. Lett. 83, 1915 (2003)] resonant mode linewidths of 0.10 nm (corresponding to a quality factor ~1.3×10⁴) were measured in a photonic crystal defect microcavity fabricated in an InAsP/InGaAsP multi-quantum-well membrane. The quality of device fabrication is of critical importance in the performance of these devices. Here, we present the results of key processing steps, including inductively coupled plasma reactive ion etching of a SiO₂ mask and the InAsP/InGaAsP membrane, and a selective undercut wet etch of an underlying sacrificial InP layer to create the freestanding membrane. The importance of etching through the membrane layer deeply into the sacrificial InP layer is highlighted, and discussed in the context of the crystallographic nature of the undercut wet etch process. The results of device processing are compared with previous work done using a chemically assisted ion-beam etch, and a discussion of the benefits of the current approach is given. ©2004 American Vacuum Society.