We report on the optimization of 2D planar photonic crystal (PhC) SiNx microcavities for the fabrication of surface-emitting thin-film organic micro-lasers with a low lasing threshold (~10 μJ/cm2). Modified H2 and L3 PhC-microcavity geometries are designed in order to achieve quality factors Q in the range of 1000–2000 with a good matching of the cavity mode spectral position with the emission range of the organic layer. 3D-Finite Difference Time Domain (3D-FDTD) simulations show that the etch depth in the SiO2 buffer layer has to be optimized in order to maintain the Q>1000Q>1000, depending on the thickness of the organic layer to be evaporated onto the SiNx/SiO2 PhC pattern. 3D-FDTD simulations also show that the optical microcavity properties are robust against typical variations of the dimensions occurring in the technological process. The PhC microcavities are designed and fabricated to be used with tris(8-hydroxyquinolinato)aluminum (Alq3) doped with 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as the gain material.