Fig. 1. Fabrication process of 3D PhCs by multi-layer photolithography

Fig. 2. Cantilever structures fabricated with absorption-enhanced exposure. The thickness of cantilever, which is measured using an environmental SEM system, reflects the effective exposure penetration depth. (a) SEM micrograph of cantilever structures that are formed using 254nm wavelength. The dose of exposure is increased from 0.9mJ/cm² (upper left) to 18.45mJ/cm² (lower right) with 0.45mJ/cm² increment; the thickness is increased accordingly from 329nm to 1860nm. (b) SEM micrograph of cantilever structures that are formed using 220nm wavelength. The dose of exposure is increased from 0.33mJ/cm² (upper left) to 6.6mJ/cm² (lower right) with 0.33mJ/cm² increment; the thickness is increased accordingly from 287nm to 1430nm. (c) Measured penetration depth as a function of varied doses. Circle dots are the result for 254nm exposure; Triangular dots are the result for 220nm exposure.

Fig. 3. Typical spin-thickness curves for different conditions. Blue lines with triangular dots are the spin result on un-crosslinked resist; pink lines with square dots are the spin result on crosslinked resist; green lines with circular dots are the spin result on patterned PhC structure with controlled airflow. In the first case, resist thickness is measured each time after a new resist layer is applied and soft baked. In the second case, every resist layer is soft baked, measured, flood exposed and post-exposure baked before the new resist layer is applied. In the third case, resist is processed exactly using the proposed 3D lithography method. The airflow rates used upon spinning are 0, 3.2m/s, 2.1m/s, 1.6m/s, 1.2m/s and 0 respectively. Solid lines represent the total resist thickness; broken lines represent the thickness result of each spin. Except the first layer in the third case, which is spun at a reduced speed to obtain desired thickness, all the resist layers are spun at 3000rpm with 500ramp for 52 seconds.

Fig. 4. SEM micrograph of multilayer “woodpile” structures that are fabricated using the proposed 3D lithography method. (a) Six-layer “woodpile” structure. (b) Eight-layer “woodpile” structure.

Fig. 5. SEM micrographs of infiltrated Zirconia “woodpile” structures. (a) A 4-layer 100µm by 100µm woodpile structure. (b) A 4-layer woodpile structure with arbitrary defects of “UD” letters embedded in the second layer from the top. (c) Sidewall view of an 8-layer woodpile structure.