Accurate determination of hole sizes in photonic crystal slabs using an optical measurement

Control and repeatability in fabrication of two-dimensional photonic crystal (PhC) slabs will become increasingly important as the technology matures into real device applications. A related problem is the determination of hole sizes in final etched devices. We have developed an optical method of measuring the hole size in PhC slabs as an alternative to the inspection of scanning electron microscope (SEM) images. The optical method relies on W1 PhC waveguides by patterning and fabricating reference W1 waveguides, the cut-off frequency of the waveguiding defect state can be measured and compared to calculations of this frequency as a function of hole size. Such calculations are relatively straightforward, and such in-situ transmission measurements are relatively cheap and fast. We show that the typical error in the measurement of hole radius is approximately 2%, or just 2–3 nm, and that this error is dominated by the uncertainty in the silicon slab thickness. Such performance is a significant improvement on current methods, which rely on the inspection via SEM. Not only is this slow and expensive, but there can be a large systematic error involved in the measurement. Different detectors, and even different settings of the same detector, will provide different contrasts between a hole and its edge, leading to different apparent hole sizes. Such errors in the absolute hole size can be of the order of 10 nm, which is as much as 5–10% for a PhC.