Accurate determination of the functional hole size in photonic crystal slabs using optical methods

Control and repeatability in the fabrication of two-dimensional photonic crystal (PhC) slabs is becoming increasingly important as the technology matures towards practical applications. A key problem in this respect is the determination of the actual hole size in finished devices. We have developed an optical method for measuring the hole size in PhC slabs as an alternative to the inspection of scanning electron microscope (SEM) images. The optical method relies on determining the cut-off frequency of W1 PhC waveguides, which is easily measured and compared to calculations as a function of hole size. We show that the typical error in the measurement of hole diameter is approximately 2%, or 5 nm. This level of accuracy is a significant improvement over current methods, which rely on the inspection by SEM. SEM inspection can introduce large systematic errors because different electron detectors, and even different settings of the same detector, will provide differing contrasts between a hole and its edge. Such errors can be of the order of 20 nm, or as much as 5–10% of the absolute hole diameter. Furthermore, our method provides the functional or effective hole size, which determines the photonic function of the device, and which may be different from the physical hole size.