Evaluation and suppression of systematic errors in optical subwavelength gratings

Optical subwavelength gratings are of growing interest for the realization of special optical effects such as artificial birefringence or antireflection layers, for example. The optical properties of such elements strongly depend on the accuracy of the fabrication technology and tools. Although e-beam lithography is known to be a high-accuracy fabrication method, even with this technology systematic grating errors may occur which affect the optical function. One example is the existence of grating ghosts (i.e. undesired propagating diffraction orders) which may occur even in the case of subwavelength grating periods. in this paper we describe how this effect is related to the address grid of the e-beam writer. Measurements of the diffraction spectrum of subwavelength gratings indicate the importance of this effect. The adaption of grating period and address grid allows the fabrication of ghost-free subwavelength gratings.