Electron beam lithography for nanofabrication of metal induced Bragg reflectors

Abstract

Electrically Induced Bragg Reflectors can be very attractive to realize programable waveguides networks. Their practical realization is nevertheless intrinsically connected to the capability to make a peculiar comb-structure electrode on the top of the waveguides with typical period of 200 nm (corresponding to the Bragg length) and a tolerance of few nanometers. In this work, the experimental fabrication of these comb-structure electrodes by means of electron beam lithography is described. We fabricated large areas 1D periodic gold structures with nanometer resolution by using a high resolution electron beam lithography (EBL) process and a post-processing technique based on lift-off. These electrodes can be employed as Induced Bragg Reflectors in a multilayer structure for a not permanent periodically modulation of the effective refractive index of the guiding structure. The desired structures are obtained with nanometric resolution by means of EBL, digging furrows of rectangular section in both a polymetilmethacrylate (PMMA) and in α-chloromethacrylate and α-methylstyrene (ZEP) layer spin-coated on silicon, then evaporating a metal layer (Au) on the top and then by lift-off of metal. The EBL technique allows a very accurate control of the dielectric distribution of the exposed area able to produce feasible, high efficiency periodic and photonic band-gap structures. The resulting 1D gratings are made of metal lines 100 nm wide with a period of 200 nm and, 120 nm wide with a period of 250 nm, respectively. Large area structures (up to 1 mm x 6 μm) have been realized with nanometre resolution and they have been characterized by scanning electron microscopy (SEM). These structures will be used in a future work of ours to realize 40 GHz switching speed modulator by inducing a Bragg Reflector with a reverse biased vertical InP/InGaAsP p-i-n diode according to the predictions of the grating reflectivity spectra and of the transient response.