Demonstration of a Monolithic Micro-Optical Bridge for Free-Space Intrachip Interconnects

Future advances in the application of photonic interconnects will involve the insertion of parallel-channel links into Multi-Chip Modules (MCMs) [1]. These will make use of new device-level components such as arrays of Vertical Cavity Surface Emitting Lasers (VCSEL's) [2] or arrays of Micro Cavity Light-Emitting Diodes (MCLED’s) [3] and low power photoreceiver circuits [4]. One of the challenges associated with the development of free-space intraand inter-MCM optoelectronic interconnects is the fabrication of manufacturable, chip-compatible, and high precision monolithic micro-optical pathway blocks. These three-dimensional modules should integrate micro-optical components to optically interconnect surface-normal transmitters and receivers. In this paper we first report on the fabrication of such a refractive micro-optical bridge by deep proton lithography [5]. Next we use this component to demonstrate the proof-of-principle of optical intra-chip interconnects by establishing a digital data link between optoelectronic transceivers positioned on the same chip. Finally we project future performances of this approach by extrapolating present-day results in the light of improved lens and emitter characteristics. Deep proton irradiation of Poly Methyl MethAcrylate (PMMA) is a deep etch lithographic technique that allows the fabrication of monolithic structures integrating refractive microlenses, microprisms, fiber positioning holes, stand-offs and alignment features [5] [6]. The technique works as follows: a proton beam in the energy range between 5 and 10 MeV, passing through a metal mask, impinges on a high molecular weight PMMA substrate. The protons break the polymer chains and decrease the material's molecular weight. Using a specific developer the irradiated zone can then be selectively removed, making it possible to structure the sample to depths of several hundreds of microns with an optical surface quality of better than 20 nm. To demonstrate an intra-chip optical interconnect between two opto-electronic transceiver elements, we have applied this deep proton lithography to fabricate a monolithic micro-optical bridge consisting of two micromirrors and two cylindrical microlenses (see Fig. 1 and Fig. 2).