Fabrication of Rod-Connected Diamond Structures at Optical Wavelengths by Micromanipulation

A rod-connected diamond structure is known to exhibit the largest full photonic bandgap. However, it´s seemingly intricate structure turned off researcher´s motivation for fabrication. In this paper, we propose a novel fabrication technology for optical rod-connected diamond photonic crystals by uniting angled dry etching method with micromanipulation. Although, diamond structures have been found to be a good candidate for a full photonic-bandgap structure, there are no cases of their accurate creation with semiconductor materials at optical wavelengths. The main reason researchers give up synthesizing the structures is their cross-linked networks. Because conventional integrated circuit fabrication techniques have been developed intensively for one-dimensional processing, experimentalists react in a negative way to structures having nonvertical frameworks. Thus, theorists have been trying to deform diamond structures to suit the convenience of experimentalists. Unfortunately, those alternative structures are often more difficult to fabricate and accompanied by a reduction of bandgaps. A few structures, such as Yablonovite, woodpile, and MIT structures- have been successfully realized for optical wavelengths using microelectronic fabrication technology. However, the Yablonovite and MIT structures seem to be technologically demanding; thus, succeeding reports have not appeared. Also, their maximum gap/midgap ratios (Deltaomega/omega_M) respectively diminish to 2-3% and 23% from the best value of 30% for a strict diamond structure (i.e., a dielectric-rod-connected diamond structure). The woodpile structure´s simpler formation (cross stack of rods) is rather stimulating for experimentalists, and various approaches have been proposed for this structure. However, deviation from the strict diamond structure diminishes the maximum Deltaomega/omega_M to 18%. Not only dielectric-rod-connected diamond structures, but their inversed structures made of ai- - r rods in dielectric background also show a large Deltaomega/omega_M, as large as 28%. We have recently realized that air-rod-connected diamond structures in dielectric materials can be relatively easily obtained by combining angled dry etching with micromanipulation. In our approach, a diamond structure is initially sliced into four planes orthogonal to the (100) direction. Each slab has openings aligned into a square lattice on a surface. Angled holes are etched twice through these openings by inductively coupled plasma reactive-ion etching (ICP-RIE) so that the neighboring rods join at the rear surface. Then each plate is assembled into 3D structures by micromanipulation. We used to use polystyrene microspheres and matching holes prepared on a plate for automatic alignment. With this alignment technique, plates are automatically placed into proper positions with a structural error of within 50 nm, and a series of assemblies can be completed within one hour. In this study, we employed a combination of square columns etched on a wafer and matching notches prepared around plates for more rapid assembly. This method reduces operating time to 15 minutes while retaining the structural errors within 50 nm. Air-rod-connected diamond photonic crystals obtainable by the proposed procedure are expected to exhibit a maximum Deltaomega/omega_M of 20% for a refractive index of the dielectric material of 3.4. The reduction of the gap seems to be caused by parts of rods sticking out from a circular joint. However, a 20% complete gap is still higher than that of the woodpile structure, and sufficient for the utilization of photonic crystals in various devices. Also, the techniques of angled dry etching and refined micromanipulation open up the possibility of designing new structures, waveguides, or defects, and high achieving productivity. These are inevitable features for the mass production of functional photonic crystal devices