Fine fabrication of gainasp-inp photonic crystal by Hi/Xe ICP etching using electron beam resist mask

Abstract

We successfully fabricate CaInAsP-lnP photonic crystuls wiih vertical hole (-90 7, swoosh sidewall (cl0 nm) and high uniformity (hi%), by inductively coupledplasrna etching using HIIXe gas with an electron beam resist mask. This can be achieved at a low-temperature condition due to the high volatility and sidewall protection of the iodide product, The etching condition is optimized by controlling the mixture ofXe gas and the applied bias.Photonic crystals (PCs) have attracted much attention due to the possibility of the versatile manipulation of photons by using a photonic band and a photonic band gap (PBG) [I]. In general, plasma etching with high anisotropy and electron beam (EB) lithography with high resolution are used to fabricate semiconductor PCs of which the dimensions are below the submicron scale. In particular, InP-based compound PCs are expected to be applicable to high-performance active photonic devices. However, at this stage, it is difficult to fabricate very fine InP PCs compared to Si and GaAs-based PCs. InP PCs with a relatively fine structure have been reported as being obtained by using Clz-based inductively coupled plasma (ICP) etching [2]-[4] or chemically assisted ion beam etching [5]- [7]. However, a high substrate temperature of over 200 "C was necessary to etch the InP compounds to desorb the etching products, due to the low volatility of the indium chloride products. Under such a high temperature condition, some extra processes, i.e., pattem transfer to dielectric andor metal layers or ion milling, were required to prevent the degradation and deformation of EB resist patterns. These complicated additional processes are considered to degrade the accuracy of the fabrication. Recently, low temperature InP etching at the scale of above 1 pm has been reported by using HVHe ICP [8] and HI/C12 TCP [9]. In this paper, we report on the successful fabrication of GaInAsP-InP PC using an EB resist mask by HI/Xe ICP at 90 "C.The fabrication target of the PC was 2D air-bridge-type GaInAsP PC slab, which is promising structure due to strong photon confinement with a simple geometry. The lattice constant and the radius of air holes were 420 nm and 120 nm, respectively. We prepared an epitaxial wafer on an InP substrate with a GaInAsP slab layer of 245 nm thickness. The etching mask of PC pattern was written on -450-nm-thick positive EB resist of ZEP520. The semiconductor layer was etched by using an ICP etching system . In this experiment, the ICP power was fixed at 80 W, to avoid any unexpected thermal effect of the ICP, The flow rate of HI and the gas pressure were reduced to be 0.4 sccm and 0.3 Pa, respectively, to suppress the excess chemical reaction of HI. However, in the case of using only HI as an etching gas, the cross-section of the holes became bow-shaped and the sidewall angle was not vertical as shown in Fig. 1Ca). This is due to the side etching inside the hole by the iodine radical, which is expected to accelerate the chemical-like etching of GaInAsP. To suppress the chemical reaction ...