Fabrication and transfer printing based integration of free-standing GaN membrane micro-lenses onto semiconductor chips

Abstract

We demonstrate the back-end integration of optically broadband, high-NA GaN micro-lenses by micro-assembly onto non-native semiconductor substrates. We developed a highly parallel process flow to fabricate and suspend micron scale plano-convex lens platelets from 6" Si growth wafers and show their subsequent transfer-printing integration. A growth process targeted at producing unbowed epitaxial wafers was combined with optimisation of the etching volume in order to produce flat devices for printing. Lens structures were fabricated with 6 − 11 µm diameter, 2 µm height and root-mean-squared surface roughness below 2 nm. The lenses were printed in a vertically coupled geometry on a single crystalline diamond substrate and with µm-precise placement on a horizontally coupled photonic integrated circuit waveguide facet. Optical performance analysis shows that these lenses could be used to couple to diamond nitrogen vacancy centres at micron scale depths and demonstrates their potential for visible to infrared light-coupling applications.

Additive GaN Solid Immersion Lenses for Enhanced Photon Extraction Efficiency from Diamond Color Centers

“…This approach is expected to require tuning of the strain profile in the epilayer including the redesign of the buffer layer. 44 Overall, the derived collection efficiency for the planar diamond surface and the monolithic diamond SIL is in good agreement with previous experimental and theoretical study, [37][38][39][40][41]54,62,63…”

“…Thus, the effective angular coverage of the lens aperture above the emitter would rise, improving the photon extraction regardless of emitter depth: compare the dark green and dark golden curves in Figure c with the light-colored curves. This approach is expected to require tuning of the strain profile in the epilayer including the redesign of the buffer layer …”

“…The micro-lens height is restricted to the 2 μm thick GaN epilayer to achieve a flat bottom surface. 44 As stated in earlier study, 44 a combination of grayscale lithography and resist reflow is used to fabricate spherical and smooth lens profiles with highly engineered lens dimensions. Figure 2c shows an atomic force microscopy (AFM) line scan and a true-scale 3D representation of the AFM data of the marked device in Figure 2b, indicating a smooth symmetrical lens with a fitted ROC = 9 μm, taking 4 μm overall device thickness into account.…”

“…A schematic of the process flow is indicated in Figure while corresponding experimental results are shown in Figure . Figure a illustrates how the GaN solid immersion lenses are defined and suspended on a strain-optimized heteroepitaxially grown GaN-on-Si chip − based on the wafer-scale compatible microfabrication process reported in earlier study . Initially, polymer resist lenses are defined by grayscale lithography and thermal reflow (1), followed by inductively coupled plasma (ICP) dry etching to transfer the lens shape into the 2 μm thick GaN top layer (2).…”

“…The GaN lenses are realized using the balanced etch selectivity of photoresist and GaN in inductively coupled plasma (ICP) etching creating high aspect ratio lenses, which cannot commonly be achieved when processing diamond with ICP etching. 44 GaN and diamond are closely index matched around the emission wavelength of the NV − center providing minimal reflection effects at the material interface. The detailed fabrication process is outlined in the following section.…”