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

TL;DR

This paper presents a novel micro-fabrication and transfer-printing method for integrating high-NA GaN micro-lenses onto various semiconductor substrates, enabling advanced optical coupling applications.

Contribution

It introduces a scalable process for fabricating and transferring free-standing GaN micro-lenses with precise placement onto non-native substrates.

Findings

Successfully fabricated 6-11 μm diameter lenses with <2 nm surface roughness.

Achieved precise placement of lenses on diamond and photonic circuits.

Demonstrated potential for efficient light coupling to NV centers and across visible to IR spectrum.

Abstract

We demonstrate the back-end integration of broadband, high-NA GaN micro-lenses by micro-assembly onto non-native semiconductor substrates. We developed a highly parallel micro-fabrication process flow to 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 to 11 $\mu$m diameter, 2 $\mu$m 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 $\mu$m-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.