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Optical Packaging of Microlens over UV-LED-Array

Markus Luetzelschwab, Dominik Weiland, Marc Desmulliez
This work is part of a project entitled “One thousand emitters per square millimetres”, an EPSRC project funded under the Basic Technology Programme. The project is jointly run between Heriot-Watt University, ... [read more]

This work is part of a project entitled “One thousand emitters per square millimetres”, an EPSRC project funded under the Basic Technology Programme. The project is jointly run between Heriot-Watt University, The Institute of Photonics at the University of Strathclyde, Imperial College London and the University of Sheffield. The overall aim of the project is to design, manufacture and package a UV-LED array for a variety of bio-medical applications. The objective of the research work presented here is the support to other partners in the assembly of a microlens array on top of a UV-LED array. This work focuses on the design, manufacture and characterisation of a packaging structure that places a microlens array on top of a micro-UV-LED array. The required lateral tolerance was defined to be less than 2µm. Two approaches were considered and systematically characterised. In the static approach, the microlens array is to be accurately placed and fixed on the LED array. In the dynamic approach, the microlens array is moveable in the vertical as well as lateral directions. Post processing of the lens and LED arrays has been carried out using a modified UV-LIGA process. The microlens array rests on four posts, to reduce lateral as well as vertical contact area for the static approach, hence reducing the probability of misalignment. Using electroplating of electrodes that are situated on the micro-UV-LED array structure, vertical alignment of the microlens array with sub-micron accuracy has been demonstrated. The dynamic approach features a vertical electrostatic and a lateral magnetic actuator that can be driven simultaneously. Both actuation methods exert a force on the microlens array without the need of a physical contact, which eases the fabrication and assembly process. As a restoring means, gel bumps are currently employed which exhibit the necessary isotropic elasticity for the lateral and vertical movement. The electrostatic actuator is able to achieve displacements of over 70µm and shows good repeatability with a standard deviation of 0.43µm at a mean value of 24.75µm. The magnetic actuator achieves lateral movement exceeding 50µm. Both actuation methods have been demonstrated to offer sub-micron displacement accuracy. Due to the viscoelastic properties of the gel bumps and with appropriate equipment, accuracies in the order of tens of nanometres are feasible.