Hygrothermal aging of flip-chip assembled MOEMS
Authors: Visser Taklo Maaike M. , Wright Daniel Nilsen , Kolberg Sigbjørn , Vardøy Astrid-Sofie B. , Hamou Faycal , Vogl Andreas , Kristiansen Helge , Kalland Erik , Bakke Thor 
Published: September 2017

Low cost and miniaturized solutions for assembly of optical microelectromechanical systems (MOEMS) are critical for increased exploitation of optical sensors and devices in consumer applications. Examples of applications based on optical sensors and devices already near or on the market are auto-focus lenses and pico-projectors for mobile phones, physiology measurement units for wearables, and high bandwidth free-space optical communication links for our homes. In addition, extensive use of photonic integrated circuits is predicted to be mandatory in order to answer to the future needs for high data transfer rates in data centers. However, the main limitation for several of these applications is the complexity, the related cost of assembly, and the unknown reliability of several lower cost assembly solutions; polymer-based solutions are e.g. prone to fail particularly fast in humid and warm conditions.

In this work, a part of a European project called Lab4MEMS II, we study the feasibility of reducing the footprint, complexity and cost of assembly of silicon micro-machined mirrors directly to organic printed circuit boards, without jeopardizing the reliability of the system. We demonstrate flip-chip assembly using novel isotropic conductive adhesives (ICAs) with a particularly low content of silver that earlier indicated stable behaviour during exposure to hygrothermal aging [1]. The new solution removes the earlier needed space around the component for wire bonds, and the assembly is simplified as electrical and mechanical joints are formed in a single step.

Dummy mirrors were designed with two different pad layouts that were either optimized for a large pitch (the pads are distributed evenly around the active area) or for mechanical compliance (the pads are concentrated into three groups that are oriented in a triangle). There are 12 pads in total for each design. The outer dimension of the dies is 7 x 7 mm and the diameter of the region reserved for an active area is 5 mm. The dummy samples represent mirrors that are controllable through the inclusion of a 2 Âμm lead zirconate titanate (PZT) piezo-electrical thin film. The PZT film and the bottom (Ti/Pt) and top (TiW/Au) electrode layers are included in the dummy samples for a good representation of the actual surfaces to be bonded, both their materials and their topography. A 100 nm layer of alumina was deposited for passivation. The passivation layer was opened in the contact holes by wet etching. The layout includes both Kelvin structures and daisy chains.

A commercially available and widely used ICA, Epotek H20E, was selected as a reference material for two novel ICAs, Mosaic A and Mosaic B. The Epotek material contains solid Ag particles whereas the Mosaic materials contain metal coated polymer spheres with a diameter of 10 Âμm and a Ag layer coating of 140 nm. The ICAs were stencil printed onto boards having ENIG surface finish. The dummy mirror dies were flipped and aligned, and bonded by applying a tool pressure corresponding to 9 MPa as calculated based on the nominal contact hole areas. Curing was performed for one or two hours at 150 Â°C, depending on the recommendations from the material providers. A total of 14 boards, with 6 dummy mirrors bonded onto each, were assembled for characterization and reliability testing.

The assemblies are presently measured electrically in-situ as they are exposed to hygrothermal aging. The humidity level is 85% RH in all tests, whereas the temperature is either 65, 75 or 85 Â°C to enable recording of lifetime distribution curves at three different temperature loads. No failures have occurred yet, illustrating already the high potential of this assembly solution, but all samples will be run to failure. Curve fitting will be used to identify distribution functions to enable lifetime predictions, but the failures will be analysed using cross-sectioning, light microscopy and scanning electron microscopy, to identify failure modes and thereby verify the correctness of accelerated testing.