Holger Rank

Eutectic wafer bonding for 3-D integration

Successful commercialization of MEMS products extremely depends on cost factors. Especially the role of integration technologies like packaging at different levels, combining MEMS with integrated circuits, and to realize 3-dimensional packaged devices is more important than ever. Bonding technologies at wafer level are key factors for 3-d integration, realizing the mechanical bond and fulfilling certain requirements like strength, hermeticity, and reliability as well as the electrical interconnection of the different functional components. From a great variety of bonding techniques eutectic bonding has got a special importance today because both hermetically sealed packages and electrical interconnects could be performed within one bonding process. Furthermore, there are some advantages such as low processing temperature, low resulting stress, and high bonding strength. These properties are mainly investigated up today. Since the early 90-ies eutectic wafer bonding is known from very large scale integration (VLSI) and is used very often in industry. Even before that time eutectic bond processes were already used in the field of chip bonding. Within this paper the development and investigation of at least two eutectic bonding technologies will be described and characterized. Although the mechanical and micro structural properties of the bond will be shown, the realization and test of electrical interconnects is focused very clearly. With an integration of certain test structures the bonding strength, the electrical properties, and the hermeticity of eutectic bonds could be measured and evaluated. At least it will be concluded with an outlook for the feasibility of eutectic bonding in 3-d integrated smart micro systems.

Reliability of platinum electrodes and heating elements on SiO 2 insulation layers and membranes

In this work, failure mechanisms of Pt electrodes including adhesion problems, material migration due to thermally induced compressive stress and electromigration that could occur in the platinum electrodes and heater structures at temperatures above 600 °C have been systematically studied, after the deposition. Lifetime determination, scanning electron microscopy and XRD analysis have been applied for samples which have experienced different loading conditions in order to qualitatively and quantitatively understand the phenomena. Electromigration testing is performed with the aim to enable time-to-failure prediction for sensor elements and compare different platinum layers in terms of their stability. Dedicated, application-related test structures are used so that the results are applicable to sensor lifetime estimations. Furthermore, a method for the determination of thermal conductivity of thin insulating films has been adapted for the characterization of plasma-enhanced chemical vapor deposition (PECVD) silicon oxide and successfully applied on two materials with different deposition recipes. These two materials are used for the fabrication of platinum-based heating elements with PECVD SiO2 as insulation or membrane layer. The results for the two recipes are similar but with a significant difference. A slight increase of the conductivities has been observed due to a thermal anneal of the test structures at temperatures above 700 °C.

Reliability characterization of a soot particle sensor: Analysis of stress- and electromigration in thin-film platinum

In this work we present a systematic investigation of failure mechanisms for thin-film platinum heater structures and interdigitated electrodes as components of a resistive type soot particle sensor. We study stress-migration and electromigration and effects of their interaction. Lifetime determination and SEM imaging are applied for samples which have experienced different load conditions to quantitatively and qualitatively understand the phenomena. We use dedicated, application-related test structures to ensure that the results are transferable to sensor lifetime estimations.

Hermeticity of eutectic bond layers for sensor packages on wafer-level

In MEMS industry there is a high demand for reliable hermetic wafer-level encapsulation. This paper presents a methodology to monitor the pressure inside a package after the wafer-level bonding process. Therefore, novel micro-electro-mechanical resonators designed as double ended tuning forks (DETFs) were built to measure the quality factor Q as a function of the inside cavity pressure. Obtained experimental results clearly verified the feasibility of the developed MEMS structures, providing a simple method to evaluate the hermeticity of wafer bonding technologies.

Characterization of LPCVD SiC thin films at elevated temperatures for robust MEMS sensor applications

In this work we present a systematic characterization of the mechanical properties of thin-film silicon carbide electrodes and released structures as components of a μ-contact type combustion pressure sensor. We developed, fabricated and successfully applied designated MEMS test structures for the determination of the Youngs modulus, residual stress, stress gradient and coefficient of thermal expansion of thin SiC films. In particular, a novel model, describing the capacitance-voltage characteristic of the test structures, has been developed and used for the purely electrical determination of the Youngs modulus and stress gradient of the SiC layers.

Novel Test Structures for Hermeticity Testing of Wafer Bonding Technologies

Introduction Packaging of microsensors fulfills the important task to protect the microstructure from environmental influences and mechanical impact. In the case of resonantly driven sensors, a very low pressure has to be maintained within an enclosed cavity in the package in order to ensure the required quality factor. Eutectic bonding is a promising technology for the hermetical encapsulation of surface micro-machined structures. In order to monitor the pressure inside a package after the bonding process novel test structures have been developed.