Maik Wiemer

Fabrication and characterization of reactive nanoscale multilayer systems for low-temperature bonding in microsystem technology

Reactive bonding is a still new low-temperature joining process that is based on reactive nanoscale multilayer systems. The heat required for the bonding process is generated by a self-propagating exothermic reaction within the multilayer system while the adhesive interconnect is supported by solder films. For microsystem applications, the approach is particularly useful if temperature-sensitive components and materials with high differences in coefficient of thermal expansion have to be joined. In this paper, this is successfully demonstrated for bonding a quartz strain gauge onto a stainless steel membrane and an IR-emitter onto a covar socket by using commercially available nickel/aluminum NanoFoils©. The quality of the bond interface of both demonstrators was investigated by scanning electron microscopy and the strength was determined by a tensile test. On the other hand, integrated microsystem applications beyond die attachment require patterned bond structures, e.g. to form bond frames. Thus, alternative materials were additionally considered that can be directly deposited on silicon substrates by magnetron sputtering, such as aluminum/titanium as well as titanium/amorphous silicon (Ti/a-Si) bilayer systems. The properties of these basic multilayer systems and their reaction products were characterized by differential scanning calorimetry and high-resolution electron microscopy. It is shown that specifically the Ti/a-Si system has substantial potential for direct microsystem technology integration provided the remaining open technological issues can be addressed during future research. In general, the results obtained in this study demonstrate the high potential of the reactive bonding process as a new advantageous assembly technology for the fabrication of future microsystems.

Investigations of thermocompression bonding with thin metal layers

In this study we successfully bonded silicon wafer substrates with metal based thermocompression technology. This technology has the advantage of inherent possibility of hermetic sealing and electrical contact. We used three different kinds of metals: gold, copper and aluminum. We will show the hermeticity, bonding strength and reliability of the different processes and compare the results.

Wafer Bonding with BCB and SU-8 for MEMS Packaging

In this paper, intermediate layer bonding technologies using SU-8 and BCB are successfully demonstrated. The bonding process, which consists of only several simple steps such as material deposition, exposure and development, as well as contact and bonding, can be carried out in a bonder at low temperature, e.g., somewhere between 120degC and 350degC. Benefits from this, integration of metal electrodes and wires between the bonding interfaces becomes possible. Moreover, since adhesive bonding does not necessitate extremely smooth contact surface, nor does it rely on the cleanliness of ambient environment, it is possible to carry out this process in a standard chemistry lab, and join different substrates without any pre-treatment. Initial inspection results showed that this method has a satisfactory yielding rate of more than 90%, and an acceptable bonding strength of above 2 MPa. The minimal thickness of the adhesive layer, which should retain the chips together after dicing, can be reduced to values between 6-10mum. For low-cost capacitive transducers, this is an attractive packaging technology. On the other hand, because SU-8 epoxy is an innovative building block for polymer devices, this method can also be used to construct complex microsystems

Room-temperature reactive bonding by using nano scale multilayer systems

This paper focuses on room-temperature reactive bonding by using nano scale multilayer systems. The exothermic reaction within the Ti/a-Si multilayer is used as an internal heat source for bonding. Herein, we used lithographical, wet etching and electro-plating procedures to generate very thin integrated reactive multilayer systems. The reaction in the very thin films generated enough heat to self-propagate and to melt tin films. High-speed camera imaging was used to characterize the reaction kinetics and the propagation of the reaction front onto substrates. Furthermore, we demonstrated a strong dependence on substrate material for adhesion after the reaction front has passed.

Wafer-Level Fabrication of Microcube-Typed Beam-Splitters by Saw-Dicing of Glass Substrate

This letter reports on the development of an integrated micro-optical beam splitter that can be array-arranged. The proposed wafer-level fabrication, based on 45 ° saw-dicing of glass substrates, allows rapid and low-cost processing. In particular, it leads to high compactness and possibility of wafer-level alignment/assembly, suitable for vertically integrated imaging micro-instruments. The device, including additional out-of plane reflection for extraction of sensing beam, can be as small as 1 mm3.

Low-temperature approaches for fabrication of high-frequency microscanners

Within this paper novel applications of low temperature silicon wafer bonding technologies for the fabrication of high frequency silicon microscanners are presented. Two technological approaches are discussed, both using low temperature bonding as a key technological step. Results of the integration of a special low temperature bonding process within the bulk technology approach are shown. Micromirror arrays fabricated with this technology are presented and show promising results for optical applications.

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.

Low-Temperature Wafer Bonding Using Solid-Liquid Inter-Diffusion Mechanism

The low temperature joining of semiconductor substrates on wafer level by solid-liquid inter-diffusion bonding using the Cu/Ga and Au/In systems is investigated regarding the bonding parameters and their influence on bond interface properties. The focus is on temperature dependence and composition of interface. In the case of Cu/Ga bonding, a phase transition from CuGa2 to Cu9Ga4 was found to be primarily responsible for an increase in bonding strength. After the temperature treatment of 90°C, a shear strength of up to 90 MPa could be achieved. Furthermore, the combination of Au and In with composition ratios suitable for AuIn2 and AuIn intermetallic phase formation was investigated. In the case of AuIn shear strength, 96 MPa was achieved using a bonding temperature of 200°C.

Strength and long-term reliability testing of wafer-bonded MEMS

Waferbonding techniques are frequently used for MEMS/MOEMS fabrication. In this paper, the potential application and methodical limitations of different strength testing approaches including tensile testing and double-cantilever- beam testing for wafer-bonded components are investigated. Special attention is given to the influence of the interfacial atomic bonding strength, the role of interface voids and notches caused by chemical or physical etching steps prior to bonding on the fracture limit. A particular aim of the paper is to discuss the potential of the Micro Chevron-Test for the assessment of the wafer bonding process with particular respect to the quality control during MEMS fabrication. In addition, the methods can also be applied to investigate the lifetime and fatigue properties of wafer- bonded samples exposed to constant or cyclic stresses.

Micro-optical design of a three-dimensional microlens scanner for vertically integrated micro-opto-electro-mechanical systems

This paper presents the optical design of a miniature 3D scanning system, which is fully compatible with the vertical integration technology of micro-opto-electro-mechanical systems (MOEMS). The constraints related to this integration strategy are considered, resulting in a simple three-element micro-optical setup based on an afocal scanning microlens doublet and a focusing microlens, which is tolerant to axial position inaccuracy. The 3D scanning is achieved by axial and lateral displacement of microlenses of the scanning doublet, realized by micro-electro-mechanical systems microactuators (the transmission scanning approach). Optical scanning performance of the system is determined analytically by use of the extended ray transfer matrix method, leading to two different optical configurations, relying either on a ball lens or plano-convex microlenses. The presented system is aimed to be a core component of miniature MOEMS-based optical devices, which require a 3D optical scanning function, e.g., miniature imaging systems (confocal or optical coherence microscopes) or optical tweezers.