Dicing of Fragile MEMS Structures
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Dicing of Fragile MEMS Structures Peter Lange1, Norman Marenco1, Sven Gruenzig1, Stephan Warnat1, and Thilo Semperowitsch2 1 Fraunhofer Institute for Silicon Technology, Itzehoe, Germany 2 Accretech GmbH, Munich, Germany ABSTRACT Stealth dicing appears to be the appropriate method to singulate surface sensitive microelectromechanical devices, without producing dicing debris or using water jet. The operating mode of the stealth dicing will be explained. A three dimensional integration project with open micromachined structures is taken as a reference to evaluate the advantages and limitations of this new dicing method. The preconditions for successful singulations will be discussed. Optical and SEM inspection results after successful separation will be discussed. INTRODUCTION Conventional singulation of MEMS (Micro Electro Mechanical System) devices is carried out after protection of the fragile structure. Particles generated during dicing can block mechanical movements, mechanical load or vibration can deteriorate hyperfragile structures and, for example for bio-MEMS, cutting fluid can interact with sensitive layers. Protection is mostly done either by a capping process or by a resist, both on a wafer level. Membrane sensors for flow and pressure are usually protected by a temporarily layer, which is removed afterwards. However, this procedure can cause yield loss. Inertial sensors, micromirrors and rf-MEMS are permanently protected by a capping layer initially for functional reasons. Devices which have open structures on both sides, generally afford new techniques because sensitive structures cannot be enclosed completely since it could hinder functionality. Maho IR laser technology for (stealth) dicing is claiming to be the ultimate separation technology for open and fragile structures. Separation takes place by generating a polycrystalline area with a high dislocation density within the wafer along the dicing line and subsequent tape expansion thereby avoiding any particle generation. Typical examples are microphones as well as ink jet printheads, which are already fabricated in mass-production. Recently a chip to wafer assembly has been proposed in the European project DAVID (Downscaled Assembly of Vertically Interconnected Devices), in which a sensor chip is bonded face down on top of an ASIC wafer (3-D integration of MEMS and ASIC) [1]. Therefore it is needed to singulate before the MEMS is placed on the ASIC structure. This MEMS is surface micromachined with features in the submicron range and thus very sensitive against particle contamination. For a characterization of the dicing process with respect to particle generation, chipping occurrence and environmental conditions, the complete process flow has to be taken in mind: the dicing and the separation of chips by expansion has to carried out in environments with at least a cleanroom class 100. The further transport to the pick and place tool and the die placement has to be done also under controlled conditions.
EXPERIMENT In this section we
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