Low Temperature MEMS Manufacturing Processes: Plasma Activated Wafer Bonding
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Low temperature MEMS manufacturing processes: plasma activated wafer bonding
Viorel Dragoi, Sharon Farrens and Paul Lindner EV Group, DI Erich Thallner Str. 1, 4780 - Schaerding, Austria ABSTRACT This paper introduces a new technology: low temperature plasma activated wafer bonding. In this process, the wafers are submitted to a plasma treatment prior to bringing them into contact for bonding. The surface activation allows process temperatures ranging from room temperature to maximum 400°C. For Si direct bonding using plasma activation the Si bulk fracture strength is reached after a thermal annealing of 1 hour at 300°C, much lower than the annealing temperature used for the standard process without plasma activation (~1100°C). Experimental results illustrating the main benefits of the process are presented. The process was successfully applied also for bonding other materials than silicon.
INTRODUCTION The extensive use of MEMS devices in an increasing number of electronic applications recommends the MEMS fabrication as a well-defined, stand-alone field. Initially driven by the applications in automotive area, Micro-Electro-Mechanical Systems (MEMS) or Micro Systems Technology (MST) main focus moved nowadays to consumer products like IT applications, microfluidics, Micro-Opto-Electro-Mechanical Systems (MOEMS) or RF MEMS. The new MEMS applications raised important challenges to the existing manufacturing technologies and lead to the development of new processes able to fulfill the new demands. MEMS microfabrication technologies using silicon, quartz and other advanced materials offer a multitude of advantages for integration of mechanical, electrical, optical and thermal parameters in order to create multifunctional sensor platforms. If in the early development stages of MEMS the fabrication process was based on technology coming from microelectronics industry, the specific needs of new generations of MEMS devices lead to development of alternative technologies to meet the new challenges. All standard technologies such as photolithography, diffusion, oxidation, ion implantation, metal coating etc. used in microelectronics manufacturing can be applied in MEMS devices manufacturing. New techniques such as wafer bonding bring additional strength to micromachining-microelectronics integration technologies enabling development of new devices. Wafer bonding became a key technology in various MEMS devices manufacturing. In this respect, wafer bonding is a very important technology as far as it enables not only 3D structure building but also wafer level packaging. Different wafer bonding approaches are currently used in MEMS manufacturing: direct bonding [1], surface activatied wafer bonding [2 - 4], eutectic bonding [5], thermo-compression bonding [6], anodic bonding [7] or adhesive bonding [8]. As direct bonding process requires a high temperature annealing which is not always suitable for the devices, a high interest is focused on developing low temperature processes (maximum temperature of 400°C).
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