Advancement in low melting solder deposition by pulsed Magnetron Sputter-PVD process for microsystemtechnology

Abstract

Micro-Electro Mechanical Systems (MEMS) are in general use today, either as product upgrading like airbag sensors or as independent microsystem like ink jet printer heads. The main application fields of microsystems are information technology, medicine, biochemistry, automotive industries and telecommunication. The components of these MEMS consist of various materials, which have to be joined. To join materials like ceramics, synthetic materials or metals to a hybrid MEMS established joining technologies have to be adjusted. For the assembling and mounting of temperature sensitive micro-components, a low temperature joining process, like the Transient Liquid Phase (TLP) bonding, is needed.The TLP-Bonding process combines the characteristics of liquid-phase joining (soldering and brazing) with diffusion bonding. The TLP solder system is a binary multilayer system of thin films. One film possesses a high (e.g. Cu, Au) and the other a low (e.g. Sn, In) melting point. During heating, the low melting component diffuses into the high melting component and forms an intermetallic phase, raising the remelting temperature of the system. The multilayer system was deposited by a pulsed Magnetron Sputter (MS)-PVD process.In former investigations, the TLP systems Cu–Sn and Cu–In showed high potential for joining operations in microsystems. The benefit of these systems is the combination of the low melting elements to the eutectic alloy SnIn. For this low melting alloy, the substrate temperature as well as the target temperature is of interest. The substrate temperatures were set and controlled by an additional cooling unit which was integrated into the sputtering facility. The target temperature is minimized by use of a pulsed power supply for deposition. The effects of different pulse rates and substrate temperatures to the microstructure and the soldering suitability of the TLP solder systems were investigated by scanning electron microscopy (SEM), nanoindentation and soldering tests.