Development of high temperature diffusion barriers and a transient liquid-phase wafer bonding for thermoelectric MEMS energy harvester
In order to convert heat up to 600 °C into electrical power, a poly-SiGe semiconductor could be a suitable thermoelectric (TE) material in a µTEG. The interconnection between TE structures can be fabricated using a wafer bonding technique. This technique requires a bond solder, the components of which are deposited on a plating base. In respect to high temperature applications, the bond solder must not melt and must remain conductive up to 600 °C, and the plating base must prevents diffusion (diffusion barrier) into the semiconductor layer and must also remain conductive. For this purpose, TiW-based and Ta-based diffusion barriers and a Ni-Sn TLP wafer bonding are developed. The TiW-based and Ta-based diffusion barriers are deposited in form of stacked thin films with a Ni top layer on 8”-wafers. In order to investigate the high thermal stability of both diffusion barriers, several methods; XRD, TEM, EFTEM, EDX analyses, and 4-probe sheet resistance measurement, are performed before and after annealing. The annealing is performed in a vacuum chamber at given temperatures up to 600 °C for 24 h. In the Ni-Sn TLP wafer bonding, Ni and Sn layers are subsequently electroplated on a 6”-waferpair using a self-made lab-scale electroplating tool and a pulsed electric current. The plating bases are the TiW-based and Ta-based diffusion barriers with the Ni top layer. After wafer bonding, shear tests are performed on a number of selected dies to investigate the bond solder’s mechanical strength and daisy chain dies are measured electrically to investigate the bond solder’s electrical resistance. These investigations are performed on both types of bonded dies before and after annealing (in a vacuum ambient at 600 °C for 24 h). Contact resistivity between both diffusion barriers and the poly-Si layer is measured using a TLM. For this measurement, "ladder" structures were fabricated on 6"-wafers and then measured using a 4-probe measurement method.
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