Performance of planar μTEG as a function of polySi properties and device membrane-based topology
Résumé
Silicon based planar micro thermoelectric generators (μTEGs) were realized by CMOS compatible technology then were evaluated to harvest low to high level of heat. The μTEGs are build up with a thermopile periodically suspended onto dielectric membranes. The thermopile is based on a poly-Silicon (pSi) TE layer; it is a series of pSi/Au or Al thermocouples (TC) patterned into a zig-zag strip that covers a surface about a third of cm². This planar topology with membranes and long TC result in modules with a high thermal resistance [1] and a performance slightly impacted by the TE thermal conductivity but rather efficiently improved by any increase of the TE power factor (PF=α².σ, α is the Seebeck coefficient, σ the electrical conductivity). The presented modules integrate different kinds of pSi layers, with a PF varying from 3 μW/K²/cm, for standard P doped pSi, to 24 μW/K²/cm for “optimum” pSi obtained by very high dose implantation doping with B or As and annealing. The evaluation of their performance is carried out using a heat concentrator placed above the μTEG and periodically in thermal contact with the thermopile at half of the junctions. The characterisation is done by injecting power to a resistance printed on the upper surface of the concentrator. The power is varied from 0 to 10 W/cm². A periodic gradient of temperature ΔT is created at each TC. This results in a Seebeck open-circuit voltage Vs that is seen to vary linearly with the input power. Typically, for an input heat flux density of 4W/cm², ΔT experienced by each TC varies up to 30, 100 and 150K for respectively 10-, 5- and 2-membranes based modules, which is consistent with the whole structure modelling. The optimum behaviour is experimentally obtained for 5-membranes based μTEGs. In these cases, Vs is in the range 6-15V depending on the pSi properties and corresponds to a maximum output power Pmax in the range 49 – 306 μW/cm² on adapted loads. The best performance is obtained for μTEGs integrating highly B doped pSi layer (large PF). Surprisingly, the As+P codoped-based μTEGs are only moderately better than those based on standard pSi (Pmax is 4 times better though the PF is a decade higher). This can be partly explained by an increase of the thermal conductivity associated to a microstructure modification via codoping. On the other hand, microstructure changes in highly B doped pSi has already been reported to improve the PF [2]. In this work we show that it is also effective to make performant planar modules.
----- References: [1] Z. Yuan et al., Sensors and Actuators A 221 (2015) 67-76. [2] For instance D. Narducci et al, Phys. Stat. Sol. A. 211 (2014) 1255-1258.