Thermal Test of an Improved Platform for Silicon Nanowire-Based Thermoelectric Micro-generators

Abstract

This work reports on an improved design intended to enhance the thermal isolation between the hot and cold parts of a silicon-based thermoelectric microgenerator. Micromachining techniques and silicon on insulator substrates are used to obtain a suspended silicon platform surrounded by a bulk silicon rim, in which arrays of bottom-up silicon nanowires are integrated later on to join both parts with a thermoelectric active material. In previous designs the platform was linked to the rim by means of bulk silicon bridges, used as mechanical support and holder for the electrical connections. Such supports severely reduce platform thermal isolation and penalise the functional area due to the need of longer supports. A new technological route is planned to obtain low thermal conductance supports, making use of a particular geometrical design and a wet bulk micromachining process to selectively remove silicon shaping a thin dielectric membrane. Thermal conductance measurements have been performed to analyse the influence of the different design parameters of the suspended platform (support type, bridge/membrane length, separation between platform and silicon rim,) on overall thermal isolation. A thermal conductance reduction from 1.82 mW/K to 1.03 mW/K, has been obtained on tested devices by changing the support type, even though its length has been halved.

Transitioning from Si to SiGe Nanowires as Thermoelectric Material in Silicon-Based Microgenerators

“…An interesting engineering approach to that problem is to design an arbitrary long bridging distance between the platform and the surrounding silicon rim and to populate it with appropriated equi-spaced silicon trenches, which will be occupied by silicon NWs at the same time in a single well-optimized CVD process for that reasonable length target. Additional thermal performance improvements have been added to the presented architecture by modifying the ancillary supports for the metal legs in order to increase also their thermal resistance [ 30 , 31 ]. Such ancillary supports were long and narrow bulk silicon bridges in earlier generations and wide, short and thin nitride supports in the later generations.…”

Silicon Nanowires: A Breakthrough for Thermoelectric Applications

“…On-chip thermoelectric devices can be fabricated by CVD growth (bottom-up approach) of Si, or SiGe, nanowires between a suspended silicon platform, which can be used as hot part of the device, and the body of the chip [ 52 , 53 , 54 , 55 , 56 ] (heat sink, cold part of the device). Millions of nanowires can be simultaneously grown by VLS-CVD [ 57 ] through catalyzing gold seeds, deposited by galvanic displacement [ 58 , 59 ]. Therefore, this technique does not require high resolution lithography for the definition of nanowires: it requires only almost standard micromachining techniques, with relaxed lithography, for the fabrication of the platforms and of the electrical contacts.…”