Thermal Penetration Depth Analysis and Impact of the BEOL Metals on the Thermal Impedance of SiGe HBTs

In this letter, we present a detailed investigation on how dynamic thermal phenomena take place in state-of-the-art SiGe HBTs when excited by sinusoidal power dissipation. To give a better insight into the mechanisms leading to the thermal impedance (
$\text{Z}_{{\textsf {th}}})$
decay, we introduce the concept of thermal penetration depth; then, with the help of 3-D thermal simulations, we illustrate its effect on the spatial distribution of the temperature variations within the transistor structure, according to the frequency of operation. In order to experimentally analyze the impact on a real device, dedicated HBT structures are designed; they consist of multi-finger SiGe HBTs realized in B55 technology from STMicroelectronics, for which modifications are made in the back-end-of-line (BEOL) metallization or in the transistor layout, increasing its deep trench isolation enclosed area. For these transistors,
$\text{Z}_{{\textsf {th}}}$
measurements are carried out in the frequency range 10kHz–1GHz; the results show that the metal connections configuration in the BEOL or layout modifications can considerably impact the
$\text{Z}_{{\textsf {th}}}$
decay at low frequencies. An identical
$\text{Z}_{{\textsf {th}}}$
trend is instead measured above 1–2 MHz, demonstrating that at higher frequencies just the region close to the heat source is concerned by dynamic thermal phenomena.