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

Abstract

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 a sinusoidal power dissipation. To give a better insight into the mechanisms leading to the thermal impedance (Zth) decay, we introduce the concept of thermal penetration depth; then, with the help of 3D 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 (DTI) enclosed area. For these transistors, Zth 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 Zth decay at low frequencies. An identical Zth 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.

Validation of Thermal Resistance Extracted From Measurements on Stripe Geometry SiGe HBTs

“…Apparently this confirms that for f > 3 MHz, HBTs with fixed W E show equivalent thermal penetration depth despite their differences in L E . This frequency value beyond which the |Z T H,pd | curves are identical depends on the doping and transistor layout [11]. However, for any change in W E there is no corresponding high-frequency overlap at non-zero |Z T H,pd | in Fig.…”

“…Interestingly, in [11] we studied the frequency-dependent thermal behavior of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) to facilitate accurate thermal model development. The study was done with the help of 3D simulations and dedicated test structure measurements in two different state-of-the-art technology nodes (B11HFC of Infineon and B55 of STMicroelectronics).…”

“…In principle, one can use this frequency-dependent trend of Z T H among the DUTs to validate the R T H measurements. However, the analysis based on high-frequency Z T H overlap reported in [11] is limited only to the DUTs of same emitter area (A E ).…”

“…In this work, we propose different normalization techniques for Z T H to extend the idea of [11] to a set of SiGe HBTs with different A E and eventually show that high-frequency overlap of normalized versions of Z T H is possible if and only if the measured R T H of all the transistors are consistent. All the Z T H normalization methods proposed in this work hold true for transistors with shallow trench isolation (STI) technology and stripe emitter geometry with an emitter length (L E ) at least five times the emitter width (W E ).…”

“…Therefore, from (11) it is clear that the effect of L E on STI enclosed area (A ST I ) is insignificant. At high frequencies, when the thermal oscillations are in close proximity to the heat source within STI[11], Z T H,pd is governed by the percentage of the heat source area enclosed by STI (A E /A ST I ). This ratio remains almost constant for any change in L E but not for the change in W E .…”