Innovative SiGe HBT Topologies With Improved Electrothermal Behavior

Abstract

This paper investigates alternative topologies of silicon germanium heterojunction bipolar transistors designed and fabricated in the state-of-the-art BiCMOS process from STMicroelectronics for improved safe-operating characteristics. Electrical and thermal behaviors of various structures are analyzed and compared, along with a detailed discussion on drawbacks and advantages. The test structures under study are different in terms of emitter-finger layouts as well as the metal stacks in the back-end-of-line. It is observed that the multifinger transistor structures having nonuniform finger lengths with wider area enclosed by the deep trench and higher metallization stacks yield an improved thermal behavior. Therefore, the safe-operating area of multifinger transistors can be extended without degrading the RF performances.Index Terms-Back-end-of-line (BEOL), deep trench, emitter layouts, multifinger transistors, safe-operating area (SOA), silicon germanium heterojunction bipolar transistors (SiGe HBTs), thermal resistance. 0018-9383

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

“…An accurate design of the metal connections can thus lead to improved SOA specifications [21] and better RF performances [22]. Moreover, as it is suggested by equation 1, δ is inversely proportional to the square root of the Pdiss frequency, which means that, at low frequencies, the thermal oscillations can significantly penetrate the structure, both downwards (towards the back of the wafer) and upwards, till eventually reaching the higher levels of the metal in the BEOL.…”

“…The HBTs under study in this work consist of five-fingers SiGe HBTs realized in STMicroelectronics B55 technology, of which a complete thermo-electrical characterization has been reported in [21]; in this section we will further extend our study, showing the thermal impedance extracted from LF S-parameter measurements in the range 10kHz-1GHz. In particular, we will now focus on the set of test structures named VM, for which copper metal dummies, acting as heat spreaders, are present upon the emitter contacts till metal-3 (VM3), metal-6 (VM6) and metal-8 (VM8); in a reference transistor named VM1 the metallization on the emitter contacts stops instead at metal-1 level (refer to Fig.3).…”

“…In particular, we will now focus on the set of test structures named VM, for which copper metal dummies, acting as heat spreaders, are present upon the emitter contacts till metal-3 (VM3), metal-6 (VM6) and metal-8 (VM8); in a reference transistor named VM1 the metallization on the emitter contacts stops instead at metal-1 level (refer to Fig.3). It must be pointed out that the Rth value of these transistors is lowered as higher levels are reached for the metallization in the BEOL [22]- [24], since the heat flux can find an alternative path in the copper of the BEOL (the results for the Rth of these HBTs are detailed and reported in [21]).…”

“…This means that, at f1 the thermal penetration depth has reached the abscissa corresponding to metal-1; the same assumption can be made as the frequency decreases, and it is also possible to identify f3 and f6, where δ reaches respectively the levels 3 and 6 of metallization. The set of test structures under study in [21] also allows an investigation of the penetration depth in the horizontal plane: the thermal diffusion phenomena under study have in fact an isotropic nature. Among the structures presented in [21], the transistor named HL1 contains metal connections reaching the same level as VM3, but has an increased Deep Trench Isolation (DTI) enclosed area.…”

“…The set of test structures under study in [21] also allows an investigation of the penetration depth in the horizontal plane: the thermal diffusion phenomena under study have in fact an isotropic nature. Among the structures presented in [21], the transistor named HL1 contains metal connections reaching the same level as VM3, but has an increased Deep Trench Isolation (DTI) enclosed area. HL1 thus allows a wider angle for the lateral heat spreading, which in turn ensures a better evacuation of the heat generated from the HS towards the silicon substrate, yielding a lower Rth [21].…”