Hydrogen induced degradation in GaInP/GaAs HBTs revealed by low frequency noise measurements

Abstract

ABSTRACTOne of today's challenges to enable the improved electrical performances and reliability of microelectronic devices consists in controlling impurities contamination: hydrogen appears to be present in most (if not all) the processes steps of the devices making (ambient atmosphere, or associated with AsH3 -VPE or AsCl3 -VPE for example in GaAs based devices,…). Hydrogen induced reliability has already been investigated for many Si or GaAs based technologies ((C)MOS, FET, HEMT, PHEMT as well as HBT devices). These effects of hydrogen on electrical behavior and on long term reliability are very difficult to understand because of the different nature and ionic association of hydrogen (H, H+, H-, H2, or associated with impurities (Ge-H, Be-H, C-H,…). Most of these studies make use of IR, SIMS, Hall measurements: in this paper, we use low frequency noise measurements, associated with static as well as dynamic characterization to identify the degradation process in GaInP/GaAs Heterojunction Bipolar Transistors (HBT supplied by Thomson LCR). The presence of Hydrogen has been identified by DLTS at the LPSC laboratory (Meudon, France). The influence of passivation (SiN and GaInP ledge) on the reliability associated with Hydrogen has been one of the first improvements on HBT devices. Low-frequency noise measurements have been performed in the range of 250Hz to 100 kHz. The noise spectra evolutions (current and voltage noise sources at the input of the devices) allowed us to identify the activation process responsible of the static and dynamic rise and fall of the HBT's current gain. Chemical reactions of C-H complexes have been proved to be the processes responsible of this degradation. Additive reliability tests have been performed on two sets of devices (featuring different emitter length) under two distinct stocking conditions (temperature and biasing of the devices) leading to different junctions temperatures: low-frequency (LF) noise measurements, associated with static and dynamic S parameter measurements led to the same conclusion about the involved chemical reaction. We found that C-H complexes break, and H diffusion towards the extrinsic surface of the device has been observed on the measured leakage currents. Sealed devices have proved to get the same degradation signature than on wafer devices: Hydrogen is assumed to be present in high concentration levels in the device layers, and reacts under thermal and electrical stress.