Considerations for the Reliability Estimation of SiGe HBTs

Authored by: Fernando Guarin

Extreme Environment Electronics

Print publication date:  November  2012
Online publication date:  November  2012

Print ISBN: 9781439874301
eBook ISBN: 9781439874318
Adobe ISBN:

10.1201/b13001-47

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Abstract

The reliability of SiGe HBTs is fundamentally the same as that of the silicon bipolar transistor. It benefits from the many years of experience and cycles of learning, not only in the transistor itself but also in the interconnect technology. The only structural difference of SiGe HBTs from conventional epitaxial base silicon bipolar transistors is the addition of small percentage of Ge in the base region, leveraging the performance advantages of bandgap engineering. From the reliability perspective, this means that we can treat the SiGe HBT as an extension of the Si bipolar transistor without any major effect due to the introduction of Ge. This has been validated after several generations of production-qualified SiGe BiCMOS technology, where Ge composition has been increased with each new generation and now approaches 25%. To this date, there have not been any reliability issues detected that can be attributed to the addition of Ge. This fact has been validated by the reports from similar SiGe technologies commercially available today [1]. Due to the stability of the silicon crystal, the boron out-diffusion from the base layer is further suppressed with the addition of Ge, resulting in the improvement of the base width control and stability. The SiGe HBTs inherits the many advantages, infrastructure and maturity of Si bipolar transistors. These many advantages over HBTs based on III–V materials also translate into higher reliability, ease of manufacturing, and ability to scale further achieving higher levels of integration. The reasons for the higher reliability also include the use of less exposed surface area with planar structure, higher quality oxides with lower interface state densities, superior surface control with oxidation, and far more stable metal contacts thanks to silicidation, lower levels of self-heating due to silicon's higher thermal conductivity, and lower out-diffusion [24]. These are all important factors that translate into higher reliability as evidenced by the absence of the catastrophic “sudden beta collapse” that is prevalent in III–V technologies. The relatively high current density of SiGe HBTs was one of the major concerns during the qualification phase of SiGe technologies. Figure 40.1 shows the beta stability for IBM's 0.18 μm SiGe technology over several thousands of hours of stress at a 140°C ambient temperature and close to a thousand hours of stress at 225°C (junction temperature of 235°C). These results clearly demonstrate the robustness for SiGe HBTs and their ability to operate reliably over long periods of time. Further validation was provided by collecting 14,000 h of stress at an ambient temperature of 125°C and three different current densities in IBM's 0.13 μm technology. These results are shown in Figure 40.2. These very long-term experiments were conducted to verify that there were no low activation energy mechanisms in SiGe similar to those observed in other material systems.

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