Identification of Humidity Related Failure Mechanisms in GaAs Based HBT’s

Humidity related failure mechanisms in GaAs based HBT power amplifiers have been investigated. Both electrical and photoemission techniques are used to characterize and identify the degradation of the base-collector junction. Moisture penetration to the HBT surface reduces the junction breakdown voltage and can lead to catastrophic short failure. Observation of biased side emission from cross-sectioned HBT’s demonstrated that the breakdown can be surface related. INTRODUCTION In recent years, GaAs HBT has emerged as a dominant technology for power amplifiers (PA) in wireless communication applications, such as cellular phones. Product performance, price, and volume are critical metrics used in the selection process by end product manufacturers with razor thin margins. Margins are also impacted, to a significant degree by quality and reliability, and extremely low failure rates are a requirement for long-term success. HBT reliability characterization typically quantifies lifetime based on drifts in transistor parametric values such as beta, Vbe, or emitter resistance. The failure mode most commonly characterized is beta degradation [1]. In a PA, beta degradation can result in gain degradation and therefore would appear to be a valid parametric value to quantify for product reliability. However, with over 100 million PA’s in the field, Conexant has never had a PA returned for gain degradation. A more important failure mode that occurs during cellular phone operation is an electric short failure [2]. This failure mode is more likely to occur due to the high power operation of the PA. In most cases the short failure occurs on the output transistor. The failure can be observed as a single melted transistor cell or massive melting across many cells. Mechanisms for the failure mode vary from handling, such as ESD or EOS, to severe mismatch of the output load. In addition, process or starting material point defects can contribute as well. ESD and EOS can be minimized with customer education and use of ESD protection circuitry. Effects of mismatch can be minimized in the phone design and through careful chip design [3]. Process defects can be managed with defect monitoring and reduction programs. The above listed mechanisms seem reasonable and rather obvious contributions to the short failure mechanism. However, uncontrolled outdoor environmental factors causing HBT PA short failure have not been documented in the literature. Outdoor usage of cellular phones exposes HBT PA’s to a wide range of environmental conditions. These environmental factors require higher quality standards for the components used inside the cellular phone. One important environmental factor is humidity or moisture penetration. Studying changes in HBT PA characteristics under extreme environmental conditions can help identify failure modes and mechanisms unique to moisture specific acceleration factors. In this work, humidity related failure mechanisms for GaAs HBT PA’s is investigated. Specifically, the collector-base breakdown related properties are studied and characterized. EXERIMENTAL HAST, autoclave, and 85/85 stress are the most common tests employed to evaluate humidity resistance of final products. Identification of root cause for failure in plastic packaged parts is often compromised by difficulties in decapsulation. Many of the new molding compounds are virtually inert materials that can only be removed with very hot fuming nitric or sulfuric acid. Success of subsequent failure analysis is limited by the degree of damage to the die during plastic removal. To identify failure modes and mechanisms uniquely associated with exposure to humidity, product was assembled without the plastic overmold material. Nineteen of these unmolded parts were placed in an autoclave and exposed to 100% humidity at 130C and 30psig for an extended period of time. The output HBT cell array of the PA can be accessed independent of the rest of the circuit, allowing the HBT collector-base junction to be characterized. The I-V curves of the collector-base junction on the output HBT were measured using a Tektronix 577 Curve Tracer Copyright © 2001, GaAs MANTECH, Inc.