Au-Free Fully Cu-Metallized InGaP/GaAs HBTs Using Pd/Ge/Cu Ohmic Contact to N-type GaAs
Edward Yi Chang
在低溫退火(250°C)下，鈀/鍺/銅歐姆接觸即具備了低接觸電阻(5.73 x 10-7 Ω-cm2)。經由X光繞射儀、穿透式電子顯微鏡、二次離子質譜儀和原子力顯微鏡的分析結果，鈀/鍺/銅歐姆接觸形成機制和微結構反應都已被研究了解；且鈀/鍺/銅歐姆接觸經過250°C、24小時的熱穩定測試後，其接觸電阻幾乎沒有變化。
In this study, a low contact resistivity Pd/Ge/Cu ohmic contact to n-type GaAs has been successfully developed. The Au-free fully Cu-metallized InGaP/GaAs HBTs using Pd/Ge/Cu ohmic contact to n-type GaAs also has been successfully fabricated for the first time. The Pd (150 Å)/Ge (1500 Å)/Cu (1500 Å) ohmic contact exhibits a very low contact resistivity of 5.73 x 10-7 Ω-cm2 at a low annealing temperature (250 °C). The ohmic contact formation mechanisms and microstructure evolution were investigated using x-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), atomic force microscopy (AFM) and energy dispersive spectrometer (EDX). The thermal stability test of the Pd/Ge/Cu ohmic contact was also performed at 250 °C for 24 hours and showed no obvious degradation on Pd/Ge/Cu ohmic contact after the annealing. The Pd/Ge/Cu ohmic contact was applied to fully Cu-metallized InGaP/GaAs HBTs. In this fully Cu-metallized HBT, Pt/Ti/Pt/Cu was used as the base metal, SiNx was used for passivation, and Ti/Pt/Cu was used for interconnect metals with Pt as the diffusion barrier. The common emitter I-V curves and Gummel plot of these Cu-metallized HBTs using Pd/Ge/Cu ohmic contact showed similar electrical characteristics as those for HBTs metallized with conventional Au-metallized HBTs. The cutoff frequency (fT) of 3x20-μm2-emitter-area devices was about 38GHz. During both the current-accelerated stress test (110 kA/cm2 stress for 24h) and the thermal stability test (annealing at 250°C for 24 hours), for the fully Cu-metallized HBT with Pd/Ge/Cu ohmic contact showed almost no obvious degradation in electrical characteristics. The results show that the novel Pd/Ge/Cu ohmic contact can be used on Au-free fully Cu-metallized InGaP/GaAs HBTs, and exhibit good device performance.
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