Regrown Extrinsic Base InP HBT with Sub-100nm Emitter Contact
This work presents the efforts pursued to optimize InP HBTs for power amplifiers above 100 GHz. Emitter width We reduction to sub-100nm dimension is achieved using a novel cosputtered Ti4wt%W emitter metal, and high power ICP dry etching. The cosputtering process enables fine-tuning of TiW alloy composition for vertical dry etch profile along sidewalls of the 600nm tall emitter metal, retaining sub-100nm emitter width from top to bottom. Base contacts are formed by low-temperature MOCVD regrowth (490 ◦C) of thick p-GaAs (> 4 × 20 cm−3) extrinsic base on the intrinsic p-InGaAs or pGaAsSb base layer, and subsequent UV i-line lift-off of e-beam deposited Pt/Ti/Pd/Au metal stack. Low overall base contact resistivity ρb,c is extracted by TLM on scaled sub100nm We DC “large area” devices to be 0.98 ± 0.4Ω-um2, which meets the requirement for >2THz fmax scaling. The larger bandgap of p-GaAs allows direct abutment of the regrown extrinsic base against sides of the n-InP emitter semiconductor, while blocking undesired electron injection into the extrinsic base. The extended regrown extrinsic base, thus, lowers Rgap by a factor of >2 between the base contact metal and emitter semiconductor, a significant contributor to Rbb in deep submicron HBTs, thanks to themuch lower sheet resistance of the extrinsic base (ρs,ex < 300Ω/sq). Hydrogen passivation of carbon dopants in the p-InGaAs intrinsic base layer is found, and partially reversed with an in-situ N2 anneal in MOCVD before temperature ramp down. Lateral hydrogen out-diffusion is believed to limit carbon dopant reactivation as the smallest We devices showed the lowest apparent intrinsic base sheet resistance (ρs,in = 1900Ω/sq) after the N2 anneal. While the added base spreading resistance Rspread underneath emitter semiconductor is manageable in sub-100nm We devices, DC devices with a GaAsSb intrinsic base are studied as a passivation-proof alternative to InGaAs for maximum Rbb scaling. Collector-base capacitance Ccb scaling is intentionally excluded in this work, so is vertical epitaxial scaling for higher fT , as both face challenges in terms of lithographic and semiconductor doping limits. RF device integration faced tremendous logistic difficulties due to the pandemic lockdown. Nevertheless, working RF devices with fmax in excess of 300GHz are demonstrated. In addition, a detailed review of conventional InP HBT scaling roadmap shows drawbacks of continued Ccb, and fT scaling in sub-100nm We process (e.g., high Rgap, and stagnating Ccb) previously overlooked due to simplified assumptions. It can be shown that conventional beyond-130-nm technology nodes offer comparable or worse device performance, a trend that can be reversed by the insertion of the regrown extrinsic base process module already a reality in SiGe HBT.