MOCVD Emitter Regrowth Technology for Scaling InGaAs/InP HBTs to Sub-100nm Emitter Width

By scaling semiconductor thicknesses, lithographic dimensions, and contactresistivities, the bandwidth of InGaAs/InP Hetero-junction Bipolar Transistors(HBTs) has reached 550/1100 GHz ft/fmax at 128 nm emitter width (wE). Primary challenges faced in scaling the emitter width are: developing high aspectratio emitter metal process for wE < 100nm, reducing base contact resistivityρb,c, and maintaining high DC current gain β.The existing W/TiW emitter process for RF HBTs cannot scale below 100nm. Process modules for scaling the emitter width to 60 nm are demonstrated.High aspect ratio trenches are etched into a sacrificial Si layer and then filledwith metal via Atomic Layer Deposition (ALD). Metals with high melting pointsare chosen to withstand high emitter current densities (JE) at elevated junctiontemperatures without suffering from electromigration or thermal decompositionand is thus manufacturable. ALD deposition of TiN, Pt, and Ru are explored.Novel base epi designs are proposed for reducing Auger recombination current(IB,Auger). A dual doping layer in the base is proposed with a higher doping in theupper 5 nm of the base for lower ρb,c and a lower doping in the remainder of thebase for reducing IB,Auger. Presence of a quasi-electric field (4EC) in the upperdoping grade accelerates electrons away from the region towards the collector,thus further reducing IB,Auger.Selective regrowth of the emitter semiconductor via Metal-Organic ChemicalVapour Deposition (MOCVD) is proposed for decoupling the extrinsic base regionunder the base metal from the intrinsic region under the emitter-base junction,for increasing β,ft, and improving ρb,c. Carbon p-dopants in the InGaAs base arepassivated by H+ during regrowth. Annealing to reactivate carbon induces surfacedamage and increases base sheet resistance (Rb,sh) and ρb,c. Process techniquesfor minimizing Rb,sh and ρb,c in an emitter regrowth process are demonstratedand compared. ρb,c of 5.5 Ω.µm2 on p-InGaAs is demonstrated on TransmissionLine Measurement (TLM) structures after regrowth and anneal, by protecting thesemiconductor surface with tungsten. This is comparable to 2.9 Ω.µm2 measuredon TLM structures that do not undergo regrowth and anneal.Feasibility of emitter regrowth is demonstrated on Large Area Devices (LADs)with SiO2 as regrowth mask, and W cap during anneal. Emitter-regrowth andnon-regrowth devices of identical dimensions and epi design are compared. Emitterregrown HBTs yield higher β of 28 as compared to 13 for non-regrowth devices.Benefits of emitter regrowth cannot be ascertained on LADs due to high seriesresistance and large gap spacings between base metal and emitter-base junction.A process flow is proposed for scaling regrown HBTs to 60 nm emitter widths.The process incorporates ALD emitter metal technology that is demonstrated inthe first half of the dissertation. New epi designs for regrown-emitter HBTs areoptimized for maximizing β, ft. Scaling regrown-emitter HBTs is essential forrealizing their benefit over non-regrowth HBTs