Investigation of Disilane-Based SiGe-HBT Layers for Low External Base Resistances

Abstract

In the past decade there has been increased interest in utilizing SiGe-heterojunction bipolar transistors (SiGe-HBT) and in particular SiGe-BiCMOS technologies for applications in the mm-and sub-mm-wave range. High data rate communication systems and optical networks for >100Gb/s, industrial and automotive sensors at 120 GHz for velocity and position control as well as imaging systems at >160 GHz for medical and security systems are some examples for these fields of application. Key elements for the continuous improvement of the high frequency performance of SiGe-HBTs are the scaling of vertical and lateral dimensions as well as design optimization to achieve reduced parasitics. In particular the maximum oscillation frequency fmax could be increased in the last years above 0.5 THz. Beside a sufficient high cut-off frequency fT, a reduced base-collector capacitance CBC and low base resistance RB are beneficial for the fmax performance of a SiGe-HBT. In this work we investigate the behavior of the external base resistance RBext for two different epitaxial processes used in SiGe-HBTs with a silane- (SiH4) and disilane-based (Si2H6) precursor. The SiGe-layers were treated by various rapid thermal annealing (RTA). We processed different wafers with an oxide stack usually used in a standard SiGe-BiCMOS process. After the release of certain silicon regions by reactive-ion-etching the epitaxial layers were deposited. First a selective epitaxial silicon buffer was grown within oxide-free windows followed by non-selective growth of a silicon layer, the in-situ boron doped SiGe base layer and an on top silicon cap layer. For the disilane-based process the SiGe layer grows in an amorphous phase whereas for the silane-based process a poly-crystalline film is formed. The growth of the whole Si-SiGe-Si stack was adapted for disilane process in order to reach the same layer thicknesses for both variants inside oxide free windows (single-crystalline regions) as for the original silane-based process. Sheet resistance (Rs) measurements were performed on 49 points across a wafer on the poly crystalline SiGe layer on an oxide. Figure 1 shows the Rs of silane-based (#1) and disilane-based (#3) SiGe-layer after a rapid thermal annealing at 1010°C for 12s. We observe a decrease of Rs by 60% in comparison to the silane-based SiGe-layer. Even for a reduced annealing temperature of 950°C (#2) the Rs dropped down around 30% in comparison to the silan-based variant. Moreover the standard deviation of the sheet resistance in the poly-crystalline regions across the wafer becomes significantly smaller and decreased by a factor of six down to approximately 50 Ohm/square for the disilane-based SiGe stack. Secondary-ion-mass-spectroscopy (SIMS) measurements were used to characterize the boron doping and SiGe-profile inside the crystalline and poly-crystalline regions of the wafers. In the poly crystalline region the germanium and boron concentration of the disilane-based process corresponds to the distribution of the single crystalline region. However we obtain a difference in the doping distribution after epitaxy (Fig. 2a) in comparison to the silane-based process. This difference becomes more significant after the RTA and we observe a quite homogeneous distribution of Ge and boron for the silane-based SiGe-stack whereas a steeper doping is within poly-crystalline layers for the disilane-based process (Figure 2b). We address this disparity in the Ge and boron distribution to differences in the diffusion behavior for the disilane-based layer stack due to i) the deposition as an amorphous Si/SiGe/Si layer and ii) the grain size of the poly-crystals after the final RTA. For our operation point the loading effect during the disilane-based epi-process leads to a 40% increased boron dose in the external base region compared to the silane-based process and without increasing the B doping inside the single crystalline region. This is beneficial for the behavior of RBext. Finally we applied both variants in a standard SiGe-BiCMOS flow. The overall performance of the SiGe-HBT depends of course on several other aspects like base-emitter and base-collector capacitances and the influence of changing one parasitic component has to be carefully evaluated. Nevertheless the external base resistance as well as the contact resistance dropped by 55% and 45%, respectively, for the disilane-based process and show the benefits for the use of this type of epitaxial layer depositions in advanced SiGe-HBTs. Figure 1