Simulation of base boron out-diffusion in SiGe HBT

Abstract

The diffu sion process and its influence on the broadening of the base bounded from both sides by undoped spacer layers of an npn SiGe HBr have been investigated by process simulator ISE reAD. Boron diff usion into strained SiGe layers was studied for diff erent times of annealing, Ge content in SiGe and widths of undoped spacer layers. 1.· Introduction 255Silicon germanium epitaxy is a key material technology that allows band gap engineering in the silicon process, and provides extremely interesting electronic and optical intrinsic device properties. Used to produce SiGe heterojunction bipolar transistors (HBT), it can yield devices with higher operational frequencies, lower noise and lower power consumption. Incorporation of gennanium in a SiiSil-xGexlSi HBT significantly changes the properties of the base region and of the base-emitter and base-collector junctions. Addition of Ge reduces the band gap of Si, leading to a narrow band gap SiGe base of the HBT. The lattice constiUlt of SiJ-xGex alloy differs considerably from that of Si. Hence, incorporation of Ge causes strains in SiGe layers (compressive in-plane), modifying the energy band structure and the density of states in the conduction and valence bands [1, 2J. Many properties of the SiGe layer were described and calculated, however, the kinetic mechanisms of layer relaxation during thermal annealing are still poorly understood. A major problem is to separate the effect of strain and composition (chemistry) on the diffusion of boron.Most previous worb on SiGe layer relaxation have focused on the growth, nucleation and subsequent thermal annealing [3]. Retarded boron diffusion has been reported in compressively strained Si-rich SiGe layers [4,5]. Authors [4] succeeded in determining the activation energies of pure Si and compressively strained Si1_XGex• Furthermore, they found out that by increasing the Gc content causes vacancy-mediated diffusion to take over at a certain point. The authors of [5] made a critical assumption that boron diffusion is mediated by positively charged point defects but did not distinguish between the effects of composition and strain. However, authors [3J write that the assumptions involved in [5] are consistent with early diffusion data results [6,3], and this appears to be incorrect. Additional studies proved that that the contributions of charged and neutral point defects to intrinsic boron diffusion are of similar magnitude. However, a strong reduction in intrinsic boron diffusion due to band gap narrowing is ruled Qut, and a more drastic strain-related phenomenon is supposed.O�7803-8535-7/04!$20.00 ©2004 IEEE 256 Authors [3J introduced the parameter Q' '" -kT 0 In( DID '0 10E as strain dependence of the activation free energy, which is assumed to be linear. This parameter controls the feed back between boron diffusivity and germanium concentration. Here, D 1S diffusion coefficient, the pre factor D'o includes quantities that should not be altered much by varying the composition or the strain. In SiOe the strain is calculat...