J F W Schiz

Activation energy for fluorine transport in amorphous silicon

The transport of ion implanted F in amorphous Si is studied using secondary ion mass spectroscopy and transmission electron microscopy. Significant redistribution of F is observed at temperatures in the range 600°C to 700°C. The measured F depth-profiles are modelled using a simple Gaussian solution to the diffusion equation, and the diffusion coefficient is deduced at each temperature. An activation energy of 2.2eV±0.4eV for F transport is extracted from an Arrhenius plot of the diffusion coefficients. It is shown that the F transport is influenced by implantation induced defects.

Leakage current mechanisms in SiGe HBTs fabricated using selective and nonselective epitaxy

SiGe heterojunction bipolar transistors (HBTs) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p/sup +/ SiGe base and n-Si emitter cap. DC electrical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with the epitaxy in two different types of transistor. In the first type, the polysilicon emitter is smaller than the collector active area, so that the extrinsic base implant penetrates into the single-crystal Si and SiGe around the perimeter of the emitter and the polycrystalline Si and SiGe extrinsic base. In these transistors, the Gummel plots are near-ideal and there is no evidence of emitter/collector leakage. In the second type, the collector active area is smaller than the polysilicon emitter, so the extrinsic base implant only penetrates into the polysilicon extrinsic base. In these transistors, the leakage currents observed depend on the base doping level. In transistors with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through of the base caused by thinning of the SiGe base at the emitter perimeter. The emitter/base leakage is shown to be due to a Poole-Frenkel mechanism and is explained by penetration of the emitter/base depletion region into the p/sup +/ polysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are observed, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole-Frenkel and band to band tunneling. These results are explained by the presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter.

Selective and non-selective growth of self-aligned SiGe HBT structures by LPCVD epitaxy

Silicon-germanium (SiGe) heterojunction bipolar transistors (HBT) have become increasingly important for high speed applications. Novel device structures are often required to fully exploit the advantages from incorporation of a heterojunction. In this work, a growth technique is described which uses both selective and non-selective growth of Si and SiGe to produce an advanced SiGe HBT structure. The surface morphology of the material grown is examined using Nomarski contrast optical microscopy and scanning electron microscopy (SEM), and the surface of the epitaxial areas appears smooth with a low defect density. The growth surface is reasonably planar, as needed for further device processing, which suggests the required thicknesses of both selective and non-selective epitaxy were achieved. The Ge and B profiles of the material are measured using secondary ion mass spectroscopy (SIMS), and the layer thicknesses are found to meet the device specification. The crystallinity and defects in the material are examined by transmission electron microscopy (TEM). The material produced is shown to be suitable for fabrication into the proposed device.

Characterisation of emitter/base leakage currents in SiGe HBTs produced using selective epitaxy

Abstract SiGe heterojunction bipolar transistors have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by non-selective epitaxy for the SiGe base and Si emitter cap. E/B leakage currents are compared with cross-section TEM images to identify sources of leakage currents associated with the epitaxy. In addition, the influence of the position of the extrinsic base implant with respect to the polysilicon emitter on the leakage currents is studied. The emitter/base leakage currents are modelled using Shockley–Read–Hall recombination, trap-assisted tunnelling and Poole–Frenkel (PF) generation. The position of the extrinsic base implant is shown to have a strong influence on the leakage currents. The PF effect dominates the emitter/base leakage current in transistors in which the collector area is smaller than the polysilicon emitter. This result is explained by penetration of the emitter/base depletion region into the p + polysilicon extrinsic base at the perimeter of the emitter. These leakage currents are eliminated when the collector area is increased so that the extrinsic base implant penetrates into the single-crystal silicon at the perimeter of the emitter.

A novel self-aligned SiGe HBT structure using selective and non-selective epitaxy

A novel self-aligned SiGe HBT structure using selective and nos-selective epitaxy is proposed. It is shown that damage in the silicon substrate induced by reactive ion etching can be effectively removed and that this allows the growth of high quality selective and non-selective epitaxy. It is also shown in that selective epitaxy followed by non-selective epitaxy can be performed in one process step while controlling the base thickness and doping.

Increased current gain and reduced emitter resistance in F-implanted, low thermal budget polysilicon emitters for SiGe HBTs

Low thermal budget polysilicon emitter contacts are reported for application in SiGe HBT processes. A fluorine implant into the polysilicon is shown to enhance the breakup of the interfacial oxide layer, and to significantly reduce the emitter resistance, when combined with anneals compatible with the fabrication of SiGe HBTs. If is shown using TEM that a 5/spl times/10/sup 15/ cm/sup -2/ fluorine implant, combined with an interface anneal of 30 seconds at 950/spl deg/C prior to emitter implant, and an emitter drive-in of 30 seconds at 900/spl deg/C, produces devices with a broken interfacial oxide layer. Similar control devices which did not receive a fluorine implant showed no signs of interfacial oxide break up. Emitter resistance measurements show that the specific interfacial resistivity falls from 114 for the control device to 17 /spl Omega/ /spl mu/m/sup 2/ for the device implanted with fluorine. Finally processing conditions are identified that allow low emitter resistance to be combined with a suppression of the base current.