Optimized blocking impurity placement for SiGe HBTs

Abstract

A high performance SiGe HBT that has a SiGe layer with a peak Ge concentration of at least approximately 20% and a boron-doped base region formed therein having a thickness. The base region includes diffusion-limiting impurities substantially throughout its thickness, at a peak concentration below that of boron in the base region. Both the base region and the diffusion-limiting impurities are positioned relative to a peak concentration of Ge in the SiGe layer so as to optimize both performance and yield.

Description

TECHNICAL FIELD[0001] The invention relates to silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs).[0002] It is generally known to form HBTs by using wafers that include one or more layers of silicon germanium (SiGe) on a silicon substrate. On such substrates, the germanium atoms create mechanical strain in the composite film due to the difference in lattice constant between the SiGe film and the silicon substrate. In the plane of the silicon substrate the larger lattice constant of the SiGe lattice is compressed onto the smaller lattice constant of the silicon substrate. In the plane perpendicular to the the silicon substrate, the SiGe layer lattice constant is greater than that of the silicon substrate and thus is under tensile stress. This strain together with the Ge atom itself, creates a bandgap offset between the SiGe film and the underlaying native Si substrate. This bandgap offset provides the unique advantages of the SiGe HBT by creating a grading field in th...

AI Technical Summary

Benefits of technology

[0007] It is thus an object of the present invention to provide a high performance SiGe HBT with a boron-doped base region and a boron-diffusion-limiting impurity region.

[0009] The foregoing and other objects of the invention are realized, in a first aspect, by a high performance SiGe HBT that has a SiGe layer with a peak Ge concentration of at least approximately 20% and a boron-doped base region formed therein having a thickness, wherein said base region includes diffusion-limiting impurities throughout said thickness at a concentration below that of boron in said base region, and wherein said diffusion limiting impurities are physically located relative to both said base region and a portion of said SiGe layer having a relatively high concentration of Ge to optimize performance and yield of said SiGe HBT

[0011] A further aspect of the invention is a method for forming a high performance SiGe layer on a Si substrate, comprising the steps of introducing germanium atoms during formation of a Si layer; introducing diffusion-limiting impurities and boron atoms during formation of said Si layer, while said germanium atoms are still being introduced; and terminating both said diffusion-limiting impurities and said boron atoms approximately simultaneously, said diffusion limiting impurities being introduced at a concentration and for a duration that optimizes both performance and yield.

Problems solved by technology

SiGe enhances charge mobility by introducing mechanical strain due to the lattice mismatches inherent in the Si--Ge compound; if there is too much Ge, or if the SiGe layer is too thick, the accepted wisdom in the art is that the resulting crystal dislocations will reduce both performance and yield.

The performance penalty would be due to dislocations relieving the mechanical stresses that create the bandgap offsets that SiGe provides.

The yield penalty would be due to the defects disturbing the crystallography of the substrate.

Carbon) suffered from yield issues.

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