Spectroscopic ellipsometry for in-line process control of SiGe:C HBT technology

Spectroscopic ellipsometry (SE) was successfully applied for in-line thickness and composition control of graded SiGe:C heterojunction bipolar transistors (HBTs). We have calculated a thickness of Si-cap and SiGe:C base, split into the gradient and plateau part and Ge content in the plateau. The procedure included the creation of databases for the refractive index dispersion of all components of HBT stacks using simple one-layer structures, with thickness and composition calibrated by X-ray diffractometry (XRD). These databases (e.g. SiGe:C optical constants vs. Ge content) were applied for thickness and composition characterization of graded HBTs with different profile shapes. The difference between SE and XRD for the estimation of the main structural parameters is discussed. Finally, the suitability of SE for measuring wafer uniformity of HBT layer thickness and composition was demonstrated, allowing a proper and efficient fine-tuning of the epitaxial growth process.