Architecture of source-drain cavity of a p-channel field effect transistor for embedding with epitaxial SiGe for enhanced performance

摘要

Increase in power consumption in field effect transistors has been curtailed in recent years by introduction of mechanical stress to achieve device speed gain over and above the traditional speed vs. power tradeoffs achieved only by scaling gate lengths. Increasingly, the source-drain region of p-channel field effect transistors are etched and epitaxial SiGe re-grown in the cavity to enhance hole mobility. However, the addition of stress as a method to improve performance would add to the process variability beyond the traditional source of lithography, now related to structure and dimension of the cavity and composition of SiGe. In this paper, compressive stress induced in the channel was directly measured using synchrotron x-ray diffraction. The samples were a set of gratings designed to map the transistor performance with varying design space. The x-ray beam was systematically stepped across the gratings at an interval of 200 nm and diffraction data collected to assess the extent of stress field. Diffraction space maps were created around the symmetric (004) and asymmetric (115) planes. Strain was deduced from Si peak shift and stress calculated from the Si elastic constants. Diffraction space maps around the asymmetric plane were used to deduce the mechanism and subsequent relaxation of strain. Diffraction data collected with x-ray beam placement close to the Si-SiGe vertical interface provided information from lateral SiGe epitaxy on the (110) plane. The presence of strained SiGe peak exhibiting tilt as well as "relaxed" SiGe peak surrounded by diffuse scattering due to dislocations were observed. The use of non-selective etch process resulted in cavity formation with multiple crystallographic planes. The subsequent relaxation mechanism that was dependent on the formation of misfit dislocations was perturbed, possibly due to pinning of the dislocations at the intersection of two crystallographic planes and served as the source of variability. Measured stress variation from 90 to 220 MPa was seen that resulted in estimated drive current enhancement variability from 5% to 15%. The maximum strain was seen where the SiGe film saw no relaxation and the energy formed due to hetero-epitaxy was transferred elastically as channel stress. The elastic relaxation was also accompanied by formation of tilted boundary. Based on these findings, the design of an ideal cavity that would maximize strain and minimize variability with layout was proposed.