Nanoscale quantitative characterization of 22nm CMOS transistor using Scanning Transmission Electron Microscopy ( STEM )

Abstract

22nm Silicon‐On‐Insulator (SOI) complementary metal‐oxide semiconductor (CMOS) technology has a number of performance boosters, such as third generation embedded DRAM, embedded stressor technology and 15 levels of copper interconnect. [1] In addition to geometric scaling, strain engineering in CMOS transistors has provided another enabler for device performance improvement. A compressive strain in PMOS channel can increase the hole mobility, and a tensile strain in NMOS channel can increase the electron mobility. In PMOS of 22nm SOI technology, epitaxial SiGe source/drain (S/D) is used to introduce compressive strain in the channel, and SiGe layer in the channel is used to control threshold voltage of device. Hence obtaining the nanoscale chemical composition and strain information of them is vital during the semiconductor development. In this work, energy‐dispersive X‐ray spectroscopy (EDX) in STEM is used to determine the Ge atomic concentration (%Ge) based on the Cliff‐Lorimer ratio method. [2] The Cliff‐Lorimer factor is calibrated by measuring a standard Si 0 .664 Ge 0.336 blanket sample with a relative error smaller than 1%. An improved high angle annular dark field (HAADF) STEM image is obtained by STEM with Drift Corrected Frame Integration (DCFI). DCFI technique integrates successive STEM images via calculating and correcting the drift from cross correlation. The produced STEM image has minimal drift and a high signal‐to‐noise ratio. It is analyzed by Geometrical Phase Analysis (GPA) to extract strain information. [3] Figure 1(a) shows a typical PMOS transistor with <110> in‐plane direction and <001> out‐of‐plane direction. SiGe channel and dual‐layer embedded SiGe source/drain (S/D) can be clearly observed in HAADF‐STEM mode. Since HAADF image intensity is proportional to atomic number Z 1.7 , [4] SiGe layers with different %Ge are clearly seen, and can be further related to %Ge measured from EDX. In Figure 1(b), EDX map shows that channel SiGe has around 22% Ge. In S/D the buffer layer has 20‐21% Ge, and the main layer has 25 ‐ 30% Ge. HAADF micrograph and the corresponding deformation maps are shown in Figure 2. A compressive strain of 0.3% in the <110> direction is observed in the channel Si. There is no deformation between channel SiGe and channel Si in the <110> direction, suggesting that channel SiGe is compressed in this direction to match underneath Si lattice. This agrees with the finding that a deformation as high as 1.9% in channel SiGe is observed in the <001> direction. In addition the channel Si shows a tensile strain of 0.3% in the <001> direction. Results show that a combination of STEM‐based techniques, including HAADF‐STEM, STEM‐GPA and STEM‐EDX, can reveal the nanoscale chemical composition and strain distribution of a transistor. These information are used to monitor and control the process.