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Strain mapping of tensile strained transistors by dark-field off-axis electron holography

: Sickmann, J.; Schuster, J.; Richter, R.; Wuerfel, A.; Geisler, H.; Engelmann, H.-J.; Lichte, H.

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15th European Microscopy Congress 2012. Online Proceedings : Manchester Central, United Kingdom, 16th - 21st September 2012
Manchester, 2012
2 pp.
European Microscopy Congress (EMC) <15, 2012, Manchester>
Abstract, Electronic Publication
Fraunhofer ENAS ()

Semiconductor devices experienced an impressive performance boost by the incorporation of strained silicon technology over the past ten years [1,2]. The induction of strain into the channel of a metal-oxide-semiconductor field-effect transistor (MOSFET) significantly improves the transistor channel mobility and the electrical performance of logic devices. Since even small derivations of the strain field in the transistor channel can have a strong impact on the device performance, the determination of the exact two-dimensional strain distribution at nanometre scale resolution is of major interest for semiconductor device characterization. Dark-field off-axis electron holography in a transmission electron microscope (TEM) has successfully proven to fulfil the desired task [3]. The technique is based on the interference of diffracted waves from adjacent sample areas using the dark-field off-axis holography configuration [4]. By superposition of a diffracted wave emanating from an unstrained crystal region, which serves as the reference, and the same diffracted wave from the region of interest containing the strained crystal, the complete diffracted electron wave hence amplitude and phase can be reconstructed. The phases of the diffracted waves include all information of the local geometric variation of the lattice. They give direct access to local changes in the lattice parameter, leading to a two-dimensional strain map [3]. Figure 1 shows a dark-field off-axis hologram of the (220) diffracted beam of a particular transistor test structure manufactured by GLOBALFOUNDRIES Dresden. Tensile strain is induced in the transistor channel by deposition of a stress material on top of the gate stack. The specimen preparation has been performed by focused-ion-beam (FIB) at facilities of GLOBALFOUNDRIES Dresden. All specimens were prepared by FIB lift-out and finally milled to a uniform thickness of about 150 nm. The holographic experiments have been performed by means of a FEI Tecnai F20 microscope equipped with an aberration corrected Lorenz lens at the Triebenberg Laboratory. We used different holographic set-ups [5] that lead to significant improvements in lateral resolution and signal resolution of the two dimensional strain maps. In particular, we are able to adjust the field of view of 200..1000 nm at respective lateral resolution over a nearly variable range of 3..10 nm. In order to compare and verify the holographic results, stress and strain fields in the transistor structures have been computed using finite element simulations at Fraunhofer ENAS. All dimensions and parameters used in the simulation model have been taken from TEM cross sections and available process data. Figure 2 shows the map of the (110) lattice strain derived from the phase of the (220) diffracted wave in Figure 1(b). Three line profiles A, B, C of the (110) lattice strain have been evaluated from the strain map strain along the y-axis. The maximum (110) lattice strain at the gate estimates ε xx = (0.27±0.05)%. The gradients of the strain profiles are in good agreement with the simulation, which predicts a maximum strain value of ε xx = 0.3% directly below the gate. Although these results emphasize the benefits of the dark-field off-axis holographic technique for strain mapping in semiconductor devices, we are going to point out the still remaining challenges of the technique like sample preparation, lateral resolution and signal resolution.