Strained silicon layers are patterned for fundamental analysis by electron beam lithography in combination with dry etching (cryo-etching (RIE) at -110°C using a SF6/O2 chemistry). Different patterns down to 50 nm feature size are prepared. Examples are shown in Fig. 1.
Fig. 1: SEM images of patterned strained silicon layers. The bare indicates 1µm.
There is a large number of methods to measure the strain in thin layers but only a few are applied to strained silicon. For characterization of strain in non-patterned layers X-ray diffraction methods and UV- Raman spectroscopy are generally used. Especially UV- Raman spectroscopy is the method of choice because it is a non-destructive method, easy to handle, allows fast measurements, and can be applied in CMOS process lines. Because the penetration depth of the laser light mostly used for excitation (λ = 325nm) is about 10 nm in silicon, UV Raman spectroscopy is applicable also for very thin strained silicon layers required for fully depleted device fabrication. The application of the conventional UV-Raman technique, however, is limited to patterned device layers. Fig. 2 shows a TEM cross-sectional image of the patterned strained silicon layer of a sSOI substrate. The blue area in the image corresponds to the diameter of the laser beam (about 800 nm) which is larger than an individual structure. Therefore the strong Si-Si vibration mode of the silicon base wafer appears and the Si-Si vibration mode of the thin strained silicon structures cannot clearly be identified anymore.
Fig. 2: TEM cross-sectional image of a patterned strained silicon layer of a sSOI substrate. The blue area indicates the diameter of the laser beam using a “conventional” UV µRaman setup.
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