Strain Mapping in SiGe by Visible Raman Spectroscopy
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Strain Mapping in SiGe by Visible Raman Spectroscopy Vasil Pajcini Charles Evans & Associates 810 Kifer Road, Sunnyvale, CA 94086-5203 ABSTRACT Heterogeneous strain in Si/SiGe for CMOS mobility enhancement may result in device yield losses or IC performance limitations. This lateral heterogeneity in strain can be uniquely measured by Raman spectroscopy. UV Raman and Visible Raman can measure this heterogeneity respectively in the thin Si cap and in the SiGe underlayer. Both UV and Visible Raman are useful. While UV Raman is optimized for the Si channel layer, Visible Raman of the SiGe underlayer provides diagnostic information when trying to understand and reduce the heterogeneity in the Si channel. In principle Visible Raman can also measure the heterogeneity of the strain in the Si channel, but in practice this is very time consuming and not as sensitive as UV Raman. This paper focuses on the kind of heterogeneity strain information available from Visible Raman.
INTRODUCTION Structures of strained Si on a relaxed SiGe buffer layer are of interest for CMOS application as the charge carrier mobility is significantly enhanced in these structures. However, the lattice mismatch in these structures causes the formation of the misfit dislocations and associated threading dislocations, which are vertically developed up to the free surface. As a result an undulated surface morphology, known as “cross-hatch” pattern, is developed with lateral strain fluctuations. Lateral heterogeneity of strain in strained Si/SiGe materials may be a serious problem for the manufacturability of high mobility ICs that use these materials. Though the cross-hatch pattern can be visualized by AFM and TEM, the magnitude of the strain fluctuations, whose reduction is of importance, can be measured only by micro-Raman spectroscopy. Xie et al. reported a 10x10 µm2 UV-Raman map of the stress in the thin Si cap of a Si/SiGe sample where the stress map could correspond to the cross-hatch pattern seen by AFM [1]. The UV (325 nm) source results in a shallow penetration of the Si cap so that the stress measured is in the Si cap. Visible Raman measurements of strain fluctuation in cross-hatch patterns have been reported on SiGe films without strained Si on top [2] and in strained-Si /SiGe heterostructures with relatively thick layers (60 nm) of strained-Si [3].
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We will report here lateral strain fluctuations measured in the SiGe underlayer (and/or strained-Si thin layer) by using visible Raman spectroscopy for samples with different structures, and with or without CMP.
EXPERIMENTAL A square area (30x30 µm2) for samples with different SiGe structures was mapped using a visible Raman spectrometer (LabRam from JY-Horiba) excited with an Ar+ ion laser (514.5 nm wavelength, 1 mw in the sample, 1µm spatial resolution). Backscattering geometry was used for the measurements. The sample structures were: Sample A: s-Si (20 nm)/ Ge0.30Si0.70(200nm)/ s-Si (20 nm)/ Ge0.30Si0.70(1µm)/ GexSi(1-x) (graded 2µm) )/Sisubstrate, where the top layers were g
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