Si-based reflection elements for ATR-FTIR investigations

In addition to the already mentioned materials. Si is used for IREs in ATR-FTIR spectroscopy. It is an inert material and solely affected by strong bases and oxidizing agents in combination with fluorine compounds only. Major drawbacks of Si for MIR optical appUcations with traditional IREs are (1) its high refractive index, which causes reflection losses at interfaces and (2) some strong absorption bands in the range between 1500 and 300 cm caused by phonon vibrations (Hind et al., 2001 Lau, 1998). 

Crystalline Si as the most important substrate material in the semiconductor industry is fabricated in high quality and purity for relatively low prices compared to other ATR materials over recent decades. Nevertheless, IREs in standard shape are still quite expensive and not as common as applications of Si IREs in standard 

In this section, advantages and disadvantages of commonly used Si IREs are briefly summarized. Finally, a new development is presented, which is able to overcome some major drawbacks of conventional Si IREs and provides numerous helds of applications that are exemplary presented in Section 14.6. 

ATR-IR analysis within the fingerprint range of organic compounds and to record the first order vibrations of Si oxide and nitride (Collins and Fan, 1954 Chabal and Raghavachari, 2002 Ogawa et al., 2003 Schumacher et al., 2010a). Additionally, they are quite expensive and not compatible with common tools used in semiconductor fabrication. 

Similar setups with shorter optical path lengths based on common 100-mm Si wafers with 100 or 111 orientation with sizes of approximately 37.5 mm x 15 mm x 0.5 mm are described (Chabal et al., 1989 Pietsch et al., 1994 Weldon et al., 1996). In order to avoid confusion, these sometimes-called MIR elements should be named wafer-based multi-reflection elements (wMREs). The use of standard Si wafers allows processing in conventional semiconductor equipment, for example in order to deposit a specific material. With the geometry specified above, up to 75 reflections (Chabal et al., 1989 Pietsch et al., 1995) occur giving a higher signal fraction of the total reflection. The optical path in the wMRE is still relatively long, and so they are subjected to similar restrictions as standard MREs in case of applications with wavenumbers 1000 cm (Weldon et al., 1996). Additionally, the preparation of the 45° bevel has to be done individually/separately for each wMRE and accessories are needed to focus/collect the IR radiation onto/from the small bevelled faces. 

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