Substrate reflectivity

Fig. 4.1. Interference of incoming and the reflected X-ray waves inthe triangular region above a flat and thick reflecting substrate. The strength ofthe electromagnetic field is represented on the gray scale by instantaneous crests (white) andtroughs (black). Inthe course of time, the pattern moves from the left to the right [4.21].

Infrared spectroscopy, including Fourier-transform infrared (FTIR) spectroscopy, is one of the oldest techniques used for surface analysis. ATR has been used for many years to probe the surface composition of polymers that have been surface-modified by an etching process or by deposition of a film. RAIR has been widely used to characterize thin films on the surfaces of specular reflecting substrates. FTIR has numerous characteristics that make it an appropriate technique for... 

In order to obtain the infrared spectrum of a thin film on a reflecting substrate, a transmission experiment is out of the question since infrared radiation cannot be transmitted through any significant thickness of a reflecting material. Instead,... 

In order to understand RAIR spectroscopy, it is convenient to model the experiment (see Fig. 4). Consider a thin film with refractive index n =n ik and thickness d supported by a reflecting substrate with refractive index ni = ri2 — iki- The refractive index of the ambient atmosphere is o- Infrared radiation impinges on the film at an angle of incidence of 6 . The incident radiation can be polarized parallel to or perpendicular to the plane of incidence. 

This characteristic of RAIR can be observed experimentally. Fig. 8 shows the transmission spectrum of polydimethylsiloxane (PDMS) while Fig. 9 shows the RAIR spectrum of a thin film of PDMS spin-coated onto a chromium substrate. It can be observed that the bands near 1024 and 1095 cm have similar intensities in the transmission spectra but the band at higher frequencies is clearly much more intense in the RAIR spectrum. This change in relative intensity when PDMS is deposited onto a reflecting substrate is related to optical effects and is not related to orientation effects. 

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

The first hormonal signal found to comply with the characteristics of both a satiety and an adiposity signal was insulin [1]. Insulin levels reflect substrate (carbohydrate) intake and stores, as they rise with blood glucose levels and fall with starvation. In addition, they may reflect the size of adipose stores, because a fatter person secretes more insulin than a lean individual in response to a given increase of blood glucose. This increased insulin secretion in obesity can be explained by the reduced insulin sensitivity of liver, muscle, and adipose tissue. Insulin is known to enter the brain, and direct administration of insulin to the brain reduces food intake. The adipostatic role of insulin is supported by the observation that mutant mice lacking the neuronal insulin receptor (NDRKO mice) develop obesity. 

For fundamental studies of nanoparticles in IRAS measurements, a very important issue is how to attach metal nanoparticles onto a conducting substrate without changing their physical and chemical properties. Recently, we have developed a new method for anchoring metallic nanoparticles on reflective substrates of gold and/or glassy carbon, which we have termed a temperature-induced deposition (TID) method [Stamenkovic et al., 2004]. A key advantage of this method is that the catalysts... 

Stamenkovic V, Arenz M, Ross PN, Markovic NM. 2004. Temperature-induced deposition method for anchoring metallic nanoparticles onto reflective substrates for in situ electrochemical infrared spectroscopy. J Phys Chem B 108 17915-17920. 

When 0 [i.e., when reaction (2) becomes rate-determining], the plateau current reflects substrate control ... 

Figure 23. Plot of exposing light intensity as a function of depth in a photoresist on a reflecting substrate. (Reproduced with permission from Ref.

Figure 24. Electron micrographs of 1.2 pm lines in a positive photoresist exposed on a reflective substrate and developed in standard photoresist developer. (Reproduced with permission from Ref. 32)...

Fig. 10.3 Colour contrast of human erythrocytes on different solid reflective substrates Al -polycrystalline aluminum Cu - polycrystalline copper Mo - monocrystalline molybdenum Ni - polycrystalline nickel Pt - chemically polished platinum Si(m) - monocrystalline silicon Si(p) - polycrystalline silicon Ti - chemically polished titanium W- monocrystalline tungsten...

The Dye-in-Polymer concept was first proposed and reduced to practice in our laboratory (12). It involves the ablative marking with an appropriate laser, of a polymer film containing a dissolved dye. The dye-in-polymer film can be easily prepared by spin-coating a dye/polymer solution onto a reflective substrate. The structure of the DIP recording medium is shown in Figure 8. 

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

Optically smooth samples with 0.3-10 xm thicknesses are placed on a gold covered surface or other high reflecting substrate. Similar to the transmission method except that the pathlength is twice as long. The physical properties are uncompromised. In some cases, optically smooth protein and starch films can be prepared (without cryogenic slicing). 

Using the surface of liquid water as the reflective substrate, infrared reflection-absorption spectra of phospholipid layers have been obtained (123-125). The infrared beam is taken external to the spectrometer with two CaF2 lenses and a flat mirror, with another flat mirror directing the reflected beam to the detector. A Langmuir film balance was designed (124) which allowed compression of the phospholipid layers to over 50 dyne/cm. 

Fig. 13.5. Frontal irradiation of photoresist film coated on a non-reflective substrate.

Figure 13.5 shows a diagram of photoresist film coated on a non-reflective substrate and irradiated from the front. The thickness of the photoresist is h. It is convenient to introduce a dimensionless variable z measuring the position in the film, with z — 0 on the top of the film and z = 1 on the photo-resist/substrate interface. The photoresist with density p consists of crosslinkable units with molecular weight M0, molar absorptivity e, and concentration m. At the onset of irradiation, concentration of the cross-linkable units is uniform through the film, m — m0 — p/M0, and the initial optical density of the film is D0, which is given by... 

As long as the film is not reflective (i.e., specular aluminum) and is deposited on a reflective substrate (i.e., Si02 on silicon), optical techniques are available. It was recognized early that the color of a thin film could be correlated to its thickness. Although not very precise, such information is very useful for quick evaluation in the laboratory. For example, silicon dioxide films on silicon substrates can be evaluated with the data of Table 1. In fact, one of the more useful aspects of this technique is that one can make rapid judgements as to film uniformity. 

---