Reflectance-based optical techniques

Reflectance-based optical characterization techniques offer the advantages of high energy resolution and sensitivity to both macrostructural and microstructural effects while nondestructively providing real-time information with the sample in any transparent ambient. Experimental and analytical methods are discussed, and examples are given to illustrate representative applications to problems of current interest in semiconductor technology. 

This paper is intended to give a brief overview of reflectance-based optical characterization techniques and their applications to determining sample properties. The next section deals with general principles, and includes comments about Instrumentation and analytic methods. The rest of the paper consists of representative examples. Other applications can be found in several recent reviews and symposium proceedings (1-5). Length limitations preclude extensive discussions references should be consulted for further details. 

The above examples are representative of the present capabilities of reflectance-based optical characterization techniques. Other applications can be found in the general references given in the introductory paragraphs. Ir reflectance has not been discussed, not from lack of examples (31. 32), but because the major fraction of reflectance characterization has been done in the v-uv. Additional progress and new applications can be expressed in all areas. 

Improvements in the single side-band performance of a mixer-based receiver can be made by filtering the unwanted side band before it is down-converted in the mixer. Such a scheme, which is described in detail by Goldsmith (1982) is based on interferrometric techniques. We will not discuss single side-band filtering any further, except to note that it is a particularly apposite demonstration of the use of optical techniques to process the radiation in the spectrometer. We will discuss the use of interferometric techniques in Section IX as a means to realize a reflection mode spectrometer. These few examples indicate the flexibility of application of optical techniques to problems of instrument design in the FIR. 

Biosensors are also classified according to the parameter that is measured by the physicochemical transducer of the biological event. Thus, classically biosensors are grouped into optical, electrochemical, acoustic and thermal ones. Optical transducers of most common enzyme biosensors are based on optical techniques such as absorption, reflectance, luminescence, chemi-luminescence, evanescent wave, surface plas-mon resonance, and interferometry. 

SPR refers to an optical technique which is a very popular detection scheme in biosensors. It is most commonly not used to study the surface itself, but rather to study the interaction of a solute with a reaction partner that has been immobilized at the surface. The principle of SPR is illustrated in Fig. 2. A laser beam is coupled into a high refractive index glass prism and totally internally reflected off its base. The reflected beam is then detected outside the third side of the prism. The base is coated with a thin metal layer (usually gold) and SPR exploits the peculiar physics of total reflection. Instead of being strictly reflected at the edge of the glass prism,... 

Ellipsometry is another optical technique widely used for studying bioparticle adsorption. The method is based on the principle that the state of polarization of light changes upon reflection from an interface. Jonsson et al. [159] consfructed the flow cell, enabling the ellipsometric measurements to be performed with a support adsorbing surface exposed to a... 

In this chapter we shall consider the optical response of a gas in the close vicinity of a solid surface. We shall become aware of different optical techniques which allow us to study various aspects of interactions between gas atoms or molecules and a surface. They are based on reflection of light from a gas-solid interface. Depending on whether the incidence angle is less or greater than the critical angle, one distinguishes between selective reflection spectroscopy (SRS) and evanescent wave spectroscopy (EWS). 

---