Optical sampling of thin films is conventionally performed using either specular reflectance or transmittance. Conventionally, reflectance measurements and transmittance measurements are made separately. Specular reflectance measurements are based on the fraction of light intensity that is reflected from a sample surface. FIG. 1A shows a simplified schematic diagram of a conventional reflectance measurement of a sample 10. It should be understood that sample 10 typically includes one or more thin film layers (not shown). As shown in FIG. 1A, the sample 10 is exposed to a light beam 12 having an intensity Io. Part of the light beam 12 is reflected off sample 10 as light beam 14, which has an intensity Ir. The light beam 14 is collected with a detector (not shown). The reflectance, R, can then be measured as the ratio of the intensities of the reflected and incident light beams, as follows:                     R        =                              Ir            Io                    .                                    eq        .                                   ⁢        1            The reflected intensity Ir of light beam 14 is less than the incident intensity Io of light beam 12 so that R<1.
Transmittance measurements are based on the fraction of light intensity that is lost as a beam passes through a sample. FIG. 1B shows a simplified schematic diagram of a conventional transmittance measurement of a sample 20. Again, it should be understood that sample 20 typically includes one or more thin film layers (not shown) that are to be measured. As shown in FIG. 1B, the sample 20 is exposed to a light beam 22 having an intensity Io. Part of the light beam 22 is transmitted through sample 20 as light beam 24, which has an intensity It. The transmitted light beam 24 is collected by a detector (not shown). The transmittance, T, can then be measured as the ratio of the intensities of the transmitted and incident light beams, as follows:                     T        =                              It            Io                    .                                    eq        .                                   ⁢        2            As with reflectance, light intensity is lost upon transmittance so that the transmitted intensity It of light beam 24 is less than the incident intensity Io of light beam 22 so that T<1.
On example of transmittance measurement is found in a technique known as Mirror Backed Infrared Reflection Absorption Spectroscopy (MBIRRAS) that prescribes placing a mirror a fixed distance behind an absorbing film sample and which is described in “Reflectance FT-IR for monitoring chemical reactions in chemically amplified photoresist for 0.25 μm X-ray lithography”, Christopher Gamsky, Ph.D. Dissertation, University of Wisconsin-Madison, 1995. The mirror and sample are spaced by a Teflon ring and held fixed by pressure from front and back plates. Incident light passes through the sample, reflects off the mirror and passes back through the sample. Measurements are explicitly performed at an oblique angle of incidence, e.g., 40° from normal, in order to avoid collecting light reflected from the surface of the sample. The air gap, which is the width of the Teflon ring, is selected to minimize interference fringes caused by collecting both the sample reflected and mirror reflected beams.
Typically, reflectance and transmittance measurements are preformed over some continuous range of wavelengths such as the mid-IR (400 cm−1 to 4000 cm−1). These spectra will normally contain features that become more or less pronounced as material properties, e.g., concentrations, change. It is therefore possible to correlate feature strength with these material properties and thereby measure these material properties. Typically, there is some range of the feature strength over which either reflectance or transmittance is usable and a single sample may contain some features within the range of a transmittance measurement and some features within the ranges of a reflectance measurement. In addition, some samples may have features that cannot be measured well by either reflectance or transmittance. In these cases it becomes necessary to modify the sample, such as the film thickness, for monitoring purposes—a modification that would be costly and require additional correlation to the originally unmeasurable sample.
Accordingly, what is needed is a metrology device that is easily configurable to operate in reflectance mode, transmittance mode or a mix of reflectance and transmittance mode.