Combined infrared and visible spectroscopic ellipsometry study of thin polymer layers

Articles

01 Dec 2007

K. Hinrichs^a and K.-J. Eichhorn^b
^aISAS–Institute for Analytical Sciences, Department Berlin, Albert-Einstein-Str. 9, 12489 Berlin, Germany
^bLeibniz Institute of Polymer Research Dresden, Hohe Str. 6, 01069 Dresden, Germany

Introduction

Thin polymer layers on solid substrates are of high technological importance due to their increasing potential for applications in electronics, sensors, nanotechnology and biotechnology. Appropriate characterisation methods are necessary for the design and analysis of devices made using such materials. This review article focuses upon presenting the many analytical possibilities for quantitative evaluation of the optical constants and thickness of polymer layers by combined application of spectroscopic ellipsometry (SE) in the visible (vis) and infrared (IR) spectral range. In general, ellipsometry1 is the method of choice for the correct determination of optical constants; it provides polarisation degree and phase as well as the amplitude information of the optical response. These quantities may be derived within one experiment and thereby optical simulations and quantitative interpretation are improved for many applications. For many polymers, no characteristic absorption bands are observed in the vis spectral range. Therefore, extension of the spectral range to the IR and ultraviolet (UV) regions is desirable because characteristic vibrational or electronic absorptions are available within these ranges. However, it must be considered that many polymers might be degraded by intense UV irradiation.

Vis ellipsometry is a standard method for determination of the optical constants, roughness and film thickness. Bare polymer surfaces, single polymer films on reflecting solid substrates, interphases in bilayers, adsorbed mono- and multilayers on polymer films can be analysed in vacuum, in different atmospheres or under liquids as well. Layers may be formed using spin- and dip-coating, grafting to or grafting from procedures, self-assembly and Langmuir–Blodgett techniques. Kinetics of layer formation, swelling and deswelling and adsorption and desorption processes can all be studied in situ. Recent results have been reported for polyelectrolytes, thermo responsive hydrogels and proteins. Only few applications are known for detailed spectroscopic ellipsometry studies, including electronic transitions of organic films. One reason might be that measurements have to be performed in the UV or vacuum ultraviolet (VUV) range, since very few electronic transitions occur in the vis. On the other hand, ellipsometry in the IR spectral range has become established over recent years because of the large amount of valuable information that can be derived from the analysis of characteristic vibrational bands. As with vis ellipsometry, optical constants and structural properties of thin films and layered systems can reliably be derived using IR ellipsometric measurements by strict correlation with optical theory. The extensive analytical potential of IR ellipsometry is based on:

a contactless and non-invasive measurement,
monolayer sensitivity,
identification of chemical bonds of the film and interface by characteristic vibrational absorption bands, and
optical modelling with respect to molecular orientations, composition, miscibility, inter-diffusion and interactions at interfaces, single films and in multilayers.