Volume 24, number 1 OPTICS COMMUNICATIONS January 1978 PHOTOACOUSTIC REFLECTION-ABSORPTION SPECTROSCOPY (PARAS) OF THIN OXIDE FILMS ON ALUMINIUM Per-Erik NORDAL* and Svein Otto KANSTAD* zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ Norwegian De.fence Research Est., P.O. Box No. 2.5, N-2007 Kjeller, Norway Received 10 October 1977 Infrared absorption spectra of thin films on metal surfaces are commonly obtained by measuring specular multireflec- tion losses for obliquely incident radiation. Using photoacoustic techniques and a tunable CO* laser, we demonstrate that the absorbed radiation may instead be directly recorded to high accuracy from a single reflection. Spectra are shown for several oxide films on aluminium, with sensitivity presently limited to approximately 1/20th of the natural oxide layer. Details and advantages of the PARAS technique are discussed, pointing out improvements expected to increase the sensitiv- ity by a factor of 100 or more. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. Introduction Reflection-absorption spectroscopy (RAS) in the middle infrared (MIR) is an established tool to study thin dielectric films on metal surfaces [l-lo]. Close to the surface of a perfect conductor, electromagnetic boundary conditions do not allow E-field components parallel to the surface. RAS of thin films on good con- ductors, therefore, is carried out using obliquely inci- dent light polarized in the plane of incidence. Francis and Ellison [l] found that the fractional change of a metal’s reflectivity due to absorption by a surface film is given by 6R,, = *sf@d? 0) - k; cos4 t9 I (1) 6R 1 (2) for 11 (polarized in the plane of incidence) and 1 radia- tion, respectively. Here X is the (vacuum) wavelength of the light incident at angle 8, d is the film thickness, n& kd and n,, k, are the real and imaginary zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED p a rts of the complex refractive indices Nd = nd t ik,, N,=n,tik, * Present address: Central Institute for Industrial Research, P.O. Box No. 350, Forskningsveien 1, Blindern, Oslo 3, Norway. Of film and substrate, respectively, and f(nd, 0) is a function whose value is of the order of unity. The imaginary part k, of Nd is related to the absorption co- efficient o(X) of the film material by kd(A) = Xcr(xy47r. Eqs. (1) and (2) are valid for 0” 5 0 ,< 80”, assum- ing highly conducting substrates with kf S nf and kf cos2 8 S 1 (good reflectors like Au, Ag, Cu and Al in the MIR have kz - 1000). For typical film materi- als of thickness 10 - 100 A then, 6R,, = 10e2 per reflec- tion at large angles of incidence, while 6R, remains ne- gligible (< 1O-5). T 0 increase sensitivity, conventional RAS methods use multireflection arrangements. Typi- cally two similar sample plates are placed - 0.5 mm apart and are illuminated with light incident at opti- mum angles 0 = 65” (for moderately absorbing films) to 0 x 88” (for strongly absorbing films) [4,5]. Corre- spondingly, optimum numbers of reflections (30-5) may be found. Near optimum conditions, reflection spectra differ little from transmission spectra of sim- ilar bulk compounds [4]. In principle, however, the spectrum of SR,, in particular differs from that of the film material (kd or 0~)due to the anomalous disper- sion that enters through the factor nd3in the leading term of eq. (1); this may cause band shifts and distor- tions. We present here the first evidence to show how photoacoustic spectroscopy (PAS) can improve and simplify such measurements. In PAS the sample, being situated inside a closed chamber, is illuminated by a 95