Elsevier

Optical Materials

Volume 32, Issue 9, July 2010, Pages 1146-1153
Optical Materials

Optical anisotropy of tilted columns thin films of chromium deposited at oblique incidence

https://doi.org/10.1016/j.optmat.2010.03.022Get rights and content

Abstract

Several chromium thin films with thicknesses comprised between 840 nm and 440 nm have been deposited by magnetron sputtering at glancing angle on silicon substrates. The deposition angle was varied between 0° and 80° resulting in nanosculptured thin films with column angles comprised between 0° and 25°. The surface topography of the films has been characterized by atomic force microscopy. It revealed different structurations of the surface originating from: the cross-section of the tilted columns with the surface, lateral ordering of the columns and faceting. The optical properties of the films have been determined from generalized ellipsometric measurements performed along different azimuths. The inspection of the off-diagonal elements of the Jones matrix as a function of columns tilt angle showed that both in-plane and out-of-plane anisotropy were present, associated with the columnar growth.

Introduction

Shadowing effects during growth of thin films at oblique angles together with limited adatoms diffusion induce the formation of well spaced columns with nanometric diameter [1], [2], [3]. This deposition technique, referred to as Glancing Angle Deposition (GLAD), allows design of a wide range of structures when the deposition angles or azimuth are varied during growth [4]. The nanosculptured thin films obtained may find applications in optics thanks to the optical anisotropy engineering allowed by GLAD [5], [6], [7]. The optical properties of nanosculptured thin films of various materials have been extensively studied because of their possible use in angle and polarization selective filters. For Cr GLAD thin films, the structural, mechanical and electrical properties of nanosculptured thin films have been reported [1], [8], [9]. Chromium structures with lateral periodicity have also been obtained by nucleating the structures on a template substrate [10]. Transmission measurements of polarized light have been performed on 54 nm thick Cr films obtained by GLAD and containing column at 45° from the normal to the surface [11], [12]. Very recently, the effective optical properties of one 160 nm thick sample containing Cr columns tilted by 45° with respect to the normal of the substrate have been interpreted using a monoclinic model [13]. However, the optical properties of thick Cr nanosculptured thin films and their evolution with deposition angle are still not well understood. In this work we use generalized spectroscopic ellipsometry to show the influence of columnar growth on the optical properties of nanosculptured Cr thin films. We will pay a particular attention to the off-diagonal elements of the Jones matrix.

Section snippets

Experiments

Chromium thin films with columnar microstructures were sputter deposited from a chromium target (50 mm diameter and purity 99.6%) with an argon pressure PAr = 0.53 Pa in a chamber with a base pressure lower than 10−6 Pa. The Si(1 0 0) substrates were ultrasonically cleaned in acetone and alcohol before introducing them into the reactor. The target was dc powered with a constant current density J = 66 A m−2. The target-to-substrate holder distance was fixed at about 10 cm, and the growth was carried out at

Results and discussion

Before measuring the optical properties of the films, the chemical and structural properties of the films were determined using complementary methods.

The surface compositions of the films were determined from X-ray Photoelectron Spectroscopy (XPS) measurements (not shown here). Two different contributions for the Cr2p core level were observed. Their relative positions and splitting corresponded to the emissions of Cr2p in metal and in oxide. The O1s-Cr2p3/2 oxide distance was found equal to

Conclusions

Thin films of chromium have been deposited using GLAD, resulting in columnar growth with column angles comprised between 0° and 25° with respect to the normal to the surface. The structural properties of the films have been determined using X-ray diffraction and reflectivity, XPS and AFM. The optical properties of the films have been investigated using generalized ellipsometric measurements. A simple multilayer model using effective medium approximation allowed extracting physical values of the

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