Elsevier

Thin Solid Films

Volume 339, Issues 1–2, 8 February 1999, Pages 88-94
Thin Solid Films

Self-shadowing and surface diffusion effects in obliquely deposited thin films

https://doi.org/10.1016/S0040-6090(98)01154-7Get rights and content

Abstract

The production of highly porous films by oblique deposition has attracted recent attention because of the possible applications of such films. The morphology of obliquely evaporated films is thought to be determined mainly by the mechanisms of self-shadowing and surface diffusion. The thin film process simulator GROFILMS has been used to verify the importance of these effects, and clarify some aspects of how they interact to determine the final film morphology. Good agreement between simulations and actual films has been achieved. Temperature control of the film during deposition is shown to be an important consideration for the production of structurally engineered films.

Introduction

The recently developed technique of glancing angle deposition (GLAD) allows the production of highly porous thin films which exhibit densities significantly below those of bulk [1], [2], [3]. Such films may engender applications in the production of catalytic surfaces, microsensor elements, micro sieves, low dielectric materials, and optical circuitry components [4]. With reference to the latter application, the demonstration of optical rotation in GLAD films is particularly encouraging [5]. The design and engineering of GLAD films for specific applications will require a thorough understanding of the dependence of the film morphologies on the details of the deposition process. Pertinent parameters include the deposition rate, the angular distribution of the incident vapour flux, the film and substrate temperature, the energetics of the surface–substrate interface, and the partial pressure of residual gases in the deposition chamber. In order to investigate the relative importance of a number of these parameters on the morphology of films grown by GLAD, we are currently employing the two-dimensional (2D) thin film process simulator GROFILMS. In this work, we present simulation results, together with scanning electron microscope (SEM) images of experimentally grown films, in support of the thesis that the essential features of GLAD film morphology may be explained by the two competing mechanisms of self-shadowing and temperature-dependent surface diffusion.

It has long been recognized that thin films grown under conditions of oblique deposition exhibit columnar morphology characterized by elongated grains that are tilted away from the substrate normal and toward the incoming flux [6]. Experimental documentation of the optical anisotropy of such films [7] and modelling of their optical properties [8] has appeared in the literature. In a key study performed at Toyota, a number of metal oxide films were deposited over the range 0°≤θ≤80°, where the angle θ is the angle subtended by the incoming flux and the outward substrate normal [9]. Birefringence was characterized in these films and exploited to construct quarterwave plates. The authors concluded that the characteristic packing density of the columns was the primary factor influencing the birefringence of the films, with column orientation playing a secondary role. More recently, films exhibiting periodically bent grain structure have been produced by dynamically altering the parameter θ during deposition [10].

If the angle remains highly oblique (θ≥80°) during deposition of a film, significant porosity may be achieved. For traditional thin film applications such as optical coatings, porosity is to be avoided since the absorption of water and other foreign gases into the film leads to degradation of the desired physical and optical properties. It has been only recently that a practical method of controlling the microstructure of the film and its mean density has been demonstrated and exploited to engineer a wide variety of morphologies [3]. Using this technology, for example, it is possible to manufacture the helicoidal bianisotropic mediums (HBMs) envisioned by Lakhtakia and Weiglhofer [11], [12].

Currently GLAD films are grown in physical vapour deposition (PVD) systems in which the evaporant sources are formed either by resistive heating of tantalum, tungsten or molybdenum boats, or by electron beam bombardment. A key feature of the deposition system is computer controlled substrate motion which allows continuous in situ control of the orientation of the substrate with respect to the vapour source throughout an entire deposition run. Equivalently, the controller allows one to program the angular coordinates (θ(t), φ(t)) of the direction of the source in that reference frame which is fixed to the substrate and whose z axis is oriented normal to the substrate surface. All films presented in this work were grown at nominal deposition rates (normal incidence) of 20–40 Å s−1 and base chamber pressures of ∼5×10−4 Pa. During each deposition, the chamber pressure remained low enough to preclude scattering of the vapour atoms; consequently the angular spread of the deposition as seen from a point of the substrate is determined by the solid angle subtended by the source aperture. Thus the flux is highly collimated with an angular spread Δθ not exceeding 2°.

Section snippets

Simulation

The GROFILMS simulator (grain oriented film microstructure simulation) was developed in order to model fundamental film growth processes such as nucleation, shadowing and grain competition, internal microstructure, substrate wetting, crystal facetting, grain boundary grooving, and high temperature copper reflow [13], [14], [15]. A detailed description of the physical models and algorithms used in the simulator may be found elsewhere [16]. Briefly, films are modelled on a 2D domain with fixed

Modelling of evaporated films

The characteristic morphology discussed above is exemplified by the chromium film shown in the SEM image of Fig. 2. The film was evaporated onto a 1 mm thick glass substrate with deposition parameters α=86°, Δα=1°. Separated, aligned columns not exceeding 100 nm in width are clearly visible in this film, as in the simulation of Fig. 1c. The narrow columns are indicative of limited adatom diffusion length which is characteristic of high melting point materials (for chromium, Tm=2163 K). A feature

Conclusions

In summary, good agreement may be achieved between predicted and observed film morphology using reasonable physical parameters in the simulations. The simulation results presented here indicate that self shadowing and surface diffusion are the primary mechanisms at work during the growth of GLAD films. Column broadening during growth can be explained by dynamic heating of the film surface. The same mechanisms of shadowing and diffusion are thought to be responsible for the numerous 3D

Acknowledgements

This work was supported by the Alberta Microelectronic Centre and the Natural Sciences and Engineering Research Council of Canada. The authors wish to thank George Braybrook for the SEM work, and Jason Parks and Jeremy Sit for assistance with the film depositions.

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