Microstructure of thermal hillocks on blanket Al thin films

doi:10.1016/S0040-6090(00)00971-8

Thin Solid Films

Volume 371, Issues 1–2, 1 August 2000, Pages 278-282

https://doi.org/10.1016/S0040-6090(00)00971-8 Get rights and content

Abstract

The microstructure of thermal hillocks on blanket Al thin films has been studied for the first time by several techniques, including sectioning and imaging in a focused ion beam system. It is found that the new material in the hillock area lifts the original film up and in some cases penetrates it. The micrographs also reveal the grain structures and give valuable insight into the mechanisms of hillock growth.

Introduction

Thin metallic films are widely used as components in microdevices. Because of their potential limitations on device reliability, mechanical stresses in such thin-film systems have been studied extensively [1]. It is generally found that thin films support much higher stresses than bulk materials of similar composition; this observation has been attributed to constraints on lattice defects due to the fine microstructures and the influence of the substrate. Consequently, stress relaxation, which requires the movement of dislocations is more difficult than in large-scale materials and is, despite recent modeling attempts, not fully understood.

An important source of mechanical stress in thin films is the thermal mismatch between the film and the substrate material. Depending on the sign of the mismatch and of the temperature change, tensile or compressive stresses can develop in the film. One mechanism of compressive stress relaxation which is specific to thin films is the formation of hillocks, i.e. extrusions of material out of the plane of the film. Such hillocking, which results in considerable roughening of the film surface, has frequently been reported in the literature [2], [3], [4], [5], [6]. However, the structure of the hillocks has not been investigated in detail. Also the exact mechanism of hillock growth, apart from some suggestions involving condensation of atoms along dislocation lines, remains unclear.

The purpose of this paper is to report the microstructure of thermal hillocks on Al films. The micrographs were obtained by several methods, i.e. side-view scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM). The most detailed insight resulted from sectioning and viewing selected hillocks with a focused ion beam (FIB). Based on the microstructural information, a possible mechanism for the growth of these hillocks is suggested. A more thorough quantitative analysis which includes a mathematical modeling will be published elsewhere [7].

Section snippets

Experimental

Pure Al films of 1 μm thickness were magnetron sputter-deposited at room temperature onto Si (100) wafers coated with 0.2 μm LPCVD SiO₂. The Al films were first passivated with a 0.3-μm thick TiW layer, which was subsequently removed by plasma etching. The purpose of the TiW films was to suppress hillock formation in the experiment that we originally planned (not described here). The films were annealed for 2 h at 450°C in forming gas which is composed of 5% H₂ and 95% N₂. Because of the higher

Results and discussion

Whereas the as-deposited films were planar, the annealing resulted in extensive hillocking as can be seen by optical and scanning electron microscopy (Fig. 1, Fig. 2). The hillocks are homogeneously distributed and have a typical spacing of approximately 70 μm. Typical dimensions of a hillock are 4 μm in width and in height. Cross-sectional TEM reveals the microstructure of the film between the hillocks and of an individual hillock (Fig. 3). Note that the film consists of columnar grains (Fig. 3

Conclusion

We have characterized the microstructure of thermal hillocks in Al films which had been annealed at 450°C. At the sites of the hillocks, the original films are found to be displaced by material inserted under them to give hillocks with a conical shape. The micrographs also reveal the grain structures and give valuable new insight into mechanisms of hillock growth. Based on the microstructural observations, we propose that the hillocks grow by diffusion of atoms from the vicinity to the

Acknowledgements

This work was funded by Sematech, SIA, and DARPA under the MARCO Interconnect Focus Center Program.

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