Constraint Effects on Thin Film Channel Cracking Behavior
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Constraint effects on thin film channel cracking behavior Ting Y. Tsui and Andrew J. McKerrow Silicon Technology Development, Texas Instruments Inc., Dallas, Texas 75246
Joost J. Vlassak Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received 23 March 2005; accepted 31 May 2005)
One of the most common forms of cohesive failure observed in brittle thin film subjected to a tensile residual stress is channel cracking, a fracture mode in which through-film cracks propagate in the film. The crack growth rate depends on intrinsic film properties, residual stress, the presence of reactive species in the environments, and the precise film stack. In this paper, we investigate the effect of various buffer layers sandwiched between a brittle carbon-doped-silicate (CDS) film and a silicon substrate on channel cracking of the CDS film. The results show that channel cracking is enhanced if the buffer layer is more compliant than the silicon substrate. Crack velocity increases with increasing buffer layer thickness and decreasing buffer layer stiffness. This is caused by a reduction of the constraint imposed by the substrate on the film and a commensurate increase in energy release rate. The degree of constraint is characterized experimentally as a function of buffer layer thickness and stiffness, and compared to the results of a simple shear lag model that was proposed previously. The results show that the shear lag model does not accurately predict the effect of the buffer layer.
I. INTRODUCTION
To reduce device size and power consumption, advanced optical and electronic devices are often made of thin-film composite structures. They can be deposited using a range of techniques, such as plasma-enhanced chemical vapor deposition (PECVD), high-density plasma deposition (HDP), spin-on coating, and a variety of sputtering methods. Most films are subject to residual stresses. Often films are deposited at temperatures greater than the ambient environment and have thermal expansion coefficients that are different from the substrate material. This mismatch in thermal expansion creates a compressive or tensile residual film stress that sometimes leads to delamination or cohesive fracture.1–6 In addition to thermal mismatch, residual stresses may also arise from the actual deposition process or as a result of lattice mismatch for epitaxial films. a)
Address all correspondence to this author. e-mail: [email protected] This paper was selected as the Outstanding Meeting Paper for the 2005 MRS Spring Meeting Symposium B Proceedings, Vol. 863. DOI: 10.1557/JMR.2005.0317 2266
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J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 14 Mar 2015
One common cohesive failure mode for thin films under tension is channel cracking, where through-film cracks propagate in the film.6 The energy release rate G for a channel crack can be calculated using the following equation 2h , (1) G=Z 2 E where h, , and E¯ ⳱ E/(1 − 2) represent the film thickness, residual stress, and
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