Abstract
In this work, we experimentally measure fracture toughness of free-standing zirconium ZrSiₓ thin films using the crack-on-a-chip method. In this method, fracture toughness is determined from the analysis of cracks, which propagate and arrest in specially designed free-standing test structures. The test structures use a well-known double cantilever beam geometry, which enables crack arrest, and don't require any external force actuation, but instead rely on the internal tensile stress of the tested thin film. To produce the ZrSiₓ test structures, a universal fabrication process was developed and used, which avoids typical issues related to etch selectivity and that can be readily applied for other thin film materials. Unlike in previous studies, which used the crack-on-a-chip method, in this work crack initiation was triggered only after the test structures were fully fabricated, which allowed to avoid the influence of the fabrication process on the extracted toughness values. For this, blunt pre-cracks included in the structures were ``sharpened'' using focused ion beam, which resulted in rapid crack propagation and subsequent crack arrest. Mechanical analysis done by a finite element method to extract the values of fracture toughness, showed that buckling of the free-standing thin film test structures has a strong influence on the results of fracture toughness calculations and therefore cannot be ignored. The fracture toughness of ZrSiₓ thin films was determined to be 2.1±0.13 MPa*m <formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex> $^{0.5}$ </tex> </formula> .
Highlights
A DVANCES in thin film technology and microfabrication have been made continuously over the past decades, which allows for fabrication and use of free-standing thin films in various applications, such as MEMS-based sensors, Manuscript received August 5, 2021; revised October 21, 2021; accepted November 1, 2021
While the crack-on-a-chip method has clear advantages over other toughness measurement techniques, several challenges remain, such as avoiding the influence of the fabrication process on crack propagation, which occurs while the test structures are being released from the substrate, and the need to improve the mechanical analysis by taking into account the out of plane deformations of the free-standing thin film test structures
In this work, the crack-on-a-chip test method was used to study the fracture of free-standing ZrSix thin films
Summary
A DVANCES in thin film technology and microfabrication have been made continuously over the past decades, which allows for fabrication and use of free-standing thin films in various applications, such as MEMS-based sensors, Manuscript received August 5, 2021; revised October 21, 2021; accepted November 1, 2021. Still, applying mechanical load to the tested film remains challenging, as it requires the use of special actuator devices fabricated onchip [6], [7] or external actuation [9], which must be carefully aligned with the sample Another novel method of thin film toughness characterization that does not require any external actuation or introduction of atomically sharp pre-cracks was recently proposed by Jaddi et al [10], [12]. While the crack-on-a-chip method has clear advantages over other toughness measurement techniques, several challenges remain, such as avoiding the influence of the fabrication process on crack propagation, which occurs while the test structures are being released from the substrate, and the need to improve the mechanical analysis by taking into account the out of plane deformations of the free-standing thin film test structures.
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