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Aluminum oxide (Alumina, Al2O3) and silicon dioxide (silica, SiO2) freestanding thin films are extensively used in a large number of electronic applications as protective layers, dielectric materials and sacrificial release layers during a device micromachining stage. Al2O3 films were deposited by electron beam-evaporation technique while SiO2 were thermally grown. The fracture toughness and environmentally assisted subcritical crack growth have been investigated using a new on chip fracture mechanics test. This technique consists of two long actuators beams pulling on a notched specimen thanks to the release of the residual stress inside the actuators after etching the material beneath. A crack is initiated from the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value [1]. Both freestanding films are susceptible to stress corrosion especially, in the presence of moisture. The mechanisms responsible for this slow cracking behavior are investigated as well as the subcritical crack growth rate in laboratory air and dry environments under various temperature conditions. The combined experimental and finite element studies provide an insight on the variation of the crack propagation rate (v) as a function of the stress intensity factor (K) in different environments. This variation follows a power law exhibiting a higher cracking rate under high K loading configurations. The fracture toughness value is around 1 MPa√𝑚 for both films. This toughness value is compared to the value obtained by push-to-pull technique (PTP) inside transmission electron microscopy (TEM) using Al2O3 and SiO2 specimens that contain a real sharp pre-crack generated during the crack-on-a-chip test.

Introduction/Purpose: Thin film materials are the main building blocks in many fields of application like flexible electronics, microelectromechanical /nanoelectromechanical systems (MEMS/NEMS) and functional coatings. However, the determination of the intrinsic fracture toughness of these materials, which is the key property controlling the failure resistance remains a challenge. Methods: In this work, a new on-chip testing method for freestanding films is developed. The technique consists of two long actuators beams pulling on a notched specimen by exploiting the residual stress inside the actuators. The residual stress upon release by chemical etching leads to the actuator contraction, hence pulling on the central notched specimen. A crack is initiated at the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value. This crack arrest measurement avoids the problem of introducing a sufficiently sharp precrack. Further subcritical crack growth can be investigated by this fracture on-chip testing technique through repeating crack length measurement over time, without monopolizing any test equipment. Results: The understanding of the mechanisms causing the subcritical crack growth is crucial to define the reliability of devices made with thin films that are showing time-dependent failure. Low-pressure chemical vapor deposition (LPCVD) silicon nitride (SiN) and silicon dioxide (SiO2) films deposited by electron beam-evaporation technique were studied with a variety of thicknesses. The specimens were tested in laboratory air and dry nitrogen environments under various temperature conditions. Conclusions: Based on both experimental data and finite element simulation results (FE), the stress intensity factor (K) and the crack velocity (v); K-v curve in different environments is determined, following classical exponential law.