A COMPARISON OF 3 TECHNIQUES FOR MEASURING STRAIN AT FRACTURE OF THIN-FILM MATERIALS

Good knowledge of the mechanical properties of thin film materials is important for reliable design of advanced electronic packages such as multi-chip modules (MCMs). Strain at fracture is one of the important properties which has not been well characterized for some of the candidate thin film materials for MCMs. In part, this is due to the lack of simple and accurate techniques to measure strain at fracture of thin film materials. Two new techniques have been developed to alleviate this problem. The first is an acoustic technique in which an accelerometer is attached to the film. The film is then gradually strained and when the film fractures. an acoustic wave propagates through the film thereby creating a transient signal from the accelerometer. This signal is used to trigger an oscilloscope. The second technique is similar except that it utilizes a laser probe to detect the acoustic wave. In this photo-acoustic method, the laser is reflected from the film onto a bisectional photodetector while the film is gradually strained. Upon film fracture, the acoustic wave modulates the laser beam and a transient voltage pulse is produced by the photodetector. Both techniques are capable of detecting the formation of extremely small cracks. The two techniques are compared with a previously reported electrochemical technique. All three techniques are accurate, simple to implement, and inexpensive but have relative advantages and disadvantages.