1849 Research Article Received: 3 February 2009 Accepted: 28 March 2009 Published online in Wiley Interscience: 26 May 2009 (www.interscience.wiley.com) DOI 10.1002/jrs.2332 Measuring stresses in thin metal ﬁlms by means of Raman microscopy using silicon as a strain gage material Thomas Wermelinger, Christophe Charpentier, M ¨ uge Deniz Y ¨ uksek and Ralph Spolenak * Mechanical stresses in microelectronics and micro-electromechanical systems may inﬂuence the reliability of applications and devices. The origin of the stresses can be because of the joining of dissimilar materials with regard to the thermal expansion coefﬁcient, electromigration or the deposition process utilized. Stresses can lead to delamination, crack formation and stress migration and therefore to a failure of the device. Identifying the locations of highest stresses in a device is crucial for reliability improvement. Currently, both Laue X-ray micro diffraction and convergent-beam electron diffraction are able to locally determine the stresses in thin metal ﬁlms. Here, we propose a modiﬁed method of indirect Raman microspectroscopy to measure stresses with a lateral resolution in the submicrometer range at a laboratory scale. The method encompasses the crystallization of an amorphous silicon layer by local laser annealing and its subsequent usage as a strain gage. Stresses in an aluminum thin ﬁlm were determined as a function of temperature. In addition to the average stress, the stress distribution could be monitored. Copyright c  2009 John Wiley & Sons, Ltd. Keywords: Raman microscopy; grain growth; thin ﬁlm; silicon; thermal stresses Introduction The local measurement of mechanical stresses is motivated by reliability improvement as well as fundamental research. Firstly, it is known that stresses and stress gradients in micro- electromechanical systems (MEMS) can lead to failure by electro- migration, fatigue, creep and fracture. Mechanical stresses can originate from differences in the coefﬁcients of thermal expan- sion of the different materials. Consequently, cyclic temperature changes have been shown to lead to fatigue. [1] Other causes of stresses in thin metal ﬁlm are electromigration [2] or deposition process. [3] Large stresses can also be induced in the substrate near embedded structures such as trenches, which can cause residual stresses or stresses can be applied externally during operation and deployment. [4–6] The problems associated with these stresses are various. One of the most serious issues is a stress-induced trigger of nucleation and propagation of dislocations and the formation of voids and cracks, which can lead to short-circuits and failure. To know why and where high stresses occur, with a lateral resolution as high as possible, could help improve the reliability of MEMS. Secondly, micro-X-ray studies have shown that stresses ob- served in thin metal ﬁlms are strongly nonuniform. [7–9] Interactions between neighboring grains lead to highly nonuniform stresses in an individual grain. It was found that grains with diameters much bigger than the ﬁlm thickness deform in a highly inhomogeneous way. For a better understanding of mechanical behavior of thin metal ﬁlms a method for stress measurements with a high lateral resolution has to be available. Several techniques are commonly used for stress measurements but none of them is without shortcomings when applied to materials used in microelectronics. One method is micro-X-ray diffraction with a spot size of less than 1 μm in diameter. [10] The drawback of this method is that it is only available at a few sources worldwide. Another technique to investigate thin ﬁlm plasticity is convergent-beam electron diffraction. [11] Although the method has a very high lateral resolution of only a few nanometers, it requires samples that are thinned to electron transparency. The sample production signiﬁcantly alters the original stress state. [4,12] Another technique to measure stress is Raman spectroscopy. Since the ﬁrst reports of Anastassakis and Burstein, [13] it is known that Raman peaks are sensitive to stresses. Ossikovski et al. [14] showed that it is possible to determine all six components of the silicon strain tensor by off-axis illumination and polarization of the incident and scattered light. Bonera et al. [15] presented another method to measure three components of the strain tensor by using a micro-Raman microscope. The lateral resolution is deﬁned by the optics and the wavelength of the laser light, and can be in the submicrometer range. [16] Ma et al. [17] applied the method to measure thermal stress in metallic interconnects by measuring the peak shift in the silicon substrate. Furthermore, from the Raman spectra the information about the microstructure such as the phase or the grain size and the temperature is also accessible. [18 – 21] In transparent materials, a confocal microscope allows the 3D mapping of peak shifts. [22] This paper presents a method for measuring stresses in thin aluminum ﬁlms using a silicon thin ﬁlm as a stress gage. Amorphous silicon was sputtered onto a silicon nitride membrane. Laser irradiation was used to crystallize the thin silicon ﬁlm. On the ∗ Correspondence to: Ralph Spolenak, Laboratory for Nanometallurgy, De- partment of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland. E-mail: ralph.spolenak@mat.ethz.ch Laboratory for Nanometallurgy, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, 8093 Zurich, Switzerland J. Raman Spectrosc. 2009, 40, 1849–1857 Copyright c  2009 John Wiley & Sons, Ltd.