Strain analysis of planarized bulk materials and thick films is often performed using optical techniques such as Moiré interferometry or digital image correlation (DIC). These methods are contactless, capable of measuring strain fields in-situ, with a sample under test strained via mechanical or thermal loading. While both techniques are effective at mapping micro-scale strains, such resolution may be insufficient for observing strain near inference regions of two bonded materials with different coefficients of thermal expansion. Attempts to detect nano-scale strain in interface regions using DIC and Moiré techniques have seen some success, but suffer from limitations not typically encountered at larger length scales.
Practical digital image correlation techniques can effectively observe strain with a resolution of 0.2 pixels, making features 0.7-2 microns in size ideal for nano-strain measurements using white light illumination. However, resolving such feature sizes requires the use of high magnification optics or an SEM. High magnification optics significantly restrict the field of view which can be imaged, while use of an SEM contributes considerable complexity to each measurement. Moiré interferometry techniques have a much wider field of view, but a maximum resolution equal to λ/2 where λ is the wavelength of light used to generate the projected reference grating. In addition to a small field of view and modest strain resolution, both DIC and Moiré interferometry rely on resolving a single image plane, limiting the detection of out-of-plane interface deformations.
An alternative technique to DIC and Moiré interferometry based on laser diffraction will be presented. This technique has the potential to detect out-of-plane sample deformations and nano-scale strain simultaneously over a large field of view. A thin, high-efficiency diffraction grating made of evaporated gold was fabricated on the planarized cross-section of a microelectronics package. Such packages contain microstructures with many combinations of metal, semiconductor, and polymer interfaces arranged in various geometries. Focused light from a He-Ne laser was then directed at the grating, generating a far-field diffraction pattern. The m=1 and m=-1 diffraction angles were determined using two wide area camera sensors and resulting diffraction angle data used to map the surface normal of the interface by stepping the incoming laser beam across the grating area. Strain data was obtained by measuring the difference in observed grating pitch at two sample temperatures. Sample preparation, optical measurement configuration, determination of diffraction angle, and accuracy/error of the surface normal and strain measurements will be presented in detail.