Diffraction from Periodic Arrays of Oxide-Filled Trenches in Silicon: Investigation of Local Strains
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0913-D05-02
Diffraction from Periodic Arrays of Oxide-Filled Trenches in Silicon: Investigation of Local Strains Michel EBERLEIN1,2, Stephanie ESCOUBAS1, Marc GAILHANOU1, Olivier THOMAS1, Pascal ROHR2, and Romain COPPARD2 1 Laboratoire TECSEN UMR 6122 CNRS Case 262, Université Paul Cézanne, 54 Avenue Normandie-Niemen, 13397 MARSEILLE Cedex 20, France 2 ATMEL ROUSSET, Zone Industrielle, 13106 ROUSSET Cedex, France
ABSTRACT The experimental evaluation of stresses at the nanometer scale is a real challenge. We propose an innovative use of High Resolution X-Ray Diffraction to measure local strains induced in silicon by periodic arrays. This technique is non-destructive and allows for the measurement of periodic strain fields in monocrystalline silicon, created in particular by Shallow Trench Isolation process. A 0.58µm-period array of trenches filled with SiO2 gives rise to satellites in reciprocal space maps around the Si substrate peak. The intensity and envelope of these satellites depend on the local strain. The experimental reciprocal space maps are compared to those computed using the kinematical theory from the elastic displacement field calculated with finite element modelling. This technique allows us to study the generation of strain during the main steps of the STI process. During the process, after trenches get filled with oxide and top layers removed, a second diffraction peak appears with a lower intensity than the substrate one. Thanks to finite element modelling, we validate that this peak is caused by an almost constant strain in the silicon active areas. Typical values of strains after trench filling are εxx = -1.68*10-3 and εzz = 1.56*10-3 where x and z refer to transverse and perpendicular directions. INTRODUCTION With the constant decrease of critical size in microelectronics, mechanical stresses in thin films and nanostructures have become an important matter. They may be used as a benefit, for example by increasing the electron mobility [1] or on the opposite have real hazardous effects like dislocation generation or leakage current increase [2]. In order to improve integration and specification of components, Shallow Trench Isolation (STI) technology is used in many microelectronics applications like non-volatile memories [3]. Deep trenches are etched in the silicon substrate and filled with SiO2 in order to isolate electrically the memory cells. The many steps of the process generate high mechanical stresses, which may damage the device and reduce its reliability, more particularly with the decrease in critical dimensions. This is the reason why it is crucial to measure stress at the local scale. While many techniques can yield average stress, it is a real challenge to get local strain fields with a nanometer spatial resolution. For instance, microdiffraction or micro-Raman spectroscopy are limited to a spatial resolution of 0.3µm [4,5], Convergent Beam Electron Diffraction has the nanometer resolution but needs a painful preparation that can modify the strain state in the sample [6]. High Res
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