Investigation of Biaxial Strain in Strained Silicon on Insulator (SSOI) Using High-Resolution X-ray Diffraction
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Investigation of Biaxial Strain in Strained Silicon on Insulator (SSOI) Using HighResolution X-ray Diffraction Yeongseok Zoo1, N. D. Theodore2, and Terry L. Alford1 1
School of Materials, Arizona State University, tempe, AZ, 85287-8706
2
Freescale Semiconductor Inc., tempe, AZ, 85284
ABSTRACT Intrinsic biaxial strain values of strained Si on insulator (SSOI) layers were measured using symmetric Bragg-Brentano configuration (i.e., {004} θ-2θ scans) and asymmetric {224} rocking curves. We confirmed that the twist angle between the layer and substrate can be incorporated into the biaxial strain equations for epitaxial layers. Moreover, as the samples were annealed up to 1200 oC, the tensile parallel strains increased from 0.56% to 0.7%. Since both the overlying strained Si and underlying substrate maintained a stressed state in the buried SiO2, the compressively strained oxide retained the lattice expansion of the overlying strained Si and resulted in the increasing parallel strains after annealing. INTRODUCTION One of the methods being developed to enhance carrier transport in silicon-based transistors is to introduce a finite amount of strain into the silicon lattice [1]. Introduction of biaxial tensile strain into the Si layer is achieved by epitaxial deposition of a thin Si film onto a substrate with a larger lattice constant. This strained Si layer on SiGe is bonded onto a thermally grown oxide layer on a Si substrate to achieve the desired strain state. The crystallographic properties, especially the strain characteristics of the SSOI layers are investigated using high resolution X-ray diffraction (HRXRD) techniques. Besides being a nondestructive technique, HRXRD is advantageous for crystallographic analysis of single crystalline films because coherent scattering of X-rays occurs from the crystalline lattices of the strained Si and Si substrate and not from the amorphous SiO2 layer. Moreover, the perpendicular and parallel strains can be discerned by measuring relative peak positions of the SSOI layer from asymmetric X-ray scans. Based on the results using HRXRD techniques the strain behavior of the SSOI layer and the relation between strained Si and SiO2 layers are also discussed for annealed samples.
EXPERIMENTAL DETAILS Figure 1 presents a schematic of the main process steps of the wafer-bonding method. Initially a strained Si is epitaxially grown on a relaxed SiGe buffer layer. Hydrogen ions are implanted through the oxide and into the SiGe layer. After growing thermal oxide on a handle substrate, both wafers are bonded using conventional wafer bonding procedures. The bonded wafers are then annealed to induce coalescence of the bonded interface. At sufficient temperatures and times, the H+ implanted layer expands and cleaves, leaving the strained Si on the handle substrate. The residual SiGe layer on the strained Si is polished after wafer cleavage. In order to investigate any high temperature effects on the SSOI strain relaxation, the samples were annealed in a Jipelec-RF induction furnac
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