Growth stresses and viscosity of thermal oxides on silicon and polysilicon
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Stresses in thermally grown SiO2 films on silicon have traditionally been determined by substrate curvature measurements. This technique is useful for studying stresses in thin films, but it cannot be used to investigate stresses generated in the substrate during film growth. In the present work, we used microelectromechanical systems-based microstrain gauge devices fabricated from single-crystal and polycrystalline silicon (henceforth silicon and polysilicon, respectively) to measure oxidation-induced stresses in both dry and wet oxidizing ambients. Our microstrain gauges had thicknesses on the micrometer scale, and were themselves used as the substrates to be oxidized. Stresses could be detected in both the SiO2 scales and the silicon and polysilicon substrates. In the SiO2 scales, the stresses were compressive and exhibited viscoelastic relaxation. The as-grown compressive stresses were greater for wet oxidation than they were for dry oxidation, and greater in scales grown on polysilicon than they were in scales grown on silicon. The viscosity of thermally grown SiO2 was the same whether scales formed by wet or dry oxidation, and the same for oxide scales on silicon and polysilicon. Significant compressive stresses were also generated in polysilicon during oxidation, but not in silicon.
I. INTRODUCTION
Thermal oxidation of silicon and polysilicon is essential to the processing of integrated circuits and has been the subject of active research for decades. The kinetics of thermal oxidation of silicon were first described by Deal and Grove in 19651 as linear-parabolic. The growth of SiO2 on silicon involves a large inherent volume expansion; the Pilling–Bedworth ratio2 (the molecular volume of SiO2 compared to the atomic volume of Si) is 2.04, and adding an oxygen atom within a Si–Si bond involves a linear expansion of 31%.3 The SiO2 layer is free to expand normal to the oxidizing surface but is constrained within the plane of the substrate, giving rise to large compressive stresses; furthermore, a substantial number of silicon interstitials is created at the scale-substrate interface during oxidation. On single-crystal substrates, most of the silicon interstitials flow into the oxide where they react with the incoming oxidizing species and become incorporated into the oxide scale, while a small fraction of the interstitials are injected into the bulk silicon.4 These self-interstitials can aggregate to form
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0017 J. Mater. Res., Vol. 21, No. 1, Jan 2006
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extrinsic stacking faults5 and also lead to oxidationenhanced diffusion of substitutional dopants.6 The compressive stresses generated in the thermal oxide on silicon decrease with increasing oxidation temperature and increasing oxidation time (i.e., with increasing oxide thickness) due to viscoelastic relaxation.7–14 While oxidation-induced stacking faults have been well studied, the oxidation-induced stresses gener
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