Study of Stress-Induced Leakage Current in Thin Oxides Stressed by Corona Charging in Air: Relationship to Goi Defects
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ABSTRACT Corona charging in air combined with non-contact oxide charge measurement with a contact potential difference probe provides an unique possibility for fast monitoring of electron tunneling characteristics without preparation of MOS capacitors. It has also been found tha corona charging of thin oxides in the tunneling range is very effective in generating stressinduced leakage current. In this work we demonstrate the sensitivity of the corona stressinduced leakage current magnitude to gate oxide integrity defect density. The experimenta results cover three of the most common gate oxide integrity defects, namely: 1- the defect induced by heavy metals (Fe.Cu) at a practically important low concentration range of lxlI(J to lx10 11 atoms/cm 3: 2 - the defects originating from interface roughness and 3 -the defects related to crystal originated particles. At low corona stress fluence, these defects play no role in the tunneling characteristics which follow ideal Fowler-Nordheim characteristics for oxides 50A or thicker and a contribution from a direct tunneling current for thinner oxides. At high corona stress fluences, gate oxide integrity defects control the magnitude of stress-induced leakage current measured at constan oxide field. It is suggested that the gate oxide integrity role is associated with the enhanced rate of the trap generation during stress. It is noted that the present findings employ a novel methodology for gate oxide integrity monitoring based on corona charging and contact potential difference measurement. INTRODUCTION Corona Oxide Characterization of Semiconductor (COCOS) [ 1] belongs to methods using a deposition of electric charge on dielectric by corona discharge, followed by contact potential difference (CPD) measurement. Historically, such an approach was introduced in 1968 by Williams and Willis [2]. They used a needle-type corona electrode to charge a small surface site. After charging the site was positioned under a transparent Kelvin probe for measurement of CPD and surface photovoltage versus deposited charge or versus time after charging [2,3]. In an attempt to follow MOS-type measurements, Verkuil and Fung proposed a version of this approach producing a localized corona induced junction analogous to a biased MOS capacitor [4.5]. Verkuil'". approach is used in commercial Quantox tools and is referred to as Corona Oxide Semiconductor (COS). The whole wafer corona charging method used in the COCOS method was introduced b Lagowski and coworkers [1,6,7]. It gives the possibility for whole wafer mapping of electrica parameters and eliminates surface charge gradients that are inherent in COS which cause charge dissipation due to surface conductance. Experimental data on the surface potential barrier, VsB, and the oxide voltage, Vox, versus the corona charge dose, available in COCOS, made it possible to develop straightforward 345 Mat. Res. Soc. Symp. Proc. Vol. 592 ©2000 Materials Research Society
procedures for determining many parameters without the complexity inherent in MOS method
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