Detection and Characterization of Stress-Induced Defects in Gate SiO₂ Films by Conductive Atomic Force Microscopy

Abstract

We have demonstrated the detection of nanometer-scale current-leakage sites in electrically stressed gate SiO2 films using a conductive atomic force microscope (C-AFM). Prior to C-AFM observations, the gate SiO2 films in metal-oxide-semiconductor capacitors were subjected to constant-current Fowler-Nordheim (FN) stress. Details of image contrasts and the relationship between the surface topography and the current image of the SiO2 films were examined. Two types of contrast were clearly observed in the current image: a sharp bright spot reflecting local current leakage were caused by hole trapping at stress-induced defects and a fuzzy bright contrasts originating from the SiO2 thickness fluctuation. The dependence of C-AFM images on the electron injection direction during FN stress application and the SiO2 film thickness clearly reveals that the stress-induced defects are distributed in the region within 2.6 nm from the SiO2/Si substrate interface.

Local Current Leakage Characterization in La2O3-Al2O3 Composite Films by Conductive Atomic Force Microscopy

“…This phenomenon suggests current conduction via defects existing in the film. On the other hand, the leakage currents from the spots (4) and (5) drastically increase with increasing the oxide voltage. The I-V characteristic at the high voltage region can be seen as a F-N tunneling conduction with a shift to the low voltage, as shown in Fig.…”

“…Conductive atomic force microscopy (C-AFM) is a powerful tool to analyze the local morphological and electrical properties in gate dielectric films [1,2] . We have elucidated thus far local current leakage and degradation processes in stressed SiO 2 films using C-AFM [3][4][5][6][7] .…”

Correlation Between Surface Topography and Static Capacitance Image of Ultrathin SiO2 Films Evaluated by Scanning Capacitance Microscopy

“…To date, oxide thickness studies have been typically related to current-sensing AFM (C-AFM) measurements. 2,23,25,26) Compared with the C-AFM method, our dC=dZ imaging technique has an advantage of obviating any need of DC offset bias for imaging. Thus we can evaluate the variation in oxide film thickness without considering deteriorating effects, such as additional anode oxidation or surface roughening due to charge injection.…”

“…For ultrathin SiO 2 or SiON gate dielectrics, the direct tunneling current through the oxide film cannot be neglected as a source of leakage current. 1) Furthermore, a small variation in SiO 2 film thickness induces local gate leakage spots 2) as well as a significant change in oxide capacitance (C ox ) thus deteriorating carrier mobility within a channel region. Therefore, the uniform thickness of an ultrathin SiO 2 film is crucial for assuring the reliability of the future generations of MOSFETs.…”

Behavior of Local Charge-Trapping Sites in La2O3–Al2O3 Composite Films under Constant Voltage Stress

“…10) The distribution of trapped holes and their trapping and detrapping phenomena have been revealed by observing the time dependence of local leakage current of the gate SiO 2 films by C-AFM. [11][12][13] The influence of trapped holes on the breakdown processes has been also clarified. 14) La 2 O 3 -Al 2 O 3 composite films are one of the promising candidates for next-generation gate high-k films because of their comparatively large dielectric constants and electron barrier height for Si.…”