EBIC and XTEM Analysis of High Voltage SMOS Reliability Failures
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EBIC and XTEM Analysis of High Voltage SMOS Reliability Failures Larry Rice Process and Materials Characterization Laboratory Digital DNA Lab, Motorola, Inc., 2200 W. Broadway Rd. , Mesa, AZ 85202 ABSTRACT Microscopists are faced with many challenges in locating and examining failure sites in the ever-shrinking semiconductor device. The site must be located using electrical characterization techniques like electron beam induced current (EBIC), photo emission microscopy (PEM) or liquid crystal (LC) and then cross-sectioned with a focused ion beam (FIB). Both PEM and LC require the semiconductor circuit to be running near operating conditions which has been observed to locally melt the area of interest, frequently destroying evidence of the failure mechanism. In contrast, EBIC typically can be accomplished at low or no applied voltage eliminating further damage to the circuit. EBIC has been applied to locate leakage sites in high voltage metal oxide semiconductor (MOS) electro static discharge (ESD) reliability failures. In addition to a brief revisit of the basic principles of EBIC and describing a technique to successfully cross section ‘hot spots’ for transmission electron microscopy (TEM) observation, focus will be placed on a case study of the reliability testing failure analysis of ESD power transistors using EBIC, SEM, focused ion beam (FIB), and XTEM. INTRODUCTION Electron beam induced current (EBIC) is a useful technique for locating defects in MOS and bipolar diodes, transistors, and capacitors where the scanning electron microscope (SEM) beam induces a current in the sample. Though the technique outlined in this paper may be correctly termed tunneling current microscopy [1], EBIC is the common term used in the semiconductor industry for the failure analysis of semiconductor devices. Often during yield enhancement efforts the failure analyst is asked to determine the mechanism for PC (process control) structure (which may contain perhaps thousands of individual devices in one structure) failures. Blind cross sections rarely give meaningful evidence. EBIC can be used to pinpoint the bad site which is then precision cross-sectioned using the FIB. In some cases, examination of the defect crosssection with the SEM only reveals where the failure occurred, either by localized enhanced etching or no etching at the site, and gives no materials information. This is where TEM observation becomes critical. However defects in the 1000Å range are very difficult to successfully cross section for TEM. This paper outlines a method of using the FIB and SEM to cross section defects for thinning the sample for TEM imaging. When an electron beam interacts with a semiconductor such as silicon, electron-hole pairs are created when the incident beam transfers enough energy to eject an electron from the silicon atom thereby creating ‘free’ electrons. The lifetime of the ‘free’ electrons is very short (in terms of distance they may be free to move about the area a couple of microns) [4]. They recombine with a neighboring silicon ato
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