Applications of Transmission Electron Microscopy and Secondary Ion Mass Spectrometry on Crystal Defect Analysis and Electronic Characterisation of Advanced 512Mb DRAM

摘要

In memory technology, the dynamic random access memory (DRAM) is one of the most frequently used integrated circuits. The reason for this is its lower price per bit by comparison with other semiconductor memories. In general, a DRAM cell consists of a MOSFET and capacitor to be a switch and charge storing unit, respectively. The cell is able to hold binary information in the form of stored charges on the capacitor. Fig. 1 shows the bit line (M0), borderless contact (CB), deep trench (DT) capacitor, transfer gate (GC) and active area (AA) structures (n-type) of a 110nm technology 512Mb trench DRAM. The basic DRAM constituents are shown in [1]. The capabilities of analytical transmission electron microscopy (TEM), such as high spatial resolution, micro-chemical analysis, etc. have led to an increasingly essential role for TEM-based analysis in process development, defect identification, yield enhancement, and root-cause failure analysis with the DRAM industry. Other authors [2] reported that more and more device leakage problems are related to crystal defects, such as dislocations, dislocation loops and stacking faults, due to high implant doses, shallow junctions, and feature size shrinkage, in particular, those defects which cross pn-junction or source/drain areas of transistors. It was reported that grain boundaries and intragranular defects act as electrical potential barriers and scattering sites, which decrease the carrier transport mobility and also serve as midgap states to increase the leakage currents [3]. Normally, these crystal defects are believed to be induced by implantation, poly etching, oxidation gate stack formation, and spacer nitride deposition processes. TEM is the most effective tool for detailed analysis of dislocations and crystal defects. In addition, it is well known that the device's performance and characteristics are highly dependent on the implanted dosage and resulting dopant depth distribution. For instance, the V_(t) of MOSFETs decreases as the channel length is shortened and also as the drain bias is increased [4]. The influencing factors of V_(t) performance are device geometry doping, gate oxide thickness and interface charge fluctuations [5]. Recently, the influences of doping profile and implantation on V_(t) behavior have been studied [6-7]. Secondary Ion Mass Spectrometry (SIMS) has the same sensitivity as the electrical test data of V_(t) on the dose variation [8]. Therefore, for new technology research and development of ULSI devices, SIMS also plays a very important role and has been widely used, especially to characterize ion implantation. SIMS applications are mainly to obtain quantitative information about contamination levels, dopant depth profiles and chemical composition information. Furthermore, SIMS has been used for manufacturing tool performance control and in-fab equipment troubleshooting. In this paper, we report several examples to carry out the applications of TEM and SIMS on crystal defect analysis and electronic characteristics of advanced 512 Mb DRAMs.