Ultrathin oxide-nitride dielectrics for rugged stacked DRAMcapacitors

Ultrathin dielectric materials that provide high capacitance values are needed for 64- and 256-Mb stacked DRAMs. It is shown that capacitance values as high as 12.3 fF/μm² can be obtained with ultrathin nitride-based layers deposited on rugged polysilicon storage electrodes. These films present the reliability and low leakage current levels required for 3.3-V applications. The nitride thickness, however, cannot be scaled much below 6 nm to avoid the oxidation-punchthrough mechanisms that appear when too-thin films are unable to withstand the reoxidation step     

X-Ray Radiation Effect in DRAM Retention Time

For quality verification, an X-ray inspection process is commonly being used for evaluating obscured and defective solder joints in surface-mount technologies, such as ball grid arrays and flip chips. Integrated circuits subjected to any form of radiation, i.e., ionizing or nonionizing, may incur some amount of damage depending on the absorbed dose. Though most X-ray inspections for high-quality imaging require ionizing dose amounts that are considered inconsequential for device failure or non-functionality, the degree of latent damage must be carefully considered. This paper discusses X-ray-induced vulnerabilities of high-density dynamic random access memory exposed to low ionizing radiation levels typical in X-ray inspection systems. We look at critical parameters and their sensitivity in relation to varying dose amounts of X-ray irradiation. In consideration of different methodologies of reducing radiation dose amounts and limiting device exposure, we propose a procedure for attenuating potentially harmful X-ray radiation levels while preserving quality images.     

Hot-carrier reliability in double-implanted lightly doped drain devices for advanced DRAMs

Hot-electron degradation in short-channel (0.50 mu m and 0.83 mu m) double-implanted lightly doped drain (DI-LDD) devices was characterised using DC stress tests. Compared to lightly doped drain (LDD) devices of the same effective channel length L/sub eff/, the measurements indicate that channel hot-electron injection is more prevalent in devices with the p/sup +/-pocket implant due to a higher peak channel electric field. Degradation is more severe in both the drain current and transconductance. However, an improvement in short-channel effects was seen in DI-LDD devices over LDD devices. For the same L/sub eff/, the punch-through voltage was higher and the subthreshold swing lower for the DI-LDD devices.<>     

Uniqueness in activation energy and charge-to-breakdown of highly asymmetrical DRAM Al/sub 2/O/sub 3/ cell capacitors

Al/sub 2/O/sub 3/ cell capacitors for dynamic random access memory (DRAM) applications were tested using constant voltage, time-dependent dielectric breakdown (TDDB) tests. The capacitors had area-enhancing, hemispherical grain (HSG) polysilicon as bottom electrodes (BEs). These electrodes acted as points of high electric field, and eased charge injection into the Al/sub 2/O/sub 3/. As a result, the capacitors had highly asymmetric current-voltage (I-V) characteristics. Time-to-fails (TTFs) were polarity-dependent, and, thus, much worse for HSG injection. However, activation energy (E/sub a/) and charge-to-breakdown (QBD) obtained from conducting stress under opposite polarities were a unique function of the electric field only. The results point to a common, polarity-independent mechanism responsible for final breakdown, and the possibility that only the kinetics of degradation is electrode controlled. Good correlation with the thermochemical E model suggests that the breakdown mechanism in Al/sub 2/O/sub 3/ might be similar to SiO/sub 2/.     

Dynamic-stress-induced dielectric breakdown in ultrathinnitride/oxide stacked films deposited on rugged polysilicon

The dielectric breakdown characteristics of thin reoxidized Si₃N₄ films on both smooth and rugged poly-Si have been studied under dynamic stressing (unipolar and bipolar) with frequencies of up to 500 kHz. For capacitors with smooth poly-Si the time to breakdown (t_BD) increases with frequency under unipolar stressing with positive gain bias, whereas it slightly decreases with frequency under unipolar and bipolar stressing with negative gate bias. For capacitors with rugged poly-Si, t_BD increases with frequency in all cases     

Consequence analysis in LPG installation using an integrated computer package

This paper presents the prototype of the computer code, Atlantide, developed to assess the consequences associated with accidental events that can occur in a LPG storage plant. The characteristic of Atlantide is to be simple enough but at the same time adequate to cope with consequence analysis as required by Italian legislation in fulfilling the Seveso Directive. The application of Atlantide is appropriate for LPG storage/transferring installations. The models and correlations implemented in the code are relevant to flashing liquid releases, heavy gas dispersion and other typical phenomena such as BLEVE/Fireball. The computer code allows, on the basis of the operating/design characteristics, the study of the relevant accidental events from the evaluation of the release rate (liquid, gaseous and two-phase) in the unit involved, to the analysis of the subsequent evaporation and dispersion, up to the assessment of the final phenomena of fire and explosion. This is done taking as reference simplified Event Trees which describe the evolution of accidental scenarios, taking into account the most likely meteorological conditions, the different release situations and other features typical of a LPG installation. The limited input data required and the automatic linking between the single models, that are activated in a defined sequence, depending on the accidental event selected, minimize both the time required for the risk analysis and the possibility of errors. Models and equations implemented in Atlantide have been selected from public literature or in-house developed software and tailored with the aim to be easy to use and fast to run but, nevertheless, able to provide realistic simulation of the accidental event as well as reliable results, in terms of physical effects and hazardous areas. The results have been compared with those of other internationally recognized codes and with the criteria adopted by Italian authorities to verify the Safety Reports for LPG installations. A brief of the theoretical basis of each model implemented in Atlantide and an example of application are included in the paper.     

Hot-carrier-induced degradation of gate dielectrics grown innitrous oxide under accelerated aging

Gate oxides grown with partial and complete oxidation in N₂ O were studied in terms of hot-carrier stressing. The DC lifetime for 10% degradation in g_m had a 15×improvement over control oxides not grown in a N₂O atmosphere. Further improvement in g_m degradation was observed in oxides that received partial oxidation as compared with control oxides. This improvement is due to the incorporation of nitrogen that reduces strained Si-O bonds at the Si/SiO₂ interface, leading to lower interface state generation (ISG). Improvements were also observed in I_g-V_g characteristics, indicating a reduction of trap sites both at the Si/SiO₂ interface and in the bulk oxide. Improved gate-induced drain leakage (GIDL) characteristics as a function of hot-carrier stressing for partial N₂O oxides were observed over control oxides. However, severe drain leakage that masked GIDL was observed on pure N ₂O oxides and is a subject for further study