Paul Mertens

Determination of tunnelling parameters in ultra-thin oxide layer poly-Si/SiO2/Si structures

Abstract In this work the electron tunnelling in device grade ultra-thin 3–6 nm n + poly-Si/SiO 2 /n-Si structures has been analysed. The well known analytic expression for the Fowler-Nordheim tunnelling current was adapted to include the case of direct tunnelling of electrons, which becomes important for oxide layers thinner than 4.5 nm. For these ultra-thin oxide MOS structures it is necessary to take the band bending in the Si substrate and in the poly-Si layer into account to determine the oxide electrical field strength and to derive the tunnelling parameters of the measured current-voltage characteristic. A method is explained to derive the tunnel barrier height φ s and the effective mass of the tunnelling electron m ox from the experimental tunnel current characteristics. It is shown that both the direct tunnelling and the Fowler-Nordheim tunnelling current can be quantitatively explained by a WKB approximation using m ox as the single fitting parameter.

Defining the Transmurality of a Chronic Myocardial Infarction by Ultrasonic Strain-Rate Imaging Implications for Identifying Intramural Viability: An Experimental Study

Background—In a correlative functional/histopathologic study, we investigated the regional deformation characteristics of both chronic nontransmural and transmural infarctions before and after a dobutamine challenge. Methods and Results—After stenosing copper-coated stent implantation to produce circumflex artery endothelial proliferation, 18 pigs were followed up for 5 weeks. Posteuthanasia histology showed 10 to have a nontransmural and 8 a transmural infarction. Eight nonstented animals served as controls. Regional radial function was monitored by measuring ultrasound-derived peak systolic strain rates (SRSYS) and systolic strains (&egr;SYS) (1) before stent implantation and (2) at 5 weeks, at baseline (bs) and during an incremental dobutamine infusion. In controls, dobutamine induced a linear increase in SRSYS (dobutamine: bs, 4.8±0.4 s−1; 20 &mgr;g · kg−1 · min−1, 9.9±0.7 s−1;P <0.0001) and an initial increase of &egr;SYS at low dose (bs, 58±5%; at 5 &mgr;g · kg−1 · min−1, 78±6%;P <0.05) but a subsequent decrease during higher infusion rates. In the nontransmural group, bs SRSYS and &egr;SYS were significantly lower than prestent values (SRSYS, 2.9±0.5 s−1 and &egr;SYS, 32±6%, P <0.05 versus prestent). During dobutamine infusion, SRSYS increased slightly at 5 &mgr;g · kg−1 · min−1 (4.7±0.6 s−1, P <0.05) but fell during higher infusion rates, whereas &egr;SYS showed no change. For nontransmural infarctions, transmural scar extension correlated closely with &egr;SYS at bs (r =0.88). For transmural infarctions, SRSYS at bs was significantly reduced and &egr;SYS was almost not measurable (SRSYS, 1.8±0.3 s−1; &egr;SYS, 3±4%). Both deformation parameters showed no further change during the incremental dobutamine infusion. Conclusions—Ultrasonic deformation values could clearly differentiate chronic nontransmural from transmural myocardial infarction. The transmural extension of the scar could be defined by the regional deformation response.

Model for the current-voltage characteristics of ultrathin gate oxides after soft breakdown

The current–voltage characteristics of metal-oxide-semiconductor capacitors with a 4.2 nm SiO2 gate oxide are investigated. After the occurrence of soft breakdown, which is observed during constant current stress of the devices, the gate current is shown to behave like a power law of the applied gate voltage. We propose that this power law behavior is due to the formation of a percolation path between the electrons traps generated in the SiO2 layer during current stress of the capacitor. We describe a simple model which accounts for the current–voltage characteristics between two neighbor trapping sites, as well as a distribution of percolation thresholds in these (finite size) ultrathin SiO2 layers. The prediction of the model is in fair agreement with the experimental results in a large voltage range, and leads to a better description of the data than previously reported models. Furthermore, it is shown that this percolation model can also explain the temperature dependence of the gate current after the...

Soft breakdown in ultrathin gate oxides: Correlation with the percolation theory of nonlinear conductors

The dielectric breakdown under constant current stressing of 4.2 nm SiO2 gate oxides is investigated. After soft breakdown, which corresponds to an anomalous increase of the stress-induced leakage current of metal–oxide–semiconductor capacitors, the current behaves like a power law of the applied gate voltage VG. After soft breakdown, charge is further injected into the SiO2 layer in order to extract the effective resistivity ρeff of the system as a function of the density of oxide traps D generated in the layer. It is found that ρeff behaves like a power law of (D−Dc) where Dc is the critical density of traps generated at soft breakdown. These results are in fair agreement with the predictions of the percolation theory of nonlinear conductor networks. Besides, the value of the critical exponent related to the resistivity is close to the one expected in two dimensions.

Non-Gaussian behavior and anticorrelations in ultrathin gate oxides after soft breakdown

The time dependence of the gate voltage VG(t) after soft breakdown of metal-oxide-semiconductor capacitors with a 2.4 nm SiO2 layer has been measured. It is found that the VG(t) fluctuation distributions are non-Gaussian, but can be described by a Levy stable distribution. The long-range correlations in VG(t) are investigated within the detrended fluctuation analysis. The Hurst exponent is found to be H=0.25±0.04 independent of the value of the stress current density J. It is argued that these are universal features of soft breakdown and are due to trapping–detrapping of electrons in and away from the primary percolation path.

Cost-effective cleaning and high-quality thin gate oxides

Some recent findings in the area of wafer cleaning and thin oxide properties are presented in this paper. Results are shown for a practical implementation of a simplified cleaning concept that combines excellent performance in terms of metal and particle removal with low chemical and DI-water consumption. The effect of organic contamination on ultrathin gate-oxide integrity is illustrated, and the feasibility of using ozonated DI water as an organic removal step is discussed. Metal outplating from HF and HF/HCI solutions is investigated. Also, the final rinsing step is critically evaluated. It is demonstrated that Si surface roughness without the presence of metal contaminants does not degrade gate-oxide integrity. Finally, some critical remarks on the reliability measurements for ultrathin gate oxides are given; it is shown that erroneous conclusions can be drawn from constant-current charge-to-breakdown measurements.

Electrical properties of thin SiON/Ta2O5 gate dielectric stacks

The electrical characteristics of metal–oxide–semiconductor capacitors with SiON/Ta2O5 gate dielectric stacks with thin Ta2O5 layers (6–10 nm) are investigated. From the field and temperature dependence of the current of the gate stacks, it is shown that the main conduction mechanism at low bias is tunneling through the stack and that Poole–Frenkel conduction in the Ta2O5 layer becomes important at larger bias and temperature. From the analysis of the data in the high voltage and temperature range, taking into account the field distribution in both layers, the refractive index n of Ta2O5 and the energy level φB of traps involved in Poole–Frenkel conduction are found to be 2.3 and 0.85 eV, respectively. It is also shown that the gate current density of the stack is reduced by one to three orders of magnitude as compared to SiO2 layers with equivalent electrical thickness (2.5–3 nm). The temperature acceleration effect on the time-dependent dielectric breakdown is shown to be much reduced in the SiON/Ta2O5 ...

Evaluation of megasonic cleaning for sub-90-nm technologies

Cleaning of nanoparticles (< 50nm ) is becoming a major challenge in semiconductor manufacturing and the future use of traditional methods, such as megasonic cleaning, is questioned. In this paper the capability of megasonic cleaning to remove nanoparticles without inflicting damage to fragile structures is investigated. The role of dissolved gas in cleaning efficiency indicates that cavitation is the main cleaning mechanism. Consequently gas mass-balance analyses are needed to optimize the performance of cleaning tools. When gas is dissolved in the cleaning present tools can remove nanoparticles down to about 30 nm using dilute chemistries at low temperature. Ultimate performance is limited by cleaning uniformity, which depends on tool design and operation. However no tool reached the target of high particle removal efficiency andlow damage. Significantly lower damage could only be obtained by decreasing the power, at the cost of a lower cleaning efficiency for nanoparticles. The development of damage-free megasonic is discussed.

Impact of Organic Contamination on Thin Gate Oxide Quality

The impact of organic contamination on the quality of 5-nm-thick gate oxide structures, both before and after gate oxidation, is studied. Sources of organic contamination are chemical surface modification (i.e. hexamethyldisilazane priming), wafer box storage and extended vacuum exposure. Gate oxide integrity is evaluated electrically. The origin and/or nature of the organic contamination is seen to have different effects on the electrical breakdown. Care should be taken when exposing silicon wafers to organic contamination prior to processing. Especially when contamination occurs at the SiO2/polysilicon interface, i.e. prior to a non-oxidizing process step, organics can be extremely deleterious.

Saturation effects in TXRF on micro-droplet residue samples

Total reflection X-ray fluorescence spectrometry (TXRF) is a well-accepted technique for ultra-trace analysis of ultra-pure reagents and silicon wafers. Nevertheless, the technique’s linear range is not well characterized. In this paper, the upper limits of the linear range of TXRF on micro-droplet residues are identified and the origin of the non-linear effect is investigated. It is observed experimentally that a systematic decrease in the accuracy occurs as a function of the metallic content, starting from amounts above 3–10 ng, depending on the sample composition. A mass-absorption model for thin films is re-formulated for the micro-droplet residue samples and the model parameters are tested on the experimental observations. The calculations are in good agreement with the experimental data. Finally, solutions to deal with these saturation effects are discussed and a method to extend the dynamic range of TXRF is proposed.