Glenn B. Alers

Intermixing at the tantalum oxide/silicon interface in gate dielectric structures

Metal oxides with high dielectric constants have the potential to extend scaling of transistor gate capacitance beyond that of ultrathin silicon dioxide. However, during deposition of most metal oxides on silicon, an interfacial region of SiOx can form that limits the specific capacitance of the gate structure. We have examined the composition of this layer using high-resolution depth profiling of medium ion energy scattering combined with infrared spectroscopy and transmission electron microscopy. We find that the interfacial region is not pure SiO2, but is a complex depth-dependent ternary oxide of Si–Tax–Oy with a dielectric constant at least twice that of pure SiO2 as inferred from electrical measurements. High-temperature annealing crystallizes the Ta2O5 film and converts the composite oxide to a more pure SiO2 layer with a lower capacitance density. Using low postanneal temperatures, a stable composite oxide structure can be obtained with good electrical properties and an effective SiO2 thickness of...

Ultra-thin gate dielectrics: they break down, but do they fail?

We study breakdown in high-quality 2-7 nm gate dielectrics, and find that soft breakdown becomes more likely for thinner oxides and for oxides stressed at lower voltages. For 2 nm oxides, an increase in gate noise is the only precise indication of soft breakdown. For many applications, devices should remain functional with the level of gate noise we have observed, after soft breakdown.

Defect dominated charge transport in amorphous Ta2O5 thin films

Ta2O5 is a candidate for use in metal–oxide–metal (MOM) capacitors in several areas of silicon device technology. Understanding and controlling leakage current is critical for successful implementation of this material. We have studied thermal and photoconductive charge transport processes in Ta2O5 MOM capacitors fabricated by anodization, reactive sputtering, and chemical vapor deposition. We find that the results from each of these three methods are similar if one compares films that have the same thickness and electrodes. Two types of leakage current are identified: (a) a transient current that charges the bulk states of the films and (b) a steady state activated process involving electron transport via a defect band. The transient process involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films <300 A thick, is dominated by a large number of defects, ∼1019–1020 cm−3, located near the metal–oxide interfaces. The interi...

Nitrogen plasma annealing for low temperature Ta2O5 films

A low temperature oxygen/nitrogen plasma process is reported that substantially reduces leakage currents in chemical vapor deposited (CVD) and physical vapor deposited (PVD) films of tantalum oxide. We show that a combination of nitrogen and oxygen in a remote downstream microwave plasma source reduces leakage currents in CVD films of tantalum oxide and also reduces trap densities as measured by charge pumping. The as deposited CVD films show a high level of photoluminescence that is substantially lowered by the plasma anneal due to a reduction in the density of midgap states. For films deposited by PVD in the thickness range of 100 nm we find low leakage currents with a substantial improvement from the introduction of nitrogen into the plasma. However, PVD films in the thickness range of 20 nm show larger relative leakage currents and less of an improvement from the addition of nitrogen. The role of nitrogen in lowering leakage currents and charge trapping is thought to occur from a reduction in the dens...

Interlevel dielectric failures in copper/low-k structures

Failure modes for inter-level dielectric layers under accelerated test conditions have been evaluated for a range of dielectric diffusion barriers in copper/low-k structures. The dominant failure mechanism for both constant voltage tests and ramped voltage tests was mechanical cracking at the dielectric barrier/low-k interface. Few occurrences of copper diffusion through the bulk ILD were observed. A simple model for the dominant failure mechanism is proposed which hypothesizes crack formation due to the electrostatic force between interdigitated lines followed by copper extrusion into the cracks. The proposed model is consistent with measurements of interfacial adhesion strengths in Cu/low-k stacks.

Tunneling current noise in thin gate oxides

We have examined fluctuations in the tunneling current of 3.5 nm SiO2 barriers for voltages in the direct tunneling regime. We find a 1/f power law for the spectral density of the fluctuations where f is the frequency. This 1/f noise can be attributed to fluctuations of a trap assisted tunneling current through the oxide that causes current noise but is not evident in the I–V curves. We suggest that this noise may be a more sensitive probe of trap assisted tunneling and degradation in thin oxides than other measures. At voltages above a threshold of 2.5 V, we observe the reversible onset of non‐Gaussian current transients in the noise. The onset of these current transients can be related to a transition in the spacial uniformity of the tunneling current density that may result in eventual breakdown of the oxide.

THERMAL STABILITY OF TA2O5 IN METAL-OXIDE-METAL CAPACITOR STRUCTURES

Using x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, we have studied structures used in metal–oxide–metal capacitors including Ta2O5/TiN/Ti, Ta2O5/Ti, Ta2O5/TaN/Ti, Ta2O5/WN/Ti, and Ta2O5/M, where M=Ta, Pt, W, Al, and Si. We find that Ti and Al are able to reduce the Ta2O5 to Ta, forming oxides of Ti and Al, respectively. The diffusion barriers TiN, TaN, and WN hamper the diffusion of oxygen and therefore postpone the reduction of Ta2O5 to higher temperatures. As judged by the temperatures at which the reduction of Ta2O5 occurs, TaN and WN are more effective oxygen-diffusion barriers than TiN. We observe no oxygen remaining in the diffusion barrier when a Ti layer is present underneath. We observe no reduction of Ta2O5 when M=Pt, W, or Si.

Advanced dielectrics for gate oxide, DRAM and RF capacitors

A new class of high dielectric constant materials is presented based on binary and ternary amorphous metal oxides. These films are compatible with low temperature processing and can be used with conventional metal electrodes like TiN. Metal alloys of both tantalum oxide and titanium oxide will be discussed with dielectric constants as high as 62 and leakage currents of less than 10/sup -8/ A/cm/sup 2/ at 1 MV/cm.

1/f resistance noise in the large magnetoresistance manganites

Resistance noise has been measured in thin films of La0.6Y0.07Ca0.33MnO3 at the ferromagnetic metal to paramagnetic‐insulator transition where a large magnetoresistance is observed. Fluctuations in resistance were found to have a 1/f power law form throughout the transition where f is frequency. This intrinsic 1/f noise is much larger than what is typical for metallic films and is consistent with the fluctuation‐dissipation theorem and magnetization fluctuations in the ferromagnetic phase. The implications of this large low frequency noise for practical magnetoresistive devices will be addressed.

Trap assisted tunneling as a mechanism of degradation and noise in 2-5 nm oxides

The mechanism of stress induced leakage current and dielectric breakdown is examined through 1/f noise in the tunneling current of 1.7-5 nm oxides. Before breakdown occurs, we find a linear relationship between low frequency 1/f noise and the increased current due to stress. This behavior can be described by a model of trap assisted tunneling for both phenomena. We develop a quantitative new model for the noise in terms of fluctuations in a trap assisted tunneling current through the oxide and show that the traditional charge-state fluctuation model is inconsistent with the voltage scaling of the noise. Our results demonstrate that noise can be used as a very sensitive measure of interface states, with a higher sensitivity than conventional capacitance-voltage relations. We show that the conduction mechanism in stressed and unstressed oxides is fundamentally different with the tunneling current in the unstressed oxides dominated by the fundamental limit of direct tunneling. Finally, noise in the post-breakdown state is used to understand the softening of breakdown at lower stressing conditions.