R. M. Fleming

Discovery of a useful thin-film dielectric using a composition-spread approach

The continuing drive towards miniaturization of electronic devices is motivating the search for new materials. Consider, for example, the case of the much-used dynamic random-access memory. The minimum capacitance per cell that can be tolerated is expected to remain at 30–40u2009fF, but as the cell area decreases, the corresponding reduction in geometric capacitance has to be compensated for. So far, this has been achieved by resorting to complex non-planar structures and/or using much thinner films of the dielectric insulator, amorphous silicon dioxide (a-SiOx), although the latter approach is limited by the electric fields that can be supported by a-SiOx before its insulating properties break down. An alternative strategy is to develop thin-film insulators that have a dielectric constant significantly greater than that of a-SiOx, reducing the size of the fields required for device operation. Here we show that a composition-spread technique allows for the efficient evaluating of materials with both a high dielectric constant and a high breakdown field. We apply this approach to the Zr–Sn–Ti–O system, and we find that compositions close to Zr0.15Sn0.3Ti0.55O2−δ are better thin-film dielectrics than high-quality deposited a-SiOx. Although detailed tests of the performance of these materials have not yet been carried out, our initial results suggest that they are likely to be comparable to the best alternatives (such as (Ba, Sr)TiO3) currently being considered for integrated-circuit capacitors.

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, ∼1019–1020u200acm−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...

Diffraction Symmetry in Crystalline, Close-Packed C60

We have grown crystals of the carbon structure C60 by sublimation. In contrast to solution-grown crystals, the sublimed crystals have long range order with no evidence of solvent inclusions. Sublimed C60 forms three dimensional, faceted crystals with a close-packed, face-centered cubic unit cell. We have refined a crystal structure using the soccer ball model of the C60 molecule. The results indicate that the C60 molecule has the expected spherical shape, however the data are not sufficiently accurate to unambiguously determine atomic positions.

HIGH DIELECTRIC CONSTANT HF-SN-TI-O THIN FILMS

High dielectric constant Hf–Sn–Ti–O thin-film materials were identified using a compositional-spread approach. Thin films of composition Hf0.2Sn0.05Ti0.75O2 prepared at 250u200a°C have excellent dielectric properties: 40–70-nm-thick films with a dielectric constant of 40–60 were obtained, depending on the processing conditions, yielding a specific capacitance of 9–17 fF/μm2. Breakdown fields were measured to be about 3–4 MV/cm, yielding a figure of merit ee0Ebr∼19u200aμC/cm2. Leakage currents, measured at 1 MV/cm, were in the range 10−7−10−6u200aA/cm2.

Band structures of organic thin-film transistor materials

We report extended Huckel theory (EHT) band structure calculations for the high-temperature polymorph of α-6T (α-sexithiophene) and C60, two organic materials that have shown promising device characteristics, as the active element in thin-film transistors (TFTs). We also report calculations for α-3T and κ-(ET)2Cu(NCS)2[where ET = bis(ethylenedithio)tetrathiafulvalene] for comparison purposes. Both α-6T/HT and C60 show well developed band dispersions. Whereas C60 is an isotropic three-dimensional (3D) molecular solid, α-6T/HT shows a strongly 2D electronic structure, and the band structure is quite similar to that of the ET organic superconductors such as κ-(ET)2Cu(NCS)2. Factors important in determining the mobilities of organic semiconductors are reviewed.

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.

Material and electrical characterization of carbon-doped Ta2O5 films for embedded dynamic random access memory applications

This work is a systematic study of carbon incorporation in Ta2O5 and its effect on the material and electrical properties of Ta2O5, a promising replacement for silicon oxide in embedded dynamic random access memory applications. Using pulsed-dc reactive and rf-magnetron sputtering of Ta2O5 performed in an argon/oxygen/carbon-dioxide plasma, we have methodically doped the Ta2O5 films with carbon. In thick (70 nm) Ta2O5 films, an optimal amount (0.8–1.4 at.u200a%) of carbon doping reduced the leakage current to 10−8u200aA/cm2 at +3 MV/cm, a four orders of magnitude reduction compared to a leakage current of 10−4u200aA/cm2 in an undoped Ta2O5 film grown in similar conditions without CO2 in the plasma. This finding suggests that carbon doping can further improve the dielectric leakage property at an optimal concentration. X-ray Photoemission Spectroscopy analysis showed the presence of carbonate (carbon bonded to three oxygen) in these electrically improved carbon-doped films. Analysis by high-resolution transmission ele...

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.

Growth and measurements of ferroelectric lead zirconate titanate on diamond by pulsed laser deposition

Pb(Zr0.53Ti0.47)O3 (PZT) on diamond is a potentially robust structure for surface acoustic wave (SAW) device applications. We have studied the growth and physical characteristics of PZT on diamond and other substrates by pulsed laser deposition. Under a broad range of processing conditions we explored, PZT deposited directly on diamond is almost exclusively pyrochlore-type, which is nonferroelectric. Growth of ferroelectric perovskite PZT is promoted via the use of a PbTiO3 buffer layer within a narrow window of processing parameters [i.e., P(O2)=100–200u200amTorr, T=550–650u200a°C, 1–2 J/cm2]. Similar results were also obtained for deposition of PZT on Si, Pt-coated Si, and Pt-coated diamond substrates. The dielectric constants of the perovskite PZT films are 500–650 at 1 V and 100 kHz. The piezoelectric coefficients of these films are in the range of 50×10−12–350×10−12u200am/V. The SAW velocity of perovskite PZT films is similar to that of highly oriented sputter deposited ZnO films. The acoustic attenuation in per...