We describe: i) nitride-layer optimization; ii) device fabrication: and iii) electrical properties of a-Si:H thin film transistors, TFTs, that integrate oxide/nitride dielectrics into an inverted, staggered gate structure. We have systematically changed the concentrations of Si-Si, Si-H and Si-NH bonding groups within the deposited nitride layers by varying the source gas flow ratio, R = NH3/SiH4 from 2.5 to 12.5. The electrical characteristics of the TFTs improve significantly as the gas phase ratio R is increased from 2.5 to approximately 10, and then decrease as R is further increased. The performance of the TFTs peaks for a source gas ratio of -10, where the channel mobility is ∼1.4 cm2/V-s, the threshold voltage is 2.3 V; and the Ion to Ioff current ratio is > 105.These increases in performance can only realized in devices in which the back of the Si channel region is passivated with an oxy-nitride interfacial region.

The asymmetric amorphous silicon thin film transistors are fabricated and exposed to various stress environments. A visible light illumination of 200,000 Ix and gate bias of 30 V are applied to both asymmetric and widely used symmetric a-Si TFT's. It is observed that the leakage current of asymmetric structure, where only one electrode is fully overlapped by gate electrode, is much less than that of symmetric one. The visible light illumination as well as gate bias stress do not degrade the leakage current of the asymmetric a-Si TFT's, while the leakage current in die symmetric TFT's are increased considerably due to the stress. Also, the degree of degradation in the threshold voltage, the field effect mobility and the subthreshold slope of asymmetric TFT's are relatively much less than that of conventional symmetric TFT's.

A detailed in situ study, by Infrared Phase Modulated Ellipsometry (IRPME), of interfaces between amorphous silicon (a-Si:H) and silicon nitride (a-SiNx) is presented. A behaviour compatible with a sharp interface is observed when a-SiNx is deposited on top of a-Si:H, the underlayer material being very weakly influenced by the deposition of the overlayer. In contrast a graded transition is observed when a-SiNx is deposited first. In the latter case, the IR measurements directly reveal a nitrogen incorporation in the first monolayers of a-Si:H together with an increase of intensity of the SiH bonds at the interface.

The electrical characteristics of n- and p-channel poly-Si thin film transistors having a double layer gate dielectric structure are reported. The gate dielectric consists of a silicon dioxide layer and a nitrogen-rich silicon nitride layer, both deposited by PECVD at low temperatures (≥400° C). When the silicon nitride was in contact with the poly-Si film, the effective carrier mobility (μeff), threshold voltage (Vth and subthreshold swing (St) for n-channel devices were 36 cm2/Vsec, -1.8 V and 1.65 V/decade, respectively, while for p-channel devices were 6 cm2/Vsec, -37 and 2.47 V/decade, respectively. These devices were not stable under negative gate bias stress, due to the injection of holes into the silicon nitride. When silicon dioxide was in contact with the poly-Si film, the μeff, Vth and St for n-channel devices were 26 cm2/Vsec, 3 V and 1.63 V/decade, respectively, while for p-channel devices were 10 cm2/Vsec, -22 V and 1.52 V/decade, respectively. These devices were stable under d.c. bias stress.

Silicon dioxide films have been grown on polysilicon films at low temperatures in dry ambients by utilizing NF3 - enhanced oxidation, which affords markedly enhanced oxidation kinetics. This facilitates thin film transistor applications, for which low thermal budget processing is essential. The grown oxide films are thicker than corresponding oxides grown on (100) single crystal silicon. Thin film transistors having a polysilicon self-aligned gate structure were fabricated on polycrystalline silicon. The gate oxide films were thermally grown by NF3 -enhanced oxidation at 650°C or 800°C. Electrical characterization of the TFTs having 800°C gate oxides showed on/off current ratios up to 5×107, effective electron mobilities (μneff) of 26 to 38 cm2/Vsec, threshold voltage (VT) values of 0±0.7 V and subthreshold swing (S) values as low as 0.3 V/dec. The devices were stable under dc electrical stressing at a field of 3 MV/cm. Further, the source to drain current activation energy was determined as a function of the gate voltage. The 650°C gate oxide TFTs exhibited on/off current ratio of 105, VT of 3.5 V, Uneffof10 cm2/V.sec and S of 0.7 V/dec.

Oxide-nitride-oxide (ONO) stacked films deposited by plasma enhanced chemical vapor deposition (PECVD) have been studied for use as the gate dielectric in active matrix displays. Good electrical properties were obtained for cadmium selenide thin film transistors (TFTs) with either ONO or single layer SiO2 gate insulation. However, the dielectric strength was up to twice as large for the ONO films. This improvement was observed with both Al and Cr electrodes.

The individual oxide layers had compressive stress, while the individual nitride layers had tensile stress. When combined in an ONO structure, a net tensile stress of relatively low magnitude was obtained. Index of refraction and infrared absorption measurements indicated that the oxide films were oxygen deficient. Hydrogen incorporation was present for both the oxide and nitride films.

This work describes process development of various kinds of compound sidewall spacer in submicron CMOS technology to improve gate oxide (GOX) leakage characteristics. This is attained by minimizing the impact of gate-level defects (GLDs) caused by in-process particle incorporation. Transistor characteristics did not suffer from this new sidewall spacer process.

SiBN ternary and SiOBN quaternary films prepared by rf-plasma and/or ECR-plasma CVD have been proposed, and this paper evaluates the relationship between film structure and dielectric constant as well as the other film properties. The SiBN films contain Si-N and B-N bonds, and the dielectric constant of the SiBN films reduces with increasing proportion of B-N bonds in the films. The SiBN films are less hygroscopic than BN films and they are therefore more stable. For the SiOBN films, the effects of oxygen doping on the dielectric constant and on breakdown strength are discussed in terms of the film composition and bonding configuration. The dielectric constant decreases with increasing oxygen atomic ratio and reaches a minimum when the amount of oxygen is equal to the amount of silicon. The effectiveness of this low-dielectric-constant material as an interlay-er is confirmed in actual VLSIs with planarized two-level metallization using the SiBN interlayer.

The porosity of sol-gel thin films may be tailored for specific applications through control of the size and structure of inorganic polymers within the coating sol, the extent of polymer reaction and interpenetration during film formation, and the magnitude of the capillary pressure exerted during the final stage of drying. By maximizing the capillary pressure and avoiding excessive condensation, dense insulating films may be prepared as passivation layers on silicon substrates. Such films can exhibit excellent dielectric integrity, viz., low interface trap densities and insulating properties approaching those of thermally grown SiO2. Alternatively, through exploitation of the scaling relationship of mass and density of fractal objects, silica films can be prepared that show a variation in porosity (7–29 %) and refractive index (1.42 ndash;1.31) desired for applications in sensors, membranes, and photonics.

The molecular structure of precursors in sol-gel processing of PbTiO3 is modified via β-diketone substitution for the isopropyl groups of titanium isopropoxide. Crystallite size and crystallization behavior are studied using x-ray diffractometry and differential thermal analysis. The crystallite size decreases with increasing amount of β-diketone substitution. Whereas the weight percent of crystalline phase first decreases and then increases with increasing β-diketone substitution. Our results indicate that the β-diketone substitution reduces the size of the structural units in the gel network resulting in smaller crystallite size and more nucleation sites in the powders produced.

In this paper we will present recent experimental results, using electron holography and electron interferometry, of studies of ferroelectric domain walls in BaTiO3 thin films. Unlike conventional TEM diffraction contrast imaging of ferroelectrics, the new technique that we have developed not only allows direct visualization of ferroelectric domain walls and electrostatic field distributions in the vicinity of the domain wall, but also enables quantitative measurement of domain wall width and local polarization. We have measured 90° domain wall width to be between 20 to 50 Å for a BaTiO3 thin specimen. The variation of polarization across the domain wall will be shown to be close to that predicted by the Zhirnov model. The value of the measured spontaneous polarization is about 1.5×10−5 C/cm2, which is closed to the bulk macroscopically measured value.

Electron paramagnetic resonance (EPR), photo-thermal deflection spectroscopy (PDS), and electrical measurements have been used to characterize as-received and UV-lluminated lead lanthanum zirconate titanate (PLZT) and PZT ceramics. Following optical illumination we observe the activation of positively charged Pb+3 and negatively charged Ti+3 ions, as well as an increased broad absorption peak around 2.6 eV. The spatial distribution of the induced absorption correlates well with the location of the absorbed UV, suggesting that photo-produced carrier pairs are trapped at Ti+4 and Pb+2 producing the observed paramagnetism. The Ti+3 EPR spectra were successfully fit using the crystal field parameters derived from the PDS spectra. Preliminary results suggest that these charged paramagnetic ions can influence the electrical properties of PZT thin films. We investigated the effects of UV light and dc bias sequences by polarization-electric field (PE) measurements and find that the coercive fields are dependent on the sign of the applied bias during UV exposure. It is reasonable to expect that Pb+3 and/or Ti+3 ions act as the charge trapping sites which cause the threshold voltage shifts in the PE loop.

Ta2O5 films suitable for DRAM use have been deposited on silicon, polysilicon, and SiO2 by LP-CVD from Ta (OC2H5)5 and O2. Uniformity, reproducibility, and conformality are excellent. Annealed films are polycrystalline, and their surfaces are characterized by 2 nm high, 1 mgr;m diameter nucleation centers surrounded by circular crystallization fronts. The electrical properties of annealed films are dominated by a SiO2-rich layer which forms between the Ta2O5 and the silicon or polysilicon substrate.

Ta2O5 films of high reliability and low leakage current density were obtained by low temperature deposition and subsequent high temperature oxygen anneal. At higher temperatures than 410°C, growth was governed by the formation of radicals in gas phase and oxidation on the surface, while at lower temperatures by the dissociation of reactant on the surface of substrates. As a result, the films deposited at lower temperatures had undensified structures, and contained more carbon that might be a leakage current source in Ta2O5 film. During post-deposition heat treatment in 800°C oxidating ambient, carbon was removed away and silicon was diffused from the substrate into the Ta2O5 film efficiently for its as-grown porous structure. After oxygen anneal, low temperature films get denser and are crystallized to mixed phase of orthorhombic and hexagonal Ta2O5, while high temperature films crystallized to orthorhombic single phase. Ta2O5 capacitor with low temperature films showed superior leakage characteristics applicable to sub-half micron memory devices.

Tantalum oxide is a widely used insulator in electronic applications requiring high permittivity. When deposited at low temperature, tantalum oxide films, however, often exhibit large leakage current. A common way to reduce leakage is to anneal the films in an ambient containing extremely reactive oxygen species, or at high temperature in dry oxygen. A different approach is to use composite oxide materials. We have studied layered tantalum based oxide films deposited by Atomic Layer Epitaxy, and observed that the leakage current is decreased by several orders of magnitude when a fraction of tantalum oxide is replaced by another oxide, such as aluminum or hafnium oxide. Leakage current density of 40 nA/cm2 in the electric field of 1 MV/cm is attained for unannealed Ta-Hf-O film deposited at 300°C. Layered composite insulators are an interesting new class of materials, and ALE proves to be a useful method for depositing them.

Effects of the growth conditions on the growth and the quality of thin films of Al2O3 were investigated in the ALE-like process. It has been found that the growth rate remains nearly independent of the growth conditions when the pulse heights are 80–120 mTorr and 20–160 mTorr for H2O2 and TMA respectively, the pulse duration longer than 0.3 s, the base pressure below 1 mTorr, and the growth temperature higher than 150°C. Also, we found that the density, the chemical bonding strength, and the electrical properties improved markedly with raising the growth temperature although the growth rate remained nearly constant. In addition, area-selective growth and partly polycrystalline films were favorably obtained by using pulsed molecular beams of TMA and H2O2 in the growth.

Ferroelectric BaTiO3 thin film capacitors were fabricated using different designs. Silicon wafers with an oxide layer were used as the substrates. Conductive metallic oxide of RuO2 was reactively sputtered on SiO2/Si as a bottom electrode. The BaTiO3 thin films with a thickness in the range of 150–300nm were deposited by RF magnetron sputtering. Different capacitor structures, including single layer amorphous, single layer polycrys-talline, and bi-layer amorphous on polycrystalline, were investigated in this study. The clear interface between BaTiO3 and RuO2 as demonstrated by cross-sectional scanning electron microscopy implies little interdiffusion. The DC conducticity of a film with a thickness of around 200nm was as low as 1×10−13OHgr-cm at 4V. The dielectric constant of the composite structure was controlled in a range from 30 to 130. Breakdown voltage varied from 5×105V/cm to 1×106V/cm.

Thin films of (BaxSrj-x)TiO3 (BST) deposited by RF sputtering have been studied for the application to the capacitor material of dynamic random access memory (DRAM). The effects of film structures on the dielectric constant and the leakage current were investigated. The films deposited at 550–600°C consist of granular grains, and the films deposited above 650°C contain columnar grains. The dielectric constant is related to the grain size in the direction of thickness. The leakage current increases as the film structure changes from granular to columnar. The low leakage current (less than 10−8 A/cm2 at +2 and -2 V) and the small value of equivalent SiO2 thickness (0.9 nm) were attained in the (Ba065Sr035) TiO3 thin films.

Ferroelectric PZT (Pb (Zr,Ti)O3) from 100 nm down to 50 nm thick was deposited on Pt/SiO2/Si substrates using MOCVD (Metal Organic Chemical Vapor Deposition) under reduced pressure at 550°C. Using Pb (DPM)2, Zr (DPM)4, and Ti(i–OC3H7)4 as precursors made it possible to control the composition of CVD films and to produce pure perovskite crystalline structure in the range of thickness less than 100 nm. Electrical measurements of the capacitors revealed that 50-nm PZT films typically had a dielectric constant of 500, resulting in the same capacitance as 0.4-nm SiO2.

Thin films of pyroelectric PZT deposited by Jet Vapor Deposition are considered here for application in IR detector arrays. PZT films of 0.5 μm thickness were deposited on platinum coated insulating substrates and then annealed to increase the perovskite grain size. Capacitor test structures were used to measure the dielectric constant, loss, AC resistivity, and pyroelectric coefficient. We compare the results with known values for bulk PZT and discuss their relevance to detector design and performance.