CO2laser annealing of Si3N4, Nb2O5, and Ta2O5thin-film optical waveguides to achieve scattering loss reduction

Significant reductions in the optical scattering losses of Si3N4, Nb2O5, and Ta2O5waveguides fabricated on SiO2/Si substrates have been measured following CO2laser annealing. The largest improvements were observed for Si3N4waveguides, where waveguide attenuation values of about 6.0 dB/cm before laser annealing were reduced to as low as 0.1 dB/cm afterwards. An improvement of more than an order of magnitude was obtained for a Nb2O5waveguide upon laser annealing, the attenuation coefficient decreasing from 7.4 to 0.6 dB/cm. In the case of one Nb2O5waveguide no improvement was obtained upon laser annealing. The attenuation coefficient of a reactively sputtered Ta2O5waveguide was found to decrease from 1.3 dB/cm before laser annealing to 0.4 dB/cm afterwards. In the case of a thermally oxidized Ta2O5waveguide a small initial improvement in waveguide attenuation was followed by degradation upon further laser annealing.     

Ultra-thin Ta2O5dielectric film for high-speed bipolar memories

A new capacitor technology, with extremely thin (5.3-20 nm) Ta2O5film deposition and weak-spot oxidation, is developed to realize high capacitance and high reliability. The Ta2O5film was reactively sputtered, followed by weak-spot oxidation. The weak-spot oxidation is achieved by placing the Ta2O5film on Si in a high-temperature dry O2ambient. The oxidation significantly improves the time-dependent-dielectric-breakdown (TDDB) characteristics and reduces the defect density of Ta2O5capacitors without reducing the capacitance by selectively oxidizing the Si surface at weak spots where the Ta2O5is locally thin or missing. The technology is based on the new discovery that Ta2O5film less than 20 nm thick shows no reduction in dielectric breakdown strength after dry O2high-temperature annealing up to 1000° C. The Ta2O5(7.5-nm) capacitor with a capacitance of 8.5 fF/µm²is applied to a high-speed bipolar memory. This makes it possible to reduce the memory cell area to one-third that of a conventional bipolar memory. The memory provides high-speed operation; access time is less than 5 ns, and sufficient soft error immunity is provided.     

p-Channel TFT's using magnetron-sputtered Ta2O5films as gate insulators

Metal-gate thin-film transistors (TFT's) have been fabricated in layers of laser-recrystallized polycrystalline silicon on fused quartz substrates at processing temperatures below 625°C. Tantalum pentoxide (Ta2O5) was used as a gate insulator instead of a conventional thermally grown silicon dioxide (SiO2). Ta2O5gate insulator was deposited onto the recrystallized silicon layer at room temperature, using an RF-magnetron sputtering system. The reactive ion etching method, using CF4as a reactive gas, was employed in patterning deposited Ta2O5. These TFT's have exhibited p-channel depletion-mode characteristics with a threshold voltage of 2.5 V and a transconductance of 70 µS at Vg= - 2 V. An on-off current ratio exceeding 10⁵has been obtained.     

Optical and electrical properties of thermal Tantalum oxide films on silicon

Tantalum pentoxide is of interest as an alternative dielectric to SiO2for MOS devices. In the present work, RF-sputtered tantalum films on silicon substrates were thermally oxidized at 500°C. Ellipso-metric measurements showed that films made in this way were uniform in refractive index except for a narrow region of tapered index at the silicon interface. Capacitance measurements gave a relative permittivity of about 26, in the range shown by anodic Ta2O5films. The conduction currents depended on the oxidation period but were comparable to or slightly better than reported for CVD Ta2O5.     

Ion-sensitive field-effect transistors with inorganic gate oxide for pH sensing

Ion-sensitive field-effect transistors (ISFET's) have been fabricated by using silicon films on sapphire substrates (SOS). Using this structure SiO2, ZrO2, and Ta2O5films are examined as hydrogenion-sensitive materials, and Ta2O5film has been found to have the highest pH sensitivity (56 mV/pH) among them. The measured pH sensitivity of this SOS-ISFET's is compared with the theoretical sensitivity based on the site-binding model of proton dissociation reaction on the metal oxide film and good agreement between them is obtained.     

Electrical and charge storage characteristics of the tantalum oxide-silicon dioxide device

The electrical characteristics of the tantalum oxide-silicon dioxide double-dielectric structure are described. The MTOS structure (metal-tantalum oxide-silicon dioxide-silicon) is similar to the MNOS double dielectric which is used as a nonvolatile memory element except tantalum oxide (Ta2O5) is used to replace the silicon nitride as the second dielectric. Capacitance voltage measurements show a negative QSSwith magnitudes smaller than those in compatable MNOS devices. Conduction characteristics of both anodic and thermally grown Ta2O5have been studied and both have been found to follow a Poole-Frenkel mechanism. The memory characteristics of the MTOS have been investigated and preliminary data are presented. Where MNOS data are available, the MTOS characteristics have been compared with those of the MNOS structure. It is shown that the threshold voltage of the MTOS device can be shifted using lower gate voltages than are needed for a comparable MNOS device. It thus appears that the MTOS device has some decided potential advantages over the MNOS structure as a nonvolatile memory element.     

A two-stage monolithic IF amplifier utilizing a Ta2O5capacitor

A two-stage monolithic IF amplifier incorporating a sputtered Ta2O5capacitor has been fabricated. The monolithic capacitor is based on a composite layer structure consisting of Au, Ta, Ta2O5, Ta, and Au. This layered structure is sequentially deposited in a single sputtering run, which eliminates particulate contamination. As a result, a thin pinhole-free dielectric layer can be deposited over large areas, and 140-pF capacitors have been fabricated with excellent yields. The large unit area capacitance of 1500 pF/mm²available with the present process has the potential for reducing the size of matching and bias circuits in microwave monolithic circuits and hybrid thin-film circuits. The monolithic amplifiers exhibit a gain of 17.5 ± 1.0 dB from 1.2 to 2.6 GHz and a minimum noise figure of ∼ 2.7 dB, with an associated gain of 17.5 dB at 1.7 GHz.     

Monolithically-integrated hybrid heterostructure diode laser with dielectric-film waveguide DBR

In the monolithically-integrated hybrid (MIH) DBR diode laser, the five-layer Ga(Al)As-GaAs heterostructure waveguide of the gain region was monolithically butt-joined on a common GaAs substrate with a highly-transparent corrugated dielectric-film waveguide consisting of sputtered SiO2, Ta2O5, and evaporated (corrugated) As2S3layers. The laser operated with the first-order grating under the pulsed current pumping at 300 K. The efficient resonant mode conversion (70 percent in power) has been obtained at the interface between the heterostructure and dielectric waveguides. The fundamental transverse and single-longitudinal mode output emission was obtained up to 160 mW (Ith = 120mA) with external differential quantum efficiencyeta simeq 32percent. The advantages of a dielectric-film waveguide DBR are demonstrated. The use of such a DBR results in a high degree of sidemode suppression and stability of the spectral position of the emission line under the temperature variation, the corresponding spectral shift beinglsim 0.01Å/K.     

Interrelation of pyroelectric and nonlinear optical coefficients in ferroelectric crystals

An experimental relation is found between the pyroelectric and electrooptic coefficients of several ferroelectric materials. The physical basis of this relation is explained using Garrett's anharmonic oscillator model, and the pyroelectric coefficients of Sr0.25Ba0.75Nb2O6, Sr2NaNb5O15,Ba2NaNb5O15, KSr2Nb5O15, K3Li2- Nb5O15, and Ag3AsS3are predicted from the known electrooptic coefficients.     

A tantalum-on-sapphire microelectrode array

The design and fabrication are described of a new microelectrode array for the production of localized electrical excitation in nerve fibers, in particular in the auditory nerve. The sapphire substrate was chosen for its electrical (insulating) and mechanical properties, tantalum was chosen as the conductor metal because of its self-passivating characteristics, and platinum as the stimulation electrode material because of its resistance to electrolysis. Experimental measurements are presented to show the effectiveness of Ta2O5as an insulator in biphasic charge delivery systems, and the effectiveness of conformal dielectric overcoating of the Ta2O5in reducing capacitive signal leakage is demonstrated. The electrochemical properties of the platinum-electrolyte interface are shown to provide a satisfactory model on which to estimate the charge delivery capabilities of the electrodes. Finally, the sequence of process-compatible steps for fabricating the microelectrode array, from the first tantalum deposition to the final overcoating are detailed.     

Charge centroid in MIOS nonvolatile memory structures

The experimental charge-centroid relations in MNOS, MAOS (A = Al2O3), and MANOS nonvolatile memory structures were analyzed and compared. New data on trap density, trapping length, and capture cross section for Al2O3and Si3N4were derived from these measurements. The trapping length in Al2O3is about 5 times larger than in Si3N4. The charge-detrapping rate in Al2O3is smaller than in Si3N4. The experimental charge-centroid relation during charge trapping and charge detrapping is described by two expressions which include the capture cross section as the only physical parameter. The calculated capture cross section is constant for MNOS and also during charge trapping in MAOS, but decreases linearly with propagating centroid position after onset of charge detrapping within Al2O3. Trap-assisted tunneling over multiple deep trap levels in Al2O3is proposed to interpret the results for MAOS. Oxygen annealing of Al2O3enhances storage capability of injected negative charge within the lower bandgap Si3N4region of the MANOS structure. This behavior is related to blocking of detrapped nitride charge at the oxygen densified energy barrier created at the Si3N4- Al2O3boundary.     

C2F6reactive ion-beam etching of LiNbO3and Nb2O5and their application to optical waveguides

Properties of C2F6(freon 116) reactive ion-beam etching (RIBE) of LiNbO3, Ti-indiffused LiNbO3, and sputter-deposited Nb2O5film on LiNbO3are reported. A maximum differential etching ratio of approximately 5:1 has been measured for LiNbO3and the AZ 1350B Shipley photoresist. We have used this etching technique to fabricate diffraction gratings on both Ti-indiffused LiNbO3and Nb2O5- LiNbO3waveguides with measured throughput efficiencies in excess of 85 percent.     

Design feasibility of a single-mode optical isolator

Isolator conditions are derived by a perturbation method for optical slab isolators in which magnetooptical crystal and anisotropic dielectric crystal films are integrated for reciprocal and non-reciprocal TE-TM mode conversion functions. The general expressions show that isolator conditions can be satisfied for any combination of wavelength, normalized frequency, waveguide thickness, and gyrotropic constant of the magnetooptical crystal by appropriate choice of the magnitude of the birefringence of the top layer film and its crystal orientation angle. Design examples of single-mode isolators with waveguide thicknesses of 4, 4.5, and 5 μm are given at 1.55 μm wavelength. It is also pointed out that isolators of the above design can be fabricated with Y3Fe5O12, Y1.5Gd1.5Fe3O12, and Tm3Fe5O12crystal films as magnetooptical materials and a solid solution of LiNbO3and LiTaO3as an anisotropic film. Tolerance in the required film thickness becomes less stringent with this design. Isolation of more than 20 dB with an insertion loss of less than 0.12 dB is feasible without any parameter adjustment, even when there is as much as a ten percent deviation in the film thickness.     

Growth of Ba2NaNb5O15single crystals for optical applications

The items discussed are: developments leading to the efficient nonlinear optical materials K3Li2Nb5O15and Ba2NaNb5O15; the relation of the stability to visible radiation shown by these materials to their, respectively, "completely filled" and "filled" tetragonal tungsten-bronze-like structures; a comparison of their physical properties to those of LiNbO3; the techniques and problems of growing optically satisfactory single crystals of Ba2NaNb5O15by pulling from the melt; processing required to obtain single domain sections and eliminate microtwinning; and the effects of excess Ba, Na, or Nb in the melt and the effects of substitutions by homologous cations.     

Broad-band emission from chromium doped germanium garnets

By varying the composition of a series of germanium garnets, it is possible to change the lattice constant of the structure over a wide range, and thus the crystal field. Three Cr-doped germanium garnets (Ca3Al2Ge3O12, CaY2Mg2Ge3O12, and MgY2Mg2Ge3O12) have been grown as single crystals. Spectroscopic results show that in these garnets the broad-band emission at room temperature is so strong that it totally dominates theR-line emission. The spectral region covered by emission from these crystals is from 700 nm to more than 1 μm. The room-temperature lifetimes range from 50 to 290μs.     

Quadruply self-aligned stacked high-capacitance RAM using Ta2O5high-density VLSI dynamic memory

A new one-transistor, one-capacitor RAM cell structure called a Quadruply Self-Aligned Stacked High Capacitance (QSA SHC) RAM is proposed as a basic cell for a future one-million-bit VLSI memory. This cell consists of a QSA MOSFET and a Ta2O5capacitor stacked on it. By this cell, the ultimate cell area3F times 2Fcan be realized with sufficient operating margin. Here,Fis the minimum feature size. The basic cell was fabricated and its operation was experimentally verified. The leakage current of Ta2O5film was small enough for the storage capacitor dielectric. Using a3F times 4Fcell and a4Fpitch sense amplifier, a one-million-bit memory was designed with a 2-µm rule. A cell size of 6.5 × 8 µm², and a chip size of 9.2 × 9.5 mm²were obtained. The access time, neglecting the RC time constant of the word line, was estimated to be about 170 ns. Based on this design, it is argued that a future one-million-bit memory can be realized by QSA SHC technology with a 2-1-µm process. The mask set of the 1-Mbit RAM was actually fabricated by an electron-beam mask maker. A photomicrograph of the 1-Mbit RAM chip patterned by the mask set is shown. This chip was patterned not to get an operating sample but to show an actual chip image of the future 1- Mbit RAM. The area of each circuit block including storage array can be seen in this chip image.     

Electrooptic guided-to-unguided mode converter

Experimental demonstration of electrooptic polarization conversion from a guided TE wave to an unguided TM wave in a thin-film light guide is reported. In the specific device we fabricated utilizing Ta2O5film and LiTaO3substrate, we have observed a 40-percent modulation when 500 V were applied across two electrodes spaced 50 μm apart. We present here a ray-optics analysis of the conversion process. With the inclusion of the Goos-Haenchen shifts, the analysis agrees well with the experiment.     

A new graded electrode for forming intimate contact with ferroelectrics

A method for forming intimate contact with ferroelectrics and other dielectric materials is proposed. The method, involving the use of a "graded" electrode structure, is shown to be very effective for making a stable, intimate contact to ferroelectric bismuth titanate (Bi4Ti3O12). In terms of stability, no distortion in the hysteresis pattern of bismuth titanate samples has been observed in more than three years of observation when a graded electrode is used. In terms of the information of an intimate contact, the minimization of various "contact effects" is demonstrated by the marked reduction of observed coercive field strength when a graded electrode (as opposed to a conventional metal electrode) is used. For Bi4Ti3O12, the graded electrode structure used consists of a thin semiconducting tin dioxide film placed between a metal and the ferroelectric body.     

Multilayer Al2O3—SiO2combination (MASC) films as a partial H2diffusion barrier for use on Ge surfaces

Multilayer Al2O3-SiO2combination (MASC) films are shown to be superior to SiO2films for Ge surface passivation during a forming-gas annealing cycle. The Al2O3is apparently much less permeable to H2than is SiO2. Hydrogen easily permeates SiO2, producing a high density of surface acceptor states on the Ge. Even in the absence of hydrogen exposure, the surface-state and fixed-charge densities under MASC films are reduced by a factor of 2 or more as compared to pyrolytic SiO2films.     

Characteristics of pulse excited electroluminescence from ZnS films containing rare earth fluroide

The transient behavior of the electrical and the optical properties under pulse excitation has been studied and analyzed in detail on the electroluminescent devices made of Au-ZnS: NdF3- Ta2O5-Ta composite films. The ZnS-Ta2O5interface is capable of accommodating a large number of trapped electrons, up to 10¹³cm^-2, with the majority of the interfacial states located at a fixed energy below the conduction band of ZnS. Carrier injection is achieved by electrons tunneling through the Au-ZnS Schottky barrier at a field of 2.6×10⁶V/cm. The threshold field of impact excitation for 1 percent NdF3in ZnS was found to be 1.5×10⁶V/cm; while the coefficient of impact excitation at 2.6×10⁶V/cm was estimated to be 1750 cm^-1. In addition, the emission time constants of several rare earth fluorides were also studied.