ISFET's using inorganic gate thin films

The characteristics of various types of ISFET's using inorganic gate films are described. The pH and pNa selectivities are investigated for SiO2, Si3N4, Al2O3, alumino-silicate, and sodium-aluminosilicate gate dielectrics. The transient response and device stability are also studied for different values of solution pH. The Al2O3gate shows a nearly ideal pH response, excellent stability, and selectivity to other cations. On the other hand, the Si3N4gate is also a good pH sensor, but it is proved by the studies of SiO2and SiOxNyfilms that the oxygen content in its surface degrades its properties as a pH sensor. Sodium-alumino-silicate, which is generally known as a material for pNa selective glass electrodes, is utilized as a gate film for the pNa ISFET. The pNa selectivity of this device is comparable to that of the conventional glass electrode. The alumino-silicate gate has also a pNa selectivity, but it is inferior to the sodium-alumino-silicate gate.     

Characteristics of MOSFET prepared on Si/MgO.Al2O3/SiO2/Si structure

A new silicon on insulator (SOI) wafer with epitaxial-Si/ epitaxial-MgO.Al2O3(0.1 µm)/SiO2(0.5 µm)/     

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.     

Heat transfer analysis of beam-lead transistor chip

The program package which is applicable to solve problems of heat transfer in transistors or integrated-circuit chips or in their stems has been developed with a three-dimensional lumped network model. The thermal resistances of beam-lead transistors calculated with this program are found to be in good agreement with the observed values. For the better design of beam-lead devices, thermal resistances are evaluated with this program for various geometries of the chips and for the passivation films of 1-micron thick SiO2, 2-micron SiO2, and 0.2-micron Al2O3or Si3N4on 1-micron SiO2. The results calculated indicate that the thermal resistance is mainly dependent on the thickness of electroplated Au in the beam-lead structure, and that the heat dissipation is especially sensitive to the distance, which is measured along the beam lead from the chip edge to the nearest end of the joining part of the beam lead to the metallized conductor on the ceramic stem.     

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.     

High-power high-reliability operation of 1.3 µm p-substrate buried crescent laser diodes

Stable high-power CW operation of 1.3-μm InGaAsP/InP p-substrate buried crescent laser diodes (PBC-LD'S) has been realized, by controlling the front and rear facet reflectivities of the laser diode chips. The front facet reflectivity is reduced to 17 percent and the rear facet reflectivity is increased to 90 percent, by evaporating multilayer dielectric films (Si/Al2O3SiO2:17 percent, SiO2/Si/SiO2/Si/SiO2:90 percent) on each facet. CW light output power of 50 mW is achieved up to 60°C. Aging tests have been carried out under automatic power control (APC) mode conditions of 50°C-30 mW, 40 mW, 50 mW, and 30°C-50 mW. All samples are operating stably in spite of junction-up configuration. The lifetimes are estimated to be more than2 times 10^{4}h for all conditions.     

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.     

High-voltage planar structure using SiO2-SIPOS-SiO2film

The BVCB0leakage current hFEwere studied for high-voltage planar transistors which had three kinds of passivation films; SiO2-semi-insulating polycrystalline silicon (SIPOS)-SiO2; SIPOS-SiO2; and SiO2-phosphosilicate glass (PSG)-SiO2. The SiO2-SIPOS-SiO2type had a lower leakage current (surface generation current) and higher hFEthan the conventional SIPOS-SiO2type. The SiO2-SIPOS-SiO2type also had the highest BVCB0due to the field-plate effect.     

Effect of thermally nitrided SiO2thickness on MIS characteristics

The flatband voltage of metal-insulator-semiconductor (MIS) structures with thermally nitrided SiO2(nitroxide) insulators has been studied as a function of the nitroxide thickness in the range of 10-60 nm, for different nitridation conditions. The most striking result is that 10-nm nitroxide films are far less susceptible than thicker ones to the degradation of the electrical properties of the starting SiO2films induced by nitridation, as revealed by a negative shift of the flatband voltage. For nitridation cycles at relatively low temperatures (900° 120 min or 1000°C 60 min) the shift in the 10-nm films is practically negligible.     

Reactive sputter etching of silicon with very low mask-material etch rates

Directional etching of deep structures in silicon is often made difficult by a high mask erosion rate. Recent results have given a Si/SiO2etch rate ratio of up to 8 without the undercut problems associated with other selective etches. In this paper a new selectivity mechanism is described which can reproducibly give Si/SiO2etch rate ratios of more than 100 with a nonloading target, and more than about 50 with a loading target. Similar etch ratios are also obtained with masks of MgF2, Al2O3, Al, and Cr. The inherently high Si/SiO2etch rate ratio obtained in Ar/Cl2discharges is here enhanced by causing selective deposition of SiO2onto slowly etched materials. The silicon may be obtained from the target, or, for easier control, from input gases such as SiCl4. The deposition rate is controlled by the oxygen concentration. The results of etching deep grooves in Si are presented. Etch-mask faceting and Si surface decoration appear to limit the attainable etch rate ratios with fine structures; however, 18-µm-deep gratings of 4.5-µm period have been etched in Si.     

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.     

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.     

Transmission-loss characteristics of Al2O3-doped silica fibers

Transmission-loss characteristics of Al2O3-doped silica fibers have been investigated to develop an Al2O3alternative dopant for VAD silica-based optical fibers. Optical properties of Al2O3-doped silica bulk glass and fibers were measured in the0.11-30-mum spectral region. From the experimental results, the intrinsic loss due to glass material was estimated for Al2O3-doped silica fibers. The calculated intrinsic loss minima for Al2O3-doped silica fibers with 0.2- and 1.0- percent relative refractive-index differences were 0.17 dB/km at 1.548 μm and 0.28 dB/km at 1.565 μm, respectively.     

Radiation resistance of Al2O3MOS devices

Integrated circuits employing MOS devices will play a vital role in tomorrow's civilian and military electronics if their degradation in a radiation environment can be eliminated. One possible approach toward alleviating radiation effects in MOS devices is to use a material with a defect structure that does not allow predominant trapping of either holes or electrons as a gate insulator. This has been done by constructing MOS devices with plasma-grown aluminum oxide. The Al2O3films are formed by first depositing aluminum on freshly cleaned and properly prepared silicon wafers. Subsequently this aluminum is oxidized in an oxygen plasma and device fabrication is then completed. The devices have excellent characteristics and stability, and their fabrication is not restricted by the conditions of the ultra-clean procedures necessary for SiO2-Si devices. Exposure to 1-MeV electron bombardment at various fluence levels and bombardment-bias conditions shows that these structures possess remarkable radiation resistance. Up to a fluence of 1 × 10¹³e/cm², under positive or negative bias, no oxide charge buildup or interface state generation is detectable. Above that fluence, only small shifts are observed. This indicates that an order of magnitude improvement in device hardening can be achieved by the use of this material.     

Analysis of the threshold voltage and its temperature dependence in electrolyte-insulator-semiconductor field-effect transistors (EISFET's)

A first-order theoretical model is developed that allows the temperature dependence of the threshold voltage of an electrolyte-insulator-semiconductor field-effect transistor (EISFET) to be determined. Specifically, a detailed analysis is presented for a representative cell consisting of a Ag/AgCI reference electrode, a simple 1:1 electrolyte, and an ISFET. In addition, an insulator is assumed for which the site-binding model is applicable. All temperature-dependent parameters are identified and quantitatively described. Results are computed for SiO2and Al2O3insulators over a pH range from 1 to 12, and a temperature range from 20 to 80°C. Graphs of the surface-site occupancies versus the pH are shown to provide useful physical insight in interpreting the results. The threshold-voltage temperature coefficient is shown to be highly dependent on the pH; however, for Al2O3, over a 20-80°C range, the variation is roughly linear.     

Choice of dielectrics for TFEL displays

The commonly used dielectrics, Y2O3, Al2O3, Si3N4, and Ta2O5, were evaluated for use in ac thin-film electroluminescent displays, along with a composite dielectric SiAlON. Adhesion, stress cracking, charge storage capacity, and display brightness and efficiency were studied for devices with ZnS: Mn layers fabricated under identical conditions. A combination of SiAlON and Ta2O5appears to be optimum as a double-layer dielectric system for ACTFEL displays.     

Characterization of thermally oxidized n+polycrystalline silicon

The properties of thermally grown silicon dioxide films on n⁺polysilicon are studied using cross-sectional TEM, and electrical measurements to evaluate conduction, electron trapping, destructive breakdown and wearout mechanisms. All of the above electrical parameters are found to be degraded by any increase in the degree of surface roughness at the oxide-polysilicon interface. Our results suggest that a significant improvement in the insulating properties of the SiO2films can be achieved if the polysilicon is initially deposited in the amorphous phase at 560°C rather than the polycrystalline phase at 620°C. For example, for dry-oxidized diffusion-doped films, there is an increase in oxide breakdown field from 3.0 MV . cm^-1to 6.2 MV . cm^-1, and a reduction in leakage (Fowler-Nordheim) current of two orders of magnitude. Furthermore, it is shown that the long-term reliability of n⁺polysilicon/SiO2/n⁺polysilicon structures is directly related to the degree of interface texture; i.e., a smoother interface will result in a significant reduction in electrical wearout and an increase in time to failure.     

InP MISFET's with Al2O3/Native Oxide double-layer gate insulators

Enhancement-mode InP MISFET's with anodic Al2O3/ native oxide double-layer for gate insulator are fabricated by anodization processes in electrolyte and in oxygen plasma. Such gate structure greatly improves the device performance; high effective electron mobilities of 1500-3000 cm²/V . s and marked reduction of drain current instability were simultaneously achieved. This device performance is consistent with the interface properties obtained by C-V measurements. InP MISFET inverters as well as ring oscillators are also fabricated to demonstrate the stability of the circuit at low frequency and to show the capability of the process employed.     

Characteristics of sealed-off waveguide CO2lasers

Gain and output power of sealed-off waveguide CO2lasers are presented as a function of gas mixtures and total gas pressure. Experimental data on circular-bore and square-channel waveguide lasers are presented. Output power per unit length of 0.2 W/cm is achieved for both types of lasers in agreement with gas-discharge scaling laws which are presented. Saturation intensities as high as 24 kW/cm²are inferred from the data. The effects of the optical properties of the waveguide wall material on the waveguide losses are discussed and theoretical waveguide loss versus wavelength is presented for BeO, Al2O3, and fused silica.     

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.