Electrical characteristics of very thin SiO2deposited at low substrate temperatures

Very thin (≲ 100-Å) films of SiO2have been deposited by a modified plasma-enhanced chemical-vapor deposition (PECVD) process at very low substrate temperatures (≲ 350°C). Low flow rates of reactive gases and a high flow of inert carrier gas were used to lower the deposition rate, ensuring improved dielectric properties and good control over film thickness. Measurements made on MOS capacitors of current-voltage characteristics, electrical breakdown, interface trap density, and mobile ion drift indicate that these very thin PECVD films are approaching thermally grown SiO2in quality and may be suitable as gate dielectrics in device applications.     

Controlling void formation in WSi2polycides

Polycides, composite wiring/electrode films formed by depositing a refractory metal silicide such as WSi2, MoSi2, or TaSi2atop a polysilicon film [1]-[4], are finding their way into IC technologies as low-resistivity electrodes/interconnects. One of the desirable features of polycide composite films is their ability for self-passivation through thermal oxidation. In some cases, however, oxidation of the two-layer materials results in the formation of large "voids" in the polysilicon film (bottom layer in the polycide) [5], [6]. A method for preventing this void formation has been found. The solution involves deposition of a thin silicon layer onto the existing two-layer material. The additional layer is designed to provide some of the silicon required for oxidation during the initial stages of self-passivation. In cases where over 200 nm of SiO2were grown atop a WSi2polycide, a silicon layer as thin as 15-nm prevented void formation.     

Thermal nitridation of Si and SiO2for VLSI

This paper presents an extensive review of our work on thermal nitridation of Si and SiO2. High-quality ultrathin films of silicon nitride and nitrided-oxide (nitroxide) have been thermally grown in ammonia atmosphere in a cold-wall RF-heated reactor and in a lamp-heated system. The growth kinetics and their dependence on processing time and temperature have been studied from very short to long nitridation times. The kinetics of thermal nitridation of SiO2in ammonia ambient have also been studied. In nitroxide, nitrogen-rich layers are formed at the surface and interface at a very early stage of the nitridation. Then the nitridation reaction mainly goes on in the bulk region with the surface and near interface nitrogen content remaining fairly constant. Our results also indicate the formation of an oxygen-rich layer at the interface underneath the nitrogen-rich layer whose thickness increases slowly with nitridation time. The nitride and nitroxide films were analyzed using Auger electron spectroscopy, grazing angle Rutherford backscattering, and etch rate measurements. MIS devices were fabricated using these films as gate insulators and were electrically characterized usingI - V, C - V, time-dependent breakdown, trapping, and dielectric breakdown techniques. Breakdown, conduction, andC-Vmeasurements on metal-insulator semiconductor (MIS) structures fabricated with these films show that very thin thermal silicon nitride and nitroxide films can be used as gate dielectrics for future highly scaled-down VLSI devices. The electrical characterization results also indicate extremely low trapping in the nitride films. The reliability of ultrathin nitride was observed to be far superior to SiO2and nitroxide due to its much less trapping. Studies show that the interface transition from nitride to silicon is almost abrupt and the morphology and roughness of the interface are comparable to the SiO2-Si interfaces.     

Single-crystal silicon transistors in laser-crystallized thin films on bulk glass

High-performance thin-film transistors (TFT) have been fabricated in single-crystal silicon thin films on bulk fused silica. Deposited films of polycrystalline silicon were patterned to control nucleation and growth of single-crystal material in pre-selected areas and encapsulated with a dielectric layer (e.g., SiO2) in preparation for laser crystallization. Patterning also minimized microcracking during crystallization. The patterned silicon layer was crystallized with a scanning CO2laser, which produced islands with preferred crystal orientation. The single crystallinity of the islands was established with transmission electron microscopy after transistor evaluation. The silicon islands were processed with conventional microelectronic techniques to form metal-oxide-semiconductor-field-effect transistors operating in the n-channel enhancement mode. The devices display exceptional electrical characteristics with "low-field" channel mobilities > 1000 cm²/V sec and leakage currents < 10 pA, for a Channel length of 12 µm and width of 20 µm. Achievement of high-performance TFT's with the combined features of microcrack suppression, preferred orientation, and selected-area crystallization render CO2- laser processing of silicon films a viable and versatile basis for a silicon-on-insulator technology.     

Recrystallization of Si films on thermal SiO2-coated Si substrates using a high-speed e-beam line source

Lateral zone melting from a fast-scanning (5-30-cm/s) e-beam line source has been used to grow single-crystal films with an area limited only by the e-beam scan field. Electron backscattering contrast and etch pit techniques have been used to study the crystallographic orientation and the extent of single-crystal silicon film growth on thermal SiO2-coated silicon (SOI) wafers.     

Si–C multilayer quasi crystals preparation by DC magnetron sputtering

Silicon carbide (SiC) is becoming one of the most important electronic materials in recent years. Single crystalline SiC is a wide-bandgap semiconductor, which finds a wide range of applications in high temperature, power consuming, and fast-acting electron devices. Common methods applied for silicon carbide films deposition are: plasma-enhanced CVD under plasma decomposition of organic compounds such as CH₄, C₂H₂, C₃H₈. These methods are complicated and expensive.

In this work we grew silicon–carbon films as Si–C thin film multilayer system with successive layers of Si and C both of equal thicknesses. The Si–C systems grown in our experiments consisted of 40 sub-layers, deposited by DC magnetron sputtering on silicon, on glass, and on Au substrates in argon plasma environment. Sputtering was provided continuously from two targets: graphite and single-crystalline silicon. Optical and electro-physical properties of the deposited thin film systems were investigated. Relative permittivity of the grown thin film systems was found to be the main and most important parameter of the Si–C system.     

Dielectric films for capacitor applications in electronic technology

In microelectronic applications, various approaches have been used to provide high capacitance thin film components. None of these has been widely accepted for reasons that are both technical and economic. Limited applications have been made of silicon and other simple oxides, e.g., of aluminum or tantalum, as dielectric media. Particularly in the area of microwave integrated circuits, requirements for high frequency, high specific capacitance, low to moderate loss, and integrability call for new thin film approaches. These requirements will only be met by using more complex compounds, possibly of ferroelectric materials. Aspects of material preparation, composition evaluation, and dielectric performance of thin film bismuth titanates are treated in a manner which should be of value in considering other complex dielectrics for similar use. Of particular technological importance is the use of thin silica barrier layers at the metal electrodes to permit achieving low dielectric loss.     

Multiferroic Bi 0.7Dy/ 0.3 FeO3 films as high k dielectric material for advanced non-volatile memory devices

Gate dielectric materials having high dielectric constant, low interface state density and good thermal stability are needed for advanced CMOS applications. In this letter, the electrical properties of novel multiferroic Bi_0.7Dy_0.3FeO₃ (BDFO) thin films deposited using the pulsed laser deposition technique on p-type (100) silicon substrate are reported. Using high frequency capacitance-voltage (C-V) measurements, the dielectric constant, effective oxide charge density and interface state density were estimated. The results suggest the potential application of multiferroic BDFO films as gate dielectric material for novel memory devices that can be electrically written and magnetically read.     

The dielectric reliability of intrinsic thin SiO2films thermally grown on a heavily doped Si substrate—characterization and modeling

The dielectric reliability of the intrinsic thin SiO2films (∼ 110 Å) thermally grown on heavily doped n-type Si substrates has been studied by using the time-zero-dielectric-breakdown (ramp-voltage-stressed I-V) and time-dependent-dielectric-breakdown (constant-voltage-stressed I-t and constant-current-stressed V-t) tests. These experiments have been performed to investigate the variations of trapped electron density, interface state density, and field enhancement in a thin SiO2film stressed with different amounts of Charges. Moreover, the temperature effects on these parameters in a thin SiO2film have also been studied. A theoretical model considering the effects of dynamic trapping (i.e., trapping and detrapping), positive charge generation, weak spots, and robust area is proposed to describe the conduction mechanism and dielectric breakdown of a thin SiO2film. Important physical parameters such as barrier height, trapped electron density, trap capture cross section, and trap generation rate have been analyzed to interpret the temperature effects.     

Dynamic micromechanics on silicon: Techniques and devices

New fabrication procedures ate described for constructing thin, electrostatically deflectable SiO2membranes on a silicon wafer in a very controllable manner. Performance parameters of these membranes are analyzed and three examples of typical applications for the micromechanical structures are discussed: a light modulator array, a micromechanical voltage-controlled switch, and the measurement of the mechanical properties of thin insulating films unconstrained by the substrate. Since the lifetimes of the membranes can be very long (>10¹⁰cycles), their dimensions very small (8.3 µm long, 950 Å thick have been demonstrated), and the fabrication technique is simple and versatile, the potential applicability of such devices seems promising.     

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.     

A selected-area CVD method for deposition of sensing films onmonolithically integrated gas detectors

A novel method for deposition of thin transducing films on geometrically well-defined areas is reported. The films represent the front-end of monolithically integrated microelectronic gas detectors. These devices contain a 1 μm thick dielectric window equipped with four-point probe electrodes and a boron-diffused silicon heater underneath the window. The central region of the window can be instantaneously switched from room temperature to several hundred °C while maintaining excellent thermal uniformity within a 350×350 μm area. Thermally activated CVD of Pt is realized on the heated region by raising the temperature of the central region into the regime of thermal decomposition of Pt precursors such as Pt(PF₃)₄. The growth of thin Pt films is monitored in situ by measuring film resistance, and film growth is terminated once desired film resistance values are reached by cutting off the heater current. This allows to optimize film properties for gas sensing applications. The deposited films are characterized ex situ by optical microscopy and scanning electron microscopy to correlate film morphology with resistance. This method can be easily extended to deposition of other films of interest such as TiO₂     

Thermal-sensitive BST thin film capacitors for dielectric bolometer prepared by RF magnetron sputtering

We have been developing a monolithic microbolometer technology for uncooled infrared focal plane arrays (Uncooled IRFPAs) along the route from fabricating pixels of thin-film dielectric bolometers on micromachined silicon substrates. In the paper, the thermal-sensitive barium strontium titanate (BST) thin film capacitors for that objective prepared by Radio-Frequency Magnetron sputtering have been investigated focusing on the condition of fabrication of BST thin films. Capacitor-Temperature properties of the thermal-sensitive BST thin film capacitors have been measured with impedance analyzer. According to the Capacitor-temperature curves, these indicated that the temperature coefficient of dielectric constant (TCD) within the ambient temperature region highly depended on the Radio-Frequency Magnetron sputtering condition of fabrication of BST thin films. BST thin film capacitors with TCD-value more than 21%/K have been prepared on the optimized condition. That is a good base for preparation of dielectric bolometer mode of uncooled IRFPAs.     

Tera Tool [terahertz time-domain spectroscopy]

The terahertz differential time-domain spectroscopic method is applied to characterize the dielectric and optical properties of a variety of thin films at terahertz frequency. The results of several samples including silicon dioxide, parylene-n polymer film, tantalum oxide film, and protein thin layer samples were presented. The dielectric property of silicon dioxide thin film is well fitted to that of a bulk. The dielectric properties of parylene-n thin films show good agreement with the result measured by the goniometric terahertz time-domain spectroscopy. The dielectric and optical properties of the tantalum oxide show reasonable data with previously available data. Some properties in thin films are slightly different from the bulk materials. The origin of this discrepancy is considered due to fine grain formation, mechanical stresses, formation of interfacial layers, or rough interfaces during thin-film deposition process. The terahertz differential time-domain spectroscopy may be applied to the measurement of the dielectric and optical properties of thin films (nanometer to micrometer) of several materials, which cannot be done by any other method.     

Tapered windows in phosphorus-doped SiO2by ion implantation

Phosphorus-doped SiO2is frequently used as a dielectric coating in silicon integrated circuits. It is important that windows in this dielectric have sufficiently tapered walls so that the subsequent metallization has good step coverage. It is shown here that tapered windows can be made in both Nitrox-deposited ∼ 1-percent phosphorus-doped SiO2and Silox-deposited ∼ 7-percent phosphorus-doped SiO2as well as undoped SiO2by an ion implantation which produces a thin damaged layer at the top of the oxide. The damaged layer etches at a faster rate than the undamaged oxide. This fast-etching layer undercuts the photoresist which serves as the etching mask and results in window walls having slopes in the range of 30-40° with respect to the wafer surface. Tapering windows by ion implantation is a dependable process that gives reproducible results without having to rely on the art of photoresist liftoff methods.     

Quantum Size Effects on Dielectric Constants and Optical Absorption of Ultrathin Silicon Films

The size dependence of the dielectric constants and optical absorption for silicon nanostructured films are investigated using density-functional theory. A critical thickness of 4.3 nm is observed for Si (100)-oriented thin films. Within this critical thickness, the dielectric function and optical absorption show remarkable size dependence, and a large reduction of static dielectric constant (from 10.8 to 4.4) is observed. This is in contrast to the weak dependence of dielectric constant on film thickness in silicon dioxide thin films. The pronounced dependence and large critical thickness demonstrate a quantum-confinement effect on optical properties, which is of great importance to nanophotonics.      

A multiwafer plasma system for anodic nitridation and oxidation

An experimental, high throughput and clean multiwafer system for plasma anodization is described. The applications of this system towards growing anodic silicon nitride and anodic silicon dioxide films, as well as the anodic nitridation of SiO2films are demonstrated. Pure nitride films thicker than 15 nm may be produced at 950°C. Oxide films are grown at temperature as low as 600°C. SiO2films can be converted to a high percentage of nitride by anodic treatment.     

Fabrication of planar dielectric waveguides with high optical damage threshold

We realized with a simple inexpensive process planar dielectric waveguides for the visible with the following improved properties: The waveguiding layers are mixed oxide films (∼40 percent TiO2in SiO2). They are hard and durable against mechanical and chemical attack. The losses are low and comparable to other planar waveguide materials. The films withstand high energy densities and intensities for pulsed and CW Laser irradiation, respectively.     

High-temperature rapid thermal nitridation of silicon dioxide for future VLSI applications

The use of nitrided SiO2for very large scale integration (VLSI) applications is becoming increasingly attractive. Nitridation can convert a thin surface region of SiO2into a nitroxide film which is a diffusion barrier that allows the use of thin dielectrics in MOS structures and a variety of gate metals without contaminating the interfacial region. We propose a two-activation-energy model of nitridation and suggest a structure for MOS gate insulator applications. We achieved this structure using rapid thermal nitridation at 1300°C for 20 s in 1 atm. of ammonia.     

Planarization of gold and aluminum thin films using a pulsed laser

Micrometer-thick gold and aluminum films have been planarized by momentarily melting them with optical pulses from a dye laser. Submicrosecond pulses were used in order to minimize the temperature rise in the substrate, reduce the energy required for melting, and prevent undesirable metallurgical reactions. Planarization of two-level gold metallization structures insulated by SiO2(including interlevel vias) has been achieved. Aluminum metallization on integrated circuits (IC's) has also been planarized. The use of a thin (<200 Å) silicon overcoating greatly aids the aluminum planarization process by passivating the aluminum and increasing its initial optical absorbance.