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Using CMOS, poly-Si gate, single-level metal, gate-array chips, techniques have been developed to reconfigure the interconnect metallization on individual circuits without degradation of device or circuit performance. These techniques involve a laser-assisted capillary wet-etch process for highly selective removal of Al-alloy interconnects and laser CVD of doped poly-Si links. This technique may be useful for prototyping, testing and optimization of gate-array and standard-cell designs and layouts.
The effects of 193 and 248 nm UV surface irradiation on the properties of Al2O3 films produced by UV laser-initiated deposition have been investigated using ellipsometry and electrical measurements. The UV irradiated region shows a higher refractive index than the unirradiated regions, indicating that densification has occurred. Energy fluences as low as 1 mJ/cm2, too low to cause significant transient heating, produce measurable effects. The results suggest that UV irradiation may be used to obtain lower deposition temperatures in a variety of deposition systems.
Si3N4 films have been deposited on Si by using 193 nm ArF excimer laser radiation to initiate the reaction of SiH4 and NH3 at substrate temperatures between 200–600°C. Stoichiometric films having physical and optical properties comparable to those produced using low-pressure chemical vapor deposition (LPCVD) have been produced. The dielectric properties of the films are at present inferior to those of LPCVD material.
N- and p-channel enhancement-mode MOSFETs have been fabricated in Si films prepared by zone-melting recrystallization of poly-Si deposited on SiO2-coated Si substrates. The transistors exhibit high surface mobilities, in the range of 560–620 cm2/V−s for electrons and 200–240 cm2/V−s for holes, and low leakage currents of the order of 0.1 pA/μm (channel width). Uniform device performance with a yield exceeding 90% has been measured in tests of more than 100 devices. The interface between the Si film and the SiO2 layer on the substrate is characterized by an oxide charge density of 1–2 × 1011 cm−2 and a high surface carrier mobility. N-channel MOSFETs fabricated inSi films recrystallized on SiO2-coated fused quartz subtrates exhibit surface electron mobilities substantially higher than those of single-crystal Si devices because the films are under a large tensile stress.
The properties of PtSi-Si Schottky barrier contacts formed by a new technique employing multilayer metallization are compared with those of contacts prepared by the conventional single-layer metallization method. The multilayer technique permits the formation of very shallow contacts without any limitation being placed on the thickness of the PtSi layer. For a PtSi layer of given thickness the PtSi-Si contact interface obtained by this technique is more uniform than the interface formed by annealing a single layer of platinum on silicon. The interfacial uniformity is independent of PtSi thickness for shallow PtSi-Si contacts produced by the multilayer technique, while for conventional contacts the uniformity decreases with increasing PtSi thickness. Large-area (9.4 × 10−3 cm2) diodes utilizing shallow PtSi-Si contacts about 200 Å deep have been fabricated without guard rings. These diodes exhibit near-ideal forward current-voltage characteristics, low reverse leakage currents (less than 5 nA at −10 V) and high breakdown voltages (over −90 V). These characteristics are superior to those of diodes using conventional PtSi-Si contacts.
Device-quality Si films have been prepared by using graphite strip heaters for zone melting poly-Si films deposited on SiO2-coated substrates. The electrical characteristics of these films have been studied by the fabrication and evaluation of thin-film resistors, Mosfets and MOS capacitors. High yields of functional transistor arrays and ring oscillators with promising speed performance have been obtained for CMOS test circuit chips fabricated in recrystallized Si films on 2-inch-diameter Si wafers. Dualgate Mosfets with a three-dimensional structure have been fabricated by using the zone-melting recrystallization technique.
An investigation of the formation of refractory metal (tungsten, molybdenum and tantalum) silicides by reaction of the metal with crystalline and polycrystalline silicon at temperatures above 900°C indicates that WSi2 formation can be inhibited by certain processing techniques. These techniques utilize the growth of an SiO2 diffusion barrier about 20 Å thick on crystalline silicon and the deposition of tungsten films in vacuum ambients that will ensure oxygen incorporation (P ≥ 2 × 10−7 Torr). The reactivity of vacuum-deposited tungsten films results in the formation of an isolating oxide between the deposited silicon and tungsten films and the maintenance of the stability of the SiO2 diffusion barrier between tungsten films and a crystalline silicon substrate. This barrier is effective up to 1050°C in hydrogen ambients containing 15–20 ppm H2O. These procedures, however, are ineffective in preventing the formation of MoSi2 or TaSi2 at or above 900°C. A possible explanation for these results is that the tungsten film contains some level of oxygen due to the gettering of residual oxygen in the vacuum. In addition, the outgassing of the silicon source may enhance the level of oxygen at the evaporated W-Si interface. When the composite layer is annealed at high temperatures in a hydrogen ambient, the oxygen diffuses readily out of the bulk of the tungsten film to the interface, probably forming Sio2 which is more stable than WO3.The resulting thin SiO2 film has sufficient integrity to prevent silicon diffusion into the tungsten, thereby preventing WSi2 formation. This property of tungsten makes it inherently useful for buried-metal films in silicon devices.
The use of zone melting recrystallization (ZMR) to prepare large-grain(and in some cases single-crystal) semiconductor films is reviewed, with emphasis on recent work on Si on SiO2. Encapsulants are generally required to minimize contamination and decomposition, induce a crystalline texture,improve surface morphology and prevent agglomeration. In the case of Si, the solid-liquid interface is faceted, which gives rise to subboundaries. These can be entrained by laterally modulating the temperature through the use of an optical absorber on top of the encapsulant. Control of thermal gradients and in-plane crystallographic orientation are important for reliable entrainment.
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