Laser-Enhanced sputter or vapor deposition of thin metallic films on ceramic substrates

Laser-assisted sputter deposition has been used to deposit thin metallic films onto ceramic substrates. This process allows the building of a film of arbitrary thickness by sequential deposition of 5- to 150-nm-thick layers alternating with laser melting. Highly adherent films of copper on sapphire and on quartz were obtained. Pulsed-laser treatment also enhances the adhesion of nickel films to sapphire substrates. This critical step in the process is the laser irradiation following each of the initial depositions. In these early stages, an interfacial reaction between film and substrate takes place during laser irradiations. An interfacial compound forms whose nature has been studied by transmission electron microscopy. The morphological features of the film, as well as the amount of film removed during these first irradiations, were analyzed as a function of laser energy density by scanning electron microscopy and by energy dispersive X-ray spectroscopy. The results were correlated with computer simulations of the thermal response of the two-media system to laser heating. The role of the interfacial thermal conductivity during laser processing is analyzed. The state of the substrate,e.g., annealed or as-polished, influences the morphology of the irradiated film. This effect is related to an enhancement of interfacial thermal conductivity. This invited paper is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presented as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.     

The epitaxial deposition of silicon on insulating substrates for MOS circuitry

Parasitic capacitances associated with large junction areas in bulk silicon MOS transistors can be virtually eliminated if MOS devices can be fabricated in silicon films on insulating substrates. The problem has been not only one of achieving suitable electrical properties in the thin films, but also of maintaining the “as-deposited” properties during the thermal treatments used in device processing. The changes in the electrical properties have made it particularly difficult to reproducibly fabricate deep depletion MOS devices, which require the deposition of thin films (less than 1.5μ) with carrier concentrations as low as ～5 × 10¹⁴ per cu cm. Modifications in the film deposition procedures and in the pregrowth treatment of the substrate have enhanced the thermal stability of the electrical properties sufficiently for the fabrication of deep depletion MOS devices on 1.0 μ thick films. Procedures have also been developed for the deposition of bothn-andp-type silicon on the same insulating substrate by a “two-stage” epitaxial process. Thus, both components of the complementary pair circuit may be enhancement mode devices. Since relatively high carrier concentrations can be employed in the discreten andp-type films, the doping requirements and the processing of the device structures have been considerably eased. The deposition conditions and the ability to deposit high quality silicon on a substrate surface from which silicon has been previously removed are interrelated and are important considerations in performing the two-stage epitaxial process.     

Small cylindrical ultrasound sources for induction of hyperthermia via body cavities or interstitial implants

In this study, small (outside diameter 1 mm) cylindrical ultrasound sources were investigated for induction of hyperthermia in tumours. These ultrasound transducers could be placed in small-diameter body cavities, or they could be used interstitially in brachytherapy catheters. The ultrasound field measurements showed that the field is fairly uniform as a function of the length of the applicator except at the ends where sharp peaks were located. However, there were significant field variations as a function of rotation angle around the transducers. The degree of these non-uniformities varied from transducer to transducer, and also as a function of frequency. The temperature measurements in vitro perfused kidneys showed that therapeutic temperature elevations could be induced in perfused tissues. The radial extent of the therapeutic zone could be increased by circulating water around the applicators, thus avoiding high temperatures on the applicator surface. It was also shown that some control over the temperature distribution along the length of the applicator could be achieved by using a two-element applicator. An array of four applicators implanted in a square pattern with the spacing of 25 mm between the catheters, was able to heat the tissue volume inside of the implant. The results showed that these small ultrasound applicators may offer significant improvement over existing techniques by increasing the penetration depth and the control over the power deposition pattern.