M. J. Godbole

Smooth polycrystalline ceramic substrates with enhanced metal adhesion by pulsed excimer laser processing

Multishot pulsed XeCl (308 nm) excimer laser irradiation of commercial fine‐grained polycrystalline alumina substrates is found to significantly improve their properties for metal film‐bonding applications. A smoother surface finish is obtained, and the adhesion strength of subsequently deposited copper films to the laser‐treated alumina surface is increased by a factor of 3–5 (200%–400%) under optimum laser conditions. Smoothing occurs when the alumina melts and undergoes molten flow before resolidifying. XPS measurements suggest that electrical activation of the near‐surface region also may contribute to the enhanced copper adhesion.

Excimer laser ablation and activation of SiOx and SiOx‐ceramic couples for electroless copper plating

A XeCl (4.0 eV photon energy) pulsed excimer laser was used to study the ablation behavior of substoichiometric silicon oxide (SSO), SiOx with x∼1.0. The SSO ablation rate was quite high and its ablation threshold quite low (≤0.3 J/cm2), thereby making it an interesting material for pulsed laser patterning without the use of deep‐UV radiation. Surface activation, as illustrated by subsequent copper deposition by the electroless process, was observed along well‐defined narrow (∼10–20 μm) lines just beyond the edges of ablated trenches in SSO deposited on XeCl‐transparent fused silica substrates. When a thin layer of SSO was deposited on polycrystalline Al2O3 or AlN substrates and subsequently laser treated, surface activation of these ceramics occurred on the laser‐irradiated regions at much lower fluences and with fewer exposures than are required to activate the bare ceramic substrates. In both types of experiment, activation is believed to result from redeposition of elemental silicon, an ablation product.

The crystal structures of stainless steel films sputter-deposited on austenitic stainless steel substrates

Sputter deposited austenitic stainless steels, AISI 304 and 316, almost always result in films having crystalline structures different from the parent material when the deposition temperature is below 648 K. The earlier investigation showed when 316L-SS films were sputter-deposited on glass substrates at room temperature, they contained a mixture of two phases, a ferrite like bcc phase and a hexagonal phase bearing a resemblance to [epsilon]-martensite. Sputter deposition of 304-SS performed at 77 K on rock salt resulted in an amorphous structure; however, when deposited at room temperature, it yielded a structure that could not be indexed either as bcc or as fcc. Thus, in general for AISI 304 or 316 compositions, low temperature depositions result in films with bcc crystal structure, whereas high temperature depositions yield fcc films. In addition to the temperature dependence, it is also relevant to question whether the substrate can influence the nature of the phases present in the films. In this paper, the authors address this question by changing the surface condition of the substrate. With this purpose they have selected a substrate of a similar nature to the target to be sputtered, 316L-SS, and varied the surface preparation.

Laser-induced interface reactions of copper thin films on sapphire substrates

The interface of a copper-sapphire couple that was irradiated with a nanosecond pulsed-excimer laser was studied by transmission electron microscopy. Deposited layers of 30 or 100 nm thickness were laser treated with energy densities in the range of 0.5 to 0.75 J/cm{sup 2}. Two different atmospheres were used during these treatments, viz., air or a mixture of argon-4 vol.% hydrogen. The copper film and a thin alumina layer were melted by the laser pulse. Two well differentiated regions could be observed in the modified layer. The region closer to the unmodified substrate consisted of epitaxially regrown alumina with crystallites misoriented up to 10{degree} relative to the substrate sapphire orientation, while precipitate particles could be seen closer to the resolidified copper. The nature of the precipitates generated in the second region was dependent on the atmosphere present during the treatment. In air a trirutile-like compound was obtained which is either oxygen or copper deficient. In an argon atmosphere a compound having a hexagonal structure closely related to sapphire was observed, where copper substituted for some aluminum. These observations are in agreement with a previously developed mathematical model that predicts melting of a thin substrate layer.

Enhanced metal-ceramic adhesion by sequential sputter deposition and pulsed laser melting of copper films on sapphire substrates

A study is presented of the effect of pulsed XeCl (308 nm) laser treatment on the adhesion between sputter-deposited copper films and sapphire substrates. Laser treatment (LT) of individual 80 nm thick copper films results in adhesion enhancement, relative to the assputtered film for XeCl energy densities >0.35 J cm−2. Thicker (∼ 1μm), strongly adherent copper films can be built up by alternating discrete and sequential sputter deposition with pulsed laser irradiations carried out in air. This sequential process yields smooth films whose adherence, as measured by the scratch test, is a factor of more than two to three greater than for as-sputtered films. The only way to remove the copper layer after irradiation was by cutting through the sapphire. Although formation of a metal oxide is a common consequence of LT in air, adhesion tests reveal no significant effect of carrying out LT in oxidizing or reducing atmospheres. During the earliest stages of the sequential process, the laser-melted film tends to break into small clusters. It is concluded that this process is driven by a surface energy gradient generated by lateral thermal gradients in the melt. These gradients, in turn, are due to the early establishment of isolated regions of good bonding and thermal contact with the substrate. One of the characteristic features of the sequential process is that this good bonding, once established in a given region, is maintained throughout successive meltings of the region. Adhesion mechanisms under LT are discussed.

Laser enhanced adhesion of copper films to sapphire substrates

Strongly adherent films of copper on sapphire can be grown by discrete and sequential sputter depositions alternating with pulsed laser treatment (PLT). In this work the influence of various processing parameters on the film adhesion and morphology are analyzed. Ultraviolet excimer lasers produce less damage to metallic films than do other visible or infrared pulsed lasers. However, the laser energy density must be carefully controlled in order to prevent excessive evaporation or film damage. Film damage and separation from the substrate are likely to occur during PLT if the initial copper films are thicker than 80 nm. Transmission electron microscopy reveals the formation of an intermediate copper/alumina compound in a 30‐nm‐thick film grown in two steps (10 nm+PLT+20 nm+PLT). The regions of strong copper–sapphire bonding and good thermal contact that are established in the early stages of the sequential deposition process, and that are maintained during the subsequent process of building up a thick, adh...

Detection of a metallic glass layer by x-ray diffraction

A simple, precise method for obtaining the average thickness of an amorphous layer formed by any surface treatment has been developed. The technique uses an x-ray diffractometer to measure the reduction in the integrated intensity of several diffracted x-ray lines due to the near surface amorphous layer. The target material for generation of x rays is selected so that the emitted x rays are strongly absorbed by the specimen. This method permits thickness measurements down to --100 nm. It has been tested on a specimen of Fe/sub 80/B/sub 20/ on which an amorphous layer was produced by pulsed XeCl (308 nm) laser irradiation; the amorphous layer thickness was found to be 1.34 ( +- 0.1) ..mu..m.

Use of sputter-deposited 316L stainless steel ultrathin films for microbial influenced corrosion studies

Ultra thin films (12nm) were sputter deposited onto cylindrical germanium internal reflection elements pre-coated with a thin (2 nm) layer of Cr{sub 2}O{sub 3}. Two crystals were inserted into Circle cell flow-through chambers and mounted on the optical bench of an Fourier Transform Infrared (FT-IR) spectrometer. One chamber was maintained as a sterile control while the other was sequentially inoculated with four bacterial species: Psudomonas aeruginosa, Bacillus subtillis, Hafnia alvei, and Desulfovibrio gigas, in that order. The water absorption band (1640cm{sup -4}) was monitored and used to follow that deterioration of the ultra thin films. In this respect, the sterile control and inoculated films exhibited only slight differences during the 1000h course of the experiment. Assay of the visible biofilm that has accumulated on the surface of the inoculated crystal after 1000h revealed that the film incorporated viable cells from all four strains.

The Effect of Cooling Rate During Rapid Solidification on the Structure and Texture of NiTi

A study has been conducted on the effects of increasing cooling rate during rapid solidification of NiTi upon the phases that are produced. The hammer and anvil rapid solidification technique and laser melting with a nanosecond excimer laser were used, which allow the cooling rate to be varied by three to four orders of magnitude. Although 1/3 (110) superlattice reflections are seen in the selected area diffraction (SAD) patterns of the splat quenched (SQ) specimens, x-ray diffraction analyses show the presence of only B2 phase and martensite. On the other hand, laser treatment (LT) of the specimens produces a layer that has a Ll/sub 0/ structure with a slight monoclinic distortion. This phase can be envisaged as a small distortion of a B2 unit cell with a volume per atom approx.3.3% lower than the equilibrium B2 phase. Also martensite is present in the layer. SQ alloys exhibited a marked (200) texture due to columnar growth opposite to the direction of heat extraction, while LT produces epitaxial regrowth of the melted layer. No substantial disordering is obtained in NiTi rapidly solidified alloys.

Substrate surface effects on the properties of sputter-deposited and laser-irradiated films

Sapphire substrates, mechanically polished to an optical finish, were annealed for two days at either 1000{degrees}C or 1350{degrees}C. The near surface condition of as-polished and of the annealed substrates was analyzed by Rutherford backscattering/channeling (RBS-C) and by scanning electron microscopy/channeling (SEM-C), by transmission electron microscopy (TEM), and by scanning tunneling microscopy (STM). The polished substrates were found to be RBS-amorphous up to 100 nm, and heavily damaged at larger depths. In agreement with these results, no electron channeling was obtained from polished samples. TEM, however, showed that the damaged region was crystalline, and the only defects detected were microtwins. Both RBS-C and SEM-C analyses revealed that the damage is removed when the sapphire substrates are annealed for 48 hrs. at 1350{degrees}C. The condition of the near-surface region, viz., as-polished or annealed, is found to strongly affect the morphology of the laser-irradiated copper films deposited on sapphire substrates. A correlation is found between the threshold for film evaporation and for film rupture upon laser irradiation, both being a function of the substrate condition. It is concluded that the near-surface damaged layer acts as a thermal barrier for heat transport across the substrate.