Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
1999-06-15
2003-02-25
Beck, Shrive P. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S125000, C427S304000, C427S305000, C427S259000, C427S282000, C427S229000, C427S098300, C427S437000, C427S443100
Reexamination Certificate
active
06524663
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to metallisation of materials, which may be electrically insulating, electrically semiconducting, or electrically conducting and relates in particular to electroless plating, and more particularly to a method for producing a patterned surface activation film on a substrate for receiving a layer of conducting material in a subsequent electroless plating step, and the use of organometallic compounds for surface activation in the metallisation of materials.
BACKGROUND ART
High specification metallisation of materials, especially electrically insulating and semiconductor materials, is increasingly required in modern electronics and microsystems. A variety of techniques are used in industrial processes, but all have limitations. Electroplating cannot be used on an insulating substrate unless an electrically conducting coating is applied to it. Evaporation techniques require a high vacuum and high temperature and give poor resolution and waste a lot of metal, and are characterised by low volumes. Radio frequency (RF) sputtering requires a high vacuum, gives poor resolution, wastes a lot of metal and requires specialised equipment. Chemical vapour deposition employs toxic and flammable gases. Thick film screen printing is typically restricted to a minimum of 80 &mgr;m line and space widths. Laser or ion beam lithography is expensive, requires scanning to cover large areas, substrate and coating damage can result from localised heating, and in the case of ion beam lithography, a high vacuum is required.
Electroless plating has yielded good quality results, and has the advantage that it can be used to plate electrical insulators and electrical semiconductors as well as electrical conductors, but is expensive because it involves a large number of steps. In particular, electroless plating selectively on some parts of a substrate only involves preparing a patterned surface activation film on the substrate to initiate the subsequent electroless reaction. The current technique for producing a patterned surface activation film on an substrate for receiving a layer of conducting material in a subsequent electroless plating step comprises the steps of:
a) preparing a solution of the film compound;
b) forming a coating of the solution on the insulating substrate and allowing the solvent to evaporate so as to leave a film of the compound;
c) covering the film with a patterned mask;
d) irradiating the film through the patterned mask under vacuum conditions;
e) rinsing the film whereby the non-irradiated parts are removed, leaving a patterned film.
U.S. Pat. No. 4,900,581 to Stuke et al. dated Feb. 13, 1990 identifies two disadvantages of the current technique, namely that the irradiation step which uses a scanning argon laser is slow and that the irradiation step does not yield sharp edges. Stuke proposes the use of sheet wise irradiation predominantly in the ultra-violet spectral range, greatly speeding up the irradiation step and also improving the sharpness of the edges. Both in the prior art acknowledged by Stuke and in the teaching of Stuke, use is made of palladium acetate as the compound of the surface activation film. The Stuke patent claims embrace metal acetates, metal acetylacetonates and metal formiates.
Further work by Esrom et al., (some of whom are co-inventors with Stuke) published in Journal de Physique Colloque C5, Supplement au No. 5, Tome 50, 5/1989, indicates that, in the case of palladium acetate, the compound mentioned in Stuke, the ultraviolet wavelengths used are in a wavelength range shorter than 190 nm, in which wavelength range light is strongly absorbed by quartz and other common optical glasses, as well as by air. In addition, Esrom states that the optimum processing conditions use a concentration of 0.47×10
−3
M palladium acetate solution, in trichloromethane (chloroform) to produce a dip coated film of 750 Å thickness. This film is then photolysed by placing the coating and substrate material under vacuum, and by exposing the coating to ultraviolet light from an excimer discharge lamp in narrow wavelength range about 172 nm at an unspecified optical intensity for 3-30 minutes, resulting in the production of a 50 Å maximum thickness film of palladium at saturation fluences. The surface roughness of the alumina substrates used in that work is not published.
Further work by Esrom et al., published in Chemtronics 4 (1989) pages 202-208 “VUV light-induced deposition of palladium using an incoherent Xe
2
* excimer source” indicates that, in the case of palladium acetate, using ultraviolet radiation at a wavelength of 172 nm from a silent discharge excimer lamp source in a vacuum chamber containing the sample, the rate of decomposition is enhanced dramatically when the exposure is performed at a pressure of approximately 1 torr. Esrom et al. in this paper explain this effect as enhancement of the removal of the volatile reaction products from the palladium acetate coating, without presenting experimental evidence of this. Esrom states in this work that “patterning was achieved by using metal contact masks”. As metals are opaque to ultraviolet and visible radiation, it is believed that these masks must have had physical apertures made in them at the sites to be irradiated.
Further work by Esrom et al., published in Applied Surface Science Vol. 54 (1992) pages 440-444 “Metal Deposition with a windowless VUV excimer source” has demonstrated the process using irradiation in the wavelength ranges 126±12 nm, and 146±12 nm.
Additionally, a review paper by Esrom and Kogelschatz published in Thin Solid Films (Switzerland) vol. 218 nos. 1-2 (1992) pp 231-246 “Surface Modification with Excimer UV Sources”, demonstrates the process with palladium acetate using incoherent excimer lamp radiation at 222 nm wavelength.
A palladium acetate based selective activation process has been developed by the Technische Universität Berlin in collaboration with the Fraunhofer Gesellschaft Institut für Zuverlassigkeit und Mikrointegration, Berlin. The process is based on the incorporation of palladium acetate within a photopolymerisable (wavelength 366 nm) precursor layer. This coating is selectively exposed, and the unexposed material is rinsed off. The exposed areas form a so-called palladium (II) network, consisting of a palladium acetate impregnated selective polymer coating. The palladium acetate is then reduced by a wet chemical immersion to palladium metal, and electroless plating is performed. The UV exposure does not reduce the palladium (II) acetate to palladium, buy merely links it into a polymer.
Palladium acetate is a metallic salt. It is not an organometallic compound, in which a metal atom is linked directly to one or more carbon atoms.
Specific difficulties with the Stuke process are:
That the process does not produce the required activation at atmospheric pressure or with a monolithic deep-ultraviolet-transmitting quartz or fused silica plate in contact with the coated substrate unless the workpiece is at a minimum temperature and is exposed to a minimum ultraviolet intensity, thereby precluding its practical use with a monolithic aperture-free mask of the type commonly used in photolithography processes and mask aligners, consisting of a transmitting plate of quartz or fused silica with a selective metal coating at the sites on the coating which are not to be exposed to ultraviolet radiation.
The practical limitation of the use of excimer lamp sources to those practically realisable excimer lamps in the air-transmissible ultraviolet (only those centred at 222 nm and 308 nm, which are based on inert gas chloride excited dimer complexes), which practically limits photochemistry with these sources to compounds having a photosensitivity in wavelength ranges close to these centres.
The limitation of the process to the use of metallic salts whose photoactivity has not been optimised for the narrow wavelength range of intense ultraviolet emission from practically realisable excimer lamps in the air
Crean Gabriel M.
Kelly Patrick V.
Macauley Daniel J.
Beck Shrive P.
Kolb Michener Jennifer
Morrison & Foerster / LLP
University College Cork--National University of Ireland
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