Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2001-06-27
2003-02-18
Barr, Michael (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S271000, C427S272000, C427S287000, C427S304000, C427S305000, C427S306000, C427S404000, C427S437000, C427S443100
Reexamination Certificate
active
06521285
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for electroless deposition of conductive material on a substrate, using a stamp having a surface onto which ink is applied, preconditioning the substrate by providing a seed layer having enhanced affinity between the ink and the preconditioned substrate and bringing the surface of the stamp into contact with the preconditioned substrate. After removing the stamp from the substrate, the printed substrate is immersed in a plating bath wherein metal-ions are dissolved and deposit on the printed regions of the substrate to form metallic structures.
BACKGROUND OF THE INVENTION
Electroless deposition of conductive material, such as metals, is a well-known process for producing fine metal patterns in printed boards. Electroless deposition occurs by means of an autocatalytic redox process, in which the cation of the metal to be deposited is reduced from a plating bath by a reducing agent dissolved in this bath onto the surface of a catalyst used to initiate the deposition. Noncatalytic surfaces therefore have to first be activated with the catalyst, such as for example palladium, before metalization can occur.
Selective electroless deposition can be achieved either by the selective deactivation of a catalytic substrate or by the selective activation by a catalyst of a non-reactive surface. Current electroless deposition processes rely on photopatterning for defining the catalytic pattern on the substrate. This approach however requires expensive lithographic tools and facilities. Moreover, the coating of large substrates by deposition of the catalytic layer to be patterned is not trivial and very expensive.
Another approach for producing a patterned catalyst is described in P. C. Hidber's article, “Microcontact Printing of Palladium Colloids: Micron-Scale Patterning by Electroless Deposition of Copper”, Langmuir 1996, 12, pp. 1275-1380. In this work, a stamp having a micron-scale pattern is used for microcontact printing the catalyst on a surface. First the stamp having a micro-scaled pattern is inked with a solution of colloids serving as a catalyst for the subsequent selective electroless deposition of, for example, copper. In this example, the stamp is made of an elastomeric material and is inked with a solution of toluene in which Pd-colloids are dissolved. Before contacting the surface of the substrate with the inked micropatterned surface of the stamp covered with Pd-colloids, the surface of the substrate is pretreated to increase the interaction between the catalyst and the target substrate. The pretreatment of the substrate includes cleaning the surface, oxidation of the surface in order to form surface hydroxyl groups and silanization of the surface by immersing it in a solution of organosilanes, in ethanol or heptane. After the pretreatment of the surface of the substrate, the stamp is applied onto the pretreated substrate transferring Pd-colloids in the regions of contact to the surface of the substrate thereby forming the Pd catalytic pattern. After removing the stamp from the substrate surface, the substrate is immersed in a plating bath containing dissolved metallic ions such as cuprate. Metalization only occurs where the substrate is activated with the palladium colloid so that metallic copper structures are formed where the catalyst has been printed.
The above mentioned technique suffers from important drawbacks, however. First, colloids are particles aggregated in a liquid and held together by attractive forces which form an entity of a much larger size than their individual components and on which, consequently, gravity has an effect so that uncontrolled deposition to solid-liquid interfaces (bottom of a chemical flask and surfaces of stamps for example) can occur. Second, the undefined nature of these attractive forces precludes to long lifetime of the colloid solution; ink solutions having catalytic colloids must be used freshly after their preparation. The two points previously mentioned adversarily affect homogeneous distribution of palladium colloids on stamps after their inking resulting in an inhomogeneous distribution of palladium colloids resulting in an irregular thickness and density of the colloidal catalyst printed on the substrate surface which then leads to irregular electroless deposition of a metal layer onto the areas of the substrate covered with the catalyst. Furthermore, typical microscopic size of palladium colloids limits the resolution of the copper electroless plated pattern since the average size of the colloidal species can be up to 0.5 &mgr;m.
Furthermore, the Pd-colloids are not well-soluble in most of commonly used solvents, such as water or ethanol. However, if toluene is used as a solvent for palladium colloids, the suspension is stable and active for months in the solution but toluene is a strong solvent for many types of polymers and, for example, destroys stamps made of PDMS (Polydimethylsiloxane) after one single use when the stamp is dipped in or covered with toluene. Experiments show that if the elastomeric stamp is covered with a thin film of toluene during inking of the stamp toluene dramatically swells the stamp even for a short inking time which induces local and long range distortions of the micropatterns and reduces the quality of contact between the stamp and the substrate during printing.
It is an object of the present invention to provide a method for electroless deposition of conductive material on a substrate using a stamp having a surface onto which ink is applied, preconditioning the substrate by providing a seed-layer having enhanced affinity between the ink and the preconditioned substrate, and bringing the surface of the stamp into contact with t the preconditioned substrate, thereby preventing all the disadvantages listed above in describing the state of the art. The invented method will prevent any deformation of the stamp caused by the solvent in which the stamp is inked. In order to achieve high quality electroless deposition of a metal in patterns on the substrate, it is also an object of the invention to deposit on the substrate the patterned layer of a catalyst with a homogeneous thickness, high purity and high density.
SUMMARY OF THE INVENTION
The present invention relates to a method for electroless deposition of conductive material on a substrate. The substrate can be a conductive or non-conductive material preferably made of glass, Si doped or not, SiO
2
, or polymers. In the following, the substrate will be patterned by means of microcontact printing using an inked stamp to provide catalytic particles for electroless deposition of metals from the ink on the stamp to the target substrate. Before printing, the substrate first will be preconditioned with a seed layer to enhance the affinity between the catalytic particles from the ink and the substrate. Suited for the seed layer is a thin evaporated film of titanium having a thickness of at least 0.5 nm. A thin self-assembled monolayer formed from a solution or a sol attached from a liquid phase to the target substrate can also be used for the seed layer. Altematively, substrates rendered hydrophilic by simple oxidation by means of O
2
-based plasma treatment or immersion in a solution containing oxidants like HSO
4
/H
2
O
2
is possible. Roughening the surface of the substrate with mechanical polishing and gas phase or wet chemistry etches are also possible to enhance the interaction between the catalyst to be printed and the substrate.
The surface of the stamp is also pretreated to render its surface wettable by any polar ink. This directly allows the use of polar catalysts or polar catalytic precursors in the ink and solves problems associated to the use of colloidal catalysts like Pd colloids. In order to render the surface of the stamp wettable by the ink, the surface of the stamp is first exposed to an O
2
-plasma or is oxidized by means of wet chemistry. This operation is very favorable because catalysts are usually polar and the best material to form patterned stamps is polydimethylsi
Biebuyck Hans
Delamarche Emmanuel
Geissler Matthias
Kind Hannes
Michel Bruno
Barr Michael
Connolly Bove & Lodge & Hutz LLP
International Business Machines - Corporation
Trepp Robert M.
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