Investigation on a new silver photoimageable conductor

Abstract

A new thick film photoimageable silver paste of improved properties has been elaborated. The paste is less sensitive to the visible light. Certain disadvantages of most commercially available materials, such as necessity of operating with the paste in the room illuminating with yellow, green or red light has been eliminated. Further improvement of conductive silver pastes to enable reducing lines width down to 10 μm has been done.

Introduction

Photoimageable pastes have been existing on the market since many years. The most popular are Du Pont FODEL series and ESL 9904 silver conductor [1], [2], [3], [4]. The pastes enable to produce fine lines of 50 μm width and 50 μm space, but the pastes are sensitive to visible light. Lately some research on photoimageable pastes has been done by C-MET in India [5], [6], [7], [8], [9]. Their compositions enable to obtain lines of 100 μm.

This paper describes a new photoimageable silver paste of improved properties. The paste is less sensitive to the visible light. Handling the paste is easier – does not require room illuminated with yellow, green or red light. It is also beneficial for the shelf life of the paste. After the paste is screen printed and dried needs to be exposed to the UV light. The exposure time has been substantially shortened. The development process has been improved by employing a friendly basic solution. Finally, the technical characteristic of the lines made of the paste have been upgraded by achieving better electric properties and reducing the widths of the conductive lines, down to 10 μm. This gives rise to higher integration and performance of electronic components.

Photoimageable pastes are very much like conventional ones. They contain some active ingredients, glass (a permanent binder) and screening agent, organic vehicle (temporary binder). Unlike the traditional vehicle, photosensitive one is a fairly complicated composition. Its role is twofold, from one hand all the powders are suspended in this medium to form screen printable paste, from the other hand, it becomes (after drying) a solid layer sensitive to UV light capable of undergoing photolithography process. In general, photoimageable vehicle is a multi-ingredient composition containing inert and active polymers: multifunctional monomers and oligomers, photo-initiators, sensitizers, antioxidants, plasticizers, dispersing agents and solvents. All the components are commercially available except the active polymers. The latter were synthesized in-house (patent in preparation). The outstanding properties of the new active polymers gave a possibility to adjust properties of the photosensitive vehicles.

Each of the components plays a special role. Multifunctional monomers and oligomers are supposed to be capable of undergoing radical polymerization. Making the paste less sensitive to the visible light was done by selecting a photo-initiator compatible with the new active polymer, with maximum absorption shifted towards lower wave lengths, and reduced absorption above 400 nm. Well selected proportions of active polymer, photo-initiator and other additives to the elaborated photoimageable silver paste substantially shortened the exposure time to 1–3 s and development time to 6–11 s. The use of more powerful UV light source was helpful as well. The exposure time applied to similar products from other suppliers is 15 s or more and the developing time – is in the range of 15–50 s [1], [2], [3], [4].

The stability of photosensitive paste depends mostly on the degradation caused by auto-oxidation (that is oxidation by oxygen present in the ambient). A longer shelf life of the paste was achieved by the use of a full range of antioxidants: hydrogen donors (primary antioxidants), hydro-peroxide decomposers (secondary antioxidants) and radical scavengers (radical traps). The application of carefully selected antioxidants, led to a longer shelf life of the elaborated paste above 6 months.

Alkaline solutions, such as sodium hydro-carbonate, necessary to develop UV irradiated layer, if not washed perfectly, bring a danger of leaving some unwanted inorganic salts in the fired layer. Instead of alkaline solution, a basic solution of organic amine, fully convertible into gasses during firing has been applied. The applied ethanolamine is harmless to human beings.