We have developed a selective process for printing a metal catalyst doped ink and electroless plating a conductor on patterned flat surfaces as well as microlens embossed polymer films. The process involves control of surface energy of the films and patterns so as to channel the ink and the conductor between the patterned structures leaving the remaining areas unaffected and transparent. This is a low temperature process and very compatible with flexible substrates and R2R processing.
Objective and BackgroundConventional method for developing conductive traces is photolithography which involves multi-steps processes, high cost of tools and materials. Photolithographic processes for flexible and roll to roll (R2R) scale up are lot more challenging. Selective printing of conductors within patterned structures, especially for features <50µm, has been an unresolved problem. The main objective of this work is to develop conductor film within patterned 3D structures and an inexpensive, scalable process for narrow conductive traces. The approach described in this paper involves tailoring and using surface energy as the control knob to achieve the objective and is applicable to flexible circuits such as RFID tags, printed circuit boards, solar cells, flexible displays, EMI shielding, touch-screens, electroluminescent lights, MEMS devices, etc.Non-lithographic patterning techniques such as screen printing, gravure, flexography, inkjet printing have been implemented for specific applications. Among them, inkjet printing [1] has been widely explored mainly due to its benefits such as easy integration in existing manufacturing lines, in-line roll-to-roll setups, high resolution, printing speed, low material consumption. Such processes usually require thermal curing to achieve the conductivity. This limits the applicability to flexible substrates where process temperatures <100ºC are preferred. Moreover, scale up of such processes on large area such as roll to roll (R2R) is difficult and poor throughput. Fine line conductor metallization and selective printing on structured surfaces such as microlenses or gratings is a challenge. Inkjet printing technology can be categorized based on two methodologies: micro-droplets and drop-on-demand printing using nanoparticle based inks. The resolution of printed conductors reported in literature is ≥20µm line width range [2] on flat surfaces. The key requirements for a good conductor for flexible circuits are, in addition to high conductivity, good adhesion and flexibility / fatigue strength.At Unipixel we develop TMOS © (Time Multiplexed Optical Shutter) displays [3] which operate using a flexible polymer film with microlens structures. The pixel control and actuation, as achieved by electrostatic attraction, requires a selective coating of a conductor film in the valley between microlens structures. More importantly, the lens tips need to be left transparent for optical coupling and light emission. Figs.1a and 1b show a cross-section of the TMOS display in the pixel "off" and "on" stat...
