PRINTING TEchniques

Semiconductors are typically produced by depositing a layer of material, spinning a photo sensitive resist over that layer, exposing the resist to light by projecting a pattern on the photoresist, developing the photo resist, etching the material to produce the desired pattern, and then to remove the resist. This is an inefficient and expensive process compared to printing, a simple additive process. However, this is required when dealing with inorganic materials which are typically blanket deposited at high temperature.

Circuits consisting of organic semiconductors, dielectrics, and conductors can be patterned using print technology at a far lower cost. For the most part, these electronic inks can be printed using conventional print equipment. The particular equipment that can be used for any given layer will depend on considerations such as space/feature resolution targets, thickness control required, and the viscosity and base solution of the electronic inks. Conventional printing techniques that are available for consideration in developing a high volume process are gravure, screen, flexography, and ink jet.

While any of these printing choices are contenders for the 30µ design rules (or greater), other techniques are required to achieve smaller dimensions. The OrganicID process has design rules compatible with print equipment for all layers other than one- the source/drain metal which has a target design rule of 2µ. This can be achieved using one of a number of techniques including micro-contact printing, surface energy modification, and laser ablation.

In micro-contact printing, an ink consisting of suspended palladium particles is transferred to the substrate with a PDMS (silicone) stamp. The stamp is inked with the palladium solution, and is then pressed onto the surface to be patterned. The soft PDMS stamp makes conformal contact with the surface and molecules are transferred directly from the stamp to the surface in the space of a few seconds. The substrate is then allowed to soak for about 1 minute in an electroless solution which builds up the pattern of palladium particles to a layer of metal that is about 40 nanometers thick.

In surface energy modification, the substrate is patterned with hydrophilic features and hydrophobic spaces. When the ink is deposited, it is repelled from the hydrophobic areas and is deposited on the hydrophobic areas.

In laser ablation, a light image is projected though a mask and focused on the substrate. The light image is produced with a laser tuned to the right amount of energy to vaporize the non-image areas. This energy setting takes into account the type of material and the depth of the removal.