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Process for ITO-coating of polymer

ITO is the most widely used transparent electrode. It is an inorganic layer deposited mainly on glass using high temperature process deposition which is widely used in opoelectronics devices and more specifically for flat display (LCD, FED). This commercial ITO is mainly available on planar glass substrates, the deposition process can then use a high temperature annealing step to crystallize the ITO deposited layer.

With the evolution of large display to organic materials (OLED), polymer substrates are beginning to be used so a considerable research effort has been made to deposit low temperature ITO on polymer foils used for the OLED technology (PET, PC for example) but this process is still at a development stage and not commercially available.

The ITO specification chart for OLED applications lead to a deposition process compatible with planar substrates, the key parameter for this application being an optimised transparency in the visible spectrum.

For bio-fluidic applications where the detection is very often optical observation or measurement, ITO is also the best candidate as a transparent electrode. In this field of applications pulled by the need of low cost disposable devices, the last years have seen a strong development of polymeric microsystems complementarily with well established silicon and glass micro-devices. It can be foreseen that for the production of large volume, low cost disposable devices (for diagnosis for example) replicated polymer micro-fluidic devices would be the commercial efficient solution.

Therefore there is a need of a low temperature ITO process compatible with polymer micro-system production (ie. with non planar substrates), optimised foremost for its electrical characteristics and, to a second extent, for its optical transparency.

In the Medics device, the observation of cell manipulation is done using an optical fluorescence microscope through the fluidic cap. The cap therefore has to be transparent for visible light and non fluorescent. A counter electrode deposited on the fluidic cap is used to create the DEP field. So this electrode has also to fulfil the transparency requirement for cell observation. Biological and fluidic compatibilities have to be respected.

Thus we had to develop a specific process to fulfil our requirements:
- A low temperature ITO process, compatible with polymer substrates (Tmax < 150°C);

- Deposition on a non planar substrate keeping an electrical continuity of the ITO layer at the vertical edges;

- Compatibility of ITO layer with the biologic liquids;

- Good electrical characteristics for DEP.

The deposition is made using a DC magnetron sputtering process in an MRC equipment.

In this equipment the samples to be covered are moved above the target, the translation speed giving the thickness of the layer. This translation also leads to different deposition incidence angles of the species on the substrate and gives very good covering characteristics on a vertical plane.

Electrical characteristics of the ITO layer are given by the layer's crystalline / amorphous structure and thickness.

The higher thicknesses lead to lower sheet resistance but also to lower optical transmission.

Another difficulty is the mechanical behaviour of the ITO layer on the polymeric substrate. Polymers have a much higher dilatation coefficient than the ITO inorganic layer, so the temperature crystallisation step of the ITO induces stress in the ITO layer, which can generate cracks and adhesion failure. This effect is enhanced when the layer is in contact with liquids: either alcohol for the cleaning and sterilization protocols or aqueous solutions used for the biological experiments.

We have developed a process, which gives unreached electrical and optical qualities of ITO on polymer as well as exceptional mechanical adhesion behaviour of the ITO layer on a polymer surface.

Stabilized Results:
- ITO layer characteristics on Polycarbonate (PC) and a SU8 photoresist.

- Rsq= 25-30Wsq or a R=4 10-4 Ohm.cm.

- Optical transmission > 90% in the visible range.

- With a lifetime in biological fluid conditions (ie no cracks or adhesion failure) > 30 days.

This layer also shows no damage after the dicing step, allowing a complete collective fabrication process of the fluidic caps.

The process has been developed and validated on a 4¿¿ wafer.

More information on the MEDICS project can be found at: http://www.siliconbiosystems.com/MeDICS/

Informazioni correlate

Reported by

Silicon Biosystems srl
Via S. Stefano 132
40125 Bologna
Italy
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