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Microfabrication of Polymer Polarization Filters

One of the key components of the polarization image sensor is the design and fabrication of micro-polarization arrays. We have developed pixel-pitch-matched polarization filters using micro fabrication techniques optimized for polymer thin films. In this research effort, two layers of thin film polarization filters are designed and fabricated separately and merged together with 1 micron precision. The final polarization filter is placed on top of the CMOS imaging sensor, demonstrating the synergy between CMOS and polymer technology to realize compact, low power polarization image sensors.

A commercially available thin film polarizer is used to create an array of micropolarizers. The thin film polarizer consists of an iodine-doped Polyvinyl Alcohol (PVA) layer, which acts as a dense array of thin microscopic wires. These microscopic wires are formed by mechanically stretching the polymer film, allowing the molecules of the PVA to align in the direction of stretching.

The microfabrication steps necessery for patterning the PVA are shown bellow:

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Figure 1 :  Microfabrication steps for creating a dual layer polarization structures





 

 

 

 

 

 






 

The final two-tier micro polarization array was tested for its structural and optical properties. A scanning electron microscope (SEM) was used to evaluate the photoresist and the etched PVA structures. The right image in Figure 2, which was recorded under 52 degree angle tilt, presents a single PVA micro structure of size 8 micron by 8 micron and 10 micron thickness. We can observe that the top of the PVA structure is not smooth, which indicates that it has been partly etched. This is a result of small variations in the PVA thickness as well as variations in the SU-8 photoresist. Hence, precise etching time for the PVA can not be determined. The top of the PVA structures was spin coated with an adhesive promoter AD420 since it has an index of refraction similar to the PVA (n=1.41). This prevented any undesirable diffraction which might occur at the surface of the PVA. Although the top portion of the PVA has been etched (around 0.3mm from the 10 mm total thickness), the optical measurements of the PVA structures have shown that the extinction ratios have been preserved during the etching procedure.

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Figure 2 : Scanning electron microscope images of the top PVA tier. Left image shows an array of 8mm by 8mm PVA structures. Right panel shows a single PVA structure with 8mm pitch and 10mm thickness, which was recorded under 52 degree angle tilt.

 

 

 

 








 

 

 

 

References:
[1] Viktor Gruev, Alessandro Ortu, Nathan Lazarus, Jan Van der Spiegel, and Nader Engheta, "Fabrication of a dual-tier thin film micropolarization array," Optics Express, Vol. 15, Iss. 8, pp. 4994-5007, April 16, 2007. (Upenn)
[2] V. Gruev, Van der Spiegel and N. Engheta, "Low Power Image Sensor With Polymer Polarization Filters," (invited) Proc. IEEE ISCAS, Seattle, USA, May 2008.
[3] V. Gruev, Van der Spiegel and N. Engheta, "High-Resolution Integrated Image Sensor with Polymer Micropolarization Array," Proc. Frontiers in Optics 2007, San Jose, California, September 2007.
[4] V. Gruev, K. Wu, J. Van der Spiegel and N. Engheta, "Fabrication of Thin Film Micro Polarization Array," Proc. IEEE ISCAS, Kos, Greece, May 2006. (IEEExplore)