Next generation cosmic microwave background (CMB) polarization anisotropy measurements will feature focal plane arrays with more than 600 millimeter-wave detectors. We make use of high-resolution photolithography and wafer-scale etch tools to build planar arrays of corrugated platelet feeds in silicon with highly symmetric beams, low cross-polarization and low side lobes. A compact Au-plated corrugated Si feed designed for 150 GHz operation exhibited performance equivalent to that of electroformed feeds: ~-0.2 dB insertion loss, <-20 dB return loss from 120 GHz to 170 GHz, <-25 dB side lobes and <-23 dB cross-polarization. We are currently fabricating a 50 mm diameter array with 84 horns consisting of 33 Si platelets as a prototype for the SPTpol and ACTpol telescopes. Our fabrication facilities permit arrays up to 150 mm in diameter.
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