Antennas at millimeter and submillimeter wavelengths present formidable challenges. One approach is to scale up lower-frequency microwave systems of hollow-metal waveguides and horns. The problem with this is that as the frequency goes up, the waveguides get smaller, and the fabrication becomes difficult \udand expensive. Another way is to follow the lead of low-frequency integrated circuits and develop monolithic integrated circuits. This is attractive because there is the possibility of integrating thin-film metal antennas, solid-state devices, and low- frequency processing circuits, all on the same chip. Moreover, a large \udnumber of devices can be made together simultaneously to \udform an array or imaging system. This means we should develop antennas that work on the common integrated-circuit substrates: quartz, silicon, and gallium arsenide.
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