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Neuroscience and neuroprosthetic devices are increasingly in need of more compact less invasive 3-D electrode arrays for interfacing with neural tissue. To meet these needs, a folding 64-site 3-D array architecture has been developed. The microstructure, in which four probes and two platforms are fabricated as a single planar unit, results in a low-profile (<; 350-μm) narrow-platform (0.604-mm2 silicon footprint) implant for cortical use. Signals are routed from 177-μm2 iridium sites through polysilicon lines to the probe back end and then across 4-μm-thick parylene-encased electroplated-gold folding lead transfers to the associated platform. Three levels of interconnect with a 10-μm minimum pitch are utilized for the 32 leads that traverse the platforms. After rapid microassembly, micromachined latches are used to fasten the folded device. Two flexible parylene cables with gold leads at a 20- μm pitch are monolithically integrated with the probes to minimize tethering and avoid any need for lead bonding within the array, and these cables carry the neural signals to a remote circuit module or percutaneous connector. With thin (~15-μm) boron-doped shanks at a ~ 200-μm pitch, the array displaces only 1.7% of the 0.64-mm2 instrumented tissue area, assuming a 100-μm recording range. Neural signals were recorded in vivo from the guinea pig auditory cortex.