From Halopedia: The process of creating an A.I. is different to what most people realize. Instead of simply "programming" an A.I., the A.I. matrix is actually created by sending electric bursts through the neural pathways of a human brain which are then replicated in a superconducting nano-assemblage. This destroys the original brain tissue however, and so the brain being used could only be obtained after the host had died. However, in some instances, as in the case of the A.I. Cortana, the candidate brain was flash-cloned and had its memories transferred to the receptacle organs so that the host, in this case Dr. Catherine Halsey, could remain alive. However, this is extremely inefficient. In the case of Cortana out of twenty of Dr Halsey's cloned brains, only two of them took.