Structured ASIC
Beginner
A half-made chip. The bottom layers are the same for everyone. Only the top wires are made for your design.
Novice
A chip built on a prefabricated base of logic and memory shared by many customers. Each design customizes only one or a few top metal and via layers, so it needs only those masks: cheaper and faster than a full custom chip, more efficient than an FPGA.
Expert
Metal- or via-programmable base array. NRE falls with the number of custom layers, while delay and power improve with a few more (Ahmed et al.). Sits between FPGA and cell-based ASIC on unit cost, power and NRE; adoption has stayed narrow.