Depending upon the chosen foundry, there are two methods of prototyping silicon devices. The first is an MPW (Multi Project Wafer) which is usually started every two or three months by a foundry. The customer purchases real estate on the wafer, usually 50 or 100 devices, and these are then used as your prototype silicon. The second way is an MLM (Multi Layer Mask) which is proprietary to each customer and is started when required, thus if you miss the tape out date it does not matter. This method is more expensive but it is an excellent development tool. Engineering wafers are also purchased and around a thousand devices per wafer can be yielded. Redesigns are also quicker as wafers can be held at either pre metal or poly. The MPW is the lower cost but the MLM route is far more flexible as a development tool.