A programmable read-only memory (PROM), a field programmable read-only memory (FPROM), and a one-time programmable non-volatile memory (OTP NVM) are forms of digital memory where the setting of each bit is locked by a fuse or an anti-fuse. These PROMs may be used to store programs permanently. One difference between a read-only memory (ROM) and a PROM is that with a PROM the programming is applied after the device is constructed.
PROMs are often manufactured blank and depending on the technology can be programmed on a wafer, during final test, or in a system. The availability of this technology allows companies to maintain a supply of blank PROMs in stock, and program them at the last minute to avoid a large volume commitment. These types of memories are frequently seen in video game consoles, mobile phones, radio-frequency identification tags, implantable medical devices, high-definition multimedia interfaces and in many other consumer and automotive electronic products.
An anti-fuse is an electrical device that performs the opposite function to a fuse. A fuse starts with a low resistance. When a fuse is “blown” (a blown fuse typically occurs when the current through the low resistance path exceeds a specified current limit), a permanent break (open) in the previously electrically conductive path occurs. An anti-fuse starts with a high resistance and is designed to permanently create an electrically conductive path. For example, an anti-fuse may consist of a thin gate oxide transistor laid out in such a way that when the thin gate oxide is ruptured (programmed), a channel diode-connected transistor is formed between a word line and a bit line. The thin gate oxide is ruptured by applying a high voltage on the gate of the channel diode-connected transistor.
When an anti-fuse is programmed, the rupture may occur in a location on the thin gate oxide that shorts a word line to a bit line instead of forming a channel diode-connected transistor between the word line and the bit line. In another example, when an anti-fuse is programmed, the rupture may occur in a location on the thin gate oxide that creates a highly resistive gate-to-drain diode connection rather than a low resistance gate-to-drain connection for the channel diode-connected transistor that is desired. Therefore, it is important that the rupture occur at a location on the thin gate oxide that creates a low resistance gate-to-drain diode connection for the channel diode-connected transistor.