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A programmable read-only memory ( PROM) is a form of digital memory where the contents can be changed once after manufacture of the device. The data is then permanent. It is one type of read-only memory (ROM). PROMs are usually used in digital electronic devices to store low level programs such as firmware or microcode. PROMs may be used during development of a system that will ultimately be converted to ROMs in a mass produced version. These types of memories are used in , video game consoles, mobile phones, radio-frequency identification (RFID) tags, implantable medical devices, high-definition multimedia interfaces (HDMI), and in many other consumer and automotive products.
PROMs are manufactured blank and, depending on the technology, can be programmed at the wafer, final test, or system stage. Blank PROM chips are programmed by plugging them into a device called a PROM programmer. A typical PROM device has an array of memory cells. The bipolar transistors in the cells have an emitter that is connected to a fuse called a polyfuse. To program a PROM is to strategically blow the polyfuses.
Commercially available semiconductor antifuse-based OTP memory arrays have been around at least since 1969, with initial antifuse bit cells dependent on blowing a capacitor between crossing conductive lines. Texas Instruments developed a MOS gate oxide breakdown antifuse in 1979.See US Patent 4184207 - High density floating gate electrically programmable ROM, and US Patent 4151021 - Method of making a high density floating gate electrically programmable ROM A dual-gate-oxide two-transistor (2T) MOS antifuse was introduced in 1982. Chip Planning Portal. ChipEstimate.com. Retrieved on 2013-08-10. Early oxide breakdown technologies exhibited a variety of scaling, programming, size and manufacturing problems that prevented volume production of memory devices based on these technologies.
Another form of one-time programmable memory device uses the same semiconductor chip as an ultraviolet-EPROM (UV-EPROM), but the finished device is put into an opaque package, instead of the expensive ceramic package with transparent quartz window required for erasing. These devices are programmed with the same methods as the UV EPROM parts but are less costly. Embedded controllers may be available in both field-erasable and one-time styles, allowing a cost saving in volume production without the expense and lead time of factory-programmed mask ROM chips. Ken Arnold, "Embedded Controller Hardware Design", Newnes, 2004, ISBN 1-878707-52-3, page 102
Although antifuse-based PROM has been available for decades, it wasn’t available in standard CMOS until 2001 when Kilopass Technology Inc. patented 1T, 2T, and 3.5T antifuse bit cell technologies using a standard CMOS process, enabling integration of PROM into logic CMOS chips. The first process node antifuse can be implemented in standard CMOS is 0.18 um. Since the gate oxide breakdown is less than the junction breakdown, special diffusion steps were not required to create the antifuse programming element. In 2005, a split channel antifuse deviceSee US Patent 7402855 split channel antifuse device was introduced by Sidense. This split channel bit cell combines the thick (IO) and thin (gate) oxide devices into one transistor (1T) with a common polysilicon gate.
The bit cell is programmed by applying a high-voltage pulse not encountered during a normal operation across the gate and substrate of the thin oxide transistor (around 6V for a 2 nm thick oxide, or 30MV/cm) to break down the oxide between gate and substrate. The positive voltage on the transistor's gate forms an inversion channel in the substrate below the gate, causing a tunneling current to flow through the oxide. The current produces additional traps in the oxide, increasing the current through the oxide and ultimately melting the oxide and forming a conductive channel from gate to substrate. The current required to form the conductive channel is around 100μA/100nm and the breakdown occurs in approximately 100μs or less.
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