Antifuse

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An antifuse is an electrical device that performs the opposite function to a fuse. Whereas a fuse starts with a low resistance and is designed to permanently break an electrically conductive path (typically when the current through the path exceeds a specified limit), an antifuse starts with a high resistance and is designed to permanently create an electrically conductive path (typically when the voltage across the antifuse exceeds a certain level).

[edit] Antifuses in integrated circuits

Antifuses are widely used to permanently program integrated circuits (ICs). ICs that use antifuse technology employ a thin barrier of non-conducting amorphous silicon between two metal conductors. When a sufficiently high voltage is applied across the amorphous silicon it is turned into a polycrystalline silicon-metal alloy with a low resistance, which is conductive.

Certain programmable logic devices (PLDs) use antifuse technology to configure logic circuits and create a customized design from a standard IC design. Antifuse PLDs are one time programmable in contrast to other PLDs that are SRAM based and which may be reprogrammed to fix logic bugs or add new functions. Antifuse PLDs have advantages over SRAM based PLDs in that like ASICs, they do not need to be configured each time power is applied. They may be less susceptible to alpha particles which can cause circuits to malfunction. Also circuits built via the antifuse's permanent conductive paths may be faster than similar circuits implemented in PLDs using SRAM technology. QuickLogic Corporation refers to their antifuses as "ViaLinks" because blown fuses create a connection between two crossing layers of wiring on the chip in the same way that a via on a printed circuit board creates a connection between copper layers.

Antifuses may be used in programmable read-only memory (PROM). Each bit contains both a fuse and an antifuse and is programmed by triggering one of the two. This programming, performed after manufacturing, is permanent and irreversible.

The antifuse is usually triggered using an approximately 5 mA current. With a poly-diffusion antifuse, the high current density creates heat, which melts a thin insulating layer between polysilicon and diffusion electrodes, creating a permanent resistive silicon link.

[edit] Macro-scale antifuses

Antifuses are also seen in the mini-light (or miniature) style low-voltage Christmas tree lights. Ordinarily (for operation from mains voltages), the lamps are wired in series. (The larger, traditional, C7 and C9 style lights are wired in parallel and are rated to operate directly at mains voltage.) Because the series string would be rendered inoperable by a single lamp failing, each bulb has an antifuse installed within it. When the bulb blows, the entire mains voltage is applied across the single blown lamp. This rapidly causes the antifuse to short out the blown bulb, allowing the series circuit to resume functioning, albeit with a larger proportion of the mains voltage now applied to each of the remaining lamps. The antifuse is made using wire with a high resistance coating and this wire is coiled over the two vertical filament support wires inside the bulb. The insulation of the antifuse wire withstands the ordinary low voltage imposed across a functioning lamp but rapidly breaks down under the full mains voltage, giving the antifuse action. Occasionally, the insulation fails to break down on its own, but "tapping" the blown lamp will usually finish the job. Often a special bulb with no antifuse and often a slightly different rating (so it blows first as the voltage gets too high) known as a "fuse bulb" is incorporated into the string of lights to protect against the possibility of severe overcurrent if too many bulbs fail.

In a similar fashion, before the advent of high-intensity discharge lamps, street light circuits using incandescent light bulbs were often operated as high-voltage series circuits. Each individual street-lamp was equipped with a film cutout; a small disk of insulating film that separated two contacts connected to the two wires leading to the lamp. In the same fashion as with the Christmas lights described above, if the lamp failed, the entire voltage of the street lighting circuit (thousands of volts) was imposed across the insulating film in the cutout, causing it to rupture. In this way, the failed lamp was bypassed and illumination restored to the rest of the street. Unlike Christmas lights, the circuit usually contained an automatic device to regulate the electrical current flowing in the circuit, preventing the current from rising as additional lamps burned out. When the failed lamp was finally changed, a new piece of film was also installed, once again separating the electrical contacts in the cutout. This style of street lighting was recognizable by the large porcelain insulator that separated the lamp and reflector from the light's mounting arm; the insulator was necessary because the two contacts in the lamp's base may have routinely operated at a potential of several thousands of volts above ground/earth.

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