Phase-change materials exhibit at least two different states. The states of phase-change material may be referenced to as amorphous and crystalline states. The states may be distinguished because the amorphous state generally exhibits higher resistivity than does the crystalline state. Generally, the amorphous state involves a more disordered atomic structure, while the crystalline state is an ordered lattice. Some phase-change materials exhibit two crystalline states, e.g. a face-centered cubic (FCC) state and a hexagonal closest packing (HCP) state. These two crystalline states have different resistivities. In the following description, the amorphous state generally refers to the state having the higher resistivity, and the crystalline state generally refers to the state having the lower resistivity.
Phase change in the phase-change materials may be induced reversibly. In this way, the phase-change material may change from the amorphous state to the crystalline state, and from the crystalline state to the amorphous state, in response to temperature changes. The temperature changes to the phase-change material may be achieved in a variety of ways. For example, a laser can be directed to the phase-change material, current may be driven through the phase-change material, or current can be fed through a resistive heater adjacent the phase-change material. With any of these methods, controllable heating of the phase-change material causes controllable phase change within the phase-change material.
Typically, semiconductor chips, such as memories, use fuses to configure the chip or deactivate failing portions of the chip. There are two types of fuses, laser fuses and e-fuses. Laser fuses are opened with a laser and e-fuses are opened with an electrical pulse. Alternatively, electrical antifuses can be used in place of fuses. Antifuses break down a thin dielectric to provide a current path. These solutions use a significant amount of chip space and are therefore costly to implement. Laser fuses are limited by the laser focus spot size and e-fuses and antifuses are limited by minimum size requirements for reliable operation. In addition, these fuses and antifuses are only one time programmable (OTP).