1. Field of the Invention
The invention relates to a writing system and method for a phase change memory.
2. Description of the Related Art
With the growth in the use of portable electronic devices, demand for non-volatile memory has increased. Among the various kinds of non-volatile memory, phase change memory is the most competitive next generation non-volatile memory due to its higher speed, lower power consumption, higher capacity, reliability, easier process integration and lower cost.
The operation of a phase change memory is mainly achieved by inputting two current pulses with different current magnitudes to the phase change memory to switch the phase change memory between an amorphous state and crystalline state. According to Ohm-Joule's Law, when the current is input to the phase change memory, the phase change memory is heated. The phase change memory may thus be crystallized or melted based on different currents. Based on the described illustration, the logic state of the phase change memory can be switched by inputting different currents, enabling data storage. FIG. 1 is a schematic diagram showing the writing current pulse and the reading current pulse of the phase change memory. When a RESET operation is applied to the phase change memory, a reset current IRESET with high amplitude and short pulse width is applied, the phase change memory is thus melted because the temperature of the phase change memory exceeds the melting temperature of the phase change material of the phase change memory, Tm. When the temperature of the phase change memory decreases, the state of the phase change memory is transformed to the amorphous state due to the insufficient cool down period. Thus, the phase change memory has high resistance. When a SET operation is applied to the phase change memory, a set current ISET with lower amplitude and longer pulse width is applied. The phase change memory is heated by the set current ISET, and the temperature of the phase change memory is held substantially between the melting temperature Tm and a crystallizing temperature Tc of the phase change material used by the phase change memory. During the SET operation, the melted phase change memory has sufficient time for crystallizing and the phase change memory thus has a low resistance.
As described, the phase change memory respectively stores data with logic state 1 and 0 by the RESET operation and the SET operation. When reading the phase change memory, a read current IREAD the amplitude of which is less than the set current ISET, is applied to the phase change memory to determine the logic state of the data stored in the phase change memory based on the sensed resistance.