Phase Change Memory: Fundamentals and Measurement Techniques March 2010 1 PHASE CHANGE MEMORY (variously abbreviated as PCM, PRAM, or PCRAM) is an emerging non-volatile computer memory technology. It may some day replace flash memory because it is not only much faster and scaleable to smaller dimensions than flash memory, but it’s also more resilient, offering up to 100 million write cycles. This article will address the underlying technology of phase change memory devices and the latest techniques for testing them. What is PCM and how does it work? A PCM cell is a tiny chunk of a chalcogenide alloy that can be switched rapidly from an ordered crystalline phase (with low resistance) to a disordered, amorphous phase (with much higher resistance) through the focused application of heat in the form of an electrical pulse. These same materials are also widely used in the active layers of re-writable optical media such as CDs and DVDs. The switch from the crystalline to the amorphous phase and back is triggered by melting and quick cooling (or a slightly slower process known as re-crystallization). One of the most promising PCM materials is GST (germanium, antimony, and tellurium), which has a melting temperature in the range of 500º–600ºC. The differing levels of resistivity of the crystalline and amorphous phases of these alloys are what allow them to store binary data. The high resistance amorphous state is used to represent a binary 0; the low resistance crystalline state represents a 1. The newest PCM designs and materials can achieve multiple distinct levels , for example, 16 crystalline states, not just two, each with different electrical properties. This allows a single cell to represent multiple bits, and to increase memory density substantially, which is currently done in flash memory. The amorphous state vs. the crystalline state A brief overview of the differences between the amorphous and crystalline states may help clarify how PCM devices work. In the amorphous phase, the GST material has short-range atomic order and low free electron density, which results in higher resistivity. This is sometimes referred to as the RESET phase, because it is usually formed after a RESET operation, in which the temperature of the DUT is raised slightly above the melting point, then the GST is suddenly quenched to cool it. The rate of cooling is critical for the formation of the amorphous layer. The typical resistance of the amorphous layer can exceed one mega-ohm. In the crystalline phase, the GST material has long-range atomic order and high free electronic density, which results in lower resistivity. This is also known as the SET phase because it is formed after a SET operation, in which the temperature of the material is raised above the Phase Change Memory: Fundamentals and Measurement Techniques Alexander Pronin, Lead Applications Engineer Keithley Instruments, Inc. A G R E A T E R M E A S U R E O F C O N F I D E N C E Top electrode Crystalline GST /crystalline GST Thermal insulator Resistor (heater) I Bottom electrode Figure 1. Typical PCM device structure.