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A G R E AT E R M E AS U R E O F CO N F I D E N C E alloys are what allow them to store binary
data. The high resistance amorphous state is
used to represent a binary 0; the low resist-
ance crystalline state represents a 1. The new-
est PCM designs and materials can achieve
multiple distinct levels [1], for example, 16
crystalline states, not just two, each with
different electrical properties. This allows
Phase Change
a single cell to represent multiple bits, and
to increase memory density substantially,
which is currently done in flash memory.
Memory: The amorphous state vs.
the crystalline state
Fundamentals A brief overview of the differences between
the amorphous and crystalline states may
and Measurement
help clarify how PCM devices work.
In the amorphous phase, the GST mate-
rial has short-range atomic order and low
Techniques
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
Alexander Pronin, Lead Applications Engineer above the melting point, then the GST is sud-
Keithley Instruments, Inc. denly quenched to cool it. The rate of cooling
is critical for the formation of the amorphous
P
layer. The typical resistance of the amor-
HASE CHANGE MEMORY (vari- by melting and quick cooling (or a slightly phous layer can exceed one mega-ohm.
ously abbreviated as PCM, slower process known as re-crystallization). In the crystalline phase, the GST mate-
PRAM, or PCRAM) is an One of the most promising PCM materials is rial has long-range atomic order and high
emerging non-volatile computer GST (germanium, antimony, and tellurium), free electronic density, which results in
memory technology. It may which has a melting temperature in the range lower resistivity. This is also known as
someday replace flash memory because it is of 500