Article on Phase changing tech

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 [1], 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.
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