An in situ phase-change memory cell study using nanometer scale x-ray optics
Jan H. Richter (jan.richter@aist.go.jp),1 Milos Krbal,1 Alexander V.
Kolobov,1, 2 Paul Fons,1, 2 Robert E. Simpson,1 and Junji Tominaga1
1Nanoelectronics Research Institute, National Institute of Advanced Industrial
Science and Technology 1-1-1 Higashi, Tsukuba 305-8562, Ibaraki, Japan
2SPring8, Japan Synchrotron Radiation Institute (JASRI),
Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan

We present for the first time an x-ray absorption spectroscopy study focusing on individual phasechange material(Ge2 Sb2 Te5 ) memory cells in fully integrated PC-RAM structures. Devices wereinvestigated employing an x-ray nanobeam of only about 300 nm diameter, which could be fully
contained within the spatial extent of the heater element within a single cell and enabled us toinvestigate individual devices without interference from non-switching material surrounding thearea of interest. By monitoring the fluorescence signals of tungsten and germanium at a photon
energy corresponding to the Ge K-edge absorption edge white line position, we were successful inproducing 2D area maps of the active cell region clearly showing the imbedded tungsten heater
element and the switched region of the phase change material. Additionally, position dependentchanges in the phase change material could be traced by taking an array of XAFS spectra of theGe K-edge on and in the vicinity of individual devices.

Due to the continuous development in consumer electronics towards increased performance and smaller device
sizes, Si technology will soon reach its physical limit asa memory material. The unsatiated demand for higher
density memory material will ultimately lead to the emergence of new types of memory storage systems from novelmaterials. Phase change materials are a promising candidate for fulfilling this role[1] and have already successfully
been used for years in optical storage medium (DVD andBlu-ray) technology. Phase change random access memory (PCRAM) utilizes a different data storage mechanism from Si technology with inherent scaling ability toa much lower ultimate scaling limit[2, 3]. In PCRAM materials information is stored in the local crystal structureof the material as opposed to charge trapping in Si based
technology[4]. Phase change materials can be reversibly
and stably switched between an amorphous and a crystalline phase on a timescale less than 50ns[5, 6] by injecting energy into the system, which is usually achieved
through either laser or electrical pulses. Originating fromthe atomic scale structural differences the two phases display large optical and electrical differences[7], with thecrystalline phase usually possessing higher refractive index, optical absorption and electrical conductivity. For
phase change material scaling down, reduces the power
input demand as the active volume decreases[8]. Once set
the material will reliably remain in its current state for
decades, which makes it suitable for long term data storage, but is also interesting from a different perspective.
This means PCRAM would retain information even if thedriving device was turned off unlike Si based technology,
where the information needs to be constantly recycled
and is immediately lost upon turning off.
The pseudobinary (GeTe)x (Sb2 Te3 )1−x system has
been intensively studied due to its current application in
optical data storage technology, particularly Ge2 Sb2 Te5

is widely used in DVD technology. Despite its tried andtested usage and extensive research being carried out,
the basic switching mechanism is still not so well understood with the umbrella flip model[9] currently being thebest model to simulate the dynamics of phase change.
In order to gain a more complete understanding of theswitching process it would be most instructive to performin situ real time measurements of single devices on fully
device integrated phase change memory materials as thephase change takes place. In this work we present x-ray
spectroscopy based measurements on single devices employing a nano beam of only about 300nm dimensions,which in the future will enable us to perform these realtime in situ measurements of the switching process onindividual devices.

FIG. 1: Schematic drawing of lance cell type device

The Ge2 Sb2 Te5 phase change layer was deposited on asample chip of about 10mm x 10mm and fully device integrated. The chip consisted of an array of device structures of lance cell type (see fig. 1), i.e. a device consists of the Ge2 Sb2 Te5 phase change layer sandwiched
between tungsten counter and bottom electrodes. The
bottom electrode and phase change layer are separated

2by a dielectric layer with only a small tungsten heater
element defining the active region. A Keithley 4200 SCS
semiconductor characterization system capable of sending electrical pulses ranging from nanoseconds to milliseconds pulse duration and amplitudes up to 10V wasused for the preparation of the initially crystalline cells
performed at AIST, i.e. various cells were switched intoa predetermined state (SET, RESET, as deposited).
Synchrotron radiation based x-ray spectroscopy experiments were carried out at beamline BL39XU at SPring8, Hyogo, Japan. X-ray spectra were taken at high energies at and around the Ge-K edge (11.105keV). For
rough alignment of the sample with the x-ray beam a microscope system was used that permitted viewing of thesample under incident x-rays by allowing the x-ray beam
to pass though an aperture in the microscope mirror optics. The x-ray beam was focused using Kirkpatrick-Baez
x-ray mirror optics into a spot of about 250nm x 350nm
spot size as confirmed by a knife edge scan[10, 11], which
was small enough to fully contain the extent of the x-ray
spot within the dimensions of one heater element, thus
enabling us to exclusively measure the active (switching)region of a single individual device with only minor interference from the surrounding matrix. Due to chip structure dependent variations in the tungsten thickness, theposition of the x-ray beam on the chip could be identified by monitoring the tungsten fluorescence signal ata fixed photon energy while performing line scans, i.e.
translating the sample under the x-ray beam along oneof the natural sample stage directions. In more detailfirst line scans across the entire counter electrode wereperformed locating the center of the counter electrode.
Subsequently more detailed scans of the central area wereperformed enabling us to locate the exact position of themuch smaller heater device. In order to gain insight into
the spatial properties of the amorphous mark created,arrays of XAFS scans were taken at various positions on
and in the vicinity of the heater device. 2D maps of thecontrast between amorphous and crystalline regions ofthe active cell area were generated using two-dimensional
XY scans recording the germanium and tungsten fluorescence response at a fixed photon energy corresponding tothe Ge K-edge absorption edge at about 11.105keV (Ge”white line”).
In this work it was attempted to gain insight into theswitching properties of phase change memory materials
by focusing on the changes occurring in single cells offully device integrated phase change memory material.
We generated 2D maps of the individual cells identifying the tungsten device structures, such as the positionand dimension of the heater element, while simultaneously gaining information on the state of the containedphase change material. In order to gain a more complete
understanding of the switched area spatially overlapping
EXAFS scans along a line crossing the switched area wereperformed. The data presented here was acquired froman amorphized memory cell with heater element dimension of 290 nm x 290 nm and from a crystalline cell withdimensions of 2 microns x 2 microns.

FIG. 2: Ge K-edge XAS spectra; red (a), blue (b) and black(c) colours represent spectra taken directly on, partly on andfar away from the heater element respectively.

Figure 2 shows XAS spectra of the Ge K-edge recorded
on a cell containing an amorphous region in an otherwise crystalline background. The three different spectra
correspond to data collected directly on the heater element (red trace), partly on the heater element (bluetrace) and significantly separated form the heater element (black trace). We should thus expect to find evidence for the amorphous phase in the first, a superposition of crystalline and amorphous features in the second and a purely crystalline spectra in the third spectra.
In Ge2 Sb2 Te5 we expect to find a reduction of about
10% in fluorescence intensity response from germanium
for the amorphous phase as compared to the crystalline
phase at a photon energy corresponding to the Ge Kedge white line at about 11.105keV[9]. A slight change
in the spectral shape of the Ge K-edge spectra is also
expected in the form of an more pronounced shoulder at
photon energies just above the main absorption feature(approximately 11.108keV to 11.116keV). In figure 2 wecan clearly observe the expected differences. The spectral
shape for traces (a) and (b) contains amorphous contributions, while trace (c) is indicative of the crystalline
structure. This is supported by the decrease in whiteline intensity from the spectra recorded in the crystalline
background over the one taken partially on the heater
element to the one fully contained in the heater element.
This indicates we have successfully managed to spatiallytrace the change of phase with trace (c) showing the crystalline background and trace (a) showing the amorphous
region upon the heater element. Trace (b) originates froma superposition of the two different phases.
In the following 2D maps of the active region of the3device and the surrounding area are presented. To thisend the germanium and tungsten fluorescence intensity
response at a fixed photon energy of 11.105keV wasrecorded in an array of 30x30 data points.

FIG. 3: 2D tungsten map of an amorphous cell (290 nm heaterelement): Tungsten x-ray fluorescence intensity as a functionof position recorded at a photon energy of 11.105keV. The
heater element is visible as an area of elevated intensity inthe bottom left corner of the map.

FIG. 4: 2D germanium map of an amorphous cell (290 nmheater element): Germanium x-ray fluorescence intensity as afunction of position recorded at a photon energy of 11.105keV.
The amorphous region of Ge2 Sb2 Te5 is visible as reduced intensity in the bottom left area of the map.

For the amorphous cell two 800 nm x 800 nm area maps
for tungsten and germanium are displayed in figures 3and 4, respectively. Red pixels correspond to low intensity and blue pixel to high. In both figures we observea rather uniform background surrounding an anomaly inthe bottom left region of the array. In figure 3, the tung-sten signal, a region of about 15% higher intensity than
the average background is clearly visible in the bottom
left region of the scan indicating the presence of additional tungsten. The dimension of this anomaly supports
the conclusion that this elevated intensity is caused by
the buried tungsten heater element. Figure 4 shows asimilar anomaly however with decreased intensity rather
than elevated one. As we recall we expect to find about
10% less fluorescence intensity response from germanium
for the amorphous phase as compared to the crystalline
phase when excited with a photon energy corresponding
to the Ge K-edge absorption edge white line position.
This is in good agreement with the observed decrease inthis image. As the measured Ge thickness is constantover the entire chip this change of intensity must result from a change of phase of the material. It might
be argued that the decrease of intensity originates froma migration and thus a decrease of Ge2 Sb2 Te5 in theobserved area either by lateral displacement or by destructive evaporation upon receiving the electrical pulse.
This however is ruled out as a mere displacement of material upon amorphization would either lead to a wall or
ring like region of enhanced intensity around the area of
decreased intensity or in case of catastrophic failure of
the device and mass ejection in the form of evaporation
would lead to a breakdown of the tungsten counter electrode observable with our microscope optics. As bothare not observed the change in intensity is attributed toa change of phase of the PCM material.

FIG. 5: 2D germanium map of a crystalline cell (2 micronheater element) : Germanium x-ray fluorescence intensity as afunction of position recorded at a photon energy of 11.105keV

For comparison and validation of our technique a 10
micron x 10 micron area map of a device containing a 2
micron x 2 micron heater element with the phase changematerial in the crystalline state is shown in figure 5. Inorder to facilitate comparison between the two germa-

4nium signal maps (Figs 4,5) this graph has been rescaled
to match the scale of the graph containing the amorphousarea. A rather featureless relatively uniform fluorescencesignal is found in this graph indicating a purely crystalline state for the phase change material over the entire
area. The simultaneously recorded tungsten signal (notshown here) map for this device clearly established theposition of the heater element and confirmed its presencein the examined area.
In this study we have demonstrated the feasibility ofinvestigating individual devices of fully device integrated
phase change memory materials by successfully locating
and measuring nanometer sized structures using an x-ray
nanobeam. It has been possible to confirm the changes
in the material as a function of position and identify thephase of the material (crystalline or amorphous) in theareas under investigation.

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