Figure 3. TUNA images map the conductivity of HfO2 as deposited (left) and post-anneal at
700C (middle) and 800C (right). These maps allows visualizing, and quantifying the size anddistribution of weak spots in the dielectric. Darker color means higher current through the tip,
indicating higher conductivity, and thus the weaker spots in the dielectric. Weak spots emerge
and proliferate upon annealing, more at the higher anneal temperature. 500nm scans courtesy
Jasmin Petry, IMEC, Belgium.

Figure 3a. TUNA Spectroscopy: Currentversus-Voltage (IV) plots with AFM tip
positioned on weak spots of thin film (HfO2).
(Averages from different locations) reveal
background leakage current dropped withannealing, and average current at a given
voltage increased after annealing, withincrease annealing temperature.

Conductive AFM

An I-Z spectrum is displayed in figure
4. Here the CAFM module was used
to study the current on a conductive
polymer sample as a function of theapplied force, or cantilever deflection.
The electrical current, sensed by the
CAFM module, is monitored while
performing a standard forcedisplacement ramping cycle. The
current spectrum displays the detailsof the change in conductivity for the
point contact as the probe makes
contact with the sample and the force
is increased.

CAFM is a powerful current measuring
technique for mapping variations inelectrical conductivity of materials(figure 3). CAFM can be applied tomaterials with medium conductivity(1pA to 1µA). The CAFM application
module can be operated in either
imaging or spectroscopy mode. In theimaging mode, images of the electricalcurrent are obtained, while in the
spectroscopy mode one can collectcurrent-voltage (I-V) or current-force(I-Z) spectra.
In the imaging mode, an electricallyconductive probe is scanned over the
sample surface in contact mode as afeedback loop keeps the deflection of
the cantilever constant while local
height of the sample is measured.
During scanning, the user can apply
a DC bias between the tip and thesample. A low-noise linear current
amplifier senses the resulting current
passing through the sample as the
topography image is simultaneously
obtained (see figure 2). The observed
current can be used as a measure forthe local conductivity or electricalintegrity of the sample under study.

In addition to the imaging mode,
CAFM also measures local currentvoltage (I-V) or current-force (I-Z) spectrausing the spectroscopy mode. In orderto obtain I-V spectra, the imaging scanis stopped and the tip is held in a
fixed location while the sample bias isramped up or down. The resulting
current through the sample is plottedversus the applied bias (figure 3a).
User-selectable parameters include startand end voltage of the ramp, ramp
direction, ramp rate, and delay times
between individual ramps. The
software can either record a single
spectrum or average over multiple
spectra. For some measurements, it isdesirable to limit the current passing
through the sample. In this case, thesoftware provides the user with a“trigger” option, which stops thevoltage ramp as soon as the userselected current value is reached. Inorder to obtain I-Z spectra, the sample
bias is kept constant, while the scanner
is moved in the Z-direction, similar to
the measurement of force-displacement
curves. The resulting current throughthe sample is plotted versus the
Z-position of the scanner. Again,
several parameters allow the user toperform and control specific I-Z
ramping experiments.

CAFM can be applied in many
research and manufacturing areas
for the analysis of a wide range ofmaterials. It can also be used to
localize and image electrical defects
in semiconductor and data storage
devices. In addition, CAFM can beapplied to characterize conductive
polymers and other materials withnonuniform conductivity, such as
semimetals, semiconductor materials,
conductive organic materials,
nanotubes and others.

Figure 4. Force-displacement (top) and I-Z
spectrum (bottom) measured simultaneouslywith the CAFM module.

Tunneling AFM
TUNA measures ultralow currents on
low-conductivity samples. As with
CAFM, a DC bias is applied between
the sample and the conductive tip as
the tip is scanning the sample incontact mode. A linear current
amplifier with a range of 60 fA to
120 pA senses the resulting current
passing through the sample. In thisway, the sample’s topography andcurrent are measured simultaneously,
enabling direct correlation of a sample
location with its electrical properties.
The noise level of the TUNA module(typ. 50 fA) allows one to perform
extremely sensitive current measurements. In addition, the TUNA module
also allows local measurement of
current-voltage spectra on the sample.
The TUNA technique is especially
useful for evaluation of thin dielectricfilms such as gate oxides (often SiO2)
in transistors. In TUNA, the current
tunneling from the tip through thedielectric film strongly depends on film
thickness, leakage paths (possibly
caused by defects) and charge traps.
All of these may significantly affectthe properties and the integrity of thewhole film, thus compromising anentire device’s performance.

TUNA can be applied in many
research or manufacturing areas
and on a wide range of materials. It
can be used to study the thickness
uniformity or interface roughness of thindielectric films, such as gate oxides.
It can also be applied to localize
and image electrical defects insemiconductor or data storage
devices. In addition, TUNA can beused for the study of conductive
polymers or organics, and otherlow-conductive materials (such as
semimetals, semiconductor materials,etc.). Some typical examples follow.
Characterization of thinsilicon oxide (SiO2) films
One of the most demanding steps in
the manufacturing of semiconductor
devices is the gate dielectric. The thindielectrics films (often SiO2 or high-k
dielectrics) are used as gate oxides infield effect transistors and as tunnelingoxides and dielectrics for memory
capacitors like dynamic random
access memory (DRAM) andelectrically erasable andprogrammable read only memory(EEPROM) devices. A structurally andelectrically homogeneous oxide is ofprimary importance in order to comply
with the requirements for reliability andlong-term stability of the gate andtunneling oxides. Otherwise,
degradation and breakdown lead toearly device failure. Even variation ofoxide thickness in the angstrom range
can have a large impact on theelectrical behavior of the transistor andmemory devices. With decreasingoxide thickness this problem becomesmuch more severe. Surface andinterface roughness result in increasingoxide fields and enhance leakage
currents and Fowler-Nordheim
tunneling, leading to fast degradationand limiting the scaling of oxides
for metal-oxide-semiconductor(MOS) devices.

Conventional measurement
techniques—macroscopic currentvoltage (I-V) and capacitance-voltage(C-V) spectroscopy, emission
microscopy (EM), and transmission
electron microscopy (TEM)—areinadequate for locating and measuring
the degree of oxide thinning. Thesemethods either lack the required
resolution or measure only structural(not electrical) information. TUNA on
the other hand, provides the spatial
resolution and sensitivity required tomap effective thickness variations ofthin dielectric films. A bias voltage,
applied between sample and probe,
gives rise to a tunneling current (hence
the name Tunneling AFM) that strongly
depends on the local electricalproperties, i.e., the effective electrical
thickness of the oxide. The conductive
probe, the oxide film underinvestigation, and the semiconductor
substrate form a local MOS structure.
Generally, the current(often Fowler-Nordheim) increases
exponentially with a linear change
in film thickness, thus providing avery sensitive technique to monitor
thickness variations.

Figure 5. TUNA current measurements taken
on a 5nm thick SiO2 gate oxide at increasing
sample bias voltages from left to right: (top)6V, 7V, (bottom) 8V, and 9V. 0.5µm scans.

spectrum shows the exponential
dependence of the current on theapplied bias voltage.

Figure 6. I-V spectrum taken on a 5nm thickgate oxide. The spectrum was recorded
while ramping the bias from the start voltage(0V) to the end voltage (2V) forward, andthen back.

Figure 5 shows a series of TUNA
current images taken at different bias
voltages on the bare surface of theoxide film. One can clearly observe
the increase in current upon increasing
the bias voltage. The current datataken at the highest voltage (9V)shows localized high-current spots inthe scanned area, indicating electricalbreakdown of the oxide film inthese locations.
Specialized software also permits themeasurement of I-V spectra in a single
location on the sample surface. Figure
6 shows a typical I-V spectrum
measured on the 5nm thick gate
oxide, obtained by ramping the bias
voltage from 0V to 2V. The

Figure 7. Topography (left) andsimultaneously-obtained tunneling current
image (right) at the transition from a field oxide
to a 40nm thick gate oxide. At the transition
the tunneling current is increased, whichindicates thinning of the silicon oxide. 1µm
scans, 0.5 pA current scale. Image courtesy
A. Olbrich, Infineon, Munich, Germany.

TUNA can also be employed tomonitor weak spots andinhomogeneities of thin dielectric films
or oxides. Structural defects on thesurface, as well as structural andelectrical inhomogeneities within theoxide (e.g., SiO2) and at the interface
to the substrate (e.g., Si), can beinvestigated. Figure 7 shows the
topography and simultaneouslyobtained tunneling current image (atconstant sample bias) at the transition
from a field oxide to a 40nm thickgate oxide. The topography image
shows (from left to right) the gate
oxide, the interface region, and thefield oxide. At the edge between thetwo regions an increased FowlerNordheim tunneling current ismeasured. This indicates local
structural thinning of the gate oxideduring the fabrication of the fieldoxide, which serves as an insulatingarea between adjacent active regions.

show any measurable current at thegiven sample bias, the tunnel oxideshows inhomogeneities in current
density, which indicates variations in
the effective oxide thickness.

In figure 8, topography and tunnelingcurrent images of an 8.5nm thicktunnel oxide for an EEPROM device
are displayed. The tunnel oxide (SiO2)
is enclosed by a thicker field oxide to
the left and right. The data was
obtained at a sample bias of 10V.
Whereas the field oxide is too thick to

Imaging of ultrathin aluminumoxide (Al2O3) films
As an alternative to traditional
nonvolatile memory devices, magnetic
random access memories (MRAMs) arebeing investigated and developed.
MRAMs operate based on thetunneling magneto-resistive (TMR)
effect, and their essential part is ametal-insulator-metal (MIM) tunnel
junction. Successful operation ofthese structures requires a chemicallyhomogeneous (free of impurities)
insulating barrier, as well as minimal
fluctuations of the barrier thickness.
Therefore it is important to spatiallyresolve the tunnel barrier and relate itto the macroscopic tunnel magnetoresistance. While conventionaltransmission electron microscopy (TEM)
and x-ray photoelectron spectroscopy(XPS) studies provide global
information on the atomic
organization, surface-interface
structure, and chemical composition,these techniques give incomplete
information on the tunnel barrier quality
at the atomic scale because they
average over depth and surface. To
date, TUNA is the only method that

Figure 8. Topography (left) and tunneling(right) current images of an 8.5nm thick tunneloxide (SiO2) measured at a sample bias of
10V. 2µm scans, 200 fA current scale.
Image courtesy A. Olbrich, Infineon,
Munich, Germany.

Figure 9. Topography (left) and tunnelingcurrent (right) images taken on a 1.2nm thinaluminum oxide (Al2O3) film at a sample bias
voltage of 0.14V. 500nm scans, 5 pA current
range. Data courtesy A. Olbrich, Infineon,
Munich, Germany.

data. Upon increasing the bias voltage
from 1V to 5V, more dots appear
in the TUNA current images. This canbe related to the size of the defects
and the depth of the defects
underneath the top surface. This
example illustrates the possibility ofusing the TUNA technique to image
and localize subsurface electricaldefects, as well as to measure thedefect size and density.

Figure 10. Sequence of TUNA images
obtained on a SiO2 film with embedded
defects. The sample bias voltage was fromleft to right: (top) 1V, 2V, (bottom) 3V,
and 5V. 1µm scan, 1 pA current scale.
Samples courtesy S. Madhukar, Motorola,
Austin, Texas.

allows characterization of the local
electrical properties of these films withvery high lateral resolution. Figure 9shows the topography and thetunneling current image of a 1.2nm
thick Al2O3 layer, which lends itself asa very good insulator for MRAMs due
to its large band gap (about 8 eV).
Local variations in effective electrical
thickness result in elevated currents—
up to several orders of magnitude. It isnoticeable that most often, areas withincreased tunneling current seem tocorrespond with topographicallyelevated features.
Imaging of defects embedded insilicon oxide (SiO2) films
An important application of TUNA is
the localization and identification ofelectrical defects in thin dielectric films.
In figure 10, a thin SiO2 film wasembedded with a controlled amount of
quantum dots, which can be viewed
as small electrical defects. The small
dots could not be observed whenusing standard SPM topography, but
clearly show up in the TUNA current

Imaging of diamond-like carbon(DLC) films in data storage
A different application for TUNA canbe found in the characterization of thindielectric films used in the data-storage
industry. A continuous effort is aimed
at improving the performance andreliability of magneto-resistive (MR)read/write heads and disc media. Forprotection against corrosion and wear,
the discs and heads are commonly
coated with a thin non-conductive DLC
film. TUNA can be used to determine
the quality of these films. When
applying a bias to the disc or head,
the tunneling current is an excellent
indicator of leakage paths, small
inhomogeneities or defects (electricalcontaminants, short circuits, DLC
thinning, etc.) in the DLC coating.
Figure 11 shows the tunneling current
images of two MR-heads with uniform

Figure 11. Tunneling current scans (left)
measured on a magneto-resistive read/write
head covered with a thin diamond-likecarbon (DLC) film. Tunneling current image(right) of a similar head with defective DLC
coating. 20µm scans. Samples courtesy T.
Ahmed, Seagate, Minneapolis, Minnesota.

coating. The topography image shows
little detail of the MR head, whereas
the TUNA current data looks throughthe DLC film and shows the different(metallic) regions of the MR headrevealing defective coating. The
tunneling current images clearly shows
weak spots in the coating of thedefective MR head. The application of
TUNA on magnetic disc media,
covered with a thin DLC film, isillustrated in figure 12. The
topographic and TUNA current data
are displayed for two samples, with adifferent DLC-coating thickness. The left
image displays the data obtained on adisc with a thin DLC film. The TUNA
current data vary between about 0
and 20 pA and show a strong spatial
variation correlating with the polishingscratches present on the discs. The
right image displays the data obtainedon a disc with a slightly thicker DLC
coating.
The average tunneling current is much
lower, corresponding to the change infilm thickness. Also, the correlation to
the surface morphology is much lesspronounced. This example illustrates
how the TUNA technique cansuccessfully assist in optimizing the
thickness, composition and processing
conditions of the DLC films.

Figure 12. Tunneling current scans measured
on two magnetic disc media covered
with a thin (left) and slightly thicker (right.)
DLC film. 0.5µm scans, 20 pA current scale.
Sample courtesy J. Leigh, Seagate,
Fremont, California.

Figure 13. Topography (left) and tunneling current (right) imagesof a thin BaTiO3 ferroelectric film. 2µm scan, 2 pA current scale.
Sample courtesy H. Ruda, University of Toronto, Canada.

Imaging of thin ferroelectric andpiezoelectric films
Another important group of materials
for applications in both MEMS andmicroelectronics are piezoelectric andferroelectric materials. Of particularinterest is BST (BaXSr1-XTiO3), which
serves as a high-epsilon dielectric involatile memory devices (DRAM) andPZT (PbZrXTi1-XO3), which may be usedin ferroelectric memory devices. Theseoxides are polycrystalline, and so far
the correlation between microscopic
structure and their electrical properties
are not well understood. Again, TUNA
can be a useful technique to analyze
local properties of these films.
Enhanced current flow is noticeable
along the grain boundaries, whereas
less current is observed on individual
grains. This behavior can, for example,
explain the undesired leakage currentof the ferroelectric capacitors
fabricated with this type of film.
The TUNA data obtained on a 500nm
thick BaTiO3 layer, is displayed infigure 13. A higher current is observed
at some of the grain boundaries, aswell as along some longer lines. Oneof these crack-like lines is displayed in
the current image. The presence of
these high-leakage current lines may
be explained by stress phenomena in
the imaged film. This effect is only
observed in the TUNA current data.

Figure 14. Topography (left) and TUNA current (right) imagesof a 100nm PPV layer on a conductive PANI layer. The sample
bias voltage was -6V. 50µm scan. Sample courtesy C. Zhang,
Uniax, Goleta, California.

TUNA and CAFM combined imaging
of conductive polymers
In addition to the previously described
inorganic materials, TUNA has also
proven to be very useful for organic
materials like conductive polymers.
The data displayed in figure 14 was
obtained using low spring constantprobes on a 100nm thick polyphenylene vinylene layer (PPV) on topof a 200nm thick poly-aniline (PANI)
layer. The PPV is a light-emitting
polymer, which is spin-cast on top ofa conductive buffer layer (PANI) to
facilitate the transport of charge
carriers. This particular sample had the
PPV layer partially peeled off to enablemeasurements of the PANI layer
underneath. The PPV part of thesample is slightly higher and visible on
the left in the topography image. The
corresponding current image clearlyshows a uniformly lower conductivityof the PPV compared to theinhomogeneous PANI buffer layer.
A second conductive polymer
example, with higher spatial resolution,is shown in figure 15. The CAFM
technique was used to map the spatial
variation of the conductivity in a thinpoly-analine film deposited on an
Indium Tin Oxide substrate. The
relatively high conductivity of thissample required the use of the CAFM
module, instead of the TUNA module.

The observed currents varied between
0 and 200 pA. The CAFM image
shows the high conductivity of thelarge areas covered with PANI, anda slightly lower conductivity for thesmaller, isolated spots covered with
PANI. The substrate appears as apoorly conductive region. The low
spring constant probe employedallows scanning at very low contact
forces, hence minimizing thedeformation or wear of these relatively
soft samples.

Figure 15. Topography (top) and CAFM
current (bottom) images of apoly-analine film on an indium tin oxide
substrate. 2µm x 1µm scan, 200 pA current
scale. Sample courtesy S. Rane, University of
Chicago, Illinois.

Scanning CapacitanceMicroscopy
The continuing miniaturization ofsemiconductor devices has createda serious challenge for traditional
materials analysis techniques, suchas secondary ion mass spectrometry(SIMS), spreading resistance profiling(SRP), and capacitance voltage (C-V)
measurements. While the accuracy,
reliability, and improved capabilitiesof these instruments provide the basis
for current materials characterization
data, their one-dimensional limitation,
inability to measure sub-0.1-mmfeatures, and limited characterization
repertoire have increased the valueof scanning probe techniques.
SCM was one of the first SPM
techniques to find its way into theadvanced semiconductor analysis
world. SCM instruments can show thecarrier concentration profiles in two
dimensions in actual semiconductor
devices, as well as the relationshipof these profiles to critical device
structures. This capability makes the
SCM module useful in thedevelopment, manufacturing,
testing, and failure analysis ofsemiconductor devices.
In SCM, the metalized probe forms a
metal-insulator-semiconductor (MIS)
capacitor with the semiconductor
sample. An AC bias applied between
the scanning contact AFM tip and thesample generates capacitancevariations, which are monitored using
a gigahertz resonant capacitancesensor. This system has been shown
to be sensitive to variations smaller
than attofarads (<10–18 F). The
capacitance variation (dC/dV) isa measure of the local carrier
concentration density and type (n-typeor p-type), and can therefore be used
for high-resolution two-dimensional
carrier profiling.
2-D carrier profiling in semiconductor
device structures
One of the most important applicationsof SCM is two-dimensional carrier
profiling of semiconductor device
structures. Both silicon and compound
semiconductors are of great interest.
Two-dimensional dopant profiling isa high priority on the International
Technology Roadmap for
Semiconductors, and it is expected to
become an enabling technology fornext-generation device manufacturing.
SCM provides the spatial resolution(about 10–20nm), and dynamic range(1015–1020 atoms/cm3) to answer
these needs.
Figure 16 shows the topography (left)
and SCM dC/dV (right) images
obtained simultaneously on a cross-

Figure 16. Topography (left) and SCM dC/dV (right) of a crosssectioned transistor of a Pentium II processor. 1.25µm scan.

sectioned transistor from a Pentium IIprocessor. The topography shows thegate region with the two spacers(bright areas), but shows no details of
the carrier profiles. The SCM dC/dV
image shows the differently doped
areas of the transistor: source, drain(both high- and low-dose implants arevisible) and gate. The SCM image
can be used to extract valuable
information such as the effective gate
length, junction depth, or details on
the lateral and vertical extension of
the doped regions. With the
exception of SSRM, this crucial
information is not accessible throughany other technique.
In addition to imaging, SCM can also
be used to measure dC/dV versus V
curves in selected positions on thesample. In figure 17, the DC sample
bias is ramped between two userselected values, and the SCM sensor
output (dC/dV) is monitored andplotted. The top curve was measured
on an n-type sample, while the bottom
curve was measured on a p-typesample. As expected, different types
result in a different sign of the dC/dV
signal, and different dopant levels
result in a different intensity of thedC/dV signal.

Figure 17. dC/dV versus V curves measured with SCM on
two different locations of a sample: n-type, 2x1017 atoms/cm3(top curve) and p-type, 3x1019 atoms/cm3 (bottom curve).

well implant) are touching, whereas
they should be separated from eachother. This “short circuit“ resulted in avery high leakage current of thisparticular device.

Figure 18. Height or topography (left), Capacitance (SCM) dC/dV amplitude (middle), andCapacitance (SCM) dC/dV phase (right) images of a conventional Silicon DRAM cell. Top metaland dielectric layers were etched to expose Silicon. The SCM dC/dV phase image differentiates ptype (bright color) and n-type (dark color) doped areas. npn and pnp transistors in DRAM cell arethen clearly visualized, allowing extraction of critical parameters, e.g., effective gate length, andvisualizing defects. The SCM dC/dV amplitude image shows relative magnitude of dopant
concentration: bright color means larger depletion capacitance, and therefore, lower doped areas(e.g., well implant areas).

Figure 19. Capacitance (SCM) dC/dV
phase (left) and height/topography (right)
images of an ion-implanted Silicon structure
consisting of an n-type implant (dark areas)
into a p-type substrate. An additional(shallower) p-type implant can be observed.
The structure has been cross-sectioned bystandard polishing techniques.

Figure 20. SCM image of a good (top) andfailed (bottom) silicon device. The bottom
image displays a short circuit between the two
doped regions. 8µm x 2µm scan.

Figure 18 illustrates topography (left),
amplitude (middle), and capacitance(right) of top metal and dielectricetched layer of a Silicon DRAM cell.
Figure 19 illustrates capacitance(left) and topography (right) of an
ion-implanted Silicon structure
consisting of an n-type implant intoa p-type substrate.
SCM is also a very valuable tool for
failure analysis on semiconductor
devices. The SCM technique allows
visualization of whether a particularimplantation is present, whether it is
the correct type (n or p), the expecteddimensions (both in-depth and lateral),and other characteristics. A typicalfailure analysis example is shown infigure 20. A cross-sectional surface
was made through a number ofdevices, including one known to havebad electrical device characteristics.
The SCM image of a good device isshown in the top image; the SCM
image of the corresponding area ofthe bad device is shown in the bottom
image. The bottom image indicatesthat the two implanted regions (i.e. thebright areas: n-type implant and n-type

Ferroelectric films
Ferroelectric thin films are very
attractive for their possible applications
in nonvolatile memories andmicroelectromechanical devices(MEMS). The decrease in size (downto tens of nanometers) of these devices
requires an appropriate description of
the material properties and processesin ferroelectric films. For example, it isfundamental to investigate whether
ferroelectric structures with nanometer
dimensions still exhibit ferroelectric andpiezoelectric properties, and to study
how these properties are affected by
the overall size. SCM is a possible
technique for these studies. For
example, SCM provides a method formeasuring the sign of the C-V slope forthe ferroelectric sample and, hence,the domain or polarization state in thethin film. SCM has been used to image
and manipulate the domain structure ina thin Pb1.0(Nb0.04Zr0.28Ti0.68)O3(PNZT) ferroelectric film. On an area of
25x25µm, the tip was scanned with a
DC sample bias voltage of -12V andsubsequently smaller areas were writtenwith DC sample bias voltages of
opposite polarity. A small AC bias
voltage was used to image thepolarized regions, i.e., to study themagnitude and sign of the slope of theC-V curve at zero DC bias. Figure 21
shows the SCM image on the rightand the corresponding AFM
topography image on the left. The dark
and light contrast regions indicate
oppositely polarized regions, where
the dC/dV signal is of high strength
but opposite sign.
In addition, SCM can also measure thepolarization (or C-V) curves of small
ferroelectric capacitors, or even of

Figure 21. Topographic (left) and SCM dC/dV image (right) ofa PNZT ferroelectric film. Dark and bright areas correspond to
oppositely polarized regions. The different polarization regions
were written using the SCM at different scan sizes and sample
bias voltages prior to this SCM scan. 25µm scan. Sample
courtesy Ch. Ganpul and M. Ramesh, University of Maryland.

single ferroelectric grains. This is
impossible with conventional probing
techniques. Figure 22 illustrates
topography (left) and capacitance(right) of FerroElectric thin film on top of
platinum electrode. Whereas figure 23
shows approximately the same areas
as in above images, but now with a
5V DC applied between tip andsample. The DC voltage changes thepolarization such that all grains havethe same polarization state. And lastly,
figure 24 shows hysteresis typical of aferroelectric domain on a single grainwith localized capacitancespectroscopy versus applied DC
bias (V).

Scanning Spreading ResistanceMicroscopy
Like SCM, SSRM is often used tomeasure dopant profiling insemiconductors, but it does so by
quantifying electrical conductivity or
resistivity. SSRM provides twodimensional information on theelectrical conductivity or resistivity ofthe sample under study. Veeco has

Figure 22. Height/topography (left) and capacitance (SCM)
dC/dV phase images (right) of FerroElectric thin film on top of
platinum electrode. Height image shows granular structure of thinfilm with 20-100nm sized grains. The SCM dC/dV phase image
shows individual grains’ polarization states; it is obtained byapplying a small amplitude AC voltage between tip and sample
that modulates the capacitance at the same frequency (DC voltage
is kept at 0V not to change the polarization state by the presenceof AFM tip). Ferroelectric polarization state is determined by charge
measurement performed in SCM. The resolution of the SCM
technique allows one to observe variations within single grains.

developed and patented this techniquein collaboration with IMEC, Belgium.
In SSRM, an electrically conductive
probe is used to measure the sample’s
local resistivity. When the probe isscanned in contact mode over regionswith different resistivity r, the electricalresistance R formed by the probesample contact will vary
proportionally. If the contact isassumed to be circular and of Ohmic
nature, the relation between R and r isgiven by the basic spreading
resistance formula: R = ρ / 4r,
whereby r is the radius of the contact.
Since the resistance can vary over
several orders of magnitude, alogarithmic current amplifier is used for
SSRM. The logamp has a current
range of seven orders of magnitude
from 10 pA up to 0.1 mA. A major
application of SSRM is themeasurement of the two-dimensional
distribution of electrical carriers inside
semiconductor structures.

Figure 23. Approximately the same areas asin above images, but now with a 5V DC
applied between tip and sample. The DC
voltage changes the polarization such that all
grains have the same polarization state.

Figure 24. Localized capacitancespectroscopy (exclusive to SCM) dC/dV
versus applied DC bias (V) on a single grainshows the hysteresis typical of a ferroelectricdomain. The data can be integrated once toobtain a Capacitance-versus-DC bias curve,
and twice to obtain a relative Polarizationversus-DC bias curve.

Figure 25. Topography (left) and resistance (right) scans of across-sectioned Si DMOS transistor. 12µm scan courtesy
IMEC, Belgium.

2-D carrier profiling in semiconductor
device structures
While the probe is scanned acrossthe cross-section of the semiconductor
device, the electrical resistance ismeasured between the conductive tip
and a large current collecting backcontact. When the applied force
exceeds a certain threshold force,the measured resistance is dominatedby the spreading resistance. On Si
structures, high forces (typically, a fewµN) are required in order to penetrate
the native oxide and to establish astable electrical contact. Since
standard AFM probes deform at thesehigh forces, doped diamond or
diamond-coated silicon probes are
employed. The extreme hardness, highYoung’s modulus, and the electricalconductivity obtained through doping
make diamond particularly suitable foruse as SSRM tip coating material.
In figure 25, analysis has been
performed on a Si DMOS transistor
structure. The transistor structure wascross-sectioned to expose thedifferently doped regions, and thenpolished using standard polishingtechniques. The topographical image(left) shows quite clearly the Al-contact(black region), the underlying oxide(brown) and the polysilicon andunderlying gate oxide. The SSRM
resistance image (right) shows theelectrically active regions. The different

Figure 26. SSRM height and resistance images of crosssectioned capacitor. The capacitor has metal thin film area with
dielectric between them. If there were leakage paths between
the different electrical contacts, the resistance image would
show them (none found in this image).

colors reflect different levels of
resistivity: dark indicates highly
conductive regions, and brightindicates low conductivity. Clearlyobservable are the highly doped
n+-substrate (black); the lower doped
n-epilayer (dark brown); the p+-body(which appears as highly resistive), then+-implant (black), the metal and theoxide regions, as well as the highly
conductive polysilicon material.
Junction positions correspond to the
sharp transition between the various
color levels.
Figure 26 illustrates an SSRM imageof a cross-sectioned capacitor
featuring a metal think film area with
dielectric between them. SSRM
images are often complementary to

SCM images, as SCM images giveno signal on dielectrics and metals,
whereas SSRM shows a big contrast
between the two.
In a second example (figure 27),
SSRM imaging was performed on asilicon MOSFET with a gate length of
0.25µm. The image (left) shows thedifferently doped regions – source,
drain, and gate – as highly conductive
areas (dark), dielectric and substrate
as low-conductivity areas, as well as
the intermediately conductive area.
The source and drain junctions areobserved as thin, bright lines of low
conductivity. A section made throughthe source region of the transistor isalso shown in figure 27 (right). The
section shows from left to right: theFigure 27. SSRM resistance scan of a cross-sectioned 0.25µm silicon MOSFET transistor.
2µm scan courtesy IMEC, Belgium.

Figure 28. Topography (left) and SSRM resistance (right) scanof an InP-based heterostructure. 7µm scans. Sample courtesy of
M. Geva, Lucent Technologies, Breinigsville, PA.

dielectric top layer, source implant(p-type), junction peak, well (n-type),and substrate. The junction depth caneasily be extracted from this section
as the distance between the junction
peak and the dielectric top layer, andis found to be 184nm.
In addition to silicon, compound
semiconductors are also of greatinterest. LEDs, photodetectors, anddiode lasers are only a few of themany devices fabricated from III-V and
II-VI semiconductors. Knowledge of thetwo-dimensional distribution of
conductivity (the electrical-active
dopant atoms in particular) is
important for process development andmonitoring. For compound
semiconductors, sample preparation isminimal; simple cleaving provides thebest surface on complex samples, andallows imaging of critical device
properties after only minutes of sample
preparation. Metal and metal-coatedsilicon tips prove to be sufficiently rigid
for stable and reproducible SSRM
measurements. For good signal-tonoise ratio, a relatively high bias
voltage (several volts) in combination
with a medium probe pressure (subµN) is required. An example of thehigh-resolution carrier density profiling
available with SSRM can be seen in
the cross-sectioned InP-based
heterostructure shown in figure 28.
The resistance image shows the

Figure 29. Topography (left) and SSRM resistance (right) scanof a granular metal film. 500nm scans.

different regions of the heterostructure:
alternating Zn-doped p-type andS-doped n-type layers with different
thickness values. The image reveals
the two-dimensional nature of thelayers toward the mesa area. This
example demonstrates SSRM
analytical power in two-dimensional
imaging of carrier distributions incompound semiconductor structures,and in particular, for the analysis of
InP hetero-structures, an area that is
gaining high interest.
Conductivity mapping ofnonsemiconductor materials
SSRM can also be used for studying
the electrical properties of nonsemiconductor materials. This includes
applications for metals, semimetals,
conductive polymers, and otherintermediately conductive materials.
For optimum performance, different
materials often require different probe
material and force set points.
In the example in figure 29, a Cu
film is imaged. The granular structureof the film is clearly observed in the
topographic data. The SSRM
resistance data show that the resistivityof the grains is higher towards theedges, as compared to the center ofthe grains. Note that the averagegrain size is 30nm, and the spatial
resolution is on the order of 5nm.

Summary
The Digital Instruments MultiMode,
Dimension 3100/5000 and CP-II
SPMs set the standard for ultrahighresolution topographical profiling andmapping of sample hardness,elasticity, friction, adhesion, andmagnetic or electrical field strength.
Now, with the addition of add-on
application modules for CAFM,
TUNA, SCM, and SSRM, theseindustry-leading instruments can beenhanced to map nanometer-scale
resolution on a wide range ofadditional properties on various
materials including low- and midstrength electrical currents, resistance,and capacitance. These enabling
technologies, which are already being
used successfully in both industry andscience, promise to play everincreasing roles in future processoptimization and scientific research.