Photoelectron
spectroscopy

Research method

Establishment of Method for Analyzing Next-Generation
Metal-Oxide-Semiconductor Field-Effect Transistors
Nondestructive analysis of internal states of semiconductor laminated structures

Role of SPring-8

SPring-8 synchrotron radiation has improved the detection sensitivity for internal
property of a material by approximately fivefold compared with conventional
X-ray photoelectron spectroscopy.*
For a laminated electrode/insulating-film/semiconductor structure, therelationship between the change in the photoelectron spectrum and the applied
voltage has been clarified to calculate the voltage applied to the insulating film.
An analysis technique that can be used to develop next-generation
metal-oxide-semiconductor field-effect transistors (MOSFETs)** has been
established.
R&D facility: Toshiba Corporation*X-ray photoelectron spectroscopy is a method of analyzing the elements of a sample surface and their electronic states by measuring the energy ofphotoelectrons emitted upon X-ray irradiation of the sample. The depth to which the sample can be analyzed depends on the energy of the irradiated X-rays. In
laboratories, soft X-rays such as Al-Kα (1.4867 keV) are used. In contrast, at SPring-8, synchrotron radiation hard X-rays with an energy of 4-10 keV are used, enabling
the analysis of samples at a greater depth. This method of using hard X-rays is called hard X-ray photoelectron spectroscopy (HAXPES). Most of the measurement
and analysis techniques used for soft X-ray photoelectron spectroscopy and HAXPES are the same; only the energy of the X-rays is different.
**A metal-oxide-semiconductor field-effect transistor (MOSFET) is a field-effect transistor that controls the current between the source and drain by applying
different voltages to the gate electrode. A laminated structure consisting of a metal (as the gate electrode between the source and drain), an oxide (as agate-insulating film), and a semiconductor (a silicon substrate) is adopted in a MOSFET. Many current large-scale integrated circuits (LSIs) adopt a MOSFET structure.

Structure and operation of MOSFETA MOSFET has a laminated structure in which agate-insulating film is sandwiched between a silicon
substrate (Si-sub) and a gate electrode. Silicon dioxide(SiO2) is generally used for the gate-insulating film. Whena voltage is applied to the gate electrode, carriers are
generated immediately below the SiO2 film to allow
current to flow between the source and drain.

SourceGateelectrode

Drain

Gate-insulating film (SiO2 film)
Si-subElectron movement

Application of voltage to gate electrode

BackgroundResults

With the aim of developing next-generation
MOSFETs, the introduction of new devices, such as
ferroelectric gate-insulating films and metal gateelectrodes, and the fabrication of multilayered
insulating films have been examined. Therefore,
methods of analyzing such new materials andtheir inner structure should be established.
Conventionally, X-ray photoelectron spectroscopy
has been used for such analysis; however, only thesurface of a sample can be observed with thismethod, and the inner structure of a sample
cannot be observed without physically destroying
the sample. In addition, it may not be possible toanalyze spectra because the number of observable
photoelectron peaks is small and the peaks of thespectra corresponding to different coexistingelements may interfere with each other. In somecases, satisfactory accuracy cannot be obtained
due to the complicated shape of the peakspectrum.

In the case of HAXPES using intense SPring-8
synchrotron radiation, the depth of observation is
as high as 20-30 nm and the number of observable
photoelectron peaks is increased. Furthermore,
optimum observation conditions were determined
for the total reflection method, in which grazing
incident X-rays are totally reflected on the sample
surface, to obtain the distribution of elements inthe depth direction by changing the observation
depth with increased detection sensitivity.
In addition, the behavior of the photoelectron
spectra was observed by changing the voltage in a
stepwise manner to evaluate the potential
distribution induced in the semiconductor
laminated structure when a voltage is applied. The
voltage applied to the insulating film was preciselydetermined. Using the results obtained, thedevelopment of next-generation MOSFETs is being
carried out.

Observation results of photoelectron spectra
under total-reflection condition

Measurement of photoelectron spectra undervoltage application

When the incident angle of hard X-rays is 0.4°, the intensityof the photoelectron peak corresponding to a SiO2 film on
the surface side is smaller than that corresponding to theSi-sub, whereas when the incident angle is 0.2°, the relative
intensities are reversed. When the incident angle is 0.1°, thepeak corresponding to the Si-sub is negligible. From thisresult, it was found that the smaller the incident angle, theshallower the depth of observation using X-rays.

With HAXPES, the measurement of photoelectron spectra with a voltage
applied to the electrode is possible because a sample with a thick laminatedstructure can be observed nondestructively. The positions of thephotoelectron peaks corresponding to each laminated layer were measured
while applying a voltage to the sample with a gate stack structure. The results
indicate that the relative shift of the photoelectron peak of each layer
depends on the applied voltage and that the voltage applied to the insulating
film depends on the polarity.

Total reflection is induced when X-rays are irradiated at asurface at a very small incident angle. Even in the case oftotal reflection, hard X-rays can penetrate into a sample
to approximately several nanometers below the surface,
thus efficiently exciting photoelectrons. The penetration
depth can be varied by controlling the incident angle,
meaning that the depth of the layer in whichphotoelectrons are excited can be changed.
To examine the increase in detection sensitivity and thechange in penetration depth for the case of totalreflection using HAXPES, a SiO2 film was observed by thismethod.

Energy analyzer
Hard X-ray

Photoelectrons

Sample surface
X-ray incident angle
X-ray penetration depth

SiO2 film 9.4 nm
Si-sub

Photoelectron intensity (Arb. unit)

Total-reflection HAXPES

1850

1s photoelectron spectrum of silicon (Si)Si-sub
X-ray incident angle
0.4°
0.2°
0.1°

SamplePhotoelectrons

SiO2

1.5

Hard X-ray

Molybdenum (Mo)

10 nm

SiO2 film

5 nm

Si-sub
Application of voltage
on back side of sample(-1.2 V～+1.2 V, 0.2 V step.)
1845

1840

1835

Relative shift ofphotoelectron peak (eV)

Achievements

1.0

Dependence of position ofphotoelectron peak on voltage
Mo3pSi1s(SiO2)
Si1s(Si-sub.)

0.5
0.0
-0.5
-1.0
-1.5
-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Applied voltage (V)Binding energy (eV)3031