zone-plate-based scanning photoelectron microscopy...

Surface Review and Letters, Vol. 9, No. 1 (2002) 213–222c© World Scientific Publishing Company

ZONE-PLATE-BASED SCANNING PHOTOELECTRONMICROSCOPY AT SRRC: PERFORMANCE AND

APPLICATIONS

RUTH KLAUSER,∗ I.-H. HONG, T.-H. LEE, G.-C. YIN, D.-H. WEI and K.-L. TSANGSynchrotron Radiation Research Center, No. 1 R&D Road VI, Hsinchu 300, Taiwan

∗[email protected]

T. J. CHUANGCenter for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan

Institute of Atomic and Molecular Sciences, Academia Sinica,Taipei 106, Taiwan

S.-C. WANGInstitute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan

S. GWODepartment of Physics, National Tsing-Hua University, Hsinchu 300, Taiwan

MICHAEL ZHARNIKOVAngewandte Physikalische Chemie, Universitat Heidelberg,Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany

J.-D. LIAODepartment of Biomedical Engineering, Chung Yuan Christian University, Chung-Li,

Taoyuan 32023, Taiwan

Adapting classical spectroscopic methods to the new challenge of studying nanomaterials, imagingtechniques are the trendsetter in recent years. Among them is scanning photoelectron microscopy(SPEM) with submicron spatial resolution, where the sample surface is raster-scanned by a focusedsoft X-ray beam, and the emitted photoelectrons are collected at each point by the input optics ofan electron energy analyzer. We have constructed such a station at SRRC in Taiwan, which is nowfully in operation. In this paper, we introduce the specific features of the instrument and discussapplication examples on the characterization of scanning-probe-induced Si3N4 to SiOx conversion andelectron- and plasma-induced chemical changes in alkanethiols self-assembled monolayers. In combin-ing two-dimensional imaging and micro-photoemission, SPEM can reveal valuable information on thechemical and electronic properties of structured and multiphase materials.

1. Introduction

More than ten years have passed since the first

submicron-resolved scanning photoelectron micros-

copy (SPEM) images using Fresnel zone plate (ZP)

focusing optics were published.1 The advances of

synchrotron light sources with higher coherent flux

from undulator beamline and the need for diagnostic

tools, which can provide spectroscopic information

on a local microscopic scale, initiated the construc-

tion of several advanced SPEM experimental stations

around the world. SPEM became an indispensable

member of the spectromicroscopy family.2 Still, as a

∗Corresponding author.

213

214 R. Klauser et al.

noncommercial system, the experimental setup and

operation of such a station are quite demanding even

with moderate resolution in the order of 100 nm for

routine operation. The spatial resolution depends

ultimately on the performance (outermost zone

width, diameter and efficiency) of the zone plate

and the brightness of the light source. Further

improvements in resolution and efficiency of the zone

plates are expected in the next few years. It should

be mentioned that zone-plate-based scanning trans-

mission X-ray microscopy (STXM) for absorption

spectroscopy can achieve smaller spatial resolution

of about 50 nm.3 In transmission mode with the

detector in downstream position, high-resolution

zone plates with smaller focal length can be used

and positioned close to the sample.

Owing mainly to the efforts of Kiskinova and her

coworkers,4–6 SPEM has finally entered the juvenile

age in moving from the pure technical aspects to

an experimental method tackling scientific problems.

Exploring the potential and limitation of this method

will certainly define its future applications. The

variation of systems studied so far has already been

quite impressive, ranging from heterogeneous multi-

phase systems in metal/semiconductors5 to recent

investigations of dynamical processes in chemical

waves.6 The strength of SPEM lies certainly in the

combination of two operation modes, namely two-

dimensional imaging and micro-photoelectron spec-

troscopy (µ-PES). This is usually done by locating

and mapping different chemical environments on the

surface, followed by measurements of the photo-

electron energy distribution curves at selected points

for detailed chemical state analysis.

A SPEM permanent end station has been set up

at the U5 undulator beamline at the Synchrotron

Radiation Research Center in Hsinchu, Taiwan, and

is now in full operation and open to the outside

user community. Details of undulator, beamline and

end station design can be found in our previous

publications.7–9 The collimated beam is focused

by Fresnel amplitude zone plates with diffraction-

limited resolutions of 90 nm and 120 nm, respec-

tively. A photon energy range of 250–800 eV

can be utilized to collect photoelectrons from the

sample by a hemispherical sector electron analyzer

with multichannel detection (MCD) capability. The

two-dimensional maps are formed by scanning the

sample with respect to the beam. The MCD

scheme allows one to monitor different kinetic en-

ergies simultaneously. The first part of this paper

will introduce the specific features of our instrument

based on test samples. In the second part, we will

discuss two applications, namely the characterization

of scanning-probe-induced Si3N4 to SiOx conversion

and electron- and plasma-induced chemical changes

in alkanethiols self-assembled monolayers. Our

research programs focus on the growth mecha-

nism of carbon- and nitrogen-containing structured

materials, such as SiC, Si3N4, SiCN, GaN and carbon

nanotubes, as well as on the microchemistry of

hydrocarbons on solid surfaces.

2. SPEM System at SRRC

The three major components of the system are (1)

the zone-plate focusing optics and an order-sorting

aperture (OSA) to cut undesired diffraction orders,

(2) the manipulator with motorized x–y stage as

coarse scanning unit and an attached piezo-driven

flexure stage for fine sample scan with a scanning

range of 100 µm × 100 µm, and (3) the detectors,

namely an electron energy analyzer, total electron

yield, sample current and transmitted photon flux

detectors.

Figure 1 shows the control scheme and the

image/data acquisition system. For alignment of the

photon beam and focusing optics, the zone plate and

OSA can be either positioned separately or together

through their common base plate by PC or manual

handset control. If the ZP and OSA are perfectly

aligned, a symmetric doughnut-shaped pattern can

be observed on a phosphor screen behind the sample

stage. The focal length of the ZP depends on the

photon energy and is, for a ZP of 200 µm diameter

and 100 nm outermost zone width, in the range of

4–13 mm for photon energies between 250 and

800 eV. The OSA, however, is only about a quarter

of the focal length away from the sample, which can

be 1 mm or even less for smaller-sized ZPs. Before

measurement, the scanned image of a reference mesh

attached to the same holder as the sample is used to

optimize the focal length. The acquisition system has

to synchronize between scanning motion and data

input. The signals from the detectors are converted

to frequencies or TTL signals and collected by a

multichannel scaler (MCS). An electronic handshak-

ing system between motion control and the MCS

Zone-Plate-Based Scanning Photoelectron Microscopy at SRRC 215

Fig. 1. Block diagram of SRRC-SPEM control and data acquisition system.

Fig. 2. Si 2p SPEM images of a section between the source, drain and gate of a FETransistor. Enhanced intensitiesshow areas of SiO2, poly-Si and Tisi2, respectively. Below : Si 2p photoelectron spectra from respective points in theimages.


has been adapted. Each MCS channel can form an

image and 32 images can be collected simultaneously.

Among them, there are 16 channels allocated to the

MCD signals of the analyzer. The data acquisition

and processing programs are written in C-language

with LabWindowr compiler. The acquisition time

of an image of 40 × 40 pixels is about 4 min, with

a dwell time of 100 ms for each point and a count

rate of up to 50 k counts/s per pixel. The infor-

mation obtained from the 16 channels corresponds

to 16 different kinetic energies. We utilize the im-

age of a gold #1000 reference mesh to optimize the

focal length and to estimate from the intensity profile

across the mesh edges the spatial resolution, which

is about 0.2 µm at present. Improvement in mea-

sured resolution can be expected using a knife edge

for calibration.

The chemical selectivity of the images is demon-

strated in Fig. 2. A field effect transistor (FET)

with the gate area between drain and source

was imaged by collecting the Si 2p photoelec-

trons excited by 392 eV photons. The 16-channel

imaging mode exhibits areas with different chemical

environment, such as silicide (TiSi2) from drain and

source, polysilicon from gate and silicon dioxide from

the substrate. The selected channels (channels 8, 11,

13) have maximal enhanced intensity for the respec-

tive electronic states of Si. The acquisition program

allows zooming in regions of interest. Those images

of a smaller area with higher resolution scan are

depicted in the lower part of Fig. 2, together with the

Si 2p photoelectron spectra from respective points

in the images. The region where the gate crosses

the source-drain connection has low intensity even

for the polysilicon image. This is due to the height

difference of several microns and therefore an out-of-

focus position of this area.

Both sample and zone plates can be transferred in

situ to adjunct chambers. Additional sample prepa-

ration and surface analytical tools can be attached

to the SPEM system.

3. Oxidation States in Scanning-Probe-Induced Silicon Oxide Patterns onSi3N4 Film

SiO2 and Si3N4 are widely used in microelectro-

nics as dielectric materials. Due to their drastic

differences in physical and chemical properties, the

possibility of locally converting Si3N4 to SiO2 can

open up new opportunities for nanofabrication.

Gwo and coworkers recently succeeded in atomic

force microscope (AFM) nanolithography of silicon

nitride.10 Due to the intense local electrical field

between tip and sample, AFM-induced anodic oxi-

dation can convert Si3N4 films to SiOx with a high

oxidation rate. For instance, a negative bias of 10 V

can result in a conversion rate of about 30 000 nm/s

for a 5-nm-thick Si3N4 film in ambience. In the case

of Si anodic AFM oxidation,11 it is widely accepted

that the mechanism for the process is essentially

controlled by the adsorption of water from the en-

vironment on the surface. The negatively charged

oxygen ions are injected from the water and trans-

ported by the high electric field through the oxide

layer to the Si/SiO2 interface, where they react with

the positively charged holes and Si to form SiO2.

For Si3N4 conversion, the reaction between oxyan-

ions, silicon nitride and the hole accumulation at the

Si3N4/oxide interface should cause a replacement of

nitrogen by oxygen. To understand the origin and

mechanism of such a replacement, it is necessary to

know if a complete conversion to SiO2 is at all possi-

ble or only partially to SiOx<2 with excess Si in the

oxide layer or to SiOxNy with the remaining nitro-

gen. Previous scanning Auger electron microscopy

measurements of AFM-induced oxide on Si3N4 film

showed the disappearance of the N-KLL peak and an

enhancement of the O-KLL signal.12 This indicates

that the conversion to oxynitrides with a substan-

tial amount of nitrogen in the layer is not the case.

However, neither AFM nor scanning Auger electron

microscopy can determine the degree of conversion,

depending on the initial parameters such as sample

bias voltage or scanning speed. We applied SPEM

imaging, micro-XPS and micro-UPS to tackle this

issue.

The silicon nitride films were grown on p-type

Si(001) wafers with the low pressure chemical va-

por deposition (LPCVD) technique. The original

film thickness was 5 nm, which was then reduced

to about 2.5 nm by HF (1%) etching to guarantee

the complete oxidation of the Si3N4 film during the

subsequent AFM patterning. After this pattern-

ing, the samples were mounted on a sample holder,

inserted into the UHV chamber and imaged by

SPEM without any further surface treatment.

Various oxidized patterns of stripes and pads with

bias voltages ranging from 5 to 10 V have been


Fig. 3. Si 2p, O 1s and N 1s SPEM images of SiOx stripes with different widths, patterned by AFM-induced Si3N4

to SiOx conversion (image size — 24 × 30 µm2; photon energy — 622 eV).

investigated. Figure 3 shows an example of three

oxidized stripes written on Si3N4 film with widths of

1 µm, 2 µm and 3 µm, respectively. The bias volt-

age was in this case 10 V and the structure height

is typically 2 nm. All images are taken at 622 eV

photon energy with an image size of 24 × 30 µm2.

The Si 2p signal inside the stripes is maximal en-

hanced at channel 7. This energy represents the Si 2p

state of silicon oxide. Channel 9, with 1.5 eV higher

kinetic energy than channel 7, exhibits reversed

contrast and reflects the second chemical state of Si,

i.e. Si3N4. The O 1s image shows weaker contrast

between stripes and nitride film due to the fact that

oxygen from native oxide contamination also exists

on Si3N4. The N 1s signal is large on the nitride

film surface. After locating the oxidized pattern,

we choose particular points on the image to perform

micro-photoelectron spectroscopy of various core and

valence levels.

The results are displayed in Fig. 4 for an oxidized

pad of 8 µm × 10 µm, selecting a point at the center

of the pad and a point outside on the Si3N4 film and

recording the photoelectrons of the Si 2p core level

and valence band. The photon energy of 622 eV has

been calibrated to the Fermi edge of a gold sample at-

tached to the same sample holder. After subtraction

of the secondary electron background, the Si 2p

spectra have been fitted by doublets of Voigt function

with a standard spin–orbit splitting and branching

ratio of the Si 2p3/2 and Si 2p1/2 peak constituents

of 0.61 eV and 2, respectively. The Si 2p spectrum

of the silicon oxide pad shows a clear asymmetry to

lower BE. The three deconvoluted components have

Si 2p3/2 peak maxima at 103.3 eV, 102.4 eV and

101.6 eV with full-width-at-half-maximum (FWHM)

of 1.3 eV, 1.1 eV and 1.1 eV, respectively. The

binding energy and width of the major component

agree well with reference values found in the lite-

rature for SiO2.13 Si 2p core level spectroscopy of

thermally grown SiO2 film on Si(100) and Si(111)

surfaces shows four oxidation states of silicon.13,14

Besides SiO2 (Si4+), intermediate states of Si1+

(Si2O), Si2+ (SiO) and Si3+ (Si2O3) apparently exist

at the interface. The energy positions of the two de-

convoluted shoulder peaks of SiOx in Fig. 4 relative

to the SiO2 peak agree with the suboxides Si3+ and

Si2+ for the oxidized Si film, but with a slightly larger

peak width. The Si3+ species decreases in intensity

for smaller probing depth, i.e. at lower photon

energy.15 We conclude that those chemical states

are due to intermediate oxidation produced during

the nitride conversion and located in the layer with

gradient depth distribution. However, no remaining

nitrogen incorporated in the film as indicated by


Fig. 4. Micro-photoemission spectra (dotted curves) ofSi 2p and valence band outside on the Si3N4 film andinside the AFM-oxidized pad for photon energy of 622 eV.Solid curves are the peak analysis of Si 2p. The valenceband is referenced to the Au Fermi edge.

the absence of N 1s and N 2s signals (see Ref. 15

and the valence band spectrum in Fig. 4) could

be observed. The Si 2p peak of Si3N4 film is es-

sentially broader, with contributions from native

oxide contamination at the surface with the Si 2p3/2

peak maximum at 102.6 eV, from stoichiometric

Si3N4 at 101.6 eV and from the Si3N4/Si interface at

100.7 eV BE. The lower panel of Fig. 4 shows the va-

lence band spectra for silicon nitride film and the

AFM-oxidized pad. The main contributions in these

spectra between 15 and 35 eV are from O 2s and N

2s, and those below 15 eV are mostly from Si 3s,

Si 3p, O 2p and N 2p. The AFM-induced oxida-

tion results in a strong increase in the O 2s intensity

and the absence of the N 2s signal. The shape of

Fig. 5. Micro-photoemission spectra at 386 eV photonenergy for Si 2p outside on the Si3N4 film and of threeAFM-oxidized pads using different AFM bias voltages of5 V, 6 V and 7 V, respectively.

the valence band from SiOx resembles the spectra

of thermal dioxide.16 The slightly larger increase in

background intensity between 1 and 4 eV illustrates

the presence of the suboxides in the layer.

The Si 2p spectra for different bias voltages

used in the conversion process are shown in Fig. 5

for 386 eV photon energy and the reference Si3N4

spectrum displayed on the bottom of the figure. The

7 V spectrum corresponds to a completely oxidized

sample with no remaining nitrogen in the layer. The

5 V and 6 V spectra, however, are a combination of

the contributions from converted oxide and noncon-

verted Si3N4. The clear separation of the oxide and

nitride peaks indicates that no significant amount

of oxynitride SiOxNy has been formed, which would

result in a single peak. From cross-sectional profile

measurements of the AFM image, it is known that

the protruded and buried parts of the oxide patterns


are in proportion to the bias.10 The reduced depth

of the buried oxide for lower bias voltage causes the

incomplete oxidation of the nitride film.

In summary, we have found that a well-prepared

sample can be completely converted to silicon oxide,

with no remaining nitrogen left. Within our prob-

ing depth, oxidation states of Si4+, Si3+ and Si2+

are observed. The major oxide is SiO2. Suboxides

containing excess Si are also produced in the layer

with gradient depth distribution.

4. Characterization of Electron- andPlasma-Induced Chemical Changesin Alkanethiol Self-AssembledMonolayers

During the last two decades, self-assembled mono-

layers (SAMs) have attracted considerable interest

from both the scientific and practical viewpoints.17

Considering the practical aspect, these films pro-

vide a means to tailor surface properties, such as

wetting, lubrication, adhesion and corrosion. These

properties can be controlled by both the selection of

suitable molecules and the physical modification of

the respective SAMs through their exposure to ions,

X-ray photons, UV light or electrons. Depending

on the SAM choice, such an exposure results in

either partial or complete damage of the pristine

film, its quasi-polymerization, and chemical modi-

fication of the tail group attached to the molecular

chain.18,19 From the viewpoint of the lithography,

films formed from the alkanethiols CH3(CH2)n−1SH

(Cn) on noble metal surfaces represent a positive

resist because the irradiated areas become soluble in

specific solvents. This enhanced solubility is related

to the irradiation-induced chemical change of the

adlayer, the loss of the orientational and conforma-

tional order, the pronounced desorption of the film

fragments, and the destructed anchoring of the Cn

molecules to the substrate. All these processes occur

during the irradiation followed by the air exposure

before the wet etching procedure.

Provided that the SAM modification is performed

through a mask and a focused electron or photon

beam is scanned in a desired way over the sample

surface, micro- and even nanostructuring of a

SAM can be achieved. The structured SAM

can then be used as a resist within the “conven-

tional” lithography approach or as a template in the

chemical lithography framework. We have studied

this potential method of structuring SAMs by soft

X-ray scanning photoelectron microscopy. Several

kinds of SAM films on Au substrates were patterned

by low energy (10 eV) electrons through a gold mesh

mask with 10 µm square opening placed on the top

of the films. After removal of the Au mesh, it was

expected that the irradiated parts of the SAM films

were partially damaged and disordered.

The Au 4f , C 1s and C KLL SPEM images of

the C12 and C18 films exposed to 10 eV electrons

through the mask are presented in Fig. 6. In

contrast to the initial expectation that the irradiated

areas (quadrates) should be thinner than the non-

treated regions (because of the irradiation-induced

desorption of the hydrocarbon fragments of the alkyl

chains), these areas reveal a smaller Au 4f signal

Fig. 6. Au 4f , C 1s and C KLL SPEM images ofCH3(CH2)11S/Au and CH3(CH2)11S/Au after exposureto 10 eV electrons through a mask.


and higher C 1s and C KLL intensities. This can

only be explained by the adsorption of the airborne

carbon-containing molecules on the irradiated areas.

An enhanced affinity of these areas to the airborne

molecules is apparently related to their high rough-

ness and the reactivity of free radicals created by

the e−-beam. A poor contrast in the C 1s images

stems from the concurring contributions from the C

1s emission and an inelastic background predomi-

nantly originated from the Au 4f electrons. As

shown in Fig. 6, the contrast provided by the inelastic

background practically mimics the contrast in the

Au 4f images. However, the C KLL image acquired

at the resonance excitation condition reveals a much

better contrast. The differences between the SPEM

images for C12/Au and C18/Au are related to the

different extent of the irradiation-induced desorption

processes with respect to the initial film thickness.

In particular, the C 1s emission and inelastic back-

ground contributions practically cancel each other in

the C 1s SPEM image for C18/Au, so that almost no

contrast is visible. Besides the lithographic patterns

based on conventional alkanethiols, some other

thiol-derived SAM resists with different functional

groups and modifications of the alkyl chain have

been studied.

Another potentially powerful method involving

physical modification of SAMs is plasma processing.

While this method is widely applied in microelec-

tronics for chemical modification, etching, and

patterning of organic surfaces and thin films, there

exist relatively few studies of plasma-treated SAMs.

We have used the nitrogen downstream microwave

plasma (containing some oxygen) to treat SAMs of

alkanethiols (AT) on polycrystalline (111) Au and Ag

substrates. The plasma contains low density and low

kinetic energy of charged particles, such as electrons

and ions.20 It is therefore assumed that long-lived

nitrogen and oxygen radicals provided the major

impact of the plasma treatment. Synchrotron ra-

diation photoelectron spectroscopy is utilized to

examine the modification of the alkanethiols C18

SAMs on the gold and silver substrates. It is found

that such a treatment results in essential damage and

disordering of the initially well-ordered and chemi-

cally homogeneous monomolecular AT layers. The

most pronounced effects are the complete (C18/Au)

or the partial (C18/Ag) oxidation of pristine thio-

late species, the partial desorption of hydrocarbon

Fig. 7. S 2p spectra and peak assignments of CH3

(CH2)17S/Au and CH3(CH2)17S/Ag before (a) andafter plasma treatments of 2 min (b) and 4 min (c).

fragments, and the appearance of nitrogen-containing

entities in this layer. The plasma-induced effects in

the alkyl matrix and at the S–substrate interface

can, however, be only partly correlated.

The rate and extent of the oxidation effect at

the S–Au interface are noticeably larger for C18/Au

as compared with C18/Ag. For instance, no signi-

ficant change is observed on C18/Ag after a short

(2 min) plasma treatment, whereas oxidation clearly

takes place on C18/Au after the same exposure. This

suggests a stronger S–metal bonding in AT/Ag. This

result is shown in Fig. 7, which depicts the photoe-

mission spectra and peak assignments of the S 2p

core level for C18/Au and C18/Ag before and after

plasma treatments of 2 min and 4 min, respectively.

The damage of the alkyl matrix in C18/Ag occurs

also more slowly than the respective process in

C18/Au. This can be explained by the difference


in the oxidation effect at the S–substrate interface,

and by the difference in packing density and ori-

entational order of the alkyl matrix in these two

systems. Further investigations in using plasma-

induced modification of SAMs for patterning with

subsequent exposure to biomolecules are ongoing.

As shown for the electron-irradiated SAM patterns,

SPEM can provide valuable information in their

characterization.

5. Conclusions and Outlook

The fast-growing fields of molecular electronics,

multiphase ultrafine structured materials and novel

quantum devices are potential subjects for scan-

ning photoelectron microscopy as a diagnostic tool

for spatially and chemically resolved studies. It

has been proven through many examples in recent

years that this technique can provide direct informa-

tion on chemical states and electronic properties at

submicron lateral resolution. In the coming years,

we will focus for the technical part on improving

the spatial resolution down to the order of a few

hundred angstroms at variable sample temperature

and for the scientific part on studying more complex

chemical networks including biological specimens. In

addition, we plan to utilize the focused SR beam

itself in fabrication and modification of structured

surfaces.

The SRRC-SPEM station, which was open to

public users a few months ago, has demonstrated

its performance in spatial resolution and chemical

microanalysis capability. Selected examples on

structured semiconductor materials and organic

overlayers on metal surfaces performed in collabo-

ration with user groups have shown the scientific

potential of zone-plate-based SPEM.

Acknowledgments

This work is partially supported by the National

Science Council of Taiwan under Grant Nos. NSC 89-

2112-M-213-016, NSC 90-2112-M-213-003, NSC89-

2112-M-007-077 and 89-2320-B-033-005-Y, by the

Ministry of Education of Taiwan under Grant

No. 90-N-FA01-2-4-5, by the German Bundesmin-

isterium fur Bildung und Forschung through Grant

No. 05 SF8VHA 1, and by the DAAD-NSC exchange

program under PPP Grant No. 10. We acknowledge

Dr. C.-H. Ko for his initial contributions to the

SPEM project.

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