Performances of the TXRF Beamline for Trace Element
Mapping at the European Synchrotron Radiation Facility.
F. COMIN and R. BARRETT
ESRF, BP 220 - 38043 Grenoble cedex, France
Abstract. The pursuit of smaller and smaller circuitry in device manufacturing imposes tight limits on the surface
contamination of the wafers. TXRF, Total Reflection X-ray Fluorescence, is the leading technique in device industry for
detecting surface impurities. However, laboratory equipments have reached their limit, and the common practice then for
reaching the desired sensitivities of ~109 atoms per cm square, equivalent to 10~6 monolayers, is then to proceed to preconcentration of the impurities. This operation erases any information on the localisation of the impurities, making much more
difficult the identification of their origin.
Centralised facilities based on Synchrotron Radiation extend the reach of classical TXRF machines offering new opportunities
both in terms of ultimate sensitivity, lateral resolution and detectable range of elements. In this contribution we will describe a
dedicated TXRF instrument operational at the European Synchrotron Radiation Facility in Grenoble along with its
performances, limitations and future developments.
INTRODUCTION
The development pace in semiconductor world is
marked by the assessments of the International
Technology Roadmap for Semiconductors (ITRS) that
every two years identifies the technological challenges
and the needs facing the device industry over a stretch
of 15 years. Since long the ITRS has identified the
concentration of residual impurities at the surface of
silicon wafers as a major problem to overcome and has
promoted the lowering of the limits of detection of the
techniques used in quantifying impurity concentration
as the necessary step to get a hand over their control.
THE TXRF TECHNIQUE
The most widely used analysis technique in
microelectronic industry is Total Reflection X-ray
Fluorescence (TXRF). In TXRF a beam of X-rays is
impinging on the wafer surface at an angle lower than
the critical angle for total reflection, thus limiting the
penetration of the X-rays below the wafer surface to
few nanometer. The X-ray beam excites the impurities
present at the surface and a solid state detector detects
the fluorescence that they emit. The TXRF technique
is well adapted to the environment of clean rooms,
allow analysis on specific areas of the wafer and can
reach low detection limits because the background
from the bulk silicon is limited.
At the end of the 90's, however, it became clear that
standard TXRF could not cope with future ITRS
requirements and that new strategies for lowering the
detection limits should be identified.
A straightforward way to increase the sensitivity of
any impurity detection technique is by preconcentrating all the impurities of an entire wafer on a
single spot. This has been, in fact, the first approach to
the reduction of the impurity detection limits. In the
Vapour Phase Decomposition pre-concentration
technique (VPD) the surface layer of the wafer is
etched away with all its impurities and all dissolved
material collected by a droplet made walking over the
surface. TXRF analysis on the droplet increases then
the detection limits because integrates over the entire
wafer surface. The localisation of the impurities or
their spatial distribution is however definitively lost.
Synchrotron Radiation TXRF
The orders of magnitude in brilliance of Synchrotron
Radiation beams with respect to rotating anode X-ray
generators provide an alternative solution for reaching
lower detection limits without recurring to preconcentration. At the SSRL in Stanford Pianetta et al.
[1] showed the possibility of attaining detection limits
in the range of 109 at/cm2 without any need of preconcentration of the impurities. At the European
Synchrotron Radiation Facility (ESRF) in Grenoble, a
feasibility test in 1996 [2] showed that the detection
limit could be lowered even more to the 108 at/cm2
range. These lower limits are the combined result of
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in the beam path after the multilayer monochromator;
this monochromator selects a bandpass of few eV for
performing
X-ray
Absorption
Spectroscopy
measurements around selected absorption edges for
analysing chemical and geometrical atomic structure
around the contaminant species.
Finally, a mirror for rejecting the higher harmonics
transmitted by the monochromator is installed
upstream the wafer analysis station. This mirror is of
the bimorph [3] type and can be piezoelectrically bent
meridionally to focus the radiation in the vertical
direction and thus increase the photon density on a
particular area of the wafer.
the increased X-ray photon flux that can be made
incident on the wafer surface and of the linear
polarisation of the radiation.
Starting from these evidences a collaborative action
between the European Synchrotron Radiation Facility
in Grenoble and some semiconductor companies under
the patronage of the European program MEDEA has
been started in 1997 for developing at the ESRF a
facility capable of sustaining the future challenges in
the pursuit of the ultimate cleanliness of silicon wafer
surfaces by allowing mapping of impurity
concentrations of the order of 108 at/cm2 on the surface
of 200 and 300 mm silicon wafers.
The facility has been built and opened to industrial
users in 2000.
ESRF EXPERIMENTAL LAYOUT
The layout of Fig. 1 shows the essential elements that
compose the 50-meter long installation: the X-ray
beams produced by the insertion devices from the right
of the figure and not shown are conditioned in an
Optics Hutch by slits and photon absorbers. The total
power emitted by the undulators (hundreds of Watts on
few mm2) would not be directly exploitable without
any further conditioning. Consequently, out of the
energy spectrum of the emitted radiation, a water
cooled multilayer monochromator selects a wide
energy bandpass with a central energy continuously
tuneable between 800 eV and 20000 eV. The typical
flux at the output of this monochromator is of the order
of 1014 photons/s.
The X-ray beam passes into the Analysis Hutch: a lead
shielded enclosure within which is embedded a class
100 clean room that hosts the TXRF measurement
chamber and the ancillary wafer handling devices for
automatic loading and unloading of the wafers.
Because of the need to transport photons with energies
as low as 800 eV, the entire beamline, TXRF chamber
and detection system are in-vacuum with no window
intercepting the beam.
Wafer handling
robot
\
TXRF station
Beam Uncooled
Experimental shutter slits
hutch
Control room
\
/
FIGURE 2. the TXRF chamber with its main
components.
The TXRF Station
The TXRF end-station is where the wafer are
analyzed. The station encompasses an atmospheric
wafer-handling robot that transfers the wafers from
standard cassettes to a pre-aligner to azimuthally orient
and centre wafers before introduction into the airlock
vessel. The airlock can host up to five 200 mm and
five 300 mm wafers. After pump-down of the airlock,
one wafer at the time can be transferred to the main
TXRF chamber for analysis. The TXRF system is
schematised in Fig.2: a hexapod actuator installed in
air just below the vacuum chamber is coupled through
a bellow and a rotary feedthrough to an electrostatic
chuck that flattens and holds the wafer in vacuum. All
Cooled
slits
A
Fixed exit
Si channel-cut
Optics hutch
FIGURE 1. Optical layout of the SR TXRF beamline
at the ESRF. The undulators 9not shown) are on the left.
The optical layout of the installation can be completed
by a silicon post-monochromator that may be inserted
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the alignment procedures of the wafer relative to the
X-ray beam are performed via the hexapod and
transmitted through the bellows and the rotary feed
hrough to the in-vacuum chuck.
below 50"; this avoids the continuous readjustment of
the angle of incidence during the rotation scan.
Of course it is always possible to trade lateral
resolution with detection limit by adding spectra from
contiguous pixels.
Performances and Limitations
The Detection Scheme.
With standard acquisition times of the order of 1000s
the attainable Lower Limit of Detection (LLD) for
transition metals is in the range of 108 at/cm2. Much
longer integration times have been chosen to analyse
an ultra-clean wafer borrowed from the SSRL facility,
in order to assess the stability of the instrument and the
absence of any stray signal from the vacuum system
environment. The result obtained of LLDs in the lower
107 at/cm2 range is comparable with the SSRL findings
(see Fig. 4).
The configuration of the station was designed on the
basis of a number of geometrical constraints; it was
desirable to have the wafer loading, unloading and
alignment procedures to be performed in a horizontal
plane for ease of operation. Furthermore the
fluorescence detectors should look along the horizontal
polarisation direction of the radiation to minimise the
collection of elastically scattered photons. A third
important constraint is that the radiation beam cannot
be focussed down into a small region of the wafer
because any single element detector would be easily
saturated. As a result the unfocussed beam of radiation
is left to impinge on the wafer surface at the desired
angle of incidence along an entire diameter. The wafer
stays then in an essentially horizontal geometry: the
side tilt of about 5 degrees toward two arrays of six
Si:Li solid state detectors permits the silicon elements
to be approached close to the illuminated diameter of
the wafer. A PTFE collimator limits the detection
footprint seen by each element of the arrays to a
segment of about 17 mm of the illuminated diameter.
The geometry is shown in Fig. 3.
4
6
Energy Eke*/]
FIGURE 4. SR TXRF of an ultra clean wafer. The
MDL is of few 107 at/cm2.
The detection of low Z atoms presents additional
problems that have been addressed with the
development of Synchrotron Radiation TXRF [4] and
that establish new, intrinsic limits to the lowest
detectable concentration of impurities. When detecting
elements lighter than silicon, in fact, it is necessary to
use exciting photon energies below the Si K-edge
threshold in order to eliminate the intense silicon
fluorescence that with its low energy tail would bury
the emission from lighter elements. However, when
exciting below the silicon threshold, the emission
spectra is dominated by a resonant Raman-Compton
background contribution in which after absorption of
an incoming photon an inelastically scattered one is reemitted with an energy decrement equivalent to the
energy necessary to excite a 2s electron into the
continuum energy levels. This process is responsible
for a triangular shaped background peaked at the
excitation energy decreased by 100 eV and with a tail
extending far into the low energy spectrum. This
Sp atial Re s o lution *>
FIGURE 3. The detection geometry.
A mapping of the contaminant distribution along this
line can then be performed in parallel by the detector
arrays and a complete rotation of the wafer around its
axis gives a complete mapping of the impurity
distribution. Special care has been placed on the initial
alignment of the chuck with respect to the external
axis of rotation in order to keep the precession angle
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beamlines [5]. The program has a scripting capability
which allows straightforward programming of
repetitive tasks such as standard analysis recipes. This
has minimised the necessary software development
and allows common software tools to be used for
control of the beamline parameters such as the
undulator pole gap and the monochromator operating
energy. In addition, dialogue with instruments using
common interfaces such as serial lines or GPIB is
rather straightforward. The high level Spec program
runs on a Unix workstation and communicates via
Ethernet using the TACO object oriented control
protocol [6] with distributed device servers running on
diverse
platforms
(VME/OS9,
PC/Linux,
PC/Windows, ...) to control the various components.
This system is readily adaptable to operation either in
an interactive or in semi- or fully-automatic modes.
Automation of the wafer handling process is in
principle straightforward. The main efforts have been
dedicated to detecting and reacting to unforeseen
events such as equipment failure. Wherever possible
these interlocks are implemented at a hardware level to
render them independent of the software configuration.
The automation at this level must control the
atmospheric robot, the vacuum pumping sequence and
the in-vacuum wafer transfer between the airlock and
analysis chamber. The adopted solution uses a
combination of device servers running on
PC/Windows 9X, PC104/Linux and VME/OS9
systems. To eliminate the risk for wafer collisions it is
necessary to track the wafers in the system. This
capability is implemented at the Spec program level
and is being further developed to allow remote
monitoring of the status of the measuring process by,
for example, a web client.
background, in contrast with the elastic scattering,
does not present strong geometrical anisotropies and is
therefore relatively insensitive to the detection
geometry. This limits the LLD for Al to a value of ~7
109 at/cm2. Fig. 5 shows the spectra from two
reference wafers with ~10n and ~1012 Al at/cm2 as
compared to the spectrum from a clean wafer. The
implication of the Raman effect in limiting the LLD
for Al is quite evident.
In the case that the required mapping detection limits
for impurities of low Z elements were lower than 7 109
at/cm2 it would be necessary to move from energy
dispersive detection towards wavelength dispersive
methods, since the higher energy resolution of optical
elements such as multilayers would increase the signal
to background ratios of low energy fluorescence peaks.
1200
1300
1400
1500
1600
1700
1800
1900
FIGURE 5. TXRF of Al contaminated wafers. The
Raman peak centered at 1600 eV buries the Al fluorescence
peak for the lowest impurity levels.
The control of the acquisition process requires the
development of robust algorithms capable of reliably
aligning the wafer in the beam and reacting
intelligently to events such as the periodic electron
refills of the ESRF storage ring.
These are
implemented directly within the Spec control program.
Furthermore the measurement of a highly
contaminated wafer may easily saturate the detector
arrays and may require a reduction of the incoming
photon flux to allow valid analysis.
AUTOMATION
The facility has been designed to accept a high degree
of automation to maximise the wafer throughput with a
minimum waste of human resources. The automation
scheme can broadly be broken down into 3 key areas;
wafer handling, data acquisition and spectrum
processing. Although these are distinct processes some
interaction is required, for example it may be
necessary to trigger further data collection following a
decision taken on the basis of the output from the
spectrum processing. Any approach to automation
must retain a high degree of flexibility in order to
allow the beamline capabilities to be fully exploited.
Spectrum processing for quantification of the surface
contamination levels is performed by multivariate
fitting of the individual spectra using pre-established
fluorescence peak and background models to
determine the fluorescence intensity contributions of
individual elements.
The fitting routines are
implemented as a library based on the AXIL
algorithms from Antwerp University running in a
Linux environment. The output from the fitting
The whole beamline instrumentation (optics and
analysis chamber) is controlled using the Spec control
program which is universally used by the ESRF
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process can be immediately published in html format
for remote assessment.
1. P.Pianetta et al., Rev. ScL Instrum. 66, 1293 (1995).
3. J. Susini, D. Labergerie, O. Hignette, in Optics for HighBrightness Synchrotron Beamlines II (ed. L. Berman).,
SPIE 2856,130-144(1996)
4. K. Baur , J. Kerner, S. Brennan, A. Singh, and P.
Pianetta, J. of Appl. Phys., 6, (2000).
A cknowledgem ents
The construction of the facility has been made possible
by the dedication of Monique Navizet, the work of
Paolo Mangiagalli and Giorgio Apostolo. Important
contributions to the software and hardware
developments for automation have been made by
Emmanuel Papillon and Ricardo Hino respectively.
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REFERENCES
1. P.Pianetta et al., Rev. ScL Instrum. 66, 1293 (1995).
2. L.Ortega, F. Comin, V. Formoso and A. Stierle, Journal of
Synch. Radiation, 5 1064-1066 (1998).
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