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SEMI M33-0998 © SE MI 1998 4 of th e described method as reporte d in Sect ion 15.13 of this docume nt. 7 Apparatu s 7.1 The VPD treatm ent a nd contam i n a tion collectio n par ticula rly, b ut also the handli ng and m…
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microanalysis, as originally described in Sections 15.2,
15.4, or 15.5 (also see Sections 5.1 and 11 of this
document).
5 Summary of Method
5.1 The native or thermally grown oxide layer of the
silicon surface is converted with HF vapor into fluid
droplets that contain the impurities of the oxide layer.
With a scanning droplet the fluid reaction products are
collected in one microdroplet. That microdroplet is
dried on the wafer under controlled conditions and
analyzed with TXRF.
5.2 Similarly, a microdroplet of process chemicals or
media can be dried on hydrophobic polished or
epitaxial silicon wafer under controlled conditions and
analyzed with TXRF.
5.3 Preferably, monochromatic and collimated X-rays
irradiate a planarized and chemi-mechanically polished,
monocrystalline silicon wafer surface. The X-rays
impinge the surface at a glancing angle that is below the
angle for total reflection of the X-rays, preferably, at an
incident angle 70% of the angle of total reflection [1.3
mrad (or 0.07 degrees) for Mo target and 2.0 mrad (or
0.11 degrees) for W target].
5.4 The evanescent waves excite the fluorescence
energy levels of the surface atoms, which then emit
fluorescence X-rays characteristic of their atomic
number. The emitted X-rays are detected by a solid
state detector that is an energy dispersive spectrometer.
In the range of specified areal density (compare with
Section 2.3 of this document) the integrated count rate
(cps) is linearly proportional to the elemental areal
density.
5.5 For quantification, the linear re gression must be
established (c.f., Section 15.6) or a linear
proportionality is anticipated between the cps data that
are measured above the certified reference
microdroplet(s) and the cps data that are measured
above the microdroplet of the unknown analytes
without changing the anglescan conditions, according
to Sections 9.1, 13.6, and 15.7.
6 Interferences
6.1 The known interferences in X-ray fluorescence
spectroscopy also affect TXRF. Thus, overlapping
fluorescence lines, escape peak, energy gain drift, X-ray
source stability, beam path background contamination
must be evaluated according to Section 15.7 of this
document.
6.2 Baseline corrections due to va rying background
contamination must be controlled by the rules of
statistical analysis (e.g., as described in Section 15.8 of
this document).
6.3 Under the specified conditions, no corrections are
required for secondary fluorescence or for oscillations
or for matrix absorption as described in Sections 15.9–
15.11 of this document.
6.4 Accuracy of the standard refer ence specimen and
positioning accuracy and precision of the detector
define the bias in the assigned areal density.
6.5 Mechanical vibration may degrade the detector
resolution and it can also decrease the selectivity.
6.6 Multielement contamination d egrades the LOD
compared with monoelement contamination.
6.7 Increased surface microroughness and/or high-
total signal count rates result in high deadtime and can
lead to non-linearity of detected fluorescence signal
versus areal density (i.e., to degradation of LOD).
6.8 Under optical conditions satisf ying the Bragg
reflections the background noise depends upon the
azimuthal orientation of the sample. Before
quantification, a determination of the azimuthal angular
range, that shows minimum Bragg reflection
background, is recommended. Otherwise the LOD may
degrade due to high background and spurious peaks.
6.9 During handling and measurem ent particles or
volatile contamination (e.g., NH
3
) from the analytical
environment must be controlled and avoided.
6.10 During measurement Ar must be excluded from
the analytical ambient (e.g., by evacuating the chamber
or flushing it with He).
6.11 Curve smoothing and evaluatio n algorithms with
controlled Fourier parameters or Digital Filtering are
preferred to direct count rate evaluation because these
algorithms provide a higher level of statistical
confidence than a software that directly quantifies cps
as described in Section 15.12 of this document.
6.12 Recovery rates as defined in Section 4.13 of this
document depend upon the distribution of the analytes
between the scanning solution (solubility) and silicon
surface (adsorption and plating). Therefore, recovery
rates depend on the:
— applied scanning solution (Sections 4.5, 8.3),
— chemical nature of the different
analytes/elements, and
— physical and physicochemical state of the silicon
surface.
6.13 Automated contamination coll ection procedure
(scanning) increases the wafer-to-wafer reproducibility
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of the described method as reported in Section 15.13 of
this document.
7 Apparatus
7.1 The VPD treatment and contamination collection
particularly, but also the handling and measurement of
the specimen wafer is to be carried out in a specified
and controlled ambient (e.g., Cl. 10 (U.S. Federal
Standard 209)).
7.2 TXRF system equipped with:
— an X-ray source,
— a monochromator (preferable),
— a sample stage capable of manipulating in the x-,
y-, and z-direction,
— automated test specimen handling,
— an energy-dispersive spectrometer X-ray
detector,
— software sub-routine for glancing angle
calibration,
— software for baseline setting and for peak-fitting
and/or range-of-interest (ROI) peak finding
identification and evaluation, and
— analysis ambient without Ar background (see
Sections 6.10 and 7.1 of ASTM F 1526). The
system is preferably equipped with a flat/notch-
finder and quick-search option. For details, see
Section 6.10 of this document and Section 7.1 of
ASTM F 1526.
7.3 The VPD and the advisable dr ying chamber(s) will
have opening(s) made of polyvinylidenfluoride
(PVDF), polyfluoroalkoxyethylene (PFA), polyfluoro-
ethylene (PTFE) or similar resistant and pure polymer
materials that will not be attacked by HF. The
chamber(s) may contain one or more wafers on stacks.
The use of a drying chamber is advisable for the
preparation of the calibration reference microdroplet
(c.f., Section 10.1), but optional for the analysis
procedure. When a drying chamber is used, it must be
evacuable to below 1 kPa. After evacuation the
chamber is to be flushed with filtered N
2
until the
complete drying of the microdroplet residue is achieved
(see Section 9.1 of this document).
7.4 For the aliquots of standard stock and scanning
solutions validated micropipettes must be used.
Validation procedure can follow the requirements of
DIN 12650 Part 6.
8 Reagents and Materials
SAFETY PRECAUTIONS — Handling HNO
3
, HF and
H
2
O
2
is dangerous. Operators must comply with X-ray
safety regulations and be trained to wear protective
garments and glasses when handling HNO
3
, HF and
H
2
O
2
under efficient exhaust.
8.1 Ultra Pure Water, HNO
3
, HF, H
2
O — As
specified in ASTM D 5127, SEMI C7.3, C7.5, and
C7.6, respectively.
8.2 Standard Stock Solution — Ce rtified and traceable
standard reference “stock” solution with known
amount(s) of nitrate salt of the metals and sodium salt
of the non-metallic elements to be analyzed. Dilutions
have to be acidified with HNO
3
at pH ≤ 2. Note that the
shelf life of diluted solutions in the ppb-range of µg/L
or ng/L is less than 2 days.
8.3 Tested Scanning Droplet (Sections 4.5 and 6.12)
— 50 to 100 µL of ultra pure water or other scanning
solutions (e.g., aqueous HF (1 volume %) and H
2
O
2
(30
volume %)). The composition of the scanning droplet
must provide a controlled recovery rate above 90% for
each analyte, including Cu.
8.4 Blank Scanning Solution — Th e composition and
the amount of a scanning droplet without surface
impurities.
8.5 Microdroplet Residue — Microdroplet calibration
standard reference solution and/or scanning or
microdroplet of a liquid process medium dried at room
temperature under controlled conditions according to
Section 7.3. Above the microdroplet residue the
fluorescence count rate remains independent of
azimuthal position and of varying incident angle for
angles below 80% of the critical angle as described in
Section 15.6 of this document.
8.6 Surrogate Wafers — Polished or epitaxial wafers
used as carrier plates for scanning droplets collected
from other specimen wafers.
9 Preparation of Certified Reference
Microdroplet for Calibration Standard
9.1 Tested Preparation Conditions — Deposit
microdroplets containing 0.01 ng, 0.1 ng and 1 ng Ni in
Ni (NO
3
)
2
in 100 µL of a diluted standard stock
solution onto (a) hydrophobic (see Section 4.12 of this
document) polished or epitaxial wafer(s). Dry it (them)
under controlled conditions (e.g., in a drying chamber
at room temperature). When a drying chamber is used,
flush the chamber with a slow flow of N
2
for 20
minutes, then evacuate it (see Section 7.3 of this
document). The droplets must not explode or extend
during drying. They should not exceed an areal
dimension of 1 mm in any direction. Elements forming
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volatile compounds must not be applied (see the note in
Section 14.7).
10 Calibration Procedure
SAFETY PRECAUTIONS — X-ray irradiation is
dangerous. It is the responsibility of the user of this
standard to establish and maintain appropriate safety
and health practices and comply with the local
regulatory ordinance. The X-ray source must be
inactivated when beam path is unshielded. Operators
must be trained to avoid exposure to X-ray irradiation.
10.1 The calibration standard is a microdroplet of 100
µL of a diluted certified standard reference “stock”
solution. Note that the shelf life of diluted solutions in
the sub-ppb-range of µg/L or ng/mL is less than 2 days.
10.2 Locate the calibration standard microdroplet
residue and place it under the detector. Adjust the
glancing angle into the range where the fluorescence
count rate is independent of varying incidence angle
(see Section 8.5 of this document).
10.3 Position the detector window a s follows. Set
mapping or quick search parameters for covering the
residue(s) with 3 × 3 detection spots (see Figure 1),
which are placed around the residue(s) as described in
Section 15.6 of this document. The mapped surface
must cover at least 95% of the square around the
residue(s) that is lying about in the center of the
detection spot. The analyzed part of microdroplet
residue under the detection spot must be higher than
95%.
Figure 1
VPD-Search VPD/TXRF
10.4 Set for mapping around the microdroplet residue
with a 50-sec integration time program at each
detection spot. Integrate at the maximum position(s) for
at least 1000 sec. These data provide the final result of
calibration due to the algorithm given in Section 13.6 of
this document.
10.5 Repeat calibration cycle (load-a nalyze-unload) at
least 3-times.
10.6 In order to distinguish relevan t results from
particle contamination from the environment and from
inefficient contamination collection, repeat
measurement on blank surface adjacent to the
microdroplet solution. The data are to be considered as
background data in the quantification algorithm (see
Section 15.12 of this document). For valid calibration,
background fluorescence counts must remain less than
20% of the fluorescence counts of microdroplet
residues, particularly, at lower areal densities (< 1 × 10
9
atoms/cm
2
).
10.7 The integrated fluorescence counts measured
with the specified Ni calibration standard can be
converted by known sensitivity factors into other
elements of interest as described in the References
section (see Sections 15.14 and 15.15 of this
document).
10.8 Keep the calibration standards in closed,
identifiable wafer holders in a clean room at room
temperature under conditions that will not change the
fluorescence reproducibility.
10.9 Optional linearity tests should deploy the
methods described in References section (see Section
15.16 of this document). Similar statistical linearity
tests can also be applied.
10.10 Upon the users request, the ab solute
instrumental calibration factor can be obtained. Due to
the Fresnel theory of X-ray absorption and
enhancement on pure metal (Ni) surfaces, the absolute
calibration can be carried out under specific optical
conditions. (See Referenced Documents section and
specifically Section 15.17.). Then, that absolute
calibration can be correlated to the external
microdroplet calibration.
11 Preparation Procedure
11.1 VPD treatment of the wafer su rface (Section
4.16) and the collection of contamination collection
with a scanning microdroplet (Section 4.5).
SAFETY PRECAUTIONS — Handling HF is
dangerous. It is the responsibility of the user of this
standard to establish and maintain appropriate safety
and health practices and comply with the local
regulatory ordinance. Operators must be trained to wear
protective garments and glasses when handling HF
under efficient exhaust.
11.2 In the VPD chamber wafers are treated with HF
vapor (e.g., isothermally distilled at room temperature
from an aqueous solution (20–50 volume %) within the
VPD chamber). The analytical specimen, the calibration