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SEMI M33-0998 © SE MI 1998 7 • me asurem ent t ime , • analysis results on t he blank m i crodroplet, • compo siti ons a nd a mount o f the sc anni ng dro plet , • running time since last calibration or freque ncy of SPC…

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droplet and the scanning droplet must be handled,
prepared and dried under specified and controlled
conditions. Drying means the evaporation of the
solvents on the wafer surface in a clean and controlled
environment without loosing analytes from the
microdroplet residue.
11.3 The wafers are exposed to the HF vapor in the
VPD chamber at room temperature. The exposure time
depends on the concentration of the HF used and on the
preceeding wafer treatment. The time has to be long
enough to ensure that the wafer surface will become
hydrophobic. After the HF treatment the solubilized
reaction products are collected by scanning a
microdroplet over the whole surface. Automatic
scanning is preferable (see Section 6.13 of this
document). The composition of the scanning solution is
optional but its volume and drying conditions should be
the same as under the preparation of calibration
standards (see Sections 8 and 15.18 of this document).
11.4 An example for tested preparation conditions
according to Section 15.18 of this document. A PTFE
or PFA petri dish of a diameter > 25 cm is filled with
20 volume % HF by mixing DI ultra pure water and 40
volume % HF of ULSI grade in a clean room ambient
of Cl 10 by U.S. Federal Standard 209. The petri dish is
positioned in the bottom of the VPD chamber, loaded
with specimen wafers. The wafers are exposed to the
HF wafer at room temperature. Exposure time is
between 30 minutes and 6 hours. Longer exposure
times can lead to deliberate etching. After the HF
treatment the solubilized, reaction products are
collected by rolling a scanning microdroplet over the
whole surface. Automatic scanning is preferable. Please
refer also to Section 4.5 and Section 8.3 of SEMI E45.
11.5 Rinse the validated micropipette at least 5 times
with ultra pure water or with the selected scanning
solution. Then fill the micropipette with the required
amount of ultra pure water or with the scanning
solution. For scanning surfaces with thicker silicon
oxide (> 300 nm) only about 50 µL of scanning
solution is sufficient. The microdroplet to be dried for
analysis should not extensively (+10%) exceed 100 µL
together with the VPD reaction products.
11.6 Put the scanning microdroplet on the wafer
surface and roll the scanning microdroplet around the
wafer edge 2 times and then over the whole surface in a
zigzag pattern. Automatic equipment can scan in a
spiral pattern with overlapping paths (see Section 6.13).
An edge exclusion of less than 1 mm is attainable for
manual or automatic scanning. Edge exclusion is well
below 1 mm at automatic scanning.
11.7 Position the scanning microdro plet in the center
of the wafer.
11.8 For monitoring the cleanliness of the VPD
preparation and contamination collection and that of the
analytical ambient, put the same volume of the blank
scanning solution, as applied to the preparation of the
calibration standard (100 µL under tested preparation
conditions), with the validated micropipette onto the
scanned specimen surface at least 3 cm off the position
of the scanning solution. For valid results fluorescence
counts above the blank must remain less than 20% of
the fluorescence counts above the droplet residues. Use
ultra pure water (100 µL) for blanks of process
chemical and media samples. If the scanning solution
consists only of ultra pure water of controlled quality,
no blanks are required.
NOTE: In the absence of oxidation agent(s), Cu recovery
rates can be reduced (see Section 15.18).
11.9 Dry the wafers as specified in Sections 7.3, 9.1,
and 11.2 of this document.
12 Analysis Procedure
12.1 Localize the microdroplet residues of the
collected scanning solution or the microdroplet residues
of the liquid process medium and the microdroplet
residue of the blank scanning solution. Detect
fluorescence counting rates above these microdroplet
residues and above the scanned blank surface under the
instrumental parameter adjusted for calibration. The
quantification algorithm is given in Section 13.6.
Integrated counts above both the blanks and the VPD
prepared surface must not exceed 20% of the integrated
counts above the microdroplet residue of the collected
scanning solution (see Section 9.6 of this document).
13 Quantification Procedure
13.1 The instrument must run unde r established
statistical process control (e.g., as described in Section
15.19 of this document).
13.2 Before releasing results, quote instrumental
parameters such as:
• rotating or sealed anode,
• voltage and current applied to the X-ray source,
• characteristic excitation line(s) of the incident X-
ray,
• glancing angle(s),
• type of monochromator,
• amount [ng] or [number of atoms] of e.g., Ni in the
calibration standard reference microdroplet,
• location of the microdroplet(s) analyzed,
• peak evaluation technique (ROI or peak-fitting),
SEMI M33-0998 © SEMI 19987
• measurement time,
• analysis results on the blank microdroplet,
• compositions and amount of the scanning droplet,
• running time since last calibration or frequency of
SPC measurement,
• lab environment classification by U.S. Federal
Standard 209,
• edge exclusion, if any (c.f., Sections 4.2 and 11.6).
13.3 Quantification of areal density is in units of 10
10
atoms/cm
2
.
13.4 Detection spot area is the surfa ce area where
above the fluorescence counts are integrated.
13.5 Scanned surface area is the sur face area where
the impurities are collected from, according to Section
9.1.
13.6 Calculate the areal density of i mpurity i
according to the following algorithm:
c
i
,
VPD
=
c
i
A
w
A
m
⋅
R
i
(atoms/cm
2
)
=
n
i
A
w
⋅
R
i
(atoms/cm
2
)
where;
c
i
,
VPD
=
density of impurity (
i
) at scanned
area of wafer surface in [atoms/cm
2
]
c
i
=
measured concentration of impurity
i
at measured spot [atoms/cm
2
]
A
w
=
VPD - scanned wafer area in [cm
2
]
A
m
=
measuring spot area in [cm
2
]
R
i
=
recovery rate of the collected impurity
(0
<
R
i
<
1) c.f., Section 4.13
n
i
=
measured number of atoms of analyte
i
The measured concentration c
i
of impurity (i) can be
related to the reference standard by means of the
following expression.
NOTE: The reference standard element is assumed to be Ni in
this section.
ci
=
R
S
F
i
⋅
c
N
i
I
N
i
⋅
I
i
[atoms/cm
2
]
where;
c
N
i
=
n
N
i
/
A
m
n
N
i
= number of impurity atoms (Ni)
in the standard reference specimen
I
i
= measured fluorescence intensity
of impurity (
i
) in counts per second [cps]
I
N
i
= measured fluorescence intensity
of the standard reference specimen (Ni)
in counts per second [cps]
R
SF
i
=
instrumental sensitivity factor
of the analyte
i
relative to the standard
element (Ni)
This formula provides the areal density for the impurity
of interest with LOD as given in Section 13.7.
13.7
Calculation of LOD
i
, the lowe st detectable
number of impurity atoms i from a scanned surface, is:
LOD
i
(t) ≡ 3 ⋅c
i ,VPD
⋅
N
bg
(t)
N
netto,i
(t)
where;
N
bg
(t) denotes the background fluorescence
cps, integrated over time (t)
N
bg
(t) = I
bg
⋅t,
with
I
bg
= intensity of the background in cps, and
where;
N
netto,i
(t) denotes the netto number of the
impurity fluorescence [cps], integrated
over the time,
N
netto,i
(t) = I
i
⋅t
The equation for the LOD
i
(t) can be rewritten as :
LODi(t ) = 3⋅
c
i,VPD
I
i
⋅
I
bg
t
14 Bias and Precision
14.1 Relative error of the described VPD-TXRF
method must be assessed according to Section 15.11 of
this document. Under the given measurement
conditions the accuracy of the results is limited by the
error summarized in Section 14.2.
14.2
Under given solute amounts an d measurement
conditions the accuracy of the results is limited only by
SEMI M33-0998 © SEMI 1998 8
the recovery rate of the VPD treatment and the
scanning solution as described in Section 15.19 of this
document. The relative error is to be calculated by:
dc
i ,VPD
c
i ,VPD
=
dR
i
R
i
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
+
dRSF
i
RSF
i
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
+
dA
w
A
w
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
+
dn
Ni
n
Ni
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
+
dI
i
I
i
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
+
dI
Ni
I
Ni
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
12
14.3 Relative error of the reference droplet standard is
determined by the error of the micropipette aliquot (V)
and of the stock solution (c
S
).
dn
N
i
n
N
i
=
dV
V
ℜ
ℜ
ℜ
ℜ
2
+
dc
S
c
S
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
2
ℜ
ℜ
ℜ
ℜ
ℜ
ℜ
12
14.4 The relative error of areal concentrations below
the concentration of the calibration standard reference
are strongly dependent upon the relative error of the
micropipette aliquots. In the given range bias due to
crystallization and/or mass absorption can be
anticipated to be less than 1% as described in Sections
15.6 and 15.11 of this document.
14.5
Reproducibility of the measure ment system must
be tested with calibration standard reference in five
complete analysis cycles (load-analyze-unload). The
standard deviation of the 5 results shall not exceed
more than 10% of the theoretical value of the standard
deviation of the respective Poisson statistics
Nx. For
long term reproducibility, please refer to Section 2.6.
14.6 The minimum sample size for controlling the
wafer-to-wafer reproducibility of the complete
procedure must consist of a group of 3 wafers of the
very same polishing and/or cleaning batch as described
in Section 15.20 of this document. Tolerated standard
deviation of 3 groups should be defined by the
interested parties.
14.7
In interlaboratory tests (round robin), the
reproducibility of the method can preferably be
evaluated in accordance with DIN ISO 5725 or ASTM
E 691.
NOTE: Report reproducibility in accordance with this
document. Calibration accuracy was found to be within 10%
relative standard deviation for K, Ca, Ti, Cr, Fe, and Cu
among five TXRF stations as stated in Section 15.21 of this
document. W and other compounds forming volatile fluorides
shall not be analyzed after VPD because of low recovery
rates.
15 References
Analytical Determination of Fe in Thin SiO(2) Layers
on Si Wafers by Atomic Absorption Spectroscopy — M.
Briska, in Technical Disclosure Bulletine (IBM), 06–
1977, p. 227
Application of Total Reflection X-Ray Fluorescence
Analysis for Metallic Trace Impurities on Silicon Wafer
Surfaces
— P. Eichinger, H. J. Rath, and H. Schwenke,
ASTM STP 990, ASTM 1989, pp. 305–13
Automated TXRF Analysis in Silicium Manufacturing
— S. Pahlke, L. Kotz, E. Heindl, and P. Eichinger, PV
98–1, The Electrochemical Society, 1998
Basic Features of Total Reflection X-Ray Fluorescence
Analysis on Silicon Wafers — W. Berneike,
Spectrochimica Acta 48B (2), 269–75, 1993
Calibration Accuracy of Different ATOMIKA TXRF
8010 Instruments — P. W. Mertens, S. De Gendt, and
K. Kenis, IIAP-UCP Meeting, IMEC, September 26,
1996, Leuven, Belgium
Calibration of TXRF Equipment — J. Knoth, H.
Schwenke, and P. Eichinger, Proceedings 2nd
International Symposium Ultra-clean Processing of Si
Surface, IMEC-Acco 1994, pp. 107–10
Chemical Analysis of Metallic Impurity on the Surface
of Silicon Wafers — T. Shiraiwa, N. Fujino, S. Sumita,
and Y. Tanizoe, AST STP No. 850 NBS/IEEE, 1987, p.
314
Determination of the Critical Thickness and the
Sensitivity for Thin-Film Analysis by Total Reflection
X-Ray Fluorescence Spectrometry
— R.
Klockenkämper and A. von Bohlen, Spectrochimica
Acta 48B (5), 461–9, 1989
Introduction to Control Charts in the Analytical
Laboratory — SPC, E. Mullins, Analyst, 119, 369–75,
1994
Is My Calibration Linear? — Analytical Methods
Commission — Analyst 119, 2363-6, 1994
A Modification of the Linear Least-Squares Fitting
Method which Provides Continuum Suppression — F.
H. Schamber, X-ray Fluorescence Analysis, Editor, T.
Doubay Ann Arbor Science Publication Ann Arbor MI,
1977, pp. 241–257
Origins of Spurious Peaks of Total Reflection X-Ray
Fluorescence Analysis of Si Wafers Excited by
Monochromatic X-Ray Beam W-Lß
— K. Yakushiji, S.
Ohkawa, A. Yoshinaga, and J. Harada, Japan Journal
Applied Physics, 33 (2), Part 1, 1130–5, 1994
Standardization of TXRF Using Microdroplet Samples
— L. Fabry, S. Pahlke, L. Kotz, Y. Adachi, S.