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SEMI E45-1101 © SEMI 1995 , 2001 3 and con trolled ambi ent (e.g., ISO Class 4 (as defi ned in ISO 14644- 1)). 7.2 T he VPD and the advi sable dry ing chamber(s) shall hav e opening(s ) m ade of PVD F, PFA, PTFE or sim i…

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SEMI E45-1101 © SEMI 1995, 2001 2
3.3 DIN Standards
2
DIN 12650 Part 6 — Mechanical, physical and
electrical laboratory apparatus; Piston operated
volumetric apparatus; Gravimetric assessment of
metrological reliability
NOTE 1: Unless otherwise indicated, all documents cited
shall be the latest published versions.
4 Terminology
4.1 Abbreviations and Acronyms
4.1.1 GFAAS — Graphite Furnace Atomic Absorption
Spectroscopy
4.1.2 ICP-MS — Inductively Coupled Plasma – Mass
Spectroscopy
4.1.3 PFA — Perfluoroalkoxy
4.1.4 PTFE — Polytetrafluoroethylene
4.1.5 PVDF — Polyvinylidene fluoride
4.1.6 TXRF — Total Reflection X-Ray Fluorescence
Spectroscopy
4.1.7 ULSI — Ultra Large Scale Integration
4.1.8 VPD — Vapor Phase Decomposition
4.2 Definitions
4.2.1 box — a protective portable container for a
cassette and/or substrates.
4.2.2 cassette — an open structure that holds one or
more substrates (e.g., wafer, masks).
4.2.3 DI water — deionized water (specified with
specific resistivity ≥ 18 MΩcm, cations: Na, Cu, Fe, Ca
≤ 0.2 µg/L).
4.2.4 minienvironment — a localized environment
created by an enclosure to isolate the product from
contamination and people.
4.2.5 pod — a box having a Standard Mechanical
Interface (SMIF) (See SEMI E19).
4.2.6 reference wafer — a cleaned wafer (see Section
8.2).
4.2.7 sampling wafer — a cleaned wafer (see Section
8.2), which will be or was exposed to the
minienvironment for a certain time.
4.2.8 standard mechanical interface (SMIF) — the
interface plane between a pod and another
minienvironment (see SEMI E19).
2 Available from Deutches Institut für Normung e.V., Beuth Verlag
GmbH, Burggrafenstrasse 4-10, D-10787 Berlin, Germany. Website:
http://www.din.de
4.2.9 vapor phase decomposition — a method in which
impurities on the surface are collected by the so-called
VPD procedure, i.e., the non-volatile products formed
by acid decomposition of the oxide at the wafer surface
are collected by a droplet of collecting agent, usually
ultra-pure hydrofluoric acid or other reagent or
combination of reagents, and the droplet subsequently
being analyzed by AAS or ICP-MS, or dried in a
manner which gives the least environmental
contamination, the residue from the droplet
subsequently being analyzed by TXRF.
5 Interferences
5.1 For worst cases, preconditioning of wafers can
result in different surface properties indicated by
different sensitivities for contamination absorption.
5.2 Non-linearity effects of the TXRF detector are
significant at higher concentration levels (> 10
13
atoms/cm
2
under the detector area).
5.3 The collection efficiency of VPD depends on:
• the chemistry of the collecting solution
• the bonding of the metal impurities to the silicon
surface
• the speed of the droplet, which is rolled over the
wafer surface
5.3.1 Careful control of contamination and all other
factors affecting the results such as solution
concentrations, scanning methods and other procedures
are necessary to obtain reproducible analytical results.
5.4 The measured TXRF intensity depends on the
accuracy of the procedure to localize and to adjust to
the sampled residue. It also depends on the distribution
of different elements in and around the residue.
5.5 The detection of Fe or Ca using ICP-MS can be
interfered with by background ions originated from the
plasma unless some controlled measures are taken to
minimize these interferences to acceptable levels.
6 Safety Precautions
6.1 Handling hydrofluoric acid is dangerous and shall
be performed according to local regulations for
laboratories. Operators shall be trained to deal with
dangerous chemicals and vapors, especially
hydrofluoric acid and HF vapor. Protective clothes and
glasses must be worn when handling hydrofluoric acid.
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
SEMI E45-1101 © SEMI 1995, 20013
and controlled ambient (e.g., ISO Class 4 (as defined in
ISO 14644-1)).
7.2 The VPD and the advisable drying chamber(s) shall
have opening(s) made of PVDF, PFA, PTFE or similar
resistant and pure polymer materials that are not
attacked by HF. The chamber(s) may contain one or
more wafer stacks. After evacuation, the chamber shall
be flushed with filtered N
2
until the complete drying of
the microdroplet residue is achieved.
7.3 For the aliquots of standard stock and scanning
solutions, validated micropipettes shall be used. DIN
12650 Part 6 provides an applicable validation
procedure.
8 Procedure
These procedures show the determination of Na, Ca,
Cu, and Fe on a silicon wafer surface with
VPD/GFAAS, VPD/ICP-MS and VPD/TXRF. For
achieving best detection limits, VPD/GFAAS and
VPD/ICP-MS shall be used for NA and Cu whereas
VPD/TXRF shall be used for Ca and Fe.
8.1 Test Requirements — The evaluation of the
minienvironment and the analysis shall be carried out
under appropriate clean conditions. Any potential for
cross contamination shall be checked in advance.
Possible contamination sources are:
• VPD preparation
• storage
• contaminated GFAAS or ICP-MS vessels
• environment
• measurement methods and collection efficiency of
VPD
• handling
8.1.1 The capability of the analytical lab has to be
checked carefully for compliance with Sections 8.2,
8.6, and 9.
8.2 Surface Conditions and Cleaning Procedure —
Polished silicon wafers with the following
specifications must be used:
• Specific resistivity = 1–100 Ωcm
• CZ crystal growth method
• Cleaned to leave a native oxide with hydrophilic
surface conditions and with Na, Ca, Cu, and Fe
concentrations lower than 1 × 10
10
atoms/cm
2
8.2.1 Wafer cleaning must be done less than ten
minutes before any further processing. This restricted
time limitation is necessary to ensure cross
contamination avoidance.
8.3 VPD Preparation — The vessel for (opening > 25
cm
2
) inside the VPD box is filled with 25 vol-% HF by
mixing DI water with 50 vol-% VLSI-grade
hydrofluoric acid (see SEMI C28) allowing
hydrofluoric acid of between 25 to 50 wt % to be used.
The wafers are then exposed to the hydrofluoric acid
vapor. Allow wafers exposure to hydrofluoric acid
vapor for 15 to 30 minutes, at which time the wafer
should become hydrophobic due to oxide removal. The
liquid reaction products are collected by rolling a DI
water droplet over the whole wafer surface using up to
100 µL for advisable machine operation or an
appropriate volume for manual operation. Any cross
contamination is minimized by using DI water as
solvent.
8.4 Collection Procedure — An automatic scanning
procedure is preferable, but if the collection procedure
is manual the following procedure should be used.
8.4.1 Use appropriate method to exclude the wafer
edge.
8.4.2 A droplet of collecting agent, usually ultra-pure
hydrofluoric acid or another reagent or a combination
of reagents, is rolled over the whole surface of the
wafer in a parallel pattern.
8.4.3 The same droplet is then moved over the whole
surface, this time in a pattern orthogonal to the first.
8.4.4 Finally, the droplet is rolled in a spiral pattern
from the wafer periphery to its center.
8.5 Pre-analysis Procedure
8.5.1 GFAAS Analysis — The wafer droplet is diluted
to 500 µL.
8.5.2 TXRF Analysis — The droplet is evaporated on
the wafer surface in a clean environment at room
temperature under a nitrogen purge.
8.5.3 ICP-MS Analysis — The wafer droplet is diluted
to 500 µL.
8.6 Sodium and Copper Analysis by GFAAS
8.6.1 Calibration Standards — 1 µg/L for Na.
8.6.2 The calibration frequency and procedures shall be
in accordance with the requirements of the ISO 9001
quality system. The temperature program for the
graphite furnace (dry, ash, atomize) is optimized for
maximum sensitivity. Volatility (e.g., NaF) should be
avoided by spiking the liquid samples with nitric acid
(VLSI grade—see SEMI C35). Prior to analysis of the
liquid sample, a three-point calibration of the element
SEMI E45-1101 © SEMI 1995, 2001 4
with an elemental standard is carried out (blank — 0.5
µg/L – 1 µg/L).
8.6.3 A detection limit for sodium and copper better
than 5 × 10
9
atoms/cm
2
(approximately 0.1 µg/L for
GFAAS analysis) is recommended.
8.6.4 The detection limit is defined by 3 • σ • R (σ =
standard deviation, R = reciprocal slope of the
calibration curve).
8.6.5 The surface concentration is calculated as
follows:
C
s
= C
1
×
V × NA × 10
-9
/(W × A)
with:
C
s
(atoms/cm
2
): surface concentration
C
1
(µg/L): analyzed concentration
V(mL): volume of diluted sample
NA = 6.023 × 10
23
mol
–1
: Avogadro number
A (cm
2
): wafer surface area
W (g/mol): atomic weight of element (22.99 for
sodium, 86.54 for copper)
8.7 Sodium and Copper Analysis by ICP-MS
8.7.1 Calibration Standards — 1 µg/L for both sodium
and copper.
8.7.2 A detection limit for sodium and copper better
than 5 × 10
9
atoms/cm
2
(e.g., for 200–300 mm wafers)
is recommended.
8.7.3 The detection limit is defined by 3 • σ • R (σ =
standard deviation, R = reciprocal slope of the
calibration curve).
8.7.4 The surface concentration is calculated as
follows:
C
s
= C
1
×
V × NA × 10
-9
/(W×A)
with:
C
s
(atoms/cm
2
): surface concentration
C
1
(µg/L): analyzed concentration
V(ml): volume of diluted sample
NA = 6.023 × 10
23
/mol : Avogadro number
A (cm
2
): wafer surface area
W (g/mol): atomic weight of element (22.99 for
Na, 86.54 for copper)
8.8 Calcium and Iron Analysis by TXRF
8.8.1 Calibration Standards — The ISO 9001 quality
system shall be applied to the calibration. The
instrument is calibrated by analyzing the calibration
wafer. The calibration wafer is prepared by dropping a
solution of metal standard (e.g., Ni or Co) on a clean
wafer surface. The volume of the droplet is the same as
used for VPD preparation (up to 100 µL), and the
resulting surface concentration must be in the lower
10
11
atoms/cm
2
range. The droplet is then evaporated on
the wafer surface at room temperature under nitrogen
purge until the liquid matrix is removed. Its residue is
analyzed by TXRF. The detection limit is defined by 3 •
σ • R (σ = standard deviation, R = reciprocal slope of
the calibration curve). Note that the droplet area must
be smaller than the spot area of the detector.
8.8.2 Reproducibility shall be first established
following the procedure below.
8.8.3 Define a grid of 3 × 3 measurement points with
an inter distance of 3 mm. The reside position, found by
optical inspection, shall be located at the center of the
matrix.
8.8.4 Determine the intensity by short analyses of each
of the nine measurement locations. The analysis area is
at maximum intensity for the standard element of the
calibration wafer.
8.8.4.1 Calibration shall be checked weekly and after
each equipment service (e.g., after an exchange of
filament, anode, or repair of anode) with the same
calibration wafer.
8.8.4.2 The surface concentration is calculated as
follows:
C
s
= RSF
m
× (C
0
/I
0
) × I
with:
C
s
(atoms/cm
2
): surface concentration of analyzed
metal M
I (counts/s): analyzed intensity of metal M
C
0
(atoms
/
cm
2
): surface concentration of standard
metal S (Ni or Co) on calibration wafer
I
0
(counts/s): analyzed intensity of standard metal
S on calibration wafer
RSF
m
: relative sensitivity factor of investigated
metal M to standard metal S
8.8.4.3 The constants RSF
m
are implemented in the
software of the TXRF equipment.
8.9 Sampling Procedure — If the average elemental
concentration for the sampling wafers is higher than the
average elemental concentration plus three standard
deviations for the analyzed reference wafers, then the
sampled minienvironment is considered to cause
significant contamination.