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SEMI MF1982-1103 © SEMI 2003 2 4.1.6 MS — m ass spectrometer 4.1.7 NPD — nitrogen/phosphorus thermionic ionization detect or 4.1.8 TBP — trib utyl phosphate, (C 4 H 9 O) 3 PO 4.1.9 TCEP — tris (2-chloroet hyl) phosphate,…
SEMI MF1982-1103 © SEMI 2003 1
SEMI MF1982-1103
TEST METHODS FOR ANALYZING ORGANIC CONTAMINANTS ON
SILICON WAFER SURFACES BY THERMAL DESORPTION GAS
CHROMATOGRAPHY
This standard was technically approved by the Global Silicon Wafer Committee and is the direct
responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by
the North American Regional Standards Committee on July 27, 2003. Initially available at www.semi.org
October 2003; to be published November 2003. Originally published by ASTM International as ASTM
F 1982-99. Last previous edition ASTM F 1982-99
ε1
.
1 Purpose
1.1 Organics are present in many materials, such as
plastics, lubricants, cleansers, soaps, and living tissues.
Some of these compounds are volatile and others can
become airborne through chemical reactions, heating,
abrasion, or outgassing. Also they can transfer to
wafers by direct contact or be left behind from solvent
residues. Once present in clean facilities, they can
deposit on wafer surfaces. Organics deposited on
wafers can cause degradation haze, wafer surface
tension changes, irregular oxidation rates, and other
effects, such as counter-doping by organophosphorus
compounds. Identification of trace level organic
contaminants is important in determining the source of
the particular contamination. These test methods use
the TD-GC technique to characterize and quantify
organics deposited on wafer surfaces.
1.2 Monitoring of organic contamination on wafer
surfaces also can be used to measure material
outgassing for proper selection of cleanroom,
construction, and wafer packaging materials.
2 Scope
2.1 These test methods cover the identification and
quantification of organic contaminants on silicon wafer
surfaces using a gas chromatograph interfaced to a mass
spectrometer (GC-MS) or a phosphorus selective
detector, or both.
2.2 These test methods describe the apparatus and
related procedures for sample preparation and analyses
by thermal desorption gas chromatography (TD-GC).
1
,
2
2.3 The range of detection limits of these test methods
depends on the target organic compounds, for example,
the range of detection limits is from the subpicogram to
the nanogram level of hydrocarbons (C
8
to C
28
) per
square centimeter of silicon wafer surface.
1 Fergason, L.A., “Analysis of Organic Impurities on Silicon Wafer
Surfaces,” Microcontamination, 4(4), 33-37, April 1986.
2 Saga, K. and Hattori, T., “Identification and Removal of Trace
Organic Contamination on Silicon Wafers Stored in Plastic Boxes,”
Journal of Electrochemical Society, 143, 3270-3284 (1996).
2.4 These test methods can be used for polished silicon
wafers, or silicon wafers with oxide films.
2.5 Two methods are described. Method A is
performed on cleaved wafers. Method B is performed
on full wafers. The detailed procedures of Method A
and Method B as well as the differences between them,
are described in Sections 5 and 7 .
2.6 Suitable safety precautions must be followed when
handling organic solvents and compounds, hot
materials subjected to propane flame, the propane flame
itself, wafer thermal desorption systems, rapid thermal
annealer, or a high temperature furnace.
NOTICE: This standard does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the user of this standard to establish
appropriate safety and health practices and determine
the applicability of regulatory or other limitations prior
to use.
3 Referenced Standards
3.1 ASTM Standard
D 6196 — Practice for Selection of Sorbents and
Pumped Sampling/Thermal Desorption Analysis
Procedures for Volatile Organic Compounds in Air
3
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
4 Terminology
4.1 Abbreviations and Acronyms
4.1.1 AED — atomic emission detector
4.1.2 C
16
— n-hexadecane, n-C
16
H
34
4.1.3 FID — flame ionization detector
4.1.4 FPD — flame photometric detector
4.1.5 GC — gas chromatography
3 Published in Volume 11.03 of Annual Book of ASTM Standards.
Available from ASTM International, 100 Barr Harbor Drive, West
Conshohoken, PA 19428-2959, USA. Telephone: 610.832.9585,
Fax: 610.832-9555, Web site:
www.astm.org
.
SEMI MF1982-1103 © SEMI 2003 2
4.1.6 MS — mass spectrometer
4.1.7 NPD — nitrogen/phosphorus thermionic
ionization detector
4.1.8 TBP — tributyl phosphate, (C
4
H
9
O)
3
PO
4.1.9 TCEP — tris (2-chloroethyl) phosphate,
(ClCH
2
CH
2
O)
3
PO
4.1.10 TD — thermal desorption
4.2 Definitions of Term Specific to This Standard
4.2.1 blank wafer — a thermally-treated wafer
desorbed of any surface organic contaminants.
4.2.1.1 Discussion — In Method A, surface organic
contaminants are desorbed with a rapid thermal
annealer or a high temperature furnace. In Method B
surface organic contaminants are desorbed by purging
them out in the heated quartz chamber unit while
helium gas is flowing as a purge gas.
5 Summary of Test Methods
5.1 Method A
5.1.1 Desorption and GC Analysis — The volatile
organic contaminants on a wafer surface are desorbed
thermally from the wafer surface in a wafer desorption
oven and swept into a sample thermal desorption tube.
The sample thermal desorption tube then is heated for a
set period in the thermal desorption unit and the volatile
organic contaminants desorbed from the sample thermal
desorption tube are swept by a stream of helium to a
cold trap where they are preconcentrated. At the end of
this period, the cold trap is heated rapidly to release the
trapped organics to the GC column head. Sample
components then are separated and eluted out of the GC
column. Then, a portion goes to a phosphorus selective
detector and the remainder goes to a mass spectrometer
(MS). Blank wafers are prepared by purging out any
surface organic contaminants in a rapid thermal
annealer or a high temperature furnace while a purge
gas is flowing.
5.1.2 Identification of Contaminants — Identification
of individual unknown compounds is performed by
correspondence of retention time of their peaks with
that of known compounds. Correspondence of reten-
tion time on a single column should not be regarded as
proof of identity. More precise identification of
individual unknown compounds is performed with MS
by matching their fragmentation patterns with mass
spectra of known compounds in the spectral library.
5.1.3 Quantification — Quantification of total organics
is based on the comparison of the integrated total peak
area of the sample peaks with the area of the external
standard compound, n-C
16
H
34
. Total organophosphorus
content in a sample is quantified by comparing the
sample signal integrated from the phosphorus selective
detector with the signal of the phosphorus standard
compound, tris (2-chloroethyl) phosphate (TCEP) or
tributyl phosphate (TBP). Specified range of standards
is measured periodically, and the result is reported with
blank wafer data.
5.2 Method B
5.2.1 Desorption and GC Analysis — The volatile
organic contaminants on a wafer surface are desorbed
thermally from the wafer surface in a quartz chamber
unit and swept into a glass TD tube. The glass TD tube
then is heated for a set period and a stream of helium
sweeps the volatile organic contaminants desorbed from
the glass TD tube to a cold trap where they are
preconcentrated. At the end of this period, the cold trap
is heated rapidly to release the trapped organics to the
GC column head. Sample components then are sepa-
rated and eluted out of the GC column. Then, a portion
goes to a phosphorus selective detector and the
remainder goes to a mass spectrometer (MS). Blank
wafers are prepared by purging out any surface organic
contaminants in the heated quartz chamber unit, while
helium gas is flowing as a purge gas.
5.2.2 Identification of Contaminants — Identification
of individual unknown compounds is performed by
correspondence of retention time of their peaks with
that of known compounds. Correspondence of
retention time on a single column should not be
regarded as a proof of identity. More precise
identification of individual unknown compounds is
performed with MS by matching their fragmentation
patterns with mass spectra of known compounds in the
spectral library.
5.2.3 Quantification — Quantification of total organics
is based on the comparison of the integrated total peak
area of the sample peaks with the area of the external
standard compound, n-C
16
H
34
. Total organophosphorus
content in a sample is quantified by comparing the
sample signal integrated from a phosphorus selective
detector with the signal of the phosphorus standard
compound, tris (2-chloroethyl) phosphate (TCEP) or
tributyl phosphate (TBP). Specified range of standards
is measured periodically, and the result is reported with
blank wafer data.
6 Apparatus
6.1 Method A
6.1.1 GC Instrument — Utilizes a capillary column to
separate a wide variety of organic compounds coupled
to a mass spectrometer (MS), or a phosphorus selective
detector, or both. Examples of phosphorus selective
detectors are flame photometric detector (FPD), and
SEMI MF1982-1103 © SEMI 2003 3
atomic emission detector (AED). The
nitrogen/phosphorus thermionic ionization detector
(NPD) also responds to nitrogen containing
compounds.
NOTE 1: A nitrogen/phosphorus thermionic ionization
detector (NPD) may also be used as a phosphorus selective
detector. This type of detector also responds to nitrogen
containing compounds. If an NPD is used, the total
organophosphorus reported should exclude any signals due to
nitrogen containing compounds. Often, identification from
the mass spectra can be used to determine whether compound
contains nitrogen or phosphorus, or both.
6.1.2 Sample Thermal Desorption Tubes — Stainless-
steel tubes packed with adsorbent medium, are used to
trap compounds of interest and release them onto a
thermal desorption unit.
NOTE 2: Note that stainless steel is catalytically active and
can corrode with time, affecting recovery for some
compounds. In this case, deactivated stainless steel, glass, or
quartz tubes also can be used. Several adsorbent materials
can be used for trapping organic compounds desorbed from
silicon wafer samples. Some examples are activated carbon,
graphitized carbon, and poly (2,6-diphenyl-p-phenylene
oxide).
6.1.3 Thermal Desorption Unit — Used to desorb
organics from sample thermal desorption tubes. The
thermal desorption unit is coupled to the GC instrument
via a heated transfer line, for example, heated to 225°C
or above.
6.1.4 Thermal Annealer — Or a high temperature
furnace used to obtain a blank wafer (see Section
7.1.1.2 ).
6.1.5 Temperature Controllable Wafer Desorption
Oven — Used to hold and heat a wafer desorption tube.
6.1.6 Wafer Desorption Tube — Stainless-steel tube
with an approximate dimension: 12.7 mm (0.5 in.) in
outer diameter, 9.53 mm (0.375 in.) in inner diameter
and 254 mm (10 in.) in length (see Figure 1), used to
desorb organics from cleaved wafers. Larger tubes may
be used for larger wafer sizes and to increase the
sensitivity of the test (see Note 2).
6.1.7 Analytical Balance
6.1.8 Quartz Plate and Carbide-Tipped Scribe — Used
to cleave wafer samples.
6.1.9 Quick Connect — Used to connect a nitrogen line
to the wafer desorption tube.
6.2 Method B
6.2.1 GC Instrument — Utilizes a capillary column to
separate a wide variety of organic compounds, coupled
to a mass spectrometer (MS) or a phosphorus selective
detector, or both. Examples of phosphorus selective
detectors are flame photometric detector (FPD), atomic
emission detector (AED), or nitrogen/phosphorus
thermionic ionization detector (NPD). The
nitrogen/phosphorus thermionic ionization detector
(NPD) also responds to nitrogen containing compounds
(see Note 1).
6.2.2 Quartz Chamber Unit — Used to desorb organic
contaminants from the wafer surface and transfer them
to a glass TD tube (see Figure 2).
6.2.3 Quartz Chamber — May be used also to purge
out any organic contaminants from a blank wafer.
6.2.4 Glass TD Tube — Packed with adsorbent
medium used to trap compounds of interest and release
them onto a cold trap. Several adsorbent materials can
be used for trapping organic compounds desorbed from
silicon wafer samples. Some examples are activated
carbon, graphitized carbon, and poly (2,6-diphenyl-p-
phenylene oxide).
6.2.5 Cold Trap — Used to concentrate organics
desorbed from glass TD tube and release the organics to
a GC instrument by rapid heating.
7 Procedure
7.1 Method A
7.1.1 Sample Handling and Preparation
7.1.1.1 At all times, avoid manual handling of samples
for analysis to prevent any secondary contamination of
samples. Use stainless steel tweezers for sample
preparation. Use a propane torch to flame stainless
steel tweezers, weigh boats, and other accessories that
come into direct contact with the sample before their
usage.
7.1.1.2 Thermally debsorb any organics from blank
wafers in a rapid thermal annealer or a high temperature
furnace. In order to desorb any organics from blank
wafers, the temperature of a rapid thermal annealer is
kept at 900°C for 15 s, or the temperature of a high
temperature furnace is kept at 700°C for 30 min.
Heating under air or oxygen will make a consistent
organic-free SiO
2
surface. Place the thermally treated
blank wafers directly into petri dishes and wrap the
dishes with organic-free aluminum foil.
7.1.2 Wafer Desorption
7.1.2.1 Clean and precondition sample thermal
desorption tubes in accordance with ASTM Practice
D 6196 prior to attaching to the wafer desorption tube.
7.1.2.2 Turn on the temperature controller for the wafer
desorption oven and wait for the temperature to reach
275°C.