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SEMI E108-0301 © SEMI 2001 3 a) coolin g the trap down to a minim u m of -150°C does not requ ire any adsorbent (glass w ool may be used) or b) a Tenax ® fille d trap has to be cooled down t o a mini mum o f -25°C. 8.1.1…
SEMI E108-0301 © SEMI 2001 2
NOTE 1: As listed or revised, all documents cited shall be the
latest publications of adopted standards.
5 Terminology
5.1 analytical environment — environment where all
analytical measurements are taking place.
5.2 headspace sampling — in this standard defined as:
collecting volatile organic compounds in an enclosed
volume by means of a silicon wafer or silicon wafer
chips.
5.3 minienvironment — A localized environment for
transport and storage created by an enclosure to isolate
the product from contamination and people.
5.4 sample — wafer or wafer chip s used for the
headspace sampling of organic contaminants.
5.5 static storage conditions — co nditions excluding
any active movement of test specimens.
5.6 thermal desorption tube — Analytical equipment
capable of collecting organic compounds of interest
(i.e., adsorbent filled glass tube).
5.7 wafer — Object made of semiconducting material
to be processed, handled or stored in the
minienvironment to be tested (i.e., prime or processed
silicon wafer).
6 Summary of Method
6.1 This test method comprises sto rage tests of wafers
in minienvironments under static storage conditions.
The contamination is directly measured from the silicon
wafer surface. Three important aspects are covered:
a) Contamination due to the minienvironment alone,
b) Contamination from the use of minienvironments for
wafer processing, and
c) Contamination from future materials to be used in
semiconductor technology.
6.2 The setup for each test compri ses an analytical
equipment in the analytical environment as well as a
test equipment in the test environment. Each
environment has to fulfill the following requirements:
6.2.1 Analytical Environment — shall be clean enough
with respect to airborne organic contamination:
recommendation is less than 100 pptM for organic
compounds with boiling points > 150°C (Class MC-
100, according to SEMI F21).
6.2.2 Test Environment — shall equ al the environment
in which the test specimen will be used.
6.3 Silicon wafers are placed in the minienvironment
or used for headspace sampling experiments. These
wafer samples are then analyzed by gas chromatogra-
phy/mass spectrometry to determine the amount of
contaminants.
6.4 The wafers or wafer chips to b e used for the test
method are decontaminated as described in ASTM
F1982 for bare silicon wafers. The surface condition of
the wafer (hydrophobic or hydrophilic) has to be
adjusted in a reproducible way and has to be the same
for all comparative measurements.
6.5 The wafer or the wafer chips are placed in the
minienvironment and are left there under static
conditions for the chosen static storage time (depending
on the intended use of the minienvironment to be tested,
a time between 1 h and 28 d may be used). When
loaded into the minienvironment the wafer may have a
temperature which reflects production circumstances
(i.e., 80°C when unloading from furnace processes). In
some situations wafers may be exposed to severe
thermal conditions. The minienvironment should be
evaluated under the conditions exposed to. Be sure that
the material to be tested does not undergo phase
transitions under the chosen temperature conditions.
6.6 After the static storage test per iod, the wafer is
returned to the analytical equipment and the organic
contamination on the wafer is measured immediately.
6.7 The quantitative value for the total amount of
outgassed and adsorbed organics from the minienviron-
ment is calculated as the difference between the
detected amount of contamination on a blank sample
vs. the amount on the exposed sample.
7 Interferences
7.1 The presence of organic conta mination in the
atmosphere of the test environment may lead to a
significant contribution to the detected total amount of
organic compounds on the test wafers or wafer chips.
Care has to be taken when subtracting the blank value
from the test value. Transport times from test
environment to analytical environment have to be as
short as possible.
8 Apparatus
8.1 For the analysis of organic compounds on wafers a
gas chromatography/mass spectroscopy apparatus with
thermal desorption unit is required: A gas
chromatography (GC) instrument, which utilizes a
capillary column to separate a wide variety of organic
compounds, combined with mass spectrometer (MS). A
thermal desorption unit is used to desorb organics from
sample thermal desorption tubes and collect them in a
trap. Two types of trapping may be used:
SEMI E108-0301 © SEMI 20013
a) cooling the trap down to a minimum of -150°C does
not require any adsorbent (glass wool may be used) or
b) a Tenax® filled trap has to be cooled down to a
minimum of -25°C.
8.1.1 The thermal desorption unit is coupled to the gas
chromatography instrument via a heated transfer line.
This apparatus may be used directly for the desorption
from wafer chips which are put in empty glass (or
stainless steel) thermo desorption tubes (method A).
8.1.2 A desorption unit for complete wafers is
specified in ASTM F1982. In this case sample thermal
desorption tubes packed with adsorbent have to be used
to trap compounds desorbed from the wafer. These
thermal desorption tubes are analyzed within the
thermal desorption unit (method B).
8.2 The analysis of outgassing org anic contamination
from minienvironments requires an extremely sensitive
analytical equipment. Recovery and limit of detection
for the different classes of compounds must be
evaluated carefully in order to ensure reliable results.
9 Reagents and Materials
9.1 For the materials used for hand ling and transport
of wafers clean, decontaminated equipment has to be
used, e.g. tweezers heated in a propane flame. Refer to
the reagent and materials described in SEMI E46 and
ASTM F1982.
9.2 Test Mixture — For the calibra tion of equipment
and the quantification of the amount of contamination a
test mixture (”cocktail”) is used. This mixture was
chosen in order to give an ”average” over typical
substance classes present in semiconductor production
lines, which have high sticking factors to the wafer
surface. The response factor of the MS-detection for
organics varies significantly. As this is true for all
contaminants, the method gives a realistic trustworthy
quantitative correlation to a ”typical average compound
mixture” of contaminants on wafer surfaces.
9.3 Preparation of Test Mixture
9.3.1 The mixture consists of triethyl phosphate,
ε-caprolactam, palmitic acid and diethylhexyl phthalate
each 0.5 µg/µl in isooctane (as solvent). The
preparation has to be done according to SEMI E46.
NOTE 2: A similar test mixture is suggested by the working
group WG 031 of the Institute for Environmental Science and
Technology (IEST)
10 Safety Precautions
10.1 All preparation and analytical work has to be
done according to local safety regulations.
11 Preparation of Minienviro nment and
Sample
11.1 Minienvironment — The minie nvironment to be
tested has to be used as received from the supplier. A
cleaning step can be added but there must be no
contribution from the cleaning procedure to the
outgassing of organic compounds from the
minienvironment. The cleaning method has to be
defined by the supplier or agreed upon between supplier
and user of the tested minienvironment.
11.2 Sample — The wafers shall be made organic-free
before using them to monitor organic contamination.
The surface condition of the wafer (hydrophobic or
hydrophilic) has to be adjusted in a reproducible way
and has to be the same for comparative measurements.
Refer to SEMI E46 and ASTM F1982.
12 Procedure
12.1 The procedure described below is used for
obtaining the baseline value of the method (method
blank) as well as the test value for outgassing organic
compounds of the minienvironment within a defined
static storage time (storage time t
S
= 1 h - 28 d).
12.1.1 Choose that static storage time according to the
intended use of the minienvironment (i.e., if the
maximum sit time of wafers in a process where the
minienvironment is to be used is 4 h then choose a
static storage time of 4 h). If no static storage time
related to processes can be defined, a recommended
value for the first static storage time is 1 day.
NOTE 3: Composition of adsorbed compounds may vary with
static storage time.
12.2 For transport store the wafers or wafer chips used
for testing in decontaminated, organic-free petri-dishes
wrapped in organic-free aluminum foil. Refer to SEMI
E46.
12.3 Loading Procedure
12.3.1 For tests at room temperature: open a
minienvironment to be tested and load it with a
decontaminated wafer using decontaminated handling
tools (i.e., tweezers). Default storage location is the
center slot of the minienvironment. Close the
minienvironment. Leave the minienvironment closed
for the chosen static storage time.
12.3.2 For tests at elevated temperatures using a
wafer furnace: place the decontaminated wafers in a
clean furnace used for production processes and heat
the wafers under inert gas to a temperature > 120°C.
Turn off the heating. When the temperature of the
furnace has reached (90±10)°C open the
minienvironment to be tested and fully load it with the
SEMI E108-0301 © SEMI 2001 4
hot wafers from the furnace. Close the
minienvironment. Leave it closed for the chosen static
storage time. For analysis use only one wafer. Default
wafer is the one in the center slot.
12.3.3 For tests at elevated temperatures using a
heating chamber: open the minienvironment to be
tested and load it with a decontaminated wafer using
decontaminated handling tools (i.e., tweezers). Default
location is the center slot. Close the minienvironment.
Wrap the minienvironment with decontaminated
aluminum foil. Place the minienvironment in an inert
and clean heating chamber heated to 70°C. Leave it for
1 h and then remove it from the heating chamber and
place it at the test minienvironment. Leave it there
closed for the chosen static storage time.
NOTE 4: The purpose of the aluminum foil is to prevent
direct contact between the recirculating hot air inside the
heating chamber and the minienvironment.
12.4 Unloading Procedure — Open the minienviron-
ment and unload the wafer using decontaminated
handling tools into decontaminated petri-dishes and
wrap them into organic-free aluminum foil. Transport
the wafer immediately to the measurement equipment
and analyze the organic contamination on the wafer
according to the standard ASTM F1982.
12.5 Method Blank — Perform the test sequence using
a container made completely of glass or quartz instead
of the minienvironment. Use the same static storage
time for the method blank but perform this blank test at
room temperature. The container has to be
decontaminated with respect to organics inside by heat
treatment (refer to SEMI E46). With this blank method
the baseline contribution from the cleanroom air on the
adsorption of organic contamination on the silicon
wafer surface is determined.
12.6 Analyzing Procedure — Put the wafer or wafer
chips in the precleaned desorption unit or thermal
desorption tube and heat it for 10 min to a minimum of
275°C (but 400°C is better). The desorbed contam-
inants have to be trapped directly with the cold trap of
the thermodesorption unit (method A) or first by
adsorbent filled desorption tubes and then by the cold
trap (method B). Desorption parameters for adsorbent
filled desorption tube and cold trap may be taken from
ASTM F1982 (see also Section 12.6.1).
12.6.1 The substances desorbed shou ld be separated
by an appropriate column temperature program. A
recommended temperature program for standard
analysis uses a polydimethylsiloxane/polydiphenyl-
siloxane (95/5) coated column (30 m × 0.25 mm × 0.25
µm), heated from 50
o
C to 250
o
C at a rate of 10
o
C/min
followed by a temperature hold at 250
o
C for 10 min.
Column flow should be about 1 ml/min He at constant
flow.
12.6.2 All parameters (thermo desorp tion unit, gas
chromatograph, mass spectrometer) should be set to
yield the recommended detection limits and recovery
rates (see Calibration Procedure Section 13.2).
12.7 Materials Testing — Materials testing can be
done, using the sample preparation described in SEMI
E46, but using gas chromatography/mass spectroscopy
instead of IMS for the analysis of contaminants on the
test wafers or wafer chips.
12.8 Perform the procedure for cali bration, method
blank and test (including sample preparation) in
triplicate in order to obtain mean value and standard
deviation for the analysis.
13 Calibration and System P erformance
13.1 Calibration — The test mixtur e (2 µl liquid,
equals 1 µg of each substance; see Section 9.3) is
applied to the wafer or wafer chips as described in
SEMI E46. The so produced reference wafer or wafer
chips are handled in the same way as the samples. That
means, they are put in the precleaned desorption unit or
thermal desorption tube and heated for 10 min to
minimum 275°C (but 400°C is better) and so on as
described in Section 12.6.
13.2 System Performance — All parameters
(thermodesorption unit, gas chromatograph, mass
spectrometer) should be set to yield the specified
detection limits and recovery rates (method B) for the
components of the reference mixture. The limit of
detection (3σ) must equal or be better than 250 ng for
each of the four reference substances. The standard
deviation (inaccuracy) of the calibration must be
≤ 10%. Sample measurements are not allowed, unless
these requirements are fulfilled. For sample
measurements the same parameters as for the
calibration measurements have to be used.
14 Quantification
14.1 Integration — After the analysis measure the
surface area (A
s
) of the tested wafer or wafer chips.
Summing up all the peak areas from gas
chromatography/mass spectroscopy chromatogram of
the wafer or wafer chips gives the total integral (I
s
).
Determine the total integral (I
b
) of the blank wafer and
the total integral of the four peaks of the gas
chromatography/mass spectroscopy chromatogram of
the test mixture (I
r
) with the same technique.