ASTM F1212-89(1996)e1
(Test Method)Standard Test Method for Thermal Stability Testing of Gallium Arsenide Wafers
Standard Test Method for Thermal Stability Testing of Gallium Arsenide Wafers
SCOPE
1.1 This destructive test method determines whether a given sample of semi-insulating gallium arsenide (GaAs) will remain semi-insulating after exposure to the high temperatures normally required for the activation of implanted layers.
1.2 The underlying assumption is that other wafers of GaAs, whose manufacturing history was the same as the wafer from which the test sample was taken, will respond to high temperatures in like manner.
1.3 The emphasis in this test method is on simplicity and safety of apparatus, and on securing a measurement that is independent of the apparatus used.
1.4 This test method is directly applicable to uncapped and unimplanted samples of GaAs. However, users of this test method may extend it to capped or implanted samples, or both, in which case a controlled test of capped versus uncapped samples, or implanted versus unimplanted samples, is recommended.
1.5 This test method detects impurities "from the bulk" (that is, from within the GaAs wafer) that will likely affect the electrical behavior of devices formed on the surface of the wafer. This test method is not sensitive to surface impurities or process-induced impurities, except as interferences (see Interferences).
1.6
General Information
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Standards Content (Sample)
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e1
Designation: F 1212 – 89 (Reapproved 1996)
Standard Test Method for
Thermal Stability Testing of Gallium Arsenide Wafers
This standard is issued under the fixed designation F 1212; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Keywords were added editorially in February 1997.
1. Scope 3.1.1 annealing—The process of heating a sample of GaAs
in a furnace to a specific temperature, under a reducing
1.1 This destructive test method determines whether a given
atmosphere and with a means to reduce the loss of arsenic via
sample of semi-insulating gallium arsenide (GaAs) will remain
sublimation from its surface.
semi-insulating after exposure to the high temperatures nor-
3.1.2 capped annealing—The process of placing a protec-
mally required for the activation of implanted layers.
tive layer (usually silicon nitride or silicon dioxide) on the
1.2 The underlying assumption is that other wafers of GaAs,
GaAs sample surface, thereby reducing the loss of arsenic
whose manufacturing history was the same as the wafer from
vapor from the sample’s surface during annealing. It is not
which the test sample was taken, will respond to high tempera-
described further in this test method, since the capping process
tures in like manner.
introduces several variables that can affect the test results.
1.3 The emphasis in this test method is on simplicity and
3.1.3 proximity annealing—The process of placing the
safety of apparatus, and on securing a measurement that is
GaAs sample between two similar pieces of GaAs, thereby
independent of the apparatus used.
reducing the loss of arsenic vapor from the sample’s surface
1.4 This test method is directly applicable to uncapped and
during annealing.
unimplanted samples of GaAs. However, users of this test
3.1.4 thermal stability—The ratio between the sample’s
method may extend it to capped or implanted samples, or both,
apparent bulk resistivity after the annealing test, and an
in which case a controlled test of capped versus uncapped
identical sample’s bulk resistivity without annealing.
samples, or implanted versus unimplanted samples, is recom-
mended.
4. Summary of Test Method
1.5 This test method detects impurities “from the bulk’’
4.1 The sample is heated in a manner similar to the heating
(that is, from within the GaAs wafer) that will likely affect the
process that an ion-implanted wafer would undergo. Then the
electrical behavior of devices formed on the surface of the
bulk resistivity of the sample is compared to the bulk resistivity
wafer. This test method is not sensitive to surface impurities or
of an identical sample (control) that did not undergo heat
process-induced impurities, except as interferences (see Inter-
treatment. The difference between the resistivities, if any, is a
ferences).
measure of the sample’s sensitivity to heat treatment, or in
1.6 This standard does not purport to address all of the
other words its “thermal stability’’.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
5. Significance and Use
priate safety and health practices and determine the applica-
5.1 Devices that involve ion implantation into a monocrys-
bility of regulatory limitations prior to use.
talline semi-insulating GaAs wafer are designed with the
2. Referenced Documents assumption that the wafer will remain semi-insulating during
manufacture. However, ion implantation always damages the
2.1 ASTM Standards:
crystal lattice of the wafer’s surface, and the damaged surface
F 76 Test Methods for Measuring Resistivity and Hall
layer tends to collect impurities from the bulk of the wafer
Coefficient and Determining Hall Mobility in Single-
2 when the wafer is heated. Those impurities can become
Crystal Semiconductors
unwanted dopants: they can render the surface layer conduc-
3. Terminology tive, or interfere with the implanted species in various ways.
The net effect in either case is a nonfunctioning device.
3.1 Definitions:
5.2 No spectroscopic method is sensitive enough to detect
all possible bulk impurities; their presence in the wafer itself
This test method is under the jurisdiction of ASTM Committee F-1 on
cannot be predicted in advance. This test method serves to
Electronics and is the direct responsibility of Subcommittee F01.15 on Gallium
concentrate them in the surface layer of a sample taken from
Arsenide.
one of the wafers, so that a semiquantitative estimate of their
Current edition approved Feb. 24, 1989. Published April 1989.
Annual Book of ASTM Standards, Vol 10.05. electrical behavior may be made.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
F 1212
5.3 It is important to understand the main assumption that 7.1.2 A quartz tube more than twice the length of the total
underlies this test method. By its use of Test Methods F 76 to heat zone of the furnace, and of sufficient diameter to hold both
measure the stability of the sample, this test method makes the the sample holder and all the samples that will be tested each
tacit assumption that the resistivity in the bulk of the heat- time;
treated test wafer is being measured. That is true, though only 7.1.3 A means to slide the tube rapidly into and out of the
indirectly. After the heat treatment of this test method, it is the hot zone of the furnace.
test sample’s surface that typically contributes the most to the 7.1.4 One removable end cap;
measured change in resistivity. That surface resistivity, in turn, 7.1.5 A supply of forming gas or Palladium-purified hydro-
is a measure of what conductive impurities were present in the gen, and of purified argon or nitrogen to one end;
bulk, prior to the anneal test. 7.1.6 A means to vent the exhaust gases (which are flam-
5.4 Measurement units of ohms per square are the theoreti- mable, and which will also contain a trace of arsenic vapor)
cally correct units for measuring the resistivity after this safely away from the work area.
thermal conversion test. The alternative units of V-cm (“bulk’’ 7.2 Sample Holder—A small dish or boat which will hold
as opposed to “sheet’’ resistivity) imply that the thickness of the GaAs samples in the furnace is needed. It must be stable
the sample’s conducting layer is known. Its thickness is known enough to hold the “stack’’ of test samples with their protective
before the heat treatment of this test method, but not after. cover pieces, and resistant to heat to the extent that it does not
Nevertheless, this test method uses the units of V-cm after the transfer impurities to the samples. Alumina or quartz is the
heat treatment, as well as ohms per square, so the“ apparent recommended material for this sample holder. Three of the
bulk’’ resistivities before and after the test may be compared. several possible types of sample holder are:
5.5 This test method is suitable for use in specifications, as 7.2.1 A simple rectangular tray, (Fig. 1) that will hold
well as in manufacturing control, research, and development. several small rectangular samples.
7.2.2 A larger tray, that will hold several half-wafer
6. Interferences
samples—if that is the size of sample that will be annealed.
6.1 The chief interference in this test method is surface 7.2.3 A “leaky boat’’ holder. This holder takes the form of
contamination on the specimens being measured for resistivity.
a quartz capsule that sits within the furnace tube. Inside the
Minute amounts of, for example, dried solvent residues or capsule are (a) some pieces of Indium arsenide to create a few
fingerprints may cause a heat-treated sample to appear ther-
torr of local arsenic overpressure, and (b) the GaAs samples.
mally unstable when it actually is not. For this reason, the The capsule is open enough to allow some of the furnace
sample cleaning steps in 11.1.6-11.1.8 must be followed
gas-flow to pass through.
scrupulously. 7.3 Resistivity Measurement Apparatus—An apparatus of
6.2 A less common interference arises from using too long
the type described in Test Methods E 76, sufficiently sensitive
a cool down time in 11.2.5-11.2.7. The maximum allowable to measure the resistivity of the GaAs being tested shall be
cool down time is not known, but a cool down time that brings
used.
the samples’ temperature to under 200°C in 30 min or less is 7.4 Miscellaneous—Two clean plastic tweezers (of cleanli-
known to remove the potential for interference.
ness suitable for the handling of semiconductor wafers) and a
source of clean, submicron-filtered pressurized gas are also
7. Apparatus
required for the drying step.
7.1 Furnace—A means to heat the test pieces to 850°C,
7.5 Sampling Equipment—A means to prepare samples both
maintain them at that temperature in a controlled atmosphere,
for the anneal furnace’s holder, and later for the resistivity test
and then cool them to below 100°C within 30 min ,is
kit shall be used. This apparatus normally consists of tools to
required, consisting of: (One embodiment of the apparatus is
scribe and cleave the GaAs wafers. It is also permissible to use
shown in Fig. 1.)
a sandblaster. Refer to Test Methods F 76 for the required
7.1.1 A regulated heat source of the clamshell or tube
shapes of resistivity specimens.
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