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SEMI C3-0699 © SEMI 19 86, 1999 11 7.2.6.6 Of the three aforementioned te c h niques, Atomic A bsorption Spectrophotometry , I nductively Coupled Plasma Emission Spectro metry, and Ion Chrom a tography, the most sensitiv…
SEMI C3-0699 © SEMI 1986, 1999 10
2. What is the effect of deep levels and intracenter
transitions on the intensity ratios?
3. What is the effect of crystallographic defects? Of
point defects? And of surface defects?
4. Interference from other shallow impurities may
mask the detection of very low-level impurities.
For instance, PLS signal of Al at 1078.5 nm may
be affected by strong signals from As at 1079.0 nm
or at 1078.2 nm.
7.2.4.2 CVD-deposited films in single crystalline form
can be used for elemental detection of donors and
acceptors for gaseous silicon compounds. However,
PLS does not detect carbon, oxygen, or nitrogen. It is
also not useful for metallic impurities. That is, in
general, PLS is useful only for a very limited number of
impurities.
7.2.5 Based on this analysis, it was r ecommended that
the SEMI Gases Analytical Procedures Subcommittee
should wait for further development of innovative and
improved analytical techniques for the quantitative
determination of low-level elemental impurities based
on FTIR and PLS. Meanwhile, techniques such as ICP,
AA, and Ion Chromatography may be applicable to a
very large number of impurities of interest to the silicon
semiconductor industry. Development and adaptation of
these technologies should be encouraged.
7.2.6 AA (Atomic Absorption Spectrophotometry),
ICP (Inductively Coupled Plasma Emission
Spectrometry), and IC (Ion Chromatography) are
probably the most valuable instrumental techniques for
the detection and analysis of trace elemental impurities
in aqueous media. These techniques usually require that
the elements be dissolved in an aqueous medium. Due
to this fact, and the fact that the elemental impurities
are typically present in silane and the chlorosilanes at
the ppb and sub-ppb levels (which is beyond the
detection limits for these techniques for most elements)
it is usually necessary to concentrate the impurities
prior to analysis, such as by passing the gas through a
series of impingers (i.e., gas bubblers) containing either
DI water or, more commonly, acidic solutions. From
analyses of the impinger solutions together with
knowledge concerning impinger solution volume(s) and
both the flow rate and period of flow of the gas, the
concentration of the respective elements in the original
gas should be readily calculated to a fairly high degree
of accuracy.
7.2.6.1 Atomic Absorption Spectroph otometry can be
used for elemental analyses for a multitude of elements
in the periodic table at detection limits ranging from
ppm to ppb, depending both on the exact configuration
or setup and the element being analyzed. The principle
of the technique involves conversion of an incoming
aerosol into an atomic vapor which can absorb light
from a primary light source, usually a hollow cathode
lamp or an electrodeless discharge lamp (EDL), the
latter providing improved sensitivity and lower
detection limits for certain elements. In addition, a
flameless (i.e., graphite furnace) technique can be used
to increase sensitivities and detection limits 50 to 100
times better.
7.2.6.2 Complementary to Atomic Ab sorption Spec-
trophotometry is Flame Emission Spectrophotometry,
which differs in the absence of a primary light source.
Again, a flame is utilized to convert an incoming
aerosol into an atomic vapor. However, the flame also
thermally elevates the atoms to an excited electronic
state, and, when the atoms return to the ground state,
they emit light detectable by a photomultiplier. This
technique is ideal for the analysis of Li, Ha, and K at
ppm and sub-ppm levels.
7.2.6.3 Inductively Coupled Plasma E mission
Spectrometry (ICP) is rapidly replacing Atomic
Absorption Spectrophotometry for elemental analyses
of liquids, since approximately 70% of the elements in
the periodic table can be determined with better
precision and equal or better sensitivity with minimal
sample preparation. Detection limits, in fact, for this
technique range from ppm to sub-ppb levels, depending
both on instrumental and/or experimental conditions
and the element being analyzed. ICP involves
nebulizing liquid samples either directly or in diluted
form into a spray chamber where a stream of argon gas
carries the smaller sample droplets into the axial
channel of an argon plasma which reaches temperatures
of 4000-8000°K. The common technique employs an
optical system for detection. Newer techniques are
being investigated using a mass spectrometer as a
detection system coupled to an ICP excitation source.
This is referred to as ICP/MS.
7.2.6.4 Ion Chromatography (IC) uses the principle of
ion exchange to separate anionic and cationic species at
detection limits ranging from ppm to ppb, depending
again both on the instrumental and experimental
conditions and on the element being analyzed.
7.2.6.5 Detection of ionic species in IC is most
commonly accomplished by an electrical conductivity
detector capable of measuring most non-transition
metal cations and anions to sub-ppm levels. An
electrochemical detector (for amperometric detection)
is available for measuring any electroactive species
having an oxidation/reduction potential near the applied
electrochemical cell potential. This latter detector is
capable of measuring many ionic species at ppb levels
(e.g., Br
–
, CN
–
, S
–
, etc.).
SEMI C3-0699 © SEMI 1986, 199911
7.2.6.6 Of the three aforementioned techniques,
Atomic Absorption Spectrophotometry, Inductively
Coupled Plasma Emission Spectrometry, and Ion
Chromatography, the most sensitive method of
analyzing for a multitude of elements is, without doubt,
graphite furnace (i.e., flameless) Atomic Absorption
Spectrophotometry. However, this instrumental
technique suffers the disadvantage of being slow and
tedious. Consequently, Inductively Coupled Plasma
Emission Spectrometry is rapidly replacing Atomic
Absorption Spectrophotometry for elemental analyses
of liquids since it is a reasonably fast technique
requiring minimal sample preparation with practical
detection limits being superior to those of standard (i.e.,
flame) Atomic Absorption Spectrophotometry for most
elements and approaching those of graphite furnace
Atomic Absorption Spectrophotometry for many of the
elements. An added attractive feature of Inductively
Coupled Plasma Emission Spectrometry is that
eventually it may be used to analyze for elements
directly in gases. Preliminary work, in fact, is currently
being conducted in this area by several research groups.
Although Ion Chromatography can be used to analyze
for a multitude of non
-
transition metal cations (e.g., Li
+
,
Na
+
, K
+
, Rb
+
, Cs
+
Ba
+2
, Mg
+2
, Ca
+2
, Sr
+2
, etc.) it is
limited by the fact that currently only a few transition
metals (e.g., Ni
+2
, Cu
+2
, Co
+2
, Zn
+2
, Fe
+2
, Fe
+3
, Cd
+2
,
etc.) can be analyzed by this instrumental technique. IC,
however, has the advantage in that it can be used for
analyzing for complex cationic and especially anionic
species directly (e.g., NH
4
+
, NO
2
+
, NO
3
-
, SO
3
-
, SO
4
-2
,
PO
4
-3
, OC1
-
, SCN
-
, CN
-
, CO
3
-2
, etc.) and for different
oxidation states for certain elements (e.g., Fe
+2
and
Fe
+3
), unlike the other techniques, which are strictly
limited to elemental analyses only. Furthermore, IC is
extremely valuable for analyzing ionic species,
particularly anions (e.g., F
-
, Cl
-
, etc.) of many common
elements which otherwise cannot be analyzed directly
by either Atomic Absorption Spectrophotometry or
Inductively Coupled Plasma Emission Spectrometry.
8 Standards
8.1 Certified Standards shall be m ade by weight
traceable to the National Institute of Standards and
Technology.
9 Determination of Precisio n
9.1 To Be Determined.
10 Safety
10.1 Because of the continuing evolution of safety
precautions, it is impossible for this publication to
provide definite statements related to the safe handling
of individual chemicals. The user is referred to product
labels, product data sheets, government regulations, and
other relevant literature.
SEMI C3-0699 © SEMI 1986, 1999 12
11 Gas Use Table (see Notes 1, 2)
Gas Use Table
Ammonia
Argon
Arsine
Boron Trichloride
Boron Trifluoride
Boron-11-Trifluoride
Carbon Tetrfluoride
Chlorine
Diborane
Dichlorosilane
Disilane
Heliu
m
Hexafluoroethane
Hydrogen
Hydrogen Bromide
Hydrogen Chloride
Hydrogen Fluoride
Methyl Fluoride
Nitrogen
Nitrogen Trifluoride
Nitrous Oxide
Oxygen
Perfluoropropane
Phosphine
Silane
Silicon Tetrachloride
Sulfur Hexafluoride
Trichlorosilane
Trifluoromethane
Tungsten Hexafluoride
Annealing X X
Carrier Gas for Bubblers X X
Chamber Cleans X X X X
CVD Carrier Gas X X
CVD Source X
CVD B Source X
CVD P Source X
CVD Epitaxial X
CVD Nitride X
CVD Oxides X X X X
CVD Polysilicon X X X
CVD Silicides X
Epitaxial Silicon X X X
CVD Silicon Nitride X X X X
CVD Tungsten/Silicide X
Dopants X X
Etching X X X
Metal Etching X X X X X X X X
Nitride Etching X X X X X X X X
Oxide Etching X X X X X X X X X
Silicon Etching X X X X X X X X
Ion Implant X X X X X
Metal Gettering or CVD Tube Cleans X
Nitridation X
Oxide Gettering X X
Oxynitrides X
Plasma Ashing X
Pressurizing Systems X X
Reactive Ion Etch X X
Reducing Gas X
Sputtering X
Inert Gas Blanketing/Purging X X X