semi合集-English.pdf - 第5362页
SEMI M54-0304 © SEMI 2003, 2004 3 strongly depe nd on the post growth annealing procedures (see Sectio n 4.4). 4.10 To obtain S I GaAs, complete ionizati on of shallow donors and acceptors and partial single ionization o…
SEMI M54-0304 © SEMI 2003, 2004 2
3.2 ASTM Standards
1
ASTM F76 — Standard Test Methods for Measuring
Resistivity and Hall Coefficient and Determining Hall
Mobility in Single-Crystal Semiconductors
ASTM F1404 — Test Method for Crystallographic
Perfection of Gallium Arsenide by Molten Potassium
Hydroxide (KOH) Etch Technique
3.3 DIN Standards
2
DIN 50448 — Testing of materials for semiconductor
technology - Contactless determination of the electrical
resistivity of semi-insulation semiconductor slices using
a capacitive probe
DIN 50449-1 — Testing of materials for semiconductor
technology - Determination of impurity content in
semiconductors by infrared absorption - Part 1: Carbon
in gallium arsenide
DIN 50449-2 — Testing of materials for semiconductor
technology - Determination of impurity content in
semiconductors by infrared absorption - Part 2: Boron
in gallium arsenide
DIN 50454-1 — Testing of materials for semiconductor
technology - Determination of dislocations in
monocrystals of III-V-compound semiconductors - Part
1: Gallium arsenide
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
4 SI GaAs
4.1 SI GaAs as described in this specification has n-
type conductivity and an electrical resistivity ρ in the
range 1 × 10
6
– 5 × 10
8
Ωcm. It is characterized by
fabrication procedures and material properties.
4.2 Synthesis of the compound GaAs from the
elements gallium (Ga) and arsenic (As) may be done
• in the crystal growth chamber immediately prior to
growth (in-situ synthesis)
• in separate equipment, independent of crystal
growth (ex-situ synthesis)
4.3 The growth of SI GaAs single crystals involves one
of the following procedures:
• Liquid Encapsulated Czochralski (LEC)
1 Available from American Society for Testing and Materials, 100
Barr Harbor Drive, West Conshohocken, Pennsylvania 19428-2959,
USA. Telephone: 610.832.9585, Fax: 610.832.9555 Website:
www.astm.org
2 Available from Deutches Institut für Normung e.V., Beuth Verlag
GmbH, Burggrafenstrasse 4-10, D-10787 Berlin, Germany, website:
www.din.de
• LEC with controlled As vapor pressure (VCZ)
• Vertical Gradient Freeze (VGF)
• Vertical Bridgman (VB)
4.4 SI GaAs single crystal ingots are thermally treated
after crystal growth. The annealing procedures are
defined by heating rates, hold temperatures, hold times
and cooling rates. Single step ingot annealing (one hold
time at one temperature) or multi step ingot annealing
(several hold times at different temperatures) may be
applied. Wafers may be treated similarly by single step
wafer annealing and multi step wafer annealing.
4.5 SI GaAs contains the intrinsic double donor point
defect EL2, which involves an As atom on a Ga site
(As
Ga
). At room temperature the first ionization level
EL2
0
/EL2
+
is approximately at ∆E = 0.69 eV below the
conduction band. The concentration [EL2
0
] of the neu-
tral defect is in the range of 5 × 10
15
to 3 × 10
16
cm
-3
.
4.6 SI GaAs contains unintentionally incorporated
impurities acting as donors. It may also contain
intrinsic donor defects other than EL2. The total
concentration [D] of donors other than EL2 should be
low for optimal control of ion implantation activation
and high electron mobility µ (see Section 4.12).
4.7 SI GaAs contains impurities acting as acceptors. It
may also contain intrinsic acceptor defects. Usually the
total concentration [A] of acceptors is adjusted by
controlled incorporation of carbon (C) in the
concentration range [C] 4 × 10
14
cm
-3
– 2 × 10
16
cm
-3
.
4.8 SI GaAs contains extrinsic and intrinsic point
defects that do not participate in the compensation
process (see Sections 4.10 and 4.11). These defects
may be isoelectronic centers (e.g. boron (B)
incorporated on the Ga site, B
Ga
) or extremely deep
donors and acceptors below and above the Fermi level,
respectively. They may influence the material quality,
for instance by degrading the activation efficiency of
implanted dopants or by acting as transient carrier traps
and nonradiative recombination centers (NRRC).
4.9 SI GaAs generally is nonstoichometric, containing
excess As with a concentration of the order of
10
18
cm
-3
. The excess As is partly incorporated as
lattice antisite defects (As
Ga
), see Section 4.5. Other
forms of point defect incorporation (e.g. interstitial As)
are likely, but have not been positively identified.
Excess As is attracted by dislocations (see Section 4.13)
and is concentrated along these, forming decoration
precipitates. Precipitates may also be generated away
from dislocations, e.g. in the inner part of dislocation
cells (see Section 4.14). They are referred to as matrix
precipitates and are generally smaller than dislocation
precipitates. The concentration and size of precipitates
SEMI M54-0304 © SEMI 2003, 2004 3
strongly depend on the post growth annealing
procedures (see Section 4.4).
4.10 To obtain SI GaAs, complete ionization of
shallow donors and acceptors and partial single
ionization of EL2 must be achieved, requiring that
[EL2] > [A] - [D] > 0 (1)
The Fermi level is then pinned approximately at ∆E
below the conduction band.
4.11 The resistivity of SI GaAs is given by
ρ = (e [n] µ
n
+ e [p] µ
p
)
-1
(2a)
where e is the electron charge, [n] and [p] the
concentration of electrons and holes and µ
n
, µ
p
the
respective mobilities. For material with ρ < 5 × 10
8
Ωcm and meeting condition (1), the second term may
be neglected, hence
ρ = (e [n] µ
n
)
-1
(2b)
The electron concentration is given by
[n] ∝ [EL2
0
] / [EL2
+
] exp ( -∆E / kT) (3)
where T is temperature, k is Boltzmann' s constant and
the concentration [EL2
+
] of the singly ionized defect is
given by
[EL2
+
] = [A] - [D] (4)
Equation (3) may be used to normalize resistivity data
ρ
M
taken at a measurement temperature T
M
. The
normalized resistivity ρ
S
at a standard reference
temperature T
S
is calculated according to
ρ
S
= ρ
M
exp (α ∆T) (5)
where ∆T = T
M
- T
S
(6)
and α depends on ∆E at T = 0 K (0.75 eV) and on T
S
.
For T
S
= 296 K (23° C), one has α = 0.0994.
4.12 Due to ionized impurity scattering µ decreases
with the concentration [I] of ionized centers, given by
[I] = 2 [A] = 2 ( [EL2
+
] + [D]
). (7)
Relations (3) and (7) imply that µ decreases with
increasing resistivity. Relation (7) further implies that,
for a given resistivity, low [D] is desirable to maximize
µ .
4.13 SI GaAs ingots usually contain dislocations
generated by thermal stress during crystal growth and
postgrowth annealing. The dislocation density (DD),
although a volume property, is evaluated by measuring
the area density of dislocations threading a substrate
surface. Structural etching with molten KOH generates
characteristically shaped etch pits at the threading
points. Hence the DD is characterized by quoting the
etch pit density (EPD).
4.14 Depending on the growth method and the ingot
diameter, the EPD may vary from essentially zero up to
2 × 10
5
cm
-2
. The lateral variation of the EPD generally
forms a pattern of globular dislocation-free “cells”
surrounded by high DD “walls”. The cell dimensions
are on the order of 100 µ m ∅ for LEC grown material
and about an order of magnitude larger for VGF grown
material.
4.15 State-of-the-art SI GaAs substrates do not contain
polycrystalline structure or twins.
4.16 The intensity of band-to-band and shallow
donor/acceptor-related radiative carrier recombination
luminescence depends on the minority carrier lifetime,
which in turn is related to the distribution of NRRCs.
These centers are generally believed to be intrinsic
defects and appear to have a minor influence on the
electrical properties.
4.17 The quality of the front surface of the wafer is
mainly determined by global and local flatness (not
addressed here in detail), surface contamination, light
point defect (LPD)
3
density and micro-roughness
(haze). Other surface irregularities to be considered in
a supplier-purchaser agreement include stain, scratches,
pits, orange peel and dimples as defined in SEMI M10.
4.18 The thickness and structure of the oxide on the
surface of SI GaAs wafers to be used for epitaxy may
be prepared to allow layer deposition without chemical
pre-cleaning by the user. Generally this material
property is guaranteed for a certain time interval only.
5 Parameter Verification
5.1 The material specification of SI GaAs according to
Section 6 requires agreements on specification
verification. Existing standard test methods are
suggested and referenced in Section 3.
5.2 If a standard test method is not available,
alternative verification methods are recommended and
described in order to identify practicable
characterization procedures that are consonant with
industry practice and cost considerations.
5.3 Some characterization methods, while intensively
used for exploratory material investigations, yield
qualitative information only and are, therefore,
inadequate for a specification verification. They are
nevertheless included, with appropriate comment, to
ensure comprehensiveness of this guideline and to
facilitate respective supplier-purchaser agreements.
3 For silicon wafers, and sometimes for GaAs, the acronym LLS
(localized light scatterer) is used.
SEMI M54-0304 © SEMI 2003, 2004 4
5.4 The concentration of C incorporated on the As
lattice site, [C
AS
], is measured using the local
vibrational mode (LVM) absorption with Fourier
Transform Infrared Spectroscopy (FTIR). Presently
two standard test methods (SEMI M30 and DIN 50449-
1) are available. SEMI M30 is valid for measurement
at room temperature and [C] > 10
15
cm
-3
. It requires
individual instrument calibration with a set of
secondary reference samples. DIN 50449-1 defines
standardized FTIR measurement parameters and
calibration factors for measurement at room
temperature and at 77K, the latter having a detection
limit [C] ≥ 10
13
cm
3
. For practically relevant
concentrations (see Section 4.7) there is no evidence for
carbon incorporation other than on the As lattice site.
Hence the calibration factors quoted in SEMI M30 and
DIN 50449-1 imply that [C] = [C
As
]. The results
obtained when using the two standard test methods
presently are conflicting, hence a harmonizing activity
is considered necessary.
5.5 The electrical resistivity ρ can be evaluated with
contacting and noncontacting techniques. The
contacting van der Pauw measurement is performed
according to ASTM F76 or SEMI M39, the latter
specifically addressing the measurement of SI GaAs.
The noncontacting measurement using a capacitive
probe is performed according to DIN 50448. This
technique enables measurement of lateral variations of
ρ. Resistivity will be quoted for an agreed standard
temperature; 296K (23° C) is recommended.
Measurements done at a different temperature must be
normalized to the standard temperature as described in
Section 4.11. The difference between the standard and
measurement temperatures must not exceed 5° C.
5.6 The electron mobility µ is measured using the Hall
effect and a van der Pauw structure. The evaluation is
performed according to ASTM F76 or SEMI M39.
5.7 The etch pit density (EPD) of LEC grown SI GaAs
is evaluated according to ASTM F1404 or DIN 50454-
1. The documents describe the etching procedure and
define test location plans linked to the crystallographic
axes.
5.8 The EPD of VGF, VCZ and VB grown SI GaAs is
10
4
cm
-2
or below. The evaluation is performed
according to DIN 50454-1 or SEMI M36. The test
locations are defined by a fixed grid (SEMI M36) or by
an adaptable grid generated by a standardized
procedure (DIN 50454-1). Guidelines are given to
assess the lateral variation of EPD.
5.9 The light point defect (LPD) density is a measure
for the density of surface irregularities that are above a
certain size limit (usually 300 nm in diameter). These
irregularities may be foreign material deposited on the
wafer surface (particles) or so-called crystal originating
pits (COPs) caused by volume material
inhomogeneities generated during the growth and/or
annealing procedures and can be revealed by
application of a light scattering technique. Depending
on the mechano-chemical polishing processes and the
light scattering technique employed, COPs may or may
not be observed. At present no standard test method for
LPD evaluation is available. Therefore, a supplier-
purchaser agreement on e.g. the used measurement
system, size restriction procedure, edge exclusion,
inclusion or exclusion of COPs is necessary. To
exclude an influence of different measuring systems the
size of LPD’s should be given in units of diameter, not
of a scattering cross section.
5.10 The concentration of the neutral double donor
[EL2
0
] is measured using the optical absorption at about
1 µ m. An absolute determination is impossible because
neither a generally accepted calibration standard nor a
standardized test procedure is presently available
4
.
Hence a supplier-purchaser agreement is necessary to
ensure reproducibility. The relative lateral variation of
[EL2
0
] can be quantitatively assessed with high
precision. The evaluation of [EL2
+
] using optical
absorption is presently considered unreliable.
5.11 The total concentration of impurities acting as
donors is measured by Spark Source or Glow Discharge
Mass Spectroscopy (SSMS, GDMS). The dominant
contributions are Si, S, O, and Te. The analytical
procedure is time-consuming and costly, hence it is
generally confined to e.g. biannual control
measurements of the supplier to ensure that raw
material supply, synthesis and crystal growth
procedures are stable. By virtue of the compensation
process (see Section 4.11) the donor concentration [D],
including intrinsic defects, is implicitly controlled by
specifying [C], [EL2] and ρ.
5.12 The total concentration of impurities acting as
acceptors is measured by SSMS or GDMS. The
analytical procedure is time-consuming and costly,
hence is generally confined to regular control
measurements of the supplier to ensure that raw
material supply and the synthesis and crystal growth
procedures are stable. The dominant contribution
usually is C, intentionally doped to control ρ. Hence it
is generally sufficient to verify that the total
concentration of acceptor impurities other than C is
small compared to [C].
5.13 The concentration of B is measured using the
local vibrational mode (LVM) absorption according to
DIN 50449-2. Established, but non-standardized
4 A DIN standard test method to measure [EL2] is in preparation and
is scheduled for publication in 2002.