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SEMI MF1723-1104 © SEMI 2004 6 12.2 Core Etching 12.2.1 Do all operations in th e etch bench clean room and zoner cl ean room with op erators in full clean room attire, including gloves, hood, and mask. 12.2.2 Make a fre…
SEMI MF1723-1104 © SEMI 2004 5
10.2.1 Parallel Cores — As shown in Figure 1, cores
are taken parallel to the filament at a minimum length
of 100 mm and a diameter of 20 mm. Two different
cores, a filament core and a growth layer core, are
required to calculate the total polysilicon rod impurity
content.
10.2.1.1 Parallel Filament Cores — Cores including
the filament are representative of the filament and the
initial deposition layer on the filament. These cores are
float zoned, analyzed, and the values combined with the
growth layer cores to calculate a total value.
10.2.1.2 Parallel Growth Layer Cores — Cores not
including the filament, only the growth layer, are
representative of the poly deposited onto the filament.
These cores are float-zoned, analyzed, and the values
combined with the filament core values to calculate a
total value.
10.2.1.3 Parallel Core Sampling Locations
10.2.1.3.1 Radial Location — Cores can be taken
across the entire rod diameter to check radial uniformity
of the deposition layer. Since the outer surface may
contain surface cracks or surface roughness, the outer 5
mm should not be sampled.
10.2.1.3.2 Axial Location — For an entire U-rod of
polysilicon, cores usually are taken from the short
bridging section or from a region on the long rod within
50 mm of either end, but may be taken at any location
to check axial uniformity of deposition.
10.2.2 Perpendicular Cores — As shown in Figure 2,
20-mm diameter cores are taken through the diameter
of the polysilicon rod with length same as the diameter
of the poly rod. Cores are taken so that the filament
and material from all parts of the deposition layer are
included. For accurate calculation of the impurities at
various growth layers, at least one end of the perpen-
dicular core should include the outer skin layer. For
rods less than 60-mm diameter, it is not possible to
produce single crystal ingots of sufficient zone length
for accurate analysis. In this case, cores parallel to the
filament are taken for analysis.
10.2.2.1 Perpendicular Growth Layer Cores — Cores
without the intersected filament, as shown in Figure 2,
can be zoned and analyzed to determine the impurities
in the deposition layer. To find the impurities in the
total rod, the filament must be analyzed separately and
combined with the growth layer results. Values are
calculated using the formulas for parallel cores.
10.2.2.2 Perpendicular Core Sampling Location —
For an entire U-rod of polysilicon, cores usually are
taken from the short bridging section, or from a region
on the long rod within 50 mm of either end. Cores may
be taken at any location to check axial uniformity of
deposition, but are not taken at the bend of the U-rod,
due to stresses in this area.
10.3 Filament Analysis — If a parallel or perpendicular
filament core can not be taken, the filament may be
analyzed separately, then combined with the growth
layer analysis. For filaments that are single crystal, or
nearly single crystal, wafers can be cut, prepared, and
analyzed by spectrophotometric techniques described in
SEMI MF1389, SEMI MF1391, and SEMI MF1630.
11 Reference Specimens
11.1 Use polysilicon control rods to monitor the purity
of the core preparation techniques, acid etch bath, and
zoning conditions. Drill a large number of deposition
layer cores, 20-mm diameter by 100-mm length from
polysilicon rods having a uniform deposition layer.
Select control rods with low impurity levels, such as
acceptor/donor values about 0.01 ppba and carbon
values about 0.05 ppma to allow the early detection of
trace impurities from interference sources. After re-
peated analysis, assign values for donor, acceptor, and
carbon concentrations. Then etch, zone, and analyze
these control rods on a periodic basis to monitor purity
of the sample preparation, etching, and zoning pro-
cesses.
11.2 Control chart values for the acceptor, donor, and
carbon concentrations from the control rods using
standard statistical process-control techniques and
statistical rules to determine if the current values are in
control. If these values exceed the statistical limits,
make corrections and repeat the analysis.
12 Procedure
12.1 Seed Preparation
12.1.1 Prepare round or rectangular seeds by core
drilling, cutting, or crystal pulling from high purity float
zone ingots. Use seed material of zero dislocation
density with acceptor content less than 0.05 ppba, donor
content less than 0.05 ppba, and carbon content less
than 0.1 ppma.
12.1.2 Select a high purity single crystal seed, 3 mm to
5 mm in diameter, for initiation of float zone crystal
growth. Orient the seed 111 within 0.5°.
12.1.3 Clean, acid etch, rinse and dry the seed to the
same procedure, using the same equipment described
for core samples. To avoid surface contamination, use
seeds within 36 h after etching, or store them in a
manner to prevent contamination.
SEMI MF1723-1104 © SEMI 2004 6
12.2 Core Etching
12.2.1 Do all operations in the etch bench clean room
and zoner clean room with operators in full clean room
attire, including gloves, hood, and mask.
12.2.2 Make a fresh acid etch mixture and fill the tanks
in the etch bench. When the proper temperatures and
water flows are achieved, place the core samples into
clean etch boats and etch, rinse, and dry the cores. Use
the HNO
3
-HF acid etching mixture, etching at least two
cycles, to remove a minimum of 100 m from the
surface of the core sample. This is necessary in order to
remove surface damage caused by the coring process.
Other acid etch mixtures can be used, but must be
evaluated and controlled to ensure effectiveness and
avoid impurity interferences.
12.2.3 After etching, float zone cores as soon as
possible to reduce the probability of contamination.
Re-etch cores if the maximum holding period is
exceeded. To extend the holding period, seal the cores
in a suitably clean material and store in a clean room
environment.
12.3 Preparation of Apparatus
12.3.1 Clean the core drilling apparatus to prevent
contamination of the core sample.
12.3.2 Clean the etch bench and check the DI rinse
water purity, temperature, total organic carbon, and
resistivity.
12.3.3 Clean the float zone chamber. Using the stain-
less steel wire brush, scrub the walls of the chamber to
loosen silicon deposits and remove the loose particles
with the vacuum cleaner. Wipe the walls, holders, and
coil with clean room wipes soaked with a high purity
solvent. Inspect cooling water reservoir, water flow,
water temperature, coil and preheater connections,
shaft, coil feedthroughs, door seals, hose connections,
travel stops, and rotations.
12.3.4 Carry out major cleaning, including acid
cleaning of coil and associated parts, and changing of
seals on a periodic basis. After cleaning, argon drying,
and vacuum pumping, perform a chamber and preheater
bakeout of at least 15 min.
12.4 Ingot Growth
12.4.1 Place the core samples and seed in the furnace
chamber so that both hang plumb and are centered and
aligned with the vertical axis of rotation.
12.4.2 Remove air from the chamber with a series of
evacuation and argon purge cycles. Fill the chamber
with argon and continue the argon purge throughout the
growth cycle, maintaining a positive pressure of argon
in the chamber.
12.4.3 Position the seed end of the core sample in the
coil, and the preheater close to the seed end of the core.
Adjust the preheater power to make the initial induction
coupling of the preheater and sample, and heat until the
sample begins to glow, about 600 to 700°C. Move the
preheater away from the sample and move the core
sample close to the coil opening to establish a molten
zone controlled by power to the coil.
12.4.4 After a small molten zone has been established
at the seed end, move the seed vertically until it touches
the molten zone. Withdraw the seed to form a conical
melt, making sure the seed has melted in, and begin the
necking phase in order to form a zero dislocation
crystal.
12.4.5 Adjust top and bottom travel and rotation rates
to complete the necking phase, check for the three-
growth facet lines to ensure the ingot is single crystal,
and adjust travel and power level to form the final
diameter of the ingot. Adjust the travel and rotations
and grow a single crystal zero dislocation ingot.
12.4.6 When the desired ingot length has been attained,
pull the ingot from the melt, making sure the ingot and
melt separate without freezing. After separation,
reduce power, stop all travels and rotation, turn off
power, and allow to cool.
12.5 Ingot Evaluation
12.5.1 Visual Examination — Examine the ingot
visually for uniformity of diameter, continuous
uniform-growth facet line, and color, to determine
whether the ingot is zero dislocation single crystal and
whether oxide deposits are present as a result of air
leaks.
12.5.2 Structural and Electrical Examination
12.5.2.1 Determine crystallographic orientation in ac-
cordance with SEMI MF26 on a sampling basis to
confirm the visual examination.
12.5.2.2 Determine the crystallographic perfection of
the grown ingot in accordance with SEMI MF1725 on a
sampling basis to confirm the visual examination.
12.5.2.3 Test for uniform distribution of
acceptor/donor impurities by plotting a resistivity
profile along the length of the ingot in accordance with
SEMI MF397. Note that resistivity values should vary
along the length consistent with the net acceptor/donor
values measured at the various points and that sharp
changes in the profile indicate point contamination or
nonuniformity of the sample deposition layer.
12.6 Ingot Sampling
12.6.1 After the ingot is inspected for crystallographic
perfection, appearance, and uniformity, and judged to
SEMI MF1723-1104 © SEMI 2004 7
be acceptable, choose sampling points for acceptor,
donor, and carbon sample wafers. Cut the sample
wafers at the proper points, then analyze for acceptor
and donor content by SEMI MF1389 and/or SEMI
MF1630, and for carbon by SEMI MF1391. Cut wafers
about 2-mm thick from the ingot for these analyses and
prepare the samples in accordance with the method
being used. Choose the ingot sampling plan
appropriate to the type of core being sampled in
accordance with the procedure in Sections 12.6.2 or
12.6.3 as appropriate.
12.6.2 Parallel Cores — For parallel cores (see
Section 10.2.1), single crystal ingot size is about 10 mm
diameter by about 200 mm in length. Select the
sampling points according to the individual segregation
coefficients for the specific impurities so these are
representative of more than 90% of the impurity
concentration as follows:
12.6.2.1 Segregation Effects — During the growth of
the crystal, crystallization from the melt, the impurity
concentration in the solid phase is different from that of
the liquid phase due to segregation.
1,2
The different
impurities have different segregation coefficients,
defined as:
l
s
C
C
K
0
(1)
where:
K
0
= equilibrium segregation coefficient,
C
s
= concentration of the impurity in the solid phase, in
atoms/cm
3
, and
C
l
= concentration of the impurity in the liquid phase, in
atoms/cm
3
.
12.6.2.2 Do not use the equilibrium segregation
coefficient for calculations since it is applicable only
for solidification at a negligibly slow growth rate. For
higher solidification rates, the impurity atoms are
rejected by the advancing melt at a greater rate than
they can diffuse into the melt. The impurity atoms
accumulate in the melt layer near the growth interface,
developing an impurity concentration gradient. The
concentration of this gradient depends on the growth
rate, fluid flow in the melt, and diffusion behavior of
the dopant. An effective segregation coefficient, K
eff
, is
described as:
)/exp()1(
00
0
DVKK
K
K
eff
(2)
where:
V = the growth rate, in cm/s,
= the diffusion layer thickness, in cm, and
D = the diffusion coefficient of the impurity in the
melt, in cm
2
/s.
12.6.2.3 In practice, measure a doping profile to
determine the concentrations of impurities along the
length of the ingot. The zone length, shown in Figure
3, is dependent on sample diameter, coil design, and
pull rate. After initial determination of the zone length,
re-measure it only after changes occur in the method
and apparatus. The doping profile, measured for each
impurity, determines where the ingot should be cut to
provide accurate impurity values. Sample points should
be chosen to be representative of more than 90% of the
impurity concentration. In Figure 3, the zone length is
measured at 15 mm. If the doping profile indicates that
the flat portion of the curve is at 12-zone lengths for an
impurity, take the sample at 12-zone lengths (12 × 15
mm = 180 mm) from the start of solidification on the
ingot.
Fi
g
ure 3
Zone Length Measurement and Ingot Sampling
Point
12.6.2.4 Ingot Profiling— For the type of float zone
furnace, coil design, pull rates, and ingot/core sample
diameter used, the effective segregation coefficient for
the impurities can vary. To calibrate the individual
zoner parameters and technique used, measure the
actual segregation coefficient profile. For example, to
determine the carbon profile, cut an ingot into wafers
along its length, and measure the carbon content in each
of the wafers. Then make a plot of carbon
concentration as a function of zone length. The number