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SEMI MF1708-1104 © SEMI 2004 3 7.6 Melter/Fl oat Zone Apparatus — A radio frequency (rf) generat or operating between 2.0 a nd 3.0 MHz with a copper, water-coole d working c o il for rf cou pling to the silicon (see Figu…
SEMI MF1708-1104 © SEMI 2004 2
SEMI MF1630 — Test Method for Low Temperature
FT-IR Analysis of Single Crystal Silicon for III-V
Impurities
4.2 ASTM Standard
D 5127 — Guide for Ultra Pure Water Used in the
Electronics and Semiconductor Industry
1
4.3 Federal Standard
209-E — Airborne Particulate Cleanliness Classes in
Cleanrooms and Clean Zones
2
4.4 ISO Standard
ISO 14644–1 — Cleanrooms and associated controlled
environments—Part 1: Classification of airborne
particulates
3
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Acronym
5.1.1 PTFE — polytetrafluoroethylene, a chemically
resistant polymer.
5.2 Definitions
5.2.1 Most terms used in this practice are defined in
SEMI MF1241.
5.2.2 Other Definitions
5.2.2.1 granular polysilicon, n — nearly spherical,
granules (200 to 3000 m) of polysilicon as produced
in a fluidized bed reactor.
5.2.2.2 melter/zoner, n — an apparatus designed to
melt granular polysilicon to a solid rod and then convert
the polycrystalline rod to a single crystal ingot by an rf
coupled coil.
5.2.2.3 silicon pedestal, n — a piece of single crystal
silicon cut from a high purity silicon ingot.
1 Annual Book of ASTM Standards, Vol 11.01, ASTM International,
100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone:
610-832-9500, Fax: 610-832-9555, Website:
www.astm.org
2 Standardization Documents Order Desk, Bldg. 4 Section D, 700
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS. (This
standard has been superseded by ISO 14644-1 and may no longer be
available.)
3 International Organization for Standardization, ISO Central
Secretariat, 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20,
Switzerland. Telephone: 41.22.749.01.11; Fax: 41.22.733.34.30
Website:
www.iso.ch
; also available in the US from American
National Standards Institute, New York Office: 11 West 42nd Street,
New York, NY 10036, USA. Telephone: 212.642.4900; Fax:
212.398.0023 Website:
www.ansi.org, and in other countries from
ISO member organizations.
6 Summary of Practice
6.1 Granular polysilicon is converted into a single
crystal silicon rod in a two-step procedure.
6.1.1 First, the silicon granules are consolidated into a
polysilicon rod by melting fluidized granules into the
molten (bottom) end of a silicon pedestal during a
downward pass of the coil of the zone furnace. After
about 15 g of polysilicon has been melted and cooled in
the zone process, a polysilicon rod about 8 to 12 mm in
diameter and up to 70 mm in length is obtained.
6.1.2 In the second step, a single crystal silicon seed is
melted into the tail end of the polycrystalline rod and a
single zone pass is done in the upward direction to level
the impurities and to convert the silicon to a single
crystal rod. This produces a single crystal silicon ingot
about 8 to 12 mm in diameter and up to 70 mm in
length from which a section is sliced for measurement
of impurities. The entire consolidation and zoning
requires about 30 min to accomplish.
6.1.3 A slice 2 to 4 mm thick is taken from the center
one-third of the single crystal silicon ingot for
measurement of impurities by infrared or
photoluminescence spectroscopies.
7 Apparatus
7.1 Acid Exhaust Fume Hood — To provide for exhaust
of acid fumes, a clean air environment (ISO Class 6
minimum, as defined in ISO 14544-1), a drain for acids
and water, and a deionized water supply. This hood
provides for the cleaning of quartz containers, funnels,
and tubes used in the melter/zoner as well as a place to
etch silicon pieces and samples used in this practice.
NOTE 1: ISO class 6 is about the same as Class 1000 as
defined in Federal Standard 209E).
7.2 Laminar Flow Hood — To provide a flow of clean
(ISO Class 6 minimum) air for drying of components
and the etched and clean silicon pieces.
7.3 Quartz Sample Containers — 200 to 300 mL
capacity quartz bottles to contain and transport granular
polysilicon samples.
7.4 Quartz Funnel — A funnel of sufficient size to
transfer granular polysilicon from the sample container
to the 15 mm inside diameter quartz tube.
7.5 Quartz Tube — A section of high purity quartz tube
with 18 mm outside diameter, 15 mm inside diameter,
and a length of 556 mm. The inside diameter and
outside diameter variances should be small to avoid
problems with clearances both internally and externally.
This tube provides the working enclosure for both the
consolidation and the zone leveling within the confines
of the working coil of the melter/float zone apparatus.
SEMI MF1708-1104 © SEMI 2004 3
7.6 Melter/Float Zone Apparatus — A radio frequency
(rf) generator operating between 2.0 and 3.0 MHz with
a copper, water-cooled working coil for rf coupling to
the silicon (see Figure 1). The coil shall have an inside
diameter of 20 mm to accommodate the 18 mm outside
diameter quartz tube and shall have sufficient power to
sustain a molten zone of at least 2 cm. Controls to
adjust the power output of the rf generator must be
readily available to the operator. The apparatus shall
have a carriage to vertically support and move the
quartz tube through the coil in a smooth and continuous
manner. The upper and lower endpieces of the carriage
shall be designed with chucks to hold silicon pedestals
as well as the quartz tubing. These endpieces shall
provide a seal to exclude air from the inside of the tube
and have connections for argon entry at the bottom
endpiece and exhaust at the top. Manual as well as
motorized movement of the carriage in the vertical
direction while minimizing horizontal motion is
essential. This entire apparatus is set up and utilized
inside a Class 6 clean room.
Figure 1
Melter/Float Zone Apparatus
7.7 PTFE Plunger-Diffuser — A cylindrical piece of
PTFE machined to close tolerance that fits snugly but
can be moved freely inside the quartz tube. This PTFE
plunger is drilled with an array of small holes for
diffusion of argon gas from the bottom endpiece of the
zoner carriage, through a bed of polysilicon granules,
and finally exhausted through the top endpiece. The
bottom of this plunger is machined to fit on the end of a
stainless steel rod that is extended through and
supported by the bottom endpiece. This rod provides a
way to vertically move the PTFE plunger/diffuser plate
within the quartz tube.
7.8 Chucks — Designed to hold silicon pedestals and
seed rods.
7.8.1 Upper Chuck — A three-jaw chuck designed to
hold a 6-mm silicon pedestal rod inside the quartz tube
and supported vertically from the top carriage piece.
7.8.2 Lower Chuck — A cylindrical piece of stainless
steel that fits inside the quartz and upon the upper end
of the stainless steel rod extending from the lower
endpiece with a hole drilled in the top about 3 mm
inside diameter and 1 cm deep (see Figure 1). This
supports a 2.5 mm single crystal silicon seed rod in a
near vertical position while allowing some (about 2
mm) horizontal movement at the top of an 80 mm rod.
7.9 Hydrogen Torch — Constructed by restricting the
end of a
¼ in. outside diameter piece of stainless steel
tubing and drilling a 1.5 mm hole as the orifice. This
torch is used to preheat the silicon to the point where rf
coupling occurs.
7.10 Diamond Saw — Suitable for cutting a sample 2 to
4 mm thick from the single crystal ingot.
7.11 Clean Room Garb — Including gowns, gloves, lint
free paper, etc.
7.12 Polyethylene Forceps — Size to hold 6 to 12-mm
silicon slice.
7.13 PTFE Beakers — 100 mL in size for handling
acids used in etching or cleaning.
8 Reagents and Materials
8.1 Hydrogen Gas — Low purity gas cylinder for
hydrogen torch.
8.2 Argon Gas — In accordance with SEMI C3.42.
8.3 Hydrofluoric (HF) Acid (49 %), in accordance with
Grade 1 of SEMI C28.
8.4 Nitric (HNO
3
) Acid (70 %) — In accordance with
Grade 1 of SEMI C35.
8.5 Mixed Acid Etchant (MAE) 57:18:25 — Composed
of HF:HNO
3
:Acetic Acid, in accordance with Grade 1
of SEMI C34.
8.6 Methanol — In accordance with Grade 1 of SEMI
C31.
SEMI MF1708-1104 © SEMI 2004 4
8.7 Deionized (DI) Water — Electronic grade Type E-2
as described in ASTM Guide D 5127.
8.8 Silicon Pedestals — 6 mm by 6 mm by 100 mm
single crystalline silicon cut from high purity silicon
ingots.
8.9 Silicon Seed Rod — 2.5 mm by 2.5 mm by 100 mm
single crystal <100> silicon rod cut from high purity
<100> silicon ingot.
9 Hazards
9.1 The acids used in this practice are potentially
harmful and must be handled with the utmost of care at
all times. Hydrofluoric acid (HF) solutions are
especially hazardous to the eyes, skin, mucous
membranes, and the lungs. Anyone using HF and other
acids must be familiar with the potential hazards and
must employ proper techniques and preventive
measures to avoid injury.
9.2 The rf generator and coil of the melter-zoner
apparatus can be injurious to the operator if the operator
is not properly trained in working with electrical
connections, rf fields, and hot parts.
9.3 Use eye protection to protect the operator from the
bright light of the molten silicon in the melter-zoner
apparatus.
10 Sampling
10.1 Granular polysilicon samples are best handled in
clean, dry quartz bottles. Samples are always taken
with clean silicon or quartz scoops or devices.
10.2 Granular polysilicon is commercially produced by
a continuous process in a fluidized bed reactor.
Samples for evaluation by this practice can be taken in
process, while packaging (normally in 270–300 kg
specially lined drums), or at the point of use. The
sampling point is therefore determined by the purpose
of the evaluation.
11 Reference Specimen
11.1 Use a large homogeneous sample of granular
polysilicon reference material to monitor the process of
consolidation and conversion to a single crystal silicon.
Periodically and repeatedly convert this reference
material to single crystal silicon to establish the
consistency and cleanliness of this practice.
11.2 Collect and statistically evaluate the values
obtained by low-temperature Fourier transform infrared
spectroscopy for carbon and the acceptor and donor
impurities in the analytical slices to establish and keep
the process in control.
12 Preparation of Apparatus
12.1 Take the utmost care to ensure the cleanliness of
all parts of the melter/zoner, especially the quartz tube,
sample containers, transfer funnel, and the carriage
endpieces.
12.1.1 Clean the inner part of the quartz tube and etch
it with 3:1 HF:HNO
3
. Following the acid etch, rinse the
tube with copious amounts of DI water and allow it to
dry in a laminar air flow hood.
12.1.1.1 Warning: Acids, especially HF, are very
hazardous to the eyes, skin, mucous membranes, and
lungs. Anyone using these acids must be familiar with
and use precautionary means to avoid injury.
12.1.2 Etch, clean and dry other quartz parts, including
sample containers and funnels as in Section 12.1.1
except that the etchant used is 1:1 HF:H
2
O.
12.1.3 Carefully clean the carriage endpieces, stainless
steel rods, and chucks of the melter/zoner by rinsing
with methanol and allowing them to thoroughly dry in
the clean room environment.
12.2 Prepare the silicon pedestals and single crystal
seed rods for use by etching with a continuous stream
of the MAE for 15 to 20 s, followed quickly with a
copious rinse of DI water. Air dry or blot dry with lint-
free paper.
13 Procedure
13.1 Assemble a freshly etched and dried quartz tube
into the carriage endpieces.
NOTE 2: Freshly, as referred to here and the following
paragraphs, refers to a time of less than 1 h.
13.2 Mount a freshly etched and dried silicon pedestal
(6 by 6 by 100 mm) in the upper chuck and place in the
upper endpiece. Center the silicon pedestal within the
quartz tube. Adjust the carriage so that the bottom of
the silicon pedestal is inside but not below the working
rf coil. Remove the pedestal and chuck and set aside
for later use.
13.3 Mount a freshly cleaned PTFE plunger-diffuser
part on the lower rod assembly and place it inside the
bottom of the quartz tube. Secure the rod assembly to
the lower endpiece and carefully move the rod and
plunger to a point of about 100 to 120 mm below the rf
coil. Start the argon purge and diffuser gas flow at a
rate of about 0.1 standard cubic feet per minute (SCFM)
through the lower portal, PTFE diffuser, and exhaust
out the top of the open quartz tube through the upper
endpiece.
13.4 Introduce the granular polysilicon sample through
the upper end piece with a quartz funnel. Fill with