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SEMI MF1708-1104 © SEMI 2004 5 about 25 g of polysilicon until the upper bed o f granules is near the lower part of th e rf coil. Re duce the argon flow until only the top layer of granu les are fluidized. 13.5 Remove th…

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

. 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

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

SEMI MF1708-1104 © SEMI 2004 5

about 25 g of polysilicon until the upper bed of granules

is near the lower part of the rf coil. Reduce the argon

flow until only the top layer of granules are fluidized.

13.5 Remove the funnel and replace the upper chuck

and silicon pedestal. Reposition the carriage so that the

pedestal is midway between the rf coil.

13.6 Ignite the hydrogen-air torch and position it

slightly above and about 30 mm from the working coil

impacting the outside of the quartz tube. Turn on and

increase the rf power to about 80% of the operating

power needed for melting. Watch for rf coupling with

the silicon rod, which will occur in about 2 min as is

evident from the red glow of the pedestal. Turn off the

torch and move the carriage upward at a rate that the

red hot zone follows the rf coil until the hot zone is at

the bottom of the silicon pedestal.

13.7 Increase the rf power until the bottom of the

pedestal melts. Move the position of the granules

upward with the lower rod and PTFE plunger until the

top of the fluidized bed begins to melt into the upper

pedestal. As the granules melt into the upper pedestal,

move the entire carriage upward.

NOTE 3: The melt freezes as it leaves the working zone of

the rf coil. The consolidated rod diameter is smaller than the

inside diameter of the quartz tube so it does not contact the

tube walls.

13.8 Continue consolidation growth with minor

adjustments until a consolidated polysilicon rod of

about 9 mm in diameter and 60 mm in length is

obtained. Lower the polysilicon granules bed and

reduce the rf power permitting the tail end of the rod to

solidify. Note the time required for the melting process

(normally about 12 min).

13.9 Remove the lower rod and plunger from the quartz

tube by removal of part of the lower endpiece. Let the

excess polysilicon granules fall out the bottom of the

tube and discard them.

13.10 Remove the PTFE plunger/diffuser from the

lower stainless steel rod and replace it with the lower

seed chuck (see Figure 1).

13.11 Mount a freshly etched and dried single crystal

silicon seed (2.5 by 2.5 by 100 mm) in the lower chuck.

Place the silicon seed rod, chuck, and lower rod into the

quartz tube through the bottom as before. Move the

lower rod upward until the seed crystal is about 5 mm

below the tip of the consolidated polysilicon rod

previously produced. Purge the entire tube for a

minimum of 1 min at a flow rate of 0.5 SCFM.

13.12 Reduce the argon purge to about 0.1 SCFM.

Reignite the hydrogen torch and increase the rf power

to 80% needed to melt silicon. Watch for the glow of

the consolidated polysilicon rod as rf coupling begins.

Shut off the torch.

13.13 Increase rf power until the consolidated

polysilicon rod tip is molten and then raise the seed to

penetrate into the melt. Hold in this position until the

seed has taken sufficient heat to melt and becomes one

with the melt from the consolidated polysilicon rod.

13.14 In a trial and error mode, increase the rf power as

needed to carry out the one pass zone leveling. When

the proper rf power is established, activate the

motorized carriage for movement downward (floating

zone movement is upward) at a rate to match that

employed during the consolidation process, typically a

zone rate of about 5.0 mm per minute. Continue the

zoning until the entire consolidated polysilicon rod has

been converted to a single crystal as is evident by the

four growth facet lines. Now, slightly reduce the rf

power and slowly move the lower rod downward

separating the single crystal ingot from the polysilicon

rod. Turn off the rf power. Allow about 2 min for the

ingot to cool.

13.15 Remove the single crystal ingot and break off the

seed crystal after scoring with a diamond scribe. Save

the remaining seed for growth of the next crystal.

Examine the crystal to be sure the growth facet lines

extend the entire length of the ingot.

13.16 Mount the crystal in the saw chuck and cut an

analytical slice 2 to 4 mm thick as needed from the

center one-third of the crystal.

NOTE 4: Since segregation of the impurities occurs both

during the consolidation step and the conversion to single

crystal, the one pass zone during the conversion to single

crystal at the same rate as the consolidation step effectively

levels these impurities. Thus, the concentration of the

impurities in the ingot is essentially constant except at both

extremes.

13.17 Prepare the analytical slice for infrared or

photoluminescence spectroscopic measurements with

application of a bright chemical etch or a mechanical

polish.

13.17.1 To etch, hold the slice with polyethylene

forceps and suspend it into a PTFE beaker filled with

the mixed acid etchant as described in Section 8.5.

Gently stir the solution with a magnetic stir bar while

immersing the silicon slice for 4 min. Remove and

quickly rinse with copious amounts of DI water. Air

dry the silicon slice, which is now ready for analysis.

13.18 Utilize the appropriate test methods for the

desired analysis. For carbon content, test by infrared

spectrophotometry in accordance with SEMI MF1391.

For determination of donor and acceptor contents, test

either by low-temperature FT-IR analysis in accordance

with SEMI MF1630 or by photoluminescence analysis

in accordance with SEMI MF1389, or both.

14 Interpretation of Results

14.1 The results obtained from spectroscopic

techniques on the analytical slice as obtained, directly

represent the impurity concentrations in the granular

polysilicon provided no impurity contamination is

found during the process. In reality, there is always

small impurity contamination during the consolidation

and conversion to a single crystal. This contamination

leads to results higher than actually present in the

granular polysilicon.

15 Precision

15.1 In the mid-nineties, the precision of this practice

was established by one laboratory as follows: A large

sample of granular polysilicon was evaluated repeatedly

and periodically by this practice for over more than a

year. Low temperature Fourier transform infrared

spectroscopy was used to measure the impurity

concentrations. The combined errors of sample

handling, consolidation to polysilicon rod, conversion

to a single crystal ingot, and the infrared determinations

gave a standard deviation of 0.03 ppba at the 0.06 ppba

concentration level for boron and phosphorus and a

standard deviation of 0.03 ppma for interstitial carbon

at the 0.07 ppma level.

15.2 Analysis of more recent similar data indicates that

the standard deviation is less than 0.02 ppba at the 0.04

ppba concentration level for boron and phosphorus and

0.02 ppma for interstitial carbon at the 0.05 ppma level.

16 Keywords

16.1 granular polysilicon; polycrystalline silicon;

polysilicon consolidation; polysilicon evaluation;

polysilicon impurities

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