During the reporting period, a successful 100 kilogram run was performed. Six ingots of 13 cm diameter were grown, ranging in size from 15.5 kg to 17.7 kg. Melt replenishment methods included both poly rod and lump feed material. Samples from each ingot were prepared for solar cell fabrication and analyses, impurity analysis, and structural studies. The furnace was converted to the 14-inch hot zone and preliminary heat runs were performed. Two sucessful runs were demonstrated, by growing 25 kg ingots from 30 kg melts. Also, a 100 kg run was attempted, utilizing the 14 inch crucible hot zone, but was prematurely terminated due to excessive monoxide which accumulated on the viewports and a seed failure

Merz, F.

English

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19790018325 2020-03-21T21:58:21+00:00Z
DOE/JPL 954888 79/(
DISTRIBUTION CATEGORY UC-63
.D NO. SE-7
DRI, NO. 60
1♦"i^. l
+I
CONTINUOUS CZOCH RALSKI GROWTH
SILICON SHEET GROWTH DEVELOPMENT
OF THE LARGE AREA SILICON SHEET TASK
OF THE LOW COST SILICON SOLAR ARRAY PROJECT
S1X7H QUARTERLY PROGRESS REPORT
JANUARY  1 - MARCH 31, 1979
PRJGRAM MANAGER: R. L. LANE
PF.INCIPAL INVESTIGATOR: F. MERZ
(`JASA-CR-158730;	 CON'ITNUCUS C2C01rAISKI	 N79-26496
l;Rr, W'IH.	 SILICON SiI['. F 'I GE,CWTH rFVFICPMFN1 CF	 ^^01
THE LARGE AREA SILICON 3PEF1 `TASK CF 'THE ICI; 	 PV^
COST SILTCCN SCIAF AFFAY rFC.IFCT CvartcVly ^C	 tJncl.^s
Progress Report, 1 (Kay*-x Cac1., CChestFr, 63/44 27821
G70
	
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KAYEX CGRPORATION
1000 MILLSTEAD WAY
ROCHESTER, NEW YORK 14624
"The JP L Low-C,-:-f Silicon Solar Array Project is sponsored by the, U.S. Department
of :nergy and forms part of the Solar Photovultaic Conversion Program to initiate
a major effort toward the development of low-cost solar arrays. This work was
performed fnr the Jet Propulsion Laboratory, California Institute of Technology,
by agreement between NASA and DoE."
DRD NO. SE-7	 DOE/JPL 954888 79/2
DRL NO. 60	 DISTRIBUTION CATEGORY UC-63
CONTINUOUS CZOCH RALSKI GROWTH
SILICON SHEET GROWTH DEVELOPMENT
OF THE LARGE AREA SILICON SHEET TASK
OF THE LOW COST SILICON SOLAR ARRAY PROJECT
SIXTH QUARTERLY PROGRESS REPORT
JANUARY 1 - MARCH 31, 1979
PROGRAM MANAGER: R. L. LANE
PRINCIPAL INVESTIGATOR: F. MERZ
KAYEX CORPORATION
1000 MILLSTEAD WAY
ROCHESTER, NEW YORK 14624
"The JPL Low-Cost Silicon Solar Array Project is sponsored by the U.S. Department
of Energy and forms part of the Solar Photovoltaic Conversion Program to initiate
a major effort toward the development of low-cost solar arrays. This work was
performed for the Jet Propulsion Laboratory, CaliFornia Institute of Technology,
by agreement bet—een NASA and DoE."
"This report was prepared as an account of work sponsored by the United States
Government. Neither the United States nor the United States Department of
Energy, nor any of their employees, nor any of their contractors, subcontractors,
or their employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness or usefulness
if any information, apparatus, product or process disclosed, or represents that
Its use would not infringe privately owned rights."
ABSTRACT
During the reporting period, a successful 100 kilogram run was performed.
Six ingots of 13 cm diameter were grown, ranging in size from 15.5 kg to 17.7 kg.
Melt replenishment methods included both poly rod and lump feed material.
Samples from each ingot were prepared for solar cell fabrication and analyses,
impurity analysis, and structural studies.
The furnace was converted to the 14-inch hot zone and preliminary heat runs
were performed. Two successful runs were demonstrated, by growing 25 kg ingots
from 30 kg melts. Also, a 100 kg run was attempted, utilizing the 14-inch
crucible hot zone, but was prematurely terminated due to excessive monoxide
which :.accumulated on the viewports and a seed failure.
i
CONTENTS
PAGE
ABSTRACT i
1.0 INTRODUCTION 1
2.0 PROGRESS 2
2.1	 HOT MELT REPLENISHMENT 2
2.2	 100 KILOGRAM RUN 3
2.3
	
SAMPLE PREPARATION AND ANALYSES 7
2.4	 14-INCH CURCIBLE/HOT ZONE 13
3.0 CONCLUSIONS AND DISCUSSION 14
4.0 PLANS 14
5.0 PROGRAM PLAN, COSTS, AND MAN-HOURS 14
ii
ILLUSTRATIONS
PAGE
FIGURE 1 METHOD OF SAMPLE PREPARATION FROM GROWN INGOTS 8
FIGURE 2. 3	 PROGRAM PLAN 15,	 16
FIGURE 4 COST 17
FIGURE 5 DIRECT LABOR 18
TABLE 1 COMPARISON BETWEEN ROD AND LUMP RECHARGING METHODS 4
TABLE 2 SUMMARY OF RUN 30 5
TABLE 3 RECHARGE AND INGOT GROWTH TIMES ( RUN 30) 6
TABLE 4 DISLOCATION DENSITY (RUN 30) 9
TABLE 5 AM-0 SOLAR CELL EFFICIENCY (RUN 30) 11
TABLE 6 DIFFUSION LENGTH MEASUREMENTS 12
iii
1.0 INTRODUCTION
The purpose of this program is to demonstrate the growth of at least
100 kilograms of single crystal ingot from one crucible by the Czochralski (CZ)
method.
T7ie approach to the continuous growth process being pursued in this
effort relies on conventional CZ technology combined with new equipment
designs which allow repeated alternate cycles of crystal growth and hot melt
replenishment by methods which are suitable for use in a high volume
production facility.
A Hamco Model CG2000 crystal grower was modified with	 special chamber
for the storage of a supply of polycrystalline silicon and a vacuum-tight
isolation valve to permit retrieval of crystals and melt replenishment
without contamination. A number of additional modifications to the facility
have been completed in the program, and the process study phase is now
under way, with a number of multi-ingot runs having been performed.
1
1.0 PROGRESS
2.1 Hot Melt Replenishmei,c--several runs were performed by hot filling the
crucible using the lump feeling mechanism to establish the reliability
of the method. The method consists of lowering a hopper through the
isolation valve and automatically releasing the polysilicon into the
crucible.
In order to prevent splashing of liquid silicon, the residual melt
was allowed to freeze over partially before recharging. It was learned
subsequently, however, that splashing was not a problem, even if the
poly is released directly into the liquid, provided the distance from
the hopper to the melt is controlled. A methoa was devised to control
this distance by proper positioning of the crucible height prior to
recharging. The position of the crucible depends upon the amount of
residual melt left in the crucible.
A minor problem that arose was the tendency of fine particles
of silicon to scatter and attach to the crucible wall or to fall in other
parts of the hot zone. This problem was due to the relatively large
quantity of fine particles in the recharge material, most of which was
prepared by hand methods from previously grown ingots. Commercial "nugget"
or "niblet" polysilicon has very few fine particles, and thus the
problem was minimized when commercial lump material was used. Solar
grade silicon, if in granular or lump form, would preferably have fine
particles removed. Of course, a small quantity of fines will be generated
in shipping, handling, and feeding lump material and therefore this
problem should be considered in production equipment design.
A comparison between the poly rod and lump methods is presented
44
2
in Table 1. A general conclusion that one may make is that the poly rod
method has less; tendency to add contaminants to the melt because the silicon
does not contact or abrade a metal c-ntainer. The lump feeder is faster and
allows the crucible to remain cooler during recharging. If a combination of
the two methods is used, the lump material is added first and, while this is
melting, the rod is preheated, thus speeding up the recharge process.
2.2 100 Kg Run Results--Tables 2 and 3 summarize the results of the 100 kg run
(Run No. 30). Table 2 lists the general conditions of crucible size, ingot
size, throughput and yield. Table 3 is a summary of the relative amounts of
time used in melting, recharging, growth, reseeding, etc.
It is interesting to note that only 500 of the total time is devoted
to actual crystal growth at full diameter. About one fourth of the run time
was used for the recharge operation. The balance was used in crown growth
and final crystal taper.
Although the throughput of 1.2 kg/hr as predicted in the economic model(')
was achieved, it is apparent that, not only increased growth rate should be
attempted, but that faster recharging will be very effective in reducing the
cost of crystal growth.
The yield of zero dislocation material from Run No. 30 was disappointing.
The growth of high quality ingot material was difficult to achieve throughout
the run. Several attempts had to be made on the first ingot to achieve a
zero dislocation structure. This problem became worse as the run progressed,
with all subsequent crystals dislocating part way through the growth cycle.
It is not clear why the dislocation problem was encountered, especially
on the first ingot, becaiisr that operation was essenti .^ Ay the same as the
state of art crystal growth process being used for semiconductor materials.
The exceptions to the standard process are: (1) hot filling to about 25 kg in
R_
3
COMPARISON OF LUMP AND ROD REVURGING
Rod
	
Lump
Recharge Time	 Relatively slow - about
	
Faster than rod.
4 hours to recharge because
	
2 hours to recharge
rod must be heated slowly at	 18-20 kg.
first.
Contamination of	 None, except for handltng	 Lump contacts metal
Melt
	
rods.	 feeding mechanism.
Effect on Crucible	 More reactive with crucible	 Crucible temperature
due to higher temperature.	 lower. Particles may
Crucible walls remain clean	 stick to crucible wall.
and particle-free.
Convenience	 Rod must be notched to hold 	 Convenient.
it.
Capacity of Recharge	 20 kg rods (18-20 kg recharge	 10 kg hopper must be
Mechanism	 capacity). I filled at East twice to
1 recharge.
TABLE 1
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.1. 
.
a 12-inch diameter crucible and (2) the growth of 130 mm diameter ingots.
The formation of dislocation and structure loss mechanisms were
discussed in more detail in the last quarterly report (2).
2.3 Sample Preparation and Analyses--Samples were taken from all ingots grown
for the purpose of measuring the following:
a. resistivity
b. dislocation density
c. grain size (in poly crystalline ingots)
d. solar cell efficiency and related parameters.
Resistivity measurements as well as dislocation densities and grain
size were na ap at Kayex. Solar cells were prepared and measured at Optical
Coating 1 urmt:ory, Inc. in City of Industry, California.
A 2-inch section was cut from the top, center, and bottom of each
ingot. These cylindrical sections were then quartered by two ver`ical cuts
in the direction of growth (see Figure 1).
2.3.1 Resistivity--Dopant was added to the melt through the dopant fixture after
each recharge was complete. The dopant was in the form of silicon/boron
alloy pellets. The resistivity of the ingots remained fairly constEmt
throughout the run varying from 2.02 ohm-cm to 2.59 ohm-em.
2.3.2 Dislocation Density (Table 4)--All ingots had a zero dislocation count at
the top. Dislocation density increased as expected, toward the bottom,
eventually causing grain boundaries to form and poly crystalline growth.
In the poly crystalline material, dislocations seemed to accumulate near
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grain boundaries, leaving the centers of grains often nearly dislocation-
free.	 T
Ingot Nu. 1 remained high quality throughout with only some dislocations
near the bottom and had no grain boundaries.
2.3.3 Grain Size--Ingots 2 through 6 were single crystal at the top but gradually
degraded to poly crystalline growth. Grain size became -smaller as ingot
growth progressed. Grain size distribution was wide and quantitative
measurements were very difficult to determine.
2.3.4 Solar Efficiency and Related Parameters--Table 5 gives the solar efficiency
at AM-0 of the ingots grown on Run 30.
Table 6 shuws measured diffusion lengths on samples from the top of
ingots 1, 3. 5 and 6. Diffusion length was measured using the short circuit
response at long wavelength (.8-1.0Um) method. Cell thickness was .015"
+ .002" (0.38 nun + .05) .
The highest solar efficiency (11.5% )V1-0) 	 ever achieved in this
program was measured on a cell prepared from the center of the first ingot
of the 100 kg run. This is approximately equi 3alent to 16% AM-1.
Solar efficiencies of cells made from the 100 kilogram run stayed
essentially constant from ingot to ingot throughout the run. This is
er juraging, as it indicates that degradation of solar efficiency due to the
expected impurity build-up will probably not be sufficient to degrade cell
performance. This conclusion is supported also by the fact that the bottom
of the last ingot had significantly higher solar efficiency than any of the
other ingot bottoms.
Solar efficiency degraded from 15Z to 30% from the top to the bottom
of each ingot. This would seem to be due to the steadily degrading crystal
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DIFFUSION LENG'T'H MEASUREMENTS
Ingot No.	 Diffusion
:op	 Length GO
1	 160
3	 80
5	 70
6	 40
Method: Short Circuit Current Response at long (.8-1.Oum)
Cell Thickness: .015" + .002"
TABLE 6
12
structure.
Ingot 1 maintained high quality single crystal zero dislocation structure
nearly to the bottom where it dislocated. Thus, the bottom samples had
approximately 20 x 10  dislocation per cm2 . This appears to explain the lower
efficiency for this sample. All of the other ingot tops were zero dislocation
single crystal. The centers and bottoms were large grain poly crystal material,
and accordingly very dislocated, especially near grain boundaries. This would
appear to	 ,lain the lower solar efficiency, although the mechanism of
efficiency loss (grain boundaries, dislocations, impurity traps, etc.) cannot
be determined easily.
2.4 !4-Inch Crucible/Hot Zone--The furnace was converted to the 14-inch hot zone
after the 100 kg run was completed.
After a number of test runs utilizing up to 30 kg melts, a 100 kg run was
attempted. Some difficulty was encountered with SiO build-up on the viewports;
however, the run was terminated when a seed broke on the third ingot. It was
concluded that a Letter seed holding design was needed for growing > 25 kg ingots.
13
3.0 Conclusions and Discussion
'llie feasibility of growing 100 kg from a 12-inch diameter crucible
has been demonstrated. Solar cell efficiency goals have been met for the
ingots grown that have good crystal structure. Efficiency degrades from
15 to 30%, however, if the ingot crystallinity degrades by dislocating
or polycrystalline growth.
Thus the major problem is to improve the yield of high quality material.
A number of possibilities have been identified as structure loss
mechanisms (2) These all relate' primarily to contaminants entering the
melt and therefore future work will include investigation in these areas.
4.0 Plans
The original 18-month program is concluded with this report; however,
JPL has granted an extension of the work. The goal of the continued effort
is to develop and optimize the process parameters for the growth of 100 kg
of high quality ingot from a single melt container.
5.0 Program Plan, Costs, and Man-Hours
The updated Program Plan is shown in Figures 2 and 3; Cost and Direct
Labor are shown in Figures 4 and 5.
Prior Reported	 Current
	
Total
Man-Hours	 7,445.5	 1,670
	
9,115.5
Costs	 $ 267,325	 66,643
	
$ 333,968
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R1: FE REN C ES
(1) R. L. Lane, "Continuous Czochralskf Growth", First Annual Progress Report
DOE/JPL 954888, September 30, 1978.
(2) R. L. Lane and F. Merz, "Continuous Czochralski Growth", Fifth Quarterly
Progress Report DUE/JPL 954888, December 31, 1978.
19


Continuous Czochralski Growth. Silicon Sheet Growth Development of the Large Area Silicon Sheet Task of the Low Cost Silicon Solar Array Project

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