51 01 -296 

Flat-Plate 

Solar Array Project 

DOEIJPL-1012-123 

Distribution Category UC-63b 

Silicon Production in a Fluidized 

Bed Reactor: Final Report 

Naresh K. Rohatgi 

April 1986 

Space Admin is tratiin 

JPL Publication 86-1 7 

TECHNICAL REPORT STANDARD TITLE PAGE 

1 4. Title and Subtitle 1 5, Report Date 

Silicon Production in a Fluidized Bed 

Reactor: Final Report I April 1986 

6, Performing Orgaritation Code 

JET PROPULSION LABORATORY 

California Institute of Technology 

4800 Oak Grove Drive 

12. Sponsoring Agency Name d Mess I JPL Publication 

14, Sponwxing Agency 6de NATIONAL AERONAUTICS AND SPACE ADMINISTBATION 

Uashington, D.C. 20546 I 

15. %PlemtaV l'+bf8s Sponsored by the U.S. Department of Energy through Interagency 

Agreement DE-AI01-85CE89008 with NASA; also identified as DOE/JfL-1012-123 and as 

JPL Project No. 5101-296 (RTOP or Customer Code 776-52-61). I 

t?fi5' Part of the development effort of the JPL in-house technology involved 

in the Flat-Plate Solar Array (FSA) Project was the investigation of a 

low-cost process to produce semiconductor-grade silicon €or terrestrial 

photovoltaic cell applications, 

of silane in a fluidized-bed reactor (FBR), Following initial investigations 

involving 1- and 2-in. diameter reactors, a 6-h. diameter, engineering-scale 

FBR was constructed to establish reactor performance, mechanism of silicon 

deposition, product morphology, and product purity, The overall mass balance 

for a11 experiments indicates that nutre than 90% of the total silicon fed into 

the reactor is deposited 09 silicon seed particles and the remaining 10% 

becomes erutriated fines. Silicon production rates were demonstrated of 

1.5 kg/h at 30% silane concentration and 3.5 kg/h at 80% silane concentration, 

The mechanism of si?icoa deposition is described by a six-path pro&ss: 

heterogeneous deposition, homogeneous decomposition, coalescence, coagulation, 

scavenging, and heterogeneous growth on fines, 

layer appears to be made up of small diameter particles. 

mrphology lends support to the concept of the scavenging of homogeneously 

nucleated silicon, 

The process selected was based on pyrolysis 

The bulk of the grown silicon 

This product 

ea6IfJ A,'bky 

I 

17. Key Worcb (Selected by AUthor(s1) 118. Dirtributicm Stutement 

Energy Conversion 

Engineering I Unclassif ied-unl irnited I 
5 101 -296 

Flat-Plate 

Solar Array Project 

DOEiJPL-1012-123 

Distribution Category UC-63b 

Silicon Production in a Flui 

Bed Reactor: Final Report 

Naresh K. Rohatgi 

April 1986 

Prepared for 

U.S. Department of Energy 

Through an Agreement with 

National Aeronautics and Space Administration 

by 

Jet Propulsion Laboratory 

California Institute of Technology 

Pasadena, California 

JPL Publication 86-17 
Prepared by the Jet Propulsion Laboratory, California Institute of Technology, 

for the U.S. Department of Energy through an agreement with the National 

Aeronautics and Space Administration. 

The JPL Flat-Plate Solar Array Project is sponsored by the U.S. Department of 

Energy and is part of the National Photovoltaics Program to initiate a major 

effort toward the development of cost-competitive solar arrays. 

This report was prepared as an account of work sponsored by an agency of the 

United States Government. Neither the United States Government nor any 

agency thereof, nor any of their employees, makes any warranty, express or 

implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process 

disclosed, or represents that its use would not infringe privately owned rights. 

Reference herein to any specific commercial product, process, or service by trade 

name, trademark, manufacturer, or otherwise, does not necessarily constitute or 

imply its endorsement, recommendation, or favoring by the United States 

Government or any agency thereof. The views and opinions of authors expressed 

herein do not necessarily state or reflect those of the United States Government 

or any agency thereof. 

This document reports on work done under NASA Task RE-152, Amendment 

419, DOE/NASA IAA NO. DE-AI01-8SCE89008. 
ABSTRACT 

Part of the development effort of the JPL in-house technology involved 

in the Flat-Plate Solar Array (FSA) Project was the investigation of a 

low-cost process to produce semiconductor-grade silicon for terrestrial 

photovoltaic cell applications. 

of silane in a fluidized-bed reactor (FBR). Following initial investigations 

involving 1- and 2-in. diameter reactors, a 6-in. diameter, engineering-scale 

FBR was constructed to establish reactor performance, mechanism of silicon 

deposition, product morphology, and product purity. The overall mass balance 

for all experiments indicates that more than 90% of the total silicon fed into 

the reactor is deposited on silicon seed particles and the remaining 10% 

becomes elutriated fines. Silicon production rates were demonstrated of 

1.5 kg/h at 30% silane concentration and 3.5 kg/h at 80% silane concentration. 

The mechanism of silicon deposition is described by a six-path process: 

heterogeneous deposition, homogeneous decomposition, coalescence, coagulation, 

scavenging, and heterogeneous growth on fines. The bulk of the grown silicon 

layer appears to be made up of small diameter particles. This product 

morphology lends support to the concept of the scavenging of homogeneously 

nucleated silicon. 

The process selected was based on pyrolysis 

Excellent purity of the silicon product was obtained from an FBR 

equipped with a quartz liner. 

less than the detection limits of spark source mass spectroscopy (e.g., 

Fe 5 0.6 ppma, Cr < 0.02 ppma, Zn < 0.04 ppma, Co 5 0.1 ppma). 

fabricated from this pure silicon product showed encouraging results and the 

data are described in this report. 

The product silicon had metallic contaminants 

Solar cells 

iii 

FOREWORD 

The Flat-Plate Solar Array (FSA) Project, initiated in 1975 to develop 

the technology for low-cost terrestial photovoltaic arrays, has been managed 

by the Jet Propulsion Laboratory (JPL) and funded by the Energy Research and 

Development Agency (ERDA) and by its successor, the U.S. Department of Energy 

(DOE). 

of produced electrical energy, the Silicon Material Task was assigned a 

responsibility of developing a process to produce silicon at a 1986 market 

price of less than $14/kg in 1980 dollars. 

To meet the Project's market price goal of $0.5O/peak watt (1975 dollars) 

As one of several parallel technology development efforts, the Project 

selected for development a high-throughput, two-step process for the 

production of silicon from silane. A preliminary economic analysis of the 

output of a 1000-MT/yr silicon production plant involving this two-step silane 

process indicated an achievable projected price of $9.50/kg in 1980 dollars. 

The first step in the two-step silane process, involves the 

transformation of metallurgical grade silicon into extremely pure silane. 

This process was developed by the Union Carbide Corporation (UCC) under a 

JPL/DOE contract. The process involving this step now is in commercial 

production. 

The second step, involving the conversion of silane into pure silicon, 

takes place in a fluidized-bed reactor (FBR). This process involves pyrolytic 

decomposition of silane followed by deposition of the silicon formed onto a 

fluidized bed of pure silicon seed particles at 650OC. 

Both UCC and JPL worked to develop this second step involving the 

conversion of silane to pure silicon. The objective of the UCC work was to 

develop a description of the operating requirements and characteristics for 

long-duration, steady-state operations, while JPL was involved in 

investigating a fundamental understanding of the process. 

Major accomplishments of the JPL in-house experimental work on the 

fluidized-bed process, which was initiated in early 1976, are summarized as 

follows : 

(1) The pyrolysis of silane was demonstrated successfully in 1-in., 

2-in., and 6-in. diameter FBRs. Reactor performance was 

characterized with respect to various operating parameters. It 

was demonstrated successfully that up to 100% concentration of 

silane in hydrogen can be used in an FBR. Such concentrations are 

necessary for silicon production at a high throughput. 

(2) Mechanisms were established that described silane pyrolysis and 

silicon deposition in FBRs. 

(3) Important. processing steps, including silicon-seed particle 

generation, silicon-seed particle cleaning, and FBR operation with 

a quartz liner, were developed to produce pure silicon product. 

V 
The feasibility of the silane-to-silicon conversion in an FBR clearly has )@en 

demonstrated on an experimental scale. The technology now is available to 

industry for carrying out tests on a pilot-plant scale prior to its 

commercialization. 

vi 
ACKNOWLEDGMENT 

The author especially thanks Dr. George C. Hsu for his scientific and 

engineering guidance. The author also thanks Jim Lloyd, Don Feller, and 

George Malczewski for their valuable laboratory assistance. 

vii 

CONTENTS 

I. INTRODUCTION ........................... 1 

A. DEVELOPMENT OF A SILICON PRODUCTION PROCESS ........ 1 

B. GOALS FOR THE JPL IN-HOUSE FBR PROCESS 

DEVELOPMENT PROGRAM .................... 3 

C. PREVIOUS WORK REPORTED ON THE FBR PROCESS 

CONDUCTED AT JPL ...................... 3 

D. OBJECTIVES OF REPORT .................... 4 

11. EQUIPMENTS AND OPERATING PROCEDURES ............... 7 

A. FLUIDIZED-BED REACTOR SYSTEM ................ 7 

1. BasicDesign ..................... 7 

2. Silicon Purity Considerations Toward 

Reactor Design ................... 16 

3. Quartz Liner ..................... 17 

B. SILICON-SEED GENERATION DEVICE ............... 20 

C. SILICON CLEANING DEVICE .................. 20 

111. EXPERIMENTAL ........................... 25 

A. RAW MATERIALS ....................... 25 

B. FLUIDIZED-BED REACTOR EXPERIMENTS ............. 25 

IV. EVALUATION OF VARIOUS ANALYTICAL TECHNIQUES ........... 29 

V. RESULTS AND DISCUSSIONS ..................... 31 

A. PERFORMANCE OF SILICON-SEED GENERATION DEVICE ....... 31 

B. PERFORMANCE OF THE SILICON CLEANING DEVICE ......... 31 

C. PERFORMANCE OF THE SILICON WITHDRAWAL SYSTEM ........ 36 

D. FLUIDIZED-BED REACTOR EXERPIMENTS ............. 36 

1. Reactor Performance ................. 36 

ix 
2 . Mechanism of Silicon Deposition ........... 

3 . Product Morphology .................. 

4 . Experiments for Further Control of 

Fines Formation ................... 

5 . Performance of a Quartz Liner ............ 

6 . Product Purity .................... 

7 . Solar Cells ..................... 

VI . CONCLUSIONS ........................... 

VI1 . RECOMMENDATIONS ......................... 

VIII.REFERENCES. ........................... 

APPENDIX: TECHNOLOGY TRANSFER ..................... 

Figures 

1 . Schematic Diagram for 6.in . Diameter 

Fluidized-Bed Reactor System ................ 

2 . Engineering Sketch of 6.in . Diameter 

Fluidized-Bed Reactor ................... 

3 . Six.in . Diameter Fluidized-Bed Reactor ............ 

4 . Gas Distributor for Fluidized-Bed Reactor ......... 

5 . Silicon-Product Withdrawal System ............. 

6 . Overall Silicon-Product Withdrawal and Collection System . . 7 . Quartz Liner for the Fluidized-Bed Reactor ......... 

8 . Diagram of Silicon-Seed Particle Generator ......... 

9 . Silicon-Seed Particle Generation Device .......... 

38 

42 

45 

45 

47 

49 

57 

59 

61 

A- 1 

9 

11 

12 

13 

14 

15 

19 

21 

22 

10 . Fluidized-Bed Silicon Cleaning Device ........... 24 

11 . Effect of Carrier Gas Flow Rate on Jet Mill Grinding .... 32 

12 . Effect of Silicon Feed Rate on Jet Mill Grinding ...... 33 

X' 
Tables 

13. Scanning Electron MicrograFh of Silicon Seed Particles 

from Seed Generation Device . . . . . . . . . . . . . . . . 34 

14. Performance of Silicon-Product Withdrawal System . . . . . . 37 

15. Experimental Silicon Production Rate 

Versus Silane Concentration . . . . . . . . . . . . . . . . 39 

16. Scanning Electron Micrographs of the 

Silicon Product from a Fluidized-Bed Reactor . . . . . . . 40 

17. Reaction Paths for Silane Pyrolysis 

in a Fluidized-Bed Reactor . . . . . . . . . . . . . . . . . 41 

18. Morphology of Deposited Layer of Silicon 

and the Elutriated Fines . . . . . . . . . . . . . . . . . . 44 

19. 

20. 

21. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

Temperature Profile in 6-in. Fluidized-Bed 

Reactor Fitted with Quartz Liner . . . . . . . . . . . . . . 48 

Czochralski Ingot from a Fluidized-Bed 

Reactor Product, JPL Run 115 . . . . . . . . . . . . . . . . 52 

Czochralski Ingot from a Fluidized-Bed 

Reactor Product, JPL Run 503 . . . . . . . . . . . . . . . . 54 

Various Wall Materials for the Fluidized-Bed Reactor . . . . 

Summary of Experiments with the 6-in. 

Fluidized-Bed Reactor . . . . . . . . . . . . . . . . . . . 

Evaluation of Various Techniques for Analysis of Silicon . Impurities in Acid-Cleaned Silicon Seed . . . . . . . . . . 

Scavenging Effect of a Secondary Silane Injection . . . . . 

Purity of Product from a Fluidized-Bed Reactor not 

Equipped with a Liner . . . . . . . . . . . . . . . . . . . 

Purity of Product from a Fluidized-Bed Reactor Equipped 

withaQuartz Liner . . . . . . . . . . . . . . . . . . . . 

Performance of Solar Cells Fabricated from 

the Silicon Produced in a Fluidized-Bed 

Reactor not Equipped with a Liner . . . . . . . . . . . . . 

Performance of Solar Cells Fabricated from 

the Silicon Produced in a Fluidized-Bed 

Reactor Equipped with a Quartz Liner . . . . . . . . . . . . 

18 

26 

30 

35 

46 

50 

51 

53 

55 

xi 
SECTION I 

INTRODUCTION 

The Flat-Plate Solar Array (FSA) Project was initiated in 1975 to 

develop the technology for the production of low-cost terrestrial photovoltaic 

arrays. FSA has been managed by the Jet Propulsion Laboratory (JPL) and 

funded by the Energy Research and Development Agency (ERDA) and by its 

successor, the U.S. Department of Energy (DOE). The Project was divided up 

into several tasks dealing with diverse technical fields. Each task was 

assigned a price goal such that the Project's market-price goal of $.50/peak 

watt (1975 dollars) of produced electrical energy could be achieved. The 

responsibility of the Silicon Material Task was to demonstrate a capability of 

producing silicon of a purity suitable for the manufacture of terrestrial 

solar arrays that met the Project's performance requirements for energy 

conversion. In addition, the silicon produced had to have a 1986 market price 

of less than $14/kg (in 1980 dollars) (References 1,2). 

In a conventional silicon-producing technology (Siemens process), the 

hydrogen reduction of trichlorosilane deposits silicon heterogenenously 

(gas-solid phase) on a hot filament at a very slow rate. 

silicon rod is obtained as the product in a batch mode. Homogeneous (gas 

phase) decomposition is suppressed by limiting the trichlorsilane feed to a 

low concentration (<.e., <5%). The silicon produced from this process is of 

semiconductor purity and can be used for solar cell fabrication. 

silicon produced by this process, however, is of the order of $60 to $100/kg, 

which is much higher than the projected Project goal to produce silicon at 

less than $14/kg. 

A coherent and dense 

The price of 

For this reason, the Project funded various low-cost silicon-production 

processes for preliminary experimental evaluation (References 3,4). 

high-throughput silicon-production process that involves fluidized-bed silane 

pyrolysis was selected for development, both at the Union Carbide Corporation 

(UCC) and JPL. 

A 

A. DEVELOPMENT OF A SILICON PRODUCTION PROCESS 

A two-step process involving the pyrolysis of silane was developed 

(References 5,6). 

In the first step, metallurigical-grade silicon is transformed into 

extremely pure silane (SiHq). 

operations: 

This transformation involves six separate 

(1) The production of trichlorosilane (SiHC13) from the 

hydrochlorination reaction of H2 and Sic14 with MgSi in a 

fluidized-bed reactor (References 7,8); 

Si + 2 H2 + 3 Sic14 + 4 SiHC13 (500°C and 515 lb/in.2) 

1 
The purification of the SiHC13 produced by the removal of both 

high-volatility gases and metals as chlorides. Residual Sic14 

is removed by distillation; . 

The disproportionation reaction of SiHC13 into dichlorosilane 

(SiH2C12) and SiCl4; 

2 SiHC13 + SiH2C12 + Sic14 (70°C) 

The purification of the dichlorosilane; 

The disproportionation reaction of dichlorosilane into silane 

(SiH4) and trichlorosilane; 

The purification of the SiH4 produced by distillation. 

UCC has demonstrated the feasibility of this first step by operating a 

LOO MT/year silane pilot plant. This reaction now is a commercial process. 

In the second step, silane is pyrolytically decomposed to produce pure 

si1 icon: 

SiH4 + Si + 2 H2 

The approach selected for investigation was the thermal decomposition of 

silane in an FBR. In this process, the silicon released from the silane is 

deposited on the surface of pure silicon seed particles. The seed particles 

grow in size at a temperature of 650°C. The FBR process has the following 

several advantages: 

It can allow both continuous withdrawal of large silicon particles 

from the reactor while also permitting the addition of seed 

particles to the reactor. 

It can be operated at a high-throughput because the large surface 

area of the silicon seed particles can take advantage of the use 

of high silane concentrations (e.g. >SO%). 

Preliminary calculations indicated the power consumption of less 

than 20 kWh/kg of silicon produced. 

An economic analysis of a 1000-metric tons/yr silicon-production 

plant, involving both steps, indicated a price of $9.50/kg in 1980 

dollars (Private comunication, Union Carbide Corporation, 

Washougal, Washington). 

A research program was conducted both at UCC and JPL to develop an 

FBR process. The objective of the UCC work was to develop the 

process empirically and to demonstrate long-duration operations. 

JPL, however, was involved in the attainment of fundamental 

understanding of the process. 

2 
B. GOALS FOR THE JPL IN-HOUSE FBR PROCESS GWELOPMENT PROGRAM 

The JPL in-house goals were: 

(1) To gather experimental data for operating-window and deposition 

kinetics. 

(2) To develop FBR technology as a means to prepare low-cost 

photovoltaic material. 

(3) Based on experimental findings, to establish a fundamental 

understanding of fluidized-bed silicon deposition in terms of 

homogeneous nucleation and chemical vapor deposition. 

6. PREVIOUS WORK REPORTED ON THE FBR PROCESS CONDUCTED AT JPL 

Initial FBR-process development work at JPL was conducted using a 1-in. 

diameter quartz reactor. Silane concentrations of less than 4% in hydrogen 

were used to study both the feasibility of silane pyrolysis in an FBR and the 

morphology of the silicon product. A fluidized-bed particle-growth model was 

developed based on mass transfer and the controlled steps of both homogeneous 

and heterogeneous chemical reactions (References 9,101. Further experimental 

work was conducted in 2-in. and 6-in. diameter FBRs to investigate various FBR 

subsystems and also to establish FBR operating-windows (References ll,12,13). 

The important experimental findings are: 

(1) The fluidized bed of silicon particles can be adequately heated by 

an external resistant heater. Silicon deposition on the reactor 

wall is not a problem if the reactor is operated according to a 

proper bed-temperature profile (Reference 13). 

(2) Gas distributors made of porous carbon, multilayer screen, and 

single-layer 325-mesh screen were all found to be satisfactory for 

the fluidized-bed operation. 

(3) A water-cooled, copper, cooling coil, located beneath the 

distributor screen-support plate, was found to be the most 

effective system to keep the distributor temperature below 300°C. 

The FBR was operated for 8 h at a 30% silane concentration without 

any sign of plugging the distributor screen. 

(4) The continuous withdrawal of product particles was successfully 

demonstrated with a withdrawal rate of 3 kg/h for 8 hours. 

(5) A U/Umf ratio1 of 3 to 5 was established as an ideal fluidization 

condition to carry out the silane pyrolysis reaction without 

formation of excessive fines (<lo%). 

greater than 5, excessive silicon fines (>lo%) are formed. 

When the U/Umf ratio is 

IWhere U is the superficial gas velocity and U,f is the minimum fluidization 

velocity . 

3 
The efficiency of the conversion of silane to silicon in the FBR 

is almost 100% at a 65OOC bed temperature and a U/Umf ratio of 5. 

A bed temperature of 630 to 670°C was established as an ideal 

condition to carry out the silane pyrolysis reaction without 

formation of excessive silicon fines (<lo%). 

Up to 100% silane can be fed into the FBR with fines formation of 

less than 10%. 

The results, at a bed temperature of 650°C and a U/Umf ratio 

of 5, showed that more than 90% of the silicon feed as silane was 

retained in the bed as deposited material and less than 10% was 

collected in the filter as elutriated fines. With an increase in 

silane feed-concentration from 20 to 80%, the rate of silicon 

product formation increases from 0.9 to 3.5 kg/h. 

of 15 cm diameter, to produce 100 MT/yr of silicon, at a 

production rate of 3.5 kg/h (reactor) and a 70% on-stream 

time-factor, are needed. In comparison, a commercial Siemens 

process, in which silicon is produced strictly by heterogeneous 

decomposition of trichlorosilane on a heated rod uses a Siemens 

bell-jar reactor 50 cm diameter x 1.2 m high. Its typical product 

rate is 0.5 kg/h. To produce 100 MT/yr of silicon by this Siemens 

technique, 33 bell-jar reactors would be needed compared to only 

five 15-cm FBRs. 

Five FBRs, each 

(10) Based on experimental data, a model was developed to predict rate 

of silicon particle growth. The rate of growth is a first-order 

reaction with respect to the concentration of silane in the 

hydrogen in the feed. The rate constant of the overall particle 

diameter growth was 13 pm/h for the given experimental 

conditions. 

D. OBJECTIVES OF REPORT 

Since the operating-window and silicon-deposition kinetics are well 

established, research was continued to establish silicon deposition mechanisms 

and silicon product purity. This report, representing the latest experimental 

work, discusses the following activities: 

(1) 

(2) 

Performance of JPL-developed silicon-seed generation device. 

Performance of JPL-developed silicon-seed cleaning device. 

(3) Performance of an FBR with a quartz liner required to produce a 

high-purity silicon product. 

(4) Latest experimental work on fluidized-bed silane pyrolysis. 

(5) Mechanism of silicon deposition. 

4 
(6) Data on silicon product morphology. 

(7) Data on silicon product purity. 

(8) Fabrication and characterization of solar cells from the 

fluidized-bed silicon product. 

5 

SECTION I1 

EQUIPMENTS AND OPERATING PROCEDURES 

A. FLUIDIZED-BED REACTOR SYSTEM 

1. Basic Design 

The 6-in. FBR system is depicted in Figure 1. The detailed 

description of an FBR system is discussed in detail elsewhere (Reference 13). 

The FBR consists of a reactor constructed from 6-in., schedule 40 stainless 

steel 316 pipe, and has dimensions of 6.065 in. inside diameter (1.D.) x 48 

In. high. The reactor has an expanded head of 24 X 24 in. to allow entrained 

particles to drop back into the bed (Figures 2 and 3). Particles in the bed 

are fluidized by a gas distributor fabricated from one layer of a 325 mesh 

stainless steel screen supported by a 1/20-in.-thick plate with 

1/8-in.-diameter multiple holes. The gas distributor is water cooled to keep 

its temperature below 350°C. There is water cooling at the periphery of the 

distributor in addition to a water-cooled copper cooling coil located beneath 

the gas distributor. 

1/20-in.-thick plate with multiple holes. 

welded to the support plate to ensure firm contact (Figure 4). 

The copper cooling coil was silver soldered to a 

The distributor screen is spot 

To maintain a constant bed height in the reactor, the 6-in. diameter FBR 

also is equipped with a continuous silicon withdrawal system (Figures 5 and 6). 

It is designed to withdraw silicon particles at a rate up to 3 kg/h. It 

consists of an externally water-cooled, silicon withdrawal tube, 15 in. long x 

0.87 in. I.D. The particles in the silicon withdrawal tube were fluidized at 

a U/Umf ratio of one to: 

(1) Keep silane out of the withdrawal tube. 

(2) Permit accumulation of large particles in the tube. 

(3) Minimize the disturbance caused by the gas velocity on the 

fluidization characteristics in the bed. 

(4) Cool the tube more effectively by preventing a sudden fall 

downward of hot bed particles. 

Located at the bottom of the withdrawal tube is an electronically 

controlled pinch valve. This valve is opened for a duration of Is to allow 

particles to drop into a tee. The opening of the valve is carried out at a 

predetermined frequency at 10 to 15 s intervals to allow cooling of the hot 

silicon particles in the tube. The particles in the tee are transported 

pneumatically to a cyclone separator and collected in a silicon holding tank 

of 10 kg capacity (Figure 6). 

The reactor was. externally heated by a two-zone, fast-response, 

12-in.-high, silicon carbide heater with a total output of 9 kw. Both zones 

are independently controlled to achieve an isothermal condition in the bed 

within 3 in. from the distributor. Atop the silicon carbide heater is a 

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COOLING CHANNEL 

DISTRIBUTOR 

WITHDRAWAL TUBE 

COOLING COIL 

SILANE INLET II - 

A THERMOCOUPLE 

9 DIFFERENTIAL PRESSURE PORT 

Figure 2. Engineering Sketch of 6-in. Diameter Fluidized-Bed Reactor 

P 11 
Figure 3. Six-in. Diameter Fluidized-Bed Reactor 

12 
Figure 4. Gas Distributor for Fluidized-Bed Reactor 

13 
.SAMPLING 

TUBE 

I ---r---””””1t-::1u 

TO LW / 

“‘STRIBUTOR 

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STORAGE 

HOPPER \ 

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COIL 

SILICON TRANSPORT CHAMBER 

Nz INLET 

CYCLONE 

Figure 5. Silicon-Product Withdrawal System 

14 
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15 
single-zone, independently controlled, 18-in. high, ceramic clamshell heater 

with a heating element made of an alloy of iron (72%), chromium (22%), cobalt 

(0.5%), and aluminum (5.5%). It has a total power output of 9 kw. The main 

purpose of the clamshell heater is to provide thermal insulation around the 

reactor and to distribute the heat input throughout the bed height. By 

helping to keep the temperature difference between the reactor wall and the 

bed within 5OoC, the risk of deposition of silicon on the reactor wall is 

minimized. 

The reactor temperature was continuously monitored by type K 

iron-constantan thermocouples located at various heights within the bed, on 

the reactor wall, and on the gas distributor. The temperature was 

continuously recorded by a Leeds & Northrup recorder. 

across the gas distributor and the bed as well as the total pressure were 

monitored. The flow rates of silane and hydrogen were monitored by Brooks 

rotameters. 

mixtures were analyzed by a Hewlett-Packard gas chromatograph equipped with a 

Porapack-Q column. 

The pressure drop 

The compositions of both the inlet and outlet silane-hydrogen gas 

The two silicon carbide heaters in the 6-in. diameter FBR were 

independently controlled by silicon-controlled rectifier (SCR) power 

controllers. The temperature was controlled by an OMEGA Dual Set-point 

controller. The electrical resistance heater in the 6-in. FBR was regulated 

by a Thermcraft Controller. 

The detailed operating procedure for the FBR system is discussed 

elsewhere (Rohatgi, N.K., "Six-inch FBR Operating Procedure," JPL Internal 

Document, 1982). The reactor was charged with clean silicon seed and 

preheated to the desired bed temperature with a hydrogen gas velocity 

equivalent to five times the minimum velocity required to fluidize the 

particles. The silane was gradually fed into the FBR, followed by a gradual 

decrease in the hydrogen flow rate to obtain the desired silane 

concentration. The reactor was operated a 5 psig pressure for a planned 

duration. Experiments were conducted both in a batch mode as well as the 

continuous-withdrawal mode to maintain a constant bed height in the reactor. 

The pressure-drop fluctuation across the bed was continuously monitored by a 

differential pressure transducer. The pressure-fluctuation signal was used to 

monitor the fluidization behavior of the bed. 

The exit gas from the 6-in. FBR was passed through 1 p porosity GUN0 

filters before being flared off. The purpose of these filters is to determine 

the extent of silicon fines elutriation. At the end of an experiment, the 

silicon in the bed was recovered, weighed, and screened to obtain the average 

particle size of the product. The silicon fines in the filters and the 

expanded head, including the lines to the filters, were also recovered and 

weighed to establish complete mass-balance. The percentages of silicon 

recovered in the bed and as fines were based upon the total silicon input to 

the reactor. 

2. Silicon Purity Considerations Toward Reactor Design 

Fluidized-bed processing of silicon particles poses a serious 

problem of reactor wall erosion that can lead to metallic impurity 

16 
contamination of the produced silicon. Major reactor components that have 

been identified as sources of metallic contamination are: 

(1) The 4 ft. tall FBR section. 

(2) The 2 ft. tall expanded head 

(3) The gas distributor 

(4) Silicon-withdrawal assembly 

Because the FBR component is exposed to a 650°C temperature it is 

anticipated that maximum wall erosion will occur in this section. The other 

reactor components are exposed to temperatures less than 250OC. In these 

sections, silicon particles are not violently fluidized. With these criteria 

in mind, the following various materials were considered for walls of the 

reactor: quartz liner, silicon carbide liner, silicon liner, silicon carbide 

coated graphite reactor, zirconium oxide protective coating on stainless steel 

reactor, and liquid nitriding of various stainless steel reactor components 

(Table 1). Each of these wall materials has certain advantages and 

disadvantages with respect to heat transfer, mechanical strength, cost, and 

potential for metallic contamination. After a thorough evaluation, it was 

decided to liquid nitride the reactor's expanded head, gas distributor, and 

silicon withdrawal assembly to increase the hardness of exposed stainless 

steel surfaces. This process involves the treatment of various stainless 

steel parts in a bath of liquid cyanide. The exposed stainless steel surface 

of the FBR section is isolated from the silicon particles by insertion of a 

mechanically sealed quartz liner. 

3. Quartz Liner 

Quartz is an excellent choice for a reactor liner from the point 

of view of silicon purity. Breakage of the liner during thermal cycling, 

however, is a serious problem. The deposition of silicon on either the inside 

or outside surface of quartz will break the liner because of the differential 

thermal expansion between silicon and quartz. Since the inside surface of the 

quartz liner is exposed to the hot silicon particles, silicon deposition can 

be controlled by maintaining a proper temperature gradient between the 

particles and reactor wall. If silane is permitted to flow in between the 

quartz liner and the stainless steel wall, the silicon will deposit on the 

quartz tube and eventually break it during thermal cycling. The design 

criterion, therefore, was to establish seals for the quartz liner between the 

expanded head flange and the gas distributor. This would prevent silane from 

flowing into the annular space between the quartz liner and the stainless 

steel reactor wall. The quartz liner design, shown in Figure 7, consists of a 

5.75-in. outside diameter x 4-ft-long quartz tube with ll8-h. wall 

thickness. The top end of the quartz liner is supported against a ring with a 

1/4-in. groove and a Graphoil gasket. The bottom end of the quartz liner 

stands on a piston gas-distributer assembly with 1/4-in. groove and a quartz 

sleeve used as a gasket. The outside surface of the piston 

pneumatic cylinder mounted against the reactor flange. The 

vacuum and pressure sealed with a viton O-ring. The entire 

cooled to maintain integrity of the O-ring. 

consists of a 

interface is 

assembly is water 

17 
Table 1. Various Wall Materials for the Fluidized-Bed Reactor 

FBR WALL 

MATERIALS DESCRIPTION ADVANTAGES DISADVANTAGES DECISION 

QUARTZ LINER 

IN STAINLESS 

STEEL REACTOR 

0 AVOIDS METAL 

INEXPENSIVE 

CO NTAMl NATl ON 

CONSTRUCTION 

LINER FOR 

FLU1 DIZED-BED 

REACTOR 

SECTION 

D POOR HEAT 

TRANSFER 

D QUARTZ MAY 

CRACK 

0 POOR HEAT 

TRANSFER 

0 POSSIBLE 

CARBON 

CONTAMINATION 

0 EXPENSIVE 

CONSTRUCTION 

'LACED A 

MECHANICALLY 

SEALED QUARTZ 

rUBE INSIDE THE 

SS REACTOR 

SAME (WITH Sic TUBE) 

SILICON 

CARBIDE LINER 

IN STAINLESS 

STEEL REACTOR 

0 AVOIDS METAL 

CONTAMI NATlO N 

X 

SILICON LINER 

IN STAINLESS 

STEEL REACTOR 

SAME (WITH 

Si TUBE) 

0 AVOIDS METAL 

CONTAMINATION 

0 POOR HEAT 

TRANSFER 

0 RELATIVELY 

EXPEN SlVE 

CONSTRUCTION 

X 

SILICON 

CARBIDE 

COATED 

GRAPHITE 

REACTOR 

INSIDE SURFACE 

OF GRAPHITE 

TUBE IS COATED 

WITH SILICON 

CARBIDE 

0 GOOD HEAT 

TRANSFER 

0 NON-UNIFORM 

COATING 

0 POSSIBLE 

CARBON 

CON TAM 1 NATION 

0 EXPENSIVE 

CONSTRUCTION 

X 

ZIRCONIUM 

OXIDE 

COATING ON 

SS REACTOR 

INSIDE SURFACE 

OF THE SS 

REACTOR IS 

COATED WITH 

ZIRCONIUM 

OXIDE 

0 GOOD HEAT 

TRANSFER 

0 POSSIBLS 

ZIRCONIUM 

CONTAMINATION 

0 CRACKING OF 

COATING DUE 

X 

TO DIFFERENTIAL 

THERMAL 

EXPANSION 

MIGRATION 

OF METALLIC 

INPURITIES 

FROM HOT 

SS SURFACE 

LIQUID 0 POSSfBLE 

NITRIDING 

OF SS 

HARDNESS OF 0 INEXPENSIVE 

PROCESS 

GAS DISTRIBUTOR, 

EXPANDED HEAD, 

AND WITHDRAWAL 

ASSEMBLY WILL 

BE TREATED BY 

THIS PROCESS 

SS SURFACE 

IS INCREASED 

BY TREATING 

IT IN A LIQUID 

CYANIDE BATH 

X = NOT SELECTED FOR THIS STUDY 

SS = STAINLESS STEEL 

18 
@ HYDROGEN EXHAUST 

@ HYDROGEN INLET 

@ DIFFERENTIAL PRESSURE GAGE 

@ CLAMSHELL HEATER 

@ THERMOCOUPLE 

@ PYROMETER 

0 SILICON CARBIDE HEATER 

@ PISTON 

@ PNEUMATIC CYLINDER 

@ SILANE INLET 

0 NITROGEN INLET 

@I QUARTZ LINER 

-0 

Figure 7. Quartz Liner for the Fluidized-Bed Reactor 

19 
The quartz-lined reactor, filled with silicon particles, is closed and 

evacuated to remove oxygen prior to heating. The silicon particles are 

fluidized with a flow of nitrogen or helium. A pressure of 200 psig is 

applied at the pneumatic cylinder to mechanically seal the top end of the 

quartz liner against the Graphoil gasket. The pressure at the pneumatic 

cylinder compensates, in the axial direction, for the differential thermal 

expansion between the stainless steel reactor wall and the quartz liner. The 

bottom end of the quartz liner is not completely sealed and hydrogen is 

permitted to flow at a rate of less than 1 SLPM. During the entire heating 

cycle, as well as during the silicon deposition experiment, a differential 

positive pressure of a 20-in. water column is maintained between the annular 

space and the inlet port for the silane/hydrogen gas mixture. This procedure 

ensures that hydrogen can leak into the reactor but that the silane/hydrogen 

mixture cannot leak into the annular space. 

B. SILICON-SEED GENERATION DEVICE 

A prior technique for preparing fluidized-bed seed involved grinding 

bulk, semiconductor-grade silicon with a mechanical crusher followed by a 

leaching treatment with acids. Not only are these efforts tedious and costly, 

but the grinding tends to introduce metallic contaminants. A jet-milling 

technique for seed-particle generation, using impinging jets of gas to grind 

silicon was studied in this work. Specifically, this technique involved the 

grinding of silicon particles through the impingement of two opposing nitrogen 

jets that carry silicon particles. The jet-milling device and its dimensions 

are shown in Figures 8 and 9. The device is lined with polyurethane and 

high-density polypropylene to avoid exposure of silicon to metallic surfaces. 

It has an impact chamber 0.7s-in. diameter x 2-in. long. The device is 

equipped with two opposing nitrogen jets, each with an orifice diameter of 

0.078 in. Large silicon particles (<2mm) are fed in at the top of the 

device and are carried into the impact chamber by the two opposing nitrogen 

jets at a supersonic velocity. Collisions of the large particles result in 

their fracture into smaller-sized particles. The fractured particles, carried 

by the effluent gas, are discharged through a 1-in. diameter pipe into a 

cyclone separator and are collected in a polyethylene jar at the bottom of the 

separator. The small particles (<SO p) in the discharged nitrogen gas 

stream are collected in an air bag filter. Using screens constructed out of 

polyethylene and polypropylene, the fractured particles are separated into 

three categories of sizes: undersize (<111 pm), desired size (111 to 

400 p), and oversize (>400 pm). The undersized particles are discarded 

while the oversized particles are recycled back into the seed generation 

device. After screening, the desired-size seed particles require acid 

cleaning. The use of a fluid-grinding technique, results, however, in 

particles with a much lower level of metallic impurities. 

c. SILICON CLEANING DEVICE 

The large silicon particles (<2 mm), as well as the silicon-seed 

particles, of 111 to 400 pm in diameter, are contaminated with metallic 

impurities in the ppma range. The removal of these impurities is necessary 

prior to conducting an experiment to grow semiconductor-grade silicon 

particles. The average diameter of a seed particle is approximately 250 pn 

20 
GROUND SILICON 

SILICON FEED 

t 

SILICON FEED 

NITROGEN NITROGEN 

.2 crn ORIFICE IMPACT CHAMBER 

Figure 8. Diagram of Silicon-Seed Particle Generator 

21 
22 
and it has a huge surface area per unit weight of the bed. Acid leaching is 

used to remove metallic impurities from individual silicon particles. Because 

the cleaning acid is a mixture of concentrated nitric and hydrochloric acids 

that reacts very fast with metallic impurities to form soluble complexes, it 

is logical to assume that the silicon cleaning process is mass-transfer 

limited. The most important design criterion for a silicon cleaning device, 

therefore, is to provide very good mixing. 

In the silicon cleaning device, depicted in Figure 10, silicon particles 

are fluidized by the cleaning solution. The entire system, in contact with 

either silicon or acid, is constructed out of polyethylene and polypropylene. 

It consists of three plastic bottles, each 3.5 in. diameter x 8 in. high that 

can hold up to 500 grams of silicon (4-in. bed height). The bottom of each 

bottle is equipped with a 50 pm-pore diameter plastic screen and distributor 

to provide a tangential flow of cleaning solution for good mixing. The top of 

the bottle is equipped with another 75 ym-pore diameter screen to retain the 

particles within the bottles. The three bottles are installed in a plastic 

tank filled with up to 2 gallons of cleaning solution that is circulated 

through the system via a plastic pump (Figure 10). The flow rate of the 

cleaning solution is adjusted by a plastic needle valve to obtain desired 

fluidization of silicon particles. 

preparation of cleaned silicon seed by this device includes the 

steps: 

A wash with deionized water, in a fluidized-bed mode, to remove 

fine silicon particles of less than 75 p in diameter. 

Cleaning with a mixture of two parts of 12N HC1 and one part of 

16N HNO3 in a fluidized-bed mode for 20 min. 

Acids are drained after 20 min and the particles are washed with 

deionized water in a fluidized-bed mode until the effluent water 

is neutral. 

The silicon particles are etched with 48% hydrofluoric acid (HF) 

in a fluidized-bed mode for 20 min. 

The HF is drained after 20 min and particles are washed with 

deionized water in a fluidized-bed mode until the effluent water 

is neutral and has a resistivity of 16 Ma. 

The wet particles are dried in a diffusion furnace at 150°C under 

a nitrogen gas blanket. 

After drying, particles are transferred into a plastic bag that 

then is sealed. 

The effectiveness of this cleaning technique will be discussed later. 

23 
I' 

r 

I- \; ;;) \I 

i ; 1 'I'. 

a 

a) 

I 

a 

a) 

N 

Ti 

a 

*rl 

3 

rl 

L4 

a) 

SI 

1 

M 

.rl 

L4 

24 
SECTION I11 

EXPERIMENTAL 

A. RAW MATERIALS 

Electronic grade silane was purchased from the Union Carbide Corporation 

(UCC), Washougal, Washington. The silane-production process was developed as 

part of the FSA Project. The silicon epitaxial layer obtained by using this 

silane as a source of silicon has a resistivity of 200 ohms-cm N type. 

High-purity nitrogen and helium were purchased from Linde, a division of UCC. 

Silicon particles of less than 2 nun diameter were purchased from Dynamit 

Nobel, Italy. These particles were prepared by Dynamit Nobel using a 

technique that involves the mechanical breaking of a Siemens silicon rod. 

This silicon has a resistivity greater tl-.an 70 ohm-cm N type and greater than 

1000 ohm-cm P type. 

B. FLUIDIZED-BED REACTOR EXPERIMENTS 

A summary of all recent FBR experiments is presented in Table 2. The 

Experiments were 

FBR was operated at a bed temperature of 650°C, a U/Urnf ratio of 4 to 5, and 

silane concentrations ranging from 30 to 50% in hydrogen. 

designed to meet the following objectives: 

(1) Conduct experiments to evaluate reactor performance. 

(2) Conduct experiments to further the control of the formation of 

fines: Run Numbers 201 to 206. 

(3) Conduct experiments to evaluate performance of a quartz liner: 

Run. Numbers 300, and 402 to 408. 

(4) Establish purity of the silicon product: Run Numbers 502 and 503. 

25 
U 

v) 

Y 

2 

3 

w 

BO B a 0 

Y 

\ 

t 

Q : 

0 K 

K K 3 m 

2 

N 

d x v) 

P) m N d 

5: 0 m 5: 3 3 0 c? 

w 

Q 

w 

Q 

0 10 Q 

0 

z 0 

z 

0 

z 

0 

z 

0 

z 

0 N v) c? N a, cv N 

R. 

0 - 

c9 

N - c. 

W 

0 

z 

0 

z 

0 Q 36 -t CYZ 

.o 0 N 0 0 m N 0 * 

26 
c ‘9 

0. 

c 

c9 

10 

c 

ro 10 

N. 

0. W 

h 

I 

N 0. a 

z 

v! m c 

-t 

m 

0 10 0 m 0 m 

5 0 10 

ro 0 10 \o 5 

0 

z 

0 

z 

0 N \o h N ro 

0. 0. c 

-7 

m 

51 m 

51 

27 

SECTION IV 

EVALUATION OF VARIOUS ANALYTICAL TECHNIQUES 

To establish the purity of silicon, involving metallic impurities in 

silicon in the ppba range, it is essential to have the measurements made with 

a reliable analytical technique by a commercial analytical laboratory with 

experience in analyzing silicon. Initially, atomic absorbtion, inductivelycoupled plasma spectroscopy, neutron activation analysis (NAA), and spark 

source mass spectroscopy (SSMS) were selected as analytical techniques for 

evaluation. The first two analytical techniques suffer from an inherent 

drawback in that they require sample preparation by dissolving silicon in acids 

prior to analysis. This poses a serious problem to the analysis of metallic 

impurities in the ppba range because impurities will be diluted by at least a 

factor of 10 during sample preparation. Metallic impurities in silicon 

directly can be analyzed by NAA, and SSMS techniques and the dissolution of 

silicon samples is not required. For this reason, the SSMS and NAA techniques 

were finally selected to evaluate both these analytical techniques and the 

various commercial analytical laboratories that performed them. 

Two different silicon samples were selected for evaluation of the 

analytical techniques: 

(1) The 111 to 508 pm diameter silicon particles prepared at JPL by 

the previously discussed jet-milling and acid-cleansing 

techniques. 

(2) Komatsu silicon supplied by UCC. It is produced by the same 

procedure as is the high-purity Siemens silicon, except that the 

silicon rod is grown from the decomposition of silane instead of 

trichlorosilane. This semiconductor-grade Komatsu silicon rod 

was mechanically broken (UCC proprietary technique) into 

approximately 400 pm diameter particles and the particles were 

used as a control sample for these evaluations. 

For the SSMS analyses, identical samples were sent to the Northern 

Analytical Laboratory (NAL), Commercial Testing and Engineering (CT&E), and to 

the Wyoming Analytical Laboratory (WAL). For NAA, identical samples were sent 

to the Oak Ridge National Laboratory (ORNL), Activation Analyses (AA), and 

Lawrence Livermore Laboratory (LLL). The results are shown in Table 3. The 

data clearly indicate that measurements of the metallic impurities in an 

identical sample varies significantly from one type of analysis to another. 

Only the SSMS analysis, conducted by NAL, and the NAA analysis, conducted by 

ORNL, provided sensitivities consistent with the requirements. 

29 
30 
SECTION V 

RESULTS AND DISCUSSIONS 

A. PERFORMANCE OF SILICON-SEED GENERATION DEVICE 

The effectiveness of silicon grinding by the impingement of two opposing 

nitrogen jets was studied with respect to nitrogen-carrier flow rate with 

different initial silicon particle sizes. As shown in Figure 11, the average 

particle sizes of 600 pm and 850 pm (anticipated product sizes from a 

commercial reactor) were reduced to 330 and 430 pm, respectively, after one 

pass-through at a nitrogen flow rate of 20 SCFM and a silicon feed rate of 48 

gm/min. In another experiment, the one-pass yield for the desirable siliconparticle sizes of +75 to -355 p range (obtained from the feed size of +500 

to 707 p) was 15.8% at 12 SCFM N2 feed rate and 40 gm/min silicon feed 

rate. The two-pass yield incresed to 28.3%. The effect of the silicon feed 

rate, shown in Figure 12, indicates that beyond the 100 gm/min silicon feed 

rate, further particle size reduction seems insignificant. 

The combination of two possible mechanisms seems to control the nature 

of the silicon grinding brought about by the impingement of two opposing jets: 

(1) The chipping of a particle at the edges will produce very small 

size particles (1 to 30 pm) of approximately spherical shape. 

(2) The fracturing of flake-shaped particles produces smaller-size 

particles with the same flake-shape. 

The silicon-grinding mechanism was verified by conducting a mass balance 

before and after the operation of the silicon seed generation device. It also 

was verified by examining the scanning electron micrographs of feed and product 

particles. Figure 13 indicates that the flake-shaped feed particles simply 

have been fractured into irregularly shaped smaller-sized flake-shaped 

particles. Particles with diameters less than 111 p were collected in the 

air bag filter. These particles were less than 1 wt 9, of the total silicon 

fed during one pass. This indicates that silicon grinding is mostly 

controlled by the second possible mechanism. 

B. PERFORMANCE OF THE SILICON CLEANING DEVICE 

To produce semiconductor-grade silicon particles, it is necessary to 

start with clean silicon seed material. The deposition of pure silicon via 

silane pyrolysis will dilute metallic impurities that already exist in the 

seed material, but will not be enough to produce pure silicon product. For 

this reason, silicon particles of less than 2 mm diameter were purchased 

from Dynamit Nobel with a minimum impurities level as indicated in Table 4 

(e.g., Fe=20 ppma, Cr=0.5 ppma, Ni=lO ppma, Cu=0.06 ppma, Mn=O.5 ppma, and 

Al=Zppma). 

employing the seed-particle generation device and then are cleaned in the 

These particles are reduced to 200 to 300 prn diameter by 

31 
900 

800 

700 

E 

1 

5 

w 

600 ln 

w 

V - 

2 500 

2 

$ 3 400 w 

> 

Q 

300 

TOTAL NITROGEN FEED RATE, SCFM 

5 10 15 20 

I I I 

0 = +707 TO -lo00 pm INITIAL SILICON 

x = 1500 TO -707 pm INITIAL SILICON 

PARTICLE SIZE 

- PARTICLE SIZE 

TOTAL SILICON FEED RATE: 48 gm/min 

3 

I I I 

0 1 42 284 42 6 568 100 

TOTAL NITROGEN FEED RATE, d/min 

Figure 11. Effect of Carrier Gas Flow Rate on Jet Mill Grinding 

32 
900 

800 

700 

I 

. 

W 

600 v) 

w 

-I 

u 

I- 500 2 

n 

w 2 400 

e5 

> 

Q: 

300 

200 

x =+500 TO -707 pm INITIAL SILICON 

PARTICLE SIZE 

TOTAL NITROGEN FLOW RATE: 340 P/min 

- 

100 I I 1 

0 100 200 300 400 

TOTAL SILICON FEED RATE, grn/rnin 

Figure 1.2. Effect of Silicon Feed Rate on Jet Mill Grinding 

33 
34 
Table 4. Impurities in Acid-Cleaned Silicon Seed* (ppma) 

ELEMENTS 

P 

Fe 

Cr 

Ni 

cu 

Zn 

co 

Mn 

Na 

Mg 

AI 

S 

K 

Ca 

RAW PARTICLES 

"AS PURCHASED" 

0.2 

20 

0.05 

10 

0.06 

< 0.02 

50.1 

0.5 

50.1 

51 

2 

<1 

5 0.07 

0.6 

JET MILLED AND 

ACID CLEANED 

"SEED FOR FBR" 

0.2 

10.6 

0.03 

C0.5 

10.02 

5 0.04 

10.1 

5 0.02 

10.1 

<1 

0.05 

(1 

(0.1 

0.1 

SPARK SOURCE MASS SPECTROSCOPY 

35 
JPL-developed fluidized-bed silicon cleaning device. As indicated by the 

following equations, the mixture of HNO3 and HC1 will remove metallic 

impurities as soluble chlorides: 

Fe + Fef3 (oxidation by HNO3) 

Fe*3+ 4 HC1 + FeCl4- (soluble) + 4H+ 

The silicon oxide layer coating on the particle is removed by etching with 

concentrated HF. 

Si + 2HNO3 + Si02 + 2 AN02 

Si02 + 4 HF + SiF4 (gas) + 2H20 

The acid-cleaned particles have impurity levels below the detection limits of 

the spark source mass spectroscopy analytical technique as shown in Table 4 

(e.g., Fe CO.6 ppma, Ni <0.5 ppma, Cu <0.02 ppma, and Mn c0.02 ppma). 

C. PERFORMANCE OF THE SILICON WITHDRAWAL SYSTEM 

The purpose of the withdrawal tube is selectively to take large 

particles out of the bed in a continuous or semicontinuous fashion. The 

results on withdrawal performance are shown in Figure 14. Half of the 

particles in the withdrawal boot have sizes greater than 300 prn, although 

the average particle size in the bed is about 220 pm. About 75% of the 

withdrawn particles are larger than 200 pm. If economically necessary, the 

undersized particles (after product particle screening) can be recycled to the 

reactor for further growth. If 10% of the bed particles are withdrawn, the 

distribution ratio of 400 pm particles in the withdrawal system compared to 

those in the bed is 1.4. In comparison, the distribution ratio of 1 

represents almost uniform distribution of bed particles. Thus, this indicates 

that the selective withdrawal of larger particles takes place in the mildly 

fluidized column. 

D. FLUIDIZED-BED REACTOR EXPERIMENTS 

The experimental results from all key experiments (Series 200, 300, 400, 

and 500) are summarized in Table 2. Discussions of the most important results 

are presented in the following sections. 

1. Reactor Performance 

The overall mass balance data are similar to those reported 

earlier (Reference 13). More than 90% of the total silicon fed into the 

reactor is deposited on the silicon seed particles, and the remaining 10% 

ended up as elutriated fines. The fines formation is influenced by the 

fluidization characteristics of the bed. If the bed is partially plugged, the 

fines formation are higher than 10% as in the case for Run. Number 202 and 203. 

36 
Z 

AVERAGE PARTICLE SIZE, pm 

Figure 14. Performance of Silicon-Product Withdrawal System 

37 
The rate of siliccn production is a function of silane concentration, 

seed particle size, and the U/Umf ratio. Figure 15 shows that in the 6-in. 

FBR, silicon production rates of 1.5 kg/h at 30% silane concentration, and 

3.5 kg/h at 80% silane concentration, are achievable. 

Most of the JPL experiments with the 6-in. FBR were of short duration 

(4 h). The particles were grown with deposition thicknesses of approximately 

10 to 20 p. 

Typical surface and deposition layer morphologies of the silicon 

products are shown in Figure 16. The SEM pictures show that the grown layer 

appears to be made up of small diameter particles with a large porosity or 

vo idage. 

2. Mechanism of Silicon Deposition 

In an FBR, silane is decomposed and silicon is formed via two 

major paths (Figure 17): 

(1) Heterogeneous chemical vapor deposition (CVD) takes place on the 

surface of seed particles in the emulsion phase of the bed. 

(2) Homogeneous gas-phase pyrolysis occurs forming condensable silicon 

species in the bubble phase of the bed leading to the formation of 

nuclei (embryos). The general size of the initial embryos, 

according to nucleation theory, was estimated to be 50 A. 

IOM CP-381-76-31), Hsu/Praturi to Lutwack, "Application of 

Nucleation Theory to Silicon Seed Growth," March 1976). 

(JPL 

The tiny silicon nuclei move about within the bubbles by Brownian motion 

and fuse by the action of van der Waals forces (path 3 in Figure 17). 

According to Levenspiel (Reference 14), the growth rate of a nucleus with a 

radius r, in this regime, is found to be: 

where : 

k is the Boltzmann constant 

p is the viscosity of a nucleus in the gas stream 

fs is the volume fraction of solids in the gas-solids mixture 

T is temperature in OK. 

By the time it takes nuclei to grow to 0.3 pm size (cO.1 SI, the 

nuclei are so few and far between (from the standpoint of Brownian Motion) 

that the growth rate drops by a factor of The mass of fine particles 

that results at this stage of nuclei coalescence is called coccal silicon. As 

a result of bubble distortion and the kneading action of fluid eddies in the 

fluidization medium, these coccal silicon fines may come out of the gas bubble 

into the emulsion phase. 

38 
4.0 

3.0 

'z u) 

5 

8 

e5 

w 

z 2.0 

0 

V 

3 

- 

Ie 

P 

1 .c 

C 

0 

I I I I 

/ PARTICLE SIZE = 250 pm 

U/Umf = 4-5 

0.2 0.4 0.6 0.8 1 .o 

FEED SILANE CONCENTRATION 

Figure 15. Experimental Silicon Production Rate Versus Silane Concentration 

39 
ORIGINAL PAGE IS 

OF POOR QUALITY 

40 
INCORPORATION 

OF 

.ON SEED 

CVD HOMOGENEOUS FINES EMU LS ION 

ON 

SEED 

ON 

F lNES 

Si 0 

COALESCENCE 

Figure 17. Reaction Paths for Silane Pyrolysis in a Fluidized-Bed Reactor 

41 
Within the bubbles. the coccal silicon fines coagulate into clusters. 

The size or length of a cluster grows as the residence time of the bubble in 

the bed increases. 

collision, the product is not densely packed. The resulting cluster is light 

in density and travels with the gas bubble. At the top of the bed, bubbles 

burst and a fraction of the silicon, typically <lo% of the silicon input in 

the forms of clusters and residual coccal silicon fines, is elutriated and 

collected in the exit filter. Limited by the terminal velocity of the 

operation of the fluidized bed, the maximum size of the elutriated clusters is 

about 10 pm under these experimental conditions. 

Since coagulation is a consequence of solid particle 

In the emulsion phase, the seeds sweep up the fine particles made up of 

both coccal silicon and initial clusters released from the bubbles. This 

phenomenon is called scavenging and has been reported in the FBR-literature as 

an effective means for the removal of fine particulates in gas-solid fluidized 

beds (References 15,16 1. 

Based upon our data, it appears that the following criteria must be met 

to incorporate fines: 

produc 

ground 

t-si1 

and 

The temperature of the scavenging region must be higher than the 

Tammann temperature2 of silicon (defined as 0.52 times the 

melting point of silicon in OK). This threshold temperature is 

610°C, or 40°C below the 650°C empirically-determined, optimum 

operating temperature of the FBR. It represents the minimum 

seed-particle surface temperature required to incorporate fines. 

As shown by SEM pictures in earlier work (Reference 171, 

scavenging of fines at temperatures below the Tammann temperature 

leads only to loose adhesion. 

The CVD reaction serves to cement the homogeneous fines into the 

growth. This surface adsorption of homogeneously nucleated 

particles and simultaneous heterogeneous deposition is similar to 

the model proposed in the carbon deposition literature 

(References 19,201. 

Fines, mostly coccal silicon, grow by heterogeneous chemical vapor 

deposition in the emulsion phase as represented by path 6 in 

Figure 17. Because the rate of scavenging by the silicon seeds is 

relatively fast and the surface area of coccal silicon versus that 

of the seed particles is small, the CVD growth of coccal silicon 

is generally considered to be an unimportant deposition process. 

Product Morphology 

To determine product morphology, the fluidized bed fines and 

.icon particle samples were mounted in resin and (1) ground, (2) 

etched, (3) polished, and (4) polished and etched before SEM 

ZTammann temperature is an index for surface mobility. It is the minimum 

temperature at which a solid undergoes solid-solid interactions such as a 

transformation from a non-wetting to a wetting condition (Reference 18). 

42 
examination. Etch compos:tions used were the Sirtl, Dash, and a proprietary 

Schwuttke formulation (personal communication, G.H. Schwuttke). The x-ray 

diffraction method was used to ascertain the structure of the product and the 

fines. Sharp lines confirmed the crystallinity of both. The samples evaluated 

were produced in runs in which the feed gas composition ranged from 6.24% 

SiHq in H2 to pure silane. 

all feed gas concentrations except the lowest concentration of 6.24%. The 

latter was run at 920°C. 

Bed temperature was controlled at 650°C for 

The surfaces of the product particles are characterized by a coating of 

0.1 to 4 pm diameter particles and continuous nodular growth features: 

15-20 pm diameter nodules are observed in the sample deposited at 920°C 

(Figure 18a). The nodules propagate outward from the surface of the growing 

layer. The spacing between the nodules extends downward as continuous pores 

or channels into the deposited layer. A fractured cross-section of the 

product particles shows the nodules to be continuous with the grown layers 

(Figure 18b). The cross-section also reveals that the continuous porosity in 

the grown layer decreases from the particle surface inward toward the 

grown-layer/seed interface. The channel diameter is smaller (typically <1 p) 

near the particle surface , and larger (1.5 to 2 pm) further into the 

deposited layer. The walls of the channels also exhibit the nodular growth 

habit (Figure 18c). In some samples, the dense bulk of the deposited layer 

reveals no apparent structure as cleaved (Figure 18b), while in others a 

granular structure is revealed by a more dispersed reflectivity. When etched 

with Dash etch, the deposited layers appear to be made up of 0.1 to 0.2 pm 

diameter particles cemented together. The Schwuttke etch also reveals 0.2 pm 

diameter particles in the channel walls. A distinct discontinuity occurs at 

the seed/grown-layer contact (Figure 18C). None of the seed structure 

propagates into the deposited material. Etch rates at the contact are 

exaggerated. Nearly continuous horizons of pores parallel to the original 

seed surface are occasionally observed in the deposited material suggesting an 

interruption and subsequent resumption of the deposition process. 

The appearance of the surface of the fines is the same as that of the 

deposited layers on the seeds (Figure 18d). A sample of fines was mounted, 

lapped, polished and etched for 5 s in a standard Sirtl etch. After etching, 

the sample was identical in appearance to growth areas on the seed material. 

The sample is porous with the matrix between the holes appearing to be made up 

of particles 0.2 to 0.4 pm in diameter. The size of the fines ranged from 5 

to 15 p in diameter. Approaching the particle surface, the pores in the 

fines sample increased in number and decreased in diameter. In one analyzed 

sample, the largest pore, 6 p in length x 1.5 pm in width, was near the 

center. The average diameter of the pores is about 1.5 p near the surface 

of the sample. 

The continuity of the nodular growth with the grown layer supports the 

conclusion that the surface features are the products of a heterogeneous 

nucleation and deposition process. That the bulk of the grown layer appears 

to be made up of small diameter particles supports the concept of scavenging 

of homogeneously nucleated material. Thus, to explain all the textures 

observed, the concept can be invoked of particle capture with subsequent 

cementation and nodular growth. 

43 
n 

W 

u 

w 

w 

0 

4-1 

0 

44 
4. Experiments fcr Further Control of Fines Formation 

A small portion of homogeneous nuclei formed from the silane 

pyrolysis is not scavenged by the seed particles and escapes the bed as 

fines. This partly may be caused by the unavilability of silane at the top of 

the bed to cement the absorbed homogeneous nuclei by chemical vapor 

deposition. A series of experiments was run to demonstrate that: 

(1) Fines formation can be reduced further by providing added 

cementation by a secondary source of silane admitted at a suitable 

bed location. 

(2) The scavenging phenomenon does exist under fluidized-bed 

conditions. 

An ideal bed height of 8 in. was calculated to be necessary for 99% of 

the silane to react. This calculation is based on published homogeneous 

(Reference 21) and heterogeneous (Reference 22) rate constants, and the 

reactor conditions of 650' bed-temperature, 50% primary silane feed and a 

total volumetric flow rate of 40 Qlmin. A 50% increase in reaction length 

was assumed because of the non-ideality of the fluidized bed. This resulted 

in an actual 99% reaction-length of 12 in. The secondary silane injection 

point was located 13 in. above the distributor (Figure 2). Total static bed 

depth was 25 in. Based on the cluster growth rate equation (Reference 171, 

the cluster size is calculated to be approximately 0.8 pn at the 13 in. 

level above the distributor. 

Control and scavenging experiments were conducted using the reactor 

conditions given above. A bed height of 25 in. was used to allow for 

secondary scavenging of silicon clusters. The secondary silane injection for 

the control run consisted of 12 Q/min of hydrogen. Two Q/min of silane 

and 8 Q/min of hydrogen were used in the scavenging experiment. The data 

are shown in Table 2 as Run Numbers 204 and 206, and in Table 5. The data 

indicate that the elutriated fines were reduced from 10% (control) to 6% 

(scavenging) with a corresponding increase in silicon deposition efficiency in 

the bed from 92 to 95.5%. 

increased by 0.1% in the control experiment, while they were reduced by 1% in 

the scavenging experiment. The product morphology studies, and these 

experiments, strongly support the proposed silicon-deposition mechanisms. 

The particles of less than 75 pn in diameter were 

5. Performance of a Quartz Liner 

The performance of a quartz liner mounted inside the stainless 

steel reactor was evaluated based on the following criteria: 

(1) The time required to reach a steady-state bed temperature of 

650°C, when an 11 kg batch of 250 pm average diameter silicon 

particles is fluidized with hydrogen at a U/Umf ratio of 4 

(Table 2, Run Number 300). 

(2) The maintenance of the bed temperature during pyrolysis using 50% 

silane in hydrogen (Table 2, Run Numbers 402, 406 to 408). 

45 
I3 

n 

0 

Z 

& 

h 

.. p. 

- E 

3. n 

m 

Z 

Lu 

8 

46 
(3) The maintenance of the proper bed-temperature profile necessary to 

eliminate deposition of silicon on the inside quartz liner wall. 

If this parameter is not properly established, the quartz liner 

will break during thermal cycling due to the differential thermal 

expansion between quartz and silicon. 

It takes approximately 3 h to reach the steady-state bed temperature of 

650°C. This is about 1 h longer than is required if silicon is directly 

heated through the stainless steel wall (no quartz liner). Once the required 

bed temperature is reached, however, its maintainability is fairly good, and 

is complemented by the exothermic reaction (-5 kcal/mole) of the silane 

pyrolysis. 

The bed-temperature profile is shown in Figure 19. Bed temperatures are 

400°C and 650°C at 1/2 in. and 2 in. from the gas distributor, respectively. 

The temperature of the outer wall of the quartz liner was within 5°C higher 

than the bed temperature. This temperature profile is very desirable because 

it facilitates deposition of silicon on the seed particles, but not on the 

liner wall. During our experiments, quartz liners were exposed to silane 

pyrolysis at 650°C for a duration of up to 4 h. The quartz liner did not 

crack 90% of the time. 

or the outside of the liner wall. 

No silicon deposition was found on either the inside 

The silicon mass balance data, involving both silicon in the bed and 

elutriated as fines, were similar to the ones obtained using no quartz liner 

(Table 2, Run. Numbers 406 to 408). 

6. Product Purity 

The purity of the FBR product was determined for two cases: 

(1) The FBR was operated without a liner, and silicon particles were 

exposed to the hot, stainless steel-reactor wall. 

(2) The FBR was operated with a quartz liner. This isolates the 

silicon particles from the stainless steel-reactor wall. 

For the non-liner case, the concentrations of metallic impurities in the FBR 

product are shown in Table 6. These data indicate that the silicon product is 

heavily contaminated with metallic impurities. The iron, chromium, and nickel 

impurities are approximately 70, 17, and 12% of the total identified 

impurities. Interestingly, these proportions are similar to the composition 

of the stainless steel 316 reactor-wall material. It is evident, therefore, 

that hot silicon particles physically removed the stainless steel wall 

materials and the metallic impurities are incorporated onto the seed silicon 

durhg the silane pyrolysis. 

For the case with the quartz liner, the metallic impurities in the FBR 

product are shown in Table 7. The use of a quartz liner as a barrier between 

the silicon particles and the stainless steel reactor wall helped to eliminate 

metallic contamination in the product silicon during fluidized-bed 

processing. The metallic impurities in the product silicon generally are less 

47 
700 

600 

500 

v 

0 

W 

oc 

4 400 

2 

4 fx 

ul 2 300 

W 

I200 

100 

0 

GAS DI: 

LINER: QUARTZ MOUNTED INSIDE 

STAINLESS STEEL REACTOR 

IN HYDROGEN 

GAS: HYDROGEN AND 50% SlLANt 

I I I I I I I I I I I I 

2 4 6 8 10 12 14 16 18 20 22 24 

'RIBUTOR 

BED HEIGHT, IN. 

Figure 19. Temperature Profile in 6-in. Fluidized-Bed Reactor 

Fitted with Quartz Liner 

48 
than the detection capability limits of spark source mass spectroscopy. The 

purity of the deposited layer of silicon is equal to or better than that of 

the starting silicon seed material. 

7. Solar Cells 

For both the non-liner and quartz liner cases, a Czochralski (CZ) 

single-crystal ingot was pulled and sliced to fabricate 2 x 2 cm baseline 

solar cells. For the case without the quartz liner, the ingot size is 4-in. 

diameter x 2-in. high (Figure 20). The resistivity of wafers obtained from 

this ingot is 1.98 R-cm at the top and 1.83 R-cm at the bottom. The 

performance data of these cells are shown in Table 8. The average efficiency 

of these undoped antireflective (AR) coated solar cells is 12.1 to 12.5% 

compared to the 13.9% obtained by using semiconductor-grade silicon of 

2 O-cm resistivity. 

For the case with the quartz liner, the ingot size is 2-in. diameter x 

3-in. high (Figure 21). The 18 mil wafers prepared from this ingot have 

resistivities varying from 23 to 34 ohm-cm. 

are 11.6 and 7.4 ppma, respectively. X-ray diffraction analysis shows this 

ingot has a single crystal structure and orientation of [lo01 type. The 

performance data of the 2 x 2 cm cells prepared from this material are shown 

in Table 9. The average efficiency of these undoped AR coated solar cells is 

approximately 11 to 12%. The control solar cells from 30 Q-cm 

semiconductor-grade silicon were not prepared. 

The oxygen and carbon impurities 

49 
Q tr) 

3000 

?77Y 

n 

w. 

09 0 ?hoqw 

3om99 

000 

Tc?Y 

W 

tr)h 

hCOO\ 

n-- 

I I I 

z 

0 

rn 

I6 -0 

-I50 
Table 7. Purity of Product from a Fluidized-Bed Reactor Equipped 

with a Quartz Liner (ppma) 

ELEMENTS 

P 

Fe 

Cr 

Ni 

cu 

Zn 

co 

Mn 

Nu 

Ms 

AI 

S 

K 

Ca 

JET MILLED AND ACID 

CLEANED SEED FOR FBR 

SSMS2 

0.2 

10.6 

0.03 

~0.5 

5 0.02 

10.04 

50.1 

IO .a2 

10.1 

1 

0.05 

C1 

CO.1 

0.1 

FBR PRODUCT' 

SSMS 

0.1 

10.6 

< 0.02 

0.5 

< 0.02 

c 0.04 

50.1 

< 0.02 

50.1 

C1 

0.05 

<1 

(0.1 

0.1 

1 ' RUN NO. 502 AND 503 

SPARK SOURCE MASS SPECTROSCOPY CONDUCTED BY THE NORTHERN 

ANALYTICAL LAB 

51 
Ln 

rl 

rl 

0 

3 

H 

52 
co n 

Rl x R R 

I? 

n 

cu 

n 

0 

0: 

b M 

n 

9 " 9 co " 

53 
54 
cv 

'? 

P 

e 

I 

0: 

0 

e 

9 

c 

c 

I 

f 

c 

c 

c 

0 

n 

n -=I. 

0 0 

R. 

P 

co 

N 

I 

00 

n 

cv 

@? 

v! 

n 

m 

10 

I 

9 

cv 

m 

'? 

9 

cr) 

lr) 

I 

c'! 

cr) 2 

c? 

: 

I 

9 

n 

m 

0 

cv. 

x 

R 

co 

cv 

c v) 

cr) 

N 

c'! c 

P 

c3 

m 

55 

SECTION VI 

CONCLUSIONS 

The following conclusions are drawn from the recent FBR experimental 

Silicon-seed particles can be prepared from 2 rmn silicon particles 

by using the JPL-developed seed-generation device. A one-pass 

yield of 15.8% (-75 to +355 pm particle diameter) has been 

achieved. 

Because the prepared seed particles require cleaning, a fluidizedbed, silicon-seed cleaning device has been developed and the 

purity of the seed particles has been established. 

The mechanism of the deposition of silicon from the pyrolysis of 

silane in an FBR is described by a six-path process: 

heterogeneous deposition, homogeneous decomposition, coalescence, 

coagulation, scavenging, and heterogeneous growth on fines. 

The scavenging of homogeneously nucleated silicon by silicon 

particles was experimentally achieved by using a secondary silane 

injection. 

The selective removal of large particles of product silicon has 

been demonstrated. 

It has been demonstrated that more than 90% of the silicon fed to 

the reactor is deposited on the silicon seed particles. Less than 

10% of the silicon feed ends up as elutriated silicon fines. 

The beneficial performance of a quartz liner in an FBR is 

established. With operating conditions defined (temperature, 

profile, etc.), the quartz-lined FBR was successfully operated at 

up to 50% silane concentration for a duration of 4 h. 

The purity of the silicon produced in a quartz-lined FBR was 

established. The metallic impurities are less than the detection 

limits of spark source mass spectroscopy. 

Solar cells fabricated from the silicon produced in a FBR with no 

quartz liner had an efficiency of approximately 12.5% and a 

resistivity of 2 ohm-cm. 

Solar cells fabricated from the silicon produced in a quartz-lined 

FBR had a resistivity of 23 to 34 ohm-cm and an efficiency of 

approximately 11 to 12%. 

57 

SECTION VI1 

RECOMMENDATIONS 

Although the JPL in-house research program demonstrated a better 

understanding of the fluidized-bed silane process, the following engineering 

problems remain to be solved: 

(1) How to operate an FBR for a long duration to reach a steady-state 

with respect to the particle-size produced. In this mode, seed 

particles of 200 pm diameter would be fed continuously to the 

FBR and product particles of 1 mm diameter would be withdrawn 

continuously from the FBR. 

(2) A demonstration is needed that the silicon product produced in the 

FBR is of semiconductor-grade. 

(3) A pilot-plant demonstration of the FBR process is needed. 

59 

SECTION VI11 

REFERENCES 

1. 

2. 

3. 

4. 

5, 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

DOE/JPL Low-Cost Solar Array Project Annual Operating Plan for Fiscal 

Year 1980, JPL Document 5101-127, Jet Propulsion Laboratory, Pasadena, 

California, August 31, 1979. 

"National Photovoltaics Program Five Year Research Plan,: 1984-1988; 

Photovoltaic Energy Technology Division, U.S. Department of Energy, 

Washington, D.C., May 1983. 

Lutwack, R. "A Review of the Silicon Material Task," JPL Publication 

84-24, February 1, 1984. 

"Proceedings of the Flat-Plate Solar Array Workshop on the Science of 

Silicon Material Preparation," JPL Publication 83-13, Jet Propulsion 

Laboratory, Pasadena, California, February 1, 1983. 

"Feasibility of the Silane Procesi for Producing Semiconductor- Grade 

Silicon," Final Report Phases I and 11, DOE/JPL 954334- 79/10, Union 

Carbide Corporation, Washougal, Washington, June 1979. 

"Experimental Process Development Unit for Producing Semiconductor-Grade 

Silicon Using the Silane-to-Silicon Process," Final Report, Phase 111, 

DOE/JPL 954334-21, Union Carbide Corporation, Washougal, Washington, 

1983. 

Mui, Y.P., "Investigation of the Hydrogenation of SiCl4," Final 

Report, DOE/JPL 955382-79/8, Massachusetts Institute of Technology, 

Cambridge, Massachusetts, April 1981. 

Mui, Y.P., "Investigation of the Hydrogenation of SiC14," Final 

Report, DOE/JPL 956061, Solarelectronics, Inc., Bellingham, 

Massachusetts, 1982. 

Kim, K., et al., "Modeling of Fluidized-Bed Silicon Deposition Process," 

JPL Publication 77-38, Jet Propulsion Laboratory, Padadena, California, 

June 1977. 

Praturi, A., et al., "Modeling of Silicon Particle Growth," JPL Document 

5101-105, Jet Propulsion Laboratory, Pasadena, California, April 15, 

1979. 

Hogle, R., et al., "In-House Study: Fluidized-Bed Silane Pyrolysis," JPL 

Document 5101-89, Jet Propulsion Laboratory, Pasadena, California, 

December 1, 1978. 

Rohatgi, N.K., e.t al., "Silane Pyrolysis in a Fluidized-Bed Reactor," 

Proceedings of the 3rd Symposium on Material of New Processing 

Technologies for Photovoltaics, J. Electrochem. SOC., Vol. 82-8, 

pp. 477-487, 1982. 
13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

Rohatgi, N., and Hsu, G., "Silicon Production in a Fluidized-Bed 

Reactor: A Parameter Study," JPL D-1283, Jet Propulsion Laboratory, 

Pasadena, California, October 1983. 

Levenspiel, O., "The Chemical Reactor Omnibook," Oregon State University 

Bookstore, Inc., Corvallis, Oregon, 1979. 

Peters, M.H., Fan, L.S., and Sweeney, T.L., AIChE J, Vol. 28, No. 1, 39, 

1982. 

Tardos, G., Gutfinger, C., and Abuaf, N., Isr. J. Technol. Vol 12, 184, 

1974. 

Hsu, G., Hogle, R., Rohatgi, N., and Morrison, A., J. Electrochem. SOC., 

Vol. 131, No. 3, p. 660, March 1984. 

Baker, R.T.K., J. Catal., Vol. 78, 473, 1982. 

Bokros, J.C., chapter entitled "Deposition, Structure and Properties of 

Pyrolytic Carbon," in Vol. 5, Chemistry and Physics of Carbon: P.L. 

Walker, Jr. (Ed.), Marcel Dekker Inc., New York, 1969. 

Kaae, J.L., Gulden, T.D., and Liang, S., Carbon, Vol. LO, p. 701, 1972. 

Hogness, T., Wilson, T.L., and Johnson, W.C., J. Am. Chem. SOC., Vol. 

58, 108, 1936. 

Iya, S., Flagella, R.N., and dePaolo, F., J. Electrochem. SOC., 129. 

No. 7, p. 1531, 1982. 

62 
APPENDIX 

TECHNOLOGY TRANSFER 

The JPL in-house technology developed for the fluidized-bed 

silicon-deposition process was transferred to the interested technical 

community outside of JPL by making informed presentations to visitors, 

presenting papers at technical meetings, and publishing reports and papers in 

technical journals. A brief summary of these activities is outlined as 

follows : 

A. VISITOR AND VISITS 

1985 

Three Japanese scientists, S. Shigeta of the New Energy 

Development Organization, K. Kaneko of Shin-Etsu Chemical Co., 

Ltd., and T. Saitoh of Hitachi, attended the 25th Project 

Integration Meeting (PIM) and held discussions with Project 

personnel on the direction of the Japanese and American 

crystalline-silicon solar energy programs. 

On February 21, R.G. Boykin and Pete Browning of the Great Lakes 

Chemical Corp. visited JPL's in-house fluidized-bed reactor lab 

and discussed its current status with George Hsu (JPL). They 

indicated that Great Lakes Chemical is actively investigating its 

expansion into the silicon business. 

T. Noda, T. Shirai, and M. Tshi of Osaka Titanium Corp. (OTC), of 

Amagasaki Hyogo, Japan, visited R. Lutwack, et al., on February 13 

to discuss Japanese technology for polysilicon production. 

1984 

On August 27, JPL was visited by Dr. J. Brian Clark, Chief 

Director of the Council for Scientific and Industrial Research, 

National Institute for Materials Research, Pretoria, South 

Africa. The purpose of his visit was to discuss the status of 

silicon-refinement technology development within the FSA project 

and in general. During his visit, Clark met with A.D. Morrison 

and R. Lutwack, FSA Advanced Materials Research Task. 

Professors S. Morimoto and Dr. Y. Shimizu, both of Tokushima 

University, Japan, visited JPL on August 21. Professor Morimoto 

described catalyst research for chlorosilane redistribution, and 

Dr. Shimizu discussed fluidized-bed reactor technology. 

A-l 
foshio Noda, Osaka Titani 0. (OTC), Japan, visited JPL on 

May 24 

production of 10% efficient cells. In 1985, th 

raised to 12% efficient cells. 

Toshio Shraiwa and Michi Ishiguro of the Osaka Titanium Co. (OTC), 

Japan, visited the JPL facilities. 

T. Fitzgerald, Rossmore, California, a consultant to JPL, and 

J. Routbort, Argonne National Laboratory, Argonne, Illinois, 

visited the JPL laboratories. 

Four visitors from Union Carbide Corp., Washougal, Washington, 

were J. Young, Director of Marketing, Polysilicon Dept., 

J. Morihara, Manager of Technology, Electronics Division, S. Iya, 

Program Manager for the JPL contract for FBR research, and 

J. Lorenz, a consultant to UCC from Scottsdale, Arizona. 

On November 7, J. Truitt and A. Carter of Texas Instruments met 

with FSA personnel to discuss FBR technology for the silane and 

trichlorosilane systems. Reactor designs, operating conditions, 

problems, and successes of these two systems were discussed. 

Texas Instruments is supporting in-house research on the 

trichlorosilane FBR process. 

T. Knuuttila, Director of Kemira of Finland, and E. Kariola, 

Attache of the Consulate General of Finland in Los Angeles, met 

with Project personnel on November 8 for a general discussion of 

polysilicon technology and market projections, One of the main 

topics was silicon production involving the use of fluosilicic 

acid as the raw material. 

A-2 
?our representatives o 

FSA facilities and lab 

held with numerous FSA person 

representatives were B. Rus 

P. Meyer, Manager of P 

On February 17, at the req 

members of the FSA Pro 

current state of the a 

expected advancement in the next decade. The visitors included 

Dr. Dominguez of the Spanish Embassy in Washington, D.C., 

Drs. Tanarro and Ortin, Ministry of Industry and Energy, and 

Dr. Oistrach of the National Institute of Industry. 

A discussion of the silicon-refinement processes that use FBR 

technology was held on May 20 with representatives of OTC, Japan. 

OTC is using-trichlorosilane in a quartz FBR and is experiencing 

problems with silicon deposition on the walls. In both the 

FSA-DOE-sponsored efforts at JPL and UCC, silane is used in 

metal-wall reactors. Under the proper operating conditions, 

silicon deposition on the walls has not been a problem. The 

visitors toured JPL's FBR research facility, where the silane 

pyrolysis work is being conducted as part of the FSA Silicon 

Materials Thrust. Participants in the discussions were 

Toshio Noda, OTC's chief operating officer, Michi Ishiguro, OTC's 

international affairs manager, and R. Lutwack, G. Hsu, and 

N. Rohatgi of the FSA Advanced Materials Research Task. 

On March 31, F. Yager of the Fraunhofer Institute for Systems and 

Innovation (ISI) Research, Karlsruhe, and A. Rauber of the 

Fraunhofer Institute for Solar Energy Systems (ISE), Freiburg, 

Federal Republic of Germany, visited the following JPL personnel: 

W. Callaghan, Manager of the FSA Project; P. Henry, Manager of the 

PAM Area; and R. Lutwack, Advanced Materials Research Task. They 

discussed prospects for photovoltaic applications in the United 

States and Europe. During the year, IS1 and ISE will be working 

on an assessment of the future of photovoltaic solar-energy 

conversion, including a review of the various photovoltaic 

development efforts, and the technical and economic aspects of 

Governmental support measures. The visitors toured the laboratory 

where JPL's FSA-supported in-house work on pyrolysis of silane in 

FBRs is being conducted. 

S. Timsit, of Alcan Research Laboratory, Kingston, Ontario, 

Canada, visited R. Lutwack, Advanced Materials Research Task, on 

April 13 for an exchange of information on the t 

crystal growth and the physics of surfaces. Alc 

planning stage of setting up a laboratory devote 

research, which Dr. Timsit will direct. K. Duma 

D. Burger, Photovoltaic Components Research Area, led a tour of 

ratories for solar cell fabrication and material 

ion. 

A-3 
Stephen Lord, J.C. Schumacher Company, Oceanside, California, 

visited R. Lutwack and G. Hsu, Advanced Materials Research Task, 

on April 15 to discuss silicon refinement using FBR technology. 

Schumacher is a producer of dopant chemicals for the semiconductor 

industry and has been developing a bromosilane process for 

polysilicon production. FBRs have been his first choice for both 

the bromosilane formation and silicon deposition steps. Lard 

toured the JPL laboratory where FSA-supported work on silane 

pyrolysis in FBRs is being conducted. 

1982 

J. Bezuidenhoudt and A. Hobkirk of the South African Industrial 

Development Corp., accompanied by G.H. Schwuttke of IBM, visited 

JPL on May 3 to discuss the two silicon-production processes 

(Hemlock's dischlorosilane process, and UCC's silane-to-silicon 

process) that are under development as part of the FSA Silicon 

Material Task. 

On May 29, three representatives of OTC, Japan, T. Noda, Chief 

Operating Officer and First Executive Vice-president, M. Ishiguro, 

Overseas Manager, and T. Shirai, Manager of Patent and Technical 

License, met with R. Lutwack, A. Briglio, and G. Hsu, of the FSA 

Silicon Material Task, and discussed silicon-production processes. 

A miniworkshop on "Science of Silicon Material Preparation" was 

held on December 9 and 19 at San Diego State University, 

California, to discuss the structure of a research program on the 

phenomena of silane decomposition. The workshop was chaired by 

R. Lutwack, FSA Advanced Materials Research Task. A series of 

experiments was proposed to identify the chemical path by which 

silicon is deposited from silane, and to identify the conditions 

under which fines are formed and collected in FBRs. Possible 

methods for preparing reactor seed particles were also proposed. 

1981 

No reported visits or visitors. 

1980 

Representatives of Allied Chemical Corporation met with members of 

the Low-Cost Silicon Solar Array (LSA) Project's Silicon Material 

Task to discuss polysilicon process technology. 

Prior to 1980 

Visits or visitors were not documented. 

A-4 
B. TECHNICAL MEETINGS 

A paper titled "Chemical Vapor Depositi 

Pyrolysis," by A.K. Praturi was present 

Conference on Chemical Vapor Deposition, 

A paper titled, "Silane Pyrolysis in a Fluidiz 

Rohatgi, G. HSU, and R. Lutwack was presented 

Electrochemical Society, Montreal, Canada, May 

"Fluidized-Bed Silicon Deposition," by G. Hsu, A. Morrison, N. Rohatgi, 

R. Lutwack, and T. MacConnell, was presented at a poster session at the 

17th IEEE Photovoltaic Specialists Conference, Orlando, Florida, May 

1-4, 1984. 

A paper titled "Silicon Particle Growth in a Fluidized-Bed Reactor,'' 

written by G. Hsu, N. Rohatgi, and J. Houseman, was presented at the 

AICHE Annual Meeting, San Francisco, California, held November 25-30, 

1984. The paper described models for silicon particle growth and their 

good correlation with experimental data. 

Progress reports were presented at every DOE FSA-PIM. These reports are 

available from Solar Data Center, Jet Propulsion Laboratory, Pasadena, 

California. 

W. Callaghan and R. Lutwack attended the Sixth European Photovoltaic 

Solar Energy Conference on April 15-19, 1985, in London, England. W. 

Callaghan participated in a panel discussion on "Material Problems of 

the PV Industry," and presented a paper titled "Flat-Plate Solar Array 

Project: Progress and Plans." R. Lutwack cochaired a session on 

crystalline silicon and presented a paper titled "New Polycrystalline 

Silicon Refining Technology and the Silicon Supply Outlook for the 

Photovoltaics Industry." 

C. PUBLISHED REPORTS AND TECHNICAL PAPERS 

"Modeling of Fluidized-Bed Silicon-Deposition Process," by K. Kim, G. 

Hsu, R. Lutwack, and A. Praturi, JPL Publication 77-38, Jet Propulsion 

Laboratory, Pasadena, California, June 15, 1977. 

"Modeling of Silicon Particle Growth," by A. Praturi, G. Hsu, and 

R. Lutwack, JPL Document 5101-105, Jet Propulsion Laboratory, Pasadena, 

California, April 15, 1979. 

"Chemical Vapor Deposition of Silicon from Silane Pyrolysis," by 

A. Praturi, Proceedings of the Sixth International Chemical Vapor 

Deposition Conference, L.F. Donaghey, P. Raichoudhery, and R.N. Tanber, 

(Eds.), pp. 2024, The Electrochemical Society, Inc., Princeton, New 

Jersey, 1977. 

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"In-House Study: Fluidized-Bed Silane Pyrolysis," by R. Hogle, G- Hsu, 

and R. Lutwack, JPL Document, 5101-89, Jet Propulsion Laboratory, 

Pasadena, California, December 1, 1978. 

A paper titled "Silane Pyrolysis in a Fluidized-Bed Reactor," written by 

N. Rohatgi, G. Hsu, and L. Lutwack, FSA Advanced Materials Research 

Task, was published in the Proceedings of the Third Symposium on 

Materials and New Processing Technologies for Photovoltaics, Vol. 82-8, 

p. 477-487, 1982. This symposium was held in connection with the 161st 

Electrochemical Society Meeting at Montreal, Canada, May 10-14, 1982. 

JPL Publication 83-13, dated February 1, 1983, titled "Proceedings of 

the Flat-Plate Solar Array Workshop on the Science of Silicon Materials 

Preparation," documented the workshop held August 23-25, 1982. The 

report was edited by R. Lutwack, Advanced Materials Research Task, and 

is part of the Silicon Materials Thrust. 

"Silicon Production in a Fluidized-Bed Reactor: A Parameter Study," by 

N. Rohatgi and G. Hsu, JPL D-1283, Jet Propulsion Laboratory, Pasadena, 

California, October 1983. 

A Review of the Silicon Material Task," R. Lutwack, JPL Publication 

84-24, Jet Propulsion Laboratory, Pasadena, California, February 1, 1984. 

"Fines in Fluidized-Bed Silane Pyrolysis," by G. Hsu, R. Hogle, N. 

Rohatgi, and A. Morrison, published in the Journal of the 

Electrochemical Society, Vol. 131, No. 3, March 1984. 

"Fluidized-Bed Silicon Deposition," by G. Hsu, A. Morrison, N. Rohatgi, 

R. Lutwack, and T. MacConne11, published in the Proceedings of the 

Seventh IEEE Photovoltaic Specialists Conference, pp. 553-557, May 1-4, 

1984. 

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