Table of Contents

Certificate i

Acknowledgements ii

Abstract iii

Table of Contents iv

List of Figures v

List of Tables v

1 INTRODUCTION 1

1.1 An Overview on Chemical Vapor Deposition 1

1.2 Silicon and its Allotropes 2

1.3 Silicon as a Semiconductor 2

1.4 Motivation 3

1.5 Objectives 3

1.6 Scope of the Project 3

1.7 Structure of Research Proposal Report 3

2 LITERATURE SURVEY 4

2.1 Introduction 4

2.2 Precursors for Polycrystalline Silicon Production 4

2.3 Metallurgical Grade Silicon and its Conversion to Electronic Grade 4

2.4 Alternative Methods to Produce Polycrystalline Silicon 6

2.4.1 Fluidized Bed Reactor 6

2.4.2 Metallurgical Method 6

2.5 CVD Reactor for the Deposition of Polysilicon 6

2.6 Design Considerations for CVD Reactor System 10

2.6.1 Effect of Pressure Inside the Reactor 10

2.6.2 Effect of Reactor Wall Temperature 11

2.6.3 Effect of Flow Rate 11

2.7 CFD Modeling of CVD Reactor 12

3 RTD STUDIES OF BELLJAR REACTOR Error! Bookmark not defined.

3.1 Introduction 13

3.2 RTD Studies for Various Reactor Configurations 13

4 PROJECT OBJECTIVES AND RESEARCH PLAN 28

4.1 Research Plan 28

4.2 Experimental Validation 28

4.3 List of Objectives 29

4.4 Yearly Research Plan 30

4.4.1 Research Plan for First Year 30

4.4.2 Research Plan for Second Year 30

4.4.3 Research Plan for Third Year: 30

References 32

List of Figures

Figure 2.1: Block diagram of EGS production 5

Figure 2.2: Schematic diagram of Siemens bell-jar CVD reactor for production of polysilicon 7

Figure 2.3: The mechanism of dissociation of TCS and deposition of silicon on the substrate 9

Figure 3.1: Behavior of various mixed flow reactors due to non-idealities 13

Figure 3.2: Top view of base plate for four configurations 14

Figure 3.3: Contours of velocity 15

Figure 3.4: Mass fraction of tracer at the reactor outlet for step input 16

Figure 3.5: Reactor with 1 inlet and 1 outlet in parallel with U Rod legs 16

Figure 3.6: Reactor with 1 inlet and 1 outlet in perpendicular to U Rod legs 17

Figure 3.7: Reactor with 1 inlet and 2 outlets in parallel with U Rod legs 17

Figure 3.8: Reactor with 1 inlet and 2 outlets Perpendicular to U Rod legs 18

Figure 4.1: Schematic diagram of experimental set-up for CVD reactor system Error! Bookmark not

defined.

List of Tables

Table 3-1: Comparison of residence times of various configurations 18

Table 4-1: Process Flow data for experimental set-up 29

1

1 INTRODUCTION

1.1 An Overview on Chemical Vapor Deposition

Chemical Vapor Deposition (CVD) is a chemical process which involves reactions that create

solid from gases. In a CVD reactor, gaseous reactant (precursor) is sent over a heated surface

(substrate) where the gaseous precursor gets transformed into solid. If the reaction takes place

on the substrate surface then it forms a film. If the reaction takes place near the substrate

surface then it forms a powder. Among all the deposition techniques, CVD has the ability to

coat a uniform film even on complex substrate surfaces involving holes and trenches. Major

applications of CVD are in thin-film surface coating, producing high purity bulk materials,

wafers and powders. Our present study focuses only on producing high purity bulk material

(polycrystalline silicon). For a CVD reactor to become commercially useful in producing high

purity bulk materials, it must have high deposition rates, uniform deposition of films and less

formation of powder. CVD reactors can be categorized into Hot Wall CVD and Cold Wall

CVD based on the mode of substrate heating (Habuka, 2001). In Hot Wall CVD reactor, the

chamber containing the substrate is surrounded by a furnace which heats the system. The

chamber is loaded with the substrate, heated to the desired temperature and then it is fed with

the precursor gas. It has the advantage of uniform temperature throughout the system and thus

uniform film growth but has the disadvantages of product deposition on walls, higher energy

usage and limitation on furnace material of construction. In Cold Wall CVD reactor, the

substrate is heated (resistive heating) and the chamber walls are cooled. Cold Wall CVD

reactors have the advantages of reduced product deposition on reactor walls, lower thermal

loads on the substrate because of faster heat-up and less energy consumption. It has the

disadvantage of non-uniformities in substrate temperature which leads to non-uniformities in

film thickness. In our present study, these non-uniformities are studied and minimized by

proper temperature control over the substrate surface.

Along with the advantages, CVD has some limitations. The precursor used for the

production of pure material need to be volatile at near room temperature which is non-trivial

for a number of elements in periodic table. The cost of the precursor may be very high. The

precursor and/or the byproducts of CVD may be highly toxic, explosive or corrosive.

2

1.2 Silicon and its Allotropes

Silicon is the second most abundant element on earth’s crust but it rarely occurs in pure

elemental form. Amorphous Silicon (a-Si), Single Crystal Silicon (c-Si) and polycrystalline

silicon are the various forms of silicon. Amorphous silicon lacks in long range order in

structural arrangement of atoms as some atoms in the structure remain un-bonded. Single

crystal silicon will have continuous and unbroken lattice structure with no grain boundaries.

Polycrystalline silicon consists of small grains of single crystal silicon. Single Crystal Silicon

is obtained from polycrystalline silicon through crystal pulling technique. Single Crystal

Silicon (c-Si) is used in electronics and solar cells. The most common solar cell types used in

solar panels are the silicon forms. Single crystal silicon is the most expensive, but is also the

most efficient, so less area is needed. Single crystal cells work better in hot weather and low

light conditions than polycrystalline or amorphous cells. Because they can sometimes be

smaller for the same wattage, they can reduce the wind and gravity load on systems that track

the sun (as opposed to systems that are fixed in one position). Polycrystalline panels, while

having lower efficiencies than single crystal panels, can nonetheless have the same wattage

per square foot as some single crystal panels, because the cells can be rectangular, filling all

of the available surface without the gaps left by round or octagonal cells used in some single

crystal panels. For the application in microelectronics, polycrystalline silicon of purity in the

range of 99.9999999% is necessary. Amorphous silicon (a-Si) is the non-crystalline form of

silicon. One of the main advantages of amorphous silicon over crystalline silicon is that it is

much more uniform over large areas (http://en.wikipedia.org/wiki/Silicon, 2010).

1.3 Silicon as a Semiconductor

Silicon has remained as the most widely used semiconductor till now because of its unique

qualities. Among these are

i) It is elemental, so there is no problem with composition,

ii) It forms a tough, adhering oxide which can be used for isolation and protection,

iii) A large variety of impurity atoms are available for controlling its conduction

properties.

iv) It can withstand to high temperatures

v) It is cheap and abundantly available compared to many other materials

3

Many attempts have been done to find out materials with better properties than silicon

but all of them ( for example Sapphire, silicon carbide, diamond and group III-V elements)

lack in some essential ingredients like ease of growing large perfect crystals, freedom from

extended and point defects, existence of native oxide etc.

1.4 Motivation

The design, manufacturing and operational knowhow of CVD reactor is one of the top

guarded secret of semiconductor industries. Worldwide only few companies have this

technology and they show interest in offering only the product not the reactor. Considering

the huge demand of ultra pure silicon of domestic semiconductor industries, it is high time to

develop this process indigenously.

1.5 Objectives

The main aim of this project is to find out the design parameters and kinetic data for the

production of high purity polycrystalline silicon. This is achieved by the following steps.

First, the flow and thermal analysis of the reactor and manipulation of its design to get the

desired results is done. For this, both CFD studies and experiments will be performed to

compare their results. After this step, reaction enabled CFD will be carried out to find out the

parameters such as feed composition, flow rate, feed temperature, silicon deposition rates etc.

With this idea, reactions will be performed in the pilot plant scale CVD reactor to deposit

polycrystalline silicon. Next step is to analyze the quality of the product polysilicon.

1.6 Scope of the Project

This project proposal is for modeling and simulation studies on CVD reactor system using

CFD (computational fluid dynamics) tools for its optimum design data generation. The

validation of the design data is also in the scope of study.

1.7 Structure of Research Proposal Report

This report is organized into several chapters. A brief overview on chemical vapor deposition

is given in this chapter. The next chapter includes the literature survey on the production of

polycrystalline silicon, various precursors available to deposit polycrystalline silicon in

chemical vapor deposition reactor and the reactions involved in this process. Chapter 3

discusses about the RTD studies of bell jar reactor with variations such as the positions of

inlet and outlet, number of inlets and outlets, feed flow rates etc. Objectives to be achieved in

this project and the yearly research plan are discussed in chapter 4.

4

2 LITERATURE SURVEY

2.1 Introduction

Early polysilicon production processes have been developed by various manufacturing

companies independently. Every company had the concern that if they reveal their optimized

process then the competitors might get the advantage of combining this process with their

own process to develop even better optimized process. Thus the process of producing

Electronic Grade Silicon has become a closely guarded industrial secret and has not been

disclosed till now. So very less literature is available on the production process of polysilicon.

2.2 Precursors for Polycrystalline Silicon Production

Silane (SiH4), Dichlorosilane (SiH2Cl2), Trichlorosilane (SiHCl3) and Silicon tetrachloride

(SiCl4) are the precursors mostly used for the production of polycrystalline silicon. Silane (or

Monosilane, SiH4) has the advantage of easiness in purification to higher degree but has

drawbacks of lower silicon deposition rate due to its low temperature and pressure conditions

which must be used to prevent gas phase nucleation and porous morphology of produced

silicon. Dichlorosilane has the disadvantage of significant polysilicon deposition on reactor

walls if quartz bell-jar is used as a material of construction. Metal bell-jar can be an option but

polysilicon purity is not guaranteed when using metal bell-jar. Silicon tetrachloride is also an

option for precursor but it requires the highest temperature (1200-12500C) and the deposition

rate is low. Among all the precursors, trichlorosilane is the major precursor of industrial

importance (Williams, 2000). Trichlorosilane can be purified to a high degree by fractional

distillation because there is a significant difference in its boiling point from that of impurity

chlorides. It also meets the ppb level purity which is required for polysilicon used in

microelectronics. Boiling point of Trichlorosilane is 31.90C which make it easy to convert it

into gaseous form and use it in CVD reactor. The reaction temperature for Trichlorosilane is

11500C which makes its deposition rate more than that of silane and lesser energy than what

is required for silicon tetrachloride conversion.

2.3 Metallurgical Grade Silicon and its Conversion to Electronic Grade

Metallurgical Grade silicon will have a purity of 98% and is produced in the electric arc

furnaces through carbo-thermal reduction of silica using carbon at a temperature close to

1900oC (Luque and Hegedus, 2003). The major reactions are

5

SiO2 + C Si + CO2 (2.1)

SiO2 + 2C Si + 2CO (2.2)

Formation of SiC and SiO can be avoided by maintaining high concentrations of SiO2. Silicon

thus produced must be purified to much higher level to use it in the manufacture of

semiconductor devices such as integrated circuits, microprocessors, semiconductor detectors,

transistors, solar cells etc. Silicon with a purity of 99.9999999% is said to be of electronic

grade. The schematic process flow diagram for manufacturing of Electronic grade silicon is

shown in figure 2.1. The process consists of hydro-chlorination of metallurgical grade silicon

(MGS) to produce trichlorosilane (TCS), purification of this material via distillation,

preparation of polycrystalline silicon rod by reduction of TCS in a CVD reactor and

conversion of poly silicon ingot into single crystal by float zone crystal pulling (O’Mara et al.,

1990). CVD reactor is the heart of the complete process. Its operation involves handling of

very explosive and hazardous gases like hydrogen and TCS at 1100-1200oC with a very

complex slim rod heating and reactor cooling mechanism.

Figure 2.1: Block diagram of EGS production (Source: http://cnx.org/content/m31994/latest/,

module By Andrew R Barron)

Presence of electrically active elements such as phosphorous and boron greatly reduce

the resistivity of silicon. Boron is a P-type impurity and 1 ppb of boron in silicon can reduce

its resistivity from 230,000 Ω-cm to 285 Ω-cm. Phosphorous is a N-type impurity and 1ppb of

6

it can reduce the resistivity of silicon to 85 Ω-cm (Moltzan and Fyffe, 1970). So these must be

removed from trichlorosilane to prevent the change in resistivity of product silicon.

2.4 Alternative Methods to Produce Polycrystalline Silicon

2.4.1 Fluidized Bed Reactor

Fluidized bed reactor is considered as an alternative to produce high purity polycrystalline

silicon and is exploited industrially in early 80s (Kojima et al., 1989). In this method, heated

silicon granules are released from the top and the precursor is introduced from the bottom of

the reactor. The precursor reacts on the surface of the granules, deposits silicon on them and

thus these granules grow in size. Fluidized Bed Reactors produce silicon with much lower

energy consumption, require less capital investment, crushing of final product is not necessary

and the production process is continuous which makes them much more cost-effective. But

this process also has some drawbacks. Silicon granules need to be heated to high temperature

which is required for trichlorosilane decomposition. It is desirable to have large, dense,

spherical, metallic beads with a monodisperse particle size distribution. Many problems result

from not having this type of product morphology (Stephen and Milligan, 1998). They can be

summarized as impurity gas inclusions, surface oxide impurities, particle frangibility etc.

These result in melting difficulty, developing a tendency to pop like popcorn during melting

and crystal growth reactor contamination.

2.4.2 Metallurgical Method

Apart from the chemical methods to obtain ultra pure silicon, some physical methods are also

available based on the application extractive metallurgy. Many metallic species are highly

soluble in silica but are sparingly soluble in solid silicon (Morita and Miki, 2003). Hence

during the solidification of metallurgical grade silicon these metallic impurities get deposited

along the grain boundaries of polycrystalline silicon. It is possible to remove these impurities

by crushing the MG-Si to a particle size of about 150-400µm and exposing these impurities to

the action of acids like HF or aqua regia. These acids extract out the metal impurities on

prolonged treatment.

2.5 CVD Reactor for the Deposition of Polysilicon

The process of producing polysilicon in bell shaped reactor using trichlorosilane-hydrogen

system is known as Siemens process. The schematic diagram of Siemens CVD reactor is

shown in figure 2.2. The gaseous feedstock is made up of ultra-pure trichlorosilane and

hydrogen gas (TCS+H2) with the TCS mole ratio ranging from 5-15%. The quartz bell-jar

7

encompasses a starting U-shaped slim rod made up of pure silicon. Upon closure of the bell-

jar and beginning of the reactor operation, the jar is purged with pure N2 gas. With the purge

gas remaining, infrared heaters are energized outside the quartz bell-jar to heat the

intrinsically pure seed rods to near 400° C. At this temperature, the slim-rods become

conductive due to thermally generated electrons within silicon. Then, high voltage, typically

thousands of volts, is applied directly to the slim-rods, which are connected to power supply

via electric contacts located at the base plate. By 800° C, the silicon rods have reached the

avalanche breakdown temperature and free electrons in the silicon permit the high-voltage

input to be replaced by a low-voltage and higher current power supply. External heating is

also stopped at this stage. The electrical power supply is maintained continuously, with the

rods surface temperature kept at 1100° C.

Figure 2.2: Schematic diagram of Siemens bell-jar CVD reactor for production of polysilicon

Another option to heat the silicon rods, initially, is via passing of hot gas through the

bell-jar to raise the temperature to 400° C. This method is reported to be slow and expensive.

A careful control of temperature, of the growing rods, is necessary to control the deposition

on silicon rods. This is done via monitoring the temperature using an optical pyrometer,

through a window in the base plate or the bell-jar itself, and using manual or programmed

method to control the same. This is necessary to prevent abnormalities during the long growth

run of the rods, and other extreme events like hot spots, rod core meltdown. At the end of the

run, the electrical supply is stopped and feedstock is turned off, the reactor chamber is purged

8

with N2 gas and the bell-jar removed. Finally, the silicon rods are removed from the reactor.

The reactor pressure is generally kept at 1 atm or lower, although larger pressures ranging up

to 6 bar is utilized for faster deposition rates.

The deposition of silicon on seed rods occurs following dissociation of the TCS on the

silicon rods itself as given by reaction,

SiHCl3 + H2 Si(s) + 3HCl (2.3)

Additional reactions can also occur, both in gas phase and on the ingot surface, during

the deposition process owing to high temperatures and high reactivity of the chlorosilanes.

Some of these are,

2SiHCl3 Si(s) + SiCl4 + 2HCl (2.4)

SiHCl3 SiCl2 + HCl (2.5)

SiCl2 + H2 SiH2Cl2 (2.6)

SiHCl3 + H2 Si + 3HCl (2.7)

SiHCl3 SiCl3 + ½H2 (2.8)

SiHCl3 + H2 SiH2Cl2 + HCl (2.9)

SiHCl3 + 2H2 SiH3Cl + 2HCl (2.10)

SiHCl3 + 2H2 SiH4 + 3HCl (2.11)

As can be seen from these reactions, actual reaction chemistry inside the CVD reactor

is complex. Apart from this, the overall poly-silicon deposition process have a complex

deposition profile because of involvement of multiple steps in the reactions, each of which

can be rate limiting under different surface conditions. First, the reacting species have to

diffuse to the surface, where they are chemisorbed and reaction takes places. After the

reaction, the product (HCl) is released to the bulk, again via diffusion. The kinetics of the

surface reactions was studied by Habuka et al (1999) in much detail. The key processes are

represented in the following steps, as shown in Figure 2.3.

9

Figure 2.3: The mechanism of dissociation of TCS and deposition of silicon on the substrate

(Habuka et al, 1999)

1. Chemisorption of TCS on the silicon surface followed by decomposition to *SiCl2 or,

gas phase decomposition of TCS to SiCl2 followed by physisorption of SiCl2 on the

surface,

SiHCl3→ *SiCl2 + HCl (2.12)

2. Decomposition of the chemisorbed species *SiCl2 leading to deposition of silicon on

the surface,

*SiCl2 +H2→ Si + 2HCl↑ (2.13)

3. Additional reactions, as shown in Figure 2.3 are also possible.

The TCS dissociation is known to occur above 575°C, but typically rod temperature of

1100 ° C is used, to enhance overall reaction rates and to minimize other reactions leading to

formation of chlorosilane byproducts. The TCS mole ratio of 5-15% and excess flow rate of

the feed gas is maintained to achieve highest possible growth rate in industrial CVD bell-jar

reactors. The high flow rate also assists in sweeping of HCl generated post dissociation, thus

preventing backward reaction which leads to leaching of silicon. High flow rates also imply

low single-pass conversion efficiency for input TCS, so reactor exhaust gas recovery systems

need to put in place for cost-effective and environmentally friendly operation. After

separation from the byproducts of reactions, the recovered TCS+H2 feedstock needs to be

purified to the same purity levels as the initial feedstock before it can be used again in the

10

CVD reactor. The exhaust gases leaving the reactor chamber represent a changing blend of

gases as the single-pass deposition efficiency goes from less than 1% at the beginning of the

run to about 25% or so as the rods diameter grow. This increase is a direct result of increase in

the rod surface area which provides more area for TCS to dissociate.

2.6 Design Considerations for CVD Reactor System

The CVD process is essentially a molecular level process and to control the deposition

process at the molecular level we have to achieve macro-scale control in the systems outside

the reactor. There are three main performance aspects of the CVD process – quality,

uniformity and throughput (Jones and Hitchman, 2009). The precursor conversion efficiency

and the consumption of high purity gases are important cost considerations. The geometry of

the reactor, the flow and temperature patterns inside the reactor all have to be designed in

such a way so as to minimize the loss of the precursor by deposition on any hot surface other

than the slim rod or the substrate or by participation in unwanted gas phase reactions. The

operation of the reactor has to be stable to ensure uniformity and quality. Uniformity is

largely governed by the gas velocity and the pressure. Temperature affects the morphology of

the deposited layers. The main processes take place at the solid surface but they can only be

indirectly controlled by manipulating the temperature, pressure and flow rate. These in turn

are related to the flow regime, the flow velocities and residence times and the temperature

fields.

2.6.1 Effect of Pressure Inside the Reactor

The mean free path of the vapor or gas molecules is a very important parameter in the CVD

process. Lower the pressure in the reactor, higher is the mean free path of the molecules. This

gives rise to certain regimes of gas flow in the reactor and these regimes are characterized by

the value of a dimensionless parameter called the Knudsen number (Kn), defined as λ/D,

where λ is the gas mean free path and D is the characteristic length scale associated with the

CVD reactor (Jones and Hitchman, 2009). Different regimes are

Kn < 0.01: Viscous flow regime, this regime prevails at atmospheric and higher pressures.

0 < Kn < 1: Transition regime

Kn > 1: Free molecular regime, this exists in the low pressure and vacuum conditions.

a) High Vacuum Region: This implies the free molecular regime. Here the mass transport rate

onto the substrate where deposition has to take place is essentially given by the Knudsen

equation:

11

J = NAP/√(2πMRT) (2.14)

The precursor and the carrier gas are assumed to form a well mixed zone inside the reactor.

The substrate experiences a flux proportional to the reactant partial pressure. There are two

ways in which a residence time can be defined for the system, one based on the flow rate (tres)

and one based on the reaction rate (tcon) at the substrate surface (Jones and Hitchman, 2009).

If tres >> tcon, then we have surface reaction growth taking place and conversion efficiency will

be lower. On the other hand if tcon >> tres we have mass transport limited growth which

provides high conversion efficiency but slow processing times (Jones and Hitchman, 2009).

So the reactor geometry and the flow rate have to be chosen to maximize the growth rate and

minimize the wastage. For molecular flow regime, film quality is controlled by controlling

contamination. Mass transport is uniform since the motion of the gas molecules is random.

Throughput has to be low since there is a need to maintain low pressures. But thermal

uniformity has to be balanced against reduced heated surface area to maximize the efficiency

of deposition.

b) Viscous flow regime: In this case the molecules are carried along with the bulk flow and

then they diffuse to the surface under the presence of a concentration gradient. So the reactor

in this regime has to be designed for uniformity since the flow is not inherently uniform. The

Reynolds number is typically low in the CVD reactor so the flow is laminar mostly.

Turbulence is not required or desired. The boundary layer thickness in this regime has to be

kept to a minimum by properly orienting the substrate against the flow of the gas stream. We

may rotate the substrate or keep it in an inclined position inside the reactor. The effect of

natural convection also has to be taken care of.

2.6.2 Effect of Reactor Wall Temperature

Wall temperature of quartz bell jar should be maintained in the range of 300 to 8000C.

Temperature less than 3000C leads to deposition of silicon halogen (SiClx) oils on the bell jar

walls. Temperature more than 8000C leads to the deposition of silicon on the bell jar walls

(Heinrich, 1962). Either of these depositions makes the wall being opaque and makes it

unable to observe the condition of u rod. This is objectionable because of the danger of carrier

rod melting at the beginning or during the course of the reaction.

2.6.3 Effect of Flow Rate

Conversion of trichlorosilane to silicon is a reversible reaction. So the feed flow rate must be

sufficiently high to remove the product HCl from the silicon rod as and when it is formed.

12

Otherwise this HCl will react with silicon and convert it back to trichlorosilane. Also the flow

rate must be the same at all points of the rod surface to ensure uniform deposition of silicon

on the surface of the u rod.

2.7 CFD Modeling of CVD Reactor

Computational Fluid Dynamics (CFD) is becoming more and more popular in the modeling of

flow systems in many fields, including chemical reaction engineering. CFD makes it possible

to numerically solve flow and energy balances in complicated geometries and the results show

specific flow and heat transfer patterns that are hard to obtain with conventional modeling

methods. In the CFD approach of modeling first the continuum domain of the problem is

replaced by a discrete domain (i.e. discrete representations of the solution domain and discrete

locations at which variables are to be calculated) using grids. Thereafter, the Navier-Stokes

and energy balance equations are applied over the entire control volumes/elements thus

generated. The set of partial differential equations for flow and energy balances generated for

the entire system is solved numerically using the boundary conditions of the system. The size

and number of control volumes (mesh density) is normally user defined and it influences the

accuracy of the solutions. By using CFD in the simulation of CVD reactor a detailed

description of the flow behavior within the reactor can be established. The CVD reactor

performance for the poly-silicon growth can be simulated after incorporating reaction

parameters over the flow field in the reactor.

13

3 PRESENT WORK

3.1 Introduction

Flow patterns in real reactors do not be like those of ideal reactors. The conversion of

trichlorosilane to silicon is a gas solid reaction so the performance of the reactor depends on

the parameters like extent of mixing and flow pattern of reaction fluid. To characterize the

mixing conditions and flow patterns inside the real reactor, RTD study is necessary.

Levenspiel (1962) has explained about how to use RTD curves to find out the type of non-

ideality present in the reactor. These curves can be used in the analysis of the bell-jar reactor.

Figure 3.1: Schematic diagram of experimental set-up for CVD reactor system

3.2 Procurement of Experimental Setup

A model reactor made of mild steel with the dimensions of actual CVD reactor setup (figure

3.1) is fabricated and RTD experiments were performed. These experiments were performed

under cold isothermal conditions and the results were compared with the FLUENT simulation

results (figure 3.10). For the actual reactor setup, we approached TEXOL, pune and got a

schematic diagram (figure 3.11) and quotation from them. An indent form with the

description of actual CVD Reactor setup is sent to the Material Management Division of IIT

14

Bombay with the estimated total cost given by TEXOL. On the other front, a U shaped

electrical heating element with temperature controller is also ordered so as to perform RTD

studies in non isothermal conditions.

3.3 RTD Studies for Various Reactor Configurations

3.3.1 Isothermal Conditions

Simulations are done in FLUENT, version 6 for various reactor configurations under

isothermal conditions. Four configurations are shown in figure 3.2. In first configuration one

inlet and one outlet are placed in line with the u rod legs. In the second configuration, the

outlet is placed perpendicular to the u rod legs. In third configuration, two outlets both in line

with the u rod legs are placed and in fourth configuration these two outlets are placed

perpendicular to u rod legs. In all these configurations the outlets are at the same distance of

100 mm from the center of the reactor base plate.

Figure 3.2: Top view of base plate for four configurations

15

Figure 3.3: Contours of velocity for isothermal case

Nitrogen is used as a carrier gas and is sent inside the reactor at a flow rate of 1.5 m/s.

The simulation is run for isothermal conditions and steady state flow condition until the

velocity profile is converged. The velocity contours of the converged flow are shown in figure

3.3. Once convergence is achieved the solution is stored and then a step input of tracer is

given with argon as a tracer. The outlet concentration is recorded until the outlet tracer mole

fraction becomes equal to the given inlet mole fraction of tracer. If this is achieved, then we

can take that the simulation results are reliable. After this, the saved file of converged velocity

profile is loaded and a pulse input is given. The outlet concentration is recorded until the

outlet tracer mole fraction becomes zero. The resultant C curves from these simulations are

analyzed to understand the flow distribution inside the reactor and to find the configuration

that best suits the requirement to get uniform solid deposition rate. From the C curve,

residence time and E curve are evaluated using the equations below.

Residence time = ∫tCdt / ∫Cdt (3.1)

E(t) = C(t) / ∫Cdt (3.2)

16

Figure 3.4: Mass fraction of tracer at the reactor outlet for step input

In the present study, inlet mass fraction of tracer is taken as 1 and for a step input the

exit tracer mass fraction has reached to 1 after 1600 s (figure 3.4). So it is proved that the

solution of the simulation is correct. Now the stored data file of converged velocity profile is

loaded and RTD study is done for a pulse input of tracer. The resultant C curve is shown in

figure 3.5. Similar study is done for the other three configurations and results shown in figures

3.6, 3.7 and 3.8.

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

0 200 400 600 800 1000 1200 1400 1600

C (kmol/m3)

t (s)

Figure 3.5: Reactor with 1 inlet and 1 outlet in parallel with U Rod legs

17

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

0 200 400 600 800 1000 1200 1400 1600

C (kmol/m3)

t (s)

Figure 3.6: Reactor with 1 inlet and 1 outlet in perpendicular to U Rod legs

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

0 200 400 600 800 1000 1200 1400 1600

C (kmol/m3)

t (s)

Figure 3.7: Reactor with 1 inlet and 2 outlets in parallel with U Rod legs

18

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

0 200 400 600 800 1000 1200 1400 1600

C (kmol/m3)

t (s)

Figure 3.8: Reactor with 1 inlet and 2 outlets Perpendicular to U Rod legs

Table 3.1: Comparison of residence times of various configurations

Configuration Residence time (s) Residence time of ideal CSTR (s) % error

a 278.7801 269.69 3.26

b 272.8628 269.69 1.16

c 274.3897 269.69 1.71

d 276.2217 269.69 2.36

In all these configurations there is a time lag in the buildup of concentration profile. This

makes it clear that there is plug flow in some part of the reactor. The residence times for all

these configurations are given in table 3.1. From the table, one can notice that all the

residence times for the actual configurations are greater than those of ideal CSTR. There

might be some strong internal recirculation and thus leading to an increase in residence time

(Figure 3.9). These results are compared with the experimental results (3.10).

19

Figure 3.9 Path lines inside bell-jar for isothermal case

Figure 3.10 RTD curves of bell-jar outlet perpendicular to U rod legs

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 10000 20000 30000 40000 50000

C (mass fraction)

Time (sec)

experimental

simulation

20

Figure 3.11 Schematic diagram of set-up for CVD reactor system given by TEXOL

21

3.3.2 Non-isothermal Conditions

Some guidelines are give in FLUENT user guide to select appropriate model for CFD

simulations. Based on these guidelines the following models are being selected for simulation

under radiation enabled non isothermal case.

Density – Incompressible ideal gas

Radiation – Discrete Ordinate (DO) Model

Pressure – PRESTO!

Flow – Laminar

Mach number is less than 0.3 so the flow is incompressible and incompressible ideal gas law

is used for density calculation. Optical thickness is less than 1 so DO model is used for

radiation. When the flow is in strongly curved domain it is recommended to use PRESTO!

model for pressure discretization. The results are shown below for an inlet flow rate of 1 m/s,

base plate temperature of 373 K, bell jar wall temperature of 573 K, U rod temperature of

1423 K and inlet fluid (nitrogen) temperature of 473 K. First, Discrete Ordinate (DO) Model

is selected for radiation and then the same simulation is run for P1 radiation model. On

observing these two results, it is clear that both are giving almost similar profiles.

Figure 3.12 Contours of temperature for DO model

22

Figure 3.13 Contours of radiation temperature for DO model

Figure 3.14 Contours of density for DO model

23

Figure 3.15 Contours of velocity for DO model

Figure 3.16 Contours of velocity for P1 model

24

Figure 3.17 Contours of temperature for P1 model

Figure 3.18 Contours of radiation temperature for P1 model

Figure

3.4 Characterization of Silicon

There are various methods available for characterization of silicon such as

(XRD), Transmission Electron Microscopy (TEM)

etc. These analysis techniques give crystal orientation

quantitative composition. XRD results for polycrystalline and single crystal silicon are shown

in figures 3.20 and 3.21 respectively. TEM results for powdered single crystal silicon are

shown in figure 3.22. TEM EDX(

figure 3.23 and elemental composition is shown in

Table 3.2 Elemental analysis of powdered single crystal silicon

25

Figure 3.19 Contours of density for P1 model

Characterization of Silicon

There are various methods available for characterization of silicon such as X ray

Transmission Electron Microscopy (TEM), Scanning Electron Microscopy

analysis techniques give crystal orientation and elemental analysis with

XRD results for polycrystalline and single crystal silicon are shown

respectively. TEM results for powdered single crystal silicon are

. TEM EDX(Energy Dispersive X ray Spectrometer) results

elemental composition is shown in table 3.2.

Elemental analysis of powdered single crystal silicon from EDX

X ray Diffraction

Microscopy (SEM)

elemental analysis with

XRD results for polycrystalline and single crystal silicon are shown

respectively. TEM results for powdered single crystal silicon are

Spectrometer) results are shown in

from EDX

Figure

Figure

26

Figure 3.20 XRD Result for polysilicon

Figure 3.21XRD Result for single crystal silicon

27

Figure 3.22TEM result for powdered single crystal silicon

Figure 3.23TEM EDX result for powdered single crystal silicon

28

4 PROJECT OBJECTIVES AND RESEARCH PLAN

4.1 Research Plan

It is proposed to model the CVD reactor using two commercially available CFD softwares,

‘FLUENT’ and ‘COMSOL’ and compare their results. Initially the CFD based model for

investigation of flow field inside the CVD reactor will be developed and the developed model

will be validated in hot & cold condition using inert gas and residence time distribution

(RTD) technique using radiotracer. After successful development of flow model, reaction

enabled CFD model for the CVD reactor system will be developed. For validation of reaction

enabled CVD model, the silicon deposition experiments will be carried out on a silicon

carbide slim rod using trichlorosilane and hydrogen gas. As the experimental study involve

deposition of silicon with trichlorosilane and hydrogen at 1150°C, the experimental facility

should have following features: (i) The system should be designed for very high leak integrity

with all the required safety features (interlocks, safety relief valves etc.) (ii) It should be of

skid mounted and portable type; (iii) the electric heating system should be equipped with high

precision temperature measurement (two color IR pyrometers) and control system (iii) it

should have trichlorosilane (TCS) supply and TCS evaporator system; (iv) it should be

equipped a ratio flow controller to control the TCS to hydrogen ratio in the feed.

4.2 Experimental Validation

The experimental setup proposed to use for validation of the CFD model of CVD reactor

system consists of a quartz bell jar reactor of diameter 300 mm and height 350 mm. The base

plate will be of water cooled type to keep its temperature below 50-60 °C. The SS bell jar

used for safety purpose as well as a heat sink to maintain the reactor gas temperature below

500 °C will be of nitrogen cooled type having diameter 450 mm and height 450 mm. The

system should be suitable for following applications: (i) For validation of flow field of CVD

reactor under cold and hot conditions using radiotracer technique and with inert gases. (ii) For

validation of reaction enabled CFD model with trichlorosilane and hydrogen gas as feed and

silicon carbide as slim rod. The flow parameters considered for the experimental set-up is

given in table 4.1

29

Table 4.1: Process Flow data for experimental set-up

Location →

Components↓

1 2 3 4 5 6 7

H2 (mol/hr) 13.39 13.39

N2 (mol/hr) 13.39 13.39

C. W. (lpm) 6 6

Temp (°C) 30 200 1150 (rod temp.) 30 35

Pressure (bar g) 0.6 3 0.6 0.6

4.3 List of Objectives

1. Determination of flow field and temperature distribution inside the CVD reactor

system using CFD (‘FLUENT’ & ‘COMSOL’), considering the following,

a. Nozzle configuration and its location effects.

b. Preheater configuration and its location effects.

c. Reactor size effects

d. Slim rod location effects

e. Slim rod growing boundary effects

f. Feed gas pre-heater effects

g. Enclosure and base plate temperature effect

h. Feed flow and composition effect

2. Design and development of experimental test facility for validation of the CFD model

developed.

3. Experimental validation of CFD model under non-reacting conditions for the flow and

temperature field in hot and cold conditions using radiotracer technique (Kr-79 as

tracer) having inert feed. CFD predicted RTD should match with the experimentally

predicted RTD at different operating conditions.

4. Development of reaction enabled CFD model to study the following parameters of the

silicon ingot CVD reactor,

a. Growth rate.

b. Composition of outgoing gas.

c. Temperature profile inside and outside of the Silicon slim rod.

30

d. Gas phase concentration profile inside the reactor.

e. Stress analysis of the silicon slim rod.

5. Experimental validation of reaction enabled CFD model with trichlorosilane and

hydrogen gas as feed and silicon carbide as slim rod.

6. Selection of process design data for development of a pilot scale CVD reactor system.

4.4 Yearly Research Plan

4.4.1 Research Plan for First Year

1. Literature survey

2. CFD model development

3. Reviews of the thermodynamic model developed and verification of the suitability of

the kinetic, heat and mass transfer model selected.

4.4.2 Research Plan for Second Year

1. CFD simulation studies for determination of flow field and temperature distribution

inside the CVD reactor considering the following,

a. Nozzle configuration and its location effects.

b. Pre-heater configuration and its location effects.

c. Reactor size effects

d. Slim rod location effects

e. Slim rod growing boundary effects

f. Feed gas pre-heater effects

g. Enclosure and base plate temperature effect

h. Feed flow and composition effect

2. Design and procurement of experimental setup for validation of CFD model.

3. Validation of the flow field in cold isothermal conditions.

4. Validation of CFD model in hot conditions.

5. Development of reaction enabled CFD model and simulation studies (based on the gas

phase and surface reaction model available in the literature).

6. Safety analysis of the experimental set-up.

4.4.3 Research Plan for Third Year:

1. Experimental validation of reaction enabled CFD model with trichlorosilane and

31

hydrogen gas as feed and silicon carbide as slim rod after installation of the

experimental set-up.

2. Optimization and selection of process design conditions for a pilot scale CVD reactor

system.

3. Analysis of safety and process design proposed for development of a pilot scale CVD

reactor system.

32

References

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