The Temperature Control System of Continuous Diffusion Furnace

Xianxin Ke, Zhitong Luo, Yujiao Zhu and Yang Liu

Department of Mechanical and Electrical Engineering, Shanghai University, No. 149, Yanchang Road, Shanghai, China

Keywords: Continuous Diffusion Furnace, P-N Junction, Smith Estimating Pre-Compensation, Temperature Control

System.

Abstract: The diffusion furnace is an important and indispensable equipment in the production process of solar cell，

it plays a crucial role in the photoelectric conversion efficiency of the solar cell. Production efficiency and

temperature control precision of traditional closed diffusion furnace is low. To solve this problem，this

paper presents a temperature control system of continuous diffusion furnace. The system uses PID cascade

control algorithm based on Smith estimating pre-compensation to achieve the temperature control of furnace

and ensure the uniformity and stability of the temperature, so as to ensure the uniformity of the dopant

diffusion. The simulation results verify the effectiveness of the control algorithm; and practical experiments

prove the feasibility of the temperature control system.

1 INTRODUCTION

Solar panel as the core part of the solar photovoltaic

power generation (Cao et al., 2013), it can convert

the solar energy into electric energy stored in

batteries and promote the work load. In the whole

production process of solar cells, P-N junction

process (Wang et al., 2012) is a crucial step, it is

related to the conversion efficiency of crystalline

silicon solar cells and the precision and stability of

the process temperature control directly affects the

quality of P-N junction. So the diffusion furnace is

indispensable important equipment in solar cell

production process, the temperature control accuracy

and the stability of the system are very important.

According to the requirements of the automatic

control temperature, PID control, cascade control

(Gervini and Perondi, 2014), Smith estimating pre-

compensation (Ma et al., 2014) and compound

control among them have gradually replaced the

traditional PID control. At present, temperature

control technology of diffusion furnace equipment in

some foreign companies is more advanced, such as

Germany’s BTU (Anonymous, 2008), the United

States’ CENTROTHERM (Yan et al., 2012) and so

on. In recent years, the domestic diffusion furnace

equipment develops quickly in upgrading and

technology, but compared with the international

advanced diffusion furnace equipment, there is still

gap.

Diffusion furnace mostly adopts the method of

resistance heating to heating; the temperature control

system contains one order inertial link with pure

hysteresis. The link has characteristics with big

inertia, pure lag and nonlinear, and it is difficult to

use mathematical method to establish model and

determine the parameters. And now the temperature

control precision requirement of diffusion furnace is

higher, the traditional furnace temperature control

system cannot meet the requirements. This paper

designs a continuous diffusion furnace that uses PID

cascade control algorithm based on Smith estimating

pre-compensation to guarantee the uniformity and

stability of the temperature, so as to ensure the

uniformity of the dopant diffusion, and to improve

photoelectric conversion efficiency of silicon (Olsen

2012).

2 DESIGN OF CONTINUOUS

DIFFUSION FURNACE

As shown in Figure 1 is diffusion process of

continuous diffusion furnace, it is divided into two

steps:

1) Quartz boat 4 carrying silicon wafers is put

on the left paddle 1 and loaded into furnace tube 2 of

diffusion furnace from left. Right paddle 5 goes into

furnace tube 2 from the right and stop go forward

when it arrives constant temperature area 3. Left

Ke, X., Luo, Z., Zhu, Y. and Liu, Y.

The Temperature Control System of Continuous Diffusion Furnace.

DOI: 10.5220/0005996402270233

In Proceedings of the 13th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2016) - Volume 1, pages 227-233

ISBN: 978-989-758-198-4

227

paddles 1 stop go forward when quartz boat arrives

at central region of constant temperature area 3.

2) The left paddle 1 failing by elevating

mechanism and handover quartz boat on it to right

paddle 5. Then left paddle 1 exits form furnace tube

2 and rises to the original position to reload the

silicon wafer. And right paddle 5 exits slowly from

constant temperature area 3, when the left side

silicon wafer on the quartz boat 4 exits from the

constant temperature area 3, the right paddle 5 exits

from diffusion furnace tube 2 and waiting to unload

the quartz boat 4.

Figure 1: Diffusion process of continuous diffusion

furnace.

Where 1-Left paddle

2- Furnace tube

3- Constant temperature area

4- Quartz boat

5- Right paddle

According to working process, the mechanical

structure design of the equipment is shown in Figure

2. The overall system of the continuous diffusion

furnace comprises 5 parts: a purification cabinet, a

gas source cabinet, a control cabinet, a resistance

heating furnace cabinet and a transfer system. The

equipment profile dimension is about 7480 (mm) *

1200 (mm) * 1500 (mm).

Figure 2: The mechanical structure of continuous diffusion

furnace.

3 TEMPERATURE UNIFORMITY

ANALYSIS OF CONTINUOUS

DIFFUSION FURNACE

In order to design temperature control system,

determine distribution ways of heating furnace wire

and whether need air curtain in the ends of furnace

tube, we need further analyze temperature

distribution of the diffusion furnace. The FLUENT

is used to modeling and simulation, the numerical

simulation analysis for the furnace temperature of

continuous diffusion furnace to get a better

temperature control method.

3.1 Establish Geometric Model of the

Continuous Diffusion Furnace

In this paper, we mainly study the temperature

distribution of continuous diffusion furnace, so

select reaction chamber of the diffusion furnace as

the research object. The geometric model of

continuous diffusion furnace as shown in Figure 3, it

mainly consists a feeding mechanism, the furnace

cavity, air curtain device, windpipe etc. And there

are three airway distributed in same angle along the

axial direction of diffusion furnace.

Figure 3: Geometric model of the continuous diffusion

furnace.

Grid division is the first step, the areas of the

oven cavity and furnace door all using tetrahedral

meshes, the Tet/Hybird grid type and internal grid

size is 10mm, as shown in Figure 7. The equations

of the model are solved by using the continuous

equation, momentum equation and energy

conservation equation.

Figure 4: Grid division of the continuous diffusion

furnace.

3.2 Set Physical Parameters and

Boundary Conditions

There are two types of heat exchange between the

inner wall of the furnace tube and the air inside the

furnace tube: one is the heat convection between

1 2 3 4 5

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each other; another is heat transfer by radiation from

inner wall of furnace tube to air inside furnace tube.

Thus ， according to the actual situation of the

continuous diffusion furnace, the boundary

conditions and physical parameters are adopted as

follows：

1) Set outer wall of the continuous diffusion

furnace as coupled wall; Set inner wall surface as

convection heat transfer and radiation wall; Set

Internal and external wall material as silicon dioxide.

2) Set temperature of furnace mouth without air

curtain as 730K.

3) Set air intake velocity of air curtain as 1m/s,

temperature is 730K; the relative pressure of outlet

surface is 0Pa;

4) Air density is 1.225K/ m3, the coefficient of

viscosity is 1.789×e-0.5kg/m·s, the coefficient of

thermal conductivity is 0.1242W/m·k, the specific

heat of air is 1006.43J/kg·K.

3.3 Analysis of Numerical Simulation

Results of Temperature Field

In order to realize better distribution of temperature

field in furnace cavity of continuous diffusion

furnace, conditions that furnace mouth without air

curtain or with air curtain are compared respectively

by 3-zone heating and 5-zone heating. The

temperature distribution in the furnace is simulated

by using the FLUENT to simulate the three-

dimensional temperature of the geometry model, as

shown in figure 5.

In Figure 5, (a) is temperature simulation result

of continuous diffusion furnace without air curtain

and heated by 3-zone hennaing; (b) is temperature

simulation result of continuous diffusion furnace

with air curtain and heated by 3-zone hennaing; (c)

is temperature simulation result of continuous

diffusion furnace without air curtain and heated by

5-zone hennaing; (d) is temperature simulation result

of continuous diffusion furnace with air curtain and

heated by 5-zone hennaing.

(a) Temperature simulation result of continuous diffusion

furnace without air curtain and heated by 3-zone hennaing

(b) Temperature simulation result of continuous diffusion

furnace with air curtain and heated by 3-zone hennaing

furnace without air curtain and heated by 5-zone hennaing

(d) Temperature simulation result of continuous diffusion

furnace with air curtain and heated by 5-zone hennaing

Figure 5: Temperature simulation results of continuous

diffusion furnace in different conditions.

From the figure (a) and figure (b) we can see that

when diffusion furnace is heated by 3-zone

hennaing, if there is no air curtain, the temperature

change in the furnace chamber like a parabola,

which cannot meet basic requirement of diffusion; if

adding air curtain, the relative temperature

difference of the middle area of about 1m in the

furnace chamber is small but not constant, so it

cannot meet the basic needs of the diffusion. From

the figure (c) and figure (d) we can see that when it

is heated by 5-zone hennaing, if there is no air

curtain, temperature difference exists in the middle

area of about 1m in the furnace chamber is small but

not constant, this situation cannot meet the basic

needs of the diffusion; When adding air curtain at

furnace mouth, the temperature of the middle area of

about 1m in the furnace chamber is constant, so this

case can meet basic needs of the diffusion.

The Temperature Control System of Continuous Diffusion Furnace

229

4 TEMPERATURE CONTROL

ALGORITHM DESIGN

The traditional PID control system can satisfy

requirements of the system in most of cases because

of its advantages that performance mature, easy to

implement, eliminate the static error of advantages

in most cases performance; but in process that

control nonlinear, time variable, coupling and

complex structure parameters, its effect is not very

good. For this problem, this paper presents a PID

cascade temperature control system based on Smith

estimating pre-compensation compensation, the

system can form cascade control by monitoring

temperature of the furnace chamber and the furnace

wall at the same time to meet performance

requirements with small temperature overshoot,

soon stabilization, high control precision.

4.1 Algorithm Design of the Control

System

The purpose that design high precision temperature

control system of diffusion furnace is mainly to

ensure diffusion uniformity of the phosphorus

element to silicon wafer. At present, main problems

of the temperature control system of diffusion

furnace is time of heat transfer process is too long

from furnace wall to furnace chamber, for which we

design a PID cascade control algorithm based on

Smith estimating pre-compensation. Thought of

Smith predictor is that regulated variable delayed is

responded to controller in advance. Thought of

cascade control is that adding a new control object -

furnace wall temperature based on the original

control object -furnace chamber temperature, it

could response interference in advance; and increase

the furnace wall temperature control can improve the

quality of the whole control system. Cascade control

system with Smith estimating pre-compensation as

shown in Figure 6.

Figure 6: Transfer function schematic of cascade control

system with Smith estimating pre-compensation.

In figure 6, Gc1(s) and Gc2(s) denote

respectively the transfer functions of principal and

subordinary regulators. Gv(s) denotes the transfer

function of actuator; Go2 (s) denotes the transfer

function of subloop controlled object; Go1(s)e-τs

denotes the transfer function of t main loop

controlled object; Gm1(s) and Gm2(s) denote

respectively the transfer functions of main and

auxiliary transmitter; Gv(s) denotes the transfer

function of actuator; Go(s)(1-e-τs) is the Smith

predictor of main loop.

In the Smith predictor:

(s)

(s)G

(s)

(s)G

2211

(s)

(1)

The equivalent transfer function of the subloop

is:

)()()()(

)()()(

)(

2221

′

(2)

So, transfer function of cascade control system

with Smith estimating pre-compensation is:

−

′

)(1)(1)(

)(1

)(1)(

)(

(3)

The characteristic equation of the system can be

seen from the above formula:

1 =

′

+ )()()()(

(4)

The characteristic equation does not contain e

-τs

so Smith predictor can make the temperature

hysteresis quality does not affect the system.

4.2 System Simulation and Result

Analysis

In this paper, the Simulink module of Matlab

software is used to simulate the system control

algorithm. In order to verify the control effect of

cascade control method with Smith estimating pre-

compensation, the conventional PID cascade control

system is compared with it. The simulation model of

the PID cascade control system with Smith

estimating pre-compensation and the conventional

PID cascade control system as shown in Figure 7.

The output response curve of the system under

normal circumstances without interference as shown

in Figure 8.

In the case of adding interference, for example,

adding interference signal in the subloop when

system stability time is 7000s. Set interference

signal is the step signal, the output response curve of

system as shown in figure 9.

Gc1(s)

Gc2(s) Gv(s)

Go2(s)

Go1(s)e

-τs

Go(s)(1-e

-τs

)

Gm2(s)

Gm1(s)

N2(s) N1(s)

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230

Figure 7: The simulation model of the PID cascade control system with Smith estimating pre-compensation.

Figure 8: Output response curve of the system under

normal circumstances.

Figure 9: Output response curve of the system under

adding interference.

As can be seen from figure 8, compared with the

conventional PID cascade control system, the PID

cascade control system with Smith predictor is more

stable and no overshoots, thus it can protect the

diffusion furnace and the control effect is better.

As can be seen from Figure 9, the PID cascade

control system with Smith predictor is better in anti-

interference ability than the conventional PID

cascade control system.

To sum up we know, no matter in normal

circumstances, or in the case of interference, the PID

cascade control system with Smith estimating pre-

compensation is better than the conventional PID

cascade control system in control precision, stability

and anti-interference ability.

5 EXPERIMENT

5.1 Temperature Control System

Design of Continuous Diffusion

Furnace

As shown in Figure 10, temperature control system

is composed with temperature cascade control

system and temperature overtemperature protection

system. The principle of temperature cascade control

system is that use subloop composed of the furnace

wall outer thermocouple (A', B', C', D', E'),

thermostat 2, trigger circuit, and power elements to

control the heating current of furnace wire. And

thermocouple (A, B, C, D, E) in the furnace chamber

will measure temperature and compared with setting

value, then together with thermostat 1, thermostat 2,

triggering circuit and power elements compose main

The Temperature Control System of Continuous Diffusion Furnace

231

circuit to monitor furnace temperature of constant

temperature area. The principle of overtemperature

protection system is that overtemperature protection

thermocouple, thermostat 3 (PLC), contactor KM1

compose control system, when the furnace

temperature is higher than the maximum

temperature that equipment can withstand in long

time, the PLC (thermostat 3) output signal to make

contactor KM1 power failure, and to stop heating

furnace wire, at the same time PLC (thermostat 3)

output signal to alarm indicator lamp to

produce sound-light alarm.

Figure 10: Temperature control system of Continuous

diffusion furnace.

The interface of temperature control system is

written by LabVIEW software, as shown in figure

11. The upper computer interface mainly composes

display of real-time temperature, settings of

parameters, establishment of the EXCEL table, etc.

Figure 11: The interface of temperature control system.

5.2 Experimental Result Analysis of

Continuous Diffusion Furnace

The prototype of the continuous diffusion furnace

used in the experiment as shown in Figure 12, which

including the control cabinet, gas source cabinet,

purification cabinet, resistance heating furnace

cabinet and so on.

Figure 12: The prototype of the continuous diffusion

furnace.

Using the LabVIEW record data of continuous

diffusion furnace in heating process, as shown in

Figure 13, (a), (b), (c), (d), (e) denote respectively

the temperature heating curves of point A, B, C, D,

E in figure 13.

(a) The temperature heating curve of point A

(b) The temperature heating curve of point B

(d) The temperature heating curve of point D

Figure 13: The actual furnace temperature curves of

different position.

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(e) The temperature heating curve of point E

Figure 13 (cont.): The actual furnace temperature curves

of different position.

From the above temperature curves can be seen

that heating process is roughly same in point A and

point E, point B and D but there exists some

difference among point A, B, C, D, E during

heating process. The reasons that cause difference of

temperature curves between A, E and B, C, D may

be due to the influence of the air curtain at the

furnace mouth, or caused duo to during heating

process, first heating temperature to 500℃and then

heating temperature 800℃. There is fluctuate after

temperature stable at 800 ℃ , the season is that

paddle enter in furnace from the furnace door cause

air fluctuation and then cause temperature

fluctuation.

Compared temperature simulation curve of

system shown in figure 9 with the actual temperature

heating curves shown in figure 13, we can know that

the heating rate and time that reach to stable

temperature both are roughly same. In the case of

external interference, the system can restore to a

stable state effectively.

6 CONCLUSIONS

In this paper, the temperature of continuous

diffusion furnace is the research object. Aim at the

temperature control characteristics of diffusion

furnace with large inertia and pure hysteresis, this

paper proposes a furnace temperature control system

with PID cascade control algorithm based on Smith

prediction compensation. Through the simulation

analysis and actual experiments of furnace

temperature control system, the paper verifies the

effectiveness of the control algorithm and the

effectiveness of the furnace temperature control

system.

ACKNOWLEDGEMENTS

This research is supported by National Key

Scientific Instrument and Equipment Development

Projects of China (2012YQ150087), National

Natural Science Foundation of China (61273325).

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