The temperature and flow rate control of diffusing chamber is one of the key technologies in the production of poly-crystal silicon thin film. As there exist some modeling uncertainties and errors in the actual system, it is difficult to guarantee the chamber variable temperature conditions and the flow rate of diffusion gas being controlled within its targeted range in the rapid thermal processing (RTP). In this paper, the control applies the programmable logic controller (PLC) to configure control hardware system, proposes expert proportional integral derivative (PID) control method to regulate the gas flow rate and H∞ control strategy to attenuate chamber modeling uncertainties and disturbances, respectively, to steer the chamber rapid variable temperature very close to the expected product temperatures. Furthermore, it designs human-machine integrated user control interface (HMI) and achieves rapid and accurately control performances for user operating production. The designed control system are simulated and tested in the application, which demonstrates that the control method has strong robustness when the modeling uncertainties, errors, parameters perturbation and disturbances, the temperature and flow rate meet the requirements of precisely trajectory following.
The poly-crystal silicon thin films have excellent photo-electric properties and production of low cost in the field of energy and informational industry, become a very important electronic materials, widely used in the integrated circuits, semiconductor devices and solar energy cells production. The manufacturing technique includes oxidation, diffusion, alloy and etc. [
The diffusing chamber is axis symmetric and separates the inner part, which contains the deposited wafers and the diffusion gas, and the lamp heaters are uniform annular which distributed on the inner part; also the chamber internal wall is coated with reflective layer. The deposited wafers are partitioned into concentric elements and are coaxial with the chamber. The temperature is sensed at twelve locations across the wafers along the cylindrical axis of the chamber.
In the production, the chamber temperature should be easily operated with given control parameters of the gas temperature and flow rate, and its product process satisfies the following technical requirements:
1) The diffusing chamber is vacuum condition before ventilating diffusion gas;
2) To ensure the purity of the diffusion gas, the ventilating pipelines and chamber need purify after an operating cycle, meanwhile, it satisfies the function of gas automatic to be replenished in the production;
3) Achieving rapid variable temperature and flow rate adjustment, satisfying the technical indicators of poly-crystal silicon thin film production;
4) The control process of all the electric actuators can be operated with manual and automatic control modes, and the control processes are conveniently, safety and reliable for user operating.
In
and two complex coupling variables with modeling uncertainties, errors and disturbances. There is a need to develop a mathematical model that takes into accounts the combined dynamics of diffusion gas flow rate and heaters power, and a method for high-performance rapid variable temperature control with strong robustness. For the chamber control theory and application, there were many results in the past few decades [
Recently, scholars and engineers have presented some novel control techniques for thermal processing in the industry application [
The remaining parts of this paper are organized as follows:
Section 2 detailedly introduces the system control hardware configuring design, which applies programmable logic controller and its function modules to configure control hardware system for the control. Section 3 describes the control strategies and programs design for the chamber control processing, the pro- cessing control simplified flow chart is shown in
system performances are simulated and analyzed in the actual production.
Notations: The notation used throughout the paper is fairly standard.
In accordance with the system devices (
According to
Considering the system cost, in order to reduce the number of input modules, the temperature analogue sampling and driving circuit design, which uses of 12 high-precisely thermal couple AD590s monitoring the temperatures of wafers, applies a piece of MC74HC4514N decoder chip constructing analogue signals switches, then through a precisely 1 kΩ resistance and an isolating amplifier converting the analogue signals to current (0 - 20 mA) sent to the 14th and 15th ports of module C1_1, the signal sampling detection uses periodic refreshing mode, the circuit design is shown in
In this section, the system control strategies and program flow chart design will be presented. The system adopts manual and automatic operating modes in terms of the requirements of production process, this paper only presents the automatic control mode design.
Based the control flow chart (
the designed main control program flow include temperature and flow rate analogue detecting control, Modbus communication, valves control, heating control, exhaust purifying, leakage testing and diffusion gas filling subroutine modules, the systems applies circular scanning output to carry out work, the main program flow chart is shown in
In
model of the diffusing chamber is
where
where
where
where H and E are known constant matrices,
Considering the actual structure of chamber and the heating mode in
Assuming
set
where
In order to reduce the control cost to be enough small with the uncertainties and disturbances, that is
where Q and R are positive weight matrices, respect to the important degree of attenuating temperature error and control cost. Then defining the temperature control optimal object evaluating equation as
where
Then from (10) to (11), the control optimal object equation is equivalent to
where
where K is the controller gain, which will be designed, from (7) to (13), obtains
where
Remark 3.1: In system (7), the is depicted the time varying unknown thermal energy which caused by the diffusing chamber’s temperature modeling uncertainties and errors, without loss of generality, which the uncertainties and errors have several aspects, such as the diffusion gas modeled uncertainties, the gas thermal decomposition energy, convection heat, unknown heat energy disturbance and etc..
The temperature dynamic mode (8), as an applied control systems design [
Definition 3.1: For diffusing chamber’s temperature system (8), under the controller
Proof: considering the following Lyapunov function
with
where
so the system (8) is asymptotically stability.
when
where
In terms of (15) and (18), applies matrix schur complements theorem [
with the zero initial condition, there have
the (20) implies
This completed the proof.
Then based on the definition 3.1, the controller parameter solving procedure is presented as the following theorem.
Theorem 3.1: The system (8) is H∞-
where
Proof: In terms of the definition 3.1, applies linear matrix inequalities theory [
In the aforementioned temperature interrupt H∞ control, as the 12 power driver boards and 12 lamp heaters have the same physical and electrical properties, respectively, therefore, by Modbus communication mode, the temperature analogue signals sampled-data sent to module C1_1 data area, then adopting the H∞ control strategy obtains temperature control signals in the interrupt routine organization block (OB35) at every 100 ms time and outputted from the 15th port of C_2 module, sent to the 12 power driving boards with the same pulse wave modulating form, achieves the 12 lamp heaters power control.
For the flow rate control, there are several important influence functions, such as the impulse disturbance of the adjusting valve’s starting and stopping, control signals noise [
In
where
There are five cases for the expert decision processing of flow rate control:
Case 1: if
Case 2: if
Case 3: if
Case 4: if
Case 5: if
Remark 3.2: In
In this control system, applies the Siemens MP277 10# HMI as user operating interface, according to the user operating and requirements of production processing, the designed main operating interface is showed in
In this section, the system control simulating and effective application are given and analyzed. The deposited wafers are assumed to be opaque.
Example 1. One type of poly-crystal silicon thin film which deposited on the glass substrate, its production need the SiH4 gas temperature maintained in 200˚C (Celsius degrees) and flow rate in 0.085 liters per second. The chamber temperature need be controlled error within 1˚C and the flow rate controlled within 5 seconds to achieve the set values of production. Some physical parameters of the diffusing chamber and SiH4 gas are given in
Set Q = 2.5, R = 1.0,
The temperature and flow rate control simulating performances which compared with traditional control methods [
Example 2. The other kind of poly-crystal silicon thin film production need the temperature in 630˚C and flow rate in 0.15 liters per second, the chamber temperature should be controlled error within 2˚C and the flow rate controlled within 5 seconds, deposited on the Si substrate. Some physical parameters of the chamber and SiCl4 gas are given in
when Q = 25 and R = 10,
Symbol | Physical parameters | Value |
---|---|---|
CQ | heat capacity matrix coefficient matrix of heat conduction gas specific heat capacity wafer specific heat capacity wafer mass heat transfer area norm bounded uncertainties | 2.80 kw・sec・˚C 0.55 kw・m2・˚C 1.355 kJ・kg・˚C 0.84 kJ・kg・˚C 405.8 g 1.54 m2 0.1 sin(t) × 10−3 kJ・s−1 |
Symbol | Physical parameters | Value |
---|---|---|
CQ | heat capacity matrix coefficient matrix of heat conduction gas specific heat capacity wafer specific heat capacity wafer mass heat transfer area norm bounded uncertainties | 2.80 kw・sec・˚C 0.55 kw・m2・˚C 0.526 kJ・kg・˚C 0.70 kJ・kg・˚C 410.5 g 1.54 m2 0.11 sin(t) × 10−3 kJ・s−1 |
In
In this case, we check the control capability of the proposed controller when the chamber temperature varies from 630˚C to 800˚C at 20 seconds scale in the production. The temperature control trajectory is shown as
Furthermore, in example 2, considering the system physical parameters perturbed by 10%, and the temperature varies from 630˚C to 1100˚C at 15 seconds scale, then decreases to 660˚C at 30 seconds scale in the production, we apply the H∞ control scheme and expert PID to the perturbed system.
From the
In this paper, the diffusing chamber’s gas temperature and flow rate control are
designed for poly-crystal silicon thin film production, and we apply Siemens PLC modules to build the mainly control hardware system. For diffusing chamber temperature, this paper designed expert PID controller to adjust the diffusion flow rate and H∞ control strategy to control 12 zones heaters power, to achieve rapid variable temperature. The user can easily and conveniently operate the production control by designed HMI. Lastly, the designed control system are simulated and applied in the actual production, and results show that the system control satisfies the requirements of production techniques and indicators for poly crystalline silicon thin film.
This work is supported by the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle aged Teachers & President (2012- 2015).
Lai, Y.B., Lu, G.P. and Wang, Z.W. (2017) Temperature and Flow Rate Control of Diffusing Chamber. World Journal of Engineering and Technology, 5, 58-76. https://doi.org/10.4236/wjet.2017.51006