JP2007099531A - System for controlling molecular beam source cell, method for controlling the same, and molecular beam epitaxial apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system for controlling a molecular beam source cell, by which the deterioration of the temperature control performance of the molecular beam source cell can be suppressed and film deposition can be stably continuously performed even when the heat capacity is changed by the decrease of the remaining amount of a molecular beam material in the molecular beam source cell. <P>SOLUTION: The system for controlling the molecular beam source cell is a temperature control system of a molecular beam epitaxial apparatus. The temperature control system comprises: a decision means for deciding whether it is needed or not to regulate the feedback gain for a period until the film deposition is started after completing film deposition; and a regulating means for regulating heating and cooling of a crucible by a temperature varying means and the feedback gain in the temperature control means based on the information on the temperature change of the crucible from the temperature measuring means. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a control system for a molecular beam source cell used in a molecular beam epitaxy (hereinafter referred to as “MBE”) apparatus. The present invention also relates to a molecular beam source cell control method and an MBE apparatus including such a molecular beam source cell control system.

  One method for epitaxially growing a desired thin film on a substrate is the MBE method. In this method, an evaporation source of various atoms as a molecular beam material is put in a crucible in a molecular beam source cell, the crucible is heated in a vacuum, and the molecular beam generated therefrom is also placed in a vacuum and properly This is a method of crystal growth by irradiating a substrate held at a certain temperature.

  An example of the MBE apparatus is shown in FIG. As shown in FIG. 10, in this MBE apparatus, a manipulator 101 that holds a substrate 104 and a substrate heater 102 is arranged in a vacuum vessel 103, and a plurality of molecular beam materials filled with various molecular beam materials in the lower portion of the vacuum vessel. Molecular beam source cells 106a and 106b. Cell shutters 105a and 105b are provided between the molecular beam source cells 106a and 106b and the substrate 104. The molecular dose irradiated to the substrate 104 is changed by opening and closing the cell shutters 105a and 105b, and the film is formed on the substrate surface. The composition and thickness of the thin film are controlled.

  An example of a molecular beam source cell is shown in FIG. As shown in FIG. 9, the molecular beam source cell includes a crucible 901 filled with a molecular beam material 902, and a cell heater 903 for heating the crucible 901 is disposed around the crucible. In addition, at least one thermocouple 904 that measures the temperature of the crucible 901 is disposed in close proximity to or in contact with the crucible 901. A radiation shield 909 is disposed around the cell heater 903 so as to surround the cell heater 903, and these components are fixed to a flange 905 serving as a base of the molecular beam source cell. A power introduction terminal 907 is attached to the flange 905, and power is supplied to the cell heater 903 via a conducting wire 908. A thermocouple introduction terminal 906 is attached to the flange 905, and the thermocouple 904 is connected to a temperature control device 911 outside the vacuum vessel. The temperature control device 911 controls the power supply device 910 of the cell heater 903 so that the output value of the thermocouple 904 becomes a set value, and the power supply device 910 supplies power to the cell heater 903 according to the control instruction.

  Since the MBE apparatus uses a molecular beam material filled in the molecular beam source cell to grow a thin film on the surface of the substrate, the molecular beam material decreases every time the thin film is grown. The heat capacity in the crucible decreases as the material in the crucible decreases. As the heat capacity decreases, the amount of heat required to raise the temperature of the crucible by the same amount decreases, so in order to keep the temperature controllability constant, it is necessary to change the control arithmetic expression according to the decrease in the material, Since a control arithmetic expression mounted on a conventional temperature control device such as PID control (Proportional Integral Derivative control) cannot be changed in accordance with the decrease in material, the temperature control performance is deteriorated. When film formation by an MBE apparatus is repeatedly performed in a thin film mass production process, the molecular beam material is filled with the maximum capacity of the crucible, and film formation is continuously performed until the material is used up. It becomes driving. In order to continuously perform stable film formation from the time of filling the material until the material runs out, there is a problem of a decrease in temperature control performance due to a decrease in the remaining amount of the material.

On the other hand, a method for estimating the remaining amount of material in the crucible has been proposed. For example, the crucible filled with molecular beam material is heated or cooled according to the profile by operating the power supply unit, etc., the heat capacity is calculated from the temperature change at that time, and the remaining amount of material is estimated from the calculated heat capacity (See Patent Document 1).
JP 2004-189539 A

  However, this method only estimates the remaining amount of the molecular beam material, and does not show a method for preventing a decrease in the temperature control performance of the molecular beam source cell due to a decrease in the material in the molecular beam source cell. In addition, the method of estimating the heat capacity by changing the heating state of the molecular beam source cell requires an operation to change the heating state, and therefore the cell heater receives a sudden temperature change and is burdened. This sudden temperature change occurs when power is cut off from the energized state of the power, conversely, when the power is cut off from the off state, or when the two power amounts are switched in steps. When you try to wake up. Furthermore, in order to execute this estimation method, it is necessary to perform work when the MBE apparatus is in a standby state, so that the amount of work necessary for operating the MBE apparatus increases.

  The problem of the present invention is that even if the heat capacity changes due to a decrease in the remaining amount of molecular beam material in the molecular beam source cell, an excessive load is not applied to the heater and power supply device of the molecular beam source cell, and the amount of work is reduced. An object of the present invention is to provide a control system for a molecular beam source cell that can suppress a decrease in temperature control performance of the molecular beam source cell without increasing it and can continuously perform stable film formation. Furthermore, it is providing the control method and MBE apparatus.

  The molecular beam source cell control system of the present invention includes a crucible for putting a molecular beam material in a molecular beam source cell used in a molecular beam epitaxial apparatus, a temperature changing means for heating or cooling the crucible, and a temperature of the crucible. A temperature control system for a molecular beam epitaxial apparatus comprising a temperature measurement means, a temperature change means and a temperature control means connected to the temperature measurement means for feedback control of the temperature of the crucible. Between the temperature control means based on the judgment means for judging whether or not the feedback gain needs to be adjusted, heating or cooling of the crucible by the temperature changing means, and the temperature change of the crucible from the temperature measuring means. Adjusting means for adjusting the feedback gain.

  The feedback control by the temperature control means can be implemented by PID control. Further, the determination means can determine whether or not the feedback gain needs to be adjusted based on the temperature information from the temperature measurement means and the set target temperature. The molecular beam epitaxial apparatus of the present invention is characterized by including such a molecular beam source cell control system.

  The molecular beam source cell control method of the present invention includes a crucible for putting a molecular beam material in a molecular beam source cell used in a molecular beam epitaxial apparatus, a temperature changing means for heating or cooling the crucible, and measuring the temperature of the crucible. A temperature control method for a molecular beam epitaxial apparatus comprising a temperature measurement means, a temperature change means, and a temperature control means connected to the temperature measurement means for feedback control of the temperature of the crucible. Until the determination of whether or not the feedback gain needs to be adjusted, heating or cooling of the crucible by the temperature changing means, and information on the temperature change of the crucible from the temperature measuring means. And an adjusting step for adjusting the feedback gain.

  According to the present invention, in the MBE apparatus, even if the remaining amount of the molecular beam material in the molecular beam source cell is reduced and the heat capacity is changed, an excessive load is not applied to the heater and power supply device of the molecular beam source cell. In addition, it is possible to suppress the temperature control performance of the molecular beam source cell from being lowered and continuously perform film formation.

  FIG. 1 illustrates the configuration of the MBE apparatus of the present invention. In this MBE apparatus, as shown in FIG. 1, a manipulator 1 that holds a substrate 4 and a substrate heater 2 is disposed in a vacuum vessel 3, and various molecular beam materials are placed under the vacuum vessel 3. It has a plurality of molecular beam source cells 6a and 6b filled. Cell shutters 5a and 5b are provided between the molecular beam source cells 6a and 6b and the substrate 4, and the molecular beam irradiated to the substrate 4 is changed by opening and closing the cell shutters 5a and 5b, so that the composition and film thickness of the thin film are changed. Control.

  The molecular beam source cells 6a and 6b are provided with heaters, and power is supplied to the heaters by the power supply devices 7a and 7b. The power supply devices 7a and 7b are connected to the temperature control device 8. The temperature of the molecular beam source cell is measured by a thermocouple or the like, the temperature information is transmitted to the temperature control device 8, and the temperature of the molecular beam source cells 6a and 6b is controlled based on this measurement data. Details of the molecular beam source cell will be described later. The manipulator 1 is provided with a thermocouple in the vicinity of the substrate heater 2 and is controlled so that the substrate 4 has a desired temperature during film formation. The MBE apparatus is connected to a control computer 9 for executing a film forming recipe, and temperature measurement data and an instruction value from the control computer 9 are input to the temperature control apparatus 8. In FIG. 1, a temperature control device for the substrate heater 2, various measuring devices such as a vacuum gauge other than the thermocouple, an exhaust device, and the like are omitted.

  FIG. 2 illustrates a molecular beam source cell of the present invention and components around the molecular beam source cell. As shown in FIG. 2, the molecular beam source cell includes a crucible 201 filled with a molecular beam material 202, and includes a cell heater 203 around the crucible 201 as temperature changing means for heating or cooling the crucible 201. In addition, a thermocouple 204 is disposed in proximity to or in contact with the crucible 201 as a temperature measuring means for measuring the temperature of the crucible 201. Further, a radiation shield 209 is disposed so as to surround the cell heater 203. The above-described components are fixed to a flange 205 serving as a base of the molecular beam source cell, and a power introduction terminal 207 is attached to the flange 205, and power is supplied from the power supply device 210 to the cell heater 203 through the conductive wire 208. Yes. Further, a thermocouple introduction terminal 206 is attached to the flange 205, and temperature information from the thermocouple 204 is transmitted to the external temperature control device 211.

As shown in FIG. 2, the temperature control device 211 is connected to a cell heater 203 that is a temperature changing unit and a thermocouple 204 that is a temperature measuring unit, and feedback-controls the temperature of the crucible 201. Feedback control is one method of automatic control, and is a control method in which an input is changed according to an output in a system having an output and an input. Specifically, the following operations (1) to (4) are repeated to control the temperature of the crucible 201 to the set temperature.
(1) The temperature of the crucible 201 is measured by the thermocouple 204, the measured temperature signal is input to the temperature control means 211, and the temperature difference from the set temperature is calculated.
(2) The temperature control means 211 further calculates the amount of power supplied to the cell heater 203 according to this temperature difference, and outputs the calculation result to the power supply device 210.
(3) The power supply device 210 supplies the cell heater 203 with power corresponding to the output value from the temperature control means 211.
(4) When power is supplied to the cell heater 203, the temperature of the crucible 201 changes.

  Control is performed by repeatedly feeding back signals such as temperature measurement → control calculation → output of manipulated variable → temperature change → temperature measurement →. The feedback control can be performed by PID control, and the temperature control device 211 performs a feedback control calculation using a PID control algorithm so that the measured temperature becomes a target value, and supplies it to the power supply device 210 of the cell heater 203. The control signal is calculated. The PID control algorithm is expressed by the following equation.

Here, t is the processing time, u (t) is the control input, e (t) is the deviation between the measured value and the target value, K _P is the proportional gain, T _I is the integration time, and T _D is the derivative time. is there. By setting these control parameters, the control characteristics of the PID calculation are determined. Hereinafter, these control parameters are described as PID gain. The temperature control device 211 is connected to a calculation processing device 212, and the calculation processing device 212 records various data such as temperature data measured by the thermocouple 204, a control signal to the power supply device 210 and output power. And the optimization process of the temperature control system of the molecular beam source cell described below is executed. The calculation processing device 212 can also be incorporated in the control computer 9 in FIG.

  The control process in the temperature control system of this invention is shown in FIG. The MBE apparatus performs film formation by batch processing, and the temperature control process is started after completion of film formation of the previous batch (step (hereinafter abbreviated as “S”) 301). First, the control performance of the current molecular beam source cell control system is evaluated (S302). Based on the evaluation result, it is determined whether the feedback gain needs to be adjusted (S303). If it is determined that adjustment is necessary (YES in S303), the process proceeds to S304. On the other hand, if it is determined that adjustment is not necessary (NO in S303), the next batch of film formation is started without performing the PID gain optimization process (S309). When it is determined that optimization of the temperature control system is unnecessary, the molecular beam source cell and the MBE apparatus can be set in a standby state until the next film formation is started. The evaluation of the control performance of the temperature control system (S302) and the necessity of adjustment (S303) will be described later.

  If it is determined that adjustment is necessary, recording of the amount of power input to the cell heater and the temperature of the molecular beam source cell measured by the thermocouple is started (S304). Recording of these data can be executed in the calculation processing device. Next, a test signal for changing the amount of power according to a certain pattern is input to the power supply apparatus (S305). As the test signal pattern, for example, a pattern in which the power amount is changed stepwise from the first power amount in which the temperature is in an equilibrium state to the second power amount having a certain difference can be used. The data of the test signal and the temperature of the molecular beam source cell that fluctuates depending on the input of the test signal are continuously recorded from the previous step. Next, for example, after confirming that the temperature of the molecular beam source cell is in an equilibrium state at the second electric energy, the test is terminated (S306), and the data recording is stopped (S307). Subsequently, an optimization calculation of a feedback gain such as a PID gain is executed from the above-mentioned recorded data by a calculation processing device or the like, and the calculation result is input to the temperature control device (S308). This optimization calculation will be described later. Thereafter, film formation of the next batch is started (S309).

  FIG. 4 shows the procedure for evaluating the control performance of the temperature control system (S302) and determining whether the feedback gain needs to be adjusted (S303). First, the temperature of the molecular beam source cell measured during film formation and the data of the target value are sequentially recorded (S401). Next, the recorded data is read into a calculation processing device or the like (S402), and for the read data, for example, 2 of the difference between the set temperature of the molecular beam source cell and the measured temperature in a certain section during film formation. The integral calculation of the multiplier value is performed, and this calculated value is set as the evaluation value of the control performance of the temperature control system (S403), and the preset reference value is compared with the evaluation value (S404). The evaluation value increases when the control performance of the control system is poor. Further, an evaluation value obtained by another calculation method different from the method of integrating the square value of the temperature deviation may be employed.

  Next, after comparing the evaluation value with a preset reference value (S404), it is determined whether or not adjustment for optimization is necessary. If the evaluation value exceeds the reference value (YES in S405) In this case, optimization of the control system is started (S406). On the other hand, if the evaluation value does not exceed the reference value (NO in S405), it is possible to wait until the next batch, assuming that the control system does not need to be optimized (S407). For example, if the evaluation value is compared with an initial state in which the PID gain is optimally set, and the difference between the evaluation value in the initial state and the current evaluation value is greater than a preset value, temperature control is performed. Determine that system optimization is required. In this way, the necessity of adjusting the feedback gain can be determined based on the temperature information from the temperature measuring means and the set target temperature. Moreover, a series of these procedures can be executed in the calculation processing apparatus.

The procedure of the PID gain optimization calculation step (S308) in FIG. 3 will be described with reference to FIG. First, the time t ₀ when the amount of power input from the power supply device to the cell heater is changed from the first power amount to the second power amount is detected. Next, based on the temperature 501 of the molecular beam source cell, a time t ₁ when the temperature of the molecular beam source cell starts to rise due to a change in the amount of electric power is detected. Subsequently, the dead time L is calculated by the following equation from t ₀ and t ₁ .
L = t ₁ −t ₀
Next, the difference between the temperature T ₁ measured at the time of the first electric energy and the temperature T ₂ measured at the time of reaching the equilibrium state at the second electric energy, that is, the temperature rise value dT (= T ₂ −T ₁ ) is calculated, and a time t _{2 at} which the measured temperature becomes T ₁ +0.632 dT is calculated. Subsequently, the time constant T is calculated by the following equation.
T = t ₂ -t ₁
Finally, using the dead time L and the time constant T, the PID gain is calculated by the following equation.
K _P K = 0.6T / L
T _I = T
T _D = 0.5L
Here, K is a steady gain, which is the ratio of the amount of change in temperature to the change in electric energy. These formulas for calculating the PID gain are known as the CHR method. However, the calculation formula of the PID gain is not limited to these, and it is possible to change each coefficient and further change the form of the formula. A series of procedures can be executed in the computing device. As described above, the molecular beam source cell control system according to the present invention is based on the temperature control means based on the information on the heating or cooling of the crucible by the temperature changing means including the power supply device and the temperature change from the temperature measuring means. And adjusting means for adjusting the feedback gain. The feedback gain is a parameter for feedback control and varies depending on the control method. For example, in the case of PID control, the PID gain corresponds to the feedback gain. By setting the feedback gain, the characteristics of the feedback control are determined.

  Next, the effect of improving the temperature control performance of the molecular beam source cell by the temperature control system will be described. The temperature control state of the molecular beam source cell before the optimization of the temperature control system is executed is shown in FIG. On the other hand, FIG. 7 shows a temperature control state after the optimization means is implemented by the temperature control system of the present invention. 6 and 7, the vertical axis represents the deviation between the measured temperature of the molecular beam source cell and the target temperature. In FIG. 6, when the evaluation value of the control performance is calculated to be 33.8446 and the reference value is set to 25, the optimization means of the temperature control system is executed, and the evaluation value after optimization is 23.0376. became. FIG. 8 shows changes in the evaluation value due to the optimization process. As is apparent from FIG. 8, the control performance is improved by the optimization processing of the present invention. Therefore, according to the present invention, in the MBE apparatus, even if the heat capacity changes due to a decrease in the molecular beam material, the temperature control performance of the molecular beam source cell is suppressed, and stable film formation is continued even in the next batch. Can be performed. In addition, because the optimization of the temperature control system is being evaluated, it is not necessary to frequently apply a test signal that causes a rapid temperature change to the heater, reducing the load on the heater and operating the MBE device. It is also possible to minimize the increase in the amount of work to be performed.

  Whether or not the feedback gain needs to be adjusted is determined between the end of film formation and the start of film formation. This determination can be made based on the temperature information from the temperature measuring means and the set target temperature. A mode in which an evaluation value is calculated from the measured temperature and the target temperature and controlled by the evaluation value is preferable. As the evaluation value, for example, the integral value of the square of the temperature deviation can be used, but the calculation method of the evaluation value is not limited to this, for example, the integral value of the absolute value of the temperature deviation, constant It is also possible to use variations in control output to the power supply apparatus when the temperature is set as a target value. Further, by using a plurality of such evaluation values, more precise control is possible.

  Further, the calculation formula for the PID gain is not limited to that exemplified above, and it is preferable that the calculation formula for the PID gain is changed according to the pattern of the test signal input to the power supply apparatus. In addition, the molecular beam source cell has been described by exemplifying an aspect in which the thermocouple and the heater are configured. However, in the molecular beam source cell configured by two or more thermocouples and the heater, It is possible to optimize the control system in the same procedure in the thermocouple and heater set.

  The MBE apparatus of the present invention includes the above-described molecular beam source cell control system. Further, the molecular beam source cell control method of the present invention includes a crucible for putting a molecular beam material in the molecular beam source cell, a temperature changing means for heating or cooling the crucible, a temperature measuring means for measuring the temperature of the crucible, A temperature control method for an MBE apparatus comprising a temperature control means connected to a temperature change means and a temperature measurement means and feedback-controlling the temperature of the crucible, wherein a feedback gain between the end of film formation and the start of film formation A determination step for determining whether or not adjustment of the crucible is necessary, and an adjustment step for adjusting the feedback gain in the temperature control means on the basis of information on the heating or cooling of the crucible by the temperature changing means and the temperature change of the crucible from the temperature measuring means It is characterized by providing. Therefore, even if the heat capacity changes due to the remaining amount of the molecular beam material, the temperature control performance of the molecular beam source cell can be maintained, and an excessive burden is not imposed on the cell heater or the like.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The present invention suppresses a decrease in temperature control performance due to a decrease in material in the molecular beam source cell without applying an excessive load to the heater and power supply device in the molecular beam source cell in the epitaxial growth using the MBE apparatus. it can. Therefore, it is useful because stable film formation can be continuously performed using the MBE apparatus.

It is a schematic diagram which shows the structure of the MBE apparatus of this invention. It is a schematic diagram which shows the periphery structure of the molecular beam source cell of this invention, and a molecular beam source cell. It is a figure which shows the control process in the control system of this invention. It is a figure which shows the process of judgment of the necessity of the evaluation of the control performance of the control system of this invention, and adjustment. It is a figure explaining the process of the optimization calculation of PID gain in the control system of this invention. It is a figure which shows the temperature control state before the optimization by the control system of this invention. It is a figure which shows the temperature control state after the optimization by the temperature control system of this invention. It is a figure which shows the change of the evaluation value by optimization in the control system of this invention. It is a schematic diagram which shows the structure of the conventional molecular beam source cell. It is a schematic diagram which shows the structure of the conventional MBE apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Manipulator, 2 Substrate heater, 3 Vacuum container, 4 Substrate, 5a, 5b Cell shutter, 6a, 6b Molecular beam source cell, 7a, 7b, 210 Power supply unit, 8, 211
Temperature control device, 9 control computer, 201 crucible, 202 molecular beam material, 203 cell heater, 204 thermocouple, 205 flange.

Claims (5)

A crucible for placing a molecular beam material in a molecular beam source cell used in a molecular beam epitaxial apparatus;
A temperature changing means for heating or cooling the crucible;
Temperature measuring means for measuring the temperature of the crucible;
A temperature control system for a molecular beam epitaxial apparatus, comprising a temperature control means for feedback control of the temperature of the crucible connected to the temperature changing means and the temperature measuring means,
Judgment means for judging whether or not the feedback gain needs to be adjusted between the end of film formation and the start of film formation;
Molecule comprising heating or cooling of the crucible by the temperature changing means, and adjusting means for adjusting a feedback gain in the temperature control means based on information on temperature change of the crucible from the temperature measuring means. Source cell control system.   2. The molecular beam source cell control system according to claim 1, wherein the feedback control by the temperature control means is PID control.   2. The molecular beam source cell control system according to claim 1, wherein the determination unit determines whether or not a feedback gain needs to be adjusted based on temperature information from the temperature measurement unit and a set target temperature. A crucible for placing a molecular beam material in a molecular beam source cell used in a molecular beam epitaxial apparatus;
A temperature changing means for heating or cooling the crucible;
Temperature measuring means for measuring the temperature of the crucible;
A temperature control method for a molecular beam epitaxial apparatus comprising temperature control means for feedback control of the temperature of the crucible connected to the temperature change means and the temperature measurement means,
A determination step for determining whether or not the feedback gain needs to be adjusted between the end of film formation and the start of film formation;
A molecule comprising: heating or cooling of the crucible by the temperature changing means, and an adjusting step of adjusting a feedback gain in the temperature control means based on information on a temperature change of the crucible from the temperature measuring means. Source cell control method.   A molecular beam epitaxial apparatus comprising the molecular beam source cell control system according to claim 1.

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