Patent Publication Number: US-2022238394-A1

Title: Temperature control method, temperature control device, and optical heating device

Description:
TECHNICAL FIELD 
     The present invention relates to a temperature control method, a temperature control device, and an optical heating device. 
     BACKGROUND ART 
     In a semiconductor manufacturing process, various heat treatments such as a film formation treatment, an oxidation and diffusion treatment, a reforming treatment, and an annealing treatment are performed on a substrate to be treated such as a semiconductor wafer. In these treatments, a heat treatment method using photoirradiation is often employed to enable noncontact treatment. Patent Document 1 described below discloses an optical heating device that includes a light-emitting diode (LED) as a light source for heating, for example. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: JP-A-2020-009927 
       
    
     SUMMARY OF INVENTION 
     Heat treatment in semiconductor manufacturing processes, or more specifically, factors such as a temperature and time maintained for heat treatment and a rate at which the temperature goes up or down, influence the quality of manufactured semiconductor devices. Thus, a heat treatment process for the substrate to be treated is required to be controlled such that a temperature of the substrate to be treated reaches and converges to a target temperature quickly and accurately. 
     For instance, Patent Document 1 discloses a method of controlling a temperature of a semiconductor wafer, a substrate to be treated, by measuring the temperature of the semiconductor wafer with a temperature measurement instrument of a noncontact-type such as a thermograph and concurrently controlling an electric current supplied to the LED in response to the temperature of the semiconductor wafer measured with the temperature measurement instrument. 
     Although the temperature of the semiconductor wafer is controlled by the temperature control method as described above so as to reach a predetermined target temperature, the temperature of the semiconductor wafer, in some cases, converges to a temperature different from the target temperature or is slow in converging to the target temperature. 
     In view of the above problem, it is an object of the present invention to provide a temperature control method, a temperature control device, and an optical heating device that enable a temperature of a substrate to be treated with heat through photoirradiation to be measured and controlled with increased accuracy. 
     Solution to Problem 
     A temperature control method according to the present invention is a method of controlling a temperature of a substrate to be treated with heat using light emitted from a light source part including a plurality of solid-state light sources. The method includes: 
     a step (A) of causing the light source part to repeatedly switch between a light-on state and a substantially light-off state; 
     a step (B) of measuring the temperature of the substrate to be treated by observing infrared light radiated from the substrate to be treated while the light source part is kept in the substantially light-off state in the step (A); and 
     a step (C) of determining either of a level of electricity supplied to the light source part in a next round of the light-on state and a time for which the light source part is kept in the next round of the light-on state based on the temperature of the substrate to be treated measured in the step (B) and a predetermined target temperature. 
     The “substantially light-off state” used in this specification means a state in which the light source part is turned off as well as a state in which radiance of the solid-state light sources of the light source part in a measurement wavelength range of a radiation thermometer is as low as less than or equal to 3 mW/sr/m 2  to avoid occurrence of an error in measuring the temperature of the substrate to be treated in the step (B). 
     The “predetermined target temperature” is, for example, a temperature that the substrate to be treated reaches for heat treatment and a reference temperature at which control processes are switched. 
     The inventors of the present invention have extensively studied and found that the temperature control method described above cannot successfully enable the measurement and control of the temperature of the substrate to be treated owing to factors described below. 
     Controlling the temperature of the substrate to be treated by measuring the temperature of the substrate to be treated and concurrently controlling the electric current supplied to the LED in response to the measured temperature of the substrate to be treated means that light is always emitted by a light source part toward the substrate to be treated when the temperature is measured. 
     Even if the light is light in a wavelength range different from a sensitive wavelength range that is a range of wavelengths of light the measurement instrument such as the thermograph observes to measure temperatures, the light contains light, albeit slightly, of wavelengths within the sensitive wavelength range for the measurement instrument such as the thermograph. Hence, such a temperature measurement instrument, which is designed to measure temperatures by receiving infrared light radiated from an object to be measured, inevitably receives part of light emitted from the light source and transmitted though the semiconductor wafer and light traveling after being reflected off an inner wall surface of a chamber together with the light radiated from the semiconductor wafer. 
     In other words, because of an error occurring between the measured temperature of the substrate to be treated and an actual temperature of the substrate to be treated due to a quantity, an intensity, and other properties of unnecessary light the measurement instrument receives together with the light radiated from the substrate to be treated, the temperature of the substrate to be treated cannot be successfully controlled. 
     In view of the factors described above, possible methods for measuring and controlling the temperature of the substrate to be treated with increased accuracy are temperature control methods as described below. Conceivably, a temperature control method, for example involves keeping the light source part in a light-on state until the temperature of the substrate to be treated reaches a target temperature, causing the light source part to switch to a light-off state to measure the temperature of the substrate to be treated when the temperature of the substrate to be treated exceeds the target temperature, and causing the light source part to switch again to the light-on state when the temperature falls below the target temperature. 
     However, with the method, the light source part does not switch to the light-off state and thus a time when the temperature of the substrate to be treated is checked does not come until attainment of the target temperature is confirmed. It is conceivable that the light source part can switch to the light-off state at a predetermined time to measure the temperature of the substrate to be treated. However, it is not realistic to specify appropriate timing on every occasion in response to an output status of the light source part and a size of the substrate to be treated or a material from which the substrate is made. 
     Conceivably, another method for temperature control involves a temperature control method of changing a duty cycle based on a difference between the measured temperature of the substrate to be treated and the target temperature under lighting control of the light source part whereby the light source part repeatedly switches between the light-on state and the light-off state in a certain cycle. 
     However, with the control method, a very short period of the light-off state may take place, and in some cases, the light source part may be controlled so as to be always kept in the light-on state. In other words, with the control method, in the same way as the above control method, a temperature control device can possibly get into a state in which a time when the temperature of the substrate to be treated is checked does not come. 
     To address this problem, the method described above ensures that the temperature of the substrate to be treated is measured while the light source part is in the light-off state. Thus, the quantity and intensity of light emitted from the light source part and incident on a light-receiving area of a thermometer are reduced compared to cases where the temperature of the substrate to be treated is measured while the light source part is in the light-on state. This reduces an error between the actual temperature of the substrate to be treated with heat and the temperature measured with the thermometer. 
     With the method described above, the light source part switches between the light-on state and the light-off state irrespective of whether the temperature of the substrate to be treated has reached the target temperature and thus a time when the thermometer can measure the temperature of the substrate to be treated without being affected by light emitted from the light source part comes. 
     In the temperature control method described above, the step (C) may include controlling the level of the electricity supplied to the light source part in the next round of the light-on state by a proportional control based on a difference between the temperature of the substrate to be treated measured in the step (B) and the target temperature and by an integral control based on a change in the temperature of the substrate to be treated measured in the step (B) over time. 
     In the temperature control method described above, the step (C) may include controlling the level of the electricity supplied to the light source part in the next round of the light-on state by the proportional control based on a difference between the temperature of the substrate to be treated measured in the step (B) and the target temperature and by the integral control and a derivative control based on a change in the temperature of the substrate to be treated measured in the step (B) over time. 
     The level of the supplied electric power is less susceptible to a driving capacity of a power supply device and a parasitic capacity of connected wires than timing with which the supply and cut-off of the electric power are switched. Thus, controlling the level of the electricity helps control the temperature of the substrate to be treated with improved precision. 
     Accordingly, with the method described above, the temperature of the substrate to be treated rises efficiently to the target temperature owing to control by the proportional operation (termed “proportional control” or “P control”). At the same time, an offset against the target temperature, which cannot be reduced by only the proportional control, is reduced owing to control by the integral operation (termed “integral control” or “I control”). The method of control combining proportional control and integral control in this way is generally termed PI control. 
     Further, the method includes controlling the level of the electricity by the derivative operation (termed “derivative control” or “D control”) and thus suppresses a sudden change in temperature of the substrate to be treated caused by external perturbations and other factors. The method of control combining proportional control, integral control, and derivative control is generally termed PID control. 
     With the method of control described above, the temperature of the substrate to be treated rises by converging to the target temperature with improved quickness and immediately returns to the target temperature in response to the occurrence of an abrupt change in temperature of the substrate to be treated caused by external perturbations and other factors. 
     In the temperature control method described above, the temperature of the substrate to be treated may be measured with a radiation thermometer in the step (B). 
     The radiation thermometer displays quick response compared to thermal cameras and other instruments. Thus, the time for which the light source part is kept in the light-off state that is set in the step (A) can be shorten. The radiation thermometer provides high accuracy in temperature measurement compared to thermal cameras and other instruments. Thus, with the method described above, the radiation thermometer helps measure and control the temperature of the substrate to be treated with increased accuracy without a decrease in heating efficiency. 
     A temperature control device according to the present invention is a device for controlling a temperature of a substrate to be treated with heat. The device includes: 
     a light source part including a plurality of solid-state light sources to emit light toward the substrate to be treated; 
     a controller to control the device to cause the light source part to repeatedly switch between a light-on state and a substantially light-off state; and 
     a thermometer to measure the temperature of the substrate to be treated by receiving infrared light radiated from the substrate to be treated while the light source part is in the substantially light-off state under control of the controller, 
     wherein the controller is configured to determine either of a level of electricity supplied to the light source part in a next round of the light-on state and a time for which the light source part is kept in the next round of the light-on state based on the temperature of the substrate to be treated measured with the thermometer and a predetermined target temperature. 
     In the temperature control device described above, the controller may be configured to control the level of the electricity supplied to the light source part in the next round of the light-on state by a proportional control based on a difference between the temperature of the substrate to be treated measured with the thermometer and the target temperature and by an integral control based on a change in the temperature of the substrate to be treated measured with the thermometer over time. 
     In the temperature control device described above, the controller may be configured to control the level of the electricity supplied to the light source part in the next round of the light-on state by the proportional control based on a difference between the temperature of the substrate to be treated measured with the thermometer and the target temperature and by the integral control and a derivative control based on a change in the temperature of the substrate to be treated measured with the thermometer over time. 
     In the temperature control device described above, the thermometer may be a radiation thermometer. 
     An optical heating device according to the present invention includes: 
     the temperature control device described above; 
     a chamber to house the substrate to be treated; and 
     a supporter inside the chamber to support the substrate to be treated. 
     With the above configuration, the temperature of the substrate to be treated is measured while the light source part is in the light-off state. Thus, the quantity and intensity of light emitted from the light source part and incident on a light-receiving area of a thermometer are reduced compared to cases where the temperature of the substrate to be treated is measured while the light source part is in the light-on state. This reduces an error between the actual temperature of the substrate to be treated with heat and the temperature measured with the thermometer. 
     With the above configuration, the light source part switches between the light-on state and the light-off state irrespective of whether the temperature of the substrate to be treated has reached the target temperature and thus a time when temperature measurement is allowed comes. Further, the time for which the light source part is kept in the light-off state is not controlled depending on the temperature of the substrate to be treated and thus, under control action, is not controlled to such an extent that temperature measurement cannot be completed. 
     The present invention can implement a temperature control method, a temperature control device, and an optical heating device that enable a temperature of a substrate to be treated with heat through photoirradiation to be measured and controlled with increased accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross-sectional view of a configuration of an optical heating device according to an embodiment, viewed along a Y direction; 
         FIG. 1B  is a plan view of a chamber in  FIG. 1A , viewed from +Z side; 
         FIG. 1C  is a schematic cross-sectional view of a configuration of an optical heating device according to an embodiment, viewed along the Y direction; 
         FIG. 2  is a schematic block diagram showing a configuration of a controller; 
         FIG. 3  shows graphs of an example showing how the controller controls an electric current supplied to a light source part and a measurement trigger signal; 
         FIG. 4  is a graph showing a result of verification in Example 1; 
         FIG. 5  is a graph showing a result of verification in Comparative Example 1; 
         FIG. 6  shows graphs of an example showing how the controller controls an electric current supplied to a light source part and a measurement trigger signal; 
         FIG. 7  shows graphs of an example showing how the controller controls an electric current supplied to a light source part and a measurement trigger signal; and 
         FIG. 8  shows graphs of an example showing how the controller controls an electric current supplied to a light source part and a measurement trigger signal. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A temperature control method, a temperature control device, and an optical heating device of the present invention will now be described with reference to the attached drawings. All the drawings shown below regarding a temperature control device and an optical heating device are schematic views, and the size proportion and the number of components on each drawing do not necessarily coincide with an actual proportion and number. 
     [Optical Heating Device] 
     First, a configuration of an optical heating device  1  will be described.  FIG. 1A  is a schematic cross-sectional view of the configuration of the optical heating device  1  according to an embodiment, viewed along a Y direction.  FIG. 1B  is a plan view of a chamber  10  in  FIG. 1A , viewed from +Z side. As shown in  FIG. 1A , the optical heating device  1  of a first embodiment includes the chamber  10  to house a substrate to be treated W 1  and a temperature control device  20 . The temperature control device  20  includes a light source part  21 , a radiation thermometer  22 , and a controller  30 . 
     The present embodiment is described on the assumption that the substrate to be treated W 1  is a silicon wafer. However, the substrate to be treated may be a semiconductor wafer made of a material other than silicon or may be a substrate such as a glass substrate. The substrate to be treated W 1  has a first principal surface W 1   a  on which a pattern (not shown) is formed and a second principal surface W 1   b  on which no pattern is formed, such that the principal surfaces are distinguished from each other. The same applies to the substrate to be treated W 1  that is a semiconductor wafer made of a material other than silicon or that is a glass substrate. 
     In the description given below, as shown in  FIGS. 1A and 1B , a direction in which a light-emitting diode (LED) substrate  21   b  and the substrate to be treated W 1  face each other is defined as a Z direction, a direction in which a pair of supporters  11  described later are opposed to each other is an X direction, and a direction orthogonal to both the X direction and the Z direction is a Y direction. 
     When positive and negative sides are distinguished from each other in representing a direction, a plus or minus sign is added as in “+Z direction” and “−Z direction. For representing a direction without distinction between positive and negative sides, a direction like “Z direction” is simply written. 
     The chamber  10 , as shown in  FIGS. 1A and 1B , includes the pair of supporters  11  to support the substrate to be treated W 1 , a light-transmissive window  10   a  to allow light emitted from the light source part  21  to come inward, and an observation window  10   b  to allow the radiation thermometer  22  to measure a temperature of the second principal surface W 1   b  of the substrate to be treated W 1 . In  FIG. 1B , an area where the light-transmissive window  10   a  is formed is not hatched such that an inner configuration of the chamber  10  can be observed. 
     As shown in  FIG. 1A , the light source part  21  includes a plurality of LED elements  21   a  on the LED substrate  21   b  and is disposed so as to emit light toward the first principal surface W 1   a  of the substrate to be treated W 1  supported by the supporter  11 . A solid-state light source included in the light source part  21  may be another light source such as a laser diode (LD), a fluorescent light source, or a combination of these light sources. In the present embodiment, the LED elements  21   a  emit light that has a peak intensity at a wavelength of 405 nm. However, the light emitted by the light source part  21  may be light that has a spectral peak in any of wavelength ranges of ultraviolet light, visible light, infrared light, and other radiation, with proviso that the light has a heating effect on the substrate to be treated W 1 . 
     The light-transmissive window  10   a  is a window through which at least the light emitted from the LED elements  21   a  is transmitted. The observation window  10   b  is a window through which infrared light is transmitted and observed by the radiation thermometer  22 . The light-transmissive window  10   a  and the observation window  10   b  are not necessarily windows that display transparency to all beams of light emitted from the LED elements  21   a  and all beams of light in a sensitive wavelength range for the radiation thermometer  22 , respectively, with proviso that heat treatment for the substrate to be treated W 1  and measurement by the radiation thermometer  22  are performed without problems. 
       FIG. 1C  is a schematic cross-sectional view of a configuration of an embodiment different from the optical heating device  1  of  FIG. 1A , viewed along the Y direction. Whereas the radiation thermometer  22  in the present embodiment, as shown in  FIG. 1A , is disposed so as to measure the temperature of the second principal surface W 1   b  of the substrate to be treated W 1 , a radiation thermometer  22  may be disposed, as shown in FIG.  1 C, so as to measure a temperature of a first principal surface W 1   a  of a substrate to be treated W 1 . As shown in  FIG. 1C , a light source part  21  and the radiation thermometer  22  may be disposed on the same side relative to the substrate to be treated W 1 , and the radiation thermometer  22  may be disposed so as to measure the temperature of the substrate to be treated W 1  through infrared light radiated in a direction leaning from the Z direction. 
     The controller  30 , as shown in  FIG. 1A , supplies an electric current al to the light source part  21  and outputs a measurement trigger signal b 1  to the radiation thermometer  22  to control timing with which the substrate temperature is measured. The controller  30  receives an electric signal b 2  based on the measured temperature of the substrate to be treated W 1  from the radiation thermometer  22 . 
       FIG. 2  is a schematic block diagram showing a configuration of the controller  30 . As shown in  FIG. 2 , the controller  30  includes a lighting control circuit  31 , an input circuit  32 , an arithmetic circuit  33 , a storage part  34 , and an output circuit  35 . 
     The lighting control circuit  31  outputs a lighting control signal c 1  to the output circuit  35  to switch between a light-on state and a light-off state and outputs a sync signal c 2  to the input circuit  32  to synchronize a time when the radiation thermometer  22  measures the temperature of the substrate to be treated W 1  with the light-on state and the light-off state. 
     In response to the sync signal c 2  input from the lighting control circuit  31 , the input circuit  32  generates the measurement trigger signal b 1  to control the timing when the temperature of the substrate to be treated W 1  is measured and outputs the measurement trigger signal b 1  to the radiation thermometer  22 . The input circuit  32  receives the electric signal b 2 , which contains information on the measured temperature of the substrate to be treated W 1 , from the radiation thermometer  22 , converts the electric signal b 2  into arithmetical temperature data c 3  that can be processed by the arithmetic circuit  33 , and outputs the arithmetical temperature data c 3  to the arithmetic circuit  33 . 
     Upon receiving the arithmetical temperature data c 3  output from the input circuit  32 , the arithmetic circuit  33  generates storage-purpose temperature data c 5  from the arithmetical temperature data c 3  and stores the storage-purpose temperature data c 5  in the storage part  34 . 
     The arithmetic circuit  33  reads out comparison-purpose temperature data c 6  stored in the storage part  34  and performs a proportional operation, an integral operation, and a derivative operation based on a change in temperature of the substrate to be treated W 1  over time in reference to the arithmetical temperature data c 3  and the comparison-purpose temperature data c 6  (the three operations are hereinafter collectively abbreviated as a “PID operation” except that the operations are each individually described). Based on an operation result determined by the PID operation, the arithmetic circuit  33  generates a current value signal c 4  that contains information on a current value supplied to the light source part  21  and that can be processed by the output circuit  35  and outputs the current value signal c 4  to the output circuit  35 . 
     In the present embodiment, the comparison-purpose temperature data c 6  is data containing target temperature data used to treat the substrate to be treated W 1  with heat and temperature data stored previously. 
     Upon receiving the lighting control signal c 1  that is output from the lighting control circuit  31  and that causes the light source part to switch to the light-on state, the output circuit  35  supplies the electric current al of a current value based on the current value signal c 4  to the light source part  21 . 
       FIG. 3  shows graphs of an example showing how the controller  30  controls the electric current al supplied to the light source part  21  and the measurement trigger signal b 1 . The graph (a) is an enlarged view showing a part of a waveform of the electric current al immediately after control start, and the graph (b) is an enlarged view showing a part of a waveform of the measurement trigger signal b 1  immediately after control start. 
     With reference to  FIG. 3 , a temperature control method provided by the temperature control device  20  will now be described based on the configuration of the optical heating device  1 . 
     When the substrate to be treated W 1 , as shown in  FIGS. 1A and 1B  is disposed inside the chamber  10  so as to be supported by the supporter  11 , the controller  30  starts supplying the electric current al to the light source part  21  (step S 1 ). 
     As shown in  FIG. 3 , after supplying the electric current al to the light source part  21  throughout a predetermined time interval T 1 , the controller  30  stops supplying the electric current al to the light source part  21  (step S 2 ). The time interval T 1  is equivalent to a time for which the light source part  21  is kept in the light-on state. 
     After stopping the supply of the electric current al to the light source part  21 , the controller  30  outputs the measurement trigger signal b 1  to the radiation thermometer  22  to start temperature measurement (step S 3 ). 
     In response to an input of the measurement trigger signal b 1 , the radiation thermometer  22  measures the temperature of the second principal surface W 1   b  of the substrate to be treated W 1  (step S 4 ). Since the supply of the electric current al to the light source part  21  stops in step S 2  and step S 14  described later, step S 4  is executed while the light source part is in the light-off state. This step S 4  corresponds to a step (B). 
     Upon completing the measurement of the temperature of the substrate to be treated W 1 , the radiation thermometer  22  outputs the electric signal b 2  to the controller  30  (step S 5 ). The electric signal b 2  is input into the controller  30  and is input as-is into the input circuit  32 . 
     The input circuit  32  converts the electric signal b 2 , which is input from the radiation thermometer  22 , into the arithmetical temperature data c 3  and outputs the arithmetical temperature data c 3  to the arithmetic circuit  33  (step S 6 ). 
     Upon receiving the arithmetical temperature data c 3  from the input circuit  32 , the arithmetic circuit  33  generates the storage-purpose temperature data c 5  based on the arithmetical temperature data c 3  to store the storage-purpose temperature data in the storage part  34  and stores the storage-purpose temperature data c 5  in the storage part  34  (step S 7 ). 
     Upon receiving the arithmetical temperature data c 3  from the input circuit  32 , the arithmetic circuit  33  reads out the comparison-purpose temperature data c 6 , which is stored in advance, from the storage part  34  (step S 8 ). 
     Upon reading out the comparison-purpose temperature data c 6  from the storage part  34 , the arithmetic circuit  33  performs a proportional operation based on a difference between the arithmetical temperature data c 3  containing information about the temperature of the substrate to be treated W 1  measured with the thermometer and the target temperature data contained in the comparison-purpose temperature data c 6  read out from the storage part  34  (step S 9 ). 
     Upon reading out the comparison-purpose temperature data c 6  from the storage part  34 , the arithmetic circuit  33  also performs an integral operation and a derivative operation based on a change in temperature of the substrate to be treated W 1  over time in reference to the arithmetical temperature data c 3  containing information about the temperature of the substrate to be treated W 1  measured with the thermometer and previously stored temperature data contained in the comparison-purpose temperature data c 6  read out from the storage part  34  (step S 10 ). Step S 9  and step S 10  correspond to a step (C). 
     In cases such as a time of first measurement where the previously stored storage-purpose temperature data c 5  is not present, the integral operation and the derivative operation may not be performed. In step S 10 , only the proportional operation and the integral operation may be performed without the derivative operation. 
     Based on data determined by the PID operation, the arithmetic circuit  33  generates the current value signal c 4  that contains information on a current value supplied to the light source part  21  and that can be processed by the output circuit  35  and outputs the current value signal c 4  to the output circuit  35  (step S 11 ). 
     The output circuit  35  prepares to output the electric current al of a current value based on the current value signal c 4  and waits for an input of the lighting control signal c 1  that is output from the lighting control circuit  31  and that causes the light source part to switch to the light-on state (step S 12 ). 
     As shown in  FIG. 3 , a time interval T 2  for which the supply of the electric current al to the light source part  21  is stopped is equivalent to a time for which the light source part  21  is kept in the light-off state. During the time interval T 2 , steps S 3  to S 12  are executed. In other words, the temperature of the substrate to be treated W 1  is measured by the radiation thermometer  22  while the light source part  21  is kept in the light-off state. The time interval T 2  is preferably, for example, within a range of 0.001 second or more and 2 seconds or less, and more preferably within a range of 0.01 second or more and 1 second or less, and even more preferably within a range of 0.05 second and 0.5 second. If the time interval T 2  is too short, the thermometer may be affected by the light emitted from the light source and the temperature may not be measured accurately, so the time interval T 2  should be somewhat longer. 
     At a time when the output circuit  35  receives the lighting control signal c 1  that is output from the lighting control circuit  31  and that causes the light source part to switch to the light-on state, the output circuit  35  starts supplying the electric current al of a current value based on the current value signal c 4  to the light source part  21  (step S 13 ). 
     After continuing supplying the electric current al to the light source part  21  throughout a predetermined time interval T 3 , the output circuit  35  stops supplying the electric current al to the light source part  21  at a time when the output circuit receives the lighting control signal c 1  from the lighting control circuit  31  (step S 14 ). The time interval T 3  is equivalent to a time for which the light source part  21  is kept in the light-on state. After that, steps S 3  to S 14  are repeated. 
     By repetition of steps S 3  to S 14 , the light source part  21  repeatedly switches between the light-on state and the light-off state. Based on feedback about the temperature value measured with the radiation thermometer  22 , PID control is executed on the current value of the supplied electric current al. In this way, the temperature of the substrate to be treated W 1  is controlled to converge to a target temperature. An action of the light source part  21  repeatedly switching between the light-on state and the light-off state by repetition of steps S 3  to S 14  corresponds to a step (A). 
     [Verification] 
     Experimental verification was conducted with the optical heating device  1  to check an error occurring when the temperature of the substrate to be treated W 1  was measured by the radiation thermometer  22  with the light source part kept in the light-on state and an effect of the error on control over the temperature of the substrate to be treated W 1 . This experimental verification was conducted with the optical heating device  1  having the configuration shown in  FIG. 1C . 
     Example 1 
     A case in which the temperature of the substrate to be treated W 1  was measured and controlled using the temperature control method described above is referred to as Example 1. 
     Comparative Example 1 
     A case in which the temperature of the substrate to be treated W 1  was measured and controlled using the same method as in Example 1 except that measurement and control were conducted with the light source part  21  kept in the light-on state without stopping the supply of the electric current al to the light source part  21  is referred to as Comparative Example 1. 
     Experimental Method 
     The light source part  21  was disposed such that the substrate to be treated W 1  and a light-emitting surface of each LED element  21   a  were separated by a distance of 75 mm. 
     A time for which the light source part was kept in the light-on state was 1,050 milliseconds, and a time for which the light source part was kept in the light-off state was 50 milliseconds. 
     The radiation thermometer  22  used was the FLHX-PNE0220-0300B005-000 radiation thermometer made by Japan Sensor Corporation. Main characteristics of the product are shown below: 
     Sensitive wavelength range: 0.8 μm to 1.6 μm 
     Measuring range: 220° C. to 1650° C. 
     The substrate to be treated W 1  and the radiation thermometer  22  were separated by a distance of 300 mm. 
     A target temperature to be reached was 300 degrees. A verification time was 300 seconds from control start. 
     In both Example 1 and Comparative Example 1, the temperature of the substrate to be treated W 1  was measured with a thermocouple attached to the second principal surface W 1   b  of the substrate to be treated W 1  concurrently with the radiation thermometer  22  to check an actual temperature of the substrate to be treated W 1  under control action. A place to which the thermocouple was attached, as shown in  FIG. 1C , was on a side opposite to a zone M 1 , which was measured with the radiation thermometer  22  and in the first principal surface W 1   a  of the substrate to be treated W 1 . 
     (Results) 
       FIG. 4  is a graph showing a result of verification in Example 1, and  FIG. 5  is a graph showing a result of verification in Comparative Example 1. In Example 1, as shown in  FIG. 4 , the temperature of the substrate to be treated W 1  is measured with the radiation thermometer  22  so that substantially coincide with data acquired with the thermocouple after a neighborhood of 40 seconds when the temperature reaches 220 degrees or higher, a measuring range of the radiation thermometer  22 , following control start. The temperature converges to the target temperature 300 degrees. 
     In Comparative Example 1, as shown in  FIG. 5 , the temperature value measured with the radiation thermometer  22 , immediately after control start, changes to higher than or equal to the target temperature 300 degrees to be reached. Conceivably, this is because the radiation thermometer  22 , immediately after control start, receives a part of light emitted from the light source part  21  and transmitted through the substrate to be treated W 1 . 
     By the control method of Comparative Example 1, as shown in  FIG. 5 , the temperature measured with the radiation thermometer  22  reaches 300 degrees immediately after control start, and thus the controller  30  falsely recognizes that the temperature of the substrate to be treated W 1  has reached a neighborhood of the target temperature immediately after control start. Hence, although the temperature of the substrate to be treated W 1  has not reached 300 degrees, the controller  30  controls the output circuit so as to supply the light source part  21  with a minimum level of the electric current al required for maintaining the temperature of the substrate to be treated W 1  at 300 degrees. 
     Thus, the light source part  21  is unable to emit light necessary for increasing the temperature of the substrate to be treated W 1  to the target temperature immediately after control start and afterward. As a result, as shown in  FIG. 5 , the temperature of the substrate to be treated W 1  does not readily rise and does not reach the target temperature after all. 
     Consequently, with the configuration and the method described above, the light source part  21  is in the light-off state at a time when the temperature of the second principal surface W 1   b  of the substrate to be treated W 1  is measured. The radiation thermometer  22  receiving light radiated from the substrate to be treated W 1  scarcely receives light emitted from the light source part  21 . This reduces an error between the temperature of the substrate to be treated W 1  that is under heat treatment and that is measured with the radiation thermometer  22  and the actual temperature of the substrate to be treated W 1 , and thus the temperature of the substrate to be treated W 1  converges to the target temperature quickly and accurately. 
     The light source part  21  switches between the light-on state and the light-off state irrespective of whether the temperature of the substrate to be treated W 1  has reached the target temperature and thus a time when the radiation thermometer  22  can measure the temperature of the substrate to be treated W 1  without being affected by light emitted from the light source part  21  comes. Further, the time for which the light source part  21  is kept in the light-off state is not controlled depending on the temperature of the substrate to be treated W 1  and thus, under control action, is not controlled to such an extent that measurement of the temperature of the substrate to be treated W 1  cannot be completed. 
     In the present embodiment, the radiation thermometer is used as a thermometer to measure the temperature of the substrate to be treated W 1 . However, another thermometer may be used, with proviso that the thermometer is able to measure temperatures in a noncontacting manner by observing infrared light in accordance with requirements including the measuring temperature range and the time for which the light-off state is kept. Such a thermometer is, for example, a thermal camera. 
     In the present embodiment, the light source part  21 , as shown in  FIG. 1A , is disposed so as to emit light toward the first principal surface W 1   a  of the substrate to be treated W 1 . However, the light source part may be disposed so as to emit light toward the second principal surface W 1   b  of the substrate to be treated W 1 . 
     In the present embodiment, the arithmetic circuit  33  included in the controller  30  is configured to perform the PID operation and execute PID control on the current value of the electric current al supplied to the light source part  21 . However, the arithmetic circuit may be configured to execute any feedback control other than PID control. For instance, the feedback control may be control executed to switch current values of the electric current al supplied to the light source part  21  depending on whether the temperature measured with the thermometer is higher or lower than the target temperature to be reached by the temperature of the substrate to be treated W 1 . 
     The chamber  10  in the present embodiment, as shown in  FIG. 1A , has the light-transmissive window  10   a  and the observation window  10   b . If the light source part  21  and the radiation thermometer  22  are housed inside the chamber  10 , the chamber  10  may not be provided with the light-transmissive window  10   a  and the observation window  10   b.    
     The supporter  11  supporting the substrate to be treated W 1  may have any structure with proviso that the first principal surface W 1   a  is disposed on an XY-plane. As shown in  FIG. 1C , a supporter  11  may have, for example, a plurality of pin-shaped protrusions whereby the substrate to be treated W 1  is supported at their tips. 
       FIG. 6  shows graphs of an example, which is different from those in  FIG. 3 , showing how the controller  30  controls the current value of the electric current supplied to the light source part  21 . The electric current al shown in  FIG. 3  is constant when the light source part is in the light-on state. However, as shown in  FIG. 6 , the current value of the electric current al supplied to the light source part  21  during a time such as the time interval T 1  and the time interval T 3  may change over time. The current value of electric current al at time interval T 3  may be controlled by PID control based on the temperature measured at time interval T 2 . 
     OTHER EMBODIMENTS 
     Other embodiments will now be described. 
     &lt;1&gt;  FIG. 7  shows graphs of an example, which is different from that in  FIGS. 3 and 6 , showing how the controller  30  controls the current value of the electric current al supplied to the light source part  21 . The graph (a) is an enlarged view showing a part of a waveform of the electric current al immediately after control start, and the graph (b) is an enlarged view showing a part of a waveform of the measurement trigger signal b 1  immediately after control start. The electric current al supplied to the light source part  21  is not necessarily fully stopped during the time interval T 2  as shown in  FIG. 3 . The electric current al that is very low in intensity may be kept supplied during the time interval T 2  so that the light source part  21  continues emitting feeble light as shown in  FIG. 7 . 
     Under such control, the supply of the electric current al to the light source part  21  is not stopped and the light source part  21  is not turned off even in the light-off state. Hence, with the control described above, a time taken until the light source part  21  starts emitting light in response to resupply of the electric current al to the light source part  21  is shorter, albeit slightly, compared to the case in which the supply of the electric current al is stopped and the light source part  21  is turned off. This helps improve efficiency with which the substrate to be treated W 1  is heated. 
     In the present embodiment, the current value of the electric current al supplied to the light source part  21  during the time interval T 2  is adjusted to meet the light-off state condition described above such that, during the time interval T 2 , radiance of the LED elements  21   a  in a measurement wavelength range of the radiation thermometer (e.g., a wavelength range from 0.8 μm to 1.6 μm when the radiation thermometer used in the above verification is used) is less than or equal to 3 mW/sr/m 2 . 
     &lt;2&gt;  FIG. 8  shows graphs of an example, which is different from those in  FIGS. 3, 6 and 7 , showing how the controller  30  controls the current value of the electric current al supplied to the light source part  21 . The graph (a) is an enlarged view showing a part of a waveform of the electric current al immediately after control start, and the graph (b) is an enlarged view showing a part of a waveform of the measurement trigger signal b 1  immediately after control start. As shown in  FIG. 8 , the temperature of the substrate to be treated W 1  may be controlled such that the time for which the light source part  21  is kept in the light-on state varies. 
     &lt;3&gt; The configurations of the optical heating device  1  and the temperature control device  20  described above are merely examples, and the illustrated configurations should not be construed to limit the present invention.