JP5876348B2 - Heater wire inspection method - Google Patents

Description

  The present invention relates to a method for inspecting a heater wire.

  Some semiconductor manufacturing apparatuses include a processing container that accommodates a substrate and a heating device that heats the substrate accommodated in the processing container. In a semiconductor manufacturing apparatus capable of holding a plurality of substrates in a vertical direction inside a processing container, a heating device is disposed so as to surround the processing container. Inside the heating device, for example, a heater element wire made of nickel-chromium alloy or iron-nickel-chromium alloy and wound at a predetermined interval is disposed.

  In such a heating device, it is inevitable that the heating performance is deteriorated due to aging of the heater wire or the like. Further, when the heater wire is disconnected due to deterioration, it is necessary to stop using the semiconductor processing apparatus and replace the heater wire.

  In order to prevent disconnection in advance, it is desired to detect an abnormality of the heater element wire. In Patent Document 1, an effective voltage and an effective current of a heater are obtained using a pulse wave, and a resistance value obtained from these is obtained. A method for predicting disconnection from the above is disclosed.

JP 2009-281837 A (paragraphs 0011 and 0042)

  When the above-described heating apparatus is used, the heater element wire thermally expands and contracts as the temperature rises and lowers, so that stress is generated and the heater element wire may be damaged or cracked. Further, since the heater wire extends and curves over time, the space between the heater wires is narrowed, and the two adjacent heater wires may be in contact with each other.

  Such an abnormality of the heater wire can be detected, for example, when it takes longer time to raise the temperature than usual or when the temperature does not rise compared to the electric power supplied to the heater wire. It is also possible to discover the deformation (extension) of the heater wire by visual inspection.

  However, when an object is heated by a heater wire, for example, temperature adjustment is performed by PID control, and thus it is not so easy to detect an abnormality in the heater wire due to a temperature rise time, electric power, or the like. Further, since the heater element wire is often incorporated in the heater unit, visual inspection of the heater element wire requires a long time and much labor for disassembling and assembling the heater unit.

  In view of the above circumstances, the present invention provides a heater element inspection method, a heating apparatus, and a substrate processing apparatus including the same that can easily detect an abnormality of a heater element.

According to the first aspect of the present invention, a step of outputting a high frequency signal having a predetermined frequency to the heater element wire from one end portion of the heater element wire, and the high frequency signal from the other end portion of the heater element wire. A step of inputting a signal, a step of obtaining transmission characteristics of the heater element based on the output high-frequency signal and the input high-frequency signal, and detecting an abnormality of the heater element based on a change in the transmission characteristic A step of changing the interval between the parallel portions where at least a part of the heater strands are arranged in parallel, and applying a second high frequency while changing the frequency from one end of the heater strand to the heater strand. Based on the step of outputting, the step of inputting the second high frequency from the other end of the heater wire, and the output of the high frequency and the input second high frequency, transmission characteristics of the heater wire A step of repeating the step of obtaining a frequency dependency a plurality of times, a step of determining a frequency region in which the transmission characteristic depends on the interval from the relationship between the interval and the frequency dependency of the transmission characteristic, and the frequency region And a step of setting the included frequency to the predetermined frequency of the high-frequency signal .

  According to the second aspect of the present invention, a step of outputting a square wave to the heater element wire from one end of the heater element, and a reflected wave of the square wave is input from the one end, or the heater A step of inputting one of the square wave transmitted waves from the other end of the element wire, and a step of inspecting the heater element wire for abnormality based on a time change of the reflected wave or the transmitted wave. A heater strand inspection method is provided.

  According to the third aspect of the present invention, a high-frequency signal is output to a heater wire arranged in a predetermined shape and one end of the heater wire, and the high-frequency signal is output from the other end of the heater wire. There is provided a heating apparatus including a high-frequency power source that inputs a high-frequency power source, and an analysis unit that acquires transmission characteristics of the heater element wire based on the output high-frequency signal and the input high-frequency signal.

  According to the fourth aspect of the present invention, a processing container in which a substrate is accommodated and processing is performed on the substrate, and a heating apparatus according to the third aspect that heats the substrate accommodated in the processing container. A substrate processing apparatus is provided.

  According to the embodiment of the present invention, there is provided a heater wire inspection method capable of easily detecting an abnormality of a heater wire.

It is a schematic diagram which shows an example of the substrate processing apparatus by the 1st Embodiment of this invention. It is a schematic diagram which shows the inner surface of the heating apparatus of the substrate processing apparatus of FIG. It is a partial cross section figure of the heating apparatus of FIG. It is a graph which shows the result of the experiment conducted along the heater strand inspection method by the 2nd Embodiment of this invention. It is a graph which shows the result of the experiment conducted along the heater strand inspection method by the 3rd Embodiment of this invention. It is a graph which shows the result of the experiment conducted along the heater strand inspection method by the 4th Embodiment of this invention.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. In all the accompanying drawings, the same or corresponding members or parts are denoted by the same or corresponding reference numerals, and redundant description is omitted. Also, the drawings are not intended to show the relative ratios between members or parts, and therefore specific dimensions should be determined by those skilled in the art in light of the following non-limiting embodiments.

(First embodiment)
First, a substrate processing apparatus according to an embodiment of the present invention will be described with reference to FIGS. 1 to 3. Referring to FIG. 1, a substrate processing apparatus 2 includes a processing container 4 in which a plurality of substrates (hereinafter referred to as wafers W) are stacked and accommodated at predetermined intervals in the vertical direction, and a side surface and an upper surface of the processing container 4. And a heating device 48 for covering.

  The processing container 4 includes an outer tube 6 having a covered cylindrical shape and an inner tube 8 having a covered cylindrical shape and disposed concentrically inside the outer tube 6. The outer tube 6 and the inner tube 8 are made of a heat resistant material, for example, quartz. Further, the outer tube 6 and the inner tube 8 are held from the lower end thereof by a manifold 10 made of a metal such as stainless steel or aluminum. The manifold 10 is fixed to the base plate 12.

  A disc-shaped cap portion 14 made of, for example, a metal such as stainless steel or aluminum is airtightly attached to the opening at the lower end portion of the manifold 10 via a seal member 16 such as an O-ring. In addition, a rotating shaft 20 that is airtight and rotatable by a magnetic fluid seal 18 is inserted through the center of the cap portion 14. The lower end of the rotating shaft 20 is connected to the rotating mechanism 22. A table 24 made of metal, for example, is fixed to the upper end of the rotary shaft 20.

  On the table 24, a heat insulating cylinder 26 made of, for example, quartz is provided. Further, a quartz wafer boat 28 made of, for example, quartz is placed on the heat insulating cylinder 26 as a support. For example, 50 to 150 wafers W are accommodated in the wafer boat 28 at a predetermined interval, for example, a pitch of about 10 mm. The wafer boat 28, the heat insulating cylinder 26, the table 24, and the cap unit 14 are loaded and unloaded integrally in the processing container 4 by an elevating mechanism 30 that is a boat elevator, for example.

  A gas nozzle 34 that penetrates the manifold 10 in an airtight manner at one end and bends upward along the inner peripheral surface of the inner pipe 8 is provided at the lower portion of the manifold 10. The other end of the gas nozzle 34 is connected to a gas supply source (not shown) via a predetermined pipe. Further, the pipe is provided with a flow rate control device such as a mass flow controller and an open / close valve (not shown), for example, so that the supply of gas supplied from the gas nozzle 34 into the processing container 4 is started, stopped, and the flow rate. Etc. are controlled. In FIG. 1, one gas nozzle 34 is shown, but a plurality of gas nozzles 34 may be provided depending on the type of gas used. For example, when a silicon oxide film is formed on the wafer W by the substrate processing apparatus 2, a gas nozzle 34 for a silicon-containing gas, a gas nozzle 34 for an oxidizing gas, and a gas nozzle 34 for a purge gas may be provided.

A gas exhaust port 36 is provided in the upper part of the manifold 10, and an exhaust system 38 is connected to the gas exhaust port 36. The exhaust system 38 includes an exhaust passage 40 connected to the gas exhaust port 36, and a pressure adjustment valve 42 and a vacuum pump 44 that are sequentially connected in the middle of the exhaust passage 40. The exhaust system 38 exhausts the gas supplied to the processing container 4 and adjusts the pressure in the processing container 4.
Note that the entire processing container 4 may be made of, for example, quartz without providing the manifold 10.

  The heating device 48 includes a heat insulator 50 having a cylindrical shape with a lid. The heat insulator 50 is formed of, for example, a mixture of amorphous silica and alumina having low thermal conductivity. The thickness of the thermal insulator 50 is typically about 30 mm to about 40 mm. In addition, the inner diameter of the heat insulator 50 is larger than the outer diameter of the processing container 4 by a predetermined length, thereby forming a predetermined space between the inner surface of the heat insulating body 50 and the outer surface of the processing container 4. Further, a protective cover 51 made of, for example, stainless steel is attached to the outer peripheral surface of the heat insulator 50 so as to cover the entire heat insulator 50.

  A heater wire 52 is wound around the inner surface of the heat insulator 50 in a coil shape. Referring to FIG. 2, a plurality of holders 54 extending in the vertical direction of the heat insulator 50 are provided on the inner surface side of the heat insulator 50. As shown in FIG. 3, the holder 54 has a plurality of recesses 56 that are desired on the inner surface of the heat insulator 50, and the heater wires 52 are held by the recesses 56. Note that the holder 54 is preferably made of a ceramic material having excellent heat resistance and insulation. Moreover, it is preferable that the holder 54 is arrange | positioned by the space | interval L (refer FIG. 2) from about 100 mm to about 150 mm according to the internal diameter of the heat insulating body 50, for example.

  Note that the pitch of the heater wires 52 (recesses 56) may be, for example, about 10 mm to about 30 mm. The outer diameter of the heater wire 52 may be about 1 mm to 10 mm, for example. Furthermore, a distance L1 (see FIG. 3) between the center of the heater wire 52 and the heat insulator 50 is, for example, about 6 mm to about 20 mm.

  Referring again to FIG. 1, the heating device 48 is provided with two current introduction terminals 52 </ b> I that penetrate the heat insulator 50 and the protective cover 51. The current introduction terminal 52I is connected to the heater element wire 52 on the inner surface side of the heat insulator 50, and is connected to the wiring 52C on the outer side of the protective cover 51. The wiring 52C is connected to the power supply device 55, and power is supplied from the power supply device 55 to the heater element wire 52 through the current introduction terminal 52I. Based on the result of temperature measurement by a thermocouple (not shown), the power from the power supply device 55 is adjusted by a temperature regulator (not shown), whereby the wafer W in the processing container 4 is heated to a predetermined temperature.

  A coil 57 is provided on the wiring 52C. That is, the coil 57 is provided so that the wiring 52C passes through the center of the coil 57. The coil 57 is connected to an inspection power source 58. The inspection power source 58 can output a high-frequency signal in a range from about 300 kHz to about 500 MHz, for example. Further, the inspection power source 58 can output a pulse signal having a rise time from about 10 ps to about 35 ps, for example.

  The high-frequency signal output from the inspection power supply 58 to the one coil 57 flows into the wiring 52C by inductance coupling, and flows to the heater wire 52 through the one current introduction terminal 52I. The high-frequency signal that has flowed through the heater wire 52 returns from the wiring 52C to the inspection power supply 58 via the other coil 57.

  Further, an analyzer 59 is connected to the inspection power source 58, so that loss or the like is based on the high frequency signal output from the inspection power source 58 to the heater element wire 52 and the high frequency signal input from the heater element wire 52. Transmission characteristics are required.

  Advantages and the like of the heating device 48 configured as described above and the substrate processing apparatus 2 including the same can be understood from the heater wire inspection method described below.

(Second Embodiment)
Next, a heater strand inspection method according to the second embodiment of the present invention will be described by taking as an example a case where the heater strand 52 of the substrate processing apparatus 2 according to the first embodiment is inspected. In the following description, FIGS. 1 to 3 will be referred to as appropriate.

  First, the experiment and the results that led to the completion of the heater wire inspection method of this embodiment will be described. In this experiment, a part of the heater wire 52 arranged on the inner surface side of the heat insulator 50 is intentionally damaged, and the transmission characteristic of the high-frequency signal in the heater wire 52 is examined according to the depth of the damage. It was.

  Specifically, first, a scratch having a depth of 2 mm from the side surface of the heater element wire 52 was formed in a part of the heater element wire 52. Next, a high frequency signal was output from the inspection power source 58 to the heater wire 52, and the frequency of the high frequency signal was changed (swept) from 80 MHz to 90 MHz. The frequency dependence of the transmission characteristics (loss) at that time was acquired by the analyzer 59. Next, the above-mentioned scratch was deepened to 4 mm and the frequency dependence of the transmission characteristics was obtained in the same manner, and the scratch was further deepened to 6 mm and the frequency dependence of the transmission characteristics was obtained similarly. In addition, the frequency dependence of the transmission characteristic of the heater element wire 52 was also acquired before scratching (scratch 0 mm).

The frequency dependence of the transmission characteristics obtained in this way is shown in FIG. From this figure, it can be seen that the transmission characteristics vary greatly depending not only on the frequency of the high-frequency signal but also on the depth of the flaw. Such a change is considered to be because the impedance of the heater wire 52 changes due to scratches formed on the heater wire 52.
Here, paying attention to the frequency of 84.55 MHz in FIG. 4A, the relationship between the transmission characteristic (loss) at this frequency and the depth of the flaw was examined. The result is shown in FIG. As shown in the figure, a substantially linear relationship is recognized between the transmission characteristics (loss) and the depth of the flaw. Therefore, by acquiring the transmission characteristics of the heater element wire 52 using a high-frequency signal having a frequency of 84.55 MHz, it is possible to know the occurrence of flaws and the depth of the flaws in the heater element wire 52. (Once a usable frequency is determined, it may be fixed to that frequency without sweeping the frequency.)
From the above, in the heater element inspection method according to the second embodiment of the present invention, the frequency of the high frequency signal that depends on the depth of the flaw of the heater element 52 is obtained in advance, and the high frequency signal having that frequency is obtained. The inspection power source 58 outputs the signal to one end of the heater wire 52, the high-frequency signal is input from the other end of the heater wire 52, the transmission characteristic (loss) of the high-frequency signal is obtained, and the temporal change in the transmission characteristic is observed. As a result, it is possible to inspect the occurrence and depth of the scratches on the heater wire 52.

  The high frequency signal can be output when the heater power from the power supply device 55 becomes zero, for example, when the temperature of the heating device 48 is lowered. Therefore, it is only necessary to output a high-frequency signal every time processing in the substrate processing apparatus 2 is completed. Further, not only when the temperature is lowered, but also when the heater power from the power supply device 55 becomes zero, a high frequency signal may be output. Regardless of temperature rise, temperature drop, or steady state, transmission characteristics can be obtained even when high-frequency power is output within a few nanoseconds, for example, at the timing when power output becomes zero during PID control. it can.

  As described above, according to the heater element inspection method according to the second embodiment of the present invention, the heater element 52 is damaged by outputting a high-frequency signal to the heater element 52 and acquiring transmission characteristics. Therefore, it is not necessary to stop the substrate processing apparatus 2. Further, it is not necessary to disassemble the heating device 48 and visually inspect the heater wire 52 and then assemble the heating device 48. Furthermore, since a high-frequency signal can be output between processes in the substrate processing apparatus 2, the heater wire 52 can be inspected without intentionally providing inspection time.

  Further, as shown in FIG. 4B, since the depth of the flaw of the heater element wire 52 can be known, the disconnection time of the heater element wire 52 can be predicted, for example, in accordance with the periodic inspection of the apparatus. The advantage that the heater can be replaced is provided. In particular, by accumulating data, the prediction accuracy of the disconnection time can be improved. In addition, it is possible to avoid the heater element wire 52 from being disconnected while the product processing wafer W is being processed in the substrate processing apparatus 2.

(Third embodiment)
Next, a heater strand inspection method according to the third embodiment of the present invention will be described by taking as an example a case where the heater strand 52 of the substrate processing apparatus 2 according to the first embodiment is inspected. In the following description, FIGS. 1 to 3 will be referred to as appropriate.

  First, the experiment and the results that led to the completion of the heater wire inspection method of this embodiment will be described. In this experiment, in a part of the heater wire 52 arranged on the inner surface side of the heat insulator 50, the interval between the heater wires 52 was changed, and the transmission characteristic of the high-frequency signal in the heater wire 52 was examined. That is, a change in the interval resulting from the irreversible extension and bending of the heater wire 52 during use is generated in a pseudo manner, and the transmission characteristics of the high-frequency signal in the heater wire 52 with respect to the interval between the heater wires 52 are examined. It was.

Specifically, by bending a part of the heater wire 52, the interval between the adjacent heater wires 52 is narrowed to 6 mm, a high-frequency signal is output from the inspection power supply 58 to the heater wire 52, and the high-frequency signal is output. The frequency of the signal was changed (swept) from 100 MHz to 150 MHz. The frequency dependence of the transmission characteristics (loss) at that time was acquired by the analyzer 59. The spacing between the heater wires 52 can be adjusted using, for example, a ceramic jig.
Subsequently, the interval between the heater wires 52 was narrowed to 2 mm, 1 mm, and 0 mm, and the frequency characteristics were obtained in the same manner. An interval of 0 mm between the heater strands 52 corresponds to that the parallel portions of the heater strands 52 are in contact with each other.

  The frequency dependence of the transmission characteristics obtained in this way is shown in FIG. As shown in the figure, in this experiment as well, the transmission characteristics vary greatly depending not only on the frequency but also on the interval between the heater wires 52. Such a change is considered to occur because the impedance of the heater wire 52 changes due to a change in the interval.

  Here, paying attention to the frequency of 129.75 MHz in FIG. 5A, the relationship between the transmission characteristics at this frequency and the distance between the heater wires 52 was examined. The results are shown in FIG. 5 (b) (FIG. 5 (b) also shows results not shown in FIG. 5 (a)). As shown in the figure, a substantially linear relationship is recognized between the transmission characteristics (loss) and the interval. Therefore, by acquiring the transmission characteristic of the heater element wire 52 using a high frequency signal having a frequency of 129.75 MHz, the change in the interval between the heater element wires 52, that is, the occurrence of the extension in the heater element wire 52 and the extent of the extension. It becomes possible to know. As in the second embodiment, once a usable frequency is determined, it may be fixed to that frequency without sweeping the frequency thereafter.

As described above, in the heater element inspection method according to the third embodiment of the present invention, the frequency of the high frequency signal that depends on the interval between the heater elements 52 is obtained in advance, and the high frequency signal having that frequency is obtained as the inspection power source. 58, the high frequency signal is output from one end of the heater wire 52, the high frequency signal is input from the other end of the heater wire 52, the transmission characteristic (loss) of the high frequency signal is obtained, and the temporal change in the transmission characteristic is observed. It is possible to inspect the extension and the extent of the heater wire 52.
Note that the output timing of the high-frequency signal is as described in the second embodiment.

  As described above, according to the heater element inspection method according to the third embodiment of the present invention, the high-frequency signal is output to the heater element 52 and the transmission characteristics are acquired, whereby the heater element 52 is bent. Therefore, it is not necessary to disassemble the heating device 48 for visual inspection or assembly. Furthermore, since a high-frequency signal can be output between processes in the substrate processing apparatus 2, the heater wire 52 can be inspected without intentionally providing inspection time. The effects and advantages described in the second embodiment can also be obtained in this embodiment.

(Fourth embodiment)
Next, a heater strand inspection method according to the fourth embodiment of the present invention will be described by taking as an example a case where the heater strand 52 of the substrate processing apparatus 2 according to the first embodiment is inspected. In the following description, FIGS. 1 to 3 may be referred to. The heater wire inspection method according to the present embodiment is suitable for detecting the position of a flaw generated on the heater wire 52. Further, this heater wire inspection method may be performed independently. For example, when combined with the heater wire inspection method according to the second embodiment, only the occurrence of a flaw and the depth of the heater wire 52 are detected. Without knowing its position.

  In the present embodiment, a pulse signal having a rise time of about 11.1 ps is output from the inspection power source 58 to one end of the heater element wire 52, and the pulse signal from the other end of the heater element wire 52 is output from the inspection power source. 58. The time change of the input pulse signal is acquired by the analyzer 59, and the time change of the impedance of the heater wire 52 is obtained based on the pulse signal. FIG. 6 is a graph showing an example of the time change of the impedance of the heater wire 52. When this graph was acquired, the intentionally damaged heater element wire 52 used in the experiment in the second embodiment was used.

  Referring to FIG. 6, a small peak is observed in the impedance curve at about 0.8 ns after the output of the pulse signal (see arrow A). This peak is considered to be caused by the reflection of the pulse signal due to the scratch on the heater wire 52. Further, since a peak occurs at about 0.8 ns after the output of the pulse signal, it can be seen that there is a flaw at a position where the pulse signal has reached about 0.4 ns after the output of the pulse signal.

  As described above, in the heater element inspection method according to the present embodiment, the pulse signal is output to one end of the heater element 52 and the reflected wave is measured to obtain the time change in the impedance of the heater element 52. The position of a flaw generated in the heater wire 52 can be specified based on the peak found in the impedance curve. In addition, when the heater 48 is started to be used, it is preferable to acquire a time change in impedance when the heater wire 52 is not damaged and to identify the peak by comparison with this.

Further, since the transmitted wave changes when the reflected wave of the pulse signal is generated, the position of the flaw of the heater wire 52 can be specified by the transmitted wave. In this case, it is possible to specify with higher sensitivity than the reflected wave.
Furthermore, when the distance between the heater wires 52 changes, the impedance also changes, which may cause a reflected wave, so that it is possible to detect not only scratches but also (local) extension of the heater wires 52. It is.

Although the present invention has been described with reference to the above-described embodiments, the present invention is not limited to the disclosed embodiments, and various modifications and changes can be made within the scope of the appended claims.
For example, in the substrate processing apparatus 2 according to the first embodiment, a high frequency signal is output to the heater element wire 52 via the coil 57, but the wiring 52C and the inspection power source 58 may be connected via a capacitor. The wiring 52C and the inspection power source 58 may be connected via a predetermined switch.

Moreover, said frequency is an illustration, Comprising: The frequency of the high frequency signal used for an inspection of a heater strand is not limited to said frequency. Since the frequency to be used differs depending on the type, interval, and length of the heater element wire to be used, and the inner diameter of the heat insulator 50, it is needless to say that it should be obtained for each heating device to be used. The frequency of the high-frequency signal (frequency sweep range) may be in a range from about 300 kHz to about 500 MHz, for example.
Moreover, it is not limited to the pulse wave, but may be a square wave in a range in which the time change of the impedance of the heater wire 52 can be measured. In other words, the voltage may be maintained for a predetermined period after rising to a predetermined voltage as well as rising to a predetermined voltage and immediately falling. Needless to say, the above rise time can be appropriately set for each heating device to be used.

  Further, it may be divided into a plurality of zones (for example, four zones) along the height direction of the heating device 48, and the heater wire 52 may be wound around each zone. In this case, it is preferable to provide a power supply device 55, an inspection power supply 58, a coil 57, and an analyzer 59 for the heater wire 52 for each zone. In this case, of course, a thermocouple should be provided at a position corresponding to each zone along the inner surface of the inner tube 8 of the processing container 4.

  The heater element wire 52 is wound in a coil shape along the inner surface of the heat insulator 50 of the heating device 48. For example, a plurality of heater elements that are folded back at a predetermined length (zigzag shape) are formed in a panel shape. Also in a heating device arranged with a predetermined holder on a heat insulating material or a heating device in which a heater wire wound in a (two-dimensional) spiral shape is arranged with a predetermined holder on a disk-shaped heat insulator The heater element detection method of the present invention can be applied. Moreover, the heater strand test | inspection method by embodiment of this invention is applicable also to the heating apparatus which does not have a heat insulating body and a heater strand is hold | maintained with a predetermined holder.

  The substrate processing apparatus 2 according to the embodiment of the present invention may be not only a film forming apparatus but also a thermal oxidation apparatus or a film forming apparatus.

  DESCRIPTION OF SYMBOLS 2 ... Substrate processing apparatus, 4 ... Processing container, 6 ... Outer tube, 8 ... Inner tube, 10 ... Manifold, 12 ... Base plate, 28 ... Wafer boat, 34. ..Gas nozzle, 48 ... heating device, 50 ... insulator, 51 ... protective cover, 52 ... heater wire, 54 ... holding tool, 55 ... power supply device, 58 ...・ Power supply for inspection, 57 ... Coil, 59 ... Analyzer.

Claims (2)

Outputting a high-frequency signal having a predetermined frequency from one end of the heater wire to the heater wire;
Inputting the high-frequency signal from the other end of the heater wire;
Obtaining transmission characteristics of the heater wire based on the output high-frequency signal and the input high-frequency signal;
Detecting an abnormality of the heater wire based on a change in the transmission characteristics ;
Changing the interval between the parallel portions in which at least some of the heater wires are parallel; and
Outputting a second high frequency while changing the frequency from one end of the heater wire to the heater wire;
Inputting the second high frequency from the other end of the heater wire;
Repeating the step of determining the frequency dependence of the transmission characteristic of the heater wire based on the output second high frequency and the input second high frequency, a plurality of times;
From the relationship between the interval and the frequency dependence of the transmission characteristic, determining a frequency region in which the transmission characteristic depends on the interval;
Setting the frequency included in the frequency region to the predetermined frequency of the high-frequency signal . The heater element inspection method according to claim 1 , wherein the outputting step is performed at a time other than when the heater element is energized.