JP4980568B2 - Method and apparatus for in-situ monitoring of substrate temperature by emitted electromagnetic radiation - Google Patents

Description

  The present invention relates generally to substrate manufacturing techniques, and more particularly to a method and apparatus for in-situ monitoring of wafer temperature due to emission of electromagnetic radiation.

  Plasma is often used in the processing of substrates such as semiconductor wafers or glass panels used in the manufacture of flat panel displays. For example, as part of substrate processing (chemical vapor deposition, plasma enhanced chemical vapor deposition PECVD, physical vapor deposition, etc.), the substrate is divided into a plurality of dies or rectangular regions, each of which becomes an integrated circuit. Next, the substrate is processed in a series of steps, and a plurality of materials are selectively removed (etched) and deposited (grown) to form electrical components on the top surface thereof.

In the plasma treatment example, the substrate is coated with a thin film of emulsion (ie, a photoresist mask, etc.) that has been cured prior to etching. Next, the areas of the cured emulsion are selectively removed to expose portions of the lower layer. Thus, the substrate is placed in a plasma processing chamber on a substrate support structure having a monopolar or bipolar electrode called a chuck. Next, a suitable etching source gas (eg, C ₄ F ₈ , C ₄ F ₆ , CHF ₃ , CH ₂ F ₃ , CF ₄ , CH ₃ F, C _{2 is used} to etch the exposed area of the substrate. H ₄ , N ₂ , O ₂ , Ar, Xe, He, H ₂ , NH ₃ , SF ₆ , BF ₃ , Cl ₂ , etc.) flow into the processing chamber to form plasma.

  Among the set of processing parameters that can be adjusted to optimize plasma processing are gas composition, gas phase, gas flow, gas pressure, high frequency power density, voltage, magnetic field strength, and wafer temperature. is there. Theoretically, it is useful to optimize each parameter for each processing step, but it is often difficult to implement in practice.

  For example, the temperature of the substrate is important because, on the wafer surface, by changing the growth rate of a polymer film such as polymerized fluorocarbon, the selectivity of the plasma is substantially affected. Careful monitoring can minimize temperature changes, widen the window of processing for other parameters, and improve processing control. However, in practice, it is difficult to obtain the temperature directly without being affected by the plasma treatment.

  For example, in one technique, the temperature of the substrate is measured with a temperature probe. FIG. 1A is a simple cross-sectional view of a plasma processing system, in which a temperature probe is used to measure the temperature of a wafer. In order to etch an exposed region of the substrate 104 such as a semiconductor wafer or glass panel, a plasma 102 is typically formed by bombarding a suitable set of etching source gases into the process chamber 100. In general, the substrate 104 is disposed on the chuck 106. The electromagnetic radiation generated by the plasma 102 combines with the kinetic energy converted by the plasma itself, causing the substrate 104 to absorb thermal energy. In order to measure the temperature of the substrate, the probe 108 extends from the underside of the substrate 104 and contacts the substrate. However, since the probe 108 sometimes drops the wafer from the chuck, the expensive wafer is destroyed.

  Another technique is the measurement of infrared (IR) radiation from a substrate with a conventional pyrometer. In general, the heated material emits electromagnetic radiation in the IR region. In general, this region includes a wavelength range from 8 to 14 μm or a frequency range from 400 to 4000 cm −1. Here, cm-1 is well known as a wave number (1 / wavelength) and is equivalent to a frequency. The measured IR radiation can then be used to calculate the substrate temperature by using Planck's radiation law for blackbody radiation.

  FIG. 1B is a simple cross-sectional view of a plasma processing system, in which a conventional pyrometer is used to measure the temperature of the wafer. As in the case of FIG. 1A, in order to etch an exposed region of the substrate 104, a suitable set of etching source gases is typically flowed into the process chamber 100 and excited to form the plasma 102. . In general, the substrate 104 is disposed on the chuck 106. The plasma 102 generates a spectrum of electromagnetic radiation, generally some of which is IR radiation. It is such radiation (along with the kinetic energy transmitted by the plasma itself) that makes the substrate 104 absorb the thermal energy. The substrate 104 also generates IR radiation corresponding to that temperature. However, the IR radiation of the substrate 104 is generally much smaller than the IR radiation of the plasma, so the pyrometer cannot distinguish between the two IR radiations. For this reason, most of the calculated temperature is the temperature of the background plasma itself, not the temperature of the substrate.

  Yet another technique uses an interferometer to measure changes in substrate thickness due to absorbed thermal energy. In general, interferometers measure physical displacement by sensing the phase difference of an electromagnetic beam reflected between two surfaces. In a plasma processing system, an electromagnetic beam may be transmitted at a frequency at which the substrate is translucent and positioned at an angle below the substrate. The first portion of the beam is reflected from the bottom surface of the substrate, while the remaining portion is reflected from the top surface of the substrate.

  FIG. 1C is a simple cross-sectional view of a plasma processing system, in which an interferometer is used to measure the temperature of the wafer. As in the case of FIG. 1A, in order to etch an exposed region of the substrate 104, such as a semiconductor wafer or glass panel, generally an appropriate set of etching source gases are flowed into the process chamber 100 to cause shock excitation. Thus, plasma 102 is formed. In general, the substrate 104 is disposed on the chuck 106. The plasma 102 generates electromagnetic radiation, typically some of which is IR radiation. This radiation (along with the kinetic energy transmitted by the plasma itself) causes the substrate 104 to absorb the thermal energy and spread the thermal energy to the amount 118. An electromagnetic beam transmitter 108, such as a laser, transmits the beam 114 at a frequency that makes the substrate 104 translucent. A portion of the beam reflects light 114 at point 124 on the bottom surface of the substrate, while the remaining portion 116 of the beam reflects at point 122 on the top surface of the substrate. Since the same beam 112 is reflected at the two points 124 and 122, the result is that the beam 114 and the beam 116 are in different phases, but the others are the same. At this time, the interferometer 130 can determine the thickness 118 of the substrate by measuring the phase shift. By making continuous measurements, the change in thickness of the substrate is measured. However, since the change in the thickness of the substrate is only measured corresponding to the change in temperature, it is not a measurement corresponding to a specific temperature. In addition, because the transmitter is also located in the plasma processing system, it can be damaged by the plasma 112 and generates contaminants that affect manufacturing yield.

  Because of these difficulties, the temperature of the substrate will always be inferred from the thermal emissivity from the plasma processing system. Once the plasma is highly heated, several types of cooling systems are typically coupled to the chuck to maintain thermal equilibrium. That is, generally, although the temperature of the substrate is stabilized within a certain range, the exact value is often unknown. For example, when creating a set of plasma processing steps for manufacturing a given substrate, a corresponding set of process parameters or recipe is set. Since the temperature of the substrate cannot be measured directly, it is difficult to optimize the recipe. The cooling system itself usually includes a cooler that feeds coolant through the cavity and into the chuck, and helium gas is fed between the chuck and the wafer. In addition to removing the generated heat, helium gas also causes the cooling system to rapidly calibrate thermal radiation. That is, if the pressure of helium gas increases, the heat transfer coefficient also increases.

  FIG. 1D is a simple characteristic diagram of temperature with respect to time in the substrate after the plasma is highly heated. Initially, the substrate is at ambient temperature 406. When the plasma is highly heated, the substrate absorbs thermal energy in a stable period 406. After a certain period, the substrate temperature stabilizes at 410. While the stabilization period 408 continues, it is a major part of all plasma processing steps, so reducing the stabilization period 408 directly improves yield. If the temperature of the substrate can be measured directly in the plasma processing system, the cooling system can be optimized to minimize the stabilization period 408.

  Furthermore, depending on the activity of the plasma treatment, its duration, or the order in relation to other processes, different amounts of heat are generated and subsequently dissipated. As explained previously, the substrate temperature directly affects the plasma processing, so measuring the substrate temperature first and then adjusting the temperature can better optimize the plasma processing process. become.

  Furthermore, the physical structure of the processing chamber in which plasma processing is performed may change. For example, contaminants are removed from the plasma processing system by bombarding the plasma without the substrate. However, since the chuck is no longer covered by the substrate, it is etched after this. Each time the decontamination process is repeated, the surface roughness of the substrate increases and the heat transfer efficiency of the substrate varies. Finally, the cooling system will not be able to compensate for the temperature sufficiently and the recipe parameters will be invalid. In practice, it is often impossible to measure temperature when this point is reached, so in general, the chuck is replaced after a certain amount of operating time, and in fact its useful life is only negligible. Is normal. This unnecessarily replaces an expensive chuck, resulting in an increase in manufacturing cost and a reduction in yield because the plasma processing system must be taken off-line for several hours when the chuck is replaced.

  In addition, it may be necessary to adjust the parameters of the recipe, since in the manufacturing equipment otherwise the same parts may be installed at different times or used to different degrees. This is because the maintenance cycle may not necessarily coincide with the maintenance cycle of other parts. Recipe parameters need to be adjusted when the process is transferred to a new version of the plasma processing system or when the process is transferred to a plasma processing system that can handle larger substrate sizes (eg, 200 mm to 300 mm). Ideally, it is useful to maintain the same recipe parameters (eg, chemistry, power, temperature). However, since the temperature of the wafer is estimated without being directly measured, in order to carry out the same manufacturing mode, it is necessary to greatly adjust the process by trial and error.

  In view of the above, improvements in methods and apparatus for in-situ wafer temperature monitoring are desired.

  In one embodiment, the present invention relates to a method in a plasma processing system for measuring a temperature of a substrate. The method includes providing a substrate composed of a set of materials, wherein the substrate absorbs electromagnetic radiation including a first set of electromagnetic frequencies, and causes the first set of electromagnetic frequencies to be thermally vibrated. It converts to a set and transmits electromagnetic radiation including a second set of electromagnetic frequencies. The method also includes positioning a substrate on a substrate support structure having a chuck; flowing an etching gas mixture into a plasma reaction vessel of a plasma processing system; Generating a plasma having a first set of electromagnetic frequencies. Further, the method includes treating the substrate with a plasma that generates a second set of electromagnetic frequencies; calculating a magnitude of the second set of electromagnetic frequencies; and converting the magnitude to a temperature value. Includes converting.

  In another embodiment, the present invention relates to an apparatus in a plasma processing system for measuring a temperature of a substrate. The apparatus is provided with a substrate made of a set of materials that absorbs electromagnetic radiation, including a first set of electromagnetic frequencies, and causes the first set of electromagnetic frequencies to be absorbed by thermal vibrations. It converts to a set and transmits electromagnetic radiation including a second set of electromagnetic frequencies. The apparatus also includes positioning the substrate on a substrate support structure having a chuck, means for flowing an etching gas mixture into a plasma reaction vessel of the plasma processing system; and exciting the etching gas mixture to Means for generating a plasma containing a set of electromagnetic frequencies; The apparatus further includes means for treating the substrate with a plasma that generates a second set of electromagnetic frequencies; means for calculating the magnitude of the second set of electromagnetic frequencies; and the magnitude of the electromagnetic frequencies as a temperature. Means for converting to a value of.

  These and other aspects of the present invention are explained in detail in the following detailed description of the invention and the associated accompanying drawings shown below. In addition, this invention is not limited to the example of illustration, The same reference number shows the same element in the form of attached drawing.

  The invention will be described in detail with reference to a few reference embodiments, with the figures shown in the accompanying drawings. In the following description, numerous specific details are presented to provide an understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without some or all of these details. In other instances, well known process steps and / or configurations have not been described in detail in order not to unnecessarily obscure the present invention.

  The fact that the phonon can be used to monitor the temperature of the substrate in situ in a plasma processing system is the inventor's confidence as described in this document, and thus the theory It is not subject to any restrictions. In general, a phonon is a vibration of thermal energy that generates an electromagnetic wave in a substrate. Individual materials bonded within a substrate, particularly those present in a crystal structure, generally emit electromagnetic radiation at a frequency inherent to that material and are closely correlated to the total amount of thermal energy absorbed in the substrate. have. By measuring the magnitude of the radiation at a frequency characteristic of the substrate material, which is a non-trivial technique and at a frequency commonly found anywhere else in a plasma processing system, the temperature of the substrate is substantially reduced. Can be calculated in an accurate way. In one embodiment, this calculation is accomplished using Planck's radiation law for blackbody radiation and is corrected by a predetermined .radiance of the substrate.

  Multiple frequencies can preferably be used in the infrared and far infrared regions. The selected frequency must substantially correspond to the region of the spectrum where the substrate has a strong absorption coefficient. A very large number of spectral regions are used. The optimum phonon is in the region between 6 μm and 50 μm. In one embodiment, measurable radiation is generated by Si-Si vibration of 16.4 μm for a Si (silicon) substrate. In other embodiments, 9.1 μm Si—O—Si oscillations can generate monitored radiation, where interstitial oxygen is involved in atomic motion. Other spectral regions can be used utilizing the rich Si-Si, Si-O, and Si-C (substituted carbon) vibrational spectra.

  FIG. 2A is a simplified diagram of a process according to one embodiment of the present invention, in which phonons are shown. In a plasma processing system, a plasma 201 is generated to produce electromagnetic radiation 202 across the entire spectrum from the X-ray region to the microwave region. Radiation 202a passes through the substrate substantially unaffected. This is the transmitted light. An example is X-rays, most of which are in the infrared spectrum. The second part, radiation 202 b, is partially absorbed by substrate 206 and partially transmitted as radiation 212. Examples are light of the appropriate frequency for substrates with low extinction or extinction coefficients in the near infrared and infrared. Some of the absorbed light is substantially converted into thermal energy. All of the remaining portion 202c is absorbed and converted into heat energy. Subsequently, the collective thermal energy generates phonons 210 in the material bound in the lattice structure of the substrate. The phonon substantially produces a specific frequency of radiation 214 that can be measured.

  FIG. 2B is a simplified diagram of a process according to one embodiment of the invention in which the temperature of the substrate is measured. As in FIG. 2A, in the plasma processing system, plasma 201 is generated to produce electromagnetic radiation 202. Some of the absorbed radiation is substantially converted into thermal energy. This thermal energy subsequently causes phonons 210 in the material bound in the lattice structure of the substrate. The phonon substantially produces a particular frequency of radiation 214 that can be measured by the detector 212. Radiation 214 is in thermal equilibrium with the radiating substrate. The detector 212 is 1) a device that can identify the frequency (or wavelength) corresponding to the emitted electromagnetic radiation, and 2) a device that measures the intensity of the electromagnetic radiation at the frequency (or wavelength) selected by the device 1). It consists of In one embodiment, the detector 212 is a light dispersive element (eg, a multilayer dielectric interference filter) as a monochromator that is optimized to deliver radiation intensity for a band of the electromagnetic spectrum corresponding to the selected material. , Prism, grating, Fabry-Perot interferometer). In another embodiment, a suitable bandpass filter is used to select the radiation of interest. Any photoelectric device that can measure the intensity of the radiation selected by the monochromator can be used in the detector. Examples include thermal detectors (thermopiles), photoconductive detectors, and photovoltaic detectors.

  FIG. 2C is a more detailed view of FIG. 2B according to one embodiment of the present invention. As in FIG. 2A, plasma 201 is generated in the plasma processing system to generate electromagnetic radiation 202. A portion of the absorbed radiation is substantially converted to thermal energy, which in turn generates phonons in the substrate 206. Calculate the temperature of the substrate 206 by measuring the radiation 214 at a frequency corresponding to the material selected by the detector 220 (ie, 16.4 μm Si—Si, 9.1 μm Si—O—Si, etc.). it can.

  In addition, the plasma processing system 200 includes any type of cooling system coupled to the chuck to achieve thermal equilibrium. This cooling system typically includes a cooler that feeds coolant through a cavity in the chuck, and helium gas is fed between the chuck and the wafer. In addition to removing generated heat, helium gas also allows the cooling system to rapidly calibrate heat dissipation or heat radiation. That is, as the pressure of the helium gas increases, the heat transfer coefficient also increases.

  Unlike the conventional example, the temperature of the substrate 206 is maintained in a sufficiently stable manner during the plasma processing by adjusting the temperature setting of the cooler 220 and the pressure of the helium 220. In particular, during the period in which the plasma cleaning continues, the heat transfer coefficient of the chuck decreases, so that the pressure of the helium 220 decreases to compensate, thereby maintaining the substrate temperature sufficiently. This allows the chuck to be used for a sufficiently long period of time and reduces the cost of chuck replacement. Moreover, since the plasma processing system 200 operates for a much longer period until maintenance is required, the yield is further maintained or improved.

  In addition, certain plasma processing steps can optimize a narrow range of substrate temperatures, as opposed to partial optimization of the substrate temperature to a wide window. In addition, some processing steps can be replaced more easily because the processing heat remaining from the previous step is rapidly reduced.

  3A-3E illustrate an excelan HPT plasma processing system according to one embodiment of the present invention. Although an Excelan HPT plasma processing system is shown here, other plasma processing systems may be used as well, regardless of this example. The etching process is performed according to the following processing conditions.

Pressure: 50mT
Power: 1800W (2MHz) / 1200W (27MHz)
Plasma configuration: Ar: 270 sccm; C ₄ F ₈ : 25 sccm; O ₂ : 10 sccm
Temperature: 20C
Duration: 300sec
FIG. 3A illustrates signal strength with respect to time within a plasma processing system according to one embodiment of the present invention. There is no substrate during the execution of this test. In general, when plasma is bombarded, the walls of the processing chamber absorb thermal energy over time 316 and generate phonons. In this example, the generated electromagnetic radiation will be measured for 16.4 μm Si—Si. In another embodiment, the radiation generated by Si—O—Si also draws a similar figure at 9.1 μm. This shape indicates that the intensity of the electromagnetic radiation increases as the walls of the plasma processing chamber gradually heat up due to plasma activity. When the plasma is turned off at 320, the walls of the process chamber begin to cool, so the corresponding signal strength also decreases. This shape indicates that, if not handled properly, electromagnetic radiation emitted by the walls of the processing chamber interferes with substrate temperature measurement.

  FIG. 3B shows a simplified characteristic diagram of wave number versus absorbance in a plasma processing system according to one embodiment of the present invention. Three graphs are shown. A graph 324 represents the absorbance of the substrate in the 20C substrate. A graph 326 represents the absorbance of the substrate in the 70C substrate. A graph 328 represents the absorbance of the substrate in the 90C substrate. In general, the higher the substrate temperature, the greater the corresponding absorbance becomes negative. In the spectrum of IR radiation generated in the plasma processing system, a 16.4 μm first peak 330 generated by Si—Si and a 9.1 μm second peak generated by Si—O—Si. Two absorption peaks at 332 appear. The largest spectral changes are observed at two peaks at 16.4 μm and 9.1 μm. The signal intensity is most sensitive at these wavelengths. Graph 324 shows positive absorption at both 16.4 μm and 9.1 μm, indicating that electromagnetic radiation absorbs more than it radiates at these wavelengths. Graphs 326 and 328 show negative absorption at both 16.4 μm and 9.1 μm, indicating that electromagnetic radiation radiates more than it absorbs at these wavelengths. The radiation emitted by the substrate and measured by the detector is thermally balanced with respect to the substrate and is independent of the radiation emitted by the plasma and the walls of the processing chamber being processed.

  FIG. 3C shows a simple diagram of absorbance versus wavelength in two temperature ranges within a plasma processing system according to one embodiment of the present invention. Over the spectrum 340 of IR radiation generated in the 20C plasma processing system, the temperature of the substrate is similar to the amount of radiation emitted by the substrate, and thus no peak appears. However, at a substrate temperature of 90 C, there are two absorptions: a 16.4 μm first peak 330 generated by Si—Si and a 9.1 μm second peak 332 generated by Si—O—Si. The peak appears again.

  FIG. 3D shows a diagram of signal strength versus temperature within a plasma processing system according to one embodiment of the present invention. Graph 346 measures signal strength 342 for temperature 307, while graph 348 measures signal strength 342 for temperature 307. Similar to FIG. 3B, the higher the substrate temperature, the higher the corresponding signal strength.

  FIG. 3E shows a characteristic diagram of absorbance versus temperature in a plasma processing system according to one embodiment of the present invention. The first graph 330 measures the generated 16.4 μm Si—Si, while the second graph 332 measures the generated 9.1 μm Si—O—Si. The higher the temperature 307, the corresponding absorbance 305 decreases in a substantially linear fashion.

  While the invention has been described in terms of various preferred embodiments, there are alterations, substitutions, and equivalent embodiments that fall within the scope of the invention. For example, although the present invention has been described in connection with an Excelan HPT plasma processing system, other plasma processing systems can be used. It should also be noted that there are many other ways of implementing the method of the present invention.

  Advantages of the present invention include in-situ measurement of substrate temperature in a plasma processing system. Further advantages include optimizing the exchange of plasma processing structures such as chucks, improving the yield of the plasma processing itself, and from the first plasma processing system to the second plasma processing system. Includes facilitating decision and migration. Since specific embodiments and best modes have been disclosed, changes and modifications have been disclosed insofar as there are remaining embodiments within the scope and spirit of the invention as defined by the claims. Can be adapted to

FIG. 2 shows a simplified cross-sectional view of a plasma processing system including a temperature probe used to measure the wafer temperature. 1 shows a simplified cross-sectional view of a plasma processing system including a conventional pyrometer used to measure the temperature of a wafer. FIG. 3 shows a simplified cross-sectional view of a plasma processing system including an interferometer used to measure wafer temperature. Figure 3 shows a simple diagram of temperature versus time in a substrate after the plasma is heated at high temperature. FIG. 6 shows a simplified diagram illustrating phonons in a process according to one embodiment of the present invention. FIG. 4 shows a simple diagram in which the temperature of a substrate is measured in a process according to one embodiment of the invention. FIG. 2B shows a more detailed view of FIG. 2B according to one embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating measurement of phonons on a substrate in a plasma processing system according to an embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating measurement of phonons on a substrate in a plasma processing system according to an embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating measurement of phonons on a substrate in a plasma processing system according to an embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating measurement of phonons on a substrate in a plasma processing system according to an embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating measurement of phonons on a substrate in a plasma processing system according to an embodiment of the present invention.

Explanation of symbols

200: Plasma processing system 201: Plasma 202: Electromagnetic radiation 202a: Radiation 202b: Radiation 202c: Remaining radiation 206: Substrate 210: Phonon 212: Detector 214: Radiation 220: Detector 302: Signal intensity 305: Absorbance 307: Temperature 316: Time 324: Graph 326: Graph 328: Graph 330: First graph 332: Second graph

Claims (20)

Positioning the substrate on a substrate support structure;
Flowing an etching gas mixture into the plasma reaction vessel of the plasma processing system;
Generating a plasma by collisionally exciting the etching gas mixture;
Treating the substrate with plasma;
By using a detector to measure the intensity of radiation having a selected frequency corresponding to a predetermined frequency based on the material composition of the substrate absorbed by the substrate during substrate processing with the plasma, Measuring the intensity of radiation emitted from phonons generated on the substrate by the substrate treatment with the plasma;
Converting the measured intensity of radiation emitted from phonons generated in the substrate into a temperature value ;
It is characterized by having ,
A method for determining a temperature of a substrate in a plasma processing system , wherein the detector is disposed within the substrate support structure to protect the substrate from contaminants generated by the detector .   The method of claim 1, wherein the detector comprises an electromagnetic radiation measurement device.   The method of claim 2, wherein the substrate is positioned between the plasma and the electromagnetic radiation measurement device.   The method of claim 1, further comprising converting the magnitude to the temperature value using Planck's radiation law for blackbody radiation.   The method of claim 2, wherein the electromagnetic radiation measurement device comprises an interferometer.   The method according to claim 2, wherein the electromagnetic radiation measuring device comprises a monochromator.   The method according to claim 2, wherein the electromagnetic radiation measuring device comprises a grating.   The method according to claim 2, wherein the electromagnetic radiation measuring device comprises a bandpass filter.   The method of claim 1, wherein the plasma is associated with electromagnetic radiation, the electromagnetic radiation comprising an infrared spectrum.   The method of claim 1, further comprising protecting the detector from the plasma by positioning the substrate between the plasma and the detector. The method of claim 1, further comprising identifying radiation emitted from phonons generated on a substrate of the selection signal by the selected frequency using a first device.   The method of claim 11, wherein the detector includes the first device therein.   The method of claim 1, wherein the detector comprises an optical dispersive element. 2. The method of claim 1, further comprising displaying a signal intensity over time, wherein the signal intensity relates to the intensity of radiation emitted from a phonon generated on a substrate of the selection signal. the method of. Plotting in a chart a first graph showing the wave number versus absorbance of the substrate at a first temperature;
Plotting in a chart a second graph showing the wave number versus absorbance of the substrate at a second temperature;
The method of claim 1, comprising: Plotting a third graph on the chart showing the wave number against the absorbance of the substrate at a third temperature;
The method of claim 15, comprising: Plotting a first graph showing the temperature of the substrate against the first absorbance of the substrate at a first wavelength in a chart;
Plotting a second graph showing the temperature of the substrate against the second absorbance of the substrate at a second wavelength in a chart;
The method according to claim 1, wherein:   The method of claim 1, wherein the etching gas mixture includes chlorine.   The method of claim 1, wherein the etching gas mixture includes boron. The method of claim 1, wherein the selected frequency corresponds to a frequency at which the signal strength of the selected frequency is most sensitive to temperature changes in the substrate.

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