THEREOF

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to methods and apparatus of non-contact temperature measurement based on dielectric constant of materials. It relates to spectrometer and ellipsometry measurement technology.

Description of Related Art

Temperature of a material is a basic and one of the most important physical parameters to be measured in materials fabrication, treatment, characterization, and other processes. Currently, the fastest commercial thermometers, such as the Marathon IR sensor by Fluke Raytek, which are based on infrared spectrometry, can determine temperature in millisecond time frame.

However, in some situations such as micro-laser processing, where the size of the area to be measured is small (such as less than one hundred micrometer), a less than a microsecond measurement time is required if real time temperature measurement is necessary, and the temperature to be measured is low (such as one hundred degrees Celsius or lower), infrared thermometry approach is less satisfactory because the number of photons emitted from the target material is too few [Rev. Sci. Instrum., 62, 392 (1991)] .

The optical reflectivity of material has also been used for temperature determination [Nature Mater., 3, 298 (2004)] . A pump laser is employed to heat up the material and a probe laser to measure the reflectivity simultaneously. For easier operation, the sample material is deposited with an Al layer whose reflectivity as a function of temperature is well calibrated before. Since the changing ratio of reflectivity with temperature AR /R for Al and other metals is usually in the order of 10 ^"4 /K, a lock-in amplifier must be used to extract low signal from high background, causing slow time response on the order of one second.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to apparatus and related method for rapid temperature measurement that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. To achieve the above objects, the present invention provides a system for temperature measurement of a sample of a material, which includes: a stage configured to hold the sample of the material; an ellipsometry system, including a light source configured to shine a polarized light on the sample and a detection system configured to measure a polarization state of a light reflected by the sample; and a controller connected to the ellipsometry system, and programmed to operate the ellipsometry system and to receive data from the ellipsometry system, to determine an imaginary part of a dielectric constant of the material based on the measured polarization state of the light reflected by the sample, and to determine a temperature of the sample based on the imaginary part of the dielectric constant of the material and a temperature dependency relationship of the imaginary part of the dielectric constant of the material.

The system further includes a thermometer configured to measure a temperature of the sample; wherein the controller is connected to the stage and the thermometer, and is further programmed to operate the stage to successively change the temperature of the sample, to operate the thermometer to successively measure the temperature of the sample, to operate the ellipsometry system and to receive data from the ellipsometry system and the thermometer at each of the plurality of temperatures, and to determine the temperature dependency relationship of the imaginary part of the dielectric constant of the material based on the data received from the ellipsometry system and the thermometer at the plurality of temperatures.

In another aspect, the present invention provides a method for temperature measurement of a sample of a material, which includes: holding the sample of the material by a stage; heating the sample using a heating laser; using a controller to operate an ellipsometry system, which includes a light source configured to shine a polarized light on the sample and a detection system configured to measure a polarization state of a light reflected by the sample, and to receive measurement data from the ellipsometry system; using the controller to determine an imaginary part of a dielectric constant of the material based on the polarization state of the light reflected by the sample measured by the ellipsometry system; and using the controller to determine a temperature of the sample based on the imaginary part of the dielectric constant of the material and a temperature dependency relationship of the imaginary part of the dielectric constant of the material.

The method further includes, before the step of using the controller to determine the temperature of the sample: holding a sample of the same material on a heating stage and heating the sample using the heating stage successively to a plurality of different temperatures;

successively measuring the plurality of temperatures of the sample using an infrared

thermometer or a contact thermometer; simultaneously with successively measuring the plurality of temperatures, using the controller to operate the ellipsometry system and to receive measurement data from the ellipsometry system at each of the plurality of temperatures; and using the controller to determine the temperature dependency relationship of the imaginary part of the dielectric constant of the material based on the data received from the ellipsometry system and the thermometer at the plurality of temperatures.

In another aspect, the present invention provides a system for temperature measurement of a sample of a material, which includes: a stage configured to hold the sample of the material; a broadband light source configured to shine a broadband light on the sample; a spectrometer configured to measure a reflected light from the sample; and a controller connected to the spectrometer, and programmed to operate the spectrometer and to receive data from the spectrometer, to determine a reflectivity of the sample at a plasma frequency of the material based on the data received from the spectrometer, and to determine a temperature of the sample based on the reflectivity of the sample at the plasma frequency and a temperature dependency relationship of the reflectivity of the sample at the plasma frequency.

The system further includes a thermometer configured to measure a temperature of the sample; wherein the controller is connected to the stage and the thermometer, and is further programmed to operate the stage to successively change the temperature of the sample, to operate the thermometer to successively measure the temperature of the sample, to operate the spectrometer and to receive data from the spectrometer and the thermometer at each of the plurality of temperatures, and to determine the temperature dependency relationship of the reflectivity of the sample at the plasma frequency at the plurality of temperatures.

In another aspect, the present invention provides a method for temperature measurement of a sample of a material, which includes: holding the sample of the material by a stage; heating the sample using a heating laser; shining a broadband light on the sample; using a controller to operate a spectrometer to measure a reflected light from the sample and to receive measurement data received from the spectrometer; using the controller to determine a reflectivity of the sample at a plasma frequency of the material based on the data from the spectrometer; and using the controller to determine a temperature of the sample based on the reflectivity of the sample at the plasma frequency and a temperature dependency relationship of the reflectivity of the sample at the plasma frequency.

The method further includes, before the step of using the controller to determine the temperature of the sample: holding a sample of the same material on a heating stage and heating the sample using the heating stage successively to a plurality of different temperatures;

successively measuring the plurality of temperatures of the sample using an infrared

thermometer or a contact thermometer; simultaneously with successively measuring the plurality of temperatures, using the controller to operate the spectrometer and to receive measurement data from the spectrometer at each of the plurality of temperatures; and using the controller to determine the temperature dependency relationship of the reflectivity of the sample at the plasma frequency based on the data received from the spectrometer and the thermometer at the plurality of temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the reflectivity and the sensitivity of reflectivity as functions of wavelength around plasma frequency according to a Drude model simulation for Al.

Figure 2 shows the sensitivity of reflectivity as function of electron scattering rate according to a Drude model simulation in for Al.

Figure 3 is a schematic diagram of the temperature measurement apparatus of one embodiment of the present invention.

Figure 4 is a schematic diagram of a two-dimensional CCD or CMOS detectors.

Figure 5 shows the real refractive index and its sensitivity as functions of wavelength around plasma frequency according to a Drude model simulation for Al.

Figure 6 shows the temperature dependency of both real and imaginary parts of the dielectric constant and the reflectivity according to a Drude model simulation for Cu.

Figure 7 is a schematic diagram of a temperature calibration apparatus of an embodiment of the present invention.

Figure 8 is a schematic diagram of a temperature measurement apparatus of an embodiment of the present invention.

Figure 9 is a schematic diagram of integrated polarizers and detectors of an embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The dc-resistivity of metals follows a linear relation of temperature:

p _L (T) = p _a + p _b T (1).

The optical constants and dc-resistivity are related as shown in the Drude model: η(ω = , ω = ; (2).

Where, η and k are the real and imaginary part of refractive index, ε _χ and ε ₂ are the real and imaginary part of dielectric constant, ω _ρ is the plasma frequency, γ is the scattering rate of conducting electron, and R is the reflectivity. The plasma frequency ω _ρ follows

UnNe ²

ωρ = J— ( ⁵⁾ ·

Then dc-resistivity follows

mY ^4π Υ

where N is the number density of electrons, e is the electric charge, and m is the effective mass of the electron. According to equation (3) to (6), the temperature dependency of all optical constants and dc-resistivity of the material are caused by the temperature dependency of electron scattering rate γ. So according to the Drude model, the behavior of all optical constants can be obtained by tuning the electron scattering rate γ ( or in fact temperature) and wavelength.

The details of the present invention are as follows:

Part I: Method and apparatus based on reflectivity at plasma frequency

The optical properties of the material are temperature dependent. The plasma frequency is a singularity in optical properties. A thermometer based on reflectivity is usually measured at a wavelength 483 nm or 532 nm far from the plasma frequency of most materials [Nature Mater., 3, 298 (2004)]. Figure 1 shows the reflectivity and the changing ratio of reflectivity with

temperature (or sensitivity) as function of wavelength according to the simulation of Drude model of metal Al. The wavelength where the reflectivity suddenly increases is the plasma frequency. The peak in the sensitivity curve indicates the sensitivity of reflectivity is the largest at the plasma frequency, which may be about one order higher than at other wavelengths. So a thermometer based on reflectivity at plasma frequency may provide more sensitivity and faster speed.

An apparatus according to one embodiment of the present invention is shown in figure 3.

The system of a thermometer based on reflectivity at plasma frequency includes a white beam source 301, a material sample 302, a heating stage 303, a standard thermometer 304, a spectrometer 305 , a computer 307 and a heating source 306. The sample 302, heating stage 303, and thermometer 304 are in a vacuum environment. The spectrometer 305 may be one with integrated grating consisting of 10 subgratings [Optics Express, 13, 10049 (2005)] or others, which can increase the measurement speed.

The temperature measurement in this embodiment includes three steps:

Step 1: find the plasma frequency of the material with the white beam source 301, the sample 302 and the spectrograph 305 at a staring temperature. In this step, the spectrograph 305 measures the reflectivity as a function of wavelength. The plasma frequency may be easily determined based on a sudden increasement of reflectivity. In good metals, such as Al, Cu et al., the plasma frequency is usually less than 200 nm. In this case, the UV source generated by a synchrotron might be used. But for semimetals and semiconductors, such as ΓΤΟ, the plasma frequency may be in visible or infra light range. Then a general white lamp source is suitable.

Step 2: establish a temperature-reflectivity calibration curve at the plasma frequency. The relationship between temperature and the reflectivity at the plasma frequency is established on the spectrograph with a controlled hot stage.

Step 3: In-situ measurement of the reflectivity at plasma frequency using a rapid spectrometer. One kind of rapid spectrometer is introduced in Optics Express, 13, 10049 (2005) without any mechanical movement. In a general case, the measurement speed is around millisecond because of the slow reading speed of the detector. Figure 4 shows a schematic diagram of a two-dimensional CCD or CMOS detector 401 (e.g. Gpixel, GSENSE400BSI) with pixels 402. The detector has the function that allows for collection of only selected pixels, for example pixels 403. Then the collecting and reading speed may dramatically increase, up to one microsecond or even nanosecond. Step 4: Determination of the temperature of the material using the data from Step 3 and the relationship from Step 2.

The changing ratio of reflectivity with temperature is related to the plasma frequency as scattering rate γ as shown in figure 2. It concludes that the larger the dc-resistivity, the lower the sensitivity of reflectivity, which indicates that the method and apparatus described here perform well for metals, but worse for insulators.

In summary, the embodiment with method and apparatus for temperature measurement based on reflectivity at the plasma frequency improves the sensitivity by one order and increases measurement speed from second to microsecond. And it is suitable thermometer for small samples (e.g. less than 200 μιη) and rapid measurement for metals.

Part II: Method and apparatus based on dielectric constant and Ellipsometry

Figure 5 shows the real part of the refractive index and its sensitivity as a function of wavelength around the plasma frequency according to a Drude model simulation in Al. The sensitivity of the refractive index n/n is about 4x10 ^" /K when the wavelength is longer than the plasma wavelength. This sensitivity is at least one order of magnitude higher than the sensitivity of reflectivity at the same wavelength. Meanwhile, the sensitivity of the imaginary part of the dielectric constant is even larger as shown in figure 6.

According to the Drude model as well, in the visible range, the scattering probability of electrons in metals satisfies y ² « ω ² . So the real and imaginary parts of the dielectric constant can be simplified as

ω 1

ω ² Ι+Γν/ω ^" ) ² ε,- = ω ³ 1+(κ/ω) ² 2 ω ³ ' = 5 ω ³ ^ ^υ o = 5 ω - ( _i + 0 - r ) ( 8 ) where ω is the angular frequency of the incoming light, ω _ρ is the plasma frequency

Ne ²

(note that here the plasma frequency is expressed in the SI units, whereas it is m _e £ ₀

expressed in equation (5) in the cgs unit, and the two expressions are equivalent), γ =—— is

εοΡο the effective life of the electrons, p ₀ is the zero frequency resistivity of the material, pi is the residual resistivity, b is the temperature coefficient of the resistivity, T is temperature, and ε ₀ is the dielectric constant of vacuum, respectively. It can be seen that, the temperature dependency of the plasma frequency ω _ρ is weak, but the temperature dependency of the zero frequency resistivity p ₀ is strong. For metals, typically only the imaginary part of the dielectric constant ε^ω, T) is strongly temperature dependent, and the real part of the dielectric constant is not. For example, as shown in Figure 6, the temperature dependency of the imaginary part of the dielectric constant is two hundred times higher than that of real part of dielectric constant for metal copper. According to equation (8), the relationship between temperature and dielectric constan

So the temperature of the material can be rapidly determined according to equation (9) if the imaginary part of the dielectric constant is known.

Figure 6 shows that the real part, the imaginary part of the dielectric constant, and the reflectivity of metal copper change as functions of temperature. From room temperature to 1500 °C, the real part of the dielectric constant changes only by 1.8%. The reflectivity changes by about 2.5%, and both are not very sensitive to temperature. In comparison, the change of the imaginary part of the dielectric constant is as high as 500% under the same condition, about 200 times higher than that of reflectivity. This means that the imaginary part of the dielectric constant is a far better parameter to be used for temperature measurement. Although the conclusion that the sensitively of the imaginary part of the dielectric constant is much higher than reflectivity is obtained from Drude model, in fact, the principle of determining temperature by optical constants relationship ε^ω, T) works well in semimetals, semiconductors, and materials with well established dielectric-temperature relationship.

Ellipsometry is a mature technology for measurement of dielectric constant of materials. For monochromic light, the fastest measurement speed ellipsometry is on the order of milliseconds using common rotating or photoelastic (PEM) analyzer , such as RC2 from J. A. Woollam.

In practice, the correlation between temperature and the imaginary part of the dielectric constant must be determined experimentally to avoid error caused by theoretical models. As a result, the temperature measurement process according to embodiments of this invention includes three steps: Step 1: establishment of temperature-imaginary part of dielectric constant calibration curve. The relationship between temperature and the imaginary part of the dielectric constant are to be established on an EUipsometry system with a controlled hot stage;

Step 2: In-situ measurement of the dielectric constant using an EUipsometry system. In this case the measurement speed is about millisecond. And

Step 3: Determination of the temperature using the data from Step 2 and the relationship from Step 1.

The embodiment of calibration apparatus and measurement apparatus are shown in figure 7 and figure 8, respectively.

The system for calibration in figure 7 contains an ellipsometry system, a heating stage 104 on which a material sample 103 is mounted, standard thermometers (the infrared spectrometer 108 or the contact thermometer 107) and computer 109. Here, the Ellipsometry system includes a laser source 101, a polarizer 102, polarization analyzers 105, and photon detectors 106.

The calibration processes according to embodiments of current invention are as follows: (1) Using the computer 109 to operate the heating stage 104 to heat the sample 103 to a staring temperature (for example, 20 °C); operate the infrared thermometer 108 or the contact thermometer 107 to measure the sample temperature at the same time; operate the ellipsometry system including the light source 101, the polarizer 102, the polarization analyzer 105 and the detector 106 to measure the imaginary and real parts of the dielectric constant of the sample. The dielectric constant ε(ω) at the starting temperature is obtained.

(2) Increase the sample temperature at a selected interval (such as 10 °C) using the heating stage 104, and repeat step (1). Step by step, the temperature is raised from room temperature to a high temperature (such as 1000 °C). The temperature dependency of the dielectric constant ε(ω, T) is thus established.

The system for temperature measurement in figure 8 uses an ellipsometry system including a laser source 201, a polarizer 202, integrated polarization analyzers and photon detectors 206, a sample stage 204 for mounting a sample 203, a pulsed heating source 210, and computer 208. The components 201, 202, 204, 205, 206 and 208 of the system of Figure 8 may be common or shared equipment as components 101, 102, 104, 105, 106 and 109 of the system of Figure 7.

The temperature measurement processes according to embodiments of the present invention include the following steps: (1) During treatment of a material sample such as rapid heating (such as by pulsed laser heating) and quenching, operate the EUipsometry system simultaneously to measure the dielectric constant of the sample.

(2) Determine the temperature of the sample by comparing measured dielectric constants with values obtained in the calibration process.

In summary, the method and apparatus of temperature measurement of a material sample based on dielectric constant and an EUipsometry system improve the sensitivity by two orders of magnitude. Such a thermometer is well suited for small samples (e.g. less than 200 μιη) and rapid temperature, because it measures the signal from a laser source with plenty of photons. And it can be widely used for metals, semimetals, semiconductors and other materials.

Part III: Method and apparatus based on dielectric constant and rapid EUipsometry As mentioned above, the fastest measurement speed ellipsometry is on the order of milliseconds using common rotating or photoelastic (PEM) analyzer , such as RC2 from J. A. Woollam. Embodiments of the present invention uses an ellipsometry system with integrated polarization analyzer and photon detectors [OPTICS EXPRESS, 17 , 8641 (2009);

CN201710588162.5], which includes 12 or more parallel running polarizers and photon detectors to measure polarization of various angles without any mechanical movement or modulation. The schematic diagram of the polarizers and detectors system is shown in figure 9. Here the integrated polarization analyzer and detector has ten different orientation polarizers 801 with one photon detector 802 behind each polarizer 801. Such an Ellipsometry can determine the polarization of a polarized light in nanosecond.

Replacing the Ellipsometry system in part II with this rapid Ellipsometry system, a much faster thermometer can be established. The method and apparatus are similar as in part II.

In summary, the embodiment with method and apparatus based on dielectric constant and a rapid Ellipsometry system improves the sensitivity by two orders of magnitude and increases measurement speed from second to nanosecond. Such a thermometer is well suited for small samples (e.g. less than 200 μιη) and rapid temperature measurement, because it measures the signal from a laser source with plenty of photons. And it can be widely used for metals, semimetals, semiconductors and other materials.

EXEMPLARY EMBODIMENTS OF CURRENT INVENTION In practice, a method that embodies this invention includes two steps: first, calibration: the relationship between temperature and optical constants (the reflectivity at plasma frequency or the imaginary part of the dielectric constant) is established experimentally; second, measurement: determine the temperature rapidly with a rapid spectrometer or Ellipsometry system and the established optical constant-temperature relationship.

Part I: Embodiment based on reflectivity at plasma frequency

The embodiment of apparatus based on reflectivity at plasma frequency in shown in figure 3. The details of an embodiment for a thermometer based on the reflectivity at plasma frequency are as follows:

See figure 3. A pulsed light (e.g. 1064 nm laser, pulse width 100 ns) emitted from the pulsed laser 306 shines on the material sample 302 (e.g. a thin film sample). Sample 302 is heated rapidly by the laser light. Simultaneously, the white light emitted from a source 301 (e.g. Energetiq EQ-99) shines on sample 302 (e.g. ITO), which is preferably a thin film mounted on a stage 303. Both the sample and the stage are located in a vacuum environment. The reflected light from the sample is measured by a rapid spectrometer (spectrograph) 305 (infinite materials). The spectrometer 305 determines and measures the reflectivity at the plasma frequency of sample 302. The standard thermometer 304 is used during calibration process. The white beam source 301, the pulsed laser 306, the sample stage 303, the standard thermometer 304, the spectrometer 305 are connected to a computer 307 to achieve automatic control, measurement and analysis using software (e.g. programmed by Labview).

Operation of the thermometer according to this embodiment includes four steps:

Step 1: find the plasma frequency of material with the white beam source 301, the sample 302 and the spectrograph 305 at a staring temperature. In this step, the spectrograph 305 measures the reflectivity as a function of wavelength. The plasma frequency can be easily determined based on observing a sudden increase of reflectivity.

Step 2: Increase the sample temperature at a given interval using heating stage 303 and measure the temperature by a standard thermometer 304, and measure the reflectivity at the plasma frequency. The temperature is raised from room temperature to a high temperature (such as 1000 °C) at given steps (such as every 10 °C). The temperature dependency of the reflectivity R(it) _p , r) is established. Step 3: Operate the pulsed heating laser 306 and the rapid spectrometer 305 simultaneously. The rapid spectrometer 305 only collects the signal around the plasma frequency for faster speed. Determine and collect the reflectivity at the plasma frequency of sample as a function of time with microsecond time steps.

Step 4: Determine the temperature of each time point by comparing measured reflectivity with values obtained in the calibration process.

Part II: Embodiment based on dielectric constant and rapid Ellipsometry

The embodiments of calibration apparatus and measurement apparatus based on the imaginary part of the dielectric constant are shown in figure 7 and figure 8, respectively.

The details of an embodiment for calibration are as follows:

See figure 7. The light (e.g. 1064 nm laser) emitted from a laser source 101 becomes linearly polarized after passing through a polarizer 102. Then it shines on a material sample 103 (e.g. a thin film sample), which is preferably a thin film mounted on a heating stage 104. Both the sample and the heating stage are located in a vacuum environment. The light becomes elliptically polarized when it is reflected by the sample, and the polarization may be determined by a polarization analyzer 105 and a photon detector 106 (Newport). The real and imaginary parts of the dielectric constant at a given temperature are determined from the ratio of reflectance values of the p and s polarized lights. The polarization analyzer 105 is an integrated analyzer including multiple (e.g. 12) polarizers of different orientations, each corresponding to a photon detector 106. Thus, the elliptical polarization state can be rapidly measured without any mechanical rotation. Meanwhile, the given temperature of the sample can be well determined by an infrared thermometer 108 (infinite materials) or a contact thermometer 107. The heating stage 104, the photon detector 106, and the infrared spectrometer 108 or the contact thermometer 107 are connected to a computer 109 to achieve automatic control, measurement and analysis using software (e.g. programmed by Labview).

The specific calibration processes are as follows:

(1) Through computer 109 to operate the heating stage 104 to heat the sample 103 to a staring temperature (e.g. 20 °C); operate the infrared thermometer 108 or the contact thermometer 107 to measure the sample temperature; operate the Ellipsometry system including the light source 101, the polarizer 102, the polarization analyzer 105 and the detector 106 to measure the imaginary and real parts of the dielectric constant of the sample. The dielectric constant ε(ω) at the starting temperature is obtained.

(2) Increase the sample temperature at a given interval using heating stage 104, and repeat step (1). The temperature is raised from room temperature to a high temperature (such as 1000 °C) at given steps (such as every 10 °C). The temperature dependency of the dielectric constant ε(ω, T) is established.

The details of an embodiment for temperature measurement are as follows:

See figure 8. A pulsed light (e.g. 1064 nm laser, pulse width 100 ns) emitted from the pulsed laser 210 shines on the material sample 203 (e.g. a thin film sample). Sample 203 is heated rapidly by the laser light. Simultaneously, the light (e.g. 1064 nm laser) emitted from a laser source 201 becomes linearly polarized after passing through a polarizer 202. Then it shines on sample 203, which is preferably a thin film mounted on a stage 204. Both the sample and the stage are located in a vacuum environment. The light becomes elliptically polarized when it is reflected by the sample, and the polarization may be determined by a polarization analyzer 205 and a photon detector 206 (Newport). The real and imaginary parts of the dielectric constant at a given temperature are determined from the ratio of reflectance values of the p and s polarized lights. The polarization analyzer 205 is an integrated analyzer including multiple (e.g. 12) polarizers of different orientations, each corresponding to a photon detector 206. Thus, the elliptical polarization state can be rapidly measured without any mechanical rotation. The stage 204, the photon detector 206, and the heating laser 210 are connected to a computer 208 to achieve automatic control, measurement and analysis using software (e.g. programmed by Labview).

The specific measurement processes are as follows:

(1) Operate the pulsed heating laser and the rapid Ellipsometry system

simultaneously. Determine and collect the dielectric constants of the sample as a function of time with nanosecond time steps.

(2) Determine the temperature of each time point by comparing measured dielectric constants with values obtained in the calibration process.

In summary, the reflectivity at plasma frequency β(ω _ρ , Γ) and dielectric -temperature relationship ει(ω, Τ) can be established first and be used to determine temperature rapidly with embodiments of the current invention. And it is especially suitable for small samples (e.g. less than 200 μηι) and rapid heat treatment on materials. But it can be widely used for other applications as well.

It will be apparent to those skilled in the art that various modification and variations can be made in the rapid temperature measurement apparatus and related method of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents.