Patent Publication Number: US-2022214221-A1

Title: Phosphor Thermometry Imaging System and Control System

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a Continuation of PCT Application No. PCT/CA2020/051623 filed on Nov. 26, 2020 and claims priority to U.S. Provisional Patent Application No. 62/940,504 filed on Nov. 26, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The following relates generally to thermal imaging using phosphor thermometry, and more particularly to systems and methods for generating high resolution 2-D thermal images of phosphor-coated surfaces. 
     BACKGROUND 
     In many semiconductor manufacturing steps, wafer temperature is an important process parameter. For example, in plasma etching, small temperature variations can cause considerable changes in etching rates or critical dimension (CD) uniformity, thereby resulting in yield loss. 
     A common technique for measuring semiconductor wafer temperature is phosphor thermometry. Phosphor thermometry generally includes three steps, namely: excitation, phosphor decay, and analysis. The excitation phase involves stimulating phosphor with light from an external light source to cause luminescence of the phosphor. During the phosphor decay phase, the external light source is switched off, and the phosphor releases energy absorbed from the external light source. This release process takes place in an exponential manner with a time constant known as “decay time” which is a function of temperature. In the analysis phase, the decay time can be observed and translated into a temperature. 
     Phosphor thermometry is often carried out using contact phosphor-based temperature sensors. These sensors operate by remote, optical excitation of the phosphor and subsequent analysis of the re-emitted, temperature-dependent optical signal. A single, point-based measurement can be implemented using, for example, a fiber optic delivery system with a single photodetector. Multiple single point measurements can be used to build a temperature profile across a surface such as on a wafer chuck and thus the water itself. However, the need for physical installation of such probes can result in space constraints, and thus can limit the number of accessible measurement points on the chuck. A method of addressing this issue is to implement 2-D thermal imaging. 
     To create a 2-D temperature profile of an object, the decay time of phosphor is measured at as many points on the object surface as possible. Typically, decay times are calculated by measuring signal intensity multiple times and fitting an exponential curve to the acquired data points. Decay times can depend on the phosphor used, and can range from, for example, 2000 μs-4000 μs. Such short decay times can necessitate the use of high-speed cameras and can require considerable data processing power. In known methods, the camera, or image capturing device (ICD) is independent of the lighting system, thereby causing a number of complexities. For example, this independence can necessitate additional data processing time to determine the status of the illumination system, perform manual calibration of the illumination system and ICD with no feedback, and manual activation of the ICD. This, in turn, can result in the capture of unnecessary data which typically needs to be filtered out during data processing, further increasing processing time. 
     In view of the foregoing, an object of the following is to develop a method and system for 2-D thermal imaging of phosphor-coated objects that addresses one or more of the above-noted issues or drawbacks. 
     SUMMARY 
     The following provides a system and method for 2-D thermal imaging of phosphor coated surfaces. The system and method enable increased temperature measurement accuracy and speed of data analysis by implementing a control system that controls simultaneously an illumination system and an image capture device including a high speed camera. More particularly, the control system can control the illumination system and the camera to acquire images when emitted light intensity ranges are in a desired range to improve temperature measurement accuracy. 
     In one aspect, there is provided a method for two-dimensional (2-D) thermal imaging of a surface having phosphor thereon, the method comprising: illuminating the surface with light having an excitation intensity, to induce phosphorescence of the phosphor to generate emitted light; measuring an intensity of the emitted light; if the intensity of the emitted light is less than a pre-determined threshold intensity, repeating the illuminating operation, or increasing the excitation intensity and repeating the measuring operation; if the intensity of the emitted light is equal to or greater than the pre-determined threshold intensity, turning off the light source; capturing a plurality of images after a delay time and/or when the intensity is less than a pre-determined maximum returned intensity; calculating, from the plurality of images, a decay lifetime of the phosphor at a number of points on the surface; and translating the decay lifetime for each point into a temperature to create a 2-D thermal image of the surface. 
     In another aspect, there is provided a phosphor thermometry system for carrying out the above method. The phosphor thermometry system includes an image capture device (ICD) positioned to capture the plurality of images of the surface; a computing device configured to receive the plurality of images from the ICD and translate data from the images into a 2-D thermal image; an illumination system including at least one light source positioned to illuminate the surface; a control system connected to the illumination system and the ICD, the control system configured to determine the intensity of the emitted light by operating the camera store and compare the pre-determined threshold intensity and/or the pre-determined maximum returned intensity to the emitted light intensity provide power to the illumination system based at least on the intensity of the emitted light; and operate the ICD to capture the plurality of images. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described with reference to the appended drawings wherein: 
         FIG. 1  is a schematic diagram of a system for 2-D thermal imaging of a semiconductor wafer by light-emitting diode (LED)-induced luminescence phosphor thermometry. 
         FIG. 2A  is an example illustration of the ICD shown in  FIG. 1 . 
         FIG. 2B  is an example illustration of the ICD shown in  FIGS. 1 and 2A  but including light intensity detectors. 
         FIG. 3  is a block diagram showing a control system for simultaneously operating the ICD and the illumination system. 
         FIG. 4  is a graph showing an example embodiment of the illumination system and ICD being controlled by the control system of  FIG. 3 . 
         FIG. 5  is a basic flow chart illustrating a method for controlling the imaging system shown in  FIG. 1  with the control system shown in  FIG. 3 . 
         FIG. 6  is a flow chart illustrating a method for controlling the imaging system shown in  FIG. 1  with the control system shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The following provides a 2-D thermal imaging system for carrying out LED-induced luminescence phosphor thermometry. The system described herein includes a control system that can control an illumination system and an ICD simultaneously to provide increased accuracy and data analysis speed as compared to known systems. 
     The 2-D thermal imaging system of the present disclosure is discussed in the context of measuring semi-conductor wafer temperature; however, it can be appreciated that the system can be applied to applications that involve measuring temperature or other attributes of other surfaces coated with phosphor. For example, the phosphor composition can be tuned to be sensitive to concentrations of certain gases and to ambient pressure, and therefore such attributes can be measured without needing to physically access the wafer. Avoiding physically accessing the wafer can help to maintain the environmental conditions in the processing chamber. For example, yttrium oxide doped with europium (Y2O3:Eu) shows a strong sensitivity to oxygen concentration in the surrounding gas phase (e.g., the chamber environment). 
     Turning to  FIG. 1 , illustrated is an example embodiment of a phosphor thermometry imaging system  100  for measuring the temperature of a semiconductor wafer  22  having a phosphor coating thereon  20 . The system  100  includes a data analysis system  10 , an ICD  12 , an illumination system  16 , and a semiconductor etching process chamber  24 . The data analysis system  10  can be provided using a general purpose or specialized computing device (e.g., personal computer) and/or can include or otherwise provide other computing functionality such as a control system, calibration system, network connectivity, programming capabilities (e.g., for the ICD  12 ), etc. The ICD  12  is preferably a high-speed camera that incorporates a charge-coupled device (CCD) detector. The ICD  12  comprises a lens  14  positioned to receive, through a window  18  provided in the top of the chamber  24 , light emitted by the phosphor coating  20 , and to adjust the focus of the emitted light on a photoactive region within the ICD  12 . A filter (not shown) can be provided between the lens  14  and the window, or between the lens  14  and the ICD  12 . Such a filter can be used to filter out unwanted light, such as ambient or reflected light, and thus prevent same from reaching the detector in the ICD  12 . It can be appreciated that the inclusion of a filter can be preferable if excitation light intensity is substantially (e.g., orders of magnitude) higher than that of light emitted by the phosphor, as discussed in greater detail below. The illumination system  16  can include a number of light sources such as LEDs  26  for emitting high-intensity visible light or ultraviolet (UV) light. The LEDs  26  can emit light having wavelengths of, for example, between approximately 380 nm to approximately 450 nm. 
     It can be appreciated that other ICDs  12 , which may have different architectures and/or working mechanisms, can also be used. Additionally, other narrow band illumination systems including, but not limited to, lasers, vertical-cavity surface-emitting lasers (VCELs), and high pressure gas bulbs with notch filters can be used to illuminate the phosphor coating, 
     As shown in  FIG. 2A , the illumination system  16  includes an annular portion  17  which can include a printed circuit board (PCB) on which the LEDs  26  can be located. An aperture, passage, or hole  19  within the annular portion  17  can be adapted to receive and connect to the ICD  12 , thereby physically integrating the illumination system  16  with the ICD  12 . The location, intensity and/or output distribution pattern of light emitted from the LEDs  26  can be tuned to, for example, provide uniform illumination over the surface of the phosphor coating  20 . 
       FIG. 2B  illustrates an illumination system  116  similar to that shown in  FIG. 2A . Similar features are therefore identified with the same reference characters, but with the prefix “1” added. In addition to a number of LEDs  126 , the illumination system  116  in this example includes at least one (preferably a plurality of) light intensity detectors  127  provided on the annular portion  117 . Such detectors  127  can be sensitive mainly to the wavelengths emitted by the phosphor  20 , and can enable a control system to determine if the light emitted by the phosphor is of sufficient intensity for the ICD  12  to begin capturing images, as discussed in greater detail below. 
     Turning to  FIG. 3 , a control system  34  can be included in the system  100  to provide and receive signals to and from, respectively, the illumination system  16  and the ICD  12 . In this example, the control system  34  is shown as being separate from the analysis system  10  for ease of illustration. Optionally, the illumination system  16  can be integrated physically into the control system  34  and/or the control system  34  can include or be integrated with the data analysis system  10 . The control system  34  can include an LED driver, or drive circuit and thus can provide energy control to the illumination system  16  to, for example, tune the intensity of light emitted from the LEDs  26  as mentioned above. The control system  34  can simultaneously control the ICD  12  and illumination system  16  to capture 2-D images that accurately reflect the temperature of the phosphor coating  20 , and that require relatively short post-processing times. The LED driver can use constant current or constant voltage topologies. The control system  34  can include suitable circuitry to perform the intended operations and can be suitably interfaced with external hardware. The control system  34  can include other known elements to ensure reliable operation including, but not limited to, microprocessors, microcontrollers, FPGA, DC-DC converting elements, and current limiting and light strobe topologies. 
       FIG. 4  illustrates a graph depicting intensity of light  42  and  42   a  emitted by the phosphor coating  20 , resulting from LED light pulses  44  and  44   a,  respectively. The intensities of the LED light pulse  44   a  and resulting emitted light  42   a  are depicted solely to illustrate a second repetition of the cycle discussed with respect to  FIG. 6 , and thus are only partially shown. As shown in the graph, the LED light pulse  44  causes the phosphor coating  20  to luminesce, more particularly to phosphoresce and thereby emit light  42 . The emitted light  42  increases in intensity until the LED light pulse  44  ends (i.e., throughout the duration of the LED pulse time  40 ). At the end of the LED light pulse  44 , the emitted light  42  can be at a “threshold returned intensity”  43 , the significance of which is explained further below. After the LED pulse duration  40 , the phosphor coating  20  continues to luminesce, but with decreasing intensity. After the LED light pulse  44  ends, the ICD  12  can capture multiple images, at a high frame rate, at a trigger time  48 . The duration of the trigger time can be very short and can vary based on the frame rate of the high speed camera in the ICD  12  being used and/or the number of images desired. The time period between the end of the LED light pulse  44  and the trigger time  48  is referred to herein as a trigger delay time  38 . At the trigger time  48 , the emitted light  42  can be of a “maximum returned intensity”  45 , as further discussed below. 
     It can be appreciated that the trigger delay time can optionally have a negative value (i.e. to occur prior to the completion of the LED light pulse  44 ). Such a negative trigger delay time could be desirable if there exists an intrinsic delay in the operation of the camera or other imaging device used with the control system  34 . 
     It has been shown from experimental trials that the performance of the system  100  (e.g., measurement accuracy and data analysis speed) can depend on the intensity of the light received by the ICD  12 . Thus, a desirable threshold intensity of emitted light, or threshold returned intensity can be established for certain operating conditions (e.g., the type of thermographic phosphor used, the involved temperatures etc.). It can be appreciated that the maximum returned intensity  45  level can, in some cases, be the same as the threshold returned intensity. As discussed in greater detail below, maximum returned intensity  45  and threshold returned intensity can be programmed into the control system  34  such that temperature measurements are calculated from consistent emitted light intensity ranges. 
     Thus, the trigger time  48 , which is positive in this example, can be used to allow the emitted light  42  to fall below the maximum returned intensity  45 . After the trigger time  48 , the emitted light  42  continuously decreases in intensity until the intensity reaches zero or nearly zero and/or until the next LED pulse  44   a  begins. The time period between LED pulses  44  and  44   a  is referred to as a cycle time, or frequency of operation  50 . It can be appreciated that reaching the threshold, or minimum returned intensity can be of particular importance since low emitted light intensities can result in low signal to noise ratios, reducing temperature measurement accuracy, or preventing the ability to accurately measure at all. In the absence of a filter, timing alone could be used to prevent saturation of the detector in the ICD  12 , in which case maximum returned intensity would correspond to the saturation intensity of the detector. However, this is unlikely to occur practically since returned light intensity will generally be orders of magnitude less than the excitation light from the LEDs. 
       FIG. 5  is a flow chart illustrating a computer executable process for controlling the imaging system  100  using, for example, the control system  34 . First, at step  51 , the LEDs  26  are activated by the control system  34  (i.e., the control system  34  provides power to the illumination system  16 ). While power is provided to the illumination system  16 , the phosphor coating  20  emits light  42 . Next, at step  52 , the illumination system  16  is turned off by the control system  34  (step  52 ), and after the trigger delay time  38  (step  53 ), the ICD  12  is triggered to capture a number of images (step  54 ). This process can then be repeated. 
     Another computer executable process for operating the system  100  using, for example, the control system  34 , is shown by the flow chart in  FIG. 6 . First, at step  60 , the control system  34  provides power to the LEDs  26  to generate the LED pulse  44  having wavelengths of between approximately 380 nm to approximately 450 nm. This, in turn, causes the phosphor coating  20  to emit light  42 . Next, at step  62 , the LEDs  26  remain activated for a time period  40 . As shown in  FIG. 4 , the emitted light  42  increases in intensity throughout the time period  40 . At step,  64 , if the threshold intensity  43  of emitted light is reached, the process proceeds to step  66  and the control system  34  turns off the LEDs  26 . If not, the process returns to step  62 . Here, the control system  34  can provide the LEDs  26  with power until the threshold intensity  43  is reached. However, the control system  34  can alternatively and continuously increase the power provided to the LEDs  26  until the threshold returned intensity  43  is reached. At step  64 , by controlling the ICD  12 , the control system  34  can measure the intensity of the emitted light  42 . In particular, the control system  34  can cause the ICD  12  to take a number of images, from which the control system  34  can measure the intensity of the emitted light  42 . Alternatively, the control system  34  can measure the intensity of the emitted light  42  using intensity detectors  127  provided on the illumination system  116 , as shown in  FIG. 2B . After step  66 , the process can proceed to step  68  wherein the LEDs remain off for the trigger delay time  38 . At step  70 , the duration of the trigger delay time  38  can be determined based on whether the intensity of the emitted light  42  is below the maximum returned intensity  45 . If the intensity of the emitted light  42  is below the maximum returned intensity  45 , the process proceeds to step  72  wherein the ICD  12  captures a number of images, which are processed by the data analysis system  10  (i.e., temperature measurement begins). If not, the process returns to step  68 . The process can be repeated for N cycles, where N is an integer selected in accordance with the particular application or environment. 
     For simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the examples described herein. However, it will be understood by those of ordinary skill in the art that the examples described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the examples described herein. Also, the description is not to be considered as limiting the scope of the examples described herein. 
     It will be appreciated that the examples and corresponding diagrams used herein are for illustrative purposes only. Different configurations and terminology can be used without departing from the principles expressed herein. For instance, components and modules can be added, deleted, modified, or arranged with differing connections without departing from these principles. 
     The steps or operations in the flow charts and diagrams described herein are just for example. There may be many variations to these steps or operations without departing from the principles discussed above. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified. 
     Although the above principles have been described with reference to certain specific examples, various modifications thereof will be apparent to those skilled in the art as outlined in the appended claims.