In Situ Surface Temperature Measurement in a Conveyor Belt Furnace via Inline Infrared Thermography

Summary

This protocol describes how to install an infrared camera into a conveyor belt furnace, conduct a customer correction of a factory calibrated IR camera, and evaluate the spatial surface temperature distribution of an object of interest. The example objects are industrial silicon solar cells.

Abstract

Measuring the surface temperature of objects that are processed in conveyor belt furnaces is an important tool in process control and quality assurance. Currently, the surface temperature of objects processed in conveyor belt furnaces is typically measured via thermocouples. However, infrared (IR) thermography presents multiple advantages compared to thermocouple measurements, as it is a contactless, real-time, and spatially resolved method. Here, as a representative proof-of-concept example, an inline thermography system is successfully installed into an IR lamp powered solar firing furnace, which is used for the contact firing process of industrial Si solar cells. This protocol describes how to install an IR camera into a conveyor belt furnace, conduct a customer correction of a factory calibrated IR camera, and perform the evaluation of spatial surface temperature distribution on a target object.

Introduction

Process control and quality assurance of objects processed in conveyor belt furnaces¹ is important and accomplished by measuring the surface temperature of the object. Currently, the temperature is typically measured by a thermocouple¹. As thermocouple measurements require contact with the object, thermocouples inevitably damage the object. Therefore, it is common to choose representative samples of a batch for temperature measurements, which are not further processed since they become damaged. The measured temperatures of these damaged objects are then generalized to the remaining samples from the batch, which are further processed. Accordingly, production must be interrupted for thermocouple measurements. Furthermore, the contact is local, needs to be readjusted after each measurement, and influences the local temperature.

Infrared (IR) thermography² has a number of advantages over classic thermocouple measurements and represents a contactless, in-situ, real-time, time-saving, and spatially resolved temperature measurement method. Using this method, each sample of the batch, including those that are further processed, can be measured without interrupting production. In addition, the surface temperature distribution can be measured, which provides insight into temperature homogeneity during the process. The real-time feature allows correction of temperature settings on-the-fly. So far, the possible reasons for not using IR thermography in conveyor belt furnaces are 1) unknown optical parameters of hot objects (especially for nonmetals³) and 2) parasitic environmental radiation in the furnace (i.e., reflected radiation detected by the IR camera in addition to the emitted radiation from the object), which leads to false temperature output².

Here, as a representative proof-of-concept example of IR thermography in a conveyor belt furnace, we successfully installed an inline thermography system into an IR lamp powered solar firing furnace (Figure 1), which is used during the contact firing process of industrial Si solar cells (Figure 2A,B)^4,5. The firing process is a crucial step at the end of industrial solar cell production⁶. During this step, the contacts of the cell are formed^7,8, and surface passivation is activated⁹. To successfully achieve the latter, the time-temperature profile during the firing process (Figure 2C) must be accurately realized. Therefore, sufficient and efficient temperature control is required. This protocol describes how to install an IR camera into a conveyor belt furnace, conduct a customer correction of a factory calibrated IR camera, and evaluate the spatial surface temperature distribution of a target object.

Protocol

1. Installation of IR camera into a conveyor belt furnace

1. Decide which part of the furnace should be measured by the IR camera.
   NOTE: Here, the peak zone of the firing process is chosen (see the orange highlighted zone in the firing area of Figure 1A).
2. Define the temperature range of interest that the IR camera should detect (e.g., 700−900 °C, the typical peak temperature range of the firing process).
3. Determine, or at least estimate (through experiments or literature), the temperature, spectral, and angular dependent emissions of the object(s) of interest (e.g., silicon solar cell) to identify the wavelength range(s) of highest emission for the temperature range of interest (under a specific camera angle).
   NOTE: Here, the emission is estimated based on previous literature³ and a software called RadPro¹⁰, which calculates the spectral, angular, and temperature-dependent emissivity for materials of interest.
4. Deciding on the IR camera type
   NOTE: Here, a midwave infrared (MWIR) indium antimonide (InSb) camera (Table of Materials) is used.
   1. Choose a camera that can detect the temperature range of interest.
   2. Select a camera whose detection wavelength range matches the wavelength range of highest emission of the object of interest in the temperature range of interest.
   3. Avoid as much parasitic radiation detection by the camera as possible by avoiding objects that emit or reflect radiation into the camera field of view (e.g., IR lamps in a furnace). 
   4. Decide on the necessary spatial and temporal resolution of the camera (e.g., 640 px x 512 px and 125 Hz [full image] for the used camera here).
5. Realize a sufficient optical path from the IR camera to object (see Figure 1B).
   1. Avoid disturbing objects in the optical path (e.g., IR lamps causing direct or reflected light).
   2. Position the camera outside of the furnace chamber, if possible.
      NOTE: Most cameras have low operating temperatures (e.g., up to 50 °C). Make sure in advance that the camera position can be changed, if desired.
   3. Remove the furnace wall and isolation at the location where the optical path should be and replace the hole with an insulating IR window.
      1. Choose the appropriate material for the window that meets the following demands: 1) as transparent as possible for the detection wavelength (λ) range of the camera (e.g., quartz glass window for ~0.2 µm < λ < 3 µm, sapphire window for ~0.4 µm < λ < 4.2 µm) and 2) able to isolate the furnace chamber thermally.
         NOTE: The resulting temperatures of the window may influence the window transmission.
      2. Avoid damage of the IR window. Do not tighten the window to avoid breakage during heat expansion.
         NOTE: The window material should have a sufficient amount of space to expand when heated up.
6. Check the resulting field of view (FOV) of the IR camera by examining the thermography image via the IR camera software. Identify the targeted object and its temperature in the thermography image. Adjust the FOV, if necessary.

2. Global customer temperature correction of a fabrication calibrated IR camera

CAUTION: The fabrication of the IR camera is assumed to include a radiometric calibration.

1. Spot local optical artifacts, such as reflection and background radiation.
2. Conduct classic thermocouple measurements of the object while simultaneously recording the wafer including thermocouple with the IR camera.
   1. Check the validity of the used thermocouples. Search for known characteristic temperature points in the temperature profile of the processed object that can be clearly visibly detected (e.g., disruption in a smooth line). If the thermocouple measures these temperature points correctly, the thermocouple is most likely correctly calibrated.
   2. Example using silicon solar cells
      1. Place the thermocouple on the rear aluminum side of the wafer. Take a temperature profile for a standard firing process¹¹.
      2. Validate the thermocouples by determining whether there is a disruption in the temperature profile from step 2.2.2.1 around the Al-Si eutectic temperature of 577 °C in the form of a flatter curve (as is the case in Figure 2D).
         NOTE: If the disruption occurs at the temperature around 577 °C, it is a sign that the temperature measurement by the thermocouple is accurate. Use only validated thermocouples for the following steps.
   3. Conduct thermocouple measurements in the temperature range of interest at the same object spot (multiple times for statistical reasons), then at spatially various random spots (for statistical reasons) to obtain time-temperature profiles.
3. Determine the local uncorrected thermography object temperature underneath the thermocouples from the thermocouple measurements from step 2.2.3 while placing the thermocouple on the upper side of the object.
   1. Check for a possible local temperature drop around the contacting thermocouple (due to heat dissipation and shading). Assume the temperature in the vicinity of the thermocouple as the object temperature directly under the thermocouple, if a local temperature drop is not present.
   2. Perform the following steps if a local temperature drop is present.
      1. Determine the spatial temperature gradient of the present temperature drop in the part that is not covered by the thermocouple.
         NOTE: It is recommended to determine the gradient at multiple spots around the temperature drop (radially) and determine an average gradient.
      2. Estimate the contribution of possible optical artifacts induced by the thermocouple (example protocol for a case in which homogenous temperature along the cell depth direction is assumed, such as in Si solar cells).
         1. Place the thermocouple on the surface opposite to the measured surface and repeat the thermocouple and thermography measurement in this configuration (as shown in Figure 3A). Turn the object, including the thermocouple, around so that the thermocouple is not in the optical path between the camera and object.
            NOTE: If the gradient of the local temperature drop is the same for the thermocouple being inside and outside of the optical path (i.e., attached to the measured or opposite surface), it is a sign that the thermocouple most likely does not induce optical artifacts.
         2. Extrapolate the gradient of the temperature drop in the case of the thermocouple contacting the measured surface (i.e., inside optical path) to the area covered by the thermocouple to obtain the temperature of the object underneath the thermocouple.
         3. Repeat 2.3.2.2.2 for each measurement from step 2.2.3.
4. Alternative to 2.3: Determine the local uncorrected thermography object temperature underneath the thermocouples from the thermocouple measurements from step 2.2.3 while placing the thermocouple on the lower side of the object. To determine the local uncorrected thermography solar cell temperature under the thermocouple, extract the local temperature at the position of the thermocouple. 
   NOTE: Keeping the thermocouple on the rear side prevents the thermocouple from blocking the sight on the object by the camera. Therefore, on the one hand, the temperature correction is significantly simpler. On the other hand, thermocouples are usually not positioned on the lower side of the object during the firing process, thus might lead to operational complications, which is why this alternative needs to be carried out extra carefully. 
5. Correct the uncorrected thermography image with respect to the thermocouple measured temperatures with the data generated from steps 2.3 or 2.4.
   1. Plot the measured temperatures via thermocouples against the determined temperatures via uncorrected IR thermography. Conduct a curve fitting.
   2. Apply the obtained curve fit as a general uniform global correction formula for the uncorrected thermography image.
6. Repeat the temperature correction for each new object type or configuration, especially when the optical parameters differ.

3. Evaluation of spatial surface temperature distribution via IR thermography

NOTE: The firing conditions are assumed to be identical for this section.

1. Creation of a two-dimensional peak temperature distribution map (see Figure 4A)
   1. Write a script with an appropriate programing language to track the surface object temperature for each object surface spot along the entire camera FOV, i.e. acting as a "virtual thermocouple" placed at all object spots simultaneously.
      NOTE: Here, the script is written in MATLAB.
   2. Extract the maximum value, i.e. the peak temperature, for each object spot and plot these temperatures in a corresponding 2D distribution map.
2. Average temperature distribution in and perpendicular to the object throughput direction (see Figure 4B)
   1. In throughput direction: average the 2D temperature distribution in the dimension which is opposite to the throughput direction. What remains, is the average 1D temperature distribution in throughput direction.
   2. Perpendicular to the throughput direction: average the 2D temperature distribution in the dimension which is in throughput direction. What remains, is the average 1D temperature distribution perpendicular to the throughput direction.
      NOTE: It is recommended to leave out the last centimeter (at least) of the edge for the averaging since optical artifacts at the object edge might falsify the resulting temperature average.

Results

As shown in Figure 3B−D, the example object (here, a silicon solar cell; strictly speaking, a passivated emitter and rear cell [PERC]¹²; Figure 2A,B) can be clearly detected by the IR camera in different configurations⁴. The different configurations are monofacially metallized (Figure 3B), bifacially metallized¹³ (

Discussion

Commonly, thermography temperature is corrected via measuring and adapting the optical parameters of the object, transmissive window and path, and environmental temperature of the object and transmissive window². As an alternative method, a temperature correction technique based on thermocouple measurements is described in this protocol. For the latter method, knowledge of the parameters mentioned above is not required. For the application shown here, this method is sufficient. However, it cannot ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Datalogger incl. Thermal barrier	Datapaq Ltd.
IR thermography camera "Image IR 8300"	InfraTec GmbH
IR thermography software "IRBIS Professional 3.1"	InfraTec GmbH
Solar cells	Fraunhofer ISE
Solar firing furnace "RFS 250 Plus"	Rehm Thermal Systems GmbH
Sheath thermocouples type K	TMH GmbH
Thermocouple quartzframe	Heraeus Noblelight GmbH