Technology Focus: SolR for inline and roll-to-roll applications
Technology Focus:
SolR for inline and roll-to-roll applications LayTec has developed a new in-line monitoring system that is capable of measuring the properties of the layers throughout the solar cell manufacturing process: layer thickness of each layer, conductivity of transparent oxide layers, effective absorption, roughness and texture. LayTec’s contactless optical monitor SolR helps to keep the processes tightly within the specification limits by direct feedback to the growth control system and statistical process control, identify and correct anomalies, accelerate development cycles, transfer established processes to new lines and re-establish conditions after maintenance. SolR is based on specular spectroscopic reflectance measurements (500-1600 nm) and is applicable basically to all major PV thin-film structures: CIGS- and CdTe-based thin-film solar cells, a-Si/μc-Si tandem cells and anti-reflective coatings on mc-Si and c-Si solar wafers. Since light reflected from the surface and all interfaces within a layer stack interferes, the spectrum of the reflected light shows an interference pattern bearing information on the refractive index n, the index of absorption k and the thickness d of all layers so far
the production line control to assure that measurements after each deposition step relate to the same position; to overcome the scattering of rough materials, we have developed focusing optical heads with large aperture for detecting specular reflection, the heads additionally Figure 1. Optical head of the SolR installed in a roll-toroll system for investigation suppress artifacts of CIGS-based structure on foil. caused by substrate bow. deposited in the PV thin-film process. A Figure 3 shows special communicating system of metrology typical thickness measurements of the stations takes reflectance spectra after every CIGS absorber layer and CdS buffer layer single deposition step with the optical measured in the same roll-to-roll process. heads positioned in the transfer lines The thickness of the CIGS absorber layer between the deposition chambers (Figure is measured between 1900 and 2000 nm 2). with the accuracy of ± 1 % (Fig. 3a). The SolR is adaptable to virtually any thickness of the CdS buffer layer varies process system. The small optical heads can between 50 and 65 nm and is measured be installed even inside vacuum chambers; with an accuracy of ± 2 %. the control computer communicates with Apart of the layer thickness, the spectra from transparent conducting oxide layers (TCOs) like ZnO:Al or SnO2:F give information about the conductivity of the layer. The light absorption by free carriers extends to the near infrared range of the spectrum. The absorption modifies the refractive index dispersion (between 1000 nm and 1600 nm). This, in turn, causes a decrease of the infrared reflected light intensity and of the amplitude of Fabry-Perot oscillations (Figure 4). The effect increases with the concentration of free carriers in the material. Hence, based on ex-situ pre-calibration, the infrared reflectance provides on-line, real-time information about the TCO conductivity. For superstrate structures this is Figure 2. Set-up of SolR integrated into a CIGS-based production line. quite straightforward because the in-line
18 – Global Solar Technology – January 2010
www.globalsolartechnology.com