The KLA Instruments comprehensive portfolio of Zeta Optical Profiler products, software, analysis, services, and expertise is designed to help scientists, researchers, and engineers address their toughest measurement challenges by generating reliable, precise data. Through the use of these products and services, they are better able to deliver the breakthroughs that impact the expanding field of photovoltaic solar cell production.
Nano Technology Solutions is the exclusive distributor of KLA Instruments supplies, installs, and supports surface metrology products and accessories across Australia and New Zealand.
Contact us to discuss your specific requirements or objectives regarding metrology for solar cell applications.
As the solar cell industry matures and solar power gains wider adoption, it is important to employ a metrology strategy that provides critical feedback at various stages to facilitate overall yield improvement and reliability of solar cell modules in the field.
Optical profilers offer a good balance between speed and flexibility and, therefore, can be a good solution for solar cell production which includes the following steps in process control:
- Inspection of the topography of the diamond wire used for silicon wafer dicing.
- Quantification of surface roughness, wafer bow, and roll-off of the sliced wafers;
- Measurement of the post-texture height, size, and pitch of the texture pyramid structure, as well as the surface area ratio using Zeta solar texture recipes;
- Measurement of the anti-reflectance coating film thickness as well as the absolute reflectance using the Zeta ZFT technique;
- Characterisation of the laser grooving for silver printing; and
- Measurement of the height, width, area, and volume of the contact printed structures (solar finger and busbar) using Zeta solar finger recipes.
KLA’s Zeta™ 3D optical profilers are flexible and accurate tools for non-destructive and high-throughput measurement of solar cells. With Z-Dot technology, HDR capability, and Zeta software with specialised analysis algorithms, Zeta Optical Profilers can perform an automatic and comprehensive characterisation of solar cell texture, laser grooves, solar fingers, and solar busbars. The Zeta systems include solar industry-specific recipes to quantify the key geometrical features of the solar cell, allowing process and quality control engineers to optimise their fabrication processes and monitor the results to produce the most cost-effective and high-performing solar cells.
The Zeta-20 and Zeta-300 systems are widely used in solar cell production. Zeta™-Solar is a dedicated optical profiler specifically designed for the unique requirements of advanced metallisation finger and busbar solar process metrology, as well as next generation R&D development. This new Zeta model leverages advances in 3D imaging technology to simplify the imaging system and improve the cost of ownership to the highly competitive solar cell market. This optical profiler includes XY stage options to accommodate large solar cells with the 230mm x 230mm format for next generation solar process lines in addition to legacy process lines. The Zeta-Solar software has been developed to seamlessly combine easy-to-use Profilm software with the well-established metallisation measurements developed for other Zeta systems. Enhanced measurement mode analysis provides solar industry-specific metallisation measurements.
Solar Texture Analysis
Pyramid structures are etched on the wafer surface to maximise solar cell light absorption by reducing reflectance. These structures can be easily characterised using a custom Zeta recipe that automatically measures, counts, and analyses the texture, reporting the following:
- Histogram of pyramid size
- Histogram of pyramid height
- 2D and 3D True Colour images
- Statistics of pyramid size, height, count, and surface area ratio.
As part of production line metrology, by defining a Zeta measurement sequence, the measurement and analysis can be automatically repeated for multiple sites across the solar cell to check the uniformity of the product.
Laser Groove Analysis
After scanning with the ZDot technique, a 3D image of the laser groove is obtained, and the depth and width of the laser groove can be further calculated from the 2D cross-section.
The Feature Find recipe function can also be applied to the 3D image to automatically determine the edge of the laser groove and to report the depth and width, making the Zeta optical profiler a fast and non-destructive tool.
The Zeta optical profilers incorporating the patented ZDot™ technology can measure deep trenches with widths ranging from 10 μm to 200 μm and even larger.
Solar Finger Characterisation
Solar fingers are used for conducting the electrical energy generated in the texture region to the busbars, which in turn conduct the energy to the edge of the wafer to reach external circuits. They are commonly made of silver, which accounts for a significant cost in the production of a solar cell wafer. The process engineer must pay attention to the contact line height and width to optimise the geometry by minimising the amount of silver used, ensuring good conductivity at low cost.
Solar fingers also have very high reflectivity, while the adjoining nitride regions have extremely low (< 5%) reflectivity. Imaging such a surface in a single scan requires the optical metrology system to have a large dynamic range.
Zeta’s high dynamic range (HDR) imaging system enables precision 3D profiling of the highly reflective metal fingers on the very dark (non-reflective) textured surfaces. The recipe automatically calculates the individual and statistical average of the height and width of multiple cross sections, returning the following parameters:
- Average height of the solar finger
- Average width of the solar finger
- Area of the solar finger
- Volume of the solar finger
Solar Busbar Characterisation
Unlike the solar finger, which is usually < 100 μm wide, the solar busbar is a much larger structure, typically of width 1500μm. Zeta optical profilers utilise automatic image stitching technology to measure the width and height along the entire busbar.
Recently, busbars are also replaced by 1x1mm silver pads in 0BB metallisation to reduce material cost and increase the light-absorption surface area. Zeta-Solar implements an advanced measurement mode (Zeta-Solar Pad Measurement) to characterise this large region while automatically measuring the average height.
Other Analyses
The roughness of the raw silicon wafers can be measured at the beginning of the production line with Z-Dot mode and Phase Shifting Interferometry (PSI) mode to calculate both 2D and 3D roughness of unpolished silicon wafers.
The bow and radius of curvature can also be measured by the software. After the contact lines are printed on the surface, it is important to measure the bow of the wafer prior to incorporating it into the solar cell module. Given that the wafers are typically thinner than 200μm, excessive bow or warpage increases the internal stresses in the wafer and it can lead to cracking or breakage. For quick production monitoring and process development it is enough to take a quick cross-sectional measurement across the wafer.